Preparation And Friction And Wear Properties Of Plasma Sprayed Remanufactured Coatings For Locomotive Cylinder Liners

With the rapid development of the economy,rail way transportation is more important than before.Service condition of the locomotive parts is harsher,which increases the failure speed and shortens the life of spare parts.Therefore,the high-performance locomotive parts are needed.Cylinder liners and piston rings are the most important friction pairs in internal combustion engines.The cylinder liner needs to be replaced frequently due to wear and cavitation.The use of plasma spraying technology in the inner wall of the cylinder liner wear to achieve green production of the cylinder,is a green and environment friendly manufacturing technology.Based on the literature research on cylinder liner remanufacturing technology,this thesis proposed to reconstruct the scrap cylinder liner through thermal spraying coating technology,to improve the tribological properties of the cylinder liner.The effects of Al2O3 and Mo on the structure and properties of the NiCrBSi coatings.The microstructure,phase structure,microhardness and tribological properties of the composite coatings were also characterized.The tribological properties of Mo-NiCrBSi coatings under dry,oil and boundary lubrication conditions were compared.The involved friction and wear mechanisms were analyzed.Firstly,the NiCrBSi coating was prepared on the 304 stainless steel substrate by plasma spraying.The coating consists y-Ni,CrB,Cr2B and Ni3B phases,and the coating had defects such as amorphous structure and crystal dislocation.The plasma sprayed particles were melted well and the molten particles formed in the coating layer due to impact spreading.The results showed that the flat thickness is 5-10 ?m,the porosity of the coating is 1.72%,and the hardness is about 850 HV.Then,Al2O3 and NiCrBSi powders were mixed by adhesive mixing,while Mo and NiCrBSi powders were mixed by mechanical blending.Al2O3-NiCrBSi and Mo-NiCrBSi composite coatings were prepared by plasma spraying.The influence of Al2O3 and Mo content on the microstructure and tribological properties of the coatings was analyzed.The friction and wear mechanisms of the coating were analyzed based on the wear and tear morphology.The results showed that the Al2O3-NiCrBSi composite coating consists of Al2O3 reinforcement and NiCrBSi matrix.The main components of the coating were ?-Al2O3,a small amount of P-Al2O3,and the amorphous structure of NiCrBSi.The content of Al2O3reinforce had a significant effect on the microstructure and wear resistance of the coating.When the content of Al2O3 was too small,the strengthening phase were not enough.On the contrary,when the Al2O3 content is too high,the brittleness will destroy the coating,shedding and forming abrasive grains.The Mo particles in the Mo-NiCrBSi coating were melted and spreaded well.Phases of Ni3B,Cr2B and Cr3C2 appeared in the coating,and the phenomenon of amorphous was also obvious.The coating hardness decreased with the increases of Mo content.The tribological test showed that the MoO2 lubricating layer was formed between the coating and the grinding ball in the friction process under dry friction,which reduced the friction coefficient and inhibit the formation of abrasive grains.The main wear mechanism in Mo-NiCrBSi coating was adhesive wear,and itswear resistance was better than Al2O3-NiCrBSi coating.At last,the oil-lubricated friction test was carried out on Mo-NiCrBSi coating.The friction coefficient of Mo-NiCrBSi coating was significantly decreased under the oil lubrication,and the stability of Mo-NiCrBSi coating was improved obviously.The Mo content had a significant effect on the friction coefficient of Mo-NiCrBSi coating.EDS results showed that the abrasive surface after oil friction had 0.34 wt%of S.In contrast,the S content of the non-abrasive surface under the same conditions was zero,indicating that Mo reacted with the oil to produce a MoS2 lubricating film.With the increase of Mo content,the peeling of particles were less.When the Mo content was 30 wt%,the wear volume of Mo-NiCrBSi coating were only 4%of the pure NiCrBSi coating.The friction coefficient curve of the Mo-NiCrBSi coating under the boundary lubrication condition are mainly composed of four parts 1)the initial oil lubrication stage;2)the boundary lubrication state;3)almost dry friction state;4)dry friction state.With the increment of Mo content,the abrupt change of friction coefficient increased from 50 min for NiCrBSi coating to 660 min for 30%Mo-NiCrBSi coating.The results showed that Mo can effectively prolong the time from boundary to dry friction.