Microscratch Testing

(Scratch Testing and Coating Interfacial Adhesion Evaluation)

Scratch resistance, mar resistance, interfacial adhesion measurements through microscratch tests.

Typical Experimental Results

Optical micrograph of a scratch made on paint coating of a Pepsi can

Test data for the scratch made on paint coating of a Pepsi can

Applications

For more information please read our application notes:
Interfacial Adhesion Evaluation of Paint Coatings on a Pepsi Can through Scratch Testing, PDF
Scratch Failure Characteristics of DLC Coating on M2 Steel, PDF

Instrument: The Micro-Combi Indentation and Scratch Tester

Key Specifications

Interfacial Adhesion Evaluation of Paint Coatings on a Pepsi Can through Scratch Testing

Surface coatings are frequently used to improve the surface properties of the material that they are applied to, also known as the substrate. Such surface property improvements include corrosion resistance, oxidation resistance, thermal barrier, wettability, aesthetics, wear resistance, and scratch resistance. Performing a scratch test on a surface coating can help characterize the mar and scratch resistance, interfacial adhesion strength, and coefficient of friction of such coatings.

One of the most commonly used surface coatings is paint, a two-phase composite material consisting of pigment particles and other additives dispersed in a continuous polymer matrix that adheres to the substrate, protecting it from the surrounding environment. Paint is used in the soft drink industry to increase the aesthetic appeal of the aluminum cans used to store the beverages. It needs to have good cohesive and adhesive strength, as excess scratching or wear during transportation will diminish the aesthetic appeal provided by the paint coating before it reaches its consumer.

Scratch testing has been widely accepted and used as a way of evaluating interfacial adhesion of coatings/substrate systems. Failure events may be found where the scratch probe produces delamination, debonding, crack, fracture, or breakthrough at the coating/substrate interface. The failure events of the coatings are normally symbolized by a combination of sudden changes in the lateral force, normal displacement, and/or normal force data. The critical load of coating interfacial failure is defined as the normal force applied to the scratch probe at the time when interfacial failure is detected. The critical load of coating adhesion failure is a good indication of interfacial adhesion strength. Normally, a higher critical load represents a higher interfacial adhesion. There are a few other ways to help identify critical loads when performing scratch tests. The most common technique involves monitoring acoustic emission from the sample during scratch, which works very well for hard and brittle coatings. Other techniques involve recording and monitoring coefficient of friction, or examining the scratch path with optical microscopy.

Scratch test data for the paint coatings on an aluminum Pepsi can is shown in Figure 1. This data was obtained on Ebatco NAT Lab’s Micro Indentation and Scratch Combi Tester (CSM Instruments, Switzerland).

From Figure 1, it can be seen that there is a large change in frictional force around the 1 mm displacement mark. Figure 2 shows the corresponding location of the scratch as viewed from an optical microscope. The failure location corresponds to a critical load of coating interfacial adhesion failure at 118.88 mN.

Scratch Failure Characteristics of DLC Coating on M2 Steel

Diamond-Like Carbon (DLC) coatings are frequently used on parts in high performance applications to prevent surface damage or reduce friction of moving parts. DLC coatings can be applied to almost any material that is compatible with vacuum environments and have many applications including electronics, automobiles, tools, shaving razors, and biomedical implants. The hardness and strength of the coating can vary greatly depending on how it is deposited and its intended applications. Due to this high variability, characterizing the properties of such coatings can provide very useful information for the preparation process optimization as well as performance evaluation.

One method for characterizing the properties of DLC coatings is scratch testing. In this method, a diamond stylus that is subjected to an increasing normal force is drawn across the coating surface. At some point, the coating will fail due to increased normal and tangential stresses applied by the moving diamond stylus. The normal forces applied to the scratching stylus at the points that a coating fails are called the critical loads of failure.

Figure 1. Scratch test data for a DLC coating on M2 steel; the vertical grey lines mark the critica failure points and the corresponding normal forces are critical loads: L_{c 1}, L_{c 2} and L_{c 3}.

A DLC-coated bar of M2 steel was tested in Ebatco’s Nano Analytical Test Lab using a MicroScratch Tester made by CSM Instruments (Switzerland). The scratch data is presented in Figure 1. There are several ways to identify critical loads when performing scratch tests. The most common techniques involve monitoring acoustic emission from the sample, looking for changes in coefficient of friction and scratch normal displacement, or post facto observation via reflected light microscopy on the scratch track.

As can be seen in Figures 1 and 2, the DLC coating exhibited distinctive scratch failure characteristics along the scratch. Three critical loads at 0.59N, 2.33N and 2.83 N corresponding to onsets of adhesive cracking, adhesive spallation and gross adhesive spallation were determined from the scratch data and optical microscopic images. The changes in coefficient of friction and normal displacement in the test data correlate well with the optical observations at the three critical points of coating failure.

Figure 2. Optical micrographs of the scratch track showing the failure characteristics of the DLC coating on M2 steel; the centers of the scratch track images corresponding to L_{c 1} (left image, cohesive cracking), L_{c 2} (center image, adhesive spallation), L_{c 3} (right image, gross adhesive spallation).