CVD coating
Chemical vapor deposition families, substrate and geometry limits, and coating qualification.
Tags: processes, finishing, coatings
On this page
An ideal additive 5 µm film per wall leaves a 20.000 mm bore at 19.990 mm, or 0.005 mm below an assumed 19.995 mm minimum. Verify coating coverage deep in the actual bore, and account for thermal dimensional change separately.
What CVD specifies
CVD uses gaseous precursors that react or decompose at the substrate. Ionbond lists TiC, TiCN, TiN and aluminum oxide, alone or in combinations, for tools and components. Ionbond CVD technology
Physical vapor deposition (PVD) creates vapor from a physical source: Ionbond describes metal-target sputtering and cathodic-arc evaporation, followed by film formation, potentially with reactive gases. Thus, the presence of a reactive gas does not by itself identify a process as CVD. Specify the actual deposition route and coating system when comparing offers. Ionbond PVD technology
Process variants and thermal compatibility
| Named process example | Published process information | Selection consequence |
|---|---|---|
| Ionbond thermal CVD | High-temperature process 900–1050 °C; medium-temperature process 720–900 °C; typical coating thickness 5–12 µm | Confirm substrate condition and subsequent heat treatment, rather than treating a finished hardened part as automatically compatible. |
| SilcoTek proprietary CVD | Parts in a vacuum vessel are heated near 400 °C | This is a different coating family and thermal exposure, not a lower-temperature substitute with identical properties. |
| Oxford Instruments PlasmaPro 100 PECVD | Plasma-enhanced deposition uses RF-generated reactive species; process conditions control film stoichiometry and stress | Qualify the film and substrate together; “plasma enhanced” does not define one universal deposition temperature. |
These are supplier-specific descriptions, not operating windows for arbitrary equipment. Ionbond explicitly states that tempered-steel substrates need post-coating heat treatment to restore hardness. Oxford describes lower-temperature processing than conventional CVD, but offers different heated-electrode capabilities; a machine's maximum electrode temperature is not a required recipe or finished-part service limit. Ionbond CVD, SilcoTek coating process, Oxford PECVD
For procurement, identify alloy, existing heat treatment, required final hardness, previous coatings, brazed or welded joints, and any assembled nonmetallic components. Ask the processor to approve the complete thermal sequence, including any restoration treatment and final dimensional inspection. A coating temperature should be evaluated against the particular part condition, not simply against the alloy's melting point. This is a planning requirement, not a prediction of distortion. SilcoTek specifically requires removal of incompatible seals, O-rings and plastics from assemblies before its process. SilcoTek, “What Makes a Part Difficult to Coat?”, September 25, 2026
Coverage, preparation and geometry
Gas access can make internal surfaces accessible to CVD, but “conformal” should not be translated into guaranteed equal thickness everywhere. SilcoTek identifies blind holes, narrow bores and porous components as locations where thickness can vary. Support contacts can also affect local appearance or thickness. Supply a sectioned drawing showing internal passages, coating boundaries and critical contact surfaces, and agree how inaccessible locations will be qualified. SilcoTek difficult-part guidance
Masking is process-dependent: Ionbond describes masking for its CVD process as very difficult. Its PVD service instead requires part rotation for homogeneous coverage because deposition is directional. These observations support discussing geometry with the processor; they do not establish a universal maximum CVD hole depth or a universal PVD exclusion rule. Ionbond CVD, Ionbond PVD
CVD is not a leveling operation. SilcoTek warns that scratches, roughness and other surface imperfections remain visible through its coating, and that contamination from machining can be hidden from visual inspection. Provide cleaning and exposure history, including machining fluids, rather than specifying only a cosmetic finish. Its preparation guidance makes a contaminant-free starting surface important to the result. SilcoTek coating preparation
Coating identity also matters. Oxford lists silicon nitride and silicon dioxide PECVD for dielectric layers, photonics and passivation, alongside other films. Those uses do not establish wear, corrosion, electrical or optical performance for an unrelated tool coating. Put the required functional test on the order instead of relying on the acronym CVD. Oxford PECVD applications
Thickness, adhesion and acceptance
Specify local thickness requirements, measurement locations and the agreed method. ISO 26423:2009 describes crater grinding followed by microscopic examination for ceramic-coating thickness. Its scope excludes cases where coating or substrate surface roughness exceeds 20% of coating thickness because of measurement uncertainty. For a hypothetical 5 µm coating, 20% is 1 µm: this arithmetic is a method-suitability consideration, not a permissible roughness specification for every coating. Do not silently substitute a particular roughness parameter for the standard's requirements. ISO 26423:2009 public scope
ISO 20502:2005 describes diamond-stylus scratch testing under constant or increasing normal force to produce adhesive and/or cohesive failure of a ceramic coating–substrate system. Its stated range is coatings up to 20 µm, with possible applicability beyond that range. A reported scratch result therefore needs the test method, substrate condition and failure interpretation; it should not be relabeled as a universal bond strength or as proof of service life. Contractual testing requires the applicable full standard and agreed acceptance criterion. ISO 20502:2005 public scope
Original dimensional allowance example
Assume an ideal cylindrical bore is 20.000 mm before coating and receives a uniform, additive 5 µm layer on its entire internal surface. Assume no substrate dimensional change and no subsequent material removal:
- Radial build: 5 µm = 0.005 mm.
- Final bore diameter: 20.000 − 2 × 0.005 = 19.990 mm.
- If the drawing requires a final minimum of 19.995 mm, this assumed result is 0.005 mm undersize.
This is geometry, not a predicted process result. Use the actual local thickness distribution and final inspection, and account separately for thermal dimensional changes. A coupon can characterize a run, but the inspection plan must explain how it represents the bore being accepted. The need for location-specific qualification follows from the supplier's confined-feature coverage warning. SilcoTek difficult-part guidance
Information to release with the part
Provide the substrate and condition, coating system, approved process route, surfaces to coat or exclude, final dimensional requirements, functional acceptance tests and representative inspection locations. Resolve any post-coating heat treatment or finishing before release. Record whether a requirement concerns the coating alone or the complete coated component; a named deposition method does not define the component's allowable load, temperature or chemical environment.
Source versions: the supplier technology pages above do not display controlled revision dates. The SilcoTek geometry article is dated September 25, 2026. ISO 26423 is edition 1, January 2009, confirmed in 2025. ISO 20502 is edition 1, August 2005, confirmed in 2019 and marked for revision on the ISO record; use its applicable corrections with the controlled text. These public scopes support method selection, not an invented acceptance specification. ISO 26423 record, ISO 20502 record