Surface and Coatings Technology 182 (2004) 227–236 0257-8972/04/$ - see front matter  2003 Elsevier Science B.V. All rights reserved. doi:10.1016/j.surfcoat.2003.08.081 Characterization of hydroxyapatite y nano-zirconia composite coatings deposited by high velocity oxy-fuel (HVOF) spray process H. Li , K.A. Khor *, R. Kumar , P. Cheang a a, b b School of Mechanical and Production Engineering, Nanyang Technological University, 50, Nanyang Avenue, Singapore 639798, Singapore a School of Materials Engineering, Nanyang Technological University, 50, Nanyang Avenue, Singapore 639798, Singapore b Received 15 April 2003; accepted 11 August 2003 Abstract Hydroxyapatite (HA)ynano-zirconia composite coatings were successfully produced through high velocity oxy-fuel (HVOF) spraying of radio frequency (RF) plasma synthesized powders. Microstructure characterization and morphology analysis on the nano-structured coatings were conducted using scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffractometry (XRD), and differential scanning calorimetry (DSC). Elastic modulus of the coatings was determined using nano-indentation on polished cross-sections, which revealed Young’s modulus value of ;130 GPa. Results also showed that the nano-sized zirconia particles (-90 nm) did not coarsen drastically after HVOF deposition, and were uniformly distributed throughout the coating. The crystallite size of tetragonal zirconia in the coating was found to be less than 13 nm. Furthermore, HA phase decomposition and chemical reaction between HA and zirconia was virtually undetected, which is beneficial towards the biological performance of the coatings. XRD analysis together with Rietveld refinement quantitatively revealed the content of the phases present in the coatings, including ;1.2 wt.% CaZrO . The conversion temperature of amorphous calcium phosphate 3 to HA was found to be ;750 8C and the ancillary influence of the heat treatment on coating structure and properties were revealed.  2003 Elsevier Science B.V. All rights reserved. Keywords: Hydroxyapatite; Zirconia; Nanostructured coatings; Ultrafine powders; Rietveld; Plasma synthesis; High velocity oxy-fuel (HVOF) 1. Introduction The incorporation of bioinert ceramics, such as yttria stabilized zirconia (YSZ), alumina, and titania into a hydroxyapatite (HA) matrix has demonstrated signifi- cant improvement in mechanical properties w1–3x with- out substantial compromise in biocompatibility w4x. However, mutual reaction between calcium phosphates (CP) and YSZ seemed inevitable w2,3x and phase trans- formation of zirconia invariably took place during ther- mal spray coating deposition w5x, which is detrimental on the basis that crystallographic transformation gener- ated residual tensile stresses in the coating. In recent years, nanostructured coatings attracted intense interest due to their enhanced mechanical prop- erties w6,7x. The improvement can be related to the classical Hall–Petch relationship where decrease in par- *Corresponding author. Tel.: q65-790-5526; fax: q65-791-1859. E-mail address: mkakhor@ntu.edu.sg (K.A. Khor). ticle size enhances the strength and toughness of ceram- ics w8,9x. Researchers have found that the main toughening mechanism in particulate reinforced com- posites was either crack inhibition or crack deflection w10x. Furthermore, a previous in vitro study has shown that nano-sized ceramics (Al O , TiO ) possess signifi- 2 3 2 cant capability of enhancing osteoblast adhesion on them w11x. Therefore, nano-sized particles within a biocom- patible coating could bring about positive effects. How- ever, available reports rely mostly on the rapid resolidification of sprayed materials upon their impinge- ment on the prior deposited material, or substrate to form nano grains within the coating w7x. Even though nano-sized particles are produced through advanced powder synthesis techniques, the nano particles cannot be fed directly into a thermal spray gun to generate a nano-structured coating. Recent attempts involved form- ing agglomerates of nano particles into 30 mm aggre- gates w7,12x; however, excessive grain growth was