Extreme pressure (EP) gear oils are used to minimize wear and scuffing in automotive and industrial applications under high load conditions, especially where high peak or shock loads are encountered. Modern EP gear oils are formulated with additives that contain active sulfur, frequently combined with phosphorus, to provide a thermally stable, noncorrosive lubricant with high load-carrying capacity.
Proper EP gear oil analysis is important in the formulation of the finished product to be sure the specified amount of additive is present and to monitor the condition of the active agent in use. Similarly, in used oil analysis, it may be necessary to evaluate additive performance under certain circumstances, especially if the ability of the additive to function as expected appears to be compromised. For this reason, it is important to understand EP gear oil chemistry, applications of EP gear oils, and the analytical tools used to measure additive concentrations.
What is a Gear Oil?
Gear lubricants perform a multitude of functions. Among other things, they are designed to reduce friction and wear, often under boundary lubrication conditions, act as heat transfer agents, and protect against corrosion and rust. In addition, they contain additives to minimize oil oxidation, inhibit foaming and separate water readily. Gear oils for on-road applications may also contain shear stable VI polymers for wide operating temperature ranges.
By contrast, industrial gear oils are generally monograde lubricants. The viscosity is selected based on the system’s requirements. It should be high enough to “cushion” gear teeth and protect against failure, but not so high as to generate excessive heat and power loss from churning. When selecting the required viscosity grade, it is important to consider not the oil’s ISO viscosity grade per-se, but rather the viscosity of the oil at the operating temperature of the gear box, because it is this parameter that determines the ability of the oil to provide an appropriate oil film.
Industrial gear oils are formulated to meet the operating requirements of the gear set. They are formulated for:
How Do EP Gear Oils Protect Against Wear?
EP gear oils contain additives that prevent metal surfaces from cold welding under the extreme pressure conditions found in situations where boundary lubrication prevails. At the high local temperatures associated with metal-to-metal contact, an EP additive combines chemically with the metal to form a surface film that is ductile enough to prevent the welding of opposing asperities and prevent scuffing or scoring that is destructive to sliding surfaces under high loads (Figure 1). Chemically reactive compounds of sulfur, phosphorus and sometimes chlorine, are used to form these inorganic films.
Figure 1. Mechanical Action of Boundary Lubrication Films
EP additives typically work by adsorbing onto the metal surface either by physical or chemical attraction. Once attached, they react with gear tooth surface material at the high, local temperatures formed when asperities (microscopically small rough spots) come into contact under boundary lubrication conditions. The additives form a low melting point eutectic with the general formula FeSxPyOz that is softer than the metal itself. This surface deforms on contact and prevents the metal surfaces from welding at the contact points.
Historically, EP additives were made with lead soaps, active sulfur and later chlorinated compounds. By the mid-1950s, lead soaps were eliminated from additive packages and replaced with zinc and phosphorus additives. Modern gear lube additives contain sulfur-phosphorus components that are thermally stable, and are noncorrosive to bronze and other nonferrous metals.
Active vs. Inactive Sulfur
Sulfur occurs in lubricants in several forms. The petroleum distillate fraction of solvent-refined base oils contains organosulfur compounds that carry through the refining process. By contrast, hydrocracked base stocks contain little or no sulfur as a result of the high severity catalytic process by which they are made. Similarly, synthetic base oils also do not typically contain sulfurous components. Many lubricant additives, however, do contain sulfur compounds. These are found in numerous different chemical states as indicated in Table 1.
The term active sulfur refers to the relative ability of a sulfur-containing compound to chemically react with a metal surface to form a metal sulfide. Inactive sulfur compounds do not form metal sulfides under test or operating conditions. The tendency of a sulfur-containing additive to react with a metal surface depends on the composition of the additive, the reaction temperature and the oxidation state of the sulfur in the compound.
Sulfides and elemental sulfur are reduced forms of sulfur and will form metal sulfides readily at sufficiently high temperatures. Sulfates are oxidized forms of sulfur and are less reactive with metals to form metal sulfides. The active vs. inactive designation depends on the chemical reactivity of the compound and the test conditions used to classify the additives.
Sulfurized Mineral Oils
Sulfurized mineral oil, made by dissolving elemental sulfur in oil by heating, is the most active form of sulfur because of the relatively weak S-S bond in elemental sulfur. Elemental sulfur exists in eight-member rings or long chains. Sulfurized mineral oil will readily stain copper under the conditions of the Copper Corrosion Test (ASTM D130), which uses a copper test strip, submerged in the test oil at 100°C for three hours as an indication of chemical reactivity to yellow metals.
Sulfurized Fatty Acids
Sulfurized fatty acids are made by chemically reacting sulfur with long chain fatty acids. They have a higher thermal decomposition temperature than elemental sulfur, typically around 265°C. As such, they do not readily stain the copper strip in the copper strip corrosion test.
Sulfur-Phosphorus EP Additives and Naturally Occurring Sulfides
Many sulfur-phosphorus EP additives thermally decompose at 250°C to 275°C and again will readily pass the copper strip corrosion test.
Sulfur compounds in solvent-refined base oils, which are also typically sulfides, are even more chemically stable. Both EP additives and base stocks are inactive based on the standard copper corrosion test. However, at the much higher localized temperatures generated under boundary lubrication, both sulfurized fatty acids and sulfur-phosphorus EP additives will decompose and form metal sulfides. Under these conditions, both additives are typically active.
Sulfurized mineral oil and other highly reactive forms of sulfur have the advantage of forming protective metal films at lower temperatures than other sulfur-containing additives. On the downside, they are corrosive to certain metal and metal alloys, particularly yellow metals, which contain copper as a major alloying element.
Modern EP gear oils contain thermally stable additives that promote system cleanliness and do not corrode yellow metals under moderate operating conditions. The goal in formulating top-tier EP gear oils is to develop additive chemistries that will carry high loads under boundary conditions and protect mating surfaces from wear while minimizing corrosiveness to yellow metals and keeping steel gear components clean.
Tests That Determine Active Sulfur Components in EP Gear Oils
Most oil analysis tests measure a lubricant’s physical or chemical property or a bulk property such as elemental composition. For example, analytical chemists can choose from 17 different ASTM test methods to determine total sulfur in fuel or lubricant.1 However, relatively few tests directly measure the concentration of specific additives and how they change with time.
Advantages and Disadvantages of Tests to Measure Active Sulfur
Direct Measurement of EP Additives
ASTM D1662 is used to determine the amount of active sulfur in a lubricant by measuring total sulfur before and after the reactive sulfur is removed by reaction with copper metal. The sample is treated with excess copper powder at 150°C until the oil shows no stain on a copper strip (ASTM D130). Active sulfur reacts with the copper powder to form copper sulfide. This test is used typically with cutting oils and may not be useful when measuring the reactive sulfur in EP gear oils that is activated at much higher temperatures.
Reserve EP Capability Test
The Reserve EP Capability Test is a nonstandard test that has been used by a lubricant company to demonstrate the performance of its industrial gear lubricants. The test measures a lubricant’s load-carrying ability over time, based on the ability of the EP additive(s) to resist oxidation. Initially, the total concentration of an indicator element in the EP package, phosphorus in this case, is measured. Then the lubricant is run through the modified U.S. Steel S-200 Oxidation Test (U.S. Steel specification 224). At the conclusion of the oxidation test, the amount of phosphorus is remeasured to check for depletion. According to the lube manufacturer, the higher the phosphorus content at the end of the test, the higher the reserve EP capability.
Oxidation stability is an important feature of any lubricant component. However, this test does not establish the primary requirement of an EP additive - its load-carrying ability as a function of time in use. While the absence of phosphorus in gear lube at the end of a severe oxidation test indicates additive depletion, the presence of phosphorus at end of test doesn’t prove that it is in the form of an effective EP agent.
Fourier Transform Infrared Spectroscopy
Fourier transform infrared spectroscopy (FTIR) is a nondestructive, instrumental analytical technique that can be used to detect individual components in a lube oil quantitatively or semiquantitatively. It is also used in “fingerprint” analysis to compare an unknown lubricant with a known product or to identify contaminants in a finished lube. FTIR replaces more expensive tests to rapidly trend build up of soot, fuel dilution and oxidation products in oil.
The technique is based on the fact that chemically bonded atoms absorb radiation in the infrared region of the spectrum from about 2.5 to 17 microns in wavelength or frequency in the 4000 cm-1 to 450 cm-1 range. The energy of the radiation absorbed depends on the chemical structure and bonds present in each molecule. As such, FTIR can be used to help identify the presence of specific molecules, such as sulfurous additives, in the oil and help determine if appreciable additive depletion has occurred.
For an excellent summary of the theory and practice of FTIR, see the article titled “Fourier Transform Infrared Spectroscopy” that appeared in Practicing Oil Analysis magazine’s March-April 2002 issue.2 A related article by Jay Powell, which also appeared in Practicing Oil Analysis magazine, discusses trending additive depletion using FTIR, and identifies some of the benefits and pitfalls of this technique.5
Obtaining the spectrum of fresh oil containing the additive of interest is the first step in tracking additive depletion by FTIR. The analyst can determine which specific infrared frequencies to monitor by making a solution of the EP additive in the base oil used in the finished oil or by obtaining this information from the oil manufacturer. Next, the analyst performs the FTIR spectrum of the used oil. The difference between the used oil’s and the new oil’s spectrum reference gives the depletion of EP additive.
This can be useful information if it is coupled with other information about the condition of the lubricant and the contaminants that have built up in the oil. Additive depletion alone is not a measure of the oil’s loss of EP properties. For example, the original EP additive may be altered by the system’s operating conditions (thermal decomposition, reaction with oxygen in the air and other components in the lubricant, reaction with metal surfaces) and, yet, may still retain some or all of its EP properties. Determining the depletion of EP additive by FTIR may provide opportunities to set up alarms for more frequent analysis or more extensive analysis (such as atomic emission spectroscopy, particle count analysis, ferrography and X-ray fluorescence spectroscopy on large particles) to establish a safe condemning limit for the gear oil or to identify mechanical problems with the gear box or transmission.
Another complication of using FTIR to track additive depletion is that the reference (new) oil spectrum should be run on oil made by the same manufacturer, ideally from the same batch of oil, as the used oil. A new baseline reference should be run whenever a new batch of gear lube is added to the oil sump or if lubricant is purchased from another supplier.
Phosphorus-31 Nuclear Magnetic Resonance
Phosphorus-31 nuclear magnetic resonance (NMR) is similar to FTIR spectroscopy in that the technique determines the chemical environment of an element, phosphorus in this instance, in individual compounds. NMR differs from FTIR in several ways. FTIR detects information related to chemical bonds while NMR identifies how the chemical environment affects the nucleus of an atom. FTIR spectra give information about almost all atom pairs in chemical bonds of interest in organic molecules. NMR is restricted to specific isotopes of elements that have an overall spin. Hydrogen-1, carbon-13, nitrogen-15 and phosphorus-31 are the isotopes most frequently probed by NMR. From a practical point of view, FTIR is far more accessible to the oil analysts than NMR because of the lower initial cost of the instrumentation, as well as lower operating and maintenance costs.
Like FTIR, NMR can be used to measure the concentration of an additive in lube oil by quantifying a particular resonance in the NMR spectrum. It is probably more sensitive in identifying the decomposition products of the additive, but this requires intimate knowledge of the additive chemistry and the decomposition products. This is generally proprietary knowledge closely held by the additive manufacturer. Finally, like FTIR, NMR data on used oils still needs to be related to a measure of the used oil’s EP or antiwear properties. In sum, NMR will likely remain a research tool rather than a routine diagnostic technique. For more information on the use of NMR to monitor additive performance, see the article in the May-June 2003 issue of Practicing Oil Analysis magazine.6
Indirect Measurement of EP Additives
Copper Corrosion Test
The copper corrosion test, ASTM D130, is a qualitative, nonspecific measure of a lubricant’s corrosivity toward copper. It can be used to indicate the presence of relatively active sulfur if the analyst or equipment operator knows that other lube components or contaminants that corrode copper are absent. It is important to understand that there is no correlation between a positive reaction in the copper corrosion test and the effectiveness of the EP additive in a gear oil.
Thermal Stability Test
The thermal stability test, originally developed as the Cincinnati Milacron Thermal Stability Test - A, is a more quantitative corrosion test than ASTM D130. When conducting this test, a beaker of the test fluid containing copper and iron rods is heated to 135°C for 168 hours. At the end of the test, the rods are rated visually for discoloration. Additionally, the change in viscosity of the oil, the amount of sludge formed in the oil and the weight loss of the copper rod are determined. The procedure is more useful for characterizing the corrosive tendencies of new EP gear oil than used oil. This test’s seven-day duration limits its use in trend analysis of used oil. Like the copper strip corrosion test, ASTM D2070 does not necessarily correlate with the amount of effective EP additive in used oil.
Friction and Wear Tests
Friction and wear tests, identified in Table 2, are used more commonly to characterize fresh gear oil’s EP and antiwear performance.
They are relatively time consuming and expensive to run and are not used routinely in lubricant trend analysis. However, it may be useful to track EP and antiwear properties of lubricants used in critical applications to establish correlations between the EP additive concentration in used oil with other oil properties and contaminants that build up in the system. Likewise, they can be instructive tests when trouble- shooting specific problems related to EP performance.
Sequential Four-Ball Test
Scientists at Penn State University developed the sequential four-ball test (SQFBT), a variant of the established ASTM four-ball friction and wear tests.4 This three-step procedure begins with an initial 30-minute run on the test fluid. After this period, the initial wear scar is measured. The fluid is then tested for an additional 30 minutes and the change in scar size recorded. The difference is a measure of the lubricant’s antiscuffing performance in the absence of the initial “run-in” wear scar that normally occurs. In the last step, additive-free white oil replaces the test lubricant for a final 30-minute run after which the scar is again measured. This stage evaluates the ability of additive deposited on the steel surfaces to protect the metal from wear or scarring. An effective AW or EP lubricant should provide lasting protection by depositing additive onto the metal surface.
This test appears to offer some advantages over the traditional ASTM friction and wear tests in that it evaluates the lasting performance of gear oil antiwear and EP components. Additional development is needed to determine if the test correlates with other indicators of useful lubricant life.
However, the four-ball wear tests should not be relied on too heavily when determining EP additive content and useful life. In one study using a ball-on-disc apparatus at slow sliding speeds, investigators found that products of oil oxidation, likely carboxylic acids, contributed to lower levels of scuffing. These acids, however, may increase corrosion, accelerate additive attrition and form sludge and deposits.