Preparation Of Al₂O₃ Matrix Coating On Silica Based Ceramic Core And Its Effects On Interface Reaction

Silica-based ceramic core is used for produce hollow,precise,and complex-shaped superalloy turbine blades in investment casting.Ceramic core directly decides the shape and size of hollow blade chamber.Though,silica-based ceramic core can meet the use requirments,it has interface reaction with the active metal elements of superalloy at high temperature due to its poor chemical stability.Aiming at the interface reaction between T silica-based ceramic core?composed of75-80%Silica,20-25%calica stabilized zirconia?and DD6 superalloy,this paper prepared Al₂O₃-Y₂O₃ composite coating and Al₂O₃ coating on ceramic core surface to restrain interface reaction and then improve inner surface roughness of castings.This paper discussed the coating preparation technology and coating effects on interface reaction.This research choose Al₂O₃ and Y₂O₃ as coating material based on the result of thermodynamic analyses.This paper discussed coating prepration method and coating slurry solid content and their effects on coating structure and morgraph.The coting preparation methods are dip-coating method and low pressure air spraying method.The Al₂O₃-Y₂O₃ composite coating and Al₂O₃ coating prepared by low pressure air spraying method,which solid content is 20%,thickness are 6 um and 10 um,respectively.The coating surface is smooth with few microcracks.The Al₂O₃-Y₂O₃composite coating and Al₂O₃ coating can cover the ceramic core matrix without influence on its usage.T silica-based ceramic core have severe interface reaction with DD6 superalloy.There are reaction layer about 10 um thick and reaction pit about 50 um deep.The inner surface roughness Ra value of the alloy panel prepared by ceramic core without coating is 6.28 um.The interface reaction can be divided into three stages,which is elements diffusion stage,replacement reaction stage and liquid phase flow stage.Al₂O₃-Y₂O₃ composite coating can't restrain interface reaction and deteriorate alloy panel surface roughness.The reason is that Y element reacted with SiO₂ and Al₂O₃and induced low temperatyre liquid phase below 1500?,which aggravated reaction of the replacement reaction stage.Then there generated low temperature liquid phase,which made Al₂O₃-Y₂O₃ composite coating failure.Al₂O₃ coating can restrain the interface reaction.Al₂O₃ coating hinder the diffusion of O,Si,Ca,and Al and then successfully restrain the interface reaction,which means Al₂O₃ coating prevent the diffusion stage of interface reaction.The Al₂O₃ coating interface is smooth without reaction zone.The Al₂O₃ coating area is composed by O and Al element and can improve surface roughness.The surface roughness Ra value of alloy panel prepared by ceramic core with Al₂O₃ coating is2.58 um.