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Fracture toughness of plasma-sprayed zirconia coatings
Surface and Coatings Technology, 34 (1988) 15
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23
15
FRACTURE TOUGHNESS OF PLASMA-SPRAYED ZIRCONIA COATINGS* G. N. HEINTZE and R. McPHERSON Department of Materials Engineering, Monash University, Clayton 3168 (Australia) (Received April 24, 1987)
Summary Plasma-sprayed ceramic coatings are formed by introducing a powder into a plasma jet to produce a stream of molten particles which then impact, spread and rapidly solidify onto a substrate to form a lamellar microstructure. The cohesive and adhesive fracture toughness of zirconia coatings, fully or partially stabilized with Y203 or CeO2, plasma sprayed onto steel, have been determined using the double cantilever beam (DCB) technique. The results show that cohesive fracture toughness is greater than adhesive toughness and that tetragonal coatings have significantly higher toughness than fully stabilized (cubic) zirconia coatings. The highest toughness was achieved with Zr02—Ce02 coatings which contained a significant proportion of transformable tetragonal phase. The results are interpreted in terms of the influence of grain size and composition on the tetragonal-to-monoclinic phase transformation.
1. Introduction Plasma-sprayed zirconia coatings have been used for some time as thermal barriers because of their refractoriness and low thermal conductivity. The zirconia is alloyed with other oxides (stabilizers) so that the coating consists entirely of either the cubic or the tetragonal form at all temperatures to avoid degradation due to volume changes arising from transformations during heating and cooling. The improvement in toughness achieved in sintered ceramics which contain tetragonal Zr02 particles of a critical size that transform under stress is well known [1]. This raises the possibility of preparing partially stabilized zirconia coatings with enhanced toughness for applications at ambient temperatures. A current hypothesis of the toughening mechanism is that a propagating crack interacts with the metastable tetragonal particles in a zone *paper presented at NTSC87, the National Thermal Spray Conference and Exposition, Orlando, FL, U.S.A., September 14- 17, 1987. 0257-8972/88/$3.50
© Elsevier Sequoia/Printed in The Netherlands
16
at the crack tip and causes them to transform martensitically into the monoclinic structure. The 4% volume expansion associated with the transformation effectively compresses the crack tip and opposes its motion. The extra energy which must be supplied to the crack to maintain propagation appears as enhanced fracture energy [2]. A high proportion of transformable tetragonal Zr02 is achieved if the martensite start temperature M5 (the temperature at which tetragonal Zr02 spontaneously begins to transform to monoclinic on cooling) is around room temperature. The M5 temperature is a function of grain size, stabilizer type and concentration. It has been observed that Zr02—(10 15)wt.%Ce02-plasma-sprayed coatings contain a large proportion of tetragonal Zr02 which transforms to monoclinic on grinding [3]. The question then arises as to whether these coatings have superior toughness to coatings which do not contain transformable Zr02. The present paper is concerned with the assessment of fracture toughness of various Zr02-based coatings using the double cantilever beam (DCB) technique first reported by Berndt and McPherson [4]. -
2. Structure of plasma-sprayed zirconia coatings It is important to consider firstly the microstructure and phase constitution of plasma-sprayed ceramic coatings before considering their fracture toughness. These coatings are composed of lamellae formed by the impact and rapid solidification of individual molten particles. The lamellae are typically 1 2 jim thick with columnar grains 0.1 0.4 jim in cross-section, extending through their thickness (Fig. 1) [31.This small grain size is deter1) [5] and cannot be mined by the rapid (106 iø~°Cs controlled. Thus the solidification only means ofrate obtaining transformable tetragonal Zr0 2 is by suitable choice of the type and concentration of stabilizer to vary M8. Rapidly quenched Zr02-stabilizer solution droplets will crystallize to tetragonal Zr02 up to a certain concentration of stabilizer beyond which -
-
-
_____
IT
Fig. 1. Columnar grains within lamellae,
17
cubic Zr02 is formed. In general the cubic structure, if formed, will persist to room temperature giving a fully stabilized coating, for example Zr02— 2Owt.%Y203. Tetragonal Zr02, if formed, will transform to monoclinic on cooling if M~ is above room temperature, but will persist as “nontransformable” tetragonal if Ms is below room temperature. Since the grain size will vary over a certain range within the coating, there will be a range of M8, and the final structure observed may consist of a mixture of phases. Unmelted particles may also contribute to the final structure. The phase constitution of some Zr02 coatings, determined by X-ray diffraction [31, are shown in Table 1. M5 of plasma-sprayed Zr02—lOwt.%Ce02 and Zr02—l5wt.%Ce02 is approximately 300 °C and 200 °C respectively whereas Ms of Zr02— 6wt.%Y203 is well below room temperature [3]. Thus 6 wt.% Y203 coatings consist predominantly of non-transformable tetragonal whereas the Zr02— CeO2 coatings contain transformable tetragonal which may contribute to their fracture toughness parameters. TABLE 1 Phase constitution of plasma-sprayed zirconia coatings
Composition
Zr026wt.%Y203 ZrO2—2Owt.%Y203 ZrO2—lQwt.%Ce02 ZrO2—l5wt.%Ce02
Per cent
Per cent
Per cent
Per cent
cubic (F)
transformable tetragonal (T)
non-transformable tetragonal (T’)
monoclinic (M)
—
85
—
—
15 32
7
78
37
23
12 100 —
8
3 —
3. Fracture toughness measurements DCB specimens (Fig. 2) comprise two arms of steel (CS1O2O). One arm is grit blasted and plasma sprayed with the selected coating (no bond coat) and then glued with a heat-curable epoxy resin film (FM-73) to the other arm, also grit blasted. The curing time is 1 h at 120 °C.The distance from the loading point to the adhesive strip denotes the precrack length which is approximately 35 mm for each DCB. Coatings were sprayed using a Plasmadyne SG-100 torch at 900 A, 40 V. Powders sprayed were Zr02—lOwt.%CeO2, Zr02—l5wt.%Ce02 (prepared in the laboratory), Zr02—6wt.%Y203 and ZrO2—2Owt.%Y203 (commercial powders). Coating thickness ranged between 0.2 and 0.5 mm. The DCB was loaded via a computer-controlled MTS 810 servohydraulic testing machine, as detailed by Ostojic [6]. The extension (endpoint displacement) and load were recorded and automatically plotted at selected time intervals. When cracking began, as determined by a decrease in
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0 Fig. 2. DCB configuration (all dimensions in millimetres).
load with increasing extension, the DCB was unloaded. Between 8 and 15 loading—unloading cycles could usually be obtained before complete failure. The crack growth resistance R may be calculated from the area enclosed by the loading path of each cycle divided by the increment in area on the DCB arm owing to the additional crack extension [7]. Hence each coating tested yields several values. The crack length was determined from the inverse gradient of the unloading portion of the curve (compliance is equal to extension divided by load) using an experimentally determined calibration curve of compliance as a function of known ërack length. 4. Results A typical series of loading—unloading cycles obtained from one DCB is shown in Fig. 3(a). For each coating type there is permanent offset of approximately 0.02 mm over the crack length range approx. 35 110 nm. For clarity each cycle of Fig. 3(a) is separated by 0.01 mm in Fig. 3(b). Both the Zr02—6wt.%Y203 and the ZrO2—2Owt.%Y203 coatings failed in either -
one of three modes, namely cohesive (failure entirely with the coating), adhesive (close to or at the substrate interface) or mixed (large islands of coating remaining on the DCB arm). Zr02—lSwt.%Ce02 coatings failed either adhesively or cohesively (very limited data for the latter) whilst only adhesive failure was observed in the Zr02—lOwt.%Ce02 coatings. Probability plots of crack growth resistance for adhesive failure only are given in Fig. 4. Crack growth resistance is highest for Zr02—lOwt.%Ce02 followed, in decreasing order, by Zr02—l5wt.%Ce02, Zr02—6wt.%Y203 and Zr02—2Owt.%Y203. Cohesive and mixed failure on separate probability plots (Fig. 5 and Fig. 6 respectively) indicate the same trends of coating order as observed for adhesive failure. Scanning electron microscopy of fracture faces showed that failure occurred by separation at lamellae—lamellae or lamellae—substrate interfaces (Fig. 7).
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5. Discussion Assessment of a contribution to coating toughness by transformation of tetragonal ZrO2 has been based on DCB fracture toughness (crack growth resistance R) measurements. This test gives a number of independent measures of R from each specimen so that a statistical assessment of the
22
difference between the different types of coatings can be made. Previously, Ostojic [6] and Berndt and McPherson [4] calculated G ( R) from the compliance of the unloading curve where F2 dC G=—-— 2B dl where F is the load, B is the DCB arm width and dC/dl is the rate of change in compliance with crack length (linear elastic fracture mechanics). This procedure was originally adopted in the present work to yield an increasing and then a decreasing G with increasing crack length, a trend also observed by Mai and Hakeem [8] in cellulose fibre cements and Swain [9] in Mg—PSZ (PSZ, partially stabilized zirconia). Mai and Hakeem attempted to correct for the permanent offset through a term additional to G. The significant extension offset observed on unloading in the present study of coatings means that linear elastic fracture mechanics is not applicable. The results have therefore been analysed by the work of fracture approach, the work consumed per unit area of crack surface, by dividing the area under the load—extension diagram by the area created during crack extension. Since originally it was envisaged that compliance and not area was required, the DCB was not usually completely unloaded, thus omitting load (and extension) values below 0.1 kN where the unloading curve is non-linear. Areas were therefore calculated above 0.1 kN which reduces R by about 10%. The crack extension per cycle was also small, making the area estimates relatively inaccurate. The results of this analysis, however, do indicate general trends and suggest that the highest values of adhesive toughness are observed with coatings which contain transformable tetragonal Zr0 2. Comparison of crack growth resistance R for the different fracture modes shows that highest R was observed for mixed failure. The more tortuous crack path of this mode could account for this. Similarly Berndt and McPherson [4] postulated that adhesive failure has the lowest crack resistance because there is a geometrical restriction on crack path in this mode. This requires a more detailed investigation. Further work is also required to provide more accurate data through larger crack extensions per cycle and to determine the microstructural features which control coating toughness.
6. Conclusions A transformation toughening effect appears to be operative in coatings containing the transformable tetragonal phase. ZrO2—lOwt.%CeO2 has higher adhesive crack growth resistance than ZrO2—l5wt.%Ce02 which would be expected because Ms is approximately 100 °Chigher and the grains are therefore more likely to transform under stress. However, the role of other aspects of coating microstructure on fracture toughness, such as pore size
23
distribution, must be assessed before more definite conclusions may be drawn.
References 1 R. C. Garvie, R. H. J. Hannink and C. Urbani, Ceramurgia mt., 6 (1) (1980) 19. 2 E. Hornbogen, Acta Met., 26 (1978) 145. 3 G. N. Heintze and R. McPherson, Advances in Ceramics, Vol. 24, American Ceramic Society, 1987, in the press. 4 C. C. Berndt and R. McPherson, Mater. Sci. Res., 14 (1981) 619 -628. 5 R. McPherson and B. Shafer, Thin Solid Films, 97 (1982) 201. 6 P. Ostojic, Ph. D. Thesis, Monash University, Australia, 1986. 7 M. Sakai, K. Urashima and M. Inagaki, J. Am. Ceram. Soc., 66 (12) (1983) 868. 8 Y. W. Mai and M. I. Hakeem, J. Mater. Sci., 19 (1984) 501. 9 M. V. Swain, in R. C. Brandt, E. P. H. Hasselman and F. F. Lange (eds.), Fracture Mechanics of Ceramics, Vol. 6, Plenum, New York, 1983, p. 355.
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23
15
FRACTURE TOUGHNESS OF PLASMA-SPRAYED ZIRCONIA COATINGS* G. N. HEINTZE and R. McPHERSON Department of Materials Engineering, Monash University, Clayton 3168 (Australia) (Received April 24, 1987)
Summary Plasma-sprayed ceramic coatings are formed by introducing a powder into a plasma jet to produce a stream of molten particles which then impact, spread and rapidly solidify onto a substrate to form a lamellar microstructure. The cohesive and adhesive fracture toughness of zirconia coatings, fully or partially stabilized with Y203 or CeO2, plasma sprayed onto steel, have been determined using the double cantilever beam (DCB) technique. The results show that cohesive fracture toughness is greater than adhesive toughness and that tetragonal coatings have significantly higher toughness than fully stabilized (cubic) zirconia coatings. The highest toughness was achieved with Zr02—Ce02 coatings which contained a significant proportion of transformable tetragonal phase. The results are interpreted in terms of the influence of grain size and composition on the tetragonal-to-monoclinic phase transformation.
1. Introduction Plasma-sprayed zirconia coatings have been used for some time as thermal barriers because of their refractoriness and low thermal conductivity. The zirconia is alloyed with other oxides (stabilizers) so that the coating consists entirely of either the cubic or the tetragonal form at all temperatures to avoid degradation due to volume changes arising from transformations during heating and cooling. The improvement in toughness achieved in sintered ceramics which contain tetragonal Zr02 particles of a critical size that transform under stress is well known [1]. This raises the possibility of preparing partially stabilized zirconia coatings with enhanced toughness for applications at ambient temperatures. A current hypothesis of the toughening mechanism is that a propagating crack interacts with the metastable tetragonal particles in a zone *paper presented at NTSC87, the National Thermal Spray Conference and Exposition, Orlando, FL, U.S.A., September 14- 17, 1987. 0257-8972/88/$3.50
© Elsevier Sequoia/Printed in The Netherlands
16
at the crack tip and causes them to transform martensitically into the monoclinic structure. The 4% volume expansion associated with the transformation effectively compresses the crack tip and opposes its motion. The extra energy which must be supplied to the crack to maintain propagation appears as enhanced fracture energy [2]. A high proportion of transformable tetragonal Zr02 is achieved if the martensite start temperature M5 (the temperature at which tetragonal Zr02 spontaneously begins to transform to monoclinic on cooling) is around room temperature. The M5 temperature is a function of grain size, stabilizer type and concentration. It has been observed that Zr02—(10 15)wt.%Ce02-plasma-sprayed coatings contain a large proportion of tetragonal Zr02 which transforms to monoclinic on grinding [3]. The question then arises as to whether these coatings have superior toughness to coatings which do not contain transformable Zr02. The present paper is concerned with the assessment of fracture toughness of various Zr02-based coatings using the double cantilever beam (DCB) technique first reported by Berndt and McPherson [4]. -
2. Structure of plasma-sprayed zirconia coatings It is important to consider firstly the microstructure and phase constitution of plasma-sprayed ceramic coatings before considering their fracture toughness. These coatings are composed of lamellae formed by the impact and rapid solidification of individual molten particles. The lamellae are typically 1 2 jim thick with columnar grains 0.1 0.4 jim in cross-section, extending through their thickness (Fig. 1) [31.This small grain size is deter1) [5] and cannot be mined by the rapid (106 iø~°Cs controlled. Thus the solidification only means ofrate obtaining transformable tetragonal Zr0 2 is by suitable choice of the type and concentration of stabilizer to vary M8. Rapidly quenched Zr02-stabilizer solution droplets will crystallize to tetragonal Zr02 up to a certain concentration of stabilizer beyond which -
-
-
_____
IT
Fig. 1. Columnar grains within lamellae,
17
cubic Zr02 is formed. In general the cubic structure, if formed, will persist to room temperature giving a fully stabilized coating, for example Zr02— 2Owt.%Y203. Tetragonal Zr02, if formed, will transform to monoclinic on cooling if M~ is above room temperature, but will persist as “nontransformable” tetragonal if Ms is below room temperature. Since the grain size will vary over a certain range within the coating, there will be a range of M8, and the final structure observed may consist of a mixture of phases. Unmelted particles may also contribute to the final structure. The phase constitution of some Zr02 coatings, determined by X-ray diffraction [31, are shown in Table 1. M5 of plasma-sprayed Zr02—lOwt.%Ce02 and Zr02—l5wt.%Ce02 is approximately 300 °C and 200 °C respectively whereas Ms of Zr02— 6wt.%Y203 is well below room temperature [3]. Thus 6 wt.% Y203 coatings consist predominantly of non-transformable tetragonal whereas the Zr02— CeO2 coatings contain transformable tetragonal which may contribute to their fracture toughness parameters. TABLE 1 Phase constitution of plasma-sprayed zirconia coatings
Composition
Zr026wt.%Y203 ZrO2—2Owt.%Y203 ZrO2—lQwt.%Ce02 ZrO2—l5wt.%Ce02
Per cent
Per cent
Per cent
Per cent
cubic (F)
transformable tetragonal (T)
non-transformable tetragonal (T’)
monoclinic (M)
—
85
—
—
15 32
7
78
37
23
12 100 —
8
3 —
3. Fracture toughness measurements DCB specimens (Fig. 2) comprise two arms of steel (CS1O2O). One arm is grit blasted and plasma sprayed with the selected coating (no bond coat) and then glued with a heat-curable epoxy resin film (FM-73) to the other arm, also grit blasted. The curing time is 1 h at 120 °C.The distance from the loading point to the adhesive strip denotes the precrack length which is approximately 35 mm for each DCB. Coatings were sprayed using a Plasmadyne SG-100 torch at 900 A, 40 V. Powders sprayed were Zr02—lOwt.%CeO2, Zr02—l5wt.%Ce02 (prepared in the laboratory), Zr02—6wt.%Y203 and ZrO2—2Owt.%Y203 (commercial powders). Coating thickness ranged between 0.2 and 0.5 mm. The DCB was loaded via a computer-controlled MTS 810 servohydraulic testing machine, as detailed by Ostojic [6]. The extension (endpoint displacement) and load were recorded and automatically plotted at selected time intervals. When cracking began, as determined by a decrease in
18
FOREE
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~‘
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0 Fig. 2. DCB configuration (all dimensions in millimetres).
load with increasing extension, the DCB was unloaded. Between 8 and 15 loading—unloading cycles could usually be obtained before complete failure. The crack growth resistance R may be calculated from the area enclosed by the loading path of each cycle divided by the increment in area on the DCB arm owing to the additional crack extension [7]. Hence each coating tested yields several values. The crack length was determined from the inverse gradient of the unloading portion of the curve (compliance is equal to extension divided by load) using an experimentally determined calibration curve of compliance as a function of known ërack length. 4. Results A typical series of loading—unloading cycles obtained from one DCB is shown in Fig. 3(a). For each coating type there is permanent offset of approximately 0.02 mm over the crack length range approx. 35 110 nm. For clarity each cycle of Fig. 3(a) is separated by 0.01 mm in Fig. 3(b). Both the Zr02—6wt.%Y203 and the ZrO2—2Owt.%Y203 coatings failed in either -
one of three modes, namely cohesive (failure entirely with the coating), adhesive (close to or at the substrate interface) or mixed (large islands of coating remaining on the DCB arm). Zr02—lSwt.%Ce02 coatings failed either adhesively or cohesively (very limited data for the latter) whilst only adhesive failure was observed in the Zr02—lOwt.%Ce02 coatings. Probability plots of crack growth resistance for adhesive failure only are given in Fig. 4. Crack growth resistance is highest for Zr02—lOwt.%Ce02 followed, in decreasing order, by Zr02—l5wt.%Ce02, Zr02—6wt.%Y203 and Zr02—2Owt.%Y203. Cohesive and mixed failure on separate probability plots (Fig. 5 and Fig. 6 respectively) indicate the same trends of coating order as observed for adhesive failure. Scanning electron microscopy of fracture faces showed that failure occurred by separation at lamellae—lamellae or lamellae—substrate interfaces (Fig. 7).
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0,
0
0
CD
0
0
0
55 A41)IqC’qOJd
aAI4a)rlwnj
.i~ ~ —
—
.0 a .0 0
.0 a .0 0
C.I
0 (0/,)
0,
~
00
C
00011
21
99
90 95 90
0
0
~180 ~-70 ~6O ~50 060
0
0
0
‘
00
0
0
di 0
/
0
~20
1g)
-
~10
00
Mtxed Failure
0
CD
ZrO2-20Y203
0
1
0Zr02—61203
01 0
20
1,0
60
80
100
120
140 160 R (JIah
180
200
220
240
260
280 300
Fig. 6. Probability of crack growth resistance for mixed failure.
~
___
—~
~1Opm (a)
—
___
(b)
Fig. 7. Example of adhesive failure in a Zr02—lOwt.%Ce02 coating where a few areas of lamellae remain adhering to the DCB arm: (a) fracture face; (b) zirconium X-ray map to help discern lamellae.
5. Discussion Assessment of a contribution to coating toughness by transformation of tetragonal ZrO2 has been based on DCB fracture toughness (crack growth resistance R) measurements. This test gives a number of independent measures of R from each specimen so that a statistical assessment of the
22
difference between the different types of coatings can be made. Previously, Ostojic [6] and Berndt and McPherson [4] calculated G ( R) from the compliance of the unloading curve where F2 dC G=—-— 2B dl where F is the load, B is the DCB arm width and dC/dl is the rate of change in compliance with crack length (linear elastic fracture mechanics). This procedure was originally adopted in the present work to yield an increasing and then a decreasing G with increasing crack length, a trend also observed by Mai and Hakeem [8] in cellulose fibre cements and Swain [9] in Mg—PSZ (PSZ, partially stabilized zirconia). Mai and Hakeem attempted to correct for the permanent offset through a term additional to G. The significant extension offset observed on unloading in the present study of coatings means that linear elastic fracture mechanics is not applicable. The results have therefore been analysed by the work of fracture approach, the work consumed per unit area of crack surface, by dividing the area under the load—extension diagram by the area created during crack extension. Since originally it was envisaged that compliance and not area was required, the DCB was not usually completely unloaded, thus omitting load (and extension) values below 0.1 kN where the unloading curve is non-linear. Areas were therefore calculated above 0.1 kN which reduces R by about 10%. The crack extension per cycle was also small, making the area estimates relatively inaccurate. The results of this analysis, however, do indicate general trends and suggest that the highest values of adhesive toughness are observed with coatings which contain transformable tetragonal Zr0 2. Comparison of crack growth resistance R for the different fracture modes shows that highest R was observed for mixed failure. The more tortuous crack path of this mode could account for this. Similarly Berndt and McPherson [4] postulated that adhesive failure has the lowest crack resistance because there is a geometrical restriction on crack path in this mode. This requires a more detailed investigation. Further work is also required to provide more accurate data through larger crack extensions per cycle and to determine the microstructural features which control coating toughness.
6. Conclusions A transformation toughening effect appears to be operative in coatings containing the transformable tetragonal phase. ZrO2—lOwt.%CeO2 has higher adhesive crack growth resistance than ZrO2—l5wt.%Ce02 which would be expected because Ms is approximately 100 °Chigher and the grains are therefore more likely to transform under stress. However, the role of other aspects of coating microstructure on fracture toughness, such as pore size
23
distribution, must be assessed before more definite conclusions may be drawn.
References 1 R. C. Garvie, R. H. J. Hannink and C. Urbani, Ceramurgia mt., 6 (1) (1980) 19. 2 E. Hornbogen, Acta Met., 26 (1978) 145. 3 G. N. Heintze and R. McPherson, Advances in Ceramics, Vol. 24, American Ceramic Society, 1987, in the press. 4 C. C. Berndt and R. McPherson, Mater. Sci. Res., 14 (1981) 619 -628. 5 R. McPherson and B. Shafer, Thin Solid Films, 97 (1982) 201. 6 P. Ostojic, Ph. D. Thesis, Monash University, Australia, 1986. 7 M. Sakai, K. Urashima and M. Inagaki, J. Am. Ceram. Soc., 66 (12) (1983) 868. 8 Y. W. Mai and M. I. Hakeem, J. Mater. Sci., 19 (1984) 501. 9 M. V. Swain, in R. C. Brandt, E. P. H. Hasselman and F. F. Lange (eds.), Fracture Mechanics of Ceramics, Vol. 6, Plenum, New York, 1983, p. 355.