Enhanced Surface Integrity of a Biomedical Grade Polyetheretherketone through Cryogenic Machining

Abstract

Polyetheretherketone (PEEK) belongs to a group of thermoplastic polymers widely used in bioengineering applications such as trauma, orthopedic and spine implants, whose mechanical and geometrical characteristics, when put in service, must be the highest possible. However, machining operations, usually carried out for their manufacturing, can assure these characteristics uniquely when they are performed in a restricted temperature window because of the PEEK mechanical behavior change nearby its glass transition temperature.To this aim, the present paper investigates the feasibility of using cryogenic machining to enhance the surface integrity of a biomedical grade PEEK. Turning trials were carried out under dry and cryogenic machining conditions at varying feed and cutting speed. The temperature reached during the process was measured by means of a thermocouple embedded in the cutting tool, whereas the surface integrity assessed in terms of surface defects, surface roughness and hardness. Additionally, the degree of crystallinity was evaluated via Differential Scanning Calorimetry (DSC). To correlate machinability results with the PEEK mechanical behavior, tensile tests were performed in the temperatures range between -100°C and 50°C.The obtained results showed that the application of liquid nitrogen always made possible achieve an enhanced surface integrity compared to the corresponding dry condition, which represent the currently used strategy for the manufacture of biomedical polymer implants. A proper temperature control during machining assured indeed the formation of lower amounts of defects as well as reduced surface roughness, by increasing the PEEK degree of crystallinity.