Thermal Barrier Coating Bond Layer (Substrate) Material

       The bonding layer is one of the most critical components in the thermal barrier coating system. It can relieve the stress caused by the mismatch between the thermal expansion coefficients of the ceramic coating and the substrate and improve the oxidation resistance of the substrate alloy. Substrate to improve physical compatibility of coating and base alloy. The composition of the bonding layer plays a decisive role in the growth rate, composition, integrity, bonding force with the substrate and exfoliation behavior of the thermal oxide of the bonding layer during thermal cycling. The bonding layer material should not form a brittle phase and form a good interfacial diffusion resistance with the metal matrix to reduce the degradation of the performance of the matrix and the bonding layer during service.



The metal bonding layer material is usually MCrAlY (M is the transition metal Fe, Ni, Co or Ni + Co). Because Fe2O3, CoO, etc. are prone to chemical reactions with the monoclinic phase or cubic phase of ZrO2 at high temperatures, the stability of ZrO2 ceramics is reduced. Therefore, FeCrAlY and CoCrAIY are not suitable for the bonding bottom layer of thermal barrier coating. Because the NiCoCrAIY bonding layer has good comprehensive properties of oxidation resistance and hot corrosion resistance, most of the bonding layer materials used in aero-engine turbine blades currently use this alloy system. MCrAIY coating is a cladding coating, and its alloy composition can be adjusted according to different usage conditions. The anti-oxidation mechanism of the coating is that in a high temperature environment, Al in the MCrAIY layer will diffuse outward, and selective oxidation will occur on the surface of the bonding layer, forming a dense Al2O3 protective film to prevent further oxidation of the bonding layer. , so as to achieve the purpose of protecting the substrate. Al is an essential element for the formation of Al2O3 oxide film. High Al content can prolong the life of the coating under high temperature oxidation conditions, but at the same time reduce the toughness of the coating. Therefore, in order to maintain the fatigue resistance of the coating, the Al content should be reduced as much as possible on the premise of ensuring the oxidation resistance of the coating. Usually the content of AI in NiCoCrAIY alloy is controlled at 8% ~ 12% (mass fraction). The component Cr is mainly used to improve the oxidation resistance and vulcanization resistance of the bonding layer. Under high temperature conditions, after the Al in the bonding layer is preferentially oxidized, Cr will continue to form a Cr2O3 film between the aluminum oxide film and the bonding layer, which plays the role of shielding the base alloy, and it can also promote the formation of Al2O3 film. . However, if the oxide film is too thick, the bonding force between the layers will be reduced. When the thickness of the oxide film between the MCrAIY layer and the ceramic layer reaches 3 μm to 5 μm, the ceramic layer will be peeled off. At the same time, adding 0.3%~1% (mass fraction) of trace rare earth element Y can play the role of oxide pinning and grain refinement, thereby improving the bonding force between Al2O3 film and substrate under thermal cycling conditions, and improving the resistance of the coating. Thermal shock performance. In addition, other alloying elements such as Si, Hf, Ta, Zr, etc. can be added to the coating to improve the mechanical properties and oxidation resistance of the coating.



In order to improve the oxidation resistance of adhesive layer materials, researchers have taken a series of measures to deposit or prefabricate a thin film with oxidation resistance or a protective oxide layer on the surface of the adhesive layer that is easy to form at high temperature. Schmitt-Thomas et al. used CVD or sputtering to prepare a 2μm ~ 5μm a-Al2O3 barrier layer on the surface of the MCrAlY coating, which greatly reduced the oxidation rate of the bonding layer. Since the bonding strength of YSZ surface layer and a-Al2O3 is better than that of YSZ and other phases, it helps to improve the life of thermal barrier coating. At the same time, Schmitt-Thomas et al. also used the electroplating method to prepare (Ni,Pt)Al with a thickness of about 28 μm as an oxygen barrier layer on the bonding layer. This improvement can significantly improve the oxidation resistance of the thermal barrier coating. The bonding layer forms Al2O3, PtAl2, Pt2Al3 and PtAl phases in sequence on the surface, which significantly improves the oxidation resistance of the coating. Lih et al. performed pre-oxidation or surface aluminizing treatment on the MCrAIY coating, which made it easier for the coating to form an Al2O3 protective oxide film on the surface of the MCrAIY during the oxidation process, thereby improving the thermal cycle life of the coating. In addition, Tolpygo et al. prepared the (Ni,Pt)Al bonding layer by CVD method. The Pt-rich phase on the surface restricts the diffusion of matrix elements, especially transition metal elements and active elements Hf to the coating surface. Hf promotes the selective oxidation of Al to form Pure Al2O3, and reduce the oxidation rate, so that the bonding layer has better adhesion.