High temperature oxidation behavior of CrN/AlN superlattice films

The oxidation behavior of CrN/AlN superlattice films with different bilayer periods (Λ), Al/(Cr + Al) ratios, and crystal structures of the AlN layer was investigated. The films were deposited using a pulsed dc closed field unbalanced magnetron sputtering system. The oxidation tests were carried out in the ambient air at elevated temperatures from 700 to 1100 °C for 1 h. The changes in the crystal phase, microstructure and hardness of the films after the oxidation tests were characterized using X-ray diffraction, scanning electron microscopy and nanoindentation, respectively. When both CrN and AlN layers were in the NaCl cubic structure, the film with Λ = 3.8 nm and an Al/(Cr + Al) ratio of 0.6 exhibited a superior oxidation resistance than the film with Λ = 12.4 nm and an Al/(Cr + Al) ratio of 0.19. The film with Λ = 3.8 nm maintained the nanolayered structure with an oxidation temperature up to 1000 °C by the protection of a thin and dense X-ray amorphous oxide layer. In contrast, when the AlN layers were in the Wurzite hexagonal structure, the film with Λ = 22.5 nm and an Al/(Cr + Al) ratio of 0.67 exhibited poor oxidation resistance. The film lost the superlattice structure at 800 °C and was completely oxidized at 1000 °C due to the formation of a porous crystalline oxide layer on the surface.     

Enhancement of mechanical and tribological properties in AISI D3 steel substrates by using a non-isostructural CrN/AlN multilayer coating

Enhancement of mechanical and tribological properties on AISI D3 steel surfaces coated with CrN/AlN multilayer systems deposited in various bilayer periods (Λ) via magnetron sputtering has been studied in this work exhaustively. The coatings were characterized in terms of structural, chemical, morphological, mechanical and tribological properties by X-ray diffraction (XRD), electron dispersive spectrograph, atomic force microscopy, scanning and transmission electron microscopy, nanoindentation, pin-on-disc and scratch tests. The failure mode mechanisms were observed via optical microscopy. Results from X-ray diffraction analysis revealed that the crystal structure of CrN/AlN multilayer coatings has a NaCl-type lattice structure and hexagonal structure (wurtzite-type) for CrN and AlN, respectively, i.e., made was non-isostructural multilayers. An enhancement of both hardness and elastic modulus up to 28 GPa and 280 GPa, respectively, was observed as the bilayer periods (Λ) in the coatings were decreased. The sample with a bilayer period (Λ) of 60 nm and bilayer number n  =  50 showed the lowest friction coefficient (∼0.18) and the highest critical load (43 N), corresponding to 2.2 and 1.6 times better than those values for the coating deposited with n = 1, respectively. The best behavior was obtained when the bilayer period (Λ) is 60 nm (n = 50), giving the highest hardness 28 GPa and elastic modulus of 280 GPa, the lowest friction coefficient (∼0.18) and the highest critical load of 43 N. These results indicate an enhancement of mechanical, tribological and adhesion properties, comparing to the CrN/AlN multilayer systems with 1 bilayer at 28%, 21%, 40%, and 30%, respectively. This enhancement in hardness and toughness for multilayer coatings could be attributed to the different mechanisms for layer formation with nanometric thickness such as the Hall–Petch effect and the number of interfaces that act as obstacles for the crack deflection and dissipation of crack energy.     

Comparison of microstructure and phase transformation for nanolayered CrN/AlN and TiN/AlN coatings at elevated temperatures in air environment

CrN/AlN and TiN/AlN multilayer coatings with modulation period of 4 nm and thickness ratio equal to 1.0 were manufactured by RF magnetron sputtering. Both films were annealed at temperatures of 800 °C in air for 1 h and then for an additional 9 h. Both coatings in as-deposited and after heat treatment were evaluated with a transmission electron microscope (TEM) equipped with EDS. After heat treatment at 800 °C for 1 h, a thick oxide layer around 260 nm was formed on the surface of the TiN/AlN coating. The oxide layer which formed on the coating was composed of three different regimes, including Al-enriched oxide with excess oxygen on the top surface, a crystalline Al-depleted TiO₂ layer 30-80 nm thick above the nitride coating and in between, mixed nano-crystalline Al₂O₃ and TiO₂ films. In comparison, only one oxide layer smaller than 50 nm in thickness was found in the annealed CrN/AlN coating. This amorphous or nanocrystalline oxide layer identified by EDS was a metal-deficient oxide, in which Al₂O₃ and Cr₂O₃ were mixed together forming a solid solution. As a result, the CrN/AlN coating exhibited superior stability compared to the TiN/AlN coating at elevated temperatures.     

Characterizations of CrN/a-CNx nanolayered coatings deposited by DC reactive magnetron sputtering of Cr and graphite targets

CrN/a-CN_x nanolayered coatings have been deposited by DC reactive magnetron sputtering of pure Cr and graphite targets. The total thickness is 1 μm and that of a-CN_x layers is kept constant at 3.5 nm. The period (bilayer thickness) is in the range 8-16 nm. CrN and a-CN_x layers are crystalline and amorphous respectively. The decrease of CrN layers’ thickness (decrease of period) in the stack leads to refinement of CrN microstructure associated with (200) preferred orientation. The hardness of nanolayered films is independent of the period’s thickness, while internal compressive stress, which remains between that of each elementary layer, follows an evolution close to that of the law of mixtures. The best tribological behaviours are reached for a periods’ thickness of 8 nm.     

Structure, hardness and thermal stability of nanolayered TiN/CrN multilayer coatings

Nanolayered TiN/CrN multilayer coatings were deposited on silicon substrates using a reactive DC magnetron sputtering process at various modulation wavelengths (Λ), substrate biases (V_B) and substrate temperatures (T_S). X-ray diffraction (XRD), nanoindentation and atomic force microscopy (AFM) were used to characterize the coatings. The XRD confirmed the formation of superlattice structure at low modulation wavelengths. The maximum hardness of the TiN/CrN multilayers was 3800 kg/mm² at Λ=80  Å, V_B=−150 V and T_S=400°C. Thermal stability of TiN, CrN and TiN/CrN multilayer coatings was studied by heating the coatings in air in the temperature range (T_A) of 400-800°C. The XRD data revealed that TiN/CrN multilayers retained superlattice structure even up to 700°C and oxides were detected only after T_A?750°C, whereas for single layer TiN and CrN coatings oxides were detected even at 550°C and 600°C, respectively. Nanoindentation measurements showed that TiN/CrN multilayers retained a hardness of 2800 kg/mm² upon annealing at 700°C, and this decrease in the hardness was attributed to interdiffusion at the interfaces.     

Microstructure and mechanical properties of nanostructure multilayer CrN/Cr coatings on titanium alloy

Five different nanostructured, multilayer coatings (CrN/Cr)x8 with different thickness ratio of Cr and CrN layers were deposited by PAPVD (Plasma Assisted Physical Vapour Deposition) vacuum arc method on Ti6Al4V titanium alloy. The microstructure, chemical and phase composition of the CrN and Cr sub-layers were characterized by SEM with EDX and Cs-corrected dedicated STEM on cross-sections prepared by focus ion beam. Besides, hardness and Young's modulus of the (Cr/CrN)x8 coatings has been measured. The adhesion has been tested by scratch test method. The obtained (CrN/Cr) multilayer coatings, 5-6 μm in thickness, have homogeneous and nanocrystalline structure, free of pores and cracks. The microstructures of Cr and CrN layers consist of columnar grains below 100 nm in diameter. The hardness and Young's modulus of these coatings depend linearly on thickness ratio of Cr and CrN layers. The decrease of the thickness ratio Cr/CrN 0.81 to 0.15 results in the increase of hardness from 1275 HV to 1710 HV and Young's modulus from 260 GPa to 271 GPa.     

Characterizations of CrN/a-CNx nanolayered coatings deposited by DC reactive magnetron sputtering of Cr and graphite targets

CrN/a-CN_x nanolayered coatings have been deposited by DC reactive magnetron sputtering of pure Cr and graphite targets. The total thickness is 1 μm and that of a-CN_x layers is kept constant at 3.5 nm. The period (bilayer thickness) is in the range 8–16 nm. CrN and a-CN_x layers are crystalline and amorphous respectively. The decrease of CrN layers’ thickness (decrease of period) in the stack leads to refinement of CrN microstructure associated with (200) preferred orientation. The hardness of nanolayered films is independent of the period’s thickness, while internal compressive stress, which remains between that of each elementary layer, follows an evolution close to that of the law of mixtures. The best tribological behaviours are reached for a periods’ thickness of 8 nm.     

Influence of bi-layer period thickness on the residual stress, mechanical and tribological properties of nanolayered TiAlN/CrN multi-layer coatings

TiAlN/CrN nanoscale multi-layered coatings have been deposited using cathodic arc evaporation system. The coatings were deposited using one Ti₅₀Al₅₀ alloy target and one Cr target with a fixed target power in all the processes, while the bi-layer thickness was varied by various rotation speeds of the substrate holder in order to produce different nanoscale multi-layered period thickness. The texture structure, residual stress, and nanoscale multi-layer period thickness of the coatings were determined by X-ray diffraction using both Bragg-Brentano and glancing angle parallel beam geometries. Hardness and adhesion strength of the coatings were measured by Nano-indentation and Rockwell-C indentation methods, respectively. It has been found that the structural and mechanical properties of the films correlate with nano-scaled bi-layer thickness and crystalline texture. The maximum hardness of nano-scaled TiAlN/CrN multi-layered coatings was approximately 36 GPa with highest residual stress of −6.2 GPa, for a bi-layer thickness ranging from 6 to 12 nm.     

Effect of heat treatment on mechanical properties and microstructure of CrN/AlN multilayer coatings

Polycrystalline CrN/AlN multilayer coatings were deposited by RF magnetron sputtering on silicon (001) substrates. The bilayer periods of CrN/AlN were controlled from 4 nm to 20 nm by the use of shutters, which were adjusted by a programmable logic control (PLC). To evaluate the thermal stability, the films were annealed at 500 °C, 600 °C, 700 °C, 800 °C, and 850 °C, for 1 h in both vacuum and air environments. The phase transformation during thermal evolution was studied by X-ray diffraction (XRD). The microstructure of CrN/AlN multilayer coatings as-deposited and after annealing was observed by transmission electron microscopy (TEM). The hardness of as-deposited CrN/AlN coating with a period of 4 nm was 28.2 GPa, which was 60% higher than that predicted by the rule of mixtures. The hardness of CrN/AlN multilayer coatings annealed at 850 °C in vacuum remained similar to the as-deposited state, and the nano-layered structure still persisted. The thermal stability of CrN/AlN coatings was better than that of CrN coating. The hardness degradation ratio of CrN/AlN coating with modulation period of 4 nm was only 8.1% at 700 °C, which was superior to that of a simple CrN coating.     

Deposition of nano-scaled CrTiAlN multilayer coatings with different negative bias voltage on Mg alloy by unbalanced magnetron sputtering

In a magnetron sputtering system, the negative substrate bias voltage has been used as a basic process parameter to modify the deposition structure and properties of coatings. In this paper we report the effect of bias voltage ranging from −40 V to −90 V on nano-scaled CrN/TiN/CrN/AlN (CrTiAlN) multilayer coatings synthesized on a Mg alloy by a closed-field unbalanced magnetron sputtering ion plating system in a gas mixture of Ar + N₂. The technological temperature and atomic concentration in the multilayer coatings were controlled by adjusting the current density of different metal magnetron targets and the plasma optical emission monitor. The composition, crystallographic structure, deposition model and friction coefficient of multilayer coatings were characterized by X-ray photoelectron spectrometry (XPS), X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM) and ball-on-disc testing. The experimental results show that the deposition model and friction coefficient of nano-scaled CrTiAlN multilayer coatings were significantly affected by the negative bias voltage (V_b). The nitride species in multilayer coatings mainly involve CrN, AlN and TiN, and XRD analysis shows that the crystallographic structure was face-centered cubic. Under different bias voltage conditions, the multilayer coating composition shows a fluctuation, and the Al and Cr concentrations respond in the opposite sense to the bias voltage, attaining their greatest values at V_b = −70 V. The surface and cross-sectional morphology shows deposition model change from a columnar model into non-columnar model with the increase in negative bias voltage. The friction coefficient of the nano-scaled multilayer coatings at V_b = −55 V stabilize after 10 000 cycles.     

Structural and photoluminescence study of thin GaN film grown on silicon substrate by metalorganic chemical vapor deposition

GaN epitaxial layer was grown on Si(111) substrate by metalorganic chemical vapor deposition (MOCVD). The structure consists of 50 nm thick high-temperature grown AlN buffer layer, 150 nm thick AlGaN layer, 30 nm low-temperature grown AlN layer, 300 nm GaN layer, 50 nm AlGaN superlattice layer, followed by 100 nm GaN epitaxial layer. The low-temperature AlN interlayer and AlGaN superlattice layer were inserted as the defect-blocking layers in the MOCVD grown sample to eliminate the dislocations and improve the structural and optical properties of the GaN layer. The dislocation density at the top surface was decreased to ∼ 2.8 × 10⁹/cm². The optical quality was considerably improved. The photoluminescence emission at 3.42-3.45 eV is attributed to the recombination of free hole-to-donor electron. The observed 3.30 eV emission peak is assigned to be donor-acceptor transition with two longitudinal optical phonon side bands. The relationship of the peak energy and the temperature is discussed.     

Corrosion behaviour of CrN/Cr multilayers on stainless steel deposited by unbalanced magnetron sputtering

This paper reports the results of the influence of bilayer period (Λ) and total thickness (f) on the corrosion resistance of magnetron-sputtered CrN/Cr multilayers. Corrosion tests were carried out by potentiodynamic polarization with 0.5 M H₂SO₄ + 0.05 M KSCN solution and electrochemical impedance spectroscopy (EIS) with 3% NaCl solution. Measurements were also taken on the uncoated substrate and hard chromium coatings for comparison. Multilayer microstructure and morphology were studied by X-ray diffraction (XRD), atomic force microscopy (AFM) and scanning electron microscopy (SEM) and chemical composition was studied by energy dispersive X-ray analysis (EDX).The experiments showed that CrN/Cr coatings having lower bilayer period and lower thickness increased their efficiency as a barrier and improved the corrosion resistance of all coatings evaluated.     

High temperature oxidation resistance of multilayered AlxTi1 − xN/CrN coatings

Al_xTi_1 − xN and CrN have been widely used as a protective coating material in many types of tools and mechanical components because of high wear performance and high temperature resistance. In this study, the high temperature oxidation behavior of Al_0.63Ti_0.37N and multilayered Al_0.63Ti_0.37N/CrN coatings was studied. These coatings were synthesized by a cathodic-arc deposition system with plasma enhanced duct equipment. The nanolayer thickness and alloy content of the deposited multilayered coating were correlated with the emission rate of alloy cathode materials. The multilayered Al_0.63Ti_0.37N/CrN coating revealed a laminate structure with stacking of Al_0.63Ti_0.37N and CrN layers, and the periodic thickness (Λ) was 16 nm. For the high temperature oxidation test, the coated samples were annealed in the temperature range of 700-1000 °C in air for 2 h. The multilayered Al_0.63Ti_0.37N/CrN possessed much thinner oxide layer thickness than Al_0.63Ti_0.37N. Even after oxidation at 1000 °C, the multilayered Al_0.63Ti_0.37N/CrN still retained their crystalline structure. An interface effect served as a barrier, and retarded the diffusion of oxygen into the multilayered Al_0.63Ti_0.37N/CrN. Thus, the multilayered Al_0.63Ti_0.37N/CrN showed a high temperature oxidation resistance superior to the Al_0.63Ti_0.37N.     

Effect of AlN layer thickness on structure and magnetic properties of FePt/AlN multilayers

FePt (50 nm) and [FePt(xnm)/AlN(1, 2, 3 nm)]₁₀ (x=2, 3 nm) films were prepared by RF magnetron sputtering technique, then were annealed at 550 °C for 30 min. This work investigates the effect of AlN layer thickness on structure and magnetic properties of FePt/AlN multilayers. Superlattice (0 0 1) peaks can be found in the grazing incidence X-ray diffraction of FePt and [FePt (3 nm)/AlN (1, 2, 3 nm)]₁₀ films, which indicate that the FCC phase has been partially transformed into ordered L1₀ phase. Compared with the single layer FePt film, superlattice (0 0 1) peaks of FePt/AlN multilayers are weak and wide, which indicates that the introducing of AlN hinders the growth of FePt particle, and also shows the introducing of AlN is not beneficial to the transformation from FCC phase to L1₀ phase. In addition, the low-angle XRD spectra show the layered structure of FePt/AlN has been broken after annealing. The coercivities, particle size, intergrain exchange interactions of FePt/AlN films are decreased with increasing AlN layer thickness.     

Corrosion protection of CrN/TiN multi-coating for bipolar plate of polymer electrolyte membrane fuel cell

The electrochemical corrosion cells will be generated from the possible pinholes of the promising CrN and TiN coatings in a PEMFC environment. To prevent the elution of possible pinholes, CrN/TiN multi-coatings on SS have been considered. This study examined the electrochemical behavior of three CrN/TiN coatings on 316L stainless steel deposited at different CrN/TiN thickness ratios by rf-magnetron sputtering as potential bipolar plate materials. Potentiodynamic tests of CrN/TiN-coated 316L stainless steel carried out in a 1 M H₂SO₄ + 2 ppm HF solution at 70 °C revealed a significantly lower corrosion current density than that of uncoated 316L SS, as well as a decrease in the corrosion current density with decreasing inner-layer CrN thickness. Electrochemical impedance spectroscopy also showed that the CrN/TiN-coated 316L SS sample had higher charge transfer resistance than the uncoated 316L SS sample, which increased with decreasing inner-layer CrN thickness. This was attributed to the crystalline-refined CrN/TiN(200).     

Nanoindentation study of magnetron-sputtered CrN and CrSiN coatings

CrN and CrSiN coatings were deposited on stainless steel substrate by reactive magnetron sputtering. The coatings were characterized for phases, chemical composition, microstructure, and mechanical properties by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM)/energy dispersive spectroscopy (EDS), atomic force microscopy (AFM), and nanoindentation technique, respectively. The cubic phase was the only phase observed in both the coatings as observed in XRD results. A dense morphology was observed in these coatings deposited with high nitrogen and Si contents, 50:50 and 18.65 at.%, respectively. Nanoindentation measurement of CrN coatings, with Ar + N₂ proportions of 60:40, showed maximum hardness (H) and modulus (E) of 21 ± 0.85 GPa and 276 ± 13 GPa, respectively. The CrN coatings deposited in pure N₂ atmosphere showed H and E values of 27 ± 1.62 and 241 ± 10 GPa, respectively. The measured H and E values of CrSiN coatings were found to be 28 ± 1.40 GPa and 246 ± 10 GPa, respectively. The improved hardness in both the coatings was attributed mainly to a reduction in crystallite size, decrease in surface roughness, and dense morphology. The incorporation of Si into the CrN coatings has improved both hardness and Young’s modulus.     

Comparison of microstructure and mechanical properties of chromium nitride-based coatings deposited by high power impulse magnetron sputtering and by the combined steered cathodic arc/unbalanced magnetron technique

Sliding, abrasive, and impact wear tests were performed on chromium nitride (CrN)-based coatings deposited on mirror-polished M2 high speed steel substrates by the novel high power impulse magnetron sputtering (HIPIMS) utilising high peak cathode powers densities of 3000 W cm⁻². The coatings were compared to single layer CrN and multilayer superlattice CrN/NbN coatings deposited by the arc bond sputtering (ABS) technique designed to improve the coating substrate adhesion by a combined steered cathodic arc/unbalanced magnetron (UBM) sputtering process. The substrates were metal ion etched using non-reactive HIPIMS or steered cathodic arc at a substrate bias voltage of −1200 V. Subsequently a 2- to 3-μm thick CrN or CrN/NbN coating was deposited by reactive HIPIMS or UBM. No bias was used during the HIPIMS deposition, while the bias during UBM growth was in the range 75-100 V. The ion saturation current measured by a flat electrostatic probe reached values of 50 mA cm⁻² peak for HIPIMS and 1 mA cm⁻² continuous during UBM deposition. The microstructure of the HIPIMS coatings observed by transmission electron microscopy was fully dense in contrast to the voided columnar structure observed in conventional UBM sputtered CrN and CrN/NbN. The sliding wear coefficients of the HIPIMS CrN films of 2.3×10⁻¹⁶ m³ N⁻¹ m⁻¹ were lower by a factor of 4 and the roughness of the wear track was significantly reduced compared to the UBM-deposited CrN. The abrasive wear coefficient of the HIPIMS coating was 2.2×10⁻¹³ m³ N⁻¹ m⁻¹ representing an improvement by a factor of 3 over UBM deposited CrN and a wear resistance comparable to that of the superlattice CrN/NbN. The adhesion of the HIPIMS deposited CrN was comparable to state-of-the-art ABS technology.     

Characterization and properties of CrN films deposited by ion-source-enhanced middle frequency magnetron sputtering

CrN films with deposition rates of 130-180 nm/min were deposited on Si (111) and carbamide alloy substrates by an ion-source-enhanced middle frequency magnetron sputtering system. Increasing of ion source voltages promoted the growth of CrN films with preferred orientation of (200). The deposited CrN films are composed of nanocrystalline particles with sizes of ∼20 nm embedded in polycrystalline matrix. The hardness of the CrN films increases from 1300 Kg/mm² without ion source bombardment to 2400 Kg/mm² with ion source voltages of 1000 V. Origins for the increasing of hardness can be attributed to dislocation strengthening and densification effects.     

Microstructure and corrosion characteristics of CrN/NiP sputtering thin films

The CrN top layer and NiP interlayer were sequentially deposited to form a CrN/NiP composite coating through sputtering technique. The CrN/NiP coating systems deposited at 350 °C, 450 °C, and 550 °C, showed amorphous/nanocrystalline, nanocrystallize with precipitations, and fully crystallized microstructure respectively for the NiP interlayers. With the introduction of NiP interlayer, the coating assemblies exhibited superior corrosion characteristics than single CrN coatings. The amorphous NiP interlayer deposited at 350 °C revealed a lower corrosion current as compared to those with crystallized NiP layers owing to their structural defects in the alloy layer. With the combination of CrN and NiP layers the corrosion attach was retarded and a better corrosion resistance was found for the CrN/NiP composite coating.     

Influence of silicon content on the microstructure and hardness of CrN coatings deposited by reactive magnetron sputtering

CrN and CrSiN films were deposited on the stainless steel and silicon substrates by DC magnetron sputtering and their microstructural features were investigated by X-ray diffraction (XRD), scanning electron microscope (FE-SEM/EDS), and atomic force microscopy (AFM). The influence of Si content along with process parameters such as power on the microstructural characteristics of Cr–Si–N and CrN films were investigated and compared between each other. The power and increasing Si contents strongly influence the microstructural and hardness of the deposited films. XRD analysis of the coatings indicates a grain refinement with increase in Si content during deposition of coatings, which is tandem with AFM and SEM results. Also, the surface roughness and particle size are decreasing with addition of Si in the films. The hardness of CrN and CrSiN was measured by microhardness tester and found that introduction of Si content in the CrN system increases its hardness from 1839 Hv to 2570 Hv.