Thermal and thermo-mechanical properties of Ti–Al–N and Cr–Al–N coatings

Metastable Ti–Al–N and Cr–Al–N coatings have been proven to be an effective wear protection due to their outstanding mechanical and thermal properties. Here, a comparative investigation of mechanical and thermal properties, for Ti–Al–N and Cr–Al–N coatings deposited by cathodic arc evaporation with the compositions (c-Ti_0.52Al_0.48N, c/w-Ti_0.34Al_0.66N and c-Cr_0.32Al_0.68N) widely used in industry, has been performed in detail. The hardness of Ti_0.52Al_0.48N and Ti_0.34Al_0.66N coatings during thermal annealing, after initially increasing to the maximum value of ~ 34.1 and 38.7 GPa with T_a up to 900 °C due to the precipitation of cubic Al-rich and Ti-rich domains, decreases with further elevated T_a, as the formation of w-AlN and coarsening of precipitated phases. A transformation to Cr₂N and finally Cr via N-loss in addition to w-AlN formation during annealing of the Cr_0.32Al_0.68N coating occurs, and thus results in a continuous decrease in hardness. Among our coatings, the mixed cubic-wurtzite Ti_0.34Al_0.66N coating exhibits the highest thermal hardness, but the worst oxidation resistance. The Cr_0.32Al_0.68N coating shows the best oxidation resistance due to the formation of dense protective α-Al₂O₃-rich and Cr₂O₃-rich layers, with only ~ 1.4 μm oxide scale thickness, after thermal exposure for 10 h at 1050 °C in ambient air, whereas Ti–Al–N coatings are already completely oxidized at 950 °C.     

Phase segregation and its effect on the adhesion of Cr-Al-N coatings on K38G alloy prepared by magnetron sputtering method

Cr_1 − xAl_xN (0 < x < 1) coatings were fabricated by a reactive magnetron sputtering method on a K38G alloy. The composition and microstructure of the coatings were investigated. Phase segregation of cubic AlN was considered in Cr_0.65Al_0.35N using X-ray diffraction analyses. This segregation of cubic AlN from CrAlN matrix might be induced by the high micro-stress. The critical failure load determined by scratch tests of the coating with c-AlN segregation was highest among all the coatings studied in the present work, which indicated that the coating has the best adhesion.     

Structure and properties of selected (Cr–Al–N,TiC–C,Cr–B–N) nanostructured tribological coatings

The paper will present the state-of-art in the process, structure and properties of nanostructured multifunctional tribological coatings used in different industrial applications that require high hardness, toughness, wear resistance and thermal stability. The optimization of these coating systems by means of tailoring the structure (graded, superlattice and nanocomposite systems), composition optimization, and energetic ion bombardment from substrate bias voltage control to provide improved mechanical and tribological properties will be assessed for a range of coating systems, including nanocrystalline graded Cr_1−xAl_xN coatings, superlattice CrN/AlN coatings and nanocomposite Cr–B–N and TiC/a-C coatings. The results showed that the superlattice CrN/AlN coating exhibited a super hardness of 45 GPa when the bilayer period Λ was about 3.0 nm. Improved toughness and wear resistance have been achieved in the CrN/AlN multilayer and graded CrAlN coatings as compared to the homogeneous CrAlN coating. For the TiC/a-C coatings, increasing the substrate bias increased the hardness of TiC/a-C coatings up to 34 GPa (at −150 V) but also led to a decrease in the coating toughness and wear resistance. The TiC/a-C coating deposited at a −50 V bias voltage exhibited an optimized high hardness of 28 GPa, a low coefficient of friction of 0.19 and a wear rate of 2.37 × 10⁻⁷ mm³ N⁻¹ m⁻¹. The Cr–B–N coating system consists of nanocrystalline CrB₂ embedded in an amorphous BN phase when the N content is low. With an increase in the N content, a decrease in the CrB₂ phase and an increase in the amorphous BN phase were identified. The resulting structure changes led to both decreases in the hardness and wear resistance of Cr–B–N coatings.     

Thermal stability and age hardening of supersaturated AlCrN hard coatings

Abstract

During advanced machining processes (high speed and dry cutting), the temperature at the cutting edge can exceed 1000°C. For modern protective hard coatings, thermal stability is of major interest. Equally important are superior mechanical properties, such as hardness, remaining at a high level over a wide temperature range. AlCrN coatings perform well in cutting tests and show excellent oxidation resistance as well as good tribological behaviour. In this work, supersaturated cubic Al_0.7Cr_0.3N coatings deposited by cathodic arc evaporation are studied. The phase and microstructure evolution of the material is investigated up to 1450°C using a combination of differential scanning calorimetry, thermal gravimetric analysis, mass spectrometry, X-ray diffraction and analytical transmission electron microscopy. During annealing up to 925°C, hexagonal AlN precipitates are formed at grain boundaries. At higher temperatures, a transformation of the remaining cubic AlCrN matrix into Cr via Cr₂N takes place, accompanied by a release of nitrogen. After annealing up to 1450°C, the AlN grains coarsen and coalesce around the Cr and Cr₂N grains. The results explain the superior cutting performance by the formation of precipitates, but also demonstrate the limitations in usage at high temperature regimes due to decomposition. Nevertheless, the substitution of Cr in the CrN lattice by Al has proven to increase the decomposition resistance significantly. Finally, nanoindentation experiments reveal that AlCrN coatings retain hardness beyond the stage of residual stress recovery up to 900°C, demonstrating an age hardening process.     

Effect of Annealing Temperature on Tribological Properties and Material Transfer Phenomena of CrN and CrAlN Coatings

This study evaluates the effects of annealing temperature and of the oxides produced during annealing processes on the tribological properties and material transfer behavior between the PVD CrN and CrAlN coatings and various counterface materials, i.e., ceramic alumina, steel, and aluminum. CrAlN coating has better thermal stability than CrN coating in terms of hardness degradation and oxidation resistance. When sliding against ceramic Al₂O₃ counterface, both CrN and CrAlN coatings present excellent wear resistance, even after annealing at 800 °C. The Cr-O compounds on the coating surface could serve as a lubricious layer and decrease the coefficient of friction of annealed coatings. When sliding against steel balls, severe material transfer and adhesive wear occurred on the CrN and CrAlN coatings annealed at 500 and 700 °C. However, for the CrAlN coating annealed at 800 °C, much less material sticking and only small amount of adhesive wear occurred, which is possibly due to the formation of a continuous Al-O layer on the coating outer layer. The sliding tests against aluminum balls indicate that both coatings are not suitable as the tool coatings for dry machining of aluminum alloys.     

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.     

High temperature corrosion behavior of a multilayer CrAlN coating prepared by magnetron sputtering method on a K38G alloy

A multilayer CrAlN coating of Cr_0.58Al_0.42N/Cr_0.84Al_0.16N/Cr_0.51Al_0.49N has been fabricated by a reactive magnetron sputtering method. It consists of a bonding layer, a Cr-rich intermediate layer and an Al-rich outer layer. The multilayer structure provides the coating with good protection against different types of high temperature corrosion, i.e., high temperature oxidation and hot corrosion. The outer Al-rich layer gives the coating good oxidation resistance at 1000 and 1100 °C due to the formation of a continuous alumina scale. The parabolic rate constants of the coated samples decrease by about 2 orders of magnitude compared with that of the bare alloy samples. The intermediate Cr-rich layer can form a Cr₂O₃ scale to provide good protection under the hot corrosion conditions in the Na₂SO₄ salt fluxing at 900, 950 and 1000 °C. The incubation period of the hot corrosion extends several times longer when the alloy was coated by the multilayer coating at the three selected temperatures.     

Mechanical properties and oxidation resistance of CrAlN/BN nanocomposite coatings prepared by reactive dc and rf cosputtering

CrAlN/BN nanocomposite coatings were deposited through reactive cosputtering, i.e., pulsed dc and rf sputtering, of CrAl and h-BN targets, respectively. X-ray diffraction (XRD) and selected area electron-diffraction (SAED) analysis indicated that the CrAlN/BN coating consists of very fine grains of B1 structured CrAlN phase. With an increasing BN volume fraction of over 8 vol.%, the nanocrystalline nature of the grains is revealed through a dispersion of fine grains in the CrAlN/BN coating. A cross-sectional observation using a transmission electron microscope (TEM) clarified that the coating demonstrating the highest level of hardness has a fiber-like structure consisting of grains that are ~ 20 nm in width and ~ 50 nm in length. X-ray photoelectron spectroscopy (XPS) analysis revealed that the coating consists mainly of CrAlN and h-BN phase. The indentation hardness (H_IT) and effective Young's modulus (E*) of the coatings increased with the BN phase ratio, reaching a maximum value of ~ 46 and ~ 440 GPa at ~ 7 vol.% of BN phase; it then decreased moderately to ~ 40 and ~ 350 GPa at 18 vol.% of BN, respectively. Furthermore, CrAlN/BN coatings showed superior oxidation resistance compared with CrAlN coatings. After annealing at 800 °C in air for 1 h, the indentation hardness of CrAlN coatings decreased to 50% of the as-deposited hardness; in contrast, the hardness of CrAlN/BN nanocomposite coatings either stayed the same or increased, attaining a value of about 46 GPa. After annealing at 900 °C for 1 h, the hardness of all the coatings decreased to about 40%.     

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.     

Influence of phase transition on the tribological performance of arc-evaporated AlCrVN hard coatings

V-alloyed AlCrN hard coatings were synthesised by reactive arc-evaporation in an industrial-sized deposition system at bias voltages ranging from − 40 to − 150 V. X-ray diffraction analysis has shown that higher energetic growth conditions stabilise the desired metastable face-centered cubic (fcc) crystal structure of Al_0.70Cr_0.05V_0.25N even at very high Al/Cr ratios resulting in hardness values comparable to Al_0.70Cr_0.30N. Ball-on-disc tests were used to assess the friction at 700 °C where Al_0.70Cr_0.05V_0.25N coatings revealed a generally lower coefficient of friction due to the formation of a tribolayer containing the lubricious oxide V₂O₅ as evidenced by Raman spectroscopy. The single-phase fcc-Al_0.70Cr_0.05V_0.25N coating appears to be more oxidation resistant leading to a reduced formation of V₂O₅ and, hence, an increase in friction. The cutting performance was evaluated by conducting side milling tests under dry conditions, where Al_0.70Cr_0.05V_0.25N coatings showed a competitive performance regardless of the growth conditions.     

Structure, mechanical properties and oxidation behaviour of arc-evaporated NbAlN hard coatings

Nb_1 − xAl_xN hard coatings were synthesised by cathodic arc-evaporation in order to study the influence of the Al concentration on crystal structure, mechanical properties and oxidation resistance. Structural investigations by X-ray diffraction revealed a transition from the face-centered cubic structure of δ-NbN to the wurtzite structure of AlN at x = 0.45… 0.56 depending on the deposition parameters. The maximum values of the mechanical properties like hardness and residual stress obtained by nanoindentation and biaxial stress temperature measurements, respectively, were found for the coatings with cubic structure and generally decrease with increasing Al content. On the other hand, higher Al concentrations are beneficial in terms of oxidation resistance as shown by annealing experiments in ambient air. The onset temperature for oxidation rises from 600 to 700 °C for Nb_0.73Al_0.27N to above 800 °C for Nb_0.29Al_0.71N regardless of changes in the crystal structure.     

Machining of depleted uranium using coated cutting tools

The machining of depleted uranium and its alloys are discussed in this article. Traditionally, these materials have been machined, with limited success, using uncoated cutting tools. New developments in titanium-based coatings such as cation-substituted Ti_1-x-y-z Al _x Cr _y Y₂N alloys, with y=0.03 and z=0.02, have been shown to offer enhanced high-temperature oxidation resistance compared with the presently used TiN and Ti_1-x Al _x N films that are deposited to cutting tool surfaces. Layers (3 μm thickness) were deposited by unbalanced magnetron sputter (UBM) deposition to small-grain WC-Co unused cutting tools that had been ion-etched in situ using a steered Cr-metal-ion cathodic arc (CA) discharge at an Ar pressure of 6 × 10⁻⁴ mbar (0.45 m Torr). The metal ion etching promoted initial local epitaxy on individual substrate grains while the overall film texture evolved through competitive growth to a (111) plane in Ti_0.44Al_0.53Cr_0.03N alloys and (200) plane in Ti_0.43Al_0.52Cr_0.03Y_0.02N alloys. Although Ti_0.04Al_0.53Cr_0.03N layers exhibited a columnar microstructure that was similar to that previously observed in Ti_1-x Al _x N alloys, the addition of 2 mol% YN resulted in significant grain refinement, giving rise to an equiaxed structure. The Knoop microhardness of Ti_0.43Al_0.52Cr_0.03Y_0.02N alloys was HK_0.025=2700 kg/mm compared with 2400 kg/mm for Ti_0.44Al_0.53Cr_0.03N alloys. The onset of rapid oxidation, as determined from thermogravimetric measurements, ranged from ≈600 °C for TiN; to 870 °C for Ti_0.466Al_0.54N; to 920 °C for Ti_0.44Al_0.53Cr_0.03N; to 950 °C for Ti_0.43Al_0.522Cr_0.03Y_0.02N. Machining experiments indicated that cutting tool life is improved significantly using Y-doped Ti-based coatings when machining uranium alloys. This paper was presented at the fourth International Surface Engineering Congress and Exposition held August 1–3, 2005 in St. Paul, MN.     

Structural and mechanical properties of corundum and cubic (AlxCr1−x)2+yO3−y coatings grown by reactive cathodic arc evaporation in as-deposited and annealed states

Coatings of (Al_xCr_1?x)_2+yO_3?y with 0.51 ? x ? 0.84 and 0.1 ? y ? 0.5 were deposited on hard cemented carbide substrates in an industrial cathodic arc evaporation system from powder-metallurgy-prepared Cr/Al targets in pure O₂ and O₂ + N₂ atmospheres. The substrate temperature and bias in all the deposition runs were 575 °C and ?120 V, respectively. The composition of the coatings measured by energy dispersive X-ray spectroscopy and elastic recoil detection analysis differed from that of the facing targets by up to 11%. Microstructure analyses performed by symmetrical X-ray diffraction and transmission electron microscopy showed that corundum, cubic or mixed-phase coatings formed, depending on the Cr/Al ratio of the coatings and O₂ flow per active target during deposition. The corundum phase was promoted by high Cr content and high O₂ flow per target, while the cubic phase was observed mostly for high Al content and low O₂ flow per active target. In-situ annealing of the cubic coatings resulted in phase transformation from cubic to corundum, completed in the temperature range of 900–1100 °C, while corundum coatings retained their structure in the same range of annealing temperatures. Nanoindentation hardness of the coatings with Cr/Al ratio <0.4 was 26–28 GPa, regardless of the structure. Increasing the Cr content of the coatings resulted in increased hardness of 28–30 GPa for corundum coatings. Wear resistance testing in a turning operation showed that coatings of Al–Cr–O have improved resistance to crater wear at the cost of flank wear compared with TiAlN coatings.     

Microstructure, mechanical and tribological properties of Cr1−xAlxN films deposited by pulsed-closed field unbalanced magnetron sputtering (P-CFUBMS)

Nanocrystallized Cr_1−xAl_xN films with various Al contents (0 to 68 at.%) were deposited by pulsed closed field unbalanced magnetron sputtering (P-CFUBMS). The effects of aluminum content on the microstructure, mechanical and tribological properties of the Cr_1−xAl_xN films have been investigated. It was found that the hardness and elastic modulus of Cr_1−xAl_xN films increased with increasing Al contents in the films and reached the highest value of 36 GPa and 370 GPa, respectively, at an Al content of 58.5 at.%. Addition of Al beyond 64.0 at.% resulted in a change in crystal structure from B1 cubic to B4 hexagonal phase. The wear resistance improved gradually with the increase of Al in the Cr_1−xAl_xN films. A combination of the abrasive and adhesive wear mechanism was proposed based on the SEM and EDS analysis of the wear track. The steady state dry coefficient of friction measured against a WC ball for the Cr_1−xAl_xN films were in the range of 0.36-0.55, and the wear rate was in the 10^− 6 mm³ N^− 1 m^− 1 range.     

Interplay of microstructural features in Cr1−xAlxN and Cr1−x−yAlxSiyN nanocomposite coatings deposited by cathodic arc evaporation

The interplay between the deposition geometry, the chemical and phase composition, the crystallite size, the lattice strain and the direction and the degree of the preferred orientation of crystallites was investigated in the Cr_1−xAl_xN and Cr_1−x−yAl_xSi_yN nanocrystalline coatings and nanocomposites, which were deposited in cathodic arc evaporation process at different positions of substrates in the deposition apparatus. The different positions of the substrates affected primarily the distance between the samples and the cathodes and consequently the chemical and phase composition of the coatings, the crystallite size, the lattice strain and the preferred orientation of crystallites. In the Cr_1−xAl_xN coatings, the dominating cubic crystallites were preferentially oriented with their 〈111〉 direction perpendicular to the sample surface; this out-of-plane preferred orientation of crystallites was accompanied by a strong in-plane texture. In the Cr_1−x−yAl_xSi_yN coatings, a strong inclination of the {111} texture from the normal direction and a decay of the in-plane preferred orientation were observed in cubic crystallites with increasing silicon (and aluminium) contents.     

Experimental and computational study on the effect of yttrium on the phase stability of sputtered Cr–Al–Y–N hard coatings

The effect of Y incorporation into cubic Cr–Al–N (B1) was studied using ab initio calculations, X-ray diffraction and energy-dispersive X-ray analysis of sputtered quaternary nitride films. The data obtained indicate that the Y incorporation shifts the critical Al content, where the hexagonal (B4) structure is stable, to lower values. The calculated critical Al contents of x ≈ 0.75 for Cr_1?xAl_xN and x ≈ 0.625 for Cr_1?x?yAl_xY_yN with y = 0.125 are consistent with experimentally obtained values of x = 0.69 for Cr_1?xAl_xN and x = 0.68 and 0.61 for Cr_1?x?yAl_xY_yN with y = 0.02 and 0.06, respectively. This may be understood based on the electronic structure. Both Cr and Al can randomly be substituted by Y. The substitution of Cr by Y increases the phase stability due to depletion of non-bonding (anti-bonding) states, while the substitution of Al by Y decreases the phase stability mainly due to lattice strain.     

Einfluß betriebsnaher thermischer Belastung auf die Eigenschaften von Y2O3-teilstabilisierten ZrO2-Wärmedämmschichten

Influence of close to practice thermal loads on the properties of thermal barrier layers of ZrO₂ partially stabilized with Y₂ O₃ The use of plasma sprayed thermal barrier coatings offers the possibility to protect thermally stressed components in engines against overstressing. A suitable material for that is Y₂O₃-partially stabilized zirconium oxide. Considering the high temperatures as they occur in use the thermal barrier coatings were examined for their resistance concerning phase composition, porosity, hardness and structure. It turned out that changes in structure lead to a considerable increase of porosity and thus to a decline of hardness. Concerning the phase composition there was only a slight change after heat treatment in favour of monoclinic and cubic modification.     

Structure and properties of Ti–Al–Y–N coatings deposited from filtered vacuum-arc plasma

Ti_0.5Al_0.5N coatings with a small amount of Y (up to 1 at.%) were deposited by filtered vacuum arc plasma at pulsed high voltage negative substrate bias potential with amplitude up to 2.5 kV and their microstructure was studied. X-ray fluorescence analysis showed that this deposition method allows ensuring well the conformity of the elemental composition of the metallic components of cathodes and films. X-ray diffraction measurements of the films with yttrium revealed a solid solution (Ti,Al)N phase with a cubic NaCl-type structure as the only crystalline phase. The films deposited with an amplitude of the substrate bias potential in the range of 1–1.5 kV were characterized by a strong axial texture [110]. In these films an increase of the yttrium content leads to the reduction of the nitride lattice parameter and growth of coherent scattering zone dimension as well as to a decrease of the surface roughness. Coatings containing 1 at.% Y exhibited high hardness of 32–36 GPa and oxidation resistance.     

Influence of Al Contents on the Microstructure, Mechanical, and Wear properties of Magnetron Sputtered CrAlN Coatings

CrAlN (0 < x < 0.1) coatings were deposited on SA304 substrate by a reactive magnetron sputtering. The microstructure and composition of the as-deposited coatings were systematically characterized by field emission scanning electron microscopy/EDS and atomic force microscopy, and the phase formation by x-ray diffraction (XRD). The hardness of the coatings was investigated using nanoindentation, while wear properties were investigated using pin-on-disk tribometer. XRD study reveals that the deposited CrAlN coatings crystallized in the cubic B1 NaCl structure. The minimum and maximum hardness of the coatings are found to be 15.28 and 18.81 GPa, respectively. The COF and wear rate are found to be 0.48 and 2.25 × 10^?5 mm³/N · m, which is lower than the CrN coatings deposited and characterized under the same environment (0.63 and 2.25 × 10^?5 mm³/Nm).     

A study of the oxidation behavior of CrN and CrAlN thin films in air using DSC and TGA analyses

Freestanding CrN_x and Cr_1 − xAl_xN films with two different Al atomic percentages with respect to the metal sublattice (x = 0.23 and x = 0.60) were produced by pulsed closed field unbalanced magnetron sputtering (P-CFUBMS). The dynamic oxidation behavior of the films has been characterized by thermal analysis using differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). The structure of the films at different thermal-annealing temperatures were investigated by X-ray diffraction (XRD) and scanning electron microscopy (SEM) in an effort to understand different phase transitions and oxidation reactions observed on the DSC curves. The peak temperatures of the main exothermic/endothermic oxidation reactions in the DSC signals at different heating rates were applied to the Kissinger model for determination of activation energies. The mechanical properties of the films at different heat-annealing states were measured by nano-indentation.It was found that the CrN_x films oxidized in air after 600 °C by the dissociation of fcc (face center cubic)-CrN to h(hexagonal)-Cr₂N and nitrogen and, after 900 °C by the dissociation of h-Cr₂N to Cr and nitrogen in the film. The addition of Al to CrN film can further improve the oxidation resistance, especially for the high temperature above 800 °C. The oxidation degradation in two Cr-Al-N films started with dissociation of fcc-CrAlN to h-Cr₂N and nitrogen in the film. The presence of thermally stable Al-N bonding in the fcc-CrAlN structure can suppress the reduction of nitrogen in the film. A dense (Cr,Al)₂O₃ layer (either amorphous or crystalline) formed at early oxidation stage (< 700 °C) can act as an effective diffusion barrier slowing down the inward diffusion of the oxygen at high temperatures. Precipitation of h-AlN phase in Cr_0.77Al_0.23N and Cr_0.40Al_0.60N films were found at 900 and 1000 °C respectively, accompanied with crystalline Al₂O₃ formation. After that, both Cr-Al-N films oxidized rapidly after the dissociation of h-Cr₂N to Cr and nitrogen. In addition, Cr_0.40Al_0.60N films exhibit higher oxidation resistance than Cr_0.77Al_0.23N films. The fcc-CrAlN was retained up to 900 °C and the precipitation of h-AlN phase took place after 1000 °C in Cr_0.40Al_0.60N films. Cr_0.40Al_0.60N films also retained a hardness of 25 GPa after annealing at 800 °C in ambient air for 1 h. The activation energies of the final oxidation exothermic peaks in CrN_x, Cr_0.77Al_0.23N and Cr_0.40Al_0.60N films in the current study were found to be 2.2, 3.2 and 3.9 eV atom^− 1 respectively.