Structure and mechanical properties of Fe1−xMnx/TiB2 multilayer coatings: Possible role of transformation toughening

The objective of this research is to characterize the elastic modulus, hardness, fracture toughness, and structure of multilayer coatings consisting of alternating nanolayers of Fe1 − xMnx and TiB2 coatings (x = 0, 0.18, and 0.35). These coatings were synthesized by dc magnetron sputtering. X-ray diffraction showed that Fe0.82Mn0.18 contains 79% bcc and 21% fcc phases, while Fe0.65Mn0.35 contains close to 100% fcc phase. The hardness of these multilayer coatings was found to exhibit a small enhancement (~ 2 GPa) over the rule-of-mixture values. The most striking finding is that the fracture toughness of Fe0.82Mn0.18/TiB2 is about twice of that for Fe0.65Mn0.35/TiB2 and Fe/TiB2 with comparable hardness. In addition, Fe0.82Mn0.18/TiB2 exhibits a quasi-elastic response in nanoindentation experiments. Given that Mn addition to Fe is known to result in the formation of metastable fcc phases at room temperature (with the degree of metastability controlled by the Mn content) and that the fcc phase may transform to the bcc phase under stress, such a transformation is likely to play a role in the increased toughness and quasi-elastic nanoindentation response observed in these coatings and may provide a strategy in the synthesis of hard coatings with improved toughness.