Improvement of thermal shock resistance for thermal barrier coating on aluminum alloy with various electroless interlayers

NiCoCrAlY/8YSZ coating was firstly directly deposited on aluminum alloy 5A06 by atmospheric plasma spray to make it applicable to short-time high temperature condition. The failure after thermal shock test was mainly due to the stress caused by thermal expansion mismatch between the bond coat and the substrate as well as the galvanic corrosion of the aluminum alloy. Ni–P, Ni–W–P and Ni–Cu–P as interlayers were electrolessly deposited on the substrate in order to mitigate the thermal stress. The composition and thermal transformation of the interlayers were investigated. Thermal shock resistance and bonding strength of multilayer coatings (interlayer/NiCoCrAlY/8YSZ) were tested. Diffusion layers mainly composed of AlNi, Al3Ni2 and Al3Ni were observed between the interlayers and the substrate after thermal shock test. The oxidation of the substrate was effectively inhibited. Ni–P interlayer obtained at lower pH value was superior to the other two interlayers and enhanced the thermal shock life from 38 to more than 200 cycles. With the application of the Ni–P and Ni–Cu–P interlayers, the bonding strength examined by pull-off test was also largely improved from 11.7 MPa to 18.8 and 19.0 MPa, respectively.