Adhesive and cohesive properties by indentation method of plasma-sprayed hydroxyapatite coatings

Adhesive and cohesive properties of the plasma-sprayed hydroxyapatite (HA) coatings, deposited on Ti-6Al-4V substrates by varying the plasma power level and spray distance (SD), were evaluated by an indentation method. The crystallinity and the porosity decreased with increasing both of these two parameters. The microhardness value, Young's modulus (E) and coating fracture toughness (K_C) were found to increase with a combinational increase in spray power and SD. The Knoop and Vickers indentation methods were used to estimate E and K_C, respectively. The critical point at which no crack appears at the interface was determined by the interface indentation test. This was used to define the apparent interfacial toughness (K_Ca) which is representative of the crack initiation resistance of the interface. It was found that K_Ca reaches to a maximum at a medium increase in both spray power and SD, while other mechanical properties of the coatings reaches to the highest value with further increase in these two plasma parameters. The tensile adhesion strength of the coatings, measure by the standard adhesion test, ISO 13779-4, was shown to alter in the same manner with K_Ca results. It was deduced that a combinational increase in spray power and SD which leads to a higher mechanical properties in the coatings, does not necessarily tends to a better mechanical properties at the interface.