Room temperature de-bonding and sliding of fibres coated with La-monazite is assessed using a composite with a polycrystalline alumina matrix and fibres of several different single crystal (mullite and sapphire) and directionally solidified eutectic compositions. These fibres provided a range of residual stresses and interfacial roughness. Sliding occurred over a debond crack at the fibre-coating interface when the sliding displacmenet and surface roughness were relatively small. At large sliding displacements with relatively rough interfaces, the monazite coatings were deformed extensively by fracture, dislocations, and occasional twinning, whereas the fibres were undamaged. Dense, fine-grained areas (10nm grain size) resembling recrystallized microstructures were also observed in the most heavily deformed regions of the coatings.
