ROUGHNESS CHARACTERIZATION PROCEDURE FOR ELECTROPOLISHED BRASS

Transcription

1 XXX International Conference on Surface Modification Technologies (SMT30) 29TH JUNE - 1ST JULY, 2016, MILAN, ITALY ROUGHNESS CHARACTERIZATION PROCEDURE FOR ELECTROPOLISHED BRASS Chloé Rotty a, Audrey Mandroyan a*, Marie-Laure Doche a, Sandrine Monney a, Séverine Lallemand a, Jean-Yves Hihn a a Institut UTINAM, UMR CNRS 6213, Université de Bourgogne Franche-Comté, 30 avenue de l Observatoire, Besançon, France. Abstract Brasses are widely used in watch manufacturing and jewelry as based materials, thanks to their good mechanical properties and corrosion resistance. Smooth and bright surfaces are usually obtained by mechanical polishing requiring a succession of manual operations. Industrially, polishing quality is visually controlled by the polisher himself. Electropolishing (EP) is generally employed either to replace mechanical polishing entirely or as a finishing treatment for complex shapes. This is an anodic dissolution phenomenon allowing a mirror finish to be obtained. EP process development at laboratory scale requires setting up of surface physical characterization techniques so as to appreciate the visual aspect. In the present study, several grades of brass (lead-free) were electropolished in a H3PO4 electrolyte using a Rotating Disk Electrode (RDE). After an electrochemical study, several specimens were treated at different potentials to study roughness evolution and to determine the best conditions. Roughness characterization was performed by combining a set of methods (SEM, AFM, mechanical roughness meter), revealing that roughness parameters are clearly dependent on the measurement technology. To confirm such results, a calibration sample was prepared by mechanical polishing (diamond suspension of 1µm) and compared to the others. On perfectly electropolished samples, roughness parameters (Ra, Rq, Rt) exhibit apparent discrepancies between mechanical roughness meter and AFM for example. This is partly explained by the measurement window as well as by the roughness profile. Assessment of an electropolished surface is not related to an absolute roughness value but must be defined relatively to the technique used to measure it. Keywords: Electropolishing; Surface roughness; Atomic Force Microscopy; Brass; Scanning Electron Microscopy. Nomenclature EP Electropolishing MP Mechanical polishing Ra Arithmetic average roughness (nm) Rq Root-mean square roughness (nm) Rt Peak-to valley (nm) Skew Skewness: asymmetric factor (-) * Corresponding author. Tel.: ; fax: address:audrey.mandroyan@univ-fcomte.fr

2 1. Introduction Material surfaces consist of an addition of macro and micro-roughness at different scales, while roughness surface characteristics depend on the process used to treat the surface. In several industrial processes, smooth and bright surfaces are required and, usually, obtained by mechanical polishing. Electropolishing (EP) is generally employed either to replace mechanical polishing entirely or as a finishing treatment for complex shapes. This is an anodic dissolution phenomenon under which a mirror finish may be achieved. Electropolishing is widely used in industry to reduce metal surface roughness and obtain a bright and smooth aspect. It could be applied as pre-treatment or as a finishing solution for a large number of materials such as copper alloys (Gabe (1972)) in jewelry. Two surface phenomena are known to provide the electropolished aspect: leveling and brightening (Landolt (1987); Du and Suni (2004)). Anodic leveling is especially due to a different current distribution between peaks and valleys, implying a preferential dissolution of peaks. A random dissolution of atoms on the surface takes place when an anodic film is formed: this is surface brightening. A mirror surface finish is obtained when the polishing operation allows removal of asperities larger than light wave length (below 400nm) (Lin and Hu 2008). While the electropolishing process has been studied by several researchers, it is still not fully understood (Lin, Hu, and Lee (2009)). In the watch manufacturing industry, polishing quality is typically evaluated by a visual examination. However, there is a strong need for physical control techniques in manufacturing output. Several techniques can be considered to measure surface roughness (optical method: reflective or interferometric, MEB, AFM, etc.), amongst which the most widely used is the mechanical stylus profiler. Indeed, resolution of roughness measurement techniques is on the same scale as surface roughness, thus complicating measurement. In industrial applications, whatever the expected functional properties, surface roughness is characterized using the arithmetic average roughness Ra, the root-mean square roughness (RMS or Rq) and also the peak-tovalley roughness Rt. However, surface roughness could be characterized by various statistical parameters (amplitude, spacing and hybrid parameters) (Gadelmawla et al. (2002)). It is difficult to choose the right parameter since there are many possibilities according to the shape of the roughness profile, and their physical signification could be ambiguous. Consequently, there is no universal roughness parameter to describe a surface. In this study, we considered electropolishing of two brass grades. Four parameters (Ra, Rq, Rt and Skewness (Rsk or Skew: asymmetric factor)) have been selected to compare electropolished and mechanically polished surfaces. A set of methods (stylus profiler, Atomic Force Microscopy and Scanning Electron Microscopy) was applied on electropolished samples to discuss roughness evaluation. 2. Experimental details 2.1. Electrochemical analysis and sample preparation The CuZn37 and CuZn42 samples used in this study were provided by Masson s Steel. CuZn37 is a single phase alloy (where the phase is the richer phase in Cu), while CuZn42 is biphasic ( + phases). Electropolishing was carried out in 10M phosphoric-acid solution. Electrochemical measurements and EP trials were performed using a Radiometer Analytical PGZ301 potentiostat monitored by VoltaMaster 4 software. A rotating disc electrode (RDE) with a 0.283cm² active surface was used at a constant speed of 500rpm. Use of a jacketed glass reactor made it possible to work at a constant temperature of 25 C. For the voltammetric study, a three-electrode system was used with a saturated calomel reference electrode and platinum coated titanium mesh as the counter electrode. Linear Sweep Voltammograms were performed at a scan rate of 2mV/s. Electropolishing treatments were carried out under potentiostatic control (1V/SCE) for 10 minutes. Prior to EP treatments, all samples were first pre-polished on SiC 1200-grit sandpaper, in order to ensure good uniformity of the initial surface and better reproducibility. The reference sample (CuZn37) was mechanically polished using diamond paste up to a 1µm grain size. Samples used for electrochemical measurements were also prepared by mechanical polishing Surface characterization All samples were characterized by several methods: Scanning Electron Microscopy (SEM), mechanical roughness analysis and Atomic Force Microscopy (AFM). SEM images were obtained at x500 magnification with an Environmental FEI Quanta 450 W microscope.

3 Surface roughness was measured with a mechanical roughness meter (Dektak Bruker) and its software. A 2.5µm radius stylus with 3mg load was used in accordance to the NF EN ISO 4288:1998 standard. Five measurements were performed on each sample. An AFM analysis was conducted with a Nano-Observer CSI AFM. An AppNano Fort Part tip (tip radius <10nm) and cantilever (K: N/m) was used in contact mode. The scan speed used was 1ln/s and the scan size was 50*50µm. Roughness measurements and picture treatment were performed using MountainsMap software from Digital Surf. For each material, six profiles were treated to calculate average roughness parameters. 3. Results 3.1. Voltammetry Figure 1 shows the anodic polarization curves for CuZn37 and CuZn42 copper alloys in phosphoric acid solution plotted at 25 C at a scan rate of 2mV/s. These curves are typical of an electropolishing process and agree with literature results on the same substrate (Huang et al. (2014); Gabe (1972); Iskander et al. (1980)). At the beginning of the polarization curve, a current increase is first observed corresponding to brass oxidation, a typical feature in strongly acidic media. Then the curve presents a large current plateau characteristic of the electropolishing process, where dissolution is under diffusion limiting control. At higher potentials, the current increase corresponds to the oxygen evolution reaction due to solvent electrolysis. CuZn37 and CuZn42 voltammograms have similar shape regardless of alloy composition. The best EP condition is known to be at the middle of the limiting-current plateau for copper and copper alloys (Huang et al. (2014)). For further roughness characterization, all the samples were then electropolished at 1V/SCE (Fig.1). Figure 1: Anodic polarization curves for CuZn37 and CuZn42 in phosphoric acid solution (10M), RDE speed fixed at 500rpm. Scan rate 3.2. Morphological study by SEM =2mV/s. After a preliminary investigation, specimens of CuZn37 and CuZn42 were electropolished for 10 minutes in the above experimental conditions (H 3PO 4 10 M, 25 C, 500 rpm). A mirror finish aspect is obtained in these conditions for both brass samples. They are compared to a mechanically polished sample of CuZn37 (1 m granulometry). Figure 2: SEM images of (a) mechanically polished sample, (b) CuZn37 electropolished surface and (c) CuZn42 electropolished sample. SEM images are presented in Figure 2. The mechanically polished sample (Fig. 2a) is characterized by linear grooves parallel to the polishing direction, as grooves are formed by abrasive diamond particles.

4 The surface morphology of the two electropolished samples (Fig. 2b-c) is related to their phase composition. The CuZn37 sample presents polygonal grains of only one phase (the microstructure is only slightly visible indicating a good polishing quality), whereas CuZn42 appears diphasic due to its phase composition. It is in accordance with the phase diagram: under 37% of zinc, only alpha (α) phase is observed (Fig. 2b). Above 37% of zinc, alpha (α) and beta (β) phases are present (Fig. 2c). For CuZn42, a typical Widmanstätten phase structure is observed. Both samples appear smooth on SEM pictures, even on the CuZn42 one. As no relief difference can be highlighted between α and β phases, they are considered to dissolve at the same rate. Therefore, a brightened and leveled surface could be achieved when brass-rde is polarized at the middle of the current-limiting plateau AFM images AFM images have been taken on the same specimens (Fig. 3). Image size is 50µm * 50µm. As for SEM analysis, the mechanically polished sample presents small linear grooves due to diamond particles. The CuZn37 electropolished sample (Fig. 3b and 3e) is smooth, and grain boundaries are virtually invisible. On the AFM picture, the and phases of the CuZn42 sample (Fig. 3c and 3f) are not exactly at the same level on this scale. A preferential dissolution of β-phase (richer in Zn) is observed in accordance with the corrosion potential of the two phases, as copper is indeed more noble than zinc. From the AFM analysis, a quantitative determination of roughness was also performed. Figure 3: AFM images of (a,d) mechanically polished surface, (b,e) CuZn37 electropolished sample and (c,f) CuZn42 electropolished 3.4. Roughness evaluation sample Stylus profiler analysis Following the morphological evaluation of the several surfaces, mechanical roughness parameters were determined. A series of 5 measurements was taken on each sample in accordance with the standard. Roughness parameters show several differences between samples (Table 1). Considering Ra and Rq values, the best results are obtained on CuZn37 MP. However, both CuZn37 samples (MP and EP) present a roughness lower than 50 nm, while CuZn42 is about 10 times rougher. Regarding the Rt parameter, the electropolished CuZn37 surface presents higher peak-to-valley amplitude than the MP sample, revealing a less homogeneous surface with some uneven defects. This is especially true for the CuZn42 EP sample as the Rt value is approximately ten times higher than the others. The Skewness parameter is more or less 0 for all materials, which means that they have a symmetrical relief: i.e. the number of points is maximum on both sides of the average line of the profile. Table 1. Mechanical roughness parameters Roughness parameter CuZn37 MP CuZn37 EP CuZn42 EP (nm) Average Std Dev Average Std Dev Average Std Dev Ra Rq Rt Skew Visual Aspect

5 Amplitude The roughness profile analysis allows interpretation of discrepancies between single-phase (CuZn37) and diphasic (CuZn42) brass: even if samples have the same brightened visual aspect, mechanical roughness profiles do not have the same shape (Fig.4). Electropolished CuZn37 presents some low amplitude periodic oscillations (period of about 200µm), which can explain the slightly higher Ra values. The CuZn42 sample exhibits an irregular profile with a higher mean amplitude and quite frequent sharp topographic variations. We can conclude that stylus profiler measurements could not be easily correlated to brightness and visual observation. This could be explained by the size of radius stylus (2.5µm), which only gives access to macroroughness. If brightness is related to a residual micro-roughness, then the AFM with a better spatial resolution allows reduction in measurement scale and provides more accurate roughness measurement (Poon and Bhushan (1995)) 1 µm Mechanical polishing CuZn37 EP CuZn42 EP Evaluation length (µm) Figure 4: Representative profiles of mechanically polished samples and electropolished samples (CuZn37 and CuZn42) AFM roughness measurements Based on the three-dimensional images obtained by AFM, roughness parameters have been determined from 6 profiles, but only one is presented for each material (Table 2 and Fig.5). Table 2. AFM roughness parameters Roughness parameter CuZn37 MP Average (nm) CuZn37 EP Average (nm) CuZn42 EP Average (nm) Ra Rq Rt Skew Visual Aspect Figure 5: AFM profiles for (a) mechanically polished sample, (b) CuZn37 electropolished surface and (c) CuZn42 electropolished sample. AFM roughness parameter values are of the same order of magnitude irrespective of the samples. Ra average values vary between 0.3nm for the CuZn37 electropolished sample and 1.2nm for the CuZn42 EP sample, meaning that they are 20 to 50 times lower than what was obtained with the mechanical profiler. Rq values are very close to Ra values. The Rt parameter of CuZn42 EP sample is twice the CuZn37 EP one.

6 In these conditions, the CuZn37 EP sample presents a lower roughness than the CuZn37 MP specimen, thus implying that EP offers a smoother surface than MP. Moreover, the measurement length is shorter than periodic oscillations observed on the mechanical roughness profile. The CuZn42 EP sample exhibits an irregular roughness profile related to the surface modification between α and β phase grains. Each period on the AFM profile is about 10µm, corresponding to the average width of α phase grains. 2D profiles extracted from AFM images highlight the same trend as roughness parameters: the CuZn37 electropolished sample is smoother and more regular than the others. The AFM analysis confirms the visual inspection results: all samples are bright and relatively smooth. The same trend has been observed on the flat and smooth surface of a copper foil by Wang et al. (2015). For a bright surface, AFM results yield a Ra equal to 45nm and a Rz of 69nm (scan size of 100*100µm), which are, respectively, 6 times and 11 times lower than matte surface roughness parameters. 4. Discussions Based on the surface aspect inspection by SEM and AFM and the roughness parameter determination, it has been demonstrated that a bright sample could be characterized by a micro-roughness. All the samples studied presented a bright and smooth aspect after mechanical polishing in one case and after electropolishing in the other two cases. However, roughness differences persist. The stylus profiler analysis of the CuZn42 electropolished sample reveals a high degree of roughness (Ra 300nm) and a lack of homogeneity, whereas the CuZn37 EP exhibits roughness of the same order of magnitude as the mechanically polished sample (Ra<50 nm). AFM measurements yielded values 20 to 50 times lower than the mechanical profiler (around 1 nm). The lowest roughness value is obtained on EP CuZn37 (Ra = 0.3 nm). CuZn42 EP sample roughness parameters are of the same order of magnitude, although slightly higher than the others. This confirms that visual perception of brightness partly depends on micro-roughness irrespective of macro-roughness (providing that macro-roughness is in the range of magnitude of visible light wavelength: 400nm). Moreover in literature, Hoart and Rothwell (1964) or Landolt (1987) describe a crystallographic plane random dissolution to achieve surface brightening. This could be the other factor allowing a high degree of brightness to be attained even if macro-roughness remains considerable (> 100nm). 5. Conclusion Even although we used several surface characterization methods, it is difficult to correlate the visual aspect of various unleaded grades of brass to absolute roughness values. SEM and AFM morphological observations provide the same information: as predicted by the brass phase diagram, the CuZn37 EP sample is monophasic whereas the CuZn42 EP sample is diphasic. The anodic dissolution treatment was comparable to a mechanical treatment in terms of smoothness. On the other hand, the comparison between mechanical roughness and AFM measurements is more difficult. This is because they do not provide the same information due to the measurement scale and the spatial resolution of instruments: macro-roughness for the stylus profiler and microroughness for AFM. Therefore, brightness is mainly dependent on micro-roughness, which is why these analyses should be used in combination to provide a better description of the electropolished surface. Acknowledgements The authors extend their thanks to the program MoMeQa Innovation Stratégique Industrielle ISI and BPI France for their financial support. We are also grateful to Mr. N. Rouge for his help on SEM images. We would like to thank the FEMTO ST Institute for the use of their SEM unit. References Du, B., and Suni, I.I., Mechanistic Studies of Cu Electropolishing in Phosphoric Acid Electrolytes. Journal of The Electrochemical Society 151, 6, C375. doi: / Gabe, D. R Electropolishing of Copper and Copper-Based Alloys in Ortho-Phosphoric Acid. Corrosion Science 12, Gadelmawla, E.S., Koura M.M., Maksoud T.M.A., Elewa I.M., and Soliman H.H Roughness Parameters. Journal of Materials Processing Technology 123, 1, doi: /s (02)

7 Hoart, T.P., and Rothwell G.P., The Influence of Solution Flow on Anodic Polishing. Copper in Aqueous O-Phophoric Acid. Electrochimica Acta 9, Huang, C.A., Chang J.H, Zhao, W.J., and Yu Huang, S., Examination of the Electropolishing Behaviour of 73 Brass in a 70% H3PO4 Solution Using a Rotating Disc Electrode. Materials Chemistry and Physics 146, 3, doi: /j.matchemphys Iskander, S. S., Mansour I.A.S., and Sedahmed G.H., Electropolishing of Brass Alloys in Phosphoric Acid. Surface Technology 10, 5, Landolt, D., Fundamental Aspects of Electropolishing. Electrochimica Acta 32, 1, Lin, C.C., and Hu, C.C., Electropolishing of 304 Stainless Steel: Surface Roughness Control Using Experimental Design Strategies and a Summarized Electropolishing Model. Electrochimica Acta 53, 8, doi: /j.electacta Lin, C.C., Hu, C.C. and Lee, T.C., Electropolishing of 304 Stainless Steel Interactive Effects of Glycerol Content Bath Temperature and Current Density on Surface Roughness and Morphology. Surface and Coatings Technology 204, Poon, C.Y. and Bhushan, B., Comparison of Surface Roughness Measurements by Stylus Profiler, AFM and Non- Contact Optical Profiler. Macro and Micro-Tribology and Mechanics of Magnetic Storage Systems 190, 1, doi: / (95) Wang, X., Liu, X., Shi, L., Li, J. and Xie, J., Characteristic and Formation Mechanism of Matt Surface of Double- Rolled Copper Foil. Journal of Materials Processing Technology 216, doi: /j.jmatprotec