Mechanical and Optical Profilometry
Profilometry results are the most instructive result in addressing the research question: what is the relative impact of chemical polishing, electropolishing, or dual-stage liquid-based surface finishing on the surface roughness of AM components? We performed a roughness and surface topography study on untreated, C, E, CE, and EC treated samples (Fig. 2). The mechanical profilometer study utilizes contact profilometry to directly compare the surface deviations of differently treated AM surfaces (Fig. 2a-b). Untreated AM samples' average Rz and Rt values were 20.83 µm and 21.61 µm, respectively (Fig. 2a). C and E decreased Rz and Rt between the 5 and 10 µm range. Interestingly, EC and CE further improved Rz and Rt below 5µm. CE appeared to produce the best result. CE also was approaching the Rz and Rt parameters associated with the Best sample surface (Fig. 2a). The untreated samples have average Ra (average roughness) and Rq (root-mean-square roughness) surface roughness values of 7.37 µm and 7.16 µm, respectively (Fig. 2b). Comparatively, C treatment showed a reduction in Ra value to 1.93 µm and Rq value to 2.07 µm (Fig. 2b). E treatment showed less decline in roughness in both parameters; Ra and Rq for the E sample smoothed down to 3.31 µm and 3.49 µm, respectively (Fig. 2b). Higher roughness after the E treatment is due to the larger variations of morphologies along with ~ 1000 µm long scan. It seems C produced smoother morphologies over a long scan length than variations observed after E surface treatment. However, CE and EC treatment showed the most reduction in surface roughness- more than any individual process examined. EC treatment exhibited an average Ra value reduced to 1.59 µm and average Rq value reduced to 1.73 µm. These results are ~ 75% lower in surface roughness than that from the as-produced samples. CE treated AM samples showed the most considerable reduction in average Ra value of surface roughness. The CE samples Ra roughness value was reduced to 1.36 µm and Rq value down to 1.76 µm. Hence, it is beneficial to utilize dual-stage liquid-based surface finishing strategies for more robust surface roughness to be achieved in AM components.
The non-contact optical Keyence profilometer also provides a similar trend in areal measurements (Fig. 2c-d). It is noteworthy that areal measurements represent the average of line scans over a 1 mm2 area. The untreated AM sample's average Sz, Sp, and Sv values were 226.44 µm, 166.35 µm, and 60.12 µm, respectively (Fig. 2c). All four treatments significantly improved the smooth surface topography (Fig. 2c). CE produced the best results. The average Sz, Sp, Sv values for the CE-treated AM samples were 27.44 µm, 9.72 µm, and 17.22 µm, respectively (Fig. 2c). CE produced an 825% reduction in Sz. On the other hand, CE reduced the average hill-like feature's height from 166.3 to 9.72 µm (Fig. 2c). At the same time, EC reduced the average hill-like feature's size from 166.3 to 10.43 µm (Fig. 2c). E and C treatments decreased the hill-like features to 16.38 and 13.48 µm, respectively (Fig. 2c). Interestingly, valley-like features depth was impacted less for all the treatments. The untreated sample showed an average valley depth (Sv) of 60.12 µm. The C, E, EC, and CE reduced Sv to 35.55 µm, 28.12 µm, 21.45 µm, and 17.72 µm, respectively (Fig. 2c). Hence, the Sv for C, E, EC, and CE were 58%, 47%, 36%, and 30% as compared to untreated samples. CE appears to produce the highest reduction in valley depth (Fig. 2c). The Sa and Sq roughness parameters were recorded for all the pieces (Fig. 2d). Sa for Sq for untreated AM part were 13.88 and 17.37, respectively (Fig. 2d). Sa and Sq for C alone were 5.22 µm and 7.29 µm, respectively. Interestingly, Sa for outer surface after E alone and on CE and EC was ~ 3 µm (Fig. 2d). However, E step alone or as part of CE or EC do not improve internal surfaces where counter electrode cannot reach (Fig. 2d). Figure 3: (a) Microstructure of as-built sample, (b) Microstructure of Electropolished only region of EC sample (c) Interfacial region of EC sample, (d) EC region of EC sample Wherever, C treatment alone or as a part of CE and EC occurs the internal surface inaccessible to counter electrodes of E step will be dominated by C treatment.
We also measured the kurtosis (Sku) parameter that measures the sharpness of the roughness profile. For a typical Gaussian distribution of hill-like feature height Sku magnitude is 3. Sku for E and C was 6.0 and 6.02, respectively. The Sku above 3 suggests height distribution is significantly spiked. Interestingly, Sku for EC was 1.77 suggesting that height distribution is spread over a big range. On the other hand, Sku for CE was 4.35 suggesting that CE treatment brought the feature height distribution to the smaller range and was close to Gaussian distribution.
Roughness measurements are the result of microstructural features on AM samples. We conducted SEM to understand the microstructure of AM samples after CE and EC treatments. For this study, we prepared samples in such a way that CE and EC both have a section of the initially treated surface, a transition zone, and a surface treated with both methods. SEM of EC was performed to gain comparative insights about the difference among final microstructure, E treated surface, the transition zone between E and EC, and untreated surface (Fig. 3). Untreated AM surface exhibited steep hills and valleys of several tens of µm (Fig. 3a). The drastic change in surface topology is consistent with the profilometer study discussed in Fig. 2. Typically, deep valley has sharp trenches that are ideal for crack initiation during dynamic stress. Such microcrack features are already shown to yield poor fatigue properties and poor strength. E step yields a smooth surface (Fig. 3b). According to our prior work, electropolishing could produce a very smooth surface after removing ~ 200 µm thick material from the surface . E step appears to create columns of microscopic regions. One can observe the microscopic regions after the excessive electropolishing step leading to selective material removal from the boundary regions . We submerged a portion of the E-treated area in the chempolishing solution to accomplish the C step on the E-treated surface, i.e., cumulative EC treated area. The region between E and EC treated area provided insight into the transitory microstructure (Fig. 3c). Microscopic regions became prominent due to the chempolishing tendency to selectively etch material away from the boundary regions (Fig. 3c). The microstructure of the EC treated end was full of microscopic zigzag serrations. However, the typical microscopic regions < 1000 nm (Fig. 3d). Hence, the C step produces micro-nano scale roughness (Fig. 3d). However, it is important to note that this roughness level is still very smooth and acceptable for many applications. More importantly, C step-induced surface roughness will be dominant on the internal surface of AM component.
SEM of the CE sample revealed insights about microstructures in a different part of the CE-treated sample. The C-treated end showed microstructure with microscopic craters and canals (Fig. 4a). As a common theme, C treatment appears to remove materials from within the region, not from the edges. However, E treatment completely removed the microscopic craters and canals and brought the lamellas rich microstructure (Fig. 4b). It is expected that the physiochemical properties of CE will be different ha the C-treated surface. The interfacial region between C and CE ends was studied (Fig.4c-d). The interfacial region near the C-treated end showed deeper microscopic craters and canals (Fig. 4b). The interfacial region near the CE end showed the emergence of lamella features and dissolution of the microscopic regions with nanoscale features (Fig. 4d). The microstructure of interfacial regions demonstrates the expected microstructure of incomplete E treatment. We envision that the porous microstructure in figure 4d can have its own advantages and may be more suitable for developing high adhesion between AM surface and different coatings intended for the specific application. It is worth noting that extremely smooth surface with mirror like finishing obtained by different approaches may not be best in the form of microstructures (Fig. 5). We were able to obtain highly smooth surfaces by excessive electropolishing. However, the cost of attaining a very smooth surface by excessive electropolishing only was that the resultant surface showed sub-microscopic cavities (Fig. 5a). We are unaware whether such nanoscopic cavities may compromise mechanical strengths. Also, we produced a sample with Ra <1 µm by combining mechanical polishing and excessive electropolishing, and this sample is referred to as Best on the roughness scale in this study. Interestingly, the SEM of this sample showed highly smooth surface morphology with a high density of sub-micron circular cavities (Fig. 5b). The average circular cavity was 300±80 nm. SEM study in Fig. 5 also suggests that excessive electropolishing in CE may result in microscopic cavities, as seen in Fig. 5. A major take-away from this study is that smoothest surfaces may still have a high population of defects that may compromise the AM parts' mechanical strength.
The surface reconstruction feature in SEM obtains topography to reveal the difference in textural elements after EC and CE surface finishing (Fig. 6). In mapping the individual surface's topography, extremes are indicated with red and blue for peaks and valleys, respectively (Fig. 6). The relative visual spectrum of light and colors reflected fills the respective magnitude between the two extremes. This height map scale is used to create the reference color values for surface deviations in the treated samples. Untreated AM surface clearly showed a dramatic change in local surface texture, revealing microscopic hills and Figure 6: SEM ESD Topography surface height heat map of (a) as-produced surface (b) EC surface (c) CE surface
valleys (Fig. 6a). In the unfinished stage, the topography result is characterized by a large propagation of peaks and a rugged surface as expected (Fig. 6a). On the EC sample difference in valley and peaks decreased significantly, and sub-microscopic craters filled the whole surface (Fig.6b). The microscopic roughness measurements along different line profiles varied from 0.5 to 2 µm Ra (Fig. 6c). We utilized a quantitative roughness measurement feature. EC and CE samples produced Ra roughness of 1.5±0.6 µm and 1.5±0.9 µm, respectively. SEM-enabled microscopic roughness measurement suggests that CE and EC roughness are comparable at the microscopic level. These results also agree with the stylus-based mechanical roughness meter data reported in Figure 2. We also found profilometry and 2-dimensional SEM result in that the chemical electropolished (CE) sequenced samples exhibit the best results. SEM enabled Ra roughness of Best samples was 0.75±0.27 µm. It means CE and EC were roughly twice as rough as the smoothest samples we could produce.
Contact Angle—Surface Energy
We envisioned that after surface treatment, AM samples are expected to be coated with different thin films or coatings. Coatings on AM components can determine if an AM part will qualify for a challenging application environment, such as high corrosion, high temperature, and high wear. Adhesion of coating after different surface treatments may vary dramatically based on the surface microstructure. A general understanding of the compatibility of different AM surfaces with prospective materials may be generated via a wettability experiment involving a test liquid and solid surface.
We conducted liquid-AM surface contact angle to investigate the effect of different surface treatments. The liquid-solid surface contact angle is an insightful study of the surface energies of the material surfaces. The surface energy of a material can be influenced by the chemical composition, surface microstructure, roughness, etc. Contact Angle is a known parameter for the analysis of the wettability of a surface, which has a direct relationship with the smoothness or lack of surface deviations a surface. Here data is taken for each of the six surfaces introduced and compared to provide an additional metric for evaluation of the relative performance of the CE and EC methods. Given the large values of contact angle results in our study and the stated precision of the contact angle meter, contact angle measurement by the Sessile drop technique proved to be a very reliable measurement. AM sample surface energy is a concern of the research in this study. Typically, this surface energy is quantified as the energy associated with the work done per unit area to create a new surface (cutting bulk material or solidifying outfacing surfaces) in an as-produced sample. Whereas, in the CE and EC samples, this surface energy is then quantified as a more complex application of conservation of energy that considers the unbalanced set of bonds/interactions at the outfacing surfaces of material and also includes the energy associated with work done by the CE and EC sequenced processes to stabilize the atomic surface. As has been shown in research to date, a surface with low surface energy will result in a high contact angle and poor wettability statistics that are often undesirable in the field of Additive Manufacturing. Wettability allows for a very common mechanism of material improvement, i.e., surface coating, to be performed more successfully. The increase in surface energy of a surface will allow for the bonding and interaction with coated materials, such as nickel or chromium, that will result in an isotropic layer of the desired material. Decreasing contact angle means an increase in surface energy. In this research, an increase in surface energy is an improvement in AM surface performance. The results of the contact angle study show that the untreated surface shows very undesirable surface energy (Fig. 7). This is an expectation and further validation of the usefulness of studying dual-stage surface finishing. Interestingly, electropolished samples also show considerably poorer surface energy performance results than one-stage chempolished samples, supporting our previous theory that electropolishing as a first stage has drawbacks that are apparent when comparing the visual microstructures of the two processes of most concern, CE and EC. Additionally, the CE sample has outperformed the six-sample set in overall surface performance again. When compared even with the sample with the 'Best' surface deviation reduction (Fig. 4), the CE sample has lower water contact angle results and thus considerably higher surface energy.
We also investigated the interaction between a wide range of light wavelengths and AM surfaces. This study is a means to understand the difference in physical surface properties of AM components not possible by the aforementioned measurement strategies. Additionally, this study is of direct interest to organizations where AM parts are being researched for space applications involving exposure to light radiations. In the analysis of each procedure's impact on AM finished surfaces, we have conducted a reflectance study, investigating the specular or diffuse reflectance of light from the sample. The specular reflectance can be described as the reflectance from a surface that is directed in a single outgoing direction. The quality of the surface will directly impact the secondary beam, which is the beam that will be characterized in these results.
The wavelength of the incident light is the independent variable in this method, and the magnitude of reflectance provides an analysis metric dependent on the amount of light absorption on the surface. As light reflects from a surface with irregularities, the absorption of the surface increases in that the strength of the reflected beam is reduced. A special opportunity to measure these irregularities exist due to the ability to manipulate the wavelength of this incident light. We can use the wavelength of light to measure the height of surface irregularities directly. In a mode very similar to an optical microscope, only features larger than the wavelength of light used to image them can impact the reflection of the light used. In this way, we began with wavelengths of light of 0.2 µm and iteratively stepped up to the value of 1 µm, measuring the reflectance value at each wavelength.
The reflectance measured in this study, also commonly known as reflectivity, is a measure of its ability to reflect radiant energy. The measure is a function of the wavelength of light used or the surface character of the plane of reflectance. In many reflectance studies, the reflectance for specular surfaces (very smooth) is very near zero. This is because the researchers use a method where the incident measurement is taken at any angles except the reflectance angle, where all light has been reflected and therefore should be close to no light reflection in this diffuse area. Researchers have used a reduced failure mode method for reflectance measurement for this study.
For each surface, the range of wavelengths is reflected from the treated surfaces of interest at a direction completely normal to the surface and reflected back to the light source. The magnitude of reflectance is higher for smooth surfaces, and the results presented (Fig. 8) show the measured reflected light reflectance. The reflectance behavior of untreated samples gives some initial validation to the reflectance results, as do the best sample results. Two features of the results, in particular, stand out. Firstly, the dual-stage sequenced processes outperform either of the individual processes in surface performance. In the case of electropolishing specifically, the outperformance is considerable, whereas chempolished samples are showing reflectance comparable to the best performing surface thus far, CE samples. EC samples have, for the first time in this study, shown results that are better than CE samples for the first time in this study, and this reality warrants further investigation surely. The second feature of this result is the shared regions of linear increase in reflectance as well as shared regions of stabilized reflectance. As we move from the ultraviolet radiation range into the visible range, the measured reflectance changed for AM surfaces treated by different methods.