Abstract
This work investigated the influence of the main process parameters on the preferred orientation, porosity, grain size and corrosion behavior of TiN films deposited by Hollow Cathode Discharge Ion-Plating system (HCDIP).Within the investigated ranges, the preferred orientation, porosity, grain size of TiN films vary mainly with the change of deposition temperature and the ion bombardment, which is expressed with an index of SE. High deposition temperature together with low ion bombardment helps to deposition a (200)-oriented TiN coating. On the contrary, low deposition temperature and high ion bombardment lead to a TiN film with (220) preferred orientation. Between these two extreme deposition conditions, (111) is the preferred orientation of deposited TiN films. Increasing deposition temperature or the degree of ion bombardment can reduce the porosity of TiN films. The grain size of TiN films generally increases with increasing temperature. The influence of ion bombardment is more evident at high temperature. Grain size initially increases with ion bombardment but decreases at later stage. The relative importance of the effects induced by the temperature and the ion bombardment determines the specific microstructure characteristics of TiN coatings. Roughly speaking, the denser TiN coatings either have a large grain size or a highly (111)-preferred texture with smaller grain.The electrochemical corrosion studies reveal that high deposition temperature and high negative bias voltage are beneficial to deposit TiN coatings with better corrosion resistance. Generally, TiN-coated stainless steels with high porosity have lower corrosion resistance. However, the variation of corrosion extent may be more than one order of magnitude for TiN coatings with the same level of porosity, especially at low level of porosity. At early stage of corrosion, micro-cracks appear on the TiN coating. The corrosion of substrate will be accelerated after the extension of the micro-cracks on TiN film.In this study, two combined indexes were used to describe the correlation between the preferred orientation, the porosity of TiN films and the deposition temperature, bias voltage and deposition power. Within the investigated ranges, the preferred orientation of TiN films changes from (200) to (111) and then to (220) with the increasing SE / T(K) index. And the porosity of TiN coatings decreases with increasing SE ×T index.This work also proposed a methodology, which is based on the corrosiveness of electrochemical test process, to measure the porosity of TiN coatings. By decreasing the corrosiveness of test process the problem of pinhole enlargement can be avoided and accurate measurement can be achieved. Based on this methodology, different electrochemical porosity test techniques can achieve a similar result of porosity measurements.