The role of α″ orthorhombic phase content on the tenacity and fracture toughness behavior of Ti-22Nb-10Zr coating used in the design of long-term medical implants

Abstract Tenacity and fracture toughness of a novel β/α″ Ti-22Nb-10Zr (wt.%) coating processed by magnetron sputtering were modified as a result of the martensitic transformation (β → α″) activated by the presence of compressive residual stresses when the coating deposition is performed at high bias voltage values. Mechanical properties, such as hardness, H , and Young’s modulus, E , values, and therefore elastoplastic response of the coating were characterized through H/E , and H 3 /E 2 ratios as a function of the extent of the martensitic transformation. These ratios were correlated to the elastic response and to the resistance to plastic deformation of a surface subjected to sliding mechanical contact, respectively. The usefulness of both ratios to design “hard and tough” coatings, suitable for enhancing of its wear resistance, is compared with the tenacity, G , the semi-quantitative, FT , and the quantitative, K I , fracture toughness values obtained from nano-scratch characterization. Results show that Ti-22Nb-10Zr (wt.%) coating with the highest and lowest hardness and Young’s modulus values, and therefore the highest H/E and H 3 /E 2 , has the highest cracking resistance and fracture toughness. Under linearly ramped loading from 0.1 to 5 and 100 mN it was impossible to produce fracture of the coating when it was deposited with a bias voltage of −63 V. In return, the coating deposited with a bias voltage of −148 V shows an almost complete elastic recovery until the moment of its fracture and delamination, which is an evidence of its high tenacity and superior fracture toughness. The K I value is ∼21 MPa∗m 1/2 , which is higher than typical values of bio-ceramics (Al 2 O 3 and ZrO 2 ) used in medical applications, demonstrating that this coating could be used in components subjected to high wear and cyclic impacts, e.g. on femoral heads in artificial hip joints.