Sapphire fibre surface damage caused by a polycrystalline zirconia coating has been analysed using scanning electron microscopy (SEM) and electron probe microanalysis (EPMA). "Pitting" of the sapphire surface was partially attributed to formation of a transient liquid phase (interphase) capable of local dissolution of alumina at zirconia grain contacts. Chemical etching was used to verify that the interphase material was silicate-based and resided at triple points between zirconia grains and the fibre surface. An additional crystalline calcium hexaluminate phase (hibonite) was found on some fibres. Origin of these im purities and resulting consequences of their presence are rationalized in view of observed surface modification.