ICMCTF2012 Session D3-1: Coatings for Mitigating Bio-Corrosion, Tribo-Corrosion and Bio-Fouling

Tribocorrosion covers the material degradation process resulting from the combined interaction of wear and corrosion phenomena on surfaces subjected to a relative contact movement. Bio-tribocorrosion is the designation used to describe the tribocorrosion behavior of materials in contact with biological environments, as it happens in dental implants.

In fact, dental implants are under a complex congregation of mechanical solicitations and chemical aggressive substances, which can change considerably in magnitude or nature over the day and between individuals. Moreover, dental implants have a strong interaction with cells (soft and hard tissue cells and/or microorganisms) resulting in the local modification of the surrounding environment, both from the chemical and mechanical points of view. For instance, while the adhesion of bone cells to the material is desirable to provide osseointegration, colonization by microbes and the consequent formation of biofilms should be avoided. Surface modification routes are being investigated in order to provide multifunctional properties to the surface of dental implants.

In this work, an overview of the current knowledge of the bio-tribocorrosion mechanisms of Ti alloys used in dental implants will be presented. The effects of biofilms or cultured osteoblastic cells on the bio-tribocorrosion response will be addressed. Special focus will be given to surface modifications techniques which presently appears as promising to provide a good combination of biological and bio-tribocorrosion responses.

Research for alternatives to electroplated hard chromium coatings is continued because of their tremendous environmental and health hazard concern. Among the various possibilities, recently, electroplated Co-W alloys show promising results like abrasive wear and corrosion resistance close to and even better than electroplated hard chromium. In the present study, the Co-W alloy matrix is further strengthened by incorporating nano-size WC particle via electrochemical codeposition technique. The matrix cobalt helps in binding the WC particles and in return particles strengthen the matrix along with host tungsten as alloy element. In order to understand corrosion resistance of these alloys, potentiodynamic polarization was carried out in 0.5M NaCl solution at room temperature and compared with similar composition CoW alloys. Indeed, it was found that nano-size WC particle helps in improving the corrosion resistance of the composites. The differences in corrosion behaviour were understood by analyzing the corroded surface morphology and composition analysis.

Further importance has been given on corrosion behaviour under mechanical loading conditions (known as tribocorrosion) in the same solution to extend the possibility of industrial applications of these coatings. Under mechanical loading and unloading conditions, open-circuit potential (OCP) of a few coatings was measured in order to estimate the material loss under mechano-chemical environment. In this presentation, details of tribocorrosion of these coatings will be discussed correlating with corroded wear scar morphology.

Fretting corrosion is one of the most deleterious mechanisms for the degradation of metallic biomaterials, especially in the orthopedic field. Around 1.5 million of hip prostheses are implanted worldwide. This work is dedicated to study the synergistic effect of proteins and ions concentration on the wear of stainless steel, 316L, against a polymer sample under fretting-corrosion conditions.

A specific device allows reproducing fretting corrosion, i.e. relative displacements of microns between materials in contact. In order to compare with previous investigations, the amplitude was equal to ± 40 µm with sinusoid dal amplitude. The duration of each test was 14400 s, 4 hours.

The 316L sample size was the same for all samples. 3D roughness of 316L, Ra (3D), was 10 ± 2 nm. PMMA (PolyMethyl Methacrylate) is cylindrical with a roughness of 35 ± 5 nm. 4 solutions of NaCl were selected in order to study the effect of the ionic strength: 10^-3 mol.L^-1, 10^-2 mol.L^-1, 10 ^-1 mol.L^-1 and 1 mol.L^-1. Additionally on each solution described previously, concentrations of proteins (pure albumin) were: 0, 1 and 20 g.L^-1. Experiments were investigated at temperature of 22 ± 1 °C. The potential was equal to -400 mV/SCE and the current density was measured with a particular attention to the device insulation.

The synergistic formalism is:

W = W_c +W_m + (DW_cm + DW_mc); W: total wear volume; Wc: wear volume due to corrosion; Wm: wear volume due to mechanics; DWcm: synergistic wear volume, corrosion enhances wear due to mechanic; DWmc: synergistic wear volume, mechanic enhances wear due to corrosion.

As expected, the main wear volume of stainless steel is due the synergistic terms for a salts concentration close to the one of human physiological liquid. 10^-1 mol.L^-1 is the threshold concentration for increasing wear and the part of the synergy effect. At 10^-3 mol.L^-1 of NaCl, the content of albumin has no significant impact on the wear volume. For fixed salts concentration higher than the threshold, albumin does not promote the wear of stainless steel surface, it decreases (divided by 3 from 0 g.L^-1 to 20 g.L^-1). The mechanical wear, calculated at applied potential of -800 mV/SCE, does not change according to the albumin concentration.

The most interesting point is the albumin changes the mode of synergy. At 0 g.L^-1 of albumin, the main synergistic term is the influence of mechanics on corrosion, DWmc. On the contrary, at 20 g.L^-1, the main synergistic term is the influence of corrosion on mechanics, DWcm. One might suggest albumin promotes the corrosion after mechanical damage of the passive film. However albumin prevents from mechanical degradation.

In orthopedics, the use of metal-based alloys is a reoccurring practice because of their biocompatibility, low-wear rates and post-operative stability. However, metal-on-metal (MoM) hip joint bearings endure a constant load during articulation while immersed in synovial fluid. This environment has shown to be detrimental because of leaching metal ions and particulate debris in the neighboring tissue that has lead to implant failure and revision surgery. Currently, MoM joints are facing serious challenges with market usage declining from 35% to less than 10%. Tribochemical reactions potentially generate a type of tribolayer on the surface, which has shown a better electrochemical and tribological behavior. However, this layer is very unstable, nonhomogeneous and patchy in appearance. A defined micro-topography of the surface could be a solution facilitating strong film growth. To address these concerns, the aim of the current work is to investigate electrochemical properties of 5 types of topographies on a CoCrMo alloy surface.

Five different topographies were prepared using electrochemical etching. The topographies were adjusted by applying a different current during the etching process. The textured samples were than compared to a control sample that was polished using a conventional polishing process. Corrosion tests were conducted in an electrochemical cell with bovine calf serum (30 g/L protein) as the electrolyte. A standard corrosion test protocol was used that included monitoring of the open circuit potential for 1 hr and an electrochemical impedance spectroscopy test with a frequency range of 100K Hz to 0.005 Hz. Samples were then polarized during the cyclic polarization test from -0.8 V to -0.8 V with a peak voltage of 1.8 V. The surfaces were then characterized using a white light interferometer and scanning electron microscope.

The surfaces with a micro-topography demonstrated an improved corrosion behavior when compared to the control sample. Texturing the surface results in an even distribution of surface depressions. It appears that this topography enhances the adherence of protein resulting in a decelerated corrosion rate. Thus, a micro-topography of the surface is ideal for creating a heterogeneous proteinaceous layer that is analogous to a tribolayer, which is already reported. To further comprehend the properties of a micro-topography it is essential to expose a tribocorrosive environment, combining the influence of mechanical wear and corrosion. It can be assumed that such topography may reduce the influence of abrasion by particle entrapment and enhances the influence of tribolayer formation resulting in lower wear.