Green Silicon Carbide (SiC) is a synthetic, ultra-hard abrasive. The “green” color denotes its high purity (over 99%), which results in sharper, harder, and more brittle crystals compared to its black silicon carbide counterpart.
Properties of the green silicon carbide:
Extreme Hardness: Mohs hardness of 9.5, This allows it to easily scratch and cut glass (Mohs ~5.5).
Sharp and Brittle Crystals: The grains fracture to form new sharp edges during use, a property known as self-sharpening. This ensures consistent cutting performance.
High Thermal Conductivity: Helps dissipate the heat generated during the polishing process.
Its Role in the Glass Polishing Process
It’s crucial to understand that “polishing” in manufacturing often involves two distinct stages, and green SiC is primarily used in the first one:
1. Primary Application: Lapping (or Grinding)
This is the critical step before the final fine polish. The goal of lapping is to remove significant surface damage, create a flat surface, and prepare the glass for the final polish.
Function: Green SiC performs micro-cutting or micro-fracturing. Its sharp, hard grains act like countless tiny chisels, mechanically removing material from the glass surface.
Process: The powder is mixed with water to form a slurry. This slurry is fed between the glass workpiece and a flat lapping plate (typically made of cast iron or tin). As the plate rotates, the suspended SiC grains abrade the glass surface.
Grit Sizes Used: A range of grits is used in a sequential order.
Coarse Grits (e.g., F400, F500): For rapid stock removal and leveling.
Medium Grits (e.g., F800, F1000): To remove the scratches left by the coarse grit.
Fine Grits (e.g., F1200, F1500): To create a smooth, semi-transparent surface with very fine scratches, making it ready for the final polish.
2. Limited Application: Fine Polishing
While very fine grades of green SiC (e.g., W7, W5) can be used for a preliminary polish, it is less common for achieving a true, optical-quality finish. The mechanical cutting action of SiC, even at fine sizes, can leave a microscopic “haze” or sub-surface damage.
Comparison with Other Common Glass Polishing Abrasives
| Abrasive | Hardness | Primary Mechanism | Best For | Pros & Cons |
|---|---|---|---|---|
| Green Silicon Carbide | 9.5 | Mechanical Micro-cutting | Lapping & Edging | Pro: Very fast cutting, cost-effective for stock removal. Con: Can leave sub-surface damage. |
| Cerium Oxide (CeO₂) | ~6-7 | Chemo-Mechanical Polishing (CMP) | Final Optical Polishing | Pro: Produces exceptional, scratch-free clarity and transparency. Con: Slower, more expensive. |
| Aluminum Oxide (Al₂O₃) | 9.0 | Mechanical Micro-cutting | General Lapping | Pro: Softer and less likely to cause deep scratches than SiC. Con: Less cutting efficiency, wears faster. |
| Diamond | 10 | Mechanical Micro-cutting | Hard Materials, Fast Lapping | Pro: The hardest, most aggressive cutter. Con: Very expensive, can cause deep scratches if not controlled. |
Advantages and Disadvantages of Green SiC for Glass Polishing
Advantages:
High Efficiency: Cuts glass very quickly, making it ideal for stock removal.
Cost-Effective: Offers an excellent balance of performance and price for the lapping stage.
Consistent Performance: Self-sharpening nature maintains a consistent cutting rate.
Wide Availability: Comes in a wide range of standardized grit sizes.
Application Best Practices
Sequential Grit Sizing: Always progress through grits in sequence (e.g., 400 -> 600 -> 1000). Skipping steps will leave deep scratches that are very time-consuming to remove.
Proper Slurry Consistency: A well-mixed slurry ensures even cutting and prevents the powder from settling.
Pressure and Speed: Optimal pressure and rotation speed are critical. Too much can cause cracking; too little results in slow processing.
Cleanliness: Thorough cleaning of the workpiece and equipment between each grit change is essential to prevent contamination from larger, stray grains.