A Servoelectric Apparatus with Potentiostat to Study the Fretting Corrosion of Cobalt-Chromium-Titanium Alloy Couples

摘要

The study of fretting degradation in prosthetic modular junctions requires a combination of tribological testing and electrochemical measurements. The goal of this study was to design a testing apparatus that offers this combination using servoelectric actuator technology because such technology provides ease of installation and handling and is cost-effective. A line-contact configuration that more closely simulates in vivo conditions than the usual point-contact geometry was used. The operation of the apparatus was demonstrated in a study of fretting of a high carbon CoCrMo-Ti-6Al-4V couple. A custom-made fretting corrosion apparatus using a servoelectric universal testing machine (THELKIN, Winterthur, Switzerland) in combination with a standard three-electrode electrochemical system was built. Line contact between the test surfaces was achieved by pressing the flat surfaces of two CoCrMo pins against a vertical Ti-6Al-4V rod attached to the machine actuator. The pins and rod served as the working electrode in an electrochemical cell controlled by a potentiostat. Fretting corrosion conditions were as follows: ±50μm, 17 N normal force, diluted bovine serum (30g/l proteins) medium at pH 4.5 or 7.6, 5000 cycles test duration, under potentiostatic conditions (-0.25 V versus SCE). The servoelectric actuator system was shown not to interfere with the electrochemical measurements. The fretting loops were consistent with a gross slip regime and indicated good system rigidity. The amount of Co and Cr released to the medium was significantly greater at pH 4.5 than at pH 7.6, whereas release of Ti was independent of pH. The reason for the higher wear and corrosion rate of the CoCrMo under acidic conditions appears to be connected with the decreased formation or stability of a chromium-rich protective tribofilm. Significance: A novel fretting corrosion apparatus that goes beyond point contact was constructed and validated to advance the study of fretting degradation in prosthetic modular junctions.