Studies of Corrosion and Tribocorrosion Behavior of Surface Modified CoCrMo Alloy

表面改性後鈷鉻鉬合金的耐蝕及耐摩蝕性能研究

Student thesis: Doctoral Thesis

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Award date20 Apr 2017

Abstract

The average lifetime of artificial joint is 15 years, while patients nowadays demand implant with longer service life due to the increase in life expectancy. Aseptic loosening (migration of stem), which is accompanied with bone loosening and limb-length discrepancy, accounts for 55% of the total hip joint failure. The reason for aseptic loosening could be complex, and the main material-related reason is that the wear particles activate macrophages and osteoclast, which leads to osteolysis, and subsequently aseptic loosening. Furthermore, the combined effect of wear and corrosion, known as tribocorrosion, could induce a higher material loss rate than corrosion and wear alone. The present research aims at improving the corrosion and tribocorrosion resistance of CoCrMo biomedical alloy by using surface treatments. Firstly, tribocorrosion test protocol has been established after comprehensive literature review and extensive baseline studies. The tribocorrosion resistance of untreated CoCrMo alloy has been examined on the purposely-built tribocorrosion tester. The effect of potential and load on the tribocorrosion behavior of CoCrMo alloy has been studied systematically.

After thorough study about the tribocorrosion behavior of as-received CoCrMo alloy, amorphous carbon (a-C) coating has been deposited on alloy by using closed-field unbalanced magnetron sputtering (CFUBMS) system. The role of interlayer on mechanical and corrosion properties of a-C coating has been discussed, and Cr/CrCx has been selected as an interlayer due to its good corrosion resistance and mechanical properties. In addition, the deposition parameters play an important role in the quality of coatings. The a-C coatings deposited at bias ranging from -120 V to -200 V favor the nanocomposite structure where nanoclusters with diameter ranges from 2 to 4 nm are embedded in the amorphous matrix. These nanoclusters show lattice distance of 0.21 nm, which is consistent with (111) diamond. Subsequently, the a-C coatings possess higher sp3 content, better corrosion and tribocorrosion behavior.

Ion implantation can lead to chemical and structural evolution of material surface without an abrupt interface. Thereby, the physical, chemical and mechanical properties of material can be modified without the risk of coating delamination. In this study, carbon ion implantation has been adopted by using a metal vapor vacuum arc (MEVVA) ion source with a solid cathode. It is found that carbon ion implantation promotes the formation of an amorphous layer (composed of C, Co, Cr, and Mo), and nanocomposite layer with nanocrystals embedded in the amorphous matrix. The formation mechanism of amorphous layer and nanocrystals has been explained from a thermodynamic viewpoint. The influence of structural evolution on corrosion and tribocorrosion behavior of ion-implanted CoCrMo alloy has been studied comprehensively. Both the corrosion and tribocorrosion resistance has been significantly improved by carbon ion implantation. The corrosion current density and material loss due to tribocorrosion has been reduced to only 3%, and 2% of that for the untreated alloy. It is expected that the working life of artificial joint could be increased by one order of magnitude under the best of circumstances. The TEM observation provides direct evidence that corrosion mechanism of CoCrMo alloy has been altered from localized corrosion to uniform corrosion after surface amorphization. An amorphous passive layer that is thicker than the surface layer removed by corrosion is formed. The film thickness has been increased by 10 nm. The self-passivation process is believed to be responsible for the decrease in corrosion rate. Such study could serve as guidance for material preparation that, improve the corrosion resistance by facilitating an amorphous layer with good passivation ability.

Lastly, Al2O3/a-C duplex coating has been applied on CoCrMo alloy by combination with physical vapor deposition (PVD) and atomic layer deposition (ALD). The Al2O3/a-C duplex coated alloy exhibits the best corrosion resistance in the present study. However, the Al2O3 layer shows unsatisfactory wear resistance. In the future work, the potential application of ALD Al2O3 layer could be further explored by applying a-C/Al2O3 duplex layer.

To sum up, though the reasons for implant failure are complex, the present work has demonstrated that appropriate surface treatment of the CoCrMo alloy could offer significant potential to increase the service life of implants.

    Research areas

  • CoCrMo, Magnetron sputtering, Ion implantation, Atomic layer deposition, Tribocorrosion, Corrosion mechanism