Adhesion Measurement
A mechanical evaluation procedure used to quantify the adhesion and fracture resistance of thin film coatings on a microstructural scale provides a direct measure of interfacial strength. Utilizing a micro scratch test, a spherical or conical stylus is drawn across the film under a progressive normal force. This test reveals the critical load at which the film undergoes cohesive or adhesive failure.
By systematically increasing the force, the stylus drives a wedge into the layer, separating the film from the substrate and allowing for the calculation of the shear stress at the boundary.
Stylus Operation
Linear movement of the diamond stylus is coupled with a controlled loading rate to generate a continuous path of increasing strain on the film. As the stylus penetrates the coating, the normal load and lateral friction force are recorded simultaneously. This relationship allows researchers to calculate the coefficient of friction and associate specific force values with physical failures.
Failure Identification
Microscopic observation of the resulting scratch track reveals the specific modes of coating failure, such as cracking, chipping or complete delamination. Acoustic emission sensors are used to detect high frequency stress waves generated by these failure events. These events are matched with the load data to pinpoint the exact critical force that causes debonding.
Metrological Calibration
Reference standards and calibration routines are utilized to maintain test reproducibility across different instruments. Standard test methods specify the stylus radius and the loading rate to minimize variation in the critical load results. This procedure is verified regularly to ensure that scratch test measurements remain comparable between production batches.
