This article is one of eight papers to be presented exclusively on the web as part of the January 2000 JOM-e—the electronic supplement to JOM.
	The following article appears as part of JOM-e, 52 (1) (2000), http://www.tms.org/pubs/journals/JOM/0001/Blue/Blue-0001.html JOM is a publication of The Minerals, Metals & Materials Society

Functional Coatings: Overview

High-Density-Infrared Transient Liquid Coatings

Craig A. Blue, Vinod K. Sikka, Evan K. Ohriner, P. Gregory Engleman, and David C. Harper

A video of plasma arc melting


TABLE OF CONTENTS
INTRODUCTION RADIANT-ENERGY THEORY HIGH-DENSITY INFRARED TRANSIENT-LIQUID COATING HIGH-DENSITY-INFRARED PROCESSING FACILITY AT ORNL COATING METHODS DIE-CASTING PINS THICK COATINGS, RAPID INFILTRATION HIGH-DENSITY-INFRARED HEAT TREATING CONCLUSIONS ACKNOWLEDGEMENTS References

A high-density-infrared, transient-liquid coating process has been developed to produce wear- and corrosion-resistant coatings on a variety of surfaces that are of commercial interest. The process combines infrared heating with power densities up to 3.5 kW/cm² with a room-temperature spray process to quickly form wear- and/or corrosion-resistant coatings in seconds. This process has been demonstrated using Cr₂C₃ and WC-reinforced coatings with nickel-based binders. Coating densities as high as 98-100 percent of theoretical density have been achieved with coating thickness of 10 mm to 2 mm. The same processing techniques have also been shown to be capable of performing localized and selective heat treatment of surfaces.

INTRODUCTION

Infrared technology is used in a wide range of industrial applications. It is used for drying solder resists in the electronics industry, browning and sterilization in food processing, stoving and curing in finishing, drying and sealing in textiles, softening in plastics, drying in printing, and preheating and shrink fitting in engineering.¹ The types of equipment used for these applications are typically limited to processing temperatures of 537-760°C.

Infrared heating has many advantages over other heating techniques. Infrared heating provides an inherently clean, noncontact heating method; rapid-response energy fluxes capable of heating rates in excess of 500°C/s (state-of-the-art equipment provides excellent spatial and tempered control that allows sample-only heating unidirectionally over large areas); rapid power-level changes (low thermal mass [halogen lamps] or no thermal mass [plasma lamp]); rapid cooling rates, due to the cold-wall nature of the process in which only the sample is heated; and controllable temperature-gradient processing with flux densities up to 3,500 W/cm². Thus, infrared provides a versatile and flexible answer to heat-transfer problems throughout the industrial spectrum.² For a more detailed discussion of radiant theory, see the sidebar "Radiant-Energy Theory."




Figure 1. A schematic of a high-power density lamp and the principal operation.		Figure 2. The spectral irradiance produced by the HDI lamp.





Figure 3. The typical reflector configurations utilized with the high-density lamp showing (a-left) line focus and (b-right) uniform irradiance.

RADIANT-ENERGY THEORY

High-density infrared (HDI) provides a fast, controllable method for metal-heating applications. All bodies radiate energy as a function of their absolute temperature, as defined by Stefan-Boltzmann Law

Q = kT⁴

(A)

where Q is total emissive power (W/cm²), k is the Stefan-Boltzmann constant 5.56 × 10^-12 (W/cm²-°K⁴), and T is absolute temperature (K).

Infrared energy is the portion of the electromagnetic spectrum between 0.78 mm and 1,000 mm. The infrared electromagnetic spectrum can be divided into three divisions: short wave (0.78 mm to 2.0 mm), medium wave (2.0 mm to 5.0 mm), and long wave (5.0 mm to 1 mm). The actual emission spectrum of a given source is dependent upon its temperature; increasing the source temperature will result in shorter overall wavelengths of the energy. This also corresponds to an increase in the overall emissive power per Equation A.³

In order to understand which parameters are important in rapid-infrared heating, consider the general equation for heat transfer between the source and target

Q = (FV) × (ES) × (AT) × (k) × (TS⁴ - TT⁴)

(B)

Where Q is heat transfer between the source and target (W/cm²), FV is the view factor between the source and target, ES is the emissivity factor of the source, AT is the absorption factor of the target, k is the Stefan-Boltzmann constant, TS is the absolute temperature of the source, and TT is the absolute temperature of the target.

The view-factor term is the fraction between 0 and 1 that quantifies the amount of radiant energy emitted from the source that falls incident upon the target. Control of the heating rate is accomplished by varying the source temperature. The absorbed heat transfer (Q) results in a temperature rise of the target as defined by

T=	(Q) × (A) × (t)		(C)
	(M) (Cp)

where T is the product temperature rise (K), A is the target area (cm²), t is the heating dwell time, M is the target mass (kg), and Cp is the target specific heat (W-s/kg-K).

Increased temperature rise can be achieved by increasing the dwell time or the amount of infrared incident on the target. The wavelength of light produced by quartz-halogen lamp systems is approximately 1.2 mm, that of the plasma system is 0.2-1.4 mm. The plasma-lamp spectrum has a portion of the emitted radiation outside the infrared spectrum. Spectral distribution of radiant energy follows Plank's Law, which, as a function of temperature and the wavelength considered, gives the monochromatic emissivity M_l of a black body for wavelength l^3-5

(D)

where M_l is the radiant power in W/mm², corresponding to wavelength l; C₁ is the constant 3.741 × 108 W × mm⁴/m²; C₂ is the constant 14,388 mm × K; l is wavelength (mm); and T is the temperature of the filament (K). A depiction of Plank's Law is shown in Figure A.

Figure A. Plank's Law, revealing the variation in radiated power as a function of filament temperature and wavelength.

The emission of radiation, however, is not the emission of heat. It is only when a body absorbs radiation that it is converted into heat. The overall absorption capability, a, at a point is the ratio between the flux absorbed and the incident flux. For all known bodies, this ratio is less than one, since part of the radiation is reflected and, if the body is not opaque, part of the radiation is transmitted. If r is the reflection factor or reflectivity and t is the transmission factor or transmissivity,

r + t + a = 1

(E)

This is schematically depicted in Figure B.

Figure B. A schematic of absorption of infrared radiation on a surface.

HIGH-DENSITY-INFRARED PROCESSING FACILITY AT ORNL

The Infrared Processing Center of the Materials Processing Group in the Metals and Ceramics Division at ORNL has a variety of HDI equipment with the capability of producing heat fluxes 10-3,500 W/cm². In the HDI processing facility, the HDI lamp is mounted on a large, five-axis robotic manipulator arm. A state-of-the-art robotic controller defines arm movement; this controller is capable of using computer-aided drawing data files of large parts to generate instructions to manipulate the source over a complicated geometry in a predetermined, systematic way. The HDI processing facility is shown in Figure C.

Figure C. The HDI processing facility at Oak Ridge National Laboratory.

Test-sample processing is performed in an environmentally controlled box, which has a quartz window cover to permit processing of materials in a controlled atmosphere. The infrared reflector has a focal length that extends through the quartz and onto the material being processed. A lathe to rotate parts while heat treating or fusing coatings is also included in the processing facility.

Another feature of the plasma lamp for this facility is a water window (Figure D), which passes a thin film of water over the quartz glass covering the elliptical reflector to protect the lamp when operating in harsh environments. This window has a 3 mm water film that continuously cools the lamp quartz window. The water clings to the quartz window due to surface tension and stream momentum. Water is introduced on one side of the lamp across an air knife and removed on the opposite side through a vacuum orifice. This protects the lamp from liquid-metal splatter or hot-spalled material, which is necessary in many processing applications.

Figure D. A plasma-infrared lamp showing the internal and external water walls.

Many of the infrared systems at Oak Ridge National Laboratory's (ORNL's) Infrared Processing Center utilize tungsten-halogen lamps, which contain filaments that glow at approximately 2,900°C, resulting in a theoretical power density of approximately 500 W/cm². Design consideration typically limits the power densities to approximately 40 W/cm². This technology has been shown to be an environmentally clean alternative to salt-bath technologies for preferential tempering of large die blocks, flash annealing, joining, preheating, and fusing room-temperature-sprayed powder coatings.

Although very effective at fusing powder coatings and rapid heat treating while precisely controlling the solid/liquid interface and microstructure on small- to medium-size parts, the power densities are not high enough to rapidly fuse coatings or harden them on large structures; the plasma-infrared system, with its higher heat loads, is needed for these purposes. The infrared processing technique discussed here is a plasma-based light source rather than a resistively heated source. This higher power-density capability has demonstrated materials-processing capabilities at temperatures in excess of 3,020°C.

HIGH-DENSITY INFRARED TRANSIENT-LIQUID COATING

The high-density infrared (HDI) transient-liquid coating (TLC) process utilizes a unique technology to produce extremely high-power densities of 3.5 kW/cm² with a single lamp. Instead of using an electrically heated resistive element to produce radiant energy, a controlled and contained plasma is utilized. A schematic of the lamp is shown in Figure 1.

The lamp consists of a quartz tube 3.175 cm in diameter and 10.16 cm, 20.32 cm, or 38.1 cm long. The lamp is sealed at the ends where the cathode and anode are located. Deionized water mixed with argon or nitrogen gas enters at the cathode side through high-velocity jets impinging at a given angle. Due to the high velocities and pressure, the deionized water is impelled to the wall of the quartz tube and spirals down the length of the tube in a uniform 2-3 mm thick film. This water film serves two purposes: to cool the quartz wall and to remove any tungsten particulate that may be expelled from the electrodes. The gas moves in a spiral fashion through the center of the tube, and a capacitative circuit initiates the plasma. The plasma, which has a temperature in excess of 10,000 K,⁶ is stable and produces a radiant spectrum 0.2-1.4 mm (Figure 2). The spectrum is primarily in the infrared (0.78 mm to 1.00 mm), although substantial energy is released in the visible wavelength, similar to the appearance of natural sunlight in energy distribution and color rendition.

The spectrum is absorbed with high efficiency by metal surfaces. In contrast, the spectrum of a CO₂ laser with wavelengths near 10.6 mm is absorbed with much lower efficiency. The powder coatings discussed here are highly absorbing, because the open areas act like black bodies.

The lamp has a typical life of approximately 1,200 h, and failure occurs in the anode and cathode, which are inexpensive and can be changed in approximately 15 minutes. Furthermore, the lamp has a consistent spectral output independent of lamp life and power level. The lamp is typically configured with a reflector to produce a line focus or an area of uniform irradiance (Figures 3a and 3b).

COATING METHODS

Three different coating methods employing infrared heating have been used. In all three methods, the coating material is placed on the surface and then rapidly melted using radiant heating.

The very high-power densities achievable with the arc lamp permit the coating of almost any material. The three methods (plasma spray and fuse, powder spray and fuse, and rapid infiltration) have been demonstrated. This almost instantaneous on/off capability of the high-power density arc lamp system allows for excellent controllability. The solid/liquid phase reactions that occur on processed surfaces can be modeled, as can the effects of the process on the base material.

Plasma Spray and Fuse

Plasma spraying is a long-established coating process in which powder particles of the coating material are melted in gas plasma and propelled onto the substrate surface. Coating porosity and the interfacial properties of plasma-sprayed coatings are areas that have limited the application of these coatings due to the ability of corrosive environments to penetrate the coating. In order to correct such deficiencies, plasma-spayed coatings can be remelted and interface properties improved by utilizing the HDI process.

A plasma-sprayed coating that has been HDI-processed is shown in Figures 4a and 4b. The HDI-processed thermal-sprayed coating has dramatically reduced coating porosity, as seen in Figure 4b. The coating has only microporosity, similar to the base 4340 steel, after the HDI processing. The mechanical interface between the coating and the base material has been transformed into a metallurgical bond.

Hardness profiling from the coating to the base material reveals that approximately 200 mm of the base material is slightly overtempered (Figure 5). The HDI processing almost completely eliminates the porosity in the plasma coating. Processing time is approximately 10 cm²/s for the 20 cm lamp, and the resultant hardness of this coating is 982 HV. This post-HDI processing of plasma-sprayed coatings is presently being evaluated for the surfacing of rolling-mill rolls, for corrosion resistance in the chemical industry, and for wear/corrosion applications in the heavy equipment industry.

Powder Spray and Fuse

Coupling HDI processing with powder spraying at ambient temperature is a second coating method. Room-temperature spray processes are utilized to deposit WC and Cr₂C₃ with an alloy matrix on the surfaces of wear- and corrosion-sensitive parts. The ceramic particulate can have volume fractions as high as 70 vol.%, while the matrix is chosen to provide sufficient coefficient of thermal expansion to accommodate the differences in expansion and contraction between the coating and matrix. The matrix is also chosen to resist thermal softening and chemical effects. The process allows for selective deposition at room temperature of a coating material only in the desired areas, and fusing has little effect on the base material. Figures 6a and 6b illustrate some results of ORNL's HDI-TLC process. Large chain parts utilized in industries such as mining, where corrosion and wear can result in large down time of equipment, are shown in the figure.

The scan speed and power density used to produce the tungsten carbide/nickel-chromium coating shown in Figure 6b was 0.5 cm/s and 1,000 W/cm², respectively. These coatings have a typical hardness on the order of 1,000 HV, which is similar to the as-plated hardness of hard chromium (1,020 HV). Also, the HDI-TLC coatings are under compression after coating, which suppresses crack formation.

DIE-CASTING PINS

A substantial amount of work has been accomplished in the area of depositing an HDI coating of Cr₂C₃ on H-13 core pins, which are a serious problem for aluminum die-casting industries. This process was also used to produce a coating to eliminate the reaction of the molten aluminum with steel dies that form a low-temperature eutectic at 652°C. This reaction is also called soldering. Work accomplished by ORNL in the HDI-TLC of die pins has increased the life of the pins by an order of magnitude. Further refinement in fusing cycles and metal-matrix improvements may extend the life of high-aspect ratio pins, which are most susceptible to soldering due to the higher temperatures experienced, by two orders of magnitude or more.

The key to the success of this work is the ability of the HDI-TLC process to fuse the coating to the base material without excessive dissolution of the iron. If iron is present at the coating surface, soldering will persist.

Figure 4. Hardfacing alloy (a-left) as-thermal sprayed and (b-right) after HDI-fused at 1,000 W/cm²-0.5 cm/s. Figure 5. The hardness profile of a plasma-sprayed hardfacing alloy, which has been HDI-processed.

Figure 6. (a-left) HDI-TLC steel parts and (b-right) 15 mm, tungsten-carbide/nickel-chromium coating metallography. Figure 7. An HDI-TLC 3 mm tungsten carbide/nickel-chromium coating on 4340 steel.

THICK COATINGS, RAPID INFILTRATION

If thicker coatings are necessary, a third coating method may be utilized in which a precursor carbide mat is first applied in the area to be coated, and a metallic matrix is rapidly infiltrated into the ceramic precursor and wetted to the substrate. This is almost identical to the described spray process, but it allows for thicker coatings up to 3 mm. A typical thick coating of this type is shown in Figure 7.

With this type of application technique, hard phase loading of 80 vol.% is possible. These types of coatings can be utilized for a variety of applications, including rollers, shafts, turbines, conveyors, drilling tooling, mining equipment, textile guides, pipes, compressors, dies, paper rolls, pumps, tooling, cutters, metal-working equipment, and lawn equipment.

HIGH-DENSITY-INFRARED HEAT TREATING

Although not the focus of this paper, the use of infrared for surface heat treating can be a powerful tool. It has been shown that surface hardening of H-13 steel can be accomplished at speeds in excess of 1 m/min. and at sweep widths of up to 35 cm (video of HDI hardening of H-13 steel).³ This large sweep minimizes the area of overlap on larger structures and allows for more consistent microstructural control.

It has also been shown that when H-13 is surface hardened, the HDI process puts the surface in compression, due to the phase changes that occur just in the surface area. These steels can be highly resistant to fatigue cracking in subsequent tensile loading, as in aluminum die-casting environments.

CONCLUSIONS

HDI-TLC and HDI processing are extremely powerful ways to coat, alter coatings, and heat treat while having minimal effects on the base material. The plasma-infrared processing equipment is relatively new to the materials-processing area and is gradually being exploited in coatings applications. The water-window technology developed to protect the lamp allows the processing of materials that splatter and smoke with no detrimental effects on the lamp. Other advanced materials-processing techniques with the HDI technology are presently being explored that may bring to the market new materials that cannot be produced economically at the present time.

ACKNOWLEDGEMENTS

The authors thank Ted Huxford and Peyton Moore for reviewing the paper and Millie Atchley for preparing the manuscript. Research for this work was sponsored by the U.S. Department of Energy, Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Industrial Technologies, Advanced Industrial Materials Program and the Office of Transportation Technologies, Advanced Automotive Propulsion Materials Program, under contract DE-AC05-96OR22464 with Lockheed Martin Energy Research Corporation.

References
1. N.C. Cox and D.E. McGee, "Use of High Density IR for the Rapid Heating of Metals," Industrial Heating, 4 (1989), pp. 46-48.
2. H. Bischof, "The Answer Is Electrical Infrared," J. Microwave Power and Electron. Energy, 25 (1) (1990), pp. 47-52.
3. R. Loison, Chauffage industrial (France: Ecole nationale superieure des Mines de Paris, 1956).
4. J. Gosse, Rayonnement thermique (France: Editions Scientifiques Riber, 1975).
5. J. Scadura et al., Initiation aux transferts thermiques (France: Technique et Documentation, 1978).
6. D.M. Camm and B. Lojek, "High Power Arc Lamp RTP System for High Temperature Annealing Applications" (Paper presented at 2nd International Rapid Thermal Conference, 1994).

Craig A. Blue, Vinod K. Sikka, Evan K. Ohriner, P. Gregory Engleman, and David C. Harper are with the Infrared Processing Center at Oak Ridge National Laboratory.

For more information, contact C.A. Blue, Oak Ridge National Laboratory, Metals and Ceramics Division, One Bethel Valley Road, Oak Ridge, Tennessee 37831-6083; (423) 574-4351; fax (423) 574-4357; e-mail blueca@ornl.gov.

Copyright held by The Minerals, Metals & Materials Society, 2000
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Figure 4. Hardfacing alloy (a-left) as-thermal sprayed and (b-right) after HDI-fused at 1,000 W/cm²-0.5 cm/s.			Figure 5. The hardness profile of a plasma-sprayed hardfacing alloy, which has been HDI-processed.





Figure 6. (a-left) HDI-TLC steel parts and (b-right) 15 mm, tungsten-carbide/nickel-chromium coating metallography.			Figure 7. An HDI-TLC 3 mm tungsten carbide/nickel-chromium coating on 4340 steel.