ICMCTF 2023 Session G2-2-WeA: Surface Modification of Components in Automotive, Aerospace and Manufacturing Applications II

The aim of the present study is to investigate the influence of plasma diffusion treatments with different process gases on the surface properties of austenitic stainless steels. This is necessary to functionalize austenitic stainless steels as a material for hydrogen applications, such as bipolar plates in the proton-exchange membrane fuel cell (PEMFC).

For this purpose, the austenitic stainless steel X2CrNiMo17-12-2 (AISI 316L) was modified by low temperature plasma diffusion treatment. The experimental investigations focus on improving the corrosion behavior and the interfacial contact resistance (ICR) of the modified boundary zone. Nitrogen and carbon as well as the combination of both were used as process gas in a temperature window from 390 to 450 °C.Own preliminary work has shown that temperatures above this could lead to an increase in corrosion current, which can be explained by the formation of chromium nitrides. While lower treatment temperatures can possibly lead to inhomogeneous nitriding zones.

In order to evaluate the properties with respect to corrosion resistance, the samples were exposed to potentiodynamic polarization measurements. The ICR was determined under surface pressure between two copper electrodes coated with gold. The diffusion zone was analyzed by X-ray diffraction (XRD) and Glow Discharge Optical Emission Spectroscopy (GDOES). In addition, standardized surface analytics, like Scanning Electron Microscopy (SEM) combined with Energy Dispersive X-ray spectroscopy (EDX) have been applied.

The results show a clear influence of the treatment temperature and the process gases on the studied properties. Plasma diffusion processes based on nitrogen show a significantly higher influence of the treatment temperature than those based on carbon. The plasma carbonized samples show a corrosion rate up to two orders of magnitude lower than the nitrided samples (between 10^-4 and 10^-5 A/cm²) in potentiodynamic measurements. But the IRC is significantly increased after the corrosion measurement from lower than 20 up to more than 100 mΩ*cm². The nitrocarburized samples combine the properties of both individual processes and show a comparatively low corrosion rate at low temperatures in combination with low IRC values at increased temperatures.

Tools used in automotive and manufacturing applications come in different shapes and sizes. The deposition of a protective PVD coating is relatively straight-forward for shank tools (drills, mills, reamers) since a standard planetary mounting enables to set up a more or less uniform batch. Other types of tools (stamps, saw blades, powder compaction tools) have more complex shapes, which complicates mounting and reduces the uniformity of the batch. Thus a standard protocol for a uniform batch of shank tools may have to be adapted depending on the specific collection of tools.

There are several features that influence the coating properties on the surface of tools, despite having been coated in the same batch. The tool shape and size dictate the rotation type (single, double, triple rotation) which in turn strongly influences the microstructure. In high ionization rate processes, such as cathodic arc evaporation, electric field concentration on sharp edges can substantially locally increase the coating thickness; that can be a critical issue in reaming when micrometer-sized tolerances are required. Yet another parameter is the vertical position of tools in the chamber with border areas (top/bottom) where the thickness quickly decreases. A similar effect is observed at shaded areas which are often the working surfaces of some tools, such as dies. In addition to new tools, reground tools can be coated too, which requires more steps to prevent subsequent re-coating.

These features will be addressed based on daily experience in our job coating activities. We will show the dependence of coating thickness, roughness, growth defect density and microstructure. These observations should serve as a guide to reduce unwanted influences and optimize the local coating properties. Our own results will be supported by results published by other authors; nevertheless, these data are relatively scarce since they are often retained as secret know-how within a job coating facility.

With the increasing demand for fiber reinforced materials, chemical vapor infiltration (CVI) as a means to fabricate protective layers between the reinforcement and the matrix, is of increasing industrial interest. These layers can enhance the thermal stability of composites, for example in heat shields, or can be designed to strengthen or weaken the adhesion of the reinforcement to the matrix and thus tailor the mechanical properties. One main requirement to this technology is high infiltration rates and homogeneity. That’s why batch and continues coating concepts are developed and optimized.

In this context newly developed infiltration processes will be presented and the impact of the equipment design will be discussed. The reactor designs allows for example continuous processing of fiber tows of variable length and stable operation for complete infiltration of 3D porous bodies. Additionally, the modular concept allow a high flexibility in terms of gas feeds and deposited materials, and the extensions for industrial upscaling. CVI processes for the deposition > 1 µm thick BN, PyC and SiC films on SiC fibers were developed and show promising characteristics with regard to the infiltration rate and homogeneity.