Adhesion is an important quality indicator in coating business. Among all the accepted testing methods, Rockwell C indentation test is the one most commonly used (according to VDI RL 3198, 5.4 Pic. 11). The advantage of this test is its simplicity. As shown in figure 1, this test involves a Rockwell hardness tester that has a conical indenter with a diamond tip, by applying a set load (150N for HSS; 60N for WC) to press through the coating and causes plastic deformation to the substrate. The indentation mark is then studied under a microscope to classify the adhesion of a coating into 6 levels, 4 acceptable (i.e. HF1 – HF4; only small amount of coating delamination is allowed) and 2 unacceptable grades (i.e. HF5 & HF6; with large area coating delamination occurs). The shortcomings of this test method are that it does not provide quantitative result, and is based on the tester’s subjective judgment.

On the other hand, using scratch testing method to determine coating adhesion can obtain quantitative results. Also by studying the failure modes of the scratch mark to determine how the coating behaves under tensile and compressive forces. The results can also be analyzed by advanced software to calculate stress responses at the loading point, which gives invaluable direction to the development work of new coatings.

The working principle of a scratch tester is shown in fig. 2. It also uses a conical head stylus with a spherical diamond tip, which is pressed against the coating with an applied load that is programmed according to the user’s setting (e.g. constant mode or linear progressive mode etc.), while the sample stage moves in a chosen speed underneath. A very precise and controlled destructive scratch mark is then created on the sample surface by the diamond stylus.

For coating quality assurance tests, linear progressive loading mode is mostly used. With this mode, there exists a critical load, Lc, at which the coating starts to fail. This failing point can be precisely detected by sensor, and it can also be observed at the scratch mark (through imaging device or microscope). The ‘critical load’ is defined as the smallest load at which a ‘recognizable failure’ occurs to the coating (Fig. 3). Thus, the adhesion of a coating can be quantified by this critical load.

A good quality and high precision scratch tester has the following features:

 A platform that allows different combinations of modules; for example optical or mechanical (stylus) type profilometer, or optical microscope with video capture function.

 A good load force control system, with electronic close loop control that has active force feedback signal (correction not through software), which can detect sudden force change and make adjustment within 5 ms. This enables accurate detection of early fracture events. With less advanced equipment, the load on the stylus might experience a sudden force decrease due to the drop of the stylus position when fracture starts to appear, and not able to maintain correct load. This might stop the fracture propagation in the coating because the sudden decrease of stress. Hence Lc can not be detected accurately.

 A complete software package with convenient user interface;

 Real time display of normal force, friction force, friction coefficient, penetration depth and acoustic emission data.

 Different modes of normal load, e.g. linear, step and constant load etc.

 Easy video capture and measurements.

 Scratch mark image and data are displayed on the same screen, and cursor position allows one to one correlation between data and location at the scratch mark for convenient analysis of result.

 Convenient measurement report generator allows efficient work.

CSM is a world famous coating testing equipment manufacturing company in Switzerland, who has technology in precision engineering. The scratch testers that she developed are robust and reliable with very high reproducibility. Because of our reputation in the coating industry, Techmart is now the sole agent for CSM instruments in China. Please don’t hesitate to contact us for further information.