| Metallic titanium has numerous potential applications owing to its outstanding corrosion resistance and biocompatibility.Unfortunately,due to its high cost,it has not been widely used in anticorrosion and biomedical fields so far.Chemical vapor deposition(CVD)of titanium films is inexpensive and thus the most promising method for expanding the application of metallic titanium.However,in the main reaction system for CVD of titanium films,the CVD reaction principle of titanium subchlorides(TiCl2 and TiCl3)as titanium precursors is still not clear,and the CVD process is not optimized.Resultantly,several acute problems arise.First,the deposition temperatures are so high that the commonly used metal substrates cannot withstand.Second,the nature of the deposited films is titanium oxide rather than metallic titanium.Third,there is a lack of optimization for the basic anticorrosion performance of titanium film.These have hindered the practical application of the CVD titanium film.To address the aforementioned issues,the novel idea was proposed to synthesize titanium subchlorides,subsequently investigate the CVD reaction principle and optimize the CVD process directly using titanium subchlorides.Accordingly,the superior precursor between titanium subchlorides was selected,and its synthesis,thermal stability and vaporization behavior were systematically studied.Based on these,the CVD reaction principle of the preferred precursor was experimentally examined,and the related deposition behaviors such as low-temperature deposition and oxygen incorporation mechanism were thoroughly explored.Moreover,the anticorrosion performance of CVD titanium coatings on copper,nickel and 316L stainless steel in 3.5 wt.%NaCl solution and the anticorrosion mechanism of CVD titanium coatings were studied.The main innovative findings achieved are as follows:(1)TiCl2 was found to be a better CVD precursor of metallic titanium than TiCl3,and a selective and rapid synthesis method of TiCl2 at low temperatures was established based on fluidization.Thermodynamically,titanium can be deposited using TiCl3 only at 900-1300℃ and an extremely low deposition efficiency(approximately 2.4%),while titanium deposition using TiCl2 can reach a much higher deposition efficiency(approximately 32%-35.5%)at reduced temperatures(<900 ℃).TiCl2 was therefore selected as the preferable precursor.During the fluidized-bed synthesis of TiCl2 using TiC14 vapor and titanium powder,the partial pressure of TiCl4≤12.2 kPa prevented its excess and the resulting production of TiCl3,hence the selective synthesis of TiCl2.Reaction temperature<625℃ inhibited the defluidization due to the melting of TiCl2 on the surface of titanium powder,yielding the formation rate of TiCl2 at 625℃ twice that at 1000℃ reported.Finally,the kinetic equation for the synthesis of TiCl2 from titanium and TiCl4 under chemical reaction control was obtained,in which the activation energy was 102.92 kJ/mol,and the reaction order for partial pressure of TiCl4 was 0.55.(2)The thermal stability of TiCl2 was ascertained,and the vapor pressure data of TiCl2 in a wider temperature range was obtained.When solid TiCl2 is heated up,no obvious chemical reaction except the crystal transformation occurs,and TiCl2 begins to sublime at about 500℃ and melt at about 640 ℃.The enthalpy values of sublimation and vaporization above 640℃ were 257 kJ mol-1 and 112 kJ mol-1.The vapor pressure of TiCl2 was determined by the thermogravimetric method,expanding the temperature range for the vapor pressure data of TiCl2 from no higher than 627℃reported to 865℃.(3)The CVD reaction principle from TiCl2 to titanium was revealed,and the low-temperature deposition of low-oxygen titanium film was achieved through the optimization.TiCl2 was experimentally confirmed to be the CVD precursor of metallic titanium.Its CVD reaction principle was mainly 3TiCl2(g)→Ti(s)+2TiCl3(g),and TiCl2(g)→Ti(s)+TiCl4(g)occurred merely at 800-1400 ℃.Owing to the unique feature of TiCl2 that it disproportionates and produces titanium deposits immediately after its low-temperature vaporization,the temperature for CVD of an appreciable titanium film is substantially reduced from above 1000℃ reported to 620℃.Furthermore,the selective and efficient synthesis of TiCl2 ensures its sufficient partial pressure during the deposition,allowing the deposition rate of titanium much higher than the dissolution rate of the trace oxygen in high purity argon into titanium.Consequently,a low-oxygen titanium film is deposited(4)The pitting corrosion resistance of copper,nickel and 316L stainless steel was optimized by using CVD titanium coatings,and the corrosion protection mechanisms of CVD titanium coatings were revealed.The low porosity of the coating and its remaining high titanium content after the thermal diffusion are the fundamental causes of its superb pitting corrosion resistance.The high titanium content of the coatings forms stable passive surface films mainly composed of TiO2 in 3.5 wt.%NaCl solution.Additionally,the low porosity of the coatings generates large ion diffusion resistance and charge transfer resistance.Thus the CVD titanium coatings provide adequate protection of the substrates against pitting corrosion.For example,the titanium coating on copper has low porosity and the CuTi surface layer,reducing the corrosion rate in the passive state to about 1%that of pure copper.The titanium coating on nickel has low porosity and the NiTi or thin NiTi2 surface layer,increasing the pitting potential from-0.07 V for pure nickel to about 0.08 V or 0.26 V,respectively.The titanium coating on 316L stainless steel has the dense NiTi2 surface layer,resulting in a higher pitting potential(1.2 V)than those of the titanium coatings prepared by physical vapor deposition.These studies show that CVD titanium film can significantly improve the corrosion resistance of metals in salt water,and thus promises to be utilized in anticorrosion and biomedical fields.This study has laid a foundation for the practical application of CVD titanium film. |
