Mechanical properties of sprayed overlayers on superalloy substrates, obtained via indentation testing

This paper concerns the use of a recently-developed methodology for inferring stress-strain curves from indentation data, based on iterative FEM simulation of the procedure. A relatively large indenter (2 mm diameter) is used, with deep penetration (to about 25% of the indenter radius). This has been carried out on (polished) free surfaces of sprayed superalloy overlayers on single crystal superalloy substrates. Both load-displacement data and residual indent profiles were obtained, with the overlayers being in two different conditions (as-sprayed and annealed). The overlayers were relatively thick (∼2.5 mm), so it was also possible to carry out uniaxial compression tests on them (in the through-thickness direction). The inferred stress-strain curves were similar in each case when derived from load-displacement data and indent profiles, and also close to the plots obtained by conventional uniaxial testing. The yield stress levels in both cases were around 1000 MPa, but the work hardening rate was significantly higher for the as-sprayed material. This kind of information is of considerable potential value when attempting to optimize the properties of such overlayers. The procedure can be employed, with some confidence, to cases for which uniaxial testing is difficult or impossible.