Temperature dependent mechanical properties of metallic HVOF coatings

Abstract

The complex failure mechanisms of metallic coatings under high thermomechanical loads can be analyzed in finite element simulations. For this purpose, mechanical properties are needed to ensure a realistic modelling approach. The development of new coatings and the usage of coating processes different from other investigations in the literature require new measurements to obtain realistic material parameters.In the present study, tensile- and compression tests as well as vibrating reed experiments were carried out to determine Young's modulus as well as strength and yield stress of HVOF sprayed metallic coatings at different temperatures. Free standing specimens of NiCrAlY, NiCuCrAl and Rene80 coatings were produced to avoid an unknown influence of the substrate to the experimental results.The values of Young's modulus from the three measurement methods show a good agreement at moderate temperatures. At elevated temperatures, a discrepancy between the quasi-static tests (tensile/compression) and the vibrating reed experiments has been observed, caused by creep and plasticity due to the lower strain rates and larger strains in the quasi-static tests.After a pre-exposure of the coatings to high temperatures, an increase of Young's modulus was observed. The mechanism of this effect has not been understood yet, but may be caused for example by recovery or a closure of defects and micro-cracks in the coatings.All coatings have a relatively large compressive strength and yield stress greater than 1 GPa at moderate temperatures, but exhibit a brittle/ductile transition at elevated temperatures where a sharp reduction of the yield stress and increase of ultimate strain could be observed.