Applications and Developments of Thermal Spray Coatings for the Iron and Steel Industry.

The steel making processes involves extreme and harsh operating conditions; hence, the production hardware is exposed to degradation mechanisms under high temperature oxidation, erosion, wear, impact, and corrosive environments. These adverse factors affect the product quality and efficiency of the steel making industry, which contributes to production downtime and maintenance costs. Thermal spray technologies that circumvent surface degradation mechanisms are also attractive for their environmental safety, effectiveness and ease of use. The need of thermal spray coatings and advancement in terms of materials and spray processes are reviewed in this article. Application and development of thermal spray coatings for steel making hardware from the molten metal processing stages such as electric arc and basic oxygen furnaces, through to continuous casting, annealing, and the galvanizing line; to the final shaping process such as cold and hot rolling of the steel strips are highlighted. Specifically, thermal spray feedstock materials and processes that have potential to replace hazardous hard chrome plating are discussed. It is projected that novel coating solutions will be incorporated as awareness and acceptance of thermal spray technology grows in the steel making sectors, which will improve the productivity of the industry.

Microparticles of High Entropy Alloys Made by Laser-Induced Forward Transfer.

The controlled deposition of CoCrFeNiMo0.2 high-entropy alloy (HEA) microparticles was achieved by using laser-induced forward transfer (LIFT). Ultra-short laser pulses of 230 fs of 515 nm wavelength were tightly focused into ∼2.4 µm focal spots on the ∼50-nm thick plasma-sputtered films of CoCrFeNiMo0.2. The morphology of HEA microparticles can be controlled at different fluences. The HEA films were transferred onto glass substrates by magnetron sputtering in a vacuum (10-8 atm) from the thermal spray-coated substrates. The absorption coefficient of CoCrFeNiMo0.2α≈6×105 cm-1 was determined at 600-nm wavelength. The real and imaginary parts of the refractive index (n+iκ) of HEA were determined from reflectance and transmittance by using nanofilms.

Antibacterial Longevity of a Novel Gallium Liquid Metal/Hydroxyapatite Composite Coating Fabricated by Plasma Spray.

Hydroxyapatite (HAp)-coated metallic implants are known for their excellent bioactivity and osteoconductivity. However, infections associated with the microstructure of the HAp coatings may lead to implant failures as well as increased morbidity and mortality. This work addresses the concerns about infections by developing novel composite coatings of HAp and gallium liquid metal (GaLM) using atmospheric plasma spray (APS) as the coating technique. Five weight percent Ga was mixed into a commercially supplied HAp powder using an orbital shaker; then, the HAp-Ga particle feedstock was coated onto Ti6Al4V substrates using the APS technique. The X-ray diffraction results indicated that Ga did not form any Ga-related phases in either the HAp-Ga powder or the respective coating. The GaLM filled the pores of the HAp coating presented both on the top surface and within the coating, especially at voids and cracks, to prevent failures of the coating at these locations. The wettability of the surface was changed from hydrophobic for the HAp coating to hydrophilic for the HAp-Ga composite coating. Finally, the HAp-Ga coating presented excellent antibacterial efficacies against both initial attachments and established biofilms generated from methicillin-resistant Staphylococcus aureus and Pseudomonas aeruginosa after 18 h and 7 days of incubation in comparison to the control HAp coating. This study shows that GaLM improves the antibacterial properties of HAp-based coatings without sacrificing the beneficial properties of conventional HAp coatings. Thus, the HAp-Ga APS coating is a viable candidate for antibacterial coatings.

Baghdadite coating formed by hybrid water-stabilized plasma spray for bioceramic applications: Mechanical and biological evaluations.

This work studies the mechanical and biological properties of Baghdadite (BAG, Ca3ZrSi2O9) coating manufactured on Ti6Al4V substrates by hybrid water-stabilized plasma spray (WSP-H). Hydroxyapatite (HAp, Ca10(PO4)6(OH)2) coating was produced by gas-stabilized atmospheric plasma spray and used as a reference material. Upon spraying, the BAG coating exhibited lower crystallinity than the HAp coating. Mechanical testing demonstrated superior properties of the BAG coating: its higher hardness, elastic modulus as well as a better resistance to scratch and wear. In the cell viability study, the BAG coating presented better human osteoblast attachment and proliferation on the coating surface after three days and seven days compared to the HAp counterpart. Furthermore, the gene expression study of human osteoblasts indicated that the BAG coating surface showed higher expression levels of osteogenic genes than those on the HAp coating. Overall, this study indicates that enhanced mechanical and bioactive properties can be achieved for the BAG coating compared to the benchmark HAp coating. It is therefore concluded here that the BAG coating is a potential candidate for coating orthopedic implants.

Micro- to nano-scale chemical and mechanical mapping of antimicrobial-resistant fungal biofilms.

A fungal biofilm refers to the agglomeration of fungal cells surrounded by a polymeric extracellular matrix (ECM). The ECM is composed primarily of polysaccharides that facilitate strong surface adhesion, proliferation, and cellular protection from the surrounding environment. Biofilms represent the majority of known microbial communities, are ubiquitous, and are found on a multitude of natural and synthetic surfaces. The compositions, and in-turn nanomechanical properties, of fungal biofilms remain poorly understood, because these systems are complex, composed of anisotropic cellular and extracellular material, and importantly are species and environment dependent. Therefore, genomic variation, and/or mutations, as well as environmental and growth factors can change the composition of a fungal cell's biofilm. In this work, we probe the physico-mechanical and biochemical properties of two fungal species, Candida albicans (C. albicans) and Cryptococcus neoformans (C. neoformans), as well as two antifungal resistant sub-species of C. neoformans, fluconazole-resistant C. neoformans (FlucRC. neoformans) and amphotericin B-resistant C. neoformans (AmBRC. neoformans). A new experimental methodology of characterization is proposed, employing a combination of atomic force microscopy (AFM), instrumented nanoindentation, and Synchrotron ATR-FTIR measurements. This allowed the nano-mechanical and chemical characterisation of each fungal biofilm.

Application of High-Density Electropulsing to Improve the Performance of Metallic Materials: Mechanisms, Microstructure and Properties.

The technology of high-density electropulsing has been applied to increase the performance of metallic materials since the 1990s and has shown significant advantages over traditional heat treatment in many aspects. However, the microstructure changes in electropulsing treatment (EPT) metals and alloys have not been fully explored, and the effects vary significantly on different material. When high-density electrical pulses are applied to metals and alloys, the input of electric energy and thermal energy generally leads to structural rearrangements, such as dynamic recrystallization, dislocation movements and grain refinement. The enhanced mechanical properties of the metals and alloys after high-density electropulsing treatment are reflected by the significant improvement of elongation. As a result, this technology holds great promise in improving the deformation limit and repairing cracks and defects in the plastic processing of metals. This review summarizes the effect of high-density electropulsing treatment on microstructural properties and, thus, the enhancement in mechanical strength, hardness and corrosion performance of metallic materials. It is noteworthy that the change of some properties can be related to the structure state before EPT (quenched, annealed, deformed or others). The mechanisms for the microstructural evolution, grain refinement and formation of oriented microstructures of different metals and alloys are presented. Future research trends of high-density electrical pulse technology for specific metals and alloys are highlighted.

Influence of charged defects on the interfacial bonding strength of tantalum- and silver-doped nanograined TiO2.

A nano-grained layer including line defects was formed on the surface of a Ti alloy (Tialloy, Ti-6Al-4V ELI). Then, the micro- and nano-grained Tialloy with the formation of TiO2 on its top surface was coated with a bioactive Ta layer with or without incorporating an antibacterial agent of Ag that was manufactured by magnetron sputtering. Subsequently, the influence of the charged defects (the defects that can be electrically charged on the surface) on the interfacial bonding strength and hardness of the surface system was studied via an electronic model. Thereby, material systems of (i) Ta coated micro-grained titanium alloy (Ta/MGTialloy), (ii) Ta coated nano-grained titanium alloy (Ta/NGTialloy), (iii) TaAg coated micro-grained titanium alloy (TaAg/MGTialloy) and (iv) TaAg coated nano-grained titanium alloy (TaAg/NGTialloy) were formed. X-ray photoelectron spectroscopy was used to probe the electronic structure of the micro- and nano-grained Tialloy, and so-formed heterostructures. The thin film/substrate interfaces exhibited different satellite peak intensities. The satellite peak intensity may be related to the interfacial bonding strength and hardness of the surface system. The interfacial layer of TaAg/NGTialloy exhibited the highest satellite intensity and maximum hardness value. The increased bonding strength and hardness in the TaAg/NGTialloy arises due to the negative core charge of the dislocations and neighbor space charge accumulation, as well as electron accumulation in the created semiconductor phases of larger band gap at the interfacial layer. These two factors generate interfacial polarization and enhance the satellite intensity. Consequently, the interfacial bonding strength and hardness of the surface system are improved by the formation of mixed covalent-ionic bonding structures around the dislocation core area and the interfacial layer. The bonding strength relationship by in situ XPS on the metal/TiO2 interfacial layer may be examined with other noble metals and applied in diverse fields.

Cell response and bioactivity of titania-zirconia-zirconium titanate nanotubes with different nanoscale topographies fabricated in a non-aqueous electrolyte.

The morphology and the physical and chemical characteristics of four groups of TiO2-ZrO2-ZrTiO4 nanotubes that were fabricated via anodization in a non-aqueous electrolyte were investigated in order to examine their influence on the bioactivity of, and cell adhesion on, Ti50Zr alloy. Scanning electron microscopy (SEM) and 3D profilometry were used for the characterization. The in vitro cell responses to nanotubular surfaces with different inner diameters (Di) between 25 and 49 nm were assessed using osteoblast cells (SaOS2). The results of the MTS assay indicated that the percentage of cell adhesion on the nanotubes was influenced by the nanoscale topographical parameters including the tube inner diameter (Di), the tube wall thickness (Wt), the amplitude roughness (Sa) and the spacing roughness (Sm) of the nanotubular surface. Cell adhesion was promoted to 84.9% on nanotubes with an inner diameter of 25 nm, or 80.3% on nanotubes with a large wall thickness of 34 nm due to the accelerated integrin clustering and focal contacts of formation. A nanotubular surface with a low spacing roughness of 33 nm(3) nm(-2) led to a cell adhesion of 61.0%. Similarly, a nanotubular surface with a high amplitude roughness of 1.03 µm revealed a cell adhesion of 61.5% in instances where the inner diameters (29 nm) and wall thicknesses (24 nm) were within the critical dimensional parameters for cells to survive and thrive.

Fabrication and Characterization of Nanoporous Niobia, and Nanotubular Tantala, Titania and Zirconia via Anodization.

Valve metals such as titanium (Ti), zirconium (Zr), niobium (Nb) and tantalum (Ta) that confer a stable oxide layer on their surfaces are commonly used as implant materials or alloying elements for titanium-based implants, due to their exceptional high corrosion resistance and excellent biocompatibility. The aim of this study was to investigate the bioactivity of the nanostructures of tantala (Ta2O5), niobia (Nb2O5), zirconia (ZrO2) and titania (TiO2) in accordance to their roughness and wettability. Therefore, four kinds of metal oxide nanoporous and nanotubular Ta2O5, Nb2O5, ZrO2 and TiO2 were fabricated via anodization. The nanosize distribution, morphology and the physical and chemical properties of the nanolayers and their surface energies and bioactivities were investigated using SEM-EDS, X-ray diffraction (XRD) analysis and 3D profilometer. It was found that the nanoporous Ta2O5 exhibited an irregular porous structure, high roughness and high surface energy as compared to bare tantalum metal; and exhibited the most superior bioactivity after annealing among the four kinds of nanoporous structures. The nanoporous Nb2O5 showed a uniform porous structure and low roughness, but no bioactivity before annealing. Overall, the nanoporous and nanotubular layers of Ta2O5, Nb2O5, ZrO2 and TiO2 demonstrated promising potential for enhanced bioactivity to improve their biomedical application alone or to improve the usage in other biocompatible metal implants.

The influence of titania-zirconia-zirconium titanate nanotube characteristics on osteoblast cell adhesion.

Studies of biomaterial surfaces and their influence on cell behavior provide insights concerning the design of surface physicochemical and topography properties of implant materials. Fabrication of biocompatible metal oxide nanotubes on metallic biomaterials, especially titanium alloys such as Ti50Zr via anodization, alters the surface chemistry as well as surface topography of the alloy. In this study, four groups of TiO2-ZrO2-ZrTiO4 nanotubes that exhibit diverse nanoscale dimensional characteristics (i.e. inner diameter Di, outer diameter Do and wall thicknesses Wt) were fabricated via anodization. The nanotubes were annealed and characterized using scanning electron microscopy and 3-D profilometry. The potential applied during anodization influenced the oxidation rate of titanium and zirconium, thereby resulting in different nanoscale characteristics for the nanotubes. The different oxidation and dissolution rates both led to changes in the surface roughness parameters. The in vitro cell response to the nanotubes with different nanoscale dimensional characteristics was assessed using osteoblast cells (SaOS2). The results of the MTS assay indicated that the nanotubes with inner diameter (Di)≈40nm exhibited the highest percentage of cell adhesion of 41.0%. This result can be compared to (i) 25.9% cell adhesion at Di≈59nm, (ii) 33.1% at Di≈64nm, and (iii) 33.5% at Di≈82nm. The nanotubes with Di≈59nm exhibited the greatest roughness parameter of Sa (mean roughness), leading to the lowest ability to interlock with SaOS2 cells.

Fabrication and characterization of TiO2-ZrO2-ZrTiO4 nanotubes on TiZr alloy manufactured via anodization.

Titanium and its alloys are able to grow a stable oxide layer on their surfaces and have been used frequently as substrates for anodization in an electrochemical surface treatment. A nanotubular oxide layer is formed in the presence of fluorine anion (F-) via anodization due to the competition between oxide formation and solvatization. In this study, a highly ordered titania-zirconia-zirconium titanate (TiO2-ZrO2-ZrTiO4) nanotubular layer was formed on the surface of Ti50Zr alloy via anodic oxidation in an F- containing electrolyte. The sizes of the nanotubes (i.e., the inner and outer diameters, and wall thicknesses), morphology, crystal structure, hydrophilic properties and components of the TiO2-ZrO2-ZrTiO4 nanotubular layer before and after annealing were examined by scanning electron microscopy (SEM), thin film X-ray diffraction, X-ray photoelectron spectroscopy (XPS) analysis and water contact angle measurements. The results indicated that the inner diameter, outer diameter and wall thickness of the as-formed TiO2-ZrO2-ZrTiO4 nanotubes were distributed in the ranges of 3-120 nm, 12-165 nm and 3-32 nm, respectively, and depended on the F- concentration of the electrolyte and the applied potential during anodization. The number of smaller nanotubes increased with increasing F- concentration and the mean nanotube inner and outer diameters increased with increasing applied potential. The as-formed TiO2 and ZrTiO4 nanotubes exhibited an amorphous structure and the as-formed ZrO2 nanotubes displayed an orthorhombic structure. These phases transformed into anatase TiO2 and orthorhombic ZrO2 and ZrTiO4 after annealing. The hydrophilic properties of the TiO2-ZrO2-ZrTiO4 nanotubular layer were affected by the size distribution of the nanotubes. The surface roughnesses and the nanotubular character transformed the nanotubes to exhibit superhydrophilic properties after annealing. The TiO2-ZrO2-ZrTiO4 nanotubular surface on Ti50Zr alloy exhibited higher surface energy than that of the TiO2 nanotubular surface on commercially pure (CP) titanium.

Cell response of anodized nanotubes on titanium and titanium alloys.

Titanium and titanium alloy implants that have been demonstrated to be more biocompatible than other metallic implant materials, such as Co-Cr alloys and stainless steels, must also be accepted by bone cells, bonding with and growing on them to prevent loosening. Highly ordered nanoporous arrays of titanium dioxide that form on titanium surface by anodic oxidation are receiving increasing research interest due to their effectiveness in promoting osseointegration. The response of bone cells to implant materials depends on the topography, physicochemistry, mechanics, and electronics of the implant surface and this influences cell behavior, such as adhesion, proliferation, shape, migration, survival, and differentiation; for example the existing anions on the surface of a titanium implant make it negative and this affects the interaction with negative fibronectin (FN). Although optimal nanosize of reproducible titania nanotubes has not been reported due to different protocols used in studies, cell response was more sensitive to titania nanotubes with nanometer diameter and interspace. By annealing, amorphous TiO2 nanotubes change to a crystalline form and become more hydrophilic, resulting in an encouraging effect on cell behavior. The crystalline size and thickness of the bone-like apatite that forms on the titania nanotubes after implantation are also affected by the diameter and shape. This review describes how changes in nanotube morphologies, such as the tube diameter, the thickness of the nanotube layer, and the crystalline structure, influence the response of cells.

Transition metal-substituted cobalt ferrite nanoparticles for biomedical applications.

Transition metals of copper, zinc, chromium and nickel were substituted into cobalt ferrite nanoparticles via a sol-gel route using citric acid as a chelating agent. The microstructure and elemental composition were characterized using scanning electron microscopy combined with energy-dispersive X-ray spectroscopy. Phase analysis of transition metal-substituted cobalt ferrite nanoparticles was performed via X-ray diffraction. Surface wettability was measured using the water contact angle technique. The surface roughness of all nanoparticles was measured using profilometry. Moreover, thermogravimetric analysis and differential scanning calorimetry were performed to determine the temperature at which the decomposition and oxidation of the chelating agents took place. Results indicated that the substitution of transition metals influences strongly the microstructure, crystal structure and antibacterial property of the cobalt ferrite nanoparticles.

A review of the application of anodization for the fabrication of nanotubes on metal implant surfaces.

Metal implants are the best choice for the long-term replacement of hard tissue, such as hip and knee joints, because of their excellent mechanical properties. Titanium and its alloys, due to their self-organized oxide layer, which protects the surface from corrosion and prevents ion release, are widely accepted as biocompatible metal implants. Surface modification is essential for the promotion of the osseointegration of these biomaterials. Nanotubes fabricated on the surface of metal implants by anodization are receiving ever-increasing attention for surface modification. This paper provides an overview of the employment of anodization for nanotubes fabricated on the surface of titanium, titanium alloys and titanium alloying metals such as niobium, tantalum and zirconium metal implants. This work explains anodic oxidation and the manner by which nanotubes form on the surface of the metals. It then assesses this topical research to indicate how changes in anodizing conditions influence nanotube characteristics such as tube diameters and nanotube-layer thickness.

Selection of the implant and coating materials for optimized performance by means of nanoindentation.

Mechanical compatibility between a coating and a substrate is important for the longevity of implant materials. While previous studies have utilized the entire coating for analysis of mechanical compatibility of the surface, this study focuses on the nanoindentation of a uniformly thermally sprayed splat. Hydroxyapatite was thermally sprayed to create a homogeneous deposit density, as confirmed by microRaman spectroscopy, of amorphous calcium phosphate. Substrates were commercially pure Ti, Ti-6Al-4V, Co-Cr alloy and stainless steel. Nanoindentation revealed that splats deposited on the different metals have similar hardness and elastic modulus values of 4.2 ± 0.2 GPa and 80 ± 3 GPa, respectively. The mechanical properties were affected by the substrate type more than residual stresses, which were found to be low. It is recommended that amorphous calcium phosphate is annealed to relieve the quenching stress or that appropriate temperature histories are chosen to relax the stress created in cooling the coating assembly.

Plasma-enhanced synthesis of bioactive polymeric coatings from monoterpene alcohols: a combined experimental and theoretical study.

This paper describes the synthesis and characterization of a novel organic polymer coating for the prevention of the growth of Pseudomonas aeruginosa on the solid surface of three-dimensional objects. Substrata were encapsulated with polyterpenol thin films prepared from terpinen-4-ol using radio frequency plasma enhanced chemical vapor deposition. Terpinen-4-ol is a constituent of tea tree oil with known antibacterial properties. The influence of deposition power on the chemical structure, surface composition, and ultimately the antibacterial inhibitory activity of the resulting polyterpenol thin films was studied using X-ray photoelectron spectroscopy (XPS), water contact angle measurement, atomic force microscopy (AFM), and 3-D interactive visualization and statistical approximation of the topographic profiles. The experimental results were consistent with those predicted by molecular simulations. The extent of bacterial attachment and extracellular polymeric substances (EPS) production was analyzed using scanning electron microscopy (SEM) and confocal scanning laser microscopy (CSLM). Polyterpenol films deposited at lower power were particularly effective against P. aeruginosa due to the preservation of original terpinen-4-ol molecules in the film structure. The proposed antimicrobial and antifouling coating can be potentially integrated into medical and other clinically relevant devices to prevent bacterial growth and to minimize bacteria-associated adverse host responses.

Impact of nanoscale roughness of titanium thin film surfaces on bacterial retention.

Two human pathogenic bacteria, Staphylococcus aureus CIP 68.5 and Pseudomonas aeruginosa ATCC 9025, were adsorbed onto surfaces containing Ti thin films of varying thickness to determine the extent to which nanoscale surface roughness influences the extent of bacterial attachment. A magnetron sputter thin film system was used to deposit titanium films with thicknesses of 3, 12, and 150 nm on glass substrata with corresponding surface roughness parameters of R(q) 1.6, 1.2, and 0.7 nm (on a 4 microm x 4 microm scanning area). The chemical composition, wettability, and surface architecture of titanium thin films were characterized using X-ray photoelectron spectroscopy, contact angle measurements, atomic force microscopy, three-dimensional interactive visualization, and statistical approximation of the topographic profiles. Investigation of the dynamic evolution of the Ti thin film topographic parameters indicated that three commonly used parameters, R(a), R(q), and R(max), were insufficient to effectively characterize the nanoscale rough/smooth surfaces. Two additional parameters, R(skw) and R(kur), which describe the statistical distributions of roughness character, were found to be useful for evaluating the surface architecture. Analysis of bacterial retention profiles indicated that bacteria responded differently to the surfaces on a scale of less than 1 nm change in the R(a) and R(q) Ti thin film surface roughness parameters by (i) an increased number of retained cells by a factor of 2-3, and (ii) an elevated level of secretion of extracellular polymeric substances.

Effect of ultrafine-grained titanium surfaces on adhesion of bacteria.

The influence of the ultrafine crystallinity of commercial purity grade 2 (as-received) titanium and titanium modified by equal channel angular pressing (modified titanium) on bacterial attachment was studied. A topographic profile analysis of the surface of the modified titanium revealed a complex morphology of the surface. Its prominent micro- and nano-scale features were 100-200-nm-scale undulations with 10-15 microm spacing. The undulating surfaces were nano-smooth, with height variations not exceeding 5-10 nm. These surface topography characteristics were distinctly different from those of the as-received samples, where broad valleys (up to 40-60 microm) were detected, whose inner surfaces exhibited asperities approximately 100 nm in height spaced at 1-2 microm. It was found that each of the three bacteria strains used in this study as adsorbates, viz. Staphylococcus aureus CIP 68.5, Pseudomonas aeruginosa ATCC 9025 and Escherichia coli K12, responded differently to the two types of titanium surfaces. Extreme grain refinement by ECAP resulted in substantially increased numbers of cells attached to the surface compared to as-received titanium. This enhanced degree of attachment was accompanied with an increased level of extracellular polymeric substances (EPS) production by the bacteria.

Hydroxyapatite/polymer composite flame-sprayed coatings for orthopedic applications.

The complex biological and mechanical requirements for implant materials in the human body generally cannot be furnished by one single material. In the present study, hydroxyapatite/polymer composite coatings with different contents of hydroxyapatite were produced using a flame spray system. This processing route is intended to obtain a coating with an optimal combination of biological and mechanical properties of these two materials for skeletal implants. The composite coatings were produced from a mechanical blend of hydroxyapatite and ethylene methacrylic acid copolymer powders, which were delivered from a fluidized bed powder feeder. Characterization of the coating surface morphology, polished coating cross-sections, and fracture surface morphology was conducted by scanning electron microscopy. The dissolution behavior of the coatings was evaluated with a calcium-specific ion meter. The stress-strain behavior was investigated by tensile testing. The biological and mechanical properties were found to be related to the volume and distribution of the hydroxyapatite in the polymer matrix. This technique provides a means of preparing hydroxyapatite/polymer coatings for application as implants.

Surface characteristics and dissolution behavior of plasma-sprayed hydroxyapatite coating.

One of the most important concerns with the clinical use of plasma-sprayed hydroxyapatite (HA) coatings is the resorption of the coating, and dissolution at neutral pH is one of the two major resorption mechanisms. In this study, highly crystalline pure HA powders were atmospherically plasma sprayed using various parameters. Dissolution of both HA powders and coatings was measured using a calcium ion meter. Surface characteristics, including phase, morphology, and roughness, were compared for the coatings before and after dissolution. Pulverized HA coatings exhibited significantly higher dissolution compared with the same quantity of feedstock HA powders because of the decreased crystallinity and fine crystal size of the coating. Furthermore, the dissolution decreased with the crystallinity of the coating. Dissolution of HA coatings did not show much difference with respect to the coatings in the initial stage of immersion (4 h). However, dissolution of all coatings reached saturation in a fresh physiological solution. The saturation values were much lower compared with their counterparts in the form of powders, which may imply the stability of HA coatings in long-term use. In addition to crystallinity, the particle melting status in the coatings, i.e., the volume of nanocrystals, and porosity, was found to be another important factor for the dissolution of the HA coating. X-ray diffraction patterns of HA coatings indicated the complete dissolution of impurity phases and amorphous phase after the coatings were immersed in the solution for 4 days. Coatings sprayed at lower power (27.5 kW) exhibited a pattern of crystalline HA whereas coatings sprayed at higher power (42 kW) exhibited a pattern of bone apatite. Surface morphologies showed preferential dissolution of amorphous phase in all coatings accompanied with precipitation of bone apatite observable for coatings sprayed at higher power. Surface roughness measured after the dissolution studies increased for the two coatings sprayed at lower power level but decreased for coatings sprayed at higher power level. This decrease is attributed to the better match in solubility characteristics between the fine crystals and the amorphous calcium phosphate within the coating.