JPH11157962A - Highly abrasion-resistant alumina-based sintered compact and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain an alumina-based sintered compact with high abrasion resistance owing to the high hardness and high toughness of its surface, and to provide a method for producing the above sintered compact. SOLUTION: This highly abrasion-resistant alumina-based sintered compact bearing a surface-hardened layer dispersed with metal oxide crystal grains other than alumina each <=0.2 μm in average size in the grains and grain boundaries of the main crystal phase in the region ranging from the surface to a depth of >=0.01 mm of this sintered compact with alumina as the main crystal phase containing >=20 vol.% of plate alumina crystal >=4 in aspect ratio, is obtained by heat treatment of a molded form made from a mixture prepared by incorporating alumina powder with metal (e.g. Ti, Mg, Fe) compound(s) capable of forming solid solution in alumina crystal in such an atmosphere that the metal (s) is capable of forming solid solution in the alumina crystal, followed by treatment in such an atmosphere that the metal (s) can be deposited as the corresponding oxide (s).

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an alumina-based sintering material having a surface-hardened layer having excellent wear resistance on a surface suitably used as a structural material used particularly for wear-resistant parts and engine parts and a cutting tool. The present invention relates to a binder and a method for producing the same.

[0002]

2. Description of the Related Art Conventionally, alumina-based sintered bodies have excellent properties such as high hardness, high temperature resistance, and environmental resistance.
It has attracted attention as a wear-resistant material represented by a pump liner. However, the surface hardness is not sufficient for high wear resistance requirements when used under more severe conditions. In order to enhance the wear resistance of the sintered body, it is necessary to increase not only the hardness but also the toughness.

[0003] In order to improve the hardness of the alumina-based sintered body, it has been conventionally known to refine the alumina main crystal or to add SiC or ZrO ₂ to form a composite.
122164 and JP-A-63-139044.

In order to improve toughness, SiC whiskers are added to alumina or La-containing β-
Dispersing Al ₂ O ₃ is also disclosed in JP-A-63-30378.
And JP-A-63-134551.
It has also been proposed to increase the toughness by growing a plate of alumina main crystal.

[0005]

However, the above-described technique for improving the hardness of the alumina-based sintered body has little effect on toughness, and thus has little effect on improving wear resistance. Further, the dispersion of β-Al ₂ O ₃ and the formation of plate-like alumina have the effect of improving the toughness, but lower the hardness of the sintered body, and thus tend to lower the wear resistance. It is in. The addition of SiC whiskers can improve the hardness and toughness of the sintered body and improve the wear resistance.
There are problems in that the raw materials are expensive, the production cost is high, and when used in a high-temperature oxidizing atmosphere, the chemical stability is lacking.

Accordingly, an object of the present invention is to provide an alumina-based sintered body having excellent wear resistance due to the surface of the sintered body having high hardness and high toughness, and a method for producing the same.

[0007]

Means for Solving the Problems The present inventor has repeatedly studied methods for simultaneously improving hardness and toughness and improving wear resistance without impairing the stability of alumina at high temperatures. Along with forming a plate-like alumina crystal structure with a high aspect ratio, the surface of the sintered body contains alumina crystal grains,
Alternatively, it has been found that the above object can be achieved by forming a layer in which fine metal oxide crystals other than alumina are precipitated at the grain boundaries.

That is, the high wear-resistant alumina-based sintered body of the present invention comprises 20 plate-like alumina crystals having an aspect ratio of 4 or more.
Alumina having an average particle size of 0.2 μm or less is formed in a region from the surface of a sintered body containing alumina containing not less than 10% by volume or more to a depth of 0.01 mm or more within the grains of the main crystal phase and at grain boundaries. Characterized by comprising a surface hardened layer in which metal oxide crystal particles other than the above are dispersed, wherein the second oxide crystal particles contain at least one metal selected from Ti, Mg and Fe. desirable. In the central part of the sintered body, the metal oxide is dissolved in a main crystal phase.

The method for producing a highly wear-resistant alumina-based sintered body according to the present invention comprises the steps of: forming a compact comprising a mixture of alumina powder and a metal compound capable of forming a solid solution in alumina crystals; After a heat treatment in an atmosphere capable of forming a solid solution in the crystal, a treatment is performed in an atmosphere in which the metal can be precipitated as an oxide, and a metal oxide other than alumina having an average particle size of 0.2 μm or less is formed on the surface of the sintered body. The present invention is characterized in that a crystallized particle forms a surface hardened layer dispersed in the grains of the alumina crystal and at the grain boundaries.

[0010] Specifically, a molded body obtained by adding and mixing a Ti compound as the metal compound is heat-treated in a reducing atmosphere to dissolve Ti in the alumina crystal, and then heat-treated in an oxidizing atmosphere to contain Ti. The oxide was precipitated in the grains and at the grain boundaries of the alumina crystal, and a compact formed by adding and mixing a Ti compound and a Mg compound as the metal compound, or a composite compound thereof, was subjected to a heat treatment in an oxidizing atmosphere to form a mixture in the alumina crystal. Was dissolved in Ti and Mg, and then heat-treated in a reducing atmosphere to precipitate Mg-containing oxides in the grains and at the grain boundaries of the alumina crystal.
e, heat-treating the compact in which the compound is added and mixed in an oxidizing atmosphere to dissolve Fe in the alumina crystal, and then heat-treating in a reducing atmosphere to reduce the Fe-containing oxide to the inside of the alumina crystal and at the grain boundary. It is characterized by being deposited.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION The high wear-resistant alumina-based sintered body of the present invention has a sintered body containing 20% by volume or more of plate-like crystals having an aspect ratio of 4 or more. The dispersion of the plate-like alumina crystals is effective for improving the toughness of the sintered body. When the volume fraction of the plate crystal is small, the effect of improving the toughness is small.
A crystal having an aspect ratio of 4 or more is 20% by volume or more, particularly 3% by volume.
It is desirable that the content be 0% by volume or more.

The high wear-resistant alumina-based sintered body of the present invention has a mean particle size of a metal oxide other than alumina of 0.2 μm or less within a depth range of 0.01 mm or more from the surface. It is important to provide a surface hardened layer in which fine crystal grains are dispersed in the grains and the grain boundaries of the alumina main crystal.

By forming a surface hardened layer in which fine crystals of metal oxide are deposited on the surface of such a sintered body, the interaction of the alumina main crystals with the plate-like crystals causes the surface of the sintered body to have a high hardness. As a result, the wear resistance of the sintered body can be greatly improved.

If the particle size of the metal oxide dispersed in the surface hardened layer of the sintered body is large, the effect of increasing the hardness by the dispersion is significantly reduced. Is limited to 0.2 μm or less, and particularly preferably 0.1 μm or less.

Further, if the thickness of the surface hardened layer on which the metal oxide is deposited is small, the contribution to high hardness is small, so that the thickness is set to 0.01 mm or more.
Increasing the precipitation temperature or extending the treatment time to increase the thickness of the surface hardened layer causes grain growth of alumina main crystals and coarsening of some deposited metal oxide particles. There is a possibility that the hardness of the surface is reduced. In order to realize good wear resistance, the thickness of the surface hardened layer is 0.5 mm or less, particularly 0.02
Desirably, it is 0.4 mm. In the central part further inside the surface hardened layer where the metal oxide was deposited,
The metal oxide precipitated on the surface is formed as a solid solution in the alumina main crystal.

According to the present invention, the metal oxide other than alumina precipitated in the alumina main crystal grains and at the grain boundaries is at least one selected from the group consisting of Ti, Mg and Fe.
It is important that species elements are included. Specific precipitated oxide crystal phases include TiO ₂ , MgAl ₂ O ₄ , Fe
When at least one of Al ₂ O ₄ is dispersed in the alumina crystal grains, it is easy to have a crystalline consistency with a parent phase (alumina crystal phase), and a large strain is generated in the parent phase. The effect of suppressing dislocation movement and improving hardness is particularly large.

If the content of the metal element is small, the precipitation amount of the oxide crystal phase is reduced. As a result, the effect of improving the hardness is small. Oxide crystals are generated, and strength and hardness may be reduced. Therefore, these metal oxides, in particular, the amount of Ti in terms of TiO ₂ ,
It is desirable that the total of the reduced amount and the converted amount of Fe in terms of Fe ₂ O ₃ be 0.5 to 5 mol%, particularly 1 to 3 mol%.

In order to produce the highly wear-resistant alumina-based sintered body of the present invention, first, a Ti-containing compound, a Mg-containing compound or F-containing compound is added to alumina powder having an average particle size of 0.1 to 1 μm.
The e-containing compound is added in an amount of 0.5 to 5 mol% in terms of TiO ₂ , MgO, and Fe ₂ O ₃ , and mixed. The compound may be any of oxide powder, metal powder, organic salts, inorganic salts and a solution thereof.

The above mixture is formed into an arbitrary shape by a desired forming means, for example, a die press, a cold isostatic press, a cast molding, an injection molding, an extrusion molding and the like.

Next, the molded body is subjected to a known sintering method, for example, a hot pressing method, a normal pressure sintering method, a gas pressure sintering method, a microwave heating sintering method, and a hot isostatic pressure treatment after sintering. HIP) treatment and after glass sealing (HI
P) Sintering by various sintering methods such as treatment.

According to the present invention, when firing, first, T
After heat treatment in an atmosphere in which each metal element in a metal compound such as i, Mg, Fe, etc. can be dissolved in alumina crystals,
It is important to include a step of performing a heat treatment in an atmosphere in which each of the metal elements can be precipitated as an oxide from these solid solutions.

By simply adding the above-mentioned metal element or a compound containing these metal elements to alumina powder and firing, the metal oxide is dispersed only at the grain boundaries of the alumina crystal and grows greatly. . Therefore, according to the present invention, after a solid solution having a uniform composition is once formed by heat treatment under the condition that the solubility of the metal in the alumina crystal increases, heat treatment is performed under the condition that the solubility in the alumina crystal decreases. Thereby, the metal as an oxide,
It can be precipitated and dispersed as fine crystal particles within the grains of the alumina crystal and at the grain boundaries.

In the above-mentioned solid solution treatment step, ions having a high solid solubility have a large effect of suppressing the growth of the alumina crystal in the (001) direction. (Short diameter) can be generated. In order to further promote the plate-like crystal growth, it is possible to adjust the firing conditions.
It is desirable to add SiO ₂ or the like at a ratio of 1% by weight or less. In order to suppress the coarsening of the alumina crystal, for example, Y ₂ O ₃ , La ₂ O ₃ , Yb ₂ O ₃ , Lu ₂
It is also possible to add a rare earth element oxide such as O ₃ , zirconium oxide, or the like to such an extent that the formation of plate crystals is not hindered.

When a Ti compound is used as a metal compound, when the Ti is heat-treated in a reducing atmosphere, the ionic valence of Ti becomes 3+, the solubility in alumina crystals increases, and a solid solution is formed. By treating this solid solution in an oxidizing atmosphere, the ionic valence of Ti returns to 4+, and the solubility in alumina crystals decreases. As a result, Ti becomes
It can be deposited as TiO ₂ , Al ₂ TiO ₅ .

As the metal compound, Ti: Mg =
When mixed at a ratio of 1: 1, Ti and Mg can be simultaneously dissolved in alumina crystals at the same molar ratio when treated in an oxidizing atmosphere. Then, by treating this solid solution in a reducing atmosphere, the ionic valence of Ti becomes 3+, and the Ti alone is preferentially dissolved in alumina. Since Mg alone cannot be dissolved in alumina, it can be precipitated in the form of MgAl ₂ O ₄ .

Further, when an Fe compound is used as the metal oxide, when the metal oxide is treated in an oxidizing atmosphere, Fe has an ionic valence of 3+ and can form a solid solution in alumina crystals.
By treating the solid solution in a reducing atmosphere, the ionic valence of Fe becomes 2+, the solubility in the alumina crystal is reduced, and the solid solution can be precipitated in the form of FeAl ₂ O ₄ .

The reducing atmosphere is a hydrogen-containing atmosphere, an inert gas atmosphere, a high vacuum or the like, and an oxygen partial pressure of 10 ⁻⁶ a.
tm or less. The treatment in an oxidizing atmosphere may be performed in the air. In addition, if the temperature at the time of the solid solution or the precipitation treatment is low, the target structure cannot be formed, and if the temperature is high, the alumina crystal grains and the precipitated particles are coarsened. From such a viewpoint, solid solution,
The temperature at the time of precipitation is preferably in the range of 1100 to 1500 ° C.

The thickness of the surface hardened layer on which the metal oxide has been deposited can be arbitrarily adjusted depending on the temperature and the processing time during the deposition step. For the reason mentioned above for the thickness of the surface hardened layer,
In order to adjust it to the range of 0.01 to 0.5 mm, the processing time is preferably set to 1 to 20 hours.

[0029]

EXAMPLE Titanium oxide (TiO ₂ ) powder having an average particle diameter of 0.7 μm and magnesium hydroxide (Mg (O ₂ ) having an average particle diameter of 0.6 μm were added to alumina powder having an average particle diameter of 0.5 μm.
H) ₂ ) powder, Fe ₂ O ₃ powder having an average particle size of 0.7 μm,
Using SiO ₂ powder having an average particle diameter of 0.5 μm, the powders were weighed and mixed so as to have the composition shown in Table 1 to obtain a mixed powder. Then, the mixed powder is molded by a pressure of 1 t / cm ² ,
Further, a molded body was produced by applying a hydrostatic pressure treatment at a pressure of 3 t / cm ² , and then calcined at 1500 ° C. in various atmospheres shown in Table 1 for 2 hours to form a solid solution. Then, the deposition treatment was performed at the temperature, time and atmosphere shown in Table 1.

For each of the obtained sintered bodies, X-ray diffraction measurement of the surface of the sintered body was performed to identify the crystal phase, and the results are shown in Table 2. In addition, the cross section of the sintered body was mirror-finished, image analysis was performed on a scanning electron micrograph after etching, and the area ratio of alumina crystals having an aspect ratio of 4 or more among the observed alumina crystals was determined. The volume ratio was used. Further, the thickness of the surface layer where the precipitated phase exists and the particle size of the precipitated particles were determined from a transmission electron micrograph. Table 2 shows the results. Further, as mechanical properties, the Vickers hardness (load: 1 kg) of the mirror surface of the sintered body was measured, and the fracture toughness was calculated by an indentation method (IF method). In addition,
On-disk method (load 1kg, speed 5m / s, 5mi
n)).

[0031]

[Table 1]

[0032]

[Table 2]

From Tables 1 and 2, the sintered body obtained according to the present invention has a surface Vickers hardness of 18 GPa or more, a fracture toughness of 4.5 MPa · m ^1/2 or more, and a wear rate of 100 ×
Excellent abrasion resistance of 10 ⁻⁶ mm ³ / kg · m or less was exhibited.

On the other hand, a sample No. 1 of the conventional alumina material, a sample No. 2 having an aspect ratio of 4 or more and a small crystal ratio, a sample No. 3 having no surface hardened layer, and a hardened layer having a thickness of 0.01 mm Samples Nos. 4 and 9 below and Sample No. 8 in which the average particle size of the precipitated metal oxide crystal particles was 0.2 μm or more.
Have a wear rate of 200 × 10 ⁻⁶ mm ³ / kg ·
m or more, the wear resistance was low.

[0035]

As described above in detail, according to the present invention, fine oxide crystal particles other than alumina are formed on the surface of an alumina sintered body mainly composed of plate-like alumina crystals within the alumina crystal grains and at the grain boundaries. The alumina-based sintered body having high wear resistance can be provided by dispersing the alumina-based sintered body.

Claims (7)

[Claims] The present invention relates to a sintered body having a main crystal phase of alumina containing 20% by volume or more of plate-like alumina crystals having an aspect ratio of 4 or more, in a region from the surface to a depth of 0.01 mm or more.
The average grain size of 0.2 μm
A highly wear-resistant alumina-based sintered body comprising a surface hardened layer in which the following metal oxide crystal particles other than alumina are dispersed. 2. A highly wear-resistant alumina sintered body according to claim 1, wherein said metal oxide crystal particles contain at least one metal selected from Ti, Mg and Fe. 3. The highly wear-resistant alumina-based sintered body according to claim 2, wherein said metal oxide is dissolved in a main crystal phase in a central portion of the sintered body. 4. A molded body comprising a mixture of an alumina powder and a metal compound capable of forming a solid solution in alumina crystals is heat-treated in an atmosphere in which the metal forms a solid solution in alumina crystals. A surface in which metal oxide crystal particles other than alumina having an average particle size of 0.2 μm or less are dispersed in and within the alumina crystal grains by treating in an atmosphere in which the metal can be precipitated as an oxide. A method for producing a highly wear-resistant alumina-based sintered body, wherein a hardened layer is formed. 5. A molded body to which a Ti compound has been added and mixed as a metal compound is heat-treated in a reducing atmosphere to form a solid solution of Ti in alumina crystals, and then heat-treated in an oxidizing atmosphere to form a Ti-containing oxide. The method for producing a highly wear-resistant alumina-based sintered body according to claim 4, wherein the alumina-based sintered body is precipitated in the grains and at the grain boundaries of the alumina crystal. 6. A metal compound comprising a Ti compound and M
g compound or a composite body containing a composite compound thereof is heat-treated in an oxidizing atmosphere to dissolve Ti and Mg in alumina crystals, and then heat-treated in a reducing atmosphere to reduce the Mg-containing oxide to alumina. 5. The method for producing a highly wear-resistant alumina-based sintered body according to claim 4, wherein the sintered body is precipitated in the grains of the crystal and at the grain boundaries. 7. A molded body to which an Fe compound is added and mixed as the metal compound is heat-treated in an oxidizing atmosphere to dissolve Fe in alumina crystals, and then heat-treated in a reducing atmosphere to reduce the Fe-containing oxide. The method for producing a highly wear-resistant alumina-based sintered body according to claim 4, wherein the alumina-based sintered body is precipitated in the grains and at the grain boundaries of the alumina crystal.