Availableonlineatwww.sciencedirect.com SCIENCE DIRECT o WEAR ELSEVIER Wear256(2004)233-242 www.elsevier.com/locate/wear Solid-particle erosion and strength degradation of Si3N4/BN fibrous monoliths K C. Gorettaa F Gutierrez-Moraa Nan Chen a J. L. Routbort a, * T.A. Orlova b B.I. Smirnov b, A.R. de Arellano-LOpe Energy Technology Division, Argonne National Laboratory, Argonne, IL 60439-4838, USA loffe Physical-Technical Institute of the Russian Academy of Sciences, St. Petersburg 194021, Russia Departamento de Fisica de la Materia Condensada. Universidad de sevilla, 41080 Sevilla, Spain Received 7 November 2002; received in revised form 4 March 2003; accepted 14 March 2003 Abstract Erosive damage was studied in Si3 N4/Bn fibrous monoliths(FMs) and the individual constituents of the cells and cell boundary monolithic Si3N4 and BN. Unidirectional, 0/90, and +45 FMs were tested Specimens were subjected to impact at 90 by angular Sic particles of average diameter 143 um, traveling at 50-100 m/s. Steady-state erosion rates in the FMs were higher than predicted by a rule of mixtures based on erosion rates of the cell and cell-boundary phases. The relatively rapid FM erosion was attributed to chipping of the Si3 n4 cells caused by radial cracks. Bending strengths were measured before and after erosion testing to steady state at 100 m/s. the ength of monolithic Si3 N4 decreased 22%, the bn was not tested because insufficient material was available. Within experimental error, the strengths of the FMs were unaffected by erosion. Fracture data obtained approximately 1.5 years apart suggested that the FMs w susceptible to environmentally assisted slow crack growth O 2003 Elsevier B V. All rights reserved Keywords: Fibrous monoliths; Erosion; Strength; Silicon nitride; Boron nitride 1. Introduction BN FMs are produced commercially by Advanced Ceramics Research, Tucson, AZ. These and related fMs are being con Powder-derived ceramic fibrous monoliths(FMs) gener- sidered for aerospace applications in which foreign-object ally consist of strong ceramic cells that are surrounded by a damage is possible [11]. Resistance to impact erosion and weaker cell boundary. The cells are typically 100-500 um strength retention following damage are of concern in vide[1-10). FMs are produced by fabrication techniques aerospace and other possible applications for many FM such as extrusion or dip-coating. They are often fabricated We have previously studied erosive damage in Zrsioz as laminates that are laid up from duplex extruded filaments based FMs [12]. The cell boundary in those FMs consisted that consist of a cell phase surrounded by a sheath that forms of large-grained highly porous ZrSiO4. We found that the a continuous cell boundary. In flexure, FMs exhibit grace- FMs eroded more rapidly than would be expected from a ful failure, with energy dissipation arising from substantial simple rule-of-mixtures [13] for composite structures. The sliding of the cells[8]. FMs constitute a lower-cost alterna- anomalously rapid erosion was attributed to wholesale re- tive to conventional continuous-fiber ceramic composites in moval of cells once the supporting cell boundary had been Among ceramic FMs, those consisting of Si3 Na cells and In this study, we have conducted solid-particle erosion a continuous BN cell boundary have achieved the best over tests on Si3N4/BN FMs produced by Advanced Ceramics all mechanical properties and, therefore, have been studied Research. The cell boundary in these FMs is dense, in con most thoroughly [2-8]. Their flexural strengths can exceed trast to that of the ZrSio4 FMs. For comparison,mono- 700 MPa and work-of-fracture values, although typically 3- lithic Si3N4 and BN specimens were also tested. Strengths 6kJ/m, can exceed 10 kJ/m[3-7]. A wide variety of Si3 N4/ in flexure were measured before and after erosion testing The goals of this work were to determine the basic response ponding author. Tel:+1-603-252-5065; fax: +1-630-252-4298. of Si3 Na/BN FMs to erosion and the extent to which erosive E-jmail address: routbortaanl. gow (J.L. Routbort) damage affected strength 0043-1648/S-see front matter 2003 Elsevier B V. All rights reserved doi:10.1016S0043-1648(03)00392-2
Wear 256 (2004) 233–242 Solid-particle erosion and strength degradation of Si3N4/BN fibrous monoliths K.C. Goretta a, F. Gutierrez-Mora a, Nan Chen a, J.L. Routbort a,∗, T.A. Orlova b, B.I. Smirnov b, A.R. de Arellano-López c a Energy Technology Division, Argonne National Laboratory, Argonne, IL 60439-4838, USA b Ioffe Physical-Technical Institute of the Russian Academy of Sciences, St. Petersburg 194021, Russia c Departamento de Fisica de la Materia Condensada, Universidad de Sevilla, 41080 Sevilla, Spain Received 7 November 2002; received in revised form 4 March 2003; accepted 14 March 2003 Abstract Erosive damage was studied in Si3N4/BN fibrous monoliths (FMs) and the individual constituents of the cells and cell boundary, monolithic Si3N4 and BN. Unidirectional, 0/90◦, and ±45◦ FMs were tested. Specimens were subjected to impact at 90◦ by angular SiC particles of average diameter 143m, traveling at 50–100 m/s. Steady-state erosion rates in the FMs were higher than predicted by a rule of mixtures based on erosion rates of the cell and cell-boundary phases. The relatively rapid FM erosion was attributed to chipping of the Si3N4 cells caused by radial cracks. Bending strengths were measured before and after erosion testing to steady state at 100 m/s. The strength of monolithic Si3N4 decreased 22%; the BN was not tested because insufficient material was available. Within experimental error, the strengths of the FMs were unaffected by erosion. Fracture data obtained approximately 1.5 years apart suggested that the FMs were susceptible to environmentally assisted slow crack growth. © 2003 Elsevier B.V. All rights reserved. Keywords: Fibrous monoliths; Erosion; Strength; Silicon nitride; Boron nitride 1. Introduction Powder-derived ceramic fibrous monoliths (FMs) generally consist of strong ceramic cells that are surrounded by a weaker cell boundary. The cells are typically 100–500 m wide [1–10]. FMs are produced by fabrication techniques such as extrusion or dip-coating. They are often fabricated as laminates that are laid up from duplex extruded filaments that consist of a cell phase surrounded by a sheath that forms a continuous cell boundary. In flexure, FMs exhibit graceful failure, with energy dissipation arising from substantial sliding of the cells [8]. FMs constitute a lower-cost alternative to conventional continuous-fiber ceramic composites in some applications. Among ceramic FMs, those consisting of Si3N4 cells and a continuous BN cell boundary have achieved the best overall mechanical properties and, therefore, have been studied most thoroughly [2–8]. Their flexural strengths can exceed 700 MPa and work-of-fracture values, although typically 3– 6 kJ/m2, can exceed 10 kJ/m2 [3–7]. A wide variety of Si3N4/ ∗ Corresponding author. Tel.: +1-603-252-5065; fax: +1-630-252-4298. E-mail address: routbort@anl.gov (J.L. Routbort). BN FMs are produced commercially by Advanced Ceramics Research, Tucson, AZ. These and related FMs are being considered for aerospace applications in which foreign-object damage is possible [11]. Resistance to impact erosion and strength retention following damage are of concern in aerospace and other possible applications for many FMs. We have previously studied erosive damage in ZrSiO4- based FMs [12]. The cell boundary in those FMs consisted of large-grained highly porous ZrSiO4. We found that the FMs eroded more rapidly than would be expected from a simple rule-of-mixtures [13] for composite structures. The anomalously rapid erosion was attributed to wholesale removal of cells once the supporting cell boundary had been degraded [12]. In this study, we have conducted solid-particle erosion tests on Si3N4/BN FMs produced by Advanced Ceramics Research. The cell boundary in these FMs is dense, in contrast to that of the ZrSiO4 FMs. For comparison, monolithic Si3N4 and BN specimens were also tested. Strengths in flexure were measured before and after erosion testing. The goals of this work were to determine the basic response of Si3N4/BN FMs to erosion and the extent to which erosive damage affected strength. 0043-1648/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0043-1648(03)00392-2
K.C. Goretta et al /Wear 256(2004)233-242 2. Experimental procedures 2.1. Specimen preparation and microstructure FM specimens were obtained in three laminated forms unidirectional,o90°,and±45°. All were fabricated by Ad vanced Ceramic Research and were made from Si3 N4/BN green filaments [2 that were produced by melt coextru- sion of a blend of 52 vol. ceramic powder mixture in an ethylene-based copolymer binder [6]. The coex truded filaments contained A82 vol. Si3 N4 core(E-10 H Ube Industries, Tokyo)and a18 vol. Bn sheath(HCP Grade, Advanced Ceramics Corporation, Cleveland, OH) The Si3N4 was a sinterable composition, 92 wt %com- mercial Si3N4 powder, 6 wt% Y203, and 2 wt. Al2O3 Sheets of uniaxially aligned green filaments were wound on a cylindrical mandrel. The filaments were held in place with a spray adhesive that allowed removal upon drying of unidirectional green sheets from the mandrel. The sheets were then stacked to fabricate various laminates [2, 3]. The laminates were warm pressed at a160C to produce solid panels, which were then subjected to a binder pyrolysis 1 mm step that consisted of slow heating in flowing N2 to 600C over a period of 42 h. The Si3N4/BN panels were then hot pressed at 1740° for I h under≈28 MPa pressure, which Fig2. SEM photomicrograph of fractured(a)090°and(b)±45°sc yielded 3mm-thick billets with densities that were >98% mens, showing laminated layers of flattened SiN4 cells surrounded by of theoretical The hot-pressed FM specimens consisted of flattened Two sets of Si3N4 specimens were examined: mono- Si3N4 cells surrounded by a continuous bn cell bound- lithic Si3N4 fabricated by Advanced Ceramics Research ary(Figs. I and 2). The plate-like bn grains were highly for which the composition was the same as that of the cells textured [8, 14] of the FMs, and archival in-situ-reinforced Si N4 fabricated 50 um Si3N4 fibers BN flakes Fig. 1. Composite SEM phe graph showing basic FM structure
234 K.C. Goretta et al. / Wear 256 (2004) 233–242 2. Experimental procedures 2.1. Specimen preparation and microstructure FM specimens were obtained in three laminated forms: unidirectional, 0/90◦, and ±45◦. All were fabricated by Advanced Ceramic Research and were made from Si3N4/BN green filaments [2] that were produced by melt coextrusion of a blend of ≈52 vol.% ceramic powder mixture in an ethylene-based copolymer binder [6]. The coextruded filaments contained ≈82 vol.% Si3N4 core (E-10, Ube Industries, Tokyo) and ≈18 vol.% BN sheath (HCP Grade, Advanced Ceramics Corporation, Cleveland, OH). The Si3N4 was a sinterable composition, 92 wt.% commercial Si3N4 powder, 6 wt.% Y2O3, and 2 wt.% Al2O3 [6]. Sheets of uniaxially aligned green filaments were wound on a cylindrical mandrel. The filaments were held in place with a spray adhesive that allowed removal upon drying of unidirectional green sheets from the mandrel. The sheets were then stacked to fabricate various laminates [2,3]. The laminates were warm pressed at ≈160 ◦C to produce solid panels, which were then subjected to a binder pyrolysis step that consisted of slow heating in flowing N2 to 600 ◦C over a period of ≈42 h. The Si3N4/BN panels were then hot pressed at 1740 ◦C for 1 h under ≈28 MPa pressure, which yielded 3 mm-thick billets with densities that were >98% of theoretical. The hot-pressed FM specimens consisted of flattened Si3N4 cells surrounded by a continuous BN cell boundary (Figs. 1 and 2). The plate-like BN grains were highly textured [8,14]. Fig. 1. Composite SEM photomicrograph showing basic FM structure. Fig. 2. SEM photomicrograph of fractured (a) 0/90◦ and (b) ±45◦ specimens, showing laminated layers of flattened Si3N4 cells surrounded by BN cell boundary. Two sets of Si3N4 specimens were examined: monolithic Si3N4 fabricated by Advanced Ceramics Research, for which the composition was the same as that of the cells of the FMs, and archival in-situ-reinforced Si3N4 fabricated
K.C. Goretta et al/Wear 256(2004)233-242 by AlliedSignal (Morristown, ND)[15] and Dow Chemical (Midland, MD)[16]. BN specimens were also fabricated by Advanced Ceramics Research [17 2.2. Erosion and strength test 0.6 Solid-particle erosion tests were carried out in a slinger- type apparatus that has been described previously [18-201 Tests were conducted in vacuum(500 m Torr), and thus aerodynamic effects were negligible. The feed rate of the erodent was 8gmin, for which interactions between pa d ticles were also negligible The erodent particles were angular SiC abrasives(Norton Co., Worcester, MA, USA) with mean diameter of 143 um [12, 15, 20]. The test velocity() was 50, 70, or 100 m/s and the angle of impact was 90. Specimens were impacted par- Fig. 3. Weight loss by143μ m Sic at )m/s: BN (open ci allel to the hot-pressing direction. Steady-state erosion rate (open diamonds),#4: circles,o90°FM (ER, in mg/g)values were determined from plots of the spec triangles), and ACR imen weight loss versus weight of particles impacting the surface. At least three runs were conducted for each speci- men, and at least four runs were conducted when a transient BN specimen was by far the smallest. Each ER value was was observed in the initial erosion response Following each defined as the slope of linear least-squares fits to the data run, specimens were removed, brushed, cleaned by an air The results for the three types of FMs were similar. The blast, and weighed. The average weight-loss measurements unidirectional FM was tested only at 100 m/s. Insufficient were accurate to 2%. This uncertainty arose due to slightly material was available to allow for more testing. The two incomplete or inconsistent cleaning of the surfaces Si3N4 specimens exhibited similar erosion rates, which were Four-point fracture tests were conducted with an Instron much lower than those of the FMs. The soft BN specimen Model 4505 apparatus(Canton, MA, USA). The loading rate exhibited the highest erosion rate was 1.3 mm/min. The inner load span was 9.5 mm and the Data on erosion rate versus the velocity of impact revealed outer load span was 15 mm [20, 21]. As-received specimens interesting trends(Fig. 4). As might be expected, the ER were polished with 1 um diamond paste prior to testing. Of values for the Si3Na/BN FMs were between those of the the eroded specimens, only those eroded at 100 m/s were hard Si3 N4 and soft BN. Power-law fits to the data of Fig. 4 tested for strength; they were not polished. Four specimens are shown in Table 1. ER was a stronger function of yfor were tested per condition. The strength of only one of the the FMs than for either of the monolithic Si3N4 ceramics or Si3N4 ceramics, the one produced by Advanced Ceramics the BN Research(ACR), was tested. The BN specimens were too The single-impact sites were generally characteristic of small to test after erosion, but as-polished data were avail erosion of brittle solids. Many of the impacts evinced all able from a previous study [22, 23]. Fracture data sets on polished-and-beveled specimens were obtained in the late winter of 2000 and the summer of 2001. In between tests the samples were stored in an open laboratory environment. Eroded and fractured surfaces were examined by scanning electron microscopy(SEM). Single-impact damage sites were also examined by Sem to elucidate the weight-loss mechanism [ 18-21. All specimens were coated with Au-Pd and examined in a JEOL 5400(Peabody, MA)or Hitachi S-4700-ll microscope(Tokyo, Japan) 3. Results 3.. Erosion tests Representative data for weight loss versus dose of impact ate vs SiC erodent velocity for BN(open circles), 0/90o monds),+45. FM(filled diamonds), AlliedSignal Si3 N4 ing SiC particles are shown in Fig. 3. Differences in dose (open which are almost completely obscured by filled triangles), are a consequence of the surface area that was eroded; the and ACR Si3 N4(filled triangles)
K.C. Goretta et al. / Wear 256 (2004) 233–242 235 by AlliedSignal (Morristown, NJ) [15] and Dow Chemical (Midland, MI) [16]. BN specimens were also fabricated by Advanced Ceramics Research [17]. 2.2. Erosion and strength tests Solid-particle erosion tests were carried out in a slingertype apparatus that has been described previously [18–20]. Tests were conducted in vacuum (≈500 mTorr), and thus aerodynamic effects were negligible. The feed rate of the erodent was ≈8 g/min, for which interactions between particles were also negligible. The erodent particles were angular SiC abrasives (Norton Co., Worcester, MA, USA) with mean diameter of 143 m [12,15,20]. The test velocity (V) was 50, 70, or 100 m/s and the angle of impact was 90◦. Specimens were impacted parallel to the hot-pressing direction. Steady-state erosion rate (ER, in mg/g) values were determined from plots of the specimen weight loss versus weight of particles impacting the surface. At least three runs were conducted for each specimen, and at least four runs were conducted when a transient was observed in the initial erosion response. Following each run, specimens were removed, brushed, cleaned by an air blast, and weighed. The average weight-loss measurements were accurate to ±2%. This uncertainty arose due to slightly incomplete or inconsistent cleaning of the surfaces. Four-point fracture tests were conducted with an Instron Model 4505 apparatus (Canton, MA, USA). The loading rate was 1.3 mm/min. The inner load span was 9.5 mm and the outer load span was 15 mm [20,21]. As-received specimens were polished with 1 m diamond paste prior to testing. Of the eroded specimens, only those eroded at 100 m/s were tested for strength; they were not polished. Four specimens were tested per condition. The strength of only one of the Si3N4 ceramics, the one produced by Advanced Ceramics Research (ACR), was tested. The BN specimens were too small to test after erosion, but as-polished data were available from a previous study [22,23]. Fracture data sets on polished-and-beveled specimens were obtained in the late winter of 2000 and the summer of 2001. In between tests, the samples were stored in an open laboratory environment. Eroded and fractured surfaces were examined by scanning electron microscopy (SEM). Single-impact damage sites were also examined by SEM to elucidate the weight-loss mechanism [18–21]. All specimens were coated with Au–Pd and examined in a JEOL 5400 (Peabody, MA) or Hitachi S-4700-II microscope (Tokyo, Japan). 3. Results 3.1. Erosion tests Representative data for weight loss versus dose of impacting SiC particles are shown in Fig. 3. Differences in dose are a consequence of the surface area that was eroded; the Fig. 3. Weight loss vs. dose for specimens eroded by 143 m SiC at 100 m/s: BN (open circles), unidirectional FM (filled circles), 0/90◦ FM (open diamonds), ±45◦ FM (filled diamonds), allied signal Si3N4 (open triangles), and ACR Si3N4 (filled triangles). BN specimen was by far the smallest. Each ER value was defined as the slope of linear least-squares fits to the data. The results for the three types of FMs were similar. The unidirectional FM was tested only at 100 m/s. Insufficient material was available to allow for more testing. The two Si3N4 specimens exhibited similar erosion rates, which were much lower than those of the FMs. The soft BN specimen exhibited the highest erosion rate. Data on erosion rate versus the velocity of impact revealed interesting trends (Fig. 4). As might be expected, the ER values for the Si3N4/BN FMs were between those of the hard Si3N4 and soft BN. Power-law fits to the data of Fig. 4 are shown in Table 1. ER was a stronger function of V for the FMs than for either of the monolithic Si3N4 ceramics or the BN. The single-impact sites were generally characteristic of erosion of brittle solids. Many of the impacts evinced all Fig. 4. Erosion rate vs. SiC erodent velocity for BN (open circles), 0/90◦ FM (open diamonds), ±45◦ FM (filled diamonds), AlliedSignal Si3N4 (open triangles, which are almost completely obscured by filled triangles), and ACR Si3N4 (filled triangles)
K.C. Goretta et al /Wear 256(2004)233-242 Table I The steady-state erosion surfaces all indicated similar Velocity exponent n and linear correlation coefficient R for ER data mechanisms of material removal. Brittle, cleavage-like frac- n ture was dominant(Fig. 6). The eroded surfaces of the 2.0±0.2 BN contained many flake-like features, which are presum- SigNa(ACR) 2.2±0.2 0.9990 ably related to fracture of BNs highly anisotropic crystals 2.1±0.2 0.9997 The Si3 N4 surfaces contained fine debris, similar to that 090° 5.4士04 0.9999 observed at the single-impact sites. The FM surfaces ±45° 4.1±0.5 tained some of the flake-like features observed in the Low-magnification observations of the specimens revealed that the surfaces of the BN and Si3N4 specimens were flat, of the features that lead to material removal: indenting, and that all of the FMs were undulating. For each of the FMs, radial-crack formation, and spalling of the target caused by the undulations scaled with the size of the cells(Fig. 7) propagation of lateral cracks. Important differences were ap- parent among the targets. The bn damage craters were the 3. 2. Strength tests largest(average diameter s200 um) and probably evinced he most evidence of microplasticity. Many of the impacts Results of the four-point flexural tests, for specimens be- into the Si3 N4 caused only scuffing, but little or no material fore and after erosion testing, are shown in Table 2. The data removal. When a clear impact crater was evident, it was rel- for as-polished specimens exhibited expected trends and atively large(average diameter 50-70 um), and inevitably moderate surprises. As expected, the monolithic Si3N4 was he site was strewn with fine debris. Little difference was strongest, the monolithic bn the weakest, and the Fms were observed among the various types of FMs. Despite the com- in between. Among the FMs, the unidirectional one was the paratively high erosion rates of the FMs, the single-impact strongest. All of the strength values were, however, lower damage sites of the FMs(average diameter N50 um)were than those obtained in the year 2000 from the same panels almost always smaller than those that led to material re- [22,23]. The specimens from 2000 were, however, longer, moval in the Si3N4(Fig. 5). Their average width was nearly their outer loading span was 40 mm, and their inner loading one order of magnitude smaller than the Si3N4 cells. As ex- span was 15 mm Solid-particle erosion to steady-state con- pected, the size of the damage site scaled with I ditions for the surfaces appeared to reduce the strengths of a (b) c 50 Fig. 5. SEM photomicrographs of single impact by 143 um SiC erodent:(a)BN, (b) Si3N4, (c) FM in cell region, and(d) FM near cell/cell-boundary
236 K.C. Goretta et al. / Wear 256 (2004) 233–242 Table 1 Velocity exponent n and linear correlation coefficient R for ER data Specimen n R Si3N4 (self-reinforced) 2.0 ± 0.2 0.9999 Si3N4 (ACR) 2.2 ± 0.2 0.9990 BN 2.1 ± 0.2 0.9997 0/90◦ 5.4 ± 0.4 0.9999 ±45◦ 4.1 ± 0.5 0.9534 of the features that lead to material removal: indenting, radial-crack formation, and spalling of the target caused by propagation of lateral cracks. Important differences were apparent among the targets. The BN damage craters were the largest (average diameter ≈200m) and probably evinced the most evidence of microplasticity. Many of the impacts into the Si3N4 caused only scuffing, but little or no material removal. When a clear impact crater was evident, it was relatively large (average diameter ≈50–70m), and inevitably the site was strewn with fine debris. Little difference was observed among the various types of FMs. Despite the comparatively high erosion rates of the FMs, the single-impact damage sites of the FMs (average diameter ≈50m) were almost always smaller than those that led to material removal in the Si3N4 (Fig. 5). Their average width was nearly one order of magnitude smaller than the Si3N4 cells. As expected, the size of the damage site scaled with V. Fig. 5. SEM photomicrographs of single impact by 143 m SiC erodent: (a) BN, (b) Si3N4, (c) FM in cell region, and (d) FM near cell/cell-boundary interface. The steady-state erosion surfaces all indicated similar mechanisms of material removal. Brittle, cleavage-like fracture was dominant (Fig. 6). The eroded surfaces of the BN contained many flake-like features, which are presumably related to fracture of BN’s highly anisotropic crystals. The Si3N4 surfaces contained fine debris, similar to that observed at the single-impact sites. The FM surfaces contained some of the flake-like features observed in the BN. Low-magnification observations of the specimens revealed that the surfaces of the BN and Si3N4 specimens were flat, and that all of the FMs were undulating. For each of the FMs, the undulations scaled with the size of the cells (Fig. 7). 3.2. Strength tests Results of the four-point flexural tests, for specimens before and after erosion testing, are shown in Table 2. The data for as-polished specimens exhibited expected trends and moderate surprises. As expected, the monolithic Si3N4 was strongest, the monolithic BN the weakest, and the FMs were in between. Among the FMs, the unidirectional one was the strongest. All of the strength values were, however, lower than those obtained in the year 2000 from the same panels [22,23]. The specimens from 2000 were, however, longer; their outer loading span was 40 mm, and their inner loading span was 15 mm. Solid-particle erosion to steady-state conditions for the surfaces appeared to reduce the strengths of
K.C. Goretta et al/Wear 256(2004)233-242 237 b 0.2 mm Fig. 7. Low-magnification SEM photomicrograph of eroded surface of unidirectional FM; topology indicates loss of cells all, but the unidirectional and +45 Si3N4/BN specimens Relatively large error bars cast some doubt on each of the comparisons between before and after erosion SEM examination of the Si3 N4 and Si3 N4/BN surfaces revealed that cleavage dominated the fracti cesses. The Si3 N4 surfaces were characteristic of a dense fine-grained ceramic. The Si3N4 contained many elongate grains(Fig. &a). The FMs exhibited larger features, and some Bn was evident on the surfaces(Fig. 8b) 10 Fig. 6. SEM photomicrographs of steady-state surfaces for erosion by 10um 143 um SiC erodent at 100 m/s:(a)BN, (b) Si3N4, and (c)representative Table 2 Four-point flexural strength before(oo)and(oi) after(oer )erosion testing, including data from Refs. [21, 22]a (MPa) do(MPa) Oer(MPa) Change SinA 677±127601±128470±27-2 38士3 Unidirectional476±30194±6193±28-1 10m 0/90 379±86109±5 94±8 175±13100±21139±25+39 Fig. 8. SEM photomicrographs of fracture surfaces of (a)ACR Si3N4 a ai values are from early 2000 and ao are from mid 2001 and(b)0/90] Si3 N4/BN FM
K.C. Goretta et al. / Wear 256 (2004) 233–242 237 Fig. 6. SEM photomicrographs of steady-state surfaces for erosion by 143 m SiC erodent at 100 m/s: (a) BN, (b) Si3N4, and (c) representative FM. Table 2 Four-point flexural strength before (σo) and (σi) after (σer) erosion testing, including data from Refs. [21,22]a Specimen σi (MPa) σo (MPa) σer (MPa) Change (σo: σer) (%) Si3N4 677 ± 127 601 ± 128 470 ± 27 −22 BN 38 ± 3– – – Unidirectional 476 ± 30 194 ± 6 193 ± 28 −1 0/90◦ 379 ± 86 109 ± 5 94 ± 8 −14 ±45◦ 175 ± 13 100 ± 21 139 ± 25 +39 a σi values are from early 2000 and σo are from mid 2001. Fig. 7. Low-magnification SEM photomicrograph of eroded surface of unidirectional FM; topology indicates loss of cells. all, but the unidirectional and ±45◦ Si3N4/BN specimens. Relatively large error bars cast some doubt on each of the comparisons between before and after erosion. SEM examination of the Si3N4 and Si3N4/BN fracture surfaces revealed that cleavage dominated the fracture processes. The Si3N4 surfaces were characteristic of a dense, fine-grained ceramic. The Si3N4 contained many elongated grains (Fig. 8a). The FMs exhibited larger features, and some BN was evident on the surfaces (Fig. 8b). Fig. 8. SEM photomicrographs of fracture surfaces of (a) ACR Si3N4 and (b) 0/90◦ Si3N4/BN FM