Investigation of the effect of different bias modes applications on the corrosion behavior of Ti and TiAlN coatings produced by CA-PVD technique

Nitride based hard ceramic coatings are one of the most popular materials for tribological applications due to their high hardness and wear resistance. Nowadays, application areas of these coatings are not limited just for tribological usage, they are also highly demanded in corrosion protection, electronic circuit elements, decorative coating applications, etc. These advanced applications which require high performance make the corrosion protection ability a significant parameter for nitride based hard ceramic coatings. However, coatings produced by cathodic arc physical vapor deposition (CA-PVD) method have some inherent structural defects such as droplets, pinholes, porosities, cracks, voids, etc. which allow the aggressive media to reach to the substrate-coating interface and cause poor corrosion protection ability. At that point, these coatings can be used satisfactorily as long as substrate material is corrosion resistant. To improve corrosion behavior of nitride based ceramic hard coatings, different approaches have been proposed such as optimization of coating parameters, multilayer coating applications, alloying the coating, composite and nano-composite coating applications, increasing the coating thickness, and deposition of a metallic interlayer between the substrate and coating. In the literature, studies focused on the effect and interpretation of the coating parameters on the corrosion protection ability of nitride based hard ceramic coatings are limited. In this study, we aimed to investigate the effect of structural changes induced by application of different modes of bias on the corrosion behavior of Ti and TiAlN coatings produced by CA-PVD technique. For this purpose, Ti and TiAlN coatings were deposited on the mild steel substrate by applying DC bias and superimposed pulse on DC bias modes. During deposition process, different coating periods for Ti interlayer coatings and different pulse bias magnitudes for TiAlN coatings were selected to observe the effect of coating deposition period and pulse bias magnitude on the structural changes and corrosion ability of the coatings. The morphological and structural changes that are expected to induce on the coatings depending on the applying different bias modes were investigated by characterization process. During the characterization step, morphological observations were performed by SEM analyses and phase identifications were conducted by XRD analyses. Also, FIB analyses were combined with SEM analyses to observe morphological changes expected to occur on the coatings after electrochemical tests. To investigate the corrosion behavior of Ti interlayer and TiAlN coatings, electrochemical experiments were performed in deaerated 0.1 N HCl solution at room temperature. Three electrode cell was used for the electrochemical measurements. Electrochemical tests were divided into two steps. In the first step, time dependent electrochemical impedance spectroscopy (EIS) measurements were conducted in 45 kHz-10 mHz frequency range with 10 mV amplitude. Then in the second step, potentiodynamic tests were done by polarization of Ti coatings from cathodic region to anodic region and polarization of TiAlN coatings to the cathodic region with 0.5 mV/s scan rate. For a further support to our electrochemical test results, porosity calculations were conducted by using two different approaches. One of these methods is proposed by Elsener, et al. based on the formula including polarization resistance, corrosion potential values of the substrate material and coating and anodic tafel slope of the substrate material. The fact that the potential data used by this method represent complex reactions and create question marks in terms of the reliability and correctness of the obtained results. At that point, we propose a new method for porosity calculations. This new method is based on the idea that the open area in the coatings, which means the amount of porosity, can be calculated from the cathodic current density deviations of the coatings. It is well known that, ceramic based hard coatings are behaving cathodic with respect to steel substrates and the cathodic reaction, that is hydrogen evaluation reaction for deaerated acidic solutions, on them is kinetically limited. It means that cathodic reactions occurring on these coating systems are mainly decided by substrate material. Thus, cathodic current density values of coating systems mainly belong to the substrate which is exposed to the aggressive media by porosities on the coatings. Therefore, we think that using cathodic current density deviations can be a good way for determination of the porosity amount of the coatings. From the characterization step of the produced coatings, it is confirmed by XRD analyses that application of superimposed bias modes caused titanium peaks to shift to the left and to their expansion. This indicates that particle size decreases by applying superimposed bias modes. In TiAlN coatings, there is no new phase formation observed, but changes in the orientation are determined and also, phase peaks shift to the left and expands as in Ti interlayer coatings. SEM analyses were performed to investigate the morphological changes in the coatings. In both Ti interlayer and TiAlN coatings, the droplet density decreases dramatically by superimposed bias mode application. Also, in the case of Ti interlayer coatings, the droplet density increases with increasing deposition period regardless of the bias modes. In TiAlN coatings, the droplet density decreases with increasing the magnitude of pulse bias voltage. From electrochemical experiments performed to determine the corrosion behavior of the coatings polarization resistance, corrosion potential and current density values were obtained and investigated in aspect of the corrosion protection ability. Polarization resistance values of Ti interlayer coatings obtained by EIS measurements are higher than bare mild steel substrate which means that deposition of Ti interlayer coatings increases the corrosion resistance of the mild steel independent of the mode of bias application. Also by increasing the immersion time, polarization resistance values of the samples tend to decrease. Application of superimposed bias mode has a positive impact on the polarization resistance of the coatings, and Ti interlayer coatings produced by superimposed bias mode have higher polarization resistance than Ti coatings produced by DC bias mode. Moreover, increasing of deposition time results in production of higher polarization resistant coatings regardless on the bias mode. Besides EIS measurement results, data obtained by Tafel extrapolation method from polarization curves of the samples were investigated to determine the corrosion behavior of the coatings. According to that, corrosion potential values of all coatings have more negative values than the potential of bare mild steel. Also, all Ti interlayer coatings have lower corrosion current densities than bare mild steel which exhibits that Ti coatings improve corrosion resistance of mild steel. Ti interlayer coatings produced by superimposed bias mode have lower corrosion current density values than Ti coatings produced by DC bias mode. EIS measurement results of TiAlN coatings show that all coatings have higher polarization resistance than bare mild steel and it shows that TiAlN coatings improve the corrosion resistance of bare mild steel. Also, polarization resistance values of all samples decreases by increasing immersion time. Superimposed bias mode application causes increase in the polarization resistance values of TiAlN coatings and the increasing the magnitude of pulse bias voltage improves the polarization resistance values. Furthermore, corrosion potential values of coatings are similar to the bare steel, but some of these coatings produced by different pulse bias voltage magnitude represent more negative corrosion potential values compared to the bare steel. According to the data obtained by electrochemical tests for both Ti interlayer and TiAlN coatings, it can be seen that EIS measurements and potentiodynamic analyses results harmonize with each other. Observation of the increment in the corrosion protection ability of the coatings by applying superimposed bias mode which is determined by polarization resistance and corrosion current density values can be explained by the reduction of the droplet concentration on the coatings and denser structure of the coatings. Production of the coatings having denser and less droplet concentration by using superimposed bias mode minimizes the defect sizes such as pinholes, porosities, etc. As a result, open paths are reduced and corrosion resistance of the coatings improves. Higher corrosion resistance values obtained from Ti coating produced by longer deposition period may result from the increasing of the coating thickness. After the electrochemical experiments, the porosity amounts of the coatings were calculated by two different methods. According to results obtained from the first method proposed by Elsener et al., Ti interlayer coatings produced by superimposed bias mode have lower porosity amount. In TiAlN coatings, as in Ti coatings, application of superimposed bias mode causes a dramatic decreasing in the amount of porosity in the coatings. In addition, increasing the deposition time and the magnitude of pulse bias voltage reduce the amount of porosity. By increasing the immersion time, porosity amount of the coatings increases regardless of the bias mode for both Ti and TiAlN coatings. This is related to the progression of the corrosion of the substrate under the coatings by the increase of the exposure period. In the second method, in which the amount of porosity is calculated from the cathodic current density variations proposed in this study, the same tendency as in the first method is observed for Ti coatings, but there is some diversity in the trend of porosity amounts for TiAlN coatings. The significant difference between these two methods is that the amount of porosity obtained by the proposed method is lower than the results obtained by the Elsener et. al. method. At this point, ferroxyl test were performed to verify which method gives more reliable and realistic porosity values. According to the results, it is found that porosity calculation method based on the cathodic current density variations and ferroxyl test gave harmonized results with each other. Thus, it is confirmed that porosity calculation method proposed by us gives more reliable and realistic results.