Deposition of yttria-stabilized zirconia thermal barrier coatings by laser-assisted plasma coating at atmospheric pressure

摘要

This thesis details the design and construction of a microwave generation system, and a coaxial cylindrical plasma torch, where an atmospheric-pressure plasma (APP) can be generated using 2.45 GHz. The discharge characteristics of the atmospheric plasma, such as electron temperature Te, electron density ne, and plasma gas temperature Tg, are experimentally investigated using the optical emission spectroscopy (OES) technique. The discharge mechanisms of the atmospheric pressure plasma are also theoretically discussed. The APP generated in the atmospheric-pressure plasma torch (APPT) has various applications determined by the plasma gas type and plasma gas temperature, which can range from room temperature to as high as several thousand degrees Celsius. In this study, the APP is utilized to deposit the yttria-stabilized zirconia (YSZ) thermal barrier coatings (TBC) at atmosphere pressure. These coatings can be applied on silicon wafers, and multi-layer nickel-based superalloy substrates (Ren?? N5), by a new deposition technique called laser-assisted plasma coating at atmospheric pressure (LAPCAP). The plasma generated in the APPT has the potential to increase the vapor volume ablated from the YSZ target by a Nd:YAG laser, therefore, adhesion strength between the coatings and the substrate and deposition rate can be increased. The deposited YSZ coatings show columnar structures as can be seen by other deposition methods, such as electron beam-physical vapor deposition (EB-PVD) or the conventional pulsed laser deposition (PLD) at low pressure. Some differences in the morphology, such as column size and porosity, are compared with the coatings deposited by EB-PVD and low-pressure PLD. The substrate temperature, the target-to-substrate distance and the laser ablation energy density are considered to be the determining parameters in order to obtain thick and adhesive coatings. The structures of the YSZ coatings deposited at different substrate temperatures from 20 ??C to 1300 ??C using helium/nitrogen plasma, were investigated and compared by means of scanning electron microscope (SEM) and focused ion beam (FIB) techniques. X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) analysis have showed that although a fully stabilized cubic phase YSZ coating can be achieved at different deposition temperatures with appropriate laser energy density, the microstructures, stoichiometry and phase composition of the deposited coatings can be strongly influenced by many experimental parameters, such as the deposition temperature, microwave power and laser energy density.