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Influence of Ti on the mechanical properties, thermal stability and oxidation resistance of Al–Cr–N coatings
Vacuum 120 (2015) 127e131
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Short communication
Influence of Ti on the mechanical properties, thermal stability and oxidation resistance of AleCreN coatings Yuxiang Xu a, Li Chen a, b, *, Ziqiang Liu a, Fei Pei a, b, Yong Du a a b
State Key Laboratory of Powder Metallurgy, Central South University, Changsha, Hunan, 410083, China Zhuzhou Cemented Carbide Cutting Tools Co., Ltd., Zhuzhou, Hunan, 412007, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 24 March 2015 Received in revised form 24 June 2015 Accepted 6 July 2015 Available online 8 July 2015
Al0.68Cr0.32N, Al0.66Cr0.25Ti0.09N and Al0.62Cr0.20Ti0.18N coatings prepared by cathodic arc evaporation exhibit single phase cubic structure. Increasing Ti content from 0 to 18 at% causes a continuous hardness promotion from ~27.4 to 33.2 GPa, however, a drop in elastic modulus from ~481.8 to 337.1 GPa. Annealing of AleCreN in Ar results in a transition into stable phases of wurtzite (w-) AlN and Cr via an intermediate phase Cr2N. Alloying with Ti into AleCreN promotes the w-AlN formation, but retards the N-loss, where the finally stable phases of w-AlN, Cr and TiN after annealing are obtained. Nevertheless, the Ti-addition has a significantly inferior effect on the oxidation resistance of AleCreN coating due to the higher affinity between Ti and oxygen, where the Al0.68Cr0.32N and Al0.62Cr0.20Ti0.18N coatings after oxidation at 1100 C for 20 h reveal oxide scales of ~1.3 and 4.0 mm, respectively. © 2015 Elsevier Ltd. All rights reserved.
Keywords: AlCrN AlCrTiN Hardness Thermal stability Oxidation resistance
Hard coatings have been proven to be excellent candidates to improve wear protection, oxidation and corrosion resistance for advanced machining processes (e.g., high-speed and dry cutting). AleCreN coating is one of the most popular and universally used protective layers in machining applications due to its excellent tribological property as well as outstanding resistance to oxidation and corrosion [1,2]. The most significant advantage of Cr1xAlxN coatings is their excellent oxidation resistance, which is related to the formation of dense and adherent (Al, Cr)2O3 mixed oxide scale during exposure to air at elevated temperature [3,4]. The thermal stability of protective coatings counts for as much as their mechanical properties and oxidation resistance in machining applications [5,6]. A transformation into their stable phases of wurtzite (w-) AlN and cubic (c-) Cr via intermediate phase of Cr2N occurs, which is accompanied by N-loss due to the chemical instability of CreN bond during thermal load of metastable AleCreN coatings [7,8]. This decomposition of AleCreN coatings leads to a continuous reduction in mechanical properties and thereby is detrimental to machining applications [9,10]. Incorporation of a fourth constituent (e.g. Si, Ti, V, Zr, Y) into AleCreN to tailor the structure and properties receives
* Corresponding author. State Key Laboratory of Powder Metallurgy, Central South University, Changsha, Hunan, 410083, China. E-mail address: [email protected] (L. Chen). http://dx.doi.org/10.1016/j.vacuum.2015.07.004 0042-207X/© 2015 Elsevier Ltd. All rights reserved.
considerable interests recently [11e14]. Si-addition results in the formation of nanocomposite structure with amorphous Si3N4 interfacial phase bundling nanocrystalline CrAlN grains, and thus provokes a significant improvement in mechanical and thermal properties [15]. And lower friction and higher oxidation resistance can be achieved in AleCreN coatings by alloying with V and Y, respectively [16,17]. The previous studies indicate that Ti-addition into AleCreN coatings causes improved cutting performance in different machining applications due to their increased hardness and wear resistance [18e22]. However, the thermal stability and oxidation resistance of coatings also act as crucial factors on the machining performance of coated tools. The thermal decomposition process of metastable AleCreTieN coatings during annealing depends on the coatings composition, heat treatment condition and deposition method. Hasegawa et al. indicates the formation of w-AlN can be detected in Al0.66Cr0.24Ti0.10N and Al0.62Cr0.11Ti0.27N coatings when annealing above 900 C [23]. The effect of Ti on the thermal stability of AleCreN coating is further investigated with n et al. [12]. They found the addition of 1, 2 and 11 at% Ti by Forse that alloying with Ti can stabilize the cubic structure of AleCreN via suppressing the formation of w-AlN. Besides, AleCreTieN coatings manifested higher oxidation resistance than TieAleN coatings [21,24,25]. However, the research by Forsen et al. [12] indicates that incorporation 11 at% Ti into AleCreN coating leads to a significant drop in oxidation resistance, where the TiO2 emerged
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after heating up to 1100 C [12]. In this work, AleCreN and AleCreTieN coatings were deposited by cathodic arc evaporation to investigate the effect of Ti on the mechanical properties, thermal stability and oxidation resistance of coatings. AleCreN and two compositions of AleCreTieN coatings were prepared in a commercial cathodic arc system (Balzers Oerlikon Rapid Cooling System, RCS) from Al0.70Cr0.30, Al0.70Cr0.20Ti0.10 and Al0.65Cr0.15Ti0.20 targets (99.99% purity), respectively, which has the working N2 pressure of ~2 Pa, the substrate bias of 100 V, the target current of 180 A, and the deposition temperature of 550 C. Prior to the deposition, the substrates were ultra-sonically cleaned in acetone and ethylene, and then were cleaned with an Argon-ionetching process at an Ar pressure of 0.3 Pa and substrate bias voltage of 150 V DC for 20 min. The thickness of deposited AleCreN and AleCreTieN is ~3.0 mm. Thermal annealing was performed in the differential scanning calorimetry (DSC) instrument (Netzch-STA 409C, Germany), in which coating samples heated up to specified annealing temperatures (Ta ¼ 800e1300 each 100 C, and 1450 C), respectively, with a rate of 10 K/min and cooled down immediately with a rate of 50 K/min in flowing Ar (99.9% purity, 20 sccm flow rate). Prior to these measurements, the coatings were removed from their lowalloy steel substrates by chemical etching in 10 mol% nitric acid to avoid interference from substrate materials. Coated polycrystalline Al2O3 substrates were isothermally oxidized at 1000 and 1100 C, respectively, for 20 h in the DSC equipment with a heating rate of 10 K/min and a cooling rate of 50 K/min in synthetic air (79 vol.% N2, 21 vol.% O2 and 20 sccm flow rate). Subsequently, the oxidized coated Al2O3 substrates were investigated by the fracture cross-sectional scanning electron microscopy (SEM, LEO1525, Germany) observation with Energy Dispersive X-ray (EDX) composition analyses. The elemental compositions of the as-deposited coatings were determined using EDX attached to SEM. Quantification of the elemental composition was obtained by elemental standards and a TiN coating standard which have been quantified by Rutherford back-scattering spectroscopy. The error of measurements for the metal atoms is below 2 at%. Phase identifications were conducted by X-ray diffraction (XRD) with Cu Ka radiation using a Bruker D8 in Bragg/Brentano mode at 40 mA and 40 kV. Hardness and elastic modulus of as-deposited coatings on cemented carbide (WC-6 wt.% Co) were derived from nanoindentation with Berkovich diamond indenter using an instrumented nanoindenter (CSM Instruments, Switzerland) after the Oliver and Pharr method [26]. According to the experimental results based on the large-load (30 mN) penetration test, a smaller penetration load of 10 mN was chosen to measure the mechanical properties of the coatings to keep the indentation depth (~200 nm) below 10% of the film thickness for minimizing the influence of substrates. Elemental analysis by EDX shows that the AleCreN and AleCreTieN coatings are stoichiometric with N/metal ratios of 1 ± 0.08 and compositions of Al0.68Cr0.32N, Al0.66Cr0.25Ti0.09N and Al0.62Cr0.20Ti0.18N, respectively. XRD patterns of freestanding samples, as shown in Fig. 1, yield single phase face centered cubic (B1eNaCl type) structure for AleCreN and AleCreTieN coatings. Increasing Ti content for our coatings results in a continuous shift of the diffraction peaks position to lower 2q angles, which primarily due to the increased lattice size caused by substitution of Al and Cr atoms in the metal sublattice with Ti having a larger atomic radius. In addition, a broadening of the XRD peaks with elevated Ti contents in the coatings can be ascribed to the increased microstrains and/or decreased grain size. Grain sizes of Al0.68Cr0.32N, Al0.66Cr0.25Ti0.09N and Al0.62Cr0.20Ti0.18N coatings from (200) peaks using Scherrer formula is 20.5 ± 0.3, 16.4 ± 0.3 and 12.6 ± 0.2 nm, respectively, which actually reflect the dimensions of the
Fig. 1. XRD patterns of as-deposited Al0.68Cr0.32N, Al0.66Cr0.25Ti0.09N and Al0.62Cr0.20Ti0.18N coatings.
coherently scattering domains. The hardness (H), elastic modulus (E) and corresponding ratio of hardness to reduced modulus (H/E*, where E* ¼ E/(1-m2); m is Poisson's ratio) of AleCreN and AleCreTieN coatings are depicted in Fig. 2. Alloying with Ti into AleCreN coating causes an increase in hardness from 27.4 ± 0.4 GPa for AleCreN to 29.1 ± 1.3 GPa for 9 at% Ti-containing coating towards 33.2 ± 0.8 GPa for 18 at% Ti-containing coating, which is related to the solid solution strengthening and grain refinement. The Ti-containing coatings exhibit the significantly reduced indentation moduli, which can probably be attributed to the grain refinement. The reduced indentation moduli results in the increased H/E*-ratios and, consequently, a higher wear resistance than Ti-free coating [27]. The structural evolution of powdered Al0.68Cr0.32N, Al0.66Cr0.25Ti0.09N and Al0.62Cr0.20Ti0.18N coatings after annealing up to 1450 C is performed by XRD as displayed in Fig. 3. Only a small shift of the XRD peaks to higher 2q angles after annealing of Al0.68Cr0.32N at Ta ¼ 800 C is observed, as shown in Fig. 3a, where minute structural changes such as recovery and relaxation occur, and thereby leads to a drop in density of defects. Annealing of Al0.68Cr0.32N at 900 C results in the appearance of w-AlN reflexes, which gradually
Fig. 2. Mechanical properties of as-deposited Al0.68Cr0.32N, Al0.66Cr0.25Ti0.09N and Al0.62Cr0.20Ti0.18N coatings. The Poisson's ratio of coatings assume to 0.25.
Y. Xu et al. / Vacuum 120 (2015) 127e131
129
Fig. 3. XRD patterns of (a) Al0.68Cr0.32N, (b) Al0.66Cr0.25Ti0.09N and (c) Al0.62Cr0.20Ti0.18N coatings after heating up to 1450 C.
grow with further increase of Ta. However, the thermal decomposition process of the CreAleN has a confidential relation with the thermal treatment conditions. Willmann et al. [9] detected the diffraction signals of w-AlN for Al0.68Cr0.32N when annealing above 700 C under a longer treatment time of 60 min in comparison with our annealing condition. While Reiter's study shows that the decomposition of fcc-Al0.71Cr0.29N occurs till Ta up to 950 C with holding time of 120 min [1]. Increasing Ta to 1100 C in our study causes CreN bond to dissociate to form hexagonal (h-) Cr2N, and concomitant N-loss. The aggravated N-loss leads to the formation of thermodynamic stable c-Cr during annealing above 1100 C. After annealing at 1200 C, no intensity at the XRD peak positions of asdeposited Al0.68Cr0.32N can be detected, indicating complete decomposition of the original supersaturated matrix. Further increasing Ta to 1300 and 1450 C causes a continuous transition of h-Cr2N into c-Cr as well as an increased intensity of w-AlN diffraction peaks. We can still observe the diffraction peaks of hCr2N after annealing at Ta ¼ 1450 C. Alloying with Ti into AleCreN promotes the formation of w-AlN at Ta ¼ 800 C, where the diffraction peaks of w-AlN can be observed after annealing of Al0.66Cr0.25Ti0.09N and Al0.62Cr0.20Ti0.18N, see Fig. 3b and c. This is different from the results reported n et al. [12], which indicate the retarded w-AlN formation by Forse with Ti-addition during thermal annealing due to the increased stability of c-(Ti)AlN. The promoted formation of w-AlN for our Ticontaining coatings might be attributed to the decreased grain size caused by Ti-addition, which increases the volume fraction of grain boundary of coatings. The grain boundary acts as a short diffusion path for Al during annealing. Therefore, the reduction of coating grain size promotes the precipitation of w-AlN at grain boundary. It is worth noting that AleCreN exhibits a bit faster precipitation of w-AlN during 1000e1100 C induced by its slightly higher Al content as well as the small amount of w-AlN already at the initial stage of annealing. The increased weight fraction (calculated by reference intensity ratio method) of w-AlN for Al0.68Cr0.32N is ~8.6%, corresponding to ~6.7% for Al0.66Cr0.25Ti0.09N and ~2.9% for Al0.62Cr0.20Ti0.18N. In addition, the spinodal decomposition is undetectable in our experiments, which is disagreement with that n et al. [12]. The binodal precipitation of c-TiN reported by Forse from Al0.66Cr0.25Ti0.09N can be observed at Ta ¼ 1100 C, see Fig. 3b, as in the case of Ti0.75Al0.25N [28]. And increasing Ti content to 18 at
% results in an earlier formation of c-TiN at Ta ¼ 1000 C, as shown in Fig. 3c. Whereas, the spinodal decomposition of Al0.62Cr0.20Ti0.18N cannot be excluded absolutely under the detection limit of XRD, where the diffraction peaks of c-AlN could be overlapped by the coating signals. However, the N-loss is suppressed for our Ticontaining coatings. The h-Cr2N formation due to N-loss cannot be observed until elevated Ta to 1200 C. The XRD peaks still contain a contribution of the cubic solid solution. This improvement can be attributed to the higher binding energy of TieN bond with a value of 476.0 kJ/mol than that of CreN bond with a value of 377.8 kJ/mol [29]. Therefore, the intensity of h-Cr2N diffraction peaks for Al0.66Cr0.25Ti0.09N is higher than that for Al0.62Cr0.20Ti0.18N. Annealing of Ti-containing coatings at Ta ¼ 1300 C induces a complete decomposition of cubic solid solution to form c-TiN, hCr2N, c-Cr and w-AlN. The Ti-containing coatings almost completely transform into their stable phases c-TiN, w-AlN and c-Cr with further increasing Ta to 1450 C. Only weak h-Cr2N diffraction
Fig. 4. Synchronous (a) thermal gravimetric analysis and (b) DSC results obtained from Al0.68Cr0.32N, Al0.66Cr0.25Ti0.09N and Al0.62Cr0.20Ti0.18N powdered samples in synthetic air atmosphere.
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Fig. 5. SEM fracture cross section images of (a and b) Al0.68Cr0.32N and (d and e) Al0.62Cr0.20Ti0.18N coating after oxidation at (a and d) 1000 C and (b and e) 1100 C for 20 h (c and f) EDX line-scan composition profiles along the arrow line indicated in (b) and (e), respectively.
peaks can be detected for Al0.66Cr0.25Ti0.09N, see Fig. 3b. Fig. 4 shows the synchronous (a) mass change and (b) heat flow of Al0.68Cr0.32N, Al0.66Cr0.25Ti0.09N and Al0.62Cr0.20Ti0.18N freestanding coating samples during heating up to 1500 C. The onset temperature of mass gain curve for Al0.68Cr0.32N due to oxidation is ~1157 C, see Fig. 4a, which is related to the corresponding exothermic heat flow as shown in Fig. 4b. The onset and ending temperatures of mass gain shift to lower values with Ti-addition, where this shift becomes more pronounced with increasing Ti content. The exothermic heat flow agrees well with the above change of mass gain, where alloying with Ti induces an earlier exothermic heat flow. These results suggest that incorporation of Ti has a negative effect on the oxidation resistance of AleCreN coating, which can be attributed to the higher affinity between Ti and oxygen. Fig. 5a and b shows the fracture cross-sections of Al0.68Cr0.32N coatings after isothermal oxidation in synthetic air at 1000 and 1100 C for 20 h, respectively. Only a very thin oxide scale of ~0.25 mm with dense structure, which consists of Cr, Al and O, is observed as shown in Fig. 5a. Increasing Ta to 1100 C yields a ~1.3 mm oxide scale with layered structure consisting of Cr-oxide rich top-layer and Al-oxide rich sublayer, see Fig. 5b, which is verified by EDX line-scan shown in Fig. 5c. During the initial oxidation state of AleCreN coating, the dense (Cr, Al) oxide scale, which retards the oxidation diffusion process, forms and thus has a positive effect on the oxidation resistance [4,30]. However, with the aggravation of oxidation (e.g. extended oxidation time or elevated oxidation temperature) the outward diffusion of Cr due to the higher diffusion rate of Cr than that of Al results in the formation of Cr-oxide rich top-layer, and corresponding the Al-oxide rich sublayer. This is similar to the oxidation process of TieAleN coatings [31,32]. Incorporation of Ti into AleCreN causes a significant drop in oxidation resistance, where the Al0.62Cr0.20Ti0.18N coating reveals oxide scales with thickness of ~2.6 and 4.0 mm after oxidation at
1000 and 1100 C, respectively, see Fig. 5d and e. EDX line-scan of Ti-containing coating after oxidation at 1000 C (not shown here) indicates that this oxide scale consists of Ti-oxide rich top-layer and (Cr, Al)-oxide rich sublayer, see Fig. 5d. The outward diffusion of Ti during oxidation of Ti-containing coating, which is also detected by Polcar and Cavalerio [33] in nanolayered CrAlTiN coatings, due to the higher affinity between Ti and oxygen and the stress-induced cracks from growth of TiO2 breaks the dense structure of (Cr, Al) oxide scale, and thereby accelerates the oxidation. This (Cr, Al)oxide rich sublayer transforms into lamellate structure with further oxidation at 1100 C, see Fig. 5e and f, which is also related to the different diffusion rate of Al and Cr. In summary, incorporation of 9 and 18 at% Ti into AleCreN induces elevated hardness values of ~29.1 and 33.2 GPa, and reduced elastic modulus of ~303.2 and 337.1 GPa. Annealing of metastable Al0.68Cr0.32N results in decomposition into c-Cr and w-AlN via an intermediate phase of h-Cr2N as well as accompanying N-loss, where the onset temperatures of w-AlN, h-Cr2N and c-Cr formation are 900, 1100 and 1100e1200 C, respectively. Al0.66Cr0.25Ti0.09N and Al0.64Cr0.18Ti0.18N reveal an earlier formation of w-AlN at 800 C during annealing induced from grain refinement. However, Tiaddition retards the N-loss due to the higher binding energy of TieN bond with the onset temperatures of h-Cr2N and c-Cr formation at 1200 and 1200e1300 C, respectively. AleCreN coating is oxidized to form protective dense oxide scale with thickness of ~0.25 nm at 1000 C. Due to the different diffusion rate of Al and Cr, layered oxide scale with Cr-rich layer and Al-rich sublayer is observed when oxidation at 1100 C. While the outward diffusion of Ti due to the higher affinity between Ti and oxygen breaks the dense structure of (Cr, Al) oxide scale, and thereby causes a significant drop in oxidation resistance of AleCreTieN coating. Acknowledgements Financial support for this work by the National Nature Science
Y. Xu et al. / Vacuum 120 (2015) 127e131
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Vacuum journal homepage: www.elsevier.com/locate/vacuum
Short communication
Influence of Ti on the mechanical properties, thermal stability and oxidation resistance of AleCreN coatings Yuxiang Xu a, Li Chen a, b, *, Ziqiang Liu a, Fei Pei a, b, Yong Du a a b
State Key Laboratory of Powder Metallurgy, Central South University, Changsha, Hunan, 410083, China Zhuzhou Cemented Carbide Cutting Tools Co., Ltd., Zhuzhou, Hunan, 412007, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 24 March 2015 Received in revised form 24 June 2015 Accepted 6 July 2015 Available online 8 July 2015
Al0.68Cr0.32N, Al0.66Cr0.25Ti0.09N and Al0.62Cr0.20Ti0.18N coatings prepared by cathodic arc evaporation exhibit single phase cubic structure. Increasing Ti content from 0 to 18 at% causes a continuous hardness promotion from ~27.4 to 33.2 GPa, however, a drop in elastic modulus from ~481.8 to 337.1 GPa. Annealing of AleCreN in Ar results in a transition into stable phases of wurtzite (w-) AlN and Cr via an intermediate phase Cr2N. Alloying with Ti into AleCreN promotes the w-AlN formation, but retards the N-loss, where the finally stable phases of w-AlN, Cr and TiN after annealing are obtained. Nevertheless, the Ti-addition has a significantly inferior effect on the oxidation resistance of AleCreN coating due to the higher affinity between Ti and oxygen, where the Al0.68Cr0.32N and Al0.62Cr0.20Ti0.18N coatings after oxidation at 1100 C for 20 h reveal oxide scales of ~1.3 and 4.0 mm, respectively. © 2015 Elsevier Ltd. All rights reserved.
Keywords: AlCrN AlCrTiN Hardness Thermal stability Oxidation resistance
Hard coatings have been proven to be excellent candidates to improve wear protection, oxidation and corrosion resistance for advanced machining processes (e.g., high-speed and dry cutting). AleCreN coating is one of the most popular and universally used protective layers in machining applications due to its excellent tribological property as well as outstanding resistance to oxidation and corrosion [1,2]. The most significant advantage of Cr1xAlxN coatings is their excellent oxidation resistance, which is related to the formation of dense and adherent (Al, Cr)2O3 mixed oxide scale during exposure to air at elevated temperature [3,4]. The thermal stability of protective coatings counts for as much as their mechanical properties and oxidation resistance in machining applications [5,6]. A transformation into their stable phases of wurtzite (w-) AlN and cubic (c-) Cr via intermediate phase of Cr2N occurs, which is accompanied by N-loss due to the chemical instability of CreN bond during thermal load of metastable AleCreN coatings [7,8]. This decomposition of AleCreN coatings leads to a continuous reduction in mechanical properties and thereby is detrimental to machining applications [9,10]. Incorporation of a fourth constituent (e.g. Si, Ti, V, Zr, Y) into AleCreN to tailor the structure and properties receives
* Corresponding author. State Key Laboratory of Powder Metallurgy, Central South University, Changsha, Hunan, 410083, China. E-mail address: [email protected] (L. Chen). http://dx.doi.org/10.1016/j.vacuum.2015.07.004 0042-207X/© 2015 Elsevier Ltd. All rights reserved.
considerable interests recently [11e14]. Si-addition results in the formation of nanocomposite structure with amorphous Si3N4 interfacial phase bundling nanocrystalline CrAlN grains, and thus provokes a significant improvement in mechanical and thermal properties [15]. And lower friction and higher oxidation resistance can be achieved in AleCreN coatings by alloying with V and Y, respectively [16,17]. The previous studies indicate that Ti-addition into AleCreN coatings causes improved cutting performance in different machining applications due to their increased hardness and wear resistance [18e22]. However, the thermal stability and oxidation resistance of coatings also act as crucial factors on the machining performance of coated tools. The thermal decomposition process of metastable AleCreTieN coatings during annealing depends on the coatings composition, heat treatment condition and deposition method. Hasegawa et al. indicates the formation of w-AlN can be detected in Al0.66Cr0.24Ti0.10N and Al0.62Cr0.11Ti0.27N coatings when annealing above 900 C [23]. The effect of Ti on the thermal stability of AleCreN coating is further investigated with n et al. [12]. They found the addition of 1, 2 and 11 at% Ti by Forse that alloying with Ti can stabilize the cubic structure of AleCreN via suppressing the formation of w-AlN. Besides, AleCreTieN coatings manifested higher oxidation resistance than TieAleN coatings [21,24,25]. However, the research by Forsen et al. [12] indicates that incorporation 11 at% Ti into AleCreN coating leads to a significant drop in oxidation resistance, where the TiO2 emerged
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after heating up to 1100 C [12]. In this work, AleCreN and AleCreTieN coatings were deposited by cathodic arc evaporation to investigate the effect of Ti on the mechanical properties, thermal stability and oxidation resistance of coatings. AleCreN and two compositions of AleCreTieN coatings were prepared in a commercial cathodic arc system (Balzers Oerlikon Rapid Cooling System, RCS) from Al0.70Cr0.30, Al0.70Cr0.20Ti0.10 and Al0.65Cr0.15Ti0.20 targets (99.99% purity), respectively, which has the working N2 pressure of ~2 Pa, the substrate bias of 100 V, the target current of 180 A, and the deposition temperature of 550 C. Prior to the deposition, the substrates were ultra-sonically cleaned in acetone and ethylene, and then were cleaned with an Argon-ionetching process at an Ar pressure of 0.3 Pa and substrate bias voltage of 150 V DC for 20 min. The thickness of deposited AleCreN and AleCreTieN is ~3.0 mm. Thermal annealing was performed in the differential scanning calorimetry (DSC) instrument (Netzch-STA 409C, Germany), in which coating samples heated up to specified annealing temperatures (Ta ¼ 800e1300 each 100 C, and 1450 C), respectively, with a rate of 10 K/min and cooled down immediately with a rate of 50 K/min in flowing Ar (99.9% purity, 20 sccm flow rate). Prior to these measurements, the coatings were removed from their lowalloy steel substrates by chemical etching in 10 mol% nitric acid to avoid interference from substrate materials. Coated polycrystalline Al2O3 substrates were isothermally oxidized at 1000 and 1100 C, respectively, for 20 h in the DSC equipment with a heating rate of 10 K/min and a cooling rate of 50 K/min in synthetic air (79 vol.% N2, 21 vol.% O2 and 20 sccm flow rate). Subsequently, the oxidized coated Al2O3 substrates were investigated by the fracture cross-sectional scanning electron microscopy (SEM, LEO1525, Germany) observation with Energy Dispersive X-ray (EDX) composition analyses. The elemental compositions of the as-deposited coatings were determined using EDX attached to SEM. Quantification of the elemental composition was obtained by elemental standards and a TiN coating standard which have been quantified by Rutherford back-scattering spectroscopy. The error of measurements for the metal atoms is below 2 at%. Phase identifications were conducted by X-ray diffraction (XRD) with Cu Ka radiation using a Bruker D8 in Bragg/Brentano mode at 40 mA and 40 kV. Hardness and elastic modulus of as-deposited coatings on cemented carbide (WC-6 wt.% Co) were derived from nanoindentation with Berkovich diamond indenter using an instrumented nanoindenter (CSM Instruments, Switzerland) after the Oliver and Pharr method [26]. According to the experimental results based on the large-load (30 mN) penetration test, a smaller penetration load of 10 mN was chosen to measure the mechanical properties of the coatings to keep the indentation depth (~200 nm) below 10% of the film thickness for minimizing the influence of substrates. Elemental analysis by EDX shows that the AleCreN and AleCreTieN coatings are stoichiometric with N/metal ratios of 1 ± 0.08 and compositions of Al0.68Cr0.32N, Al0.66Cr0.25Ti0.09N and Al0.62Cr0.20Ti0.18N, respectively. XRD patterns of freestanding samples, as shown in Fig. 1, yield single phase face centered cubic (B1eNaCl type) structure for AleCreN and AleCreTieN coatings. Increasing Ti content for our coatings results in a continuous shift of the diffraction peaks position to lower 2q angles, which primarily due to the increased lattice size caused by substitution of Al and Cr atoms in the metal sublattice with Ti having a larger atomic radius. In addition, a broadening of the XRD peaks with elevated Ti contents in the coatings can be ascribed to the increased microstrains and/or decreased grain size. Grain sizes of Al0.68Cr0.32N, Al0.66Cr0.25Ti0.09N and Al0.62Cr0.20Ti0.18N coatings from (200) peaks using Scherrer formula is 20.5 ± 0.3, 16.4 ± 0.3 and 12.6 ± 0.2 nm, respectively, which actually reflect the dimensions of the
Fig. 1. XRD patterns of as-deposited Al0.68Cr0.32N, Al0.66Cr0.25Ti0.09N and Al0.62Cr0.20Ti0.18N coatings.
coherently scattering domains. The hardness (H), elastic modulus (E) and corresponding ratio of hardness to reduced modulus (H/E*, where E* ¼ E/(1-m2); m is Poisson's ratio) of AleCreN and AleCreTieN coatings are depicted in Fig. 2. Alloying with Ti into AleCreN coating causes an increase in hardness from 27.4 ± 0.4 GPa for AleCreN to 29.1 ± 1.3 GPa for 9 at% Ti-containing coating towards 33.2 ± 0.8 GPa for 18 at% Ti-containing coating, which is related to the solid solution strengthening and grain refinement. The Ti-containing coatings exhibit the significantly reduced indentation moduli, which can probably be attributed to the grain refinement. The reduced indentation moduli results in the increased H/E*-ratios and, consequently, a higher wear resistance than Ti-free coating [27]. The structural evolution of powdered Al0.68Cr0.32N, Al0.66Cr0.25Ti0.09N and Al0.62Cr0.20Ti0.18N coatings after annealing up to 1450 C is performed by XRD as displayed in Fig. 3. Only a small shift of the XRD peaks to higher 2q angles after annealing of Al0.68Cr0.32N at Ta ¼ 800 C is observed, as shown in Fig. 3a, where minute structural changes such as recovery and relaxation occur, and thereby leads to a drop in density of defects. Annealing of Al0.68Cr0.32N at 900 C results in the appearance of w-AlN reflexes, which gradually
Fig. 2. Mechanical properties of as-deposited Al0.68Cr0.32N, Al0.66Cr0.25Ti0.09N and Al0.62Cr0.20Ti0.18N coatings. The Poisson's ratio of coatings assume to 0.25.
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Fig. 3. XRD patterns of (a) Al0.68Cr0.32N, (b) Al0.66Cr0.25Ti0.09N and (c) Al0.62Cr0.20Ti0.18N coatings after heating up to 1450 C.
grow with further increase of Ta. However, the thermal decomposition process of the CreAleN has a confidential relation with the thermal treatment conditions. Willmann et al. [9] detected the diffraction signals of w-AlN for Al0.68Cr0.32N when annealing above 700 C under a longer treatment time of 60 min in comparison with our annealing condition. While Reiter's study shows that the decomposition of fcc-Al0.71Cr0.29N occurs till Ta up to 950 C with holding time of 120 min [1]. Increasing Ta to 1100 C in our study causes CreN bond to dissociate to form hexagonal (h-) Cr2N, and concomitant N-loss. The aggravated N-loss leads to the formation of thermodynamic stable c-Cr during annealing above 1100 C. After annealing at 1200 C, no intensity at the XRD peak positions of asdeposited Al0.68Cr0.32N can be detected, indicating complete decomposition of the original supersaturated matrix. Further increasing Ta to 1300 and 1450 C causes a continuous transition of h-Cr2N into c-Cr as well as an increased intensity of w-AlN diffraction peaks. We can still observe the diffraction peaks of hCr2N after annealing at Ta ¼ 1450 C. Alloying with Ti into AleCreN promotes the formation of w-AlN at Ta ¼ 800 C, where the diffraction peaks of w-AlN can be observed after annealing of Al0.66Cr0.25Ti0.09N and Al0.62Cr0.20Ti0.18N, see Fig. 3b and c. This is different from the results reported n et al. [12], which indicate the retarded w-AlN formation by Forse with Ti-addition during thermal annealing due to the increased stability of c-(Ti)AlN. The promoted formation of w-AlN for our Ticontaining coatings might be attributed to the decreased grain size caused by Ti-addition, which increases the volume fraction of grain boundary of coatings. The grain boundary acts as a short diffusion path for Al during annealing. Therefore, the reduction of coating grain size promotes the precipitation of w-AlN at grain boundary. It is worth noting that AleCreN exhibits a bit faster precipitation of w-AlN during 1000e1100 C induced by its slightly higher Al content as well as the small amount of w-AlN already at the initial stage of annealing. The increased weight fraction (calculated by reference intensity ratio method) of w-AlN for Al0.68Cr0.32N is ~8.6%, corresponding to ~6.7% for Al0.66Cr0.25Ti0.09N and ~2.9% for Al0.62Cr0.20Ti0.18N. In addition, the spinodal decomposition is undetectable in our experiments, which is disagreement with that n et al. [12]. The binodal precipitation of c-TiN reported by Forse from Al0.66Cr0.25Ti0.09N can be observed at Ta ¼ 1100 C, see Fig. 3b, as in the case of Ti0.75Al0.25N [28]. And increasing Ti content to 18 at
% results in an earlier formation of c-TiN at Ta ¼ 1000 C, as shown in Fig. 3c. Whereas, the spinodal decomposition of Al0.62Cr0.20Ti0.18N cannot be excluded absolutely under the detection limit of XRD, where the diffraction peaks of c-AlN could be overlapped by the coating signals. However, the N-loss is suppressed for our Ticontaining coatings. The h-Cr2N formation due to N-loss cannot be observed until elevated Ta to 1200 C. The XRD peaks still contain a contribution of the cubic solid solution. This improvement can be attributed to the higher binding energy of TieN bond with a value of 476.0 kJ/mol than that of CreN bond with a value of 377.8 kJ/mol [29]. Therefore, the intensity of h-Cr2N diffraction peaks for Al0.66Cr0.25Ti0.09N is higher than that for Al0.62Cr0.20Ti0.18N. Annealing of Ti-containing coatings at Ta ¼ 1300 C induces a complete decomposition of cubic solid solution to form c-TiN, hCr2N, c-Cr and w-AlN. The Ti-containing coatings almost completely transform into their stable phases c-TiN, w-AlN and c-Cr with further increasing Ta to 1450 C. Only weak h-Cr2N diffraction
Fig. 4. Synchronous (a) thermal gravimetric analysis and (b) DSC results obtained from Al0.68Cr0.32N, Al0.66Cr0.25Ti0.09N and Al0.62Cr0.20Ti0.18N powdered samples in synthetic air atmosphere.
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Fig. 5. SEM fracture cross section images of (a and b) Al0.68Cr0.32N and (d and e) Al0.62Cr0.20Ti0.18N coating after oxidation at (a and d) 1000 C and (b and e) 1100 C for 20 h (c and f) EDX line-scan composition profiles along the arrow line indicated in (b) and (e), respectively.
peaks can be detected for Al0.66Cr0.25Ti0.09N, see Fig. 3b. Fig. 4 shows the synchronous (a) mass change and (b) heat flow of Al0.68Cr0.32N, Al0.66Cr0.25Ti0.09N and Al0.62Cr0.20Ti0.18N freestanding coating samples during heating up to 1500 C. The onset temperature of mass gain curve for Al0.68Cr0.32N due to oxidation is ~1157 C, see Fig. 4a, which is related to the corresponding exothermic heat flow as shown in Fig. 4b. The onset and ending temperatures of mass gain shift to lower values with Ti-addition, where this shift becomes more pronounced with increasing Ti content. The exothermic heat flow agrees well with the above change of mass gain, where alloying with Ti induces an earlier exothermic heat flow. These results suggest that incorporation of Ti has a negative effect on the oxidation resistance of AleCreN coating, which can be attributed to the higher affinity between Ti and oxygen. Fig. 5a and b shows the fracture cross-sections of Al0.68Cr0.32N coatings after isothermal oxidation in synthetic air at 1000 and 1100 C for 20 h, respectively. Only a very thin oxide scale of ~0.25 mm with dense structure, which consists of Cr, Al and O, is observed as shown in Fig. 5a. Increasing Ta to 1100 C yields a ~1.3 mm oxide scale with layered structure consisting of Cr-oxide rich top-layer and Al-oxide rich sublayer, see Fig. 5b, which is verified by EDX line-scan shown in Fig. 5c. During the initial oxidation state of AleCreN coating, the dense (Cr, Al) oxide scale, which retards the oxidation diffusion process, forms and thus has a positive effect on the oxidation resistance [4,30]. However, with the aggravation of oxidation (e.g. extended oxidation time or elevated oxidation temperature) the outward diffusion of Cr due to the higher diffusion rate of Cr than that of Al results in the formation of Cr-oxide rich top-layer, and corresponding the Al-oxide rich sublayer. This is similar to the oxidation process of TieAleN coatings [31,32]. Incorporation of Ti into AleCreN causes a significant drop in oxidation resistance, where the Al0.62Cr0.20Ti0.18N coating reveals oxide scales with thickness of ~2.6 and 4.0 mm after oxidation at
1000 and 1100 C, respectively, see Fig. 5d and e. EDX line-scan of Ti-containing coating after oxidation at 1000 C (not shown here) indicates that this oxide scale consists of Ti-oxide rich top-layer and (Cr, Al)-oxide rich sublayer, see Fig. 5d. The outward diffusion of Ti during oxidation of Ti-containing coating, which is also detected by Polcar and Cavalerio [33] in nanolayered CrAlTiN coatings, due to the higher affinity between Ti and oxygen and the stress-induced cracks from growth of TiO2 breaks the dense structure of (Cr, Al) oxide scale, and thereby accelerates the oxidation. This (Cr, Al)oxide rich sublayer transforms into lamellate structure with further oxidation at 1100 C, see Fig. 5e and f, which is also related to the different diffusion rate of Al and Cr. In summary, incorporation of 9 and 18 at% Ti into AleCreN induces elevated hardness values of ~29.1 and 33.2 GPa, and reduced elastic modulus of ~303.2 and 337.1 GPa. Annealing of metastable Al0.68Cr0.32N results in decomposition into c-Cr and w-AlN via an intermediate phase of h-Cr2N as well as accompanying N-loss, where the onset temperatures of w-AlN, h-Cr2N and c-Cr formation are 900, 1100 and 1100e1200 C, respectively. Al0.66Cr0.25Ti0.09N and Al0.64Cr0.18Ti0.18N reveal an earlier formation of w-AlN at 800 C during annealing induced from grain refinement. However, Tiaddition retards the N-loss due to the higher binding energy of TieN bond with the onset temperatures of h-Cr2N and c-Cr formation at 1200 and 1200e1300 C, respectively. AleCreN coating is oxidized to form protective dense oxide scale with thickness of ~0.25 nm at 1000 C. Due to the different diffusion rate of Al and Cr, layered oxide scale with Cr-rich layer and Al-rich sublayer is observed when oxidation at 1100 C. While the outward diffusion of Ti due to the higher affinity between Ti and oxygen breaks the dense structure of (Cr, Al) oxide scale, and thereby causes a significant drop in oxidation resistance of AleCreTieN coating. Acknowledgements Financial support for this work by the National Nature Science
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