Non-equilibrium Microstructure and Thermal Stability of Plasma-sprayed Al–Si Coatings

Abstract

A splat-quenched, thick Al–Si deposit was manufactured by low-pressure plasma spraying (LPPS) and investigated in terms of microstructural inhomogeneity, Si solid solubility in α–Al, formation of metastable phases, and thermal stability. The LPPS Al–Si deposit had an inhomogeneous, layered microstructure consisting of splat-quenched lamellae and the incorporation of unmelted or partially melted particles. The splat-quenched Al–Si lamellae were formed by deposition of a fully liquid droplet and had an almost featureless microstructure at relatively low magnifications. There was a significant reduction in the α–Al lattice parameter in the LPPS Al–Si deposit because of extended Si solubility in the α–Al matrix. Transmission electron microscopy investigations showed that the splat quenching of liquid Al–Si droplet led to (i) columnar grain growth of α–Al(Si), (ii) formation of nano-sized Si precipitates in the Al matrix which was supersaturated with Si; and (iii) formation of amorphous Si phase embedded in the crystalline Al matrix. On reheating, the amorphous Si transformed into fine crystalline Si by interdiffusion of Al and Si atoms. Simultaneously, Si precipitation occurred in the supersaturated α–Al matrix. The overall activation energy for the Si crystallization/precipitation was estimated as ∼81 kJ/mol from a modified Kissinger analysis.