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Nanostructured Lanthanum Zirconate Coating and Its Thermal Stability Properties
Proceedings of Sino-Swedish Structural Materials Symposium 2007
Nanostructured Lanthanum Zirconate Coating and Its Thermal Stability Properties ZHAO Xiao-dong*, ZENG Ke-li, XIE Jim-gang, LI Zhen-duo (Beijing General Research Institute of Mining and Metallurgy, Beijing 100044,China)
Abstract: In this paper, nano-La2Zr2@ thermal barrier coatings (MCrAIY+ La2Zr207) were prepared by atmospheric plasma sprayed (APS).The microstructures and thermal stability properties were systematically studied to compare with their conventional counterparts by Scanning Electric Microscopy (SEM), transmission electron microscope (TEM) and X-Ray diffraction(XRD). The results showed that the nanostructured lanthanum zirconate coatings were typical lamellar structure which was composed of columnar grains about 90nm in diameter. A large quantity of micro-cracks and homogeneous distributed fine pores formed in the nanostructured zirconia coating. After ablation at 1300°C for 24 h, no apparent phase transformation was observed in lanthanum zirconate coating. The growth mechanism of the grains was subsequently discussed.
Key word: plasma spraying ;Lanthanum zirconate coating ;nanostructured coating ;thermal stability property
1 Introduction
2 Experimental Procedures
Thermal barrier coatings (TBCs) are well used to protect high-temperature metallic components. Conventional TBCs, yttria partial stabilized zirconia (Y-PSZ), are limited by operation temperature (4200 "C ), because of phase transformation from the t'-tetragonal to cubic and then to monoclinic (m). For this purpose we are developing a new series of thermal barrier coatings that consist of a MCrAlY bond coat and lanthanum zirconate top coat. It is reported that lanthanum zirconate coatings were founded as lower thermal conductivity, better chemical and thermal stability in much higher temperature"].
2.1 Preparation of nano-coatings The lanthanum zirconate powder was synthesized by the sol-gel methodL2'with the original grain size of about 40-80nm. Then the powders were agglomerated into bigger powders between 45 and 80pm in diameter as shown in Fig.la, which was used for APS, Fig.1b gives the conventional lanthanum zirconate powder produced by Sub-institute of Metal Materials of Beijing General Rearch Institute of Iining and Metallurgy(BGlUMM- SIMM), and its average size is about 70pm. The coatings were made by plasma spray systems
a) Nano- La2Zr2O7 powders
b) Conventional La2Zr207powders
Fig.1 Morphologies of powder for APS 147
Proceedines of Sino-Swedish Structural Materials SvmDosium 2007
(Metco.7M. Metco. Inc Westbuny.L.L. NEW YORK) on a superalloy with MCrAlY (NO. KF-330, by
parameters are listed in Tablel. 2.2 MicrostructuralAnalysis
BGRIMM-SIMIM) bond coat and lanthanum zirconate top coatings. The substrate was nickel based superalloy,
The coatings were first incised into cross section, then polished by sand paper in 400 , 600# , 800 # made of Sic. To characterize the changes in
and the ceramic top coatings were deposited onto bond coating to a thickness of approximately 300pm. The
microstructure during the hot ablation tests, polished
Table1 Parameters of plasma spraying for lanthanum zirconate TBCs Parameters
Bond coat ( MCrAlY )
Top coat (nano -La2Zr207)
cariier gas flow ( l/h)
48
45
Primary gas flow ( vh)
1600
1600
Second gas flow ( Ih)
3000
3Ooo
Spray distance ( mm)
70
60
Voltage ( V )
80
80
Current ( A )
520
550
Thickness ( p m )
100
300
cross section of resin impregnated samples were
h. The sample with dimension of 40mm x 40mm x
observed by scanning electron microscopy (SEM: HJTACHl S-3500,Japan). The phases present in the various samples were identified by x-ray diffraction XRD using Cu Ka radiation, a step size of 0.1 deg, and
2mm was prepared.
3 Results and Discussion 3.1 Micrstructuredof nano-coatings Fig.2 shows microstructure of the cross-section of the coatings. Fig.2a and Fig.2b show the microstructure of the cross-section of lanthanum zirconate conventional coating and lanthanum zirconate Nano-Coatings. Both coatings have a porous microstructure, which was common for the plasma-sprayed coatings. The interface between bond coat and substrate alloy was compact and no more cracks and pores were observed. The substrate is nickel based superalloy and the bond coat is MCrAlY (NO.
a 2 theta scan window from 10" to 90".The particle size and morphology were observed by transmission electron microscope (TEM: HITACHI HT-80, Japan), with accelerate voltage of 75 200KV and the multiple of 1 100.
-
-
2.3 Experiment of Ablation In order to determine the stability of lanthanum zirconate coatings in higher temperatures, the ablation experiment was carried out at l3OO"C in muffle for 24
(a) Conventional La2Zr207coatings; (b) Nano- La2Zr207coatings
Fig.2 Microstructure of cross-section of coatings
148
Proceedings of Sino-Swedish Structural Materials Symposium 2007
KF-330, by BGRIMM-SIMM),so they are similar in physical performance. The typical lamina structures of nano-lanthanum zirconate coatings were shown in Fig.3(a) and Fig.3(b). In a high magnification image, Fig.3(a) showed that more detach of the columnar structure of the splats were observable, indicating a respective nucleation of columns during deposition. Fig.3(b) showed the fine
and of homogeneous pores in the microstructure with a large quantity of small micro-cracks. Fig.3(c) and Fig.3(d) gives the morphologies in TEM of the TBCs. A majority of grain size was no more than lOOnm with a little grain size between 90 and 180nm. The Debverind of electron diffraction can also be found. Fig.3(d) shows the columnar crystal is 60- lOOnm in diameter and about 500nm in length.
a), b) Fracture and cross-section of Nano- La2Zr207coatings; c)
. d)TEM of Nano- La2Zr207coatings
Fig.3. Morphologies of TBCs:
zirconate coating was still stable without phase transformation. But the diffraction peaks after ablation becomes sharper, which can be explained by the grown up of grain size, and the size which was shown in Fig. 4 is still in the nanometer scale. Nano-lanthanum zirconate coatings show very better thermal stability than Conventional Laz&@ coatings. It has been proved by many s c h ~ l a r s [that ~-~~ some of nano-materials, especially nano-ceramic, could prevent the grown up of grains in high temperature, namely, “Fermi Level Pinning”, and they could restrain nano-grain from growing up. The
3.2 Thermal stability of nano-coatings Fig4 shows the picture of Nano-Coatings after ablation in 1300 “C for 24h with crystal size is 80-150nm, we found that the grains had grown up in diameter, which size was a little larger than those before ablation. This growth trend just can also be testified by the calculation of shalle formula as shown in Table 2. Fig.5 shows the XRD results of the Nano-Coatings of lanthanum zirconate before and after ablation. They are both typical pure pyrochlore structure compare with PDF (NO.: 71-2363). The result revealed that nanostructured lanthanum 149
Proceedingsof Sino-Swedish StructuralMaterials Symposium 2007
phenomenon that the nano-lanthanum zirconate grain in coatings only grow up in the nanometer scale could not be explained by the conventional “Theoretical of
the surface energy” and “the principle of enegy minimum” [&*I. The mechanism that nano-lanthanum zirconate grain only grow up in same range is required
2Theta
(a)=-plate
of Nan& LazZrz07before ablation; ( b ) =-plate
of Nano- LazZr2& after ablation
Fig. 5 XRD of Nan* LazZr2@ TBCs
’hble2
S i z eof nano-TBCs before and after ablation
Coatings
2Theta
d-value
FWHM
Before blation
28.96 1
3.0805
0.592
20.72
After ablation
27.847
3.201 1
0.449
32.53
4 Conclusions
Grain size(nm)
lanthanum zirconate coating was still stable without phase transformation. The detailed mechanism of the grown-up of nano-lanthanum zirconate was required to
In this paper, nano- lanthanum zirconate thermal barrier coatings( MCrAlY+ La2Zr207 1was prepared by APS. The microstructures were columnar microstructure and the columnar crystal size was 60-100nm in diameter and about 5OOnm in length. After ablation at 1300 for 24h, nanostructured
be studied. References: [ 11 X.Q. Cao, R.Vassen, W.Fischer W, et al.Lanthanum-cerium
150
Proceedings of Sino-Swedish Structural Materials Symposium 2007 oxide
as
a
thermal
barrier-coating material
Materials Science and Engineering: 2004 ,
for
248-253.
high-temperatureapplications [J]. Adv. Mater., 2003,15 (17):
[6]Wang
1438-1442. [2] K.Koteswara Rao,
366(2) :
lan ,
etal.Heat
capacity
enhancement and
thermodynamic properties of nanostructured amorphous
Taqveem Banu, M.Vitha1 et al.
Preparation and characterization of bulk and nano particles
Si&[J]. Journal of Non-Crystalline Solids , 2001 ,
of La2Zr207and La2Zr207by sol-gel method [J]. Materials
2%(1-2): 139-142. [7] Delgado-ArellanoV.G , Espitia-CabreraM.L, Reyes-Gasga
Letters, 2002 (54) :205-210.
J.
[3] C.H.Moelle, H.J.Fecht. Thermal stability of nanocrystalline
Structural study
of
zirconia
nanoclusters by
iron prepared by mechanical attrition [J]. Nanostructured
high-resolution transmission electron microscopy [J].
Materials, 1995(6): 421-424.
Materials Characterization, 2004, 52(3): 179-186.
[4] T.Inami, et al. Grain size measurement of nanocrystalline
[8]Natter H., Loffler M.S., Krill C.E. Crystallite growth of
gold by dray diffraction method [J]. Nanostructured
nanocrystalline transition metals studied in situ by high
Materials,
temperature synchrotron X-ray diffraction [J]. Scripta
1999(12): 657-660.
Materialia, 2001, 44(8-9): 2321-2325.
[5] Zhang heng, et al.Nove1 nanostructed Pd-Zr oxides [J].
151
Nanostructured Lanthanum Zirconate Coating and Its Thermal Stability Properties ZHAO Xiao-dong*, ZENG Ke-li, XIE Jim-gang, LI Zhen-duo (Beijing General Research Institute of Mining and Metallurgy, Beijing 100044,China)
Abstract: In this paper, nano-La2Zr2@ thermal barrier coatings (MCrAIY+ La2Zr207) were prepared by atmospheric plasma sprayed (APS).The microstructures and thermal stability properties were systematically studied to compare with their conventional counterparts by Scanning Electric Microscopy (SEM), transmission electron microscope (TEM) and X-Ray diffraction(XRD). The results showed that the nanostructured lanthanum zirconate coatings were typical lamellar structure which was composed of columnar grains about 90nm in diameter. A large quantity of micro-cracks and homogeneous distributed fine pores formed in the nanostructured zirconia coating. After ablation at 1300°C for 24 h, no apparent phase transformation was observed in lanthanum zirconate coating. The growth mechanism of the grains was subsequently discussed.
Key word: plasma spraying ;Lanthanum zirconate coating ;nanostructured coating ;thermal stability property
1 Introduction
2 Experimental Procedures
Thermal barrier coatings (TBCs) are well used to protect high-temperature metallic components. Conventional TBCs, yttria partial stabilized zirconia (Y-PSZ), are limited by operation temperature (4200 "C ), because of phase transformation from the t'-tetragonal to cubic and then to monoclinic (m). For this purpose we are developing a new series of thermal barrier coatings that consist of a MCrAlY bond coat and lanthanum zirconate top coat. It is reported that lanthanum zirconate coatings were founded as lower thermal conductivity, better chemical and thermal stability in much higher temperature"].
2.1 Preparation of nano-coatings The lanthanum zirconate powder was synthesized by the sol-gel methodL2'with the original grain size of about 40-80nm. Then the powders were agglomerated into bigger powders between 45 and 80pm in diameter as shown in Fig.la, which was used for APS, Fig.1b gives the conventional lanthanum zirconate powder produced by Sub-institute of Metal Materials of Beijing General Rearch Institute of Iining and Metallurgy(BGlUMM- SIMM), and its average size is about 70pm. The coatings were made by plasma spray systems
a) Nano- La2Zr2O7 powders
b) Conventional La2Zr207powders
Fig.1 Morphologies of powder for APS 147
Proceedines of Sino-Swedish Structural Materials SvmDosium 2007
(Metco.7M. Metco. Inc Westbuny.L.L. NEW YORK) on a superalloy with MCrAlY (NO. KF-330, by
parameters are listed in Tablel. 2.2 MicrostructuralAnalysis
BGRIMM-SIMIM) bond coat and lanthanum zirconate top coatings. The substrate was nickel based superalloy,
The coatings were first incised into cross section, then polished by sand paper in 400 , 600# , 800 # made of Sic. To characterize the changes in
and the ceramic top coatings were deposited onto bond coating to a thickness of approximately 300pm. The
microstructure during the hot ablation tests, polished
Table1 Parameters of plasma spraying for lanthanum zirconate TBCs Parameters
Bond coat ( MCrAlY )
Top coat (nano -La2Zr207)
cariier gas flow ( l/h)
48
45
Primary gas flow ( vh)
1600
1600
Second gas flow ( Ih)
3000
3Ooo
Spray distance ( mm)
70
60
Voltage ( V )
80
80
Current ( A )
520
550
Thickness ( p m )
100
300
cross section of resin impregnated samples were
h. The sample with dimension of 40mm x 40mm x
observed by scanning electron microscopy (SEM: HJTACHl S-3500,Japan). The phases present in the various samples were identified by x-ray diffraction XRD using Cu Ka radiation, a step size of 0.1 deg, and
2mm was prepared.
3 Results and Discussion 3.1 Micrstructuredof nano-coatings Fig.2 shows microstructure of the cross-section of the coatings. Fig.2a and Fig.2b show the microstructure of the cross-section of lanthanum zirconate conventional coating and lanthanum zirconate Nano-Coatings. Both coatings have a porous microstructure, which was common for the plasma-sprayed coatings. The interface between bond coat and substrate alloy was compact and no more cracks and pores were observed. The substrate is nickel based superalloy and the bond coat is MCrAlY (NO.
a 2 theta scan window from 10" to 90".The particle size and morphology were observed by transmission electron microscope (TEM: HITACHI HT-80, Japan), with accelerate voltage of 75 200KV and the multiple of 1 100.
-
-
2.3 Experiment of Ablation In order to determine the stability of lanthanum zirconate coatings in higher temperatures, the ablation experiment was carried out at l3OO"C in muffle for 24
(a) Conventional La2Zr207coatings; (b) Nano- La2Zr207coatings
Fig.2 Microstructure of cross-section of coatings
148
Proceedings of Sino-Swedish Structural Materials Symposium 2007
KF-330, by BGRIMM-SIMM),so they are similar in physical performance. The typical lamina structures of nano-lanthanum zirconate coatings were shown in Fig.3(a) and Fig.3(b). In a high magnification image, Fig.3(a) showed that more detach of the columnar structure of the splats were observable, indicating a respective nucleation of columns during deposition. Fig.3(b) showed the fine
and of homogeneous pores in the microstructure with a large quantity of small micro-cracks. Fig.3(c) and Fig.3(d) gives the morphologies in TEM of the TBCs. A majority of grain size was no more than lOOnm with a little grain size between 90 and 180nm. The Debverind of electron diffraction can also be found. Fig.3(d) shows the columnar crystal is 60- lOOnm in diameter and about 500nm in length.
a), b) Fracture and cross-section of Nano- La2Zr207coatings; c)
. d)TEM of Nano- La2Zr207coatings
Fig.3. Morphologies of TBCs:
zirconate coating was still stable without phase transformation. But the diffraction peaks after ablation becomes sharper, which can be explained by the grown up of grain size, and the size which was shown in Fig. 4 is still in the nanometer scale. Nano-lanthanum zirconate coatings show very better thermal stability than Conventional Laz&@ coatings. It has been proved by many s c h ~ l a r s [that ~-~~ some of nano-materials, especially nano-ceramic, could prevent the grown up of grains in high temperature, namely, “Fermi Level Pinning”, and they could restrain nano-grain from growing up. The
3.2 Thermal stability of nano-coatings Fig4 shows the picture of Nano-Coatings after ablation in 1300 “C for 24h with crystal size is 80-150nm, we found that the grains had grown up in diameter, which size was a little larger than those before ablation. This growth trend just can also be testified by the calculation of shalle formula as shown in Table 2. Fig.5 shows the XRD results of the Nano-Coatings of lanthanum zirconate before and after ablation. They are both typical pure pyrochlore structure compare with PDF (NO.: 71-2363). The result revealed that nanostructured lanthanum 149
Proceedingsof Sino-Swedish StructuralMaterials Symposium 2007
phenomenon that the nano-lanthanum zirconate grain in coatings only grow up in the nanometer scale could not be explained by the conventional “Theoretical of
the surface energy” and “the principle of enegy minimum” [&*I. The mechanism that nano-lanthanum zirconate grain only grow up in same range is required
2Theta
(a)=-plate
of Nan& LazZrz07before ablation; ( b ) =-plate
of Nano- LazZr2& after ablation
Fig. 5 XRD of Nan* LazZr2@ TBCs
’hble2
S i z eof nano-TBCs before and after ablation
Coatings
2Theta
d-value
FWHM
Before blation
28.96 1
3.0805
0.592
20.72
After ablation
27.847
3.201 1
0.449
32.53
4 Conclusions
Grain size(nm)
lanthanum zirconate coating was still stable without phase transformation. The detailed mechanism of the grown-up of nano-lanthanum zirconate was required to
In this paper, nano- lanthanum zirconate thermal barrier coatings( MCrAlY+ La2Zr207 1was prepared by APS. The microstructures were columnar microstructure and the columnar crystal size was 60-100nm in diameter and about 5OOnm in length. After ablation at 1300 for 24h, nanostructured
be studied. References: [ 11 X.Q. Cao, R.Vassen, W.Fischer W, et al.Lanthanum-cerium
150
Proceedings of Sino-Swedish Structural Materials Symposium 2007 oxide
as
a
thermal
barrier-coating material
Materials Science and Engineering: 2004 ,
for
248-253.
high-temperatureapplications [J]. Adv. Mater., 2003,15 (17):
[6]Wang
1438-1442. [2] K.Koteswara Rao,
366(2) :
lan ,
etal.Heat
capacity
enhancement and
thermodynamic properties of nanostructured amorphous
Taqveem Banu, M.Vitha1 et al.
Preparation and characterization of bulk and nano particles
Si&[J]. Journal of Non-Crystalline Solids , 2001 ,
of La2Zr207and La2Zr207by sol-gel method [J]. Materials
2%(1-2): 139-142. [7] Delgado-ArellanoV.G , Espitia-CabreraM.L, Reyes-Gasga
Letters, 2002 (54) :205-210.
J.
[3] C.H.Moelle, H.J.Fecht. Thermal stability of nanocrystalline
Structural study
of
zirconia
nanoclusters by
iron prepared by mechanical attrition [J]. Nanostructured
high-resolution transmission electron microscopy [J].
Materials, 1995(6): 421-424.
Materials Characterization, 2004, 52(3): 179-186.
[4] T.Inami, et al. Grain size measurement of nanocrystalline
[8]Natter H., Loffler M.S., Krill C.E. Crystallite growth of
gold by dray diffraction method [J]. Nanostructured
nanocrystalline transition metals studied in situ by high
Materials,
temperature synchrotron X-ray diffraction [J]. Scripta
1999(12): 657-660.
Materialia, 2001, 44(8-9): 2321-2325.
[5] Zhang heng, et al.Nove1 nanostructed Pd-Zr oxides [J].
151