The most popular and widely used technique for assessing the adhesion strength of coating–substrate systems , scratch testing consists of using a hard metal or diamond spherical tipped indenter (typically with a radius of 200 μm) to apply an increasing load on the coating surface continuously while the sample is displaced at a constant velocity. Scratching of the surface results in increasing elastic and plastic deformation until extensive spalling of the coating from the substrate occurs at some critical load (Lc). In general, the critical load is determined by acoustic emission, friction force measurements, or optical microscopy.
Applications of Scratch Testing:Commercial polymers;Paint and varnishes for the automotive industry;Multilayered systems;Application related performance;
Scratch testing is invaluable to gain insight into materials such automotive paints, varnishes, as well as coatings. Single asperity scratching allows the mechanisms behind abrasive or ductile damage to be understood without the complexity of multiple asperity contacts.
A range of conical diamond tips (apical angle from 35° to 150°) are drawn across the surface under constant, stepwise or progressively increasing load.The morphology of the scratches is examined with a combination of microscopy and profilometry techniques. Scratch hardness values are calculated to generate a scratch map that depicts the behaviour of the surface under a complete range of working conditions.Scratch testing can be automated to achieve Medium Throughput data generation.