AVS2013 Session TR-TuP: Tribology Poster Session

The experimental equipment was constructed on the basis of pin-on-disk technique in a vacuum chamber. The pin was made of metal and the disk was made of plastic material. Amount of the separated electric charges between them during sliding friction was measured with an electrometer by connecting to the metal sample.

Experiment with a stainless steel pin and a poly-ethylene terephthalate (PET) disk was performed. The charge density on the rubbed surface of PET was calculated to be about -5x10^-4 C/m² from the generated charges and the width of the friction track. In air, the charge accumulation was, however, one order of magnitude less than the value measured in a vacuum. This should be caused by the charge relaxation as a result of the micro-gap discharge [1].

[1] T. Miura and I. Arakawa, IEEE Transactions on Dielectric and Electrical Insulation, 14, 560-565 (2007).

Research on low friction materials has attracted considerable attention from the viewpoint of effective energy use for realizing a low carbon society, as these materials can substantially reduce energy loss in bearings, gears, and other mechanical drive parts. However a problem is an increase in friction due to the oxidation reaction in high-temperature environments.

We therefore tried to develop advanced high-temperature motion materials with excellent tribological properties (i.e.: little wear, low friction, good lubrication) in order to guarantee a smooth sliding motion over long periods. Our final purpose is to design coatings suitable for smooth sliding motion up to 1073K.

As a first step, In order to measure friction at high-temperatures exceeding 673Ｋ, we designed and manufactured a prototype of a high temperature tribological evaluation device, For this purpose high-temperature friction measurement system based on Bowden-Leben type system has been successfully developed that can evaluate sliding friction force under changing load from 9.8N to 0.098N and under changing heat temperature from 1073K to room-temperature.

We have investigated solid lubricant coatings in order to develop high performance motion mechanism at high-temperature. In previous work, we developed high temperature lubricating coatings by precise control of coating processes using ceramics such as copper oxide, zinc oxide, boron nitride, etc. This time, we focused Cr₂O₃, since this material has been reported to be suitable at high-temperature, but not well been investigated for industrial application to sliding parts.

We have carried out basic study of SUS304, SUS440C and SUJ2 to find out suitable friction parameter at high-temperature from 973K to room-temperature. Their tribological characteristics at high-temperature was evaluated by high-temperature friction measurement system , abrasion measurements by Stylus Surface Profiler and Micro Slurry-jet Erosion.

In static oxidation, the formed titanium oxide (TiO2) was found to have mainly anatase structure at temperatures between 500ºC to 600ºC and transform to rutile structure at temperature higher than 600ºC. Through this study, oxidation rate and activation energy of oxidation for each sample were evaluated. It is found the samples exhibited higher activation energy could have higher oxidation resistance. These results are consistent with those obtained from wear testing.

“Ohmic” RF-MEMS are radio frequency Microelectromechanical Systems (RF-MEMS) switches relying on metal-metal contact. They are of great interest to the telecommunications and defense industries due to their potential for use in switching networks, low-noise/power circuits, portable wireless systems, phased arrays, filters, and antennas. Issues with reliability, however, have prevented widespread commercial use of these devices. In an effort to better understand important degradation mechanisms in the vicinity of the contact, we simulated the complicated environment at the electrical contact through implementation a multi-scale method. This method incorporates an overlay of a finite difference mesh on top of a traditional molecular dynamics simulation. Thermal and electric transport equations are solved via finite difference part of the method and the results are coupled to an underlying atomistic simulation. In this work, contact deformation of ohmic RF-MEMS was approximated as the indentation of a single-asperity on a variety of substrates. These substrates included polycrystalline gold, polycrystalline gold with a void and polycrystalline gold with a trapped pocket of contamination. Indentation was performed for a variety of pressures and applied voltages. The different structures of the substrate result in drastically different steady state thermal profiles when voltage was applied. This significantly affected the indentation depth as compared to room temperature no voltage cases. Flow stress calculations as a function of bulk temperature were used to provide insight into trends in indentation depths as a function of load and underlying structure of the substrate.

The tribocorrosion phenomenon is present in biomedical alloys that are used in artificial implants to replace natural joints. This damage limit the service life of such implants, the hard coatings can improve the resistance of wear and corrosion. The multilayers of TiAlN/TiAl were deposited on CoCrMo alloys by magnetron sputtering. The structure of coatings was studied by means of XRD and the composition by RBS and EDS techniques. The tribocorrosion behavior of CoCrMo alloys alone and coated with TiAlN/TiAl multilayers was studied in simulated body fluid. The tribocorrosion was performed using a ball on plate reciprocating tribometer, the test was conducted in a simulated body fluid at 37 °C of temperature. The loads used were between 0.25 N to 2N, the oscillating frequencies was 1Hz. The corrosion and tribocorrosion were studied using open circuit potential (OCP), potentiodynamic polarization, cyclic polarization and potentiostatic polarization measurements. The potentiodynamic polarization was used to estimate the change in the corrosion rate due to wear and the potensiostatic polarization in the passive region to measure the change in the wear rate due to corrosion. In order to study the wear mechanisms, the topography and composition of worn surfaces were analyzed by means of profilometry, SEM and EDS. For the CoCrMo alloy the corrosion augmentation factor was greater than the wear augmentation factor. The coatings improve the corrosion and tribocorrosion resistance of CoCrMo alloys.