Influence of Substrate Surface Finish on the Wear and Corrosion Behaviour of AlTiN/AlTiCrN Coatings

This study investigates the influence of substrate surface finish on the morphological, mechanical, and microstructural properties of AlTiN/AlTiCrN coatings on AISI A8 tool steel, and how this cascade of property changes collectively dictates the wear and corrosion behaviour of the coated tool steels. The wear resistance was evaluated as per the ASTM G65 test, while the corrosion performance was assessed using electrochemical impedance spectroscopy and potentiodynamic polarization techniques. Surface roughness was measured using profilometry, while the hardness (H) and elastic modulus (E) were determined via nanoindentation. Microstructural characterization was carried out using scanning electron microscopy, energy dispersive spectroscopy, and X-ray diffraction techniques. The results showed that increasing the average substrate roughness by about 500-fold (from 0.003 to 1.7 µm) led to a relatively modest rise of about 22% in the wear rate (from 1.8 to 2.2 × 10⁻⁴ mm³/Nm). This reduction in the wear performance of the coatings on the rougher substrates was attributed to lower hardness, a reduced H/E ratio, and an elevated surface roughness of the coatings. Similarly, the coating’s corrosion resistance deteriorated with increasing surface roughness, as reflected by a decline in protective efficiency (from 77 to 58%) and an increase in porosity (from 0.27 to 0.49%). This observation was linked to the cumulative effect of the reduction in (111) crystallographic orientation, the presence of larger macroparticles, and an elevated surface roughness of the coatings on the rougher substrate finish. Overall, these findings underscore the potential of a smoother AISI A8 substrate surface finish to simultaneously improve the wear and corrosion performance of AlTiN/AlTiCrN coatings by promoting better coating morphology, mechanical strength, and electrochemical stability.