High Temperature And High Pressure Synthesis And Toughening Mechanism Of High Performance Boron Carbide Matrix Composite

Advanced ceramic boron carbide（B₄C）,has become one of the most promising lightweight structural engineering ceramics,owing to its excellent properties such as ultra-high hardness,low density and high temperature resistance.However,the high brittleness caused by high hardness greatly limits the wide application of B₄C in industrial field.Solving the contradiction between hardness and toughness and realizing the research and development of high hardness and high toughness B₄C have always been a hot and difficult point in the research of ceramic materials.Introducing metal,oxygen（carbon,boron）compounds,carbon materials and other secondary phase are effective methods to improve the toughness of B₄C.Nevertheless,The low hardness of the second phases often damage the overall hardness of B₄C composite.At present,the toughness of B₄C matrix composite is mostly lower than6 MPa·m^1/2 when the hardness is higher than 30 GPa,while the hardness is generally lower than 29 GPa when the toughness is higher than 7 MPa·m^1/2.This toughness value is much lower than that of industrial requirements.So,achieving the breakthrough of high toughness for B₄C composites with high hardness is a very challenging scientific research topic.Exploring a new strengthening and toughening mechanism is the key to realize the breakthrough of high toughness of B₄C composite with high hardness.At present,the toughening mechanisms of second phase composite include crack deflection,crack branching,crack bridging and pull out.In fact,the pull-out toughening mechanism is related to the bond strength and friction force between the two phases.So,it is possible to effectively increase the interaction between the two phases on the basis of phase boundary bonding by introducing stress into the composite material,by which the pull out mechanism can also be further strengthened.In addition,it is not only possible to explore new strengthening and toughening mechanisms,but also to develop B₄C composite with high hardness and toughness.However,the introduction of prestress requires the selection of appropriate second phase and experimental methods.Non-metallic high-strength carbon materials(Fullerene-C₆₀,carbon nanotubes-CNT,Graphene oxide-GO)possess lower density and higher strength than B₄C,which have become an ideal toughening and strengthening phase for B₄C.At the same time,the hollow carbon material can achieve pre-compression under the action of high temperature and high pressure.After pressure relief,the pre-compressed carbon material is bounded by the B₄C matrix and difficult to recover,and finally the storage of prestress can be realized.In addition,different dimensions carbon materials have different contact areas among B₄C matrix grains,which play an important role on the pull-out toughening mechanism.Therefore,it is of great scientific significance and application value to explore the toughening mechanism of B₄C-carbon composites under the action of prestress for the development of B₄C matrix composite with high hardness and toughness.In this paper,non-prestressed nano Ti powder and prestressed non-metallic carbon materials of different dimensions fullerene,single-wall carbon nanobundles and graphene oxide(C₆₀/SWNTb/GO)were selected as the second phase to synthesize B₄C matrix composite with high hardness and toughness by high temperature and high pressure method.Innovative results obtained are as following:1.Nano-Ti was chosen as the start materials to react with B₄C to form the dispersive nano TiB₂-reinforced B₄C composites by HPHT method.The optimal hardness and toughness values can be achieved to 30 GPa and 7.9 MPa·m^1/2respectively by addition of 5 mol%Ti.The hardness and toughness increased 122%and 114%respectively compared to B₄C-micro TiB₂ composite.The experimental results show that in the absence of prestress,the TiB₂ can be used as as the second composite phase for its weak mechanical properties but good toughness.Therefore,the intergranular fracture mode of crack deflection,crack branching caused by nano TiB₂can enhance the toughness effectively.The nanosize of TiB₂ in B₄C-nano TiB₂ composite reflects the Hall-petch effect,which increases the strength of the second phase itself and result in the generation of high hardness.2.Zero dimension fullerene(C₆₀)was used as the B₄C second phase binder to synthesize B₄C-Graphite（B₄C-Gr）composite by HPHT method.The optimal the hardness and toughness value reach to 29 GPa and 7.3 MPa·m^1/2 respecively by adding 2 vol%C₆₀.The results show that C₆₀ is transformed into graphite at high temperature and pressure.The graphite stored about 1.1 GPa of prestress in composite,which strengthened the crack bridging and pull-out toughening mechanism and improved the toughness.However,the large particle size of graphite and its own poor mechanical properties result in the hardness of B₄C-Gr lower than 30 GPa.3.One-dimensional single wall carbon nanotube bundles（SWNTb）was chosen as second phase binder to form B₄C-SWNTb composites by HPHT.The optimal hardness and toughness value reach to 30 GPa and 8.3 MPa·m^1/2 by adding 2 vol%SWNTb,which is 3.7 times higher than that of pure B₄C.The toughness limit(6MPa·m^1/2)of B₄C carbon matrix composites at high hardness（30 GPa）has been broken.The hollow SWNTb achieves prestressed storage of about 2 GPa,which more effectively strengthens the crack bridging and pull-out toughening mechanism.The high hardness comes from the good non-coherent and amorphous links between SWNTb and B₄C.4.Two-dimensional graphene oxide（GO）was chosen as second phase binder to form B₄C-rGO composites by HPHT.The optimal toughness and hardness values can be achieved to 30.1 GPa and 8.6 MPa·m^1/2 respectively for B₄C-2 vol%rGO composites,which is 4 times higher than that of pure B₄C.The interlayer van der Waals force stores a maximum prestress of about 3.3 GPa in rGO,which maximizes the crack bridging and pull-out toughening mechanisms and achieves high toughness.Moreover,the layered and fold rGO has higher specific surface area and interfacial friction as well as larger contact area with B₄C grains,thus achieving excellent comprehensive performance of high hardness and high toughness.The interaction between B₄C and dispersed nano TiB₂ was enhanced by high temperature and high pressure sintering,and the hardness of B₄C-nano TiB₂composite reached to 30 GPa,which was 122%higher than that of B₄C-micro TiB₂.The second phase of prestressed sequestration of hollow carbon material is further introduced.The prestress increases with the increase of the content and dimension of the second phase.Thus,the interfacial friction is gradually increased with the increase of prestress,by which enhance the bridging and pull-out toughening mechanisms,so the fracture toughness is gradually improved.The optimum values of fracture toughness are 7.3 MPa·m^1/2,8.3 MPa·m^1/2 and 8.6 MPa·m^1/2,respectively.This paper provides a new way for the preparation of B₄C matrix composites with high hardness and toughness.