The Effect of Anodizing to
Minimize Friction and Wear
of Aluminum Surfaces
Ryan LeBlanc
Rensselaer Polytechnic Institute
MANE-6960 Friction and Wear of Materials
Professor Ernesto Gutierrez-Miravete
December 15, 2009
12/15/2009
Rensselaer Polytechnic Institute MANE-6960
Friction and Wear of Materials
1
Introduction
• In the aerospace industry materials such as aluminum are critical in producing
lightweight designs
• Unfortunately, the wear resistance of materials like aluminum are not always adequate
and can result in durability issues
• The durability of the vast number of moving components in a kinematic system can
drive factors such as maintenance cost and overall customer satisfaction
• Unfortunately for low compressor bleed systems in commercial engines, the geometry
and loads applied to a rotating ring causes deflections that can and do result in contact
between the aluminum ring and aluminum case.
• This contact is detrimental to the overall health of the bleed system causing damage to
both the ring and case, increasing friction in the system, and ultimately resulting in
binding of the ring.
• As a result, for increased durability, the bleed system utilizes high strength anodized
aluminum with hard anodizing at these critical interfaces.
12/15/2009
Rensselaer Polytechnic Institute MANE-6960
Friction and Wear of Materials
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Friction and Wear in Metals
• Adhesion via metallic bonds forms at the interface of two
metals, and with similar metals, bond strength can effectively
match that of the bulk material
• Normal load drives the surfaces together and compresses
asperities, partially deforming, against one another and.
• When the load is removed, “elastic recovery” occurs as the
Aluminum wear particles on steel
materials partially spring back to an original shape
• Materials with a high elastic modulus generally exhibit lower adhesive properties due to
the decreased real area of contact.
• High ductility increases the adhesive force by elongating the contacts at the asperities
thereby creating a greater contact area
• One way of reducing the friction between two surfaces is to introduce an intermediate
lubricating layer.
• Most metals, including aluminum, oxidize readily in air producing a thin film of amorphous
Al2 03 between 1 to 10 nm thick
• This naturally occurring thin oxide film can act as a lubricant in metal to metal contact
under low load situations.
• The film has low shear strength and ductility resulting in low friction between the surfaces
• Under higher loads and sliding conditions, the film is penetrated effectively eliminating
the film’s lubricating effect due to asperity contacts.
• As a result, self-mated aluminum has a coefficient of friction between 0.8 to 1.2
12/15/2009
Rensselaer Polytechnic Institute MANE-6960
Friction and Wear of Materials
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The Anodizing Process for Aluminum
• Anodizing is a process that builds on the natural tendency of metals such as aluminum to
spontaneously oxidize
• Attach the aluminum work piece to the positive side of a DC power supply and placing it
into an electrolyte bath such as diluted sulfuric acid.
• Another electrically conductive metal, inert in the anodizing bath, is connected to the
positive side of the power supply.
• With the power supply activated, electrons are pulled from the aluminum into the
solution causing the aluminum to react with water to form a build-up of an oxide layer.
• At the cathode, hydrogen gas is formed.
• The chemical reaction is 2 Al + 3 H2O  Al2 O3 + 3 H2
• The bath electrolyte is chosen so that the oxide is insoluble or
at least dissolves at a slower rate than it deposits.
• As the oxide layer develops, the sulfuric acid in solution
continues to partially dissolve the aluminum substrate and
oxide layer.
• This is replaced by the volume of oxide growing on the surface
in the form of hexagonal cells with hollow cores.
• The porous surface permits the solution to access the
substrate aluminum continuing to form an increasingly thick
layer up to .001 inches thick for Sulfuric (Type 2) anodizing.
12/15/2009
Rensselaer Polytechnic Institute MANE-6960
Friction and Wear of Materials
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More on Anodizing…
• In order to prevent unwanted absorption of
particles, the oxidized layer is cleaned in fresh
water and sealed.
• The sealing process is, at its simplest form,
another bath in hot water around 200 degrees
Fahrenheit.
• In this bath at temperature, the water reacts with the oxidized surface to form crystals of
hydrated aluminum oxide that fill the pores.
• When sealed the surface is stable and impermeable in a range of temperatures and
conditions.
• The chemical reaction for sealing in water
Al2 O3 + 3 H2O  2 AlOOH * H2O
• The standard spec, MIL-A-8625, covers chromic (Type 1), sulfuric (Type 2) and hard
anodize (Type 3).
• Sulfuric acid (Type 2) is the most commonly used as compared to chromic acid (Type 1),
which is less frequently used due to carcinogenic concerns.
12/15/2009
Rensselaer Polytechnic Institute
MANE-6960 Friction and Wear of Materials
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Hard Anodizing
• To achieve a thicker (around .003 inches thick), denser oxide layer, known as hard anodize,
the bath temperature is dropped to around 5 degrees Celsius from 20 degrees in Type 2
• The hardness value is from 65 to 70 Rockwell C, much greater than that for bare aluminum
• In one test, the average coating thickness of a hard anodized coating was 50 ± 1.8 µm with
a surface roughness (Ra) of about 2.36 ± 0.28 µm in the as coated condition.
• Scanning electron microscope shows hard anodize coatings exhibit about 40% porosity.
• Pore size was about 15 ± 5 µm with a micro-hardness of 490 ± 15 HV.
• Bare 6061 alloy was measured with a micro-hardness of 102 HV.
• Wear tests indicated hard anodizing improves the abrasive resistance of the bare 6061
alloy by a factor of 2
• As the electrolyte temperature increases, so does the porosity of the oxide due to the
increase in aggressive dissolution of the built up oxide walls of the layer
• Higher porosity of the anodic layer means there is less material present in the oxide film to
resist the indentation load in hardness tests resulting in lower wear resistance of the oxide
12/15/2009
Rensselaer Polytechnic Institute
MANE-6960 Friction and Wear of Materials
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Improving Anodized Surface Friction and Wear Resistance
• Anodizing allows impregnation or layering of low friction
coefficient materials into the pores and onto the oxide surface.
• One test used an ultra-sonic impregnation technique to fill the
pores with PTFE.
• Results showed that for times over 600 seconds the ultrasonic
energy can enlarge and destroy the microstructure of the pores
• Longer impregnation time reduced the surface hardness and
caused cracks.
• For shorter times, PTFE impregnated surface had a reduced
friction coefficient of 0.166 from 0.575 for the basic oxide film
surface.
• Impregnating PTFE in the oxide has only been marginally
successful due to controlling pore size through the use of
electrolyte solutions that are commercially not economical as
well as the lack of strong charge on the covalent polymer
12/15/2009
Rensselaer Polytechnic Institute
MANE-6960 Friction and Wear of Materials
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Conclusion
• Aluminum has great potential as a low weight, moderate strength structural material for
application in the aerospace industry.
• Unfortunately, in sliding contact situations, aluminum has poor wear resistance and high
friction, especially in an aluminum on aluminum scenario.
• The naturally occurring oxide layer that forms will protect the base substrate from corrosion
but is not effective for wear resistance due to inadequate thickness in its natural state.
• The sulfuric anodizing process is an economical and effective way to improve the wear
characteristics of this oxide layer.
• Hard anodizing, at lower electrolyte temperatures, allows for thicker, denser build up of the
oxide due to smaller pore size.
• This greater density results in higher microhardness of the oxide layer as compared to the
base substrate.
• Brittle fracture of the oxide replaces ductile failure at the surface creating debris that
contributes to third body wear at a lower rate.
• Improvements to the oxide layer structure, wear and friction properties can be
accomplished using organic acids to control pore size and array and impregnation
techniques to incorporate low friction materials such as PTFE into the pores and as an
overlay.
• Hard anodizing is an effective method to improve the friction and wear characteristics of
aluminum in many applications.
12/15/2009
Rensselaer Polytechnic Institute
MANE-6960 Friction and Wear of Materials
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