| Summary: | 博士 === 國立清華大學 === 材料科學工程學系 === 87 === Abstract
This research program concerns itself primarily in the study of adhesion of metal nitrides on metal substrates. Metal nitride coatings that include TiN, CrN and ZrN are either prepared by hollow-cathode discharge ion plating process or arc-ion-plating process. The Taguchi methodology of design of experiment is employed for optimization of process variables. Titanium nitride (TiN), the principal nitride coating prepared for the program was put to go through a comprehensive structure characterization and property evaluation. The evaluation culminated in the study of adhesion by means of scratch testing and tensile-film-cracking technique. In scratch testing three failure modes, i.e. buckling failure with cracking, spallation/chipping failure and tensile cracking are distinguished. A two-step failure mechanism is found to be involved in the entire failure of the coating during testing. Only the spallation/chipping failure mode can be related to the adhesion strength of the coating. The other two failure modes are connected to the cohesive strength of the coating. The exiting model that relates the normal critical load of the scratch testing has been critically reviewed. The Bull-Rickerby model and its modified version by Attar et al. are deemed to be erroneous. In the evaluation of adhesion by tensile-film-cracking technique, it is concluded that a realistic interfacial shear strength of a coating to its substrate can be obtained after the residual stress of the coating is incorporated in the effective stress that triggers the fracture of coatings. In addition, a finite element analysis (FEA) is performed for the stress distribution of the coating-substrate system subjected to tensile strain. The result of FEA shows that both extreme ends and the midpoint of the inter-crack-spacing are associated with the maximum value and the zero value of the interfacial shear stress, respectively, which can be approximated by a cosine function of a half wavelength. This differs significantly from the sine function employed by the old model proposed by Agrawal and Raj, in which both extreme ends and the mid-point of the inter-crack-spacing have a zero interfacial shear stress value, and locations at 1/4 and 3/4 length-wise are associated with the maximum value. As a result, a new model relating the maximum interfacial shear stress is proposed.
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