Friction material Basics: Part 1

What are Friction materials? 

Friction Material is composite material used in brake applications such as brake linings, brake pads and clutch facings used in the automotive vehicles, brake blocks and disc pads used in railways and Sintered brake pads used in aerospace. 

The material is called as friction materials because it is exposed to very high frictional forces. Though it's called as friction material by name, it also exposed to wear and lubrication in addition to friction.

Please refer the disc brake system assembly and disc pads in the below images



 

                             Disc pads                                                Brake linings


Image references: 

6 Ways To Extend The Life Of Your Brake Pads/Brake Shoes (gomechanic.in)

Drum Brakes vs Disc Brakes | Spinny Blog

Brake shoes - Brake shoe - Wikipedia

Friction material composition:

Generally friction material composite contains 5 group of materials




Classification of friction materials products based on composition:


Asbestos type:

Type of materials that contains asbestos fibre, mostly chrysotile type with other materials like fibers, binders, lubricants and abrasives. This type of materials are predominantly used in Heavy Commercial Vehicle (HCV) brake lining because of longer life. The asbestos are naturally occurring silicates available in fibre form which has good thermal, chemical and frictional properties. Most important thing to note about this class of friction materials is the versatility of the properties and that is the reason it is called miracle material or God given material. For example, for a HCV brake lining, the asbestos based formula requires only 5 to 7 raw materials and whereas non- asbestos based formula requires around 12 to 15 raw materials to meet the same functional requirement. 

It was introduced in 60s and gradually usage has been reduced since its carcinogenic and more prone to cause lung cancer and most of the countries have already banned the usage of asbestos in friction materials. Banning of asbestos based friction material was the main thrust for the friction material manufacturers to look into an alternative materials. 

Non-Asbestos Organics (NAO) :

NAO is a type doesn't have metallic materials in the formulation. So, mostly its used in passenger car disc pads where we need lesser brake noise and it is even good for rotors also. So, whenever there is a requirements for lower noise and lesser dust, NAO type is preferred. On the other hand, the NAO doesn't have higher thermal stability and it will fade at lower temperatures compared to other types of friction materials.

Low metallic:

Low metallic type contains metallic materials, mostly steel, of around 5-30 % and its used in Passenger Car (PC) disc pads and Heavy Commercial Vehicles (HCV) disc pads with good frictional performance and longer life compared to NAO type. It has superior performance at higher temperatures compared to lower temperatures. Important thing to focus is to ensure the bonding between the backing plate and the pads because friction pads delaminate when subject to high temperature conditions.

Semi metallic:

Semi metallic type  contains metallic materials, mostly steel, of around 50-60% and mostly used in high friction requirement applications like racing and high performance vehicles. This type material gives good frictional performance, but we need to compromise on NVH and rotor wear.

Woven based:

Woven based materials are mostly used in clutch facing applications. Both the asbestos type and the viscous yarn type with copper fibers is predominantly used. The processing of woven based friction material is different from other type, first, fibers are impregnated in the resin bath and then dried in the oven and then compressed in hot mould.

Paper based:

Paper based materials are used in industrial applications in transmission systems mainly for wet system applications. In this process, cellulose fibers and other additives are made into base paper and this base paper is impregnated with the resin and then cured.

Sintered:

Sintered type materials are used in short distance aircraft brakes where the landing and taxiing is frequent and the racing applications. It contains metal powders with solid lubricants and friction modifiers and most common ingredients are copper, iron, tin, graphite and silicon carbides. Sintering is a process of combining the materials below the matrix melting point.

Carbon-carbon:

Graphitic carbon based fibers are mostly used in aircraft applications. The challenge with the carbon - carbon composites is the long manufacturing time and costly raw material prices. So, commercialization of the product is a big challenge as of now.

Challenges and future scope of friction materials:

       The dynamic changes in the material regulations are the primary challenges for the friction material manufacturers. The ban for asbestos fibers was a big challenge and now OEM's have almost moved out of it with the alternative products in the market. The ban of copper powders in US is also a huge topic for the friction material manufacturing and various options like mineral fibers, conductive graphite and ceramics fibers are potential replacements available. The ban of lead, heavy metals in EU making the manufacturers difficult to meet the REACH compliance. When we discuss on the commercial aspects, uncertainty in raw material prices due to unexpected events like COVID, war and increase in crude prices is a perennial challenge for all the friction manufacturers.

     Eco friendly / green product (Asbestos free/copper free/lead free) is an emerging requirement for which we can expect new products in premium market segments. On the other end, change from engine vehicles to electric vehicles and the usage of electromagnetic or regenerative braking system is expected to have an negative impact on the friction materials market. 


For more queries on the Friction material basics: Part 1 and consultancy services please write to hemapolymercs@gmail.com or comment in this blog.

For further information on this topic on development of friction material, please read friction material basics: Part 2

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