Process for production of a silicon nitride ceramic

Abstract

A silicon nitride ceramic of the present invention possesses excellent strength of the surface, including a silicon nitride and a rare earth oxide compound and being characterized in that the ratio of the transverse rupture strength, at a room temperature, of the fired surface used as a tensile surface to the transverse rupture strength, at a room temperature, of the worked surface used as a tensile surface subjected to the working so as to have the surface roughness of R.sub.MAX 0.8 .mu.m or less is 0.7 or more, and the strength ratio is satisfied even when any portion besides the fired surface is utilized as the tensile surface to be worked to have the surface roughness of R.sub.MAX 0.8 .mu.m or less. The present invention also provides a process for producing a silicon nitride ceramic including the steps of: (1) mixing .alpha.-Si.sub.3 N.sub.4 powder and .beta.-Si.sub.3 N.sub.4 powder to obtain a raw material powder so as to satisfy the formula indicated by 0.05.ltoreq..beta./.alpha.+.beta..ltoreq.0.50, in which a refers to the weight of .alpha.-Si.sub.3 N.sub.4 powder and .beta. refers to the weight of .beta.-Si.sub.3 N.sub.4 powder; (2) mixing at least one sintering aid to the raw material powder; (3) forming the powder mixture to give a compact; and (4) firing the compact at a temperature ranging from 1800.degree. to 2000.degree. C. under a nitrogen atmosphere having an atmospheric pressure of at least 1 atm.

Description

BACKGROUND OF THE INVENTION AND RELATED ART STATEMENT

The present invention relates to a silicon nitride ceramic having high strength, and more particularly, to a silicon nitride ceramic having high strength on its surface without working after firing, and the process for production thereof.

A silicon nitride ceramic has conventionally been drawing attention because of its high strength at high temperatures, chemical stability, etc., and therefore, various researches have been made into the application of a silicon nitride ceramic to materials to be used for heat engines such as diesel engines and gas turbine engines.

Since silicon nitride generally has low sinterability, a rare earth oxide such as Y.sub.2 O.sub.3 is first added as a sintering aid to silicon nitride powder. Then, the powder is formed to make a compact having a desired shape. Finally, the compact is subjected to firing under gas pressure to obtain a silicon nitride ceramic.

As a raw material powder of silicon nitride to produce such a ceramic, there has been used a raw material containing 90% or more of .alpha.-Si.sub.3 N.sub.4 in consideration of the sinterability, etc. It is known that firing causes phase transition from .alpha.-Si.sub.3 N.sub.4 to .beta.-Si.sub.3 N.sub.4 and forms columnar crystals.

However, firing the compact containing such a large amount of .alpha.-Si.sub.3 N.sub.4 brings about the roughness of crystallization and the like upon phase transition and does not give a minutely sintered ceramic, resulting in the deterioration of the strength and the like. From such a view point, Japanese Patent Laid-Open 62-297269, Japanese Patent Publication 2-50076, and Japanese Patent Laid-Open 2-255573 disclose a process for producing a silicon nitride ceramic, in which a large amount of .beta.-Si.sub.3 N.sub.4 is added to a raw material powder.

A silicon nitride ceramic obtained by any of these processes (i.e., a process in which a raw material contains mainly .alpha.-Si.sub.3 N.sub.4 and a process in which a large amount of .beta.-Si.sub.3 N.sub.4 is added to a raw material) possesses large pores on the surface of the ceramic just after firing (without working). (The surface without working after firing is hereinafter referred to as "fired surface".) The problem is that such a ceramic does not possess sufficient strength on the fired surface because stress concentrates in the peripheral portions of large pores.

Therefore, any of silicon nitride ceramics obtained by either of these two kinds of processes needs to be subjected to working onto the fired surface so as to produce a final products having the surface with desired strength. The step of working is laborious, time-consuming, and cost-increase.

Moreover, a ceramic obtained by any of the conventional processes does not possess satisfactory strength on the fired surface regardless of the thickness of the ceramic.

SUMMARY OF THE INVENTION

Hence, the present invention solves aforementioned problems. The present invention provides both a thin silicon nitride ceramic and a thick silicon nitride ceramic, each of which is excellent in the strength of the fired surface and each of which decreases the cost for producing the final product. The present invention also provides a process for producing such a silicon nitride ceramic.

The present inventors have earnestly studied, noting the properties of a silicon nitride ceramic such as the pore size and strength of the fired surface, and found that the above-described objects can be achieved by specifying the mixing ratio of .beta.-Si.sub.3 N.sub.4 into the raw material powder, which led to the completion of the present invention.

Accordingly, the silicon nitride ceramic of the present invention, which substantially composed of a silicon nitride and a rare earth oxide compound, is characterized in that the ratio of the transverse rupture strength of the fired surface used as a tensile surface at a room temperature to the transverse rupture strength, at a room temperature of the worked surface used as a tensile surface subjected to the working so as to have the surface roughness of R.sub.MAX 0.8 .mu.m or less is 0.7 or more, and the strength ratio is satisfied even when any portion besides the fired surface is utilized as the tensile surface to be worked to have the surface roughness of R.sub.MAX 0.8 .mu.m or less.

R.sub.MAX is a parameter for measuring surface roughness defined by JIS B 0601, which denotes the difference of the height between the highest point and the lowest point in a predetermined part of the curved line made by the cross section and the surface. Therefore, the relation between R.sub.MAX and R.sub.a, which is another parameter for measuring surface roughness, can be expressed by the formula: R.sub.MAX .apprxeq.4R.sub.a.

The process for producing a silicon nitride ceramic of the present invention is characterized in that .alpha.-Si.sub.3 N.sub.4 powder and .beta.3-Si.sub.3 N.sub.4 powder are mixed to obtain a raw material powder so as to satisfy the following formula:

0.05.ltoreq..beta./.alpha.+.beta..ltoreq.0.50 (.alpha. refers to the weight of .alpha.-Si.sub.3 N.sub.4 powder, and .beta. refers to the weight of .beta.-Si.sub.3 N.sub.4 powder.) A sintering aid is added to and mixed with the raw material powder. Then, the powder is formed to give a compact. After that, the compact is fired at a temperature ranging from 1800.degree. to 2000.degree. C. under a nitrogen atmosphere having an atmospheric pressure of at least 1 atm.

DETAILED DESCRIPTION OF THE INVENTION

According to the present invention, there is obtained a Si.sub.3 N.sub.4 ceramic, in which the difference of the surface strength between the fired surface and the surface subjected to a working (hereinafter referred to as a worked surface) is small, by controlling the compounding ratio of .alpha.-Si.sub.3 N.sub.4 powder and .beta.-Si.sub.3 N.sub.4 powder.

Therefore, the obtained ceramic already possesses sufficient strength required to be used as a product before it is subjected to working unlike a silicon nitride ceramic obtained by any of conventional processes. When the compact is formed so as to have a near net shape which excludes a margin to be removed by working after firing, the obtained ceramic does not need to be subjected to working.

Thus, the strength on the fired surface is improved since a pore size in the ceramic becomes small by controlling the mixing ratio of .beta.-Si.sub.3 N.sub.4 powder. Though the reason is not clear at present, it can be surmised as follows:

As the sintering mechanism of a Si.sub.3 N.sub.4 ceramic, first, SiO.sub.2 particles and a sintering aid such as Y.sub.2 O.sub.3 particles, existing on the surface of Si.sub.3 N.sub.4 powder particles, melt and react to be in a liquid phase in the step of firing, and as the reaction proceeds, .alpha.-Si.sub.3 N.sub.4 particles and .beta.-Si.sub.3 N.sub.4 particles, which were lying scattered at random before firing, are disposed regularly.

Then, all of the .alpha.-Si.sub.3 N.sub.4 particles and some of the .beta.-Si.sub.3 N.sub.4 particles melt, and then precipitate and grow as columnar crystals of .beta.-Si.sub.3 N.sub.4 around the remaining .beta.-Si.sub.3 N.sub.4 particles (relatively large sized .beta.-Si.sub.3 N.sub.4 particles) each of which works as a core for the crystal growth. The Si.sub.3 N.sub.4 ceramic seems to be obtained in this way.

In such a sintering mechanism, when a large amount of .alpha.-Si.sub.3 N.sub.4 powder is contained in the raw material powder, the amount of transitional .alpha.-Si.sub.3 N.sub.4 particles into .beta.-Si.sub.3 N.sub.4 particles is large, and the amount of the remaining .beta.-Si.sub.3 N.sub.4 particles, each of which works as a core for the crystal growth, is small. In this case, the strength of the fired surface of the obtained ceramic is lowered because the distribution of .beta.-Si.sub.3 N.sub.4 crystals becomes uneven, which makes the size of pores rather large.

In contrast, when a large amount of .beta.-Si.sub.3 N.sub.4 powder is contained in the raw material powder, the amount of transitional .alpha.-Si.sub.3 N.sub.4 particles into .beta.-Si.sub.3 N.sub.4 particles is small, and the amount of the remaining .beta.-Si.sub.3 N.sub.4 particles, each of which works as a core for the crystal growth, is large. In this case, the size of pores existing on the surface of the compact before firing does not become smaller as well as the diameters of the .beta.-Si.sub.3 N.sub.4 particles after sintering become too small, and as a result, the pores become rather large, giving the same disadvantage.

Accordingly, it is presumed that the strength of the fired surface can be improved by controlling the amount of .beta.-Si.sub.3 N.sub.4 powder contained in the raw material powder within the range given by the present invention because it makes the above mentioned transition from .alpha.-Si.sub.3 N.sub.4 to .beta.-Si.sub.3 N.sub.4 appropriate and makes the pore size smaller.

Next, description is made on a silicon nitride ceramic of the present invention.

A silicon nitride ceramic of the present invention is characterized in that the difference between the strength of the fired surface and the strength of the worked surface is small. For example, when the surface of the ceramic of the present invention is worked with abrasive grains having a size of #800, the surface roughness indicated by Rmax becomes about 0.8 .mu.m or less. The ratio of the four-point bending strength (transverse rupture strength) at a room temperature, of the fired surface used as the tensile surface to the four-point bending strength at a room temperature, of the worked surface used as the tensile surface is at least 0.7.

Further, this strength ratio is satisfied even when any portion besides the fired surface is utilized for the tensile surface to be worked. That is, even when a portion around the fired surface or around the center of the ceramic is utilized for the tensile surface to be worked, the above-described ratio is satisfied.

When the strength ratio is less than 0.7, the strength of the fired surface is not satisfactory, and it is not preferable to use such a fired surface as a surface of the final product from a practical view point. Further, the strength of the fired surface itself at a room temperature is preferably larger than 650 MPa, more preferably 700 MPa.

Furthermore, a pore size on the fired surface of the ceramic of the present invention is preferably not larger than 50.times.50 .mu.m. "Pore size" is the size of pores existing in the cross section generated by a transverse rupture of the silicon nitride ceramic. When a pore is oval, the pore size is expressed by "a.times.b", referring to the length of the longer axis as a and the shorter axis as b.

When a pore exists on the tensile surface of the silicon nitride ceramic, the pore size is expressed by "a.times.b", referring to the length of the longer axis on the surface as a and the depth of the pore as b.

The significance of this measurement is based on the fact that such a rupture starts at such a pore when a ceramic in which the fired surface is utilized as the tensile surface.

A pore larger than 50.times.50 .mu.m is not preferable because not only the strength but also the oxidation resistance of the ceramic deteriorate.

Further, R.sub.MAX of the fired surface is preferably 15 .mu.m or less. When R.sub.MAX is larger than 15 .mu.m, it is not preferable because the strength of the ceramic deteriorates.

Regarding the size of the columnar silicon nitride particles existing on the fired surface of the ceramic, it is preferable that the length perpendicular to the axis is 4 .mu.m or less and the axis is 50 .mu.m or less. When the size is larger than 4 .mu.m.times.50 .mu.m, it is not preferable because the strength of the ceramic is lowered since the columnar particles induce the rupture of the ceramic.

Moreover, concerning a sintered silicon nitride ceramic of the present invention, even a thick ceramic having a thickness of, for example 20-150 mm, as well as a thin ceramic, possesses excellent properties. Therefore, a silicon nitride ceramic of the present invention can be applied to various uses.

Next, the process for producing a silicon nitride ceramic of the present invention is described.

First, .alpha.-Si.sub.3 N.sub.4 powder and .beta.-Si.sub.3 N.sub.4 powder are mixed so as to satisfy the following formula:

0.05.ltoreq..beta./.alpha.+.beta..ltoreq.0.50 (.alpha. refers to the weight of .alpha.-Si3N.sub.4 powder, .beta. refers to the weight of .beta.-Si.sub.3 N.sub.4 powder.) It is preferable that the average diameter of the .alpha.-Si.sub.3 N.sub.4 powder particles is 0.3-1.0 .mu.m and that the average diameter of the .beta.-Si.sub.3 N.sub.4 powder particles is 0.3-3 .mu.m. When the average diameter of the .alpha.-Si.sub.3 N.sub.4 powder particles is larger than 1.0 .mu.m, the sinterability deteriorates. When the average diameter of the .beta.-Si.sub.3 N.sub.4 powder particles is larger than 3 .mu.m, the length perpendicular to the axis of the columnar crystals of .beta.-Si.sub.3 N.sub.4 obtained by sintering becomes at least 3 .mu.m, which is not preferable.

The amount of oxygen contained in .alpha.-Si.sub.3 N.sub.4 raw material powder can be increased by such a method that .alpha.-Si.sub.3 N.sub.4 raw material powder is fluidized in the reheating furnace the temperature of which is controlled to be, for example, 300.degree. C.-800.degree. C. for 1-20 hours and a method that .alpha.-Si.sub.3 N.sub.4 raw material powder is stirred in a slurry which has a temperature of 30.degree.-80.degree. C. and in which the amount of water is controlled.

The .alpha.-Si.sub.3 N.sub.4 is preferably treated by such a method because such a method further enhances the strength and the like of the Si.sub.3 N.sub.4 ceramic to be obtained.

To the obtained powder mixture is added 3-25 wt % of a sintering aid in proportion to the sum of the powder mixture and the sintering aid. The sintering aid is not particularly restricted, and, for example, a rare earth oxide such as Y.sub.2 O.sub.3, Yb.sub.2 O.sub.3, and Er.sub.2 O.sub.3, and a metal carbide such as MoC.sub.2 and SiC, and the like can be employed. More than one sintering aid may be employed.

Then, the mixture is subjected to a mixed grinding using, for example, silicon nitride balls, followed by drying to obtain powder for forming a compact.

Then, the obtained powder is subjected to a molding such as press molding, extrusion molding, and slip casting to obtain a compact having a desired shape.

The obtained compact is then fired at a temperature of about 1800.degree.-2000.degree. C. under a nitrogen atmosphere having an atmospheric pressure of at least 1 atm for about 2-8 hours to obtain a sintered Si.sub.3 N.sub.4 ceramic.

The atmospheric pressure and the firing temperature is preferably controlled so that silicon nitride is not decomposed during firing. Further, the fired ceramic may be subjected to a heat treatment, if desired, at a temperature of about 1000.degree.-1500.degree. C. under an ambient atmosphere for about 0.5-10 hours.

The present invention is hereinafter described in more detail by way of Examples and Comparative Examples. However, the present invention is not restricted by the following Examples.

EXAMPLES 1-6

.alpha.-Si.sub.3 N.sub.4 powder having a purity of 97 wt %, an oxygen content of 1.40 wt %, and BET specific surface area of 12.0 m.sup.2 /g and .beta.-Si.sub.3 N.sub.4 powder having a purity of 98 wt %, an oxygen content of 0.8 wt %, and BET specific surface area of 9.0 m.sup.2 /g are mixed in the ratio shown in Table 1.

To the mixture were added three kinds of sintering aids, Y.sub.2 O.sub.3 having an average particle diameter of 0.7 .mu.m and BET specific surface area of 10.0 m.sup.2 /g, MoC.sub.2 having an average particle diameter of 1.5 .mu.m and BET specific surface area of 2.0 m.sup.2 /g, and SiC having an average particle diameter of 0.2 .mu.m and BET specific surface area of 20.0 m.sup.2/ g.

The mixing ratio of each of the sintering aids, (i.e., Y.sub.2 O.sub.3, MoC.sub.2, and SiC) is respectively 8 wt %, 2 wt %, and 1 wt % in proportion to the sum of the powder mixture and the sintering aids.

The obtained mixture of 20 Kg and silicon nitride balls of 40 g, which work as a medium for stirring, and water were put into a mill for stirring, having an internal volume of 100 liter, and the mixture was subjected to grinding for 3 hours. Subsequently, the mixture was dried to vaporize the water, and thus, granulated powder particles having an average diameter of 50 .mu.m for forming a compact.

Then, the powder was subjected to press molding under the pressure of 200 kg/cm.sup.2, followed by isostatic pressing under 5 ton/cm.sup.2 to obtain a compact having dimensions of 80 mm.times.80 mm.times.6 mm (thickness). This compact was fired at a temperature of 1900.degree. C. in a nitrogen atmosphere under an atmospheric pressure of 9.5 for 3 hours to obtain a silicon nitride ceramic having dimensions of about 67 mm.times.67 mm.times.5 mm (thickness).

(Performance Evaluation)

(1) Strength of Fired Surface

The fired surface of the silicon nitride ceramic was not subjected to working and used as a tensile surface. The ceramic was measured for four-point bending strength (transverse rupture strength) on the basis of JIS R 1601, except for tensile surface roughness, at a room temperature and at 1400.degree. C. The results are shown in Table 1.

(2) Pore Size

The ruptured portions of the tesp piece which was measured for the strength of the fired surface were observed with a scanning electron microscope. At least samples were measured for the size of each pores existing on such portions. The average size of the examined pores are shown in Table 1.

(3) Size of Silicon Nitride Columnar Crystal

The fired surface of the silicon nitride ceramic was observed with a scanning electron microscope. 100 columnar crystals on the fired surface were chosen out at random. The average lengths parallel to the axis of the crystals and perpendicular to the axis of the crystals are shown in Table 1.

(4) Surface Roughness

The fired surface of the ceramic was measured for the surface roughness (R.sub.MAX) using an instrument. The results are shown in Table 1.

(5) Strength of Worked Surface

The fired surface of the ceramic was subjected to working using grains having a size of #800 so that the surface roughness indicated by Rmax becomes 0.8 .mu.m or less. The ceramic was measured for four-point bending strength at a room temperature and at 1400.degree. C. At this time, the tensile surface was taken out of the vicinity of the center of the ceramic having a thickness of 5 mm. The results are shown in Table 1. Additionally, the ratio of the strength of the fired surface to the strength of the worked surface at a room temperature (.sigma..sub.as /.sigma..sub.#800) is also shown in Table 1.

Comparative Examples 1-7

The silicon nitride ceramics were obtained in the same manner as in Examples 1-6 except that the compounding ratio of .alpha.-Si.sub.3 N.sub.4 powder and .beta.-Si.sub.3 N.sub.4 powder was varied as shown in Table 1. The ceramics were measured for the same performance evaluations as described above. The results are shown in Table 1.

EXAMPLES 7-14

Comparative Examples 8-11

The silicon nitride ceramics were obtained in the same manner as in Examples 1-6 except that the compounding ratio of .alpha.-Si.sub.3 N.sub.4 powder and .beta.-Si.sub.3 N.sub.4 powder, the kind and the amount of sintering aids, and firing conditions were varied as shown in Table 2. The ceramics were measured for the same performance evaluation. The results are shown in Table 3.

The conditions of the sintering aids used were as follows:

Yb.sub.2 O.sub.3 Er.sub.2 O.sub.3 SC.sub.2 O.sub.3 WC NbC MoSi.sub.2 WSi.sub.2

Average Diameter(.mu.m) 0.5 0.6 0.9 0.6 2.0 3.0 3.0

BET Specific

Surface Area(m.sup.2 /g) 9.0 9.0 9.0 2.5 3.0 4.0 3.0

EXAMPLES 15, 16 and 20

Comparative Examples 12 and 13

To .alpha.-Si.sub.3 N.sub.4 powder was added water having 50 wt % in proportion to the sum of the weight of the powder and the water. The mixture was subjected to stirring treatment for 5 hours using an attriter mill so that the rate of the oxygen content in .alpha.-Si.sub.3 N.sub.4 powder is increased from 1.40 wt % to 1.50 wt %. The ceramics were obtained in the same manner as in Example 1-6 except for the kind and the amount of the sintering aids, which is shown in Table 4. The ceramics were measured for the same performance evaluations, and the results are shown in Table 5. The conditions of the sintering aids utilized are the same as the conditions described above.

EXAMPLES 17-19

Comparative Examples 14 and 15

The ceramics were obtained in the same manner as in Examples 1-6 except that the rate of the oxygen content in .alpha.-Si.sub.3 N.sub.4 powder was increased from 1.40 wt % to 1.55 wt % by the treatment at 500.degree. C. in an electric furnace for 5 hours. The ceramics were measured for the same performance evaluations, and the results are shown in Table 4 and 5. The conditions of the sintering aids utilized are the same as the conditions described above.

EXAMPLES 21-24

Comparative Examples 16-18

The ceramics were obtained in the same manner as in Examples 1-6 except that there were utilized .alpha.-Si.sub.3 N.sub.4 powder having a purity of 97 wt %, an average particle diameter of 0.6 .mu.m, an oxygen content of 1.3 wt %, and BET specific surface area of 12 m.sup.2 /g (.alpha.-Si.sub.3 N.sub.4 content of 98%) and .beta.-Si.sub.3 N.sub.4 powder having a purity of 98 wt %, an average particle diameter of 0.8 .mu.m, an oxygen content of 1.0 wt %, and BET specific surface roughness of 9 m.sup.2 /g (.beta.-Si.sub.3 N.sub.4 content of 95%), and that the compounding ratio and the firing conditions were controlled as shown in Table 6.

The compacts having the dimensions of 80 mm.times.80 mm.times.24 mm (thickness), 100 mm.times.100 mm.times.60 mm (thickness), and 150 mm.times.150 mm.times.120 mm (thickness) were subjected to the treatment to obtain thick ceramics each having a thickness of 20 mm, 50 mm, and 100 mm respectively under the condition shown in Table 6. Each of the obtained ceramics were measured for the strength of the fired surface and the worked surface, and the results are shown in Table 7.

Regarding the measurement for the strength, two kinds of tensile surfaces were measured, i.e., (1) a worked surface obtained by abrasion to remove the thickness of 500 .mu.m from the fired surface (outer portion of the ceramic), and (2) a portion just in the middle of the thickness of the ceramic (inner portion of the ceramic). The ratio of the strength on the fired surface to the strength of the tensile surface in outer portion (.sigma..sub.as /.sigma..sub.out) and inner portion (.sigma..sub.as /.sigma..sub.in) are also shown in Table 7.

TABLE 1__________________________________________________________________________ Temp. of worked Temp. of fired Ratio of Dimentions Compounding Relative surface (MPa) surface (MPa) strength at Pore of columnar Surface ratio density Room Room room temp. dimentions crystal roughness .alpha. *.sup.1 .beta. *.sup.2 (%) temp. 1400.degree. C. temp. 1400.degree. C. .sigma..sub.ss /.sigma..sub.#800 .mu.m (.mu.m) R.sub.max__________________________________________________________________________Example 1 95 5 98.2 920 800 700 780 0.77 50 .times. 50 50 .times. 3 15 2 90 10 99.3 930 800 840 790 0.90 50 .times. 40 30 .times. 3 8 3 80 20 99.7 1000 830 800 800 0.80 15 .times. 15 35 .times. 4 11 4 70 30 99.8 980 830 960 790 0.98 20 .times. 15 40 .times. 5 10 5 60 40 99.3 950 790 810 780 0.85 30 .times. 20 30 .times. 3 9 6 50 50 99.4 930 800 650 740 0.70 40 .times. 30 30 .times. 3 8Comparative 1 100 0 95.3 750 730 430 710 0.57 70 .times. 60 60 .times. 3 25Example 2 99 1 96.0 780 750 500 720 0.64 80 .times. 60 65 .times. 3 17 3 40 60 99.5 900 740 570 700 0.63 60 .times. 70 20 .times. 4 6 4 30 70 98.3 800 710 440 710 0.55 80 .times. 90 20 .times. 4 5 5 20 80 99.7 760 710 430 650 0.57 70 .times. 60 10 .times. 3 7 6 10 90 98.9 700 600 410 580 0.59 80 .times. 100 10 .times. 3 7 7 0 100 99.4 650 550 300 600 0.46 105 .times. 60 10 .times. 3 7__________________________________________________________________________ *.sup.1 . . . .alpha. Si.sub.3 N.sub.4 powder *.sup.2 . . . .beta. Si.sub.3 N.sub.4 powder TABLE 2______________________________________Com-pounding Amount of Firingratio sintering aid conditions.alpha..sup.*1 .beta..sup.*2 (wt %) .degree.C. .multidot. hr .multidot.______________________________________ atmExample 7 95 5 12Y.sub.2 O.sub.3, 1Mo.sub.2 C 1900 .multidot. 6 .multidot. 10 8 80 20 14Yb.sub.2 O.sub.2 1850 .multidot. 4 .multidot. 5 9 70 30 3Y.sub.2 O.sub.2, 13Yb.sub.2 O.sub.3, 2000 .multidot. 3 .multidot. 1010 90 10 10Er.sub.2 O.sub.2, 2Mo.sub.2 C, 2SiC 1900 .multidot. 4 .multidot. 10 .sup.*311 50 50 5Sc.sub.3 O.sub.2, 2NbC 1900 .multidot. 2 .multidot. 10 .sup.*412 80 20 5Y.sub.2 O.sub.2, 5Er.sub.2 O.sub.3, 0.5WC, 1950 .multidot. 4 .multidot. 1013 70 30 5Y.sub.2 O.sub.2, 5Yb.sub.2 O.sub.2, 1800 .multidot. 5 .multidot. 314 90 10 4Sc.sub.2 O.sub.2, 4Yb.sub.2 O.sub.3, 1800 .multidot. 5 .multidot. 3Compar-ativeExample 8 0 100 6Y.sub.2 O.sub.2, 5SiC 1900 .multidot. 4 .multidot. 10 9 100 0 10Er.sub.2 O.sub.3, 2WSi2 1850 .multidot. 5 .multidot. 510 30 70 12Er.sub.2 O.sub.2, 6Yb.sub.2 O.sub.2, 3Mo.sub.2 2000 .multidot. 2 .multidot. 10 .sup.*511 10 90 3Y.sub.2 O.sub.2, 5Sc.sub.2 O.sub.3, 1900 .multidot. 3 .multidot. 10______________________________________ .sup.*1 .alpha. Si.sub.3 N.sub.4 powder .sup.*2 .beta. Si.sub.3 N.sub.4 powder .sup.*3 The test piece was worked and then subjected to heat treatment at 1300.degree. C. for 5 hours. .sup.*4 The test piece was worked and then subjected to heat treatment at 1000.degree. C. for 10 hours. .sup.*5 The test piece was worked and then subjected to heat treatment at 1500.degree. C. for 1 hour. TABLE 3__________________________________________________________________________ Temp. of worked Temp. of fired Ratio of Dimentions Relative surface (MPa) surface (MPa) strength at Pore of columnar Surface density Room Room room temp. dimentions crystal roughness (%) temp. 1400.degree. C. temp. 1400.degree. C. .sigma..sub.ss /.sigma..sub.#800 .mu.m (.mu.m) R.sub.max__________________________________________________________________________Example 7 98.0 950 790 670 780 0.70 50 .times. 40 50 .times. 3 14 8 99.2 800 760 660 780 0.82 20 .times. 20 40 .times. 3 12 9 99.5 780 780 690 780 0.89 25 .times. 25 40 .times. 3 10 10 98.6 820 810 800 800 0.98 35 .times. 30 45 .times. 3 13 11 97.3 920 780 670 760 0.73 40 .times. 50 30 .times. 3 9 12 99.5 860 800 820 790 0.95 20 .times. 30 40 .times. 3 10 13 98.5 930 740 750 750 0.81 30 .times. 20 40 .times. 4 9 14 98.1 850 800 680 800 0.80 30 .times. 30 50 .times. 4 14Comparative 8 99.4 560 550 250 500 0.45 80 .times. 70 10 .times. 3 7Example 9 95.3 830 760 520 750 0.63 70 .times. 60 70 .times. 3 20 10 99.8 940 720 520 700 0.55 80 .times. 90 20 .times. 2 5 11 98.5 700 620 280 600 0.40 80 .times. 100 10 .times. 2 8__________________________________________________________________________ *.sup.1 . . . .alpha. Si.sub.3 N.sub.4 powder *.sup.2 . . . .beta. Si.sub.3 N.sub.4 powder TABLE 4______________________________________Com-pounding Amount of Firingratio sintering aid conditions.alpha..sup.*1 .beta..sup.*2 (wt %) .degree.C. .multidot. hr .multidot.______________________________________ atmExample15 95 5 12Y.sub.2 O.sub.3, 1Mo.sub.2 C 1900 .multidot. 6 .multidot. 1016 80 20 14Yb.sub.2 O.sub.2 1850 .multidot. 4 .multidot. 5 .sup.*317 90 10 4SO.sub.2 O.sub.2, 4Yb.sub.2 O.sub.3, 1800 .multidot. 5 .multidot. 318 50 50 5Sc.sub.2 O.sub.2, 2NbC 1900 .multidot. 2 .multidot. 10 .sup.*419 70 30 5Y.sub.2 O.sub.3, 5Yb.sub.3 O.sub.3, 1800 .multidot. 5 .multidot. 320 60 40 3Y.sub.2 O.sub.3, 14Yb.sub.2 O.sub.3 2000 .multidot. 3 .multidot. 10Compar-ativeExample12 70 30 8Y.sub.2 O.sub.3, 1SiC 1700 .multidot. 5 .multidot. 113 40 60 12Y.sub.2 O.sub.3, 1Mo.sub.2 C 1900 .multidot. 6 .multidot. 10 .sup.*514 20 80 5Y.sub.2 O.sub.3, 5Yb.sub.2 O.sub.3, 1800 .multidot. 5 .multidot. 315 0 100 10Y.sub.2 O.sub.3, 1900 .multidot. 4 .multidot. 10______________________________________ .sup.*1 .alpha. Si.sub.3 N.sub.4 powder .sup.*2 .beta. Si.sub.3 N.sub.4 powder .sup.*3 The test piece was worked and then subjected to heat treatment at 1200.degree. C. for 5 hours. .sup.*4 The test piece was worked and then subjected to heat treatment at 1300.degree. C. for 5 hours. .sup.*5 The test piece was worked and then subjected to heat treatment at 1500.degree. C. for 1 hour. TABLE 5__________________________________________________________________________ Temp. of worked Temp. of fired Ratio of Dimentions Relative surface (MPa) surface (MPa) strength at Pore of columnar Surface density Room Room room temp. dimentions crystal roughness (%) temp. 1400.degree. C. temp. 1400.degree. C. .sigma..sub.ss /.sigma..sub.#800 .mu.m (.mu.m) R.sub.max__________________________________________________________________________Example 15 99.0 1020 760 850 740 0.83 40 .times. 50 45 .times. 3 13 16 99.5 1100 810 990 800 0.90 30 .times. 10 50 .times. 3 12 17 98.5 1060 820 840 800 0.79 30 .times. 40 45 .times. 3 14 18 99.5 980 760 810 740 0.83 20 .times. 40 30 .times. 2 10 19 99.4 950 780 900 790 0.95 20 .times. 30 40 .times. 2 10 20 98.3 960 900 940 840 0.98 40 .times. 50 35 .times. 3 9Comparative 12 78.5 120 -- 80 -- 0.67 -- -- --Example 13 99.6 940 740 650 730 0.69 90 .times. 80 30 .times. 2 10 14 98.7 900 720 530 710 0.59 40 .times. 70 40 .times. 2 8 15 96.3 700 650 370 600 0.53 100 .times. 40 20 .times. 3 7__________________________________________________________________________ *.sup.1 . . . .alpha. Si.sub.3 N.sub.4 powder *.sup.2 . . . .beta. Si.sub.3 N.sub.4 powder TABLE 6__________________________________________________________________________ Ceramic (5 mm in thickness) Temp. of worked Ratio ofCompounding Amount of Firing Relative surface (MPa) Temp. of fired strength atratio sintering aid Conditions density Room surface (MPa) room temp..alpha..sup.*1 .beta..sup.*2 (wt %) .degree.C. .multidot. hr .multidot. atm (%) temp. 1400.degree. C. Room temp. .sigma..sub.a s /.sigma..sub.#200__________________________________________________________________________Example21 90 10 7Y.sub.2 O.sub.3, 1WC 1900 .multidot. 6 .multidot. 10 99.0 900 780 720 0.8022 80 20 10Yb.sub.2 O.sub.3, 2Mo.sub.2 C 1800 .multidot. 3 .multidot. 5 99.5 860 750 730 0.8523 .sup. 70.sup.*3 30 5Y.sub.2 O.sub.3, 5Yb.sub.2 O.sub.3, 1SiC 1800 .multidot. 8 .multidot. 5 97.8 1000 870 840 0.8424 60 40 4Y.sub.2 O.sub.3, 6Sc.sub.2 O.sub.3, 2SiC 1850 .multidot. 3 .multidot. 10 99.0 920 830 740 0.80ComparativeExample16 100 0 8Y.sub.2 O.sub.3, 1WC 1900 .multidot. 4 .multidot. 10 96.8 800 760 440 0.5517 30 70 6Yb.sub.2 O.sub.3, 1Mo.sub.2 C 1800 .multidot. 5 .multidot. 5 99.3 800 710 470 0.5918 10 90 3Y.sub.2 O.sub.3, 8Sc.sub.2 O.sub.3, 2SiC 1850 .multidot. 3 .multidot. 10 98.7 760 680 440 0.58__________________________________________________________________________ .sup.*1 .alpha. Si.sub.3 N.sub.4 powder .sup.*2 .beta. Si.sub.3 N.sub.4 powder .sup.*3 .alpha. Si.sub.3 N.sub.4 powder was subjected to wet treatment to increase the amount of oxygen from 1.3 wt % to 1.45 wt %. TABLE 7__________________________________________________________________________ Outer portion of ceramic Inner portion of ceramic Strength Strength of Strength Strength of of fired worked surface Ratio of of fired worked surface Ratio of Relative surface (MPa) strength at Relative surface (MPa) strength at Thickness density Room temp. Room room temp. density Room temp. Room room temp. (mm) (%) (MPa) temp. 1400.degree. C. .sigma..sub.as /.sigma..sub.#out (%) (MPa) temp. 1400.degree. C. .sigma..sub.as /.sigma..sub.in__________________________________________________________________________Example 21 100 99.5 730 880 760 0.83 97.8 730 730 740 0.94 22 50 99.6 740 880 770 0.84 98.9 740 860 780 0.86 23 50 97.6 850 980 880 0.87 98.2 850 950 850 0.89 24 20 99.5 750 860 750 0.87 99.5 750 880 700 0.85Comparative 16 100 98.3 430 810 770 0.53 90.5 430 590 350 0.73Example 17 50 98.5 430 820 730 0.52 95.3 430 630 600 0.68 18 20 97.6 440 750 680 0.59 93.2 440 600 590 0.73__________________________________________________________________________

As shown in Tables 1-5, the silicon nitride ceramics in Examples 1-20, which are in the scope of the present invention, has a small difference in the strength between the worked surface and the fired surface and has small pore sizes. Moreover, the treatment to increase the rate of oxygen in .alpha.-Si.sub.3 N.sub.4 powder improves the properties of the ceramic to be obtained as shown in Table 5.

Further, a ceramic of the present invention maintains the above-described strength in both inner and outer portions, and shows excellent properties regarding the strength.

As have been described up to this point, the present invention provides a silicon nitride ceramic which is excellent in the strength of the fired surface and which decreases the cost for producing the final product by specifying the mixing ratio of .beta.-Si.sub.3 N.sub.4 to the raw material. The present invention also provides a process for producing such a silicon nitride ceramic.

Therefore, the ceramic of the present invention does not need working after sintering so as to obtain satisfactory strength. When the ceramic is produced to have a near net shape, which does not have a margin to be removed by working, the ceramic does not need working.

Further, even a thick ceramic exhibits an excellent strength, and the application of the ceramic can be expanded.

Claims

1. A process for producing a silicon nitride ceramic, comprising the steps of:

mixing a rare earth compound with .alpha.-Si.sub.3 N.sub.4 powder and .beta.-Si.sub.3 N.sub.4 powder to obtain a raw material powder so as to satisfy the formula: 0.05.ltoreq..beta./.alpha.+.beta..ltoreq.0.50 (.alpha. refers to the weight of .alpha.-Si.sub.3 N.sub.4 powder, .beta. refers to the weight of -Si.sub.3 N4 powder);

forming said raw material powder to give a compact; and

firing said compact at a temperature ranging from 1800.degree. to 2000.degree. C. under a nitrogen atmosphere having an atmospheric pressure of at least 1 atm to form a silicon nitride ceramic comprising silicon nitride and a rare earth compound, wherein the ratio of (1) the transverse rupture strength, at room temperature, of a fired surface used as a tensile surface to (2) the transverse rupture strength, at room temperature, of a worked surface used a tensile surface subjected to working so as to have a surface roughness of R.sub.MAX 0.8 .mu.m or less is 0.83 or more and wherein pores on the fired surface each have a size of 50 .mu.m.times.50 .mu.m or smaller.

2. A process for producing a silicon nitride ceramic according to claim 1, wherein an oxygen content of .alpha.-Si.sub.3 N.sub.4 powder is increased within 1.40 to 1.70 wt %.

3. A process for producing a silicon nitride ceramic according to claim 3, wherein said oxygen content is increased by fluidizing .alpha.-Si.sub.3 N.sub.4 powder in a reheating furnace the temperature of which is controlled to be 300.degree. C.-800.degree. C. for 1-20 hours.

4. A process for producing a silicon nitride ceramic according to claim 2, wherein said oxygen content is increased by stirring .alpha.-Si.sub.3 N.sub.4 in a slurry which has a temperature of 30.degree.-800.degree. C. and in which the amount of water is controlled.

5. A process for producing a silicon nitride ceramic according to claim 1, wherein said ceramic is subjected to a heat treatment at a temperature of 1000.degree.-1500.degree. C. under an ambient atmosphere for 0.5-10 hours.

6. A process for producing a silicon nitride ceramic according to claim 2, wherein said ceramic is subjected to a heat treatment at a temperature of 1000.degree.-1500.degree. C. under an ambient atmosphere for 0.5-10 hours.