The Effect of Crystal Orientation on Thermal Shock-Induced Fracture and Properties of Ion Implanted Sapphire
AU0121220
The Effect of Crystal Orientation on Thermal Shock-Induced Fracture and
Properties of Ion Implanted Sapphire
V. N. Gurarie1, P. H. Otsuka1, D. N. Jamieson1, J. S. Williams2, M. Conway2
'School of Physics, MARC, University of Melbourne, Parkville VIC. 3052
2Department of Electronic Materials Engineering, Research School of Physical Sciences and
Engineering, ANU, Canberra, 0200
Abstract
Ion beam modification of thermal shock resistance of sapphire single crystals with various crystallographic faces is experimentally investigated. The temperature threshold of fracture is determined in both implanted and unimplanted crystals by measuring the fragment contraction on cooling from fracture temperature. Optical and SEM microscopy are used to analyse fracture morphology and thermal shock behaviour on the (0001), (1 1 02) and (112 0) faces in sapphire crystals implanted with 70 keV Si" ions and then subjected to thermal stress testing using pulsed plasma. The most stable crystal faces in terms of stress resistance are established. Ion implantation is shown to reduce the temperature threshold of fracture for all faces tested. The
(1 1 02) face proved to be the most stable for both implanted and unimplanted crystals. The results are discussed on the basis of fracture mechanics principles and the implantation-induced crack nucleation process.
32/ 40 PACS: 61.82.-d; 61.72.Ww; 62.20.-x; 62.20.Mk
Keywords: Ion Implantation; Ceramics; Stress Resistance; Fracture
1. Introduction
Ceramic materials are known to be highly temperature resistant. However, an important consideration is their brittle nature, which causes them to fracture readily under sufficiently high stresses, in particular under dynamic loading. Surface modification by ion implantation has been shown to lower the fracture threshold in ceramic materials, 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 effectively raises the resistance to damage and therefore should result in higher durability following damage by thermal shock [1].
The observed modification of the fracture behaviour is due to the formation of surface energy- absorbing layers, which are known to improve the resistance to impact and thermal shock damage of ceramic materials. Ion implantation has been shown to be effective in producing such layers. Numerous fine cracks developed in the layers limit the strength of the material, but provide an effective mechanism for absorbing strain energy during thermal shock and preventing catastrophic crack propagation [2].
The crystal orientation is shown to affect the structure and mechanical properties of ion implanted crystals. In particular, the dose threshold of amorphisation in sapphire and thermal stress resistance of magnesia crystals are shown to be sensitive to crystal orientation [3,4]. There is a need to explore the surface energy-absorbing layers in crystals with different orientations.
Previous studies have mainly focused on cubic crystals, such as MgO [1,3]- In this study we
investigate the thermal stress resistance of the layers in sapphire single crystals with different
crystallographic faces. The study aims at analysing the effect of crystal orientation on thermal
stress resistance and establishing the most resistant crystal faces under thermal shock for
implanted and unimplanted sapphire crystals.
2. Experimental
The (0001), (1120) and (1102) faces of A12O3 single crystals are implanted with 70 keV Si" ions
to a dose of 1.0 x 1016 cnr2 at room temperature. Only half of the crystal surface is subjected to
implantation so that the implanted and unimplanted regions can be tested under similar thermal
conditions. This allows the response to thermal shock of implanted and unimplanted regions to be
compared.
Thermal shock is produced by exposing the sample surface of ~1.5 x 3 cm2 to a plasma jet produced by a plasma gun [1]. The plasma pulse duration is ~10 /us. The samples are placed at some distance from the gun with the plasma jet propagating perpendicular to the sample surface. The surface layer is under compressive strain during the heating stage followed by tensile stresses arising during the cooling stage of the plasma pulse. In these experiments the surface peak temperature is calibrated by measuring the size of fragments, bounded by cracks, and the gap between them [1]. 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 deformation of the fragment is
Ab/b, where Ab 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 Ablb = a (7f- To), where a is the thermal expansion coefficient, Tf is the fracture temperature, at which a crack separating adjacent fragments
originates, and To is the initial sample temperature. The fracture temperature is then determined from
the expression: Tf = To + Ab I b a. Details of this treatment are given elsewhere [1]. The gaps between fragments are often very small, particularly between small fragments and at low peak temperatures, so it is difficult to resolve them under an optical microscope and an SEM is used in these cases. Fig. 1 shows an SEM image of fracture in sapphire where the gap between the fragments is resolved.
3. Results and discussion
An optical and SEM microanalysis of fracture patterns indicates that in the region adjacent to the threshold of fracture, where the peak temperature is relatively low, cracks mostly propagate along cleavage planes. The fracture patterns are formed by the intersection of cleavage planes with the crystal face. The fracture patterns near the edge of the fracture zones for implanted and unimplanted areas in AI2O3 crystal for three faces are shown in Fig. 2 (a), (b), (c). Fig. 2 indicates that the observed fracture morphology depends on the crystal orientation. There is no distinct difference in the fracture morphology for implanted and unimplanted crystals. However, following implantation the temperature threshold of fracture is considerably changed.
The temperature calibration in the x direction has been done using the method described above.
This allows the temperature threshold corresponding to the start of fracture to be determined for implanted and unimplanted crystals for all three faces. Experimentally, the stress resistance parameter A7b was measured at the edge of the fracture zone where the temperature variation is just sufficient to originate fracture. Fig. 2 demonstrates that in the ion implanted regions fracture penetrates further into lower temperature zones. This means that on the implanted face fracture starts at lower peak temperature as compared to the unimplanted one. The experimental results of the ion beam effect on the stress resistance for different crystal faces in sapphire are given in
Table 1.
The data in the table indicate that the (1 1 02) face is most resistant to thermal shock for both implanted and unimplanted crystals. Following implantation, the (0001) and (1 T02) faces reduce their thermal shock resistance by -40%, while the (1120) face reduces its thermal shock resistance by -14%. The results suggest that the implantation effect on crack nucleation is dependent on crystal orientation in sapphire.
The results also suggest that ion implantation makes anisotropic properties less pronounced.
Indeed, the difference in A7b for the unimplanted (1102) and (11 20) crystal faces is -33%, while for the same faces in implanted crystals the difference is ~6%. The standard deviation in
A7b for unimplanted crystal faces is 64.3°, while for implanted crystals it is 26.8°.
The crack propagation over a particular cleavage plane system is also dependent on the crystal face and state of stress. The planes of easy cleavage in AI2O3 crystals belong to the following families [5,6]: {1 102}, {llOO} and {1126}. A microscopic analysis is used to identify the cleavage system along which cracks propagate and form the fracture morphology observed on different faces. However, the analysis indicates that the same fragment shape can be formed by different cleavage systems in sapphire. Therefore, a comprehensive identification of the cleavage systems, which produce fracture patterns on different faces, should be done in conjunction with a theoretical analysis of stress distribution and comparing stresses in different cleavage planes. The data are discussed in terms of the structural changes induced by ion implantation and their effect on crystal properties. The quantity ATb depends on elastic constants, thermal coefficient of expansion and the tensile strength ab [3]. All these properties are affected by ion implantation.
Ion implantation introduces lattice damage which commonly makes interatomic bonds weaker.
As a result, the elastic modulus and elastic constants are expected to be decreased, as reported in
[7]. However, in the equation that determines the quantity ATb, the elastic constants are present in both numerator and denominator [3]. As a consequence their change due to ion implantation is unlikely to significantly change the quantity ATb.
At the same time the thermal stress resistance parameter A 7b is proportional to fracture stress
Ob, which is highly sensitive to ion beam-induced lattice defects. It is known from fracture
? mechanics that for brittle solids ab = ( yE I na ) , where a is the crack length, y is the specific surface energy and E is the elastic modulus in the direction perpendicular to the cleavage plane.
All quantities y, E and a in the equation are affected by ion implantation. Since lattice defects make interatomic bonds weaker, the surface energy and elastic modulus are likely to be lowered following ion implantation. On the other hand, the implantation-induced lattice damage produces crack-nucleating defects of the size a bigger than that of the pre-existing ones [1,3]. The overall effect of ion implantation in this case is the reduction in crb and hence in ATb.
As indicated above, the extent of reduction in ATb depends on the crystal face. The microanalysis of the fracture patterns indicates that cracks propagate along different cleavage planes on different faces. This means that ion beam-induced modification of the surface energy and the elastic modulus in the direction normal to the cleavage plane are sensitive to crystallographic direction. A bigger reduction in ATb and ab is observed for faces and cleavage systems which display higher thermal stress resistance prior to implantation. Ion implantation is
V also likely to develop various lattice defects along certain preferential directions, thus producing crack nucleating defects of a different size in different cleavage planes. As a result some crystal faces that are more stress resistant before implantation become less resistant after implantation and visa versa as illustrated by the (0001) and (1120) faces in Table 1.
4. Conclusion
Optical and SEM analysis of fracture patterns indicate that the temperature threshold of fracture and crack density in sapphire crystals are substantially changed by ion implantation, while the fracture morphology remains practically unchanged. Ion implantation reduces the stress resistance parameter for all faces tested. The degree of reduction depends on the crystal face. The
(11 02) face is shown to be most resistant to thermal shock for both implanted and unimplanted sapphire crystals. The results suggest that ion implantation reduces the degree of anisotropy in sapphire as characterised by the spread in thermal stress resistance between different faces investigated.
References
[1] V.N. Gurarie, A.V. Orlov and J.S. Williams, Nucl. Instr. Meth. B 127/128 (1997) 616-620;
147, (1999) 221-225.
[2] J.B. Wachtman, Mechanical Properties of Ceramics (J. Wiley & Sons, 1996).
[3] V.N. Gurarie, P.H. Otsuka, J.S. Williams and M.J. Conway, Nucl. Instr. Meth. B 178/1-4
(2001) 138-143. [4] M.E. O'Hern, L.J. Romana, CJ. McHargue, J.C. McCallum and C.W. White, Effects of
Radiation on Materials: 15th International Symposium, ASTM STP 1125, R.E. Stoller, A.S.
Kumar and D.S. Gelles, eds, ASTM, Philadelphia (1992) 740-748.
[5] A.Azhdari and S. Nemat-Nasser, Mechanics of Materials 28 (1998) 247-262.
[6] S.M. Wiederhorn, J. Am. Cer. Soc, 52 (9) (1969) 485-491.
[7] C.W. White, CJ. McHargue, P.S. Sklad. L.A. Boatner and G.C. Farlow, in :Ion Implantation and
Annealing of Crystalline Solids (North-Holland, Amsterdam, 1989), p. 124-125. Fig. 1. SEM image of thermal shock-induced cracking in sapphire.
Fig. 2. Fracture patterns near the fracture threshold for implanted and unimplanted regions: (a) (0001) face, (b) (1120) face, (c) (1T02) face.
Table 1. Thermal shock resistance parameter for unimplanted and implanted sapphire crystals with different faces. Table 1. Thermal shock resistance, ATb (K), for unimplanted and implanted sapphire crystals with different faces. Face ATb (K), unimplanted ATb (K), implanted (0001) 650 390 (1120) 550 470 (lfO2) 820 500
G Fig. 1. ATm (K) f
700
650
600
550
500
450
400 Unimplanted t Implanted
Fig. 2 (a) ATm (K) f
700
650
600
550
500 /
75/an 450 Unimplanted t Implanted
Fig. 2 (b) ATm(K) .
Unimplanted Implanted
Fig. 2 (c)