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24The biocompatibility and osteoconductivity of hydroxyapatite (HA) coatings have led to 25 their use in a wide range of applications in dentistry and orthopaedics. One such application 26 is for the uncemented fixation of implants, where coatings are commonly applied to 27 titanium implants using a plasma thermal spraying process. The spraying process is 28 affected by a large number of parameters leading to highly complex process -property -29 structure relationships. In a step forward from one-at-a-time analyses, this study used 30Design of Experiment (DOE) methodology to investigate the simultaneous effects of key 31 plasma spray process parameters on hydroxyapatite coatings for biomedical applications. 32The effects of five plasma spray process parameters (current, gas flow rate, powder feed 33 rate, spray distance and carrier gas flow rate) on the roughness, crystallinity and purity of 34 hydroxyapatite coatings was determined using a fractional factorial design. The results of 35 this study enabled identification of consistent and competing influences within the process 36 and the identification of some first order interactions. In particular, the diffuse particle size 37 of the HA feedstock powder was found to influence the responses observed within the 38 parameter range investigated. The roughness of HA coatings was found to relate to the 39 particle velocity and the degree of particle melting occurring, withhigher coating roughness 40 resulting when current was high, gas flow rate was low and powder feed rate was high. 41Highest coating crystallinity resulted at high current, low spray distance and low carrier 42 gas flow rate. Under these conditions deposition of larger HA particles resulted leading to 43 higher amounts of bulk crystalline material and the low spray distance increased the 44 substrate temperature allowing amorphous material to recrystallise. Coating purity relates 45 directly to thermal decomposition of the particles within the plasma jet with a high purity 46 3 coating resulting at low particle temperatures i.e at the lower ranges of powder feed rate, 47 spray distance and carrier gas flow rate. This study thus brings greater clarity on the effects 48 of plasma spray process parameters on the properties of resultant hydroxyapatite coatings. 49 50 Keywords 51Plasma spraying, hydroxyapatite, Design of Experiment (DOE) 52 53

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