Powder Synthesis,consolidation And Mechanical Properties Of MB₂-MC Powders By Boron/Carbonthermal Reduction

MB₂-MC（M=Zr,Hf）ceramics have high melting point,good friction resistance,and high hardness,so they are widely used in rocket pushers,brake pads,and other high-temperature mechanical fields.Currently,MB₂-MC（M=Zr,Hf）ceramic precursor powders are generally prepared by a single MB₂,MC powder or by in situ synthesis of single noble metals（Hf,Zr）and B₄C.However,the above methods are expensive and the preparation process is more complicated.The preparation of MB₂-MC（M=Zr,Hf）by boron/carbon thermal reduction has low raw material cost and simple preparation process,which is suitable for industrial production.In this study,physical and chemical reactions within ternary system were explored by thermodynamics calculations in order to predict reaction process and guide research of process and recipe optimization.In the final composite powders,a controlled two-phase composition can be achieved.Furthermore,ZrB₂-ZrC and HfB₂-Hf C ceramics were prepared by hot pressing and spark plasma sintering,respectively.And the densification behavior and mechanical properties of MB₂-MC（M=Zr,Hf）ceramics were explored.The results demonstrated that:（1）A novel route MO₂+B₄C+C→MB₂+MC+CO（M=Zr、Hf）was used to synthesize MB₂-MC composite powders.The reaction process was demonstrated:6MO₂+5B₄C=6MB₂+4B₂O₃+5C occurred firstly.As the temperature rises,MO₂+B₂O₃+5C=MB₂+5CO occurred.When the temperature is further increased,MO₂+3C=MC+2CO.It is worth noting that part of the intermediate B₂O₃could have been reacted in Reaction（4）at 65 mbar vacuum,while the left B₂O₃ may have reacted with MC to introduce MB₂by MC+B₂O₃+2C=MB₂+3CO.（2）Based on reaction mechanism,ZrB₂-ZrC composite powders was synthesized by one-step method.ZrB₂,ZrC and some other oxygen impurities was identified by XRD,TEM,EDS and XPS.Adjusting molar ratio of raw materials led oxygen content to go down to0.97wt%and HfB₂:Hf C phase content ratios to change.Good sinterability of the powder products was demonstrated by hot pressing at 1950℃for 60min under 30 MPa pressure,which resulted in fully dense ZrB₂-ZrC composite ceramics with Vickers hardness value larger than ZrB₂ceramics.（3）Based on reaction mechanism,HfB₂:Hf C phase content ratios showed a dependence on B₄C/C additions,holding times and cold pressures.HfB₂-Hf C composite powders with low oxygen content were synthesized at 1750°C for 1 h using Hf O₂,B₄C and carbon black at a raw material ratio of m（B₄C:Hf O₂:C）=1.25:3.75:7.5.Furthermore,HfB₂-Hf C ceramic composites prepared SPS of the HfB₂-Hf C hybrid powders at 2100℃exhibited good sinterability and mechanical properties.