L Ceseracciu et al. International Journal of Refractory Metals Hard Materials 23(2005)375-381 Sheets of pure alumina(hereinafter designated"A) as well as of the composite alumina-zirconia(hereinaf- ter designated"AZ")in the volume ratio 60/40 wer prepared. The thicknesses of the green tapes were lected in order to obtain, after sintering, layers of about 200 um(A)and 250 um(AZ). After drying, laminate of 50x 34 mm were cut from the different ceramic sheets Hybrid laminates were prepared by stacking and warm pressing the green sheets at 75C at a press 30 MPa for 30 min. Samples were obtained by alter (b) nately superimposing one layer of alumina and one lay of alumina-zirconia( this structure is hereinafter desig nated A/AZ). The structures were designed in order to have always an alumina layer in both the outer surfaces Debonding was carried out with a very slow heating rate up to 600C, followed by y sintering at 1550° for 1 h. thus obtained dense samples (97% of theoretical density) with a thickness of about 3.0 mm, contain ing layers with a thickness ratio of about 1/1.3. In the hybrid samples, due to lower thermal expansion coeffi- ient and shrinkage during sintering, the alumina layers undergo residual compressive stresses. As reference material (i.e. nominally stress free), pure monolithic alu mina(Ma)was prepared by cold isostatic pressing and sintering at 1550C for I h Fig. I shows a sample of the laminated ceramic com- posite and the sem picture of the interface between two lumina a Imina-zirconia. where it can be observed that the interface is properly bonded. It can be also observed that the alumina grains inside the alu- mina-zirconia layers are smaller than the ones in the pure alumina layers. This is a consequence of the con- strain effect that the zirconia grains produce on the growing of neighbouring alumina grains by preventing the diffusion of alumina between grains Fig. 1.(a) Picture of the multilayer where the different alumina and Once the laminated plates were produced, they were alumina-zirconia layers can be appreciated. (b) SEM pictures of th cut into prismatic bars of about 4 x 3 x 20 mm with a interface between alumina and alumina/zirconia layers of an A/AZ diamond saw. The top layer of alumina(which was in laminated composites. It can be observed that the interface is well ompression) was polished with diamond suspension up to 3 um with a low applied force in order to avoid excessive loss of material (30 um at most), and to pro- als are similar at the microstructural level, and that the samples. Several samples were polished in the cross sec- tion area and thermally etched (1500 C, 30 min)for inated architecture observing the microstructure of the material nearby the interface(Fig. 1). MA samples were polished in 2.2. Mechanical characterization the same way, and their microstructure was also ob- served by SEM. The mean grain size in the alumina lay Both surface hardness and toughness were evaluated ers of the laminates and in the monolithic alumina was by Vickers indentation [15] on the top alumina layer. measured to be 1.9+0.7 um and 1.1+0.8 um respec- The hardness of the outer alumina layer in the A/AZ tively. These differences are not sufficiently large to affect laminated composite(applied load I kg) was HA/AZ ignificantly the mechanical behaviour of the different 16.9+0.5 GPa, and the hardness of the monolithic alu materials. In addition, considering that both the lami- mina was H= 16.7+0.9 GPa. It can be seen that nates and the Ma materials have the same amount of both the Ma and the alumina layer have comparable porosity, it can be assumed that, apart from a possible values of hardness. Fracture toughness was evalu grain orientation in the laminated structures, the materi ated by measuring the crack lengths produced and bySheets of pure alumina (hereinafter designated ‘‘A’’) as well as of the composite alumina–zirconia (hereinaf￾ter designated ‘‘AZ’’) in the volume ratio 60/40 were prepared. The thicknesses of the green tapes were se￾lected in order to obtain, after sintering, layers of about 200 lm (A) and 250 lm (AZ). After drying, laminate of 50 · 34 mm were cut from the different ceramic sheets. Hybrid laminates were prepared by stacking and warm pressing the green sheets at 75 C at a pressure of 30 MPa for 30 min. Samples were obtained by alter￾nately superimposing one layer of alumina and one layer of alumina–zirconia (this structure is hereinafter desig￾nated A/AZ). The structures were designed in order to have always an alumina layer in both the outer surfaces. Debonding was carried out with a very slow heating rate up to 600 C, followed by sintering at 1550 C for 1 h. We thus obtained dense samples (97% of theoretical density) with a thickness of about 3.0 mm, contain￾ing layers with a thickness ratio of about 1/1.3. In the hybrid samples, due to lower thermal expansion coeffi- cient and shrinkage during sintering, the alumina layers undergo residual compressive stresses. As reference material (i.e. nominally stress free), pure monolithic alu￾mina (MA) was prepared by cold isostatic pressing and sintering at 1550 C for 1 h. Fig. 1 shows a sample of the laminated ceramic com￾posite and the SEM picture of the interface between two layers of alumina and alumina–zirconia, where it can be observed that the interface is properly bonded. It can be also observed that the alumina grains inside the alu￾mina–zirconia layers are smaller than the ones in the pure alumina layers. This is a consequence of the con￾strain effect that the zirconia grains produce on the growing of neighbouring alumina grains by preventing the diffusion of alumina between grains. Once the laminated plates were produced, they were cut into prismatic bars of about 4 · 3 · 20 mm with a diamond saw. The top layer of alumina (which was in compression) was polished with diamond suspension up to 3 lm with a low applied force in order to avoid excessive loss of material (30 lm at most), and to pro￾duce a similar surface flaw size distribution for all the samples. Several samples were polished in the cross sec￾tion area and thermally etched (1500 C, 30 min) for observing the microstructure of the material nearby the interface (Fig. 1). MA samples were polished in the same way, and their microstructure was also ob￾served by SEM. The mean grain size in the alumina lay￾ers of the laminates and in the monolithic alumina was measured to be 1.9 ± 0.7 lm and 1.1 ± 0.8 lm respec￾tively. These differences are not sufficiently large to affect significantly the mechanical behaviour of the different materials. In addition, considering that both the lami￾nates and the MA materials have the same amount of porosity, it can be assumed that, apart from a possible grain orientation in the laminated structures, the materi￾als are similar at the microstructural level, and that the differences in the mechanical behaviour can be mainly attributed to the presence of residual stresses in the lam￾inated architecture. 2.2. Mechanical characterization Both surface hardness and toughness were evaluated by Vickers indentation [15] on the top alumina layer. The hardness of the outer alumina layer in the A/AZ laminated composite (applied load 1 kg) was H A=AZ v ¼ 16.9
0.5 GPa, and the hardness of the monolithic alu￾mina was HMA v ¼ 16.7
0.9 GPa. It can be seen that both the MA and the alumina layer have comparable values of hardness. Fracture toughness was evalu￾ated by measuring the crack lengths produced and by Fig. 1. (a) Picture of the multilayer where the different alumina and alumina–zirconia layers can be appreciated. (b) SEM pictures of the interface between alumina and alumina/zirconia layers of an A/AZ laminated composites. It can be observed that the interface is well bonded. L. Ceseracciu et al. / International Journal of Refractory Metals & Hard Materials 23 (2005) 375–381 377