Study On Preparation,microstructure Evolution And Toughening Mechanism Of TiB₂-based Cermets

TiB₂ based cermets have good performance in hardness,high temperature red hardness,metal adhesion resistance and high temperature oxidation resistance.Therefore,it has a very broad application prospect in the fields of cutting tool and wear-resisting part.However,the low strength and toughness of TiB₂ based cermets is the key problem restricting its wider applications.One of the effective methods to realize the strengthening and toughening of TiB₂ based cermets is to control the microstructure evolution of TiB₂ based cermets in liquid phase sintering through the optimization of cermet components and forming a beneficial core-rim structure.In this study,TiB₂ based cermets with typical core-rim structure were prepared by vacuum liquid phase sintering with TiB₂ as the hard phase,Co Ni as the binder phase,WC and Ti C as the hard phase additives.With the toughening effect of core-rim structure,the interface bonding strength of hard phase and binder phase is improved,and the comprehensive mechanical properties of TiB₂ based cermets are improved.The main contents and conclusions of the study are as follows:（1）During the liquid phase sintering,TiB₂ ceramic particles dissolve and precipitate,realizing the growth and spheroidization of TiB₂ ceramic particles.A coherent interface is formed between TiB₂ hard phase and Co Ni binder phase,which improves the strength and toughness of TiB₂-Co Ni cermet.The transversal rupture strength and fracture toughness of TiB₂-20wt.%Co Ni cermet sintered in vacuum at 1480℃for 1 h reached 1470 MPa and 10.43 MPa·m^1/2,respectively.（2）When Ti C is added,two kinds of“core-rim”structures are formed:TiB₂ core-（Ti,Co,Ni）（B,C）rim phase and Ti C core-（Ti,Co,Ni）C rim phase.A coherent interface is formed between the TiB₂ core-（Ti,Co,Ni）（B,C）rim phase-Co Ni binder phase,which enhances the interface bonding strength.The interface transition region between Ti C core-（Ti,Co,Ni）C rim phase-Co Ni binder phase forms an amorphous Co Ni metal thin layer.The coherent interface and amorphous metal thin layer both improve the wettability of Co Ni binder phase to TiB₂ ceramic particle,and effectively improve the comprehensive mechanical properties of TiB₂-Ti C based cermets.After vacuum sintering at 1462℃for 1 h,the relative density,Rockweel hardness,transversal rupture strength and fracture toughness of TiB₂-25 wt.%Ti C-20 wt.%（Co-Ni）cermet reached 99.61%,90.2 HRA,1725 MPa,11.35 MPa·m^1/2,respectively.（3）The addition of WC can form a core-rim structure of TiB₂ core-（Ti,W,Co,Ni）（B,C）rim phase.A coherent interface is formed between the TiB₂ core and the（Ti,W,Co,Ni）（B,C）rim phase,which is conducive to further improving the strength and toughness of the cermet.The transversal rupture strength and fracture toughness of TiB₂-7.5 wt.%WC-20 wt.%Co Ni cermet obtained by vacuum sintering reached 2108 MPa,12.21 MPa·m^1/2,respectively.（4）WC and Ti C were added at the same time to form two core-rim structures:TiB₂ core-（Ti,W,Co,Ni）（B,C）rim phase and Ti C core-（Ti,W,Co,Ni）C rim phase.A completely coherent interface is formed between TiB₂ core and（Ti,W,Co,Ni）（B,C）rim phase,and between Ti C core and（Ti,W,Co,Ni）C rim phase.A coherent interface is formed between（Ti,W,Co,Ni）（B,C）rim phase and Co Ni binder phase.Between the（Ti,W,Co,Ni）C rim phase and the Co Ni binder phase,an amorphous Co Ni metal thin layer is formed.The order of dissolution and precipitation of ceramic particles during liquid phase sintering is WC,Ti C and TiB₂.（5）WC/Ti C mass ratio has an important influence on the formation of core-rim structure in TiB₂ based cermets.When WC/Ti C mass ratio is 0,no core-rim structure is generated;When WC/Ti C mass ratio is 1:7 and 2:6,the finer Ti C core-（Ti,W,Co,Ni）C rim phase accounts for the main proportion;When WC/Ti C mass ratio is 3:5,4:4 and 5:3,the thicker TiB₂core-（Ti,W,Co,Ni）（B,C）rim phase accounts for the main proportion.（6）WC/Ti C mass ratio has an important influence on the friction and wear properties of TiB₂ based cermets.When WC/Ti C mass ratio is 0 and1:7,fatigue wear is dominant;When WC/Ti C mass ratio is 2:6 and 3:5,adhesive wear is dominant;When WC/Ti C mass ratio is 4:4 and 5:3,abrasive wear and fatigue wear coexist.Oxidation wear exists in the friction and wear process.This dissertation included 104 figures,42 table and 173 references.