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Thermal stress resistance of ion implanted sapphire crystals
Nuclear Instruments and Methods in Physics Research B 147 (1999) 221±225
Thermal stress resistance of ion implanted sapphire crystals V.N. Gurarie b
a,* ,
D.N. Jamieson a, R. Szymanski a, A.V. Orlov a, J.S. Williams b, M. Conway b
a School of Physics, MARC, University of Melbourne, Parkville, VIC 3052 Australia Department of Electronic Materials Engineering, Research School of Physical Sciences and Engineering, ANU, Canberra, 0200, Australia
Abstract Monocrystals of sapphire have been subjected to ion implantation with 86 keV Siÿ and 80 keV Crÿ ions to doses in the range of 5 ´ 1014 ±5 ´ 1016 cmÿ2 prior to thermal stress testing in a pulsed plasma. Above a certain critical dose ion implantation is shown to modify the near-surface structure of samples by introducing damage, which makes crack nucleation easier under the applied stress. The eect of ion dose on the stress resistance is investigated and the critical doses which produce a noticeable change in the stress resistance are determined. The critical dose for Si ions is shown to be much lower than that for Crÿ ions. However, for doses exceeding 2 ´ 1016 cmÿ2 the stress resistance parameter decreases to approximately the same value for both implants. The size of the implantation-induced crack nucleating centers and the density of the implantation-induced defects are considered to be the major factors determining the stress resistance of sapphire crystals irradiated with Siÿ and Crÿ ions. Ó 1999 Elsevier Science B.V. All rights reserved. PACS: 61.72Qq; 81.05Je Keywords: Ion implantation; Ceramics; Thermal stress; Fracture
1. Introduction Surface modi®cation by ion implantation has been previously shown to alter a number of strength and fracture characteristics of lithium ¯uoride, magnesium oxide and glass samples [1,2]. In particular, thermal shock testing of LiF and MgO crystals implanted with Ar or Siÿ ions has revealed that the fracture threshold is lowered by
* Corresponding author. Tel.: +61 3 9344 5439; fax: +61 3 9347 4783; e-mail: [email protected]
ion implantation, allowing fracture to be initiated at lower surface temperatures. At the same time, ion implantation produces a higher density of cracks, but such cracks penetrate smaller distances into the material. This eectively raises the resistance to fracture damage [3,4]. The observed modi®cation of fracture behaviour is due to the formation of surface energy-absorbing layers which are known to improve the impact and thermal shock resistance of ceramic materials. Fine cracks developed in such layers limit the strength of the material, but provide an eective mechanism for absorbing strain energy during
0168-583X/98/$ ± see front matter Ó 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 9 8 ) 0 0 5 7 4 - 6
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V.N. Gurarie et al. / Nucl. Instr. and Meth. in Phys. Res. B 147 (1999) 221±225
thermal shock and preventing catastrophic crack propagation [5]. Ion implantation has been shown to be eective in generating numerous crack nucleating centers in lithium ¯uoride, magnesium oxide and glass samples [1,2]. These centers are readily activated to develop multiple microcracking and thus to eectively absorb the strain energy under the applied stress. Ion implantation using 86 keV Siÿ and Crÿ ions is shown to be capable of producing highly ecient surface energy-absorbing layers in magnesium oxide crystals [6]. It was also established that the microcrack density and the eciency of strain energy absorption in the surface layer can be increased without reducing the thermal stress resistance [6]. With this in view it is important to establish and quantify the implantation conditions which limit the strength and thermal stress resistance of implanted ceramic materials. The most signi®cant reduction in the stress resistance parameter was observed in MgO and sapphire crystals following implantation with 2.9 MeV protons [7]. The eect is related to the formation of hydrogen gas bubbles. This study aims at analysing the eect of the implantation dose on the thermal stress resistance, comparing the eects for both Siÿ and Crÿ implants and establishing major factors which determine the stress resistance of ion implanted sapphire crystals. 2. Experimental method The (0 0 0 1) faces of sapphire monocrystals were implanted with 86 keV Siÿ and 80 keV Crÿ ions to doses ranging from 5 ´ 1014 to 5 ´ 1016 cmÿ2 at room temperature. Only half of the crystal surface was subjected to implantation to compare the fracture behaviour and characteristics for implanted and unimplanted regions subjected to identical thermal stress loading. Thermal shock was produced by exposing the sample surface of 1.5 ´ 3 cm2 to a plasma jet produced by a plasma gun [1,2]. The plasma pulse duration was 40 ls. The samples were placed at some distance from the gun with the plasma jet
propagating perpendicular to the sample surface. The temperature and heat ¯ux were continuously reduced from the central part of the plasma-affected area in a radial direction along the x axis which separates the implanted and unimplanted areas. This arrangement allows both implanted and unimplanted regions to be tested under similar thermal conditions with the surface temperature varying from room temperature up to the melting point and above within roughly a 10±20 mm range along the x axis [1]. The temperature gradient is estimated to be 200°C/mm in the x direction. However, due to the short duration of the plasma pulse, the temperature gradient in the direction perpendicular to the surface is about three orders of magnitude higher than that across the surface. Therefore, the thermal stresses produced under these conditions are mainly due to the temperature gradient in the direction perpendicular to the surface. This produces a planar stress, with a zero stress component normal to the surface. In these experiments the surface temperature along the x axis is calibrated by measuring the size of fragments, bounded by cracks, and the gap between them. This gap is formed as a result of the contraction of adjacent fragments on cooling from the fracture temperature. Thus, by the end of cooling the relative temperature-induced deformation of the fragment is Db/b, where Db is the gap between the fragments and b is the fragment size. On the other hand, the relative (temperature) deformation is known to be equal to Db/b a (Tf )To ), where a is the thermal expansion coecient, Tf is the fracture temperature, at which a crack separating adjacent fragments originates, and To is the ®nal (room) sample temperature. The fracture temperature is then determined from the expression: Tf To +Db/ab. Details of this treatment are given elsewhere [1]. For a pure elastic behaviour the fracture temperature is equal to the actual temperature Tm corresponding to cooling from a zero stress level. Ion implantation generally suppresses the plastic strain, so for the maximum dose of 5 ´ 1016 cmÿ2 used in the experiments we assumed the plastic strain prior to fracture to be negligible, so Tm Tf . The gaps between fragments are often very small,
V.N. Gurarie et al. / Nucl. Instr. and Meth. in Phys. Res. B 147 (1999) 221±225
223
particularly between small fragments and at low peak temperatures, so it is dicult to resolve them under an optical microscope and an SEM is used in these cases. 3. Results and discussion An optical photograph of the fracture pattern of the sapphire crystal, subjected to thermal shock in a plasma, is shown in Fig. 1. The temperature is increased along the x axis in an upward direction towards the center of the plasma aected area. The x axis separates the implanted and unimplanted regions with the right hand side of the crystal irradiated with 86 keV Siÿ ions to a dose of 1.0 ´ 1016 cmÿ2 . The low temperature edge of the fracture zone corresponds to the start of fracture where the peak surface temperature corresponds to the fracture threshold, that is, the minimum temperature variation (relative to the mean sample temperature) required to initiate fracture. The photograph illustrates that in the irradiated region the fracture extends further into a region of lower temperature (lower x values), indicating the reduction in the thermal shock resistance following implantation. Similar eects have been observed in lithium ¯uoride and magnesium oxide crystals irradiated with Ar and Siÿ , respectively [1,2]. The temperature along the x axis has been calibrated by analysing the fragment size and the gaps between them as described above. For the maximum dose of 5 ´ 1016 cmÿ2 used in the experiments the plastic strain prior to fracture was taken to be negligible and the fracture temperature is then equal to the actual peak temperature corresponding to the start of cooling. In this way the minimum temperature variation necessary to initiate fracture has been evaluated for dierent implantation doses for sapphire crystals irradiated with 86 keV Siÿ and 80 keV Crÿ ions and the data are presented in Fig. 2. The graphs show that the minimum temperature variation necessary to initiate fracture is decreased with the dose for both Siÿ and Crÿ implants. However, this eect is only noticeable above certain critical doses, which correspond to 1.0 ´ 1015 cmÿ2 and 1.0 ´ 1016 cmÿ2 for Siÿ and
Fig. 1. Implanted and unimplanted fracture zones in sapphire crystal subjected to thermal shock; implantation with 86 keV Siÿ ions, dose 1.0 ´ 1016 cmÿ2 .
Crÿ implants respectively. Below the critical dose the implanted crystals have the same thermal stress resistance parameter as the unimplanted ones. This suggests that below the critical dose ion implantation generates crack nucleating defects which are less severe than the ones preexisting in the crystal. With this in mind it is worth noting that the critical
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V.N. Gurarie et al. / Nucl. Instr. and Meth. in Phys. Res. B 147 (1999) 221±225
is the plastic strain, E is the elastic modulus, rf is the fracture stress. The fracture stress is known to depend on the length of existing crack-nucleating defects according to the expression [8]: p 2 rf K= pc; where K is the fracture toughness and c is the length of a single-ended crack. Combining Eq. (1) and Eq. (2)) give the thermal stress resistance parameter in the form: p 3 DT ep =a 1 ÿ lK=aE pc:
Fig. 2. Thermal stress resistance parameter of sapphire crystals versus dose; implantation with 86 keV Siÿ and 80 keV Crÿ ions.
dose and the corresponding lattice damage are obviously dependent on the size and distribution of the preexisting crack nucleating defects in the crystal. In this particular case, as follows from Fig. 2, below the critical dose the samples implanted with both Siÿ and Crÿ do not show any substantial variation in the stress resistance parameter and, hence, in the size and distribution of the preexisting defects. Therefore, a signi®cant dierence in the critical dose for both implantation species is mainly due to the dierent nature or depth distribution of damage they generate in the crystal. The other characteristic feature of the graphs is that the stress resistance parameter for samples implanted with Crÿ is decreased faster with dose than for those implanted with Siÿ . At the dose above 2.0 ´ 1016 cmÿ2 the stress resistance parameter for both Siÿ and Crÿ implants reduces to comparable values. To identify the properties affected by the Siÿ and Crÿ implantation and analyse the eect of ion dose, the minimum temperature variation required to initiate fracture is presented in the form: DTm Tm ÿ T0 ep =a 1 ÿ l rf =aE;
1
where the terms (1)l)(rf /aE) and (ep /a) are the temperature variations corresponding to the elastic and plastic strain prior to fracture respectively; ep
Previous estimations using Eq. (1) and Eq. (2)), TRIM and RBS data demonstrate that the size of the crack-like defects in the implanted crystals is comparable with the ion range and dimensions of the implantation-induced lattice damage [6,7]. These results suggest that the implantation-induced defects combine and grow within the damaged layer during implantation and heating to nucleate a microcrack of dimensions comparable with the damage range. The reduction of DT with increasing ion dose is due to the increase in the size of the crack-nucleating defects (to produce microcracks of size c) and the modi®cation of properties involved in Eq. (3). After crack-nucleating defects agglomerate to produce cracks of the dimensions of the damaged layer, a further reduction in DT with dose is possible by a further modi®cation of the crystal properties. According to TRIM calculations and ion channelling measurements the ion range and the damage dimensions in sapphire crystals implanted with Si ions are roughly 2.5 times greater than that in crystals implanted with Crÿ ions. Therefore, according to Eq. (3) shorter microcracks which are expected to be nucleated by the Crÿ -induced defects should provide higher values of the stress resistance parameter as compared to the Siÿ -induced defects. Accordingly, higher doses of Crÿ ions are required in order to initially reduce the temperature threshold of fracture below the critical value determined by the preexisting defects. As a result the critical dose for Crÿ ions proved to be much higher that that for Si ions as illustrated in Fig. 2. The plastic strain prior to fracture and fracture toughness are also expected to be particularly sensitive to ion dose due
V.N. Gurarie et al. / Nucl. Instr. and Meth. in Phys. Res. B 147 (1999) 221±225
to the increase in the defect density with dose that tends to suppress plastic deformation. Other properties involved in Eq. (3), in particular the thermal coecient of expansion, could also be aected by ion implantation, but such eects have not been considered in the treatment here. A sharp decrease in DT for the Crÿ -implanted crystals at the dose exceeding the critical one is due to the fact that the damage produced by Crÿ ions although localised in a much thinner layer as compared to Siÿ implantation is more severe due to heavier Crÿ ions. Indeed, TRIM calculations demonstrate that the degree of lattice damage produced by Crÿ ions in sapphire is roughly twice as high as that produced by Siÿ ions for the same dose. RBS data to be published elsewhere are consistent with TRIM calculations. This obviously results in a more rapid change in the crystal properties and, as a consequence, in the stress resistance parameter with increasing dose for Crÿ ions in comparison with that for the Siÿ ions. 4. Conclusion The eect of the implantation dose in sapphire crystals with Siÿ and Crÿ ions on the stress resistance is investigated and the critical doses which produce a noticeable change in the stress resistance are determined. The critical dose for Si ions is
225
shown to be by one order of magnitude lower than that for Crÿ ions. However, for doses exceeding 2 ´ 1016 cmÿ2 the stress resistance parameter decreases to approximately the same value for both implants. The size of the implantation-induced crack nucleating centers and the density and distribution of the implantation-induced defects are considered to be the major factors aecting the properties of a near-surface layer and determining the stress resistance of sapphire crystals irradiated with Siÿ and Crÿ ions. References [1] V.N. Gurarie, J.S. Williams, A.J. Watt, J. Mat. Sci. Eng. A189 (1994) 319. [2] V.N. Gurarie, J.S. Williams, J. of Mat. Res. 5 (6) (1990) 1257. [3] D.P.H. Hasselman, Mat. Sci. Eng. 71 (1985) 251. [4] D.P.H. Hasselman, J.P. Singh, Am. Cer. Soc. Bull. 58 (9) (1979) 856. [5] D.W. Richerson, Modern Ceramic Engineering, Chapter. 4.5, Marcel Dekker, New York, 1982. [6] V.N. Gurarie, A.V. Orlov, J.S. Williams, Instr. and Meth. in Phys. Res. B 127/128 (1997) 616±620. [7] V.N. Gurarie, D.N. Jamieson, R. Szymanski, A.V. Orlov, J.S. Williams, in: J.C. Barbour, S. Roorda, D. Ila, (Eds.), Atomistic Mechanisms in Beam Synthesis and Irradiation of Materials, vol. 504, MRS Symposium Proceedings 1997, Fall Meeting, Boston, MA, in press. [8] J.B. Wachman, Mechanical Properties of Ceramics. Wiley, New York, 1996.
Thermal stress resistance of ion implanted sapphire crystals V.N. Gurarie b
a,* ,
D.N. Jamieson a, R. Szymanski a, A.V. Orlov a, J.S. Williams b, M. Conway b
a School of Physics, MARC, University of Melbourne, Parkville, VIC 3052 Australia Department of Electronic Materials Engineering, Research School of Physical Sciences and Engineering, ANU, Canberra, 0200, Australia
Abstract Monocrystals of sapphire have been subjected to ion implantation with 86 keV Siÿ and 80 keV Crÿ ions to doses in the range of 5 ´ 1014 ±5 ´ 1016 cmÿ2 prior to thermal stress testing in a pulsed plasma. Above a certain critical dose ion implantation is shown to modify the near-surface structure of samples by introducing damage, which makes crack nucleation easier under the applied stress. The eect of ion dose on the stress resistance is investigated and the critical doses which produce a noticeable change in the stress resistance are determined. The critical dose for Si ions is shown to be much lower than that for Crÿ ions. However, for doses exceeding 2 ´ 1016 cmÿ2 the stress resistance parameter decreases to approximately the same value for both implants. The size of the implantation-induced crack nucleating centers and the density of the implantation-induced defects are considered to be the major factors determining the stress resistance of sapphire crystals irradiated with Siÿ and Crÿ ions. Ó 1999 Elsevier Science B.V. All rights reserved. PACS: 61.72Qq; 81.05Je Keywords: Ion implantation; Ceramics; Thermal stress; Fracture
1. Introduction Surface modi®cation by ion implantation has been previously shown to alter a number of strength and fracture characteristics of lithium ¯uoride, magnesium oxide and glass samples [1,2]. In particular, thermal shock testing of LiF and MgO crystals implanted with Ar or Siÿ ions has revealed that the fracture threshold is lowered by
* Corresponding author. Tel.: +61 3 9344 5439; fax: +61 3 9347 4783; e-mail: [email protected]
ion implantation, allowing fracture to be initiated at lower surface temperatures. At the same time, ion implantation produces a higher density of cracks, but such cracks penetrate smaller distances into the material. This eectively raises the resistance to fracture damage [3,4]. The observed modi®cation of fracture behaviour is due to the formation of surface energy-absorbing layers which are known to improve the impact and thermal shock resistance of ceramic materials. Fine cracks developed in such layers limit the strength of the material, but provide an eective mechanism for absorbing strain energy during
0168-583X/98/$ ± see front matter Ó 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 9 8 ) 0 0 5 7 4 - 6
222
V.N. Gurarie et al. / Nucl. Instr. and Meth. in Phys. Res. B 147 (1999) 221±225
thermal shock and preventing catastrophic crack propagation [5]. Ion implantation has been shown to be eective in generating numerous crack nucleating centers in lithium ¯uoride, magnesium oxide and glass samples [1,2]. These centers are readily activated to develop multiple microcracking and thus to eectively absorb the strain energy under the applied stress. Ion implantation using 86 keV Siÿ and Crÿ ions is shown to be capable of producing highly ecient surface energy-absorbing layers in magnesium oxide crystals [6]. It was also established that the microcrack density and the eciency of strain energy absorption in the surface layer can be increased without reducing the thermal stress resistance [6]. With this in view it is important to establish and quantify the implantation conditions which limit the strength and thermal stress resistance of implanted ceramic materials. The most signi®cant reduction in the stress resistance parameter was observed in MgO and sapphire crystals following implantation with 2.9 MeV protons [7]. The eect is related to the formation of hydrogen gas bubbles. This study aims at analysing the eect of the implantation dose on the thermal stress resistance, comparing the eects for both Siÿ and Crÿ implants and establishing major factors which determine the stress resistance of ion implanted sapphire crystals. 2. Experimental method The (0 0 0 1) faces of sapphire monocrystals were implanted with 86 keV Siÿ and 80 keV Crÿ ions to doses ranging from 5 ´ 1014 to 5 ´ 1016 cmÿ2 at room temperature. Only half of the crystal surface was subjected to implantation to compare the fracture behaviour and characteristics for implanted and unimplanted regions subjected to identical thermal stress loading. Thermal shock was produced by exposing the sample surface of 1.5 ´ 3 cm2 to a plasma jet produced by a plasma gun [1,2]. The plasma pulse duration was 40 ls. The samples were placed at some distance from the gun with the plasma jet
propagating perpendicular to the sample surface. The temperature and heat ¯ux were continuously reduced from the central part of the plasma-affected area in a radial direction along the x axis which separates the implanted and unimplanted areas. This arrangement allows both implanted and unimplanted regions to be tested under similar thermal conditions with the surface temperature varying from room temperature up to the melting point and above within roughly a 10±20 mm range along the x axis [1]. The temperature gradient is estimated to be 200°C/mm in the x direction. However, due to the short duration of the plasma pulse, the temperature gradient in the direction perpendicular to the surface is about three orders of magnitude higher than that across the surface. Therefore, the thermal stresses produced under these conditions are mainly due to the temperature gradient in the direction perpendicular to the surface. This produces a planar stress, with a zero stress component normal to the surface. In these experiments the surface temperature along the x axis is calibrated by measuring the size of fragments, bounded by cracks, and the gap between them. This gap is formed as a result of the contraction of adjacent fragments on cooling from the fracture temperature. Thus, by the end of cooling the relative temperature-induced deformation of the fragment is Db/b, where Db is the gap between the fragments and b is the fragment size. On the other hand, the relative (temperature) deformation is known to be equal to Db/b a (Tf )To ), where a is the thermal expansion coecient, Tf is the fracture temperature, at which a crack separating adjacent fragments originates, and To is the ®nal (room) sample temperature. The fracture temperature is then determined from the expression: Tf To +Db/ab. Details of this treatment are given elsewhere [1]. For a pure elastic behaviour the fracture temperature is equal to the actual temperature Tm corresponding to cooling from a zero stress level. Ion implantation generally suppresses the plastic strain, so for the maximum dose of 5 ´ 1016 cmÿ2 used in the experiments we assumed the plastic strain prior to fracture to be negligible, so Tm Tf . The gaps between fragments are often very small,
V.N. Gurarie et al. / Nucl. Instr. and Meth. in Phys. Res. B 147 (1999) 221±225
223
particularly between small fragments and at low peak temperatures, so it is dicult to resolve them under an optical microscope and an SEM is used in these cases. 3. Results and discussion An optical photograph of the fracture pattern of the sapphire crystal, subjected to thermal shock in a plasma, is shown in Fig. 1. The temperature is increased along the x axis in an upward direction towards the center of the plasma aected area. The x axis separates the implanted and unimplanted regions with the right hand side of the crystal irradiated with 86 keV Siÿ ions to a dose of 1.0 ´ 1016 cmÿ2 . The low temperature edge of the fracture zone corresponds to the start of fracture where the peak surface temperature corresponds to the fracture threshold, that is, the minimum temperature variation (relative to the mean sample temperature) required to initiate fracture. The photograph illustrates that in the irradiated region the fracture extends further into a region of lower temperature (lower x values), indicating the reduction in the thermal shock resistance following implantation. Similar eects have been observed in lithium ¯uoride and magnesium oxide crystals irradiated with Ar and Siÿ , respectively [1,2]. The temperature along the x axis has been calibrated by analysing the fragment size and the gaps between them as described above. For the maximum dose of 5 ´ 1016 cmÿ2 used in the experiments the plastic strain prior to fracture was taken to be negligible and the fracture temperature is then equal to the actual peak temperature corresponding to the start of cooling. In this way the minimum temperature variation necessary to initiate fracture has been evaluated for dierent implantation doses for sapphire crystals irradiated with 86 keV Siÿ and 80 keV Crÿ ions and the data are presented in Fig. 2. The graphs show that the minimum temperature variation necessary to initiate fracture is decreased with the dose for both Siÿ and Crÿ implants. However, this eect is only noticeable above certain critical doses, which correspond to 1.0 ´ 1015 cmÿ2 and 1.0 ´ 1016 cmÿ2 for Siÿ and
Fig. 1. Implanted and unimplanted fracture zones in sapphire crystal subjected to thermal shock; implantation with 86 keV Siÿ ions, dose 1.0 ´ 1016 cmÿ2 .
Crÿ implants respectively. Below the critical dose the implanted crystals have the same thermal stress resistance parameter as the unimplanted ones. This suggests that below the critical dose ion implantation generates crack nucleating defects which are less severe than the ones preexisting in the crystal. With this in mind it is worth noting that the critical
224
V.N. Gurarie et al. / Nucl. Instr. and Meth. in Phys. Res. B 147 (1999) 221±225
is the plastic strain, E is the elastic modulus, rf is the fracture stress. The fracture stress is known to depend on the length of existing crack-nucleating defects according to the expression [8]: p 2 rf K= pc; where K is the fracture toughness and c is the length of a single-ended crack. Combining Eq. (1) and Eq. (2)) give the thermal stress resistance parameter in the form: p 3 DT ep =a 1 ÿ lK=aE pc:
Fig. 2. Thermal stress resistance parameter of sapphire crystals versus dose; implantation with 86 keV Siÿ and 80 keV Crÿ ions.
dose and the corresponding lattice damage are obviously dependent on the size and distribution of the preexisting crack nucleating defects in the crystal. In this particular case, as follows from Fig. 2, below the critical dose the samples implanted with both Siÿ and Crÿ do not show any substantial variation in the stress resistance parameter and, hence, in the size and distribution of the preexisting defects. Therefore, a signi®cant dierence in the critical dose for both implantation species is mainly due to the dierent nature or depth distribution of damage they generate in the crystal. The other characteristic feature of the graphs is that the stress resistance parameter for samples implanted with Crÿ is decreased faster with dose than for those implanted with Siÿ . At the dose above 2.0 ´ 1016 cmÿ2 the stress resistance parameter for both Siÿ and Crÿ implants reduces to comparable values. To identify the properties affected by the Siÿ and Crÿ implantation and analyse the eect of ion dose, the minimum temperature variation required to initiate fracture is presented in the form: DTm Tm ÿ T0 ep =a 1 ÿ l rf =aE;
1
where the terms (1)l)(rf /aE) and (ep /a) are the temperature variations corresponding to the elastic and plastic strain prior to fracture respectively; ep
Previous estimations using Eq. (1) and Eq. (2)), TRIM and RBS data demonstrate that the size of the crack-like defects in the implanted crystals is comparable with the ion range and dimensions of the implantation-induced lattice damage [6,7]. These results suggest that the implantation-induced defects combine and grow within the damaged layer during implantation and heating to nucleate a microcrack of dimensions comparable with the damage range. The reduction of DT with increasing ion dose is due to the increase in the size of the crack-nucleating defects (to produce microcracks of size c) and the modi®cation of properties involved in Eq. (3). After crack-nucleating defects agglomerate to produce cracks of the dimensions of the damaged layer, a further reduction in DT with dose is possible by a further modi®cation of the crystal properties. According to TRIM calculations and ion channelling measurements the ion range and the damage dimensions in sapphire crystals implanted with Si ions are roughly 2.5 times greater than that in crystals implanted with Crÿ ions. Therefore, according to Eq. (3) shorter microcracks which are expected to be nucleated by the Crÿ -induced defects should provide higher values of the stress resistance parameter as compared to the Siÿ -induced defects. Accordingly, higher doses of Crÿ ions are required in order to initially reduce the temperature threshold of fracture below the critical value determined by the preexisting defects. As a result the critical dose for Crÿ ions proved to be much higher that that for Si ions as illustrated in Fig. 2. The plastic strain prior to fracture and fracture toughness are also expected to be particularly sensitive to ion dose due
V.N. Gurarie et al. / Nucl. Instr. and Meth. in Phys. Res. B 147 (1999) 221±225
to the increase in the defect density with dose that tends to suppress plastic deformation. Other properties involved in Eq. (3), in particular the thermal coecient of expansion, could also be aected by ion implantation, but such eects have not been considered in the treatment here. A sharp decrease in DT for the Crÿ -implanted crystals at the dose exceeding the critical one is due to the fact that the damage produced by Crÿ ions although localised in a much thinner layer as compared to Siÿ implantation is more severe due to heavier Crÿ ions. Indeed, TRIM calculations demonstrate that the degree of lattice damage produced by Crÿ ions in sapphire is roughly twice as high as that produced by Siÿ ions for the same dose. RBS data to be published elsewhere are consistent with TRIM calculations. This obviously results in a more rapid change in the crystal properties and, as a consequence, in the stress resistance parameter with increasing dose for Crÿ ions in comparison with that for the Siÿ ions. 4. Conclusion The eect of the implantation dose in sapphire crystals with Siÿ and Crÿ ions on the stress resistance is investigated and the critical doses which produce a noticeable change in the stress resistance are determined. The critical dose for Si ions is
225
shown to be by one order of magnitude lower than that for Crÿ ions. However, for doses exceeding 2 ´ 1016 cmÿ2 the stress resistance parameter decreases to approximately the same value for both implants. The size of the implantation-induced crack nucleating centers and the density and distribution of the implantation-induced defects are considered to be the major factors aecting the properties of a near-surface layer and determining the stress resistance of sapphire crystals irradiated with Siÿ and Crÿ ions. References [1] V.N. Gurarie, J.S. Williams, A.J. Watt, J. Mat. Sci. Eng. A189 (1994) 319. [2] V.N. Gurarie, J.S. Williams, J. of Mat. Res. 5 (6) (1990) 1257. [3] D.P.H. Hasselman, Mat. Sci. Eng. 71 (1985) 251. [4] D.P.H. Hasselman, J.P. Singh, Am. Cer. Soc. Bull. 58 (9) (1979) 856. [5] D.W. Richerson, Modern Ceramic Engineering, Chapter. 4.5, Marcel Dekker, New York, 1982. [6] V.N. Gurarie, A.V. Orlov, J.S. Williams, Instr. and Meth. in Phys. Res. B 127/128 (1997) 616±620. [7] V.N. Gurarie, D.N. Jamieson, R. Szymanski, A.V. Orlov, J.S. Williams, in: J.C. Barbour, S. Roorda, D. Ila, (Eds.), Atomistic Mechanisms in Beam Synthesis and Irradiation of Materials, vol. 504, MRS Symposium Proceedings 1997, Fall Meeting, Boston, MA, in press. [8] J.B. Wachman, Mechanical Properties of Ceramics. Wiley, New York, 1996.