The method and its evaluation
The scratch test is an established method in material testing. Done in a laboratory setting, it simulates the kinds of stresses that a coating will undergo in everyday use. According to ASTM C1624 and ISO 20502, as well as DIN EN 1071-3, the scratch test may be used to check the adhesive and cohesive strengths of coatings.
A diamond indenter with a rounded tip (Rockwell) is pressed onto the surface under test. As the sample is moved underneath it at constant speed, the indenter leaves a scratch mark.
The test force exerted by the indenter on the surface can either remain constant or be increased. A constant test force is suitable for determining the simple scratch hardness of a material. In most measurements, however, the progressive mode is used. Here, the force of the indenter is increased either stepwise or linearly.
At very low loads, the coating can resist the indenter. But as the force increases, so does the material stress – until a specific point when material failure sets in: the critical load LC. Damage patterns (cracks) from that are characteristic for the specific coating; finally, the coating splinters off.
Initial evaluations are always performed visually. Under a high-resolution microscope, the examiner determines at which force the cracks first formed, or when the coating started to chip. Damage patterns that are frequently seen include fishbone-like cracks, moon-shaped damage and large-area chipping.
Because the scratch test is a comparative method that relies on reference measurements, it is the user who sets the criteria for what’s considered a critical load.
Besides the optical analysis, the ST 30 and ST 200 can also evaluate other parameters such as the tangential force and the acoustic emissions.
The tangential force FT is the resistance that the material exerts against the indenter. This value can be used together with the exerted force FN to calculate the coefficient of friction.
The acoustic emissions, i.e. the scratching noise, often increases as the coating fails. Fischer scratch testers can evaluate this in conjunction with the force curve.
Factors you need to pay attention to: Wear and tear on the indenter
In scratch testing the indenter is considered a consumable part. Since it wears out quickly, especially with hard coatings such as DLC, the indenter should be checked regularly under a microscope. Rule of thumb: A Rockwell diamond indenter lasts for about 100–200 measurements when used on hard coatings.