JPH01298058A - Production of zirconia ceramic - Google Patents

Abstract

PURPOSE:To increase mechanical strength, hardness and toughness by molding raw material powder so as to contain CeO2 and MgO in a fixed amt. by sintering and thereafter by heat-treating. CONSTITUTION:A hydrated material is obtd. by coprecipitation method by mixing water-soluble salts of Ce and Mg, and basic salt of Zr, and thereafter dried to obtain a powder. The powder is mixed so as to obtain a desired molar ratio, calcined to powder having >=1m<2>/g BET specific surface area and thereafter, furthermore crushed to obtain zirconia (A) powder having <=1mum mean particle size. Then, the (A) powder is molded and sintered at 1300-1650 deg.C to obtain CeO2-MgO-TSZ(tetragonal stabilized zirconia) sintered compact (B) having >=90% relative density to the theoretical density. Then, the sintered compact (B) is heat-treated at 800-1200 deg.C for 5-20hr to produce zirconia ceramic consisting of 4-12mol% CeO2, 2-7mol% MgO and the remainder ZrO2.

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Field of Application> The present invention relates to a method for producing zirconia ceramics having excellent mechanical strength, hardness and toughness.

More specifically, cerium oxide (hereinafter CeO
It may be written as □. ) and magnesium oxide (hereinafter referred to as Mg
It may be written as o. The present invention relates to a method for producing zirconia ceramics having excellent mechanical strength, hardness, and toughness and particularly suitable for use as mechanical structural materials.

<Prior Art> It has been well known that Ce01 and MgO are used as stabilizers for zirconium oxide (hereinafter sometimes referred to as Z r O2).

For example, if 20 mol% or more of CeO is contained relative to ZrO□, zirconia ceramics consisting of tetragonal and cubic crystals can be obtained, and if 16 to 20 mol% of CeO□ is contained, it is composed of tetragonal crystals. Completely stabilized zirconia ceramics can be obtained, and if the amount is less than 16 mol%, unstabilized zirconia ceramics consisting only of monoclinic crystals can be obtained at room temperature.
al of Materials 5cienc
e) 17 (1982), p. 265, Fig. 1] In either case, although the toughness is high, the mechanical strength and hardness are insufficient.

Zirconia ceramics containing MgO in ZrO□ are also known, but in this case, the temperature
The tetragonal product precipitated at °C decomposes into monoclinic crystals and MgO during cooling.

Therefore, for magnesia partially stabilized zirconium, a method is adopted in which the zirconium is fired in a cubic solid solution region, rapidly cooled at an appropriate cooling rate, and tetragonal crystals are precipitated in a cubic matrix. [For example, Journal of American Ceramic Society
ricanCeramic 5ociety) No. 50
Vol. 6, No. 6 (1967), pp. 288-290] However, in this case, the resulting ceramic has a large cubic grain size of about 10 to 100 μm, and a zirconia ceramic with excellent mechanical strength cannot be obtained.

In addition, the strength of zirconia ceramics using CeO and MgO in combination as stabilizers <M, V, Swain [Aust Ceram 86. August 1986, Procedaig ltU
sTcERAM 86. ＾ug, 1986. PROCE
EI) INGS p371-378) etc., and according to this, the crystal grain size of a sintered body of 1 μm or less has a strength of Max 700 Mfla, and a strength of 8 MNio+
A sponge-like sintered body with a sintered density of 95% or less and a toughness value of "-" is described.

This method uses stabilizers CeOz and MgO as Y, O
Although it has the advantage of being cheaper than ceramics, the ceramics obtained from the disclosed items have excellent strength, toughness, hardness, etc. required as mechanical structural materials, and have well-balanced properties. The present inventors conducted intensive studies to obtain zirconia ceramics with excellent mechanical strength, hardness, and toughness using Cent and MgO, which are cheaper than Y2O. It was discovered that a sintered body that satisfies the above physical properties can be obtained if the compacted body after sintering is heat-treated under specific conditions.That is, the method of the present invention involves heat-treating the raw material powder after shaping and sintering. Features Cerium oxide 4-12 mol%
The present invention provides a method for producing zirconia ceramics containing 2 to 7 mol% of magnesium oxide.

The method of the present invention will be explained in more detail below.

The zirconia ceramics used in the method of the present invention are Cent
about 4 to about 12 mol%, preferably about 6 to about 10 mol%,
About 2 to about 7 mol% MgO, preferably about 3 to about 6 mol%
The remainder is mainly composed of ZrO□.

If Ce01 is outside the above range, the stabilizing effect of the tetragonal crystal will be small and a ceramic with sufficient mechanical strength will not be obtained, and even if MgO is outside the above range, zirconia ceramics with desired physical properties will not be obtained. do not have.

In addition, in the zirconia ceramic according to the method of the present invention, about 50% by weight or more of the crystal grains constituting the sintered body are tetragonal, 1
0% by weight or less is monoclinic, and the sum of tetragonal and cubic is 9
0% by weight or more and its average crystal grain is about 1 to about 5 μm
It consists of

If the average crystal grains of the ceramic are outside the above range, even if the molded body is heat-treated after sintering, the effect of improving toughness will be small.

The method for producing zirconia ceramics of the present invention includes:
Ce Ox M g OT S Z (square stabilized zirconia) The relative density to the theoretical density of the sintered body is 90
% or more (i.e., the porosity is 10% or less), preferably about 95
It is essential to adjust the raw material particle size, particle size distribution, and further sintering conditions so that the sintered body exceeds %, and after sintering, heat-treat the obtained sintered body.

The above specific manufacturing conditions cannot be determined unambiguously depending on the method of obtaining raw materials, calcination conditions, pulverization conditions, crushing conditions, and mixing conditions of CeQ and MgO, etc., but it facilitates sintering at low temperatures. In order to achieve this, the raw zirconia powder may have an average particle size of about 1 μm or less, preferably about 0.5 μm or less.

More specifically, a hydrate obtained from water-soluble salts of cerium and magnesium and a basic salt of zirconium by a coprecipitation method, or a powder obtained by drying the hydrate, is mixed with a desired molar ratio. The components are mixed uniformly and then calcined to give a BET specific surface area of about 1 m2/g or more, preferably about 10 m2/g.
m"/g to about 50 m"/g, and further pulverized with a vibration mill or the like to obtain zirconia powder with an average particle diameter of about 1 μm or less.

Next, this powder is molded by known molding methods such as mold molding, rubber press, extrusion molding, and injection molding, or by using some of these molding methods in combination, and then heated in a heating furnace for 130 min.
Raise the temperature to 0 to 1650°C and then heat it for several hours, e.g.
It can be obtained by sintering for 5 hours and heat-treating under specific conditions.

The heat treatment is performed by sintering the above molded body at a temperature of 1,300 to 1,650°C, and then at a temperature of 1,250°C or less, preferably 800 to 1,000°C.
What is necessary is just to hold|maintain in a furnace at 200 degreeC for 1 hour or more, Preferably 5 to 20 hours.

The atmosphere during sintering and heat treatment is not particularly limited, but
Since degassing occurs from the ceramic in a significantly reducing atmosphere, firing can be performed in air, in a non-oxidizing atmosphere of nitrogen or argon, or in a non-oxidizing atmosphere after firing in air, depending on the purpose.

The zirconia ceramic thus obtained in the method of the present invention usually has a bending strength of 70 kg/mm or more,
Fracture toughness value 10MNm-” or more, Vickers hardness 100
It has extremely excellent mechanical properties with physical properties of 0 Kg/mm2 or more.

Furthermore, the zirconia ceramics obtained by the method of the present invention, which normally contain 3 mol% yttrium oxide as a stabilizer and have a tetragonal crystal structure, spontaneously change from tetragonal to monoclinic in air at 100 to 300°C. Although it has a fatal defect that mechanical strength decreases due to transformation, it does not undergo any transformation from tetragonal to monoclinic even after being left at 100 to 300"C for 1000 hours, and has excellent thermal stability. .

Although it is not clear why the zirconia ceramics of the present invention becomes highly tough due to heat treatment of the sintered body,
According to the X-ray diffraction method, the zirconia ceramic NOX obtained by the method of the present invention is often observed to have a small amount of monoclinic and/or cubic crystals reduced by heat treatment compared to the ceramic before heat treatment. This is presumed to be because the transition from tetragonal to monoclinic occurs more easily, and the volume expansion at that time absorbs fracture energy, increasing the ability to prevent crack propagation.

<Effects of the Invention> As detailed above, the method of the present invention uses Y2O as a stabilizer.
By using CeO□ and MgO, which are cheaper than , in a specific proportion, and heat-treating the resulting sintered body under specific conditions, compared with commercially available zirconia ceramics,
We provide zirconia ceramics that are excellent not only in mechanical strength but also in hardness and toughness, and compared to ceramic nox obtained using a stabilizer in the same composition range, the heat-treated one is superior to other ceramics. It can provide ceramics with significantly improved toughness without deteriorating physical properties, and can be used for cutting/cutting tools, extrusion/drawing dies, engine parts,
Machine structural materials such as bearing balls, ballpoint pen poles, mechanical seals, shafts, nozzles, pistons, etc.
It is extremely useful as a solid electrolyte material for oxygen sensors, etc., and its industrial value is enormous.

Although only zirconia powder and a stabilizer have been described as the raw materials used in the example of the method for producing zirconia ceramics of the present invention, these are just the main ingredients, and other materials may be used as long as they do not impair the effects of the present invention. silica, alumina,
It is of course possible to use sintering accelerators and grain growth inhibitors known in the art, such as titania kaolin and mullite.

<Examples> The present invention will be explained in more detail with reference to Examples below, but these Examples are not intended to limit the present invention.

Example I Z r OCj! x ・8 The precipitated hydrate obtained by precipitating H, O with aqueous ammonia was passed through d & cerium nitrate and magnesium chloride, and the composition after firing was as shown in Table 1.
0z -MgO-ZrOz was added to the precipitated hydrate of zirconia in the form of an aqueous solution, wet-mixed uniformly in a ball mill, filtered, dried, and crushed, and the resulting powder was
The mixture was calcined for 2 hours at °C and further pulverized in a vibration mill to obtain ceria- and magnesia-containing zirconia powder with a BET surface area of 20 m2/g and an average particle size of 0.1 μm.

This powder was heated at 35X using a rubber press (IT/cm”).
It was molded into a size of 5 x 5 mm, sintered in an electric furnace in air at a temperature of 1650'C for 2 hours, and then heat treated at a temperature of 1200C for 10 hours.

The physical properties of the obtained zirconia ceramics are also shown in Table 1.

In the Examples, the sintered density was determined by the Archimedes method, the crystal structure was determined by the X-ray diffraction method, and the fracture toughness and hardness were determined by the V, I (Vickers I) proposed by Evans et al.
dentation) method (measurement conditions: load 30kg x 1
5sec, Young's modulus E=1.86x 1015MN/m
” ) Bending strength was measured using the 3-point bending test method (span length 30 mm,
It was measured using a load application acceleration of 0.5 mm/min).

Further, the particle size of the sintered body was measured by a section method using scanning electron micrographs.

Claims (1)

[Claims] (1) A method for producing zirconia ceramics containing 4 to 12 mol% of cerium oxide and 2 to 7 mol% of magnesium oxide, which comprises molding and sintering raw material powder and then heat-treating it.