INVESTIGATION OF CHEMICAL VAPOR DEPOSITED ALUMINUM AS A REPLACEMENT FOR CADMIUM SERDP Project WP-1405 FINAL REPORT Draft - Revision 0 9 February, 2008 Prepared by Dr. Elizabeth S. Berman - Co-Principal Investigator Dr. Eric W. Brooman - Co-Principal Investigator Air Force Research Laboratory Acquisition Systems Support Branch Materials & Manufacturing Directorate WPAFB, OH Approved for public release; distribution is unlimited. INVESTIGATION OF CHEMICAL VAPOR DEPOSITED ALUMINUM AS A REPLACEMENT FOR CADMIUM Table of Contents List of Figures ............................................................................................................................. vii List of Tables............................................................................................................................... xii List of Acronyms ......................................................................................................................... xv Acknowledgments.....................................................................................................................xviii Executive Summary ................................................................................................................... xix 1.0 Objective and Approach....................................................................................................1 1.1 Objective ...........................................................................................................................1 1.2 Technical Approach ..........................................................................................................3 2.0 Background to Alternative Coating and Process Selection...............................................7 2.1 Problem Addressed by this Technology............................................................................7 2.2 Prior Work on Cadmium Replacement..............................................................................8 2.3 Best Cadmium Alternative Candidates .............................................................................9 2.4 Selection of Aluminum for Study .....................................................................................10 2.5 Selection of Chemical Vapor Deposition as the Deposition Method ...............................10 2.6 Selection of Atmospheric Pressure CVD as Preferred Deposition Method.....................13 3.0 Materials, Coating Deposition, and Test Methods ..........................................................14 3.1 Materials..........................................................................................................................14 3.1.1 Substrates................................................................................................................14 3.1.1.1 Low Carbon, Mild and High-strength Steels.....................................................14 3.1.1.2 Other High-strength Aerospace Alloys .............................................................15 3.1.1.3 Coupon, Panel, and Test Specimen Designs...................................................15 3.1.2 Cleaning Chemicals.................................................................................................19 3.1.3 Precursor Chemicals................................................................................................19 3.1.3.1 Tri-ethyl Aluminum............................................................................................19 3.1.3.2 Tri-isobutyl Aluminum.......................................................................................19 3.1.3.3 Proprietary Blend 1...........................................................................................19 3.1.3.4 Proprietary Blend 2...........................................................................................19 3.1.4 Carrier Gases ..........................................................................................................19 3.1.5 Post-treatment Chemicals........................................................................................20 3.1.6 Reference Materials.................................................................................................20 3.1.6.1 Aluminum Metal................................................................................................20 3.1.6.2 IVD Aluminum...................................................................................................20 3.1.6.3 Electroplated Aluminum....................................................................................20 3.2 Coating Deposition Methods ...........................................................................................21 3.2.1 Deposition in Tubular Reactor .................................................................................22 3.2.2 Deposition in Rotating Barrel Reactor......................................................................24 3.2.3 Deposition in Commercial Barrel Reactor................................................................26 iii Draft Final Report Project WP-1405 INVESTIGATION OF CHEMICAL VAPOR DEPOSITED ALUMINUM AS A REPLACEMENT FOR CADMIUM Table of Contents (continued) 3.3 Testing Methods..............................................................................................................26 3.3.1 Coating Structure.....................................................................................................28 3.3.1.2 X-ray Diffraction................................................................................................28 3.3.2 Thickness and Surface Roughness .........................................................................28 3.3.3 Throwing Power and Conformal Coverage..............................................................28 3.3.4 Density and Porosity................................................................................................28 3.3.4.1 Optical Microscopy ...........................................................................................29 3.3.4.2 Electrochemical Impedance Spectroscopy.......................................................29 3.3.5 Chemical Composition.............................................................................................29 3.3.5.1 Energy Dispersive Analysis with X-rays ...........................................................29 3.3.5.2 Auger Electron Spectroscopy...........................................................................29 3.3.5.3 Nuclear Reaction Analysis................................................................................29 3.3.5.4 X-ray Photoelectron Spectroscopy...................................................................30 3.3.5.5 Glow Discharge Optical Emission Spectroscopy..............................................30 3.3.5.6 Electron Probe Microanalysis...........................................................................30 3.3.6 Electrochemical and Corrosion Properties...............................................................30 3.3.6.1 Open Circuit (Galvanic) Potential and Polarization Behavior ...........................30 3.3.6.2 Salt Fog Exposure Corrosion Resistance.........................................................31 3.3.6.3 Alternating Immersion Corrosion Resistance ...................................................32 3.3.6.4 Exposure to Aircraft Maintenance Fluids..........................................................33 3.3.6.5 Stress-Corrosion Cracking Resistance.............................................................34 3.3.7 Mechanical Properties .............................................................................................35 3.3.7.1 Adhesion...........................................................................................................35 3.3.7.2 Hardness ..........................................................................................................36 3.3.7.3 Young’s Modulus..............................................................................................36 3.3.7.4 Coefficient of Friction........................................................................................37 3.3.7.5 Tensile and Notch Fracture Strength................................................................37 3.3.7.6 Hydrogen Embrittlement and Re-embrittlement Resistance ............................38 3.3.7.7 Fatigue Resistance...........................................................................................39 3.3.8 Electrical Properties.................................................................................................39 3.3.8.1 Contact Resistance ..........................................................................................40 3.3.8.2 Electrical Resistivity..........................................................................................40 4.0 Test Results and Accomplishments ................................................................................42 4.1 Test Results ....................................................................................................................42 4.1.1 Coating Structure.....................................................................................................42 4.1.1.1 Optical Microscopy and Metallography.............................................................42 4.1.1.2 X-ray Diffraction................................................................................................44 4.1.2 Thickness and Surface Roughness .........................................................................47