Nano-structured alloy and composite coatings for high temperature applications

Abstract

Nano-structured materials often possess special properties that materials with identical compositions but ordinary grain size do not have. This paper reports our work on the surface nano-crystallisa-tion and nano-structured alloy and composite coatings. A number of processing methods including magnetron sputtering, thermal spray and pulse electro-spark deposition have been used to produce surface nano-crystalline structure. The compositions and microstructures can be well controlled by using different targets or electrodes, nano-structured composites and adjusting processing parameters. Surface nano-structured coatings can provide special chemical, mechanical and electronic properties such as high temperature corrosion and corrosive wear resistance. It has potential applications such as turbine blades, engine parts for petrochemical, aerospace and electronic device industries. This paper is focused on the study of the interrelations between processing, microstructure and properties. Physical models have been established to explain the effects of nano-crystalline structure on the properties. Keywords: Nano-materials, coating, high temperature corrosion, metal-oxide composite coatings , Ti and Ti-Al based materials (PVD), thermal spray, electrochemical deposition, sol-gel methods, electro-spark deposition, and laser and electron beam surface treatment 4. Processing parameters can be carefully controlled to obtain a fine grain size and processing temperature should be kept low. Magne-tron sputter deposition (a PVD technique) has been used to produce alloy coatings with a wide range of alloy compositions 5. When an alloy coating is deposited with an unbalanced magnetron sputter, the grain size and deposition rate are affected by the working Ar pressure, substrate bias voltage , target current and the distance between target and substrate 6-7. When Ni-Cr-Al alloy coatings are deposited on alloy substrate, the coating grain size is mainly a function of the Ar pressure. Grain size as small as ~50 nm was obtained with a low Ar pressurez of 2-5 mTorr 7,8. Doping can also change the grain size and shape of the deposited thin films. Figure 1 shows an example in which the Zn and ZnO thin films were deposited on a glass substrate by magnetron sputtering with DC or RF sources 9. It can be seen that the Zn grains are much smaller toward the substrate surface (Fig. 1a); and the Al doping reduced the grain size