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 H3/E2 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, KI, 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 H3/E2, 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 KI value is ∼21 MPa∗m1/2, which is higher than typical values of bio-ceramics (Al2O3 and ZrO2) 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.