ICMCTF2014 Session D2-1: Coatings for Bio-corrosion, Tribo-corrosion and Bio-tribology

Recently graphene appears to be an inexpensive material available in large quantities with a well-known potential effectiveness for improving mechanical, electrical, thermal and other properties; which make it promising in many applications such as solar cells, hydrogen storage, sensors, batteries, super-capacitors and nanocomposites. Graphene is well dispersed in polymer matrix by solution mixing, melt mixing, and in situ polymerization methods. The ability of graphene to form chemical bonds in polyurethane (PU) chains, it can be effective nanofiller for the PU matrix. Moreover, due to the disk-like shape of the graphene, it is believed that graphene can be choice for improving the gas-barrier property of the nanocomposite protective coating. Waterborne polyurethane (WBPU) consists of mainly urethane and urea groups. Water is the main solvent during synthesis of WBPU. Only water evolved from WBPU at drying stage of coatings. Thus this process is environmentally friendly and free from pollution. In this report we synthesized environmentally friendly WBPU/graphene dispersions through a pre-polymer process using different graphene content (up to 3 wt%). The synthesized dispersions were used as a coating material on mild steel and evaluate the corrosion efficiency of WBPU/graphene coatings.

Metal-on-metal hip replacements have experienced a sharp decline in the last two years due to biocompatibility issues related to wear and corrosion products. Despite some excellent clinical results, the release of wear and corrosion debris and the adverse response of local tissues have been of great concern. There are many unknowns regarding how CoCrMo metal bearings interact with the human body. In this study we investigated the corrosion of CoCrMo alloys with different heat treatments. Solution annealing the wrought alloys resulted in significant grain growth. After annealing at 1230 ^oC, the second phase, i.e. chromium carbide, was partially dissolved into the matrix. The corrosion current density (i_corr) dropped from ~1400 nA/cm² for the original wrought alloy to ~300 nA/cm² for solution annealed specimens, indicating that grain coarsening improves corrosion resistance. Pitting corrosion preferentially attacked high-angle grain boundaries with higher energy. All low-angle boundaries were immune to corrosion in the tests. We also evaluated metal-on-metal hip replacement retrieved at revision surgery using electron microscopy and Raman spectroscopy. Thin layers of films are frequently generated in-vivo on CoCrMo MoM surfaces, which are known as tribological films or tribofilms. The EELS results show that tribofilm consist of over 82% sp²-bonded carbon while no discernible nitrogen was found, suggesting that the tribofilm is primarily graphitic carbon. High-resolution TEM micrographs showed short-range ordered fringes in the tribofilm. The results were further confirmed by the Raman spectrum. The tribofilm may be protective against the degradative processes taking place in-vivo in the host body. The composition and ultrastructure is unique and can provide insights into the basic mechanisms of tribocorrosion in metal on metal bearings.

Modern hip prostheses feature a modular implant design with a tapered junction between the femoral neck and head, which gives the surgeon flexibility in implant assembly and reduces inventory. Despite their apparent benefits, modular junctions also introduce complications in the hip implant system by introducing additional interfaces. The implant is subjected to various loads and body movements, which may produce micromotion movement at the interface. This can lead to mechanically assisted corrosion (Fretting-Corrosion) and increased release of wear debris and metal ions. This study has two objectives. (1) To study the electrochemical behaviour of commonly used metal alloys (Ti6Al4V and CoCrMo) in a simulated joint fluid evironment with two different pHs (7.6 and pH 3.0), and (2) to implement a new fretting-corrosion apparatus in order to study the fretting-corrosion behavior of Ti-CoCrMo couple in a proteinous solution (BCS) and a saline solution (0.9% NaCl).

For corrosion tests, six Ti6Al4V discs (Mac-Master Carr, Elmhurst, IL) and six wrought high-carbon CoCrMo discs (ATI Allvac, Pittsburgh, PA) were milled from rods. The samples were divided into 4 groups (n=3) as a function of disc type (Ti6Al4V or CoCrMo) and pH level (3.0 and 7.6). For tribocorrosion tests, Ti6Al4V alloy rods (exposed surfaces: 11mm by 11mm) and 2 CoCrMo pins (exposed Surfaces: 12mm diameter x 7 mm thickness) were used. A sinusoidal fretting motion (amplitude of +150 um) was applied to the Ti alloy rod, which was generated through an Instron 8800 load frame (Canton, MA). A load of 200N was applied axially from both sides and the tangential load was monitored. The evolution of potential was monitored during test.

Significant differences were noted within both metals for Ecorr and Icorr (p<0.05; ANOVA). Ti6Al4V exhibited significantly lower Icorr values than CoCrMo in all pHs (p<0.0001; t-test). The Ecorr value for Ti6Al4V was significantly higher than CoCrMo at pH 3.0 (p=0.004; t-test). During the fretting motion, the potential suddenly decreased (0.15V BCS and 0.2V-NaCl) and then slowly recovered during the final stabilization period. Fluctuations of the potentials were evident in both solutions. Further studies will be done by simulating other conditions at the modular junction (pH decrease, varying protein concentrations) to understand the mechanisms driven by the synergistic interaction of wear and corrosion at the modular junction.

Interest in biodegradable magnesium (Mg) implants has increased dramatically in the last decade due to their desirable mechanical properties that are close to the human bone, thus avoiding stress shielding effect. In addition, Mg is resorbed in the body and surgery to remove the implant after healing is not needed. However, the high corrosion rate of magnesium and its alloys severely limit their practical applications as implants. To control the corrosion rate of magnesium, biocompatible and bio-resorbable calcium-phosphate (Ca-P) coatings were deposited in this study using AZ31 magnesium substrate and hydrothermal deposition process. Coatings obtained at various temperatures were found to mainly consist of a mixture of monetite [CaHPO₄] and tricalcium phosphate [Ca_2.86 Mg_0.14 (PO₄)₂] with partially magnesium substituted in tricalcium phosphate structure as determined by X-ray diffraction (XRD) and Fourier transform infrared (FTIR) spectroscopy methods. Coating morphology was examined with scanning electron microscopy (SEM) and the results showed that coatings became denser with increasing temperature. Coating adhesion was examined by pull-out adhesion test indicating only coating cohesive failure at 5.2-5.8 Mpa. It was clearly observed that significant portion of the coating still remained on the substrate after adhesion test. Potentiodynamic polarization conducted in simulated body fluid (SBF) solution confirmed that coating significantly enhanced the corrosion performance of Mg substrate. The corrosion current density of Mg substrate decreased approximately 10,000-fold in the presence of coating. The mass loss for the coated samples was only around 1/3 of that observed for bare substrate, and the amount of Mg ions released from the coated samples was significantly lower than in case of bare substrate after immersion in SBF solution. The described hydrothermal method provided crystalline and compact coatings with excellent adhesion strength and significantly improved corrosion resistance with good prospects for biomedical application.

Keywords: Magnesium, Calcium-Phosphate coating, Corrosion resistance