In this study, the formation processes of molybdenum dithiocarbamate (MoDTC) and zinc diallyldithiophosphate (ZDDP) tribofilms were investigated using an in situ Raman-space layer imaging method (SLIM) tribometer under boundary lubrication. According to the results, the ratio of the POx and P-O-P bonds in the tribofilms was almost the same as the film thickness behavior calculated from the SLIM images. Hence, the the time-dependence chemical composition change of the tribofilms of phosphate related bonds in the tribofilms is considered a factor in the growth of ZDDP tribofilms. Moreover, the presence of MoDTC in the ZDDP solution contributed to the formation of phosphate compounds with a relatively high ratio of P-O-P links, such as ultra-phosphate glasses. This mechanism is attributable to the friction-reduction effects of MoS2 on the tribofilm, which can contribute to maintaining a suitable condition for the formation of relatively high P-O-P-link-ratio tribofilms.
This paper presents a simple approach for the use of pyrene as fluorescence dye for the observation of the EHL contacts and behavior of lubricants without natural fluorescence properties. Dependence of the light emission of pyrene on its concentration was studied and also, specific emission and excitation spectrum for pyrene dissolved in PAO was found. A mercury lamp was used as the source of the excitation light with proper filter set according to the fluorescence spectrum. Pyrene’s excimer proved to be very resistive against photobleaching and stable in time. This paper also describes experimental methodology on how to improve fluorescence properties of greases by adding pyrene and this approach was also verified by experiments to ensure that there was no influence on grease or pyrene’s emission. This paper also discusses the calibration procedure for valid quantitative experiments.
To cope with the increasing demands of high-performance materials, efforts were made using Laser Powder Bed Fusion technique (L-PBF), taking advantage of its extremely short solidification time to fabricate copper alloy with highly refined microstructure. Copper and Copper alloys are reputed for their high reflectivity resulting in low laser absorption, which makes their 3D fabrication process very challenging. This study aims to provide high density samples in order to obtain high mechanical and tribological properties. This objective is realized through the optimum energy density value which is obtained by monitoring laser power P, scan speed v, hatch space h and layer thickness t. Upon mapping of optimum parameters, the best combination is adopted to manufacture the samples of this study. Mechanical and tribological characterization of nickel aluminium bronze alloy confirmed the efficiency of laser powder-bed fusion (L-PBF) in providing high performance materials, higher properties such as wear resistance, tensile strength and hardness were obtained compared to other manufacturing techniques.
Research interests recently increased in studying polymer-type friction modifiers (polymer FM) consisting of a polymer backbone and a functional group because of their good friction reduction capability. However, there is no clear verification of their working mechanisms. Therefore, the polymer FM behavior at the contact region must be understood to provide mechanistic information and design further-improved polymer FMs. For this purpose, in situ observation techniques have been applied to the chemical analysis of the oil film formed from a polymer FM using a micro-Fourier transform infrared spectrometer. The experimental results indicate that the polymer concentrations at the Hertzian contact drastically increased, exceeding the bulk concentration of oil. The following new friction reduction mechanism is proposed based on the clarified polymer FM concentration: the concentrated adsorption layer of the polymer FM prevents direct contact of the surface asperities.
The cooling rate during fabrication affects the microstructural features and interface strength in metal matrix composites (MMCs). Thus, the present investigation is focused on characterizing the effect of quenching medium on wear and friction response of self-lubricating Al-Cu dual matrix composites. Mica was reinforced in Al-Cu metal matrix composites with the variation of weight percentage as 1%, 2%, 3%, 5%, 8% and 10%. A standard powder metallurgy route was adopted for fabrication. The sintered composites were normalized, and quenched in water, and oil. A pin on disc study was carried to investigate the friction and wear behavior of normalized, oil quenched, water quenched and as-prepared (green) composites. The results revealed that oil quenched composites exhibited the lowest friction coefficient and highest wear loss. The highest friction coefficient of ~0.8 was observed for normalized composites, whereas the highest friction coefficient for water quenched composites was ~0.6. The oil quenched composites exhibited about 8 ~ 15 times higher wear loss as compared to normalized and water quenched composites. Worn surfaces were analyzed under a scanning electron microscope to know the dominant wear mechanisms. The developed composites find potential applications in automotive industry such as pistons for petrol and diesel engines.
Carbon nanomaterials have been studied for their promising tribological properties under boundary lubrication. However, the behavior of carbon nanomaterials upon entering narrow friction interfaces under boundary lubrication is poorly understood. In this study, to clarify how graphene oxide (GO), which is one of the carbon nanomaterials, enters and works on a friction interface under boundary lubrication, in situ observations of a friction interface between an SUJ2 ball and soda glass-lime disk in a GO water dispersion, with a load of 1 N, were conducted using optical microscopy. Isolated dispersed GO accumulated only in front of the contact areas. GO accumulations formed, grew, and peeled off due to the friction. Although the GO accumulations were significantly unstable, they functioned as friction reduction and anti-wear. We firstly found that the unstable GO lubricating accumulations that formed in front of the contact areas were able to widen between the ball and the disk similar to a wedge; this wedge effect would cause a decrease in friction and wear. Unlike the wedge effect under hydrodynamic lubrication, the GO accumulations that functioned as the wedge stayed on the ball surface.
To improve the rolling–sliding contact fatigue strength of a case-carburized steel, the effect of the wear surface morphologies of manganese phosphate (MnP) coated steel and the growth and removal of phosphorus-containing tribofilm on surface-initiated crack formation was investigated. In order to modify the wear surface morphologies, two types of surface textures (ground and shot blasted) were prepared, followed by the MnP coating process. The tribological properties of the coated steel, tribofilm growth and removal, and surface-initiated crack formation were evaluated using a ball-on-disk tribometer with a rolling–sliding mode. The MnP coating on both the ground and shot blasted steel had nearly the same thickness and surface roughness. However, for the ground surface sample, the interface morphology between the coating and steel substrate was more irregular than the shot blasted surface sample, resulting in a larger number of exposed steel areas with smaller sizes after the MnP was almost worn away on tribological tests. During the running-in period, phosphorus-containing tribofilm growth and removal on the smaller exposed steel areas were observed. The surface-initiated crack formation on the smaller exposed steel areas was suppressed compared with larger exposed steel areas.
With the increasing usage of high-strength steel, tool wear becomes a major challenge in sheet metal forming. As tribological properties are in consequence of the system characteristics, wear prediction is extremely difficult. The wear resistance characteristic curve is introduced in a former publication of the authors to predict wear in sheet metal forming. With the help of this curve, the evaluation of the tool life spans in strip drawing tests under different contact pressures is possible. However, other influencing factors on tool wear should also be taken into account. In this paper, two types of tools made from cold working steel and cast iron are used to investigate the influence of the tool characteristics hardness and surface roughness on the wear behavior and the life span in strip drawing tests with high strength steels. Moreover, the dominating wear mechanisms with respect to different tool properties are also discussed. As a consequence, the wear resistance characteristic curves are derived and new values to characterize the wear resistance in terms of hardness and surface roughness are presented. With the results, more knowledge about the influencing factors on wear is given and the life spans of different tribological systems can be quantitatively predicted. Finally, the wear behavior of the tools after rework is also discussed.
Energy losses and friction locking are decisive factors in the conceptual design and sustainable realization of machine elements. Thus, the improvement of the tribological properties of rolling bearings by ceramic coatings on bearing surfaces represents a promising approach. These coatings are to be optimally adapted to the load case by minimizing the slip and resulting wear by rolling elements. For this purpose, molybdenum-based coatings were applied by means of magnetron sputtering in a vacuum atmosphere at controlled and adjusted oxygen partial pressure on 100Cr6 axial bearing washers. The effect of diffusing oxygen at near surface areas can be achieved during the physical vapor deposition (PVD) process itself as well as under adequate loading cases, so that a regenerative separation layer prevents high tribological wear at running surfaces. The generated layers were then characterized by high-resolution analysis with regard to morphology, attachment to the substrate and stoichiometry. The adjusted process parameters yielded pure molybdenum, as well as molybdenum oxide, dioxide and trioxide as a function of corresponding oxygen partial pressure. Scanning electron microscopy
(SEM) was used for topographical evaluation, X-ray diffraction (XRD) for the characterization of stoichiometry and focussed ion beam cutting (FIB) for coating thickness determination. From selected surfaces, additional energy-dispersive X-ray spectroscopy (EDX) mappings were performed to quantify local oxygen contents at the border area of generated molybdenum layers. To record tribological characteristics, the layers were analyzed for their mechanical properties subsequently. Therefore, nanoindentational studies were carried out, which could provide information on the wear behavior in point contact in the form of nanoclay experiments. The results showed lower coefficients of friction for oxidized surfaces and thus a better resistance against sliding wear than uncoated specimen surfaces.
A heat-assisted magnetic recording (HAMR) is being implemented to increase the density of hard disks. However, adhesion of smear to the head slider is a drawback. We investigated a smear generation mechanism under laser irradiation in different environments (air and helium) using a disk coated with perfluoropolyether lubricant D-4OH. The amount of smear was smaller in air than in helium at all temperatures. An analysis of smears at 258°C using time-of-flight secondary ion mass spectrometry (TOF-SIMS) suggested that the end group was oxidatively decomposed and the hydrocarbons were burned. And the differences in the end group/main chain ratio and the hydrocarbon ratio between the two environments were smaller at 540°C than at 258°C. It suggested that the lubricant molecules were thermally decomposed owing to the high temperature. It was observed that there was a small difference in the smear amounts between the two environmental gases at 540°C due to the thermal decomposition of the lubricants, but the presence of oxygen was effective in reducing the amount of smear when the heating temperature was 200–300°C.