Thermally sprayed ceramic coatings have a layered structure with a limited interface bonding. Substrate surface temperature prior to droplets impact during deposition significantly influences the microstructure and properties of the coatings. Through the controlling of substrate surface temperature, the lamellar interface bonding of the deposits could be possibly improved. Al 2 O 3 coatings were deposited by atmospheric plasma spraying at the surface temperatures of 100, 275, 375, 480, 530 and 660°C. The fractured cross-section morphology was characterized by scanning electron microscopy. X-ray diffraction was used to analyse the phase contents. Micro-hardness, Young’s modulus and thermal conductivity of the deposits were measured. It was observed that the interface area with columnar grain growth across splat-splat interfaces was increased with increasing deposition temperature. Consequently, micro-hardness, Young’s modulus and thermal conductivity were increased with the increase of deposition temperature. The improvement of properties suggest that the lamellar interface bonding in the deposits was improved as the substrate surface temperature is increased.

The development of new hardmetal coating applications such as fatigue-loaded parts, structural components and tools for metal forming is connected with improvement of their performance and reliability. For modelling purposes the knowledge of thermophysical, mechanical and other material data is required. However, this information is still missing today. In the present work the thermophysical data of a WC-17Co coating sprayed with a liquid-fuelled HVOF-process from a commercial agglomerated and sintered feedstock powder from room temperature up to 700 °C was determined as an example. The dependence of the heat conductivity on temperature was obtained through measurement of the coefficient of thermal expansion, the specific heat capacity and the thermal diffusivity. Heat conductivities ranging from 29.2 W/(mK) at 50°C to 35.4 W/(mK) at 700 °C were determined. All measurements were performed twice (as-sprayed and after the first thermal cycle) in order to take into account the structural and compositional changes. Extensive XRD and FESEM studies were performed in order to characterize the phase compositions and microstructures in the as-sprayed and heat-treated states. Bulk samples obtained by spark plasma sintering from the feedstock powder were studied for comparison.

In the present work, APS and HVOF processes have been used to prepare alumina (Al 2 O 3 ) and magnesium aluminate spinel (MgAl 2 O 4 ) coatings designed for electrical insulating applications. The microstructures and the phase compositions of the sprayed coatings were evaluated by microscopic and XRD analysis. The electrical characteristics electrical resistance, electrical resistivity and dielectric breakdown strength were investigated using different methods: direct current (DC) measurements, electrochemical impedance spectroscopy (EIS) and dielectric breakdown testing. The electrical resistance was measured at room temperature at different humidity levels. Differences in the insulating properties due to the different natures of the coating materials, microstructures and the measurement methods used for electrical characterisation are discussed.

This is the first of two papers concerning the intrinsic mechanical properties of arc-sprayed WC-FeCSiMn coatings. In part 1 the elastic and plastic forming behavior of the layers are investigated by indentation, bending and tensile tests. They were performed on coated mild steel substrates as well as freestanding as-sprayed samples with different geometries. Considering the coatings microstructure, element and pore distribution, as well as the local microhardness the results of the indentation, bending, and tensile tests were evaluated. The critical role of pores and inhomogeneities within the sprayed coating was examined in detail. Micro- and macrocracking were investigated by SEM after the indentation and tensile tests. In-situ surface observation by optical 3D-microscopy was used to study the onset of cracking during the 3-point bending test.

The various thermal spraying methods available include the plasma process, which uses a plasma flame to melt a fine powder before it is sprayed onto a substrate, and the High Velocity Oxy-Fuel (HVOF) spray process, in which the flame is made from the combustion of oxygen. These methods differ both in the temperature and velocity with which the molten particles impact the substrate, leading to different coating characteristics. This includes differences in splat morphology and the nature of microstructural interactions at the splat-substrate interface. That is, features such as local melting of the substrate, the existence of porosity and the presence of oxides. For this study a nickel-chromium powder was sprayed onto mirror-polished stainless steel substrates using both plasma spray and HVOF to form single splats. These splats, and their interface with the substrate, were characterized using a range of microstructural characterization techniques and the observed differences were correlated to the spray conditions used.

In this study, we investigated microstructures of thermal sprayed coatings and single deposited splats using two types of ion beam milling: one is argon ion beam for the cross-sectioning of thermal sprayed coatings in a cross section polisher, the other is gallium focused ion beam for the cross-sectioning and TEM sample preparation of single deposited splats. The cross section of WC-Co coatings fabricated by the polisher showed that it created a mirrored surface with minimizing artifacts such as pull-outs of ceramic particles or smearing of pores during conventional metallographic preparations. A thin and locally re-thinned membrane of single warm-sprayed nickel splat was feasible to observe the internal interface of particle/substrate in high resolution electron images. The substrate was heavily deformed by the impact of nickel particle with high kinetic and thermal energies. The particle and the substrate were intimately bonded without any voids or gaps.

Functionally graded coatings (FGC), in which the composition and properties vary gradually from the bond layer to the top layer, are introduced in this study. Using a mixing nozzle, a graded coating was generated employing twin wire spraying (TWAS) process. The deposition started with a base layer of massive metallic wires for better adhesion to the substrate. At following top layers hard material, in form of powders, was injected to the massive wires to enhance the wear protection. The results show that microstructure, porosity, and compositions are gradually varied in the coatings. This is a clear indication for the better performance in as-sprayed FGC than coatings sprayed by means of cored wires. The approach was to grade the coating composition from pure metallic to composite with higher content of hard material particles. The goal of the study is to articulate the needed coating performances by customizing the layers deposited regarding to their position.

Pores and cracks have significant influence on the structural rigidity and mechanical behavior of the thermally sprayed materials. For some applications dense coatings are needed, while for others (e.g. thermal barriers) some level of porosity is desirable. Recent development in thermal spraying focuses on the tailored design of pores and cracks for specific applications. In this project, ceramic coatings with different level and morphology of pores and cracks were plasma and HVOF sprayed on titanium alloy substrates. Coating microstructures were observed using scanning electron microscopy. Mechanical behavior of the prepared coatings was evaluated using four-point bending test in terms of changing coating stiffness with increasing mechanical load both in compression and tension. Significant level of coating non-linearity and hysteresis were observed. Tests carried out for coatings with the same chemical composition but different microstructure proved strong dependency of coatings mechanical properties on pores and cracks morphology. Microstructure features relevant for the applied loading are discussed.