From APS to HVOF spraying of conventional and nanostructured titania feedstock powders: a study on the enhancement of the mechanical properties

Nanostructured and conventional titania (TiO2) feedstocks were thermal sprayed using air plasma spray (APS) and high-velocity oxy-fuel (HVOF). The HVOF-sprayed coatings made from the nanostructured feedstock exhibited superior abrasion resistance, bond strength and crack propagation resistance when compared to the coatings made from the conventional feedstocks sprayed using HVOF and APS. The enhancement of the mechanical properties was due to (i) the processing (HVOF) and (ii) the nanocharacter of the feedstock. It was found that the HVOF-sprayed coating made from the nanostructured feedstock exhibited isotropic characteristics and microstructure with tiny zones of agglomerated nanostructured particles randomly spread throughout the coating structure. It was observed that these nanostructured zones acted as crack arresters by blunting and branching crack tips, enhancing the crack propagation resistance of the coating. Due to the isotropic characteristics of mechanical properties, the HVOF-sprayed coating made from the nanostructured feedstock exhibited uniform crack propagation under Vickers indentation, i.e., four cracks emanating from the corners of the Vickers indentation impression.