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 CETRUMT 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
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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.
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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
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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)
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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.
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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.
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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.
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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.
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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).
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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
COF vs.
Boundary
Hydrodynamic
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.
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Tribological Test of Lubricants
The following tests can be performed for evaluation of tribological
properties of lubricants using CETR-UMT:
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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
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CETR-UMT Test Schematics
Ball/Pin-on-plate
Ball/Pin-on-disk
Piston ring
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Disk/Ring-on-disk/ring
Block-on-ring
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CETR-UMT Test Schematics
Cross cylinder
Four-ball Test
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Spiral Wear Track
Ball/Pin-on-ring
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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).
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ASTM Standard Lubricant Tests
D2266:
D2509:
D2625:
D2670:
D2714:
D2981:
D3233:
D3704:
D4172:
D5001:
D5183:
D5620:
D5706:
D5707:
D6078:
D6079:
D6425:
G181 :
Wear Preventive Characteristics of Lubricating Grease (4-Ball Method)
Measurement of Load-Carrying Capacity of Lubricating Grease
Endurance Life Solid Lube (Pin and Vee Method)
Wear Properties of Fluid Lube (Pin and Vee Method)
Calibration and Operation of a Block-on-Ring
Test Method for Solid Lubricants in Oscillating Motion
Extreme Pressure Properties of Fluid Lube (Pin and Vee Method)
Wear preventive properties of Lubricating Greases using Block-on-ring
Wear Preventive Characteristics of Lubricating Fluid (4-Ball Method)
Lubricity of Aviation Turbine Fuels (Ball-on-Cylinder Configuration)
Coefficient of Friction of Lubricants (Four-Ball Method)
Thin Film Lube in a Drain and Dry Mode Test (Pin and Vee Method)
Extreme Pressure Properties of Lubricating Greases
Friction and Wear Properties of Lubricating Grease
Lubricity of Diesel Fuels (Ball-on-Cylinder Configuration)
Lubricity of Diesel Fuels (Fast Reciprocating Configuration)
Friction and Wear Properties of Extreme Pressure (EP) Lubricating Oils
Friction Tests of Piston Ring and Cylinder Liner in Lubricated Conditions
September 11, 2012
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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
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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
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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
September 11, 2012
Ring-on-disk test can also be
performed in this module
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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
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Block-on-Ring Test
Low load (3N), High speed (3000 rpm), 2 hrs
0.5
0.4
0.3
0.2
Z [um]
0.1
0
-0.1
-0.2
-0.3
-0.4
-0.5
-200
Optical Image of wear
scar on block
September 11, 2012
-100
0
X [ um ]
100
200
A depth profile of the wear scar on
block
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Block-on-Ring Test
High load (900N), slow speed (600 rpm), 2 hrs
Optical Image of wear
scar on block
September 11, 2012
A depth profile of half cross-section
of the wear scar on block
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Pin-on-Vee Test
ASTM D2625, D2670, D3233, D5620
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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
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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.
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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.
Load Profile
September 11, 2012
Piston ring holder
Piston ring
Segment and
cylinder liner
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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.
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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.
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.
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Four-Ball Test Results
(a)
(c)
(b)
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
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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.
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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
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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
Sliding/Rolling (%)
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90
0
80
17
70
34
60
50
32
High Temperature Drives
1500C Rotary
3500C Rotary
10000C Rotary
1500C Reciprocating
3500C Reciprocating
10000C Reciprocating
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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
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May 8, 2012
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