Wear-Corrosion Synergism Influences the Performance of High-Carbon and Low-Carbon CoCrMo

INTRODUCTION Recently, the popularity of Metal-on-Metal (MoM) hip bearings have diminished due to the concerns with generated wear debris and metal ion release that cause a negative response in the surrounding tissues. MoM bearings are typically manufactured from two types of metal alloys, lowcarbon (LC) and high-carbon (HC) CoCrMo. However, little is known about the in-vivo behavior of the two alloys [1]. Therefore, it is critical to understand and distinguish between the performances of the alloys in a simulated hip-joint environment. While corrosion and wear have been studied for both materials, there is no reported comparative study about the combined effect of corrosion and wear (tribocorrosion) [2, 3]. Hence the aim of this study was to evaluate the tribocorrosion behavior of a HC and LC CoCrMo alloy in a simulated physiological environment. It was hypothesized that the HC CoCrMo alloy would have a greater tribocorrosion resistance. METHODS Cylindrical pins of HC CoCrMo (0.24 % carbon) and LC CoCrMo (0.02% Carbon) (ATI Allvac, 12 mm diameter) were articulated against a 28 mm diameter alumina ball in a specially designed pin-on-ball set up which incorporated an electrochemical cell. The flat surface of the pins was polished to a surface roughness of 9.4 ± 2.6 nm Ra using standard metallographic methods. The wear tests were performed using a normal load of 16 N with ball rotation of ± 15° at 1 Hz for 100k cycles. Diluted bovine calf serum (30 g/L protein) was used as the lubricant and electrolyte. Two series of tests were conducted under electrochemicallycontrolled conditions: (1) with the pin at the free corrosion potential (Ecorr) and (2) under potentiostatic conditions, with the pin at a fixed potential of -0.28V vs. SCE. Electrochemical impedance spectroscopy (EIS) measurements were taken before and after each test. Using the ZView software (Scribner Associates), the Randle’s EIS equivalent circuit was used to determine the polarization resistance (Rp) and double layer capacitance (Cdl). The total material loss (Kwc) was determined by 3-D profilometry and the loss due to corrosion (Kc) was calculated with Faraday’s law. The total weight loss (Kwc) can be represented as Kwc=Kw +Kc, where Kw is weight loss due to mechanical wear [3]. RESULTS The evolution of current under potentiostatic conditions for HC (Fig.1a) and LC (Fig.1b) alloys indicate high fluctuations that are evident of the depassivation and repassivation mechanism associated with mechanical articulations. A run-in-period was observed in the evolution of the friction coefficient values for both conditions, the friction being higher initially for the HC alloy.