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Oxidation resistance and micromechanical properties of a Ti–46Al–8Nb (at.%) alloy with Cr–Si magnetron-sputtered coatings
Accepted Manuscript Oxidation resistance and micromechanical properties of a Ti–46Al–8Nb (at.%) alloy with Cr–Si magnetron-sputtered coatings
Marzena Mitoraj-Królikowska, Elzbieta Godlewska PII: DOI: Reference:
S0257-8972(18)30768-0 doi:10.1016/j.surfcoat.2018.07.073 SCT 23639
To appear in:
Surface & Coatings Technology
Received date: Revised date: Accepted date:
13 May 2018 18 July 2018 20 July 2018
Please cite this article as: Marzena Mitoraj-Królikowska, Elzbieta Godlewska , Oxidation resistance and micromechanical properties of a Ti–46Al–8Nb (at.%) alloy with Cr–Si magnetron-sputtered coatings. Sct (2018), doi:10.1016/j.surfcoat.2018.07.073
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ACCEPTED MANUSCRIPT Oxidation resistance and micromechanical properties of a Ti-46Al-8Nb (at.%) alloy with Cr-Si magnetron-sputtered coatings
Marzena Mitoraj-Królikowska, AGH University of Science and Technology, Faculty of Materials Science and Ceramics, Al. Mickiewicza 30, 30-059, Krakow, Poland, phone
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number: +48 126172813, e-mail: [email protected]
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Elzbieta Godlewska, AGH University of Science and Technology, Faculty of Materials
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Science and Ceramics, Al. Mickiewicza 30, 30-059, Krakow, Poland, phone number: +48
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126172536, e-mail: [email protected]
Abstract:
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Samples of a Ti-46Al-8Nb (at.%) alloy with Cr-0.5Si, Cr-5Si or Cr-66Si (CrSi2) coatings deposited by magnetron sputtering were oxidised in air under thermal cycling
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conditions at 700 C and 800 C for 80 hours. Mass changes over time were measured
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periodically to evaluate reaction progress as well as susceptibility of the coating or scale to spallation. Vickers hardness, elastic modulus and adhesion of the coatings were determined
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before and after oxidation. Selected samples were oxidised for up to 1500-2000 hours. The nature and extent of coating degradation in each case was evaluated through systematic
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analyses of surfaces and cross-sections using different techniques, including scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDS) and X-ray diffraction (XRD). It has been found that coatings with the composition Cr-5Si (at.%) had the most promising combination of oxidation resistance and micromechanical properties.
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ACCEPTED MANUSCRIPT Keywords: titanium aluminides, magnetron sputtering, Cr-Si coatings, oxidation, mechanical testing
Abbreviations: µ - friction coefficient [-]
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E - Young’s Modulus [GPa]
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HIT - indentation hardness [MPa] Hmax - maximum penetration depth under a load of 10mN [nm]
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HV –Vickers hardness [HV]
Lc1 – critical load for cohesive failure [N]
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Lc2 – critical load for adhesive failure [N]
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I – cathode current intensity [A]
P – cathode emission power [W]
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pAr - argon pressure [Pa]
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Pmax – maximum load applied in scratch test [N] t – coating deposition time [s]
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1. Introduction
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T – temperature [C]
Titanium aluminides have the potential to be used as high-temperature structural materials due to their low density (3.7 - 3.9 g/cm3), high melting point, good mechanical properties at elevated temperatures as well as good creep and oxidation resistance [1-3]. Because of these unique properties Ti-Al alloys find application as components of motor vehicles [4], industrial turbines [5] and aero-engines [6]. In particular, they can replace heavier Ni or Fe based superalloys operating at moderate temperatures and loads, thus
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ACCEPTED MANUSCRIPT contributing to more than 50% total mass reduction. This is especially important for the rotating parts of aero-engines. However, despite many advantages titanium aluminides have some drawbacks, e.g. insufficient low-temperature ductility [7], insufficient oxidation resistance at temperatures higher than 750-800 C [8] and reduction in tensile elongation after exposure to oxidizing environments at elevated temperatures (“embrittlement”) [9,10]. Even a
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few hours exposure to air at 700 C can cause significant deterioration of mechanical
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properties of titanium aluminide alloys. The embrittlement is induced by dissolution of small
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atoms, such as oxygen, nitrogen or hydrogen. The attempts to improve oxidation resistance and prevent dissolution of gases [11-13] included alloying and surface treatment. Numerous
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coating systems and deposition techniques (PVD, CVD, ion implantation, etc.) were investigated [14-16] to develop a possibly thin, compact and adherent coating which would
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protect the alloy against oxidation and penetration of gases. Among the deposition techniques, magnetron sputtering seems particularly attractive because of good reproducibility and high
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purity of coatings and eco-friendly process conditions [17].
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The aim of this study was to develop a protective coating for the Ti-46Al-8Nb (at.%) alloy and evaluate its adhesion, ductility and oxidation resistance. It has been assumed that the
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coating should prevent dissolution of gases in the alloy during the short-term exposure at 700 C and should enhance its oxidation resistance at 800 C. The chemical composition of
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coatings (Cr, Si) was selected on the basis of the results described in the literature [18-20] and the well-known properties of chromium and silicon which form slow-growing protective oxides.
2. Experimental procedure The investigated alloy, Ti-46Al-8Nb (at.%), was produced by horizontal centrifugal casting (ACCESS e.V. Aachen). It had fully lamellar microstructure consisting of -TiAl 3
ACCEPTED MANUSCRIPT (75mol.%) and 2-Ti3Al (25mol.%). Ingots, 16.0 mm in diameter were cut into pellets, 1.0 – 1.5 mm thick, by means of a diamond saw. The specimens were ground with emery papers up to 2000 grit and polished with diamond suspensions (9, 3, 1 m grain size). Before coating deposition they were ultrasonically cleaned in water with detergents, next in acetone and finally in double-distilled water. The deposition process was conducted in a vacuum chamber
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equipped with a WMK-50 magnetron gun and medium frequency pulsed power supply
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(DORA Power System, PL). Argon flow was controlled by MKS flowmeter. Temperature of
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the samples during heating, surface treatment in the glow-discharge plasma and deposition was measured by thermocouple. Deposition was performed under different argon pressures
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and different cathode current intensities. Coating thickness (about 1-2 µm) was controlled by
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adjusting the deposition time. All deposition parameters are collected in Table 1.
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Table 1 Parameters of coating deposition (pAr-argon pressure; I–cathode current intensity; t– deposition time; T–temperature, P–cathode emission power)
pAr [Pa]
I [A]
t [s]
T [oC] P [W]
Cr-0,5Si
0.4
0.4
1800
270
360
0.4
0.5
1800
300
460
1.5
0.2
1800
300
240
Cr-5Si
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CrSi2
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Target composition [at.%]
Targets, 50 mm in diameter, with compositions: Cr-0.5Si (at.%) or Cr-5Si (at.%) were made from elemental powders by hot pressing at 25 MPa and 1600 C. The Cr-66Si (CrSi2) targets were purchased from Kurt J. Lesker Company. Samples with magnetron sputtered coatings were oxidised for 80 hours in air under thermal cycling conditions at 700 or 800 C. One-hour cycles (1-h) were used. For selected samples the exposure time was extended to 1500 - 2000 hours. Each thermal cycle consisted 4
ACCEPTED MANUSCRIPT of rapid heating (50 C/min) to the desired temperature, maintaining at the constant temperature for 1 hour and rapid cooling (50 C/min) to the room temperature. During the test, the specimens were kept in alumina crucibles which enabled collection of oxidation products that might be lost by spallation. Once a day the samples were weighed with an accuracy of 10-4g. Net mass changes (sample alone without the spalled matter ) and gross
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mass changes (sample with the crucible and the spalled matter inside ).
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Micromechanical tests were performed in conformity with ISO 14577-1, ISO 14577-2,
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PN-EN ISO 6507-1:2005 before and after the oxidation test using the CSEM Micro-CombiTester (MCT). The Vickers hardness measurement was done in six different spots on each
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sample, maximum load being 10 mN. The loading rate was 20mN/min. For each loading/unloading cycle the corresponding maximum penetration depth of the indenter (Hmax),
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Vickers hardness (HV), indentation hardness (HIT) and Elastic (Young’s) Modulus (E) were determined using the established models. All samples were submitted to scratch tests. The
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Rockwell C diamond intender was used with a linearly increasing load (10 mN/min) in the
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range from 0.05 to 30 N (Pmax, maximum load) until delamination or chipping of the coating occurred. Coating failure was detected by three methods – measurement of friction and
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acoustic emission and by microscopic inspection of the scratch track. Critical normal force at which the first cohesive or adhesive failure of the coating occurred was referred to as critical
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load Lc1 or Lc2, respectively.
3. Results 3.1 Cr-Si coatings Net mass changes of Ti-46Al-8Nb (at.%) samples with Cr-Si coatings during oxidation at 700 C and 800 C for up to 80 h are shown in Fig. 1.
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Ti-46Al-8Nb (at.%), net mass changes
0.3 0.2 0.1 700oC 0.0
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2
m/A [mg/cm ]
800oC
uncoated
Cr-0.5Si
Cr-5Si
CrSi2
60
80
20
40 Time [h]
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-0.2 0
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-0.1
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Fig. 1 Net mass changes of Cr-Si coated and uncoated Ti-46Al-8Nb (at.%) samples during oxidation in air at 700 C or 800 C (1-h cycles). As can be seen in Fig. 1, at 700 C the net mass changes of the Cr-0.5Si and Cr-5Si
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coated samples were slightly higher while at 800 C similar to those of uncoated Ti-46Al-8Nb
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(at.%) alloy. The CrSi2 – coated samples had the lowest net mass changes. The negligible mass losses measured at 700 C were probably caused by microdamages upon handling and
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weighing. No solid debris were found inside the crucible.
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Micrographs in Fig. 2 show polyhedral crystals of Cr2O3 on the surface of samples after oxidation, larger in size at the higher oxidation temperature. The contribution of chromium oxide decreased with the concentration of chromium in the coating. In the case of CrSi2 coating, the Cr2O3 crystals protruded from the fine-crystalline and partly amorphous SiO2 layer. According to the XRD analysis in Fig. 3, at 700 C nitridation of titanium and chromium could take place along with oxidation. Oxidation products of the Cr-0.5Si coating contained traces of Al2O3. At 800 C, Cr2O3 was the predominant oxide phase on all samples
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ACCEPTED MANUSCRIPT (XRD in Fig. 4). In the case of CrSi2 coatings, SiO2 and Ti3N were also found on the surface
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after oxidation.
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Fig. 2 Surfaces of Ti-46Al-8Nb (at.%) samples with Cr-Si coatings after 80 hours of oxidation in air at 700 C and 800 C (1-h cycles) - SEI/BEI
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700oC, 80h, 1-h cycles Cr2O3 Cr Al2O3
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Cr-5Si
Cr2N
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Intensity [a.u.]
Cr-0.5Si
Ti3N SiO2 CrSi2
CrSi2
10
20
30
40
50
60
70
80
90
2 Fig. 3 XRD patterns from the surface of Ti-46Al-8Nb (at.%) samples with Cr-Si coatings after oxidation at 700 C for 80 h (1-h cycles) 7
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800oC, 80h, 1-h cycles Cr2O3
30
40
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50
60
70
80
Ti3N
90
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20
TiAl SiO2
CrSi2
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Cr-5Si
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Intensity [a.u.]
Cr-0.5Si
2
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Fig. 4 XRD patterns from the surface of Ti-46Al-8Nb (at.%) samples with Cr-Si coatings after oxidation at 800 C for 80 h (1-h cycles)
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Additional information about coating performance was obtained from the cross-sections
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(Fig. 5). All the investigated Cr-Si coatings had good protective properties at 700 C. There were no signs of interdiffusion with the substrate or phase transformations in the alloy. At
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800 C the best protection was provided by CrSi2. The coating was only partially oxidised.
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However, at 800 C some interaction between the CrSi2 coating and the substrate was already visible. Silicon from the coating reacted with titanium to form Ti-Si phases. In the case of chromium-rich coating, Cr-0.5Si, some inward diffusion of chromium took place. As can be seen in Fig. 5, thickness of the chromium oxide layer (Cr2O3) on the surface increased with temperature.
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Fig. 5 Cross-sections of Ti-46Al-8Nb (at.%) with Cr-Si coatings after 80 hours of oxidation in air at 700 C and 800 C - BEI
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Hardness of the Cr-0.5Si coating decreased after oxidation at 700 C (about 3 times) and increased after oxidation at 800 C (about 1.2 times) – see Table 2. In the case of Cr-5Si, HV and HIT decreased about 4 times after oxidation at 700 C and about 2 times at 800 C. HV and HIT were about 2 times lower for CrSi2 oxidised at 800 C than at 700 C. Young’s modulus only slightly decreased for Cr-0.5Si after oxidation whereas for Cr-5Si it decreased at least two times . The value of Young’s modulus for CrSi2 oxidised at 800 C was lower than at 700 C. 9
ACCEPTED MANUSCRIPT Adhesion of the Cr-Si coatings was evaluated on the basis of scratch test (Table 3). Critical loads (Lc2) for delamination were high and ranged from 20 to 30N. The lowest Lc2 value was recorded for the CrSi2 coating oxidised at 700 C. The as-received Cr-0.5S and Cr5Si layers were rather brittle, which is reflected in relatively low Lc1 values. The chevron-like cracks observed in Cr-0.5Si before oxidation indicated very low coating ductility. However,
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ductility of the investigated Cr-Si coatings generally increased after oxidation, the most
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pronounced effect was seen in the case of Cr-5Si.
Hmax [nm]
HV
HIT [MPa]
E [GPa]
Cr-0.5Si as-received
165 8
1744 187
18446 1982
296 30
Cr-0.5Si oxidised for 80 h at 700 C
251 5 3
632 227
6681 2406
217 88
147 11
2098 185
22200 1952
273 53
159 6
1811 113
19157 1190
315 45
292 28
452 93
4782 980
122 9
Cr-5Si oxidised for 80 h at 800 C
238 21
767 105
8111 1112
130 49
CrSi2 oxidised for 80 h at 700 C
186 9
1184 140
12528 1486
210 28
CrSi2 oxidised for 80 h at 800 C
248 11
628 34
6644 357
152 19
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Coating and surface condition
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Table 2 Maximum penetration depth, Vickers hardness, indentation hardness and Young’s modulus for Ti-46Al-8Nb [at.%] samples with Cr-0.5Si, Cr-5Si, and CrSi2 coatings
Cr-5Si as-received
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Cr-0.5Si oxidised for 80 h at 800 C
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Cr-5Si oxidised for 80 h at 700 C
Table 3 Scratch test results for Ti-46Al-8Nb (at.%) samples coated with Cr-0.5Si, Cr-5Si, or CrSi2 under maximum load Pmax = 30 N
Coating and surface condition
Lc1 [N]
Lc2 [N]
10
2.0 ± 0.2 (1)
19.3 ± 3.1 (2)
0.18 ± 0.02
Cr-0.5Si oxidised for 80 h at 700 C
10.4 ± 0.8 (3)
>30 (4)
0.23
Cr-0.5Si oxidised for 80 h at 800 C
8.4 ± 0.7 (5)
20.4 ± 0.8 (6)
0.24 ± 0.2
Cr-5Si as-received
1.9 ± 0.1 (7)
>30 (8)
0.20 ± 0.01
Cr-5Si oxidised for 80 h at 700 C
18.9 ± 2.1 (5)
27.0 ± 0.8 (9)
0.24 ± 0.02
Cr-5Si oxidised for 80 h at 800 C
21.8 ± 2.2 (5)
26.9 ± 1.5 (10)
0.24 ± 0.00
CrSi2 oxidised for 80 h at 700 C
9.1 ± 1.5 (5)
16.6 ± 3.1 (2)
0.21 ± 0.01
CrSi2 oxidised for 80 h at 800 C
16.9 ± 3.0 (5)
20.1 ± 3.4 (11)
0.20 ± 0.01
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Cr-0.5Si as-received
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(1) chevron-like cracks inside the scratch track, at the load of 15N cracks propagating outward (2) cracks and delamination on both sides of the scratch track (3) small crack inside the scratch track (4) large crack at the load of 25 N, no delamination (5) cohesive cracks inside the scratch track (6) cracks and occasional delamination on the scratch track sides, at 25 N continuous delamination on the scratch track sides (7) small cracks inside the scratch track, curved in the direction opposite to the motion of the indenter and propagating outward (8) the number of cracks increasing with the load, no delamination; (9) cracks and occasional delamination on the scratch track sides (10) small abrasion inside the scratch track (11) cracks and occasional delamination on scratch track sides, at the load of 26 N continuous delamination on the scratch track sides
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Basing on the oxidation behaviour and the mechanical properties of the investigated
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coatings, we selected one composition, i.e. Cr-5Si, for long-term oxidation tests.
3.2 Long-term oxidation of Ti-46Al-8Nb (at.%) samples coated with Cr-5Si The oxidation runs at 700 and 800 C were continued for more than 1000 hours. Over the entire oxidation time the events of scale spallation or cracking were not observed. Surfaces of the samples after exposure are shown in Fig. 6. It can be seen that the higher temperature promoted crystal growth. The large polyhedral crystals formed at 800 C were composed of Cr2O3. These were accompanied by some oxides or nitrides of alloy or coating components. 11
ACCEPTED MANUSCRIPT According to EDS, after oxidation at 800 C the surface was slightly richer in titanium and aluminium than at 700 C (Fig.6). Niobium was not detected on the surface after oxidation at 800 C, which suggests that the Ti and Al counts were not collected from the substrate but from the scale (Ti, Al oxides or nitrides). In the case of samples oxidised at 700 oC, the concentrations of Al, Ti and Nb determined by EDS (Fig. 6) might originate from both, the
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scale and the substrate because the scale was rather thin (2.0-3.0 µm, Fig. 7).
Fig. 6 Surface of the Cr-5Si coating on Ti-46-8Nb (at. %) substrates (BEI) after oxidation at 700 C for 1600 hours and at 800 C for 1100 hours with average compositions (in at.%) determined by EDS and the corresponding XRD patterns from the surface
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ACCEPTED MANUSCRIPT The XRD analysis (Fig. 6) from the surface of the oxidised coatings confirmed that Cr2O3 was the main component of the scale. At 700 C it was the only oxide phase detected together with intermetallic phases from the substrate and titanium nitride. Composition of the surface after oxidation at the higher temperature was more complex (Fig. 6). Oxides (Cr2O3, SiO2 and TiO2) were accompanied by nitrides (TiN and AlTi2N).
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A cross-section of the sample oxidised at 700 C revealed very thin scale (Fig. 7). On
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the surface there were relatively large Cr2O3 crystals protruding from a mixed-oxide layer
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containing Al, Si, Ti, and Cr. Some nitride phases might be present close to the alloy/scale
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interface.
Fig.7 Micrograph of the cross-section (BEI) and chemical composition in at. % (EDS) in the marked points of a Ti-46Al-8Nb (at.%) sample with a Cr-5Si coating after oxidation in air at 700 oC, for 1600 hours
Further information about the scale microsturcture and composition was obtained from STEM and high resolution EDS maps, in Fig. 8. The layer beneath the polyhedral Cr2O3 crystals was fine-grained and contained mainly chromium and oxygen. The subjacent layer 13
ACCEPTED MANUSCRIPT containing Cr, Si and O seemed to be partially oxidised coating. Chromium and silicon were not uniformly distributed in this layer. The zone below the Cr- and Si-rich layer contained alloy components: a continuous thin layer rich in Al was followed by a Ti-rich layer.
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Underneath, close to the alloy/coating interface there were Nb-rich precipitates.
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Fig. 8 STEM micrograph of scale formed on a Ti-46Al-8Nb (at.%) sample with a Cr-5Si coating after 1600 hours of oxidation in air at 700 C (1-h cycles) along with EDS elemental maps (Ti, Al, Nb, O, Cr, Si, O)
For comparison the STEM image of an uncoated Ti-46Al-8Nb (at.%) sample oxidised in the same conditions (700 C , 1600 hours) is shown in Fig. 9. The chemically modified zone had thickness of about 1 µm and was multilayer: Al2O3 on the top, TiO2 below, a mixed oxide/nitride (TiO2, Al2O3, TiN) layer underneath and Nb-rich precipitates close to the alloy/scale interface. The details regarding scale formation on this alloy were reported previously [21-23]. 14
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Fig. 9 STEM micrograph of the scale formed on Ti-46Al-8Nb (at.%) after 1600 hours of oxidation in air at 700 C (1-h cycles) along with EDS elemental maps (Ti, Al, Nb, O)
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The scale formed on the coated alloy after 1100 hours of oxidation at 800 C was thicker (compare Fig. 7 and Fig. 10). The following consecutive layers could be
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distinguished: Cr2O3 on the surface followed by a layer (point 2) containing titanium,
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aluminium, chromium and silicon, uniform in thickness and probably built of oxide or nitride phases. According to the EDS analysis in point 2, the layer was richer than the surface in Ti and Al and still contained some Si. Underneath (point 3, 4, 5) three layers could be recognised
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– rich in titanium, aluminium or niobium, respectively. Opposite to what is shown in Fig. 8,
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titanium-rich layer is under the aluminium-rich one.
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Fig. 10 Cross-section (BEI) and chemical composition in at. % (EDS) in the marked points of a Ti46Al-8Nb (at.%) sample with a Cr-5Si coating after oxidation in air at 800 C, for 1100 hours (1-h cycles)
4. Discussion
As it is known, both components of the Cr-Si coatings form slow-growing protective
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oxides. Depending on the proportion of Cr and Si, one of the oxides grows preferentially.
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Oxidation behaviour of Cr-Si coatings has not been systematically studied. More information can be found on Cr-Si-N coatings deposited on silicon [24] and steels [25, 26] to improve wear resistance of these materials. Oxidation resistance of Cr-N and Cr-Si-N coatings on STS 304 steel was investigated in air in the temperature range 800-1000 C [27]. It has been reported that Cr2O3 on the CrN coatings grew by an outward diffusion of chromium and an inward diffusion of oxygen (confirmed by the marker method). In the case of Cr-Si-N coatings (Cr0.78Si0.22N, and Cr0.67Si0.33N), the scale contained amorphous SiO2 together with crystalline Cr2O3 and it has been concluded that SiO2 reduced the rate of diffusion (Cr, Si, and 16
ACCEPTED MANUSCRIPT N outward and O inward) and increased the activation energy of oxidation. A similar conclusion can be drawn on the basis of the results obtained in this work. SiO2 (quartz) was detected by XRD only in the case of CrSi2 coatings (Figs. 3, 4). On the other hand the increased background level (Fig. 4) for the higher silicon content can be attributed to amorphous SiO2. The presence of SiO2 can be confirmed indirectly. In Fig. 1 it is visible that
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the net mass changes of the Ti-46Al-8Nb (at.%) samples with the Cr-5Si coating were lower
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than with the Cr-0.5Si coating at 700 C and slightly lower at 800 C after 45 hours of
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oxidation. It seems that the higher Si content reduced the oxidation rate. The more effective reduction of the oxide scale growth on coatings with higher amounts of Si is visible in Fig. 2
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especially at 800 C. This is consistent with the results of oxidation of a Cr-Si alloy [28]. It has been reported that an addition of only 3 at% Si significantly improved the oxidation and
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nitridation resistance of the alloy even at a very high temperature of 1473 K [28]. When oxidation takes place in air the formation of nitrides is also possible. Chromium nitride, Cr2N,
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and titanium nitride Ti3N, were detected in the samples oxidised at 700 C (Fig. 3). Studies on
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oxidation-nitridation of chromia formers have shown that nitride phases were located underneath the chromia scale [28] in a form of precipitates or a continuous layer [29]. It has
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been also reported [28,30] that Cr2N grew by an inward diffusion of nitrogen. As can be seen in Fig. 15 all investigated Cr-Si coatings effectively protected the alloy
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from oxidation and penetration of gases at 700 C. The initial interface between the coating and the substrate was well preserved, there was no evidence of phase transformations or interdiffusion. At the temperature of 800 C the scale was more complex. In the case of Cr0.5Si coating, an interdiffusion zone was observed at the coating/substrate interface. Chromium enrichment under the coating was not found in the samples coated with Cr-5Si or CrSi2. Interdiffusion between the coating and the alloy was also observed in samples coated with CrSi2 at 800 C where Ti-Si phases could form. 17
ACCEPTED MANUSCRIPT Among the investigated Cr-Si coatings the most promising were those containing 5 at.% Si. The Cr-0.5Si coatings did not produce enough SiO2 and large crystals of Cr2O3 were formed on the surface. The Si-rich intermetallic, CrSi2, although very stable in the oxidation conditions appeared brittle. Samples with the Cr-5Si coating were oxidised for an extended time. As can be seen in
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Figs. 7, 8, and 10, the scale had good adhesion both at 700 C and 800 C and the maximum
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thickness of the oxide/nitride layer with niobium rich precipitates did not exceed 2-3 µm (Fig. 7) or 4-5 µm (Fig. 10). This indicates very good oxidation properties. As follows from Fig. 6
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after 1600 hours of oxidation at 700 C Cr2O3 was the only crystalline oxide on the surface.
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Some amorphous SiO2 might be present also. It is interesting to note that titanium and aluminium were found in the coating (Figs. 6, 7) as a result of the outward diffusion of these
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elements during the exposure at 700 C-800 C. Similar effect was observed in a Cr-Si coating deposited on a near-alpha titanium alloy, TA15, by a double glow plasma alloying at
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850-950 C for 4 hours [31], where titanium and chromium reacted to form Cr-Ti
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intermetallic. In the case of Cr-5Si coating investigated in this work, titanium from the substrate probably reacted also with nitrogen (TiN in Fig. 6) owing to the inward diffusion of
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this element. At 700 C the very thin (initially less than 1 µm) Cr-5Si coating sufficiently well
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protected the alloy against oxidation (Fig. 7). Oxidation at 800 C for 1100 hours caused more advanced degradation of the coating. Traces of TiO2 were found by XRD (Fig. 6) in the nearsurface region. SiO2 was also detected by XRD which proves that it was at least partly crystalline. The scales in Figs. 8, 10 and 5 had similar morphology. As follows from Fig. 8, layers rich in Al, Ti and Nb were present beneath the Cr/Si oxide scale. According to the EDS analysis in Fig. 10, the continuous layers are probably composed of Ti and Al nitrides. The presence of nitrides is confirmed by XRD in Fig. 6.
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ACCEPTED MANUSCRIPT 5. Summary Protective coatings deposited by magnetron sputtering had many advantages. Compared with the common processes based on diffusion, the deposition temperature was relatively low (< 300 C), owing to which the microstructure and mechanical properties of the alloy were not affected. For coating deposition two-component targets were used, which enabled very
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good control of the coating chemical composition. All coatings were smooth and adherent
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without macro-defects and cracks. These relatively thin coatings effectively protected the
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alloy against oxidation at 700 C and 800 C. The Cr2O3 defected within the cation sublattice was formed accompanied by nanocrystalline/amorphous SiO2. The Cr-5Si coating protected
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the alloy against oxidation at 700 C for at least 1600 hours but after 1100 hours of oxidation in air at 800 C traces of TiO2 were found on the surface.
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Depending on the coating composition micromechanical properties were different. The Cr-0.5 Si coating after 80 hours of oxidation in air at 800 C (Table 2) had the highest
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indentation hardness (22.2 GPa). As regards Young’s modulus (E), very low values (high
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ductility of coating) were found in the case of the Cr-5Si coatings after oxidation at 700 C and 800 C (Table 2). This can be ascribed to very fine-grained structure of the scale.
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Generally it could be concluded that for all investigated samples the value of E decreased
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after oxidation. In order to evaluate the resistance to plastic deformation and crack propagation some authors use the ratios HIT/E and HIT3/E2 as a criterion [32]. The higher the values of HIT/E or HIT3/E2, the better the wear resistance. The HIT/E and HIT3/E2 ratios of the investigated samples are collected in Table 4.
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HIT3/E2 [MPa]
Cr-0.5Si as-received
0.062
71.63
Cr-0.5Si oxidised for 80 h at 700 C
0.031
6.33
Cr-0.5Si oxidised for 80 h at 800 C
0.081
146.80
Cr-5Si as-received
0.061
Cr-5Si oxidised for 80 h at 700 C
0.039
Cr-5Si oxidised for 80 h at 800 C
7.35 31.57
0.060
44.59
0.044
12.69
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CrSi2 oxidised for 80 h at 800 C
70.85
0.062
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CrSi2 oxidised for 80 h at 700 C
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Coating and surface condition
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HIT/E [-]
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Table 4 HIT/E and HIT3/E2 ratio for the investigated coatings on a Ti-46Al-8Nb (at.%) substrate
It can be seen that the H/E values before oxidation are relatively high and almost the
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same for all Cr-Si coatings. After oxidation at 800 C the best wear resistance could be expected in the case of Cr-0.5Si. In general the H/E and H3/E2 values calculated for the Cr-Si coatings in this work were similar to those reported for the Cr-Si coatings deposited on
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titanium alloy TA15 [31]. Adhesion of the coating is another very important parameter, which
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could be estimated from the scratch test (Table 3). The as-received Cr-5Si coatings had the best adhesion. Critical loads for the cohesive (Lc1) and adhesive (Lc2) failure were 1.9 N and > 30N , respectively (Table 3). After oxidation at 700 C and 800 C the highest Lc1 and Lc2 values (considering both loads) were found for the Cr-5Si coating, probably because of very fine-grained oxide scales. It is worthwhile to note that the critical loads for adhesive failure, close to 30-40 N, are typical of TiN layers deposited on cutting tools [33]. In addition, the investigated Cr-Si coatings had low values of friction coefficient, close to 0.2 (Table 3), which indicates that their surface was smooth and could have satisfactory anti-wear and anti20
ACCEPTED MANUSCRIPT friction properties. The friction coefficients of these coatings were about two times lower compared with Cr-N and Cr-Si-N anti-wear coatings deposited on Si-wafers and stainless steel [24].
4. Conclusions
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1. Magnetron sputtering technique appeared useful for coating deposition on the Ti-
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46Al-8Nb (at.%) alloy.
2. All the investigated Cr-Si coatings provided good protection of the Ti-46Al-8Nb
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(at.%) alloy against oxygen and nitrogen absorption at 700 C and 800 C in short-
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term oxidation tests (80 hours) in thermal cycling conditions. 3. The Cr-5Si coating had the best combination of oxidation resistance and
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micromechanical properties and provided very good protection in a long-term oxidation test at 700 C (up to 1600 hours). The protective properties and the lifetime
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of the coating at 800 C are limited because of some interdiffusion around the
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interface with the substrate.
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Acknowledgement
The authors gratefully acknowledge financial support from AGH-UST statutory funds
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(badania statutowe: 11.11.160.438) and as well as valuable contributions of Dr. S. Zimowski, Dr. M. Kot and Dr. R. Mania to the measurements of micromechanical properties and coating deposition.
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Tie-Gang Wang, Yu Dong, Belachew Abera Gebrekidan, Yan-Mei Liu, Qi-Xiang Fan, Kwang Ho Kim, Microstructure and Properties of the Cr–Si–N Coatings Deposited by Combining High-Power Impulse Magnetron Sputtering (HiPIMS) and Pulsed DC Magnetron Sputtering, Acta Metall. Sin. (Engl. Lett.) 30/7 (2017) 688–696.
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[32]
Wangyang Ni, Yang-Tse Cheng, Michael J. Lukitsch, Anita M. Weiner, Lenoid C. Lev, David S. Grummon, Effects of the ratio of hardness to Young’s modulus on the friction and wear behaviour of bilayer coatings, Appl. Phys. Lett. 85 (2004) 40284030.
[33]
J. Valli, U. Mäkelä, A. Matthews, V. Murawa, TiN coating adhesion studies using the scratch test method, J. Vac. Sci. Technol. A3 (1985) 2411-2414.
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[26]
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pAr [Pa]
I [A]
t [s]
T [oC] P[W]
Cr-0,5Si
0.4
0.4
1800
270
360
Cr-5Si
0.4
0.5
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300
460
CrSi2
1.5
1800
300
240
1800
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0.2
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Target composition [at.%]
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Table 1 Parameters of coating deposition (pAr-argon pressure; I–cathode current intensity; t– deposition time; T–temperature, P–cathode emission power)
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Hmax [nm]
HV
HIT [MPa]
Cr-0.5Si as-received
165 8
1744 187
18446 1982
296 30
Cr-0.5Si oxidised for 80 h at 700 C
251 5 3
632 227
6681 2406
217 88
Cr-0.5Si oxidised for 80 h at 800 C
147 11
2098 185
22200 1952
273 53
Cr-5Si as-received
159 6
1811 113
19157 1190
315 45
Cr-5Si oxidised for 80 h at 700 C
292 28
452 93
4782 980
122 9
Cr-5Si oxidised for 80 h at 800 C
238 21
767 105
8111 1112
130 49
CrSi2 oxidised for 80 h at 700 C
186 9
1184 140
12528 1486
210 28
CrSi2 oxidised for 80 h at 800 C
248 11
628 34
6644 357
152 19
E [GPa]
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Coating and surface condition
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Table 2 Maximum penetration depth, Vickers hardness, indentation hardness and Young’s modulus for Ti-46Al-8Nb [at.%] samples with Cr-0.5Si, Cr-5Si, and CrSi2 coatings
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Table 3 Scratch test results for Ti-46Al-8Nb (at.%) samples coated with Cr-0.5Si, Cr-5Si, or CrSi2 under maximum load Pmax = 30 N
Lc2 [N]
Cr-0.5Si as-received
2.0 ± 0.2 (1)
19.3 ± 3.1 (2)
0.18 ± 0.02
Cr-0.5Si oxidised for 80 h at 700 C
10.4 ± 0.8 (3)
>30 (4)
0.23
Cr-0.5Si oxidised for 80 h at 800 C
8.4 ± 0.7 (5)
20.4 ± 0.8 (6)
0.24 ± 0.2
Cr-5Si as-received
1.9 ± 0.1 (7)
>30 (8)
0.20 ± 0.01
Cr-5Si oxidised for 80 h at 700 C
18.9 ± 2.1 (5)
27.0 ± 0.8 (9)
0.24 ± 0.02
Cr-5Si oxidised for 80 h at 800 C
21.8 ± 2.2 (5)
26.9 ± 1.5 (10)
0.24 ± 0.00
9.1 ± 1.5 (5)
16.6 ± 3.1 (2)
0.21 ± 0.01
16.9 ± 3.0 (5)
20.1 ± 3.4 (11)
0.20 ± 0.01
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CrSi2 oxidised for 80 h at 800 C
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CrSi2 oxidised for 80 h at 700 C
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Lc1 [N]
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Coating and surface condition
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(1) chevron-like cracks inside the scratch track, at the load of 15N cracks propagating outward (2) cracks and delamination on both sides of the scratch track (3) small crack inside the scratch track (4) large crack at the load of 25 N, no delamination (5) cohesive cracks inside the scratch track (6) cracks and occasional delamination on the scratch track sides, at 25 N continuous delamination on the scratch track sides (7) small cracks inside the scratch track, curved in the direction opposite to the motion of the indenter and propagating outward (8) the number of cracks increasing with the load, no delamination; (9) cracks and occasional delamination on the scratch track sides (10) small abrasion inside the scratch track (11) cracks and occasional delamination on scratch track sides, at the load of 26 N continuous delamination on the scratch track sides
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Table 4 HIT/E and HIT3/E2 ratio for the investigated coatings on a Ti-46Al-8Nb (at.%) substrate HIT3/E2 [MPa]
Cr-0.5Si as-received
0.062
71.63
Cr-0.5Si oxidised for 80 h at 700 C
0.031
Cr-0.5Si oxidised for 80 h at 800 C
0.081
146.80
0.061 0.039
7.35
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CrSi2 oxidised for 80 h at 700 C
0.062
31.57
0.060
44.59
0.044
12.69
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CrSi2 oxidised for 80 h at 800 C
70.85
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Cr-5Si oxidised for 80 h at 700 C
6.33
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Cr-5Si as-received
Cr-5Si oxidised for 80 h at 800 C
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HIT/E [-]
Coating and surface condition
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Figure captions: Fig. 1 Net mass changes of Cr-Si coated and uncoated Ti-46Al-8Nb (at.%) samples during oxidation in air at 700 C or 800 C (1-h cycles).
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Fig. 2 Surfaces of Ti-46Al-8Nb (at.%) samples with Cr-Si coatings after 80 hours of oxidation in air at 700 C and 800 C (1-h cycles) - SEI/BEI
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Fig. 3 XRD patterns from the surface of Ti-46Al-8Nb (at.%) samples with Cr-Si coatings after oxidation at 700 C for 80 h (1-h cycles)
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Fig. 4 XRD patterns from the surface of Ti-46Al-8Nb (at.%) samples with Cr-Si coatings after oxidation at 800 C for 80 h (1-h cycles)
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Fig. 5 Cross-sections of Ti-46Al-8Nb (at.%) with Cr-Si coatings after 80 hours of oxidation in air at 700 C and 800 C – BEI
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Fig. 6 Surface of the Cr-5Si coating on Ti-46-8Nb (at. %) substrates (BEI) after oxidation at 700 C for 1600 hours and at 800 C for 1100 hours with average compositions (in at.%) determined by EDS and the corresponding XRD patterns from the surface
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Fig.7 Micrograph of the cross-section (BEI) and chemical composition in at. % (EDS) in the marked points of a Ti-46Al-8Nb (at.%) sample with a Cr-5Si coating after oxidation in air at 700 oC, for 1600 hours
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Fig. 8 STEM micrograph of scale formed on a Ti-46Al-8Nb (at.%) sample with a Cr-5Si coating after 1600 hours of oxidation in air at 700 C (1-h cycles) along with EDS elemental maps (Ti, Al, Nb, O, Cr, Si, O Fig. 9 STEM micrograph of the scale formed on Ti-46Al-8Nb (at.%) after 1600 hours of oxidation in air at 700 C (1-h cycles) along with EDS elemental maps (Ti, Al, Nb, O) Fig. 10 Cross-section (BEI) and chemical composition in at. % (EDS) in the marked points of a Ti46Al-8Nb (at.%) sample with a Cr-5Si coating after oxidation in air at 800 C, for 1100 hours (1-h cycles) 29
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0.4
Ti-46Al-8Nb (at.%), net mass changes
0.3
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0.2
700oC
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0.1 0.0 -0.1 Cr-0.5Si
-0.2 0
20
40
Cr-5Si
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uncoated
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2
m/A [mg/cm ]
800oC
60
CrSi2 80
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Time [h]
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Fig. 1
Fig. 2
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ACCEPTED MANUSCRIPT 700oC, 80h, 1-h cycles Cr2O3 Cr Al2O3
Cr-0.5Si
Intensity [a.u.]
Cr-5Si
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Cr2N
Ti3N SiO2 CrSi2
CrSi2
20
30
40
50
60
70
80
90
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2
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10
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Fig. 3
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800oC, 80h, 1-h cycles
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Cr2O3
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Cr-5Si
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Intensity [a.u.]
Cr-0.5Si
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20
30
TiAl SiO2
CrSi2
40
50
60
70
80
Ti3N
90
2
Fig. 4
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Fig. 5
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Fig. 6
33
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Fig.7
Fig. 8
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Fig. 10
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Fig. 9
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Graphical abstract
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- Cr-Si coatings with good adherence and micromechanical properties deposited by magnetron sputtering on Ti-48Al-8Nb substrates; - satisfactory protection of the alloy against oxidation and gas absorption in a short-term test; - excellent performance of a Cr-5Si coating confirmed in a long-term oxidation test at 700 C
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Marzena Mitoraj-Królikowska, Elzbieta Godlewska PII: DOI: Reference:
S0257-8972(18)30768-0 doi:10.1016/j.surfcoat.2018.07.073 SCT 23639
To appear in:
Surface & Coatings Technology
Received date: Revised date: Accepted date:
13 May 2018 18 July 2018 20 July 2018
Please cite this article as: Marzena Mitoraj-Królikowska, Elzbieta Godlewska , Oxidation resistance and micromechanical properties of a Ti–46Al–8Nb (at.%) alloy with Cr–Si magnetron-sputtered coatings. Sct (2018), doi:10.1016/j.surfcoat.2018.07.073
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ACCEPTED MANUSCRIPT Oxidation resistance and micromechanical properties of a Ti-46Al-8Nb (at.%) alloy with Cr-Si magnetron-sputtered coatings
Marzena Mitoraj-Królikowska, AGH University of Science and Technology, Faculty of Materials Science and Ceramics, Al. Mickiewicza 30, 30-059, Krakow, Poland, phone
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number: +48 126172813, e-mail: [email protected]
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Elzbieta Godlewska, AGH University of Science and Technology, Faculty of Materials
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Science and Ceramics, Al. Mickiewicza 30, 30-059, Krakow, Poland, phone number: +48
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126172536, e-mail: [email protected]
Abstract:
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Samples of a Ti-46Al-8Nb (at.%) alloy with Cr-0.5Si, Cr-5Si or Cr-66Si (CrSi2) coatings deposited by magnetron sputtering were oxidised in air under thermal cycling
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conditions at 700 C and 800 C for 80 hours. Mass changes over time were measured
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periodically to evaluate reaction progress as well as susceptibility of the coating or scale to spallation. Vickers hardness, elastic modulus and adhesion of the coatings were determined
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before and after oxidation. Selected samples were oxidised for up to 1500-2000 hours. The nature and extent of coating degradation in each case was evaluated through systematic
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analyses of surfaces and cross-sections using different techniques, including scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDS) and X-ray diffraction (XRD). It has been found that coatings with the composition Cr-5Si (at.%) had the most promising combination of oxidation resistance and micromechanical properties.
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ACCEPTED MANUSCRIPT Keywords: titanium aluminides, magnetron sputtering, Cr-Si coatings, oxidation, mechanical testing
Abbreviations: µ - friction coefficient [-]
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E - Young’s Modulus [GPa]
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HIT - indentation hardness [MPa] Hmax - maximum penetration depth under a load of 10mN [nm]
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HV –Vickers hardness [HV]
Lc1 – critical load for cohesive failure [N]
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Lc2 – critical load for adhesive failure [N]
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I – cathode current intensity [A]
P – cathode emission power [W]
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pAr - argon pressure [Pa]
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Pmax – maximum load applied in scratch test [N] t – coating deposition time [s]
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1. Introduction
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T – temperature [C]
Titanium aluminides have the potential to be used as high-temperature structural materials due to their low density (3.7 - 3.9 g/cm3), high melting point, good mechanical properties at elevated temperatures as well as good creep and oxidation resistance [1-3]. Because of these unique properties Ti-Al alloys find application as components of motor vehicles [4], industrial turbines [5] and aero-engines [6]. In particular, they can replace heavier Ni or Fe based superalloys operating at moderate temperatures and loads, thus
2
ACCEPTED MANUSCRIPT contributing to more than 50% total mass reduction. This is especially important for the rotating parts of aero-engines. However, despite many advantages titanium aluminides have some drawbacks, e.g. insufficient low-temperature ductility [7], insufficient oxidation resistance at temperatures higher than 750-800 C [8] and reduction in tensile elongation after exposure to oxidizing environments at elevated temperatures (“embrittlement”) [9,10]. Even a
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few hours exposure to air at 700 C can cause significant deterioration of mechanical
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properties of titanium aluminide alloys. The embrittlement is induced by dissolution of small
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atoms, such as oxygen, nitrogen or hydrogen. The attempts to improve oxidation resistance and prevent dissolution of gases [11-13] included alloying and surface treatment. Numerous
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coating systems and deposition techniques (PVD, CVD, ion implantation, etc.) were investigated [14-16] to develop a possibly thin, compact and adherent coating which would
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protect the alloy against oxidation and penetration of gases. Among the deposition techniques, magnetron sputtering seems particularly attractive because of good reproducibility and high
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purity of coatings and eco-friendly process conditions [17].
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The aim of this study was to develop a protective coating for the Ti-46Al-8Nb (at.%) alloy and evaluate its adhesion, ductility and oxidation resistance. It has been assumed that the
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coating should prevent dissolution of gases in the alloy during the short-term exposure at 700 C and should enhance its oxidation resistance at 800 C. The chemical composition of
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coatings (Cr, Si) was selected on the basis of the results described in the literature [18-20] and the well-known properties of chromium and silicon which form slow-growing protective oxides.
2. Experimental procedure The investigated alloy, Ti-46Al-8Nb (at.%), was produced by horizontal centrifugal casting (ACCESS e.V. Aachen). It had fully lamellar microstructure consisting of -TiAl 3
ACCEPTED MANUSCRIPT (75mol.%) and 2-Ti3Al (25mol.%). Ingots, 16.0 mm in diameter were cut into pellets, 1.0 – 1.5 mm thick, by means of a diamond saw. The specimens were ground with emery papers up to 2000 grit and polished with diamond suspensions (9, 3, 1 m grain size). Before coating deposition they were ultrasonically cleaned in water with detergents, next in acetone and finally in double-distilled water. The deposition process was conducted in a vacuum chamber
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equipped with a WMK-50 magnetron gun and medium frequency pulsed power supply
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(DORA Power System, PL). Argon flow was controlled by MKS flowmeter. Temperature of
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the samples during heating, surface treatment in the glow-discharge plasma and deposition was measured by thermocouple. Deposition was performed under different argon pressures
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and different cathode current intensities. Coating thickness (about 1-2 µm) was controlled by
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adjusting the deposition time. All deposition parameters are collected in Table 1.
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Table 1 Parameters of coating deposition (pAr-argon pressure; I–cathode current intensity; t– deposition time; T–temperature, P–cathode emission power)
pAr [Pa]
I [A]
t [s]
T [oC] P [W]
Cr-0,5Si
0.4
0.4
1800
270
360
0.4
0.5
1800
300
460
1.5
0.2
1800
300
240
Cr-5Si
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CrSi2
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Target composition [at.%]
Targets, 50 mm in diameter, with compositions: Cr-0.5Si (at.%) or Cr-5Si (at.%) were made from elemental powders by hot pressing at 25 MPa and 1600 C. The Cr-66Si (CrSi2) targets were purchased from Kurt J. Lesker Company. Samples with magnetron sputtered coatings were oxidised for 80 hours in air under thermal cycling conditions at 700 or 800 C. One-hour cycles (1-h) were used. For selected samples the exposure time was extended to 1500 - 2000 hours. Each thermal cycle consisted 4
ACCEPTED MANUSCRIPT of rapid heating (50 C/min) to the desired temperature, maintaining at the constant temperature for 1 hour and rapid cooling (50 C/min) to the room temperature. During the test, the specimens were kept in alumina crucibles which enabled collection of oxidation products that might be lost by spallation. Once a day the samples were weighed with an accuracy of 10-4g. Net mass changes (sample alone without the spalled matter ) and gross
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mass changes (sample with the crucible and the spalled matter inside ).
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Micromechanical tests were performed in conformity with ISO 14577-1, ISO 14577-2,
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PN-EN ISO 6507-1:2005 before and after the oxidation test using the CSEM Micro-CombiTester (MCT). The Vickers hardness measurement was done in six different spots on each
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sample, maximum load being 10 mN. The loading rate was 20mN/min. For each loading/unloading cycle the corresponding maximum penetration depth of the indenter (Hmax),
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Vickers hardness (HV), indentation hardness (HIT) and Elastic (Young’s) Modulus (E) were determined using the established models. All samples were submitted to scratch tests. The
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Rockwell C diamond intender was used with a linearly increasing load (10 mN/min) in the
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range from 0.05 to 30 N (Pmax, maximum load) until delamination or chipping of the coating occurred. Coating failure was detected by three methods – measurement of friction and
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acoustic emission and by microscopic inspection of the scratch track. Critical normal force at which the first cohesive or adhesive failure of the coating occurred was referred to as critical
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load Lc1 or Lc2, respectively.
3. Results 3.1 Cr-Si coatings Net mass changes of Ti-46Al-8Nb (at.%) samples with Cr-Si coatings during oxidation at 700 C and 800 C for up to 80 h are shown in Fig. 1.
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Ti-46Al-8Nb (at.%), net mass changes
0.3 0.2 0.1 700oC 0.0
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2
m/A [mg/cm ]
800oC
uncoated
Cr-0.5Si
Cr-5Si
CrSi2
60
80
20
40 Time [h]
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-0.2 0
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Fig. 1 Net mass changes of Cr-Si coated and uncoated Ti-46Al-8Nb (at.%) samples during oxidation in air at 700 C or 800 C (1-h cycles). As can be seen in Fig. 1, at 700 C the net mass changes of the Cr-0.5Si and Cr-5Si
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coated samples were slightly higher while at 800 C similar to those of uncoated Ti-46Al-8Nb
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(at.%) alloy. The CrSi2 – coated samples had the lowest net mass changes. The negligible mass losses measured at 700 C were probably caused by microdamages upon handling and
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weighing. No solid debris were found inside the crucible.
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Micrographs in Fig. 2 show polyhedral crystals of Cr2O3 on the surface of samples after oxidation, larger in size at the higher oxidation temperature. The contribution of chromium oxide decreased with the concentration of chromium in the coating. In the case of CrSi2 coating, the Cr2O3 crystals protruded from the fine-crystalline and partly amorphous SiO2 layer. According to the XRD analysis in Fig. 3, at 700 C nitridation of titanium and chromium could take place along with oxidation. Oxidation products of the Cr-0.5Si coating contained traces of Al2O3. At 800 C, Cr2O3 was the predominant oxide phase on all samples
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after oxidation.
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Fig. 2 Surfaces of Ti-46Al-8Nb (at.%) samples with Cr-Si coatings after 80 hours of oxidation in air at 700 C and 800 C (1-h cycles) - SEI/BEI
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700oC, 80h, 1-h cycles Cr2O3 Cr Al2O3
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Cr-5Si
Cr2N
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Intensity [a.u.]
Cr-0.5Si
Ti3N SiO2 CrSi2
CrSi2
10
20
30
40
50
60
70
80
90
2 Fig. 3 XRD patterns from the surface of Ti-46Al-8Nb (at.%) samples with Cr-Si coatings after oxidation at 700 C for 80 h (1-h cycles) 7
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800oC, 80h, 1-h cycles Cr2O3
30
40
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50
60
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80
Ti3N
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TiAl SiO2
CrSi2
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Cr-5Si
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Intensity [a.u.]
Cr-0.5Si
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Fig. 4 XRD patterns from the surface of Ti-46Al-8Nb (at.%) samples with Cr-Si coatings after oxidation at 800 C for 80 h (1-h cycles)
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Additional information about coating performance was obtained from the cross-sections
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(Fig. 5). All the investigated Cr-Si coatings had good protective properties at 700 C. There were no signs of interdiffusion with the substrate or phase transformations in the alloy. At
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800 C the best protection was provided by CrSi2. The coating was only partially oxidised.
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However, at 800 C some interaction between the CrSi2 coating and the substrate was already visible. Silicon from the coating reacted with titanium to form Ti-Si phases. In the case of chromium-rich coating, Cr-0.5Si, some inward diffusion of chromium took place. As can be seen in Fig. 5, thickness of the chromium oxide layer (Cr2O3) on the surface increased with temperature.
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Fig. 5 Cross-sections of Ti-46Al-8Nb (at.%) with Cr-Si coatings after 80 hours of oxidation in air at 700 C and 800 C - BEI
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Hardness of the Cr-0.5Si coating decreased after oxidation at 700 C (about 3 times) and increased after oxidation at 800 C (about 1.2 times) – see Table 2. In the case of Cr-5Si, HV and HIT decreased about 4 times after oxidation at 700 C and about 2 times at 800 C. HV and HIT were about 2 times lower for CrSi2 oxidised at 800 C than at 700 C. Young’s modulus only slightly decreased for Cr-0.5Si after oxidation whereas for Cr-5Si it decreased at least two times . The value of Young’s modulus for CrSi2 oxidised at 800 C was lower than at 700 C. 9
ACCEPTED MANUSCRIPT Adhesion of the Cr-Si coatings was evaluated on the basis of scratch test (Table 3). Critical loads (Lc2) for delamination were high and ranged from 20 to 30N. The lowest Lc2 value was recorded for the CrSi2 coating oxidised at 700 C. The as-received Cr-0.5S and Cr5Si layers were rather brittle, which is reflected in relatively low Lc1 values. The chevron-like cracks observed in Cr-0.5Si before oxidation indicated very low coating ductility. However,
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ductility of the investigated Cr-Si coatings generally increased after oxidation, the most
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pronounced effect was seen in the case of Cr-5Si.
Hmax [nm]
HV
HIT [MPa]
E [GPa]
Cr-0.5Si as-received
165 8
1744 187
18446 1982
296 30
Cr-0.5Si oxidised for 80 h at 700 C
251 5 3
632 227
6681 2406
217 88
147 11
2098 185
22200 1952
273 53
159 6
1811 113
19157 1190
315 45
292 28
452 93
4782 980
122 9
Cr-5Si oxidised for 80 h at 800 C
238 21
767 105
8111 1112
130 49
CrSi2 oxidised for 80 h at 700 C
186 9
1184 140
12528 1486
210 28
CrSi2 oxidised for 80 h at 800 C
248 11
628 34
6644 357
152 19
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Table 2 Maximum penetration depth, Vickers hardness, indentation hardness and Young’s modulus for Ti-46Al-8Nb [at.%] samples with Cr-0.5Si, Cr-5Si, and CrSi2 coatings
Cr-5Si as-received
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Cr-0.5Si oxidised for 80 h at 800 C
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Cr-5Si oxidised for 80 h at 700 C
Table 3 Scratch test results for Ti-46Al-8Nb (at.%) samples coated with Cr-0.5Si, Cr-5Si, or CrSi2 under maximum load Pmax = 30 N
Coating and surface condition
Lc1 [N]
Lc2 [N]
10
2.0 ± 0.2 (1)
19.3 ± 3.1 (2)
0.18 ± 0.02
Cr-0.5Si oxidised for 80 h at 700 C
10.4 ± 0.8 (3)
>30 (4)
0.23
Cr-0.5Si oxidised for 80 h at 800 C
8.4 ± 0.7 (5)
20.4 ± 0.8 (6)
0.24 ± 0.2
Cr-5Si as-received
1.9 ± 0.1 (7)
>30 (8)
0.20 ± 0.01
Cr-5Si oxidised for 80 h at 700 C
18.9 ± 2.1 (5)
27.0 ± 0.8 (9)
0.24 ± 0.02
Cr-5Si oxidised for 80 h at 800 C
21.8 ± 2.2 (5)
26.9 ± 1.5 (10)
0.24 ± 0.00
CrSi2 oxidised for 80 h at 700 C
9.1 ± 1.5 (5)
16.6 ± 3.1 (2)
0.21 ± 0.01
CrSi2 oxidised for 80 h at 800 C
16.9 ± 3.0 (5)
20.1 ± 3.4 (11)
0.20 ± 0.01
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Cr-0.5Si as-received
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(1) chevron-like cracks inside the scratch track, at the load of 15N cracks propagating outward (2) cracks and delamination on both sides of the scratch track (3) small crack inside the scratch track (4) large crack at the load of 25 N, no delamination (5) cohesive cracks inside the scratch track (6) cracks and occasional delamination on the scratch track sides, at 25 N continuous delamination on the scratch track sides (7) small cracks inside the scratch track, curved in the direction opposite to the motion of the indenter and propagating outward (8) the number of cracks increasing with the load, no delamination; (9) cracks and occasional delamination on the scratch track sides (10) small abrasion inside the scratch track (11) cracks and occasional delamination on scratch track sides, at the load of 26 N continuous delamination on the scratch track sides
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Basing on the oxidation behaviour and the mechanical properties of the investigated
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coatings, we selected one composition, i.e. Cr-5Si, for long-term oxidation tests.
3.2 Long-term oxidation of Ti-46Al-8Nb (at.%) samples coated with Cr-5Si The oxidation runs at 700 and 800 C were continued for more than 1000 hours. Over the entire oxidation time the events of scale spallation or cracking were not observed. Surfaces of the samples after exposure are shown in Fig. 6. It can be seen that the higher temperature promoted crystal growth. The large polyhedral crystals formed at 800 C were composed of Cr2O3. These were accompanied by some oxides or nitrides of alloy or coating components. 11
ACCEPTED MANUSCRIPT According to EDS, after oxidation at 800 C the surface was slightly richer in titanium and aluminium than at 700 C (Fig.6). Niobium was not detected on the surface after oxidation at 800 C, which suggests that the Ti and Al counts were not collected from the substrate but from the scale (Ti, Al oxides or nitrides). In the case of samples oxidised at 700 oC, the concentrations of Al, Ti and Nb determined by EDS (Fig. 6) might originate from both, the
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scale and the substrate because the scale was rather thin (2.0-3.0 µm, Fig. 7).
Fig. 6 Surface of the Cr-5Si coating on Ti-46-8Nb (at. %) substrates (BEI) after oxidation at 700 C for 1600 hours and at 800 C for 1100 hours with average compositions (in at.%) determined by EDS and the corresponding XRD patterns from the surface
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ACCEPTED MANUSCRIPT The XRD analysis (Fig. 6) from the surface of the oxidised coatings confirmed that Cr2O3 was the main component of the scale. At 700 C it was the only oxide phase detected together with intermetallic phases from the substrate and titanium nitride. Composition of the surface after oxidation at the higher temperature was more complex (Fig. 6). Oxides (Cr2O3, SiO2 and TiO2) were accompanied by nitrides (TiN and AlTi2N).
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A cross-section of the sample oxidised at 700 C revealed very thin scale (Fig. 7). On
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the surface there were relatively large Cr2O3 crystals protruding from a mixed-oxide layer
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containing Al, Si, Ti, and Cr. Some nitride phases might be present close to the alloy/scale
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interface.
Fig.7 Micrograph of the cross-section (BEI) and chemical composition in at. % (EDS) in the marked points of a Ti-46Al-8Nb (at.%) sample with a Cr-5Si coating after oxidation in air at 700 oC, for 1600 hours
Further information about the scale microsturcture and composition was obtained from STEM and high resolution EDS maps, in Fig. 8. The layer beneath the polyhedral Cr2O3 crystals was fine-grained and contained mainly chromium and oxygen. The subjacent layer 13
ACCEPTED MANUSCRIPT containing Cr, Si and O seemed to be partially oxidised coating. Chromium and silicon were not uniformly distributed in this layer. The zone below the Cr- and Si-rich layer contained alloy components: a continuous thin layer rich in Al was followed by a Ti-rich layer.
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Underneath, close to the alloy/coating interface there were Nb-rich precipitates.
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Fig. 8 STEM micrograph of scale formed on a Ti-46Al-8Nb (at.%) sample with a Cr-5Si coating after 1600 hours of oxidation in air at 700 C (1-h cycles) along with EDS elemental maps (Ti, Al, Nb, O, Cr, Si, O)
For comparison the STEM image of an uncoated Ti-46Al-8Nb (at.%) sample oxidised in the same conditions (700 C , 1600 hours) is shown in Fig. 9. The chemically modified zone had thickness of about 1 µm and was multilayer: Al2O3 on the top, TiO2 below, a mixed oxide/nitride (TiO2, Al2O3, TiN) layer underneath and Nb-rich precipitates close to the alloy/scale interface. The details regarding scale formation on this alloy were reported previously [21-23]. 14
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Fig. 9 STEM micrograph of the scale formed on Ti-46Al-8Nb (at.%) after 1600 hours of oxidation in air at 700 C (1-h cycles) along with EDS elemental maps (Ti, Al, Nb, O)
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The scale formed on the coated alloy after 1100 hours of oxidation at 800 C was thicker (compare Fig. 7 and Fig. 10). The following consecutive layers could be
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distinguished: Cr2O3 on the surface followed by a layer (point 2) containing titanium,
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aluminium, chromium and silicon, uniform in thickness and probably built of oxide or nitride phases. According to the EDS analysis in point 2, the layer was richer than the surface in Ti and Al and still contained some Si. Underneath (point 3, 4, 5) three layers could be recognised
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– rich in titanium, aluminium or niobium, respectively. Opposite to what is shown in Fig. 8,
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titanium-rich layer is under the aluminium-rich one.
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Fig. 10 Cross-section (BEI) and chemical composition in at. % (EDS) in the marked points of a Ti46Al-8Nb (at.%) sample with a Cr-5Si coating after oxidation in air at 800 C, for 1100 hours (1-h cycles)
4. Discussion
As it is known, both components of the Cr-Si coatings form slow-growing protective
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oxides. Depending on the proportion of Cr and Si, one of the oxides grows preferentially.
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Oxidation behaviour of Cr-Si coatings has not been systematically studied. More information can be found on Cr-Si-N coatings deposited on silicon [24] and steels [25, 26] to improve wear resistance of these materials. Oxidation resistance of Cr-N and Cr-Si-N coatings on STS 304 steel was investigated in air in the temperature range 800-1000 C [27]. It has been reported that Cr2O3 on the CrN coatings grew by an outward diffusion of chromium and an inward diffusion of oxygen (confirmed by the marker method). In the case of Cr-Si-N coatings (Cr0.78Si0.22N, and Cr0.67Si0.33N), the scale contained amorphous SiO2 together with crystalline Cr2O3 and it has been concluded that SiO2 reduced the rate of diffusion (Cr, Si, and 16
ACCEPTED MANUSCRIPT N outward and O inward) and increased the activation energy of oxidation. A similar conclusion can be drawn on the basis of the results obtained in this work. SiO2 (quartz) was detected by XRD only in the case of CrSi2 coatings (Figs. 3, 4). On the other hand the increased background level (Fig. 4) for the higher silicon content can be attributed to amorphous SiO2. The presence of SiO2 can be confirmed indirectly. In Fig. 1 it is visible that
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the net mass changes of the Ti-46Al-8Nb (at.%) samples with the Cr-5Si coating were lower
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than with the Cr-0.5Si coating at 700 C and slightly lower at 800 C after 45 hours of
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oxidation. It seems that the higher Si content reduced the oxidation rate. The more effective reduction of the oxide scale growth on coatings with higher amounts of Si is visible in Fig. 2
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especially at 800 C. This is consistent with the results of oxidation of a Cr-Si alloy [28]. It has been reported that an addition of only 3 at% Si significantly improved the oxidation and
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nitridation resistance of the alloy even at a very high temperature of 1473 K [28]. When oxidation takes place in air the formation of nitrides is also possible. Chromium nitride, Cr2N,
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and titanium nitride Ti3N, were detected in the samples oxidised at 700 C (Fig. 3). Studies on
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oxidation-nitridation of chromia formers have shown that nitride phases were located underneath the chromia scale [28] in a form of precipitates or a continuous layer [29]. It has
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been also reported [28,30] that Cr2N grew by an inward diffusion of nitrogen. As can be seen in Fig. 15 all investigated Cr-Si coatings effectively protected the alloy
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from oxidation and penetration of gases at 700 C. The initial interface between the coating and the substrate was well preserved, there was no evidence of phase transformations or interdiffusion. At the temperature of 800 C the scale was more complex. In the case of Cr0.5Si coating, an interdiffusion zone was observed at the coating/substrate interface. Chromium enrichment under the coating was not found in the samples coated with Cr-5Si or CrSi2. Interdiffusion between the coating and the alloy was also observed in samples coated with CrSi2 at 800 C where Ti-Si phases could form. 17
ACCEPTED MANUSCRIPT Among the investigated Cr-Si coatings the most promising were those containing 5 at.% Si. The Cr-0.5Si coatings did not produce enough SiO2 and large crystals of Cr2O3 were formed on the surface. The Si-rich intermetallic, CrSi2, although very stable in the oxidation conditions appeared brittle. Samples with the Cr-5Si coating were oxidised for an extended time. As can be seen in
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Figs. 7, 8, and 10, the scale had good adhesion both at 700 C and 800 C and the maximum
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thickness of the oxide/nitride layer with niobium rich precipitates did not exceed 2-3 µm (Fig. 7) or 4-5 µm (Fig. 10). This indicates very good oxidation properties. As follows from Fig. 6
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Some amorphous SiO2 might be present also. It is interesting to note that titanium and aluminium were found in the coating (Figs. 6, 7) as a result of the outward diffusion of these
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elements during the exposure at 700 C-800 C. Similar effect was observed in a Cr-Si coating deposited on a near-alpha titanium alloy, TA15, by a double glow plasma alloying at
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850-950 C for 4 hours [31], where titanium and chromium reacted to form Cr-Ti
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intermetallic. In the case of Cr-5Si coating investigated in this work, titanium from the substrate probably reacted also with nitrogen (TiN in Fig. 6) owing to the inward diffusion of
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this element. At 700 C the very thin (initially less than 1 µm) Cr-5Si coating sufficiently well
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protected the alloy against oxidation (Fig. 7). Oxidation at 800 C for 1100 hours caused more advanced degradation of the coating. Traces of TiO2 were found by XRD (Fig. 6) in the nearsurface region. SiO2 was also detected by XRD which proves that it was at least partly crystalline. The scales in Figs. 8, 10 and 5 had similar morphology. As follows from Fig. 8, layers rich in Al, Ti and Nb were present beneath the Cr/Si oxide scale. According to the EDS analysis in Fig. 10, the continuous layers are probably composed of Ti and Al nitrides. The presence of nitrides is confirmed by XRD in Fig. 6.
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ACCEPTED MANUSCRIPT 5. Summary Protective coatings deposited by magnetron sputtering had many advantages. Compared with the common processes based on diffusion, the deposition temperature was relatively low (< 300 C), owing to which the microstructure and mechanical properties of the alloy were not affected. For coating deposition two-component targets were used, which enabled very
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good control of the coating chemical composition. All coatings were smooth and adherent
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without macro-defects and cracks. These relatively thin coatings effectively protected the
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alloy against oxidation at 700 C and 800 C. The Cr2O3 defected within the cation sublattice was formed accompanied by nanocrystalline/amorphous SiO2. The Cr-5Si coating protected
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the alloy against oxidation at 700 C for at least 1600 hours but after 1100 hours of oxidation in air at 800 C traces of TiO2 were found on the surface.
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Depending on the coating composition micromechanical properties were different. The Cr-0.5 Si coating after 80 hours of oxidation in air at 800 C (Table 2) had the highest
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indentation hardness (22.2 GPa). As regards Young’s modulus (E), very low values (high
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ductility of coating) were found in the case of the Cr-5Si coatings after oxidation at 700 C and 800 C (Table 2). This can be ascribed to very fine-grained structure of the scale.
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Generally it could be concluded that for all investigated samples the value of E decreased
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after oxidation. In order to evaluate the resistance to plastic deformation and crack propagation some authors use the ratios HIT/E and HIT3/E2 as a criterion [32]. The higher the values of HIT/E or HIT3/E2, the better the wear resistance. The HIT/E and HIT3/E2 ratios of the investigated samples are collected in Table 4.
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HIT3/E2 [MPa]
Cr-0.5Si as-received
0.062
71.63
Cr-0.5Si oxidised for 80 h at 700 C
0.031
6.33
Cr-0.5Si oxidised for 80 h at 800 C
0.081
146.80
Cr-5Si as-received
0.061
Cr-5Si oxidised for 80 h at 700 C
0.039
Cr-5Si oxidised for 80 h at 800 C
7.35 31.57
0.060
44.59
0.044
12.69
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CrSi2 oxidised for 80 h at 800 C
70.85
0.062
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CrSi2 oxidised for 80 h at 700 C
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HIT/E [-]
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Table 4 HIT/E and HIT3/E2 ratio for the investigated coatings on a Ti-46Al-8Nb (at.%) substrate
It can be seen that the H/E values before oxidation are relatively high and almost the
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same for all Cr-Si coatings. After oxidation at 800 C the best wear resistance could be expected in the case of Cr-0.5Si. In general the H/E and H3/E2 values calculated for the Cr-Si coatings in this work were similar to those reported for the Cr-Si coatings deposited on
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titanium alloy TA15 [31]. Adhesion of the coating is another very important parameter, which
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could be estimated from the scratch test (Table 3). The as-received Cr-5Si coatings had the best adhesion. Critical loads for the cohesive (Lc1) and adhesive (Lc2) failure were 1.9 N and > 30N , respectively (Table 3). After oxidation at 700 C and 800 C the highest Lc1 and Lc2 values (considering both loads) were found for the Cr-5Si coating, probably because of very fine-grained oxide scales. It is worthwhile to note that the critical loads for adhesive failure, close to 30-40 N, are typical of TiN layers deposited on cutting tools [33]. In addition, the investigated Cr-Si coatings had low values of friction coefficient, close to 0.2 (Table 3), which indicates that their surface was smooth and could have satisfactory anti-wear and anti20
ACCEPTED MANUSCRIPT friction properties. The friction coefficients of these coatings were about two times lower compared with Cr-N and Cr-Si-N anti-wear coatings deposited on Si-wafers and stainless steel [24].
4. Conclusions
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2. All the investigated Cr-Si coatings provided good protection of the Ti-46Al-8Nb
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(at.%) alloy against oxygen and nitrogen absorption at 700 C and 800 C in short-
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term oxidation tests (80 hours) in thermal cycling conditions. 3. The Cr-5Si coating had the best combination of oxidation resistance and
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micromechanical properties and provided very good protection in a long-term oxidation test at 700 C (up to 1600 hours). The protective properties and the lifetime
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Acknowledgement
The authors gratefully acknowledge financial support from AGH-UST statutory funds
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(badania statutowe: 11.11.160.438) and as well as valuable contributions of Dr. S. Zimowski, Dr. M. Kot and Dr. R. Mania to the measurements of micromechanical properties and coating deposition.
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Jong Hyun Park, Won Sub Chung, Young-Rae Cho, Kwang Ho Kim, Synthesis and mechanical properties of Cr–Si–N coatings deposited by a hybrid system of arc ion plating and sputtering techniques, Surf. Coat. Tech. 188–189 (2004) 425– 430.
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[33]
J. Valli, U. Mäkelä, A. Matthews, V. Murawa, TiN coating adhesion studies using the scratch test method, J. Vac. Sci. Technol. A3 (1985) 2411-2414.
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pAr [Pa]
I [A]
t [s]
T [oC] P[W]
Cr-0,5Si
0.4
0.4
1800
270
360
Cr-5Si
0.4
0.5
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300
460
CrSi2
1.5
1800
300
240
1800
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0.2
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Target composition [at.%]
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Table 1 Parameters of coating deposition (pAr-argon pressure; I–cathode current intensity; t– deposition time; T–temperature, P–cathode emission power)
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Hmax [nm]
HV
HIT [MPa]
Cr-0.5Si as-received
165 8
1744 187
18446 1982
296 30
Cr-0.5Si oxidised for 80 h at 700 C
251 5 3
632 227
6681 2406
217 88
Cr-0.5Si oxidised for 80 h at 800 C
147 11
2098 185
22200 1952
273 53
Cr-5Si as-received
159 6
1811 113
19157 1190
315 45
Cr-5Si oxidised for 80 h at 700 C
292 28
452 93
4782 980
122 9
Cr-5Si oxidised for 80 h at 800 C
238 21
767 105
8111 1112
130 49
CrSi2 oxidised for 80 h at 700 C
186 9
1184 140
12528 1486
210 28
CrSi2 oxidised for 80 h at 800 C
248 11
628 34
6644 357
152 19
E [GPa]
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Table 2 Maximum penetration depth, Vickers hardness, indentation hardness and Young’s modulus for Ti-46Al-8Nb [at.%] samples with Cr-0.5Si, Cr-5Si, and CrSi2 coatings
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Table 3 Scratch test results for Ti-46Al-8Nb (at.%) samples coated with Cr-0.5Si, Cr-5Si, or CrSi2 under maximum load Pmax = 30 N
Lc2 [N]
Cr-0.5Si as-received
2.0 ± 0.2 (1)
19.3 ± 3.1 (2)
0.18 ± 0.02
Cr-0.5Si oxidised for 80 h at 700 C
10.4 ± 0.8 (3)
>30 (4)
0.23
Cr-0.5Si oxidised for 80 h at 800 C
8.4 ± 0.7 (5)
20.4 ± 0.8 (6)
0.24 ± 0.2
Cr-5Si as-received
1.9 ± 0.1 (7)
>30 (8)
0.20 ± 0.01
Cr-5Si oxidised for 80 h at 700 C
18.9 ± 2.1 (5)
27.0 ± 0.8 (9)
0.24 ± 0.02
Cr-5Si oxidised for 80 h at 800 C
21.8 ± 2.2 (5)
26.9 ± 1.5 (10)
0.24 ± 0.00
9.1 ± 1.5 (5)
16.6 ± 3.1 (2)
0.21 ± 0.01
16.9 ± 3.0 (5)
20.1 ± 3.4 (11)
0.20 ± 0.01
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Lc1 [N]
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(1) chevron-like cracks inside the scratch track, at the load of 15N cracks propagating outward (2) cracks and delamination on both sides of the scratch track (3) small crack inside the scratch track (4) large crack at the load of 25 N, no delamination (5) cohesive cracks inside the scratch track (6) cracks and occasional delamination on the scratch track sides, at 25 N continuous delamination on the scratch track sides (7) small cracks inside the scratch track, curved in the direction opposite to the motion of the indenter and propagating outward (8) the number of cracks increasing with the load, no delamination; (9) cracks and occasional delamination on the scratch track sides (10) small abrasion inside the scratch track (11) cracks and occasional delamination on scratch track sides, at the load of 26 N continuous delamination on the scratch track sides
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Table 4 HIT/E and HIT3/E2 ratio for the investigated coatings on a Ti-46Al-8Nb (at.%) substrate HIT3/E2 [MPa]
Cr-0.5Si as-received
0.062
71.63
Cr-0.5Si oxidised for 80 h at 700 C
0.031
Cr-0.5Si oxidised for 80 h at 800 C
0.081
146.80
0.061 0.039
7.35
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0.062
31.57
0.060
44.59
0.044
12.69
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70.85
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Cr-5Si oxidised for 80 h at 700 C
6.33
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Cr-5Si as-received
Cr-5Si oxidised for 80 h at 800 C
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HIT/E [-]
Coating and surface condition
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Figure captions: Fig. 1 Net mass changes of Cr-Si coated and uncoated Ti-46Al-8Nb (at.%) samples during oxidation in air at 700 C or 800 C (1-h cycles).
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Fig. 2 Surfaces of Ti-46Al-8Nb (at.%) samples with Cr-Si coatings after 80 hours of oxidation in air at 700 C and 800 C (1-h cycles) - SEI/BEI
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Fig. 3 XRD patterns from the surface of Ti-46Al-8Nb (at.%) samples with Cr-Si coatings after oxidation at 700 C for 80 h (1-h cycles)
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Fig. 4 XRD patterns from the surface of Ti-46Al-8Nb (at.%) samples with Cr-Si coatings after oxidation at 800 C for 80 h (1-h cycles)
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Fig. 5 Cross-sections of Ti-46Al-8Nb (at.%) with Cr-Si coatings after 80 hours of oxidation in air at 700 C and 800 C – BEI
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Fig. 6 Surface of the Cr-5Si coating on Ti-46-8Nb (at. %) substrates (BEI) after oxidation at 700 C for 1600 hours and at 800 C for 1100 hours with average compositions (in at.%) determined by EDS and the corresponding XRD patterns from the surface
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Fig.7 Micrograph of the cross-section (BEI) and chemical composition in at. % (EDS) in the marked points of a Ti-46Al-8Nb (at.%) sample with a Cr-5Si coating after oxidation in air at 700 oC, for 1600 hours
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Fig. 8 STEM micrograph of scale formed on a Ti-46Al-8Nb (at.%) sample with a Cr-5Si coating after 1600 hours of oxidation in air at 700 C (1-h cycles) along with EDS elemental maps (Ti, Al, Nb, O, Cr, Si, O Fig. 9 STEM micrograph of the scale formed on Ti-46Al-8Nb (at.%) after 1600 hours of oxidation in air at 700 C (1-h cycles) along with EDS elemental maps (Ti, Al, Nb, O) Fig. 10 Cross-section (BEI) and chemical composition in at. % (EDS) in the marked points of a Ti46Al-8Nb (at.%) sample with a Cr-5Si coating after oxidation in air at 800 C, for 1100 hours (1-h cycles) 29
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0.4
Ti-46Al-8Nb (at.%), net mass changes
0.3
PT
0.2
700oC
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0.1 0.0 -0.1 Cr-0.5Si
-0.2 0
20
40
Cr-5Si
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uncoated
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m/A [mg/cm ]
800oC
60
CrSi2 80
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Time [h]
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Fig. 2
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Cr-0.5Si
Intensity [a.u.]
Cr-5Si
PT
Cr2N
Ti3N SiO2 CrSi2
CrSi2
20
30
40
50
60
70
80
90
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Fig. 3
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800oC, 80h, 1-h cycles
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Cr2O3
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Cr-5Si
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Intensity [a.u.]
Cr-0.5Si
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20
30
TiAl SiO2
CrSi2
40
50
60
70
80
Ti3N
90
2
Fig. 4
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Fig. 8
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Fig. 10
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Fig. 9
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Graphical abstract
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- Cr-Si coatings with good adherence and micromechanical properties deposited by magnetron sputtering on Ti-48Al-8Nb substrates; - satisfactory protection of the alloy against oxidation and gas absorption in a short-term test; - excellent performance of a Cr-5Si coating confirmed in a long-term oxidation test at 700 C
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