Plasma-sprayed coating of hydroxylapatite on super austenitic stainless steels

Ti, Ti ahoy, Co-Cr alloy and 316L SS are used for metallic prostheses, and other various types have been developed. However, the use of 316L SS is decreasing, owing to its low pitting resistance. Ti and Ti alloy are not used in the head part of hip joints because of their low corrosion-wear resistance. However, despite this problem, Ti and Ti6Al4V are reported to be used most widely today. In this study, super austenitic stainless steel (SASS) was used as a substrate with a very high corrosion resistance and good mechanical properties. Also, the manufacturing process of SASS is easier than that of Ti and Co-Cr alloy. HA coatings on these substrates are produced using the plasma-spraying technique. The interacting parameters have been shown to contribute complicatedly to coating properties. As the current was increased, the coating thickness gradually increased, and with an increasing number of splats. The increased spray distance augmented the melting degree of the powder that, in turn, decreased the surface roughness of its coating layer. The Ca/P weight ratio of the coating layers increased from 2.162 in the powder state to about 2.8-3.2 in the coating. Although unobserved in the powder, CaO and Ca4P2O9 were seen in the coating layer. Also a decrease in HA phase fraction along with crystallinity took place. In the cases of CaO and Ca4P2O9 phases, the increase in current and spray distance brought about an increase in melting amount, producing an increase in high-temperature phases other than the HA phase. From the immersion test, it was found that the longer the immersion time, the more gradual is the active reaction from the interface of the coating layer and the substrate. It was also noted that pitting was generated from an exposed part of the substrate through the connected pores. Therefore, it can be deduced that for bone ingrowth and high corrosion resistance, a graded coating that maintains a high level of surface roughness on the coating and dense around the interface is required. (C) 1998 Elsevier Science S.A. All rights reserved.