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Investigation of infrared spectral emissivity of low emittance functional coating artefacts

https://doi.org/10.1016/j.infrared.2020.103454Get rights and content

Highlights

  • The validation of a spectral emittance measurement facility with a clearly established uncertainty budget and clear traceability to a National Metrology Institute was reported.

  • An indirect comparison between NIM and CCT was also carried out using the CVD-SiC sample that was used to validate the uncertainty claimed by NIM.

  • The consistent performance of CVD-SiC and the low emittance functional coating artefacts, in terms of stability and repeatability, demonstrated that they could serve as potential emittance artefacts for performing infrared scale comparison.

Abstract

Accurate infrared spectral emissivity measurements play a vital role in many thermal and optical applications of high value industries. In this paper, we report the experimental investigation of the infrared spectral emissivity of low emittance functional coating artefacts. The functional coating uses 35% aluminum particles as filler to reduce the surface emissivity. Fourier transform infrared spectrometer based infrared emissivity measurement systems, developed by NIM, were used in this investigation. The normal spectral emissivity of the coating was investigated both at NIM and BIAM in the spectral infrared range 3–14 μm, and the temperature range 300–600 °C. The artefacts’ stability and the infrared emittance scale comparison between BIAM and NIM are reported in this paper. An indirect comparison between NIM and CCT was also carried out using the CVD-SiC sample that was used to validate the uncertainty claimed by NIM. The infrared emittance scales’ comparison results demonstrate the scales are clearly in agreement well within the overall comparison uncertainty.

Introduction

Infrared spectral emissivity plays a vital role in many thermal and optical applications. From radiation thermometry, energy conversion, industrial material processing, remote sensing and non-contact defect detection to aerospace applications, accurate infrared emissivity measurements are urgently needed. Recently, under the auspice of the Consultative Committee on Thermometry (CCT) Task Group on Thermophysical Quantities (TG-ThQ), an inter-laboratory comparison of the major NMIs’ infrared spectral emittance scales was carried out. A 2% level of agreement of the scales was found – within the individual institute scale realization uncertainty – by using silicon carbide as the transfer comparison artefact [1].

Even many major National Measurement Institutes (NMIs) [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], universities and industrial research institutes [11], [12], [13], [14] have developed measurement systems to support the industrial need for accurate infrared spectral emissivity measurements in these areas, very few results for low emissivity artefacts were published. In this paper, we report the experimental investigation of the infrared spectral emissivity of a low emittance functional coating. Low infrared emissivity aeronautical materials, especially function coatings, are widely used to reduce the infrared signature difference between the aircraft and the environment. Aluminum particles are usually used as the main fillers [15], [16], [17]. Generally, the high-volume content of aluminum powders is necessary to achieve the low infrared emissivity of coatings. The samples studied in this paper were developed by BIAM, using 35% weight percent Al as filler to reduce the surface emissivity.

At NIM, a grating monochrometer based infrared emissivity measurement system was developed in 2008 [18]. The directional spectral emissivity of a heated sample could be measured at temperatures from 200 °C to 723 °C and spectral range between 2 μm and 15 μm. The signal-to-noise ratio of comparison measurement between the spectral radiances of the sample and blackbody was improved by a lock-in amplifier and statistical measurement method. Recently, the system was upgraded by employing a sodium heat-pipe sample heater to improve the temperature range up to 1000 °C and a Fourier transform infrared spectrometer (FTIR) to improve the IR wavelength accuracy. The spectral emissivity of the low emittance coating samples were experimentally investigated using NIM and BIAM FTIR based emissivity measurement systems. The details of the systems can be seen in Section 3.

The construction of the low emittance functional coating artefacts, the measurement process and the results at BIAM and NIM are described in this paper. The local infrared spectral emissivity values were assigned to low emittance functional coating artefacts by BIAM and NIM respectively. The difference from the mean value of the BIAM and NIM values for the artefacts was derived. This allowed us to compare the infrared spectral emittance scales as realized by the two institutes.

To validate the uncertainty claimed by NIM, an indirect comparison was performed using the same CVD-SiC sample. The difference from the infrared spectral emittance scales realized by NIM and the CCT Supplementary Comparison S1 participants is also reported in this paper.

Section snippets

The low emittance functional coating artefacts

Two low emittance functional coating artefacts, BIAM-LEC-1#, BIAM-LEC-2#, were constructed at BIAM. The low emittance functional coatings were formed on a copper substrate for BIAM-LEC-1# and a superalloy substrate for BIAM-LEC-2# by means of a compressed air spraying method. The thicknesses of the coatings were both 50 μm. The thermal conductivities of the coatings are 2 W/(m·K) perpendicular to surface. The design features of the substrate are shown in Fig. 1. The diameter of the substrate is

The infrared spectral emissivity measurement system

The BIAM and NIM infrared spectral emissivity measurement systems had the identical design, and manufactured by NIM The both systems employed FTIR to compare the spectral radiance of the sample and blackbody at the same temperature, which is called the direct measurement method. An overview of the system can be seen in Fig. 3. The temperature range covered from 100 °C to 1000 °C, the spectral range covered from 3 μm to 14 μm.

The main components include the sample heating system, the blackbody

Stability

Stability tests were performed by comparing the spectral radiation of the BIAM-LEC −2# artefact and NIM’s VTBCF at 600 °C in terms of the radiance temperature, by means of the TRT II, on three different days. In this way the emissivity of the artefact at the TRT II waveband can be obtained. The results are listed in Table2.

The results obtained indicated that the BIAM-LEC −2# artefact was very stable, the waveband emissivity values obtained by the TRT II were well within 0.004 over the 3 days

Conclusions

Two low emittance functional coating artefacts with different substrates were constructed at BIAM, and were evaluated by infrared spectral emissivity measurement at BIAM and NIM respectively. The temperature range covered was (300–600) °C, the spectral range covered was (3–14) μm. The artefacts’ stability, the typical infrared spectral emissivity curves at different evaluated temperatures, and the scale comparison between BIAM and NIM were experimentally investigated and are reported in this

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

DW acknowledges the support from National Natural Science Foundation of China (NSFC) (NO. 11772318).

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