Effects Of PCS And ZrO₂ On The Densification, Microstructure And Properties Of Hot Pressed Boron Carbide

As a promising material, boron carbide(B4C) has been used in light weight armor, abrasive materials, absorbent nuclear materials, atomic reactor control and nuclear shield materials as well as high temperature thermoelectric conversion due to its excellent properties such as high hardness(>30 GPa), high melting point(2450 °C), high neutron absorption cross-section and good chemical stability and thermoelectric properties. However, the widespread applications are restricted by the poor fracture toughness and poor sinterability. Dense boron carbide ceramics can be made by pressureless sintering at very high temperatures, usually beyond 2100 °C, even if using additives, and the strength and toughnes of ceramics are relavitely low. Hot-pressing is therefore an essential technique for obtaining dense and high performance boron carbide ceramics, but at present much attention are focused on the relationship between process, microstructure and properities of boron carbide. Unfortunately, scare publications had been covered as yet on the hot-pressing kinetics and densification mechanisms of boron carbide. The densification mechanisms of hot-pressed boron carbide and boron carbide with addtives are still not well understood.Based on general densification equation for hot pressing and pore-dragged creep model, the densification mechanisms and grain growth kinetics of commercial boron carbide were studied in the paper, and the effects of particle size on the densification of hot-pressed boron carbide also were investigated. On this basis, the effects of polycarbosilane(PCS) and Zr O2, which represents non-interface reacted additives and interface reacted additives respectively, on the densification mechansims of boron carbide were further studied, and the relationships between microstructure and properties of composites were also investigated deeply.(1) The densification processes, microstructure and mechanical properties of commercial boron carbide can be markly improved by enhancing pressure and temperature. At the initial stage of sintering, the rapid densification may be dominated by plastic flow and by grain boundary diffusion at the final stage. With densification proceeding, grain coarsening is unavoidable at the high temperatures due to pore-controlled boundary migration and ecaporation/condensation mechanisms when the relative density is beyond 93%. When dwell time is more than 40 min, further extending dwell time has no significant effect on the increasing of density, but facilitate grain growth. And reducing B4 C particle size can decearse the sintering viscosity and resistance, which is beneficial to the rapid completion of plastic flow, and reduce the dwell time greatly.(2) During sintering, the introduction of in-situ Si C derived from PCS can reduce the sintering activation energy of boron carbide and improve the densification. The densification may be dominant by grain boundary diffusion. The fine Si C grains derived from PCS are dispersed homogeneously in B4 C matrix, forming a kind of complicated intragranular/intergranular microstructure, which effectively inhibite the grain growth of B4 C and improve the fracture toughness greatly. The main toughening mechanisms of B4 C matrix by the in situ Si C grains are crack bridging at the wake of crack tip by micron-sized Si C grains and crack deflection induced by Si C nanoparticles due to residual tensike stress resulting from thermal expansion mismatch between B4 C and Si C nanoparticles. Besides, layer structure and defects in the composites also contributed to enhancing toughness due to partial absorption of crack propagation energy. In addition, the further addition of Si not only improved the densification process due to the formation of liquid phase Si, but also greatly enhanced the mechanical properties of B4 C ceramics. However, the added Si showed no significant effect on fracture toughness. The improvement in mechanical properties was attributed to the decrease of porosity and the elimination of free carbon derived from polycarbosilane and B4 C powder. The addition of Si improved the microstructure homogeneity and further refined the layer structure of in-situ Si C grains. The relative density, flexural strength, Vickers hardness, and fracture toughness of the sample prepared with 11.4 wt% Si respectively reached 99.2%, 390 MPa, 33.5 GPa, and 5.54 MPa·m1/2.(3) Zr O2 can initially react with B4 C at 1100 °C, and a higher temperature is needed to drive reaction. Zr O2 can be compoletely coverted to Zr B2 at 1400 °C. The introduction of Zr O2 strengthens the plastic flow deforation mechanism during the initial sintering stage, markedly enhance the densification rate and reduce the sintering activation energy, thus improveing the sintering behavior of B4 C significantly and promoting the densification. Meanwhile, the in-situ fine Zr B2 grains, evenly distributed between B4 C grains, improve the microstructure and mechanical properities of ceramics. The improvement in mechanical properties may be mainly attributed to the decrease of porosity and refinement of grain size. And the introduction of in-situ Zr B2 can change the fracture model of B4 C ceramics from single transgranular fracture to the mixed fracture containing transgranular and intergranular fracture, enhancing the fracture toughness of composites. The great improvement in toughness may be attributed to crack split crack deflection induced by residual tensike stress resulting from thermal expansion mismatch. Thus, the fracture toughness of composites can increase with the addition of Zr O2. When the amount of added Zr O2 was 15 wt%, the relative density, hardness, elastic modulus, strength and toughness of composite respectively reached 99.3%, 32.1GPa, 426.7 GPa, 517.6 MPa and 5.23 MPa·m1/2.