Structure and stability of synthetic interphases in CMCs

In a research programme focused on the development of high-temperature CMCs for engineering application, a broad range of fibre-matrix interphase chemistries have been studied with a view to combining appropriate micromechanical properties with thermal and environmental stability above 1000 degrees C. Various oxide and phosphate interphases have been prepared via PVD and liquid precursor routes, their reaction with mainly oxide fibres assessed and interfacial debond stresses measured. Saphikon (Al2O3) monofilaments have been used in the initial development of coating techniques and in the fabrication of model CMCs. The following types of coating have been prepared: a) Porous oxides of simple constitution (ZrO2 with stabilising cations) with an open columnar structure controlled by the PVD (magnetron sputtering) conditions. b) Complex oxides in which the interface debond requirement is satisfied by an intrinsically low debond energy with the fibre or matrix. Examples are provided by a range of stoichiometric rare earth phosphates of constitution REPO(4) and complex oxides of type MXO(4) such as LaVO4. These interfaces have been prepared from liquid precursors and demonstrate the importance of stoichiometric composition in avoiding the formation of beta-alumina reaction products which are detrimental to the debond requirement. c) Rare earth beta-aluminas, with weak 'layer-plane' bonding, formed either via liquid precursor coating or by in-situ reaction from the rare earth doped PVD coatings. Preferred orientation and fine grain size of the layered crystals is important in achieving moderate debond stresses and retention of fibre strength. These varied interface compositions and synthesis methods have also been studied in relation to a potentially new generation of oxide fibres and matrices containing garnet, spinel, mullite and other complex oxide phases.