JPH0442864A - Highly strong sialon sintered product - Google Patents

Abstract

PURPOSE:To provide a sialon sintered product comprising the crystal of alpha--sialon modified with a rare earth element, the crystal of beta-sialon and small amounts of a crystal phase and/or a glass phase containing the rare earth element and having excellent high temperature strength, fractural strength and oxidation resistance. CONSTITUTION:RE alpha-sialon powder containing RE alpha-sialon of formula I (RE is modifying rare earth element selected from Ho, Er, Tm, Yb and Lb; 0<x <=2) as a main phase and containing <=8wt.% of excessive oxygen based on the theoretical oxygen amount restricted by formula I is mixed with alpha-silicon nitride powder, heated and then sintered to prepare a highly strong sialon sintered product comprising the crystal phase of the RE alpha-sialon of formula I, the crystal phase of beta-sialon of formula II (0<z<=4.2) and small amounts of crystal phase and/or glass phase containing the RE and suitable for gas turbine rotors.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a high-strength sialon-based sintered body useful for producing various engineering ceramics having high temperature and high strength, high hardness, and high toughness.

(Prior art and its problems) α-Sialon has Af at the Si position of α-type silicon nitride,
It is a substance in which ○ is substituted and solid-solved at the N position, and at the same time, the modifying cation M enters and solid-solves at the interstitial position, and has the formula Mx (S i, Af) Iz (0, N) +b
CI) (where M represents Li, Mg, Ca, Y, and lanthanide elements (excluding La and Ce), and χ is 0<x
The number satisfies ≦2. ). This Mα-sialon has characteristics as an engineering ceramic, such as high hardness, low coefficient of thermal expansion, and excellent corrosion resistance.

However, one reason may be that the sintered body of Mα-sialon crystal single phase has a granular crystal shape, but it cannot be said that the properties such as strength and fracture toughness are sufficient for engineering ceramics. Therefore, in order to improve this drawback, Japanese Patent Application Laid-open Nos. 58-185484 and 58-204
No. 875, No. 63-233082, etc., M
α-sialon crystal phase and A at the Si position of β-type silicon nitride
A needle represented by the formula S i6-gAfgoyoN, -, (n) (in the formula, 2 is a number satisfying Q<z≦4.2) where l is a solid solution substituted with 0 at the N position. A sialon-based sintered body has been proposed in which a β-sialon crystal phase is composited with a β-sialon crystal phase.

However, the mechanical properties of this sialon-based sintered body are not practically sufficient for use as a high-temperature material. In addition, as the modifying cation M, yttrium Y is generally used, but the sialon-based sintered body containing Y is
It is said that oxidation resistance at high temperatures is poor, and it is expected that it will be difficult to use it for gas turbine parts used under harsh conditions.

(Objective of the Invention) An object of the present invention is to solve the above-mentioned problems and provide a novel sialon-based sintered body having excellent oxidation resistance, high temperature strength, and high toughness.

(Summary of the Invention) According to the present invention, a crystal of α-silicon nitride, with the formula REx(S
i, Al) Iz (0, N) lb (III) (
In the formula, RE represents a modifying rare earth element selected from Ho, Er, Tm, Yb, and Lu, and X represents 0 < x≦2.
is a number that satisfies ) A high-strength sialon-based sintered body composed of a crystal of RE α-sialon represented by the formula (II), a crystal of β-sialon represented by the formula (II), and a small amount of crystal phase and/or glass phase containing RE is provided. be done.

(Detailed Description of the Invention) The content ratio of each phase in the sialon-based sintered body of the present invention is usually within the range shown below.

1〈α-Silicon nitride crystal phase〈40% by weight 5〈RE
Crystalline phase of α-sialon: 50% by weight 30% Crystalline phase of β-sialon: 90% by weight 1<20% by weight or glass phase The crystal phases of have compositions represented by formula (III) and formula (n), respectively, and each phase does not necessarily have to have a constant composition between each crystal grain, and may have a different composition. good. Ho, Er, Tm, Yb, or Lu, which has an ionic radius smaller than that of Y, enters into a solid solution between the lattices of α-sialon, thereby improving the oxidation resistance of the obtained sialon-based sintered body, and improving the oxidation resistance of the obtained sialon-based sintered body in a high-temperature oxygen-containing atmosphere. Above all, it can be used stably. In particular, when the element entering the solid solution is Er, Tm, Yb or Lu, the oxidation resistance is excellent.

If the proportion of the crystalline phase of α-silicon nitride is smaller than the above range, grain growth during sintering will be significant and the properties such as toughness of the obtained sialon-based sintered body will be insufficient; If it is larger than this range, sintering becomes insufficient and the mechanical strength of the obtained sialon-based sintered body decreases, which is not preferable.

Further, in the sialon-based sintered body of the present invention, the major axis of the α-silicon nitride crystal is 0.02 to 2 μm, the major axis of the REα-sialon crystal is 0.05 to 10 μm, and the major axis of the β-sialon crystal is 1 It is preferable that it is 100 micrometers.

Examples of crystal phases containing RE include melilite-type RE.
zSi: +0sNa, apatite type REx. (SiO
a) bNz, wollastonite type RESiOzN, wallerite type REaSizOJz, garnet type REx81sO+z, mullite type RExA1□0. and RE3AISi ZO7NZ, RE2si2o,,R
Examples include EzSi05, but the crystal group is not limited to these crystal groups.

In the sialon-based sintered body of the present invention, β-sialon crystals, α-silicon nitride fine grain crystals that have not undergone a phase transition, and a crystal phase and/or a glass phase containing RE
- Exists with Sialon crystals.

The fine grain crystals of α-silicon nitride present in this sintered body improve the strength characteristics of the sialon-based sintered body. Furthermore, various phases having different compositions, crystal phases, particle shapes, and coefficients of thermal expansion are present in the sintered body, and after sintering, microcracks occur in the sintered body due to microstrains. This, together with the strengthening effect of the matrix due to the three-dimensional intermingling of needle-like crystals and granular crystals, is thought to improve the mechanical properties such as fracture toughness of the obtained sialon-based sintered body.

The method for producing the sialon-based sintered body of the present invention includes:
Any method may be used as long as a sintered body having the above structure can be obtained.

An example of a method for manufacturing the sialon-based sintered body of the present invention is shown below.

The sialon-based sintered body of the present invention has REα-sialon represented by the above formula (III) as a main phase, and has the formula (
I It is obtained by heating and sintering in the range of ~2100°C.

As the REα-sialon powder, any powder may be used as long as it has REα-sialon represented by the formula (III) as its main phase. Powders prepared according to the invention of Sho 62-223009 are preferred.

This proposed method consists of (a) amorphous silicon nitride powder, (b) metallic aluminum or aluminum nitride, (c)
RE (R
(E is a rare earth element for modification selected from Ho, Er, Tm, Yb or Lu), and if necessary, (d) an oxygen-containing compound of aluminum or silicon, as desired. The mixture was mixed under a nitrogen-containing atmosphere to have a REα-Sialon composition of
This is a method for producing REα-sialon powder by heating to a temperature in the range of 0°C.

The REα-Sialon powder obtained by this method is a fine and uniform powder with a secondary particle size of 0.2 to 2 μm, and contains almost no free carbon and metal impurities, so it is free from pores and abnormal grain growth. It is possible to provide a sintered body without

In order to improve the sinterability of the REα-sialon powder and at the same time obtain a high-strength sialon-based sintered body, it is necessary to increase the R of the sintering raw material.
It is necessary that the Eα-sialon powder contains 8% by weight or less of excess oxygen with respect to the theoretical oxygen amount defined by the formula [■].

As a method for containing excess oxygen in the REα-sialon powder, for example, in the preparation stage of the REα-sialon powder, an excessive amount of silicon, aluminum, or an oxygen-containing compound of a specific rare earth element RE is added to amorphous silicon nitride. Method,
A method is employed in which REα-sialon powder is heated in an oxygen-containing atmosphere. As an example of the latter, REα-Sialon powder is heated at 800 to 1200° in an oxygen-containing atmosphere.
The excess of oxygen over the stoichiometric amount is heated to a temperature in the range of R.
An example is a method of incorporating it into Eα-sialon powder.

Heating time is usually 0.5 to 5 hours.

This treatment is carried out by, for example, placing a thin layer of REα-Sialon powder on a holding plate and leaving it in an oxygen-containing atmosphere;
This can be carried out by a method in which α-sialon powder is fluidized in an oxygen-containing atmosphere.

The amount of excess oxygen is 8% by weight or less, preferably 1 to 6.5% by weight, particularly preferably 2 to 4% by weight.

If the amount of excess oxygen is too large, many phases with low melting points remain in the sintered body, and mechanical properties at high temperatures are impaired.

The α-silicon nitride powder preferably has an average particle size of 1 μm or less in terms of sinterability, and furthermore, it impairs the strength, corrosion resistance, and oxidation resistance of the obtained sintered body at high temperatures. It is preferable that the content of impurities is 0.1% by weight or less.

The blending ratio of α-silicon nitride powder in the mixture of REα-sialon powder and α-silicon nitride powder is 40% by weight or more, preferably 50 to 90% by weight, more preferably 60 to 8% by weight.
It is 0% by weight. As the blending ratio of α-silicon nitride powder increases within the above range, the ratio of β-sialon phase in the produced sialon-based sintered body increases. When the blending ratio of α-silicon nitride powder exceeds 90% by weight, the sinterability of the mixture decreases, and the densification of the sintered body does not proceed.

Further, in the present invention, the α-silicon nitride powder may further contain aluminum or a modifying rare earth element RE as a component. In this case, aluminum or the rare earth element RE for modification may be present inside or on the surface of the α-silicon nitride powder, or oxides, nitrides, oxynitrides, etc. of aluminum or the rare earth element RE for modification may be present inside or on the surface of the α-silicon nitride powder. Powders may be mixed. Further, it is preferable that the content of metal impurities other than the main metal components is 0.1% by weight or less.

There is no particular restriction on the method of mixing REα-sialon powder and α-silicon nitride powder, and methods known per se can be used, such as dry mixing the two, wet mixing the two in an inert liquid, and then mixing the two in an inert liquid. A method of removing the liquid, etc. can be adopted as appropriate.

As a mixing device, a type mixer, a ball mill, a vibration mill, etc. are conveniently used.

For heating and sintering of mixed powder, for example, the mixed powder is directly shaped into a predetermined shape using a dry or wet method, and if it is formed using a wet method, it is dried and then heated with a nitrogen-containing non-oxidizing gas under normal pressure or pressurized. A method of sintering in an atmosphere, a method of filling a graphite die of a predetermined shape with raw material powder, and hot-pressing it, etc. can be adopted. Further, by further subjecting the sintered body obtained by the above method to hot isostatic pressing, the physical properties of the sintered body can be further improved.

The mixed powder may be shaped prior to normal pressure or atmospheric pressure sintering by a known method, such as a rubber press method, a -shaft molding method, a cast molding method, an injection molding method, an explosive compression molding method, and the like.

The sintering temperature is typically 1600-2100'C and the sintering time is typically 0.5-10 hours. If the sintering temperature is too low, sintering will not proceed, and if the sintering temperature is too high,
Composition changes occur in the sintered body due to thermal decomposition.

By the method, β-sialon crystals that are thought to be produced by the reaction between REα-sialon and α-silicon nitride, α-silicon nitride fine crystals that have not undergone phase transition, and crystal phases and/or glass containing RE are obtained. R whose phase is the raw material
From the composition of Eα-sialon, a sialon-based sintered body can be obtained which exists together with crystals of REα-sialon in which X in the formula (I [[) is slightly lower).

In particular, it is important to precipitate highly heat-resistant wallerite-type REaSizOJz microcrystals at the grain boundaries of the sialon-based sintered body in order to improve the oxidation resistance.

(Effects of the Invention) Compared to conventional sialon-based sintered bodies, the sialon-based sintered body obtained by the present invention not only has excellent mechanical properties such as high-temperature strength and fracture toughness, but also has particularly remarkable oxidation resistance. As a result, it can be suitably used as a highly reliable structural material, particularly as heat-resistant parts such as rotors, stators, and combustors for gas turbine engines.

(Example) Examples and comparative examples are shown below.

Examples 1 to 15 and Comparative Examples 1 to 7 Amorphous silicon nitride powder, rare earth oxide (RE z Os ) powder, and metal A! in the blending ratios shown in Table 1! The powder was mixed for 1 hour in a vibratory mill under nitrogen gas atmosphere. The mixed powder was filled into a carbon crucible, set in a resistance heating high temperature furnace, and heated from room temperature to 1200"C for 1 hour, from 1200°C to 1400"C for 4 hours, and then at 1400°C in a nitrogen gas atmosphere. The REα-sialon powder was crystallized by heating from 1,600″C to 1600″C on a 2-hour heating schedule.Table 1 shows the characteristic values of the obtained REα-sialon powder.

REα-sialon powder synthesized as above and α-silicon nitride powder having the following characteristics: specific surface area: 11.5 bo/g particle shape bi-equiaxed crystal formation phase: α phase>95% metal impurities: <500 ppm (Made by Ube Industries)
was wet-milled for 48 hours using ethanol as a medium at the blending ratio shown in Tables 2 and 3, and then
It was vacuum dried at C.

The obtained powder mixture was preformed into a rectangular shape using a mold with a cross section of 50 x 80 s + m squares, and then a pressure of 1.5 to
Rubber pressed with n/cj. The obtained molded body was heated from room temperature to 1750° in a nitrogen atmosphere at normal pressure using an electric furnace.
The temperature was raised to C at a rate of 2° C./min and maintained at the same temperature for 4 hours.

Tables 2 and 3 show the results of measuring the bulk density and proportion of the formed phase of the obtained sialon-based sintered body. Note that the proportion of the formed phase was calculated from the X-ray diffraction peak intensity. FIG. 1 shows an X-ray diffraction chart of the sintered body obtained in Example 10.

Further, 100 test pieces of 3 x 4 x 40 mm were cut out from the produced sintered body, set in a 4-point bending test jig with an outer span of 30 mm and an inner span of 10 mm, and the bending strength at room temperature and 1300°C was measured. The fracture toughness value +c was measured by the 5EPB method. Further, as an oxidation resistance test of the obtained sintered body, the test piece was heat-treated in air at 1350° C. for 100 hours, and the weight increase due to oxidation and the bending strength at room temperature after oxidation were measured.

These results are shown in Tables 2 and 3.

[Brief explanation of the drawing]

FIG. 1 is an X-ray diffraction diagram of the sintered body obtained in Example 10 of the present invention.

Claims (1)

[Claims] α-Silicon nitride crystal, formula: RE_x(Si,Al)_1_2(O,N)_1_
6 (wherein, RE represents a modifying rare earth element selected from Ho, Er, Tm, Yb, and Lu, and x represents 0<x≦2
is a number that satisfies ) REα-Sialon crystal represented by ▲There are mathematical formulas, chemical formulas, tables, etc.▼ (In the formula, z is a number that satisfies 0<z≦4.2.) β-Sialon crystal represented by and a high-strength sialon sintered body composed of a small amount of crystal phase and/or glass phase containing RE.