Adhesion testing by the circular scoring method, also known as the circle-cut adhesion test, is a classical bench procedure for evaluating how firmly a dry paint film bonds to a metal or other rigid substrate. Standardized in GB/T 1720, the test drives a steel needle through the coating in a pattern of overlapping circular loops and rates the resulting film detachment on a seven-grade scale. This article explains the principle, apparatus, sample requirements, test procedure, grading criteria, and typical industrial applications of the method.
Technical Overview
The circular scoring method is one of the most widely used and longest-established techniques for assessing the adhesion of organic coatings, particularly paint films on flat metal substrates. In this method, a motorized adhesion tester guides a hardened steel needle along a continuous path of overlapping circular loops, cutting completely through the coating down to the substrate. The scribing action imposes a combination of cutting, shearing, and lifting stresses on the film, especially at the points where consecutive loops intersect. These intersections act as stress concentrators that efficiently expose weak bonding at the film-substrate interface or between coating layers.
After scribing, the scored pattern is examined under magnification, and the degree of coating detachment, flaking, or lifting is compared against standardized grading descriptions and reference photographs. The result is expressed as an adhesion grade from 1 (best, film completely intact) to 7 (worst, severe detachment). In China the method is standardized as GB/T 1720-79 (reconfirmed 1989), and it remains a routine quality-control test for paints, varnishes, and related coatings. Accredited third-party laboratories and in-house coating laboratories commonly perform this test as part of coating qualification and batch release programs.
- Delivers a semi-quantitative adhesion rating (Grade 1 to Grade 7) in a short test time.
- Best suited to thin and medium-thickness paint films on hard, flat, rigid substrates.
- Requires a dedicated motorized circle-cut adhesion tester with interchangeable needle loads.
- Complements, but is not numerically interchangeable with, the cross-cut test and the pull-off test.
Relationship to Other Adhesion Test Methods
The circle-cut method belongs to the same family of cutting-based adhesion tests as the cross-cut test (ISO 2409 and ASTM D3359 Method B) and the X-cut tape test (ASTM D3359 Method A), while the pull-off test (ISO 4624 and ASTM D4541) measures adhesion as a tensile stress value in megapascals. The circle-cut method differs in that the cutting path is a continuous curved line with repeated self-crossings rather than a lattice of straight cuts, and no pressure-sensitive tape is applied to lift the detached film. Because the stress state and evaluation scales differ, grades obtained by the circle-cut method cannot be directly converted to ISO 2409 or ASTM D3359 classifications.
Testing Samples
Laboratory-Prepared Coated Panels
The most common specimens are flat panels coated under controlled conditions, such as tinplate sheets, cold-rolled steel panels, galvanized steel panels, or aluminum alloy sheets. The coating is applied by drawdown, spraying, or dip coating at the film thickness specified by the product standard, and the panels are cured and dried according to the coating system's technical data sheet before testing.
Finished Coated Components
Production parts such as appliance panels, automotive body or refinish panels, metal furniture, hardware, and coil-coated sheet can also be tested, provided the part presents a sufficiently flat and rigid area that accommodates the full scribing pattern and can be clamped securely on the instrument stage.
Specimen Conditioning and Preparation
Specimens must be fully cured, clean, and free of visible surface defects in the test area. Typical requirements include the following:
- Conditioning at 23 ± 2 °C and 50 ± 5 % relative humidity for a defined period (commonly at least 16 hours) before testing.
- Measurement and recording of dry film thickness in the test area, since needle load selection depends on thickness and hardness.
- Selection of a test area free from edges, welds, scribe marks, runs, or other pre-existing defects.
- Provision of at least two separate test positions per sample to allow duplicate determinations.
Testing Items
The primary output of the test is the adhesion grade of the coating, supported by observations of how the film fails. Typical reporting items include:
- Adhesion grade per GB/T 1720 on the 1-to-7 scale, including the mean of replicate determinations.
- Description of the failure mode, such as flaking, lifting, cracking, or chipping at loop intersections, and whether failure occurs at the substrate interface (adhesive failure) or between coats (cohesive or intercoat failure).
- Scribing needle load used, dry film thickness, substrate type, and surface pretreatment.
- Coating identification, curing schedule, and conditioning conditions.
Testing Methods
Apparatus
The test is performed with a motorized circle-cut adhesion tester of the type commonly designated QFZ in laboratory practice. The instrument consists of a synchronous motor drive, an eccentric needle-carrying arm, a height-adjustable specimen stage, and a lever system onto which calibrated weights are placed to control the force pressing the needle onto the film. The scribing element is a hardened steel needle that must be straight and sharp; a bent or worn needle produces false results and must be replaced. Auxiliary equipment includes a magnifier (typically around 4x to 10x), a soft brush for removing debris, and film-thickness measurement instruments.
Test Procedure
The general sequence of a determination is as follows:
- Condition the specimen and measure the dry film thickness at the intended test position.
- Select the needle load, starting with a light weight and increasing stepwise until the needle just penetrates through the film to the substrate without excessively gouging the metal; record the load used.
- Clamp the specimen securely on the stage and lower the needle gently onto the coating surface.
- Start the motor so that the needle traces a continuous row of overlapping circular loops, each loop approximately 10 mm in diameter, with adjacent loops intersecting one another along a scribed length of roughly 70 to 80 mm.
- Stop the instrument, lift the needle, remove the specimen, and carefully brush loose debris from the pattern without disturbing adherent film.
- Inspect the pattern immediately under a magnifier in good, oblique lighting, and grade the detachment against the standard's criteria.
Grading Criteria
The adhesion grade is judged primarily on the extent of film detachment within the scored pattern, with the grading photographs of the standard serving as the reference for borderline cases. The grades are characteristically described as follows:
- Grade 1: the scribe line is smooth and continuous and the film is completely intact, with no detachment anywhere in the pattern.
- Grade 2: the film is essentially intact, showing only minute lifting or chipping at isolated loop intersections.
- Grade 3: small flakes detach at the intersections, with the detached area typically below about 5 percent of the scored region.
- Grade 4: detachment is clearly visible along the intersections and parts of the loops, on the order of roughly 5 to 10 percent of the region.
- Grade 5: substantial detachment affecting roughly 10 to 25 percent of the scored area.
- Grade 6: heavy detachment affecting roughly 25 to 50 percent of the scored area.
- Grade 7: more than half of the film within the scored pattern detaches or peels away.
The percentage figures above are indicative descriptions of typical practice; the official grading illustrations in the standard are decisive for classification. Most product specifications call for Grade 1 or Grade 2 as the acceptance criterion.
Result Validation and Reporting
At least two determinations should be performed on each coating. If the two results differ by more than one grade, the test should be repeated on fresh areas of the specimen. The final report states the individual and mean grades, the needle load, the film thickness, the substrate and surface preparation, the curing and conditioning history, and any observed anomalies such as intercoat delamination or cohesive failure within a layer.
Applications
Because of its speed, low specimen cost, and sensitivity to interfacial weakness, the circular scoring method is used across the coatings value chain, from raw-material screening to finished-goods inspection.
- Formulation research and development, evaluating the influence of binder type, pigment volume concentration, crosslinker level, and additives on film adhesion.
- Assessment of surface pretreatments such as degreasing, phosphating, anodizing, and abrasive blasting, and of primer compatibility with topcoats.
- Batch quality control in paint manufacturing and application lines, verifying that production coatings meet specified adhesion grades.
- Qualification of coating systems for appliances, automotive components, metal furniture, architectural panels, and coil-coated products.
- Failure analysis of flaking, peeling, or delamination complaints, distinguishing poor adhesion from cohesive film defects.
- Routine testing services offered by accredited third-party laboratories for supplier verification and regulatory compliance submissions.
FAQ
What is the difference between the circle-cut method and the cross-cut test?
The circle-cut method scribes a continuous row of overlapping circular loops with a motorized needle and grades detachment on a 1-to-7 scale without using tape, whereas the cross-cut test cuts a lattice of straight lines, often applies a pressure-sensitive tape, and rates the result from 0 to 5 in ISO 2409 or ASTM D3359. The two methods stress the film differently, and their grades are not interchangeable.
Which grade is considered acceptable?
Most specifications require Grade 1 or Grade 2. Grade 1 indicates an intact film with a smooth scribe line, and Grade 2 allows only negligible lifting at intersections. Grades of 3 or worse generally indicate inadequate adhesion or a coating, surface preparation, or curing problem that requires investigation.
What film thickness is suitable for this method?
The method is best suited to thin and medium paint films in which the needle can reach the substrate under a reasonable load without tearing or plowing the coating. For very thick or highly elastic coatings, the cross-cut test or pull-off adhesion test is usually more appropriate, and the choice of method should follow the relevant product standard.
Why is needle condition and load selection important?
A dull, bent, or corroded needle cuts unevenly and can artificially lower the grade, while an excessively heavy load gouges the substrate and distorts the failure mode. The load must be just sufficient for the needle to reach the substrate, and the needle should be inspected and replaced regularly.
Can the test be performed on curved or assembled parts?
The specimen must present a flat, rigid area large enough for the complete scribing pattern and must clamp firmly onto the instrument stage, so the test is generally limited to flat panels and flat regions of parts. Curved surfaces, tubular parts, and very thin or flexible sheets are poor candidates for this method.
How many measurements should be taken?
A minimum of two determinations per coating is standard practice. Results that agree within one grade can be averaged for reporting; a larger disagreement calls for repeat testing, since it usually indicates non-uniform film thickness, incomplete curing, or inconsistent surface preparation.