A new technique to measure through film thickness fracture toughness

Abstract

A new experimental procedure was developed to measure the in plane, through film thickness, fracture toughness of hard thin films deposited on soft substrates. Pre-cracks were fabricated in thin film specimens using focused ion beam (FIB) milling techniques. The crack opening force was generated by means of the indentation sinking-in effect. The effect creates a bending moment and tensile stress on the thin film near the pre-cracks and promotes crack extension. The amount of crack tip blunting prior to the critical failure was characterized by inspecting the crack cross-sections. The fracture toughness of the thin film can be qualitatively evaluated by using the crack tip opening displacement model (CTOD). The fracture toughness results of two hard thin film/soft substrate systems are presented; hard nickel phosphorus films on soft aluminum substrates and titanium/aluminum multilayered films on aluminum substrates. Finite element analysis (FEA) techniques were also used to understand the mode of mixity near the crack tip. The results indicated that the crack tip mode of mixity is dominated by the Mode I opening, provided that the indentation location is sufficiently far from the pre-cracks.