- Email: [email protected]
Thermal fatigue of ion implanted magnesium oxide crystals
Nuclear Instruments and Methods in Physics Research B 148 (1999) 773±777
Thermal fatigue of ion implanted magnesium oxide crystals V.N. Gurarie
a,*
, D.N. Jamieson a, A.V. Orlov a, J.S. Williams b, M. Conway
b
a
b
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 The present work deals with developing a novel technique suitable for thermal fatigue testing of implanted materials and investigating the cycling fatigue properties of implanted magnesium oxide crystals. Single crystals of MgO were implanted with Siÿ 90 keV ions with a dose of 5.0 ´ 1016 cmÿ2 and then subjected to thermal cycling in a discharge plasma. The peak cycle temperature and thermal ¯ux were measured using a calorimetric method. The number of thermal cycles prior to an appearance of ®rst cracks and fatigue limit were evaluated for both the implanted and unimplanted regions. The data demonstrate that ion implantation increases the fatigue life and fatigue limit of MgO crystals. The analysis suggests the implantation-induced lattice damage retards the dislocation motion and, thus, reduces the plastic strain during thermal cycling that increases the resistance to the development of fatigue cracks. Ó 1999 Elsevier Science B.V. All rights reserved. PACS: 61.72Qq; 81.05Je Keywords: Ion implantation; Ceramics; Fatigue properties modi®cation
1. Introduction The cycling fatigue is often considered as a major cause of failure of ceramics and presents one of the major concerns in the design of reliable ceramic components [1,2]. These eects are reported to be of signi®cance in thermal barriers [3], nuclear reactors [4], plasma-facing components [5], electronic devices and packaging [6], ceramic composites [7], multilayered materials [8] and others. A
* Corresponding author. Tel.: 613 9344 7670; fax: 613 9347 4783; e-mail: [email protected]
number of mechanisms of fatigue crack development in ceramics are proposed [1±7]. A low-cycle thermal fatigue at high cycle temperatures is of particular problem in ceramics since high temperatures during cycling reduce the resistance to plastic deformation and creep, thus, producing cycling plastic strain that can lead to the development of fatigue cracks. Ion beam modi®cation of a wide variety of mechanical properties of ceramic materials is a subject of experimental study. However, the fatigue properties of ion implanted materials have rarely been measured. The paper aims at both developing a technique suitable for thermal fatigue testing of implanted materials and
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 7 2 1 - 6
774
V.N. Gurarie et al. / Nucl. Instr. and Meth. in Phys. Res. B 148 (1999) 773±777
investigating the ion beam modi®cation of fatigue cycling properties of magnesium oxide crystals. 2. Method and technique Magnesium oxide single crystals used in the experiments have the following dimensions: surface area 15 ´ 20 mm2 and thickness 6 mm. For all samples the freshly cleaved (1 0 0) surface was irradiated with Siÿ 90 keV ions with a dose of 5.0 ´ 1016 cmÿ2 . Only half of the sample surface was implanted to compare the fatigue properties of implanted and unimplanted regions under identical thermal cycling condidtions. To monitor the appearance of ®rst cracks and their propagation the sample surface is periodically examined under an optical microscope after each ®ve cycles. The experimental arrangement used for thermal fatigue cycling is shown in Fig. 1. The capacitor battery is discharged between two electrodes in the air-®lled chamber. A vacuum pump is used to trigger the discharge by reducing pressure in the chamber to a breakdown value. The interelectrode distance is 5±6 mm. The tungsten electrodes have a rectangular cross section of the area of 4 ´ 8 mm2 .
Fig. 1. Experimental arrangement.
The longer 8 mm dimension of the electrodes is parallel to the sample surface. Such con®guration provides a relatively uniform plasma stream over the sample surface. The intensity of plasma jet varies by placing the sample at dierent distances from the discharge space. The capacitor battery of 5 kV and 200 lF is discharged under pressure in the chamber between 40 and 80 mbar depending on the electrode distance. An oscilloscope connected to a coil placed near the discharge chamber shows heavily damped oscillations with a period of 40 ls. Damping indicates that 98% of the capacitor energy is released in 20 ls of the ®rst half period. Magnesium oxide samples are placed at 20 mm from the electrodes in the sample holder made of te¯on. The sample surface is covered with the te¯on mask except for a circular area of 9 mm in diameter which is exposed to the plasma as shown in Fig. 1. This area is divided into two semicircles with one of them being implanted. This allows both semicircular regions, implanted and unimplanted, to be tested under identical thermal cycling conditions. The plasma temperature under the experimental conditions is estimated to be 5 ´ 104 K which corresponds to ion energy of 10 eV. This energy is too low to produce any noticeable implantation eect in terms of the penetration depth and the degree of lattice damage in magnesium oxide crystals as compared to 90 keV Siÿ ions. Such plasma is not expected to produce any surface modi®cation and damage comparable with the implantation-induced and preexisting defects in the crystals used. A thermocouple is attached to the back surface of the sample to measure its temperature change. Thermal cycling is produced by multiple discharges with time interval between them of 30 s. During this time the heat transferred through the plasma aected area is uniformly distributed over the sample. Thus, the sample is practically free of a temperature gradient and thermal stresses by the time of the next thermal pulse. This prevents the build-up of a large-scale temperature gradient across the sample thickness due to accumulation of heat by successive thermal cycles. Thus, the stress is independently produced by each thermal
V.N. Gurarie et al. / Nucl. Instr. and Meth. in Phys. Res. B 148 (1999) 773±777
775
r ks ; p
shock consisting of the 20 ls duration heating stage followed by immediate cooling. A typical example of the temperature variation for MgO crystals during and after thermal cycling is shown in Fig. 2. During thermal cycling the rate of temperature increase as measured by the thermocouple is gradually slowed down because with the sample temperature increasing the heat losses into the surrounding ambient are increased. The graph indicates that the rate of cooling during the post-cycling stage, which starts after 300 s of cycling, is much lower than the rate of heating during thermal cycling in particular at a low sample temperature. For the sample temperature change of less than 3°C, which corresponds to the ®rst ®ve cycles, the heat losses can be ignored compared to the heat supply. In the experiments the sample temperature has been measured for the ®rst 5 cycles. The mean temperature variation due to one cycle has been determined as DT DTn /n, where DTn is the temperature change due to the n number of cycles. In determining the peak surface temperature per cycle DTm , the sample is considered as an in®nite half-space. A plane heat source acts on the sample surface during the time s. This assumption is justi®ed by the fact that the thickness of the heated layer is immeasurably less than the specimen thickness and the linear dimensions of the surface exposed to the plasma. For this case [9]:
The fatigue life has been measured as a number of thermal cycles, made at the same peak cycle temperature, prior to an appearance of fatigue fracture. The measurements made for various peak cycle temperatures are presented in Fig. 3. The curves indicate that with the cycle peak temperature decreasing the number of cycles required to produce fracture increases. This is a characteristic feature of fatigue failure. The results demonstrate that ion implantation of MgO crystals with the Siÿ ions increases their fatigue life and fatigue limit. For the ion implanted area 400 thermal cycles proved to be insucient to reach fracture at the cycle peak temperature of 700°C which is
Fig. 2. Variation of sample temperature during thermal cycling and subsequent cooling.
Fig. 3. Fatigue curves for implanted and unimplanted MgO crystals.
2F0 DTm K
where k is the thermal diusivity, K is the thermal conductivity and F0 is the heat ¯ux. The heat ¯ux averaged over the time s is determined using the expression: F0 qVcDT =sA; where q, V and c are the sample density, volume and speci®c heat respectively and A is the area exposed to plasma. 3. Results and discussion
776
V.N. Gurarie et al. / Nucl. Instr. and Meth. in Phys. Res. B 148 (1999) 773±777
apparently close to the fatigue limit of the ion implanted material. At the same time, for the unimplanted area 300 cycles are sucient to produce fatigue fracture at the peak cycle temperature of 675°C that is obviously higher than the fatigue limit of the unimplanted material. In the experiments the appearance of ®rst four cracks has been used as a criterion for evaluating the number of thermal cycles prior to fracture. This criterion proved to be more reliable for the evaluation of the fatigue resistance than the one based on the appearance of a single crack. Indeed, the study of the eect of ion implantation on fatigue properties of crystals requires that both the implanted and unimplanted areas have the same distribution of preexisting crack-nucleating defects. However, crack-nucleating defects in brittle solids are known to have a nonuniform distribution over the crystal. A statistical dispersion of the preexisting defects is apparently a major reason why ®rst fatigue cracks were noticed to appear sometimes in the implanted region. The eect of a nonuniform defect distribution can be eliminated or minimized by using many samples tested under identical cycling conditions. Otherwise, this can also be achieved using one or more samples in which the fatigue life is assessed by observing the crack nucleation on many defects. The latter method is possible in these experiments since the relaxation of thermal stresses due to a single crack is limited to a relatively small volume adjacent to the crack. The observation of crack density following thermal shock indicates that the size of the volume is comparable with the depth of crack penetration, which is 30 lm [10]. Outside the volume the thermal stress is almost unaected by the appearance of the crack. The criterion based on four cracks proved to be less dependent on the statistical dispersion in the defect distribution and provided systematic data on the eect of ion implantation on the fatigue life. Higher resistance of the implanted crystals to cycling fatigue is obviously related to the nature of the implantation-induced lattice damage. A high density of dislocations and other defects is produced which suppress the dislocation mobility. For a thermal shock in a semiin®nite space a full prohibition of the strain in the planes parallel to the
sample surface gives: aDTm ee + ep , where ee and ep are the elastic and plastic strain respectively. For a plane state of stress the elastic strain is limited by the yield stress ry according to the equation: ee ry /(1 ) m)E, where m is the Poisson coecient and E is YoungÕs modulus. The decreased dislocation mobility due to ion implantation is likely to increase the yield stress and, hence, the elastic strain per cycle. Since during thermal cycling the term aDTm remains constant, then the plastic strain is expected to be lower and the elastic strain higher in the implanted crystals in comparison with the unimplanted ones. This approach is consistent with the schematic stress±temperature hysteresis diagram presented in Fig. 4 for implanted and unimplanted crystals. According to the diagram the yield stress for the implanted material is higher and, as a consequence, the plastic strain during the cycle, which is characterized by the loop width, is lower. The data suggest that the thermal fatigue failure results from the interaction of a plastic slip with the preexisting and implantation-induced crack nucleating defects. The reduction in plastic deformation is obviously one of the major reasons for the increase in the thermal fatigue resistance of MgO crystals following ion implantation. It is worth noting that a single powerful thermal shock produces much higher crack density in the Si implanted MgO crystals compared to that in the unimplanted ones. This eect is due to the fact that ion implantation
Fig. 4. Thermal cycling loops for implanted and unimplanted samples.
V.N. Gurarie et al. / Nucl. Instr. and Meth. in Phys. Res. B 148 (1999) 773±777
is capable of generating numerous crack-nucleating defects [10]. On the contrary, the fatigue-induced cracks have smaller density in the implanted crystals. This is obviously associated with a different mechanism of crack nucleation in both processes. The plastic strain is known to be noticeable in MgO above 1100°C [11]. According to the graphs in Fig. 3, the thermal fatigue in MgO crystals is observed in the temperature range between 700°C and 1100°C. The data suggest that the dislocation motion and plastic slip, possibly on a microscopic scale, obviously proceed at temperatures below 1100°C. 4. Conclusions Using a calorimetric method for the cycle peak temperature evaluation a technique is developed for thermal cycling testing of ion implanted crystals and comparing their fatigue properties with that of unimplanted ones. The fatigue life and fatigue limit are determined to be higher in the implanted MgO crystals. The eect is related to the reduction in the dislocation mobility and the
777
plastic cyclic strain caused by the implantationinduced lattice damage.
References [1] H. Kishimoto, T. Hoshide, N. Okabe, Cycling Fatigue in Ceramics, Elsevier, Amsterdam, 1998. [2] H.L. Lee, S.E. Park, B.S. Hahn, J. Mater. Sci. 30 (1995) 2521. [3] D.M. Zhu, R.A. Miller, Surface and Coating Technology 94±95 (1997) 94. [4] V. Gutzeit, H. Hoven, J. Linke, M. Rodig, Practische Metallographie 35 (1998) 136. [5] J.A. Crowell, J.P. Blanchard, Fusion Eng. and Design 27 (1995) 515. [6] M.J. Lee, M. Pecht, Trans. ASME J. Electr. Packaging 117 (1995) 225. [7] L. Vandenbulke, M. Leparoux, J. de Physique IV 5 (1995) 735. [8] A.E. Giannakopoulo, S. Suresh, M. Finot, M. Olsson, Acta Metallur. Materialia 43 (1995) 1335. [9] H.S. Carslaw, J.C. Jaeger, Conduction of Heat in Solids, 2nd ed. 1959. [10] V.N. Gurarie, A.V. Orlov, J.S. Williams, Nucl. Instr. and Meth. B 127/128 (1997) 616. [11] J.R. Hague, L.F. Linch, A. Rudnick, F.C. Holden, W.H. Duckworth, Refractory ceramics for airspace, Am. Cer. Soc. 1964.
Thermal fatigue of ion implanted magnesium oxide crystals V.N. Gurarie
a,*
, D.N. Jamieson a, A.V. Orlov a, J.S. Williams b, M. Conway
b
a
b
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 The present work deals with developing a novel technique suitable for thermal fatigue testing of implanted materials and investigating the cycling fatigue properties of implanted magnesium oxide crystals. Single crystals of MgO were implanted with Siÿ 90 keV ions with a dose of 5.0 ´ 1016 cmÿ2 and then subjected to thermal cycling in a discharge plasma. The peak cycle temperature and thermal ¯ux were measured using a calorimetric method. The number of thermal cycles prior to an appearance of ®rst cracks and fatigue limit were evaluated for both the implanted and unimplanted regions. The data demonstrate that ion implantation increases the fatigue life and fatigue limit of MgO crystals. The analysis suggests the implantation-induced lattice damage retards the dislocation motion and, thus, reduces the plastic strain during thermal cycling that increases the resistance to the development of fatigue cracks. Ó 1999 Elsevier Science B.V. All rights reserved. PACS: 61.72Qq; 81.05Je Keywords: Ion implantation; Ceramics; Fatigue properties modi®cation
1. Introduction The cycling fatigue is often considered as a major cause of failure of ceramics and presents one of the major concerns in the design of reliable ceramic components [1,2]. These eects are reported to be of signi®cance in thermal barriers [3], nuclear reactors [4], plasma-facing components [5], electronic devices and packaging [6], ceramic composites [7], multilayered materials [8] and others. A
* Corresponding author. Tel.: 613 9344 7670; fax: 613 9347 4783; e-mail: [email protected]
number of mechanisms of fatigue crack development in ceramics are proposed [1±7]. A low-cycle thermal fatigue at high cycle temperatures is of particular problem in ceramics since high temperatures during cycling reduce the resistance to plastic deformation and creep, thus, producing cycling plastic strain that can lead to the development of fatigue cracks. Ion beam modi®cation of a wide variety of mechanical properties of ceramic materials is a subject of experimental study. However, the fatigue properties of ion implanted materials have rarely been measured. The paper aims at both developing a technique suitable for thermal fatigue testing of implanted materials and
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 7 2 1 - 6
774
V.N. Gurarie et al. / Nucl. Instr. and Meth. in Phys. Res. B 148 (1999) 773±777
investigating the ion beam modi®cation of fatigue cycling properties of magnesium oxide crystals. 2. Method and technique Magnesium oxide single crystals used in the experiments have the following dimensions: surface area 15 ´ 20 mm2 and thickness 6 mm. For all samples the freshly cleaved (1 0 0) surface was irradiated with Siÿ 90 keV ions with a dose of 5.0 ´ 1016 cmÿ2 . Only half of the sample surface was implanted to compare the fatigue properties of implanted and unimplanted regions under identical thermal cycling condidtions. To monitor the appearance of ®rst cracks and their propagation the sample surface is periodically examined under an optical microscope after each ®ve cycles. The experimental arrangement used for thermal fatigue cycling is shown in Fig. 1. The capacitor battery is discharged between two electrodes in the air-®lled chamber. A vacuum pump is used to trigger the discharge by reducing pressure in the chamber to a breakdown value. The interelectrode distance is 5±6 mm. The tungsten electrodes have a rectangular cross section of the area of 4 ´ 8 mm2 .
Fig. 1. Experimental arrangement.
The longer 8 mm dimension of the electrodes is parallel to the sample surface. Such con®guration provides a relatively uniform plasma stream over the sample surface. The intensity of plasma jet varies by placing the sample at dierent distances from the discharge space. The capacitor battery of 5 kV and 200 lF is discharged under pressure in the chamber between 40 and 80 mbar depending on the electrode distance. An oscilloscope connected to a coil placed near the discharge chamber shows heavily damped oscillations with a period of 40 ls. Damping indicates that 98% of the capacitor energy is released in 20 ls of the ®rst half period. Magnesium oxide samples are placed at 20 mm from the electrodes in the sample holder made of te¯on. The sample surface is covered with the te¯on mask except for a circular area of 9 mm in diameter which is exposed to the plasma as shown in Fig. 1. This area is divided into two semicircles with one of them being implanted. This allows both semicircular regions, implanted and unimplanted, to be tested under identical thermal cycling conditions. The plasma temperature under the experimental conditions is estimated to be 5 ´ 104 K which corresponds to ion energy of 10 eV. This energy is too low to produce any noticeable implantation eect in terms of the penetration depth and the degree of lattice damage in magnesium oxide crystals as compared to 90 keV Siÿ ions. Such plasma is not expected to produce any surface modi®cation and damage comparable with the implantation-induced and preexisting defects in the crystals used. A thermocouple is attached to the back surface of the sample to measure its temperature change. Thermal cycling is produced by multiple discharges with time interval between them of 30 s. During this time the heat transferred through the plasma aected area is uniformly distributed over the sample. Thus, the sample is practically free of a temperature gradient and thermal stresses by the time of the next thermal pulse. This prevents the build-up of a large-scale temperature gradient across the sample thickness due to accumulation of heat by successive thermal cycles. Thus, the stress is independently produced by each thermal
V.N. Gurarie et al. / Nucl. Instr. and Meth. in Phys. Res. B 148 (1999) 773±777
775
r ks ; p
shock consisting of the 20 ls duration heating stage followed by immediate cooling. A typical example of the temperature variation for MgO crystals during and after thermal cycling is shown in Fig. 2. During thermal cycling the rate of temperature increase as measured by the thermocouple is gradually slowed down because with the sample temperature increasing the heat losses into the surrounding ambient are increased. The graph indicates that the rate of cooling during the post-cycling stage, which starts after 300 s of cycling, is much lower than the rate of heating during thermal cycling in particular at a low sample temperature. For the sample temperature change of less than 3°C, which corresponds to the ®rst ®ve cycles, the heat losses can be ignored compared to the heat supply. In the experiments the sample temperature has been measured for the ®rst 5 cycles. The mean temperature variation due to one cycle has been determined as DT DTn /n, where DTn is the temperature change due to the n number of cycles. In determining the peak surface temperature per cycle DTm , the sample is considered as an in®nite half-space. A plane heat source acts on the sample surface during the time s. This assumption is justi®ed by the fact that the thickness of the heated layer is immeasurably less than the specimen thickness and the linear dimensions of the surface exposed to the plasma. For this case [9]:
The fatigue life has been measured as a number of thermal cycles, made at the same peak cycle temperature, prior to an appearance of fatigue fracture. The measurements made for various peak cycle temperatures are presented in Fig. 3. The curves indicate that with the cycle peak temperature decreasing the number of cycles required to produce fracture increases. This is a characteristic feature of fatigue failure. The results demonstrate that ion implantation of MgO crystals with the Siÿ ions increases their fatigue life and fatigue limit. For the ion implanted area 400 thermal cycles proved to be insucient to reach fracture at the cycle peak temperature of 700°C which is
Fig. 2. Variation of sample temperature during thermal cycling and subsequent cooling.
Fig. 3. Fatigue curves for implanted and unimplanted MgO crystals.
2F0 DTm K
where k is the thermal diusivity, K is the thermal conductivity and F0 is the heat ¯ux. The heat ¯ux averaged over the time s is determined using the expression: F0 qVcDT =sA; where q, V and c are the sample density, volume and speci®c heat respectively and A is the area exposed to plasma. 3. Results and discussion
776
V.N. Gurarie et al. / Nucl. Instr. and Meth. in Phys. Res. B 148 (1999) 773±777
apparently close to the fatigue limit of the ion implanted material. At the same time, for the unimplanted area 300 cycles are sucient to produce fatigue fracture at the peak cycle temperature of 675°C that is obviously higher than the fatigue limit of the unimplanted material. In the experiments the appearance of ®rst four cracks has been used as a criterion for evaluating the number of thermal cycles prior to fracture. This criterion proved to be more reliable for the evaluation of the fatigue resistance than the one based on the appearance of a single crack. Indeed, the study of the eect of ion implantation on fatigue properties of crystals requires that both the implanted and unimplanted areas have the same distribution of preexisting crack-nucleating defects. However, crack-nucleating defects in brittle solids are known to have a nonuniform distribution over the crystal. A statistical dispersion of the preexisting defects is apparently a major reason why ®rst fatigue cracks were noticed to appear sometimes in the implanted region. The eect of a nonuniform defect distribution can be eliminated or minimized by using many samples tested under identical cycling conditions. Otherwise, this can also be achieved using one or more samples in which the fatigue life is assessed by observing the crack nucleation on many defects. The latter method is possible in these experiments since the relaxation of thermal stresses due to a single crack is limited to a relatively small volume adjacent to the crack. The observation of crack density following thermal shock indicates that the size of the volume is comparable with the depth of crack penetration, which is 30 lm [10]. Outside the volume the thermal stress is almost unaected by the appearance of the crack. The criterion based on four cracks proved to be less dependent on the statistical dispersion in the defect distribution and provided systematic data on the eect of ion implantation on the fatigue life. Higher resistance of the implanted crystals to cycling fatigue is obviously related to the nature of the implantation-induced lattice damage. A high density of dislocations and other defects is produced which suppress the dislocation mobility. For a thermal shock in a semiin®nite space a full prohibition of the strain in the planes parallel to the
sample surface gives: aDTm ee + ep , where ee and ep are the elastic and plastic strain respectively. For a plane state of stress the elastic strain is limited by the yield stress ry according to the equation: ee ry /(1 ) m)E, where m is the Poisson coecient and E is YoungÕs modulus. The decreased dislocation mobility due to ion implantation is likely to increase the yield stress and, hence, the elastic strain per cycle. Since during thermal cycling the term aDTm remains constant, then the plastic strain is expected to be lower and the elastic strain higher in the implanted crystals in comparison with the unimplanted ones. This approach is consistent with the schematic stress±temperature hysteresis diagram presented in Fig. 4 for implanted and unimplanted crystals. According to the diagram the yield stress for the implanted material is higher and, as a consequence, the plastic strain during the cycle, which is characterized by the loop width, is lower. The data suggest that the thermal fatigue failure results from the interaction of a plastic slip with the preexisting and implantation-induced crack nucleating defects. The reduction in plastic deformation is obviously one of the major reasons for the increase in the thermal fatigue resistance of MgO crystals following ion implantation. It is worth noting that a single powerful thermal shock produces much higher crack density in the Si implanted MgO crystals compared to that in the unimplanted ones. This eect is due to the fact that ion implantation
Fig. 4. Thermal cycling loops for implanted and unimplanted samples.
V.N. Gurarie et al. / Nucl. Instr. and Meth. in Phys. Res. B 148 (1999) 773±777
is capable of generating numerous crack-nucleating defects [10]. On the contrary, the fatigue-induced cracks have smaller density in the implanted crystals. This is obviously associated with a different mechanism of crack nucleation in both processes. The plastic strain is known to be noticeable in MgO above 1100°C [11]. According to the graphs in Fig. 3, the thermal fatigue in MgO crystals is observed in the temperature range between 700°C and 1100°C. The data suggest that the dislocation motion and plastic slip, possibly on a microscopic scale, obviously proceed at temperatures below 1100°C. 4. Conclusions Using a calorimetric method for the cycle peak temperature evaluation a technique is developed for thermal cycling testing of ion implanted crystals and comparing their fatigue properties with that of unimplanted ones. The fatigue life and fatigue limit are determined to be higher in the implanted MgO crystals. The eect is related to the reduction in the dislocation mobility and the
777
plastic cyclic strain caused by the implantationinduced lattice damage.
References [1] H. Kishimoto, T. Hoshide, N. Okabe, Cycling Fatigue in Ceramics, Elsevier, Amsterdam, 1998. [2] H.L. Lee, S.E. Park, B.S. Hahn, J. Mater. Sci. 30 (1995) 2521. [3] D.M. Zhu, R.A. Miller, Surface and Coating Technology 94±95 (1997) 94. [4] V. Gutzeit, H. Hoven, J. Linke, M. Rodig, Practische Metallographie 35 (1998) 136. [5] J.A. Crowell, J.P. Blanchard, Fusion Eng. and Design 27 (1995) 515. [6] M.J. Lee, M. Pecht, Trans. ASME J. Electr. Packaging 117 (1995) 225. [7] L. Vandenbulke, M. Leparoux, J. de Physique IV 5 (1995) 735. [8] A.E. Giannakopoulo, S. Suresh, M. Finot, M. Olsson, Acta Metallur. Materialia 43 (1995) 1335. [9] H.S. Carslaw, J.C. Jaeger, Conduction of Heat in Solids, 2nd ed. 1959. [10] V.N. Gurarie, A.V. Orlov, J.S. Williams, Nucl. Instr. and Meth. B 127/128 (1997) 616. [11] J.R. Hague, L.F. Linch, A. Rudnick, F.C. Holden, W.H. Duckworth, Refractory ceramics for airspace, Am. Cer. Soc. 1964.