Influence of abrasion on the surface characteristics of thermally sprayed hydroxyapatite coatings

The surface condition of hydroxyapatite thermal sprayed coatings can be measured in terms of smooth melted and unmelted regions. Both these areas collectively contribute to abrasion resistance of a coated implant. The purpose of this work was to determine the influence of abrasion on the surface characteristics of coatings. Coatings with three degrees of roughness (R(a) of 7, 10 and 24 microm) were produced by thermal spraying. These coatings were subjected to abrasion in a pin-on-disk test. It was found that the majority of weight loss occurs within the first minute of the abrasion. Raised areas from the coating are removed and the bone analogue pin spreads amongst the elevated areas to produce a smoother coating. Further, abrasion is dependent upon the applied load. Small loads produce a smaller or negligible increase in coating loss, however, a higher load is able to displace more material from the coating surface. It is expected that the coating loss from plasma sprayed coatings amounts to a value of less than 3 wt% during the insertion into bone. Modification of the coating surface from abrasion is dependent upon the surface roughness of the coating.