Key Engineering Materials Vol. 642

Abstract: At microscale, friction is better understood fundamentally through hydrodynamic and elastohydrodynamic lubrication. However, the mechanisms governing friction at nanoscale remains a subject of interest. With the emergence of small-scale devices such as Microelectromechanical Systems (MEMS) and Nanoelectromechanical Systems (NEMS), there is a need to improve on the fundamental understanding of friction at diminishing gaps. Therefore, the paper investigates the friction of a simple fluid (n-hexadecane 99%) using an atomic force microscope. The measurements are interpreted using modified Eyring’s thermal activation energy approach in order to examine the effect of molecular solvation at the assumed smooth summit of asperities. It is found out that solvation for a sliding contact could be observed through the shear stress activation volume due to generated thermal energy, which indicates the movement of the fluid molecules into and out of the contact.

Abstract: The paper introduces an alternative approach to predict boundary friction for rough surfaces at micros-scale through the empirical integration of asperity-like nanoscale friction measurements. The nanoscale friction is measured using an atomic force microscope (AFM) tip sliding on a steel plate, confining the test lubricant, i.e. base oil for the fully formulated SAE grade 10w40. The approach, based on the Greenwood and Tripp’s friction model, is combined with the modified Elrod’s cavitation algorithm in order to predict the friction generated by a slider-bearing test rig. The numerical simulation results, using an improved boundary friction model, showed good agreement with the measured friction data.

Abstract: This study aims to investigate the effects of castor oil and biodiesel on the tribological performance of engine oil. Five mixed types of engine oils (B0, B5, B100, 5B-5% and 10B-5%) were used as the testing oils. Tribological performance of the test oil was evaluated under different temperature and duration by Cameron Plint TE77 reciprocating rig. SEM, EDS and AES/XPS were separately to characterize the morphology of worn surface and indentify the composition of the surface films. The results show that the anti-wear ability of the engine oil mixed with castor oil is better than that of the engine oil mixed with biodiesel oil. At room temperature, the anti-wear ability of engine oil is dominated by the physical adsorption capacity and hydrodynamic effects; while at high temperature, the anti-wear ability of engine oil is depend on the moment of decomposition of ZDDP additive and its chemical reaction rate.

Abstract: Using C/C composite and chrome bronze as a friction couple, the frictional wear properties of C/C composite with electric current is studied in this paper. The results have shown that current, velocity and load are important factors to affect the frictional wear properties of C/C composite with electric current. Wear rate of C/C composite increases with the increase of arc energy .The coefficient of friction and the wear rate increase with the increase of velocity when the electric current is constant of 100A. The coefficient of friction increases but the wear rate decreases with the increase of load when the electric current is constant at 100A. The coefficient of friction decreases but the wear rate increases with the increase of current when the load is constant of 80N. Comparing with no electric current, the coefficient of friction of C/C composite with electric current decreases but the wear rate of that increases obviously. The wear mechanism of C/C composite is mainly of electric wear caused by arc erosion under the condition of current-carrying.

Abstract: Carbon nitride coatings doped with tungsten were deposited on high speed steel disks by unbalanced DC magnetron sputtering using nitrogen-argon mixture gas. The coatings were deposited on three different types of interlayer (Ti, Ti/TiN and Ti/TiC), and the tungsten target current was varied from 0 to 0.9 A. Surface morphology of the coatings were measured by roughness testing and scanning electron microscopy (SEM). In addition, the chemical composition and depth profile were analyzed by X-ray Diffraction (XRD) analysis, Raman spectroscopy, and glow discharge spectroscopy (GDS). Finally, the hardness (H) and elasticity (E) were measured by nanoindentation and a Rockwell indentation test, while the tribological properties were tested using a pin-on-disk tribometer. After all, the coatings were measured by cutting testing of tuning inserts and micro-drillers. It is found that all of the coatings are amorphous and have a thickness of approximately 1.5 μm. Moreover, the nitrogen content is around 30 at%, while the tungsten content varies in the range of 0-9 at%. In addition, the hardness values are in the range of 15-20 GPa and the elasticity varies from 236 to 274 GPa. A good correlation is observed between the wear resistance and the indentation adhesion level. Furthermore, it is found that the hardness is not significantly correlated to the tungsten content and the coatings deposited on the Ti/TiC interlayer have greater adhesion. Finally, the coatings generally have a very low coefficient of friction (0.01-0.3) and a wear coefficient as low as 10^-6 mm³/Nm, and the CN/TiC coating reduced 41% and 43% of flank wear in the cutting testing of turning inserts and micro-drillers respectively.

Abstract: The graphene containing Ni-based composite coatings with different graphene addition amounts were prepared on 45 steel surface by using dipulse composite electrodeposition technology. The tribological behavior of composite coating was tested by against GCr15 steel pin under the dry condition. The friction coefficient of composite coating is 20-30% lower than pure Cr coating, their wear rates are almost in the same magnitude. Compared with 45 steel, the friction coefficient of composite coating decreases and wear rate falls over 50%.

Abstract: With the rapid development of advanced science and technology, the performance requirements of the PTFE fabric composite is rapidly increase. In this work, the dry friction tests of PTFE braided composites were carried out on high-speed Tribo-tester analysing the influence of different working conditions to the friction temperature and humidity in the measurement of friction characteristics experiments. Scanning electron microscopy (SEM) was utilized to examine composite microstructures and study modes of failure. Energy spectrometer analysis was also performed to study the wear surface and the ingredient of the debris in an effort to get the mechanism of the composites damage.

Abstract: Using the Taguchi method, a 5-factorial, 3-level orthogonal array (L’18 (3⁵)) was used as the experimental layout for 18 kinds of frictional lining specimen. These specimens were prepared in order to investigate their performance in establish a relationship between T_s/T_d (T_s: static torque; T_d: dynamic torque) and dμ/dν_x (μ: friction coefficient; ν_x: relative velocity of frictional lining and clutch drum) parameters. The frictional torque and the rotational speeds of the driveline, clutch, and clutch drum as functions of contact time for 100 clutch cycles are experimentally obtained dμ/dν_x and T_s/T_d. In the study, a sharp rise in maximum contact pressure ratio occurred when the relative sliding velocity reached the critical velocity, V_c. This increase in maximum contact pressure ratio generally led to an increase of the (initially negative) dμ/dν_x value. The mean values of dμ/dν_x and T_s/T_d for the clutching tests with 100 cycles can be roughly divided into three groups dependent on the fluctuation intensities of these two parameters, for each of which there is a linear relationship. The maximum contact pressure ratio significantly increases even increasing the mean value of dμ/dν_x.

Abstract: A decorative and high hardness chromium-carbon coating, electrodeposited with a trivalent chromium sulfate electrolyte, could improve the environment pollutions and toxicity of hexavalent chromium system. The trivalent chromium-carbon coatings can be deposited onto substrates directly without any buffer layer. The lifetime of various mechanical parts can be improved due to their high hardness and excellent corrosion resistance. The effect of different concentration of main salt on the composition, hardness, corrosion resistance, coefficient of friction and wear resistance of the coatings were investigated. The SEM observation of surface morphology showed that the appropriate concentration of main salt can diminish the defects caused by internal stress under excessive content, As increasing the concentration of main salt from 0.05M to 0.2M, the I_corr of the coatings would be raised from 1.04×10^-6 A/cm² to 1.26×10^-5 A/cm². XRD results showed that the structure of the coatings prepared by at 0.1 M main salt with a heat treatment at 400 °C for 1 h had Cr₇C₃ and Cr₂₃C₆ phases appeared as well as the hardness of that enhanced up to 1350 Hv than 880 Hv of the as-deposited coatings. Wear test results showed that the coatings prepared by at 0.1 M main salt via a heat treatment of 400 °C had the better wear resistance.