Wear behaviour of hydrogenated DLC in a pin-on-disc model test under lubrication with different diesel fuel types

The wear behaviour of hydrogenated diamond like-carbon (DLC) coating in DLC/steel tribological contact in a pin-on-disc model test under lubrication with two diesel fuels is presented in this work. The first diesel fuel was standard EN590 that contained ester-based antiwear additives. In contrast to EN590, the second diesel fuel, called GDK650, did not contain antiwear additives. It was experimentally observed that the antiwear additives are detrimental to the DLC. The effects of load, speed and temperature on the DLC and steel counterbody wear were investigated. Steel counterbody wear volume was found to be not affected by pressure, temperature, speed and lubricant, whereas the DLC-coating revealed correlation between the parameters and wear rate. Regarding the results of the tribological tests under both diesel lubrications, new mathematical wear laws were developed.     

Boundary lubrication mechanisms of carbon coatings by MoDTC and ZDDP additives

Fuel economy and reduction of harmful elements in lubricants are becoming important issues in the automotive industry. An approach to respond to these requirements is the potential use of low friction coatings in engine components exposed to boundary lubrication conditions. Diamond-like-carbon (DLC) coatings present a wide range of tribological behavior, including friction coefficients in ultra-high vacuum below 0.02. The engine oil environment which provides similar favourable air free conditions might lead to such low friction levels.In this work, the friction and wear properties of DLC coatings in boundary lubrication conditions have been investigated as a function of the hydrogen content in the carbon coating. Their interaction with ZDDP which is the exclusive antiwear agent in most automotive lubrication blends and friction-modifier additive MoDTC has been studied. Hydrogenated DLC coatings can be better lubricated in the presence of the friction-modifier additive MoDTC through the formation of MoS₂ solid lubricant material than can non-hydrogenated DLC. In contrast, the antiwear additive ZDDP does not significantly affect the wear behavior of DLC coatings. The good tribological performances of the DLC coatings suggest that they can contribute to reduce friction and wear in the engine, and so permit the significant decrease of additive concentration.     

Extreme Pressure Lubricant Additives Interacting on the Surface of Steel- and Tungsten Carbide–Doped Diamond-Like Carbon

Certain diamond-like carbon (DLC) coatings offer excellent tribological properties under both dry and oil-lubricated sliding conditions. However, the underlying mechanisms under lubricated conditions are generally not fully understood, especially when performance depends on strong tribochemical interactions with lubricant additives. The aim of the present work is to explore the friction and wear performance of steel and tungsten carbide (WC)-doped DLC (WC-DLC) surfaces in the presence of different types of extreme pressure (EP) and nitrogen–sulfur-based (NS) additives. Tribological tests were performed on a ball-on-disc test rig, and X-ray photoelectron spectroscopy (XPS) was used for physical and chemical characterization of the tribofilms. It was observed that EP and NS additives significantly reduced the wear of WC-DLC surfaces in comparison with tests conducted on steel surfaces. XPS indicated that the additive interactions on the WC-DLC surface formed a distinctive tribofilm that promoted better friction and wear performance. The higher concentration of carbon compounds and lower concentration of oxygen compounds in the tribofilm significantly improved friction and wear characteristics.     

Friction and wear behavior of thioether hydroxy vegetable oil

This work describes the tribochemical evaluation of vegetable oil based antiwear additive obtained through chemical modification. The Sulfur was incorporated using a chemical reaction of epoxidized vegetable oil and common thiols, resulting in formation of a hydroxy thioether derivative of vegetable oils. The synthesis retains the vegetable oil structure, eliminates poly-unsaturation in the molecule, and adds polar functional groups that significantly improve adsorption on metal surfaces. These additives are obtained by chemical modification of oils originated from natural resources. The tribochemical behavior of sulfur incorporated vegetable oil was studied by measuring friction coefficient using ball-on-disk configuration and wear scar diameter using four-ball configuration. Comparative tests with commercial antiwear additives demonstrate the effectiveness of these derivatives. The derivatives were found useful as agriculturally based antiwear additives for lubricant applications.     

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.     

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.     

Friction and wear behavior of diamond-like carbon coating on plasma nitrided mild steel under boundary lubrication

The friction and wear properties of synthetic ionic liquid functionalized borate esters as additives in poly-alpha-olefin (PAO) were measured for diamond-like carbon (DLC) coating on plasma nitrided AISI 1045 steel. Results show that the borate esters gave much better friction–reduction and antiwear properties for DLC coating/steel and DLC coating/DLC coating sliding pairs than zinc dialkyldithiophosphate (ZDDP). In addition the DLC coating had much better wear resistance than the nitrided mild steel substrate, indicating that duplicate surface modification was more effective in significantly increasing the wear resistance of mild steel.     

Comparison of boundary lubrication films formed under four‐ball friction test conditions with different AW/EP Additives‐an X‐ray photoelectron spectroscopy study

The antiwear and extreme‐pressure properties of six different types of additive (molybdenum dialkyldithiophosphate, dibenzyl disulphide, molybdenum dialkyldithiocarbamate, zinc dialkyldithiophosphate, chlorinated paraffin wax, and triaryl phosphate) were evaluated by standard four‐ball friction and wear tests. This was followed by scanning electron microscopy (SEM), X‐ray photoelectron spectroscopy (XPS), and X‐ray photoelectron imaging (XPI) analyses of the worn surfaces to determine the structure of the boundary lubrication film and the mechanism of the tribochemical reaction occurring during the friction process. The presence of the additives in the base oil significantly increased the weld load and drastically reduced the wear‐scar diameter, suggesting antiwear and extreme‐pressure properties of the additives. The enhanced antiwear and loadcarrying capacity of the additive‐containing oils was attributed to the formation of a complex boundary lubrication film formed between the surfaces during the friction process as a result of the tribochemical reaction. The antiwear and extreme‐pressure properties of the additives were explained based on the XPS data. The studies indicated that the lubricating properties of the additives depend on their chemical nature and reactivity with metal surfaces.     

The Wear Resistance of HVOF Sprayed Composite Coatings

The aim of the study was to determine the interactions between standard antiwear zinc dialkyldithiophosphate (ZDTP)-type additives and composite coatings containing hard phases of Al₂O₃, SiC, and TiN in the nickel matrix. The analysis was conducted for selected ceramic materials with different structures and different tribological behavior of ionic, covalent, and metallic bonds. The composite coatings were deposited on C45 steel using the high-velocity oxygen-fuel (HVOF) process. This process efficiently uses high kinetic energy and controlled thermal output to produce dense, low porosity coatings with highly predictable chemistries that are homogeneous in structure. The coatings can operate under harsh service conditions, because they are characterized by higher durability and higher wear and corrosion resistance. It was necessary to determine the interactions between the ZDTP-type antiwear additives (zinc dialkyldithiophosphates) and the coatings. The tribological properties of nickel and nickel-based composite coatings were examined by means of a T-01 M tester functioning in the ball-on-disc configuration during technically dry friction and boundary lubricated friction with lubricants containing 1% ZDTP. The comparative analysis confirmed different tribochemical activity and, accordingly, different tribological effectiveness of the nickel and nickel-based composite coatings during friction.     

Tribological properties of non‐ferrous coating lubricated with rapeseed oil containing lubricant additives

With the increasing requirements of environmental protection, using lead, hexavalent chromium and other heavy metals will be prohibited in different surface coatings. In this paper, the non‐ferrous CrN coatings and the rapeseed oil were chosen as friction pair and biodegradable lubricant. The tribology properties were investigated using SRV‐IV reciprocating friction wear test. The worn surface of CrN coatings was observed and analysed using scanning electron microscopy and X‐ray photoelectron spectroscopy. The results indicate that the rapeseed oil containing 1 wt.% MoZn (MoDTC (molybdenum dithiocarbamate) + ZnDTP (zinc dithiophosphate) (1 : 3)) exhibits better friction‐reducing properties than the rapeseed oil containing 1 wt.% PN (an amine salt of an alkoxylphosphate) additives, whereas the rapeseed oil containing 1 wt.% PN additive exhibits better wear resistance properties than the rapeseed oil containing 1 wt.% MoZn additives. This is partly attributed to the tribophysical and tribochemical reactions between the lubricant and the CrN coating sliding surfaces. Copyright © 2012 John Wiley & Sons, Ltd.     

Tribological studies of coated pistons sliding against cylinder liners under laboratory test conditions

The presence of coatings and surface topography play an important role in the tribological performance of sliding components. Depending on the coating used, it is possible to reduce friction and/or reduce wear. However, although there may be low friction and wear‐resistant coatings suitable for use in pistons, some coatings may hinder the tribological performance by changing the lubrication regime or by preventing additives from their intended function through chemical mechanisms. In this work, piston skirt segments extracted from a commercial aluminium alloy piston were coated with a diamond‐like carbon (DLC) coating, a graphite–resin coating or a nickel–polytetrafluoroethylene (Ni–PTFE) coating and were tribologically tested using a reciprocating laboratory test rig against commercial grey cast iron liner segments. The tribological tests used commercial synthetic motor oil at a temperature of 120 °C with a 20 mm stroke length at a reciprocating frequency of 2 Hz. Results showed that the graphite–resin coating, although it may serve as a good break‐in coating, wears rapidly. The Ni–PTFE coating showed friction reduction, whereas the DLC coating wore off quickly due to its small thickness. Furthermore, the higher hardness of the DLC coating relative to the cast iron liner surface led to pronounced changes on the liner counterface by polishing. In contrast with the uncoated piston skirt segments, all of the coatings prevented the formation of a visible tribochemical film on the cast iron surface. Copyright © 2012 John Wiley & Sons, Ltd.     

Tribochemistry of silicon and oxygen doped,hydrogenated Diamond-like Carbon in fully-formulated oil against low additive oil

Diamond-like Carbon (DLC) coatings are increasingly used to reduce wear and lower friction in many applications. Doped DLCs are being produced with the goal of further enhancing the friction and wear profile as well as increasing the coating reliability.Silicon is often incorporated into DLC as it is known to affect the sp²/sp³ ratio which in turn can affect the hardness of the film. It can also improve adhesion of the DLC coating to the substrate and lower internal stress.In this study, investigations into the wear behaviour, tribochemistry and oil-formulation dependence of Si, O-doped DLC (Si-DLC) were conducted. The oxidative stability of Si-DLC was also examined.Silicon-doped DLC is able to form a protective tribofilm when a fully-formulated lubricant is used. The tribofilm is composed of S, P, Ca and Zn which are widely recognised as being important to wear reduction.A mechanism of wear repression facilitated by oil additives is proposed.     

Wear properties of DLC-coated steel rollers running with highly contaminated lubrication

The presence of hard contaminants in lubrication can lead to the premature failure of rolling bearings. To reduce the negative effect of such contaminants, hard carbon-based coatings (diamond-like carbon; DLC) can be applied to the surfaces of steel bearings. DLC coatings generate a low friction and a high sliding wear resistance to enhance the tribological properties and improve the durability of running components. This work explores the merits of DLC coatings for use in very demanding applications, such as in highly contaminated environments. The wear properties of DLC-coated bearing rollers were evaluated by comparing them with uncoated rollers. The degree of wear found on the coated rollers was serious, especially under relatively high contaminant concentrations. The three-body abrasive wear produced a relatively coarse scoring of the coating surface, which caused the corresponding disc to suffer more damage than the disc running against an uncoated roller under the same operating conditions. The results indicate that supposedly wear-protective coatings cause even more damage to running surfaces once they have been broken up by hard contaminants, and highlight the importance of keeping the bearing coating intact. In practise, it is important to eliminate contaminants from the lubricant of rolling bearings, in particular for bearings with a DLC anti-wear coating.     

Antiwear Properties of Bonded MoS2 Solid Lubricant Coating under Dual-Rotary Fretting Conditions

Bonded MoS₂ solid lubricant coatings are widely used in tribology for their friction-reducing and antiwear properties. However, such coatings have been rarely investigated in complex fretting conditions, such as dual-rotary fretting (DRF). DRF is a complex fretting wear mode that combines torsional fretting with rotational fretting. In this work, the antiwear properties of bonded MoS₂ solid lubricant coating under dual-rotary fretting conditions were studied. Results indicated that the MoS₂ coating had better friction-reducing and antiwear properties than the substrate for alleviating DRF wear. The coating can greatly influence the fretting regimes and reduce the coefficient of friction. Furthermore, the service life of the coating was strongly dependent on the competition of the two fretting components and was reduced as the rotational fretting component increased.     

Tribological characterisation of hard coatings with and without DLC top layer in fretting tests

The potential of coatings to protect components against wear and to reduce friction has led to a large variety of protective coatings. In order to check the success of coating modifications and to find solutions for different purposes, initial tests with laboratory tribometers are usually done to give information about the performance of a coating. Different Ti‐based coatings (TiN, Ti(C,N), and TiAlN) and NiP were tested in comparison to coatings with an additional diamond‐like carbon (DLC) top coating. Tests were done in laboratory air at room temperature with oscillating sliding (gross slip fretting) with a ball‐on‐disc arrangement against a ceramic ball (Al₂O₃). Special attention was paid to possible effects of moisture (relative humidity). The coefficient of friction was measured on line, and the volumetric wear at the disc was determined after the test from microscopic measurements of the wear scar and additional profiles. The friction and wear behaviour is quite different for the different coatings and depends more or less on the relative humidity. The DLC coating on top of the other coatings reduces friction and wear considerably. In normal and in moist air the coefficient of wear of the DLC top‐layer coating is significantly less than 10⁻⁶ mm³/Nm and the coefficient of friction is below 0.1. In dry air, however, there is a certain tendency to high wear and high friction. Copyright © 2006 John Wiley & Sons, Ltd.     

A comparison of the tribological behaviour of steel/steel, steel/DLC and DLC/DLC contacts when lubricated with mineral and biodegradable oils

Diamond-like carbon (DLC) coatings, which can nowadays be applied to many highly loaded mechanical components, sometimes need to operate under lubricated conditions. It is reasonable to expect that in steel/DLC contacts, at least the steel counter body will behave according to conventional lubrication mechanisms and will interact with lubricants and additives in the contact. However, in DLC/DLC contacts, such mechanisms are still unclear. For example, the “inertness” of DLC coatings raises several questions about whether they are able to provide real boundary “lubrication” or whether they are just a “passive” member in these contacts. On the other hand, biodegradable oils, in particular vegetable base oils, possess a good lubricating ability, often much better than mineral or conventional synthetic oils as a result of the large amount of un-saturated and polar components that can promote the lubricity of DLC coatings. Accordingly, in this study, we present the results of experiments under severe boundary-lubrication conditions during reciprocating sliding. We look at the effect of the type of mating surfaces - steel/DLC, DLC/DLC and steel/steel - and the type of oil on the tribological performance of DLC coatings. We compare the wear and friction behaviours of two types of DLC coatings, i.e., a “pure” non-doped a-C:H DLC coating (denoted as a-DLC) and a WC-containing multilayer coating (denoted as W-DLC) tested with a mineral oil and a biodegradable vegetable oil. These oils, which have very different chemical compositions, were used as base oils and also with mild AW and strong EP additives. Among other things, the results confirm the following: (1) coating/coating lubricated contacts can resemble metal-lubrication mechanisms; (2) additives reduce wear in coating/coating contacts by up to 80%; (3) better wear and friction performance are obtained with oils that contain large amounts of polar and un-saturated molecules; (4) a coating/coating combination generally results in less wear than a steel/coating combination.     

Improved mixed and boundary lubrication with glycerol-diamond technology

Abstract

The fuel economy and reduction of harmful elements in lubricants are becoming important issues in the automotive industry. An approach to respond 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 ultralow friction films to protect the surfaces of ductile metals for space applications are expected to fulfil this part. The main purpose of this work is to investigate the friction and wear properties of glycerol lubricated DLC coatings under boundary lubrication conditions. The DLC material consists of tetrahedral hydrogen free amorphous diamond-like carbon (denoted as ta-C) as shown by the time of flight secondary ion mass spectroscopy (ToF-SIMS) analyses and the nanoindentation measurements. The friction coefficient below 0&middot.01, called superlubricity, and no measurable wear were obtained by sliding the ta-C/ta-C friction pair in the presence of pure glycerol as a lubricant at 353 K. The mechanism by which glycerol is able to reduce the friction in the millirange was revealed by ToF SIMS analyses inside and outside wear scars formed by friction experiments using deuterated glycerol and ¹³C glycerol.     

羟基硅酸镁粉体添加剂含量对金属表面自修复膜生成的影响及机制

采用羟基硅酸镁粉体作为润滑油添加剂,在MMU-5G材料端面摩擦磨损试验机上,研究了不同添加剂含量对45#钢/45#钢摩擦副磨损表面自修复膜生成的影响及其机制,借助SEM及EDX测试分析摩擦副的表面形貌及表面成分组成。结果表明,自修复添加剂的含量对羟基硅酸镁粉体添加剂在磨损表面形成自修复膜影响显著。在添加剂质量分数为2%,3%和5%的工况条件下,试样磨损表面有自修复膜生成。添加剂质量分数为2%时,易于短时间内达到磨损-自修复动态平衡,自修复效果最为理想。自修复膜的生成过程包含磨粒磨损和摩擦化学反应2个阶段。自修复膜的生成使得试样摩擦磨损表面平整光滑,可以有效降低金属磨损。     

Tribological Behaviors of Different Diamond-Like Carbon Coatings on Nitrided Mild Steel Lubricated With Benzotriazole-Containing Borate Esters

Three synthesized benzotriazole-containing borate esters were separately added into poly-alpha-olefin (PAO) as additives, using molybdenum dithiocarbamate (MoDTC) as the comparison. The friction and wear behavior of Ti-DLC and Ti/Al-DLC coating on nitrided AISI-1045 steel sliding against AISI 52100 steel under the lubrication of PAO containing various additives was evaluated using a reciprocating ball-on-disk friction and wear tester. The morphology and chemical feature of the worn surfaces of the DLC coatings were observed and analyzed using a three dimensional (3D) surface profiler, a scanning electron microscope (SEM), and an X-ray photoelectron spectroscope (XPS). Results show that the three kinds of benzotriazole-containing borate esters as additives in PAO had much better tribological properties than MoDTC; the wear resistance of Ti/Al-DLC coating was better than Ti-DLC coating.     

Antiwear Tribofilm Formation on Steel Surfaces Lubricated With Gear Oil Containing Borate,Phosphorus, and Sulfur Additives

The formation of antiwear tribofilms plays a critical role in the longevity of automotive gears. The focus of this experimental study was on the lubrication efficacy of gear oils with different contents of borate-, phosphorus-, and sulfur-containing additives leading to the formation of protective tribofilms. Experiments were performed with AISI 52100 steel balls sliding against AISI 52100 steel disks in baths of different oils at ambient (～32 °C) and elevated (～100 °C) temperatures under load and speed conditions favoring sliding in the boundary lubrication regime. Friction coefficient responses accompanied by electrical contact voltage measurements provided real-time information about the formation and durability of the antiwear tribofilms. The wear resistance of the tribochemical films was quantified by wear rate data obtained from surface profilometry measurements of wear tracks on the disk specimens and sliding tests performed at ambient temperatures after the formation of the tribofilms during elevated-temperature sliding. Results indicate a strong dependence of tribofilm formation on temperature and type of additives. The slightly lower friction and higher wear resistance obtained at elevated temperatures with blended oils is attributed to the increased chemical reactivity of additives containing borate, phosphorus, and sulfur, leading to the formation of durable tribofilms. Relatively higher wear resistance and faster tribofilm formation were obtained with the borate-enriched gear oil formulations.