JPH0352425B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention has high mechanical strength and fracture toughness
This invention relates to a ZrO ₂ -CeO ₂ based zirconia sintered body. The sintered body according to the present invention is suitable for use as a constituent material for cutting or cutting tools, various dies, nozzles, sliding parts, and the like. Pure zirconia has a reversible phase transition between 900 and 1200℃, which causes a large volume change.
Such abnormal volume changes cause cracks in the sintered body, making it difficult to manufacture the sintered body. Therefore, when creating a sintered body, various stabilizers such as MgO, CaO, _Y2O3 , etc. are dissolved in solid solution to create a sintered body that has a cubic crystal structure that does not undergo phase transition _. So-called stabilized zirconia (hereinafter referred to as FSZ)
) is commonly used. However, such a FSZ sintered body has a drawback that it is inferior in mechanical strength such as bending strength and thermal shock strength, and is not suitable for applications requiring such properties. Recently, high strength, which has fundamentally improved this drawback,
Research is being conducted on zirconia sintered bodies with high toughness. This sintered body differs from the FSZ sintered body in its composition and crystal structure. In other words, in terms of composition, the amount of stabilizer added is smaller than that of FSZ. On the other hand, in terms of crystal structure, while FSZ has a cubic system, it has a tetragonal system or a mixed phase of a tetragonal system and a cubic system.
Such a sintered body is called a partially stabilized zirconia (hereinafter referred to as PSZ) sintered body. This PSZ
The sintered body has high strength and high toughness, and the reason for this is explained below. In other words, when mechanical stress concentration is applied to the tip of a crack in a sintered body, the surrounding tetragonal particles undergo stress-induced transformation into monoclinic particles, and the accompanying volume expansion causes the crack to open. By inhibiting the progress, fracture toughness and fracture strength are increased. Therefore, in order for the PSZ sintered body to have high strength and high toughness, it is extremely important that it contains particles consisting of a tetragonal crystal phase. However, this tetragonal crystal phase is the high temperature phase in most zirconia-stabilizing component systems and does not exist as a stable phase at room temperature. Therefore,
It is made to exist as a metastable phase at room temperature, but in this case, the method of manufacturing the sintered body and the selection of the type and amount of stabilizer added are important. Currently, as a high-strength, high-toughness PSZ sintered body,
MgO- _ZrO2- based, CaO- _ZrO2- based, and _{Y2O3 - ZrO2-} based sintered bodies are already known. The present inventors have discovered that the above stabilizer MgO,
As a result of intensive research aimed at obtaining sintered bodies with excellent mechanical properties using stabilizers other than CaO and Y ₂ O ₃ , we discovered that the ZrO ₂ -CeO ₂ system is extremely effective. He came up with an invention. According to previous research, ZrO ₂ − CeO ₂ based sintered bodies are ZrO ₂ −
It was said that it does not exhibit the high strength and toughness known for Y ₂ O ₃ systems. However, the present inventors found that even in ZrO ₂ -CeO ₂ -based sintered bodies, if the composition ratio is within a specific range and the crystal phase is tetragonal, ZrO ₂ -Y It was discovered that high strength and high toughness comparable to those of the ₂ O ₃ type were developed, and furthermore, if the powder obtained by a particular manufacturing method was sintered as a raw material, it could be made into a dense material with high strength and toughness. It was discovered that a sintered body can be obtained, and the present invention was achieved. That is, the present invention is a zirconia sintered material mainly consisting of ZrO ₂ and CeO ₂ , with a molar ratio of CeO ₂ /ZrO ₂ in the range of 8/92 to 30/70, and whose crystal phase is mainly tetragonal. It provides the body. To produce the sintered body of the present invention, ZrO ₂ −CeO ₂
The raw material powder may be formed into a desired shape by a rubber press method or the like, and then sintered by firing at a temperature of 1300 to 1600°C for several hours. ZrO ₂ -CeO ₂ raw material powder can be prepared by the so-called dry synthesis method, in which zirconia powder and ceria powder are mixed, calcined and pulverized repeatedly. In order to obtain a sintered body with high mechanical strength, a mixed powder obtained by coprecipitation from a mixed solution of a zirconium compound and a cerium compound is separated, dried, and calcined as described below. It is preferable to use a wet synthesis method using a mixed solution of a zirconium compound and a cerium compound as a starting material. For example, an aqueous solution of a zirconium salt such as zirconium oxychloride or zirconium nitrate and an aqueous solution of a cerium salt such as cerium nitrate are mixed to a desired composition, and aqueous ammonia is added thereto, and the resulting precipitate is filtered out. This method involves drying, calcining, and obtaining the desired powder. Alternatively, a method may be used in which a mixed solution of the zirconium salt and cerium salt is heated for several tens of hours to cause a hydrolysis reaction, and the resulting sol is dried and calcined to obtain the desired powder. Furthermore, the desired powder is obtained by evaporating the mixed solution of the above zirconium salt and cerium salt to dryness under normal pressure or reduced pressure, pulverizing it, and calcining it at a temperature of 500°C to 900°C. The method is also good. Powders obtained by these so-called wet synthesis methods have excellent sinterability and are suitable as raw materials for producing the sintered body of the present invention. In addition, it is not necessary to use high-purity cerium oxide or cerium salt as the cerium raw material.
If the cerium content is 80% or more, the balance may be an oxide or salt made of a light rare earth metal such as samarium or lanthanum. Such raw materials are inexpensive and suitable as raw materials for industrial products. Therefore, in the present invention, mainly ZrO ₂ and CeO ₂
A zirconia sintered body consisting of ZrO ₂ means a sintered body mainly using CeO ₂ as a stabilizer for ZrO 2 , and in addition to CeO ₂ other rare earth oxides such as Y ₂ O ₃ , Yb ₂ O ₃ , La ₂ O ₃ , Sm ₂ O ₃ , etc., or alkaline earth metal oxides MgO, CaO, etc., and metal oxides of Group 3 and Group 4 of the periodic table.
It may contain Sc ₂ O ₃ , TiO ₂ , HfO ₂ , etc. For example, in a ZrO ₂ −CeO ₂ −Y ₂ O ₃ based sintered body,
When the amount of Y ₂ O ₃ added is 2 mol % or less and partial stabilization is achieved, it can be considered that the added CeO ₂ makes an important contribution to the partial stabilization. fall within range. CeO2/ _CeO2 in the _ZrO2 - _CeO2 -based sintered body of the present invention
The molar ratio of _ZrO2 should be in the range of 8/92 to 30/70. If the CeO ₂ /ZrO ₂ molar ratio is less than 8/92, it is extremely difficult to obtain a sintered body having a tetragonal crystal phase. In this case, after the sintered body is fired, a phase transition from a tetragonal system to a monoclinic system occurs during cooling to room temperature, and cracks may even occur due to the volume expansion that occurs at this time. In addition, when the molar ratio exceeds 30/70, although a sintered body with a crystal phase consisting of coexisting tetragonal and cubic crystal phases is obtained, the proportion of the tetragonal system decreases, so The expected high fracture toughness and fracture strength cannot be obtained. Measurement of each crystal phase is performed by X-ray diffraction method,
For example, the X-ray diffraction intensities of the monoclinic <111> and <111> planes, the tetragonal <111> plane, and the cubic <111> plane are M<111>, M<111>, and T< 111
>, C<111>, the intensity ratio of (M<111>+M<111>)/(T<111>+C<111>+M<111>+M<111>) is expressed as the weight percent of monoclinic system. do. The particle size of the sintered body of the present invention is preferably 2 μm or less, 0.5 μm at a firing temperature of 1400°C, and 0.5 μm at a firing temperature of 1600°C.
It becomes 2μm. Conventionally, it has been known that PSZ sintered bodies deteriorate over time when exposed to high temperatures for long periods of time, causing the sintered bodies to break. For example, for a Y ₂ O ₃ -PSZ sintered body with a particle size of 2 μm or more, the
If left at a temperature of ℃ for a long time, the tetragonal crystal will transform to monoclinic crystal, and cracks will be observed in the sintered body. That is, the finer the particle size, the less thermal deterioration occurs over time. The particle size of the sintered body of the present invention can be easily controlled to 2 μm or less, and such a sintered body is preferable because it has excellent thermal stability. Furthermore, the sintered body of the present invention has a mechanical strength of 3-point bending strength of 50 kg/mm ^{2 or more and a fracture toughness of 50 kg/mm 2} or more.
It has characteristics of 4MN・m ^-1.5 or more. If the characteristic values are below these values, the sintered body of the present invention cannot withstand use as a useful industrial member, which is not preferable. Next, the sintered body of the present invention will be explained in detail based on Examples. Three-point bending strength and fracture toughness in Examples were measured by the following methods. Three-point bending strength measurement was performed by cutting and grinding a plate-shaped sintered body into a rectangular bar-shaped test piece measuring 3 mm x 4 mm x 40 mm.
This is carried out under the conditions specified in R 1601, with a span length of 30 mm and a load application acceleration of 0.5 mm/min. Fracture toughness is measured by the indentation method, in which a Bitkers indenter is driven into the mirror-polished sintered sample surface and the value is calculated from the ratio of the length of the indentation to the length of the crack generated from the indentation. The driving load of the indenter is 20 kg. The formula used for calculation is D.
The following formula is given in B. Marshall and AGEvans, Journal of American Ceramics Society (J. Am. Ceram. Soc. 64 (12), C-182 (1981). K _IC = 0.036 E ^0.4 P ^0.6 a ^-0.7 (C/a) ^-1.5 K _IC ; Fracture toughness (N m ^-1.5 ) E; Modulus of elasticity (N m ^-2 ) P; Load (N m ^-2 ) a; Indentation Diagonal length (m) C: Length of crack generated from indentation (m) Thermal stability was measured at 200℃, dew point 50℃
This was done by observing the presence or absence of cracks on the surface of a test piece that was held for 1000 hours under a stream of water vapor-containing air. Examples 1 to 6, Comparative Example 1 Zirconium oxychloride aqueous solution and cerium 80
%, remainder, lanthanum, samarium, neodymium, and other light rare earth metals were mixed to a desired composition and heated continuously at 100°C for 60 hours to obtain a hydrolyzed sol. The powder obtained by evaporating to dryness was calcined at 900°C and further ground in a ball mill for 48 hours to prepare a ZrO ₂ -CeO ₂ -based raw material powder. Next, the raw material powder was processed by a rubber press method to a thickness, width, and length of 4 mm and 40 mm, respectively.
A plate-shaped molded body with a diameter of 56 mm was obtained. This molded body
The present invention was baked at a temperature of 1400 to 1500°C for 2 hours.
A ZrO ₂ −CeO ₂ based sintered body was obtained. The crystal phase content, crystal grain size, bending strength, fracture toughness, and thermal stability of the seven sets of sintered bodies thus created were measured. These results are shown in Table 1. Examples 7 and 8, Comparative Example 2 An aqueous solution of zirconium oxychloride and an aqueous solution of cerium nitrate with a purity of 99% or more were mixed to give the desired composition, and the mixture was prepared according to the method described in Example 1.
A ZrO ₂ -CeO ₂ based raw material powder was prepared. After molding this powder using the rubber press method, the temperature
The desired sintered body was obtained by holding at 1400°C for 2 hours.
Measurements similar to those described in Example 1 were performed on the three sets of sintered bodies thus produced. The measurement results are shown in Table 1.

【table】

[Table] Examples 9 and 10 Aqueous solutions of zirconium oxychloride, cerium nitrate, and yttrium nitrate are mixed to give the desired composition, and aqueous ammonia is added to adjust the pH of the solution.
8 to obtain a precipitate, which was then filtered, dried, calcined at 800°C, and further ground in a ball mill for 24 hours to obtain a ZrO ₂ −CeO ₂ −Y ₂ O ₃ raw material powder. I got it. The above raw material powder was molded and fired in the same manner as described in Example 1 to obtain a sintered body of the present invention. The crystal phase content, crystal grain size, bending strength, fracture toughness, and thermal stability of the two sets of sintered bodies thus created were measured. The measurement results are shown in Table 2.

【table】

[Table] Examples 11 to 18, Comparative Example 3 An aqueous solution of zirconium oxychloride and an aqueous cerium nitrate solution were mixed to a desired composition, and aqueous ammonia was added thereto to bring the pH of the solution to 8 to form a precipitate. Separated, dried, and calcined at 800℃.
Furthermore, it was ground in a ball mill for 24 hours to form ZrO ₂ −CeO ₂
A raw material powder was prepared. This powder was molded and measured in the same manner as in Example 7. The results are shown in Table 3.

【table】

[Table] As is clear from the above results, the zirconia sintered body of the present invention mainly contains CeO ₂ as a stabilizer, and has a crystalline phase mainly composed of tetragonal or tetragonal and cubic crystals, and has a high It has strength and high toughness. Applications that require mechanical strength and thermal/mechanical durability, such as cutting tools, extrusion or wire drawing dies, cutters, spray nozzles, grinding balls, ball bearings, mechanical seals, hydraulic equipment structural parts, internal twisting It is suitable as an industrial material for engine structural parts, etc., and is extremely useful industrially. Comparative Examples 4 to 6 Sintered bodies were prepared under the same conditions as in Example 7, except that cerium nitrate was replaced with a predetermined amount of yttrium nitrate.
Similar measurements were made. The results are shown in Table 4.

【table】

【table】

Claims (1)

[Claims] 1 A composition having a CeO ₂ /ZrO ₂ molar ratio of 8/92 to 30/70.
A zirconia sintered body consisting of ZrO ₂ and CeO ₂ with a tetragonal crystal phase. 2 ZrO _{2 -CeO 2 mixed} powder with a CeO ₂ /ZrO ₂ molar ratio of 8/92 to 30/70 obtained by wet synthesis method,
A method for producing a zirconia sintered body, which comprises forming and sintering to produce a zirconia sintered body whose crystal phase is a tetragonal system.