Preparation And Friction And Wear Properties Of Co-continuous TiC/45 Steel Matrix Composites

As the rail transportation becomes faster and greater,the requirements for the brake materials are also getting higher and the performance of existing cast steel and forging steel brake materials is limited.It is urgent to develop new brake materials with both high friction coefficient and high wear resistance.In this thesis,the TiC（Ti）and TiC-Ti B₂porous ceramic preforms were prepared by the organic foam impregnation and pressureless sintering method based on the experimental study about the wettability and its influencing factors of the TiC-steel system by the sessile drop methods.The effects of Ti addition and Ti B₂ contents on the phases,microstructures,formation mechanism and mechanical properties of the TiC porous ceramics were analysed.The co-continuous TiC/45 steel matrix composites were prepared by the melt infiltration method using the prepared porous ceramic preforms as the reinforced network.The effects of the compositions and structures of the preforms on the macro and micro morphologies,phase compositions and mechanical properties of the composites were studied.The friction and wear experiments of the co-continuous TiC/45 steel matrix composites under different loads and rotational speeds were carried out,and the friction and wear characteristics and mechanism of the co-continuous composites were explored,which laid a foundation for the application of this new material in the field of high-speed rail braking materials.The nonstoichiometric carbon vacancy TiC（TiC_x）ceramic plate was prepared by spark plasma sintering through adding different content of Ti into TiC raw material.The effects of the carbon vacancy and temperature on the wettability between 45 steel and TiC_x ceramic were studied.The results showed that increasing the temperature and reducing the x value of the non-stoichiometric TiC_x ceramic could reduce the contact angle between the melt of 45 steel and the TiC_x ceramic plate and improved the wettability between the two.During the wetting process,the diffusion depth of the molten steel in the plate increased with the increase of temperature and the decrease of the x value.When the value of x was 0.7～0.8,the wetting process was dominated by the dissolution;while the value of x decreased to 0.58,the wetting process was dominated by the reaction and the Fe₂Ti phase was generated.The comparative wettability test of 45 steel and brake disc steel showed that the diffusion depth of 45 steel in the ceramic plate was greater than that of brake disc steel at lower temperature,while the the diffusion depth of the former was smaller than that of the latter because the Ti atoms in the ceramic plate were more likely to snatch the C atoms in the Cr and Mo carbides in the brake disc steel generated TiC or TiC_x at 1650°C.Porous ceramic preforms of the pure TiC（T0）,TiC uniformly mixed Ti（T1）and TiC coated by Ti（T2）were prepared by the organic foam impregnation and pressureless sintering,respectively.The results showed that both mixed Ti and coated Ti could improve the strength of TiC ceramic network.The T2 preform was composed of the TiC region in the core,TiC_x region in the middle and a dense mixture region of the TiC_x and Ti in the outer layer.As the composition of the ceramics network changed from pure TiC to Ti-coated TiC,the porosities of preforms decreased from 94.3%to 89.3%,and the compressive strength increased from 0.90 MPa to 2.33 MPa.The presence of a small amount of metallic Ti was beneficial to improve the mechanical properties of the porous ceramic preforms.Co-continuous TiC（Ti）/45 steel matrix composites CT0,CT1 and CT2 were prepared by the melt infiltration process with the three kinds of porous ceramic preforms T0,T1 and T2 as the reinforcement.Due to the different components of the preforms,the ceramic reinforcement network of the three composites were partially continuous,weakly continuous,and strong continuous,respectively.The macro and micro interlocking structure between the matrix and the reinforcing phase in CT2 enhanced the hindrance of the network to dislocation movement.A large amount of dimple morphologies appeared on the tensile fracture surface of CT2,and the strengthening and toughening mechanisms,such as crack deflection,bridge connection and secondary crack initiation were observed,which improved the mechanical properties of the composite of which the vickers hardness,tensile strength and fracture toughness were 4.11 GPa,481 MPa and 28.9 MPa·m^1/2,respectively.In addition to adding Ti to improve the strength of TiC porous preform and its wettability with steel matrix,the TiC-Ti B₂ porous ceramic preform with the three-dimensional network structure and its steel matrix composites were prepared by the high temperature reaction between Ti and B₄C.The results showed that when the mass ratio of TiC:Ti B₂ in the preform was 37:55,Ti B₂ could effectively inhibit the decomposition of TiC.The porosity and bulk density of TiC-Ti B₂ porous ceramic preforms with different B₄C contents were 87.1%～96.0%and 0.17～0.22 g/cm³,respectively.When the content of B₄C in the preform raw materials were 20 wt.%and 25 wt.%,the ceramic networks of TiC-Ti B₂/45 steel matrix composites were strong and weak continuous,respectively.The former had the best mechanical properties,and its Vickers hardness,tensile strength and fracture toughness were 2.85 GPa,382 MPa and 23.7 MPa·m^1/2 respectively.In general,the mechanical properties of TiC-Ti B₂/45 steel matrix composites were lower than that of TiC（Ti）/45 steel matrix composites.The friction and wear tests of the three co-continuous TiC（Ti）/45 steel composites CT0,CT1 and CT2 and the brake disc steel showed that the friction coefficients of them were similar,but the wear rates of the co-continuous composites were significantly reduced,which would become a new wear-resistant brake material with better comprehensive performance.Among them,the composite CT2 had the lowest specific wear rate,which was 81.6%,54.1%and 23.2%lower than that of the brake disc steel,CT0 and CT1.Especially in high speeds and high loads wear tests,CT2 showed the better wear resistance than others.The wear mechanism of the co-continuous TiC（Ti）/45 steel matrix composites was slightly oxidative wear and abrasive wear at the lower load and speed,and a mixture of abrasive wear,oxidation wear and adhesive wear at the higher load and speed.The co-continuous and double interlocking structures in the composites had a significant effect on reducing wear during friction process.