GB/T 16921-2005Metallic coatings -- Measurement of coating thickness -- X-ray spectrometric methods (English PDF)

1.4 The actual measurement range for a given coating material depends primarily on

the energy of the characteristic X-ray fluorescence being analyzed and the permissible measurement uncertainty; it varies depending on the instrumentation and operating procedures used.

3.1 Operating mechanism

There is a relationship between the mass per unit area of the coating layer (if the density is known, then it is the linear thickness of the coating layer) and the intensity of secondary radiation. For any practical instrument system, this relationship is first determined by calibration, using a standard block with a known mass per unit area of the coating layer. If the density of the coating layer material is known, meanwhile the actual density is given, then such a standard block can provide the linear thickness of the coating layer.

Note. The density of the coating layer material is the density under coating conditions, not necessarily the theoretical density of the coating layer material at the time of measurement. If this density differs from the density of the calibration standard, a coefficient reflecting this difference shall be used and noted in the test report.

Fluorescence intensity is a function of the atomic number of the element. If the surface coating layer, intermediate coating layer (if present), substrate are composed of different elements, OR if a coating layer is composed of more than one element, these elements will produce their own radiation characteristics. An appropriate detector system can be adjusted to select one or more energy bands, allowing the device to measure the thickness and composition of both the surface coating layer and some intermediate coating layers simultaneously.

3.2 Excitation

3.2.1 General requirements

X-ray spectroscopic methods for determining coating layer thickness are based on the interaction of a strong, narrow beam of polychromatic or monochromatic X-rays with the substrate and coating layer. This interaction produces secondary radiation with discrete wavelengths and energies, which possess the elemental characteristics of the coating layer and substrate.

Such radiation can be produced by a high-pressure X-ray tube generator or a suitable radioactive isotope.

3.2.2 Generation by a high-voltage X-ray tube

Under stable conditions, if a sufficient potential is applied to the X-ray tube, appropriate excitation radiation can be produced. Most thickness measurements require an applied voltage of approximately 25 kV ~ 50 kV, but for measuring low atomic number coating materials, it may be necessary to reduce the voltage to 10 kV. The measurement uncertainty is reduced by using a primary color filter installed between the X-ray tube and the specimen.

The main advantages of this excitation method are.

- A very strong radiation beam can be generated on a very small measurement surface through collimation;

- Personnel safety requirements are easily guaranteed;

- Sufficiently stable emission can be obtained using modern electronic methods.

3.2.3 Generation by radioactive isotopes

Only a few radioactive isotopes emit gamma rays in an energy band, which is suitable for coating thickness measurement. Ideally, the excitation radiation energy is slightly higher (slightly shorter wavelength) than the required characteristic X-ray energy. The advantage of radioactive isotope excitation is a more compact instrument structure, mainly because no cooling is required. Furthermore, unlike high-voltage X-ray tube generators, its radiation is monochromatic and has a low background intensity.

Compared to the X-ray tube method, its main technical disadvantages are.

- The obtained intensity is much lower, making small-area measurements impossible;

- Some radioactive isotopes have short half-lives;

- High-intensity radioactive isotopes pose personnel protection issues (high-voltage

X-ray tubes can be easily shut down).

3.3 Dispersion

3.3.1 General requirements

Secondary radiation generated on the surface of the coating layer by X-ray irradiation usually contains components other than those required for coating layer thickness measurement. The desired components can be separated using wavelength dispersion or energy dispersion.

3.3.2 Wavelength dispersion

The wavelength characteristics of the coating layer or substrate can be selected using a crystal spectrometer. Typical characteristic radiation data for commonly used crystals can be found in publications of authoritative institutions in various countries.

3.3.3 Energy dispersion

X-ray quanta are usually expressed in terms of wavelength or equivalent energy. The relationship between wavelength and energy is.

Where.

lambda - Wavelength, in nanometers (nm);

E - Energy, in kiloelectron volts (keV).

3.4 Detection

The detector used in the wavelength dispersion system consists of a gas-filled tube, a solid-state detector, or a scintillation counter connected to a photomultiplier.

The most suitable detector for receiving fluorescence photons in the energy dispersion system is selected by the instrument designer according to the application.

Measurements can be performed in a normal atmosphere within the energy band range of 1.5 keV ~ 100 keV, without the need for helium or a vacuum.

Fluorescence radiation of different characteristic energies first enters the energy dispersion detector; then enters a multichannel analyzer to control the selection of the correct energy band.

3.5 Thickness measurement

3.5.1 Emission method

If measuring the characteristic radiation intensity of the coating layer, this intensity will increase with increasing thickness before reaching saturation thickness, as shown in

Figure 1a).

When using the X-ray emission method, the instrument is adjusted to receive the characteristic energy band of the selected coating material. Thus, a thin coating layer produces low intensity, while a thick coating layer produces high intensity.

3.5.2 Absorption method

If measuring the characteristic radiation intensity of the substrate, this intensity decreases with increasing thickness, as shown in Figure 1b).

The X-ray absorption method utilizes the characteristic energy band of the basis material. Thus, a thin coating layer produces high intensity, while a thick coating layer produces low intensity. In practical applications, it is important to ensure that there is no intermediate layer.

Absorption characteristics are inversely similar to emission characteristics.

3.5.3 Ratio method

When the coating thickness is expressed as the ratio of the strength of the substrate and the coating material, it is possible to combine the X-ray absorption method and the emission method. This intensity ratio method is essentially independent of the distance between the specimen and the detector.

3.5.4 Measurement

For the two methods described in 3.5.1 and 3.5.2, many commercial instruments employ a normalized count rate system, adjusting the characteristic count rate of the uncoated substrate to 0 and the characteristic count rate of the infinitely thick coating material to 1.Therefore, the count rate for all measurable thicknesses falls within the normalized count rate range of 0 ~ 1.See Figure 2.

In all cases, the best or most sensitive measurement range is approximately between 0.3 and 0.8 on the characteristic count rate scale. Therefore, to obtain the best measurement accuracy across the entire thickness range, a calibration standard with a characteristic count rate value of 0.3 ~ 0.8 should be used. To ensure measurement accuracy at other thicknesses, some instruments may use other standards. Because the uncertainty of the calibration standard increases with decreasing thickness, a correct mathematical relationship can be established at the thinner end of the thickness range, by appropriately using a standard block with a thick coating and low uncertainty.

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