The impact of surface cracks and surface roughness in the performance of hard chromium coatings in cold rolling applications

• Surface cracks on hard chromium coatings are too small to serve as oil reservoirs. • Plate-out experiments confirm that surface cracks do not enhance oil retention, even at elevated temperatures. • Chrome plating significantly reduces high-frequency surface roughness. • The smoothing effect of chrome plating plays a crucial role in lubrication efficiency during cold rolling processes. • Faster oil spreading is observed on chrome-plated surfaces due to reduced microscale roughness. Hard chrome plating is widely utilized in industrial applications due to its high hardness, wear resistance, corrosion resistance, and low friction. However, the mechanisms responsible for its exceptional tribological performance and strip cleanliness in cold rolling remain insufficiently understood, posing a challenge to the development of viable alternatives. To design future effective alternatives, it is crucial to first comprehend why chromium coatings are so effective. This study investigates the effect of hard chromium plating on steel surface topography and its correlation with lubrication performance in cold rolling applications using oil-in-water emulsions. Optical microscopy, scanning electron microscopy (SEM), confocal microscopy, and atomic force microscopy (AFM) are employed to analyze the microstructural and topological characteristics of chrome-plated surfaces. The findings challenge the conventional assumption that surface cracks in hard chrome coatings can serve as oil reservoirs that would significantly enhance lubrication in the roll bite of a cold rolling mill with the help of exact crack width measurement and plate out test. Additionally, Power Spectral Density (PSD) analysis indicates that chrome plating reduces high-frequency roughness while preserving average roughness and larger wavelength features. Dynamic surface wetting experiments using a state-of-the-art droplet analyzer link this reduction in high-frequency roughness to enhanced wettability and faster lubricant spreading on chrome-plated surfaces compared to uncoated steel. These findings highlight the critical role of surface topography in lubrication performance and provide valuable insights to guide the development of future alternatives to hard chrome plating in cold rolling applications.