Wear with low-lubricity fuels I. Development of a wear mapping technique

Wear, I60 (1993) 325-332 325

Wear with low-lubricity fuels I. Development of a wear mapping technique

P. I. Lacey US Army Belvoir Fuels and Lubricants Research Facility, Southwest Research Institute, San Antonio, TX (USA)

(Received July 6, 1992; accepted September 10, 1992)

Abstract

Use of low-lubricity fuels such as Jet A-l is now relatively common in military ground equipment, while severe refinery processes such as hydrotreating are removing reactive components from commercially available diesel fuels. Under critical conditions, durability problems with fuel-sensitive equipment emerge with the use of Jet A- l. Consequently, a bench wear test that accurately reflects the environment within the fuel injection system is needed. However, a better understanding of the primary wear mechanisms present in the fuel injection system and their relationship with potential bench wear tests is first required.

Wear maps have previously been used to study ceramic materials and to define the wear mechanisms in metallic contacts. This technique systematically produces a data base according to a self-consistent methodology and allows the effects of each contact parameter to be precisely described. This paper, the first of two, details the initial test development required to ensure that the data base is completed according to a single well-defined procedure. The second paper, “Correlation between wear maps and pump components”, uses the test methodologies and results developed to elucidate the wear mechanisms present in fuel-lubricated contacts and to assist in bridging the gap between laboratory tests and practical applications.

1. Introduction

During the mid-1960s, improvements in petroleum

refining and treatment processes removed many of the compounds required for effective lubrication in aviation kerosene. Since that time, considerable effort produced a study of the wear mechanisms present in low-lubricity fuels [l] and a standard procedure that reflects fuel- related wear in aviation equipment [2]. Nonetheless, there currently exists no minimum lubricity requirement for aviation fuels.

Many commercially available diesel fuel injection systems depend entirely on the intrinsic lubricity of the fuel to provide resistance to sliding wear. At present, increasingly strict emission controls for compression ignition engines encourages development and produc- tion of more severely refined diesel fuels. In addition, the US Department of Defense is procuring aviation turbine fuels for ground equipment that previously operated on diesel [3]. Laboratory tests demonstrate the sensitivity of the fuel injection system to such changes in fuel composition [4, 51. While these developments stimulate an increasing interest in the wear resistance of fuel-lubricated components, relatively little research is being conducted in this area, and the minimum

lubricity requirements of the diesel injection system are largely undefined.

A durability test series was performed using rotary fuel injection pumps [4]. In general, directionally correct correlation has been achieved with the fuel lubricity tests developed for aviation. However, results are not universally accurate, and a number of effects require further investigation using a more versatile test apparatus.

A multitude of contact conditions coexist within the fuel system, and the optimum combination of test parameters must be selected to reflect these needs. Previous studies concentrate on a well-defined wear mechanism with a correspondingly narrow set of test conditions. A more comprehensive study is required to define fuel-related wear over the complete range of conditions likely to exist in practical applications.

This paper details the development of a systematic wear-mapping technique for the study of fuel lubricity. This methodology is then used in a subsequent paper to study the relationship between standard fuel lubricity tests used in aviation and the diesel fuel injection system. Wear mapping allows representation of wear rate and mechanism as a function of two simultaneous variables. Clearly, however, many variables play an

0043-1648/93/%6.00 0 1993 - Elsevier Sequoia. All rights reserved

3% F’. I. Lumy i Wear mapping technique

integral role in wear, so the wear-mapping procedure must represent more than a simple plotting technique between two independent variables. It must represent a self-consistent and unified test methodology in which the remaining variables are either accurately defined or eliminated.

Ashby and co-workers [6, 71 are among the first workers to use the concept of deformation maps at- tempting to categorize wear mechanisms as a function of contact conditions. Hsu and co-workers [8,9] further develop the technique in the study of ceramic materials. Both two- and three-dimensional plots represent wear mechanism and material removal rate, respectively. Of particular interest is the characteristic transition from mild to severe wear that delineates the useful application range of ceramic materials. A selection of wear maps plotted from tests performed with the same material allows effective comparison between different fuel and additive combinations. Materials effect may be similarly defined. Most previous studies use wear maps to in- terpret the predominant wear mechanisms, with particular reference to the physical and chemical properties in a defined family of contact materials.

The primary purpose of the current study is to develop a suitable format to evaluate fuel lubricity with particular reference to the diesel injection system. As such, the emphasis is on developing a rapid and accurate test methodology that reflects the contact parameters present in a predefined application.

2. Test development

The wear-mapping technique allows representation of wear as a function of two simultaneous variables. Many parameters have a direct influence on wear: speed, load, temperature, and test duration, for example. Hsu et al. [8] suggest that five significant three-dimensional wear maps are required for a given material/ lubricant pair: speed and load, speed and temperature, load and temperature, load and time, and speed and time. In addition to these traditional test parameters, the effects of humidity and test atmosphere must be considered with low-lubricity, highly refined fuels [lo]. The absence of polar compounds and reactive species allows formation of an oxide layer on the metallic surfaces. This oxide layer is easily removed from the opposing surfaces during sliding, producing relatively severe wear. If polar species are present, they will be preferentially adsorbed, preventing oxygen from reaching and reacting on the surface. Widespread disagree- ment is present in the literature as to which components are most beneficial in preventing wear - nitrogen, oxygen, sulfur and acid components have been suggested [ll, 121.

Data for the wear maps in the present study were generated using the Cameron-Plint high-frequency reciprocating wear apparatus, shown schematically in Fig. 1. In this configuration, an upper specimen slides on a lower flat with a pure sinusoidal motion. The scotch yoke mechanism that provides this motion is driven by a variable speed motor. This machine provides the relatively low contact stresses required for effective wear testing of fuels and had the ability to run a wide range of test metallurgies. In addition, the oscillating contact is analogous to that found in many areas of the injection system. A more complete description of the Cameron-Plint apparatus may be obtained in ref. 13. However, in its normal configuration, the Cam- eron-Plint apparatus does not facilitate provision of a controlled test atmosphere.

Development of an effective wear-mapping technique for low-lubricity fuels requires that the effects of moisture and humidity be precisely regulated. A Perspex enclosure that completely surrounds the test specimens and fuel reservoir was fabricated, with openings for load application and friction force measurement. The atmosphere within the box is controlled by a continuous gas flow of 4 1 min -I, half of which is aerated through the fuel. The remaining gas maintains a positive pressure within the test chamber to prevent atmospheric con- tamination. The test chamber is purged with the con- ditioned gas supply for 45 min prior to each test to ensure that moisture and oxygen content is stabilized. The relative humidity of the gas supply is controlled between 0% and 100% as required. The temperature inside the chamber is maintained at 38 “C, which is the minimum flash point of Jet A-l aviation turbine fuel [14]. Light fractions in the fuel may be lost at higher temperatures, with a resulting change in fuel characteristics.

Low-lubricity fuels are especially susceptible to con- tamination. Each specimen is rigorously cleaned according to the procedure detailed in ASTM D 5001 [2] immediately prior to testing. Initial repeatability tests also indicate that a fuel’s lubricity is strongly

Fig. 1. Schematic diagram of the Cameron-Plint apparatus.

P. I. Lacey / Wear mapping technique 327

dependent on its storage history. Test variation between different samples of the same Jet A-l fuel is greatly decreased by clay-treating in accordance with ASTM D 3948 [15]. All Jet A-l fuel is clay-treated immediately prior to use.

Each wear map test is performed with a 6.35 mm diameter ball sliding on a polished test flat. Wear is represented by the diameter of the scar formed on the ball, measured using optical microscopy. If the scar is not circular, an average diameter is calculated from measurements taken along the major and minor axes of the ellipse formed. The results are left in the form of the measured wear scar diameter. This is the normal reporting procedure in the ASTM tests for aviation turbine fuel lubricity studied in the second paper, “Correlation between wear maps and pump components”. Comparison between the two test techniques is easier with the data in this format. If required, the volume of material (v) corresponding to a flat of a given diameter (0) on a sphere of radius (R) may be approximately calculated [16] using the following equa- tion:

V= (Z’IxD4)l(64R) (I)

From consideration of eqn. (1) it is clear that a slight variation in the commonly reported wear scar diameter will correspond to a disproportionately large change in volumetric wear.

Each of the maps is plotted as a function of applied load. If required, the approximate pressure over the apparent contact area at the end of the test may be calculated from the wear scar diameter, assuming a circular contact. Apparent contact pressure is not used when plotting the results, as it is not necessarily an accurate representation of the contact pressure during the test. For example, poor lubricity fuels will produce a large wear scar, resulting in a comparatively low apparent contact pressure when measured at the conclusion of the test. Moreover, severe adhesion and seizure typically exist within the initial portion of the test prior to material removal and increased geometric contact area and so are best reflected by normal load. No perfect solution exists; however, previous workers have also plotted their maps as a function of applied load [8, 91.

During the present work, the required test load is attained by gradually increasing the normal force at a rate of 5 N s-’ at the beginning of each wear test. This technique is analogous to the pneumatic loading system in the standard ASTM procedure for the measurement of aviation turbine fuel lubricity [2]. Instan- taneous loading would produce a peak hertzian contact pressure of 6000 N mm-’ at only 100 N applied load. This running-in period is required to prevent transient

scuffing of the severe counterformal contact with poor lubricity fuels.

A short duration test is required to facilitate the large test matrix, while still producing an accurately quantifiable wear scar. Under ideal conditions, the contact parameters and resulting wear mechanisms remain constant throughout each wear test. In practice, accurate wear measurement and the need for a re- producible geometry require a counterformal contact. Under these conditions, the geometrical contact area and the associated contact pressure are a function of sliding distance and wear volume. The resulting wear scar is the summation of the many wear mechanisms and events that pertain throughout the test.

To determine the most effective sliding duration, a series of exploratory tests is performed to define the running-in process for the contact. The fuel is neat Jet A-l, with the characteristics given in Table 1. The upper (reciprocating) specimen is a chrome alloy steel ball manufactured from AISI standard steel No. E- 52100, with a diameter of 6.35 mm, grade 25 EP finish. This metallurgy is not widely used in the fuel injection system, but corresponds with that used in ASTM standard bench wear tests described in Part II (following paper). The opposing flat specimen is also AISI standard steel No. E-52100, polished to a mirror finish. Each of the tests is lubricated with Jet A-l at an applied load of 50 N and a stroke length of 4.7 mm. The results of these preliminary tests are shown in Fig. 2(a). Clearly, the wear mechanisms produced during the running-in process may be more accurately correlated with sliding distance than test duration. No single duration may be chosen for which tests at each speed correspond to the same point on the running-in curve.

A number of supplementary tests are performed to determine the effects of fuel lubricity and load on running in. Applied loads of 10, 75 and 150 N are used with No. 2 Reference diesel (Cat 1-H) and clay- treated Jet A-l, representing good and bad lubricity fuel, respectively. The tests are performed at an average reciprocating speed of 120 mm s-l, which represents the mean of the speed range available on the Cam- eron-Plint apparatus. Jet A-l repeatedly seized at an applied load of 150 N; however, the results obtained at the remaining loads are presented in Fig. 2(b).

TABLE 1. Characteristics of Jet A-l fuel used

Aromatics, vol.% Olefins, vol.% Sulfur, wt.%

Initial boil pt. (“C) End point (“C) Viscosity @ 40 “C (cSt)

Jet A-l Diesel

8.1 41.1 0.0 3.7 0.002 0.39

160 204 218 354 3 1.07

328 i? I. Lacey I Wear mapping technique

~233 mm/s o 60 mm/s *I20 mm/s c 240 mm/s

0 _.-._-_- _ __A /1

0 10 20 30 40 50 60 70 60 90

(4

0.5

0.45

z- 0.4

5 0.35

5 I 0.3

E .& 0.25

;ir 8 0.2

‘m 0.15

$ 0.1

0.05

0

Test Duration (min)

@I

0 50 100 150 200 250 300 350 4M)

Sliding Distance (m)

Fig. 2. Running-in curves for 52100 steel in Cameron-Plint wear tests: (a) different test speeds at 50 N load; (b) different applied loads at 120 mm s-’ sliding speed.

For the speed, load, and fuel combinations studied, a sliding distance of approximately 150 m corresponds to the onset of a more gradual increase in scar diameter. Similarly, the volumetric wear rate as calculated from eqn. (1) begins to gradually decline. This point represents the optimum compromise between test duration and wear scar diameter. To further reduce the error associated with measurement of the wear scar diameter requires greatly increased sliding distances. All subsequent wear tests used in the preparation of the wear maps are carried out at this sliding distance to ensure that tests performed over a range of sliding speeds are directly comparable.

3. Results

3.1. Wear rate maps The wear rate map shown in Fig. 3(a) is prepared

for Jet A-l as a function of applied load and humidity. The measured wear volume prior to transition is a strong function of humidity, confirming the presence of an oxidative corrosion wear mechanism. It should be noted that some of the smaller undulations are

(c) - r, 0 -0

Fig. 3. Wear maps for 52100 steel lubricated with clay-treated Jet A-l: (a) as a function of relative humidity and load in air; (b) as a function of relative humidity and load in nitrogen; (c) as a function of relative humidity and load in Jet A-l with corrosion inhibitor and air.

caused by the mathematical filtration mechanisms in- herent to the graph plotting system. The wear scar diameter increases from approximately 0.32 to 0.48 mm as moisture content is varied from 0% to 100%. At

P. I. Lacey / Wear mapping technique 329

an applied load of approximately 100 N, a sharp transition to severe scuffing occurs, accompanied by a concomitant increase in friction. The test is terminated immediately to prevent damage to the apparatus and the wear scar diameter is arbitrarily set to 1. The transition to severe wear and seizure is largely defined by applied load, although humidity also produces a measurable effect; with no moisture present, seizure occurs at a very low load of approximately 50 N; however, a slight increase in humidity increases scuffing resistance, reaching a maximum load-carrying capacity of 140 N at 50% relative humidity. It is likely that the oxide layer formed in the presence of moisture prevents adhesion and subsequent seizure between the contacting asperities. However, excess moisture is detrimental and reduces the load required for failure of the boundary film to occur. Examination of the worn surfaces with a scanning electron microscpe indicates that the decreased scuffing resistance may be due to increasing oxide thickness and metallic oxide wear particles pro- moting abrasion and increasing surface roughness.

Both oxygen and water are required for the formation of an effective surface oxide layer. Oxygen dissolves in hydrocarbons in accordance with Henry’s law, with the amount present in the hydrocarbons proportional to the partial pressure of oxygen in the atmosphere. The above test procedure was repeated in an atmosphere of pure nitrogen to produce the wear map shown in Fig. 3(b). As expected, the wear scar produced at low loads is somewhat less than that seen in the previous wear map for an oxygen-rich atmosphere, even at 0% humidity. However, tests in dry air (0% humidity in Fig. 3(a)) should also preclude the oxidative/corrosive wear mechanism to provide similar results to nitrogen. The increased wear rate in dry air as compared with nitrogen is probably due to residual moisture contained in the fuel. Tests performed in accordance with ASTM 1744 [17] show approximately 15 ppm moisture in the fuel at nominally 0% relative humidity. Increasing the prepurge time from 45 min did not appreciably reduce the measured moisture content. Similarly, wear also increases slightly with rising moisture content in a nitrogen environment, probably due to residual oxygen present in the fuel. Corrosive wear may persist in the absence of oxygen to form Fe(OH), [lo, 181. None- theless, the wear rate at very low loads in the absence of oxygen is comparable with that typically seen with a formulated engine oil.

The absence of reactive products normally produced during oxidative breakdown of the fuel does not affect wear of lightly loaded contacts in this nominally oxygen- free environment. However, the onset of seizure in fuel-lubricated contacts in a nitrogen environment is less than that seen in the presence of oxygen. The maximum load capacity of Jet A-l in nitrogen is 100

N at 50% relative humidity, compared with 140 N in air. Increasing humidity has a slight effect on scuffing load, again probably due to corrosion with residual oxygen.

Many commercial corrosion inhibitors are available to prevent formation of the oxide film that is removed during corrosive wear. A wear map for Jet A-l with 15 mg 1-l of a common dilinoleic acid (DLA) based corrosion inhibitor is shown in Fig. 3(c) as a function of moisture content. Ideally, wear should be reduced to a level similar to that seen in a nitrogen environment. In practice, wear rate under lightly loaded conditions is only marginally less than that seen in air. The dilinoleic acid-based corrosion inhibitor does not appear to eliminate oxidative wear at the concentration used. When moisture is present, the applied load required for the onset of scuffing wear and seizure is marginally increased by the corrosion inhibitor. However, at 0% humidity, the scuffing load capacity of the additized fuel is appreciably greater. This result indicates that the DLA does form an effective boundary film, but that an antagonism exists between it and water.

The test series with no moisture present is repeated several times, and seizure found to occur in the load range between 150 and 200 N. The decreased repeatability is probably due to trace quantities of moisture, which appear to greatly affect the test results. At a relative humidity of only 5%, the load-carrying capacity of the same fuel sample is reduced to approximately 125 N.

3.2. Wear mechanism maps The wear mechanisms present at the conclusion of

each test are defined using optical and scanning electron microscopy. To assist in visual interpretation, these results are then represented in a wear mechanism map. The different regions in this map define areas with similar material-removal mechanisms. Typically, a number of wear mechanisms coexist within the scar with a single wear mechanism predominating.

Figures 4(a) and 4(b) show wear maps derived from visual examination for 52100 steel lubricated with neat and additized Jet A-l, respectively (corresponding to the wear maps in Figs. 3(a) and 3(c)). At low loads in each instance (region A), the topography of the worn surface is highly polished, with very isolated darker areas of surface deformation due to mild adhesive wear. Material removed is predominantly by corrosive wear with little or no plastic deformation of the substrate material. No furrows or abrasive plowing are visible. As the applied load is increased, the darker portion of the surface gradually extends. Approximately 40% of the surface is discolored in region B; however, little severe plastic deformation of plowing is yet present.

330 P. 1. Lacey I Wear mapping technique

0 so 100 166 200 250

Ibl LOAD [N)

Fig. 4. Wear mech~ism maps as a function of relative humidi~ and applied load in air: (a) lubricated with Jet A-l; (b) lubricated with Jet A-l +corrosion inhibitor.

Further increases in applied load promote abrasive evidence of adhesive contact indicates the presence of plowing or seizure (regions C and D, respectively), an effective interfacial layer between the bulk materials. depending on the level of humidity. In region C, the This wear mechanism is most prevalent at intermediate worn surface is strongly anisotropic and formed of many moisture levels with neat Jet A-l, possibly due to an parallel plowed grooves. However, the topography does effective surface oxide film and at lower humidities not contain fractures, and visual examination indicates when corrosion is controlled using an additive. By that the opposing surface profiles conform with each comparison, the scars formed during seizure (region other, as discussed by Wang et al. [19). Such severe D) are inhomogeneous and randomly fractured due to surface defo~ation and plowing with relatively little severe plastic defo~ation and adhesive wear.

P. Z. Laccy I Wear mapping technique

TABLE 2. Standard deviation of test error prior to seizure

331

Load (N) Humidity (%) Wear sear diameter (mm)

Test No.

1 2 3 4 5

Average

Std. error

10 0 0.33 0.34 0.32 0.33 0.34 0.332 8~10-~

10 100 0.47 0.49 0.47 0.46 0.47 0.472 10x 10-j

75 0 0.37 0.39 0.35 0.36 0.39 0.372 16 x 1O-3

50 100 0.47 0.47 0.45 0.49 0.50 0.476 17x 10-3

TABLE 3. Standard deviation of test error at onset of seizure

Humidity Seizure load (N) Std.

(%) error Test No. Average

1 2 3 4 5 6

25 90 110 115 95 100 100 100 9 50 140 150 135 140 135 150 140 6 70 100 125 130 110 125 135 120 12

3.3. Test repeatability As noted previously, relatively low repeatability is

obtained for measurements of scuffing resistance during critically moisture-sensitive tests with dehydrated fuel. A number of tests from various locations on the wear map shown in Fig. 3(a) are duplicated to establish test repeatability. Results appear in Table 2. Little variation is present with consecutive tests on lightly loaded contacts, characterized by oxidative wear. Greater variability is observed in more highly loaded tests close to transition, probably due to random transient scuffing during the initial portion of the test.

A second test series is performed to determine the accuracy with which the critical load required for the onset of scuffing can be predicted. The applied load is incrementally increased by 5 N in consecutive tests until seizure terminates the test series. This procedure is repeated six times with the results given in Table 3. Scuffing load capacity appears most repeatable at approximately 50% relative humidity, probably because this region is least sensitive to small variations in moisture content. As a result, a standard humidity of 50% is chosen for future test work. This value is an effective compromise and probably reflects conditions normally found in the field.

4. Summary Acknowledgments

This paper details the development of a procedure This work was performed by the Belvoir Fuels and to systematically define both wear rate and mechanism Lubricants Research Facility (BFLRF) at Southwest

in fuel-lubricated contacts. The test procedure is de- signed to provide accurate control of all relevant test parameters and optimize test repeatability, while min- imizing test duration. The resulting test methodology delineates the useful operating range of low-lubricity fluids, with particular reference to accurate definition and control of test humidity and atmosphere.

A number of wear maps were prepared to determine the optimum parameters for future work and also demonstrate the usefulness of the concept. Wear in a completely inert atmosphere is found to be significantly less than that seen with a common corrosion inhibitor. Moreover, the additive’s resistance to both mild oxidative wear and scuffing appears reducedby the presence of moisture. It seems that corrosion inhibitors specified under MIL-I-25017 do not completely eliminate oxidative wear.

Wear mechanism maps were developed based on the observed surface topography at the conclusion of the test. A number of wear mechanisms are apparent, as expected, with a combination of oxidative corrosion and adhesive scuffing predominating. The multitude of unregulated parameters present during practical op- eration makes quantitative comparison with bench-test results difficult. However, qualitative comparison between the wear mechanism maps and the surface topography of failed components facilitates a more accurate correlation between the bench test and reality. The approximate position of the worn component on the wear map may be located both by qualitative comparison of the contact parameters in each instance and also by qualitative comparison of the wear mechanisms present. The paper that follows uses this process to assist in the development of a bench wear test to accurately represent the lubricity requirements of the fuel injection system.

332 P. I. Lacey I Wear mapping technique

Research Institute, San Antonio, TX, under Contract No. DAAK70-87-C-0043. Work is funded by the US Army Belvoir Research, Development and Engineering Center, Fort Belvoir, VA, with Mr. T. C. Bowen (SATBE-FL) serving as contracting officer’s repre- sentative. Project technical monitor is Mr. M. E. LePera (SATBE-FL).

The authors would also like to acknowledge the efforts of BFLRF personnel, especially J. J. Dozier for performing the laboratory tests and J. W. Pryor for editing the final draft of the manuscript.

References

G. Datschefski, History, Development and Status of the Ball- on-Cylinder Lubricity Evaluator for Aero Gas Turbine Fuels, A report prepared under MOD contact AE12a/193 by the Esso Research Center, Abingdon, UK. Test Method for Measurement of Lubricity of Aviation Turbine Fuels by the Ball-on-Cylinder Lubricity Evaluator (BOCLE), Method D 5001-89, ASTM, Philadelphia, PA, 1989. Department of Defense Directive 4140.43, Subject: Fuel Sta- bilization, March 1988. P. I. Lacey and S. J. Lestz, Effect of low-lubricity fuels on diesel injection pumps. Part II. Laboratory evaluation, Paper No. 920824, Society of Automotive Engineers, Warrendale, PA, 1992. P. I. Lacey, The Relationship between Fuel Lubricity and Injection System Wear, Interim Report BFLRF No. 275 (AD 247927) US Army Belvoir Fuels and Lubricants Research Facility (SwRI), San Antonio, TX, January 1992.

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H. J. Frost and M.F. Ashby, Deformation Mechanism Maps: the Plasticity and Creep of Metab and Ceramics, Pergamon, Oxford, 1982. S. C. Lim and M. F. Ashby, Wear mechanism maps, Acta Metall., 35 (1987) l-24. S. M. Hsu, Y. S. Wang and R. G. Munro, Quantitative wear maps as a visualization of wear mechanism transitions in ceramic materials, Wear, 134 (1989) l-11. S. M. Hsu, P. I. Lacey, Y. S. Wang and S. W. Lee, Wear mechanism maps of ceramics, Advances in Engineering Tri- bology, Special Publication 31, Society of Tribologists and Lubrication Engineers, Park Ridge, IL, 1990. J. K. Appeldoorn, I. B. Goldman and F. F. Tao, Corrosive wear by atmospheric oxygen and water, Trans. ASLE, 12 (1969) 140-150. L. Grabel, Lubricity Properties of High Temperature Jet Fuel, Naval Air Propulsion Test Center, NAPTC-PE-112, August 1977. D. Wei and H. A. Spikes, The lubricity of diesel fuels, Wear, 111 (2) (1986) 135-144. M. D. Kanakia, J. P. Cuellar, Jr., and S. J. Lestz, Development of Fuels Wear Tests using the Cameron-Plint High-frequency Reciprocating Machine, Interim Report BFLRF No. 262 (AD A216003), Belvoir Fuels and Lubricants Research Facility, Southwest Research Institute, San Antonio, TX, May 1989. Aviation Turbine Fuel, Grades Jet A-l/Jet A, Standard D 1655, ASTM, Philadelphia, PA 1989. Method for Determining Water Separation Characteristics of Aviation Turbine Fuels by Portable Separometer in the Field, Standard D 3948, ASTM, Philadelphia, PA, 1990. I. M. Feng, A new approach of interpreting the four ball wear results, Wear, 5 (1962) 275-288. Method to Determine Water in Liquid Petroleum Products by Karl Fischer Reagent, ASTM, Philadelphia, PA, 1989. U. R. Evans. The Corrosion and Oxidation of Metat?. Scientific Principles and Practical Applications, Arnolbs, London, 19ko. F. X. Wang, P. I. Lacey, R. S. Gates and S. M. Hsu, A study of the relative surface conformity between two surfaces in sliding contact, J. Tribal., 113 (4) (1991) 755-761.

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Wear with low-lubricity fuels I. Development of a wear mapping technique