Study On The Preparation Of Nanostructured TiB₂ Cermet Powder Used For HVOF

At present, researchers all around the world are focus on the study of ultrafine powders used in HVOF. Based on this background, this paper mainly studied various methods and processes aiming to synthesize nanostructured TiB₂ cermet powder, suitable for utilization in HVOF.In the investigation of metal reduction used for the synthesis of TiB₂ in this paper, magnesiothermic reduction and aluminothermic reduction were adopted.Self–propagation high–temperature synthesis（SHS） was induced by firing or heating the reactant with voltaic arc. Three aspects were different from the troditional process: using voltaic arc to fire the reactant; preforming transversely on the steel plate; reacting without inert gas shielding. The final powders were obtained after acidwashing, breaking and milling.Due to the high-temperature effect of the plassma arc, carbothermic method and boron carbide method were used in this paper to prepare TiB₂ with plassma arc to be the thermal source, heating up the billet on the steel plate. The final podwer was obtained after breaking and milling.Phase analysis was respectively made by X ray diffraction of the final powders botained with the three methods metioned above, morphological analysis was made with the scanning electron microscopy. Conclusion was drawed that TiB₂ can not be prapared with carbothermic method but can be generated by the other two methods, but the grain size can not reach nanosize.The method of high-energy ball milling was also introduced in the experiments, only the effects of the atmosphere in the pot and the milling time on the mechanical alloying （MA） course and reaction product were researched with other parameters invariable. X ray diffraction patterns, Scanning Electron Microscopy images and transmission electron microscope images of powders milling for 1 h, 2 h, 5 h, 10 h, 20 h, 60 h were all obtained, finding: MA took place and finished after milling for 5h, the resultant was pure, just including TiC and TiB₂. MA can be realized not just in the 99.99 % Ar protection, but also in the condition of both pure Ar before milling and no air intruding during milling. After the reaction, the following milling would make the powders continually much smaller, the fall rate of grain size was obvious from 5 h to 20 h but slow from 20 h to 60 h. After milling for 60 h, the mean grain size of TiC reached to about 10.2 nm with homogeneous distribution. But the size distribution of TiB₂ was wide, from about 50 nm to 400 nm. The synthesis mechanism of the method was considered to be self-propagating reaction induced by mechanical shock.It can be concluded from the comparaion of the four methods that the high-energy ball milling which realized the MA process was the best candicate to synthesize nanosized TiB₂ cermet powder used in HVOF and of great research value.