Developing and Testing Multilayered Coatings Using Ducom Water Droplet Erosion Tester (WDE)

Wind and rain are inseparable. In the world of tribology, the combination of wind and rain is toxic, as they affect the performance of transportation and clean energy systems that are imperative in today’s society.

Video representing the  aereo-acustic noise from blades in wind turbine (Wind power plant in Bukowsko, Poland)

Wind turbine blades that harness wind power for the production of clean energy are highly affected by rainfall. During rainfall, the repeated impact of liquid droplets on the blades’ leading edges can reach 100 m/s and beyond, and this liquid-solid interaction can cause leading edge erosion.

This has even more importance in the exteriors of fighter jets and civil airplanes (for example, French Rafale Jets, procured by India), which can travel faster than 250 m/s. The impact energy of liquid droplets is even higher here and causes even more severe erosion than the wind turbine blades (impact energy is proportional to velocity power 2).

The impact of liquid droplets on rigid surfaces generates water hammer pressure of several gigapascals. Later, the droplet depressurizes through lateral jets that can move 2 to 6 times faster than the impact velocity. Lateral jetting and water hammer pressure can exert stress beyond the solid surface’s endurance limit, resulting in failure mechanisms like increased roughness, fatigue cracks, delamination, spalling, and pitting.

Dealing with Failure Mechanisms

These failure mechanisms have an impact on wind turbine energy production due to reduced lift and an increase in drag. The blades’ service life can also be affected, with leading-edge erosion being visible within 2 to 3 years of installation.

Similar failure mechanisms must be dealt with in the fighter jets and airplanes to reduce the servicing cost and maintenance. For example, repainting a civilian airplane can be as expensive as $ 300,000 and requires as much as 50 gallons of paint.

"Droplet Generator" in Ducom Water Droplet Erosion tester, generates liquid droplets that repeatedly impact the rigid specimen at 200 m/s.

Aircraft and wind turbine blades use epoxy and elastomeric coatings to protect their surfaces against rain. These coatings should perform beyond erosion resistance, as they should not affect the designed stress and strain limits of the wind turbine blades, they should not increase the airplane weight, and they must be dust and UV resistant.

In this regard, global materials research in transportation and wind energy is concentrated on the development and testing of multilayered coatings using the Ducom Water Droplet Erosion Tester (WDE). Ducom WDE can simulate liquid-solid erosion and failure mechanisms in the lab for materials used in airplanes, fighter jets, and wind turbines.

Testing unit in Ducom Water Droplet Erosion Tester.

Ducom has carefully designed, developed, and manufactured the complicated “Droplet Generator” at the water droplet erosion tester’s core. This droplet generator can control the droplets precisely – at a size between 2 to 5 mm, a droplet impact velocity between 25 to 250 m/s, and a droplet impact frequency between 5 to 50 Hz.

Erosion wear of a elastomer coating due to repeated impact of liquid droplets of 2 mm in diameter.

Erosion wear of an elastomer coating due to repeated impact of liquid droplets of 2 mm in diameter. 

Note: Ducom is currently working on an upgrade of the droplet generator to Mach 1 and 2 for simulating the wear mechanisms of fighter jet coatings. Please contact Ducom if you want to know more about Mach 1 or 2 type droplet generators.

Ducom Water Droplet Erosion Tester is used by several research facilities in France and the Netherlands, to simulate a wide range of droplets and the wear mechanisms and 400 hours of uninterrupted testing.”

Steve Jobs once said, “Get closer than ever to your customers. So close, in fact, that you tell them what they need before they realize it themselves”. Ducom follows this philosophy at different stages of design and manufacturing of the Ducom Water Droplet Erosion Tester. This will be explained in the following few examples.

Example A

It has been indicated by research that the residue of water films after droplet impact can form a “cushion” with the ability to resist water hammer pressure. This could affect the liquid-solid impact wear mechanisms. Ducom has designed a pressure controlled “air blade” system that controls the water film cushioning effect to investigate this hypothesis (see Figure 1).

Example B

Ducom Water Droplet Erosion Tester (WDE) is equipped with a pressure controlled air blade and matrix modulator (A). Matrix modulator is used to generate a matrix of erosion wear on the same surface (B).

Figure 1. Ducom Water Droplet Erosion Tester (WDE) is equipped with a pressure-controlled air blade and matrix modulator (A). Matrix modulator is used to generate a matrix of erosion wear on the same surface (B). 

As Figure 1 shows, the process of erosion over years of rainfall exposure can be grouped into five phases - incubation, acceleration, maximum erosion, deceleration, and steady-state erosion.

These five phases of erosion can be observed in the lab within a few hours by utilizing the “Matrix Modulator” in Ducom WDE. The matrix modulator is a Y-Z stage that enables the user to position the impact of droplets on the surface accurately. This organizes erosive wear in columns and rows.

Example C

Erosion resistant coatings are made up of layers of anti-corrosion primer, polyurethane topcoat, and Clear Coat that protect the airplanes against scratch, erosion, and UV. As airplanes are flexible in mobility, the impact angle of a liquid droplet or rain droplet can vary between 15o to 90o.

Ducom LDE is flexible and can simulate the droplet impact angle with a range of 15o to 90o. As Figure 2 shows, the angle of impact affects wear, and there was severe plastic deformation of the silicone-based clear coat at an attack angle of 60o in comparison to 90o.

Ducom Water Droplet Erosion Tester with test specimen at 90o (A) and 60o (B) to the liquid droplets shot from the droplet generator. The surface topography image shows no signs of wear on the specimen at 90o, where as there is a severe plastic deformation on the same specimen at 60o.

Figure 2. Ducom Water Droplet Erosion Tester with test specimen at 90o (A) and 60o (B) to the liquid droplets shot from the droplet generator. The surface topography image shows no signs of wear on the specimen at 90o, whereas there is a severe plastic deformation on the same specimen at 60o

Example D

Customer satisfaction is now a key factor for obtaining financial success, and the product development team is greatly focused on benchmarking their coatings against their competitors. Reproducibility and repeatability are crucial for product gap analysis and its benchmarking.

Ducom WDE is used to repeatedly simulate the wear mechanisms and establish key differences between the coatings. As Figure 3 shows, the sol-gel based clear coat showed no wear in comparison to PU based clear coat or silicon-based clear coat.

Aerospace grade coatings were tested in Ducom WDE, at droplet velocity of 200 m/s and impact angle at 90o. Clear coat with PU tape was delaminated (A), silicone based clear coat was ruptured (B) and sol-gel based clear coat passed the test without any visible wear (C).

Figure 3. Aerospace-grade coatings were tested in Ducom WDE, at droplet velocity of 200 m/s and impact angle at 90o. Clearcoat with PU tape was delaminated (A), the silicone-based clear coat was ruptured (B) and sol-gel based clear coat passed the test without any visible wear (C).

Wind turbines that harness wind energy need to be protected from rain, as rain and wind are inseparable. Ducom Water Droplet Erosion Tester can allow validation and development of erosion-resistant coatings to protect wind turbines and aircraft against rain and wind.

Note: For solid-solid interaction, Ducom has designed, developed, manufactured, and deployed the Air Jet Erosion Tester. Ducom Air Jet Erosion Tester is widely used by tribologists and material scientists in Australia, India, Italy, Malaysia, Norway, Russia, Spain, Sweden, Switzerland, Turkey, and the USA.

This information has been sourced, reviewed and adapted from materials provided by Ducom.

For more information on this source, please visit Ducom.

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