Multilayer article characterized by low coefficient of thermal expansion outer layer

DETAILED DESCRIPTION The present invention is directed to a multilayer article which includes a substrate comprising a compound selected from the group consisting of a ceramic compound, a Si-containing metal alloy and combinations thereof.
In particular, the ceramic compound may be a Si-containing ceramic or an oxide ceramic with or without Si.
One preferred oxide ceramic is a mullite (3Al.
sub.
2 O.
sub.
3 2SiO.
sub.
2)-containing ceramic.
The multilayer article also includes an outer layer and at least one intermediate layer located between the outer layer and the substrate.
The outer layer comprises one of the following compounds having a low coefficient of thermal expansion: rare earth (RE) silicates; at lease one of a) hafnia (HfO.
sub.
2) and b) hafnia (HfO.
sub.
2)-containing composite oxides; and zirconia (ZrO.
sub.
2)-containing composite oxides.
Components of gas turbine engines, internal combustion engines and heat exchangers may be comprised of the multilayer article of the present invention.
Although the outer layer may be referred to as a top coat, it need not be a coating per se.
Also, other layers may be placed on top of the outer layer (i.
e.
, further from the substrate than the outer layer).
It should be understood that terms such as upper, lower, top, bottom and the like are used in this disclosure for purposes of illustration and should not be used to limit the invention, since these relative terms depend upon the orientation of the substrate.
The intermediate layer is typically a layer that is applied to the substrate or to a layer(s) on the article.
However, some substrate materials, such as mullite-containing materials, inherently form an intermediate layer (e.
g.
, mullite).
Whether applied or inherent, both constitute intermediate layers as these terms are used in this disclosure.
Also, the term composite oxide used herein means a compound or a mixture containing at least two oxides.
In one embodiment of the invention the outer layer comprises a low CTE rare earth silicate having a formula selected from the group consisting of (1) RE