Without protective coatings, surface oxidation adversely affects the performance and durability of the metal component (e.g., engine parts, turbine blades, tooling and many others). It is critical to test the protective coating chosen for a given application.
Importance of 3-D Noncontact Profilometer 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. Ensuring the quality control of such parameters will heavily rely upon quantifiable, reproducible and reliable inspection of the coating surface. The Nanovea 3-D noncontact profilometers utilize chromatic confocal technology with unique capability to measure extreme 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.
The 3-D noncontact profilometer will be used to characterize the surface of two samples with AlSiN coatings (Fig. 1) – one with a low level of silicon, the other with a high level. The AlSiN coating was deposited on H13 steel by magnetron sputtering then submitted to an oxidation-resistance test (900°C for one hour).
Measurement Setups and Tips
Measurement areas randomly selected on the sample and drastic changes in surface topography are not an issue for Nanovea profilometers. Height variation down to nanometers and up to 27 mm can easily be measured.
The axial chromatism technique uses a white-light source, where light passes through an objective lens with a high degree of chromatic aberration. The refractive index of the objective lens will vary in relation to the wavelength of the light. In effect, each separate wavelength of the incident white light will refocus at a different distance from the lens (different height). When the measured sample is within the range of possible heights, a single monochromatic point will be focalized to form the image. Due to the confocal configuration of the system, only the focused wavelength will pass through the spatial filter with high efficiency, thus causing all other wavelengths to be out of focus. The spectral analysis is done using a diffraction grating. This technique deviates each wavelength at a different position, intercepting a line of CCD that, in turn, indicates the position of the maximum intensity and allows direct correspondence to the Z height position (Fig. 2).
Nanovea optical pens have zero influence from sample reflectivity. Variations require no sample preparation and have advanced ability to measure high surface angles capable of large Z-measurement ranges. You can measure any material: transparent or opaque, specular or diffusive, polished or rough.
The results of testing for the low-silicon sample are shown in Figs. 3 and 5. Results of the same tests for the high-silicon sample are shown in Figs. 4 and 6. Peak-count histograms for both samples are shown in Figure 7.
In this article, we have shown how the Nanovea 3-D noncontact profilometer can precisely characterize both the topography and the nanometer details of an oxidized surface. By comparing the two surfaces, it has been clearly shown that the AlSiN coating with high silicon performed much better than that with lower silicon. From the 3-D surface measurements, areas of interest can quickly be identified and then analyzed with a list of endless measurements (dimension; roughness, finish, texture; shape, form, topography; flatness, warpage, planarity; volume, area; step, height, depth, thickness; and others). To further view in detail, a 2-D cross section can quickly be chosen to analyze at nanometer range. With this information, oxidized surfaces can be broadly investigated with a complete set of surface-measurement resources. IH
Here we have seen a very a unique way to use the 3-D noncontact profilometer, but there are many more applications found throughout aerospace. In particular, the precision-machined parts and components used in the industry are machined to exacting standards to meet the stringent requirements of their working environment. The 3-D noncontact profilometer provides a vital tool for not only QA and QC but for the designing process itself. From turbine blades to electronic components and everything in between, the superior white-light axial chromatism technique used by the profilometer provides the widest range of resolution on the widest range of material surface and geometries than any other technique. With this type of capability, the measurements – roughness, form, flatness, depth, volume and others – of nearly any part can be inspected at nano- through macro-range. As parts and material-characterization requirements become smaller and more demanding, aerospace is turning to a user-friendly technique with bench-top, high-speed and portable options.