ABSTRACT
This chapter discusses the efficacy of nanoindentation technique toward the measurement of residual stress. Processing of ceramic materials involved sintering, thermal-spray process, etc., all of which require high temperature. We shall be concentrating here only on the effect of the plasma-spray technique for the fabrication of HAp coating, as discussed in Chapter 9. We know that quenching and thermal mismatch-induced stresses are two prime contributors to the residual stress in plasma-sprayed ceramic coatings [1]. Basically, in plasma spraying, the flowable powder granules/particles are fed from the powder feeder into the plasma jet and then propelled at very high speed (≈200 m⋅s−1) through the plasma jet toward the metallic substrate. The molten ceramic droplets then spread as splats in the shape of a pancake over metals. As cooling is done in an open-air atmosphere, an extremely rapid heat-transfer process starts between the stack of the ceramic splats and the metal substrate. Therefore, it is understood that the heat rejection is directly dependent on thermal diffusivity of the respective metal substrates [2]. The residual stress is generated in the ceramic coating due to the sudden quenching from high temperature to room temperature and the mismatch of the coefficient of thermal expansion (CTE) between the metal and the ceramic coating (e.g., MIPS-HAp).
