# Effect of Surface Roughness on Early Stage Oxidation Behavior of Ni-Base Superalloy IN 625
## 2 Materials and Methods  
In the present work a commercially available Ni-base superalloy IN 625, with nominal composition given in Table 1, was investigated.  
From the rods of IN 625, 2 mm thick coupons with diameter of 20 mm were machined. The surfaces of the samples were prepared with two methods, namely grinding up to 220 grit SiC paper and polishing using a 1 \(\upmu\)m diamond suspension. To control the reproducibility of the test results, two samples per each surface condition were prepared. After surface preparation, samples were ultrasonically cleaned in ethanol. The roughness of the samples was measured using two methods: by traditional roughness measurement using contact profilometer HOMMEL Werke T8000 and by fractal analysis of the cross-sections produced from the ground and polished sample cross-section images. The surface roughness of each sample was evaluated by commonly used parameters such as arithmetic average height (R\({}_{\text{a}}\)), average distance between the highest peak and lowest valley (R\({}_{\text{z}}\)), and maximum height of the profile (R\({}_{\text{max}}\)), of which the calculation procedures are described in [13; 14]. The analysis by contact profilometer was done under the following parameters: the traverse length (L\({}_{\text{t}}\)) was 4 mm, linear speed of stylus (V\({}_{\text{t}}\)) was 0.5 mm/s, and cut off length was 0.8 mm. Fractal analysis of polished and ground samples was performed on the cross-sections of as-prepared alloys in accordance with the procedure given by [PERSON] et al. [15] using Sfrax 1.0 software [16]. The procedure of roughness analysis using fractal analysis included the following:  
1. Image acquisition by optical microscope,
2. Converting images into binary scale images (alloy--white, resin--black),
3. Digitalization of binary scale images into 2-D x-y data sets with constant x step,
4. Performing fractal analysis using Sfrax 1.0 software [16].  
\begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline  & & & & & & & & & & \\ \hline Ni & Cr & Co & Mo & Nb & Al & Ti & Fe & Mn & C \\ \hline BASE & 21.5 & 1 & 9 & 3.65 & 0.4 & 0.4 & 5 & 0.5 & 0.1 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Nominal chemical composition of IN 625 expressed in wt %.  
As described in [10] based on fractal analysis the plots showing the relative length as function of scale length can be drawn. Also the following characteristic parameters describing the measured surface roughness profiles can be derived [17; 18]:  
* D (Fractal Dimension), given by the Equation (1): \[\text{D}=1+\mid\text{slope}\mid,\] (1)
* LSFC (Length-Scale Fractal Complexity), given by the Equation (2): \[\text{LSFC}=1000\;(\text{D}-1),\] (2)
* Relative length at a given scale (L\({}_{\text{R}}\))
* Smooth-rough crossover (SRC)  
Rougher surfaces possess higher values of D, LSFC, and relative length at a given scale, while the value of SRC indicates the scale at which roughness starts to prevail (e.g., on \(\upmu\)m-scale). After roughness analysis, samples were cleaned in ethanol once again using ultrasonic cleaner. Such prepared alloys were oxidized at 950 \({}^{\circ}\)C for 24 h in air using tube furnace. After exposure, samples were analyzed in terms of chemical composition using glow discharge optical emission spectrometry (GD-OES). The GD-OES depth profiles were quantified according to the procedure described in references [19; 20; 21]. After GD-OES analysis, samples were electro-plated with nickel and mounted in epoxy resin. Metallographic cross-sections of the oxidized alloy specimens were prepared by a series of grinding and polishing steps, the final step being fine polishing with the SiO\({}_{2}\) suspension with the 0.25 \(\upmu\)m granulation. The cross-sections were analyzed using a Nikon Epiphot 300 optical microscope and a Hitachi S3400N scanning electron microscope (SEM). The oxidation kinetics was determined by oxide scale thickness measurement in 13 randomly selected locations on the cross-sections.