The Preparation Of YSZ Thermal Barrier Coatings With Interlayer On Mg Alloy Substrate And The Study About Its Thermal Shock Resistance And Corrosion Resistance

NiCrAlY/YSZ thermal barrier coatings(TBCs)were directly deposited on the surface of magnesium alloy with atmospheric plasma spraying(APS)method,and the thermal shock life of this as-mentioned TBCs was extremely short.In order to improve the thermal shock life of the TBCs on Mg alloy,Ni-P and Ni-P/Al,Ni-P/Al/Ni-P interlayers between the Mg alloy substrate and NiCrAlY bond coat were fabricated by electroless plating for Ni-P layer and atmospheric plasma spraying(APS)for Al layer.Subsequently,thermal barrier coatings(TBCs)comprising of NiCrAlY as bond coat and SYSZ as top coat were successively deposited on the above pre-treated AZ91D magnesium alloy substrates by APS.The thermal shock life of YSZ TBCs on Mg alloy substrate with Ni-P,Ni-P/Al and Ni-P/Al/Ni-P interlayers as well as the blank sample of YSZ TBCs on magnesium alloy were heated at 400?.The phase compositions and the microstructures of the above four as-prepared samples before and thermal shock test were analyzed by X-ray diffractometry(XRD),energy dispersive spectrometer(EDS)and scanning electron microscopy(SEM).The "cauliflower" structured Ni-P layer(?30?m)was dense and has no pores or cracks,as well as it was metallurgically bonded to the magnesium alloy or Al layer tightly.The thickness of the plasma sprayed aluminum layer is?100 ?m,there is no obvious crack but a small amount of pores exist.Moreover,the Al layer and the Ni-P layer are uniformly distributed and tightly connected.The NiCrAlY/YSZ coating has a lamellar structure and mechanically engages with the intermediate layer,and the entire coating was complete.After suffering thermal shock with 5 cycles,the YSZ TBCs without interlayer peeled off form magnesium alloy.By contrast,YSZ TBCs with Ni-P,Ni-P/Al and Ni-P/Al/Ni-P interlayers flaked after 61,91 and 226 thermal shock cycles respectively.The preparation of interlayer greatly improved the thermal shock life of YSZ TBCs on Mg alloy substrate.The intermediate layer provides anti-oxidation and anti-ablative protection for the Mg alloy substrate during the spraying process,and at the same time improves the galvanic corrosion of the coating against water in the water quenching process.Nearly 2/3 surface area of upper coating peeled off in TBCs without interlayer during the heating-water quenching thermal shock test.The thermal shock failure was mainly caused by the co-effect of the thermal oxidation of the Mg alloy during the spraying process and the mismatch thermal expansion between the coating and the substrate as well as the corrosion of the water quenching process.About 1/2 surface upper coating in YSZ TBCs with Ni-P interlayer peeled off at the interface between Ni-P and Mg alloy substrate during thermal shock test.During the water quenching process,the water would get through the interface between magnesium alloy and Ni-P interlayer,the active Mg alloy would serviced as the anode and suffered the galvanic corrosion.Meanwhile,the mismatch expansion of substrate and coatings led to the thermal stress was constantly accumulated in each coating.Finally,the corrosion stress and thermal stress caused the thermal shock failure of upper coating.The whole upper coating peeled off in YSZ TBCs with Ni-P/A1 interlayer.The micro-morphologies indicated that the active Mg alloy suffered more severe galvanic corrosion than the aluminum layer as the water medium penetrated into the interface between heterogeneous metals during the water quenching process.Meanwhile,the thermal stresses caused by the mismatch thermal expansion coefficient(TEC)of NiCrAlY bond coating and Mg alloy would constantly accumulated and acted on Al interlayer.The co-effect of corrosion stress and thermal stress induced the Al interlayer fractured,which leads to thermal shock failure of YSZ TBCs with interlayer.The whole upper coating peeled off in YSZ TBCs with Ni-P/Al/Ni-P interlayer.The thermal shock failure of this as-prepared sample was still the result of the double coupling effect of galvanic corrosion stress from Mg alloy and thermal expansion mismatch stress from each coating.But the Ni-P/Al/Ni-P sandwich intermediate layer has the best corrosion protection for the magnesium alloy and significantly reduces the thermal expansion of the coating to the substrate,so that the thermal shock life of the coating is significantly improved.