Indentation fracture and indentation delamination in ZnO film/Si substrate systems

Abstract

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 10 mN 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.

Acknowledgements

This work is fully supported by grant HKUST6013/98E from the Research Grants Council of the Hong Kong Special Administrative Region, PR China. M. H. Zhao thanks Hong Kong University of Science and Technology for the Post-Doctoral Fellowship Matching Fund. T.-Y. Zhang thanks the Croucher Foundation for the Croucher Senior Research Fellowship Award, which gave him more research time by releasing him from teaching duties.