Alumina is a relatively low-cost material and easily processable by thermal spraying into wear and corrosion resistant coatings. However, thermally sprayed alumina coatings show inferior corrosion resistance versus chromia coatings, particularly in low and high pH electrolytes. Further, alumina possesses decreased mechanical properties in the as-sprayed state. In the present study, the effect of chromia addition on the properties of the plasma and HVOF sprayed alumina coatings were studied. Pure alumina powder and four different Al 2 O 3 -Cr 2 O 3 powders were prepared in two different atmospheres and produced to coatings with APS and HVOF methods. Phase composition of the powders and coatings were studied by X-ray diffraction. Electron microscopy was used for the examination of the microstructure of the powders and coatings. The mechanical properties of the coatings were studied by hardness and abrasion resistance tests. The chemical properties of the coatings were examined by corrosion exposure tests. Results indicate that with chromia addition it is possible to improve the mechanical and chemical properties of the thermally sprayed alumina coatings.

Superior wear performance combined with excellent friction properties against metals makes chromium oxide (Cr 2 O 3 ) an interesting coating material for many industrial applications. However, Cr 2 O 3 is a challenging material for HVOF spraying due to its high melting temperature. Fracture toughness and lamella cohesion of a coating is limited and may be improved by using ceramic-ceramic –nanocomposite powders, which forms phases with improved properties. In this study Cr 2 O 3 -TiO 2 systems were selected aiming to improve the toughness and lamella cohesion of coating without reducing the excellent wear properties.

The potential of the high velocity oxy-fuel (HVOF) thermal spray process to produce coatings with reduced porosity is well known. The ability to produce high density ceramic coatings offers potential in high performance applications in the fields of wear, corrosion resistance and dielectric coatings. It has been, however, demonstrated that benefits from HVOF ceramic coatings can be obtained only if particles are melted enough and good lamella adhesion is produced. Therefore, due to the operational limits of the HVOF process, the process-structure- relationship must be well optimized. One strategy to improve melting of ceramic particles in the relatively low flame temperatures of the HVOF process is to modify particle crystal structure and composition. In this paper, the effects of the powder structure and the composition on coating microstructure and deposition efficiency of the HVOF spray process are studied. The effect of fuel gas, hydrogen vs. propane, was also demonstrated. The studied materials were agglomerated alumina- and titania-based pure and composite powders. Coating properties such as microstructure, hardness, and abrasive wear resistance, were compared to the coating manufactured by using conventional fused and crushed powders.