The mechanical properties of brittle ZnO film and ductile Au film along with the proposed characterization methodologies are presented here....[ Read more ]

The mechanical properties of brittle ZnO film and ductile Au film along with the proposed characterization methodologies are presented here.

ZnO films with thicknesses ranging from 0.202 to 1.535 μm were deposited by using the magnetron sputtering technique on Si (100) substrates 525 μm thick. Then, Vickers indentation tests were carried out on the ZnO/Si systems at room temperature, in which the applied load varied from 10mN to 2.0 N. The experimental results show that only indentation-induced radial cracking occurred in the systems with film thicknesses equal to and thinner than 0.554 μm, from which the residual stress in the films was extracted to be 387 MPa in compression. For the systems with film thicknesses equal to and thicker than 0.832 μm, only indentation-induced delamination occurred when indentation loads were low. Under high indentation loads, radial cracking concurrently occurred with delamination. The radial cracks were invisible at the film surfaces because the crack length was smaller than the delamination size. The critical film thickness for indentation-induced delamination was found to be around 0.7 μm for the ZnO/Si systems. Combining the composite hardness models with the indentation-induced delamination model, we developed a method to determine the interfacial fracture energy between a film and its substrate. The novel method is particularly useful for indentation equipment without any displacement measurement devices. Using the new method, we extracted the interfacial fracture energy to be about 12.2 J m^-2 and from 9.2 to 11.7 J m^-2 for the cases without and with buckling respectively of delaminated films. Consequently, the pure mode I interfacial fracture energy was calculated to be 10.4 J m^-2 for the ZnO/Si systems.

In particular, we explored the phase angle and a formula was derived to calculate the phase angle of interfacial mixed mode fracture in the indentation delamination test, in which the delaminated film buckles. Interfacial fracture energy depends on the phase angle and calculating the phase angle is necessary for the determination of mode I interfacial fracture toughness.

In the section of Au film, we report a microbridge testing method for microbridge beams initially buckled by residual compressive stress. A theoretical formula is derived in closed form with the consideration of substrate deformation. Measuring the profile of a buckled microbridge beam, one can evaluate Young's modulus and residual compressive stress of the beam. Alternatively, the Young's modulus and residual stress can also be evaluated from the rnicrobridge test under small loads, i.e., from the elastic load-deflection curve. Moreover, we introduce a microbridge testing approach to determine yield strength of a ductile thin film. Experimentally, microbridge samples were fabricated with a 0.48 μm thick Au film deposited on a silicon wafer by electroplating. The Young's modulus, residual stress and yield strength of the Au film were determined from the microbridge test to be 66.2 ± 2.9 GPa, -8.5 ± 1.0 MPa and 162.4 ± 5.9 MPa, respectively.

In addition, strain gradient plasticity theory of polycrystalline thin film was investigated experimentally with bending the cantilever beams of Au films of different thicknesses ranging from 0.2 - 4.8 μm. We found that the strain gradient effect on strengthening, as proposed by the theory and quantified by the characteristic length scale, vanishes when the grain size of the Au films decreases to a critical value so that the geometrically necessary dislocations can not be sustained within a grain. Although smaller film thickness increases the magnitude of strain gradient, film thickness alone does not govern the strengthening effect because the underlying mechanism of the theory requires the grain size to be large enough to accommodate dislocations. Only when the grain size exceeds the critical value, the strain gradient strengthening starts to take effect, which is also the observations of this study.