Numerical Analysis And Interfacial Performance Improvement Of YSZ Thermal Barrier Coatings On Low Heat Rejection Internal Combustion Engine

In this dissertation,8YSZ ceramic coat and CoNiCrAlY bond coat weredeposited on aluminum substrate and cast iron substrate employed in low heatrejection internal combustion engine by atmospheric plasma spray (APS) and highvelocity oxgen-fuel spray (HVOF) process due to the excellent thermal insulation andcorrosion resistance properties of thermal barrier coatings (TBCs), whose service lifetime and failure mechanism were investigated. To avoid the failure of TBCs at theinterfacial region caused by thermal mismatch between coatings and substrate, animprovement of TBCs was clarified by employing the Ni-based electroless plating asinterlayer. The effect of interlayer on the bonding strength, service life, failure modeand failure mechanism of coatings were investigated. The main research results werelisted as follows:Firstly, a3D model of piston coated with TBCs was established by ANSYS code.The temperature and stress distribution in TBCs on aluminum and cast iron pistonwere investigated, which is helpful to clarify the failure mechanisms of coatings bymechanical approach. The results indicated that the deposition of TBCs couldsignificantly raise the temperature of piston crown. As the thickness of top coatincreases, the surface temperature of ceramic coat increases while the surfacetemperature of substrate and stress amplitude both decreases. The stress amplitude atthe interface between bond coat and substrate was very high for two kinds of substratewhich may results in the failure of coatings at the interface between bond coat andsubstrate.Secondly, the thermal shock resistance and failure mode of TBCs on2A70aluminum were investigated by thermal cycling test. The failure mechanism of twolayer TBCs was discussed and the effect of electroless plating was also evaluated. Theresults indicated that the service life time of TBCs on aluminum substrate was about60cycles. The Mg and Al have relatively higher chemical activity than that of otherelements in substrate and could combined with O element at the surface of substrateand cracks forming the brittle oxide layer, which would break and led to the crackpropagation under the condition of alternative stress. The addition of interlayersignificantly improves the service life of coatings. The lifetimes of TBCs with Ni-P interlayer and Ni-Cu-P interlayer increase to approximately600cycles and300cycles,respectively. The interdiffusion between interlayer and substrate resulted in theformation of Ni-Al diffusion layers and particles, which have high brittleness and lowdeformability and would result in the failure of coatings at the interface betweendiffusion layer and substrate.Finally, the thermal shock resistance and failure mode of TBCs on QT-500castiron were studied to clarify the failure mechanism of TBCs on cast iron under waterquenching condition. The method of prolonging the thermal shock life of TBCs oncast iron by using the electroless plating as interlayer was tested. The research showedthat the service life of common TBCs was about1000cycles. The coatings withinterlayer was intact after1400cycles. An oxidation zone mainly consisting of Fe andSi oxide was located at the surface of substrate without interlayer. By contrast, for theTBCs with Ni-P interlayer, double-layer oxide layer was found at the bondcoat/interlayer interface and interlayer/substrate interface after the Ni-P interlayerfailed.The addition of interlayer could significantly improve the thermal shockresistance of TBCs on aluminum and cast iron substrate. The Ni-P interlayer couldrelieve the thermal mismatch between coatings and substrate. Besides, the Ni-Pinterlayer could also improve the corrosion resistance of cast iron substrate. Generallyspeaking, the method of employing the electroless platings as interlayer betweenTBCs and aluminum substrate or cast iron substrate is a technical solution ofimproving the thermal shock resistance of TBCs.