AVS2011 Session TR-WeA: Emerging Interfaces of Tribological Importance

Micro-electro-mechanical systems (MEMS) are considered to be an important technology for the development of several products in daily life such as electronics, medical devices, and packaging. Even after tremendous progress in fabrication of miniaturized devices based on silicon materials, the development of highly robust surfaces with low friction and resistance against wear is still a challenging subject of accomplishment . To accomplish this goal, new fluorine-containing terminal alkynes were synthesized and self-assembled onto Si(111) substrates to obtain fluorine containing organic monolayers. Such covalently bound organic monolayers have similar surface properties as polytetrafluoroethylene (Teflon), but these monolayers are more stable than traditionally coated PTFE. The combination of these properties yields a highly improved wear resistantance.

The surface chemical compositions of three major brands of silicone hydrogel (SH) contact lenses were analyzed using X-ray photoelectron spectroscopy (XPS) prior to and following treatment in a test solution of diblock copolymer of polyethylene oxide and polybutylene oxide. Atomic force microscopy (AFM) was also employed to evaluate the surface topography and frictional properties of these lenses prior to and following similar solution treatments.For surface compositional analysis with XPS, lens surfaces have been prepared through a vacuum drying procedure, in which the hydrogel is taken from a fully hydrated state directly to an ultraclean, ultrahigh vacuum environment. Contact and tapping mode AFM were used to measure the frictional and topographical properties in aqueous environments. Prior to treatment, differences in surface elemental composition of the various lenses were found to reflect known bulk compositions and/or respective surface treatments. Following solution treatment, surface chemical modifications were apparent in balafilcon A (PureVision®) and lotrafilcon B (O₂ OPTIX®), especially in the distribution chemical functionalities present at the surface. Only modest changes in surface composition were observed for the senofilcon A (ACUVUE® Oasys®) system. AFM measurements in saline revealed large disparities between the coefficients of friction of the three lenses, with balafilcon A and lotrafilcon B exhibiting coefficients of friction approximately five times greater than that of senofilcon A. Lens surface treatment with the diblock copolymer test solution produced a significant reduction in the coefficients of friction of the two lenses exhibiting higher friction, yet only a small reduction in friction was observed for senofilcon A lens. Together, these results depict a strong correlation between the surface chemistry and frictional response of the lens systems as they relate to solution treatment with this specific diblock copolymer. This study indicated that diblock copolymers containing polyethylene oxide and polybutylene oxide may have a positive impact on the lubrication and wetting properties of silicone hydrogel lenses.

The progression of local cartilage surface damage toward early stage osteoarthritis (OA) likely depends on the severity of the damage and its impact on the local lubrication and stress distribution in the surrounding tissue. It is difficult to study the local responses using traditional methods; in-situ microtribological methods are being pursued here as a means to elucidate the mechanical aspects of OA progression. While decades of research have been dedicated to the macrotribological properties of articular cartilage, the microscale response is unclear. An experimental study of healthy cartilage microtribology was undertaken to assess the physiological relevance of a microscale friction probe. Normal forces were on the order of 50 mN. Sliding speed varied from 0 to 5 mm/s, and two probes radii, 0.8 mm and 3.2 mm, were used in the study. In-situ measurements of the indentation depth into the cartilage enabled calculations of contact area, effective elastic modulus, elastic and fluid normal force contributions, and the interfacial friction coefficient. This work resulted in the following findings: 1) at high sliding speed (V=1-5 mm/s), the friction coefficient was low (µ = 0.025) and insensitive to probe radius (0.8 mm – 3.2 mm) despite the 4-fold difference in the resulting contact areas; 2) The contact area was a strong function of the probe radius and sliding speed; 3) the friction coefficient was proportional to contact area when sliding speed varied from 0.05 mm/s-5 mm/s; 4) the fluid load support was greater than 85% for all sliding conditions (0% fluid support when V=0) and was insensitive to both probe radius and sliding speed. The findings were consistent with the adhesive theory of friction; as speed increased, increased effective hardness reduced the area of solid-solid contact which subsequently reduced the friction force. Where the severity of the sliding conditions dominates the wear and degradation of typical engineering tribomaterials, the results suggest that joint motion is actually beneficial for maintaining low matrix stresses, low contact areas, and effective lubrication for the fluid-saturated porous cartilage tissue. Further, the results demonstrated effective pressurization and lubrication beneath single asperity microscale contacts. With carefully designed experimental conditions, local friction probes can facilitate more fundamental studies of cartilage lubrication, friction and wear, and potentially add important insights into the mechanical mechanisms of OA.

The 2008 International Magnetic Tape Storage Roadmap¹ projects that the total magnetic spacing between the recording head and the tape magnetic layer must decrease from the current 43 nm spacing to about 23 nm by the year 2018 in order for tape to maintain its cost advantage as an information storage medium. Because tape drives are contact recording systems, interactions between head materials and components in the tape magnetic layer can detrimentally affect the head-tape separation via deposit formation on head surfaces as well as preferential erosion of critical recording head elements. Understanding the nature of these interactions is therefore essential for mitigating undesirable increases in the magnetic spacing. This study shows that deposition and erosion phenomena in tape heads can be varied at the local level by changing the electrical configuration of adjacent pole tip structures in multichannel heads, and that the composition of the head deposits depends on the electrical configuration of the pole tips. Using atomic force and electric force microscopy, we show that conductive deposits form on the “trailing edge” of pole tips which are electrically connected to earth ground or to the head substrate. The conductive deposits become non-conductive further “downstream” from the pole tips. Deposits adjacent to electrically isolated poles are always non-conductive. Auger analysis shows that the surfaces of the conductive deposit regions contain high levels of Fe and Co, and small amounts of P and Y, whereas the surfaces of the non-conductive deposits contain predominantly P and Y, with very low levels of Fe. Because all of these elements are present in the magnetic coating of the tape, and because the compositions of the deposits on heads having NiFe pole tips is similar to those on heads having CoZrTa pole tips, these results suggest that the deposits originate from components in the tape and not from metallic structures in the tape head.

1. International Magnetic Tape Roadmap, Information Storage Industry Consortium, September, 2008.

*Supported by the Information Storage Industry Consortium Tape Program