Influence of Si segregates on the structural evolution, mechanical properties, and high-temperature fracture toughness of Cr-Si-B2±z coatings

Abstract

The impact of Si-segregates and varying deposition conditions on the structural and mechanical properties of sputter deposited, high-temperature oxidation-resistant Cr-Si-B2±z coatings is studied from ambient, to elevated temperatures. Overstoichiometric, AlB2-structured Cr-Si-B2±z thin films with Si-content up to 15 at.% were synthesized on Ti-6Al-4V by magnetron-sputtering using a substrate bias of −120 V. The enhanced surface diffusion promotes mechanically superior, (001)-oriented coatings with hardness of H∼30 GPa up to a Si-content of 3 at.%. Higher Si-concentrations result in significant hardness loss to H∼20 GPa, related to a bias-independent solubility-limit in the CrB2-structure and the formation of mechanically-weak Si grain-boundary segregates. The as-deposited hardness of all Cr-Si-B2±z compositions is maintained after annealing to 800 °C, despite the initiation of material recovery. A B/Cr-ratio-independent oxidation resistance up to 1400 °C is demonstrated, underlining a minimum Si-content of 8 at.% to form a stable SiO2-based scale. In line with the room-temperature hardness, increasing Si-contents are accompanied by decreasing fracture toughness, reducing from KIC∼2.9 (Cr0.28B0.72) to ∼1.7 MPa√m (Cr0.24Si0.10B0.66). High-temperature cantilever bending up to 800 °C revealed a brittle-to-ductile-like transition for Cr0.28B0.72, resulting in an increased fracture toughness of KIC∼3.3 MPa√m. Si-alloyed coatings show decreasing fracture resistance up to 400 °C, whereas beyond, Si-segregates enable high-temperature plasticity and thus a significantly increased damage tolerance.