近相变温度下钛表面融盐电解法渗硼的渗层强化生长机制

As an effective method for improving the surface hardness and wear resistance of the titanium metal and its alloys, molten salt electrolytic boriding on titanium surface has the advantages of low cost and effectiveness. However, the thickness of the boriding layers were limited by the diffusity of the boron atoms through the solid phase. Hence how to increase the diffusity of the boron atoms will be the key problems for the molten salt electrolytic boriding process. In this proposal, the properties of the enhanced self-dissusion for Ti near phase transition temperture will be applied to the boring of titanium with the aim to enhance the diffusity of the boron atoms and to enhance the growth rate of the boriding layer. . In this proposal, the material characterizing methods of SEM, TEM and XRD etc. will be applied and the theory of crystallography and solid state diffusion be referred to in carrying out the following research work: first, influences of the surface morphology and crystal defect of titanium metal on the microstructure of the boriding layers during the molten salt electrolytic boriding process will be investigated, and the diffusion behavior of the boron atom in the crystal defect of metal will be analyzed; then the crystal structure of the boriding layers near the phase transition temperature will be invstigated, the enhanced diffusion behaviour of boron atom near phase trasition temeprature and then the growth mechansim of the TiB2, TiB crystals will be analyzed; finaly the enhanced growth mechansim of the dual boride layers and grow kinetics will be summarized and the kinetic model will be constructed. The research work in this proposal aims at revealing the diffusion kinetics of boron atom near phase transition temperature, seeking for methods for enhancing the growth of boriding layers, and it will provide a basis for the optimization and development of the technology of molten salt electrolytic boriding on the surface of titanium in the future.

钛表面熔盐电解法渗硼具有成本低廉、效果明显等优点，可有效提高钛及合金的表面硬度和耐磨性。但是渗层的厚度受限于B原子在固相的扩散速率，如何提高B的扩散速率是渗硼过程的关键问题。本项目拟将Ti在近相变温度下的自扩散强化特性应用于钛的表面渗硼过程，以强化B原子的扩散速率，进而强化硼化物渗层的生长速率。. 拟借助SEM、TEM、XRD等材料表征手段，运用晶体学及固体扩散理论研究钛的表面形态和晶体缺陷对渗层结构的影响，分析B原子在表面缺陷处的扩散行为；研究近相变温度下硼化物渗层的晶体结构，分析B原子在固相中的强化扩散行为，分析TiB2、TiB晶体的长大机理；研究近相变温度下双相硼化物渗层的强化生长机制及生长动力学，并建立相应的动力学模型。本项目旨在揭示钛表面融盐电解渗硼过程中近相变温度下B原子的扩散规律及扩散动力学机理，寻求渗层强化生长的方法，为后续钛表面渗硼工艺的优化和开发提供参考。

为提高钛及钛合金表面硬度及耐磨性的一种有效手段，是近年来钛合金表面改性领域的研究热点之一。熔盐电解法渗硼具有成本低廉、效果明显、渗层厚度可控的优点。.本项目对钛表面熔盐电解法渗硼的工艺参数及机理进行了研究，具体如下： 分别采用Na2B4O7-Na2CO3及CaCl2-Na2CO3熔盐体系在金属Ti表面渗硼，分析电流密度、温度及渗硼时间对渗层结构的影响，结合渗层表面硬度测试，确定了渗硼工艺参数；研究了不同熔盐体系的工艺参数对渗层结构及表面性能的影响；结合循环伏安法及示差扫描热量(DSC)-热重(TG)分析，对渗硼机理进行了分析；对近相变温度（910~920℃）下的渗层结构及渗硼基体冷却后的显微组织进行了分析；在分析渗层结构及渗层长大机理的基础上对渗层生长的动力学进行了研究。.（1）采用Na2B4O7-Na2CO3熔盐体系对工业纯钛TA2进行熔盐电解法渗硼，当电流密度为3000A/m2、温度为910℃、保温2h可在钛表面得到双相硼化物渗层，试样外表层是较为致密的TiB2，内表层形成针状TiB组织。所获得的渗层厚度为36um，渗硼后钛的表面硬度可达到805HV；.（2）采用CaCl2-Na2CO3熔盐体系，当电流密度为500A/m2、温度为920℃、保温4h获得的渗层中TiB2厚度约为8um，TiB晶须渗入到基体的最大深度为26.6μm，渗硼试样的表面硬度可达到2696HV；.（3）近相变温度（910~920℃）下由于存在着B原子的强化扩散作用，所得渗层的TiB晶须较长，同时渗硼基体冷却后得到的等轴α相组织拉伸塑性和疲劳强度都较高，综合性能良好。.（4）钛表面的渗硼机理如下：钠离子在钛表面不断被还原，得到的钠原子再与B2O3反应，在钛表面生成大量的B原子， B原子在浓度差和高温的作用下，通过β-Ti间隙扩散到钛基体中与钛发生反应，先形成TiB2晶体；然后，渗层下部的TiB2会与Ti反应而生成TiB；同时新生成的B原子扩散通过TiB2渗层，进而沿着TiB晶须扩散，与晶须间的Ti反应也会生成TiB。.（5）得到渗层生长动力学方程。.本项目的研究成果对于钛表面渗硼工艺的优化和开发具有一定的参考意义。