Strength of ductile thin films

How to measure strength of thin ductile films

For determining the mechanical properties of ductile thin films on brittle substrates as strength or toughness a testing method was developed by which the influence of the substrate on the test results can be eliminated (Fig.1). For these purposes a sharp saw-cut is introduced into the substrate to near the film interface from the specimen backside. In order to prepare a free-standing film region, the substrate ligament is subsequently broken by bending using a breakage force Fbreakage, which is sufficiently small to avoid film damage. The strength of the film is tested now by applicating a bending load F in opposite direction to open the initiated crack as shown in Fig.1a. The strength σF of the film may be calculated from the measured load FF if a homogeneous tensile stress state inside the film is justified for h<<b

(1)         σF = FF • a / (w • h • b)

(a - distance between loading point and crack, w – specimen width, h - film thickness, b - depth of the residual substrate above the notch). Application of equation (1) requires that a plane stress situation inside the film exists. The crack opening length xF at the film-substrate interface as obtained from the load point deflection sF becomes (for s<< a):

(2)      xF = sF • b / a .

The film strain ε = Δl/l0 is calculated from xF assuming xF = Δl. However, only load-deflection curves can be presented instead of stress-strain plots, because the length of the strained film strip, l0, is not exactly known. An upper estimate of l0 can be derived assuming a linear elastic behavior of the film up to the maximum load, and using the Young's modulus of the film obtained from other measurements like the laser pulse method.

For tensile test measurements (without bending) using the same apparatus as shown in Fig. 1 but with an other sample holder, the strength calculation neglecting any superimposed bending is simplified to

(3)     σF = FF / (w • h) .

 

Fig. 1a) Schematic drawing of the measuring principle
Fig. 1b) Picture of the apparatus (developed by G. Jaeger, IFW Dresden).