Application of optical profiling for scratch characterization

Test performed

Scratch testing is an effective way to perform testing for comparative values of adhesion of coatings. In this application, the scratch test is used to compare the silicon-like coatings applied by chemical vapor deposition on aluminium. One sample was anodized and one was not.

In scratch testing, a Rockwell (Sphero-conical) diamond with a known radius is dragged across the sample surface with an increasing normal load while recording the lateral (frictional) force. By observing the surface with a microscope and by studying the graph of friction, the user can find the critical loads at which failures happen in the coating (cracking, chipping, delamination, etc.)

The scratch test revealed that the sample that was not anodized showed failures at lower loads, confirming the expectations in coating scratch resistance.

Figure 1 : Scratch test friction graph (the early failure causes the change in coefficient of friction slope)

Figure 2 : Example of a failure: Delamination of the coating on the anodized sample at 12.14N under 200x magnification

For this particular application, the volume and depth of a scratch were characterized. Height profiles were extracted at the end of the scratch. From these profiles, a feature in the 3D Analysis Software was used to fill in an area, shown in red in the profile images, to compute the maximum depth and area of the shaded region. The top of the red region in all of the images were kept at the normal level of the sample/coating. In doing so, the maximum depth value is in reference to the surrounding (flat/level) surface.

Sample with anodization – 0-4N Scratch

Figure 3 : 3D image of the entire scratch

Figure 4 : Cross section of the end of the scratch and cross section area calculation

Figure 5 : Profile along scratch length and area of center profile

Sample with anodization – 0-20N Scratch

Figure 6 : 3D image of the entire scratch

Figure 7 : Cross section of the end of the scratch and cross section area calculation

Figure 8 : Profile along scratch length and area of center profile

Application

This use of the optical profiler adds useful information to the scratch test method. It allows to easily find the exact shape of the scratch at any point which gives a better understanding of the failure in the sample. It can be used to calculate the volume removed in the scratch track, and also the volume of build-up on the sides of the track.

The longitudinal profile shows depth of the scratch at any point along its length. This can be used to understand what coating or substrate is being uncovered by the scratch. This is useful in multi-coating systems where it can be difficult to know what layer was uncovered and is being observed under the microscope.

At any point along the length of the scratch, a lateral profile can be taken to provide the shape of the scratch. This is valuable information when trying to calculate the pressure between the tip and the substrate at the different failure points.