Influence of Physical Vapor Deposition on High-Cycle Fatigue Performance of Additively Manufactured Ti-6Al-7Nb Alloy

Abstract

Load-bearing permanent implants, such as hip or knee joint replacements, are permanently loaded in the human body and must withstand considerable high loading cycles. The characteristic properties of additively manufactured Ti-6Al-7Nb, manufactured by laser powder bed fusion (LPBF), such as a rough surface and high residual stresses, have a detrimental effect on the fatigue behavior of such components. Functional physical vapor deposition (PVD) coatings and heat treatments offer the possibility to influence these properties. For this reason, the effects of stress-relief heat treatment (SR; 600 °C/4 h) and three PVD coatings (titanium nitride (TiN), titanium carbonitride (TiCN), and silver-containing amorphous carbon (a-C:Ag)) on the mechanical properties, in terms of high-cycle fatigue, are identified. Wöhler curves are determined and the staircase procedure ascertains the fatigue strengths. The fatigue strengths increase compared to the as-built condition by 105.4% (SR), 44.2% (TiN), 31.1% (TiCN), and 2.6% (a-C:Ag). Fracture surfaces are analyzed by scanning electron microscopy and show LPBF characteristic defects such as pores. The surfaces are partially divided into forced and fatigue fracture, the latter characterized by fatigue striations. Overall, PVD coatings, and especially SR, lead to an improved high-cycle fatigue behavior.