Tribological behaviour of polyalphaolefins: Wear and rolling‐contact fatigue tests

Polyalphaolefin (PAO) fluids have become widely accepted as high‐performance lubricants and functional fluids due to certain inherent, and highly desirable, characteristics. One of these characteristics is their low toxicity, which, combined with excellent viscometrics and lubricity, have made low‐viscosity PAO fluids an important component in lubricant formulations. Typical data found in product specifications for lubricants are the kinematic viscosity and the viscosity index. These values do not give enough information with which to choose the optimum lubricant for a lubricated contact. In mechanical systems, rolling, sliding, and rolling/sliding contacts occur, and lubricants have to work optimally under these operating conditions. In this study the rolling‐contact fatigue lives (L₅₀ and L₁₀) of PAOs of different viscosities were experimentally determined. The tests were carried out using a four‐ball machine. Wear tests were also carried out using another four‐ball tester in order to measure the wear‐scar diameter and the flash temperature parameter. The lubricants were characterised by infrared spectroscopy, and the pitting of the balls was observed using scanning electron microscopy.     

Effect of Lubricant Viscosity and Type on Ball Fatigue Life

Two separate investigations were conducted to determine the effect of lubricants on the fatigue life of M-1 tool steel balls in the rolling contact fatigue spin rig. In the first investigation four paraffinic mineral oils with viscosities of 5 to 113 centistokes at the 100F test temperature were used. Longer life was obtained with more viscous oil, life varying approximately as the 0.2 power of lubricant viscosity. In the second investigation of methyl silicone, a paraffinic mineral oil, a sebacate, a water base glycol and an adipate, each of which had a viscosity of about 10 centistokes at the 100F test temperature, were used. The 10 per cent life was about 40 times as great with the silicone (best) as with the adipate (poorest). The life results correlated fairly well with the pressure viscosity characteristics as estimated from lubricants of the same base stocks.     

Rolling fatigue tests of three polyglycol lubricants

In this work, the rolling fatigue lives of three polyglycols (PAG-9, PAG-12 and BREOX-B-135X) are determined using IP-300 standard. A four-ball test machine was used and 10% life (L₁₀) and 50% life (L₅₀) were obtained. In addition, the stress-time curves for L₁₀ and L₅₀ were also determined. This work showed that: firstly, all polyglycols were tested under boundary lubrication regime (λ < 1) where in rolling contacts the surface mode of failures prevails; secondly, in oils of the same family, the pressure-viscosity coefficient is relatively constant, therefore an increase in viscosity improve the minimum film thickness with the consequent increase of the λ ratio; and finally, differences in λ ratio for the three polyglycols resulted in different asperity interactions and rolling contact fatigue lives.     

Effect of Lubricant Degradation on Contact Fatigue

It is well known that contact fatigue is affected by contact pressure, frictional stress, residual stress, initial distribution of material flaws, and so on. The behavior of contact pressure and, primarily, the frictional stress is determined by the viscous properties of the lubricant used. It is also recognized that lubricants degrade while passing through lubricated contacts. Degradation of lubricants causes viscosity loss that, in turn, reduces the frictional stress and raises contact fatigue life. The objective of this study was to find out the extent to which lubricant degradation may change contact fatigue life of elastic surfaces completely separated by lubricant. The analysis was performed numerically based on the models of contact fatigue and lubricant degradation recently developed by the author. The results showed that contact fatigue life of solids completely separated by lubricants with the same ambient viscosity may vary significantly due to the specific way lubricants are formulated. In particular, contact fatigue life is strongly affected by the initial molecular weight distribution of the polymeric additive (viscosity improver) in the lubricant and contact operating conditions, which in some cases promote fast lubricant degradation caused by high lubricant shearing stresses.     

Boundary Lubrication RCF Performance of 4 cSt Gas Turbine Engine Oils at 204 °C

The rolling contact fatigue (RCF) and wear performance of three qualified formulations of MIL-L-7808K (4 cSt at 100 °C) were evaluated using AISI VIM-VAR M50 steel. The RCF tester differentiated the fatigue life and wear performance of gas turbine engine lubricants in boundary lubrication regime. Tests were conducted using a ball-on-rod type RCF tester at a maximum Hertzian stress of 4.8 GPa and temperature of 204 °C. Although all the three lubricants meet the military specification, the RCF results suggest that life and wear in the boundary lubrication regime are significantly affected by the formulation. Changes in physical and chemical properties of post test lubricants such as viscosity, acid number and additive concentration were minimal. However, the lowest Fe concentration and COBRA reading, which is a measure of the electrical property of the lubricant, correlated with the highest RCF life and lowest wear.     

A study of rolling bearing fatigue life with mineral oil lubrication

Tests on mineral oil lubricated deep groove ball bearings show that there is an optimum lubricant viscosity to ensure maximum rolling contact fatigue life. The results are explained by lubricant film thickness measurements and the application of E.H.L. theory. The optimum lubricant viscosity increases with increase of load and decreases with reduction of speed.     

Behavior of perfluoropolyalkylethers under simulated dynamic bearing conditions

Perfluoropolyalkylether (PFPAE) fluids are of interest to the United States Air Force as potential high temperature liquid lubricants in gas turbine engines. PFPAE fluids have desirable thermal and oxidative stability, and favorable temperature/viscosity characteristics. However, their performance depends on the specific base fluids, additives, bearing material used as well as contact conditions and environments. Screening tests using a modified ball‐on‐rod type rolling contact fatigue (RCF) tester were conducted to study the effects of the above variables and lubricant circulation on fatigue life, wear and performance of PFPAE. Post test lubricant samples were analyzed for changes in physical and chemical properties. Traditional testing for viscosity, acid number and weight changes was performed. Fluid degradation was studied using Fourier transform infrared spectroscopy (FTIR), gas chromatography with atomic emission detector (GC‐AED) and supercritical fluid chromatography (SFC). Elemental analysis of the deposits formed at the tribocontact were determined by Auger electron spectroscopy (AES). This revised version was published online in July 2006 with corrections to the Cover Date.     

Rolling contact fatigue life of AISI 52100 steel balls with mineral and synthetic polyester lubricants with PTFE nanoparticle powder as an additive

This work evaluates the rolling contact fatigue life of AISI 52100 steel bearing balls with mineral and synthetic oil, with and without additive, using a four-ball tester. IP 300/87 was applied with a total load of 600 kg (corresponding to a maximum Hertz stress of 8.709 MPa). SN-350 (a neutral mineral solvent) and TMP-05 (an environment friendly synthetic polyester) of the same viscosity were additivated with PTFE nanoparticle powder in different percentages. The total test time for each specimen was recorded and processed on Weibull probability plots. L₁₀ and L₅₀ are given for pure base oils and oils additivated with 1, 3 and 5 wt.% of PTFE powder. The results point to an increase in the fatigue life when additive-containing oils were used. In order to provide some understanding of the protection mechanism of the additive, the contact angle and the influence of the surface wettability on the lubricant unctuosity were analysed. Scanning electron microscopy and LINK techniques were used to study pitting, crack propagation, the chemical influence of additive and fluorine content.     

Elasto-Hydrodynamic Lubrication of Rolling-Contact Surfaces

An X-ray technique is described for the measurement of the thickness and shape of thin oil films (from 5 to 50 millionths of an inch thick) formed between the rolling and rolling-sliding surfaces of hardened steel rollers as in rolling bearings and gears. The film thickness with white mineral oil, diester-base, and silicone lubricants was found to vary with temperature (viscosity), speed, and load in much the same way as expected from elasto-hydrodynamic considerations. The shape of the elastically flattened contact regions on the rollers appeared to change with rolling speed in such a way as to explain why ball bearings and gears have longer fatigue life (in revolutions) at high speed than at low speed.

The experimental results are compared with lubrication theory in which the elastic deflections of the bearing surfaces are combined with the viscous flow of the lubricant under pressure. Although there is general agreement between theory and experiment, possible modifications to the theory to account for the increasing discrepancy with experiment at high speed, load, and viscosity are discussed.     

含纳米银粉润滑剂抗接触疲劳性能研究

制备了含纳米银粉的二相流体润滑剂，利用球-棒式接触疲劳试验机研究了纳米银粉对GCr15钢球和钢棒的接触疲劳性能的影响，并借助于扫描电子显微镜分析了钢球接触表面的疲劳点蚀形貌。结果表明：与基础油(10^#机械油)相比，含1％纳米银粉的润滑剂可使GCri5钢球的接触疲劳寿命L10提高3．54倍。其主要原因在于纳米银粉吸附于接触表面形成低剪切膜，降低了摩擦系数．减小了接触表面的切向应力，延缓了疲劳裂纹扩展速度。     

Influence of load and lubricants on EHD film thickness and rolling-contact fatigue lives of AISI 52100 steel balls

This work has evaluated the influence of load and type of lubricant on the thickness of the elastohydrodynamic (EHD) film and the rolling-contact fatigue lives of AISI 52100 steel balls. The lubricants studied have various viscosities and included two mineral oils and five synthetic oils from three families. Firstly, the central film thickness was determined in order to predict the lubrication regime. The stress—number of cycles fatigue curves were then calculated by means of Weibull plots, and the fatigue mechanism was evaluated. The test machine used for the analysis was a Seta-Shell 1980 four-ball EP lubricant tester. The 12.7 mm diameter test balls were made from a single batch of carbon-vacuum-deoxidised AISI 52100 steel with hardness RC65. Elastohydrodynamic film thickness estimation was carried out using pressure—viscosity coefficients (a). In this study, to calculate (X), a new interferometric technique, ultrathin film interferometry, was employed to measure the film thickness. A practical method was developed for evaluating EHD, mixed film, and solid lubrication processes. Micrographic mapping and energy dispersive spectroscopy (EDS) were used to analyse the rolling track of the test balls.     

Evaluation of Hot Pressed Silicon Nitride as a Rolling Bearing Material

A series of tests has been conducted to evaluate the suitability of silicon nitride as a bearing material for rolling contact applications. The ability of silicon nitride to be lubricated by some conventional lubricants was found to be satisfactory. This was determined by wettability studies, lubricant film thickness and traction coefficient measurements on the optical EHD rig and friction coefficient measurements by the pin-on-disk method. The abrasive wear coefficient, measured on a lopping machine using 600 grit SiC abrasive, was found to be high compared to other ceramics. It was also dependent on the composition of the silicon nitride. Comparative rolling contact fatigue tests on steel and silicon nitride flat washers were conducted using steel rollers and balls. A high wear rate leading to grooving in the rolling track on silicon nitride was observed. The spalling resistance of silicon nitride was found to be higher than that of steel under the test conditions used. Surface interactions in the Si₃N₄-M50 steel contacts, detrimental to the life of the steel rolling elements, were recognized. Attempts were made to reduce the severity of these interactions and prolong the life of bearings containing ceramic elements.     

A naphthenic oil of hydrogenated coal tar distil- late as a lubricant with low solidification pressure

A new type of lubricant, TN, composed mainly of condensed polycyclic naphthenic hydrocarbon rings, was developed from coal tar through a complete hydrogenation process. This oil is characterised by a high pressure-viscosity coefficient and low solidification pressure. The tribological characteristics of this new oil were investigated in an elastohydrodynamic lubrication regime under sliding, rolling/sliding, and rolling contact conditions. The results for the TN oil were compared with those of several oils of different molecular structures: polyalphaolefin of paraffinic hydrocarbon, and naphthenic and paraffinic petroleum oils. It was found that the TN oil was superior to the other oils tested in its ability to form thick oil films, to reduce wear, to increase the coefficient of traction, and to prolong fatigue life.     

Some Effects of Additives on Rolling Contact Fatigue

To study the influence of lubricant physical and chemical properties on rolling contact fatigue, a variety of base stocks and additives were evaluated with a 4-ball type fatigue machine. The effect of viscosity was found to depend on the means by which a given viscosity was achieved. Although some commonly used EP and antiwear additives had a pronounced effect on fatigue life, the direction and magnitude of the additive effect depended on the particular additive and its concentration in the blend. In addition, it was observed that the additive effect depended on the choice of base oil and ball steel. These results indicate that the chemical properties as well as the physical properties of the lubricant, can be important in rolling contact fatigue.     

Lubricant Effects in the Transition from Boundary to Microelastohydrodynamic Lubrication

Lubricant effects in the friction transition from boundary to microelastohydrodynamic lubrication were investigated by using a ballon-flat tribotester at sliding speeds from 0.02 to 0.88 mm/sec. Three lubricants—cyclophosphazine (X-IP), poly-alpha-olefin (PAO) and Z-DOL—were used, in this investigation. When X-IP was used at room temperature, a drop in friction coefficient from 0.22 to 0.12 at sliding speeds ≥0.10 mm/sec. (an unusually low speed) was observed, accompanied by a rise in the contact electrical resistance across the ball-fiat interface. The friction drop did not occur at temperatures ≥100°C. The friction transition was achieved at lower speeds when sliding perpendicular to the surface roughness texture. No transition occurred when PAO and Z-DOL were substituted as the lubricant. The latter him lubricants were working in the boundary lubrication regime as indicated by the contact resistance measurement.     

Rolling contact fatigue properties of Tuffritrided 0.2% C steel

The effects of initial surface roughness and lubricant viscosity on the rolling contact fatigue behaviour of a Tufftrided plain 0.2% C steel are reported. The Tufftriding treatment substantially improves the rolling contact performance. Improvement in the surface finish before Tufftriding is beneficial. Changes in lubricant bulk viscosity have no significant effect on the fatigue properties.     

Micropitting performance of nitrided steel gears lubricated with mineral and ester oils

This study compares the gear micropitting performance of high pressure nitriding (HPN) steel gears, lubricated with three different gear oils: a standard mineral lubricant, containing a special micropitting additive package, and two biodegradable esters with low toxicity additivation. The physical, chemical and wear properties of the three lubricants were determined, as well as their biodegradability and toxicity characteristics. The gear material and the corresponding heat treatment are presented.Gear simulation tests were performed in a Falex machine, using a roller-disc geometry, in order to evaluate the lubricant temperature and friction coefficient corresponding to each gear oil.Gear micropitting tests were performed on the FZG machine, using type C gears, and lubricant samples were collected during the tests for wear particle analysis. Post-test analysis included the mass loss measurement of the gear (pinion and wheel), the ferrometric analysis of the lubricant samples and the teeth flank roughness measurement below and above the pitch line. The teeth flanks were inspected using scanning electron microscopy (SEM) and surface topography measurements to assess the number and depth of micropits. Metallurgical cuts were done to observe the size and depth of micropits as well as contact fatigue crack initiation and propagation.The ester lubricants show better micropitting performance than the mineral oil, confirming the potential of environmental friendly fluids as high-performance gear oils.     

Lubricant Film Thickness and Wear in Rolling Point Contact

Lubricant film thickness in the partial elastohydrodynamic range was measured as a function of rolling speed in a rolling 4-ball configuration, for four mineral oils, two esters and a polyphenylether covering a viscosity range of 8–360 cs. Film thickness is shown to vary with speed as a power function. At the point where a full elastohydrodynamic film is formed, the product of viscosity and speed is constant for most lubricants.

Wear rate w after run-in follows a simple wear law w = Ka where a is the total area of asperity contact and K is a constant characteristic of the lubricant, of the order of 1 μg/inch³ for compounded ester type lubricants rating these as best, and 4 μg/inch³ for ester base stock, 6 μg/inch³ for polyphenylether and 13 μg/inch³ for mineral oil base stock, rating these lubricants progressively poorer in the order given, regarding their wear-preventing ability.

A hypothesis regarding size and shape of wear particles is derived, suggesting platelet shaped particles. Limited data appear to confirm an exponential distribution of wear particle diameters with a mean of the order of 10^?4 inch.     

Pressure viscosity coefficients and traction properties of synthetic lubricants for wind turbine gear systems

Synthetic lubricants are increasingly used to provide equipment reliability for wind turbine gear boxes. The majority of synthetic lubricants used today are based on polyalphaolefins. In gear systems where contact pressures are high, the pressure viscosity coefficient and traction values of the lubricant are important fundamental properties. A comparison of these properties for a wind turbine lubricant based on a polyalphaolefin and two lubricants based on polyalkylene glycols has been undertaken. Pressure viscosity coefficients were calculated from viscosity measurements made using an ultra‐high pressure falling needle viscometer at pressures up to 50 000 psi. Significant differences in properties were observed with both polyalkylene glycol lubricants showing lower pressure viscosity coefficients and much lower traction values. A calculation of the film thickness values in the Hertzian contact zone suggests that polyalkylene glycol lubricants may provide elastohydrodynamic films that are approximately 25% thicker than polyalphaolefin lubricants. Copyright © 2012 John Wiley & Sons, Ltd.     

Lubricant and additive effects on spur gear fatigue life

Spur gear endurance tests were conducted with six lubricants using a single batch of consumable-electrode vacuum melted (CVM) AISI 9310 spur gears. The sixth lubricant was divided further into four batches, each of which had a different additive content. Lubricants tested with a phosphorus-type load carrying additive showed a statistically significant improvement in life over lubricants without this type of additive. The presence of sulphur-type antiwear additives in the lubricant did not appear to affect the surface fatigue life of the gears. No statistical difference in life was produced with those lubricants of different base stocks but with similar viscosity, pressure-viscosity coefficients and antiwear additives. Gears tested with a 0.1 wt % sulphur and 0.1 wt % phosphorus EP additives in the lubricant had reactive films that were 200 to 400 Å (0.8 to 1.6 μin) thick.