Buchbesprechungen

Ohne Zusammenfassung     

Structure, mechanical properties and oxidation behaviour of arc-evaporated NbAlN hard coatings

Nb_1 − xAl_xN hard coatings were synthesised by cathodic arc-evaporation in order to study the influence of the Al concentration on crystal structure, mechanical properties and oxidation resistance. Structural investigations by X-ray diffraction revealed a transition from the face-centered cubic structure of δ-NbN to the wurtzite structure of AlN at x = 0.45… 0.56 depending on the deposition parameters. The maximum values of the mechanical properties like hardness and residual stress obtained by nanoindentation and biaxial stress temperature measurements, respectively, were found for the coatings with cubic structure and generally decrease with increasing Al content. On the other hand, higher Al concentrations are beneficial in terms of oxidation resistance as shown by annealing experiments in ambient air. The onset temperature for oxidation rises from 600 to 700 °C for Nb_0.73Al_0.27N to above 800 °C for Nb_0.29Al_0.71N regardless of changes in the crystal structure.     

Influence of phase transition on the tribological performance of arc-evaporated AlCrVN hard coatings

V-alloyed AlCrN hard coatings were synthesised by reactive arc-evaporation in an industrial-sized deposition system at bias voltages ranging from − 40 to − 150 V. X-ray diffraction analysis has shown that higher energetic growth conditions stabilise the desired metastable face-centered cubic (fcc) crystal structure of Al_0.70Cr_0.05V_0.25N even at very high Al/Cr ratios resulting in hardness values comparable to Al_0.70Cr_0.30N. Ball-on-disc tests were used to assess the friction at 700 °C where Al_0.70Cr_0.05V_0.25N coatings revealed a generally lower coefficient of friction due to the formation of a tribolayer containing the lubricious oxide V₂O₅ as evidenced by Raman spectroscopy. The single-phase fcc-Al_0.70Cr_0.05V_0.25N coating appears to be more oxidation resistant leading to a reduced formation of V₂O₅ and, hence, an increase in friction. The cutting performance was evaluated by conducting side milling tests under dry conditions, where Al_0.70Cr_0.05V_0.25N coatings showed a competitive performance regardless of the growth conditions.     

In Situ Studies of TiC<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>N<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> Hard Coating Tribology

TiC_1−x N _x hard coatings present time-dependent tribological behavior with an initial running-in period (500–2000 cycles) marked by an elevated friction coefficient, followed by >10000 cycles with low-friction and wear at room temperature (RT) in ambient air. The mechanisms behind this behavior are not completely understood. Tribological tests performed at RT and at different relative humidity (RH) levels revealed that a minimum value between 15 and 25% RH is needed to trigger the low-friction regime at a sliding speed of 100 mm s⁻¹. By in situ observations of transfer film growth, it could be observed that third body material is formed during this running-in period by plowing of the coating and shearing of the removed material. The appearance and thickening of the transfer film marks the beginning of the steady-state low-friction regime where the velocity is accommodated by interfacial sliding. At this stage in the tribological test, the recorded Raman spectra indicated the presence of C–H bonds in the wear track. Use of in situ analytical tools during wear tests provided insights with respect to tribological phenomena that were not available by conventional, post-mortem analysis methods.     

Tribological Properties of Arc-Evaporated NbAlN Hard Coatings

Nb_1−x Al _x N hard coatings were synthesised by cathodic arc-evaporation with different Al contents to study its influence on the tribological properties. Ball-on-disc tests at temperatures up to 700 °C were performed and the recorded coefficient of friction was generally in the range from 0.8 to 1.0. Subsequent analysis of the coating wear track and the counterpart wear scar by optical profilometry and Raman spectroscopy revealed details on the wear behaviour of the tested coatings. The best wear performance for the Nb-rich coatings was in the temperature range of 300–500 °C, whereas at the maximum testing temperature the higher oxidation resistance with increasing Al content was beneficial in terms of wear resistance.