Preparation Of TiB₂and ZrB₂ Coatings By Electrophoretic Deposition In Molten Inorganic Salts Containing Nanoparticles

Nanoparticles suspended in molten inorganic salts can form stable colloids.Therefore,the availability of electrophoretic deposition of nanoparticles in molten inorganic salts is explored in this work.It is well-known that transition-metal diborides,characterized by high melting point,high hardness,high thermal and electrical conductivity as well as good high-temperature oxidation resistance,are suitable for various applications such as coatings against wearing and liquid metal corrosion,and cathode material for the Hall-Héroult cell.In this work the electrophoretic deposition of transition-metal diboride nanoparticles in molten inorganic salts were investigated.Firstly,the availability of electrophoretic deposition of various types of nanoparticles in two types of molten salts（Na Cl-KCl-Al Cl₃and Na F-Al F₃）was explored.It has been found that in molten Na Cl-KCl-Al Cl₃system（710℃）,WC,TiB₂and ZrB₂nanoparticles can be electrophoretically deposited on the cathode substrate and in molten Na F-Al F₃salts（980℃）,the electrophoretic deposition of Ti O₂,Ti N,TiB₂and ZrB₂can be achieved;while the electrophoretic deposition of Ti C,Zr O₂,Si and carbon nanotubes failed.In molten Na F-Al F₃salts（61:39mol）containing TiB₂nanoparticles（mean size:50 nm）at 980℃,the electrophoretic deposition of TiB₂resulted in formation of dense TiB₂coatings on Mo or graphite substrates with an applied cell voltage of 1.2 V.The TiB₂coating on Mo substrate had a thickness of around 12μm,and its surface consisted of large nanoplates with a thickness of 100-200 nm and length of around500 nm.The existence of a large amount of low-angle（less than 5°）grain boundaries was conformed by TEM.The hardness and elastic modulus of this TiB₂coating were calculated as 43.7±2.8 GPa and 597±41 GPa assessed by nano-indentation tests.The electrophoretically deposited TiB₂coating on a graphite substrate,with a thickness of approximately 50μm,had micro-hardness of 3585±139HV_0.1（36.6±1.4GPa）.In molten Na Cl-KCl-Al Cl₃（10 mol%）salts containing ZrB₂nanoparticles（mean size:50 nm）at 710℃,compact ZrB₂coating with a thickness of around 2μm on a graphite substrate has been prepared by electrophoretic deposition with an applied cell voltage of 1.5 V.The broadening of diffraction peaks observed from the XRD result and high resolution TEM characterization,indicated the existence of its nanocrystal structure.Its hardness and elastic modulus was obtained as 20.8±1.7GPa and146.4±20.6GPa,respectively,by assessment of nano-indentation tests.In addition,in the same system,different cell voltages（1.2V,1.4V,1.5V,1.6V）were applied to investigate the influence of the cell voltage on the electrophoretic deposition.With the increase of the voltage,the thickness of the ZrB₂coating increased;while at 1.6 V the thickness decreased.According to EDS analysis,the ZrB₂coating prepared by electrophoretic deposition with an applied cell voltage of 1.2V and 1.4V did not contain chlorine element.