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° C. to 1400° C. or sintered with a product of sintering temperature (° C.) and sintering time (sec) below 600000 (° C.·sec) at a temperature of 1400° C. to 1900° 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° 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 α&#39;-sialon (general formula: M x  (Si, Al) 12  (O, N) 16 , wherein M represents Mg, Ca, Li, and rare-earth elements) and columnar β&#39;-sialon (including β 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 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 μm and an aspect ratio of 3 to 20. The composite materials indeed show a strength of 167 kg/mm 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 μm to 3.5 μm in shorter-axis diameter and 10 μm to 14 μm in longer-axis diameter, such that the matrix itself can make a fracture source. The strength in its embodiment is 158 kg/mm 2  at a maximum, and the baking temperature is above 1800° 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 α, β-Si 3  N 4  and α, β-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° C. to 1400° C. Otherwise, for temperatures of 1400° C. to 1900° C., the method comprises a product of sintering temperature (° C.) and sintering time (sec) below 600000 (° C.·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 2  and a fracture toughness of 15 MPa.m 1/2   at temperatures lower than 1100° 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° C. to 1400° C. Temperatures lower than 1200° C. could not allow the sintering to progress, while temperatures exceeding 1400° 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 (° C.) and sintering time (sec) is below 600000 (° C.·sec) in a temperature range of 1400° C. to 1900° C. The products below 600000 (° C.·sec) could not allow the grain growth to progress even at temperatures above 1400° C., so that a longer-axis diameter not more than 200 nm can be obtained. Also, temperatures exceeding 1900° C. would make it impossible to suppress grain growth even with short time sintering, so that temperatures below 1900° 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 μm 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 μm, (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 μm, with sintering aids added thereto, was sealed with Ar gas in a metal pot (SUS304, inner diameter: 100 mmφ) by a ZrO 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 μm and the mean longer-axis diameter of silicon nitride crystal grains in the powder was 10 nm. The sintering aids were 0.2 μm in mean particle size and weighted as 2 wt % of Al 2  O 3 , 1 wt % of MgO, and 5 wt % of Y 2  O 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 3×4×40 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 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° 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.   (°C.)          time     method  method (°C. · 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° C.              Bending                    Fracture                           Bending                                 FractureSample    Relative     Mean longer-axis              strength                    toughness                           strength                                 toughnessNo. density (%)     diameter (nm)              (kg/mm.sup.2)                    (MPa · m.sup.1/2)                           (kg/mm.sup.2)                                 (MPa · 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° 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° 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 μm 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 2  even under sintering conditions of 1300° C.×5 min. 
     (EXAMPLE 2) 
     An amorphous silicon nitride powder whose mean diameter was 0.5 μm, 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 (°C.)               Sintering method                            time (°C. · 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° C.             Bending                   Fracture                          Bending                                FractureSample    Relative     Mean    strength                   toughness                          strength                                toughnessNo. density (%)     grain size (nm)             (kg/mm.sup.2)                   (MPa · m.sup.1/2)                          (kg/mm.sup.2)                                (MPa · 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 μm, 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 2  O 3 , Al 2  O 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 2  O 3 , 80 nm for Al 2  O 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 (°C.)               Sintering method                            time (°C. · 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° C.             Bending                   Fracture                          Bending                                FractureSample    Relative     Mean    strength                   toughness                          strength                                toughnessNo. density (%)     grain size (nm)             (kg/mm.sup.2)                   (MPa · m.sup.1/2)                          (kg/mm.sup.2)                                (MPa · 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 μm 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° 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 2 , a Weibull coefficient of 25, and a mean toughness of 20 MPa.m 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 (°C.)                      ing time (°C. · sec)______________________________________31       110032       115033       120034       130035       1400              58800036       1500              630000______________________________________ 
    
     
                                           TABLE 8__________________________________________________________________________             Room temperature                          1100° C.             Bending                   Fracture                          Bending                                FractureSample    Relative     Mean    strength                   toughness                          strength                                toughnessNo. density (%)     grain size (nm)             (kg/mm.sup.2)                   (MPa · m.sup.1/2)                          (kg/mm.sup.2)                                (MPa · 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.