The hardness of crystalline materials is one of the most critical factors determining the selection of cutting, grinding, and polishing processes. It directly influences machining efficiency, tool wear, process parameters, cost control, and final product quality. Differences in hardness significantly affect tool selection, machining stress, defect control, and achievable surface quality.
This article analyzes four typical crystalline materials—sapphire, silicon carbide, silicon wafers, and quartz—from the perspective of hardness characteristics, and systematically examines how these properties influence machining processes and the corresponding optimized processing methods.

1. Core Hardness Characteristics of Four Crystal Materials

Material	Mohs Hardness	Vickers Hardness (HV)	Key Hardness Characteristics	Additional Properties Affecting Machining
Sapphire (Al₂O₃ single crystal)	9.0	2000–2300 HV	Extremely high hardness, second only to diamond; moderate brittleness; relatively low fracture toughness	Excellent chemical stability, corrosion resistance, and strong hardness retention at high temperatures
Silicon Carbide (SiC single crystal)	9.5	2400–2800 HV	Hardness close to diamond; highly brittle; fracture toughness slightly higher than sapphire	High mechanical strength, high-temperature resistance, excellent thermal shock resistance; prone to stress cracks during machining
Silicon Wafer (single-crystal Si)	6.5–7.0	1100–1300 HV	Moderate hardness; combination of brittleness and plasticity; strong anisotropy	Good machinability; easily achieves high flatness through CMP; surface prone to oxidation
Quartz (amorphous SiO₂)	7.0	800–1000 HV	Moderate-to-low hardness; highly brittle; hardness uniformity lower than single crystals	Extremely low thermal expansion coefficient, strong resistance to temperature variations, resistant to most chemicals except hydrofluoric acid

2. Impact of Hardness on Cutting, Grinding, and Polishing Processes

(1) Sapphire: “Specialized Tooling + Low-Stress Processing” Under Extremely High Hardness

Cutting

Due to sapphire’s extremely high hardness, conventional cutting tools wear rapidly. Standard cutting methods often cause edge chipping and cracks.
Recommended process:
Electroplated diamond wire saw
Wire diameter: 0.12–0.18 mm
Diamond particle size: 3–5 μm
Wire speed: 10–15 m/s
Feed rate: 0.05–0.1 mm/min
Specialized coolant containing polyethylene glycol and SiC micropowder is used to reduce cutting temperature and stress, controlling edge chipping within ≤50 μm.
Laser cutting is generally not recommended due to the formation of thick heat-affected layers. Only femtosecond laser cutting is suitable for complex shapes, followed by grinding to remove the damaged layer.

Grinding

The main challenge is removing cutting damage while minimizing tool wear.
Typical staged grinding process:
Coarse grinding: 15–20 μm diamond slurry, cast iron plate, pressure 0.1–0.2 MPa
Medium grinding: 5–8 μm diamond slurry, cemented carbide plate
Fine grinding: 1–3 μm diamond slurry, pressure <0.05 MPa
Adequate cooling is essential with slurry circulation ≥5 L/min to prevent thermal cracking.

Polishing

Sapphire polishing requires both material removal efficiency and nanometer-level surface quality.
Typical method: Chemical Mechanical Polishing (CMP)
Parameters:
Diamond colloidal slurry: 0.1–0.5 μm
Polyurethane polishing pad
Pressure: 0.03–0.08 MPa
Rotation speed: 30–50 rpm
Chemical reactions form a thin oxide film that is mechanically removed, enabling surface roughness Ra ≤ 0.2 nm without subsurface damage.

(2) Silicon Carbide: Ultra-High Hardness and High Brittleness

Silicon carbide presents one of the most difficult machining challenges among semiconductor materials.

Cutting

The key challenge is suppressing crack propagation.
Recommended parameters:
Diamond wire diameter: 0.10–0.15 mm
Diamond particle size: 2–4 μm
Wire speed: 15–20 m/s
Feed rate: 0.03–0.08 mm/min
A high-viscosity coolant containing antioxidants and stress-dispersing agents maintains cutting temperatures below 50°C and limits edge chipping to ≤30 μm.
For 6-inch or larger wafers, dual-wire synchronous cutting technology improves flatness and reduces wafer warping.

Grinding

Grinding pressure must be strictly controlled to avoid brittle fracture.
Process example:
Coarse grinding: 20–30 μm diamond slurry, pressure 0.08–0.15 MPa
Intermediate grinding: 8 μm → 3 μm → 1 μm diamond abrasives
Defects must be inspected after each stage using laser confocal microscopy.
Because SiC is highly corrosion-resistant, grinding fluids typically contain no chemical additives, but abrasive dispersion must remain uniform.

Polishing

Due to extreme hardness, SiC polishing efficiency is low and scratches easily occur.
Recommended two-step polishing process:
Diamond mechanical polishing
0.1 μm diamond colloid
Pressure: 0.05 MPa
Speed: 40 rpm
Plasma-Assisted Polishing (PAP)
Plasma performs atomic-level surface etching to correct micro-topography.
Final results:
Surface roughness: Ra ≤ 0.1 nm
Subsurface damage layer: ≤50 nm
Compared with traditional CMP, this method improves efficiency by more than 30% while reducing tool wear.

(3) Silicon Wafers: Efficient and Cost-Effective Conventional Processing

Cutting

Silicon wafers have moderate hardness and excellent machinability.
Typical wire saw parameters:
Wire diameter: 0.18–0.25 mm
Diamond size: 5–8 μm
Wire speed: 8–12 m/s
Feed rate: 0.15–0.3 mm/min
Standard emulsified coolant is sufficient, and edge chipping can be controlled within ≤40 μm.
Because silicon is anisotropic, cutting direction must align with the crystal orientation, typically <111> or <100>, to maintain surface uniformity.

Grinding

Grinding is relatively simple and uses low-cost abrasives.
Typical process:
Coarse grinding: SiC abrasive (10–20 μm), resin plate, pressure 0.2–0.3 MPa
Fine grinding: Al₂O₃ abrasive (3–5 μm), soft plate, pressure ~0.1 MPa
Silicon’s partial plasticity reduces brittle fracture risk, and cooling requirements are moderate.

Polishing

Silicon wafers benefit from highly mature CMP technology, enabling high efficiency and low cost.
Typical CMP parameters:
Silica slurry: 0.05–0.1 μm
Foam polyurethane pad
Pressure: 0.1–0.15 MPa
Speed: 50–60 rpm
The chemical reaction forms a thin SiO₂ film, which is mechanically removed to achieve:
Surface roughness Ra ≤ 0.1 nm
Polishing time ≤30 minutes per wafer
Post-polishing cleaning is essential to remove residual silica slurry and prevent oxidation.

(4) Quartz: Precision-Oriented Processing for Brittle Materials

Cutting

Although quartz is slightly softer than silicon, it is extremely brittle.
Two main cutting methods:

Diamond wire sawing
Wire diameter: 0.15–0.20 mm
Diamond size: 4–6 μm
Wire speed: 10–14 m/s
Feed rate: 0.08–0.15 mm/min
CO₂ laser cutting for complex shapes
Wavelength: 10.6 μm
Cutting speed: 0.5–1 m/min
Laser processing generates a 1–2 μm damaged layer, which must be removed by grinding.

Grinding

Soft abrasives and low-pressure processes are preferred.
Coarse grinding: Al₂O₃ (15–20 μm), resin plate, pressure 0.08–0.12 MPa
Fine grinding: CeO₂ (3–5 μm), felt pad, pressure <0.05 MPa
Neutral coolant prevents chemical reactions with quartz.

Polishing

Quartz polishing focuses on optical transparency and surface flatness.
Typical optical polishing process:
Cerium oxide slurry: 0.05–0.1 μm
Pitch polishing pad
Pressure: 0.03–0.06 MPa
Speed: 30–40 rpm
Optimizing slurry pH improves chemical-mechanical synergy, achieving:
Surface roughness Ra ≤ 0.05 nm
Optical transmittance ≥90% (UV–IR range)
No scratches or stress cracks

3. Comparative Summary of Processing Characteristics

Processing Aspect	Sapphire	Silicon Carbide	Silicon Wafer	Quartz
Tool requirements	High-hardness diamond tools	Ultra-performance diamond tools	Standard diamond or alumina tools	Soft abrasive tools
Machining efficiency	Low	Very low	High	Medium
Main defect risks	Edge chipping, subsurface cracks	Stress cracks, surface scratches	Crystal orientation deviation, oxidation	Chipping, laser damage layer
Processing cost	Medium–high	Very high	Low	Medium
Recommended process mode	Specialized equipment + CMP	High-precision equipment + PAP	Conventional equipment + mass CMP	Optical equipment + soft polishing

Conclusion

The hardness characteristics of crystalline materials fundamentally determine their machining strategies and processing challenges.
Ultra-hard materials such as silicon carbide and sapphire require high-performance tools, low-stress processes, and precision equipment. Processing efficiency is relatively low, costs are high, and defect control focuses on cracks and scratches.
Moderate-hardness materials such as silicon wafers and quartz benefit from mature machining technologies, allowing higher efficiency and lower cost. The primary concerns shift to crystal orientation deviation, edge chipping, and surface damage layers.
In practical manufacturing, process optimization must be based on material hardness, target precision, production scale, and cost constraints. By carefully selecting tools, process parameters, and cooling strategies, manufacturers can achieve the optimal balance between processing efficiency and product quality.