Table Of Content

AD-A265 352 I,,,'l l1 11111 III ' CORROSION BEHAVIOR OF METAL MATRIX COMPOSITES I FINAL REPORT FOR THE PERIOD July 1, 1989 through September 1992 CONTRACT NO. r. 0014-89-J-3116 Principal Investigator Harovel G. Wheat Mechanical Engineering Center for Materials Science and Engineering The University of Texas at Austin Austin, TX 78712 -. DTIC S ELECTE MAY 2 81993 E Office of Naval Research Atm: Dr. A. John Sedriks JANUARY 1993 93-12493 a iA',- C.ASSIFC.ATON OF !-1Ir ýIAGi REPORT DOCUMENTATION PAGjE f~mApl~~ RE~pPOT SECURITY' C..ASS5 ,CA';ON 'b QSTR:C;vE NAAP~NrS '~SEC,,R'Y CLA5ýIIQCAIACN OE6P '. i. ; 2 PERoq~i;NG OQGANI2ATION REPORT %tjMqER(S S MON:T0RjNGOCA",,ZA!.0r g p .ý E'; aNAME OF PERFORMING ORGANIZATION 6b OFFCE SYvqOL. 74 NAME OF MO-II-OpIN G OGN:ZA-iO.,% The Uni vers ity of Texas at aI0chcab'C) Of ceo Austin Ofc OfNaval Research ADDRESS (COrY. Pa~re, 'I'd ZIP COde) 70 AOOR[SS(C,ry. Sr..rr jrvC ,:pCo~e Mechanical- Enaineerina 800 N. tuincy Street The University of Texas at Austin Arlington, VA 22217-5000 Austin, TX 787-12 ______________________ NME OF FIJNOINGISPONSORNr, 8b OFFICE SYVBC, 9 PROCJREMENT ASTRUMENT CEN7 FýCAr'Of. N,.%BE ORGCANIZATION (it applicable) ,ffjce of Naval Research AD Ri SS (C~tY. Strer, and ZIP Code) 10 SOURCE OF FUNDING NUBR TACPt -Ni7 PROGRAM PROJECT TASK Arlington, VA 22217-5000 E8L9E-MJE-N T3 1N 16I No NoCCESSCN 'N LiE (Include Secvf~tY C`JaSs~f,CJt10n) Corrosion Behavior of Metal Matrix Composites PERSONAL AUTHOR ~S) anN..4ht lyp w WEORT 13b tlME COvERED 14 DATE Of REPORT (yfjr,44onAr. y ? PCECOj Final Report _ I fnRaO 2 09092 1993/Jan)/29 7 SUPPLEMENTARY NOTATION COSATI COOES 18 SUBE CT T ERMS (Continue on 'eveie of ,,eCesse, and .dentsf'y by block nubr Corrosion, Copper Metal Matrix Composites, Graphite, FIELD GROUP ISUS-GROUP 1 1L ' Dispersion Strengthened Copper ABSSRACT (Continue onAfi everSe of necessary ond odenrd'y by block nunm bed A research program was conducted to assess the corrosion behavior of two categories of copper metal matrix composites; commercially available composites and composites in development The commercially available composite studied was dispersion strengthened copper (2.7 vol% A1 0 /Cu) which was found to possess comparable corrosion resistance to copper. In addition, it 2 3 did not appear to be susceptible to stress corrosion cracking when U-bend specimens were tested in 3.5 wt % NaGl or artificial seawater for a period of 4 months. Stress corosion cracking was also not observed under proof ring loading in 3.5 wt % NaCI for a period of 8 months at an applied stress of 95% of the yield stress. T'he composites in development studied were HEH-R GrP'ICu composites containing 1.2, 5, 15, * ~25 and 40 vol % graphite; 50 vol % Grf/Cu; and 70 vol %W f/Cu. These composites experienced varying degrees of galvanic corrosion during polarization. Benzotriazole, however, offers excellent potential as a corrosion inhibitor for these composites. S.YRISI,:ONi AVAIIA8iý1Y Of ABSTRACT 2'. .ABTRACT, SECý.R:Ty CI.ASS5%CAO\0 CD otcifa Unrestricted C~SIEO0NMIE SAME AS RP NAME OF RESPONSIBLE INDIVIDUAL 22b TELEPHONE (include A ea Coe 22 y . ST%'sO A. J. Sedriks (20 2) 69 6 -44 01 1131 M F011-~ 1473, JUN 86 Sj"cý4AoiT~ 'b oc~. V.CATCIý' I~ "'ACE S/N, 0102-ILF-CI'4-b603 1. INTRODUCTION Metal matrix composites (MMCs) basically consist of a reinforcement incorporated into a metallic matrix. By adding elemental, metallic or ceramic reinforcements in vawious forms, including whiskers, particulates, and fibers to a metal matrix, a composite material is created that may possess superior properties over the monolithic material. In fact, the most important reascn to develop MMCs is that they can be tailored to meet the critical performance requirements for advanced engineering applications. These requirements include high strength and stiffness, lo,. densities, and low coefficients of thermal expansion. Although the incorporation of the second phase into a matrix material can enhance the physical and mechanical properties of that materxd. it could also significantly change the corrosion behavior (1, 2). Composites, by their nature, combine materials having considerably different corrosion properties. With regard to corrosion resistance, the dual natures of metal matrix composites make them susceptible to three adverse processes (3); localized galvanic corrosion due to coupling of the metal and reinforcement: crevice attack at the metal/reinforcement interface; and preferred localized attack at structural and compositional inhomogeneities within the metal matrix. The work described in this report focuses on aluminum metal matrix composites and copper metal matrix composites. The work on SiC/6061 Al has been published (2) and is not included in this report. The work on the effect of thermal history on the corrosion behavior of Gr/Al is not complete and will be presented at the Corrosion/93 Poster Session sponsored by the National Association of Corrosion Engineers (4). There is increasing interest in the utilization of Cu metal matrix composites for marine applications. Unfortunately, very limited corrosion studies of Cu metal matrix composites in chloride environments have been performed to date (5). The primary goal of this research has been to understand the corrosion behavior of metal matrix composites in neutral chloride solutions. In particular, the effects of dissolved oxygen on the reduction and oxidation kinetics, the effect of type and form of reinforcement, and the effect I of increasing the volume fraction of reinforcement have been investigated. Particular attention has been given to the corrosion morphology, the corrosion mechanisms involved, and the corrosion susceptibility of the copper composites. The corrosion behavior of copper metal matrix composites that are either commercially available or in development was studied in neutral open to air, aerated and deaerated 3.5 %kt% sodium chloride solutions using electrochemical techniques, scanning electron microscopy (SEM), energy dispersive x-ray spectroscopy (EDX), and in some cases a rotating disk electrode, U-bend specimens, proof ring loading, corrosion inhibitors, ionic solution analysis :A and Auger electron spectroscopy (AES). The materials under investigation were dispersion '-aA- strengthened copper (AI 03/Cu or DSCu); high energy-high rate (HEHR) consolidated 2 particulate graphite reinforced copper metal matrix composites consisting of 1.2, 5, 15, 25, and 40 vol% graphite; 50 vol% ± 30" angle-plied continuous P100 graphite fiber copper metal matrix composites; and 70 vol% unidirectional continuous type 218-CS tungsten wire reinforced copper metal matrix composites. Electrochemical techniques were used to determine the effects of Di(cid:127)t, ibi tio n chloride on the corrosion behavior of the various composites. Availability Codes Dit Avail a;-d ior 2. MATERIALS Spuciat 2.1 Commercially Available Composites 2.1.1 Dispersion Strengthened (DS) Copper The DS copper was obtained from SCM Metal Products, Inc. DS Cu is produced from a blend of pure copper and aluminum powders and the finely distributed (2.7 vol% A 1203) particles are achieved by an internal oxidation process. Properties for DS Cu are given in Table 1. The properties are quite impressive. In fact, in a report to Naval Sea Systems Command, Aylor (4) irdicated that of the following materials AI 03/Cu, TiC/Cu, Si N4/Cu, Si/Cu, B.ViCu 2 3 and Gr/Cu, dispersion-strengthened (DS) copper or (A1203Cu) was the most promising material for high-strength marine applications. In addition, the material's sea water corrosion behavior in 2 low flow conditions was similar to that of pure copper, while the mechanical properties and modules were significantly increased over pure copper. 2.2 Composites In Development 2.2.1 Particulate Graphite Reinforced Copper MMCs The particulate graphite reinforced copper metal matrix composites were fabricated at the Center for Electromagnetics at The University of Texas at Austin. High energy-high rate (HEHR) consolidation techniques were utilized in this process. This process involves the internal heating of a composite powder blend through the rapid discharge of a homopolar generator (HPG). Details on the pulsed homopolar generator used in this novel processing technique and information on the tribological properties of these composites are discussed elsewhere (7,8). HEHR processing employs a 10 Megajoule homopolar generator that supplies a 100 kiloamp pulse to rapidly heat and solidify the composite powder compact. This short time a: high temperature and the preferential heating and melting at the graphite-copper interface serve to encapsulate the graphite reinforcement, thereby providing a highly dense composite product. A schematic detailing the HEHR consolidation assembly is shown in Figure 1 and a typical HPG consolidation pulse shape is shown in Figure 2. An in depth discussion on the fundamental approaches to high energy-high rate processing has been described by Marcus et al, (9). The Grp/Cu composites are processed in the following manner. A ceramic coating of boron nitride is sprayed onto the electrodes, die, and backing ring contact surfaces to prevent arcing during processing. The composite powder blend is loaded into a cold wall die made from seamless stainless steel hydraulic tubing. The powder blend which is contained by electrodes in the top and bottom, is compacted with the desired pressure (275 MPa for these specimens). The HPG discharges a 100 kiloAmp pulse in 200 milliseconds to consolidate t he compact via internal heating and welding of the powder particles. The pressure is maintained for five minutes to permit conductive heat transfer through the electrodes to the large copper platens. The die is unloaded and J;e consolidated Grp/Cu product removed. These processing parameters have been 3 developed and refined to optimize the tribological properties of these binderless Grp/Cu composites. Particulate reinforced graphite copper metal matrix composites are currently used as sliding electrical contacts for generator parts in the automotive, utility, and aerospace industries (10). Sliding electrical contacts transfer current between stationary and moving parts of electrical machinery. Commercially available brushes incorporate low temperature binders such as lead or tin to improve the strength and interfacial bonding of the brush material. These binders decrease the conductivity of the electrical contact and tend to melt in high velocity-high current applications, resulting in decreased conductivity and excessive wear of the electrical contact. Sliding electrical contacts for high velocity-high current applications must demonstrate high conductivity and wear resistance at high sliding interface temperatures (11). Therefore, binderless graphite reinforced copper composites offer an attractive alternative for high velocity- high current applications. The composites studied were manufactured using dendritic copper powders (325 mesh) and graphite flakes (230 mesh). Dendritic copper powders are used over spherical powders because the dendritic powders increase the interfacial contact between the copper and the graphite reinforcement. This increase in interfacial contact provides a more highly dense microstructure due to more efficient and thorough interfacial heating. HEHR consolidation techniques were utilized to fabricate 1.2, 5, 15, 25, and 40 volume percent Grp/Cu composites; HEHR consolidated dendritic copper, conventionally processed powder metallurgy (P/Ni) copper, and electrolytic tough pitch (ETP) copper specimens were also included in this study as control specimens.. All specimens, with the exception of the ETP samples, were pressed at 275 NIPa during their respective processing techniques and manufactured as circular coupons 22 mm in diameter. ETP copper was obtained from Farmers Copper. 4 2.2.2 Graphite Fiber Reinforced Copper MMCs The angle-plied continuous fiber reinforced graphite/copper MMCs studied in this projet: were manufactured at NASA Lewis Research Center in Cleveland, OH. All Grf/Cu composites tested were manufactured using pitch based P-100 graphite fibers, 10 Prm in diameter, produced by Amoco Performance Products. Currently, processing and manufacturing of these composites is labor intensive. The graphite fibers are cut from a spool and manually spread and positioned with a scalpel to insure proper alignment. Commercially available Cu coated P-100 graphite fibers from American Cyanamide Co. are utilized to manufacture the Grf/Cu composites. Liquid infiltration techniques are used in this process. Essentially, copper coated graphite yarns are hand cut from a spool. soaked in water, examined for defects, and manually positioned within an open-ended die. Proper alignment and distribution of the fibers is essential. Multilayer, angle-plied or unidirectional composites are manufactured in this manner simply by placing and orienting layers of yams appropriately. The resulting sandwich structure is placed in a vacuum furnace and heated to 399"K until the water has boiled and the pressure reaches 10-5 torr. The fiber sandwich is then consolidated into the finished composite panel by hot pressing in an argon-3.75 % hydrogen atmosphere. The complete details of this consolidation technique and mechanical and thermal properties are given by McDanels and Diaz (12). The excellent specific thermal conductivity, high modulus of elasticity, and high tensile strength make Gif/Cu composites ideal candidates for high heat flux structures such as space power radiator panels where component density is a critical concern. 2.2.3 Tungsten Fiber Reinforced Copper MMCs The unidirectional continuous fiber reinforced tungsten/copper MMCs studied in this project were also manufactured at NASA Lewis Research Center in Cleveland, OH. Commercially available Type 218CS tungsten filament wire from General Electric Corp., 180 p.m in diameter, was used as the reinforcement. Oxygen free, high conductivity copper (99.994% Cu) was selected as the matrix material for the following reasons: it has a 5 melting point below the recrystallization temperature of tungsten, copper is insoluble in .uags-ct- (13) and molten copper wets tungsten (14). The Wf/Cu MMCs were manufactured using liquid phase infiltration techniques. Continuous unidirectional tungsten fibers were placed in ceramic tubes to assure axial orientation and infiltrated with copper. Clean slugs of copper were placed above the tungsten fiber bundles. The ceramic tube and its contents were placed in a vacuum of a hydrogen furnace and heated at 1478"K for one hour. The copper melted and flowed over and around the tungsten wires -o for-, a sound, dense MMC. After this time, the specimen rods were removed from the ceramic tubes and stored in a dessicator. Details of this process and the mechanical properties of these composites are discussed by McDanels et al (15,16). The ease of manufacturing these composites allowed for the mechanical properties to be investigated over a wide range of \'oiume percent reinforcement. A 70vol% Wf/Cu MMC was studied in particular. Tungsten reinforced composites are ideal for high strength, high thermal conductivity applications where component weight is not a limiting criteria. The density of tungsten is 19.26 g/cm3, resulting in a composite density of 16.16 g/cm3 for a 70vol% Wf/Cu MMC. Due to the high strength and conductivity of these composites, they offer potential as components in high temperature aircraft and rocket engine turbines. 2.3 Microstructural Properties of MMCs The nature of any surface prior to any electrochemical testing must be understood and characterized in order to determine the types of corrosion initiation sites present. In particular, the reinforcement/matrix interface must be examined to evaluate it as a potential initiation site for localized galvanic corrosion or crevice corrosion. Variations in the surface can also affect the continuity and integrity of a protective film and promote increased localized attack. Structural flaws from processing, such as crevices or pores, can also result in increased localized corrosion. These structural flaws can result from the incomplete wetting of the reinforcement by the molten copper during processing and tend to create differential aeration cells, thus enhancing localized 6 corrosion. Considering these possibilities, the microstructural properties of copper MMCs must be identified at the onset of corrosion testing. Figures 3 through 6 show micrographs of ETP copper or the copper composites pricr to electrochemical testing. (The micrograph of ETP Cu is included for comparison.) 2.3.1 ETP Cu Figure 3 shows an SEM micrographs of the Cu surface prior to electrochemical testing. 2.3.2 Dispersion Strengthened (DS) Copper A representative as-polished microstructure of DS Cu prior to electrochemical testing is shown in Figure 4a. Dilute nitric acid solution was used to etch the polished DS Cu sample surface to reveal the particles shown in Figure 4b. 2.3.3 Grp/Cu MMCs A representative as-polished microstructure of one of the graphite particulate reinforced copper composites, a 25 vol% Grp/Cu composite, prior to electrochemical testing is shown in Figure 5a. The samples displays relatively uniform distribution of the graphite particles in the copper matrix although graphite pullout resulting from metallurgical preparation is evident. The dark areas in this figure are graphike particles and the brighter areas that are rough in appearance are surface defects due to graphite pullout. Black areas at the graphite-copper interface are voids where the molten copper did not completely wet the graphite reinforcement. These surface defects and voids may set up differential aeration cells and act as preferential initiation sites for localized corrosion. The nature of the graphite-copper interface was investigated by heavily etching virgin specimens in 50% NH4OH + 50% H 0 . An SEM micrograph of such a heavily etched 25 volc% 2 2 Grp/Cu composite in shown in Figure 5b. Note the good mechanical bonding at the interface. This microencapsulation of the graphite flake results from the rapid heating and cooling of the copper matrix during HEHR processing. Tight microencapsulation of the graphite particle leads to a more dense composite and provides fewer preferential anodic sites to participate in localized corrosion processes. The as-processed residual porosity measurements for all Grp/Cu HEHR 7 consolidated composites and P/M (powder metallurgically prepared) copper control specmer:;(cid:127) are given in Table 2. These values are averages of measurements performed on at least ::re different specimens using the water displacement method. All HEHR consolidated Gr;,,Cu composites have less residual porosity than their F/M procez sed counterparts. 2.3.4 Grp'Cu MMCs A representative as-polished, unetched microstructure of a 50 vol% Grt!Cu composilte prior to electrochemical testing is shown in Figure 6. Note the uniform fiber distribution throughout the panel, although a few areas of fiber clusters and strings of touching are obse,- c,. The grain structure and the nature of the fiber-matrix interface were examined. Fiber-matrix debonding does not occur. A sound, fully dense composite results from these consolidation techniques. 2.3.5 Wr/Cu MMCs A typical cross section of an as-polished unetched Wf/Cu composite is shown in Figure The fibers are properly oriented and evenly distributed throughout the copper matrix. A fe%ý regions of incomplete penetration of the molten copper into the tungsten tows weti c noted. These areas may act as initiation sites for localized corrosion. The interface i sound and voids are no, 5 observed. 3. EXPERIMENTAL PROCEDURE All electrochemical tests were conducted on planar electrodes using an EG&G Model 2-3 Potentiostat/Galvanostat, EG&G model 342 software, a five iiouth flask, and a flat specimen holder. The specimen holder exposed a 1.0 cm2 surface area unless otherwise noted. For example, in the case of the HEHR composites, the five mouth flask and flat specimen holder were specially designed and manufactured to accommodate the 22 mm diameter of the HEHR composite specimens. Potentials were measured using a saturated calomel electrode (SCE) with a Luggin capillary equipped with a Vycor fitted tip. The tip was located approximately I mm from the exposed specimen. Two graphite counter electrodes were used to assure current symmetry. 8