Phase composition and microstructure of hydroxyapatite (HA) significantly affects the biological and mechanical properties of final hydroxyapatite (HA) coating. In the present study, HA coatings were deposited on Ti-6Al-4V by micro-plasma spraying (MPS) using different spray parameters. The influence of spray parameters on the composition and microstructure of the coatings were investigated. To understand the formation mechanism of HA coatings, the in-flight particles and splats were examined as well. The morphologies of coatings surface, cross-sections, initial powder, in-flight particles and splats were characterized by scanning electron microscopy (SEM). Xray diffraction (XRD) was employed to analyze the phase composition. Three typical HA coatings were fabricated. The results indicated that the coating composition and microstructure were tightly related to the melting state of inflight particles. And this was influenced by the spraying parameters. The formation mechanisms of these coatings were discussed.

To manufacture a protective coating with low thermal conductivity and good frictional wear performance, a Fe 59 Cr 12 Nb 5 B 20 Si 4 coating was designed and produced by high velocity oxygen fuel (HVOF) spraying; the properties and performance of this coating where then compared with those of a commercially available AISI 316L stainless steel coating. In the as-deposited state, both coatings exhibit dense layered structures with porosity below 1% and slight oxidation. The microstructure of the Fe-based coating has an amorphous matrix and some precipitated nanocrystals. The result is that the designed Fe-based coating has a thermal conductivity (2.66 W/m·K) that is significantly lower than that of the 316L stainless steel coating (5.87 W/m·K). Based on its advantageous structure, the Fe-based coating exhibits higher microhardness, reaching 1258±92 HV. The friction coefficient and wear rate of the Fe-based coating show an increase at 200°C followed by a decrease at 400°C, due to the evolution of the wear mechanism at different temperatures. The dominant wear mechanism of the Fe-based coating at room temperature is fatigue wear accompanied by oxidative wear. At 200°C, due to the existence of “third body” abrasive wear, the wear process was accelerated. The large-area oxide layer is likely responsible for the decrease of friction of the coating at 400°C.

In this work, micro-plasma spraying is used to produce hydroxyapatite coatings on Ti-6Al-4V substrates. To understand coating formation mechanisms, in-flight particle velocity and surface temperature were monitored under different spraying conditions. XRD measurements show that the resulting coatings have a high degree of crystallinity with little amorphous or metastable phases. Some of the coatings were also found to have a uniformly distributed columnar structure, corresponding to a strong (002) texture and excellent stability in Hanks’ salt solution even after 14 days of immersion.

In this work, gas-atomized FeCr powders were deposited on aluminum substrates by HVOF spraying, forming dispersion strengthened coatings with a dense layered structure and low porosity. SEM, TEM, and XRD analyses show that the coatings primarily consist of amorphous matrix (40%) with precipitated nanocrystals and hard boride phases. A number of coating properties, including microhardness, bonding strength, and thermal conductivity, were measured and are correlated with spraying conditions.

In this study, FeCrNbBSiC coatings are deposited on aluminum alloy 4042 substrates by wire arc spraying and their microstructure, phase composition, hardness, bonding strength, and thermal conductivity are evaluated in comparison with a commercial 316L stainless steel coating. The FeCrNbBSiC samples were found to have a dense and homogeneous lamellar microstructure with much less oxide inclusions than the stainless steel layer. The iron-based coatings were also much harder, mainly due to their composite structure with amorphous and nanocrystalline phases, and their bonding strength was slightly higher. In thermal conductivity testing, the FeCrNbBSiC layers were found to have thermal insulation properties close to that of conventional (YSZ) TBCs, making them a good candidate for engineering use.

In this study, Fe-Cr-Al and Fe-Cr-Al-B cored wires were produced and deposited on steel substrates by wire arc spraying. The microstructure, hardness, and high-temperature corrosion behavior of the cored-wire deposits were evaluated in comparison to Fe-Cr and commercial Fe-Cr-Al solid-wire coatings. All coating samples exhibited lamellar microstructures with oxide inclusions, the fewest being in the Fe-Cr-Al-B deposits. Microhardness was measured along coating cross-sections at various distances from the coating-substrate interface. The Fe-Cr coatings were the hardest, followed by the Fe-Cr-Al-B deposits. Thermogravimetric analysis was used to evaluate high-temperature corrosion behavior in a molten salt environment under cyclic conditions, with the Fe-Cr-Al-B cored-wire deposits performing the best.

In this study, FeCrB coatings are deposited by wire arc spraying using powder cored wires to investigate the factors that affect thermal conductivity. Experimental results show that increasing boron content in the wires reduces oxide content in the coatings, which increases thermal conductivity. Annealing also increases thermal conductivity, which can be explained by grain growth and a reduction in porosity.

A series of Ni-based cored wires with different boron contents were designed to prepare corrosion-resistant coatings by two-roll wire-arc spraying. These coatings were evaluated for their potential to provide added protection and reduced maintenance for applications in waste-to-energy (WTE) plants. The as-deposited coatings, which primarily are composed of nanocrystalline particles, exhibit uniform and dense layered structures with porosity of about 3%. The investigators selected thermo-gravimetric techniques to evaluate the high-temperature corrosion behavior of the coatings in molten salt environment (Na 2 SO 4 -10 wt% NaCl) at 800°C. The coated surfaces exhibited significantly reduced corrosion rates in comparison to those of the SA 213-T 2 substrate during all tests. These results were due to the formation in the coatings of composite surface oxide films, including Cr 2 O 3 and NiCr 2 O 4 , which serve to prevent the diffusion or penetration of corrosive species. Furthermore, the boron content appears to have a significant influence on the corrosion behavior of the designed coatings: the coating with the best performance had 16 at. % B added. The wire-arc sprayed Ni-based coatings could be an effective and economical treatment to prevent corrosion and extend the lifetime of super-heater tubes in WTE plants.

A series of FeCrBCSi coatings were prepared by two-roll wire-arc spraying to investigate the influence of the boron content on the microstructure and properties of the coatings. Microstructural studies show that the as-deposited coatings present a dense layered structure with the porosity around 3%, and are primarily composed of a composite structure of amorphous phase and nanocrystalline phase. It is found that the addition of boron content within the composition range examined exhibits a significant effect on the phase component, as well as the microhardness and wear resistance of the coatings. The optimal composite phase structure, in terms of higher amorphous fraction and lesser nanocrystalline size distribution, leads to the relatively best performance of the coating with 26% boron added. The relative wear resistance of this coating is about 18 and 4 times higher than that of the Q235 steel and commercially available SHS 7170 coating, respectively, therefore wire-arc sprayed FeCrBCSi coating could be an effective and economic approach to withstand wear environment.

A Fe48Cr15Mo14C15B6Y2 alloy with high glass forming ability (GFA) was selected to prepare amorphous metallic coatings by atmospheric plasma spraying (APS) process. The as-deposited coatings present a dense layered structure and low porosity. Microstructural studies show that some nanocrystals and a fraction of yttrium oxides formed during spraying process, which induced the amorphous fraction of the coatings decreasing to 69% compared with fully amorphous alloy ribbons of the same component. High thermal stability employs the amorphous coatings to work below 910K temperature without crystallization. Corrosion behavior of the amorphous coating was investigated by electrochemical measurement. The results show that the coatings exhibit extremely wide passive region and low passive current density in 3.5% NaCl and 1mol/L HCl solutions, which illustrate their superior ability to resist localized corrosion. Moreover, the corrosion behavior of the amorphous coatings in 1mol/L H 2 SO 4 solution is similar to their performance in chlorine ions contained conditions, which manifests their flexible and extensive ability to withstand aggressive environments.

Thermal spraying technique has been widely applied for the production of ceramic protection layer on metals that used in various hostile environments. However, all sprayed coatings have defects such as connected pores and unmelted particles, which deteriorate coating properties. To improve the properties of sprayed coatings, a lot of approaches have been undertaken such as laser irradiation, seal sintering with liquid alloys and sol-gel infiltration technique. Lasers are promising technological tools due to its speedy treatment and simplicity of process control. Moreover, laser treatment technology enables not only the post-treatment but also the pre and simultaneous treatment by combining with spraying process. Generally, wide beams of as uniform as possible are preferred for use in laser surface treatment to obtain a uniform depth of melting, alloying or cladding and to cover a large area by partially overlapping of tracks. However, it is not easy to produce a uniformly treated coating by conventional laser treatment method as desired. To obtain a near-uniform beam intensity for practical laser irradiation, a kaleidoscope was installed in a conventional YAG laser. In this research, laser beam properties of YAG laser equipped with a kaleidoscope and its effect on surface modification of plasma sprayed zirconia coatings and WC-Co system coatings prepared by HVOF spraying was investigated.

The silicon coating was sprayed on titanium substrate by low pressure plasma spraying and the subsequent coating was heat-treated in vacuum. It is found that a titanium silicide coating with the composition changed gradually can be formed through thermal diffusion treatment of silicon coating sprayed by low pressure plasma on titanium substrate. The formed silicide coatings are characterized by optical microscopy, scanning electron microscopy, EPMA analysis and X-ray diffraction (XRD). The forming process of the silicide coating is investigated by examining the relationship between silicide coating thickness and thermal diffusion parameters. The results show that the composition of silicide coating changes gradually from TiSi, at the silicon coating side through TiSi and Ti5Si4, to Ti5Si4, near substrate side. The thickness of such graded silicide coating is determined by temperature and holding time during heat-treatment. The diffusion of silicon into titanium substrate is mainly responsible for the formation of silicide. Moreover, the investigation of oxidation behavior of silicide coating shows that the formation of silicide coating on the titanium substrate can improve the oxidation resistance of titanium.

Densification of plasma-sprayed NiCrAlY coatings was studied from the viewpoint of hybrid spraying combined with YAG laser irradiation. The laser beam was varied while performing low pressure plasma hybrid spraying and the microstructure of each coating was examined and compared with a conventional plasma sprayed coating. Of the various hybrid spraying methods tested, simultaneous irradiation produced the hardest microstructure with the least amount of porosity. The characteristics of the microstructures observed in each coating are explained with respect to their thermal hysteresis behaviors.

Hybrid plasma spraying combined with YAG laser irradiation was studied in order to obtain the optimum zirconia coatings for thermal barrier use. Zirconia coatings of approximately 150 ;Um in thickness were formed on the NiCrAlY under coated steel substrates both by means of conventional plasma spraying and hybrid plasma spraying under a variety of conditions. Post-laser irradiation was also conducted on the plasma as-sprayed coating for comparison. The microstructure of each coating was studied in detail and, for some representative coatings, thermal barrier properties were evaluated by hot erosion and a hot oxidation test. With hybrid spraying, performed under optimum conditions, it was found that a microstructure with appropriate partial densification and without connected porosities was formed and that cracks, which are generally produced in the post-laser irradiation treatment, were completely inhibited. In addition, hybrid spraying formed a smooth coating surface. These microstructural changes resulted in improved coating properties with regard to hardness, high temperature erosion resistance and oxidation resistance. Keywords: Hybrid spraying, YAG laser, Plasma-spraying, Thermal barrier coating, Zirconia, NiCrAlY, Densification, Hot-erosion resistance. Oxidation resistance, Roughness.

As an ideal zirconia thermal barrier coating, the coating microstructure with the strong bonding among coating particles and with certain amount porosities in the coating without connected porosity is desirable. However, it is difficult for usual plasma-spraying to create a zirconia coating with these characters. In order to obtain an excellent zirconia coating with these characters, we prepared zirconia coating on steel substrate under coated with NiCrAlY alloys by means of a hybrid spraying (that is, YAG laser combined plasma spraying) and studied the effect of this hybrid spraying process and plasma and laser conditions on the microstructure of coating, and further compared the microstructure of the coating in this hybrid spraying with that in post-laser irradiation of as-sprayed zirconia coating. It is known that the microstructure with densification is formed by the post-laser irradiation of as-sprayed coating and that microcracks are newly produced in the process of rapid cooling. However, by using this newly-developed hybrid spraying, the microstructure with partial densification in the coating without connected porosities was formed and cracks which is generally produced in the post-laser irradiation treatment were inhibited completely. In addition, this hybrid spraying can be done without the post-treatment of coating. Furthermore, the coating properties, such as hardness and wear resistance associated with bonding state among the coating particles in the zirconia coating created by this hybrid spraying were improved.