Combating CMAS Attack on Thermal Barrier Coatings

As the most immediate and devastating effects of COVID are beginning to subside corporations and private individuals around the world are seeking to move away from virtual interactions to in-person interactions.

What this means for the airline and aerospace industry is more planes flying to more destinations than in 2020 or 2021 — many of which are known for adverse environmental conditions that can wreak havoc upon aerospace components.

Ensuring that critical components can withstand abusive operating conditions is the job of Innovative Test Solutions.

One such instance of this is how molten calcium-magnesium-alumino-silicate (CMAS) interacts with thermal barrier coatings (TBC), leading to TBC damage and degradation.

The TBC is a critical element that allows gas turbine engines to operate at temperatures as high as 1200 degrees Celsius. Under these conditions, CMAS attacks become a factor in TBC failure.

In turn, it can lead to lower strain tolerance and potentially catastrophic equipment failure that can significantly affect aerospace components. Ensuring that critical components can withstand abusive operating conditions is the job of ITS.

CMAS Attacks: A Significant Factor in Thermal Barrier Coating Damage

Much of the advancement in aerospace technology over the past 30 years can be attributed to lessons learned during the conflicts in the Persian Gulf and the ongoing Global War on Terror.

Operations were conducted in some of the most environmentally inhospitable regions around the world. Extreme conditions placed great stress on airframes and vital engine components susceptible to foreign object damage and premature wear from airborne materials sucked into engine intakes.

As the aerospace industry seeks to make gas turbine engines more efficient and powerful through greater thermodynamic efficiency to reduce pollution and improve fuel economy, a corresponding increase in engine operating temperatures is occurring. One of the greatest enemies to any component or mechanical system is heat, which can cause a breakdown of alloys, leading to warping, cracking, and ultimately life-threatening failures.

During the military operations of the past three decades, military aircraft experienced a phenomenon in which jet engines created so much heat that fine particulates in the air melted, coated, and penetrated turbine engine vanes with molten particulates. Sand proved to be one of the most common particulates in these operating environments. When exposed to highly heated engine components, a CMAS attack occurs due to superheated silica and molten materials penetrating microscopic fissures in turbine engine vanes. Under the best of circumstances, this results in increased maintenance costs. In the worst cases, it can cause a catastrophic failure.

CMAS Erosion: Beyond the Military

Until recently, CMAS attack and erosion concerns were largely relegated to military transports and fighter jets operating from remote and austere bases where environmental conditions subject airframes and components to enhanced stress. With the ever-evolving world economy, including the emergence of economic powers and their interests in Asia, Africa, and the Middle East, the world is experiencing an increase in commercial flights of all varieties to these locations.

This puts increased stress on airline and transport companies who are contending with increased fleet maintenance and decreased service life. As a result, it is imperative to identify higher-performing TBCs to protect engines from failure and the associated loss of life and property.

TBCs are highly advanced ceramics that help protect against erosion and foreign object damage when applied to superalloys. These specialized coatings also provide thermal stability while simultaneously reducing thermal conductivity. In turn, TBCs provide greater fuel efficiency and decreased pollution for aircraft of all types.

Military aircraft typically experience failure from short exposure to high concentrations of particulates. The commercial air transport and airline industries face the challenge of exposure to lower concentrations of particulates over a longer duration. The result of these two different types of exposures is the same CMAS attack phenomenon outlined above.

Many of the markets now being served in places like Africa, Asia, and the Middle East are all undergoing their own industrial revolution.

Local atmospheres frequently contain an increased volume of particulates that over an extended exposure window are likely to result in this type of CMAS attack. To address this concern, significant investment has taken place in rapid research and development of advanced thermal barrier coatings.

Benefits of CMAS Erosion Testing

Jet engines are the most expensive single component of an airliner. For commercial jet engines, typical costs range from $7 to $8 million for an Airbus A320 engine to as much as $12 million per engine for a Boeing 747.

Given the prices, airlines and air transportation companies have reason to be concerned about the increased cost of maintenance associated with a CMAS attack.

The research and testing of TBCs will help prevent the loss of life and aircraft. It can also protect the profit margins for an industry that is under increasing financial pressure from skyrocketing energy costs and a worldwide economic slowdown. Now more than ever, investments in prevention could pay substantial dividends in the long-term.

How CMAS Erosion Testing Works

To determine the effectiveness of a particular TBC, a properly designed test rig must be engineered to expose the TBC materials to constant and consistent jet blasts. When done correctly, this testing can determine the level of CMAS erosion that the TBC material in question prevented.

The effectiveness of the TBC is determined by the cold temperature of the substrate beneath the TBC material.

Typically, TBC coatings are tested to a surface temperature between 700 and 1600 degrees Celsius. ITS, however, has created test rigs that allow for jet blasts as high as 1700 degrees Celsius.

ITS CMAS erosion testing frequently exposes test materials to this level of extreme heat for a predetermined length of time as required. These Jet Engine Test (JETS) rigs have also been modified to allow the introduction of CMAS into hot gas stream.

The axial introduction of sand particulates allows near identical testing of the actual environment these coatings will face in commercial use. If the substrate superalloy maintains a cool temperature during such extreme exposure, it indicates that the TBC is providing the heat resistance necessary to prevent a CMAS attack. Data like this is essential for preventing CMAS-related TBC damage and failure.

The utility of thermal barrier and other ceramic coatings extends far beyond the aerospace industry. It can play a vital role in automotive manufacturing, oil and gas exploration and extraction, power generation, medical devices and prosthetics, and industrial applications, including the manufacture of steel. The advanced ceramics contained in TBCs offer a number of benefits, including resistance to heat, low electrical and thermal conductivity, and corrosion and wear resistance, including dry lubrication as well as thermal shock resistance. Perhaps the most important benefit is the ability to economically apply the coating precisely across a non-even surface.

As material development further advances and manufacturing depends increasingly on superalloys and other revolutionary materials, it is critical to thoroughly test them using the most demanding and exacting standards to safeguard lives, the environment, and property. TBCs are a constantly improving technology that require evolving testing as we seek to push the limits of performance for the benefit of mankind and the environment. Innovative Test Solutions is setting the standard for TBC testing today.

Learn more about JETS testing by contacting the ITS team with any questions.

Our engineering and testing laboratory specializes in the mechanical behavior of structures and structural material with a particular emphasis in the areas of TBCs, vibration testing, fatigue testing, fracture mechanics, creep/rupture, in addition to friction and wear testing.

In addition to maintaining ASTM test protocols, ITS has the ability to develop custom, one-of-a-kind test rigs coupled with advanced analytical testing procedures that allow the properties of various metal alloys and composites to be evaluated.