薄膜微桥拉伸法基体梯度裂纹的制备及其稳态扩展

Preparation of Gradient Substrate Pre-crack and Its Steady Propagation During Tensile Testing for Microbridge Method

16 mm×32 mm单晶硅片基体紧凑拉伸测试样品上尖锐裂纹的可控制备,及其在拉伸载荷作用下的稳态扩展,是微桥法薄膜断裂韧性测试得以实现的关键。相较于紫外激光切割,采用红外激光切割方法在基体背面制备出锯齿状的背槽,并通过控制位移加载步幅和球形压头均衡加载,实现了三点弯曲法基体尖锐裂纹的可控制备。该基体裂纹具有梯度形貌特征,对加载过程中的"载荷-位移"曲线以及表面裂纹扩展过程进行了考察,揭示了梯度形貌对裂纹失稳扩展的抑制作用。基体尖锐裂纹的可控制备及其在拉伸测试中的稳态扩展,为微桥法的标准化测试奠定了基础。

Compact tension(CT)specimen of 16 mm×32 mm rectangular silicon wafer was used as the substrate for the microbridge tensile testing method.A proper way is required to produce a sharp pre-crack in the CT specimen.And a stable extension of the crack is also desired during the tensile testing for fracture toughness of thin films.However,both of them have not been successfully achieved yet since microbridge tensile testing method was proposed in 2011.In this paper,a narrow groove with a serration morphology was produced with infrared laser cutting method along central line of the silicon wafer CT specimen.And then three-point bending test was conducted to prepared a sharp crack with a self-made mini device.A linear actuator was adopted serving as the loading bar.Load are applied through a0.8 mm spherical indenter mounting on the tip of the loading bar.The balance loading was studied,and a sharp crack of desired length can then be produced in a standard procedure.More importantly,the substrate crack possesses a type of gradient morphology.That is,there is a long distance that silicon wafer fractures along the groove,while instead of a visible crack,a crease mark appear on the surface.Tensile testing of the CT specimen with the gradient substrate crack was then conducted.Results showed that the gradient crack extends a short distance of about 474μm in average in a single increment step of 0.5μm of the actuator.Though the crack extends to a distance varying in a wide range,it is evidently not catastrophic.Theoretical analysis showed that the gradient substrate crack consumes an ever increasing energy during its fracture,which restrains a sudden extension of substrate crack to a long distance.Innovations that have been made in this paper,a special laser cutting procedure,the serration morphology of the groove,a gradient crack as well as its stable extension within a single displacement increment of the loading,provide the base of the microbridge tensile testing method for fracture toughness of thin films.