End Mill Coatings: TiN, TiCN, TiAlN, AlTiN, AlCrN, nACo, DLC and Diamond
Which coating belongs on which material? Hardness, oxidation temperature and friction coefficient tables, a compatibility matrix for 18 materials and the 6 coating mistakes made most often on the shop floor.
Short answer
- Coating choice follows the material, not the budget. A wrong match does not extend tool life, it shortens it.
- In aluminium a TiAlN/AlTiN coating gives 3–5 times shorter life than a polished uncoated tool. In aluminium a DLC-coated or uncoated tool is preferred.
- Diamond (CVD) and PCD cannot be used on ferrous materials, because carbon diffuses into iron and the coating graphitises.
- AlTiN and AlCrN are designed for dry/MQL work. Running with heavy emulsion prevents the protective Al₂O₃ barrier from forming.
A coating is a ceramic layer a few micrometres thick, deposited onto a carbide tool by the PVD or CVD method. It has three functions: to increase hardness, to insulate against heat and to reduce friction. This effect is not valid on every material, however; in a wrong match the coating shortens tool life.
Short answer
- General steel, wet cutting: TiAlN
- General steel, dry/MQL, high speed: AlTiN
- Stainless steel and Inconel: AlCrN
- Cast iron: TiCN or AlCrN
- Hardened steel (45+ HRC): nACo / AlTiSiN / TiSiN
- Aluminium, copper, plastic: Polished uncoated or DLC
- Graphite, high-silicon aluminium, CFRP: CVD diamond or PCD
Coating properties
| Coating | Hardness (HV) | Oxidation temperature | Friction coef. | Colour | Typical thickness | Application |
|---|---|---|---|---|---|---|
| Uncoated (polished) | 1,600–1,800 | – | 0.40–0.60 | Grey | – | Aluminium, copper, plastic |
| TiN | 2,300 | 600 °C | 0.55–0.65 | Golden yellow | 1–4 µm | General purpose, HSS tools |
| TiCN | 3,000 | 400 °C | 0.20–0.40 | Blue-grey | 1–4 µm | Cast iron, abrasive materials; wet cutting mandatory |
| TiAlN (Ti dominant) | 3,200–3,300 | 800 °C | 0.45–0.50 | Purple-black | 2–4 µm | The general-purpose standard for steel/cast iron |
| AlTiN (Al dominant) | 3,400–3,600 | 900 °C | 0.45–0.70 | Black-purple | 2–4 µm | The dry/MQL standard; forms an Al₂O₃ barrier |
| AlCrN | 3,000–3,200 | 1,100 °C | 0.30–0.35 | Blue-grey | 2–4 µm | Best oxidation resistance; stainless steel, superalloys |
| nACo / AlTiSiN | 4,000–4,500 | 1,100–1,200 °C | 0.40–0.45 | Violet-blue | 2–3 µm | The standard for hard machining at 45–65 HRC |
| TiSiN | 4,000–4,200 | 1,100 °C | 0.40 | Copper-violet | 2–3 µm | nACo alternative, high thermal shock resistance |
| CrN | 1,700–1,900 | 700 °C | 0.30–0.50 | Silver-grey | 2–6 µm | Copper, titanium, plastic injection moulds |
| ZrN | 2,700–2,900 | 550–600 °C | 0.40 | Light gold | 1–3 µm | Reduces adhesion on aluminium and titanium |
| DLC | 2,000–4,000 | 350–400 °C | 0.05–0.15 | Black | 1–3 µm | Aluminium, copper, plastic, composite; not heat resistant |
| CVD diamond | 8,000–10,000 | 600 °C | 0.10–0.20 | Matt grey-white | 6–20 µm | Graphite, CFRP, aluminium with Si > 12% |
| PCD (brazed tip) | ~8,000 | 700 °C | 0.10 | – | – | 50–100 times the life in aluminium/composite |
Critical point: CVD diamond and PCD cannot be used on ferrous materials (steel, cast iron, stainless steel). At high temperature carbon atoms diffuse into the iron, the diamond graphitises and disappears within minutes. This is not a matter of hardness but of chemical diffusion.
Material × coating compatibility matrix
Rating: ●●● best · ●● suitable · ● only when unavoidable · ✕ not suitable
| Workpiece material | Uncoated | TiN | TiCN | TiAlN | AlTiN | AlCrN | nACo | DLC | Diamond |
|---|---|---|---|---|---|---|---|---|---|
| Structural steel (S235, St37) | ● | ●● | ●● | ●●● | ●● | ●● | ● | ✕ | ✕ |
| Carbon steel (C45) | ● | ●● | ●● | ●●● | ●●● | ●● | ● | ✕ | ✕ |
| Alloy steel (42CrMo4) | ✕ | ● | ●● | ●●● | ●●● | ●● | ●● | ✕ | ✕ |
| Pre-hardened mould steel (1.2312, 1.2738) | ✕ | ● | ●● | ●●● | ●●● | ●●● | ●● | ✕ | ✕ |
| Hardened steel 45–55 HRC | ✕ | ✕ | ● | ●● | ●●● | ●●● | ●●● | ✕ | ✕ |
| Hardened steel 55–65 HRC | ✕ | ✕ | ✕ | ● | ●● | ●●● | ●●● | ✕ | ✕ |
| Austenitic stainless steel (304, 316) | ✕ | ● | ●● | ●● | ●●● | ●●● | ●● | ✕ | ✕ |
| Duplex stainless steel (2205) | ✕ | ✕ | ● | ●● | ●●● | ●●● | ●● | ✕ | ✕ |
| Grey cast iron (GG25) | ● | ●● | ●●● | ●●● | ●● | ●●● | ●● | ✕ | ✕ |
| Ductile iron (GGG50) | ● | ●● | ●●● | ●●● | ●●● | ●●● | ●● | ✕ | ✕ |
| Wrought aluminium (6061, 7075) | ●●● | ● | ●● | ● | ● | ● | ✕ | ●●● | ●● |
| Cast aluminium (Si > 10%) | ●● | ● | ●● | ● | ● | ● | ✕ | ●●● | ●●● |
| Copper, brass, bronze | ●●● | ● | ●● | ● | ● | ●● | ✕ | ●●● | ●● |
| Titanium (Ti6Al4V) | ● | ● | ● | ●● | ●●● | ●●● | ●● | ●● | ✕ |
| Nickel based (Inconel 718) | ✕ | ✕ | ● | ●● | ●●● | ●●● | ●● | ✕ | ✕ |
| Plastic (POM, PA, PMMA) | ●●● | ● | ● | ✕ | ✕ | ✕ | ✕ | ●●● | ●● |
| Graphite (electrode) | ● | ✕ | ✕ | ● | ● | ● | ✕ | ●● | ●●● |
| CFRP / glass fibre composite | ● | ✕ | ●● | ● | ● | ● | ✕ | ●●● | ●●● |
Dry or wet? The rule by coating
| Coating | Dry | Emulsion | MQL | Reason |
|---|---|---|---|---|
| AlTiN / TiAlN | preferred | thermal shock risk in interrupted cuts | ideal | The Al₂O₃ barrier only forms when hot |
| AlCrN | preferred | requires care | ideal | Same mechanism |
| nACo / TiSiN | preferred | thermal cracking risk, not recommended | ideal | In hard machining heat makes cutting easier |
| TiCN | not recommended | mandatory, with generous flow | may be insufficient | Above 400 °C its hardness falls rapidly |
| TiN | only at low Vc | preferred | suitable | 600 °C limit |
| DLC | only at low Vc | preferred | suitable | Graphitises at 350–400 °C |
| Uncoated | limited | mandatory | suitable | Adhering chips have to be washed away |
| Diamond (CVD/PCD) | suitable | suitable | suitable | Dust extraction mandatory in graphite |
In interrupted cutting (face milling, parts with holes) dry or MQL work is always required. In wet cutting the tool heats and cools on every revolution; thermal cracks (comb cracks) form and the edge fails through particles breaking out.
The 6 mistakes made most often on the shop floor
- Choosing a coating independently of the material. In aluminium a TiAlN-coated cutter gives 3–5 times shorter life than a polished uncoated one. The coating rounds the edge and aluminium sticks to the rounded edge.
- Using AlTiN with heavy coolant. Thermal shock plus the failure of the protective barrier to form → micro cracks at the edge.
- Running TiCN dry. The coating is gone in 5 minutes and the carbide is left bare.
- TiN on hardened steel. The 600 °C limit is exceeded within the first two minutes when machining at 55 HRC.
- Trying diamond coating on steel because it is "the hardest coating". The issue is not mechanical hardness but chemical diffusion.
- Continuing to use a reground tool uncoated. A reground tool that is not recoated gives roughly 40% of the life of the original.
Coating selection decision tree
Is the workpiece material ferrous?
├── NO (Al, Cu, plastic, graphite, composite)
│ ├── High-Si aluminium / graphite / CFRP → CVD diamond or PCD
│ ├── General aluminium, copper, plastic → Polished uncoated or DLC
│ └── Titanium → AlTiN or AlCrN
└── YES
├── Hardness > 45 HRC → nACo / TiSiN / AlCrN
├── Stainless steel or Inconel → AlCrN or AlTiN
├── Cast iron (abrasive, cool) → TiCN or AlCrN
├── General steel, wet cutting → TiAlN
└── General steel, dry/MQL, high Vc → AlTiN
How does a coating affect tool cost?
A coating generally adds 15–40% to the price of a tool but, in a correct match, increases tool life by a factor of 2–8. In a wrong match (AlTiN on aluminium) it shortens life.
The decision is made according to the material, not to the budget.
The Tool and Coating Selector, which gives a direct recommendation once the material and the operation are entered, can be used for this purpose.
Frequently Asked Questions
Are TiAlN and AlTiN the same thing?
No. The order in the name indicates the element that dominates by weight: titanium dominates in TiAlN, aluminium in AlTiN. As the aluminium content rises, oxidation temperature and hardness increase while toughness drops somewhat. In practice TiAlN is for wet/medium-speed steel work, AlTiN for dry/high-speed work and hard materials.
What coating thickness is appropriate?
On an end mill 2–4 µm is ideal. A thicker coating rounds the cutting edge, sharpness is lost and on small diameters chip evacuation deteriorates. On tools with D1 ≤ 3 mm, 1–2 µm is preferred.
Does the colour of the coating indicate its quality?
No. The colour is only the optical result of the coating chemistry and the shade varies from manufacturer to manufacturer. What is decisive is the layer architecture (single layer / multilayer / nanocomposite), the substrate pre-treatment and the edge preparation.
Does a reground tool have to be recoated?
Regrinding removes the coating from the cutting edge. A reground tool that is not recoated gives roughly 40% of the life of the original. On carbide tools, regrinding plus recoating usually costs 35–50% of the price of a new tool and makes sense.
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