The Al <sub> 2 </ Sub> O <sub> 3 </ Sub> / Al Nano-wrapped Powders And Metal-based Gradient Coating Research

With the development of modern science and teehnology,there are higher requirements for the surficial performance of materials,and many methods of surface treatment come forth.The characteristics,the preparation methods,development, application of metal-based ceramic gradient coatings and nano-coatings were overviewed in the first part of this thesis.The advantages and disadvanges of the preparation methods and materials of the gradient coating were summarized.On this basis,it was proposed that metal-based Al₂O₃/Al gradient coating was prepared by using slurry method.In this coating,the Al powders was used for transitional material, and the nano-Al₂O₃/Al was used for reinforcement particles.In this paper,Al₂O₃ coated Al nanometer powders were synthesized by calcining the dry gel prepared from Al(NO₃)₃·9H₂O,nanometer Al and ammonium acid carbonate through heterogeneous precipitation method.The optimal solution concentration of raw materials and ratio were determined by experiments.TG-DSC, XRD,SEM,TEM,infrared spectrum analysis and particle size analysis were used to characterize the coating nano-powders.The Al₂O₃/Al composite powders with good dispersibility and uniform spherical particles were obtained when Al(NO₃)₃ solution concentration was 0.2mol/L,and the ratio of Al(NO₃)₃/NH₄HCO₃ was 1/8.The metal-based Al₂O₃/Al gradient coating was obtained through heat treatment by using aluminium dihydrogen phosphate as binder,sodium molybdate as corrosion inhibitor,aluminum powders as transition material,alumina as surface coatting material.The adhesion,bond strength,the number of thermal shock resistance, microhardness of the gradient coatings were tested by using multi-techniques.The result show that the optimal ratio of binder/aggregate was 0.55,the proper amount ofγ-Al₂O₃ in the aggregate was 10%,the optimal temperature of heat treatment is 700℃.The microstructure,composition and phase distribution were characterized by using SEM,EDS,XRD.It can be seen that chemical composition changes continuously,there was no remarkable interface because that the aluminum in the transition layer reacted with the ferrum in the metal substrate forming ferroaluminium which make the bonding strength higher because of element's interpenetration,it was found that the bond strength and thermal shok resistance of the gradient coating were better than single-Al₂O₃ ceramic coating.On the basis of Al₂O₃ coated Al nanometer powders and metal-based Al₂O₃/Al gradient coating,the gradient coating containing the Al₂O₃/Al nanometer panicles was prepared.It were found that the microhardness and thermal shock resistance of the gradient coating with 0.7wt%nanometer powder were better than those of the coating without nanometer podwer.