Intermetallic TiAl-based matrix composites: Investigation of the chemical and mechanical compatibility of a protective coating adapted to an alumina fibre

The investigation of both chemical and mechanical compatibility of a TiAl-based matrix with a ceramic fibre has led to the choice of the "Saphikon" single crystal alumina fibre and to develop a double coating concept consisting of an external yttria layer acting as a diffusion barrier and an internal tungsten coating enabling decohesion at the fibre/matrix interface. Tensile tests performed on monofilament composites, i.e. alumina fibres successively coated with W, Y2O3 and TiAl, revealed that after the heat treatment (1 h at 1100 degreesC) simulating the consolidation cycle, the double coating provides an 80% increase in the mean tensile strength as compared with a monofilament composite without coating. SEM and TEM observations did not show any evidence of interfacial reaction. Moreover, the absence of cracking in a bulk composite specimen confirms the predictions of the internal stresses calculations and proves the good fibre-matrix thermomechanical compatibility. Finally, push-out tests were used to validate the possibility of fibre/W interface debonding at rather low stresses (tau (d)approximate to 100 MPa). (C) 2000 Acta Metallurgica Inc. Published by Elsevier Science Lid. All rights reserved.