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 phaseThe SEVNB method (see, e.g., [6]) is usually com￾pared 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 deter￾mined from stress–strain curves obtained in four-point flexure of solid (without stress concentrators) speci￾mens. 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 mate￾rials (’<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 dia￾grams 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 per￾formed 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