Microstructure and properties of sintered Co-Cr-Mo alloy powder under different processing conditions

Abstract

Co-Cr-Mo (ASTM F-75) alloy is one of the most important metallic biomaterials that are commonly used for surgical implant due to its mechanical properties, good wear resistance and biocompatibility. This study has focused on the effect of sintering time and sintering temperature on the microstructure development and properties of sintered Co-Cr-Mo alloy powder, sintering mechanism and activation energy, and corrosion behaviour. In the fabrication of PM Co-Cr-Mo alloy, five weight percentages (wt. %), 1.0, 1.5, 2.0, 2.5 and 3.0 of binder (stearic acid) were studied to determine the optimum amount of binder content based on the results of linear shrinkage, bulk density, apparent porosity and Vickers microhardness of the sintered samples. Then the Co-Cr-Mo alloy powder were blended with the selected amount of optimum binder (2wt. % of stearic acid) using a rotation mill at 16SRPM for 30 minutes, uniaxially pressing at SOOMPa and sintering in a furnace at three different sintering temperatures (12S0°C, 1300°C and 13S0°C) for five sintering times (30, 60, 90, 120 and 150 minutes) in argon atmosphere. The characterisation on sintered samples were carried out based on microstructure, grain size, bulk density and apparent porosity, Vickers microhardness test and followed by compressive strength. The study of sintering mechanism was carried out in order to determine the activation energy of Co-CrMo alloy. The corrosion behaviour of selected samples was analysed based on the minimum value of compressive strength. For corrosion test, the selected samples were immersed in simulated body fluid, 0.9% sodium chloride (NaCt) solution at 37°C for 90 days. From this study, the values of bulk density and grain size increased with increasin~ sintering temperature and sintering time. The bulk density values are in the range 7.04g/cm to 7.21 g/cm3 , 7.16 g/cm3 to 7.28 g/cm3 and 7.45 g/cm3 to 7.54 g/cm3 for sintering temperature of 1250°C, 1300°C and 1350°C, respectively for five sintering times. Meanwhile, the grain sizes for five sintering times are 25.6~m to 37.7~m, 36.6~m to 44.5~m and 80.4~m to 89.9~m respectively for the three sintering temperatures. However, opposite results were obtained for apparent porosity, hardness and compressive strength. The samples sintered at 13S0°C have the highest values of hardness (303HV -294HV) and compressive strength (329MPa-206MPa) for 30 to 150 minutes of sintering times. Based on the fracture mode, all samples show the fracture with a shear mode and occurred close to an angle of 45° from the compressive axis. The samples sintered at 12S0°C and 1300°C exhibited smooth transgranular fracture mode. Meanwhile, the step-like transgranular fracture mode was observed in the samples sintered at 1350°C. The results of corrosion test showed that sample sintered at 1300°C gives the highest value of corrosion rate (0.07Smpy) meanwhile sample sintered at 1350°C has the lowest corrosion rate (0.006mpy). From this study, the samples sintered at 13S0°C with 120 minutes of sintering times showed the compressive strength close to the bone strength and better corrosion properties.