Synthesis and characterization of Mo-B based protective coatings

Achenbach, Jan-Ole; Schneider, Jochen M. (Thesis advisor); Mayrhofer, Paul H. (Thesis advisor)

Aachen (2020)
Book, Dissertation / PhD Thesis

In: Materials Chemistry dissertation 33 (2020)
Page(s)/Article-Nr.: 1 Online-Ressource (XV, 80 Seiten) : Illustrationen, Diagramme

Dissertation, Rheinisch-Westfälische Technische Hochschule Aachen, 2020

Abstract

In the first part of this thesis, the angle-resolved composition evolution of Mo-B-C thin films deposited from a Mo2BC compound target is investigated experimentally and theoretically. Depositions were carried out by direct current magnetron sputtering (DCMS) in a pressure range from 0.09 to 0.98 Pa in Ar and Kr. The substrates were placed at specific angles α with respect to the target normal from 0 to ± 67.5°. A model based on TRIDYN and SIMTRA was used to calculate the influence of the sputtering gas on the angular distribution function of the sputtered species at the target, their transport through the gas phase and film composition. Experimental pressure and sputtering gas dependent thin film chemical composition data were in good agreement with simulated angle-resolved film composition data. In Ar, the pressure-induced film composition variations at a particular α were within the error of the EDX measurements. Contrary, an order of magnitude increase in Kr pressure resulted in an increase of the Mo concentration measured at α = 0° from 36 to 43 at. %. It is shown that the mass ratio between sputtering gas and sputtered species defines the scattering angle within the collision cascades in the target, as well as for the collisions in the gas phase, which in turn define the angle- and pressure-dependent film compositions. In the second part, the low temperature oxidation behavior of crystalline Mo2BC thin films was systematically investigated. Mo2BC exhibits a unique combination of high stiffness and moderate ductility, enabling the application as protective and wear resistant coating. As the low temperature oxidation behavior of Mo2BC coatings is unexplored, direct current magnetron sputtered Mo2BC coatings were oxidized at temperatures ranging from 500 to 100°C for up to 28 days. Time-of-Flight elastic recoil detection analysis reveals that the onset of oxidation takes place at approximately 300°C as a significant increase in the O content was observed. Crystalline oxide scales containing orthorhombic MoO3 were identified after oxidation for 15 min at 500°C and 10 days at 200°C. Isothermal oxidation at 200 and 100°C exhibit oxide scale thicknesses of 401 nm ± 33 nm and 22 nm ± 10 nm after 14 days. Oxidation for 28 days at 100 °C exhibits an oxide scale thickness of 13 nm ± 3 nm which is comparable to the aforementioned oxide scale thickness after oxidation for 14 days at 100°C. Based on the combination of mechanical properties and the here reported low temperature oxidation behavior Mo2BC coatings qualify for application in solid wood machining and low temperature forming processes at temperatures close to 100°C or below. In the third and last part of this thesis, the first thin film synthesis of MoAlB coatings is communicated. MoAlB was reported to be the only oxidation resistant transition metal boride as it forms a dense, adherent alumina scale. While, based on X-ray diffraction investigations, the formation of phase pure orthorhombic MoAlB coatings was observed, energy dispersive X-ray spectroscopy carried out in a scanning transmission electron microscope revealed the presence of Al-rich and O-rich regions within the MoAlB matrix. The oxidation kinetics of coatings and bulk was comparable as the scale thickness formed on the MoAlB coating after oxidation at 1200°C for 30 minutes was similar to the one extrapolated for bulk MoAlB. Furthermore, the oxidation kinetics of MoAlB coatings was significantly lower as the one reported for bulk Ti2AlC. Finally, the elastic properties measured for the as deposited coatings were consistent with ab initio predictions.

Identifier

Downloads