ICMCTF2000 Session E3: Coatings Resistant to Severe and Unusual Environments
Friday, April 14, 2000 8:30 AM in California
E3-1 Special Considerations for Lubrication in Space Environments
J.H. Sanders (Air Force Research Laboratory); P.J John (University of Dayton Research Institute); J.N. Cutler (Systran Federal Corporation); J.S. Zabinski (Air Force Research Laboratory)
The unique environments of space require special considerations with regards to lubricated systems. Of particular interest are differences in tribological mechanisms at low orbital pressures relative to atmospheric pressure. For example, inter-plate shear of solid lubes, evaporation/ decomposition rates of liquid lubes, and anti-wear film formation of liquid lubricant additives are dependent on atmospheric conditions including water and/or oxygen partial pressures and the absolute pressure. These environmental effects must be considered in the design of lubricated systems and in the selection of component materials to ensure reliable operation in space. This presentation will highlight some of the unique differences between lubrication phenomena in space and terrestrial environments. Particular emphasis will be given on recent high speed bearing research and development activities investigating the performance and tribochemistry of new materials and ceramic coatings on components lubricated with low volatility liquid lubes. Air and inert atmosphere tests showed little difference between material systems while vacuum tests showed a three-order magnitude increase in lifetime using selectively paired ceramic surfaces.
E3-3 Alumina and AlON Matrixes for Solid Lubricant Composites
J.S. Zabinski (Air Force Research Laboratory); A.A. Voevodin (Systran Federal Corp.); J.H. Sanders (Air Force Research Laboratory)
Alumina thin films were used as hard, thermally stable matrixes for thin film solid lubricant composites. MoS@sub 2@, PTFE, and CF@sub x@ were added as inclusions to enable lubrication in both terrestrial (moist) and space (vacuum) environments. In this system, alumina provided abrasion resistance, MoS@sub 2@ provided vacuum lubrication, and the carbon-based materials provided lubrication in moist atmospheres. Magnetron assisted pulsed laser deposition (MSPLD) was used to grow the different composites. Alumina coatings were bombarded with nitrogen from a Kauffman ion source to create a dense AlON matrix. N@sub 2@ pressure and ion energy were varied to control the nitrogen content in the coatings. Bombardment improved coating adhesion and scratch resistance. Past efforts have shown that DLC matrixes combined with carbide and dichalcogenide inclusions work well as wear resistant lubricious coatings in humid and dry environments at moderate temperatures. However, high temperature use was limited by DLC degradation in air, partially due to oxidation and phase transformation above 300@super o@C. From this perspective, efforts were focussed on the use of oxidation resistant matrixes to encapsulate phases for lubrication in extreme environments. The deposition methodology, chemical and mechanical properties, and tribological response of the coatings to dry, vacuum, and moist environments will be discussed.
E3-4 Evaluation of TiB@sub 2@ Coated Carbide in Sliding Contact Against Aluminium
M. Berger, S. Hogmark (The Ångström Laboratory, Sweden)
A major problem with machining of aluminium alloys are the very abrasive hardphase particles (oxides, carbides) embedded in the metal. The strong tendence to form built-up edges is another problem. @paragraph@Bulk titaniumdiboride (TiB@sub 2@) is both hard and known to have a low solubility in liquid aluminium. These characteristics make it an interesting candidate in applications involving machining of aluminium alloys. Especially in machining operations involving both shaping (removal of large volumes) and surface finish, diamond coated tools tend to produce too rough surfaces. @paragraph@In this paper the transfer of aluminium to the surface of magnetron sputtered TiB@sub 2@-coatings, has been studied in sliding tests against an aluminium alloy (AA7075). It was found that TiB@sub 2@ coated carbides performed better than TiN coated or uncoated carbides.
E3-5 Mo-N Coatings on Al and Al-Si Alloys
Y. Kilic, M. Ürgen, M.K. KazmanliI, A.F. Cakir (Istanbul Technical University, Turkey)
In recent years deposition of hard coatings on soft substrates such as aluminum alloys drew some attention. The use of aluminum alloys as lightweight construction material for various machine parts are also increasing. Coating of hard, lubricious films on aluminum alloys represent the possibility of extending the application fields of aluminum alloys. Modern physical vapor deposition processes are able to produce high-quality coatings for many applications if the process control is adapted. In this study, Mo-N films were deposited on Al-Si (17% Si ) alloy substrates by using arc-PVD technique in order to modify surface properties that are important for tribological applications. Under linear relative displacement condition fretting wear behavior of these coatings is studied and the experimental results are compared to uncoated samples. The coated surfaces were analyzed by X- Ray Diffraction (XRD). Scanning electron microscopy (SEM) observations are also conducted both on the surface and cross-section to understand morphology and microstructure of Mo-N layer and its relation with the substrate material.
E3-7 Applications and Characterization of the Electrospark Alloying Process
S. Hammond, L. Brown (Rolls- Royce Allison); J.E. Kelley (Advanced Surfaces and Processes, Inc.)
E3-9 Effect of Sputtering Parameters on Ta Coatings for Gun Bore Applications
D.W. Matson, E.D. McClanahan, J.P. Rice (Pacific Northwest National Laboratory); S.L. Lee, D. Windover (Benet Laboratories)
Sputtered tantalum is currently under development as a replacement for electrodeposited chromium in medium- and large-diameter gun tubes. Tantalum offers a number of attractive properties for this application, including a high melting temperature, high ductility, and an environmentally friendly deposition method. However, vapor-deposited tantalum can appear in both the characteristic bcc phase found in the bulk material, and in a very brittle and less desirable "beta" phase. The beta phase is often observed in coating defects, as well as at the interface of thick sputtered tantalum. Presence of the beta phase at the interface of protective bore coatings is considered undesirable because of its brittleness and resulting failure as the coating is stressed. Sputtered tantalum is currently under development as a replacement for electrodeposited chromium in medium- and large-diameter gun tubes. Tantalum offers a number of attractive properties for this application, including a high melting temperature, high ductility, and an environmentally friendly deposition method. However, vapor-deposited tantalum can appear in both the characteristic bcc phase found in the bulk material, and in a very brittle and less desirable "beta" phase. The beta phase is often observed in coating defects, as well as at the interface of thick sputtered tantalum. Presence of the beta phase at the interface of protective bore coatings is considered undesirable because of its brittleness and resulting failure as the coating is stressed. @paragraph@A high-rate triode sputtering system with a cylindrical coating geometry was used to produce 125 micrometer thick tantalum coatings on 4340 steel smooth bore cylindrical substrates. A systematic series of tests were performed to evaluate the effects of sputtering gas species and substrate temperature during deposition on the phase and microstructure of the coatings. Argon, krypton, and xenon were evaluated as sputtering gases, both in terms of the operating characteristics of the sputtering system and in the quality of the resulting coatings. Krypton was used as the sputtering gas to test the effects of substrate pre-heating at temperatures from 100@super o@C to 300@super o@C. Results of these studies suggest that at substrate temperatures above 150@super o@C, the bcc phase predominates regardless of the gas species used for the sputtering process. Sputtering gas species did, however, affect microhardness and stress values measured for the sputtered coatings.
E3-10 Functional Metal Based Coatings on Ceramic Substrates
F.H.W. Loeffler (Physikalisch-Technische Bundesanstalt, Germany)
For applications in the fields of metrology e. g. for the evaluation of calorimetric parameters or in electrical measurements the combination of a ceramic substrate with a metal based coating is necessary. The applications require coatings with a good adhesion and a high purity which can withstand a corrosive medium at various temperatures. Typical coatings are copper, silver, gold and platinum. These coatings can be achieved up to 1 mm thickness and with a homogeneous structure by electroplating of electric conductive substrates. The paper describes three possibilities for the metallising of non conductive substrates. First example is the metallising of the substrate with a chemical-silver-spray system followed by electroplating. Prior to the electroplating process PVD-sputter technique will be used for the metallising as a second possibility. The third possibility is a PVD coating with one or more layers. The used substrate material, an aluminium oxide disc, will be coated only at one front side. The samples will be evaluated not only with conventional methods but also in practical use.
E3-11 Environmentally Induced Alterations in the Surface Chemistry of High Integrity Coatings
T.H. Dugall (UWM-Milwaukee); T.L. Barr (University of Wisconsin-Milwaukee); S. Seal (University of Central Florida); E. Benko (Institute of Metal Cutting, Poland)
Nitride coatings are often added to various types of surfaces in order to alter the hardness, wearability, and aesthetic qualities of those regions. In particular, boron nitride coatings (often in conjunction with other nitrides and carbides) are commonly placed on the surface of metal cutting tools. It is traditional to regard these nitride coatings as units of extreme integrity, but recent surface analyses using XPS have demonstrated that these nitride coatings are as susceptible as other solid surfaces to oxidation attack by the principal actors in the earth's gas phase environment, oxygen and water vapor. Previous studies from our group and others have documented the inevitability of these chemical alterations, but little effort has been devoted to the controllability and impact on these features, i.e., do they alter key physical features or are they always superficial adducts of no consequence. In the present study, we document the problem and report the results of controlled chemical alterations, in which variations in surface texture, chemistry, temperature, vapor exposure, and tribological properties are employed as a means to modify and gauge these effects.