Deposition and characterization of nanocomposite Cr-based coatings

• Main Author: Cheong, Jhun Yew.
• Other Authors: Tay Beng Kang
• Format: Theses and Dissertations
• Language: English
• Published: 2012
• Subjects: DRNTU::Engineering::Materials::Composite materials; DRNTU::Engineering::Materials::Material testing and characterization; DRNTU::Engineering::Materials::Ceramic materials; DRNTU::Engineering::Materials::Functional materials
• Online Access: http://hdl.handle.net/10356/50921
• Institution: Nanyang Technological University

Hard coatings have been applied onto the surfaces of different tools to improve their lifetime, performance and efficiency. For high-speed machining and other high temperature applications, thermal stability and oxidation resistance of the hard coatings are crucial. Currently, TiAlN coatings represent state-of-the-art coating in the tools coating industry by providing good surface wear protection under operation temperatures up to 800 ºC. In many cases, however, this is still insufficient to meet the application requirements. In sight of this, research work for developing better performing coatings for high temperature applications has been a hot research topic. In this research, nanocomposite CrAlSiN coating was deposited by a lateral rotating cathode arc process. It was characterized and benchmarked with TiAlN coating which was deposited using the same technique and similar parameters. In order to investigate the oxidation behavior and thermal stability of the coating, the as-deposited coatings were annealed at different temperatures, for different time durations and in various atmospheres. The as-deposited and high temperature annealed coating microstructure was studied by X-ray diffraction (XRD) and scanning electron microscope (SEM). The chemical composition was measured by glow discharge optical spectroscopy (GDOS) and energy dispersive X-ray (EDX). Mechanical testing was carried out using nanoindentation hardness measurements. It was observed that after annealing at 1000 ºC, the CrAlSiN coating was partially oxidized but the XRD pattern revealed that the nitride peak of the CrAlSiN coating was still present. Compared to the TiAlN coating, the coating was dominated by a large amount of oxygen and little residual nitrogen was detectable in the coating indicating that the coating was almost fully oxidized. Cross-section images and GDOS depth profile of the CrAlSiN coating revealed that two oxide layers had been formed. In-depth analysis showed that the outmost layer consists of a Cr rich oxide layer followed by an inner Al rich oxide layer. The formation of these dense oxide layers was responsible for the excellent oxidation resistance of the CrAlSiN coating. Hardness measurements showed that both the as-deposited CrAlSiN and TiAlN coatings had similar hardness at 34 GPa and 33 GPa, respectively. Results showed that upon completion of the high temperature annealing process, the CrAlSiN coating was able to maintain its high hardness of 33 GPa up to 900 ºC. After annealing at 1000 ºC, the hardness of the CrAlSiN coating dropped down to 16 GPa that was still much higher when compared to only 10 GPa for the TiAlN coating.