M. Pavese et al. /Ceramics International 34(2008)197-203 100um Fig 8. Silica layer on the surface of the oxidised samples. 1 mm300kU 685E1 1449/01 DIPSMI C Fig. 10. Fracture surface of as-prepared SiC buckle. higher number of interfaces and a better residual strength distribution. The best materials are the one with 0. 6 mm layers since 0.4 mm layers are so thin that are more prone to suffer damage during the preparation phase. On the contrary the oxidation has a more marked effect on mechanical properties of thinner layers, even if the overall mechanical strength after b oxidation remains significantly greater for thinner layers than for thicker ones Crack deflection is another interesting issue that helps to xplain the mechanical behaviour of multilayers. Vickers indentation tests were performed on the polished section of the multilayers, and a typical result is presented in Fig. 11: the Fig.9. Stress/displacement curves of Sic multilayer buckles as-prepared (a) radial cracks formed during the indentation are very short and after heat treatment in air at 1600C for 100h(b) while tangential ones are free to move along the layer; the crack deflection is also visible for radial cracks. This suggests the presence of residual stresses. A confirmation of their presence effect, is most evident when the fracture surfaces are examined was given by performing residual stress measurements by (Fig.10) microdiffraction XRd with DRaSt method It is evident from Fig 9 that the oxidation treatment(100 h at stresses from 600 to 1200 MPa were meas the 00C)does not appreciably change the failure mode of multilayer section. multilayers These results corroborate the expected fracture mechanism The layer thickness on the other side has a rather marked the cracks cannot easily propagate from one layer to another, so influence on the mechanical properties of such materials. that each layer fails singularly and, rather than sudden fracture, Table 1 shows the radial compression strength of samples a structured curve is observed obtained from layers of different thickness(tape thickness, as Thermal shock tests were carried out on samples with measured from the blade height during tape casting, 0.4, 0.6 and 0.6 mm thick layers. The system described in Section 2.2 was used. and the results are shown in Table 2. These results shows that thinner layers give better No significant differences were observed after 10 or 50 mechanical properties to the multilayers, probably due to a thermal shock cycles, thus suggesting that thermal shock from Table I Buckles compression tests results: compression strength for different layer thickness samples 0.4 mm tape thickness 0.6 mm tape thickness 0.8 mm tape thickness As-prepared Heat treated Heat treated Heat treated Compression strength(MPa 235 184 356 210 4effect, is most evident when the fracture surfaces are examined (Fig. 10). It is evident from Fig. 9 that the oxidation treatment (100 h at 1600 8C) does not appreciably change the failure mode of multilayers. The layer thickness on the other side has a rather marked influence on the mechanical properties of such materials. Table 1 shows the radial compression strength of samples obtained from layers of different thickness (tape thickness, as measured from the blade height during tape casting, 0.4, 0.6 and 0.8 mm). These results shows that thinner layers give better mechanical properties to the multilayers, probably due to a higher number of interfaces and a better residual strength distribution. The best materials are the one with 0.6 mm layers, since 0.4 mm layers are so thin that are more prone to suffer damage during the preparation phase. On the contrary the oxidation has a more marked effect on mechanical properties of thinner layers, even if the overall mechanical strength after oxidation remains significantly greater for thinner layers than for thicker ones. Crack deflection is another interesting issue that helps to explain the mechanical behaviour of multilayers. Vickers indentation tests were performed on the polished section of the multilayers, and a typical result is presented in Fig. 11: the radial cracks formed during the indentation are very short, while tangential ones are free to move along the layer; the crack deflection is also visible for radial cracks. This suggests the presence of residual stresses. A confirmation of their presence was given by performing residual stress measurements by microdiffraction XRD with DRAST method. Compressive stresses from 600 to 1200 MPa were measured on the multilayer section. These results corroborate the expected fracture mechanism: the cracks cannot easily propagate from one layer to another, so that each layer fails singularly and, rather than sudden fracture, a structured curve is observed. Thermal shock tests were carried out on samples with 0.6 mm thick layers. The system described in Section 2.2 was used, and the results are shown in Table 2. No significant differences were observed after 10 or 50 thermal shock cycles, thus suggesting that thermal shock from M. Pavese et al. / Ceramics International 34 (2008) 197–203 201 Fig. 8. Silica layer on the surface of the oxidised samples. Fig. 9. Stress/displacement curves of SiC multilayer buckles as-prepared (a) and after heat treatment in air at 1600 8C for 100 h (b). Fig. 10. Fracture surface of as-prepared SiC buckle. Table 1 Buckles compression tests results: compression strength for different layer thickness samples 0.4 mm tape thickness 0.6 mm tape thickness 0.8 mm tape thickness As-prepared Heat treated As-prepared Heat treated As-prepared Heat treated Compression strength (MPa) 235 184 356 210 143 123