Silicon nitride based sintered body

Abstract

Discloses a silicon nitride based sintered body composed only of uniform, fine crystal grains, and improved in both strength and fracture toughness in the middle and low temperature ranges. A crystalline silicon nitride powder composed of crystal grains whose longer-axis diameter is not more than 200 nm or an amorphous silicon nitride powder is used as material powder. The silicon nitride powder is sintered at a temperature of 1200.degree. C. to 1400.degree. C. or sintered with a product of sintering temperature (.degree. C.) and sintering time (sec) below 600000 (.degree. C..multidot.sec) at a temperature of 1400.degree. C. to 1900.degree. C. Thus, a silicon nitride based sintered body in which the longer-axis diameter of silicon nitride and/or sialon crystals is not more than 200 nm is obtained.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a silicon nitride based sintered body having excellent mechanical properties in middle and low temperature ranges from room temperature to 1100.degree. C. as structural ceramic materials to be used for automobile parts, wear-resistant tools, and the like. The invention also relates to methods for producing the same.

2. Description of the Prior Art

Silicon nitride is a material well balanced in strength, fracture toughness, corrosion resistance, wear resistance, thermal shock resistance, oxidation resistance, and the like, so that it is used over a wide range of fields such as cutting tools and engine parts. In particular, it has recently been drawing industrial attention as a structural material of automobile engines, gas turbines, and the like. However, to apply silicon nitride based sintered bodies to those fields which demand high reliability to materials such as in automobile engines, it is essential to further improve their fracture toughness so as to overcome their brittleness, and to improve their strength at the same time.

For example, as disclosed in Japanese Patent Laid-Open Publication No. SHO 61-91065 and Japanese Patent Laid-Open Publication No. HEI 2-44066, there have been proposed silicon nitride single-phase materials that show improved mechanical characteristics such as strength by generating a combination of crystal phases, or a composite crystal phase, of equiaxial particulate .alpha.'-sialon (general formula: M.sub.x (Si, Al).sub.12 (O, N).sub.16, wherein M represents Mg, Ca, Li, and rare-earth elements) and columnar .beta.'-sialon (including .beta. type silicon nitrides). However, as apparent from their embodiments, the methods for producing a sintered body that show a strength characteristic of their resulting sintered bodies stably exceeding 100 kg/mm.sup.2 by flexural strength are in all cases by the hot press process, such that high strength characteristic has not yet been attained stably from the industrial point of view.

Also, there have been proposed composite materials, such as disclosed in Japanese Patent Laid-Open Publication No. HEI 4-202059, in which 1 to 500 nm fine particles are dispersed in silicon nitride and sialon having a longer-axis diameter of 0.05 to 3 .mu.m and an aspect ratio of 3 to 20. The composite materials indeed show a strength of 167 kg/mm.sup.2 at a maximum in the embodiments, but may incur deterioration in strength because they may contain coarse silicon nitride. Further, the composite materials only have a Weibull coefficient of 9 or so. Thus, high strength characteristic could not be attained with stability, also in this case.

Yet further, as disclosed in Japanese Patent Laid-Open Publication No. HEI 4-295056, there has been proposed a sintered body in which different types of particles are dispersed in a grain boundary phase of silicon nitride having a columnar structure. This sintered body has maximum particle sizes as much as 2 .mu.m to 3.5 .mu.m in shorter-axis diameter and 10 .mu.m to 14 .mu.m in longer-axis diameter, such that the matrix itself can make a fracture source. The strength in its embodiment is 158 kg/mm.sup.2 at a maximum, and the baking temperature is above 1800.degree. C. Thus, the sintered body could not be sufficient in terms of productivity and cost.

in the prior art attempts to improve the strength and toughness of silicon nitride based sintered bodies, as described above, it has been very difficult to improve the strength and the fracture toughness at the same time, due to the fact that enhancing the strength by forming the fine grained structure would result in lowered fracture toughness and, conversely, enhancing the fracture toughness by presenting columnar crystals through addition of whiskers or grain growth of silicon nitride would incur deterioration in strength.

SUMMARY OF THE INVENTION

Accordingly, in view of the above conventional problems, the object of the present invention is to provide a silicon nitride based sintered body which is composed of uniform, very fine crystal grains and which is improved in bending strength and fracture toughness at the same time, and moreover to provide a method for producing the same.

To achieve the above object, the silicon nitride based sintered body according to the present invention comprises at least one of .alpha., .beta.-Si.sub.3 N.sub.4 and .alpha., .beta.-sialon whose longer-axis diameter is not more than 200 nm, more desirably not more than 50 nm.

Further, the method for producing a silicon nitride based sintered body according to the present invention comprises the step of sintering silicon nitride powder at a temperature of 1200.degree. C. to 1400.degree. C. Otherwise, for temperatures of 1400.degree. C. to 1900.degree. C., the method comprises a product of sintering temperature (.degree. C.) and sintering time (sec) below 600000 (.degree. C..multidot.sec).

The silicon nitride powder is desirably a crystalline silicon nitride powder composed of crystal grains whose longer-axis diameter is not more than 200 nm or an amorphous silicon nitride powder. Also, the method for producing a crystalline silicon nitride powder is desirably mechanical alloying process. The method of sintering is desirably spark plasma sintering, microwave sintering, ultra-high pressure sintering, or the like.

The silicon nitride of the present invention, as a matrix phase, is composed of crystal grains whose longer-axis diameter is not more than 200 nm. Attaining such fine, uniform structure as has not been attained hitherto makes it possible to attain a high strength that has not been expected so far.

Further, in the present invention, unlike the conventional counterpart, refining the grain size leads to enhancement in fracture toughness. This could be attributed to the fact that while the conventional sintered body whose longer-axis diameter is more than 200 nm would result in brittle fracture of the material, reducing the longer-axis diameter to not more than 200 nm would develop a ductile state of ceramics due to the grain boundary slide phenomenon or the like.

As a result of these, the sintered body of the present invention shows high bending strengths and high toughnesses more than a bending strength of 500 kg/mm.sup.2 and a fracture toughness of 15 MPa.m.sup.1/2 at temperatures lower than 1100.degree. C.

One method for producing the sintered body of the present invention may be the step of sintering silicon nitride powder at a temperature of 1200.degree. C. to 1400.degree. C. Temperatures lower than 1200.degree. C. could not allow the sintering to progress, while temperatures exceeding 1400.degree. C. would cause grain growth to progress such that the longer-axis diameter exceeds 200 nm.

As another method, it is desirable that the product of sintering temperature (.degree. C.) and sintering time (sec) is below 600000 (.degree. C..multidot.sec) in a temperature range of 1400.degree. C. to 1900.degree. C. The products below 600000 (.degree. C..multidot.sec) could not allow the grain growth to progress even at temperatures above 1400.degree. C., so that a longer-axis diameter not more than 200 nm can be obtained. Also, temperatures exceeding 1900.degree. C. would make it impossible to suppress grain growth even with short time sintering, so that temperatures below 1900.degree. C. are desirable.

The silicon nitride material powder to be used is not particularly limited, but is desirably a crystalline silicon nitride powder composed of crystal grains whose longer-axis diameter is not more than 200 nm with a view to improving the sinterability as much as possible. As this material powder, an ultrafine powder whose mean diameter is not more than 200 nm and which is fabricated by the CVD process or the like may be used, but in general the finer the powder becomes, the more it tends to aggregate and becomes difficult to treat. Therefore, as the material powder to be used, it is desirable that the diameter is in the range of 0.5 to 2 .mu.m and that the powder is composed of sintering aids and crystal grains of silicon nitride whose longer-axis diameter is not more than 200 nm, as shown in FIG. 1. The method for fabricating such a powder is desirably a mechanical alloying process in which powder is milled at high acceleration and thereby alloyed. Although the mechanical alloying has conventionally been considered as a phenomenon that can occur in metals, a phenomenon has been found that a plurality of ceramics grains are refined and simultaneously fused by milling at higher acceleration than ever so as to be formed into a powder.

As the silicon nitride material powder, it is also desirable to use amorphous silicon nitride powder. The reason is that amorphous powder will be divided into fine crystal grains when crystallized. As the method for reducing the grain size of the amorphous powder in its crystallization, it is further desirable that a fine oxide whose mean particle size is not more than 100 nm is used as sintering aids in addition to the above-described method. Although the reason is not well known, it may be that when amorphous powder is divided into fine crystal grains, the fine oxide acts like a crystal nucleus. To add such fine sintering aids, the conventional method may be used where fine sintering aids whose mean particle size is not more than 100 nm is mixed with amorphous powder by a ball mill or the like. However, generally, the finer the powder becomes, the more it tends to aggregate and the more difficult to uniformly mix. Accordingly, it is preferable to use the sol-gel process in which an oxide is synthesized by hydrolyzing a solution having metal alkoxide or the like dissolved in an organic solvent, so that the amorphous powder and the oxide sintering aids will be uniformly mixed.

Further, it is desirable to use spark plasma sintering or microwave sintering as the sintering method for the above processes. The spark plasma sintering is preferred since the oxidized layer on the particle surface is removed by spark plasma during the sintering process so that particles are activated while electric-field diffusion is developed among grains by spark, whereby sintering and crystallization are carried out at low temperatures. Similarly, the microwave sintering is also preferred since sintering progresses from the interior of the sintered body, whereby sintering and crystallization are carried out at low temperatures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of a composite powder fabricated by mechanical alloying.

According to the drawing, (1) is a composite grain having a diameter of 0.5-2 .mu.m, (2) represents sintering aids having a diameter of 200 nm or less, and (3) represents silicon nitride grains in which the longer-axis diameter is 200 nm or less.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(EXAMPLE 1)

A silicon nitride powder whose mean particle size is 0.5 .mu.m, with sintering aids added thereto, was sealed with Ar gas in a metal pot (SUS304, inner diameter: 100 mm.phi.) by a ZrO.sub.2 ball mill and then subjected to mechanical alloying for one hour at room temperature by a planetary ball mill. The number of self-rotations of the mill was 500 rpm and its number of orbit-revolutions was 200 rpm. When the powder was evaluated by a transmission electron microscope, its particle diameter was 1 .mu.m and the mean longer-axis diameter of silicon nitride crystal grains in the powder was 10 nm. The sintering aids were 0.2 .mu.m in mean particle size and weighted as 2 wt % of Al.sub.2 O.sub.3, 1 wt % of MgO, and 5 wt % of Y.sub.2 O.sub.3, by weight per cent relative to 100 of silicon nitride powder.

Next, a mixed powder was pressed, and sintered under conditions as shown in Table 1. The resulting sintered body was cut into bending strength test pieces of 3.times.4.times.40 mm, and finished with cutting work by a #800 diamond grindstone. Thereafter the test pieces were checked for three-point bending strength in accordance with JIS R 1601. Further, relative density, fracture toughness value K.sub.IC, and mean longer-axis diameter of each sintered body were determined. Crystal grains were lapping-processed by one arbitrary cross section of the sintered bodies and then etching-processed for 30 min. with a 80.degree. C. etching solution (HF: HNO=2:1). Thereafter their mean grain size was observed by a scanning electron microscope at a magnification of 100000. The results are shown in Table 2.

Also, as comparative examples, samples were fabricated by wet mixing the above powder by a common ball mill (Nos. 15, 16), and another sample was fabricated by sintering the powder not by plasma sintering but by hot-press sintering (hereinafter abbreviated as H. P.) (No. 4). These samples were also evaluated.

TABLE 1______________________________________ Product of temp. Sintering and sinter-Sample temp. Sintering Mixing Sintering ing timeNo. (.degree.C.) time method method (.degree.C. .multidot. sec)______________________________________1 1100 5 min mechanical plasma alloying sintering2 1150 5 min mechanical plasma alloying sintering3 1200 5 min mechanical plasma alloying sintering4 1300 2 hr mechanical H.P. alloying sintering5 1300 5 min mechanical plasma alloying sintering6 1300 15 min mechanical plasma alloying sintering7 1400 4 min mechanical plasma 336000 alloying sintering8 1500 4 min mechanical plasma 360000 alloying sintering9 1500 10 min mechanical plasma 900000 alloying sintering10 1600 2 min mechanical plasma 192000 alloying sintering11 1750 2 min mechanical plasma 210000 alloying sintering12 1850 1 min mechanical plasma 111000 alloying sintering13 1900 1 min mechanical plasma 114000 alloying sintering14 1950 1 min mechanical plasma alloying sintering15 1200 5 min ball mill plasma sintering16 1300 5 min ball mill plasma sintering______________________________________ TABLE 2__________________________________________________________________________ Room temperature 1100.degree. C. Bending Fracture Bending FractureSample Relative Mean longer-axis strength toughness strength toughnessNo. density (%) diameter (nm) (kg/mm.sup.2) (MPa .multidot. m.sup.1/2) (kg/mm.sup.2) (MPa .multidot. m.sup.1/2)__________________________________________________________________________1 90 10 50 6.5 40 6.22 95 10 120 7.0 100 7.03 100 10 700 25 680 244 100 100 20 5.5 10 5.05 100 20 650 20 630 196 100 30 650 19 630 187 100 40 600 18 580 178 100 50 600 17 580 169 100 250 350 4.8 300 4.210 100 150 520 16 515 1411 100 180 520 15.5 510 1412 100 190 510 15 500 1413 100 195 505 15 500 1414 100 250 300 4.4 120 4.015 100 500 150 4.8 140 4.016 100 500 130 4.4 120 4.0__________________________________________________________________________

From the results of Table 2, it can be understood that the samples of the embodiments of the present invention have bending strengths and fracture toughnesses equivalent to or more than those of the conventional comparative example (No. 4) by H. P. sintering and those of the comparative examples (Nos. 15, 16) by mixing by a ball mill, in either case of room temperature and 1100.degree. C. In particular, samples of 50 nm or less particle sizes (Nos. 5, 6, 7, 8) can be found to have been improved substantially in bending strength and fracture toughness.

The other comparative examples (Nos. 1, 2, 9, 14) are found to have substantially deteriorated in bending strength and fracture toughness due to improper sintering temperature or time. Temperatures higher than 1900.degree. C. caused occurrence of grain growth, with deteriorated characteristics.

Next, with the time of mechanical alloying reduced to 10 min, a composite powder whose powder diameter was 2 .mu.m and whose mean longer-axis grain diameter of fine crystal grains in powder was 300 nm was prepared and used. Then the mean longer-axis diameter of sintered body was only 300 nm and the bending strength was as low as 200 kg/mm.sup.2 even under sintering conditions of 1300.degree. C..times.5 min.

(EXAMPLE 2)

An amorphous silicon nitride powder whose mean diameter was 0.5 .mu.m, with sintering aids added thereto at the same ratio as in Example 1, was mixed for 100 hours by a nylon ball mill. Then the mixed powder was pressed and sintered each for 7 min. under conditions as shown in Table 3. As in Example 1, characteristics of the resulting sintered bodies are shown in Table 4.

TABLE 3______________________________________ Product of temperature Sintering tem- and sinteringSample No. perature (.degree.C.) Sintering method time (.degree.C. .multidot. sec)______________________________________17 1100 plasma sintering18 1150 plasma sintering19 1200 plasma sintering20 1300 plasma sintering21 1300 H.P. sintering22 1400 plasma sintering 58800023 1500 plasma sintering 630000______________________________________ TABLE 4__________________________________________________________________________ Room temperature 1100.degree. C. Bending Fracture Bending FractureSample Relative Mean strength toughness strength toughnessNo. density (%) grain size (nm) (kg/mm.sup.2) (MPa .multidot. m.sup.1/2) (kg/mm.sup.2) (MPa .multidot. m.sup.1/2)__________________________________________________________________________17 90 amorphous 50 3.5 40 3.018 95 amorphous 80 4.8 70 4.319 99 50 650 18 620 1620 100 100 550 16 520 1521 90 100 50 5 10 4.022 100 200 500 15 440 1323 100 300 300 4.8 200 4.0__________________________________________________________________________

From the results of Table 4, it can be understood that the samples of the embodiments of the present invention have bending strengths and fracture toughnesses higher than those of the conventional comparative example (No. 21) sintered by H. P. The other comparative examples (Nos. 17, 18, 23) are found to have been insufficiently crystallized or substantially deteriorated in bending strength and fracture toughness due to improper sintering temperature.

(EXAMPLE 3)

To an amorphous silicon nitride powder whose mean particle size was 0.5 .mu.m, metal alkoxides of yttrium, aluminium, and magnesium were added to 5 wt %, 2 wt %, and 1 wt %, respectively, by conversion to oxides, and ultrasonically mixed by adding ethanol to them. The mixed solutions were hydrolyzed by adding ammonium and water thereto, whereby fine particles of Y.sub.2 O.sub.3, Al.sub.2 O.sub.3, and MgO were synthesized. Then the mixed solutions were dried and thus mixed powders of the amorphous powder and the oxides were prepared. When the oxides were evaluated for particle size by TEM (Transmission Electron Microscope), the results were 50 nm for Y.sub.2 O.sub.3, 80 nm for Al.sub.2 O.sub.3, and 30 nm for MgO.

Next, the mixed powders were pressed and sintered under conditions as shown in Table 5 each for 7 min. As in Example 1, characteristics of the resulting sintered bodies are shown in Table 6.

TABLE 5______________________________________ Product of temperature Sintering tem- and sinteringSample No. perature (.degree.C.) Sintering method time (.degree.C. .multidot. sec)______________________________________24 1100 plasma sintering25 1150 plasma sintering26 1200 plasma sintering27 1300 plasma sintering28 1300 H.P. sintering29 1400 plasma sintering 58800030 1500 plasma sintering 630000______________________________________ TABLE 6__________________________________________________________________________ Room temperature 1100.degree. C. Bending Fracture Bending FractureSample Relative Mean strength toughness strength toughnessNo. density (%) grain size (nm) (kg/mm.sup.2) (MPa .multidot. m.sup.1/2) (kg/mm.sup.2) (MPa .multidot. m.sup.1/2)__________________________________________________________________________24 90 amorphous 50 3.5 40 3.025 95 amorphous 80 4.8 70 4.326 99 10 800 20 680 1727 100 20 750 18 580 1528 90 30 20 5 10 4.029 100 100 550 16 500 1530 100 250 450 4.8 400 4.0__________________________________________________________________________

From the results of Table 6, it can be understood that the resulting sintered bodies are finer in grain size and improved in bending strength and fracture toughness, as compared with the results of Table 4 in which oxide sintering aids with mean particle size 0.2 .mu.m was used. Also, samples of Nos. 24 and 25 had not been crystallized due to low sintering temperatures, with the result of substantially low strengths.

(EXAMPLE 4)

The powders fabricated in Example 1 were plasma-sintered at a sintering temperature of 1300.degree. C. for 5 min. From these plasma-sintered samples, 20 test pieces were cut out, and evaluated for grain size, bending strength, and fracture toughness. Then results with remarkably high reliability were obtained as a mean grain size of 20 nm, a mean bending strength of 650 kg/mm.sup.2, a Weibull coefficient of 25, and a mean toughness of 20 MPa.m.sup.1/2.

(EXAMPLE 5)

The compacts fabricated in Example 1 were sintered for 7 min. under conditions as shown in Table 7. Microwave sintering conditions other than sintering temperature and sintering time were a nitrogen gas pressure of 1 atm and a microwave frequency of 20 GHz. On the resulting sintered body samples, evaluation was made in the same way as in Example 1. The results are shown in Table 8.

TABLE 7______________________________________ Product of tem- perature and sinter-Sample No. Sintering temperature (.degree.C.) ing time (.degree.C. .multidot. sec)______________________________________31 110032 115033 120034 130035 1400 58800036 1500 630000______________________________________ TABLE 8__________________________________________________________________________ Room temperature 1100.degree. C. Bending Fracture Bending FractureSample Relative Mean strength toughness strength toughnessNo. density (%) grain size (nm) (kg/mm.sup.2) (MPa .multidot. m.sup.1/2) (kg/mm.sup.2) (MPa .multidot. m.sup.1/2)__________________________________________________________________________31 90 10 50 3.5 40 3.032 95 10 80 4.8 70 4.333 99 10 700 18 680 1734 100 20 600 16 580 1535 100 50 550 15 540 1536 100 100 350 4.8 330 4.0__________________________________________________________________________

From the above results, it can be understood that the samples other than those of Nos. 31, 32, and 36 whose sintering temperature and sintering time were improper have remarkably superior bending strength and fracture toughness.

According to the present invention, silicon nitride ceramics can be offered which are superior in both bending strength and fracture toughness. The silicon nitride ceramics are very useful for a wide range of applications from automobile engine members that demand high reliability to structural ceramic materials for wear-resistant tools and the like.

Claims

1. A silicon nitride based sintered body consisting essentially of silicon nitride and/or sialon crystals whose mean longer-axis diameter is not more than 200 nm.

2. The sintered body of claim 1, wherein the mean longer-axis diameter of the silicon nitride and/or sialon crystals is not more than 50 nm.