EFFECTIVE DENSIFICATION OF B4C-SiC COMPOSITES WITH Al2O3-Y2O3 ADDITIVE: FRACTURE TOUGHNESS, WEAR AND BALLISTIC BEHAVIOUR

Abstract

Boron carbide (B4C) ceramic stands out as a primary candidate for ballistic and wearresistant applications due to its attractive combination of properties such as high hardness, low density, and chemical stability. However, challenges in sintering dense B4C and its relatively low fracture toughness hinder the widespread use of monolithic B4C. Silicon carbide (SiC) offers superior high temperature strength and oxidation resistance compared to B4C. While exceptional properties of B4C-SiC composites position them as potential candidates for various structural applications, practical utility remains constrained. Preparation of B4C-SiC composites needs high sintering temperature due to predominant covalent bonding, and tendency to develop oxide rich layer on the particle surface. Various additives are helpful in sintering dense B4C-SiC composites at lower temperature, and enhancing fracture toughness. Oxide additives are particularly preferred for sintering of B4C-SiC composites due to the formation of liquid phase at lower temperatures. Al2O3-Y2O3 additive, form eutectic with surface oxides such as SiO2 or B2O3 at lower temperatures, and facilitate densification. Despite the significance, only limited literature exists on the effect of oxide additives on densification, phase evolution, microstructural development, and mechanical properties of B4C-SiC composites. Additionally, there is a scarcity of studies investigating the effect of oxide additives on microstructure, mechanical properties and tribological behaviour of B4C-SiC composites. Furthermore, the effect of SiC and Al2O3 additive on the ballistic performance of B4C-SiC composite is not yet reported. In the initial part of the study, powders of B4C, 10wt% SiC, (0wt%, 1.5wt%, 3wt%, or 6wt%) Al2O3 additives were spark plasma sintered at 1600°C-1800°C for 10 min under 40 MPa pressure in argon atmosphere. When sintered at 1600°C, addition of 6wt% Al2O3 additive improved density to 99.6% due to the formation of Al2SiO5 liquid phase. Transmission electron microscopy (TEM) analysis of the composites revealed clean boundaries between B4C and SiC grains and liquid phase precipitates at the grain boundary junctions. The addition of Al2O3 marginally affected the hardness of the composites but significantly augmented the fracture toughness due to dominant crack deflection and crack bridging mechanisms. In view of the possible application of the B4C-SiC composites in wear conditions, the subsequent part of the study investigated the sliding wear behaviour of B4C-SiC composites as a function of Al2O3 additive content (0 to 6wt%) and sliding load (5 to 15 N). Wear volume exhibited a direct relation with load and an inverse relation with Al2O3 content. Despite the variation in Al2O3 content, the wear mechanisms of brittle fracture, pull-outs, and grooving remain the same. The third part of the study investigates the effect of Al2O3-Y2O3 additive composition and sintering temperature on the densification, microstructure, and mechanical properties of B4C-SiC composites. A relative density of ~98% is obtained with Al2O3-Y2O3 additive at 1600°C due to the presence of liquid phase. Optimum hardness and fracture toughness obtained with only Al2O3 additives, compared to the mixed (Al2O3-Y2O3) additives or only Y2O3 additive, owing to alteration in the composition of grain boundary phase. TEM analysis corroborated presence of liquid phase precipitates and clean rain boundaries between B4C and SiC grains. In the fourth part of the study, influence of Al2O3-Y2O3 additive composition and applied load on the sliding wear behaviour of B4C-SiC composites was examined. Wear volume is high for composite sintered with Y2O3 additive due to the presence of variety of liquid phase precipitates as possible stress risers in the microstructures. As B4C ceramics are preferred for armour protection system, an attempt is made in the last part of the investigation to understand the effect of SiC and Al2O3 addition on the ballistic performance of B4C ceramics against WC-Co projectile. The addition of SiC resulted in a reduction of both depth of penetration in the aluminum alloy backing plate and residual velocity of the projectile. This effect can be attributed to the increased absorption of ballistic energy with addition of SiC. Conversely, the addition of Al2O3 led to the weak bonding of liquid phase precipitates with B4C or SiC grains. Effect of composition change on the fracture mechanisms is also studied. Overall, the experimental findings from the present doctoral investigation demonstrate successful fabrication of dense (~98%) B4C-SiC composites at the lowest sintering temperature of 1600°C with the help of Al2O3-Y2O3 additive. The composites sintered with Al2O3 additive at 1700°C exhibited exceptional fracture toughness of 6.4 MPa.m0.5 due to dominant crack deflection and crack bridging. The composite sintered with only Al2O3 additive exhibited the least wear volume. However, the composite sintered without Al2O3 additive outperformed others in the ballistic test. The results obtained from the present research study of B4C-SiC composite underscore the importance of the judicious selection of additive composition and sintering temperature as per the requirement in mechanical, wear or ballistic conditions.