Lesson 11.8CoatChapter 11 · Coatings and surface finishing

PVD coating (thin films in vacuum)

Atoms knocked off a target coat parts in vacuum

PVD (physical vapor deposition) coats parts with a metal or ceramic film a few microns thick, built atom by atom in a vacuum. The gold-colored layer on many drill bits is a PVD coating of titanium nitride. It makes tools last several times longer and gives watches and faucets a hard, lasting color.

Photo: Cetko, CC0, via Wikimedia Commons

PVD coating · sputtering in a vacuum chamber, with a close-up of the filmThe circled numbers match the steps below; the box on the right zooms in on one coated surface.

How it works

Clean and load. Parts are cleaned spotless, often in ultrasonic baths, then mounted on fixtures that turn during coating. Any trace of oil or dust would stop the film from sticking.
Pump down and heat. Pumps remove the air until the pressure is roughly a millionth of normal. Heaters bring the parts to typically 200–500 °C, and a short blast of argon ions scrubs their surfaces clean.
Knock atoms off the target. Argon gas is turned into a plasma (a glowing, electrically charged gas). Its ions slam into the target, a plate of the coating metal, and knock atoms loose: this is sputtering. Other machines free the atoms with an electric arc or by simple evaporation.
Fly and react. The freed atoms fly in straight lines and land on whatever they can see. Nitrogen fed into the chamber reacts with them, so titanium lands as hard, gold-colored titanium nitride (TiN).
Turn and build up. The fixtures rotate so every face gets its share. The film typically grows 1–5 µm thick, then the chamber cools and is vented.

Key facts

Materials: Coatings: TiN, TiAlN, CrN, DLC (diamond-like carbon), aluminum, chromium. Parts: tool steel, carbide, stainless steel, glass, some plastics
Typical parts: Drills, end mills and inserts, molds, watch cases, faucets, lenses, chips
Part size: From tiny inserts to parts tens of centimeters long; limited by the chamber
Precision: Film typically 1–5 µm, down to fractions of a micron; the part's size barely changes
Surface finish: Copies the surface below (polished in, polished out); colors from gold (TiN) to black (DLC)
Batch size it suits: Hundreds to thousands of small parts per load; coating services take small runs
Tooling and machine cost: High: an industrial system typically costs hundreds of thousands to over a million dollars; cost per part is low in full batches
Speed: A batch typically takes a few hours from loading to unloading

Strengths and limits

Strengths

Very hard, thin film that keeps sharp edges sharp
Many hard and decorative colors, such as gold, black or bronze
Moderate heat: most tool steels keep their hardness
Clean, dry process with no liquid waste

Limits

Line of sight: deep holes and hidden faces get little or no coating
Needs a vacuum chamber: high machine cost and batch work
The part must be spotless, or the film peels
Too thin to fill scratches or rebuild worn parts

Where it's used

Cutting toolsGold TiN drills, dark TiAlN end mills, carbide inserts
Molds and diesWear-resistant films on injection molds and forming tools
Watches and jewelryBlack, gold or rose-gold finishes on steel cases and bands
Home and bathBrushed-nickel or brass-look faucets and door handles
Optics and electronicsAnti-reflection layers on lenses, metal films on chips and displays
AutomotivePiston rings and engine parts with CrN or DLC coatings

How to recognize it

A thin, even color on metal: gold on drill bits, violet-black or bronze on cutting tools.
The color covers outer faces and cutting edges perfectly but fades inside deep holes and grooves.
No drips, runs or thick edges: the metal itself seems to have changed color.
Very hard to scratch, unlike paint of the same color.