Comparison between shot peening and abrasive blasting processes as deposition methods for hydroxyapatite coatings onto a titanium alloy

Recent studies have shown that combining a compressed air jet with entrained hydroxyapatite
(HA) particles with a jet of abrasive particles can be used to deposit a well adhered crystalline
HA coatings onto titanium substrates. A similar particle bombardment process utilising a
flow of shot peen particles and a flow of suitable powder particles has been used to deposit a
range of coatings, though the deposition of bioceramic powders have not yet been reported by
this method. In this study a direct comparison between the shot peen and abrasive
bombardment processes has been undertaken to determine which technique yields coatings
exhibiting higher levels of adhesion on titanium alloy substrates. Both processes were shown
to effectively deposit a layer of crystalline apatite onto the titanium substrates over a range of
pressures and jet to substrate heights. It was observed that for both processes that an increase
in particle kinetic energy producing corresponding enhancements in both deposition rate and
surface roughness. The shot peen process however produced a smooth layer of laminar
apatite, which was readily removed from the surface using a scratch adhesion test technique.
In contrast the combination of a jet of HA and abrasive powders resulted in an increase in
surface abrasion and increased mechanical interlocking of the HA into the metal surface was
observed. The mechano-chemical affect achieved resulted in a better adhered HA layer. The
surface morphology obtained using the two treatments was significantly different with an
increase in the average roughness (Ra) of ≈ 70 and 80 % for samples treated with abrasive
particles over shot peen. This difference in surface treatment is further highlighted by the
removal of the HA using an acid etch. The roughness (Ra) of the underlying titanium layer
after this removal is, on average, >175 % higher for the surface treated with the abrasive
particles during HA deposition.