S.S. Pathak; S. Supakul; K. Jacob; G.J. Tucker
SSRN
We report on the first successful deposition of a four-layered Ti2AlC/Ti MAX/metal nanolaminate system using physical vapor deposition techniques. Unlike previous unsuccessful attempts that had failed due to possible diffusion occurring at the elevated deposition temperatures of 800–900 C, our current work utilizes a combination of a TiC seed layer (to act as a diffusion barrier) and alternating deposition temperatures of 800 C and room temperature for the MAX and metal layers respectively in order to mitigate diffusion in the nanolaminate. The resultant 4-layered Ti2AlC/Ti nanolaminate showed minimal diffusion across the layers, and it also preserved a continuous multilayered microstructure with~ 170 nm individual layer thicknesses and nanocrystalline grain sizes. Nanoindentation and micro-pillar compression of the 4-layered Ti2AlC/Ti nanolaminate showed an impressive combination of strength (yield, instability strength and hardnesses of 3.56±0.14 GPa, 5.57±0.03 GPa and 7.15±0.5 GPa respectively) and strain to failure (plastic strain at instability 0.072±0.005, which is 7x higher than other ceramic/metal nanolaminates of comparable layer thicknesses). The high strength values of the nanolaminate had a predicted correspondence to the grain size strengthening of their respective components. On the other hand, the significant 7x improvement in instability strain in our Ti2AlC/Ti nanolaminate is postulated to be caused by the nano-size grains present in the Ti2AlC layers. These nanocrystalline grains enable the MAX layers to avoid brittle cracking or kink banding along basal planes since the wide distribution of the nano-grain orientations allows for multiple operative deformation mechanisms under compressive deformation. Thus, the incorporation of a nano-grained MAX phase layer in our Ti2AlC/Ti MAX/metal nanolaminate was instrumental in promoting toughness in the nanolaminate without compromising its high strength, something not readily achievable in other ceramic/metal nanolaminates.