Investigation on the microstructural damage mechanisms in hydrogenated amorphous carbon coating systems (a-C:H)

达姆施塔特工业大学的物理冶金学（PhM）和弗赖堡的弗劳恩霍夫材料力学研究所（IWM）之间的当前项目的目的是将涂层工艺参数与氢化非晶碳涂层（a-C：H）的结构和性能相关联。该项目的重点是韧性金属基底上a-C：H涂层的变形和损伤行为以及优化涂层系统的新方法的开发，特别感兴趣的是结构与界面处的机械性能以及复杂应力状态下的附着力之间的关系。所使用的镀膜工艺是在IWM开发的，是一种低温低压PECVD工艺，将通过专门构建的等离子体监测器进行分析。由于低温，可以用细晶粒尺寸涂覆钢和铝，而不会引起微观结构的任何变化。一个挑战是通过冷加工和表面处理来提高铝合金的承载能力。此外，我们的目标是将界面结构和残余应力分布与不同基底涂层系统的损伤行为联系起来。在第一个资助期内，开发了标准涂层工艺，通过等离子体监测和基底制备进行工艺控制以及表征方法，并成功测试了不同系统。这里的一个主要部分是工艺和等离子体参数与粘附层[Gro 13]的机械和化学性质之间的相关性，现在将转移到功能层。为了研究机械性能对化学结构的依赖性，采用了小角度截面法（SACS），该方法提供了非常高的空间分辨率[Sch 13]。用基于离子束的切割（FIB）方法和数字图像相关（DIC）[Kro 13，Ahm 13]测定涂层内的残余应力。此外，铝基板的负载能力增加累积辊压焊（ARB），而使用喷丸，表面下的残余应力和加工硬化被引入到钢基板。在第二个资助期内，将从系统特性和应力状态方面研究甲苯基功能层的损伤行为。因此，涂层分层的临界剪切应力将使用多长度尺度方法确定，并得到FE模拟的支持。该程序将适用于通过喷丸、冷轧或表面锤击实现的不同基材改性。对于表现出足够附着力的基材涂层系统，将在接触载荷下进行单轴疲劳试验。变形实验将伴随有限元模拟，以预测涂层系统在复杂应力状态下的行为并对其进行优化。

Aim of the present project between the Physical Metallurgy (PhM) at TU Darmstadt and the Fraunhofer Institute for Mechanics of Materials (IWM) in Freiburg, is to correlate the coating process parameters with the structure and the properties of hydrogenated amorphous carbon coatings (a-C:H). Focus of the project is the deformation and damage behavior of a-C:H coatings on ductile metal substrates together with the development of new methods to optimize coating systems.The relation between the structure and the mechanical properties at the interface as well as the adhesion under complex stress states is of peculiar interest. The used coating process, developed at the IWM, is a low temperature and low pressure PECVD process which will be analyzed via a specially built plasma monitor. Because of the low temperatures, it is possible to coat steels as well as aluminum with a fine grain size without causing any changes in the microstructure. One challenge is to improve the load capacity of the Al alloys by cold working and surface treatments. Furthermore, we aim to correlate the interface structure and residual stress distribution with the damage behavior of the different substrate coating systems.Within the first funding period, a standard coating process, the process control via plasma monitoring and the substrate preparation as well as characterization methods were developed and successfully tested for different systems. One main part here was the correlation between process and plasma parameters with the mechanical and chemical properties of the adhesion layer [Gro13], which will now be transferred to the functional layer. In order to investigate the dependencies of the mechanical properties on the chemical structure, a small angle cross-section method (SACS) was applied which provides a very high spatial resolution [Sch13]. The residual stress within the coating was determined with an ionbeam- based cutting (FIB) method and digital image correlation (DIC) [Kro13, Ahm13]. Further, the load capacity of the aluminum substrates was increased by accumulative roll bonding (ARB), while using shot peening, subsurface residual stresses and work hardening was introduced into the steel substrates. Within the second funding period, the damage behavior of the toluene- based functional layer will be investigated with respect to the system properties and the stress state. Therefore, the critical shear stress for coating delamination will be determined using a multiple length scale approach, supported by FE simulations. This procedure will be applied to different substrate modifications achieved by either shot peening, cold rolling or surface hammering. For substrate-coating systems showing a sufficient adhesion, fatigue tests will be performed, uniaxial and under contact load. The deformation experiments will be accompanied by FEM simulation, in order to predict the coating systems behavior under complex stress state and to optimize it.