Process optimization and performance of nanoreinforced HVOF-sprayed ceramic coatings

Improved mechanical properties have widely been demonstrated for bulk nanocrystalline materials. Especially with ceramic materials decreasing of grain size has been found to be favourable. Nanocrystalline materials offer better thermal shock resistance, lower thermal conductivity and better wear resistance than their conventional counterparts. For nanostrucutured bulk composites with nanosized metal precipitations in the nanocrystalline ceramic matrix improved fracture toughness properties have also been reported. An increasing effort has been made to transfer such improvements also into thermal sprayed ceramic coatings. Mainly work has been carried out with plasma spray systems, but recently it has been shown that HVOF (High Velocity Oxy-Fuel) process can produce much denser coatings and hence better environmental protection capacity. In this paper we describe the development of HVOF sprayed nanocrystalline Al2O3-composite coatings, where the grain size of Al2O3 has been decreased and a few percents of alloying elements has been added in order to toughen the coating