Young's modulus and fracture toughness determination of high velocity oxy-fuel-sprayed bioceramic coatings

Indentation tests along with three- and four-point bend tests were utilized for the determination of Young's modulus (E) of high velocity oxy-fuel (HVOF) sprayed hydroxyapatite (HA) and HA/titania (TiO₂) coatings. In addition, fracture toughness (K_1c) and strain energy release rates (G) with reference to the coating/substrate interface were investigated using the indentation technique and four-point bend test, respectively. Results showed that the E values of the bioceramic coatings were significantly dependent upon microstructure and phase composition. The incorporation of titania (10 and 20 vol.%) as reinforcements, which had higher stiffness than HA, was found to effectively improve the overall E values and fracture toughness of the composite coatings. It was revealed that the splats' interface played a substantial role in determining the fracture toughness on the assumption that cracks propagated predominantly along the interface. The present study claimed that the indentation test was essentially a local-phase-dependent method and was markedly influenced by surface roughness of the samples. The three- and four-point bend tests were found to be relatively more reliable for the evaluation of overall bulk property of the coating. And typical E values obtained from the three-point bend test were notably close to those obtained from the four-point bend test. It was found that the four-point bend test was not suitable for the determination of fracture energy at the coating/substrate interface due to the poor cohesion of the bioceramic coatings.