Oxidation of thin chromium nitride films: kinetics and morphology

Abstract

Magnetron-sputtered, 150 - 500 nm thick chromium nitride films of CrN and stoichiometry were exposed to air at 500 - . Oxide scale growth and morphology and the phase transformations during oxidation were investigated by means of Rutherford backscattering spectrometry, resonant nuclear reaction analysis, scanning force microscopy (SFM) and x-ray diffraction. A combined logarithmic and parabolic scale growth was found and interpreted as due to inwards diffusion of oxygen via grain boundaries (logarithmic) and outwards lattice diffusion of Cr (parabolic). Pt-marker experiments confirmed this interpretation. The activation energies of the two processes were determined to be Q = 60(20) and . During oxidation of a transformation into CrN was observed. A linear increase in the surface roughness and surface grain size with the oxide layer thickness was found by SFM.

Effect of the oxygen content on the structure, morphology and oxidation resistance of Cr–O–N coatings

“…This means that the oxidation rate tends to increase with increasing the fraction of grain boundaries, i.e. with decreasing the grain size [19]. Although this is in contrast to our results, the increasing oxidation resistance can be explained by increasing the thermal compressive stress with increasing the oxygen content.…”

“…According to previous reports, the oxygen diffusion at grain boundaries is the fastest process in the oxidation of polycrystalline CrN coatings [19]. This means that the oxidation rate tends to increase with increasing the fraction of grain boundaries, i.e.…”

“…The coefficient of thermal expansion (CTE) depends on the anharmonic vibrational amplitude of the chemical bond and not on the crystal structure [20]. Since the CTE of WC (~3.8·10 − 6 K − 1 , [19]) is larger than that of CrN (~2.3…10 − 6 K − 1 , [21]) and lower than the one of Cr 2 O 3 (~9.6·10 − 6 K − 1 , [20]), an increase of CTE, observed in other oxynitrides [22,23], would lead to thermal tensile stress for pure CrN and progressively higher thermal compressive stress with increasing O rel . The resulting open structure for the CrN and more compact morphology for the oxygen-rich coatings promote and hinder, respectively, the O 2 diffusion through the films.…”

Oxidation Resistance of a Cr0.50Al0.50N Coating Prepared by Magnetron Sputtering on Alloy K38G

“…It can be seen from the figure that reaction (3) has a much more negative Gibbs free energy; therefore alumina prefers to form. However, many previous results [27][28][29][30] indicated that the scales of CrAlN coatings consisted mainly of Cr 2 O 3 with some amounts of a-Al 2 O 3 dissolved in it. The reason for the formation of the oxide scales containing less a-Al 2 O 3 was explained by the fact that the aluminum content is low, and the activity of aluminum element is not high enough to forman a-Al 2 O 3 enriched scale.…”

“…The oxidation processes of CrAlN coatings have been studied widely by other researchers [24,[27][28][29][30], and the following two reactions are considered during oxidation of the coatings in an oxygen-containing atmosphere, Figure 8 gives the dependence of DG 0 on temperature. The thermodynamic data in the calculation are from Rao [31] and extrapolated to a wider temperature range.…”