ICMCTF2015 Session D3: Coatings for Bio-corrosion, Tribo-corrosion and Bio-tribology

The NbTiN coatings are considered promosing candidate materials for biomedical applications because of their potential to improve the wear life and corrosion resistance of bearing components. In this paper, we present a systematic study of Nb_1-xTi_xN system with x = 0 – 1. The films are deposited onto CoCr substrates by magnetron sputtering and cathodic arc, where the nitrogen partial pressure is varied and the substrate bias is adjusted to modify the crystal structure, stoichiometry, and growth morphology of the films.

Chemical composition and microstructures are characterised using RBS, XPS, XRD, SEM, TEM, and mechanical properties by nanoindentation. Wear properties and friction coefficients are studied using a ball-on flat tribometer with polyethylene ball as the counterface material.

Although the presence of (Ti) favours the formation of cubic phase over a vide range of N₂/Ar ratio, the very low and the very high nitrogen partial pressure lead to the occurrence of hexagonal phases either alpha or delta prime, respectively.

Grazing incidence XRD under different angles shows that the initial stage of growth is accompanied by the formation of hexagonal phase, which changes to the cubic phase at higher thickness. On the basis of XPS, most of the films are N over stoichiometry. Hardness and modulus are very sensitive to Nb/Ti ratio, and both parameters increase with the Nb content in the cubic coatings. In this presentation, structural and mechanical properties will be discussed in terms of deposition parameters and growth kinetics.

Magnetron sputtering has been recognized as a suitable surface modification technique to improve the performance of a material by using a wide spectrum of coatings, which also can provide additional capabilities. In this study, Zr-C-N-Ag coatings were deposited onto stainless steel 316L aiming to evaluate the corrosion resistance and chemical changes of the coatings after immersion in simulated body fluids containing proteins, in order to assess the possibility of these coatings to be used for orthopedic devices. Samples were prepared by DC unbalanced reactive dual magnetron sputtering using two targets, Zr and Ag, in Ar, C₂H₂, N₂ atmosphere. The materials were characterized in terms of composition and structure using electron probe microanalysis, X-ray photoelectron spectroscopy, X-ray diffraction and Raman spectroscopy.

The results revealed that the coatings are composed by a variety of phases among which ZrCN, Ag and a:C phases have the predominant content. The electrochemical assessment evidenced a large dependence of the corrosion resistance on the silver content in the films, deteriorating the electrochemical stability of the material as the silver content increases. The amorphous carbon phase also negatively influenced the polarization resistant, behaviour ascribed to the induced morphological changes. The immersion test revealed a progressive increment of the polarization resistance as the time evolves, attributed to the passivation of both the surface and the pores, due to either ZrO₂ formation on the top or a protective layer of albumin on the surface, result corroborated by X-ray photoelectron spectroscopy.

In order to improve the wear resistance and the friction properties of materials for the head / cup contact of hip prostheses, we studied the effect of the presence of dimples on one of the contacting bodies. Our research focused on CoCr – UHMWPE and CoCr – CoCr bearing couples, in a contact between a textured flat specimen made of CoCr and a polished ring (in metal or polymer), on a laboratory tribometer, at 37°C in calf serum. The dimples (of circular shape in a hexagonal network) were produced with a femtosecond laser and the effects of dimples’ parameters (diameter - from 15 µm to 40 µm -, height - from 10 µm to 25 µm -, and pitch - from 60 µm to 100 µm -) on friction and wear were studied.

Steady state friction coefficients are analyzed vs. dimples’ parameters, area ratio (% of dimples’ surface on the whole contacting surface) and aspect ratio (diameter / height). Worn surfaces of both counterbodies are observed by optical microscope, digital microscope and SEM. Interferometry measurements are performed to measure the wear volume. Dimples’ filling by wear debris is analyzed.

In MoP contact, micro-texturing cannot decrease friction, compared to smooth polished surface. In MoM contact, friction coefficient of textured surface can be lower than the one of polished reference surface for high value of area ratio (≥10%) and low value of aspect ratio (≤2). Moreover, concerning wear of MoM contact, for all values of area ratio (from 4% to 23%), if the aspect ratio is large enough (≥2), wear volume of textured surface is lower than the one of the reference polished surface. It is possible to divide the wear amount by more than 2 by putting dimples with a diameter of 30 µm, a height of 10 µm and a spacing of 60 µm, in MoM contact.

Recently, plasma electrolytic oxidation (PEO) process has been widely studied and applied in industries due to its ability to create functional oxide layers on Al, Ti and Zr alloys. In this work, a pulsed DC power supply was adopted to grow the zirconia coating on the pure Zr metal by PEO treatment. Three duty cycle values, 25%, 50%, 75% at a fixed frequency of 1000 Hz and direct current (DC) of power supply control were used, respectively, to fabricate the zirconia coatings in the aqueous solution containing K₃PO₄ and KOH in three different concentration ratios. The processing time was kept at 10 min. The structures of PEO coating was determined by the X-ray diffractometer (XRD). The surface and the cross-section morphologies of the PEO coating were examined by scanning electron microscopy (SEM). The HRC-DB method was used to evaluate the adhesion quality of the coating. The surface roughness was examined by a profilometer. The biocompatibility of the zirconia coating was examined by the 3T3 cell cultivation and the MTS assay . The corrosion resistance of the PEO coating was evaluated by the potentiodynamic polarization test in Hank’s solution. Effects of duty cycles and electrolyte concentration on the microstructure, adhesion property and biocompatibility of the zirconia coatings by PEO treatment were discussed in this work.

Metallic materials for the design and construction of biomedical implants (i.e. hip and knee joint prostheses) are still one of the considered materials because of their excellent mechanical properties, corrosion resistance and biocompatibility. However, they are also subjected to the interaction between wear and corrosion during the operating conditions, thus degrading through a tribocorrosion mechanism. Fundamental understanding of the involved phenomena will serve as the basis for specific material design under bio-tribocorrosion situations. Mechanistic approach of tribocorrosion has leaded the development of new models for describing this interaction. Consideration of corrosion principles together with tribological laws has allowed establishing the basis for new tools in tribocorrosion modeling. This work introduces the fundamental concepts of tribocorrosion for biomedical applications and describes the experimental techniques available for those tribocorrosion studies (in-vitro and in-vivo) together with the analysis of the existing models for interpretation