Micro-Finish Hard Anodized Coatings on Aluminum
Transcription
Micro-Finish Hard Anodized Coatings on Aluminum
. Micro-Flnish Hard Anodized Coatings on Aluminum # Materials Fabrication Division Lawrence Livermore National Laboratory For AESF SUR/FIN 92 Atlanta, GA June 22-25,1992 ABSTRACT The production of thin hard anodized coatings on Single Point Diamond Turned (SPDT) 6061-T6 aluminum has been studied. The investigation centered on producing a surface finish of less than 10 microinch after anodizing. By starting with a 2 microinch (AA) surface finish and controlling time, temperature, current density and solution chemistry, coatings with surface finishes of 8 microinch and a thickness of .0003”,are obtained. Surface roughness from several anodizing solutions is compared. The operational life of a PTFE sliding seal against a coated cylinder bore is used as verification of I ‘ h i s work was performed under the auspices of the U. S.Department of Energy by Lawrence Livermore National Laboratory under contract No. W-7405-Eng-48. 123 AESF Annual Technical Conference SUWFR'NB 'sz 1. Atlanta, Georgia The American Electroplaters and Surface Finishers Society, Inc. (AESF) is an international, individualmembership, professional, technical and educational society for the advancement of electroplating and surface finishing. AESF fosters this advancement through a broad research program and comprehensive educational programs, which benefit its members and all persons involved in this widely diversified industry, as well as govemment agencies and the general public. AESF disseminates technical and practical information through its monthly joumal, Plathg and Surface Finishing, and through reports and other publications, meetings, symposia and conferences. Membership in AESF is open to all surface finishing professionals as well as to those who provide services, supplies, equipment, and support to the industry. According to the guidelines established by AESF's Meetings and Symposia Committee, all authors of papers to be presented at SUWFIN@havebeen requested to avoid commercialism of any kind, which includes references to company names (except in the title page of the paper), proprietary processes or equipment. Statements of fact or opinion in these papers are those of the contributors, and the AESF assumes no responsibility for them. All acknowledgments and references in the papers are the responsibility of the authors. Published by the American Electroplaters and Surface Finishers Society, Inc. 12644 Research Parkway Telephone: "31-6441 Orlando, FL32826-3298 Fa: 4071281-6446 Copyright 1992 by American Electroplaters and Surface finishers Society, Inc. All rights reserved. Printed in the United States Of America. -@ispublication mav not be reproduced, stored in a retrieval system, or transmitted in whole or part, in my form or by my means, electronic, mechanical, photocopying, recording, or otherwise without the prior written permission of AESF, 12644 Research Parkway, Orlando, FL 328263298. Printed by AESF Press SUf%FINeisa registered trademark of the American Electroplaters and Surface Finishers Society. Inc. DVTRODUCTION Aluminum alloys are used extensively for various aerospace assemblies and components because of a combination of high strength and low density. In addition i t has the ability to be machined or formed to a wide range of configurations. Some of the assemblies made from aluminum include reciprocating pumps used to transfer liquid or gaseous materials. During pump use, friction occurs from a combination of surface contact and the forces required t o maintain a seal between piston and cylinder wall. This results i n piston and/or cylinder wall wear and eventually component failure. Because aluminum sliding against aluminum has a high coefficient of frictionl. lubricants and coatings are used t o minimize this wear. To further reduce friction and improve the seal between piston and cylinder wall, rings, seals and wipers are used. These may be composed of various materials that include elastomers, polymers, metals, or intermetallic compounds such as oxides or carbides. There are several problems associated with the use of friction reducing seals and coatings. Some applications require ultra clean operation which prohibit most lubricants. Certain types of seals can produce compatibility problems, i.e., corrosion o r galling. One technique for increasing the wear and corrosion resistance of aluminum, is the application of an inert oxide coating, This surface treatment combines oxygen and aluminum to form one of the hardest substances known: A l 2 . 0 3 . This oxide coating has many advantages, including resistance t o heat, erosion, and moderate chemical solutions2. It also has a very low coefficient of friction. These anodic coatings are formed by electrolytic oxidation of an aluminum surface while immersed in a suitable electrolyte. Many formulations have been used, but the most prevalent are the sulfuric, sulfuric-oxalic, and the oxalic. In some instances electrical current manipulation is used t o increase the density, thickness and/or hardness of the coating3. The surface finish of these coatings depend on the condition of the surface before anodizing and the processing conditions*. One method of reducing surface finish after anodizing is t o lap or hone the coating. This has produced finishes down t o a less than 2 microinch. An aluminum pistmi conventionally machined to a finish of 32 microinch was fitted with PTFE sealing ring@. This piston was then placed into a matching aluminum cylinder, as part of a reciprocating pump. The assembly was then leak checked using a vacuum pump equipped with a helium mass spectrometer. The leak rate was initially excessive (>80 Vmin) and grew successively worse with continued piston travel. 2 I 24 To reduce the leak rate, a smoother finish was machined into the internal cylinder bore using a lathe fitted with a diamond tool. This reduced the surface finish to 1-2 microinch, which was well below the seal manufacturers recommendations. A pump assembly was then tested. The results showed an acceptable initial leak rate, but after one piston pass leakage became excessive. At this point the surface of the cylinder was visually inspected and was found to have numerous scratches. The leading edge of the PTFE seal was also inspected and found to contain minute particles of aluminum. The source of the particles was thought to be a residue from the machining process. A new cylinder was machined and then cleaned using an ultrasonic tank and aqueous cleaning agents. The piston was fitted with new seals and installed in the cylinder. Leak testing this cleaned assembly still showed excessive leak rates after one piston cycle. An microscopic evaluation of the diamond turned surface showed scratches from contact with PTFE sealing surface. These scratches, running horizontally down the cylinder bore, provided leak paths. To make the surface less susceptible to damage from the sealing surface, a coating of some type was envisioned. The first choice was to coat the internal surfaces with electroless nickel and then SPDT. This has provided excellent surface finishes on a variety of components6. After zincating, copper striking, and deposition of .002" of electroless nickel, the can was SPDT. Leak test results looked excellent (<8 Umin). At this time it was discovered that copper of any kind was incompatible with the material that would be used in the pump. Without a copper strike, the only alternative to provide adequate adhesion on this alloy was heat treatment'. The heat treatment step could not be done because the 6061-T6 aluminum cylinder had to remain in the T6 condition, to retain its mechanical properties. The next choice was t o coat the internal surfaces of the cylinder by hard anodizing. The first coating was applied t o a rough machined cylinder using a chilled (32°F)sulfuric acid solution (15%weight) to a thickness of .002". This increased the 32 microinch finish to 82. Since this finish was too rough, a hand lapping operation followed. The time required to decrease the surface finish in the cylinder bore t o an acceptable level was over 4 hours. The remaining lapping compound and alumina particles were removed by ultrasonic cleaning. It wzs decided t o cliscontinue this approach because of the time required to produce the required finish. Another WOW was the lapping residue, which if not completely removed, could clog the small pump delivery lines during operation. 3 In an effort to reduce the surface roughness caused by the hard anodizing, several test specimens were anodized for various lengths of time. As expected, Figure 1 shows that shorter anodizing time produces much less surface roughness and of course a thinner coating. The project designers felt that the coating thickness should be no less than .0003”, which should withstand moderate contact w e e . This thickness was obtained after 10 minutes a t 36 ASF. This produced a surface finish of about 21 microinch, twice the seal manufacturers recommended value. In an attempt t o reduce this finish, other chemistries were examined. Because of the myriad of solutions that will electrolytically form oxide coatings on aluminum, the next most widely used chemistry was chosen for investigation. This is a sulfuridoxalic process that was developed commerciallp. Several test specimens were hard coated in this bath for various times. The surface finish results are shown in Figure 2. This process increased the surface roughness from 2 to 14 microinch in 10 minutes. With these results a new cylinder was prepared and run for 10 minutes. The internal surface finish increased t o 15 microinch after anodizing. After assembly the piston was able to make several passes before the leak rate became excessive. Because of the success of this chemistry change, another process, that had originated in Germany and Japan, was tried. This solution contains (45 gm/l) oxalic acid and is operated a t 60°F and 25 ASF. From this formula a new solution was prepared and test samples were processed. Several differences were noted including higher starting voltage (45 volts vs. 20 volts) and a coating color of translucent yellow instead of grey. The results (Figure 3) show that this produced a 12 microinch finish in 14 minutes. With this data, a cylinder was SPDT and then anodized for 14 minutes. This produced a coating that averaged .0002” thick which had a surface finish that varied from 8 to 18 microinch. This variation was due to the coating being thicker a t the open end of the cylinder. The cathode up to this point had been a 1”diameter aluminum tube, which was also the source of air agitation. To try and remedy the thickness disparity, a “conical” cathode was fabricated. The conical shape was to increase the current density where the coating was thin, and reduce the surface finish where it was thick. After anodizing the surface was inspected and the conical cathode had produced an elliptical coating. A new parallel cathode was fabricated allowing only 1/2” of clearance between electrode surfaces. The results of using this “parallel” cathode provided a uniform coating. Variations in the surface finish of the cylinder were noted after the use of the 1” cathode. These were attributed to current density variations. To find the optimal current density, samples were run at current densities varying from 40 t o 10 ASF. To minimize any thickness disparity all samples were run for 360 amp min/ft2. The results, shown in Figure 4, show an optimal anode current 4 126 density of 18 ASF. The .0003" thickness is reached in 18 minutes a t this current density. Solution temperature is known td have an effect on the porosity of anodized coatingslo. To determine its impact, another set of samples was coated using various solution temperatures ranging from 70 to 30"F, with a constant 360 amp min/ft2. The results in Figure 5, show a decrease in surface finish as the solution is cooled to 32°F. The voltage required t o maintain a constant current density also increased with reduced temperature. This and the fact that the coating is less soluble at reduced temperature is most likely why the surface finish is reduced". The three anodizing methods are compared in Figure 6. The large reduction in surface finish is easily seen on this chart. When the coating thickness is factored in, the decrease in roughness between the suluric and the oxalic is about 65%, and between the sulfuridoxalic and the oxalic 46%. With these results a SPDT cylinder was coated using 45 gm/l oxalic acid solution cooled to 32°F.The anode current density of 18 ASF with an anode to cathode distance of U2". Air agitation was supplied to circulate the solution and prevent localized overheating. After processing for 20 minutes the coating was found to have a surface finish of 8 microinch. The thickness was .0003". This cylinder was then assembled with a matching piston and subjected to the cyclic pump test. The leak rate was very low and the pump cycled over 70 cycles before any seal degradation. Most of the degradation was from flattening of the seal face as opposed to scratching. Microphotographs of the anodized surface, showed a visible reduction in the surface roughness. This is attributed to the reduced solvent action of the cooled oxalic process. To determine coating density, several coated samples were weighed, stripped and reweighed. This mass, divided by the volume of the oxide was used to determine density. A value of 3.2 gm/cms, is greater than that obtained from a sulfuric acid solution12. 5 i Thin oxide coatings on SPDT aluminum surfaces are produced in oxalic acid. When produced on a SPDT surface, the finish obtained is much smoother that from a sulfuric acid or sulfuridoxalic solution. The sliding life of a PTFE sealing ring inside a coated cylinder can be greatly enhanced by this coating. The optimal anodizing conditions are: gmfl oxalic acid 18 ASF anode current density Y2" anode to cathode distance (minimum cathode current density) 32 degree F. solution temperature 20 minute run time 45 \ 6 128 90 - 50 - 30 - 20 - 10 - 0 - 40 I ./ I 5 0 10 15 20 25 30 35 40 45 Minutes Fiaurs 2 Surface Flnish vs Anodizing Time (12 % sulfuric Acid +2 % Oxalic Acid- 36 ASF) 100 1 90 EO 70 -5 z -ev 2 s 60 50 40 30 20 io 0 0 5 10 15 25 20 Minutes 7 30 35 40 45 / Surface Finish vs Anodizing Tim. (4.5% Oxalic Acid- 25 ASF-60 deg. F.) 100 -, 90 -. - 80 .-0 L 2 I 40 I 0 5 15 10 20 25 30 35 40 45 Minutes Surface Finish vs Anode Current Density (4.5 % Oxalic Acid-360 AmpMin.) 30 25 - / 20 Q J 5 0 , 5 10 15 I 1 20 25 ASF 8 130 i 30 35 40 45 EinuLs Surface 'finish vs Solution Temperature (4.5 % oxalic Acid25 ASF) 25 - 20 J r -2 -t C 15 0 10 5 25 30 35 40 45 I 1 50 55 Degrees 60 F. EkuhLfi Surface Finish vs Anodizing Method (360 Ampere Minutes) Microinch Minute8 2 9 65 70 E 75 References . 1 Handbook of Tables for Applied Engineering Science, 2nd Ed., R.E. Bolz and G.L. Tuve, Ed., CRC Press, 1987. 2 Electroplating Engineering Handbook,3rdEdition, A.K.Graham Editor, (461464). 3 Sanford Process Co., US.Pat. 4,128,461 (1978),4,133,725 (1979). 4 Aluminum Company of America, Bulletin 14-A. 5 Ball Seal Company. J.W. Dini, "Electroless Nickel: An Important Coating for Diamond Turning Applications". Proceedings Electroless Nickel Conference II, Products Finishing Magazine, Cincinnati, OH. 7 J.W. Dini, H.R. Johnson, "Quantitative Adhesion Data for Electroless Nickel Deposited on Various Substrates" UCRL Pub. 9 ANSVASTM B 580 - 73, Anodic oxide coatings on aluminum. 9 ALUMALITE 225, Aluminum Company of America. 10 Miyata, A., Isawa, K., Furuichi, A. and Takamura, K., Rept. Inst. Phys. Chen. Res., Tokyo, 1961,37, 142-48. 11 G. Bailey, G.C. Wood, "The Morphology of Anodic Oxide Films Formed on Aluminum in Oxalic Acid.", Truns. Inst. Met. Fin., Vol. 52, (1974). 12 Spooner, R.C., "Anodic Treatment of Aluminum in Sulfuric Acid Solutions"., J. Electrochemical SOC.,102, (4), 156-62, (1955). 10 132