S. Tariolle et al /Journal of the European Ceramic Sociery 25(2005)3639-3647 Therefore, Eq (2)can be expressed in relation with porosity F G 0.57 Ga(I-p where Gp is the fracture energy of the porous layer and Gd that of the dense laver Fig. 1. Schematic representation of notched specimen tested in 3 point According to Eq. (3), a porosity of 37 vol. is red bending fracture test Span L=20mm; W: thickness; b: width to initiate crack deflection at the interface between porous depth. F indicates the direction of the applied load. The layers are perpen- and dense layers, this value of porosity was experimentally dicular to this direction confirmed in SiC4 and alumina specimens 2.2 Technical characterizations In this context. we have studied boron carbide laminar opposites for their potential applications in armor and in nu- Density of materials was calculated from the measured clear energy fields 'Different boron carbide composites with weight and the geometrically determined volume. Image various weak interlayers or weak interfaces were elaborated analysis was used for characterization of grain and pore to study their reinforcement properties by crack defection. size using micrographs obtained by optical microscopy (for The energetic criterion and the level of porosity reported by porosity)and SEM(for grain size) Clegg and coworkers, to achieve crack deflection in the case of porous weak interlayers have been verified for boron 2.2.I. Description of the technique of observation of carbide laminar composites ed using 3 point-bending fracture tests on notched speci 2. Experimental procedure e o Crack propagation in multi-layered materials was evalu- hens(Fig. 1). A mechanical testing machine was used (IN STRON 8562)with a cross-head speed of0.025 mm/min The 2.. Preparation displacement was measured by an LVDT sensor The different composites have been elaborated using the 2.2.2. Measure of the work of rupture and of the crack tape-casting technique. The complete description of the pro- deviation cess has already been described. 9, 0 Composites were com- he work of rupture was evaluated using load-displace- posed of alternate dense boron carbide layers(94% of theo- ment curves and the crack deviation was measured on frac etical density )and weak layers or weak interfaces. Starting tographies of composites from boron carbide powder(Tetrabor 3000F, Wacker Ceran The work of rupture WR was calculated using Eq (4) ics, mean diameter 0.75 um), solid state sintering of boron where C()corresponds to the hatched area below the curve carbide was performed by pressureless sintering(2150C/1h (Fig. 2)until the load reached a plateau for a load C under argon)using phenolic resin as sintering aid.8, I0Com posites with different types of interlayers and interfaces have WR- Lb(w -a. c(x)dx been elaborated (Table 1). Porous interlayers were obtained using corn starch as pore forming agent, under-sintered in- An apparent friction stress can be calculated using Eq (5) terlayers using no sintering aid and weak interlayers using a mixture of boron carbide and boron nitride(55/45 in vol Fe ume). Weak interfaces were obtained using different sprays b(w-ae) (graphite or boron nitride) which have been pulverized on dense layers before thermocompression 0 Work of rupture Table I Denomination of the various composites elaborated Composite with Friction stress denomination B,C-BN Interlayers with mixing B, C-BN (55/4 Interlayers prepared with 50 vol. of( erlayers prepared with 55 vol % of 000,020,040.060,080,100.12014 IBN Interfaces with boron nitride Fig. 2. Load-displacement curve obtained on lamellar composites3640 S. Tariolle et al. / Journal of the European Ceramic Society 25 (2005) 3639–3647 Therefore, Eq. (2) can be expressed in relation with porosity p: Gp Gd(1 − p) ≤ 0.57 (3) where Gp is the fracture energy of the porous layer and Gd that of the dense layer. According to Eq. (3), a porosity of 37 vol.% is required to initiate crack deflection at the interface between porous and dense layers, this value of porosity was experimentally confirmed in SiC4 and alumina5 specimens. In this context, we have studied boron carbide laminar composites for their potential applications in armor and in nu￾clear energy fields.7 Different boron carbide composites with various weak interlayers or weak interfaces were elaborated to study their reinforcement properties by crack deflection.8 The energetic criterion and the level of porosity reported by Clegg and coworkers4,5 to achieve crack deflection in the case of porous weak interlayers have been verified for boron carbide laminar composites. 2. Experimental procedure 2.1. Preparation The different composites have been elaborated using the tape-casting technique. The complete description of the pro￾cess has already been described.9,10 Composites were com￾posed of alternate dense boron carbide layers (94% of theo￾retical density) and weak layers or weak interfaces. Starting from boron carbide powder (Tetrabor 3000F, Wacker Ceram￾ics, mean diameter 0.75
m), solid state sintering of boron carbide was performed by pressureless sintering (2150 ◦C/1 h under argon) using phenolic resin as sintering aid.8,10 Com￾posites with different types of interlayers and interfaces have been elaborated (Table 1). Porous interlayers were obtained using corn starch as pore forming agent, under-sintered in￾terlayers using no sintering aid and weak interlayers using a mixture of boron carbide and boron nitride (55/45 in vol￾ume). Weak interfaces were obtained using different sprays (graphite or boron nitride) which have been pulverized on dense layers before thermocompression. Table 1 Denomination of the various composites elaborated. Composites denomination Composite with NSA Interlayers with no sintering aid B4C-BN Interlayers with mixing B4C-BN (55/45 in voume) CS45 Interlayers prepared with 45 vol.% of corn starch CS50 Interlayers prepared with 50 vol.% of corn starch CS55 Interlayers prepared with 55 vol.% of corn starch I-BN Interfaces with boron nitride I-G Interfaces with graphite Fig. 1. Schematic representation of notched specimen tested in 3 point￾bending fracture test. Span L = 20 mm; W: thickness; b: width; ae: notch depth. F indicates the direction of the applied load. The layers are perpen￾dicular to this direction. 2.2. Technical characterizations Density of materials was calculated from the measured weight and the geometrically determined volume. Image analysis was used for characterization of grain and pore size using micrographs obtained by optical microscopy (for porosity) and SEM (for grain size). 2.2.1. Description of the technique of observation of crack propagation Crack propagation in multi-layered materials was evalu￾ated using 3 point-bending fracture tests on notched speci￾mens (Fig. 1). A mechanical testing machine was used (IN￾STRON 8562) with a cross-head speed of 0.025 mm/min. The displacement was measured by an LVDT sensor. 2.2.2. Measure of the work of rupture and of the crack deviation The work of rupture was evaluated using load–displace￾ment curves and the crack deviation was measured on frac￾tographies of composites. The work of rupture WR was calculated using Eq. (4) where C(x) corresponds to the hatched area below the curve (Fig. 2) until the load reached a plateau for a load C. WR = 1 Lb(W − ae)
C(x)dx (4) An apparent friction stress can be calculated using Eq. (5). Ff = C b(W − ae) (5) Fig. 2. Load–displacement curve obtained on lamellar composites