Wear assessment of a Ti–6Al–4V motion-preserving porous artificial-cervical-joint fabricated by SLM after surface carburization

Abstract

Adjacent segment degenerative diseases are a major clinical concern for anterior cervical corpectomy with fusion surgery. In this study, a hybrid motion-preserving cervical prosthesis consists of two articulating joints and one porous fusion body is designed to provide motion ability and reduce the occurrence of degenerative diseases. The hybrid design is fabricated from Ti–6Al–4V (TC4) alloy using selective laser melting (SLM) technology. After carburization pretreatment, two types of bearing couple SLM-TC4 self-mating and SLM-TC4/UHMWPE were studied in terms of lubrication regime, contact mechanics and wear performance. According to the ISO 18192-1 standard, five million cycles (Mc) of in vitro spine simulator wear tests were conducted in bovine serum lubricated condition.Hamrock-Dawson minimum lubrication film thickness analysis stated that both bearing was likely under boundary lubrication only, with extensive surface asperities contact. And, the contact stress encountered during in vitro condition was not enough to cause surface fatigue failure of any bearing couple. In term of wear performance, the obtained steady state wear rates were 0.160 ± 0.004 mm3/Mc for SLM-TC4 self-mating and 8.57 ± 0.06 mm3/Mc for SLM-TC4/UHMWPE bearing couples, respectively. According to these outcomes, SLM-TC4 self-mating couple was more promising for the articulating joint bearing design.For SLM-TC4 self-mating couple, the initial wear mechanism was abrasion with potential third-body abrasion wear. As the wear cycle proceeding, the TiC cermet layer was gradually damaged and exposed the local bare metal. Under this circumstance, material loss was consisted of abrasion wear, adhesion wear, and tribo-corrosion. In comparison, the wear mechanism of SLM-TC4/UHMWPE bearing was always abrasion-adhesion of UHMWPE. The surface of UHMWPE socket was severely plowed by the hard micro bulge of the TiC layer, and material loss and plastic deformation occurred only on the UHMWPE socket. In conclusion, carburization pre-treatment causes a hard TiC cermet layer formation on the SLM-TC4, which enhance its hardness and protect underling material from corrosion damage. The thickness and quality of the TiC layer directly affect the longevity of the joint prosthesis in vivo.