Brake Pads: Understanding Friction And Formulations

Brake Pads: Understanding Friction And Formulations

How the components in the friction material shear, break and interact during braking can determine a pad's friction level, noise and wear characteristics.

Brake pad arrangementFriction is the force resisting the relative motion of elements sliding against each other. In the case of cars and trucks, it is the brake pads pushing against a rotor that changes kinetic energy into heat.

If you could mount a microscope on a brake pad, you would see bits and pieces of the pad and rotor breaking away from the surfaces as they contacted the rotor. As this is happening, the heat is physically and chemically changing the exposed friction material, and bits and pieces are being torn or sheared from the rotor and pad.

Some particles become part of the friction surface of the rotor, while others are cast off to stick to wheels and eventually be washed down the drain, potentially winding up in rivers and streams. 

 The bottom line is that for the brakes to function, the rotors and pads have to wear. Even a brake rotor’s metallurgy can determine how a pad wears.

Brake pad manufacturingTwo Types of Friction

So friction is friction right? Wrong. There are two types of friction when it comes to brakes.

Abrasive friction is the breaking of bonds of both the pad material and the cast iron of the disc when the caliper pushes them together. Adherent (or adhesive) pad material forms a very thin transfer layer of pad material on the surface of the rotor. The two surfaces are the same materials and generate friction by breaking or shearing the bonds in the pad.

Abrasive friction is the wearing of the pad and rotor to change forward motion into heat. Both components wear. Semi-met pads and some non-asbestos organics (NAO) use this type of friction.

Brake pad manufacturingAdherent (or adhesive) pad material transfers a very thin layer of pad material onto the surface of the rotor. Ceramic and some NAO pads use this type of friction. The transfer layer is bonded to the rotor’s surface and cannot be washed away by water or wheel cleaners. The only way to remove it is by removing it with a brake lathe or abnormal heat.

The layer is always being worn and replenished by the brake pad during braking. These pads produce dust. Adherent friction is easier on rotors, but the pads become the primary wear component. 

With this type of pad, it is critical to machine the rotor with the correct surface finish and follow the recommended break-in procedure so the transfer layer can be established. 

With both types of friction, it is critical for the rotor to have minimal runout. Abrasive friction materials will wear away at high spots, creating disc thickness variation and pulsation. Adhesive or adherent friction material could deposit the friction material unevenly and cause brake judder. 

Brake pad manufacturingThe Secret Sauce of Friction

How the components in the friction material shear, break and interact during braking can determine a pad’s friction level, noise and wear characteristics.

A brake pad may require up to 20 different raw materials. Some raw components of a friction material are abrasive, while other components lubricate. Some components, like structural fibers and resins, hold the pad together, while other components tune the friction levels through various temperature ranges.

Tuning the components in a brake pad mix is like tuning a graphic equalizer on a stereo for the best sound. This is the secret art of friction material formulation and why some pad manufacturers protect their recipes like they’re Coke’s formula or KFC’s seven secret herbs and spices.

Ceramic

Ceramic pads’ structural properties are very stable under high temperatures, much like CorningWare cookware, but that is where the comparison ends. Ceramic materials that go into a brake pad are very small strands that are engineered to be a certain length and width.

There are three advantages of ceramic pads in certain applications. First, since the ceramic materials offer stable performance under a wide range of temperatures, they can offer quiet performance. Second, ceramic brake pads manage heat in the caliper better on some vehicles than some non-ceramic applications. Third, ceramic brake dust doesn’t show up on, or stick to, wheels like some other brake pad formulations. This could be an important factor if you have an expensive set of custom wheels.

Brake pad and rotor setNAO

Non-asbestos organic (NAO) friction materials typically wear more than harder semi-metallic compounds. Generalizing about the wear characteristics of NAO and ceramic-based compounds is difficult because there are so many variations. Wear varies depending on the formula the friction supplier chooses for a particular application. Different vehicles require different coefficients of friction, so formulas are often “application engineered” to deliver the best combination of stopping power, wear resistance, pedal feel and noise control. Most premium-quality NAO and ceramic-based linings will provide long life and wear less than an equivalent set of NAO pads on the same application.

Semi-Metallic

Semi-metallic pads use metal fibers to give structure and provide friction. The metals used are typically high-quality steel, copper and other exotic metals.

The other part of the semi-metallic mix is comprised of a variety of materials like glues, lubricants and structural fibers. The manufacturer will blend the components together to give the best performance for that application.

Hybrids

Some friction manufacturers are picking and choosing the best properties of ceramic, NAO and semi-met to make hybrid brake pads that are quiet, but still have aggressive performance. Some of these pads are problem-solvers for known noise-prone vehicles.

A Word On Dust

Many ceramic brake pads claim to have “low dusting” formulation. But, how is this accomplished? The materials found in some ceramic brake pads are more compatible with today’s popular aluminum, open-wheel designs.

Also, some manufacturers have carefully chosen raw materials that are also used to help eliminate the appearance of dusting. The color of the material is a light gray, so it is less visible on wheels (unlike some NAO pad materials that produce a dark brown or black dust that clings to wheels).

Ceramic particles are less abrasive to the rotor surfaces. In addition, some ceramic pads tend to contact the rotor in a more adhesive fashion to slow the wheel, as compared to semi-metallic and other pads that are much more abrasive.

The abrasive action wears away at the rotor and creates rotor dust that can stick to the wheel. This approach can reduce rotor wear by as much as 80%, increasing rotor life by as much as 400% versus semi-metallic pads on the same application.

Every brake manufacturer uses its own ceramic-based or ceramic-enhanced compounds. The type of ceramics used, the particle size, distribution, hardness and other ingredients that go into a ceramic type of friction material can all vary, even from one vehicle application to another. So, ceramics should not be stereotyped or viewed as some type of generic product.

You May Also Like

Charging More for Brake Jobs

Here’s why charging more for brake jobs keeps customers coming back.

Why is everyone chasing the cheapest brake service? It often seems that new-car dealers, chains and independent shops are locked in a race to the bottom when it comes to brake service. 

Some shops see brake jobs as the bottomless breadsticks or happy hour specials that get people in the door so that they spend money on entrees. The difference is a consumer has never needed to use an appetizer to keep their family safe. 

Tapered Wheel Bearings

Here’s what you should know as a technician when servicing tapered wheel bearings.

Understanding Passive Wheel Speed Sensor Operation

Passive types of wheel speed sensors are still used in many applications so understanding their operation is important.

Threadlocking Compounds For Vehicle Corners

To get the most out of these “liquid” tools, you first need to know how they work.

How Regenerative Brakes Operate

Regenerative braking is a hybrid’s first choice for braking.

Other Posts

Replacing Master Cylinders

The most common problems that occur in the master cylinder are wear in the piston bore and piston seal failure.

Akebono Expands Severe Duty Disc Brake Pad Kits

Akebono said it expanded its severe-duty ultra-premium disc brake pad line by 14 new part numbers.

Benefits of Using Brake Parts Designed to Work Together

When replacing brake system components, it might seem like using compatible parts from any manufacturer – regardless of brand – should get the job done. However, for the safest brake job with the best performance, it’s always best to use parts from the same manufacturer, like ADVICS, where our brake pads, brake rotors, hydraulics, calipers

Ceramic vs. Semi-Metallic Brake Pads: How They Work and What They’re Made Of

Your customers rely on their vehicles to get them safely from point A to point B, and they also rely on their brake pads and brake rotors to provide reliable stops each and every time. So how does the vehicle’s friction system work and what role do disc brake pads play? Once you understand how