Typical as-plasma-sprayed hydroxyapatite (HA) coatings along the thickness have an internal gradient structure and chemistry [Rs]. The thickness gradient structure extends from amorphous base to crystalline surface. During plasma spraying, the amorphous calcium phosphate (ACP) base is formed by rapid cooling of external molten part of flying HA spray-powder particles upon impact on cold substrate. If the flying HA spray-powder particles are heated in plasma jet above 1843 K, but below 1973 K, the ACP, tricalcium phosphate (TCP) and tetracalcium phosphate (TTCP) will co-exist in close proximity within rapidly quenched structure [Rs]. If overheating temperatures are higher than 1973 K, the CaO will be additionally present. The CaO-containing ACP phase can be considered as hydroxyl-deficient phase because it is formed from a melt which has experienced the highest temperatures in plasma jet (>1973 K) [Rs]. The ACP, TTCP and CaO produce an increase of the Ca/P molar ratio in the near-interface region of the as-plasma-sprayed HA coatings, which is known as the Ca/P ratio gradient [Rs].
Considering the applications of HA coatings as medical implants, the thickness gradient structure is unfavorable. The crystalline surface promotes poor initial bone fixation and the ACP base imparts low structural integrity of the HA coatings [Rs]. The last is of particular importance because a brittle nature of the ACP phase has been frequently held responsible for deterioration of the HA coating’s