Abstract In this study, a novel strategy to manufacture high strength cold-sprayed Al coating by using powder with wide size distribution is proposed. The microstructure and mechanical properties of deposited coating sprayed at three typical impact velocities before and after heat treatment are investigated. Furthermore, the deposition and strengthening mechanisms of the coating sprayed at various impact velocities are clarified. The results show that the coating with higher density and mechanical properties can be successfully fabricated by cold spray at comparatively low particle impact velocity. The mechanical properties were enhanced with the contribution of heat treatment process. It is the in-process tamping effect induced by larger powder that results in the severe plastic deformation thus leads to densification and excellent mechanical properties of the cold-sprayed Al coating.

Abstract Ni-Al intermetallics have excellent corrosion and oxidation resistance, but their use in thermal spraying has been limited due to issues with in-flight oxidation. In this study, a novel approach is proposed to remove oxide from Ni-Al droplets in-flight by adding a deoxidizer (diamond) to the feedstock powder. A mixture of nickel, aluminum, and diamond powders was mechanically alloyed using a combination of cryogenic and planetary ball milling. The resulting Ni/Al/diamond composite powder was then plasma sprayed via the APS process, forming Ni-Al coatings on Inconel 738 substrates. Phase composition, microstructure, porosity, and microhardness of the coatings were characterized by X-ray diffraction, scanning electron microscopy, image analysis, and hardness testing, respectively. Oxygen content measurements showed that the coatings contained significantly less oxygen than coatings made from ordinary Ni/Al powders. In-flight particle temperatures were also measured and found to be higher than 2300 °C. The low oxygen content in the coatings is attributed to the in-situ deoxidizing effect of ultrahigh temperature droplets which are also oxide-free.

Abstract Cascaded plasma torches are becoming increasingly common, but the influence of geometry, notably that of the anode, is relatively unexplored. This work investigates the relationship between anode-cathode distance and plasma voltage fluctuations. The study was conducted using cascaded torches that can be configured with different numbers of neutrodes and commercially available Al2O3 powders. The powders were sprayed at different gas flow rates and current intensities while monitoring voltage fluctuations as well as in-flight particle temperature and velocity. The resulting alumina coatings were characterized based on microstructure, phase composition, porosity, and hardness. A frequency analysis of the arc voltage fluctuations revealed well-defined peaks at 60, 120, and 180 kHz that vary in intensity based on the number of neutrodes. The more neutrodes, the sharper and higher the peak. In contrast, the power spectra of the arc voltage generated by a conventional plasma torch contains no such peaks, indicating a random displacement of the arc root leading to less stability of the arc.

Abstract A previous study showed that Cu can be cold sprayed onto carbon fiber-reinforced polymers (CFRPs) if a Cu interlayer is deposited prior to low-pressure cold spraying. In this present study, Cu was cold sprayed onto CFRP substrates that were coated with either Sn (cold spray) or Ni electroplating. Two layers of Cu powder were also cold sprayed onto a Cu-plated CFRP substrate to investigate the effect of a second particle layer on impacting particles. Test results showed that the relative hardness between the particle and substrate has a major effect on deformability, impact mode, and deposition efficiency (DE), which explains why Cu could not be cold sprayed onto Sn or Ni interlayers and why the deposition efficiency of Cu-on-Cu substrates is lower than that of one pass spraying. In summary, the results suggest that Cu can be successfully cold sprayed at low pressures onto electroplated Cu due to their similarity in hardness.

Abstract Hydroelectric turbines are strongly affected by cavitation and the damage it can cause to critical part surfaces and profiles. The study of thermal spray processes and materials is thus relevant to improving turbine performance. The main objective of this work is to evaluate the influence of fuel-oxygen ratio on tungsten- and chromium-carbide cermet coatings deposited by HVOF. Particle velocity and temperature were measured as were coating hardness, porosity, and cavitation resistance. Higher particle velocities were obtained at higher fuel ratios, producing harder, denser coatings with better cavitation resistance. Based on test results, the wear mechanism starts with the nucleation of the cavitation that occurs in the pores, resulting in the formation of craters and the eventual detachment of lamellae as indicated by the smoothness of the surface.

Abstract Two kinds of cermet powders, WC-10Co4Cr and WC-20CrC-7Ni, were deposited on 1040 steel via high velocity air fuel (HVAF) spraying to evaluate resistance in cavitation erosion conditions with additional electrochemical effects. Coating microstructure, phase composition, and microhardness were examined along with the topography of eroded surface layers. The cavitation resistance of the WC-20CrC-7Ni coating was found to be approximately 1.3 times greater than that of the other coating, which can be attributed to its finer grain structure, lower pore density, and the presence of high Cr and Ni content in the feedstock powder which serves to strengthen the matrix.

Abstract Cu and Cu-MoS2 coatings were fabricated by cold gas dynamic spray and the fretting wear performance of the two coatings was compared. A mixture (95 wt.% Cu + 5 wt.% MoS2) was used as feedstock for the composite coating. Coatings were sprayed with identical gas flow conditions on the substrates preheated to approximately 170°C. The cross section of the coatings was analyzed by scanning electron microscope (SEM) and MoS2 concentration was measured, as well as coating microhardness. Fretting tests were carried out under gross slip conditions in ambient environment. SEM observation on wear scars and counterspheres revealed the development of third bodies, by which the sliding was accommodated. For the Cu-MoS2 coating, solid lubrication effects in the form of friction drops occurred in early cycles (< 5k), but eventually (> 5k) the coating's friction behavior was similar to the pure Cu coating. Third body morphology and wear of the two coatings were distinctly different, which could largely be attributed to the hardness reduction of the Cu-MoS2 composite due to poorly bonded interfaces induced by the effect of MoS2 during particle impact and coating formation.

Abstract Titanium aluminium based nitride (Ti, Al)N coatings possess excellent tribological behaviour with respect to metal cutting and polymer forming contacts. In the present work TiAlN coatings were deposited by plasma spray process. Three coatings of TiAlN were deposited on AISI-347 grade boiler steel substrate out of which two were thin nano coatings deposited at different temperatures of 500°C and 200°C and one conventional coating was deposited by plasma spraying. The as sprayed coatings were characterized with relative to coating thickness, microhardness, porosity and microstructure. The optical microscopy (OM), the XRD analysis and field mission scanning electron microscope (FESEM with EDAX attachment) techniques have been used to identify various phases formed after coating deposited on the surface of the substrate. Subsequently the sliding wear behaviour of uncoated, PVD sprayed nanostructured thin TiAlN coatings deposited at 500°C and 200°C and plasma sprayed conventional coated AISI-347 grade boiler steel were investigated according to ASTM standard G99-03 using pin on disk wear test rig. Cumulative wear volume loss and coefficient of friction, μ were calculated for the coated as well as uncoated specimens for 10, 15 and 20 N normal loads at a constant sliding velocity of 1 m/sec. The worn out samples were analysed with SEM/EDAX. Wear rates in terms of volumetric loss (mm³/g) for uncoated and coated alloys were compared. The nanostructured TiAlN coatings deposited at 500°C and 200°C has shown minimum wear rate as compared to conventional TiAlN coating and uncoated AISI-347 grade boiler steel. Nanostructured TiAlN coatings were found to be successful in retaining surface contact with the substrate after the wear tests.

Abstract A typical feature of plasma sprayed zirconia-based coatings is that their depostion efficiencies are low. Thus, from the economic view of reducing cost, how to improve the depostion efficiency of plasma sprayed zirconia-based coatings is very important for practical application. In this paper, spray-dried nanostructured powders and conventional powders were used as feedstocks to deposit nanostructured and conventional zirconia coatings by atmospheric plasma spraying(APS). Deposition efficiencies for both nanostructured and conventional zirconia coatings deposited under different spraying parameters were comparatively measured. The obtained results revealed that the spray-dried nanostructured powders possessed higher deposition efficiency than that of conventional powders. In addition, it was found that spraying parameters had strongly influence on microstructure and microhardness of zirconia coatings. Therefore, in order to obtain high quality zirconia coating, proper spraying parameters must be considered.

Abstract In the Cold Gas Dynamic Spray (CGDS) process, coatings are deposited by the virtue of the high particle velocity achieved by the use of converging-diverging (de Laval) nozzle along with suitable particle characteristics and process parameters. In this study copper coatings were deposited on aluminium substrates using helium as the accelerating gas. The influence of the CGDS conditions, primarily driving gas temperature and pressure, on the nature of the deposited coatings and the deposition efficiency of the process were investigated. The results indicate that it is possible to deposit copper coatings at a wide range of process conditions, with successful deposition being observed with the driving gas at room temperature and 11 bar pressure (a condition where the nozzle is still choked). However, the nature of the coatings is strongly dependent upon the processing conditions. With room temperature driving gas, an increase in pressure lead to an increase in deposition efficiency, and increase in substrate deformation and an increase in microhardness in the deposit due to higher levels of work hardening. The use of driving gas at temperatures as low as 473 K resulted in recrystallisation in the deposit and a decrease in tendency to debond due to stress relief during recrystallisation. Recrystallisation also manifested itself in reduced hardness. The sensitivity of the recrystallisation conditions to the traverse speed of the jet over the substrate indicated that these processes are initiated by the impingement of the hot gas jet onto the deposit following deposition and not by changes in velocity or temperature of the particles upon impact.

Abstract A microplasma spraying torch with a hollow cathode electrode is designed to melt completely the refractory materials and deposit coatings at plasma power level up to several kilowatts. The designed torch permits spray material to be fed into plasma arc jet through axial powder injection. In the present study, molybdenum is used as a typical refractory spray material. The effects of the main processing parameters including plasma arc power, plasma gas flow and spray distance on the particle velocity during spraying, and the microstructure and properties of the coatings are investigated. The microstructure of coating is characterized with optical microscopy and scanning electron microscopy. The properties of the coating are characterized by microhardness and abrasive wear tests. The particle velocity during in-flight is carried out using a particle velocity/temperature measurement system based on thermal radiation. The comparison of the microstructure and property of micro-plasma sprayed Mo coatings with those of the coating deposited by the conventional plasma spraying operated at a power of 42 kW is performed. The results show that the abrasive wear loss of the Mo coatings deposited by the micro-plasma spray torch is comparable to that of the coating deposited by the conventional plasma spraying disregarding the one order difference in the plasma operating power.

Abstract TiO 2 and TiO 2 -Al composites coatings were prepared by plasma spraying using a reconstituted nanosized feedstock via a spray drying method. Effects of various spray conditions on the microstructure, porosity, microhardness and wear resistance related to the mechanical performance of coatings were evaluated. The coatings sprayed at relatively low plasma power were composed of two distinct microstructures of well defined lamellar structure, similar to microstructure of conventional plasma sprayed coatings typically observed from fully melted particles, and embedded nano or sub-micron particles originated from partial/non-molten particles of feedstock materials. The fraction of partial/non-molten particles in coating layers was increased by Al additive. Such a characteristic of blended microstructure of coatings was clearly confirmed from a bimodal distribution of microhardness described by Weibull plots. The optimized addition of Al into TiO 2 improved mechanical properties such as microhardness and wear resistance.

Abstract A supersonic flame was produced in HVO/AF through the combustion of kerosene and the mixture of oxygen and air. The flame Temperature can be adjusted between 1400 and 2800 °C through changing the flux ratio of oxygen and air in the mixture. Three kinds of nanostructure WC-12Co coatings were sprayed by HVO/AF under three different spray conditions. Experiments results show that the coatings bond well with the substrates with very dense structure. The decarburization reduces with the decrease of flame temperature. Nanostructure coatings were much denser than microstructure coatings with about 0.4 to 0.5 times increase in microhardness.

Abstract Various processes are used to manufacture the nanostructured materials, such as the sol-gel process, the DC-MSIP process, and the HF-MSIP process. It is also possible that the nanostructures can also be produced in a short time using a thermally activated coating process. Zirconium oxide and aluminum oxide are the most important oxide ceramics. Both have a wide range of applications. In this paper, nanostructured layers of zirconium and aluminum oxide are produced with atmospheric plasma spraying. The layers are characterized with a scanning electron microscope, a transmission electron microscope, and an X-ray phase analyzer. Dense nano-structured oxide layers (zirconium oxides and aluminum oxides) with high microhardness are produced using the atmospheric plasma spray process. Paper includes a German-language abstract.

Abstract In this paper, the industrial application study is carried out to optimize the robot-controlled plasma spraying process of the thermal insulation layer for the hot gas flow of a GE-Frame-6B gas turbine. Due to the need for the off-line programming procedure, a precise 3D model of the automated coating system and the workpiece is created. In preliminary tests without a workpiece, basic spraying parameters such as spraying distance and spraying angle are optimized to achieve a sufficient layer quality. In the spray experiments, yttria stabilized zirconia Thermal Barrier Coating is used as the coating material and (NiCr)-6Al (Metco 443NS) as the bondcoat. The microstructure is examined using a light microscope, and microhardness measurements are also carried out. The results of the injection experiments are used to set the injection parameters and the travel path for the real component. Paper includes a German-language abstract.

Abstract The high velocity oxygen fuel process is a powerful tool to produce WC-based coatings with minimum carbide degradation through a limited temperature and a reduced time of interaction between the powder and the flame. WC-10Co-4Cr cermets have been modified by adding carbon-based materials to protect the WC phase from decarburization during the spraying process. The novel powders were made by ball milling and spray drying. The layers were characterized by means of SEM, X-ray diffractometer, microhardness, and wear tests. Structural properties were correlated with the wear behavior and the microhardness. In order to protect the WC-10Co-4Cr powder, this paper evaluates the effectiveness of the addition of a carbon-based material (graphite and amorphous carbon), as attempted by Moreau et Dallaire to protect TiC powders produced by plasma spraying. It discusses the effects resulting from the agglomeration process. Paper includes a German-language abstract.

Abstract In this paper, a study on the spraying of Hastelloy C-276 is carried out using the Axial III plasma spray method. The aim is to develop a layer with a low oxide content. The oxide content is related to the microhardness of the spray coating. In various test series, a Hastelloy C-276 layer with low hardness could be achieved with the help of plasma spraying in a protected and unprotected atmosphere. Paper includes a German-language abstract.

Abstract This article reports on a method for densifying thermally sprayed layers by arc plasma sintering (SPS). Thermal spray ceramic coatings consist from relative high percentage of porosity compared than that of metallic coatings. An investigation of the metallurgical and mechanical properties of flame-sprayed zirconia coatings after SPS at various temperatures and loadings condition was performed. The results obtained in the present work can be summarized as follows: bond strength and microhardness of coating after SPS is increased by three times compared than as-sprayed coating, and X-ray diffraction analysis indicates that the phase of zirconium dioxide-25MgO coating was influenced the physical properties of coating. Paper includes a German-language abstract.

Abstract In this paper, various nickel-based coatings (NiCr, NiAl, and NiCrAlFe) are applied using flame and plasma spraying. The coatings are post-treated with a 6 kW carbon dioxide cw laser. Four different laser powers are used to fuse the layers together. The structure of the post-treated layers and the heat-affected area are examined with the aid of light and scanning electron microscopy. The microhardness profile of the layer and the depth of the heat affected area are measured. The wear resistance is tested by means of the rubber wheel wear test. It is observed that the plasma-sprayed NiCrAlMo-Fe layers, which have been post-treated with a laser power of 1000 W, have the highest wear resistance. Paper includes a German-language abstract.

Abstract In this paper, binary FeAl layers on steel substrates are produced by detonation spraying and examined using a microscope and X-ray diffractometry. Lamellar layers showed the phases FeAl, iron-aluminum alloy, and aluminum oxide. It is observed that with increasing amounts of fuel gas (oxygen and acetylene) and decreasing detonation frequency, the proportion of FeAl in the layers decreases. It is also observed that the microhardness of the layers changed irregularly with reduced proportions of lamellar FeAl phases. Paper includes a German-language abstract.