Tungsten carbide – cobalt powders (WC-17wt.%Co) were plasma sprayed by a water-stabilized system WSP. A matrix of experiments with variable feeding distances and spray distances was carried out. Thinner coatings were carried out on carbon steel substrates and thicker coatings on stainless steel substrates to compare fast cooling conditions – the former with slower cooling conditions. Basic characterization of coatings was done by XRD, SEM and light microscopy plus image analysis. Microhardness was measured on polished cross sections. The main focus of investigation was on resistances against wear in dry as well as wet conditions. The appropriate tests were performed with set-ups based on ASTM G65 and G75, respectively. The influence of spray parameters onto coating wear performance was observed. The results of mechanical tests are discussed in connection with changes of phase composition and with the character of the coating’s microstructure. The results show that for obtaining of the best possible WC-17Co coating with WSP process, from the viewpoint of wear resistance, the desired parameters combination is long feeding distance combined with short spray distance.

The shear test in accordance with EN 15340 is a new test method for testing the bond between substrate and thermally sprayed coatings. It has been developed in order to enable a test method without the disadvantages of the method currently in use, the adhesion test. In the adhesion test the adhesives influence the test results; therefore in the shear test no adhesives are used. The adhesives are among others the reason for a large deviation of the test results using the pull-off test. Using the shear test the mode of the test results depend on the ratio between adhesion and cohesion; this ratio causes three different fracture modes. In order to investigate the deviation depending on the fracture mode samples have been coated by wire flame spraying, arc, plasma and HVOF spraying. Different ceramics, carbides and metals have been used as thermal spraying materials. For each material-process combination between 30 and 120 samples have been coated. This report describes the interpretation of the results of the shear test depending on the fracture mode and the coating materials applied by different thermal spraying processes. The deviation of the results depending on the fracture mode has been investigated using a shear test device by which the shear force is recorded over the displacement of the coating. The knowledge of the deviation and the distribution of the results is necessary to decide on the required number of samples to reach a result with a defined reliability.

For the promising erosion and oxidation resistance of carbide cermet coating, Cr39Ni7C cermet coatings were deposited by Diamond Jet spray process using a commercial Sulzer Metco 5241 powder in this study. The microstructure, phase composition and high temperature oxidation behavior of the deposited coatings were investigated. The speed and temperature of in-flight particles were measured by DPV-2000. The results revealed that the speed of in-flight particles decreased from 229 m/s to 150 m/s with the increasing of spraying distance from 100 mm to 300 mm, whereas the average temperature of in-flight particles increased from 1926 to 2245 K. The decarburization of Cr 3 C 2 increased with the increasing of fuel gas flow from 30 FMR to 40 FMR for higher heat enthalpy of flame. Due to the formation of Cr 2 O 3 on/in the coatings at high temperature, the sprayed coatings had good oxidation resistance at 1073 K in air atmosphere. For the lower porosity of the coating deposited under the spraying distance of 150 mm, its oxidation resistance was better than that sprayed under the spraying distance of 200 mm.

Residual stress build up in thick thermal spray coatings is a property of concern. The adhesion of these coatings to the substrate is strongly influenced by the residual stress generation during the coating deposition process. In the HVOF spray process, due to lower processing temperature and higher particle velocity as compared to plasma spraying, significant peening stresses are generated during the impact of semi molten particles on the substrate. The combination of these peening stresses together with quenching and cooling stresses that arise after deposition can be of significant importance. In this paper both a numerical finite element analysis (FEA) method, to calculate peening, quenching and cooling residual stresses, and experimental methods, as Modified Layer Removal Method (MLRM) and Neutron Diffraction analysis, are applied. The investigation is performed for thick Inconel 718 coatings on Inconel 718 substrates. Combined, these numerical and experimental techniques yield a deeper understanding of residual stress formation and a tool for process optimisation. The relationship between the stress state and deposit/substrate thickness ratio is given particular interest.

Nanostructured and conventional Alumina–13wt.% Titania powders were thermally sprayed using air plasma spray(APS) process. Scanning electron microscopy (SEM) was used to examine the morphology of the agglomerated powders and the cross section of the alumina-titania coatings. The microstructure and phase composition of the coatings were characterized by X-ray diffraction (XRD), and scanning electron microscopy (SEM).The fatigue and mechanical properties of the coatings were investigated. SEM analyses were also carried out on the fracture surfaces of fatigue-tested samples to assess the mechanisms of deformations. The experimental data indicated that the nanostructured coated samples exhibited higher stiffness, hardness, and fatigue strength compared to the conventional coated samples.

Thermal spray coatings from several different coating families have been metallographically prepared using traditional and modern metallographic techniques. The different recipes used were intended to demonstrate the effect of abrasive on coating appearance. Traditional metallographic recipes, which rely heavily on the use of silicon carbide (SiC) abrasive papers, were found to produce a notably different appearance than those prepared using modern recipes. Modern recipes, which incorporate extended diamond grinding and polishing steps, were found to produce what appears to be a more representative coating structure. Other variables, including mounting media and use of vacuum impregnation, were also found to influence coating appearance.

In the recent decade, considerable numerical models have been built up to simulate the thermal spray process. However, much less work has focused on the prediction of thermo-physical properties of the thermal spray coating, in particular the heat insulation properties. In this paper, a microstructure integrated finite element model is developed to investigate the heat insulation behavior of the thermal spray coating. A two-layer model is used to calculate thermal conductivity of the coating, where one layer stands for the coating by a unit cell, while another one for a standard material with known thermal conductivity. In the proposed unit cell model, pores and unmelted particles are assumed spherical and randomly distributed, and the interface between the coating and the unmelted particles is perfectly debonding. Based on the predictions, the effect of the pores, unmelted particles, cracks and their respective distributions on the heat insulation behavior of the coating has been further discussed in the paper.

Air plasma sprayed thermal barrier coatings, which reduce the temperature in the underlying substrate material, are an essential requirement for the hot section components of an industrial gas turbine. For TBC systems, the adherence of the top coating is one of the most important parameter for the durability of TBC system. In this work, the thermal fatigue behaviour of an air plasma sprayed thermal barrier coating was investigated. In addition, the residual interfacial strength was also evaluated by means of the 4-point bending test. From the measurement of the AE signals during the thermal fatigue tests, micro-cracking occurred in each cooling stage of the thermal fatigue cycles and then such damage depends on the number of thermal cycle. In addition, TGO grew at the interface with the exposed time at elevated temperature (the time dependent damage). Thermal barrier coating undergoes both time dependent damage and cycle dependent one under thermal fatigue condition. The life of thermal cycle with high temperature dwell time is shorter than not only that of isothermal exposure but also that of thermal cycle without dwell time.