O.N. Grigoriev et aL./Composites: Part B 37(2006)530-541 531 composites requires the determination of their mechanical Grinding and mixing of the batch components were carried characteristics such as of elastic moduli, strength, stiffness, out in a planetary ball mill. The powder particles after mixing stress-strain diagrams, distribution of phases of different have a sufficiently small grain size for hot pressing(2.5 and materials in the structure of a composite, and the magnitude 1. 1 m, respectively), ensuring optimum dispersion. The B4C and distribution of residual stresses in their volume. There are additives and in some cases. TiO, additives were introduced several analytical and experimental methods to solve these into SiC-and TiB2-based mixtures In the presence of TiO2, the problems [12-21] etc. The work presented here is based on the reactionary hot pressing with formation of secondary TiB analysis of experimentally obtained physical-mechanical during reaction T1O2+B. 2+Co took place. The characteristics of a composite [14-22]. This approach creates additives were introduced for reducing and matching of hot possibilities for theoretical investigation of a deformation pressing temperatures of various composition layers behavior of the developed composites We used a slip casting method for manufacturing lamina The effect of structure and residual stresses on strength of tapes. The ceramic tapes with a thickness 50 um were multilayer SiC(B. C)/MeB, composites was explored. A layer prepared from powders of various compositions. To remove structure (grain size, porosity etc. )was altered by using casting defects the tapes were folded in rolls and then rolled up different sintering additives, various raw materials (a- and to the thickness of 400 um. Fro B-SiC powders) as well as by changing the manufacturing required size were cut out and the packages containing 11-13 conditions. Residual thermal stresses are controlled by the pairs of alternating layers were obtained for the chosen composition of layers. Experimental studies on the mechanical compositions strength of the ceramic composite specimen were performed Hot pressing was carried out using a pilot induction hot using procedures and equipment described in [23-24] ress in graphite dies without chamber. The 2. Materials and procedures 1600-2150C, pressure 26-30 MP of isothermal densification 7-20 min, and heating rates were up to Two kinds of a-Sic powders were used: (1) technical 100%/min. Samples for testing of the mechanical properties abrasive powders, M5 grade, produced by the Zaporo abrasive plant, Ukraine, and(2)powders of UFO5 and UF10 sectioned dies grades from the Starck company, Germany. Both powders were ne specimens for mechanical tests in the form of a mixtures of polytypes: mainly 6H, 15R and 3C. TiB2 powders rectangular beam(Fig. 1a) were produced by sawing the (TC 6-09-03-7-75)from the Donetsk factory of chemical package into billets and polishing them with a diamond tool in reagents (Ukraine), and abrasive B4C powders from such a way that the layers throughout the thickness of the beam ere located as symmetrically as possible relative to the Zaporozhye abrasive plant(GOST 5744-74), were used as middle plane(Fig. 1b). Specimens of preset sizes were tested in sintering additives. Some properties of as-received powders are given in four-point bending(pure flexure, Fig. la)at room temperature Table 1 with a deformation rate(displacement of the cross-head of a The B-SiC powder, produced by Institute for Problems of testing machine)of 0.005 mm/min Vickers hardness was determined under the load of 5n. the Materials Science, had the content of 3C-polytype up to 100%o Sic powders were very different in their defectiveness and microstructure of composites was investigated by optical and sinterability. The powders UFO5 and M5 were characterized scanning electron microscopy(SEM), and the phase compe with low width of X-ray diffraction peaks and good resolution sIton--by X-ray diffraction(XRD) of Ka doublets and, therefore, had a high degree of structural perfection. XRD peaks of UF10, as well as of B-Sic powder 3. Results and discussion were very broad due to high density of defects(stacking faults, 3. 1. Characteristics of monolithic ceramics polytypes interlayer, and nonhomogeneous microstrains accordance with TEM data) which, apparently, facilitated an increase in their activity during sintering The bending strength of single-phase and heterogeneous ramics with composition similar to the ones in the layered composites is shown in Table 2. Single-phase silicon carbide Table 1 ceramics had high porosity (5-10%), average grain size 5-10 The characteristic of powder and up to 100 um for raw powders of a-SiC and B-SiC, Powder Size of particles, Content of oxy- Free carbon respectively. In the latter case the high grain size is due to grain dso(um) gen(wt%) (wt%) growth during B-a transformation of silicon carbide at hot pressing Hot pressing of pure silicon carbide without sintering 0.17 additives leads to the formation of porous coarse-grained B-Sic 0.1-0.2 ≤0.5×10 materials with the low strength(110-190 MPa). Introduction <0.1 of boron carbide allows to reduce porosity of ceramics to 1-3% with the relevant increasing of strength up to 300-370 MPa.composites requires the determination of their mechanical characteristics such as of elastic moduli, strength, stiffness, stress–strain diagrams, distribution of phases of different materials in the structure of a composite, and the magnitude and distribution of residual stresses in their volume. There are several analytical and experimental methods to solve these problems [12–21] etc. The work presented here is based on the analysis of experimentally obtained physical–mechanical characteristics of a composite [14–22]. This approach creates possibilities for theoretical investigation of a deformation behavior of the developed composites. The effect of structure and residual stresses on strength of multilayer SiC(B4C)/MeB2 composites was explored. A layer structure (grain size, porosity etc.) was altered by using different sintering additives, various raw materials (a- and b-SiC powders) as well as by changing the manufacturing conditions. Residual thermal stresses are controlled by the composition of layers. Experimental studies on the mechanical strength of the ceramic composite specimen were performed using procedures and equipment described in [23–24]. 2. Materials and procedures Two kinds of a-SiC powders were used: (1) technical abrasive powders, M5 grade, produced by the Zaporozhye abrasive plant, Ukraine, and (2) powders of UF05 and UF10 grades from the Starck company, Germany. Both powders were mixtures of polytypes: mainly 6H, 15R and 3C. TiB2 powders (TC 6-09-03-7-75) from the Donetsk factory of chemical reagents (Ukraine), and abrasive B4C powders from the Zaporozhye abrasive plant (GOST 5744-74), were used as sintering additives. Some properties of as-received powders are given in Table 1. The b-SiC powder, produced by Institute for Problems of Materials Science, had the content of 3C-polytype up to 100%. SiC powders were very different in their defectiveness and sinterability. The powders UF05 and M5 were characterized with low width of X-ray diffraction peaks and good resolution of Ka-doublets and, therefore, had a high degree of structural perfection. XRD peaks of UF10, as well as of b-SiC powder, were very broad due to high density of defects (stacking faults, polytypes interlayer, and nonhomogeneous microstrains, in accordance with TEM data) which, apparently, facilitated an increase in their activity during sintering. Grinding and mixing of the batch components were carried out in a planetary ball mill. The powder particles after mixing have a sufficiently small grain size for hot pressing (2.5 and 1.1 mm, respectively), ensuring optimum dispersion. The B4C additives and, in some cases, TiO2 additives were introduced into SiC- and TiB2-based mixtures. In the presence of TiO2, the reactionary hot pressing with formation of secondary TiB2 during reaction TiO2CB4C/TiB2CCO took place. The additives were introduced for reducing and matching of hot pressing temperatures of various composition layers. We used a slip casting method for manufacturing lamina tapes. The ceramic tapes with a thickness w50 mm were prepared from powders of various compositions. To remove casting defects the tapes were folded in rolls and then rolled up to the thickness of 400 mm. From the sheets, plates of the required size were cut out and the packages containing 11–13 pairs of alternating layers were obtained for the chosen compositions. Hot pressing was carried out using a pilot induction hot press in graphite dies without a vacuum chamber. The temperature of isothermal sintering was in the range of 1600–2150 8C, pressure 26–30 MPa, time of isothermal densification 7–20 min, and heating rates were up to 1008/min. Samples for testing of the mechanical properties having sizes 45!45!5 mm were produced in the multi￾sectioned dies. The specimens for mechanical tests in the form of a rectangular beam (Fig. 1a) were produced by sawing the package into billets and polishing them with a diamond tool in such a way that the layers throughout the thickness of the beam were located as symmetrically as possible relative to the middle plane (Fig. 1b). Specimens of preset sizes were tested in four-point bending (pure flexure, Fig. 1a) at room temperature with a deformation rate (displacement of the cross-head of a testing machine) of 0.005 mm/min. Vickers hardness was determined under the load of 5 N. The microstructure of composites was investigated by optical and scanning electron microscopy (SEM), and the phase compo￾sition—by X-ray diffraction (XRD). 3. Results and discussion 3.1. Characteristics of monolithic ceramics The bending strength of single-phase and heterogeneous ceramics with composition similar to the ones in the layered composites is shown in Table 2. Single-phase silicon carbide ceramics had high porosity (5–10%), average grain size 5–10 and up to 100 mm for raw powders of a-SiC and b-SiC, respectively. In the latter case the high grain size is due to grain growth during b/a transformation of silicon carbide at hot pressing. Hot pressing of pure silicon carbide without sintering additives leads to the formation of porous coarse-grained materials with the low strength (110–190 MPa). Introduction of boron carbide allows to reduce porosity of ceramics to 1–3% with the relevant increasing of strength up to 300–370 MPa. Table 1 The characteristic of powders Powder Size of particles, d50, (mm) Content of oxy￾gen (wt%) Free carbon (wt%) SiCM5 5 1.5 1–2 SiCUF05 1.47 0.55 – SiCUF10 0.7 1.2 0.17 b-SiC 0.1–0.2 %0.5!10K2 – TiB2 30 0.3 !0.1 B4C 20 1.5 2 O.N. Grigoriev et al. / Composites: Part B 37 (2006) 530–541 531