Researches On Microstructures And Properties Of Self-lubricating Si₃N₄ Matrix Composites

Si₃N₄ ceramic has been widely used in aerospace bearings on account of its exceptional properties,such as high strength,thermal shock resistance,high hardness,high fatigue toughness,wear resistance and acid and alkali resistance,however,its intrinsic drawbacks of inferior toughness,poor sinterability and high dry friction coefficient have greatly limited its application and development.This work aimed at synthesizing self-lubricating Si₃N₄ matrix composites by adding onion-like carbon?OLC?and non-stoichiometric TiN_0.3.OLC and TiN_0.3 were added as lubricating phase and sintering aid,respectively,in addition,TiN_0.3 could also enhance toughness of sintered body.OLC were synthesized by annealing detonation nanodiamond?⁵ nm?.In order to obtain OLC with specific microstructure added as lubricating phase,the influences of different annealing temperature,holding time and heating rate on the microstructures of OLC were investigated in this work.Results show that the conversion degree of nanodiamond to OLC gradually deepens by raising temperature and prolonging holding time,which is more complete graphite shells and smaller diamond core;OLC take on microstructures of large diamond core and incomplete graphite shells when raising heating rate to 45 °C/min at 1300 °C.The effect of TiN_0.3 on structure and phase transition of OLC was found in the TiN_0.3-OLC binary composites sintered at high temperature and high pressure?HTHP?.N vacancies destroyed the structure of OLC and induced C atoms to diffuse into TiN_0.3 which gave rise to the formation of Ti?NC?,meanwhile,the rest part of OLC transformed to graphites.Graphite still existed in sintered body when the TiN_0.3 content reduced to 5 vol.%,indicating that high pressure suppressed the long-distance migration of C atoms.Graphite and OLC were found coexisted in Si₃N₄-TiN_0.3-OLC ternary composites sintered at high temperature and high pressure,suggesting Si₃N₄ matrix composites with different lubricating phases had been synthesized.The fracture toughness of Si₃N₄-TiN_0.3-OLC ternary composites sintered body had been significantly improved compared with that of Si₃N₄-OLC binary composites sintered body.The grain size of TiN did not abnormally grow but maintained at nanoscale of approximately 100 nm and TiN dispersed throughout the sintered body contributing to the improvement of toughness.For the 10 vol.% TiN_0.3-10 vol.%OLC-80 vol.% of ?-Si₃N₄ composites sintered at 5 GPa,1500 °C,holding 15 min,the fracture toughness and vickers hardness are 6.6 MPa·m^1/2 and 18.6 GPa,respectively.