Syntheses and mechanical properties of Cr-Mo-N coatings by a hybrid coating system

Effects of Mo content up to 30.4 at.% on the microstructure and mechanical properties of CrN coatings are reported in this study. Ternary Cr-Mo-N coatings were deposited onto steel substrates (AISI D2) using a hybrid coating method of arc ion plating (AIP) using Cr target and DC magnetron sputtering technique using Mo target in N₂/Ar gaseous mixture. The synthesized Cr-Mo-N coatings formed a substitutional solid solution of (Cr,Mo)N where larger Mo atoms replaced Cr in CrN crystal. The Cr-Mo-N coatings showed increased hardness value of approximately 34 GPa at 21 at.% Mo, compared with 18 GPa for pure CrN. The friction coefficient decreased from 0.49 for pure CrN coating to 0.37 for Cr-Mo-N with 30.4 at.% Mo. This result is believed to be due to tribo-layer formation of MoO₃ which is known to function as a solid lubricant.     

Stainless Steel Bipolar Plates Deposited with Multilayer Films for PEMFC Applications

A chromium nitride (CrN, Cr₂N)/chromium (Cr)/indium-tin-oxide (ITO) system and a gold (Au)/titanium (Ti) system were separately deposited using a sputtering method and an E-beam method, respectively, onto stainless steel 316 and 304 plates. The XRD patterns of the deposited stainless steel plates showed the crystalline phase of typical indium-tin oxide and of metallic phases, such as chromium, gold, and the metal substrate, as well as those of external chromium nitride films. The nitride films were composed of two metal nitride phases that consisted of CrN and Cr₂N compounds. The surface morphologies of the modified stainless steel bipolar plates were observed using atomic force microscopy and FE-SEM. The chromium nitride (CrN, Cr₂N)/chromium (Cr)/indium-tin-oxide (ITO) multilayer that was formed on the stainless steel plates had a surface microstructural morphology that consisted of fine columnar grains 10 nm in diameter and 60 nm in length. The external gold films that were formed on the stainless steel plates had a grain microstructure approximately 100 nm in diameter. The grain size of the external surface of the stainless steel plates with the gold (Au)/titanium (Ti) system increased with increasing gold film thickness. The electrical resistances and water contact angles of the stainless steel bipolar plates that were covered with the multilayer films were examined as a function of the thickness of the ITO film or of the external gold film. In the corrosion test, ICP-MS results indicated that the gold (Au)/titanium (Ti) films showed relatively excellent chemical stability after exposure to H₂SO₄ solution with pH 3 at 80 °C.     

Nanolayer (Ti,Cr)N coatings for hard particle erosion resistance

This paper discusses the synthesis and characterization of titanium chromium nitride ((Ti,Cr)N) thin films deposited onto AM355 stainless steel by multi-source cathodic arc physical vapor deposition (PVD) for improved erosion resistance. The effect of Cr evaporator current and substrate bias on the erosion resistance of the (Ti,Cr)N coating were investigated. The coatings were characterized using X-ray diffraction, scanning electron microscopy, electron probe microanalysis, scanning transmission electron microscopy, scratch adhesion testing and erosion testing. The (Ti,Cr)N coatings deposited using multisource mode were determined to be nanolayered structures consisting of TiN rich and CrN rich layers. EPMA showed that the atomic percentage of Cr within the coating increased (increased Cr:Ti ratio) with increasing Cr evaporator current and that the (Ti,Cr)N chemical composition did not appear to change with varying bias. Using XRD and STEM, it was determined that all nanolayer (Ti,Cr)N coatings were multi-phased consisting of a B1 NaCl crystal structure. XRD also revealed that as the Cr evaporator current was increased, there was an increase in the CrN phase volume. Macroparticle incorporation increased with an increase in Cr evaporator current and decreased with an increase in bias. The nanolayer (Ti,Cr)N coatings ranged in Vickers hardness from 1700 to 2800 VHN_0.050. Coating adhesion increased as Cr:Ti ratio increased. In regards to erosion, (Ti,Cr)N coatings with a high number of TiN/CrN interfaces performed poorly against alumina media. As the Cr evaporator current was varied, the coating deposited with the highest Cr:Ti ratio (evaporator current of 85 A) and when bias was varied, the lowest substrate bias of - 50 V had the best erosion performance.     

Erosion performance and characterization of nanolayer (Ti,Cr)N hard coatings for gas turbine engine compressor blade applications

(Ti,Cr)N nanolayer coatings were deposited on Ti–6Al–4V, 17-4PH and Inconel 718 substrates using cathodic arc physical vapor deposition for improved erosion and corrosion resistance. Coating corrosion performance was highly dependent on the coating thickness and packing factors and correlated with increased chromium content within the (Ti,Cr)N nanolayer coatings. The change in cathode current predominantly affected coating thickness and the bias affected the packing factor. Erosion tests of the coated and uncoated substrates at both 30° and 90° erodent impingement angles were conducted using angular aluminum oxide media at particle velocities up to 145 m/s. Chromium evaporator current and substrate bias were varied to change film stoichiometry and microstructure for erosion performance evaluation. When chromium evaporator current was varied, the increase in chromium content led to an increase in binary CrN phase volume and a decrease in TiN phase volume. The increase in CrN phase volume decreased both hardness and erosion performance at both impingement angles. Lower bias values resulted in better erosion performance. At 30° erodent impingement, all coated samples outperformed the uncoated substrate; whereas, for 90° impingement, only coatings deposited at low bias values (? 25 V, ? 50 V, and ? 100 V) and high Ti:Cr ratios (> 2.4) outperformed the uncoated substrate. The primary coating failure mechanism was microchipping.     

Microstructure and mechanical properties of CrN coating deposited by arc ion plating on Ti6Al4V substrate

CrN coatings have been grown by arc ion plating (AIP) onto Ti6Al4V alloy substrate at various nitrogen pressures (PN₂). The goals of this investigation are to study the influence of nitrogen pressure content on the composition, structure and mechanical properties of AIP CrN coatings, as well as their tribological properties. With an increase of PN₂, the main phases in the coatings changed from CrN + Cr₂N + Cr to CrN, and the texture of CrN was transformed from CrN (111)-oriented to (220)-oriented. Furthermore, the multi-layers including a metal Cr layer, a Cr₂N layer and a CrN layer were observed by cross-sectional TEM (XTEM), besides an “unbalanced” state transition layer at the interface of CrN/substrate which was analyzed by nucleation thermodynamics subsequently. An increase in nitrogen pressure also resulted in a change of micro-hardness due to the variation in composition and structure. Finally, the tribological properties of the Ti6Al4V substrate and the CrN/Ti6Al4V coating system have also been explored, which shows that CrN coatings can act as good wear resistance layer for Ti6Al4V substrate.     

Effect of bilayer period on properties of Cr/CrN multilayer coatings produced by laser ablation

Multilayers of alternate Cr/CrN coatings deposited on AISI 301 steel by a pulsed Nd:YAG laser have been tested by microindentation to measure both hardness and Young's modulus. Critical load L_C has been determined by scratch test. The total thickness of multilayers was 1 µm, and the bilayer period Λ varied from between 60 nm and 1000 nm. TEM and EDS techniques were used for investigating and analyzing the microstructure of the multilayers. The results of these measurements show that the mechanical properties of Cr/CrN multilayers depend on the bilayer period. The maximum improvement in performance over a single layer of CrN has been found to be Λ at 250 nm, particularly in terms of critical load and Young's modulus.     

Effects of nitrogen pressure and pulse bias voltage on the properties of Cr-N coatings deposited by arc ion plating

Cr-N coatings were deposited on 1Cr18Ni9Ti stainless steel in the pure N₂ atmosphere by arc ion plating (AIP). The relationships between deposition parameters and coating properties were investigated. X-ray diffraction showed a phase transformation from CrN + Cr₂N + Cr → CrN + Cr → CrN and the CrN preferred orientation changed from (200) to (220) as N₂ pressure increased. Increasing bias voltage led to CrN preferred orientation changed from (200) to (220) and the formation of Cr₂N. XPS results indicated that chemical composition of the coatings changed as N₂ pressure increased but it changed little with bias voltage. The lower melting point of chromium nitride formed on target surface induced the increase of macroparticles and deposition rate with increasing N₂ pressure; and bias voltage had an obvious effect on reducing macroparticles of the Cr-N coatings. Residual stresses were measured by substrate curvature technique, and the changing tendency coincided with the microhardness of the coatings.     

Preparation and characterization of Cr-P coatings by electrodeposition from trivalent chromium electrolytes using malonic acid as complex

Cr-P coatings were prepared by electrodeposition from trivalent chromium plating bath using malonic acid as complex. The influences of bath composition on the trivalent chromium electrodeposition process and deposited coating properties were studied. The effects of plating parameters such as current density, bath pH and plating time on structure and morphology of deposited coatings were investigated in detail. XRD, SEM, EDAX and XPS techniques were used to characterize the Cr-P deposited coatings. Results show that the composition, microstructure and surface morphology of the Cr-P coatings depend on bath composition and plating conditions including bath pH, current density, plating time, etc. Results of EDAX and XPS indicate that the deposited coatings contain Cr(s), Cr(III), phosphorus, oxygen and carbon. The optimum bath composition was obtained using malonic acid as complex and the mechanism of Cr-P electrodeposition was analyzed. The optimum plating parameters for good-quality chromium deposited coating are pH 2-3, current density 3-12 dm², temperature 35 °C and Ti/IrO₂ as anode. These results may be of great practical and theoretical significance for further improvement of trivalent chromium plating process.     

Chromium-nitride in situ composites with a compositional gradient formed by reactive DC plasma spraying

Nitrides of transition metals have good wear- and corrosion-resistant properties because of their high hardness and chemical stability. Chromium-nitride coatings can be deposited by ion plating; however, the thin thickness due to the slow deposition rate must be improved for severe wear-resistant applications. The main objective in this paper is to realize good structural control in the processing of chromiumnitride in situ composite coatings formed at a high deposition rate. They were synthesized by reactive low-pressure plasma spraying using elemental chromium powder as a spray material. The transferred arc between the gun electrode and the substrate was used to accelerate the nitriding reaction. The sprayed coatings consist of chromium, Cr₂N, and CrN, which have a composition gradient from the substrate interface to the surface. The volume fraction of Cr₂N increases with transferred arc current, and nonreacted chromium concurrently decreases, except close to the substrate. The CrN phase, however, only exists as a surface layer of 20 to 30 μm because it is decomposed to Cr₂N above 1420 K. The hardness of the composite coatings depends on the volume fraction of Cr₂N, and it increases to 1300 HV at a Cr₂N volume fraction of 0.98. The seizure stress with lubricant depends on the coating hardness. The maximum seizure stress of 24.9 MPa is obtained at a hardness of 1300 HV. The composite coatings also show a superior wear resistance. Hence, the Cr₂N in situ composite coatings synthesized by reactive plasma spraying with transferred arc are expected to be good candidates for wear-resistant applications.     

Tribological behaviour of pulsed magnetron sputtered CrB2 coatings examined by reciprocating sliding wear testing against aluminium alloy and steel

Among the number of attractive properties that transition-metal diborides (TiB₂, CrB₂, etc.) possess, high resistance to wear and chemical inertness are the most important when considering diboride coatings for dry machining of nonferrous materials, such as aluminium and its alloys. Due mostly to the problematic deposition of chromium diboride (preparation of targets, target cracking during the deposition process, control of stoichiometry etc.), these coatings remain comparatively less studied than, for example, titanium diborides, regarding their tribological performance.In this paper we report on the tribological behaviour of pulsed magnetron sputtered (PMS), smooth and fully dense, crystalline, 21-38 GPa hard CrB₂ coatings examined by reciprocating sliding wear testing in ambient air (20 ± 2 °C, 20-30% humidity) against EN AW-2017A aluminium alloy and AISI 52100 chrome steel. The results are compared to those of pulsed magnetron sputter deposited TiN and CrN coatings. It is demonstrated that pulsed magnetron sputtered chromium diboride coatings exhibit the best tribological performance, in terms of amount of aluminium adhered on the surface of the wear track, during testing against aluminium alloy. When slid against AISI 52100 steel PMS CrB₂, CrN and TiN coatings exhibited coefficients of friction of 0.6, 0.6-0.7 and 0.43-0.45 respectively. The tribological behaviour of coatings was found to be dependent on the transfer film formation and its properties. Wear rates were up to ten times lower for pulsed magnetron sputtered CrB₂ coatings, compared to DC sputtered Cr-B films.     

Deposition and characterization of CrN/Si3N4 and CrAlN/Si3N4 nanocomposite coatings prepared using reactive DC unbalanced magnetron sputtering

Nanocomposite coatings of CrN/Si₃N₄ and CrAlN/Si₃N₄ with varying silicon contents were synthesized using a reactive direct current (DC) unbalanced magnetron sputtering system. The Cr and CrAl targets were sputtered using a DC power supply and the Si target was sputtered using an asymmetric bipolar-pulsed DC power supply, in Ar + N₂ plasma. The coatings were approximately 1.5 μm thick and were characterized using X-ray diffraction (XRD), nanoindentation, X-ray photoelectron spectroscopy and atomic force microscopy. Both the CrN/Si₃N₄ and CrAlN/Si₃N₄ nanocomposite coatings exhibited cubic B1 NaCl structure in the XRD data, at low silicon contents (< 9 at.%). A maximum hardness and elastic modulus of 29 and 305 GPa, respectively were obtained from the nanoindentation data for CrN/Si₃N₄ nanocomposite coatings, at a silicon content of 7.5 at.%. (cf., 24 and 285 GPa, respectively for CrN). The hardness and elastic modulus decreased significantly with further increase in silicon content. CrAlN/Si₃N₄ nanocomposite coatings exhibited a hardness and elastic modulus of 32 and 305 GPa, respectively at a silicon content of 7.5 at.% (cf., 31 and 298 GPa, respectively for CrAlN). The thermal stability of the coatings was studied by heating the coatings in air for 30 min in the temperature range of 400-900 °C. The microstructural changes as a result of heating were studied using micro-Raman spectroscopy. The Raman data of the heat-treated coatings in air indicated that CrN/Si₃N₄ and CrAlN/Si₃N₄ nanocomposite coatings, with a silicon content of approximately 7.5 at.% were thermally stable up to 700 and 900 °C, respectively.     

Oxidation tuning in AlCrN coatings

In this work, we have studied the influence of the coating design and composition on the oxidation behavior of Al_xCr_1−xN (x = 0.70) coatings. In particular, we have studied the effect brought about by the deposition of an additional subsurface titanium nitride barrier layer as well as by the doping of the AlCrN-based coatings by tungsten, boron and silicon. The coatings studied have been deposited using the cathodic arc vacuum (CAV) technique. The multilayered AlCrN/TiN coatings with TiN sublayer were oxidized in air at 900 °C over 3 h and then analyzed by Glow Discharge Optical Emission Spectroscopy (GDOES) and X-ray photoelectron spectroscopy (XPS). Oxidation tests were performed in air at 900 and 1100 °C for the AlCrN and AlCrWN, AlCrSiN, and AlCrBN coatings. In each case weight gain was measured and the surface morphology of the oxidized samples were studied using Secondary Electron Microscopy (SEM) and Energy Dispersive Spectroscopy (EDS). The results obtained showed that the oxidation behavior of the aluminum rich AlCrN-based coatings could be improved in two ways: (1) by controlling the chromium outward diffusion rate in multi-layered coatings and (2) by alloying the AlCrN-based coatings with Si. Both improvements are related to the enhancement of the protective oxide film formation.     

Comparative investigation of TiAlC(N), TiCrAlC(N), and CrAlC(N) coatings deposited by sputtering of МАХ-phase Ti2 − хCrхAlC targets

A comparative investigation of the structure and properties of TiAlC(N), TiCrAlC(N), and CrAlC(N) coatings deposited by sputtering of МАХ-phase Ti_2 − хCr_хAlC targets (where x = 0, 0.5, 1.5, and 2) in an Ar atmosphere or in a gaseous mixture of Ar + N₂ is presented. The coatings were characterized in terms of their structure, elemental and phase composition, hardness, elastic modulus, elastic recovery, thermal stability, friction coefficient, wear rate, corrosion, and high-temperature oxidation resistance. The structure of the coatings was studied by means of X-ray diffraction, scanning and transmission electron microscopy, X-ray photoelectron spectroscopy, glow discharge optical emission spectroscopy, electron energy loss spectroscopy, and Raman spectroscopy. To evaluate the thermal stability and oxidation resistance, the coatings were annealed either in vacuum or in air at temperatures 600-1200 °C. The results obtained show that the TiAlCN coatings possess high hardness of 32-35 GPa, low friction coefficient against WC-Co well below 0.25, high thermal stability up to 1200 °C, and superior performance in dry milling tests against high Cr steel. Meanwhile, the coatings with high Cr content demonstrated improved oxidation resistance up to 1000 °C and superior electrochemical behavior, but their mechanical and tribological properties were deteriorated.     

A comparative study of reactive direct current magnetron sputtered CrAlN and CrN coatings

Approximately 1.5 μm thick CrN and CrAlN coatings were deposited on silicon and mild steel substrates by reactive direct current (DC) magnetron sputtering. The structural and mechanical properties of the coatings were characterized using X-ray diffraction (XRD) and nanoindentation techniques, respectively. The bonding structure of the coatings was characterized by X-ray photoelectron spectroscopy (XPS). The surface morphology of the coatings was studied using scanning electron microscopy (SEM) and atomic force microscopy (AFM). The XRD data showed that the CrN and CrAlN coatings exhibited B1 NaCl structure. Nanoindentation measurements showed that as-deposited CrN and CrAlN coatings exhibited a hardness of 18 and 33 GPa, respectively. Results of the surface analysis of the as-deposited coatings using SEM and AFM showed a more compact and dense microstructure for CrAlN coatings. The thermal stability of the coatings was studied by heating the coatings in air from 400 to 900 °C. The structural changes as a result of heating were studied using micro-Raman spectroscopy. The Raman data revealed that CrN coatings got oxidized at 600 °C, whereas in the case of CrAlN coatings, no detectable oxides were formed even at 800 °C. After annealing up to 700 °C, the CrN coatings displayed a hardness of only about 7.5 GPa as compared to CrAlN coatings, which exhibited hardness as high as 22.5 GPa. The potentiodynamic polarization measurements in 3.5% NaCl solution indicated that the CrAlN coatings exhibited superior corrosion resistance as compared to CrN coatings.     

多弧离子镀制备Cr/CrN多层厚膜的微观结构和力学性能

通过多弧离子镀（MAIP）在室温下将具有不同调制比（1:2、1:3、1:5）的多层Cr/CrN厚涂层沉积在A100钢基底上。腔室温度在沉积过程中由室温逐渐升高到160~170 ℃。设计调制结构是为了使膜/基结合强度和机械性能最大化。调制比为1:2的Cr/CrN多层涂层表现出最高的膜/基结合强度（L_c=63.8 N）,这可能归因于最高的材料硬度(H)/弹性模量(E)和H³/E²数值比（分别为0.083和0.138）。Cr层越厚,多层Cr/CrN厚涂层的塑性和摩擦学性能越好。干摩擦试验表明,与单层CrN相比,Cr/CrN多层涂层的平均摩擦系数和比磨损率分别最高降低了24%和94%。随着Cr层变厚,磨损机理从表面疲劳磨损转变为磨料磨损,这种现象可归因于硬度和塑性的协调变化。     

Mechanical and tribological properties of multilayered TiSiN/CrN coatings synthesized by a cathodic arc deposition process

The monolayered TiSiN and multilayered TiSiN/CrN were synthesized by cathodic arc evaporation. The Ti/Si (80/20 at.%) and chromium targets were used as the cathodic materials. With the different I_[TiSi]/I_[Cr] cathode current ratios of 1.8, 1.0, and 0.55, the multilayered TiSiN/CrN coatings possessed different multilayer periods (Λ) of 8.3 nm, 6.2 nm, and 4.2 nm. From XRD and TEM analyses, both the monolayered TiSiN and multilayered TiSiN/CrN revealed a typical columnar structure and B1-NaCl crystalline, no peaks of crystalline Si₃N₄ were detected. Among the multilayered TiSiN/CrN coatings, the multilayered coating with Λ = 8.3 nm possessed higher hardness of 37 ± 2 GPa, higher elastic modulus of 396 ± 20 GPa and the lower residual stress of − 1.60 GPa than the monolayered (Ti_0.39Si_0.07)N_0.54 coating(− 7.25 GPa). Due to the higher Cr/(Ti +Cr + Si) atomic ratio, the multilayered TiSiN/CrN with Λ = 5.5 nm possessed the lowest friction coefficient. But the lowest of wear rate was obtained by the multilayered TiSiN/CrN with Λ = 8.3 nm, because of higher H³/E^?2 ratio of 0.323 GPa. The monolayered TiSiN possessed the highest wear rate of 2.87 μm²/min. Therefore, the mechanical and tribological property can be improved by the design of multilayered coating.     

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′-Fe₄N → 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.     

High temperature oxidation of a (Cr2N−Mo2S3)/Cr double-layered coating deposited on steel by D.C. magnetron sputtering

A coating consisting of (Cr₂N−Mo₂S₃) overlay coating and an underlying Cr coating was deposited on a steel substrate by D.C. magnetron sputtering. The oxidation characteristics of the deposited double-layered coating were studied at temperatures ranging from 400 to 900 °C in air. The oxidation product was primarily Cr₂O₃. The unreacted coating beneath the oxide scale had some dissolved oxygen, sulfur, and iron. Oxidation of the coating occurred via complex routes such as the outward diffusion of chromium and nitrogen from Cr₂N and iron from the substrate, and the inward transport of oxygen from air, chromium from Cr₂N, and S from Mo₂S₃. This counter diffusion of various ions occurred easily via fine crystallites that constituted the coating, which had some solubility of S, O, and Fe.     

Comparative investigation of Al- and Cr-doped TiSiCN coatings

The aim of this work was a comparative investigation of the structure and properties of Al- and Cr-doped TiSiCN coatings deposited by magnetron sputtering of composite TiAlSiCN and TiCrSiCN targets produced by self-propagating high-temperature synthesis method. Based on X-ray diffraction, scanning and transmission electron microscopy, X-ray photoelectron spectroscopy, and Raman spectroscopy data, the Al- and Cr-doped TiSiCN coatings possessed nanocomposite structures (Ti,Al)(C,N)/a-(Si,C) and (Ti,Cr)(C,N)/a-SiC_xN_y/a-C with cubic crystallites embedded in an amorphous matrix. To evaluate the thermal stability and oxidation resistance, the coatings were annealed either in vacuum at 1000, 1100, 1200, and 1300 °C or in air at 1000 °C for 1 h. The results obtained show that the hardness of the Al-doped TiSiCN coatings increased from 41 to 46 GPa, reaching maximum at 1000 °C, and then slightly decreased to 38 GPa at 1300 °C. The Cr-doped TiSiCN coatings demonstrated high thermal stability up to 1100 °C with hardness above 34 GPa. Although both Al- and Cr-doped TiSiCN coatings possessed improved oxidation resistance up to 1000 °C, the TiAlSiCN coatings were more oxidation resistant than their TiCrSiCN counterparts. The TiCrSiCN coatings showed better tribological characteristics both at 25 and 700 °C and superior cutting performance compared with the TiAlSiCN coatings.     

Short‐term oxidation and hot corrosion resistance of a gradient CrN/Cr1−xAlxN coating

A CrN/Cr_1?xAl_xN coating comprised of an inner layer of CrN and an outer layer of Cr_1?xAl_xN with a gradient distribution of Al was deposited on two different alloys by a reactive sputtering method. Oxidation and hot‐corrosion tests of the gradient CrN/Cr_1?xAl_xN coating were performed at different temperatures. The phase compositions and morphologies of the as‐deposited coating and the corrosion products were investigated by using XRD and SEM/EDS. The results showed that the gradient CrN/Cr_1?xAl_xN coating exhibited good oxidation resistance at temperatures above 1000 °C owing to the formation of an α‐Al₂O₃‐rich oxide scale. The coating possessed good hot‐corrosion resistance in molten sulfate because the inner CrN layer could supply enough Cr to form a relatively protective Cr₂O₃ after the Al₂O₃‐enriched scale failed due to its dissolution in the molten sulfate.