# Thermal spraying

Spray-process families, substrate preparation, finishing and coating-system acceptance boundaries.

Reviewed: 2026-11-01

Tags: processes, finishing, coatings

A hypothetical shaft prepared at **49.600 mm** with **0.300 mm radial spray buildup** reaches **50.200 mm**. Grinding to **50.000 mm** removes **0.100 mm radially** and leaves **0.200 mm radial coating** in the ideal model. Verify eccentricity, local minimum remaining coating and final geometry.

## Process and intended function

Conventional thermal spray builds overlapping splats from molten or semi-molten particles. TWI distinguishes coatings made from the same material by different spray processes. [TWI thermal spray](https://www.twi-global.com/what-we-do/research-and-technology/technologies/coating-and-surface-engineering/thermal-spraying)

The substrate and coating remain different parts of the engineering system. Struers describes the substrate as providing bulk strength and the coating as supplying selected surface properties. For repair, determine whether the route restores a functional surface or is separately qualified to carry load; deposited material does not automatically restore the original component's design allowables. [Struers thermal-spray metallography](https://www.struers.com/en/Knowledge/Materials/Thermal-spray-coatings)

## Process families are not interchangeable

| Route | Basic distinction |
| --- | --- |
| Combustion flame spray | Flame heats wire or powder feedstock. |
| Electric-arc wire spray | An arc between feed wires supplies the heat for metallic feedstock. |
| Plasma spray | A plasma jet heats feedstock; ceramic coatings are among its applications. |
| High-velocity oxy-fuel, HVOF | Combustion and high particle velocity produce a different thermal/kinetic history from conventional flame or plasma spraying. |
| Cold spray | Accelerated particles deposit in the solid state, rather than through melting and solidification. |

Oerlikon Metco's introductory guide describes the conventional thermal-spray families. TWI separately describes cold spray as impact-driven plastic deformation below the particles' melting point. Its gas can be heated: “cold” identifies the deposition mechanism and does not mean every part experiences room temperature. Keep the exact process, feedstock and approved parameter set in the specification. [Oerlikon introduction, Issue 7, May 2022](https://www.oerlikon.com/ecoma/files/BRO-0005_Thermal_Spray_Brochure_EN.pdf?download=true), [TWI cold spray](https://www.twi-global.com/what-we-do/research-and-technology/technologies/coating-and-surface-engineering/cold-spraying)

Do not silently equate spraying with spray-and-fuse. Oerlikon's guide describes an additional fusing step for self-fluxing alloys, with diffusion into the substrate and altered porosity. That additional thermal exposure and bonding route need their own substrate review. [Oerlikon introduction, section 2.7.3](https://www.oerlikon.com/ecoma/files/BRO-0005_Thermal_Spray_Brochure_EN.pdf?download=true)

## Preparation, access and thermal exposure

For the conventional processes described by TWI, bonding is primarily mechanical and depends on particle impact and surface condition. Cleaning and roughening by an appropriate process are therefore consequential. Specify substrate alloy/condition, existing coatings, contamination, preparation, any bond coat and the topcoat. This description should not be generalized into a claim that every spray route has an identical interface mechanism. [TWI thermal spray](https://www.twi-global.com/what-we-do/research-and-technology/technologies/coating-and-surface-engineering/thermal-spraying)

Spraying is directional. ASM's discussion of porosity explains that incoming material cannot necessarily fill voids hidden beside trapped particles because deposition is line-of-sight. At part scale, review recesses, bores, edges, masking and fixture contacts with the processor. Agree gun access and representative inspection locations rather than transferring a flat-coupon result to an inaccessible passage. [ASM, “Sources of coating porosity”, March 6, 2020](https://www.asminternational.org/spray-tips-sources-of-coating-porosi-1/)

The hot gas or particle temperature is not the substrate temperature. Conversely, a process that avoids melting the substrate is not proof of compatibility with every temper, braze, assembly or polymer. Establish a qualified thermal budget, cooling and monitoring arrangement for the actual component, including subsequent fusing, sealing or finishing. This is a process-planning requirement, not a generic allowable temperature.

## Porosity, finishing and the complete system

Porosity and splat boundaries can matter independently of nominal thickness. Oerlikon's sealer guide identifies penetration through coating defects and resulting substrate corrosion as possible failure paths. A sealer is another specified material with its own cure and service compatibility, not a universal correction for an unsuitable spray deposit. Its **Metcoseal LP-40** example is directed to low-porosity carbide and oxide coatings for general wet-corrosion protection, including hydraulic rods and industrial rolls; qualify the actual environment and coating combination. [Oerlikon sealers, FLY-0109.0, ©2024](https://www.oerlikon.com/ecoma/files/FLY-0109_Sealers.pdf?download=true)

Oerlikon's introduction describes machining, grinding or lapping where required to achieve final dimensions and finish. Specify both as-sprayed allowance and the remaining finished coating. Final size alone cannot prove that a minimum layer remains at every high spot, transition or edge. Where sealing is required, define its position in the approved finishing sequence and the condition in which inspection occurs. [Oerlikon introduction, section 2.7](https://www.oerlikon.com/ecoma/files/BRO-0005_Thermal_Spray_Brochure_EN.pdf?download=true)

### Original bounded restoration example

Use the hypothetical **49.600 mm prepared shaft** and **0.300 mm uniform radial deposit**, with no other dimensional change:

`as-sprayed diameter = 49.600 + 2 × 0.300 = 50.200 mm`.

Grinding to **50.000 mm** removes `(50.200 − 50.000) / 2 = 0.100 mm` radially and leaves **0.200 mm** radial coating. These are planning assumptions, not an advertised process capability. Incoming eccentricity, local deposit variation and grinding setup can make the minimum remaining layer smaller than this ideal value. Establish substrate size, final geometry and local coating acceptance separately.

## Testing and qualification boundaries

**ISO 14916:2017** measures tensile adhesive or cohesive strength normal to a thermally sprayed surface, including interfaces within multilayer systems. Its scope explicitly says the result is not an absolute durability value for operational use and identifies fused self-fluxing coatings as inappropriate for this tensile method. Preserve the coating stack and failure location with any reported result. [ISO 14916 scope](https://www.iso.org/standard/60995.html)

**ASTM C633-24** also uses a normal tensile test with an adhesive-bonded loading fixture. Its scope limits the conventional method to coatings thicker than **0.015 in.**, with **0.38 mm** given as the informational conversion, because bonding-agent penetration can invalidate results. It discusses possible exceptions for suitable dense coatings or other bonding agents; do not apply an exception without qualification. It also warns against using the result as an intrinsic strength for environmental-stress design calculations. [ASTM C633-24](https://store.astm.org/standards/c633)

Cross-section examination can assess thickness, pores, oxides, cracks, unmelted particles and interface condition. Preparation itself can create misleading evidence: Struers warns that smeared metal may conceal pores and brittle-particle pullout may exaggerate them. Record the preparation and evaluation method, locations and acceptance criteria rather than treating an arbitrary micrograph as a universal porosity measurement. [Struers preparation guidance](https://www.struers.com/en/Knowledge/Materials/Thermal-spray-coatings)

Record substrate condition, feedstock, preparation, masking, spray route, finishing and applicable acceptance results.

Source versions: Oerlikon **BRO-0005.7** prints **Issue 7 – May 2022**; its sealer guide prints **FLY-0109.0, ©2024**. ISO 14916 is edition 2, March 2017, confirmed in 2022. ASTM identifies **C633-24**, updated November 19, 2024. Undated TWI and Struers web guidance is distinguished from controlled product instructions and contractual acceptance limits. [ISO record](https://www.iso.org/standard/60995.html), [ASTM record](https://store.astm.org/standards/c633)

## Related reference

[Surface-finishing processes](index.md)