Application Note | Coating Adhesion Testing
Multi-Scratch Automation of Similar Samples Using the PB1000 Mechanical Tester
Critical Loads of Six Coatings from One Automated Scratch Matrix for High-Volume Quality Control
Research & Experimental Testing
Xavier Herrera-Keehn & Jocelyn Esparza
Visual Design & Editorial
Andrew Shore
Introduction
Coatings are widely used in various industries because of their functional properties. A coating’s hardness, erosion resistance, low friction, and high wear resistance are just some of the many properties that make coatings important. A commonly used method to quantify these properties is scratch testing, this allows for a repeatable measurement of a coating’s adhesive and/or cohesive properties. By comparing the critical loads at which failure occurs, the intrinsic properties of a coating can be evaluated.
Why Automated Multi-Scratch Testing Matters for Quality Control
A scratch test is performed by sliding a stylus over a sample’s surface while applying a specific load that is either constant or progressive. While the scratch test is conducted, the coating will begin to fail at specific applied loads, these points along the scratch are known as the critical loads. The critical load is a function of many factors, including stylus shape and radius, sliding speed, loading rate, coating thickness, and much more. By using identical test parameters for comparative evaluation between samples, variance in test results are reduced which leads to a higher degree of accuracy and consistency between samples.
Another important data set that can be recorded is a sample’s acoustic emissions (AE). AE are elastic waves generated by the release of internal energy, which can give the operator insight towards what is occurring during the testing1. Automating the testing process can reduce errors from test set up and user operation. This is an indispensable feature for efficient and reliable quality control testing where multiple identical samples are to be tested.
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Measurement Objectives
In this study we showcase the NANOVEA PB1000 Mechanical Tester’s ability to perform automated scratch testing on multiple samples. The tests were conducted on six different coated samples to exhibit the PB1000’s efficiency and reproducibility.
NANOVEA PB1000 Large Platform
Mechanical Tester
Measurement Parameters
The NANOVEA PB1000 Mechanical Tester was used to test six samples mounted in a puck holder using the test parameters summarized in Table 1. Three scratches were performed on each sample with the scratch map feature.
Fig. 2: Six samples mounted on a puck holder.
| Parameter | Value |
|---|---|
| Load type | Progressive |
| Initial load | 5 N |
| Final load | 30 N |
| Sliding speed | 5 mm/min |
| Sliding distance | 10 mm |
| Indenter geometry | Rockwell (120° cone) |
| Indenter material (tip) | Diamond |
| Indenter tip radius | 100 µm |
| Atmosphere | Air |
| Temperature | 24 °C (room temperature) |
Table 1: Test parameters used.
Results and Discussion
With the high precision position control of the NANOVEA PB1000 Mechanical Tester, specific areas can be targeted for testing. A summary of the results from the six coatings can be found below in Table 2, where the Initial Delamination, LC1, is defined as the critical load at which the first signs of adhesive failure occurred. While LC2, the Repetitive Delamination, is defined as the load at which repetitive adhesive failures had taken place. Looking at the data, it is evident that Sample B delaminated at a much lower load than its counterparts. While Sample F fared best in the testing, having the highest LC1 and LC2 values.
| Sample | Initial delamination (LC1) [N] | Repetitive delamination (LC2) [N] |
|---|---|---|
| A | 10.7 ± 0.6 | 15.0 ± 0.9 |
| B | 9.4 ± 0.1 | 10.7 ± 0.3 |
| C | 12.2 ± 0.7 | 15.4 ± 0.3 |
| D | 12.4 ± 1.0 | 15.6 ± 0.3 |
| E | 11.6 ± 1.0 | 15.8 ± 0.6 |
| F | 13.0 ± 1.3 | 17.9 ± 0.1 |
Table 2: Summary of critical loads of the tested samples.
Fig. 3 below shows a visual side-by-side comparison of the LC1 and LC2 microscope images for samples B and F. Before adhesive failure occurs, chevron cracking takes place. This is defined as small cracks with a 45° orientation relative to the scratch direction. Sample B’s initial delamination occurred at a load of 9.4 N, catastrophic failure by way of annular cracks can be seen beyond the scratch width. This can be attributed to high compressive stress from the stylus and poor coating adhesion. Once the load reached 10.7 N, repetitive wedge failures can be seen on sample B. In comparison, Sample F exhibits a small buckling failure at LC1 and repetitive buckling failures at its respective LC2.
The AE for each sample was also recorded during testing. Fig. 4 below depicts the scratch test graphs for both sample B and F. When looking at the graph for sample B, the very first spike of acoustic emissions were found at an early stage of the scratch test (~0.78 mm). This is then followed by repetitive AE peaks, indicating continual similar failures as the test proceeded. Now looking at the graph for sample F, there is an initial spike at ~1.1 mm, this corresponds with the first visual sign of chipping. As the load increased, the AE spikes continued to increase, correlating to continuous failures in the coating.
Fig. 4: Normal force, penetration, and acoustic emissions of Sample B (top) and Sample F (bottom).
The scratch test matrix was finished within minutes with minimum operator handling and superior efficiency and repeatability, thanks to the high precision sample stage and user-friendly software interface of the NANOVEA Mechanical Tester. Compared to conventional scratch test procedure, the multi sample scratch test in this study can substantially cut the time and possible errors in sample handling and test setup.
Conclusion
In this study we were able to showcase the ability of the NANOVEA PB1000 Mechanical Tester to perform multi sample scratching in a repeatable manner. Compared to conventional scratch test procedure, the multi scratch test in this study can substantially cut the time and error in sample handling and test setup. Demonstrating that the NANOVEA PB1000 Mechanical Tester is the perfect option for large volume quality control, substantially reducing test set up time and increasing test efficiency.
References
1. Tomastik, Jan; Ctvrtlik, Radim; Bohac, Petr; Dráb, Martin; Koula, Václav; Cvrk, Karel; Jastrabík, L. (2015). Utilization of Acoustic Emission in Scratch Test Evaluation. Key Engineering Materials, 662, 119–122.
Frequently Asked Questions About Automated Multi-Sample Scratch Testing
Why automate scratch testing across multiple samples?
Using identical test parameters for every sample reduces variance between results and improves accuracy and consistency. Automation also reduces errors from test setup and operator handling, which matters for efficient, reliable quality control of many identical samples.
What is a critical load in a scratch test?
As the stylus slides under a progressive load, the coating begins to fail at specific loads called critical loads. Here LC1 (initial delamination) is the load at the first signs of adhesive failure and LC2 (repetitive delamination) is the load at which repetitive adhesive failures take place.
How were the six coated samples tested?
Six samples were mounted in a puck holder and three scratches were made on each with the scratch map feature: a progressive load from 5 to 30 N over 10 mm at 5 mm/min with a Rockwell 120° diamond cone of 100 µm tip radius.
Which coating performed best and worst?
Sample F had the highest critical loads (LC1 13.0 ± 1.3 N, LC2 17.9 ± 0.1 N). Sample B delaminated at much lower loads (LC1 9.4 ± 0.1 N, LC2 10.7 ± 0.3 N), with annular cracks beyond the scratch width attributed to high compressive stress and poor adhesion.
What does acoustic emission add to a scratch test?
Acoustic emissions are elastic waves released by internal energy, such as cracking. On Sample B the first AE spike appeared early (~0.78 mm), followed by repetitive peaks; on Sample F the first spike at ~1.1 mm matched the first visible chipping.
Which NANOVEA instrument performs automated multi-sample scratch tests?
The NANOVEA PB1000 Mechanical Tester, whose high-precision stage and scratch map feature completed the six-sample test matrix within minutes with minimal operator handling.
