Cyclic oxidation behavior of plasma surface chromising coating on titanium alloy Ti–6Al–4V

Applied Surface Science 261 (2012) 800–806

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Cyclic oxidation behavior of plasma surface chromising coating on titanium alloy Ti–6Al–4V Dong-Bo Wei ∗ , Ping-Ze Zhang, Zheng-Jun Yao, Jin-Tang Zhou, Xiang-Fei Wei, Peng Zhou College of Materials Science and Technology, Nanjing University of Aeronautics and Astronautics, Nanjing 211100, China

a r t i c l e

i n f o

Article history: Received 5 July 2012 Received in revised form 24 August 2012 Accepted 24 August 2012 Available online 30 August 2012 Keywords: Ti–6Al–4V alloy Metal coatings High-temperature oxidation Diffusion

a b s t r a c t The cyclic oxidation behavior of plasma surface chromising coating on titanium alloy (Ti–6Al–4V) was researched in air at 650 ◦ C, 750 ◦ C and 850 ◦ C. A NiCrAlY coating was prepared by multi-arc ion plating as a comparison. The surface morphologies, microstructures and phases of both coatings before and after oxidation were investigated using scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and X-ray diffractometry (XRD). The results show that the chromising coating consisted of an outer layer of loose Cr deposition, an intermediate layer of compact Cr deposition and an inner Ti–Cr mutual diffusion layer. The multilayer oxide scales formed in the oxidation process, which has the better cyclic oxidation resistance compared with NiCrAlY thermal barrier coating. However, the brittleness of Ti(Cr, Al)2 laves phase resulted in spallation of oxide scales at 750 ◦ C and 850 ◦ C. © 2012 Elsevier B.V. All rights reserved.

1. Introduction TC4 alloy (Ti–6Al–4V) has been widely used as a structural titanium alloy in many industries, because of its stable mechanical properties and good cost performance [1,2]. Initially, TC4 alloy is designed for use under the normal temperature (mainly −60 to 300 ◦ C). In recent years, with the development of aerospace, oil and chemical industries, the working temperature of structure titanium alloy has been rising up (>400 ◦ C) [3,4]. The TC4 alloy has been unable to meet the requirement of high temperature work environment due to its poor oxidation resistance [5]. Some investigations indicate that a serious “Oxygen brittle” has been detected at the surface of TC4 alloy after oxidized at 400 ◦ C for 10–20 h, which result in a rapid decline of mechanical properties [6]. Our previous study shows that the spalling of oxidation film has been observed on TC4 after 3 cycles at 650 ◦ C (1 h as a cycle). Therefore, A high temperature protective coating is necessary for TC4 alloy in high temperature condition. The traditional method to prepare high temperature protective coating on titanium alloy is based on diffusing, such as pack cementation, laser cladding processing. Some research works have been doing and they provide good protective effect [7–9]. However, the high processing temperature causes the brittle phases formed in matrix alloy, which are quite harmful to the mechanical properties of matrix alloy [10–12]. It is the inevitable disadvantage of diffusion technology. Recent researches suggest that some

∗ Corresponding author. Tel.: +86 158 5065 8479; fax: +86 025 5211 2626. E-mail address: [email protected] (D.-B. Wei). 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2012.08.102

emerging surface engineering technologies, such as PVD, CVD, micro-arc oxidation and ion plating, are very effective ways to improve the oxidation resistance of TC4 alloy [13–15]. They can prepare a protective coating at less than 300 ◦ C, which will not influence the mechanical properties of matrix alloy. Unfortunately, their common shortcoming is that there are a huge difference in thermal expansion coefficient between coating and matrix, which results in the bad antistrip performance of coating at high temperature [16,17]. At present, a lot of researches of preparing high temperature protective coating on TC4 are focusing on how to decrease the process temperature and increase the adhesion of coating with matrix alloy in the meantime [18–20]. The double glow plasma surface alloying process [21], known as Xu-Tec process in the western countries, provides a new way to achieve the goal. This technology has been proven to be an effective method to improve the surface performance of metal, such as hardness, wear resistance, and oxidation resistance [21–23]. Its prominent advantage is that a gradient metallurgical bonding coating is obtained in process, which has good adhesion with the substrate. In addition, with the aid of ion bombarding, the diffusion temperature is lower than the traditional diffusion technology, which will reduce the impact on mechanical performance of matrix alloy. Previous research has revealed that double glow plasma surface chromising was an effective way to increase the surface hardness and improve the high temperature oxidation resistance of TC4 titanium alloy [23,24]. In this paper, it’s cyclic oxidation behavior of plasma surface chromising coating on TC4 alloy was investigated at 650 ◦ C, 750 ◦ C and 850 ◦ C. The NiCrAlY thermal barrier coating,

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Table 1 The chemical composition of the TC4 alloys (at%). Ti

Al

V

Fe

N

C

H

O

The rest

5.8–6.0

3.2–3.5

0.21–0.23

0.02–0.04

0.02–0.05

0.008–0.01

0.13–0.17

Table 2 Process parameters of plasma chromising and multi-arc ion plating.

2.4. Coating evaluation

Items

Plasma chromising

Multi-arc ion plating

Processing temperature ( ◦ C) Processing time (h) Work-piece pressure (Pa) Distance between source and cathode (mm) Source voltage (V) Cathode (work-piece) voltage (V)

750 3 35–40 18–22 900 400–500

180–200 2 1.3 × 10−2 80 650–250 –

as a kind of high temperature resistant coating, has been implemented in some situation [25]. In this paper, in order to evaluate the anti-oxidation performance of chromising coating, a NiCrAlY coating (about 15 ␮m thick) as a comparison is prepared on TC4 alloy by multi-arc ion plating technology.

Cyclic oxidation tests were performed in high temperature box resistance furnace in air at 650 ◦ C, 750 ◦ C and 850 ◦ C. One cycle consisted of 1 h at temperature and 10 min cooling down to 60 ◦ C. Coating specimens were periodically weighed and visually inspected during testing up to an exposure time period of 100 1h cycles. The investigations of the microstructure before and after oxidation were performed using a scanning electron microscope (SEM) with energy-dispersive X-ray spectrometer (EDS) analysis. Elemental compositions were determined using semi-quantitative analysis for spot and line scan measurements. X-ray diffraction (XRD) using a Cu K␣ source was used to identify the phases of the coating and oxide scale. 3. Results and discussion

2. Experimental

3.1. Morphology and composition of coating

2.1. Specimen preparation

Fig. 1 was the scanning electron micrographs of plasma surface chromising coating: (a) the top view; (b) the cross-sectional view. As shown in Fig. 1(a), the top of plasma surface chromising coating was segmented owing to cracks, which were perpendicular to the surface. Fig. 1(b) showed that the middle layer (II) with columnar structure grew perpendicular to the free surface. A diffusion layer (III) was based on the matrix, and there was a clear bounds between them. A layer of loose Cr deposition (I) was at the surface of coating. The small angle X-ray diffraction (Fig. 2) and EDS analysis showed the main phases of chromising coating consisted of Cr, Cr1.97 Ti1.07 , CrTi4 and Ti. The mole ratios of the phases were calculated using expressions from the work of Miller et al. [27]. The phase mole fractions were summarized in Table 3. A more-refined XRD and EDS analysis to the diffusion layer suggested that the mole fractions of each phase showed some small changes along the depth in diffusion layer. Along with the depth direction, the mole fractions of Ti and CrTi4 showed a tiny increase, the mole fractions of Cr and Cr1.97 Ti1.07 accordingly showed a tiny decrease. The micro-structure characterization was described in detail in a previous study [24].

The chemical composition of the TC4 alloys used in this experiment was in Table 1. Each substrate specimen was prepared by cutting the rolled plate into coupons of approximate dimension 15 mm × 15 mm × 5 mm, grinding their surface to a mirror finish using a series of SiC polishing paper. The polished specimens were ultrasonically washed in an acetone bath and dried in air before experiment. 2.2. NiCrAlY thermal barrier coating Prior to deposition, the ion plating chamber was flooded with high pure argon and evacuated to a background vacuum of 5 × 10−3 Pa. Then, the surface of specimens was further cleaned by ion bombardment for 5 min with a negative bias up to 300V. The deposition was carried out with a Ni–33Cr–8Al–0.7Y alloy target (wt%). The parameters of deposition are showed in Table 2. 2.3. Plasma chromising coating The principle of double glow plasma surface alloying process was as follows: a target electrode and a substrate electrode were mounted in a vacuum chamber (anode). The chamber was evacuated to a pressure of about 5 × 10−2 Pa and then filled with argon to 20–50 Pa. The anode and the target were connected to pulse power supplies, and the anode and the substrate were connected to DC power supplies. The potential difference between the substrate and the target resulted in an unequal electronic potential hollow cathode effect. On heating to a given temperature, the ions or atoms sputtered from the target were deposited on the substrate due to the negative bias which was lower than that of the target. The ions and atoms diffused into the matrix at the elevated temperature; thus, an alloyed coating was developed on the substrate. The full explanation of double glow plasma surface alloying process was in Ref. [26]. In this study, the sputtering target of chromium plate (˚ 100 mm × 5 mm) with purity of 99.9 at% prepared by powder metallurgy was used as the source electrode to supply alloying elements. The parameters of plasma chromising are showed in Table 2.

3.2. 650 ◦ C oxidation behavior analysis Fig. 3 shows the oxidation kinetics of TC4 alloy, plasma chromising coating and NiCrAlY thermal barrier coating which were exposed to air at 650 ◦ C under cyclic oxidation condition. The TC4 specimen failed after 32 cycles due to extensive oxide scale spallation, while both the coated specimens were perfect without any defects, such as crack and spalling after 100 cycles. The oxidation kinetic curves of both coatings followed parabolic rate law, indicating that the diffusion process prevailed during oxidation [28]. In the early oxidation stage, the higher mass gain of the plasma chromising coating in comparison to NiCrAlY thermal barrier coating was due to its larger specific surface area, as shown in Fig. 1(a). A continuous and compact scale was observed on the surface of both coated specimens after 17 cycles. XRD and EDS analysis suggested that the oxidation films of both coated specimens mainly consisted of Cr2 O3 . However, after 34 cycles, some TiO2 were detected under the Cr2 O3 scale of the chromising coating, as shown in Fig. 4. After 44 cycles, a large amount of Ti(Cr, Al)2 laves phases precipitated at

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Table 3 The phase mole fractions of plasma chromising coating. Structure of chromising coating

Thickness (␮m)

The top loose layer The middle layer The diffusion layer

20 10 4

The main phase distribution (mol%) Cr

Cr1.97 Ti1.07

CrTi4

Ti

87 79 24–20

– – 38–33

– – 20–17

12 10 17–14

the substrate/oxide film interface. In the meantime, a Al depletion layer with 4 ␮m thickness was detected beneath the Ti(Cr, Al)2 laves phase layer, as shown in Fig. 5(a). There was no oxygen detected beneath the Ti(Cr, Al)2 laves phase layer in the whole oxidation process (Fig. 5(b)). Unlike isothermal oxidation at 650 ◦ C, there were more Ti(Cr, Al)2 laves phases and thicker Al depletion layer developed in cyclic oxidation condition [24]. Some researchers found that the compact and inform Cr2 O3 scale had a good resistance to fluctuating thermal stress [28]. In this study, it did not only prevent the process of oxygen diffusion, but also depressed the external diffusion of Cr. It made more Cr diffused inward. At last, the mutual diffusion between Ti, Al and Cr resulted in more Ti(Cr, Al)2 laves phases and thicker Al depletion layer developed in cyclic oxidation condition. Oxidized after 32 cycles, remarkable breakage and massive peeling-off were observed on the surface of TC4 specimens. It

Fig. 2. The XRD pattern of plasma chromising coating.

indicated poor cyclic oxidation resistance of Ti–6Al–4V alloy at this exposure temperature. In the first 7 cycles, TiO2 was the mainly oxidation product. Numerous researchers found that nonstoichiometric titania was a semi-protective oxide with a high growth rate, which resulted in a significantly rising of the mass gain [13]. XRD and EDS analysis showed a TiO2 &Ti2 O3 mixed oxidation scale was formed in the following 7 cycles, which slowed down the oxygen invasion. However, the TiO2 &Ti2 O3 mixed oxidation scale was not a compact protective film [16]. A large amount of oxygen diffused across the oxidation scale, which resulted in the second soaring of the mass gain in 15–32 cycles. Oxidized after 18 cycle, XRD and EDS analysis indicated that a large amount of TiO2 were formed on the surface of porous mixed oxidation scale. This structure obviously couldn’t defend the invasion of oxygen. Oxidized to 32 cycle, the rapid spalling of oxidation scale suggested

-2

Mass Gain ((mg· cm ))

0.75 TC4 NiCrAlY coating plasma Cr coating 0.50

0.25

0.00

0

10

20

30

40

50

number of cycles (h) Fig. 1. The scanning electron micrographs of plasma chromising coating: (a) the top view; (b) the cross-sectional view.

Fig. 3. The oxidation kinetics of TC4 alloy, plasma chromising coating and NiCrAlY thermal barrier coating at 650 ◦ C under cyclic oxidation condition.

D.-B. Wei et al. / Applied Surface Science 261 (2012) 800–806 10000

1.00

● ▲ ● Cr2O3 ▲ TiO2 -2

Mass Gain ((mg ¤ cm ))

■ Ti(Cr Al)2

▲ 6000

● ●

2000

0

20

● ▲ ■ ●

40

●

▲ ● ●

60

NiCrAlY coating plasma Cr coating

0.75

,

intensity (CPS)

8000

4000

803

■

80

■

■

■

100

2θ (°) Fig. 4. The XRD pattern of plasma chromising coating at 650 ◦ C under cyclic oxidation condition for 50 h.

0.50

0.25

0.00

-0.25

0

20

40 60 number of cycles (h)

80

Fig. 6. The oxidation kinetics of plasma chromising coating and NiCrAlY thermal barrier coating at 750 ◦ C under cyclic oxidation condition.

the big difference of thermal expansion coefficient between matrix alloy and oxidation scale. It caused the continuous rapid spalling every 2 cycles after the 32 cycle. 3.3. 750 ◦ C oxidation behavior analysis

Fig. 5. The SEM&EDS analyses of the chromising coating after oxidation at 650 ◦ C for 50 h. (a) Cross-sectional image; (b) line profiles across A and B of (a).

Fig. 6 shows the oxidation kinetics of Ti–6Al–4V specimens coated with plasma chromising coating or NiCrAlY thermal barrier coating at 750 ◦ C. As the same with the oxidation at 650 ◦ C, the oxidation kinetic curves of both coatings followed parabolic rate law. And the plasma chromising coating revealed the higher mass gain in the early oxidation stage. However, the NiCrAlY thermal barrier coating was beginning to bubble and flaking at 73 cycles. There was no spalling detected on the surface of plasma Cr coating in the whole oxidation process. The SEM and EDS analysis suggested that a continuous and compact Cr2 O3 layer has formed on the surface of NiCrAlY coated specimens after 18 cycles, which suppressed the inward diffusion of oxygen. Oxidized after 68 cycles, a small amount of nickel oxide was detected under the Cr2 O3 layer. In the following 5 cycles, it was oxidized to a layer of nickel oxide. Studies show that a nickel oxide film has good tenacity and excellent adhesion [29]. In this study, it didn’t only absorb the oxygen diffused inward, but also provided a excellent mesosphere between the Cr2 O3 layer and TC4 substrate. Oxidized after 73 cycles, the nickel in NiCrAlY coating was used up. As the increase of the oxidation time, the Ti of matrix alloy was begging to oxidize, which leaded to a layer of titanium oxide formed under the mesosphere. It explained why the oxidation scale peeled off quickly and completely in the following 5 cycles. Fig. 8(a) shows the cross-sectional morphology of plasma chromising coating after oxidized at 750 ◦ C for 80 cycles. There was no evident spalling or crack observed in the whole oxidation process. The oxide scale appeared a complex overlapping structure consisting of the outer Cr2 O3 layer, the middle TiO2 layer and the inner Cr2 O3 layer, as show in Figs. 7 and 8(b). XRD&EDS analysis suggested a large number of Cr2 O3 particles were dispersed in the outer Cr2 O3 layer. The inner Cr2 O3 layer was compact, and with some fine dispersed TiO2 particles. The middle TiO2 layer was mainly caused by the high activity of Ti at high temperatures [29]. In the meantime, the “carried diffusion” phenomenon helped the outward diffusion of Ti [30]. It thought that the outwardly diffusion of Cr carried Ti and piled-up continuously ahead of the inner Cr2 O3 layer.

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4

14000

● ▲ ●

8000

■ Ti(Cr Al)2

▲ ●

▲

-2

▲ TiO2

●

6000

●

4000

▲■

2000 0

20

40

▲● ● 60

NiCrAlY coating plasma Cr coating

3

,

intensity (CPS)

10000

● Cr2O3

Mass Gain ((mg cm ))

12000

■ 80

■ ■

2

1

■

0

100

2θ (°) Fig. 7. The XRD pattern of plasma chromising coating at 750 ◦ C under cyclic oxidation condition for 80 h.

Near the oxide scale/substrate boundary, mutual diffusion of Ti, Al and Cr had happened and resulted in a Ti(Cr, Al)2 laves phase layer with more than 5 ␮m in thickness. Meanwhile, a lot of kirkendall voids were left in the depletion area. EDS analysis indicated that there were no O detected in the Ti(Cr, Al)2 laves phase layer, as shown in Fig. 8(b). The previous isothermal oxidation experiment

0

10

20 30 number of cycles (h)

40

50

Fig. 9. The oxidation kinetics of plasma chromising coating and NiCrAlY thermal barrier coating at 850 ◦ C under cyclic oxidation condition.

suggested that the Ti(Cr, Al)2 laves phase layer could effectively prevent the inward diffusion of oxygen [31]. Some studies revealed that Ti(Cr, Al)2 laves phase was a brittle phase [32]. In this study, a evident gap emerged between the oxide scale and the Ti(Cr, Al)2 laves phase layer. It indicated huge difference of thermal expansion coefficients between them. 3.4. 850 ◦ C oxidation behavior analysis Fig. 9 shows the oxidation kinetics of Ti–6Al–4V specimens coated with plasma chromising coating and NiCrAlY thermal barrier coating at 850 ◦ C. The plasma chromising coating exhibited about threefold lifetime improvement relative to the NiCrAlY thermal barrier coating. Before spallation, a duplex Cr2 O3 oxide scales consisting of a porous outer portion and a compact inner portion were observed on the plasma chromising coated specimens after 34 cycles, as show in Fig. 10. EDS analysis indicated that a porous TiO2 layer was at the surface of oxide film, as show in Fig. 11(a). As the oxidation reaction proceeds, growth stresses were generated within the oxide film. In the meantime, more Cr cations were migrating outwards. Frequently this loss of Cr cations caused the formation of a porous zone of scale. In order to maintain contact with the substrate, the Cr2 O3 relaxed to two different forms: the 14000

▲

▲ ●

12000

▲ TiO2

●

10000

intensity (CPS)

● Cr2O3 ■ Al2O3

▲ ●

8000

■

6000

■ ■

4000

▲ ●

■

●

2000

0 25

30

35

40

45

50

■ ▲ 55

60

65

■

70

2θ (°) Fig. 8. The SEM&EDS analyses of the chromising coating after oxidation at 750 ◦ C for 80 h. (a) Cross-sectional image; (b) line profiles across A and B of (a).

Fig. 10. The XRD pattern of plasma chromising coating at 850 ◦ C under cyclic oxidation condition for 38 h.

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2. In the cyclic oxidation process, the inward diffusion of Cr was superior to the outward diffusion. As the outward diffusion of Ti and Al, the Ti–Cr mutual diffusion layer changed into the Ti(Cr, Al)2 laves phase layer, which prevented the inward diffusion of oxygen. However, the brittleness of Ti(Cr, Al)2 laves phase resulted in spallation of oxide scale at 750 ◦ C and 850 ◦ C. 3. After 38 cycles at 850 ◦ C, multilayer oxide scales were observed on the plasma chromising coating, which released the cyclic thermal stress. The middle flexible Al2 O3 layer adjusted the difference of thermal expansion coefficients between the outer column Cr2 O3 layer and matrix alloy. In the meantime, dispersed oxides of Ti in the inner Cr2 O3 layer improved adhesion of oxide scales. Acknowledgements This project was supported by National Natural Science Foundation of China (Grant No. 51175247), the Funding of Jiangsu Innovation Program for Graduate Education (Grant No. CXZZZ11 0203) and the Opening Project of Jiangsu Key Laboratory of Advanced Metallic Materials (Grant No. AMM201102). References

Fig. 11. The SEM&EDS analyses of the chromising coating after oxidation at 850 ◦ C for 38 h. (a) Cross-sectional image; (b) line profiles across A and B of (a).

outer porous column structure and the inner compact malleable structure. The voids at the outer zone of the scale made a rising of oxygen partial pressure in local equilibrium with the scale inner surface, which finally resulted in more severe oxidation. In the meantime, more Ti and Al diffused outwardly through the voids. After 38 cycles, a continuous Al2 O3 layer formed, and TiO2 particles filled up the porous zone of scale„ as show in Fig. 10. In the meantime, a large number of kirkendall voids and Ti(Cr, Al)2 laves phases formed between the oxide scale and the metal, as show in Fig. 11(a). However, the SEM observation revealed that the duplex oxide scales and a large number of inner voids had formed after 26 cycles. In the 39–50 cycles, there were no evident structure change and elements diffusion detected by SEM and EDS. There was no oxygen detected in the Ti(Cr, Al)2 laves phase layer. It indicated that the cyclic thermal stresses was the leading cause of spallation on of oxide scale, not the growth stresses of oxide scales. In a word, the brittleness of Ti(Cr, Al)2 laves phase was still the mainly reason for the thermal cycle failure. 4. Conclusions 1. A plasma chromising coating with Ti–Cr mutual diffusion layer was obtained on TC4 alloy by the double glow plasma alloying technology, which has the better cyclic oxidation resistance compared with NiCrAlY thermal barrier coating.

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