J Mater Sci(2008)43:5942-5947 DOI10.1007/10853-008-28620 Effects of rolling and hot pressing on mechanical properties of boron carbide-based ceramics Nina Orlovskaya. Sergey Yarmolenko Jag Sankar· JAkob Kuebler· Mykola Lugovy Received: 25 July 2007/Accepted: 3 July 2008/Published online: 27 July 2008 e Springer Science+Business Media, LLC 2008 Abstract A study of hot pressed B4C-based laminates, superior hardness, and high compressive strength values after rolling and without rolling, has been performed relative to metals [1-3. Ceramic laminates with strong elucidate the existence of fracture resistance/crack length interfaces, combined with excellent fracture toughness and anisotropy induced by this processing technique. While the damage tolerance, can potentially provide an even better crack lengths/fracture resistance was affected significantly ballistic performance than traditional single phase ceramics by the presence of the residual stresses in B.C/BC-ZrB 2 or particulate ceramic composites laminates, no differences in Vickers crack lengths were One of the most important lightweight body armor observed in B.C/B4C laminates prepared by rolling and hot materials is boron carbide-based ceramic composites [4, 5 pressing, as compared to the crack lengths seen in pure B4C Polycrystalline BC ceramics have high hardness in the ceramics prepared by hot pressing without rolling. X-ray range of 32-35 GPa [6-8], a high Youngs modulus in diffraction analysis confirmed that no texture has been the range of 430-450 GPa[9], and a bending strength in the formed during the rolling and hot pressing of B4C range of 400-600 MPa [10]. Room temperature isotropic ceramIcs elastic moduli of B.C show that its bulk, shear and Youngs moduli are substantially higher than those of most solids Consequently, B4C belongs to the so-called strong solids classification. Conversely, BC ceramics have a relatively Ceramics offer a number of attractive properties, including toughness(up to 8.2 MPa me 1 pa m/2[11].There is a Introduction low fracture toughness of 28-33 M possibility of a significant increase in the apparent fracture or higher) via design of high specific stiffness, high specific strength, low thermal residual stresses in multilayered B C-based composites[ 12, conductivities, and chemical inertness in many environ- 13]. Non-linear stress-strain behavior of B. C/B C laminates ments. Ceramics and ceramic composites are attractive with weak interfaces was reported earlier in [14]. A decrease materials for use in armor systems due to low density, in brittleness has great potential for the realization of improved armor material system N. Orlovskaya(凶) It was recently reported that B C single crystals exhibit of Central Florida. Orlando, FL. USA a strong anisotropy of the elastic constants [15]. The elastic e-mail:norlovsk@mail.ucfedu;nil2903@gmail.com constants for B4C single crystals were determined in the coordinate system where xi was parallel to [10101 S. Yarmolenko. J. Sankar North Carolina A&T State University, Greensboro, NC, USA crystallographic direction, x2 was parallel to [12101 tallographic direction, and x3 was parall J. Kuebler crystallographic direction. The reported values of room Material Science and Technology, EMPA, Duebendorf, temperature elastic constants for BC single crystals are cI1=542.81,c33=534.54,c13=63.51,c12=130.59, and c44=164.79 GPa. Accordingly, the maximum ratio of ute for Problems of Materials Science, Kyiv, Ukraine elastic constants cu and ci3 is equal to 8.55. Based on the 2 Springer
Effects of rolling and hot pressing on mechanical properties of boron carbide-based ceramics Nina Orlovskaya Æ Sergey Yarmolenko Æ Jag Sankar Æ Jakob Kuebler Æ Mykola Lugovy Received: 25 July 2007 / Accepted: 3 July 2008 / Published online: 27 July 2008 Springer Science+Business Media, LLC 2008 Abstract A study of hot pressed B4C-based laminates, after rolling and without rolling, has been performed to elucidate the existence of fracture resistance/crack length anisotropy induced by this processing technique. While the crack lengths/fracture resistance was affected significantly by the presence of the residual stresses in B4C/B4C–ZrB2 laminates, no differences in Vickers crack lengths were observed in B4C/B4C laminates prepared by rolling and hot pressing, as compared to the crack lengths seen in pure B4C ceramics prepared by hot pressing without rolling. X-ray diffraction analysis confirmed that no texture has been formed during the rolling and hot pressing of B4C ceramics. Introduction Ceramics offer a number of attractive properties, including high specific stiffness, high specific strength, low thermal conductivities, and chemical inertness in many environments. Ceramics and ceramic composites are attractive materials for use in armor systems due to low density, superior hardness, and high compressive strength values relative to metals [1–3]. Ceramic laminates with strong interfaces, combined with excellent fracture toughness and damage tolerance, can potentially provide an even better ballistic performance than traditional single phase ceramics or particulate ceramic composites. One of the most important lightweight body armor materials is boron carbide-based ceramic composites [4, 5 Polycrystalline B4C ceramics have high hardness in the range of 32–35 GPa [6–8], a high Young’s modulus in the range of 430–450 GPa [9], and a bending strength in the range of 400–600 MPa [10]. Room temperature isotropic elastic moduli of B4C show that its bulk, shear and Young’s moduli are substantially higher than those of most solids. Consequently, B4C belongs to the so-called strong solids classification. Conversely, B4C ceramics have a relatively low fracture toughness of 2.8–3.3 MPa m1/2 [11]. There is a possibility of a significant increase in the apparent fracture toughness (up to 8.2 MPa m1/2 or higher) via design of residual stresses in multilayered B4C-based composites [12, 13]. Non-linear stress–strain behavior of B4C/ B4C laminates with weak interfaces was reported earlier in [14]. A decrease in brittleness has great potential for the realization of improved armor material systems. It was recently reported that B4C single crystals exhibit a strong anisotropy of the elastic constants [15]. The elastic constants for B4C single crystals were determined in the coordinate system where x1 was parallel to ½1010 crystallographic direction, x2 was parallel to ½1210 crystallographic direction, and x3 was parallel to [0001] crystallographic direction. The reported values of room temperature elastic constants for B4C single crystals are c11 = 542.81, c33 = 534.54, c13 = 63.51, c12 = 130.59, and c44 = 164.79 GPa. Accordingly, the maximum ratio of elastic constants c11 and c13 is equal to 8.55. Based on the N. Orlovskaya (&) University of Central Florida, Orlando, FL, USA e-mail: norlovsk@mail.ucf.edu; nil2903@gmail.com S. Yarmolenko J. Sankar North Carolina A&T State University, Greensboro, NC, USA J. Kuebler Material Science and Technology, EMPA, Duebendorf, Switzerland M. Lugovy Institute for Problems of Materials Science, Kyiv, Ukraine 123 J Mater Sci (2008) 43:5942–5947 DOI 10.1007/s10853-008-2862-0
j Mater sci(2008)43:5942-5947 5943 nalysis of Cauchys relationships, Poissons ratios, and hot pressing temperature of 2170C, a pressure of 30 MPa, elastic anisotropic factors for the single crystal elastic and a dwell time of 1 h. As a result, dense laminate sam- constants, it was concluded that B4C is more anisotropic in ples(98-99% relative density) were obtained and further Anisotropy of elastic properties, if detected, should result bending experiments. During hot pressing of the e elasticity and interatomic bonding than st solids. machined into 4.5x5x 45 mm' bars for four-poin in differences in the mechanical properties of boron carbide shrinkage of the individual layers occurred, he ceramics and can significantly affect the ballistic perfor- resulting thicknesses were about 0. 15 mm post-pressing mance of the material. The ballistic performance depends The interfaces between individual layers of the same on the hardness and elastic modulus of the material, composition completely disappeared during hot pressing therefore materials with low elastic modulus will under- The main manufacturing steps for B4C ceramics without perform relative to the materials with high elastic modulus. rolling included grinding of raw powders and hot pressing In this way anisotropy can significantly affect the ballistic The hot pressing conditions were the same as previously performance and other mechanical properties described for the laminates It is important to know if anisotropy can be introduced Standard X-ray diffraction technique has been used to manufacturing procedures, such as rolling or hot pressing. if any texture appears as a result of processing steps l ate into the polycrystalline B4C composites via different evaluate the phase composition of the materials and estin It was reported that rolling and hot pressing induced texture fracture resistance was measured by an indentation in Si3N4 laminates, which resulted in anisotropy in the technique. Indent locations on the sample surface are fracture toughness values [16]. It is known that the contact shown in Fig. la. It follows from Fig. la that a top face is pressure in the deformation zone during rolling can be very perpendicular to the hot pressing direction and parallel to high; therefore, the texturing of the polycrystalline BC can the rolling direction, while a side face is parallel to the occur during rolling. If texturing occurs with the prefer- hot pressing direction. The indentation of surfaces was ential alignment in the direction, which has the lowest c13 done in a way to produce cracks in planes parallel and elastic constant, it will dramatically affect both ballistic perpendicular to the hot pressing direction. Twenty performance and fracture toughness of the material. have been made on the faces of the B4C/BC The goal of the present research is to determine if there laminates and the BC ceramics without rolling. Care was any anisotropy in the fracture resistance of B. C ceramics taken to place the impressions far enough away from each introduced during manufacturing processes such as rolling other to ensure no interactions occurred between cracks Ind hot pressing. To achieve this goal, both hot pressed generated from the corners of each Vickers impressions BC ceramics without rolling and B. C/B4C laminates Spatial resolution of the microhardness tester was about prepared by rolling and hot pressing without thermal I um, which creates an uncertainty of about 10%o for the residual stresses were investigated. For comparison B4C/ hardness value. Digital images of micrographs obtained c-30 wt %ZrB 2 with thermal residual stress and B4 C- from an optical microscope at a magnification of 100x 30 wt%SiC/B4C-30 wt%Sic laminates without thermal were used for calculations. Measurement accuracy was esidual stresses have been also tested about 0. 12 um resulting in 0.5% accuracy of measured fracture resistance and hardness values. Hardness H [GPa was calculated using following equatio Experimental 1854P H Three types of the laminates, (1)BC/B C,(2)B4C- 30 wt%SiC/BC-30 wt%SiC, and (3) B4C/B,C-30 wt% where P is the indentation load (N) and d is the diagonal ZrB2, have been produced using the techniques described (um) of a Vickers indent [19]. The fracture resistance was in detail elsewhere [17]. In the case of the first two com calculated as composition For the first laminate, it is pure b, c com- K=0.016(5)a2 position, for the second laminate it is B4C-30 wt%SiC composition. In the case of the third laminate the layers where E is the Youngs modulus, H is the hardness, and c is were made using two compositions--pure BAC and B4 C- the crack length from the center of impression to the crack 30 wt%ZrB2. The main manufacturing steps included end [20] grinding of raw powders, plasticizing ground powders with Elastic modulus and strength were determined from load a crude rubber, rolling of ceramic tapes, stacking tapes displacement plots that were recorded together, and hot pressing of laminates. The hot pressing ing tests. Displacement was measured using a special three conditions were as follows: a heating rate of 100oC/min a point deflection gauge. Specimens were loaded parallel and 2 Springer
analysis of Cauchy’s relationships, Poisson’s ratios, and elastic anisotropic factors for the single crystal elastic constants, it was concluded that B4C is more anisotropic in elasticity and interatomic bonding than most solids. Anisotropy of elastic properties, if detected, should result in differences in the mechanical properties of boron carbide ceramics and can significantly affect the ballistic performance of the material. The ballistic performance depends on the hardness and elastic modulus of the material, therefore materials with low elastic modulus will underperform relative to the materials with high elastic modulus. In this way anisotropy can significantly affect the ballistic performance and other mechanical properties. It is important to know if anisotropy can be introduced into the polycrystalline B4C composites via different manufacturing procedures, such as rolling or hot pressing. It was reported that rolling and hot pressing induced texture in Si3N4 laminates, which resulted in anisotropy in the fracture toughness values [16]. It is known that the contact pressure in the deformation zone during rolling can be very high; therefore, the texturing of the polycrystalline B4C can occur during rolling. If texturing occurs with the preferential alignment in the direction, which has the lowest c13 elastic constant, it will dramatically affect both ballistic performance and fracture toughness of the material. The goal of the present research is to determine if there is any anisotropy in the fracture resistance of B4C ceramics introduced during manufacturing processes such as rolling and hot pressing. To achieve this goal, both hot pressed B4C ceramics without rolling and B4C/B4C laminates prepared by rolling and hot pressing without thermal residual stresses were investigated. For comparison B4C/ B4C–30 wt%ZrB2 with thermal residual stress and B4C– 30 wt%SiC/B4C–30 wt%SiC laminates without thermal residual stresses have been also tested. Experimental Three types of the laminates, (1) B4C/B4C, (2) B4C– 30 wt%SiC/B4C–30 wt%SiC, and (3) B4C/B4C–30 wt% ZrB2, have been produced using the techniques described in detail elsewhere [17]. In the case of the first two compositions, all layers have been prepared with the same composition. For the first laminate, it is pure B4C composition, for the second laminate it is B4C–30 wt%SiC composition. In the case of the third laminate the layers were made using two compositions—pure B4C and B4C– 30 wt%ZrB2. The main manufacturing steps included grinding of raw powders, plasticizing ground powders with a crude rubber, rolling of ceramic tapes, stacking tapes together, and hot pressing of laminates. The hot pressing conditions were as follows: a heating rate of 100 C/min, a hot pressing temperature of 2170 C, a pressure of 30 MPa, and a dwell time of 1 h. As a result, dense laminate samples (98–99% relative density) were obtained and further machined into 4.5 9 5 9 45 mm3 bars for four-point bending experiments. During hot pressing of the laminates, shrinkage of the individual layers occurred, and the resulting thicknesses were about 0.15 mm post-pressing. The interfaces between individual layers of the same composition completely disappeared during hot pressing. The main manufacturing steps for B4C ceramics without rolling included grinding of raw powders and hot pressing. The hot pressing conditions were the same as previously described for the laminates. Standard X-ray diffraction technique has been used to evaluate the phase composition of the materials and estimate if any texture appears as a result of processing steps [18]. Fracture resistance was measured by an indentation technique. Indent locations on the sample surface are shown in Fig. 1a. It follows from Fig. 1a that a top face is perpendicular to the hot pressing direction and parallel to the rolling direction, while a side face is parallel to the hot pressing direction. The indentation of surfaces was done in a way to produce cracks in planes parallel and perpendicular to the hot pressing direction. Twenty impressions have been made on the faces of the B4C/ B4C laminates and the B4C ceramics without rolling. Care was taken to place the impressions far enough away from each other to ensure no interactions occurred between cracks generated from the corners of each Vickers impressions. Spatial resolution of the microhardness tester was about 1 lm, which creates an uncertainty of about 10% for the hardness value. Digital images of micrographs obtained from an optical microscope at a magnification of 1009 were used for calculations. Measurement accuracy was about 0.12 lm resulting in 0.5% accuracy of measured fracture resistance and hardness values. Hardness H [GPa] was calculated using following equation H ¼ 1854P d2 ; ð1Þ where P is the indentation load (N) and d is the diagonal (lm) of a Vickers indent [19]. The fracture resistance was calculated as KR ¼ 0:016 E H 1=2 P c3=2 ; ð2Þ where E is the Young’s modulus, H is the hardness, and c is the crack length from the center of impression to the crack end [20]. Elastic modulus and strength were determined from load displacement plots that were recorded in four-point bending tests. Displacement was measured using a special threepoint deflection gauge. Specimens were loaded parallel and J Mater Sci (2008) 43:5942–5947 5943 123
5944 J Mater Sci(2008)43:5942-5947 Hot pressing direction perpendicular to the rolling plane to reveal possible dif- ferences in mechanical properties(Fig. 1b, c). The strength Rolling and elastic modulus were then determined using standard procedure EN 843-1/EN 843-2 [21, 22]. Fracture surfaces of B4 C/B C laminates fractured parallel and perpendicular Crack to the hot pressing direction were investigated by scanning electron microscopy. Side face mall face Results and discussion Fracture is parallel to the hot Indentation crack length anisotropy was observed in B4C/ B4C-30 wt %ZrB 2 laminates manufactured by rollin hot pressing techniques. The micrograph of the Vickers impression placed in the center of thin B4C layer is shown in Fig. 2. It is possible to see differences in the crack lengths originating from the corners of the Vickers impression, and extending in directions parallel and perpendicular to the interface. As a result, a significant difference in calculated BC layers after fracture resistance values(5.8+0.9 MPa m for cracks (b) perpendicular to the interface and parallel to the hot allel to the interface and perpendicular to the hot pressing Fracture is perpendicular to direction), using the same elastic modulus value for the the hot pressing direction calculations in both directions, is reported. Crack anisot- ropy can be caused by the existence of thermal residual stresses introduced during the cooling of the laminate due to the mismatch in the coefficients of thermal expansion of B.C and B4C-30 wt%ZrB2 layers. However, the rolling ar hot pressing can also introduce elastic anisotropy and, as result, fracture resistance anisotropy in the B4C ceramics These processes will also affect the crack length of the materials due to the differences in the stiffness and fracture resistance of the B,c in different directions. Fig. 1 A schematic presentation of the indent locations on the sample To determine what other parameters, besides thermal (a)and the sample orientation for four point bending tests(b and c) residual stresses, can affect the crack length of Vickers side face of the B4 C/B.C-ZrB Cracks perpendicular to layer Cracks parallel to layer interfaces laminate. Note the difference in interfaces and parallel to the hot and perpendicular to the hot the length of vickers cracks pressing direction parallel and perpendicular to the 2 Springer
perpendicular to the rolling plane to reveal possible differences in mechanical properties (Fig. 1b, c). The strength and elastic modulus were then determined using standard procedure EN 843-1/EN 843-2 [21, 22]. Fracture surfaces of B4C/B4C laminates fractured parallel and perpendicular to the hot pressing direction were investigated by scanning electron microscopy. Results and discussion Indentation crack length anisotropy was observed in B4C/ B4C–30 wt%ZrB2 laminates manufactured by rolling and hot pressing techniques. The micrograph of the Vickers impression placed in the center of thin B4C layer is shown in Fig. 2. It is possible to see differences in the crack lengths originating from the corners of the Vickers impression, and extending in directions parallel and perpendicular to the interface. As a result, a significant difference in calculated fracture resistance values (5.8 ± 0.9 MPa m1/2 for cracks perpendicular to the interface and parallel to the hot pressing direction, and 2 ± 0.7 MPa m1/2 for cracks parallel to the interface and perpendicular to the hot pressing direction), using the same elastic modulus value for the calculations in both directions, is reported. Crack anisotropy can be caused by the existence of thermal residual stresses introduced during the cooling of the laminate due to the mismatch in the coefficients of thermal expansion of B4C and B4C–30 wt%ZrB2 layers. However, the rolling and hot pressing can also introduce elastic anisotropy and, as result, fracture resistance anisotropy in the B4C ceramics. These processes will also affect the crack length of the materials due to the differences in the stiffness and fracture resistance of the B4C in different directions. To determine what other parameters, besides thermal residual stresses, can affect the crack length of Vickers Hot pressing direction Top face Small face B plane C plane Crack C C C C A A A A B B B B Rolling direction A plane (a) Side face B plane C plane Indent k C C C C A A A A B B B B A plane pressing direction Fracture is perpendicular to the hot pressing direction (c) (b) B4C layers after rolling Fracture is parallel to the hot Fig. 1 A schematic presentation of the indent locations on the sample (a) and the sample orientation for four point bending tests (b and c) Cracks parallel to layer interfaces and perpendicular to the hot pressing direction Cracks perpendicular to layer interfaces and parallel to the hot pressing direction 60 microns 200 microns Fig. 2 Indentation cracks on a side face of the B4C/B4C–ZrB2 laminate. Note the difference in the length of Vickers cracks parallel and perpendicular to the layer interfaces 5944 J Mater Sci (2008) 43:5942–5947 123
j Mater sci(2008)43:5942-5947 Fig 3 Indentation cracks on the side face of rolled B,C/B, c Crack laminate without compositional perpendicular to hot gradient. No difference in the rack length after indentation was noticed impressions, indentation tests were performed on the hot surface (Fig 1b) the Youngs modulus was equal to pressed BC ceramics prepared both with and without 442+ 19 GPa(presents the standard deviation of the rolling. The typical Vickers impression on the side surface measurements). In the second case when the fracture of rolled B4C/ B, C laminate without compositional gradient started from the side surface(Fig. lc)the Youngs modu is shown in Fig 3. It is possible to see in Fig 3 no signif- lus was equal to 486+ 35 GPa. The strength values for icant anisotropy of the crack length was observed in the both cases were 529+ 38 MPa and 478+ 46 MPa, laminates. In other words, no thermal residual stresses were respectively present after cooling the hot pressed sample from 2473 K to To verify that the mechanical properties do not depend room temperature. Hardness and fracture toughness mea- on the hot pressing and rolling directions, the four-point surements performed on the three different surfaces of the bending tests were also performed on different B4C hot pressed and rolled samples are summarized in Table 1. 30 wt %SiC/B40-30 wt%SiC laminates. In the case of As follows from Fig I and Table 1, the impression placed loading parallel to the hot pressing direction(Fig. 1b) the top face produced cracks in planes only parallel to elastic modulus was 443 +8 GPa and in the case of hot pressing direction, while the impression placed into loading perpendicular to the hot pressing direction the side surface generated cracks in planes parallel and (Fig. Ic)elastic modulus was equal to 441+ 17 GPa. The perpendicular to the hot pressing direction. Therefore, there strength values for both cases were measured to be are indentation cracks on different faces produced in a 469+96 MPa and 548+ 61 MPa, respectively. parallel plane, but no significant difference in fra acture The fractured surfaces of B.C/B, C laminates which resistance was detected for any of the cracks. Both fracture fractured perpendicular and parallel to the hot pressing resistance and hardness of B.C ceramics and BC/ B4c direction are shown in Fig. 4a and b In both B.C/BC and laminate have very similar values to those reported in the B C-30 wt %SiC/B4C-30 wt%SiC laminates it was not literature [7,9, 111 possible to detect interfaces between rolled layers after hot The Youngs modulus of B.C/ B4C laminates was pressing. A fully transgranular fracture mode was observed laminates exhibited ordinary linear deformation behavior. axial shape(Fig. 4c). The grale o srains retaining the equi- measured using the four-point bending technique. The in both cases, with most of the The laminate ceramics were loaded in two different ways. 3-6 um. The majority of the grains had a smooth fracture In the first case, when the fracture started from the top surface without any cleavage. However, certain grains did Table 1 fracture toughness in different planes of B4C/BC Sample Hardness Fracture toughness( MPa m) Plane A Plane c hot pressing to hot pressing hot pressing direction) direction) Not rolled Top face B 31±2 2.6士0.2 3±04 2.6士0.3 2.7士0.2 Small face C 32士2 2.5士0.3 2.6±0.5 Rolled Top face B 3士0.7 3.3士1 Side face a 29士2 2.5士04 2.8±0.3 Small face C 29士2 2.5±0.5 24士0.3 2 Springer
impressions, indentation tests were performed on the hot pressed B4C ceramics prepared both with and without rolling. The typical Vickers impression on the side surface of rolled B4C/ B4C laminate without compositional gradient is shown in Fig. 3. It is possible to see in Fig. 3 no significant anisotropy of the crack length was observed in the laminates. In other words, no thermal residual stresses were present after cooling the hot pressed sample from 2473 K to room temperature. Hardness and fracture toughness measurements performed on the three different surfaces of the hot pressed and rolled samples are summarized in Table 1. As follows from Fig. 1 and Table 1, the impression placed into the top face produced cracks in planes only parallel to the hot pressing direction, while the impression placed into the side surface generated cracks in planes parallel and perpendicular to the hot pressing direction. Therefore, there are indentation cracks on different faces produced in a parallel plane, but no significant difference in fracture resistance was detected for any of the cracks. Both fracture resistance and hardness of B4C ceramics and B4C/ B4C laminate have very similar values to those reported in the literature [7, 9, 11]. The Young’s modulus of B4C/ B4C laminates was measured using the four-point bending technique. The laminates exhibited ordinary linear deformation behavior. The laminate ceramics were loaded in two different ways. In the first case, when the fracture started from the top surface (Fig. 1b) the Young’s modulus was equal to 442 ± 19 GPa (±presents the standard deviation of the measurements). In the second case when the fracture started from the side surface (Fig. 1c) the Young’s modulus was equal to 486 ± 35 GPa. The strength values for both cases were 529 ± 38 MPa and 478 ± 46 MPa, respectively. To verify that the mechanical properties do not depend on the hot pressing and rolling directions, the four-point bending tests were also performed on different B4C– 30 wt%SiC/B4C–30 wt%SiC laminates. In the case of loading parallel to the hot pressing direction (Fig. 1b) elastic modulus was 443 ± 8 GPa and in the case of loading perpendicular to the hot pressing direction (Fig. 1c) elastic modulus was equal to 441 ± 17 GPa. The strength values for both cases were measured to be 469 ± 96 MPa and 548 ± 61 MPa, respectively. The fractured surfaces of B4C/B4C laminates which fractured perpendicular and parallel to the hot pressing direction are shown in Fig. 4a and b. In both B4C/B4C and B4C–30 wt%SiC/B4C–30 wt%SiC laminates it was not possible to detect interfaces between rolled layers after hot pressing. A fully transgranular fracture mode was observed in both cases, with most of the grains retaining the equiaxial shape (Fig. 4c). The grain size was in the range of 3–6 lm. The majority of the grains had a smooth fracture surface without any cleavage. However, certain grains did Crack parallel to hot pressing direction Crack perpendicular to hot pressing direction 40 microns Fig. 3 Indentation cracks on the side face of rolled B4C/B4C laminate without compositional gradient. No difference in the crack length after indentation was noticed Table 1 Fracture toughness in different planes of B4C/B4C laminate sample Sample Indent location Hardness (GPa) Fracture toughness ( MPa m1/2) Plane A (parallel to hot pressing direction) Plane B (perpendicular to hot pressing direction) Plane C (parallel to hot pressing direction) Not rolled Top face B 31 ± 2 2.6 ± 0.2 3 ± 0.4 Side face A 31 ± 2 2.6 ± 0.3 2.7 ± 0.2 Small face C 32 ± 2 2.5 ± 0.3 2.6 ± 0.5 Rolled Top face B 29 ± 1 3 ± 0.7 3.3 ± 1 Side face A 29 ± 2 2.5 ± 0.4 2.8 ± 0.3 Small face C 29 ± 2 2.5 ± 0.5 2.4 ± 0.3 J Mater Sci (2008) 43:5942–5947 5945 123
5946 J Mater Sci(2008)43:5942-5947 Fig 4 Fracture surfaces of B,C/B,C laminates fractured parallel and perpendicular to the hot pressing direction.(a)a sample fractured parallel to the hot pressing direction;(b)a sample fractured perpendic a fracture surface of B,C/B,c nates:(d)a fracture surface showing a twinned BC grain ●B4C exhibit cleavage steps and in some grains twinning was Iso detected(Fig. 4d). A small amount of intergranular porosity was present in the triple points of the grain boundary(Fig. 4c). Some insignificant amount of closed porosity within the BC grains has also been observed. XRD patterns of B4 C/BC laminates, taken from the top and side surfaces of a bending bar, are presented in Fig. 5, and they correspond to the rhombohedral B4C structure published in other works [23, 24]. A small amount of C impurity was also detected. One can see there is no remarkable difference between the patterns, which indi 40 80 cates that no crystallographic texture is present in the samples. This serves as an additional evidence for the absence of anisotropy in mechanical properties [23] No crack length anisotropy was found in rolled B4C/B4C ind B4C-30 wt%SiC/ BC-30 wt%SiC layered compos- ites without a compositional gradient. Further, XRD analysis does not show crystallographic texture formation in B. C/B4C laminates after rolling and hot pressing Therefore, it is possible to conclude that the Vickers crack Io 20 30 40 50 60 70 80 length anisotropy in B.C/B C-30 wt%ZrB2 is a result of 2 Theta the thermal residual stresses originated during the cooling Fig 5 XRD Patterns of rolled B. C/B4C laminates taken from the top of the composite due to differences in the thermal expan- (a) and side (b) faces of a bending bar sion coefficients between the layers of the laminate. 2 Springer
exhibit cleavage steps and in some grains twinning was also detected (Fig. 4d). A small amount of intergranular porosity was present in the triple points of the grain boundary (Fig. 4c). Some insignificant amount of closed porosity within the B4C grains has also been observed. XRD patterns of B4C/B4C laminates, taken from the top and side surfaces of a bending bar, are presented in Fig. 5, and they correspond to the rhombohedral B4C structure published in other works [23, 24]. A small amount of C impurity was also detected. One can see there is no remarkable difference between the patterns, which indicates that no crystallographic texture is present in the samples. This serves as an additional evidence for the absence of anisotropy in mechanical properties [23]. Conclusions No crack length anisotropy was found in rolled B4C/B4C and B4C–30 wt%SiC/ B4C–30 wt%SiC layered composites without a compositional gradient. Further, XRD analysis does not show crystallographic texture formation in B4C/B4C laminates after rolling and hot pressing. Therefore, it is possible to conclude that the Vickers crack length anisotropy in B4C/B4C–30 wt%ZrB2 is a result of the thermal residual stresses originated during the cooling of the composite due to differences in the thermal expansion coefficients between the layers of the laminate. Fig. 4 Fracture surfaces of B4C/B4C laminates fractured parallel and perpendicular to the hot pressing direction. (a) a sample fractured parallel to the hot pressing direction; (b) a sample fractured perpendicular to the hot pressing direction; (c) a fracture surface of B4C/B4C laminates; (d) a fracture surface showing a twinned B4C grain 10 20 30 40 50 60 70 80 Intensity, a.u 10 20 30 40 50 60 80 2 Theta,° 2 Theta,° Intensity, a.u. (a) (b) 70 B4C Carbon inclusion Fig. 5 XRD patterns of rolled B4C/B4C laminates taken from the top (a) and side (b) faces of a bending bar 5946 J Mater Sci (2008) 43:5942–5947 123
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Acknowledgements This work was supported by NSF project 0748364 ‘‘CAREER: Hard and tough boron rich ceramic laminates designed to contain thermal residual stresses,’’ the European Commission INCO-Copernicus Grant ICA2-CT-2000-10020 ‘‘LAMINATES,’’ Swiss Federal Office for Education and Science Grant BBW 99.0785, AFOSR, the project # F49620-02-0340, and NATO Collaborative Linkage Grant ‘‘Layered ceramic sensors for biological and chemical detection.’’ References 1. Johnson GR, Holmquist TJ (1999) J Appl Phys 85:8060. doi: 10.1063/1.370643 2. Orphal DL, Franzen RR, Charters AC, Menna TL, Piekutowski AJ (1997) Int J Impact Eng 19:15. doi:10.1016/S0734-743X(96) 00004-8 3. Chen M, Mccauley JW, Hemker KJ (2003) Science 299:1563. doi:10.1126/science.1080819 4. Bourne NK (2002) In: Proceedings of the Royal Society of London series A—mathematical, physical and engineering sciences, 458(2024), p 1999 5. Vogler TJ, Reinhart WD, Chabildas LC (2004) J Appl Phys 95:4173. doi:10.1063/1.1686902 6. Niihara K, Nakahira A, Hirai T (1984) J Am Ceram Soc 67:13 7. Champagne B, Angers R (1979) J Am Ceram Soc 62:149. doi: 10.1111/j.1151-2916.1979.tb19042.x 8. Yamada S, Sakaguchi S, Hirao K, Yamauchi Y, Kanzaki S (2003) J Ceram Soc Jpn 111:53. doi:10.2109/jcersj.111.53 9. Thevenot F (1990) J Eur Ceram Soc 6:205. doi:10.1016/0955- 2219(90)90048-K 10. Abzianidze TG, Eristavi AM, Shalamberidze SO (2000) J Solid State Chem 154:191. doi:10.1006/jssc.2000.8834 11. Lee H, Speyer R (2002) J Am Ceram Soc 85:1291 12. Orlovskaya N, Lugovy M, Ko F, Yarmolenko S, Sankar J, Kuebler J (2006) Compos B 37:524. doi:10.1016/j.compositesb. 2006.02.022 13. Orlovskaya N, Lugovy M, Kuebler J, Mechanical performance of 3 layered B4C/B4C-SiC laminates (in preparation) 14. Tariolle S, Reynaud C, Thevenot F, Chartier T, Besso JL (2004) J Solid State Chem 177:487. doi:10.1016/j.jssc.2003.02.007 15. Mcclellan KJ, Chu F, Roper JM, Shindo I (2001) J Math Sci 36:3403. doi:10.1023/A:1017947625784 16. Radchenko A, Subbotin V, Lugovy M, Orlovskaya N, Fracture toughness anisotropy of hot pressed silicon nitride (in preparation) 17. Orlovskaya N, Lugovy M, Subbotin V, Radchenko O, Adams J, Cheda M et al (2005) J Math Sci 40:5483. doi:10.1007/s10853- 005-1923-x 18. Kakazey M, Vlasova M, Gonzalez-Rodriguez JG, DominguezPatino M, Leder R (2006) Mater Sci Eng A 418:111. doi: 10.1016/j.msea.2005.11.018 19. Tabor D (1951) Hardness of metals. Clarendon Press, Oxford 20. Antis GR, Chantikol P, Lawn BR, Marshall DB (1981) J Am Ceram Soc 64:533. doi:10.1111/j.1151-2916.1981.tb10320.x 21. European Standard EN 843-1, Advanced technical ceramics— Mechanical properties of monolithic ceramics at room temperature—Part 1: determination of flexural strength, December 2006 22. European Standard EN 843-2, Advanced technical ceramics— Mechanical properties of monolithic ceramics at room temperature—Part 2: determination Young’s modulus, shear strength and Poisson’s ratio, December 2006 23. Anselmi-Tamburini U, Ohyanagi M, Munir ZA (2004) Chem Mater 16:4347. doi:10.1021/cm049195q 24. Lemis-Petropoulos P, Kapaklis V, Peikrishvili AB, Politis C (2003) J Mod Phys B 17:2781. doi:10.1142/S0217979203018582 J Mater Sci (2008) 43:5942–5947 5947 123
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