Biotribological behavior of ta-C coatings for load-bearing implants

Amorphous/diamond-like carbon (DLC) coatings deposited on the articulating surfaces of total hip (THR) or total knee replacements (TKR) have the potential to enhance the overall biotribological behavior and longevity [1-2]. This requires excellent adhesion to soft and hard implant materials, biocompatibility as well as superior mechanical properties and wear resistance [3].

Medical grade Co28Cr6Mo (CoCr), Ti6Al4V (Ti64) and ultrahigh molecular weight polyethylene (UHMWPE) were studied as substrate materials. Tetrahedral amorphous carbon (ta-C) coatings were deposited by plasma-filtered pulsed laser-arc technique using a commercial PVD coating system. The surface topography, cytocompatibility as well as mechanical properties and adhesion of ta-C coatings were analyzed. The biotribological behavior was studied in pin-on-disk sliding experiments under substitute synovial fluid lubrication. The wear behavior was investigated through laser scanning microscopy (LSM), Raman spectroscopy as well as focused ion beam scanning electron microscopy (FIB-SEM). [4]

The ta-C coatings exhibited typical morphology and composition and featured higher roughness compared to the substrates. Contact angle measurement as well as indirect and direct cell testing suggested sufficient biocompatibility and suitability for biomedical applications. The ta-C coatings demonstrated excellent mechanical properties with improved hardness- to-elasticity ratio. The adhesion between the coatings and CoCr, Ti64, and UHMWPE substrates was excellent. Despite higher friction, the ta-C coatings significantly reduced wear on both contacting bodies. The intact coating effectively protected the UHMWPE disk from adhesive or abrasive wear. However, crack networks and near-surface fatigue were observed after prolonged testing. Particle analysis revealed minimal release of nanometer-sized particles compared to the reference groups. The biotribological behavior remained consistent over time. [4]

Cracking and folding in the ta-C coating on UHMWPE substrates were attributed to the mismatch in thermal expansion coefficients between the coating and substrate materials, which led to varied expansion and shrinkage during temperature changes, along with the high intrinsic compressive stress typical of ta-C coatings. The compressive stress in the coating, combined with time-dependent plastic deformation of the PE substrate, resulted in fold formation, coating shearing, crack formation, and subduction. Furthermore, cyclic tribological stress exceeded the compressive stress resistance of the ta-C coating, leading to the formation and propagation of microcracks near the surface (near-surface fatigue). Anyways, the durability of the ta-C coating is expected to extend the service life of metallic and polymeric implant materials. [4]

[1] B. Rothammer et al.: Amorphous Carbon Coatings for Total Knee Replacements—Part II: Tribological Behavior, Polymers, 13, 2021, 1880, DOI: 10.3390/polym13111880

[2] B. Rothammer, K. Neusser, M. Bartz, S. Wartzack, A. Schubert, M. Marian: Evaluation of the Wear-Resistance of DLC-Coated Hard-on-Soft Pairings for Biomedical Applications, Wear, 523, 2023, 204728, DOI: 10.1016/j.wear.2023.204728

[3] R. Shah et al.: Enhancing mechanical and biomedical properties of protheses - Surface and material design, Surfaces and Interfaces, 27, 2021, 101498, DOI: 10.1016/j.surfin.2021.101498

[4] B. Rothammer et al.: Wear mechanism of superhard tetrahedral amorphous carbon (ta-C) coatings for biomedical applications, Advanced Materials Interfaces, 2023, 2202370, DOI: 10.1002/admi.202202370