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Adhesion strength is the resistance of an interface between a coating and its substrate to separation, and it is reported either as a stress at failure or, more often in practice, as the load at which damage appears in a standardised test. No single test measures an intrinsic interfacial property, because the quantity obtained depends on the stress state the test imposes and on the deformation of both layers. The scratch test is the most widely adopted: a loaded tip is drawn across the surface and the critical load at which the coating detaches is read from the acoustic or the frictional signal, and its interpretation has been examined in detail for a range of coating-substrate pairs ^[1]. Compressive stress in the coating contributes substantially to that critical load, so a mechanical model is needed to separate the residual stress from the true interfacial toughness ^[2]. Impact and pendulum variants load the interface in a different way and come into their own where the coating is too hard or too thin for a progressive scratch ^[3]. Combining scratching with acoustic microscopy lets the detached area be imaged rather than inferred ^[4], and in multilayer systems the same approach ranks the individual interfaces ^[5]. Values obtained with different methods are not directly comparable.