Although research has been reported on pre and post processes for cold spraying, quantitative results are lacking. This study aims to quantify the effect of pre-coat grit blasting and post-spray stress relief and annealing treatments on cold-spray coating microhardness, bond strength, and microstructure. It was found that stress relief treatments reduce hardness, but have little effect on adhesion. Annealing also reduces hardness, but is shown to significantly improve adhesive bond strength. Grit blasting, on the other hand, was found to have a detrimental effect on tensile adhesion strength with little impact on coating microhardness.

Deposition of amorphous aluminium powder using cold spray technology as a corrosion prevention measure was studied. Amorphous aluminium (Al-Ni-Ce) powder was successfully deposited on 7000-series aluminium substrates using cold spray parameters of 1.7 MPa under compressed air and temperature of 450°C. The coatings were subjected to tensile bond strength measurement and comparative studies with cold sprayed pure Al6061 coatings were conducted. The results obtained showed that the amorphous aluminium coatings exhibited better adhesive strength. In addition, salt-water immersion test was conducted. The Al-Ni-Ce coating not only demonstrated better corrosion resistance but also exhibited evidence of passivation of surface imperfections such as scratches in the coatings.

Cold spray is a material deposition process that uses a high pressure, high velocity gas jet for the deformation and bonding of particles. However, deposition of brittle or hard materials such as ceramics has not been successful: unless they are co-deposited with a ductile matrix material. This paper examines the WC particle size and its influence on the deposition of Co-based cermets. Micro- and nano-structured powders with similar Co content were employed. Varying the WC particle size influenced significantly the deposition efficiency of the coating process. Micrometer-structured WC-Co feedstocks did not permit coating build up when processed under comparable or elevated thermal spray parameters used for the nanostructured WC-Co feedstocks. In addition, micrometer-structured WC-Co coatings exhibited a conjoint erosion and deposition effect on the surface. Fine WC particles (<1 μm) were observed near to the substrate interface and larger WC particles (1-2 μm) in the vicinity of the coating surface. These observations indicate the existence of a critical WC particle size for deposition by the cold spray method and that the size criteria arises due to the formation and cohesion mechanisms within the coating layer.

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.

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.

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.

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.

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.

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.

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.

As ceramic materials, the use of hydroxyapatite (HA) in clinical applications is severely limited by its intrinsic poor mechanical properties. The incorporation of some bioinert ceramics is believed to be a way to improve the mechanical reliability of HA matrix. HA coatings with titania addition were produced by using high velocity oxy-fuel (HVOF) spraying process in the present study. The mechanical properties of the as-sprayed coatings in terms of adhesive strength, shear strength and fracture toughness were investigated aiming to reveal the reinforcing effect of the titania addition in HA coatings. Qualitative phase analysis through X-ray diffraction (XRD) showed that mutual chemical reaction between TiO 2 and HA occurred during coating formation, from which CaTiO 3 was resulted. Totally unmelted titania powders were observed which suggests that the mutual reaction locates at HA/TiO 2 splats' interface. Significant influence of coating microstructure on mechanical properties was revealed. As the content of titania in HA coatings reached 20vol%, the adhesive strength decreased largely. As the content of titania reached 30vol%, the further augmentation of the adhesive strength of is possibly resulted from the improved coating microstructure. The fracture toughness exhibited the values of 0.48 Mpa ⋅ m ½ 0.60 Mpa ⋅ m ½ and 0.67 Mpa ⋅ m ½ for pure HA coating, 10vol% TiO 2 blended HA coating and 20vol% TiO 2 blended HA coating, respectively.

This paper compares two types of hydroxyapatite (HA) composite coatings, HA/Ti-6Al-4V and HA/Y-ZrO2. The powders used in the study were prepared using a slurry process then deposited by plasma spraying. The resulting coatings were characterized based on their microstructure, mechanical properties, and biocompatibility. Both composite coatings performed better than pure HA coatings in tensile adhesion and indentation tests. Testing also revealed that the HA/Y-ZrO2 coatings had favorable strength and fracture toughness and that the HA/Ti-6Al-4V coatings had good affinity to living tissue and sufficient mechanical strength.

This paper investigates the influence of spraying current and gun transverse speed on the characteristics of the hydroxyapatite (HA) and Ti-6A1-4V composite coatings. Two different kinds of feedstock are being used; HA and Ti-6A1-4V in the ratio of 80 v/t. %: 20 wt. % and 50 wt. %: 50 wt. % respectively. The paper investigates the influence of the plasma spray currents at 600 A, 800 A, and 1000 A and the influence of the feed speeds of the spray gun at 250 mm/s, 500 mm/s, and 750 mm/s. It examines the powders or coatings using an Instron machine, XRD, and scanning electron microscopy. It is found that a spraying current of l000 A, relative to 600 A and 800 A, gives a coating that is less porous and composed of a well-splayed structure. It also yields the highest bond strength among the coatings. Paper includes a German-language abstract.

The attractive bioactive properties of HA are significantly reduced upon plasma spraying because of the phase transformation that accompanied the deposition process. One major factor that influence the extent to which the transformation occur appears to be the morphology and physical states of the HA raw powders. This paper reports the study on the influence of powder morphology and property on the fracture behaviour and tensile adhesive strength of plasma sprayed HA coatings. Three types of powders were used in the study; calcined HA (CHA), spray dried HA (SDHA) and flame spheroidised HA (SHA). The particle size range of 53 - 75 μm was employed for all 3 types of powders to effect an accurate comparison of the powders. Results show that the cohesive bond strength of the SHA coating was the highest because of the denser microstructure created by well-formed lamella splats. A correspondingly lower bond strength was recorded with less coherent coatings generated by agglomerated CHA and SDHA powders.