This paper focuses on the ZnDTP-derived tribofilm formation process in the early stage of sliding, mainly on the formation of a sulfide layer and a polyphosphate layer, in studying the formation ...process on a steel surface with roughness equivalent to that of an actual machine. Unlike the results of ZnDTP single-formulated oil using a regular mirror-finished steel test piece, the sulfur-based tribofilm was preferentially formed at the initial sliding stage on the steel test piece with relatively large roughness equivalent to the actual machine level. It was also found that it took longer to form a phosphorus-based tribofilm that exhibited wear resistance. Thus, the behavior of tribofilm formed on the test pieces with roughness equivalent to the actual machine level differed significantly from that of the mirror-finished test pieces. In particular, it was found that the sulfurization reaction in the early stage of sliding is important through the corrosive wear of the top of roughness, and played an important role in controlling the subsequent tribological behavior.
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 MoS
2 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 MoS
2 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.
Organophosphates are well-known as the canonical additives for lubricants. Thus, understanding of the additive behaviour is a key aspect in the design of films on metal surfaces. Different types of ...phosphates are added to improve their antiwear properties, but the contributions of individual esters to these properties has not been studied using a combination of practical and theoretical approaches. In this study, organophosphates were isolated with high purity and their tribological characteristics were evaluated by using a Bowden-type reciprocating friction tester and a four-ball wear tester. Mono-oleylphosphate had a lower friction than di-oleylphosphate and exhibited excellent antiwear characteristics. Analysis of the sliding surfaces using desorption electrospray ionization-mass spectrometry (DESI-MS) and X-ray photoelectron spectroscopy (XPS) indicated that the film structure could predict the occurrence factor of the tribological characteristics of the oleylphosphates. Then the adsorption energies of the monoester on iron and iron oxide surfaces were higher than those of the diester, as assessed using density functional theory (DFT) calculations, owing to the difference in their chemisorption processes, as confirmed by further DFT analysis. Studies on the reactivity of additives and their interactions with surfaces are important for understanding the tribochemistry of additives.
Tantalum is known to be a highly corrosion resistant refractory metal. Boronized tantalum has potential use in machinery components, structures and as a bearing material in joint implants due to its ...high corrosive resistance and hardness. In this research, the friction and wear characteristics of boronized tantalum were studied against bearing steel E52100 under dry and simulated body fluid (SBF) conditions. Results showed that SBF caused tribochemical reactions leading to an increase in friction corresponding with amorphous debris. Fatigue cracks formed under dry conditions and tribochemical wear cracks formed under SBF conditions.
Overbased detergents are well known in the tribology field as anti-wear additives. In boundary lubrication, they generate a quite thick tribofilm on rubbing surfaces. They were studied by coupling ...XPS and AES depth profiles with XANES and ToF-SIMS analyses. Under friction, we show by ToF-SIMS analysis that detergent molecules are split into smaller structural units. Moreover, ionic bonds do not resist high pressure and shearing, and sulfur disappears from the contact zone. The overbased calcium carbonate core finally collapses and crystallizes to give a good anti-wear film between rubbing surfaces.
The new approach to the kinetics of triboreactions, based on a mathematical model denoted as
α
i
model implies energy emitted by the surface as impulses in the range of 1–5 eV. This communication ...looks for a better understanding of the mechanism of triboreactions by explaining the
α
i
model, proposed previously in terms of the thermodynamics of irreversible processes. The reaction rate
J
c
found by non-equilibrium thermodynamics is also consistent with two superimposed pathways for tribochemical reactions. In the former, the reaction follows the thermal mechanism typical of no friction conditions, in the latter, direct transfer of energy from triboelectrons to molecules generate special excited or activated molecules A*, such as radicals or ions which react rapidly to form the products, enhancing the global reaction rate.
X-ray absorption near-edge structure spectroscopy (XANES) has been used to investigate the chemistry and thickness of thermal and antiwear (AW) films generated on steel from oil solutions containing ...phosphate ester additives. DPP, a diaryl phosphate, reacted with steel to form a thermal phosphate film at lower temperatures than TPP, a triaryl phosphate and Irgalube 349, an amine phosphate. This phosphate film formation at lower temperatures resulted in better wear protection to the metal in tribochemical experiments, as indicated by a smaller wear-scar measurement for oil solutions containing the DPP additive. For TPP, a brief period of wear to the metal was necessary to initiate the tribochemical reaction between the additive and substrate. Once the tribochemical reaction begins, TPP is able to generate a tribochemical film of relatively the same thickness and chemistry as DPP. Irgalube 349 generated the thickest thermal films at temperatures greater than 150 °C, significantly thicker than any of the films generated from DPP and TPP. The substantial difference in thickness is believed to be due to the availability of alkyl/ammonium cations which enables continued growth of the phosphate film.
Over the decades, the application of mechanical force to influence chemical reactions has been called by various names: mechanochemistry, tribochemistry, mechanical alloying, to name but a few. The ...evolution of these terms has largely mirrored the understanding of the field. But what is meant by these terms, why have they evolved, and does it really matter how a process is called? Which parameters should be defined to describe unambiguously the experimental conditions such that others can reproduce the results, or to allow a meaningful comparison between processes explored under different conditions? Can the information on the process be encoded in a clear, concise, and self-explanatory way? We address these questions in this Opinion contribution, which we hop
The antiwear properties of zinc dialkyldithiophosphate (ZDDP), dispersants, and mixtures of ZDDP and different dispersants have been evaluated using a pin-on-flat Plint wear machine. Tribochemical ...interactions between ZDDP and dispersants have been investigated under boundary lubrication conditions by means of X-ray absorption near edge structure (XANES) spectroscopy, probing the phosphorus, sulfur and nitrogen absorption edges. The results show that the dispersants do not give any wear protection by themselves in the base oil. The dispersants also do not affect the antiwear property of ZDDP under the given testing conditions. The N K-edge XANES analysis indicates that dispersants contribute to the chemical composition of the tribofilms and form mixed ammonium/zinc polyphosphates. Phosphorus in the tribofilms is present mainly in the form of medium-chain polyphosphate on the surface and short-chain polyphosphate in the bulk. Sulfur appears in the tribofilms mainly as sulfide S-II, possibly zinc sulfide. The presence of dispersants in oil blends does not disturb the polyphosphate (and sulfide) formation, but it does decrease the chain length of the polyphosphate in the tribofilms.
Triboelectrochemical techniques use an electrochemical set-up (mainly of the three-electrode type) for controlling the potential of the surface of a conducting material subjected to rubbing in a ...tribometer. In this way it is possible to carry out friction and wear tests in electrolytic solutions under well-defined chemical conditions determined by the applied electrode potential. In addition, triboelectrochemical techniques offer the possibility of following in-situ and in real time the kinetics of electrochemical oxidation reactions (corrosion) by the simple measure of an electrical current. In the present study triboelectrochemical experiments were carried out on sputter deposited TiN coatings sliding against alumina in a borate solution (pH 8.4). The surface of selected worn coatings was characterised by X-ray photoelectron spectroscopy (XPS) and the topography by scanning electron microscopy (SEM). Results show that the rate of wear critically depends on the prevailing (electro)chemical conditions which determine the chemical surface state of the TiN coating. The behaviour is attributed to the lubricating properties of surface oxide films having a thickness in the nanometre range.
