A tool having a silicon nitride sintered body and method of producing same

Abstract: A tool having a silicon nitride sintered body excellent in toughness as well as wear resistance can be provided in economical manner. The feature of the tool consists in that the silicon nitride crystal grains comprise 50 to 90% by volume of hexagonal pillar crystals having a minor axis of at most 1.0 µm and 5 to 30% by volume of hexagonal pillar or columnar crystals having a minor axis of at least 1.5 µm. As the silicon nitride starting powder, there is preferably used a mixture of at least two silicon nitride powders prepared by the imide decomposition method, direct metallic silicon nitriding method and silica reducing method.

Description: This invention relates to a tool having a silicon nitride sintered body which has a good wear resistance as well as excellent toughness, which is particularly useful in the field of cutting tools or wear resistant tools and a method of producing same. Up to the present time, in tool parts of sintered aluminum oxides, it has been proposed to increase the transverse rupture strength and improve the wear resistance by rendering the crystal grain size of the sintered body as small as possible and simultaneously reducing the grain size distribution. Specifically, this proposal has been carried by adding a sintering aid capable of lowering the sintering temperature eg. NiO, ZrO₂, Y₂O₃ or by adding a sintering aid capable of suppressing the grain growth itself, eg. TiC. Increasing the transverse rupture strength and improving the wear resistance by rendering the crystal grain size of the sintered body as small as possible and reducing the grain size distribution is also an effective means for improving the transverse rupture strength as well as the wear resistance in sintered silicon nitrides. Thus, some techniques have been developed, for example, synthesis of fine grain high α- silicon nitride powder for the purpose of forming uniform nuclei of β- silicon nitride in the transformation of α→β, optimization of the sintering curves, uniform mixing of an sintering aid such as Y₂O₃, etc. As a result of these efforts for the technique development, silicon nitride sintered bodies have been employed as a tool material in the fields of cutting and machining cast irons in which aluminium oxide sintered bodies are hardly used, for example, in the field of rough cutting cast irons. As described above, the method comprising rendering the crystal grain size of the sintered body as small as possible and reducing the grain size distribution, thereby improving the transverse rupture strength as well as the wear resistance, is effective and has increasingly been used in the field of rough cutting cast irons, in which it difficult to use aluminum oxide sintered bodies. However, it is found by the inventors that when a silicon nitride sintered body is actually subjected to cutting of cast iron, wear of the silicon nitride sintered body rapidly proceeds by such a mechanism that the crystal grains fall through abrasion wear due to the presence of a very hardened layer (chilled layer) on the surface of the cast iron or due to the so-called sand-containing phenomenon. That is, it is apparent that there is a limitation for improving the transverse rupture strength and the wear resistance of a sintered silicon nitride tool by the method comprising rendering the crystal grain size of the sintered body as small as possible and reducing the grain size distribution. EP-A-0356244 discloses a silicon nitride sintered body having a composition of 1.5 - 3.0 weight % of Y₂O₃, 0.1 - 1.0 weight % of Al₂O₃ and the balance Si₃N₄, formed by sintering at a temperature of 1900 - 2100°C. EP-A-0277753 discloses a silicon nitride sintered body having a composition of 1.5 to 3.0% by weight of Y₂O₃ and 96 to 98.4% by weight of Si₃N₄, formed by sintering at a temperature of 1850 to 2000°C. Neither of these documents disclose a silicon nitride sintered body which has an improved transverse rupture strength and wear resistance. Accordingly, the present invention provides a tool having a silicon nitride sintered body containing crystal grains comprising 50 to 90% by volume of hexagonal pillar crystals having a minor axis of at most 1.0µm and 5 to 30% by volume of hexagonal pillar crystals having a minor axis of at least 1.5µm, and further containing from greater than 1% to 10% by weight of Al₂O₃ and 1% to 10% by weight of Y₂O₃ as a sintering aid. The present invention can thus provide a tool having a silicon nitride sintered body capable of exhibiting excellent toughness and wear resistance in use. An embodiment of the present invention can also provide a tool having a silicon nitride sintered body which has a longer life as compared with prior art tools having silicon nitride sintered body consisting of only fine crystal grains. Furthermore, the present invention provides a process for the production of said tool comprising the steps of using a silicon nitride powder having a grain size of 1.0µm or smaller, mixing the material with from greater than 1% to 10% by weight of Al₂O₃ and 1% to 10% by weight of Y₂O₃ as a sintering aid, pulverizing the resulting mixture, subjecting the pulverized mixture to press molding and then subjecting the resulting molded article to sintering in a nitrogen atmosphere at a pressure of 1 to 300 atm and a temperature of from 1700°C to less than 1850°C. As the imide, there is for example used Si(NH₃)₂. In one embodiment the method includes further subjecting the product after the sintering to HIP treatment at a temperature of 1600 to 1900°C and at a nitrogen gas pressure of 10 to 3000 atm. In a preferred embodiment of the present invention, the above described tool having a sintered silicon nitride body can be produced by using raw material powders prepared by at least two of imide decomposition methods, direct silicon nitriding methods and silica reducing methods. Examples of the present invention will now be described in more detail. In the development of materials for the silicon nitride sintered tools, it has been tried to increase the transverse rupture strength and improve the wear resistance by rendering the crystal grain size of the sintered body as small as possible and reducing the grain size distribution. However, it is generally known in ceramic sintered bodies or compacts that the smaller is the grain size of crystal grains composing the matrix, the more increased is the transverse rupture strength of the sintered bodies or compacts, but the smaller is the fracture toughness. It has been found by our studies that in cutting cast iron by a tool of a sintered silicon nitride body, abrasion wear tends to occur according to such a mechanism that the crystal grains fall due to the presence of a very hardened layer (chilled layer) and sand-containing layer on the surface of the cast iron and the wear of the tool rapidly proceed. Therefore, it is desired that a material for a cast iron cutting tool has a high fracture toughness and in this respect, the above described tool of the prior art is insufficient for the abrasion wear and has a limitation. To this end, tile inventors have made studies to develop a material having an improved transverse rupture strength and improved wear resistance, in particular, high resistance to the abrasion wearing from a different standpoint from that of the prior art and thus, have found that such a material as to resist to the abrasion wear can be obtained by composing a silicon nitride sintered body of fine crystal grains and coarse crystal grains. That is, a silicon nitride sintered body is composed of a mixture of fine crystal grains and coarse crystal grains aiming at providing a high strength and hardness by the fine grains and a high toughness by the coarse grains, and is applied to cutting cast irons, thus obtaining good results. Specifically, the silicon nitride sintered body is composed of crystal grains comprising 50 to 90 % by volume of fine hexagonal pillar or columnar crystal grains having a minor axis of at most 1.0µm, preferably 0.1 to 1.0µm and 5 to 30 % by volume of coarse hexagonal pillar or columnar crystal grains having a minor axis of at least 1.5 µm, preferably 1.5 to 3.0µm, since 50 % or more of fine crystal grains having an average minor axis of at most 1 µm are necessary for achieving a strength for cutting cast irons and 5 to 30 % by volume of coarse crystal grains having an average minor axis of at least 1.5µm are necessary for increasing the resistance to the abrasion wear. If the amount of the fine crystal grains having a minor axis of at most 1µm exceeds 90 % by volume, the object of the present invention, i.e. improving the abrasion wear resistance cannot be attained, while the amount of the coarse crystal grains having a minor axis of at least 1.5 µm exceeds 30 % by volume, the transverse rupture strength is lowered. Furthermore, it is impossible to prepare a sintered body of grains having a minor axis of at most 0.1 µm from commercially available silicon nitride powders and using grains having a minor axis of at least 3.0 µm, the transverse rupture strength is markedly lowered. In the silicon nitride sintered body, from greater than 1 to 10 % by weight of Al₂O₃ and 1 to 10 % by weight of Y₂O₃ are contained as a sintering aid. Since α-silicon nitride has such a property that it is dissolved in a liquid phase to reprecipitate β- silicon nitride nuclei and tends to be grown as a long hexagonal pillar crystal in the C axis direction of hexagonal system, if more, there can readily be obtained a high strength silicon nitride sintered body in which hexagonal pillar crystals are entangled with each other. By incorporation of the above described sintering aids, β- silicon nitride tends to grow in a hexagonal pillar form and thus a high strength sintered body can more readily be obtained (Cf. Japanese Patent Publication No. 21091/1973). The reason for limiting the quantities of Al₂O₃ and Y₂O₃ respectively to 1 to 10 % by weight is that in any case, if less than 1 % by weight, the effect of addition thereof is little, while if more than 10 % by weight, the wear resistance is remarkably lowered. The silicon nitride sintered body can be produced by the use of a silicon nitride powder obtained by the imide decomposition method, that obtained by reducing and nitriding SiO₂ powder or that obtained by directly nitriding metallic silicon, as a raw material. In particular, two or more of these powders are preferably used in combination. Since grain growth of the silicon nitride powder take place during sintering, it is naturally required to use, as a raw material, a silicon nitride powder having a grain size of 1.0 µm or smaller. The fine-coarse grains-mixed structure of the present invention can be prepared by controlling the conditions in the step of sintering these silicon nitride powders, or by mixing a fine powder and coarse powder. As to the raw material powders, there are powders tending to grow readily and being hard to grow, depending upon the history and grain size thereof. As the powder to be used, the powder hard to grow is preferable since control of the grain size thereof is relatively readily carried out. When using raw material powders prepared by at least two kinds of methods selected from the group consisting of imide decomposition methods, direct metallic silicon nitriding methods and silica reducing methods, a sintered body or compact can readily be produced in which fine grains and coarse grains of silicon nitride are uniformly dispersed, which is preferred. This is probably due to that the liquidus temperature and the β-nuclei forming temperature during transformation of α-silicon nitride into β-type are different, depending upon the history of preparation of the silicon nitride, so that the degrees of the grain growth thereof be different and the fine-coarse grainsmixed structure can readily be obtained. The silicon nitride sintered body can be produced by the following methods. Firstly, a silicon nitride powder, α- Al₂O₃ powder and Y₂O₃ powder are mixed it, a predetermined proportion in a ball mill, then granulated, press molded and finally sintered In a nitrogen atmosphere under pressure. As occasion demands, the thus resulting sintered product is subjected to a HIP treatment. During the same time, the sintering should preferably be carried out at a temperature of 1700 to 1850°C and a pressure of 1 to 300 atm, since if lower than 1700°C, the crystal grains are too fine and if higher than 1850°C, the crystal grains are too coarse, while if lower than 1 atm, the silicon nitride tends to be decomposed during sintering and if higher than 300 atm, such a high pressure is commercially meaningless. The HIP treatment should preferably be carried out at a temperature of 1600 to 1900°C and a nitrogen pressure of 10 to 3000 atm, since if lower than 1600°C, there is little HIP effect and if higher than 1900°C, the grain growth is too remarkable, while if lower than 10 atm, there is little pressure effect and if higher than 3000 atm, such a high pressure is commercially meaningless. As illustrated above, a tool having a silicon nitride sintered body is described which has both excellent strength and wear resistance as a result of the silicon nitride sintered body body having a fine-coarse grains-mixed structure, the crystal grains being composed of 50 to 90% by volume of hexagonal pillar crystals having a minor axis of at most 1.0 µm and 5 to 30% by volume of hexagonal pillar crystals having a minor axis of at least 1.5 µm. The silicon nitride sintered body can favorably be compared with the prior art tool of the sintered silicon nitride body consisting of only fine grains, in particular, in respect of the lengthened life. This largely contributes to the industry. Specific examples of the present invention will now be described. Example 1 A commercially available α- silicon nitride powder prepared by the imide decomposition method was mixed with α- Al₂0₃ powder and Y₂o₃ powder in a proportions as shown in Table 1, Sample Nos. 1 to 6, ball milled, pulverised and then subjected to press molding at a pressure of 1 ton/cm². The resulting molded products were sintered in a nitrogen atmosphere at a pressure of 5 atm and a temperature of 1800 °C for 2 hours to obtain Sample Nos. 1 to 3, and further subjected to a HIP treatment at a temperature of 1750 °C and a pressure of 1000 atm to obtain Sample Nos. 4 to 6. The crystal grains of the resulting sintered bodies were examined by subjecting the fractures thereof to measurement by SEM, ion etching or hot NaOH etching. These sintered bodies (Sample Nos. 1 to 6) were worked into throwaway inserts of JIS SNG 433 by grinding, which were then subjected to the following two cutting tests. For comparison, other throwaway inserts (Sample Nos. 7 to 10) outside the scope of the present invention were subjected to the similar cutting tests. Cutting Test 1 Machining System: turning Workpiece: FC 25 (H B = 180) Cutting Speed: 600 m/min Feed:0.4 mm/rev Cutting Depth: 1.5 mm Cutting Test 2 Machining System: milling Workpiece: FC 25 (H B = 200) Cutting Speed: 200 m/min Feed:0.25 mm/edge Cutting Depth: 2.5 mm Sample No. Silicon Nitride Powder Al₂O₃ Powder (wt %) Y₂O₃ Powder (wt %) Silicon Nitride Crystals ≦ 1 µm ≧ 1.5 µm 1 imide decomposition method 2 5 60 10 2 -do- 5 8 70 20 3 -do- 3 3 80 8 4 -do- 2 5 65 8 5 -do- 5 8 75 12 6 -do- 3 3 85 5 7 -do- 3 3 70 2 8 -do- 5 0 80 0 9 -do- 15 5 20 30 10 direct Si nitriding method 5 5 40 25 Note: Sample Nos. 1 to 6 our invention Sample Nos. 7 to 10 comparison The test results are shown in Table 2. As is evident from Table 2, the inserts of the present invention (Sample Nos. 1 to 6) each have a longer life than the comaparative inserts (Sample Nos. 7 to 10). The Insert life was measured at a flank wear width of 0.3 mm. Life of Insert Sample No. Test No. 1 (min) Test No. 2 (min) 1 34 55 2 38 48 3 35 59 4 35 70 5 38 60 6 33 80 7 20 25 8 17 23 9 13 19 10 16 15 Example 2 Commercially available α-silicon nitride powders prepared by the imide decomposition method, direct metallic silicon nitriding method and silica reducing method, α - Al₂O₃ powder and Y₂O₃ powder were used, mixed In proportions as shown in Table 3, Sample Nos. 11 to 15, ball milled, and then subjected to press molding at a pressure of 1 ton/cm². The resulting molded products were sintered in a nitrogen atmosphere under pressure at a temperature of 1800 °C for 2 hours and further to a HIP treatment at a temperature of 1750 °C and a pressure of 1000 atm to obtain Sample Nos.11 to 15. These sintered bodies (Sample Nos. 11 to 15) were worked into throwaway inserts of JIS SNG 433 by grinding, which were then subjected to the similar cutting tests to Example 1. Sample No. Silicon Nitride Powder Al₂O₃ Powder (wt %) Y₂O₃ Powder (wt %) Silicon Nitride Crystals (1) (2) (3) ≦ 1 µm ≧ 1.5 µm 11 70 30 0 2 5 55 30 12 85 15 0 2 5 60 17 13 95 5 0 2 5 65 10 14 60 10 30 2 5 62 15 15 0 20 80 2 5 55 20 The test results are shown in Table 4. As is evident from Table 4, the inserts of the present invention (Sample Nos. 11 to 15) obtained by the use of two or are of silicon nitride starting powders differing in the production methods thereof have more excellent cutting properties than the other ininserts of the present invention (Sample Nos. 1 to 6) using the single silicon nitride starting powders. Life of Insert Sample No. Test No. 1 (min) Test No. 2 (min) 11 48 63 12 45 59 13 38 65 14 41 61 15 40 55

Claims: A tool having a silicon nitride sintered body containing crystal grains comprising 50 to 90% by volume of hexagonal pillar crystals having a minor axis of at most 1.0µm and 5 to 30% by volume of hexagonal pillar crystals having a minor axis of at least 1.5µm, and further containing from greater than 1% to 10% by weight of Al₂O₃ and 1% to 10% by weight of Y₂O₃ as a sintering aid. A tool having a silicon nitride sintered body as claimed in Claim 1, wherein the sintered body is obtained from a silicon nitride powder prepared by the imide decomposition method. A tool of silicon nitride sintered body as claimed in Claim 1, wherein the sintered body is obtained from at least two silicon nitride powders prepared by the imide decomposition method, direct metallic silicon nitriding method and silica reducing method. A process for the production of a tool as claimed in Claim 1 comprising the steps of using a silicon nitride powder having a grain size of 1.0µm or smaller, mixing the powder with from greater than 1% to 10% by weight of Al₂O₃ and 1% to 10% by weight of Y₂O₃ as a sintering aid, pulverizing the resulting mixture, subjecting the pulverized mixture to press molding and then subjecting the resulting molded article to sintering in a nitrogen atmosphere at a pressure of 1 to 300 atm and a temperature of from 1700°C to less than 1850°C. A process as claimed in Claim 4 including the further step of subjecting the product after sintering to HIP treatment at a temperature of 1600 to 1900°C and at a nitrogen gas pressure of 10 to 3000 atm. A process as claimed in Claims 4 or 5, wherein the silicon nitride powder is prepared by at least two of imide decomposition methods, direct silicon nitriding methods and silica reducing methods.

Inventors: KOBAYASHI MITSUNORI C O ITAMI, MORIGUCHI HIDEKI C O ITAMI WOR, NOMURA TOSHIO C O ITAMI WORKS, KOBAYASHI, MITSUNORI, C/O ITAMI WORKS OF SUMITOMO, MORIGUCHI, HIDEKI, C/O ITAMI WORKS OF SUMITOMO, NOMURA, TOSHIO, C/O ITAMI WORKS OF SUMITOMO
Applicants: SUMITOMO ELECTRIC INDUSTRIES, SUMITOMO ELECTRIC INDUSTRIES, LTD.
Application year: 1991
Main classification: C04B35
IPC codes: C04B35, B23B27