Case StudyBlack Cr-C-N on plated ABS

The colours and the peeling are the same fracture.

Five interference tints on a black coating looked like a chemistry defect. One object explains both complaints: a fracture inside the coating, above a transparent film whose thickness reproduces every colour.

The engineering question

Why does a black coating show interference colours and lift away?

Evidence available

A program table, one control-screen photograph, one hand record and the defect description. No film measurement exists.

How Theorema Coatings solved it
  1. 01
    Reconstructed the flux

    The program table turned into per-step C/Cr ratios and ion energies.

  2. 02
    Inverted the colours

    224 candidate films screened; one class reproduces all five tints.

  3. 03
    Weighed the fracture

    The failing plane’s toughness against every interface below.

  4. 04
    Separated cause from confound

    Recipe and geometry ruled out; two runs isolate the rest.

What the failure revealed

The film grew with too much carbon and too little ion energy; it is soft, transparent, and fractures within itself.

First-order interference colours of a transparent film on chromium, by thickness A continuous colour scale from 0 to 140 nanometres; the five reported tints sit in windows between 16 and 136 nanometres, while the full coating at 1080 nanometres or more lies far off this scale. showing all five needs 84–104 nm of variation 04080120140 nm coating ≥ 1080 yellow red violet blue green 16–28 52–54 56–60 62–110 122–136
Computed from the defect record. First-order colours of a transparent film on chromium. The whole colour run ends at 136 nm; the coating the gate requires is eight times thicker.
  1. 01

    The recipe could not form a hard film

    Carbon per chromium runs 1.77→7.08 against the Cr₃C₂ ceiling of 0.667. Acetylene ions arrive at 24–48 eV; the 55 eV threshold needs 137.5 V.

  2. 02

    The fracture runs inside the coating

    The failing plane holds 4.94 J/m²; every interface beneath it holds 83–400. A tougher film is the only change that acts on the failing plane.

  3. 03

    The difference between chambers lies outside the recipe

    The same recipe passes on the old chamber, fails on the new. Three suspects lie outside the recipe; the plating and the line are the most likely.

Recommendation

Localize the cause with two runs before touching the recipe.

Run 1 · old machine

today’s blanks · same recipe

bad · expected
The plating and the line are the cause.

Fix the plating; the new chamber is not the cause.

good
Run 2 · clean unplated metal

held at 60 °C

holds
Qualify and season the chamber.

Review the plating prep.

fails
Harden the base recipe.

140 V / 50% on steps 07–13 — the only recipe change.

Acceptance gate, fixed before the run

A tape test delivers 1–3 J/m² against driving forces of 10–40; the gate replaces it.

RunC/CrThicknessΔE*abPressurePart tempRecord
140 V / 50% base≤ 0.67≥ 1080 nm≤ 3≤ 1 Pa< 60 °Call logged