Organizational and Tribological Properties of Laser-melted NiCoCrTaAI-TiC Composite Coatings
Ni-based alloys exhibit low density,good plasticity,high strength,and excellent corrosion and wear resistance under high-temperature conditions.Therefore,they are often preferred in high-temperature and harsh environments.They are widely used in various military engines and civil equipment fields such as thermal power generation,petrochemicals,and metallurgical industries.However,they are more prone to fatigue and creep damage in high-temperature environments,which seriously affect the working efficiency,reliability,and durability of equipment utilizing Ni-based alloys.To improve the service life of Ni-based alloys in harsh environments,NiCoCrTaAl-TiC composite powders were prepared via vacuum-mixed ball milling,and metal/ceramic composite coatings were successfully deposited on the surface of K418 nickel-based alloys via laser cladding technology.The phase compositions and microstructures of the coatings were examined using an X-ray diffractometer and metallographic microscope.The effects of different Al contents(0,5,10,and 15％)on the mechanical and tribological properties of the NiCoCrTaAl-TiC composite coatings were examined using a micro-Vickers hardness tester,scanning electron microscope,high-speed reciprocating friction and wear tester,and ultra-depth-of-field microscope.Actual operating environments,such as rainwater environment(pH6.2),seawater immersion(pH8),and lubricating oil were simulated for the coating with the best wear resistance,and the corrosion and wear resistances of the coating in different environments were further examined.The results show that the composite coating is mainly composed of TiC,Cr2Ni3,Al2O3,and AINi3 phases,and intermetallic compounds such as Al4CrNi15 and Al4Ni15Ta.The internal structure of the coating is dense and composed of dendrites in the middle and equiaxed grains at the top.As the Al content increases,the average hardness of the coating initially decreases and then increases.The strengthening mechanism of the hardness corresponds mainly to the joint strengthening of TiC,Al2O3,and AlNi3 phases.Under dry friction conditions,with increasing Al content,the wear loss of the coating initially increases and then decreases.Furthermore,the main wear form changes from adhesive to abrasive wear.In summary,when the Al content is 15wt.％,the composite coating exhibits the best microhardness,microstructure,and tribological properties,and its wear resistance is approximately 25％higher than that of the coating with 0wt.％Al content.Subsequently,the 15wt.％Al composite coating was immersed in rainwater and seawater for 2 h,and its friction coefficient was:lubricating oil＜rainwater＜seawater.The depth of the wear scar and amount of wear were essentially the same as those of the coating without corrosion treatment,indicating that the addition of Al can improve the corrosion resistance of the composite coating.
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