Plastic and nonmetallic article shaping or treating: processes – Including step of generating heat by friction
Patent
1983-01-21
1985-01-08
Andrews, R. L.
Plastic and nonmetallic article shaping or treating: processes
Including step of generating heat by friction
501 97, 501 98, C01B 2168, C04B 3558
Patent
active
044926650
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a process for the production of mouldings from materials based on silicon nitride in accordance with the preamble of patent claim 1.
As a first step in this process a powder mix is prepared by grinding/mixing silicon nitride with silicon powder containing a high percentage of Si, preferably 40-85 percent by weight in relation to the amount of Si.sub.3 N.sub.4 added, and a sintering aid, whereby the silicon nitride powder serves as a dispersing agent for silicon and enables grinding down of the mix to a mean particle size of less than 1 .mu.m. After preparation of a moulding by means of known moulding methods (pressing, casting, injection moulding etc.) the silicon content of this body is, by way of the next step in this process, partly or fully nitrided to silicon nitride, thus bringing about an internal increase in mass and, accordingly, an increase in the so-called green density (i.e. compact density prior to sintering) of the body produced from powder. After machining, if applicable, the moulding is subjected to final nitriding and is then sintered, preferably by conventional means (without simultaneous application of mechanical pressure) with the assistance of the sintering aid added, until a final required density is achieved.
Materials based on silicon nitride are regarded as belonging to the group of high-temperature ceramics. The main characteristics of these materials are their elevated high-temperature strength, wear resistance and corrosion resistance, which ensure that they are suitable for components of internal combustion engines as well as for parts subject to wear and for machining.
When manufacturing products from these materials use is most often made of traditional ceramic or powder-metallurgical moulding methods such as slip casting, injection moulding, pressing or a combination of these processes. Temporary binders, injection moulding plastics and/or pressing aids are removed in the normal way by burning off (fuming off). Dense materials can be achieved by applying two types of sintering methods, i.e. by sintering (in a conventional way or subject to simultaneous application of mechanical pressure--pressure-sintering) of bodies made from Si.sub.3 N.sub.4 -powder with an addition of sintering aid, or by nitriding bodies formed from Si-powder (reaction-sintering). As a rule, sintering is effected in a nitrogen atmosphere (possibly subject to overpressure) at temperatures above 1500.degree. C., whereas reaction-sintering is generally effected in several stages close to the melting point for pure silicon.
However, these methods are subject to various disadvantages which hinder their commercial application. It is for instance possible to achieve an entirely non-porous material by pressure-sintering, but only uncomplicated shapes can be produced by this method. With reaction sintering it is possible to produce complex shapes with high dimensional tolerance owing to the fact that the amount of shrinkage which occurs in the course of nitriding is low. The nitriding process does however result, on the one hand, in a material with a residual porosity of normally 15-20% which entails a relatively low strength level, and, on the other hand, necessitates a treatment period amounting to one or several days. It is assumed that this is due to the low rate at which nitrogen is diffused within the generated microstructure of the silicon nitride. A fine-grained microstructure with an increased proportion of grain boundaries can result in increased nitrogen permeability. In order to bring about such a microstructure it is desirable to start with a finely ground Si-powder, preferably with a grain size of less than 1 .mu.m. However it is hard to grind down silicon to the said grain size owing to the difficulty of dispersing the ground powder.
With conventional sintering (often described in the literature as pressureless sintering) a completely dense material can be achieved, with a strength comparable to that of pressure-sintered material. Complex mouldings can, in addition,
REFERENCES:
patent: 3591337 (1971-07-01), Lumbey
patent: 4125952 (1978-11-01), Ezis et al.
patent: 4264547 (1981-07-01), de Pous
Andrews R. L.
Chapman Terryence
Swedish Institute for Silicate Research
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