Abstract This paper compares two methods for determining the composition of Ti/TiN coatings deposited by reactive plasma spraying. The coatings were obtained by spraying titanium powder in a low-pressure N2/Ar atmosphere. The resulting film has a variable nitrogen content in the form of titanium nitrides, depending on gas partial pressure, total pressure, sample-source distance, and other parameters. The composition of the film was determined using X-ray diffraction and X-ray photoelectron spectroscopy. The two techniques provide similar results and either can be used for the compositional characterization of these coatings.

Abstract Aluminum coatings reinforced with either Al 2 O 3 or SiC particles were deposited onto aluminum substrates and subjected to various tests. The coatings were made with mechanically alloyed powders via atmospheric plasma spraying (APS). Both types of coatings had uniformly distributed hard particles, porosities in the range of 4 to 5%, and bond strengths of around 20 MPa. The wear resistance of the SiC-reinforced coatings, however, was almost 35% higher than the coatings containing Al 2 O 3 . X-ray examination (XRD) showed that the Al 2 O 3 particles undergo partial phase transformation during spraying, making them more prone to wear.

Abstract Effects of spray parameters, such as spray distance, SD, and substrate temperature, Ts, and post heat treatment on the structure and properties of plasma-sprayed zircon coatings were investigated. Zircon was totally decomposed by plasma spray; the coatings were composed of tetragonal zirconia (t-ZrO2) and amorphous silica (a-SiO2), because of the rapid cooling of molten particle right after the impingement to the substrate. Porosity of the as-sprayed coatings was highly affected by both of substrate temperature and spray distance. In all range of the spray distance which had been tried in this study, higher substrate temperature resulted in lower porosity of the coatings; the coating with porosity of 2% was obtained at Ts = 1573K with SD = 95mm. Porosity also decreased with decrease of spray distance. By the heat treatment at 1473K, t-ZrO2 transformed to monoclinic zirconia (m-ZrO2) and a-SiO2 crystallized to cristobalite, respectively. Cracks in the coating disappeared, and open porosity decreased. This can be attributed to sintering of SiO2 and phase transformation of ZrO2. After the heat treatment at 1673K, the coating was composed of ZrSiO4 with dispersed fine m-ZrO2 particle. Open porosity of all the coatings increased up to 10% at this temperature. This is because of volume shrinkage during the formation of zircon.

Abstract Titanium carbide cermet spray powder was produced by the SHS process (Self-propagating High-temperature Synthesis) using elemental Ti, C, Mo and prealloyed CrNiMo powders as starting materials. The powder was characterised (particle size distribution, phase structure, morphology) and the internal structure of each cermet particle was found out to be dense consisting of fine distribution of carbides embedded in a metallic matrix. The particle size range suitable for thermal spraying was obtained by sieving and air classifying. The coatings were prepared by HVOF spraying (DJH2600 and DJH2700). The dry abrasion wear resistance was evaluated by the rubber wheel abrasion wear test and electrochemical corrosion behaviour by open circuit potential measurements. According to the XRD analysis the amount of retained carbides in the coatings is high and the carbide phase has a spherical shape also in the coatings. The microstructure of coatings obtained is dense and the coatings possess good properties in wear and corrosion tests. WC-Co-Cr and Cr3C2-NiCr powders were used for comparison.

Abstract Quasicrystals are materials whose structure cannot be understood within classic crystallographic methodology. Quasiperiodic structures have a long-range orientation order but lack transitional periodicity. This non-periodic structure gives quasicrystals a unique range of physical properties including: poor thermal conductivity; excellent crystallographic stability up to their melting point; can be thermally sprayed to display either wear resistant or abradable characteristics; possess excellent release properties (low coefficient of friction); and, they are non-work hardenable. This effort describes parametric conditions utilized to deposit coatings of identical parent metal chemistry but with vastly different end properties.

Abstract Calcined spray-dried hydroxyapatite (Ca10(PO4)(OH)6; i.e., HA) powders were atmospherically plasma sprayed (APS) using various process parameters. The resulting phases within the coating surface and the interface between the coating and the substrate were analyzed using X-ray diffraction (XRD) methods. This XRD revealed the presence of both amorphous (i.e., amorphous calcium phosphate: ACP) and crystalline phases. The crystalline phases included both HA and some impurity phases from the decomposition of HA, such as tricalcium phosphate (α-TCP and β-TCP), tetracalcium phosphate (TTCP) and calcium oxide (CaO). The crystallinity of HA decreased with increasing spray power and stand-off distance (SOD). The percentage of all impurity phases increased with the spray power. The percentage of both TCP and TTCP decreased with the SOD while the CaO percentage increased. In addition, the percentage of ACP and CaO were higher in the interface than at the surface of the coating while the percentage of TCP and TTCP exhibited the opposite effect.

Abstract Tungsten Carbide (WC) thermal spray coatings have had increased acceptance in commercial aircraft applications driven by the desire to replace chromium electroplate due to environmental and economic considerations [1]. In order to confidently replace chromium electroplate by WC thermal spray coatings, evaluation of wear and fatigue characteristics of the WC thermal spray coatings is necessary. For WC thermal spray coatings to replace chromium electroplate in aircraft applications, the coatings must demonstrate wear and fatigue characteristics as good as or better than those of chrome plating. Previous research in this area has shown that the fatigue life of the WC thermal spray coatings can be improved by inducing compressive stresses in the coating. This paper compares the wear characteristics of several types of WC thermal spray coatings with those of chromium electroplate in sliding wear tests following the "block-on-ring" procedures described in ASTM G77 standard. Wear results are interpreted in terms of coating residual stresses and in terms of X-ray diffraction and Scanning Electron Microscope (SEM) analysis.

Abstract FeAl iron-aluminide based materials with the ordered B2 structure are excellent candidates for use in high temperature applications because of the combination of good mechanical properties, low density, low cost and availability of raw materials, and improved oxidation resistance. The aim of this article is to produce an ultra-fine grained FeAl coating by HVOF thermal spraying of milled powders and characterize the fine scale features of its microstructure. Comparison is made with a more conventional coating obtained by projection of powders obtained by atomization. Starting powders and coatings were investigated using X-ray diffraction and transmission electron microscopy. It was observed that the coating obtained from milled powders had a microstructure essentially characterised by a nanometer grain size and the presence of a disordered FeAl phase.

Abstract A reaction-formed NiAl intermetallic compound (IMC) powder has been deposited as a coating onto low carbon steel test coupons by the High Velocity Oxy-Fuel (HVOF) process using both gaseous and liquid fuels. The microstructure of this coating has been examined using scanning electron microscopy and x-ray diffraction and was found to depend on spraying conditions. Oxidation tests on the coating in air, between the temperatures of 800°C-1200°C, revealed that an α-alumina (Al 2 O 3 ) scale formed on the coating's surface. At 1200°C, a nickel spinel (NiO/NiAl 2 O 4 ) and haematite (Fe 2 O 3 ) phases were observed. Diffusion studies were performed to calculate an activation energy for iron ion diffusion in NiAl.

Abstract The present paper describes the synthesis of nanocrystalline 316-stainless steel coatings by high velocity oxy-fuel (HVOF) thermal spraying. The feedstock powders were synthesized by mechanical milling to produce flake-shaped agglomerates with an average grain size of less than 100 nm. The powders were introduced into the HVOF spray to successfully produce nanocrystalline coatings. X-ray diffraction analysis and transmission electron microscopy were used to determine the average grain size of the milled powders. Scanning electron microscopy and transmission electron microscopy were used to study the morphology of the nanometric particles and the microstructure of the as-sprayed coatings. The properties of various coating materials were characterized by microhardness measurements performed on the polished surface of the cross section.

Abstract The corrosion characteristics of two bespoke Ni-Cr-Mo-B alloy powders sprayed by the high velocity oxy-fuel (HVOF) process have been studied using potentiodynamic and potentiostatic corrosion analysis in 0.5M H2SO4. The deposits have also been microstructurally characterised using X-ray diffraction (XRD), scanning electron microscopy (utilising both secondary electron (SE) and backscattered electron (BE) modes), and transmission electron microscopy (TEM). Results from the microstructural examination of the two alloys have revealed a predominantly amorphous/nanocrystalline (fcc) matrix containing submicron boride precipitates as well as regions of martensitically transformed laths. Apparent recrystallisation of the amorphous matrix has also been observed in the form of cellular crystals with an fcc structure. The oxide stringers observed at splat boundaries were found to be columnar grained α-Cr 2 O 3 , though regions of the spinel oxide NiCr 2 O 4 with a globular morphology were also observed. The coatings of the two alloys exhibited comparable resistance to corrosion in 0.5M H 2 SO 4 , as revealed by potentiodynamic tests. They both had rest potentials approximately equal to -300mV(SCE) and passive region current densities of around 1mAcm-2. Microstructural examination of samples tested potentiostatically revealed the prevalence of degradation at splat boundaries, especially those where significant oxidation of the deposit had occurred.

Abstract Hydroxyapatite (HA) was plasma sprayed onto titanium alloy substrates. One set of samples had a crystalline content of 25%, the other set, 30%. The coatings were then subjected to CO2 laser treatments at different power levels and scanning rates while surface temperatures ware monitored with a pyrometer. XRD analysis verified the crystallinity of HA and the content of amorphous calcium phosphate as well as foreign phases. Optical microscopy revealed the microstructure and depth of the laser modified zone.

Abstract Perovskite-type LaMnO 3 powders and coatings have been prepared by a novel technique, the reactive suspension plasma spraying (SPS) using an inductively coupled plasma of about 40 kW plate power and an oxygen plasma sheath gas. Suitable precursor mixtures were found on the basis of solid state reactions, solubility and the phases obtained during the spray process. Best results were achieved by spraying a suspension of fine MnO 2 powder in a saturated ethanol solution of LaCl 3 with a 1:1 molar ratio of La and Mn. Low reactor pressure was helpful in order to diminish the amount of corrosive chlorine compounds in the reactor. As-sprayed coatings and collected powders showed perovskite contents of 70-90%. After a post-treatment with an 80% oxygen plasma an almost pure LaMnO 3 deposit was achieved in the center of the incident plasma jet.

Abstract As part of a characterization and mechanical research about hydroxyapatite (HA) plasma deposits for hip prosthesis, we addressed the problem of determining their crystallinity. A traditional normalization method employed by several laboratories is based on X-ray diffraction by a powder mixture of the investigated HA sample with a standard of crystalline powder, namely Al 2 O 3 . This method is quite unsatisfactory, as very often delivers unreasonable results. In order to overcome these difficulties we investigated some new methods for determining the crystallinity of HA sample, which are based on X-ray diffraction. All these methods provide reasonable results.

Abstract Processing induced residual stresses play an important role in the performance of thermally sprayed coatings. Their precise determination is a key to influence the coating properties by modification of process variables and to understand the processing-property relationship. Among various methods for residual stress measurement, x-ray diffraction holds a specific position by being nondestructive, phase distinctive, localized and applicable for real parts. The sin 2 ψ method is commonly applied for bulk materials as well as coatings. However, the results are often reported without sufficient experimental details and the method is used in its simplified form without justification of certain assumptions. In this investigation, the sin 2 ψ x-ray diffraction method was used to measure residual macrostress in plasma sprayed Ni, NiCrAlY and ZrO 2 +8%Y 2 O 3 coatings. Reproducibility of the method was tested and the assumptions allowing its use are discussed and experimentally verified. For Ni coatings, a comparison with blind hole and neutron diffraction measurements is presented. The results are discussed with respect to processing, structure and properties of the coatings.

Abstract Two boron-rich Fe/Cr based gas atomised powders (Armacor M and Armacor C) have been thermally sprayed using the HVOF process and the resultant deposits subsequently characterised, using X-ray diffraction, scanning electron microscopy (SEM), plan view transmission electron microscopy (TEM), and microhardness measurements. The wear and corrosion characteristics of the two alloy coatings have also been investigated by three body abrasive wear (utilising cross-sectional TEM to examine the worn surfaces) and potentiodynamic corrosion testing respectively. Results from microstructural analysis of the as-sprayed deposits revealed the presence of small chromium-iron boride precipitates within a predominantly amorphous matrix in the Fe-based Armacor M coating. The Fe-Cr-based Armacor C coating, however, consisted mainly of regions of nano- and microcrystalline material interspersed with chromium boride precipitation. Iron-chromium oxides have been observed within both of the alloy coatings studied. Both of these alloys exhibit good abrasive wear resistance when compared with other metallic based HVOF sprayed coatings. Both Armacor M and Armacor C also exhibit extensive passivation on exposure to an acidic solution. The wear and corrosion test results are related to the microstructural observations.

Abstract X ray diffraction and transmission electron microscopy studies and measurements of hardness and void content were carried out for WC-20% Co coatings produced by detonation flame spraying at various oxygen/acetylene ratios in the detonating gas mixture. It was demonstrated that successive transition from (WC+Co) to (W 2 C+Co 3 W 3 C) to (W+CO 7 W 6 ) occurs as the oxygen content in the mixture is increased, and that amorphous-nanocrystalline structures form in the coating. Two types of these hybrid structures were revealed, one including an amorphous metallic matrix containing precipitates of intermetallic nanocrystals, the other having an amorphous oxide matrix and nanocrystalline precipitates of CO 3 O 4 and WO 3 . The hybrid structures were shown to improve coating density and hardness.

Abstract The technology of thermal spraying is approaching maturity, and in the quest to reduce production costs whilst maintaining coating quality, attention is turning increasingly to more cost-effective routes for the manufacture of the starting powders. One such route is self-propagating high-temperature synthesis (SHS), which reduces the required energy input for powder production. In this work, TiC-Ti+Ni and (Ti, W)C-Ni powders produced by the SHS process have been studied in the as-received and as-sprayed states, to evaluate the suitability of SHS powders for the production of wear-resistant coatings. The starting powders and the coatings produced by atmospheric-plasma and HVOF spraying have been characterised using analytical (XRD, EDS) and microscopical techniques (optical, SEM). The technological properties of the as-sprayed coatings have also been characterised, including hardness, wear resistance (using a Rubber-Wheel test (ASTM G-65)) and corrosion resistance (in marine water environment).

Abstract To maintain surface roughness of process rolls in cold rolling steel plants, WC-Co coatings have been known to be effective ones. In this study, a high pressure/high velocity oxygen fuel (HP/HVOF) process was used to obtain WC-Co coatings. To get the best quality of coatings, WC-Co coatings are sprayed with numerous powders made by various processes. These powders include agglomerated sintered powders, fused-crushed powders, extra high carbon WC-Co powders and (W 2 C, WC)-Co powders. After spraying, properties of coatings such as hardness, wear resistance. X-ray diffraction, and microstructures were analyzed. For coatings produced by agglomerated-sintered powders, hardness of the coating increased as power levels and the number of passes were increased. In case of the coatings produced by fused-crushed powders, a very low deposition rate was obtained due to a low flowablity of the powders. In addition, the WC-Co coatings sprayed with extra carbon content of WC-Co did not show improved hardness and wear resistance. Also, some decomposition of WC was observed in the coating. Finally, the coatings produced by (W 2 C, WC)-Co powders produced higher hardness and lower wear resistance coating.

Abstract Cermet (WC-Co) coatings have been produced on steel substrates by plasma spraying in vacuum and in air. These have been examined microstructurally and characterised in terms of porosity content, stiffness, microhardness and abrasion resistance. Particular attention has been paid to the phase constitution, as revealed by X-ray diffraction and scanning electron microscopy. High precision densitometry has been used to study porosity levels. Coatings with three different metal contents (9, 12 and 17wt.%Co) have been examined. There is a strong tendency for chemical reactions to occur within the plasma plume, particularly for spraying in air. These reactions can result in the formation of various carbides and even of metallic tungsten. Thermodynamic and kinetic aspects of the reactions involved are briefly examined. Such reactions are strongly promoted by the presence of oxygen, and are much less marked during vacuum plasma spraying. Plasma power and substrate temperature have secondary effects on the degree of reaction which occurs. A marked correlation was observed between degree of reaction and resistance to abrasive wear. This is consistent with the reaction products being brittle and causing poor interfacial cohesion. It was also found that wear resistance was greater for the coatings with lower metal contents. This behaviour can be attributed to the wear occurring predominantly by ploughing of the metallic phase and consequent release of ceramic particles. This occurred more readily when the metal content was higher. In coatings which had undergone pronounced chemical reaction, however, metal had been replaced by reaction products which conferred poor cohesive strength, leading to poor wear resistance.