SILICON NITRIDE TYPE SINTERED BODY AND PROCESS FOR ITS PRODUCTION

Abstract: A silicone nitride type sintered body consists of a Si3N4-Y2O3-MgO-Al system. The stoichiometry of the system satisfies (I) and where A1, B1 and C1 represent mols of Y, Mg and Al, respectively per mol of Si3N4. The crystallised phase of Si3N4 comprises an alpha-Si3N4 phase in which a part of Si3N4 is replaced by Al and O, and a binding glass phase consisting of Si, Al, O, N, Y and Mg. The amts. of alpha-phase and beta-phase satisfy (II). Particles of carbides and carbonitrides of Ti, having a particulate size 1 microns or less, are dispersed in the Si3N4-Y2O3-MgO-AlN system and amt. to 0.5-5 wt.%, of the system. The process for producing this sintered body comprises (i) mixing the following composite powder: Y2O3 (A2 mol), AlN (B2 mol), MgO (C2 mol) and Si3N4 (1 mol), (ii) mixing the powder with an organic solvent and (iii) subjecting the powder to a heat-pressurising sinter, pressurising in an N2 gas sinter, or heat-static water pressurising in an N2 gas sinter. A2, B2 and C2 satisfy predetermined conditions.

Description: The present invention concerns a silicon nitride type material with much less deterioration of strength at high temperature and suitable for super-high speed cutting tools or high temperature structural material. Heretofore, as Si₃N₄ material, those described in, for example, Japanese Patent Unexamined Publication (KOKAI) Sho 49-21091, Sho 57-95873, Sho 59-182276, etc. have been known. As the production process for them, there are a production method for β-Si₃N₄ in Si₃N₄-Y₂O₃-Al₂O₃ system adding Y₂O₃ and Al₂O₃ or in Si₃N₄-Y₂O₃-MgO-Al₂O₃ by adding Y₂O₃-MgO-Al₂O₃ as the sintering aid, as well as a production method for α'-Si₃N₄ and β-Si₃N₄ in Si₃N₄-Y₂O₃-AlN series α-sialon or Si₃N₄-Y₂O₃-Al₂O₃-AlN series by adding Y₂O₃-AlN. However, a glass phase comprising Si, N and additive ingredient is present between Si₃N₄ particles, which bind crystal grains of Si₃N₄ in any of the sintered bodies. The glass phase is softened at high temperature to reduce the strength. That is, the deterioration of the strength is remarkable at a temperature higher than 800°C and the strength is reduced to about one-half, in view of normal temperature bending strength, at a temperature higher than 1200°C. For preventing the strength deterioration at high temperature, Si₃N₄ series material free from glass phase, for example, solid-solution type Si₃N₄ such as β-sialon or reaction sintered Si₃N₄ compound of Si₃N₄ alone may be satisfactory. However, although these materials show less deterioration of strength at high temperature, the strength itself is low at normal temperature and they are not suitable for structural material and tool material. As for other means, although it is possible to prevent softening at high temperature by crystallizing the glass phase, this requires heat treatment in addition to sintering to make the steps complete. EP-A-0 124 989 discloses a Si₃N₄ sintered body of the Si₃N₄-Y₂O₃-Al₂O₃-AlN type, optionally further comprising less than 5% by weight of at least TiO₂/MgO/ZrO₂. The Si₃N₄ sintered body has α' and β phases or, optionally, α', β and Si-rare earth element-Al-O-N phases (referred to as "x" phases) wherein the α' to β phase ratio is 0.05-0.7 : 0.95-0.3 or in the three-phase option the ratio of the α'phase to the total of a', β and "x" is 0.01-0.7 : 1. The principal object of the present invention is to obtain silicon nitride material having bending strength at normal temperature comparable with that of usual silicon nitride having a glass phase and, in addition having much less deterioration of strength at high temperature. According to the present invention, the foregoing object has been attained by using a Y₂O₃-MgO-AlN system as a sintering aid and controlling the stoichiometrical ratio of Y, Mg, Al in the sintered body within a specific range. That is, in accordance with the present invention, it is possible to obtain silicon nitride material having a bending strength at normal temperature of not less than 9.806·10⁸ Pa (100 kg/mm²), which is comparable with that of usual β-Si₃N₄ having a glass phase and with much less deterioration of the strength such as deterioration of the strength at high temperature of not greater than 10% at 1000°C and 20% at 1200°C as compared with usual material, by controlling the number of moles of Y, Mg and Al present in the sintered body of Si₃N₄-Y₂O₃-MgO-AlN series. As the specific condition in the stoichiometrical ratio of Y, Mg and Al in the sintered body described above, the following three conditions are to be satisfied: (1) 0.1 ≦ A₁ + B₁ + C₁ ≦ 0.4 (2) 0.2 < B₁/A₁ < 1.2 (3) -0.03 ≦ C₁ - (A₁ + B₁) ≦ 0.03 where A₁, B₁ and C₁ represent the number of moles of Y, Mg and Al, respectively, per one mole of Si₃N₄. In the silicon nitride type sintered body according to the present invention, the strength of the sintered body can be improved by admixing and dispersing the particles of titanium carbide, nitride and carbonitride. Since they cause no reactions such as solid solution with Si₃N₄ or sintering aids, they function as a dispersion improver, by which toughness is improved. The grain size is, desirably, less than 1 µm. For production process according to the present invention, while a sintered body excellent in high temperature strength and easy sinterability can be obtained by a specific composition range, there are several conditions also in the production. They are use of an organic solvent as a solvent upon mixing pulverization and use of starting powder with the grain size of less than 1 µm. According to the present invention, it is possible to obtain a silicon nitride sintered body having a bending strength at normal temperature comparable with that of usual silicon nitride having a glass phase and less deterioration in the strength at high temperature. Figs. 1 - 4 are diagrams illustrating the characteristics of the sintered body according to the present invention. Fig. 1 is a diagram showing the effect to the bending strength at room temperature, of molar ratio for each of Y, Mg and Al defined as: Y = A₁, Mg = B₁ and Al = C₁ in a Si₃N₄-Y₂O₃-MgO-AlN sintered body applied with HIP processing at 1650°C after preliminary sintering in N₂ gas at 1700°C. The ordinate indicates the bending strength (Pa) [(kg/mm²)] at a room temperature. That is, as shown in Fig. 1, it can be seen that the bending strength at a room temperature is high within a range of C₁ - (A₁ + B₁) of not less than -0.03 mole and not greater than 0.03 mole. That is, the sinterability is improved within the range making ordinary sintering possible and increasing the strength at a room temperature. Contrarily, if it exceeds 0.03 mole, the sinterability is remarkably reduced making it impossible for ordinary sintering but sintering is possible only by hot pressing. Further, if it is less than 0.03 mole, although the sinterability is preferred, the strength at a room temperature is low to deteriorate the characteristics of high strength as the merit of Si₃N₄ series and it is not suitable as the structural material. Within the range of the present invention, it is considered for the behavior of Y, Mg and Al in the sintered body that β-Si₃N₄ phase is formed in Si₃N₄ + Y₂O₃ + MgO + (Al in AlN) and α-Si₃N₄ phase is formed in Si₃N₄ + Y₂O₃ + MgO + AlN, in which the respective sintering aids are partially solid solubilized and partially forms, a glass phase. In the course of the formation, if MgO is used in excess ( B₁/A₁ > 1.2 ), α-Si₃N₄ phase in which Mg is excessively solid-solubilized is formed and to reduce the strength since this phase comprises granular crystals not acicular crystals of β-Si₃N₄ phase with high strength. Contrarily, if MgO is insufficient ( B₁/A₁ > 0.2 ) sinterability is lowered. Accordingly, the range should be 1.2 ≧ B₁/A₁ ≧ 0.2 . Further, the total amount of the sintering aids is from 0.1 to 0.4 mole based on one mole of Si₃N₄. The sinterability is poor if it is less than 0.1 mole and, on the other hand, no sufficient strength can be obtained in the mechanical property, in particular, bending strength due to the increase of bind glass phase at the crystal grain boundary if it exceeds 0.4 mole. In the region of the composition as described above, the state of the crystal phase as expressed by the ratio by volume of α-Si₃N₄ phase and β-Si₃N₄ phase is: In the present invention, the addition amount of Al₂O₃ often used as the sintering aid is zero. Fig. 2 shows the effect, to the mechanical property, in a case where Al₂O₃ is added to 95Si₃N₄-5Y₂O₃-(1.16MgO)-(3AlN)-(1TiN) (here and hereinafter based on weight % and the value in each of the blanks shows a ratio exceeding 100 (external ratio). In this experiment, Si₃N₄-Y₂O₃ is assumed as 100 and other additives are added by the ratio as indicated. As can be seen from the figure, the relative density is decreased and the bending strength is reduced along with the increase of the addition amount. That is, the presence of Al₂O₃ is not referred in the present composition and it is considered that Al₂O₃ forms β-Si₃N₄ while forming a glass phase with Si₃N₄, Y₂O₃, MgO, etc. proceeding to AlN. As a result, a ratio and relative density are lowered and thereby reducing the hardness and the strength. Further, strength of the sintered body can be improved by adding and blending the particles of titanium carbide, nitride and carbonitride. Since they cause no reactions such as solid solution with Si₃N₄ or sintering aids, they function as a dispersion improver thereby improving the toughness. Their grain size is preferably of not greater than 1 µm. If the grain size is not greater than 1 µm it shows an effect of suppressing the grain growth of Si₃N₄ and contributes to the improvement of the strength and hardness as the dispersion improver. However, if the grain size exceeds 1 µm, the function as the dispersing improver is reduced to provide only poor improvement for the strength and the hardness. Fig. 3 shows the effect of the addition amount in a case where TiN is added, as a dispersant, to 95Si₃N₄-5Y₂O₃-(1.16MgO)-(3AlN) as the base. As shown in the figure, the bending strength is improved if the addition amount is from 0.5 to 5 % by weight. The composition of the present invention contains AlN, which reacts with water to form ammonia and, at the same time, it is partially oxidized to form Al-oxide. That is, if the composition is mixed with an aqueous solvent, the effect of AlN addition is eliminated to show the same nature as the addition of Al₂O₃. Accordingly, the suitable solvent is an organic solvent, preferably, methanol or ethanol for preventing oxidation of AlN. Referring to the grain size of the starting powder, there is certain a relationship between the strength and the starting powder in the composition according to the present invention and the smaller grain size is preferred for all of Si₃N₄, Y₂O₃, MgO and AlN. In particular, the average grain size of Si₃N₄ and Y₂O₃ is not greater than 1 µm and the grain size of Si₃N₄ is, preferably, not greater than 0.5 µm, because the dispersibility of the powdery particles, solid solubilization and formation of the glass phase are considered to be promoted when the grain size is smaller. Fig. 4 shows the dependency of the bending strength on the grain size of Si₃N₄. As can be seen from Fig. 4, the effect to the bending strength due to the difference of the average grain size of the Si₃N₄ starting powder is not remarkable in No. 3 as usual β-Si₃N₄, but the bending strength is abruptly reduced if the average grain size exceeds 1 µm in No. 4 as the product according to the present invention. Accordingly, within the composition range of the present invention, the average grain size of the Si₃N₄ starting powder has to be not greater than 1 µm, and, preferably, not greater than 0.5 µm. A predetermined amount of Y₂O₃-MgO-AlN was added to an α-Si₃N₄ powder with an average grain size of 0.2 µm, to which TiN or TiC powder with an average grain size of 0.4 µm was further added and they were charged together with ZrO₂ balls into a rubber-lined pot and pulverized and mixed for 40 hours using methanol as a solvent. Then, the resultant slurry was spray dried with the addition of organic binder by a closed type spray drier. The resultant pelletized powder was press molded into 10 × 50 × 5 mm, removed with the binder and then maintained at 1.013 bar (1 atm.) of N₂ gas, at a sintering temperature of 1650 - 1750°C for 2 hours to obtain a sintered body having a theoretical density of not less than 95%. The product was further treated by hot isostatic pressing (HIP) under N₂ gas at 1600°C - 1519.5 bar (1500 atm.) and served for various evaluations for physical property and characteristics. Hot press sintering was applied to those impossible for usual sintering under the conditions at a temperature of 1720 ± 20°C, under a pressure of 1.961·10⁷ Pa (200 kg/cm²), for a time of 100 min in N₂ gas. In the same way, a cut tip of a shape according to JIS SNGN432 was prepared and used for cutting test. The compositions of the test specimens and the results of the test are shown in the appended table. As can be seen from the table, remarkable improvement was recognized in the high temperature characteristics of the sintering material according to the present invention as compared with conventional Si₃N₄-Y₂O₃-Al₂O₃ series or Si₃N₄-MgO-Al₂O₃ series or β-sialon series. Nos. 17 - 20 used for comparison have the crystal phase ratio in the sintered body of: within a range: and have rather improved strength. However, as a result of measuring the high temperature strength with those impossible for normal pressure sintering and sintered by hot pressing, although the strength at high temperature was excellent as compared with Si₃N₄-Y₂O₃-Al₂O₃ series or Si₃N₄-MgO-Al₂O₃ series, sintering was impossible under normal pressure sintering and sintering was applied by hot pressing. It can also be seen from the above that the smaller addition amount of Al₂O₃ is preferred. This examples show the addition of 1 % by weight of TiN but it has also been found that the strength could be improved similarly by the use of TiC. Since the silicon nitride type sintered body according to the present invention has a bending strength at normal temperature comparable with that of the usual silicon nitride having glass phase and shows much less deterioration of strength at high temperature, it is excellent when compared with conventional Si₃N₄ type tools in view of wear resistance and chipping resistance for tools suitable to high speed cutting of high tip temperature and can be applied generally as a high temperature structural material.

Claims: A silicon nitride type sintered body prepared from Si₃N₄ and a sintering aid of Y₂O₃-Mgo-AlN, having A₁ moles of Y, B₁ moles of Mg and C₁ moles of Al based on one mole of Si₃N₄ in the sintered body, wherein each amount of A₁, B₁ and C₁ is greater than zero and chosen to satisfy the following conditions:- -0.03 ≦ C₁ - (A₁ + B₁) ≦ 0.03 moles; and 0.1 ≦ A₁ + B₁ +C₁ ≦ 0.4 moles; and 0.2 ≦ B₁/A₁ ≦ 1.2 and wherein the crystal phases in the sintered body of Si₃N₄ comprise the following:- an α-Si₃N₄ phase in which a part of the Si₃N₄ phase is replaced by Y, Mg, O and Al; and a β-Si₃N₄ phase in which a part of the Si₃N₄ phase is replaced by Al and O; and a binding glass phase comprising Si, Al, O, N, Y and Mg, and wherein the ratio between the α-Si₃N₄ phase and the β-Si₃N₄ phase is: 5 ≦ [ α / ( α + β)] ≦ 50 expressed as percent by volume. The silicon nitride type sintered body of claim 1, wherein particles of titanium carbide, nitride and carbonitride with a grain size of not greater than 1 µm are added as dispersing improver by 0.5 to 5% by weight with respect to the Si₃N₄-Y₂O₃-MgO-AlN of claim 1 wherein the amount of Si₃N₄-Y₂O₃ is assumed as 100% by weight. A process for producing a silicon nitride type sintered body which comprises adding A₂ moles of Y₂O₃, B₂ moles of AlN and C₂ moles of MgO per mole of Si₃N₄ to a Si₃N₄ powder containing not less than 90% by volume of α-Si₃N₄ phase with an average grain size of not greater than 1.0 µm, in such a powder blend composition as capable of satisfying the conditions : 0.08 ≦ A₂ + B₂ + C₂ ≦ 0.35 (mole) (1) -0.28 ≦ B₂ - (A₂ + C₂) ≦ 0.035 (mole) (2) A₂ > 0, B₂ > 0, C₂ > 0 (mole) (3) 0.4 ≦ A₂ ≦ 2.4 (mole) (4) pulverizing and mixing them in an organic solvent, and then sintering the powder by means of one of hot press sintering, press sintering in a nitrogen gas atmosphere, normal pressure sintering or hot isostatic pressing in nitrogen gas. The process of claim 3, wherein particles of titanium carbide, nitride or carbonitride with a grain size of not greater than 1 µm are added as dispersing improver by from 0.5 to 5% by weight with respect to the powder blended powder composition wherein the amount of Si₃N₄-Y₂O₃ is assumed as 100% by weight.

Inventors: FURUKAWA MITSUHIKO NIPPON TUNG, NAGANO MITSUYOSHI NIPPON TUNGS, SHIROYAMA MASAHARU NIPPON TUNG, TAKANO YASUMI NIPPON TUNGSTEN, FURUKAWA, MITSUHIKO, NIPPON TUNGSTEN CO., LTD., NAGANO, MITSUYOSHI, NIPPON TUNGSTEN CO., LTD., SHIROYAMA, MASAHARU, NIPPON TUNGSTEN CO., LTD., TAKANO, YASUMI, NIPPON TUNGSTEN CO., LTD.
Applicants: NIPPON TUNGSTEN, NIPPON TUNGSTEN CO., LTD.
Application year: 1989
Main classification: C04B35
IPC codes: C04B35