Scratch and Wear Behaviour of Co-Cr-Mo Alloy in Ringer's Lactate Solution

Abstract

Cobalt-chromium-molybdenum (Co-Cr-Mo) alloy is a material recommended for biomedical implants; however, to be suitable for this application, it should have good tribological properties, which are related to grain size. This paper investigates the tribological behaviour of a Co-Cr-Mo alloy produced using investment casting, together with electromagnetic stirring, to reduce its grain size. The samples were subjected to wear and scratch tests in simulated body fluid (Ringer's lactate solution). Since a reduction in grain size can influence the behaviour of the material, in terms of resistance and tribological response, four samples with different grain sizes were produced for use in our investigation of the behaviour of the alloy, in which we considered the friction coefficient, wear, and scratch resistance. The experiments were performed using a tribometer, with mean values for the friction coefficient, normal load, and tangential force acquired and recorded by the software. Spheres of Ti-6Al-4V and 316L steel were used as counterface materials. In addition, to elucidate the influence of grain size on the mechanical properties of the alloy, observations were conducted via scanning electron microscopy (SEM) with electron backscatter diffraction (EBSD). The results showed changes in the structure, with a reduction in grain size from 5.51 to 0.79 mm. Using both spheres, the best results for the friction coefficient and wear volume corresponded to the sample with the smallest grain size of 0.79 mm. The friction coefficients obtained were 0.37 and 0.45, using the Ti-6Al-4V and 316L spheres, respectively. These results confirm that the best surface finish for Co-Cr-Mo alloy used as a biomedical implant is one with a smaller grain size, since this results in a lower friction coefficient and low wear.

Keywords: cobalt–chromium–molybdenum alloy; hip replacements; orthopaedics; scratch test; wear.

Figure 1

Scheme for tribological testing of the Co-Cr-Mo alloy samples.

Figure 2

SEM images of polished surfaces of the Co-Cr-Mo alloy samples. (a) Dendritic structure, with an inter-dendritic carbide phase and (b) grain structure.

Figure 3

Schematic illustration of the UMT wear test device, with immersion of the sample in Ringer’s lactate solution.

Figure 4

Determination of the scratch groove material removal factor (f_ab) by evaluating three positions along the length of the scratch.

Figure 5

Schematic illustration of the theoretical model for calculating the ratio, using the f_ab material removal factor and the abrasive mechanisms: (a) ploughing, (b) microfatigue, (c) wedging formation, (d) microcutting, and (e) pile-up/sink-in. Crystallographic identification was performed using electron backscatter diffraction (EBSD) (FEI Quanta 400 FEG ESEM), in different regions of the samples, prior to the wear tests. The main objective was to identify the phases, carbide, and the alloy matrix.

Figure 6

Optical microscopy images of the Co-Cr-Mo alloy samples, showing the grain size: (a) AM1 (coarse grains, no electromagnetic field application), (b) AM2 (fine grains), (c) AM3 (fine grains), and (d) AM4 (fine grains).

Figure 7

Optical microscopy images with two distinct scale bar 200µm and 50µm of the microstructure of the Co-Cr-Mo alloy: (a) cobalt-rich matrix phase, in blue square emphasis (AM1, no frequency applied); (b) abundance of M₂₃C₆ carbides (AM2, 15 Hz); (c) carbides formed at grain boundaries and thin grain contour lines (AM3, 75 Hz); (d) lamellar carbides phases at grain boundaries (AM4, 150 Hz).

Figure 8

(a) SEM image showing the 8 points evaluated. (b) Kikuchi lines and the diffraction pattern. (c) Marking of higher lines. (d) Grain boundaries, identified as M₂₃C₆. (e) Lamellar constituent.

Figure 9

Friction coefficients variation from the Co-Cr-Mo alloy samples (AM1-AM4), and counterbody (a) a 316L steel sphere and (b) a Ti-6Al-4V sphere.

Figure 10

Images obtained from 3D profilometer analysis of eight cross-section wear tracks of Co-Cr-Mo samples AM1, AM2, AM3, and AM4, together with the corresponding depth profiles obtained using the interferometry technique (CCI).

Figure 11

SEM images of the scratch grooves and profiles at different positions of the scratch tracks for samples (a) AM1, (b) AM2, (c) AM3, and (d) AM4.

Figure 12

Experimental results for the ramp-load scratch testing of the Co-Cr-Mo alloy samples (a) AM1, (b) AM2, (c) AM3, and (d) AM4, using a Berkovich diamond indenter, showing the normal force (depth) and COF, together with images of the scratch tracks.

Figure 13

(a) Mean profile cross-section widths, as a function of depth, obtained via 3D optical profilometry. (b) Images showing the experimental results for the ramp-load scratch testing of Co-Cr-Mo samples AM1-AM4.

Figure 14

Results obtained using the Berkovich indentation technique. (a) Image of the end of the scratch performed on the surface of the sample. (b) Schematic interpretation of the indentation test in the sample (c) Hardness Hardness test results, Young’s modulus, and H/E ratio.