Research On High-temperature Oxidation Property Of Silicon-modified Aluminizing Coatings On Nickel-base Superalloy DZ4

Fail analysis of aircraft turbo-charger blades shows that most of cracks form and expand along the grain boundries which are vertical with principal stress. Therefore, eliminating these grain boundries can improve the resistance of turbine vane cracks, based on which directly solidification was developed. But, in high performance aircraft turbine engine manufacturing, only depending on advanced refrigeration, development of new superalloy and improvement of turbine vane manufacture techniques, are difficult to meet with the need of high temperature worm intensity and resistance of high temperature oxidation and hot corrosion. At present, the universal method of resolving these problems home and abroad is to exert protective coatings on the surface of turbine vane. Some researchers point out that preparation of Al-Si binary coating is simple and it has excellent corrosion resistance which can be compared with Pt2CrAl and MCrAlY coatingsMoreover, the lifetime of aircraft turbine engine is limited, so in order to prolong the lifetime, we re-prepares coatings on surface of superalloy DZ4. According to GB/T13303-91, the high temperature oxidation experiments were conducted at 1100℃for 300h. High temperature oxidation behaviors of the alloy with and without coating in static air have been investigated by X-ray diffraction (XRD) and scanning electron microscopy with an energy-dispersive X-ray analysis attachment (SEM/EDS). The oxidation kinetic curves of specimens were drawn, and the surface morphologies and cross-sections of specimens were examined. The phase structures of specimens were identified; the microstructure and chemical composition after longtime oxidation were also investigated to determine the degradation mechanism of the Al-Si coating.The main results are summarized as following: abundant NiO and a few acicularθ-Al₂O₃ formed in the surface of the parent cast alloy in earlier oxidation. After 300 hours, discontiguous spinal Al₂O₃ was observed in the surface, whereas NiO appeared in the fundus. Some Ni(Al,Cr)₂O₄ aslo appeared. Transition layer composed ofβ-NiAl andγ'-Ni₃Al formed between substrate and oxide scale in the process of diffusion. A mass of acicular carbide precipitated along grain boundary, and these carbide largely influenced mechanics performance of alloy.θ-Al₂O₃, NiO andα-Al₂O₃ formed fastly on the surface of Al-Si coatings in earlier oxidation at 1100℃. Inferior protective Cr₂O₃ and Ni(Al,Cr)₂O₄ formed with time prolongating. Almost allθ-Al₂O₃ transformed toα-Al₂O₃ at 300h and a continuousM₅SiC carbide zone formed between bond layer and substrate. The presence of continuous M₅SiC carbide zone in the interdiffusion zone slowed the inward diffusion of Al and outward diffusion of Ni in the coating, which prolongated the lifetime of this alloy. 50%β-NiAl transformed toγ'-Ni₃Al. An integrated and continuous alumina scale was detected on the surface of the Al-Si layer, inner oxidation was not observed.