Choosing tool substrate and coating by application
Exactalloy · Updated
Substrate controls the tool body and coating controls the interface. Match the coating to the workpiece family and match the substrate grain size to the operation, not the other way round.
Substrate: grain size follows the operation
Grain size trades toughness against wear resistance. Finer grain resists abrasive wear and holds an edge, which suits hardening work and finishing; coarser grades absorb more load and suit heavy roughing.
- Sub-micrograin (0.4–0.8 µm): high hardness and finishing work.
- Fine grain: general purpose with a balanced life.
- Coarser grain: heavy roughing and interrupted cuts.
- PCD and PCBN are separate cutting materials rather than coatings.
Coating families and where they fit
A harder coating is not automatically better. The coating has to be compatible with the workpiece chemically as well as mechanically, so selection follows the material group.
- TiAlN / AlTiN: steel and cast iron, good hot hardness.
- Nano-composite AlTiN: hardened steel and high-speed finishing.
- ZrN: aluminium, reduces built-up edge.
- DLC: aluminium, copper and composites, low friction.
- Diamond: graphite, CFRP and high-silicon aluminium.
Why diamond fails on steel
Diamond coatings are chemically unstable in contact with ferrous material at cutting temperature. Diamond belongs on non-ferrous, composite and graphite work; it is not a universal upgrade.
- Diamond on steel accelerates wear instead of preventing it.
- For hardened steel use high-hardness carbide or CBN instead.
- For high-silicon aluminium, diamond resists the abrasive silicon phase.
Balancing life against cost
A more expensive tool is only better if it reduces cost per part. Compare on total cost rather than purchase price, and include regrinding in the calculation.
- Compare cost per part using measured tool life in the same operation.
- A premium tool that survives longer can cut tool changes and scrap.
- Regrinding can restore much of the life at 30–50% of new-tool cost.
- Keep the same grade and coating across a batch so results are comparable.