TABLE II Three layered composite design. A total thickness of a tile- 10.5 mm Thickness of Layers (um) Apparent Composition B4C-30wt%SiC (MPa) FIens(MPa) le(MPam/2 B4C-30wt %SiC/B4C 32 TABLE III Nine layered composite design. A total thickness of a tile-10.35 mm Thickness of Layers(um) B4C-30wt%SiC a comp(MPa) Ptens(MPa) Kle(MPam/-) B4C-30wt%SiC/B4C 2250 TABLE IV Some properties of the powders and green tapes after rolling Additive density Poured density Poured density Relative Thickness Comp f the powder f the granulas density 0. 0.095 0.45 B4C-30wt %SiC/B4C 1.5 0.186 0.14 allows optimizing the process of roll compaction [36]. Powders are continuously supplied in the bunker and In our case there is a challenging problem to produce furtherinto the deformation zone in between rolls. Pow- thin tapes with a small amount of plasticizer and suffi- ders are supplied to the deformation zone due to both cient strength and elasticity to handle green layers after the gravitational force and friction between rolls and rolling Crude rubber(1-3 wt%)has to be added to the powders. The relative density of the tape (pr)can be mixture of powders as a plasticizer through a 3% solu- calculated from tion in petrol. Then the powders were dried up to the 2 wt% residual amount of petrol in the mixture. After C2r sieving powders with a 500 um sieve, granulated pow ders were dried up to the 0.5 wt% residual petrol. The schematic presentation of rolling is shown in Fig 3. where p, is a relative powder density, i is a drawing coefficient, a is an intake angle, and R is a roll diame- ter. A roll mill with 40 mm rolls was used for rolling The velocity of rolling was in the range of 1-1. 2 m/min Working pressure was varied from O I ton/cm- for rela- tive density of tapes 64%to l ton/cm- for 74% density propertie rolling ed in Table iv Samples of ceramics were prepared by hot press ing of the rolled tapes stacked together. The hot press- ing conditions were as follows:(a) a heating rate was 100C/min;(b)a hot pressing temperature was kept at 2150oC during hot pressing of a majority of the tiles, Bunker. 2. Powders 3 Rolls 4. Transmission. 5. Motor. 6 Bottom support 7. Tape and some hot pressing was done at 2200.C to ensure that fully dense materials were obtained; (c)a pressure B4C·30wt% SiC tapes B was kept at the level of 30 MPa; and(d)a dwell time at hot pressing temperature was 50-60 min Graphite dies were used for the hot pressing of laminates with graphite surfaces coated by B layer in order to prevent a direct contact between graphite and ceramic material. 90 x 90 x 10 mm tiles were produced as a result of hot pressing. Dense(97-100% of density)laminate sam- ples were obtained 100mm 5. Microstructure of laminates During hot pressing of laminates the shri and B,- 30 wt%SiC rolled tapes. The thickness of an individual tape individual layers occurred, and their thickness become after rolling is between 0.4-0.5 mm. 0. 15 mm after hot pressing. The interfaces betweenTABLE II Three layered composite design. A total thickness of a tile – 10.5 mm Thickness of Layers (µm) Apparent Composition B4C-30wt%SiC B4C σcomp (MPa) σtens (MPa) KIc (MPam1/2) B4C-30wt%SiC/B4C 900 8700 632 131 44 TABLE III Nine layered composite design. A total thickness of a tile – 10.35 mm Thickness of Layers (µm) Apparent Composition B4C-30wt%SiC B4C σcomp (MPa) σtens (MPa) KIc (MPam1/2) B4C-30wt%SiC/B4C 150 2250 662 99 32 TABLE IV Some properties of the powders and green tapes after rolling Green Tape Additive density Poured density Poured density Relative Thickness Composition d50 (µm) (g/cm3) of the powder of the granulas density (mm) B4C 2.5 2.52 0.111 0.095 0.71 0.45 B4C-30wt%SiC/B4C 1.5 2.69 0.186 0.146 0.74 0.47 allows optimizing the process of roll compaction [36]. In our case there is a challenging problem to produce thin tapes with a small amount of plasticizer and suffi- cient strength and elasticity to handle green layers after rolling. Crude rubber (1–3 wt%) has to be added to the mixture of powders as a plasticizer through a 3% solu￾tion in petrol. Then the powders were dried up to the 2 wt% residual amount of petrol in the mixture. After sieving powders with a 500 µm sieve, granulated pow￾ders were dried up to the 0.5 wt% residual petrol. The schematic presentation of rolling is shown in Fig. 3. Figure 3 (A) Schematic presentation of rolling. (B) Photograph of B4C and B4C-30 wt%SiC rolled tapes. The thickness of an individual tape after rolling is between 0.4–0.5 mm. Powders are continuously supplied in the bunker and further into the deformation zone in between rolls. Pow￾ders are supplied to the deformation zone due to both the gravitational force and friction between rolls and powders. The relative density of the tape (ρr) can be calculated from ρr = ρp λ
1 + α2R hs (3) where ρp is a relative powder density, λ is a drawing coefficient, α is an intake angle, and R is a roll diame￾ter. A roll mill with 40 mm rolls was used for rolling. The velocity of rolling was in the range of 1–1.2 m/min. Working pressure was varied from 0.1 ton/cm2 for rela￾tive density of tapes 64% to 1 ton/cm2 for 74% density. The properties of the powders and green tapes after rolling are presented in Table IV. Samples of ceramics were prepared by hot press￾ing of the rolled tapes stacked together. The hot press￾ing conditions were as follows: (a) a heating rate was 100◦C/min; (b) a hot pressing temperature was kept at 2150◦C during hot pressing of a majority of the tiles, and some hot pressing was done at 2200◦C to ensure that fully dense materials were obtained; (c) a pressure was kept at the level of 30 MPa; and (d) a dwell time at hot pressing temperature was 50–60 min. Graphite dies were used for the hot pressing of laminates with graphite surfaces coated by BN layer in order to prevent a direct contact between graphite and ceramic material. 90 × 90 × 10 mm tiles were produced as a result of hot pressing. Dense (97–100% of density) laminate sam￾ples were obtained. 5. Microstructure of laminates During hot pressing of laminates the shrinkage of the individual layers occurred, and their thickness become 0.15 mm after hot pressing. The interfaces between 5487