What each coating resists
The useful question is what the tool is dying from, because that is what the coating has to address.
| Coating | Application temperature | Resists | Typical use |
|---|---|---|---|
| TiAlN | Up to about 800 C | Abrasion and heat | General steel and stainless, milling and drilling under coolant |
| AlTiN | Up to about 900 C | Heat with a harder surface | Hard milling, dry cutting, titanium |
| AlCrN | Up to about 1100 C | Oxidation at high temperature | Hardened steel, nickel alloys, hard milling |
| TiCN | Up to about 400 C | Abrasion with high toughness | Cast iron, interrupted cuts, tapping |
| DLC | Low temperature | Adhesion and friction | Aluminium, copper, composites, non-ferrous |
| CVD Al2O3 | High temperature process | Crater and flank wear | Continuous turning of steel and cast iron |
CVD and PVD are different processes
CVD is applied at high temperature, which produces a thick, well-bonded coating with excellent wear resistance. The trade-off is that the high temperature slightly affects the substrate and the coating is blunter, so the edge is less sharp.
PVD is applied at a much lower temperature, producing a thinner coating that preserves a sharp edge. That is why milling, threading and stainless work use PVD: the edge has to cut rather than rub, and the coating has to survive thermal cycling on every revolution.
Why a hard coating can make aluminium worse
Aluminium fails tools by adhesion rather than by abrasion, so the answer is a low-friction surface and a sharp edge. A hard coating such as TiAlN blunts the edge and adds a rough surface for material to key into, which makes built-up edge more likely rather than less.
DLC or uncoated polished carbide is the correct answer, and the polished surface does more for tool life than any coating hardness could.
Coating thickness and edge sharpness
A coating adds thickness to the edge, which increases the effective edge radius. On a 6 mm end mill that is negligible; on a 0.5 mm micro drill it can be a significant fraction of the edge radius, and it may be the difference between cutting and rubbing.
This is why micro tools are frequently supplied uncoated or with a very thin coating, and why a coating chosen for its hardness can destroy the performance of a small tool.
Frequently asked questions
What is the difference between TiAlN and AlCrN?
TiAlN resists heat and abrasion well and is the general-purpose choice for steel and stainless. AlCrN resists oxidation at higher temperature, which suits hardened steel and nickel alloys where the edge runs hotter and would otherwise oxidise rather than wear.
Why is TiAlN not recommended for aluminium?
Because aluminium fails tools by adhesion rather than abrasion, and a hard coating makes the edge blunter and rougher, which encourages built-up edge. DLC or uncoated polished carbide is the correct choice.
Which is better, CVD or PVD?
Neither in general: CVD gives better wear resistance in continuous turning of steel and cast iron, and PVD gives a sharper edge that suits milling, threading, stainless and any interrupted cut. The operation decides.
Can a coating be too thick?
Yes, particularly on small tools. A coating increases the effective edge radius, so on a micro drill it can be a significant fraction of the radius and can prevent the tool cutting cleanly. Micro tools are often supplied uncoated or thinly coated for that reason.
Do I need a coating at all?
For aluminium, cast iron and some composite work, uncoated can outperform a coated tool. For steel, stainless and heat-resistant alloys, a coating is what makes the tool economically viable, because uncoated carbide wears too quickly at the temperatures involved.