Localized plasma electrolytic oxidation of aluminum by focusing the discharges in the electrolytic free jet process (Jet-PEO)

等离子体电解氧化（PEO）是一种生产氧化物转化涂层的工艺，非常适合作为腐蚀屏障和促进金属-塑料复合材料界面的附着力。到目前为止，在工业规模中，PEO方法仅在电解质浴中进行。关于在无毒无氟电解质中用于Al合金的PEO的约500 V的高工艺电压和约30 A/dm²的电流密度，通过将PEO工艺限制于选定的功能表面（例如，连接表面）。根据现有技术，这只能通过用极粘性、高度电绝缘和极耐化学和热的覆盖物进行掩蔽来实现，这些覆盖物难以应用和移除。使用封闭的电解质自由射流（Jet-PEO）的PEO代表了一个有前途的可能性，无掩模，局部有限的生产PEO层。因此，通过电解液射流的受控运动，以前无法在电解槽工艺中处理的部件可以局部或大面积进行PEO处理。与浴法相比，涂层性能可以局部调节。Jet-PEO的可行性已经在无氟电解质的初步工作中得到了证明。然而，目前不可能有针对性地生产具有低厚度和明显多孔外层（其特征在于许多孔和底切）的粘附促进PEO层。因此，本项目旨在科学地理解Jet-PEO的工艺特性、氧化物微观结构和表面性质之间的关系。首先，使用电化学极化和点火测试在电解质设计内获得具有最低可能点火电压和高层形成速率的稳定Jet-PEO过程的必要基础知识。建立了基于电化学参数、放电特性与PEO膜微观结构和形貌之间关系的有限元模拟模型。验证和校准的仿真模型的层的微观结构和形态的时间和空间分辨的发展进行使用的实验装置与线状反电极。多物理有限元模拟用于设计自由射流过程中的流体力学，射流几何形状和电气过程控制。利用Jet-PEO工艺，最终在衬底表面上实现点状和线状氧化物结构。氧化物结构的特征在于，特别是相对于使用微毛细管池的腐蚀保护，并通过使用分形算法评估的表面形貌。

Plasma electrolytic oxidation (PEO) is a process for the production of oxidic conversion coatings, which are perfectly suitable as a corrosion barrier and for adhesion promotion in the interface of metal-plastic composites. In the industrial scale, the PEO process has only been carried out in an electrolyte bath, so far. Regarding the high process voltages of around 500 V and current densities of around 30 A/dm² for the PEO of Al alloys in non-toxic, fluoride-free electrolytes, it makes sense to save energy by limiting the PEO process to selected functional surfaces (e.g., joining surfaces). According to the state of the art, this is only possible by masking with extremely adhesive, highly electrically insulating and chemically and thermally extremely resistant covers, which are difficultly applicable and removable. PEO using a closed electrolyte free-jet (Jet-PEO) represents a promising possibility for the mask-free, locally limited production of PEO layers. Components that previously could not be handled in the bath process can thus be PEO treated locally or over a large area through the controlled movement of the electrolyte jet. In contrast to the bath process, the coating properties can be adjusted locally. The feasibility of Jet-PEO has already been proven in preliminary work with fluoride-free electrolytes. However, the targeted production of adhesion-promoting PEO layers with a low thickness and a pronounced porous outer layer, which is characterized by numerous pores and undercuts, is currently not possible. Therefore, the present project aims at the scientific understanding of the relationships between the process characteristics, the oxide microstructure and the surface properties in Jet-PEO. Firstly, the necessary basic knowledge for a stable Jet-PEO process with the lowest possible ignition voltage and high layer formation rate is acquired within the electrolyte design using electrochemical polarization and ignition tests. An FEM simulation model is developed based on the relationships between electrical parameters, discharge characteristics and microstructure and morphology of the PEO layers from an instrumented bath process. Validation and calibration of the simulation model with regard to the temporally and spatially resolved development of the layer microstructure and morphology are carried out using an experimental setup with a wire-shaped counter electrode. Multiphysical FEM simulations are used to design the free-jet process with regard to fluid mechanics, jet geometry and electrical process control. With the Jet-PEO process, punctiform and linear oxide structures are finally realized on substrate surfaces. The oxide structures are characterized in particular with respect to the corrosion protection using a microcapillary cell and by evaluating the surface topography using fractal algorithms.