www.bruker-axs.com Characterization of Lubricants for Quality Control and R&D Suresh Kuiry, PhD Bruker Nano Surfaces Division Tribology and Mechanical Testing, 1717 Dell Ave, Campbell, CA 95008, U.S.A. May 8, 2012 1
Introduction Lubrication Science and Engineering - lubrication regimes: Boundary-Mixed-Hydrodynamic; Stribeck curve Importance and advantages of testing lubricants using CETR- UMT at ambient and elevated temperatures Tests using various contact modes: Point, Line, and Plane ASTM standard tests for tribological evaluation of lubricants using CETR-UMT including Piston ring test, Four-ball test, Twist Compression test; Pin and Vee test, Test at various Rolling-to-Sliding ratios Q & A September 11, 2012 2
Lubrication Science and Engineering To improve the smoothness of relative motion of one surface with respect to another and to prevent damage by using thin (1-100 mm) layers, Lubrication Science and Engineering deals with knowledge and technique that help enhancing or diagnosing the effectiveness of such films to prevent damage in solid contacts. Damage of solid contacts: Wear - adhesive, abrasive, erosive, oxidative, corrosive, fretting, impact, melting etc. Surface Fatigue Such damage incurs enormous costs in terms of machine down-time, replacement parts, and loss of productivity. Hence, it is important to perform tribological tests of lubricants for Quality Control and for development of more effective lubricants. September 11, 2012 3
Functions of Lubricants -Primary Functions of Lubricants: Reduce Friction and Wear -Secondary Functions of Lubricants: To facilitate cooling - Liquid lubricants are the most effective Remove contaminants/or debris or to prevent contaminants from entering the system To keep moving parts apart and reduce surface fatigue, vibrations etc. Prevent corrosive and oxidative damages of moving parts To act as a sealant for gases September 11, 2012 4
Types of Lubricants Lubricants are characterized by their formulations: Minerals Oils - Derived from Crude oil Bio-lubricants - Triglyceride esters from plants (canola, rapeseed, castor, palm) or animals (Lanolin-sheep wool s grease, animal fat etc.) Synthetic Oils - Poly-alpha Olefin; Synthetic-, Phosphate- and silicate esters; Alkylated Napthalene; Ionic fluids Solid Lubricants - PTFE; Hex-B-nitride, MoS 2, WS 2 ; Cd, Au, Pb, Sn, Zn, Cd, Bronze Aqueous Lubricants - Brush polymers (PEG) September 11, 2012 5
Lubrication Regimes Three regimes: Boundary Film Lubrication: Load is carried by the surface asperities rather than by the lubricant. Fluid Film Lubrication: Load is carried entirely by the lubricating film. -Hydrodynamic - the motion of contacting surfaces, and the bearing design pump lubricant around the bearing and maintain the lubricating film. -Hydrostatic- an external pressure is applied to the lubricant in the bearing to maintain the fluid lubricant film. Mixed Lubrication (Elastohydrodynamic)- Elastic deformation of the asperity contact enlarges the load-bearing area, whereby the viscous resistance of the lubricant helps support the load. September 11, 2012 6
Boundary Film Lubrication Boundary or Boundary and Extreme Pressure (E.P.) lubrication deals with direct asperities contact between two surfaces. Under this condition, COF is the ratio of effective shear stress and the plastic flow stress of the contact materials. Lubricant additives help reducing friction by forming a low-shear strength interface on hard metal contacts. At low temperature (100-150 o C) and high pressure (up to 1 GPa), covering the contacting surfaces with adsorbed mono-molecular (such as fatty acids, Silane etc.) low-shear layer minimizes the friction. At high temperature, the formation of sacrificial films of inorganic materials due to reaction between lubricant additives containing sulphur (dibenzyldisulphide), chlorine or phosphorus and the metal surface prevents metallic contact from severe wear. The lubrication under such condition depends on working temperature at which the rapid film forms that protects the contacts from damage. The presence of oxygen and water can influence the sacrificial film formation and thereby the lubrication. September 11, 2012 7
Hydrodynamic Lubrication The two contacting surfaces are completely separated by a lubrication film thereby reducing friction and wear. COF is maintained at a very low level of about 0.005 and failure of such bearings rarely occurs. There are two conditions for the occurrence of hydrodynamic lubrication: (a) relative motion of two contacting surfaces with sufficient velocity to generate a load carrying lubricating film (b) the surfaces must be inclined to create a pressure field to support the liquid Any liquid or gas can be used for this form of lubrication provided there is no chemical reaction on the bearing surfaces. Despite problems of damage during start-stop and vibration during operation, hydrodynamic lubrication is the preferred form of lubrication in most bearing systems. September 11, 2012 8
Hydrostatic Lubrication The two contacting surfaces are completely separated by a lubrication film thereby reducing friction and wear - similar to hydrodynamic lubrication. Unlike hydrodynamic lubrication, the pressure is generated by an external pump that maintains a continuous supply of pressurized lubricant. Also, the friction force is minimum at a very slow sliding speed. This can be used for design and operation of precision control system. Major disadvantage of hydrostatic lubrication is that the process depend upon the reliability of the pump; if the pump is not reliable the lubrication system is likely to fail causing damages. September 11, 2012 9
Elastohydrodynamic Lubrication (EHL) In this region, the lubricating film is very thin (0.1 o 1 mm) but it manages to separate the contacting surfaces, reducing friction and wear. The contacting surface are deforming elastically due to the presence of hydrodynamic pressure in the film. The mechanism of EHL involves a rapid change in the lubricant property from nearly an ideal liquid state outside the contact to an extremely viscous state within the contact. Mostly mineral or synthetic oil could perform such function for they exhibit pressure dependent viscosity (piezoviscous). September 11, 2012 10
Stribeck Curve More than 100 years ago, Richard Stribeck worked on resolving long-standing questions over bearing friction characteristics as a function of load, speed, and lubrication. Mixed Boundary Hydrodynamic COF vs. ηv F Z plot is called Stribeck curve, where, η = viscosity of lubricant, V = sliding velocity, and F z = normal load. Boundary and Mixed Lubrication Regimes represent situations in which bearing surfaces are not completely separated by a lubricant film but experience some degree of solid-to-solid contact. This situation can happen at high load or low velocity. September 11, 2012 11
Tribological Test of Lubricants The following tests can be performed for evaluation of tribological properties of lubricants using CETR-UMT: Stribeck curve Ball-on-disk/plate Pin-on-disk/plate Ring-on-disk/plate Disk-on-disk Cylinder-on-plate Block-on-ring Pin-on-vee Roller-on-roller Piston-ring-on-cylinder-liner Four-ball test Twist compression test Test at various Sliding-to-Rolling ratios September 11, 2012 12
CETR-UMT Test Schematics Ball/Pin-on-disk Ball/Pin-on-plate Disk/Ring-on-disk/ring Piston ring Block-on-ring September 11, 2012 13
CETR-UMT Test Schematics Cross cylinder Spiral Wear Track Four-ball Test Ball/Pin-on-ring September 11, 2012 14
Test Variables CETR-UMT can monitored the following tests variables: Programmable normal load (Fz) Friction force (Fx), torque Electrical Contact Resistance (ECR) Acoustic Emission (AE) Speed Humidity Temperature CETR-UMT also has capability for imaging and metrology of the wear scar using Optical microcopy, AFM, and 3D-Optical Microscopy (interferometry). September 11, 2012 15
ASTM Standard Lubricant Tests D2266: Wear Preventive Characteristics of Lubricating Grease (4-Ball Method) D2509: Measurement of Load-Carrying Capacity of Lubricating Grease D2625: Endurance Life Solid Lube (Pin and Vee Method) D2670: Wear Properties of Fluid Lube (Pin and Vee Method) D2714: Calibration and Operation of a Block-on-Ring D2981: Test Method for Solid Lubricants in Oscillating Motion D3233: Extreme Pressure Properties of Fluid Lube (Pin and Vee Method) D3704: Wear preventive properties of Lubricating Greases using Block-on-ring D4172: Wear Preventive Characteristics of Lubricating Fluid (4-Ball Method) D5001: Lubricity of Aviation Turbine Fuels (Ball-on-Cylinder Configuration) D5183: Coefficient of Friction of Lubricants (Four-Ball Method) D5620: Thin Film Lube in a Drain and Dry Mode Test (Pin and Vee Method) D5706: Extreme Pressure Properties of Lubricating Greases D5707: Friction and Wear Properties of Lubricating Grease D6078: Lubricity of Diesel Fuels (Ball-on-Cylinder Configuration) D6079: Lubricity of Diesel Fuels (Fast Reciprocating Configuration) D6425: Friction and Wear Properties of Extreme Pressure (EP) Lubricating Oils G181 : Friction Tests of Piston Ring and Cylinder Liner in Lubricated Conditions September 11, 2012 16
Rotary Test A Ball or pin is installed under the load sensor and a standard disk is installed inside a liquid holder and the Ball/Pin-on-disk test is performed using CETR-UMT. Fx and Fz are measured to obtain COF. This set up can also be used to obtain an automated Stribeck curve. Rotary Test Module ECR and AE can also be monitored. Tests at ambient or elevated temperatures September 11, 2012 17
Reciprocating Test A plate is installed inside the liquid holder. Ball-on-plate and pin-onplate tests can be performed in reciprocating motion using this module. Piston ring-on-cylinder test can also be performed. Fx and Fz data are measured to obtain COF. Reciprocating Test Module ECR and AE can also be monitored. Test can be performed at ambient or elevated temperatures September 11, 2012 18
Disk-on-disk Test A disk is installed inside the liquid holder. Disk-on-disk tests can be performed in rotary motion using this module. Fx data is obtained from the torque (Tx). Tx and Fz data are measured to obtain COF. Disk-on-disk Test Module Ring-on-disk test can also be performed in this module September 11, 2012 19
Block-on-Ring Tests Medium Load BOR High Load BOR A block holder is used to hold a block. A standard ring is installed on the arbor which rotates in horizontal axis. Fx and Fz data are measured to obtain COF. Block will have wear scar that can be measured by using either an Bruker Optical Microscope or a 3D-microscope September 11, 2012 20
Z [um] Block-on-Ring Test Low load (3N), High speed (3000 rpm), 2 hrs 0.5 0.4 0.3 0.2 0.1 0-0.1-0.2-0.3-0.4 Optical Image of wear scar on block -0.5-200 -100 0 100 200 X [ um ] A depth profile of the wear scar on block September 11, 2012 21
Block-on-Ring Test High load (900N), slow speed (600 rpm), 2 hrs Optical Image of wear scar on block A depth profile of half cross-section of the wear scar on block September 11, 2012 22
Pin-on-Vee Test ASTM D2625, D2670, D3233, D5620 September 11, 2012 23
Pin-on-Vee Test Results A B C ASTM D5620 Test: Lubricants A (red), B (green), and C(blue) are tested as at 1170 N load and 290 rpm for 1 hr: Lube A failed after 185 s; B and C didn t fail even after 1 hr of test, hence B and C are good lubricants for anti-seizure applications; Lubricant C is the best because it maintains the lowest COF. September 11, 2012 24
Pin-on-Vee Test Results A B ASTM 3233 test : run-in for 5 min at 1174 N load with 290 rpm; raise the load to 1832 N and run for 1 min at 290 rpm; raise the load 2828 N run for 1 min; raise the load to 3824 N run for 1 min like that till failure of oil occurs. Failure of the oil is characterized by sharp increase of friction or the motor will be unable to hold the torque. Report the last load level where it fails. Lubricant A and B failed at 1832 and 2828 N (motor stalled) load, respectively. September 11, 2012 25
Piston ring-on-cylinder Liner Test Friction test of Piston ring and cylinder liner materials can be performed under lubricated conditions as per ASTM G181-05. The test parameters are: temperature 100 ± 2 o C; loading from 20 N to 200 N with a step of 20 N with holding time in each load is 1 min; unloading from 200N to 20N with 20 N step and a holding time of 1 s in each load; Stroke of 10 mm; Frequency of 10 Hz. The load profile is shown below. The average value of friction during loading and unloading corresponding to the same load is reported as a function of load. Load Profile Piston ring holder Piston ring Segment and cylinder liner September 11, 2012 26
Piston ring-on-cylinder Liner Test Piston ring-cylinder liner friction test (ASTM G181) set-up (Left) and results of friction test of two lubricants (right). Lube B (blue) exhibited a rise in friction with the increase of load due to change to towards boundary regime at higher load. Lube A (red) showed not only higher COF but also less increase in COF from 20 to 200 N load. September 11, 2012 27
Four-Ball Test This test is performed in CETR-UMT using a 4-ball test set-up with rotary drive and a heating chamber up to 150 0 C as per ASTM D4172 Standard test method. Top View Load SAE 52100 balls with 0.5 inch diameter are used. Three balls are assembled inside a 4-ball liquid holder. The test lubricant is poured inside the 4-ball holder. The fourth ball engages at the center from the top of the three balls with a load of 392 N. The liquid holder with three balls rotate at 1200 rpm for 1 hour. A torque sensor is used to measure friction and normal force to calculate the COF. After the test, the wear scar on all the three balls are measured and average wear scar diameter value is reported. Smaller the wear scar diameter better is the performance of the lubricant. September 11, 2012 28
Four-Ball Test Results (a) (c) (b) Fig. (a) Optical image of the wear scar on a ball after 4-ball test at 75 0 C, 392 N load, and 1200 rpm for 60 min, (b) 3D-profile of the wear scar, and (c) COF vs. time plot September 11, 2012 29
Twist Compression Test Schematic Test Set-up Typical Twist Compression test plots The friction torque plot usually exhibit four distinct stages: initial break-in (I), effective lubrication (II), depletion of lubricant (III), and failure of lubricant (IV). The time taken for Tz and AE to rise due to the failure of the lubricant is reported as the durability of the lubricant. Greater the durability better is the performance of the lubricant. September 11, 2012 30
Test at Varying Sliding-to-Rolling Ratios in Rotary Motion 0% : sliding when three balls are used 15% : sliding when three short rollers are used 35% : sliding when three Long rollers are used 65% : sliding when three Long rollers are installed on three slanted slots September 11, 2012 31
Test at Varying Sliding-to-Rolling Ratios in Reciprocating Motion A: upper disk B: bearing cages C: rollers or balls D: lower disk E: lower disk holder F: upper pin or ball holder for loading by servo-controlled carriage. G: Liquid holder Angle, degee 90 80 70 60 Sliding/Rolling (%) 0 17 34 50 September 11, 2012 32
High Temperature Drives 150 0 C Rotary 350 0 C Rotary 1000 0 C Rotary 150 0 C Reciprocating 350 0 C Reciprocating 1000 0 C Reciprocating September 11, 2012 33
Summary For comprehensive tribological characterization, lubricants are required to be tested and compared in all three lubrication regimes, various sliding-to-rolling ratios, and in various conditions of humidity and temperatures. CETR-UMT can perform standard and customized tests to characterize the tribological properties of lubricants for improved research, formulation, application development and quality control in the widest ranges of loads, speeds and temperatures. September 11, 2012 34
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