The Study Of Molten Salt Electrolytic Boriding On Titanium

Being an important new structural material, Titanium is known as the ?third metal? with the research and development of new technology and the implement of energy conservation policy in the 21 st century. The characteristics of low hardness and wear resistance of titanium limit its application in the relevant industry. However, as Ti B2 has a series of advantages i.e. high melting point, high hardness, high strength and so on, boriding of Titanium was adopted in practice to improve surface hardness and wear resistance of titanium and its alloys and is thought to be one of economical and effective methods, which is also becoming one of the hot points in the field of metal surface engineering. In this thesis, the boriding of titanium via molten salt electrolysis was applied to improve the surface properties of titanium and the relevant process parameters and mechanism were investigated.Firstly, the molten salt with composition of 80%Na2B4O7: 20%Na2CO3(weight ratio) was used for the electrochemical boriding process. The cross-section morphology of the boride layer and the elements contents were measured by scanning electron microscope(SEM) and energy dispersive spectroscopy(EDS) respectively. X-ray diffraction(XRD) technique was applied to determine the phases of the boride layer. The surface micro-hardness of the boring sanple was tested by microhardness tester. A continuous Ti B2 phase and Ti B whiskers were obtained on the Ti substrate under the following operating parameters: boriding temperature of 910?, duration of 2 h and current density of 3000A/m2. The Ti B2 was formed on the outer layer of the sample and Ti B was formed on the inner layer. It was also found that the thickness of the borides layer obtained under the above condition is about 36 um, where the thickness of Ti B2 is 9um. After boronizing treatment, the surface hardness of the sample was 3 to 4 times higher than the matrix, significantly improving the surface hardness of the material and achieving the basic purpose of surface modification. And the influences of such processing parameters as current densentity, temperature and duration on the microstructure of the boriding lagyer were investigated.Secondly, as the Ti B2 layer was loose and was not bonded with the next layer tightly, the addition of rare earth oxide into the electrolyte was tried to solve the problem,which makes the boriding layer denser. The results show that when the composition of the molten salt was 80%Na2B4O7:16%Na2CO3:4%Ce O2 in weight ration, homogeneous and dense boriding layer was obtained.Thermodynamic calculations were carried out on the oxidation reactions of titanium and the borides and the related reactions of the broriding process and then the mechanism of the boriding on titanium was analyzed considering the experiment results. It shows that under an inert atmosphere, Ti B2 was firstly formed on the surface of the metal titanium, which then gradually generated to acicular whisker Ti B by reaction with Ti; in addition, some B atoms will diffuse through the Ti B2 lyaer, along the Ti B crystal to react with Ti and form Ti B; the Ti B whiskers penetrate into the matrix of Ti and closely ?pin? on it.The relationship between the reaction constant and the temperature was deduced to be Log10D=-1.95-4572/T according to the kinetic analysis of the boronizing process.