Microstructure,High Temperature Oxidation And Wear Behavior Of TiB₂（p）/FeCr Composites

With the growing demand for high-performance materials,there is an urgent need to design and develop more new composite materials.At the same time,extending the service life of materials has become one of the important tasks to enhance the overall competitiveness of the country’s manufacturing industry.The development of high-performance heat-resistant and wear-resistant materials will produce huge economic and social benefits.At present,TiB₂/Fe composites have received extensive attention,showing good mechanical properties and excellent wear resistance,but few studies on oxidation resistance in high temperature environments.Therefore,this paper aims to prepare TiB₂（p）/FeCr composites with different matrix compositions by in-situ synthesis.By adding alloying elements,the microstructure can be controlled to improve its mechanical properties,high temperature oxidation resistance and wear resistance.The main research contents and results are as follows:（1）FeCr-based composites are mainly primary and eutectic TiB₂,primary TiB₂ is hexagonal,petals and small hexagonal particles,eutectic TiB₂ includes irregular dendrites,stamens(1^st eutectic)and secondary eutectic stripes,etc.,whose morphology,size and distribution are affected by chemical composition.When the Cr content exceeded 16 wt.%,thin striped eutectic TiB₂ increased and coarsened,and the primary TiB₂ particles aggregated and grew,showing the characteristics of lath and petal.When the Cr content was 22 and 28wt.%,long strips of（Fe,Cr）₂B appeared.Excessive addition of 2,4 and 6 wt.%Ti（T2,T4 and T6）to TiB₂/Fe22Cr,the primary TiB₂ morphology of the samples changed from petal-like to hexagonal particles,striped secondary eutectic TiB₂ and gray strips（Fe,Cr）₂B phase elimination.During the solidification process of TiB₂（p）/FeCr composites,the two phases of TiB₂ and（Fe,Cr）₂B compete for nucleation and growth.Increasing the Ti content（such as the T2 sample）effectively inhibits the formation of（Fe,Cr）₂B and increases the Cr content in the matrix at the same time.（2）As the Cr content increased from 0 wt.%to 28 wt.%,the microhardness increased from 205 HV_0.5 to 401 HV_0.5,and the fracture toughness first increased and then decreased gradually,and stabilized between 2.97 and 3.09 MPa·m^1/2.With the addition of excess Ti（2～6wt.%）,the microhardness increased from 358 HV_0.5（0 wt.%Ti）to 765 HV_0.5（6 wt.%Ti）,and the fracture toughness K_c1 increased from 3.03 MPa·m^1/2（0 wt.%Ti）increased to 5.55MPa·m^1/2（6 wt.%Ti）.（3）The constant temperature oxidation weight gain test shows that the addition of more than 10 wt.%Cr can improve the high-temperature oxidation resistance at 800℃,and the high-chromium samples（16-28 wt.%Cr）have little difference in the oxidation rate,and all have complete oxidation resistance.When the Cr content was between 16 and 28 wt.%,the average oxidation rates at 900℃were 0.106 g/m²·h（16Cr）,0.096 g/m²·h（22Cr）and 0.095g/m²·h（28Cr）,respectively.The average oxidation rates of 22Cr and 28Cr samples at 1000℃were 0.149 g/m²·h and 0.193 g/m²·h,respectively,and both had oxidation resistance.With excessive addition of 2,4 and 6 wt.%Ti,the oxidation rate of the samples at 900℃is much higher than that of the original composition（22Cr）,and the oxidation amount gain rate is increased by 202.1%,217.1%and 241.3%,respectively.It could be seen that addition of excessive Ti deteriorates the high temperature oxidation resistance of TiB₂（p）/FeCr-based composites.（4）During the high temperature oxidation of TiB₂（p）/FeCr composites,a continuous and dense Cr₂O₃ protective layer was formed on the surface of the samples,which blocked the diffusion of O^-ions and Fe⁺ions in the oxide layer.When the Cr content increased from 22wt.%to 28 wt.%,the surface oxide layer evolved from a three-layer structure（rutile Ti O₂ outer layer+dense Cr₂O₃ transition layer+Cr₂O₃/Ti O₂ mixed inner layer）to a double-layer structure（rutile Ti O₂ outer layer）+dense Cr₂O₃ inner layer),the possible mechanism is:Cr-rich striped TiB₂ disappears,inhibiting the formation of continuous Cr₂O₃/Ti O₂ inner layer;Ti⁺ions diffuse outward through the Cr₂O₃ layer,promoting the growth of the Ti O₂ surface layer.Similarly,excessive Ti addition leads to the transformation of the surface oxide layer structure from rutile Ti O₂ outer layer+dense Cr₂O₃ transition layer+Cr₂O₃/Ti O₂ mixed inner layer to rutile Ti O₂ outer layer+dense Cr₂O₃ transition layer+Ti O₂ inner layer,the main reason may be the coarse primary The interface between TiB₂ particles and the substrate provides a channel for the diffusion of O^-ions,which weakens the blocking ability of the Cr₂O₃ transition layer to O^-ions.（5）Dry friction and wear experiments show that under the load of 30 N,the wear amount of TiB₂（p）/FeCr composite decreases first and then increases with the Cr content,and the wear resistance of 4Cr and 22Cr samples is the best（wear amount:0.015 mm³）and the worst（wear amount:0.18 mm³）,respectively.With increasing Ti content,the wear amount increased from0.18 mm³（T0）to 0.26 mm³（T2）,and then decreased to 0.132 mm³（T4）and 0.072 mm³（T6）,the samples with excess addition of 6 wt.%Ti had the excellent friction and wear properties.（6）The indoor temperature dry friction and wear of TiB₂/FeCr matrix composites includes three stages:slight wear,mixed wear and severe spalling.The slight wear stage is manifested as fine ploughing caused by abrasive wear.The mixed wear stage is dominated by adhesive wear,forming mechanical mixtures.In the severe spalling stage,plastic deformation leads to crack initiation and large exfoliation of the mechanically mixed layer.