Tribological characterization of NCD in physiological fluids M. Amaral a , C.S. Abreu b , F.J. Oliveira a , J.R. Gomes c , R.F. Silva a, ⁎ a CICECO, Glass and Ceramics Eng. Deparment, Univ. of Aveiro, Portugal b Physics Department, Porto Superior Eng. Institute, ISEP, Portugal c Mech. Eng. Deparment, CIICS, Univ. of Minho, Portugal Available online 18 January 2008 Abstract NCD films deposited on silicon nitride (Si 3 N 4 ) ceramic substrates by hot-filament chemical vapour deposition (HFCVD) technique were biotribologically assessed under lubrication of Hank's balanced salt solution (HBSS) and dilute fetal bovine serum (FBS), using a pin-on-flat test configuration. The reciprocating tests were conducted under an applied load of 45 N during 500,000 cycles using a NCD coated Si 3 N 4 biocompatible ceramic substrates with two different surface preparations: i) polished (P) and ii) polished and plasma etched (PE). Friction coefficient values of 0.02 and 0.12 were measured for the P samples under HBSS and FBS lubrication, respectively. PE samples showed increased adhesion relatively to P ones and withstood 6 km of sliding distance without any evidence of film fracture but with friction coefficients of 0.06 for HBSS and 0.10 for FBS experiments. Evidences of protein attachment and salt deposition were found, being the responsible for the enhancement of friction under FBS relatively to HBSS. The wear rates measured for the NCD films are in the range of ~10 − 9 –10 − 8 mm 3 ·N − 1 m − 1 , values that are similar to the best values found for ceramic-on-ceramic combinations. © 2008 Elsevier B.V. All rights reserved. Keywords: Nanocrystalline; Diamond film; Hot filament CVD; Biotribology 1. Introduction “Total hip replacement is one of the most challenging types of human implants from the materials science's point of view” [1]. In fact, the selection of materials for artificial hip joint is not always straightforward and mechanical properties (strength, elasticity, toughness and ductility), tribological features (low friction and low wear), biocompatibility and corrosion resis- tance are just some aspects that must be considered [2,3]. For hip prosthesis, the femoral head is usually made of metallic alloys (stainless steel, CoCr or some Ti-based alloys) or ce- ramics (Al 2 O 3 or ZrO 2 ). The acetabular cup (socket) is made of ultra-high molecular weight polyethylene (UHMWPE), alu- mina (Al 2 O 3 ) or zirconia (ZrO 2 ), or CoCr alloys. In this kind of prostheses, one of the main causes of failure is the production of wear debris [1,3–6], which enhance the wear rate through a three-body wear mechanism when deposited between the two bearing surfaces. These residues are generally responsible for osteolysis and aseptic loosening of the implant [4–6]. The search for new materials that potentially improve the wear resistance and minimize the generation of harmful debris of artificial hip joints has been an objective in the past few years. Coating the implants with protective functional films may be a successful way of reducing wear and corrosion of the com- ponents of hip prosthesis and of avoiding adverse biological reactions with the surrounding tissues. Nanocrystalline diamond (NCD) coatings have the required properties to play this role. The super hardness of this film ensures low wear rates while the innocuous nature of any wear debris prevents inflammatory responses. In this work, silicon nitride (Si 3 N 4 ) ceramics were used as substrate material due to the well-proven adhesion of NCD to this ceramic [7]. A screening analysis is performed using pin-on-flat tribological tests under lubrication with simulated physiological fluids: Hanks's balanced salt solution (HBSS) and dilute fetal bovine serum (FBS). The experiments are self-mated, considering that articulating surfaces, acetabular cup and femoral head, can both be coated with NCD films. Available online at www.sciencedirect.com Diamond & Related Materials 17 (2008) 848 – 852 www.elsevier.com/locate/diamond ⁎ Corresponding author. Department Ceramics & Glass Eng., University of Aveiro, 3810-193 Aveiro, Portugal. Tel.: +351 234370243; fax: +351 234425300. E-mail address: rsilva@cv.ua.pt (R.F. Silva). 0925-9635/$ - see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2007.12.070