Characterizing the true stress-strain curve of CrAlN coatings by nanoindentation using finite element-reverse analysis

This paper proposes a methodology to convert thin film nanoindentation load-displacement curves into tensile stress-strain curves. An optimization procedure, based on the trust-region reflective algorithm, is established to identify a unique set of three elastoplastic properties: Young’s modulus, yield stress and the strain hardening coefficient of the coating materials. The uniqueness issue of the optimization results is addressed by choosing initial guess parameters closer to the target values. The initial guess parameters are provided by the Jönsson and Hogmark model. The optimal elastoplastic properties of the coatings including, Young’s modulus (E), yield stress (σy) and work hardening exponent (n), are used to generate the true stress-strain curve. The presented methodology is validated on CrAlN monolayer coatings with different aluminum content. The results show good agreement between the simulated and the experimental nanoindentation load-displacement curves and as a result, the true stress-strain curve of the studied monolayer coatings is built successfully. These findings provide valuable insight for evaluating the elastoplastic properties of monolayered systems in order to optimize their structure.