Process for forming inorganic skin film

Abstract

A slide surface construction which is an inorganic skin film formed of an aggregate of Fe crystals having a body-centered cubic structure. The aggregate includes a large number of {222} oriented Fe crystals with their {222} planes (by Miller indices) oriented toward a slide surface. At least some of these {222} oriented Fe crystals are hexangular pyramid-shaped Fe crystals having six ridge lines. These Fe crystals having six ridge lines ensure that the oil retention of the slide surface construction can be improved, and a slide load acting on the slide surface can be finely divided. The inorganic skin film may be formed by a plating process such as electrolytic plating, chemical vapor deposition, or sputtering. During plating the output of a plating energy source is controlled to produce a pulsed waveform.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an inorganic skin film and, in particular, to an inorganic skin film formed of an aggregate of crystals and having at least one of a large number of pyramid-shaped metal crystals and a large number of truncated pyramid-shaped metal crystals in a surface thereof, and to a process for forming such inorganic skin film.

2. Description of the Prior Art

One example of such conventionally known inorganic skin films is an Fe-plated layer which is provided on outer peripheral surfaces of a land portion and a skirt portion of a base material of aluminum alloy, for example, of a piston for an internal combustion engine in order to provide an improved wear resistance.

However, under existing circumstances where a high speed and a high output of the internal combustion engine are desired, the prior art Fe-plated layer suffers from problems of insufficient oil-retaining property, i.e., oil retention, and poor initial conformability and thus a poor seizure resistance because the slide surface thereof is relatively smooth.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a process for forming an inorganic skin film of the type described above, which can meet the above demand.

To achieve the above object, according to the present invention, an inorganic skin film may be formed of an aggregate of crystals having a cubic crystal structure, at least some of these crystals being hexangular pyramid-shaped or truncated hexangular pyramid-shaped crystals having six ridge lines in a surface of the skin film.

The hexangular pyramid-shaped or truncated hexangular pyramid-shaped crystals, i.e., crystals having six ridge lines, are small in average grain size and substantially uniform in grain size, as compared with triangular pyramid-shaped or truncated triangular pyramid-shaped crystals, i.e., crystals having three ridge lines. In the crystals having six ridge lines, there is a correlation between the grain size and the height. Thus, the grain size is substantially uniform, which indicates that the height is also substantially equal. Moreover, two adjacent metal crystals having six ridge lines assume mutually biting states.

For example, if the inorganic skin film of metal crystals having six ridge lines is used as a slide surface construction, the slide surface thereof has an enlarged surface area, as compared with the case where the slide surface is formed of crystals having three ridge lines. In addition, the slide surface takes on a very intricate morphology having a large number of extremely fine crests, a large number of extremely fine valleys defined between the crests, and a large number of extremely fine swamps defined due to the mutual biting of the crests, and hence, the slide surface construction has an extremely good oil retention. The slide surface construction also has an enhanced initial conformability by preferential wearing of tip ends of the crystals having six ridge lines.

Further, by finely dividing the crystals having six ridge lines uniformly, a local increase in surface pressure can be avoided, and a fine division of a slide load can be achieved. Thus, the slide surface construction exhibits an excellent wear resistance not only with lubrication but also without lubrication.

The process for forming an inorganic skin film according to the present invention contemplates plating the inorganic skin film which is formed of an aggregate of crystals and including at least one of a large number of pyramid-shaped crystals and a large number of truncated pyramid-shaped crystals in a surface of the skin film, the process having the steps of: controlling the output of a plating energy source to produce a pulse waveform such that the output is increased from a minimum value to reach a maximum value and then is reduced to the minimum value; and setting a ratio T.sub.ON /T.sub.C of an output time T.sub.ON to a cycle time T.sub.C into a range of T.sub.ON /T.sub.C .ltoreq.0.45, wherein the output time T.sub.ON is from the starting point of increasing the output to a starting point of reducing the output, and the cycle time T.sub.C is a time required for one cycle which is from an output increasing start point to a next output increasing start point of the output.

If the output of the plating energy source is controlled in the above manner and the ratio T.sub.ON /T.sub.C of the output time to the cycle time is set in the above manner, it is possible to form, on a surface, an inorganic skin film including a large number of pyramid-shaped crystals which are finely divided uniformly. In addition, it is possible to set the content S of the pyramid-shaped crystals and/or the like in the surface in a range of S.gtoreq.40%.

As a result, for example, in a piston having an inorganic skin film of the type described above, the inorganic skin film exhibits an excellent slide characteristic with lubrication. If the content S is set in a range of S.gtoreq.90%, it is possible to enhance the slide characteristic of the inorganic skin film even without lubrication.

However, if the ratio T.sub.ON /T.sub.C of the output time to the cycle time is larger than 0.45, the content S of the pyramid-shaped crystals and the like is less than 40%, and the benefits of providing the crystals is lost.

The above and other objects, features and advantages of the invention will become apparent from the following detailed description of preferred embodiments, taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side view of a piston;

FIG. 2 is an enlarged sectional view taken along line 2--2 in FIG. 1;

FIG. 3 is a perspective view illustrating a body-centered cubic structure and its (hhh) plane;

FIG. 4A is a diagrammatic plan view of a portion of one example of a slide surface construction of this invention;

FIG. 4B is a plan view of a metal crystal having three ridge lines;

FIG. 5 is an enlarged sectional view taken along line 5--5 in FIG. 4A;

FIG. 6 is a diagram illustrating an inclination of the (hhh) plane in a body-centered cubic structure;

FIG. 7 is an X-ray diffraction pattern for the slide surface construction;

FIG. 8A is a photomicrograph showing a crystal structure of a slide surface of the slide surface construction;

FIG. 8B is an enlarged photomicrograph of FIG. 8A;

FIG. 9 is an X-ray diffraction pattern for a slide surface construction;

FIG. 10A is a photomicrograph showing a crystal structure of a slide surface of the slide surface construction;

FIG. 10B is an enlarged photomicrograph of FIG. 10A;

FIG. 11 is a photomicrograph showing a crystal structure of a slide surface of a slide surface construction;

FIG. 12A is a photomicrograph showing a crystal structure of a slide surface of a slide surface construction;

FIG. 12B is an enlarged photomicrograph of FIG. 12A;

FIG. 12C is a photomicrograph showing a crystal strucutre of a section of the slide surface according to FIGS. 12A and 12B;

FIG. 13A is a photomicrograph showing a crystal structure of a slide surface of a slide surface construction;

FIG. 13B is an enlarged photomicrograph of FIG. 13A;

FIG. 14 is a graph illustrating the relationship between the area rate of Fe crystals having six ridge lines and the seizure generating load;

FIG. 15 is a graph illustrating the relationship between the area rate of Fe crystals having six ridge lines and the wear amount occurring without lubrication;

FIG. 16 is a graph illustrating the relationship between the content of {222} oriented Fe crystals and the seizure generating load;

FIG. 17 is a waveform of an output from an electrolytic plating power source;

FIG. 18 is a perspective view of a metal crystal having six ridge lines;

FIG. 19 is a perspective view of the metal crystal having six ridge lines formed by a method that differs from the method used in forming the metal crystal of FIG. 18;

FIG. 20 is an X-ray diffraction pattern for a slide surface construction;

FIG. 21 is a photomicrograph showing a crystal structure of a slide surface of a slide surface construction;

FIG. 22 is an X-ray diffraction pattern for the slide surface construction;

FIG. 23 is a photomicrograph showing a crystal structure of a slide surface of a slide surface construction;

FIG. 24 is a graph illustrating the relationship between the time ratio T.sub.ON /T.sub.C and the content S of {222} oriented Fe crystals;

FIG. 25 is a graph illustrating the relationship between the concentration Fe.sup.2+ of Fe ions in a plating bath and the content S of the {222} oriented Fe crystals;

FIG. 26 is a graph illustrating the relationship between the pH of the plating bath and the content S of the {222} oriented Fe crystals;

FIG. 27 is a graph illustrating the relationship between the content S of the {222} oriented Fe crystals and the seizure generating load;

FIG. 28 is a graph illustrating the relationship between the content S of the {222} oriented Fe crystals and the wear amount;

FIG. 29 is an X-ray diffraction pattern for a slide surface construction;

FIG. 30 is a photomicrograph showing a crystal structure of a slide surface of the slide surface construction;

FIG. 31 is an X-ray diffraction pattern for a slide surface construction;

FIG. 32 is a photomicrograph showing a crystal structure of a slide surface of the slide surface construction;

FIG. 33 is a graph illustrating the relationship between the minimum current density CDmin and the seizure generating load;

FIG. 34 is a graph illustrating the relationship between the minimum current density CDmin and the wear amount;

FIG. 35 is a perspective view illustrating a face-centered cubic structure and its (3hhh) plane;

FIG. 36 is a plan view of a metal crystal having four ridge lines;

FIG. 37 is a diagram illustrating an inclination of the (3hhh) plane in a face-centered cubic structure;

FIG. 38 is an X-ray diffraction pattern for a slide surface construction;

FIG. 39 is a photomicrograph showing a crystal structure of a slide surface of the slide surface construction;

FIG. 40 is a plan view illustrating crystal planes present in slants of a metal crystal having three ridge lines;

FIG. 41 is a plan view illustrating crystal planes present in slants in one example of a metal crystal having six ridge lines;

FIG. 42 is a plan view illustrating crystal planes present in slants in another example of a metal crystal having six ridge lines; and

FIG. 43 is a plan view illustrating crystal planes present in slants in one example of a metal crystal having four ridge lines.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

First Embodiment

Referring to FIGS. 1 and 2, a piston 1 for an internal combustion engine includes a base material 2 of an aluminum alloy, and a stratified slide surface construction (inorganic skin film) 4 formed on an outer peripheral surface of each land portion 3.sub.1 and a skirt portion 3.sub.2 of the base material 2 by plating.

As shown in FIG. 3, the slide surface construction 4 is formed of an aggregate of metal crystals having a cubic crystal structure, e.g., a body-centered cubic structure (which will be referred to as a bcc structure hereinafter) in this embodiment. The aggregate includes a large number of (hhh) oriented metal crystals grown from the base material into a columnar form with their (hhh) planes (by Miller indices) oriented toward a slide surface (skin film surface) 4a for sliding on an inner wall 5 of a cylinder bore.

When the aggregate of the metal crystals having the bcc structure includes the large number of (hhh) oriented metal crystals with their (hhh) planes (by Miller indices) oriented toward the slide surface 4a, as described above, at least some of the (hhh) oriented metal crystals can be formed into hexangular pyramid-shaped (or truncated hexangular pyramid-shaped) metal crystals in the slide surface 4a, i.e., metal crystals 7.sub.1 each having six ridge lines 6, as shown in FIG. 4A. The metal crystals 7.sub.1 having six ridge lines 6 are small in average grain size and substantially uniform in grain size, as compared with a triangular pyramid-shaped (or triangular truncated pyramid-shaped) (hhh) oriented metal crystal shown in FIG. 4B, i.e., metal crystals 7.sub.2 having three ridge lines 6. There is a correlation between the grain size and the height of the (hhh) oriented metal crystals. Therefore, the substantial uniformity of the grain size means that the height is substantially equal.

Moreover, two adjacent metal crystals 7.sub.1 having six ridge lines 6 are in a mutually biting relation. Thus, the slide surface 4a has an enlarged surface area as compared with the case where the slide surface is formed of crystals having three ridge lines, and takes on an intricate morphology having a large number of extremely fine crest portions 8, a large number of extremely fine valley portions 9 provided between the crest portions 8, and a large number of extremely fine swamps 10 (see FIG. 5) provided by mutual biting of the crest portions 8. This leads to extremely good oil retention of the slide surface construction 4. In addition, the tip end of each metal crystal 7.sub.1 having six ridge lines 6 will be worn preferentially, thereby providing an improved initial conformability of the slide surface construction 4.

Further, by finely dividing the metal crystals 7.sub.1 having six ridge lines 6 uniformly, a local increase in surface pressure can be avoided, and the slide load can be finely divided. Thus, the slide surface construction 4 exhibits an excellent wear resistance not only with lubrication but also without lubrication.

As shown in FIG. 6, an inclination of the (hhh) plane with respect to a phantom plane 11 along the slide surface 4a causes an inclination of the metal crystal 7.sub.1 having six ridge lines 6 which influences the oil retention, the initial conformability and the wear resistance characteristics of the slide surface construction 4. It is preferred that inclination angle .theta. formed by the (hhh) plane with respect to the phantom plane 11 is in a range of 0.degree..ltoreq..theta..ltoreq.15.degree.. The direction of the inclination of the (hhh) plane is not limited. If the inclination angle .theta. is larger than 15.degree., the oil retention, initial conformability and wear resistance characteristics of the slide surface construction 4 are adversely effected.

The metal crystals having the bcc structure include those of simple metals such as Fe, Cr, Mo, W, Ta, Zr, Nb, V, etc., and alloys thereof.

In the plating treatment for forming the slide surface construction 4, the basic compositions and conditions for electrolytic Fe-plating are as given in Tables 1 and 2.

TABLE 1______________________________________Plating bath composition (g/liter)Ferrous Boric Ammonium Organicsulfate acid sulfate additive(s)______________________________________100 .about. 400 0 .about. 50 0 .about. 200 0 .about. 150______________________________________

The organic additives used are urea, saccharin, etc.

TABLE 2______________________________________Treating conditionsPlating Plating bath Cathode currentbath pH temperature (.degree.C.) density (A/dm.sup.2)______________________________________3 .about. 6.5 10 .about. 60 0.1 .about. 10______________________________________

A direct current or pulse current energization may be applied. If the area rate A of the metal crystals 7.sub.1 having six ridge lines 6 in the slide surface 4a is to be in a range of A.gtoreq.60% under application of the pulse current energization, the average current density CDm in a cathode is set in a range represented by 1 A/dm.sup.2 .ltoreq.CDm.ltoreq.10 A/dm.sup.2, and a ratio T.sub.ON /T.sub.C of output time T.sub.ON to cycle time T.sub.C is set in a range represented by T.sub.ON /T.sub.C .ltoreq.0.45, as described hereinafter.

In the electrolytic deposition of the Fe-plating treatment under the above-described conditions, the precipitation and content of the (hhh) oriented Fe crystals are controlled by the cathode current density or the average current density CDm, the pH of a plating bath and the like.

In addition to the electrolytic plating processes, examples of other plating treatments that may be used are PVD processes, CVD processes, sputtering processes, ion plating and the like, which are gas-phase plating processes. Conditions for W-plating or Mo-plating by sputtering include, for example, an argon pressure of 0.2 to 1 Pa, an average argon acceleration power of DC 1 to 1.5 kW, and a temperature of the base material of 150.degree. to 300.degree. C. Conditions for W-plating by a CVD process include, for example, a WF6 starting material, a gas flow rate of 2 to 15 cc/min., a pressure of 50 to 300 Pa within a chamber, a temperature of the base material of 400.degree. to 600.degree. C., and an average output power of ArF excimer laser of 5 to 40 W.

Particular examples will be described below.

A plurality of pistons 1 for internal combustion engines were produced by subjecting the outer peripheral surfaces of the land portions 3.sub.1 and the skirt portion 3.sub.2 of a base material 2 of an aluminum alloy to an electrolytic Fe-plating to form a slide surface construction 4 having an aggregate of Fe crystals.

Tables 3 and 4 show conditions for the electrolytic Fe-plating in examples 1 to 16 of pistons 1 with the slide surface constructions 4. The plating time was varied in a range of 10 to 60 minutes, so that the thickness in the examples 1 to 16 was set at 15 .mu.m. In Table 4, the term "DC" means the utilization of a direct current energization process. The same is true in subsequent Tables.

TABLE 3______________________________________Plating bath composition (g/liter)Example Ferrous Boric AmmoniumNo. sulfate acid sulfate Urea Saccharin______________________________________1 400 0 0 0 02 400 0 0 0 03 400 0 0 0 04 400 0 0 0 05 400 0 0 0 06 300 0 0 0 07 230 30 100 100 18 230 30 100 100 19 400 0 0 0 010 400 0 0 0 011 200 0 0 0 012 400 0 0 0 013 400 0 0 0 014 200 0 0 0 015 230 30 100 100 0.416 230 30 100 100 0.4______________________________________ TABLE 4______________________________________Treating conditions Temperature of CathodeExample Plating plating currentNo. bath pH bath (.degree.C.) density (A/dm.sup.2)______________________________________1 6 50 42 5.7 50 43 5.5 50 44 5 50 45 4.5 50 46 6 50 47 6 50 DC 0.28 6 50 DC 19 6 50 3.510 5.5 50 3.511 6 50 3.512 6 50 713 5.5 50 714 6 50 715 4 50 DC 516 2.7 50 DC 7______________________________________

Table 5 shows the crystal shape of the slide surface 4a, the area rate A and the average grain size d of the Fe crystals having six ridge lines, the content S of the oriented Fe crystals and the hardness of the oriented Fe crystals.

TABLE 5__________________________________________________________________________ Fe crystals having six ridge lines Crystal Average shape of Area grain Hard-Example slide rate size d Content S (%) of oriented Fe crystals nessNo. surface A (%) (.mu.m) {110} {200} {211} {310} {222} (Hv)__________________________________________________________________________1 Hexagonal 99 3 0 0.3 2.4 0 97.3 380 pyramid2 Hexagonal 96 3 0 0 4.8 0 95.2 370 pyramid3 Hexagonal 90 3 0 0 9.9 0 90.1 365 pyramid4 Hexagonal 87 3 0.5 0.2 13.9 0 85.4 348 pyramid Trigonal pyramid5 Hexagonal 70 3 0.6 3.2 17.9 1.6 76.7 336 pyramid Trigonal pyramid6 Hexagonal 60 3 5.5 0.4 28.5 0.4 65.2 331 pyramid Trigonal pyramid7 Hexagonal 30 3 16.3 1.6 29.4 2.3 50.4 328 pyramid Trigonal pyramid8 Trigonal 0 -- 32.8 1.2 20.8 2.2 43 302 pyramid9 Hexagonal 99 2 0 0.2 1.2 0 98.6 390 pyramid10 Hexagonal 90 2 0 0.3 8.7 0 91 370 pyramid11 Hexagonal 60 2 9.1 3.6 28.3 1.6 57.3 340 pyramid Trigonal pyramid12 Hexagonal 98 5 0 0 2.9 0 97.1 375 pyramid13 Hexagonal 90 5 0 0.5 9.2 0 90.3 360 pyramid14 Hexagonal 50 5 11 1.9 31.7 1.3 54.1 335 pyramid Trigonal pyramid15 Fine grain -- -- 15 27 15 13 30 290 pyramid in part16 Fine grain -- -- 16 34 10 19 21 280__________________________________________________________________________

The area rate A of the Fe crystals having six ridge lines was determined according to A=(C/B).times.100 (%), wherein B represents an area of the slide surface 4a, and C represents an area occupied by all the Fe crystals having six ridge lines in the slide surface 4a. The grain size of the Fe crystals having six ridge lines is an average value of distances between the mutually opposed corners on the opposite sides of an apex, i.e., three distances.

The content S was determined in the following manner on the basis of X-ray diffraction pattern (X-ray was applied in a direction perpendicular to the slide surface 4a) for the examples 1 to 15. By way of one example, the properties of Example No. 1 will be described in detail. FIG. 7 is an X-ray diffraction pattern for the example 1. The content S of the oriented Fe crystals was determined from the following expressions. It should be noted that, for example, the term "{110} oriented Fe crystal" means an oriented Fe crystal with its {110} plane oriented toward the slide surface 4a.

{110} oriented Fe crystals: S.sub.110 =((I.sub.110 /IA.sub.110)/T).times.100 {200} oriented Fe crystals: S.sub.200 =((I.sub.200 /IA.sub.200)/T).times.100 {211} oriented Fe crystals: S.sub.211 =((I.sub.211 /IA.sub.211)/T).times.100 {310} oriented Fe crystals: S.sub.310 =((I.sub.310 /IA.sub.310)/T).times.100 {222} oriented Fe crystals: S.sub.222 =((I.sub.222 /IA.sub.222)/T).times.100 wherein each of I.sub.110, I.sub.200, I.sub.211, I.sub.310 and I.sub.222 is a measurement (cps) of intensity of X-ray reflected from each crystal plane, and each of IA.sub.110, IA.sub.200, IA.sub.211, IA.sub.310 and IA.sub.222 is an intensity ratio of X-ray reflected from each crystal plane in an ASTM card, i.e., IA.sub.110 =100, IA.sub.200 =20, IA.sub.211 =30, IA.sub.310 =12 and IA.sub.222 =6. Further, T=(I.sub.110 /IA.sub.110)+(I.sub.200 /IA.sub.200)+(I.sub.211 /IA.sub.211)+(I.sub.310 /IA.sub.310)+(I.sub.222 /IA.sub.222).

FIGS. 8A and 8B are photomicrographs showing a crystal structure of the slide surface 4a in the example 1 (at different magnifications). In FIGS. 8A and 8B, a large number of Fe crystals having six ridge lines of a hexangular pyramid shape are observed. As shown in Table 5, the area rate A of the Fe crystals having six ridge lines is 99%, and the average grain size d thereof is 3 .mu.m. This Fe crystal having six ridge lines is a {222} oriented Fe crystal with its (hhh) plane, i.e., {222} plane oriented toward the slide surface 4a. In this case, the content S of the {222} oriented Fe crystals is 97.3%.

FIG. 9 is an X-ray diffraction pattern of the Example No. 6. FIGS. 10A and 10B are photomicrographs showing a crystal structure of the slide surface 4a in the Example No. 6 (at different magnifications). In FIGS. 10A and 10B, a large number of Fe crystals having six ridge lines and a large number of Fe crystals having three ridge lines are observed. As shown in Table 5, the area rate A of the Fe crystals having six ridge lines is 60%, and the average grain size d thereof is 3 .mu.m. In this case, the content S of the {222} oriented Fe crystals is 65.2%., as shown in Table 5 and FIG. 9. This content S is a value including Fe crystals having six ridge lines and three ridge lines.

FIG. 11 is a photomicrograph showing a crystal structure of the slide surface 4a in the Example No. 8. In FIG. 11, a large number of Fe crystals having three ridge lines are observed. The content S of the Fe crystals having three ridge lines, i.e., {222} oriented Fe crystals is 43%, as shown in Table 5.

FIGS. 12A and 12B are photomicrographs showing a crystal structure of the slide surface 4a in the Example No. 9 (at different magnifications). FIG. 12C is a photomicrograph showing a crystal structure of a section in the Example No. 9. In FIGS. 12A and 12C, a large number of Fe crystals having six ridge lines are observed. As shown in Table 5, the area rate A of the Fe crystals having six ridge lines is 99%, and the average grain size d thereof is 2 .mu.m. In this case, the content S of the {222} oriented Fe crystals is 98.6%, as shown in Table 5.

FIGS. 13A and 13B are photomicrographs showing a crystal structure of the slide surface 4a in the Example No. 12 (at different magnifications). In FIGS. 13A and 13B, a large number of Fe crystals having six ridge lines are observed. As shown in Table 5, the area rate A of the Fe crystals having six ridge lines is 98%, and the average grain size d thereof is 5 .mu.m. In this case, the content S of the {222} oriented Fe crystals is 97.1%., as shown in Table 5.

A seizure test was carried out in a chip-on-disk manner for the Example Nos. 1 to 14 and 16 to determine the relationship between the area rate A of the Fe crystals having six ridge lines and the seizure generating load, thereby providing the results shown in Table 6 and FIG. 14. Conditions for the test were as follows: the material of the disk was an Al-10% by weight of Si alloy; the rotational speed of the disk was 15 m/sec.; the amount of oil supplied was 0.3 ml/min.; and the area of the slide surface of the chip made from the slide surface construction was 1 cm.sup.2.

TABLE 6______________________________________Example No. Seizure generating load (N)______________________________________1 14102 14003 13804 11005 9206 8707 8608 8509 143010 140011 88012 117013 111014 85016 300______________________________________

FIG. 14 is a graph taken from Table 6, wherein points (1) to (14) and (16) correspond to the Example Nos. 1 to 14 and 16, respectively.

As apparent from Table 6 and FIG. 14, in the examples 1 to 6 and 9 to 13 in which the area rate A of the Fe crystals having six ridge lines is equal to or more than 60%, the slide surface 4a has a good oil retention and a good initial conformability and therefore, the seizure generating load is increased considerably, as compared with the Example Nos. 7, 8, 14 and 16. It is apparent that particularly in the Example Nos. 1 to 3, 9 and 10 in which the average grain size d of the Fe crystals having six ridge lines is set equal to or less than 3 .mu.m when the area rate A thereof is equal to or more than 90%, the seizure generating load is increased drastically, as compared with the examples 12 and 13 in which the average grain size d is set at a value larger than 3 .mu.m.

A wear test was carried out in a chip-on-disk manner without lubrication for the Example Nos. 1 to 14 to determine the relationship between the area rate A of the Fe crystals having six ridge lines and the amount of wear of the chip, thereby providing the results shown in FIG. 15. Conditions for the test were as follows: the material of the disk was an Al-10% by weight of Si alloy; the rotational speed of the disk was 0.5 m/sec.; the load was 100N; the slide distance was 1 km; and the area of the slide surface of the chip made from the slide surface construction was 1 cm.sup.2. The wear amount of the chip is a decrement (mg) per area (1 cm.sup.2) of the chip.

TABLE 7______________________________________Example No. Wear amount (mg)______________________________________1 0.82 0.83 0.94 1.15 1.36 1.47 1.58 1.59 0.710 0.811 1.312 1.013 1.114 1.5______________________________________

FIG. 15 is a graph taken from Table 7, wherein points (1) to (14) correspond to chips in the Example Nos. 1 to 14, respectively.

As apparent from Table 7 and FIG. 15, in each of the chips in the examples 1 to 6 and 9 to 13 in which the area rate A of the Fe crystals having six ridge lines is in a range represented by A.gtoreq.60%, the wear amount of the chip is smaller than those of the chips in the Example Nos. 7, 8 and 14, and hence, each of these chips has a good wear resistance even without lubrication.

FIG. 16 illustrates the relationship between the content S of the {222} oriented Fe crystals and the seizure generating load in the Example Nos. 1 to 8, 15 and 16. The seizure generating loads are as given in Table 6 for the Example Nos. 1 to 8 and 16. For the Example No. 15, it was ascertained in the seizure test under the same conditions as those described above that the seizure generating load was of 500N. Points (1) to (8), (15) and (16) in FIG. 16 correspond to the Example Nos. 1 to 8, 15 and 16, respectively.

In FIG. 16, a region with the content S of the {222} oriented Fe crystals equal to or more than 90% is a region in which the entire slide surface is formed substantially of Fe crystals having six ridge lines. A region with the content S in a range of 43%<S<90% is a region in which the entire slide surface is formed of a combination of Fe crystals having six ridge lines and Fe crystals having three ridge lines. Further, a region with the content S equal to 43% is a region in which the entire slide surface 4a is formed substantially of Fe crystals having three ridge lines, as in the Example No. 8. In a region with the content S<43%, the Fe crystals having three ridge lines are decreased and the fine grain-shaped Fe crystals are increased, with a reduction in the content S. In a region with content S.ltoreq.21%, the entire slide surface 4a is formed substantially of fine grain-shaped Fe crystals, as in the Example No. 16.

As apparent from FIG. 16, in the Example Nos. 1 to 3 in which the content S of the {222} oriented Fe crystals is equal to or more than 90%, the slide surface 4a has extremely improved oil retention and initial conformability and hence, the seizure generating load is increased drastically, as compared with the Example Nos. 4 to 7 and 8. In the Example Nos. 15 and 16, the oil retention or the like is poor and hence, the seizure generating load is reduced extremely.

Second Embodiment

In this embodiment, as in the first embodiment, a stratified slide surface construction 4 is formed by plating on outer peripheral surfaces of land portions 3.sub.1 and a skirt portion 3.sub.2 on a base material 2 of an aluminum alloy of a piston 1 for an internal combustion engine, as shown in FIGS. 1 and 2.

The slide surface construction 4 is likewise formed of an aggregate of metal crystals having a body-centered cubic structure (which will be referred to as a bcc structure). The aggregate includes a large number of (hhh) oriented metal crystals likewise grown from the base material 2 into a columnar form with their (hhh) planes (by Miller indices) oriented toward the slide surface 4a for sliding on an inner wall 5 of a cylinder bore.

In forming the slide surface construction 4, an electrolytic plating process is utilized. In this electrolytic plating process, the electric current (output) I from a plating power source (energy source) is controlled to produce a pulse waveform with a lapse of time T, so that the electric current I is increased from a minimum current value Imin to reach a maximum current value Imax and then reduced to the minimum current value Imin, as shown in FIG. 17.

If the energization time (output time) from the starting point of the increase in the current I to the starting point of the decrease in the current I is represented by T.sub.ON and the cycle time T.sub.C where one cycle is defined to be from the starting point of the previous increase to the starting point of the next increase is represented by T.sub.C, the ratio of the energization time T.sub.ON to the cycle time T.sub.C, i.e., the time ratio T.sub.ON /T.sub.C is set in a range represented by T.sub.ON /T.sub.C .ltoreq.0.45.

In this case, in the cathode, the content S of the (hhh) oriented metal crystals can be set in a range of S.gtoreq.40% by setting the minimum current value CDmin at 0 (zero) and setting the average current density CDm, for example, at a value in a range of CDm.gtoreq.2.5 A/dm.sub.2.

On the other hand, if the minimum current value CDmin is set at 0 (zero); the time ratio T.sub.ON /T.sub.C is set in a range of T.sub.ON /T.sub.C .ltoreq.0.35 and the average current density CDm is set in a range of CDm.gtoreq.3.5 A/dm.sub.2, the content S of the (hhh) oriented metal crystals can be set in a range of .gtoreq.90%.

If the content S of the (hhh) oriented metal crystals is set in the range of S.gtoreq.90% under the above-described forming conditions, then substantially the entire slide surface 4a can be likewise formed of the aggregate of metal crystals 7.sub.1 having six ridge lines 6, as shown in FIG. 4A.

If the content S of the (hhh) oriented metal crystals is in a range represented by 40%.ltoreq.S.ltoreq.90%, the slide surface 4a is formed of an aggregate of metal crystals 7.sub.1 having six ridge lines 6 and metal crystals 7.sub.2 having three ridge lines 6. In this case, the slide surface construction 4 has a slide characteristic lower than that of a slide surface construction 4 in which substantially the entire slide surface 4a is formed of the metal crystals 7.sub.1 having six ridge lines 6.

In the above-described forming process, if the minimum current density CDmin is set in a range represented by CDmin.gtoreq.0.2 A/dm.sub.2 ; the time ratio T.sub.ON /T.sub.C is set in a range represented by T.sub.ON /T.sub.C .ltoreq.0.35 and the average current density CDm is set in a range represented by CDm.gtoreq.3.5 A/dm.sup.2, metal crystals 7.sub.1 having six ridge lines 6 each having a substantially flat slant 12 can be precipitated, as shown in FIG. 18.

This metal crystal 7.sub.1 having six ridge lines 6 has a high hardness, which is effective for increasing the wear resistance of the slide surface construction 4.

On the other hand, if the minimum current density CDmin is set in a range represented by CDmin.ltoreq.-0.2 A/dm.sup.2 ; the time ratio T.sub.ON /T.sub.C is set in a range represented by T.sub.ON /T.sub.C .ltoreq.0.35 and the average current density CDm is set in a range represented by CDm.gtoreq.3.5 A/dm.sup.2, metal crystals 7.sub.1 having six ridge lines 6 each having a relatively deep valley portion 13 between adjacent ridge lines 6 can be precipitated, as shown in FIG. 19.

This metal crystal 7.sub.1 having six ridge lines 6 has a good oil retention attributable to such valley portion 13 and therefore is effective for increasing the seizure resistance of the slide surface construction 4.

The metal crystals having the bcc structures usable in this invention include those of simple metals such as Fe, Cr, Mo, W, Ta, Zr, Nb, V, etc., and the alloys thereof.

In addition to the electrolytic plating, other plating treatments that may be used are PVD processes, CVD processes, sputtering processes, ion plating and the like, which are gas-phase plating processes. When a W-plating or Mo-plating is carried out by sputtering process, the conditions are controlled to provide, for example, an argon pressure of 0.2 to 1 Pa, a temperature of the base material of 150.degree. to 300.degree. C. and an average argon acceleration power (energy source) of 1 to 1.5 kW. When a W-plating is carried out by a CVD process, conditions are controlled to provide, for example, a WF6 (a starting material) flow rate of 2 to 15 cc/min., a pressure of 50 to 300 Pa within a chamber, a temperature of the base material of 100.degree. to 400.degree. C., and an average output power of ArF excimer laser (energy source) of 5 to 40 W.

Group-A

A slide surface construction 4 having a thickness of 15 .mu.m and formed of an aggregate of Fe crystals was formed on the outer peripheral surfaces of the land portions 3.sub.1 and the skirt portion 3.sub.2 of the aluminum alloy base material 2 by the utilization of an electrolytic Fe plating process, thereby producing a piston 1 for internal combustion engines. The following various considerations were made for slide surface constructions of such pistons 1:

(a) With regard to time ratio T.sub.ON /T.sub.C :

Table 8 shows plating bath conditions for Example Nos. 1 to 24 and 50 to 54 of the slide surface constructions 4.

TABLE 8______________________________________Plating bath conditions Concentration of ferrous ConcentrationExample sulfate of Fe.sup.2+ ions Tempera-No. (g/liter) (mol/liter) pH ture (.degree.C.)______________________________________ 1 .about. 24, 400 1.4 6 5050 .about. 54______________________________________

Table 9 shows plating conditions for the Example Nos. 1 to 14. In this case, the average current density CDm was set at 7 A/dm.sup.2 for the Example Nos. 1 to 6, and at 4 A/dm.sup.2 for the Example Nos. 7 to 14, and the time ratio T.sub.ON /T.sub.C was varied while keeping the average current density constant.

TABLE 9______________________________________Treating conditions of plating Minimum Maximum Average current current current Output density density density Time timeExample CDmin CDmax CDm ratio T.sub.ONNo. (A/dm.sup.2) (A/dm.sup.2) (A/dm.sup.2) T.sub.ON /T.sub.C (msec)______________________________________1 0 140 7 0.05 22 0 35 7 0.2 23 0 23 7 0.3 24 0 17.5 7 0.4 25 0 14 7 0.5 26 0 DC DC .infin. .infin. 7 77 0 80 4 0.05 28 0 20 4 0.2 29 0 20 4 0.2 110 0 20 4 0.2 111 0 13 4 0.3 212 0 10 4 0.4 213 0 8 4 0.5 214 0 DC DC .infin. .infin. 4 4______________________________________

Table 10 shows plating conditions for the Example Nos. 15 to 24. In this case, the average current density CDm was set at 3.5 A/dm.sup.2 for the Example Nos. 15 to 19, and at 3 A/dm.sup.2 for the Example Nos. 20 to 24, and the time ratio T.sub.ON /T.sub.C was varied while keeping the average current density CDm constant.

TABLE 10______________________________________Treating conditions of plating Minimum Maximum Average current current current Output density density density Time timeExample CDmin CDmax CDm ratio T.sub.ONNo. (A/dm.sup.2) (A/dm.sup.2) (A/dm.sup.2) T.sub.ON /T.sub.C (msec)______________________________________15 0 70 3.5 0.05 216 0 17.5 3.5 0.2 217 0 11.5 3.5 0.3 218 0 8.5 3.5 0.4 219 0 7 3.5 0.5 220 0 60 3 0.05 221 0 15 3 0.2 222 0 10 3 0.3 223 0 7.5 3 0.4 224 0 6 3 0.5 2______________________________________

Table 11 shows plating conditions for Example Nos. 50 to 54. In this case, the average current density CDm was set at 2.5 A/cm.sup.2, and the time ratio T.sub.ON /T.sub.C was varied while keeping the average current density CDm constant.

TABLE 11______________________________________Treating conditions of plating Minimum Maximum Average current current current Output density density density Time timeExample CDmin CDmax CDm ratio T.sub.ONNo. (A/dm.sup.2) (A/dm.sup.2) (A/dm.sup.2) T.sub.ON /T.sub.C (msec)______________________________________50 0 50 2.5 0.05 251 0 12.5 2.5 0.2 252 0 8.5 2.5 0.3 253 0 6.5 2.5 0.4 254 0 5 2.5 0.5 2______________________________________

Table 12 shows the crystal shape of the slide surface 4a, the area rate A and average grain size d of Fe crystals having six ridge lines, the content S of the oriented Fe crystals, and hardness of the oriented Fe crystals in the Example Nos. 1 to 24 and 50 to 54, respectively.

TABLE 12__________________________________________________________________________ Fe crystals having six ridge lines Crystal Average shape of Area grain Hard-Example slide rate size d Content S (%) of oriented Fe crystals nessNo. surface A (%) (.mu.m) {110} {200} {211} {310} {222} (Hv)__________________________________________________________________________1 Hexagonal 100 5 0 0.2 1.8 0 98 370 pyramid2 Hexagonal 100 5 0 0 2.9 0 97.1 375 pyramid3 Hexagonal 98 5 0.4 0.8 2 0.3 96.5 370 pyramid Trigonal .ltoreq.1 -- pyramid4 Hexagonal 75 5 1.1 2.6 9.7 1.3 85.3 325 pyramid Trigonal 10 -- pyramid5 Granular -- -- 10.2 23.1 17 25.4 24.3 2706 Granular -- -- 13 27.7 14.8 26.2 18.3 2507 Hexagonal 97 3 0 0.9 3.2 0 95.9 385 pyramid Trigonal .ltoreq.1 -- pyramid8 Hexagonal 96 3 0.5 1.2 3.3 0.7 94.3 380 pyramid Trigonal .ltoreq.1 -- pyramid9 Hexagonal 98 3 0.6 1.2 2.1 0.4 95.7 380 pyramid Trigonal .ltoreq.1 -- pyramid10 Hexagonal 94 3 0.5 1.4 4.2 0.5 93.4 360 pyramid Trigonal .ltoreq.1 -- pyramid11 Hexagonal 94 3 0.6 1 4.4 0.5 93.5 360 pyramid Trigonal .ltoreq.1 -- pyramid12 Hexagonal 70 3 2.6 4.3 10.5 2.5 80.1 330 pyramid Trigonal 10 -- pyramid13 Granular -- -- 10.4 21.1 16.8 25.3 26.4 26014 Granular -- -- 11.2 26.4 15 27.1 20.3 25015 Hexagonal 93 2 0.6 1.5 4.6 0.6 92.7 390 pyramid Trigonal .ltoreq.1 -- pyramid16 Hexagonal 92 2 0.8 1.6 5.1 1 91.5 390 pyramid Trigonal .ltoreq.1 -- pyramid17 Hexagonal 90 2 1 2.1 5.3 1 90.6 370 pyramid Trigonal .ltoreq.1 -- pyramid18 Hexagonal 65 2 3.8 3.3 15.4 3.6 73.9 350 pyramid Trigonal 10 -- pyramid19 Granular -- -- 8.5 19.8 17.1 26 28.6 26020 Hexagonal 70 2 2.6 3 12.9 2 79.5 350 pyramid Trigonal 10 -- pyramid21 Hexagonal 65 2 2.8 4.1 13.8 2.5 76.8 330 pyramid Trigonal 10 -- pyramid22 Hexagonal 60 2 3.1 5.3 18.5 2.8 70.3 320 pyramid Trigonal 10 -- pyramid23 Hexagonal 10 3 11.9 3.5 30.2 3.3 51.1 280 pyramid Trigonal 45 -- pyramid24 Granular -- -- 8.7 14 19.8 25.2 32.3 27050 Hexagonal 50 1.5 5.7 5.9 17.3 4 67.1 340 pyramid Trigonal 15 -- pyramid51 Hexagonal 40 1.5 5.1 6.8 18.8 4.8 64.5 330 pyramid Trigonal 25 -- pyramid52 Hexagonal 10 1.5 10.2 5.4 25.3 3.8 55.3 300 pyramid Trigonal 45 -- pyramid53 Hexagonal 5 1.5 20.1 4.7 26.3 2.5 46.4 270 pyramid Trigonal 40 -- pyramid54 Granular -- -- 9.9 13.1 17.3 23.2 36.5 270__________________________________________________________________________

The area rate A of the Fe crystals having six ridge lines was determined according to A=(C/B).times.100 (%), wherein B represents an area of the slide surface 4a, and C represents an area occupied by all the Fe crystals having six ridge lines in the slide surface 4a. The grain size of the Fe crystals having six ridge lines is likewise an average value of distances between the mutually opposed corners on the opposite sides of an apex, i.e., three distances.

The content S was determined in the same manner as described above on the basis of X-ray diffraction pattern (X-ray was applied in a direction perpendicular to the slide surface 4a) for the Example Nos. 1 to 24 and 50 to 54.

FIG. 20 is an X-ray diffraction pattern for the Example No. 8. For measurements (cps) I.sub.110, I.sub.200, I.sub.211, I.sub.310 and I.sub.222 of intensity of X-ray reflected from each crystal plane, I.sub.110 =0.8K, I.sub.200 =0.4K, I.sub.211 =1.7K, I.sub.310 =0.15K and I.sub.222 =9.8K. For intensity ratios IA.sub.110, IA.sub.200, IA.sub.211, IA.sub.310 and IA.sub.222 of X-ray reflected each crystal plane in an ASTM card, IA.sub.110 =100, IA.sub.200 =20, IA.sub.211 =30, IA.sub.310 =12 and IA.sub.222 =6. Hence, T=(I.sub.110 /IA.sub.110)+(I.sub.200 /IA.sub.200)+(I.sub.211 /IA.sub.211)+(I.sub.310 /IA.sub.310)+(I.sub.222 /IA.sub.222)=1.73K.

FIG. 21 is a photomicrograph showing a crystal structure of the slide surface 4a in the Example No. 8, in which a large number of Fe crystals having six ridge lines of a hexangular pyramid shape are observed. As shown in Table 12, the area rate A of the Fe crystals having six ridge lines is 96%, and the average grain size d thereof is 3 .mu.m. This Fe crystal having six ridge lines is a {222} oriented Fe crystal with its (hhh) plane, i.e., {222} plane oriented toward the slide surface 4a. In this case, the content S of the {222} oriented Fe crystals is 94.3%, as shown in FIG. 20.

FIG. 22 is an X-ray diffraction pattern of the Example No. 14. FIG. 23 is a photomicrograph showing a crystal structure of the slide surface 4a in the Example No. 14, in which a large number of granular Fe crystals are observed.

FIG. 24 shows the relationship between the time ratio T.sub.ON /T.sub.C and the content S of the {222} oriented Fe crystals in the Example Nos. 1 to 24 and 50 to 54. In FIG. 24, points (1) to (24) and (50) to (54) correspond to the Example Nos. 1 to 24 and 50 to 54. As apparent from FIG. 24, the content S of the {222} oriented Fe crystals can be provided in a range represented by S.gtoreq.40%, at respective average current density CDm by setting the time ratio T.sub.ON /T.sub.C in a range represented by T.sub.ON /T.sub.C .ltoreq.0.45.

In this case, if the average current density CDm is set in a range of CDm.gtoreq.3.5 A/dm.sup.2 and the time ratio T.sub.ON /T.sub.C in a range T.sub.ON l/T.sub.C .ltoreq.0.35, the content S of the {222} oriented Fe crystals is in a range of S.gtoreq.90%. In a region of 90%.ltoreq.S.ltoreq.100% and 0.ltoreq.T.sub.ON /T.sub.C <0.35 in FIG. 24, most of the {222} oriented Fe crystals are in the form of Fe crystals having six ridge lines.

On the other hand, a region in FIG. 24 obtained by eliminating a region in which the Fe crystals having six ridge lines are present alone from a region of S.gtoreq.40% and T.sub.ON /T.sub.C .ltoreq.0.45 is a region in which both the Fe crystals having six ridge lines and the Fe crystals having three ridge lines are present.

(b) With regard to concentration (Fe.sup.2+) of Fe ions in the plating bath:

Table 13 shows plating conditions for the Example Nos. 25 to 36 of the slide surface constructions 4.

TABLE 13______________________________________Plating bath conditions Concentration of ferrous ConcentrationExample sulfate of Fe.sup.2+ ions Tempera-No. (g/liter) (mol/liter) pH ture (.degree.C.)______________________________________25 300 1.05 6 5026 200 0.7 6 5027 100 0.35 6 5028 500 1.75 6 5029 300 1.05 6 5030 200 0.7 6 5031 100 0.35 6 5032 500 1.75 6 5033 300 1.05 6 5034 200 0.7 6 5035 300 1.05 6 5036 200 0.7 6 50______________________________________

Table 14 shows plating conditions for the Example Nos. 25 to 36. In this case, the average current density CDm was set at 7 A/dm.sup.2 in the Example Nos. 25 to 27, at 4 A/dm.sup.2 in the Example Nos. 28 to 31, at 3.5 A/dm.sup.2 in the Example Nos. 32 to 34 and at 3 A/dm.sup.2 in the Example Nos. 35 and 36. The concentration of Fe ions (Fe.sup.2+) was varied while keeping the average current density CDm as shown in Table 13 constant.

TABLE 14______________________________________Treating conditions of plating Minimum Maximum Average current current current Output density density density Time timeExample CDmin CDmax CDm ratio T.sub.ONNo. (A/dm.sup.2) (A/dm.sup.2) (A/dm.sup.2) T.sub.ON /T.sub.C (msec)______________________________________25 0 35 7 0.2 226 0 35 7 0.2 227 0 35 7 0.2 228 0 20 4 0.2 229 0 20 4 0.2 230 0 20 4 0.2 231 0 20 4 0.2 232 0 17.5 3.5 0.2 233 0 17.5 3.5 0.2 234 0 17.5 3.5 0.2 235 0 15 3 0.2 236 0 15 3 0.2 2______________________________________

Table 15 shows the crystal shape of the slide surface 4a, the area rate A and average grain size d of the Fe crystals having six ridge lines, the content S of the oriented Fe crystals, and the hardness of the oriented Fe crystals for the Example Nos. 25 to 36, respectively.

TABLE 15__________________________________________________________________________ Fe crystals having six ridge lines Crystal Average shape of Area grain Hard-Example slide rate size d Content S (%) of oriented Fe crystals nessNo. surface A (%) (.mu.m) {110} {200} {211} {310} {222} (Hv)__________________________________________________________________________25 Hexagonal 96 5 0.8 1.3 2 0.5 95.4 365 pyramid Trigonal .ltoreq.1 -- pyramid26 Hexagonal 65 5 3.1 4.8 15.2 2.2 74.7 280 pyramid Trigonal 10 -- pyramid27 Hexagonal 10 5 10.4 3.6 26.7 2.7 56.6 210 pyramid Trigonal 45 -- pyramid28 Hexagonal 95 3 0.7 1.4 2.3 0.7 49.9 380 pyramid Trigonal .ltoreq.1 -- pyramid29 Hexagonal 92 3 1.5 2.3 3.2 0.9 92.1 365 pyramid Trigonal .ltoreq.1 -- pyramid30 Hexagonal 60 3 5.8 3.6 19.1 2.5 69 300 pyramid Trigonal 10 -- pyramid31 Trigonal 45 -- 20.8 3.5 28.8 2.4 44.5 270 pyramid32 Hexagonal 92 2 1.5 2 3.5 1 92 390 pyramid Trigonal .ltoreq.1 -- pyramid33 Hexagonal 90 2 1.8 2.4 4.6 1 90.2 380 pyramid Trigonal .ltoreq.1 -- pyramid34 Hexagonal 40 2 11.3 3.8 22.2 2 60.7 320 pyramid Trigonal 20 -- pyramid35 Hexagonal 60 2 6 3.9 17.6 2.6 69.9 300 pyramid Trigonal 10 -- pyramid36 Trigonal 40 -- 17.5 9.7 20.3 11.8 40.7 275 pyramid__________________________________________________________________________

FIG. 25 shows the relationship between the concentration of Fe ions (Fe.sup.2+) in the plating bath and the content S of the {222} oriented Fe crystals in the Example Nos. 25 to 36 and the above-described Example Nos. 2, 8, 16 and 21. In FIG. 25, points (2), (8), (16), (21), (25) to (36) correspond to the Example Nos. 2, 8, 28, 21, 25 to 36. As apparent from FIG. 25, the content S of the {222} oriented Fe crystals can be set in a range of S.gtoreq.40% by setting the concentration of Fe.sup.2+ ions in a range of Fe.sup.2+ .gtoreq.0.6, for example, at an average current density Cdm.gtoreq.A/dm.sup.2. In order to set the content S of the {222} oriented Fe crystals in a range of S.gtoreq.90%, it is necessary to set the concentration of Fe.sup.2+ ions in a ranges of Fe.sup.2+ .gtoreq.1 at an average current density Cdm.gtoreq.3.5 A/dm.sup.2.

(c) With regard to pH of plating bath:

Table 16 shows plating conditions for the Example Nos. 37 to 47 of the slide surface constructions 4.

TABLE 16______________________________________Plating bath conditions Concentration of ferrous ConcentrationExample sulfate of Fe.sup.2+ ions Tempera-No. (g/liter) (mol/liter) pH ture (.degree.C.)______________________________________37 400 1.4 5.5 5038 400 1.4 5 5039 400 1.4 4 5040 400 1.4 7 5041 400 1.4 5.5 5042 400 1.4 5 5043 400 1.4 7 5044 400 1.4 5.5 5045 400 1.4 5 5046 400 1.4 5.5 5047 400 1.4 5 50______________________________________

Table 17 shows plating conditions for the Example Nos. 37 to 47. In this case, the average current density CDm was set at 7 A/dm.sup.2 for the Example Nos. 37 to 39, at 4 A/dm.sup.2 in the Example Nos. 40 to 42, at 3.5 A/dm.sup.2 for the Example Nos. 43 to 45 and at 3 A/dm.sup.2 for the Example Nos. 46 and 47. The pH value was varied while keeping the average current density CDm constant as shown in Table 16.

TABLE 17______________________________________Treating conditions of plating Minimum Maximum Average current current current Output density density density Time timeExample CDmin CDmax CDm ratio T.sub.ONNo. (A/dm.sup.2) (A/dm.sup.2) (A/dm.sup.2) T.sub.ON /T.sub.C (msec)______________________________________37 0 35 7 0.2 238 0 35 7 0.2 239 0 35 7 0.2 240 0 20 4 0.2 241 0 20 4 0.2 242 0 20 4 0.2 243 0 17.5 3.5 0.2 244 0 17.5 3.5 0.2 245 0 17.5 3.5 0.2 246 0 15 3 0.2 247 0 15 3 0.2 2______________________________________

Table 18 shows the crystal shape of the slide surface 4a, the area rate A and average grain size d of the Fe crystals having six ridge lines, the content S of the oriented Fe crystals, and the hardness of the oriented Fe crystals in the Example Nos. 37 to 47, respectively.

TABLE 18__________________________________________________________________________ Fe crystals having six ridge lines Crystal Average shape of Area grain Hard-Example slide rate size d Content S (%) of oriented Fe crystals nessNo. surface A (%) (.mu.m) {110} {200} {211} {310} {222} (Hv)__________________________________________________________________________37 Hexagonal 96 5 1.2 0.5 2.2 0.3 95.8 370 pyramid Trigonal .ltoreq.1 -- pyramid38 Hexagonal 80 5 1.2 1.1 8.3 1.4 88 330 pyramid Trigonal 10 -- pyramid39 Hexagonal 5 5 11.1 2.8 31.5 3.5 51.1 280 pyramid Trigonal 45 -- pyramid40 Hexagonal 95 3 0.5 1.1 3 0.3 95.1 380 pyramid Trigonal .ltoreq.1 -- pyramid41 Hexagonal 93 3 0.8 1.2 4.5 0.8 92.7 370 pyramid Trigonal .ltoreq.1 -- pyramid42 Hexagonal 80 3 0.5 0.2 13.9 0 85.4 348 pyramid Trigonal 5 -- pyramid43 Hexagonal 92 2 1.0 1.5 4.6 0.8 92.1 390 pyramid Trigonal .ltoreq.1 -- pyramid44 Hexagonal 90 2 1.7 2.6 4.2 1.5 90 380 pyramid Trigonal .ltoreq.1 -- pyramid45 Hexagonal 70 2 3 4.1 10.7 2.7 79.5 360 pyramid Trigonal 10 -- pyramid46 Hexagonal 60 2 3.8 5.6 17.3 3 70.3 300 pyramid Trigonal 10 -- pyramid47 Hexagonal 40 2 5 6.8 18.6 4.6 65 295 pyramid Trigonal 25 -- pyramid__________________________________________________________________________

FIG. 26 shows the relationship between the pH value of the plating bath and the content S of the {222} oriented Fe crystals in the Example Nos. 37 to 47 and the above-described Example Nos. 2, 8, 16 and 21. In FIG. 26, points (2), (8), (16), (21) and (37) to (47) correspond to the Example Nos. 2, 8, 16, 21 and 37 to 47, respectively. As apparent from FIG. 26, the content S of the {222} oriented Fe crystals can be set in a range of S.gtoreq.40% by setting the pH value in a range of pH.gtoreq.4.5, for example, at an average current density CDm.gtoreq.3 A/dm.sup.2. In order to set the content S of the {222} oriented Fe crystals in a range of S.gtoreq.90%, it is necessary to set the pH value in a range of pH.gtoreq.5.5 at an average current density CDm.gtoreq.3.5 A/dm.sup.2.

(d) With regard to seizure and wear resistance:

A seizure test was carried out in a chip-on-disk manner with lubrication for the Example Nos. 1, 7, 8, 11 to 14, 22 to 24 and 31 of the slide surface constructions 4 to determine the relationship between the content S of the {222} oriented Fe crystals and the seizure generating load. Test conditions were as follows: the material of the disk was Al-10% by weight of Si alloy; the rotational speed of the disk was 15 m/sec.; the amount of oil supplied was 0.3 ml/min.; and the area of the slide surface of the chip made from the slide construction was 1 cm.sup.2.

A wear test was also carried out in a chip-on-disk manner with lubrication for the Example Nos. 1, 7, 8, 11 to 14, 17, 22 to 24 and 31 to determine the relationship between the content S of the {222} oriented Fe crystals and the wear amount of the chip. Test conditions were as follows: the material of the disk was an Al-10% by weight of Si alloy; the rotational speed of the disk was 0.5 m/sec.; the load was 100N; the slide distance was 1 km; and the area of the slide surface of the chip made from the slide construction was 1 cm.sup.2. The wear amount of the chip is decrement (mg) per area (1 cm.sup.2) of the chip.

Table 19 shows the results of the seizure and wear tests. FIGS. 27 and 28 are graphs taken from Table 19. In both FIGS. 27 and 28, points (1), (7), (8), (11) to (14), (17), (22) to (24) and (31) correspond to the Example Nos. 1, 7, 8, 11 to 14, 17, 22 to 24 and 31, respectively.

TABLE 19______________________________________ Content S (%) of {222} Seizure WearExample oriented Fe generating amountNo. crystals load (N) (mg)______________________________________1 98 1420 0.77 95.9 1410 0.88 94.3 1400 0.811 93.5 1380 0.912 80.1 1080 1.213 26.4 300 2.414 20.3 300 2.517 90.6 1380 0.922 70.3 880 1.423 51.1 850 1.524 32.3 350 2.331 44.5 800 1.5______________________________________

As is apparent from Table 19 and FIG. 27, when the content S of the {222} oriented Fe crystals is in a range of S.gtoreq.40%, the slide surface 4a has a good oil retention and a good initial conformability and hence, the seizure resistance is increased considerably. It is apparent that particularly when the content S of the {222} oriented Fe crystals is in a range of S.gtoreq.90%, the seizure resistance is increased drastically due to an increase in density of the Fe crystals having six ridge lines.

In addition, as apparent from Table 19 and FIG. 28, when the content S of the {222} oriented Fe crystals is in a range of S.gtoreq.40%, the wear amount of the chip becomes smaller and therefore, even without lubrication, a good wear resistance is obtained. Particularly when the content S of the {222} oriented Fe crystals is in a range of S.gtoreq.90%, the seizure resistance is extremely good due to an increase in density of the Fe crystals having six ridge lines.

Group-B

Although the minimum current density CDmin has been set at 0 (zero) A/dm.sup.2 in the above-described Group-A, the minimum current density CDmin is set in a range of CDmin.gtoreq.0.2 A/dm.sup.2 or in a range CDmin.ltoreq.-0.2 A/dm.sub.2 in this Group-B.

Table 20 shows plating conditions for Example Nos. 1 to 7 of slide surface constructions. It should be noted that the Example No. 4 is the same as the Example 8 in Group-A.

TABLE 20______________________________________Plating bath conditions Concentration of ferrous ConcentrationExample sulfate of Fe.sup.2+ ions Tempera-No. (g/liter) (mol/liter) pH ture (.degree.C.)______________________________________1 .about. 7 400 1.4 6 50______________________________________

Table 21 shows plating conditions in the Example Nos. 1 to 7.

TABLE 21______________________________________Treating conditions of plating Minimum Maximum Average current current current Output density density density Time timeExample CDmin CDmax CDm ratio T.sub.ONNo. (A/dm.sup.2) (A/dm.sup.2) (A/dm.sup.2) T.sub.ON /T.sub.C (msec)______________________________________1 -1 24 4 0.2 22 -0.5 22 4 0.2 23 -0.2 21 4 0.2 24 0 20 4 0.2 25 0.2 19 4 0.2 26 0.5 18 4 0.2 27 1 16 4 0.2 2______________________________________

Table 22 shows the crystal shape of the slide surface 4a, the area rate A and average grain size d of the Fe crystals having six ridge lines, and the content S of the oriented Fe crystals, and hardness of the oriented Fe crystals in the Example Nos. 1 to 7, respectively.

TABLE 22__________________________________________________________________________ Fe crystals having six ridge lines Crystal Average shape of Area grain Hard-Example slide rate size d Content S (%) of oriented Fe crystals nessNo. surface A (%) (.mu.m) {110} {200} {211} {310} {222} (Hv)__________________________________________________________________________1 Hexagonal 95 6 0.8 1.5 4.2 0.7 92.8 370 pyramid Trigonal .ltoreq.1 -- pyramid2 Hexagonal 93 6 0.8 1.7 3.7 0.7 93.1 370 pyramid Trigonal .ltoreq.1 -- pyramid3 Hexagonal 94 5 0.7 1.6 3.5 0.7 93.5 375 pyramid Trigonal .ltoreq.1 -- pyramid4 Hexagonal 96 3 0.5 1.2 3.3 0.7 94.3 380 pyramid Trigonal .ltoreq.1 -- pyramid5 Hexagonal 95 5 0.4 1.4 3.2 0.4 94.6 400 pyramid Trigonal .ltoreq.1 -- pyramid6 Hexagonal 96 6 0.3 1.3 2.9 0.4 95.1 400 pyramid Trigonal .ltoreq.1 -- pyramid7 Hexagonal 97 6 0.3 1.2 2.9 0.3 95.3 400 pyramid Trigonal .ltoreq.1 -- pyramid__________________________________________________________________________

FIG. 29 is an X-ray diffraction pattern for the Example No. 1, and FIG. 30 is an electronic photomicrograph showing a crystal structure of a slide surface 4a in the Example No. 1. As apparent from FIG. 30, if the minimum current density CDmin is set in a range CDmin.ltoreq.-0.2 A/dm.sup.2, the {222} oriented Fe crystals are Fe crystals having six ridge lines having relatively deep valleys between the adjacent ridge lines.

FIG. 31 is an X-ray diffraction pattern for the Example No. 7, and FIG. 32 is an electronic photomicrograph showing a crystal structure of the slide surface 4a in the Example No. 7. As apparent from FIG. 32, if the minimum current density CDmin is set in a range CDmin.gtoreq.0.2 A/dm.sup.2, the {222} oriented Fe crystals are Fe crystals having six ridge lines and substantially flat slants between adjacent ridge lines.

A seizure test with lubrication and a wear test without lubrication for the Example Nos. 1 to 7 were carried out under the same conditions as those described above to provide results given in Table 23. FIGS. 33 and 34 are graphs taken from Table 23, wherein points (1) to (7) correspond to the Example Nos. 1 to 7.

TABLE 23______________________________________ Content S (%) of {222} Seizure WearExample oriented Fe generating amountNo. crystals load (N) (mg)______________________________________1 -1 1420 0.92 -0.5 1420 0.93 -0.2 1415 0.854 0 1400 0.85 0.2 1390 0.76 0.5 1390 0.77 1 1390 0.7______________________________________

As is apparent from Table 23 and FIG. 33, if the minimum current density CDmin is set in a range of CDmin.ltoreq.-0.2 A/dm.sup.2, each Fe crystal having six ridge lines has an Improved oil retention due to each valley portion thereof and hence, has an enhanced seizure resistance as in the Example Nos. 1 to 3.

As is apparent from Table 23 and FIG. 34, if the minimum current density CDmin is set in a range CDmin.gtoreq.0.2 A/dm.sup.2, each Fe crystal having six ridge lines has an increased hardness and hence, has an enhanced wear resistance.

Group-C

As shown in FIG. 35, the slide surface construction 4 may be formed of an aggregate of metal crystals having a face-centered cubic structure (which will be referred to as an fcc structure). The aggregate includes (3hhh) oriented metal crystals grown into a columnar form from a base material 2 with their (3hhh) planes (by Miller indices) oriented toward a slide surface 4a, wherein the content S of the (3hhh) oriented metal crystals is set in a range of S.gtoreq.40%. As shown in FIG. 36, the (3hhh) oriented metal crystal is a metal crystal of a quadrangular pyramid (or truncated quadrangular pyramid) shape in the slide surface 4a, i.e., a metal crystal 7.sub.3 having four ridge lines 6.sub.3.

As shown in FIG. 37, the inclination of the (3hhh) plane with respect to a phantom plane 11 extending along the slide surface 4a appears as an inclination of the quadrangular pyramid and therefore, an influence is imparted to the oil retention and the initial conformability of the slide surface construction 4. Preferably, the inclination angle .theta. formed by the (3hhh) plane with respect to the phantom plane 11 is set in a range represented by 0.degree..ltoreq..theta..ltoreq.15.degree. for the same reason as that described above. In this case, the direction of inclination of the (3hhh) plane is not limited.

Metal crystals having the fcc structure includes those of simple metals such as Pb, Ni, Cu, Pt, Al, Ag, Au and the like and alloys thereof, particular examples of which now will be described below.

Cam shafts for internal combustion engines were produced by subjecting the outer peripheral surfaces of a base material of a cast iron to an electrolytic Ni-plating to form a slide surface construction 4 comprised of an aggregate of Ni crystals. Plating bath conditions were as follows: the concentration of Ni ions (Ni.sup.2+) was 1 mol/liter; the pH was 5.5 and the temperature was 55.degree. C. Plating conditions were as follows: the minimum current density CDmin was 0 A/dm2; the maximum current density CDmax was 20 A/dm.sup.2 ; the average current density CDm was 4 A/dm.sup.2 ; the time ratio T.sub.ON /T.sub.C was 0.2; and the output time T.sub.ON was 2 msec.

The resultant slide surface was of a combined quadrangular pyramid/granular shape, wherein the area rate A of Ni crystals having four ridge lines was 60% and the average grain size d of the Ni crystals having four ridge lines was 4 .mu.m; the content S of {111} oriented Ni crystals was 29%; the content S of {222} oriented Ni crystals was 15.2%; the content S of {220} oriented Ni crystals was 4.7%; and the content of {311} oriented Ni crystals was 51.1%; and the hardness Hv was 250.

The contents S were determined from the following expressions on the basis of an X-ray diffraction pattern (the X-ray was applied in a direction perpendicular to the slide surface) of a slide surface construction shown in FIG. 38. For example, the term "{111} oriented Ni crystal" means an oriented Ni crystal with its {111} plane oriented toward the slide surface.

{111} oriented Ni crystal: S.sub.111 =((I.sub.111 /IA.sub.111)/T).times.100 {200} oriented Ni crystal: S.sub.200 =((I.sub.200 /IA.sub.200)/T).times.100 {220} oriented Ni crystal: S.sub.220 =((I.sub.220 /IA.sub.220)/T).times.100 {311} oriented Ni crystal: S.sub.311 =((I.sub.311 /IA.sub.311)/T).times.100 wherein each of I.sub.111, I.sub.200, I.sub.220 and I.sub.311 is a measurement (cps) of intensity of X-ray reflected from each crystal plane, I.sub.111 =78.6K, I.sub.200 =17.4K, I.sub.220 =2.7K, and I.sub.311 =27.7K, and each of IA.sub.111, IA.sub.200, IA.sub.220 and IA.sub.311 is an intensity ratio of X-ray reflected from each crystal plane in an ASTM card, IA.sub.111 =100, IA.sub.200 =42, IA.sub.220 =21 and IA.sub.311 =20. Hence, T=(I.sub.111 /IA.sub.111)+(I.sub.200 /IA.sub.200)+(I.sub.220 /IA.sub.220)+(I.sub.311 /IA.sub.311)=2.71K.

FIG. 39 is a photomicrograph showing a crystal structure of the slide surface. In FIG. 39, a large number of quadrangular pyramid-shaped Ni crystals having four ridge lines are observed. This Ni crystal having four ridge lines is a {311} oriented Ni crystal with its (3hhh) plane, i.e., {311} plane oriented toward the slide surface, and has a content S equal to 51.1%.

A seizure test with lubrication and a wear test without lubrication were carried out for this slide surface construction under the same conditions as those described above. The results showed that seizure generating load was 650N and the wear amount was 1.6 mg.

In the (hhh) oriented metal crystals having the body-centered cubic structure, i.e., the metal crystals having three or six ridge lines, the crystal shape in the slide surface, the crystal planes present in each slant and the like are as given in Table 24.

TABLE 24______________________________________Crystal Crystalshape in plane Characteristicslide present in of Referentialsurface each slant slant drawing______________________________________Triangular (hh0) High hardness, FIG. 40pyramid plane--- good close- wettability packed and wear plane resistanceHexangular (hhh) Excellent FIG. 41:Pyramid plane: 50% wettability slant (5hhh) due to (hhh) having deep plane: 50 plane having a valleys large surface energy (hh0) High hardness, FIG. 42: plane: 50% good flat slant (2hhh) wettability plane: 50% and wear (hh0) resistance plane--- close- packed plane______________________________________

In the (3hhh) oriented metal crystals having the face-centered cubic structure, i.e., the metal crystals having four ridge lines, the crystal shape in the slide surface, the crystal planes present in each slant and the like are as given in Table 25.

TABLE 25______________________________________Crystal Crystalshape in plane Characteristicslide present in of Referentialsurface each slant slant drawing______________________________________Quadrangular (h00) Good FIG. 43pyramid plane--- wettability plane and wear having high resistance atom density______________________________________

In a slide surface construction formed of an aggregate of metal crystals having an fcc structure, when the aggregate includes a large number of (hhh) oriented metal crystals with their (hhh) planes (by Miller indices) oriented toward the slide surface, at least some of the (hhh) oriented metal crystals can be formed into metal crystals having six ridge lines similar to those described above in the slide surface. The (hhh) oriented metal crystals may be {111} oriented Ni crystals, when Ni is used.

The slide surface construction of this invention is applicable, for example, to a slide portion of any of following parts for internal combustion engines: pistons (ring grooves), piston rings, piston pins, connecting rods, crank shafts, bearing metals, oil pump rotors, oil pump rotor housings, cam shafts, springs (end faces), spring seats, spring retainers, cotters, rocker arms, roller bearing outer cases, roller bearing inner cases, valve stems, valve faces, hydraulic tappets, water pump rotor shafts, pulleys, gears, transmission shaft portions, clutch plates, washers, and bolts (bearing surfaces and threaded portions).

It will be understood that the present invention is not limited to the formation of the slide surface construction and is applicable to the formation of a light absorbing member utilizing an absorbance of a skin film having metal crystals of the type described above, the formation of a magnetic shield material utilizing a high permeability of the skin film, the formation of a primary coat and the like. In this case, the light absorbing members embrace a light absorbing film used for an infrared laser processing, a light receiving plate in a heat exchanger utilizing a solar heat, a reflection preventing film in a solar cell, and the like. The primary coats embrace those formed on an outer surface of a cast-in insert member, a coating surface of a member, and the like.

Further, the material for forming the skin film are not limited to the described metals, and an inorganic skin film may be formed of ceramics such as carbonates, oxides, nitrides, etc., having a cubic crystal structure.

Claims

1. A process for forming an inorganic skin film using a plating process selected from the group consisting of electrolytic plating, chemical vapor deposition, and sputtering, said inorganic skin film being formed of an aggregate of crystals, said aggregate including at least one of a plurality of hexangular pyramid-shaped crystals or a plurality of truncated hexangular pyramid-shaped crystals in a surface of said skin film, said process comprising the steps of:

controlling the output of a plating energy source to produce a pulse waveform such that said output is increased from a minimum value to reach a maximum value and then is reduced to said minimum value; and

setting a ratio T.sub.ON /T.sub.C of an output time T.sub.ON to a cycle time T.sub.C within a range of T.sub.ON /T.sub.C .ltoreq.0.45, wherein said output time T.sub.ON is from a start point of increasing said output to a start point of reducing said output, and said cycle time T.sub.C is the time required for one cycle from an increasing-start point to a next increasing-start point of said output.

2. A process for forming an inorganic skin film according to claim 1, wherein said ratio T.sub.ON /T.sub.C is set in a range of T.sub.ON /T.sub.C .ltoreq.0.35.

3. A process for forming an inorganic skin film according to claim 2, wherein said crystals are metal crystals, and said plating process is an electrolytic plating process in which the average current density CDm is set in a range of CDm.gtoreq.3.5 A/dm.sup.2.

4. A process for forming an inorganic skin film according to claim 3, wherein the minimum current density CDmin is set in a range CDmin.gtoreq.0.2 A/dm.sup.2.

5. A process for forming an inorganic skin film according to claim 3, wherein the minimum current density CDmin is set in a range CDmin.ltoreq.-0.2 A/dm.sup.2.

6. A process for forming an inorganic skin film according to claim 2, wherein said crystals are Fe metal crystals.

7. A process for forming an inorganic skin film according to claim 6, wherein said plating process is an electrolytic plating process in which the average current density CDm is set in a range of CDm.gtoreq.3.5 A/dm.sup.2.

8. A process for forming an inorganic skin film according to claim 7, wherein the minimum current density CDmin is set in a range CDmin.gtoreq.0.2 A/dm.sup.2.

9. A process for forming an inorganic skin film according to claim 7, wherein the minimum current density CDmin is set in a range CDmin.ltoreq.-0.2 A/dm.sup.2.

10. A process for forming an inorganic skin film according to claim 1, wherein said crystals are metal crystals, and said plating process is an electrolytic plating process in which the average current density CDm is set in a range of CDm.gtoreq.3.5 A/dm.sup.2.

11. A process for forming an inorganic skin film according to claim 10, wherein the minimum current density CDmin is set in a range CDmin.gtoreq.0.2 A/dm.sup.2.

12. A process for forming an inorganic skin film according to claim 10, wherein the minimum current density CDmin is set in a range CDmin.ltoreq.-0.2 A/dm.sup.2.

13. A process for forming an inorganic skin film according to claim 1, wherein said crystals are Fe metal crystals.

14. A process for forming an inorganic skin film according to claim 12, wherein said plating process is an electrolytic plating process in which the average current density CDm is set in a range of CDm.gtoreq.3.5 A/dm.sup.2.

15. A process for forming an inorganic skin film according to claim 14, wherein the minimum current density CDmin is set in a range CDmin.gtoreq.0.2 A/dm.sup.2.

16. A process for forming an inorganic skin film according to claim 14, wherein the minimum current density CDmin is set in a range CDmin.ltoreq.-0.2 A/dm.sub.2.

17. A process for forming an inorganic skin film according to claim 11 said pyramid-shaped crystals being hexagonal pyramid-shaped crystals and said truncated pyramid-shaped crystals being truncated hexangular pyramid-shaped crystals.

18. A process for forming an inorganic skin film according to claim 1, wherein said plating process is an electrolytic plating process performed in a plating bath, and the pH of said plating bath is set in a range of pH.gtoreq.4.5.

19. A process for forming an inorganic skin film according to claim 18, wherein said crystals are metal crystals, and the average current density CDm of said plating process is set in a range of CDm.gtoreq.3.5 A/dm.sup.2.

20. A process for forming an inorganic skin film according to claim 1, wherein said plating process is an electrolytic plating process performed in a plating bath, and the pH of said plating bath is set in a range of pH.gtoreq.5.5.

21. A process for forming an inorganic skin film according to claim 20, wherein said crystals are metal crystals, and the average current density CDm of said plating process is set in a range of CDm.gtoreq.3.5 A/dm.sup.2.

22. A process for forming an inorganic skin film using a plating process selected from the group consisting of electrolytic plating, chemical vapor deposition, and sputtering, said inorganic skin film being formed of an aggregate of crystals, said aggregate including a plurality of (hhh) oriented crystals having a cubic crystal structure with their (hhh) planes (by Miller indices) oriented toward the surface of the skin film, at least some of said (hhh) oriented crystals being hexangular pyramid-shaped crystals or truncated hexangular pyramid-shaped crystals, said process comprising the steps of:

controlling the output of a plating energy source to produce a pulse waveform such that said output is increased from a minimum value to reach a maximum value and then is reduced to said minimum value; and

setting a ratio T.sub.on /T.sub.C of an output time T.sub.on to a cycle time T.sub.c within a range T.sub.ON /T.sub.C .ltoreq.0.45, wherein said output time T.sub.ON is from a start point of increasing said output to a start point of reducing said output, and said cycle time T.sub.C is the time required for one cycle from an increasing-start point to a next increasing-start point of said output.

23. A process for forming an inorganic skin film according to claim 22, wherein said ratio T.sub.ON /T.sub.C is set in a range of T.sub.ON /T.sub.C .ltoreq.0.35.

24. A process for forming an inorganic skin film according to claim 23, wherein said crystals are metal crystals, and said plating process is an electrolytic plating process in which the average current density CDm is set in a range of CDm.gtoreq.3.5 A/dm.sup.2.

25. A process for forming an inorganic skin film according to claim 24, wherein the minimum current density CDmin is set in a range CDmin.gtoreq.0.2 A/dm.sup.2.

26. A process for forming an inorganic skin film according to claim 24, wherein the minimum current density CDmin is set in a range CDmin.ltoreq.-0.2 A/dm.sup.2.

27. A process for forming an inorganic skin film according to claim 23, wherein said crystals are Fe metal crystals.

28. A process for forming an inorganic skin film according to claim 27, wherein said plating process is an electrolytic plating process in which the average current density CDm is set in a range of CDm.gtoreq.3.5 A/dm.sup.2.

29. A process for forming an inorganic skin film according to claim 28, wherein the minimum current density CDmin is set in a range of CDmin.gtoreq.0.2 A/dm.sup.2.

30. A process for forming an inorganic skin film according to claim 28, wherein the minimum current density CDmin is set in a range of CDmin.ltoreq.-0.2 A/dm.sup.2.

31. A process for forming an inorganic skin film according to claim 22, wherein said crystals are metal crystals, and said plating process is an electrolytic plating process in which the average current density CDm is set in a range of CDm.gtoreq.3.5 A/dm.sup.2.

32. A process for forming an inorganic skin film according to claim 31, wherein the minimum current density CDmin is set in a range of CDmin.gtoreq.0.2 A/dm.sup.2.

33. A process for forming an inorganic skin film according to claim 31, wherein the minimum current density CDmin is set in a range of CDmin.ltoreq.-0.2 A/dm.sup.2.

34. A process for forming an inorganic skin film according to claim 22, wherein said crystals are Fe metal crystals.

35. A process for forming an inorganic skin film according to claim 34, wherein said plating process is an electrolytic plating process in which the average current density CDm is set in a range of CDm.gtoreq.3.5 A/dm.sup.2.

36. A process for forming an inorganic skin film according to claim 35, wherein the minimum current density CDmin is set in a range CDmin.gtoreq.0.2 A/dm.sup.2.

37. A process for forming an inorganic skin film according to claim 35, wherein the minimum current density CDmin is set in a range CDmin.ltoreq.-0.2 A/dm.sup.2.

38. A process for forming an inorganic skin film according to claim 22, wherein the content S of said (hhh) oriented crystals in the surface of the skin film is set in a range of S.gtoreq.40%.

39. A process for forming an inorganic skin film according to claim 38, wherein said plating process is an electrolytic plating process performed in a plating bath, and the pH of said plating bath is set in a range of pH.gtoreq.4.5.

40. A process for forming an inorganic skin film according to claim 39, wherein said crystals are metal crystals, and the average current density CDm of said plating process is set in a range of CDm.gtoreq.3.5 A/dm.sup.2.

41. A process for forming an inorganic skin film according to claim 22, wherein the content S of said (hhh) oriented crystals in the surface of the skin film is set in a range of S.gtoreq.90%.

42. A process for forming an inorganic skin film according to claim 41, wherein said plating process is an electrolytic plating process performed in a plating bath, and the pH of said plating bath is set in a range of pH.gtoreq.5.5.

43. A process for forming an inorganic skin film according to claim 42 wherein said crystals are metal crystals, and the average current density CDm of said plating process is set in a range of CDm.gtoreq.3.5 A/dm.sup.2.