Plasma And Particle Temperature Measurements In Thermal Spray Using Spectroscopy

Introduction

Plasma thermal spray coating is a process that utilizes the high-energy state of plasma to melt and propel particles onto a substrate, creating a robust coating. This technique can generate plasma temperatures that exceed the surface temperature of the Sun, which is around 5,778 K. The precise measurement of plasma temperature is critical and is often achieved through spectroscopy, a method that analyzes the light emitted by the plasma to determine its characteristics. Spectroscopy can provide accurate temperature readings and is essential for ensuring the quality and consistency of plasma-sprayed coatings.

Figure 1: Illustration of thermal spray technology.

Principle of plasma temperature measurement

The principle behind using spectroscopy to measure plasma temperature is based on the relationship between the emitted light spectrum and the temperature of the plasma. Here's a breakdown of the process:

1. Thermal Equilibrium: Plasmas in many applications, especially those in laboratory settings, are often in a state of thermal equilibrium. This means that the particles within the plasma (electrons, ions, and neutrals) have a common temperature distribution.

2. Emission Spectrum: When a plasma is excited, its constituent particles emit light at specific wavelengths. The intensity of this emitted light depends on the temperature of the plasma.

3. Boltzmann Distribution: The distribution of excited states of the particles in the plasma follows a Boltzmann distribution. This distribution is directly related to the temperature of the plasma.

4. Spectral Analysis: By analyzing the intensity ratios of different spectral lines emitted by the plasma, we can determine the temperature distribution. The more highly excited states of the particles will be more populated at higher temperatures, leading to a greater intensity of the corresponding spectral lines.

5. Calibration: To obtain accurate temperature measurements, it is essential to calibrate the spectrometer using a known temperature source. This involves comparing the measured spectrum of the known source to the spectrum of plasma and establishing a relationship between intensity ratios and temperature.

6. Temperature Calculation: Once the calibration is complete, the temperature of the plasma can be calculated based on the measured intensity ratios of the spectral lines using the Boltzmann distribution.

Result

In one study, the researchers collected the emission spectra from the plasma using a spectrometer and plasma temperatures can be determined by the atomic Boltzmann distribution method. 

Optical Emission Spectroscopy: Determines plasma temperature by analyzing the optical emission spectrum. It identifies specific peaks corresponding to different elements.

Boltzmann Distribution: Uses the intensity of spectral lines to calculate temperature, assuming local thermal equilibrium.

Figure 2: Optical emission spectra at 20 mm axial distance for the three cases, (a) without any injection (no injection), (b) two-phase injection of ethanol and air (air + EtOH injection), and (c) two-phase injection of air and YSZ ethanol suspension (injection of air + YSZ EtOH suspension)

Figure 2 shows the optical emission spectra at a 20 mm axial distance from the nozzle for three different cases: 

• No Injection: Baseline spectra without any additional substances.
• Air + Ethanol Injection: Spectra showing peaks from decomposed ethanol (C and H peaks).
• Air + YSZ Ethanol Suspension Injection: Spectra with additional peaks from yttria-stabilized zirconia (Zr and Y peaks), indicating the presence of evaporated YSZ material.

These results highlight how different injections affect the plasma composition and temperature.

Figure 3: Developments of plasma excitation temperatures for two low pressure plasma spraying conditions.

Figure 3 shows the axial development of plasma excitation temperatures determined by Boltzmann plots for two different conditions: 

• First Condition (Ar and He as plasma gases): The temperature decreases continuously in the downstream direction, dropping below 10,000 K at 435 mm axial distance.
• Second Condition (Ar, He, and H_2 as plasma gases): The temperatures are lower compared to the first condition due to the dissociation of hydrogen molecules (H_2 → 2H), which consumes energy.

This comparison highlights how the addition of hydrogen affects the plasma temperature, leading to a more significant temperature drop along the axial direction.

By analyzing the emitted light from plasma, optical emission spectroscopy allows for the identification of gas species and their temperatures. The Boltzmann distribution method further refines this measurement by using the intensities of spectral lines to determine the excitation temperature. This combination of techniques provides accurate and detailed insights into plasma characteristics, essential for optimizing thermal spray processes and ensuring high-quality coatings.

Measurement system

The Ocean SR6 spectrometer is a high-sensitivity device designed for a variety of applications, including the measurement of plasma temperature in thermal spray techniques. It is equipped with a CCD array of 2048 elements, which allows for a detailed analysis of the spectral information. The SR6's wavelength range spans from 180 to 1100 nm, making it suitable for capturing a broad spectrum of light and providing valuable data on the thermal properties of plasma. This range is particularly useful for optical emission spectroscopy, a method often employed to determine the in-flight temperature of particles in thermal spray processes. The spectrometer's high signal-to-noise ratio (SNR) ensures accurate readings, which are crucial for optimizing the spraying process and ensuring the quality of the coatings produced. The SR6's versatility and precision make it a valuable tool for researchers and engineers working to advance the field of thermal spray technology.

Figure 4: Ocean SR spectrometer.

The Ocean Insight UV-Vis XSR fiber is a critical component in measuring the plasma temperature of thermal spray techniques. Its ability to cover a wavelength range of 180-800nm allows for accurate detection of the spectral lines necessary to determine the plasma's temperature. This range is particularly useful for capturing the broad spectrum of light emitted by plasma, which is essential for precise temperature measurements. The data collected by the Ocean Insight UV-Vis fiber can help optimize the thermal spray process, ensuring better coating quality and adherence.

Figure 5: UV-VIS XSR fiber.

Conclusion

Plasma thermal spray coating, with its ability to achieve extremely high temperatures, is a powerful technique for creating robust coatings. Spectroscopy plays a crucial role in this process by providing accurate plasma temperature measurements, ensuring quality and consistency. Emission spectrum offers detailed insights into plasma characteristics. Advanced tools like the Ocean SR6 spectrometer and UV-Vis XSR fiber enhance the precision of these measurements, optimizing the thermal spray process and improving coating quality. Continued advancements in these technologies will further refine and enhance thermal spray applications.