Effect of niobium onto the tribological behavior of cathodic arc deposited Nb-Ti-N coatings

This investigation addresses a need for higher quality surfaces and wear resistance of coatings used on load bearing medical implants. Multilayered cathodic arc deposited coatings are candidates for such applications and were the subject of this study. Nb-Ti-N coatings were deposited by cathodic arc using TiNb compound cathodes. The microstructure and properties of the coatings were characterized using XRD, TEM/STEM, EDS, nanoindentation, scratch tests and pin-on-disc testing. Throughout the coating, macroparticles consisting of a Nb rich core and a nitrided titanium porous shell were evidenced by STEM and EDS. It was shown that inhomogeneities related with the soft and malleable metallic Nb inclusions alter the tribological behavior of the coatings. The underlying mechanism of wear in hip-simulator test liquid against UHMWPE was investigated. During pin-on-disc tests, the top layers of the coatings were removed and the embedded niobium droplets became visible. The wear of UHMWPE during pin-on-disks tests was increased on Nb-Ti-N counter surfaces compared to the wear of UHMWPE on TiN itself. It is inferred that the wear of the coatings is initiated by the growth discontinuities and pores around the Nb droplets. The debris released from the coating acts as third body wear particles, grinds the remaining coating and causes the wear of UHMWPE pins.