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Evidence of ferroelasticity in Y-tetragonal zirconia polycrystals
MATERIALSLETTERS
Volume 9, number 9
EVIDENCE
OF FERROELASTICITY
IN Y-TETRAGONAL
May 1990
ZIRCONIA
POLYCRYSTALS
M.G. CAIN and M.H. LEWIS Physics Department. University of Warwick, Coventry CV4 7AL, UK
Received 29 January 1990; in final form 28 February 1990
A ferroelastic transition has been observed in yttria-stabilised tetragonal zirconia polycrystalline ceramics (TZP) via X-ray diffraction and ultrasound non-destructive testing (NDT), measured as a function of applied uniaxial stress. Analysis of the ultrasound data indicates that the transition, occurring within the bulk ofthe material at a stress of = I .6 GPa, is largely reversible and as such can act as an energy absorption mechanism within these materials.
1. Introduction Zirconia is polymorphic existing in three well defined crystallographic forms [ 1 ] ; cubic, tetragonal and the room temperature monoclinic variant. The two high temperature forms can be stabilised at room temperature by appropriate solid solution doping with either di- or tri-valent metal ion oxides. It is possible, by judicious control of particle size, alloying chemistry and processing, to generate a fully tetragonal zirconia polycrystalline ceramic (TZP), with superior mechanical properties over conventional ceramics [ 2,3]. The mechanisms that are responsible for the impressive properties are well documented [ 4-7 ] and originate from the martensitic tetragonal to monoclinic transformation occurring within the stress field of a propagating crack. Recently, however, an additional toughening mechanism has been proposed by Virkar [ 81 and is based on a ferroelastic-type transition of the tetragonal zirconia in the presence of external stress. A crystal is ferroelastic if it has two or more stable orientation states and can be reproducibly transformed from one state to another by the application of mechanical stress [ 91. By analogy to ferromagnetic and ferroelectric crystals a ferroelastic material is characterised by the existence of a permanent strain following compressive loading and a hysteresis between the applied stress and resulting strain. Virkar has shown that Ce-stabilised TZP exhibits these phenomena [ 8 1. The area enclosed by the hysteresis loop 0167-577x/90/$
represents mechanical energy dissipated in a single compressive/tensile cycle. This energy absorption mechanism is thought to contribute to the fracture toughness of the material. In the case of tetragonal zirconia application of a compressive stress in excess of the coercive stress, a, (the stress required for ferroelastic transition to occur), along the c-axis of the tetragonal unit cell will transform it into an a-axis and conversely the a-axis into a c-axis. This effectively results in a rotation of the [ 0011 direction through 90”. An indirect indication of this ferroelastic transition can be accomplished by analysing the relative intensities of the tetragonal X-ray diffraction peaks of the (002)/(200) doublet and the (I 13)/( 131) doublet. The relative intensities of the (200) and the (002) planes can be determined by considering the relevant multiplicity factors and the corresponding structure factors. The ratio of the integrated intensities corresponding to the (200) set of planes is thus found to be about twice that of the (002) set. Subsequent to a ferroelastic-type transition, in which the [ 0011 direction is switched through 90’, a form of preferential alignment can occur in which the diffracted intensity from the (200) set of planes is decreased while that from the (002) set is equally increased. The resulting anisotropy has been observed in Ce-stabilised TZP ceramics [ 81 both on ground and fractured surfaces. This paper reports some experimental observations on yttria-stabilised TZP, using X-ray diffrac-
03.50 0 Elsevier Science Publishers B.V. (North-Holland )
309
Vofume9, number9
May 1990
MATERIALS LETTERS
tion similar to related work on ceria-stabilised TZPs [ 8 ] and transitions occurring within the bulkof the material were observed using ultrasound non-destructive testing measured as a function of applied uniaxial stress. Any transitions that occur within the bulk would be indicated by discontinuous changes in the ultrasound velocity.
few seconds and later correlated with the stress.
3. Results The surface crystallography of a mirror finish polished 3Y-TZP ceramic was compared to that of a coarsely ground surface using X-ray diffraction, figs. la and lb. No monoclinic content was detected on the polished surface and a barely detectable level on the ground surface indicated that a negligible tetragonal to monoclinic martensitic transformation had occurred. As expected the ratio of the (200) / (002) doublet for the polished surface was very nearly 2. The same ratio for the ground surface, however, was reduced to a value of %0.3, indicative of an anisotropic surface reorientation of the tetragonal crystallites. The X-ray diffraction pattern of a fracture surface of the same material, fig. 1c, reveals similar features to that of the ground surface, i.e. a reduction in the (200) / (002) ratio to a value of = 1.0. Interestingly, there is also a small monoclinic content which indicates that some stress-induced martensitic transformation had occurred. The ( 113 ) / ( 13 1) ratios further augment these observations. The ultrasound results are shown in fig. 2. Here, the square of the ultrasonic velocity is plotted against
2. Experimental details X-ray diffraction studies were conducted using a Philips type 1130/00 powder diffractometer (A=: 1.5405 A) which was operated at 28=l”/min. A 3 mol”~ Y-TZP ceramic was ground flat to dimensions of 10x 4 x 5 mm. The stress was applied in compression using an Instron universal testing machine with a constant imposed strain rate. A small lead zirconate (PLZT) ultrasound transducer was attached to the sampie using vacuum grease. An ultrasonic signal of 15 MHz was generated and transmitted through the sample, reflecting off the opposite wall and was received by the same transducer. In this way the time of flight between transmission and reception could be used to determine the ultrasonic velocity. Longitudinal waves were analysed and effects such as side wall reflection were assumed to be negligible. The ultrasound results were recorded every
a) Polished
b) Ground
c) Fractured
;_r, t200
t111
too2
mllT A
e
e
e
Fig. 1. X-ray traces identifying the change in crystallite orientation.
310
Volume 9, number
9
MATERIALS
Ultrasound
May 1990
LETTERS
99
Velocity2 (Normalised) x 100%
98
\, ,
97 500
1000 Stress
Fig. 2. Ultrasound
velocity squared
stressed material
applied uniaxial stress (the elastic moduli are very nearly proportional to the square of the velocity). Any significant change in the velocity may indicate a structural change within the bulk of the ceramic. Two such transitions occur at stress levels, a,, = 400? 50 MPa and acz= 1.6t- 0.2 GPa. A hysteresis between ultrasound velocity and stress was observed in the ultrasound/stress map, both after a=~,, and after a=~,,. Further analysis of fig. 2 indicates that the second transition, Q, is more pronounced than the first and, additionally, exhibits a smaller separation between the loading line and unloading line than does the first transition. The larger separation between the loading and unloading characteristics associated with the first transition (a,, ) is indicative of the irreversibility of this particular transition. The resulting structure of a surface that was po-
2000
/ MPa
plotted against stress indicating
Fig. 3. X-ray trace for the resulting
1500
identifying
the two transitions
the crystalline
uCI and 0,:.
changes.
sitioned normal to the application of stress was measured using X-ray diffraction, fig. 3. A small amount of monoclinic zirconia was observed on the surface ( < 5%) and an alteration in the (200) / (002) peak ratio implies some surface reorientation had occurred.
4. Discussion
and conclusions
The data obtained from X-ray analysis of polished and ground Y-stabilised TZP are similar to related work by Virkar on Ce-stabilised TZP [ 8 1. Furthermore, analysis of the fracture surfaces of Y-TZP reveals similar characteristics to that of the ground surface. It is apparent that significant reorientation of the tetragonal crystallites had occurred on the sur311
Volume 9, number 9
MATERIALS LETTERS
face of the ceramic such that the grains with a favourable orientation had their [ 00 1 ] directions switched through an angle of 90”. It is probable that the data obtained during the ultrasound measurements are not a surface effect and the observed transitions do not result from alternative stress-induced phenomena such as shear cracking. This is indicated by the high degree of reversibility in the ultrasound response at ocz when the zirconia was repeatedly loaded and unloaded. The ultrasound velocity decreased with increasing applied load as expected from the Poisson ratio of the material. From considerations of the strain energy, it is clear that, if any reorientation of the tetragonal grains were to occur, then their c-axes would align in a direction perpendicular to the applied stress. Since the ultrasound was measured in a direction perpendicular to the applied stress, then the relative increase in the ultrasound velocity at G= ac2 indicates that the ultrasound time of flight along the c-axis of tetragonal zirconia is longer than that along the a-axis. This implies a difference in elastic modulus along the two orthogonal axes. It is generally accepted that the tetragonal phase in TZP is stabilised with respect to the tetragonal to monoclinic martensitic transformation by virtue of the surrounding matrix constraint. The martensitic transformation may be initiated when sufficient constraint is removed and additional strain energy is available as found, for example, in the vicinity of a propagating crack or at a fractured or ground surface. It is clear that, as external stress is applied, the probability of such a transformation is more likely to occur on the surface of the ceramic rather than within the bulk. Since the transformation would be limited to the first few tens of microns (several layers of grains) at the surface then the change in ultrasound time of flight as a function of applied stress
312
May 1990
would not be particularly pronounced, as observed. This assumes, of course, that the sample dimensions were an order of magnitude larger than this surface depth. Furthermore, the martensitically transformed crystallites would be stable against retransformation when the material was unloaded. This would show up as a non-reversibility in the loading/unloading ultrasound characteristic, as observed. The second transition is more pronounced than the first and is largely reversible, indicative of a bulk ferroelastic transition with matrix constraint. This work indicates that Y-stabilised TZP is ferroelastic, the transition may occur in the bulk as well as on fracture surfaces where it may contribute significantly to toughness increments. Confirmation of the bulk transition is being sought from direct neutron diffraction measurements in stressed material.
Acknowledgement The authors are grateful to Richard Melville for assistance with the ultrasound measurements. This investigation was supported by Alcan Chemicals Ltd. and the Science and Engineering Research Council.
References [ 1 ] R.C. Garvie, in: High temperature oxides, Part 2, ed. A. Alper (Academic Press, New York, 1970). [2] T.K. Gupta, J. Mater. Sci. 12 (1977) 2421. [3] K. Tsukuma, Am. Ceram. Sot. Bull. 65 (1986) 1386. [4] A.G. Evans, Advan. Ceram. 3 (1983) 193. [ 51F.F. Lange, J. Mater. Sci. 17 (1982) 225. [ 61 R.M. McMeeking, J. Am. Ceram. Sot. 65 (1982) 242. [ 71 W. Pompe, Advan. Ceram. 3 ( 1983) 283. [ 81 A.V. Virkar and R.L.K. Matsumoto, Commun. Am. Ceram. Sot. 69 ( 1986) C224. [9] K. Aizu, J. Phys. Sot. Japan 27 (1969) 387.
Volume 9, number 9
EVIDENCE
OF FERROELASTICITY
IN Y-TETRAGONAL
May 1990
ZIRCONIA
POLYCRYSTALS
M.G. CAIN and M.H. LEWIS Physics Department. University of Warwick, Coventry CV4 7AL, UK
Received 29 January 1990; in final form 28 February 1990
A ferroelastic transition has been observed in yttria-stabilised tetragonal zirconia polycrystalline ceramics (TZP) via X-ray diffraction and ultrasound non-destructive testing (NDT), measured as a function of applied uniaxial stress. Analysis of the ultrasound data indicates that the transition, occurring within the bulk ofthe material at a stress of = I .6 GPa, is largely reversible and as such can act as an energy absorption mechanism within these materials.
1. Introduction Zirconia is polymorphic existing in three well defined crystallographic forms [ 1 ] ; cubic, tetragonal and the room temperature monoclinic variant. The two high temperature forms can be stabilised at room temperature by appropriate solid solution doping with either di- or tri-valent metal ion oxides. It is possible, by judicious control of particle size, alloying chemistry and processing, to generate a fully tetragonal zirconia polycrystalline ceramic (TZP), with superior mechanical properties over conventional ceramics [ 2,3]. The mechanisms that are responsible for the impressive properties are well documented [ 4-7 ] and originate from the martensitic tetragonal to monoclinic transformation occurring within the stress field of a propagating crack. Recently, however, an additional toughening mechanism has been proposed by Virkar [ 81 and is based on a ferroelastic-type transition of the tetragonal zirconia in the presence of external stress. A crystal is ferroelastic if it has two or more stable orientation states and can be reproducibly transformed from one state to another by the application of mechanical stress [ 91. By analogy to ferromagnetic and ferroelectric crystals a ferroelastic material is characterised by the existence of a permanent strain following compressive loading and a hysteresis between the applied stress and resulting strain. Virkar has shown that Ce-stabilised TZP exhibits these phenomena [ 8 1. The area enclosed by the hysteresis loop 0167-577x/90/$
represents mechanical energy dissipated in a single compressive/tensile cycle. This energy absorption mechanism is thought to contribute to the fracture toughness of the material. In the case of tetragonal zirconia application of a compressive stress in excess of the coercive stress, a, (the stress required for ferroelastic transition to occur), along the c-axis of the tetragonal unit cell will transform it into an a-axis and conversely the a-axis into a c-axis. This effectively results in a rotation of the [ 0011 direction through 90”. An indirect indication of this ferroelastic transition can be accomplished by analysing the relative intensities of the tetragonal X-ray diffraction peaks of the (002)/(200) doublet and the (I 13)/( 131) doublet. The relative intensities of the (200) and the (002) planes can be determined by considering the relevant multiplicity factors and the corresponding structure factors. The ratio of the integrated intensities corresponding to the (200) set of planes is thus found to be about twice that of the (002) set. Subsequent to a ferroelastic-type transition, in which the [ 0011 direction is switched through 90’, a form of preferential alignment can occur in which the diffracted intensity from the (200) set of planes is decreased while that from the (002) set is equally increased. The resulting anisotropy has been observed in Ce-stabilised TZP ceramics [ 81 both on ground and fractured surfaces. This paper reports some experimental observations on yttria-stabilised TZP, using X-ray diffrac-
03.50 0 Elsevier Science Publishers B.V. (North-Holland )
309
Vofume9, number9
May 1990
MATERIALS LETTERS
tion similar to related work on ceria-stabilised TZPs [ 8 ] and transitions occurring within the bulkof the material were observed using ultrasound non-destructive testing measured as a function of applied uniaxial stress. Any transitions that occur within the bulk would be indicated by discontinuous changes in the ultrasound velocity.
few seconds and later correlated with the stress.
3. Results The surface crystallography of a mirror finish polished 3Y-TZP ceramic was compared to that of a coarsely ground surface using X-ray diffraction, figs. la and lb. No monoclinic content was detected on the polished surface and a barely detectable level on the ground surface indicated that a negligible tetragonal to monoclinic martensitic transformation had occurred. As expected the ratio of the (200) / (002) doublet for the polished surface was very nearly 2. The same ratio for the ground surface, however, was reduced to a value of %0.3, indicative of an anisotropic surface reorientation of the tetragonal crystallites. The X-ray diffraction pattern of a fracture surface of the same material, fig. 1c, reveals similar features to that of the ground surface, i.e. a reduction in the (200) / (002) ratio to a value of = 1.0. Interestingly, there is also a small monoclinic content which indicates that some stress-induced martensitic transformation had occurred. The ( 113 ) / ( 13 1) ratios further augment these observations. The ultrasound results are shown in fig. 2. Here, the square of the ultrasonic velocity is plotted against
2. Experimental details X-ray diffraction studies were conducted using a Philips type 1130/00 powder diffractometer (A=: 1.5405 A) which was operated at 28=l”/min. A 3 mol”~ Y-TZP ceramic was ground flat to dimensions of 10x 4 x 5 mm. The stress was applied in compression using an Instron universal testing machine with a constant imposed strain rate. A small lead zirconate (PLZT) ultrasound transducer was attached to the sampie using vacuum grease. An ultrasonic signal of 15 MHz was generated and transmitted through the sample, reflecting off the opposite wall and was received by the same transducer. In this way the time of flight between transmission and reception could be used to determine the ultrasonic velocity. Longitudinal waves were analysed and effects such as side wall reflection were assumed to be negligible. The ultrasound results were recorded every
a) Polished
b) Ground
c) Fractured
;_r, t200
t111
too2
mllT A
e
e
e
Fig. 1. X-ray traces identifying the change in crystallite orientation.
310
Volume 9, number
9
MATERIALS
Ultrasound
May 1990
LETTERS
99
Velocity2 (Normalised) x 100%
98
\, ,
97 500
1000 Stress
Fig. 2. Ultrasound
velocity squared
stressed material
applied uniaxial stress (the elastic moduli are very nearly proportional to the square of the velocity). Any significant change in the velocity may indicate a structural change within the bulk of the ceramic. Two such transitions occur at stress levels, a,, = 400? 50 MPa and acz= 1.6t- 0.2 GPa. A hysteresis between ultrasound velocity and stress was observed in the ultrasound/stress map, both after a=~,, and after a=~,,. Further analysis of fig. 2 indicates that the second transition, Q, is more pronounced than the first and, additionally, exhibits a smaller separation between the loading line and unloading line than does the first transition. The larger separation between the loading and unloading characteristics associated with the first transition (a,, ) is indicative of the irreversibility of this particular transition. The resulting structure of a surface that was po-
2000
/ MPa
plotted against stress indicating
Fig. 3. X-ray trace for the resulting
1500
identifying
the two transitions
the crystalline
uCI and 0,:.
changes.
sitioned normal to the application of stress was measured using X-ray diffraction, fig. 3. A small amount of monoclinic zirconia was observed on the surface ( < 5%) and an alteration in the (200) / (002) peak ratio implies some surface reorientation had occurred.
4. Discussion
and conclusions
The data obtained from X-ray analysis of polished and ground Y-stabilised TZP are similar to related work by Virkar on Ce-stabilised TZP [ 8 1. Furthermore, analysis of the fracture surfaces of Y-TZP reveals similar characteristics to that of the ground surface. It is apparent that significant reorientation of the tetragonal crystallites had occurred on the sur311
Volume 9, number 9
MATERIALS LETTERS
face of the ceramic such that the grains with a favourable orientation had their [ 00 1 ] directions switched through an angle of 90”. It is probable that the data obtained during the ultrasound measurements are not a surface effect and the observed transitions do not result from alternative stress-induced phenomena such as shear cracking. This is indicated by the high degree of reversibility in the ultrasound response at ocz when the zirconia was repeatedly loaded and unloaded. The ultrasound velocity decreased with increasing applied load as expected from the Poisson ratio of the material. From considerations of the strain energy, it is clear that, if any reorientation of the tetragonal grains were to occur, then their c-axes would align in a direction perpendicular to the applied stress. Since the ultrasound was measured in a direction perpendicular to the applied stress, then the relative increase in the ultrasound velocity at G= ac2 indicates that the ultrasound time of flight along the c-axis of tetragonal zirconia is longer than that along the a-axis. This implies a difference in elastic modulus along the two orthogonal axes. It is generally accepted that the tetragonal phase in TZP is stabilised with respect to the tetragonal to monoclinic martensitic transformation by virtue of the surrounding matrix constraint. The martensitic transformation may be initiated when sufficient constraint is removed and additional strain energy is available as found, for example, in the vicinity of a propagating crack or at a fractured or ground surface. It is clear that, as external stress is applied, the probability of such a transformation is more likely to occur on the surface of the ceramic rather than within the bulk. Since the transformation would be limited to the first few tens of microns (several layers of grains) at the surface then the change in ultrasound time of flight as a function of applied stress
312
May 1990
would not be particularly pronounced, as observed. This assumes, of course, that the sample dimensions were an order of magnitude larger than this surface depth. Furthermore, the martensitically transformed crystallites would be stable against retransformation when the material was unloaded. This would show up as a non-reversibility in the loading/unloading ultrasound characteristic, as observed. The second transition is more pronounced than the first and is largely reversible, indicative of a bulk ferroelastic transition with matrix constraint. This work indicates that Y-stabilised TZP is ferroelastic, the transition may occur in the bulk as well as on fracture surfaces where it may contribute significantly to toughness increments. Confirmation of the bulk transition is being sought from direct neutron diffraction measurements in stressed material.
Acknowledgement The authors are grateful to Richard Melville for assistance with the ultrasound measurements. This investigation was supported by Alcan Chemicals Ltd. and the Science and Engineering Research Council.
References [ 1 ] R.C. Garvie, in: High temperature oxides, Part 2, ed. A. Alper (Academic Press, New York, 1970). [2] T.K. Gupta, J. Mater. Sci. 12 (1977) 2421. [3] K. Tsukuma, Am. Ceram. Sot. Bull. 65 (1986) 1386. [4] A.G. Evans, Advan. Ceram. 3 (1983) 193. [ 51F.F. Lange, J. Mater. Sci. 17 (1982) 225. [ 61 R.M. McMeeking, J. Am. Ceram. Sot. 65 (1982) 242. [ 71 W. Pompe, Advan. Ceram. 3 ( 1983) 283. [ 81 A.V. Virkar and R.L.K. Matsumoto, Commun. Am. Ceram. Sot. 69 ( 1986) C224. [9] K. Aizu, J. Phys. Sot. Japan 27 (1969) 387.