CERAMICS INTERNATIONAL ELSEVIER Ceramics International 33(2007)315-320 www.elsevier.com/locate/ceramint Evaluation of fracture resistance of ceramics Edge fracture tests George Gogotsi, Sergey Mudrik, Vasily Galenko Pisarenko Institute for Problems of Strength, 2, Timiryazevskaya St, 01014 Kiev, Ukraine Received 17 June 2005: received in revised form 27 July 2005: accepted 20 September 2005 Available online 28 February 2006 Abstract Advanced oxide and nonoxide ceramics were tested by edge flaking with different indenters. Flaking toughness values obtained with the Rockwell indenter were shown to be practically proportional to fracture toughness values measured by the single edge V-notched beam(SEVNB) method. The discussed edge fracture(EF)method enabling the determination of fracture resistance of ceramics in small-size specimen tests was demonstrated to be appropriate for materials technology as well as for the evaluation of ceramics for dentistry, cutting tools, and like uses. Emphasis was placed on the results of investigation of chip scars formed upon flaking off the specimens with the Rockwell, Vickers, and Knoop C 2006 Elsevier Ltd and Techna Group S.r.l. All rights reserved. Keywords: C. Fracture; D. Conventional ceramics; Edge fracture; Flaking toughness; Indentation 1. ntroduction chosen very precisely. For different brittle materials under study (from Crown glass to tungsten carbide) the edge toughness M Fracture of brittle materials upon concentrated loads versus critical plane strain energy release rate Glc relationship attracted attention already in the Paleolithic Period, when was also established Authors [4]used the rockwell and Knoop stone tools were first developed. This approach was also used indenters to investigate not only ceramics and hard metals but for the choice of the best stones for tool production, which also brittle tool steels. They confirmed the validity of the M-GIo became the starting point for fracture resistance tests. relationship and noticed that the Klc-GIe relationship was not A study [1] was a new step in this direction; its authors observed. Several dental ceramics were extensively studied revealed the linear relationship between chip sizes(chip width- elsewhere [5]. The tests were performed under small loads to-height and thickness-to-heig pproximately using a diamond cone. The initial portions of Prd diagrams constant)at the same loads applied to the edge of the material. were nonlinear Sintered Si3 N4 ceramics for valves of internal They proposed [2] to indent a rectangular specimen with the combustion engines were also investigated [6,7]. Studies of the polycrystalline Rockwell indenter using increasing loads at effect of specimen edge shapes on edge toughness resulted in different distances from the edge until a chip breaks off, the modification of their design determined the linear relation between the load Pr required to Known data, while very interesting and useful, demonstrate produce a chip and the distance d of the indenter from the edge, that the M-GIc relationship has no physical substantiation. It is and suggested the ratio Pd as a measure of the edge toughness not observed for ceramics [8, 9] but holds for different brittle M of the material. One of the authors [3 proposed to use the materials(e.g, test results [2, 3)). A known procedure does not lope of the fracture load Pr versus edge distance d diagram to take account of differences in the mechanical behavior of determine this value. These tests would require a special test ceramics [10] and other brittle materials, fracture patterns in the machine because the loading point on indentation must be zone of the indenter contact with the specimen as well as the real fracture surface area formed upon flaking. Therefore,a more advantageous edge fracture method was developed [9]. Its Conmepondingauto te is s0 4 28. 6 at +30 4 256 16 4. wfis veny ed d tita bilit ft ivig diferent ceramic materials 2-8842/$3200C 2006 Elsevier Ltd and Techna Group S.r.L. All rights reserved
Evaluation of fracture resistance of ceramics: Edge fracture tests George Gogotsi *, Sergey Mudrik, Vasily Galenko Pisarenko Institute for Problems of Strength, 2, Timiryazevskaya Str., 01014 Kiev, Ukraine Received 17 June 2005; received in revised form 27 July 2005; accepted 20 September 2005 Available online 28 February 2006 Abstract Advanced oxide and nonoxide ceramics were tested by edge flaking with different indenters. Flaking toughness values obtained with the Rockwell indenter were shown to be practically proportional to fracture toughness values measured by the single edge V-notched beam (SEVNB) method. The discussed edge fracture (EF) method enabling the determination of fracture resistance of ceramics in small-size specimen tests was demonstrated to be appropriate for materials technology as well as for the evaluation of ceramics for dentistry, cutting tools, and like uses. Emphasis was placed on the results of investigation of chip scars formed upon flaking off the specimens with the Rockwell, Vickers, and Knoop indenters. # 2006 Elsevier Ltd and Techna Group S.r.l. All rights reserved. Keywords: C. Fracture; D. Conventional ceramics; Edge fracture; Flaking toughness; Indentation 1. Introduction Fracture of brittle materials upon concentrated loads attracted attention already in the Paleolithic Period, when stone tools were first developed. This approach was also used for the choice of the best stones for tool production, which became the starting point for fracture resistance tests. A study [1] was a new step in this direction; its authors revealed the linear relationship between chip sizes (chip widthto-height and thickness-to-height ratios are approximately constant) at the same loads applied to the edge of the material. They proposed [2] to indent a rectangular specimen with the polycrystalline Rockwell indenter using increasing loads at different distances from the edge until a chip breaks off, determined the linear relation between the load Pf required to produce a chip and the distance d of the indenter from the edge, and suggested the ratio Pf/d as a measure of the edge toughness M of the material. One of the authors [3] proposed to use the slope of the fracture load Pf versus edge distance d diagram to determine this value. These tests would require a special test machine because the loading point on indentation must be chosen very precisely. For different brittle materials under study (from Crown glass to tungsten carbide) the edge toughness M versus critical plane strain energy release rate GIc relationship was also established. Authors [4] used the Rockwell and Knoop indenters to investigate not only ceramics and hard metals but also brittle tool steels. They confirmed the validity of the M–GIc relationship and noticed that the KIc–GIc relationship was not observed. Several dental ceramics were extensively studied elsewhere [5]. The tests were performed under small loads using a diamond cone. The initial portions of Pf–d diagrams were nonlinear. Sintered Si3N4 ceramics for valves of internal combustion engines were also investigated [6,7]. Studies of the effect of specimen edge shapes on edge toughness resulted in the modification of their design. Known data, while very interesting and useful, demonstrate that the M–GIc relationship has no physical substantiation. It is not observed for ceramics [8,9] but holds for different brittle materials (e.g., test results [2,3]). A known procedure does not take account of differences in the mechanical behavior of ceramics [10] and other brittle materials, fracture patterns in the zone of the indenter contact with the specimen as well as the real fracture surface area formed upon flaking. Therefore, a more advantageous edge fracture method was developed [9]. Its efficiency and suitability for testing different ceramic materials was verified in the present investigation. www.elsevier.com/locate/ceramint Ceramics International 33 (2007) 315–320 * Corresponding author. Tel.: +380 44 285 44 64; fax: +380 44 286 16 84. E-mail address: ggogotsi@ipp.kiev.ua (G. Gogotsi). 0272-8842/$32.00 # 2006 Elsevier Ltd and Techna Group S.r.l. All rights reserved. doi:10.1016/j.ceramint.2005.09.017
G. Gogotsi et al. /Ceramics International 33(2007)315- 2. Materials and methods Fracture point 2. Ceramics Gas-pressure-sintered silicon nitrides GPSSN, sintered silicon carbides ekasic c as well as sintered zirconia ceramics of different structure and composition were the major materials investigated in this study ( Table 1). To compare the Point of loading P Specimen edge above method with the single edge v-notched beam (SEVNB) one, linear elastic ceramics were tested (similar ceramics are Fig. 1. Edge flaking of a hot-pressed silicon nitride [9] specimen(Rockwell usually used in Round Robin fracture toughness tests [13]. indenter, fracture load, Pr=83N fracture distances, L=0. 18 mm): l-spec All these ceramics were fine-and coarse-grained materials men, 2-chip f rather high homogeneity. The exception was commercial EKasicC ceramics (ESC Ceramics gmbh Co. KG Germany) with grain sizes up to 1500 um and inelastic TS- not appear on indentation as well as chips flaked off from the grade ceramics(Nilcra Incorp, Australia). specimens did not hack to pieces SEVNB fracture toughness values were determined accord 2.2. Test method ing to a known procedure [14]. According to our procedure, a loading point on the specimen 3. Results and discussion is first chosen by the naked eye(or by means of a magnifyin glass), edge flaking of ceramics is effected by indentation, and The first step of our investigation based on the use of the the peak load Pr at the moment of chip formation is determined. Rockwell indenter was confirmed the existence of the linear Pr Then the fracture distance L(Fig. 1), which is equal to the chip L relationship for different ceramic materials. Sintered Al2O3/ thickness, is accurately measured on the specimen using the (Y, Nb, Fe)-TZP(Klc=5.9 MPa m"), self-bonded SiC 20% micrometer eye-piece of an Olympus BX 5IM optical Si(KIc=2.4 MPa m") as well as gPssn ceramics were used binocular microscope(1000x). The slope of the regression in these experiments. In accordance with the obtained results line for data points(PrL diagram) obtained under constant (Fig 3), the PrL relationship is independent of Pr values and experimental conditions was used as the characteristic of the therefore flaking toughness Et can be related to the mechanical flaking toughness Et properties of the material. It should be noted that the edge The Rockwell, Knoop, and Vickers diamond indenters were toughness M was proposed using a similar approach [2] used to apply the load at the edge of the polished rectangular The second step of our investigation consisted in the specimen with an edge radius of about 10-20 um and cross- comparison of experimental and published results of flaking sections from 3 mm x 3 mm to 9 mm x 10 mm. The speci- toughness determinations. Ceramics similar to those invest mens were tested at varying fracture distances L from the edge gated by other authors were tested using the Rockwell indenter (Fig. 2) and Pr values, which changed accordingly, were and closely related data for several materials were obtained [9] recorded using a computer. The indenters were placed in the The studies on Prozyr zirconia ceramics, for instance oving rod of a Ceram Test device of our design [14, mounted demonstrate that our L-based measurement results can be on a multipurpose compression/tension test machine(cross- sufficiently consistent with data derived by measuring the value head speed in experiments was constant and equal to 0.5 mm/ d on a commercially available device designed for this purpose min). The specimens were fixed in place with the holder of an(Fig 4) X-Y table installed on the rigid load cell of this device. Lateral The third step involved the evaluation of the effect cracks and a heave of the specimen surface near their edges did indenter types on test results. To do this, the majority of Table Characteristics of ceramic materials Ceramics Nomenclature and reference Brittleness Elastic modulus Strength Scandia Sc2O3 [11 Al2O32[121 Zirconia Prozyr(Y-PSz)[5,9] 210 4.90 Y-TZP 19 Silicon nitride GPSSN9,13,14] 5.40 Si2N4[12 Silicon carbide EKasicC 19 430 2.8
2. Materials and methods 2.1. Ceramics Gas-pressure-sintered silicon nitrides GPSSN, sintered silicon carbides EKasic1 C as well as sintered zirconia ceramics of different structure and composition were the major materials investigated in this study (Table 1). To compare the above method with the single edge V-notched beam (SEVNB) one, linear elastic ceramics were tested (similar ceramics are usually used in Round Robin fracture toughness tests [13]. All these ceramics were fine- and coarse-grained materials of rather high homogeneity. The exception was commercial EKasic1 C ceramics (ESC Ceramics GmbH & Co. KG, Germany) with grain sizes up to 1500 mm and inelastic TSgrade ceramics (Nilcra Incorp., Australia). 2.2. Test method According to our procedure, a loading point on the specimen is first chosen by the naked eye (or by means of a magnifying glass), edge flaking of ceramics is effected by indentation, and the peak load Pf at the moment of chip formation is determined. Then the fracture distance L (Fig. 1), which is equal to the chip thickness, is accurately measured on the specimen using the micrometer eye-piece of an Olympus BX 51M optical binocular microscope (1000). The slope of the regression line for data points (Pf–L diagram) obtained under constant experimental conditions was used as the characteristic of the flaking toughness Et. The Rockwell, Knoop, and Vickers diamond indenters were used to apply the load at the edge of the polished rectangular specimen with an edge radius of about 10–20 mm and crosssections from 3 mm 3 mm to 9 mm 10 mm. The specimens were tested at varying fracture distances L from the edge (Fig. 2) and Pf values, which changed accordingly, were recorded using a computer. The indenters were placed in the moving rod of a CeramTest device of our design [14], mounted on a multipurpose compression/tension test machine (crosshead speed in experiments was constant and equal to 0.5 mm/ min). The specimens were fixed in place with the holder of an X–Y table installed on the rigid load cell of this device. Lateral cracks and a heave of the specimen surface near their edges did not appear on indentation as well as chips flaked off from the specimens did not hack to pieces. SEVNB fracture toughness values were determined according to a known procedure [14]. 3. Results and discussion The first step of our investigation based on the use of the Rockwell indenter was confirmed the existence of the linear Pf– L relationship for different ceramic materials. Sintered Al2O3/ (Y, Nb, Fe)-TZP (KIc = 5.9 MPa m1/2), self-bonded SiC + 20% Si (KIc = 2.4 MPa m1/2) as well as GPSSN ceramics were used in these experiments. In accordance with the obtained results (Fig. 3), the Pf–L relationship is independent of Pf values and therefore flaking toughness Et can be related to the mechanical properties of the material. It should be noted that the edge toughness M was proposed using a similar approach [2]. The second step of our investigation consisted in the comparison of experimental and published results of flaking toughness determinations. Ceramics similar to those investigated by other authors were tested using the Rockwell indenter and closely related data for several materials were obtained [9]. The studies on Prozyr zirconia ceramics, for instance demonstrate that our L-based measurement results can be sufficiently consistent with data derived by measuring the value d on a commercially available device designed for this purpose (Fig. 4). The third step involved the evaluation of the effect of indenter types on test results. To do this, the majority of 316 G. Gogotsi et al. / Ceramics International 33 (2007) 315–320 Table 1 Characteristics of ceramic materials Ceramics Nomenclature and reference Brittleness measure x [9] Elastic modulus (GPa) Strength (MPa) Fracture toughness (SEVNB) (MPa m1/2) Scandia Sc2O3 [11] 1 218 110 1.49 Alumina Al2O3-2 [12] 1 232 322 2.93 Zirconia Prozyr (Y-PSZ) [5,9] 1 210 1300 4.90 Mg-PSZ (TS-grade) [9,10] 0.42 206 521 10.40 Y-TZP [9] 1 206 521 7.40 Silicon nitride GPSSN [9,13,14] 1 320 920 5.40 Si3N4 [12] 1 273 486 4.32 Silicon carbide EKasic1 C [9] 1 430 389 2.80 Fig. 1. Edge flaking of a hot-pressed silicon nitride [9] specimen (Rockwell indenter, fracture load, Pf = 83 N, fracture distances, L = 0.18 mm): 1—specimen, 2—chip.
G. Gogotsi et al./Ceramics Intemational 33(2007)315-320 317 Fig. 2. Chip scars on GPSSN(a) and Prozyr specimens (3 mm x 4 mm cross-section) .d values Al2O3/(Y, Nb, Fe)-TZP z.3 ErR=567N/mm Our results E,R=534 N/mm Self-bonded SiC+20%SI Er=214 N/mm Quinn et al M=522 N/mm Fracture distance L,m Fig. 3. Fracture load Pt vs fracture distance L diagrams for different cerami Edge distance d(Fracture distance L),mm Fig. 4. Comparison of fracture load Pr vS. edge distance d(fracture distance L) diagrams for Prozyr ceramics tested in [5] and in our laboratory. materials under study were tested with different indenters. Y-PSZ ceramics were not observed, although flaking toughness These results demonstrate that flaking toughness values values were different. On this basis, it is suggested to use the decreased with stress concentrations in the fracture zone following terms: Rockwell flaking toughness EIr, Knoop (Fig. 5a). For silicon carbide ceramics, characterized by the flaking toughness EtK, and Vickers faking toughness E wide scatter of the data, it appeared to be narrower. Such a result The fourth step referred to the examination of the does not correspond to [4]. Therefore, an additional investiga- relationship between chip scar sizes on the specimens. The tion of EKasic C ceramics was undertaken. To obtain data results of this investigation for elastic homogeneous ceramics ( Fig 5b), these ceramics were studied on the specimens with a demonstrated a linear proportionality between maximum cross-section of about 3 mm x 3 mm. therefore, one can infer width-to-fracture length and height-to-fracture length ratios that the wide scatter of experimental data is associated with size for them(Fig. 7). It means that the EF method is effective at any effects (chip scars were higher than half the specimen chip sizes, when flaking toughness values are measured. Thus, thickness, as is shown in Fig. 2), hence, thicker specimens it is appropriate to substitute w measurements for Lones (9 mm x 10 mm)were tested. As is seen in Fig. 6, the picture because such measurements are easier to perform more does not change: the same EuR value and the same scatter of the precisely by an optical microscope results. But it should be noted that the scatter of the data for In addition to ceramics listed in Table l, a hot-pressed silicon other similar ceramics [4] was wider than that observed in our nitride particulate ceramic composite with 10% tin (elastic experiments, which can be associated with the specific features modulus 320 GPa)[14] was also studied. The fracture toughness of the test method used(see Section 1). It is significant that any of this material was different(4.8 and 5.15 MPa m)in the changes in the scatter of the results for GPSSN (Fig 5a) and direction A(parallel to the hot pressing axis)and the direction B ER=417 N/mm ◆EK=470N/m EIR=583 N/mm 100 ◆EK-274Nmn o Enre4sD Nmm Fracture distance L, mm Fracture distance L. mm Fig. 5. Fracture load Pr vs. fracture distance L diagrams for EKasic C(a)and GPSSN (b) specimens obtained with different indenters
materials under study were tested with different indenters. These results demonstrate that flaking toughness values decreased with stress concentrations in the fracture zone (Fig. 5a). For silicon carbide ceramics, characterized by the wide scatter of the data, it appeared to be narrower. Such a result does not correspond to [4]. Therefore, an additional investigation of EKasic1 C ceramics was undertaken. To obtain data (Fig. 5b), these ceramics were studied on the specimens with a cross-section of about 3 mm 3 mm, therefore, one can infer that the wide scatter of experimental data is associated with size effects (chip scars were higher than half the specimen thickness, as is shown in Fig. 2), hence, thicker specimens (9 mm 10 mm) were tested. As is seen in Fig. 6, the picture does not change: the same EtR value and the same scatter of the results. But it should be noted that the scatter of the data for other similar ceramics [4] was wider than that observed in our experiments, which can be associated with the specific features of the test method used (see Section 1). It is significant that any changes in the scatter of the results for GPSSN (Fig. 5a) and Y-PSZ ceramics were not observed, although flaking toughness values were different. On this basis, it is suggested to use the following terms: Rockwell flaking toughness EtR, Knoop flaking toughness EtK, and Vickers flaking toughness Etv. The fourth step referred to the examination of the relationship between chip scar sizes on the specimens. The results of this investigation for elastic homogeneous ceramics demonstrated a linear proportionality between maximum width-to-fracture length and height-to-fracture length ratios for them (Fig. 7). It means that the EF method is effective at any chip sizes, when flaking toughness values are measured. Thus, it is appropriate to substitute W measurements for L ones because such measurements are easier to perform more precisely by an optical microscope. In addition to ceramics listed in Table 1, a hot-pressed silicon nitride particulate ceramic composite with 10% TiN (elastic modulus 320 GPa) [14] was also studied. The fracture toughness of this material was different (4.8 and 5.15 MPa m1/2) in the direction A (parallel to the hot pressing axis) and the direction B G. Gogotsi et al. / Ceramics International 33 (2007) 315–320 317 Fig. 2. Chip scars on GPSSN (a) and Prozyr specimens (3 mm 4 mm cross-section). Fig. 3. Fracture load Pf vs. fracture distance L diagrams for different ceramics. Fig. 4. Comparison of fracture load Pf vs. edge distance d (fracture distance L) diagrams for Prozyr ceramics tested in [5] and in our laboratory. Fig. 5. Fracture load Pf vs. fracture distance L diagrams for EKasic1 C (a) and GPSSN (b) specimens obtained with different indenters.
G. Gogotsi et al. /Ceramics International 33(2007)315-320 Morrell et al. (d values Our results(L values) Ex=397 N/mm Direction B ER=481 N/mm t之 o specimens9×l0mm Fracture distance /. mm Fig.6. Fracture load P, vs. fracture distance L diagrams for EKasic" c directions i um niobe Fig. 8. Fracture Pr vs. fracture distance L diagra silicon Edge distance d( Fracture distance L), composite (different loading 0.1 Fig. 7. Maximum chip scar width W(a)and chip scar depth D(b)vs fracture distance Diagrams of GPSSN specimens tested with different indenters(O: Rockwell, ◇: Vickers,and◆: Knoop) (normal to the hot pressing axis), accordingly. Such an effect is specimen is rectangular, its surface is polished, its loading also typical of flaking toughness, as is shown in Fig. 8. It is plane is perpendicular to the indentation direction. However, interesting to note that in case of a silicon nitride/silicon nitride- in a lot of cases chip scars look differently. For instance, chip titanium nitride laminar ceramic composite with a compressed scars formed on hot-pressed Si3N4(Fig. 1)were similar i3N4 outer layer [14], the relationship between faking those shown in [4] for PM tool steel(64 HRC), the shape of toughness in the A and B directions is opposite. Tensile stresses grains governed fracture patterns of EKasicC ceramics in the compressed outer layer contribute to its fracture under (Fig. 9), etc. Based on the results of this investigation and loading in the direction B published data, it is impossible to examine all the factors Considerable study has been given to chip scars on the controlling edge fracture patterns in different ceramic tested specimens because they control the accuracy of the materials data obtained in this investigation. As is seen, symmetrical Mention should also be made of some unintelligible effects chip scars (Fig. 2)can be obtained if the edge of the observed during the experiments, e.g., nonlinearity of fracture Fig 9. Chip scars on an EKasic C specimen
(normal to the hot pressing axis), accordingly. Such an effect is also typical of flaking toughness, as is shown in Fig. 8. It is interesting to note that in case of a silicon nitride/silicon nitridetitanium nitride laminar ceramic composite with a compressed Si3N4 outer layer [14], the relationship between flaking toughness in the A and B directions is opposite. Tensile stresses in the compressed outer layer contribute to its fracture under loading in the direction B. Considerable study has been given to chip scars on the tested specimens because they control the accuracy of the data obtained in this investigation. As is seen, symmetrical chip scars (Fig. 2) can be obtained if the edge of the specimen is rectangular, its surface is polished, its loading plane is perpendicular to the indentation direction. However, in a lot of cases chip scars look differently. For instance, chip scars formed on hot-pressed Si3N4 (Fig. 1) were similar to those shown in [4] for PM tool steel (64 HRC), the shape of grains governed fracture patterns of EKasic1 C ceramics (Fig. 9), etc. Based on the results of this investigation and published data, it is impossible to examine all the factors controlling edge fracture patterns in different ceramic materials. Mention should also be made of some unintelligible effects observed during the experiments, e.g., nonlinearity of fracture 318 G. Gogotsi et al. / Ceramics International 33 (2007) 315–320 Fig. 7. Maximum chip scar width W (a) and chip scar depth D (b) vs. fracture distance L diagrams of GPSSN specimens tested with different indenters (*: Rockwell, ^: Vickers, and ^: Knoop). Fig. 6. Fracture load Pf vs. fracture distance L diagrams for EKasic1 C ceramics (specimens of different sizes) and data [4]. Fig. 8. Fracture load Pf vs. fracture distance L diagrams for a silicon nitride/titanium nitride particulate ceramic composite (different loading directions). Fig. 9. Chip scars on an EKasic1 C specimen
G. Gogotsi et al. /Ceramics Intenational 33(2007)315-320 319 Table 2 Comparison of test methods Choice of a Tool for fracture Analys Results of the tested specimen Paleolithic Naked eye Stone in hand Choice Production of of chip scar high-quality Known[] Microscope of a Indenter in a c ulation of edge Data of edge fracture resistance evaluation dgc Naked eye Indenter Measurement of Calculation of Data for fracture (or a magnifying glass) standard hip scar toughness evaluation Ear=878 N/mm (linear approximation) Sc: O3(11 VSi3N4(154) ★Y-TZP(18) Fig. 10. Fracture load Pr vS fracture distance L diagram for Mg-PSZ ceramics Fracture toughness Kc, MPam2 Fig. 11. Flaking toughness ER vS. fracture toughness Kie diagram for the load Pr versus fracture distance L diagrams(Fig. 10). As a result of their linear approximation, EtR =878 N/mm can be obtained. If the PrL diagram is divided into 0.15-0.30 mm, 0.30-0.45mm,and0.45-0.60 mm pieces,ER=550,891 and 1040 N/mm are derived. If the PrL diagram is divided 4. Conclusions into a larger number of pieces or differentiated, the obtained diagram would be similar to the R-curve for these The edge fracture(EF) method can easily be put to practical ceramics [10]. This problem will be investigated in the use in any laboratory performing mechanical tests immediate future but it is already clear that the ef method It has been shown that the rockwell flaking toughness etr can provide new reliable data on the fracture resistance of versus fracture toughness Klc relationship is almost linear Before proceeding to the discussion of the results, it would the fracture resistance of ceramic material ive evaluation of Thus, Er values can be used for the comparative evaluation of be appropriate to compare the EF method with those described elsewhere [2] and probably used in the Paleolithic Period Acknowledgments Summarizing the investigation results, it might be well to The authors are thankful to Professor E Almond(Nation evaluate the flaking toughness EtR versus fracture toughness Klc Physical Laboratory, UK) for his idea of studying the edge lationships for linear elastic and single phase ceramics under fracture of ceramics, Dr J. Quinn(American Dental Associa Idy(Fig. 11). In this case the validity of the SEVNB method is tion Foundation, USA) for her support in launching this obvious since it makes use of fracture mechanics concepts [15] investigation, Dr J. Kubler(EMPA, Switzerland) and Dr. A based on the model solid body which is homogeneous and Rendtel(ESK Ceramics gmbh Co. KG, Germany) for linear elastic. These diagrams demonstrate that the flaking ceramic specimens as well as Eng. B. Ozersky(Pisarenko toughness EIR versus fracture toughness Klc relationship is Institute for Problems of Strength, Ukraine) for his assistance in almost linear he performance of experiment
load Pf versus fracture distance L diagrams (Fig. 10). As a result of their linear approximation, EtR = 878 N/mm can be obtained. If the Pf–L diagram is divided into 0.15–0.30 mm, 0.30–0.45 mm, and 0.45–0.60 mm pieces, EtR = 550, 891, and 1040 N/mm are derived. If the Pf–L diagram is divided into a larger number of pieces or differentiated, the obtained diagram would be similar to the R-curve for these ceramics [10]. This problem will be investigated in the immediate future but it is already clear that the EF method can provide new reliable data on the fracture resistance of ceramics. Before proceeding to the discussion of the results, it would be appropriate to compare the EF method with those described elsewhere [2] and probably used in the Paleolithic Period (Table 2). Summarizing the investigation results, it might be well to evaluate the flaking toughness EtR versus fracture toughness KIc relationships for linear elastic and single phase ceramics under study (Fig. 11). In this case the validity of the SEVNB method is obvious since it makes use of fracture mechanics concepts [15] based on the model solid body which is homogeneous and linear elastic. These diagrams demonstrate that the flaking toughness EtR versus fracture toughness KIc relationship is almost linear. 4. Conclusions The edge fracture (EF) method can easily be put to practical use in any laboratory performing mechanical tests. It has been shown that the RoFkwell flaking toughness EtR versus fracture toughness KIc relationship is almost linear. Thus, EtR values can be used for the comparative evaluation of the fracture resistance of ceramic materials. Acknowledgments The authors are thankful to Professor E. Almond (National Physical Laboratory, UK) for his idea of studying the edge fracture of ceramics, Dr. J. Quinn (American Dental Association Foundation, USA) for her support in launching this investigation, Dr. J. Kubler (EMPA, Switzerland) and Dr. A Rendtel (ESK Ceramics GmbH & Co. KG, Germany) for ceramic specimens as well as Eng. B. Ozersky (Pisarenko Institute for Problems of Strength, Ukraine) for his assistance in the performance of experiments. G. Gogotsi et al. / Ceramics International 33 (2007) 315–320 319 Table 2 Comparison of test methods Method Choice of a loading point Tool for fracture Characterization of the tested specimen Analysis Results Paleolithic Naked eye Stone in hand Visual examination of chip scars Choice of more fracture-resistant stones Production of high-quality stone tools Known [2] Microscope of a special test unit Indenter in a special test unit – Calculation of edge toughness Data of edge fracture resistance evaluation Edge fracture Naked eye (or a magnifying glass) Indenter in a standard test unit Measurement of chip scar sizes Calculation of flaking toughness Data for fracture toughness evaluation Fig. 10. Fracture load Pf vs. fracture distance L diagram for Mg-PSZ ceramics. Fig. 11. Flaking toughness EtR vs. fracture toughness KIc diagram for the ceramics under study (characters in brackets are the number of tests)
G. Gogotsi et al. /Ceramics International 33(2007)315- References [8] F Petit, P Descamps, J.P. Erauw, F Cambier, Toughness(Kle)measure- ment by a sliding indentation method, Key Engineering Materials, vols. [I] E. Almond, N. McCormick, Constant geometry edge flaking of brittle 206-213. Transaction Technical Publications. Switzerland, 2002 materials. Nature 321(1986)53-55 629632. [2] N. McCormick, E. Almond, Edge flaking of brittle materials, J. Hard [9] G.A. Gogotsi, S. Mudrik, A. Rendtel, Sensitivity of silicon carbide and Mater.l(1990)25-51 other ceramics to edge fracture: method and results. in: E. Lara- Curzio. M [3] N. McCormick, A Method for Edge Toughness Testing, Crown Copy. eadey(Eds ) 28th International Conference on Advanced Ceramics and ht, Nottingham. England, 199 opposites: A, Ceramic Engineering Science Proceedings, American [4] R Morrell, A Gant, Edgechipping-what doesittellus? in: J.R. Varuer, G D Ceramic Society, vol 25(3), 2004, pp. 237-246 Quinn(Eds ) Fractography of Glasses and Ceramics IV, Ceramic Transac- [10] G.A. Gogotsi, Deformational behaviour of ceramics, J.Eur. Ceram Soc. 7 tion. vol. 122. American Ceramic Westerville, 2001 pp 23-41 991)87-92 [5]J. Quinn, I. Lloyd, Flake and scratch size ratios in ceramics, in: J.R. [11] G Gogotsi, Thermal stress behaviour of yttria, scandia and AIN ceramics, Varuer, G D. Quinn(Eds ) Fractography of Glasses and Ceramics IV, Ceran.n.6(1980)31-35 Ceramic Transaction, vol. 122, American Ceramic Society. Westerville, 112] G.A. Gogotsi, V.L. Galenko, B.L. Ozerskty, T.A. Khristewich, Edge 2001,pp.57-72. Strength of materials, 2005, No 5, pp. 84-92 [6 M. Hang, R Danzer, R Paar, Edge toughness of brittle m in: GN [13] J Kubler, Fracture Toughness of Ceramics Using the SEVNB Method: Babini. M. Haviar, P. Saigalik(Eds ) Engineering Ceramics 96: Higher ound Robin, VAMAS Report No37/ESIS Document D2-99, EMPA Reliability Through Processing, Kluwer Academic Publishers, 1997. pp Swiss Federal Laboratories for Materials Testing and Research, Duben- 327-335 dorf, Switzerland, September 1999. [71 R. Danzer, M. Hangl, R Paar, Edge chipping of brittle materials, in: J.R. [14] G. Gogotsi, Fracture toughness of ceramics and ceramic composites, Varuer, G.D. Quinn(Eds ) Fractography of Glasses and Ceramics IV, Ceram.Int.2902003)777-784 Ceramic Transaction, voL. 122, American Ceramic Society, Westerville, [15] T L. Anderson, Fracture Mechanics: Fundamentals and Applications. 43-55 CRS Press LLC. Boca Raton. 1995
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