S. Guicciardi et al. / Journal of the European Ceramic Society 27 (2007)351-356 Fig. 1.(a) Schematic diagram of the multilayer preparation and (b) bars preparation. Circular samples with thickness of 250 um and diameter of fracture toughness, Kle, was calculated according to the SenB 40 mm were punched out from the as-cast green tapes. Both formula: 7 monolithic and layered samples were produced. Monolithic PS amples were prepared by individually stacking I or C layers. KI Bw3/2/(a) The layered samples were produced by stacking 21 layers with quence I//C/l. /C//l. A schematic diagram is presented were warm-pressed at 75C for 15 min with an applied pressure f(a) 3a1/(1.99-a(1-a)(2.15-3.93a +2. Fig. 1. All the laminate materials, composite and monolithic, of 17 MPa, and then heated at 80C for 15 min without pres 2(1+2a)(1-a)2 sure. The burnout stage(150 C/h from 25 to 600oC, 30 min Pc is the critical load at fracture, S the span, B the width of holding time) was followed by sintering in a graphite furnace the bar, W the thickness of the bar and a the ratio between notch processing are reported in Refs. s trogen. More details of the length(a) and bar thickness w. nin) under lowing n The relative densities of the sintered samples were measured 3. Results and discussion by Archimede's method. On the polished cross section of the samples, the microstructure was analysed by scanning elec- 3. 1. Microstructure of the materials tron microscopy(SEM, Cambridge S360)and energy dispersive microanalysis(EDS, INCA Energy 300, Oxford instruments, The relative density of the laminate composite after sinter UK). The linear thermal expansion coefficient(CTE) of the ing was about 98%. A very good adhesion was found between monolithic materials was measured with dilatometric tests(Net- the layers as visible delamination or large structural defects at zsch Geraetebau Dil E 402, Germany)up to 1400C in air, the interface were not found. A polished cross section of the vith a heating rate of 5C/min. The Youngs modulus(E)of the monolithic materials was measured by the resonance fre quency method on 28 mm x 8 mm x 0.8 mm specimens using an H&P gain-phase analyzer. The flexural strength(o) of the monolithic materials and the laminated composite was mea- ured on a 4-pt bending fixture(outer span: 20 mm, inner span O mm)with a crosshead speed of 0.5 mm/min using chamfered bars 25 mm x 2.50mm x 1.75 mm, length x width x thickness respectively. The fracture toughness of the monolithic materials and laminated composite was measured by Single Edge V- Notched Beam(SE VNB). From the laminated composite, bars of about25mm×4mm×3 mm(length× thickness x width) were cut with the bar thickness corresponding to the thick ness of the sintered disc, as shown in Fig. lb. A notch was first introduced at the centre of the bending bar with a 500 um thick diamond saw, then this notch was sharpened using a razor blade sprinkled with 3 um diamond paste. Care was taken in 200um positioning the notch tip well within a tensile or compressive yer,Fig. 2. The notched bars were fractured in a 3-pt bend- Fig. 2. Optical micrograph showing an example of the notch introduced in the ing device with a crosshead of 0.5 mm/min and the appa 21-layers bars352 S. Guicciardi et al. / Journal of the European Ceramic Society 27 (2007) 351–356 Fig. 1. (a) Schematic diagram of the multilayer preparation and (b) bars preparation. Circular samples with thickness of 250 m and diameter of 40 mm were punched out from the as-cast green tapes. Both monolithic and layered samples were produced. Monolithic samples were prepared by individually stacking I or C layers. The layered samples were produced by stacking 21 layers with sequence I/I/C/I...I/C/I/I. A schematic diagram is presented in Fig. 1. All the laminate materials, composite and monolithic, were warm-pressed at 75 ◦C for 15 min with an applied pressure of 17 MPa, and then heated at 80 ◦C for 15 min without pres￾sure. The burnout stage (150 ◦C/h from 25 to 600 ◦C, 30 min holding time) was followed by sintering in a graphite furnace (1850 ◦C/30 min) under flowing nitrogen. More details of the processing are reported in Refs.15,16 The relative densities of the sintered samples were measured by Archimede’s method. On the polished cross section of the samples, the microstructure was analysed by scanning elec￾tron microscopy (SEM, Cambridge S360) and energy dispersive microanalysis (EDS, INCA Energy 300, Oxford instruments, UK). The linear thermal expansion coefficient (CTE) of the monolithic materials was measured with dilatometric tests (Net￾zsch Geraetebau Dil E 402, Germany) up to 1400 ◦C in air, with a heating rate of 5 ◦C/min. The Young’s modulus (E) of the monolithic materials was measured by the resonance fre￾quency method on 28 mm × 8 mm × 0.8 mm specimens using an H&P gain-phase analyzer. The flexural strength (σ) of the monolithic materials and the laminated composite was mea￾sured on a 4-pt bending fixture (outer span: 20 mm, inner span: 10 mm) with a crosshead speed of 0.5 mm/min using chamfered bars 25 mm × 2.50 mm × 1.75 mm, length × width × thickness, respectively. The fracture toughness of the monolithic materials and laminated composite was measured by Single Edge V￾Notched Beam (SEVNB). From the laminated composite, bars of about 25 mm × 4 mm × 3 mm (length × thickness × width) were cut with the bar thickness corresponding to the thick￾ness of the sintered disc, as shown in Fig. 1b. A notch was first introduced at the centre of the bending bar with a 500m￾thick diamond saw, then this notch was sharpened using a razor blade sprinkled with 3m diamond paste. Care was taken in positioning the notch tip well within a tensile or compressive layer, Fig. 2. The notched bars were fractured in a 3-pt bend￾ing device with a crosshead of 0.5 mm/min and the apparent fracture toughness, KIc, was calculated according to the SENB formula:17 KIc = PcS BW3/2 f (α) (1) where f (α) = 3α1/2[1.99 − α(1 − α)(2.15 − 3.93α + 2.7α2)] 2(1 + 2α)(1 − α) 3/2 (2) Pc is the critical load at fracture, S the span, B the width of the bar, W the thickness of the bar and α the ratio between notch length (a) and bar thickness W. 3. Results and discussion 3.1. Microstructure of the materials The relative density of the laminate composite after sinter￾ing was about 98%. A very good adhesion was found between the layers as visible delamination or large structural defects at the interface were not found. A polished cross section of the Fig. 2. Optical micrograph showing an example of the notch introduced in the 21-layers bars.