Th. Le Mogne

MoS2 single sheet lubrication by molybdenum dithiocarbamate

The mechanisms by which Modtc reduces friction in the centirange under boundary lubrication have been investigated using analytical tribometry. First, the SRV friction test was coupled with energy-filtering TEM on wear fragments and spatially-resolved XPS inside the wear scars. Second, we performed UHV friction tests on Modtc tribofilms previously created on a large area. The overall data demonstrate that the mechanisms of friction-reduction by Modtc is attributed to the effect of sliding between single layers of MoS2 only, and not to intra-sliding in MoS2 3-D crystal. Highly-dispersed MoS2 sheets are present in a carbon matrix in the tribofilm material. The growth of the 2-D MoS2 single sheets is thought to be formed by degradation of the Modtc molecule by electron transfer mechanisms activated by the friction process. The lubrication of the uncoated, stationary counterface is attributed to successive transfer of individual sheets towards the friction surface. Practically, in these conditions only a few per cent of dispersed MoS2 is sufficient to lubricate at the same level as pure MoS2.

Chemistry of the interface between aluminium and polyethyleneterephthalate by XPS

Abstract X-ray photoelectron spectroscopy (XPS) has been used to study the interface between in-situ thermally evaporated aluminium (Al) and a biaxially oriented film of polyethyleneterephthalate (PET). The results of metallization show the formation of an intermixing layer interface, which is composed of Al-O-C(PET) compounds, Al-C(PET)-like species and Al clusters. A reaction mechanism of Al on PET is presented. The chemical attack is preferentially towards the sites of stronger basicity. First it takes place on the most basic groups (carbonyl groups) and then on the ether linkages, both lead to the formation of Al-O-C(PET) compounds. The further reaction is ended by the attack of the benzene rings, giving rise to Al-C(PET)-like species.

Friction reduction by metal sulfides in boundary lubrication studied by XPS and XANES analyses

In this work, the antiwear action of zinc dithiophosphate (ZnDTP) in combination with friction modifier agent molybdenum dithiocarbamate (MoDTC) has been investigated. A Cameron–Plint friction machine was used to generate large tribofilm areas in mild tribological conditions. Two analytical techniques, X-ray photoelectron spectroscopy (XPS) and X-ray absorption near edge structure (XANES) spectroscopy, have been employed to characterize the chemical species in tribofilms. XANES spectroscopy at the P K and L-edge, Mo L and M-edge and S K-edge was carried out in order to investigate phosphates and sulfides species in the tribofilms This was followed by XPS analysis in the same location in the tribofilms. Special attention has been paid to the peak fitting of Fe 2p, Zn 2p, S 2p, Mo 3d and O 1s photopeaks and Auger ZnLMM lines. The combined analyses have shown that the MoDTC+ ZnDTP containing oil produces mainly shorter chain metal polyphosphate material in addition to ZnS and MoS2. No zinc sulfate has been detected. XPS confirms that the formation of MoS2 is enhanced by the presence of ZnDTP. For all additives combinations; no iron has been detected in the tribofilms. Analytical results are discussed and compared with theoretical predictions from the Chemical Hardness model for ZnDTP, MoDTC and ZnDTP + MoDTC tribofilms formation.

Super-low friction of MoS2 coatings in various environments

The ultra-low friction coefficient (typically in the 10−2 range) of MoS2-based coatings is generally associated with the friction-induced orientation of ‘easy-shear’ planes of the lamellar structure parallel to the sliding direction, particularly in the absence of environmental reactive gases and with moderate normal loads. We used and AESXPS ultra-high vacuum tribometer coupled to a preparation chamber, thus allowing the deposition of oxygen-free MoS2 PVD coatings and the performance of friction tests in various controlled atmospheres. Friction of oxygen-free stoichiometric MoS2 coatings deposited on AISI 52100 steel was studied in ultra-high vacuum (UHV: 5 × 10−8 Pa), high vacuum (HV: 10−3 Pa), dry nitrogen (105 Pa) and ambient air (105 Pa). ‘Super-low’ friction coefficients below 0.004 were recorded in UHV and dry nitrogen, corresponding to a calculated interfacial shear strength in the range of 1 MPa, about ten times lower than for standard coatings. Low friction coefficients of about 0.013–0.015 were recorded in HV, with interfacial shear strength in the range of 5 MPa. Friction in ambient air leads to higher friction coefficients in the range of 0.2. Surface analysis performed inside the wear scars by Auger electron spectroscopy shows no trace of contaminant, except after friction in ambient air where oxygen and carbon contaminants are observed. In the light of already published results, the ‘super-low’ friction behaviour (10−3 range) can be attributed to superlubricity, obtained for a particular combination of cystallographic orientation and the absence of contaminants, leading to a considerable decrease in the interfacial shear strength.

Superlubricity of MoS2: crystal orientation mechanisms

Abstract We have investigated the origin of the extraordinary low friction coefficient (in the 10 -3 range or even less) of pure and stoichiometric sputtered MoS 2 coatings, in ultrahigh vacuum. In these conditions, shear strengths of the interface as low as 1 MPa were measured. Importantly, the tribometer was operating in macroscopic contact conditions, typically at a long time-length scale. Friction-induced orientation of (0001) basal planes of MoS 2 grains parallel to the sliding direction was first verified by means of electron diffraction. Friction-induced rotation of these crystals around the c axis, during intercrystallite slip in the contact, was investigated by high resolution transmission electron microscopy performed on selected wear fragments. Atomic force microscopy at atomic resolution was also carried out on the surface inside and outside the wear scar. The data indicated that the vanishing of the friction force was due to frictional anisotropy in the interface between nanometre-scale domains in rotational disorder (intercrystallite slippage of incommensurate sulphur-rich hexagonal lattices). The term superlubricity was used here because of the zero friction state that could be theoretically predicted in these conditions. Finally, the mechanisms of MoS 2 superlubricity are thought to depend on the proper combination of the grain size, the two crystal orientation effects and the absence of contaminants.

Friction of Hexagonal Boron Nitride in Various Environments

The mechanims of friction of hot-pressed h-BN sliding against itself in an AES/XPS analytical tribometer have been studied under high vacuum of 10 -8 Pa; under a low partial pressure of 10 -3 Pa of different gases, i.e., air, CO, C 3 H 8 and H 2 O; under a higher partial pressure of 10 Pa of N 2 , O 2 , air and C 3 H 8 ; and in ambient air, 50 percent humidity, at an atmospheric pressure of 10 5 Pa, respectively.

Superlow friction of oxygen-free MoS2 coatings in ultrahigh vacuum

Abstract Low friction of MoS 2 -based coatings in the absence of reactive gases and particularly water vapour is generally attributed to friction-induced orientation of “easy shear” planes of the lamellar structure, parallel to the sliding direction. It has been suggested that the substitution of sulphur by oxygen in the MoS 2 structure could improve its tribological performance by increasing the basal plane distance. To check the role of the presence of oxygen in the friction of MoS 2 , we have developed an Auger electron spectroscopy-X-ray photoelectron spectroscopy ultrahigh vacuum tribometer, coupled with a preparation chamber, which allows the investigation of oxygen-free MoS 2 sputter-deposited coatings and the performing of in-situ friction measurement in an ultrahigh vacuum. MoS 2 coatings (120 nm thick) were deposited on (100) Si substrates. No trace of oxygen contamination was detected by X-ray photoelectron spectroscopy or Auger electron spectroscopy. In these conditions, reciprocating friction of the film against SiC spherical pins (normal load, 1 N; maximum hertzian pressure, 0.66 GPa; vacuum state, 50 nPa) gave extraordinary low friction coefficients below 0.005, which were often difficult to measure with the equipment available. In light of wear debris and surface analyses, the mechanisms of this superlow friction of MoS 2 are discussed.

In situ MoS2 formation and selective transfer from MoDPT films

Abstract UHV friction tests and complementary analyses using AES, XPS, TEM and EELS give a global view of the friction reduction mechanisms of MoDTP antiwear and friction-reducing additive. The study was performed with MoDTP tribofilms composed mainly of an amorphous phosphate glass containing flexible and highly dispersed MoS 2 single sheets. The transfer films created under UHV friction were studied by AES. The hard and soft acids and bases (HSAB) principle was used to exploit the data. We observed that relatively high friction ( μ ∼0.3) was associated with the transfer of the whole tribofilm from the flat to the pin by a chemical reaction between the phosphates present in the tribofilm and the native oxide layer on the pin. When the oxide layer was removed, the MoS 2 sheets reacted with the nascent metal surface of the pin. The transfer film observed on the pin is very thin (4 nm) and it can clearly be seen that only a few single sheets of MoS 2 are needed to achieve friction at the centirange.

UHV friction of tribofilms derived from metal dithiophosphates

The friction‐reduction mechanisms of Modtp and Zndtp were highlighted by submitting tribofilms to friction in ultra‐high vacuum (UHV). The use of an UHV tribometer to understand these phenomena is justified by the fact that the friction coefficient recorded in UHV is close to the friction coefficient obtained in traditional tests in oil. After UHV friction, the transfer films on the pin were analyzed by in situ AES, XPS and AES mapping. Low friction is associated with the transfer to the pin of a sulfur‐rich film. In the case of Modtp, we observe a very thin MoS2 film. The UHV friction coefficient approaches 0.04. In the case of Zndtp, the transfer film contains ZnS together with some phosphates. Because of the poor capacity of ZnS to reduce friction, the UHV friction coefficient recorded is near 0.15. A global model of the action of dithiophosphates in reducing friction is described on the basis of the hard and soft acids and bases (HSAB) principle.

Tribochemistry of ZDDP and MoDDP chemisorbed films

By means of a UHV analytical AES/XPS pin-on-flat tribotester, we have investigated tribochemical change as well as transfer mechanisms in static reaction films on steel from ZDDP (anti-wear) and MoDDP (friction modifier) lubricant additives. The results show that solid ZDDP reaction films are able to strongly reduce friction to a value of 0.1–0.3 (in the absence of a film, steel-on-steel gives a friction coefficient value of up to 2). MoDDP reaction films produce ultra-low friction (below 0.05) after an induction period. In situ AES analysis and energy-filtered XPM gave definitive evidence of frictioninduced chemical changes and selective material transfer from the flat to the pin. In the case of MoDDP, the thiophosphate film is decomposed and a “MoS2” residual film is found in the wear scar on the pin.