Brake noise is one of the most persistent customer complaint issues in the automotive industry. According to statistics, approximately 30% of in-warranty vehicle complaints are related to braking system abnormal sounds, with brake pad squeal and high-frequency shriek being the most common noise types. Molybdenum disulfide (MoS₂), a layered-structure solid lubricant material, reduces noise in brake pad formulations through mechanisms such as stabilizing friction coefficient and suppressing interfacial vibration transmission. This article examines the causes of brake noise, analyzes the noise-reduction mechanism of MoS₂, and discusses practical application effects based on SAE J2521 and GB/T 5763 standard test data.

Causes and Classification of Brake Noise

The fundamental nature of brake noise is friction-induced self-excited vibration. When a brake pad contacts the brake disc, friction forces generate periodic fluctuations at the interface. These fluctuations transmit through the caliper, bracket, and suspension system to the vehicle body, producing audible noise.

Based on frequency range and generation mechanism, brake noise is primarily classified into three categories:

- **High-frequency squeal (>1kHz)**: The most common type, accounting for over 60% of brake noise complaints. It is primarily caused by normal-direction vibration coupling at the friction interface, directly related to the negative slope characteristic of the friction coefficient-relative velocity curve.

- **Low-frequency groan (100-500Hz)**: Occurs predominantly during low-speed braking and start-up, caused by the stick-slip phenomenon during the transition from static to kinetic friction.

- **Creep-groan (<100Hz)**: Occurs during very low-speed braking (<5km/h), related to the microscopic surface morphology of the brake disc and the creep characteristics of the friction material.

The SAE J2521 standard specifies testing and evaluation methods for brake system noise, requiring noise rating based on loudness and duration under defined temperature, humidity, and deceleration conditions.

Relationship Between Friction Coefficient Fluctuation and Noise

One of the core mechanisms of brake noise generation is friction coefficient instability. An ideal friction material should maintain a constant friction coefficient across different temperatures, pressures, and speeds. However, in actual operating conditions, friction coefficient fluctuations are inevitable.

According to dynamometer test data, standard semi-metallic brake pads exhibit friction coefficient fluctuation amplitudes of ±0.08 (nominal friction coefficient 0.40) within the 100-300°C range, while brake pads containing molybdenum disulfide show reduced fluctuation amplitudes of ±0.04 in the same temperature range. The fluctuation amplitude of the friction coefficient is directly proportional to the vibration excitation intensity—larger fluctuations make self-excited vibration more likely to reach audible thresholds.

GB/T 5763 standard specifies the allowable friction coefficient fluctuation range for automotive brake linings. At designated temperature test points (100°C, 150°C, 200°C, 250°C, 300°C), qualified products should have friction coefficient deviations not exceeding ±0.10. Formulations with 2-5% molybdenum disulfide demonstrate smaller deviation ranges across multiple temperature test points, contributing to reduced noise occurrence probability.

Noise Reduction Mechanism of MoS₂

### Stabilizing the Friction Interface

In the hexagonal layered crystal structure of molybdenum disulfide, S-Mo-S tri-layer stacks are bonded by weak van der Waals forces, with interlayer shear strength approximately 1/30 of the intralayer chemical bond strength. This structure causes MoS₂ to preferentially slip along interlayer planes at the friction interface, forming a uniform transfer film. The transfer film transforms the friction contact from "hard-hard" to "hard-soft-hard," effectively smoothing local stress concentrations and reducing the instantaneous engagement-release cycles of micro-asperities.

### Suppressing Stick-Slip Vibration

Stick-slip vibration is the primary source of low-frequency brake noise. When the static friction coefficient significantly exceeds the kinetic friction coefficient, periodic "stick-slip" alternation occurs at the interface, generating low-frequency vibration. The interlayer slip characteristic of molybdenum disulfide reduces the difference between static and kinetic friction coefficients (μs-μk). Experimental data shows that friction materials without solid lubricant additives have a static-kinetic friction coefficient difference of approximately 0.06-0.10, while adding 3% molybdenum disulfide reduces this difference to 0.03-0.05, significantly decreasing the stick-slip vibration tendency.

### Damping and Vibration Reduction

Molybdenum disulfide has a Mohs hardness of 1.0-1.5, classifying it as a soft material. In friction composite materials, the addition of soft phases increases the material's loss factor (tan δ). Dynamic Mechanical Analysis (DMA) testing shows that phenolic resin-based friction materials with 5% molybdenum disulfide exhibit an increase in tan δ from 0.035 to 0.048 (approximately 37% increase) across the room temperature to 250°C range. Higher internal dissipation means more vibration energy is converted to heat rather than radiated as acoustic waves.

### Thermal Stability Assurance

During braking, friction interface temperatures can reach 300-400°C. Conventional organic lubricants (such as graphite in humid environments) may fail at these temperatures. Molybdenum disulfide withstands sustained temperatures up to 350°C in air without decomposition, with TGA testing showing mass loss below 2% at temperatures up to 400°C. This means MoS₂ maintains transfer film integrity and lubrication effectiveness under high-temperature braking conditions, preventing friction coefficient surges and noise intensification caused by lubrication failure.

Dynamometer Test Data

Brake noise dynamometer tests conducted according to SAE J2521 standard compared brake pads with and without 3% molybdenum disulfide in the same base formulation:

In 20 cold braking tests (50°C initial temperature, 3m/s² deceleration), brake pads without molybdenum disulfide produced audible noise (>70dB) 8 times, while those with molybdenum disulfide produced noise only 3 times. In hot braking tests (250°C initial temperature), the control group produced noise 12 out of 20 times, while the additive group decreased to 5 out of 20 times.

Regarding noise loudness, the maximum noise level of brake pads with molybdenum disulfide decreased from 86dB in the control group to 78dB, a reduction of approximately 8dB. While an 8dB reduction corresponds to approximately 84% reduction in acoustic energy, it should be noted that brake noise perception depends not only on loudness but also on frequency characteristics and duration.

Formulation Application Considerations

When incorporating molybdenum disulfide into brake pad formulations, the following technical factors should be considered:

**Addition amount**: The typical addition range for MoS₂ is 2-5%. Amounts below 1% produce insignificant noise reduction effects, while amounts above 8% may cause excessive friction coefficient reduction, affecting braking torque. GB 5763 standard requires automotive brake linings to maintain a friction coefficient of no less than 0.25 under specified conditions.

**Particle size selection**: For brake pad applications, MoS₂ with D50 in the 3-8μm range (medium or fine powder) is recommended. Overly coarse particles (D50>15μm) affect transfer film uniformity, while ultrafine powder (D50<1μm) has high specific surface area but poor dispersibility, potentially causing formulation inhomogeneity.

**Compatibility with other components**: Molybdenum disulfide has good compatibility with phenolic resin, rubber, and other binders, but attention should be paid to synergistic effects when used simultaneously with certain metal sulfides (such as antimony sulfide). Differential Thermal Analysis (DTA) should be performed during formulation design to confirm that no adverse reactions occur between components at processing temperatures.

Conclusion

The core issue of brake noise lies in unstable vibration at the friction interface. Molybdenum disulfide plays a practical role in brake pad noise reduction through multiple mechanisms: stabilizing friction coefficient, reducing stick-slip vibration tendency, increasing material internal damping, and maintaining high-temperature lubrication performance. Dynamometer test data demonstrates that properly dosed MoS₂ significantly reduces both the frequency and loudness of brake noise. In practical formulation design, addition amount, particle size, and compatibility with other components must be comprehensively considered to achieve a balance between noise reduction and braking performance.

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