Finally, the hardness results presented in Fig. 9c indicate that an increase in the WO3 filler loading resulted in an initial increase in the hardness of the composite, but a further increase in filler loading at 10 vol% caused a reduction in hardness probably due to the undesirable agglomeration of the fillers. The initial increase in the hardness of the composites observed for both the nano-fillers and the micro-fillers may be attributed to their uniform dispersion within the epoxy matrix, together with their strong interaction with the epoxy chains to form good interfacial bonding.