Degree Name

Master of Science (MS)

Semester of Degree Completion

1997

Thesis Director

Ping Liu

Abstract

Understanding the effect of machining on the structure and property of subsurface layer of ultra high molecular weight polyethylene (UHMWPE) can significantly facilitate the increase in longevity of artificial joints. This study was performed to investigate the machining effects on the polymeric structure and tribological properties. Machining parameters include cutting speed, tool feed rate and depth of cut. Temperature rise was studied during machining to assess the extent of thermal degradation due to machining. Polymeric structure of the semicrystalline polymer was characterized using differential scanning calorimetry (DSC) in terms of melting enthalpy, crystallinity and melting temperature. Surface texture and hardness were also related to machining conditions. Changes in structure and surface texture caused by machining were related to the tribological behavior of machined UHMWPE.

The temperature rise observed under the studied machining conditions was no more than 6°C and the half peak duration for temperature rise was less than 13 s. Therefore, thermal degradation of the polymer caused by machining could be marginal. When the same ratio of cutting speed and tool feed rate was kept, higher cutting speed and tool feed rate showed lower temperature rise on the UHMWPE.

Melting temperature of UHMWPE decreased due to decrease in crystallinity and molecular chain length caused by machining. The higher the cutting speed, the more damage was caused on the polymeric structure. The change in tool feed rate and depth of cut had no significant effects on structure of machined UHMWPE. Surface roughness of machined UHMWPE was improved as the cutting speed increased. As the tool feed rate increased, surface roughness (Ra) increased. There was an optimum depth of cut for the best surface finish, which was 0.2 mm in this research. Durometer hardness did not change significantly as machining parameters varied.

Coefficient of friction of metal pin on UHMWPE had static and dynamic characteristics during the reciprocal dry sliding test. At the beginning of each half cycle, coefficient of friction was higher than average. At the end of each half cycle, coefficient of friction was lower than the average. The average coefficient of friction increased quickly within the first 60 seconds. Then the coefficient of friction increased very slowly. The initial average coefficient of friction was in the range of 0.12 to 0.15. After one hour of sliding, the average of coefficient of friction was in the range from 0.17 to 0.23. No significant correlations were found between depth of cut and coefficient of friction or between cutting speed and friction coefficient. However, coefficient of friction decreased as cutting speed increased when the same ratio of cutting speed to tool feed rate was maintained.

Increase in cutting speed caused more damage on the structure of machined UHMWPE. Wear factor decreased as cutting speed increased if tool feed rate was kept unchanged. Wear factor increased as cutting speed increased when the ratio of cutting speed and tool feed rate was kept constant. There was an optimum depth of cut for the best surface roughness and wear resistance, which was about 0.2 mm under the studied condition. Optical micrograph analysis showed severe plastic deformation and material flow occurred in subsurface of machined UHMWPE at an early wear stage.

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