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Abstract:Residual stresses have been a major source of concern, as they are an inevitable consequence of manufacturing and fabrication processes. The magnitude of these stresses is often as high as, or at least, comparable to the yield strength of the material. In terms of arc sprayed coatings, the utilization of bore hole drilling methods presents some practical disadvantages as mechanical parameters (Poisson's ratio, Young's modulus) need to be identified in order to determine the residual stress distribution. Curvature techniques using Almen strips are cost-and time-effective methods that can be used for analytical quality assurance. Within the scope of this work, a quantitative study of the amount of residual stresses induced in a twin wire arc spraying (TWAS) process for a given combination of process parameters was conducted using the incremental bore hole drilling method, as well as the curvature method including Almen strips. Therefore, the effect of the primary gas pressure, substrate preheating temperature, and handling parameters, such as the spray angle and gun velocity, which influence the coating deposition as well as the heat input into the substrate, are examined. The experiments were carried out by using an iron-based cored wire with cast tungsten carbides as filling. The results of both methods are in an acceptable accordance with each other. Different stress fields were observed depending on the parameter settings.

Metal structures in offshore facilities are usually protected from corrosion using Zn-Al coatings even though they are subjected to collective stress conditions. This paper evaluates a post-treatment called machine hammer peening and its effect on surface finish, induced residual stresses, and near-surface microstructure of thermally sprayed ZnAl4 coatings. As expected, coating roughness was reduced from about Rz = 53.5 μm in the as-sprayed condition to 10.4 μm after treatment and coating densification was revealed in the near-surface zone. Residual stresses, which were surprisingly compressive in the as-sprayed condition, were likewise affected by the peening process, reaching a maximum of 200 MPa. The influence of peening direction and other such parameters were also investigated as part of the study.

Current developments in different industrial sectors show an increasing demand of thermally sprayed internal diameter (ID) coatings. The most recent research and development is mainly focused on commercial applications such as arc spraying (AS), atmospheric plasma spraying (APS), and plasma transferred wire arc spraying, especially for cylinder liner surfaces. However, efficient HVOF torches are meanwhile available for ID applications as well, but in this field, there is still a lack of scientific research. Especially, the compact design of HVOF-ID and APS-ID spray guns, the need of finer powders, and the internal spray situation leads to new process effects and challenges, which have to be understood in order to achieve high-quality coating properties comparable to outer diameter coatings. Thus, in the present work, the focus is on the ID spraying of bond coats (BC) and thermal barrier coatings (TBC) for high-temperature applications. An HVOF-ID gun with a N 2 injection was used to spray dense BCs (MCrAlY) coatings. The TBCs (YSZ) were sprayed by utilizing an APS-ID torch. Initially, flat steel samples were used as substrates. The morphology and properties of the sprayed ID coating systems were investigated with respect to the combination of different HVOF and APS spray parameter sets. The results of the conducted experiments show that the HVOF-ID spray process with N 2 injection allows to adjust the particle temperatures and speeds within a wide range. CoNiCrAlY bond coats with a porosity from 3.09 to 3.92% were produced. The spray distance was set to 53 mm, which leads to a smallest coatable ID of 133 mm. The porosity of the TBC ranged from 7.2 to 7.3%. The spray distance for the APS-ID process was set to 70 mm, which leads to a smallest coatable ID of 118 mm.