Sapphire deformation

Figure 6.18 Dislocation structure in sapphire deformed 4% by basal glide at 1400°C, consisting of A) glide dislocations, B) edge dipoles, C) faulted dipoles and D, E) dislocation loops. Basal foil, 650 kV. (Micrograph from B. J. Pletka and T. E. Mitchell, Case Western Reserve University, reproduced courtesy of The American Ceramic Society, Westerville, OH.)...

Figure 9.15 Dislocations in sapphire deformed on the basal plane to 3.6% shear strain at 1400°C. Examples of glide dislocations (C), regular dipoles (D), faulted dipoles (F),...

Figure 9.24 Stress-strain curves of sapphire deformed in basal glide at various temperatures [201].

Figure 9.25 Critical resolved shear stress versus mol% of oxide solute (for cations in the form of Cr +, Ti andTi ) in sapphire deformed at 1500°C. The curves through the data correspond toa c dependence. Data from Ref [203].

Fig. 3.73 Dislocation substructure in -doped sapphire deformed 2.5 % on basal plane at 1520 °C. Strings of loops resulting from breakup of dipoles by self-climb are apparent. Basal foil [38]. With kind permission of John Wiley and Sons...

The beam bending viscometer is depicted in Figure 10.10. A glass beam of uniform cross section is extended across an alumina muffle. Using a sapphire or fused silica hook, a load is applied at the center of the beam. The deformation rate of the center of the beam is measured, and the viscosity is determined... 

The shift of the A line in the epilayers has been connected with the variation of the lattice parameters of GaN [1,11,12], The shift of this line was also measured in samples subjected to hydrostatic pressure (see Datareview A3.1). Combination of all these data permits one to obtain the whole series of excitonic deformation potentials [6,16], Two sets of data are available which are consistent with each other and are given in TABLE 1. The discrepancies between them are linked to the differences in the values of the stiflhess coefficients of GaN used by the authors. Gil and Alemu [6] in their work subsequent to the work of Shan et al [16] used data not available when Shan et al calculated their values. The notations are the same and are linked to the relationship with the quasi cubic model of Pikus and Bir [17], Deformation potentials as and a6 have been obtained by Alemu et al [8] who studied the anisotropy of the optical response in the growth plane of GaN epilayers orthorhombically distorted by growth on A-plane sapphire. For a detailed presentation of the theoretical values of deformation potentials of GaN we refer the reader to Suzuki and Uenoyama [20] who took the old values of the stiflhess coefficients of GaN [21]. 

AIN buffer layer on (0001) sapphire. Composition was determined by help of X-ray diffraction. A continuous increase of the mode energy with x was observed. An AlxGai.xN/GaN/sapphire heterostructure grown with the AIN buffer layer technique was studied in infrared reflection and Raman spectroscopy by Wetzel et al [2] (FIGURE 2). From an X-ray analysis of the c-axis an AlN-ftaction of x = 0.15 was derived. Recently, however, it was shown that AIN layers in heterostructures with GaN are coherently strained up to a thickness of at least 350 nm. This leads to misinterpretation of the AIN fraction [8], Including the deformation of the unit cell in the pseudomorphic structure above, a value 50% smaller is concluded (x = 0.08). In backscattering off the c-plane the Ai(LO) mode was determined at 752 cm 1 (square with cross symbol) in excellent agreement with the infrared reflection data [2],... 

The effects of very high stresses and strain-rates have been investigated in microhardness experiments. In these experiments, loads of 50-500 g (corresponding to stresses as high as 2 GPa) are exerted by a diamond or sapphire Vickers indenter for about 20 seconds at temperatures up to 1,(X)0°C. Clearly, steady-state flow is never achieved but such experiments have provided important information about the dislocations involved in the deformation of olivine, for example. 

M.L. Kronberg, Plastic deformation of single crystals of sapphire Basal slip and twinning, Aeta Met. 5, 507-529 (1957). 

OMCL-TR800PS A is designed for contact-mode operation and has a conical probe tip. The probe was scanned over a sapphire surface to estimate the effective radius of curvature, R, determined to be about 20 nm. This value is not valid for larger sample deformation. To utilize JKR analysis, the probe tip must approximate a spherical shape. Thus, we tried to keep the deformation value to be as small as possible. R150FM-10 is a commercially available spherical probe tip with R= 150 nm. 

A difference appears only when, on increasing the load, the indentation size becomes much larger than the grains whereas in sapphire the deformability increases with the growing plastic zone, in the polycrystals this size effect is partly offset by the hindrance of dislocation activity due to the close spacing of the grain boundaries. 

Diamond and sapphire differ in that they have lower than normal ju.f values in the area of 0.1, and its value depends on the load, as might be expected for materials that deform elastically rather than plastically. Such materials also begin to show surface damage beyond a certain load. Under very clean conditions, m for diamond has been found to rise to 0.6, suggesting that some mechanism such as the adsorption of a monomolecular water layer or slight surface oxide formation may act to lubricate the diamond surface naturally. 

The centerpiece is sealed by pressure between two windows made of quartz or of sapphire. Quartz windows are good for transmitting light in the far-ultraviolet (UV). Sapphire, which is less susceptible to stress deformation in high force fields, is preferred with interference optics. 

The availability of sizable single crystals has led to a significant literature on the deformation of sapphire of various orientations, and at various temperatures. As already noted, the first such study was by Wachtman and Maxwell in 1954 [6], who activated (0001) 1/3 (1120) basal slip at 900 °C via creep deformation. Since that time, it has become clear that basal slip is the preferred slip system at high temperatures, but that prism plane slip, 1120 (1100), can also be activated and becomes the preferred slip system at temperatures below 600°C. Additional slip systems, say on the pyramidal plane 1012 1/3 (1011), have very high CRSSs and are thus difficult to activate. Both, basal and rhombohedral deformation twinning systems, are also important in AI2O3 (these are discussed later in the chapter). 

Dislocation-dissociation has been an important issue in studies of basal deformation in sapphire since the seminal studies of Kronberg [7], who suggested that basal dislocations would dissociate into half-partials via the reaction ... 

Among the oxides described in this chapter, deformation twinning is most important in the case of sapphire. Two twinning systems are known, on the rhombohedral and basal planes, and dislocation models for each have been suggested and confirmed using TEM. 

Rhombohedral Twinning Rhombohedral twinning is the deformation mode with the lowest CRSS in sapphire [105], and can occur at temperatures as low as —196 °C [106]. 

Parthasarathy, T.A. and Mah, T. (1993) Deformation behavior of an AljCh-YsAlsOu eutectic composite in comparison with sapphire and YAG. J. Am. Ceram. Soc., 76 29-32. 

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