The relative basicity or acidity of slurry quantified on the pH scale is a parameter often ignored in optical polishing. However, an optimum pH level has to be determined and maintained to achieve optimum production yields for any given combination of optical material and abrasive.
This article discusses the impact of the pH level of the slurry on the following optical polishing factors:
- Material removal rate
- Surface finish
- Staining and etching
- Process control
Material Removal Rate (MRR)
The addition of extra abrasive is a common activity when there is low MRR. However, this “brute force” approach has limited efficacy and could have a negative impact on the surface finish of polished optics, thereby eventually wasting money. MRR can be controlled by adjusting the pH level of the slurry.
Figure 1 illustrates the dramatic change in the MRR of fused silica glass during shifting of the slurry pH from acidic to alkaline at constant abrasive concentration. This drastic change is not possible with abrasive concentration alone. This means it is necessary to maintain the abrasive concentration and pH level of the slurry in order to maximize MRR.
Figure 1. Effect of pH on polishing rate of Fused Silica: 5wt% ceria from CE- 6750 with 2.0 PSI downforce, 51 RPM, & 80 mL/min flow rate on a Suba X non-embossed Pad using a PR-1 double sided polisher
The pH level, at which the particles dispersed in a fluid exhibiting a net zero charge, induces agglomeration of particles into larger bodies of variable size. Particle size variability is an issue in polishing slurries as it affects surface finish, resulting in higher RMS and peak to valley measurements.
Visible scratches and sleeks will also be the result of agglomerated particles (Figure 2). In either case, productivity and process yield are affected. The agglomeration of abrasive particles can be prevented by changing the pH away from the point of zero charge.
Figure 2. Scratching caused by agglomerates in an unstabilized slurry at the point of zero charge as seen on N-BK7 using a Zygo NewView 8000 Profilometer
Staining and Etching
The exposure to alkaline or acidic conditions makes many substrate materials, soft glasses in particular, susceptible to staining. In some cases, substrate materials like crystalline calcite will be dissolved under acidic pH conditions. Etching and staining in susceptible materials can be reduced by maintaining a controlled slurry pH.
Ions are released by substrates into the slurry in the process of grinding and polishing. The pH level of the slurry is either increased or decreased due to the release of these free ions, thereby affecting all of the aforementioned optical polishing factors.
Hence, it is essential to dynamically monitor and control the slurry pH using the appropriate tools in order to offset the pH changes induced by the polishing process (Figure 3).
Figure 3. The photo on the left is a true “in-process” look at how we use our ISFET pH probe. The photo on the right depicts benchtop pH measurement.
Most substrates, especially glasses containing silica, exhibit a negative surface charge. As a result, positively charged particles will be attracted by these glasses. This causes problems for abrasive particles as they can embed into the substrate surface. Large embedded particles can lead to scratching or failed coatings during fine polishing.
This, in turn, increases scrap rate and lowers productivity. This can be addressed by changing the slurry pH to match the slurry charge to that of the substrate. By this way, abrasive particles can be prevented from binding to the substrate.
Selecting the appropriate slurry and maintaining stability throughout the polishing process helps achieving time and cost savings through faster removal rates, improved slurry longevity, consistent surface quality, and better yields (lower scrap rate).
Besides improving profitability for existing processes, controlling pH helps entering into new markets with tighter tolerances and challenging substrates. Nanophase™ Cerium Oxide polishing slurries reduce pH drift during polishing, thanks to their formulation to maintain consistency across a broad range of pH levels.
About Nanophase Technologies Corporation
Nanophase develops, manufactures and sells an integrated family of nanomaterial technologies. Nanophase produces engineered nanomaterial products for use in a variety of markets including Surface Finishing, Exterior Coatings, Personal Care, Plastics, Scratch Resistant Coatings, and Textiles. The Company was founded in 1989, went public in 1997 and the common stock currently trades on the OTC Market (OTCQB) under the symbol NANX.
This information has been sourced, reviewed and adapted from materials provided by Nanophase Technologies Corporation.
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