Research On Reaction Synthesis Of TiO_2-x Ceramic-Metal Composite Conductive Wear-resistant Coating By Plasma Jet

As a special electrical contact element that transmits electric energy signals and control signals between relative moving surfaces of equipment,current-carrying tribo-pairs is very important for safe and efficient operation of equipment.Adding wear-resistant ceramic phase to metal materials can realize the synergistic improvement of tribological properties and current-carrying quality,but the electrical properties of composites can be significantly reduced due to the poor conductivity of ordinary ceramic particles.Magnéli phases titanium oxide ceramics(TiO_2-x)not only have high wear resistance and corrosion resistance,but also have metal-like conductive properties,which is an ideal reinforcement phase for ceramic metal composites of current-carrying tribo-pairs.However,the traditional preparation process of TiO_2-x ceramic materials is complicated and costly,and there are some problems such as excessive sintering and particle agglomeration.The in-situ synthesis of TiO_2-x conductive ceramics using strong reducing media H₂ in high-temperature plasma jet and their composite preparation on the surface of equipment components can ensure the overall conductivity of the components and improve their surface wear resistance,providing an important technical solution for their long service life.The objective of this paper is to explore the preparation of a new type of TiO_2-x ceramic-metal composite conductive wear-resistant coating.Therefore,experimental studies on TiO₂conversion and synthesis of TiO_2-x under different process conditions are firstly carried out to explore the physicochemical reactions in plasma jet and the formation mechanism of the TiO_2-x coating.Then,based on the study of the influence of TiO₂ agglomerated powder structure on the microstructure and properties of the TiO_2-x coating,a Cu-TiO₂ core-shell structure powder with micron-scale Cu particles coated by nano-scale TiO₂ particles is designed and synthesized to obtain a controllable preparation of the Cu/TiO_2-x composite coatings with a ceramic-metal network interconnection structure.The internal construction mechanism and organization structure of the Cu/TiO_2-x coating are systematically studied,and its mechanical,electrical and tribological properties are tested,providing a new idea and scientific basis for the development and application of high-performance Cu/TiO_2-x ceramic metal coating.（1）Combined with theoretical analysis and experimental characterization,it is found that the melting state of droplets is obviously improved with increasing spraying power.The spreading morphology of droplets after hitting the substrate changes from broken and poor melted at low spraying power to radial,petal and disc shape at higher spraying power.Subsequently,the droplet spreading characteristics and coating microstructure at different spraying powers are analyzed,and the coating forming mechanism is revealed.Finally,the competitive mechanism of TiO₂deoxidation reaction led by H₂ and TiO_2-x reoxidation reaction led by O₂in plasma jet is studied.（2）Nano TiO₂ agglomerated,micron TiO₂ agglomerated,and nano/micron TiO₂ composite agglomerated powders are synthesized through powder material ratio design,and the TiO_2-xcoating with different structure are prepared using these powders as feedstocks.The microstructure,mechanical,electrical and tribological properties of the TiO_2-x coating are significantly affected by the structure of TiO₂ agglomerated powders.The increase of micron-scale TiO₂ particle content makes more TiO₂ phase convert to TiO_2-x phase in the TiO_2-x coating,but the coating forming quality deteriorates due to the increase of internal defects and porosity of the coating.The microhardness and conductivity of the TiO_2-x coating decrease with the increase of micron-scale TiO₂ particle content,with a decrease of 9.1%and 61.7%,respectively.However,the composite coatings show enhanced fracture toughness,bonding strength,and wear resistance,among which coating B75 have the highest toughness(0.98 MPa·m^1/2),strength（44.0 MPa）and wear resistance.（3）a Cu-TiO₂ core-shell structure powder with micron-scale Cu particles coated by nano-scale TiO₂ particles is designed and synthesized.Nano-scale TiO₂ particles are evenly dispersed in slurry by means of ultrasonic dispersion and PH adjustment.The surface modification of micron-scale Cu particles with PAANH₄ has good electrostatic stability and steric hindrance effect,which improves the dispersion stability of Cu particles in slurry.Adding emulsifier can increase slurry viscosity and improve the stable suspension of Cu particles.The effects of slurry composition such as dispersant,binder and solid content on coating effect,particle size and loose density of Cu-TiO₂ powders during spray granulation are studied.A stable preparation process of Cu-TiO₂ core-shell structure powders for plasma spraying is formed.（4）Based on the above design of Cu-TiO₂ core-shell structure powders,the Cu/TiO_2-xcomposite coating with ceramic-metal mesh interconnecting structure is successfully obtained by adjusting the spraying parameters.Spray Watch-2i is used to monitor the flight characteristics of droplets in plasma jet,and the melting state of droplets is quantitatively analyzed with the melting index（M.I.）.Detailed characterization of droplet spreading and solidification characteristics and coating organization structure at different spraying powers is conducted.It is found that the phases of the Cu/TiO_2-x composite coating include elemental Cu,TiO₂,TiO_2-x,Tiand Cu oxides,as well as amorphous phases dominated by Cu and Cu O.With the increase of spraying power,the content of TiO_2-x phases,Tiand Cu oxide phases in the coating increases,while the content of amorphous phase decreases.The internal construction mechanism of the Cu/TiO_2-x coating is revealed,and the influence of substrate surface temperature on the microstructure of the Cu/TiO_2-x composite coating is analyzed.（5）The effect of organization structure on mechanical,electrical and current-carrying tribological properties of the Cu/TiO_2-x composite coatings is further studied.When the spraying power increases from 41.4 k W to 54.0 k W,the microhardness of the Cu/TiO_2-xcomposite coating increases from 399 HV0.5 to 637 HV0.5,while the fracture toughness and conductivity decrease from 1.25 MPa·m^1/2 and 4.7×10⁶ S/m to 0.92 MPa·m^1/2 and 1.1×10⁶ S/m,respectively.The friction coefficient of the Cu/TiO_2-x coating increases with the increase of spraying power,but the wear degree of the Cu/TiO_2-x coating decreases.The friction coefficient and wear rate of the Cu/TiO_2-xcoating decrease firstly and then increase under different current-carrying condition.Although the increase of electric current leads to the increase of contact surface roughness,the volume loss of coating matrix material is relatively little under current-carrying conditions,which is mainly related to the increase of the number of wear debris distributed on the contact surface and the formation of polycrystalline tribo-layer.