Effect of Hydrogen Exposure on Mechanical and Tribological Behavior of CrxN Coatings Deposited at Different Pressures on IN718

Abstract

In the current study, the properties of the CrxN coatings deposited on the Inconel 718 superalloy using direct current reactive magnetron sputtering are investigated. The influence of working pressure on the microstructure, mechanical, and tribological properties of the CrxN coatings before and after high-temperature hydrogen exposure is studied. The cross-sectional scanning electron micrographs indicate the columnar structure of the coatings, which changes from dense and compact columns to large columns with increasing working pressure. The Cr/N ratio increases from 1.4 to 1.9 with increasing working pressure from 300 to 900 mPa, respectively. X-ray diffraction analysis reveals a change from mixed hcp-Cr2N and fcc-CrN structure to approximately stoichiometric Cr2N phase. After gas-phase hydrogenation, the coating deposited at 300 mPa exhibits the lowest hydrogen absorption at 600 °C of all investigated coatings. The results indicate that the dense mixed cubic and hexagonal structure is preferential for hydrogen permeation resistance due to the presence of cubic phase with higher packing density in comparison to the hexagonal structure. After hydrogenation, no changes in phase composition were observed; however, a small amount of hydrogen is accumulated in the coatings. An increase of coating hardness and elastic modulus was observed after hydrogen exposure. Tribological tests reveal that hydrogenation leads to a decrease of the friction coefficient up to 20%–30%. The best value of 0.25 was reached for hydrogen exposed CrxN coating deposited at 300 mPa.

Hydrogen Permeation, and Mechanical and Tribological Behavior, of CrNx Coatings Deposited at Various Bias Voltages on IN718 by Direct Current Reactive Sputtering

“…Additionally, the (110) peak of Cr 2 O 3 oxide phase was detected that probably was associated with the formation of the thin oxide layer on the surface during cooling of the sample after hydrogenation. Similar to our previous work, the lowest hydrogen absorption was found for the coating with mixed hcp-Cr 2 N and c-CrN phase composition due to the higher packing density of the cubic structure [13]. Zhou et al [43] observed for titanium nitride coatings that hydrogen impermeability of the coating also depends on the crystal orientation of the films.…”

“…The difference in hydrogen absorption is associated with the modification of crystalline structure and morphology of the coatings. On the one hand, the formation of mixed CrN + Cr2N phase in coatings deposited at 60 V leads to lower hydrogen permeation through the coating than the Cr2N phase formed at higher bias voltages [13,18,25]. On the other hand, the denser structure of the coatings deposited at higher bias voltages reduces the rate of hydrogen diffusion through the coating due to reduced intergranular space and pores in the coating.…”

“…It has been reported that at a high hydrogen concentration, the δ-phase in IN718 dramatically reduces the ductility of the alloy [12]. Furthermore, hydrogen embrittlement of IN718 occurs preferably at the grain boundaries even at low hydrogen concentration [13].…”

“…In our previous work the effect of hydrogenation on mechanical and tribological properties of Cr-N coatings deposited at different argon pressures was investigated with the result that a mixed cubic and hexagonal structure is preferential for hydrogen permeation-resistant coatings due to the presence of a cubic phase with high packing density [13]. Along with argon pressure, bias is one the most important parameters, which controls the deposition process [13,22].…”

“…Along with argon pressure, bias is one the most important parameters, which controls the deposition process [13,22]. A negative bias voltage applied to the substrate can influence nucleation and growth kinetics during coating growth, and will subsequently modify the microstructure and mechanical properties of the coatings [17,[23][24][25][26][27][28].…”