Two different mechanisms
Electroplating deposits a metal layer that is chemically similar to the substrate and, in the case of zinc, deliberately less noble. When corrosion starts, the zinc sacrifices itself to protect the steel — the coating is consumed so the screw is not. That is why a zinc-plated screw can show white rust and still be structurally sound.
PVD deposits a ceramic film — a nitride or carbide — that is harder than the steel beneath it but chemically inert. It does not sacrifice itself. If the film is breached, corrosion proceeds at the breach with no protection from the surrounding coating. PVD is a wear and appearance solution, not a corrosion solution, unless it is combined with an interlayer.
Where PVD wins
- Drive recesses that see tens of thousands of automated engagements — TiN and TiCN resist deformation far better than zinc.
- Visible decorative fasteners where a durable black or gold is required without paint or plating chemistry.
- Low-friction joints where a DLC film gives µ around 0.10 without liquid lubricant.
- Optical and vacuum environments where a dry, low-outgassing film is preferable to oil.
Where electroplating wins
- Corrosion resistance as the primary requirement — zinc-nickel delivers 480–720 h salt spray at a fraction of the PVD cost.
- High-volume parts where the cost per piece matters more than wear resistance.
- Threaded joints where a slight diametral growth is acceptable and the coating can be gauged after plating.
- Applications where the fastener is buried inside a housing and appearance is irrelevant.
The thread-fit trap
Both processes change dimensions. A 10 µm zinc coating adds up to 20 µm diametrally; a 3 µm PVD film adds 6 µm. For Class 6g this is usually tolerable, but for a tight 6f fit it can turn a good design into an interference fit.
The practical answer is to specify the fit class, the coating thickness and the gauging point together. If the thread is critical, gauge after coating or mask the thread. Do not assume the drawing tolerance covers the coating.
| Property | Zinc-nickel plating | TiN PVD | DLC PVD |
|---|---|---|---|
| Thickness | 8 – 15 µm | 2 – 4 µm | 2 – 3 µm |
| Hardness | HV 250 – 350 (film) | HV 2300 | HV 2000 – 4000 |
| Salt spray | 480 – 720 h | 96 – 240 h with interlayer | 96 – 240 h with interlayer |
| Friction (µ) | 0.15 – 0.22 | 0.20 | 0.10 |
| Colour | Silver / blue | Gold | Near-black |
| Cost index | 1.0 | 3 – 4 | 4 – 6 |
A common combined specification
For a visible screw on a high-cycle assembly line, the best answer is often both: a zinc-nickel or nickel underlayer for corrosion, then a thin DLC or CrN top film for wear and colour. The underlayer provides sacrificial protection at a breach; the top film provides hardness and the appearance the product needs.
This is more expensive than either alone, so reserve it for positions where both requirements genuinely exist. On an internal screw, zinc-nickel alone is almost always the better engineering and commercial choice.