METALS PROCESSING

The properties of metals and their alloys depend strongly on their processing history. For example, the distributions of phases, grain structure, alloy compositional segregation, and defects in final commercial products depend on the conditions under which materials are processed and fabricated. These distributions in turn are crucial in determining the alloy strength, ductility, homogeneity, and other properties important for industrial applications. The Metals Processing Program focuses on measurements and predictive models needed by industry to design improved processing conditions, provide better process control, develop improved alloy and coating properties, tailor material properties for particular applications, and reduce costs.

Major successes in applying measurements and modeling to processing applications have been achieved in interactions with the aerospace, powder metallurgy, electroplating and electronics industries. Predictive models for solidification and microstructural evolution during processing have been incorporated by industry into design systems for casting of aerospace alloys and production of defect-free electronic materials, helping to reduce rejection rates arising from defective parts. Cooperative research and development projects with industry have resulted in significant improvements in process control for atomization of steel and superalloy powders. Standard reference materials, certified for coating thickness, microhardness or chemical composition, are being fabricated by electrodeposition techniques and powder metallurgy. Critical mechanistic, chemical and process variables controlling the structure/ property relationships of coatings and thin films produced by electrodeposition are being developed to take further advantage of this electrochemical process, which does not require high purity starting materials and is readily adaptable to large scale production.

Measurements and predictive models for processing being pursued in this program are of three kinds:

• Measurements, data, and models are developed to help design materials production processes, such as measurements and evaluations to provide alloy phase diagrams, which are the roadmaps that alloy designers use to predict the alloy phases that can be produced under specific processing conditions. These evaluations are playing key roles in NIST collaborations with industrial consortia on electronic solders and casting of superalloys for aerospace applications.
  
  

• Measurements are made under dynamic conditions to monitor, in real time, properties of materials while they are actually being produced and to determine difficult-to-measure process parameters while the process is occurring. Special fast response sensors, simulations and imaging techniques have been developed for application to powder atomization and thermal spray processes, and workshops have been held to transfer these techniques to industry. Here, dynamic models of the process are important both for design of manufacturing procedures and for applications of real time feedback and control.
  
  

• To evaluate the adequacy of process models, it is important to measure the properties of the final materials and relate them to the process conditions. Current work in this respect includes evaluation of methods used to optimize properties of electrodeposited coatings and corrosion resistance of rapidly solidified nitrogenated steels.

In all of this work, the goal is to help U.S. industry apply measurements and predictive modeling to produce improved materials at reduced cost.

Project Title: PROCESSING OF ADVANCED MATERIALS

Investigators: F. S. Biancaniello, R. D. Jiggetts, U. R. Kattner, S. D. Ridder, R. J. Schaefer, M. E. Williams, J. L. Fink and R. E. Ricker

Objectives:

Objectives of this project are to provide industry with measurements, sensors, predictive models, methodologies and standards needed to apply intelligent processing techniques to production of advanced alloys. To aid industry, techniques are developed to prepare improved standard reference materials and reference samples, relate processing conditions to final properties of materials, and provide measurements that can be used for feedback and control.

Technical Description:

State of the art techniques are employed in the processing and synthesis of high performance materials. Predictive models and thermodynamic assessments are developed to aid in microstructure, composition, porosity and property control. This research is part of a long-term research effort on advanced processing, emphasizing rapid solidification and powder metallurgy. One outgrowth of the program was a highly successful NIST/Industry Consortium project on applying intelligent processing concepts to rapidly-solidified nickel-based superalloy powders produced by atomization techniques. Current research is focussed on three main areas. The first area is collaboration with powder metallurgy companies to apply NIST- developed techniques for nozzle optimization and control of industrial atomizers. These techniques are also being extended to thermal spray processes used in producing coatings for automotive, aerospace and other industrial applications. The second is the application of rapid solidification processing and powder metallurgy methods to produce state-of-the-art standard reference materials with enhanced chemical homogeneity. The third area is research on atomized high nitrogen stainless steel, including support for an on-going industrial ATP project involving studies of thermodynamic and kinetic effects on nitrogen solubility, and methods of measuring corrosion properties of these highly corrosion-resistant alloys.

In addition, the Metallurgy Division's alloy preparation facility is critical to maintaining a world-class materials science and engineering program at NIST. Advanced processing equipment and methods are used to produce specimens for measurements within NIST and also for collaborations with industry and academia.

Planned Outcome:

This activity is designed to produce measurements, diagnostics, and sensors for feedback and control of advanced processing techniques. The plan is to develop predictive models for metals processing and to acquire data and measurements for expert systems development. This work is planned to help industry produce more reliable, higher quality material at lower cost.

The activity endeavors to produce fully-dense standard reference materials with enhanced chemical homogeneity for a wide variety of users, including but not limited to automotive, aerospace, powder producer and the metals casting industries.

An understanding of the effects of processing conditions and final microstructures on the properties of metal alloys is essential to achieving reproducible properties and accurate models of metals. Having an in-house NIST fabrication facility allows us to explore processing- structure and property relationships in a meaningful way.

External Collaborations:

Collaborations are being conducted with Crucible Materials Corporation on

(1) thermodynamic predictions, (2) corrosion measurements and kinetic models through an ATP CRADA on enhanced corrosion properties of high nitrogen stainless steels produced by atomization and (3) hot isostatic pressing (HIP). A CRADA project with Carpenter Technology Corporation is underway to apply NIST nozzle design optimization techniques to the production of fine powder for the metal injection molding industry. Cooperative work is being done with Johns Hopkins University to investigate short range order in metallic glasses.

Accomplishments:

A system has been installed to perform diagnostics on gas flows in industrial-sized powder atomization systems. Optical sensors applicable to analyses of advanced powder production systems have been developed through SBIR and NIST interactions.

Two new standard reference materials, C1150b (white cast iron) and 1267a (446 ferritic stainless steel) have been produced by rapid solidification of gas atomized powders and HIP consolidation. These more homogeneous SRM's, requested by ASTM Committee E1, will allow improved measurement of industrial materials.

A predictive model has been developed for predicting nitrogen solubility, phase stability and enhanced properties in gas atomized high nitrogen stainless steels.

Impact:

Control techniques and melt practice developed at NIST for production of superalloys and corrosion-resistant nitrogenated stainless steel have been adapted by industry to improve commercial products and reduce production costs.

More homogeneous standard reference materials have been produced, allowing improved measurement of industrial materials.

Outputs:

Publications:

Biancaniello, F. S., Ricker, R. E., and Ridder, S. D. "Structure and Properties of Gas Atomized, HIP-Consolidated High Nitrogen Stainless Steel," Advanced Particulate Materials & Processes, (Metal Powder Industries Federation, Princeton, NJ, 1997) pp. 309-316.

Swartzendruber, L. J., Hicho, G. E., Biancaniello, F., Shull, R. D., and Shapiro, A. J. "Determination of Austenite/Ferrite Ratios in Stainless Steels Using The Mossbauer Effect," Proceedings of International Conference on Applications of the Mossbauer Effect, in press.

Presentations:

Biancaniello, F. S. "Gas Atomized High Nitrogen Steel for Armor Applications," Aberdeen Proving Grounds, Aberdeen, MD., June, 1997.

Biancaniello, F. S. "Recent Studies on Powder Processed Nitrogenated Stainless Steel," TMS Symposium on P/M Current Research and Industrial Practices, Indianapolis, Indiana, Sept., 1997.

Biancaniello, F. S. "New Process Control Agents for Mechanical Alloying and Ball Milling," TMS Symposium on P/M Current Research and Industrial Practices, Indianapolis, Indiana, Sept., 1997.

Biancaniello, F. S. "Structure and Properties of Gas Atomized, Hip-Consolidated High Nitrogen Stainless Steel," 5th International Conference on Advanced Particulate Materials and Processes, West Palm Beach, Florida, April, 1997.

SRM's in production:

SRM #C1150b White Cast Iron

SRM #C1267a Ferritic Stainless Steel (446 SS)

SRM's under development:

SRM #1245a Inconel 625

Project Title: SOLIDIFICATION MODELING

Investigators: S. R. Coriell, J. A. Warren, and W. J. Boettinger

Objectives:

Analytical and numerical models of solidification processes are developed by NIST with special emphasis on solute segregation during alloy solidification and crystal growth. Such models will allow the prediction of microstructure and segregation as a function of processing conditions, for example, solidification velocity, thermal conditions, and alloy composition.

Technical Description:

The properties of solidified materials, e. g. castings and electronic materials, depend on the distribution of solutes or dopants, on the phases present, and on the defect structures. Modeling of the solidification process involves solution of the heat flow, diffusion, and fluid flow equations with boundary conditions on external surfaces and at the solid-liquid interface, which is a free boundary. The role of fluid flow on interface stability and microsegregation is investigated with application to possible microgravity experiments which would help explain the role of fluid flow in terrestrial processes. Dendritic growth is always present in castings and determines the scale of microsegregation; phase field models are being implemented which allow the calculation of solute distribution for complex dendritic morphologies. These calculations allow studies of tip kinetics, solute redistribution, and coarsening.

Planned Outcome:

Models for alloy solidification and crystal growth processes will be tested and made available to industry and academia. These models will improve our ability to determine the optimum processing conditions for a given material. In addition, they will define critical experiments for determining thermophysical properties necessary for accurate modeling.

External Collaborations:

Modeling and interpretation of experiments on the directional solidification of lead bromide doped with silver bromide has continued in collaboration with scientists at Northrop Grumman Science and Technology Center. A model was developed for the growth of in-situ composites in the monotectic aluminum-indium system in collaboration with Prof. J. B. Andrews of the University of Alabama at Birmingham. NIST is participating in an experiment on the onset of cellular growth in bismuth-tin alloys with Prof. R. Abbaschian of the University of Florida. Collaboration with Marshall Space Flight Center on interface instabilities during melt and solution growth has continued.

Phase field calculations of solute trapping are being conducted with Prof. A. Wheeler, Southampton University, and of grain boundary formation with Prof. R. Kobayashi, Hokkaido University.

Accomplishments:

The role of convection and interface instability in causing inhomogeneities in dopant distribution in the acousto-optical material lead bromide has been analyzed in collaboration with N. B. Singh. Measurements of the diffusion coefficient of silver bromide in lead bromide were carried out at Northrup-Grumman Science and Technology Center; the diffusion coefficient is an important parameter in modeling crystal growth in this system. An analysis of the effect of convection and container walls on the growth of dendrites into pure supercooled melts has been developed. The results indicate the processing conditions needed to avoid both convective and wall effects.

A previous model of eutectic solidification has been extended to monotectic solidification in which aligned rods of a liquid phase are formed in a solid matrix. The new model accounts for diffusion in rod phase and has been applied to the aluminum-indium system.

Analytic and numerical calculations have been performed in order to determine the fluid flow conditions required to avoid step bunching during crystals growth of KDP, large crystals of which are required for laser fusion applications. Realistic modeling of the growth process required extension of a previous theory to include nonlinear anisotropic interface kinetics. Bunching of the elementary steps during layerwise growth causes impurity segregation resulting in inhomogeneous optical properties that lead to decreased damage thresholds in the crystals.

The phase field technique has been extended to treat directional solidification. Microstructure prediction with a specified growth velocity and temperature gradient is now possible and the formation of arrays of cells and dendrites is being studied. An understanding of the factors that determine the characteristic spacing of these arrays will lead to more robust models for segregation spacing in cast materials.

Impact:

Optimized commercial growth conditions identified in the NIST research can now be used to produce quality lead bromide-silver bromide crystals as a non-linear optical material.

Supercomputer calculations of a single dendrite will permit construction of improved models of microsegregation processes for castings that will lead to improved mechanical properties.

Outputs:

Publications:

Boettinger, W. J. and Warren, J. A., "Prediction of Solidification Microstructure using the Phase-Field Method," Proceedings of the General COST512 Workshop on Modeling in Materials Science and Processing, Ed. by M. Rappaz and M. Kedro, European Commission, Brussels, 1996, p. 11-20.

Warren, J. A. and Boettinger, W. J., "Numerical Simulation of Dendritic Alloy Solidification Using a Phase Field Method," Solidification Processing, 1997, edited by J. Beach and H. Jones, Department of Engineering Materials, University of Sheffield, UK 1997, p. 442.

Singh, N. B., Mani, S. S., Adam, J. D., Coriell, S. R., Glicksman, M. E., Duval, W. M. B., Santoro, G. J., and DeWitt, R., "Direct Observations of Interface Instabilities," J. Crystal Growth 166, 364 (1996).

Singh, N. B., Glicksman, M. E., Coriell, S. R., Duval, W. M. B., Santoro, G. J., and DeWitt, R. "Measurement of Diffusion Coefficient Using a Diaphragm Cell: PbBr₂-AgBr System," J. Crystal Growth 167, 107 (1996).

Coriell, S. R., Murray, B. T., Chernov, A. A., and McFadden, G. B., "Step Bunching on a Vicinal Face of a Crystal Growing in a Flowing Solution," J. Crystal Growth 169, 773 (1996).

Sekerka, R. F., Coriell, S. R., and McFadden, G. B., "The Effect of Container Size on Dendritic Growth in Microgravity," J. Crystal Growth 171, 303 (1997).

Coriell, S. R., Mitchell, W. F., Murray, B. T., Andrews, J. B., and Arikawa, Y., "Analysis of Monotectic Growth: Infinite Diffusion in the L₂ Phase," J. Crystal Growth 179, 647 (1997).

Coriell, S. R., Chernov, A. A., Murray, B. T., and McFadden, G. B., "Step Bunching: Generalized Kinetics," J. Crystal Growth, in press.

Presentations:

Boettinger, W. J., "Prediction of Dendritic Growth Microstructures using the Phase-Field Method," Supercomputer Seminar Series, University of Minnesota, Minneapolis, MN, November 1996.

Boettinger, W. J.,"Prediction of Dendritic Growth Microstructures using the Phase-Field Method," at Institute for Theoretical Physics Workshop, Quantitative Methods in Materials Research, University of California at Santa Barbara, Jan. 1997.

Boettinger, W. J., "Numerical Simulation of Dendritic Alloy Solidification Using a Phase Field Method," TMS-AIME, Sept. 1997.

Boettinger, W. J., "Numerical Simulation of Dendritic Alloy Solidification Using a Phase Field Method," Materials Science and Engineering Dept. Seminar, Carnegie-Mellon University, Oct. 1997.

Coriell, S. R., "Morphological Stability of Faceted Interfaces," Ames Laboratory, Iowa State University, Ames, Iowa, August, 1997.

Coriell, S. R., " Morphological Stability of a Vicinal Face: Effects of Anisotropic Kinetics and Fluid Flow," The Eastern Regional Conference on Crystal Growth and Epitaxy, Atlantic City, NJ, September, 1997.

Warren, J. A., "Phase Field Simulations of the Solidification of a Binary Alloy," Materials Research Society, San Francisco, April 2, 1997.

Warren, J. A., "Phase Field Simulations of the Solidification of a Binary Alloy," Second SIAM Conference on Mathematical Aspects of Materials Science, Philadelphia, PA, May 14 1997.

Warren, J. A., "Phase Field Simulations of the Solidification of a Binary Alloy," Seventh Conference on Computational Research on Materials, Morgantown WV, May 16th, 1997.

Warren, J. A., "Phase Field Simulations of the Solidification of a Binary Alloy," Solidification Processing 1997, Sheffield England, July 7th, 1997.

Warren, J. A.,"Phase-field Simulations of Solidification, Collision and Grain Boundary Formation" TMS, Materials Week, Indianapolis, September 1997.

Project Title: SENSORS AND DIAGNOSTICS FOR THERMAL SPRAY PROCESSES

Investigators: S. D. Ridder and F. S. Biancaniello

Objectives:

The primary focus of this project is to develop tools for the measurement and control of process conditions for plasma spray systems. This includes off-line analysis tools (e.g. high- speed cinematography, holography) and real-time sensors suitable for process control. In addition, mathematical modeling techniques will be used to provide predictive calculations of process variables and product characteristics. Appropriate process sensors and controls will then be incorporated into an expert system driven process controller with generic applicability to a wide range of metal processing equipment and computer platforms.

Technical Description:

The focus of the thermal spray project, once installed, will be the development of measurement tools to provide diagnostic and control capabilities for the production of industrially important spray coatings such as ceramic-based Thermal Barrier Coatings (TBC's) and metallic based corrosion and wear reducing layers.

The industrial use of thermal spray processes is as old as "fuel gas" welding and has become an integral part of the manufacturing art. In practice, thermal jets, generated by oxygen/fuel flames or plasmas (DC or AC) within a spray "gun" are used to melt or soften feed-stock materials and then propel the resulting particles onto various substrates. The geometry and operating parameters of the gun depend on the intended function of the resulting coated part. Due to the more reliable feed characteristics, there is growing interest in systems that use wire for the material feed stock, especially for the production of metallic coatings. However, many industrially important coatings are produced from ceramics and friable metallic materials that cannot easily be processed into a suitable "wire" shape.

Instrumentation packages for plasma spray systems currently incorporate user interface panels that provide indication and control of several process parameters: e.g., voltage, current, gas flow rates, powder flow rate, substrate position, and live video displays. Most commonly applied by skilled technicians, plasma spray is now being adapted for automatic control using robotics. NIST has capabilities in this area.

The switch to automatic control and robotics has created increasing demands for new sensors, diagnostics, modeling, and expert system controls. This project is designed to address these industrial needs. A DC plasma, powder feed, thermal spray system will be used to study new sensor and diagnostic systems. High-speed cinematography, multi-exposure laser holography, and high-speed video cameras will be developed to provide diagnostic tools for thermal spray systems. New Infra-Red (IR) thermal imaging sensors, currently capable of measuring the temperature of rough, variable emissivity surfaces, will be improved to provide on-line measurement of particle temperature and velocity.

Intelligent process control requires detailed understanding of the effects of process variables or parameters on the resulting coating characteristics. Process parameters must be identified, reduced (dimensional analysis and parameterization is used to identify the dependent and independent variables), and measured. A process model must be determined that provides a mapping of the process parameter space to the resulting coating properties and process efficiency. Finally, a control system is developed incorporating the process model, sensors, and actuators that provides the necessary heuristics and response time for achieving the product goal. This will ultimately allow US industry to produce the advanced materials that this process can provide with reliable performance and acceptable cost.

Planned Outcome:

Robust process sensors will be developed and provided to industry for monitoring and control of atomizers and plasma spray systems. New mathematical modeling tools will be developed to aid in equipment design and improve process efficiencies. Expert-system-driven process controllers will be developed by NIST and its industrial and academic partners with hardware and software supplied and supported by third party companies which have established national distribution networks.

External Collaborations:

Current collaborative work on thermal spray processing includes the NIST SBIR funded research with Stratonics, Inc., The Cooke Corporation, and North Dancer Labs, Inc. all aimed at developing new sensor and diagnostics technology. A cooperative exchange of expertise has been initiated with scientists at Los Alamos National Laboratory. NIST has provided assistance in the design and operation of an inert gas atomization system for beryllium alloys and Los Alamos has provided technical assistance in the operation of the NIST D.C. plasma thermal spray equipment.

Accomplishments:

The NIST thermal spray system has been installed and initial trials have been conducted. This system will be used to provide experimental thermal spray coatings for the NIST project on process modeling and control of thermal spray.

A Phase I SBIR on developing a high speed video camera for spray diagnostics and control was awarded to The Cooke Corporation. During initial trials of this device in the NIST thermal spray system, multiple exposure images of thermal spray droplets in flight were demonstrated. The measured droplet sizes ranged from 40 to 100 m with speeds from 80 to 190 ms^-1.

A Phase II SBIR for developing a thermal imaging sensor for spray processing equipment was awarded to Stratonics, Inc.. This sensor, as currently configured, provides "real-time" temperature, emissivity, and surface roughness measurements on material coatings as they are formed. Development work is in progress on the use of image intensifying components with the intent of providing temperature measurements of particles/droplets "in-flight".

A Phase I SBIR on developing a holographic diffuse light source for spray diagnostics was awarded and work was initiated with North Dancer Labs, Inc. This sensor, if successful, will provide illumination suitable for specularly reflective materials such as metal powder, droplets, and coatings. These measurement tools will be valuable for many materials processing systems. In particular, they will provide thermal and coating quality data for NIST process modeling and control software that is to be developed as part of the thermal spray project.

Impact:

Ongoing NIST SBIR funded research has resulted in a new imaging pyrometer with wide applicability in the materials processing area. This "Thermal Spray Imaging" sensor, available from Stratonics, Inc. of Laguna Hills, CA uses special IR optics to produce a high resolution two-color image of the material under test. This approach provides both temperature and emissivity data with spatial resolution as high as 15 m. Equipped with a standard video CCD array, this device can measure real-time accurate surface temperatures (10 K), emissivity, and roughness of such objects as plasma spray coatings, spray deposition substrates, and the surface of hot or molten materials. An image intensifying camera is currently under development that will provide similar results from thermal spray and atomization droplets in flight.

Outputs:

Publications:

Rosenthal, P. A., Cosgrove, J. E., Haigis, J. R., Markham, J. R., Solomon, P. R., Farquharson, S., Morrison Jr., P. W., Ridder, S. D., and Biancaniello, F. S. "FT-IR process monitoring of metal powder temperature and size distribution," Optical Sensors for Environmental and Chemical Process Monitoring, SPIE, 2367, 183, (1995).

Presentations:

Ridder, S. D. "Metal Powder Research at NIST," TMS Symposium on P/M Current Research and Industrial Practices, Indianapolis, Indiana, Sept., 1997.

Project Title: ELECTRODEPOSITION OF ALUMINUM ALLOYS

Investigator: G.R. Stafford

Objectives:

This project seeks to develop an understanding of electrolyte behavior, morphological development and crystal structure operative during the electrodeposition of aluminum alloys from both organic halide and alkali halide based chloroaluminate electrolytes.

Technical Description:

Aluminum and many of its alloys can impart excellent corrosion protection when applied as a thin coating to other materials. Typical coating technologies include hot-dipping, flame spray and physical vapor deposition (PVD). Electrodeposition may offer an inexpensive method for producing homogeneous and fine-grained aluminum-based thin films. Unfortunately, aluminum can only be electrodeposited from aprotic, nonaqueous solvents or molten salts. One of the more widely explored molten salt electrolytes consists of a mixture of AlCl₃ and an alkali chloride such as NaCl. The electrodeposition of alloys such as Al-Ti, Mn, Cr, Ni, Co and Cu have been demonstrated. Recent reports in the literature indicate that niobium may be electrochemically dissolved in chloroaluminate electrolytes and thus provide a means for electrodepositing Al-Nb alloys. Al-Nb alloys produced by PVD have shown excellent resistance to pitting in chloride media. Part of our effort this year has focused on the electrodeposition of Al-Nb alloys from the AlCl₃ - NaÇl electrolyte.

The AlCl₃ - alkali chloride systems have been widely explored and a continuous process for the electrodeposition of Al-Mn alloys onto sheet steel has been developed. Even so, physical properties such as high vapor pressure or high melting temperature make them unsuitable for many technological applications. Organic chloroaluminates, which are obtained when certain anhydrous organic chloride salts such as 1- methyl-3ethyl-imidazolium chloride (MeEtimCl) are combined with AlCl₃, are viable alternatives since they are liquids at room temperature and exhibit negligible vapor pressure. Our remaining activities in this program have focused on the electrodeposition of pure aluminum and some of its alloys from the AlCl₃-MeEtimCl room temperature electrolyte.

Planned Outcome:

There are three primary goals to this work. The first is to verify reports in the literature indicating that niobium may be electrochemically dissolved in chloroaluminate electrolytes. This may be a significant result since refractory metals have generally been considered to be stable in these melts. The formation of an electroactive niobium species in solution may provide a means for electrodepositing Al-Nb alloys. We expect to gain information on the electroactive niobium species and demonstrate the deposition of Al-Nb alloys.

We plan to characterize the microstructure, morphology and chemical purity of pure aluminum, electrodeposited at room temperature from an AlCl₃-MeEtimCl molten salt electrolyte. It is likely that these deposits will have some chloride contamination. We anticipate that the addition of benzene will reduce the viscosity, increase the ionic conductivity of the melt and eliminate the chloride in the deposits. In addition, we expect to demonstrate the electrodeposition of Al-Ni, Co and Cu alloys from the room temperature AlCl₃-MeEtimCl molten salt electrolyte.

External Collaborations:

We are working with Geir Martin Haarberg of the Norwegian University of Science and Technology in Trondheim, Norway to understand the electrochemical behavior of niobium in chloroaluminate electrolytes. An understanding of niobium complex ion formation in this electrolyte is essential in our effort to electrodeposit Al-Nb alloys.

We are working with Professor Charles Hussey of the University of Mississippi to develop a chloroaluminate electrolyte which will allow us to electrodeposit pure aluminum and aluminum alloys at room temperature.

Accomplishments:

A study of the electrochemical behavior of niobium in a 52:48 mole ratio AlCl₃ - NaCl molten salt electrolyte was initiated. Niobium can indeed be electrochemically dissolved, however the solubility and electrochemical properties of the electroactive species have yet to be determined. Aluminum-niobium alloys, containing a Nb atomic fraction of up to 13.5 %, have been electrodeposited. Deposits containing less than 5 % Nb are face centered cubic (fcc) aluminum. As the Nb content increases, an amorphous phase is introduced into the structure. The exact composition of the amorphous phase as well as the phase distribution in the 5 % to 15 % Nb alloys have yet to be determined.

Pure aluminum electrodeposited from AlCl₃:MeEtimCl electrolytes is often contaminated with chloride. The viscosity of this room temperature system is higher than that of the high temperature analogs and electrolyte entrapment appears to be a significant adverse consequence of this high viscosity. We have demonstrated that the addition of benzene to the AlCl₃:MeEtimCl electrolyte reduced the viscosity and increased the ionic conductivity of the melt. Deposits made from benzene containing electrolytes are chloride-free and have much improved morphologies over those made from pure melt. It is likely that similar benefits can be obtained from the use of less hazardous aromatic solvents.

We have determined that Al-Ni, Al-Co and Al-Cu alloys can be electrodeposited from AlCl₃:MeEtimCl electrolytes at potentials positive of the aluminum deposition potential. The mechanism appears to be similar to that observed in the inorganic chloroaluminates where aluminum incorporation is driven by the free energy of alloy formation.

Outputs:

Publications:

Mitchell, J. A., Pitner, W. R., Hussey, C. L., and Stafford, G. R., "Electrodeposition of Cobalt and Cobalt-Aluminum Alloys from a Room-Temperature Chloroaluminate Molten Salt," J. Electrochem. Soc. 143, 3448 (1996).

Liao, Q., Pitner, W. R., Stewart, G., Hussey, C. L., and Stafford, G. R., "Electrodeposition of Aluminum from the Aluminum Chloride-1-Methyl-3-Ethylimidizolium Chloride Room- Temperature Molten Salt + Benzene," J. Electrochem. Soc. 144, 936 (1997).

Stafford, G. R., and Moffat, T. P., "The Electrodeposition of Corrosion Resistant Aluminum Alloy Coatings," Proc. of NACE Topical Symposium on Coatings and Surface Modification, Corrosion '97, New Orleans, LA (1997).

Stafford, G. R., and Haarberg, G. M., "The Electrodeposition of Al-Nb Alloys from Chloroaluminate Electrolytes," Proc. NATO Advanced Research Workshop on Refractory Metals in Molten Salts, Apatity, Russia, August, 1997.

Presentations:

Stafford, G. R., "The Electrodeposition of Corrosion Resistant Aluminum Alloy Coatings," Norwegian University of Science and Technology, Trondheim, Norway, October, 1996.

Stafford, G. R., and Moffat, T. P., "The Electrodeposition of Corrosion Resistant Aluminum Alloy Coatings," NACE Topical Symposium on Coatings and Surface Modification, Corrosion '97, New Orleans, LA (1997).

Stafford, G. R., and Haarberg, G. M., "The Electrodeposition of Al-Nb Alloys from Chloroaluminate Electrolytes," NATO Advanced Research Workshop on Refractory Metals in Molten Salts, Apatity, Russia, August, 1997.

Project Title: ELECTRODEPOSITED COATING THICKNESS STANDARDS

Investigators: C. R. Beauchamp, H. B. Gates, and D. R. Kelley

Objectives:

The objective of this work is to re-supply SRM Coating Thickness Standards numbers 1357, 1358, 1359, 1361a, 1362a, and 1363a used by the organic and inorganic coating industries.

Technical Description:

These standards have proven to be popular due to the diversity of industries which use them. The inventory kept by SRMP is low since the Electrochemical Processing Group last produced these SRMs in 1994. This production shutdown was necessary in order for us to replace obsolete primary standards and completely revise the production and certification protocols.

The uncertainties due to deficiencies in the certification methodology have been reduced by replacing the stage used for the certification of the secondary standards with one that completely automates the measurement process. Consequently, operator intervention and bias have been minimized. In addition, the operating range for each of the instruments as well as the mathematical models used to fit the data have been reviewed and optimized.

The production and thickness assignment of the primary standards, used for in-house calibration, represents the final task prior to continuing the production of the revised coating thickness standards. Assigning certified thickness values for these primary standards allows the certification of the secondary standards, which are sold to the general public, to proceed.

Planned Outcome:

The goal of this work is to replenish the stock of the following SRM's by a total of 953 units distributed in the following manner:

SRM 1358 479 units

SRM 1359 20 units

SRM 1362a 319 units

SRM 1363a 90 units

SRM 1364a 45 units

SRM's Total 953 units

Accomplishments:

Work on the replacement primary standards used for the certification of the coating thickness SRM's listed above has been completed. Each of these primary standards has an average standard deviation which is less than 1.5% of the certified coating thickness.

All of the 953 outstanding units of SRM's 1358, 1359, 1362a, 1363a, and 1364a have been fabricated. The thickness measurement has been completed on 550 of these 953 units.

The packaging design for the new SRM's has been completed.

Outputs:

SRM's in production:

SRM#1357 Cu & Cr Coating on Steel

SRM#1358 Cu & Cr Coating on Steel

SRM#1359 Cu & Cr Coating on Steel

SRM #1362a Cu & Cr Coating on Steel

SRM#1363a Cu & Cr Coating on Steel

SRM# 1364a Cu & Cr Coating on Steel

Project Title: GOLD MICROHARDNESS STANDARDS

Investigators: D. R. Kelley and C. E. Johnson

Objectives:

The objective of the proposed work is to develop a gold (Au) microhardness standard which will be used to verify the calibration of microhardness instruments when used for measurements of soft materials at low loads.

Technical Description:

The request for this standard has come mainly from the electronics industry where gold is electrodeposited on printed circuit board contacts. Also, the general plating industry for precious metals has requested the standard for process control of addition agents to Au electrolytes. This microhardness standard is expected to fill a void in the low hardness, low load standards presently offered. It will allow the electronics and precious metals plating industry to verify the proper operation of the microhardness measuring devices presently used for quality assurance.

Steps are now being taken to scale-up the fabrication of a low load microhardness standard prototype. This requires a scale-up of the Au electrodeposition process, a means to cut the material uniformly and accurately, a system to mount the samples in the mounting media and a jig to diamond turn up to eight samples at a time.

A non-uniform current density during Au deposition results in non-uniform grain size and hardness. We expect to reduce the hardness variation across the sample surface by scaling up the electrodeposition process and electroplating a large panel, perhaps eight inches square. This is analogous to a method successfully implemented to reduce the thickness variation in our electrochemically produced coating thickness standards.

In order to cut the larger panels into 1.5 cm square SRM samples, we propose to use a diamond saw blade on a table saw with a traversing table. Traditional methods of cutting using a silicon carbide abrasive wheel are unacceptable since these methods often introduce abrasive media into the gold electrodeposit.

Planned Outcome:

We expect to produce a 24K gold, low load microhardness standard, with an average Knoop hardness of 75 HK and overall uncertainty of less than 10%.

Accomplishments:

The Electrochemical Processing Group has produced a 24K gold, low load microhardness standard prototype. The surface area is 2.25 cm² and the deposit thickness is 200 m. At a load of 25 grams, more than 1,000 indentations can be made on its surface. The average Knoop hardness is 75.5 +/- 10%. Conventional metallographic procedures were utilized to prepare the gold surface but proved unsuccessful. Large amounts of grinding and polishing media became imbedded in the gold and therefore rendered the surface unacceptable. An alternative method, diamond turning was developed. A single point diamond is used to turn the gold deposit to a mirror finish having a surface roughness of 63.5 nanometers peak to valley.

Outputs:

The Electrochemical Processing Group has produced a 24K gold, low load microhardness standard prototype with an average hardness of 75.5 Knoop +/- 10%.

SRM's under development:

SRM#1870 Gold Microhardness Standard

Project Title: ELECTROGALVANIZED COATINGS ON STEEL

Investigators: G. Stafford, C. Beauchamp, and D. Kelley

Objectives:

The object of this work is to develop the electrochemical expertise that will enable the production of mass per unit area standards to be used by the steel industry to calibrate on-line x-ray fluorescence instruments for process control of continuous strip plating of electrogalvanized coatings.

Technical Description:

The domestic electrogalvanizing market is approximately 2.5 billion dollars per year. Pure zinc still maintains about 82% of the domestic electrogalvanizing market, followed by alloy plating of Zn-Ni and Zn-Fe systems at 9% each. At the present, there is little accountability among sheet steel manufacturers with respect to zinc electrodeposited due to discrepancies between measurement methods. There is a critical need for standards of mass per unit area and composition for the electrogalvanizing industry, this is particularly accentuated by the industry's push to become compliant with regulations such as those self imposed under ISO 9000. The prototypes under development will serve to fill a void in the standards which are currently available, particularly for those manufacturers which require much improved reference coatings for on-line quality assurance.

Planned Outcome:

Zn/steel coating thickness standards will be developed that are applicable to the Zn coated sheet steel currently produced by the electrogalvanizing industry. These coatings must have a controlled microstructure including crystal orientation (for suitable mechanical properties) and uniform thickness (mass per unit area) to make them suitable for use as calibration standards for x-ray fluorescence and gravimetric test methods. These standards will be certified with respect to the coating mass per unit area. Prototype coupons, having an overall coating mass uncertainty of less than 5% will be fabricated in configurations which are acceptable to both the x-ray fluorescence and gravimetric measurement communities.

Accomplishments:

Zinc electrodeposited coatings with mass per unit area uniform enough to comply with the 5% uncertainty required for gravimetric testing methods (1 cm square coupons) were fabricated. Improvements to the cell geometry employed in the deposition process of these coatings are still being made with the objective of obtaining an overall coating distribution that is compliant with the tighter uncertainties required for the x-ray fluorescence instrumentation (10 cm x 15 cm panel). In addition, the algorithm employed for the selection of the coupons was optimized to increase the yield of coupons per plate while maintaining a low uncertainty.

Outputs:

Prototype plates capable of complying with the 5% uncertainty required for the gravimetric technique were prepared. In addition, the algorithm employed for the selection of the coupons was optimized to increase the yield of coupons per plate while maintaining uncertainties low.

Project Title: ELECTRODEPOSITED CHROMIUM FROM TRIVALENT ELECTROLYTES

Investigators: J. L. Mullen and C. E. Johnson

Objectives:

The program is primarily focused on determining the effects of electrolyte composition and operating parameters on the composition, structure and properties of chromium electrodeposits in which a trivalent electroactive chromium species is used. The structure and properties of the chromium coatings from trivalent electrolytes will be compared to coatings from hexavalent electrolytes.

Technical Description:

Chromium is widely used as an electrochemically applied coating on metal for wear resistance, for reduced friction, or for a desired appearance. In present commercial electroplating processes, chromium is deposited from electrolytes in which it is in the toxic hexavalent (Cr⁺⁶) state. Present commercial deposition of chromium from non-toxic trivalent electrolytes (Cr⁺³) is limited solely to decorative applications where the coating thickness is on the order of 0.5 m to 5.0 m. The thicker deposits required for functional applications cannot be obtained from the commercial bath chemistry. A Cr⁺³ -based electrolyte, recently developed at NIST (U.S. Patent 5,415,763), allows one to electrodeposit chromium coatings which are thick enough (50 m to 250 m) to be suitable for engineering applications; however, the wear resistance is somewhat lower than coatings made from hexavalent electrolytes. This program focuses on the structural characterization of chromium coatings electrodeposited from the NIST trivalent electrolyte, paying particular attention to structural features which may lead to the observed lower wear resistance.

Planned Outcome:

The processing conditions which cause the properties of chromium deposited from trivalent electrolytes to be equal or superior to deposits from hexavalent electrolytes will be identified. The properties, which may be improved by heat treatment, include hardness and wear resistance. The commercial success of the use of trivalent electrolytes as an alternative to hexavalent electrolytes for depositing chromium coatings for engineering applications will be further enhanced by the understanding of the effects of processing conditions.

External Collaborations:

As a result of the NIST "Workshop on Electrodeposition of Thick Chromium Coatings from Trivalent Electrolytes," informal collaborations were initiated with Dr. John Dash of Portland State University to study the effects of chloride additions to a sulfate-catalyzed trivalent electrolyte on the adhesion of the chromium deposit on heat-treated steels.

An investigation into the use of amorphous alloy coatings of Co/Cr, deposited from a modified trivalent chromium electrolyte, as potential bearing surfaces for orthopedic implants is being carried out in collaboration with the Biomaterials Group of the NIST Polymers Division which is being partially supported by NIH.

Accomplishments:

It has been shown that chromium coatings from trivalent electrolytes are amorphous with a lamellar structure when viewed in cross-section, compared to a very small grained crystalline structure for deposits from hexavalent electrolytes. The conjecture is that the low wear resistance for the as-deposited coatings from the trivalent electrolyte is due to fracture along the lamellae. To support this conjecture, it is known that: (1) amorphous autocatalytic nickel deposits have a lamellar structure and when subjected to a bend test fail along the lamellae, and (2) the wear debris, generated by high load, unlubricated pin on disk type wear testing of tri- chromium deposits, is in flake form indicative of fracture normal to the applied load. It also appears that the wear resistance of heat-treated tri-chromium deposits does not exceed the hex-chromium deposits until the lamellar structure is minimized or lost even though the tri- chromium hardness is 1.3 times higher than the hex-chromium deposits. The influence of electrolyte chemistry and mass transport on the lamellar structure formation will be investigated.

An investigation into the effect of heat treatment on the microstructure and properties of trivalent chromium deposits from one electrolyte chemistry was completed. Significant results were: a maximum hardness of 1900 HK₅₀, compared to 1000 HK₅₀ for deposits from hexavalent electrolytes, was obtained after heat treatment at 600 ^oC; DTA scans revealed a double exotherm around the glass devitrification temperature; and, TEM revealed, possibly for the first time, a structure modulated material consisting of alternating 20 nm layers of a BCC and amorphous structure.

Outputs:

Presentations:

Mullen, J. L., and Johnson, C. E., "Workshop on Electrodeposition of Thick Chromium Coatings from Trivalent Electrolytes," NIST, January, 1997.

Johnson, C. E., and Mullen, J. L., "Structure and Properties of Functionally Thick Chromium Electrodeposits from a Trivalent Electrolyte-A "Green" Technology," 126th TMS Meeting, Orlando, Florida, February, 1997.

Patents Granted:

Methods and Electrolyte Compositions for Electroplating Metal-Carbon Alloys
Christian E. Johnson, et al.
U.S. Patent 5,672,262 issued 09/30/97

Project Title: ELECTROCHEMICAL PROCESSING OF NANOSCALE MATERIALS

Investigator: T. P. Moffat

Objectives:

The objective of this project is to develop an understanding of physical phenomena and processing parameters required for producing complex materials via electrochemical processing.

Technical Description:

A variety of nanostructured materials may be synthesized by electrochemical deposition. Currently, our effort is focused on producing low dimensional structures, such as strained- layer superlattices, with an eye towards possible application in magnetic and mechanical devices. Producing these materials requires an understanding of heteroepitaxial deposition of iron group metals on a variety of different substrates ranging from metal and semiconductor single crystals, to highly oriented thin films. Understanding the linkage between the processing parameters, the dynamics of nucleation and growth processes and morphological stability is central to providing well-defined materials. In order to develop a deeper understanding of some of these issues, in-situ characterization of the structure and dynamics of solid/electrolyte interfaces is being pursued by scanning probe microscopy, and morphology during homo- and heteroepitaxial growth has been explored.

Planned Outcome:

Electrodeposition is a convenient, low temperature, inexpensive process for producing thin films for a variety of technological applications ranging from metallization of semiconductor devices to the synthesis of magnetic materials. Our studies using STM to characterize metal deposition processes promise to contribute valuable information on the relevant physical processes, kinetics and morphological evolution during film growth. In a generic sense, the success of the electroplating industry stems largely from the remarkable influence of electrolyte additives on the physical properties of the deposited film. Chloride ion is a ubiquitous species in most commercial copper electroplating processes, thus our STM studies contribute fundamental information to the subject. This is likely to be of some importance as submicron copper metallization is introduced into the fabrication of semiconductor devices via electrochemical or CVD processes. Our specific findings are that the surface of the copper electrode is covered by an ordered layer of oxidatively adsorbed chlorine at electrode potentials typically associated with copper deposition and dissolution. The adlayer exerts a strong influence on the adatom binding and activation energy at steps and thus plays a dominant role in determining the evolution of surface morphology. Our scanning probe microscopy studies promise to provide considerable insight into the way these adsorbates influence microstructural evolution.

External Collaborations:

We are working with Professor L. Salamanca-Riba of the University of Maryland to explore the magnetic properties of electrodeposited strained-layer superlattices. Mr. Matsuhira Shima, a Ph.D. student at the University of Maryland, is involved in the synthesis and characterization of Cu/Co multilayers.

We are working with Dr. David van Heerden of Johns Hopkins University who is investigating the structure of electrodeposited Cu-Ni multilayers using cross-sectional TEM.

Accomplishments:

A firm scientific foundation for electrochemical deposition of alloys and multilayers requires a one to one correlation between coulometry and film thickness. This demands a knowledge of the current distribution and the current efficiency. In the last year we performed investigations into the current efficiency of iron group metal deposition as a function of film thickness (0-100 nm) and have designed a new electrochemical cell that optimizes the primary and tertiary current distributions so that coulometric control of film growth is directly related to film microstructure.

A scheme for depositing highly oriented copper seed layers on Si(100) and Si(111) has been adopted as a substrate for electrochemically growing oriented Cu-Co strained-layer superlattices for magnetic property investigations. In collaborations with the Magnetic Materials Group, M. Shima and L. Salamanca-Riba of the University of Maryland, [Cu/Co]_NSi(100)films were shown to exhibit a dependence of the GMR on in-plane orientation, due to magnetocrystalline anisotropy of the epitaxy structure.

The capability was developed of using in-situ STM to study the structure and dynamics of the deposition/dissolution of Cu, and the influence of anion adsorption and metal underpotential deposition on step dynamics. Studies to date have focused on chloride adsorption and lead underpotential deposition on Cu(100), Cu(111) and more recently Cu(110). Step faceting due to the formation of an ordered, commensurate adlayer has been demonstrated. Likewise, the impact of adsorption on step-step interactions is being explored. Studies have been initiated for heteroepitaxial deposition of nickel and cobalt deposition on Cu(100).

An effort has been initiated to directly deposit metals onto semiconductor substrates.

Outputs:

Publications:

Moffat, T. P., "Electrodeposition of Strained-Layer Superlattices," V-451, Electrochemical Synthesis and Modification of Materials, MRS Pittsburgh (1997).

Moffat, T. P., "STM Study of the Influence of Adsorption on Step Dynamics," V-451, Electrochemical Synthesis and Modification of Materials, MRS Pittsburgh (1997).

Presentations:

Moffat, T. P., "Electrochemical Synthesis and Characterization of Novel Materials," Swiss Federal Institute of Technology, Lausanne, Switzerland, September, 1997.

Moffat, T. P., "Electrodeposition of Strained-Layer Superlattices," 192nd Electrochemical Society Meeting, Fundamental Aspects of Electrochemical Deposition and Dissolution Including Modeling, Paris, France, September, 1997.

Moffat, T. P., "STM Study of Adsorption and Electrodeposition Processes on Copper Single Crystals," 191st Electrochemical Society Meeting, Third International Symposium on In-Situ Characterization of Electrochemical Processes, Montreal, Canada, May, 1997.

Moffat, T. P., "STM Study of the Influence of Adsorption on Step Dynamics," MRS Symposium on Electrochemical Synthesis and Modification of Materials, Boston, Massachusetts, December, 1996.

Moffat, T. P., "Electrodeposition of Strained-Layer Superlattices," MRS Symposium on Electrochemical Synthesis and Modification of Materials, Boston, Massachusetts, December, 1996.

Moffat, T. P., "Electrochemical Synthesis and Characterization of Novel Materials, Louisiana State University, Baton Rouge, Louisiana, October, 1996.

U.S. Department of Commerce
Technology Administration
National Institute of Standards and Technology
Materials Science & Engineering Laboratory
Metallurgy Division

For further help finding information about specific NIST programs and publications,
please contact the Public Inquiries Unit, (301) 975-3058

Revised March 09, 1998