This work investigates the influence of plasma-sprayed deposits on the fatigue life of coated specimens. Hydroxyapatite (HA) and TiO 2 were deposited on dog-bone shaped substrates under different spraying conditions while measuring in-flight particle temperature and velocity. The coated specimens were then subjected to cyclic bending with constant deflection and the number of cycles to failure was recorded. It was found that the higher the temperature and velocity of particles during spraying, the greater the improvement in fatigue life up to a maximum of 46% compared to uncoated samples.

Changes in phase composition of titania (TiO 2 ) coatings were studied with respect to in-flight particle characteristics during atmospheric plasma spraying. A CCD camera was used to record the average temperature and velocity of TiO 2 particles within the plasma jet for six combinations of spraying parameters selected according to Taguchi design of experiments. Phase composition of the coatings was assessed by means of X-ray diffractometry and numerically specified using Rietveld analysis. Changes in composition between the powders and coatings were observed with respect to the in-flight properties of the titania particles during spraying. In general, it was shown that the amount of impurity phases (hematite, quartz) present in the powder decreased after deposition and that in-flight temperature has only a moderate effect on the phase composition of TiO 2 coatings. For comparison purposes, additional coatings were deposited by cold spraying and phase composition changes were assessed.

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.

The major problems with plasma sprayed hydroxyapatite (HA) coatings for hard tissue replacement are severe HA decomposition and insufficient mechanical properties of the coatings. The loss of crystalline HA after high temperature spraying is due mainly to the loss of OH- in terms of water. The present study employed steam to treat HA droplets and coatings during both in-flight and flattening stages. The microstructure of the HA coatings and splats was characterized using scanning electron microscopy, Raman spectroscopy, Fourier transform infrared spectroscopy, and X-ray diffractometry (XRD). Results showed that a significant increase in crystallinity of the HA coating was achieved through the steam treatment (e.g., from 58% to 79%). The Raman spectroscopy analyses on the individual splats and coatings indicate that the mechanism involves entrapping of water molecules by the individual HA droplets upon their impingement. It further suggests that the HA decomposition has already taken place before the impingement of the droplets on pre-deposited materials or the substrate. The improvement in crystallinity and phases, e.g., from tricalcium phosphate and amorphous calcium phosphate to HA, was achieved by reversing the HA decomposition through providing extra OH-ions. Furthermore, the steam treatment during the spraying also accounts for remarkably increased adhesion strength from 9.09 MPa to 23.13 MPa.

Nanostructured titania (TiO 2 ) coatings were produced by high velocity oxy-fuel (HVOF) spraying. They were engineered as a possible candidate to replace hydroxyapatite (HA) coatings produced by air plasma spray (APS) on implants. They exhibited mechanical properties, such as hardness and bond strength, much superior to those of APS HA coatings. In addition to these characteristics, the surface of the nanostructured coatings exhibited regions with nanotextured features originating from the semi-molten nanostructured feedstock particles. This nanotexture is considered an asset, due to its better interaction with the adhesion proteins of the osteoblast cells, such as fibronectin, which exhibit dimensions in the order of nanometers. Osteoblast cell culture demonstrated that this type of coating supported osteoblast cell growth and did not negatively affect cell viability.

A large variety of bioceramics have been successfully utilized as implant materials for promoting fixation of bony tissues. Different bioceramics exhibited markedly different proliferation rates of the osteoblast cells in vitro. Clarification of the mechanism about the attachment and proliferation/differentiation of the cells would contribute to selecting suitable biomaterials for hard tissue replacement. Nanostructured hydroxyapatite (HA) coatings have been produced and they have shown promising mechanical performances. The present study employed the 2-dimentional gel electrophoresis assay for comprehensively studying the proteomics of the cells proliferated on the nanostructured HA coating. Results showed that the nanostructured HA coatings promoted proliferation of the osteoblast cells. Alkaline phosphatase (ALP) assay revealed an increased ALP activity of the proliferated viable cells, and obviously the presence of the nanosized pores can enhance the anchoring and stretching of the cells. No obvious difference in the 2-D gel maps taken for the cells proliferated on the HA coating and for control can be found. This in turn suggests that the nanostructured HA coating induces minor changes in the cells. The promoted cell proliferation by the HA coating could then be possibly explained by the affected apoptosis/cell cycle of the cells.

A good adhesion of plasma sprayed hydroxyapatite (HA) coating on Ti-based alloy is crucial for ensuring highly-reliable non cemented implants in the biomedical industry. In the present work, the laser shock adhesion test, namely LASAT, has been applied to investigate the interface strength of plasma sprayed HA coatings. This contact less method allowed a rapid assessment of the HA coating adhesion on simple coated plates. Varying the laser energy to impact the substrate and to generate the interface decohesion, a LASAT adhesion threshold can be determined for the highest laser fluence (J/m²) for which no debonding of the coating occurred. This qualitative and discerning LASAT procedure has been carried out on HA coatings to investigate the role of various interfaces on the adhesive property of the HA/Ti bond. According to the LASAT analysis, a surface roughness prepared with medium or coarse grit-blasting did not influence drastically the adhesion threshold while smooth pre-oxidized specimens LASAT threshold were near to those obtained with a Ti bond-coat. These thresholds also corresponded with the highest adhesion measured in this study. In addition, pre-heating treatment of substrates just prior to spraying up to 270°C did not exhibit a significant difference with grit-blasted HA/Ti interface. Further investigations (SEM, XRD) was also achieved to investigate the interface characteristics before and after the laser treatment. Sample cross-sections of laser shocked specimens were examined in detail, right at the impact location and within the debonding area to assess the fracture feature. This complementary materials analysis permitted to establish the relevance of the LASAT test as a fast and easy-to-use method devoted to the design or the control of highly adhesive HA coatings. Preliminary experiments to apply the LASAT method in liquid environment is described. Further work is on progress to implement an in situ adhesion testing of HA coating in simulated body fluid.

Nanostructured hydroxyapatite (HA) and HA/nano-zirconia powders were sprayed by both plasma and HVOF spraying. Microstructure characterization on the nanostructured coatings were conducted using scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffractometry (XRD), and Raman spectroscopy. Results showed that the nanostructures of the HA feedstock retained to some extent after the thermal spraying depending on the melt state of the powders. The microstructural features of individual HA splats were also characterized through TEM observing both as-sprayed and ion-milled splats. A nanostructure (with ~ 30nm grains) within the surrounding parts of the HA splats was revealed, while significant grain growth (a size up to 5 µm) depending on flattening state was found at the center of the splats. It also revealed that the nano-sized zirconia particles (< 90 nm) retained their fine size after HVOF deposition, and were evenly distributed within the coating. The crystallite size of tetragonal zirconia in the coating was found to be less than 13 nm. The biocompatibility of the coatings was characterized using in vitro incubation testing in simulated body fluid and osteoblast cell culturing. It showed that the presence of the nanostructures in the coatings improved the stability of the coatings (delayed the dissolution). The addition, the presence of the nanostructures contributes to improved mechanical properties.

The adhesion strength of a ceramic coating deposited through direct spraying on a roughened substrate is a key issue in the manufacture of high-quality coatings on industrial components. The purpose of this work was to develop a rapid and discerning procedure for establishing adhesion level of a ceramic coating on a metallic substrate. The Laser Shock Adhesion Test, namely LASAT, was successfully applied to ceramic coatings with irradiation impact on the metallic side. Suitable parameters were found to determine the LASAT adhesion threshold using a standard Nd:YAG laser source. With a laser-irradiated area of several millimetres in diameter, it allowed assessment of the coating threshold on several areas of a coated plate sample. A control procedure for a qualitative assessment of coating adhesion was developed. This testing procedure could be easily used in industry, with possible location of the LASAT unit near to the spraying booth, for a direct production control on coated sample to improve the tracability of manufactured parts. Additional work was carried out to investigate a quantitative approach of the LASAT test to ceramic coating. The purpose was to simulate the shock wave propagation with the RADIOSS® code (a 3D software originally developed for car crash simulation). This code was implemented to calculate the velocity of the material and corresponding pressure throughout the substrate and the coating during the shock wave release (less than 2 ms). Experimental VISAR profiles ('Velocity Interferometer System for Any Reflector') were monitored in the straight direction of the laser-irradiated area on the rear side. These experimental signals (velocity measures) of the ceramic coating could be fitted and compared with a fairly good agreement with simulated profiles obtained by RADIOSS®. This modelling work was the first step towards a more comprehensive coating adhesion strength calculation in the future.

A novel thermal plasma process, based on inductively coupled plasma torch is employed for producing nano-sized calcium phosphate powders from spray-dried hydroxyapatite (HA) feedstock. The phases during plasma process of HA feedstock under different working conditions have been studied. It is revealed that amorphous calcium phosphate is predominant in the nano-sized powders. HA, α-TCP and CaO are also detected in the nano-sized powders. After heat treatment at 800 °C in air, β-Ca 2 P 2 O 7 (β-DCP) and HA are found to dominate in the powders. The presence of β-DCP is attributed to the HA feedstock directly decomposed into DCP in the plasma flame, and this phase formed amorphous calcium phosphates region by the rapid quenching process. This region crystallized into β-DCP after heat treatment.

Spark-plasma sintering (SPS) was adopted in this study as a rapid post-spray treatment for yttria stabilized zirconia (YSZ) electrolytes prepared through the direct current (dc) plasma spray process. The lamellar microstructure in the as-sprayed samples was found to significantly reduced the ionic conductivity of the YSZ electrolytes. However, the ionic conductivity (at 1053 °C) increased sharply from 0.065 S/cm for the as-sprayed electrolyte to 0.122 S/cm for electrolyte post-spray treated through SPS at 1400°C for 3 minutes. These phenomena were attributed to the microstructure transformation from lamellar structure of the as-sprayed samples to equi-axial type granular structure of the post-spray samples treated by SPS. Correspondingly, porosity reduced from ~ 10.72 % in as-sprayed coating to ~ 1.38% in the SPS sample treated at 1400°C for 3 min. Majority of the pores in the SPS sample were also found to have contracted to a narrow size range 0.03 – 1 µm. AC impedance spectroscopy demonstrated the effect of the microstructure modification between samples treated in the SPS at temperature 1200, 1300 and 1400°C for 3 minutes with the concomitant reduction of the resistivity, which is consistent with the microstructure changes of the YSZ electrolyte from lamellar structure to granular structure. Overall, the results show that spark-plasma-sintering (SPS) is an effective and rapid post-spray treatment to improve the relative density and electrical properties of plasma sprayed YSZ electrolytes.

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.

Nanostructured and conventional hydroxyapatite (HA) feedstocks were evaluated to determine the effects of feedstock structure on the processing, properties and performance of coatings produced by atmospheric plasma spraying. The structure of the feedstocks was characterized using scanning electron microscopy (SEM). It was found that the nanostructured feedstock particles were formed by an agglomeration of nanostructured fibers having dimensions of less than 500 nm in length and below 100 nm in width. The average particle temperatures and velocities were measured during plasma spraying and found to be ~2650°C and ~315 m/s for the both feedstocks. The mechanical, microstructural and biocompatibility characteristics of coatings deposited on Ti-6Al-4V substrates were evaluated. The hardness was measured using Vickers indentation. The bond strength was determined via a tensile adhesion test. Microstructural characteristics of the coatings and their porosity levels were evaluated using SEM and image analysis. Phase analysis was carried out via X-ray diffraction (XRD) and aided by energy-dispersive X-ray analysis (EDS). The in-vitro behavior of these coatings was investigated in a simulated physiological solution in an attempt to simulate the environment of an implant in the human body.

Hydroxyapatite (HA)/titania composite coatings were deposited on titanium alloy substrates using the high velocity oxy-fuel (HVOF) technique. Chemical reactions between the mechanically blended HA and titania particles in the HVOF stream were analyzed. Qualitative phase analysis through X-ray diffraction (XRD) on the composite coatings showed that the chemical reaction between titania and HA occurred during the impingement stage. High temperature differential scanning calorimeter (DSC) analysis revealed that the reaction temperature was 1410 C. The activation energy of the chemical reaction between HA and titania demonstrated a value of 5441.46 kJ/mol obtained through the multiple-heating-rate method. Chemical bonding caused by the reaction between the components was suggested, which may be mainly responsible for the trapping of titania particles during the impingement. Transmission electron microscope (TEM) observation identified the reaction zone and phase distribution area within the HA/titania composite coatings. It demonstrated that the reaction products located around titania were beneficial for the improvement of coating structure. Furthermore, in vitro bioactivity of the HA/titania composite coatings in simulated body fluid (SBF) was revealed. Results showed that the coatings were fully covered by a bone-like apatite layer after 7 days’ incubation.

Plasma spraying is a fast and inexpensive process for fabricating YSZ electrolyte for SOFCs. In this investigation, free-standing plasma sprayed YSZ disks are treated by spark plasma sintering at different temperatures, soak times, and loading cycles. SEM examination shows that the lamellar microstructure of the as-sprayed zirconia is converted to a predominantly granular structure with no significant phase changes as per XRD analysis. Microhardness and laser flash diffusivity measurements show that the SPS treatments also improve YSZ layer density, tensile modulus, and thermal conductivity. Paper includes a German-language abstract.

This paper examines the influence of spark plasma sintering (SPS) on plasma-sprayed hydroxyapatite (HA) coatings. For comparison purposes, a conventional heat treatment is also carried out. The surface microstructure as well as the crystallinity of each layer is determined by means of SEM and XRD analysis. Test results show that the crystallinity of the layers increases with increasing SPS temperature up to 800 °C and a large amount of β tricalcium phosphate is formed. Paper includes a German-language abstract.

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.

In the present study, hydroxyapatite coatings were deposited on Ti-6Al-4V alloy substrate by high velocity oxy-fuel (HVOF) spray technique. The as sprayed HA powders and coatings were analyzed with the aim to reveal the melting state of HA powders and its influence on coating properties. Scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR) and X-ray diffraction (XRD) were employed for the characterization of the starting powders and as-sprayed coatings. Differential scanning calorimetry (DSC) was performed to determine the recrystallization temperature of the amorphous phase in HVOF HA coating. Results show that different melting state of HA powders can be achieved through altering HA powder size and/or spray parameters. XRD result reveals that the as sprayed HA coating made from large powders with size of ~50 µm is composed of crystalline HA and very small amount of a-tricalcium phosphate (TCP). While the coatings deposited using fine powders around 30 μm demonstrated a lot of amorphous phase besides crystalline HA and small amount of a-TCP. The recrystallization temperature of the amorphous phase in HA coating is ~720°C. The adhesive strength of the HVOF sprayed HA coatings is ~31MPa and is largely dependent on the melting state of HA powders. This suggests that the fully melted state of the feedstock can result in the formation of amorphous phase, and simultaneously decrease the adhesive strength. It also suggests that the melted fraction of the powders is the most critical factor influencing the adhesive strength and phase composition of HVOF HA coatings. The partial melting state of HA powders is beneficial in terms of adhesive strength and crystallinity.

The aim of this paper is to study the behavior of two different sets of as-sprayed coatings obtained from two different starting HA powders, in a defined simulated body fluid (SBF) for a period of 28 days. Spray-dried hydroxyapatite (SDHA) and spheroidized hydroxyapatite (SHA) were the two selected powders. A Controlled Atmosphere Plasma Spraying system (CAPS) was use for the production of the coatings. Effect of pressure and surrounding atmosphere during the spraying of the coatings, were also evaluated. During the in-vitro test, dissolution of the coatings and precipitation of a poorly crystallized apatite in a preferential crystallographic orientation ([001] direction) was observed for the two sets of coatings. Dissolution of the coatings was measured by: 1) weighing the specimen before and after soaking and 2) by measuring the calcium ion concentration in the SBF solution with the Inductively Coupled Plasma-Atomic Emission Spectrometer (ICP-AES). Scanning Electron Microscopy (SEM) exposed the morphology of the coatings and X-Ray diffraction (XRD) and Fourier Transform Infrared Spectrometer (FTIR) revealed their structure and composition.

The thermal shock resistance of thermal barrier coating depends strongly on the shear stress generated by the thermal expansion mismatch between the ceramic and bond coat layer. Applying a functionally graded structure composing of NiCoCrAlY and YSZ along the coating can mitigate this effect. The paper studied the improvement of thermal shock properties with different number of intermediate layers (2 - 4) added over the temperature cooling range 900 - 30 °C. Acoustic emission (AE) technique was utilised to determine the moment of occurrence of damage within the coatings, and thus help to identify the corresponding failure mechanisms. Cross section analysis of the coatings after thermal shock tests revealed that the coatings generally failed by two mechanisms: edge delamination and segmentation of zirconia topcoat. Failures in the coatings with 2 and 3 intermediate layers (total of 3 and 4 layers respectively in the overall coating) were dominated by edge delamination while the coating with 4 intermediate layers exhibited only segmentation of the top zirconia layer. This points to the fact that interfacial stresses were not critically affecting the integrity of the 5-layer coating (4 intermediate layers plus the ceramic top layer). The cumulative and rate energy results showed that the energy released by the coatings during the thermal shock tests were in the order of 3-layer coating > 4-layer coating > 5-layer coating. The 5-layer coating had demonstrated the best thermal shock resistance among the four coatings.