Nanoscale Multilayer Coatings Research Articles

Hydrogen-Induced Microstructure Changes in Zr/Nb Nanoscale Multilayer Structures

Zr/Nb nanoscale multilayer coatings (NMCs) were studied after hydrogenation in a gaseous environment at 400 °C. The hydrogen distribution and content were determined by pressure and hydrogenation time. Increasing the pressure from 0.2 to 2 MPa resulted in different hydrogen distribution within the Zr/Nb NMCs, while the concentration remained constant at 0.0150 ± 0.0015 wt. %. The hydrogen concentration increased from 0.0165 ± 0.001 to 0.0370 ± 0.0015 wt. % when the hydrogenation time was extended from 1 to 7 h. The δ-ZrH hydride phase was formed in the Zr layers with Zr crystals reorienting towards the [100] direction. The Nb(110) diffraction reflex shifted towards smaller angles and the interplanar distance in the niobium layers increased, indicating significant lateral compressive stresses. Despite an increase in pressure, the nanohardness and Young’s modulus of the Zr/Nb NMCs remained stable. Increasing the hydrogen concentration to 0.0370 ± 0.0015 wt. % resulted in a 40% increase in nanohardness. At this concentration, the relative values of the Doppler broadening variable energy positron annihilation spectroscopy (S/S0) increased above the initial level, indicating an increase in excess free volume due to hydrogen-induced defects and changes. However, the predominant positron capture center remained intact. The Zr/Nb NMCs with hydrogen content ranging from 0.0150 ± 0.0015 to 0.0180 ± 0.001 wt. % exhibited a decrease in the free volume probed by positrons, as demonstrated by the Doppler broadening variable energy positron annihilation spectroscopy. This was evidenced by opposite changes in S and W (S↓W↑). The microstructural changes are attributed to defect annihilation during hydrogen accumulation near interfaces with the formation of hydrogen–vacancy clusters and hydrides.

Exploring the potential of nano technology: A assessment of nano-scale multi-layered-composite coatings for cutting tool performance

Nano-scale Multi-layered Composite Coatings (Nano scale MLCC) offer potential benefits for cutting tool performance. Such coatings can be designed to improve tool durability and cutting efficiency by providing additional wear protection and improved heat dissipation. The Nano coatings can also be tailored to reduce friction, which can enhance productivity and reduce tool wear. The unique Nano-scale topography of MLCC helps to reduce the contact area between the cutting tool and the workpiece, thus reducing friction and heat. The coatings also include specific additive components that enhance the properties of the coating, such as corrosion resistance, lubricant retention, and improved thermal stability.This critical review explores the potential of Nano-scale multi-layered-composite coatings for improving the performance of cutting tools. The literature in this field is reviewed and discussed with respect to the performance benefits and challenges associated with the implementation of these nanotechnology coatings. The latest progress in nanofabrication, nanomaterials, and other technologies are discussed in relation to their ability to provide new and improved cutting tool applications. The review summarizes the current state of the art on Nano-scale multi-layered-composite coatings and their influence on cutting tool performance, with particular focus on machining productivity, tool life, cutting forces and wear rates. Recommendations are provided on which Nano-scale coatings may best fit the demands of particular machining operations, and on future research directions in this field. The review also discusses the challenges and future advancements of Nano-scale coatings and other methods for improving cutting tool performance. In conclusion, this review provides a comprehensive insight into the future potential of MLCC for cutting tool performance enhancement.

Effects of bias voltage on the microstructure and properties of AlCrN/AlTiN nanoscale multilayer coatings

Bias voltage has great influence on the microstructure and properties of multilayer coatings, but it is endowed less attentions compared with modulation periods. In this work, effects of bias voltage ranging from −120 V to −210 V on the microstructure and properties of the AlCrN/AlTiN multilayer coatings are investigated. Results show that the multilayer coatings possess fcc-(Al,Cr,Ti)N singular phase, and grow preferentially along the (111) plane, but the texture coefficient of the (111) plane drops greatly at −210 V. Surface roughness decreases firstly and then increases with the bias voltage. Conversely, the hardness increases gradually to the maximum value 33.0 GPa at −180 V, and then decreases to 26.4 GPa at −210 V caused by the texture variation and coarse structure. The adhesion strength decreases gradually with the bias voltage due to augmented residual stress. The friction coefficient and wear rate drop firstly and then increase at −210 V. Abrasion, oxidation and adhesion wear are primarily responsible for the failure of the coatings at mediate bias voltage, while spallation accelerates the failure of the coatings at −120 V and −210 V caused by their poor microstructure and mechanical property.

Effect of Proton Irradiation on Zr/Nb Nanoscale Multilayer Structure and Properties

The effect of proton irradiation on the structure, phase composition, defect state and nanohardness of Zr/Nb nanoscale multilayer coatings was investigated. Preservation of the Zr/Nb layered structure with 50 and 100 nm thick layers, was observed after irradiation with protons at 1720 keV energy and 3.4 × 1015, 8.6 × 1015 and 3.4 × 1016 ions/cm2 fluences, and the interfaces remained incoherent. In the Zr/Nb nanoscale multilayer coatings with individual layer thicknesses of 10 and 25 nm, there were insignificant fluctuations in interplanar distance, which were influenced by changes in irradiation fluence, and the interfaces were partially destroyed and became semicoherent. Changing irradiation fluence in the investigated ranges led to a decrease in the nanohardness of the Zr/Nb nanoscale multilayer coatings with individual layer thicknesses of 10–50 nm. Variable-energy positron Doppler broadening analysis revealed that these changes are primarily caused by peculiarities of the localization and accumulation of the embedded ions and do not cause a significant increase in the S-parameters of Zr/Nb nanoscale multilayer coatings with a layer thickness less than 100 nm.

The Microstructure of Zr/Nb Nanoscale Multilayer Coatings Irradiated with Helium Ions

The effect of helium ion irradiation on the microstructure and properties of composites based on Zr/Nb nanoscale multilayer coatings (NMCs) was studied. X-ray diffraction (XRD), transmission electron microscopy (TEM), and variable-energy Doppler broadening spectroscopy (DBS) were used for the in-depth analysis of defects in the irradiated NMCs. After irradiation of the Zr/Nb NMCs with helium ions at a 1017 ion/cm2 dose, the layered structure was generally retained, but the internal stresses in the layers were increased, which caused wave-like distortion in the ion deposition zone. The Zr/Nb NMCs with an individual layer thickness of 25 nm were characterized by the smallest microstress changes, but single blisters were formed in the near-surface region. The microstructure of the Zr/Nb NMCs with a layer thickness of 100 nm exhibited relatively smaller changes upon helium ion irradiation. The prevailing positron-trapping center was the reduced-electron-density area at the interfaces before and after irradiation of the Zr/Nb NMCs regardless of the layer thickness. However, the layer thickness affected the DBS parameter profiles depending on the positron energy, which was probably due to the different localization of implanted ions within the layers or at the interfaces.

Distribution of Hydrogen and Defects in the Zr/Nb Nanoscale Multilayer Coatings after Proton Irradiation.

Radiation damage is one of the significant factors limiting the operating time of many structural materials working under extreme conditions. One of the promising directions in the development of materials that are resistant to radiation damage and have improved physical and mechanical properties is the creation of nanoscale multilayer coatings (NMCs). The paper is devoted to the experimental comprehension of changes in the defect structure and mechanical properties of nanoscale multilayer coatings (NMCs) with alternating layers of Zr and Nb under irradiation. Series of Zr/Nb NMCs with different thicknesses of individual layers were fabricated by magnetron sputtering and subjected to H+ irradiation. The evolution of structure and phase states, as well as the defect state under proton irradiation, was studied using the methods of high-resolution transmission electron microscopy (HRTEM), X-ray diffraction analysis (XRD), glow discharge optical emission spectroscopy (GDOES), and positron annihilation spectroscopy (PAS). The layer-by-layer analysis of structural defects was carried out by Doppler broadening spectroscopy (DBS) using a variable-energy positron beam. To estimate the binding energy and the energy paths for the hydrogen diffusion in Zr/Nb NMCs, calculations from the first principles were used. When the thickness of individual layers is less than 25 nm, irradiation causes destruction of the interfaces, but there is no significant increase in the defect level, the S parameter (open volume defects amount) before and after irradiation is practically unchanged. After irradiation of NMC Zr/Nb with a thickness of layers 50 and 100 nm, the initial microstructure is retained, and the S parameter is significantly reduced. The GDOES data reveal the irregular H accumulation at the interface caused by significant differences in H diffusion barriers in the bulk of Zr and Nb multilayers as well as near the interface’s region.

Evaluation of Novel Combined Plasma Nitriding / Physical Vapor-Deposited Cr/Crn Nanoscale Multilayer Coatings for the Application of Bipolar Plates in Proton Exchange Membrane Fuel Cells

Evaluation of Novel Combined Plasma Nitriding / Physical Vapor-Deposited Cr/Crn Nanoscale Multilayer Coatings for the Application of Bipolar Plates in Proton Exchange Membrane Fuel Cells

Design of nano-scale multilayered nitride hard coatings deposited by arc ion plating process: Microstructural and mechanical characterization

The objective of the present study was to investigate some transition metal nitride to determine the relationship between microstructural evolution and mechanical properties such as hardness and adhesion strength of the hard coating. Results showed that it was possible to form nano-scale multilayers of Ti(CuNiZr)N/AlCr(SiW)N with chemical fluctuation of Ti-rich and Al-rich nitrides. It was believed that these nano-scale multilayers with chemical fluctuation could form during table rotation of the arc ion plating process, causing difference in mean free path of chemical elements from the target. Microstructural investigation confirmed that the multilayer consisted of three layers with nanostructured periodic stripe patterns on the multilayer coating based on detailed individual layers analysis. Furthermore, Ti(CuNiZr)N/AlCr(SiW)N multilayer showed mechanical properties, including hardness of 36.0 ± 3 GPa and adhesion strength represented by the critical load of 93 ± 5 N with the highest H/E and H3/E2 ratios.

TiN/NbN Nanoscale Multilayer Coatings Deposited by High Power Impulse Magnetron Sputtering to Protect Medical-Grade CoCrMo Alloys

This study describes the performance of nanoscale multilayer TiN/NbN coatings deposited on CoCrMo medical-grade alloys by utilising novel mixed high power impulse magnetron sputtering (HIPIMS) and direct current unbalanced magnetron sputtering (UBM) technique in an industrial size vacuum coater. Scanning electron microscopy analysis showed that these coatings were extremely dense without any intercolumnar voids. The coating exhibited high hardness of 28 GPa, as well as low friction and wear coefficient of 0.7 and 1.4 × 10−14 m3·N−1·m−1, respectively, as compared to the bare material. Scratch tests revealed superior coating to substrate adhesion due to the HIPIMS etching prior to coating deposition. Energy-dispersive X-ray analysis of the wear debris generated during the impact test together with focused ion beam cross-section analysis in different locations of the impact crater revealed the coating failure mechanism and further confirmed the excellent coating to substrate bonding strength. Potentiodynamic polarisation tests in NaCl and Hank’s solutions revealed the clear passivation behaviour, several orders of magnitude lower corrosion currents, and high pitting potentials of the coating, which guarantee excellent protection to the base alloy in such aggressive environments. Inductively coupled plasma mass spectrometry analysis of Hank’s solution containing corrosion debris of the coated sample revealed that the leaching of harmful metal ions from the base material was reduced to below the detection limit of the technique, thus demonstrating the high barrier properties of the coating.

Atomic insights on intermixing of nanoscale nitride multilayer triggered by nanoindentation

Mechanical properties of nanoscale multilayer coatings are to a large extent governed by the number of interfaces and their characteristics. While for a reduced layer thickness, increasing strength and toughness values have been reported, properties degrade for layer thicknesses of just several nanometers. Here, we report on an entirely overlooked phenomenon occurring during the indentation of nanolayers, presumably explaining the degradation of properties. Nanoindentation, commonly used to determine properties of hard coatings, is found to disrupt and intermix the multilayer structure due to the deformation imposed. Detailed electron microscopy studies and atomistic simulations provide evidence for intermixing in an epitaxial transition metal nitride superlattice thin film induced by nanoindentation. The formation of a solid solution reduces the interfacial density and leads to a sharp drop in the dislocation density. Our results confirm that plastic deformation causes the microstructure instability of nitride multilayer, which may further improve our understanding of multilayer strength mechanisms.

Effect of Proton Irradiation on the Defect Evolution of Zr/Nb Nanoscale Multilayers

Nanoscale multilayer coatings (NMCs) with different crystal structures are considered as capable of self-healing after radiation damage due to the recombination of vacancies and interstitials. This work is focused on a defect distribution study of NMCs based on Zr/Nb layers (25/25 nm and 100/100 nm) after proton irradiation. Coatings with a total thickness of 1.05 ± 0.05 µm were irradiated by 900-keV protons using a pelletron-type electrostatic accelerator with an ion current of 2 µA for durations of 60 min to 120 min. The influence of the irradiation effect was studied by X-ray diffraction analysis (XRD), glow discharge optical emission spectrometry (GD–OES), and Doppler broadening spectroscopy using a variable energy positron beam. The results obtained by these methods are compatible and indicate that defect concentration of Zr/Nb NMCs remains unchanged or slightly decreases with increasing irradiation time.

Comparing electrochemical behavior of applied CrN/TiN nanoscale multilayer and TiN single-layer coatings deposited by CAE-PVD method

ABSTRACT The main goal of this study is to compare the electrochemical behavior of CrN/TiN multilayer coating and TiN single-layer coating applied by cathodic arc evaporation (CAE) -physical vapor deposition (PVD) on the substrate of Ti-6Al-4 V in Hank’s solution. So, a TiN single-layer coating was formed using PVD method as well as CrN/TiN multilayer coating under totally the same conditions including deposition temperature, working pressure, bias voltage, cathode current and deposition time. To identify the phases in the coatings, X-ray diffraction (XRD) was utilized in order and field emission scanning electron microscopy (FE-SEM) was used in order to study the microstructure of coating. Potentiodynamic polarization (PDP) and electrochemical impedance spectroscopy (EIS) tests were also carried out for corrosion measurements. In the corrosion evaluation, the PDP and EIS tests were performed for the samples after immersion in Hank’s solution for 24 hours to reach the steady state. The obtained results from PDP curves showed that the specimen with CrN/TiN multilayer coating exhibited better corrosion behavior than the TiN single-layer coatings. This improvement in corrosion resistance is probably due to the high density of the layers in the CrN/TiN multilayer coating that prevents the penetration of the corrosive solution to the substrate.

Electrochemical Corrosion Behavior of TiAlN/CrN Nanoscale Multilayer Coatings by Multi-Arc Ion Plating in 3.5% NaCl Solution

Multi-arc ion plating was used to deposit TiAlN/CrN multilayer coatings with different bilayer thicknesses on Type 316L stainless steel. The impacts of the bilayer thickness on the electrochemical performance of the TiAlN/CrN multilayer coatings were explored in a neutral saline (3.5% NaCl) solution. The fracture cross-sectional morphology of the as-deposited coatings were investigated by scanning electron microscope. The crystal structure and preferred orientation were analyzed by x-ray diffraction. The interface characteristic was detected by transmission electron microscopy. The potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) were used for as-deposited coatings. Corroded surface morphology and EIS of the 15-day immersion coatings and substrate were examined to analyze the anticorrosion performance. EIS showed an increase of polarization resistance (Rp) and a decrease in constant-phase element (CPE) for multilayer coatings with a decrease in bilayer thickness. After 15 days of immersion, the EIS plot showed that the C3 coating (with 20 bilayers) had the lowest electrical double-layer capacitance (CPEdl) and highest charge transfer resistance (Rct) in all samples. The relative decay of the Rp of the C3 coating was the smallest compared with the as-deposited coatings, which shows a satisfactory corrosion resistance. The electrochemical tests and immersion corrosion morphology showed that the existence of a large number of interfaces between individual layers in a multilayer structure inhibits the pitting propagation significantly and enhances the corrosion resistance.

Corrosion Behavior of TiN/CrN Nanoscale Multi-layered Coating in Ringer's Solution

In this article, corrosion behavior of the single-layered TiN, single-layered CrN and multi-layered TiN/CrN coatings have been fabricated on 420C stainless steel (SS) by Cathodic Arc Evaporation (CAE) method in Ringer's solution was studied. The electrochemical appraisal was done by the Potentiodynamic Polarization (PDP) and Electrochemical Impedance Spectroscopy (EIS) tests. Results of PDP test revealed that all coated specimens have an increment in the polarization resistance (Rp) values (ranges from 438.2 to 593.6Ω.cm2) compared to bare SS (268.1Ω.cm2). Corrosion rate values for bare 420C SS and CrN, TiN and multi-layered TiN/CrN coatings were measured 0.086, 0.042, 0.050 and 0.068mpy, respectively. Furthermore, EIS results were showed that the single-layer CrN coating well depicted by an equivalent circuit with three time constants and has larger impedance and maximum logarithmic variation of C2. Finally, the corrosion performance results confirmed that single-layered CrN coating is an appropriate choice for deposition on 420C SS with the aim of application in general surgical instruments.

Self-Cleaning Properties of TiN/CrN Nanoscale Multi-layer Deposited on Surgical 420C Stainless Steel

The present paper focuses on the investigation of self-cleaning properties based on studding of water repellency and blood repellency for TiN- and CrN single-layer and TiN/CrN nanoscale multi-layer coatings deposited via Cathodic Arc Evaporation (CAE) method on medical grade 420C stainless steel substrate. X-ray diffraction (XRD) method and Field Emission Scanning Electron Microscope (FESEM) was used to characterize microstructure. Surface roughness parameters were measured by using Taylor-Habsson method. Blood contact angle and water contact angle measurements test were applied to characterize the self-cleaning properties of the specimens. The analysis of sample data shows that coated specimens have more water contact angle values in comparison to bare steel. Among the coated samples, CrN single-layer has the highest water contact angle (80°) due to its lower surface roughness (Ra=0.189µm) among the other samples. Moreover, the findings of the paper prove that samples had inverse behavior against blood and water in the contact angle measurement test. Bare steel has higher blood contact angle (76.1°) and more blood repellency than the coated specimens. It seems that the different behavior of samples against water and blood in contact angle tests is due to the nature of two fluids. TiN/CrN multi-layer coating results had minor differences in water and blood contact angle tests (67.3° and 62.2° respectively).

Microstructure and mechanical properties evaluation of cathodic arc deposited CrCN/ZrCN multilayer coatings

For the development of advanced nanoscale multilayer protective coatings, proper design of microstructure and chemical composition of carbon containing sequential transition metal nitride nanolayers is an important issue. In this work, five different nanostructured CrCN/ZrCN multilayer coatings were deposited periodically by cathodic arc evaporation. The bilayer period of the CrCN/ZrCN multilayer coatings was kept at 20 nm. The C2H2 gas flow ratio was adjusted from 6.3 to 20.0% for achieving CrCN/ZrCN multilayer coatings with 2.3–4.2 at.% carbon content. Nanolaminated CrCN and ZrCN nitride layers and thin amorphous carbon nitride mixed nanolayers ∼5 nm thick were obtained as the carbon content reached 4.2 at.%. It was found that the hardness and adhesion quality were strongly improved by the carbon addition to the CrCN/ZrCN multilayer coatings. An increase of 2.6–4.6 GPa in hardness was found for the CrCN/ZrCN multilayer coatings due to the balance of solution hardening effect of carbon atoms and the softening by the amorphous mixed nanolayer. An optimal combination of high hardness, 28.9 GPa, and good adhesion, 41 N of upper critical load were achieved when the carbon content was 4.2 at.% for the CrCN/ZrCN multilayer coatings.

The study of the tribological properties of TiAgN/W2N coatings in a wide temperature range

TiAgN/W2N nanoscale multilayered coatings with modulation periods (Λ) ranging from 2 to 7 nm were deposited by magnetron sputtering. The mechanical and wide-temperature tribological properties of the coatings were investigated. All the multilayer coatings exhibited enhanced hardness as compared with TiAgN nanolayer, which was attributed to the decrease in the disclination dipole motion and the grain boundary sliding. With the increase of the modulation period, the hardness decreased due to the reduced stress-accumulated interfaces. The maximum hardness of 27.6 GPa was revealed in TiAgN/W2N multilayer (Λ = 2 nm). In the temperature range of 25 °C–500 °C, compared with TiAgN nanolayer, all the multilayer coatings exhibited improved friction properties due to the formation of lubricated oxides on the wear track. The lowest friction coefficient of 0.26 was revealed at 500 °C in TiAgN/W2N multilayer (Λ = 2 nm), whereas the wear rate was 6.7 × 10−6 mm3·N−1 mm−1. With the increase of the sliding temperature, the reduced friction coefficient and the increased wear rate for multilayer coatings were due to the more lubricative oxides. Sliding against A2O3 ball, the main wear mechanism of the coatings was considered to be abrasive wear at room temperature, change to abrasive and oxidation wear at high temperature.

Cavitation erosion performance of CrAlYN/CrN nanoscale multilayer coatings deposited on Ti6Al4V by HIPIMS

Water droplet erosion (WDE) protection of Ti6Al4V turbofan blades is of paramount importance to the aviation industry. A novel CrAlYN/CrN nanoscale multilayer coating deposited by the HIPIMS technique was evaluated as a potential candidate for this application. Literature suggests a strong correlation in performance ranking under WDE and cavitation erosion (CE) tests. Hence, the WDE performance of the CrAlYN/CrN coating on Ti6Al4V was investigated with an ultrasonic cavitation device.The results show excellent adhesion and superior erosion resistance of the CrAlYN/CrN coating (erosion rate lower by a factor of 14 compared to the bare Ti6Al4V substrate) and compared to coatings reported in the literature with spallation as their main erosion mechanism. Cross-sectional FIB studies revealed formation of substrate cracks underneath the coating when CE generated stresses exceeded the fatigue strength of the Ti6Al4V alloy. The interfaces of the nanoscale multilayers protected the substrate by forming an effective barrier against shock waves, delayed fatigue crack formation, deflected and arrested any cracks formed impeding the overall coating damage. The research shows that the CE resistance is influenced by the coating's texture and elastic properties, (Young's modulus).The paper discusses the erosion mechanisms of the coating and the excellent CE protection it offers.

Microstructure, phase composition and mechanical properties of novel nanocomposite (TiAlSiY)N and nano-scale (TiAlSiY)N/MoN multifunctional heterostructures

This paper reports on the deposition and characterization of (TiAlSiY)N nanocomposite and (TiAlSiY)N/MoN nano-scale multilayer coatings obtained by means of arc-PVD method. The investigation of structural-phase composition and mechanical properties is carried out by means of scanning electron microscopy (SEM), equipped with energy dispersive spectrum (EDS), High resolution transmission electron microscopy (HRTEM), Raman spectroscopy, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and nanoindentation. The microstructure of the (TiAlSiY)N nanocomposite coating shows preferential (111) growth of fcc-(Ti, Al)N grains. Multilayers coating of (TiAlSiY)N/MoN with a bilayer period of 10 nm was synthesized, showing that the fcc-MoN layer grows coherently with the fcc-(Ti,Al)N, due to the template effect of the latter, which results in a local cube on cube fcc(Ti,Al)N||fcc(MoN) epitaxial growth with (200) preferential orientation. The hardness, reduced elastic modulus, elastic strain prior to plastic deformation and resistance to plastic deformation of the nanocomposite (TiAlSiY)N coating are determined as 24.6 GPa, 243 GPa, ~0.09 and 0.29 GPa, respectively. The enhancement of the mechanical properties of the multilayer coating up to 38.37, 392.5 GPa, ~0.09 and 0.38 GPa is also observed.

Long-term behaviour of Nb and Cr nitrides nanostructured coatings under steam at 650 °C. Mechanistic considerations

There is an increasing demand for steam power plants to operate in super-critical conditions i.e. temperatures in excess of 600 °C. Under these conditions creep resistant ferritic steels oxidize and therefore require coatings in order to last. Physical vapor deposition and especially High Power Impulse Magnetron Sputtering deposited CrN/NbN nano-scale multilayer coatings with a 2.45 Cr/Nb ratio showed excellent performance when exposed to 650 °C in pure steam environment up to 2000 h. However the role of Nb in offering protection is unclear. In order to study the long term behaviour of this type of coatings as well as to determine the influence of Nb on their oxidation resistance, a CrN/NbN coating with a 1.16 Cr/Nb ratio was studied for 12,650 h. The coating is hard, well adhered and resistant to environmental corrosion, which are properties required in particular for coatings to be applied on turbine blades. The coating also protects P92 from steam oxidation at 650 °C, however coating growth defects influence significantly the oxidation resistance. The long-time exposure allowed to study the protection/degradation mechanisms provided by this type of ceramic coatings. It was found that oxide nodules grow due to the presence of coating defect originated from substrate defects. Moreover, the higher Nb CrN/NbN coating slowly oxidizes, consuming the coating to a large extent after 12,650 h. As a result, protective oxides containing Cr and Nb are developed, remaining well attached to the substrate for at least the test duration, and preventing further substrate oxidation by steam. Interestingly, thin voids present in the as deposited coating self-heal by forming Cr rich oxides, which block steam to reach the substrate.