HIGH TOUGHNESS ZRO2 SINTERED BODY AND METHOD OF PRODUCING SAME

Abstract: A high-toughness ZrO₂ sintered body contains Nd₂O₃ and Y₂O₃ as stabilizers of the crystal structure of ZrO₂. The sintered body consists essentially of ZrO₂ containing 0.1 to 3 mol% of Nd₂O₃ and 0.5 to 3.5 mol% of Y₂O₃. Alternatively, the material composition contains in addition 10 to 60 volume% of at least one of Al₂O₃, SiC, TiC, B₄C and TiB₂. Preferably, the sintered body has a stress-induced transformation ratio of 25% or more.

Description: The present invention relates to a ZrO₂ sintered body containing Y₂O₃ and Nd₂O₃ as stabilizers of the crystal structure of ZrO₂ (hereinafter referred to simply as "stabilizers"), and more particularly to a ZrO₂ sintered body having high toughness for use in tools such as dies and cutters, structural parts, decorating parts, etc. Recently, partially stabilized zirconia (simply "PSZ") has been getting much attention as high-strength, high-toughness ceramics. This PSZ undergoes stress-induced transformation, that is a phenomenon in which tetragonal ZrO₂ metastable at a room temperature is transformed into a stable monoclinic ZrO₂ under the influence of an outside force. Specifically, an outside force is absorbed as an energy for transformation, and ZrO₂ undergoes volume increase, preventing the generation of cracks which cause fracture. This provides ZrO₂ with high strength and toughness. PSZ which is mainly used presently is Y₂O₃-PSZ which contains a small amount of Y₂O₃ as a stabilizer. This Y₂O₃-PSZ is fully described in "Microstructurally Developed Toughening Mechanisms in Ceramics," Technical Report No. 3, F. F. Lange, Rockwell International, July 1978. In addition, Japanese Patent Publication No. 61-21184 (Japanese Patent Laid-Open No. 56-134564) discloses that Y₂O₃-PSZ has high strength and high durability at 200-300°C when satisfying the conditions that a Y₂O₃ content is 2-7 mol%, that its crystal structure is mainly composed of a tetragonal phase, and that its average crystal grain size is 2 µm or less. Further, "Microstructurally Developed Toughening Mechanisms in Ceramics, Technical Report No. 7, F. F. Lange, Rockwell International, October 1979 discloses a ZrO₂-Y₂O₃-Al₂O₃ sintered body, and Japanese Patent Publication No. 61-59265 (Japanese Patent Laid-Open No. 58-32066) discloses a zirconia sintered body containing Y₂O₃, etc. as stabilizers and consisting essentially of 40-99.5 wt% of tetragonal ZrO₂ and 0.5-60 wt% of Al₂O₃. These zirconia sintered bodies have improved hardness and strength because the solution and dispersion of Al₂O₃ in ZrO₂ lowers a temperature at which tetragonal ZrO₂ is transformed into monoclinic one, preventing the grain growth of ZrO₂, thereby increasing the content of tetragonal ZrO₂ and also enhancing a sliding resistance of ZrO₂ grains along the grain boundaries. On the other hand, various attempts have been made to provide methods of producing such zirconia sintered bodies. For instance, Japanese Laid-Open No. 60-54972 discloses a method of producing ZrO₂ ceramics having improved strength by pressing ZrO₂ powder containing a certain amount of stabilizers such as Y₂O₃, etc. and sintering it to a relative density of 93% or more, and then subjecting the sintered body to hot isostatic pressing (simply HIP). Japanese Patent Publication No. 61-59267 (Japanese Patent Laid-Open No. 58-36976) gives consideration to starting material powder, proposing the use of starting material powder produced by a coprecipitation method in which each component of ZrO₂, stabilizers and Al₂O₃ can ideally be dispersed, and sintering it to provide a sintered body having a uniform structure with substantially no micropores. Further, Japanese Patent Laid-Open No. 60-86073 discloses the use of the above HIP method using starting material powder produced by a coprecipitation method. Various attempts have been made to use stabilizers other than Y₂O₃. For instance, Technical Report of Tohoku University Metal Materials Laboratory, No.12, pp. 19-21 (March, 1987) shows the effects of stabilizing tetragonal ZrO₂ by a ZrO₂-Ln₂O₃ system (3.5 mol% Ln₂O₃=Sc₂O₃, Y₂O₃, La₂O₃, Ce₂O₃, Pr₂O₃, Nd₂O₃.) It is indicated that among the above oxides, Nd₂O₃ shows the largest stabilizing effects next to Y₂O₃. However, with respect to the use of the conventional PSZ in tools such as dies and cutters, it is inferior to competing cemented carbide in toughness and hardness. Accordingly, PSZ has been used in extremely limited applications, and it has been considered difficult to find sufficient applications. Particularly, when Al₂O₃ is added, the toughness of ZrO₂ ceramics rather decreases, although its strength and hardness increase. Accordingly, it has been needed to develop ZrO₂ ceramics having excellent strength, toughness and hardness. OBJECT AND SUMMARY OF THE INVENTION In view of the above circumstances, the present invention is aimed at increasing the toughness of Y₂O₃-PSZ ceramics. As a result of intense research, the inventor has found that conventional Y₂O₃-PSZ can be improved to have better toughness by adding both Y₂O₃ and Nd₂O₃ in certain amounts as stabilizers, thereby providing a ZrO₂ sintered body with a crystal structure mainly composed of a tetragonal phase or a mixture of tetragonal one and cubic one, and having an average crystal grain size of 2 µm or less, particularly 1 µm or less. Thus, the high-toughness ZrO₂ sintered body according to one embodiment of the present invention contains 0.1-3 mol% of Nd₂O₃ and 0.5-3.5 mol% of Y₂O₃ as stabilizers. The high-toughness ZrO₂ sintered body according to another embodiment of the present invention contains Nd₂O₃ and Y₂O₃ as stabilizers, and has a crystal structure mainly composed of a tetragonal phase or a mixture of tetragonal one and cubic one and further an average crystal grain size of 1 µm or less. The method of producing a high-toughness ZrO₂ sintered body containing 0.1-3 mol% of Nd₂O₃ and 0.5-3.5 mol% of Y₂O₃ as stabilizers according to the present invention comprises the steps of pressing starting material powder having the above composition to provide a green body, and after sintering, subjecting it to hot isostatic pressing at a temperature of 1300-1600°C and a pressure of 100 kg/cm² or more. The high-toughness ZrO₂ sintered body according to a further embodiment of the present invention consists essentially of 40-90 volume% of ZrO₂ containing 0.1-3 mol% of Nd₂O₃tand 0.5-3.5 mol% of Y₂O₃ as stabilizers and 10-60 volume% of at least one of Al₂O₃, SiC, TiC, B₄C and TiB₂. The high-toughness ZrO₂ sintered body according to a still further embodiment of the present invention contains Nd₂O₃ and Y₂O₃ as stabilizers and has a crystal structure substantially composed of a tetragonal phase or a mixture of tetragonal one and cubic one, with an average crystal grain size of 2 µm or less, the sintered body consisting essentially of 40-90 volume% of ZrO₂ and 10-60 volume% of at least one of Al₂O₃, SiC, TiC, B₄C and TiB₂. Further, the method of producing a high-toughness ZrO₂ sintered body consisting essentially of 40-90 volume% of ZrO₂ containing 0.1-3 mol% of Nd₂O₃ and 0.5-3.5 mol% of Y₂O₃ as stabilizers and 10-60 volume% of at least one of Al₂O₃, SiC, TiC, B₄C and TiB₂ according to a still further embodiment of the present invention, comprises the steps of pressing starting material powder having the above composition to provide a green body, and after sintering, subjecting it to hot isostatic pressing at a temperature of 1300-1600°C and a pressure of 100 kg/cm² or more. DETAILED DESCRIPTION OF THE INVENTION The improvement of toughness of a ZrO₂ sintered body can be achieved by stress-induced transformation from tetragonal structure to monoclinic structure as mentioned above, but detailed investigation of various stabilizers has led to the discovery that the addition of both Nd₂O₃ and Y₂O₃ to ZrO₂ gives a maximum stress-induced transformation ratio from tetragonal ZrO₂ to monoclinic ZrO₂. Thus, the inventor has found that the addition of both Nd₂O₃ and Y₂O₃ gives substantially higher toughness to a ZrO₂ sintered body than the conventional stabilizers do. The term "substantially high toughness" used herein has the following meaning: A ZrO₂ sintered body containing a monoclinic phase, for instance, a ZrO₂ sintered body containing about 2 mol% of Y₂O₃ has high apparent toughness because of microcracks due to the existance of a monoclinic phase, but when it is used for tools such as dies and cutters it does not show enough wear resistance and chipping resistance that would be expected from the measured toughness. From this aspect, a ZrO₂ sintered body containing no monoclinic phase, which contains about 2.5-3 mol% of Y₂O₃ is conventionally used as a high-toughness material practically. With respect to toughness required for tools, it is important that ZrO₂ ceramics have microcracks; in other words, they have a mechanism of exhibiting high toughness without containing a monoclinic phase. Why a conventional ZrO₂ sintered body having microcracks, in other words, conventional ZrO₂ with a monoclinic phase shows high toughness is not by its material but by its structure, contributing to the toughness. On the other hand, the sintered body according to the present invention is characterized by containing no monoclinic phase other than that existing on the surface of the sintered body which may be formed by stress-induced transformation while working, such as grinding, and by having a crystal structure substantially composed of a tetragonal phase or a mixture of tetragonal one and cubic one with a high stress-induced transformation ratio of the tetragonal phase which leads to high toughness. Incidentally, as mentioned above, the addition of Y₂O₃ or Nd₂O₃ individually to ZrO₂ as a stabilizer is already known. However, as described in Technical Report of Tohoku University Metal Materials Laboratory, the effects of stabilizing tetragonal ZrO₂ by Nd₂O₃ do not exceed those by Y₂O₃, and there has been no disclosure about stress-induced transformation ratio of a tetragonal phase. Accordingly, the addition of both Nd₂O₃ and Y₂O₃ has never been taught. In the present invention, Nd₂O₃ is limited to 0.1-3 mol% and Y₂O₃ is to 0.5-3.5 mol%, both based on the amount of sintered body, because when they are less than the lower limit, a monoclinic phase appears, and when they exceed the upper limits the percentage of a cubic phase increases, while decreasing the content of a tetragonal phase. The preferred amount of Nd₂O₃ is 0.5-1.5 mol% and the preferred amount of Y₂O₃ is 1-2 mol%. To achieve high toughness, the crystal structure of a ZrO₂ sintered body is substantially composed of a tetragonal phase, or a tetragonal phase and a cubic phase in combination. The term "substantially tetragonal phase or a mixture of tetragonal one and cubic one" used herein means that the sintered body does not contain the monoclinic phase other than that existing in the surface of the sintered body generated by a stress-induced transformation caused by working, etc., and that it has a crystal structure substantially composed of a tetragonal phase, or if a cubic phase exists, the percentage of the cubic phase to the all crystal structure is 30 mol% or less. To achieve the crystal structure of a sintered body which is substantially composed of a tetragonal phase or a mixture of tetragonal one and cubic one, it is important that an average crystal grain size of the sintered body is 1 µm or less. According to the present invention, the stress-induced transformation ratio can be increased to as high as 25% or more, or preferably as high as 30% or more, while it is 20% or so for the conventional Y₂O₃-PSZ. Incidentally, the stress-induced transformation ratio in the present invention is defined by a method shown in Examples later. Next, to provide the ZrO₂ sintered body with high hardness, it is necessary that the ZrO₂ sintered body has a second phase by adding 10-60 volume% of Al₂O₃, SiC, TiC, B₄C and TiB₂ alone or in combination. When the amount of these compounds is less than 10 volume%, sufficient improvement in hardness cannot be achieved. On the other hand, when it exceeds 60 volume%, the toughness decreases remarkably. Al₂O₃, SiC, TiC, B₄C and TiB₂ are selected because they have high hardness and their fine powder is easily available. When Al₂O₃ is added, sintering in the air is possible. However, when SiC, TiC, B₄C or TiB₂ is added, hot pressing in vacuum or in the non-oxidizing atmosphere should be conducted to achieve high density. Incidentally, any of Al₂O₃ SiC, TiC, B₄C and TiB₂ may be in the form of fiber or whisker. The resulting sintered body should be subjected to hot isostatic pressing at a temperature of 1300-1600°C and the pressure of 100 kg/cm² or more to achieve further higher density and strength. To disperse the whisker in ZrO₂ to provide it with higher toughness, it is necessary to use the whisker having higher elasticity and smaller thermal expansion coefficient than ZrO₂. Any of Al₂O₃, SiC, TiC, B₄C and TiB₂ satisfy these requirements, and their whisker in a proper size can be dispersed in a ZrO₂ matrix uniformly to prevent cracks from propagating by changing and dividing their directions to absorb internal energy, thereby achieving high toughness. The size of the whisker is suitably 0.5-3 µm in diameter and 5-100 µm in length. As starting material powder, salts which can form ZrO₂, Y₂O₃, Nd₂O₃, or if necessary Al₂O₃ after burning may be used, and preferably powder produced by a wet method such as a coprecipitation method can be used to provide a ZrO₂ sintered body with more uniform composition. The present invention will be explained in further detail by means of the following Examples. Example 1 Commercially available ZrO₂ - 1.5-2.5 mol% Y₂O₃ powder prepared by a coprecipitation method and neodymium nitrate [Nd(NO₃)₃·6H₂O] were mixed in various after-sintering compositions shown in Table 1 and ball-milled in a wet state. A binder was added to each mixture to prepare a slurry which was then formed into granules by a spray drier, and the granules were rubber-pressed at a pressure of 3 ton/cm². The resulting green body was sintered at a temperature of 1500°C in the atmosphere and then subjected to a HIP treatment in an Ar gas at 1450°C and 1500 atm. Each test piece thus prepared (simply "TP") was evaluated with respect to various properties by the following methods. Bending strength was measured according to JIS R1601. Fracture toughness was calculated by the following Niihara's Equation from data obtained by a Vickers hardness test (load 20 kg). (K IC /Ha 1/2 )(H/E) 2/5 =0.018(ℓ/a) -1/2 K IC : Fracture toughness, H: Vickers hardness E: Young's modulus, a: Half of dent ℓ: Half of (Crack length - Indentation diameter) A stress-induced transformation ratio is expressed by a ratio of the proportion of a monoclinic phase generated after a surface of TP is ground by a diamond grinder of #100 at a pressure of about 0.5 kg/cm² for 10 minutes, to the proportion of a tetragonal phase before grinding: Ta: Proportion of tetragonal phase before grinding (as annealed at 1200°C after near finishing) (mol%) Ma: Proportion of monoclinic phase before grinding (mol%) Mb: Proportion of monoclinic phase after grinding (mol%) Hardness was measured by Vickers method under a load of 500g. The crystal structure was determined by calculating the proportion (mol%) of a monoclinic phase, a tetragonal phase and a cubic phase by the following equations using peak values of X-ray diffraction (It, Im, Ic). TP was annealed in the air at 1200°C after mirror finishing. The results are shown in Table 1. Test No. 3 shows a test piece containing 2 mol% of Y₂O₃ alone and having a fracture toughness of 10.5 (MN/m 1.5 ), but it contains a monoclinic phase. Thus, it cannot be said that it has practically high toughness. Test No. 7 shows a test piece of a conventional composition having 2.5 mol% Y₂O₃, which shows a fracture toughness of 7.1 (MN/m 1.5 ). On the other hand, Test Nos. 1, 2, 4, 5, 6, 8 and 9 show test pieces containing no monoclinic phase and having fracture toughness greater than 8.5 (MN/m 1.5 ). It is concluded that when the crystal grain size is small the tetragonal phase is stabilized, making the stress-induced transformation less likely, but the addition of both Nd₂O₃ and Y₂O₃ evidently increases the stress-induced transformation ratio even when the crystal grain size is small compared to a ZrO₂ sintered body containing only Y₂O₃, thereby contributing to increase of toughness. Example 2 Commercially available ZrO₂(2 mol% Y₂O₃)-30 vol% Al₂O₃ powder prepared by a coprecipitation method and neodymium nitrate [Nd(NO₃)₃·6H₂O] were mixed in various after-sintering compositions shown in Table 2, and ball-milled in a wet state. A binder was added to each mixture to prepare a slurry which was then formed into granules by a spray drier, and the granules were rubber-pressed at a pressure of 3 ton/cm². The resulting green body was sintered at a temperature of 1500°C in the atmosphere and then subjected to a HIP treatment in an Ar gas at 1450°C and 1500 atm. Each test piece thus prepared (simply "TP") was evaluated with respect to various properties by the same methods as in Example 1. The results are shown in Table 2. Test No. 10 shows a test piece containing 2 mol% of Y₂O₃ alone and having a fracture toughness of 9.1 (MN/m 1.5 ), but it contains a monoclinic phase. Thus, it cannot be said that it has practically high toughness. Test No. 11 shows a test piece of a conventional composition having 3 mol% Y₂O₃ alone, which shows a fracture toughness less than 6.0 (MN/m 1.5 ). On the other hand, Test Nos. 12-16 show test pieces containing no monoclinic phase and having fracture toughness higher than 8.5 (MN/m 1.5 ). It is concluded that when the crystal grain size is small the tetragonal phase is stabilized, making the stress-induced transformation less likely, but the addition of both Nd₂O₃ and Y₂O₃ evidently increases the stress-induced transformation ratio even when the crystal grain size is small compared to a ZrO₂ sintered body containing only Y₂O₃, thereby contributing to increase of toughness. Example 3 Commercially available ZrO₂-2 mol% Y₂O₃ powder prepared by a coprecipitation method, neodymium nitrate and one of SiC (average particle size: 0.5 µm), TiC (average particle size: 0.7 µm), B₄C (average particle size: 2 µm) and TiB₂ (average particle size: 0.9 µm) were mixed in various after-sintering compositions shown in Table 2. In each mixture, each of SiC, TiC, B₄C and TiB₂ was 30 vol% (Nos. 17-20 in Table 2) based on the total amount. Each mixture was formed into a green body in the same manner as in Example 2. The resulting green body was placed in a graphite mold and sintered by hot pressing at a temperature of 1500°C and a pressure of 250 kg/cm² in vacuum for 1 hour, and then subjected to a HIP treatment in an Ar gas at 1500°C and 1500 atm for 1 hour. Each test piece thus prepared (simply "TP") was evaluated with respect to various properties as in Example 1. The results are shown in Table 2 (Nos. 17-20). It is verified that the present invention can provide ZrO₂ ceramics with excellent toughness and hardness. Example 4 Commercially available ZrO₂(2 mol% Y₂O₃)-38 vol% Al₂O₃ powder prepared by a coprecipitation method, neodymium nitrate and SiC whisker (average size: 0.8 µm in diameter x 20 µm in length) were mixed in various after-sintering compositions (SiC whisker: 20 vol%) shown in Table 2 (Nos. 21 and 22). Each mixture was sintered in the same manner as in Example 3. The properties of the resulting sintered bodies are shown in Table 2. The data show that the addition of SiC whisker contributes to improving the stress-induced transformation ratio, and that the addition of both Y₂O₃ and Nd₂O₃ (No. 22) remarkably increases the stress-induced transformation raito as compared with a case where only Y₂O₃ is added as a stabilizer (No. 21). Therefore, the ZrO₂ sintered body of the present invention (No.22) has excellent fracture toughness. As mentioned above, the addition of both Y₂O₃ and Nd₂O₃ increases the toughness of a ZrO₂ sintered body which is conventionally insufficient. Therefore, the ZrO₂ sintered body of the present invention may be used in wide varieties of applications including tools such as dies and cutters. The present invention has been explained referring to the Examples, but it should be noted that any modifications may be made unless they deviate from the scope of the present invention defined by the claims attached hereto.

Claims: A high-toughness ZrO₂ sintered body containing 0.1 to 3 mol% of Nd₂0₃ and 0.5 to 3.5 mol% of Y₂O₃ as stabilizers of the crystal structure of Zr0₂. The Zr0₂ sintered body of claim 1 having a crystal structure mainly composed of a tetragonal phase or a mixture of a tetragonal phase and a cubic phase with an average crystal grain size of 1 µm or less. The Zr0₂ sintered body of claim 1 consisting essentially of 40 to 90 volume % of Zr0₂ and 10 to 60 volume % of at least one of Al₂0₃, SiC, TiC, B₄C and TiB₂. The Zr0₂ sintered body of claim 3 having a crystal structure substantially composed of a tetragonal phase or a mixture of a tetragonal phase and a cubic phase with an average crystal grain size of 2 µm or less. The Zr0₂ sintered body according to claim 3 or 4, wherein Al₂0₃, SiC, TiC, B₄C and TiB₂ are in the form of fiber or whisker. The Zr0₂ sintered body according to any of claims 1 to 5, having a stress-induced transformation ratio of 25% or more. A method of producing a high-toughness Zr0₂ sintered body containing Nd₂0₃ and Y₂0₃ as stabilizers of the crystal structure of Zr0₂, comprising the following steps: pressing a starting material powder consisting essentially of Zr0₂ containing 0.1 to 3 mol% of Nd₂0₃ and 0.5 to 3.5 mol% of Y₂0₃ to provide a green body; and subjecting the green body, after sintering, to hot isostatic pressing at a temperature of 1300 to 1600° C and a pressure of 100 kg/cm² or more. The method of claim 7, wherein the starting material powder contains essentially 40 to 90 volume% of Zr0₂ and 10 to 60 volume% of at least one of Al₂0₃, SiC, TiC, B₄C and TiB₂. The method of claim 8, wherein Al₂0₃, SiC, TiC, B₄C and TiB₂ are in the form of fiber or whisker.

Inventors: INOUE, RYOJI
Applicants: HITACHI METALS, LTD.
Application year: 1988
Main classification: C04B35
IPC codes: C04B35