ICMCTF2016 Session E1-1: Friction, Wear, Lubrication Effects, and Modeling

New electrical contact material is crucial for suitable product development in the smart grid. The new materials should combine several, sometimes contracting properties such as low contact resistance, high wear resistance and low friction. A widely used material for commercial contact product in electric grid is Ag, which has excellent electrical properties. A disadvantage of Ag is too soft and, in a sliding contact application, the friction coefficient between two sliding Ag surfaces is far too high (>1), and thus giving a high wear rate.

There are several approaches to improve the tribological properties of Ag contact: silver alloying, surface engineering of Ag, and addition of lubricants. In this work, we demonstrate the concept of silver alloying with other elements, e.g. Nb, Cr, W, Zr, etc, to improve tribological properties but also keep low electrical contact resistance. It is very time-consuming to select appropriate alloying elements with correct composition and structure. This problem can be solved by using an approach based on combinatorial materials science, which can rapidly screen different elements. We have constructed a combinatorial platform including a combinatorial sputtering system, which can deposit thin films with large composition gradients in a single experiment. Rapid screenings of XRD, XPS, XRF, Raman, EDS and SEM were employed to determine the chemical composition and structure. The friction coefficients as well as the electrical contact resistances can rapidly be measured in custom-designed equipment directly on the gradient films.

Screening results with several binary and ternary alloys including e.g. Nb, Zr and Cr show a complicated pattern of solid solutions, immiscible multiphases of alloys, or even amorphous phases for some compositions. A dramatic decrease in friction coefficient from >1 to < 0.5 was observed for Ag-Nb alloys at the composition of 13-20at% of Nb. The contact resistance of Ag-Nb at these compositions was only slightly higher than for pure Ag. Combined with an increased hardness and lower wear rate this suggests a potential alloy composition window for a new electrical contact material. The results will be discussed based on comparison of properties related to phase evolution during alloying.

Considerable efforts to improve the hardness and wear resistance of a zinc coating (only) have been widely researched but not fully understood. Zn-based alloy coatings have been considered as a good replacement for improved surface properties. This article reports the wear resistance of Zn-based alloy coating unto mild steel. In this study, three Zn-Al based composite coatings were deposited onto mild steel substrates using plasma spayed techniques. Coating characterization was carried out prior to and after wear studies via scanning electron microscopy (SEM), and energy disperse X-ray spectroscopy (EDX) elemental analysis. The results showed that the different chemical composition plays a crucial role in influencing the wear behaviour of plasma pray Zn-Al alloy coatings. The influence of residual stresses measured by X-ray diffraction on the wear resistance was also investigated and distinctly discuss in details.

To enhance sliding wear resistance of Fe₃Al-based coatings via mechanical properties improvement, commercial TiC particles were incorporated into the feedstock powder and projected on steel substrate using the High-Velocity Oxy-Fuel (HVOF) technique. The effects of the incorporation of TiC particles on the composition, microhardness, elastic modulus, friction and dry sliding wear resistance against alumina counterpart were investigated. Compositional analyses showed that the composite coatings mainly consisted of TiC and Fe₃Al phases with no detectable decomposition or oxide phases. Microstructural analyses confirmed that the TiC particles were uniformly dispersed surrounded by the Fe₃Al lamellar structure. The average microhardness of the coatings was improved from HV300 for the unreinforced Fe₃Al to HV780 for the composite coatings. In addition, the average elastic modulus was slightly increased from 120 GPa for the matrix to roughly about 180 GPa for the composite coatings. In consequence, the H/E ratio of the coatings increased by incorporation of TiC particles into the iron aluminide matrix. The composite coatings showed excellent resistance to dry sliding wear at 10 m.s^-1 and under the load of 5 N. The wear rate of the unreinforced coating is on the order of 10^-3 mm³.N^-1.m^-1. On the other hand, the wear rate of the 50 vol.% TiC coating can be three orders of magnitude lower. Friction coefficient of the Fe₃Al-TiC coatings was approximately 10% less in comparison to unreinforced coating. Analyses of the initial worn surfaces of the unreinforced coating showed microcracks and plowing marks. The worn surfaces of the reinforced coatings were considerably smoother than that of the unreinforced Fe₃Al coatings and the wear scare width was also smaller in the case of the composite coatings.

Continuous contact of hard materials leads to high friction between the two surfaces sliding against each other, which can result in high temperatures, thus influencing the life and quality of the parts under sliding. The existence of lubricants can offer some possible reduction in friction and wear of sliding contact in various applications. To achieve improved friction coefficient and wear resistance under sliding condition, bonded solid lubricants have an edge over the conventional choice. Solid lubricants in sufficient thickness can effectively work under extreme conditions of temperature, loads and speed condition. To apply lubricants more effectively with sufficient thin film thickness in the sliding zone, its lubrication characteristics depends on particle size. In view of this, the current research work aim to investigate the effect of various particle size on the lubricating effectiveness of molybdenum disulphide & the tribological properties of solid lubricants as an additive in SAE 40 oil was investigated with a four-ball tester. In addition, to understand its lubrication mechanism, the morphology and main elements of the worn surface were characterized using an optical microscope. Results also indicate that with finer particles at aggravating sliding conditions, the lubricating effectiveness was improved. This result may be clearly illustrate that for a given concentration, finer particles more easily will form a complete and continuous film rather than coarser particles.

The Tribological response of Ti-6Al-4V alloy and laser-clad TiNIZrO₂ composite coatings were studied by considering the effect of different applied loads (5, 15, 25 and 35 N) in both dry and wet conditions. The studies were performed on a ball-on-disk tribometer. The microindentation hardness of the clad layers was calculated. The surface morphology and the composition of the coatings were characterized by scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDX) respectively. Results show an increase in microhardness values obtained for TiNiZrO₂ composite coatings. Considerable improvement in the tribological response for the laser clad coatings was obtained when compared with Ti-6Al-4V during sliding wear tests against WC ball in both dry and wet conditions. The microstructural evolution after cladding as a result of ZrO₂ addition played a significant role in the wear performance of the laser-clad TiNi alloy.