Many engineering drawings written 15–20 years ago still carry a specification that looks like this:
ISO 10683:2000 — FLZNNC-480-L
At first glance it seems straightforward: a zinc flake coating with a lubricated top coat. But when that specification reaches a modern coating supplier, one of the first questions asked is often:
"What coefficient of friction (CoF) is required?"
That question regularly surprises buyers, because the original drawing never mentions friction at all. The reason lies in how ISO 10683 has evolved over the past two decades — and why the friction class is no longer something that can simply be assumed.
What Is ISO 10683?
ISO 10683 is the international standard covering non-electrolytically applied zinc flake coatings for threaded fasteners.
Unlike electroplating, zinc flake coatings provide corrosion protection without introducing hydrogen embrittlement, which makes them the preferred choice across:
- Automotive
- Mining
- Offshore
- Rail
- Defence
- Heavy structural applications
Typical coating systems include Delta-Protekt, Delta-Seal, Geomet, Magni, Dörken and NOF Metal Coatings. These systems usually consist of:
- A zinc/aluminium flake basecoat
- An optional lubricated topcoat
- An optional colour or sealer
Reading the Legacy Designation: FLZNNC-480-L
Many legacy drawings specify a designation in the FLZNNC family. Breaking it down:
| Code | Meaning |
|---|---|
| FL | Zinc flake coating |
| ZN | Zinc |
| NC | Non-chromate |
| 480 | Minimum corrosion resistance (480 h neutral salt spray) |
| L | Lubricated topcoat |
Under ISO 10683:2000, this designation was perfectly acceptable — and complete. Notice what is missing: there is no friction coefficient specified anywhere.
What Changed in ISO 10683:2014 and 2018?
Beginning with the 2014 revision (retained in the 2018 edition), ISO 10683 introduced a more complete designation system. The standard now explicitly recognises that:
Different lubricants produce different tightening behaviour.
The coating designation therefore allows an additional identifier to define the required friction class. Instead of only:
`` FLZNNC-480-L ``
the specification may become something equivalent to:
`` FLZNNC-480-LX ``
where the additional character identifies the friction category defined by the purchaser. The exact friction class is no longer assumed — it must be agreed between customer and supplier.
Why Does the Friction Coefficient Matter?
Many people assume zinc flake coatings exist only to prevent corrosion. In reality, the lubricant has a second, equally critical function:
It controls installation torque.
When a bolt is tightened, the applied torque is converted into preload — but only a small proportion of that torque actually stretches the bolt. A typical torque distribution is approximately:
- 50% consumed by thread friction
- 40% consumed by under-head friction
- 10% converted into bolt preload
The implication is stark:
Small changes in friction produce large changes in bolt tension.
A Worked Example
Assume a bolt is tightened to 250 Nm.
- If friction increases unexpectedly, less preload is generated: clamp force drops, and the joint may loosen in service.
- If friction is too low, preload becomes excessive: the bolt may yield, and fatigue life decreases.
Two coatings that both pass 480-hour salt spray resistance can therefore deliver very different mechanical performance. Corrosion resistance alone tells you nothing about how the fastener will behave when it is tightened.
Why Coating Suppliers Ask the Question
A coater cannot simply guess which lubricant should be applied. Consider two legitimate, corrosion-compliant systems:
| Coating | Typical Friction Coefficient |
|---|---|
| KL105 | 0.12–0.18 |
| Delta Seal GZ Silver | 0.09–0.14 |
Both satisfy corrosion requirements. Both are zinc flake technology. But they behave differently during tightening. Selecting the wrong lubricant can result in:
- Incorrect installation torque
- Bolt preload outside design limits
- Assembly failures
- Customer rejection
That is why responsible coaters will always ask the question up front.
What If the Drawing Only References ISO 10683:2000?
This is extremely common in defence and infrastructure projects, where drawings may be decades old. Since the original specification predates the newer designation system, there are generally three ways to handle it.
Option 1 — Confirm with the Design Authority (Recommended)
If possible, go back to the customer and ask:
- Is a specific coefficient of friction required?
- Is a preferred coating system already approved?
- Is torque validation required?
This is the safest engineering approach, because it removes the ambiguity at the source.
Option 2 — Use the Supplier's Standard Lubricant
If the customer has no preference, many suppliers propose their standard system — for example KL105 or Delta Seal GZ Silver. The chosen friction range should then be documented in the coating records and communicated to the customer for sign-off.
Option 3 — Match Existing Production
For maintenance or repeat contracts, the most practical route is often to use the same coating system previously supplied, so that tightening behaviour stays consistent with what is already in service.
Can the Friction Coefficient Be Left Unspecified?
Technically, yes. For a drawing written to ISO 10683:2000, the omission is not an error — that edition simply did not require this level of designation.
However, modern coating companies generally prefer clarification, because today's quality systems place much greater emphasis on:
- Repeatability
- Installation performance
- Traceability
- Torque consistency
Procurement Considerations
When reviewing an RFQ that references ISO 10683:2000, buyers should check:
- Which revision of ISO 10683 is contractually required?
- Is the drawing intentionally using the 2000 edition?
- Does the customer specify installation torque?
- Is bolt preload critical to the joint design?
- Does the coating supplier require a friction class?
- Will the chosen lubricant affect compatibility with existing assemblies?
Clarifying these questions before production starts can prevent costly rework later.
Best Practice Recommendation
For legacy drawings referencing ISO 10683:2000, avoid making assumptions. Instead:
- Verify whether the customer requires a specific coefficient of friction.
- If not specified, agree on the coating system with the supplier.
- Record the selected friction range in the manufacturing documentation.
- Inform the customer of the proposed coating system before production begins.
This approach protects both supplier and purchaser, while ensuring consistent installation performance.
Final Thoughts
The evolution of ISO 10683 reflects an important shift in engineering philosophy. Earlier editions focused primarily on corrosion protection. Modern revisions recognise that surface coatings also influence joint performance through friction control.
Two zinc flake coatings may look identical after passing a 480-hour salt spray test, yet produce significantly different bolt preload during installation. For that reason, specifying — or confirming — the required coefficient of friction has become just as important as specifying corrosion resistance.
For procurement and quality engineers, understanding this seemingly small detail can prevent installation issues, warranty claims and costly field failures.