- Email: [email protected]
Microstructural evaluation of ZrO2–MgO coatings
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 9 ( 2 0 0 9 ) 488–496
journal homepage: www.elsevier.com/locate/jmatprotec
Microstructural evaluation of ZrO2 –MgO coatings A. Nusair Khan ∗ , I.N. Qureshi Metallurgy Division, Rawalpindi, Pakistan
a r t i c l e
i n f o
a b s t r a c t
Article history:
Zirconium oxide, stabilized with magnesium oxide, as a topcoat a coat and Ni–5Al as a
Received 14 June 2007
bondcoat were air plasma sprayed onto a nickel base alloy substrates. Microstructural and
Received in revised form
phase changes were observed during the thermal treatment. Formation of nickel-oxide was
18 January 2008
noticed during the experimentation. Chemical composition profile for the given system,
Accepted 12 February 2008
at high temperature, were determined and discussed in this paper. Microstructural characterization was carried out by scanning electron and optical microscopes, whereas phase analysis was carried out by X-ray diffractometer (XRD). © 2008 Elsevier B.V. All rights reserved.
Keywords: Ceramic coatings High temperature ceramics Air plasma spraying
1.
Introduction
Thermal barrier coatings (TBCs) are widely used in gas turbines for propulsion and power generation (Brindley and Miller, 1989; Bennett, 1986; Nesbitt, 1990; Zhu and Miller, 2000). The TBCs can be considered as three layer materials systems, consisting of (1) a superalloy substrate, (2) an oxidation-resistant metallic bondcoat, and (3) the ceramic top coating deposited either by plasma spraying or electron beam physical vapor deposition The zirconia topcoat has excellent thermal shock resistance, low thermal conductivity and a relatively high coefficient of thermal expansion (CTE). The bondcoats provide a rough surface for mechanical bonding of the ceramic topcoat, protect the underlying alloy substrate against high temperature oxidation corrosion, and minimize the effect of CTE mismatch between the substrate and ceramic topcoat materials. Pure zirconia transforms from tetragonal to monoclinic phase and thus volumetric changes associated with this transformation. This transformation is not desirable in the application of coating. According to Bennett (1986), it can be stabilized with either CaO (5 wt.%), MgO (15–24 wt.%), whereas
∗
Corresponding author. E-mail address: [email protected] (A.N. Khan). 0924-0136/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2008.02.032
Brandon and Taylor (1989) and Steffens and Fisher (1987) shown that it can also be stabilized with Y2 O3 (6–12 wt.%). The fully or partially stabilized zirconia is usually deposited by plasma spraying method on the metal surface. These coatings are bad heat conductors and have good thermal shock resistant properties. Among them ZrO2 –MgO are relatively cheaper than the ZrO2 –Y2 O3 that is why utilized in those regions where the temperature intensity is relatively low, e.g. in the exhaust of the jet engines. Further, MgO stabilized system can be used for the development of intermediate coating in a three part graded coating system with magnesium zirconate as a topcoat. In this study, ZrO2 –MgO system was undertaken. The microstructural and phase changes were observed after different thermal treatments.
2.
Experimental
The coating system composed of a bondcoat and a top coating was air plasma sprayed onto a nickel-base super alloy plate, having the chemical composition given in Table 1. The powder
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 9 ( 2 0 0 9 ) 488–496
Table 1 – Chemical composition of substrate metal Weight percentage Substrate
Hastelloy G-50 alloya
51.0 20.0 2.0 19.0 9.0 0.5 0.5 0.3 0.005 0.010
≥50 19–21 2.50 15–20 8–10 1.0 1.0 1.0 0.015 0.015
Ni Cr Co Fe Mo W Mn Si S C a
Nearest standard.
used for the bond coating was Ni–5Al with grain size 50–90 m. The topcoat material grain size was 20–90 m and the chemical composition was 76ZrO2 –24MgO (wt.%). A rectangular nickel base superalloy plate with thickness 5 mm was grit blasted. A 7MC Metco Sulzer Plasma gun was utilized for the deposition of both the bond and top coating. The bond coating was sprayed to a total thickness of 95 ± 10 m, whereas the top coating was sprayed to the total thickness of 180 ± 20 m. The cross section of coating system is shown in Fig. 1. During the spraying of both bond and top coatings, the substrate temperature was controlled by air jets. After thermal spraying, the specimens were cut by diamond cutter for different experimentation. The chemical composition profile was made with the help of energy dispersive spectrometer (EDS) attached with scanning electron microscope (SEM). For this purpose, the cross section of the specimens grinded and polished carefully and a gold layer was sputtered to make the specimen conductive. The phase analyses were done on the surface of the top coating and on the backside of the top coating (obtained after etching of bondcoat in 35% HNO3 solution) after exposing to high temperatures for different timings. The purpose was to examine the formation of any monoclinic phase and the analysis of the phases formed near the interface of the bondcoat and the topcoat, during the heat treatment. The diffraction measurements were performed using an X-ray diffractometer
Fig. 1 – Cross section of as-sprayed coating system with topcoat, bondcoat and substrate.
489
by utilizing the cobalt K␣ radiation. The diffraction of main peaks for the 1¯ 1 1 monoclinic phase, 1 1 1 tetragonal phase, and the 1 1 1 monoclinic phase were all found in the 2 range between 32◦ and 38◦ . The volume fraction, Vm of the monoclinic phase was calculated according to the Toraya et al. (1984) method, where Vm is: Vm =
PXm 1 + (P − 1)Xm
(1)
where the value of P was calculated by Toraya et al. (1984), for the present composition, as 1.3 and for the monoclinic–tetragonal system, Xm is the integrated intensity ratio defined by: Xm =
Im (1¯ 1 1) + Im (1 1 1) Im (1¯ 1 1) + Im (1 1 1) + IT (1 1 1)
where I is the integrated intensity from the monoclinic (m) and tetragonal (T ) phases. Samples were heat treated in tube type furnace at 750, 900 and 1000 ◦ C for 10, 30, 60 and 100 h. After heating and holding of samples at high temperature, the samples were allowed to cool down into the furnace, while the tube remained closed. This long heat treatment was accomplished in different stages. The heat treatment was carried out only during the day time for 8–10 h while during the night the furnace remained switched off.
3.
Results and discussion
3.1.
Microstructure
The cross section of the samples was observed under the SEM and optical microscope. The as-sprayed coating samples showed a typical APS microstructure, with crack network and porosity in the topcoat (of ZrO2 –24MgO). The most prominent observation for all the samples treated at high temperature was the formation of an intermediate layer at the interface of topcoat and bondcoat. The thickness of this light gray color layer varied by different exposure time and temperatures. From here-on, we call this layer between the topcoat and bondcoat as “Heat Affected layer” (HAL). The samples treated at 750 ◦ C showed that the topcoat and bondcoat remained stabled through out the treatment, however, it was observed that a HAL appeared after 30 h at this temperature (Fig. 2(b)). The thickness of this layer was about 1–2 m. No cracking or debonding observed at this temperature (Fig. 2). After 10 h of treatment at 900 ◦ C, it was observed that the HAL was about 30 m. This layer became thicker with more exposure time (Fig. 3), which was 45 and 56 m after 30 and 60 h, respectively. Further, it was observed that the vertical cracks appeared in the topcoat after 60 h (Fig. 4(b and c)). It appeared that HAL grew at the expense of bondcoat which is demonstrated in Fig. 5. In the bondcoat after long oxidation at 1000 ◦ C, two types of phases were observed (Fig. 6), i.e. light gray and dark phases richer in chromium and aluminum contents, respectively. They might be the oxides of chromium and aluminum.
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j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 9 ( 2 0 0 9 ) 488–496
Fig. 2 – Coating system treated at 750 ◦ C: (a) 10 h with no HAL and (b) 30 h with HAL appeared between topcoat and bondcoat.
Coatings treated at 1000 ◦ C also demonstrated pitted interface at bondcoat and HAL (Fig. 7). This pitted surface actually richer in aluminum and leads to the detachment of the topcoat from bondcoat. Further, at 1000 ◦ C for 60 h, it was also observed that the substrate surface closed to the bondcoat oxidized (Fig. 6).
3.2.
Chemical composition profile
The chemical composition profile for the samples treated at 1000 ◦ C for 60 h is given in Figs. 8–9. It is evident that the elements like Cr, Fe, Co and Mo depleted from the substrate closed to the bondcoat and highly increased in bondcoat. The original composition of bondcoat was Ni–5Al, which became rich in Cr up to 28%. Similarly, the presences of Fe, Co and Mo was also observed. The HAL, which is rich in nickel (>85%) and oxygen (>15%) indicating the presence of nickel-oxide with small
amounts of Fe (0.5–0.8%). The presence of Zr (2–10%) was also detected closed to the topcoat end. Similar trend of elemental distribution was observed for the samples treated at 900 ◦ C for 30 and 60 h. The EDS analysis revealed that the HAL close to the topcoat having the concentration of Zr, whereas it was riched in Ni contents close to the bondcoat (Fig. 9). In some samples, this HAL was broken from both the bondcoat interfaces (Fig. 4(a)). Further, at the interface of HAL and bondcoat revealed richer in aluminum contents (Fig. 10), this indicates the formation of the alumina at the interfaces of HAL. This oxide may induce stresses at the interface (Nusair Khan and Lu, 2003). After the thermal exposure at 1000 ◦ C for 60 h, the chemical analysis of topcoat’s from backside, i.e. the portion which touched the bondcoat is also analyzed by EDS. The chemical composition is given in Table 2, which is almost the same as observed in the cross section of the sample.
Fig. 3 – Coating system at 900 ◦ C treated for: (a) 10 h, (b) 30 h, (c) 60 h and (d) 90 h.
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 9 ( 2 0 0 9 ) 488–496
491
Fig. 5 – Change in bondcoat and HAL thickness as a function of thermal exposure at 900 ◦ C.
3.3.
Fractrography
Further details of the microstructural features were examined through fracture surfaces of topcoat. For this purpose, the topcoat was removed by immersing the coated samples in 65% HNO3 . It was observed that the as-sprayed topcoats removed just in 24 h immersion time, whereas the samples treated at 1000 ◦ C for 60 h remained unaffected and no topcoat detached from the substrate. Thus, the samples were left in fresh solution but nothing happened even after 15 days time. It was then decided to try in boiling HNO3 and after 15 min treatment we obtained the topcoat. The removed topcoat then fractured and observed in SEM. The as-sprayed microstructure was composed of lamellae that fallow the geometry of underlying material. Each lamella has a thickness of about 3–5 m. Between the two lamellae, horizontal delaminations
Fig. 4 – Coating system treated at 1000 ◦ C treated at: (a) 30 h, (b) 30 h and (c) 60 h.
Table 2 – Chemical composition of backside of topcoat after 1000 ◦ C, 60 h Elements Ni O Al Cr Fe
wt. (%) 76 18 3 0.5 2.50
Fig. 6 – Bondcoat and substrate after oxidation at 1000 ◦ C for 60 h.
492
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 9 ( 2 0 0 9 ) 488–496
Fig. 7 – Aluminum rich pitted surface at the interface of bondcoat and HAL. Fig. 9 – Chemical composition profile for Ni and Zr after heat treatment at 1000 ◦ C for 60 h.
Fig. 8 – Chemical composition profile for Cr, Fe, Co and Mo after heat treatment at 1000 ◦ C for 60 h.
Fig. 10 – Change in aluminum contents after 1000 ◦ C for 60 h.
were observed which refers towards the low cohesion between the splats (Fig. 11). Some cracked lamellae (Fig. 11) were also observed in the as-sprayed samples, which were believed to be cracked during the cooling of topcoat. This may be due to the tensile stresses which were developed during the cooling of the coating, and some times exceeds the yield limits of the material causing cracks. The HAL region consists of oxides which remained attached to the topcoat after chemical etching of bondcoat. These oxides were confirmed as Ni oxides (NiO) and are associated with high porosity (Fig. 12).
3.4.
Phase analysis
The room temperature equilibrium phases for 24% stabilized zirconia coating are expected to be a monoclinic phase
Fig. 11 – Fractured surface of topcoat in as-sprayed condition.
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 9 ( 2 0 0 9 ) 488–496
493
Fig. 12 – Nickel-oxide on the backside of the topcoat observed on detached topcoat.
(m). However, the rapid cooling of the top coating material during the plasma spraying process gives a non-equilibrium tetragonal phase, denoted by T . This phase is referred to as non-transformable since it does not transform into the monoclinic phase during the rapid cooling after spraying. After heating at high temperature, during cooling to room temperature, the tetragonal phase, T, transforms to the monoclinic phase. This transformation (T to m) is associated with a volume expansion to the order of 4–5% (Bengtsson, 1995). It is believed that this volumetric expansion will cause damage to the integrity of the top coating, which could result in failure during the application of the coating. X-ray diffraction phase analysis shows the as-sprayed samples to consist almost completely of tetragonal T phase
Fig. 14 – Change in percent volume of monoclinic phase with thermal treatment.
(Fig. 13(a)). Table 3 demonstrating the obtained d-values for different conditions of samples with their corresponding searched cards. Following the quantitative phase analysis described in Section 2, the monoclinic phase fraction at different temperatures for different time is shown in Fig. 14. It was observed that the %age of monoclinic phase increases with both temperature and time (at a particular temperature). After 10 h of exposure at 750, 900 and 1000 ◦ C, the volume percent of monoclinic was 7, 20 and 38, respectively. At 900 ◦ C, where
Fig. 13 – X-ray diffraction patterns of topcoat without any thermal exposure: (a) top surface and (b) back surface.
494
1000 ◦ C, 60 h
As sprayed
Top surface
Tetragonal (ZrO2 )
Monoclinic (ZrO2 )
NiO
ZrO2 –MgO
14–0534
37–1484
4–835
80–0967
Top surface
Back surface
Back surface
d
I/I0
d
I/I0
d
I/I0
d
I/I0
d
I/I0
d
I/I0
d
I/I0
d
I/I0
2.928 2.538 2.118 1.79 1.526
100 20 10 35 25
3.8625 3.0442 2.9045 2.8967 2.5034 2.4103 2.4103 2.0907 1.9450
20 100 80 90 25 10 10 10 10
3.7 3.6484 3.1703 2.9603 2.8508 2.7082 2.5985 2.55 2.4423
10 8 60 100 50 15 15 30 5
1.7848 1.6546 1.6196 1.6111 1.5605 1.5395 1.4925 1.4821 1.3249
10 10 10 8 15 50 15 15 5
2.453 2.399 2.079 1.472 1.257 1.203 1.200
5 70 100 60 35 30 25
2.949 2.584 2.542 1.804 1.551 1.535 1.474 1.291 1.27
100 20 60 100 40 80 40 40 60
3.698 3.639 3.165 2.841 2.623 2.606 2.54 2.2138 1.8481
14 10 100 68 21 11 13 12 18
2.41 2.088 1.476 1.259 1.206 1.044 0.958 0.934 0.853
91 100 57 16 13 8 7 21 17
1.7741 1.7689 1.5265 1.5173
30 30 15 25
2.263 2.118 1.9956 1.8521 1.8178 1.8034
10 30 5 15 60 50
1.2726 1.219 1.178
10 8 10
1.8187 1.8038 1.6937 1.6571
22 13 11 11
0.804
7
d 2.9266 2.9111 2.5244 1.7886 1.7815 1.5261 1.5217 1.5195 Other d-values are with less intensity
I/I0 35 100 25 30 30 10 20 15
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 9 ( 2 0 0 9 ) 488–496
Table 3 – Results for as-sprayed coating along with samples treated at 1000 ◦ C for 60 h, the matching cards are also mentioned (JCPDS, 1999)
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 9 ( 2 0 0 9 ) 488–496
495
Fig. 15 – X-ray diffraction patterns of topcoat after thermal exposure at 1000 ◦ C for 60 h: (a) top surface and (b) back surface.
the experimentation was conducted up to 100 h, in the start, i.e. up to 30 h the volume percent of monoclinic phase was up to 20–23, which gradually increases up to 37% after 60 h. The trend at this temperature seems became constant and only slight increase in monoclinic phase was observed up to 100 h of exposure. Similar trend was observed in samples treated at 1000 ◦ C, where volume percent of monoclinic phase after 30 h was 52 and almost remains constant after 60 h (Fig. 15(a)). It was observed that the samples treated at 750 ◦ C initially demonstrated small amount of monoclinic phase but then no increase in this phase was observed up to 60 h of treatment (Fig. 13). The opposite side of the topcoat (which was closed to the bondcoat) was also checked for possible phases. In case of assprayed samples, the rhombohedral magnesium–zirconium oxide peaks were observed (Table 3; Fig. 13(b)). Similarly, at 1000 ◦ C for 60 h the phase analysis of the opposite side of the topcoat revealed as nickel-oxide (NiO) (Fig. 15(b)). The intensities of the peaks with d-values for both the above phases are given in Table 3, with their corresponding matching cards. This nickel-oxide phase is believed to be of HAL, which remained stuck with the topcoat after the etching of bondcoat.
4.
Conclusions
MgO stabilized zirconia thermal barrier coatings were characterized after the thermal exposure at different temperatures and times. Microstructural and phase changes were observed in this respect. It was observed that the thickness of HAL increased with time and temperature. The HAL was identified as nickel-oxides. These oxides were found spongy in nature. The chemical composition profile revealed that at high temperature the elements from the substrate were diffused out to the bondcoat. Volume percent of the monoclinic phase was determined against the temperature. It was found that up to a certain limit, at particular temperature, the volume percent of the monoclinic phase became constant. MgO-stabilized zirconia coatings were observed stable up to 750 ◦ C and beyond this became deteriorated quickly with exposure.
references
Brindley, W.J., Miller, R.A., 1989. Thermal barrier coatings for better engine efficiency. Advance Materials Processing 136, 29.
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j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 9 ( 2 0 0 9 ) 488–496
Bennett, A., 1986. Properties of thermal barrier coatings. Material Science and Technology 2, 257–261. Brandon, J.R., Taylor, R., 1989. Thermal properties of ceria and yttria partially stabilized zirconia thermal barrier coatings. Surface and Coatings Technology 39/40, 143–151. Bengtsson, P., 1995, Microstructural studies and residual stress evolution in zirconia based thermal barrier coatings. Thesis. Linkoping University, S-581 83 Linkoping, Sweden. JCPDS, 1999. Powder diffraction files. JCPDS-International Center for Diffraction Data, USA. Nesbitt, J.A., 1990. Thermal response of various thermal barrier coatings in a high heat flux rocket engine. Surface and Coatings Technology 43–44, 458–469.
Nusair Khan, A., Lu, J., 2003. Behaviour of air plasma sprayed thermal barrier coatings, subject to intense thermal cycling. Surface and Coatings Technology 166, 37–43. Steffens, H.D., Fisher, U., 1987. Characterization and thermal shock testing of yttria stabilized zirconia coatings for heat engine. Surface and Coatings Technology 32, 327–338. Toraya, H., Yoshimura, M., Somiya, S., 1984. Calibration curve for quantitative analysis of the monoclinic tetragonal ZrO2 system by X-ray diffraction. Journal of American Ceramics Society 67 (6), c119–c121. Zhu, D.M., Miller, R.A., 2000. Thermal conductivity and elastic modulus evolution of thermal barrier coatings under high heat flux. Journal of Thermal Spray Technology 9, 175–180.
journal homepage: www.elsevier.com/locate/jmatprotec
Microstructural evaluation of ZrO2 –MgO coatings A. Nusair Khan ∗ , I.N. Qureshi Metallurgy Division, Rawalpindi, Pakistan
a r t i c l e
i n f o
a b s t r a c t
Article history:
Zirconium oxide, stabilized with magnesium oxide, as a topcoat a coat and Ni–5Al as a
Received 14 June 2007
bondcoat were air plasma sprayed onto a nickel base alloy substrates. Microstructural and
Received in revised form
phase changes were observed during the thermal treatment. Formation of nickel-oxide was
18 January 2008
noticed during the experimentation. Chemical composition profile for the given system,
Accepted 12 February 2008
at high temperature, were determined and discussed in this paper. Microstructural characterization was carried out by scanning electron and optical microscopes, whereas phase analysis was carried out by X-ray diffractometer (XRD). © 2008 Elsevier B.V. All rights reserved.
Keywords: Ceramic coatings High temperature ceramics Air plasma spraying
1.
Introduction
Thermal barrier coatings (TBCs) are widely used in gas turbines for propulsion and power generation (Brindley and Miller, 1989; Bennett, 1986; Nesbitt, 1990; Zhu and Miller, 2000). The TBCs can be considered as three layer materials systems, consisting of (1) a superalloy substrate, (2) an oxidation-resistant metallic bondcoat, and (3) the ceramic top coating deposited either by plasma spraying or electron beam physical vapor deposition The zirconia topcoat has excellent thermal shock resistance, low thermal conductivity and a relatively high coefficient of thermal expansion (CTE). The bondcoats provide a rough surface for mechanical bonding of the ceramic topcoat, protect the underlying alloy substrate against high temperature oxidation corrosion, and minimize the effect of CTE mismatch between the substrate and ceramic topcoat materials. Pure zirconia transforms from tetragonal to monoclinic phase and thus volumetric changes associated with this transformation. This transformation is not desirable in the application of coating. According to Bennett (1986), it can be stabilized with either CaO (5 wt.%), MgO (15–24 wt.%), whereas
∗
Corresponding author. E-mail address: [email protected] (A.N. Khan). 0924-0136/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2008.02.032
Brandon and Taylor (1989) and Steffens and Fisher (1987) shown that it can also be stabilized with Y2 O3 (6–12 wt.%). The fully or partially stabilized zirconia is usually deposited by plasma spraying method on the metal surface. These coatings are bad heat conductors and have good thermal shock resistant properties. Among them ZrO2 –MgO are relatively cheaper than the ZrO2 –Y2 O3 that is why utilized in those regions where the temperature intensity is relatively low, e.g. in the exhaust of the jet engines. Further, MgO stabilized system can be used for the development of intermediate coating in a three part graded coating system with magnesium zirconate as a topcoat. In this study, ZrO2 –MgO system was undertaken. The microstructural and phase changes were observed after different thermal treatments.
2.
Experimental
The coating system composed of a bondcoat and a top coating was air plasma sprayed onto a nickel-base super alloy plate, having the chemical composition given in Table 1. The powder
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 9 ( 2 0 0 9 ) 488–496
Table 1 – Chemical composition of substrate metal Weight percentage Substrate
Hastelloy G-50 alloya
51.0 20.0 2.0 19.0 9.0 0.5 0.5 0.3 0.005 0.010
≥50 19–21 2.50 15–20 8–10 1.0 1.0 1.0 0.015 0.015
Ni Cr Co Fe Mo W Mn Si S C a
Nearest standard.
used for the bond coating was Ni–5Al with grain size 50–90 m. The topcoat material grain size was 20–90 m and the chemical composition was 76ZrO2 –24MgO (wt.%). A rectangular nickel base superalloy plate with thickness 5 mm was grit blasted. A 7MC Metco Sulzer Plasma gun was utilized for the deposition of both the bond and top coating. The bond coating was sprayed to a total thickness of 95 ± 10 m, whereas the top coating was sprayed to the total thickness of 180 ± 20 m. The cross section of coating system is shown in Fig. 1. During the spraying of both bond and top coatings, the substrate temperature was controlled by air jets. After thermal spraying, the specimens were cut by diamond cutter for different experimentation. The chemical composition profile was made with the help of energy dispersive spectrometer (EDS) attached with scanning electron microscope (SEM). For this purpose, the cross section of the specimens grinded and polished carefully and a gold layer was sputtered to make the specimen conductive. The phase analyses were done on the surface of the top coating and on the backside of the top coating (obtained after etching of bondcoat in 35% HNO3 solution) after exposing to high temperatures for different timings. The purpose was to examine the formation of any monoclinic phase and the analysis of the phases formed near the interface of the bondcoat and the topcoat, during the heat treatment. The diffraction measurements were performed using an X-ray diffractometer
Fig. 1 – Cross section of as-sprayed coating system with topcoat, bondcoat and substrate.
489
by utilizing the cobalt K␣ radiation. The diffraction of main peaks for the 1¯ 1 1 monoclinic phase, 1 1 1 tetragonal phase, and the 1 1 1 monoclinic phase were all found in the 2 range between 32◦ and 38◦ . The volume fraction, Vm of the monoclinic phase was calculated according to the Toraya et al. (1984) method, where Vm is: Vm =
PXm 1 + (P − 1)Xm
(1)
where the value of P was calculated by Toraya et al. (1984), for the present composition, as 1.3 and for the monoclinic–tetragonal system, Xm is the integrated intensity ratio defined by: Xm =
Im (1¯ 1 1) + Im (1 1 1) Im (1¯ 1 1) + Im (1 1 1) + IT (1 1 1)
where I is the integrated intensity from the monoclinic (m) and tetragonal (T ) phases. Samples were heat treated in tube type furnace at 750, 900 and 1000 ◦ C for 10, 30, 60 and 100 h. After heating and holding of samples at high temperature, the samples were allowed to cool down into the furnace, while the tube remained closed. This long heat treatment was accomplished in different stages. The heat treatment was carried out only during the day time for 8–10 h while during the night the furnace remained switched off.
3.
Results and discussion
3.1.
Microstructure
The cross section of the samples was observed under the SEM and optical microscope. The as-sprayed coating samples showed a typical APS microstructure, with crack network and porosity in the topcoat (of ZrO2 –24MgO). The most prominent observation for all the samples treated at high temperature was the formation of an intermediate layer at the interface of topcoat and bondcoat. The thickness of this light gray color layer varied by different exposure time and temperatures. From here-on, we call this layer between the topcoat and bondcoat as “Heat Affected layer” (HAL). The samples treated at 750 ◦ C showed that the topcoat and bondcoat remained stabled through out the treatment, however, it was observed that a HAL appeared after 30 h at this temperature (Fig. 2(b)). The thickness of this layer was about 1–2 m. No cracking or debonding observed at this temperature (Fig. 2). After 10 h of treatment at 900 ◦ C, it was observed that the HAL was about 30 m. This layer became thicker with more exposure time (Fig. 3), which was 45 and 56 m after 30 and 60 h, respectively. Further, it was observed that the vertical cracks appeared in the topcoat after 60 h (Fig. 4(b and c)). It appeared that HAL grew at the expense of bondcoat which is demonstrated in Fig. 5. In the bondcoat after long oxidation at 1000 ◦ C, two types of phases were observed (Fig. 6), i.e. light gray and dark phases richer in chromium and aluminum contents, respectively. They might be the oxides of chromium and aluminum.
490
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 9 ( 2 0 0 9 ) 488–496
Fig. 2 – Coating system treated at 750 ◦ C: (a) 10 h with no HAL and (b) 30 h with HAL appeared between topcoat and bondcoat.
Coatings treated at 1000 ◦ C also demonstrated pitted interface at bondcoat and HAL (Fig. 7). This pitted surface actually richer in aluminum and leads to the detachment of the topcoat from bondcoat. Further, at 1000 ◦ C for 60 h, it was also observed that the substrate surface closed to the bondcoat oxidized (Fig. 6).
3.2.
Chemical composition profile
The chemical composition profile for the samples treated at 1000 ◦ C for 60 h is given in Figs. 8–9. It is evident that the elements like Cr, Fe, Co and Mo depleted from the substrate closed to the bondcoat and highly increased in bondcoat. The original composition of bondcoat was Ni–5Al, which became rich in Cr up to 28%. Similarly, the presences of Fe, Co and Mo was also observed. The HAL, which is rich in nickel (>85%) and oxygen (>15%) indicating the presence of nickel-oxide with small
amounts of Fe (0.5–0.8%). The presence of Zr (2–10%) was also detected closed to the topcoat end. Similar trend of elemental distribution was observed for the samples treated at 900 ◦ C for 30 and 60 h. The EDS analysis revealed that the HAL close to the topcoat having the concentration of Zr, whereas it was riched in Ni contents close to the bondcoat (Fig. 9). In some samples, this HAL was broken from both the bondcoat interfaces (Fig. 4(a)). Further, at the interface of HAL and bondcoat revealed richer in aluminum contents (Fig. 10), this indicates the formation of the alumina at the interfaces of HAL. This oxide may induce stresses at the interface (Nusair Khan and Lu, 2003). After the thermal exposure at 1000 ◦ C for 60 h, the chemical analysis of topcoat’s from backside, i.e. the portion which touched the bondcoat is also analyzed by EDS. The chemical composition is given in Table 2, which is almost the same as observed in the cross section of the sample.
Fig. 3 – Coating system at 900 ◦ C treated for: (a) 10 h, (b) 30 h, (c) 60 h and (d) 90 h.
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Fig. 5 – Change in bondcoat and HAL thickness as a function of thermal exposure at 900 ◦ C.
3.3.
Fractrography
Further details of the microstructural features were examined through fracture surfaces of topcoat. For this purpose, the topcoat was removed by immersing the coated samples in 65% HNO3 . It was observed that the as-sprayed topcoats removed just in 24 h immersion time, whereas the samples treated at 1000 ◦ C for 60 h remained unaffected and no topcoat detached from the substrate. Thus, the samples were left in fresh solution but nothing happened even after 15 days time. It was then decided to try in boiling HNO3 and after 15 min treatment we obtained the topcoat. The removed topcoat then fractured and observed in SEM. The as-sprayed microstructure was composed of lamellae that fallow the geometry of underlying material. Each lamella has a thickness of about 3–5 m. Between the two lamellae, horizontal delaminations
Fig. 4 – Coating system treated at 1000 ◦ C treated at: (a) 30 h, (b) 30 h and (c) 60 h.
Table 2 – Chemical composition of backside of topcoat after 1000 ◦ C, 60 h Elements Ni O Al Cr Fe
wt. (%) 76 18 3 0.5 2.50
Fig. 6 – Bondcoat and substrate after oxidation at 1000 ◦ C for 60 h.
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Fig. 7 – Aluminum rich pitted surface at the interface of bondcoat and HAL. Fig. 9 – Chemical composition profile for Ni and Zr after heat treatment at 1000 ◦ C for 60 h.
Fig. 8 – Chemical composition profile for Cr, Fe, Co and Mo after heat treatment at 1000 ◦ C for 60 h.
Fig. 10 – Change in aluminum contents after 1000 ◦ C for 60 h.
were observed which refers towards the low cohesion between the splats (Fig. 11). Some cracked lamellae (Fig. 11) were also observed in the as-sprayed samples, which were believed to be cracked during the cooling of topcoat. This may be due to the tensile stresses which were developed during the cooling of the coating, and some times exceeds the yield limits of the material causing cracks. The HAL region consists of oxides which remained attached to the topcoat after chemical etching of bondcoat. These oxides were confirmed as Ni oxides (NiO) and are associated with high porosity (Fig. 12).
3.4.
Phase analysis
The room temperature equilibrium phases for 24% stabilized zirconia coating are expected to be a monoclinic phase
Fig. 11 – Fractured surface of topcoat in as-sprayed condition.
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Fig. 12 – Nickel-oxide on the backside of the topcoat observed on detached topcoat.
(m). However, the rapid cooling of the top coating material during the plasma spraying process gives a non-equilibrium tetragonal phase, denoted by T . This phase is referred to as non-transformable since it does not transform into the monoclinic phase during the rapid cooling after spraying. After heating at high temperature, during cooling to room temperature, the tetragonal phase, T, transforms to the monoclinic phase. This transformation (T to m) is associated with a volume expansion to the order of 4–5% (Bengtsson, 1995). It is believed that this volumetric expansion will cause damage to the integrity of the top coating, which could result in failure during the application of the coating. X-ray diffraction phase analysis shows the as-sprayed samples to consist almost completely of tetragonal T phase
Fig. 14 – Change in percent volume of monoclinic phase with thermal treatment.
(Fig. 13(a)). Table 3 demonstrating the obtained d-values for different conditions of samples with their corresponding searched cards. Following the quantitative phase analysis described in Section 2, the monoclinic phase fraction at different temperatures for different time is shown in Fig. 14. It was observed that the %age of monoclinic phase increases with both temperature and time (at a particular temperature). After 10 h of exposure at 750, 900 and 1000 ◦ C, the volume percent of monoclinic was 7, 20 and 38, respectively. At 900 ◦ C, where
Fig. 13 – X-ray diffraction patterns of topcoat without any thermal exposure: (a) top surface and (b) back surface.
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1000 ◦ C, 60 h
As sprayed
Top surface
Tetragonal (ZrO2 )
Monoclinic (ZrO2 )
NiO
ZrO2 –MgO
14–0534
37–1484
4–835
80–0967
Top surface
Back surface
Back surface
d
I/I0
d
I/I0
d
I/I0
d
I/I0
d
I/I0
d
I/I0
d
I/I0
d
I/I0
2.928 2.538 2.118 1.79 1.526
100 20 10 35 25
3.8625 3.0442 2.9045 2.8967 2.5034 2.4103 2.4103 2.0907 1.9450
20 100 80 90 25 10 10 10 10
3.7 3.6484 3.1703 2.9603 2.8508 2.7082 2.5985 2.55 2.4423
10 8 60 100 50 15 15 30 5
1.7848 1.6546 1.6196 1.6111 1.5605 1.5395 1.4925 1.4821 1.3249
10 10 10 8 15 50 15 15 5
2.453 2.399 2.079 1.472 1.257 1.203 1.200
5 70 100 60 35 30 25
2.949 2.584 2.542 1.804 1.551 1.535 1.474 1.291 1.27
100 20 60 100 40 80 40 40 60
3.698 3.639 3.165 2.841 2.623 2.606 2.54 2.2138 1.8481
14 10 100 68 21 11 13 12 18
2.41 2.088 1.476 1.259 1.206 1.044 0.958 0.934 0.853
91 100 57 16 13 8 7 21 17
1.7741 1.7689 1.5265 1.5173
30 30 15 25
2.263 2.118 1.9956 1.8521 1.8178 1.8034
10 30 5 15 60 50
1.2726 1.219 1.178
10 8 10
1.8187 1.8038 1.6937 1.6571
22 13 11 11
0.804
7
d 2.9266 2.9111 2.5244 1.7886 1.7815 1.5261 1.5217 1.5195 Other d-values are with less intensity
I/I0 35 100 25 30 30 10 20 15
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Table 3 – Results for as-sprayed coating along with samples treated at 1000 ◦ C for 60 h, the matching cards are also mentioned (JCPDS, 1999)
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Fig. 15 – X-ray diffraction patterns of topcoat after thermal exposure at 1000 ◦ C for 60 h: (a) top surface and (b) back surface.
the experimentation was conducted up to 100 h, in the start, i.e. up to 30 h the volume percent of monoclinic phase was up to 20–23, which gradually increases up to 37% after 60 h. The trend at this temperature seems became constant and only slight increase in monoclinic phase was observed up to 100 h of exposure. Similar trend was observed in samples treated at 1000 ◦ C, where volume percent of monoclinic phase after 30 h was 52 and almost remains constant after 60 h (Fig. 15(a)). It was observed that the samples treated at 750 ◦ C initially demonstrated small amount of monoclinic phase but then no increase in this phase was observed up to 60 h of treatment (Fig. 13). The opposite side of the topcoat (which was closed to the bondcoat) was also checked for possible phases. In case of assprayed samples, the rhombohedral magnesium–zirconium oxide peaks were observed (Table 3; Fig. 13(b)). Similarly, at 1000 ◦ C for 60 h the phase analysis of the opposite side of the topcoat revealed as nickel-oxide (NiO) (Fig. 15(b)). The intensities of the peaks with d-values for both the above phases are given in Table 3, with their corresponding matching cards. This nickel-oxide phase is believed to be of HAL, which remained stuck with the topcoat after the etching of bondcoat.
4.
Conclusions
MgO stabilized zirconia thermal barrier coatings were characterized after the thermal exposure at different temperatures and times. Microstructural and phase changes were observed in this respect. It was observed that the thickness of HAL increased with time and temperature. The HAL was identified as nickel-oxides. These oxides were found spongy in nature. The chemical composition profile revealed that at high temperature the elements from the substrate were diffused out to the bondcoat. Volume percent of the monoclinic phase was determined against the temperature. It was found that up to a certain limit, at particular temperature, the volume percent of the monoclinic phase became constant. MgO-stabilized zirconia coatings were observed stable up to 750 ◦ C and beyond this became deteriorated quickly with exposure.
references
Brindley, W.J., Miller, R.A., 1989. Thermal barrier coatings for better engine efficiency. Advance Materials Processing 136, 29.
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Bennett, A., 1986. Properties of thermal barrier coatings. Material Science and Technology 2, 257–261. Brandon, J.R., Taylor, R., 1989. Thermal properties of ceria and yttria partially stabilized zirconia thermal barrier coatings. Surface and Coatings Technology 39/40, 143–151. Bengtsson, P., 1995, Microstructural studies and residual stress evolution in zirconia based thermal barrier coatings. Thesis. Linkoping University, S-581 83 Linkoping, Sweden. JCPDS, 1999. Powder diffraction files. JCPDS-International Center for Diffraction Data, USA. Nesbitt, J.A., 1990. Thermal response of various thermal barrier coatings in a high heat flux rocket engine. Surface and Coatings Technology 43–44, 458–469.
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