Microtribology of silicon,oxide, and carbide surfaces

This paper presents an overview of the microtribological properties of silicon oxide, sapphire, and titanium carbide surfaces as well as a self‐assembled monolayer with respect to their application in microsystems. Testing was performed with a reciprocating microtribometer with normal loads in the micronewton to millinewton range. Silicon and titanium carbide balls were used as counterbodies. For silicon oxide, sapphire and a perfluorodecyltrichlorosilane self‐assembled monolayer (FDTS), the microfriction corresponds to the water contact angle when the smoother titanium carbide ball or the relatively rougher silicon ball was used as a counterbody. Microfriction measurements performed on tribopairs of the same material, but having different roughnesses, showed that the friction of the rougher tribopairs is lower than that of the smoother ones. Interestingly, in the microforce regime, reduction in friction was significant and almost as much as when hydrophobic self‐assembled monolayers are applied on smooth surfaces. This investigation showed that comparative microtribological investigations between different material systems can be very challenging due to the fact that comparable roughness values on samples and countersamples are difficult to realize. Copyright © 2006 John Wiley & Sons, Ltd.     

Friction and Wear Behavior of Austenitic Stainless Steel: Influence of Atmospheric Humidity,Load Range,and Grain Size

Research conducted on steels is motivated by a technological need to further improve their properties. Among the different steel types, austenitic stainless steels possess good corrosion resistance and formability. However, they also have a relatively low yield strength. One way of increasing the yield strength is by grain refining. This work presents a study on the effect of relative humidity and applied load range on the friction and wear of AISI 304 austenitic stainless steel characterized by two different grain sizes: 2.5 and 40 m. Using a precision microtribometer, with applied loads in the N regime, it was found that capillarity plays a dominant role. At the same loads, in high humidity environments, both the fine and coarse grain steels exhibit high friction relative to measurements performed under dry conditions. At loads greater than 2 mN a reversal in microfrictional behavior occurs in that microfriction was greater under dry conditions than under moist conditions. At loads of 2 N, using a standard ball-on-disk tribometer, severe wear was evident at low humidities while relatively lower wear was observed at high humidities indicating a lubricating effect of water.     

Wear Resistance of Fine-Grained High Nitrogen Austenitic Stainless Steel Coated with Amorphous Carbon Films: The Soft X-ray Spectroscopy Approach

The wear resistance dependence on grain size of a high-nitrogen-alloyed austenitic stainless steels (HN) is investigated and compared to measurements for the same samples coated with amorphous carbon (a-C:H) and nitrogenated amorphous carbon (a-C:H(N)) films, deposited by means of plasma-enhanced chemical vapour deposition. A synergic effect between the grain refining and the film in the case of nitride amorphous carbon overcoats is observed in terms of increased low-friction performance duration. The temperature dependence of the wear resistance of a micro crystalline HN stainless steel coated with carbon films is also investigated. An overall decrease of the films' wear resistance is found with increasing temperature. Furthermore, a higher wear resistance is found in the a-C:H coated steel with respect to the a-C:H:N material. High-lateral-resolution photoemission microscopy reveals that inhomogeneities within the film after wear testing are correlated to an increase of the number of N-sp² C-bonded sites. The study of energy-distribution curves and high-lateral-resolution images on the nitrogenated samples shows that a modification of the surface chemistry occurs by mechanical action; in particular this implies that existing N-sp² C sites are beginning to cluster as temperature increases.