Jean-Michel Martin

Tribologically transformed structure in fretting

Abstract Both fretting wear and fretting fatigue suffer from particle detachment and cracking induced damage. Wear induced by fretting is related to a three-stage phenomenon: (1) accommodation of the displacement in the upper layers of the two counterbodies; (2) detachment of particles from material with a modified or transformed structure; and (3) third-body behaviour, i.e. accommodation of the velocity in the powder bed. The specific transformed structure from which debris is made is called the tribologically transformed structure or TTS. TTS has been shown to form in the first accommodation stage within a very few initial fretting cycles. Understanding of its formation and degradation are required to control and predict wear generated by fretting. Extensive studies focussed on the nature of TTSs depending on several metallic contacts (steels, aluminium alloys, α-, β-, or α+β-titanium alloys,xa0…) and different testing conditions (load, sliding amplitude, number of cycles, environment). Powerful analytical tools were utilized to determine the TTS composition and structure. TTS appeared as a nanocrystalline structure, corresponding to the chemical composition of the initial material and made of the more stable structure in accordance with the equilibrium diagram. No specific effects of oxygen or hydrogen was detected. In the present article, experimental results will be discussed to point out possible mechanisms of the formation of the TTS. An analogy with butterflies which form under rolling fatigue will be outlined. At last, an energy approach to explain formation will be presented for the case of low-alloy steel. It is demonstrated that TTS formation is related to a critical cumulative plastic deformation associated with a specific threshold dissipated energy.

Mechanism of boundary lubrication with zinc dithiophosphate

Abstract Experiments were carried out with pure zinc dithiophosphate in paraffin as a lubricant. Different friction tests using smooth and rough surfaces were studied by surface analysis (Auger electron spectroscopy and secondary ion mass spectroscopy) and lubricant analysis (thin layer chromatography). Four types of surface transformation were detected: soft track, cutting track, delamination and adherent film. The wear rate was minimal when an adherent film was formed on steel and cast iron surfaces. The role of the additive in the formation of the film is outlined. Surface products play an important part in the production of a paste in the contact zone. This paste, which is compressed in the interface, is the dominant factor affecting wear behaviour.

A Computational Chemistry Study on Friction of h-MoS2. Part I. Mechanism of Single Sheet Lubrication

In this work, we theoretically investigated the friction mechanism of hexagonal MoS(2) (a well-known lamellar compound) using a computational chemistry method. First, we determined several parameters for molecular dynamics simulations via accurate quantum chemistry calculations and MoS(2) and MoS(2-x)O(x) structures were successfully reproduced. We also show that the simulated Raman spectrum and peak shift on X-ray diffraction patterns were in good agreement with those of experiment. The atomic interactions between MoS(2) sheets were studied by using a hybrid quantum chemical/classical molecular dynamics method. We found that the predominant interaction between two sulfur layers in different MoS(2) sheets was Coulombic repulsion, which directly affects the MoS(2) lubrication. MoS(2) sheets adsorbed on a nascent iron substrate reduced friction further due to much larger Coulombic repulsive interactions. Friction for the oxygen-containing MoS(2) sheets was influenced by not only the Coulomb repulsive interaction but also the atomic-scale roughness of the MoS(2)/MoS(2) sliding interface.

Transfer films and friction under boundary lubrication

The role of transfer phenomena in the mechanisms of friction reduction by organic molybdenum compounds is studied with the aid of ultrahigh vacuum (UHV) analytical tribometry. Additives used are zinc dithiophosphate (Zndtp), molybdenum dithiophosphate (Modtp), molybdenum dithiocarbamate (Modtc) and Modtc/Zndtp combinations. Experiments involve UHV friction tests on tribofilms formed previously and in situ surface analyses by Auger electron spectroscopy (AES) and imaging. In the presence of Modtc, friction reduction was found to be associated with the transfer of highly dispersed MoS2 as isolated sheets, from the tribofilm to the slider. In the presence of Modtp (or the mixture Modtc/Zndtp), a two-step tribochemical reaction is generally observed. First, phosphate from the film is transferred to the oxide on the pin and the friction coefficient is about 0.3, after an induction period of a few cycles, pure MoS2 single sheets are transferred to the pin and the phosphate is eliminated as wear debris from the contact zone. Friction is then at its lowest value (0.02). The chemistry of the transfer phenomena is modeled using the hard and soft acid and base (HSAB) principle as described by Pearson. The overall data suggest that friction behavior under boundary lubrication with additives may be directly related to molecular scale transfer mechanisms in general.

Synergistic effects in binary systems of lubricant additives: a chemical hardness approach

Tribochemical interactions between antiwear zinc dithiophosphate (Zndtp), friction modifier molybdenum dithiocarbamate (Modtc) and detergent overbased calcium borate (CB) lubricant additives have been investigated by coupling analytical TEM and micro‐spot XPS in the tribotester Optimol of SRV GmbH (mild wear conditions in boundary lubrication). Synergistic effects have been observed on both friction and wear data, especially in the Modtc/Zndtp combination. Results have been interpreted on the basis of a chemical hardness concept: the hard and soft acids and bases (HSAB) principle, stabilisation of hard–hard pairs and the maximum hardness principle. The performance of the Modtc/Zndtp mixture is mainly due to the generation of MoS2 single sheets and the digestion of MoO3, which is also formed, by the zinc polyphosphate glass. The final result of the tribochemical reaction is a tribofilm composed of MoS2 sheets embedded in a mixed Mo/Zn polyphosphate glass. The CB/Modtc mixture has a similar mechanism except that the oxide is not completely eliminated, due to the softer borate anion compared with the phosphate one.

A Computational Chemistry Study on Friction of h-MoS2. Part II. Friction Anisotropy

In this work, the friction anisotropy of hexagonal MoS(2) (a well-known lamellar compound) was theoretically investigated. A molecular dynamics method was adopted to study the dynamical friction of two-layered MoS(2) sheets at atomistic level. Rotational disorder was depicted by rotating one layer and was changed from 0° to 60°, in 5° intervals. The superimposed structures with misfit angle of 0° and 60° are commensurate, and others are incommensurate. Friction dynamics was simulated by applying an external pressure and a sliding speed to the model. During friction simulation, the incommensurate structures showed extremely low friction due to cancellation of the atomic force in the sliding direction, leading to smooth motion. On the other hand, in commensurate situations, all the atoms in the sliding part were overcoming the atoms in counterpart at the same time while the atomic forces were acted in the same direction, leading to 100 times larger friction than incommensurate situation. Thus, lubrication by MoS(2) strongly depended on its interlayer contacts in the atomic scale. According to part I of this paper [Onodera, T., et al. J. Phys. Chem. B 2009, 113, 16526-16536], interlayer sliding was source of friction reduction by MoS(2) and was originally derived by its material property (interlayer Coulombic interaction). In addition to this interlayer sliding, the rotational disorder was also important to achieve low friction state.

The nature and origin of wear particles from boundary lubrication with a zinc dialkyl dithiophosphate

Abstract It is well known that in many tribological situations antiwear additives act by reaction film formation on contacting surfaces. In the present paper we describe our observations on the reaction film formed with a zinc dialkyl dithiophosphate and the wear particles. Detailed structural and chemical information has been obtained by using optical microscopy, scanning electron microscopy, transmission electron microscopy and X-ray energy spectrometry. The results show a clear connection between the reaction film material and the nature of the wear particles present in the lubricant; both a highly dispersed phase system and a layered structure are present. The origin of the wear particles is discussed in terms of crack initiation mechanisms.

Friction-Induced Amorphization with ZDDP—An EXAFS Study

Many experimental works are concerned with the microinvestigation of interface bodies created during the wear process (especially wear particles). Previous works wing STEM have shown that ZDDP wear protection is associated with the structureless nature of wear debris, so that friction-induced amorphization was a key factor to obtain the antiwear conditions. In this paper, the synchrotron radiation, EXAFS (Extended X-ray Absorption Fine Structure) is used to probe the local environment of atoms (Fe and Zn) in interface ZDDP films. Results clearly show the fully amorphous character of these materials. Consequences on wear models are then discussed. Presented as an American Society of Lubrication Engineers paper at the ASLE/ASME Tribology Conference in Atlanta, Georgia, October 8–10, 1985

Real Time TEM Imaging of Compression and Shear of Single Fullerene-Like MoS2 Nanoparticle

The deformation and degradation behavior of single inorganic fullerenes nanoparticles of MoS2 under compression and shear has been observed in real time using a high-resolution transmission electron microscope equipped with a nanoindentation holder. The MoS2 nanoparticles were compressed using a nanoindenter and a truncated diamond tip. For the first time, real time imaging of the deformation of individual nanoparticles clearly shows first orientation changes in the particle shape during loading process followed by a large deformation and the exfoliation of the outer sheets of the fullerene nested structure. Exfoliation was observed for a contact pressure estimated at 1xa0GPa. Additional sliding tests performed with the nanoindenter gave evidence for a rolling process for lower contact pressures up to 100xa0MPa.

Triboscopy, a new approach to surface degradations of thin films

Abstract The microscopic description of an interface and its degradations under sliding give access to phenomena involved in wear processes. More precisely, on a length scale of 10 −3 –10 −6 m, many different mechanisms can occur: formation of scratches and cracks, deformation, emission of debris, their recirculation and reattachment etc. This information is particularly important for knowledge of the resistance of a coated material subjected to wear. We have developed a new method based on the coupling of a tribometer and a numerical image processor, here called “triboscopy”. Its major interest is the synthetic representation of the behaviour of an interface during reciprocating sliding, using one datum or several data from the test. We present the first results obtained on two different polymer-coated surfaces. We show the different degradation modes occurring and how these are demonstrated by the triboscopic images.