Jiping Ye

Evaluation of Local Mechanical Properties in Depth in MoDTC/ZDDP and ZDDP Tribochemical Reacted Films Using Nanoindentation

Local mechanical properties in depth and near the surface of MoDTC/ZDDP and ZDDP tribofilms, which exhibited obviously different friction coefficients in a pin-on-disc test, were determined by using a nanoindentation technique combined with in-situ atomic force microscopy (AFM) observation. Tapping-mode AFM observation revealed that the MoDTC/ZDDP film was much rougher than the ZDDP film. Nanoindentation measurement revealed that the MoDTC/ZDDP and ZDDP tribofilms possessed different elasto-plasticities around a depth of several nanometers from the surface, although both films showed the same hardness and modulus depth distributions except in the surface area. The same mechanical depth distributions indicated that both kinds of tribofilm were functionally graded materials; that is, they consisted of a layer near the surface with lower hardness and modulus and providing lubrication and a base layer with higher hardness and modulus and serving to modify property differences at the interface. Most importantly, the different elasto-plasticities near the tribofilm surfaces revealed that the MoDTC/ZDDP tribofilm possessed lower shearing yield stress than the ZDDP tribofilm. The results of this study suggest that the presence of some solid lubricants such as MoS2 just below the MoDTC/ZDDP film surface reduced the boundary friction coefficient.

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.

Determination of nanostructures and mechanical properties on the surface of molybdenum dithiocarbamate and zinc dialkyl-dithiophosphate tribochemical reacted films using atomic force microscope phase imaging technique

Nanostructures and mechanical properties on the surface of two kinds of tribofilm formed from zinc dialkyl-dithiophosphate (ZDDP) and molybdenum dithiocarbamate (MoDTC) additives, which exhibited obviously different friction coefficients in a pin-on-disc test, were determined by using an atomic force microscopy (AFM) phase imaging technique. The level of interactive force between the tip and sample was modulated for distinguishing well-defined structures and mechanical properties of individual components not only on the uppermost surface but also in the underlying area near the surface in the AFM tapping mode. It was found that the MoDTC/ZDDP tribofilm possessed a lower surface modulus than the ZDDP film in the elastic deformation range. Most importantly, nanostrips oriented in the sliding direction were found in the MoDTC/ZDDP tribofilm at a depth of around 10 nm from the surface. These nanostrips possessed lower shearing stress than the surface matrix and formed the inner skin layer, which exhibited low...

Nanometer-scale mechanical/structural properties of molybdenum dithiocarbamate and zinc dialkylsithiophosphate tribofilms and friction reduction mechanism

Nanometer-scale differences in mechanical and structural properties between the molybdenum- dithiocarbamate/zinc-dialkylsithiophosphate (MoDTC/ZDDP) tribofilm and ZDDP tribofilm were successfully evaluated by using atomic force microscopic phase-image techniques, Auger electron spectroscopy and X-ray photo spectroscopy. It is well known that the MoDTC/ZDDP tribofilm exhibits markedly lower friction behavior than the ZDDP tribofilm. To elucidate the mechanism of friction reduction originating from the MoDTC additive, attention was focused on property differences in the surface area in particular, from the uppermost surface to an underlying region of less than 10 nm in depth. It was found that the friction reduction due to the MoDTC/ZDDP additives originates from an inner skin layer formed by MoS2 nanostrips just below the surface. The MoS2 nanostrips were oriented in the sliding direction, had low yield strength and acted as a solid lubricant in lowering the friction coefficient of the MoDTC/ZDDP tribofilm.

Ultra-low friction properties of DLC lubricated with ester-containing oil —Part 1: Pin-on-disc & SRV friction tests—

This paper presents a material combination that reduces the friction coefficient significantly to an ultra-low regime (below 0.05) under boundary lubrication, as indicated by the results of pin-on-disc toric sliding tests and reciprocating sliding tests (SRV). This ultra-low friction performance was obtained by sliding hardened steel pins on a hydrogen-free diamond-like carbon (DLC) film (a-C) lubricated with a poly-alpha-olefin (PAO) oil containing an ester additive. Moreover, the friction coefficients of a-C couples were substantially lower than those of hydrogen-containing DLC couples and that of an a-C/steel combination.

Super Low Friction Property of DLC Lubricated With Ester-Containing Oil: Part 2 — Nanometer-Scale Morphological, Structural and Frictional Properties

We have succeeded for the first time anywhere in lowering the friction coefficient of a diamond-like-carbon (DLC) coating to less than 0.01 under boundary lubrication in engine oil [1–3]. This anomalous super-lubrication behavior has been observed for a hydrogen-free DLC-coated (ta-C) disc in an ester-containing oil but not for a hydrogenated DLC (a-C:H) coating. It is thought that some chemical adsorbent may form only on the ta-C sliding surface due to some tribochemical reactions. Our recent studies have suggested that the macro-scale reduction of friction is dependent on nanometer-scale tribological properties [4–6]. The superlow friction behavior seen in a pin-on-disc friction test was taken as the object of this investigation with an eye toward elucidating the mechanism of the anomalous friction reduction. Pin-on-disc tests were conducted by sliding a ta-C/ta-C pair in the presence of poly alpha-olefin based oil containing a modifier additive of glycerol monooleate ester (PAOES1 oil). Nanometer-scale tribological properties were investigated by using atomic force microscopy (AFM), the AFM phase-image technique, and nanoscratch measurements. Attention was focused on the differences in surface roughness, nanostructure and nanofriction coefficient between the sliding and non-sliding areas in an effort to find the origin of the super-lubrication behavior.Copyright

Super Low Friction Property of DLC Lubricated With Ester-Containing Oil: Part 1 — Friction Properties Evaluated in Rig Tests

This paper presents a material combination that reduces the friction coefficient markedly to a super low friction regime (below 0.01) under boundary lubrication. Friction tests were conducted with a test rig consisting of three pins pressed against a rotating disc, as shown in Fig. 1. The pins were made of bearing steel AISI52100 and the disc was made of carburized steel SCM415, which was coated with a diamond-like-carbon (DLC) film. The test conditions were as follows. Pins: Fixed, not rotating; DLC: CVD a-C:H, PVD ta-C; Lubricant: 5W-30 API SG Engine oil; Ester-containing oil (PAOES1): Poly alpha-Olefin containing 1 mass% of glycerol mono-oleate; Pressure: 0.7 Gpa; Sliding speed: 0.03–1m/s; Oil temperature: 353K (80 deg. Celsius).Copyright

Ultralow friction properties of DLC lubricated with ester containing oil —Part 2: Nanoscratch analysis of surface friction—

Ultralow friction behavior was exhibited by a hydrogen-free DLC-coated disc lubricated in an ester-containing engine oil in a pin-on-disk test. Nanoscratch analysis was used to elucidate the macro-scale mechanism of the ultralow friction behavior in relation to nanoscale tribological properties. Local friction coefficient distributions relative to the nanoscale depth showed that the sliding areas exhibited much lower friction coefficients than the non-sliding areas regardless of whether or not the surfaces were cleaned with hexane. It was found that the nanoscale friction coefficient revealed the real nature of the surface materials; it was independent of not only the loading conditions in the nanoscratch system but also surface roughness. These observations indicated that the surface chemical states between the sliding and non-sliding areas were different. An ultrathin tribofilm with a low friction characteristic formed on the sliding surface of the DLC-coated disc during the pin-on-disk test in the ester-containing oil, which presumably reduced the macro-scale friction to an ultralow level.