Multi-objective particle swarm optimisation of multilayer functionally graded coating systems for improved interfacial delamination resistance

In this paper a multilayer coating system was considered for a 316 L stainless steel substrate with potential application for hydrogen storage. The coating system was comprised of a 316 L stainless steel/Al2O3 functionally graded (FG) layer and a SiC top coat. The composition gradation and thicknesses of the FG layer and SiC top coat were considered as parameters to be optimized for higher edge interfacial delamination resistance within the system. A novel method of delamination energy variation was first used for finite element analysis (FEA) of the system and the best set of parameters were defined as well as an interfacial delamination criterion (maximum stable edge delamination crack length in the system). Analytical solutions of thermal residual stress, strain and strain energy density were also studied for the system. Two new multi-objective functions were derived based on the analytical equations and further investigated to maximize interfacial delamination resistance of the system using multi-objective particle swarm optimisation (MOPSO). The optimized conditions achieved by different methods were finally compared and concluded to be in good agreement. Overall, the use of the new objective functions for MOPSO, combined with FEA, increased the maximum stable delamination crack length from 7 mu m (obtained by a thermal residual stress criterion used by previous researchers) to 50 mu m (using the new method). Therefore, FEA based on energy variation and MOPSO with the new objective functions were found to be applicable for optimisation with respect to interfacial delamination resistance.