Combined in situ (ATR FT-IR) and ex situ (XPS) Study of the ZnDTP-Iron Surface Interaction

Attenuated total reflection infrared (ATR FT-IR) and X-ray photoelectron spectroscopy (XPS) have been used for the in situ and ex situ characterization of thermal and tribological films formed on iron from a commercial zinc dialkyldithiophosphate (ZnDTP). From in situ ATR FT-IR analysis, information on the chemical changes occurring at the iron/lubricant additive interface was obtained during heating and sliding at high temperatures. Different mechanisms and chemical compositions have been found for the thermal and tribochemical reactions between the ZnDTP and the iron surface under the experimental conditions used in this work. Both the ATR FT-IR and the XPS results show the decomposition of ZnDTP with the formation of polyphosphates following thermal testing at 150°C. However, after tribological testing at the same temperature an inorganic phosphate film has been detected on the iron surface instead.     

X-ray Photoelectron Spectroscopy Analysis of Tribostressed Samples in the Presence of ZnDTP: A Combinatorial Approach

The influence of load on the chemistry of tribofilms formed on a steel surface in solution of pure di-isopropyl zinc dithiophosphate (i-ZnDTP) in n-decane has been investigated by means of a combinatorial tribological experiment involving X-ray photoelectron spectroscopy. The experiment consisted of the preparation of a set of spatially separated areas, produced under various tribological test conditions, and the subsequent spectroscopic probing of the chemical composition of the tribofilm. The experiment was carried out at room temperature under boundary-lubrication conditions and revealed a physically adsorbed layer of the additive in the non-contact area and a thin (ca. 5nm), inhomogeneous phosphate film covering the tribostressed areas. The total amount of phosphate present in the tribostressed area was found to increase with increasing load. In the contact areas, iron oxides and metal sulfides have also been detected.     

Boundary Lubrication by Pure Crystalline Zinc Orthophosphate Powder in Oil

The aim of this study is to assess the tribological behavior of pure crystalline zinc orthophosphate under boundary lubrication in order to model zinc phosphate-based anti-wear additives. Boundary films were generated from α-Zn₃(PO₄)₂ powder dispersed in poly-alpha-olefin oil, at ambient temperature, by means of a steel sphere-on-flat contact in reciprocating motion. Electrical contact resistance and friction coefficient evolutions enable an understanding of the tribological behavior of crystalline zinc orthophosphate at the sliding interface. A conductive atomic force microscope (C-AFM) equipped with a current sensing setup, Raman spectroscopy, and nano-indentation were used to characterize the resulting film. When involved in a tribological contact, zinc orthophosphate powder forms a continuous patchy adherent film, changing its structure to amorphous orthophosphate, on both sliding steel surfaces. Morphological and mechanical properties of the film are discussed with respect to the ZDTP tribofilm models.     

Studies on ZDDP Anti-Wear Films Formed Under Different Conditions by XANES Spectroscopy,Atomic Force Microscopy and 31P NMR

Antiwear (AW) films, generated from a mineral base oil containing a zinc dialkyl dithiophosphate (ZDDP) additive, were studied as a function of formation temperature, load and rubbing time. The surface morphology of these films was investigated using atomic force microscopy (AFM), and surface roughness calculated for the observed differing surface morphologies. The morphology of the films is heterogeneous for all the tested conditions, but the surface roughness is dependent on the rubbing condition. X-ray absorption near edge structure (XANES) spectroscopy has been used to characterize the chemistry of these films, and the intensity of the phosphorus K-edge was also used to monitor their thickness. The thickness of these films is in the range of 10–90 nm depending on the running conditions. Phosphorus L-edge spectra show that these films have a similar chemical nature with variable polyphosphate chain-lengths. 31P NMR was used to study the decomposition of ZDDP in the residual oils. The spectra show that the primary and secondary ZDDP react differently under the various conditions. The tribological characteristics of these AW films were probed by measuring the coefficients of friction (μ) and the wear scar width (WSW) of the counter faces. μ is highly related to the applied load and the results of WSW measurements show that the wear performance is related to all the tested parameters, temperature, load and rubbing time.     

Tribochemistry of monomolecular lubricant films of ethanolamine oligomers

The aim of present study was to find out the influence of oxygen and nitrogen containing ethanolamine oligomers on the tribological behavior. X-ray photoelectron spectroscopy (XPS) was used to obtain insight into the molecular mechanisms leading to the macroscopic lubricity. Monomolecular lubricant films were deposited onto ultra thin copper films sputtered onto silicon wafers. Surfaces covered with the three ethanolamine oligomers were investigated by XPS before and after tribological tests, performed with the translatorily oscillating test machine and Falex micro-tribometer.The structure of the molecular film is elucidated using angular resolved X-ray photoelectron spectroscopy with a prototype preparation chamber, permitting the transfer of samples from liquid to the analysis chamber under Helium protective gas preventing exposure to ambient conditions. Solutions with a concentration of 250 ppm of the respective ethanolamine oligomers in double distilled water were transferred into the adsorption device, which is an extension of the spectrometer. Results show that compounds bond to copper and steel surface by N atom present in ammonia group and hydroxyl group are oriented on the top layer of tribofilm, this highest group influence lubricity properties.     

Development of amorphous carbon coating with luminescent silica/CdSe/ZnS quantum dots underlayer for wear monitoring

Amorphous carbon (a-C) coating with luminescent wear-sensing underlayer is developed for achieving the tribological coating with wear monitoring capability. Wear monitoring of a-C coating by luminescent spectroscopy to determine the remaining thickness of the coating after the wear test is also demonstrated. The coating structure consists of a-C films deposited by R.F. magnetron sputtering method onto the luminescent layer, which is made from silica coating containing CdSe/ZnS quantum dots (Silica/QD). A thin Si-intermediate layer is added between a-C and silica/QD layer for adhesion improvement. The physical as well as tribological properties of the coatings are analysed. Furthermore, wear monitoring of a-C films is also demonstrated to determine the remaining coating thickness after the tribological test. The demonstration is carried out by firstly formulating a relation between luminescent intensity detected from the coating and coating thickness. Then the luminescent intensity is measured again from the wear track. The remaining thickness is finally determined by using the relationship between luminescent intensity and coating thickness. The fabricated coating exhibits a smooth surface with the average surface roughness of 1.35 nm and a friction coefficient of 0.1. The demonstration of wear monitoring shows that the remaining thickness of the coating after the tribological test determined by luminescent spectroscopy is compared well with the thickness measured by profilometry. This suggests that wear monitoring of a-C films by luminescent spectroscopy technique is feasible.     

A tribological study of a ternary complex of zinc with dioxyethylenated octylphenol phosphate in water

A novel aqueous antiwear additive, a ternary complex of zinc with dioxyethylenated octylphenol phosphate and triethanolamine (TXOZ + TEA), has been synthesised. Its tribological behaviour in water was evaluated using a four‐ball tester. The results indicate that the complex in water exhibits higher load‐carrying capacity than mineral oil plus ZDTP, and excellent antiwear behaviour. The elemental composition of its rubbed surface was investigated with Auger electron spectroscopy (AES), and the results of this analysis are presented in the paper. The action mechanism of the additive is also discussed.     

Characterization of Tribofilms Generated from Serpentine and Commercial Oil Using X-ray Absorption Spectroscopy

The chemical and tribological properties of serpentine particles suspended in lubricating oil were investigated using a pin-on-disk high frequency friction machine at 100 °C. The wear scar width of the upper steel pins was measured by an optical microscope. The tribofilm was characterized by scanning electron microscopy (SEM), energy dispersive X-ray (EDX) elemental mapping, and X-ray absorption near-edge structure (XANES) spectroscopy. It was found that the addition of serpentine to commercial engine oil improves its tribological properties. The SEM and EDX elemental mapping shows that a tribofilm formed by the commercial oil with serpentine contains silicon, magnesium, oxygen, phosphorus, sulfur, zinc, calcium, and carbon on the worn surface, which is different from the tribofilm formed by the commercial oil without serpentine. The results of the XANES analysis show that the addition of serpentine to the commercial oil changes the chemical compositions of the tribofilms. This change may account for the better tribological properties of the lubricating oil containing serpentine. The formation mechanism of the tribofilm is discussed.     

Tribochemistry of Bulk Zinc Metaphosphate Glasses

Zinc polyphosphate glasses are the principal component of the antiwear tribofilms formed on steel surfaces in the presence of additives, such as zinc dialkyldithiophosphates. In this work, amorphous, zinc metaphosphate glasses have been synthesized and characterized by means of X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FT-IR), elemental analysis, and X-ray photoelectron spectroscopy (XPS). Tribological tests were performed by rubbing steel balls against the zinc metaphosphate discs in a poly-α-olefin (PAO) bath at room temperature. XPS was used in order to characterize the tribostressed areas on both metaphosphate discs and steel balls. A transfer film, constituted of iron and zinc polyphosphates, was formed on the contact area of the balls. This transfer film was found to reduce friction and prevent ball wear. A reduction in the relative intensities of XPS signals related to bridging-oxygen species and a binding energy shift of 0.4 eV of the P 2p toward lower values demonstrated the presence of shorter-chain-length phosphates inside the tribo-tracks on the discs. Furthermore, iron was transferred to the glass during the tribological tests. A tribochemical reaction between zinc metaphosphate and iron oxide has been proposed as an explanation for the depolymerization of the glass and the formation of iron phosphate.     

The Effectiveness of the ZDDP Tribofilms in Lubrication of Sol–Gel Titania in Technical Dry Friction Conditions

The objective of this study was the investigation of the tribological properties of the sol–gel derived titania modified by physically deposited zinc dialkyldithiophosphate (ZDDP) films. Titania coatings were prepared on silicon wafers Si(100) using sol–gel dip-coating method. Amorphous, anatase, and rutile titania were obtained in the post-preparation annealing process conducted at 100, 500, and 1000 °C, respectively. Deposition of ZDDP having butyl- (C4) or dodecyl- (C12) alkyl chain was performed by means of dip-coating (DC), self-assembly (SA), and Langmuir–Blodgett (LB) methods. The effectiveness of the modification was monitored by the wetting contact angle measurement. An increase of the surface hydrophobicity was observed upon modification. The surface topography, imaged with the use of Atomic Force Microscopy (AFM), revealed the presence of island-like agglomerates having different size of ZDDP films deposited using DC and SA method. Smooth and compact C12ZDDP films were observed when LB deposition was applied. The tribological performance of the ZDDP films on titania coatings was tested with the use of microtribometer operating in the normal loads range of 30–100 mN in the technical dry friction conditions. It was found that ZDDP tribofilms effectively decrease the coefficient of friction and effectively reduce the wear of titania coatings.     

Enhancing the Tribological Behavior of Lubricating Oil by Adding TiO2, Graphene,and TiO2/Graphene Nanoparticles

This article investigates the tribological behavior of nanoparticles (NPs) of titanium dioxide anatase TiO₂ (A), graphene, and TiO₂ (A) + graphene added to the pure base oil group ΙΙ (PBO-GΙΙ). The morphology of these two nanostructures of TiO₂ (A) and graphene was characterized by transmission electron microscopy (TEM). Oleic acid (OA) was blended as a surfactant into the formulation to help stabilize the NPs in the lubricant oil. A four-ball test rig was used to determine the tribological performance of six different samples, and an image acquisition system was used to examine and measure the wear scar diameter of the stationary balls. Field emission–scanning electron microscopy (FE-SEM) was used to examine the wear morphology. Energy-dispersive X-ray spectroscopy (EDX), element mapping, and Raman spectroscopy were employed to confirm the presence of (TiO₂ (A) + graphene) and the formation of a tribolayer/film on the mating surfaces. Moreover, a 3D optical surface texture analyzer was utilized to investigate the scar topography and tribological performance. The experiments proved that adding (0.4?wt% TiO₂ (A) + 0.2?wt% graphene) to the PBO-GΙΙ optimized its tribological behavior. These excellent results can be attributed to the dual additive effect and the formation of a tribofilm of NPs during sliding motion. Furthermore, the average reductions in the coefficient of friction (COF), wear scar diameter (WSD), and specific wear rate (SWR) were 38.83, 36.78, and 15.78%, respectively, for (0.4?wt% TiO₂ (A) + 0.2?wt% graphene) nanolubricant compared to plain PBO-GΙΙ lubricant.     

Effect of Surface Oxide Layer of Steel on the Tribological Characteristics of Load-bearing Additives for Multiply-Alkylated Cyclopentane Oil under High Vacuum

The tribological performance of two types of additives—alkylated phenyl phosphate and lead naphthenate—dissolved in multiply-alkylated cyclopentane was evaluated under a high vacuum using two types of ball-on-disk tribometers: a reciprocating motion tribometer under mild loading conditions and a unidirectional rotating motion tribometer under heavy loading conditions. A ball and a flat disk made of SUS440C stainless steel were used as specimens for both tribometers. The surface of the as-received flat disk specimen was covered with a thick (>40 nm) oxide layer. For the examination of the effect of the surface oxide layer on the tribological performance of the liquid lubricants under a high vacuum, another specimen with an oxide layer (approx. 4 nm thick) was prepared. The alkylated phenyl phosphate additive showed better lubrication performance with the specimen with the thicker oxide layer, but the lead naphthenate additive showed superior performance with the thinner oxide layer specimen. It is also shown that these opposite tribological characteristics are explained by the hard and soft acids and bases principle.     

Study of the Interaction of ZDDP and Dispersants Using X-ray Absorption Near Edge Structure Spectroscopy—Part 2: Tribochemical Reactions

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.     

Alkyl Imidazolium Ionic Liquids as Friction Reduction and Anti-Wear Additive in Polyurea Grease for Steel/Steel Contacts

Five room temperature ionic liquids (ILs), 1-butyl-3-methylimidazolium hexafluorophosphate (L-P104), 1-hexyl-3-methylimidazolium hexafluorophosphate (L-P106), 1-octyl-3-methylimidazolium hexafluorophosphate (L-P108), 1-decyl-3-methylimidazolium hexafluorophosphate (L-P110), and 1-hexyl-3-methylimidazolium tetrafluoroborate (LB106) were studied as 1 wt% additives of polyurea grease for steel/steel contacts. Their tribological behaviors as additives of polyurea grease for steel/steel contacts were evaluated on an Optimol SRV-IV oscillating reciprocating friction and wear tester and an MRS-1J (G) four-ball tester at room and high temperatures. The friction test results showed that the ILs, as 1 wt% additives in polyurea grease for steel/steel contacts, had better friction reduction and anti-wear properties at high temperature than at room temperature, and ILs can significantly improve the friction reduction and anti-wear properties of polyurea grease compared with base grease containing 1 wt% of zinc dialkyldithiophosphate (T204). The excellent tribological properties are attributed to the formation of a surface protective film composed of FeF₂, nitrides, and compound containing the P–O bonding on the lubricated metal surface by a tribochemical reaction. The ordered adsorbed films and good miscibility of ILs with the base grease also contributed to the excellent tribological properties. Wear mechanisms and worn steel surfaces were studied by a PHI-5702 multifunctional X-ray photoelectron spectrometer and a JSM-5600LV scanning electron microscope.     

Improvement of the tribological properties of Al6061–T6 alloy under dry sliding conditions

The tribological properties of ultrasonic nanocrystalline surface modification (UNSM) treated Al6061–T6 alloy were investigated at various normal loads under dry sliding conditions. Electron backscattering diffraction (EBSD) analysis revealed a microstructure alteration of about 70 μm in thickness generated by the UNSM technique. The friction test results showed that the friction coefficient and wear rate of the UNSM-treated specimen reduced by about 25 and 20% compared to that of the UNSM-free specimen, respectively. X-ray photoelectron spectroscopy (XPS) results showed that the oxide percentage on the worn surface increased, but that of carbon percentage decreased after the UNSM treatment.     

Effect of superconducting transition on tribological properties of materials

The results of the tribological study of metal superconductors (lead, niobium, vanadium, tantalum) and high-temperature superconducting ceramics YBa₂Cu₃O_7−δ at temperatures lower and higher than the critical temperature of the superconducting transition are presented and discussed using background literature data. New information is given on the effect of the superconducting transition on the friction coefficient and the friction surface properties of lead and superconducting ceramics. Some general rules have been determined and the differences in friction behaviour of the materials studied have been observed for the superconducting and normal states. It was shown that it would be advantageous to carry out subsequent studies under similar conditions to reach superconducting and normal states in order to exclude the temperature and environmental effects on the friction behaviour of various superconducting materials.     

Effects of linear PFPAE decomposition products on the rolling contact fatigue performance of M50

In bench tribology tests, the influence of lubricant decomposition products on the lubricated system and on lubricant performance are often overlooked, primarily because testing is performed in vented, or open, systems. However, the lubricant in a gas turbine engine might be expected to more closely follow the principles of interaction that are found in unvented, or closed, systems. Results reported here comparing vented and unvented bearing housings in rolling contact fatigue (RCF) tests with a linear perfluoropolyalkylether (PFPAE) lubricant and VIM-VAR M50 steel at 316°C and a stress of 4.8 GPa clearly show that significantly more wear, corrosion, and fluid breakdown occur in the closed system than in the open system. Under these conditions, PFPAEs catalytically decompose to corrosive products. These corrosive products are partially vented in the open system, but retained in the closed system, causing more extensive corrosion of bearing materials, and significantly affecting the lubricant's tribological performance. Post test lubricant was analyzed for viscosity, acid number, and metals to assess changes in the lubricant's physical and chemical properties. Changes were more severe in the closed system. The nuclear magnetic resonance (NMR) and Fourier transform infrared (FTIR) spectra also showed significantly more carboxylic acid buildup in the stressed fluid from the closed system. The films formed in the tribojunction were analyzed using X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES). Organic films formed in the open system reduced wear while inorganic films formed in the closed system resulted in higher wear. Consequently, we conclude that more attention needs to be given to the effect of decomposition products during bench type tribological testing of high temperature lubricants. This revised version was published online in July 2006 with corrections to the Cover Date.     

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,electrochemical, and spectral analysis of zinc dialkyldithiophosphates (ZDDP) of different thermal stabilities

The antiwear properties of five zinc dialkyldithiophosphates, which differed in thermal stability, were evaluated in a 4-ball machine by a preliminary wear track formation method. After testing the additives, the wear resistance of the films formed was measured by the increase in preliminary wear scar diameter over that of the additive-free oil. IR spectroscopy was then used to study the transformation products of additives after thermostatic testing at between 100 and 240°C on steel surfaces, where the interaction products were evaluated by electrode potential method. The interaction of the decomposition products of dithi-ophosphates with metal surfaces was found to be followed by a change in electrode potential. For the tested additives, a relationship is shown between antiwear effectiveness of the film on a friction track during tribological tests and the electrode potential value of the steel sample. Different parts of the relationship between the electrode potential and temperature relate to different stages of dithiophosphate decomposition measured by IR spectroscopy. It can be said that chemical modification of the steel surface which produces an improvement of antiwear properties, starts at the decomposition temperature of the dithiophosphate neutral salt.     

Industrial Gear Oils: Tribological Performance and Subsurface Changes

This study examined the tribological performance of three gear oils (Oils A, B and C), in relation to surface and microstructural changes. Oil A contains molybdenum dithiophosphate friction modifier, Oil B contains amine molybdate combined with zinc dialkyl dithiophosphate antiwear additive, while Oil C contains phosphonate and a commercial gear oil package. Following sliding tests of a hardened AISI 52100 steel ball on a spheroidized AISI 52100 steel disc, the worn surfaces were chemically studied using Raman and energy-dispersive X-ray spectroscopy. The tribological performance for each oil was different, likewise the nature of the tribofilm formed. After a 5 min sliding test, the hardness-depth profile of the worn surfaces was measured; also the cross-sectional microstructure was examined using scanning electron microscopy combined with focused ion beam preparation and transmission electron backscattered diffraction (t-EBSD) techniques. With Oil A, there was a relatively small increase in surface hardness (33% greater than that of the unworn surface), whereas with Oils B and C, the average hardness near the surface was 100% greater than that of the unworn surface. The cross-sectional microstructure using Oil A also differed from Oils B and C, which were quite similar. The result shows that with Oil A refinement of the ferrite grains spreads deeper into the material (>?10 µm), whilst with Oils B and C it was largely limited to 2–3 µm below the surface. It is concluded that the lubricant formulations and their associated tribofilms influenced the extent of deformation in the subsurface layers and consequently influenced the wear performance.