Macro- and micro-scale friction behaviors and underlying mechanisms of HfO₂ film fabricated by electron beam evaporation

As a high-performance optical and dielectric material, Hafnium dioxide (HfO₂) films prepared by electron beam evaporation occupy an important position in the field of surface protection for optical and microelectronic devices due to their combination of high hardness and high stability. Particularly in high-energy laser systems, any minor defects caused by fretting wear or mechanical scratching can lead to severe component failure under continuous high-energy laser bombardment. Therefore, deeply investigating the friction failure mechanism of HfO₂ films is essential. Accordingly, based on the mechanical properties of films with different thicknesses, this study investigates the macro- and micro-friction and wear mechanisms. This is achieved through ball-on-disk friction and nano-scratch tests to explore the behavioral differences of HfO₂ films with different thicknesses. Research findings indicate that the significant intrinsic hard-brittle characteristics of the 300 nm film cause rapid, large-area fragmentation during ball-on-disk friction tests. This results in a shorter running-in stage for the 300 nm film, and the initial friction rapidly rises to a high level. In contrast, the 200 nm film maintains an initial friction coefficient of around 0.2 and fully exhibits a typical three-stage friction evolution process. In 0-500 mN nano-scratch experiments, both films display three-stage evolution characteristics. However, the 300 nm film shows extensive film spallation in the middle stage, leading to premature entry into the substrate-dominated friction stage. In contrast, the 200 nm film developed a debris-rich third-body-like layer during steady wear, which helped stabilize the interface and delay substrate exposure. Nanoscratch tests under 0-50 mN reveal that the 200 nm film exhibits more pronounced plastic morphological features. The early scratch process presents a damage mode involving both crack propagation and local deformation. Meanwhile, the high hardness and residual stress of the 300 nm film make it more prone to early micro-crack propagation and fracture. Therefore, its frictional characteristics are primarily dominated by high-energy brittle damage. This study provides an experimental basis for the structural optimization and the regulation of friction and wear behaviors of HfO₂ films. Additionally, it offers a reference for their subsequent application research in related optical protection scenarios.