dc.description.abstract
Global warming, along with climate change, pose some key challenges in the automotive industry, including social pressure and regulatory requirements. One alternative, to reduce the emission of harmful pollutants, is the optimization of the combustion process, e.g. higher pressures and temperatures during the combustion. One well established way of facing those high demands, is the coating of highly loaded parts with CrN. The excellent wear behavior of CrN, along with a good corrosion resistance, are already highly advanced and the topic of various studies. To further enhance the properties of the thin film, the adhesion properties of the coating on a substrate are of high interest. This thesis was conducted, to enhance the adhesion properties of CrN coatings on two different steel alloys (later referred to as nitrided and hardened steel due to their surface pre-treatment). To determine a baseline coating for further adhesion experiments, several CrN coatings were deposited and analyzed in terms of mechanical properties (hardness, indentation modulus, residual stresses) and structure. Furthermore, to investigate the influence of the surface pre-treatment of the samples on the adhesion properties, several etching procedures (Ar, N2, Ar+H2, Cr+-ion), with modified BIAS and cathode configurations, were conducted and analyzed using SEM, TEM and HRC-adhesion tests. Since Cr-ion etching proofs to be very promising, the etching rate and the implantation depth of Cr into Si and austenite substrates were determined by ERDA, RBS and SIMS measurements, respectively. Beforehand, a calculation of the theoretical implantation depth, using SRIM, was conducted. The HRC- adhesion test revealed only excellent adhesion results (HF1) of the coating on the nitrided steel substrates. Overall, the adhesion properties of the coating on the hardened steel substrates were worse than for the nitrided samples. An increase in the applied BIAS potential, as well as synchronizing the BIAS to the cathode signal, results in improved adhesion properties. In contrary, the generation of Cr-ions using a two-cathode configuration, resulted in the existence of surface contaminants at the interface. These contaminants, along with cavities, could also be observed for the samples showing subpar adhesion properties. The highest etching rate (~46 nm/h) was observed for a Cr-ion etching process, utilizing a combination of Ar and H2 as a working gas. This high etching rate, results from the combination of sputter etching (Ar) with chemical etching (H2). In comparison, the other Cr-ion processes using only Ar as working gas showed etching rates between 12 nm/h (DC-BIAS) and 23 nm/h (synchronized BIAS). The calculated implantation depth of Cr-ions into Si samples (3.4 nm) is approx. two times higher, than for iron or stainless steel. This implantation in Si samples could be verified using SIMS for Cr-ion etched samples, but a deeper implantation is visible. At a BIAS voltage of -1000 V a net implantation depth of roughly 20 nm could be observed. Furthermore, at -700 V an implantation zone of ~16 nm is visible whereas for -300 V a net deposition of a ~150 nm thick Cr layer is detected. ERDA measurements of austenite samples also revealed a net deposition of the etched samples with -300 V and -700 V BIAS of 115 nm and 24 nm, respectively. At -1000 V an ~17 nm thick implantation zone is observed. These results were verified by RBS and TEM analyses. These findings show, that even advanced coating systems can be further enhanced by applying the proper surface pre-treatment method. Especially Cr-ion etching has the potential to improve the adhesion of the coating and should be further investigated.
en