Evaluation of Pinhole Defects and Corrosion Resistance of TiN-coated Stainless Steels

TiN薄膜作为干性涂层的代表，其硬度很高，因此具有很高的耐磨损和耐腐蚀性能，而且其外观具有吸引人的金色。然而，由于微缺陷的存在，它们通常不适合耐腐蚀应用。特别是所谓的穿孔针孔缺陷影响了TiN薄膜的环境屏蔽效果，在提高基体的耐蚀性方面还存在许多问题。为了使TiN薄膜中的针孔缺陷密度最小化，必须对工艺参数进行充分的控制，为此，必须开发一种快速、定量的针孔缺陷评价方法。因此，本研究采用临界钝化电流密度（CPCD）方法，定量评价了采用射频反应溅射法制备tin涂层不锈钢的针孔缺陷面积比。主要结果如下：在N2流量与总流量比F_<N2>/F_<总>为0.4及以上的条件下，沉积的-0.5mum厚tin膜具有典型的金样颜色。它们呈柱状结构，随着F_<N2>/F_<total>的增大，优先取向由<220>向<111>方向变化。本文将针孔缺陷的电化学评价集中在F_<N2>/F_<total> = 0.4时沉积的TiN薄膜上。随着膜厚的增加，镀锡不锈钢的临界钝化电流密度i_<crit>显著降低，镀锡后不锈钢的耐蚀性显著提高。厚度在1.5m以上，i_<crit>呈增加趋势，并出现开裂和/或剥落。针孔缺陷的面积比通过涂层与未涂层试样的i_<临界>的比值，即CPCD法来评价。阳极极化前后的光学显微图与实际缺陷面积比吻合较好。因此，这种电化学测量方法作为耐蚀涂层针孔缺陷的评价方法是优越的。少

TiN films as a representative dry coating are very hard, so they are highly resistant against wear and corrosion, and furthermore their appearance is attractive gold-like color. Due to the presence of micro-defects, however, they are usually unsuited for corrosion-resistible applications. In particular, the so-called perforating pinhole defect impairs the environment shielding effect of TiN film, and there are many problems remaining in connection with improving the corrosion resistance of the substrat. In orter to minimize pinhole defect density in a TiN film, the process parameters must be adequately controlled, for this to happen, a quick and quantitative evaluation method of pinhole defect must be developed. In this study, therefore, the critical passivation current density (CPCD) method was used to evaluate pinhole defects quantitatively as the defect area ratio for the TiN-coated stainless steels prepared by the r.f.reactive sputtering. The main results obtained can be summarized … More as follows :The -0.5mum thick TiN-films deposited at the N_2 flow-to-total flow ratio F_<N2>/F_<total> of 0.4 and above were with the characteristic gold-like color. They exhibited the columnar structure, of which the preferential orientation changed from<220>to<111>direction with an increasing F_<N2>/F_<total>. Here, the electrochemical evaluation of pinhole defects is focused on the TiN films deposited at F_<N2>/F_<total> of 0.4. The critical passivation current density i_<crit> in the TiN-Coated stainless steels decreased considerably with an increasing film thickness, and the resulting corrosion resistance was improved remarkably by TiN-coating. Above 1.5mum in thickness, however, there was an increasing tendency in i_<crit> with cracking and/or peeling. The area ratio of pinhole defects was evaluated by the ratio of i_<crit> of a coated and a non-coated specimen, i.e., by the CPCD method. The result coincided well with the true defect area ratio based on the optical micrographs before and after polarized anodically. Consequently, such electrochemical measurement is superior as an evaluation method for the pinhole defects of corrosion-resistible coating. Less