Products Finishing

SEP 2018

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Page 25 of 59

24 SEPTEMBER 2018 — Before the etching process, the specimens were degreased by dipping in a 45 g/l Alumal Clean 115 solution. The first set was etched at 40°C for four minutes in an acidic etch bath followed by the low caustic etch process. To investigate the effect of dissolved aluminum on the surface morphology, various amount of dissolved aluminum containing solutions were added. The caustic baths contained 60 g/l sodium hydroxide (NaOH), 35 g/l Alumal Etch LMA 235, and 10, 50 or 100 g/l of dissolved aluminum. The etching time was two minutes for post-treatment at 55°C (Samples A, B and C). Additionally, samples were pre-treated using conventional Results The R values and the three-dimensional surface topogra- phies of the first set samples (A-E) were investigated using 3D profilometry. The results are presented in Table 2 and Figure 1 on page 23. The results indicate that increasing the dissolved aluminum amount in the low caustic step creates a surfaces with lower average roughness but more homogeneously distributed pits, thus causing a more consistent surface appearance for samples B and C. Compared to the control samples, these exhibited a unique distribution of these pits in which the average roughness is closer to control Sample E (long-life caustic etch control sample), but the surface gloss was closer to Sample D (acid etch control sample), as shown in Table 3. Furthermore, the lack of dissolved aluminum in the low caustic bath for the Sample A caused the surface to be etched more than samples B and C, and caused die and streak marks to return to the surface. The aggressive nature of the low caustic bath as a result of the low dissolved aluminum content caused a rise in etching rate on Sample A. This increase created a decrease in masking capability of the acid etch by resurfacing the defects on the sample surface. The difference in the dissolved aluminum content in the low caustic step for Samples B and C led to a small difference in surface gloss in which Sample C had a higher roughness and Figure 2. The first set of samples: (a) Sample A, 100X; (b) Sample A, 500X; (c) Sample B, 100X; (d) Sample B, 500X; (e) Sample C, 100X; (f) Sample C, 500X; (g) Sample D, 100X; (h) Sample D, 500X; (i) ) Sample E, 100X; and (j) Sample E, 500X. Table 2. R Values of Samples (A-E) SAMPLE Ra (μm) Rq (μm) A 1.39 1.73 B 0.936 1.18 C 1.06 1.33 D 1.37 1.71 E 1.06 1.33 Table 3. Surface Gloss of First of Set Samples at 60°C (A-E) SAMPLE GLOSS Rq (μm) A 23 1.73 B 27.8 1.18 C 22 1.33 D 14.2 1.71 E 39.8 1.33 alkaline etch and acidic etch parameters as control samples for comparison (Samples D and E). The surface gloss of the samples was measured using a Novo-Gloss Lite glossmeter. In the second phase of the study, new specimens with the same surface gloss were produced by using different low caustic baths at various dissolved aluminum concen- trations. The gloss of the samples was adjusted by control- ling the process time to enable a relevant comparison of the masking properties after being subjected to a similar etching. The surface morphology of the samples was characterized by using SEM/SEI (JEOL JSM_7000F), 3D surface profiling (WYKO NT1100 Veec) and a glossmeter. a) b) c) d) e) f) g) h) i) j) PLATING

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