Soda-Lime Glass

Soda-lime glass (~550 HV) is a hard, brittle, amorphous material. Being amorphous, there is no crystalline microstructure to reveal; preparation is focused on cross-section analysis of surface features, coatings, lamination interfaces, or fracture surfaces. The extreme brittleness is the primary preparation challenge: the material chips and fractures easily during sectioning and grinding. Edge retention for coating analysis requires careful mounting. Fracture surfaces for failure analysis should be preserved and examined before any cross-sectioning.

Use a low-speed precision diamond wafering saw with a thin diamond blade (0.3-0.5 mm) and continuous coolant. Cutting speed: 100-150 RPM with very low feed rate. The glass is extremely brittle; aggressive cutting causes chipping and fracture propagation. For thin glass (windshields, float glass), score and snap along a controlled line for rough sectioning before precision cutting. Laminated glass (with PVB interlayer) can delaminate during cutting if heat is excessive. Leave 1-2 mm allowance for grinding.

Cold mounting with a clear or hard epoxy resin is standard. Glass is thermally stable but thermal shock from hot mounting can crack thin specimens. Vacuum impregnation is recommended for laminated glass to fill any delamination gaps. For coating analysis, edge-retaining mounting with mineral-filled epoxy is essential; the coatings are typically only 10-500 nm thick and extremely fragile. Orient the surface of interest face-down. For transparent glass specimens, clear epoxy allows transmitted light and cross-polarized examination for stress patterns.

The extreme brittleness requires gentle grinding to prevent edge chipping and subsurface fracture.

Grinding sequence:

• 320 grit SiC: Remove sectioning damage (30-60 seconds). Light pressure (15-25 N). Glass chips easily; monitor edges continuously.
• 400 grit SiC: Remove previous scratches (20-40 seconds). Maintain light pressure.
• 600 grit SiC: Refinement (20-40 seconds).
• 800 grit SiC: Fine grinding (20-30 seconds).
• 1200 grit SiC: Final grinding (15-30 seconds). The surface should be uniformly ground without visible chips.

Disc speed: 150-250 RPM. Use complementary rotation. Water lubrication is essential. Grind with the glass edge trailing (trailing the direction of disc rotation) to minimize chipping force. If chips persist at the leading edge, reduce pressure further or start at a finer grit. Diamond grinding discs (15-6 μm) can be used as an alternative that produces less chipping.

Standard diamond polishing works well on glass. The hard, homogeneous amorphous structure polishes to a very smooth finish.

Diamond polishing sequence:

• 6 μm diamond: 3-5 minutes on a medium-hard synthetic pad (20-25 N). Good scratch removal.
• 3 μm diamond: 3-5 minutes on a synthetic pad (15-20 N). Surface should become transparent in cross-section.
• 1 μm diamond: 2-3 minutes on a synthetic pad (12-18 N).

Final polishing:

• 0.05 μm colloidal silica: 1-3 minutes on a soft pad with light pressure. The colloidal silica provides a slight CMP action on glass that gives an excellent surface. The polished cross-section should be optically transparent (glass-like) when examined at low magnification.

For fracture surface examination, do NOT cross-section. Examine the fracture surface directly under a stereo microscope and SEM to identify the fracture origin, mirror, mist, and hackle zones.

Soda-lime glass is amorphous, so there is no microstructure to reveal by etching. However, etching can reveal stress patterns and compositional variations.

HF-Based Etchants (Chemical Etching) - For stress/composition analysis:

• Composition: 1-5% HF in water (1-5 ml HF, 95-99 ml water)
• Application: Immerse for 15-60 seconds.
• Reveals: Stress patterns in tempered glass (differential etch rate in compression vs. tension zones). Compositional variations from ion exchange or weathering. Tin-side vs. air-side distinction in float glass (tin side etches differently).
• Safety: HF is acutely toxic and penetrates skin causing deep tissue damage. Calcium gluconate gel must be immediately available. Full PPE mandatory. Work in fume hood.

Buffered HF (NH4F/HF) (Chemical Etching) - Gentler alternative:

• Composition: Buffered oxide etch (BOE), commercially available
• Application: Immerse for 30-120 seconds. More controlled etch rate than dilute HF.
• Reveals: Same features as HF but with better control and reproducibility.

Note: For most glass cross-section analysis (coating evaluation, lamination, surface defects), etching is not performed. The as-polished cross-section is examined directly.