Sample Preparation for Nanoindentation and Scratch testing

Results from mechanical testing can only be as good as the sample used for testing. Therefore, it is a good idea to consider sample preparation prior to sending samples. Four things to consider for sample preparation are surface roughness, film thickness, sample tilt, and sample size.  Some comments are listed below for each of these topics.

Surface Roughness

A good rule-of-thumb to keep in mind is that the surface roughness of a sample should not be more than 5% of the indentation depth. This means that if you want mechanical properties at a depth of 100 nm into the surface, then the surface roughness should be less than 5 nm; measured properties at 40 nm of penetration into a surface should have a sample with a surface roughness less than 2nm. This is just a guideline; if your surface is rougher you will just have more scatter in your results – it does not mean that no results will be measured.

Film Thickness

It is a common rule-of-thumb that material properties can be taken anywhere below 10% of the film thickness without excessive substrate effects. While this is commonly true for hardness results, it often does not hold well for results of elastic modulus. For thin films, it is recommended that a thin film test method be used that accounts for the substrate influence in the results. The Continuous Stiffness Measurement (CSM) technique also provides clear evidence of when substrate influences are affecting the data and allows the mechanical properties to be evaluated in the most appropriate regions.

Sample Tilt

The ISO standard for nanoindentation (ISO 14577) provides a guideline that the surface of the sample should be within 1 degree of perpendicular alignment with the vertical axis of the nanoindentation tip. This guideline is more important with hard films and surfaces than it is for softer surfaces – hard surfaces will cause an offset angle to be exacerbated due to the finite lateral stiffness of the transducers. If a sample with a hard surface has more than a 1 degree tilt, an angled puck should be used in mounting the sample; an angled puck is pictures in the figures below. The sample tray is specifically designed with a 3-pt contact to ensure the best sample puck alignment possible. The sample tray is shown in the figures below.

Sample Size

Standard samples are usually mounted to sample pucks that are 1.25” (approx. 30 mm) in diameter and 1” (25.4 mm) in height. If the sample is more than 0.157” (4 mm) in height, then it needs to be less than 1.25” in diameter. Sample up to 2” tall can be accommodated. In addition, sample pucks as large as 1.5” (38 mm) can be accommodated using an alternate sample tray. Shown below are a few standard samples mounted and ready for testing. Many other non-standard sample sizes can also be accommodated.

Sample tray that holds standard 1.25” (31 mm) sample pucks. The outset shows the 3-pt contact that ensures proper puck alignment.

Standard sample mounts: (Top Row) SiC samples on Si wafers; Sections of O-ring samples mounted; Kapton® samples epoxied to a glass slide; Angled puck used for mounting a razor blade. (Bottom Row) Metallographic epoxy mount and polished sample; 1.5” tire cross-section in an epoxy metallographic mount; Several low-κ samples mounted to one sample puck; DLC coatings on polycarbonate.