ICMCTF2010 Session H3: Surface Engineering of Coatings: Tribo, Bio and Nano-Corrosion Effects

Aluminum coatings are widely used for the protection of steel structures, but nowadays, structural materials are subjected to more severe conditions combining multi-physical parameters. A reinforcement of the properties of PVD aluminum coatings is necessary to check the new operating conditions of the materials. One problem of aluminum in corrosive media is the susceptibility to localized corrosion, and particularly to pitting corrosion in saline solution that drastically reduces the lifetime of the coatings. So it could be interesting to modify the corrosion mechanism from a localized type to a uniform mode.

Aluminum based alloys were deposited on glass substrates by magnetron sputtering PVD technique. Rare earth (RE) alloying elements (Ce, Y or Gd) were added in order to improve the properties of the aluminum coatings. The influence of the addition of different RE elements is discussed. The incorporation of RE elements has a very limited effect on the mechanical properties. For the 3 RE elements, a shift of the corrosion potential towards more negative values was observed when the content of RE elements increased. Extended immersion tests revealed that the corrosion potential of these alloys slightly evolves towards more positive values due to the formation of a passive film. The composition of this passive film will affect the mechanism of pitting corrosion, that could be less harmful for the lifetime of the Al based coatings.

The evaluation of the corrosion resistance provided by insulating coatings to stainless steel is challenging due to the insulating properties of the coatings. Potentiodynamic studies are possible when the coatings have defects (micro pores) that allows the flow of the carriers (electrons or ions) between the substrate and the electrolyte. However, the interpretation using standard Tafel analysis is not accurate for quantitative evaluation of the corrosion properties. The ac techniques, such as, electrochemical impedance spectroscopy (EIS) provides an appropriated interpretation with detailed information of the processes occurring at both the coating-electrolyte interface and the localized corrosion through the small pores.

Niobium oxide ceramic coatings deposited on medical grade stainless steel, could be used as protective coatings in biomedical applications, such as, orthopaedical or dental implants. As a ceramic, it is a very stable material with good mechanical properties and previous studies have shown that it is not-toxic to human cells and tissues. Niobium oxide might occur in different crystalline phases; Nb2O5, NbO2 and NbO and the production of these phases by physical vapor deposition (PVD) methods require deposition at substrates temperature above 300°C. This will lead to a crystalline but columnar microstructure, as common for PVD coatings, where the inter-columnar space always constitute an open porosity where the electrolyte can reach the substrate with the subsequent failure of the coating. Therefore, in this work, we decide to evaluate the corrosion resistance of amorphous niobium oxide (a-NbOx) films using NaCl 0.89% solution, which are known as simulated biological solutions. The films were deposited by magnetron sputtering on stainless steel (AISI 316L) and the corrosion resistance was evaluated using potentiodynamic, polarization resistance and EIS. The deposition conditions and thicknesses were varied to find the ideal coating that could significantly improve the corrosion resistance of stainless steel to a chloride solution. The results showed that 500 nm constitutes a good thickness where there is no interconnected porosity and therefore the a-NbOx film forms a barrier between the electrolyte and the substrate.

In this study, the electrochemical behavior of a TiN film deposited by cathodic arc method (substrate: Ti6Al4V) was investigated in bovine calf serum (BCS, protein: 30 g/L) and ringer solution at 37^oC. The corrosion behavior was studied by measuring open current potential (OCP) and Potentiodynamic test. Further, the electrochemical behavior was investigated as a function of immersion time (1hr, 15 hrs, 40 hrs, 115 hrs) in both solutions. Electrochemical impedance spectroscopy (EIS) technique was used to characterize the electrochemical double layer and corrosion kinetics. Optical and SEM images were used to characterize the corroded surface. Results indicate that the presence of the protein in the solution increases the possibility of pit formation on the surfaces and may adversely affect the overall corrosion resistance of the TiN coated surface.

Usage of implants in human body has a long history of more than thousand years, and it is accepted as an effective clinical method, particularly in dentistry and orthopedics. Recently the implant usage is dramatically increased, for example, 500,000 stainless steel cardiac stents are now implanted in the US, every year. In dentistry, implant supported overdenture has been accepted as a standard of care in many countries.

However, in addition to the functional requirement, the nature of implant metals in the body environment always invites some concerns, particularly the issues such as the interactions with adjoining tissues and release of metal ions and its transport to the remote organs. Due to the clear clinical evidences (tumors, hyper sensitivity etc.) of such influence, during last 10 years, many studies were reported on the corrosion behaviour of the implant metals, based on the specific applications and environments, (For example, Cp Ti, in artificial saliva as a function pH). The main challenges in the area of Bio-corrosion is the need of researchers from materials science, physicians, biochemists, electrochemist, mechanical engineers, tribologist and microbiologist and evaluating their new finding in the light of clinical experience and effective practical implementations.

Hence this presentation addresses the progress made in the area of bio-corrosion, specifically, in the dentistry and orthopedics and some insights in methodologies in studying together electrochemistry, surface chemistry and cell culture. The influence of protein content in a biofluids on corrosion kinetics is highlighted. Further, importance of research area called tribocorrosion which simulates natural conditions, such as mastication or artificial joint, where the implants are exposed to movements in the chemical environment is outlined. Keeping this view, Dental school at UIC and Tribology Section, Dept of Orthopedic surgery (RUSH University, Chicago) established a strong research collaboration to address such issues and some recent outcomes and a road map and scope of this research area are included.

Aluminum based alloys were deposited on glass substrates by electron-beam evaporative PVD technique. Alloying elements (Ti or Mg) were added in order to improve both mechanical and corrosion behaviors of aluminum films. This approach aims to develop sacrificial protection of flat steel products. Since their vapour pressures are different, aluminum and alloying elements were co-evaporated from two separate sources. Different contents of transition metals were examined

The electrochemical characterizations were carried out in saline solution. It is shown that aluminum can be mechanically reinforced with preserving sacrificial behaviors. The evolution of the properties is strongly linked to the nature of the incorporated element. A comparison with a previous study on the electrochemical and mechanical behaviors of Al-Cr and Al-Gd EBPVD coatings will permit to complete the analysis of the influence of the incorporation of transition metal in aluminum.

In this study, the tribocorrosive nature of titanium (CpTi) was investigated in artificial saliva as a function of pH (3,6,9) and load (20N). The sliding duration (2000 cycles), frequency (1.24 Hz) and load parameters were selected to closely mimic the oral environment and mastication process. The evolution of the current (at potentiostatic condition of Ecorr (E vs SCE)) and friction coefficients were measured during the test. Electrochemical impedance spectroscopy (EIS) measurements were made before and after the sliding (wear test) to comprehend the changes in the corrosion kinetics and surface chemistry. The results indicate that there is a major influence of pH on the tribocorrosion mechanisms and the synergistic interactions of wear and corrosion. Particularly, the high weight loss at pH 6.5 invites some concerns on the implant behavior in a normal oral environment.

In studies of tribo-corrosion, the performance of dental materials is becoming of increasing interest. This is because in oral environments, corrosion, together with wear arising from oral processing, provide a tribo-corrosion environment which can vary significantly in pH, load, and size of wear debris. In such conditions, the performance of materials may be difficult to predict.

In tribo-corrosion, there has been significant research into descriptions of regimes of degradation. More recently the distinctions of wear affected corrosion (“additive behaviour) and “corrosion affected wear (“synergistic behaviour”) have also been made. Such categories enable a mechanistic description to be assigned for the various tribo-corrosion processes.

In this work, the performance of a candidate dental material and a hydroxyapite coating were compared with a reference material-steel- in various micro-abrasion-corrosion environments. The environments included acidic conditions i.e. in orange juice and neutral solutions i.e. milk. Regimes of tribo-corrosion were identified based on the results enabling a new mechanistic description to be defined for dental materials and coatings