Microstructure And Properties Of NiAl And Its Composite Coatings Fabricated By Mechanical Alloying

During the process of mechanical alloying, the metal powder undergoes the repeated crushingand cold welding. Due to the repeated collisions and cold welding, a fraction of powder tends toadhere to the surface of the inner wall of chamber and balls, so as to form a coating layer of certainthickness. Based on this phenomenon, a planetary high-energy ball milling was applied to produceNiAl coating and NiAl-TiC-Al₂O₃composite coating on carbon steel substrates. Then the certaincoatings were annealing treatment. The phase composition microstructures and elements distributin ofmechanically alloyed coatings at different milling durations and different milling speed were studiedusing XRD, SEM, EDS and TEM. Micro-hardness test, scratch adhesion test, high temperatureoxidation experiment, electrochemical corrosion technique and friction and wear experiment werecarried out to inspect the properties of the coating layer.The results show that in the preparation of NiAl intermetallic compound coating, prolongedmilling time increased the thickness and dense of the coating, but milling speed to the thickness, thereis an optimal middle value. The coating with the largest thickness, about150μm, were obtained bysetting a bowl rotation speed of350r/min and a ball milling time of5h. Coating performance testsshow that, the microhardness of NiAl coating decreased with increasing depth gradient-likedistribution. The maximum hardness of the coating surface up to559.8HV_0.1, improved significantlycompared to the original substrate. The adhesion strength between the coating and the substratesreached the maximum value74.2N, not just a mechanical bond, but also the role of chemical force. Inaddition, the oxidation resistance and erosion resistance were higher than that of the substrate.The NiAl-TiC-Al₂O₃composite coating was successfully deposited on the steel substrate. Thecoating with the largest thickness and micro-hardness was obtained by setting a bowl rotation speed of400r/min and a ball milling time of9h. The diffusion of Al and Fe atoms at the interface led to a Fe-Aldiffusion layer possessing a metallurgical bonding characteristic. With the addition of TiC and Al₂O₃,the oxidation resistance and wear resistance was improved greatly.Very dense and homogeneous structures have been obtained by annealing of the selectivecoatings.The annealing of the NiAl coating has resulted in significant formation of the （Fe, Ni） solidsolution due to inter diffusion of atoms, and disappearance of the hardness gradient in the coating.During the annealing NiAl-TiC-Al₂O₃composite coating, there was an solid state reaction withproducts of Ni3Al and AlN. However, cracks appeared in the interface after annealing treatment, and the density and uniformity has decreased because of a large number of holes in the coating.Based all the results, the author put forwards thereaction mechanism during ball milling and theformation mechanism of the NiAl coating.