Microstructure And Wear Behavior Of Cold-sprayed Cu-BNNSs Composite Coating

Copper-based composites have been widely used in the field of self-lubricating materials.it is expected that their properties can be further improved under the more demanding working conditions.As an emerging remanufacturing technology,cold spraying is increasingly employed to prepare and repair various metallic parts,including Cu ones,due to its outstanding characteristics such as low oxidation,low energy consumption and high efficiency.In order to meet the requirements of remanufacturing worn Cu parts,Cu-BNNSs composite coating was prepared on TU1 copper substrate by mechanical alloying and cold spraying.The powder and cold spraying coating were characterized by SEM,EDS and XRD as well as other inspection techniques.The microstructure,mechanical properties and tribological behavior of the composite coating were systematically studied.The specific research contents are as follows:(1)The preparation procedure and microscopic characteristics.BNNSs(thickness: 20-50nm)was successfully stripped from h-BN;the planetary ball mill was used to mix the electrolytic copper powder and BNNSs.The huge energy in the ball milling process promoted the cold welding of the electrolytic copper powder,and made some BNNSs embedded inside the copper particles.After optimizing the spraying process parameters,a densely structured composite coating(porosity: ?1.2%)was prepared.Except the typical peaks of the cubic structure of Cu,there were no observable peaks of other phases in the XRD patterns.The lower processing temperature of cold spraying results in little oxidation of the coating during the cold spraying process,which is confirmed by XRD of cold sprayed coatings.The addition of BNNSs reduces the deposition efficiency and surface roughness to a certain extent.Compared with the pure Cu coating,the surface of the composite coating is flatter and tighter.This is because some of the BNNSs hindered the adiabatic shear.Some particles could not bond to the existing coating,but played the role of "shot peening" to make the coating surface flatter.(2)The physical properties.The average microhardness of the pure Cu coating(138.9 HV)is higher than that of the TU1 copper substrate(100.8 HV)because the effect of work hardening,while the average microhardness of the Cu-BNNSs composite coating(119.2 HV)is slightly lower than that of pure Cu coating;this is due to the poor bonding between BNNSs and Cu in the Cu-BNNSs composite coating.The composite coating has a good bond with the substrate,and its average bond strength can reach about 71.2 MPa.(3)The friction performance.Under the dry friction test conditions,the composite coating exhibits excellent wear resistance.In the stable wear stage,the friction coefficient of the Cu-BNNSs composite coating is about 0.51,which is much lower than that of the pure Cu coating(0.76).And the wear rate of Cu-BNNSs composite coating was about 20% lower than that of pure Cu coating.(4)The wear mechanism.The wear mechanisms of the TU1 copper matrix were mainly adhesive wear and fatigue wear,while Cu coating mainly experienced severe abrasive wear and oxidative wear and Cu-BNNSs composite coating slight abrasive wear.The existence of BNNSs has two main effects on the wear trajectory: first,the weak bonding between the BNNSs layers enables the interlayer sliding induced by the shearing between the countersurfaces and correspondingly reduces the friction coefficient;second,the flat BNNSs adhered to the worn surface can isolate the contact of ball and sample: this "protective film" improved the wear resistance effectively.Thus,there were few deep grooves on the composite coating.