ABSTRACT
Hydroxyapatite (HAp) is used as a bioactive ceramic coating on metallic implants in (a) total joint prostheses as an alternative to PMMA (polymethyl methacrylate) cement-based fixation, (b) dental implants for bioactive fixation, and (c) fillers for repairing bone defects. Despite the wealth of literature [1-15], however, there has not been much of a systematic study on the nanohardness (H) at the local microstructural level of HAp or HAp composite coatings on metallic substrates as measured by the nanoindentation technique. For HAp thin (≈350-650 nm) films [3, 4] on Ti and Si substrates, nanohardness data was a strongly sensitive function of film thickness and the chemical composition of the substrate. For high-velocity oxy fuel (HVOF) sprayed HAp coatings [5], the nanohardness of the coating was greater than that measured at the coatingsubstrate interface. Just the opposite trend was reported for laser-deposited HAp coatings [10, 11]. Even for laser-deposited HAp coatings, a large variation of nanohardness was observed [6, 10, 11]. HAp-CNT composites were about 30% harder than the HAp matrix material [11]. Flame-sprayed HAp coating also displayed a great variation of nanohardness (≈8-4.5 GPa) measured on both plan and cross sections [13, 14]. The microhardness of macroplasma-sprayed HAp (MAPS-HAp) [1, 2, 7, 8, 12] and MAPS-HAp composite [8, 9] also have been reported. The data were sensitive to choice of indenter shape, e.g., a Vickers [7] or a Knoop indenter [1, 12]. Recently, the author and coworkers reported synthesis characterization along with nanomechanical properties evaluated on the plan section of MIPS-HAp coating on surgical-grade SS316L substrates [15-18].
