The effect of chromium on the gamma to alpha phase transition of alumina coating formed on 316L SS by a cathodic micro arc deposition (CMAD) process

• Cr doped Al2O3 ceramic coating was fabricated by cathodic micro-arc deposition (CMAD).
• The introduction of Cr can promote the formation of α-Al2O3 in CMAD process.
• Cr doping effects in CMAD and wet chemical process were compared.
• A novel phase transition mechanism based on crystal defects is proposed.
• The mechanism effectively explains the inductive role of Cr on Al2O3 phase transition.

Alumina ceramic coatings are widely used because of their favorable physical and chemical stability. However, the formation of stable alpha phase alumina on metallic substrates at lower temperatures still presents technical challenges. In this work, an Cr doped Al2O3 ceramic coating was successfully fabricated on 316L stainless steel by a new cathodic micro-arc deposition (CMAD) technique using Al(NO3)3 as the precursor. We studied the effect of chromium on its phase transition from gamma to alpha as well as the underlying mechanism. Phase and chemical composition distribution, surface morphology, and photoluminescence data were characterized by X-ray diffraction (XRD), electron probe microanalysis (EPMA), a scanning electron microscope (SEM) with an energy dispersive spectroscopy (EDS), and a microscopic confocal Raman spectrometer (MCRS). The results showed that the ceramic coatings have a porous microstructure and were mainly composed of Cr doped α-Al2O3 and γ-Al2O3. This suggests that the introduction of Cr in the electrolyte solution can promote the nucleation and growth of the corundum phase of α-Al2O3. Based on the comparison of conventional heat treatment of alumina precipitates with and without Cr dopant via a sol–gel process, we proposed that the reducing atmosphere and local high sparking temperature that subsequently enhances the diffusion of defects in oxides may be contributed to this transition. This finding might provide a simple, cost efficient way to fabricate corundum alumina coatings on metallic substrates under mild temperatures without sacrifice of its mechanical properties.