Tungsten carbide (WC-Co) cemented carbides dominate modern metal cutting. Understanding substrate composition, ISO classification, and coating technologies is essential for optimizing tool life and machining productivity.
Cemented Carbide Composition
Cemented carbides consist of hard tungsten carbide (WC) particles bonded by a ductile cobalt (Co) matrix. The WC grain size and cobalt content determine the fundamental properties:
- **Low cobalt (3–6%)**: High hardness (91–93 HRA), high wear resistance, but lower toughness. Used for finishing operations and cast iron machining.\n- **Medium cobalt (6–10%)**: Balanced properties. General-purpose turning and milling grades.\n- **High cobalt (10–15%)**: Maximum toughness and impact resistance. Heavy roughing, interrupted cuts, and difficult materials.
Grain size ranges from ultra-fine (<0.5 µm) for maximum hardness and edge sharpness to coarse (3–5 µm) for maximum toughness. Modern nano-grain carbides (<0.2 µm) achieve hardness above 94 HRA.
ISO Application Classification
ISO 513 defines application groups based on workpiece material and chip type:
- **P-group (blue)**: For steel and steel castings producing long chips. Contains TiC/TaC additions to resist crater wear. Grades P10–P40 (lower number = harder, higher number = tougher).\n- **M-group (yellow)**: For stainless steel, manganese steel, and difficult-to-machine materials. Versatile but specialized grades needed for austenitic stainless.\n- **K-group (red)**: For cast iron, non-ferrous metals, and non-metallic materials producing short chips. Straight WC-Co without TiC/TaC additions.\n- **N-group (green)**: Non-ferrous metals (aluminum, copper). Sharp uncoated or PVD-coated grades.\n- **S-group (brown)**: Heat-resistant superalloys and titanium. Maximum toughness substrates with PVD coatings.\n- **H-group (grey)**: Hardened steels (>45 HRC). Ultra-fine grain substrates or CBN.
CVD Coating Technology
Chemical Vapor Deposition coats carbide inserts at 900–1050°C with multi-layer ceramic films:
- **TiCN**: First layer, provides adhesion and wear resistance. Thickness 3–8 µm.\n- **Al₂O₃ (alumina)**: Chemical barrier and thermal insulation. Enables higher cutting speeds. 2–8 µm.\n- **TiN**: Top layer, gold color for wear detection. 0.5–2 µm.\n- **Total CVD thickness**: 8–25 µm. Provides excellent crater and flank wear resistance.
CVD coatings are inherently under tensile stress and slightly round the cutting edge, making them better suited for turning than milling in most cases.
PVD Coating Technology
Physical Vapor Deposition applies thin, hard coatings at 400–600°C:
- **TiAlN**: The most versatile PVD coating. Oxidation resistant to 800°C. 1–5 µm.\n- **AlCrN**: Superior oxidation resistance (900°C). Excellent for dry machining and hardened steel.\n- **TiSiN (nanocomposite)**: Hardness up to 40 GPa. For hardened steel and high-speed finishing.\n- **Total PVD thickness**: 1–8 µm. Compressive stress maintains sharp cutting edges.
PVD coatings are preferred for milling (interrupted cutting), small inserts, drilling, and machining sticky materials like stainless steel and titanium where a sharp edge is critical.
Coating Selection Guide
**General steel turning**: CVD TiCN/Al₂O₃/TiN — maximum tool life at moderate to high speeds. **Stainless steel**: PVD TiAlN — sharp edge reduces work hardening. **Cast iron**: CVD Al₂O₃ (thick) — thermal barrier at high speeds. **Titanium/superalloys**: PVD TiAlN or uncoated — sharp edge, low cutting forces essential.