Friction response of ZDDP thermal film and tribofilm

Abstract

An original dynamic tribotest was used to study the friction response of two kinds of tribological surface: zinc-dialkyl-dithio-phosphate (ZDDP) thermal film and ZDDP tribofilm. This apparatus allows friction to be determined with no need for measuring tangential force. A velocity dependent friction contribution was found in each case in addition to a velocity independent one, showing the viscous character of such films. Furthermore, during test on ZDDP thermal film, the viscous damping represents 18% of the total dissipated energy as it is 16% for ZDDP tribofilm.     

Non-ferrous coating/lubricant interactions in tribological contacts: Assessment of tribofilms

Use of low friction non-ferrous coatings for engine tribo-components exposed to boundary lubrication is becoming popular in automotive industries. The excellent tribological behaviour of some non-ferrous coatings also reduces dependence on some harmful components of lubricants. In this work, hydrogenated diamond like carbon (HDLC) and chromium nitride (CrN) coatings sliding against cast iron counterbody have been used to study the interaction with friction modifiers (Moly dimer and Moly trimer) and antiwear additive zinc dialkyldithiophosphate (ZDDP) under boundary lubrication condition. The tribological results of the non-ferrous coatings are compared with those of uncoated steel. Tribofilms are formed using a reciprocating pin-on-plate tribometer. The chemical analysis of the tribofilms has been accomplished using X-ray photoelectron spectroscopy (XPS). The XPS analysis shows that the friction modifiers form a low friction tribofilm on the non-ferrous coatings. No antiwear tribofilm derived from ZDDP was observed on the HDLC coating but a stable antiwear tribofilm was found on the CrN coating. Moly dimer together with ZDDP+Base Oil showed the lowest friction coefficient for the CrN coating while Moly trimer along with ZDDP+Base Oil gave the lowest friction for the HDLC coating. This study will investigate the generic differences between the tribofilms formed on the DLC and CrN coatings by two additive-containing oils.     

Understanding the composition and low friction tribofilm formation/removal in boundary lubrication

The thickness of a tribofilm formed from additives in a lubricant is determined by the nature of a dynamic process which involves film formation and film removal. In this work, the removal rate of tribofilms is studied using a distinctive testing procedure. Tribofilms are produced using a pin-on-reciprocating plate tribometer with lubricants comprising a combination of base oil (PAO6), zinc dialkyldithiophosphate (ZDDP) and molybdenum dialkyldithiocarbamate (MoDTC) additives. In some tests the same lubricant is used throughout the test to assess the formation of the tribofilm. However, an important aspect of the study is the measurement of the tenacity of the tribofilm and to do this the lubricant was changed after a tribofilm had formed, to a lubricant free from additives. Tribofilms (fully developed and partially removed) have been chemically characterised using energy dispersive X-ray analysis (EDX). Qualitative and quantitative information regarding the tenacity of the low friction tribofilms is obtained. Surface analyses have shown that there is a distinct link between chemical composition of the tribofilms formed and their tenacity. The low friction tribofilm formed from the lubricant with ZDDP and MoDTC was shown to be more durable than the one formed from the lubricant with only MoDTC. The mechanism of how the ZDDP tribofilm provides this durability is discussed.     

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.     

Tribological performances of ZDDP and ashless triphenyl phosphorothionate (TPPT) as lubricant additives on Ti–N and Ti–Al–N coated steel surfaces

Abstract

In recent years, there has been much attention on the effects of lubricant additives on the friction and wear properties of surface coatings. However, little research has been conducted to investigate the influence of antiwear additives on the tribological performances of titanium nitride (Ti–N) and titanium aluminium nitride (Ti–Al–N) coatings. It has been reported that introducing aluminium into Ti–N coatings enhanced their oxidation resistance. In this study utilising a pin on cylinder tribometer, lubricants containing zinc dialkyl dithiophosphate (ZDDP) or a more environmentally friendly alternative, ashless triphenyl phosphorothionate (TPPT), were used. Experimental results revealed that ZDDP and TPPT helped to reduce wear on both coatings through the formation of a tribofilm, although it was also found that both additives increased the friction coefficient on both surfaces. Based on overall findings, this paper suggests the use of TPPT as a suitable ZDDP replacement for providing wear protection on Ti–N and Ti–Al–N coatings.     

Experimental simulation of chemical reactions between ZDDP tribofilms and steel surfaces during friction processes

Friction tests are performed in a controlled environment (Ultra High Vacuum), between steel surfaces and a ZDDP tribofilm at different contact severities. According to AES analyses, evidence of chemical reactions activated by friction is given. The reaction of the ZDDP tribofilm with the native iron oxide could partially explain its antiwear behavior.     

Influence of Dispersant and ZDDP on Soot Wear

Diesel engines and gasoline direct injection (GDI) engines both produce soot due to incomplete combustion of the fuel and some enters the lubricant where it accumulates between drain intervals, promoting wear of rubbing engine components. Currently the most favoured mechanism for this wear is that the anti-wear additives present in engine oils, primarily zinc dialkyldithiophosphates (ZDDPs), react very rapidly with rubbing surfaces to form relatively soft reaction products. These are easily abraded by soot, resulting in a corrosive-abrasive wear mechanism. This study has explored the impact of engine oil dispersant additives on this type of wear using combinations of dispersant, ZDDP and carbon black, a soot surrogate. It has been found that both the concentration and type of dispersant are critical in influencing wear. With most dispersants studied, wear becomes very high over an intermediate dispersant concentration range of ca 0.1–0.4 wt% N, with both lower and higher dispersant levels showing much less wear. However a few dispersants appear able to suppress high wear by ZDDP and carbon black over the whole concentration range. A series of experiments have been carried out to determine the origin of this behaviour and it is believed that high levels of dispersant, and, for a few dispersants, all concentration levels, protect the iron sulphide tribofilm initially formed by ZDDP from abrasion by carbon black.     

Durability of ZDDP Tribofilms Formed in DLC/DLC Contacts

The tribological properties of diamond-like carbon (DLC) coatings have drawn much attention of OEMs and lubricant manufacturers in recent years. It is important to know whether conventional friction modifier and antiwear additives can form durable films and work as effectively with DLCs as they normally do on steel surfaces. In this study, the film-forming and friction properties of the antiwear additive ZDDP and the strength of tribofilms formed by this additive on five widely used DLC types, namely a-C:H, a-C:H:W, a-C:H:WC, Si-DLC and ta-C, have been investigated. It is found that ZDDP-derived tribofilms form on all the DLCs but exhibit different friction characteristics based on DLC type. With all DLCs, the amount of tribofilm elements measured after durability tests was less than that measured initially. Over 90 % of thiophosphate and 70 % of sulphide/sulphate were lost during durability tests. ZDDP tribofilms were found to be strongly adhered on Si-DLC and a-C:H compared with the other DLCs. The ZDDP tribofilms formed in DLC/DLC contact appear to be similar in structure to those formed in steel/steel contact but not to exhibit the antiwear performance seen in steel/steel contacts.     

Tribological behavior and nature of tribofilms generated from fluorinated ZDDP in comparison to ZDDP under extreme pressure conditions—Part II: Morphology and nanoscale properties of tribofilms

Tribofilms generated using lubricants containing zinc dialkyl dithiophosphate (ZDDP) and fluorinated zinc dialkyl dithiophosphate (F-ZDDP) were examined. Nanoindentation yielded reduced elastic modulus and hardness as a function of the penetration depth of the nano-indenter for different loading conditions. The mechanical strength of the tribofilm was evaluated by conducting scanning wear and nano-scratch tests. Results indicate that load used to create the tribofilms plays an important role in determining their hardness with the tribofilms formed at higher loads exhibiting higher hardness. Tribofilms formed from F-ZDDP were more resistant to scratch and wear compared to ZDDP tribofilms. FIB measurements indicate F-ZDDP tribofilms were thicker than ZDDP tribofilms.     

Interaction of ZDDP with Borated Dispersant Using XANES and XPS

The thermochemical reaction and tribochemical reaction of zinc dialkyldithiophosphate (ZDDP), a borated dispersant, and the mixture of ZDDP and borated dispersant on steel surfaces were investigated. Both pin-on-disk and ball-on-disk were used to generate tribofilms. The chemical state of nitrogen, boron, phosphorus, and sulfur in heated oil solutions, thermal films, and tribofilms were analyzed by X-ray absorption near edge structure (XANES) spectroscopy to obtain the chemical nature of species on the surface and in the bulk of the films. High-resolution X-ray photoelectron spectroscopy (XPS) has also been used to analyze boron (B) in tribofilms.

The borated dispersant in base oil by itself yields good anti-wear behavior. This can be attributed to the presence of boron in the dispersant. The wear scar widths (WSW) for ZDDP alone, and in combination with the dispersant, yield similar results within the experimental error. It was found that the borated dispersant facilitates the decomposition of ZDDP and the formation of phosphate in tribofilms and thermal films. B K-edge XANES shows that boron has a trigonal coordination in the untreated additive, but the coordination changes partially to a tetrahedral coordination in the tribofilm upon rubbing. No BN was detected in the film analyzed by B K-edge or N K-edge. Boron 1s XPS also did not show the presence of BN in the film.     

ZDDP and MoDTC interactions in boundary lubrication—The effect of temperature and ZDDP/MoDTC ratio

Tribofilms formed under boundary lubrication from ZDDP and MoDTC additives alone or in different ratios in the lubricant have been studied. The tribological performance is linked to the tribofilm properties and consequently to the lubricating conditions. Tribofilms are formed using a reciprocating pin-on-plate tribometer. Surface sensitive analytical techniques, such as energy dispersive X-ray analysis (EDX) and X-ray photoelectron spectroscopy (XPS) have been used for tribofilm characterisation. The XPS peaks have been deconvoluted to characterise the species formed in the wear scar. The formation of species with different tribological properties, due to the decomposition of ZDDP and MoDTC molecules as a result of testing temperature, is shown. Surface analyses have shown that MoDTC decomposes, even in low-lubricant bulk temperature tests (30 °C), forming the same species as in high-lubricant bulk temperature tests (100 and 150 °C) but the tribofilms give different tribological performance. The effectiveness in friction reduction is shown to depend on the ratio between what are defined as high- and low-friction species in the tribofilm.     

Lubrication of carbon coatings with MoS2 single sheet formed by MoDTC and ZDDP lubricants

The fuel economy and reduction of harmful elements of lubricants are becoming important issues in the automotive industry. One approach to these requirements is the potential use of low‐friction coatings in engine components exposed to boundary lubrication conditions. Diamond‐like carbon (DLC) coatings, extensively studied as ultra‐low friction films to protect ductile metals surfaces for space applications, are expected to fit the bill. The main purpose of this work is to investigate the friction and wear properties of DLC coatings lubricated with molybdenum dithiocarbamate (MoDTC) and zinc dithiophosphate (ZDDP) under boundary lubrication conditions. The mechanisms by which MoDTC reduces the friction in the centirange were studied using ultra‐high vacuum (UHV) analytical tribometer. The UHV friction tests were performed on a tribofilm previously formed on selected DLC material with MoDTC and ZDDP containing oil. Ex‐situ characterizations show that the composition of this tribofilm is similar to that of a tribofilm obtained on steel surfaces in the same lubrication conditions with MoS₂ single sheets dispersed inside zinc phosphate zones. However, analyses by X‐ray photoelectron spectroscopy (XPS) indicate that MoDTC and ZDDP additives seem to be more active on steel surfaces than carbonaceous ones. After UHV friction with the tribofilm formed on selected DLC and steel pin counterpart, the wear scars of both sliding surfaces were characterized by in‐situ analytical tools such as Auger electron spectroscopy, scanning Auger microscopy and micro‐spot XPS. Low friction is associated with the transfer of a thin MoS₂ film to the steel pin counterpart. Copyright © 2006 John Wiley & Sons, Ltd.     

Tribological characteristics of greases with and without metallo-organic friction-modifiers

ABSTRACT

An important focus of grease development is to minimize friction and wear while improving load bearing capacity. ASTM D2266 test method is commonly used to evaluate performance of grease at 75°C, 40?kg and 1200?rpm for 1 hour. However, actual applications may require bearings to be subjected to cyclic loading and variable frequency conditions wherein rotations per minute (rpm), load and duration of test are variables. Five different blends of greases were formulated using ZDDP (3?wt.%), PTFE (2?wt.%), MoDTC (2?wt.%), combination of ZDDP/PTFE in a weight ratio of 3:2 and a combination of ZDDP/PTFE/MoDTC in 3:2:2 weight ratios. They were tested under ASTM D2266 test method as well as under cyclic loading and variable frequency conditions where loads, frequency and duration of the tests were treated as variables. It was found that the combination of ZDDP/PTFE/MoDTC results in significant improvement in the wear and friction under cyclic loading as well as ASTM D2266 test conditions. It was also demonstrated that MoDTC accelerated the tribochemical degradation of ZDDP that resulted in the formation of a protective tribofilm layer on the interacting surfaces. The analysis of the tribofilm formed indicated that when MoDTC was used together with ZDDP and PTFE, a combination of MoS₂, phosphates and sulfates of Zn and Fe are formed whereas when only ZDDP and PTFE was used the tribofilms were largely composed of phosphates and sulfates of Zn and Fe.     

Evaluation of Nanoscale Friction Depth Distribution in ZDDP and MoDTC Tribochemical Reacted Films Using a Nanoscratch Method

The distributions of local friction coefficients relative to the depth and near the surface of MoDTC/ZDDP and ZDDP tribofilms were successfully evaluated by using a nanoscratch method combined with in situ AFM observation. It was found that both tribofilms were friction-functionally graded materials. The friction coefficients decreased from 0.35 to 0.16 with a decrease in the scratch depth from 60 to 10 nm. It was observed that the MoDTC/ZDDP and ZDDP tribofilms possessed different shear strength levels near the surface as evidenced by the different valley-shaped friction coefficient distributions they exhibited for scratch depths ranging from 2 to 10 nm. Based on our recent nanomechanical measurements, this observation indicated that both tribofilms possessed an ultra-low friction inner skin layer at a depth of about 10 nm below the surface. Most importantly, the inner skin layer of the MoDTC/ZDDP tribofilm possessed a lower friction coefficient than that of the ZDDP tribofilm (0.084 versus 0.104) and was thinner (about 3.2 nm versus 6.4 nm). These results thus revealed that the reduction in friction attributed to the MoDTC additive originates from the different friction behavior of the inner skin layers of the MoDTC/ZDDP and ZDDP tribofilms. These nanoscratch results agree with the findings of our recent work on detecting differences in mechanical properties between these tribofilms by nanoindentation measurements.     

ZDDP and MoDTC interactions and their effect on tribological performance – tribofilm characteristics and its evolution

In this work, the interactions between two key additives in current lubricants (ZDDP and MoDTC) and the effect on tribofilm formation and tribofilm evolution under boundary lubrication are studied. The chemical and tribological characteristics of the tribofilms are probed using measurement of friction, wear and film characteristics. Tribofilms have been examined by energy dispersive X-ray analysis (EDX) and X-ray photoelectron spectroscopy (XPS). In order to investigate the morphology of the reaction films formed, atomic force microscopy (AFM) was used. In this work, for the first time, a link between a proposed MoDTC breakdown mechanism and MoDTC tribofilm characteristics, measured on experimentally derived tribofilms, is made.     

Lubrication of CrN Coating With Ethyl-Dimethyl-2-Methoxyethylammonium Tris(pentafluoroethyl)Trifluorophosphate Ionic Liquid as Additive to PAO 6

This paper studies ethyl-dimethyl-2-methoxyethylammonium tris(pentafluoroethyl)trifluorophosphate ionic liquid [(NEMM)MOE][FAP] as 1 wt% additive to a polyalphaolefin (PAO 6) in the lubrication of CrN PVD coating. The tribological behavior of this mixture has also been compared with a traditional oil additive, such as zinc dialkyldithiophosphate (ZDDP). Friction and wear tests were performed by means of a ball-on-plate reciprocating tribometer, and XPS was used to analyze wear surfaces. The experimental results showed that both additives substantially improve the anti-friction and anti-wear performance of the base oil. However, the tribological behavior of the ionic liquid as oil additive does not reach that of ZDDP. The interactions of each additive with the surface and tribofilm formation contributed to improve the tribological behavior of the lubricants.     

Action Mechanism of WS2 Nanoparticles with ZDDP Additive in Boundary Lubrication Regime

The effect in the tribological performance of WS₂ fullerene-like nanoparticles in PAO base oil when adding a ZDDP additive was studied at 100 °C in the boundary lubrication regime. The tribological properties of the dispersion surpass those obtained without one of the two additives. The friction modifier properties of the particles are improved in the presence of ZDDP, while the anti-wear properties of the ZDDP are increased when the particles are added to the dispersion. The composition of the formed tribofilm was investigated. Results show that a 50–60 nm tribofilm is formed on the steel surface composed by WS₂ mixed on the ZDDP chemical tribofilm. A WS₂-rich layer is observed at the top of the tribofilm. A correlation between the chemical composition of the tribofilm and the tribological properties of the “PAO + WS₂ + ZDDP” dispersion was made. Synergy between the two additives was proven.     

Tribological properties of Mg/Al–CO3 layered double hydroxide as additive in base oil

Abstract

The tribological performance of Mg/Al–CO₃–layered double hydroxide (LDHs) nanoparticles in base oil was studied as a sole additive and in combination with zinc dialkyldithiophosphate (ZDDP) at 100°C. Results show that LDHs could improve antifriction and antiwear properties of base oil. The blend with LDHs alone shows better antifriction properties than that of the mixture of LDHs and ZDDP, but the addition of ZDDP helps to stabilise the friction coefficient. Surface analyses have been performed to study the morphology, nanoparticle distribution and chemical species in the tribofilm. The results show that tribofilms contain Mg, Al, C, O, Zn, P and S, and the structure of Mg/Al–CO₃–LDHs changes under shearing stress and this process can help reduce friction coefficient. Thus, LDHs could be chemically incorporated into tribofilm to reduce the polyphosphate chain length originated from ZDDP decomposition resulting in a medium chain polyphosphate, which provides very good wear protection.     

Characterization of tribofilms derived from zinc dialkyldithiophosphate and salicylate detergents by X-ray absorbance near edge structure spectroscopy

Phosphorous and sulfur L- and K-edge X-ray absorption near edge structure (XANES) spectra were recorded to characterize the surface chemistry of the tribofilms derived from a commercial zinc dialkyldithiophosphate (ZDDP) and its blends with overbased calcium and magnesium salicylate detergents. The results show that the chemical structure of the tribofilms generated from the mixture of ZDDP and detergents is different from that of the tribofilm derived from ZDDP alone. However, the two kinds of detergents inhibit ZDDP from forming tribofilm, producing thinner polyphosphate films. Though XANES analysis does not provide definite evidence to the existence of magnesium element in the tribofilms, it is likely that calcium or magnesium has been incorporated therein. Moreover, ion exchange reaction and the formation of zinc polyphosphate film may occur simultaneously during sliding. However, calcium or magnesium (poly)phosphates do not contribute to reduce friction and wear of a steel–steel pair.     

Influence of temperature and ZDDP concentration on tribochemistry of surface-capped molybdenum sulfide nanoparticles studied by XANES spectroscopy

The tribological behavior of surface-capped MoS₃ nanoparticles (nano-MoS₃) in hydrocarbon oils was studied in combination with ZDDP at test temperatures in the range of 100–160 °C and at ZDDP content of 0–1.0 wt% in oil. It was demonstrated that this combination of additives demonstrates high antiwear and antifriction efficiency, especially at high temperatures and low ZDDP content. X-ray Absorption Near Edge Structure (XANES) spectroscopy at the sulfur, molybdenum, and phosphorus edges was used to identify the chemical species in the tribochemical films. It was established that the tribofilms formed by combination of ZDDP and nano-MoS₃ contain phosphate-based layers incorporating MoS₂-type fragments. An increase in temperature and ZDDP content results in an increase in tribofilm thickness, while the relative Mo content in tribofilm decreases. Under the tested conditions, the best tribological properties are demonstrated by the composition comprising 500 ppm Mo and 0.1 wt% ZDDP in oil.