高载下单晶铜和单晶硅的径向纳动与损伤行为研究

采用纳米压痕仪研究了单晶铜和单晶硅径向纳动的运行特点和损伤过程.结果表明:径向纳动的残余压痕深度随循环次数增加急剧减小,而纳动循环中载荷-位移曲线在闭合前表现为1个迟滞环;试样在首次径向纳动循环中耗散的能量最大,其后逐渐减小并趋于稳定;材料的接触刚度和弹性模量在最初几次纳动循环中增加较快,随后变化趋于平缓;尽管2种材料的压痕投影面积均随纳动循环次数增加而增大,但由于损伤机制不同,使其径向纳动损伤显示出各自不同特点,其中单晶铜主要表现为压痕边缘的皱褶堆积,而单晶硅表现为塑性区边界裂纹的萌生与扩展.

Radial Nanofretting of Single Crystal Copper and Silicon under High Load

As a new tribological movement mode,nanofretting refers to a cyclic movement of contact interfaces with the relative displacement amplitude in nanometer scale.Since it widely exists in microelctromechanical systems(MEMS),nanofretting damage may become a key tribological problem besides microwear and adhesion.Using a nanoindenter,the radial nanofretting behaviors of single crystal copper and Si(110) were studied under various peak indentation forces.It was found that the residual deformation depth decreased quickly with the increase in the number of nanofretting cycles.The indentation curves of copper and silicon exhibited a hysteresis loop especially in the initial nanofretting cycles,which indicated an energy dissipative process in nanofretting.The energy dissipation in nanofretting may be attributed to the friction between diamond indenter and sample,the plastic deformation of sample,as well as the stress-induced phase transition process in nanofretting.The energy dissipation was the highest in the first cycle and then decreased dramatically to a constant after 20 cycles.Both the contact stiffness and elastic modulus increased sharply in the first several cycles and attained constants thereafter.Even the projected area of the indents in copper and silicon showed an increase with the increase in the number of nanofretting cycles,the radial nanofretting damage exhibited their unique behaviors.The radial nanofretting damage in copper was mainly identified as the pileup of the wrinkles around indents.Whereas,the radial nanofretting damage in silicon was characterized as the initiation and propagation of the cracks on the edges of plastic zone.