CVD Preparation Process Of BN Coating On Si₃N₄ Fiber Surface And Toughening Mechanism In Composites

In continuous silicon nitride fibers reinforced ceramic matrix composites,due to the strong bonding force formed at the fiber/substrate interface,the load is quickly transferred to the reinforcement fibers when the composite is loaded,which weakens the fibers reinforcement and toughening effects,and leads to brittle fracture of composite materials.In order to modify the toughness of continuous fibers reinforced ceramic matrix composites,the preparation of BN interface phase coating on Si3N4 fibers surface and its effects on the toughening mechanism of composites were studied.The effects of different process parameters on the preparation,deposition mechanism and properties of CVD BN coating on Si3N4 fiber surface were investigated.Studies have shown that the main component of the coatings obtained by deposition is BN and a small amount of BNx Oy,and the surface is smooth and dense,showing granules of pebbles.The coatings have the layered structure under high temperature and low pressure,and the crystallinity is high.There is no obvious deposition induction period on the surface of Si3N4 fibers.The deposition temperature has an important influence on the morphology and structure of the coatings.The particle size increases with the deposition temperature in the range of 1100℃～1500℃,and the secondary fine particles are generated near the large particles at high temperature.The coating growth rate is positively correlated with temperature.At lower temperatures,it is mainly controlled by mass transfer rate,resulting in particle packing structure.When the temperature rises,the coating growth is controlled by surface chemical reaction,and the atomic diffusion rearrangement rate increases at high temperature.The coatings are converted into the layered structure.The effects of deposition pressure and precursor concentration on the deposited coatings are mainly achieved by changing the bulk volume fraction of the precursor.The surface particles of the coatings at low pressure and low concentration are smaller than those on the surface of the coatings under high pressure and high concentration.Under the lower partial volume fraction of the precursor,the coating mainly forms a layered structure with good orientation and tight structure.Under high volume partial pressure,the coatings growth include surface reaction and gas phase nucleation growth,which lead to form a tight particle packing structure.The coating growth rate increases first and then reaches equilibrium after the volume fraction of the precursor increases.The effects of continuous deposition time on coating growth was investigated.As the deposition time increases,the BN coating structure changes from particle packing growth to layered growth,and deposition for a long time,the coating structure of the original particle packing growth transforms into a layered structure due to sufficient diffusion rearrangement.The crystal structure of the CVD BN coatings increases with the deposition time,and the crystallinity increases.The deposition temperature has a great influence on the mechanical strength of the fibers.The retention rates at 1200℃ and 1300℃ both are more than 90%,and the best at 1200℃;while under 1400℃ the retention rate is only 56.7%,and which is even lower,only 41.8% at 1500℃.The deposition temperature has little effects on the modulus of the fibers.The deposition pressure has little effects on the mechanical properties of the fibers.At 1200℃,the fiber strength retention rate exceeds 90% when the deposition pressure is 50 Pa,100 Pa,200 Pa,and 500 Pa,respectively.The effects of coating thickness on fiber strength increased first and then decreased,around 400 nm the retention rate was the best.Si3N4f/BNc/BN bundle composites were prepared by PIP process,and Si3N4f/BNc/Si O2-BN bundle composites were prepared by sol-gel and PIP.In the BN-based composites,as the pyrolysis temperature increases,the products crystallinity rises.At low temperature,the matrix component has a small amount of by-products besides BN,but the matrix component is only BN when cracked at 1200℃.The interface phase coating has obvious toughening on the tensile strength of the composites.The introduction of the coatings effectively modifies the strong bonding force between the fibers and the matrix.When cracked at 1200℃,the toughening effect of the interface phase coating has increased by 140% when the coatings thickness is about 400 nm.In the Si O2-BN matrix composites,the matrix reacts with the coatings to form a strong bonding force,which inhibits the weakening of the fiber/matrix interface,and the fibers pull-out is weak.The strength of the composites is lower than that of the BN-based composites.