TITLE PAGE
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
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TABLE OF CONTENTS
TITLE PAGE ............................................................................................................................................................. 1
TABLE OF CONTENTS.............................................................................................................................................. 2
TABLE OF FIGURES ................................................................................................................................................. 3
TABLE OF EQUATIONS............................................................................................................................................ 3
INTRODUCTION ..................................................................................................................................................... 4
ALUMINUM ........................................................................................................................................................... 5
TRIBOLOGICAL PROPERTIES OF METALS ................................................................................................................ 5
ANODIZING ............................................................................................................................................................ 7
FRICTION AND WEAR STUDIES OF ALUMINUM INTERFACES .................................................................................. 9
CONCLUSION ....................................................................................................................................................... 16
BIBLIOGRAPHY ..................................................................................................................................................... 17
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TABLE OF FIGURES
FIGURE 1: IDEALIZED STRUCTURE OF ANODIC POROUS ALUMINUM OXIDE (ASOH ET AL.) ...................................................................7
FIGURE 2: CROSS-SECTION OF POROUS OXIDE IN THE VICINITY OF METAL/OXIDE INTERFACE. (THOMPSON AND WOOD) ..........................7
FIGURE 3: DEVELOPMENT OF ANODIZED OXIDE LAYER FROM BARE METAL....................................................................................8
FIGURE 4: SEM IMAGE OF ENDS OF WEAR SCARS AND WEAR DEBRIS FOR NODULAR FE VS. 6061 T6. (WYCLIFFE, 577) .......................10
FIGURE 5: SEM OBSERVATIONS OF SCRATCHES: (A) TREATED, (B) NON-TREATED ALUMINUM ALLOY (MEZLINI, 2855) .........................11
FIGURE 6: EVOLUTION OF WEAR VOLUME VERSUS DISPLACEMENT AMPLITUDES FOR THE NON-TREATED AND ANODIZED A357 (MEZLINI,
2856) ...................................................................................................................................................................12
FIGURE 7: EVOLUTION OF THE POROSITY, DETERMINED AT THE OXIDE SURFACE, IN FUNCTION OF THE ELECTROLYTE TEMPERATURE. FE-SEM
IMAGES OF THE SURFACE OF POROUS ANODIC FILMS FORMED AT ELECTROLYTE TEMPERATURES OF A) 5 °C, B) 35 °C AND C) 55 °C.
D) CONVERTED BINARY IMAGE OF B), THE WHITE SPOTS CORRESPOND TO PORE MOUTHS. (AERTS, 7313-7314) ........................ 13
FIGURE 8: EVOLUTION OF THE MICROHARDNESS OF THE ANODIC OXIDE LAYER IN FUNCTION OF THE ELECTROLYTE TEMPERATURE.
EVOLUTION OF THE WEAR VOLUME OF THE ANODIC OXIDE LAYER (I.E. THE VOLUME REMOVED BY THE CORUNDUM BALL DURING
WEAR TESTING) IN FUNCTION OF THE ELECTROLYTE TEMPERATURE. (AERTS, 7312-7313) ..................................................... 13
FIGURE 9: AFM IMAGES OF FILM FABRICATED IN H3PO4 + C2H2O4 ELECTROLYTE WITH SOME ORGANIC ACID (ZHANG, 2672) ..........14
FIGURE 10: MICROSTRUCTURE OF COMPOSITE UNDER THE DIFFERENT IMPREGNATING TIME (ZHANG, 2673) .....................................15
FIGURE 11: HARDNESS OF THE COMPOSITE WITH REGARD TO ULTRA-SONIC IMPREGNATING TIME. WEAR RATE OF SURFACE SELFLUBRICATING COMPOSITES AND NON-IMPREGNATING ANODIC OXIDE FILM WITH REGARD TO LOAD. RESULT OF MEASUREMENTS OF
BALL-ON-DISK TYPE ROTATION FRICTION TEST: (A) NONIMPREGNATED ANODIC OXIDE FILM AND (B) IMPREGNATED ANODIC OXIDE
FILM. (ZHANG, 2673) .............................................................................................................................................. 15
TABLE OF EQUATIONS
EQUATION 1: WEAR RATE....................................................................................................................................................6
EQUATION 2: OXIDATION REACTION OF ALUMINUM .................................................................................................................7
EQUATION 3: SEALING OF THE OXIDE PORES BY HOT WATER ......................................................................................................8
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Introduction
In the design of aircraft engines, the durability of the vast number of moving components can
drive factors such as maintenance cost and overall customer satisfaction. One way to improve the
durability of the various components is to evaluate and understand the materials, operating conditions,
and loads which inherently control the friction and wear that a system will encounter during its life.
There are two types of kinematic systems driven by single actuators that control the variable stator
vanes and low pressure compressor bleed system operation that are greatly affected by the friction and
wear of critical interfaces. For variable stator vane systems, legacy designs have incorporated translating
and rotating rings to control vane arm operation and angular positioning of variable vanes in the
compressor. A similar ring design with mechanically retained seals has proven effective for sealing in
compressor air for the low pressure compressor bleed system. In both applications, maintaining
concentricity of the ring to evenly spaced case bosses around the circumference helps maintain accuracy
in the SVA and helps maximize sealing in bleed systems. To maintain proper gaps between ring and case,
carbon fiber based polymer composite bearing pads are mounted to the ring or case. The replaceable
bumpers work effectively for the operating temperature ranges of these systems and also provide a low
friction surface that can help reduce the loads required to actuate the system.
Unfortunately for the bleed systems, due to the geometry and loads applied to the ring,
deflections 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. For increased durability,
the bleed system utilizes high strength anodized aluminum with hard anodizing at these critical
interfaces. Other Teflon impregnated coatings are used on surfaces where the aluminum is expected to
contact other aluminum surfaces. The goal of this project is to research aluminum and the process
anodizing for reduced friction and improved wear characteristics.
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Aluminum
Aluminum is a readily available metal that suits many of the requirements of today’s industry. It
has a low density of .098 lb/in3 compared to steel that is about 0.29 lb/in3 or titanium that is about 0.16
lb/in3, which is a great benefit when trying to maintain low weight designs in fields such as the
aerospace industry. Also, aluminum can be tempered to satisfy the structural needs of a variety of
applications. When medium to high strength aluminum is required, one of the most commonly used
6000 series aluminum is alloy 6061. Alloy 6061 aluminum is easily heat treatable to a variety of tempers,
where ultimate and yield tensile strengths values can range from 22 ksi and 16 ksi in the annealed state
(designation “–O”) to 38 ksi and 35 ksi in a solution heat treated and artificially aged state (designation
“-T6”). Although aluminum has a much lower effective maximum use temperature than metals such as
steel and titanium, other advantages such as good machinability, weldability and general corrosion
resistance make this a very versatile material (Alloy 6061, 1-2).
Tribological Properties of Metals
For any interface, particularly those with clean surfaces, the adhesive force acting between the
two mating surfaces can have a significant impact on the frictional force required to generate relative
motion. Interactions occur at asperities forming what is called the real area of contact. It is these
locations that are compressed into each other and transfer external loads. Depending on the material
combinations and duration of contact, the adhesive force for metals is driven by the distance between
the two surfaces to form strong metallic bonds at the atomic scale or weak van der Waals bonds at
greater distances. When metallic bonds are formed at the interface of the two materials, and if they are
like materials, bond strength can effectively match that of the bulk material (Bhushan, 175-177). Other
factors that can affect the adhesive force between two surfaces are the modulus of elasticity and
ductility due to their effect on the real area of contact. The normal load driving the surfaces together
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will compress asperities against one another and partially deform. When the load is removed, “elastic
recovery” occurs as the materials partially spring back to an original shape (Bhushan, 172-174).
Therefore, materials with a high elastic modulus generally exhibit lower adhesive properties due to the
decreased real area of contact. High ductility, on the other hand, increases the adhesive force by
elongating the contacts at the asperities thereby creating a greater contact area (Bhushan, 172-177).
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 (Bhushan, 252). 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 (Bhushan, 252). 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
(Bhushan, 255).
In terms of durability, annealed 6061 aluminum has Brinell hardness of 30 whereas the T6
temper has a Brinell hardness of 95 (Alloy 6061, 1-2). Although this is a great improvement, it is
significantly lower than that of steel such as A286 and titanium such as AMS 4928 (alloy 6-4) at a Brinell
hardness of 304 (A286) (Allvac). In the case of plastic contacts, low hardness results in an increase in the
wear rate at the interface due to the inverse proportionality expressed in Equation 1, where  is the
wear rate, H is the hardness, k is a non-dimensional coefficient based on the materials at the contact
interface (Bhushan, 336).
𝜐=
𝑘𝑊𝑥
𝐻
Equation 1: Wear Rate (Bhushan, 336)
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Anodizing
Anodizing is a process that builds on the natural tendency of metals such as aluminum to
spontaneously oxidize. The process can be further assisted with the incorporation of temperature, an
electrolytic solution, and electrical current. Anodizing works by attaching the aluminum work piece to
the positive side of a DC power supply and placing it into the bath. 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 listed below (Equation 2) (Alwitt).
2 Al + 3 H2O  Al2 O3 + 3 H2
Equation 2: Oxidation Reaction of Aluminum (Kopeliovich)
Typically 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 (Alwitt). This is replaced by the volume of oxide growing on the
surface in the form of hexagonal cells with hollow cores (Figure 1, Figure 2, Figure 3) (Alwitt). It is this
porous surface that permits the solution to access the substrate aluminum and continue to form an
increasingly thick layer up to .001 inches thick for Sulfuric (Type 2) anodizing (Alwitt). Another benefit of
the pores is that they allow the absorption or impregnation by another substance, such as dyes and/or
materials like PTFE (Alwitt).
Figure 1: Idealized structure of anodic porous aluminum
oxide (Asoh et al.)
Figure 2: Cross-section of porous oxide in the vicinity of
metal/oxide interface. (Thompson and Wood)
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Figure 3: Development of Anodized Oxide Layer from Bare Metal (Kopeliovich)
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. Once sealed, the surface is stable and impermeable in a
range of temperatures and conditions. The chemical reaction for sealing in water is shown below
(Equation 3) (Kopeliovich).
Al2 O3 + 3 H2O  2 AlOOH * H2O
Equation 3: Sealing of the Oxide Pores by Hot Water (Kopeliovich)
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. In order to achieve a thicker, denser oxide layer,
known as hard anodize, the bath temperature is dropped to around 5 degrees Celsius from the 20
degrees Celsius used in Type 2 anodizing (Alwitt). There is great benefit to hard anodize for improved
wear resistance and corrosion resistance. With hard anodizing, the oxide cells are larger and have
smaller pores, which results in a denser layer than regular anodizing. The hardness value is from 65 to 70
Rockwell C, much greater than that for bare aluminum. Thicknesses are generally around .002 inches
thick (Alwitt).
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Friction and Wear Studies of Aluminum Interfaces
In an attempt to characterize and understand the friction and various modes of wear that a
material surface will exhibit it is necessary to define certain test criteria upon which valid conclusions
can be drawn without the influence of a wide array of uncontrolled variables. In many cases,
understanding the way a surface or substrate will react in a given environment under specific operating
conditions can be determined in a single experiment, but with a small change the results can be much
different than expected. As a result, the following research is a summary of a sample of technical papers
that have characterized the general interaction of bare and anodized aluminum against a variety of
different conditions as a comparative study with a specific focus on the anodize and hard anodize
process with respect to friction, hardness and wear resistance.
One experiment of interest investigates the effect of sliding distance on the resulting friction
coefficient and wear. As expected, it was discovered that the friction coefficient µ is dependent on both
sliding conditions such as “test load, sliding velocity, environment, temperature and geometry of sliding
surfaces” and changes to the sliding contact area caused by “metal transfer, debris generation and cyclic
surface deterioration” (Mahdavian & Mai, 221). Geometry of the surfaces was controlled by grinding
aluminum alloy 2011-T3 (hardness of 67 HRB) to an average surface roughness of 5 µm (Mahdavian &
Mai, 223). The friction coefficient starts from an initial value µi of 0.34, increases to a maximum value µm
of 0.64, and then drops to a steady state value µs of 0.44 for the remainder of the sliding distance. Test
results noted that “aluminum debris generated in the sliding process were initially small but became
larger in size with increasing sliding distance” and that “severe damages were observed on the sliding
surfaces of the specimen and there was transfer of aluminum wear particles in large lumps to the scar
surface of the stationary slider” (Mahdavian & Mai, 223). The conclusion drawn from these results
indicate that the transfer of material between two softer like-metals such as aluminum can be
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significant due to adhesive forces but that ultimately the generation rate of wear debris being lower
than the removal rate drives the steady state friction value (Mahdavian & Mai, 231).
Understanding the interactions of dissimilar metals can also shed light on the types of wear
damage potentially seen in the field. Another study discusses a process of wear called galling “defined as
a severe form of wear characterized by localized, macroscopic material transfer, removal or formation of
surface protrusions when two solid surfaces experience relative sliding under load” (Wycliffe, 575). For
the aluminum on steel sliding contact, at first there is a formation of surface protrusions as the
aluminum is transferred to the iron slider driven by adhesion. From then on out, two-body abrasion
occurs due to generated wear particles producing “machining chip” type debris as shown in Figure 4
(Wycliffe, 576). The initiation of galling of aluminum on steel appears to be driven by a “minimum
amount of deformation at a small number of points of asperity contact” at low loads. The load drives
adhesion of asperities causing welding of the joints where resulting “deformation breaks the natural thin
oxide on the aluminum and extrudes the aluminum substrate between the cracks thus formed”
(Wycliffe, 578).
Figure 4: SEM image of ends of wear scars and wear debris for nodular Fe vs. 6061 T6. (Wycliffe, 577)
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Many other tests focus on the effect of hard anodizing on wear resistance. Since aluminum is a soft
metal that does not fare well in wear situations, the development and understanding of ways to
improve the wear characteristics and durability of this protective oxide layer is critical. Before delving
into the improvements offered by hard anodizing, it is important to understand the limitations.
Generally, it is understood that abrasive wear resistance increases with hardness, but this is not always
true. Scratch tests of hard anodized (thickness of about 20 µm) versus bare aluminum indicated the
thicker oxide treatment “induces the transition of wear mechanism from ductile to brittle damage” as
can be seen in Figure 5. In the end, this test shows that increasing hardness by hard anodizing aluminum
decreases abrasive wear resistance (Mezlini, 2855).
Figure 5: SEM observations of scratches: (a) treated, (b) non-treated aluminum alloy (Mezlini, 2855)
In another test, fretting wear of hard anodized aluminum was compared to bare aluminum,
both against steel. “Fretting is a process in which material is removed from one or both of two
contacting surfaces when the surfaces undergo cyclic tangential displacement with respect to each
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other” (Mezlini, 2852). In this case, hard anodizing results in a reduction of adhesion, especially at the
beginning of fretting tests. In addition, wear debris from the anodized layer is trapped between the
surfaces as a third body filler and further reduces friction between the surfaces (Mezlini, 2855). Also
above a certain displacement threshold, the generation of debris from the oxide layer contributes
significantly to the reduction of the overall wear rate as compared to the bare aluminum, Figure 6.
Figure 6: Evolution of wear volume versus displacement amplitudes for the non-treated and anodized A357 (Mezlini, 2856)
Investigation into the generation of the oxide layer has shown the benefit of controlling factors
such as electrolyte temperature and composition. 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. Further analysis via scanning electron microscope shows that the hard anodize
coatings generally 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. Here, wear tests indicated
that “hard anodizing improves the abrasive resistance of the bare 6061 alloy by a factor of 2” (Krishna,
1097). As the electrolyte temperature increases, so does the porosity of the oxide resulting in the
reduction of the hardness and wear resistance of the oxide, Figure 7 and Figure 8. Higher porosity of the
anodic layer means there is less material present in the oxide film to resist the indentation load in
hardness tests. The porosity at higher temperatures is due to the increase in aggressive dissolution of
the built up walls of the layer (Aerts, 7316).
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Figure 7: Evolution of the porosity, determined at the oxide surface, in function of the electrolyte temperature. FE-SEM
images of the surface of porous anodic films formed at electrolyte temperatures of a) 5 °C, b) 35 °C and c) 55 °C. d) converted
binary image of b), the white spots correspond to pore mouths. (Aerts, 7313-7314)
Figure 8: Evolution of the microhardness of the anodic oxide layer in function of the electrolyte temperature. Evolution of
the wear volume of the anodic oxide layer (i.e. the volume removed by the corundum ball during wear testing) in function of
the electrolyte temperature. (Aerts, 7312-7313)
Composition of the electrolyte bath alters the structure of the oxide layer and opens the door
for other methods of friction and wear reduction. Adding organic acids to the solution significantly
affects the morphology of the oxide film. “The organic acid added in the phosphoric acid and oxalic acid
electrolyte can be adsorbed on the surface of oxide film, and restrict the diffusion of H3PO4 and C2H2O4
molecules toward the anode face” (Zhang, 2672). This process occurs at locations of high electric-field
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intensity, which “reduces the current density of the active region, and makes the electric-field intensity
of each section of the oxide film tend to be identical” (Zhang, 2672). As a result the film growth becomes
more stable and “the porous structure of the film gets more regular and has a highly ordered array”,
Figure 9 (Zhang, 2672).
Figure 9: AFM images of film fabricated in H3PO4 + C2H2O4 electrolyte with some organic acid (Zhang, 2672)
This highly ordered pore structure with more controlled pore size allows for impregnation and
or layering of low coefficient of friction materials into the pores and onto the surface of the oxide. 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 the pores and destroy the microstructure of
anodized template” (Zhang, 2672). Therefore, longer impregnating 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, Figure 10 and Figure 11 (Zhang, 2674). Unfortunately,
impregnating PTFE in the oxide has only been marginally successful. This is partially 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” (Gabe, 56).
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Figure 10: Microstructure of composite under the different impregnating time (Zhang, 2673)
Figure 11: Hardness of the composite with regard to ultra-sonic impregnating time. Wear rate of surface self-lubricating
composites and non-impregnating anodic oxide film with regard to load. Result of measurements of ball-on-disk type
rotation friction test: (A) non-impregnated anodic oxide film and (B) impregnated anodic oxide film. (Zhang, 2673)
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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.
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