| In this dissertation,Ti-Si-C composite powders were prepared by a spray drying/precursor pyrolysis technology using Ti powders,Si powders,and sucrose(the precursor of carbon)as raw materials.The powders were deposited by atmosphere plasma spraying to obtain Ti-Si-C composite coatings for wear resistance and high-temperature oxidation resistance.On this basis,the spray drying/precursor pyrolysis technology of the Ti-Si-C composite powders was optimized;the microstructure and properties of the coatings were analyzed;the formation mechanism of the reactive plasma spraying was investigated.The Ti-Si-C composite powders by spray drying/precursor pyrolysis technology consisted of Ti powders,Si powders,and carbon from the pyrolyzed sucrose.Spray drying endowed the composite powders with high sphericity and excellent flowability.The pyrolyzed carbon inherited the high cohesive strength of sucrose and prevent the raw particles from separating during plasma spraying.Ti-Si-C composite powders with high sphericity,excellent flowability,and high bonding strength could be obtained using 14 wt%sucrose as the precursor and process parameters of Ti average diameter of 10 μm,Si average diameter of 5 μm,ball milling time of 6 hours,and solid content of 50%.Ti-Si-C composite coatings had a typical thermal spraying layered microstructure,mainly composed of TiC,Ti5Si3,and Ti3O in two kinds of lamellae.Submicron spherical TiC particles aggregated to form a Si-poor lamella,while the nanosized Ti5Si3 and Ti3O coexisted to form a Si-rich lamella.As the Si content increased,the Ti5Si3 content in the composite coatings increased,while TiC and Ti3O content decreased.In the reactive plasma spraying process,each Ti-Si-C composite powder participates in the reaction as an independent reaction unit.After the composite powder enters the plasma jet,the Ti powder and Si powder rapidly heat up and melt to form a liquid phase that wraps the solid carbon.The solid C reacts with the Ti element in the liquid phase to generate TiCs which grow into submicron particles,forming droplets containing submicron spherical TiC aggregates.During the impact,the submicron spherical TiC aggregation zone deforms into Si-poor lamellae,and the liquid phase rapidly cools and solidifies to form a Si-rich lamella of nanoscale Ti5Si3 and Ti3O.The Ti-Si-C composite coating has high hardness,excellent wear resistance,and good high-temperature oxidation resistance far exceeding that of TC4 alloy.With the increase of Si content in the composite coating,the hardness of the coating increases first and then decreases.The surface Rockwell hardness,Vickers microhardness,and nanoindentation hardness can reach 86.86 ± 1.36 HR15N,1980.92± 310.47 HV0.1,and 22.9 GPa,respectively.As the Si content increases,the oxidation resistance of the coating increases,and the higher the temperature,the more obvious improvement of the oxidation resistance.With the increase of Si content,the brittle Ti5Si3 phase in the coating increases,and the wear resistance of the coating decreases.The overall performance of the coating with Si content of 6wt%is the best.Its room temperature wear resistance,high temperature wear resistance(600℃),and oxidation resistance(800℃)performance are 169 times,45 times and 5 times that of TC4 titanium alloy.Ti-SiC-C composite powders were prepared with sucrose as the precursor,SiC as the source of Si and part of C,and Ti-Si-C atomic ratio of 3:1:2.The Ti-SiC-C composite coatings were deposited by these powders and compared with coatings prepared with element Ti and element Si as raw materials.Both coatings formed a submicron spherical TiC aggregated Si-poor lamella and a Ti5Si3-Ti3SiC2 symbiotic Si-rich lamella structure.In the coatings using SiC as the raw material,SiC was not completely reacted and remained in the form of particles,resulting in a lower Ti3SiC2 phase content,but the hardness was slightly higher. |
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