Scratch tester - An easy method of testing coating adhesion

> The scratch tester - A simple coating adhesion test One of the primary
> requirements of a PVD coating is excellent adhesion of the coating to
> the substrate, a property that can be defined as the interfacial
> forces between two surfaces. The most common method of assessing PVD
> coating adhesion is the scratch tester.
>
> scratch tester
> *Scratch tester equipment*
>
>
> *Scratch test - basics
> *The scratch tester moves a Rockwell diamond tip with a radius of
> 200µm across the coated surface of a substrate at a constant velocity
> while an increasing normal force is applied with a constant loading
> rate. The scratch test introduces stresses to the interface between
> coating and substrate causing delamination or chipping of the coating.
> The critical normal force at which the first failure of the coating is
> detected is termed the critical load Lc.
>
>
>
> *Scratch tester - Failure detection *
> The typical scratch tester has three methods of detecting coating
> failure; a load cell to measure the change in friction, acoustic
> emission or observation of the scratch channel using an attached
> optical microscope. The best scratch adhesion testers use all three
> methods of coating failure detection. The intensity of the acoustic
> emission is dependent on the type of coating failure during the
> adhesion test e.g. cracking, chipping (cohesive failure) and
> delamination (adhesive failure). It is therefore important to observe
> the coating failure after the adhesion test using an optical
> microscope to confirm the critical load.
>
> *Scratch test - Possible substrates*
> The scratch tester can be used to assess the coating adhesion on many
> different types of substrates such as metal alloys, semiconductors,
> glass, and refractive materials but is the most accurate when
> assessing the adhesion of hard coatings onto relatively hard
> substrates >54 HRC.
>
> scratch channel on a TiN coating
> *Example of the end of a scratch channel after a scratch test *
> From http://www.pvd-coatings.co.uk/scratch-adhesion-tester.htm
>