Plasma generated by an SG-100 torch was applied to a spray suspension formulated with the use of ZrO 2 +8 wt% Y 2 O 3 (8YSZ) solid phase. The solids had a mean size of about 4.5 μm and were obtained by milling of commercial Metco 204 NS powder. The suspension was formulated with 20 wt% solid phase, 40 wt% water and 40 wt% ethanol. The plasma spray parameters were optimized with the electric power equal to 40 kW, working gases composition Ar 45 slpm and H 2 5 slpm, spray distance varying from 40 to 60 mm, and torch scan linear speed varying from 300 to 500 mm/s. Coatings with thicknesses ranging from 51 to 106 μm were sprayed onto stainless steel substrates. The porosity of the samples was found from the image analysis of metallographically prepared cross-sections of the samples to be in the range of 8 to 12%. Thermal diffusivity was measured with the use of the commercial NanoFlash system in the temperature range from room temperature to 523 K. The measurements were made with the use of the coatings sprayed on the substrate, and a 2-layer numerical model was developed to determine thermal diffusivity of the coatings. The diffusivity was in the range from 0.196 × 10 -6 to 0.352 × 10 -6 m 2 /s in room temperature depending on the spray parameters. The obtained data were then associated with the literature data of density and specific heat and experimental porosity to find thermal conductivity, which was in the range of 0.47 to 0.86 W/(mK) at room temperature, depending on the spray run.

Inductively coupled radio frequency (RF) plasma spraying, powered by high-frequency oscillating electrical current, performed an important role in fine powder manufacture. It was used in the present study to prepare fine spherical bioceramic powders of hydroxyapatite (HA) whose chemical composition similar to those of natural bone. The as-sprayed powders consisted of both micron-sized spherical particles and nano-sized particles. In addition to the spheroidization effect, rf plasma treatment led to the decomposition of HA into secondary calcium phosphate phases including tri-calcium phosphate (TCP), tetra-calcium phosphate (TTCP) and calcium oxide (CaO). The microstructure investigation showed that the spheroidized particles were either fully dense or hollow structure with a shell. The reason for the formation of hollow spheres was contributed to the higher density of the solidifying surface layer compared with the molten phase during solidification.

Ultra-fine hydroxyapatite (HA) powders were produced with radio frequency (RF) suspension plasma spraying (SPS). This novel technique utilizes the inherent properties of the RF plasma enabling axial feeding of the suspension into the plasma producing spherical ultra-fine HA powders. These powders were examined by XRD and Rietveld analysis using the Rietquan 2.3 Quantitative Analysis software package. The aim of the analysis was to determine the various amounts of decomposed phases and amorphous content after SPS of HA. Results showed that the amount of decomposed phases rose up to a plate power of 15 kW there after decreasing at higher plate powers. The amorphous phase however kept increasing with plate power reaching about 35 wt.% in the powders sprayed at 21 kW. These trends have led to the belief that the phase content and hence, the characteristics of the powders are controlled mainly by the competitive processes of decomposition and melting and evaporation within the plasma. The morphology of the powders was also observed through TEM and changes in molecular structure were investigated by FTIR. DSC was carried out to observe the crystallisation of amorphous calcium phosphate into HA.

Ultra-fine hydroxyapatite powders were successfully synthesized using radio frequency (RF) suspension plasma spraying (SPS). This novel technique utilises the inherent characteristics of the RF plasma to axially feed and spheroidise a liquid suspension to produce spherical ultra-fine HA powders. This offers an alternative approach over conventional D.C. and flame spheroidising techniques which are better suited for solid feed stocks. Rietveld analysis was subsequently applied using Rietquan Quantitative Analysis software package to determine the amount of decomposed phases and amorphous content of the as-sprayed powder. This was also compared against quantitative XRD analysis employing internal and external standards. However, pure phases needed for calibration is scarce and amorphous calcium phosphate (ACP) is virtually impossible to isolate. In addition, the long and laborious task of obtaining calibration curves makes this technique unpopular. Nevertheless, conventional quantitative phase analysis (QPA) was carried out, using relative peak height ratios of HA and the phase involved, but the calculated decomposition only shows relative trends for a particular parameter variation. Determining the actual phase content is critical because of possible variations in biological responses when used as coatings and inserts in restorative orthopaedic implants. Varying tissue responses can arise from decomposed phases such as α and β-tricalcium phosphate (TCP) and tetra-calcium phosphate (TTCP) as well as ACP which generally have higher solubility as compared to crystalline. QPA via the Rietveld method provides a powerful tool that offers the user simultaneous quantitative phase determination of multiphase systems containing amorphous content. Unlike XRD QPA, the amorphous content could be indirectly calculated using crystalline alumina standard. XRD QPA results showed that decomposition generally rose with plate power without considering the amorphous content. With Rietveld QPA, the results showed an initial rise in decomposition before decreasing at higher plate powers. The amorphous phase content was calculated at different plate powers and concentration of suspension with the aid of alumina as an external standard. Results showed that the amorphous content increased substantially at higher powers. This study demonstrates the ability of Rietveld analysis to completely quantify all associated amorphous and crystalline phases within a multiphase system for any thermally treated material.