JPS6168372A - Y2o3 containing partially stabilized zirconia - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to a novel Y2O3 partially stabilized zirconia with excellent hardness and toughness.

BACKGROUND OF THE INVENTION Ceramics are inherently brittle materials because of the bonding patterns between their constituent atoms. In other words, compared to metal materials, it is difficult for dislocations to multiply and move within the material, and generally ductility does not appear. The brittleness can be improved to some extent if there is another defect that disperses the stress at the surface.

Therefore, in recent years, one method that has attracted attention as a way to improve the brittleness of ceramics is to make them tougher by utilizing transformation (hereinafter referred to as transformation-toughening).
, T). This improves toughness by utilizing the fact that the transformation of Zr (h) from the tetragonal phase (hereinafter abbreviated as one phase) to the monoclinic phase (hereinafter abbreviated as m-phase) is a martensitic transformation. The principles and methods of T and T are briefly described below.

Other oxides suitable for ZrO2 such as Y2O3, CaO
.

A solid solution containing MgO, etc. is converted into a high-temperature cubic phase (hereinafter referred to as C-
(abbreviated as "phase"), then lower the temperature a little and cook at C-
Annealing is performed in a two-phase region of phase and t-phase. Then, a t-phase rich in Zr2O3 is precipitated in the C-phase.

This precipitated phase has a specific Ms point (temperature at which martensitic transformation begins) that depends on its composition, size, and consistency with the parent phase, but usually the annealing temperature is adjusted so that the MS point is below room temperature. Set a time. The zirconia solid solution having a seven-phase precipitate that is stable even at room temperature and thus produced is called partially stabilized zirconia (hereinafter abbreviated as PSZ).

Now, one phase precipitated in this manner transforms into a martensitic phase when stress is applied from the surroundings.

In other words, the stress applied to the material is relieved by the transformation. This also holds true for cracks that have propagated through the ceramic material, and the stress at the tip of the crack is felt and the t-phase precipitate undergoes martensitic transformation to the m-phase (stress-induced martensitic transformation). The stress at the tip of the crack is then relaxed. In other words, the crack temporarily stops there. Moreover, the m-phase exerts stress on the crank in a direction that limits the growth of the crank due to its volume change effect. Therefore, unless a stress greater than the stress acting upon transformation from L-phase to M-phase is applied, cracks will not develop any further.

The first paper on PSZ T and T using such phenomena was published in Nature, 258, 703 (1975
) was announced by Garvie et al., but thereafter T.
,T, has been published in many academic papers, patents, etc.

On the other hand, in 1978, C1aussen et al. mixed and sintered Zr02 (with a small amount of Y2'03 added) and M2O3, which is not normally a solid solution, and mixed ZrO2 particles into M2O3.
We succeeded in improving the toughness of the material by dispersing ZrO2 into the matrix of ZrO2 and utilizing the transformation of ZrO2 from t-phase to m-phase. There, the Zroz particles dispersed within the grains and at the grain boundaries undergo martensitic transformation from the t-phase to the m-phase during cooling after sintering, but at that time, a large number of microscopic or racks are formed in the matrix around the ZrO2 particles. occurs. When a large rack propagates, this small rack disperses the stress at its tip, improving the toughness of the material. This method of increasing toughness by indirectly utilizing transformation is applied not only to materials whose matrix is only AQ2O3 but also to various ceramic materials.

Normally, the toughness of ceramics is expressed by the fracture toughness value Krc, and for example, while the toughness value of Si3N4 obtained by reaction sintering is about 2 MN/m'', the toughness value of the above PSZ reaches as much as 15 MN/m''. However, the current situation is that it still falls short of AQ gold alloy 35>IN/m″7.

As mentioned above, the methods of improving toughness using stress-induced martensitic transformation and microcracks accompanying the transformation involve heat-treating precipitated or dispersed particles containing ZrO2 as a main component in the matrix. It is characterized by being generated by

Problems to be Solved by the Invention On the other hand, the present invention improves the hardness and toughness of zirconia containing Y2O3.

Conventionally, a 7-phase can be made to appear in a matrix having a C-phase by precipitation heat treatment, or a t-phase Zr0t can be created by sintering.
The rhombohedral phase (hereinafter r
It differs from conventional PSZ in that martensite (abbreviated as ``-phase'') and/or jin-phase martinsite appears.

It should be noted here that the transformation from a C-phase to a single phase is also a martensitic transformation according to the method of the present invention, and is different from the conventional precipitation by diffusion of atoms. The point is that they are different. Therefore, the seven-phase produced by martensitic transformation, which is the object of the present invention, is referred to as t-phase martinsite.

This is also the first report to discover an r-phase in the bulk of zirconia ceramics, and furthermore, it has been revealed that this is due to martensitic transformation, which had not been combined prior to this application, and this t-phase martinsite and The appearance of r-phase martinsite is the P of the present invention.
This is the cause of the improvement in hardness and toughness in SZ.

The present invention differs from the conventional heat treatment method in PSZ, that is, by cooling from the C-phase at a reasonable cooling rate without intermediate annealing, the t-phase martinsite and/or
Alternatively, it is Y2O3 partially stabilized zirconia which has the appearance of r-phase martinsite and has hardness and toughness equivalent to or higher than conventional PSZ materials.

A child actor for solving the problems The present invention uses a method described in detail below to make a t-phase and/or an r-phase appear in a ZrOz -Y20y solid solution having a C-phase by martensitic transformation, and A PSZ material with high hardness and toughness is obtained.

The chemical composition of the PSZ of the present invention is 1-10 mol% of Y2O]
, the remainder is essentially Z r 02, and other impurities include 5102 and Fl! 203, TiO2, etc.

When Y203 is less than 1 mol%, the material made by the above method has only the m-phase at room temperature, and this m-phase
The -phase is formed by transforming the C-phase to the one-phase to the m-phase during cooling.

When the content of Y2O3 exceeds 1 mol %, the -phase martinsite begins to mix, and the m-phase martinsite decreases relatively. At 2 mol% Y20x, the structure at room temperature is almost seven-
Although the phase is bruchinsite, some r-phase is present. At 2.5 mol%, the hepta-phase and r-phase coexist as martensite. When the mole % of Y2O3 exceeds this composition, an untransformed parent phase (c-phase) remains in addition to the martensitic phase. As the mole % of Y2O3 increases, the relative size of the parent phase to the martensitic phase generally increases. However, its value depends on the size of the crystal grains; in the case of 2.5% Y2O3, about 80% of grains with an average diameter of 1 mm are martensite, but about 50% of grains with a diameter of several pm are martensite. It becomes a site. Y2O
When 3% is 3%, martensite is mostly r-phase,
Sometimes a small amount of one phase exists. At this time as well, the matrix C-
The amounts of phases are present in approximately the same proportions as stated above. 3
．． Even at 5 mol %, the situation where the r-phase and one phase coexist is the same, but the amount of the parent phase is relatively increased. At 4 to 10 mol%, most of the t-phase and only rutensanito are observed as martensitic phase, but the relative amount is Y2O3 mol%.
At 10 mol %, martensite is only rarely observed.

If Y203 exceeds 10 mol%, martensite is not produced and the structure at room temperature is similar to the C-phase produced at high temperatures. Therefore, in the present invention, t-phase martinsite and r-phase martinsite are stable at room temperature. Existing Yz Oi 1”
It was limited to a range of 0 mol%.

To produce the PSZ of the present invention, powdered ZrO2 and Y2
It can be produced by mixing O1 and then melting it at a high temperature to form a C-phase, followed by cooling.

The important point is i! The purpose is to obtain a C-phase with uniform X formation as a high-temperature summation, and the method is not limited, but argon arc melting, plasma heating, or a method of sintering a solid solution powder of Yz (h-ZrO2 in the C-phase region) You can achieve your goal by using .

Within the scope of the present invention, when ZrO2 and Y2O3 are mixed to a desired composition and melted, for example at a high temperature, Y201 and Zr0z form a solution with a homogeneous composition above about 2750°C. When this is cooled, zirconia with this composition can be obtained with almost no fluctuation in composition. When the sample after cooling was analyzed by EPMA, the composition was uniform, and there was no sign that Y203 was precipitated or segregated at grain boundaries.

A C-phase solid solution with a uniform composition melted at a high temperature is produced relatively quickly.
Specifically, when cooled at a cooling rate of approximately ℃/sec or more, C-
Precipitation of t-phase does not occur in the phase, but martensitic transformation occurs at a relatively low temperature from C-phase to one phase and from C-phase to r-phase. As a result, when the surface of the material is observed with an optical microscope, surface undulations characteristic of martensite are observed, and when observed with a polarizing microscope, martensite plates and twin crystals as internal defects are observed inside the martensite.

Furthermore, it is possible to identify the shape of martensite, internal defects, martensite phase, etc. using an electron microscope, an X-ray diffraction method, or the like.

Next, the hardness and toughness of PSZ having r-phase martinsite and/or rhino-phase martinsite of the present invention will be explained.

For PSZ in which ZrO2 contains 1 to 25 mol% of Y2O3 and is cooled from the high-temperature C-phase to produce 1-phase martinsite and r-phase martinsite through martensitic transformation, the room temperature hardness was measured using a micro Vickers hardness tester. The results of the measurements are shown in Figure 1.

The hardness increases with the appearance of -phase martinsite, and most becomes r-phase martinsite at 3 mol% Y2O.
The hardness shows a peak value at 3.

After that, the hardness decreases as the amount of Y2O3 increases, but 2
Even if it becomes 5 mol% Y2O3, m when Y203 is not added
-The value is larger than the hardness of the phase. This is considered to be due to solid solution hardening.

An even more significant relationship is observed with respect to toughness. Second
The figure shows the toughness value measured by the indentation method and the phases existing at room temperature relative to the mole% of Y201.As is clear from the figure, the toughness value is proportional to the amount of the r-phase. The toughness value increases at about 3.096 Y203 and takes a maximum m-c value of 20 MN/m'', that is, in the region where the r-phase exists, its toughness value is significantly higher than that of the surrounding composition. In other words, it can be said that the toughness is higher when the r-phase is present than when only the r-phase or m-phase is present.

Example Examples of the present invention are shown below.

5.0 g of ZrO2 powder (average particle size 1 m) and mol %
An appropriate amount of Y2O3 powder (average particle size IBm) was sufficiently kneaded using a ball mill and one-rotation mixer so that the amount of Y2O3 powder was 1 to 10%. The kneaded powder is subjected to primary molding under a pressure of 10 kg/crn' and pre-fired at 1400° C. for 30 minutes. The pre-fired body is melted in an argon arc and allowed to cool on a water-cooled copper mold. In the case of water-cooled copper molds, the cooling rate is quite fast as shown in Table 1.

The cooling rate was not measured from the melting temperature to 3000K, but below 3000K, the cooling rate becomes slower as the temperature decreases. When cooled at a cooling rate that can prevent long-range diffusion of atoms, the uniform composition at the time of melting is carried over to a low temperature, and when it reaches the Xs point, it undergoes martensitic transformation, and the -phase martinsite and r-phase martinsite. The composition after cooling was examined by EPMA, and it was confirmed that the concentration variation depending on location was 0.5 mol% or less.

The formation of martensite during cooling was confirmed by the surface undulations after cooling and observation using a polarizing microscope after polishing the sample.

Table 1. Cooling rate of PSZ that was left to cool on a water-cooled copper mold after argon arc melting. Figure 3 is a photograph of the result of observing the sample thus obtained with a transmission electron microscope. The phase was identified from the electron beam diffraction pattern.

In the figure, (a) is a material consisting only of ZrO2 and is in m-phase (monoclinic). In the figure, adjacent parts in a layered manner have a twin crystal relationship.

(b)-(1) is a material containing 2.5 mol % Y2O3, and plate-shaped martensitic ht-phase and r-phase appear in the cubic crystal.

(b)-(2) shows 3 mol% Y2O3 ter-phase. The martensite in materials with this composition is mostly r-phase at room temperature, but sometimes hepta-phase martensite (b
)-(1).

(c) and (d) are 5 and 6 mol% Y203 materials, respectively; in these cases, tetragonal crystals appear in cubic crystals, and martensite is a heptaphase.

(e) shows 25 mol% Y2O3, but the figure shows C-
It is phase. When Y2O3% becomes 10% or more, martensite is no longer observed.

Regarding the single-phase martinsite and r-phase martinsite in the PSZ of the present invention, the fact that they are generated by martensitic transformation is due to the presence of surface undulations as described above.
This is obvious from the shape, the presence of internal defects, the presence of habit planes, and the absence of compositional changes. X-ray diffraction is also used to identify the surface phase.

The hardness of the sample prepared by the above method was measured using a micro Vickers. The results are shown in Table 2. This is the first
As shown in the figure, 3 horses with the largest amount of r-phase O1%Y2
Maximum hardness 14.0 for O3. Take OGPa. That is, r-
It is clear that the formation of phase martinsite causes an increase in hardness. -Table 3 shows the relationship between the toughness value determined by the indentation method and Y2O3 mol%. Again, it is clear that the presence of the r-phase increases the toughness value.

Effects of the Invention As described above, the PSZ of the present invention is useful, for example, as a ceramic material, and the t-phase and r-phase generated by martensitic transformation provide high toughness and high hardness, and are superior to conventional precipitation type or Dispersed PSZ-ZrO2
Compared to ceramics, it does not require heat treatment and provides higher hardness and toughness. This makes it easier to design ceramic materials and expands their use as various industrial materials.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the relationship between hardness and Y2Ox concentration for PSZ having a uniform composition produced by a dissolution method. FIG. 2 is a diagram showing the relationship between Y2O3 degree of elasticity, toughness value, and phases existing at room temperature. FIG. 3 is a transmission electron micrograph (magnification: X8000 times) showing changes in the structure depending on the amount of Y2O3.

Claims (2)

[Claims] (1) ZrO_2 containing 31 to 10 mol% of Y_2O_2 generates rhombohedral crystals (r-phase) and/or tetragonal crystals (t-phase) by cooling from a uniform high-temperature cubic phase (c-phase). Partially stabilized zirconia containing Y_2O_3. (2) Cool the homogeneous high temperature cubic phase (c-phase) with ZrO_2 containing 1 to 10 mol% of Y_2O_3, and transform it into rhombohedral (r-phase) and/or tetragonal (t-phase) by martensitic transformation. ) is produced from partially stabilized zirconia containing Y_2O_3 according to claim (1).