Tool Coatings
Tool coatings are micrometre-thin films of a hard compound — often a nitride of a metal — deposited onto a cutting tool to change how its surface behaves. A coating does not make the tool material harder underneath; it puts a harder, slicker, more heat-resistant skin where the cutting happens. A coated drill or insert runs faster, lasts longer and sticks less than the same tool uncoated, which is why the great majority of production cutting tools are coated, and why coating type is part of the specification of nearly every insert and end mill sold.
What a coating actually does
A coating earns its keep four ways at once. It resists wear: the film is harder than the steel or carbide it covers, so abrasive wear eats the coating instead of the tool. It lowers friction: a slicker surface means less heat generated at the edge and less force needed to push the chip, and a low-friction surface is also far less likely to pick up workpiece material and build a built-up edge. It insulates and shields against heat: certain coatings form a thin oxide scale at cutting temperature that keeps oxygen away from the tool and reflects some heat, letting the edge survive speeds that would soften or oxidise it bare. And it protects against chemical attack and galling on materials that like to weld to a tool. All of this happens in a film a few millionths of a metre thick — which is why the film must sit on a correct substrate. A coating cannot save the wrong tool grade, a blunt geometry or a dull edge; it multiplies the performance of a good tool and does little for a bad one. It also wears through where it is doing its job, and regrinding a coated tool grinds the coating off the cutting edge — which is why cobalt high-speed steel, which carries its hot-hardness benefit through the whole body, stays valuable where tools are resharpened.
PVD or CVD: how the film gets there
Two processes put coatings on tools, and the choice between them shapes what a coating can be. PVD — physical vapour deposition — condenses the coating material onto the tool in a vacuum at low to moderate temperature. Because the tool never gets hot enough to soften, PVD suits high-speed steel as well as carbide, and because the film is thin, it lands on a sharp edge and keeps it sharp. PVD is therefore the process for drills, taps, end mills and anything with a keen edge that must stay keen. CVD — chemical vapour deposition — grows the coating by chemical reaction on a much hotter tool, typically at around a thousand degrees. The result is a thicker, very well-adhered film that can be built in layers, with outstanding heat resistance — but only on a substrate that can take the heat (carbide, not HSS), and with an edge that is rounded rather than razor-sharp. CVD is the process of heavy turning and roughing inserts, where a robust, heatproof edge matters more than sharpness. The rule of thumb follows the cut: sharp-edged, interrupted and precision tools take PVD; continuous, heavy, hot cuts take CVD.
The common coatings
The coatings a machinist actually meets are a small, standard family, most of them recognisable by colour. Titanium nitride (TiN) — the familiar gold — is the economical workhorse: general-purpose wear resistance on drills, taps and mills for steels and non-ferrous metals. Titanium carbonitride (TiCN), the blue-grey, is harder and slicker, for abrasive work and cast iron. Titanium aluminium nitride (TiAlN and AlTiN), the dark violet-grey, is the modern standard for steel and stainless at speed: its aluminium oxidises into an alumina skin at cutting temperature that gives it its heat resistance, and it is the coating of choice for dry and high-speed machining. Chromium nitride (CrN), the silver coating, resists adhesion and corrosion rather than extreme heat, and is chosen for aluminium and other non-ferrous work where built-up edge is the enemy. Diamond-like carbon (DLC) has the lowest friction of all and the non-stick behaviour for aluminium, plastics and composites, but it cannot take high temperature. And real coatings are often not one film but a stack — a base for adhesion, a hard middle layer for wear, a top layer for heat or low friction — which is how a single insert gets more than any one compound could give.
Choosing a coating
The first rule of coating selection is chemical, not mechanical: titanium-based coatings must not be used on titanium or aluminium work, because the tool and the workpiece share an affinity and the edge picks up material and fails by adhesion and built-up edge. For those metals the anti-stick coatings — CrN, DLC, or an uncoated sharp tool — are the answer. Given that, the choice runs by workpiece and cut. Steels and stainless at speed and temperature want TiAlN or AlTiN; abrasive cast iron and graphite want the harder TiCN family; heavy, continuous, high-temperature turning wants a thick CVD multilayer on carbide; sharp, flexible, interrupted milling wants a thin PVD film on a sharp edge; and a coated tool being resharpened will need re-coating, or the shop plans for the reground edge to run bare.
A coating is specified together with the substrate, not instead of it: the high-speed steel or carbide body supplies toughness and edge, and the coating adds the surface performance on top — the reason coated grades sit at the top of each carbide family. Together they set how fast a tool can go, which is where feeds and speeds take over, inside the everyday practice of CNC machining.