CrN Grains Research Articles

In-plane Cr2N−CrN metal-semiconductor heterostructure with improved thermoelectric properties

Epitaxial metal-semiconductor heterostructures with suitable Schottky barrier can lead to high thermoelectric figure-of-merit (zT) due to selective filtering of low-energy electrons as well as reduced thermal conductivity from phonon scattering at the interfaces. Lattice-matched vertical metal-semiconductor multilayer/superlattices as well as metallic nanoparticles embedded inside semiconducting hosts have been studied intensively to explore their thermoelectric properties. However, development of in-plane metal-semiconductor heterostructures and exploration of their physical properties have remained elusive primarily due to the growth and fabrication challenges. In-plane heterostructures are expected to be more suitable for planar integration and should exhibit unique properties. In this work, we demonstrate an in-plane ${\mathrm{Cr}}_{2}\mathrm{N}\text{\ensuremath{-}}\mathrm{CrN}$ metal-semiconductor heterostructure that exhibits an improved thermoelectric power factor. The in-plane heterostructure is deposited by controlling the Cr-flux during deposition that leads to an in-plane phase separation between the metallic-${\mathrm{Cr}}_{2}\mathrm{N}$ and semiconducting CrN grains. Temperature-dependent electrical transport exhibits an Arrhenius-type thermal activation behavior with an activation energy of 70 meV, and an in-plane electrical conductivity that is about two orders of magnitude higher than that of CrN. The Seebeck coefficient also remained moderately large at \ensuremath{-}150 \ensuremath{\mu}V/K at 700K leading to a very large power factor of $2.1\phantom{\rule{0.16em}{0ex}}\mathrm{mW}/\mathrm{m}{\mathrm{K}}^{2}$ at 700 K.

Fabrication of Cr-Si-N coatings using a hybrid high-power impulse and radio-frequency magnetron co-sputtering: The role of Si incorporation and duty cycle

Cr-Si-N thin films were successfully fabricated by a hybrid deposition system containing a high power impulse magnetron sputtering (HiPIMS) and a radio frequency (RF) magnetron in an Ar and N2 mixture. It can be found that the deposition rate of Cr-Si-N films increased with increasing power of RF and duty cycle of HiPIMS. Varying both HiPIMS duty cycle and RF power lead to a variation in Si content in the resulting Cr-Si-N films, indicating that the Si content in thin films is a crucial parameter. The Cr-Si-N film grown at a duty cycle of 2.5%, a repetition frequency of 1000 Hz, a HiPIMS average power of 500 W, and RF power of 450 W exhibits the highest Si content of 14.2 at.%, hardness of 31.5 GPa, elastic modulus of 292 GPa, the highest resistance to failure value of 0.1079, and the smallest grain size of 8 nm, average surface roughness of 0.7 nm and a wear rate of 0.50 × 10−6 mm3/N-m. The improved film properties can be attributed mainly to the incorporation of Si element. More importantly, the existence of Si atoms in CrN grains for forming solid solution (Cr,Si)N phase or extra Si atoms reacting with N atoms for forming amorphous Si3N4 (Si-N) phase segregated at the boundaries of the columnar CrN grains has strong influence on the microstructure and mechanical properties of thin films. The lower duty cycle of HiPIMS and higher Si content show synergistic effects on the grain refinement and enhanced mechanical properties of Cr-Si-N films in this work.

A proper assessment of TEM diffraction patterns originating from CrN nitrides in a ferritic matrix

Upon nitriding of a Fe-3%Cr alloy at 550 °C during 48 h at a nitriding potential KN = 5 atm−1/2, finely dispersed CrN grains precipitate. Considering that the nitrides developing within the nitrided layer adopt a cubic, rock-salt structure, Transmission Electron Microscopy (TEM) investigations revealed the Bain orientation relationships (also called Baker-Nutting) existing between the matrix and the precipitates. Given the latter and the structures of both ferrite (bcc) and nitrides (fcc), we have shown that group theory predicts the existence of three nitrides variants, orthogonal to each other, developing in ferrite along the 〈001〉α-Fe directions, which is in agreement with our experimental observations. The nitrides adopt a very thin platelet-like morphology, energetically corresponding to an absolute extremum. Diffraction patterns obtained along the [001]α-Fe result from the simultaneous diffraction of the ferritic matrix and the three nitrides variants. The platelet-like nitride precipitates give birth to diffuse intensity lines, originating from the two families developing along the (100) and (010) ferrite planes, as well as possible appearance of additional reflections, located at the forbidden positions {100}α-Fe and {110}CrN of the remaining variant, driving the interpretation of such diffraction patterns complicated. Foil thickness has proven to play a key role in the occurrence of this phenomenon. When the geometric diffraction conditions are favorable (i.e. very thin foil along the zone axis and very fine precipitates platelets), TEM analysis permitted to attribute these extra reflections to the nodes located in the first layer of the third variant's reciprocal space.

Microstructure and mechanical properties of Cr–Ni–N coatings deposited by HiPIMS

Cr–Ni–N coatings, the Ni content of which was altered from 0 to 6·3 at-%, were deposited by a hybrid coating system consisting of high power impulse magnetron sputtering and radio frequency magnetron sputtering. The effects of Ni addition to Cr-N coatings on the microstructure and mechanical properties of the coatings were investigated in this study. The instrumental analysis revealed that the Ni element was incorporated into Cr-N crystals as solid solutions, while excess Ni was precipitated as nanocrystalline phases at the Cr-N grain boundaries. The toughness of the Cr–Ni–N coatings was significantly improved with Ni content increased. In addition, the hardness of the coatings slightly increased from 30·4 to 32·6 GPa due to the grain refinement effect and solid solution hardening. From the tribological tests, the highest wear resistance was obtained from the sample having a Ni content of 2·7 at-%.

Understanding Local Bonding Structures of Ni-Doped Chromium Nitride Coatings through Synchrotron Radiation NEXAFS Spectroscopy

CrN has widespread applications as protective coatings, for example, in aircraft jet engines whereby their high hardness and good oxidation resistance render metal components resistant to harsh operating conditions. Alloying elements are commonly incorporated (doped) into the coatings to further enhance their thermomechanical properties. However, the effect of dopants on the electronic properties and their roles in modifying the grain boundary configurations remain unclear. Lack of such critical knowledge has hindered the development of design strategies for high performance CrN-based coatings. To address this challenging issue, in the present study near-edge X-ray absorption fine structure (NEXAFS) investigations of Cr1–yNiyN coatings at the Cr L3,2-edge (570–610 eV), Ni L3,2-edge (840–890 eV), and N K-edge (380–450 eV) regions were conducted using synchrotron radiation soft X-ray (SXR) spectroscopy in both Auger electron yield (AEY) and total fluorescence yield (TFY) modes. The chemical states in CrNiN were found to change with the increase of Ni content, manifested as a small chemical shift and moderate change of shapes of various absorption edges. The CrN grain size also became smaller with increasing Ni concentration. These findings help improve our understanding of local bonding structures, which could potentially lead to improved coating designs for highly demanding applications.

Designing superhard, self-toughening CrAlN coatings through grain boundary engineering

One of the toughest challenges that hinders the application of ceramic coatings is their poor damage tolerance. Addressing this problem requires the development of novel micro- or nanostructures that would impart to these coatings both high hardness and high toughness. In this paper, CrAlN coatings, with varying Al contents up to 30at.%, were engineered onto steel substrates using the magnetron sputtering technique. Whilst the addition of Al does not significantly alter the columnar microstructure, it does change the preferred grain orientation and increase the compressive residual stress. Moreover, the hardness, elastic strain to failure (H/E) and plastic deformation resistance (H3/E2) of the resultant CrAlN coating with the highest Al content were found to increase ∼47, ∼29 and ∼140%, respectively, as compared to CrN. Evidence collected from transmission electron microscopy and X-ray photoelectron spectroscopy experiments shows that AlN, existing in an amorphous state at the columnar CrN grain boundaries, has a crucial role in providing the unusual combination of high hardness and exceptional damage resistance. The results provide a new pathway to developing durable ceramic coatings suitable for applications involving severe loading conditions.

MICROSTRUCTURE AND MECHANICAL PROPERTIES OF DLC FILMS DOPED WITH HIGH CRYSTALLINITY CrN NANOPARTICLES

DLC films with dispersed high crystallinity CrN nanoparticles were prepared by high power pulsed magnetron discharge plasma ion implantation deposition(HPPMS-PIID) combined with DC magnetron sputtering(DCMS).The surface morphology,structure and properties of CrN-DLC films with the different currents of C target were studied.The results show that the C content increases as the raise of the current of the C target and clear characteristics of DLC films are found at a higher C content.CrN doped in DLC exists as nanoparticles with highly 200 preferred orientation, and the smallest size of the CrN grains is 42.39 nm.The C1s peak primarily consists of the three peaks that correspond to C-sp~2,C-sp~3 and CN-sp~3,and the ratio of total sp~3 to sp~2 is 44.8%.Excellent adhesion between film and substrate with critical load of 66.8 N and high nanohardness up to 24.3 GPa are achieved due to highly energetic ion bombardment and implantation in HPPMS-PIID.

Factors governing the mechanical behaviour of CrSiN coatings: Combined nanoindentation testing and transmission electron microscopy

A direct link between the microstructure and mechanical properties of CrSiN coatings with varying Si contents was established by examining their microstructural evolution and deformation behaviour, enabled by cross-sectional transmission electron microscopy. A transformation from a columnar-grained structure, in the binary compound, to a nanocomposite structure, consisting of CrN nanocrystallites embedded in an amorphous matrix, was observed in the coating having a Si content of 28.5 at.%. The columnarstructured coatings were observed to deform by grain boundary shear sliding, whilst cracks occurred in nanocomposite coatings. Moreover, the coating having a Si content of 14.3 at.% exhibited the highest hardness as a result of grain boundary strengthening facilitated by Si3N4 segregation, together with solid solution strengthening of Si in CrN, the Hall–Petch effect for the finer CrN grains and a high compressive residual stress. A further increase of Si content to 28.5 at.% led to a reduction in hardness due to the high volume fraction of the amorphous phase, together with the presence of cracks in the as-deposited coating. The findings provide deeper insight into the origin of the mechanical behaviour of CrSiN coatings.

Structure-property-performance of nanocomposite Cr–Si–N films synthesized by a duplex surface treatment technique

Abstract

Microstructure and mechanical property evaluation of pulsed DC magnetron sputtered Cr–B and Cr–B–N films

CrB2 and four Cr–B–N films with high Cr/B ratio and various nitrogen contents were deposited by a co-sputtering process using a bipolar asymmetric pulsed DC reactive magnetron sputtering system. The structures and BN bonding nature of the thin films were characterized by X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR), respectively. The surface and cross sectional morphologies of the thin films were examined by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The surface roughness of the thin films was explored by atomic force microscopy (AFM). Nanoindentation, microscratch and ball-on-disk wear techniques were used to evaluate the hardness, and tribological properties of the thin films, respectively.The microstructure of the Cr–B–N thin films changed from a coarse columnar structure to a glassy and featureless morphology as the nitrogen content increased from 15.2at.% to 54.5at.%, whereas the corresponding structure developed from an amorphous state to a nanocomposite structure consisting of CrN nanograins and amorphous BN phases. It was found that high hardness, good tribological and brittle properties were obtained for the CrB2 coating. The hardness and elastic modulus of the Cr–B–N thin films decreased with increasing nitrogen content until the nanocomposite structure of nanocrystalline CrN grains and an amorphous BN matrix was formed. However, the hardening effect induced by the nanocomposite structure was limited due to the fact that the small CrN nanograins were surrounded by a thick intergranular soft amorphous BN layer. On the other hand, the fracture toughness and resistance against elastic strain to failure of the Cr–B–N coatings were effectively enhanced by the addition of nitrogen.

Formation of a nanostructured CrN layer on nitrided tool steel by low-temperature chromizing

A duplex chromizing process involving the salt bath chromizing of a plasma-nitrided tool steel was carried out at 773 K for 3–24 h. A single-phase CrN layer with a grain size ranging from 20 to 50 nm and an underlying layer consisting of α-Fe and nanosized CrN grains with a grain size ranging from 40 to 80 nm were formed by a 24 h chromizing at 773 K. The duplex chromizing mechanism was discussed.

High temperature phase changes and oxidation behavior of Cr–Si–N coatings

Composition, structure and morphology of Cr–Si–N coatings deposited by cathodic arc ion-plating on cemented carbide substrates were studied. A systematic variation of the deposition parameters resulted in relative Si atomic concentrations Si/(Cr + Si) within 0 and 15at.%, which affect significantly the properties of the coatings, their phase stability and oxidation resistance. All Cr–Si–N coatings consist of nanocrystalline CrN grains with diminishing crystallite sizes at increased Si content. The microstructure of the samples, observed by scanning electron microscopy, is columnar for the coatings with low Si concentration, and becomes denser for specimens with higher Si content. The hardness and Young's modulus of the coatings increase with increasing Si concentration up to a maximum value of 26GPa and 430GPa, respectively, for the Cr 0.94Si 0.06N coatings, followed by a progressive decrease. X-ray powder diffraction studies were performed in situ up to 1000°C in vacuum and in air. Annealing in vacuum resulted in the decomposition of CrN into Cr 2N and N 2 and the subsequent oxidation. The annealing performed in air proved an excellent oxidation resistance of the coatings, which strongly depends on their composition and morphology. The best oxidation resistance was obtained for the hardest samples with intermediate Si concentration, for which the formation of Cr 2O 3 has not been observed even at 1000°C. Recrystallization, which occurs at elevated temperatures both in vacuum and in the air, becomes significant above approximately 800°C.

Phase transformations in low-temperature chromized 0.45 wt.% C plain carbon steel

The phase transformations occurring in a 0.45 wt.% C plain steel subjected to plasma nitriding at 540–560 °C for 5.5 h, followed by a salt bath thermoreactive deposition and diffusion (TRD) chromizing process at 500 °C or 550 °C (a process referred to as low-temperature chromizing or duplex chromizing) was investigated by means of optical microscopy(OM), scanning electron microscopy(SEM), X-ray energy dispersive spectroscopy(EDS), and X-ray diffraction. It was found that a CrN compound layer with an average thickness of 7.4 μm and an average micro-hardness of 1476 HV 0.01 was formed in the prior plasma nitrided compound layer by low-temperature chromizing at 550 °C for 6 h. The chromized coating as a whole was found consisting of three sub-layers, namely the outer CrN layer, the intermediate diffusional layer, and the inner residual nitrided compound layer, all formed in the prior nitrided compound layer, and with the inner sub-layer vanishing by prolonging the chromizing time. The intermediate diffusional layer formed at the initial stages of TRD was seen “black” under OM (hence is called “black zone”), and found consisting of α-Fe as a major phase. The self-exhaustion of the “black zone” promoted the chromium atom diffusion deeper into the substrates. The transformation paths involved in the decomposition of the prior nitrided compound layer was likely to be ε-Fe 2–3N → γ′-Fe 4N → α-Fe; and the high hardness of the chromized coating was attributed to a large amount of nano-sized and evenly distributed CrN grains generated in the compound layer.

Growth of epitaxial CrN on MgO(001): Role of deposition angle on surface morphological evolution

CrN layers, 6 to 500 nm thick, were grown on MgO(001) at 600 °C by ultra-high-vacuum magnetron sputter deposition in pure N 2 discharges at 2.6 Pa. The deposition angle α with respect to the surface normal was varied from 0° to 80° in order to directly probe the effect of atomic shadowing on the surface morphological evolution. Layers grown with α = 0° are single crystals which develop a regular surface mound structure. At low layer thicknesses, t ≲ 25 nm, the surface mounds grow primarily vertically, due to kinetic roughening, and form square-shapes with edges along low-energy 〈100〉, directions. Continued growth at t ≳ 25 nm is dominated by mound-competition and coalescence which leads to a self-similar growth mode with increases in both mound height and width. Layers deposited from oblique angles α = 80° also nucleate as single crystals with a cube-on-cube epitaxial relationship with the substrate. However, rough surfaces with cauliflower-type morphologies cause the nucleation of misoriented CrN grains that develop into cone-shaped grains that protrude out of the epitaxial matrix to form triangular faceted surface mounds. Atomic shadowing exacerbates the growth rate of these misoriented grains, causing a dramatic increase in the root-mean-square surface roughness, which is ∼ 16× higher for layers grown at α = 80° than at α = 0°. The roughening follows a power-law with a roughening exponent β that increases from 0.37 ± 0.04 to 0.57 ± 0.15 as α is increased from 0° to 80°. This increase is attributed to a transition from kinetic roughening to roughening caused by atomic shadowing effects.

Surface morphological evolution of epitaxial CrN(001) layers

CrN layers, 57 and 230 nm thick, were grown on MgO(001) at Ts=600–800°C by ultrahigh-vacuum magnetron sputter deposition in pure N2 discharges from an oblique deposition angle α=80°. Layers grown at 600 °C nucleate as single crystals with a cube-on-cube epitaxial relationship with the substrate. However, rough surfaces with cauliflower-type morphologies cause the nucleation of misoriented CrN grains that develop into cone-shaped grains that protrude out of the epitaxial matrix to form triangular faceted surface mounds. The surface morphology of epitaxial CrN(001) grown at 700 °C is characterized by dendritic ridge patterns extending along the orthogonal ⟨110⟩ directions superposed by square-shaped super mounds with ⟨100⟩ edges. The ridge patterns are attributed to a Bales-Zangwill instability while the supermounds form due to atomic shadowing which leads to the formation of epitaxial inverted pyramids that are separated from the surrounding layer by tilted nanovoids. Growth at 800 °C yields complete single crystals with smooth surfaces. The root-mean-square surface roughness for 230-nm-thick layers decreases from 18.8 to 9.3 to 1.1 nm as Ts is raised from 600 to 700 to 800 °C. This steep decrease is due to a transition in the roughening mechanism from atomic shadowing to kinetic roughening. Atomic shadowing is dominant at 600 and 700 °C, where misoriented grains and supermounds, respectively, capture a larger fraction of the oblique deposition flux in comparison to the surrounding epitaxial matrix, resulting in a high roughening rate that is described by a power law with an exponent β>0.5. In contrast, kinetic roughening controls the surface morphology for Ts=800°C, as well as the epitaxial fraction of the layers grown at 600 and 700 °C, yielding relatively smooth surfaces and β⩽0.27.

アークイオンプレーティング法により形成したＣｒ‐Ｓｉ‐Ｎ皮膜の機械的特性およびバイアス電圧の影響

Cr-Si-N coatings were prepared by incorporating several atomic percentage of Si to CrN, using an arc ion plating method. In order to change the Si content in the coatings, CrSi targets of 5, 10, 15at%Si were used. The bias voltages were changed from 0 to −250V. Silicon wafers and carburized Chromium Molybdenum Steels (SCM415) were used as the substrates. The aim of this study is to investigate the influence of Si incorporation and bias voltage on the structure and mechanical properties of the coatings. The hardness of the coatings increased drastically up to Hk4000 at 3at%Si with only a very small quantity of Si incorporated. The hardness was also influenced strongly by the bias voltage and increased with increasing bias voltage, attaining maximum at around −60V and then decreased with increasing bias voltage. This can be explained by the change of internal stress of the coatings. The adhesion of the Cr-Si-N coatings was a little smaller than that of the CrN coatings, but within a lower bias voltage range of 0˜−30V. The critical load on scratch test was 60N, the same as that of the CrN coating. The X-ray diffraction (XRD) profiles showed that the coatings with a low content of Si incorporated to CrN presented a polycrystalline structure consisting of nano-crystalline CrN which seemed to be separated from each other by an amorphous Si-N(a-Si-N) phase. It was assumed that the high hardness of the CrSiN coatings was mainly attributed to the formation of a composite structure of nc-CrN and a-Si-N and the decrease of the CrN grain size.

High temperature oxidation of ion-plated CrN films

The high-temperature oxidation of CrN films which were deposited onto stainless steel substrates using an arc ion plating apparatus was studied at temperatures ranging from 1023 to 1173 K for 0.6 to 480 ks in air. The oxidation rate obtained from mass gain as a function of time was found to fit well to a parabolic time dependence. Formed oxide layers were analyzed by XRD, SEM, and SAM. An activation energy of the oxidation of CrN was slightly lower than that of the self-diffusion coefficient of Cr ion in Cr2O3. It is concluded that the oxidation of CrN is controlled by the outward diffusion of Cr ions through the Cr2O3 layer formed on each CrN grain.