Preparation And Properties Of B₄C/Al Composites

B₄C has high hardness,low density,excellent chemical stability and neutron absorption performance,but the bulk and dense B₄C usually needs to be prepared under high temperature and pressure by hot pressing,which is expensive to produce,and the low fracture toughness of B₄C inhibits its wide application.The study found that B₄C and metal or alloy composites were prepared.Metal composite materials can effectively solve the brittleness problem of B₄C.Due to aluminum（alloy）material light,good toughness and good chemical compatibility with B₄C,the research of B₄C/A1 composite material has attracted much attention.The study in this paper is mainly divided into two parts.In the first part,B₄C/Al composites are prepared by low pressure cold spraying technology.The effects of spraying distance,carrier gas pressure,powder particle size,B₄C powder content,spraying temperature and heat treatment temperature on the microstructure and properties of B₄C/Al composites are studied.In the process of preparing B₄C/Al composites by cold spraying,the hardness of B₄C/Al composites prepared by cold spraying is 69.6HV₅,72.1HV₅ and 68.3HV₅,and the bending strength is increased from 234.8MPa to 289.5 MPa with the increase of the particle size of B₄C powder from 13μm and 25μm to 48μm.Its fracture toughness is3.74～4.43MPa·m^1/2,and its fracture mode presents typical brittle fracture characteristics.When the particle size of B₄C powder is 25μm,the wear rate reaches the minimum3.72mm³/N·m.In the process of preparing B₄C/Al composites by cold spraying,the hardness of B₄C/Al composites by cold spraying increases from 68.3HV₅ to 77.0HV₅ with the increase of B₄C powder content from 50wt%and 60wt%to 70wt%.The bending strength increased from 289.6MPa to 315.8MPa.The fracture toughness is 4.43～4.56MPa·m^1/2,and the fracture mode is typical brittle fracture.In the process of preparing B₄C/Al composites by cold spraying,the hardness of B₄C/Al composites by cold spraying decreases from 72.4HV₅ to 69.0HV₅ with the increase of spraying distance from 10mm and 15mm to 20mm.Its bending strength decreased from 240.2MPa to 225.8MPa.The fracture toughness of the B₄C/Al composites is between 3.80MPa·m^1/2 and 3.91MPa·m^1/2,and the fracture mode presents typical brittle fracture characteristics.When the spraying distance increases from 15mm to 20mm,the wear rate of B₄C/Al composites increases from 3.72mm³/N·m to6.40mm³/N·m.The reason for these phenomena is that the increase of spraying distance leads to the decrease of particle velocity and kinetic energy,and finally leads to the loose structure of B₄C/Al composites.The hardness of B₄C/Al composites prepared by cold spraying increases from69.0HV₅ to 76.6HV₅ with the increase of carrier gas pressure from 0.6MPa and 0.8MPa to 1.0MPa.The bending strength increased from 257.7MPa and 236.9MPa to 287.5MPa.The fracture toughness is 3.89～4.21MPa·m^1/2,and the fracture mode is typical brittle fracture.The wear rate of B₄C/Al composites increases from 3.87mm³/N·m to9.46mm³/N·m when the carrier gas pressure drops from 0.8MPa to 0.6MPa.The reason for these phenomena is that the low air pressure of carrier gas makes it difficult for particles to be accelerated to the critical speed,which ultimately leads to the loose structure of B₄C/Al composites.In the process of preparing B₄C/Al composites by cold spraying,the hardness of B₄C/Al composites prepared by cold spraying decreases from 81.0HV₅ to 68.3HV₅,and the bending strength increases from 352.9MPa to 313.9MPa with the increase of spraying temperature from 300℃to 500℃.The fracture toughness of B₄C/Al composites ranges from 4.56 MPa·m^1/2 to 4.90MPa·m^1/2.When the heat treatment temperature reaches 400℃and above,the deformation before fracture of B₄C/Al composites increases significantly.With the increase of spraying temperature,the thermal conductivity of B₄C/Al composites increases from 114.2W/（m·K）to 132.0W/（m·K）.In the process of preparing B₄C/Al composites by cold spraying,the hardness of B₄C/Al composites by cold spraying decreases from 72.1HV₅ to 52.6HV₅ with the increase of heat treatment temperature from 300℃to 500℃.With the increase of heat treatment temperature,the bending strength of B₄C/Al composites increases slightly at first and then decreases,and the bending strength of B₄C/Al composites is 227.1MPa,241.5MPa and 214.2MPa at 300℃,400℃and 500℃.The fracture toughness ranges from 3.91 MPa·m^1/2 to 4.21MPa·m^1/2,and the fracture mode varies with heat treatment.With the increase of heat treatment temperature,the thermal conductivity of B₄C/Al composites increases from 89.6W/（m·K）to 100.0W/（m·K）.In the second part,the effects of different raw material ratios on the microstructure and properties of B₄C/Al composites prepared by pressureless infiltration were studied.The results show that the porosity of porous B₄C ceramics increases with the increase of B₂O₃ content in the raw material ratio.This results in a corresponding increase in the porosity of the B₄C/Al composites prepared by pressure infiltration.When the content of B₂O₃ increases from 15wt%to 25wt%,the bending strength decreases from338.3±26.0MPa to 240.6±14.2MPa.The fracture toughness decreased from7.02±0.22MPa·m^1/2 to 6.27±0.40MPa·m^1/2.Its hardness decreased from 284.2HV₅ to225.6HV₅.Meanwhile,the wear resistance of B₄C/Al composites decreases,and the wear rate increases from 0.316mm³/N·m to 0.649mm³/N·m.The friction coefficient increased from 0.452 to 0.539.