Mineral Base Oil Research Articles

The lubrication of DLC coatings with mineral and biodegradable oils having different polar and saturation characteristics

Due to improved performance over the last decade, diamond-like carbon (DLC) coatings are more frequently used in highly loaded mechanical components that sometimes need to operate under boundary- or mixed-lubrication conditions. However, DLC coatings are considered as “inert” coatings with a low surface energy and their lubrication ability according to conventional metal-lubrication mechanisms is therefore questionable. In order to investigate whether the base oil polarity and saturation characteristics play a role in these processes, a tribological investigation of the a-C:H coating lubricated with natural (sunflower) and synthetic (saturated and un-saturated) biodegradable oils that posses different amount of polar components and double bonds was performed. For a comparison we also used mineral base oil with low polar component. The DLC/DLC and steel/steel contacts were tested with base oils without additives and in combination with AW and EP additives. Despite the higher wear compared to steel/steel contacts, the results suggest that the wear of DLC/DLC contacts can be importantly improved by using oils with more polar groups and non-saturated molecules. These findings, together with well-known and even proverbial “low-friction properties”, suggest a great potential for the use of DLC coatings in combination with biodegradable oils, particularly when additives are present.

Rheological properties of high‐viscosity‐index mineral base oils for automotive engine lubricants

AbstractViscosity‐pressure‐temperature relations for paraffinic mineral base oils at pressures up to 0.7 GPa and temperatures between 30 and 90°C were determined using a falling‐ball‐type viscometer. The oils used were solvent refined oils, hydrocracked oils, and an oil produced by a wax isomerisation process. The viscosity at pressures higher than those possible with the viscometer was then derived by applying a simplified solution to the traction curves determined using an elastohydrodynamic disc‐on‐ball tester. When the measured viscosity and the calculated viscosity were plotted against pressure, for the oils with a viscosity index higher than 120 the viscosity derived from traction measurements followed the curve extrapolated to the high‐pressure region using either the Yasutomi or Roelands equations (the parameters for which were obtained using the viscometer). However, the calculated viscosity for the lower‐viscosity‐index oils deviated upwards from the extrapolated curve.

Particle Emissions from a Small Two-Stroke Engine: Effects of Fuel, Lubricating Oil, and Exhaust Aftertreatment on Particle Characteristics

The effects of fuel and lubricating oil formulation and exhaust catalytic aftertreatment on physical and chemical characteristics of two-stroke engine exhaust particles were studied. The exhaust particles were produced with a professional chainsaw engine. The employed fuels were a 98-octane oxygenated, low-sulfur, low-aromatic reformulated gasoline, which served as a reference, and a 95-octane nonoxygenated alkylate gasoline that had no aromatics and olefins. The applied lubricating oils were a semisynthetic mineral-based oil and a biodegradable ester-based oil. In total eight fuel–lubricating oil–catalyst combinations were studied. The test runs were conducted on a test bench and exhaust was diluted in a full-flow dilution tunnel. The size and number emissions of the exhaust particles were measured with a scanning mobility particle sizer (SMPS). The organic carbon (OC) and elemental carbon (EC) composition of the particles were analyzed with a thermal–optical transmission analyzer (TOT). In addition, the inorganic ion and metal composition of the particles were quantified, and the gaseous total hydrocarbon (THC), carbon monoxide (CO), and nitrogen oxide (NOx) emissions were measured. The volatility characteristics of the exhaust particles were studied with a thermal desorption unit combined with the SMPS. The particle mass (PM) emissions ranged without catalyst from 2.9 to 3.4 g/kWh and with catalyst from 1.7 to 2.4 g/kWh, the catalytic converter thus reducing PM emissions by 19–50%. Without catalyst the alkylate fuel–biodegradable oil combination gave the highest particle mass emissions, but with catalyst with the same fuel–oil mixture the emissions were the lowest. The count median diameter (CMD) of the particles ranged from 57 to 123 nm. Without catalyst, the alkylate fuel–biodegradable oil combination gave the lowest number emissions, but with catalyst with the same fuel–oil combination the emissions were the highest. The catalytic converter reduced the particle size by 22–56 nm, but it also increased the number emissions by a factor of 1.3–2.6. In thermal analysis 88–98% of the exhaust particle volume proved to be volatile, the solid fraction consisting of elemental carbon and metal residues. With the mineral-based lubricating oil, the metal residues appeared in two modes at the evaporation temperatures of 350°C and higher, while in the particles produced with the biodegradable oil the residues were unimodally distributed.

Transmission Electron Microscopy of Boundary-Lubricated Bearing Surfaces. Part II: Mineral Oil Lubricant with Sulfur-and Phosphorus-Containing Gear Oil Additives

Transmission electron microscopy (TEM) was performed on cross-sectional samples of tapered roller bearing cone surfaces that were tested at two levels of local boundary lubrication severity, Λ ∼ 1.1 and 0.3. Unlike our previously reported work in which a base mineral oil was used, the bearing tests were conducted in mineral oil with sulfur- and phosphorus-containing gear oil additives. Structural and compositional characterization of undetached antiwear surface layers on the base steel (cone raceway) revealed that the films contained crystalline and amorphous regions. A sharp interface (<∼10 nm) that separated the surface layer and base steel was imaged. The surface layer for the cone tested at Λ ∼ 1.1 consisted of Fe, O, and P, whereas that for the cone tested at Λ ∼ 0.3 consisted of Fe, O, P, C, Ca, and S. Various TEM analytical techniques were used to study the segregation of these elements throughout the antiwear surface layer volume.

Tribological performance of three advanced piston rings in the presence of MoDTC-modified GF-3 oils

The piston assembly (piston ring and cylinder bore) is one of the key parts of the internal combustion (IC) engine. Its performance will directly determine the performance of the whole engine. The piston assembly’s tribological performance will be influenced by both its mechanical properties and the tribochemical interactions that take place on their surfaces. In this paper, three kinds of advanced stainless steel piston rings with a single nitrided layer, CrN coating on the nitrided layer and a B4C and CrN binary-layer coating on the nitrided layer, respectively, were employed. Their frictional behavior and wear performance, when sliding against the cast iron cylinder bore materials lubricated with two kinds of GF-3 category mineral-based engine oils (one of them blended with MoDTC friction modifier), were investigated on a SRV tribotester. The test conditions were set and maintained to simulate engine-operating conditions. SEM (Scanning Electron Microscopy) and EDX (Energy Dispersive X-ray spectroscopy) were employed to characterize the morphology and elemental composition of the wear tracks. Tribotests and analysis results indicate that changes in both the mechanical properties of the tribomate (piston coatings) and tribochemical interactions (formulation of engine oils) have an impact on the tribological performance of the piston assembly. Tribochemical interactions will have a more obvious influence on friction coefficients while the mechanical properties of the tribomate have a more obvious influence on wear.

Effect of additives on friction during plane strain compression of aluminium strip

The paper describes an experimental investigation of the tribology during plane strain compression testing (PSCT) of aluminium strip with steel tools, lubricated by mineral base oils containing either stearic acid or aluminium stearate as additive in concentrations up to 0.5%. The present study is a follow-up to the work of Sutcliffe et al. [Boundary lubrication of aluminium strip during plane strain compression, Wear 254 (2003) 65–79]. The close conformance of the strip and tool indicate that boundary lubrication should play a significant role in the tribology. The new work supports the conclusions of the previous study, that for the first indent hydrodynamic influences are important, while additives do not affect friction. After a number of indentations a transfer layer builds up on the tool. Depending on the additive concentration and tool roughness, this may either lead to a rise or fall in friction factor. Scanning electron microscope and optical observations show how the transfer layer can either form a smooth mottled appearance which is easily removed using tissue paper for higher additive concentrations, or takes the form of strongly attached particles for lower additive concentrations or in the absence of additives. Similar behaviour is observed for the two types of additive, indicating that the stearate chain in the soap is able to react with the Fe-based tool as effectively as when it is part of the acid—and probably also with the Al strip.

Influence of carbon coatings on lubricating properties of boundary layers

The friction behaviours of two types of diamond-like carbon coating (amorphous and monocrystalline) were studied under boundary lubrication. As lubricants, liquid paraffin with and without surfactant additives (stearic and oleic acids) were used as well as base mineral oil with viscosity corresponding to SAE 30. Tests were conducted on a ball-on-disc tribometer and on a tribometer where two rings were rubbing against each other. It was found that in all types of test the monocrystalline coating ensures minimal values of friction coefficient in a wider range of temperatures, whereas the amorphous coating has maximum values of friction coefficient compared with the monocrystalline coating and with the reference uncoated steel pair. This effect takes place for all lubricants studied. The observed experimental data can be explained by the strong orienting effect of the monocrystalline carbon coating on the structural order of molecules in boundary layers and correspondingly on their lubricating ability. The effect of the amorphous coating on molecular orientation in boundary layers is rather poor.

Primary and ultimate biodegradabilities of mineral base oils and their relationships with oil viscosity

We have determined the primary and ultimate biodegradabilities of a series of paraffinic base oils representative of the main classes that are used for the production of lubricating oils, such as engine, industrial, and marine oils. Primary and ultimate biodegrability (B) data were determined using the CEC test and the modified Sturm test (OECD 301B), respectively. There was a strong relationship between the two B values. For a given oil, biodegradation in the modified Sturm test was logically lower than that given by the CEC test. The B values of mineral oils were compared to oil (kinematic) viscosities (KV) known to affect significantly oil biodegradability. Both ultimate and primary B values were linearly related to log(KV).

The temperatures at scuffing and seizure in a four‐ball contact

AbstractThe temperature of surface asperities affects lubricant‐surface tribochemical interactions. It is important to know the nature of this to identify ways of preventing scuffing and seizure under extreme‐pressure (EP) conditions. A new model for the determination of the temperature of contacting asperities is presented in this paper. It assumes the superposition of thermal processes occurring on the macroscale and thermal phenomena in the contact of asperity tips (microscale). Numerical results have been obtained for conditions of four‐ball testing of various lubricating oils — a mineral base oil with and without antiwear and EP additives. To calculate the scuffing and seizure temperatures, knowledge of the mechanical and physical properties of the test ball material (bearing steel) and lubricants, as well as the parameters describing the surface topography of the balls, was necessary. Friction coefficient curves were also needed; they were determined during four‐ball tests with a continuously increasing load. For the base oil with lubricating additives, the temperature of contacting surface asperities at the moment of scuffing initiation was calculated to be about 230°C and increased to over 1000°C at the highest loading of the four‐ball tribosystem. This suggests the possibility of tribochemical reactions of the lubricating additives with the steel surface, and diffusion of some elements, a modified surface layer having good antiseizure properties being produced. Such a layer prevents seizure of the tribosystem. For the base oil without lubricating additives, scuffing initiated at about 150°C and the temperature exceeded 1200°C at seizure. The temperature values obtained agree with results in the literature.

Performance evaluation of vegetable-based oils in drilling austenitic stainless steel

The efficiency of six cutting oils was evaluated in drilling AISI 316L austenitic stainless steel using conventional HSS-Co tools by measurements of tool life, tool wear, cutting forces and chip formation. Seven tools were tested with each fluid to catastrophic failure. Cutting forces and chip breaking were recorded for each bore, and tool wear was measured at constant intervals. A commercial mineral-based oil was taken as reference product, and five vegetable-based cutting fluids at different levels of additivation were tested. All measured parameters were in agreement, confirming that a performing fluid produces longer tool life, better chip breaking, lower wear and cutting forces. In particular, good correlation was found between tool life and cutting forces. Differences in cutting forces due to the fluid could be measured with a higher repeatability than tool life, thus resulting in a measurement capability comparable to that obtained using tool life as a performance criterion. As a consequence, it is suggested that drilling thrust can be used to assess the performance of cutting fluids in drilling stainless steel, resulting in considerable time savings and cost reduction with respect to tool life testing. All vegetable-based fluids performed better than the reference product. The best performance was obtained with a cutting fluid yielding 177% increases in tool life and 7% reduction in thrust force.

Charging Ability of Pure Hydrocarbons and Lubricating Oils

An experiment using a purpose-built test rig to examine the effect of oil chemistry on the charging ability of various oils revealed that the charge produced depends on two extrinsic properties; the electric field (used to charge the droplet) and the droplet radius. A normalization parameter was developed from these dependencies and used to compare the results from the tests described herein. Four classes of liquids were tested. These were pure hydrocarbon liquids (hexadecane, decane, and squalane), base oils (both mineral and synthetic stocks), formulated oils (mineral base oil with additives), and commercial oils (fully formulated mineral and synthetic oils, available over-the-counter). The charging for both pure hydrocarbons and base oils indicates that the charged species have a low mobility and thus are likely to be large, bulky molecules. For this reason, the charging of the base oils is noticeably time dependent and the charge imparted on the oil did not always reach saturation. The charging (rate) of base oils appears to follow the level of impurities (particularly sulfur). Generally, both pure hydrocarbons and base oils tend to charge more when a positive field is applied. Conductivity values help to explain the measured charge levels. This would be expected as conductivity is related to the ionic species (charge carriers) present. The majority of the formulated and commercial oils tested reached charge saturation for all conditions tested, resulting in a normalized value of around 0.1 pC mm− 1 V− 1 , comparable with a theoretical calculated value. The formulated and commercial oils that did not reach saturation have conductivities comparable with base oils and therefore behave in a fashion similar to the base oils.

X-ray absorption spectroscopy of tribofilms produced from zinc dialkyl dithiophosphates on Al–Si alloys

X-ray absorption near edge structure (XANES) analysis has been used to characterize the chemistry of tribologically derived phosphorus films from a mineral base oil with a zinc dialkyl dithiophosphate (ZDDP) additive. These species were detected in the wear scars found on both counter-surfaces from a reciprocating boundary contact between different aluminum–silicon alloys and either the same alloy or with a steel counter-surface. Two different Al–Si alloys were used which had significantly different silicon contents, 0.7% (aluminum 6061) and 7% (aluminum 319), respectively. Couples between A319/A319, A319/52100 steel, 52100 steel/A319, A6061/A6061, A6061/52100 steel, and 52100 steel/A6061 were investigated. The phosphorus K- and L-edge spectra show that with the onset of wear of the aluminum matrix an aluminum phosphide species is found in the wear scar. Furthermore, unreacted-ZDDP, a linkage isomer, and polyphosphate species were detected in many of the wear scars. The spectra showed little chemical-variation with increase in rubbing time for each series studied. Excluding the 52100 steel/Al alloy couples, it was found that for the various A319 couples the A319/52100 steel performed the best. For the A6061 couples, the A6061/A6061 couple was the best. In all cases the modes of wear are strongly dependent on the yield strength of the materials used.

Tribological behaviour of S‐esters and amine salts of the O, O‐polybutoxyglycol diester of dithiophosphoric acid in mineral oil solution

AbstractThe tribological properties of derivatives of the polyalkoxyglycol esters of dithiophosphoric acid were tested using four‐ball and ball‐on‐disc machines. Polybutoxyglycol dithiophosphoric acid derivatives were found to be effective load‐carrying additives in mineral base oil. Four‐ball and ball‐on‐disc data of some of these additives showed them to have very good antiwear/extreme‐pressure and antifriction properties. These properties were dependent upon the additive's chemical nature, concentration, sliding speed, and load. The presence of polybutoxy groups decreased the concentration of phosphorus in the additive in comparison to zinc dialkyldithiophosphates.

Respiratory Toxicology of Mineral Oils in Laboratory Animals

Early subchronic and chronic inhalation toxicology studies on various petroleum mineral oils and formulated lubricants supported the ACGIH TLV of 5 mg/m(3) for mineral oil mist. Additional subchronic studies with aerosolized mineral base oils and lubricants during the last 15 years demonstrated that exposures to aerosols of mineral base oils (often >100 mg/m(3)) resulted mainly in concentration-related accumulation in the lung of alveolar macrophages laden with oil droplets. Inflammatory cells were observed with higher aerosol concentrations, consistent with the clinical literature from highly exposed workers. These pulmonary changes appeared to be a nonspecific response to the presence of deposited aerosol. Studies on sensory irritation during exposures of lab animals to mineral oils indicate possible effects only with very high aerosol concentrations. Coupled with changes in refining to remove carcinogenic polynuclear aromatic hydrocarbons from mineral oils several decades ago, this information indicates that current aerosols of mineral oils have a profile of low toxicity from acute to long-term exposures. Available information suggests that additives in some formulated products and/or maintenance of mineral-based metalworking fluids may play a much more significant role in potential health effects.

Development of a superlattice (Ti,Hf,Cr)N coating for cold metal forming applications

Tribosystems in mechanical engineering put high demands towards tools, surface technology and lubrication. Cold forming processes in particular represent a significant challenge for wear reduction. Mineral-based oils with high contents of ecologically harmful additives are state of the art due to high abrasive and adhesive wear. In order to reduce the ecological impact and to increase tool life, biodegradable lubricants and hard coatings have been developed, which are deposited by modern vacuum technologies. In the collaborative research center ‘Environmentally friendly tribological systems’ (SFB 442), which has been established by the German Science Foundation (DFG), a new arc-PVD Ti–Hf–Cr–N coating has been developed. This nanolayer system with single layers of different hardness is supposed to fulfill the requirements of high hardness and abrasive wear resistance while possessing low Young's modulus values at the same time. In this paper, the results of coating characterization based on various standardized tests and on microscopic analysis are shown. The influence of the structure and the coating process on the performance is evaluated using nanoindentation and pin-on-disc tests.

Composition and Analysis of Mineral Oils and Other Organic Compounds in Metalworking and Hydraulic Fluids

Metalworking fluids (MWFs) are the primary source of organic contaminants in oily wastewater at many manufacturing facilities. Such facilities, including those in the automotive industry, face the continuing challenge of improving wastewater treatment for organics. Hydraulic fluids (HFs), chemically similar to MWFs, have been a major source of costly soil contamination at industrial and commercial facilities.A better understanding of these organics is essential to the treatment of oily wastewater and HF decontamination. Therefore, two aspects were reviewed: (1) current and historical organic chemical compositions of MWFs and HFs and (2) existing and emerging analytical methods. Base mineral oils were emphasized, as they are the primary ingredient of these fluids and the main contaminant in oily wastewater and HF-contaminated soil. Hydrocarbon compositions of “naphthenic” (cycloalkanes) and “paraffinic” (straight and branched noncyclic alkanes) mineral oils were described, as were the chemical nature of the various MWF and HF additives.Analytical methods for gross organic measurements, individual organics, and structure- and sizebased fractions and distributions were reviewed. Most promising for characterizing the base oil fraction were gas chromatography (GC) coupled with two soft ionization mass spectrometry detection methods and comprehensive 2-dimensional GC. These methods can provide complete distributions of hydrocarbon structures and sizes and are potentially useful to monitor hydrocarbon fate in wastewater treatment, soil/sediment remediation, or other applications involving complex hydrocarbon mixtures.

Viscosity-Pressure-Temperature Characteristics of High Viscosity Index Mineral Base Oils and Their EHL Traction

Viscosity-pressure-temperature relations at a pressure up to 0.7 Gpa and a temperature between 30 and 90°C were determined with a falling ball type viscometer for some paraffinic mineral base oils including solvent refined oils, hydrocracked oils and the oil produced by a wax isomerization process. Then, the viscosity at a higher pressure condition than that of the viscometer was derived by applying the simplified non-Newtonian solution to the traction curves determined with a ball on disk type EHD tester. When the measured viscosity and the calculated viscosity were plotted against pressure, for the oils with higher viscosity index than 120, the viscosity derived from traction measurements followed the extrapolated curve to the high pressure region by either the Yasutomi formula or the Roelands formula using the parameters obtained with the viscometer. On the other hand, the calculated viscosity for the higher viscosity index oils deviated upwards from the extrapolated curve.

Multiple regression modelling of mineral base oil biodegradability based on their physical properties and overall chemical composition

A set of 38 mineral base oils was characterized by a number of chemical (i.e., overall chemical composition) and physical parameters used routinely in industry. Their primary biodegradability was evaluated using the CEC L-33-A-93 test. Multiple (stepwise) linear regression (MLR) analyses were performed to describe the relationships between the biodegradability values and the chemical or physical properties of oils. Chemical, physical, and both types of parameters were successively used as independent variables. Using chemical descriptors as variables, a four-variable model equation was obtained that explained only 68.2% (adjusted R-squared statistic=68.2%) of the variability in biodegradability. The fitting was improved by using either the physical or the whole parameters as variables. MLR analyses led to three-descriptor model equations involving kinematic viscosity (as log), Noack volatility (as log) and either the viscosity index (pure physical model) or the paraffinic carbon percentage (mixed chemical–physical model). These two models displayed very similar adjusted R-squared statistics, of approximately 91%. Their predicting ability was verified using 25 additional base oils or oil blends. For 80% of oils on a total of 63, the absolute percentage error on biodegradability predicted by either model was lower than 20%. Kinematic viscosity was by far the most influential parameter in the two models.

Wear in metal/silicon nitride sliding pairs

Ball-on-rotating disc tests were conducted to assess the relative wear behaviour of various disc materials in contact with silicon nitride under lubricated environment at room temperature high load and high speed conditions. Some selected tests were also performed under unlubricated conditions for comparison. The discs were made of grey cast iron, Al-bronze, bronze, aluminium and stainless steel. The lubricant was commercial additive free mineral base oil having a viscosity grade of 68. The applied normal load ranged between 16 and 300 N. The sliding speed changed from 0.5 to 4.0 m/s. The behaviour of grey cast iron and Al-bronze in sliding contact with silicon was characterised by a decrease in wear with an increase in sliding speed. Bronze and aluminium experienced very high wear rates as sliding speed increased. Stainless steel exhibited high wear and also caused significant damage to silicon nitride. Severity of wear between stainless steel and silicon nitride increased with the speed. High wear occurred at both low and high sliding speeds, which were accompanied by high wear track temperatures on the disc surface. This phenomenon resulted in considerable wear on both mating surfaces.

A method for testing lubricants under conditions of scuffing. Part II. The anti‐seizure action of lubricating oils

AbstractThis paper describes an investigation of lubricating oils under extreme‐pressure (EP) conditions in a specially modified four‐ball tester. A new test method developed at the Tribology Department of ITeE described in Part I of this paper was used. In this, during a test run, the applied load is increased continuously and the friction torque is measured. A sudden increase in the friction torque indicates the collapse of the lubricating film — where scuffing is initiated. The load at this moment is called the scuffing load. If the load is increased further, it is possible to observe scuffing propagation until seizure occurs, i.e., a defined, maximum friction torque is reached. Thus, scuffing is considered as a process leading to seizure.Using the method, tribological experiments were performed employing various lubricating oils consisting of viscosity‐index improvers and antiwear (AW) and extreme‐pressure (EP) additives added to a base oil. Mineral and synthetic base oils of different kinematic viscosities were used. The aim was to investigate the influence of such lubricants on scuffing initiation and propagation with the present methodology.In Part I it was shown that scuffing initiation depends strongly on the kinematic viscosity of the lubricant; the higher the viscosity, the greater the scuffing load. The presence of AW and EP additives in the lubricant increases the scuffing load significantly. It was also shown that the kinematic viscosity of the lubricant oils has no effect on scuffing propagation. However, scuffing propagation is significantly mitigated by AW and, to a greater extent, by EP additives. The results of surface analyses show the decisive nature of the chemical reactions of AW and EP additives with the steel ball surface under scuffing conditions, as well as the possible diffusion of sulphur and phosphorus. Chemical reactions and diffusion lead to the creation of an inorganic surface layer (probably iron sulphide), the good anti‐seizure properties of which limit scuffing propagation.