S Guicciardi et al. /Journal of the European Ceramic Sociery 27(2007)351-356 multilayer composite is shown in Fig 3a. The layers'thickness was in the range of 170-200 um. Small variations in thickness were unavoidable since the discs were produced from differ ent tapes with the same compositions. In Fig. 3b and c, detailed views of the single layers are reported. A small amount of poros- ty was found in both layers(1-2%). Further details on the microstructure are reported elsewhere. The monolithic mate rials presented similar microstructural features. No trace of the junction between the stacked layers was found, such that these pecimens resembled bulk materials 3. 2. Properties of the monolithic materials Some properties of the monolithic materials are reported in 100um Table 1. The CTEs were linear function of the temperature in the measured range. The Cte of material C was higher than that of material l, due to the higher content of MoSi,, which is the phase with the highest CTE. Typical values reported in the literature for the pure phases are: 8 AIN: 5.59 x 10-6oC-I iC:5.12×10-6°C-1,MoSi2:9.1×10-6°C-1 The higher value of Youngs modulus of material C is mainly e to the higher amount of MoSi2 and Sic, which are very stiff phases(440 GPa). The fracture toughness of material C was higher than that of material I(Table 1), very likely as a result of the higher content of MoSi,. Due to its high value of Cte, the higher content of moSi, increased the residual stress in the alN-Sic matrix which acted as a toughening mechanism. I The fexural strength of both materials was relatively high 10 um with a low dispersion around the mean value, see Table 1. Mate rial I was slightly stronger than material C. 3.3. Calculation of the residual stresses in the laminated The residual stresses in the various rigidly bonded layers can be estimated according to the lamination theory G+a1△T= constant E o;;=0 where Ei is the elastic residual deformation and oi the stress developed in the layer of thickness t Respectively, ai, Ei and vi are the thermal expansion coefficient, the Youngs modulus and Fig. 3. SEM micrographs of the microstructure of the multilayer saI the Poissons ratio of layer i. AT is the temperature-range over Panoramic view of the layered material. (b)Detailed view of the I layer, and(c) which elastic stress develops due to thermal strain mismatch. of the C layer. The MoSi2 phase is visible as bright contrast grains di the AlN-Sic matrix Table 1 Compositions and properties of the constituent materials Material Composition(vol %) E(GPa) CTE(25-1000°)(×10-6°C Kle(MPam.S) a(MPa) 80AIN +10SiC+10MoSi 27 620 2.1±0.1 571士25 55AIN +15SiC+30MoSiz 23 2.8±0.4 513±23 Apparent fracture toughness and flexural strength of the laminated composite material. E= Youngs modulus, v=Poissons ratio(calculated), CTE=linear thermal expansion coefficient, Kle-fracture toughness, a=4-pt bending strengthS. Guicciardi et al. / Journal of the European Ceramic Society 27 (2007) 351–356 353 multilayer composite is shown in Fig. 3a. The layers’ thickness was in the range of 170–200m. Small variations in thickness were unavoidable since the discs were produced from differ￾ent tapes with the same compositions. In Fig. 3b and c, detailed views of the single layers are reported. A small amount of poros￾ity was found in both layers (∼1–2%). Further details on the microstructure are reported elsewhere.16 The monolithic mate￾rials presented similar microstructural features. No trace of the junction between the stacked layers was found, such that these specimens resembled bulk materials. 3.2. Properties of the monolithic materials Some properties of the monolithic materials are reported in Table 1. The CTEs were linear function of the temperature in the measured range. The CTE of material C was higher than that of material I, due to the higher content of MoSi2, which is the phase with the highest CTE. Typical values reported in the literature for the pure phases are:18 AlN: 5.59 × 10−6 ◦C−1, SiC: 5.12 × 10−6 ◦C−1, MoSi2: 9.1 × 10−6 ◦C−1. The higher value of Young’s modulus of material C is mainly due to the higher amount of MoSi2 and SiC, which are very stiff phases (∼440 GPa18). The fracture toughness of material C was higher than that of material I (Table 1), very likely as a result of the higher content of MoSi2. Due to its high value of CTE, the higher content of MoSi2 increased the residual stress in the AlN–SiC matrix, which acted as a toughening mechanism.19 The flexural strength of both materials was relatively high with a low dispersion around the mean value, see Table 1. Mate￾rial I was slightly stronger than material C. 3.3. Calculation of the residual stresses in the laminated composite The residual stresses in the various rigidly bonded layers can be estimated according to the lamination theory:3 εi = 1 − νi Ei σi + αiT = constant (3) i σiti = 0 (4) where εi is the elastic residual deformation and σi the stress developed in the layer of thickness ti. Respectively, αi, Ei and νi are the thermal expansion coefficient, the Young’s modulus and the Poisson’s ratio of layer i. T is the temperature-range over which elastic stress develops due to thermal strain mismatch. Fig. 3. SEM micrographs of the microstructure of the multilayer samples. (a) Panoramic view of the layered material. (b) Detailed view of the I layer, and (c) of the C layer. The MoSi2 phase is visible as bright contrast grains dispersed in the AlN–SiC matrix. Table 1 Compositions and properties of the constituent materials Material Composition (vol.%) E (GPa) ν CTE (25–1000 ◦C) (×10−6/ ◦C) KIc (MPa m0.5) σ (MPa) I 80AlN + 10SiC + 10MoSi2 325 0.27 6.20 2.1 ± 0.1 571 ± 25 C 55AlN + 15SiC + 30MoSi2 348 0.23 6.87 2.8 ± 0.4 513 ± 23 Apparent fracture toughness and flexural strength of the laminated composite material. E = Young’s modulus, ν = Poisson’s ratio (calculated), CTE = linear thermal expansion coefficient, KIc = fracture toughness, σ = 4-pt bending strength.