Papers by Author: Fabian Hoffmann

Abstract: It has been established that hardness and density of diamond-like carbon (DLC) layers can be raised by increasing ion energy during deposition, decreasing H-content and by increasing sp³-fraction. To confirm differences in hydrogen content of hydrogen containing and hydrogen free DLC films deposited at different bias voltages, layers were etched in oxygen atmosphere in a capacitively coupled plasma device. By employing real-time ellipsometry measurements, the H-content of the hydrogen containing a-C:H layers were estimated by determining the optical constants n and k (n-real part and k-imaginary part of the refractive index). In addition, DLC layers were analyzed by X-ray photoelectron spectroscopy to estimate the ratio of sp^²- and sp^³-hybridization. The mechanical and tribological properties of the coatings were evaluated by means of nanoindentation and ball-on-disc-tests. Finally correlations between these properties, H-content and sp³/sp²-ratio were obtained in an effort to explain different tribological behaviors of DLC-layers.

Abstract: Since diamond like carbon layers feature excellent mechanical and tribological behavior under defined environmental circumstances, they are well established in a wide field of industrial and automotive applications in the last decade. However, the pretreatment of the substrate plays also an important role in supporting and enforcing the excellent properties of the coatings. This work analyses the effect of the plasma nitrided cold working steel substrate (80CrV2) on the adhesion, friction and wear resistance of DLC-coatings and compares it to the performance of DLC-coatings applied on a non-hardened substrate material. Therefore the grinded and polished specimens were nitrogen-hardened in an Arc-PVD (Physical Vapor Deposition)-device before the DLC-coating was applied in a Magnetron Sputter-PVD-process. In order to measure the hardness of the thin film coating, a nanoindenter was used. The adhesion was tested with a scratch tester and the wear resistance was measured by using a Ball-on-disc-tester. A 3D-profilometer and a SEM (Scanning Electron Microscope) were utilized to analyze the scratches and wear tracks on the samples. With these results correlations between the substrate nitriding and the mechanical and tribological performance of the DLC-coating were made.

Abstract: Age hardening is one of the most important processes to strengthen aluminium alloys. It usually consists of the steps solution annealing, quenching and aging. For heat treatment simulations as well as for the appropriate choice of quenching processes in heat treatment shops, knowledge of the temperature- and time-dependent precipitation behaviour during continuous cooling is required. Quenching should happen as fast as necessary to reach high strengths, but also as slow as possible, to reduce residual stresses and distortion. This optimal quenching rate of an aluminium component depends on its chemical composition, initial microstructure and solution annealing parameters as well as on its dimensions. Unfortunately continuous cooling transformation (CCT) diagrams of aluminium alloys do almost not exist. Instead isothermal transformation (IT) diagrams or given average quenching rates are used to estimate quenching processes, but they are not satisfying neither for heat treatment simulations nor for heat treatment shops. Thermal analysis, especially Differential Scanning Calorimetry (DSC) provides an approach for CCT-diagrams of aluminium alloys, if the relevant quenching rates can be realized in the DSCequipment. The aluminium alloy Al-4.5Zn-1Mg (7020) is known for its relatively low quenching sensitivity as well as for its technical importance. The complete CCT-diagram of 7020 with cooling rates from a few K/min to some 100 K/min has been recorded. Samples have been solution annealed and quenched with different cooling rates in a high speed DSC. The resulting precipitation heat peaks during cooling have been evaluated for temperature and time of precipitation start, as well as their areas as a measure for the precipitate amount. Quenched samples have been further investigated regarding their microstructure by light and electron microscopy, hardness after aging and precipitation behaviour during re-heating in DSC. The CCT-diagram correlated very well with the microstructure, hardness and re-heating results. A critical cooling rate with no detectable precipitation during continuous cooling 155 K/min could be determined for 7020. A model to integrate the CCT-diagram in heat treatment simulation of aluminium alloys is under development.

Abstract: For quenching of age hardenable aluminum alloys today predominantly aqueous quenching media are used, which can lead due to the Leidenfrost phenomenon to a non-uniform cooling of the parts and thus to distortion. In relation to the conventional quenching procedures in aqueous media, gas quenching exhibits a number of technological, ecological, and economical advantages. The quenching intensity can be adjusted by the variable parameters gas pressure, gas velocity as well as the kind of gas and thus can be adapted to the requirements of the component. By the higher uniformity and the better reproducibility, gas quenching offers a high potential to reduce distortion. Cost savings would be possible, because of reduced distortion and therefore less reworking. High-pressure gas quenching with nitrogen or helium, as well as air quenching at ambient pressure in a gas nozzle field was applied to the spray formed aluminum alloy Al-17Si-4Fe-3Cu-0.5Mg-0.4Zr (DISPAL S232). Hardness and tensile tests have been carried out to determine the mechanical properties after gas quenching and aging compared to water quenching. The distortion behavior of a forged aluminum component of the spray formed alloy was examined after gas quenching and after water quenching. Gas quenching showed remarkable advantages regarding distortion.