Influence of thickness on the properties of hydroxyapatite coatings deposited by KrF laser ablation.

The growth of hydroxyapatite coatings obtained by KrF excimer laser ablation and their adhesion to a titanium alloy substrate were studied by producing coatings with thicknesses ranging from 170 nm up to 1.5 microm, as a result of different deposition times. The morphology of the coatings consists of grain-like particles and also droplets. During growth the grain-like particles grow in size, partially masking the droplets, and a columnar structure is developed. The thinnest film is mainly composed of amorphous calcium phosphate. The coating 350nm thick already contains hydroxyapatite, whereas thicker coatings present some alpha tricalcium phosphate in addition to hydroxyapatite. The resulting coating to substrate adhesion was evaluated through the scratch test technique. Coatings fail under the scratch test by spallating laterally from the diamond tip and the failure load increases as thickness decreases, until not adhesive but cohesive failure for the thinnest coating is observed.