The quality and operation performance of layer composites manufactured by thermal spraying is influenced by two different processes, the activation process of the surface prior to coating deposition and the coating process itself. The activation of the substrate surface is mainly performed by grit blasting operations. Surface activation by grit blasting is used in order to improve the bonding between substrate and coating, which is strongly related to the size and nature of the surface topography and roughness generated by the blasting process. Besides the roughening effect, grit blasting induces compressive residual stresses into the substrate surface which can be critical especially for thin walled components, e.g. piston rings, where the component shape is an important factor for the operation behavior and functionality. Another effect is an increase of hardness in the surface region related to the induced compressive stresses. A variety of blasting parameters can influence the surface characteristics, like nozzle diameter, grit medium and size, blasting pressure, distance and time. The influence of these parameters on the surface roughness, hardness, component deformation and residual stresses was investigated by tactile surface metrology, universal hardness and Almen tests as well as experimental residual stress analysis with the incremental hole drilling and milling method. All investigations were performed on rectangular steel strips. The results are discussed concerning quality control features for grit blasting processes in serial production.

More than 20 years the Institute of Technical Thermodynamics of the German Aerospace Center (DLR) in Stuttgart has been active in the field of plasma spray technology with improvements of spray equipment and their application. At the beginning the DC vacuum plasma spray method was in the center of interest and knowledge gained from rocket technology helped to develop supersonic nozzles for the plasma spray torches producing plasma jets with increased velocity and improved laminarity in order to get denser coatings with higher quality. In the meantime also nozzles for subsonic conditions with controlled expansion of the plasma jet leading to considerably increased deposition efficiency were developed as compatible parts for already existing equipment and made available on the market. In the next step also the DC plasma torches themselves have been improved. Recently a modern equipment for RF plasma technology could be developed and installed, where some new ideas could be realised. Paper text in German.