Study On The Endogenous Mechanism Of Oxide Diffusion Barrier In The Aluminide Coating System

The diffusion of elements between aluminum coating and substrate is an important reason for the failure of the coating in the high temperature environment,and a layer of diffusion barrier between the aluminum coating and the substrate can play a role in inhibiting the diffusion of elements.The studies show that aluminum coating system containing oxide particles that is prepared by the“composite electrodeposition and aluminizing”process can form diffusion barrier by itself at high temperature and the diffusion barrier has a strong inhibitory effect on the diffusion of elements.But at present,there is little research on the formation mechanism of oxide particle diffusion barrier in this system.In this paper,the Ni–CeO₂ film and Ni–Al₂O₃ film were prepared by composite electrodeposition technology in which the mass percentage of nano CeO₂ particles were about 1%,2%and 3%,and the mass percentage of nano Al₂O₃ particle is about 0.5 at first;In the second,the Ni2Al3/Ni coating systems with CeO₂(particle size is about 15-30nm)and Al₂O₃(particle size is about 15-30 nm)were prepared by aluminizing treatment which means the application of high active powder coating technology and the control of aluminizing time the coating is not permeable.In the third,the samples were annealed at1000?in the vacuum condition,and the element distribution and morphology of the samples before and after annealing were analyzed by means of microscope analysis technique and scanning electron microscope analysis technique to study the effect of oxide particle content on the formation of diffusion barrier and the mechanism of the formation of diffusion barrier.The results are as follows:(1)Although the contents of oxide particles in the aluminide coating system were different,they all had the phenomenons that the boundaries that were far from the surface of aluminide coating moved into the coating,the thickness of the aluminide coating increased and CeO₂ oxide particles were gathered at the coating interface with vacuum annealing and the CeO₂-rich layers could block the interdiffusion and form oxide diffusion barriers.The effect of oxide diffusion barrier on blocking interdiffusion of elements was increased with the increase of the content of oxide particles in the aluminide coating system.(2)The aluminide coating system contained nano Al₂O₃ oxide particles also had these phenomenons that the inner boundaries moved into the coating,the thickness of the aluminide coating was increased and Al₂O₃ oxide diffusion barrier that could block interdiffusion of elements at the interface of the aluminide coating was formedin the process of vacuum annealing.(3)The formation mechanism of oxide diffusion barrier is”interfacial drag mechanism”in the aluminide coating system:the oxide particles are draged by the interface movement caused by degradation of the aluminide coating and gather at the interface to form oxide diffusion barrier.(4)The formation of oxide diffusion barrier can be divided into three stages in the aluminide coating system:(1)The transformation stage from?-Ni₂Al₃ phase to the phase of?-NiAl phase;(2)The stage of the inter-transformation between?-Ni₃Al phase and?-NiAl phase,oxide particles movement with coating interface movementand and aggregation at the interface;(3)The formation stage of the oxide diffusion barrier.