JPH01290547A - Method for toughening ceramics - Google Patents

Abstract

PURPOSE:To improve strength and toughness by producing a mold body or a presintered body with ceramics contg. a specified amt. of fine zirconia as starting material, impregnating a soln. contg. zirconium into the body, drying and sintering this impregnated body. CONSTITUTION:Zirconia of <=3mum average particle size is incorporated into ceramics such as alumina, mullite, zircon or silicon carbide as starting material by 3-20wt.% and a molded body or a presintered body is produced with the starting material. A soln. contg. zirconium or an aq. soln. of a zirconium salt such as zirconium oxychloride or zirconium nitride is impregnated into the body and this impregnated body is dried and sintered. By this method, residual compressive stress is generated in the surface layer of the resulting ceramic sintered body and ceramics having superior strength and toughness is obtd.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention aims to generate residual compressive stress in the surface layer of a ceramic sintered body to simultaneously improve strength and toughness.

Conventionally, surface pressure has been used as a method to strengthen ceramics! 8
Methods of configuring layers are known. For example, a method of dissolving chromium oxide on the surface of an alumina sintered body and a method of dissolving 1-His acid on the surface of titania have been proposed, but the strength improvement effect of the former is 2.7 to 28.9! However, it is insufficient. In addition, a method of applying compressive stress to the surface layer by a rapid cooling method has been proposed, but it has not been put into practical use because it is difficult to control and has poor reproducibility. A method has been proposed in which the zirconia in the surface layer is transformed from a tetragonal system to a monoclinic system by removing the stabilizer by high-temperature heat treatment, and applying compressive stress to the surface due to the volume expansion at that time.However, these methods In this case, the thickness to which the residual compressive stress is applied is thin and the thickness to which the residual compressive stress is applied to the surface layer is thin (distorted and thin) compared to the depth of the typical back surface deflection that governs the strength of ceramics, so the strengthening of the ceramic is In addition, there is also a method of producing a three-layer ceramic structure by press molding or casting, in which the inner layer is made of stabilized zirconia and matrix ceramic, and the outer layer is a composite sintered body of unstabilized zirconia and matrix ceramic. Ceramics made using this method can control the thickness of the surface compressed layer, making the compressed layer thicker than 1y on the surface of the ceramic, which is effective. This is because the surface layer is molded first and then the inner layer is molded, so the interface tends to contain defects, and furthermore, the surface layer has compressive stress and the inner layer has tensile stress. This is because stress discontinuity occurs at the working interface.Another drawback is that it is difficult to manufacture.

When the present inventors attempted to impregnate a ceramic molded body or a pre-sintered body (porous) with a zirconium aqueous solution, dry it, and sinter it, the ratio of tetragonal zirconia to monoclinic zirconia 4 or more, and the particle size is 0.1 to 1 μm, and zirconia is dispersed in the matrix with a concentration gradient such that the concentration at the surface of the sintered body is high and the concentration decreases toward the inside. It was found that the strength of the ceramics was significantly improved compared to ceramics composed only of the matrix phase.

After that, as a result of further intensive studies to improve fracture toughness, we found that 3 to 20 WT of zirconia with a particle size of 3 μm or less was added to the raw material of the compact or pre-sintered body to be impregnated with zirconium welding α! By including zirconia, the surface becomes high and the concentration gradient decreases toward the inside, and the monoclinic/tetragonal ratio also becomes ceramic with a structure that is high at the surface and continuously decreases toward the inside. is obtained,
The present inventors have discovered that the strength and fracture toughness are significantly improved compared to ceramics composed only of a matrix, and have completed the present invention.

The present invention will be explained in detail below.

3~20u of zirconia particles with an average particle size of 3μ or less
T! A mixture or compound of one or more of alumina, mullite, zircon, silicon nitride, silicon carbide, and sialon containing an average particle size of 3 μm or less, or a ceramic powder raw material produced by firing them. A molded body having a desired shape is formed by this process.

The reason why the average particle size is limited to 3 μm or less is that if it is larger than that, the average particle size of zirconia after sintering will increase and the volume will expand when it transforms into a monocrystalline system, which can cause uncontrollable cracks in the matrix ceramic. This is because the strength decreases, and when the particle size of the matrix ceramic exceeds 3 μm, the sintering temperature increases significantly by 1, making it impractical.

As a method for molding ceramics, commonly used methods for molding ceramics, such as a press method, a slip casting method, and an injection molding method, are used. CIP treatment is more suitable because coarse pores are removed.

The molded body is then preliminarily sintered if necessary. Preliminary sintering is performed for the purpose of imparting strength to facilitate machining and handling, but it is necessary that the bulk density is 70γ or less, which is the theoretical density.

The reason is that if the bulk density is 70% or more, the zirconia concentration in the sintered body will be low and the toughening effect will be reduced.

Thereafter, the compact or pre-sintered body is impregnated by immersing it in a solution containing zirconium such as zirconium oxychloride, zirconium nitrate, or zirconium acetate in the air or in a vacuum tank. The concentration of the solution for impregnation should be 1 to 4 mol/l, because if it is too thick, the viscosity will be high and it will be difficult to impregnate, but if it is too thin, the zirconia concentration in the sintered body will be low and the toughening effect will be reduced. is preferred. Further, in the impregnation treatment, it is also preferable to repeat impregnation and drying, whereby a highly concentrated zirconia-impregnated body can be obtained.

Thus, by drying and sintering after impregnation,
Zirconia has a concentration gradient that is high at the surface and decreases toward the inside, and a ceramic with a monoclinic/tetragonal ratio that is high at the surface and continuously decreases toward the inside can be obtained. This ceramic has significantly improved strength and fracture toughness compared to ceramics composed only of a matrix, making it tougher.

Here, the amount of zirconia blended into the raw ceramic powder before molding is 3 to 20 wtχ, preferably 7 to 15 w.
It is preferred that tχ. This means that if there is too much, monocrystalline systems will become the main component both on the surface and inside, regardless of whether or not impregnation is applied.
Strength becomes weaker. On the other hand, if it is too small, the surface layer and the interior will be mainly tetragonal, regardless of the presence or absence of impregnation treatment, and the toughness will improve.

In addition to high toughness due to the microcrank, the ceramic according to the present invention has a large amount of zirconia in the surface layer transformed into a monoclinic crystal system and expands as a body, whereas as it becomes inner, monoclinic zirconia becomes larger. The amount of zirconia decreases continuously, and the interior remains mostly tetragonal zirconia with no change in volume, so it is thought that strong compressive stress acts on the surface layer, achieving both strength improvement and toughness improvement at the same time. It will be done.

Conventionally, during the production of zirconia dispersion-strengthened ceramics, it has been difficult to prevent zirconia from agglomerating and uniformly disperse the zirconia, but according to the present invention, uniform dispersion is easily possible because the initial zirconia concentration is low.

In addition, the strength can be increased by 40° C. or higher and the toughness can be increased by 100X or more just by impregnation treatment, which is extremely effective for industry.

The present invention will be explained below with reference to Examples.

“Example 1” Sumitomo Aluminum Co., Ltd.! ! Alumina AESIICY
(average particle size 0.5 μm) 10 parts by weight of unstabilized zirconia UEP (average particle size 0.2 μm) manufactured by Daiichi Kigenso Co., Ltd. was mixed with 90°C weight part, D305 was used as a dispersant,
Added wtL stearic acid emulsion and 2.5νtX wax emulsion and heated in a ball mill for 24 hours.
Wet mixed.

After drying, it is crushed and subjected to -axial pressure molding method (molding pressure cuff 00Jf
/cm2) to produce a molded body of 4x4x36mm3, and pre-sintered at 1100°C. 80℃ pure water 100g
After dissolving 10 Jr. of zirconium oxychloride in the solution, the sintered body was placed in the zirconium salt solution and impregnated for 30 minutes. After that, it was immediately immersed in a 1/1 ammonia aqueous solution for 4 hours, and then immersed in a 1/1 ammonia aqueous solution for 2 hours at 80℃ and 2 hours at 120℃.
Dry. Raise the temperature at a rate of 10°C/min to 1650°
A toughened ceramic (sample 1) was obtained by bonding the mixture.

In addition, as a control, the firing temperature was 160℃ without impregnation treatment.
Sample 02 was prepared by performing the same treatment as in the previous step except that the temperature was 0°C. The obtained sample had a span distance of 20 at room temperature.
A three-point bending test was conducted under the conditions of 0.5mm/mir+ and a crosshead descending speed of 0.5mm/mir+. The results are shown below.
(Unit: Jf/mm2) Fracture toughness (!α) was determined by the IF method. The load at this time was 20 kgf.

The results are shown below. (Unit: MPa, Fm) "Example 2" Alumina AESIICY (manufactured by Sumitomo Aluminum Co., Ltd.)
.

Mullite was synthesized using Fukushima silica (with an average particle diameter of 0.571 m) and pulverized in a ball mill for 48 hours to prepare a raw material with an average particle diameter of 2.571 m. 10 parts by weight of Daiichi Kigenso Co., Ltd.'IEJ unstabilized zirconia EP (average particle size 1.0 μm) was mixed with 90 parts by weight of the raw material, and the dispersant was 0.
305, 2 wtL of stearic acid emulsion and 2.5 νtX of wax emulsion were added and wet-mixed for 24 hours using a ball mill.

After drying, crush it and use the -axial pressure molding method (molding pressure cuff 00 kg)
f/cm2) by 4 x 4 x 36 mm3
A molded body was prepared and pre-sintered at 1100°C. 80
1OJ of zirconium oxychloride in 100g of pure water at ℃
After dissolving r, the preliminary sintered body was placed in the zirconium salt solution and impregnated for 30 minutes. Thereafter, it was immediately immersed in l/] ammonia aqueous solution for 4 hours, then at 80°C for 2 hours, 1 hour.
It was dried at 20°C for 2 hours. The temperature was raised at a rate of 10oC/min and sintered at 1650°C for 1hr to obtain a toughened ceramic (sample 1).

In addition, as a counter a, the firing temperature is 11 degrees without impregnation treatment.
Sample 2 was prepared in the same manner as in the previous step except that the temperature was 600'C. 1. The sample asked is heated by $ and span distance j
t[20+nm, to cross? descending speed 0.5mm/
A three-point bending test was conducted under the conditions of m (n). The results are shown below. (Unit: kgf1mrr12) The fracture toughness value is
Obtained using the IF method. The load at this time was 20.3f.

Claims (1)

[Claims] 3 to 20 wt% zirconia with an average particle diameter of 3 μm or less
A method for strengthening ceramics, which comprises impregnating a molded body or pre-sintered body composed of a ceramic raw material containing a zirconium salt aqueous solution or a zirconium-containing solution, and sintering it after drying.