HVOF thermal spraying has been developed to deposit dense ceramic coatings with improved protective properties for various applications. Even though HVOF coatings are much denser as compared to ordinary plasma sprayed coatings, the coating properties are inferior as compared to bulk ceramics because of pores and microcracks, which influence adversely the coating properties, i.e. toughness, hardness and wear resistance. One strategy to improve the properties of the coatings is to decrease the grain size of the ceramic phase and to add toughening elements to the microstructure. Nanocrystalline coatings have been found to offer better thermal shock resistance, lower thermal conductivity and better wear resistance than their conventional counterparts. In this paper we describe the development of nanocrystalline ceramic composite coatings, where the grain size of ceramic has been decreased and a few percents of alloying element has been added in order to toughen the coating. Dense nanostructured alumina coatings alloyed with Ni and ZrO 2 nanosized particles were manufactured by HVOF spraying by using HV2000 spray gun. Mechanical properties, especially elastic modulus and relative fracture toughness were studied. Used techniques were instrumented nanoindentation and KIC evaluation. As a result coatings with nearly 100% improvements in relative fracture toughness were produced for nanoreinforced alumina composite coating. Results are compared with the microstructure, hardness and abrasive wear resistance of the coatings.

HVOF thermal spraying has been developed to deposit dense Al 2 O 3 -coatings with improved protective properties for various applications. Nanocrystalline coatings have been found to offer better thermal shock resistance, lower thermal conductivity and better wear resistance than their conventional counterparts. In this paper we describe the development of nanocrystalline Al 2 O 3 and Al 2 O 3 -Ni -coatings, where the grain size of Al 2 O 3 has been decreased and a few percents of nickel has been added in order to toughen the coating. Coatings were manufactured by HV- 2000 HVOF using spray parameters determined based on the on-line spray diagnostics. Parameters were selected aiming at different melting stages of the powder. The resulting microstructure of the coatings and its influence on the coating properties is discussed

The thermal spray process melts powder at very high temperatures and propels the molten material to the substrate to produce a coherent deposit. This heating produces a certain amount of vaporization of the feedstock. Upon exiting the plasma plume the fast cooling conditions lead to condensation of the vapor. An electrical low pressure impactor was used to monitor the concentration of ultra-fine particles at various radial and axial distances. Metal, namely iron powder, showed very high concentration levels which increase with distance. Ultra-fine particles from ZrO 2 -8Y 2 O 3 reached a peak concentration at 6 cm. Use of an air barrier during spraying decreases the population of ultra-fine particles facilitating the production of a stronger coating.