Individual layer thickness-dependent nanoindentation and nanotribological behaviors of Ta/Co nanolaminates

摘要

The nanohardness (H), reduced elastic modulus (E), and nanotribological properties including nanoscratch and nanowear resistance of tantalum/cobalt (Ta/Co) nanolaminates with varying individual layer thickness (h) were systematically studied using a nanoindenter. With decreasing individual layer thickness from 100 nm to 5 nm, the H of the nanolaminates increased gradually and reached a peak of similar to 7.20 GPa at h = 5 nm, while the E increased gradually with a sudden decrease at h = 25 nm. The critical point of delamination (P-c), coefficient of friction (COF), and post-scratch microstructures of the nanolaminates were assessed via nanoscratch testing. The normal load at P-c was increased gradually with a decrease in h under a ramped load. The post-scratch microstructures after delamination showed clear film chipping/crack formation and propagation of cracks/breakdown of grains at larger h, while no film chipping/cracking/breakdown of grains was observed in the nanolaminate with h = 5 nm. The wear rate of the nanolaminates increased with an increase in applied load irrespective of h and, interestingly, it started decreasing with increasing the numbers of wear cycles and became steady at higher numbers of wear cycles during nanowear tests. The nanolaminate with h = 5 nm showed the lowest wear rate with minimal plastic deformation. Nanoindentation of the worn surfaces of the nanolaminate with h = 5 nm after nanowear testing showed significant work hardening despite its negligible plastic deformation during nanowear tests. Overall, the Ta/Co nanolaminate with individual layer thickness at a few nanometers' length-scale exhibited high strength and excellent scratch and wear resistance with minimal plastic deformation.