Z Krstic, V.D. Krstic /Jounal of the European Ceramic Sociery 29(2009)1825-1829 Fig. 1.(a) Peeling and (b) delamination in the plate-form laminates. Fig. 3. Sample configuration for three-point bending test with corresponding dimensions powder was used as a raw material. Sub-micron size Al2O3 (A-16, Alcoa) and Y203(Alpha Aesar) powders were used as sintering aids BN powder(Carborundum Co., grade HPP-325) with the addition of Al,O3 and Si3 N4 was used for casting weak interlayers. After drying, pressureless sintering was done in a Fig. 2. Schematics of crack deflection in(a)circular and (b)rectangular cross- graphite resistance furnace (Vacuum Industries, USA)at tem- sectioned concentric laminate structures peratures ranging from 1740C to 1800C for I h under static primary crack is blunted and reinitiated at the next layer. This N2 gas atmosphere. Fracture toughness was measured using the three-point bend- nakes the laminate's apparent fracture toug ss Insensitive to ing test at room temperature with a straight-through notch the crack radius introduced in the mid-section of the samples( Fig 3). The notch Shanches-Herencia et al. have showed that the crack exten- was introduced by a 500 um thick diamond wheel through the sion within the layer occurs only when the layer thickness first or first two layers and its depth(o750-1220 um) was exceeds a critical value which is directly related to the critical measured under an optical microscope with 50x or 100x mag- strain energy release rate or fracture toughness cifications. The initial crack radius does not play significant role GeE 0.34 (1-2)2 2) SInce the crack has to be reinitiated on the next layer creating 2) an inherently sharp crack. The test was carried out on an Instron machine(Model 8502 FIB, Instron Co., Canton, USA)using a jig where te is the critical layer thickness, Eis the Young's modulus, with the span of 26 mm and the crosshead speed of 0.06 mm/min v is the Poison's ratio, Ge is the critical strain energy release rate Five samples were tested per data point. The fracture toughness and or is the residual stress at the surface of the layer. Also, Philips et al. I0 showed that the crack deflection in the was calculated using the equation1 interfacial critical strain energy release rate Gic and the bulk KIc= P 301/2 plate-form laminates does not occur when the ratio between the Y critical strain energy release rate GB exceeds unity. Accordin to Philips et al. 0, this condition is achieved when the interfacial where Kic is the fracture toughness, P is the maximum load at toughness is high enough and the performance of the laminate fracture, S is the span, B is the sample width, W is the sample would revert to that of the monolithic materials. The relationship height, a is the coefficient(a=a/w; a the notch depth)and r is between the interfacial fracture toughness(Gic), the numbers of the stress intensity factor coefficient, which is expressed by the layers(T)and the layer thickness(8)is given by the equation: equation Y=1.9887-1.326a-(3.49-0.68a+1.35a2( G (1+a) where oc is the critical stress for failure of the next layer and e is the Youngs modulus 3. Results and discussion 2. Experimental procedure There are several important mechanical parameters which Concentric Si3 N4/BNlaminates are fabricated by slip-casting determine the engineering application of any material. In the alternate layers of Si3 N4 and Bn employing the previously present work, the emphasis was placed on the fracture tough- developed modified slip-casting method. High purity a-Si3N4 ness. Fig 4 shows the change of the apparent fracture toughness1826 Z. Krstic, V.D. Krstic / Journal of the European Ceramic Society 29 (2009) 1825–1829 Fig. 1. (a) Peeling and (b) delamination in the plate-form laminates. Fig. 2. Schematics of crack deflection in (a) circular and (b) rectangular cross￾sectioned concentric laminate structures. primary crack is blunted and reinitiated at the next layer. This makes the laminate’s apparent fracture toughness insensitive to the crack radius. Shanches-Herencia et al.3 have showed that the crack exten￾sion within the layer occurs only when the layer thickness exceeds a critical value which is directly related to the critical strain energy release rate or fracture toughness: tc = GcE 0.34 (1 − ν2)σ2 r (2) where tc is the critical layer thickness, E is the Young’s modulus, ν is the Poison’s ratio, Gc is the critical strain energy release rate and σr is the residual stress at the surface of the layer. Also, Philips et al.10 showed that the crack deflection in the plate-form laminates does not occur when the ratio between the interfacial critical strain energy release rate GIC and the bulk critical strain energy release rate GBC exceeds unity. According to Philips et al.10, this condition is achieved when the interfacial toughness is high enough and the performance of the laminate would revert to that of the monolithic materials. The relationship between the interfacial fracture toughness (GIC), the numbers of layers (T) and the layer thickness (δ) is given by the equation: GIC = σc δ 18E  T −  (T − 1)3 T 2  where σc is the critical stress for failure of the next layer and E is the Young’s modulus. 2. Experimental procedure Concentric Si3N4/BN laminates are fabricated by slip-casting alternate layers of Si3N4 and BN employing the previously developed modified slip-casting method.8 High purity -Si3N4 Fig. 3. Sample configuration for three-point bending test with corresponding dimensions. powder was used as a raw material. Sub-micron size Al2O3 (A-16, Alcoa) and Y2O3 (Alpha Aesar) powders were used as sintering aids. BN powder (Carborundum Co., grade HPP-325) with the addition of Al2O3 and Si3N4 was used for casting weak interlayers. After drying, pressureless sintering was done in a graphite resistance furnace (Vacuum Industries, USA) at tem￾peratures ranging from 1740 ◦C to 1800 ◦C for 1 h under static N2 gas atmosphere. Fracture toughness was measured using the three-point bend￾ing test at room temperature with a straight-through notch introduced in the mid-section of the samples (Fig. 3). The notch was introduced by a 500 m thick diamond wheel through the first or first two layers and its depth (∼750–1220m) was measured under an optical microscope with 50× or 100× mag￾nifications. The initial crack radius does not play significant role since the crack has to be reinitiated on the next layer creating an inherently sharp crack. The test was carried out on an Instron machine (Model 8502 FIB, Instron Co., Canton, USA) using a jig with the span of 26 mm and the crosshead speed of 0.06 mm/min. Five samples were tested per data point. The fracture toughness was calculated using the equation11: KIC = P BW1/2 · S W · 3α1/2 2(1 − α) 3/2 · Y (4) where KIC is the fracture toughness, P is the maximum load at fracture, S is the span, B is the sample width, W is the sample height, α is the coefficient (α = a/W; a the notch depth) and Y is the stress intensity factor coefficient, which is expressed by the equation: Y = 1.9887 − 1.326α − (3.49 − 0.68a + 1.35α2)α(1 − α)(1 + α) −2 (5) 3. Results and discussion There are several important mechanical parameters which determine the engineering application of any material. In the present work, the emphasis was placed on the fracture tough￾ness. Fig. 4 shows the change of the apparent fracture toughness