Friction Material Testing

Friction material inspection mainly conducts scientific testing and analysis on the friction coefficient testing, wear rate testing, physical property testing, thermophysical property testing, noise and vibration testing, and fade and recovery property testing, objectively assessing the product quality grade and issuing a detailed test report, providing a basis for manufacturers' quality control and downstream users' product acceptance.

Testing Items

Friction coefficient testing: The dynamic friction coefficients and static friction coefficients of the material under the different temperatures, speeds and pressures are determined. It is the core indicator of evaluating the braking effectiveness of the friction material. Through the analysis of the friction coefficient-temperature curves, the thermal stability and anti-thermal-fade properties of the material are evaluated, providing the key data for the product design and working condition adaptation.

Wear rate testing: Quantifies the volume or mass losses of the friction material under the specific working conditions, usually expressed as the wear amounts per unit friction work. This testing is used to evaluate the durability and service lives of the material, and to analyse the matching with the mating parts (such as the brake discs). It is the important basis of measuring the economy and reliability of the material.

Physical property testing: Covering the material density, hardness (Rockwell or Brinell), impact strength, shear strength and compression modulus, etc. These basic physical parameters directly affect the machining performance, assembly qualities of the material, as well as its compression resistance and shear load-bearing capacities during the braking processes.

Thermophysical property testing: Mainly including the determination of the thermal expansion coefficient, thermal conductivity and specific heat capacity. These parameters are related to the heat transfer, distribution and thermal stress states of the friction pairs during the working, and are crucial for preventing the thermal cracks, thermal judder and thermal fade phenomena of the braking systems.

Noise and vibration testing: Through the bench testing or real vehicle road testing, the noise (such as the low-frequency moan and high-frequency squeal) and vibration signals generated during the braking processes are collected and analysed. This item aims to evaluate the NVH performance of the material, and to provide the basis for optimizing the formulations and structures to suppress the abnormal braking noises.

Fade and recovery property testing: Simulates the harsh working conditions such as the continuous or high-speed braking, examining the degrees of the friction coefficient decrease (thermal fade) with the temperature rise, as well as the abilities to recover the initial performances (recovery) after the cooling. It is the key testing of evaluating the high-temperature stability of the material.

Material composition and structure analysis: The means such as the spectroscopy and electron microscopy are used for the qualitative and quantitative analysis of the chemical compositions, fiber/filler distributions, porosity and interface bonding states of the friction material, establishing the relationships between the micro-structures and macro properties, used for the quality control and formulation research and development.

Testing Scope

Automotive brake friction materials: Including the disc brake pads, drum brake shoes and their linings. The testing needs to cover the differentiated requirements of the different vehicle types such as the passenger cars, commercial vehicles and racing cars for the braking performance, service lives and environmental protection (low-metal and NAO formulations).

Industrial brake and clutch friction plates: Applied to the braking and transmission systems in the fields such as the construction machinery, wind power generation, mining equipment, machine tools and ships. The testing needs to emphatically focus on the performance under the high loads, frequent start-stop and special environments (such as the humidity and dust).

Rail transit friction materials: Covering the brake shoes and brake pads of the high-speed railways, subways, locomotives and trams. The testing standards are extremely strict, especially emphasizing the friction stability, wear resistance and low damage to the brake discs under the high-speed and high-energy-density conditions.

Aviation friction materials: Used for the aircraft landing gear brake pads, requiring the extremely high thermal stability, specific heat capacity and the reliability and consistency under the extreme temperatures and pressures. The testing is usually carried out on the inertia dynamometers simulating the real landing energies.

Dry and wet friction materials: Distinguished according to the working media. The dry materials (such as the most brake pads) work in the air; the wet materials (such as the automatic transmission clutch plates) work in the oil, and their testing needs to evaluate the friction and wear characteristics under the specific lubrication conditions.

Non-asbestos organic (NAO), semi-metallic, low-metallic and ceramic-based friction materials: For the materials of the different formulation systems, the testing emphases are different. For example, the NAO materials focus on the environmental protection and low noise, and the ceramic-based materials focus on the high-temperature stability and wear resistance. The testing scope needs to cover their characteristic indicators.

Raw material and intermediate product testing: The incoming inspection is carried out on the raw materials such as the binders (resins and rubbers), reinforcing fibers, friction performance modifiers and fillers, and the process quality control is carried out on the uniformity and densities of the intermediates such as the mixtures and molded blanks.

Testing Methods

Constant speed friction testing machine method: According to the standards such as GB/T 5763 and SAE J661, under the fixed rotation speeds and pressures, the friction coefficients and wear rates are continuously determined through the programmed temperature rise. This method has the popularized equipment and is the basic and routine method of evaluating the friction properties of the material.

Inertia dynamometer testing method: Simulates the actual vehicle braking processes. The kinetic energy is stored through the flywheels to simulate the inertia, and the comprehensive performance testing such as the fade, recovery, wear and noise can be carried out. This method is closer to the actual working conditions and is the core verification means of the product research and development and certification.

Differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA): Used to analyse the thermal decomposition temperatures, glass transition temperatures and thermal weight loss behaviors of the binders (such as the phenolic resins), evaluating the thermal stability and temperature resistance upper limits of the material.

Scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS): The morphology characteristics of the worn surfaces (such as the wear debris, ploughing grooves and transfer films) are observed, and combined with the EDS analysis of the surface element distributions, the wear mechanisms (adhesive wear, abrasive wear and fatigue wear, etc.) are studied.

Hardness testing method: The Rockwell hardness testers (HR) or Brinell hardness testers (HB) are used to measure the surface hardnesses of the material. The hardness affects the wear resistance, compression collapse resistance and noise tendencies of the material, and is the important monitoring indicator of the product consistency and process stability.

Shear strength testing method: The friction material specimens are bonded to the backing plates, and the forces parallel to the backing plates are applied until the failure, determining their bonding shear strengths. This method is used to evaluate the reliability of the bonding layers between the material and the backing plates (steel backs).

Noise vibration bench testing method: In the dedicated anechoic chambers or semi-anechoic chambers, the chassis dynamometers or inertia dynamometers are used, combined with the high-sensitivity acoustic sensors and accelerometers, to collect and analyse the frequency domain and time domain characteristics of the braking noises, positioning the squeal frequencies.

Testing Standards

Domestic Standards:

1. GB/T 5763, Brake linings for automobiles (constant speed friction testing method for automotive brake linings)

2. GB/T 5764, Clutch facings for automobiles

3. GB/T 22309, Road vehicles — Brake linings — Shear test procedure for disc brake pad and drum brake shoe assemblies

International Standards:

1. SAE J661, Brake linings quality test procedure

2. ISO 26867, Road vehicles — Brake lining friction materials — Friction behaviour assessment for automotive braking systems

3. ISO 15484, Road vehicles — Brake lining friction materials — Product definition and quality assurance

Note: The applicable standard can be selected according to the product use and customer trade requirements; standards agreed in the contract take precedence over general standards.

Testing Instruments and Equipment

Constant speed friction testing machine: The core components include the spindle drive systems, hydraulic loading systems, heating and temperature control devices, friction torque transducers and data acquisition systems. Used to carry out the standardized preliminary evaluation of the friction and wear properties, and is the key equipment of the production line quality control.

Inertia brake dynamometer: The large comprehensive testing platform, including the flywheel groups with the adjustable inertia, drive motors, hydraulic servo braking systems, high-precision temperature measurement systems, as well as the comprehensive data acquisition and processing units. It can simulate the real braking working conditions, carrying out the regulatory certification and property development testing.

Thermal analyser: Mainly including the differential scanning calorimeters (DSC) and thermogravimetric analysers (TGA). Used to precisely measure the heat flow changes and weight changes of the material under the programmed temperature control, providing the data for the thermal characteristic research of the binders and organic components in the formulations.

Material microscope and electron microscope: The stereomicroscopes are used to observe the macro structures and wear appearances of the material; the scanning electron microscopes (SEM) equipped with the energy dispersive spectrometers (EDS) are used for the high-resolution micro morphology observation and micro-area composition analysis, and are the necessary tools of studying the wear mechanisms.

Universal material testing machine: Equipped with the different fixtures and transducers, it can be used for the mechanical property testing such as the compression strength, shear strength, bending strength and impact strength of the friction material, evaluating the mechanical integrity of the material.

Hardness tester: The Rockwell hardness testers (for the harder materials) or Brinell hardness testers (for the softer or non-homogeneous materials) are selected according to the material characteristics. Used for the fast and non-destructive detection of the product hardness uniformity and batch consistency.

Noise and vibration analysis system: Composed of the high-performance data acquisition instruments, microphone arrays, accelerometers, rotation speed transducers and professional noise and vibration analysis software. Used to locate, identify and quantify the braking noise and vibration problems in the bench or real vehicle testing.