The disc brake pad underlayer – the undervalued layer

Most brake pads of cars and commercial vehicles have an underlayer between the friction material and steel backplate. This underlayer ensures good bonding strength with the backplate, dampens vibration and noise generated in the braking system, and gives a thermal and moisture barrier. And, depending on the raw materials used in the underlayer, it may save costs.

Despite these key functions, the brake pad underlayer is often undervalued and underappreciated. There has been relatively little work on its development, but its properties are critical to the performance and safety of the whole brake system.

The underlayer is glued to the backplate using an adhesive (see Figure), in most cases phenolic based. 

Like the friction material in the brake pad itself, the underlayer can incorporate Lapinus mineral fibres to improve strength, thermal insulation and damping.

Mineral fibre concentrations in brake pad friction materials are typically between 0% (European low-metal formulations) and 15% (Non-Asbestos Organic brake pad formulations) by weight. In the underlayer, they can be up to 40%.

In OE disc brake pads the cost of the underlayer can be even higher than the friction material. This is to ensure that the combination of the friction material and the backplate give the best possible performance. In aftermarket brake pads the cost of the underlayer is normally lower than the cost of the friction material.

Even when underlayer material costs are lower, there must be no compromise on performance. One of the most important features of the underlayer is to provide sufficient shear strength so that the friction material cannot separate from the backplate. This must hold after years of operation in various conditions: low and high temperatures, different humidity, and in winter often in harsh conditions when roads are salted.