Importance of Nano Scratch Testing for Quality Control

A major concern for paint manufacturers is the ability of the paint to withstand cracking. Once paint begins to crack, it fails to protect the substrate that it was applied to, therefore, failing to satisfy the client. For example, if a branch happens to scratch or strike the side of a car and immediately afterward the paint begins to chip, the paint manufacturer would lose business due to their poor paint quality. Paint quality is very important because if the metal substrate becomes exposed it may begin to rust or corrode. This applies to several other areas such as household and office supplies, electronics, toys, research tools and more. Although the paint may be resistant to cracking when first applied, the properties may change over time with weathering. This is why it is very important to have the paint samples tested at their weathered stage. Although cracking under a high load of stress may be inevitable, the manufacturer must predict how the coating weakens over time and how deep the affecting scratch may be in order to provide their consumers with the best possible products.

Measurement Objective

Figure 1 Click to enlarge

Our goal was to simulate the process of scratching in a controlled and monitored manner to observe sample behavior effects. In this application, the Nanovea Mechanical Tester, in its nano scratch testing mode, was used to measure the load required to cause failure to an approximately seven-year-old, 30 to 50 µm-thick paint sample on a metal substrate. A 2 µm diamond-tipped stylus was used at a progressive load ranging from 0.015 mN to 20.00 mN to scratch the coating.

We performed a pre and post scan of the paint with a 0.2 mN load in order to determine the value for the true depth of the scratch. The true depth analyzes the plastic and elastic deformation of the sample during testing; whereas, the post-scan only analyzes the plastic deformation of the scratch. The point where the coating fails by cracking is taken as the point of failure. We used ASTM D 7187 as a guide to determine our testing parameters. Having used a weathered sample, thus testing a coating sample at its weaker stage, we can conclude that the sample presented us with lower points of failure. Five tests were done on this sample in order to determine the exact failure critical loads.

Measurement Principle

The scratch test method is a very reproducible, quan-titative technique in which critical loads at which failures appear are used to compare the cohesive or adhesive properties of coatings or bulk materials. During the test, scratches are made on the sample with a sphero-conical stylus (tip radius ranging from 1 to 20 mm), which is drawn at a constant speed across the sample under a constant load, or, more commonly, a progressive load with a fixed loading rate (Figure 1). The sphero-conical stylus is available with different radii (which describes the “sharpness” of the stylus). Common radii are from 20 to 200 mm for micro/macro scratch tests, and 1 to 20 mm for nano scratch tests.

When performing a progressive load test, the critical load is defined as the smallest load at which a recognizable failure occurs. In the case of a constant load test, the critical load corresponds to the load at which a regular occurrence of such failure along the track is observed.

In the case of bulk materials, the critical loads observed are cohesive failures, such as cracking, or plastic deformation of the material. In the case of coated samples, the lower load regime results in conformal or tensile cracking of the coating, which still remains fully adherent (which usually defines the first critical load). In the higher load regime, further damage usually comes from coating detachment from the substrate by spalling, buckling or chipping.

Table 1 Click to enlarge

Comments on the Critical Load

The scratch test gives very reproducible, quantitative data that can be used to compare the behavior of various coatings. The critical loads depend not only on the mechanical strength (adhesion, cohesion) of a coating-substrate composite but also on several other parameters; some of them are directly related to the test itself, while others are related to the coating-substrate system (Table 1).

Means for Critical Load Determination

Microscopic Observation
This is the most reliable method to detect surface damage. This technique is able to differentiate between cohesive failure within the coating and adhesive failure at the interface of the coating-substrate system.

Tangential (Frictional) Force Recording
This enables the force fluctuations along the scratch to be studied and correlated to the failures observed under the microscope. Typically, a failure in the sample will result in a change (a step, or a change in slope) in coefficient of friction. Frictional responses to failures are very specific to the coating-substrate system in study.

Acoustic Emission (AE) Detection
Detection of elastic waves generated as a result of the formation and propagation of microcracks. The AE sensor is insensitive to mechanical vibration frequencies of the instrument. This method of critical load determination is mostly adequate for hard coatings that crack with more energy.

Depth Sensing
Sudden change in the depth data can indicate delimitation. Depth information pre and post scratch can also give information on plastic versus elastic deformation during the test. 3D non-contact imaging such as white light axial chromatism technique and AFMs  can be useful to  measure exact depth of scratch after the test.

Test Parameters
Testing parameters are noted in Table 2.

Results

This section presents the data collected on the failures during the scratch test. We describe the failures observed in the scratch and define the critical loads that were reported. A summary table of the critical loads for all samples, and a graphical representation are presented, and lastly detailed results for each sample: the critical loads for each scratch, micrographs of the failure, and the graph of the test.

Failures Observed and Definition of Critical Loads

Critical failure is the point where the coating fails in such a way that cracks are visible for the remainder of the scratch track. The failure is clearly seen in the micrograph in Figure 2.

Table 3 shows the critical loads for the coating on a metal substrate and the true depth. Figure 3 illustrates the scratch at 20 mN and larger magnification, and Figure 4 shows a true depth graph for the linear scratch test.

Conclusion

The Nanovea Mechanical Tester, during the Nano Scratch Tester Mode, allows simulation of many real-life failures of coatings. By applying loads in a controlled and closely monitored fashion, the instrument allows one to identify at what load failure in the scratch occurs. This can then be used as a way to determine quantitative values for scratch resistance and true depth values amongst various samples. A clear and consistent cracking failure with also consistent true depth values may be used for further improvement of sample durability. The very small standard deviations also show the reproducibility of the instrument. This type of information can help manufacturers improve the paint quality and improve formulations to withstand the effects of weathering.

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