Silicon carbide (SiC) is a very important and common abrasive used in the manufacture of grinding wheels and cut-off discs. Its unique physical and chemical properties make it irreplaceable in specific applications. This article will focus on two main aspects: why silicon carbide is used and the production process.
1. Why Choose Silicon Carbide as an Abrasive?
Silicon carbide is a man-made, high-hardness material with a Mohs hardness of 9.5, surpassed only by diamond and cubic boron nitride (CBN). It possesses several key properties that make it ideal for abrasive tools:
High Hardness and Sharpness: Silicon carbide particles are extremely hard and sharp, enabling efficient material removal.
Brittleness: Compared to corundum (aluminum oxide) abrasives, silicon carbide is more brittle. This is an advantage during grinding, as dull particles break, revealing new sharp edges and corners, creating a “self-sharpening” effect that maintains sustained cutting power.
Good Thermal Conductivity: This helps dissipate heat generated during the grinding process.
Chemical stability: It is particularly suitable for machining materials with high hardness and low tensile strength because it does not react with these materials.
Main Application Categories:
Black Silicon Carbide (SiC): Low purity, approximately 98.5% or higher. It has excellent thermal conductivity and toughness and is primarily used for machining materials with low tensile strength, such as:
Non-metallic materials such as glass, ceramics, and stone
Cast iron, non-ferrous metals (such as copper, aluminum, and brass)
Rubber and leather
Refractory materials
Green Silicon Carbide (SiC): Higher purity (>99%), harder and more brittle. It is primarily used for machining hard and brittle materials, such as:
Carbide (tungsten steel) tools
Optical glass, jade, agate
Titanium alloys
Semiconductor materials such as silicon and germanium
1. Raw Material Preparation and Mixing
This is the most critical step, determining the performance of the grinding wheel (such as hardness, grit size, and structure).
Abrasive Grain: Choose the appropriate silicon carbide type (black or green) and grit size (e.g., 46, 60, 120, etc.) based on the material being machined. Higher grit sizes result in finer particles, resulting in a smoother surface finish, but also lower grinding efficiency.
Bond: It acts like “glue,” holding the abrasive grains together. Common types include:
Resinoid Bond: The most commonly used, typically made of phenolic resin. The resulting grinding wheels offer excellent elasticity, strength, and high-speed resistance, making them ideal for cutting, grinding, and rough grinding. Most cutting discs use resin bonds.
Vitrified Bond: Made from ceramic materials such as clay and feldspar. The resulting grinding wheels are rigid, heat-resistant, and retain their shape well, but are relatively brittle. They are primarily used for precision grinding and less commonly for cutting discs.
Other bonds, such as rubber and metal bonds, are used in specialized applications.
Fillers: Functional materials such as cryolite and pyrite are added to lubricate, cool, or improve grinding performance during the grinding process.
Precisely proportioned silicon carbide abrasive, binder powder, and fillers are placed in a mixer and mixed for a long, uniform period.
2. Molding
The mixed materials are placed into a mold of a specific size and shape.
On a large press, cold pressing is performed, applying enormous pressure (ranging from tens to thousands of tons) to compact the loose mixture into a dense, initially strong grinding wheel blank.
A core rod is placed in the center of the mold to directly form the mounting hole for the grinding wheel.
3. Curing
This is the process that gives the grinding wheel its final strength. The method varies depending on the bond:
For resin-bonded grinding wheels: The pressed grinding wheel blank is placed in a large oven (hardening furnace) and heated at a strictly controlled temperature (e.g., 180°C – 200°C) and time (tens of hours). The resin melts and undergoes a cross-linking reaction, eventually solidifying and firmly bonding the abrasive grains together.
For vitrified-bonded grinding wheels: The blank is placed in a high-temperature kiln and sintered at temperatures exceeding 1000°C. The vitrified bond vitrifies and firmly bonds the abrasive grains.
4. Processing & Inspection
Processing: After curing, the grinding wheel blank undergoes turning and static balancing to ensure accurate bore diameter, concentricity of the outer diameter, and smooth, vibration-free operation.
Inspection: This is a crucial step for safety. Each grinding wheel undergoes rigorous testing, including:
Rotation Test: Rotating at a speed exceeding its maximum operating speed (e.g., 1.5 times) to test its structural integrity and prevent cracking during use.
Appearance, hardness, and balance are inspected.
Finally, qualified cutting wheels are labeled and packaged for shipment.