Toughness measurement of ceramic thin films by two-step uniaxial tensile method

Fracture toughness of bulk materials is a topic of classical measurements, but that of thin films, especially hard and superhard thin films, is a different matter owing to specimen dimensions (thickness) and lack of convincing test procedure. Fracture toughness is the ability of a material to resist the growth of a pre-existing crack. For thin films, however, the testing usually includes crack initiation together with crack propagation thus the result is not the same as the classical “fracture toughness”. For thin films, it is better to use term “toughness” to avoid confusion. For engineering applications, it is of vital importance that the toughness of thin films be evaluated with a convincing method and procedure. This paper presents a new and relatively easy method in which two-step uniaxial tensile test is performed to characterise toughness of ceramic thin films. In the test, the film/substrate system is subjected to uniaxial tensile stress until the film fractures. After the load is removed to allow complete elastic recovering, this system is subjected to a second loading to the earlier extension. The toughness of the thin film is derived from the energy difference between the two subsequent loading extension curves. The crack initiation and propagation patterns are examined with scanning electron microscope (SEM). As an example, toughness of a nc-TiN/a-SiNx nanocomposite thin film has been tested with satisfactory results.