Sintered reaction-bonded silicon nitride components

Details Sintered reaction-bonded silicon nitride components Sintered reaction-bonded silicon nitride components

1996-12-19

1999-03-02

Derrington, James

Plastic and nonmetallic article shaping or treating: processes

Outside of mold sintering or vitrifying of shaped inorganic...

Utilizing chemically reactive atmosphere other than air, per...

264 401, 264659, 264676, 264683, C04B 3565

Patent

active

058766600

DESCRIPTION:

BRIEF SUMMARY
FIELD OF THE INVENTION

This invention is concerned with a method of manufacturing dense sintered reaction-bonded silicon nitride components and with components made by said method. The term "dense" is used herein to mean substantially non-porous and at least 95% of theoretical density.


BACKGROUND OF THE INVENTION

Silicon nitride components can be made by sintering silicon nitride powder. Such sintering can only occur in the presence of a liquid phase. Current formulations therefore include 10-20% by weight of one or more oxides including yttria, rare earths, alumina, magnesia etc. which, in combination with the oxide layer present on the surface of the silicon nitride, form a glass in which the latter can dissolve and reprecipitate to effect consolidation. Components made by this method are used as structural components intended to withstand temperatures as high as 1400.degree. C. and therefore have utilised those oxides such as yttria and the rare earths, which generate the most refractory glasses and crystalline silicates. This approach, together with the use of high purity silicon nitride powders and sophisticated control of microstructure during sintering, has resulted in the production of ceramics with flexural strengths of about 1 GPa at room temperature which retain a high proportion of this value at 1200.degree. and, sometimes, 1400.degree. C. However, this method incurs production costs which are sufficiently high to inhibit its use in high volume applications such as the substitution of metal components in the automotive and general engineering industries. These costs are due to high material costs, high sintering temperatures (often above 1800.degree. C.), and high process losses.
An alternative to starting with silicon nitride powder in the manufacture of silicon nitride components is to utilise metallic silicon powder which is available at a cost which is lower by about one order of magnitude. Nitridation of silicon powder at temperatures in the range 1150.degree.-1400.degree. C. is a well established process for the formation either of silicon nitride powder or of porous reaction-bonded silicon nitride components. Whilst the latter have a useful strength of about 200 MPa, which is retained to at least 1200.degree. C., together with excellent thermal shock characteristics, their porosity and poor resistance to wear make them unsuitable for use as engineering components.
Several authorities (including Mangels Ceram Eng Sci Proc 1981 2 589-603) have shown that silicon preforms containing sintering aids such as MgO, Al.sub.2 O.sub.3 or Y.sub.2 O.sub.3 can be nitrided at temperatures below 1400.degree. C. and sintered in nitrogen at temperatures in the range 1750.degree.-1850.degree. C. to form components having 98% of theoretical density. This sintered reaction-bonded silicon nitride has the advantage of a low overall linear shrinkage in the range 9-12% compared to a doubling of these values for components derived from a sintered silicon nitride powder. Despite this little commercial attention has been paid to the sintered reaction-bonded silicon nitride process because the reaction between silicon and nitrogen is highly exothermic and, if not conducted with care, easily results in localised heating of preforms to temperatures exceeding the melting point of silicon (1420.degree. C.). This problem increases with the size of the furnace; nitridation times of about one week are typical. Also, after this slow nitridation stage, sintering is effected in a second graphite furnace with the component embedded in a protective powder bed. Furthermore, the mechanical properties of sintered reaction-bonded silicon nitride components are generally slightly inferior to and more difficult to control than those from the more direct sintered silicon nitride route.
Pompe (U.S. Pat. No. 4,492,665) has shown that the addition of 15-50% of fine silicon nitride powder to silicon powder acts as a dispersing aid and allows the latter to be ground using a ball mill to a grain size of <1 .mu.m. The addition of Y.su

REFERENCES:
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patent: 5308561 (1994-05-01), Leimer et al.
Mangels, John A., Effect of Rate-Controlled Nitriding & Nitriding Atmospheres on the Formation of Reaction-Bonded Si.sub.3 N.sub.4 In Ceramic Bulletin, vol. 60, No. 6 (1981).

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