The tribological behavior of MoST (Ti-Mo-S) and MoS₂ coatings have been investigated by in situ tribometry, which allowed real time observation of third body dynamics. Coatings were deposited by either closed field unbalanced magnetron sputtering (MoST and MoS₂) or ion beam assisted deposition (MoS₂). Reciprocating sliding tests were performed in dry and ambient (40-50% RH) air environments at sliding speed of 1 mm/s. Transparent sapphire hemispheres were used as counterbodies to provide reasonably high contact stress (1.1 GPa mean Hertzian stress at 24N load) and allow optical observation of third body dynamic processes. The rate of third body transfer film buildup on the sapphire counterfaces was monitored in situ using Newton’s rings. Titanium containing coatings showed rapid thickening of third body transfer film on the sapphire hemisphere as compared to MoS₂ coatings in humid environments. Optical observations of third body dynamics demonstrated that for all coatings and environments the major velocity accommodation mode was interfacial sliding, where most of the sliding took place between the transfer film and the wear track. Profilometry and scanning instrumented indentation were used to evaluate hardness and modulus of the wear tracks and third body transfer films. Transfer film hardness was lower than that of the parent coatings. The role of third body chemistry and mechanics in the tribological performance of MoS₂ based coatings will be discussed.

[1] N.M. Renevier, V.C. Fox, D.G. Teer, and J. Hampshire, Surf. Coat. Tech. 127 (2000) 24-27.

The ability to actively control friction properties of a surface is of great interest to both micro/nanotechnology and macroscale applications. We present our investigations on the effect of varying thermal conditions on the micro/nanotribological properties of selected polymer films using scanning probe microscopy (SPM). The grafted films studied are based on the poly-N-isopropylacrylamide (PNIPAAM) molecule, which is known to be heat sensitive and switch from hydrophilic to hydrophobic states at and above the transition temperature (30 to 35°C)¹. PNIPAAM based surfaces have been investigated for a variety of "smart surface" applications^2,3. However, very few studies have addressed their tribological behavior. In this study, micro/nanoscale friction and adhesive forces between the films and a hydrophobically functionalized Si₃N₄ tip is measured as a function of ambient temperature to identify switchable friction states of the films. The study will provide a better understanding of lubrication mechanisms at the molecular level and advance us towards realizing surfaces for active friction control.

¹ H.G. Schild, Poly(N-isopropylacrylamide) experiment, theory and application, Prog. Polym. Sci., vol 17 (1992) pp. 165.

₂ J. Hoffmann et al., Photopatterning of thermally sensitive hydrogels useful for microactuators, Sensors and Actuators A-Physical, vol 77, (1999) pp. 139-144.

₃ L. Liang et al., Thermosensitive poly(N-isopropylacrylamide)-clay nanocomposites with enhanced temperature response, Langmuir, vol. 16, (2000) pp. 9895-9899.

Perfluoropolyethers (PFPEs) are widely used as lubricants on magnetic media in hard disk driver (HDD), and play a critical role in protecting both the carbon overcoat and the underlying magnetic layer from damage caused by slider-disk contacts during normal operation. Because the lubricant layer on the disk surface is extremely thin (1-20 nm), lubricant stability is an important factor that directly influences the long-term reliability of HDD [1]. However, one of the major tribological problems at the head-disk interface (HDI) is the decomposition of PFPE lubricants, which leads to failure of HDI. The decomposition of PFPE has been explained with some mechanisms, e.g., thermal and catalytic reactions. However, these mechanisms do not explain fully the decomposition of PFPE. On the other hand, Nakayama et al. [2] discovered microplasma in the gap of sliding contact. This suggests that the high energetic microplasma causes the decomposition of the lubricants. So, the purpose of this work is to study the decomposition of perfluoropolyether (PFPE) lubricant in the plasma produced by the electrical-discharging. Reflection-absorption Fourier transform infrared (FTIR) technique and gas-chromatograph/mass-spectroscopy (GC/MS) have been used to analyze the decomposition products of PFPE lubricant produced by the plasma. The observed results showed that PFPE lubricant is decomposed in the plasma. This suggests that PFPE lubricants are decomposed by the tribo-microplasma, which generated at head-disk sliding contact.

[1] C. M. Mate, Tribol. Lett., 4, 119 (1998).

[2] K. Nakayama, and R. A. Nevshupa, J. Phys. D: Appl. Phys., 35, L53, (2002). .