Study on the Microstructure and Properties of Arc Ion Plated NiAl/ZrO2 Thermal Barrier Coatings

• Main Authors: Jing-Tang Chang, 張景棠
• Other Authors: Ju-Liang He
• Format: Others
• Language: zh-TW
• Published: 2005
• Online Access: http://ndltd.ncl.edu.tw/handle/95852496919034854500

博士 === 逢甲大學 === 材料科學所 === 94 === In this thesis, thermal barrier coatings which consist of Ni-Al bond coats and zirconia top coats are deposited onto Alloy600 nickel-based superalloy by using arc ion plating (AIP) process. A hybrid arc/sputtering system has been developed for the doping of the yttrium or aluminum stabilizers. The influence of deposition parameters on the structural and thermal properties of the coatings is investigated. Experimental results show that using either the conventional AIP or hybrid arc/sputtering system utilizing a NiAl arc source material can deposit crystalline, highly adhesive, and highly oxidation resistant Ni-Al alloy coatings without the need for any in-situ heating or a post heat treatment. The increased energy of the ions bombarding the negative biased substrates and the incident angle of arc-emitted species are considered to enhance the sputtering effects of aluminum and thereby decrease its concentration in the deposited coatings compared to that of the source material. During the 1100°C oxidation test, the Ni-Al alloy coatings with the higher aluminum content form an Al2O3 layer on the surface to suppress further oxidation and associated mass increase. It is found that doping a small amount of yttrium both stabilizes the B2 structure of the as-deposited nickel-rich Ni-Al films and also enhances the nucleation and growth of Al2O3 during the high temperature oxidation tests. Aluminum from the bond coat is lost through interdiffusion with the substrate. This situation is suppressed by doping a trace amount of yttrium is into the Ni-Al coating. The yttrium accelerates the formation of oxides in diffusion voids that act as a diffusion barrier. A hybrid arc/sputtering system is used to produce the YSZ and aluminide-stabilized zirconia (ASZ) top coat of the TBC system at temperatures of about 400 °C, with the evaporation of zirconium from the arc source and the stabilizing yttrium and aluminum species being deposited by the sputtering source. The as-deposited YSZ coatings, that are almost entirely monoclinic after deposition, are found to develop some cubic phases after a few thermal cycles. The tensile stresses, parallel to the surface, during the heating stage of the thermal cycle test results in perpendicular micro cracks in both of the ASZ and YSZ dense columnar top coats. Cracks parallel to the surface are also formed in the top coats near to the bond coat interface as well as through the TGO between the top and bond coat. These cracks grow and eventually connect near the bond coat/top coat interfacial region resulting in the spallation of the top coat. The thermal stress and formation of cracks are considered to cause phase transformation in the top coat that leads to a decrease in the cubic and tetragonal phases. With the presence of the overlying top coat, the intergranular and internal oxidations in Ni-Al bond coat become more obvious. By enhanced formation of the internal oxides in diffusion voids, the interdiffusion between bond coat and substrate is suppressed whether or not yttrium is doped. These internal oxides delaminate the bond coat from substrate and result in the failure of Ni-Al/YSZ TBCs after 700 cycles.