How to Coat the Inside of Stainless Steel Tubes and Small Bores

ALD coating inside stainless steel tubes and small bores is possible because vapor-phase reactants can reach surfaces that line-of-sight coating methods cannot. The process builds a thin film through sequential, self-limiting surface reactions. As a result, the coating can follow an internal passage without substantially changing its diameter.

However, uniform coverage depends on several factors. Tube geometry, surface condition, coating chemistry, process temperature, and precursor exposure all affect how well the coating reaches the deepest internal surfaces.

VaporPulse Technologies develops custom atomic layer deposition processes for parts with complex geometries, internal features, and application-specific coating requirements. Therefore, a successful tube-coating project starts with understanding how reactants will move through the passage and how the finished internal coating will be verified.

Why Stainless Steel Tubes Are Difficult to Coat Internally

The inside of a narrow tube creates a high-aspect-ratio surface. As the tube becomes longer or its internal diameter becomes smaller, precursor molecules must travel farther before reaching the center or closed end.

The challenge becomes greater when a component contains:

• Blind bores
• Curved passages
• Threads
• Internal steps
• Side ports
• Diameter changes
• Welded areas
• Multiple connected channels

Line-of-sight deposition methods often concentrate material near the opening. Meanwhile, deeper or shadowed surfaces may receive less coverage.

Liquid coatings create a different set of challenges. For example, they may pool, drain incompletely, trap fluid, or make very thin film thicknesses difficult to control.

ALD avoids many of these limitations because the process introduces reactants as gases. Therefore, the gases can move through small passages and react with accessible surface sites.

However, gas-phase deposition does not guarantee uniform coverage automatically. The process must provide enough exposure for each reactant to reach and saturate the entire target area.

How ALD Coating Inside Stainless Steel Tubes Works

A thermal ALD process generally alternates between two reactants.

First, the process introduces one reactant into the chamber. It reacts with available sites on the component surface. Next, the system purges excess material and reaction byproducts from the chamber.

Then, the process introduces a second reactant. Another purge follows.

One complete sequence forms a small amount of material. By repeating this cycle, the process gradually builds the coating to the required thickness.

Inside a stainless steel tube, the area near the opening encounters the reactant first. Once available surface sites near the entrance react, additional precursor molecules can travel farther into the passage.

Therefore, the exposure must continue long enough for the reactant to reach the center or closed end of the tube. The same requirement applies during the second half of the ALD cycle.

For this reason, a recipe developed on a flat test coupon may not coat a long internal bore uniformly. Research into ALD conformality in high-aspect-ratio structures shows that geometry, gas transport, pressure, exposure time, and surface reaction kinetics all influence coating penetration.

What Controls Coating Reach and Uniformity?

Several variables determine how successfully an ALD coating reaches the inside of a tube or small bore.

Internal Diameter and Tube Length

The relationship between internal diameter and coated length is one of the first factors to evaluate.

Generally, a short, wide tube is easier to coat than a long capillary. Narrow passages create greater transport resistance. Likewise, longer passages require the reactants to travel farther before reaching the deepest surfaces.

Therefore, a dimensioned drawing should identify:

• Internal diameter
• Total tube length
• Required coated length
• Open and closed ends
• Internal transitions
• Cross holes or branches
• Areas that must remain uncoated

A photograph of the outside of the component rarely provides enough information to evaluate an internal coating project.

Open Tubes Versus Blind Bores

An open tube may allow gases to enter from both ends, depending on the reactor configuration and fixture design. Consequently, reactants may reach the interior more easily.

A blind bore creates a different challenge because it has one opening and a closed end. Reactants must enter and leave through the same path. As a result, reaching and purging the deepest surface becomes more difficult.

In addition, a vent hole or connected channel can change how gases move through the part. The coating provider should identify these features before developing the process.

Precursor Exposure

Deep or narrow features often need longer precursor exposure than flat surfaces.

The correct exposure depends on:

• Precursor chemistry
• Tube dimensions
• Operating pressure
• Gas flow
• Process temperature
• Surface reaction rate
• Total internal surface area
• Reactor configuration

However, simply increasing the number of coating cycles does not solve incomplete saturation.

More cycles may continue adding material near the entrance. Meanwhile, the deepest surfaces can remain thinner if the reactants fail to penetrate fully during each cycle.

Therefore, the process must first establish adequate precursor exposure throughout the bore.

Purge Conditions

Each purge step must remove excess reactant and reaction byproducts from the entire passage before the process introduces the next reactant.

For example, a purge that works for an exposed coupon may be too short for a long tube or interconnected manifold. Residual reactants may cause unwanted gas-phase reactions or affect film quality.

For this reason, exposure and purge timing should be developed together.

Stainless Steel Surface Condition

Stainless steel surfaces may contain machining oil, drawing lubricant, polishing compound, fingerprints, particles, cleaning residue, or oxidation from previous processing.

These contaminants can interfere with film nucleation. As a result, the coating may grow inconsistently across the surface.

Therefore, surface preparation should be discussed before shipping the parts. The appropriate cleaning method depends on the stainless steel grade, surface finish, contamination history, assembly materials, and selected coating chemistry.

For example, a bare stainless steel tube may tolerate preparation steps that would not work for an assembly containing polymers, seals, adhesives, or brazed joints.

Choosing a Coating Material for a Stainless Steel Tube

The correct ALD material depends on what the inner wall needs to withstand.

For example, a coating designed to reduce moisture exposure may differ from one intended for electrical insulation, chemical resistance, diffusion control, or surface passivation.

VaporPulse works with ALD materials and multilayer coating systems that include aluminum oxide, titanium oxide, titanium nitride, hafnium oxide, silicon oxide, nanolaminates, and other application-specific films.

Material selection should consider:

• Chemicals contacting the tube
• Operating temperature
• Moisture exposure
• Plasma exposure
• Electrical requirements
• Required film thickness
• Maximum process temperature
• Substrate and assembly materials
• Dimensional tolerances
• Expected service life

Importantly, a thicker coating is not automatically a better coating.

ALD provides the most value when a thin functional layer needs to follow internal geometry while preserving bore clearance. In addition, greater thickness increases cycle count and processing time.

Excessive thickness may also introduce stress or affect tight dimensional tolerances. Therefore, coating thickness should match the actual performance requirement rather than simply maximizing material buildup.

Fixturing Stainless Steel Tubes for Internal Coating

The fixture needs to hold the part securely while maintaining vapor access to the bore.

Whenever possible, technicians should place contact points away from critical coating surfaces. If the exterior of the tube also requires coating, the fixture should minimize areas where supports shield the part.

Parts with multiple tubes or channels require additional consideration. For example, closely packed components can restrict gas flow and increase the total reactive surface area inside the chamber.

Similarly, a process developed using one loosely loaded tube may behave differently when a full fixture contains several components. Therefore, production loading should become part of the pilot-development process.

Selective coating creates another challenge. Threads, sealing lands, electrical contacts, or mounting surfaces may need to remain uncoated.

Any masking or exclusion method must tolerate the process conditions. At the same time, it must avoid contaminating the chamber or restricting reactant access to the bore.

How to Verify Internal Coating Coverage

Inspecting the inside of a tube is usually more difficult than inspecting an exterior surface. Therefore, the verification method should be selected before processing begins.

Possible approaches include:

• Sectioning a sacrificial tube
• Placing representative witness surfaces at different depths
• Measuring coating thickness near the entrance, center, and end
• Cross-sectional microscopy
• Elemental or compositional analysis
• Electrical testing
• Corrosion or chemical exposure testing
• Application-specific performance testing

An exterior witness coupon can confirm that film growth occurred inside the chamber. However, it does not prove that the center of a long bore received the same coating thickness.

For development projects, a sacrificial component or representative test geometry often provides the clearest way to evaluate internal penetration.

In addition, the acceptance criteria should identify where measurements will occur and how much coating variation the application can tolerate.

Information to Provide Before Requesting a Tube Coating

A coating provider needs more than the outside dimensions of the part.

Therefore, include the following information when requesting an evaluation:

• Stainless steel grade
• Internal diameter
• Tube length
• Required coated length
• Open, blind, curved, or branched geometry
• Surface finish
• Known contamination or prior treatments
• Maximum allowable process temperature
• Preferred coating material, if known
• Functional goal of the coating
• Chemicals or environments the coating will face
• Areas that must remain uncoated
• Required thickness or performance criteria
• Quantity and expected production volume
• Availability of sacrificial parts for testing

VaporPulse supports prototype, pilot, and low-volume ALD projects. Consequently, this approach can work well for internal tube applications that require process development before moving to established production parameters.

When ALD May Not Be the Right Coating Method

ALD offers strong advantages for thin, conformal internal films. However, it does not suit every tube or bore.

For example, extremely long tubes with very small diameters may require impractical exposure and purge times. Likewise, closed cavities without an effective gas path may trap reactants or make complete purging difficult.

Heavy scale, loose contamination, deep surface damage, or porous weld deposits can also interfere with coating performance. In these situations, a thin film may not function as a continuous barrier.

In addition, ALD typically does not rebuild worn dimensions or create a thick structural layer. If the application requires substantial material buildup, another coating or surface-treatment method may provide a better solution.

Therefore, the part geometry, required film function, and verification plan should all be reviewed before selecting a coating method.

Planning an Internal Stainless Steel Tube Coating Project

Coating the inside of a stainless steel tube requires more than selecting a material and specifying a thickness.

The process must deliver both reactants to every required surface. It must also remove them completely between exposures and produce measurable coverage at the deepest point.

Start by providing a dimensioned drawing, stainless steel grade, operating conditions, target surfaces, and a clear description of what the coating needs to protect against.

With that information, the coating provider can evaluate geometry, process requirements, material compatibility, and verification options before developing the process.

VaporPulse Technologies works with internal features, small bores, complex geometries, and non-standard substrates. To discuss a tube, capillary, manifold, or other internally coated component, contact VaporPulse Technologies with the part drawing and application requirements.