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, /15/20091 Rensselaer Polytechnic Institute MANE-6960 Friction and Wear of Materials

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. 12/15/20092 Rensselaer Polytechnic Institute MANE-6960 Friction and Wear of Materials Introduction

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 materials partially spring back to an original shape 12/15/20093 Rensselaer Polytechnic Institute MANE-6960 Friction and Wear of Materials Friction and Wear in Metals 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 Al 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 Aluminum wear particles on steel

12/15/20094 Rensselaer Polytechnic Institute MANE-6960 Friction and Wear of Materials 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 H 2 O  Al 2 O H 2 The Anodizing Process for Aluminum 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 5 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. More on Anodizing… 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 Al 2 O H 2 O  2 AlOOH * H 2 O 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.

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 6 Hard Anodizing

12/15/2009 Rensselaer Polytechnic Institute MANE-6960 Friction and Wear of Materials 7 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 from 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 Improving Anodized Surface Friction and Wear Resistance

12/15/2009 Rensselaer Polytechnic Institute MANE-6960 Friction and Wear of Materials 8 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.