Tribological Properties of a Self-Repairing Material: Functionalized Graphene/montmorillonite Nanosheets Lubricant Additives

The functionalized graphene/montmorillonite (FG/MTT) nanosheets were synthesized through chemically bonding by a simple, green method, which has remarkable dispersion stability in oil and its lubricating performance was evaluated by a four-ball tribometer. The test results show that FG/MTT has a preeminent lubricant property when the concentration is 0.4 mg/ml. Compared with the bare oil sample, its average friction coefficient (FC) and wear scar diameter (WSD) decrease by 50.4 % and 13.2 %, respectively. The synergistic effect between FG and MTT was further explored by comparing the lubricant mechanism of the different additives. After synthetically analyzing worn surface by means of scanning electron microscopy and X-ray photoelectron spectroscopy, the lubrication mechanism of the FG/MTT nanocomposite as oil additive is discussed and postulated: The FG/MTT with weak interlayer adhesion is filled between the friction pairs to avoid contact and clinging of some asperities, and the sliding between the layers plays a role in lubrication. Furthermore, FG/MTT will react with the surface of the friction pair to form a repair layer composed of Fe2O3, SiC, SiO2, and aluminosilicate, mending the grinding surface and promoting the hardness after friction.

Investigation of Al-Mg based composite incorporated with MoS2 through powder metallurgy

In this study, powder metallurgy process has been utilized to produce the composite materials. Pure aluminium (Al) has been selected as the matrix material for the preparation of composite materials. 4% of magnesium (Mg) has been added as the alloying element, whilst molybdenum disulphide (MoS2) is reinforced with a varying wt.% (2, 4 & 6). The intent of this work is, to evaluate some basic mechanical properties (density, microhardness, compression strength and toughness), surface degradation properties (corrosion and wear), and electrical conductivity of the fabricated composite materials. Moreover, SEM mapping and EDAX analysis have been conducted to confirm the presence of reinforcement particles with homogenous distribution in the matrix material and to do the fractographic study of the compressive strength tested samples. The density of the composite material Al-4% Mg-6%MoS2 has been increased compared to the density of pure Al material. Micro Vickers hardness test shows that Al-4% Mg-6%MoS2 composite has 32.86% more hardness as compared to that of pure Al material. Compressive strength of the composite material with the higher wt.% of MoS2 is found to be 181.81 N/mm2, while Al material has only 167.52 N/mm2. The buckling formation during the compression test is avoided in the composite material owing to the existence of MoS2 particulates. The wear loss of composite materials is found low as compared with the unreinforced Al material, owing to the solid lubricant property of MoS2 particles, and hence, coefficient of friction (COF) is also lessened with the increase in the MoS2 wt.%. Also, the MoS2 reinforced materials show good resistance to corrosion due to the presence of molybdenum, which acts as a consistent layer obstacle to prevent the surface from further degradation.

Prediction of the normal contact stiffness between elastic rough surfaces in lubricated contact via an equivalent thin layer

In this work, the normal contact stiffness of lubricated rough interface is evaluated theoretically by describing the lubricated rough interface as an equivalent thin layer. Layer parameters, including equivalent thickness and effective Young’s modulus, are used to characterize the normal contact stiffness by incorporating the contributions of asperity contact and lubricant contact simultaneously. On the basis of layer parameters, the normal contact stiffness of lubricated rough interface is obtained as a function of interfacial separation, surface topography, and properties of contacting solids and lubricants. Effects of surface topographies and lubricant types on the normal contact stiffness are investigated at varying interfacial separations and contact area fractions. The proportion of solid stiffness and lubricant stiffness from the total normal stiffness is also discussed. Numerical solutions reveal that the normal contact stiffness depends sensitively on the lubricant property at initial contact, whereas the influences of surface topographies become obvious with the decrement of interfacial separation or increment of contact area fraction. Comparisons between the predicted values of normal contact stiffness and experimental data for both dry interface and lubricated interface are presented to validate the rationality of the developed model.

Experimental investigations on R430A as a drop-in substitute for R134a in domestic refrigerators

In this work, the energy performance of a domestic refrigerator has been experimentally investigated using R430A as a possible drop-in substitute to replace R134a. Experiments have been carried out in an R134a-based single evaporator domestic refrigerator of total volume 190 L. The pull-down and ON/OFF cycle performance tests were carried out at 32 ± 0.3 ℃ ambient temperature. The continuous running performance test was carried out for wide range of ambient temperatures between 24 ℃ and 43 ℃ by maintaining evaporator temperature at −12 ± 0.2 ℃. The experimental results showed that the R430A has 3.9% lower energy consumption with 3.8%–6.4% higher coefficient of performance when compared to R134a. However, the compressor discharge temperature of R430A was observed to be 2 ℃–4 ℃ higher when compared to R134a. The lubricant used in R134a compressors is physically stable and retains its lubricant property at 4 ℃ elevated temperature when operating with R430A. The total equivalent warming impact of R430A was found to be 4%–5% lower when compared to R134a due to its higher energy efficiency and lower global warming potential. The results confirmed that R430A is found to be a good drop-in substitute to phase out R134a in domestic refrigerator servicing sectors.

Study on the normal contact stiffness of the fractal rough surface in mixed lubrication

In this paper, an elastic interface model is developed to theoretically analyze the contact stiffness of a mixed lubrication surface where the solid and the lubricant contacts have to-be-determined contributions to the whole contact stiffness. The interfacial contact stiffness is composed of the solid contact stiffness and the lubricant contact stiffness, in which the two components are associated with each other via the equivalent thickness of lubricant. Based on the combination of two widely acknowledged ultrasonic measurement models and the Taylor approximating equation, the derivation of the lubricant contact stiffness is mostly affected by the material properties and the equivalent thickness of lubricant, and the equivalent thickness is determined by the solid contact properties under the mixed lubrication condition. Results of the mathematical analysis show that the contact stiffness of the mixed lubrication surface is larger than that of the dry rough surface due to the presence of lubricant. The interfacial contact stiffness of the mixed lubrication is obviously affected by the surface topography and the lubricant property. The proportions of contact stiffness contributed from the solid part and the lubricant part are varying with the contact area and the surface topography. Model predictions are compared with experiment results to verify the accuracy of proposed model. The analysis of the interfacial contact stiffness involved in mixed lubrication provides a theoretical basis for the performance prediction of machine tools, and might be useful to elucidate the contact properties by ultrasonic pulse probing in real engineering applications.

Empirical Comparison of Sliding Friction and Wear Behaviors of Gray and White Cast Iron

In this paper, sliding friction and wear behaviors of gray cast iron A35 and white cast iron manufactured by quenching from the same cast iron in water were studied and compared by employing pin-on-disk wear tests. Microstructure of the worn surfaces before and after the wear tests were investigated by optical microscope observations. These images show that flakes separated from the surface in gray cast iron due to delamination process, while in white cast iron, the separation of materials from its surface is in the form of powder. In addition, the gray cast iron had higher graphite volume fraction with Type-A graphite flake morphology. The results show that white cast iron has less rate of wear than gray cast iron due to the higher hardness. However, gray cast iron because of presenting graphite flakes in its surface (lubricant property) has lower average coefficient of friction.

A Physics-Based Heat Transfer Analysis for Aerospace Ball Thrust Bearings

Component temperatures have a significant influence on ball bearing life and durability. Critical parameters such as internal clearance, contact stress and oil film thickness are influenced by the thermal characteristics of a bearing. Previous heat transfer analyses that predict bearing ring temperatures have been typically based on empirical data. A new physics-based analysis has been developed which is derived from fundamental heat transfer and fluid flow equations. Gyroscopic torque and churning forces are balanced by oil shearing contact traction, using recent developments in new life theory [1] and lubricant property models [2]. Bearing ring temperatures and heat generation calculated by the new analysis have been successfully correlated with test data over a wide range of bearing sizes and operating conditions.