C. Reynaud et al. /Joumal of the European Ceramic Sociery 25 (2005)589-592 growing crack comes sufficiently close to a microcrack or De tip of th tic parameters of the two PFAs used increased and then, the apparent fracture energy of the liga- ment is reduced This reduction varies with the relative den- GP Blanks et al derived a minimum level of porosity equal to 37% to ensure Corn starch Graphite platelet ds0=14μm 8m×8pmx3um crack deflection, in good agreement with their experimental Aspect ratio 67 results for silicon carbide. Assuming that the fracture energy of the dense ligament is related to that of the porous layer GP, by the relation cosity and the rheological behaviour of the slurry and com- plement one another to produce handleable ceramic tapes Glig 1-P (1) The optimum concentrations for the binder and the plasti cizer were determined to be 8wt% for both additives on where P is the volume fraction of porosity, Davis et al. 8ex- the base of the ceramic powder. The third step corresponds pressed the criterion for continued crack deflection in terms to the incorporation, into the suspension, of the pore form- of easily measurable variables ing agent(PFA): corn starch(Roquette-France)or graphite platelets (Union Carbide-US)(Table 1). The PFA volume 0.57(1-P) (2) fraction, referred to the sum of the ceramic volume and pFA G volume, was 45 and 50% for graphite platelets and varied The experimental results obtained by Clegg's group with from 5 to 55% for corn starch. We were unable to fabricate ther alumina or silicon carbide were consistent with re- sound parts when larger PFA volume fraction were incor lation(2), crack deflection being observed when the ratio porated. A mixing time of 3.5 h leads to an uniform distri Gp/Gs falls under the line 0.57(1-P), that occurs for a bution of the PFA. Finally the slurry was de-aired at a low lume fraction of porosity of 0.37 rotation speed during 24 h The aim of the work presented in this paper was to deter Then the slurries were tape cast onto a Mylar" film using mine the porosity dependence of the mechanical properties a moving double blade device on a laboratory tape casting of a SiC material densified by liquid phase sintering on a bench(Elmeceram-France ). The tapes had smooth surfaces wide range of porosity, and to compare the mechanical be- and uniform thicknesses, which could be varied between haviour of dense-porous laminates to that obtained by Blanks 100 and 150 um. Sheets were punched in the green tapes et al. for silicon carbide densified by solid state sintering and stacked on each other using two types of stacking se- quence: (i) stacking of identical layers to obtain monolithic dense or porous specimens, and(ii) alternate stacking of 2. Processing and materials microstructure PFA free and PFA containing layers to obtain symmetrical dense/porous laminar composites. The outer layers were al 2. 1. Processing ways dense layers. The number of tapes was chosen to lead to a final thickness of about 3-4 mm after sintering. The The materials were made by stacking layers obtained by thickness of the layers was varied by stacking several tapes tape casting and co-sintering. The ceramic powder must of the same nature. This led to different architectures that al- ave a narrow particle size distribution and a rather low lowed to study the influence of the dense to porous thickness mean diameter dso was 1 um for the starting a-SiC powder ratio (Sika Tech FCP13, Norton-Norway) and the specific sur- After stacking, the specimens were pressed under 60 MPa face area(BEt)was 13 m/g. The densification aids(5 wt. at 65C. The burnout of organics was performed by heat- of the total ceramic content) for the liquid phase sinter ing very slowly(6 C/h)up to 550oC in air. Sintering was ing of SiC were Y203(Rhone-Poulenc-France)and Al203 conducted in a graphite furnace (VAS, France)under ar- (CR15 Baikowski-France)in a ratio corresponding to the gon at atmospheric pressure during I h at 1950C. The rel- YAG-Al2O3 eutectic composition(40 wt. Y203-60 wt ative density of the PFA free monolithic specimens was AlO3) 98% The first step of the elaboration of the suspensions for tape-casting consists in the dispersion of the ceramic pow- 2. 2. Microstructure ders in a solvent containing a dispersant. This was performed by planetary milling with alumina balls during 4 h in the a detailed description of the sintered microstructures of MEK -ethanol azeotrope containing 0.6wt. of a phosphate the monolithic and laminated Sic materials have been al ester(CP213, Cerampilot-France). Then, an acrylic binder ready published. So, only the main results will be briefly (Degalan" LP51701, Rohm and Haas-US) and a phtalate eported here plasticizer (DPB, Prolabo-France)were added to the suspen The microstructures were homogeneous. and in the case sion and mixed during 16h. They allow to optimise the vis- of the laminar composites, the layers were parallel with an590 C. Reynaud et al. / Journal of the European Ceramic Society 25 (2005) 589–597 growing crack comes sufficiently close to a microcrack11 or a pore,12 the stress intensity factor at the tip of the crack is increased and then, the apparent fracture energy of the liga￾ment is reduced. This reduction varies with the relative den￾sity, and for a cubic array of spherical pores, Blanks et al.7 derived a minimum level of porosity equal to 37% to ensure crack deflection, in good agreement with their experimental results for silicon carbide. Assuming that the fracture energy of the dense ligament is related to that of the porous layer, GP, by the relation: Glig = GP 1 − P (1) where P is the volume fraction of porosity, Davis et al.8 ex￾pressed the criterion for continued crack deflection in terms of easily measurable variables: GP GS < 0.57(1 − P) (2) The experimental results obtained by Clegg’s group with either alumina or silicon carbide were consistent with re￾lation (2), crack deflection being observed when the ratio GP/GS falls under the line 0.57(1 − P), that occurs for a volume fraction of porosity of 0.37. The aim of the work presented in this paper was to deter￾mine the porosity dependence of the mechanical properties of a SiC material densified by liquid phase sintering on a wide range of porosity, and to compare the mechanical be￾haviour of dense-porous laminates to that obtained by Blanks et al.7 for silicon carbide densified by solid state sintering. 2. Processing and materials microstructure 2.1. Processing The materials were made by stacking layers obtained by tape casting and co-sintering. The ceramic powder must have a narrow particle size distribution and a rather low mean diameter d50 was 1m for the starting
-SiC powder (Sika Tech FCP13, Norton-Norway) and the specific sur￾face area (BET) was 13 m2/g. The densification aids (5 wt.% of the total ceramic content) for the liquid phase sinter￾ing of SiC were Y2O3 (Rhone-Poulenc-France) and Al2O3 (CR15 Ba¨ıkowski-France) in a ratio corresponding to the YAG–Al2O3 eutectic composition (40 wt.% Y2O3–60 wt.% Al2O3). The first step of the elaboration of the suspensions for tape-casting consists in the dispersion of the ceramic pow￾ders in a solvent containing a dispersant. This was performed by planetary milling with alumina balls during 4 h in the MEK-ethanol azeotrope containing 0.6 wt.% of a phosphate ester (CP213, Cerampilot-France). Then, an acrylic binder (Degalan® LP51/01, Röhm and Haas-US) and a phtalate plasticizer (DPB, Prolabo-France) were added to the suspen￾sion and mixed during 16 h. They allow to optimise the vis￾Table 1 Characteristic parameters of the two PFAs used PFA CS GP Chemical nature Corn starch Graphite platelet Dimension d50 = 14 m 8m × 8m × 3m Aspect ratio 1.2 2.67 cosity and the rheological behaviour of the slurry and com￾plement one another to produce handleable ceramic tapes. The optimum concentrations for the binder and the plasti￾cizer were determined to be 8 wt.% for both additives on the base of the ceramic powder. The third step corresponds to the incorporation, into the suspension, of the pore form￾ing agent (PFA): corn starch (Roquette-France) or graphite platelets (Union Carbide-US) (Table 1). The PFA volume fraction, referred to the sum of the ceramic volume and PFA volume, was 45 and 50% for graphite platelets and varied from 5 to 55% for corn starch. We were unable to fabricate sound parts when larger PFA volume fraction were incor￾porated. A mixing time of 3.5 h leads to an uniform distri￾bution of the PFA. Finally the slurry was de-aired at a low rotation speed during 24 h. Then the slurries were tape cast onto a Mylar® film using a moving double blade device on a laboratory tape casting bench (Elmeceram-France). The tapes had smooth surfaces and uniform thicknesses, which could be varied between 100 and 150m. Sheets were punched in the green tapes and stacked on each other using two types of stacking se￾quence: (i) stacking of identical layers to obtain monolithic dense or porous specimens, and (ii) alternate stacking of PFA free and PFA containing layers to obtain symmetrical dense/porous laminar composites. The outer layers were al￾ways dense layers. The number of tapes was chosen to lead to a final thickness of about 3–4 mm after sintering. The thickness of the layers was varied by stacking several tapes of the same nature. This led to different architectures that al￾lowed to study the influence of the dense to porous thickness ratio. After stacking, the specimens were pressed under 60 MPa at 65 ◦C. The burnout of organics was performed by heat￾ing very slowly (6 ◦C/h) up to 550 ◦C in air. Sintering was conducted in a graphite furnace (VAS, France) under ar￾gon at atmospheric pressure during 1 h at 1950 ◦C. The rel￾ative density of the PFA free monolithic specimens was 98%. 2.2. Microstructure A detailed description of the sintered microstructures of the monolithic and laminated SiC materials have been al￾ready published.13 So, only the main results will be briefly reported here. The microstructures were homogeneous, and in the case of the laminar composites, the layers were parallel with an