ICMCTF2009 Session H3: Surface Engineering of Coatings: Tribo, Bio and Nano-Corrosion Effects

As orthopaedic implant manufacturers improve longevity of hip and knee prostheses, new coatings are being developed to minimize in-vivo wear against a polyethylene bearing. The rationale for development of a CVD alumina coated Co-28Cr-6Mo implant is driven by the manufacturers desire to combine the wear resistance of an alumina surface with the fracture toughness and damage tolerance of a conventional orthopaedic structural alloy. Physical and mechanical properties that are important in designing a material for use as an implant articular wear surface include scratch resistance, fatigue strength, hardness, fracture toughness, aqueous wettability, biocompatibility and corrosion resistance. In this part of the investigation, a multilayer coating consisting of chemical vapor deposition CVD Al₂O₃ with a TiCN-containing interlayer on a Co-28Cr-6Mo implant substrate is evaluated in terms of corrosion resistance in as-deposited and scratch-damaged conditions using potentiodynamic/potentiostatic testing and scanning electron microscopy. Oxidized Zr-2.5Nb, similar to Oxinium material by orthopaedic implant manufacturer Smith & Nephew, is selected for comparison because of its similar oxide surface/metal substrate composite structure, which consists of a 4-5 µm layer of ZrO₂ grown into a polished Zr-2.5Nb surface by thermal oxidation.

Single sweep potentiodynamic scans performed in non-deaerated Hanks solution at 37°C followed by SEM surface inspection suggest that at potentials up to +2.2V (vs. Ag/AgCl), the as-coated TiCN and Al₂O₃ components exhibit lower corrosion currents and better resistance to dissolution with increasing anodic potential than the Co-28Cr-6Mo substrate. Cyclic scans performed in non-deaerated Hanks solution with 25% bovine serum on scratched multilayer CVD alumina coating to reverse potentials of +1.1V and +2.2V (vs. Ag/AgCl) followed by SEM cross section inspection, exhibit no hysteresis characteristic of crevice corrosion or pitting. Potentiostatic testing of scratched CVD alumina coating and scratched oxidized Zr-2.5Nb at +0.5V and +1.0V (vs. V_oc) for 72 hours reveal current decrease to approximately 1 nA, suggesting that spontaneous passivation occurs in the scratched CVD alumina coated Co-28Cr-6Mo at these potentials in these conditions.

Titanium spontaneously forms a compact and protective oxide layer (mainly TiO₂) in the atmospheric environment, which provides anti-corrosive properties. When used as a bone implant, cells will be in direct contact with this surface layer, covered with proteins which nature are also dependent on both the environment and the surfaces properties of the material.

In this study oxide films were created combining two different techniques, anodic treatment followed by a thermal oxidation. By the anodic treatment specific Ca/P ratios were created in the oxide film, while the thermal treatment was aimed at creating a compositional/structural transition across the film, which may influence the tribological and corrosion behavior of the material. A β-GP + calcium acetate solution was used as electrolyte in the anodic treatment performed at different cell voltages.

The thickness, composition, structure and topography of the films were investigated by FE-SEM, XRD, Raman Spectroscopy and AFM. Electrochemical impedance spectroscopy (EIS) was used before and after anodic treatment in order to characterize and compare the original native oxide film with the anodic oxide film. AFM and SEM analysis demonstrate substantial differences on film topography, both at the micro and nano-scale, with an enhancement of the overall surface roughness. Adhesion of the films was evaluated by the scratch test associated to acoustic signal measurements. Furthermore, the tribocorrosion behavior of the material was evaluated by combining tribological and electrochemical tests (AC and DC).

Results show that a wide range of surface states (from the chemical composition, structural and topographical point of views) can be obtained, some of them showing good performance in terms of biocompatibility. The combination of both treatments strongly enhances the tribocorrosion behavior of the material.

Nanoscale CrN/NbN multilayer PVD coatings have proven their mettle in offering combined erosion-corrosion resistance. However growth defects (under-dense structures and droplets) in the coating reduces the ability to offer combined erosion-corrosion resistance. In this work a novel High Power Impulse Magnetron Sputtering (HIPIMS) technique has been utilised to pre-treat substrates and deposit dense nanoscale CrN/NbN PVD coatings (HIPIMS-HIPIMS technique). This new technique, rich with metal ion plasma, deposits very dense structures and offers virtually defect free coatings (free of droplets as observed in Cathodic Arc technique and under-dense structures observed in standard dc sputtering)

Plasma diagnostic studies revealed a high metal ion to gas ion ratio (Cr:Ar) of 3:1 for HIPIMS pre-treatment conditions with the detection of 14%Cr⁺² and 1%Cr⁺³ ions and J_sof 155 mAcm^-2. For coating deposition conditions the metal ion to gas ion rat io was around 1:4 which is significantly higher compared to DC at 1:30. Characterisation of coating on HSS revealed a high adhesion of L_c75 N (scratch tests), high hardness of 34 GPa and Young's modulus of 381 GPa. Friction coefficient was 0.78 and dry sliding wear resistance was very high with wear coefficients (K_c) values of 1.22 x 10^-15. The effect of superior microstructure (droplet defect and intergranular void free) on erosion-corrosion resistance has been evaluated by subjecting the coatings to a slurry impingement (Na₂CO₃+ NaHCO₃buffer solution with Al₂O₃particles of size 500-700µm) at 90° with a velocity of 5 ms^-1. Experiments have been carried at -1000 mV, +300 mV and +700 mV representing 3 different corrosion conditions. The development in the coating technology and in the performance of nanoscale CrN/NbN PVD coatings due to the novel HIPIMS technique is presented by comparing results with coatings deposited by standard UBM technique.