Tribiology of Engineered Surfaces in Aggressive Environments

To improve the performance of sliding systems, surface modifications and coatings are often applied to opposing surfaces. This thesis focuses on characterizing two tribo-systems (DLC-DLC and steel micropatterns-flat) under their predicted application environments. The first section is focused on friction testing of micropatterned surfaces for orthopaedic device design, the second section elucidates how the sliding of diamond-like-carbon (DLC) coatings changes with temperature and humidity. The experimental design and major results of these sections are as follows. (1) The use of micropatterning to create uniform surface morphologies has been cited as yielding improvements in the coefficient of friction during high velocity sliding contact. Studies have not been preformed to determine if these micropatterns could also be useful in biomedical applications, such as total joint replacement surfaces, where the lower sliding velocities are used. In this study, the effect of pattern geometry, feature size and lubricant on contact friction and surface damage was investigated using 316L steel in sliding contact with a stainless steel and polyethylene pins. Using a novel proprietary forming process that creates millions of microstructures in parallel, a variety of micropatterned surfaces were fabricated to study the influence of shape (oval, circular, square), geometry (depressions, pillars) and feature size (10, 50 and 100 mm) on both contact friction and surface damage. The coefficients of friction were measured for each surface/lubricant/pin system using a CETR scratch testing system. Results showed that round depressions with diameters of 10 μm had a significantly lower steady state coefficient of friction than the non-patterned substrates or substrates with greater diameter depression patterns. (2) The use of diamond-like carbon (DLC) has been cited as a friction and wear reducing coating during sliding contact and is widely used in the hard disk drive (HDD) industry. Studies have not shown the simultaneous effects of the temperature and humidity or temperature and load on DLC coatings. This project will show the effects on the friction and wear of non- hydrogenated DLC coatings in high temperature environments (23 to 250 °C), various humid environments (10 – 95 %RH), dependence on load (2.66 to 10 N), and the combined effects each environmental condition. The DLC coatings being used in this study are ta-C (tetrahedral amorphous-carbon) and a-C (amorphous-carbon), which were deposited onto a substrate of Al2O3-TiC (Seagate) and 440C stainless steel counterface pin. The friction for this tribosystem was monitored by a built in-house POD system, which can control the humidity levels and can reach temperatures up to 300°C. It has been shown that the a-C is less sensitive to the humidity levels, but is more sensitive to the surrounding temperature than ta-C DLC coatings