H. Liao

Influence of coating microstructure on the abrasive wear resistance of WC/Co cermet coatings

Thermally sprayed WC/Co cermet coatings are widely used for their resistance to abrasive wear. In this work the influence of the abrasive grain size on the abrasive wear resistance was investigated for WC/Co coatings sprayed with several processes: APS, high-velocity oxy-fuel (HVOF) and VPS. Results show that the larger the abrasive particle size, the lower the abrasion resistance of the coatings. Moreover, among the thermal spray techniques tested, HVOF coatings have the best wear resistance while VPS coatings present the lowest resistance. The role of the coating microstructure and residual stresses on the wear track and morphologies, as well as the abrasive wear behaviour, are discussed.

Effect of residual stresses on air plasma sprayed thermal barrier coatings

Abstract Thermal barrier coating with a CoNiCrAlY bond coating and a 0.3 mm thick zirconia, (7% yttria stabilized) top coating were air plasma sprayed onto a Hastelloy-X nickel based super alloy substrate coupons. Substrate preparation is a key initial step in the production of quality thermal sprayed coatings. Grit blasting is usually used to roughen the substrate surface so that better adhesion of the coating to the substrate is achieved. The effects of different grit blasting variables on the substrate surface roughness were examined by using the Taguchi designed experimentation. The residual stress profiles were determined for a different set of conditions by hole drill method. The Anstis et al. model was modified for the determination of indent toughness of the substrate and bond coat interface. The results showed that the combination of grit size with distance and pressure during the grit blasting were the most influential parameters. The adhesion of the coatings increases with increase of substrate roughness up to certain limits and then decreases. It was observed that the residual stresses have an impact over the adhesion of the coatings. The indentation toughness results showed that with increase of substrate roughness there is an increase in interfacial toughness due to high compressive stresses associated with high rough surfaces. Further, the final deposition temperature and heat treatment effect the residual stress profile and subsequently the interfacial toughness of the coatings.

Uniform design method for optimization of process parameters of plasma sprayed TiN coatings

Abstract Titanium nitride (TiN) coatings deposited by PVD and CVD have been increasingly used in industry for diverse applications. In this work, a plasma spray process of TiN deposition was investigated using a uniform design method. The experimental results of deposition efficiency, porosity, oxide content, microhardness and fracture toughness were regressed as the third order polynomial equations of the investigated process parameters. The relationships between the deposition efficiency, porosity, oxide content, microhardness and fracture toughness were examined in terms of coefficient of correlation. It was found that there were significant linear relationships between the deposition efficiency, oxide content, microhardness and fracture toughness. On the contrary, the linear relationships between them and the porosity were rather weak. Within the range of the uniform design of experiments, the deposition efficiency, microhardness and fracture toughness all increased, and the oxide content decreased while decreasing the argon flow rate and increasing the hydrogen flow rate. Using atomized liquid CO 2 for cooling the substrate during plasma spraying, it is possible to further improve the quality of the TiN coating.

Relationships between in-flight particle characteristics and coating microstructure with a twin wire arc spray process and different working conditions

Abstract The aim of this study is to get a better understanding of the build up of coatings obtained through twin wire arc spray process. Properties of in-flight steel particles, i.e. diameter, velocity and temperature, were determined using a dpv system, for different working conditions. Due to very large size distribution, specific experimental set-up has been developed in order to collect particles at a given spraying distance. As a result, the dpv diameter parameter was calibrated and well-marked tendencies on particle velocity and diameter have been found while changing input parameters. Then, the morphology of the splats has been studied in terms of flattening degree and shape factor. Finally, some correlations have been established between input parameters and properties of coatings in relation with particle characteristics and splats analysis.

Friction and wear mechanisms of thermally sprayed ceramic and cermet coatings

Abstract The tribological behavior of Al 2 O 3 /TiO 2 and WC/Co plasma-sprayed coatings was studied under dry sliding conditions. X-ray photoelectron spectroscopic analyses have shown the presence of graphite in WC/17xa0wt% Co coatings and a thin WO 3 layer in the contact. These phases could explain the improvement in the friction behavior of the cermet/ceramic couples versus the ceramic/ceramic couples.

Correlations between operating conditions, microstructure and mechanical properties of twin wire arc sprayed steel coatings

Abstract An experimental design matrix was set up in which carbon steel coatings were deposited with a twin wire arc spray gun (TAFA 9000™), using either compressed air or nitrogen as spraying gas. The coatings mechanical properties were studied. Some correlations were made between these properties, spraying conditions and the microstructure of the deposits. Youngs modulus was estimated by the single beam method using finite element modeling. Results show that direct relationships do exist between spray conditions, oxide content in the coating and microhardness. Youngs modulus of the coatings depends on the lamella thickness and the oxide content. When increasing the compressed air flow rate, Youngs modulus increases at first because smaller particles and finer lamellae were made and it decreases later because of a higher oxide content. The increase of nitrogen flow rate lowers the oxide content and increases Youngs modulus.

Improvement in wear resistance of plasma sprayed yttria stabilized zirconia coating using nanostructured powder

Abstract Plasma sprayed yttria stabilized zirconia coatings were prepared using nanostructured and conventional powders with optimized process parameters for the highest deposition efficiency, the smallest porosity and the highest microhardness. The tribological properties of these coatings against 100C6 steel were then tested with a ball-on-disc arrangement. Results showed that although the friction coefficients of the coatings sprayed using the nanostructured powder were slightly different from those of the coatings sprayed using the conventional powder, the former coatings were more wear resistant than the latter coatings. The wear mechanisms of all the coatings were explained in terms of adhesion-induced spallation and micro-fracturing of lamellae. The improvement in wear resistance of the coatings sprayed using the nanostructured powder could be mainly ascribed to the decrease of micrometer-sized defects such as pores and interlamellar and intralamellar cracks in the coatings.

Influence of HVOF spraying parameters on in-flight characteristics of Inconel 718 particles and correlation with the electrochemical behaviour of the coating

The high velocity oxy fuel (HVOF) process is currently used to produce metallic coatings on a wide variety of surfaces, imparting to the base material improved wear, high temperature or electrochemical corrosion resistance. However, HVOF spraying of metals in the atmosphere typically leads to the formation of embedded oxides within a coating. In many service applications, the presence of oxides results in poor coating performance. The gas dynamics of the HVOF spray are particularly important in terms of oxide contents. Therefore, the objective of the present work is to determine the effect of the oxygen/fuel ratio on the particle characteristics during their flight in the jet. A comparison between the oxide contents in Inconel 718 coatings was established as a function of the spray parameters. Electrochemical measurements were performed, giving information on the role of the oxides present in the coating in relation to its corrosion behaviour. Finally, directions are proposed to optimise coating properties obtained with the HVOF process in relation to the in-flight particle characteristics.

Interlamellar boundary characterization in Ni-based alloy thermally sprayed coating

The use of thermal spraying process for corrosion protection requires a high level of knowledge concerning the material modifications during coating deposition. The effect of the high velocity oxygen fuel process on inconel 718 thermally sprayed coating microstructure was studied by XRD, transmission electron microscopy and STEM analyses. Thermally sprayed coating exhibits an interlamellar oxidation related to the in-flight and cooling particles. The in-flight oxidation is characterised by a globular oxide which results from convective motion in the liquid phase of the molten particles before impact on the substrate. The oxide was identified as CrNbO4. The cooling step on the substrate induces a duplex oxide with an outer layer composed of columnar spinel oxide (Ni, Fe)Cr2O4 and an inner oxide corresponding to a nanocrystalline CrNbO4 oxide, which is related to the rapid cooling of the flattened particles. # 2002 Elsevier Science B.V. All rights reserved.

Modelling of impact behaviour of cold spray particles: review

Abstract Cold spraying (CS) is a coating technique, which has developed rapidly in the last two decades and shows great potential in the industrial community due to its advantages of low temperature deposition as well as no oxides forming in the coating. This review’s focus is on the behaviour of particles impacting and the prediction of critical velocity for particle deposition during CS as calculated by numerical simulations according to the open literature. The first part presents an introduction of CS and its particle bonding mechanism. The second part briefly introduces the typically employed numerical computation methods and compares these methods. The third part discusses the effect of particle parameters on particle deformation behaviour. Finally, the current problems and prospects existing in numerical simulations of the impact of CS particles are explored.