Silicon nitride based sintered material and process of manufacturing same

Abstract: There is disclosed a silicon nitride based sintered material which contains 0.1% to 20% by volume of zirconium oxide, 0.1% to 14% by volume of zirconium nitride, 3% to 15% by volume of a binder phase of an Mg-Si-O-N or Mg-Si-Zr-O-N system, and balance β-silicon nitride. The sintered material may include an oxide layer of an average thickness of 10 to 1,000µm in a surface thereof and having a zirconium oxide concentration increasing toward the surface thereof. A process specifically adapted to manufacture the above sintered material is also disclosed.

Description: The present invention pertains to a silicon nitride (Si₃N₄) based sintered material which has high toughness and strength, and is excellent in antifracture characteristics, so that it can be suitably used to manufacture cutting tools to be employed under severe cutting conditions, and further relates to a process specifically adapted to manufacture this Si₃N₄ based sintered material. As disclosed in Japanese Patent Application B-Publication No. 60-20346, there is known an Si₃N₄ based sintered material which is manufactured by preparing powders of Si₃N₄, zirconium oxide (ZrO₂), and magnesium oxide (MgO) as starting materials; blending these powders into a prescribed composition; compacting the mixture into a green compact; and sintering the green compact at a temperature of 1,600°C to 2,100°C under a pressure of 0,15 MPa (1.5 atm) to 13,2 MPa (130 atm) in a nitrogen atmosphere or an atmosphere of a mixture of nitrogen and inert gases. This prior art Si₃N₄ based sintered material exhibits great strength, but its toughness is not sufficient. Therefore, when the material is, for example, employed to manufacture cutting tools for high-speed continuous wet cutting or rough milling of cast iron, the cutting edge is susceptible to fracturing or chipping, resulting in unduly short tool life. SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide a silicon nitride based sintered material which exhibits not only great strength but also excellent toughness, and hence is suitably used to manufacture cutting tools to be employed under severe cutting conditions. Another object of the invention is to provide a process suitably adapted to manufacture the aforesaid sintered material. According to a first aspect of the present invention, there is provided a silicon nitride based sintered material essentially consisting of 0.1 to 20% by volume of ZrO₂, 0.1 to 14% by volume of zirconium nitride (ZrN), 3 to 15% by volume of a binder phase of a system selected from the group consisting of Mg-Si-O-N system and Mg-Si-Zr-O-N system, and balance β-Si₃N₄. According to a second aspect of the invention, there is provided a process of manufacturing an Si₃N₄ based sintered material comprising the steps of: (a) preparing a powder mixture essentially consisting of 1 to 27% by weight of ZrN, 0.1 to 5% by weight of silicon oxide (SiO₂), 1 to 10% by weight of MgO and balance Si₃N₄; (b) compacting the powder mixture into a green compact; (c) subjecting the green compact to a primary sintering at a temperature of 1,500°C to 2,000°C under a pressure of 0,10 to 5,06 MPa (1 to 50 atm) in a nitrogen atmosphere; and (d) subjecting the compact thus sintered to a secondary sintering at least once at a temperature of 1,700°C to 2,000°C under a pressure of 10,13 to 202,6 MPa (100 to 2,000 atm) in a nitrogen atmosphere. In the foregoing, the sintered material may further include an oxide layer of an average thickness of 10 to 1,000µm formed in a surface thereof and having a ZrO₂ concentration increasing toward the surface thereof. For producing this modified sintered material, the green compact obtained from the above compacting step (b) may be kept in an oxidizing atmosphere containing 5 to 1,000 ppm of oxygen for 30 minutes to 3 hours during a temperature elevation step from 500°C to 1,300°C as part of the primary sintering. Alternatively, the oxide layer could be formed by keeping the material subjected to the secondary sintering at a temperature of 500°C to 1,000°C in air for 30 minutes to 3 hours. DETAILED DESCRIPTION OF THE INVENTION After an extensive study on the improvement of the characteristics of silicon nitride based sintered material, the inventors have come to know that if a green compact of a specific composition is subjected to a primary sintering under a relatively low pressure in a nitrogen atmosphere, an excellent degree of sintering can be ensured, and that if a secondary sintering is subsequently effected at least one time under a relatively high pressure in the same nitrogen atmosphere, the resulting sintered material exhibits excellent characteristics which meet the aforesaid requirements. More specifically, the inventors prepared powders of ZrN, SiO₂, MgO and Si₃N₄ (including α-Si₃N₄ as a principal constituent) as starting materials, and blended these powders to prepare a mixture essentially consisting of 1 to 27% by weight of ZrN, 0.5 to 5% by weight of SiO₂, 1 to 10% by weight of MgO and balance Si₃N₄. Then, they compacted the mixture into a green compact under usual conditions, and subjected the green compact to a primary sintering at a temperature of 1,500°C to 2,000°C under a relatively low pressure of 0,10 to 5,06 MPa (1 to 50 atm) in a nitrogen atmosphere. With this treatment, α-Si₃N₄ changed into β-Si₃N₄ in which grains were somewhat of a needle-like shape. In addition, a liquid phase of Mg-Si-O-N or Mg-Si-Zr-O-N system which was less viscous and had an excellent wettability was obtained as a binder phase or grain-boundary phase, and hence excellent degree of sintering was achieved. Subsequently, the material thus sintered was further subjected to a secondary sintering one time or more at a temperature of 1,700°C to 2,000°C under a relatively high pressure of 10,13 to 202,6 MPa (100 to 2000 atm) in the same nitrogen atmosphere. With this sintering, a part of ZrN which served as a constituent of a dispersed phase changed into ZrO₂ according to the following reactions: ZrN + SiO₂ + N₂ → ZrO₂ + Si₃N₄ (1) ZrN + binder phase (primary sintering) + N₂ → ZrO₂ + binder phase (secondary sintering) (2) During the formation of ZrO₂, internal micropores were eliminated to provide a more finely grown grain structure, and grains of β-Si₃N₄ became needle-like more fully. Furthermore, as compared with the binder phase formed by the primary sintering, the oxygen content in the binder phase decreased while nitrogen content therein increased. The resulting Si₃N₄ sintered material contained 0.1 to 20% by volume of ZrO₂, 0.1 to 14% by volume of ZrN, 3 to 15% by volume of Mg-Si-Zr-O-N system or Mg-Si-O-N system, and balance β-Si₃N₄ of needle-like grains, and exhibited excellent thermal shock resistance and wear resistance. Thus, Si₃N₄ sintered material provided in accordance with the present invention essentially consists of 0.1 to 20% by volume of ZrO₂, 0.1 to 14% by volume of ZrN, 3 to 15% by volume of binder phase of Mg-Si-Zr-O-N system. and balance β-Si₃N₄ of needle-like grains. The Si₃N₄ sintered material in accordance with the invention contains ZrN which exhibits high heat conductivity and hardness, and hence is excellent in thermal shock resistance and wear resistance. In addition, ZrO₂, which is chemically stable and enhances the resistance to oxidation, is contained therein and undergoes martensite transformation to absorb fracture energy when cracking has developed. This ZrO₂ and Si₃N₄ of needle-like grains ensure high toughness and cause the structure to become more minuscule to thereby enhance the strength of the material. In this sintered material, ZrO₂ forms a solid solution with Mg (with Si and N in some cases), and has a cubic or tetragonal structure. Furthermore, the binder phase, which has relatively high nitrogen content but less oxygen content, contributes to the enhancement of heat resistance. It has been realized that this binder phase is composed of one of glass phase, forsterite phase and enstatite phase, or a mixture of at least two of these phases. The reasons why the compositions in the Si₃N₄ based sintered material are determined as recited above will hereinafter be described. (a) ZrO₂ Zr, which has existed in a solid solution of the binder phase formed in the secondary sintering, mostly forms a solid solution with Mg and occasionally with Si and N during the cooling step and is deposited as ZrO₂. When cracking has developed, ZrO₂ thus formed is subjected to martensite transformation, whereby fracture energy can be absorbed thereinto, and the material is prevented from being broken. However, if its content is less than 0.1% by volume, the aforesaid advantages cannot be expected. On the other hand, if the content exceeds 20% by volume, the hardness unduly decreases, resulting in insufficient wear resistance. Therefore, the ZrO₂ content has been determined so as to range from 0.1% by volume to 20% by volume. (b) ZrN ZrN is of two kinds: one is obtained from material powder itself, while the other existed in a solid solution formed by the secondary sintering and is deposited during the cooling step. Inasmuch as ZrN has high heat conductivity and hardness, the resulting material exhibits high thermal shock resistance and wear resistance. However, if its content is less than 0.1% by volume, these advantages cannot be achieved. On the other hand, if the content exceeds 14% by volume, toughness and strength are unduly reduced. Therefore, the content is set to the range of between 0.1% and 14% by volume. (c) Binder Phase SiO₂ powder, MgO powder, SiO₂ existing on the surface of Si₃N₄ powder, and ZrN powder partly react with one another to produce a binder phase, in which oxygen content is relatively high while nitrogen content is low. Due to such oxygen-nitrogen concentration distribution, the binder phase exists as a liquid phase of low viscosity during the sintering operation, so that it facilitates sintering to thereby enhance the strength of the material. However, if the content of the binder phase is less than 3% by volume, a desired strength cannot be ensured. On the other hand, if the content exceeds 15% by volume, hardness and heat resistance are reduced unduly. Therefore, the content is set to the range of between 0.3% and 15% by volume. (d) β-Si₃N₄ β-Si₃N₄ is deposited while a liquid phase is formed by SiO₂ existing in the surface of Si₃N₄ powder, which contains α-Si₃N₄ as a principal constituent, SiO₂ powder, MgO powder and ZrN powder used as auxiliary constituents. The growth of the Si₃N₄ grains into needle-like grains is minimal during the primary sintering, but is great during the secondary sintering according to the aforesaid reaction (1). When the needle-like grains grow, the toughness is increased due to crack deflection. Furthermore, as the grains grow, micropores existing in the sintered material are absorbed, so that the sintered material develops a fine grain structure. Thanks to these structural changes, the strength of the sintered material can be markedly enhanced. As described above, Si₃N₄ based sintered material of the invention exhibits excellent toughness, strength and antifracture characteristics, and is also superior in thermal shock resistance and wear resistance. Accordingly, when employed as cutting tools for use under the conditions where the above characteristics are required, the sintered materials of the invention exhibit excellent performance for an extremely prolonged period of time. Therefore, although the demands for high-speed operation are increasing in the field of cutting tools, the cutting tools made of the sintered material of the invention can successfully meet these demands. Next, the process specifically suitable to manufacture the above sintered material will be described. As will be understood from the foregoing, powders of ZrN, SiO₂, MgO and Si₃N₄ (including α-Si₃N₄ as a principal constituent) are first prepared as starting materials, and are blended to prepare a mixture essentially consisting of 1 to 27% by weight of ZrN, 0.5 to 5% by weight of SiO₂, 1 to 10% by weight of MgO and balance Si₃N₄. Thereafter, the mixture is compacted into a green compact, which is then subjected to a primary sintering at a temperature of 1,500°C to 2,000°C under a pressure of 0,10 to 5,06 MPa (1 to 50 atm) in a nitrogen atmosphere. Subsequently, the material thus sintered is further subjected to a secondary sintering one time or more at a temperature of 1,700°C to 2,000°C under a pressure of 10,13 to 202,6 MPa (100 to 2000 atm) in the same nitrogen atmosphere. The secondary sintering may be carried out in succession with the primary sintering, or separately from the primary sintering. In addition, in the case where two or more secondary sintering operations are carried out, the second sintering operation may be carried out intermittently apart from the first sintering operation. With these secondary sintering operations, a part of ZrN changes into ZrO₂, during which internal micropores are eliminated to cause the structure to become minute, and grains of β-Si₃N₄ became needle-like more fully. Furthermore, as compared with the binder phase formed by the primary sintering, the oxygen content in the binder phase decreases while nitrogen content therein increases. With these procedures, the heat resistance can be improved. The reason why the manufacturing conditions are determined as set forth above will now be described. A. Blend Composition (a) ZrN powder During the primary sintering, ZrN exists as it is, and ensures an excellent degree of sintering. During the secondary sintering, a part of ZrN changes into ZrO₂, and the remaining ZrN imparts excellent thermal shock resistance and wear resistance to the sintered material, while ZrO₂ formed absorbs the fracture energy to enhance the toughness of the material. Accordingly, if its blend content is less than 1% by weight, the ZrO₂ content in the sintered material becomes less than 0.1% by volume, and the remaining ZrN content becomes less than 0.1% by volume. Hence, the aforesaid advantages cannot be expected. On the other hand, if the blend content exceeds 27% by weight, the ZrO₂ content becomes excessive, so that the hardness becomes insufficient to ensure a desired wear resistance. In addition, the ZrN content exceeds 14% by volume, so that the resistance to oxidation is reduced. Therefore, the ZrN content has been determined so as to range from 1% by weight to 27% by weight. (b) SiO₂ and MgO powders SiO₂ powder and MgO powder react with SiO₂ existing on the surface of Si₃N₄, and in some cases with ZrN powder during the primary sintering to produce an Mg-Si-O-N or Mg-Si-Zr-O-N liquid phase of low viscosity, in which oxygen content is relatively high while nitrogen content is low. An excellent sintering is ensured due to this liquid phase to produce a binder phase. In addition, during the secondary sintering, the oxygen content relatively decreases while the nitrogen content increases to thereby enhance heat resistance of the sintered material. However, if SiO₂ content and MgO content are less than 0.1% by weight or 1% by weight, respectively, a desired degree of sintering cannot be ensured. On the other hand, if SiO₂ content and MgO content exceed 5% by weight or 10% by weight, respectively, the content of the binder phase exceeds 15% by volume, so that the heat resistance of the sintered material is reduced unduly. Accordingly, the blend composition is determined so as to have 0.1 to 5% by weight of SiO₂ and 1 to 10% by weight of MgO. (c) Si₃N₄ powder As SiO₂ existing in the surface of Si₃N₄ powder, which contains α-Si₃N₄ as a principal constituent, SiO₂ powder and MgO powder used as auxiliary constituents, and ZrN powder form a liquid phase, Si₃N₄ changes into β-Si₃N₄ grains. The growth of β-Si₃N₄ is minimal during the primary sintering, but is remarkable during the second sintering, so that the needle-like grains grow according to the aforesaid reaction (1) during the secondary sintering. Therefore, micropores existing in the sintered material are absorbed, and the grain structure of the sintered material become fine, so that the strength of the material is markedly enhanced. Thus, Si₃N₄ powder is essential and is blended as a principal constituent so as to balance the aforesaid other constituents. In addition, in order to ensure an excellent and uniform degree of sintering, it is preferable to use powders of minuscule particles having a narrow particle size distribution. B. Conditions for Primary Sintering During the primary sintering, there must be produced a binder phase of low viscosity, in which oxygen content is relatively high while nitrogen content is low so that an excellent sintering can be ensured. However, if the sintering temperature is less than 1,500°C or if the sintering pressure is less than 0,13 MPa (1 atm), the binder phase cannot be formed sufficiently, and hence a satisfactory degree of sintering cannot be ensured. On the other hand, if the pressure exceeds 5,06 MPa (50 atm), nitrogen gas of such high pressure is confined within the sintered body, and adversely affects the degree of sintering. In addition, it impedes the absorption of micropores to prevent the sintered body from becoming minute and further to prevent needle-like β-Si₃N₄ grains from growing. Furthermore, in order to achieve a temperature exceeding 2,000°C, a manufacturing installation must be made unduly large in scale. Accordingly, the temperature and pressure at the primary sintering have been set to the range of 1,500 to 2,000°C and the range of 0,13 to 5,06 MPa (1 to 50 atm), respectively. C. Condition for Secondary Sintering During the secondary sintering, inasmuch as the aforesaid reactions (1) and (2) occur, the heat resistance of the binder phase is enhanced due to the decrease in oxygen content and increase in nitrogen content, and ZrO₂ is formed from a part of ZrN. In addition, the growth of needle-like β-Si₃N₄ grains is remarkably promoted, and the micropores existing in the sintered body are absorbed to cause the sintered body to become minute, whereby high toughness and high strength can be ensured. However, if the sintering temperature is less than 1,700°C, or if the sintering pressure is less than 10,13 MPa (100 atm), the aforesaid reactions (1) and (2) do not occur sufficiently, so that excellent characteristics as described above cannot be imparted to the sintered material. On the other hand, if the temperature exceeds 2,000°C, or if the pressure exceeds 202,6 MPa (2,000 atm), the installation must be made unduly large in scale. Accordingly, the temperature and pressure at the secondary sintering have been set to the range of 1700 to 2000°C and the range of 10,13 to 202,6 MPa (100 to 2000 atm), respectively. In the foregoing, although the primary and secondary sintering conditions depend on the blend composition as described above, the pressure at the secondary sintering should be naturally set greater than that at the primary sintering. Furthermore, the Si₃N₄ sintered material manufactured as described above may be modified so as to include an oxide layer of an average thickness of 10 to 1,000µm in a surface thereof and having a ZrO₂ concentration increasing toward the surface thereof. When a destructive stress is exerted on the surface of the material, this oxide layer serves to subject ZrO₂ to martensite transformation to thereby absorb the stress. Therefore, the resulting material is less susceptible to fracturing or chipping. However, if the average layer thickness of the oxide layer is less than 10µm, the additional advantage as described above cannot be obtained. On the other hand, it is found that if the layer thickness exceeds 1,000µm, the wear resistance abruptly decreases. Therefore, the average layer thickness of the oxide layer has been determined to the range of between 10 µm and 1,000µm. Furthermore, for forming the aforesaid oxide layer on the surface of the material, the above manufacturing method may be modified as follows: The atmosphere in which temperature is elevated from 500°C to 1,300°C in the primary sintering is modified to an oxidizing one containing 5 to 1,000 ppm of oxygen, and the green compact is kept in the atmosphere for a period of time of 30 minutes to 3 hours. Alternatively, without modifying the primary sintering operation, the material subjected to the secondary sintering may be kept at a temperature of 500°C to 1,000°C in atmospheric for 30 minutes to 3 hours. Furthermore, these oxidation treatments may be both carried out as appropriate. The present invention will now be described in more detail by way of the following examples. Example 1 There were prepared starting powders of Si₃N₄ (α/β=97/3 by volume ratio; oxygen content: 2% by weight), ZrN (oxygen content: 3% by weight), SiO₂, MgO and ZrO₂ each having an average particle size of 0.1 to 0.5µm. These powders were blended in compositions as set forth in Tables 1-1 and 1-2, and were subjected to wet mixing in a ball mill for a period of 72 hours. After being dried, the mixtures were compacted to form green compacts each having a size of 30mm x 30mm x 10mm and green compacts each having a shape of a cutting insert in conformity with JIS (Japanese Industrial Standard) SNGN120408. These green compacts were then subjected to primary and secondary sintering under the conditions as set forth in Tables 1-1 and 1-2 to produce sintered materials 1 to 14 of the invention, comparative sintered materials 1 to 8 and prior art sintered material 1. As to the various sintered materials thus obtained, their compositions were measured. In this example, the contents of ZrO₂ and ZrN were obtained by calculation using intensity measurement of X-ray diffraction peaks for Si₃N₄, ZrO₂ and ZrN, while the content of the binder phase of Mg-Si-O-N system or Mg-Si-Zr-O-N system was obtained by an EPMA (X-ray microanalyser) observation of a specular polished surface of the specimen. Moreover, theoretical density ratio, Rockwell hardness (A scale), transverse rupture strength were measured, and fracture toughness was also measured by indentation method. Furthermore, using a vertical milling machine, the sintered materials were subjected to interrupted cutting test of cast iron under the following conditions: Workpiece: cast iron (JIS.FC25) square bar with bores having a width of 150mm and a length of 300mm Cutting speed: 200 m/minute Depth of cut: 2 mm Feed rate: 0.25 mm/revolution Chamfer honing of cutting edge: 0.15 mm x -25° In this test, it was measured how long it took until the cutting edge was subjected to chipping. The results are set forth in Table 2. As will be clearly seen from Table 2, the sintered materials 1 to 14 of the invention had theoretical density ratios of no less than 99% and had less micropores formed, so that they were very fine. Therefore, the sintered materials 1 to 14 of the invention exhibited excellent toughness, strength and antifracture characteristics as compared with the comparative sintered materials 1 to 8 and the prior art material 1, and exhibited great hardness equivalent to the prior art materials. Example 2 Starting powders similar to those in Example 1 were prepared and blended in compositions as set forth in Table 3, and the same procedures as in Example 1 were repeated to produce green compacts. These compacts were then subjected to primary and secondary sintering under the conditions as set forth in Table 3 to produce sintered materials 15 to 28 of the invention and prior art sintered materials 2 to 4. As to the various sintered materials thus obtained, the their compositions, theoretical density ratios, Rockwell hardness (A scale), transverse rupture strength and fracture toughness were measured in a manner similar to Example 1. In addition, as to the surface oxide layer of each material, its layer thickness was measured and its ZrO₂ concentration distribution was observed by X-ray diffraction. Furthermore, the sintered materials were subjected to interrupted cutting test under the same conditions as in Example 1. The results are set forth in Table 4. As clearly seen from Table 4, the sintered materials 15 to 28 of the invention had theoretical density ratios of no less than 99% and had less micropores formed, so that they were very fine. Therefore, the sintered materials 15 to 28 of the invention exhibited excellent toughness, strength and antifracture characteristics as compared with the prior art sintered materials 2 to 4, and exhibit high hardness equivalent to the prior art materials. As described above, Si₃N₄ based sintered material of the invention exhibits excellent toughness, strength and antifracture characteristics as compared with the prior art sintered materials, and exhibit high hardness equivalent to the prior art materials. Accordingly, when employed as cutting tools for use under the conditions where the above characteristics are required, the sintered materials of the invention exhibit excellent performance for an extremely prolonged period of time. Therefore, although the demands for high-speed operation are increasing in the field of cutting tools, the cutting tools of the sintered material of the invention can meet these demands successfully. Obviously, numerous modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described herein.

Claims: A silicon nitride based sintered material essentially consisting of 0.1% to 20% by volume of zirconium oxide, 0.1% to 14% by volume of zirconium nitride, 3% to 15% by volume of a binder phase of a system selected from the group consisting of Mg-Si-O-N system and Mg-Si-Zr-O-N system, and balance β-silicon nitride. A silicon nitride based sintered material according to claim 1, further comprising an oxide layer of an average thickness of 10 to 1,000µm in a surface thereof and having a zirconium oxide concentration increasing toward the surface thereof. A process of manufacturing a silicon nitride based sintered material characterized by the steps of: (a) preparing a powder mixture essentially consisting of 1 to 27% by weight of zirconium nitride, 0.1 to 5% by weight of silicon oxide, 1 to 10% by weight of magnesium oxide and balance silicon nitride; (b) compacting said powder mixture into a green compact; (c) subjecting said green compact to a primary sintering at a temperature of 1,500°C to 2,000°C under a pressure of 0,10 to 5,06 MPa (1 to 50 atm) in a nitrogen atmosphere; and (d) subjecting the compact thus sintered to a secondary sintering at least once at a temperature of 1,700°C to 2,000°C under a pressure of 10,13 to 202,6 MPa (100 to 2,000) atm in a nitrogen atmosphere. A process of manufacturing a silicon nitride based sintered material according to claim 3, further comprising keeping said green compact obtained from said compacting step (b) in an oxidizing atmosphere containing 5 to 1,000 ppm of oxygen for 30 minutes to 3 hours during temperature elevation step from 500°C to 1,300°C in said primary sintering. A process of manufacturing a silicon nitride based sintered material according to claim 3 or claim 4, further comprising keeping said material subjected to said secondary sintering at a temperature of 500°C to 1,000°C in air for 30 minutes to 3 hours.

Inventors: AIKAWA YASUTAKA C O CHUO-KENKY, KOYAMA TAKASHI C O CHUO-KENKYU, OHSHIMA HIDEO C O CHUO-KENKYUS, AIKAWA, YASUTAKA, C/O CHUO-KENKYUSHO, KOYAMA, TAKASHI, C/O CHUO-KENKYUSHO, OHSHIMA, HIDEO, C/O CHUO-KENKYUSHO
Applicants: MITSUBISHI MATERIALS CORP, MITSUBISHI MATERIALS CORPORATION
Application year: 1991
Main classification: C04B35
IPC codes: C04B41, C04B35