Abstract
This paper reports the characteristics and properties of two different sets of hydroxyapatite (HA) coatings obtained by thermal spraying two different HA powders, spray dried HA (SDHA) and spheroidisided HA (SHA), at various chamber pressures. The evaluation of the coatings following post-spray treatment at 800°C is also included. Comparisons to the results from the as sprayed coatings were made to show the effects of post-spray heat treatment. The median particle sizes for SDHA and SHA were 35 and 47 μm, respectively. The main difference between the powders is their density and morphology. The SHA powder provides higher density and spherical morphology, and a more porous and rough surface is present in the SDHA powders. These two powders were sprayed using a controlled atmosphere plasma spraying (CAPS) system in the inert plasma spray (IPS) and high pressure plasma spray (HPPS) modes. Four different pressures of 100, 150, 200, and 250 kPa were applied in an argon atmosphere. Post-spray heat treatment was performed to increase the crystallinity of the coatings, to homogenise their structure, and to convert other calcium phosphate phases to HA. Different techniques were used to characterise these coatings. Scanning electron microscopy (SEM) was used to observe surface and cross-section morphology of both powder and sprayed coatings. The phase and chemical characteristics of the samples were evaluated by X-ray diffraction (XRD) and Fourier transform infrared spectrometry (FTIR). Finally, in order to have a qualitative measure of the integrity of the coatings, Vickers hardness testing was performed on polished cross-sections of the coatings.
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