激光熔覆NiCrBSi合金涂层的裂纹形成机理

Study on Cracking Mechanism of Laser Clad NiCrBSi Coating

从微观组织研究了激光熔覆NiCrBSi镍基涂层的裂纹形成机理,分析了涂层显微组织、裂纹形貌和断口成分.结果显示:Ni60涂层具有高裂纹敏感性,即使涂层与基体热膨胀系数差异较小,涂层也容易开裂.原因是:①Ni60涂层包含大量的粗大硬质初生相和硬质共晶组织以及较少的韧性相,具有高的脆硬性;②断口聚集含Ca、S、Si等元素的熔渣;③粗大的硬质相妨碍了液体金属的自由流动,造成金属基体的不连续性;④粗大硬质相晶界处的液膜易在拉应力作用下破裂,形成缩孔或微裂纹,从而成为裂纹源.此外,裂纹在顶端起裂并向下扩展,裂纹多沿粗大硬质相晶界扩展,具有典型的凝固裂纹特征.

Cracking mechanism of laser clad NiCrBSi coating was studied through investigation on the mi-crostructure of the coating. The microstructure, crack behavior and chemical compositions of fracture sec-tion 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 ther-mal 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 ob-structs 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.