Abstract
Fretting corrosion of taper junctions is long known and of great concern, because of metal ion and particle release and their related adverse local and systemic effects on the human body (1–3). Orthopedic taper junctions are often comprised of CoCr29Mo6/TiAl6V4 pairings. Beside others the imprinting of the TiAlV-machining marks into the CoCrMo-taper is of clinical interest (4, 5). Thus, the multifactorial details and their interdependencies on the macro-, micro, and nanoscale are still a matter of research (6). This contribution presents the mechanisms of imprinting found in an in-vitro fretting corrosion test. The worn surfaces, the lubricant as well as its remains were analyzed after test and the findings brought into relation to the characteristic wear sub-mechanisms. The fretting tests were conducted by means of a cylinder-on-pin set-up. All details about the test and the sequence of analyses can be found in (7, 8). A marked tribofilm of C-rich organic matter and oxidized wear particles of both bodies was generated at the TiAlV/CoCrMo contact area (Figure 1a, c). After removing the tribofilm chemically, extremely fine scratches of sub-µm depth became visible on the CoCrMo body (Figure 1b). The TiAlV body showed shallow shelves leaving troughs filled with grainy debris (Figure 1d) mainly of Ti-oxide wear particles. The shelves stick to the surfaces and, therefore, move relatively to the counterbody. In combination with the grainy debris this brings about “Microploughing” on the CoCrMo surfaces. Microploughing is known for destroying any passive film resulting in “Tribocorrosion”. The question remains how the shelves are formed. From the surface analyses one could conclude that they point towards “Delamination”. But this would also mean that they would not stick rigidly to the surfaces but be ejected from the contact area. Focused Ion Beam (FIB) cuts were done in order to investigate the near- and subsurface structure of the shelves in order to clarify the governing mechanisms (Figure 2). Below the platinum protection layer appears a laminated structure of highly deformed nanocrystalline and amorphous areas. EDS confirmed that the lighter intermediate layers consist mainly of Ti-oxide. This microstructure is supposedly formed by severe plastic deformation and the generation of shear bands, which under fretting pile up on top of each other. This cannot be connected to “Delamination”. We therefore propose to categorize the formation mechanism of these shelves as a specific form of microploughing. Thus, imprinting is neither driven by any galvanic effects (9) nor by hardness differences of TiO2 and Cr2O3 (10) but by microploughing on the TiAlV-body leading to tribocorrosion at specific sites of CoCrMo what imprints the surface grooves of the softer TiAlV into the harder CoCrMo.
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