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Residual micro- and macrostresses in the plasma-sprayed zirconia-based TBCs
___ I!!3
s
I
March 1994
__
4._
ELSEYIER
Materials Letters 19 ( 1994) 48-52
Residual micro- and macrostresses in the plasma-sprayed zirconia-based TBCs M. Levit, I. Grimberg, B.-Z. Weiss 1 Department of Materials Engineering, Technion-Israel Institute of Technology, Haifa 32000, Israel
Received 20 December 1993; accepted 23 December 1993
Abstract Micro- and macrostresses as well as the size of coherently diffracting domains were studied with the aid of X-ray diffraction in plasma-sprayed Zr02-7Y203 thermal barrier coatings (TBCs) as functions of substrate temperature. The macrostresses were measured by the “sin?P’-technique, the microstresses and the domain size were evaluated from X-ray line broadening. The changes in the macrostresses were explained by differences in the thermal contractions of the ceramic layer and the metallic substrate, while the variations in the microstresses were attributed to the growth of the m-ZrOz phase with the increase of the substrate temperature during deposition. The increase in dislocation density associated with the formation of the m-ZrOz phase was revealed by XRD and was also confirmed by transmission electron microscopy.
1. Introduction Thermal barrier coatings (TBCs) give effective thermal protection to components working at high temperatures [l-4]. In addition, the coatings protect metallic components against erosion and hot corrosion. TBCs deposited on metallic component are subjected to high thermal stresses, especially in parts exposed to temperature cycling. These stresses mainly arise from the mismatch in thermal expansion coefficients between the ceramic coat ( z 10 x 1O-a/oC) and the metallic substrate ( x 15 x 10-6/“C) [ 31. They may result in spalling or distortion of the deposited layers, leading to failure of the TBCs. In order to reduce the effect of thermal stresses an intermediate metallic bond coat is used [ l-5 1. TBCs also contain residual stresses, which arise from the nature of the plasma-spraying process (rapid ’ Present address: Kulite Semi-Conductor (International) One Willow Tree Road, Leonia, NJ 07605, USA.
Ltd.,
cooling of molten or partially molten droplets, impacting on the cool substrate surface) [ 3,6,7]. The temperature of the substrate during deposition can markedly influence the nature and magnitude of these stresses. The presence of compressive residual stresses may have a beneficial effect on the coating, improving its durability. In addition, residual stresses may reduce the magnitude of the thermal stresses and thus significantly prolong the service life of the coating. The term, “residual stresses”, commonly refers to macrostresses. The presence of the microstresses is usually ignored when the state of stresses is considered [8,9]. However, the local stresses are strongly influenced by microstresses and may differ significantly from the average macrostress value. The microstresses act within a limited area (usually inside one grain) and result in X-ray peaks broadening, while macrostresses, operative throughout the coating, produce shifts of the peaks’ position. Microstresses may arise either from structural defects or
0167-577x/94/$07.00 0 1994 Elsevier Science B.V. All rights reserved SSDI0167-577x(94)00007-A
M. L.evit et al. /Materials Letters I9 (1994) 48-52
from local phase transformations accompanied by volume changes [ lo- 12 1. The present work is a study of the residual microand macrostresses in the plasma-sprayed Zr02-7Y203 thermal barrier coatings deposited on a Ni-based superalloy substrate held at different temperatures. The macrostresses were measured with conventional Xray diffraction (XRD) using the “sin*!J”‘-technique. XRD line-broadening analysis was applied to the estimation of the microstresses and the average size of coherently diffracting domains.
2. Experimental procedure Rectangular bar specimens, 50 X 20 x 10 mm3, of a Ni-based superalloy were plasma sprayed with a duplex TBC consisting of a Ni-22Cr- 1OAl- 1Y (wt%) bond coat of thickness x 0.1 mm and a ZQ-7Y203 ceramic coat of a thickness between 0.3 and 0.4 mm. The coatings were produced by means of the Metco EG 88 computer-controlled plasma spraying device. Four temperatures of the substrate during deposition were chosen, namely: 75, 150, 300 and 5Oo”C, which were controlled by thermocouples connected to the substrates and linked to an automatic air cooling system. The XRD measurements were conducted with assprayed specimens in flat-on geometry by a BraggBrentano type diffractometer. The X-ray APD system, a Philips PW-1820, was operated through a MicroVAX Computer using Philips PW1700 software. The diffractometer included a long fine-focus Cu X-ray tube (IzcUKa= 1.54 A) powered by generator operated at 40 kV and 40 mA. A graphite monochromator was located in the path of the diffracted beam and filtered all radiation except Ka. Residual stress measurements were carried out by the “sin*!I”‘-technique based on (620) t-Zr02 peak (28= 144.4”). The (620) peak positions were determined exactly with Philips PW-1869 profile fitting program. The peak shifts were gauged during tilting the samples through angles of 0, 20, 30, 40 and 50”. The least-squares linear approximation was applied, and the residual macrostresses were calculated from the line slope and elastic constants in the framework of a biaxial stress model [ 13 1. The macrostresses were determined in the longitudinal direction of the TBC
49
specimen (the direction of the plasma gun’s motion). The microstrains and the size of coherently diffracting domains were evaluated by X-ray line profile analysis. Two approaches to the single-line broadening analysis were employed: the approximate Voightfunction method [ lo] and the Fourier single-line method [ 111. The analysis was performed on the (400) t-ZrO, peak (28=73.4”). The experimental results of the microstrains and the domain size were then averaged. From the calculation of the penetration depth of X-rays [ 121 for the angles corresponding to (400) and (602) t-Zr02 peaks, it appears that the microstresses and domain size were evaluated in the outer ~24 urn, while the macrolayer with thickness stresses were measured in the depth of = 38 pm.
3. Results and discussion
The residual macrostresses as a function of the substrate temperature are presented in Fig. 1. The residual stresses in the ceramic surface layer ( x 40 urn thick) progressively change from tensile ( x 27 MPa at a substrate temperature of 75°C) to compressive ( = 8 MPa at 500°C). An experimental value of Young’s modulus (34 MPa) was used for macrostress calculations [ 3,6 1. This value is about a sixth of that of monolithic bulk zirconia ( = 200 GPa)
I
30 .........------.._. A
*v
0
loo
“” ” .................-
200
Substrate
3cIl
400
temperature
500
fxxl
, OC
Fig. 1. Residual macrostresses in the plasma-sprayed ZrQ-7Y203 ceramic coating as a function of the substrate temperature during deposition.
50
hf. Levit et al. /Materials Letters 19 (1994) 48-52
[ 3,141, as a result of the presence of pores and microcracks in the plasma-sprayed coating [ 15 1. Residual macrostresses in the zirconia-based TBCs mainly may arise as a result of two different causes: (i) the difference in the shrinkages of the ceramic coating and the metallic substrate during cooling and (ii) the tetragonal-to-monoclinic phase transformation, which is accompanied by a 3-4% volume expansion [ 3,6 1. The contribution of the transformation-induced stresses is small as compared with that of thermally induced stresses, since the volume of mZr02 formed is very small. The volume of the monoclinic phase gradually changes from x0.7% at the substrate temperature of 75°C through x 1% at 150°C and = 1.2% at 300°C to x 1.4% at 500°C. The quantities were established by semi-quantitative XRD analysis [ 15 1. Thus the residual macrostresses in the plasma-sprayed TBCs may be basically assumed to be of a thermal nature. Postulating that the coating adheres rigidly to the substrate, implicates that the thermal contractions of the substrate and the coating control the state of residual stresses in the TBC-substrate composite. It appears, that the shrinkage of the substrate, held at 75°C during deposition, is smaller than that of the coating. This results in high tensile stresses in the ceramic coating. At 5Oo”C, on the other hand, the contraction of the substrate is greater than that of the coating, and results in compressive stresses within the coating layer. The changes in microstrains with substrate temperature are shown in Fig. 2. The microstrains are seen to increase significantly with the increase of the substrate temperature. It seems that the t-m transformation, associated with volume expansion, is the principal cause of the formation of the microstresses within the zirconia layer. Thus, the increase in the microstrains with the substrate temperature may be explained by the moderate build-up of the monoclinic m-ZrOl phase. Changes in microstrains are slowed down with increasing substrate temperature, probably as a result of the decrease in the rate of mZr02 formation. The exact evaluation of the microstresses is difficult, but their order of magnitude may be estimated from the hydrostatic stress model [ 16 1. Calculated according to this approach, using Young’s modulus of monolithic bulk zirconia, the microstresses change from = 170 MPa at the substrate tem-
~
0d5 I 0.11
” i..
008
2
0.04
..__. ______._............._______.__ .._.._....._. +
0.00 I
0
loo
200 300 400 500 Substrate temperature , OC
t
x
Fig. 2. Residual microstrains in the plasma-sprayed ZrOz-7Y20 ceramic coating as a function of the substrate temperature during deposition.
Fig. 3. Crack deflection and branching observed in zirconia TBCs.
perature of 75°C to = 300 MPa at 5Oo”C, and are relatively high as compared with the macrostresses determined. The high values of the microstresses reflect the nonuniform character of stress distribution within the coating. Such stress distribution may cause the deflection and/or branching of cracks during their propagation (see Fig. 3) and thereby considerably
M. Levit et al. /Materials Letters I9 (1994) 48-52
increase the fracture energy and fracture toughness of zirconia TBCs [ 17 1. Thus, the formation of a small amount ( x 1%) of the m-ZrO, phase seems to explain why the plasma-sprayed Y-PSZ TBCs with 78 wt% of yttria show a maximum in the lifetime in the high-temperature cyclic conditions as compared with those of smaller or larger yttria contents [ 18 1. Zirconia containing less than 7 wt% Y203 produces larger portion of transformable t-ZrOz and, consequently, a larger amount of m-Zr02 with excessive cracking, while Zr02 containing more than 8 wt% of Y203 stabilizes the c-ZrOz phase and produces hardly any m-ZrOz phase, and therefore has no such toughening mechanisms. From the microstructural studies the average grain size of the zirconia coating was found to be independent of the substrate temperature [ 151. However, the variation in the size of coherently diffracting domains, determined from XRD line broadening, showed a significant decrease with increasing substrate temperature (see Fig. 4). It presumably can be attributed to the dislocation formation (no change in the density of t’-ZrO, twins with changing substrate temperature was observed). Dislocations were found to be ordered in a network, as shown in Fig. 5, located in highly stressed areas in the vicinity of and inside the m-ZrOz grains. The influence of the macrostresses on the formation of dislocations can be assumed to be small as compared to microstresses. Thus, the appearance of the monoclinic phase leads not only to a rise of residual microstresses but also to an increase in the number of dislocations and conse-
Substrate
temperature
, OC
Fig. 4. Size of coherently diffracting domains in the plasmasprayed Zr02-7Y203 ceramic coating as a function of the substrate temperature during deposition.
51
Fig. 5. Dislocation network in the vicinity of the m-Zr02 phase.
quently to a decrease fracting domains.
in the size of coherently
dif-
4. Conclusions Residual micro- and macrostresses as well as the size of coherently diffracting domains were studied in plasma-sprayed Zr02-7Y203 TBCs. The coatings were deposited on N&based superalloy substrates which were kept at four different temperatures. The state of residual macrostresses changes from tension ( E 27 MPa) to compression ( x 8 MPa) as the substrate temperature is increased from 75 to 500°C. The variation in macrostresses is primarily caused by differences in the thermal contractions of the ceramic layer and the metallic substrate. The microstresses increase from = 170 MPa at 75°C to = 300 MPa at 500°C. It is believed that the moderate build-up of the monoclinic m-ZrOz, by t-m transformation accompanied by volume change, with increasing substrate temperature results in an increase of microstresses in the zirconia layer. The variations in the size of coherently diffracting
52
M. Levit et al. /Materials Letters 19 (1994) 48-52
domains decrease with increasing substrate temperature, which can be attributed to the formation of dislocations and their ordering, as a result of the increase in microstresses.
5. Acknowledgement The authors would like to express their thanks to Professor H.-D. Steffens and Dr. Z. Babiak of the University of Dortmund, Germany, for supplying specimens for this research and for valuable suggestions. The financial support of the German-Israeli Foundation for Scientific Research and Development (GIF) is sincerely appreciated.
6. References [ I ] B.J. Gill and R.C. Tucker Jr., Mater. Sci. Technol. 2 ( 1986) 201. [ 21 A.R. Nicoll, H. Gruner, G. Wuest and S. Keller, Mater. Sci. Technol. 2 (1986) 214. [ 31 W.J. Lackey, D.P. Stinton, G.A. Cemy, L.L. Fehrenbacher and A.C. Schaflhauser, Ceramic coatings for heat engine materials - status and future needs (Oak Ridge National Laboratory, Oak Ridge, TN, 1984). [4] T.A. Taylor, M.P. Overs, B.J. Gill and R.C. Tucker Jr., J. Vacuum Sci. Technol. A 3 (1985) 2526.
[ 51 R. Taylor, J.R. Brandon and P. Morrel, Surface Coat. Technol. 50 (1992) 141. [6] M. Hobbs, Surf. J. 16 (1985) 101. [7] H.-D. Steffens, B. Wielage and J. Drozak, Surface Coat. Technol. 45 ( 199 1) 299. [ 81 M.K. Hobbs and H. Reiter, in: Thermal spray: advances in coatings technology, ed. D.L. Houck (American Society for Metals, Metals Park, OH, 1988) pp. 285-290. [9] SC. Gill and T.W. Clyne, Metall. Trans. B 21 (1990) 377. [ lo] Th.H. de Keijser, J.I. Langford, E.J. Mittemeijer and A.B.P. Vogels, J. Appl. Crystall. 15 (1982) 308. [ 111 R. Delhez, Th.H. de Keijser and E.J. Mittemeijer, Anal. Chem. 312 (1982) 1. [ 121 H.P. Klug and L.E. Alexander, X-ray diffraction procedures for polycrystalline and amorphous materials, 2nd Ed. (Wiley,NewYork, 1974). [ 131 I. Noyan and J.B. Cohen, Residual stress measurements by diffraction and interpretation (Springer, Berlin, 1987). [ 141 G. Samsonov, The oxide handbook (IFI-Plenum Data Company, New York, 1982). [ 151 M. Levit, S. Berger, I. Grimberg and B.-Z. Weiss, J. Mater. Synth. Process. to be published ( 1993 ) [ 161 S.P. Timoshenko and J.N. Goodier, Theory of elasticity (McGraw-Hill, New York, 1985). [ 171 P.D. Harmsworth and R. Stevens, J. Mater. Sci. 27 ( 1992) 611. [ 181 R.J. Bratton and S.K. Lau, in: Advances in ceramics, Vol. 3, Science and technology of zirconia, eds. A.H. Heuer and L.W. Hobbs (American Ceramic Society, Columbus, OH, 1982) pp. 241-253.
s
I
March 1994
__
4._
ELSEYIER
Materials Letters 19 ( 1994) 48-52
Residual micro- and macrostresses in the plasma-sprayed zirconia-based TBCs M. Levit, I. Grimberg, B.-Z. Weiss 1 Department of Materials Engineering, Technion-Israel Institute of Technology, Haifa 32000, Israel
Received 20 December 1993; accepted 23 December 1993
Abstract Micro- and macrostresses as well as the size of coherently diffracting domains were studied with the aid of X-ray diffraction in plasma-sprayed Zr02-7Y203 thermal barrier coatings (TBCs) as functions of substrate temperature. The macrostresses were measured by the “sin?P’-technique, the microstresses and the domain size were evaluated from X-ray line broadening. The changes in the macrostresses were explained by differences in the thermal contractions of the ceramic layer and the metallic substrate, while the variations in the microstresses were attributed to the growth of the m-ZrOz phase with the increase of the substrate temperature during deposition. The increase in dislocation density associated with the formation of the m-ZrOz phase was revealed by XRD and was also confirmed by transmission electron microscopy.
1. Introduction Thermal barrier coatings (TBCs) give effective thermal protection to components working at high temperatures [l-4]. In addition, the coatings protect metallic components against erosion and hot corrosion. TBCs deposited on metallic component are subjected to high thermal stresses, especially in parts exposed to temperature cycling. These stresses mainly arise from the mismatch in thermal expansion coefficients between the ceramic coat ( z 10 x 1O-a/oC) and the metallic substrate ( x 15 x 10-6/“C) [ 31. They may result in spalling or distortion of the deposited layers, leading to failure of the TBCs. In order to reduce the effect of thermal stresses an intermediate metallic bond coat is used [ l-5 1. TBCs also contain residual stresses, which arise from the nature of the plasma-spraying process (rapid ’ Present address: Kulite Semi-Conductor (International) One Willow Tree Road, Leonia, NJ 07605, USA.
Ltd.,
cooling of molten or partially molten droplets, impacting on the cool substrate surface) [ 3,6,7]. The temperature of the substrate during deposition can markedly influence the nature and magnitude of these stresses. The presence of compressive residual stresses may have a beneficial effect on the coating, improving its durability. In addition, residual stresses may reduce the magnitude of the thermal stresses and thus significantly prolong the service life of the coating. The term, “residual stresses”, commonly refers to macrostresses. The presence of the microstresses is usually ignored when the state of stresses is considered [8,9]. However, the local stresses are strongly influenced by microstresses and may differ significantly from the average macrostress value. The microstresses act within a limited area (usually inside one grain) and result in X-ray peaks broadening, while macrostresses, operative throughout the coating, produce shifts of the peaks’ position. Microstresses may arise either from structural defects or
0167-577x/94/$07.00 0 1994 Elsevier Science B.V. All rights reserved SSDI0167-577x(94)00007-A
M. L.evit et al. /Materials Letters I9 (1994) 48-52
from local phase transformations accompanied by volume changes [ lo- 12 1. The present work is a study of the residual microand macrostresses in the plasma-sprayed Zr02-7Y203 thermal barrier coatings deposited on a Ni-based superalloy substrate held at different temperatures. The macrostresses were measured with conventional Xray diffraction (XRD) using the “sin*!J”‘-technique. XRD line-broadening analysis was applied to the estimation of the microstresses and the average size of coherently diffracting domains.
2. Experimental procedure Rectangular bar specimens, 50 X 20 x 10 mm3, of a Ni-based superalloy were plasma sprayed with a duplex TBC consisting of a Ni-22Cr- 1OAl- 1Y (wt%) bond coat of thickness x 0.1 mm and a ZQ-7Y203 ceramic coat of a thickness between 0.3 and 0.4 mm. The coatings were produced by means of the Metco EG 88 computer-controlled plasma spraying device. Four temperatures of the substrate during deposition were chosen, namely: 75, 150, 300 and 5Oo”C, which were controlled by thermocouples connected to the substrates and linked to an automatic air cooling system. The XRD measurements were conducted with assprayed specimens in flat-on geometry by a BraggBrentano type diffractometer. The X-ray APD system, a Philips PW-1820, was operated through a MicroVAX Computer using Philips PW1700 software. The diffractometer included a long fine-focus Cu X-ray tube (IzcUKa= 1.54 A) powered by generator operated at 40 kV and 40 mA. A graphite monochromator was located in the path of the diffracted beam and filtered all radiation except Ka. Residual stress measurements were carried out by the “sin*!I”‘-technique based on (620) t-Zr02 peak (28= 144.4”). The (620) peak positions were determined exactly with Philips PW-1869 profile fitting program. The peak shifts were gauged during tilting the samples through angles of 0, 20, 30, 40 and 50”. The least-squares linear approximation was applied, and the residual macrostresses were calculated from the line slope and elastic constants in the framework of a biaxial stress model [ 13 1. The macrostresses were determined in the longitudinal direction of the TBC
49
specimen (the direction of the plasma gun’s motion). The microstrains and the size of coherently diffracting domains were evaluated by X-ray line profile analysis. Two approaches to the single-line broadening analysis were employed: the approximate Voightfunction method [ lo] and the Fourier single-line method [ 111. The analysis was performed on the (400) t-ZrO, peak (28=73.4”). The experimental results of the microstrains and the domain size were then averaged. From the calculation of the penetration depth of X-rays [ 121 for the angles corresponding to (400) and (602) t-Zr02 peaks, it appears that the microstresses and domain size were evaluated in the outer ~24 urn, while the macrolayer with thickness stresses were measured in the depth of = 38 pm.
3. Results and discussion
The residual macrostresses as a function of the substrate temperature are presented in Fig. 1. The residual stresses in the ceramic surface layer ( x 40 urn thick) progressively change from tensile ( x 27 MPa at a substrate temperature of 75°C) to compressive ( = 8 MPa at 500°C). An experimental value of Young’s modulus (34 MPa) was used for macrostress calculations [ 3,6 1. This value is about a sixth of that of monolithic bulk zirconia ( = 200 GPa)
I
30 .........------.._. A
*v
0
loo
“” ” .................-
200
Substrate
3cIl
400
temperature
500
fxxl
, OC
Fig. 1. Residual macrostresses in the plasma-sprayed ZrQ-7Y203 ceramic coating as a function of the substrate temperature during deposition.
50
hf. Levit et al. /Materials Letters 19 (1994) 48-52
[ 3,141, as a result of the presence of pores and microcracks in the plasma-sprayed coating [ 15 1. Residual macrostresses in the zirconia-based TBCs mainly may arise as a result of two different causes: (i) the difference in the shrinkages of the ceramic coating and the metallic substrate during cooling and (ii) the tetragonal-to-monoclinic phase transformation, which is accompanied by a 3-4% volume expansion [ 3,6 1. The contribution of the transformation-induced stresses is small as compared with that of thermally induced stresses, since the volume of mZr02 formed is very small. The volume of the monoclinic phase gradually changes from x0.7% at the substrate temperature of 75°C through x 1% at 150°C and = 1.2% at 300°C to x 1.4% at 500°C. The quantities were established by semi-quantitative XRD analysis [ 15 1. Thus the residual macrostresses in the plasma-sprayed TBCs may be basically assumed to be of a thermal nature. Postulating that the coating adheres rigidly to the substrate, implicates that the thermal contractions of the substrate and the coating control the state of residual stresses in the TBC-substrate composite. It appears, that the shrinkage of the substrate, held at 75°C during deposition, is smaller than that of the coating. This results in high tensile stresses in the ceramic coating. At 5Oo”C, on the other hand, the contraction of the substrate is greater than that of the coating, and results in compressive stresses within the coating layer. The changes in microstrains with substrate temperature are shown in Fig. 2. The microstrains are seen to increase significantly with the increase of the substrate temperature. It seems that the t-m transformation, associated with volume expansion, is the principal cause of the formation of the microstresses within the zirconia layer. Thus, the increase in the microstrains with the substrate temperature may be explained by the moderate build-up of the monoclinic m-ZrOl phase. Changes in microstrains are slowed down with increasing substrate temperature, probably as a result of the decrease in the rate of mZr02 formation. The exact evaluation of the microstresses is difficult, but their order of magnitude may be estimated from the hydrostatic stress model [ 16 1. Calculated according to this approach, using Young’s modulus of monolithic bulk zirconia, the microstresses change from = 170 MPa at the substrate tem-
~
0d5 I 0.11
” i..
008
2
0.04
..__. ______._............._______.__ .._.._....._. +
0.00 I
0
loo
200 300 400 500 Substrate temperature , OC
t
x
Fig. 2. Residual microstrains in the plasma-sprayed ZrOz-7Y20 ceramic coating as a function of the substrate temperature during deposition.
Fig. 3. Crack deflection and branching observed in zirconia TBCs.
perature of 75°C to = 300 MPa at 5Oo”C, and are relatively high as compared with the macrostresses determined. The high values of the microstresses reflect the nonuniform character of stress distribution within the coating. Such stress distribution may cause the deflection and/or branching of cracks during their propagation (see Fig. 3) and thereby considerably
M. Levit et al. /Materials Letters I9 (1994) 48-52
increase the fracture energy and fracture toughness of zirconia TBCs [ 17 1. Thus, the formation of a small amount ( x 1%) of the m-ZrO, phase seems to explain why the plasma-sprayed Y-PSZ TBCs with 78 wt% of yttria show a maximum in the lifetime in the high-temperature cyclic conditions as compared with those of smaller or larger yttria contents [ 18 1. Zirconia containing less than 7 wt% Y203 produces larger portion of transformable t-ZrOz and, consequently, a larger amount of m-Zr02 with excessive cracking, while Zr02 containing more than 8 wt% of Y203 stabilizes the c-ZrOz phase and produces hardly any m-ZrOz phase, and therefore has no such toughening mechanisms. From the microstructural studies the average grain size of the zirconia coating was found to be independent of the substrate temperature [ 151. However, the variation in the size of coherently diffracting domains, determined from XRD line broadening, showed a significant decrease with increasing substrate temperature (see Fig. 4). It presumably can be attributed to the dislocation formation (no change in the density of t’-ZrO, twins with changing substrate temperature was observed). Dislocations were found to be ordered in a network, as shown in Fig. 5, located in highly stressed areas in the vicinity of and inside the m-ZrOz grains. The influence of the macrostresses on the formation of dislocations can be assumed to be small as compared to microstresses. Thus, the appearance of the monoclinic phase leads not only to a rise of residual microstresses but also to an increase in the number of dislocations and conse-
Substrate
temperature
, OC
Fig. 4. Size of coherently diffracting domains in the plasmasprayed Zr02-7Y203 ceramic coating as a function of the substrate temperature during deposition.
51
Fig. 5. Dislocation network in the vicinity of the m-Zr02 phase.
quently to a decrease fracting domains.
in the size of coherently
dif-
4. Conclusions Residual micro- and macrostresses as well as the size of coherently diffracting domains were studied in plasma-sprayed Zr02-7Y203 TBCs. The coatings were deposited on N&based superalloy substrates which were kept at four different temperatures. The state of residual macrostresses changes from tension ( E 27 MPa) to compression ( x 8 MPa) as the substrate temperature is increased from 75 to 500°C. The variation in macrostresses is primarily caused by differences in the thermal contractions of the ceramic layer and the metallic substrate. The microstresses increase from = 170 MPa at 75°C to = 300 MPa at 500°C. It is believed that the moderate build-up of the monoclinic m-ZrOz, by t-m transformation accompanied by volume change, with increasing substrate temperature results in an increase of microstresses in the zirconia layer. The variations in the size of coherently diffracting
52
M. Levit et al. /Materials Letters 19 (1994) 48-52
domains decrease with increasing substrate temperature, which can be attributed to the formation of dislocations and their ordering, as a result of the increase in microstresses.
5. Acknowledgement The authors would like to express their thanks to Professor H.-D. Steffens and Dr. Z. Babiak of the University of Dortmund, Germany, for supplying specimens for this research and for valuable suggestions. The financial support of the German-Israeli Foundation for Scientific Research and Development (GIF) is sincerely appreciated.
6. References [ I ] B.J. Gill and R.C. Tucker Jr., Mater. Sci. Technol. 2 ( 1986) 201. [ 21 A.R. Nicoll, H. Gruner, G. Wuest and S. Keller, Mater. Sci. Technol. 2 (1986) 214. [ 31 W.J. Lackey, D.P. Stinton, G.A. Cemy, L.L. Fehrenbacher and A.C. Schaflhauser, Ceramic coatings for heat engine materials - status and future needs (Oak Ridge National Laboratory, Oak Ridge, TN, 1984). [4] T.A. Taylor, M.P. Overs, B.J. Gill and R.C. Tucker Jr., J. Vacuum Sci. Technol. A 3 (1985) 2526.
[ 51 R. Taylor, J.R. Brandon and P. Morrel, Surface Coat. Technol. 50 (1992) 141. [6] M. Hobbs, Surf. J. 16 (1985) 101. [7] H.-D. Steffens, B. Wielage and J. Drozak, Surface Coat. Technol. 45 ( 199 1) 299. [ 81 M.K. Hobbs and H. Reiter, in: Thermal spray: advances in coatings technology, ed. D.L. Houck (American Society for Metals, Metals Park, OH, 1988) pp. 285-290. [9] SC. Gill and T.W. Clyne, Metall. Trans. B 21 (1990) 377. [ lo] Th.H. de Keijser, J.I. Langford, E.J. Mittemeijer and A.B.P. Vogels, J. Appl. Crystall. 15 (1982) 308. [ 111 R. Delhez, Th.H. de Keijser and E.J. Mittemeijer, Anal. Chem. 312 (1982) 1. [ 121 H.P. Klug and L.E. Alexander, X-ray diffraction procedures for polycrystalline and amorphous materials, 2nd Ed. (Wiley,NewYork, 1974). [ 131 I. Noyan and J.B. Cohen, Residual stress measurements by diffraction and interpretation (Springer, Berlin, 1987). [ 141 G. Samsonov, The oxide handbook (IFI-Plenum Data Company, New York, 1982). [ 151 M. Levit, S. Berger, I. Grimberg and B.-Z. Weiss, J. Mater. Synth. Process. to be published ( 1993 ) [ 161 S.P. Timoshenko and J.N. Goodier, Theory of elasticity (McGraw-Hill, New York, 1985). [ 171 P.D. Harmsworth and R. Stevens, J. Mater. Sci. 27 ( 1992) 611. [ 181 R.J. Bratton and S.K. Lau, in: Advances in ceramics, Vol. 3, Science and technology of zirconia, eds. A.H. Heuer and L.W. Hobbs (American Ceramic Society, Columbus, OH, 1982) pp. 241-253.