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

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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.