This paper presents insights into how Zinc Dialkyl Dithiophosphate (ZDDP) influences the formation and progression of micropitting. Experimental investigations were conducted using a twin-disc ...tribometer in rolling-sliding contacts under mixed or boundary lubrication conditions, focusing on the impact of ZDDP on micropitting in bearing steel samples. Results show that ZDDP reduces wear and increases surface friction by forming a tribofilm. This facilitates micropitting formation, but also retards the progression of micropitting. It highlights that understanding the chemical and mechanical interactions at the tribological interface is essential for designing effective mitigation strategies for managing micropitting and avoiding related issues in bearing applications.
Effects of the structure of zinc dialkyldithiophosphates (ZDDPs) on the tribological properties of a tetrahedral amorphous carbon (ta-C) film were investigated. Friction tests were performed at ...diamond-like carbon (DLC)/DLC and steel/steel contacts under lubrication with poly-alpha olefin (PAO) and PAO containing six types of ZDDPs with different alkyl groups. The tribological properties of the ta-C film depended on the chemical composition of ZDDP-derived tribofilms on the outermost surface, which depended on the abundance ratio of neutral-ZDDP and basic-ZDDP in the ZDDP solution. Thus, the ZDDP forms play important roles in the tribological properties of the ta-C film. All these results suggest that the ZDDP forms can determine the tribological properties of the ta-C film lubricated with the ZDDP solution.
•Frictional properties of ta-C do not depend on the alkyl chain length of ZDDPs.•Tribological properties of ta-C depend on the chemical composition of tribofilms.•ZDDP forms in solution play important roles in tribological properties of ta-C.
The ongoing trend for using ever lower viscosities of lubricating oils, with the aim of improving the efficiency of mechanical systems, means that machine components are required to operate for ...longer periods under thin film, mixed lubrication conditions where the risk of surface damage is increased. For this reason, the role of zinc dialkyldithiophosphate (ZDDP) antiwear lubricant additive has become increasingly important in order to provide adequate surface protection. It is known that due to its exceptional effectiveness in reducing surface wear, ZDDP may promote micropitting by preventing adequate running-in of the contacting surfaces. However, the relationship between ZDDP tribofilm growth rate and the evolution of micropitting has not been directly demonstrated. To address this, we report the development of a novel technique using MTM-SLIM to obtain micropitting and observe ZDDP tribofilm growth in parallel throughout a test. This is then applied to investigate the effect of ZDDP concentration and type on micropitting.
It is found that oils with higher ZDDP concentrations produce more micropitting but less surface wear and that, at a given concentration, a mixed primary-secondary ZDDP results in more severe micropitting than a primary ZDDP. Too rapid formation of a thick antiwear tribofilm early in the test serves to prevent adequate running-in of sliding parts, which subsequently leads to higher asperity stresses and more asperity stress cycles and consequently more micropitting. Therefore, any adverse effects of ZDDP on micropitting and surface fatigue in general are mechanical in nature and can be accounted for through ZDDP's influence on running-in and resulting asperity stress history. The observed correlation between antiwear film formation rate and micropitting should help in the design of oil formulations that extend component lifetime by controlling both wear and micropitting damage.
•Growth of ZDDP films and evolution of micropitting are observed simultaneously.•Anti-wear formulations that result in fast tribofilm growth promote micropitting.•Inadequate anti-wear formulations suppress microptting but result in excessive wear.•Optimal formulations are able to control both wear and micropitting.•Anti-wear additives are shown to affect contact stress history and hence surface fatigue damage.
Zinc dialkyldithiophosphate (ZDDP) anti-wear lubricant additive contributes to significant harmful emissions and its replacement has become a priority. In this study, green silica nanoparticles were ...synthesized and optimized for concentration to serve as hybrid additives alongside ZDDP in lubricants, aiming to mitigate environmental concerns. A combination of 0.5 wt% silica nanoparticles, 0.5 wt% oleic acid (OA), and 0.75 wt% ZDDP demonstrated notable improvements, reducing ZDDP usage by ∼25 % while achieving significant tribological enhancements. At room temperature, coefficient of friction (COF) was reduced by ∼60 %, and wear was reduced by ∼70 %, while at 100 °C, COF was reduced by ∼80 %, and wear was reduced by ∼96 %. Advanced characterization techniques such as FESEM with EDS mappings, EPMA, and TOF-SIMS were employed to elucidate the underlying mechanisms for observed reduction in friction and wear.
•A combination of silica nanoparticles and ZDDP is explored as hybrid additive to PAO4 oil.•Conventional ZDDP addition is reduced by 25 % in hybrid additive containing base oil.•Friction reduction of ∼ 60 %, and wear reduction of ∼ 70 % at room temperature using hybrid additive.•Friction reduction of ∼ 80 %, and wear reduction of ∼ 96 % at 100 °C using hybrid additive.•Plausible mechanisms of surface protection using the hybrid additive are proposed.
The effect of atmospheric gas on zinc dialkyldithiophosphate (ZDDP) tribofilm formation was investigated. Macroscale friction tests and in-situ atomic force microscopy (AFM) observations were carried ...out in both normal air and argon. In argon, the tribofilms grew slowly although the initial formation stages were similar to those of ones formed in normal air. Furthermore, after prolonged friction, the tribofilms gradually became smooth and wore away. Oxygen in the lubricant is thus important for ZDDP tribofilm formation, and a sufficient supply of oxygen is necessary to maintain the anti-wear performance of ZDDP.
•The effect of atmospheric gas on ZDDP tribofilm formation was investigated.•ZDDP tribofilm formation in argon was visualised using in-situ AFM observation.•ZDDP tribofilms formed in argon were prone to wear.•They had longer polyphosphate chains and a high sulphur rate.•A sufficient supply of oxygen is necessary to maintain the anti-wear property of ZDDP.
The designed synthesis of ionic liquid-type organic molybdenum (IOMo) compounds was investigated for their tribological performance as additives in alkylated naphthalene base oil, and their ...performance was compared with commercial additives. When IOMo is combined with ZDDP as additive, they synergistically enhance its anti-friction properties and further improve its anti-wear performance effectively. By analyzing worn surface, coupled with the adsorption performance of the additive, the "adsorption-reaction-film formation" lubrication mechanism is proposed. IOMo forms strongly adsorbed ultra-thick adsorption film on the friction pair surface through electrostatic adsorption, effectively avoiding the running-in wear stage. During the friction process, frictional chemical reactions facilitate the formation of a complex boundary lubrication film, thereby demonstrating excellent anti-friction and anti-wear capabilities across the entire lubrication range.
•Ionic liquid-type organic molybdenum (IOMo) was designed and synthesized.•IOMo exhibits outstanding solubility and stability in the AN30 base oil.•Cations and anions in IOMo adsorb on metal surfaces, forming multilayer film.•Combining IOMo with ZDDP enhances friction reduction and antiwear synergistically.•The unique boundary lubrication mechanism boosts IOMo and ZDDP performance in AN30.
Metal oxide nanoparticles as lubricant additives have attracted much attention, thanks to their excellent anti-wear ability and load-carrying capacity. Unfortunately, organic molybdenum, a small ...molecule auxiliary of zinc dialkyldithiophosphate (ZDDP) with high reactivity, exhibits poor load-bearing capacity. Therefore, molybdenum oxide nanoparticle (denoted as MONP) was surface-capped by dodecylamine (DDA) and maleic anhydride dodecyl ester (MADE) to obtain DDA/MADE-MONP, an organic-inorganic nanohybrid as a potential replacement of sulfur- and phosphorus-free commercial organic molybdenum additive (Organic-Mo). The as-prepared DDA/MADE-MONP nanohybrid was characterized by X-ray diffraction, transmission electron microscopy, and Fourier transform infrared spectroscopy; and its effect on the load-bearing capacity of an alkylated naphthalene base oil (AN30), as used alone and combined with ZDDP, was investigated. Results show that, whether used alone or combined with ZDDP, the as-prepared DDA/MADE-MONP has better load-bearing capacity than the commercial organic molybdenum additive. The reason lies in that DDA/MADE-MONP can release MoO3 nanocores during the friction process to fill up the pits on the worn steel surface and form a dense protective tribofilm.
In this study, a bio-based ester was experimentally tested under a reciprocating contact in terms of the tribological performance to assess the potential of bio-based materials as boundary ...lubrication additives for low Sulphated Ash, Phosphorus, and Sulphur (SAPS) lubricants. A synergy in tribological performance has been observed when the ethyl oleate was used with a low concentration of ZDDP, showing both lower friction and wear loss, compared to only ethyl oleate or Zinc dialkyldithiophosphates (ZDDP) as single additives in base oil. SEM-EDX and ToF-SIMS analyses confirm that the additive combination of ester and low-concentration ZDDP generates a more evenly distributed tribo-film than that for only ZDDP as an additive. Although the tribo-film thickness of the ester/ZDDP lubricant combination, measured with AFM, issmaller than that for only low-concentration ZDDP, the combination shows superior wear protection ability over ZDDP. Mass spectra from ToF-SIMS show that the main component of the film formed by the additive combination is zinc polyphosphate, while the film generated from low-concentration ZDDP consists of short-chain iron phosphate. It is proposed that the ethyl oleate present in the additive combination alleviates severe shear in the contact, which promotes the formation of a uniform tribo-film composed of glassy zinc polyphosphate. The formed tribo-film enhances the adsorption of ester fragments, improving the tribological performance compared to using only ester as an additive. The obtained results demonstrate the potential of bio-based esters to reduce ZDDP usage and move towards greener lubricants.
•Ethyl oleate is first investigated as a lubricant additive with ZDDP under reciprocating contacts.•Synergism for the additive combination is confirmed, with low friction and wear.•Ethyl oleate promotes the formation of uniformly distributed ZDDP tribo-film.•Ester molecular fragments deposited on ZDDP film improve tribology performance further.
Micro-pitting presents a failure of the rolling/sliding contact metal asperities operating under boundary/mixed lubrication conditions. The studies have shown that micro-pitting failure competes with ...mild wear and that lubricant additives can have either detrimental or beneficial effects on micro-pitting evolution. This article describes a methodology to investigate micro-pitting damage on bearing steels using a twin-disc machine to better represent mechanical components, i.e. bearings, crankshafts, etc. In addition, effects of roughness, hardness and the ZDDP additive are presented and discussed. A sufficient hardness difference can completely eliminate micro-pitting damage mode. Furthermore, the presence of ZDDP anti-wear additive in fully formulated engine oil was shown to protect rougher surfaces and promote wear on smoother surfaces, thus completely eliminating the micro-pitting damage mode.
•Methodology to develop and assess micro-pitting damage of bearing steel surfaces.•Higher hardness leads to higher micro-pitting damage but lower wear.•Sufficient hardness difference can completely eliminate micro-pitting damage mode.•The presence of ZDDP additive in fully formulated engine oil promotes wear.
In order to overcome the mismatching between traditional additives and DLC coatings in solid-liquid lubricating system. Cu nanoparticles modified with diisooctyl dithiophosphoric acid (NPCuDDP) were ...prepared and used as nano-additives in DLC/PAO solid-liquid lubricating systems. Four kinds of DLC coatings including amorphous carbon (a-C) and amorphous carbon doped with Si (a-C(Si)), Al (a-C(Al)) and H (a-C(H)) respectively, with a hardness range from 22 GPa to 7 GPa were used to study the tribological mechanism and adaptability of NPCuDDP to the hardness and composition of DLC coatings. It was found that NPCuDDP reduced the COF of DLC coatings/PAO systems by 19%–22% regardless of their hardness and composition and made the wear rate of all DLC coatings decrease by 2–3 orders of magnitude. While the composite of tribofilm was decided by the doped elements in DLC coatings. The doped element H inhibited the oxidation of copper in tribofilm, and led to the lowest wear rate of a-C(H) coating. The doped element Al in a-C(Al) facilitated the enrichment of phosphates on friction surface. The hardness rather than composition of DLC coatings determines whether traditional additive ZDDP can form tribofilm. ZDDP can only form Zinc polyphosphate tribofilm on the steel balls rubbed with a-C, a-C(Si) and a-C(Al) coatings whose hardness was over 12 GPa. The friction coefficient of the system increases greatly because of the high strength of ZDDP tribofilm. The high activity of soft metal nano-particle in NPCuDDP urged the formation of tribofilm on friction surface easily and produced a significant antifriction and antiwear effect, and is expected to replace traditional small molecule additives as the high-efficiency special lubricating oil additives for DLC coatings solid-liquid lubrication system.
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•Copper nano-additives exhibit extensive adaptability to four kinds of DLC coatings with a hardness range of 7–22 GPa.•The wear rate of all DLC coatings decreased by 2–3 orders of magnitude due to copper nano-additives.•The doped element H inhibited the oxidation of copper in tribofilm, and led to the lowest wear rate of a-C(H) coating.•The doped element Al in a-C(Al) facilitated the enrichment of phosphates on friction surface.•The hardness rather than composition of DLC coatings determines whether traditional additive ZDDP can form tribofilms.
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