The Tribological Properties of Cu-Sn-Bi Alloy for Slipper Pair of Plunger Pump
Powder metallurgy was used to prepare two types of copper-tin-bismuth(Cu-Sn-Bi)composites with different Ni contents(mass fraction 0%and 2%)on SPCC(Steel plate cold rolled commercial).Copper alloy layer density and Brinell hardness were measured.The composite microstructure was observed by metallographic microscopy and EBSD(Electron Backscattered Diffraction).The self-lubricating properties of two samples were investigated at different environmental temperatures(25,70,100 and 150℃).Their tribological properties under oilless conditions were also evaluated.The results showed that the addition of Ni increased the density of the Cu-Sn-Bi composite from 7.94 g/cm3 to 8.19 g/cm3,and the Brinell surface hardness increased from 56.5 to 66.2.A significant increase in matrix density was the result of pore reduction on the surface of copper alloy.Copper alloy composites showed significant grain refinement,including increased small-angle grains.It was attributed to the recrystallizing reaction caused by adding Ni to the alloy,which may improve the overall mechanical properties of the material.The morphology of the pits at the tensile fracture surface of the copper layer was significantly different.Strengthening results in smaller pits in the alloy,which reduced its brittleness and improved its surface toughness.Friction tests were performed under dry conditions using a force of 50 N,a sliding speed of 0.125 m/s and at four different ambient temperatures.The Ni-reinforced Cu-Sn-Bi alloy had a more consistent friction coefficient at lower ambient temperatures and a longer service life at higher temperatures.As the temperature increased,the wear rate of the pre-strengthened sample gradually increased,while the wear rate of the strengthened alloy sample decreased.The wear rate of the pre-strengthened alloy material was 2.642×10-5 mm3/(N·m),which was 2.17 times higher than that of the strengthened material[1.217×10-5 mm3/(N·m)]at an ambient temperature of 150℃.The above phenomenon occurred due to the formation of a wear-resistant layer on the worn surface of the strengthened alloy sample.At low temperatures,the wear-resistant layer was mainly composed of a slightly plastic deformation layer and an adsorbed NiO-oxide layer on the surface of the wear track.At high temperatures,the wear-resistant layer was mainly composed of Fe2O3 and Fe2O4 uniformly coated on the surface of the wear track.Furthermore,the strengthened Cu-Sn-Bi alloy greatly reduced the amount of debris that was peeled off during the dry friction process.This significantly reduced the wear damage caused by debris on the friction pair,while minimizing debris contamination in lubricated friction.This indicated that the alloy had good application potential in closed mechanical equipment.The Stribeck curve showed that the Ni-reinforced Cu-Sn-Bi alloy sample could enter the mixed EHL(Elasto hydrodynamic lubrication)stage faster and smoother in an oilless lubrication condition.This meat that the Cu-Sn-Bi bimetallic alloy material reinforced with 2%Ni could achieve good wetting compatibility under oilless lubrication conditions.This helped to reduce wear problems and improve the service life of friction pairs.
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