Date of Award
Doctor of Philosophy (PhD)
Jeremy L Gilbert
Mechanically assisted crevice corrosion (MACC) is known to cause clinical complications in modular joint replacements, particularly at metal junctions. While several factors affect an implant’s tribocorrosion performance in vivo, a fundamental understanding of the effects of individual design and material elements is lacking. Two such factors, the interfacial compliance and the hardnesses of the contacting surfaces, were evaluated in terms of their influence on metal interfaces under simulated MACC conditions.
Interfacial compliance was first studied to determine if design modifications which alter the geometric-based compliance of a fretting interface could manipulate its contact mechanics. A series of variable-load pin-on-disk fretting corrosion tests was performed on Ti-6Al-4V and CoCrMo alloys, using pins of varying cantilever-shaped contact geometries to create three different compliances. Fretting mechanics and fretting corrosion currents acquired for each compliance were used to determine regime boundaries for stick, stick-slip, and slip. Results showed a highly correlated and material-dependent relationship between work of fretting (WOF) and fretting current. A high interfacial compliance reduced relative micromotion, induced stick and stick-slip conditions at lower loads, and remained more robust against small displacements. Using correlated mechanical and electrochemical data, fretting corrosion maps were presented as a new method of evaluating and predicting surface damage at the interface.
Next, the same interfaces were exposed to a long-term, high-damage environment to determine their viability under more clinically-relevant conditions. Significantly lower currents and displacements were measured in the most compliant group, confirming that elastic bending of the contact asperity accounted for most of the applied fretting motion. Electrochemical test methods quantified the changes in the interfaces after testing, which correlated with the mechanical energy dissipated during testing. Additional analyses revealed substantially more surface damage to CoCrMo disks than Ti-6Al-4V pins. This is thought to result from adhesive wear of mixed oxide debris to the pin and its subsequent plastic deformation of the disk. Surface damage observed on high compliance samples suggests that some abrasion is unavoidable even with geometric modifications, and that the use of an insulating layer between metal-metal interfaces may be more effective in preventing metal degradation.
Long-term test conditions were also used to evaluate the fretting corrosion resistance of a low-hardness, high-compliance interface created by a self-reinforced polymeric thin film composite (SRC-PEEK) when placed between Ti-6Al-4V/CoCrMo couples. Thin films did not prevent sliding motion, but significantly reduced metal surface damage and debris production without effectively compromising mechanical integrity. Additional benefits of a soft, compliant layer were observed with respect to damage via third body wear. Results suggest that a combination of low hardness, high strength, and compliance make SRC-PEEK an ideal interfacial layer.
Lastly, the effects of surface hardness were studied by testing variable-load fretting corrosion of metal disks by pins with hardnesses spanning >18 GPa. Pins that were below the hardness of the disks yielded at individual points of contact, preventing the onset of MACC. Pins that were at or above disk hardnesses were able to abrade the surface oxide in a load-dependent manner, demonstrating the impact of individual asperity interactions on hard-hard contacts. These observations support elasto-plastic approximations of contact mechanics rather than Hertzian estimates, which describe only purely elastic behavior.
Smith, Stephanie Marie, "Compliance, Hardness, and Fretting Corrosion in Modular Taper Junctions: The Micromechanics of Contact" (2020). All Dissertations. 2604.