Tribological behavior of amorphous carbon coatings on Ti6Al4V alloy and UHMWPE for total knee replacement using a pin-on-disk tribometer

Objectives: The aim of the endoprosthetic replacement of joints is to restore functionality and to provide patients with a painless life. Total knee replacements (TKR) usually consist of a Co28Cr6Mo (CoCr) femoral component that rubs against UHMWPE (PE) inlay. The wear particles removed from TKR are largely responsible for early prosthetic failure, primarily due to aseptic loosening. This study focused on the characterization and tribological testing of wear-reducing amorphous carbon coatings with good adhesion to Ti6Al4V (Ti64) and PE and to analyze wear mechanisms and influencing factors.

Methods: Due to promising tribologically effective behavior, a tungsten-doped hydrogen-containing amorphous carbon coating (a-C:H:W) was applied to the Ti64 (Ti64:W) pins and an a-C:H coating to the UHMWPE (PE:H) disks by physical vapor deposition (PVD). A thin adhesive layer of titanium and an intermediate layer of tungsten carbide were used to achieve a good coating-substrate adhesion. In contrast, the a-C:H coating was applied to the PE substrate by reactive PVD without intermediate layers. The coating samples were characterized regarding coating thickness and adhesion to substrate using crater-grinding method [1] and scratch testing [2], respectively. Hardness was measured by instrumented indentation test [3].

The tribological behavior of the coated material pairings in comparison to the uncoated pairings was investigated utilizing a climate-controlled pin-on-disk tribometer. Efforts were made to reproduce the boundary conditions in the peri-implant environment, whereby the diluted bovine serum used was tempered to 37°C in fully flooded lubrication. The wear rates were determined using light and laser scanning microscopy.

Results and conclusion: With an indentation hardness of about 16GPa, a-C:H:W coating (thickness of < 1 µm) was significantly harder than CoCr or Ti64 The coating systems deposited on Ti64 endured loads of about 8N in scratch tests without spalling. The a-C:H coating had a thickness of about 1.5µm and an indentation hardness of about 1.5GPa, which represented a forty-fold increase compared to substrate hardness while maintaining high adhesion. Thus, no coating applied to PE showed any damage in the form of chipping in the scratch tests.

The wear rates of uncoated and coated disks as well as uncoated and coated pins are compared in figure 1. It was found that a coated sliding combination reduces wear on PE disks by about 41% and on pins by 77%. The a-C:H coating was suitable for preventing wear on PE due to its inert character and its favorable HIT/EIT ratio. A similar behavior was observed for the a-C:H:W coated Ti64 pins compared to the uncoated Ti64 and CoCr pins. Despite the pin having permanently been in contact, the magnitude of the wear rate differed between PE and metal substrates by a factor of 10. This could be attributed to the stiffer and harder character of metals, which could be supplemented by an additional coating with a-C:H:W to achieve tribologically favorable behavior.

References

1. German Institute for Standardization (DIN). DIN EN ISO 26423. 2016.
2. German Institute for Standardization (DIN). DIN EN ISO 20502. 2016.
3. German Institute for Standardization (DIN). DIN EN ISO 14577-1. 2015.