Adhesion and Cohesion of Three-Dimensional Porous Calcium Phosphate Coatings on Titanium Deposited by Ultrasound-Assisted Micro-Arc Oxidation Method

The comprehensive approach using both pull-off tensile and scratch tests provided a detailed understanding of the adhesion and cohesion properties of the 3D-porous calcium phosphate (CaP) coatings deposited on a Ti substrate by the ultrasound-assisted micro-arc oxidation (UMAO) method. Using the pull-off tensile test which takes into account tensile stresses directed normally to the coating surface, it was found that the ultrasound (US) employing during the the micro-arc oxidation (MAO) process led to a slight decrease in the adhesion of the coating to the substrate from 23.4 to 19.3 MPa as well as to a change in the failure mechanism from the cohesive type characteristic for the control MAO coatings to the adhesive-cohesive type for the UMAO coatings. On the contrary, the results of the scratch test, which takes into account compressive stresses directed normally and tangentially to the coating surface, showed that the US employed during the MAO process led to an increase in the critical loads LC2 and LC3 to 6.7 and 30.6 N, respectively, promoting the adhesive-cohesive and adhesive failures of the coatings. Thus, the US employed during the MAO process led to an increase in the critical shear stresses of the UMAO coatings, thereby increasing their abrasion resistance.