From the appearance of high velocity oxygen fuel (HVOF) thermal spray system in the 1980s, WC cermet coatings have been used as anti-wear coatings in many industrial manufacturing applications. Recently, WC cermet spray materials were applied using new thermal spray methods such as warm spray and cold spray, which are still in the research phase. In HVOF spraying, WC-Co and WC-Ni powders are regularly used as coating materials. On the other hand, using cold spray, WC-Fe alloy series can be deposited as dense and thick coatings, better than WC-Co. In this study, WC-Fe alloy powders were sprayed by cold spray to investigate the influence of binder metal on the coating properties and compared with those of HVOF WC-CoCr coatings. It was observed that the lower metal ratio and FeCrNi chemical composition exhibited improved results.

Cavitation erosion is a mass loss process that occurs in a component subject to a liquid pressure variation. The mass loss phenomena occurred by extensive microstructure deformation with combination of shock loading and fatigue caused by the impact and collapse of the bubbles at surface. Many studies have been done to evaluate the cavitation resistance of the thermally sprayed coatings. Oxide formation, microstructure and tensile adhesion are important characteristics for coatings against cavitation. In this work some Fe-Mn-Cr-Si alloys, that is a class of steel with strain induced phase transformation, were used to evaluate the chemical composition influence on the microstructure, oxide formation, chemical composition and tensile adhesion of HVOF coatings. It was observed a significant reduction on the porosity, from 3% to 0,1%, and area fraction oxide from 15% to 7%, with Boron and Nickel addition. A combination of Nickel and Boron addition improve the wettability of the splats with increase on the tensile adhesion of the coatings, up to 60MPa, otherwise, higher levels of Boron content reduce the adhesion of the coatings.

The purpose of this work is to optimise the chemical composition of perovskite coatings prepared by injecting a suspension of submicrometric LaMnO 3 perovskite particles (d 50 =~ 1 µm) in a direct current (d.c.) plasma jet. The perovskite powder composition, the particle size and the plasma parameters were modified in order to diminish the manganese evaporation. The process consists in mechanically injecting a well dispersed stable suspension of submicrometric perovskite particles in a dc plasma jet. In the process, large suspension droplets (~300 µm) are sheared into tiny ones (a few µm) by the plasma jet flow. Then the solvent is evaporated and the particles melt resulting in perovskite droplets of about 1 µm impacting on the substrate, the coating resulting from their layering. Such coatings are to be used as cathodes for the SOFCs (Solid Oxide Fuel Cells). Best results were obtained by injecting a stable suspension containing a 10 mol% MnO 2 doped perovskite powder with 3 µm particle size in an Ar plasma forming gas and 300 A of current intensity.

In a variety of engineering applications, components are exposed to corrosive/erosive environment. Protective coatings are essential to improve the functional performances and/or extend the lifetime of the components. Thermal spraying as a cost-effective coating deposition technique offers high flexibility in coatings’ chemistry/morphology/microstructure design. However, the pores formed during spraying inherently restrict the use of coatings for corrosion protection. In view of the above gap to have a high quality coating, bi-layer coatings have been developed to boost the corrosion performance of the coatings. In a bi-layer coating, an intermediate layer is deposited on the substrate before spraying the coating. The electrochemical behavior of each layer is critical to ensure a good corrosion protection. The corrosion behavior of the layers strongly depends on coating composition and microstructure, which are affected by feedstock material and spraying process. In the present work, Cr 3 C 2 -NiCr top layer with different intermediate layers (i. e., Fe-, and Ni-based) were sprayed by high-velocity air fuel (HVAF) process. Microstructure analysis, as well as electrochemical tests, e.g., open-circuit potential (OCP) and polarization were performed. The results showed a direct link between the OCP of each layer in a bi-layer coating and corrosion mechanisms. It was found that the higher corrosion resistance of Ni-based intermediate layers than Fe-based coatings was due to higher OCP of the coating in the galvanic couple with top layers. Splat boundaries and interconnected pores reduced the corrosion resistance of the intermediate layers, however a sufficient reservoir of protective scale-forming elements (such as Cr or Al) improved the corrosion behavior.

Chemical composition differences between the feedstock powder and the final coating of a series of composite abradable coatings were investigated. Graphite filler material mass distributions in the coating, overspray powder and burned power is calculated. A preliminary mechanism of graphite loss during the deposition process is established. It is found that the graphite content in the coating is significantly lower than that in the feedstock powder. Over 70% graphite in the feedstock powder is lost during the deposition process. Melting and shrinkage of the nickel shell of the nickel cladded graphite particle as flying through the flame, which resulted in the exposure of the graphite core to the flame and substrate, is the main reason for graphite loss and chemical composition change between the feedstock powder and the final coating. A random manner of particle structure transformation in the flame and its reactions with the spray environment is concluded as an important reason for the poor process repeatability of abradable coatings.

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.

The aim of this study is to improve the surface properties of carbon fiber reinforced polymers (CFRP) to facilitate the deposition of a ceramic erosion-resistant coating by air plasma spraying (APS). To avoid mechanical damage induced by grit blasting, the CFRP substrate was chemically etched to remove the majority of superficial epoxy, which is responsible for the poor adhesion of ceramic coatings. Chemical etch times of around 5 min were found to be the most effective, although remaining regions of epoxy interfered with the formation of alumina coatings. To overcome the problem, plasma spray parameters were adjusted, resulting in high-velocity, partially melted alumina particles capable of removing epoxy left on substrate surfaces. Using a combination of chemical etching and the modified spraying process, continuous 50 µm thick alumina coatings are achievable on polymer-matrix composite substrates.

In this study, high pressure cold spray (HPCS) process was used to metallize the surface of polymeric substrates to improve their mechanical performance, such as erosion, wear, and strength. Thermoplastic polymer materials (PEEK, PEI, and ABS) were used as substrate. Commercially pure (CP) Al and 7075 Al were cold sprayed onto the polymeric substrates. Good quality defect-free coatings were achieved in all combinations except with ABS substrates, which suffered from distortion during CS process due to stored thermal energy. 7075Al coatings showed high adhesion strength but low thickness (low deposition efficiency (DE)), whereas CP Al coatings revealed high thickness (high DE) but poor adhesion strength. Based on the obtained results, the DE and bonding strength are not only highly sensitive to properties of the substrate, but also to the applied process parameters as well as powder morphology. It is concluded that two separate sets of spray parameters should be applied for 7075 Al and CP Al deposition otherwise, either more damage or less bonding is achieved to the substrate. Also, for each one of these powders, the first layer of metal/polymer should be deposited with a separate recipe than the subsequent metal/metal layers. Coefficient of thermal expansion and hardness difference between the coating material and the substrate were also found to be key factors to developing continuous coatings on the polymeric substrates with the HPCS process.

The properties of thermally sprayed coatings significantly depend on the alloy composition and the adjusted process parameters. In addition to the powder certificate it may be useful to analyse the chemical composition of the sprayed powder during the spraying process itself. The principle of composition analysis is similar to the chemical analysis in an ICP plasma but the boundary conditions are more complex because the sprayed powder should not be completely evaporated in a thermal spray process. Nevertheless all thermal spraying processes lead to a certain evaporation of the species and to excitation of atomic states. The transition into the ground state occurs under emission of characteristic lines. The intensity of these lines is influenced by the plasma temperature, the particle temperature, the temperature dependent evaporation rate of the alloying elements and the powder feed rate. In consideration of the boundary conditions and the information from a detailed analysis of the emitted spectra the lines can be used to quantify the chemical composition of the sprayed alloys online. The theory of the principle for on-line analysing the chemical composition will be deduced and the first experimental validation will be presented.

Microstructure and properties of plasma sprayed cast iron coatings are closely linked to the spray conditions such as substrate temperature, chamber pressure, particle size, and spray distance. Another factor is the chemical composition of sprayed particles, which affects the physical properties such as density, viscosity, and thermal conductivity of droplets. In spraying cast iron on aluminum alloy substrate the purpose is to deposit a superior wear resistant coating as an approach to improve its wear resistance. Presence of graphite in cast iron increases the wear resistance of cast iron coating because of its self-lubricant property. Graphite grows during droplet solidification and splat cooling and thus its appearance is related to the solidification rate of the individual droplets. Alloying elements such as Al and Si in cast iron materials promote the graphite formation because they act as strong graphitizers probably by creation active nuclei for graphite growing. The aim of this paper is to examine the influence of powder chemical composition on the features and properties of sprayed cast iron splat and coating by spraying three cast iron powders of different chemical compositions on Al-Si-Cu alloy. The effect of powder chemical composition on graphite formation and microstructure was investigated. In addition, the mechanical properties such as friction, wear resistance, and microhardness of sprayed coatings with those powders were examined. The influence of chemical composition of sprayed powders on the microstructure of cast iron coatings was examined by X-ray and SEM.

This paper presents the results of metallographic investigations of electric arc sprayed composite coatings for the manufacture or refurbishment of bearing components. The materials studied include iron aluminide and aluminum bronze, and their interface microstructure was examined by optical and environmental scanning electron microscopy (ESEM).

Plasma Transferred Arc hardfacing (PTA) is an excellent tool for surface tailoring as it allows for the manipulation of coatings chemical composition. In particular in-situ alloy development can be achieved during the deposition of different powder mixtures. In this work powder mixtures of Ni-Al, Nb-Al and Fe-Al were deposited by PTA. Coatings were characterized for their mechanical features at room temperature evaluated by Vickers microhardness under 300gf load, nano- (0.04gf) and macro- (10kgf) scratch tests and pin-on-abrasive disc tests under 1kgf. Results showed very high dilution for the processed coatings with Vickers microhardness varing with the chemical composition of the deposited powder, mixtures, with the Fe based deposits exhibiting the lower hardness (below 400Hv) and the Nb-based deposits reaching 900HV. Scratch hardness followed Vickers micro hardness only for the Nb based coatings. Abrasion mechanism also varied for each alloy system and within each alloy system, the harder the coating the better the abrasive wear resistance. However when comparing the full set of coatings the Nb based coatings exhibited a superior performance and the Ni based deposits the poorer wear resistance.

In this study, bioactive glass powders were synthesized from four different types of oxides (SiO2, P2O5, CaO and MgO). These oxides were mixed, melted, milled and sieved to produce powders with two chemical compositions of the 31SiO2-11P2O5-(58-x)CaO-xMgO system. The powders were plasma sprayed onto AISI 316L stainless steel and Ti6Al4V titanium alloy substrates using a F4MB Sulzer Metco gun. The physical and mechanical properties of coatings, as well as their bioactivity were evaluated. The bioactivity tests were carried out exposing the surface of coatings to simulated body fluid (SBF) during 1, 9 and 15 days. The thickness and hardness of apatite layer produced on the surface of each coating during bioactivity tests were evaluated. The results indicate that the thickness of apatite layer formed during 15 days in SBF is between 31 and 51 µm and its hardness is between 1.5 and 1.9 GPa according to the chemical composition of feed stock powder used to manufacture the coatings. Additionally, the harness of bioglass coatings decreased around 26% after to expose them to SBF.

Today, powder particles diameter used for thermal spraying is generally comprised between 5 and 100µm with a preferred range around 40µm for APS applications. Actually, the future trends in plasma spraying are directed to the use of fine or ultrafine powders and the reduction of the steps between raw materials and coatings. So, the present paper investigates the way to use directly spray dried ceramic powders in suppressing the sintering stage. AI2O3 based powders were obtained by the spray drying process. By optimizing the parameters (slurry composition and injection as well as drying characteristics), a narrow grain size distribution was achieved. Chemical composition and shape of synthesized powders were analyzed by scanning electron microscopy (SEM) equipped with energy dispersive spectrometry (EDS). The crystallographic structure was identified by X-ray diffraction (XRD). Demonstration was made that it is possible to obtain coatings using directly spray dried ceramic powders. The plasma spray process parameters (such as current intensity, gas flow rate, powder feed rate and injection mode, cooling stage,...) have to be managed to achieve cohesive coatings. The structure and chemical composition of these coatings were studied. In this way, the direct use of spray dried powders appears as a promising way to realize ceramic coatings.

Undesirable changes in the coating composition can be reduced if plasma spraying is performed in a special chamber with a regulated atmosphere, the process being extremely costly, though. Another method allowing changes in the structure of coatings is thermal treatment. Numerous works present and discuss the results of the research into the influence of carburizing, nitriding, and laser or electron beam treatment on the properties of plasma sprayed coatings. This work concerned composite coatings obtained by plasma spraying of the blend of Al 2 O 3 -3TiO 2 and CuO powders, which were then reduced at an atmosphere of dissociated ammonia. Their chemical composition and morphology were determined by means of the EDS microprobe and the Joel 5400 scanning microscope respectively. It has been reported that the process of reduction contributes to the homogeneity of the coating, and that the modified structure contains Al 2 O 3 and Cu. Abstract only; no full-text paper available.

The most commonly used structural materials for blades and other high temperature components of gas turbines are nickel superalloys such as Inconel 738, MAR M247M or Hastelloy. Thermal barrier coatings (TBCs) are widely used on these substrates as protection against high temperatures and oxidation. A TBC system consists of a top coat of yttria partially stabilized zirconia deposited by air plasma spray and an underlying bond coat (usually MCrAlY, where M is Ni, Co or a combination of both). MCrAlYs are normally deposited by thermal spray processes such as air plasma spray, vacuum plasma spray (VPS/LPPS) or high velocity oxygen fuel (HVOF). In general, the adhesion of the whole thermal barrier system is strongly dependent on the surface preparation of the substrate and it is generally believed that a certain degree of roughness promotes better adhesion. OEM’s (Original equipment manufacturer) procedure for preparation of substrates and analysis have been reviewed and considered as basis of this work. The scope of this work is to set up a new cleaning methodology in order to obtain a completely pollution free surface to be coated afterwards with HVOF or VPS/LPPS. The properties of this new methodology have been compared with standard surface preparation techniques such as blasting with corundum and silicon carbides. The obtained samples have been analysed by means of metallography and chemical composition of the interface in order to measure the interfacial pollution between substrate and coating. Finally adhesion of MCAlY coating have been tested and compared with specification of the main OEMs.

A continuous galvanizing line (CGL) has a zinc pot, which is filled with molten zinc for zinc coating. In a zinc pot there are pot rolls to guide steel strip. Usually WC-Co thermal sprayed coatings are used for protection of the pot rolls from severe corrosion by molten zinc. Authors analyzed WC-Co coatings used in a zinc pot of a CGL for 33 and 56 days. On the surface of a WC-Co coated roll, many kinds of deposits were observed including top dross, Fe2Al5 inter-metallic compound, which might induce dross defect on the surface of galvanized steel. Diffusion depth of zinc into the WC-Co coating used for 33 days was only within 10µm but some areas were severely attacked along cracks within the coating layer. Usually molten zinc contains small amount of aluminum about 0.12 - 0.2%. Through SEM study, we observed that not only zinc but also aluminum diffused into the WC-Co coating after service in the zinc pot for 56 days. Al-Fe rich layers were observed on the surface of the spray coating for some cases. The phase of those layers might be Fe2Al5 since their chemical compositions are similar to Fe2Al5 top dross.

Cavitation erosion frequently occurs in hydraulic components such as turbines, valves, pumps, and ship propellers. Arc thermal spray processing has the possibility to be used for maintenance recovering of hydraulic blade runners. Fe-Cr-Mn-Si is a cavitation-resistant class of steel with a high concentration of oxidation elements—which can be important for arc thermally sprayed coatings—and a strain-induced phase transformation. The influence of chemical composition on oxide formation, microstructure, and cavitation resistance of Fe- Mn-Cr-Si thermally sprayed coatings was studied, and its field performance in a Francis type runner was evaluated. Microstructures and properties were investigated by XPS, XRD, optical microscopy, and ultrasonic cavitation testing. The best cavitation resistance was obtained in Fe-Mn-Cr-Si alloy with a nickel addition; this composition has lower oxide and splash droplets content and exhibits better splat wetting than Fe-Mn-Cr-Si without nickel. Strain-induced phase transformation occurred in arc thermally sprayed coatings during cavitation tests. Better performances for Fe-Mn-Cr-Si alloys, without nickel, were obtained in alloys with higher strain induced martensite contents after cavitation tests. In field tests, after 2000 operation hours, it was verified that the recovered areas presented only a small number of eroded areas, and cavitation erosion was reduced compared with uncoated areas.

The deposition of cavitation-resistant materials coatings in turbine blades is an important way to reduce cavitation damage. Fe-Cr-Mn-Si is a cavitation-resistant steel with many deoxidation elements, which can be important for arc thermal spraying materials. The influence of air pressure, arc tension, and chemical composition on the microstructure, area fraction oxide, porosity, microhardness, and cavitation resistance were studied. Microstructures and properties were investigated by XRD, optical and electronic microscopy, microhardness testing, and ultrasonic cavitation testing per ASTM G32-93. Chromium addition promotes an increase in area fraction oxide, and reduces the porosity, changing the microhardness. An increase in air pressure raised the oxide fraction in the SMA_A and 2 alloys. The SMA_A mass loss rates were 31.8, 25.8, and 37.2 mg/h, respectively, for the samples with 280, 410, and 550 kPa of air pressure. For the SMA_3 samples, the increase in the arc voltage reduces the oxide fraction, changing the mass loss rate to 43.8, 32.4, and 29.4 mg/h for 25, 30, and 35 V, respectively. Phase transformations occurred in the arc thermal spray, for all coatings, during cavitation tests. The SEM analysis verified that the mass loss in arc thermal spray coatings occurred because of the oxide fracture and delamination of the splats.

In this investigation, binary FeAl layers are produced by detonation spraying and examined by means of microscopy and X-ray diffractometry. The examinations show that the layers have a lamellar structure consisting of FeAl and Fe 4 Al 13 phases with a small amount of alumina. It is observed that with increasing amounts of fuel gas and decreasing detonation frequency, the proportion of FeAl phases decreases and variations in microhardness throughout the coating become irregular. Paper includes a German-language abstract.