Cobalt-chromium-molybdenum alloy has high hardness, excellent wear resistance, good biocompatibility and outstanding corrosion resistance. It has a wide range of applications in the biomedical field and is commonly used for oral dentures and joint implants [1–5]. However, this alloy suffers from poor machinability and high processing costs. Conventional machining methods can damage the workpiece surface and produce large residual stresses, while seriously wearing out the tool, so achieving high-precision machining becomes exceedingly challenging [6–7]. EPP emerges as a novel surface finishing technology for metals and alloys. It can efficiently process complex shaped workpieces, while the electrolyte used for processing is a neutral salt solution, and therefore does not produce a waste solution that is difficult to dispose of [8–10]. In addition, there is no macroscopic force during the processing process, which will not leave micro cracks on the surface and impact the workpiece's lifespan. Korolyov A. et al.’s research on nickel titanium alloy vascular stents had shown that EPP technology can effectively improving the surface quality, rounding sharp edges and enhancing the corrosion resistance of the surface [11]. Aliakseyeu Y G. et al. established an EPP process mode for surface finishing of cobalt chromium alloy implants, which can smooth microrelief while removing scratches generated by pre-grinding. The roughness was reduced to 0.057µm, and the surface reflection coefficient was increased to 0.7, resulting in a smooth and wear free high-quality surface [12]. Thus, EPP technology proves capable of attaining exceptional surface finishes, even for challenging-to-machine high-temperature alloys.
There are numerous factors that affect the processing effect of EPP, including electrolyte, power supply, process parameters, pre-treatment and post-treatment, etc. [13]. Electrolyte as one of the key factors, generally according to the different metals and alloys to choose the appropriate neutral salt solution. For instance, ammonium sulfate solution is suitable for various types of steel and its alloys [14–15], ammonium chloride and copper sulfate solution are commonly used for copper alloys [16], ammonium chloride and potassium chloride solution are utilized for aluminum and aluminum alloys [17–18], while ammonium chloride and potassium fluoride solution are employed for titanium alloys [19–20]. When polishing cobalt-chromium alloys, sulfate solutions are generally used as the electrolyte, specifically one or a mixture of ammonium sulfate solution, potassium sulfate solution and ammonium bisulfate solution. Moreover, the addition of certain additives to the solution can further enhance the polishing effect and produce a higher-quality surface. For instance, Bosung Seo et al. discovered that adding strong oxidant H2O2 during plasma electrolytic polishing can expedite the oxidation of cobalt chromium alloy. This results in an increased thickness of the oxide film and an improvement in the corrosion resistance of the alloy [21].
Generally, additives include complexing agents, surfactants and corrosion inhibitors. The complexing agent can complex with metal ions in the electrolyte to prevent insoluble precipitates from being produced, thereby improving the quality of the surface finish. Zhang Wei et al. investigated the effect of using complexing agent slurry on the performance of chemical mechanical polishing of sapphire. The alumina slurry with the addition of the complexing agent EDTA showed a 52% increase in removal rate and a 51% decrease in surface roughness value compared to that without the addition of the complexing agent [22]. Surfactants can promote sufficient mixing of the components and make the surface quality more uniform. Wang Yan et al. investigated the effect of surfactant ammonium dodecyl sulfate (ADS) on wafer planarization in weak alkaline copper polishing slurry. Compared to the case without surfactant addition, the surface tension of the polishing slurry was reduced, the within-wafer non-uniformity of the removal rate was reduced to 4.15%, and the flattening of the wafer surface was improved [23]. The corrosion inhibitor can prevent the diffusion of dissolved oxygen in the electrolyte to the metal surface and slow down the corrosion rate of the material. Ma Tengda et al. compared the effect of the novel inhibitor (BIT) with the typical inhibitor (BTA) on the CMP parameters of the barrier layer. The analysis found that the corrosion inhibition efficiency of 24mM BTA and 24mM BIT was up to 51% and 48.2%, respectively, the galvanic corrosion of copper and tantalum was also reduced [24]. Therefore, choosing the appropriate additive and determining its mixing ratio can greatly improve the final processing results.
In this paper, an electrolyte preference method for adding appropriate additives to neutral salt solutions is adopted. Based on the experimental requirements, only different types of complexing agents are selected as additives in this study. A 4% mass fraction ammonium sulfate solution serves as the salt solution, to which different complexing agents are added in certain proportions to prepare the corresponding electrolytes. The experiments are conducted using a single-factor preference method. This study aims to provide a more reliable and effective electrolyte for electrolytic plasma polishing of cobalt-chromium-molybdenum alloys. Additionally, it seeks to contribute to the advancement of efficient and precise surface processing technology for metallic biomaterials in the biomedical field.