In this research, the adhesion strength of PVD TiN film film on ceramics was investigated. In order to explore the failure source of TiN film, two kinds of transparent model substrate were chosen. Based on the results of model experiments, TiN coated $Si_3N_4$ was studied in the context of effect of substrate yield properties on the adhesion strength of TiN film.
In chapter III, the failure mode of PVD TiN film on $(11\bar{2}0)$ sapphire were investigated by analyzing the adhesion strength and failure mechanism through in-situ observations of the fracture sequence during scratch tests and static normal indentation. TiN was deposited by arc ion plating on $(11\bar{2}0)$ sapphire and the thickness of the TiN film was controlled to 700 nm. Delamination of TiN film was monitored in-situ from below the contact through a transparent sapphire substrate, using zoom optics mounted into a video imaging sensor. In-situ observation of failure source enables us to detect the failure origin of TiN coating on sapphire. The failure origin of TiN film on $(11\bar{2}0)$ sapphire was identified as both rhombohedral and basal twinning of the sapphire substrate. Rhombohedral twinning was initiated first and basal twinning ensued. Twinning-induced plastic deformation of the sapphire substrate triggered the initiation of interfacial delamination of the TiN coating. The plastic deformation of the substrate ultimately induced failure of the protective coating.
In chapter IV, TiN film coated soda-lime glass were investigated. The effect of TiN film thickness as well as the effect of elastic mismatch between TiN film and a soda-lime glass was analyzed. Similar to a sapphire substrate, through in-situ observations of the fracture sequence during scratch tests and static normal indentations, failure mechanism was suggested. TiN was deposited by arc ion plating on soda-lime glass substrate and the thickness of TiN film was controlled from 1 ㎛ to 5 ㎛. Sets of static normal indentations at a maximum load from 5 to 30 N at an interval of 5 N was conducted to compare with the results of in-situ observation. The observation with top and bottom view after the indentations clearly demonstrates the failure source of TiN film. The stress distribution during loading with diamond cone indenter with a tip radius of r = 200 ㎛, on TiN coated glass was analyzed by a FEM simulation. The maximum principle shear stress located at just below the surface of glass substrate reached 7 GPa which was reported as critical load to induce plastic deformation of a soda-lime glass. During removal of applied load, the stress relaxation stored in glass substrate provokes the radial cracking on glass substrate. The failure of TiN coated soda-lime glass was originated from the radial cracking of the glass substrate. Radial cracking was initiated during removing applied load. The radial cracking on substrate surface triggered the initiation of interfacial delamination of the TiN coating. It was convinced that the plastic deformation including microcracking and radial cracking of the substrate ultimately induced failure of the protective coating.
In chapter V, based on the results of model experiments, TiN coated $Si_3N_4$ was investigated in the context of controlling the yield properties of $Si_3N_4$ substrate through governing the microstructure of $Si_3N_4$. The effect of the microstructure of silicon nitride, which was used for a substrate, on the adhesion strength of PVD TiN film on $Si_3N_4$ was investigated. Silicon nitride substrates with different microstructures were synthesized by controlling the size (fine or coarse), the phase (α or β) of $Si_3N_4$ starting powder, and sintering temperature. The microstructure of $Si_3N_4$ was characterized in terms of grain size, aspect ratio of elongated grain, and β -to- α phase ratio. Identical chemical composition but different mechanical properties, such as toughness, elastic modulus, and hardness of $Si_3N_4$ were obtained from the diverse microstructures. Hertzian indentation was introduced to estimate the yield properties of $Si_3N_4$, such as critical loads for yield $(P_y)$ and for ring cracking $(P_c)$. The effect of the microstructure of $Si_3N_4$ on adhesion strength is discussed. TiN films on $Si_3N_4$ showed very high adhesion strengths in a range of 80-140 N. Hardness and the Py of $Si_3N_4$ substrate were primary parameters influencing the adhesion strength of TiN film.
Accordingly, the adhesion strength of PVD TiN Film could be controlled by optimum design of the microstructure of $Si_3N_4$ substrates. Moreover, very high adhesion strength of TiN film on $Si_3N_4$ increases the potential for the application in the industrial fields.
