Physical and Chemical Properties of Mineral Oils Including Their Effects On Lubrication Summary This paper is a review of the physical and chemical properties of mineral oils that affect lubrication. A few properties may have both physical and chemical effects. Recognition of these properties is useful for designing lubrication systems, choosing lubricating oils, diagnosing lubrication, friction and wear problems, and selecting appropriate testing methods. The material is limited to industrial lubricating and hydraulic mineral oils and is written for people entering the field of tribology.
Herguth_Physical-Physical and Chemical Properties of Mineral Oils Douglas Godfrey, William D. Herguth Herguth Laboratories, Inc. 23 pages November 2003
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Physical and Chemical Properties of Mineral Oils
Introduction......................................................................................................................................4 Viscosity ................................................ .................................................... ..........................................................................................4 ......................................4 Units of Viscosity Measurements.............................................................................................5 Viscosity Index (VI) ........................................................ ........................................................................................................... ................................................... ......5 VI Improvers.............................................................................................................................6 Viscosity Measurements...........................................................................................................6 Viscosity Pressure Coefficient..................................................................................................6 Viscosity Shear Rates .................................................. ....................................................... .............................................................6 ......6 Pour Point.........................................................................................................................................7 Density ................................................. ................................................... .........................................7 Specific Gravity .................................................... ....................................................................................................... ................................................... ........................8 Thermal Expansion ............................................. .................................................... .............................................................................8 .........................8 Bulk Modulus or Compressibility....................................................................................................9 Gases in Mineral Oil ............................................. .................................................... ...........................................................................9 .......................9 Dissolved Gases........................................................................................................................9 Entrained Gases .................................................. ........................................................ ......................................................................9 ..............9 Foam ...................................................... .............................................................................................................. .................................................................................10 .........................10 Vapor Pressure...............................................................................................................................10 Flash Point ............................................... ........................................................ ......................................................................................10 ..............................10 Thermal Conductivity ................................................. .................................................... ...................................................................11 ...............11 Thermal Capacity, Heat Capacity and Specific Heat.....................................................................11 Electrical Conductivity / Dielectric Strength.................................................................................11 Surface Tension ................................................. ....................................................... ............................................................................12 .....................12 © 2004 SKF Reliability Systems All Rights Reserved
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Physical and Chemical Properties of Mineral Oils Contaminants ............................................. ................................................... .................................13 Gaseous...................................................................................................................................13 Liquid......................................................................................................................................13 Solid Particulates ....................................................................................................................14 Semi-Solids.............................................................................................................................15 Contaminants which Affect Chemical Properties..........................................................................15 Water.......................................................................................................................................16 Corrosive Liquids .............................................. .....................................................................16 Base Oil Composition....................................................................................................................16 Base Oil Lubricating Properties.....................................................................................................17 Additives to Improve the Chemical Properties of Oil ............................................ .......................18 Oxidation Inhibitors................................................................................................................18 Rust Inhibitors ................................................... .....................................................................19 Dispersants and Detergents.....................................................................................................19 Additives to Improve Boundary Lubrication Performance .......................................... .................19 Anti-friction ................................................. ...........................................................................19 Saponification Number...........................................................................................................20 Anti-Wear ................................................ ................................................... ............................20 Anti-Scuff ................................................ ...............................................................................20 Oxygen....................................................................................................................................21 Sulfur Compounds..................................................................................................................21 Phosphorous Compounds .............................................. ................................................... ......22 Chlorine Compounds..............................................................................................................22 References ?? .................................................................................................................................22 About Herguth Laboratories, Inc. ..................................................................................................23
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Physical and Chemical Properties of Mineral Oils
Introduction
Viscosity
This paper is a review of the physical and chemical properties of mineral oils that affect lubrication. A few properties may have both physical and chemical effects. Recognition of these properties is useful for designing lubrication systems, choosing lubricating oils, diagnosing lubrication, friction and wear problems, and selecting appropriate testing methods. The material is limited to industrial lubricating and hydraulic mineral oils, and is written for people entering the field of tribology.
Viscosity is the property of a fluid that causes it to resist flow, which mechanically is the ratio of shear stress to shear rate. Viscosity may be visualized as a result of physical interaction of molecules when subjected to flow. Lubricating oils have long chain hydrocarbon structures, and viscosity increases with chain length. Viscosity of an oil film, or a flowing column of oil, is dependent upon the strong absorption of the first layer adjacent to the solid surfaces, and the shear of adjacent layers.
In each section the authors have:
Defined each property according to the Organization For Economic Cooperation and Development [1]. Stated how the property is measured by ASTM methods [2]. Described its application to industrial lubrication. Used SI units given in "Standard Practice For The Use Of The International System of Units Modernized" [3].
The information came from many unnamed authors. However, we specifically wish to acknowledge the authors cited in the references, from which most of the information was obtained. The reader is referred to the references for further details. We also acknowledge obtaining specific lubricant properties from the bulletins of several lubricant suppliers.
Viscosity is by far the most significant property for establishing the thickness, pressure, and temperature of an oil film in hydrodynamic lubrication (HDL) and in elastohydrodynamic lubrication (EHL). Viscosity is also a significant factor in predicting the performance and fatigue life of rolling element bearings and gears that are lubricated by oil. Plastohydrodynamic lubrication accounts for the existence of hydrodynamic effects in metalworking. Calculations for oil film thickness require knowledge of the viscosity of the oil film at the temperature, pressure, and shear rate in the component. Viscosity is in the numerator of all equations predicting oil film thickness, fluid friction or hydraulic pressure. Oil film thickness increases with viscosity. Viscosity is also in equations for calculating the Sommerfeld Number, velocity in an oil film, shear stress, fluid friction force, and power loss for hydrodynamic bearings.
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Physical and Chemical Properties of Mineral Oils
Units of Viscosity Measurements The unit of absolute or dynamic viscosity is force divided by area times time. The SI unit is Pascal times second Pa s (or Ns m-2). Typically, mineral oils are between 0.02 and 0.05 Pa.s at 40 degree Celsius. 1 mPa.s = 1 centiPoise (cP) cP is commonly used for absolute viscosity. The symbol for viscosity is usually mu. When gravity is used to cause flow for the viscosity measurement, the density p of the oil is involved and kinematic viscosity is reported =mu/p. The basic SI unit is meter2/second (m2 s-1). Also; 1 cm2 s-1 = 1 Stoke (St) and 1 mm2 s-1 = 1 centiStoke (cSt) cSt is commonly used for kinematic viscosity. Viscosity of industrial lubricants is commonly classified using the International Standard Organization
Viscosity Grade (ISO VG) system, which is the average viscosity in centiStokes (cSt) at 40 degree C. For example, ISOVG 32 is assigned to oils with viscosity between 28.8 and 35.2 cSt at 40 degree C. The viscosity of oils is dependent upon temperature, pressure, and shear rate. Viscosity decreases as temperature increases because the molecules vibrate more and interact less. Conversely, the viscosity of oil increases as temperature decreases and can become grease-like at very low temperatures.
Viscosity Index (VI) VI is a commonly used expression of an oil's change of viscosity with temperature. VI is based on two hypothetical oils with arbitrarily assigned VI's of 0 and 100. The higher the viscosity index, the smaller the relative change in viscosity with temperature. Most industrial mineral lubricating oils have a VI between 55 and 100, but VI varies from 0 to "high VI" oils with VI up to 175. ViscosityTemperature - VI relationship is shown in the following table:
Industrial Oil
Viscosity, cSt
Viscosity, cSt
Viscosity
Visc-Temp
ISOVG 32
40°C
100°C
Index
Coefficient
Machine Oil
30.4
4.8
58
0.842
Turbine Oil
32
5.4
102
0.831
Hydraulic Oil
30.4
6.1
154
0.799
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Physical and Chemical Properties of Mineral Oils A less arbitrary indication of the change in viscosity with temperature is the viscositytemperature coefficient. For 40 to 100 degree C it is defined as Viscosity (cSt) at 40 degree C minus Viscosity (cSt) at 100 degrees C, divided by the Viscosity (cSt) at 40 degrees C. Calculated values of the viscosity-temperature coefficient are also shown in the table. The lower the value of the coefficient, the higher the VI. The coefficient for mineral oils can vary by a factor of 10 depending on the temperatures.
Viscosity Pressure Coefficient Viscosity increases with pressure because the molecules are squeezed together forcing greater interaction. In an EHL contact where the pressure can be 2.1 GPa (300,000 psi) the viscosity is so high that the oil is considered a plastic-like solid. Viscosity at high pressures is measured by flow through pressurized capillary tubes, or a ball falling down a pressurized tube. The higher the temperature, the lower the viscosity increase due to pressure.
VI Improvers VI improvers are used in a few industrial oils, such as gear oils, by the addition of high molecular weight polymers and are called multi-grade oils. They reduce change in viscosity with temperature. The chemistry of VI improvers, as well as other additives, is described in References 4 and 5.
Viscosity pressure coefficient is the slope of lines on graphs of the log of viscosity vs. pressure. The unit for pressure viscosity coefficient is the reciprocal of pressure. The SI units are 1/Pa or m2 N-1. Reference 7 gives the pressure viscosity coefficients of several mineral oil showing a variation from 1.6 to 2.68 X 10-8 Pa-1. The coefficient increases with viscosity, and can vary by a factor of 3.
Viscosity Measurements Viscosity is measured by ASTM method D 445 using a common cross arm viscometer. The sample is introduced into a "U" shaped, calibrated, glass tube, submerged in a constant temperature bath. The oil is warmed to the desired temperature (usually 40 degree C for industrial oils) and allowed to flow via gravity down the tube and up the opposite side. The number of seconds the oil takes to flow through the calibrated region is measured. The oil's viscosity in cSt is the flow time in seconds multiplied by the apparatus constant. Viscosity is also measured in the Brookfield viscometer by measuring the resistance to rotation of a spindle in a container of oil at a specified temperature. Brookfield viscosity is useful for low temperature measurements. For example, a gear oil for arctic use is 120,000 cP at -40 degree C. (See Reference 6 for details of viscosity temperature relations). © 2004 SKF Reliability Systems All Rights Reserved
Pressure viscosity coefficient can also be measured from oil film thickness and other parameters from a transparent disk-on-ball apparatus. Pressure viscosity coefficient is used in the calculation of oil film thickness in tribological contacts. For example, in EHL contacts, oil film thickness is directly proportional to the 0.74 power of the pressure viscosity coefficient.
Viscosity Shear Rates Mineral oil viscosity does not change much with shear rate, that is, they are Newtonian fluids. However, the viscosities of multigrade, non-Newtonian oils usually decrease with shear rate because of the temporary alignment or breaking down of long chain hydrocarbon molecules to form shorter molecules. Shear rate is speed divided by oil film thickness: Shear rate =ms-1/m = s-1, or reciprocal seconds. For example, with a speed 6
Physical and Chemical Properties of Mineral Oils of 1 ms-1 and an oil film 1 micrometer thick, the shear rate is 106 s-1. Shear stability is defined as the ability of a lubricant to withstand shearing without breaking of the long chain hydrocarbon molecules. In lubrication, the viscosity of an oil at high shear rates is important to understanding performance in high speed, thin oil film equipment. An example is a large tilting pad thrust bearing in an hydroelectric generator. Viscosity, as a function of shear rate, is measured by various rotating instruments. The instruments measure the force resisting the flow of oil films of known thickness and speeds. ASTM method D 4683-90 prescribes a tapered roller rotating in a matched tapered stator with a known oil film thickness between them. Results are reported as: viscosity in cP (at 150 degree C and a shear rate of 106 s-1). Another rotating apparatus is the Couette Rheometer, where a precision cylinder rotates at high speed in a larger cylinder with an oil film of known thickness between them. Viscosity at high shear rates is also measured with an ultrasonic shear tester, and a high shear rate capillary at specified frequency, temperature and time. Many original equipment manufacturers (OEMs) now require a minimum shear stability. Some OEMs now require a viscosity of 2.9 cP (at 150 degree C and 106 s-1).
Pour Point Pour point is a viscosity temperature phenomenon. It is defined as the lowest temperature at which a lubricant will flow under specified conditions. Most lubricant suppliers give the pour point of their oils so that the user can determine if it can be pumped and would be fluid in low temperature applications. © 2004 SKF Reliability Systems All Rights Reserved
The pour point increases with viscosity. For example, an ISOVG 46 mineral oil might ha ve a pour point of -39 degree C, whereas an ISOVG 460 would have a pour point of -15 degree C. For mineral oils, the increase in viscosity as temperature is reduced, is due to gelling of the oil by the precipitation of crystalline wax. Although this paper is a review of mineral oil properties, it should be noted that one advantage of many synthetic oils is their very low pour points because of the absence of wax. Pour point depressants for mineral oils are additives that lower the pour point by interfering with wax crystallization. Pour point is measured by ASTM D 97, which describes the procedure for cooling an oil until it will not pour out of a vessel. Pour point, flash point, VI, and other properties of 81 mineral oils are given in Reference 7. Cloud point is defined as the temperature at which a wax cloud first appears on cooling mineral oil under specified conditions.
Density Density is the mass of a unit volume of a substance. Oil density is used to determine the mass of a given volume, or the volume of a given mass. Density is used in lubrication to identify an oil, or oil fractions, and in the measurement of kinematic viscosity (absolute viscosity divided by density). Also, density is in the equations for the calculation of temperature rise in an oil film, and the equation for Reynolds Number (which determines if flow of an oil film is laminar (smooth layers) or turbulent (tumbling)). The SI unit for density is kg m-3, but usually reported as grams/ml (g/ml-1). For example, an oil could be 850 kg m-3 or 0.850 g ml-1. The density of mineral oil lubricants varies from 0.86 to 0.98 g ml-1 7
Physical and Chemical Properties of Mineral Oils (60 degrees F) and reported as A. P. I. gravity at 60 degrees F.
Specific Gravity For many liquids, specific gravity is used which is ratio of the mass of a given volume to the mass of an equal volume of water. Therefore, specific gravity is dimensionless. The specific gravity of mineral oils also varies from 0.86 to 0.98 since the specific gravity of water is 1 at 15.6 degree C. Specific gravity decreases with increased temperature and decreases slightly as viscosity decreases for similar compositions. Reference 5 (pp. 482484) gives the specific gravity of 81 mineral oils at 15.6 degree C. Most lubricant supplier's typical data bulletins give A.P.I. (American Petroleum Institute) Gravity in degrees for lubricating oils instead of specific gravity. A.P.I. gravity is an expression of density measured with a hydrometer. A.P.I. gravity has an inverse relationship with specific gravity, as shown in the following table from Reference 8. A. P. I. Gravity
Specific Gravity
15
0.97
34.9
0.85
Many mineral oil lubricants have an A.P.I. gravity value of around 27 degrees. Reference 8 gives the equation for converting A.P.I. gravity to specific gravity. Density, specific gravity, and A.P.I. gravity are measured by ASTM D-1298, using a calibrated, glass hydrometer and a glass cylinder. The cylinder is partially filled with the sample oil and the hydrometer is set into the oil and allowed to stabilize. A reading of the gravity is taken from the markings on the stem of the hydrometer at the surface of the oil. The temperature of the oil is measured and the final result is converted to 15.6 degree C © 2004 SKF Reliability Systems All Rights Reserved
Two other oil properties related to density are thermal expansion and bulk modulus or compressibility.
Thermal Expansion The volume of a given oil mass increases with temperature, therefore, its density decreases. The degree of expansion is expressed as the coefficient of thermal expansion. Thermal expansion is useful to determine the size of a container needed when the oil will be heated. Inexperienced people often have an oil overflow because of a surprising amount of thermal expansion. In HDL, the thermal expansion of the oil in the clearance of a bearing increases the hydraulic pressure. Some researchers discuss the "thermal wedge" mechanism of film formation and apply it to parallel sliding surfaces, especially flat, non-tilting, thrust bearings. The coefficient of thermal expansion is the ratio of the relative change of volume to a change in temperature. Thermal expansion is expressed as the ratio of volume change to the initial volume after heating 1 degree C. Therefore, the unit is reciprocal degree C, or degree C-1. The values of the coefficient of thermal expansion for mineral oil are near 6.4 X 10-4 degree C-1. Thermal expansion (or contraction) determinations require the measurement of the volume of a given mass of oil at various temperatures. The sample is placed in a graduated cylinder and the volume is observed as the temperature is either increased or decreased. A simplified method of calculating the thermal expansion of petroleum products can be found in ASTM D 1250, Petroleum 8
Physical and Chemical Properties of Mineral Oils Measurement Tables, "Volume Corrections Factors".
Bulk Modulus or Compressibility Bulk modulus expresses the resistance of a fluid to a decrease in volume due to compression. A decrease in volume would increase density. Compressibility is the reciprocal of bulk modulus or the tendency to be compressed. Bulk modulus varies with pressure, temperature, molecular structure and gas content. Generally, mineral oils are thought to be incompressible. In high-pressure hydraulic systems a high bulk modulus or low compressibility is required to transmit power efficiently and dynamically. In EHL, bulk modulus is a factor used in some film thickness calculations. Bulk modulus is a consideration in some viscosity-pressure relationships. (Low viscosity polysiloxane fluids have a low bulk modulus or high compressibility compared to mineral oils). Dissolved gases decrease bulk modulus of mineral oils. The unit for bulk modulus is pressure and the unit for compressibility is the reciprocal of pressure. The SI units are N m-2, and m2 N-1 respectively. Bulk modulus is determined by measuring the volume of an oil at various pressures or derived from density measurements at various pressures. Bulk modulus can also be measured by the speed of sound in oils under various pressures. A discussion of bulk modulus and values are given in References 9 and 10. Since a graph of pressure versus volume gives a curve, the secant to the curve is used and is called Isothermal Secant Bulk Modulus.
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Gases in Mineral Oil Dissolved Gases Gases are soluble in mineral oils to a limited amount. The amount varies with the type of gas and oil temperature. For example, 8 to 9% of air, by volume, is soluble in mineral oil at room temperature and is invisible. Dissolved gases affect oil viscosity, bulk modulus, heat transfer, oil and metal oxidation, boundary lubrication, foaming and cavitation. Boundary lubrication is improved by the oxygen in dissolved air because it continuously repairs the protective oxide films on metals. Dissolved oxygen is considered an important anti-scuff component. The amount of dissolved gas become evident when gases come out of solution vigorously when the oil is subjected to low pressures. The amount of soluble gas is measured by ASTM D 2780 "Solubility of Fixed Gases In Liquid Test". This method physically separates the gas through an extraction process and measures the quantity volumetrically. This method allows for subsequent qualitative analysis of the extracted gas by any appropriate technique.
Entrained Gases If the amount of a gas in oil exceeds saturation, small bubbles will form, remain suspended, and the oil will appear hazy. This is called entrained gas. The bubbles slowly rise to the surface. Bubbles of a gas, such as air, in an oil film cause holes that reduce oil film continuity and decrease the film's ability to prevent solid-to-solid contact. The relative tendency of various oils to release entrained gas is measured by a gas bubble separation method ASTM D 3427. The method uses a cylinder-like test vessel with gas inlet and outlet ports. Air, or another gas (if of interest), is introduced into the bottom of 9
Physical and Chemical Properties of Mineral Oils the vessel at a specified temperature and flow rate. At the end of seven minutes the gas flow is stopped and the change in density as measured by a densitometer is recorded. The test is complete when the total volume of entrained air is reduced to 0.20% by volume. The results are reported as the time it took for the oil to attain this value.
Foam Foaming is defined as the production and coalescence of gas bubbles on a lubricant surface. Foam may be a result of a variety of problems including air leaks, contamination, and over filling of sumps. Foaming can cause loss of oil out of a vent and serious operational problems in most lubricated systems. Excessive foam can starve bearings and pumps of liquid lubricant (pump cavitation) causing failure, and cause poor performance in hydraulic systems. The foaming characteristics of an oil are measured by ASTM D-892. Using a calibrated porous stone, air is blown into the bottom of a graduated cylinder for a specified time. Immediately upon completion of the blowing period, the foam that has formed on the top of the oil is measured. Ten minutes after the completion of the blowing period, an additional measurement is made of the remaining foam as the foam retention characteristics of the oil. The results are reported in milliliters. Most commercial lubricants contain foam inhibitors, as these inhibitors reduce foaming. A few ppm of silicone is commonly used (Reference 4).
Vapor Pressure Vapor pressure is the pressure exerted by a vapor on a liquid when it is in equilibrium with its own vapor. Volatility is an expression of vapor pressure or the tendency to evaporate. Loss of oil vapors to the © 2004 SKF Reliability Systems All Rights Reserved
atmosphere (one cause of oil consumption) is directly related to volatility. Since the vapors of mineral oils are combustible, volatility becomes important in fire safety during the handling and use of oils. The higher the concentration of low molecular weight fractions, the greater the vapor pressure. Vapor pressure is reported as a pressure at a specified temperature. Volatility is reported as percent evaporative weight loss and is measured by ASTM method D-972, where the sample is weighed into a test cell and heated air is passed over the sample for a specified period of time. At the end of the test time the sample is reweighed. The difference between the original sample weight and the after test weight is a measurement of the evaporation loss. Evaporation is reported as percent loss at 250 degree C after 1 hour.
Flash Point Flash point is an indication of the combustibility of the vapors of a mineral oil, and is defined as the lowest temperature at which the vapor of an oil can be ignited under specified conditions. Flash point is clearly related to safety. Flash point of lubricating oils is measured using ASTM D 92. An open cup of oil is heated at a specific rate while periodically passing a small flame over its surface. The flash point is considered to be the lowest temperature at which the oil vapors will ignite, but not sustain a flame. For example, the flash point of an ISOVG 32 turbine oil might be 222 degree C. If one continues to heat the oil and continues to pass the small flame across the surface, the vapors will eventually sustain a flame and this is the fire point. The fire point of the same turbine oil approaches 252 degree C.
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Physical and Chemical Properties of Mineral Oils
Thermal Conductivity Thermal conductivity is the rate of transfer of heat for a given temperature gradient through a material. The rate of transfer of heat from a hot spot to a cooler area in a bearing is a factor in controlling overheating. Therefore, an oil with a high thermal conductivity would be expected to lower the temperature of a bearing. Thermal conductivity of a lubricating oil is used in equations to calculate heat transfer in bearings. The unit of thermal conductivity is heat over a distance per degree of temperature. Most mineral oils have approximately the same thermal conductivity of near 0.1 W m-1 degree C-1. (For reference, the thermal conductivity of water and ethylene glycol are about three times higher than oils). Thermal conductivity is measured using ASTM D 2717. This method consists of a test vessel containing oil and a platinum resistance thermocouple. Thermal conductivity is determined by measuring the temperature gradient produced across the liquid sample by a known amount of heat introduced into the cell by an electrically heated platinum element. Thermal conductivity increases slightly with higher temperatures.
Thermal Capacity, Heat Capacity and Specific Heat Thermal capacity is the heat required to raise a body one unit or degree of temperature. Thermal capacity and heat capacity are synonymous. The SI unit is J kg-1 degree C-1 Specific heat is the ratio of the thermal capacity of the substance to that of water (at 15 degree C). Therefore, thermal capacity is numerically equal to specific heat. Specific heat is a function of fluid structure and density. In HDL, specific heat is used in the © 2004 SKF Reliability Systems All Rights Reserved
calculation of heat transfer, temperature rise, and other thermal factors in an oil film. In Reference 8, the thermal capacity of three materials is given as: Thermal Capacity Material (J kg-1 degree C-1) Water
4184
Mineral Oil
1966
Steel
460
Therefore, more heat is required to raise the temperature of 1 kilogram of water and oil than is required for steel. Specific heat is measured using ASTM D 3947. A sample is introduced into a differential scanning calorimeter by means of a hermetically sealed sample cell. The cell is placed on a thermocouple in a controlled environment programmed to increase temperature at a specific rate. This allows the calculation of the specific heat of the oil.
Electrical Conductivity / Dielectric Strength Electrical conductivity is the quantity of electricity per unit area transferred through a body at a given voltage gradient. The unit of electrical conductivity involves the reciprocal of resistance (1/ohm or mho) and a distance, and is mho cm-1. (Siemens/cm is a preferred international unit). A clean, dry base oil has a very low conductivity of 10-14 mho/cm. A used, wet, dirty, detergent oil could have a conductivity of 10-8 mho cm-1 and act as an electrolyte. The electrical conductivity of mineral oils increases with temperature because of the 11
Physical and Chemical Properties of Mineral Oils greater mobility of electron carrying species such as acids compared to simple hydrocarbons. Conductivity is measured by instruments that measure the current between two electrodes immersed in the oil with an applied voltage. Dielectric breakdown voltage is the voltage at which a dielectric (insulator) breaks down or allows sparks under the influence of strong electrical fields. This property is important for insulating oils, the unit is kilovolts (kV). Electrical conductivity and dielectric breakdown voltage of oils are important in lubricated components subjected to stray or self-generated electric currents. If the electrical conductivity of a lubricating oil is sufficient, the current can be dissipated or grounded without undue harm by sparks to a bearing surface. However, if the oil or its contaminants are corrosive, metal corrosion may occur because corrosion of metals by liquids requires the conduction of electrons. If the oil has high resistance, and high voltage is generated, a spark will jump across the oil film, resulting in damage to both bearing surfaces in the form of pits. Scanning electron micrographs of the pits show that the metal was once molten. Large equipment with rotating magnetic parts can build up very high electrical charges and sparks may jump across the oil film of bearings in spite of grounding efforts. Another source of electrical charge is from streaming potential of a high velocity liquid that generates currents, which corrode surfaces. Damage was found on the metering edges of aircraft servomechanism valves operating in phosphate esters that were corroded by streaming potential driven currents. The problem was solved by increasing the electrical conductivity of the phosphate ester with an additive. The same phenomenon has been observed with mineral oil based hydraulic oils at sites of high oil velocity. Oils with higher conductivity act as an electrolyte to promote corrosion. In © 2004 SKF Reliability Systems All Rights Reserved
lubricating oils, electrical conductivity and dielectric strength vary with base oil, additive composition and decomposition products. Compounds that disassociate into ionic species carry electrons and increase conductivity. Some dispersants and detergents increase base oil conductivity significantly. Electrical conductivity or its reciprocal, resistance, is measured with an apparatus that determines the current flowing through an oil between immersed electrodes at a constant distance apart.
Surface Tension Surface tension is the surface energy between a liquid and its own vapor, or air, or a metal surface. The word tension comes from the force that resists any attempt to increase the surface area. Surface tension is thought to be a factor in the ability of an oil to "wet" a surface, in emulsion stability, and in the stability of dispersed solids. However, "wetting" has been found to be a complex phenomenon involving oleo phobic and oleophilic films on the metal surface. Some additives markedly change surface tension. An example is water-containing soap for the formation of bubbles. Silicone is added to mineral oils to reduce surface tension and as result, foaming characteristics may change. The SI unit of surface tension is N m-1, but dynes/cm is commonly used. Surface tension of oils on metals can be compared by placing a small drop on a clean metal surface and observing if the drop has a high contact angle (high surface tension) or very low contact angle (spreads out, low surface tension). Surface tension between an oil and a metal surface is measured by using the Nouy ring method. The method involves placing a clean, platinum wire ring on the surface of the oil, where the force required to 12
Physical and Chemical Properties of Mineral Oils pull the ring away from the surface is measured. Interfacial tension exists between two liquid layers and is measured by ASTM D-971. The method uses a tensiometer and a platinum ring. The ring is lowered into a beaker of water and oil. It is then brought up to the water-oil interface where the actual measurement takes place. The force required to pull the ring through the interface is measured by the tensiometer and considered to be the interfacial tension of the oil. The value for mineral oil varies from 30 to 35 dynes cm-1 (Reference 8).
Contaminants Contaminants in lubricating oils or hydraulic fluids are any foreign material whether ingested or generated. Contaminants can be gases, liquids, solids or semi-solids. Contaminants have a profound affect on lubricant performance. A great majority of component tribological failures are due to contaminants. Reference 11 classifies contaminants and describes their effect.
Gaseous Gases enter an oil system in different manners and come from different sources depending upon the application. Examples include: air leaks; process gas in a pump; gas from leaking seals in a gas turbine; and freon gases in refrigeration compressors. Gases in oil decrease oil viscosity, promote foaming, interfere with oil film continuity and may lead to a catastrophic bearing failure. Combustible gases decrease the flash point and thus increase the risk of an explosion. See "Gases in Oil" section for methods of measurement.
Liquid Liquid contaminants come in four forms: © 2004 SKF Reliability Systems All Rights Reserved
1. 2. 3. 4.
additions of the wrong oil. water. solvents. process liquids.
The addition of the wrong oil with a different viscosity will affect oils film thickness, pumping, and other properties depending upon viscosity. The simplest method of detection is a viscosity measurement. If the added oil has different base oil or additives, the first visual evidence may be a precipitate due to additive interaction. Metal analysis using Emission Spectroscopy (ES) may reveal the presence of a wrong oil by detecting unexpected metalloorganic additives. For example, if one was using a Rust and Oxidation inhibited oil (R and O), and ES indicated the presence of zinc and phosphorous (possibly from the anti-wear additive zinc dialkyldithiophosphate), then contamination by a wrong oil could be suspect. In ES analysis, metallic elements in the oil sample are excited by a high temperature arc and emit light energy. Each metallic element emits light at a unique wavelength. The resulting light is focused and passed through a diffraction grating to separate the beam into component wavelengths. The intensity of light at specific wavelengths is measured using photo multiplier tubes, and compared to calibration standards, and reported as parts per million (ppm) concentration. Large (greater than 10 microns) and dense particles, such as metallic wear chips or sand, may not be measured as accurately as the soluble materials because the particles may not be effectively introduced or completely volatilized in the excitation region of the spectrometer. Other methods involving acid digestion or spark on residue are available that analyze all particles. The user should remember that the results are reported as elemental metals with no indication of the compounds of which they are 13
Physical and Chemical Properties of Mineral Oils a part. For example, silicon could be from silicone or sand (SiO2), iron could be from metallic iron wear fragments, from rust (hydrated iron oxide Fe2O3.H2O due to corrosion), or from iron sulfide. Water contamination comes from two primary sources; leaks in the system or condensation of moisture in the air space. Leaks may be from steam, seals or gaskets. Water in oil is detrimental to the formation of the oil film, and in the promotion of contact fatigue of gear teeth and rolling element bearings. In many systems, water in the oil as low as 0.01% can shorten bearing life substantially. Water contamination may be detected by observing a layer of free water in the bottom of a container of drained oil, or in oil withdrawn from the bottom of a sump. Water may be present if the oil appears hazy or milky. A useful test for water is the "crackle test", where a drop of oil is placed onto a hot surface, if it crackles, or sputters, a large quantity of water is present. Another "in the shop" test is to immerse a hot soldering iron into a container of oil and observe sputtering if the oil is wet. Water is measured quantitatively by titration or distillation tests in the laboratory. The common method is ASTM D 1533 or ASTM D 1744 "Water By Karl Fischer Method". Almost any measurable water (greater than 0.05%) is a sign of a problem and should be addressed. Levels as low as 0.01% may be a sign of impending problems in some systems, such as refrigeration compressor oils. The source of the water may be determined by ES, knowing what metal compounds are in the source water. For example, high sodium, boron, and potassium indicate cooling water may be the source. Process liquids and solvent contaminants also interfere with oil film formation as does the presence of water, but the effects are mostly © 2004 SKF Reliability Systems All Rights Reserved
chemical (see later). Process liquid contamination is common in pumps. Contamination can be indicated by changes in odor, color, flash point, or viscosity of the oil. The amount may be measured (depending on the liquid) by Total Acid Number (TAN), density, flash point, and Infrared (IR) spectrum. Interpretation of the problem is fairly straightforward since the process liquid is usually known.
Solid Particulates Hard particles which may be present in a new machine, those which invade the system, or are formed by the system are: shop debris such as welding or grinding splatter; sand and other earth materials; and wear fragments such as work hardened iron. If their size is near oil film thickness, they will embed, dent, or abrade surfaces, thus reducing component life. Abrasion can be detected on a worn part by observation of parallel scratches in the direction of sliding, or by microchips and abrasive particles in the used oil. The amount of abrasives is measured by ES showing high concentrations of metals such as silicon and aluminum from sand or clay. The amount of iron or other metals worn off is also determined by ES (as discussed previously, subject to particle size limitations) and by wear particle analysis. Wear particle analysis (using microscopic techniques) reveals the amount of iron, and the iron microchips characteristic of abrasion. One method of separating magnetic material for measurement of the amount and type passes the oil over a glass microscope slide resting on a pair of strong magnets. Magnetic particles, and any particles associated with them, are deposited on the slide. After de-oiling, the deposits are evaluated microscopically. 14
Physical and Chemical Properties of Mineral Oils Investigation into the type and amount of contamination by solids is typically carried out using an extraction process, microscopic observation, and particle counters. For microscopic observation the particles must be separated by settling (see Reference 11), or centrifuged from the oil, de-oiled and dried. Many particles such as sand, metal chips, steel shot, and weld splatter are recognizable. There are three common types of instruments for counting particles automatically. They employ light scattering using a laser beam, light interruption or blockage using a light source and a detector, or flow decay or pressure drop using different size screens and sensors.
Semi-Solids Semi-solid contaminants are generally oxidation and/or thermal polymerization products, carbonaceous material, microorganisms, or oil/ additive/water reaction products, and fragments of elastomers. In lubricated systems, these and other small particulates contribute to sludge, which collects and plugs pipes, pumps and orifices possibly leading to oil starvation. A dirty and discolored oil is a sign of the presence of these materials. If there are oxidation products present, they can be measured by an increase in TAN, pentanetoluene insolubles, IR spectrums, or the amount of filter residue. With the exception or IR and pentane-toluene insolubles, these tests do not necessarily measure the actual lubricant degradation products, but rather the presence of a semisolid material, and the evaluator makes a judgment on the source.
Contaminants which Affect Chemical Properties A contaminant is any material not in the original fresh oil, whether it is generated within the system or ingested. Mineral oils react with oxygen at elevated temperatures to first form hydroperoxides, and then organic acids. Acid Number is a measure of the acidity of an oil. A steady increase in the acidity of a lubricating oil is an indication of oxidation with use. A sudden increase would suggest contamination by an outside source. The amount of acid is measured by tests, primarily ASTM D 664 (potentiometric) or D 974 (colorometric). The results are expressed as milligrams of potassium hydroxide (KOH) per gram of sample required to titrate a sample in a specified solvent to a specified end point. Baker (Reference 7) gives the acid number of many lubricants. Most fresh Rust and Oxidation (R and O) inhibited oils have very low acid numbers, such as less than 0.05 mg KOH/g. Acidity may also increase by contamination. Contamination of roller bearings greases by an acidic metalworking fluid is known to occur. Organic acids from oil oxidation, if not neutralized, degrade oil properties and cause corrosion. Babbitt bearings are susceptible, but so is the steel in rolling element bearings. An alarm for an oil change is a steady rise in acid number by laboratory tests. A badly oxidized oil could have a acid number 2 to 3 times higher than the fresh oil reference. This oil, or the oil in a grease, would have an unusual odor compared to fresh oil. Some fresh metalworking fluids are basic due to basic additives such as calcium or magnesium sulfonate, calcium phenates, or some ashless (no metal) compounds added during manufacturing. The alkalinity is measured by base number either ASTM D-
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Physical and Chemical Properties of Mineral Oils 2896 or ASTM D-4739 whereby the basic constituents of the sample are titrated using an acid. The results are expressed as the equivalent number of milligrams of potassium hydroxide, that are required to titrate one gram of sample in a specified solvent to a specified end point. Decreasing values in the base number test are of prime importance.
Water Water in oil is detrimental to lubrication both physically, as discussed earlier, and chemically. The amount of water is expressed as ppm, percent by volume, or percent by weight. Water is soluble in base oils to 50 to 80 ppm, and in formulated oils to a few hundred ppm. Hazy oil is most often a result of undissolved water droplets. Low water content is especially important in hydraulic, gear, pneumatic tool, steam cylinder and circulating oil systems. Water catalyzes oil oxidation and acid formation, and reacts with or precipitates additives. Water corrodes many common tribological metals. For example, water reacts with ferrous alloys to form rust, the hydrated iron oxide. Rust creates solid debris in the oil and pits the metal surfaces. Pits may initiate contact fatigue in rolling element bearings, and contact fatigue or bending fatigue in gears. The literature states that water promotes contact fatigue in rolling element bearings. See Part 3 for methods of measuring water concentration in oil. Another indication of water contamination in lubricating oils may be increased sodium and chlorine content especially in marine applications, because salt (NaCl) is always present in large quantities in sea water and to some extent in all but distilled or rain water. Rust inhibitor is an additive that adsorbs on a metal surface in preference to water and therefore prevents rusting of ferrous alloys. Metal deactivators are additives that reduce metal corrosion by also preferentially © 2004 SKF Reliability Systems All Rights Reserved
adsorbing on the surface and protecting it from corrosion.
Corrosive Liquids In chemical plants, the ingestion of corrosive liquids through seals and breathers causes lubrication problems. Organic and inorganic acids, and chlorinated hydrocarbons are most common. The presence of corrosive liquids in a lubricating oil could first be indicated by a change in appearance, odor, or viscosity. Other indicators might be by stain, corrosion, pitting of metal parts, or by the presence of a precipitate. The corrosive material can be identified by laboratory analysis of the oil, or solids in the oil, by IR or X-Ray diffraction techniques. For example, the identification of a precipitate of iron sulfate by X-Ray Diffraction would indicate the corrosive liquid was sulfuric acid.
Base Oil Composition Crude oils, from which lubricating mineral oils are obtained, contain thousands of compounds, some of which remain after the refining processes. Most commercial lubricants are mixtures where the composition is chosen for the proposed use. The major mineral oil components are paraffinic, naphthenic, and a smaller amount of aromatic compounds. The variations in composition directly affect lubricant performance. Paraffinic oils are straight chain or branched aliphatic hydrocarbons belonging to the series with the general formula CnH2n+2. Paraffin's are saturated with respect to hydrogen. A typical paraffinic oil molecule with 25 carbon and 52 hydrogen atoms has a molecular weight of 352. Very high molecular weight paraffins are solid waxes, also dissolved in small amounts of mineral oils. Naphthenic or alicyclic oils have the characteristics of naphthenes, which are 16
Physical and Chemical Properties of Mineral Oils saturated hydrocarbons of which the molecules contain at least one closed ring of carbon atoms. Base oils may contain a small amount of Aromatic hydrocarbons. Aromatics are unsaturated molecules identified by one or more benzene rings or by chemical behavior similar to benzene. The concentration of the three major types of compounds in a given base oil is determined by chemical analysis. The results are given as %CP for paraffinics, %CN for naphthenics and %CA for aromatics. For example, a commercial base oil is 68%CP, 32%CN, and less than 1%CA. The amount of each type of compound in a mineral oil is calculated from equations given in ASTM D3238-90, which require values of molecular weight, refractive index and density. Mineral base oils contain many minor components, such as sulfur, nitrogen, oxygen, metal compounds, and water. The type and quantity of these compounds depends upon the degree of refining. For example, medicinal white oil contains the least amount of minor components, and a "bright stock" contains the most. The sulfur content of a pure mineral oil, medicinal white oil, is less than 20 ppm, whereas the sulfur content of a refined base oil depends on the type of refining process, and may be 200 to 6,000 ppm, while bright stock may range from 5,000 to 10,000 ppm sulfur. These sulfur compounds are surface active and affect boundary lubrication. Medicinal white oil is an extremely poor boundary lubricant often used as a reference, primarily for research. However, medicinal white oil is used in food machinery lubricants where there may be a chance of it being ingested. A bright stock is known to have better, but limited, boundary lubrication properties. Some of the minor components also provide natural oxidation inhibition for the base oils.
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Base Oil Lubricating Properties Paraffins are relatively unreactive and thus have better oxidation stability compared to naphthenes. Thus, they can be used longer at higher temperatures. For example, oil companies state that turbine oils are blended using the best paraffinic base stocks. However, because of the relative stability and other properties of paraffins, they have poor solvency for additives. For example, elemental sulfur has limited solubility in paraffinic base oils and higher solubility in naphthenic base oils. Conversely, naphthenics have good solvency for many additives. Aniline point is a measure of the solvent power of an oil. It is an indication of the solvent effect of oil on other organic compounds such as rubber, seals, gaskets, and electrical insulation. Aniline point is defined as the lowest temperature at which an oil is completely miscible, that is clear, with an equal volume of aniline. The lower the temperature, the greater the solvency and the more likely rubber and polymeric compounds will swell. A commercial "handy" oil has an aniline point of 90 degree C indicating it would cause more rubber swell than a paper machine oil with an aniline point of approximately 120 degree C. Aniline point is measured by ASTM D 611 where a sample of oil is introduced into a vial containing an equal portion of aniline. The fluids are warmed and lightly mixed until they are completely soluble, then allowed to cool. The temperature at which a haze appears, indicating the two liquids are no longer soluble, is considered to be the aniline point. In addition, naphthenic oils dissolve greater quantities of gases than do paraffinics. Also, paraffins have slightly higher flash points than naphthenics. Because of their wax content, paraffins have higher pour points than naphthenics. For example, for two oils of the same viscosity at 40 degree C, the paraffinic 17
Physical and Chemical Properties of Mineral Oils would have a pour point of -18 degree C, and the naphthenic a pour point of - 43 degree C. In general, paraffins have a higher viscosity index than naphthenics. For example, 100 and 70 VI respectively. The VI of turbine oils range from 96 for a low viscosity oil to 102 for a high viscosity oil. In contrast, a general utility oil has a low VI of 29 to 60. Paraffins generally have a lighter color than naphthenics. Pressure viscosity coefficients for paraffinic and naphthenic oils are similar, even though the naphthenic base oils are more affected by pressure than the paraffinics. The range for naphthenics is 1.3 to 2.8 X 10-8 m2 N-1, and 1.4 to 3.4 X 10-8 m2 N-1 for the paraffinics depending upon viscosity (Reference 1). Pure base oils are not suitable lubricants for industrial use because of their poor lubricating and stability properties. With oxidation and corrosion inhibitors they are suitable lubricants only under pure hydrodynamic lubrication. They are sold primarily to companies that blend their own additives into the base oils.
Additives to Improve the Chemical Properties of Oil Oxidation Inhibitors Oxidation Inhibitors Mineral oils readily react with oxygen at elevated temperatures to first form hydroperoxides, then organic acids. These compounds lead to viscosity increase, formation of sludge and varnish, discoloration, acidic odor, corrosion of metal parts, promote foaming, and tendency to emulsify. Resistance to oxidation is a critical property for all machines, but especially critical for machines requiring extended life at elevated temperatures, such as turbines, aircraft engines, and hydraulic systems. Also, good oxidation resistance prolongs storage © 2004 SKF Reliability Systems All Rights Reserved
life. Oxidation resistance may be due to natural inhibitors or commercial additives. Four types of oxidation inhibitor additives are: zinc dithiophosphates; aromatic amines; alkyl sulfides; and hindered phenols. Metal surfaces and soluble metal salts, especially copper, usually promote oxidation. Therefore, another approach to inhibiting oxidation is to reduce the catalysis by deactivating the metal surfaces. The effectiveness of the anti-oxidants in delaying oil oxidation can be measured by laboratory tests known generally as oxidation stability tests. Oxidation stability is measured in accelerated tests at high temperature, in the presence of excess oxygen, catalysts and possibly water. Results are expressed as the time required to reach a predetermined level of oxidation. Criteria can be a darkening color, the amount of sludge, gum, acids, and the amount of oxygen consumed, and in some cases by the depletion of the anti-oxidant chemical compound itself. The two most common test methods for oxidation resistance are ASTM D 943 "Oxidation Stability of Steam Turbine Oils" (TOST), and ASTM D 2272 "Oxidation of Steam Turbine Oils by the Rotary Bomb Method" (RBOT). ASTM D 943 TOST is a widely used method for comparison of a lubricating oil's ability to resist oxidation. However, it is seldom the method of choice for used oil comparisons. In method ASTM D 943 a controlled flow of oxygen is bubbled through a water, oil, and copper and iron catalysts mixture at 95 degrees C until the acid number reaches 2.0 mg KOH per gram (Reference 3). Results are given in hours. For example, a hydraulic oil with moderate oxidation resistance could be 1,000 hours, and a turbine oil could be greater than 4,000 hours. Other tests used for measuring oxidation resistance can be found in Reference 3.
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Physical and Chemical Properties of Mineral Oils ASTM D 2272 RBOT is also used to compare new oils but has also proven effective in determining the remaining useful life of used oils. A sample of oil is introduced into a high pressure bomb, heated and rotated until the onset of oxidation takes place as evidenced by a pressure drop. The results are reported as the time in minutes it took for this reaction to occur. There are a variety of new developments in the measurement of the antioxidant concentration, such as Differential Scanning Calorimetry, and Cyclic Voltametry (References 11 & 12). Caution should be used when using any accelerated oxidation test to estimate the remaining useful life of an oil because it may not represent field experience.
Rust Inhibitors Since water is a common contaminant in mineral oil lubricated systems used on earth, anti-rust additives are used. Rust inhibitors prevent the formation of rust (hydrated iron oxide) on iron surfaces by the formation of protective films, or by the neutralization of acids. Typical anti-rust compounds are highly basic compounds, sulfonates, phosphates, organic acids, esters or amines. The rusting of ferrous parts in a lubricated system is undesirable. The rust contributes to sludge, causes loss of metal, sticking of metal parts, and the formation of solid particles of rust that are abrasive. Rust indicates the presence of water in the system. The ability of a treated oil to prevent rusting may be measured by ASTM D 665, entitled Rust Prevention Characteristics. A 300 ml sample of lubricant is introduced into a beaker containing 30 ml of either salt or fresh water. A specially prepared bullet-shaped steel rod is placed in the beaker along with the oil/ water mixture. The mixture is heated and stirred for 24 to 48 hours to promote rust on the steel bullet. At the end of the test time the bullet is carefully inspected and rated for any sign of rust.
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Dispersants and Detergents These additives keep sludge, fine solid, and semi-solid contaminants dispersed in the oil rather than settling out as deposits. The compounds used are succinimides, neutral calcium and barium sulfonates, phenates, polymeric detergents and amine compounds. Detergent dispersants are also basic calcium sulfonates/phenates that neutralize sludge precursors. Ash content is the percent by weight of noncombustible residue of an oil. The metallic detergents and dispersants are the primary contributors to ash and may cause unwanted inorganic residue to form. The efficiency of some machines operating at high temperatures is reduced by a build-up of these undesirable deposits. For example, many compressor oils require very low ash, such as a trace. Ash content using ASTM D 874 Sulfated Residue is the most commonly used technique. This method consists of slowly burning the oil in a crucible. The carbonaceous residue after burning is wetted with sulfuric acid and reheated. Once the sulfuric acid is completely volatilized more sulfuric acid is introduced and the crucible is heated in a muffle furnace at 875 degree C until a constant weight of inorganic residue is obtained. This residue is considered the sulfated ash in percent by weight.
Additives to Improve Boundary Lubrication Performance Anti-friction Anti-friction, sometimes called lubricity, is defined as the ability of a lubricant to reduce friction, other than by its purely viscous properties. Anti-friction additives reduce friction below that of the base oil alone under conditions of boundary lubrication. The additives are adsorbed on, or react with the 19
Physical and Chemical Properties of Mineral Oils metal surface or its oxide to form monolayers of low shear strength material. The compounds are long chain (greater than 12 carbon atoms), alcohols, amines, and fatty acids. A classic example is oleic acid reacting with iron oxide to form a film of the iron oleate soap. The low shear strength of the soap film causes the low friction.
Saponification Number Saponification is a chemical test indicating the amount of fatty material in the oil and, therefore, one index of anti-friction. Saponification is a chemical process of converting fats to soap. Certain lubricants such as worm gear oils, steam cylinder oils, machine tool way lubricants, and pneumatic tool oils, contain fatty type additives to improve anti-friction properties. Saponification number is performed according to ASTM D 94. The saponification number indicates the amount of fatty substances in the oil. Saponification number is the number of milligrams of KOH that combines with the fat in 1 gram of oil to form the soap. Therefore, the higher the number, the greater the amount of fatty material. Anti-friction is measured directly by laboratory bench tests, where a low coefficient of friction ("f"), measured under conditions of boundary lubrication indicates good antifriction performance. Examples of bench tests are the four-ball test machine and the pin-ondisk apparatus. A pin-on-disk apparatus with steel sliding on steel, with a base oil would give an f of 0.10 to 0.15, whereas the addition of 2% oleic acid to the oil, f would be reduced to 0.05 to 0.08. In an industrial machine, antifriction reduces power requirements. No bench machine has been found to correlate satisfactorily with an industrial machine. However, if materials and operating conditions in the bench machine simulates the industrial machine as closely as possible, the © 2004 SKF Reliability Systems All Rights Reserved
results are useful for screening lubricants, revealing wear mechanism, and warning of problems. The final lubricant test is in the industrial machine itself.
Anti-Wear Anti-wear additives are those that reduce or control wear. They form organic, metalloorganic, or metal salt films on the surface. Sliding or rolling occurs on top of, or within, the films thus reducing metal-to-metal contact. Anti-wear additives only reduce the rate of wear, which still occurs, but without a catastrophic failure. The films are sacrificed so that the wear fragments in the oil are primarily the film material. Anti-wear performance is measured on numerous bench lubricant testers operating under moderate conditions, where the volume or weight of material removed is measured. An example is the 4-ball wear test. Also, the pin-on-disk apparatus is used and run under conditions described in ASTM G 99-90. The types of anti-wear additives are zinc dialkyldithiophosphates (ZDDP), carbamates, organic phosphates such as tricresyl phosphates, organic phosphates and chlorine compounds. The most common anti-wear additive is ZDDP, which decomposes to deposit metallo-organic species, zinc sulfide or zinc phosphate, or reacts with the steel surface to form iron sulfide or iron phosphate. Operating conditions control the specific film material.
Anti-Scuff Anti-scuff additives are those that prevent scuffing. Scuffing is defined as damage caused by solid-phase welding between sliding surfaces. Anti-scuff additives reduce scuffing by forming thick films of high melting point metal salts on the surface which prevent metal to metal contact which, when 20
Physical and Chemical Properties of Mineral Oils extensive, may cause scuffing. The mechanical properties of the films, such as melting point, shear strength, ductility, and adhesion to the metal surface determine the effectiveness. Common anti-scuff additives are sulfur or phosphorous compounds more chemically active than anti-wear additives. A common gear oil anti-scuff additive is a mixture of an organic sulfur compound and an organic phosphorous compound usually identified as S/P. Excessive chemical activity of anti-scuff additives creates a danger of corrosive wear. For example, an active sulfur compound may reduce the risk of scuffing of steel gear teeth, but severely tarnish any corrodible metal. Microscopically, the scuffed surface appears irregular, torn, with plastic deformation, and shows evidence of melting. The definitive test of scuffing is the evidence of metal transfer. Anti-scuffing properties of oils are also measured on lubricant testers run under severe conditions. Usually load, oil temperature, speed, or a combination are increased until scuffing occurs. Scuffing is usually accompanied by high f, such as between 0.2 and 0.5, and possible localized heat, oil smoking, and noise. Wear fragments in the oil are usually large metallic particles. Note: There is some overlapping of anti-wear and anti-scuff performance. That is, some additives have good anti-wear properties and can prevent scuffing to a limited degree. Following are some components of oil or additives that affect lubrication under boundary lubrication conditions.
Oxygen The oxygen in air dissolved in oil forms metal oxide films that have anti-wear and limited anti-scuff properties. Iron oxides, especially Fe3O4 on steel, is effective in reducing metalto-metal contact. This oxide is frequently found as wear fragments in used oil when low © 2004 SKF Reliability Systems All Rights Reserved
wear occurs. Conversely, if an oil is deaerated so that the oxide film cannot be continuously repaired, high wear and scuffing occurs immediately.
Sulfur Compounds Sulfur compounds in lubricating oils and their chemical activity are directly related to antiwear and anti-scuff properties. Elemental sulfur was a historical additive used in a gearbox to reduce oil temperature. Sulfur content is useful in understanding boundary lubrication performance. The sulfur compounds may be naturally occurring in the base oil, or added as additives. A low sulfur content would explain poor boundary lubrication performance. A very high sulfur content would explain corrosion problems where the corrosion product is found to be a metal sulfide, or where contamination by hydrogen sulfide was found. Iron sulfide films are frequently identified on undamaged ferrous surfaces in industrial equipment. The amount of sulfur in oil is reported as percent or ppm total sulfur. Therefore, one must consider the several sources of sulfur, such as in the base oil naturally, additives, ZDDP, and organo sulfur compounds such as sulfurized olefins. The source of sulfur can be narrowed down by analyzing for the stoichiometric amounts of associated elements. Examples are: analysis of zinc, sulfur and phosphorous for the ZDDP additive, sulfur and phosphorous for a S/P gear oil anti-scuff additive, or molybdenum and sulfur for a black oil possibly containing molybdenum disulfide. Usually, lubricating oil suppliers provide only physical properties and performance data, but little or no additive chemistry or elemental analysis. If the additive chemistry is of interest to a user, a laboratory might perform a series of tests for identification. 21
Physical and Chemical Properties of Mineral Oils For example, if analysis showed the presence of sulfur, and ES analysis showed zinc and phosphorous, and infrared analysis showed peak characteristic of ZDDP, then the presence of the ZDDP additive would be indicated.
Phosphorous Compounds
References [1] Organization For Economic Co-operation and Development, "Glossary of Terms And Definitions In The Field Of Friction, Wear, and Lubrication". Wear Control Handbook , Eds. Peterson, M. B. and Winer, W. O., ASME, New York, 1980, pp. 1143-1203.
Various phosphorous containing compounds are added to lubrication oils to improve antiwear properties. The most common is tricresyl phosphate (TCP). Other organo-phosphates and phosphites are used. These compounds are thought to adsorb on or react with the rubbing metal surface to form protective films of organometallic or iron phosphate. Some tribological parts, such as camshafts, are pretreated to form thick iron manganese phosphate coatings to minimize metal-tometal contact during break-in.
[2] ASTM, "Annual Book of ASTM Standards", Section 5, Petroleum Products, Lubricants, and Fossil Fuels. American Society For Testing And Materials, 1916 Race Street, Philadelphia, PA, 19103-1187.
Chlorine Compounds
[4] Liston, T. V., "Engine Lubricant Additives, What They Are and How They Work", Lubrication Engineering , Vol. 48(5), 1992, pp. 389-397.
Chlorine compounds continue to be used in some oils and commercial additives based on their reputation for reducing friction and improving anti-scuff properties. However, considerable danger of corrosion is present because of the chloride ion. Therefore, if a problem of corrosion including rusting is found, a chlorine analysis is suggested. Further, any halogen compound in oil creates disposal or re-refining problems.
Conclusions This paper reviewed the physical and chemical properties of mineral oils. As such, it provided a basis for understanding the relations between properties and their effect on lubrication. Recognition of these relations is useful for designing lubrication systems, choosing lubricating oils, diagnosing lubrication, friction and wear problems, and selecting appropriate testing methods.
[3] ASTM. "Standard Practice for Use of the SI International System of Units - The Modernized Metric System", E380-93, 1993. American Society For Testing And Materials, 1916 Race Street Philadelphia PA 191031187.
[5] Rizvi, S. Q. A., "Lubricant Additives and Their Function", Friction, Lubrication and Wear Technology, ASM., 1992, pp. 98-112. [6] Litt, F., Assoc. Editor, "Starting From Scratch: Tribology Basics", STLE, Park Ridge, IL 60068. [7] Baker, A. E., "Lubricant Properties and Test Methods", Handbook of Lubrication, Theory and Practice of Tribology, Vol. I, Ed. Booser, E. R., CRC Press, 1984, pp.481-515. [8] Klaus, E. E. and Tewksbury, E. J., "Liquid Lubricants", Handbook of Lubrication, Theory and Practice of Tribology, Vol. II, Ed. Booser, E. R. CRC Press, 1984, pp.229-254.
[9] Wright, W. A., "Prediction of Bulk Moduli and Pressure-Volume-Temperature Data for © 2004 SKF Reliability Systems All Rights Reserved 22
Physical and Chemical Properties of Mineral Oils Petroleum Oils", ASLE Trans., Vol(10), 1967, pp. 349-356. [10] Fein, R. S., "Liquid Lubricants" Friction, Lubrication, and Wear Technology, ASM, 1992, pp.81-88. [11] Kauffman, R. E., "Rapid Determination of Remaining Useful Lubricant Life", Handbook of Lubrication and Tribology, Vol. III, Ed. Booser, R. E., CRC Press, Boca Raton, FL, 1994, pp. 89-100. [12] Herguth, W.R. and Dr. Phillips, S., Comparison of Common Analytical Techniques to Voltammetric Analysis of Antioxidants in Industrial Lubricating Oil, Proceedings STLE Annual Meeting Condition Monitoring , May 19, 1995.
About Herguth Laboratories, Inc. Herguth Laboratories, Incorporated is a testing services, oil analysis, fuel analysis and grease analysis laboratory that specializes in failure analysis, tribology studies and research. Herguth Laboratories, located in Vallejo, California, has been analyzing lubrication for over 25 years. With modern, state-of-the-art facilities, a competitive pricing structure and rapid turnaround, Herguth is prepared to meet all of your lubrication analysis needs. http://www.herguth.com Herguth Laboratories, Inc. 101 Corporate Place, P.O. Box B - Vallejo Ca. 94590-6968
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