Study Of Preparation And Performance Of Micro Plasma Oxidation Ceramic Coatings On Titanium Alloy

Titanium alloy denominated The Third Metal Growing Up is progressively used in so many new and high technology fields of aerospace, petroleum, atomic energy, shipping, surgery implant and so on, because it has a series of excellent properties, such as low density, high specific strength, thermal stability, corrosion resistance, fatigue resistance, etc. But titanium alloy appears poor wearing resistance, easily conglutinating by heated, reduction of plasticity and strength easily resulted from high temperature oxidation and hydrogen uptake, crack corrosion under situations, such as high temperature or pH<2, and higher standard electrode potential –1.63V leading other materials contacted with it to erode, which limits application of titanium alloy. Therefore, it is very important that titanium alloy should be surface-treated in order to improve the performances in use. In the thesis, Micro Plasma Oxidation (MPO) was applied on preparing the ceramic coatings in-situ on TC4 titanium alloy. Three electrolytes in different systems and relevant processing parameters were mainly studied. The factors on performances of ceramic films were analyzed. Then, microstructure and phase composition were researched by SEM and XRD. At last, forming mechanism of MPO ceramic coatings was also elementarily discussed. The three optimized electrolytes and corresponding processing parameters have been obtained by study. And some laws as follows: (1)relation between different electrolyte systems and the ceramic coating's performances; (2)relation between the current density and the ceramic coating's performances; (3)relation between the ultimate voltage and the ceramic coating's performances; (4)relation between the processing time and the ceramic coating's performances. It is shown that the ceramic coatings are mainly composed of a-TiO₂, r-TiO2 and Al₂O₃·TiO₂ from electrolyte. The thickness of coating is 10⁴0μm, and the hardness is 600⁷50HV. Simultaneously, there are many micro-poles surrounded by plasma sinters on coating surface through SEM. The preparation of ceramic coatings has two stages. The first step is initial anodizing which forms the TiO₂ film under electrochemical reactions proceed to between the titanium alloy and the electrolyte, which is the essential processing for plasma discharge. And the second step is plasma discharge. Some fusants are formed by chemical, electrochemical, plasma-chemical actions, and then which are forced out by heavy pressure and shock chilled by electrolyte. So the ceramic coating is prepared.