Compositions: ceramic – Ceramic compositions – Refractory
Patent
1989-03-22
1991-06-11
Bell, Mark L.
Compositions: ceramic
Ceramic compositions
Refractory
501103, 501105, C04B 3558
Patent
active
050232169
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to an improved ceramic material and, in particular, to an improved engineering ceramic material.
Engineering ceramics are materials such as the oxides, nitrides and carbides of the metals silicon, aluminium, boron and zirconium. They are characterized by great strength and hardness; properties which in theory can be retained to very high (>10000.degree. C.) temperatures. Two of the most promising types of ceramic are the sialon family, and the zirconia family.
The sialons are based on the elements Si, Al, O, N, hence the acronym. A successful commercial sialon is the .beta.'-sialon which has the .beta.-Si.sub.3 N.sub.4 crystal structure, but with some of the silicon atoms replaced by aluminium atoms, and for valency balance some nitrogen atoms replaced by oxygen atoms. The sialons are usually formed by mixing Si.sub.3 N.sub.4, Al.sub.2 O.sub.3, AlN with a metal oxide (often Y.sub.2 O.sub.3), compacting the powder to the desired shape, and then firing the component at .about.1750.degree. C. for a few hours. The function of the metal oxide is to react with the alumina and the silica layer (which is always present on the surface of each silicon nitride particle), to form a liquid phase which dissolves the reactants and precipitates the product. The liquid phase (which still contains dissolved nitrides), cools to form a glass between the .beta.'-sialon grains. Typically, a Y.sub.2 O.sub.3 densified .beta.'-sialon contains about 15 volume percent of Y-Si-Al-O-N glass and 85 volume percent .beta.'-sialon. At temperatures above 800.degree. C. this glass begins to soften and the strength decreases. The glass/sialon can be heat treated at .about.1300.degree. C. to crystallise the glass. In the case of .beta.'-sialon and glass, the glass crystallises to give Y.sub.3 Al.sub.5 O.sub.12 (yttro garnet or YAG) and a small amount of additional .beta.'-sialon. With glass/O'-sialon the crystallisation produces Y.sub.2 Si.sub.2 O.sub.7 (yttrium disilicate) plus a small amount of additional O'-sialon. This crystallisation process reduces the room temperature strength of the material, but this reduced strength is maintained to higher temperature. The reason that crystallisation reduces strength is not completely understood, but is probably because the crystalline YAG occupies a smaller volume than the glass it replaces; crystallisation leaves small cracks. The grain boundary phase is a necessary evil in these materials, it is a remnant of the densification process.
.beta.'-Sialon has the general composition Si.sub.6-Z Al.sub.Z P.sub.Z N.sub.8-Z where 0<Z.ltoreq.4.2, whilst O'-sialon has the general composition Si.sub.2-X Al.sub.X O.sub.1+X N.sub.2-X where 0<X<0.20. O'-sialon has an expanded silicon oxynitride crystal lattice structure.
The .beta.'-sialon is a strong engineering ceramic with good oxidation resistance and creep resistance up to 1300.degree. C. The O'-sialon has approximately two thirds the strength of .beta.'-sialon, but has very much improved oxidation resistance up to 1400.degree. C. The two materials are in thermodynamic equilibrium and so composite materials can be formed. The high temperature creep resistance is determined by the grain boundary phase, which for these materials is usually YAG.
Another promising ceramic family is based on zirconia, ZrO.sub.2. The monoclinic or tetragonal zirconia is dispersed in a matrix typically mullite, alumina or cubic zirconia. The dispersed zirconia toughens by a process known as transformation toughening. Basically, the composite is fired at high temperature (at least 1100.degree. C.), when the ceramic densifies, and the zirconia is in its high temperature tetragonal form. On cooling, part of the tetragonal zirconia attempts (and fails) to transform to its low temperature monoclinic form. The matrix constrains the zirconia in its tetragonal form which at room temperature is metastable. This transformation would be accompanied by a 3-5 volume percent increase in each zirconia crystal. The effect is to put the entire matrix into
REFERENCES:
patent: 4506020 (1985-03-01), Butler et al.
patent: 4506021 (1985-03-01), Jack et al.
patent: 4748138 (1988-05-01), Watanabe et al.
patent: 4804644 (1989-02-01), Anseau et al.
Sanders and Mieskowski, "Strength and Microstructure of Si.sub.3 N.sub.4 Sintered with ZrO.sub.2 Additions" in Advanced Ceramic Materials, vol. 1; No. 2, 1986, pp. 166-173.
Anseau Michael R.
Lawson James M.
Slasor Shaun
Bell Mark L.
Cookson Group PLC
Jones Deborah
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