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-b ased matrix with a ceramic fibre has led to the choice of the "Saphikon" si ngle crystal alumina fibre and to develop a double coating concept consisti ng of an external yttria layer acting as a diffusion barrier and an interna l tungsten coating enabling decohesion at the fibre/matrix interface. Tensi le tests performed on monofilament composites, i.e. alumina fibres successi vely coated with W, Y2O3 and TiAl, revealed that after the heat treatment ( 1 h at 1100 degreesC) simulating the consolidation cycle, the double coatin g provides an 80% increase in the mean tensile strength as compared with a monofilament composite without coating. SEM and TEM observations did not sh ow any evidence of interfacial reaction. Moreover, the absence of cracking in a bulk composite specimen confirms the predictions of the internal stres ses calculations and proves the good fibre-matrix thermomechanical compatib ility. Finally, push-out tests were used to validate the possibility of fib re/W interface debonding at rather low stresses (tau (d)approximate to 100 MPa). (C) 2000 Acta Metallurgica Inc. Published by Elsevier Science Lid. Al l rights reserved.