PROCESS FOR THE PRODUCTION OF HEAT AND WEAR-RESISTANT CERAMIC MATERIALS, PRODUCT OF THE PROCESS AND STARTING MATERIAL COMPOSITION FOR USE IN THE PROCESS

Abstract: Heat and wear-resistant ceramic materials are produced by sintering a composition comprised of 5 to 40 % by weight of Al₂O₃, 0.05 to 4 % by weight of a sintering aid and 56 to 94.95 % by weight of a TiC component in which 4 to 30 % by weight of TiC are substituted with Ti until Ti cannot be detected as the metallic phase.

Description: The present invention relates to a process for the pro­ duction of ceramic materials excelling in heat resistance, wear resistance and electrical conductivity and, more particularly, to a process for the production of ceramic materials composed mainly of TiC, which are applicable as the cutting tools for ductile cast iron, etc., and are useful as the materials such as electrically conductive ceramics, for instance, as ceramic heaters as well as electrode material, etc. which are required to possess wear and corrosion resistance. TiC has heretofore been known as a high-temperature ma­ terial having excellent anti-spalling properties, since it has a high melting point and a high density, shows a small coefficient of thermal expansion and suffers no decrease in heat conductivity and high temperature. Since TiC is a difficult-to-sinter-material, however, a densely sintered body is only obtained as cermet through the addition of metals such as Co, Ni, etc. However, since the aforesaid cermet is governed by the behaviour of metallic phases due to the fact that it is a composite material, no full advantage is taken of the aforesaid properties, especially high-temperature proper­ ties of TiC per se . For instance, a cermet tip obtained from the aforesaid cermet is used for finish-cutting in the cutting of duc­ tile cast iron. However, if cutting is effected at a speed exceeding as high as 300 m/min., then wearing proceeds rapidly even in that finish-cutting with an in­ crease in crator wearing and a tendency toward chipping. At a cutting speed of no higher than 150-200 m/min., on the contrary, the finished surface becomes rough as a result of the ocurrence of fusing, although the degree of roughening is not as large as the case of tips based on super-hard metals. The present invention intends to provide a solution to the aforesaid problems, and has for its object the pro­ vision of a process for the production of densified ceramic material which takes the advantage of TiC. This object is solved by the process as described in the main claim. Advantageous further features are evident from subclaims 2 to 6. The invention also provides a product derived by this process and a starting material composition for the pro­ cess which is described in claim 8 and the following subclaims. According to the present invention, there is provided a process for the production of ceramic materials by sintering a composition until Ti cannot be detected as the metallic phase through X-ray diffraction (or optical microscopy), said composition comprising 5 to 40 % by weight of Al₂O₃, 0.05 to 4 % by weight of a sintering aid, and 56 to 94.95 % by weight of a TiC component in which 4 to 30 % by weight of TiC is substituted with Ti. The TiC component partly substituted with Ti suitable for use as the starting material in the present invention pro­ vides a TiC x component where x is 0.80 to 0.90 correspon­ ding approximately to the Ti amount of 10-20 % by weight of TiC. As to commercially available TiC, x for TiC x is usually more than 0.95. In the present invention Al₂O₃ and a sintering aid are used together with the TiC component. Al₂O₃ is a chemically stable substance that excels in oxi­ dation resistance and has a low free energy of formation. Dispersion of this material in the TiC component introduces improvements in the oxidation resistance and chemical sta­ bility of the ceramic material as a whole. This adds oxi­ dation resistance and chemical stability to the excellent properties TiC possesses. In the present invention, Al₂O₃ is used in an amount of 5 to 40 % by weight (preferably 15 - 35 %, most prefer­ ably 20 - 35 %). However, in an Al₂O₃ amount of less than 5 % by weight, the effect as mentioned above is not suffici­ ently produced, whereas in an Al₂O₃ amount exceeding 40 % by weight, the properties of TiC per se decrease. In the present invention, the reason why the sintering aid is used with Al₂O₃ is that an Al₂O₃ compound formed by the reaction between Al₂O₃ and the sintering aid serves to pro­ mote the sintering of ceramic materials with improvements in sinterability in combination with the aforesaid effective action produced by Al₂O₃. In the present inven­ tion, the wording "sintering aid" refers to MgO, CaO, SiO₂, ZrO₂ or NiO, and rare earth oxides such as Y₂O₃, Dy₂O₃, Er₂O₃ and Ho₂O₃, which are usually used for sintering of Al₂O₃ ceramics or those composed mainly of Al₂O₃ such as Al₂O₃-TiC system, Al₂O₃-ZrO₂ system or the like system. The sintering aids may be used alone or in combination. In the present invention, the sintering aid(s) is (are) used in an amount of o.o5 to 4 % by weight (preferably 1 - 4%). However, in an amount of less than 0.05, the effect as mentioned above is not fully produced. In an amount ex­ ceeding 4 % by weight, on the contrary, a large amount of the aforesaid Al₂O₃ compound gives rise to a lowering of the high-temperature properties of ceramic materials. Pre­ ferred combinations of the sintering aids embrace o.5 - 1 % by weight of (MgO, CaO) and o.5 - 3 % by weight of (Dy₂O₃, Y₂O₃). Referring then to the Ti component in TiC, it is required that Ti be incorporated in TiC as solid-solution in the course of sintering so as to substantially prevent it from remain­ ing as the metallic phase in the ceramic material. In con­ sequence, the TiC component assumes a non-stochiometrical composition, thus making the crystal structure incomplete and unstable. It is considered that this facilitates the sintering reactions such as solid-phase reactions, as a result of which the sinterability of the invented ceramic material can be improved. This phenomenon also means that the interface between the Al₂O₃ particles and the TiC par­ ticles or the interface between the TiC particles is strengthened. Although not clarified, yet, it seems that, referring to the high-temperature strength of TiC per se , the bonding state assumes the nature of metallic bonding in addition to the inherent covalent bonding, thus re­ sulting in improvements in strength and toughness. In the present invention, the amount of Ti is in a range of 4 to 30 % by weight (preferably 10 - 20 %) of TiC so as to add the aforesaid properties to the ceramic materials. However, if the amount of Ti is less than 4 % by weight of TiC, the effect as mentioned above is not fully obtained. If the amount of Ti exceeds 30 % by weight of TiC, on the other hand, there is a possibility that Ti may remain as the metallic phase, and there is a drop of wear resistance in view of cutting ability. In the course of sintering, it is required that Ti is incorporated in TiC as solid-solu­ tion to such an extent that it is not detected as the metallic phase at least through X-ray diffraction or optical microscopy. The formulation according to the present invention contain­ ing Al₂O₃, the sintering aid and the TiC component in the predetermined amounts should be sintered in a non-oxidizing atmosphere so as to prevent oxidation of TiC and Ti. The sintering should be conducted at a temperature of 1500- 1900°C (preferably 1600-1850°C, most preferably 1650-1800°C). The sintering may be effected through the normal sintering (i.e., without applying mechanical pressure on the mass to be sintered) or through press-sintering (i.e., under appli­ cation of mechanical pressure) with different atmosphere gas pressure. The press-sintering embraces hot press (H.P.) sintering or hot isostatic pressing (HIP) sintering, the latter in­ cluding preferably pre-sintering by the normal sintering. The preferred sintering temperatures are 1600-1850°C for the normal sintering, 1600-1900°C for H.P. and 1400-1900°C for HIP. The pre-sintering proceeding HIP is preferably at 1600-1850°C. The pressure for H.P. should be preferably at least 100 atm, while for HIP at least 1000 atm. Preferred atmoshpere is inert gas (Ar etc.) of a few Torr to 1 atm for the normal sintering; vacuum, inert gas or atmospheric air for H.P. By using the invented process for production of ceramic materials, it is possible to take the advantage of TiC and produce densified ceramic materials having heat and wear resistance. The ceramic materials produced by the invented process for the production of ceramic materials are useful for cutting tools for cast iron such as the aforesaid ductile cast iron, steel, high nickel, aluminium, titanium and non­ metallic materials as well as for mechanical parts re­ quired to possess wear, heat and corrosion resistances. Due to their electrical conductivity, they can be used as electrically conductive ceramics, for instance, are useful as the materials for ceramics heaters or electrode materials required to possess heat and corrosion resistance. Examples Prepared were alpha-Al₂O₃, 70 % of which had a particle size of no more than 1 µm, a commercial grade of TiC having a mean particle size of 1.1 µm and an overall carbon con­ tent of 19.4 wt % (TiC x : x = about 0,97), TiC passed through a 325-mesh sieve and a sintering aid having a purity of 99.5 % or higher, which were then formulated in the proport­ ions as specified in Table 1, and were wet-pulverized for 30 hours with acetone in a stainless ball mill resulting in a mean particle size of less than 1 µm. Thereafter, acetone was volatilized off in a drier, and the resulting products were finely pulverized in a mortar until they were all passed through a 60-mesh sieve, to thereby obtain green base powders. The Green base powders were sintered at the temperatures and by the methods, both being specified in Table 1. The sintering procedures employed in the examples were: 1. Hot-pressing at a pressure of 200 kg/cm² for a hot- pressing period of 15 minutes in a graphite mold at tempe­ ratures shown in Table 1 (designated as H.P. in the Table 1). 2. One-hour sintering of a green compact in an argon atmosphere at a reduced pressure of 300 Torr at temperatu­ res shown in Table 1 (designated as normal sintering in the Table 1). 3. Pre-sintering at temperatures specified in Table 1 for 1 hour in an argon atmosphere of reduced pressure (300 Torr), followed by sintering at 1400-1900°C at a pressure of 1500 atm by means of hot isostatic pressing (designated as H.I.P. in the Table). The thus obtained sintered bodies were polished to a tip shape of SNGN432TN and a surface state of 3S or less (according to JIS) with a diamond grinding wheel to mea­ sure the relative theoretical density and hardness there­ of, and were thereafter subjected to cutting tests under the conditions as specified in Table 2. In addition, the state of Ti in the thus obtained sintered bodies were examined by means of an X-ray diffractometer. In the comparison examples, the materials were formulated in the proportions as specified in Table 1. According to the procedure as mentioned above, sintering and shaping were carried out to measure the relative theoretical den­ sity and hardness of the thus obtained samples, which were in turn subjected to cutting tests. As was the case with the examples of the present invention, the state of Ti was examined by means of an X-ray diffractometer. The TiC cer­ met used was ordinarily available one. Measurement through X-ray diffractometer was conducted under the following conditions: From the results set forth in Table 1, it has been found that the ceramic materials increase in wear or abrasion and, in some cases, wear off in the following samples 1 to 7 inclusive. It has thus been noted that, in order to take the advantage of TiC and produce densified ceramic materi­ als, Al₂O₃ and the sintering aid(s) should be used in the given amounts and the TiC component should be substituted with Ti in the predetermined amount, and sintering should be carried out in such a manner that Ti is not detected as any metallic phase through the X-ray diffractometry, as noted from sample Nos. 1 to 19 of the examples of the pre­ sent invention. The comparative samples 20-30 relate to the following occurrences: 1. As in the case with No. 20, Al₂O₃ is contained in the formulation in an amount exceeding 40 % by weight. 2. As is the case with No. 21, Al₂O₃ is contained in the formulation in an amount of less than 5 % by weight. 3. As is the case with No. 22, the TiC component is substituted with Ti in an amount exceeding 30 % by weight. 4. As is the case with No. 23, the TiC component is substituted with Ti in an amount of less than 4 % by weight. 5. As is the case with No. 24, the sintering aid is con­ tained in the formulation in an amount exceeding 4 % by weight. 6. As is the case with No. 25, the sintering aid is only contained in an amount of less than 0.05 % by weight. 7. As is the case with Nos. 26 to 30, Ti remains to such a degree that it can be detected as the metallic phase by means of the X-ray diffractometry. The ceramic materials produced according to the invented process of the production thereof were measured for the electrical conductivity, which was found to range from about 50 to 100 × 10⁻⁶ Ohm . cm, although varying depen­ ding upon the composition. This conductivity is considered to be ones with superior electric conductivity among the practical ceramic materials used in the prior art. As shown in Table 1, the inventive examples provide high relative theoretical density of more than 99 % and high hardness (Rockwel 45N scale) of more than 90.

Claims: 1. A process for the production of heat and wear- resistant ceramic materials, characterized by sintering a composition consisting essentially of 5 to 40 % by weight of Al₂O₃ , 0.05 to 4 % by weight of a sintering aid and 56 to 94.95 % by weight of a TiC component in which 4 to 30 % by weight of TiC are substituted with Ti until Ti cannot be detected as the metallic phase. 2. A process as defined in Claim 1, in which said sintering aid is at least one selected from the group consisting of MgO, CaO, SiO₂, ZrO₂, NiO and rare earth oxide. 3. A process as defined in Claim 2, in which said rare earth oxide is at least one selected from the group con­ sisting of Y₂O₃, Dy₂O₃, Er₂O₃ and Ho₂O₃. 4. A process as defined in any of the preceding claims, in which the sintering is carried out at 1500-1900°C. 5. A process as defined in Claim 4, in which the sintering is carried out at 1600-1850°C. 6. A process as defined in any of the preceding Claims, in which the sintering is conducted in a non-oxidizing atmosphere. 7. A product of the process as defined in any of the preceding Claims. 8. A starting material composition for use in the process as defined in any of the preceding claims 1 to 6, said composition consisting essentially of 5 to 40 % by weight of Al₂O₃, 0.05 to 4 % by weight of a sintering aid and 56 to 94.95 % by weight of a TiC component in which 4 to 30 % by weight of TiC are substituted with Ti. 9. A composition as defined in Claim 8, in which said sintering aid is at least one selected from the group consisting of MgO, CaO, SiO₂, ZrO₂, NiO and rare earth oxide. 10. A composition as defined in Claim 9, in which said rare earth oxide is at least one selected from the group consisting of Y₂O₃, Dy₂O₃, Er₂O₃ and Ho₂O₃.

Inventors: KINUKAWA, TATSUJI C/O NGK SPARK PLUG CO.., LTD., SUZUKI, JUNICHIRO C/O NGK SPARK PLUG CO.., LTD.
Applicants: NGK SPARK PLUG CO. LTD.
Application year: 1985
Main classification: C04B35
IPC codes: C04B35