Bonnet Polishing of Cobalt Chrome Alloys for Artificial Implants

Cobalt chrome (CoCr) alloys are the most extensively used biomaterials for manufacturing artificial implants which need nanometre scale surface finish and micrometre scale form tolerance to allow long term survival in vivo. Traditional finishing of these devices is usually carried out by manual or simple robot polishing which are time-consuming and labour-intensive. The aim of this thesis is to investigate and develop a deterministic polishing process for improving the surface finish and form tolerance of the bearing surfaces of artificial implants.

In order to improve the surface finish for CoCr alloys, a Taguchi method with the consideration of interaction effects was applied to optimise the process parameters. By using the optimised process parameters, the surface roughness of workpieces can be improved up to 8nm Sa, which is far better than 50nm Ra, the recommended value of ISO 7206-2:2011 for metallic bearing surface of artificial implants.

The evolution of all 15 parameters of surface topography during polishing process has been investigated. In addition, the effects of the combination of polishing cloths/pads and abrasives on surface topography have been investigated as well, indicating that both polishing cloths/pads and abrasives can affect the improvement of surface topography.

A deterministic polishing process is dependent on the material removal which is controlled by process parameters. The way of which material removal and polishing forces were affected by the process parameters has been investigated. Based on the experimental data, a modified Preston equation model was created to predict the material removal rate for bonnet polishing of CoCr alloys.

Form tolerance plays a very significant role in the bearing surfaces of artificial implants. By using the method of form correction, a new design of multi-radius femoral head which is unable to be fabricated by traditional polishing process was successfully manufactured. In addition, form correction was also applied to a roughly ground freeform knee femoral component. The success of the form correction experiments indicated that bonnet polishing is a robust technology when applied to the surfaces of artificial implants.