Airflow Simulation and Measurement of Brake Wear Particle Emissions with a Novel Test Rig

Particle emissions generated by the braking systems of road vehicles represents a significant non-exhaust contributor. Fine particles such as these are transported through airborne routes. They are known to adversely affect human health and currently there are no policies in place to regulate them. Before this issue can be addressed, it is important to characterise brake wear debris which is the purpose of this study. A newly-developed test rig consisting of a closed but ventilated enclosure surrounds a brake dynamometer equipped with a cast iron rotor. A sampling probe was made in accordance with the isokinetic principles in order to withdraw a representative aerosol sample from the outlet duct. Measurements of real-time particulate numbers and mass distributions are recorded using a Dekati ELPI®+ unit and the brake materials were tested under drag-braking conditions. Prior to measurements, Computational Fluid Dynamics (CFD) simulations were performed to investigate the most suitable sampling points used in the experiments. Preliminary experimental results show that there is a noticeable increase in particle numbers, compared to background levels, with a corresponding change in the mass distribution; coarser particles become more prominent during these braking events. These results provide confidence in the performance of the test rig and its ability to measure airborne brake wear debris in order to compare emissions from various friction pairs

Wear Mechanisms of Hydrogenated DLC in Oils Containing MoDTC

Diamond-Like Carbon (DLC) coatings are well known for offering excellent tribological properties. They have been shown to offer low friction and outstanding wear performance in both dry and lubricated conditions. Application of these coatings for automotive components is considered as a promising strategy to cope with the emerging requirements regarding fuel economy and durability. Commercially available oils are generally optimised to work on conventional ferrous surfaces and are not necessarily effective in lubricating non-ferrous surfaces. Recently, the adverse effect of the Molybdenum DialkyldithioCarbamate (MoDTC) friction modifier additive on the wear performance of the hydrogenated DLC has been reported. However, the mechanisms by which MoDTC imposes this high wear to DLC are not yet well understood. A better understanding of DLC wear may potentially lead to better compatibility between DLC surfaces and current additive technology being achieved. In this work, the wear properties of DLC coatings in the DLC/cast iron (CI) system under boundary lubrication conditions have been investigated to try to understand what appears to be a tribocorrosion-type process. A pin-on-plate tribotester was used to run the experiments using High Speed Steel (HSS) plates coated with 15 at.% hydrogenated DLC (a-C:15H) sliding against CI pins or ceramic balls. The lubricants used in this study are typical examples of the same fully formulated oil with and without ZDDP. The friction and wear responses of the fully formulated oils are discussed in detail. Furthermore, Optical Microscopy (OM) and Scanning Electron Microscopy (SEM), Energy-Dispersive X-ray spectroscopy (EDX), Focused Ion Beam (FIB) and Transmission Electron Microscopy (TEM) were used to observe the wear scar and propose wear mechanisms. The X-ray Photoelectron Spectroscopy (XPS) analysis was performed on the tribofilms to understand the tribochemical interactions between oil additives and the DLC coating. Nano-indentation analysis was conducted to assess potential structural modifications of the DLC coating. Coating hardness data could provide a better insight into the wear mode and failure mechanism of such hard coatings. Given the obtained results, the wear behaviour of the hydrogenated DLC coating was found to depend not only on the presence of ZDDP in the oil formulation but also on the counterpart type. This study revealed that the steel counterpart is a critical component of the tribocouple leading to MoDTC-induced wear of the hydrogenated DLC

Tribo-oxidation of a brake friction couple under varying sliding conditions

This study explores a common parameter that is used to describe the energy input into a friction pairing in pin-on-disc investigations, the pv-value. The impacts of multiple sliding speed, v, and contact pressure, p, combinations were investigated while keeping their product, the pv-value at a constant level. The chosen tests for this study consisted of steady-state drag braking applications on a small-scale test bench. The actual contact area on the friction material's surface was measured after the tests and correlated to the steady-state temperature, T, that was reached during testing. The tribological interface showed sensitivity towards the different sliding and loading conditions including a shift in oxidising states of the iron contents of the friction couples. The sliding and loading conditions were reversed after the transition of oxidising states in order to investigate their impact. The results show that the oxidising states dynamically react to the operating conditions, but the overall frictional performance of the system can remain at an altered level due to enduring changes in the actual contact area and the thermal response of the friction couple with the transition in oxidising states

Investigation of pure sliding and sliding/rolling contacts in a DLC/Cast iron system when lubricated in oils containing MoDTC-Type friction modifier

Diamond-like carbon (DLC)/cast iron (CI) systems have been widely investigated due to their important application in engine components such as cylinders, pistons and more specifically for the cam/follower interface. The pure sliding contact of the DLC/CI system has traditionally been the focus of research; consequently less is understood about sliding/rolling contact systems. In addition, the tribological and tribochemical characteristics of the Molybdenum Dialkyl Dithiocarbamate (MoDTC) as a lubricant additive in such sliding/rolling contacts are not fully understood. In this study, a Mini Traction Machine (MTM) was used to run the experiments using alloy steel balls coated with 15 atomic percent (at. %) hydrogenated DLC (a-C: 15H) rubbing against uncoated cast iron discs. Results showed that the sliding/rolling ratio affects friction, wear and tribochemistry in CI/DLC systems; pure sliding enhances MoDTC activation. MoDTC decomposes to form MoS₂, FeMoO₄ and not MoO₃. In addition, it was observed that MoS2/FeMoO₄ ratio depends on test conditions and affects to the friction performance

Utilising H/E to predict fretting wear performance of DLC coating systems

Diamond-like carbon coatings have previously been studied as a protective coating for fretting wear protection providing low friction and low wear. H/E ratio has been used as a metric to rank coating performance in sliding wear, but this has not been applied to gross-slip fretting. Three DLC coating systems (a-C:H, Si-a-C:H, a-C:H:W top layers) on hardened M2 tool steel were studied using a bespoke electrodynamic shaker with a 10 mm 52100 steel ball as the counterface. This work has shown that H/E ratio can be used to predict wear performance in gross-slip fretting; the highest H/E ratio a-C:H performed best with low friction and wear

The effect of MoDTC-type friction modifier on the wear performance of a hydrogenated DLC coating

The application of Diamond-Like Carbon (DLC) coatings for automotive components is becoming a promising strategy to cope with the new challenges faced by automotive industries. DLC coatings simultaneously provide low friction and excellent wear resistance which could potentially improve fuel economy and durability of the engine components in contact. The mechanisms by which a non-ferrous material interacts with a variety of lubricant additives is becoming better understood as the research effort in this area increases however there are still significant gaps in the understanding. A better understanding of DLC wear may lead to lubricant additive solutions being tailored for DLC surfaces to provide excellent durability (wear) as well as similar or increased fuel economy (low friction). In this work, the wear and friction properties of DLC coating under boundary lubrication conditions have been investigated. A pin-on-plate tribotester was used to run the experiments using HSS steel plates coated with 15 at% hydrogenated DLC (a-C:15H) sliding against cast iron pins. One type of fully formulated oil with and without ZDDP and two levels of a MoDTC type friction modifier (Mo-FM) was used in this study. The friction and wear response of the fully formulated oils is discussed in detail. Furthermore, Optical Microscope and Scanning Electron Microscopy (SEM) were used to observe the wear scar and obtain wear mechanisms. Energy-Dispersive X-ray analysis (EDX) and X-ray Photoelectron Spectroscopy (XPS) analysis were performed on the tribofilms to understand the tribochemical interactions between oil additives and the DLC coating. A nano-indentation study was conducted to observe the changes in the structure of the coating, which can provide a better insight into the wear mode and failure mechanism of such hard coatings. In the light of the physical observations and tribochemical analysis of the wear scar, the wear behaviour of a hydrogenated DLC (a-C:15H) coating was found to depend on the concentration of the MoDTC friction modifier and the wear performance is much better when ZDDP is present in the oil. The tribochemical mechanisms, which contribute to this behaviour, are discussed in this paper

Measuring tappet rotation in a valvetrain rig when lubricated in a fully formulated oil containing MoDTC-type friction modifier

In a direct acting valve train configuration, tappet rotation plays a key role in improving lubrication, reducing wear and friction. However, to the best of the authors' knowledge, no studies were found to investigate the rotation of tappet under the effect of different coatings, thicknesses of tappets and formulations with Molybdenum Dialkyl Dithiocarbamate (MoDTC) which has been recently reported to be detrimental to Diamond-Like Carbon (DLC) wear. In this work, a new technique of measuring tappet rotation has been developed. A giant magnetoresistance (GMR) sensor coupled with a split pole ferrite disk magnet was used. The sensor was installed very close to the tappet/bucket while the magnet was mounted into the underside of the tappet. Experiments were performed using standard production steel tappets coated with Mn-phosphate (MnPO4) and diamond-like carbon (DLC) coatings. In general, results showed that the tappet rotation is strongly dependant on oil formulation, clearance, speed/temperature, and surface roughness of the coating. MoDTC promoted the rotation of the tappet under both coatings. In addition, DLC inserts showed an increase in tappet rotation as compared to MnPO4 inserts. Nevertheless, regardless of the type of coating, the thickest tappets showed the highest rotation

The effect of clearance between tappet insert and camlobe on the tribological and tribochemical performance of cam/follower surfaces

This paper examines the effect of tappet insert clearance on the tribological and tribochemical performance of the camlobe/follower tribopair when lubricated in a fully-formulated oil containing 1 wt% of Molybdenum Dialkyl Dithiocarbamate (MoDTC). Tests were performed on a Single Cam Rig (SCR), taken from 1.25 l FORD Zetec (SE) engine. White Light Interferometry and Talysurf contact profilometry were used to characterise the wear scar on the tappet inserts and camlobes respectively. In addition, Scanning Electron Microscopy (SEM) was used on both (i.e. camlobes and tappet inserts) for wear mechanisms assessment as well as to access the durability of coatings used on tappet inserts. Energy-Dispersive X-ray (EDX) and Raman spectroscopy analyses were also used to understand the tribochemical interactions between oil additives and the cam/follower interface. Results show that the chemistry of the tribofilm derived on camlobes and tappet inserts vary as a function of tappet insert clearance and cam profile. Also, regardless of the type of coating, the smaller clearance of tappet inserts exhibited higher friction and wear. Therefore, based on this work, the use of the thicker tappet insert would be inadvisable as this possibly can cause higher fuel consumption and inefficient performance of the intake/exhaust valves of the engine

Noise reduction of automobile cooling fan based on bio-inspired design

This study aims to minimize the noise generated by automobile cooling fans. Fan blade structures with ridged surfaces based on bio-inspired principles are 3D printed and used to replace the conventional fan blades. The effect of the bio-inspired ridge structures on the noise reduction of the cooling fan is demonstrated by orthogonal experiments in a semi-anechoic chamber. Experimental results show that with an increase in the rotational speed, the effect of the surface textures on the acoustic performance of the cooling fan becomes more significant. For example, at a fan speed of 1750 r/min, all the bio-inspired blade designs reduce noise compared with the original fan and, in particular, the sound pressure level is reduced by 3.83 dB(A) for the design with a ridge width of 4 mm and a ridge pitch of 15 mm. Through variance analysis of the measured noise, the ridge pitch distance has the most significant impact on noise reduction under low speed conditions whilst, under high speed conditions, the ridge width has the most significant influence. In addition to the experimental studies, computational fluid dynamics (CFD) simulations of the cooling fan are carried out to explain the mechanism of noise reduction for the ridged fan blades. When the fan runs, the horseshoe vortexes generated by the ridge structures disturb the flow of the boundary layer, reduce the influence of the fluid flow on the boundary layer, and delay the transition of the fan blade laminar flow to turbulence. It is also seen that there is a reduction of the intensity of the fan blade trailing edge vortices and the scale of the secondary vortices, thereby achieving the overall aim of noise reduction. This research has significance in the noise reduction design of automobile cooling fans

Tribological performance and tribochemical processes in a DLC/steel system when lubricated in a fully formulated oil and base oil

Diamond-like carbon (DLC) coatings show extremely good promise for a number of applications in automotive components as they exhibit excellent tribological properties such as low friction and good wear resistance. This can impact on improved fuel economy and durability of the engine components. Much work has been reported on the dry sliding of DLC coatings with less so in lubricated contacts and, as such, there is a need to further understand the tribochemistry of lubricated DLC contacts. Commercially-available oils are normally optimised to work on ferrous surfaces. Previous studies on DLC lubricated contacts have tended to use model oil systems rather than fully formulated lubricants and from this an interesting picture of lubrication mechanisms is emerging. Optimising compatibility between a surface and a set of lubricant additives may lead to excellent durability (wear) as well as increased fuel economy (low friction). In this work, the friction and wear properties of a DLC coating under boundary lubrication conditions have been investigated and the tribological performance compared with that of an uncoated steel system. A pin-on-plate tribotester was used to run the experiments using High speed steel (HSS) M2 grade plates coated with 15 at.% hydrogenated DLC (a-C:15H) sliding against cast iron pins. A Group III mineral base oil, fully synthetic Group IV PAO and four different fully formulated oils were used in this study. Furthermore optical and scanning electron microscopes (SEM) were used to observe the wear scar and to assess the durability of the coatings. Energy-Dispersive X-ray analysis (EDX), X-ray Photoelectron Spectroscopy (XPS) and Raman spectroscopy analyses were performed on the tribofilms to understand the tribochemical interactions between oil additives and the a-C:15H coating. This study show that the durability of the a-C:15H coating strongly depends on the selected additive package in the oils. In addition the effect of detergent, dispersant and antioxidants on the performance of the molybdenum-based friction modifier (Mo-FM) and ZDDP anti-wear additive was investigated and results are reported in this paper