High strength high toughness TiB2 ceramics

Abstract: A ceramic sintered body comprising TiB₂ as the principal component, 0.5 to 30 wt. % of SiC and 2 to 40 wt. % of ZrO₂. In some cases ZrO₂ includes stabilizers for tetragonal crystal, such as Y₂O₃, MgO or CeO₂. The sintered body has a dense structure where TiB₂, ZrO₂ and SiC unifornly disperse. ZrO₂ is located at the grain boundary of TiB₂, and inhibits the grain growth of TiB₂ as well as improves the strength of the sintered body by binding it to TiB₂ strongly. SiC is located not only at the grain boundary but also on the inside of both grains. Since the contract of SiC during cooling process of the sintered body is less than TiB₂ and ZrO₂, compressive stress is produced in SiC and around thereof. The sintered body is remarkably strengthened by the residual compressive stress, and simultaneously, the high fracture toughness is also achieved.

Description: TiB₂ ceramics are hard ceramics, and utilized for cutting tools, plastic working tools, high wear resistance parts and the like. The present invention relates to a novel TiB₂ ceramics where the strength and the fracture toughness are improved. TiB₂ has various characteristics, such as a high melting point, a high hardness and a high wear resistance. Moreover, it is excellent in high temperature anticorrosion, and is electrically conductive. Therefore, TiB₂ can be processed by electric discharge machining. Whereas, TiB₂ is inferior in flexural strength which is 196 to 294 MPa (20 to 30 kgf/mm²) and in fragility. Thereupon, various investigations were conducted in order to improve the strength of TiB₂, and for example, Japanese Patent No. 1,096,092 discloses to improve the flexural strength and the fragility by adding nickel boride, cobalt boride, iron boride or the like as binder. Japanese Patent KOKOKU No. 59-7688 discloses to improve the strength as well as oxidation resistance and heat resistance by adding zirconium oxide such as ZrO₂, ZrO, ZrO 0.35 or Zr₃O 1-x to TiB₂. Japanese Patent KOKOKU No. 58-57393 discloses that a composite ceramics sintered material having a high density, a high strength and excellent in oxidation resistance is obtained by adding 10 to 60 wt. % of TiB₂ to TiN. Austrian Patent Official Gazette No. 199,886 discloses that a composite sintered body of TiC and TiB₂ has a high hardness, wear resistance, anticorrosion and the like. However, the strongest flexural strength of the above sintered bodies is only 1078 MPa (110 kgf/mm²), and the fracture toughness is insufficient. Thus, it has been desired to develop a TiB₂ having a greater strength and fracture toughness in order that TiB₂ is widely utilized as materials for various uses. Japanese Patent Publication JP-A-58-130168 further describes a sintered ceramic material containing TiB₂, ZrO₂ and 0,001 to 10 wt% of a "cubic crystalline metallic carbide". Said document does not mention the addition of silicon carbide. SUMMARY OF THE INVENTION An object of the invention is to provide a TiB₂ ceramic sintered body having a high strength and a high fracture toughness remaining the excellent qualities of TiB₂ such as high hardness, high wear resistance and high temperature anticorrosion. The inventors have investigated in order to achieve such an object, and found that the above TiB₂ ceramic sintered body can be obtained by combining SiC and ZrO₂ with TiB₂. Thus, the present invention provides a high strength high fracture toughness TiB₂ ceramic sintered body comprising TiB₂ as the principal component, 0.5 to 30 wt. % of SiC and 2 to 40 wt. % of ZrO₂. DETAILED DESCRIPTION OF THE INVENTION The raw powder of TiB₂ has preferably less impurities, and a suitable TiB₂ has a purity of more than 99 wt. %. The particle size is preferably fine, and not more than 3 µm, particularly not more than 1 µm as the mean particle size is preferred. The content of TiB₂ in the sintered body must be the greatest, and a suitable content is about 40 to 97.5 wt. %, preferably about 60 to 94 wt. %. The raw powder of SiC has preferably less impurities, and a suitable SiC has a purity of more than 99 wt. %. The particle size is preferably fine, and not more than 3 µm, preferably not more than 1 µm as the mean particle size is preferred. The content of SiC in the sintered body is about 0.5 to 30 wt. %, and about 1 to 10 wt. % is preferred. When the content of SiC is beyond 30 wt. %, the decreases in hardness and strength are problems. While, when the content is less than 0.5 wt. %, the blending effect is insufficient. The raw powder of ZrO₂ has preferably less impurities, and a suitable ZrO₂ has a purity of more than 99 wt. %, except additives optionally added. The particle size is preferably fine, and not more than 3 um, preferably not more than 1 um as the mean particle size is preferred. The raw powder of ZrO₂ may be in a form of sol. When ZrO₂ is stabilized in a form of tetragonal crystal, the sintered body is rendered to have a higher strength. For that purpose, ZrO₂ is preferably blended with 1 to 8 %, preferably 3 to 7 % of Y₂O₃, 1 to 4 % of MgO or 10 to 21 % of CeO₂, as the per cent by weight to ZrO₂, as a stabilizer for tetragonal crystal. Two or more kinds of tetragonal crystal stabilizers may be added. The content of ZrO₂ in the sintered body is about 2 to 40 wt. %, and 5 to 30 wt. % is preferred. When a tetragonal crystal stabilizer is contained, the content includes the weight of the stabilizer. When the content of ZrO₂ is beyond 40 wt. %, the decreases in hardness and strength are problems. While, when the content is less than 2 wt. %, the blending effect is insufficient. The grain growth of TiB₂ can also be inhibited by adding a metal carbide other than SiC, a metal nitride, a carbonitride or a metal boride other than TiB₂. Such a component includes TiC, VC, ZrC, NbC, Cr₃C₂, TaC, TiN, Ti(C,N), TaB₂, ZrB₂, NbB₂, and the like. Two or more kinds of the above compounds may be combined. The content of the above compounds is usually less than 20 wt. %, and 0.5 to 20 wt. %, particularly 3 to 15 wt. %, is suitable. As the preparation method of the ceramic sintered body of the invention, first, the above raw powders are uniformly mixed. The mixer is preferably a grinding type, such as ball mill, capable of grinding the whole mixed powder. When the balls of TiB₂, SiC or ZrO₂ are used, the contamination caused by the abrasion of the balls can be avoided. Subsequently, the mixed powder is formed into a molded body having a theoretical density ratio of more than about 50 %. The molded body is placed in a sintering furnace, and sintered at 1600 to 2000°C under nonoxidizing atomosphere to produce a dense sintered body. The sintered body is preferably further densified by hot isostatic pressing (HIP). The HIP is preferably conducted at 1450 to 1800 °C at more than 101.3 MPa (1000 atm) in an inert gas such as argon. While, the molded body may be sintered by capsule HIP process where the molded body is sealed with vacuum in a metal or glass capsule and HIP is conducted in a state shielded from outer gases. In this case, a dense sintered body can be obtained by the HIP at 1450 to 1800°C at 101.3 to 202.6 MPa (1000 to 2000 atm) for 1 to 2 hours. The sintered body thus obtained has a dense structure where TiB₂, ZrO₂ and SiC are uniformly disperse. In particular, ZrO₂ is located at the grain boundary of TiB₂, and inhibits the grain growth of TiB₂. The grain size of TiB₂ is small, i.e. 1 to 3 µm, and the TiB₂ grains grow little from the raw powder. While, SiC is located not only at the grain boundary but also on the inside of both grains of TiB₂ and ZrO₂. The three-point flexural strength of the sintered body is not less than 1078 MPa (110 kgf/mm²), usually in the range of 1275 to 1570 MPa (130 to 160 kgf/mm²). The hardness is about 19620 to 29430 MPa (2000 to 3000 kgf/mm²). The fracture toughness is about 6 to 13 MPam 0.5 , usually not less than 7 Mpam 0.5 . The growth rate of TiB₂ itself is high, and in the case of TiB₂ simplex, a dense sintered body is not obtained. In the sintered body of the invention, ZrO₂ not only inhibits the grain growth of TiB₂ but also improves the stregnth of the sintered body by binding it to TiB₂ strongly. The fracture toughness of the sintered body is improved by the stress-induced transformation and the residual compressive stress after transformation of ZrO₂ itself. The greatest characteristic of the present sintered body is to contain SiC. The thermal expansion coefficient of SiC is 4 x 10⁻⁶/°C which is smaller than TiB₂ of 7 x 10⁻⁶/°C and ZrO₂ of 8 x 10⁻⁶/°C. The sintered body contracts during cooling process, and only the contract of SiC is little. Then, compressive stress is produced in SiC and the TiB₂ and the ZrO₂ around SiC, and remains in the sintered body which is remarkably strengthened by the residual compressive stress; simultaneously, the high fracture toughness is also achieved. Therefore, as shown in Examples 1 through 24, the strength and the fracture toughness of the sintered body of the invention are much greater than those of Comparative Examples 1 through 8 composed of TiB₂ and ZrO₂. Moreover, as shown in Comparative Examples 9 through 11, when the SiC content is out of the range, the effect of SiC is little. The grain growth of TiB₂ is also inhibited by adding a metal carbide, a metal nitride, a metal carbonitride, a metal boride such as TiC, VC, ZrC, NbC, Cr₃C₂, TaC, TiN, Ti(CN), TaB₂, ZrB₂, NbB₂, and they improve the strength of the sintered body. The metal carbides, metal carbonitride, and metal nitride have an action to improve sintering ability, and the blending of them is advantageous for the production of a dense sintered body. The ceramic sintered body of the invention is excellent in strength, hardness and fracture toughness, and it can be used for various uses such as cutting tools, plastic working tools and high wear resistance parts. EXAMPLES Example 1 87.5 wt. % of TiB₂ powder, 10 wt. % of ZrO₂ powder stabilized by adding 3.5 wt. % of Y₂O₃ (TZ-3.5Y) and 2.5 wt. % of SiC powder were mixed and ground by an attritor using zirconia balls in ethanol solvent. The mixed powder was dried by a rotary evaporator and rendered granules by passing through a sieve. The granules were uniaxially pressed and cold isostatically pressed to form a molded body having a density of about 60 %. The molded body was placed in a glass bottle capsule, and HIP was conducted at 1600°C at 202.6 MPa (2000 atm) for 2 hours. The sintered body thus obtained was dense, and a mean particle size of TiB₂ was less than 3 µm. The three components were uniformly dispersed in the sintered body. ZrO₂ grains were located around TiB₂, and SiC was located at the both grain boundaries and on the inside of both grains of TiB₂ and ZrO₂. As shown in Table 1, the sintered body had a three-point flexural strength of 1472 MPa (150 kgf/mm²), a Vickers hardness of 25310 MPa (2580 kgf/mm²) and a fracture toughnes of 7.0 MPam 0.5 . Examples 2 through 15 TiB₂ powder, ZrO₂ powder added with no Y₂O₃, 3.5 wt. % of Y₂O₃ (TZ-3.5Y), 5.2 wt. % of Y₂O₃ (TZ-5.2Y) or 7.0 wt. % of Y₂O₃ (TZ-7.0Y) and SiC powder were mixed and ground in the composition shown in Table 1, and molded, similar to Example 1. Then, a glass capsule HIP was conducted under the conditions shown in Table 1. The sintered bodies obtained had a dense fine structure where the three components were uniformly dispersed, similar to Example 1. The properties of the sintered bodies are shown in Table 1. Examples 16 through 22 TiB₂ powder, ZrO₂ powder added with no Y₂O₃, 3.5 wt. % of Y₂O₃ (TZ-3.5Y), 5.2 wt. % of Y₂O₃ (TZ-5.2Y) or 7.0 wt. % of Y₂O₃ (TZ-7.0Y) and SiC powder were mixed and ground in the composition shown in Table 1, and subjected to uniaxial press and CIP to form a molded body having a density of about 60 %, similar to Example 1. The molded body was sintered at 1700 to 1800°C to obtain a sintered body having a density of 95 to 99 %. The sintered body was hot isostatically pressed at 1500 to 1600°C at 202.6 MPa (2000 atm) for 2 hours to obtain a dense sintered body. The sintered bodies thus obtained had a dense fine structure where the three components were uniformly dispersed, similar to Example 1. The properties of the sintered bodies are shown in Table 1. Example 23 85 wt. % of TiB₂ powder, 10 wt. % of ZrO₂ powder added with 2.8 wt. % of MgO and 5 wt. % of SiC were mixed and ground, and subjected to uniaxial press and CIP to form a molded body having a density of about 60 %, similar to Example 1. The molded body was treated with glass encapsulation HIP under the conditions shown in Table 1. The sintered body thus obtained was dense, and the three components having a mean particle size of less than 3 µm were uniformly dispersed therein. The properties of the sintered body are shown in Table 1. The sintered body was highly tough, and the toughness was beyond 10 MPam 0.5 . Example 24 85 wt. % of TiB₂ powder, 10 wt. % of ZrO₂ powder added with 15.5 wt. % of CeO₂ and 5 wt. % of SiC were mixed and ground, and subjected to uniaxial press and CIP to form a molded body having a density of about 60 %, similar to Example 1. The molded body was treated with HIP similar to Example 23. The sintered body thus obtained had a dense and uniform fine structure similarly. The toughness was also beyond 10 MPam 0.5 . Examples 25 through 47 TiB₂ powder, ZrO₂ powder added with no Y₂O₃, 3.5 wt. % of Y₂O₃ (TZ-3.5Y), 5.2 wt. % of Y₂O₃ (TZ-5.2Y) or 7.0 wt. % of Y₂O₃ (TZ-7.0Y), SiC powder and TiC, VC, ZrC, NbC, Cr₃C₂, TaC, TiN, Ti(CN), TaB₂, ZrB₂ or NbB₂ were mixed and ground in the composition shown in Table 2, and subjected to uniaxial press and CIP to form a molded body having a density of about 60 %, similar to Example 1. The molded body was sintered and hot isostatically pressed with or without a glass capsule under the conditions shown in Table 2. The sintered bodies thus obtained had a dense fine structure where the four or more components were uniformly dispersed. The properties of the sintered bodies are shown in Table 2. Comparative Examples 1 through 11 TiB₂ powder, ZrO₂ powder added with no Y₂O₃, 3.5 wt. % of Y₂O₃ (TZ-3.5Y), 5.2 wt % of Y₂O₃ (TZ-5.2Y) or 7.0 wt. % of Y₂O₃ (TZ-7.0Y) and SiC powder were mixed and ground in the composition shown in Table 3, and subjected to uniaxial press and CIP to form a molded body having a density of about 60 %, similar to Example 1. The molded body was treated with glass encapsulation HIP or sintering and HIP under the conditions shown in Table 3. The sintered bodies had a dense structure where the three components were uniformly dispersed. Nevertheless, the strength was weak compared with Examples of the invention, and was 1078 MPa (110 kgf/mm²) at the highest, as shown in Table 3. Example Sintering Conditions HIP Conditions Properties of Sintered Body Composition (wt.%) Temperature (°C) Temperature (°C) Pressure (atm) Three-Point Flexural Strength (kgf/mm²) Hardness (kgf/mm²) Fracture Toughness (MPam 0.5 ) 1 TiB₂-10%(TZ-3.5Y)-2.5%SiC - 1600 2000 150 2580 7.0 2 TiB₂-2%(TZ-3.5Y)-2.5%SiC - 1600 2000 130 2800 6.2 3 TiB₂-5%(TZ-3.5Y)-2.5%SiC - 1600 2000 133 2750 6.8 4 TiB₂-10%(TZ-3.5Y)-2.5%SiC - 1600 2000 152 2500 7.8 5 TiB₂-10%(TZ-3.5Y)-2.5%SiC - 1600 1000 146 2450 7.4 6 TiB₂-20%(TZ-3.5Y)-2.5%SiC - 1600 2000 135 2450 7.0 7 TiB₂-20%(TZ-3.5Y)-2.5%SiC - 1600 2000 155 2400 8.2 8 TiB₂-30%(TZ-3.5Y)-2.5%SiC - 1500 2000 138 2150 9.5 9 TiB₂-30%(TZ-3.5Y)-2.5%SiC - 1500 1000 135 2000 9.5 10 TiB₂-20%(TZ-5.2Y)-2.5%SiC - 1600 2000 135 2250 8.0 11 TiB₂-20%(TZ-5.2Y)-2.5%SiC - 1600 2000 133 2200 7.8 12 TiB₂-40%(TZ-5.2Y)-2.5%SiC - 1600 2000 130 2000 9.0 13 TiB₂-10%(TZ-7.0Y)-2.5%SiC - 1600 2000 140 2500 7.1 14 TiB₂-10%ZrO₂-2.5%SiC - 1500 1000 139 2400 7.0 15 TiB₂-20%ZrO₂-5%SiC - 1600 2000 135 2250 8.8 16 TiB₂-10%(TZ-3.5Y)-2.5%SiC 1800 1600 2000 144 2450 8.2 17 TiB₂-20%(TZ-3.5Y)-2.5%SiC 1700 1600 2000 143 2400 8.8 18 TiB₂-20%(TZ-5.2Y)-2.5%SiC 1700 1600 2000 145 2350 9.0 19 TiB₂-5%ZrO₂-1%SiC 1700 1500 2000 133 2550 7.5 20 TiB₂-10%ZrO₂-5%SiC 1700 1500 2000 139 2350 7.8 21 TiB₂-20%ZrO₂-10%SiC 1700 1500 2000 137 2200 8.4 22 TiB₂-20%ZrO₂-30%SiC 1700 1500 2000 133 2100 7.0 23 TiB₂-10%(TZ-2.8M)-5%SiC - 1600 2000 115 2300 11.0 24 TiB₂-10%(TZ-15C)-5%SiC - 1600 2000 116 2300 12.5 Comparative Example Sintering Conditions HIP Conditions Properties of Sintered Body Composition (wt.%) Temperature (°C) Temperature (°C) Pressure (atm) Three-Point Flexural Strength (kgf/mm²) Hardness (kgf/mm²) Fracture Toughness (MPam 0.5 ) 1 TiB₂-10%ZrO₂ - 1600 2000 100 2500 4.2 2 TiB₂-20%ZrO₂ - 1600 2000 84 2300 6.5 3 TiB₂-30%ZrO₂ - 1600 2000 65 2100 7.4 4 TiB₂-10%(TZ-3.5Y) - 1600 2000 108 2600 4.0 5 TiB₂-20%(TZ-3.5Y) - 1600 2000 110 2450 5.5 6 TiB₂-30%(TZ-3.5Y) - 1600 2000 100 2200 6.3 7 TiB₂-10%(TZ-3.5Y) 1800 1600 2000 95 2300 4.0 8 TiB₂-30%(TZ-3.5Y) 1700 1600 2000 75 2000 6.6 9 TiB₂-30%(TZ-3.5Y) -0.1%SiC - 1600 2000 50 2900 3.2 10 TiB₂-20%(TZ-5.2Y) -35%SiC - 1600 2000 95 1700 6.5 11 TiB₂-20%(TZ-5.2Y) -0.1%SiC - 1600 2000 105 2550 4.0 TZ-3.5Y ZrO₂ added with 3.5 % by weight of Y₂O₃ TZ-5.2Y ZrO₂ added with 5.2 % by weight of Y₂O₃ TZ-2.8M ZrO₂ added with 2.8 % by weight of MgO TZ-15C ZrO₂ added with 15.5 % by weight of CeO₂

Claims: A ceramic sintered body comprising TiB₂ as the principal component, 0.5 to 30 wt% of SiC and 2 to 40 wt % of ZrO₂, the total content of TiB₂, SiC and ZrO₂ being not less than 80 wt %, the three-point flexural strength of said sintered body being not less than 1078 MPa (110 kg/mm²), and the fracture toughness being not less than 7 MPam 0.5 . The ceramic sintered body of claim 1 of which the content of SiC is 1 to 10 wt. % and the content of ZrO₂ is 5 to 30 wt. %. The ceramic sintered body of claim 1 wherein the ZrO₂ contains 1 to 8 % of Y₂O₃, 1 to 4 % of MgO and/or 10 to 21 % of CeO₂, as the per cent by weight to ZrO₂. The ceramic sintered body of claim 2 wherein the ZrO₂ contains 3 to 7 % of Y₂O₃ as the per cent by weight to ZrO₂. The ceramic sintered body of claim 1 which further contains 0.5 to 20 wt. % of one or more kinds of the compounds selected from the group consisting of metal carbides other than SiC, metal nitrides, metal carbonitrides and metal borides other than TiB₂. The ceramic sintered body of claim 5 wherein said one or more kinds of the compounds are selected from the group consisting of TiC, VC, ZrC, NbC, Cr₃C₂, TaC, TiN, Ti(CN), TaB₂, ZrB₂ and NbB₂. The ceramic sintered body of claim 5 or claim 6 wherein the ZrO₂ contains 1 to 8 % of Y₂O₃, 1 to 4 % of MgO and/or 10 to 21 % of CeO₂, as the per cent by weight to ZrO₂. The ceramic sintered body of claim 4 which further contains 0.5 to 20 wt. % of one or more kinds of the compounds selected from the group consisting of metal carbides other than SiC, metal nitrides, metal carbonitrides and metal borides other than TiB₂. The ceramic sintered body of claim 8 wherein said one or more kinds of the compounds are selected from the group consisting of TiC, VC, ZrC, NbC, Cr₃C₂, TaC, TiN, Ti(CN), TaB₂, ZrB₂ and NbB₂. A process for producing a ceramic sintered body which comprises, forming a molded body of a mixed powder comprising TiB₂ as the principal component, 0.5 to 30 wt. % of SiC and 2 to 40 wt. % of ZrO₂ having a theoretical density ratio of more than 50 %, sintering the molded body at 1600 to 2000°C under nonoxidizing atmosphere to produce a dense sintered body. The process of claim 10, which further comprises, treating the dense sintered body with hot isostatic pressing at 1450°C to 1800°C at more than 101.3 MPa (100 atm)in an inert gas. A process for producing a ceramic sintered body which comprises, forming a molded body of a mixed powder comprising TiB₂ as the principal component, 0.5 to 30 wt. % of SiC and 2 to 40 wt. % of ZrO₂ having a theoretical density ratio of more than 50 %, and sintering the molded body by encapsulation HIP process where the molded body is sealed with vacuum in a metal or glass capsule and hot isostatic pressing is conducted at 1450 to 1800°C at 101.3 to 202.6 MPa (1000 to 2000 atm). The ceramic sintered body of claim 1 wherein three-point flexural strength is not less than 1275 MPa (130 kgf/mm²). The ceramic sintered body of claim 5 or 8 wherein said one or more kinds of the compounds are selected from the group consisting of TiC, TiN, Ti(CN) and ZrB₂.

Inventors: NISHIO HIROAKI C O NKK CORPORA, TORIZUKA SHIRO C O NKK CORPORA, NISHIO, HIROAKI C/O NKK CORPORATION, TORIZUKA, SHIRO C/O NKK CORPORATION
Applicants: NIPPON KOKAN KK, NKK CORPORATION
Application year: 1989
Main classification: C04B35
IPC codes: C04B35