Solid-particle erosion of directionally solidified Al2O3–ZrO2 (Y2O3) eutectics

Wear 268 (2010) 571–578

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Solid-particle erosion of directionally solidified Al2 O3 –ZrO2 (Y2 O3 ) eutectics夽 ˜ b , V.M. Orera b , Nan Chen c , D. Singh c , J.L. Routbort c,∗ K.C. Goretta a , J.I. Pena a

Asian Office of Aerospace Research and Development, Tokyo 106-0032, Japan Instituto de Ciencia de Materiales de Aragon, CSIC-Universidad de Zaragoza, Zaragoza E-50018, Spain c Argonne National Laboratory, Argonne, IL 60439-4838, USA b

a r t i c l e

i n f o

Article history: Received 16 October 2008 Received in revised form 13 August 2009 Accepted 12 October 2009 Available online 21 October 2009 Keywords: Eutectic ceramic Directional solidification Erosion Mechanical properties

a b s t r a c t Resistance to solid-particle erosion was studied by measuring individual damage sites in directionally solidified Al2 O3 –ZrO2 (Y2 O3 ) eutectics that contained three concentrations of Y2 O3 (0.5, 3, and 9% in the ZrO2 phase). For comparison, polycrystalline Al2 O3 , sapphire, and polycrystalline and single-crystal tetragonally stabilized ZrO2 were also studied. All specimens were impacted at normal incidence by angular 63 and 143 ␮m SiC particles traveling at 100 m/s. The eutectics containing 3% and 9 mol.% Y2 O3 generally exhibited the smallest damage zones and the ZrO2 specimens exhibited by far the largest damage zones. Examination of damage features and comparison with basic mechanical properties and models for erosion of brittle materials led to conclusions that the eutectics were resistant to erosion because of the presence of large compressive residual stresses in their Al2 O3 phases and that the ZrO2 materials were susceptible to erosive damage because transformation toughening was ineffective in reducing propagation of lateral cracks that emanated from the damage sites. Published by Elsevier B.V.

1. Introduction Directionally solidified eutectic ceramics (DSECs) exhibit a wide range of useful properties. They are being considered for use in applications such as advanced gas turbines, substrates for thin films, and bioengineered systems [1–3]. Much is known about processing and microstructural development of eutectic ceramics [1–17]. Well-made oxide eutectics exhibit excellent mechanical properties to near their melting points [10,15], and many thorough studies of their mechanical properties have been published ([1] and references therein). We are especially interested in the fundamentals of directionally solidified Al2 O3 –ZrO2 (Y2 O3 ) eutectics because of the wide range of microstructures and the many mechanical properties that have been reported. Elastic modulus, hardness, fracture strength, fracture toughness, creep, resistance to sliding wear, and residual stresses have all been studied in materials for which microstruc-

夽 The submitted manuscript has been created by the University of Chicago as Operator of Argonne National Laboratory (‘Argonne’) under Contract No. DE-AC0206CH11357 with the U.S. Department of Energy. The U.S. Government retains for itself, and others acting on its behalf, a paid-up, nonexclusive, irrevocable worldwide license in said article to reproduce, prepare derivative works, distribute copies to the public, and perform publicly and display publicly, by or on behalf of the Government. ∗ Corresponding author at: Argonne National Laboratory, Energy Systems Division, 9700 South Cass Avenue, Argonne, IL 60439-4838, USA. Tel.: +1 630 252 5065; fax: +1 630 252 4798. E-mail address: [email protected] (J.L. Routbort). 0043-1648/$ – see front matter. Published by Elsevier B.V. doi:10.1016/j.wear.2009.10.003

ture has been characterized well [1,14,17–26]. Given the excellent resistances of these eutectics to sliding wear [23], one might also expect good resistance to wear by solid-particle erosion. We have studied solid-particle erosion of ceramics, composites, and brittle materials for many years [27–35]. We report here on measurements of the resistances of three directionally solidified Al2 O3 –ZrO2 (Y2 O3 ) eutectics to erosion by sharp SiC abrasives. These studies were conducted to determine the basic mechanisms of material removal and whether the eutectics are resistant to erosion. For comparison, sapphire, polycrystalline Al2 O3 , and single-crystal and polycrystalline Y2 O3 -stabilized ZrO2 specimens were also tested. 2. Background Solid-particle erosion is usually measured as mass or volume of target lost per mass or volume of erodent particles impacting the surface [27–38]. Consistent results and good fits to models of erosion of brittle materials have been obtained when the erodent particles are significantly harder than the targets they impact [39–41]. The precision of a determination for erosion rate is dependent on variables such as the area of the surface being eroded, the mass of erodent used, and the number of materials-loss tests conducted. Because only comparatively small rods of directionally solidified Al2 O3 –ZrO2 (Y2 O3 ) eutectics could be obtained, we decided to study erosion by examination of individual impact sites. Careful measurements of the sizes of damage zones, coupled with direct observation

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by scanning electron microscopy, can yield many useful insights. This approach has been used, for example, to study impact damage in soda-lime glass and polycrystalline alumina [42,43]. Impact of brittle materials by solid particles has been shown to lead to formation of radial and lateral cracks that emanate from an elastic-plastic impact site [36–38,42,44–48]. The models contend [36–38,44–48] and experiments have confirmed [27,29,32–36,42] that for impact by hard particles, lateral cracks are indeed responsible for material loss in many brittle materials. For normal impact pressure P, the lateral cracks that form have length cL , where



cL = A

F(E/H)3/4

1/2

P 5/8 ,

Kc H 1/4

(1)

and A is a constant, F is a geometric factor, E is the target elastic modulus, H is the target hardness, and Kc is the critical stress-intensity factor of the target [47]. For normal impact pressure P, the radial cracks that form have length cR , where



cR =

F ∗ (E/H)1/2 Kc H 1/4

2/3

P 2/3 ,

(2)

and F* is a geometric factor and the other terms are as defined in Eq. (1) [46]. Radial crack lengths are generally significant relative to those of lateral cracks, but they do not contribute significantly to materials-loss by erosion because they do not enclose a volume of material. This model will form the basis of our analysis of single-impact damage sites in our specimens. It can be extended to predict material loss by impact from a great many non-interacting particles of approximately uniform size. Other researchers have also developed models for impact by sharp particles arriving at normal incidence. They agree broadly, but not in the fine details. Evans et al. developed a dynamic impact model [36], in which erosion rate W defined as eroded volume per volume of erodent striking the target can be expressed as W ∝ V 3.2 R3.7 0.25 Kc−1.3 H −0.25 ,

(3)

where V is the velocity of the impacting particle, R is the particle radius,  is the particle density, and Kc and H of the target are as defined in Eq. (1). The quasi-static model of Wiederhorn and coworkers for erosion of brittle solids predicts W ∝ V 2.4 R3.7 1.2 Kc−1.3 H 0.11 ,

(4)

where all terms are as defined previously [44,45]. Ritter et al. model the problem by defining the volume X of material removed per impact event: X = E 1.25 Kc−1 H −1.4 U 1.2 ,

(5)

where  is a constant, E is the target elastic modulus, U is the kinetic energy of the impacting particle, and Kc and H are as in the previous equations [48]. These models indicate that tough, hard targets are most resistant to erosion, with toughness being the more-dominant materials property in determining steady-state erosion rate. Eqs. (1)–(5) provide a quantitative basis for comparing impact damage to the properties of the target material. 3. Experimental details 3.1. Materials Three sets of directionally solidified Al2 O3 –ZrO2 (Y2 O3 ) eutectic rods were studied; they contained 0.5, 3, or 9 mol.% Y2 O3

Fig. 1. SEM photomicrographs of microstructures of (a) Al2 O3 ceramic and (b) 3-TZY ceramic.

in the ZrO2 phase. These rods will be designated 0.5Y, 3Y, and 9Y. The ZrO2 phase was primarily monoclinic in the 0.5Y rod, tetragonal in the 3Y rod, and cubic in the 9Y rod [49]. The rods were grown at a solidification rate of 10 mm/h. Details of their processing and resulting microstructures can be found in Refs. [14,18,49]. The microstructures consisted of a homogeneous dispersion of elongated zirconia particles (degenerated lamellae with a width-to-length ratio of approximately 0.35), with interlamellar spacing of approximately 2 ␮m embedded in a continuous sapphire matrix. The diameters of the Al2 O3 –ZrO2 (Y2 O3 ) rods were all 2.2–2.3 mm. The crystallography of these samples is described in Ref. [14] and can be defined by interpenetrating bicrystals, for which alumina grew preferentially with the c-axis parallel to the growth direction and the various zirconia phases on average grew perpendicular to the {0 0 1} or {0 1 1} planes. For comparison, the erosion characteristics of various Al2 O3 and ZrO2 specimens were also studied. Sapphire and polycrystalline specimens and single-crystal and polycrystalline ZrO2 /3 mol% Y2 O3 (3-TZY) specimens were obtained from our archives. The Al2 O3 was a silica-bonded specimen with a microstructure that featured a bimodal grain-size distribution of angular grains approximately 7 and 45 ␮m in average diameter. The polycrystalline 3-TZY specimen consisted of fine, equiaxed grains approximately 0.7 ␮m in diameter (Fig. 1). A summary of the expected mechanical properties for the various targets is shown in Table 1. Vickers hardness (Hv) values were measured with a Buehler MicroMet 5103 Tester (Lake Bluff, IL). For all measurements, the load was 1 kg and the time of indentation was 10 s. Elastic modulus (E) and fracture toughness (KIC ) values were taken from the literature.

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Table 1 Specimens tested for resistance to solid-particle erosion; hardness data were measured in this work and other data are averaged over the references cited, with KIC measured with single-edge notched beams, except where noted. Specimen Alumina Sapphire (0 0 0 1) Sapphire longitudinal 3-TZY ceramic 3-TZY crystal (0 0 1) 0.5Y transverse 0.5Y longitudinal 0.5Y average 3Y transverse 3Y longitudinal 3Y average 9Y transverse 9Y longitudinal 9Y average

E (GPa)

Hv (GPa)

KIC (MPa m0.5 )

386 456 361

13.7 17.0 11.4

4 4.5 ≈2.5

≈220 ≈230

12.7 13.6

≈11 10.6

– –

11.8 11.4

– –

a

– – ≈370 – – 343

a

a

14.6 15.9 – 15.3 16.1 –

4.8b 4.4b – 7.8 – ≈5b

References [50,51] [2,18,52,53] [18,52] [55–58] [18,54,55] – – [1] [21] [21] [2] [18] [18] [18]

a Values expected to be below those of 3Y and 9Y because of absence of transformation toughening and presence of many significant microcracks [1]. b Values obtained by indentation testing.

Fig. 2. Schematic diagram of impact site, with radial cracks emanating from two points of the indentation (shaded region) and lateral cracks emerging from two sides; dimensions a, b, and depth h of the indentation define the size of the zone.

3.2. Experiments Surfaces of all specimens were polished with 1 ␮m diamond grit so that Vickers microhardness or impact damage could be studied. The erosion experiments were conducted in a slinger-type

apparatus that has been described [59]. The pressure of the test chamber was reduced to 40 Pa, and thus aerodynamic effects were minimized. Particle feed rates were sufficiently low to minimize particle–particle interactions and total feeds were small enough to minimize overlapping of impact sites. The erodent particles were angular SiC abrasives (Crystalon 37, Norton Co., Worcester, MA, USA) of nominal diameter 63 or 143 ␮m. This erodent was selected because it has been used in many of our other studies of brittle

Fig. 3. Impact by 143 ␮m SiC: (a) of polycrystalline Al2 O3 for which some material loss and radial and emergent lateral cracks were observed by SEM and (b) significant indentation was observed by profilometry; of sapphire, for which (c) and (d) the basic patterns of material removal and indentation were similar.

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materials [32] and because it is harder than any of the target ceramics [39–41,60]. For erosion of brittle materials, a sufficiently hard erodent will produce indentation types of damage. As an erodent approaches the hardness of its target, the indentation mechanism will not operate [60]. The choice of SiC as the erodent thus has advantages of producing a type of damage for which models are available and for which data can be compared with those of many other studies. It has the disadvantage of limiting application of the data and analyses to real-world erosion problems, for which softer erodents, such as silica, are often encountered. The impact velocity was 100 m/s and the impact angle was 90◦ . A velocity of 100 m/s was chosen not for any specific application, but rather because it has been used in many previous erosion studies and thus direct comparisons with other data sets can be made. The eroded surfaces were examined by scanning electron microscopy (SEM) in a Hitachi S-4700-II (Tokyo, Japan) fieldemission microscope. Surfaces were coated with a thin layer of carbon prior to being examined. Damage sites were also examined with a MicroXAM interferometric surface profiler (ADE Phase Shift, Tucson, AZ). The dimensions of the damage sites were taken as the maximum areal extent of plastic indentation and extension of cracks, and the maximum depth h of the indentation (Fig. 2). Radial cracks (R) emanating beneath the impact zone were not measured, nor need they be because these cracks do not contribute to material removal [36–38,44–48]. 4. Results The first set of measurements focused on SEM of individual impact sites and the second set on their depths determined by profilometry. The range of response to erosive impact was surprisingly wide. The data are compiled in Table 2. The error bars reflect standard deviations. Individual measurements were estimated to be accurate to within a few percent. Given the large standard deviations, this additional source of uncertainty is negligible. Only SEM of the specimens impacted by 143 ␮m SiC is shown. This choice was made simply because the larger sites were easier to image well.

Table 2 Dimensions of impact damage sites in targets for impact at 100 m/s by 63- or 143-␮m SiC particles at normal incidence. Specimen 63 ␮m SiC Alumina Sapphire (0 0 0 1) Sapphire longitudinal 3-TZY ceramic 3-TZY crystal (0 0 1) 0.5Y transverse 0.5Y longitudinal 3Y transverse 3Y longitudinal 9Y transverse 9Y longitudinal 143 ␮m SiC Alumina Sapphire (0 0 0 1) Sapphire longitudinal 3-TZY ceramic 3-TZY crystal (0 0 1) 0.5Y transverse 0.5Y longitudinal 3Y transverse 3Y longitudinal 9Y transverse 9Y longitudinal

a (␮m)

b (␮m)

± ± ± ± ± ±

8 4 5 9 7 4

6 7 13 24 32 25

22 15 11 14

± ± ± ±

5 4 4 3

36 37 40 49 179 76

± ± ± ± ± ± ± ± ± ±

15 12 18 34 47 45

± ± ± ± ± ±

3 3 4 8 6 8

6 5 5 6

± ± ± ±

3 2 1 1

16 13 7 13 60 10

17 18 30 35 131 50

± ± ± ± ± ±

7 10 6 7 41 17

11 21 10 18

30 23 19 27

± ± ± ±

11 13 5 15

a

a

a

51 42 35 49

a

h (␮m) 5.0 2.3 1.8 4.4 4.4 1.5 1.3 1.4 1.6 2.2 1.8

± ± ± ± ± ± ± ± ± ± ±

0.8 0.6 0.7 1.6 1.3 0.6 0.4 0.5 0.5 0.4 0.8

5.4 3.0 2.8 4.9 5 3.3 2.5 2.0 2.0 2.4 2.8

± ± ± ± ± ± ± ± ± ± ±

0.5 0.3 0.8 0.7 1 0.9 0.5 0.6 0.6 0.7 1.4

a Measurements could not be made reliably because of extended radial-type cracks.

Fig. 4. (a) SEM photomicrograph of damage from impact by 143 ␮m SiC on polycrystalline 3-TZY; (b) surface profilometry scan of damage by 63 ␮m SiC on single-crystal 3-TZY, for which material loss by significant propagation of lateral cracks is evident (arrows).

The polycrystalline Al2 O3 specimen exhibited a pattern that has been seen by others [29,61,62]. Plastic indentation and material removal by formation of a combination of what appeared to be wholesale removal of grains and propagation of lateral cracks were observed (Fig. 3a). There was significant scatter in the sizes of the damage zones. The damage zones were deep, often flat-bottomed, and their depths exhibited comparatively little scatter (Table 2; Fig. 3b). The damage zones in the sapphire specimens were qualitatively similar, but the damage areas were generally smaller and the depths more shallow (Table 2; Fig. 3c and d). The damage zones in the polycrystalline 3-TZY specimen were larger than in the Al2 O3 or sapphire specimens. In particular, the lateral cracks extended further and produced more material removal. Lateral cracking was especially pronounced in the single-crystal 3-TZY specimen (Fig. 4). The DSECs exhibited similar microstructures, but those of the 3Y and 9Y specimens were more uniform. Impact damage in the 0.5Y DSEC could be measured only partially. SEM images and surface profilometry were obtained for transverse cross sections, for which damage zones were characterized by indentation, along with limited cracking (Fig. 5a–c). Longitudinal section exhibited many very long cracks that appeared to extend from impact damage sites (Fig. 5d). These cracks were so long that they could not be measured reliably. The damage zones in the 3Y and 9Y DSECs were essentially identical (Figs. 6 and 7). Indentation sites all exhibited debris and clear evidence of indentation, but only limited cracking. 5. Discussion Examination of Eqs. (1)–(5) indicates damage zone size should decrease with increasing toughness. Scattergood and

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Fig. 5. Impact by 143 ␮m SiC on 0.5Y DSEC: SEM photomicrographs of (a) transverse and (b) longitudinal sections for which indentation and cracking are observed; (c) indentation and (d) indentation and extended radial cracking revealed by profilometry.

coworkers have measured erosion rates for many ceramics ([27,30,39,42,61,63–65] and references therein). They modeled effects of materials properties of both the erodent particles and the target on erosion rate. They concluded that for a given erodent particle, erosion rate was determined primarily by the fracture toughness of the target [66], with the toughest targets being as expected most resistant to erosive damage. The data taken here do not obviously follow a trend of higher fracture toughness resulting in greater resistance to erosion. More information is needed. Considerations of individual damage patterns, often by comparing directly two different ceramic materials, can allow one to extract useful insights into the erosion resistances of the ceramics measured here. For the three types of Al2 O3 specimens, the best resistance to erosive damage was exhibited by the basal plane sapphire. Sapphire and Al2 O3 have previously been shown to be comparatively resistant to solid-particle erosion [67,68], with short-crack-length fracture toughness dominating its erosion resistance. Sapphire’s hardness is highest in the basal plane [69] and, as shown in Table 1, this plane’s fracture resistance is superior to that of coarse-grained Al2 O3 or non-basal plane sapphire [69–74]. For these specimens, as expected, damage zone size scaled with basic mechanical properties. The damage zones for the polycrystalline Al2 O3 were unexpectedly deep, and they were approximately independent of erodent size. Examination by SEM and surface profilometry suggested that

the damage depth was often determined by removal of a grain of material rather than by plastic indentation. Silica-bonded Al2 O3 has comparatively weak grain boundaries and such a mechanism for material loss—grain-boundary fracture and removal of grain—has been observed previously [27]. This mechanism would explain both the measured depths and independence of depth on size of erodent particle. The polycrystalline 3-TZY specimens possessed by far the lowest elastic modulus values and the highest fracture toughness values. One would expect them to be the most resistant to erosive damage. Instead, they were the least resistant. Comparatively poor resistance to solid-particle erosion has been observed before for tetragonally stabilized zirconias [65,66]. The explanation we offer for this fact emerges from the details of its toughening mechanism. Stress fields at cracks result in transformation of tetragonal to monoclinic zirconia. The resulting volumetric expansion places closing tractions along the crack, reducing the driving force for propagation and increasing fracture toughness. Fig. 8 , adapted from Marshall et al. [47], shows an ideal impact damage zone. Below an indentation is found a plastically deformed zone (PZ), from which extend radials cracks (R) and lateral cracks (L). A residual force Pr driven by the radially expanding plastic zone determines the crackdriving force. Examination of this model damage zone indicates the effects of transformation toughening should be different for propagation of radial cracks than for propagation of lateral cracks. Transformation toughening should be quite effective in limiting the

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Fig. 7. Impact by 143 ␮m SiC on 9Y DSEC: (a) transverse and (b) longitudinal sections, for which the eutectic microstructure, indentation, and limited cracking are observed by SEM.

Fig. 6. Impact by 143 ␮m SiC on 3Y DSEC: SEM photomicrographs of (a) transverse and (b) longitudinal sections, for which the eutectic microstructure, indentation, and (c) limited cracking are observed; indentation revealed by profilometry.

extension of radial cracks. Volumetric expansion along the crack should indeed exert crack-closing tractions. The results obtained in this study meet this expectation: no evidence of significant extension of lateral cracks was observed. For extension of lateral cracks, the region designated at V in Fig. 8 must be considered. The dimensions of this volume may be quite wide, but the depth is shallow. In our specimens, the depths were typically 2–3 ␮m. For a volume expansion caused by phase transformation, there is no overlying volume of material to constrain the expansion toward the surface direction. For a polycrystalline specimen, transformation would result in volumetric expansions along the three axes of the individual lattice directions of crystals that would be randomly oriented. Some crack deflection or microcracking could occur to dissipate some of the fracture energy. For the single-crystal, resistance to propagation of lateral cracks should be much worse. Rather than creating a closing traction along the crack wake, the volumetric expansion caused by transformation to the monoclinic phase should act as a wedge, driving a lateral crack further. These cracks should propagate quite freely in the zirconia specimens, especially in the single-crystal specimen, as they do indeed appear to do.

This qualitative description of why 3-TZY specimens are highly susceptible to erosive damage in the form of propagation of lateral cracks can be tested. Breder et al. have done so quite elegantly [75]. They prepared a series of Al2 O3 specimens that contained 10% zirconia. Three different treatments were applied, resulting in three different fractions of tetragonal zirconia near the surface. They then measured erosion rates and fracture strengths after erosion testing. Erosion rates are determined by propagation of lateral cracks. Reduction in strength is determined by extent of radial cracking. They found erosion rate to be independent of fraction of tetragonal zirconia, but strength to be proportional to fraction of tetragonal zirconia. Transformation toughening reduced radial crack propagation, but not lateral crack propagation.

Fig. 8. Schematic diagram of impact site in a ceramic: a plastic zone (PZ) is present beneath the indentation and radial cracks (R) and lateral cracks (L) extend from the zone; the volume of material V is what would be removed by propagation of a lateral crack.

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The 3Y and 9Y DSEC specimens were highly resistant to erosive damage, but the 0.5Y DSEC was not. In fact, very little propagation of lateral cracks could be observed for the 3Y and 9Y specimens. Each of the three specimens possessed a lamellar microstructure. Ceramics such as thermal barrier coatings and fiber composites also exhibit lamellar microstructures. Solid-particle erosion studies of such materials have revealed effects of such microstructures on erosion rate [76–80], but they do not provide an explanation of why the 3Y and 9Y specimens would be so resistant to erosive damage. The starkest differences between 0.5Y and 3Y/9Y directionally solidified Al2 O3 –ZrO2 (Y2 O3 ) eutectics were their roomtemperature residual stress states. The residual stresses in the Al2 O3 phase were, in both the transverse and longitudinal directions, highly tensile for the 0.5Y specimen, but highly compressive for the 3Y and 9Y specimens (see Fig. 5.3 in Ref. [1]). In fact the longitudinal residual stress for the 0.5Y specimen has been shown to exceed 1 GPa [1]. The extreme crack propagation in the longitudinal direction that was observed in these studies is no doubt due primarily to the residual stress states. In contrast, compressive stress of approximately 400 MPa in both directions for the 3Y and 9Y specimens should be quite effective in resisting crack propagation [81,82]. A more-subtle difference among the DSECs was observed in comparing the results for impact by 63 and 143 ␮m SiC particles. The 3Y and 9Y specimens were the most resistant to damage when impacted by the smaller particles, but were approximately equal in resistance to basal plane sapphire when impacted by the larger particles. The trends appeared to be significant statistically (Table 2). We offer no firm explanation, but two possible reasons can be offered. It is possible that with larger crack-driving force, the effects of residual stresses were partially overcome. It is also possible that the larger kinetic energy of 143 ␮m particles induced more local heating, which could have relaxed the residual stresses. Significant local heating leading to discernible melting has been reported for low-ductility target materials [83,84]. We saw no evidence of such heating in any of the specimens, but effects of heating cannot be ruled out. Three sets of experiments to test the explanations offered here can be envisioned. Two should be comparatively easy, one more difficult.

(1) If DSECs are resistant to erosive damage because of residual stresses rather than some other property related to their complex microstructures [24], then similar DSECs that do not contain significant residual stresses can be tested. We have obtained data for steady-state erosion of a series of Al2 O3 –yttrium aluminum garnet DSECs [85]. These DSECs were in fact no more erosion resistant than were their constituent ceramics. (2) Steady-state erosion data should be obtained for directionally solidified Al2 O3 –ZrO2 (Y2 O3 ) eutectics. It may well be that repeated impact will reduce the resistance to erosion, through some combination of heating and interaction of many cracks. (3) Erosion of directionally solidified Al2 O3 –ZrO2 (Y2 O3 ) eutectics could be measured as a function of temperature [81]. Many of the mechanical properties of Al2 O3 , sapphire, and directionally solidified Al2 O3 –ZrO2 (Y2 O3 ) eutectics are comparatively insensitive to temperature, to at least 1000 ◦ C [1,72–74]. Residual stresses do, however, relax. We do not have the capability to measure erosion as a function of temperature, but others do.

This work revealed clear trends in erosive damage and plausible explanations can be offered for each of them. Proof awaits gathering additional data.

577

6. Conclusions Resistance to damage from sharp SiC particles impacting a target at 100 m/s was studied for directionally solidified Al2 O3 –ZrO2 (Y2 O3 ) eutectics that contained three concentrations of Y2 O3 (0.5, 3, and 9% in the ZrO2 phase) and for these related target ceramics: polycrystalline Al2 O3 , sapphire, and polycrystalline and singlecrystal tetragonally stabilized ZrO2 . Damage zones consisted of combinations of plastic indentation and cracking. Their sizes were measured by SEM and surface profilometry. The Al2 O3 and sapphire specimens were much more resistant to damage than were the ZrO2 specimens. The ZrO2 exhibited by far the largest damage zones because of significant extension of lateral cracks. The eutectics containing 3 and 9% Y generally exhibited minimal cracking and generally the smallest damage zones. Comparison with basic mechanical properties and models for erosion of brittle materials suggested that the ZrO2 materials were susceptible to erosive damage because transformation toughening was ineffective in reducing propagation of lateral cracks that emanated from the damage sites. Erosion and fracture studies by Breder et al. [75] support this conclusion. The eutectics that contained 3 and 9% Y were concluded to be resistant to erosion because of the presence of large compressive residual stresses in their Al2 O3 phases. This explanation is plausible, but remains unproven. Experiments to test its validity and the inherent resistances to erosion of the eutectics were suggested.

Acknowledgments Financial support was provided by the Air Force Office of Scientific Research; the Spanish Ministry of Science and Technology, through Projects MAT2006-13005-C03-1; and the U.S. Department of Energy, under Contract DE-AC02-06CH11357 at Argonne National Laboratory, managed by the UChicago, LLC. The electron microscopy was performed in the Electron Microscopy Collaborative Research Center at Argonne National Laboratory.

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