Tribological behavior of electroless Ni–P/Ni–P–TiO₂ coatings obtained on AZ91D magnesium alloy by a chromium-free process

Studies about the tribological behavior of Ni–P–TiO₂ coatings on magnesium alloys are very scarce and the wear mechanisms involved are not analyzed. In this work, Ni–P and Ni–P/Ni–P–TiO₂ nanocomposite coatings have been formed on AZ91D magnesium alloy by direct electroless technique with multiple steps, avoiding both the use of Cr(VI) compounds and the HF activation procedure. This work focused on two main aspects: (i) the formation of the composite coatings with different sizes and concentrations of TiO₂ nanoparticles, studying their morphology and chemical composition, and (ii) the study of the tribological properties of the coatings under dry sliding conditions. For tribological and mechanical evaluation, dry sliding friction and wear testing and nanoindentation measurements were performed. Scanning electron microscopy equipped with energy-dispersive X-ray (SEM/EDX) and X-ray diffraction (XRD) was used for the characterization of the coatings. Wear tracks and debris were analyzed by means micro-Raman spectroscopy and SEM/EDX. The addition of TiO₂ nanoparticles decreases the wear rate and improves the tribological behavior of the coatings. The wear mechanisms involve flattening of the nodules and abrasive wear to three bodies, accompanied by tribo-oxidation.