Tailored femtosecond-laser ramps for depth-resolved characterization of hard coatings

Abstract

Depth-resolved characterization of hard coatings is essential for understanding structure–property relationships in multilayered and graded architectures. However, the preparation of shallow-angle cross sections on coated cemented carbide remains challenging, as the cemented carbide substrate is difficult to process reproducibly by conventional mechanical routes. In this work, we present a flexible preparation strategy based on femtosecond-laser ablation for the fabrication of small-angle cross sections with freely adjustable inclination angles, followed by polishing of the laser-prepared ramp surface to obtain a surface quality suitable for advanced analytical characterization. Particular emphasis is placed on the evaluation and optimization of the femtosecond-laser process window. Initially, the influence of pulse and line distance on surface quality and roughness evolution is investigated. Building on this, laser power, repetition rate, ablation rate, and focus-shift strategies are correlated with ramp geometry and linearity. Inclination angles ranging from ~0.1° to ~10° are prepared and optimized directly on cemented carbide substrates, enabling tailored preparation routes for smooth ramps with well-defined geometry and reproducible surface quality. The approach is particularly attractive for cross-sectional nanoindentation mapping, which is demonstrated on a benchmark TiBN multilayer coating with a total thickness of ~6 µm consisting of six individual ~1 µm layers. In addition, TiN/TiB₂ multilayer systems with ~50/50 and ~100/100 nm bilayer architectures are investigated to evaluate the achievable lateral resolution and the limits of resolving increasingly fine multilayer designs. By tailoring the ramp angle and adapting the femtosecond-laser process window to the required ablation depth and ramp length, the coating thickness can be laterally expanded over defined distances. This allows the preparation geometry to be matched to the spatial-resolution requirements of the respective analytical technique.