Study On Strengthening-toughening Technology And Mechanisms Of Reaction Bonded Boron Carbide Ceramic Composites

Reaction bonded boron carbide(RBBC)composites absorbed people's attention for the advantages of lower sintering temperature,simple process and near net shape sintering,etc.However,the usage of RBBC composites has been limited because of the lower mechanical properties.In this study,chopped carbon fibers and annealing treatment have been adopted to obtain RBBC composites with good mechanical properties.Based on the influence of infiltration temperature and annealing treatment on the mechanical properties of RBBC composites,the dependence of the microstructure,residual stress and mechanical properties was studied.The mechanisms of strengthening and toughening also have been discussed.This work is thus believed to have important theoretical significance and practical application value in the improvemrnt of the mechanical properties and enlarging the applicable area of the RBBC composites.The influence of chopped carbon fibers on the microstucture and mechanical properties of RBBC composites has been studied.The results indicated that,during the infiltration process,the original chopped carbon fibers,carbon black and B4C reacted with liquid silicon to form nemaline SiC,granular SiC and B12(B,C,Si)3,respectively.The infiltrated composites consisted of four phases,namely B4C,ternary B12(B,C,Si)3 compounds,SiC and residual Si.With the increase of infiltration temperatures,both the size of SiC and boron carbide increased.The mechanical properties of fiber toughening RBBC composites increased initially and then decreased with the increase in infiltration temperatures of which the optimal temperature was 1500?.The optimum bending strength,fracture toughness and Vikers hardness of the obtained RBBC composites are 392 MPa,4.9 MPa·m1/2 and 16 GPa,respectively.The fracture toughness of fiber toughening RBBC composites significantly increased from 2.6 MPa·m1/2 to 3.2 MPa·m1/2(23%),compared with the composites without chopped carbon fibers.The formed nemaline SiC can be responsible for the increase of the fracture toughness.The effect of annealing treatment on the microsstucture and mechanical properties of the RBBC composites without chopped carbon fibers has been studied.The results indicated that the annealing treatment played an important role in improving the mechanical properties of RBBC composites.The optimum bending strength,fracture toughness and Vikers-hardness of the RBBC composites annealed at 1000? for 10 h were 398 MPa,4 MPa-m1/2 and 19 GPa,which increased of 28%,7%and 3%respectively,compared with those of RBBC composites without annealing treatment.The residual tension stress of large and small size Si zones decreased from 1750 MPa and 900 MPa to 850 MPa and 25 MPa,respectively.The decrease of residual tension stress can be responsible for the increase of the mechanical properties of the RBBC composites.