Variation in the nanoindentation hardness of platinum

Pure platinum was probed with a nanoindenter fitted with a Berkovich tip to various depths. The indent pattern was made on the as-polished specimen prior to heat treating, after heat treating at 500 °C for 30 min, and again after further heat treating at 1000 °C for 30 min. The variability in the measured hardness decreased as the indentation depth increased from 50 to 300 nm. When the sampled was annealed, the hardness variation was also greater. Increasing hardness variation with decreasing dislocation density and sampling volume indicates that dislocation density plays a critical role in the observed variation, beyond solely instrumentation uncertainty, and supports a defect-based explanation for the stochastic behavior. It appears that the stochastic behavior occurs when multiple dislocations are present in the sampled volume rather than sampling only a single dislocation.