Thin Layers with a Big Impact

When about to board a flight, we often focus on the impressive aircraft that flies us to our destination. Yet the unsung hero that helps ensure a safe and speedy journey is not as apparent. The turbine engines that propel the aircraft through the sky at speeds of up to 600mph endure punishing conditions. As a result, it’s critical that these engines are protected with the functional coatings necessary for optimum performance. Plasma spray coating technology, particularly cascading arc plasma, provides these components with the protection needed to function at the highest efficiency and lowest fuel consumption – especially important in these uncertain economic times.

A Little History
Plasma spray is a proven materials processing technique for producing protective and surface-enhancement coatings using a plasma jet. When first introduced in the early 1960s, plasma spray was celebrated as an innovative method of applying coatings of ceramic and other high-temperature melting materials. Hailed for its ability to use a wide range of feedstock materials with higher melting temperatures, such as alloys and ceramics, the plasma spray gun gained rapid success for critical coating applications in the power generation, commercial aviation, military, and other industries.

Today, plasma spray is one of the most versatile thermal coating processes available, producing high-performance coatings with superior durability and reliability – and it has been improved upon. The plasma spray process uses a combination of heat and velocity to create a coating from fine, powdered material feedstock. An arc forms between the anode and cathode, where the gas flows and becomes ionized (stripped of electrons). This creates a plasma plume into which powdered spray material is injected, melted, and accelerated toward the substrate surface.

Plasma spray has further evolved into cascading arc plasma. With its enhanced efficiency, cascading arc plasma saves costs when applying coating materials to gas turbine and airframe components, engine and drive train components, hydroelectric and steam turbine components, and paper manufacturing rolls, to name but a few. Compared to traditional plasma spray, cascading arc plasma’s reliability allows for tighter control of coating process windows, which is important for critical components and long spray runs. In particular, this technology benefits the aerospace industry, making parts last longer and function more efficiently.
 

A Comparison
Because of arc instabilities, conventional plasma spray guns are prone to producing varying temperatures within the plasma jet. However, cascading arc plasma guns are able to provide a uniform heating platform with steady temperatures. The cascade fixes the electric arc length, which provides it with a starting path over a series of electrically neutral rings within the arc chamber.

Once the gun is ignited, only the common front anode, or nozzle, is electrically connected to the power supply. The fixed arc length has the advantage of stabilizing the plasma plume and eliminating the extreme amplitude power oscillations (in the 3kHz to 5kHz range) that result from an unstable arc.

As a result, cascading arc plasma generates homogenous ionization, creating a consistent coating for a part. An everyday example is the simplicity and consistency of baking bread in a well-controlled oven, where a faulty oven with fluctuating temperatures would produce variable outcomes. Traditional plasma’s inconsistent coating also limits the amount of material that can be injected into the flame in a given period of time. As a result, cascading arc plasma has a higher efficiency than traditional plasma coatings with improvements generally measuring between 60% and 400%.

The uniform heating platform leads to higher coating throughput and productivity, or higher volumes of coating material applied per unit of time. Cascading arc plasma’s consistent stream of gas heats a greater percentage of powder compared to traditional plasma, and this allows the powder to be injected at a higher rate. With more material landing on the part, manufacturers achieve an overall reduction in material costs and processing time.

The amplitude and frequency of the oscillation is directly linked to the heating and flight path of each powder particle. As a result, the stable plume produced by cascading arc plasma guns significantly increases deposit efficiency and the consistent quality of the coating. In addition, this also creates a highly reproducible coating process, making it easier for the technician to achieve uniform results.

The process window for cascading arc plasma guns remains comparatively stable for a longer period of time, enabling extended spray runs and reduced maintenance. Due to higher process drift, which is an indication that gun parts need to be replaced, conventional plasma guns last 15 to 20 hours before needing maintenance. However, the stable arc in cascading arc plasma guns causes less damage within the gun, allows the parts to last longer, and creates a genuine, repeatable production process. Ultimately, the increased productivity depends upon the materials being sprayed.

Cascading arc plasma guns have a greater material feedstock utilization, which reduces the time and cost associated with spraying a part. Just 10 years ago, a customer may have needed several different guns to accommodate the wide range of applications. Now, a single cascading arc plasma gun, such as the TriplexPro, can cover all applications except for internal spray processes.

Also available are single-cathode guns, such as Sulzer Metco’s SinplexPro, that can be integrated into most existing plasma spray systems. The difference is that while the TriplexPro functions like a stove top that heats over a broad area, the SinplexPro is more like a Bunsen burner, with one localized heat spot.

Cascading arc plasma spray produces abradable and thermal barrier coatings that reduce fuel consumption and emissions. While coatings are generally from 0.003" to 0.02" thick, thicker coatings can be up to 0.08", causing the conventional plasma process to take 12 to 16 hours to spray the parts. However, cascading arc plasma guns are capable of long nonstop spray runs that reduce the application time to 4 to 8 hours.

Traditional plasma spray can tailor the coating thickness to suit the application and is able to cover complex geometries, such as turbine engines components. The technology also has a variety of available coating chemistries suitable for different operating conditions. However, cascading arc plasma has the added benefit of saving time, labor, gas, and electrical costs associated with shorter spray runs.

Evolving for Simplicity
Like most new technologies, cascading arc guns have progressed since first debuting on the market, with current versions functioning far more optimally than the first. A common lingering false perception is that cascading arc plasma guns are overly complex because they are composed of more elements. In reality, the guns have evolved for simplified maintenance. For example, Sulzer Metco’s TriplexPro cascading arc plasma gun often lasts for 200 hours before needing maintenance. Similar to other plasma guns, cascading arc plasma guns require only one power supply. Additionally, cascading arc plasma technology now has the flexibility to run on hydrogen gas.

As plasma spray technology continues to evolve, its applicability will broaden. Above all, there is no substitute for industry experience combined with thorough application know-how. To achieve the highest success, it is critical that users work with material and equipment suppliers that are knowledgeable about cascading arc plasma technologies to ensure a durable part that can be coated efficiently and cost-effectively.

Sulzer Metco Inc.
Westbury, N.Y.
www.sulzer.com

About the authors: Steven Ort is director of equipment product line management at Sulzer Metco (steven.ort@sulzer.com) and Dave Hawley is Sulzer Metco’s director of research and development (david.hawley@sulzer.com).