CERAMICS INTERNATIONAL ELSEVIER Ceramics International 29(2003)777-784 www.elsevier.com/locate/ceramint Fracture toughness of ceramics and ceramic composites George A Gogotsi Institute for Problems of strength, 2 Timiryazevskaya Street, Kiev 01014, Ukrain Received 22 July 2002: received in revised form 9 September 2002: accepted 10 November 2002 Abstract The fracture toughness of zirconia, alumina, and silicon nitride ceramics, zirconia and alumina single crystals, silicon carbide as well as silicon nitride ceramic particulate composites, silicon nitride laminated composites, and other ceramics materials were studied by a single edge v-notched beam (SEVNB) method Manual and mechanical procedures for V-notch polishing-out and three-and four-point flexure tests were developed. Load-deflection diagrams for V-notched specimens contributed to better understanding of the deformation behavior of ceramics at room temperature and 1300-1400C sEnb (single edge notched beam) and sEPB (single edge precracked beam) as well as micro-Raman spectroscopy data were used to analyze the sevPb results. The ratio between SevNb and senb results is about 0.6 for elastic materials, over 0.9 for inelastic ones, and about 1.0 for laminated ceramic composite. The polishing-out of a V-notch does not lead to the tetragonal-monoclinic phase transformation in zirconia C 2003 Elsevier Ltd and Techna S r l. All rights reserved Keywords: B Composites; B Spectroscopy: C. Fracture; C. Toughness; Ceramics; SEVNB-method 1. ntroduction steps in this field. Leading specialists from different countries engaged in those investigations performed Ceramics and ceramic composites are promising statistical analyses to compare the test methods sum materials having rather high strength characteristics but marized in Table 1. They were helpful for creating the luite low crack resistance properties at the same time. basis of more reliable determination of the fracture This is one of the major factors hindering the wide-scale toughness characteristics of ceramics. For a long time, application of these materials in various fields of human no particular preference was given to any of the meth activities. The crack resistance is critical not only for ods considered because each of them had appreciable ceramic products operating under extreme mechanical shortcomings, which restricted their application [2, 3] and thermal loads but also for structural components Recently, much attention has been focused on the whose brittle fracture is intolerable even under arbitrary SEVNB method [4], which is a further development of a loads simple and widely used the single edge notched beam For many years, the performance of ceramics has (SENB) method suitable only for rough fracture tough been evaluated on the basis of full-scale tests. However, ness evaluations. The test data obtained by the SEVnB their fracture toughness characteristics have not always method were found to be comparable to those obtained been the object of scientific interest. Wide-scale fracture by the single edge precracked beam (SEPB) method toughness investigations were started only in the late usually considered as providing the real values of cri- 1980s International prestandard studies(Round Robin tical stress intensity factor(Klc) but this test is often on Fracture Toughness-RRFT [ID aimed at the assess- difficult to realize in practice [5]. To evaluate the poten ment of the accuracy and reliability of the data obtained tials of the SEvnb method as a standard one for testing by commonly accepted test methods were important ceramics, its analysis was performed [6]. We verified the efficiency of our procedures and demonstrated the applicability of the SEVNB method to different ceramic E-mail address: ggogotsi @ipp. adam. kiev. ua(GA. Gogotsi). materials 0272-8842/03/$30.00@ 2003 Elsevier Ltd and Techna S.r.l. All rights reserved. doi:10.l016/S0272-8842(02)00230-4
Fracture toughness of ceramics and ceramic composites George A. Gogotsi Institute for Problems of Strength, 2 Timiryazevskaya Street, Kiev 01014, Ukraine Received 22 July 2002; received in revised form 9 September 2002; accepted 10 November 2002 Abstract The fracture toughness of zirconia, alumina, and silicon nitride ceramics, zirconia and alumina single crystals, silicon carbide as well as silicon nitride ceramic particulate composites, silicon nitride laminated composites, and other ceramics materials were studied by a single edge V-notched beam (SEVNB) method. Manual and mechanical procedures for V-notch polishing-out and three- and four-point flexure tests were developed. Load–deflection diagrams for V-notched specimens contributed to better understanding of the deformation behavior of ceramics at room temperature and 1300–1400 C. SENB (single edge notched beam) and SEPB (single edge precracked beam) as well as micro-Raman spectroscopy data were used to analyze the SEVPB results. The ratio between SEVNB and SENB results is about 0.6 for elastic materials, over 0.9 for inelastic ones, and about 1.0 for laminated ceramic composite. The polishing-out of a V-notch does not lead to the tetragonal-monoclinic phase transformation in zirconia ceramics. # 2003 Elsevier Ltd and Techna S.r.l. All rights reserved. Keywords: B. Composites; B. Spectroscopy; C. Fracture; C. Toughness; Ceramics; SEVNB-method 1. Introduction Ceramics and ceramic composites are promising materials having rather high strength characteristics but quite low crack resistance properties at the same time. This is one of the major factors hindering the wide-scale application of these materials in various fields of human activities. The crack resistance is critical not only for ceramic products operating under extreme mechanical and thermal loads but also for structural components whose brittle fracture is intolerable even under arbitrary loads. For many years, the performance of ceramics has been evaluated on the basis of full-scale tests. However, their fracture toughness characteristics have not always been the object of scientific interest. Wide-scale fracture toughness investigations were started only in the late 1980s. International prestandard studies (Round Robin on Fracture Toughness–RRFT [1]) aimed at the assessment of the accuracy and reliability of the data obtained by commonly accepted test methods were important steps in this field. Leading specialists from different countries engaged in those investigations performed statistical analyses to compare the test methods summarized in Table 1. They were helpful for creating the basis of more reliable determination of the fracture toughness characteristics of ceramics. For a long time, no particular preference was given to any of the methods considered because each of them had appreciable shortcomings, which restricted their application [2,3]. Recently, much attention has been focused on the SEVNB method [4], which is a further development of a simple and widely used the single edge notched beam (SENB) method suitable only for rough fracture toughness evaluations. The test data obtained by the SEVNB method were found to be comparable to those obtained by the single edge precracked beam (SEPB) method usually considered as providing the real values of critical stress intensity factor (KIc) but this test is often difficult to realize in practice [5]. To evaluate the potentials of the SEVNB method as a standard one for testing ceramics, its analysis was performed [6]. We verified the efficiency of our procedures and demonstrated the applicability of the SEVNB method to different ceramic materials. 0272-8842/03/$30.00 # 2003 Elsevier Ltd and Techna S.r.l. All rights reserved. doi:10.1016/S0272-8842(02)00230-4 Ceramics International 29 (2003) 777–784 www.elsevier.com/locate/ceramint E-mail address: ggogotsi@ipp.adam.kiev.ua (G.A. Gogotsi)
G.A. Gogotsi/Ceramics International 29(2003)777-784 2. Experimental 2.2. Test specimens 2. Materials Rectangular beams (3 mmx4 mm cross-section) v- notched in two stages were used as test specimens [7 Isostatically pressed silicon nitride ceramics (Si3N4) The package of 2-6 specimens were glued onto a cera- [7], sintered (Y-PSZ)[8] and sintered under pressure (Y- mic plate and then V-notched. Two additional speci PSZa) partially-stabilized zirconia ceramics [9] were the mens were fixed on both sides of the package for its major materials used in the process of verifying the protection from the edge effects due to notching. A experimental procedures. Gas-pressure sintered silicon preliminary notch 0.8 mm deep(spec simens for lou nitride(GPSSN) as well as sintered alumina(Al2O3-998) point flexure)or 1.6 mm deep(specimens for three-point and silicon carbide (ssic)[6] analyzed in the rrFt97 flexure) was cut in the package by a 150-300 um thick also became the objects of the investigation. In our diamond disk. At the next stage, the V-notch was experiments we also used zirconia and alumina single manually (convenient for comparatively"soft'cera- posites with compressed SiaN4 and tensile-stressed polished-out after filling the preliminary notch wih S) crystals as well as silicon nitride based laminated com- mics)or mechanically (necessary for"hard"ceramics) Si3N4 n wt. TiN layers [10]. This composite, layers diamond paste of grain sizes ranging from 2 to 7 um of which were prepared by rolling, was produced by hot and a razor blade 150-300 um thick. In the case of pressing at 1850C for 20 min under pressure 150 MPa. manual polishing-out, the walls of the preliminary notch For high-temperature investigations, reaction-bonded (1.6 mm deep) served as the razor blade guides. The and sintered Si3N4 30% SiC 3% Mgo [11] and purpose of the test determined the number of notches hot-pressed (in graphite molds)Sic 50%ZrB 2 (one or three) cut in a standard 45-mm long specimen 10% B4C[12] ceramic particulate composites were also (Fig. 1). The criterion for the proper V-notch sharpness used. Some data on these ceramics are given in Table 2. is its width, S, equal to two root radii p. The notch Conventional ceramics fracture toughness test methods Specimen Method and fracture Specimen Method and fracture surface of specimen surface of specimen SEPB (surface crack cracked beam) SENB i hgea (single edg (indentation IF (chevron fracture) notched beam) Fig. 1. SEVNB specimens: a, b, three-point fiexure; c, four-point flexure
2. Experimental 2.1. Materials Isostatically pressed silicon nitride ceramics (Si3N4) [7], sintered (Y-PSZ) [8] and sintered under pressure (YPSZa) partially-stabilized zirconia ceramics [9] were the major materials used in the process of verifying the experimental procedures. Gas-pressure sintered silicon nitride (GPSSN) as well as sintered alumina (Al2O3-998) and silicon carbide (SSiC) [6] analyzed in the RRFT’97 also became the objects of the investigation. In our experiments we also used zirconia and alumina single crystals as well as silicon nitride based laminated composites with compressed Si3N4 and tensile-stressed Si3N4 + n wt.% TiN layers [10]. This composite, layers of which were prepared by rolling, was produced by hot pressing at 1850 C for 20 min under pressure 150 MPa. For high-temperature investigations, reaction-bonded and sintered Si3N4 + 30% SiC + 3% MgO [11] and hot-pressed (in graphite molds) SiC + 50%ZrB2 + 10% B4C [12] ceramic particulate composites were also used. Some data on these ceramics are given in Table 2. 2.2. Test specimens Rectangular beams (3 mm4 mm cross-section) Vnotched in two stages were used as test specimens [7]. The package of 2–6 specimens were glued onto a ceramic plate and then V-notched. Two additional specimens were fixed on both sides of the package for its protection from the edge effects due to notching. A preliminary notch 0.8 mm deep (specimens for fourpoint flexure) or 1.6 mm deep (specimens for three-point flexure) was cut in the package by a 150–300 mm thick diamond disk. At the next stage, the V-notch was manually (convenient for comparatively ‘‘soft’’ ceramics) or mechanically (necessary for ‘‘hard’’ ceramics) polished-out after filling the preliminary notch with a diamond paste of grain sizes ranging from 2 to 7 mm and a razor blade 150–300 mm thick. In the case of manual polishing-out, the walls of the preliminary notch (1.6 mm deep) served as the razor blade guides. The purpose of the test determined the number of notches (one or three) cut in a standard 45-mm long specimen (Fig. 1). The criterion for the proper V-notch sharpness is its width, S, equal to two root radii . The notch Table 1 Conventional ceramics fracture toughness test methods Specimen Method and fracture surface of specimen Specimen Method and fracture surface of specimen SEPB (single edge precracked beam) SCF (surface crack in flexure) SENB (single edge notched beam) IS (indentation strength) CNB (chevron notched beam) IF (indentation fracture) Fig. 1. SEVNB specimens: a, b, three-point flexure; c, four-point flexure. 778 G.A. Gogotsi / Ceramics International 29 (2003) 777–784
G.A. Gogotsi/ Ceramics International 29(2003)777-784 length and radius were measured with optical(Olimpus For the recording of the load-deflection curves, a hi BX5IM)and scanning electron microscopes (x500 or sensitivity LVDT-based deflectometer was suspended better) on a specimen(Fig. 2)and was in no way connected to the testing arrangement [13]. The LVDT was located 23. Procedures outside the heating chamber in case of high-temperature The specimens were tested in three-point (16 mm span In all the experiments, the speed of the testing between the bearing rollers) or four-point flexure(20-40 machine crosshead was constant and equal to 0.5 mm, machine and fitted with a hard load cell, a system for Fracture toughness, Kle values were calculated in precision displacement of a loading rod, and loading accordance with ASTM standard [14](three-point flex- supports. The latter are equipped with an attachment ure tests) and Din one [15](four-point flexure tests) for the rotation of the bearing rollers during specimen Additional procedures for evaluating Klc and other loading. Three-point flexure required the precise align- mechanical characteristics are described elsewhere [16] ment of a specimen on the bearing rollers, with the axis a micro-Raman imaging m of the central bearing roller and the radius of the v- JSA)with a 514.5 nm excitation line of an argon laser notch root being in the same plane as the applied load .(100 objective and a- l-um diameter spot) was used In high-temperature tests, we used a loading block to examine the surfaces of fractured Y-PSZ specimens which is similar to that used in room temperature tests. by a procedure described in Ref [171 3. Results and discussion The results of comparative three-and four-point flex ure tests of monolithic ceramics and particular ceramic composites are summarized in Table 3, where the data obtained within the RRFT97 program are also cited The variation of Ki values as a function of notch root Fig. 2. Loading scheme for three-point flexure(a) and deflectometer radius was studied for silicon nitride and zirconia( Fig 3) Ispended on a specimen 3x4x45 mm'(b) Load-deflection diagrams for V-notched specimens are Table 2 Characteristics of some material Material Density (g/cm Strength at 20C(MPa) Hardness(GPa) Method of manufacture 14.1 GPSS 224 Y-PSZ 05 425 IP sintered Material of RRTF97 Comparative fracture toughness tests(MPa m/2) Three-point flexure(a/wa point flexure(a/Wa0. 2...0.3 Our results RRFT97 results 5.5±0.07(5 5.35±0.16(5) GPSS 5.3±0.04(5) 5.2±0.18(5) 5.36±0.34(129) Si3 N4+30%SiC+ 3% Mgo 27±0.14(4) 240±0.16(5) SSiC 266±0.20(4) 2.61±0.18(56) SiC+ 50%oRb,+10% BC 3.59±0.12(3) 3.51±0.15(3) A2O3998 3.5±0.05(5) 3.6±0.06(5) 3.57±0.22(135) 5.7±0.17(5) 59±0.19(5) ± Standard deviation. The number of specimen tested (in parentheses)
length and radius were measured with optical (Olimpus BX51M) and scanning electron microscopes (500 or better). 2.3. Procedures The specimens were tested in three-point (16 mm span between the bearing rollers) or four-point flexure (20–40 mm spans between the bearing rollers) on a home-made Ceramtest block, mounted on a universal testing machine and fitted with a hard load cell, a system for precision displacement of a loading rod, and loading supports. The latter are equipped with an attachment for the rotation of the bearing rollers during specimen loading. Three-point flexure required the precise alignment of a specimen on the bearing rollers, with the axis of the central bearing roller and the radius of the Vnotch root being in the same plane as the applied load. In high-temperature tests, we used a loading block which is similar to that used in room temperature tests. For the recording of the load–deflection curves, a highsensitivity LVDT-based deflectometer was suspended on a specimen (Fig. 2) and was in no way connected to the testing arrangement [13]. The LVDT was located outside the heating chamber in case of high-temperature tests. In all the experiments, the speed of the testing machine crosshead was constant and equal to 0.5 mm/ min. Load cell and deflectometer readings were recorded with a coordinate potentiometer. Fracture toughness, KIc values were calculated in accordance with ASTM standard [14] (three-point flexure tests) and DIN one [15] (four-point flexure tests). Additional procedures for evaluating KIc and other mechanical characteristics are described elsewhere [16]. A micro-Raman imaging microscope (Renishaw, USA) with a 514.5 nm excitation line of an argon laser (100 objective and a1-mm diameter spot) was used to examine the surfaces of fractured Y-PSZ specimens by a procedure described in Ref. [17]. 3. Results and discussion The results of comparative three- and four-point flexure tests of monolithic ceramics and particular ceramic composites are summarized in Table 3, where the data obtained within the RRFT’97 program are also cited. The variation of KIc values as a function of notch root radius was studied for silicon nitride and zirconia (Fig. 3). Load–deflection diagrams for V-notched specimens are Fig. 2. Loading scheme for three-point flexure (a) and deflectometer suspended on a specimen 3445 mm3 (b). Table 3 Comparative fracture toughness tests (MPa m1/2) Test method Three-point flexure (a/W0.5) Four-point flexure (a/W0.2...0.3) Our results RRFT’97 results Si3N4 5.50.07 (5)a 5.350.16 (5) – GPSSN 5.30.04 (5) 5.20.18 (5) 5.360.34 (129) Si3N4+30%SiC+3% MgO 2.270.14 (4) 2.400.16 (5) – SSiC 2.45 (1) 2.660.20 (4) 2.610.18 (56) SiC+50%ZrB2+10% B4C 3.590.12 (3) 3.510.15 (3) – Al2O3-998 3.50.05 (5) 3.60.06 (5) 3.570.22 (135) Y-PSZ 5.70.17 (5) 5.90.19 (5) – Standard deviation. a The number of specimen tested (in parentheses). Table 2 Characteristics of some materials Material Density (g/cm3 ) Strength at 20 C (MPa) Hardness (GPa) Method of manufacture Si3N4 3.14 700 14.1 HIP GPSSN 3.23 >920 13.5 Gas-pressure sintereda SSiC 3.15 – 22.4 Sintereda Al2O3-998 3.86 350 19.3 Sintereda Y-PSZ 6.05 425 12.1 IP + sintered a Material of RRTF’97. G.A. Gogotsi / Ceramics International 29 (2003) 777–784 779
G.A. Gogotsi/ Ceramics international 29(2003)777-784 三 Y-PSZ v月9 Y-PSZa , P__SEPB-method V-notch root radius (um) V-notch root radius(um) Fig 3. Effect of V-notch root radii on the Kls values for Si,N4(a)and Y-PSz (b)ceramics 60 z40 4 mm x 5mm cr 3mm x 4mm cross-section pecten 024681012141618 6 Deflection(um) Deflection(um) Fig 4. Load-deflection diagrams for notched Si3 N4+ 30% SiC+ 3% Mgo (a) and SiC+ 50% ZrB2+10% BC(b)specimens tested at room tem- perature(1, 3)and at 1400C(2, 4) presented in Figs 4 and 5. The results of micro-Raman racy of our test procedures. The comparison of the data analysis are given in Fig. 6. The comparative data on presented in Table 3 also points to the fact that essential SEVNB and senB results are summarized in Tables 4-6. differences between three- and four-point flexure results High-temperature test results are cited in Table 7 are absent. a similar conclusion was also made else- It is useful to start the analysis with emphasis on the where [18] for ceramic matrix composites. Conse Kle values obtained in three- and four-point flexure quently, both test methods might be considered (Table 3). The data presented in this table demonstrate identical. Moreover, four-point flexure can be more good agreement between our results and the average easily applied in practice because it does not require a results of RRFT97 [6], which confirms sufficient accu- precise placement of specimens on the bearing rollers, which is difficult to achieve without a trained operator On the other hand, three-point flexure tests can utilize small-size specimens, which is advantageous for materi- p1=20m als science research t The analysis of results(Fig. 3)shows that a decrease decrease in the Kle values for Si3 N4 and Y-PSZ ceramics (similar relation was also observed for other materials [2D. It is interesting to note that the fracture toughness- Si3N4(Fig. 3a) Deflection 8, um the vicinity of a small notch root radius as compared to Fig. 5. Load-deflection diagrams for notched SiC+50%- that of the Y-PSZ(Fig. 3b). Such an effect is probably TiB+10%BC specimens with V-notch root radiuses PI and p2 are determined by different sensitivity of these materials to qual (1 and 2)and differed (3)in values on them opposite side stress concentrations because of differences in their
presented in Figs. 4 and 5. The results of micro-Raman analysis are given in Fig. 6. The comparative data on SEVNB and SENB results are summarized in Tables 4–6. High-temperature test results are cited in Table 7. It is useful to start the analysis with emphasis on the KIc values obtained in three- and four-point flexure (Table 3). The data presented in this table demonstrate good agreement between our results and the average results of RRFT’97 [6], which confirms sufficient accuracy of our test procedures. The comparison of the data presented in Table 3 also points to the fact that essential differences between three- and four-point flexure results are absent. A similar conclusion was also made elsewhere [18] for ceramic matrix composites. Consequently, both test methods might be considered identical. Moreover, four-point flexure can be more easily applied in practice because it does not require a precise placement of specimens on the bearing rollers, which is difficult to achieve without a trained operator. On the other hand, three-point flexure tests can utilize small-size specimens, which is advantageous for materials science research. The analysis of results (Fig. 3) shows that a decrease in the V-notch radius of a specimen leads to an essential decrease in the KIc values for Si3N4 and Y-PSZ ceramics (similar relation was also observed for other materials [2]). It is interesting to note that the fracture toughnessnotch root radius curve for Si3N4 (Fig. 3a) flattens in the vicinity of a small notch root radius as compared to that of the Y-PSZ (Fig. 3b). Such an effect is probably determined by different sensitivity of these materials to stress concentrations because of differences in their Fig. 3. Effect of V-notch root radii on the KIc values for Si3N4 (a) and Y-PSZ (b) ceramics. Fig. 4. Load–deflection diagrams for notched Si3N4+30% SiC+3% MgO (a) and SiC+50% ZrB2+10% B4C (b) specimens tested at room temperature (1, 3) and at 1400 C (2, 4). Fig. 5. Load–deflection diagrams for notched SiC+50%- TiB2+10%B4C specimens with V-notch root radiuses 1 and 2 are equal (1and 2) and differed (3) in values on them opposite sides. 780 G.A. Gogotsi / Ceramics International 29 (2003) 777–784
G.A. Gogotsi/ Ceramics international 29(2003)777-784 grain sizes( 4 um for Si3 N4 and >I um for Y-PSZ) as contradict the results of sintered SiC whisker-reinforced it was mentioned elsewhere [19]. It should be empha- Si3N4[18] tests. A similar tendency is also typical of Sic sized that only for notch root radii less than 5-7 um [7] ceramics [191 e Kle values for Y-PSZ ceramics agree with SEPb data The examination of the specimens fractured in the (similar behavior is also typical of fine-grained alumina tests revealed a fracture crack that propagated from the [20]. A similar conclusion follows from [6], where an points where "additional"stress concentrators were attempt was made to relate the V-notch radius values present. This observation also confirms the assumption required for the correct determination of Kle values to that the fracture of a loaded ceramic specimen starts the averaged values of this parameter obtained in the from a small crack ahead of a machined notch root [19] RRFT97 studies. As follows from Fig 3a, the decrease In this context, it is interesting to mention that K in Klc values for Si, N4 ceramics occurs with a decrease magnitudes are influenced by the sharpness of a notch in notch radii down to about 30 um, which does not root rather than by its shape [7] W 250 150 l00 Raman shift(cm") Raman shift(cm") Fig. 6. Raman spectra of Y-PSZ specimens fractured by the SEVNB method (a)and by indentation(b): nonfractured specimen surface (1), dia- mond saw surface (2), fracture surface(3), razor blade surface(4), indentation edge(5), and indentation center(6) Table 4 Kic values for ceramics obtained by SEVNB and SENB methods(MPa m/2) Test method Index p Brittleness measure, X SEVNB SENB 5.14±0.29 1.18±0 SiC+ 50%ZrB,+ 10%B, C 3.52±0.08 6.24士 0.564 5.17±0.06 9.12±0.29 Three 0.567 SSiC 4.42±0.23 Si3 N4+ 30%SiC+ 3%Mgo 2.49±0.16 0.920 Mg- PSZ (TSgrade) 944±0.1 10.2±0.27 0.925 s& coo Three 0.25 The width Table 5 KIe values for single crystals obtained by SEVNB and SENB methods(MPa m) Single crystals Peculiarity Test method Index p modulus(GPa) measure, X SENB SEVN Zirconia Partially stabilized (3%Y203 9.33±0.95 0.33±2.171 Alumina Specimen axis 45 to optical axis of crystal 403 2.31±0.34 0.94 Specimen axis 90 to optical axis of cryst 410 3.19±0.53 85±0.50 1.12
grain sizes (4 mm for Si3N4 and >1 mm for Y-PSZ) as it was mentioned elsewhere [19]. It should be emphasized that only for notch root radii less than 5–7 mm [7] the KIc values for Y-PSZ ceramics agree with SEPB data (similar behavior is also typical of fine-grained alumina [20]. A similar conclusion follows from [6], where an attempt was made to relate the V-notch radius values required for the correct determination of KIc values to the averaged values of this parameter obtained in the RRFT’97 studies. As follows from Fig. 3a, the decrease in KIc values for Si3N4 ceramics occurs with a decrease in notch radii down to about 30 mm, which does not contradict the results of sintered SiC whisker-reinforced Si3N4 [18] tests. A similar tendency is also typical of SiC ceramics [19]. The examination of the specimens fractured in the tests revealed a fracture crack that propagated from the points where ‘‘additional’’ stress concentrators were present. This observation also confirms the assumption that the fracture of a loaded ceramic specimen starts from a small crack ahead of a machined notch root [19]. In this context, it is interesting to mention that KIc magnitudes are influenced by the sharpness of a notch root rather than by its shape [7]. Fig. 6. Raman spectra of Y-PSZ specimens fractured by the SEVNB method (a) and by indentation (b): nonfractured specimen surface (1), diamond saw surface (2), fracture surface (3), razor blade surface (4), indentation edge (5), and indentation center (6). Table 4 KIc values for ceramics obtained by SEVNB and SENB methods (MPa m1/2) Material Test method Flexure (points) Index ’ Brittleness measure, SEVNB SENBa Y-PSZ 5.140.29 9.540.47 Four 0.538 1 Soda limit glass 0.660.07 1.180.09 Four 0.562 1 SiC+50%ZrB2+10%B4C 3.520.08 6.240.37 Three 0.564 1 Si3N4 5.170.06 9.120.29 Three 0.567 1 SSiC 2.610.18 4.420.23 Four 0.590 1 Si3N4+30%SiC+3%MgO 2.270.14 2.490.16 Three 0.920 0.88 Mg-PSZ (TS-grade) 9.440.12 10.20.27 Four 0.925 0.41 La8.8Ca0.2CoO3 2.2 2.2 Three 10.25 a The width of notch is 0.2–0.3 mm. Table 5 KIc values for single crystals obtained by SEVNB and SENB methods (MPa m1/2) Single crystals Peculiarity Elastic modulus (GPa) Test method Brittleness measure, Index ’ SENB SEVNB Zirconia Partially stabilized (3% Y2O3) 245 9.330.95 10.332.17 1 0.9 Alumina Specimen axis 45 to optical axis of crystal 403 2.310.34 2.450.29 10.94 Specimen axis 90 to optical axis of crystal 410 3.190.53 2.850.50 1 1.12 G.A. Gogotsi / Ceramics International 29 (2003) 777–784 781
G.A. Gogotsi/ Ceramics international 29(2003)777-784 Table 6 Kle values for SiNa/Si3N4 n% TiN laminated composites (x =1)obtained by SEVNB and SENB methods (MPa Content(n) of TiN layers" Test method Average value SEVNB KIC(MPa m/2) KIC (MPa m) KIc(MPa m/2) 19±0.30 6.08±0.08 ±0.71 4. 601 5.81±0.07 5.91士0 8.57 8.11 8.66±0.58 931 9.48±0.24 7.87 8.99±0.75 8.37±0.47 8.90±0.41 a Average thickness of layers is 0. 185 mm. The SEVNB method(see, e.g., [6) is usually com- Table 7 pared with the SEPB, SCF, and CNB methods. But it is High temperature fracture toughness test results (SEVNB method also feasible to compare sEVNB and senb data Materials KIc(MPa m2) (Table 4), paying attention to the fact [21, 22] that the evaluation of inelastic materials gives fracture toughness 1300°C results which practically coincide with those for the SiNa 4.2士0.3 specimens with sharp and blunt stress concentrators. Si3N4+30%SiC+3%MgO 2.27+0.1 2.68±0.I Table 5 summarized the index of sensitivity to stress SiC+ 50%ZrB2+10%BaC 3.52+0.1 3.63=0.3 3.70=0. concentrations, equal to the ratio of the Klc values Si3N4[17F 5.6±0.5 5.0±0.4 obtained by the sevnb and senb methods, and the The notches were produced by diamond saw with V-shaped tip inelasticity of ceramics described by the brittleness measure x [22]. The latter is equal to the ratio of the But in some tests of ceramics particulate composites, the specific elastic energy accumulated in ceramics by the situation was different, especially if the sharpness of a moment of fracture to the total energy spent for its V-notch was not uniform(V-notch root radii on the mined from stress-strain curves obtained in four-point load-deflection diagrams imen are not equal).Several deformation. Brittleness measure values were deter- opposite sides of the spe an have an unusual shape flexure of solid(without stress concentrators) speci because fracture initiates in the vicinity of the notch mens. The analysis of these data confirms the existence root with a smaller radius, where higher stress of relationship between and x In the tests of elastic concentrations are present( Fig. 5) materials(x=1), is about 0.6, and for inelastic mate- To complete the analysis of Table 4, we should note rials(<1), it exceeds 0.9. But this conclusion is correct that in fracture toughness tests of Ts-grade zirconia only for monolith ceramics and ceramics particulate ceramics, the index was lower than unity. At the same composites. For single crystals(Table 5)and ceramic time, the comparison of SENB and SEPB data for these laminated composites(Table 6) the picture is different ceramics (if the annealing of specimens was not per and above-mentioned dependence is not observed formed at temperatures exceeding the boundary of Almost all the studies on the deformation behavior of monoclinic-tetragonal transformation) could give a V-notched ceramic specimens with a p value of about ratio of Klc values more than unity. For example, this 0.6, produced linear load-deflection diagrams or dia- ratio was 1.24 [24], which was probably caused by the grams with small nonlinearity (e. g, curves I and 3 in phase transformation in the area of the initial crack Fig 4), which is associated with a comparatively slow nucleation, when the specimen was prepared for SEPB rack growth that is permissible in accordance with [23]. Therefore, it was interesting to investigate the phase
The SEVNB method (see, e.g., [6]) is usually compared with the SEPB, SCF, and CNB methods. But it is also feasible to compare SEVNB and SENB data (Table 4), paying attention to the fact [21,22] that the evaluation of inelastic materials gives fracture toughness results which practically coincide with those for the specimens with sharp and blunt stress concentrators. Table 5 summarized the index of sensitivity to stress concentrations, ’, equal to the ratio of the KIc values obtained by the SEVNB and SENB methods, and the inelasticity of ceramics described by the brittleness measure [22]. The latter is equal to the ratio of the specific elastic energy accumulated in ceramics by the moment of fracture to the total energy spent for its deformation. Brittleness measure values were determined from stress–strain curves obtained in four-point flexure of solid (without stress concentrators) specimens. The analysis of these data confirms the existence of relationship between ’ and . In the tests of elastic materials (=1), ’ is about 0.6, and for inelastic materials (’<1), it exceeds 0.9. But this conclusion is correct only for monolith ceramics and ceramics particulate composites. For single crystals (Table 5) and ceramic laminated composites (Table 6) the picture is different and above-mentioned dependence is not observed. Almost all the studies on the deformation behavior of V-notched ceramic specimens with a ’ value of about 0.6, produced linear load-deflection diagrams or diagrams with small nonlinearity (e.g., curves 1and 3 in Fig. 4), which is associated with a comparatively slow crack growth that is permissible in accordance with [23]. But in some tests of ceramics particulate composites, the situation was different, especially if the sharpness of a V-notch was not uniform (V-notch root radii on the opposite sides of the specimen are not equal). Several load-deflection diagrams can have an unusual shape because fracture initiates in the vicinity of the notch root with a smaller radius, where higher stress concentrations are present (Fig. 5). To complete the analysis of Table 4, we should note that in fracture toughness tests of TS-grade zirconia ceramics, the index ’ was lower than unity. At the same time, the comparison of SENB and SEPB data for these ceramics (if the annealing of specimens was not performed at temperatures exceeding the boundary of monoclinic-tetragonal transformation) could give a ratio of KIc values more than unity. For example, this ratio was 1.24 [24], which was probably caused by the phase transformation in the area of the initial crack nucleation, when the specimen was prepared for SEPB. Therefore, it was interesting to investigate the phase Table 6 KIc values for Si3N4/Si3N4 + n% TiN laminated composites ( =1) obtained by SEVNB and SENB methods (MPa m1/2) Content (n) of TiN layersa (%) Test method Average value of KIC (MPa m1/2) SEVNB SENB Four-point flexure, 20/40 mm Three-point flexure, 1 6 mm Three-point flexure, 16 mm KIC (MPa m1/2) KIC (MPa m1/2) KIC (MPa m1/2) 10 6.61 6.80 6.27 6.560.19 6.45 6.37 5.74 6.190.30 5.95 6.20 6.08 6.080.08 5.26 5.59 7.03 5.960.71 5.42 5.48 8.416.431.32 5.26 5.90 4.94 5.370.36 6.45 6.03 6.016.160.19 5.87 5.715.85 5.810.07 Average 5.910.46 6.010.36 5.990.41 30 8.57 8.11 9.15 8.660.58 9.84 9.319.28 9.480.24 8.918.20 9.09 8.730.36 9.63 7.87 8.07 8.520.74 Average 8.990.75 8.370.47 8.900.41 a Average thickness of layers is 0, 185 mm. Table 7 High temperature fracture toughness test results (SEVNB method) Materials KIC (MPa m1/2) 20 C 1300 C 1400 C Si3N4 5.50.14.20.3 – Si3N4+30%SiC+3%MgO 2.270.1– 2.680.1 SiC+50%ZrB2+10%B4C 3.520.13.630.3 3.700.1 Si3N4 [17]a 5.60.5 5.00.4 – a The notches were produced by diamond saw with V-shaped tip. 782 G.A. Gogotsi / Ceramics International 29 (2003) 777–784
G.A. Gogotsi/ Ceramics international 29(2003)777-784 tested by the SEVNB method. The phz s a specimens residual stresses, phase transformations or other effects state on the fracture surfaces of zircon state was which often accompany the formation of stress analyzed by micro-Raman spectroscopy(Fig. 6), which concentrators by other procedures demonstrates. in contrast to fracture or saw notch sur- faces(Fig. 6a) or the edge of indentation(Fig. 6b),a polished V-notch surface does not contain the mono- Acknowledgements clinic phase(there is no effect inducing the phase trans- formation). This is an important advantage of the Thanks are expressed to Mr. V. Galenko and Mr. B SEVNB method in comparison with other fracture Ozersky (IPs, Ukraine)for their assistance in perform- toughness test methods for zirconia and similar ceramics. ing experiments. The investigation partially financed by The comparison of high-temperature Klc evaluations INCO-Copernicus Grants (contracts 15 CT 9607 for Si3N4 ceramics, presented both in Table 7 and in and 1CA2-CT-2000-10020) Ref [18], reveals their similarity. However, in our case, in contrast to [18, fracture of the V-notch surface did References not occur and we did not observe active oxidation in the icinity of the notch. It is noteworthy that in SEPB tests [1] G.A. Gogotsi, Fracture toughness studies on ceramics and cera- of similar ceramics [18], oxidation induced the blunting particulate composites at different temperatures, in: (healing) of the initial sharp crack and, as J.A. Salem, G D. Quinn, M.G. Jenkins(Eds ) Fracture Resis- consequence, the fracture toughness results varied tance Testing of Monolithic and Composite Brittle Materials It is necessary to emphasize the increase in the fracture (ASTM STP 1409), American Society for Testing and Material West Conshohocken, PA, 2002, pp. 199-2 toughness of ceramic particula [2J. KObler, Fracture toughness amics using the SevnB temperatures (Table 7). The o value varies moderately with ethod: preliminary results, in: J.P. Singh(Ed ) Ceramic Engi- this increase(0.81 for Si3 N4 30%SiC 3% MgO and neering& Science Proceedings, American Ceramic Society, Vol 0.71 for SiC 50% ZrB2+ 10% B,C). The changes in 3 G.A. Gogotsi, V.I. Galenko, Comparative analysis of fracture the load-deflection diagrams of these ceramics with test toughness tests methods for ceramics and crystals at room an temperatures are not so pronounced (Fig 4)and are con- lower temperatures, Strength of Materials 29(1997)287-297 nected with a certain increase in their inelastic deforma 4 Le Bac, Verfahren zum Feinkerben von Keramischen KOrpern tion. It should be noted that the load-deflection diagrams Patentschrift 146416, Deutsche Demokratische Republik- Amt for Si N4+30%SiC 3% MgO unnotched specimens (5).J. Dameny A Danzer, Method for fracture toughness testing and specimens with wide notches were linear. The oxida- n layers on fractured specimens are not strong and Riosand, K.J. Miller(Eds ) Fracture from Defects, Emas Pub. cannot affect the fracture toughness of the ceramics stud lishing,1998.pp.491-496 ied in high-temperature SEVNB tests And, probably, the 6J. Kubler, Fracture Toughness of Ceramics Using the SEVNB blunting of a stress concentrator does not occur as in [ 18], lethod: Round Robin, VAMAS Report No37/ESIS Document D2-99, EMPA, Swiss Federal Laboratories for Materials Testing where in SEPB tests crack healing and a considerable and Research. Bubendorf. switzerland increase in Kle values, even at 1200oC, were observed 7 G.A. Gogotsi, Fracture toughness matrix composites(SEVNB Method Ceramics-12(1998)7-13 4. Conclusions [8 G.A. Gogotsi, Fracture toughness tests of V-notched specimens Strength of Materials 32(2000)170-177 9 G.A. Gogotsi, V.I. Galenko, B.I. Ozerskiy, AD The test data confirm that the sevnb method can be v.I. Korban, Fracture resistance, strength, and other ch easier applied in practice and can be used for the tics of Y-TZP, Refractory and Industrial Ceramics 8 majority of advanced ceramics and ceramic particulate composites at different temperatures and in the oxida [0 G.A. Gogotsi, M I. Lugovy, V.N. Slyunyayev, N.A. Orlovskaya, Development of multilayered Si3 Na-based ceramics composite tion environment. The sevnb data for ceramics and ving an ability to arrest cracks, Proc. of Int. Conf. Science for ceramic particulate composites are independent of the laterals 2002, IPMS, Kiev, 2002, pp 247-248. flexure type and exhibit small scatter. For laminated [I G.A. Gogotsi, Several Experimental Results of High Perfor- ceramic composites them are also independent of width ance Ceramics Used in Heat Engine Components, IPS AN of the stress concentrator Therefore it commands the [12] G.A. Gogotsi, Private communication. attention of engineers involved in both certification [13] G.A. Gogotsi, V P Zavada, V 1. Nerodenko, USSR Patent No I testing and materials science research. It was found that 224551,15 December I984. the ratio between SEVNB and senb data equaled [14 ASTM C1421-99, Standard Test Method for the Determination bout 0.6 for elastic ceramics and ceramics particulate composites and over 0.9 for inelastic ones. The polish- [15] Draft Standard DIN 51 109, Testing of Advanced Technical ing-out of a V-notch root does not damage the surface Ceramics: Determination of Fracture Toughness KIc, DIN NMP layer formed on a zirconia; therefore, there are no
state on the fracture surfaces of zirconia specimens tested by the SEVNB method. The phase state was analyzed by micro-Raman spectroscopy (Fig. 6), which demonstrates, in contrast to fracture or saw notch surfaces (Fig. 6a) or the edge of indentation (Fig. 6b), a polished V-notch surface does not contain the monoclinic phase (there is no effect inducing the phase transformation). This is an important advantage of the SEVNB method in comparison with other fracture toughness test methods for zirconia and similar ceramics. The comparison of high-temperature KIc evaluations for Si3N4 ceramics, presented both in Table 7 and in Ref. [18], reveals their similarity. However, in our case, in contrast to [18], fracture of the V-notch surface did not occur and we did not observe active oxidation in the vicinity of the notch. It is noteworthy that in SEPB tests of similar ceramics [18], oxidation induced the blunting (healing) of the initial sharp crack and, as a consequence, the fracture toughness results varied. It is necessary to emphasize the increase in the fracture toughness of ceramic particulate composites with test temperatures (Table 7). The ’ value varies moderately with this increase (0.81for Si3N4 + 30% SiC + 3% MgO and 0.71for SiC + 50% ZrB2 + 1 0% B4C). The changes in the load-deflection diagrams of these ceramics with test temperatures are not so pronounced (Fig. 4) and are connected with a certain increase in their inelastic deformation. It should be noted that the load–deflection diagrams for Si3N4 + 30%SiC + 3% MgO unnotched specimens and specimens with wide notches were linear. The oxidation layers on fractured specimens are not strong and cannot affect the fracture toughness of the ceramics studied in high-temperature SEVNB tests. And, probably, the blunting of a stress concentrator does not occur as in [18], where in SEPB tests crack healing and a considerable increase in KIc values, even at 1200 C, were observed. 4. Conclusions The test data confirm that the SEVNB method can be easier applied in practice and can be used for the majority of advanced ceramics and ceramic particulate composites at different temperatures and in the oxidation environment. The SEVNB data for ceramics and ceramic particulate composites are independent of the flexure type and exhibit small scatter. For laminated ceramic composites them are also independent of width of the stress concentrator. Therefore it commands the attention of engineers involved in both certification testing and materials science research. It was found that the ratio between SEVNB and SENB data equaled about 0.6 for elastic ceramics and ceramics particulate composites and over 0.9 for inelastic ones. The polishing-out of a V-notch root does not damage the surface layer formed on a zirconia; therefore, there are no residual stresses, phase transformations or other effects, which often accompany the formation of stress concentrators by other procedures. Acknowledgements Thanks are expressed to Mr. V. Galenko and Mr. B. Ozersky (IPS, Ukraine) for their assistance in performing experiments. The investigation partially financed by INCO-Copernicus Grants (contracts 15 CT 96 07 29 and 1CA2-CT-2000-10020). References [1] G.A. Gogotsi, Fracture toughness studies on ceramics and ceramic particulate composites at different temperatures, in: J.A. Salem, G.D. Quinn, M.G. Jenkins (Eds.), Fracture Resistance Testing of Monolithic and Composite Brittle Materials (ASTM STP 1409), American Society for Testing and Materials, West Conshohocken, PA, 2002, pp. 199–212. [2] J. Ku¨bler, Fracture toughness of ceramics using the SEVNB method: preliminary results, in: J.P. Singh (Ed.), Ceramic Engineering & Science Proceedings, American Ceramic Society, Vol. 18, Issue 4, 1997, pp. 155–162. [3] G.A. Gogotsi, V.I. Galenko, Comparative analysis of fracture toughness tests methods for ceramics and crystals at room and lower temperatures, Strength of Materials 29 (1997) 287–297. [4] Le Bac, Verfahren zum Feinkerben von Keramischen Ko¨rpern, Patentschrift 146416, Deutsche Demokratische Republik- Amt fu¨r Efindungs- und Patentwesen, 1979–1981. [5] D.J. Dameny, A. Danzer, Method for fracture toughness testing of ceramics—ready for standardisation, in: M.W. Brawn, E.R. Riosand, K.J. Miller (Eds.), Fracture from Defects, Emas Publishing, 1998, pp. 491–496. [6] J. Ku¨bler, Fracture Toughness of Ceramics Using the SEVNB Method: Round Robin, VAMAS Report No.37/ESIS Document D2-99, EMPA, Swiss Federal Laboratories for Materials Testing and Research, Dubendorf, Switzerland, 1999. [7] G.A. Gogotsi, Fracture toughness of ceramics and ceramic matrix composites (SEVNB Method), Refractory and Technical Ceramics 11–12 (1998) 7–13. [8] G.A. Gogotsi, Fracture toughness tests of V-notched specimens, Strength of Materials 32 (2000) 170–177. [9] G.A. Gogotsi, V.I. Galenko, B.I. Ozerskiy, A.D. Vasiliev, V.I. Korban, Fracture resistance, strength, and other characteristics of Y-TZP, Refractory and Industrial Ceramics 8 (2000) 7– 13 (in Russian). [10] G.A. Gogotsi, M.I. Lugovy, V.N. Slyunyayev, N.A. Orlovskaya, Development of multilayered Si3N4-based ceramics composite having an ability to arrest cracks, Proc. of Int. Conf. Science for Materials 2002, IPMS, Kiev, 2002, pp. 247–248. [11] G.A. Gogotsi, Several Experimental Results of High Performance Ceramics Used in Heat Engine Components, IPS AN USSR, Kiev, 1983 (in Russian). [12] G.A. Gogotsi, Private communication. [13] G.A. Gogotsi, V.P. Zavada, V.I. Nerodenko, USSR Patent No. 1 224 551, 15 December 1984. [14] ASTM C1421-99, Standard Test Method for the Determination of Fracture Toughness of Advanced Ceramics at Ambient Temperature. [15] Draft Standard DIN 51 109, Testing of Advanced Technical Ceramics; Determination of Fracture Toughness KIc, DIN NMP 291, September 1991. G.A. Gogotsi / Ceramics International 29 (2003) 777–784 783
G.A. Gogotsi/ Ceramics International 29(2003)777-784 [16 G.A. Gogotsi, Mechanical behaviour of yttria- and ferric oxide. [20 T Nishida, Y. Hanaki, G. Pezzotti, Effect of notch-root on the doped zirconia in different temperatures, Ceramics International fracture toughness of a fine-grained alumina. Journal of the 4(1998)589-595 American Ceramic Society 77(1994)606-608 [17 G.A. Gogotsi, E.E. Lomonova. Micro-Raman spectroscopy [21]Rw. Davidge, G. Tappin, The effective surface energy of brittle study of indentation-induced phase transformation in zirconia materials. Journal of Materials Science 3(1968)165-173. rystals, Refractory and Industrial Ceramics 41(2000)191 22] G.A. Gogotsi, Deformational behaviour of ceramics, Journal of the European Ceramic Society 7(1991)87-92. [18 M. Mizuno, Y. Nagano, J -w. Cao, J.-l. Kon, VAMAS round [23] ISo DIS 15732. Fine Ceramic(Advanced Ceramics, Advanced robin on fracture toughness measurements Technical Ceramics)Test Method for Fracture Toughness of omposite, in Abstracts, 9th International Con Monolithic Ceramics at Room Temperature by Single Edge Pre- ern Materials Technologies(CIMTEC 98). cracked Beam(SEPB) Method, ISO/TC 206. 1999 June 1998,d.108. [24 G.A. Gogotsi, A.V. Drozdov, V P. Zavada, M.V. Swain [19R. Damani, R. Gstrain, R. Danzer, Critical notch-root radius parison of the mechanical behaviour of partially stabilised zirco- effect in SENB-S fracture toughness testing Journal of the eur- ia with yttria and magnesia, Journal of the Australian Ceramic opean Ceramic Society 16(1996)695-702. Society27(1991)3749
[16] G.A. Gogotsi, Mechanical behaviour of yttria- and ferric oxidedoped zirconia in different temperatures, Ceramics International 24 (1998) 589–595. [17] G.A. Gogotsi, E.E. Lomonova, Micro-Raman spectroscopy study of indentation-induced phase transformation in zirconia crystals, Refractory and Industrial Ceramics 41 (2000) 191– 195. [18] M. Mizuno, Y. Nagano, J.-W. Cao, J.-l. Kon, VAMAS round robin on fracture toughness measurements of ceramic matrix composite, in Abstracts, 9th International Conference on Modern Materials & Technologies (CIMTEC’98), Florence, Italy, June 1998, d. 108. [19] R. Damani, R. Gstrain, R. Danzer, Critical notch-root radius effect in SENB-S fracture toughness testing, Journal of the European Ceramic Society 16 (1996) 695–702. [20] T. Nishida, Y. Hanaki, G. Pezzotti, Effect of notch-root on the fracture toughness of a fine-grained alumina, Journal of the American Ceramic Society 77 (1994) 606–608. [21] R.W. Davidge, G. Tappin, The effective surface energy of brittle materials, Journal of Materials Science 3 (1968) 165–173. [22] G.A. Gogotsi, Deformational behaviour of ceramics, Journal of the European Ceramic Society 7 (1991) 87–92. [23] ISO DIS 15 732. Fine Ceramic (Advanced Ceramics, Advanced Technical Ceramics)—Test Method for Fracture Toughness of Monolithic Ceramics at Room Temperature by Single Edge Precracked Beam (SEPB) Method, ISO/TC 206, 1999. [24] G.A. Gogotsi, A.V. Drozdov, V.P. Zavada, M.V. Swain, Comparison of the mechanical behaviour of partially stabilised zirconia with yttria and magnesia, Journal of the Australian Ceramic Society 27 (1991) 37–49. 784 G.A. Gogotsi / Ceramics International 29 (2003) 777–784
