materials are made from a variety of ingredients. These include various
types of fibers for reinforcement and heat management, fillers for
friction control and wear resistance, other ingredients to suppress
noise and resin to bind all of the other ingredients together.
Up to 20
different ingredients may be used to achieve the optimum balance
between the material’s friction coefficients, stopping power, pedal
feel, wear resistance and noise characteristics. As a result, many
brake pads and shoes are marketed according to the ingredients they
contain. These include nonasbestos organic (NAO), ceramic, low-metallic
When the use of asbestos was
discontinued in the U.S. because of concerns over possible health risks
of asbestos dust, NAO friction materials were developed to replace
asbestos. NAO materials contain Kevlar and other kinds of fibers and
generally provide good braking performance and quiet operation (no
brake squeal). NAO is typically used for both “standard” and “economy”
grade replacement linings and for rear linings on most drum brakes, as
well as the pads on front disc brakes in many cars and trucks.
linings are often dark gray or black. The “softer” nature of NAO
linings helps dampen vibrations that can cause noise. But a softer
lining usually lacks the wear resistance of harder materials. Because
of this, NAO linings tend to wear faster, especially at high brake
temperatures. They also give off black brake dust as they wear. f
materials that contain ceramic (silica-based) fibers or particles are
usually referred to as “ceramic” linings. Ceramic brake pads were first
used as original equipment on a few import cars back in 1980s. Today,
nearly 75 percent of all new vehicles come factory-equipped with some
type of ceramic linings. This has created a growing demand for ceramic
replacement linings in the aftermarket, which brake suppliers have
responded to by introducing their own ceramic product lines.
fibers are a good choice for brake linings because they have stable and
predictable friction characteristics. The coefficient of friction
doesn’t drop off as quickly as semi-metallics, nor does it fade as
quickly as NAO as the brakes heat up. This is called “Mu Variability.”
The more stable the friction characteristics are, the more consistent
the brake pedal feels whether the brakes are hot or cold.
no standard definition for what constitutes a ceramic friction
material, so the actual ceramic content can vary a great deal from one
brand of ceramic friction linings to another. The ceramic content may
vary from double-digit percentages to less than a few percent. The size
of the fibers or particles may also range from 0.4 to as much as 80
microns in diameter (smaller is better say some suppliers, but others
Like any friction product, the real-world performance
will vary according to the brand, formulation and application. As a
rule, ceramic linings are marketed as premium grade linings that
provide better braking performance, longer life and quiet operation.
The lighter color of the ceramic friction material also produces less
visible brake dust, so alloy wheels stay cleaner longer.
experts recommend replacing linings with the same linings as before (or
better, in the case of NAO). On vehicles that are originally equipped
with ceramic linings, ceramic replacement linings should be installed.
On vehicles that were originally equipped with NAO linings, ceramic
linings can be used as an upgrade. But ceramic linings are usually not
recommended as a replacement for semi-metallic brake pads in larger,