高择优取向TiB2电沉积层的制备及其表征

Preparation and Characterization of Highly Preferred Orientation TiB2 Coatings

摘要:

在KF-KCl体系中通过电沉积方法（直流电沉积CCP和周期断开电流电沉积PIC），以K2TiF6和KBF4作为活性物质在石墨基体上制备了高择优取向的TiB2镀层。用X射线衍射（XRD）研究了阴极电流密度对TiB2镀层晶粒尺寸、择优取向、微观应力及晶格常数的影响。结果表明，晶粒尺寸随电流密度增加而变小，这是过电位增加提高了形核速率所致，相对CCP而言，PIC制备的TiB2镀层具有更小的晶粒，这是因为PIC通过增加扩散层内离子的浓度减弱了浓差极化，提高了形核率所致；本实验条件下制备的TiB2镀层择优取向均为（001）面，这可用二维晶核理论来解释；无论CCP还是PIC，制备的TiB2镀层的晶格常数均和理论值有所偏差，这是镀层中存在的应力引起的；另外，还通过热力学预测、XRD和界面能谱（EDS）证实了镀层和基体间的结合为物理结合，界面处的裂纹可能是镀层和基体间热膨胀系数存在差异造成的。

Abstract:

Highly preferred orientation TiB2 coatings were obtained by electrochemical techniques of continuous current plating (CCP) and pulse current plating (PCP) in fluoride-chloride electrolytes (KF-KCl) containing K2TiF6 and KBF4 as the electrochemically-active components. The effects of cathode current density on the crystal size, preferred orientation, micro stresses and lattice constants were studied by X-ray diffraction (XRD). The results indicate that the crystal size gets smaller with the increase of the cathode current density due to the increase of the nucleation rate at the over-potential. Compared with the CCP, the crystal size of TiB2 coatings prepared by PIC is finer because PIC can weaken the concentration polarization and increase the nucleation rate by increasing the ion concentration of diffusion layer. The preferred orientation of TiB2 coatings prepared above is (001) plane at the experimental conditions, which can be well explained by the two-dimension crystal nuclei theory. The lattice constant of TiB2 coatings deviates from the theoretical value due to the stress in coatings. Besides, it is confirmed that the binding between TiB2 coatings and graphite substrates is the physical binding from the results of thermodynamic predictions, XRD and EDS (Energy Dispersive X-ray Spectrum). The existence of some cracks at the interface may be attributed to the difference between the thermal expansion coefficients of TiB2 and graphite.