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

Lubrication of DLC Coatings with Two Tris(pentafluoroethyl)trifluorophosphate Anion-Based Ionic Liquids

The lubrication of a Cr-DLC coating with ethyl-dimethyl-2-methoxyethylammonium tris(pentafluoroethyl)trifluoropho-sphate [(NEMM)MOE][FAP] and 1-butyl-1-methylpyrro-lidinium tris(pentafluoroethyl)trifluorophosphate [BMP] [FAP] ionic liquids (ILs) as 1 wt% additives to a polyalphaolefin (PAO 6) was studied. Zinc dialkyldithiophosphate (ZDDP) was also used as reference in order to evaluate the effectiveness of the ILs. Reciprocating ball-on-plate tribological tests at loads of 20 and 40 N were performed. The results showed that both ILs exhibited a friction reduction, especially at the lowest load tested. Antiwear properties were also improved; the PAO 6 + 1% [BMP][FAP] mixture was slightly better, close to the values for PAO 6 + 1% ZDDP. Scanning electron microscopy (SEM) images and X-ray photoelectron spectroscopy (XPS) analysis indicated that the additive–surface interaction was responsible for the tribological improvement

Effect of debris size on the tribological performance of thermally sprayed coatings

This research aims to assess the effect of the debris particle size on the tribological performance and lubrication regime parameters of a Ni-based alloy coating. This is a key industrial problem, and its resolution can contribute to better machine endurance and proper maintenance. The debris particles are simulated by hard Al2O3 particles of size ranging from nanometers to 45 μm and dispersed in an oil lubricant. The coating studied is NiCrBSi deposited by flame spraying technique followed by the Surface Flame Melting (SFM) process. The counterpart disk sample was fabricated from quenched and tempered F-5220 steel (in line with A681(O1) ASTM). This pair was tested under linear sliding contact. Our results show that the addition of alumina particles contributes to a significant increase in wear, particularly for the largest particles (micrometric size). In the case of micrometric particles, it is possible to observe the formation of higher surface roughness, numerous microgrooves, and plastic flow of NiCrBSi coating perpendicular to the sliding direction, resulting in higher loss of volume. It was found that the actual surface roughness (obtained as a function of the debris particle size) allows better identification and prediction of the lubrication regime for wear processes instead of the traditional approach that uses the initial surface roughness as a parameter

Superlubricity mechanism of diamond-like carbon with glycerol. Coupling of experimental and simulation studies

We report a unique tribological system that produces superlubricity under boundary lubrication conditions with extremely little wear. This system is a thin coating of hydrogen-free amorphous Diamond-Like-Carbon (denoted as ta-C) at 353 K in a ta-C/ta-C friction pair lubricated with pure glycerol. To understand the mechanism of friction vanishing we performed ToF-SIMS experiments using deuterated glycerol and 13C glycerol. This was complemented by first-principles-based computer simulations using the ReaxFF reactive force field to create an atomistic model of ta-C. These simulations show that DLC with the experimental density of 3.24 g/cc leads to an atomistic structure consisting of a 3D percolating network of tetrahedral (sp3) carbons accounting for 71.5% of the total, in excellent agreement with the 70% deduced from our Auger spectroscopy and XANES experiments. The simulations show that the remaining carbons (with sp2 and sp1 character) attach in short chains of length 1 to 7. In sliding simulations including glycerol molecules, the surface atoms react readily to form a very smooth carbon surface containing OH-terminated groups. This agrees with our SIMS experiments. The simulations find that the OH atoms are mostly bound to surface sp1 atoms leading to very flexible elastic response to sliding. Both simulations and experiments suggest that the origin of the superlubricity arises from the formation of this OH-terminated surface

Lubricant Selection Manual, Phase 3

Future spacecraft must be designated to operate for very long time periods in space. For example, a target goal for the Space Station is 30 years of operation. Although the actual life may be significantly less than this optimistic goal, the life will certainly be a critical issue in design. The bearings on primary components such as the alpha and beta joints must obviously be designed and lubricated with the objective of optimum performance life. In addition to these joints, there will be numerous other tribological (rubbing or rolling) interfaced that will be required to function for the life of the spacecraft. A major key to adequate performance of tribological interface is proper lubrication. Lubricants can be divided into two basic classes: solid films and liquids. Both types have been used extensively in space applications. Both have advantages and disadvantages that must be carefully considered in their selection. The purpose here is to summarize selection criteria for liquid and solid lubricants applied to long-life spacecraft

Lubrication of space systems: Challenges and potential solutions

Future space missions will all require advanced mechanical moving components which will require wear protection and lubrication. The tribology practices used today are primarily based upon a technology base that is more than 20 years old. This paper will discuss NASA's future space missions and some of the mechanism tribology challenges that will be encountered. Potential solutions to these challenges using coatings technology will be assessed

Cryogenic gear technology for an orbital transfer vehicle engine and tester design

Technology available for gears used in advanced Orbital Transfer Vehicle rocket engines and the design of a cryogenic adapted tester used for evaluating advanced gears are presented. The only high-speed, unlubricated gears currently in cryogenic service are used in the RL10 rocket engine turbomachinery. Advanced rocket engine gear systems experience operational load conditions and rotational speed that are beyond current experience levels. The work under this task consisted of a technology assessment and requirements definition followed by design of a self-contained portable cryogenic adapted gear test rig system

Solid lubricants

The state of knowledge of solid lubricants is reviewed. The results of research on solid lubricants from the 1940's to the present are presented from a historical perspective. Emphasis is placed largely, but not exclusively, on work performed at NASA Lewis Research Center with a natural focus on aerospace applications. However, because of the generic nature of the research, the information presented in this review is applicable to most areas where solid lubricant technology is useful