Sialon-based sintered body and process for producing same

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a SiAlON-based sintered body useful for obtaining various engineering ceramics having a high mechanical strength and a high toughness, and a process for producing the SiAlON-based sintered body.
2. Description of the Related Art
M.alpha.-SiAlON is a material having a crystal structure of .alpha.-silicon nitride (Si.sub.3 N.sub.4) in which Si is partially substituted by Al and N is partially substituted by O, and a modifying cation M is solid-solved in interstices of the lattice, the M.alpha.-SiAlON being represented by the formula (I): M.sub.x (Si, Al).sub.12 (O, N).sub.16 where M stands for Li, Mg, Ca, Y or a lanthanide element and x has a value satisfying 0<x.ltoreq.2. This M.alpha.-SiAlON has characteristic properties suitable for an engineering ceramic, such as a high hardness, a low thermal expansion coefficient and an excellent corrosion resistance. Nevertheless, a sintered body of M.alpha.-SiAlON phase alone does not provide satisfactory characteristics, such as a mechanical strength or fracture toughness, for an engineering ceramic, and one reason therefor is considered to be an equiaxed crystal shape thereof.
To solve the above problems, composite SiAlON-based sintered bodies comprising a M.alpha.-SiAlON phase having a spherical crystal shape and a .beta.-SiAlON phase represented by the formula (II): Si.sub.6-z Al.sub.z O.sub.z N.sub.8-z (0<z.ltoreq.4.2), the .beta.-SiAlON phase having a crystal structure of .beta.-silicon nitride in which Si is partially substituted by Al and N is partially substituted by O and having a needle-like crystal shape, were proposed in Japanese Unexamined Patent Publication (Kokai) Nos. 58-185485, 58-204875 and 63-233082.
Nevertheless, the mechanical properties of these SiAlON-based sintered bodies are still not satisfactory for high temperature applications. Moreover, although yttrium is common as the modifying cation M, a Y-containing SiAlON-based sintered body has a low oxidation resistance at high temperatures. Therefore, an application of the Y-containing SiAlON-based sintered body to high temperature engine components such as gas turbine parts to be used under severe conditions, is not practical.
The object of the present invention is to solve the above problems and to provide a SiAlON-based sintered body having an excellent oxidation resistance as well as an excellent high temperature strength and toughness.
SUMMARY OF THE INVENTION
To attain the above and other objects, in accordance with the present invention, there is provided a SiAlON-based sintered body, comprising 5 to 50% by weight of a first crystal phase of RE.alpha.-SiAlON represented by the formula (III): RE.sub.x (Si, Al).sub.12 (O, N).sub.16 where RE stands for at least one modifying rare earth element selected from the group consisting of Ho, Er, Tm, Yb and Lu and x has a value of 0<x.ltoreq.2, 30 to 90% by weight of a second crystal phase of .beta.-SiAlON represented by the formula (II): Si.sub.6-z Al.sub.z O.sub.z N.sub.8-z (0<z.ltoreq.4.2), and up to 50% by weight of a third crystal phase containing at least one rare earth element RE selected from the group consisting of Ho, Er, Tm, Yb and Lu.
Also, there is provided a SiAlON-based sintered body, comprising 5 to 50% by weight of a first crystal phase of RE.alpha.-sailon represented by RE.sub.x (Si, Al).sub.12 (O, N).sub.16 where RE stands for at least one modifying rare earth element selected from the group consisting of Ho, Er, Tm, Yb and Lu and x has a value of 0<x.ltoreq.2, 30 to 90% by weight of a second crystal phase of .beta.-SiAlON represented by Si.sub.6-z Al.sub.z O.sub.z N.sub.8-z (0<z.ltoreq.4.2), 1 to 50% by weight of a third crystal phase containing at least one rare earth element RE selected from the group consisting of Ho, Er, Tm, Yb and Lu, and 1 to 20% by weight of a crystal phase of .alpha.-silicon nitride.

BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 1 and 2 show an X-ray diffraction pattern of the sintered body in the Examples.

DESCRIPTION OF THE PREFERRED EMBODIMENTS
In a SiAlON-based sintered body of the present invention there are essentially three crystal phases, including two of a first crystal phase of RE.alpha.-SiAlON and a second crystal phase of .beta.-SiAlON. The RE.alpha.-SiAlON phase has a composition represented by the formula (III) and the .beta.-SiAlON phase has a composition represented by the formula (II). The compositions of these phases are not necessarily the same in all crystal grains in the sintered body. As disclosed in Japanese Unexamined Patent Publication (Kokai) No. 58-185485, etc., the M.alpha.-SiAlON crystal is equiaxed and the .beta.-SiAlON crystal is needle-like, and by preparing a composite of these crystal phases having different crystal shapes, an improvement of a mechanical strength and a fracture toughness, etc., of the M.alpha.-SiAlON sintered body is obtained.
The content of the M.alpha.-SiAlON crystal phase is generally 5 to 50% by weight, preferably 10 to 40% by weight and that of the .beta.-SiAlON phase is generally 40 to 95% by weight, preferably 50 to 90% by weight. If the content of the M.alpha.-SiAlON crystal phase is less than 5% by weight, the machanical strength at a high temperature is detriorated. If the content of the M.alpha.-SiAlON crystal phase is higher than 50% by weight, the mechanical strength and fracture toughness are lowered and not sufficient for engineering applications. The grain size of the M.alpha.-SiAlON is generally about 0.1 to 5 .mu.m, preferably 0.2 to 3 .mu.m. The diameter of the .beta.-SiAlON needle crystals is also generally 0.1 to 5 .mu.m, preferably 0.2 to 3 .mu.m, and the size of the long-axis of needle crystals is 0.5 to 50 .mu.m, preferably 1 to 30 .mu.m. The grain size of the M.alpha.-SiAlON is not critical. If the size is too large, a fracture toughness and a mechanical strength of the SiAlON-based sintered body is deteriorated.
In the RE.alpha.-SiAlON phase of the present invention, since Ho, Er, Tm, Yb or Lu, whose ionic radius is smaller than that of yttrium of the prior art, is solid-solved in interstices of the .alpha.-SiAlON lattice, the oxidation resistance of the SiAlON-based sintered body is improved and the SiAlON-based sintered body may be used stably even in an oxygen-containing atmosphere at a high temperature. Particularly, the oxidation resistance of the SiAlON-based sintered body is better when the element solid-solved in the interesices is Er, Tm, Yb or Lu.
The third crystal phase of the SiAlON-based sintered body of the present invention is a crystal phase containing a rare earth element RE, i.e., Ho, Er, Tm, Yb or Lu.
In a first embodiment, the third crystal phase comprises a melilite crystal phase of RE.sub.2 Si.sub.3 N.sub.4 O.sub.3, optionally with 15R-type AlN polytype SiAlON crystal phase.
The melilite is an agglomerate of fine crystals grown in the grain boundaries of the sintered body. By the formation of the melilite crystals, the heat resistance of the grain boundaries is remarkably improved and the mechanical strength at a high temperature is increased. Thus, the melilite crystal phase allows an improvement of a mechanical strength of the sintered body. The 15R-type AlN polytype SiAlON crystal phase is not essential to an improvement of the mechanical strength of the sintered body, but it is often formed simultaneously with the melilite phase and does contribute to an improvement of the heat resistance of the grains boudary phase of the sintered body.
The content of the melilite phase together with the AlN polytype SiAlON crystal, if any, is generally up to 10% by weight, preferably 2 to 8% by weight. If the content of these phases is outside of this range, the mechanical strength of the sintered body is lowered and is not-uniform among the sintered bodies.
The sintered body may contain a glass phase, but the content of the glass phase should be less than 5% by weight. If the content of the glass phase is more than 5% by weight, the mechanical strength at a high temperature is lowered.
The thus obtained SiAlON-based sintered body mainly comprises equiaxed and needle-like crystals having a size or diameter of 0.1 to 5 .mu.m which are three-dimensionally cross-linked with each other. In the body, the crystals have thermal expansion coefficients slightly different from each other, and the content of the grain boundary glass phase, one casues for a deterioration of the high temperature characteristics, is extremely low. As a result, the mechanical properties at a high temperature are excellent, and composite effects derived from whisker reinforcement, such as a particle dispersing effect, micro-cracking effect and pull-out effect, are created to increase the fracture toughness.
The sintered body of this embodiment may be produced by preparing a powder mixture of up to 60% by weight of a RE.alpha.-SiAlON powder mainly composed of a RE.alpha.-SiAlON phase represented by the formula (III) and containing oxygen in an amount excess to the stoichiometric amount of the formula (III) by up to 8% by weight, and the remainder being a powder composed of silicon, oxygen and nitrogen and containing oxygen in an amount of 1.0 to 5.0% by weight, and then sintering said powder mixture at a maximum temperature in a range of 1600.degree. to 2100.degree. C.
The RE.alpha.-SiAlON powder may be any powder mainly composed of a RE.alpha.-SiAlON phase represented by the formula (III), but a powder prepared in accordance with the disclosure of Japanese Patent Publication (Kokai) No. 62-223009, the assignee of which is the assignee of this application, is preferred. This RE.alpha.-SiAlON powder is prepared by mixing the powders of a) amorphous silicon nitride, b) metal aluminum or aluminum nitride, c) a metal salt which will form an oxide of a rare earth element RE (RE is Ho, Er, Tm, Yb or Lu), the RE being an element to be solid-solved in interstices in .alpha.-SiAlON lattice, and optionally, d) an oxygen-containing compound of aluminum or silicon, and heating the mixture in a nitrogen-containing atmosphere at a temperature of 1300.degree. to 1900.degree. C. The thus obtained RE.alpha.-SiAlON powder has a fine and uniform primary particle size of 0.2 to 2 .mu.m and containing little free carbon and metal impurities. Therefore, the RE.alpha.-SiAlON powder allows a sintered body without pores and having an extraordinary grain growth to be obtained.
The RE.alpha.-SiAlON powder used as the starting material in the process of the present invention should contain oxygen in an amount greater than the stoichiometric amount of the formula (III) by 8% by weight or less, preferably 1 to 6.5% by weight, more preferably 2 to 4% by weight. If the amount of excess oxygen is too high, the sintered body contains a considerable amount of a phase having a low melting point, resulting in a deterioration of the mechanical properties.
The method of adding excess oxygen to the RE.alpha.-SiAlON powder may be, for example, an addition of an oxygen-containing compound of silicon, aluminum or a rare earth element RE to the amorphous silicon nitride in an excess amount in the process of preparing the powder, or a heating of the RE.alpha.-SiAlON powder in an oxygen-containing atmosphere. In the latter, for example, a RE.alpha.-SiAlON powder is heated to 800.degree. to 1200.degree. C. in an oxygen-containing atmosphere. This may be carried out, for example, by allowing the RE.alpha.-SiAlON powder placed on a supporting plate in an oxygen-containing atomosphere for a certain time period, or fluidizing the RE.alpha.-SiAlON powder in an oxygen-containing atmosphere.
The content of the RE.alpha.-SiAlON powder in the starting powder mixture is up to 60% by weight, preferably 20 to 50% by weight. If this exceeds 60% by weight, the mechanical strength and the fracture toughness are lowered.
The powder (Si--N--O powder) composed of silicon, oxygen and nitrogen and containing oxygen in an amount of 1.0 to 5.0% by weight may be a single powder containing silicon, oxygen and nitrogen in the particle or a mixture of powders of silicon nitride, silicon oxynitride and silicon dioxide. By using powders containing an adequate amount of oxygen, the volume of a liquid phase having an adequate viscosity formed during heating is increased, whereby a densification is accelerated and a sintered body having a high density is obtained.
A preferable powder is an .alpha.-silicon nitride powder containing oxygen in an amount of 1.0 to 5.0% by weight. It is also preferably that, from the viewpoint of sinter-activeness, the .alpha.-silicon nitride powder has an average particle size of 1 .mu.m or less and more than 60% of the oxygen exists in a surface layer having a depth of 50 nm of each powder, and the content of metal impurities other than Si which deteriorates the mechanical strength, corrosion resistance and oxidation resistance at a high temperature is not more than 0.1% by weight.
The Si--O--N powder may further contain aluminum or a modifying rare earth element RE. The aluminum or a modifying rare earth element RE may be contained in the Si--O--N powder, or a powder of oxide, nitride or oxynitride of aluminum or a modifying rare earth element RE may be mixed to the Si--O--N powder. The powder containing aluminum or a modifying rare earth element RE preferably does not contain metal impurities other than Si in an amount more than 0.1% by weight.
The content of Si--O--N powder in the starting powder mixture is 40% by weight or more, preferably 60 to 90% by weight. The higher the content of the Si--O--N powder, the higher the content of the .beta.-SiAlON phase in the sintered body. If the content of the Si--O--N powder in the starting powder mixture exceeds 90% by weight, the sintering of the powder mixture becomes difficult and the bulk density of the sintered body is lowered.
The method of preparing a powder mixture is not limited and may be a known method, for example, a dry mixing of the respective starting powders or a combination of a wet mixing of the starting powders in an inactive solvent and removing the solvent. Preferably, the mixing machine is a V-shape mixer (twin-cylinder blender), a ball mill or a vibration mill, etc.
The sintering of the starting powder mixture may be carried out, for example, by forming the dry or wet mixed powders into a desired shape, drying the shaped body if wet mixed, and then sintering it in a an ambient or pressurized nitrogen-containing non-oxidizing atmosphere, or by charging the starting powders in a die having a desired shape and effecting a hot press, or the like. The thus-obtained sintered body may be further subjected to a hot isostatic pressing (HIP), to obtain a further densification of the sintered body.
The forming of the starting powders before the sintering may be carried out by known methods, such as a rubber pressing, a uniaxial pressing, a slip casting, a injection molding, an impact forming, etc.
The sintering temperature is generally 1600.degree. to 2100.degree. C., preferably 1700.degree. to 1900.degree. C., and the sintering time 2100.degree. C., preferably 1700.degree. to 1900.degree. C., and the sintering time is generally 0.5 to 10 hours. If the heating temperature is too low, the sintering does not proceed, and if the sintering temperature is too high, the composition of the sintered body is changed by thermal decomposition.
Thus, in the above process, a SiAlON-based sintered body comprising a RE.alpha.-SiAlON phase, a .beta.-SiAlON crystal phase and a melilite crystal phase, optionally with a 15R-type AlN polytype SiAlON crystal phase and a glass phase, is obtained. It is considered that a .beta.-SiAlON crystal phase and a melilite crystal phase, optionally with a 15R-type AlN polytype SiAlON crystal phase and a glass phase, are formed by a reaction of the starting RE.alpha.-SiAlON powder and Si--O--N powder. The resultant RE.alpha.-SiAlON crystal of the sintered body has a composition in which the value x is slightly less than that of the starting RE.alpha.-SiAlON powder.
In a second embodiment of the SiAlON-based sintered body of the present invention, the first and second crystal phases of RE.alpha.-SiAlON and .beta.-SiAlON are the same as in the first embodiment, but preferably the grain size of the RE.alpha.-SiAlON crystal is 0.1 to 10 .mu.m in average and the long-axis size of the .beta.-SiAlON crystal is 0.5 to 50 .mu.m in average If the grain size of the RE.alpha.-SiAlON or the size of the long-axis of the .beta.-SiAlON crystal are too large, the mechanical strength and fracture toughness of the SiAlON-based sintered body are deteriorated. In a second embodiment, the third crystal phase is a fine crystal phase of defect-fluorite structure RE.sub.2 M.sub.2-u O.sub.7-2u (RE stands for at least one rare earth element selected from the group consisting of Ho, Er, Tm, Yb and Yb, M stands for at least one of Hf and Zr, and u has a value of -1<u<1), optionally together with a fine crystal phase of rock salt structure MN.sub.v where M stands for at least one of Hf and Zr and v has a value of 0.7.ltoreq.v.ltoreq.1.2.
The fine crystal phase of defect-fluorite structure RE.sub.2 M.sub.2-u O.sub.7-zu is RE.sub.2 Hf.sub.2 O.sub.7, RE.sub.2 Zr.sub.2 O.sub.7 or a solid solution thereof, or a solid solution thereof with RE.sub.2 O.sub.3, HfO.sub.2 to ZrO.sub.2. The fine crystal phase of rock salt structure MN.sub.v is HfN, ZrN or a solid solution thereof, or a compound thereof having a non-stoichiometric composition. The shape and size of RE.sub.2 M.sub.2-u O.sub.7-2u and MN.sub.v are not critical, but usually they are present in grain boundary or a triple point as equiaxed or bulky crystals having 0.1 to 10 .mu.m. These fine crystal phases, dispersed in the SiAlON-based matrix or its grain boundary, improve the mechanical properties of the grain boundary phase (a crystalline phase containing RE.sub.2 M.sub.2-u O.sub.7-2u and/or a glass phase containing RE). The content of the fine crystal phase of defect-fluorite structure RE.sub.2 M.sub.2-u O.sub.7-2u is generally 1 to 20% by weight, preferably 2 to 15% by weight. The content of the fine crystal phase of rock salt structure MN.sub.v is generally 0.5 to 10% by weight, preferably 1 to 8% by weight. If the contents of these phases are outside these ranges, the mechanical strength of the sintered body is lowered and becomes not-uniform.
The sintered body of the second embodiment of the present invention may comprise a further crystal phase containing at least one rare earth element RE selected form the group consisting of Ho, Er, Tm, Yb and Lu. This RE-containing crystal phase may be, for example, melilite-type RE.sub.2 Si.sub.3 O.sub.3 N.sub.4, apatite-type RE.sub.10 (SiO.sub.4).sub.6 N.sub.2, Wollastonite-type RESiO.sub.2 N, Woehlerite-type RE.sub.4 Si.sub.2 O.sub.7 N.sub.2, garnet-type RE.sub.3 Al.sub.5 O.sub.12, mullite-type RE.sub.4 Al.sub.2 O.sub.9, RE.sub.3 AlSi.sub.2 O.sub.7 N.sub.2, RE.sub.2 Si.sub.2 O.sub.7, RE.sub.2 SiO.sub.5, etc. but is not limited thereto. These RE-containing crystal, phases are copresent with a glass phase in the grain boundary of the sintered body. If the content of these crystal phases is increased, the heat resistant of the grain boundary is improved. Thus the RE-containing crystal phase allows an improvement of the mechanical strength of the sintered body. The content of this RE-containing crystal phase is generally up to 10% by weight, preferably 2 to 8% by weight. If the content of this RE-containing crystal phase is more than 10% by weight, the mechanical strength of the sintered body is lowered and is not uniform.
The sintered body of the second embodiment of the present invention preferably further comprises an AlN polytype crystal phase of Al and Si containing complex oxynitrides. The AlN polytype crystal phase of Al and Si containing complex oxynitrides is a group of crystals having a near-AlN composition and a long periodic crystal structure, with Ramsdell symbols such as 15R, 21R, 27R, 8H, 12H and 2H.delta.. Here, R means the rhombohedral crystal structure and H means the hexagonal crystal structure. These AlN polytype crystals are grown from the Al--Si--O--N containing liquid phase formed at high temperature sintering conditions. The grain size of these crystals is typically 0.1 to 10 .mu.m. The content of the AlN polytype crystal phase is generally up to 10% by weight, preferably 2 to 8% by weight. If the content of the AlN polytype crystal phase is more than 10% by weight, the mechanical strength of the sintered body is lowered and is not uniform.
The sintered body may contain a glass phase, but the weight. If the content of the glass phase is more than 5% by weight, the mechanical strength of the sintered body at a high temperature is lowered.
The thus obtained SiAlON-based sintered body comprises a matrix of RE.alpha.-SiAlON crystals, .beta.-SiAlON crystals, and AlN polytype crystals of Al and Si containing complex oxynitrides, a grain boundary phase of RE-containing crystals and defect-fluorite-type RE.sub.2 M.sub.2-u O.sub.7-2u fine crystals and/or a glass phase, and a dispersing phase of rock salt-type MN.sub.v fine crystals. The dispersing phase may be a part of the matrix and/or a grain-boundary phase.
As described before, the RE.sub.2 M.sub.2-u O.sub.7-2u and MN.sub.v fine crystals improve the strength of the grain boundary phase, particularly the heat resistance of the glass phase, and provide excellent mechanical properties at a high temperature. The sintered body comprises various phases different in composition, crystal or amorphous, grain shape, grain size and thermal expansion coefficient, and as a result, fine micro-cracks are formed in the sintered body due to micro-strains after sintering. The above fine cracks, in combination with the reinforcing effect and the crack propagation preventing effect by the cross linking of the needle-like and equiaxed crystals, provide an improvement of the mechanical properties, such as a fracture toughness of the obtained sintered body.
The process for producing the above sintered body is not limited, but an example thereof is a process comprising preparing a powder mixture of up to 60% by weight of a RE.alpha.-SiAlON powder mainly composed of a RE.alpha.-SiAlON phase represented by the formula (III) and SiAlON phase represented by the formula (III) and containing oxygen in an amount greater than the stoichiometric amount of the formula (III) by up to 8% by weight, and up to 15% by weight of a hafnium nitride powder and/or a zirconium nitride powder, the remainder being a .alpha.-silicon nitride powder, and then sintering said powder mixture at a maximum temperature in a range of 1600.degree. to 2100.degree. C.
The starting RE.alpha.-SiAlON powder is the same as described before.
The .alpha.-silicon nitride powder preferably has an average particle size of 1 .mu.m or less for sintering, and further preferably the content of impurities which would deteriorate that strength, corrosion resistance and oxidation resistance at a high temperature of the sintered body is less than 0.1% by weight.
The .alpha.-silicon nitride powder may further contain aluminum or a modifying rare earth element RE. The aluminum or a modifying rare earth element RE may be contained in the .alpha.-silicon nitride powder, or a powder of oxide, nitride or oxynitride of aluminum or a modifying rare earth element RE may be mixed to the .alpha.-silicon nitride powder. The powder containing aluminum or a modifying rare earth element RE preferably does not contain metal impurities other than Si in an amount more than 0.1% by weight.
The amount of the .alpha.-silicon nitride powder in the starting powder mixture is 40% or more, preferably 50 to 90% by weight, more preferably 60 to 80% by weight. In these ranges, the higher the amount of the .alpha.-silicon nitride powder, the higher the content of the .beta.-SiAlON phase in the formed sintered body. If the amount of the is more than 90% by weight, the powder mixture is difficult to sinter and the bulk density of the obtained sintered body is lowered.
The starting powder mixture optionally further contains up to 10% by weight of hafnium nitride and/or zirconium nitride. For example, since the fine crystal MN.sub.v is formed by a reaction of the hafnium oxide and/or zirconium oxide in the staring powder mixture with nitrogen gas in the atmosphere during the sintering, the amount of the fine crystal MN.sub.v in the obtained sintered body may be increased be previously adding the hafnium nitride and/or zirconium nitride powder to the starting powder mixture.
The method of mixing, shaping and sintering may be carried out in the same manner as described for the first embodiment, but the sintering temperature is generally 1600.degree. to 2100.degree. C. If the temperature is below 1600.degree. C., it is difficult to obtain a high density SiAlON-based body. If the temperature is above 2100.degree. C., the composition of the sintered body would be changed by a thermal decomposition of SiAlON.
By the above process, there is obtained a SiAlON-based sintered body which comprises a matrix of RE.alpha.-SiAlON crystals, .beta.-SiAlON crystals that are considered to be formed by a reaction of the RE.alpha.-SiAlON and .alpha.-silicon nitride, and AlN polytype crystals of Al and Si containing complex oxynitrides, a grain boundary phase of RE-containing crystals and fluorite-type RE.sub.2 M.sub.2-u O.sub.7-2u fine crystals and/or a glass phase, and a dispersing phase of rock salt-type MN.sub.v fine crystals, the RE.alpha.-SiAlON crystal having a composition represented by the formula (III) in which x has a value slightly smaller than that of the starting .alpha.-SiAlON powder.
In order to form heat resistant crystal phase and glass phase in the grain boundaries of the SiAlON-based sintered body, it is important to form AlN polytype crystals of an Al and Si containing complex oxynitride under the sintering conditions. Therefore, preferably a formation of the AlN polytype crystals is accelerated, for example, by using a RE.alpha.-SiAlON powder having an excess oxygen amount of 5% by weight or less as the starting RE.alpha.-SiAlON powder, or by adding an aluminum nitride powder, or by increasing the nitrogen pressure in the sintering atmosphere, etc.
In a third embodiment of the present invention, the SiAlON-based sintered body comprises a first crystal phase of RE.alpha.-SiAlON, a second crystal phase of .beta.-SiAlON, another RE-containing crystal phase, and a crystal phase of .alpha.-silicon nitride.
The first crystal phase of RE.alpha.-SiAlON, the second crystal phase of .beta.-SiAlON and the another RE-containing crystal phase are the same as in the first or second embodiment. Namely, the SiAlON-based sintered body of the third embodiment is very similar to the SiAlON-based sintered bodies of the first and second embodiments except that it further comprises a crystal phase of .alpha.-silicon nitride.
The .alpha.-silicon nitride crystal is a fine particulate crystal having a size of 0.02 to 2 .mu.m and would have to be converted to .beta.-Si.sub.3 N.sub.4 by keeping at a high temperature during sintering. If the sintering is carried out under such conditions that the .alpha.-Si.sub.3 N.sub.4 remains by shortering the sintering time or lowering the sintering temperature, etc., all grains constituting the sintered body (.alpha.-SiAlON, .beta.-SiAlON, etc.) are fine. In a sintered body composed of such fine grains, the size of defects, origins of a fracture, is decreased and the breaking energy is increased, so that the mechanical strength of the sintered body is improved. Further, since there are a plurality of crystal grains having different moduluses in the sintered body, the mechanical strength of the sintered body is improved.
The .alpha.-silicon nitride crystal preferably has a grain size of 0.02 to 2 .mu.m, more preferably 0.05 to 1 .mu.m. If the grain size of the .alpha.-silicon nitride crystal is larger than 2 .mu.m, the mechanical strength and fracture toughness of the SiAlON-based sintered body is deteriorated.
Note, in the third embodiment, the long axis size of the RE.alpha.-SiAlON crystal is generally 0.01 to 10 .mu.m, preferably 0.2 to 5 .mu.m in average, and the long axis size of the .beta.-SiAlON crystal is generally 0.5 to 50 .mu.m, preferably 1 to 30 .mu.m in average. The grain size of the another RE-containing crystal phase is as described in the first and second embodiments.
The content of the .alpha.-silicon nitride crystal phase is generally 1 to 40% by weight, preferably 5 to 20% by weight. If the content of the .alpha.-silicon nitride crystal phase is less than 1% by weight, the grain growth of the other phases is not suppressed and an improvement of the mechanical properties, toughness, etc. of the sintered body is not obtained. If the content of the .alpha.-silicon nitride crystal phase is higher than 40% by weight, the sintering is not proceeded sufficiently and the mechanical strength of the sintered body is lowered.
Note, in the third embodiment, the content of the first crystal phase of RE.alpha.-SiAlON is generally 5 to 50% by weight, preferably 10 to 40% by weight, the content of the second crystal phase of .beta.-SiAlON is generally 30 to 90% by weight, preferably 40 to 85% by weight, the content of the another RE-containing crystal phase is preferably 1 to 20% by weight, more preferably 2 to 15% by weight.
The SiAlON-based sintered body of the third embodiment comprises .beta.-SiAlON crystals, .alpha.-silicon nitride fine crystals (probably not converted from the starting .alpha.-silicon nitride), RE-containing crystals and/or a glass phase, as well as RE.alpha.-SiAlON crystals.
As described above, the .alpha.-silicon nitride fine crystals improve the mechanical properties of the sintered body. Further, also in the third embodiment, the sintered body comprises various phases different in composition, crystal or amorphous, grain shape, grain size and thermal expansion coefficient, and as a result, fine micro-cracks are formed in the sintered body due to micro-strains after sintering. In combination with the matrix reinforcing effect and the crack propagation suppressing effect by the three-dimensional cross-linking of the needle-like and equiaxed crystals, the above fine micro-cracks provide an improvement of the mechanical properties, such as a fracture toughness of the obtained sintered body.
The process for producing the above mentioned sintered body is not limited, but an example thereof is a process comprising preparing a powder mixture of up to 60% by weight of a RE.alpha.-SiAlON powder mainly composed of a RE.alpha.-SiAlON phase represented by the formula (III) and containing oxygen in an amount excess to the stoichiometric amount of the formula (III) by up to 8% by weight, the remainder being a .alpha.-silicon nitride powder, and then firing said powder mixture at a maximum temperature in a range of 1600.degree. to 2100.degree. C.
This process is very similar to the process described for the sintered body of the second embodiment. The difference is that, in the process of the second embodiment, the starting .alpha.-silicon nitride reacts with the RE.alpha.-SiAlON to form .beta.-SiAlON and all of the starting .alpha.-silicon nitride is consumed, but in the above process for producing the sintered body of the third embodiment, the starting .alpha.-silicon nitride also reacts with the RE.alpha.-SiAlON to form .beta.-SiAlON but a significant portion of the starting .alpha.-silicon nitride remains as an .alpha.-silicon nitride phase in the sintered body. This can be attained by lowering the sintered temperature, or shortening the soaking time at a high temperature, or raising the heating rate of the sintering, or using coarse .alpha.-silicon nitride powders, typically 0.8 to 5 .mu.m.
Namely, the starting materials, and the steps of mixing, shaping and sintering can be the same as described in the second embodiment, but the composition of the starting powder mixture is slightly changed. By slightly decreasing an amount of excess oxygen of the starting materials, the mass transport during a sintering is suppressed so that .alpha.-Si.sub.3 N.sub.4 remains.
By the above process, there is obtained a SiAlON-based sintered body which comprises a matrix of RE.alpha.-SiAlON crystals, .beta.-SiAlON crystals that is considered to be formed by a reaction of the RE.alpha.-SiAlON and .alpha.-silicon nitride, and .alpha.-silicon nitride crystals that have not been converted, a grain boundary phase of RE-containing crystals and/or a glass phase, the RE.alpha.-SiAlON crystals having a composition represented by the formula (III) in which x has a value slightly smaller than that of the starting .alpha.-SiAlON powder.
In order to improve an oxidation resistance of the SiAlON-based sintered body, it is preferred to precipitate fine crystals of Woehlerite-type RE.sub.4 Si.sub.2 O.sub.7 N.sub.2 in grain boundaries of the sintered body.
The thus obtained SiAlON-based sintered bodies of the first, second and third embodiments have improved mechanical characteristics, such as a high temperature strength and fracture toughness as well as a remarkably improved oxidation resistance, in comparison with SiAlON-based sintered bodies in the prior art. Therefore, the obtained SiAlON-based sintered body can be preferably used as highly reliable structural material, particularly in high temperature parts such as a rotor a starter and a burner of a gas turbine engine.
EXAMPLES
Reference Examples 1 to 6
Amorphous silicon nitride powders, rare earth element oxide (RE.sub.2 O.sub.3) powders and metal aluminum powders, with compositions shown in Table 1, were mixed in a nitrogen-containing atmosphere by a vibration mill for one hour. The mixed powders were charged in a carbon crucible and set in an electric heating-type furnace. In the furnace, the mixed powders were heated from the room temperature to 1200.degree. C. in one hour, from 1200.degree. C. to 1400.degree. C. in 4 hours and from 1400.degree. C. to 1600.degree. C. in 2 hours, to form crystalline RE.alpha.-SiAlON powders.
The characteristics of the obtained RE.alpha.-SiAlON powders are shown in Table 1.
EXAMPLES 1 TO 15 AND COMPARATIVE EXAMPLES 1 TO 7
The RE.alpha.-SiAlON powders obtained in Reference examples 1 to 6 were mixed with .alpha.-silicon nitride powders having the following properties:
______________________________________Specific surface area: 17.5 m.sup.2 /gParticle shape: equiaxed crystalFormed phase: .alpha.-phase > 95%Oxygen content: 2.3% by weightMetal impurities: less than 500 ppm______________________________________
The mixed powders having a composition as shown in Tables 2 and 3 were wet milled in ethanol as a medium in a ball mill for 48 hours, vacuum dried at 80.degree. C., and then formed by cold isostatic pressing at 1.5 tons/cm.sup.2. The shaped body was heated in nitrogen atmosphere at normal pressure from room temperature to 1800.degree. C. at a rate of 2.degree. C./min., and then kept at 1800.degree. C. for 4 hours.
The obtained SiAlON-based sintered body had properties shown in Tables 2 and 3.
The bending strengths of the sintered body at the room temperature and 1300.degree. C. were measured by cutting out 100 test pieces 3.times.4.times.40 mm in size from the sintered body, and setting the test pieces in a four point bending test jig.
The fracture toughness K.sub.IC of the sintered body was measured by the SEPB (single edge pre-cracked beam) method.
As a test of the oxidation resistance of the sintered body, test pieces were heated in air at 1350.degree. C. for 100 hours and the weight gain of the piece due to oxidation, and the bending strength of the piece at room temperature after the oxidation, were measured.
TABLE 1__________________________________________________________________________Rare Formed Excess Specificearth Composition of starting .alpha.-SiAlON Composition of RE.alpha.-SiAlON oxygen surfaceelement powders (g) phase powder (wt %)* content area grainRE AmorSN RE.sub.2 O.sub.3 Al (wt %) RE Si Al O N (wt %) (m.sup.2 /g) shape__________________________________________________________________________Reference Y 500 64.4 66.6 >90 7.4 44.6 9.9 4.8 33.3 2.8 2.5 equiaxedexample 1Reference Ho " 103.6 " >90 12.8 42.0 9.3 4.5 31.4 2.6 2.6 "example 2Reference Er " 104.8 " >90 13.0 41.7 9.4 4.8 31.1 2.9 2.3 "example 3Reference Tm " 105.8 " >90 13.1 41.9 9.3 4.6 31.1 2.7 2.8 "example 4Reference Yb " 108.1 " >95 13.4 41.7 9.2 4.7 31.0 2.8 2.4 "example 5Reference Lu " 109.1 " >95 13.5 41.6 9.3 4.7 30.9 2.9 2.2 "example 6__________________________________________________________________________ AmorSN: Amorphous silicon nitride *A desired composition is RE.sub.0.5 Si.sub.9.75 Al.sub.2.25 O.sub.0.75 N.sub.15.25 .delta. (.delta.: excess amount of oxygen)
TABLE 2__________________________________________________________________________ Bending strength Composition (MPa) of starting Room**Rare powders Temp. Weightearth (wt %) Bulk Formed phases (wt %)* strength grain after REelement .alpha.-Sia SN.alpha.- (g/cm.sup.3)density .alpha.-Sia .beta.-Sia liteMeli- AlN ##STR1## Temp.Room 1300.degree. C. oxidationafter (mg/cm.sup.3)**o xidation__________________________________________________________________________Example 1 Ho 40 60 3.281 >18 >67 >4 >3 7.4 1340 900 860 0.838Example 2 " 30 70 3.240 >14 >74 >3 >3 7.6 1390 920 890 0.821Example 3 Er 50 50 3.322 >26 >54 >5 >4 7.2 1200 890 970 0.323Example 4 " 40 60 3.289 >19 >66 >4 >3 8.3 1250 960 1020 0.301Example 5 " 30 70 3.248 >15 >73 >3 >3 7.9 1320 970 1070 0.284Example 6 " 20 80 3.208 >11 >80 >2 >2 7.8 1270 910 1100 0.265Example 7 Tm 40 60 3.274 >17 >69 >4 >3 7.6 1360 940 990 0.201Example 8 " 30 70 3.233 >13 >74 >3 >3 8.5 1390 990 1040 0.182Example 9 " 20 80 3.192 >9 >81 >2 >2 8.2 1260 890 1090 0.165Example 10 Yb 40 60 3.279 >21 >64 >4 >3 7.8 1330 980 1120 0.100Example 11 " 30 70 3.236 >16 >71 >3 >3 8.7 1380 1010 1180 0.093Example 12 " 20 80 3.198 >13 >77 >2 >2 8.6 1320 970 1200 0.088Example 13 " 10 90 3.160 >7 >85 >2 >1 8.0 1250 900 1090 0.084Example 14 Lu 40 60 3.294 >25 >59 >4 >3 8.9 1380 1000 1220 0.078Example 15 " 30 70 3.244 >17 >70 >3 >3 9.5 1420 1060 1250 0.070__________________________________________________________________________ .alpha.-Sia: .alpha.-SiAlON .alpha.-SN: .alpha.-silicon nitride .beta.-Sia: .beta.-SiAlON Melilite: RE.sub.2 Si.sub.3 N.sub.4 O.sub.3 AlN: 15Rtype AlN polytype SiAlON *The remainder of the formed phases is a glass phase. **Value measured after heat treating in air at 1350.degree. C. for 100 hours.
TABLE 3__________________________________________________________________________ Bending strength Composition (MPa) of starting Room**Rare powders Temp. Weightearth (wt %) Bulk Formed phases (wt %)* strength grain after REelement .alpha.-Sia SN.alpha.- (g/cm.sup.3)density .alpha.-Sia .beta.-Sia liteMeli- AlN ##STR2## Temp.Room 1300.degree. C. oxidationafter (mg/cm.sup.2)**o xidation__________________________________________________________________________Com- Y 70 30 3.267 >55 >35 0 0 6.4 860 620 570 1.147parativeExample 1Com- " 40 60 3.229 >15 >70 >3 >2 7.5 1300 900 600 0.981parativeExample 2Com- Ho 70 30 3.406 >50 >40 0 0 6.0 850 570 760 0.926parativeExample 3Com- Er 70 30 3.415 >52 >39 0 0 6.1 780 640 740 0.521parativeExample 4Com- Tm 70 30 3.408 >53 >37 0 0 6.3 830 590 770 0.431parativeExample 5Com- Yb 70 30 3.421 >56 >36 0 0 6.4 850 630 800 0.225parativeExample 6Com- Lu 70 30 3.430 >58 >34 0 0 6.2 810 650 780 0.174parativeExample 7__________________________________________________________________________ .alpha.-Sia: .alpha.-SiAlON .alpha.-SN: .alpha.-silicon nitride .beta.-Sia: .beta.-SiAlON Melilite: RE.sub.2 Si.sub.3 N.sub.4 O.sub.3 AlN: 15Rtype AlN polytype SiAlON *The remainder of the formed phases is a glass phase. **Value measured after heat treating in air at 1350.degree. C. for 100 hours.
EXAMPLES 16 TO 25 AND COMPARATIVE EXAMPLES 8 TO 14
The RE.alpha.-SiAlON powders used in Examples 1 to 15, .alpha.-silicon nitride powders having the following properties:
______________________________________Specific surface area: 10.0 m.sup.2 /gParticle shape: equiaxed crystalFormed phase: .alpha.-phas > 95%Oxygen content: 0.5% by weightMetal impurities: less than 500 ppm,______________________________________
and silicon dioxide powders having the following properties:
______________________________________Specific surface area: 8.0 m.sup.2 /gParticle shape: equiaxed crystalMetal impurities: less than 700 ppm,______________________________________
were combined in a ball mill to have the compositions shown in Tables 4 and 5. The starting powders were wet milled in ethanol as a medium in the ball mill for 48 hours, vacuum dried at 80.degree. C., and then formed by cold isostatic pressing at 1.5 tons/cm.sup.2. The shaped body was heated in nitrogen atmosphere at normal pressure from the room temperature to 1800.degree. C. at a rate of 2.degree. C./min., and then kept at 1800.degree. C. for 4 hours.
The obtained SiAlON-based sintered body had the properties shown in Tables 4 and 5.
TABLE 4__________________________________________________________________________ Bending strength Composition (MPa)Rare of starting Room**earth powders Temp. Weightele- (wt %) Bulk Formed phases (wt %)* strength grain after REment .alpha.-Sia SN.alpha.- SiO.sub.2 (g/cm.sup.3)density .alpha.-Sia .beta.-Sia liteMeli- AlN ##STR3## Temp.Room 1300.degree. C. oxidationafter (mg/cm.sup.2)**o xidation__________________________________________________________________________Exam- Ho 39 58.5 2.5 3.265 >19 >65 >4 >3 7.6 1320 910 870 0.830ple 16Exam- Er 39 58.5 2.5 3.272 >21 >63 >4 >3 7.8 1220 940 920 0.311ple 17Exam- " 28.5 68 3.5 3.254 >13 >75 >3 >3 7.5 1280 960 950 0.276ple 18Exam- " 19 78 3.0 3.220 >9 >81 >2 >2 7.7 1270 870 900 0.259ple 19Exam- Tm 39 58.5 2.5 3.277 >20 >65 >4 >3 7.4 1330 930 970 0.215ple 20Exam- " 28.5 68 3.5 3.249 >15 >72 >3 >3 8.2 1350 980 990 0.192ple 21Exam- Yb 39 58.5 2.5 3.266 >23 >61 >4 >3 8.5 1310 970 1090 0.116ple 22Exam- " 28.5 68 3.5 3.225 >14 >73 >3 >3 8.4 1370 1010 1130 0.102ple 23Exam- " 19 78 3.0 3.206 >12 >79 >2 >2 8.1 1300 960 1180 0.094ple 24Exam- Lu 39 58.5 2.5 3.278 >24 >60 >4 >3 8.8 1400 990 1200 0.087ple 25__________________________________________________________________________ .alpha.-Sia: .alpha.-SiAlON .alpha.-SN: .alpha.-silicon nitride .beta.-Sia: .beta.-SiAlON Melilite: RE.sub.2 Si.sub.3 N.sub.4 O.sub.3 AlN: 15Rtype AlN polytype SiAlON *The remainder of the formed phases is a glass phase. **Value measured after heat treating in air at 1350.degree. C. for 100 hours.
TABLE 5__________________________________________________________________________ Bending strength Composition (MPa)Rare of starting Room**earth powders Temp. Weightele- (wt %) Bulk Formed phases (wt %)* strength grain after REment .alpha.-Sia SN.alpha.- SiO.sub.2 (g/cm.sup.3)density .alpha.-Sia .beta.-Sia liteMeli- AlN ##STR4## Temp.Room 1300.degree. C. oxidationafter (mg/cm.sup.3)**o xidation__________________________________________________________________________Com- Y 40 60 0 2.367 >35 >55 0 0 6.2 800 580 350 2.873para-tiveExam-ple 8Com- " 39 58.5 2.5 3.090 >18 > 67 >3 >3 7.5 1250 920 640 0.974para-tiveExam-ple 9Com- Ho 40 60 0 2.752 >34 >57 0 0 5.9 780 500 400 2.010para-tiveExam-ple 10Com- Er 40 60 0 2.700 >31 >60 0 0 6.2 700 590 430 1.408para-tiveExam-ple 11Com- Tm 40 60 0 2.795 >33 >56 0 0 6.4 730 520 390 0.991para-tiveExam-ple 12Com- Yb 40 60 0 2.668 >36 >55 0 0 6.3 760 550 450 0.637para-tiveExam-ple 13Com- Lu 40 60 0 2.842 >38 >50 0 0 6.1 750 530 480 0.458para-tiveExam-ple 14__________________________________________________________________________ .alpha.-Sia: .alpha.-SiAlON .alpha.-SN: .alpha.-silicon nitride .beta.-Sia: .beta.-SiAlON Melilite: RE.sub.2 Si.sub.3 N.sub.4 O.sub.3 AlN: 15Rtype AlN polytype SiAlON *The remainder of the formed phases is a glass phase. **Value measured after heat treating in air at 1350.degree. C. for 100 hours.
EXAMPLES 26 TO 40 AND COMPARATIVE EXAMPLES 15 TO 21
The RE.alpha.-SiAlON powders used in Examples 1 to 15, .alpha.-silicon nitride powders (produced by Ube Industries Ltd.) having the following properties:
______________________________________Specific surface area: 11.5 m.sup.2 /gParticle shape: equiaxed crystalFormed phase: .alpha.-phase > 95%Oxygen content: 1.3% by weightMetal impurities: less than 500 ppm,______________________________________
and, hafnium oxide, zirconium oxide, hafnium nitride and zirconium nitride (hafnium oxide to zirconium nitride provided by Herman C. Starck Ltd.) were combined in a ball mill to have the compositions shown in Tables 6 and 7. The combined powders were wet milled in ethanol as a medium in the ball mill for 48 hours, vacuum died at 80.degree. C.
Then, 150 grams of the mixed powders were charged in a boron nitride coated graphite die having an inner diameter of 100 mm. The die was set in a hot apparatus device and was heated from room temperature to 1705.degree. C. at an elevation rate of 200.degree. C./hour under a pressure of 250 kg/cm.sup.2, and then kept at 1750.degree. C. and 250 kg/cm.sup.2 for one hour.
The bulk density and phase composition of the sintered bodies were measured and are shown in Table 6 and 7.
The properties of the obtained sintered bodies were evaluated in the same manner as in Examples 1 to 15, except that the bending strength at a high temperature was measured at 1400.degree. C. The properties of the sintered bodies are shown in Tables 6 and 7.
The Weibull modulus of the bending strengths at the room temperature were calculated for 50 test pieces, and were 20 to 30 in Examples 16 to 30 and 6 to 10 in Comparative examples 8 to 14.
TABLE 6 Bending strength (MPa) Room** Weight Rare Composition of starting Temp. grain earth powders (wt %) Bulk Formed phases (wt %)* strength after REelement Sia.alpha.- .alpha. SN HfO.sub.2 ZrO.sub.2 HfN ZrN (g/cm.sup.3)density Sia.alpha.- .beta. Sia REMO MN ANP ##STR5## Temp.Room 1400.degree. C. oxidationafter (mg/cm.sup.2)**oxidation Example 26 Ho 38 57 5 0 0 0 3.588 30 61 5.5 1.5 2 7.6 1530 790 980 0.868 Example 27 " 29 67 0 4 0 0 3.348 19 72.5 5 1.5 2 7.8 1580 830 1030 0.850 Example 28 Er 53 43 1 3 0 2 3.498 46 45.5 4 2 2.5 7.5 1400 760 1000 0.402 Example 29 " 39 59 0 2 0 0 3.352 32 63 2 1 2 8.5 1460 880 1110 0.378 Example 30 " 29 67 3 0 1 0 3.521 20 71 5 2.5 1.5 8.1 1520 920 1150 0.364 Example 31 " 19 77 2 2 0 0 3.398 9.5 82 4.5 2 2 8.0 1470 800 1210 0.350 Example 32 Tm 39 59 1 0.5 0.5 0 3.400 30.5 65 1.5 1.5 1.5 7.8 1560 910 1130 0.274 Example 33 " 29 67 4 0 0 0 3.492 19 73 5 1.5 1.5 8.7 1600 900 1180 0.260 Example 34 " 19 76 0 5 0 0 3.325 8 83 5.5 1.5 2 8.4 1460 810 1220 0.244 Example 35 Yb 39 59 1 1 0 0 3.376 32 63 2 1.5 1.5 8.0 1520 930 1250 0.186 Example 36 " 29 69 2 0 0 0 3.388 20 75 2 1 2 8.8 1540 960 1280 0.172 Example 37 " 19 78 2 0 1 0 3.407 13 81 2.5 1.5 2 8.7 1510 830 1330 0.165 Example 38 " 10 87 0 3 0 0 3.246 7 88 3 1 1 8.2 1460 750 1230 0.156 Example 39 Lu 39.5 59 1.5 0 0 0 3.375 33 63 1.5 1 1.5 9.1 1550 970 1350 0.134 Example 40 " 29 68 2 1 0 0 3.412 22 71 3 2 2 9.6 1600 950 1380 0.128 .alpha.-Sia: .alpha.-SiAlON .alpha.-SN: .alpha.-silicon nitride .beta.-Sia: .beta.-SiAlON MN: MN.sub.v ANP: AlN polytype crystal phase formed in Al--Si--O--N system (often mainly comprising 15Rtype crystal phase) *wt % of formed phases except for another REcontaining crystal phase and/or glass phase. **Value measured after heat treating in air at 1350.degree. C. for 100 hours.
TABLE 7 Bending strength (MPa) Room** Weight Rare Composition of starting Temp. grain earth powders (wt %) Bulk Formed phases (wt %)* strength after REelement Sia.alpha.- .alpha. SN HfO.sub.2 ZrO.sub.2 HfN ZrN (g/cm.sup.3)density Sia.alpha.- .beta. Sia REMO MN ANP ##STR6## Temp.Room 1400.degree. C. oxidationafter (mg/cm.sup.2)**oxidation Comparative Y 60 40 0 0 0 0 3.236 50 50 0 0 0 6.4 1010 380 620 1.436 Example 15 Comparative " 37 58 2 1 1 1 3.454 30 62.5 4 2 1.5 7.5 1430 850 690 1.160 Example 16 Comparative Ho 60 40 0 0 0 0 3.367 47 53 0 0 0 6.1 990 440 770 1.095 Example 17 Comparative Er 60 40 0 0 0 0 3.382 51 49 0 0 0 5.6 980 470 740 0.487 Example 18 Comparative Tm 60 40 0 0 0 0 3.363 48 52 0 0 0 6.0 1020 490 790 0.365 Example 19 Comparative Yb 60 40 0 0 0 0 3.375 53 47 0 0 0 5.8 1030 520 820 0.222 Example 20 Comparative Lu 60 40 0 0 0 0 3.392 54 46 0 0 0 6.5 1050 530 840 0.155 Example 21 .alpha.-Sia: .alpha.-SiAlON .alpha.-SN: .alpha.-silicon nitride .beta.-Sia: .beta.-SiAlON REMO: RE.sub.2 M.sub.2u O.sub.7-2u MN: MN.sub.v ANP: AlN polytype crystal phase formed in Al--Si--O--N system (often mainly comprising 15Rtype crystal phase) *wt % of formed phases except for another REcontaining crystal phase and/or glass phase. **Value measured after heat treating in air at 1350.degree. C. for 100 hours.
EXAMPLES 41 TO 55 AND COMPARATIVE EXAMPLES 22 TO 29
The RE.alpha.-SiAlON powders used in Examples 1 to 15, .alpha.-silicon nitride powders produced by UBE Industries Ltd., hafnium oxide powders, zirconium oxide powders, hafnium nitride powders and zirconium nitride powders were combined in a ball mill to obtain the compositions shown in Tables 8 and 9. The combined powders were wet milled in ethanol as a medium in the ball mill for 48 hours, vacuum dried at 80.degree. C.
The mixed powders were preliminarily formed into a rectangular shape by a mold having a cross section 50.times.80 mm in size, and a cold isostatic pressing of the preliminarily shaped body was performed at 1.5 tons/cm.sup.2. The shaped body was then heated under the conditions of the temperature and nitrogen gas pressure as shown in Tables 8 and 9, to perform a normal sintering or a gas pressure sintering in an atmosphere.
The properties of the obtained sintered bodies were evaluated in the same manner as in Examples 16 to 30 and are shown in Tables 8 and 9. The Weibull modulus (50 samples) were 18 to 28 in Examples 31 to 45 and 6 to 10 in Comparative examples 22 to 28.
TABLE 8 Bending strength (MPa) Sintering Room** Weight Rare Composition of conditions Temp. grain earth starting powders (wt %) Gas Bulk Formed phases (wt %)* strength after REelement Sia.alpha.- .alpha. SN HfO.sub.2 ZrO.sub.2 AlN kg/cm.sup.2 Gpressure (.degree.C.)Temp. (g/cm.sup.3)density Sia.alpha.- .beta. Sia REMO MN ANP ##STR7## Temp.Room 1400.degree. C. oxidati onafter (mg/cm.sup.2)**oxidation Example 41 Ho 38 57 2 3 0 1 1750 3.492 30 61 5 2 2 7.6 1370 760 850 0.890 Example 42 " 28 67 2 2 1 70 1900 3.438 20 71 3.5 2.5 3 8.3 1310 800 890 0.838 Example 43 Er 53 44 2 1 0 1 1720 3.488 44 49 3 2 2 7.1 1230 680 980 0.425 Example 44 " 28 67 0 3 2 1 1780 3.340 20 72 3 1.5 3.5 7.0 1210 700 1020 0.380 Example 45 " 39 59 2 0 0 70 1850 3.415 32 63 2 1.5 1.5 8.0 1260 790 1030 0.368 Example 46 " 19 78 0 3 0 150 1950 3.312 10.5 83 2 2.5 2 8.6 1190 810 1080 0.332 Example 47 Tm 38 57 2 1 2 1 1750 3.442 30 62 3.5 1.5 3 7.5 1330 750 970 0.285 Example 48 " 29 68 0 2 1 1 1780 3.304 20 74 2 1 3 7.7 1350 770 1040 0.268 Example 49 " 19 77 2 0 2 70 1950 3.345 10 83 1.5 1.5 4 8.4 1280 800 1080 0.240 Example 50 Yb 48 48 2 2 0 1 1720 3.478 39 54 4 2 1 7.5 1220 720 1120 0.190 Example 51 " 39 59 0 2 0 1 1780 3.338 31 64.5 2 1 1.5 7.3 1240 760 1150 0.176 Example 52 " 29 68 2 0 1 70 1800 3.383 21 72.5 2 1.5 3 8.3 1320 790 1170 0.158 Example 53 " 18 77 1 2 2 150 2000 3.332 12 79 3 1.5 4.5 9.1 1220 840 1200 0.145 Example 54 Lu 39 59 2 0 0 1 1750 3.428 32 64 2 1 1 7.3 1230 730 1180 0.140 Example 55 " 24 72 0 3 1 70 1900 3.310 17 76 2.5 2 2.5 8.6 1300 860 1240 0.090 .alpha.-Sia: .alpha.-SiAlON .alpha.-SN: .alpha.-silicon nitride .beta.-Sia: .beta.-SiAlON REMO: RE.sub.2 M.sub.2u O.sub.7-2u MN: MN.sub.v ANP: AlN polytype crystal phase formed in Al--Si--O--N system (often mainly comprising 15Rtype crystal phase) *wt % of formed phases except for another REcontaining crystal phase and/or glass phase. **Value measured after heat treating in air at 1350.degree. C. for 100 hours.
TABLE 9 Bending strength (MPa) Sintering Room** Weight Rare Composition of conditions Temp. grain earth starting powders (wt %) Gas Bulk Formed phases (wt %)* strength after REelement Sia.alpha.- .alpha. SN HfO.sub.2 ZrO.sub.2 AlN kg/cm.sup.2 Gpressure (.degree.C.)Temp. (g/cm.sup.3)density Sia.alpha.- .beta. Sia REMO MN ANP ##STR8## Temp.Room 1400.degree. C. oxidation after (mg/cm.sup.2)**oxidation Com- Y 60 40 0 0 0 1 1 750 3.198 50 50 0 0 0 5.6 880 290 570 1.337 parative example 22 Com- " 37 57 0 4 2 1 1750 3.215 30.5 60 4.5 2 3 7.0 1210 700 600 1.225 parative example 23 Com- " 27 68 2 2 1 70 1900 3.207 19.5 71 3.5 3 3 6.9 1260 810 640 1.113 parative example 24 Com- Ho 60 40 0 0 0 1 1750 3.322 44 56 0 0 0 5.8 880 360 700 1.042 parative example 25 Com- Er 60 40 0 0 0 1 1750 3.330 48 52 0 0 0 5.5 870 390 680 0.534 parative example 26 Com- Tm 60 40 0 0 0 1 1750 3.318 45 55 0 0 0 5.8 900 420 740 0.396 parative example 27 Com- Yb 60 40 0 0 0 1 1750 3.325 50 50 0 0 0 5.6 930 450 760 0.252 parative example 28 Com- Lu 60 40 0 0 0 1 1750 3.330 52 48 0 0 0 6.2 .alpha.-Sia: .alpha.-SiAlON .alpha.-SN: .alpha.-silicon nitride .beta.-Sia: .beta.-SiAlON REMO: RE.sub.2 M.sub.2u O.sub.7-2u MN: MN.sub.v ANP: AlN polytype crystal phase formed in Al--Si--O--N system (often mainly comprising 15Rtype crystal phase) *wt % of formed phases except for another REcontaining crystal phase and/or glass phase.
EXAMPLES 56 TO 70 AND COMPARATIVE EXAMPLES 30 TO 36
The RE.alpha.-SiAlON powders used in Examples 1 to 15 and .alpha.-silicon nitride powders (produced by Ube Industries Ltd.) having the following properties:
______________________________________Specific surface area: 11.5 m.sup.2 /gParticle shape: equiaxed crystalFormed phase: .alpha.-phase > 95%Metal impurities: less than 500 ppm,______________________________________
were combined in a ball mill as to obtain compositions shown in Tables 10 and 11. The combined powders were wet milled in ethanol as a medium in the ball mill for 48 hours, and vacuum dried at 80.degree. C.
The mixed powders were preliminarily formed into a rectangular shape by a mold having a cross section 50.times.80 mm in size, and a cold isostatic pressing of the preliminarily shaped body was performed at 1.5 tons/cm.sup.2. The shaped body was then heated in an electric furnace under a nitrogen atmosphere at the normal pressure from room temperature to 1750.degree. C. at an elevating rate of 2.degree. C./min., and then kept at 1750.degree. C. for 4 hours.
The bulk densities and phase compositions of the obtained sintered bodies were measured and are shown in Tables 10 and 11.
The properties of the obtained sintered bodies were evaluated in the same manner as in Examples 16 to 30 and are shown in Tables 10 and 11.
TABLE 10__________________________________________________________________________ Bending strength Composition (MPa) of starting Room**Rare powders Temp. Weightearth (wt %) Bulk Formed phases (wt %)* strength grain after REelement .alpha.-Sia SN.alpha.- (g/cm.sup.3)density SN.alpha.- .alpha.-Sia .beta.-Sia er***Anoth- ##STR9## Temp.Room 1300.degree. C. oxidationafter (mg/cm.sub.2)**o xidation__________________________________________________________________________Example 56 Ho 40 60 3.279 >8 >19 >58 >3 8.4 1390 870 890 0.796Example 57 " 30 70 3.242 >5 >14 >69 >3 8.6 1440 890 920 0.780Example 58 Er 50 50 3.320 >13 >26 >45 >4 8.2 1250 900 960 0.307Example 59 " 40 60 3.291 >9 >18 >59 >3 9.3 1300 960 1020 0.286Example 60 " 30 70 3.246 >7 >13 >68 >3 8.9 1370 950 1090 0.270Example 61 " 20 80 3.210 >4 >10 >75 >2 8.8 1320 920 1030 0.252Example 62 Tm 40 60 3.272 >10 >17 >60 >3 8.6 1410 940 990 0.191Example 63 " 30 70 3.235 >8 >13 >65 >3 9.5 1440 990 1040 0.173Example 64 " 20 80 3.190 >5 >9 >75 >2 9.2 1310 940 1020 0.157Example 65 Yb 40 60 3.281 >12 >16 >60 >3 8.8 1380 1010 1080 0.095Example 66 " 30 70 3.234 >9 >12 >65 >3 9.7 1430 1050 1130 0.088Example 67 " 20 80 3.200 >5 >8 >76 >2 9.6 1370 1020 1130 0.084Example 68 " 15 85 3.172 >3 >5 >83 >1 9.0 1300 920 1010 0.080Example 69 Lu 40 60 3.296 >15 >20 >51 >3 9.9 1430 1050 1160 0.074Example 70 " 30 70 3.242 >10 >17 >61 >3 10.5 1470 1090 1180 0.067__________________________________________________________________________ .alpha.-Sia: .alpha.-SiAlON .alpha.-SN: .alpha.-silicon nitride .beta.-Sia: .beta.-SiAlON *The remainder of the formed phases is a glass phase. **Value measured after heat treating in air at 1350.degree. C. for 100 hours. ***RE-contrary crystal phase, etc.
TABLE 11__________________________________________________________________________ Bending strength Composition (MPa) of starting Room**Rare powders Temp. Weightearth (wt %) Bulk Formed phases (wt %)* strength grain after REelement .alpha.-Sia SN.alpha.- (g/cm.sup.3)density SN.alpha.- .alpha.-Sia .beta.-Sia er***oth- ##STR10## Temp.Room 1300.degree. C. oxidationafter (mg/cm.sup.2)**o xidation__________________________________________________________________________Com- Y 70 30 3.267 0 >55 >35 0 6.4 860 620 550 1.417parativeexample 30Com- " 40 60 3.219 >5 >30 >53 >2 7.5 990 610 590 1.281parativeexample 31Com- Ho 70 30 3.406 0 >50 >40 0 6.0 850 570 760 0.926parativeexample 32Com- Er 70 30 3.415 0 >52 >39 0 6.1 780 640 740 0.521parativeexample 33Com- Tm 70 30 3.408 0 >53 >37 0 6.3 830 590 770 0.431parativeexample 34Com- Yb 70 30 3.421 0 >56 >36 0 6.4 850 630 800 0.225parativeexample 35Com- Lu 70 30 3.430 0 >58 >34 0 6.2 810 650 780 0.174parativeexample 36__________________________________________________________________________ .alpha.-Sia: .alpha.-SiAlON .alpha.-SN: .alpha.-silicon nitride .beta.-Sia: .beta.-SiAlON *The remainder of the formed phases is a glass phase. **Value measured after heat treating in air at 1350.degree. C. for 100 hours. ***RE-contrary crystal phase, etc.

Number	Date	Country
2-90140	Apr 1990	JPX
2-99253	Apr 1990	JPX
2-145301	Jun 1990	JPX

Number	Name	Date
4506020	Butler et al.	Mar 1985
4547470	Tanase et al.	Oct 1985
4563433	Yeckley et al.	Jan 1986
4818635	Ekstrom et al.	Apr 1989
5032553	Tarry	Jul 1991

Sialon-based sintered body and process for producing same

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