Spray Parameter Development

Let Our Know-How in Hybrid HVAF Tech Help You

Why is spray parameter development so important?

Spray parameter development is crucial in the thermal spray industry as it has a huge impact on the quality, efficiency, and performance of the coating.  Efficient spray parameter development ensures that the coating adheres well to the substrate, and has the desired microstructure, and meets all the performance requirements. It also helps in minimizing defects like porosity, cracks, and uneven thickness. The optimal set of spray parameters will vary depending on the specific thermal spray process, powders and substrates used.

Can you use the same spray parameters for the same powder chemistry and grain size from multiple powder manufacturers?

Based of our testing, the simple answer is No, even if all powders were manufactured using identical powder morphology, powder chemistry by mass fraction, powder size distribution by weight percentage, and powder apparent density, this still won’t guarantee that a single spray parameter could be used to obtain the exact same coating results for powders produced by different manufacturers.

During our Hybrid HVAF coating technology development and testing phase with our landing gear OEM partners Collins Aerospace, and Heroux-Devtek one of the conditions was to identify multiple powder suppliers whose powders would meet the OEM specification requirements.  The images below show hardness results on three WC-10Co4Cr (-15+5µm) powders sprayed with the same parameter set.  The minimum acceptable hardness value is 1000 HV.  Supplier #1 obtained a mean value of 998 HV, supplier #2 obtained a mean value of 873 HV, where as supplier #3 obtained a mean value of 1074 HV.

What are the key aspects to look for when developing spray parameters?

The spray parameters refer to the key variables that control the thermal spray process, including but not limited to:

  • Spray Distance: The distance between the spray gun and the substrate affects coating quality. It determines the degree of heat transfer, particle velocity, and spray pattern.
  • Spray Angle: The angle at which the spray gun is held relative to the substrate influences coating uniformity and adhesion. Different angles may be required for different substrates and coating materials.
  • Spray Rate: This refers to the rate at which the coating material is fed into the spray gun. It affects the thickness and density of the coating.
  • Particle Size and Velocity: The size and velocity of sprayed particles impact coating adhesion, porosity, and overall quality. Small particles and high velocities can result in much denser coatings, which is one of the benefits of Hybrid HVAF coating technology.
  • Gas Flow Rates: The flow rates of carrier gases and fuel gases influence the combustion process and particle acceleration.
  • Substrate Temperature: The substrate temperature is a critical parameter in thermal spray processes.
  • Cooling Rates: The cooling rates after the particles impact the substrate affect the microstructure and properties of the coating.

 

Here are typical tests performed when developing spray parameters?
  • Coating Thickness Measurement: Techniques such as ultrasonic measurement, eddy current measurement, or optical profilometry are employed to measure the thickness of the sprayed coating. This helps in adjusting spray parameters to achieve the desired coating thickness.
  • Microstructural Analysis: Metallographic techniques, including optical microscopy and electron microscopy, are used to examine the microstructure of the coating. This analysis helps in understanding the coating’s grain structure, porosity, and other features.
  • Adhesion Testing: Adhesion tests assess the strength of the bond between the coating and the substrate. Common methods include pull-off tests, tape tests, and scratch tests. Improving adhesion is often a key goal in parameter development.
  • Hardness Testing: Hardness measurements, using techniques like Vickers or Rockwell hardness testing, provide information about the coating’s resistance to wear and deformation. Adjustments to parameters may be needed to achieve the desired hardness.
  • Porosity Evaluation: Testing for porosity involves examining the coating for voids or gaps. Techniques such as dye penetrant testing or microscopic examination can be used to identify and quantify porosity levels.
  • Roughness Measurement: Surface roughness measurements help assess the quality of the coating finish. Techniques like profilometry or interferometry are employed to quantify surface roughness.
  • Residual Stress Measurement: Residual stresses within the coating can be evaluated using techniques such as X-ray diffraction or strain gage methods. Adjustments to parameters may be necessary to minimize or control residual stresses.
  • Corrosion Testing: Coated samples may undergo various corrosion tests to evaluate the coating’s resistance to corrosive environments. This is particularly important for applications where corrosion resistance is a critical requirement.
  • Performance Testing: Depending on the application, functional tests may be conducted to assess the performance of the coated components under real-world conditions. This could include wear testing, thermal cycling, or other relevant tests.
Conclusion

In order to achieve the best results, it’s important to conduct extensive testing and experimentation to find the right combination of spray parameters for a particular application. This process involves considering the properties of the coating material, the substrate material, and the desired coating characteristics. Overall, the importance of spray parameter development lies in achieving consistent and high-quality coatings, reducing rework, and enhancing the overall efficiency of thermal spray processes.

Did you like this? Share it!

No comments for “Spray Parameter Development

Comments are closed.