May 8, 2012 1
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.
September 11, 2012 2
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 3
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.
Lubrication Science and Engineering
September 11, 2012 4
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 5
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, MoS2, WS2; Cd, Au, Pb,
Sn, Zn, Cd, Bronze
• Aqueous Lubricants - Brush polymers (PEG)
September 11, 2012 6
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 7
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- 150oC) 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 8
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 9
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 10
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 (piezo-
viscous).
September 11, 2012 11
Stribeck Curve
COF vs. η𝑉
𝐹𝑍 plot is called
Stribeck curve, where,
η = viscosity of lubricant,
V = sliding velocity, and
Fz = 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.
Boundary Hydrodynamic
Mixed
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.
September 11, 2012 12
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 13
CETR-UMT Test Schematics
Ball/Pin-on-disk Ball/Pin-on-plate
Piston ring Block-on-ring
Disk/Ring-on-disk/ring
September 11, 2012 14
CETR-UMT Test Schematics
Cross cylinder
Ball/Pin-on-ring Four-ball Test
Spiral Wear Track
September 11, 2012 15
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 16
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 17
Rotary Test
Rotary Test Module
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. ECR and AE can also be monitored. Tests at ambient or elevated temperatures
September 11, 2012 18
Reciprocating Test
A plate is installed inside the liquid holder. Ball-on-plate and pin-on-plate 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. ECR and AE can also be monitored. Test can be performed at ambient or elevated temperatures
Reciprocating Test Module
September 11, 2012 19
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. Ring-on-disk test can also be performed in this module Disk-on-disk Test Module
September 11, 2012 20
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 21
Block-on-Ring Test
Optical Image of wear scar on block
A depth profile of the wear scar on block
X [ um ]
-200 -100 0 100 200
Z [
um
]-0.5
-0.4
-0.3
-0.2
-0.1
0
0.1
0.2
0.3
0.4
0.5
Low load (3N), High speed (3000 rpm), 2 hrs
September 11, 2012 22
Block-on-Ring Test
Optical Image of wear scar on block
A depth profile of half cross-section of the wear scar on block
High load (900N), slow speed (600 rpm), 2 hrs
September 11, 2012 23
Pin-on-Vee Test
ASTM D2625, D2670, D3233, D5620
September 11, 2012 24
Pin-on-Vee Test Results
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.
A
C
B
September 11, 2012 25
Pin-on-Vee Test Results
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.
B A
September 11, 2012 26
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 ± 2oC; 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.
Piston ring holder Load Profile
Piston ring Segment and cylinder liner
September 11, 2012 27
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 28
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 1500C as per ASTM D4172 Standard test method. 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.
Load
Top View
September 11, 2012 29
Four-Ball Test Results
Fig. (a) Optical image of the wear scar on a ball after 4-ball test at 75 0C, 392 N load, and 1200 rpm for 60 min, (b) 3D-profile of the wear scar, and (c) COF vs. time plot
(a)
(b)
(c)
September 11, 2012 30
Twist Compression Test
Typical Twist Compression test plots Schematic Test Set-up
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 31
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 32
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 33
High Temperature Drives
1500C Rotary
10000C Reciprocating
10000C Rotary 3500C Rotary
3500C Reciprocating 1500C Reciprocating
September 11, 2012 34
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.
