Impact of Delrin disc: lubricated and dry contacts

Abstract

A Delrin disc is used in the Björk-Shiley tilting disc heart valve (H.L. Milligan, B.M. Johnson and P.A. Engel, ASTM J. Test. Eval., 22 (5) (1994) 474–484; C.G. Montero et al., Ann Thorac. Surg., 37 (4) (1984) 328–336), where dynamic contact occurs between metallic cylindrical struts and the disc, during valve action. To investigate the wear behavior of the Delrin disc, impact wear tests were performed by using a pivotal hammer device with a spherical striking face. For the biomedical wear study (Milligan et al.), raised temperature (36.6°C) and lubrication (simulating blood flow) was always provided, but in the present work both dry and lubricated contacts were investigated in order to compare wear mechanisms. For a small (sharp) indenter radius, plastic deformations of the Delrin occurred under the initial impact cycles. These incremental plastic deformations diminished under subsequent impacts, and were later followed by completely elastic contact, contributing ‘zero wear’. Beyond this region, a measurable wear regime would start. At severe contact stress (sharp indenter), an accelerated wearing pattern was seen to start at some stage during the measurable wear range. Taking into account plastic deformations existing at the beginning of the wear process, the present work discusses an impact wear model. The combined effects of wear, shear friction and crack propagation are discussed.