Measuring coating performance
Common test methods for adhesion, hardness and corrosion resistance evaluation.
4 min
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Technical articles
Claims about a coating only mean something when they are tied to a named test method, a stated specimen preparation and a stated cure state. For thin inorganic and hybrid films the same nominal property can differ by a wide margin depending on substrate, film thickness and how long the panel was allowed to cure. This overview covers the methods most often requested in specifications for silazane-based coatings and what each one actually tells you.
Adhesion and hardness
Cross-cut testing to ISO 2409 or ASTM D3359 is the fastest qualitative screen. A lattice is cut through the coating to the substrate, tape is applied and removed, and the amount of detached coating is rated on a classification scale. It is sensitive to blade condition and to cut spacing, which is tied to film thickness in the standard, so the spacing used should always be reported alongside the rating. On very hard, thin films the cut itself can cause chipping unrelated to true adhesion, which is why the method is a screen rather than a design value.
Pull-off testing to ASTM D4541 or ISO 4624 gives a quantitative bond strength. A dolly is bonded to the coating, cut around if the standard requires it, and pulled normal to the surface until failure. The number is only interpretable together with the failure mode: adhesive failure at the interface, cohesive failure within the coating, or failure of the adhesive itself. A high value that turns out to be glue failure tells you only that the coating exceeded the adhesive.
Pencil hardness to ASTM D3363 ranks scratch and gouge resistance against graded pencil leads. It is coarse and operator-sensitive, but it is widely quoted and useful for comparing cure states of the same product. Thin films report the hardness of the coating and substrate together, so a hard film on soft aluminium will under-read.
Wear, corrosion and surface energy
Taber abrasion to ASTM D4060 measures mass loss, or in some protocols haze change on transparent substrates, after a defined number of cycles with specified wheels and load. Results are comparable only when wheel type, load and cycle count match, and for thin coatings the test can wear through to the substrate, at which point the numbers stop describing the coating at all.
Neutral salt spray to ISO 9227 or ASTM B117 remains the most requested corrosion screen. It is a comparative and reproducible exposure, not a predictor of service life, and scribed panels are usual so that creep from the scribe can be measured. Electrochemical impedance spectroscopy is a useful complement where barrier quality needs to be tracked without waiting hundreds of hours.
Accelerated weathering to ISO 16474 or ASTM G154 with fluorescent UV lamps addresses gloss retention, colour change and chalking under cycled UV and condensation. For hydrophobic and easy-clean claims, static water contact angle by sessile drop, with contact angle hysteresis or roll-off angle where relevant, is the standard measure; report drop volume and how long after cure the measurement was taken.
Reading and writing a test report
A result that cannot be reproduced is not a specification. For every value, the report should state the method and its revision, the substrate and its preparation, the dry film thickness and how it was measured, the cure schedule and the conditioning time before testing, and the laboratory conditions during the test.
Comparisons between products are only valid when those variables match. A salt-spray figure obtained on a blasted steel panel at one film thickness cannot be compared with one obtained on a pretreated panel at another thickness, and neither is a statement about service life.
When a specification is being agreed, fix the test matrix before samples are sent: which methods, which substrates, which acceptance criteria, and who performs the test. For grade-specific values, work from the technical data sheet for that grade, and request the test conditions behind any figure that will be written into a specification.
Practical application note
Prepare a full set of identical panels, typically cold-rolled steel, aluminium or glass, degrease and abrade or pretreat them identically, and apply the coating in one session with film thickness measured and recorded. Cure all panels together under the schedule specified for the grade and condition them for a stated period before testing, since adhesion and hardness on silazane films continue to develop after tack-free. Run destructive tests such as cross-cut and pull-off on separate panels from those used for salt spray or weathering. The recurring pitfalls are testing under-cured panels, comparing results across different film thicknesses or substrates, and quoting a salt spray hour count without the scribe condition and rating criteria.
In practice
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