- Email: [email protected]
Al2O3 multilayers
Vacuum 64 (2002) 267–273
Study of ZrO2/Al2O3 multilayers Pengtao Gaoa,*, L.J. Mengb, M.P. dos Santosa, V. Teixeiraa, M. Andritschkya b
a Departamento de Fisica, Universidade do Minho, 4710 Braga, Portugal Departamento de Fisica, Instituto Superior de Engenharia do Porto, Rua de Sa*o Tome!, 4200 Porto, Portugal
Abstract In the present work we present a study of ZrO2/Al2O3 multilayers prepared by DC reactive sputtering with different ZrO2 layer thicknesses. To test their thermal stability, the multilayers were annealed at high temperature. A possible reason for the tetragonal phase to be stabilised in the multilayers is discussed. X-ray diffraction and Raman spectroscopy have been used to characterise the structure and the residual stress of the multilayers. r 2002 Elsevier Science Ltd. All rights reserved.
1. Introduction ZrO2 and Y2O3 stabilised ZrO2 thin films have been extensively studied due to their numerous applications. These films have been used as bond layers of/and thermal barriers coating [1–5], dielectric films [6,7], buffer layer of YBa2Cu3O7x superconductive thin films [8,9], and protective coatings [10,11]. It is well known that pure ZrO2 exists in three phases at different temperatures in atmospheric pressure environment: the monoclinic phase is stable up to 11701C, the tetragonal phase is stable in the temperature range between 11701C and 23701C, and the cubic phase for 2370–26801C. When certain conditions are satisfied, the high temperature phases could transform to monoclinic phase. This phase transformation accompanies a 3–5% volume expansion [12]. This volume expansion may cause high residual stress and even lead to delamination in ZrO2 films. So, it is important *Corresponding author. Tel.: +351-53-604320; fax: +35153-678981. E-mail address: gao@fisica.uminho.pt (P. Gao).
to stabilise the high temperature phases at room temperature for many applications. There are two ways to stabilise the high temperature phases. One is doping by other metallic oxides, such as Y2O3, Al2O3, CeO2, and some rare earth oxides. The other way is to control the crystallite size of the high temperature phases. Small crystallites of tetragonal ZrO2 have lower free energy compared with that of small crystallites of monoclinic ZrO2 [13,14], which means that the tetragonal phase of ZrO2 could be stabilised if the crystallite size is less than a certain value (critical size). Critical sizes of 30 nm [13,14] in bulk ZrO2, 50 nm [15] in evaporated ZrO2 films, and 16.5 nm [16] in ZrO2 films prepared by the CVD method have been reported. We tried to use the control of the crystallite size to stabilise tetragonal ZrO2 at room temperature. ZrO2/Al2O3 nano-multilayers were deposited by DC reactive sputtering, and the thermal stability of these multilayers have been studied in the present work. We used Al2O3 in the multilayers because Al2O3 has almost twice the elastic constant as that of ZrO2 (390 vs. 207 GPa) [17].
0042-207X/02/$ - see front matter r 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 0 4 2 - 2 0 7 X ( 0 1 ) 0 0 3 1 1 - 6
268
P. Gao et al. / Vacuum 64 (2002) 267–273
This high elastic constant provides structure stability for the phase transformation.
down, to determine the structure of the films with the 2y angle in the range of 20–701 using Cu Ka (40 kV, 20 mA) radiation in steps of 0.021. The peak positions and the full width of peak at half of maximum intensity (FWHM) were obtained by fitting the measured peak with two Gaussian curves with an intensity ratio of 2 : 1 in order to find the true peak width (FWHM) and the position corresponding to monochromatic Ka1 radiation. The average crystallite dimension, D, was calculated by the formula of D=0.9 l/B cos y [18], where l is the X-ray (Ka1) wavelength and y is the Bragg diffraction angle and B is the FWHM after correction for instrument broadening. The residual stresses were calculated by E dn d0 s¼ ; ð1Þ n d0
2. Experimental details The multilayers were prepared by DC reactive sputtering. The NiCrAlY alloy substrates were placed on a rotary holder and moved sequentially to Zr (Y, 4% wt) and Al (purity of 99.9%) targets controlled by computer. The sputtering gas (Ar) and the reactive gas (O2) were then introduced into the chamber. The multilayers growth started with a ZrO2 layer and ended with an Al2O3 layer. The deposition rates of ZrO2 and Al2O3 were 3.5 and 3.4 nm/min, respectively. The substrate was not heated during the deposition. The distance between the substrates and the targets was 60 mm. The details of the deposition conditions for ZrO2 and Al2O3, and the multilayers are listed in Tables 1 and 2, respectively. To test the thermal stability of the multilayers, they were annealed at 10001C for 24 h twice, and then at 11001C for 24 h, once. It took 5 min for the temperature of the annealing furnace to increase from room temperature to the annealing temperature and the samples were taken out of the furnace to the room temperature environment for cooling
where s is the residual stress, E=270 GPa [19], ( and dn are the Young’s n=0.3 [19], d0=2.96 A modulus, Poisson ratio, d-spacing of tetragonal ZrO2 (1 1 1) plane from standard powder X-ray diffraction files and measured d-spacing of t(1 1 1) plane, respectively. Raman spectroscopy has also been used to characterise the residual stress of the multilayers. The Raman spectra of the multilayers were measured using a JOBIN-YVON triple T64000 Raman spectrometer with a Coherent Innova 92 Ar ion laser. The integrating time for the Raman spectra measurements was 10 min for 5 times repetition.
Table 1 Deposition conditions for the multilayer
O2 concentration (%) Sputtering pressure (mbar) Cathode current (A) Cathode potential (V) Base pressure (mbar)
ZrO2
Al2O3
25 5.2 103 1.89 270
35 4.9 103 1.99 220 3 106
3. Results and discussion Al2O3 mono-film has an amorphous structure that can be observed by X-ray diffraction. Fig. 1
Table 2 Layer number and layer thickness of the samples T1
T2
T3
T4
T5
T6
T7
T8
T9
Structure
Al2O3
ZrO2
Al2O3/ZrO2
Al2O3/ZrO2
Al2O3/ZrO2
Al2O3/ZrO2
Al2O3/ZrO2
Al2O3/ZrO2
Al2O3/ZrO2
Thickness (nm)
210
200
4/4
8/4
8/8
8/12
8/20
8/30
8/40
Number of layer
1
1
250/250
250/250
150/150
150/150
120/120
88/88
70/70
269
P. Gao et al. / Vacuum 64 (2002) 267–273
shows the X-ray diffraction patterns of the ZrO2/ Al2O3 multilayers with different ZrO2 layer thicknesses. The ZrO2 mono-film has a monoclinic phase structure. The peak at about 28.11 is attributed to the diffraction of the monoclinic phase (1 1 1) plane. The spectrum of the multilayers is absolutely different from that of the mono-ZrO2 film. Only the tetragonal phase can be found for the multilayers with ZrO2 layer thicknesses less than 20 nm. There is only one peak of tetragonal (1 1 1) for the sample with the ZrO2 layer thicknesses at 4 nm, which means that the ZrO2 growth has a preferred orientation along this plane for thicknesses less than 4 nm. The other samples have a random orientation. When the ZrO2 layer thickness is higher than 30 nm, a peak at about 28.11, which is attributed to a monoclinic phase (1 1 1) plane, arises and its intensity increases with the increase of the ZrO2 layer thickness. The intensity ratio of m(1 1 1) and t(1 1 1) also increases with the increase of ZrO2 layer thickness. The X-ray diffraction peak intensity is proportional to the mole fraction of the corresponding phase. Therefore, the increase of
Fig. 1. X-ray diffraction patterns of the ZrO2/Al2O3 multilayers.
this intensity ratio means that the fraction of monoclinic phase in the films increases with the increase of ZrO2 layer thickness. From the discussion above, it can be concluded that the tetragonal phase transformed to monoclinic phase during the film deposition for the samples with ZrO2 layer thickness above 30 nm. This phase transformation can be verified by the residual stress of the multilayers after deposition, which are shown in Table 3 and indicates that the residual stress of the two samples with monoclinic phase is higher than that of the others. As mentioned above, the phase transformation from tetragonal phase to monoclinic phase involves a volume expansion and this could lead to high stress in the films. Therefore, the high residual stress in samples with monoclinic phase indicates that the phase transformation occurred during the multilayers deposition. By fitting the measured X-ray diffraction peak, the crystallite size of the multilayers has been calculated. Fig. 2 shows the variation of the crystallite size of the multilayers with the variation of the ZrO2 layer thickness. It is found that the crystallite increases from 2 to 4.4 nm with the increase of ZrO2 layer thickness from 4 to 8 nm, and then no increase can be found with further increase of the ZrO2 layer thickness. So, the critical size of the tetragonal ZrO2 is around 4.5 nm in our films. This value is higher than those reported by Aita et al. [19], and lower than those mentioned above. About the thermal stability of the multilayers, Fig. 3 shows the X-ray diffraction patterns of the multilayers after annealing. It can be observed that the monoclinic phase transformed to tetragonal phase in the two samples with ZrO2 layer thickness of 30 and 40 nm [see Fig. 3(a), the diffraction peak (at about 281) attributed to monoclinic phase disappeared after annealing], but the tetragonal phase transformed to monoclinic phase again after
Table 3 Residual stress of samples after deposition Sample number
T3
T4
T5
T6
T7
T8
T9
Stress calculated by X-ray diffraction (GPa)
0.51
0.23
2.53
1.83
1.66
4.33
3.96
270
P. Gao et al. / Vacuum 64 (2002) 267–273
Fig. 2. The variation of the crystallite size of the multilayers with the ZrO2 layer thickness.
the third cycle of annealing in the sample with a ZrO2 layer of thickness 40 nm [see Fig. 3(b), the diffraction peak attributed to monoclinic phase reappeared]. During the first cycle of annealing, the monoclinic phase transformed to tetragonal phase in the high temperature environment, then the metastable frozen tetragonal phase was formed during the rapid cooling down process. During the three cycles of annealing, the crystallites of tetragonal ZrO2 develop. Table 4 shows the average crystallite size of the multilayers before and after annealing. The crystallite size increases after annealing, and when the crystallite size reaches a certain value, the free energy of the tetragonal phase is higher than that of the monoclinic phase, therefore the phase transformation of tetragonal to monoclinic occurs. In the multilayers with the ZrO2 thickness less than 30 nm the Al2O3 can provide enough elastic energy to constrain the tetragonal phase at room temperature. However, for the sample with the ZrO2 layer thickness of 40 nm, the fraction of Al2O3 in the multilayer is lower than that of the others, and not enough elastic energy can be provided, and then the back phase transformation occurs for this sample. From the above discussion, it can be concluded that the tetragonal phase can be satabilised in the multilayers with ZrO2 layer thickness less than 30 nm after annealing. The average crystallite size calculated from X-ray diffraction is the crystallite dimension perpendicular to the substrate. Therefore, the crystallite size
Fig. 3. X-ray diffraction patterns of the multilayers after annealing (aF10001C, 24 h, twice; bFthird time, at 11001C, 24 h).
should not be bigger than the dimension of the ZrO2 layer thickness. However, from Table 4, it can be found the average crystallite size of some samples is bigger than the dimension of the corresponding thickness of ZrO2. Schoofield et al. [20] reported the HRTEM image of ZrO2/Al2O3 multilayer, and found that the layer surface becomes rough and loses definition between the layers after more than 15 layers from the substrate, which is also our case. Then, the multilayer becomes a mixture film of ZrO2 and Al2O3 in some microregion. Thus, the ZrO2 crystallite can propagate through the Al2O3 layers. Therefore, the average crystallite size can be bigger than the corresponding ZrO2 layer thickness. Fig. 4 shows micrographies of the multilayers with different ZrO2 thickness. It can be seen that
271
P. Gao et al. / Vacuum 64 (2002) 267–273 Table 4 Average crystallite size of ZrO2 in the multilayer before and after annealing Sample number
T3
T4
T5
T6
T7
T8
T9
Thickness of ZrO2 layer (nm)
4
4
8
12
20
30
40
Crystallite size (before annealing) (nm)
2
2.3
4.4
4.2
Crystallite size (first cycle) (nm)
4.5
5.9
6.1
10.2
Crystallite size (second cycle) (nm)
5.6
6.9
6.8
Crystallite size (third cycle) (nm)
9.5
9.6
9.7
4.5
5.1
4.5
14
15.2
22.2
12
15.2
16
24.6
21.1
25.5
20.8
30.2
Fig. 4. Typical microscopy photographs of the multilayers after annealing.
the multilayers with the ZrO2 layer thickness between 8 and 20 nm showed excellent thermal stability after annealing. No cracks and delamination can be found from the surface of these multilayers. These multilayers can be used as bond layers of thermal barrier coatings. The Raman spectra of the multilayers have been measured before and after annealing. Fig. 5 shows the Raman spectra of one of our samples before and after annealing and the variation of the
Raman peak position with the variation of the residual stress. It can be shown that the Raman peaks shift to lower wavenumbers with the increase of compressive stress. Similar results have been obtained from the other samples. Due to these results, several Raman spectra around a crack on this sample have been measured. Fig. 6 shows the Raman topography at different measurement positions around the microcrack, indicating that the Raman peaks shift to higher
272
P. Gao et al. / Vacuum 64 (2002) 267–273
wavenumbers with the variation of the measurement position from the edge of the crack to the nodelamination area. This means that the residual stress decreases from the no-delamination area to the edge of the crack due to the stress relief at the edge of the crack after cracking. This allows characterisation of the local stress of ZrO2 by Raman spectroscopy.
4. Conclusions Fig. 5. Raman spectra of one sample (T8) before and after annealing (aFbefore annealing; bFafter the first cycle of annealing; cFafter the second cycle of annealing; dFafter the third cycle of annealing).
The tetragonal phase of ZrO2 can be stabilised in the multilayers with the ZrO2 layer thickness less than 20 nm during the films deposition, even after high temperature annealing. The multilayers with ZrO2 layer thicknesses between 8 and 20 nm showed excellent thermal stability after annealing and these multilayers can be used as bond layers of thermal barrier coatings. It is also concluded that the Raman peak position shifts to lower wavenumbers with the increase of compressive stress. This provides the possibility to characterise the local residual stress of ZrO2 films by Raman spectroscopy.
Acknowledgements Pengtao Gao is grateful to the Orient Foundation for providing a scholarship, and also wishes to thank A. Azevedo and Fernanda Guimar*aes, from the University of Minho for the XRD and SEM measurements, respectively.
References
Fig. 6. The Raman topography at different positions across the micro-crack (T8).
[1] Teixeira V, Andritschky M. High Temp High Pressures 1993;25:213. [2] Teixeira V, Andritschky M, Fischer W, Buchkremer HP, Stover D. J Mater Process Technol 1999;92/93:209. [3] Teixeira V, Andritschky M, Fischer W, Buchkremer HP, Stover D. Surf Coatings Technol 1999;120:103. [4] Chen HC, Pfender E, Heberlein J. Thin Solid Films 1998;315:159. [5] Schulz U, Fritscher K, Peters M. Surf Coatings Technol 1996;82:259. [6] Tan GL, Wu XJ. Thin Solid Films 1998;330:59.
P. Gao et al. / Vacuum 64 (2002) 267–273 [7] Horita S, Watanabe M, Masuda A. Mater Sci Eng B 1998;54:79. [8] Kim ET, Yoon SG. Thin Solid Films 1993;277:7. [9] Aguiar R, Sanchez F, Ferrater C, Varela M. Thin Solid Films 1997;306:74. [10] Hasuyama H, Shima Y. Nucl Instrum Methods B 1997;127:827. [11] Maggio RD, Fedrizzi L, Rossi S, Scardi P. Thin Solid Films 1996;286:127. [12] Stevens R, Zirconia and zirconia ceramics. 2nd ed. Magnesium Elektron publication No. 113, Leeds: Leeds University, 1986. p. 13. [13] Garvie RC. J Phys Chem 1965;69:1238.
273
[14] Garvie RC. J Phys Chem 1978;82:218. [15] El-Shanahoury IA, Rudenko VA, Ibbrahim IA. J Am Ceram Soc 1970;53:264. [16] Kim JS, Marzouk HA, Reucroft PJ. Thin Solid Films 1995;254:33. [17] Scanlan CM, Gaijdardziska-Josifovska M, Aita CR. Appl Phys Lett 1994;64:3548. [18] Cullity BD. Elements of X-ray diffraction. 2nd ed. Reading, MA: Addsion-Wesley, 1978. [19] Aita CR, Wiggins MD, Whig R, Scanlan CM. J Appl Phys 1996;79:1176. [20] Schofield MA, Aita CR, Rice PM, GaijdardziskaJosifovska M. Thin Solid Films 1998;326:117.
Study of ZrO2/Al2O3 multilayers Pengtao Gaoa,*, L.J. Mengb, M.P. dos Santosa, V. Teixeiraa, M. Andritschkya b
a Departamento de Fisica, Universidade do Minho, 4710 Braga, Portugal Departamento de Fisica, Instituto Superior de Engenharia do Porto, Rua de Sa*o Tome!, 4200 Porto, Portugal
Abstract In the present work we present a study of ZrO2/Al2O3 multilayers prepared by DC reactive sputtering with different ZrO2 layer thicknesses. To test their thermal stability, the multilayers were annealed at high temperature. A possible reason for the tetragonal phase to be stabilised in the multilayers is discussed. X-ray diffraction and Raman spectroscopy have been used to characterise the structure and the residual stress of the multilayers. r 2002 Elsevier Science Ltd. All rights reserved.
1. Introduction ZrO2 and Y2O3 stabilised ZrO2 thin films have been extensively studied due to their numerous applications. These films have been used as bond layers of/and thermal barriers coating [1–5], dielectric films [6,7], buffer layer of YBa2Cu3O7x superconductive thin films [8,9], and protective coatings [10,11]. It is well known that pure ZrO2 exists in three phases at different temperatures in atmospheric pressure environment: the monoclinic phase is stable up to 11701C, the tetragonal phase is stable in the temperature range between 11701C and 23701C, and the cubic phase for 2370–26801C. When certain conditions are satisfied, the high temperature phases could transform to monoclinic phase. This phase transformation accompanies a 3–5% volume expansion [12]. This volume expansion may cause high residual stress and even lead to delamination in ZrO2 films. So, it is important *Corresponding author. Tel.: +351-53-604320; fax: +35153-678981. E-mail address: gao@fisica.uminho.pt (P. Gao).
to stabilise the high temperature phases at room temperature for many applications. There are two ways to stabilise the high temperature phases. One is doping by other metallic oxides, such as Y2O3, Al2O3, CeO2, and some rare earth oxides. The other way is to control the crystallite size of the high temperature phases. Small crystallites of tetragonal ZrO2 have lower free energy compared with that of small crystallites of monoclinic ZrO2 [13,14], which means that the tetragonal phase of ZrO2 could be stabilised if the crystallite size is less than a certain value (critical size). Critical sizes of 30 nm [13,14] in bulk ZrO2, 50 nm [15] in evaporated ZrO2 films, and 16.5 nm [16] in ZrO2 films prepared by the CVD method have been reported. We tried to use the control of the crystallite size to stabilise tetragonal ZrO2 at room temperature. ZrO2/Al2O3 nano-multilayers were deposited by DC reactive sputtering, and the thermal stability of these multilayers have been studied in the present work. We used Al2O3 in the multilayers because Al2O3 has almost twice the elastic constant as that of ZrO2 (390 vs. 207 GPa) [17].
0042-207X/02/$ - see front matter r 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 0 4 2 - 2 0 7 X ( 0 1 ) 0 0 3 1 1 - 6
268
P. Gao et al. / Vacuum 64 (2002) 267–273
This high elastic constant provides structure stability for the phase transformation.
down, to determine the structure of the films with the 2y angle in the range of 20–701 using Cu Ka (40 kV, 20 mA) radiation in steps of 0.021. The peak positions and the full width of peak at half of maximum intensity (FWHM) were obtained by fitting the measured peak with two Gaussian curves with an intensity ratio of 2 : 1 in order to find the true peak width (FWHM) and the position corresponding to monochromatic Ka1 radiation. The average crystallite dimension, D, was calculated by the formula of D=0.9 l/B cos y [18], where l is the X-ray (Ka1) wavelength and y is the Bragg diffraction angle and B is the FWHM after correction for instrument broadening. The residual stresses were calculated by E dn d0 s¼ ; ð1Þ n d0
2. Experimental details The multilayers were prepared by DC reactive sputtering. The NiCrAlY alloy substrates were placed on a rotary holder and moved sequentially to Zr (Y, 4% wt) and Al (purity of 99.9%) targets controlled by computer. The sputtering gas (Ar) and the reactive gas (O2) were then introduced into the chamber. The multilayers growth started with a ZrO2 layer and ended with an Al2O3 layer. The deposition rates of ZrO2 and Al2O3 were 3.5 and 3.4 nm/min, respectively. The substrate was not heated during the deposition. The distance between the substrates and the targets was 60 mm. The details of the deposition conditions for ZrO2 and Al2O3, and the multilayers are listed in Tables 1 and 2, respectively. To test the thermal stability of the multilayers, they were annealed at 10001C for 24 h twice, and then at 11001C for 24 h, once. It took 5 min for the temperature of the annealing furnace to increase from room temperature to the annealing temperature and the samples were taken out of the furnace to the room temperature environment for cooling
where s is the residual stress, E=270 GPa [19], ( and dn are the Young’s n=0.3 [19], d0=2.96 A modulus, Poisson ratio, d-spacing of tetragonal ZrO2 (1 1 1) plane from standard powder X-ray diffraction files and measured d-spacing of t(1 1 1) plane, respectively. Raman spectroscopy has also been used to characterise the residual stress of the multilayers. The Raman spectra of the multilayers were measured using a JOBIN-YVON triple T64000 Raman spectrometer with a Coherent Innova 92 Ar ion laser. The integrating time for the Raman spectra measurements was 10 min for 5 times repetition.
Table 1 Deposition conditions for the multilayer
O2 concentration (%) Sputtering pressure (mbar) Cathode current (A) Cathode potential (V) Base pressure (mbar)
ZrO2
Al2O3
25 5.2 103 1.89 270
35 4.9 103 1.99 220 3 106
3. Results and discussion Al2O3 mono-film has an amorphous structure that can be observed by X-ray diffraction. Fig. 1
Table 2 Layer number and layer thickness of the samples T1
T2
T3
T4
T5
T6
T7
T8
T9
Structure
Al2O3
ZrO2
Al2O3/ZrO2
Al2O3/ZrO2
Al2O3/ZrO2
Al2O3/ZrO2
Al2O3/ZrO2
Al2O3/ZrO2
Al2O3/ZrO2
Thickness (nm)
210
200
4/4
8/4
8/8
8/12
8/20
8/30
8/40
Number of layer
1
1
250/250
250/250
150/150
150/150
120/120
88/88
70/70
269
P. Gao et al. / Vacuum 64 (2002) 267–273
shows the X-ray diffraction patterns of the ZrO2/ Al2O3 multilayers with different ZrO2 layer thicknesses. The ZrO2 mono-film has a monoclinic phase structure. The peak at about 28.11 is attributed to the diffraction of the monoclinic phase (1 1 1) plane. The spectrum of the multilayers is absolutely different from that of the mono-ZrO2 film. Only the tetragonal phase can be found for the multilayers with ZrO2 layer thicknesses less than 20 nm. There is only one peak of tetragonal (1 1 1) for the sample with the ZrO2 layer thicknesses at 4 nm, which means that the ZrO2 growth has a preferred orientation along this plane for thicknesses less than 4 nm. The other samples have a random orientation. When the ZrO2 layer thickness is higher than 30 nm, a peak at about 28.11, which is attributed to a monoclinic phase (1 1 1) plane, arises and its intensity increases with the increase of the ZrO2 layer thickness. The intensity ratio of m(1 1 1) and t(1 1 1) also increases with the increase of ZrO2 layer thickness. The X-ray diffraction peak intensity is proportional to the mole fraction of the corresponding phase. Therefore, the increase of
Fig. 1. X-ray diffraction patterns of the ZrO2/Al2O3 multilayers.
this intensity ratio means that the fraction of monoclinic phase in the films increases with the increase of ZrO2 layer thickness. From the discussion above, it can be concluded that the tetragonal phase transformed to monoclinic phase during the film deposition for the samples with ZrO2 layer thickness above 30 nm. This phase transformation can be verified by the residual stress of the multilayers after deposition, which are shown in Table 3 and indicates that the residual stress of the two samples with monoclinic phase is higher than that of the others. As mentioned above, the phase transformation from tetragonal phase to monoclinic phase involves a volume expansion and this could lead to high stress in the films. Therefore, the high residual stress in samples with monoclinic phase indicates that the phase transformation occurred during the multilayers deposition. By fitting the measured X-ray diffraction peak, the crystallite size of the multilayers has been calculated. Fig. 2 shows the variation of the crystallite size of the multilayers with the variation of the ZrO2 layer thickness. It is found that the crystallite increases from 2 to 4.4 nm with the increase of ZrO2 layer thickness from 4 to 8 nm, and then no increase can be found with further increase of the ZrO2 layer thickness. So, the critical size of the tetragonal ZrO2 is around 4.5 nm in our films. This value is higher than those reported by Aita et al. [19], and lower than those mentioned above. About the thermal stability of the multilayers, Fig. 3 shows the X-ray diffraction patterns of the multilayers after annealing. It can be observed that the monoclinic phase transformed to tetragonal phase in the two samples with ZrO2 layer thickness of 30 and 40 nm [see Fig. 3(a), the diffraction peak (at about 281) attributed to monoclinic phase disappeared after annealing], but the tetragonal phase transformed to monoclinic phase again after
Table 3 Residual stress of samples after deposition Sample number
T3
T4
T5
T6
T7
T8
T9
Stress calculated by X-ray diffraction (GPa)
0.51
0.23
2.53
1.83
1.66
4.33
3.96
270
P. Gao et al. / Vacuum 64 (2002) 267–273
Fig. 2. The variation of the crystallite size of the multilayers with the ZrO2 layer thickness.
the third cycle of annealing in the sample with a ZrO2 layer of thickness 40 nm [see Fig. 3(b), the diffraction peak attributed to monoclinic phase reappeared]. During the first cycle of annealing, the monoclinic phase transformed to tetragonal phase in the high temperature environment, then the metastable frozen tetragonal phase was formed during the rapid cooling down process. During the three cycles of annealing, the crystallites of tetragonal ZrO2 develop. Table 4 shows the average crystallite size of the multilayers before and after annealing. The crystallite size increases after annealing, and when the crystallite size reaches a certain value, the free energy of the tetragonal phase is higher than that of the monoclinic phase, therefore the phase transformation of tetragonal to monoclinic occurs. In the multilayers with the ZrO2 thickness less than 30 nm the Al2O3 can provide enough elastic energy to constrain the tetragonal phase at room temperature. However, for the sample with the ZrO2 layer thickness of 40 nm, the fraction of Al2O3 in the multilayer is lower than that of the others, and not enough elastic energy can be provided, and then the back phase transformation occurs for this sample. From the above discussion, it can be concluded that the tetragonal phase can be satabilised in the multilayers with ZrO2 layer thickness less than 30 nm after annealing. The average crystallite size calculated from X-ray diffraction is the crystallite dimension perpendicular to the substrate. Therefore, the crystallite size
Fig. 3. X-ray diffraction patterns of the multilayers after annealing (aF10001C, 24 h, twice; bFthird time, at 11001C, 24 h).
should not be bigger than the dimension of the ZrO2 layer thickness. However, from Table 4, it can be found the average crystallite size of some samples is bigger than the dimension of the corresponding thickness of ZrO2. Schoofield et al. [20] reported the HRTEM image of ZrO2/Al2O3 multilayer, and found that the layer surface becomes rough and loses definition between the layers after more than 15 layers from the substrate, which is also our case. Then, the multilayer becomes a mixture film of ZrO2 and Al2O3 in some microregion. Thus, the ZrO2 crystallite can propagate through the Al2O3 layers. Therefore, the average crystallite size can be bigger than the corresponding ZrO2 layer thickness. Fig. 4 shows micrographies of the multilayers with different ZrO2 thickness. It can be seen that
271
P. Gao et al. / Vacuum 64 (2002) 267–273 Table 4 Average crystallite size of ZrO2 in the multilayer before and after annealing Sample number
T3
T4
T5
T6
T7
T8
T9
Thickness of ZrO2 layer (nm)
4
4
8
12
20
30
40
Crystallite size (before annealing) (nm)
2
2.3
4.4
4.2
Crystallite size (first cycle) (nm)
4.5
5.9
6.1
10.2
Crystallite size (second cycle) (nm)
5.6
6.9
6.8
Crystallite size (third cycle) (nm)
9.5
9.6
9.7
4.5
5.1
4.5
14
15.2
22.2
12
15.2
16
24.6
21.1
25.5
20.8
30.2
Fig. 4. Typical microscopy photographs of the multilayers after annealing.
the multilayers with the ZrO2 layer thickness between 8 and 20 nm showed excellent thermal stability after annealing. No cracks and delamination can be found from the surface of these multilayers. These multilayers can be used as bond layers of thermal barrier coatings. The Raman spectra of the multilayers have been measured before and after annealing. Fig. 5 shows the Raman spectra of one of our samples before and after annealing and the variation of the
Raman peak position with the variation of the residual stress. It can be shown that the Raman peaks shift to lower wavenumbers with the increase of compressive stress. Similar results have been obtained from the other samples. Due to these results, several Raman spectra around a crack on this sample have been measured. Fig. 6 shows the Raman topography at different measurement positions around the microcrack, indicating that the Raman peaks shift to higher
272
P. Gao et al. / Vacuum 64 (2002) 267–273
wavenumbers with the variation of the measurement position from the edge of the crack to the nodelamination area. This means that the residual stress decreases from the no-delamination area to the edge of the crack due to the stress relief at the edge of the crack after cracking. This allows characterisation of the local stress of ZrO2 by Raman spectroscopy.
4. Conclusions Fig. 5. Raman spectra of one sample (T8) before and after annealing (aFbefore annealing; bFafter the first cycle of annealing; cFafter the second cycle of annealing; dFafter the third cycle of annealing).
The tetragonal phase of ZrO2 can be stabilised in the multilayers with the ZrO2 layer thickness less than 20 nm during the films deposition, even after high temperature annealing. The multilayers with ZrO2 layer thicknesses between 8 and 20 nm showed excellent thermal stability after annealing and these multilayers can be used as bond layers of thermal barrier coatings. It is also concluded that the Raman peak position shifts to lower wavenumbers with the increase of compressive stress. This provides the possibility to characterise the local residual stress of ZrO2 films by Raman spectroscopy.
Acknowledgements Pengtao Gao is grateful to the Orient Foundation for providing a scholarship, and also wishes to thank A. Azevedo and Fernanda Guimar*aes, from the University of Minho for the XRD and SEM measurements, respectively.
References
Fig. 6. The Raman topography at different positions across the micro-crack (T8).
[1] Teixeira V, Andritschky M. High Temp High Pressures 1993;25:213. [2] Teixeira V, Andritschky M, Fischer W, Buchkremer HP, Stover D. J Mater Process Technol 1999;92/93:209. [3] Teixeira V, Andritschky M, Fischer W, Buchkremer HP, Stover D. Surf Coatings Technol 1999;120:103. [4] Chen HC, Pfender E, Heberlein J. Thin Solid Films 1998;315:159. [5] Schulz U, Fritscher K, Peters M. Surf Coatings Technol 1996;82:259. [6] Tan GL, Wu XJ. Thin Solid Films 1998;330:59.
P. Gao et al. / Vacuum 64 (2002) 267–273 [7] Horita S, Watanabe M, Masuda A. Mater Sci Eng B 1998;54:79. [8] Kim ET, Yoon SG. Thin Solid Films 1993;277:7. [9] Aguiar R, Sanchez F, Ferrater C, Varela M. Thin Solid Films 1997;306:74. [10] Hasuyama H, Shima Y. Nucl Instrum Methods B 1997;127:827. [11] Maggio RD, Fedrizzi L, Rossi S, Scardi P. Thin Solid Films 1996;286:127. [12] Stevens R, Zirconia and zirconia ceramics. 2nd ed. Magnesium Elektron publication No. 113, Leeds: Leeds University, 1986. p. 13. [13] Garvie RC. J Phys Chem 1965;69:1238.
273
[14] Garvie RC. J Phys Chem 1978;82:218. [15] El-Shanahoury IA, Rudenko VA, Ibbrahim IA. J Am Ceram Soc 1970;53:264. [16] Kim JS, Marzouk HA, Reucroft PJ. Thin Solid Films 1995;254:33. [17] Scanlan CM, Gaijdardziska-Josifovska M, Aita CR. Appl Phys Lett 1994;64:3548. [18] Cullity BD. Elements of X-ray diffraction. 2nd ed. Reading, MA: Addsion-Wesley, 1978. [19] Aita CR, Wiggins MD, Whig R, Scanlan CM. J Appl Phys 1996;79:1176. [20] Schofield MA, Aita CR, Rice PM, GaijdardziskaJosifovska M. Thin Solid Films 1998;326:117.