Densification Behavior,Microstructure And Properties Of TiB₂-TiC Composites

TiB₂-TiC composite materials have a high melting point,high hardness,heat resistance and wear resistance.They have important application values in the fields of machinery and aerospace.This paper aims at the bottleneck problems of TiB₂-TiC composites such as difficult sintering,high brittleness,and poor reliability.The TiB₂-TiC composites are prepared by high-energy ball milling combined with two-step pressureless sintering,and the sintering activity of the powder is effectively improved through the high-energy ball milling process.The"two-step"sintering process of low-temperature reaction pre-sintering,high-temperature rapid densification and in-situ growth and development of TiB₂ plate crystals was improved,and the strengthening and toughening of TiB₂-TiC composite materials was successfully achieved.The author systematically studied the influence of material composition,additive content and sintering process parameters on the structure and mechanical properties of composite materials.At the same time,the author revealed its sintering densification behavior,strengthening and toughening mechanism,and influencing factors of the growth of TiB₂ plates,and the phase interface structure relationship between TiB₂ and TiC,and finally TiB₂-TiC composite material with excellent mechanical properties and friction and wear properties is obtained.The results show that the particle size of the TiB₂-TiC composite powder obtained after high-energy ball milling is about 100 nm,and the main impurity is B₂O₃,Zr O₂,WC,Co,etc.The thermodynamic calculation results show that the TiC in the composite system and the WC introduced in the high-energy ball milling are beneficial to remove the oxygen impurities introduced in the high-energy ball milling process.The phase equilibrium calculation results show that 856℃is the starting temperature of the deoxygenation reaction.When the temperature is increased to1448℃or higher,oxygen impurities are quickly removed,and oxygen removal is complete at 1613℃.The calculation results and the experimental vacuum degree versus temperature curve show the result is consistent.Compared with the traditional one-step sintering process,the"low temperature first and then high temperature"two-step pressureless sintering process is conducive to material densification and obtaining TiB₂-TiC composites with excellent comprehensive properties.The author found that long-term low-temperature holding is beneficial to the full progress of the oxygen removal reaction and the removal of pores and inhibiting the grain growth at the same time.Fast densification was acheieved at high temperature.The calculated activation energy of hot pressed TiB₂-TiC composite is 262 k J/mol,and the mass transfer in the densification process is dominated by grain boundary diffusion.By comparison,it is found that pressureless sintering is beneficial to the development of TiB₂ plates,while hot-pressing sintering and SPS sintering inhibit the development of TiB₂ plates.The main influencing factors that promote the growth of TiB₂ into plates are the oxygen content in the material system and liquid system.The activation energy for length and width direction of TiB₂ is 929 k J/mol and 1146 k J/mol,respectively.It was found that low-temperature pressureless sintering can obtain a high proportion of heterogeneous coherent interfaces.The phase relationship is TiC[11^<sub>0]∥TiB₂[1 2^<sub>10],TiC（111）∥TiB₂（0001）,as the sintering temperature increases,the proportion of phase interfaces with phase relationships decreases significantly.There are 27%heterogeneous interfaces with orientation relationships in the samples of pressureless sintering at 1500℃/4h.The TiC nanophase which easily precipitated in the TiB₂ grains has the same phase relationship with the parent phase.The first-principles calculation results show that when the interface between the two phases of TiB₂ and TiC is a titanium atomic layer in TiC,the maximum separation work calculated is 11.68 J/m²,and the interface structure is the most stable at this time.The TiB₂-TiC composite prepared by two-step pressureless sintering of low-temperature then high-temperature achieved better performances.TiB₂-TiC composite prepared by pressureless sintering at 1500℃/4 h+2000℃/1 h has excellent mechanical properties.The Vickers hardness,elastic modulus,flexural strength and fracture toughness of that TiB₂-TiC composite are 21.8 GPa and 562 GPa,746 MPa,7.31 MPa·m^1/2 respectively.The main toughening mechanism is the crack deflection and bridging of TiB₂ plates.Friction and wear tests show that the room temperature friction coefficient and volume wear rate of TiB₂-TiC composites change little with the extension of the friction distance and temperature,but increase with the increase of the load.The wear mechanism of TiB₂-TiC composites is mainly adhesive wear,accompanied by abrasive wear and fatigue wear.