Application Note | Oxidation Resistance
AlSiN Oxidation Surface Area Using 3D Profilometry
How Silicon Content Changes the Oxidized Surface of an AlSiN Hard Coating
Research & Experimental Testing
Johans Restrepo
Visual Design & Editorial
Andrew Shore
Introduction
Oxidation resistance is critical for metal components in high temperature environments. Exposure to high temperatures can deplete the metal through rapid oxidation and material loss. Without protective coatings, surface oxidation adversely affects the performance and durability of metal components, for example engine parts, turbine blades and tooling. It is critical to test the protective coating chosen for a given application.
Importance of 3D Non-Contact Profilometry for Oxidation-Resistant Coatings
To reliably understand and evaluate oxidation resistance, the surface can be measured after an oxidation resistance test. Understanding surface parameters can lead to the best selection of processing and control measures, and ensuring quality control of such parameters relies heavily upon quantifiable, reproducible and reliable inspection of the coating surface. The NANOVEA 3D Non-Contact Profilometers use chromatic confocal technology with a unique capability to measure extremely rough surfaces and steep angles. Where other techniques fail to provide reliable data due to probe contact, surface variation, angle, absorption or reflectivity, NANOVEA Profilometers succeed.
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Measurement Objective
The 3D non-contact profilometer is used to characterize the surface of two samples with AlSiN coatings, one with a low level of silicon and the other with a high level. The AlSiN coating was deposited on H13 steel by magnetron sputtering and then submitted to an oxidation resistance test (900 °C, 1 hour).
NANOVEA ST400 Standard
Optical 3D Profilometer
Results
Low Silicon
High Silicon
| ISO 25178 parameter | Low silicon | High silicon |
|---|---|---|
| Sa (µm) | 19.257 | 2.846 |
| Sq (µm) | 21.222 | 6.639 |
| Sp (µm) | 58.568 | 51.175 |
| Sv (µm) | 31.935 | 36.808 |
| Sz (µm) | 90.503 | 87.983 |
| Ssk | 0.731 | 4.612 |
| Sku | 2.014 | 24.773 |
Table 1: Roughness of the oxidized coatings (from Figs. 2 and 3).
| Coating | Maximum step height (µm) | Mean step height (µm) |
|---|---|---|
| Low silicon | 59.546, 53.814, 53.502, 51.843 | 58.134, 49.540, 52.663, 50.324 |
| High silicon | 44.426, 45.099 | 42.948, 41.351 |
Table 2: Step heights across oxide features (from Figs. 2 and 3).
Conclusion
In this application, we have shown how the NANOVEA 3D Non-Contact Profilometer can precisely characterize both the topography and the nanometer details of an oxidized surface. Comparing the two surfaces clearly shows that the AlSiN coating with high silicon performed much better than that with low silicon. From the 3D surface measurements, areas of interest can quickly be identified and analyzed with a wide list of measurements (dimension, roughness, finish and texture, shape, form and topography, flatness, warpage and planarity, volume and area, step height, depth and thickness, and others), and a 2D cross section can be chosen to analyze details at the nanometer range. With this information, oxidized surfaces can be broadly investigated with a complete set of surface measurement resources.
Frequently Asked Questions About Oxidized Coating Surfaces
Why measure surfaces after oxidation?
Oxidation at high temperature depletes metal and degrades engine parts, turbine blades and tooling; measuring the surface after an oxidation test shows how well a protective coating resisted.
What samples were tested?
Two AlSiN coatings, one with low and one with high silicon, deposited on H13 steel by magnetron sputtering and oxidized at 900 °C for 1 hour.
How did the two coatings compare?
The low-silicon coating was covered with large oxide nodules (Sa 19.257 µm), while the high-silicon coating stayed mostly flat with scattered nodules (Sa 2.846 µm), so it performed much better.
How tall were the oxide features?
Maximum step heights across nodules were 51.843 to 59.546 µm on the low-silicon coating and 44.426 to 45.099 µm on the high-silicon coating.
What does the peak count histogram show?
The distribution of peaks per mm² by height, showing many tall peaks spread over a wide range on the low-silicon surface and a narrower distribution on the high-silicon surface.
Why non-contact?
Chromatic confocal measurement handles extremely rough surfaces and steep angles where probe contact, angle, absorption or reflectivity defeat other techniques.
