ICMCTF2010 Session E3-1: Tribology of Nanostructured and Amorphous Films

Monday, April 26, 2010 10:00 AM in Room Pacific Salon 1
Monday Morning

Time Period MoM Sessions | Abstract Timeline | Topic E Sessions | Time Periods | Topics | ICMCTF2010 Schedule

Start Invited? Item
10:00 AM E3-1-1 Tribological and Mechanical Properties of Nanostructured Hydrogenated Amorphous Carbon and Titanium Diboride Films
Bo Zhao, Yip-Wah Chung (Northwestern University)
Hydrogenated amorphous carbon films are of great interest due to their favorable ultra-low friction and low wear rate properties in dry environments. Our work demonstrated that sulfur doping of hydrogenated carbon films enabled them to achieve ultra-low friction performance in both dry and humid environments. However, these films have a hardness of 7 - 10 GPa and an elastic modulus around 80 GPa, which are too low for some high stress applications. Formation of nanostructured coatings is known to improve hardness. With the aim to produce hard, low-friction coatings, we synthesized nanolayered and nanocomposite films of sulfur-doped and undoped hydrogenated carbon and titanium diboride as a hard second phase using dual-target magnetron sputtering. Titanium diboride deposited by this method had a hardness >30 GPa. This paper will discuss the film structure and how such structure correlates with its tribological and mechanical properties.
10:20 AM E3-1-3 Tribology of Nanocrystalline Diamond Coatings
Nimel Theodore (US Naval Research Laboratory); Matthias Wiora (Ulm University); Kathryn Wahl (US Naval Resarch Laboratory)

Reciprocating sliding tests of sapphire balls against the nanocrystalline diamond have been performed to examine the effects of moisture in the surrounding ambient environment. Microstructure of the diamond coatings was confirmed by X-ray diffraction (XRD) to be nanocrystalline with crystallite sizes ranging from to 20 to 70 nm, with a preferred texture in the (220) orientation. These diamond coatings had similar visible wavelength Raman absorption bands that were deconvolved into 5 peaks: a single sharp peak at 1332 cm-1 typical of crystalline diamond; broad peaks at 1340 cm-1 and 1580 cm-1 characteristic of the D and G peaks in sp2 hybridized carbon; and, additional peaks at 1135 cm-1 and 1470 cm-1 that are commonly attributed to transpolyacetylene bonding. All nanodiamond coatings initially possessed high friction (0.5 - 0.8) before running-in to a steady-state value of ~0.05; each coating had a characteristic run-in profile to low friction (<0.1) occurring between 300 to 1000 cycles. Coatings with larger crystallites and higher surface roughness were more sensitive to changes in humidity, with faster run-in occurring as humidity increased between 3 and 45%. For these larger-crystallite coatings, wear of the sapphire counterface increased by an order of magnitude between 5 and 45% RH. However, coatings having smaller grain sizes and lower roughness exhibited similar run-in and counterface wear regardless of humidity. Coating tribological performance as a function of humidity will be related to the microstructural and compositional differences among the nanocrystalline diamond coatings.

10:40 AM E3-1-4 Influence of Ultraviolet Ray Irradiation on CNx Coating's Tribological Characteristics
Takayuki Tokoroyama, Noritsugu Umehara (Nagoya University, Japan); Yoshio Fuwa (Toyota Motors Co., Ltd., Japan)
Carbon Nitride (CNx) coating is one of the promising materials as well tribological property, especially low friction coefficient. The superlow friction phenomena of CNx coating slid against Si3N4 ball in N2 gas was suggested to be taken place when the topmost surface of CNx changed to graphitic structure. It was reported that nitrogen atoms included CNx coating desorbed during the friction. From the view point of nitrogen desorption from CNx coating, it was assumed that ultraviolet ray could break C-N single bond when the ray power exceeded C-N single bonding energy. In this study, we hypothesized that ultraviolet ray irradiation could make the CNx coating topmost surface graphitic without friction in N2 gas. We carried out preparing the CNx coating irradiated ultraviolet ray as 254, 312, and 365 nm wave length, because the C-N single bond is 305 kJ/mol and the other ultraviolet rays are 469, 382, and 327 kJ/mol, then these specimens were tested in N2 gas to compare frictional property of with and without irradiation. CNx coating was synthesized with Ion beam assisted deposition method as 100 nm thickness on Si(100) substrate. Ar ion beam irradiated to carbon target, sputtered carbon was deposited on the substrate and nitrogen ion beam was irradiated to the specimen, simultaneously. Friction test was done with ball-on-disk type frictional tester which was set in vacuum chamber and the test conditions were that normal load was 0.1 N, sliding speed was approx. 0.04 m/s, and counter material was Si3N4 ball with 8 mm diameter. Before the friction test, the vacuum chamber was evacuated to approx. 0.1 Pa, then nitrogen gas was filled into the chamber approx. atmospheric pressure. All specimens did not show superlow friction coefficient at the initial of friction test, however, the coatings which were irradiated showed superlow friction phenomena gradually. We defined that the running-in period was friction coefficient became lower than 0.05, all irradiated specimens showed shorter running-in period than as-deposit CNx. The average of initial several cycle friction of as-deposit CNx showed 0.04, on the other hand, in the case of 312 nm ultraviolet ray irradiated CNx showed 0.01 or lower average friction coefficient. These results indicated that ultraviolet ray could break the C-N single bond, these broke bonds were restructured to C=C double bond, and nitrogen atoms desorbed from the coating as the cause of restructuring to N2 molecule.
11:00 AM Invited E3-1-5 Tribology of Nanocrystalline Diamond and Amorphous Carbon Films: Achieving Ultralow Friction and Wear
Robert W. Carpick (University of Pennsylvania)
Ultrastrong carbon-based films that are characterized by high amounts of sp3-bonded carbon and very little hydrogen can possess extremely low friction and wear. Examples include nanocrystalline and ultrananocrystalline diamond (NCD and UNCD, respectively), and tetrahedral amorphous carbon (ta-C). It is well-known that smooth surfaces are required to achieve this, but in addition, the vapor environment is also crucial. It has been long known that some partial pressure of water or hydrogen is needed, or else high friction and wear will result. In this talk, I will describe our experimental work which has demonstrated new insights into these phenomena. We have observed friction coefficients as low as 0.005 in atmospheres as dry as 0.7%RH. We show that the mechanism of low friction and wear in these materials has nothing to do with the formation of crystalline graphitic layers, as had been postulated previously. Rather, we present spatially-resolved surface chemical and bonding analysis obtained by synchrotron-based photoemission electron microscopy (PEEM) that proves that passivation by dissociative adsorption of water is the primary mechanism responsible for maintaining low friction and wear of self-mated contacts of UNCD and ta-C in humid environments. We propose that the dynamic competition between the rate of dissociative passivation and the rate of trans-interfacial bonding of dangling bonds produced during sliding is the key to understanding the threshold between low and high friction states. We find that the transition from low to high friction is both reversible and extremely sharp. A possible explanation for these newly observed phenomena based on the energetics and kinetics of dissociation will be discussed.
Time Period MoM Sessions | Abstract Timeline | Topic E Sessions | Time Periods | Topics | ICMCTF2010 Schedule