Stoichiometric CrN Research Articles

Influence of nitrogen and niobium incorporation in bcc-chromium coatings on microstructure and mechanical properties

Stoichiometric CrN and Cr:N with different nitrogen (N) content are of interest for hard coating applications. In the Cr-N material system, bcc-Cr rich coatings, containing a few percent of diluted N, provide tunability in microstructure and mechanical properties. Additionally, the incorporation of Nb into the CrNx coatings may further tailor the materials properties. In this work, bcc-CrNx and CrNbNx coatings were deposited by reactive magnetron sputtering, and their mechanical and microstructural characteristics were investigated as a function of N content. Depending on the N content, the phases observed by X-ray diffractometry (XRD) varied from metallic bcc-Cr, mixed bcc-Cr/h-Cr2N, to h-Cr2N/CrN. X-ray reflectivity (XRR), and scanning electron microscopy (SEM) measurements show that a dense and nearly columnar-free bcc-CrNx coatings was obtained at ~13–25 at.% of N, while CrNbNx coatings composed of dense, column-free, and featureless microstructure at a N content of ~10–20 at.%. The dense and nearly column-free microstructure composed of dispersion of Cr2N grains into the bcc-Cr matrix for both CrNx and CrNbNx coatings, as shown by high resolution transmission electron microscope analysis (HRTEM). Nanoindentation revealed a hardening of the coatings due to the grain refinement, solid solution strengthening, and variation in phase content.

Tailoring structure, morphology, and tribo-mechanical properties of HiPIMS-deposited CrxNy coatings for enhanced performance in wear and corrosion protection

During reactive sputtering of CrxNy, precise regulation of nitrogen gas is crucial to control the properties of the deposited coating. In the present work, we investigate the influence of nitrogen content on CrxNy characteristics, including morphology, crystalline structure, hardness, corrosion protection, and wear protection performances. Our findings reveal that the hexagonal Cr2N coatings, although slightly lower in hardness (19 GPa) compared to stoichiometric CrN, exhibit superior corrosion resistance with a protection efficiency of 82%, and a reduced sliding wear behavior at 1.35E-09 mm3/N/m. Conversely, stoichiometric CrN shows the highest hardness at 22.8 GPa. Using high-power impulsed magnetron sputtering as a deposition technique, we were able to design CrxNy protective coatings with desired microstructure and phase composition, enabling the production of performance-tailored CrxNy coatings suitable for a diverse range of applications.

Phase Composition and Thermoelectric Properties of Epitaxial CrMoVN Thin Films

Thin films of CrMoVN are deposited on c‐plane sapphire (Al2O3 (0001)) by direct current reactive magnetron sputtering, to investigate the effects of Mo and V addition to CrN‐based films. All films grow epitaxially, but Mo incorporation affects the crystal structure and nitrogen content. All films in the CrMoVN series are understoichiometric in nitrogen, but largely retain the NaCl B1 structure of stoichiometric CrN films. Addition of vanadium increases the phase‐stability range of the cubic phase, allowing for higher solubility of Mo than what has previously been reported for cubic CrN. The Seebeck coefficient and electrical resistivity are greatly affected by the alloying, showing a decrease of the Seebeck coefficient along with a decrease in resistivity. Cr0.83Mo0.11V0.06Nz shows a 70% increase in power factor (S2σ = 0.22 mW m−1 K−2) compared to the reference CrNz (S2σ = 0.13 mW m−1 K−2).

Thermal, electrical, and mechanical properties of hard nitrogen-alloyed Cr thin films deposited by magnetron sputtering

CrN based materials, including stoichiometric CrN and Cr:N with a wide range of nitrogen contents, are commonly used as hard and corrosion-resistant coatings. Cr-rich films in this materials system can retain the bcc structure of metallic Cr with few percent of dissolved nitrogen, which can be used for tailoring the mechanical, thermal, and electrical properties. Here, we investigated low nitrogen containing Cr thin films deposited by high ion assisted magnetron sputtering with a substrate temperature of 200 °C. With the gas flow ratio maintained at fN2/Ar = 0.02, the substrate bias and the target power allows for control of the film composition (0.03 < N/Cr < 0.34). The films comprise a mixture of bcc-Cr and hexagonal Cr2N1-δ phases. The mechanical properties studied by nanoindentation and Brillouin inelastic light scattering revealed a hardening effect due to the multiphase nanostructure. The mechanical properties of the Cr:N films depend on the residual stress, on the amount of h- Cr2N1-δ phase and on the nanostructuring nature of the coatings. A maximum hardness of 37 GPa was achieved for a dense film with a Young's modulus of 340 GPa, a shear modulus of 118 GPa, and a relatively low thermal conductivity of 7 W/mK.

Synthesis, Hardness, and Electronic Properties of Stoichiometric VN and CrN

We report synthesis of single-crystal VN and CrN through high-pressure ion-exchange reaction routes. The final products are stoichiometric and have crystallite sizes in the range of 50–120 μm. We also prepared VN and TiN crystals using high-pressure sintering of nitride powders. On the basis of single-crystal indentation testing, the determined asymptotic Vickers hardness for TiN, VN, and CrN is 18 (1), 10 (1), and 16 (1) GPa, respectively. The relatively low hardness in VN indicates that the metallic bonding prevails due to the overfilled metallic σ bonds, although the cation−anion covalent hybridization in this compound is much stronger than that in TiN and CrN. All three nitrides are intrinsically excellent metals at ambient pressure. In particular, VN exhibits superconducting transition at Tc ≈ 7.8 K, which is slightly lower than the reported values for nitrogen-deficient or crystalline-disordered samples due to unsuppressed “spin fluctuation” in the well-crystallized stoichiometric VN. The magnetostructural transition in CrN correlates with a metal–metal transition at TN = 240(5) K and is accompanied by a ∼40% drop in electrical resistivity. In addition, more detailed electronic properties are presented with new insights into these nitrides.

Structural stability and thermodynamics of CrN magnetic phases fromab initiocalculations and experiment

The dynamical and thermodynamic phase stabilities of the stoichiometric compound CrN including different structural and magnetic configurations are comprehensively investigated using a first-principles density-functional-theory (DFT) plus U approach in conjunction with experimental measurements of the thermal expansion. Comparing DFT and DFT+U results with experimental data reveals that the treatment of electron correlations using methods beyond standard DFT is crucial. The non-magnetic face-centered cubic B1-CrN phase is both, elastically and dynamically unstable, even under high pressure, while CrN phases with non-zero local magnetic moments are predicted to be dynamically stable within the framework of the DFT+U scheme. Furthermore, the impact of different treatments for the exchange-correlation (xc)-functional is investigated by carrying out all computations employing the local density approximation and generalized gradient approximation. To address finite-temperature properties, both, magnetic and vibrational contributions to the free energy have been computed employing our recently developed spin-space averaging method. The calculated phase transition temperature between low-temperature antiferromagnetic and high-temperature paramagnetic (PM) CrN variants is in excellent agreement with experimental values and reveals the strong impact of the choice of the xc-functional. The temperature-dependent linear thermal expansion coefficient of CrN is experimentally determined by the wafer curvature method from a reactive magnetron sputter deposited single-phase B1-CrN thin film with dense film morphology. A good agreement is found between experimental and ab initio calculated linear thermal expansion coefficients of PM B1-CrN. Other thermodynamic properties, such as the specific heat capacity, have been computed as well and compared to previous experimental data.

ChemInform Abstract: Synthesis of Stoichiometric and Bulk CrN Through a Solid‐State Ion‐Exchange Reaction.

AbstractPhase‐pure, stoichiometric, and well‐crystallized bulk CrN is prepared from powders of Na2CrO4 and hexagonal BN in a molar ratio of 1:2 by a solid state ion‐exchange route under high pressure (5 GPa, 1200 °C).

A comparison of the oxidation behavior of CrN films deposited using continuous dc, pulsed dc and modulated pulsed power magnetron sputtering

The study is aimed at comparing the oxidation behavior of the stoichiometric CrN films deposited by continuous dc magnetron sputtering (dcMS), mid-frequency pulsed dc magnetron sputtering (PMS), and modulated pulsed power (MPP) magnetron sputtering techniques in a closed field unbalanced magnetron sputtering system. These as-deposited CrN films exhibited a cubic structure and similar stoichiometric compositions, but with different microstructures and residual stresses. After annealing in the ambient air from 600 to 1000°C, the changes in the crystal phase, microstructure, and hardness of the films were characterized using X-ray diffraction, scanning electron microscopy, energy dispersive X-ray spectroscopy line scan, and nanoindentation. The oxidation activation energies of the films were calculated using Arrhenius equation. It was found that the MPP CrN film exhibited superior oxidation resistance than dcMS and PMS CrN films. After annealing at 900°C, the MPP CrN film exhibited an extremely dense structure and the cubic phase was well maintained. On the other hand, the dcMS and PMS CrN films were severely oxidized into a porous structure with the development of β-Cr2N and Cr2O3 phases and a rapid degradation of the cubic phase after 700 and 800°C, respectively. The results indicate that all films showed a parabolic oxidation rate below 900°C. The oxidation activation energies for the dcMS, PMS and MPP CrN films are 116kJ/mol, 141kJ/mol, and 195kJ/mol, respectively. The better oxidation resistance of the MPP CrN film is attributed to its dense microstructure and low residual stress.

Ion-beam irradiation effects on reactively sputtered CrN layers

This paper presents a study of micro-structural changes induced in CrN layers by irradiation with 120 keV argon ions. The layers were deposited on (100) Si wafers, at different nitrogen partial pressures (2?10-4, 3.5?10-4 and 5?10-4 mbar), to a total thickness of 260-280 nm. During deposition the substrates were held at 150?C. After deposition the samples were irradiated with argon ions to the fluencies of 1?1015 and 1?1016 ions/cm2, under the vacuum of 7?10-6 mbar. Characterization of the samples structure and morphology were performed by X-ray diffraction (XRD) analysis and cross-sectional transmission electron microscopy (XTEM), and the concentration profiles were determined by Rutheford backscattering (RBS) spectrometry. It was found that the layer composition strongly depends on the nitrogen partial pressure during deposition. A pure stoichiometric CrN phase was achieved for the highest nitrogen partial pressure (5?10-4 mbar). Argon ions irradiation induces micro-structural changes in the CrN layers such as variation of the lattice constants, micro-strain and mean grain size.

Electronic and magnetic phase diagram ofCr1−xVxN

We report the structural, magnetic, and electronic phase diagram of ${\text{Cr}}_{1\ensuremath{-}x}{\text{V}}_{x}\text{N}$. Stoichiometric CrN is a narrow gap, correlation-induced, semiconductor that orders antiferromagnetically below 286 K. The changes in the chemical bond associated to the magnetic order result in a nonactivated behavior of the resistivity in the antiferromagnetic state. Introducing holes into this system produces a series of inhomogeneous magnetic/electronic states, as identified through electronic and thermal conductivity, and magnetic susceptibility. The magnetic/electronic phase diagram of ${\text{Cr}}_{1\ensuremath{-}x}{\text{V}}_{x}\text{N}$ is an example of electronic complexity in a simple system from the chemical and structural point of view.

Room temperature ferromagnetism and hopping transport in amorphous CrN thin films

Magnetic and electronic properties of stoichiometric amorphous CrN thin films grown on MgO (001) substrates by radio-frequency nitrogen-plasma-assisted molecular beam epitaxy have been investigated. The magnetic property of the amorphous CrN thin films shows a ferromagnetic behavior even at room temperature, and can be interpreted by the percolation theory of magnetic polaron where we consider Cr3+ defects as magnetic impurities which lead to the formation of bound magnetic polarons. The obtained results of electrical conductivity are explained by the variable-range-hopping theory of the Mott and Davis model.

Ion-beam irradiation effects on reactively sputtered CrN thin films

The present study deals with CrN/Si bilayers irradiated at room temperature (RT) with 120 keV Ar ions. The CrN layers were deposited by d.c. reactive sputtering on Si(1 0 0) wafers, at different nitrogen partial pressures (2 × 10 −4, 3.5 × 10 −4 and 5 × 10 −4 mbar), to a total thickness of 240–280 nm. The substrates were held at room temperature (RT) or 150 °C during deposition. After deposition the CrN/Si bilayers were irradiated up to fluences of 1 × 10 15 and 1 × 10 16 ions/cm 2. Structural characterization was performed with Rutherford backscattering spectroscopy (RBS), cross-sectional transmission electron microscopy (XTEM) and grazing angle X-ray diffraction (XRD). For the highest nitrogen pressure (5 × 10 −4 mbar) a pure stoichiometric CrN phase was achieved. The results showed that Ar ion irradiation resulted in the variation of the lattice constants, micro-strain and mean grain size of the CrN layers. The observed microstructural changes are due to the formation of the high density damage region in the CrN thin film structure.

Thermoelectric properties of stoichiometric and hole-doped CrN

We report the thermoelectric figure of merit of chromium nitride, CrN, and its optimization through hole-doping. CrN is a degenerate semiconductor with large thermoelectric power, reaching −185 μV/K at 420 K. The resistivity can be reduced through hole-doping in the series Cr1−xVxN, keeping a large thermopower. The thermal conductivity of CrN is rather low compared to other transition-metal nitrides, reaching its minimum value of 1.0 W/m K at 267 K. The largest ZT=0.04 was measured for Cr0.9V0.1N at room temperature. Our results suggest that CrN could be a good starting point for the design of a thermoelectric material with optimal mechanical properties.

The influence of rotation during sputtering on the stoichiometry of TiAlN/CrN x multilayer coating

Multilayer coatings TiAlN/CrN x were prepared by magnetron sputtering with planetary rotation of samples alongside TiAl and Cr targets. Coatings were investigated by transmission electron microscope (TEM). Thickness of individual layers was 50–80 nm. Chemical composition across TiAlN and CrN x layers was measured by electron energy-loss spectroscopy (EELS) point-by-point analysis. It was found that stoichiometry in CrN x varied strongly while the composition in TiAlN was approximately constant. TEM images revealed ~ 25 nm thick lamellae inside some of the CrN x layers. It was found that lamellae correspond to hexagonal Cr 2N phase. Variations of stoichiometry in CrN x and existence of Cr 2N phase can be explained by differences in the deposition rate which occur due to rotation of sample. During the deposition process the nitrogen partial pressure was constant, therefore the only variable that could influence stoichiometry was deposition rate. In certain positions of sample the deposition rate was high enough for growth of Cr 2N phase. Changes of stoichiometry did not occur in TiAlN indicating that nitrogen partial pressure was high enough for growth of stoichiometric TiAlN layers but not high enough for growth of stoichiometric CrN. A previously developed model, in which we calculated the deposition rate in dependence of trajectory of sample, can explain observed differences in stoichiometry and growth of Cr 2N phase.

Control of microstructures and properties of dc magnetron sputtering deposited chromium nitride films

Chromium nitride coatings have been prepared by a conventional DC magnetron reactive sputtering process in nitrogen–argon mixed atmospheres. The sputtering pressure and target voltage versus nitrogen flow rate curves were established in order to control the structures and properties of chromium nitride coatings. A good correspondence among the sputtering pressure, target voltage evolutions and the phase developments with respect to nitrogen flow rate has been found. The stoichiometric Cr 2N and CrN coatings were confirmed by EPMA and XRD analysis. Cryogenic fracture cross-section SEM images show columnar growth morphologies. Stoichiometric chromium nitrides present high hardness and elastic modulus as well as high H 3/ E 2 ratio in a nano-indenter test. Adhesion and tribological properties were evaluated by scratch and pin-on-disk tests, respectively. Chromium nitrides present normal adhesion failure critical load (Lc 2) between 10 and 20 N and friction coefficients ranging from 0.5 to 0.75.

Electron-irradiation damage in chromium nitrides and chromium oxynitride thin films

The aim of this work is to monitor changes of the N–K electron energy-loss near-edge structure (ELNES) of chromium nitride layers (CrN) introduced by electron irradiation in a transmission electron microscope (TEM). These changes are different for each sample material and seem to give an indication for a particular composition. The CrN samples (CrN and Cr0.47N0.53) were prepared on silicon wafers by reactive magnetron sputtering of a metallic chromium target in nitrogen plasma. In addition, a CrON sample (Cr0.5O0.2N0.3) was also investigated. This sample was prepared by the addition of oxygen to the plasma during film deposition. The ELNES of the N–K ionization edge of stoichiometric CrN shows a typical fine structure (peaks at 399.0 and 401.1eV) and remains nearly unaffected even after high-current-density irradiation. On the other hand the N–K fine structures of Cr0.47N0.53 and Cr0.5O0.2N0.3 show a change of the ELNES with irradiation dose. This presumably arises from a 1s–π*-transition of molecular nitrogen located at interstitial positions in these samples.

Metal/semiconductor phase transition in chromium nitride(001) grown by rf-plasma-assisted molecular-beam epitaxy

Structural and electronic properties of stoichiometric single-phase CrN(001) thin films grown on MgO(001) substrates by radio-frequency N plasma-assisted molecular-beam epitaxy, are investigated. In situ room-temperature scanning tunneling microscopy clearly shows the 1×1 atomic periodicity of the crystal structure as well as long-range topographic distortions which are characteristic of a semiconductor surface. This semiconductor behavior is consistent with ex situ resistivity measurements over the range 285 K and higher, whereas below 260 K, metallic behavior is observed. The resistivity-derived band gap for the high-temperature region, 71 meV, is consistent with the tunneling spectroscopy results. The observed electronic (semiconductor/metal) transition temperature coincides with the temperature of the known coincident magnetic (para-antiferro) and structural (cubic-orthorhombic) phase transitions.

Nanostructured chromium nitride films with a valley of residual stress

A series of chromium nitride films has been prepared by reactive DC magnetron sputtering onto multiple moving substrates. The films are characterized via an array of experimental techniques, including high-resolution synchrotron radiation X-ray scattering, transmission electron microscope (TEM), atomic force microscope (AFM) and double-crystal diffraction topography (DCDT). Chromium nitride films displays distinct phase structure, stress state and growth morphology, depending on nitrogen flows. The films deposited at very low nitrogen flows show significantly refined microstructures. The macrostructural refinement, accompanied by decreased surface roughness, takes place continuously with increasing nitrogen flow until a nanocrystalline film is obtained. The nanostructured film appears to occur only at the onset of transition from the hexagonal Cr 2N to a distorted cubic lattice CrN x . Correspondingly, a deep stress valley (i.e. a sudden reversal of tensile stress to compressive stress) is observed by DCDT. Deposition at higher nitrogen flows leads to the formation of nearly stoichiometric CrN films, with slightly elongated grain structures. The elongated grains also align up crystallographically in a unique direction related to the deposition geometry, indicating the development of tri-axial texture in the CrN films.

Phase formation and structure of magnetron sputtered chromium nitride films: in-situ and ex-situ studies

The effects of deposition conditions on phase formation, crystallographic characteristics, residual stress, and morphology of chromium nitride films have been studied. The films were prepared via reactive magnetron sputtering with a confocal deposition geometry. The geometry is compatible with an X-ray diffraction system, which allows the growing films to be studied in-situ and in a quasi-real time fashion. The use of the unique film growth/characterization system indicates that in-situ X-ray diffraction can be a useful tool for in-situ monitoring film depositions at various growth stages. In addition to in-situ diffraction studies, the grazing incidence X-ray scattering (GIXS) with synchrotron radiation, pole figures, double crystal diffraction topography (DCDT) and scanning electron microscope were also used to characterize the post-deposited films. The effect of nitrogen flow on phase formation can be described based on three ranges of nitrogen flows. With low nitrogen flows, the films consist of primarily BCC Cr phase or hexagonal β-Cr 2N. Growth at moderate nitrogen flows leads to the films of N-deficient cubic CrN 1− x . With high nitrogen flow, the deposition results in films of nearly stoichiometric CrN with the (111) type texture. In particular, the processing window for Cr 2N appears to be very small, in comparison with CrN, which can be readily obtained at high nitrogen flows. Thus the films of single Cr 2N phase with well-defined β structure can only be obtained under strictly controlled conditions.

Surface chemical changes induced by low‐energy ion bombardment in chromium nitride layers

AbstractIn this work three types of CrNx layers—one close to Cr2N, the other to CrN and a third with high nitrogen content CrN1+x—were prepared by reactive magnetron sputtering. The samples were subjected to bombardment by Ar+ and N2+ ions applied in sequence. The relative atomic concentration and the chemical states of the elements in the surface layer were determined by x‐ray photoelectron spectroscopy. Alternate bombardment with Ar+ and N2+ ions (1–2 keV) resulted in significant compositional and chemical state changes. Although prolonged Ar+ bombardment did not decrease the nitrogen content of Cr2N below the nominal (CrN0.5) composition, it decreased the nitrogen content of the closely stoichiometric CrN appreciably, to about CrN0.8. At bombardment with N2+ ions nitrogen could be implanted into the Cr2N films, changing its composition up to about CrN0.8. It was shown that extra nitrogen could be incorporated also into the CrN samples, increasing the nitrogen content above the stoichiometric composition up to about CrN1.2. The CrN1+x‐type sample showed high stability towards Ar+ bombardment. Two significantly different chemical states of nitrogen were determined for the two types of nitrides: the N 1s peak at 397.5 ± 0.1 eV binding energy was characteristic of N in Cr2N and the peak at 396.5 ± 0.1 eV of N in the CrN state. The effect of Ar+ and N2+ bombardment on the surface composition discussed in this paper may provide information for elaboration of deposition conditions for CrNx coatings with predetermined chemical composition and structure. Copyright © 2002 John Wiley &amp; Sons, Ltd.