Mechanical Properties of Ceramics and Composites

the SiC and TiB2 phases (and elongation of SiC grains with the α–β transformation to 7.3 MPa·m1/2 due to enhanced crack bridging and deflection).

Other composites also show a particle size dependence of toughness. Thus Bellosi et al. [140] showed that addition of coarser (to 7 m ) TiN particles to Si3N4 increased K by 60%, while use of finer (<3 m) TiN increased K by 100% (with similar but smaller trends for strength). Nagaoka et al. [141] reported a K maximum of 7.8 MPa·m1/2 in composites of 10 v/o TiN particles in a Si3N4 matrix at a TiN D of 4 m, which was attributed to observed microcracking associated with the TiN particles (and probably assisted by the Si3N4 grain boundary phase). Crack deflection or bridging and associated R-curve effects have been shown in many of these composites [133–144]. Petrovic et al. [145] showed (I) toughness increasing to maxima at 40 v/o additions of MoSi2 particles to Si3N4 (+ MgO) and then decreasing. Composites with 10 m MoSi2 particles reached the highest maximum of > 8 versus > 5 MPa·m1/2 with 3 m MoSi2 (from 4.6 MPa·m1/2 with no MoSi2 and 1 w/o MgO). Composites with 10 m MoSi2 particles and 5 w/o MgO reached a maximum of > 6 MPa·m1/2 (from 4 at φ= 0).

Sigl and Kleebe [146] showed that additional increases in (NB) toughness of B4C + 20 or 40 v/o TiB2 particles ( 3 m) to 6.0 MPa·m1/2 occurred when excess carbon was present, e.g. versus 3–3.5 MPa·m1/2 with no excess carbon (and 2.2 MPa·m1/2 for B4C alone). They showed that the excess carbon caused microcracking, mainly at TiB2-B4C boundaries, and was the source of the additional toughness.

A potentially interesting system is that of AlN-SiC, which can form either a complete solid solution or two phase bodies, or some mixture of each depending on processing. As discussed earlier, Ruh et al. [45] showed somewhat lower E values for two-phase versus solid solution bodies (Fig. 8.11). Landon and Thevenot [147] reported toughness (I) constant over a range of compositions, i.e. 5.2 ± 0.3 and 4.7 ± 0.2 MPa·m1/2 respectively for compositions with 45–90 w/o α-SiC and 45–80 w/o β-SiC despite variations in E (and some of strength, related at least partly to changes in G, Chap. 9, Sec. III.D). Li and Watanabe [148] reported toughness increasing from 3.6 MPa·m1/2 to a maximum of 4.6 MPa·m1/2 at 10 m/o AlN (i.e. a 30 % increase, about the same as the maximum increase in strength, but this was at 5 m/o AlN, Chap. 9, Sec. III.D).

Even quite low levels of particulate addition to a matrix can have considerable impact in some cases, especially at quite fine particle sizes. Thus more limited additions of fine (e.g. nm scale) SiC particles in Al2O3 and Si3N4 matrices were noted. Addition of 1–5 v/o of submicron β-SiC particles to BaTiO3 by Hwang and Niihara [43] increased fracture toughness (and Young’s modulus and Vickers hardness) by 20–25% along with reducing G to < 1/3 of the value with no additions, i.e. from 1.4 to 0.4 m and a transition from tetragonal to pseudocubic structure at the higher loadings (Fig. 8.19).

FIGURE 8.19 Young’s modulus and fracture toughness versus volume percent (v/o) of submicron β-SiC particles to BaTiO3. Note the initial marked increase in E and K as well as the grain sizes of the BaTiO3 (in m) for each composition next to the K values, and the phase of the BaTiO3 for each composition next to the Young’s modulus value (T= tetragonal and Pc= pseudocubic). (Data after Hwang and Niihara [43].)

(Strengths increased by as much as 100–200%, but with much of this due to reduction of G at most sintering temperatures, and some effects of the tetragonal to cubic phase transformation for bodies with 3 and 5 v/o SiC, see Fig. 9.11.) Similarly, limited additions of nanoscale β-SiC particles to Si3N4 by Sasaki et al. [44] increased toughness nearly 50% from 4.1 MPa·m1/2 to a maximum of 6.1 MPa·m1/2 at 5 v/o and then decreasing back to the 0 v/o level at 20 v/o (while strengths increased < 1/2 as much, peaked at 10 v/o, and then rapidly decreased below 0 v/o levels).

Finally note three facts from these observations. First, there is substantial evidence for a particle dependence of toughness in at least several systems, indicating probable broad impact of D, e.g. K often increasing with D (and G), at least to some optimum D. Very fine particles may act as an extension of larger particle effects but may have other effects in addition or instead of larger particle effects; more research is needed in this area. Second, there is no clear evidence for the sign of the particle strains from expansion differences with the matrix determining the nature and ex-

tent of toughness effects of the dispersed particles. Thus while SiC or TiC particles have thermal expansions < for an Al2O3 matrix and these same particles have expansions > for an Si3N4 matrix, as do TiB2 particles in a SiC matrix, all showed toughening with particle additions and an optimum particle size. While microcracking is a probable factor in many and possibly all of these, as shown for the SiC-TiB2, composites, more extensive and detailed evaluations are needed, e.g. effects of particle size distributions, mixing, agglomeration, and orientation are needed along with statistically significant data. Third, while some composites developed earlier were not tested for R-curve effects, most of these composites show such effects with typical large cracks as used for most toughness measurements.

D.Ceramic Platelet and Whisker Composites

A review of the ceramic platelet composites literature shows similar results to particulate composites discussed above, in particular K frequently passing through a maximum as φ increased. Thus addition of 5–20 vol% Al2O3 platelets (e.g. 10 m dia.) to 2Yand 3Y-TZP) matrices by Heussner and Claussen [149] using isopressing, sintering, and HIPing increased K (CNB) by 15–40% at maxima at φ= 0.05, while addition of the platelets to a 12 Ce-TZP matrix gave a minimum in K at φ= 0.05 (the latter minimum attributed to effects of nontransformable particles suppressing transformation in the Ce system). Huang and Nicholson [150], using a 4.5Y-TZP and tape casting followed by sintering and HIPing showed K maxima at φ= 0.15–0.3, with IF showing a substantially greater maximum than the CNB (i.e. similar to Heussner and Claussen). They also showed that larger platelets (dia. 12 m versus 2 and 1 m) and higher aspect ratios (respectively 12, 5, and 1) gave somewhat greater K increases and that orientation of the platelets randomly, parallel, or perpendicular to the stress axis had little effect) (see also Chap. 11, Sec. III.E).

Chen and Chen [47] reported that in situ formation of various hexaluminate platelets with overall dimensions similar to those of the matrix Al2O3 grains (e.g. 2–15 m) resulted in a maximum toughness, i.e. 4.3 versus 3.0 MPa·m1/2 for the martix alone, at φ= 0.3. Fracture was mixed interand transgranular at peak toughness, and higher volume fractions had crack bridging by elongated aluminate grains. Similar but greater increases in (SEPB) toughness from 3.5 to 6 MPa·m1/2 were reported by Yasuoka et al. [151] by such in situ growth of hexaluminate platelets combined with inducing some platelet character of the Al2O3 grains via doping of 240 ppm SiO2. The latter addition enhanced intergranular fracture and hence grain bridging and thus was a major factor in this addition being the source of 3/4 the total toughness improvement, but reducing strengths by 6%. Kim et al. [152] reported nearly identical (I) toughness increases, maxima (at 1 m/o), and subsequent modest decreases in Al2O3 doped with 0.5 to 3 m/o Na2O + MgO to grow in situ beta alumina platelets (with

greater strength increases to a more pronounced maximum at 0.5 m/o, but with some of this due to G reduction). Koyama et al. [153] reported that (CNB) toughness of Al2O3 with platelet grains due to low additions of CaO + SiO2 increased nearly the same as was found by Yasuoka et al. and showed substantial R-curve effects. Toughness increased with either diameter (d) or thickness (t) to the 1/2 power (or d5/6t-1/3), which was also true for the lesser increases as G increased in bodies with equiaxed grains. However, as noted in Chap. 3, Sec. IV.A, strengths of bodies both with platelet grains and with equiaxed grains increased as toughness deceased, following typical Al2O3 σ–G-1/2 behavior. An and Chan [154] also demonstrated substantial toughening and R-curve effects in Al2O3 toughened via in situ formation of 30 v/o Al2O3·CaAl12O19 platelets in Al2O3, but noted that this came at the expense of strength, which they acknowledged was inevitable, as is now being increasingly recognized (Chap. 9, Sec. III.E).

Nischik et al. [155] evaluated composites of either Al2O3 or SiC platelets (both 15 m dia. and 1 m thick) made by variations of powder processing with a mullite matrix via sintering, with and without HF or oxidative pretreatment of the SiC platelets. Toughness (IF, and strengths) were apparently measured for cracks parallel with the pressing or pressure filtration directions. The Al2O3 platelet composites (φ= 0.1) had either somewhat lower or higher toughnesses than the matrix alone depending on processing, i.e. 1.9–2.8 versus 2.2–2.6 MPa·m1/2 for the matrix (but always somewhat to substantially lower strengths than the matrix, i.e. 150–240 versus 260–310 MPa for the matrix). SiC platelet composite toughnesses for φ= 0.1 ranged from about as low as with Al2O3 platelets to higher values, i.e. 1.9–3.3 MPa·m1/2, while strengths ranged from lower to higher values compared to Al2O3 platelet composites, i.e. 120–340 MPa, thus higher than for the matrix alone. While HIPing gave among the highest toughnesses (and strengths) it did not give the highest values; varying φ from 0.05 to 0.2 did not show clear increases in toughness (or strength), nor did HF etching or preoxidation of the SiC platelets. However, the substantial interfacial, i.e. matrix-SiC platelet, fracture with as-received SiC platelets was reduced with preoxidation that then often resulted in substantial transgranular fracture of the SiC platelets, e.g. often parallel with the plane of the platelets.

Chou and Green [156] investigated the mechanical properties of composites with 10–30 v/o of α-SiC platelets with average diameters and thicknesses (in m), and aspect ratios of 12, 2, and 6, and 24, 6, 4 in an alumina matrix made by hot pressing. Composites with the larger SiC platelet showed slight decreases of E as φ increased to 0.15 and then dropped sharply by φ= 0.2 and further by φ=0.3, while composites with the smaller SiC platelets showed that E increased continuously but modestly over this range. The differences were shown to be due to microcracking mainly in the plane of the platelets ( parallel with the hot pressing surfaces) due to expected size dependence of microcracking, but also a volume fraction dependence. Fracture toughness (IF)

measurements for cracks propagating perpendicular to the plane of hot pressing were nearly the same for the smaller platelets, increasing almost linearly from 4.3 to 7.1 MPa·m1/2 as φ increased from 0 to 0.3. The larger platelet composites’ toughnesses paralleled the Young’s modulus behavior, i.e. increasing somewhat less to 5.3 MPa·m1/2 at .φ 0.15 and then decreasing to 3.5 (φ= 0.2) and then to 2.9 MPa·m1/2 at φ= 0.3. They did not measure toughness for the third normal direction (i.e. crack propagation normal to the hot pressing axis, thus parallel with the larger surfaces of the platelets), which they recognized would produce lower toughness values. However, they also measured toughness from indent crack sizes in composites of the smaller platelets with φ= 0.3, again showing little difference for the two crack orientations parallel with the hot pressing axis (hence normal to the plane of the platelets), i.e. 6.7–7.0 MPa·m1/2 in good agreement both relatively and absolutely with their indent fracture measurements. The indent toughness for cracks normal to the hot pressing axis, i.e. parallel with the platelets, was 3.3 MPa·m1/2.

Chaim and Talanker [157] reported substantial (NB) toughness increases with addition of α-SiC platelets (50–250 m dia., 5-25 m thickness) in a cordierite glass matrix. Most of the increase occurred from 0 to 10 v/o SiC addition, e.g. from 1.6 to 2.5 MPa·m1/2, i.e. 50% increase, followed by only another 10–15% increase at 30 v/o SiC for crack propagation parallel with the hot pressing direction. Crack propagation in the normal direction gave only modestly lower K values at 10 and 20 v/o and no difference at 30 v/o SiC, so preferred orientation had limited effect. Crystallizing the glass matrix increased composite toughnesses by about the same amount it increased the toughness of the matrix alone, which was to 1.9 MPa·m1/2. While some of these toughness trends were similar to those for strength, there were also significant differences.

Cooper et al. [158] showed that WOF of alumina-graphite refractories increased linearly from 20 J/m2 at 5 v/o graphite to 80 J/m2 at 40 v/o graphite, where the increase appeared to be beginning to saturate.

Mitchel et al. [159] reported toughness (IF) increased to 6.2–7.3 MPa·m1/2 for crack propagation normal to the plane of hot pressing (hence the plane of alignment of the platelets) in composites of thin α-SiC platelets ( 25 m dia., φ=0.2) in a β-SiC matrix. The key to the toughness was coating the SiC platelets with a thin layer of Al2O3 powder before mixing into the fine β powder (with B + C sintering additions). For crack propagation parallel with the plane of hot pressing (hence also with the alignment of the planes of the platelets) the toughness was 3.8–4.6 MPam1/2, i.e. only slightly above that of the matrix alone (3.5–4.0 MPa·m1/2). (However, strengths were the same for both test orientations despite a nearly 100% difference in toughness.)

Baril et al. [25] also showed that uniform addition of up to 30% SiC platelets (11–24 m dia.) to Si3N4 increased K 40% (but left strength unchanged or reduced by up to 20%, while the Weibull modulus was increased by

up to 130%). Strength retention was distinctly best with the finest platelets. Weibull modulus results were mixed, while K results were generally independent of platelet size. Hanninen et al. [160] reported greater K increases respectively with larger vs. smaller platelets and particulates (of similar diameter as the small platelets), but with corresponding greater S decreases as the volume fraction of particles or platelets increased. Claar et al. [161] reported high strengths (450–900 MPa), K values (11–23 MPa·m1/2), and Weibull moduli (21–68) for reaction processing Zr and B4C to produce in situ formed ZrC grains and ZrB2 “platelets.” However, much and possibly all of the improvement in mechanical properties and specifically toughness is due to the residual Zr content, i.e. properties increased with increased free Zr content, and plastic deformation of Zr ligaments on fracture surfaces was observed. Also, the “platelets” were not disks, as in most of the previous cases above, but were much closer to thicker whiskers (typically several microns in thickness).

More investigation has been conducted on ceramic whisker composites, in part since whiskers were available and showed success in earlier development of Al2O3-SiC whisker composites, which are in commercial production and provide a more comprehensive starting point, especially studies of Becher and colleagues [85,162–166]. They showed fracture toughness increasing as φ increases, indicating a probable maximum at φ 0.3 (as have others, though some of this maximum may reflect increasing densification difficulty as φ increases), as well as that toughness increases were linear as a function of φ1/2 and of r1/2 (where r is the whisker radius). They also showed that toughness increased as the Al2O3 matrix grain size increased, e.g. by 1 MPa·m1/2 on going from G 1–2 to 4–8 m, and again on going to 15–20 m (and that there was considerable transgranular fracture of the Al2O3 matrix grains, especially as G increased), and that lower surface oxygen on the whisker surfaces gives higher toughnesses.

Other studies are generally consistent with these trends, e.g. Yang and Stevens [59] showed linear increases in fracture energy as φ increased (to 0.3), with greater increases with whiskers leached to remove surface oxide versus asreceived whiskers, and also showed substantial matrix transgranular fracture. Yasuda et al. [167] showed fracture energy and toughness increasing with both φ and r, but that they were linear with φ1 not φ1/2 and with r2/l (where l = whisker length) not r1/2 and that the level of the linear increases varied substantially with different sources of whiskers, in part probably reflecting the above whisker dimension effects but also possible effects of composition such as surface oxygen. Krylov et al. [168] reported (NB) toughness (and strength) increasing rapidly from 4.5 MPa·m1/2 as whisker diameters increased from 0.1 m, but saturating at 6.4 MPa·m1/2 at diameters of 2 m in composites with 20 v/o whiskers. Baek and Kim [169] reported nearly linear increases in toughness with 20 v/o whiskers as their lengths increased from 9–18 m, but with differing rates and levels, i.e. 3 to 4.5 and 4.7 to 5.2 MPa·m1/2 respectively for NB and CNB tests.

They attributed the increased benefit of longer whiskers to increased whisker pullout and reported that their data compared favorably with a model for effects of fiber (whisker) pullout.

Tiegs’s [170] compilation of Al2O3-SiC whisker composite data allows some additional observations to be made on this system. Thus the anisotropy of toughness for crack propagation nominally normal versus parallel with the whiskers (i.e. respectively normal and parallel with the plane of hot pressing) varied from 1.1 to 2.8 for mostly 30%, averaging 1.7±0.6. While much of the variation reflects different whisker and fabrication parameters, and the anisotropy is generally substantial, the toughness for crack propagation parallel with the plane of hot pressing and of the whisker alignment is still typically above that of the Al2O3 matrix alone, e.g. by as much as 20%.

Consider other oxide matrix whisker composites, where glasses, cordierite, TZP, and mullite matrices have been more common (Table 8.2). These composites also generally show toughness increasing as φ increases (e.g. to 0.2–0.3), but with considerable variation in results, much of which probably reflects variations in measurement techniques, matrix values, and processing differences. Wadsworth and Stevens [58] showed some dependence on SiC whisker dimensions similar to those seen in Al2O3 but emphasized benefits of increased whisker aspect ratio, but also noted that with similar aspect ratios larger whiskers increased toughness more than smaller ones (but had opposite effects on strength). They cited crack deflection, crack bridging, and load transfer as improving toughness, but noted there was no evidence of whisker pullout as a factor. The importance of whisker aspect ratio was also reported by Okada et al. [176] for their Y-TZP matrix composite with either in situ development, or direct addition, of mullite whiskers, where a pronounced toughness maximum was found at φ= 0.15. They showed that an important factor in (I) toughness increasing from 7 to 15 MPa·m1/2 was the aspect ratio of the whiskers increasing from 1.3 to 2.5.

Wu et al. [172] showed toughness (and strength) with a mullite matrix reaching maxima (at φ= 0.4) for tests with stresses both parallel and perpendicular to the plane of hot pressing. Particular variability was indicated with TZP matrices. Thus Claussen et al. [177] reported toughness doubling from 6 to 12 MPa·m1/2 with addition of 30 v/o SiC whiskers (but strengths were reduced by 40%, as was also the case for Yang and Stevens [59], with both possible reactions and matrix grain size reductions due to the whiskers being noted as possible reasons for some of the limitations). However, Becher and Tiegs [178] and Ruh et al. [179] both report substantial and additive increases in toughness of mullite-based composites with combined additions of TZP and SiC whiskers.

Ceramic whisker composites with nonoxide matrices have also been investigated, with Si3N4 matrices and SiC whiskers being most common with both similar and different results and thus the focus of review for such nonoxide composites. Buljan et al. [174] were apparently the only investigators to compare di-

rectly composites of either SiC particles or whiskers in a Si3N4 matrix. They showed a modest toughness decrease of 20% with fine (0.5 m) SiC particles and a more modest increase with larger (8 m) SiC particles at φ=0.3. These results were in contrast to increases of 40% with SiC whiskers (Table 8.2), but almost all of the increase occurred from φ=0.2 to 0.3, indicating greater advantage of the whisker composites (but with some probable resultant anisotropy). (Note however that while both indent and indentation fracture toughness tests of the whisker composites gave similar trends with φ, the former gave results 20 % lower.) Singh et al. [180] showed indent toughness increasing by 75% from φ=0 to 0.2 (with limited anisotropy found in the matrix itself). Shalek et al.’s [63] CNB tests showed substantial but lesser increases in toughness that were similar for different hot pressing temperatures despite significant changes in the matrix toughness. On the other hand Lundberg et al. [173] reported (I) and (IF) toughness decreasing by 18% from φ=0 to 0.2. Kandori et al. [181] showed CNB toughness for φ=0.1 decreasing as fabrication temperatures increased, but they were higher than that of the matrix, with this advantage increasing as temperature increased. Finally, Tiegs [170] showed that toughness at φ=0.2 increased 10% as the whisker diameter increased from 0.2 to 1.4 m diameter, but with most of the increase occurring below 1 m diameter, indicating a saturation of improvements with increased whisker diameter. Also, note that while toughness most commonly increased in these composites, the increases were more limited (even without the one decrease) relative to the toughness and Young’s modulus of the matrix, and further that strength changes were at best modest increases and more commonly decreases (Table 2) (Chap. 9, Sec. III.E).

Dusza and Sajgalik˘ [182] reported that Si3N4 bodies hot pressed with 10 or 20 w/o Si3N4 whiskers gave (IF) toughnesses respectively of 6.4 and 6.3 MPa·m1/2, similar to the matrix (6.1 MPa·m1/2) and I values for similar composites. They observed some whisker pullout similar to that for in situ toughened Si3N4 and noted that their results were more consistent with the model of Becher et al. [164] than that of Campbell et al. [183].

The first and most basic of two factors that should be noted is that microstructure is important in platelet and whisker composites, with more and clearer results for the latter. Thus improved toughness is indicated with increasing whisker aspect ratio and diameter as well as increased matrix grain size, though there are probable variations and limitations on the impact of these parameters (which may often be different than for strength, as is discussed in Chap. 9). The second factor, which is probably one of the factors changing or limiting microstructural effects, is stresses from thermal expansion differences between the dispersed and matrix phases. Again, while there appear to be variations in such differences, toughening is reported in platelet composites with matrices having greater expansion (TZP-Al2O3 and Al2O3-SiC ) as well as lower expansion (SiC-Si3N4) than the platelets. The same is true of whisker composites

with respectively greater matrix (Al2O3-SiC), neutral (mullite-SiC), and lower (cordierite and other low-expansion glasses, as well as Si3N4-SiC) matrix versus whisker expansion. There are also important indications that the bonding between matrices and whiskers can be important, i.e. higher toughness with whiskers with lower surface oxygen content and where glassy phases are found between whiskers and matrix.

E.Ceramic Eutectic Composites

There has been considerable past study of directionally solidified ceramic eutectics, especially on systems giving uniform lamellar or rod structures, with the matrix and the rods or lamellae being both single crystal structures of definite crystal orientations. While much of this was again before fracture mechanics and toughness evaluations were common, so most data on these is presented in Chap. 9, there are a few examples of explicit toughness effects. Thus Stubican and colleagues [184,185] showed that (I) toughness for crack propagation normal to the ZrB2 rods of the ZrC-ZrB2 eutectic initially rose very slowly from 3.2 MPa·m1/2 at larger spacing (λ) between rods (e.g. > 2.5 m) and then more rapidly to a maximum of 5 MPa·m1/2 at λ= 1.85 m; it then decreased rapidly to 3 MPa·m1/2 at λ= 1.65 m. Results for crack propagation parallel to the rods were very similar, with both showing pronounced maxima with substantial nonlinear increases and decreases versus λ-1/2. These toughnesses are in contrast to corresponding values of 1.7 and 1.9 MPa·m1/2 respectively for ZrC and ZrB2 alone. Toughness of ZrC-TiB2 directionally solidified composites over the range of spacings of 9 to 4+ m also showed no anisotropy as a function of crack propagation parallel or normal to the solidification direction [185]. However, toughness increased linearly versus λ-1/2 with no indication of reaching a maximum over this range; it started from a level at or below those for the constituents, and the ease of cracking implied that this system had lower toughness than the ZrC-ZrB2 system. Note that this use of the spacing of second phase entities, whether particles, rods, or plates, while unfortunately neglected in investigations of most other composites, is an important factor. This is shown in Chap. 9, Sec. III.F, not only for such eutectic composites but also for particulate composites with glass and possibly other matrices, along with the fact that the λ dependence has important connections to the G dependence of properties, especially strength, of monolithic ceramics.

Similarly Brumels and Pletka [186] showed that (I) toughness for crack propagation normal to the lamellar structure of directionally solidified NiO-CaO eutectic rose from 1.6 MPa·m1/2 at λ 1.3 m to 2 MPa·m1/2 at λ 2 m. For comparison purposes, the toughness of NiO crystal cleavage is 1.2 MPa·m1/2 and that for CaO is expected to be or <that of MgO ( 1 MPa·m1/2) (Table 2.1). Mah et al. [187] reported toughness of directionally solidified eutectic specimens

of alumina-YAG of 4.3 MPa·m1/2 [versus 1.4 MPa·m1/2 for YAG crystals on the (111) plane]. More recently Yang et al. [188] reported that (I) toughness for crack propagation normal to the lamellar structure of as-directionally solidified Y3Al5O12/Al2O3 eutectic was 2.4 MPa·m1/2 versus 2 MPa·m1/2 for normal crack propagation. Heat treatment that coarsened the microstructure reduced these values by respectively 30 and 15%.

Though investigated more, detailed data for the Al2O3-ZrO2 system is limited but shows higher toughnesses. Thus Rice et al. [189] obtained (DCB) toughnesses of 6.6 ± 0.6 MPa·m1/2 for as fusion cast thin (e.g. < 2 mm thick) plates ofeutectic compositions (commercially produced for abrasives that had a colony structure and substantial reduction) (Chap. 9, Sec. III.B.2). Krohn et al. [190] also hot pressed melt-derived eutectic powder that gave a toughness of 15 MPa·m1/2, while Homeny and Nick [116] conducted more detailed studies of eutectic Al2O3-ZrO2 powders made via plasma torch melting of particles and subsequent hot pressing of eutectic compositions that contained (a) 0, (b) 4.6, and (c) 9.5 m/o Y2O3, giving toughnesses of respectively 6.7, 7.6, and 7.9 MPa·m1/2. They attributed the substantial toughness in the first body (i.e with no stabilizer) to extensive microcracking (consistent with there being no Y2O3, resultant 90% monoclinic ZrO2, and resultant lower Young’s modulus and especially strength, Chap. 9, Sec. III.B). The source of the substantial toughness in the two bodies with Y2O3 additions that had 100% tetragonal ZrO2 was attributed not to transformation toughening but to crack bridging as well as substantial formation of strings of thin deformed material (much finer than grains or substantial fragments of them) bridging the cracks in the wake region. Mazerolles et al. [191] reported very similar values of 6.8 MPa·m1/2 for (I) toughness of oriented eutectics from directional solidification with 3 m/o Y2O3. Echigoya et al. [192] reported (I) toughness values for similarly directionally solidified compositions starting at 9 MPa·m1/2 with no Y2O3 and then dropping to 7, 5, and 4.5 MPa·m1/2 at respectively 3, 5, and 13 m/o Y2O3 for fracture normal to the growth direction. While the value for crack propagation parallel with the solidification direction was somewhat lower for no Y2O3, i.e. at 7.5 MPa·m1/2, this limited anisotropy rapidly approached nearly zero as the Y2O3 content increased.

Earlier work by Hulse and Batt [193] supports the substantial toughness of directionally solidified Al2O3-ZrO2 eutectics and provides other similar and useful data. Their WOF values for such eutectics of CaO·ZrO2-ZrO2, CaO-MgO, and Al2O3-ZrO2(Y2O3) eutectics were respectively 40, 90, and 90 J/m2, i.e. respectively similar to and twice the values they obtained for a commercial alumina. (These values translate to toughnesses of 4, 7, and 7.5 MPa·m1/2.) No details on the effects of eutectic microstructure were given. Kennard et al. [194] directionally solidified MgO-MgAl2O4 eutectic specimens, obtaining WOF values of 25 J/m2 (giving a toughness of 4 MPa·m1/2), noting that this had little relation to the eutectic rod spacing due to the control of mechanical properties by