Solar powered rocket engines, heat exchangers, space and missile propulsion systems, hot gas valves – even hip joint replacements – could benefit from a coating technology shown to be many times faster and at least 50 percent cheaper than other options by Penn State Applied Research Laboratory (ARL) materials scientists.
‘Electron beam physical vapour deposition (EB-PVD) is one of the oldest technologies used in applying ceramic and metallic coatings in microelectronics, turbine and other industries,’ said Dr. Jogender Singh, head of the ARL Advanced Coatings Centre. ‘However, recently we’ve shown that EB-PVD also offers a high degree of flexibility in forming components exactly-shaped to high tolerances and with tailored microstructure and chemistry for very high temperature and high corrosion environments.’
Using EB-PVD technology, the Penn State researchers are said to have successfully manufactured high purity rhenium components, including rhenium coated graphite balls, plates and tubes. Rhenium, a refractory metal with the second highest melting temperature, is difficult to turn into exact shapes. Currently, rhenium components are manufactured by either powder metallurgy (P/M) or chemical vapour deposition (CVD).
Singh said that both chemical vapour deposition and powder metallurgy manufacturing of rhenium components have drawbacks that can be surmounted by EB-PVD. For example, rhenium-coated balls used in thrusters currently are manufactured by CVD and cost about $8,000 to $10,000 each because each one has to be machined and re-coated several times. Powder metallurgy techniques also require tooling and other processing, which contributes to high cost and a limited range of commercial shape components.
Using EB-PVD, Singh can coat 18 balls simultaneously and produce a product with a fine grain size that requires no machining or re-coating. He estimates that rhenium coated balls produced via EB-PVD would cost at least 50 percent less than those produced by other methods.
Penn State’s industrial pilot EB-PVD instrument has six electron beam guns housed in a vacuum chamber. Four of the beams are used to evaporate the coating materials and two are used to pre-heat the object to be coated to facilitate adhesion.
The high-energy electron beams, 45 kilowatts each, are focused on the materials to be evaporated and, as the vapour is formed, the object to be coated is manoeuvred within the vapour cloud to facilitate uniform coating. In addition, two or more co-evaporated coating materials can be mixed in the vapour cloud to form functional graded coatings with improved properties and performance.
Coating compositions can be varied via co-evaporation and coatings comprised of alternating layers of different compositions can be made at relatively low temperatures. The process also offers relatively high deposition rates compared with other coating deposition techniques, produces dense coatings, and results in low contamination and high thermal efficiency.
‘Although we carried out these experiments with rhenium, a more challenging material, it should be possible to make titanium coated components as well in the same way,’ Singh said. ‘Solid titanium components are currently used for hip replacements and this process holds out the possibility of lighter, coated, more exact replacement joints – as well as other biomedical applications.’