Silicon Carbide Density and Hardness: Material Properties and Industrial Significance

Quick Answer

Silicon carbide (SiC) has a typical density of 3.20–3.22 g/cm³ and an exceptional hardness of Mohs 9.2–9.4, ranking just below diamond and boron carbide. These properties stem from its covalent Si–C bonds and strong crystal structure. High density provides thermal conductivity and strength, while high hardness enables cutting, grinding, and wear resistance in abrasives, ceramics, and semiconductor devices.

Table of Contents

• 1. Overview of Silicon Carbide (SiC)
• 2. Density of Silicon Carbide: Theory and Measurement
• 3. Hardness of Silicon Carbide and Comparison with Other Materials
• 4. Differences Between Black and Green SiC in Density and Hardness
• 5. Crystal Structure and Bonding
• 6. Industrial Applications Driven by Density and Hardness
• 7. Factors Influencing SiC Density and Hardness
• 8. Measurement and Testing Standards
• 9. Summary Table of Key Properties
• 10. FAQ

1. Overview of Silicon Carbide (SiC)

Silicon carbide (chemical formula SiC) is a non-oxide ceramic composed of silicon and carbon atoms bonded through a strong covalent lattice. It exhibits high mechanical strength, hardness, and thermal conductivity, as well as excellent chemical and oxidation resistance. Owing to these features, SiC is widely used in abrasive tools, high-temperature refractories, structural ceramics, and emerging semiconductor technologies.

SiC can appear as black silicon carbide (97–99% SiC) or green silicon carbide (99–99.9% SiC), depending on purity and production parameters. Although the chemical composition is similar, subtle variations in density, microstructure, and hardness affect performance in industrial applications.

2. Density of Silicon Carbide: Theory and Measurement

The theoretical density of silicon carbide, based on its crystal lattice (α-SiC, hexagonal structure), is approximately 3.21 g/cm³. However, the bulk density of SiC powders or sintered components can vary between 3.10 and 3.22 g/cm³, depending on manufacturing methods, porosity, and impurities.

2.1 Theoretical Density Formula

The theoretical density (ρ_th) is calculated using the crystallographic formula:

ρ_th = (Z × M) / (N_A × V_cell)

• Z = number of formula units per unit cell
• M = molecular weight of SiC (40.1 g/mol)
• N_A = Avogadro’s number (6.022×10²³ mol⁻¹)
• V_cell = unit cell volume

Depending on the polytype (α-SiC or β-SiC), slight variations occur. β-SiC (cubic 3C-SiC) shows ~3.21 g/cm³, while α-SiC (6H or 4H) ranges from 3.20–3.22 g/cm³.

2.2 Bulk and Apparent Density

In industrial practice, bulk density measures the ratio of sample mass to its overall volume (including pores). It depends on compaction, grain shape, and sintering quality. Typical bulk densities are:

• Loose SiC powder: 1.2–1.6 g/cm³
• Pressed SiC grit: 2.4–2.8 g/cm³
• Sintered or hot-pressed SiC ceramic: 3.15–3.22 g/cm³

Lower density implies higher porosity, which reduces mechanical strength and conductivity. Exporters usually specify density on Technical Data Sheets (TDS) as part of material identification.

3. Hardness of Silicon Carbide and Comparison with Other Materials

Silicon carbide is one of the hardest synthetic materials, ranking just below diamond and boron carbide. It is typically rated Mohs 9.2–9.4, or HV 2500–2800 on the Vickers hardness scale. The high hardness arises from strong directional covalent bonding between silicon and carbon atoms.

3.1 Comparative Hardness Table

This remarkable hardness allows SiC to be used as a cutting, grinding, and lapping medium for metals, ceramics, and composites that conventional abrasives cannot efficiently handle.

4. Differences Between Black and Green SiC in Density and Hardness

Both black and green SiC share similar theoretical density but differ slightly in practical measurements due to impurity levels and crystal perfection:

• Black SiC: Density ≈ 3.20 g/cm³; Hardness Mohs 9.2; tougher and less brittle; suitable for heavy-duty grinding and refractories.
• Green SiC: Density ≈ 3.21 g/cm³; Hardness Mohs 9.4; purer and sharper; ideal for polishing and wafer lapping.

The presence of metallic oxides (Fe₂O₃, Al₂O₃) in black SiC slightly decreases hardness but enhances impact resistance, making it more durable in bonded tools. Green SiC, with fewer impurities, offers higher cutting precision and lower contamination risk.

5. Crystal Structure and Bonding

SiC exists in over 200 polytypes, grouped into two main families:

• α-SiC: Hexagonal or rhombohedral polytypes (4H, 6H, 15R), stable at high temperatures, density ~3.21 g/cm³.
• β-SiC: Cubic 3C polytype, stable below 1700°C, slightly less dense (~3.20 g/cm³).

Each polytype maintains tetrahedral coordination—each Si atom bonded to four C atoms—creating a 3D covalent network. The short Si–C bond length (1.89 Å) produces extreme bond energy (~450 kJ/mol), explaining SiC’s high hardness and chemical inertness.

6. Industrial Applications Driven by Density and Hardness

SiC’s combined density and hardness make it a preferred choice in various industries:

• Abrasives: Grinding wheels, sandpapers, and blasting media rely on SiC’s hardness to cut metals and stones efficiently.
• Refractories: Dense SiC aggregates improve heat conduction and erosion resistance in furnaces and kilns.
• Mechanical Seals: High-density SiC ceramics resist wear and thermal shock in pumps and compressors.
• Semiconductors: 4H-SiC and 6H-SiC wafers enable high-voltage, high-temperature electronic devices.
• Armor and Aerospace: Lightweight but extremely hard SiC composites are used in ballistic and structural applications.

In each case, the combination of low weight (due to moderate density) and extreme hardness enhances efficiency and durability.

7. Factors Influencing SiC Density and Hardness

Several production factors can influence final density and hardness values:

1. Purity: Metallic impurities and oxide inclusions reduce crystal perfection, lowering hardness.
2. Porosity: Higher porosity decreases bulk density and mechanical strength.
3. Sintering Conditions: Higher sintering temperatures and additives (e.g., B, C) improve densification.
4. Particle Size Distribution: Finer powders pack more densely after compaction.
5. Polytype Ratio: Different SiC crystal types have slightly varying atomic packing densities.

8. Measurement and Testing Standards

Industry and laboratories use several standards to measure SiC density and hardness:

• Density: ASTM C373 (Archimedes method), ASTM C20 for apparent porosity and bulk density.
• Hardness: ASTM E384 (Vickers microhardness test), ISO 6507 for indentation testing.
• Microstructure: SEM imaging for grain bonding and porosity evaluation.

For powders, apparent density is tested by tapping or Hall flowmeter methods. Exporters such as CanAbrasive record results in TDS (Technical Data Sheets) and COA (Certificate of Analysis) for each batch.

9. Summary Table of Key Properties

10. FAQ

Q1: What is the density of silicon carbide?

The theoretical density of silicon carbide is about 3.21 g/cm³. Sintered components typically achieve 3.15–3.22 g/cm³, depending on porosity and purity.

Q2: What is the hardness of silicon carbide?

SiC has a Mohs hardness of 9.2–9.4 and Vickers hardness between 2500–2800 HV, making it one of the hardest known engineering materials.

Q3: Why is silicon carbide so hard?

Because its silicon and carbon atoms form strong covalent bonds in a tetrahedral lattice, which resists dislocation movement and deformation.

Q4: Does density affect SiC performance?

Yes. Higher density indicates fewer pores and greater mechanical strength, improving wear resistance, conductivity, and thermal stability.

Q5: What is the difference in density between black and green SiC?

Both have nearly identical theoretical density (~3.21 g/cm³), but green SiC often appears slightly denser due to fewer impurities and less porosity.

Q6: How does SiC compare with alumina or tungsten carbide?

SiC is harder than alumina and tungsten carbide but lighter in density, giving it superior strength-to-weight ratio and wear resistance.

Q7: How is SiC hardness measured?

By Vickers or Knoop indentation tests under microloads. For powders, indirect measures such as particle breakage or grinding tests are used.

Q8: Is higher hardness always better?

Not necessarily. High hardness provides wear resistance, but slightly lower hardness with higher toughness (as in black SiC) may perform better under impact or stress.