Al₂O₃ films grown by glow discharge plasma aluminising

Abstract

Aluminising of 9Cr steel substrates followed by heat treatment has been attempted to generate Al₂O₃ films along with Fe–Al diffusion zone at the coating/substrate interface. Effect of glow discharge plasma processing on the phase and microstructure of resultant alumina films in comparison with thermally processed samples has been reported. The thermal and plasma treated samples were characterised using X-ray diffraction, scanning electron microscopy–energy dispersive X-ray spectroscopy, electron probe microanalysis, X-ray photoelectron spectroscopy (XPS) and nanoindentation techniques. X-ray diffraction and XPS studies revealed γ-Al₂O₃ phase in both thermal and plasma processed samples. The XPS data indicated higher binding energies in plasma processed Al₂O₃ films as compared to thermally processed Al₂O₃ films. Scanning electron microscopy observations revealed cracks in thermally grown Al₂O₃ films while the same was not observed in plasma processed films. The EDX analysis revealed Fe(Al) diffusion layer of ∼3 μm in plasma processed films while the same was not observed in thermally treated samples. Nanoindentation tests on plasma grown alumina films indicated 16·96 GPa hardness while hardness for thermally grown alumina films was found to be 9·95 GPa. The role of plasma in generating a crack free alumina film has been discussed.

Keywords:

• Al₂O₃
• Plasma
• Aluminising
• 9Cr steel
• Mangetron

Acknowledgements

The reported work has been conducted using a plasma aluminising system developed under funding from Board of Research in Nuclear sciences (grant no. 2008/38/BRNS-01) and the financial support from BRNS is thankfully acknowledged. The authors are thankful to Mr E. Rajendra Kumar for his continuous support for the alumina coating development activities related to test blanket modules. The authors are also thankful to Mr Ghanshyam Jhala for supporting metallographic sample preparation activities. The XRD tests have been carried out at XRD laboratory, Dept. of Met. Eng. and Mater. Sci., IIT Bombay and support is thankfully acknowledged. The XPS studies have been conducted at ESCA lab, central facility at Deparment of physics, IIT Bombay and the support is duly acknowledged. The support from Dr Sutar of ESCA lab at IIT Bombay is thankfully appreciated. The authors are also thankful to Professor SVS Murty of PRL for his kind support in the EPMA analysis.