Nanoscratching of Optical Glass Surfaces Near the Elastic–Plastic Load Boundary to Mimic the Mechanics of Polishing Particles

Abstract

The nanomechanical deformations on glass surfaces near the elastic-plastic load boundary have been measured on various glasses by nanoscratching using an atomic force microscope (AFM) to mimic the mechanical interactions of polishing particles during optical polishing. Nanoscratches were created in air and aqueous environments using a 150-nm radius diamond-coated tip on polished fused silica, borosilicate, and phosphate glass surfaces; the topology of the nanoscratches were then characterized by AFM. Using load ranges expected on slurry particles during glass polishing (0.05-200 lN), plastic-type scratches were observed with depths in the nm range. Nanoscratching in air generally showed deeper & narrower scratches with more pileup compared to nanoscratching in water, especially on fused silica glass. The critical load needed to observe plastic deformation was determined to range from 0.2-1.2 lN for the three glasses. For phosphate glass, the load dependence of the removal depth was consistent with that expected from Hertzian mechanics. However, for fused silica and borosilicate glass in this load range, the deformation depth showed a weak dependence with load. Using a sub-T g annealing technique, material relaxation was observed on the nanoscratches, suggesting that a significant fraction of the deformation was due to densification on fused silica and borosilicate glass. Repeated nanoscratching at the same location was utilized for determining the effective incremental plastic removal depth. The incremental removal depth decreased with increase in number of passes, stabilizing after~10 passes. In water, the removal depths were determined as 0.3-0.55 nm/pass for fused silica, 0.85 nm/pass for borosilicate glass, and 2.4 nm/pass for phosphate glass. The combined nanoscratching results were utilized to define the composite removal function (i.e., removal depth) for a single polishing particle as a function of load, spanning the chemical to the plastic removal regimes. This removal function serves as an important set of parameters in understanding material removal during polishing and the resulting workpiece surface roughness.

Mechanism and Simulation of Removal Rate and Surface Roughness During Optical Polishing of Glasses

“…3. 19 In addition, the magnitude of the difference in removal rates between the three glasses increased with increase in the width of the PSD (i.e., increase in d PSD ). This is qualitatively consistent with the removal function trend measured for single particles in our previous study of nanoscratching on these glasses (see Fig.…”

“…In our previous study, we proposed that removal can occur both by molecular level chemical reaction at the Angstrom level (via condensationhydrolysis reactions) and by nano-plastic deformation at the nanometer level. 19 Figure 4 shows the determined removal function on a log-log plot of removal depth versus load per particle for fused silica, borosilicate, and phosphate glass. and glass type, while the latter is dominated more by mechanics.…”

“…9 Note the former is governed more by the chemistry of the slurry (composition of the slurry particle, pH, viscosity, colloidal charge, etc.) 19 The next key step is to determine the load on each particle at the interface during polishing. 3 As described in the Introduction, the quantitative removal function was experi- mentally determined in the nano-plastic regime for various glasses as a function of load using a unique nanoscratching technique on an atomic force microscope.…”

“…A schematic illustration of the polishing interface, outlining key parameters that influence material removal and workpiece roughness at microscopic scale lengths, is shown in Fig. 19 These results showed direct evidence of plastic flow at nm-scale lengths during polishing. A recent study described the properties and formation of the modified surface layer (called the Beilby layer) on the polished workpiece including its possible influence on surface roughness.…”

“…1. 9,19 The influence of the slurry particle size distribution (PSD), particularly the logarithmic slope of the distribution function for the largest particles in the exponential tail, was shown to strongly influence the roughness during polishing of fused silica glass. 18 In another recent study, detailed measurements of nano-plastic deformation on various glasses at ultra-low loads (50-250 lN), such as those expected on the particles during polishing, by nanoscratching were systematically measured.…”