Enhancing the high-cycle fatigue strength of Ti-Al-N coated Ti-6Al-4V by residual stress design

• Fatigue behavior of hard coated Ti-6Al-4V is dominated by residual stresses. • Sufficiently high residual compressive stress must be present in the coating. • Linear-elastic model further links fatigue strength to residual stress gradient. • Beneficial modification of residual stress gradient leads to fatigue enhancement. • Interlayer improves fatigue strength by shifting stress peak deeper into substrate. Physical vapor deposited coatings are widely utilized as surface protection for metal and ceramic components operating in harsh environments. However, research on the high-cycle fatigue (HCF) life of hard-coated metal substrates has reached contradictory conclusions, leaving it unclear whether ceramic coatings enhance or compromise their fatigue resistance. To improve reliability and extend service life, this study explores the residual stress-dependent influence of arc evaporated TiAlN-based thin films on the fatigue life of Ti-6Al-4V. Therefore, different stress-modifying approaches were implemented, including a substrate bias variation, a Tantalum based alloying strategy, and a specific interlayer design. The combination of high-cycle fatigue tests, synchrotron-based experiments providing depth-resolved stress profiles, and the formulation of a linear-elastic stress-failure model resulted in the following identified relationships: (i) A threshold level in the residual compressive stress state must be present in TiAlN-based coatings to prevent deteriorating HCF performance introduced by failure of the ceramic nitride. (ii) Once the residual compressive stress field is able to shift fatigue crack nucleation into the bulk titanium alloy, the HCF life increases. (iii) The further the residual tensile stress peak is shifted from the bulk material surface — achieved through an optimized residual stress design implementing a metallic interlayer beneath the TiAlN-based top coating — the greater the improvement in HCF strength. Overall, this approach achieved an unprecedented HCF enhancement exceeding 50 % compared to uncoated Ti-6Al-4V (from 420 MPa to 628 MPa at 10 7 load cycles), highlighting the importance of an in-depth understanding of stress gradients within coating-substrate combinations.