| The combustion chamber of the engine is usually composed of cylinder liner,cylinder head,piston and piston ring.The cylinder liner and piston ring work under unstable load,variable speed and thermal working conditions and it is one of the most important components in the engine.The failure of the inner wall of the cylinder seriously affects the dynamic performance and service life of the vehicles.It causes waste to swap the cylinder liners frequently.Plasma spray is an advanced technology for surface strengthening and has such advantages as high efficiency,energy saving and environmental protection.It is of great importance to prepare the wear-resistant coatings on the inner wall of the cylinder by plasma spray to prolong the service life of the cylinder liner.In this thesis,the thermal-sprayed inner wall coating of automotive engine cylinder and the relevant materials in literature were reviewed.Considering the failure mechanism and in-service requirements of cylinder liner,316L stainless steel,FeCrBSi and NiCrBSi coatings were prepared on the substrate by atmospheric plasma spray.Take into account the service temperature of the inner wall of an engine cylinder is 300?700?,the as-sprayed three coatings were then subjected to post-heat treatment at 300?700? for 1 hour.This thesis investigated the effects of heat treatment temperature and atmosphere on the structure and properties of the coatings.The microstructures and phase compositions of the coatings were analyzed by optical microscopy and X-ray diffractometer.The porosity,microhardness and tribological properties were studied.Worn surface of these coatings was observed by field-emission scanning electron microscopy and the wear mechanisms were discussed.The experimental results showed that the three coatings obtained by atmospheric plasma spray were composed of splats and there were defects such as unmelted particles,oxidation and pores in the coatings.The NiCrBSi coating was densest with a porosity of about 3.5%.The FeCrBSi coating showed the highest porosity which was about twice of that of the NiCrBSi coating.Interface oxidation of the 316L stainless steel coating was the most serious.The 316L stainless steel coating was mainly composed of y-Fe,(Fe,Ni)and Cr.The composition of FeCrBSi coating was mainly a(Fe,Cr)solid solution phases,and the NiCrBSi coating mainly consisted of ?-Ni solid solution phases and FeNi3 intermetallic compound phases.The wear resistance of the as-sprayed FeCrBSi and NiCrBSi coatings were better than that of the 316L stainless steel coatings due to the higher hardness.After the conventional process of heat treatment,oxides formed between the layers of the three coatings.For the 316L stainless steel coatings,the oxide hard phase was beneficial to improve the hardness of the coatings.With the increase of temperature,the pores and micro-cracks fused and the coating structure became denser.For the FeCrBSi and NiCrBSi coatings,heat treatment had little effect on the pore size and no fusion occurred between the oxide layers.However,recrystallization occurred at high heat temperature.Boride and carbide phases precipitated which had a positive effect on the hardness and wear resistance of the coatings.When the heat treatment was conducted under the protection of nitrogen atmosphere,the interface oxidation in the FeCrBSi coatings was alleviated and coating microstructure was optimized.However,there was no obvious change in the microstructures of the 316L stainless steel and NiCrBSi coatings.The effects of heat treatment temperature and atmosphere on the friction coefficients of the three coatings can be ignored,but the effect on the wear rates was obvious.With the samples heat-treated in the air,the wear resistance of 316L stainless steel coating reached the best at 700?.The wear rate was(1.149 ± 0.26)×10-5mm3/N-m which improved by 14%compared with the as-sprayed coatings.The wear mechanisms were adhesive and abrasive wear.The wear resistance of FeCrBSi coating reached the best value at 700?((0.481 ± 0.023)×10-5 mm3/N·m)too,which improved by 44%compared with the as-sprayed coatings.The friction failure was mainly thermal fatigue and abrasive wear.The NiCrBSi coating achieved the best wear resistance when heat-treated at 600? and the wear rate was(0.597 ± 0.06)×10-5 mm3/N·m.Compared with the as-sprayed NiCrBSi coating,the wear rate was reduced by 40%.The wear mechanism was adhesion and abrasive wear.When it comes to the heat treatment under the condition of the N2 atmosphere,the properties of 316L stainless steel and NiCrBSi coatings changed little.The wear rate of the FeCrBSi coatings decreased by 14?48%,which showed an excellent wear resistance.The FeCrBSi coating was expected to be a reinforced coating for the engine cylinder wall. |
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