Co-Cr alloys exhibit good mechanical properties such as high hardness, good magnetic properties, and good corrosion resistance. These alloys are mostly used such as dental and orthopedic implants, due to their good properties. Co-Cr-Mo and Co-Ni-Cr-Mo alloys are most commonly used in dentistry.
In scope of my master thesis work I analyzed two different dental alloys. The first one was Co-Cr-Mo (Argeloy Pharmal), and the other one was Co-Cr-W-Mo (Interdent IBOND NF). The difference between these two alloys was that the first one does not contain tungsten. Goal of the master thesis was to find out how, the heat treatment affected microstructures, hardness, and wear resistance of both alloys. The samples were first solution annealed at 1200 °C for 2 hours in argon atmosphere, then quenched in water, and in the last step, they were aged at 900°C in argon atmosphere for 1, 3 and 12 hours. Afterwards, the microstructures of this alloys were analysed with scanning electron microscope, with EDS analysis for microchemical analysis of chemical compositions of microstructural constituents. Vickers hardness and wear resistance were also measured on samples at different conditions.
Co-Cr-Mo and Co-Cr-W-Mo alloys were in as-cast condition when they were delivered. The microstructure of as-cast Co-Cr-Mo alloy, was composed of dendrites of γ phase, which is a solid solution of Co and Cr. In the interdendritic regions M23C6 carbides were present. During the ageing small precipitates of M23C6 carbides were precipitated and lead to additional hardening. After 12 hours of aging bright carbides most likely M6C carbides, rich on Mo, were observed on the grain boundaries. As-cast microstructure of Co-Cr-W-Mo alloy consisted of dendrites of phase, which showed directional growth from the outside to the centre of the sample. phase is a solid solution of Co and Cr. Meanwhile in the interdendritic regions M6C carbides were observed. During the ageing of this alloy ε-martensite (hgz1) and hgz2 phase were formed. Addition to this, it was also observed that inside of γ phase small precipitates were formed, which had an impact on additional hardening.
Hardness was measured by Vickers method. It was observed that both alloys had the highest hardness after 12 hours of ageing. Co-Cr-Mo alloy had the highest hardness of 432 HV and Co-Cr-W-Mo alloy had the highest hardness of 393 HV.
Wear resistance test showed that the Co-Cr-Mo alloy had better wear resistance, than Co-Cr-W-Mo alloy. In scope of wear analysis adhesive and abrasive wear were observed on both alloys. Nevertheless, the pull-out mechanism was only observed on Co-Cr-W-Mo alloy. The best wear resistance had Co-Cr-Mo alloy after 12 hours of ageing, while the Co-Cr-W-Mo alloy had the best wear resistance in quenched extinguished condition. It was concluded that higher resistance to wear occurred after the longer hours of ageing.
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