Thermal spraying of thick film systems - process optimization by means of residual stress and residual stress stability

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The residual stress (RS) distribution in coating systems in particular atthe interfaces is essential for the fatigue and failure behavior ofcoated components. The RS distribution induced by the coatingprocess can be optimized to fit the technical requirements of thecomponents by adapting the coating parameters. Within the scope ofthe first funding period of the research project a practicable measuringand evaluation strategy based on the incremental hole drilling methodfor analysis of local residual stress depth distributions for thick filmsystems was developed and provided. Subsequently, this methodicalapproach was successfully applied to thermally sprayed YSZ thermalbarrier coatings and MCF chromium vapor protective layers for solidoxide fuel cell (SOFC) applications. In the course of the continuationof the research project the measuring and evaluation strategy will beadapted and extended (a) to functional graded materials (FGM) and(b) to cold sprayed materials. In the following, the residual stressresults will be used for adjustment of the coating process parameters.For the FGM tungsten containing layers will be deposited onEUROFER steel substrates, while the tungsten content will begradually increased by the supply of the powder mixtures until puretungsten will be achieved for the top layer. Possible applications ofsuch FGM layers are e.g. materials for application in fusion reactors,where tungsten is used as protective coating of the ferritic-martensiticEURPOFER steel. Here, knowledge about residual stresses isessential for the assessment of the mechanical integrity and the lifetimeof layered components. A further application that will be analyzedusing the same analysis probe is on cold sprayed layers, which will beused for the repair of localized damaged high temperature parts (e.g.turbine blades). First successful preliminary test were performed forcold prayed nickel-base substrates. For the repair of localizeddamaged components knowledge about the residual stressdistribution induced by the repair process is of crucial importance.Application of the incremental hole drilling method can directly beapplied at the thermally sprayed component. Furthermore, in additionto residual stress analyses on thermal sprayed YSZ-coatingsperformed in the first funding period data of neutronographicdiffraction analyses, which are already carried out at the instrumentStressSpec at FRM II (Garching), will be evaluated. Based on thisevaluation a holistic assessment of the residual stress results will bemade. Since, so called ´through-surface-scans´ were performed, thedata evaluation is very elaborate und requires inter alia the correctionof surface effects which occur at the very surface as well as at theinterfaces of the layered structure.