Galling is adhesive wear that happens when two stainless steel surfaces slide against each other under load, cold-weld at the contact points, then tear material away as motion continues. In medical device manufacturing, where 304, 316L, and 17-4 PH dominate the bill of materials, a single galled thread can scrap an instrument that already carries hours of machining value. Anti-galling coatings applied by Sun Coating Company separate those mating surfaces with a low-friction film, which is why so many device builders partner with Sun Coating Company early in the process to treat coating as a core design input rather than a late-stage fix.
Why Stainless Steel Seizes While Other Alloys Slide
Stainless owes its corrosion resistance to a thin chromium oxide passive layer, typically 1 to 3 nanometers thick. Under sliding contact and moderate pressure, that film breaks down, exposing clean, chemically active metal on both faces. Austenitic grades compound the problem because they work harden aggressively and conduct heat poorly, so frictional energy stays concentrated at the asperities instead of dissipating. Applying anti-galling coatings provides a physical barrier between contacting surfaces to prevent localized micro-welding and thread seizure.
The result is localized cold welding. Measured by ASTM G98, the galling threshold stress for unlubricated 304 against 304 often falls between 2 and 8 ksi, while a hardened, galling-resistant alloy such as Nitronic 60 running against itself can exceed 50 ksi. That spread explains why identical-alloy pairs are the worst possible combination in a threaded or sliding assembly.
Where Galling Shows Up on the Medical Shop Floor
Device manufacturers rarely see galling during design review. It appears in production, assembly, and sometimes in the operating room:
- Bone screws and locking plates, where surgeons apply high torque into a stainless or titanium thread with no lubricant permitted at the interface.
- Reusable surgical instruments with box joints, ratchets, scissor pivots and threaded handles that seize after repeated autoclave cycles.
- Luer fittings, valve stems and threaded couplings in fluid-handling assemblies, where a galled thread leaks or cannot be disassembled for cleaning.
- Deep drawing and forming tooling used to produce stainless cannulae, trays and housings, where transferred metal on the punch ruins surface finish within a few hundred strokes.
- Adjustment screws and set screws in capital equipment, which gall during service calls and force a full subassembly replacement.
Design Fixes That Reduce Galling Before Any Coating Is Applied
Coating performs best on a part that was not designed to fail. Several changes cost nothing at the print stage:
- Pair dissimilar alloys or dissimilar hardness, with at least 50 HV separating the two mating surfaces.
- Use coarse threads instead of fine threads, since coarse profiles tolerate more surface damage before binding.
- Increase thread flank clearance to a 2A/2B class fit rather than a 3A/3B fit where tolerance stack allows.
- Reduce assembly speed, because rapid run-down generates the frictional heat that triggers adhesion.
- Specify a surface finish near 16 to 32 microinches Ra, since both mirror-smooth and rough surfaces behave poorly under sliding contact.
Those measures lower risk but rarely eliminate it on high-torque or repeatedly cycled hardware, and medical assemblies usually prohibit the oils and anti-seize pastes that solve the problem in other industries.
Coating Types Used Against Galling on Medical Components
Selection depends on contact pressure, sliding distance, patient contact, and the sterilization method. The common families break down as follows:
- PTFE and PFA fluoropolymers: dynamic coefficient of friction around 0.05 to 0.12 compared with 0.4 to 0.8 for bare stainless, applied at roughly 0.0005 to 0.0015 inches. Excellent release and chemical resistance, best suited to moderate loads.
- Fluoropolymer-reinforced composites: PTFE blended with binder resins or ceramic fillers, which hold up better than pure PTFE under repeated high-torque cycling.
- Electroless nickel with co-deposited PTFE: a 10 to 25 micron layer combining hardness with embedded lubricity, useful on forming tooling and valve components.
- Parylene C and parylene HT: vapor-deposited at 0.5 to 25 microns with uniform coverage inside blind holes and threads, widely used on implantable and patient-contact hardware.
- Diamond-like carbon and PVD nitrides (TiN, CrN, TiCN): 1 to 4 microns of very hard, low-friction ceramic for cutting instruments and high-contact-stress sliding surfaces.
- Dry film lubricants with MoS2 or graphite: inexpensive, effective under high load, though generally restricted to non-patient-contact and non-sterile-field hardware.
No single film wins every application. A surgical ratchet that cycles 500 times between sterilizations needs a different answer than a one-time-use bone screw.
Biocompatibility, Cleaning and Sterilization Constraints
Any coating that touches tissue, blood, or a sterile field falls under biological evaluation per ISO 10993-1, which the FDA recognizes through its biological evaluation guidance. Cytotoxicity, sensitization, and irritation testing apply to the finished device, not just the raw coating material, so documentation of the applied system matters during submission.
Sterilization adds a second filter. Steam autoclaving at 121 to 134 degrees Celsius, gamma irradiation at 25 to 40 kGy, and ethylene oxide exposure each affect polymers differently, and PTFE in particular degrades under gamma. Cleaning validation is the third constraint: a coating with pinholes or edge lift creates crevices that resist detergent flushing and complicate reprocessing claims.
Thickness Control on Threads and Close-Tolerance Fits
Coating a thread changes its pitch diameter by roughly four times the film thickness, measured across both flanks of both mating parts. A 0.0010 inch fluoropolymer layer on a male thread can consume 0.0040 inches of pitch diameter clearance, which is enough to turn a functional fit into an interference fit on fine-pitch hardware.
Three practices keep that in check: specify coated-condition dimensions on the print, machine the substrate to accommodate the film, and mask bearing surfaces or datum features that must stay bare. Cure temperature deserves equal attention, since fluoropolymer bake cycles of 370 to 400 degrees Celsius will over-temper 17-4 PH in the H900 condition and relax hardened springs.
Qualifying a Coating Before Production Release
Laboratory screening shortens the path to a reliable finish. ASTM G98 button testing establishes galling threshold stress for the coated pair, while ASTM G99 pin-on-disk testing produces a friction coefficient and wear rate under controlled load. For threaded hardware, torque-tension testing across 10 to 25 assembly cycles reveals whether the film survives real service or wears through after the third installation.
Adhesion verification by ASTM D3359 tape test or cross-hatch inspection, followed by the actual sterilization cycle the device will see, closes the loop. Parts that pass bench testing but were never autoclaved have a habit of blistering in the field.
Frequently Asked Questions
What Is the Galling Threshold Stress of 316 Stainless Steel?
Unlubricated 316 sliding against itself typically galls between 2 and 10 ksi under ASTM G98 testing, which is among the lowest of common engineering alloys. Pairing 316 with a dissimilar hardened alloy or applying a low-friction film raises that threshold substantially, often by an order of magnitude.
Can Anti-Galling Coatings Be Used on Implantable Devices?
Yes, provided the coated device passes biological evaluation under ISO 10993 for its contact category and duration. Parylene C and certain medical-grade fluoropolymers have long histories in implantable and long-term contact applications, but qualification always applies to the finished device rather than the coating alone.
How Thick Should a Coating Be on a Threaded Stainless Part?
Most threaded medical hardware runs between 0.0003 and 0.0010 inches of film, because thicker layers interfere with thread fit. Parylene offers the most control, with usable coverage down to half a micron on fine-pitch and small-diameter features.
Does Passivation Prevent Galling?
Passivation per ASTM A967 restores corrosion resistance but does almost nothing for galling, since the oxide film it builds is only a few nanometers thick and shears away immediately under load. Treat passivation and galling control as two separate engineering problems.
How Long Do Anti-Galling Coatings Last on Reusable Instruments?
Service life ranges from roughly 50 to several thousand cycles depending on contact stress, sliding distance, and sterilization method. Hard PVD films generally outlast polymer films on high-contact-stress surfaces, while polymers perform better where release and chemical resistance dominate.
Send Us Your Galling-Prone Components for Review
Sun Coating Company has spent decades applying fluoropolymer, parylene, and specialty release finishes to medical components, forming tooling, and precision hardware. Contact Sun Coating Company with your part drawings, contact stresses, and sterilization requirements, and our team will recommend coating services matched to how the assembly actually gets used.