Infrared

Described by Dr.Craig A. Blue, Oak ridge national Laboratory, Tennessee, USA.

Originally Published in "Coatings" July 2000
Introduction

A new high-density-infrared (HDI) transient-liquid coating (TLC) process has been developed to produce wear- and corrosion-resistant coatings on a variety of surfaces that are of commercial interest. The HDI TLC process combines infrared heating with power densities up to 3.5 kW/cm2  with thermal spray processes to produce near full density coatings that are metallurgically bonded to the substrate.

In this paper the process basics, the coatings type and quality, and possible industrial applications will be described.

High-density Infrared

The HDI TLC process utilizes a unique technology to produce extremely high power densities of up to  3.5 kW/cm2 with a single source, which is currently the most powerful single source in the world (figure 1). The lamp system is constructed by using 3.175 cm diameter quartz tube, which can be 10.16, 20.32 or 38.1 cm long. The lamp is sealed at the ends by the anode and cathode. Deionized water enters the cathode side through high-velocity jets impinging on the quartz envelope at a given angle causing the water to spiral down the length of the tube in a uniform 2-3 mm thick film on the wall. This water film serves two purposes: (1) to cool the quartz wall and (2) to remove any particles that may be expelled from the electrodes. The argon gas also moves in a spiral fashion through the centre of the tube and provides the medium for initiation and maintenance of a plasma in the central potion of the tube. The stabilized plasma produces a spectrum from 0.2 to 1.4 mm and has a colour temperature in excess of 10,000K. In contrast to a CO2 laser with a wavelength near 10.6 mm, the shorter wavelength radiation produced by the plasma-arc lamp is absorbed with much higher efficiency by most materials. Powder- and thermal-sprayed coatings are highly absorbing because their open porosity acts as a black body, increasing the efficiency even further.

 
Figure 1 Schematic of a high-power density plasma arc lamp and the principles of operation.

High-Density-Infrared Processing at ORNL

The ORNL HDI Processing Facility shown in Figure 2 features a six-axis robotic manipulator arm controlled by a state-of-the-art PC-based robotic controller. This robotic arm is capable of using computer-aided drawing data files to generate instructions to manipulate the source over a complicated geometry in a precise manner. Also available for use in the HDI Processing Facility is a lathe to rotate parts while heat treating to fuse coatings.

Another feature of the HDI Processing Facility is a water window used to protect the lamp when operating in harsh environments. This is shown in Figure 3. This water window passes a thin film of water over a piece of quartz glass covering the reflector. 

The water is introduced by an air knife on one side of the lamp and removed on the opposite side through an orifice by a vacuum. This water window protects the lamp from liquid metal splatter, hot-spalled material, off-gassing of corrosive materials, or materials that could deposit on the cold lamp tube.

Figure 2 High-density-Infrared Processing facility at the Oak ridge National Laboratory, Oak Ridge, Tennessee.

Figure 3 Infrared plasma-arc lamp showing (1) internal and (2) external water walls.

Plasma Spraying and Fusing

Plasma spraying is a long established coating process in which powder particles of the coating material are propelled onto a surface. It has long been known that coating porosity and the interfacial properties of plasma-sprayed coatings are areas that limit the application of these coatings because of the ability of corrosive environments to penetrate the coatings and attack the substrates. It has been found that using HDI processing, plasma-sprayed coatings can be re-melted and interface properties improved. A plasma-sprayed coating that has been HDI process is shown in Figure 4.

As seen in Figure 4 (b), the HDI-processed plasma-sprayed coating has been dramatically reduced in porosity, having only micropororsity, similar to the 4030 steel substrate. It should also be noted that the mechanical interface between the coating and substrate has been transformed into a metallurgical bond. Hardness profiling from the coating to the substrate material reveals that the coating has a hardness of 982 HV and that approximately 200 mm of the base material has been slightly over-tempered. Source scan speed of 0.5 cm/s with a power density of 1200 W/cm2 was used. HDI post-processing of plasma-sprayed coatings is presently being evaluated for the treatment of rolling-mill rolls, for corrosion-resistant coatings in the chemical industry, and in the heavy equipment industry for wear/corrosion applications.

Figure 4 (a) Hard facing alloy as plasma-sprayed 

 

Figure 4 (b) Hard facing alloy after High-density-Infrared fusing with a power density of 1200 W/cm2 and a scan speed of 0.5 cm/s.

 

Conclusions

It has been shown that HDI TLC and HDI processing are extremely powerful tools for fusing, altering, and heat-treating coatings while having minimal effects on the base material. Still relatively new to materials processing area, the infrared plasma-arc equipment is gradually being exploited in the area of coatings applications. Using the water window technology developed to protect the lamp allows for processing of materials that splatter and smoke with no detrimental effects to the lamp. Other advanced materials processing techniques are presently being explored with the HDI technology that may bring new materials and techniques to market that are new or cannot be produced economically at the present time.

Acknowledgements

The authors thank Michael L. Santella and Theodore J. Huxford for reviewing the article and Mille Atchley for preparing it.

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 Advance Automotive Propulsion Programme, DOE Office of Transportation Technologies, under contract DE-AC05-000R22725 with UT-Battelle LLC.

 

For further information about this article please contact:

Dr Craig A. Blue, Oak Ridge National Laboratories, Oak ridge, Tennessee, USA.

For other articles presented at the TSSEA conferences or printed in "Coatings" see the publications page or contact TSSEA for further information.

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