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Effect of V-addition on the thermal stability and oxidation resistance of CrAlN coatings
Ceramics International 44 (2018) 7013–7019
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Effect of V-addition on the thermal stability and oxidation resistance of CrAlN coatings
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Yu X. Xua, Chun Hua,b, Li Chena,b, , Fei Peia,b, Yong Dua a b
State Key Laboratory of Powder Metallurgy, Central South University, Changsha 410083, China Zhuzhou Cemented Carbide Cutting Tools Co., Ltd., Zhuzhou 412007, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Films CrAlVN Hardness Thermal properties
Since the superiority of tribological properties, V-containing coatings, with the potential to generate lubricious Magnéli oxides at elevated temperatures, are regarded as excellent candidates for machining applications. Here, a study was conducted to shed light on the intrinsic thermal stability, mechanical properties and oxidation resistance of V-alloyed CrAlN coatings deposited by cathode arc evaporation. The hardness of Cr0.48Al0.52N and Cr0.44Al0.50V0.06N coatings with a single-phase face-centered cubic structure is 30.8 ± 0.9 and 33.0 ± 1.3 GPa, respectively. Upon annealing in Ar atmosphere, Cr0.44Al0.50V0.06N coating reveals a retarded precipitation of wurtzite AlN as well as a postponement in Cr–N disintegration as compared to V-free Cr0.48Al0.52N coating, owing to the solid solution of V with a stronger binding capacity with nitrogen. The improved structural stability of Cr0.44Al0.50V0.06N coating contributes to the mild decline in hardness and elastic modulus upon annealing. When exposed to air, the incorporation of V into CrAlN promotes the formation of (Al, Cr)2O3 mixed oxide as well as additional V2O5, leading to a significantly lower onset temperature for oxidation. The existence of V2O5 degrades the protective of (Al, Cr)2O3 oxide layer and causes a complete oxidation of Cr0.44Al0.50V0.06N coating at 950 °C, where only a dense and thin oxide scale can be detected in Cr0.48Al0.52N coating.
1. Introduction As the modification of traditional CrN, CrAlN with outstanding resistance against wear [1], corrosion [2], and oxidation [3] has already been widely applied in wear components as well as forming and cutting tools to enhance their reliability and performance [4,5]. Especially, the exceptional oxidation resistance of CrAlN coatings, which primarily stems from the formation of dense and adherent (Al, Cr)2O3 oxide scales during exposure to air at elevated temperatures [6–9], favors its hightemperature applications, e.g., high-speed cutting. However, the supersaturated solid solution nature of conventional CrAlN coatings will cause the phase decomposition under thermal load, generally accompanied by a reduction of mechanical properties [10–12]. In order to meet the requirements of advanced cutting techniques, multi-component alloying and nano-structure architecture have been utilized to ameliorate high-temperature properties of hard coatings [13–17]. Alloying, which can be easily realized, is a valid modification method. The addition of moderate alloying elements can significantly tailor the properties of materials. In recent years, many alloying components involving metallic (Y, Zr, V, etc.) and non-metallic (B and Si) elements have also been introduced to the CrAlN system [18–20]. For
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instance, the incorporation of moderate Y into CrAlN is conducive to the preferential formation of Al2O3 over Cr2O3 and thus improves the oxidation resistance [21,22]. Furthermore, an enhanced thermal stability can be acquired by Y-addition on account of the retarded diffusion process upon annealing as demonstrated by Rovere and Mayrhofer [23]. Zr-containing CrAlZrN coatings exhibit excellent structural stability (with suppressed phase decomposition) [24] and tribological properties (with decreased friction coefficients) [25,26] at elevated temperatures. Nanocomposite CrAlBN and CrAlSiN coatings also reveal high mechanical properties and superior wear resistance [27–30]. The self-lubrication phenomenon of VN and related V-containing nitride coatings, such as TiAlVN and TiAlN/VN, motivates the further researches in the wear behavior of CrAlVN system [31–34]. A reduced friction coefficient from 0.7 at room temperature (RT) to 0.2 at 700 °C for Cr0.05Al0.67V0.28N coating is found by Franz et al. [31]. It can be attributed to the formation of V2O5 Magnéli oxide at high temperatures [35], which has easily activated crystallographic shear planes and a low melting point offering solid and liquid lubrication possibilities [36]. Further studies indicate that the tribological properties of CrAlVN coatings are related to their oxidation resistance [34]. Single-phase structured Cr0.05Al0.70V0.25N coating obtains a higher oxidation
Corresponding author at: State Key Laboratory of Powder Metallurgy, Central South University, Changsha 410083, China. E-mail address: [email protected] (L. Chen).
https://doi.org/10.1016/j.ceramint.2018.01.135 Received 22 November 2017; Received in revised form 31 December 2017; Accepted 16 January 2018 Available online 31 January 2018 0272-8842/ © 2018 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
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% N2, 21 vol% O2 and 20 sccm flow rate) from RT to 900, 1000, 1100, 1200, 1300, or 1500 °C with a heating rate of 10 K/min and a cooling rate of 50 K/min. Additionally, coated polycrystalline Al2O3 substrates were isothermally oxidized at 800, 850, and 950 °C for 10 h in the DSC instrument with a heating rate of 10 K/min and a cooling rate of 50 K/ min in the synthetic air (79 vol% N2, 21 vol% O2, 20 sccm flow rate). 2.3. Characterization Fracture cross-sections of CrAlN and CrAlVN coatings in the as-deposited and isothermal oxidized state were observed under scanning electron microscopy (SEM, Zeiss Supra 55). The chemical compositions of our coatings were determined by the integrated energy-dispersive Xray spectroscopy (EDX) system attached to the SEM. Using a Bruker D8 Advance diffractometer equipped with Cu Kα source (λ = 1.541 Å), Xray diffraction (XRD) investigations were performed to study the phase structure of as-deposited, annealed and oxidized free-standing coating specimens. Hardness (H) and elastic modulus (E) of as-deposited and annealed coatings onto W plates were evaluated by nanoindentation with the Berkovich diamond tip using an instrumented nanoindenter (Anton Paar UNHT³) following the Oliver and Pharr method [39]. According to the experimental results based on the large-load (30 mN) penetration test, a smaller penetration load of 15 mN with at least twenty indents was chosen to measure the mechanical properties of the coatings to keep the indentation depth (~ 160 nm) below 10% of the coating thickness.
Fig. 1. SEM fracture cross-sections of as-deposited (a) Cr0.48Al0.52N and (b) Cr0.44Al0.50V0.06N coatings onto cemented carbides substrates.
resistance than dual-phase ones, while has an increased friction due to the reduced formation of V2O5 [34]. Except for good tribological properties, excellent thermal stability with intact mechanical properties is also beneficial to the metal-cutting applications of transition metal nitride hard coatings [37,38]. However, a detailed investigation concerning the high-temperature structural and mechanical evolution of CrAlVN coatings is still missing. Therefore, this study aims at the influence of V-addition on the thermal stability and oxidation resistance of CrAlN coatings.
3. Results and discussion 2. Experimental details 3.1. Composition, morphology and phase structure 2.1. Coating deposition To simplify notations, both nitride coatings (slightly over stoichiometric in their nitrogen content) are normalized to 50 at% nitrogen. The nominal concentrations for CrAlN and CrAlVN coatings are Cr0.48Al0.52N and Cr0.44Al0.50V0.06N, corresponding to the Cr45Al55 and Cr40Al55V5 targets, respectively. The SEM cross-sectional morphologies of Cr0.48Al0.52N and Cr0.44Al0.50V0.06N coatings in Fig. 1 exhibit dense columnar crystal growth. The coating thickness, measured from SEM images, of Cr0.48Al0.52N and Cr0.44Al0.50V0.06N is ~ 2.9 and ~ 3.1 μm, respectively. XRD determinations reveal that both Cr0.48Al0.52N and Cr0.44Al0.50V0.06N coatings have a single-phase cubic structure (NaCltype, simply abbreviated here with c-), see Fig. 2. V tends to solid solution in the CrAlN-based lattice rather than forming an individual VN phase as in the case of TiAlVN [40]. Besides, alloying with V causes a shift of XRD peaks to higher 2θ angle.
CrAlN and CrAlVN coatings were prepared from Cr45Al55 (at%) and Cr40Al55V5 (at%) targets (99.99% purity), respectively, using cathodic arc evaporation method by a commercial deposition equipment (Oerlikon Balzers RCS). Cemented carbides (WC-6 wt% Co, 20 × 6 × 5 mm3), low-alloy steel foils (200 × 150 × 0.05 mm3), tungsten (W) plates (10 × 10 × 3 mm3), and polycrystalline Al2O3 sheets (25 × 5 × 0.2 mm3) as substrates were mounted on a two-folder rotating holder during deposition. In order to improve the adhesion between substrate and coating, the substrates were ultrasonically cleaned in ethanol and acetone, and then treated by an Argon-ion-etching process for 30 min in the deposition system with Ar pressure of 0.3 Pa and DC substrate bias of −180 V. Before the deposition, the main chamber was evacuated to less than 1.0 × 10−3 Pa. The N2 (99.99% purity) pressure and temperature during deposition were approximately 3.0 Pa and 550 °C, respectively. The target current for all deposition runs was 140 A, and the substrate bias was −40 V.
3.2. Thermal stability To further investigate the structural evolution at high temperatures, XRD studies, as shown in Fig. 3, were performed for annealed Cr0.48Al0.52N and Cr0.44Al0.50V0.06N coatings in Ar. When annealed at 800 and 900 °C, Cr0.48Al0.52N coating maintains its original phase structure with a slight XRD peak shift, see Fig. 3a, suggesting the annihilation and/or arrangement of microstructural defects in the recovery process. With increasing annealing temperature to 1000 °C, weak diffraction signals at 2θ = ~ 33.2° and 36.4° are detectable, indicative for the precipitation of a small amount of wurtzite (ZnS type, abbreviated merely here with w-) AlN. However, no apparent change except XRD-peak-shift for annealed Cr0.44Al0.50V0.06N coating at Ta = 1000 °C can be observed, see Fig. 3b, signifying a better thermal stability than Cr0.48Al0.52N. Fig. 4a depicts the lattice parameters of Cr0.48Al0.52N and Cr0.44Al0.50V0.06N coatings upon annealing in the temperature range from 700 to 1000 °C. Considering the lattice distortion in substitutional solid solutions, the incorporation of V with a larger atomic radius (0. 1338 nm [41]) than Cr (0.1267 nm [41]) should lead to a lattice expansion. While because of the compressive
2.2. Post-deposition treatment Heat treatments of CrAlN and CrAlVN coatings were performed in a differential scanning calorimetry (DSC) machine (Netzch STA 409C) from RT to specified annealing temperatures (Ta = 700–1200 °C in step of 100 °C, and 1500 °C) with a heating rate of 10 K/min and a cooling rate of 50 K/min in flowing Ar atmosphere (99.9% purity, 20 sccm flow rate) without heat preservation. Prior to the annealing, the coated lowalloy steel foils were diluted in 10 mol% nitric acid and then ground into the free-standing powdery state to avoid the interference from substrates. Also, vacuum annealing of our coatings onto W plates for 30 min was conducted at 800, 900, 1000, and 1100 °C using a vacuum furnace (COD533R, pressure ≤ 10−4 Pa) with a heating rate of 10 K/ min, and then samples were naturally cooled to RT in the furnace. DSC measurements with synchronous thermal gravimetric analysis (TGA) of coating powder samples were carried out in the same thermal analysis instrument (Netzch STA 409C) in flowing synthetic air (79 vol 7014
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and h-Cr2N. The original supersaturated Cr0.48Al0.52N matrix cannot be detected anymore (Fig. 3a), whereas there is still partial Cr0.44Al0.50V0.06N undecomposed (Fig. 3b). The retarded N-loss of Cr0.44Al0.50V0.06N coating is partly attributed to the higher V–N bonding energy of 523 ± 38 kJ/mol [42] than 377.8 ± 18.8 kJ/mol [42] for Cr–N. The calculated lattice parameters of ZnS-type wurtzite phase and V2N type hexagonal phase at 1200 °C are displayed in Fig. 4b and c, respectively. The Cr0.48Al0.52N and Cr0.44Al0.50V0.06N coatings have a similar lattice parameter for wurtzite phase, while the lattice parameter of hexagonal phase for Cr0.44Al0.50V0.06N is distinctly larger than that of Cr0.48Al0.52N. V2N has the same crystal structure (space group P31m) as Cr2N but with a larger lattice constant [43]. Within a certain temperature range, Cr2N and V2N can form a continuous solid solution as demonstrated in Cr–V–N ternary equilibrium phase diagram [44,45]. Therefore, the increased lattice constant of decomposed hexagonal phase in Cr0.44Al0.50V0.06N coating at 1200 °C can be attributed to the solid solution of V in Cr2N. The decomposition productions of Cr0.44Al0.50V0.06N coating at 1200 °C are thus assigned to w-AlN and h(Cr, V)2N. Based on the above analysis, the addition of V improves the thermal stability of CrAlN coating via retarding the w-AlN formation as well as the dissociation of Cr–N bonds. When the annealing temperature increases up to 1500 °C, the Cr0.48Al0.52N coating has already completely transformed into its stable phases of w-AlN and bcc-Cr, where the decomposition productions of Cr0.44Al0.50V0.06N coating are w-AlN, bcc-(Cr, V), and trace h-V2N (with higher thermal stability than hCr2N). The existence of V in bcc-Cr can also be inferred from an increased lattice parameter of the bcc phase, as depicted in Fig. 4d. Fig. 5 displays the hardness and elastic modulus of Cr0.48Al0.52N and Cr0.44Al0.50V0.06N coatings as a function of annealing temperature. The hardness of V-free Cr0.48Al0.52N coating in the as-deposited state is 30.8 ± 0.9 GPa. As a consequence of solid solution strengthening, the as-deposited Cr0.44Al0.50V0.06N coating obtains a higher hardness of 33.0 ± 1.3 GPa. On the other hand, the introduction of the stronger V–N bond, as well as obstructed dislocation movement from solid solution, reveals a higher elastic modulus of 489.2 ± 8.1 GPa for Cr0.44Al0.50V0.06N than 464.42 ± 12.2 GPa for Cr0.48Al0.52N. After annealing at 800 °C for 30 min, the hardness of Cr0.48Al0.52N and Cr0.44Al0.50V0.06N coatings immediately diminishes to 29.9 ± 0.7 GPa and 31.4 ± 0.8 GPa, respectively, see Fig. 5a. Nevertheless, the modulus of both coatings reveals a slight increase by ~ 20 GPa, as presented
Fig. 2. XRD patterns of as-deposited Cr0.48Al0.52N and Cr0.44Al0.50V0.06N powdered coating samples.
stress induced by ion bombardment during our deposition, the Cr0.48Al0.52N coating has a larger lattice parameter of 4.1203 ± 0.0004 Å than 4.1169 ± 0.0004 Å for Cr0.44Al0.50V0.06N according to Rietveld refinements. With the increase of annealing temperature, the lattice parameters of Cr0.48Al0.52N and Cr0.44Al0.50V0.06N coatings decrease owing to the stress relaxation from recovery effects. And then Cr0.48Al0.52N coating exhibits a slightly smaller lattice parameter than Cr0.44Al0.50V0.06N at 700–1000 °C. Further increasing annealing temperature to 1100 °C, the continuous formation of w-AlN in Cr0.48Al0.52N coating results in well-developed XRD peaks (2θ = ~ 33.2°, 36.0°, 37.9°, and 59.3°) with a higher intensity, see Fig. 3a. Meanwhile, the formation of hexagonal (V2N type, simply abbreviated here with h-) Cr2N can be observed due to the dissolution of Cr–N bonds under thermal load. Here, weak diffraction peaks of w-AlN (2θ = ~ 33.2°) and h-Cr2N (2θ = ~ 42.6°) for Cr0.44Al0.50V0.06N coating as shown in Fig. 3b, which behave significantly lower intensities than those for Cr0.48Al0.52N, indicate the retarded N-loss. After annealing of Cr0.48Al0.52N and Cr0.44Al0.50V0.06N at Ta = 1200 °C, the further N-loss results in the formation of bodycentered cubic (bcc-) Cr as well as the ongoing development of w-AlN
Fig. 3. XRD patterns of (a) Cr0.48Al0.52N and (b) Cr0.44Al0.50V0.06N powdered coating samples after annealing at given temperatures.
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Fig. 4. (a) Lattice parameters of Cr0.48Al0.52N and Cr0.44Al0.50V0.06N coatings as a function of annealing temperature (Ta = 700–1000 °C). Lattice parameters of decomposed (b) wurtzite, (c) hexagonal, and (d) bcc phase after annealing at (b and c) 1200 and (d) 1500 °C.
Fig. 6. Synchronous (a) TGA and (b) DSC curves of Cr0.48Al0.52N and Cr0.44Al0.50V0.06N powdered coating specimens in a synthetic air atmosphere.
in Fig. 5b. It can be ascribed to the recovery and recrystallization, which promotes the perfection of columnar crystal in our coatings. Besides, point defects in the cubic crystals are well known to be responsible for the decrease in elastic [46]. Hence, the elimination of defects upon annealing might also contributes to the slight enhancement of elastic modulus. Further annealing at 900 and 1000 °C causes a gradual drop in hardness and elastic modulus. Notably, because of the better structural stability as verified by XRD investigations (Fig. 3), the hardness and modulus of Cr0.44Al0.50V0.06N coating slightly decrease at 1100 °C, whereas a rapid decline in hardness and elastic modulus already occurs in Cr0.48Al0.52N coating. Fig. 5. Evolution of (a) hardness and (b) elastic modulus of Cr0.48Al0.52N and Cr0.44Al0.50V0.06N coatings onto W plates as a function of annealing temperature up to 1100 °C.
3.3. Oxidation resistance The synchronous TGA and DSC results during heating up to 1500 °C 7016
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Fig. 7. XRD patterns of (a) Cr0.48Al0.52N and (b) Cr0.44Al0.50V0.06N powdered coating specimens after oxidation at different temperatures.
as their mixed oxide (Al, Cr)2O3 (space group R3c ) for Cr0.48Al0.52N coating. Simultaneously, the decomposition of Cr0.48Al0.52N with concomitant N-loss leads to the emergence of h-Cr2N at 1200 °C, being similar to the case in Ar atmosphere (Fig. 3a). At 1300 °C, the complete failure, resulting from oxidation and decomposition, of Cr0.48Al0.52N generates (Al, Cr)2O3 and h-Cr2N. For Cr0.44Al0.50V0.06N coating, the (Al, Cr)2O3 has already been observed at 900 °C, as displayed in Fig. 7b, suggesting the earlier oxidation in comparison with V-free Cr0.48Al0.52N, which is in agreement with above TGA and DSC results. With increasing oxidation temperature to 1000 °C, more pronounced (Al, Cr)2O3 and minor V2O5 (ICDD 00-041-1426) with an orthometric structure (space group Pmmn) can be derived from the XRD pattern (Fig. 7b). Moreover, the Cr0.44Al0.50V0.06N coating has been fully oxidized to (Al, Cr)2O3 and V2O5 at 1100 °C, which almost stay the same even with oxidation temperature up to 1300 °C. Therefore, the h-Cr2N formation cannot be observed during oxidation of Cr0.44Al0.50V0.06N coating. Besides, since the thermal decomposition to α-Al2O3 and V2O5 at 775–800 °C [47], the AlVO4 compound, which is ordinarily detectable for CrAlVN as well as TiAlVN at relatively low-temperature (≤ 700 °C) oxidation and tribological tests [35,48], is absent in our oxidation studies at temperature ≥ 900 °C. Fig. 8 shows the fracture cross-sections of Cr0.48Al0.52N and Cr0.44Al0.50V0.06N coatings after isothermal oxidation with a duration of 10 h. When exposed to air at 800 °C, a very thin and dense oxide scale with a thickness of ~ 50 nm forms on the top of uninfluenced Cr0.48Al0.52N coating, see Fig. 8a. Whereas, the V-containing coating has been oxidized to a certain extent, as demonstrated in Fig. 8b, with a formation of the 0.8-μm-thick oxide layer. EDX linescan profile in Fig. 9 indicates the even distribution of Al, Cr, and V elements in the oxide scale onto Cr0.44Al0.50V0.06N coating. With increasing oxidation temperature up to 850 and 950 °C, Cr0.48Al0.52N still exhibits a thin and dense oxide layer, as outlined by arrows in Fig. 10a and c, which effectively postpones the further oxidation in accordance with previous studies [7,49]. In contrast, the aggravated oxidation of Cr0.44Al0.50V0.06N coating gives rise to a prominent thickening in the oxide layer to ~ 2.0 μm at 850 °C (see Fig. 10b), and even total oxidation at 950 °C (see Fig. 10d). An inferior oxidation resistance was also found in the higher V-content Cr0.10Al0.68V0.22N coating, which exhibits a bilayered oxide scale with V-rich outer layer and V-deplete inner layer as a result of the preferred outward diffusion of V during oxidation at
Fig. 8. SEM fracture cross-sectional morphologies of (a) Cr0.48Al0.52N and (b) Cr0.44Al0.50V0.06N coatings onto Al2O3 sheets after isothermal oxidation at 800 °C for 10 h.
in the synthetic air are indicated in Fig. 6. The oxidation onset temperature of Cr0.48Al0.52N coating is ~ 1148.5 °C, above which Cr0.48Al0.52N exhibits a rapid oxidation weight gain (see Fig. 6a) and a large number of heat release (see Fig. 6b). The oxidation of Cr0.48Al0.52N coating terminates at ~ 1386.7 °C with an ultimate mass gain of 20.0%. The addition of V remarkably advances the initial temperature of oxidation to ~ 882.8 °C. However, during oxidation in the temperature range of 882.8–1292.5 °C, Cr0.44Al0.50V0.06N coating manifests a relatively moderate gain in mass as well as oxidative exotherm. To fully understand the oxidation behavior of Cr0.48Al0.52N and Cr0.44Al0.50V0.06N coatings, XRD phase identifications of oxidative coating powder to various temperatures in the synthetic air, as presented in Fig. 7, were carried out. Cr0.48Al0.52N coating keeps its structural stability without the formation of crystalline oxides after oxidation at 900 °C, see Fig. 7a. As the oxidation temperature increases up to 1000 and 1100 °C, some weak diffraction peaks representing αCr2O3 (space group R3c ) and/or α-Al2O3 (space group R3c ) can be detected. This excellent oxidation resistance of Cr0.48Al0.52N coating is consistent with aforementioned thermal analyses, in which only a little mass gain was obtained at 1000 and 1100 °C. Further oxidation at 1200 °C leads to the formation of more evident α-Al2O3, α-Cr2O3 as well 7017
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4. Conclusions In this study, Cr0.48Al0.52N and Cr0.44Al0.50V0.06N coatings with a similar Al content were explored with respect to their structure, mechanical and thermal properties. Alloying with V results in an increased hardness from 30.8 ± 0.9 GPa for Cr0.48Al0.52N to 33.0 ± 1.3 GPa for Cr0.44Al0.50V0.06N due to solid solution strengthening. Correspondingly, the elastic modulus also increases from 464.4 ± 12.2 GPa for Cr0.48Al0.52N to 489.2 ± 8.1 GPa for Cr0.44Al0.50V0.06N. As compared to Cr0.48Al0.52N, the Cr0.44Al0.50V0.06N coating manifests a deferred phase decomposition upon annealing in Ar atmosphere. Owing to the improved thermal stability, the hardness and elastic modulus of Cr0.44Al0.50V0.06N coating show a relatively tardive reduction when annealed at high temperatures. The hardness and elastic modulus of Cr0.44Al0.50V0.06N coating are still higher than those of Cr0.44Al0.50V0.06N during annealing up to 1100 °C. However, the V-addition leads to an inferior oxidation resistance of our CrAlN coating because of the earlier formation of (Al, Cr)2O3 and the additional V2O5. A homogeneous oxide layer with a larger thickness of ~ 0.8 and 2.0 μm forms at 800 and 850 °C for Cr0.44Al0.50V0.06N coating, while Cr0.48Al0.52N coating remains a thin and dense oxide scale with the protective effect.
Fig. 9. EDX linescan profile of Cr0.44Al0.50V0.06N coatings onto Al2O3 sheets after exposure to air for 10 h at 800 °C.
Acknowledgment We gratefully acknowledge financial support by the National Natural Science Foundation of China under Grant Nos. 51371201 and 51775560. Li Chen thanks the supports of State Key Laboratory of Powder Metallurgy from the Central South University of China. References [1] X.-Z. Ding, X.T. Zeng, Structural, mechanical and tribological properties of CrAlN coatings deposited by reactive unbalanced magnetron sputtering, Surf. Coat. Technol. 200 (2005) 1372–1376. [2] X.-z. Ding, A.L.K. Tan, X.T. Zeng, C. Wang, T. Yue, C.Q. Sun, Corrosion resistance of CrAlN and TiAlN coatings deposited by lateral rotating cathode arc, Thin Solid Films 516 (2008) 5716–5720. [3] M. Kawate, A. Kimura Hashimoto, T. Suzuki, Oxidation resistance of Cr1−XAlXN and Ti1−XAlXN films, Surf. Coat. Technol. 165 (2003) 163–167. [4] W. Kalss, A. Reiter, V. Derflinger, C. Gey, J.L. Endrino, Modern coatings in high performance cutting applications, Int. J. Refract. Met. Hard Mater. 24 (2006) 399–404. [5] K. Bobzin, High-performance coatings for cutting tools, CIRP J. Manuf. Sci. Technol. 18 (2017) 1–9. [6] S. Hofmann, H.A. Jehn, Oxidation behavior of CrNx and (Cr,Al)Nx hard coatings, Mater. Corros. 41 (1990) 756–760. [7] A.E. Reiter, C. Mitterer, B. Sartory, Oxidation of arc-evaporated Al1−xCrxN coatings, J. Vac. Sci. Technol. A 25 (2007) 711–720. [8] J. Lin, B. Mishra, J.J. Moore, W.D. Sproul, A study of the oxidation behavior of CrN and CrAlN thin films in air using DSC and TGA analyses, Surf. Coat. Technol. 202 (2008) 3272–3283. [9] M. Zhu, M. Li, Y. Zhou, Oxidation resistance of Cr1−xAlxN (0.18 ≤ x ≤ 0.47) coatings on K38G superalloy at 1000–1100 °C in air, Surf. Coat. Technol. 201 (2006) 2878–2886. [10] H. Willmann, P.H. Mayrhofer, P.O.A. Persson, A.E. Reiter, L. Hultman, C. Mitterer, Thermal stability of Al–Cr–N hard coatings, Scr. Mater. 54 (2006) 1847–1851. [11] P.H. Mayrhofer, H. Willmann, A.E. Reiter, Structure and phase evolution of Cr–Al–N coatings during annealing, Surf. Coat. Technol. 202 (2008) 4935–4938. [12] H. Willimann, P.H. Mayrhofer, L. Hultman, C. Mitterer, Hardness evolution of Al–Cr–N coatings under thermal load, J. Mater. Res. 23 (2008) 2880–2885. [13] S. PalDey, S.C. Deevi, Single layer and multilayer wear resistant coatings of (Ti,Al) N: a review, Mater. Sci. Eng. A 342 (2003) 58–79. [14] A. Inspektor, P.A. Salvador, Architecture of PVD coatings for metalcutting applications: a review, Surf. Coat. Technol. 257 (2014) 138–153. [15] L. Chen, Y.X. Xu, L.J. Zhang, Influence of TiN and ZrN insertion layers on the microstructure, mechanical and thermal properties of Cr–Al–N coatings, Surf. Coat. Technol. 285 (2016) 146–152. [16] H.T. Wang, Y.X. Xu, L. Chen, Optimization of Cr–Al–N coating by multilayer architecture with TiSiN insertion layer, J. Alloy. Compd. 728 (2017) 952–958. [17] Z. Liu, L. Chen, Y. Xu, Structure, mechanical, and thermal properties of Ti1−xAlxN/ CrAlN (x = 0.48, 0.58, and 0.66) multilayered coatings, J. Am. Ceram. Soc. 101 (2018) 845–855. [18] J.L. Endrino, V. Derflinger, The influence of alloying elements on the phase stability and mechanical properties of AlCrN coatings, Surf. Coat. Technol. 200 (2005) 988–992.
Fig. 10. SEM fracture cross-sectional morphologies of (a and c) Cr0.48Al0.52N and (b and d) Cr0.44Al0.50V0.06N coatings onto Al2O3 sheets after isothermal oxidation at (a and b) 850 and (c and d) 950 °C for 10 h.
700 °C [50]. Whereas, a compositionally homogeneous oxide layer is observed at 800 and 850 °C for our Cr0.44Al0.50V0.06N coating. This difference might be due to the lower V content with weaker chemical drivers for diffusion at elevated temperatures. Consequently, alloying with 6 at% V promotes the formation of V2O5, leading to a lower onset temperature for oxidation. The existence of V2O5 reduces the protective effect of oxide scales, and thus degrades the oxidation resistance of CrAlN coating. 7018
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Effect of V-addition on the thermal stability and oxidation resistance of CrAlN coatings
T
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Yu X. Xua, Chun Hua,b, Li Chena,b, , Fei Peia,b, Yong Dua a b
State Key Laboratory of Powder Metallurgy, Central South University, Changsha 410083, China Zhuzhou Cemented Carbide Cutting Tools Co., Ltd., Zhuzhou 412007, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Films CrAlVN Hardness Thermal properties
Since the superiority of tribological properties, V-containing coatings, with the potential to generate lubricious Magnéli oxides at elevated temperatures, are regarded as excellent candidates for machining applications. Here, a study was conducted to shed light on the intrinsic thermal stability, mechanical properties and oxidation resistance of V-alloyed CrAlN coatings deposited by cathode arc evaporation. The hardness of Cr0.48Al0.52N and Cr0.44Al0.50V0.06N coatings with a single-phase face-centered cubic structure is 30.8 ± 0.9 and 33.0 ± 1.3 GPa, respectively. Upon annealing in Ar atmosphere, Cr0.44Al0.50V0.06N coating reveals a retarded precipitation of wurtzite AlN as well as a postponement in Cr–N disintegration as compared to V-free Cr0.48Al0.52N coating, owing to the solid solution of V with a stronger binding capacity with nitrogen. The improved structural stability of Cr0.44Al0.50V0.06N coating contributes to the mild decline in hardness and elastic modulus upon annealing. When exposed to air, the incorporation of V into CrAlN promotes the formation of (Al, Cr)2O3 mixed oxide as well as additional V2O5, leading to a significantly lower onset temperature for oxidation. The existence of V2O5 degrades the protective of (Al, Cr)2O3 oxide layer and causes a complete oxidation of Cr0.44Al0.50V0.06N coating at 950 °C, where only a dense and thin oxide scale can be detected in Cr0.48Al0.52N coating.
1. Introduction As the modification of traditional CrN, CrAlN with outstanding resistance against wear [1], corrosion [2], and oxidation [3] has already been widely applied in wear components as well as forming and cutting tools to enhance their reliability and performance [4,5]. Especially, the exceptional oxidation resistance of CrAlN coatings, which primarily stems from the formation of dense and adherent (Al, Cr)2O3 oxide scales during exposure to air at elevated temperatures [6–9], favors its hightemperature applications, e.g., high-speed cutting. However, the supersaturated solid solution nature of conventional CrAlN coatings will cause the phase decomposition under thermal load, generally accompanied by a reduction of mechanical properties [10–12]. In order to meet the requirements of advanced cutting techniques, multi-component alloying and nano-structure architecture have been utilized to ameliorate high-temperature properties of hard coatings [13–17]. Alloying, which can be easily realized, is a valid modification method. The addition of moderate alloying elements can significantly tailor the properties of materials. In recent years, many alloying components involving metallic (Y, Zr, V, etc.) and non-metallic (B and Si) elements have also been introduced to the CrAlN system [18–20]. For
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instance, the incorporation of moderate Y into CrAlN is conducive to the preferential formation of Al2O3 over Cr2O3 and thus improves the oxidation resistance [21,22]. Furthermore, an enhanced thermal stability can be acquired by Y-addition on account of the retarded diffusion process upon annealing as demonstrated by Rovere and Mayrhofer [23]. Zr-containing CrAlZrN coatings exhibit excellent structural stability (with suppressed phase decomposition) [24] and tribological properties (with decreased friction coefficients) [25,26] at elevated temperatures. Nanocomposite CrAlBN and CrAlSiN coatings also reveal high mechanical properties and superior wear resistance [27–30]. The self-lubrication phenomenon of VN and related V-containing nitride coatings, such as TiAlVN and TiAlN/VN, motivates the further researches in the wear behavior of CrAlVN system [31–34]. A reduced friction coefficient from 0.7 at room temperature (RT) to 0.2 at 700 °C for Cr0.05Al0.67V0.28N coating is found by Franz et al. [31]. It can be attributed to the formation of V2O5 Magnéli oxide at high temperatures [35], which has easily activated crystallographic shear planes and a low melting point offering solid and liquid lubrication possibilities [36]. Further studies indicate that the tribological properties of CrAlVN coatings are related to their oxidation resistance [34]. Single-phase structured Cr0.05Al0.70V0.25N coating obtains a higher oxidation
Corresponding author at: State Key Laboratory of Powder Metallurgy, Central South University, Changsha 410083, China. E-mail address: [email protected] (L. Chen).
https://doi.org/10.1016/j.ceramint.2018.01.135 Received 22 November 2017; Received in revised form 31 December 2017; Accepted 16 January 2018 Available online 31 January 2018 0272-8842/ © 2018 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
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% N2, 21 vol% O2 and 20 sccm flow rate) from RT to 900, 1000, 1100, 1200, 1300, or 1500 °C with a heating rate of 10 K/min and a cooling rate of 50 K/min. Additionally, coated polycrystalline Al2O3 substrates were isothermally oxidized at 800, 850, and 950 °C for 10 h in the DSC instrument with a heating rate of 10 K/min and a cooling rate of 50 K/ min in the synthetic air (79 vol% N2, 21 vol% O2, 20 sccm flow rate). 2.3. Characterization Fracture cross-sections of CrAlN and CrAlVN coatings in the as-deposited and isothermal oxidized state were observed under scanning electron microscopy (SEM, Zeiss Supra 55). The chemical compositions of our coatings were determined by the integrated energy-dispersive Xray spectroscopy (EDX) system attached to the SEM. Using a Bruker D8 Advance diffractometer equipped with Cu Kα source (λ = 1.541 Å), Xray diffraction (XRD) investigations were performed to study the phase structure of as-deposited, annealed and oxidized free-standing coating specimens. Hardness (H) and elastic modulus (E) of as-deposited and annealed coatings onto W plates were evaluated by nanoindentation with the Berkovich diamond tip using an instrumented nanoindenter (Anton Paar UNHT³) following the Oliver and Pharr method [39]. According to the experimental results based on the large-load (30 mN) penetration test, a smaller penetration load of 15 mN with at least twenty indents was chosen to measure the mechanical properties of the coatings to keep the indentation depth (~ 160 nm) below 10% of the coating thickness.
Fig. 1. SEM fracture cross-sections of as-deposited (a) Cr0.48Al0.52N and (b) Cr0.44Al0.50V0.06N coatings onto cemented carbides substrates.
resistance than dual-phase ones, while has an increased friction due to the reduced formation of V2O5 [34]. Except for good tribological properties, excellent thermal stability with intact mechanical properties is also beneficial to the metal-cutting applications of transition metal nitride hard coatings [37,38]. However, a detailed investigation concerning the high-temperature structural and mechanical evolution of CrAlVN coatings is still missing. Therefore, this study aims at the influence of V-addition on the thermal stability and oxidation resistance of CrAlN coatings.
3. Results and discussion 2. Experimental details 3.1. Composition, morphology and phase structure 2.1. Coating deposition To simplify notations, both nitride coatings (slightly over stoichiometric in their nitrogen content) are normalized to 50 at% nitrogen. The nominal concentrations for CrAlN and CrAlVN coatings are Cr0.48Al0.52N and Cr0.44Al0.50V0.06N, corresponding to the Cr45Al55 and Cr40Al55V5 targets, respectively. The SEM cross-sectional morphologies of Cr0.48Al0.52N and Cr0.44Al0.50V0.06N coatings in Fig. 1 exhibit dense columnar crystal growth. The coating thickness, measured from SEM images, of Cr0.48Al0.52N and Cr0.44Al0.50V0.06N is ~ 2.9 and ~ 3.1 μm, respectively. XRD determinations reveal that both Cr0.48Al0.52N and Cr0.44Al0.50V0.06N coatings have a single-phase cubic structure (NaCltype, simply abbreviated here with c-), see Fig. 2. V tends to solid solution in the CrAlN-based lattice rather than forming an individual VN phase as in the case of TiAlVN [40]. Besides, alloying with V causes a shift of XRD peaks to higher 2θ angle.
CrAlN and CrAlVN coatings were prepared from Cr45Al55 (at%) and Cr40Al55V5 (at%) targets (99.99% purity), respectively, using cathodic arc evaporation method by a commercial deposition equipment (Oerlikon Balzers RCS). Cemented carbides (WC-6 wt% Co, 20 × 6 × 5 mm3), low-alloy steel foils (200 × 150 × 0.05 mm3), tungsten (W) plates (10 × 10 × 3 mm3), and polycrystalline Al2O3 sheets (25 × 5 × 0.2 mm3) as substrates were mounted on a two-folder rotating holder during deposition. In order to improve the adhesion between substrate and coating, the substrates were ultrasonically cleaned in ethanol and acetone, and then treated by an Argon-ion-etching process for 30 min in the deposition system with Ar pressure of 0.3 Pa and DC substrate bias of −180 V. Before the deposition, the main chamber was evacuated to less than 1.0 × 10−3 Pa. The N2 (99.99% purity) pressure and temperature during deposition were approximately 3.0 Pa and 550 °C, respectively. The target current for all deposition runs was 140 A, and the substrate bias was −40 V.
3.2. Thermal stability To further investigate the structural evolution at high temperatures, XRD studies, as shown in Fig. 3, were performed for annealed Cr0.48Al0.52N and Cr0.44Al0.50V0.06N coatings in Ar. When annealed at 800 and 900 °C, Cr0.48Al0.52N coating maintains its original phase structure with a slight XRD peak shift, see Fig. 3a, suggesting the annihilation and/or arrangement of microstructural defects in the recovery process. With increasing annealing temperature to 1000 °C, weak diffraction signals at 2θ = ~ 33.2° and 36.4° are detectable, indicative for the precipitation of a small amount of wurtzite (ZnS type, abbreviated merely here with w-) AlN. However, no apparent change except XRD-peak-shift for annealed Cr0.44Al0.50V0.06N coating at Ta = 1000 °C can be observed, see Fig. 3b, signifying a better thermal stability than Cr0.48Al0.52N. Fig. 4a depicts the lattice parameters of Cr0.48Al0.52N and Cr0.44Al0.50V0.06N coatings upon annealing in the temperature range from 700 to 1000 °C. Considering the lattice distortion in substitutional solid solutions, the incorporation of V with a larger atomic radius (0. 1338 nm [41]) than Cr (0.1267 nm [41]) should lead to a lattice expansion. While because of the compressive
2.2. Post-deposition treatment Heat treatments of CrAlN and CrAlVN coatings were performed in a differential scanning calorimetry (DSC) machine (Netzch STA 409C) from RT to specified annealing temperatures (Ta = 700–1200 °C in step of 100 °C, and 1500 °C) with a heating rate of 10 K/min and a cooling rate of 50 K/min in flowing Ar atmosphere (99.9% purity, 20 sccm flow rate) without heat preservation. Prior to the annealing, the coated lowalloy steel foils were diluted in 10 mol% nitric acid and then ground into the free-standing powdery state to avoid the interference from substrates. Also, vacuum annealing of our coatings onto W plates for 30 min was conducted at 800, 900, 1000, and 1100 °C using a vacuum furnace (COD533R, pressure ≤ 10−4 Pa) with a heating rate of 10 K/ min, and then samples were naturally cooled to RT in the furnace. DSC measurements with synchronous thermal gravimetric analysis (TGA) of coating powder samples were carried out in the same thermal analysis instrument (Netzch STA 409C) in flowing synthetic air (79 vol 7014
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and h-Cr2N. The original supersaturated Cr0.48Al0.52N matrix cannot be detected anymore (Fig. 3a), whereas there is still partial Cr0.44Al0.50V0.06N undecomposed (Fig. 3b). The retarded N-loss of Cr0.44Al0.50V0.06N coating is partly attributed to the higher V–N bonding energy of 523 ± 38 kJ/mol [42] than 377.8 ± 18.8 kJ/mol [42] for Cr–N. The calculated lattice parameters of ZnS-type wurtzite phase and V2N type hexagonal phase at 1200 °C are displayed in Fig. 4b and c, respectively. The Cr0.48Al0.52N and Cr0.44Al0.50V0.06N coatings have a similar lattice parameter for wurtzite phase, while the lattice parameter of hexagonal phase for Cr0.44Al0.50V0.06N is distinctly larger than that of Cr0.48Al0.52N. V2N has the same crystal structure (space group P31m) as Cr2N but with a larger lattice constant [43]. Within a certain temperature range, Cr2N and V2N can form a continuous solid solution as demonstrated in Cr–V–N ternary equilibrium phase diagram [44,45]. Therefore, the increased lattice constant of decomposed hexagonal phase in Cr0.44Al0.50V0.06N coating at 1200 °C can be attributed to the solid solution of V in Cr2N. The decomposition productions of Cr0.44Al0.50V0.06N coating at 1200 °C are thus assigned to w-AlN and h(Cr, V)2N. Based on the above analysis, the addition of V improves the thermal stability of CrAlN coating via retarding the w-AlN formation as well as the dissociation of Cr–N bonds. When the annealing temperature increases up to 1500 °C, the Cr0.48Al0.52N coating has already completely transformed into its stable phases of w-AlN and bcc-Cr, where the decomposition productions of Cr0.44Al0.50V0.06N coating are w-AlN, bcc-(Cr, V), and trace h-V2N (with higher thermal stability than hCr2N). The existence of V in bcc-Cr can also be inferred from an increased lattice parameter of the bcc phase, as depicted in Fig. 4d. Fig. 5 displays the hardness and elastic modulus of Cr0.48Al0.52N and Cr0.44Al0.50V0.06N coatings as a function of annealing temperature. The hardness of V-free Cr0.48Al0.52N coating in the as-deposited state is 30.8 ± 0.9 GPa. As a consequence of solid solution strengthening, the as-deposited Cr0.44Al0.50V0.06N coating obtains a higher hardness of 33.0 ± 1.3 GPa. On the other hand, the introduction of the stronger V–N bond, as well as obstructed dislocation movement from solid solution, reveals a higher elastic modulus of 489.2 ± 8.1 GPa for Cr0.44Al0.50V0.06N than 464.42 ± 12.2 GPa for Cr0.48Al0.52N. After annealing at 800 °C for 30 min, the hardness of Cr0.48Al0.52N and Cr0.44Al0.50V0.06N coatings immediately diminishes to 29.9 ± 0.7 GPa and 31.4 ± 0.8 GPa, respectively, see Fig. 5a. Nevertheless, the modulus of both coatings reveals a slight increase by ~ 20 GPa, as presented
Fig. 2. XRD patterns of as-deposited Cr0.48Al0.52N and Cr0.44Al0.50V0.06N powdered coating samples.
stress induced by ion bombardment during our deposition, the Cr0.48Al0.52N coating has a larger lattice parameter of 4.1203 ± 0.0004 Å than 4.1169 ± 0.0004 Å for Cr0.44Al0.50V0.06N according to Rietveld refinements. With the increase of annealing temperature, the lattice parameters of Cr0.48Al0.52N and Cr0.44Al0.50V0.06N coatings decrease owing to the stress relaxation from recovery effects. And then Cr0.48Al0.52N coating exhibits a slightly smaller lattice parameter than Cr0.44Al0.50V0.06N at 700–1000 °C. Further increasing annealing temperature to 1100 °C, the continuous formation of w-AlN in Cr0.48Al0.52N coating results in well-developed XRD peaks (2θ = ~ 33.2°, 36.0°, 37.9°, and 59.3°) with a higher intensity, see Fig. 3a. Meanwhile, the formation of hexagonal (V2N type, simply abbreviated here with h-) Cr2N can be observed due to the dissolution of Cr–N bonds under thermal load. Here, weak diffraction peaks of w-AlN (2θ = ~ 33.2°) and h-Cr2N (2θ = ~ 42.6°) for Cr0.44Al0.50V0.06N coating as shown in Fig. 3b, which behave significantly lower intensities than those for Cr0.48Al0.52N, indicate the retarded N-loss. After annealing of Cr0.48Al0.52N and Cr0.44Al0.50V0.06N at Ta = 1200 °C, the further N-loss results in the formation of bodycentered cubic (bcc-) Cr as well as the ongoing development of w-AlN
Fig. 3. XRD patterns of (a) Cr0.48Al0.52N and (b) Cr0.44Al0.50V0.06N powdered coating samples after annealing at given temperatures.
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Fig. 4. (a) Lattice parameters of Cr0.48Al0.52N and Cr0.44Al0.50V0.06N coatings as a function of annealing temperature (Ta = 700–1000 °C). Lattice parameters of decomposed (b) wurtzite, (c) hexagonal, and (d) bcc phase after annealing at (b and c) 1200 and (d) 1500 °C.
Fig. 6. Synchronous (a) TGA and (b) DSC curves of Cr0.48Al0.52N and Cr0.44Al0.50V0.06N powdered coating specimens in a synthetic air atmosphere.
in Fig. 5b. It can be ascribed to the recovery and recrystallization, which promotes the perfection of columnar crystal in our coatings. Besides, point defects in the cubic crystals are well known to be responsible for the decrease in elastic [46]. Hence, the elimination of defects upon annealing might also contributes to the slight enhancement of elastic modulus. Further annealing at 900 and 1000 °C causes a gradual drop in hardness and elastic modulus. Notably, because of the better structural stability as verified by XRD investigations (Fig. 3), the hardness and modulus of Cr0.44Al0.50V0.06N coating slightly decrease at 1100 °C, whereas a rapid decline in hardness and elastic modulus already occurs in Cr0.48Al0.52N coating. Fig. 5. Evolution of (a) hardness and (b) elastic modulus of Cr0.48Al0.52N and Cr0.44Al0.50V0.06N coatings onto W plates as a function of annealing temperature up to 1100 °C.
3.3. Oxidation resistance The synchronous TGA and DSC results during heating up to 1500 °C 7016
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Fig. 7. XRD patterns of (a) Cr0.48Al0.52N and (b) Cr0.44Al0.50V0.06N powdered coating specimens after oxidation at different temperatures.
as their mixed oxide (Al, Cr)2O3 (space group R3c ) for Cr0.48Al0.52N coating. Simultaneously, the decomposition of Cr0.48Al0.52N with concomitant N-loss leads to the emergence of h-Cr2N at 1200 °C, being similar to the case in Ar atmosphere (Fig. 3a). At 1300 °C, the complete failure, resulting from oxidation and decomposition, of Cr0.48Al0.52N generates (Al, Cr)2O3 and h-Cr2N. For Cr0.44Al0.50V0.06N coating, the (Al, Cr)2O3 has already been observed at 900 °C, as displayed in Fig. 7b, suggesting the earlier oxidation in comparison with V-free Cr0.48Al0.52N, which is in agreement with above TGA and DSC results. With increasing oxidation temperature to 1000 °C, more pronounced (Al, Cr)2O3 and minor V2O5 (ICDD 00-041-1426) with an orthometric structure (space group Pmmn) can be derived from the XRD pattern (Fig. 7b). Moreover, the Cr0.44Al0.50V0.06N coating has been fully oxidized to (Al, Cr)2O3 and V2O5 at 1100 °C, which almost stay the same even with oxidation temperature up to 1300 °C. Therefore, the h-Cr2N formation cannot be observed during oxidation of Cr0.44Al0.50V0.06N coating. Besides, since the thermal decomposition to α-Al2O3 and V2O5 at 775–800 °C [47], the AlVO4 compound, which is ordinarily detectable for CrAlVN as well as TiAlVN at relatively low-temperature (≤ 700 °C) oxidation and tribological tests [35,48], is absent in our oxidation studies at temperature ≥ 900 °C. Fig. 8 shows the fracture cross-sections of Cr0.48Al0.52N and Cr0.44Al0.50V0.06N coatings after isothermal oxidation with a duration of 10 h. When exposed to air at 800 °C, a very thin and dense oxide scale with a thickness of ~ 50 nm forms on the top of uninfluenced Cr0.48Al0.52N coating, see Fig. 8a. Whereas, the V-containing coating has been oxidized to a certain extent, as demonstrated in Fig. 8b, with a formation of the 0.8-μm-thick oxide layer. EDX linescan profile in Fig. 9 indicates the even distribution of Al, Cr, and V elements in the oxide scale onto Cr0.44Al0.50V0.06N coating. With increasing oxidation temperature up to 850 and 950 °C, Cr0.48Al0.52N still exhibits a thin and dense oxide layer, as outlined by arrows in Fig. 10a and c, which effectively postpones the further oxidation in accordance with previous studies [7,49]. In contrast, the aggravated oxidation of Cr0.44Al0.50V0.06N coating gives rise to a prominent thickening in the oxide layer to ~ 2.0 μm at 850 °C (see Fig. 10b), and even total oxidation at 950 °C (see Fig. 10d). An inferior oxidation resistance was also found in the higher V-content Cr0.10Al0.68V0.22N coating, which exhibits a bilayered oxide scale with V-rich outer layer and V-deplete inner layer as a result of the preferred outward diffusion of V during oxidation at
Fig. 8. SEM fracture cross-sectional morphologies of (a) Cr0.48Al0.52N and (b) Cr0.44Al0.50V0.06N coatings onto Al2O3 sheets after isothermal oxidation at 800 °C for 10 h.
in the synthetic air are indicated in Fig. 6. The oxidation onset temperature of Cr0.48Al0.52N coating is ~ 1148.5 °C, above which Cr0.48Al0.52N exhibits a rapid oxidation weight gain (see Fig. 6a) and a large number of heat release (see Fig. 6b). The oxidation of Cr0.48Al0.52N coating terminates at ~ 1386.7 °C with an ultimate mass gain of 20.0%. The addition of V remarkably advances the initial temperature of oxidation to ~ 882.8 °C. However, during oxidation in the temperature range of 882.8–1292.5 °C, Cr0.44Al0.50V0.06N coating manifests a relatively moderate gain in mass as well as oxidative exotherm. To fully understand the oxidation behavior of Cr0.48Al0.52N and Cr0.44Al0.50V0.06N coatings, XRD phase identifications of oxidative coating powder to various temperatures in the synthetic air, as presented in Fig. 7, were carried out. Cr0.48Al0.52N coating keeps its structural stability without the formation of crystalline oxides after oxidation at 900 °C, see Fig. 7a. As the oxidation temperature increases up to 1000 and 1100 °C, some weak diffraction peaks representing αCr2O3 (space group R3c ) and/or α-Al2O3 (space group R3c ) can be detected. This excellent oxidation resistance of Cr0.48Al0.52N coating is consistent with aforementioned thermal analyses, in which only a little mass gain was obtained at 1000 and 1100 °C. Further oxidation at 1200 °C leads to the formation of more evident α-Al2O3, α-Cr2O3 as well 7017
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4. Conclusions In this study, Cr0.48Al0.52N and Cr0.44Al0.50V0.06N coatings with a similar Al content were explored with respect to their structure, mechanical and thermal properties. Alloying with V results in an increased hardness from 30.8 ± 0.9 GPa for Cr0.48Al0.52N to 33.0 ± 1.3 GPa for Cr0.44Al0.50V0.06N due to solid solution strengthening. Correspondingly, the elastic modulus also increases from 464.4 ± 12.2 GPa for Cr0.48Al0.52N to 489.2 ± 8.1 GPa for Cr0.44Al0.50V0.06N. As compared to Cr0.48Al0.52N, the Cr0.44Al0.50V0.06N coating manifests a deferred phase decomposition upon annealing in Ar atmosphere. Owing to the improved thermal stability, the hardness and elastic modulus of Cr0.44Al0.50V0.06N coating show a relatively tardive reduction when annealed at high temperatures. The hardness and elastic modulus of Cr0.44Al0.50V0.06N coating are still higher than those of Cr0.44Al0.50V0.06N during annealing up to 1100 °C. However, the V-addition leads to an inferior oxidation resistance of our CrAlN coating because of the earlier formation of (Al, Cr)2O3 and the additional V2O5. A homogeneous oxide layer with a larger thickness of ~ 0.8 and 2.0 μm forms at 800 and 850 °C for Cr0.44Al0.50V0.06N coating, while Cr0.48Al0.52N coating remains a thin and dense oxide scale with the protective effect.
Fig. 9. EDX linescan profile of Cr0.44Al0.50V0.06N coatings onto Al2O3 sheets after exposure to air for 10 h at 800 °C.
Acknowledgment We gratefully acknowledge financial support by the National Natural Science Foundation of China under Grant Nos. 51371201 and 51775560. Li Chen thanks the supports of State Key Laboratory of Powder Metallurgy from the Central South University of China. References [1] X.-Z. Ding, X.T. Zeng, Structural, mechanical and tribological properties of CrAlN coatings deposited by reactive unbalanced magnetron sputtering, Surf. Coat. Technol. 200 (2005) 1372–1376. [2] X.-z. Ding, A.L.K. Tan, X.T. Zeng, C. Wang, T. Yue, C.Q. Sun, Corrosion resistance of CrAlN and TiAlN coatings deposited by lateral rotating cathode arc, Thin Solid Films 516 (2008) 5716–5720. [3] M. Kawate, A. Kimura Hashimoto, T. Suzuki, Oxidation resistance of Cr1−XAlXN and Ti1−XAlXN films, Surf. Coat. Technol. 165 (2003) 163–167. [4] W. Kalss, A. Reiter, V. Derflinger, C. Gey, J.L. Endrino, Modern coatings in high performance cutting applications, Int. J. Refract. Met. Hard Mater. 24 (2006) 399–404. [5] K. Bobzin, High-performance coatings for cutting tools, CIRP J. Manuf. Sci. Technol. 18 (2017) 1–9. [6] S. Hofmann, H.A. Jehn, Oxidation behavior of CrNx and (Cr,Al)Nx hard coatings, Mater. Corros. 41 (1990) 756–760. [7] A.E. Reiter, C. Mitterer, B. Sartory, Oxidation of arc-evaporated Al1−xCrxN coatings, J. Vac. Sci. Technol. A 25 (2007) 711–720. [8] J. Lin, B. Mishra, J.J. Moore, W.D. Sproul, A study of the oxidation behavior of CrN and CrAlN thin films in air using DSC and TGA analyses, Surf. Coat. Technol. 202 (2008) 3272–3283. [9] M. Zhu, M. Li, Y. Zhou, Oxidation resistance of Cr1−xAlxN (0.18 ≤ x ≤ 0.47) coatings on K38G superalloy at 1000–1100 °C in air, Surf. Coat. Technol. 201 (2006) 2878–2886. [10] H. Willmann, P.H. Mayrhofer, P.O.A. Persson, A.E. Reiter, L. Hultman, C. Mitterer, Thermal stability of Al–Cr–N hard coatings, Scr. Mater. 54 (2006) 1847–1851. [11] P.H. Mayrhofer, H. Willmann, A.E. Reiter, Structure and phase evolution of Cr–Al–N coatings during annealing, Surf. Coat. Technol. 202 (2008) 4935–4938. [12] H. Willimann, P.H. Mayrhofer, L. Hultman, C. Mitterer, Hardness evolution of Al–Cr–N coatings under thermal load, J. Mater. Res. 23 (2008) 2880–2885. [13] S. PalDey, S.C. Deevi, Single layer and multilayer wear resistant coatings of (Ti,Al) N: a review, Mater. Sci. Eng. A 342 (2003) 58–79. [14] A. Inspektor, P.A. Salvador, Architecture of PVD coatings for metalcutting applications: a review, Surf. Coat. Technol. 257 (2014) 138–153. [15] L. Chen, Y.X. Xu, L.J. Zhang, Influence of TiN and ZrN insertion layers on the microstructure, mechanical and thermal properties of Cr–Al–N coatings, Surf. Coat. Technol. 285 (2016) 146–152. [16] H.T. Wang, Y.X. Xu, L. Chen, Optimization of Cr–Al–N coating by multilayer architecture with TiSiN insertion layer, J. Alloy. Compd. 728 (2017) 952–958. [17] Z. Liu, L. Chen, Y. Xu, Structure, mechanical, and thermal properties of Ti1−xAlxN/ CrAlN (x = 0.48, 0.58, and 0.66) multilayered coatings, J. Am. Ceram. Soc. 101 (2018) 845–855. [18] J.L. Endrino, V. Derflinger, The influence of alloying elements on the phase stability and mechanical properties of AlCrN coatings, Surf. Coat. Technol. 200 (2005) 988–992.
Fig. 10. SEM fracture cross-sectional morphologies of (a and c) Cr0.48Al0.52N and (b and d) Cr0.44Al0.50V0.06N coatings onto Al2O3 sheets after isothermal oxidation at (a and b) 850 and (c and d) 950 °C for 10 h.
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