Insights on the fracture and fatigue resistance of physical vapor deposited thin films

Abstract

Fatigue of structural components is classified among the most common forms of mechanical failure. For ductile materials, fatigue damage is correlated with the irreversible motion of dislocations, which manifests in surface stress concentrations followed by crack nucleation and growth. Surface engineering based on physical vapor deposited ceramic materials constitutes a promising solution to suppress this natural crack initiation and contributes to an increased fatigue life. While research has strived to unravel the mechanisms governing the coating to substrate behavior on a macro scale, limited information has been gathered on the independent response of nanostructured thin films under cyclic loads. Therefore, this thesis contributes to a fundamental understanding on the life cycle of PVD-deposited thin films targeted to improve the fatigue resistance in mechanically demanding environments. This framework ranges from the synthesis and detailed characterization of advanced thin film materials, to their micro-scale analysis under cyclic mechanical loads.A novel methodology, connecting cyclic-bending of free-standing microcantilevers with in-situ synchrotron nanodiffraction, is used to investigate the intrinsic fracture and fatigue properties of thin film materials. This systematic analysis focusses on a model system of metallic bcc-Cr and Cr-based compounds (fcc-CrN, hcp-CrB2, and rh-Cr2O3), to address the impact of altered bonding characters and crystal structures. Cyclic bending experiments performed up to the high cycle fatigue regime (N = 107) demonstrate that the fatigue strength of nanostructured thin films is governed by their inherent fracture resistance. In particular, cantilever cycling close to the critical stress intensity threshold is sustained without noticeable material damage, structural or stress-state changes. Within this journey, Cr1.03B2 was found with exceptional fracture and fatigue resistance, revealing a particular ductile character and a toughness value of KIc = 4.3±0.3 MPa√m.Finding the impetus in these results, a Si alloying route is followed to evolve CrB2 as a future protective coating material. Density functional theory calculations combined with atom probe tomography reveal a synthesis-independent Si solubility limit of ∼3-4 at.% in AlB2-structured solid solutions. The addition of up to 17 at.% Si entails refinement of the columnar morphology, accompanied by the growth of mechanically weak Si grain boundary segregates. This microstructural separation is reflected in degraded mechanical properties, with the film hardness and fracture resistance decreasing from H ∼ 30 to 17 GPa and KIc = 2.9 to 1.7 MPa√m with increasing Si-content, respectively. In contrast, outstanding oxidation resistance up to 1400 °C is revealed when alloying 8 at.% Si, owing to the thermally activated precipitation of Si and the B/Cr-ratio-independent growth of a stable SiO2-based scale. Additional in-situ fracture toughness measurements up to 800 °C expose a brittle-to-ductile-like transition for CrB2, increasing the fracture toughness to KIc = 3.3 MPa√m. Similarly, Si-precipitates in higher alloyed compositions enable extensive high-temperature plastic deformation combined with enhanced crack growth resistance.The final part of this thesis is devoted to investigations on the synthesis of metastable Ti-Al-N thin films using high-power impulse magnetron sputtering and the particular role of increased ion-bombardment during film growth. A systematic variation of the synthesis and discharge parameters performed on Ti1-xAlx composite targets cumulates in optimized deposition conditions for the low-temperature growth of cubic structured Ti0.37Al0.63N. The thin films obtain exceptional hardness of H ∼ 36 GPa and simultaneously a low compressive stress state, with age-hardening capacity up to H ∼ 40 GPa after annealing for 1h at 700 °C. Achieving this high Al solubility – being close to the theoretical limit of ∼67% – is seen in the preferred bombardment with process gas- and Al+-ions, while the contribution of detrimental Tin+ (n = 1, 2) ions is inherently low. In addition, particular sensitivity of the wurtzite phase formation for the nitrogen-to-argon flow ratio and substrate bias potential is revealed, both significantly influencing the arriving ion flux and adatom mobility on the substrate surface.