Study On The Organization And Properties Of Supersonic Plasma Sprayed NiCr-Mullite Coatings

Mullite has good physical and chemical properties,such as high temperatureresistance,oxidation resistance,high strength,little coefficient of thermal expansion and thermal conductivity,etc.Furthermore,Mullite has abundant raw materials,simple preparation and cheap price.Mullite ceramic coating is applied to industrial flue gas turbine blades,steelmaking oxygen gun nozzles of converter and a large number of parts heated in the furnace as functional coating to further improve its surface performance and more protection foe the substrate.In this paper,five groups of NiCr-Mullite ceramic composite coatings with different proportions of NiCr-Mullite were prepared by supersonic plasma spraying technology.Test the coating properties,Characterize the microstructure and phase of the coatings.Research the influencing factors of diverse distribution ratios on coating‘s construction and performance.Five groups of composite powders with different ratios of NiCr-Mullite were mixed by QM-3SP04 planetary ball mill,and use the DH-X2 supersonic plasma spraying equipment to spray ceramic composite coatings on the surface of 45 steel substrate with the same spraying process parameters.Use DRE-Ⅲcoefficient of thermal conductivity tester,HT-1000 type high temperature ball disc friction and wear tester,FEG-450 thermal field emission scanning electron microscope,D8 ADVANCE X-ray diffraction and ect to test the coatings’hardness,porosity,thermal shock resistance,thermal barrier property,friction and wear performance,characterize and analyze the coatings’microstructure of surface and cross section of organization.The morphology and XRD phases of the coatings’cross section showed that,the five group coatings all exhibit relatively simple phase structure characteristics.The crystal diffraction peaks are dominated by the NiCr solid solution phase and the mullite enhanced phase,meanwhile some of the AlNi₃,AlNi₄ and AlNi₂Si compound phase characteristic peaks also exist.The section morphology of the composite coating is layered structure that is stacked in layers,the coating is tightly combined.The combination of coating and substrate is mainly metallurgical.The results of coatings hardness and cracking toughness showed that,the microhardness of the five group coatings are higher than that of the substrate 45 steel,and the hardness values increased with the increase of mullite enhancement phase.When the mullite‘s volume fraction reached 75%,the microhardness values of the coatings reached the maximum value of 339.2HV.The porosity’s results showed that,the porosity of the coating decreases gradually with the addition of mullite enhancement phase,and the minimum porosity is0.9858%when the mullite’s volume fraction is 75%.The thermal shock resistance of the five group coatings showed that,the thermal shock resistance of the coatings became better and better with the increase of the mullite enhancement phase,and the thermal shock resistance of the coatings was the best when the mullite volume fraction reached 75%.The thermal conductivity test results showed that,five group coating’s heat insulation performance is good,and with the increase of the content of mullite,the ceramic coated particles would increased,and a variety of compounds could producted.So it can clear cut off heat transfer and coating’s thermal conductivity would decrease.When the mullite on the coating’s volume fraction was 75%,the coating thermal conductivity reached the lowest 6.162 W·m^-1·K^-1,and coating’s heat insulation performance is the best.The results and analysis of tribological properties showed that,the wear amount of the five groups of coatings decreases with the increase of Mullite’s content,and the friction coefficient of the coatings also decreases with the increase of Mullite’s content.When the Mullite’s content is 75%,the coating has the best comprehensive high temperature friction performance.The wear morphology showed that the wear pattern of the five groups of coating is mainly abrasive wear,adhesive wear is complementary.The analysis of coating spraying process showed that the ratio of Mullite powder on the actual coating is inversely proportional to the volume fraction of Mullite in the ball-milling composite powder.SEM and EDS of the coating surface showed that the volume fraction of Mullite on the coating increased with the increase of NiCr in the composite powder.When the NiCr powder content reached 20%,the maximum content of Mullite in the actual coating is 75%,until the content of NiCr powder reached 22%,the content of Mullite in the actual coating decreased to 61%.Through comprehensive performance test results and analysis,use the supersonic plasma spraying process to spray NiCr-Mullite composite coating,the coating has significant thermal shock resistance,thermal barrier property,and high hardness and wear resistance.When the Mullite volume fraction reaches 75%,the metal ceramic coating’s comprehensive mechanical properties can be best.