Structure and mechanical properties of Fe_1-xMn_x/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 Fe_1-xMn_x and TiB₂ coatings (x=0, 0.18, and 0.35). These coatings were synthesized by dc magnetron sputtering. X-ray diffraction showed that Fe_0.82Mn_0.18 contains 79% bcc and 21% fcc phases, while Fe_0.65Mn_0.35 contains close to 100% fcc phase. The hardness of these multilayer coatings was found to exhibit a small enhancement (~2GPa) over the rule-of-mixture values. The most striking finding is that the fracture toughness of Fe_0.82Mn_0.18/TiB₂ is about twice of that for Fe_0.65Mn_0.35/TiB₂ and Fe/TiB₂ with comparable hardness. In addition, Fe_0.82Mn_0.18/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.