industry news, news 17/08/2026 1
Evaluating the adhesion of a surface coating on zig zag wire is critical for ensuring its long-term performance in applications subject to mechanical stress and environmental exposure. A scratch test provides a semi-quantitative measure of this adhesion by applying a controlled, increasing load to a moving stylus until coating failure occurs. This method assesses the bond strength at the coating-substrate interface, which is essential for predicting resistance to delamination during repeated bending, handling, or service.
Specimen Preparation and Environmental Conditioning
The test’s validity begins with proper sample preparation. A representative section of the coated zig zag wire must be securely mounted on a rigid, flat backing using a suitable adhesive or clamping fixture to prevent movement during testing. The test area, typically an apex or straight section, should be thoroughly cleaned with a solvent like isopropyl alcohol to remove any oils or contaminants that could affect results. Prior to testing, condition the specimens in a controlled environment (e.g., 23±2°C and 50±5% relative humidity) for at least 24 hours, unless testing under specific environmental conditions (elevated temperature, humidity) is required to simulate service conditions. Documenting this conditioning is part of a standardized protocol.
Selecting the Scratch Tool and Defining Test Parameters
The choice of scratch tool (stylus) is fundamental. A Rockwell C diamond indenter with a sphero-conical tip (typically 200 μm radius) is commonly used for hard coatings, while a sharper tip may be selected for thinner or softer layers. The critical parameters to define before the test are the scratch length, the loading rate, and the load range. The test is performed by drawing the stylus across the coated surface at a constant speed (e.g., 10 mm/min) while the normal force applied to the stylus is progressively increased from a pre-set minimum to a maximum value (e.g., 1 N to 30 N) over the chosen scratch length. This creates a scratch with a linearly increasing load.
Executing the Test and Identifying Critical Failure Points
During the scratch, multiple data channels are monitored: the applied normal force (Fn), the tangential (frictional) force (Ft), the acoustic emission (AE) signal, and the depth of penetration. The point of coating failure is identified by a distinct change in one or more of these signals. The first critical failure is often cohesive (within the coating), observed as fine conformal cracking. The more significant adhesive failure, where the coating is completely removed from the substrate, is marked by a sharp increase in acoustic emission, a change in the friction coefficient, and often visible spallation or chipping at the edges of the scratch under optical examination. The load at which this adhesive failure occurs is recorded as the critical load (Lc).
Post-Test Analysis and Interpreting Results for Quality Control
After the scratch is completed, the track must be examined using optical microscopy or scanning electron microscopy (SEM). This visual inspection confirms the failure modes identified by the instrumental data and measures the exact scratch width and morphology at the critical load point. The critical load (Lc) is the primary quantitative result. However, for comprehensive quality assessment, the failure mode is equally important. Brittle chipping, ductile ploughing, or interfacial delamination each indicate different coating-substrate interface properties. These results are compared against internal acceptance criteria or industry standards (often referenced from methods like ASTM C1624 or ISO 20502) to determine if the coating adhesion is sufficient for the intended application, such as the repeated bending fatigue environments previously discussed for zig zag wire. Consistent application of this test protocol provides empirical evidence of manufacturing quality and supports material specification.