On the mechanism of ZDDP antiwear film formation

Zinc dialkyldithiophosphate additives are used to control wear and inhibit oxidation in almost all engine oils as well as many other types of lubricant. They limit wear primarily by forming a thick, protective, phosphate glass-based tribofilm on rubbing surfaces. This film formation can occur at low temperatures and is relatively indifferent to the chemical nature of the substrate. There has been considerable debate as to what drives ZDDP tribofilm formation, why it occurs only on surfaces that experience sliding and whether film formation is controlled primarily by temperature, pressure, triboemission or some other factor. This paper describes a novel approach to the problem by studying the formation of ZDDP films in full film EHD conditions from two lubricants having very different EHD friction properties. This shows that ZDDP film formation does not require solid-solid rubbing contact but is driven simply by applied shear stress, in accord with a stress-promoted thermal activation model. The shear stress present in a high pressure contact can reduce the thermal activation energy for ZDDP by at least half, greatly increasing the reaction rate. This mechanism explains the origins of many practically important features of ZDDP films; their topography, their thickness and the conditions under which they form. The insights that this study provides should prove valuable both in optimising ZDDP structure and in modelling ZDDP antiwear behaviour. The findings also highlight the importance of mechanochemistry to the behaviour of lubricant additives in general

Influence of Succinimide Dispersants on Film Formation, Friction and Antiwear Properties of Zinc Dialkyl Dithiophosphate

ZDDP (zinc dialkyldithiophosphate) is arguably the most successful antiwear additive ever employed in crankcase engine lubricants. It was originally used as an antioxidant and shortly afterwards recognized for its antiwear and extreme pressure properties. Unfortunately, another critical additive polyisobutylsuccinimide-polyamine (PIBSA-PAM), which is used as a dispersant in engine oils, is known to be antagonistic to ZDDP in terms of film formation, friction and wear. The mechanisms of this antagonism have been widely studied, but they are still not well understood. Furthermore, in order to protect engine exhaust catalysts from sulphated ash, phosphorus and sulphur (SAPS) and extend drain intervals of engine lubricants, a progressive reduction in ZDDP quantity but a growth in the use of PIBSA-PAM is required. The aim of this study is to explore the mechanisms and practical effects of the antagonism between ZDDP and PIBSA-PAM. Of particular interest is the impact on performance of the ratio of ZDDP to PIBSA-PAM, as measured by P:N ratio. Since ZDDP is a very effective antiwear additive, it produces only very low or "mild" rates of wear. To study this requires a new way to measure mild wear behaviour of formulated oils. Several techniques have been applied in this study to investigate the film formation, friction and wear properties of ZDDP- and/or PIBSA-PAM-containing oils. These include a new mild wear testing method, which is tested and developed using a range of different types of additives. It is found that the ratio of P:N plays a strong role in determining tribofilm formation and friction of ZDDP/PIBSA-PAM blends. However it plays a much weaker role in determining wear behaviour. It is found that some PIBSA-PAMs have considerable friction-reducing properties in their own right. The results suggest that PIBSA-PAM may interfere with the behaviour of ZDDP in several ways: by forming a ZDDP/ PIBSA-PAM complex at the metal surfaces to reduce the local activity of ZDDP; by PIBSA-PAM partially removing the ZDDP film; possibly also by PIBSA-PAM blocking ZDDP from metal surfaces. The newly-developed wear testing method can be used conveniently and effectively to study mild wear properties not just of ZDDP but of a wide range of other additives

Optical and other property changes of M-50 bearing steel surfaces for different lubricants and additive prior to scuffing

An ester lubricant base oil containing one or more standard additives to protect against wear, corrosion, and oxidation was used in an experimental ball/plate elastohydrodynamic contact under load and speed conditions such as to induce scuffing failure in short times. Both the ball and the plate were of identically treated M-50 steel. After various periods of operating time the wear track on the plate was examined with an interference microscope of plus or minus 30 A depth resolution and sometimes also with a scanning ellipsometer and an Auger spectrometer. The optically deduced surface profiles varied with wavelength, indicating the presence of surface coatings, which were confirmed by the other instruments. As scuffing was approached, a thin (approximately A) oxide layer and a carbide layer formed in the wear track in particular when tricresylphosphate antiwear additive was present in the lubricant. The rates of the formation of these layers and their reactivity toward dilute alcholic HCl depended strongly on the lubricant and additives. Based on these results suggestions for improved formulations and a test method for bearing reliability could be proposed

The Effect of Dissolved Water on the Tribological Properties of Polyalkylene Glycol and Polyolester Oils

The effect of water dissolved in polyalkylene glycol and polyolester oils on the tribological behavior of two material contact pairs in three test environments is evaluated. The material contact pairs are M2 tool steel against 390 aluminum and M2 tool steel against gray cast iron. The three oils are a polyalkylene glycol (PAG) and two polyolester (PEl and PE2) oils. The test environments are R134a, air and argon. The tests are conducted in a specially designed high pressure tribometer which provides an accurate control of the test variables. The results indicate that the P AG oil performed better than the esters for both material contact pairs. The wear on the aluminum plates for the tests conducted with the P AG oil in all three environments is greatest at the lowest moisture content levels. From the stand point of friction and wear, it is beneficial to have a water content level of 5000 ppm or greater in the PAG oil when the plate material is 390 aluminum. The wear on the cast iron plates, when using a PAG oil as the lubricant showed a slight increase with water content in a R134a environment. This trend is opposite when air is the test environment. Both ester oils lubricated aluminum much better than the cast iron . The difference in the amount of wear can be as high as two orders of magnitude. This is probably due to the ability of the esters to form bidentate bonds with aluminum. Esters do not form such bonds with iron. The plate wear is greater for the PEl tests than for the PE2 tests for both material contact pairs. This is most likely due to the difference in the viscosity of the oils. In PE2 oil, water does not seem to affect the friction and wear of both aluminum/steel and cast iron/steel contacts when R134a is the test environment. On the contrary, for the aluminum/steel contacts, the water content significantly influences wear when argon or air is the test environment. For the cast iron/steel contacts, the wear is strongly influenced by the water content when the test is conducted in argon, but it is not influenced by the water content when the test is conducted in air.Air Conditioning and Refrigeration Center Project 0

Surface topographical changes measured by phase-locked interferometry

An electronic optical laser interferometer capable of resolving depth differences of as low as 30 A and planar displacements of 6000 A was constructed to examine surface profiles of bearing surfaces without physical contact. Topological chemical reactivity was determined by applying a drop of dilute alcoholic hydrochloric acid and measuring the profile of the solid surface before and after application of this probe. Scuffed bearing surfaces reacted much faster than virgin ones but that bearing surfaces exposed to lubricants containing an organic chloride reacted much more slowly. The reactivity of stainless steel plates, heated in a nitrogen atmosphere to different temperatures, were examined later at ambient temperature. The change of surface contour as a result of the probe reaction followed Arrhenius-type relation with respect to heat treatment temperature. The contact area of the plate of a ball/plate sliding elastohydrodynamic contact run on trimethylopropane triheptanoate with or without additives was optically profiled periodically. As scuffing was approached, the change of profile within the contact region changed much more rapidly by the acid probe and assumed a constant high value after scuffing. A nonetching metallurgical phase was found in the scuff mark, which was apparently responsible for the high reactivity

Characterization of lubricated bearing surfaces operated under high loads

The composition and surface profiles of M-50 steel surfaces were measured after operation at high loads in a bearing contact simulator. An ester lubricant (trimethyolpropane triheptanoate) was used with and without various additives. Optical profiles were obtained + or - to 30 depth resolution with a phase-locked interference microscope in 10 micron diameter areas within and outside the wear tracks. Optical constants and surface film thickness were measured in the same areas with an electronic scanning ellipsometer. Film composition was measured with a scanning Auger electron spectrometer. It is concluded that metal oxide formation is accelerated within the wear tracks

The influence of zinc dialkyldithiophosphate and other lubricant additives on soot-induced wear

Diesel engines are becoming increasingly popular in both passenger and commercial vehicles because they offer better fuel efficiency than their gasoline counterparts. However, a disadvantage of this type of engine is the high volumes of soot it produces, which can contribute to an increase in wear of the engine components. A number of wear mechanisms have been proposed to explain wear by soot, of which the abrasive mechanism is still the most widely accepted. Lubricant additives such as zinc dialkyldithiophosphate (ZDDP) are frequently used as antiwear (AW) agents but the combined effect of soot and additives such as ZDDP on wear is not well understood. The aim of the work described in this thesis is to explore the impact of soot on wear both in the absence and presence of lubricant additives, of particular interest is the additive ZDDP. A unidirectional sliding/rolling test is used to explore the impact of film-thickness on wear while a reciprocating wear tester enables accurate wear measurements for individual test lubricants. The AW additives studied, most especially ZDDP, show excellent antiwear behaviour in the absence of carbon black (CB) – used as a substitute for soot. However, once CB is combined with AW additives, an unusual wear pattern emerges and an increase in wear is observed. A new mechanism of wear by soot is therefore suggested and discussed to explain this effect

Tribochemical investigation of ZDDP tribofilm

The current trend for using lower-viscosity lubricants with the aim of improving fuel efficiency of mechanical systems means that machine components are required to operate for longer periods in thin oil film, mixed and boundary lubrication conditions, where the risk of surface damage is increased. For this reason, the role of tribofilms generated from the antiwear additive zinc dialkyldithiophosphate (ZDDP) in providing surface protection has become increasingly important. However, the properties, performance and the mechanisms of tribofilm formation are not fully understood. Therefore, this thesis aims to further understand the tribochemical behaviour of ZDDPs. Several inter-connected areas of research are described in this thesis. These all investigate the formation of tribofilms by ZDDP and the impact of tribofilm formation on wear. Taken together they contribute to our understanding of the mechanisms of tribological behaviour of ZDDPs and should assist in the design of lubricants and rubbing components. Firstly, the evolution of ZDDP tribofilm properties, in particular, tribofilm durability and the origins of this durability, are examined. It is found that ZDDP tribofilms undergo a structural transformation during rubbing from a predominantly amorphous structure to one that is nanocrystalline, resulting in the tribofilm becoming much stronger and more durable. Secondly, the reaction mechanisms of tribofilm formation on various non-ferrous metal and non-metallic materials are studied, both by ion-implanting various alloying elements into steel surfaces, and by using non-metallic rubbing materials. It is found that a potentially important factor in the formation of ZDDP tribofilms is the presence and concentration of ferrous and/or non-ferrous metal atoms at the surface. Non-ferrous metals may act as adsorption sites for ZDDP in a similar manner to Fe in steel, enabling formation of ZDDP tribofilms. Thirdly, to further understand the antiwear performance of ZDDP, its impact on the wear of a-C:H DLC in the presence of molybdenum dialkyldithiocarbamate (MoDTC), which is known to be problematic, is investigated. It is well known that MoDTC can produce very high wear of DLC and that this can be mitigated by the presence of ZDDP in the lubricant. From this research it is shown that ZDDPs reduce DLC wear in the presence of MoDTC mainly by forming thick antiwear tribofilms and reducing the ratio of MoO3/MoS2 in the tribofilm. Fourthly, although ZDDP is very effective for reducing most types of wear, it is known that ZDDP can promote one particular form of wear, micropitting wear. In this research a new method of studying micropitting that enables both micropitting and tribofilm formation to be studied in parallel is developed which is key to assessing the impact of lubricant chemistry on micropitting. This is used to show that the influence of ZDDP on micropitting originates from its tendency to prevent running-in of the contacting surfaces. The mechanism by which a widely-used black oxide coating limits micropitting is explored and it is shown that this relatively soft coating provides adequate running-in even in the presence of ZDDP-containing oils. Finally, the impact of friction on micropitting is studied by isolating its effects from those of running-in, by controlling separately the formation of ZDDP and MoDTC tribofilms. Results show that friction has a very significant impact on micropitting.Open Acces

Effect of fullerene containing lubricants on wear resistance of machine components in boundary lubrication

Fullerenes, a new form of carbon nanomaterials, possess unique physical and mechanical properties that make their use as additives to liquid lubricants potentially beneficial. The goal of this study was to investigate the effect of fullerene containing lubricants on wear resistance of steel-bronze couples operating under boundary lubrication conditions. A mathematical model of deformed asperity contact was built to calculate real contact area and real contact pressure. Computer controlled wear friction testing methodology and equipment were designed, developed and implemented for obtaining reliable and objective experimental data. In addition, optical and scanning electron microscopy and standard surface texture analysis were employed. Heavy duty motor oil SAE 10 was modified by admixing fullerenes C60, a fullerene mixture of C60 and C70, fullerene containing soot, and graphite powder. The experiments showed that all of the selected fullerene additives dissolved in liquid lubricants reduce wear of the tested materials. In addition, it was found that despite improvements in wear resistance, the selected modified lubricants did not significantly change friction characteristics. Improvement of wear resistance of contact surfaces operating with fullerene modified lubricants can be explained by the presence of fullerenes in real contact while the liquid lubricant is squeezed out. Fullerenes are considered to function as minute hard particles that do not break down under applied normal force, and tend to separate direct contact of functional surfaces of selected materials

Enhancement of tribological properties of mineral oil by addition of sorbitan monostearate and borate

Thesis (Doctoral)--Izmir Institute of Technology, Chemical Engineering, Izmir, 2010Includes bibliographical references (leaves: 225-235)Text in English; Abstract: Turkish and Englishxxviii, 248 leavesThe development of modern automobile and engine industries requires lubricants that can withstand high temperatures and pressures. Recent advances made in the chemistry provide the use of inorganic particles as lubricant additives. Therefore inorganic boron-based additives have been the focus of much attention, as they posses a good combination of properties, such as wear resistance, friction-reducing ability. In this study, the state of art in the field of inorganic particle, zinc borate synthesis and its employment in tribology were investigated. The synthesis of zinc borate was achieved not just by precipitation, but also production methods such as inverse emulsions. The products were characterized by SEM, FTIR, TGA, DSC, EDX. In lubrication part, the friction reducing and antiwear ability of the particles as an additive in the mineral oil was focused. Sorbitan monostearate was used to cover the surfaces of inorganic particles in order to provide better dispersion of additives in the oil. Friction and wear behavior of the lubricants were measured by four-ball wear test machine. The effects of dispersing agent, zinc borate type as well as surfactant concentration on the tribological properties of the lubricants were investigated. Sorbitan monostearate not only outperformed as a dispersing agent of inorganic particles, but also it proved to be an efficient antiwear agent. The lowest wear scar diameter was obtained by the lubricant containing zinc borate synthesized via coordination precipitation method. The addition of this sample with the surfactant in the oil reduced the wear scar diameter from 1.402 mm to 0.550 mm