Infrared technologies are 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. However, the process can often be time consuming and not suitable for applying coatings to all materials such as steel and its alloys. Continuing research by Dr Craig Blue and his team in the Metals and Ceramics Division at Oak Ridge National Laboratory, Tennessee, USA, has led to the development and installation of the world’s most powerful plasma lamp, which can reportedly apply coatings 10 times faster than conventional techniques without damaging the wear-resistant coating.
Built by Vortek Industries, Vancouver, Canada, to meet ORNL’s specifications, the 300,000 watt plasma arc lamp was originally developed as a floodlight and used to light huge areas. Now, the adapted plasma source is an effective tool for coating aluminium and other alloys, and is capable of delivering up to 3.5kW/cm2 to treat areas up to 35cm by 3.175cm. ‘Using this lamp is like painting with a roller instead of a brush,’ says Blue. ‘We can treat an area 35 centimetres wide with one pass instead of the industry standard of 3 millimetres.’
Figure 1. The Plasma-arc lamp installed at Oak Ridge Laboratory.
The lamp consists of a quartz tube 3.175cm in diameter and 10.16cm, 20.32cm, or 38.1cm long. The lamp is sealed at the ends in which the cathode and anode are located. De-ionised water mixed with argon or nitrogen gas enters at the cathode side through high-velocity jets impinging at the given angle. Owing to the high velocities and pressure, the water is impelled on to the wall of the tube and spirals down the length of the tube in a uniform 23mm thick film. This water is used to cool the quartz wall and remove any tungsten particles that may be expelled from the electrodes. The gas moves in a spiral fashion through the centre of the tube and a capacitative circuit initiates the plasma. The plasma is stable and produces a radiant spectrum of 0.2-1.4μm. The spectrum is primarily in the infrared (0.78-1.00µm), although substantial energy is released in the visible wavelength, similar to the appearance of natural sunlight in energy distribution and colour rendition.
Mounted on a Cincinnati-Milacron model T3-776 robot and using a universal robot controller developed and built by Robot Workspace Technologies in Fort Meyers, Florida, USA, a variety of experiments can be performed in a short amount of time.
The typical lifespan of the lamp is 1,200 hours, and because failure only occurs in the anode and cathode these can be replaced cheaply and within 15 minutes.
The high-power densities achievable using the arc lamp means coatings can be applied to almost any material. Applications range from coating parts for automobiles and construction equipment to coating the inside of large-calibre gun barrels and applying coatings to aircraft parts.
Protective Coatings for Steel
Using the plasma source, Blue and colleagues have developed a chromium carbide coating that protects H-13 steel from attack by liquid aluminium. ‘We are finding that by using our powerful plasma lamp to precisely and rapidly heat a precursor material on the H-13 steel,’ says Blue, ‘we can make coatings that fuse with the substrate without changing the base material properties. We are testing these coating dies at Tennessee Tools to see if the coating allows the dies to last longer.’
Since its installation, ORNL’s Infrared Processing Centre has attracted even more industrial interest from companies such as Caterpillar and B F Goodrich. Native American Technologies of Golden, Colorado, USA, has designed a computer-aided system to guide the lamp and produce thermally formed shapes from flat metal plates, and Vortek is working with the semiconductor industry to improve the processing of wafers using the plasma source.
While plasma-infrared processing equipment is relatively new to the materials processing area, this lamp is definitely shining its light on new ways to coat materials quickly and easily without damaging the base material.