Friction materials are essential components in a wide range of industrial applications. But what do we mean when we talk about “friction” materials? Basically, these are materials designed to generate a controlled resistance force, known as friction, between surfaces in relative contact. This force is essential to control movement, transmit power and ensure safety in various mechanical systems. Imagine a world without brakes in cars, clutches in industrial machinery, or landing systems in airplanes. Better with friction, right?
However, nowadays there is a wide variety of friction materials, so the one we need for each application must be properly selected, taking into account its fundamental properties:
- Coefficient of friction: This property is key, as it indicates the relationship between frictional force and normal force. A high coefficient is essential in braking systems, where the aim is to stop movement efficiently. On the other hand, a low and stable coefficient is desirable in clutches and transmissions, where energy losses due to friction are to be minimized.
- Wear resistance: Wear, an unavoidable phenomenon in systems subject to friction, results in the gradual loss of material from the surfaces in contact. A friction material with high wear resistance will extend the life of the component, reducing maintenance costs and improving system reliability. Think about your car’s brake pads: The more resistant they are to wear and tear, the less often you’ll need to replace them.
- Thermal stability: Friction generates heat, and a friction material must maintain its mechanical and tribological properties (friction, wear, and lubrication) even at high temperatures. This property is crucial in high-performance applications, such as racing brakes or aircraft braking systems, where temperatures can reach critical levels that could compromise performance and safety if the material is not stable.
- NVH (Noise, Vibration and Harshness): The generation of noise, vibration, and harshness during braking or power transmission can significantly affect user comfort and experience. Friction materials should be designed to minimize NVH, especially in automotive and transportation applications.
In addition, corrosion resistance and compatibility with other system components, such as brake discs or drums, must be taken into account to prevent damage and ensure optimal performance. Compatibility also involves considering aspects such as heat transfer and the generation of noise and vibrations, which can affect the efficiency and comfort of the system.
Types of friction materials
Friction materials are mixtures of various materials, fundamentally classified as:
- Binders: Mainly phenolic resins. Its function is to hold all the ingredients together. Its high-temperature behaviour is critical to prevent “fading” (loss of brake efficiency due to overheating).
- Reinforcing Fibers: They provide structural integrity and mechanical strength. Examples: aramid fibers, glass, cellulose, and the steel fibers mentioned.
- Fillers and Fillers: Materials such as barite, aluminum oxide or barium sulfate. They are used to improve processability, reduce cost, and provide specific properties such as compressive strength.
- Friction Modifiers: The most “secret” part of the formulation. They are a set of abrasives (hard) and solid (soft) lubricants that control the coefficient of friction, wear, and NVH. Examples: petroleum coke, graphite, antimony sulfide (although its use is in decline due to regulations).
Industrial applications are becoming more diverse, which in turn has driven the development of a wide variety of friction materials, each with specific characteristics for different situations. What major typologies can we find?
Organic Friction Materials (Resin Matrix)
With asbestos:
Mention its prohibition and historical legacy.
Non-Asbestos Organic (NAO): This is the top category today.
- NAO (proper): Formulations with low or no steel fiber content, using aramid fibers, glass, cellulose, and mineral fillers. They are usually less aggressive with the disc and have good NVH (Noise, Vibration and Roughness).
- Low-Steel / Low-Met: They contain between 10% and 30% steel fibers or steel wool. They offer better heat transfer and greater “bite” at the cost of increased disc wear and NVH potential.
- Semi-Metallic: They contain a high percentage of steel (typically >50%). Excellent durability and high-temperature performance, but can be noisier and more aggressive with the back side.
Metal Friction Materials (Metal Matrix)
Sintered
Correctly described as compacted and sintered metal powders (copper, bronze, iron). They are ideal for extreme applications due to their very high thermal and wear resistance.
Advanced Inorganic Friction Materials
Ceramic Matrix Composites (CMCs)
They are distinct from NAO materials that use “ceramic fiber.” True ceramics, such as Carbon-Ceramics, are ceramic matrix composites. Their description as “inorganic” is correct, but they deserve a category of their own for their performance and cost.
Carbon/Carbon (C/C)
Correctly identified for extreme applications such as aeronautics and competition.
Industrial applications of friction materials
Depending on the environment and application, different types of friction materials are used, each designed to withstand specific conditions of use, such as temperature, pressure, and wear. Which are the most common?
Automotive industry
In the automotive industry, friction materials are essential to ensure the proper functioning of braking and clutch systems. Disc and drum brakes, clutches, pads, and brake bands are critical components in vehicles, from cars to heavy-duty trucks.
Disc brakes, for example, require materials that withstand high temperatures and offer stable performance even in extreme situations, such as emergency braking or during prolonged use on slopes. In clutches, friction materials ensure a smooth and effective coupling between the moving parts, improving the transmission of power from the engine to the wheels.
Railway industry
In the railway sector, friction materials play a vital role in the safe and effective braking of trains, especially high-speed trains. Brake shoes and brake pads for trains must be able to stop huge masses at impressive speeds, which requires materials with high wear resistance and superior heat dissipation capacity.
In addition, clutch systems also rely on these materials to ensure reliable power transmission in all types of conditions, including the most extreme.
Aerospace
In the aerospace industry, friction materials play a key role in aircraft landing and braking systems. Carbon brakes, for example, are used in commercial and military aircraft due to their ability to withstand extremely high temperatures without losing efficiency. Not only must these brakes stop multi-ton aircraft at very high speeds, but they must also be light enough not to compromise the aircraft’s performance.
In helicopters, clutch discs must ensure precise and safe operation during critical flight and landing maneuvers.
Heavy machinery
Heavy machines, such as excavators, cranes, and other construction or mining equipment, also rely on friction materials to operate safely and efficiently. The disc and drum brakes used in this machinery must be robust and durable, capable of withstanding the harsh conditions of the terrain and heavy loads.
In addition, clutches in this machinery are essential for transferring power from the engine to the motion systems, and the friction materials that compose them must withstand the constant wear and heat generated during long-duration operations.
Other applications
In addition to the industries mentioned, friction materials are also used in a variety of other applications. In elevators, for example, they ensure smooth and safe braking. On conveyor belts, friction systems help control the speed and movement of products. Even in sports equipment, such as bicycles and gym machinery, friction materials enable efficient braking and safe control. Also in textile machinery, where precise movement is key, these materials ensure continuous and reliable operation.
Benefits of friction materials
Durability and resistance
High-quality friction materials, such as ceramic or sintered metal, are designed to withstand demanding operating conditions, characterized by high temperatures, pressures, and speeds. Its exceptional resistance to wear and thermal deformation ensures long service life, reducing the need for frequent replacements and minimizing maintenance costs. This translates into long-term savings and increased system reliability.
Cost-Efficiency
While some advanced friction materials may have a higher initial cost, their increased durability and long-term performance can lead to significant savings in operating and maintenance costs. Choosing the right material should consider not only the initial price, but also the total cost of ownership over its lifetime. In many cases, investing in a high-quality friction material can be more economical in the long run, thanks to reduced unplanned downtime and replacement costs.
Versatility and design
The wide range of friction materials available, each with its own unique properties, allows them to be adapted to a wide variety of designs and specific requirements. This gives engineers the flexibility to optimize the performance, efficiency and safety of systems, tailoring them to the particular needs of each application, from high-power brakes for racing vehicles to smooth, quiet clutches for industrial machinery.
Innovation and new trends in friction materials
Innovations and Future Trends in Friction Materials
The world of friction materials is constantly evolving, driven by three major forces: the electrification of transport, environmental sustainability and the relentless pursuit of superior performance. Below, we explore the most important trends that are defining the future of braking systems.
1. The Electric Vehicle (EV) Challenge: Beyond Regenerative Braking
Electric vehicles have introduced a new dynamic in the use of brakes. Thanks to regenerative braking, where the electric motor acts as a generator to recover energy and slow the vehicle down, traditional friction brakes are used much less frequently. While this extends the life of pads and discs, it has uncovered a new technical challenge: corrosion.
When not used regularly, ambient humidity and external agents (such as salt in winter) remain on the surface of the disc and tablet, causing rapid oxidation. This oxide layer can cause:
- Noise and Vibration (NVH): A noticeable increase in squeaks and harshness, especially during the first braking after a period of inactivity.
- Inconsistent Answer: On the first application of the brake, the oxide layer must be removed by friction, which can result in a poor or uneven initial braking sensation until the surface is clean.
To combat this problem, the industry is actively developing pad formulations with increased corrosion resistance and brake discs with advanced protective coatings (such as zinc-aluminum coatings or nitriding technologies) that keep surfaces clean and ready to perform at all times.
2. Sustainability and Regulations: The Copper-Free Era
Environmental awareness has focused on the composition of the brake pads. For decades, copper has been a key ingredient for its excellent ability to manage heat and stabilize the coefficient of friction. However, the dust worn from the tablets, which contains copper particles, is carried by rainwater into rivers and oceans, where it is toxic to aquatic life.
In response, pioneering regulations such as the “Better Brake Rule”, implemented in the states of Washington and California (USA), have forced the progressive elimination of copper in friction formulations. This legislation has acted as a global innovation engine, forcing manufacturers around the world to develop new “Copper-Free” materials. The challenge is to find alternative material combinations that can replicate the valuable properties of copper without compromising safety, performance or comfort.
3. Advances in Manufacturing: “Positive Molding”
The way a brake pad is made is just as important as the materials that make it up. The dominant trend in high-quality manufacturing is the positive molding process.
Unlike older methods, this process uses a precisely pre-measured amount of friction mixture, which is fed into a mold and subjected to extremely high pressure and temperature. This method ensures that the entire pickup is compressed with an even density from the center to the edges. The result is a much more consistent end product, which directly translates into predictable and stable braking performance throughout the life of the pad, eliminating variations that could occur with less controlled molding processes.
Technical Considerations of Friction Materials
The choice of a suitable friction material depends not only on its physical or chemical properties, but also on compliance with a number of international regulations and standards. These regulations not only seek to ensure the safety and performance of products, but also facilitate global trade and ensure the quality of components used in various industries.
To ensure that friction materials meet the necessary requirements in terms of safety, quality and performance, different international organizations have established specific regulations and standards. Among the main ones are SAE (Society of Automotive Engineers), ISO (International Organization for Standardization) and JASO (Japanese Automotive Standards Organization).
For example, braking performance is often evaluated following the SAE J2522 standard (known as AK Master), which defines a series of braking at different speeds and temperatures to characterize the coefficient of friction, temperature sensitivity, and fading.
In terms of particulate emissions, the new Euro 7 standard and WLTP (Worldwide Harmonized Light Vehicles Test Procedure) procedures are imposing strict limits on PM10 and PM2.5 particles generated by brakes, driving the development of low-emission materials.
Durability is measured in tests such as the SAE J2707, which simulates wear under taxi fleet conditions to predict material life.
Safety: Preventing accidents
One of the main objectives of the regulations is to ensure the safety of systems in which friction materials are used. A faulty or low-quality component can lead to serious failures in systems such as brakes, clutches, and other key mechanisms, which could trigger fatal accidents, especially in critical industries such as automotive, aeronautics, and rail. Safety tests, such as heat and pressure resistance, help prevent these risks, ensuring that friction materials maintain their integrity under extreme conditions.
Performance: Ensure proper braking, wear, and noise
Performance is another critical aspect that these regulations seek to control. Brakes, for example, must be able to stop a vehicle or machine efficiently and consistently, regardless of external conditions. Regulations such as SAE or JASO include wear and noise tests to ensure that friction materials not only offer reliable braking, but also minimize noise and maintain their effectiveness over time. These tests also verify the material’s ability to handle high temperatures without losing performance, which is essential in applications such as high-speed brakes or aircraft landing systems.
Quality: Promote durable and reliable products
Durability and reliability are essential for friction materials, particularly in industrial applications where downtime for maintenance or replacement can be extremely costly. Quality standards set by organizations such as ISO ensure that products not only meet technical specifications but also offer a long service life under conditions of heavy use. Fatigue and mechanical strength testing are critical to ensure that friction materials maintain their properties throughout their operational life.
Trade: Facilitating the global sale of friction materials
International regulations and standards also play an important role in the global trade in friction materials. By complying with regulations such as ISO or SAE, manufacturers can ensure that their products are accepted in markets around the world. This makes it easy to export and ensures that customers in different regions receive quality products that meet the same levels of safety and performance, regardless of their origin. Complying with these regulations also allows manufacturers to obtain certifications that increase customer confidence and the reputation of their products in the global market.
In short, friction materials are essential components in a wide range of industries, from automotive to aerospace to heavy machinery. Their ability to withstand high temperatures, pressure and wear makes them indispensable in braking and clutch systems, guaranteeing the safety and efficiency of the equipment. In addition, it is a sector in constant evolution, due to its growing applications.
FAQ
What are friction materials and why are they important?
Friction materials are those designed to generate a controlled resistance force, called friction, between two surfaces in contact. This friction is essential for controlling motion, transmitting power, and ensuring safety in countless applications.
Their importance lies in the fact that without them, it would be impossible to brake a vehicle, transmit the power of an engine to the wheels or control the movement of an industrial machine. In short, friction materials are critical to the safe and efficient operation of many mechanical systems we use every day.
What are the advantages of organic materials compared to metallics?
- Cost: Organic materials are generally cheaper than metallics, making them an attractive option for applications where budget is a determining factor.
- Adaptability: Organic materials typically exhibit a more stable coefficient of friction over a wider range of temperatures and operating conditions. This makes them more versatile and able to adapt to various situations, from cold start to intense braking.
- Comfort (NVH): They tend to generate less noise, vibration, and harshness during braking, which significantly improves comfort and user experience, especially in passenger vehicles.
- Less wear on the back side: Organic materials are usually less aggressive with contact surfaces (brake discs or drums), which reduces their wear and extends their service life, contributing to a lower maintenance cost.
- Good cold performance: They show good braking performance even at low temperatures, which is crucial in situations where the braking system has not reached its optimum operating temperature, such as in cold start or cold weather.
How to select the right friction material for a specific application?
Choosing the ideal friction material depends on several key factors:
- Operating conditions: It is critical to consider the temperature, pressure, speed, and environment in which the system will operate. For example, high-temperature applications will require materials with excellent thermal stability, such as ceramics.
- Performance Requirements: The coefficient of friction, wear resistance, NVH and other properties must meet the specific requirements of the application.
- Cost: The initial cost of the material and its useful life must be considered to evaluate the total cost of ownership.
- Compatibility: The material must be compatible with the other components of the system to prevent damage and ensure optimal performance.
- Regulations and standards: It is essential to comply with the regulations and safety and quality standards applicable to the industry and the specific application.
What recent innovations have improved the performance of friction materials?
The friction materials industry faces challenges such as stricter regulations on chemicals and the need to adapt to new electric vehicle powertrains. Innovation is key to developing more sustainable and efficient materials. An important trend is the use of bio-based and recycled materials, which reduce environmental impact and promote the circular economy.
What are the key considerations for the implementation of friction materials in industrial projects?
In addition to choosing the right material, implementing friction materials in industrial projects requires considering:
- Design and sizing: Adapt the system design to material properties, loads, and usage conditions.
- Manufacturing and processing: Use specific techniques for each material and achieve the desired properties.
- Maintenance and replacement: Inspect and replace components periodically to ensure safety and performance.
- Environmental impact: Assess the material lifecycle and prioritize sustainable options and clean manufacturing processes.
