Micro-abrasion-corrosion of cast CoCrMo-Effects of micron and sub-micron sized abrasives

The abrasion damage on retrieved CoCrMo based hip joints is reported to be influenced by the entrainment of micron and sub-micron sized debris/hard particles. This paper represents the first attempt to look into the effects of relatively soft abrasives with micron and sub-micron dimensions on the abrasion mechanisms and the abrasion-corrosion performance of the cast CoCrMo in simulated hip joint environments. A modified micro-abrasion tester incorporating a liquid tank and a three-electrode electrochemical cell was used. Al O (300 nm and 1 μm) and sub-micron sized BaSO abrasives were chosen as being comparable in the size and hardness to the wear particles found in vivo. Results show that the specific wear rates of cast CoCrMo are dependent on the abrasive particle size, hardness and volume concentration. Larger particle size, higher hardness and greater abrasive volume fractions gave greater wear rates. The wear-induced corrosion current generally increases with increasing wear rates, and the presence of proteins seems to suppress the wear-induced corrosion current especially when abrasive volume fractions were high. This study shows that the nature of abrasives and the test solutions are both important in determining the wear mechanisms and the abrasion-corrosion response of cast CoCrMo. These findings provide new and important insights into the in vivo wear mechanisms of CoCrMo. © 2009 Elsevier B.V. All rights reserved.

Micro-abrasion mechanisms of cast CoCrMo in simulated body fluids

The abrasion seen on some of the retrieved CoCrMo hip joints has been reported to be caused by entrained hard particles in vivo. However, little work has been reported on the abrasion mechanisms of CoCrMo alloy in simulated body environments. Therefore. this study covers the mapping of micro-abrasion wear mechanisms of cast CoCrMo induced by third body hard particles under a wide range of abrasive test conditions. This study has a specific focus on covering the possible in vivo wear modes seen on metal-on-metal (MoM) surfaces. Nano-indentation and nano-scratch tests were also employed to further investigate the secondary wear mechanisms-nano-scale material deformation that involved in micro-abrasion processes. This work addresses the potential detrimental effects of third body hard particles in vivo such as increased wear rates (debris generation) and corrosion (metal-ion release). The abrasive wear mechanisms of cast CoCrMo have been investigated under various wear-corrosion conditions employing two abrasives, SiC (similar to 4 mu m) and Al(2)O(3) (similar to 1 mu m), in two test solutions, 0.9% NaCl and 25% bovine serum. The specific wear rates, wear mechanisms and transitions between mechanisms are discussed in terms of the abrasive size, volume fraction and the test solutions deployed. The work shows that at high abrasive volume fractions, the presence of protein enhanced the wear loss due to the enhanced particle entrainment, whereas at much lower abrasive volume fractions, protein reduced the wear loss by acting as a boundary lubricant or rolling elements which reduced the abrasivity (load per particle) of the abrasive particles. The abrasive wear rate and wear mechanisms of the CoCrMo are dependent on the nature of the third body abrasives, their entrainment into the contact and the presence of the proteins. (C) 2009 Elsevier B.V. All rights reserved.

Interpretation of electrochemical measurements made during micro-scale abrasion-corrosion

This paper brings together and analyzes recent work based on the interpretation of the electrochemical measurements made on a modified micro-abrasion-corrosion tester used in several research programmes. These programmes investigated the role of abradant size, test solution pH in abrasion-corrosion of biomaterials, the abrasion-corrosion performance of sintered and thermally sprayed tungsten carbide surfaces under downhole drilling environments and the abrasion-corrosion of UNS S32205 duplex stainless steel. Various abrasion tests were conducted under two-body grooving, three-body rolling and mixed grooving-rolling abrasion conditions, with and without abrasives, on cast F75 cobalt-chromium-molybdenum (CoCrMo) alloy in simulated body fluids, 2205 in chloride containing solutions as well as sprayed and sintered tungsten carbide surfaces in simulated downhole fluids. Pre- and post-test inspections based on optical and scanning electron microscopy analysis are used to help interpret the electrochemical response and current noise measurements made in situ during micro-abrasion-corrosion tests. The complex wear and corrosion mechanisms and their dependence on the microstructure and surface composition as a function of the pH, abrasive concentration, size and type are detailed and linked to the electrochemical signals. The electrochemical versus mechanical processes are plotted for different test parameters and this new approach is used to interpret tribo-corrosion test data to give greater insights into different tribo-corrosion systems. Thus new approaches to interpreting in-situ electrochemical responses to surfaces under different abrasive wear rates, different abrasives and liquid environments (pH and NaCl levels) are made. This representation is directly related to the mechano-electrochemical processes on the surface and avoids quantification of numerous synergistic, antagonistic and additive terms associated with repeat experiments. (C) 2010 Elsevier Ltd. All rights reserved.