Study on Cracking Mechanism of Laser Clad NiCrBSi Coating

Cracking mechanism of laser clad NiCrBSi coating was studied through investigation on the microstructure of the coating.The microstructure,crack behavior and chemical compositions of fracture section were investigated.The results show that the Ni60 coating has high crack susceptibility,which still has numerous cracks due to high brittle brittleness and high thermal gradient even if the difference of thermal expansion coefficient between the coating and substrate is low.The reasons are: the coating has a large amount of coarse hard primary phases,hard eutectics and a few of tough phase,resulting in high brittle hardness;the fracture section has slag containing Ca,S,Si elements;the coarse hard phase obstructs the flow of metal liquid,resulting in the discontinuity of the metal matrix;the grain boundary of the coarse hard phase is crack source due to the separation of the liquid films formed on the grain boundary under the action of tensile stress induced in cooling process.In addition,cracks initiate and propagate downwards along grain boundary of coarse hard phase,which has a typical characteristic of hot crack.