Process for producing .beta.-sialon based sintered bodies

Details Process for producing .beta.-sialon based sintered bodies Process for producing .beta.-sialon based sintered bodies

1991-10-21

1994-04-12

Derrington, James

Plastic and nonmetallic article shaping or treating: processes

Including step of generating heat by friction

501 89, 501 93, 501 98, C04B 3558

Patent

active

053023293

DESCRIPTION:

BRIEF SUMMARY
DESCRIPTION

1. Technical Field
This invention relates to a process for producing .beta.-sialon based sintered bodies having significantly high values of fracture toughness (which is hereunder abbreviated as K.sub.IC) using three starting materials, i.e., a .beta.-sialon powder, a sintering aid such as MgO and an additive such as SiC and by specifying such factors as the fineness of the individual starting materials and their mixing proportions.
2. Background Art
Sialon is known to occur in many forms such as .alpha.-, .beta.-, o'-, x- and polycrystalline types depending upon the composition and crystalline form. The .beta.-sialon which is to be used in the present invention as a starting material is represented by the chemical formula Si.sub.6-z Al.sub.z O.sub.z N.sub.8-z (0<z.ltoreq.4.2). Sintered bodies of .beta.-sialon have many advantages such as high strength at both room temperature and high temperatures, high resistance to oxidation at high temperatures and high resistance to erosion by molten metals and, therefore, they are used in machine parts such as engines and gas turbines, various refractories and cutting tools.
There are two basic processes for producing .beta.-sialon sintered bodies. One method is well established and has been studied for many years; starting materials such as Al.sub.2 O.sub.3, SiO.sub.2, Si.sub.3 N.sub.4 and AlN are appropriately mixed and immediately sintered (this method is generally referred to as the reactive sintering method). Many patents have issued on this method as exemplified by U.S. Pat. Nos. 4,066,468, 4,506,021 and Japanese Patent Publication No. 1763/1978. The other method comprises the following steps: synthesizing a .beta.-sialon powder by firing the premix that has been prepared in the process of the reactive sintering method, mixing the powder with a sintering aid such as MgO or Y.sub.2 O.sub.3 and either hot pressing the resulting mixture or shaping it into a compact which is then subjected to normal sintering. This second method is hereinafter referred to as the powder sintering method for convenience' sake.
With a view to making the .beta.-sialon sintered bodies more suitable for use in the aforementioned applications, active studies are being made to develop composite sintered bodies that are provided with enhanced wear resistance and toughness by incorporating other compounds. For instance, Journal of the Ceramic Society of Japan, Vol. 95(4), pp. 450-452, 1987 describes a modified reactive sintering method, in which starting materials, i.e., .alpha.-Si.sub.3 N.sub.4, .alpha.-Al.sub.2 O.sub.3, AlN and SiC, are mixed and hot pressed to produce a sintered body composed of .beta.-sialon and SiC (50 wt %). Modified powder sintering methods are also described in several patents. According to the examples described in Japanese Patent Publication Nos. 15505/1987 and 41193/1987, a .beta.-sialon powder having a maximum particle size of 60 mesh (250 .mu.m) is mixed with appropriate amounts of powders on a submicron order such as Y.sub.2 O.sub.3, ZrO.sub.2, TiC, TiN and TiCN and the resulting mixture is hot pressed in a nitrogen atmosphere at 1750.degree. C. to produce a sintered body. According to the examples of Japanese Patent Public Disclosure No. 73472/1984, a .beta.-sialon powder having an average particle size of 1.0 .mu.m is mixed with a sintering aid (Y.sub.2 O.sub.3 powder and/or MgO powder) and an additive [(Ti,W)CN compound] and the resulting mixture is shaped into a powder compact at a pressure of 1000 kgf/cm.sup.2, with the compact being then subjected to normal sintering in a nitrogen atmosphere at 1600.degree.-1750.degree. C. to produce a sintered body.
However, most of the sintered bodies produced by those conventional methods have K.sub.IC values lower than 6.0 MPam.sup.1/2, so when they are used in the aforementioned applications, particularly in sliding machine parts, surface flaws develop, which will grow to cracks and frequently lead to the destruction of the machine part.
Recognizing the need to solve these problems, the present invent

REFERENCES:
patent: 3895219 (1975-07-01), Richerson
patent: 3903230 (1975-09-01), Kamigaito
patent: 4066468 (1978-01-01), Kamigaito
patent: 4365022 (1982-12-01), Tabata
patent: 4506021 (1985-03-01), Jack
Abstract of Japanese Pat. Pub. Disclosure No. 73472/1984.
N. E. Cother et al, "The Development of Syalon Ceramics and their Engineering Applications, " Trans. J. Br. Ceram. Soc. 1982, p. 141.

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