| Author: | Wei, Long |
| Title: | A study of tribology performance, airborne particle emissions and brake squeal noise of copper-free friction materials |
| Advisors: | Choy, Yat Sze (ME) Cheung, Chun Shun (ME) |
| Degree: | Ph.D. |
| Year: | 2020 |
| Subject: | Motor vehicles -- Brakes Brakes Friction materials Hong Kong Polytechnic University -- Dissertations |
| Department: | Department of Mechanical Engineering |
| Pages: | xix, 186 pages : color illustrations |
| Language: | English |
| Abstract: | Disc brakes are the most common type of passenger vehicle brakes. In a braking process, both the brake pad and the disc are worn, generating airborne wear particles. Moreover, braking squeal noise is generated from the friction induced vibration of the brake system. Brake wear particles contribute up to 55% by mass of the total non-exhaust PM10 emissions and 21% by mass of the total road traffic emissions. Brake squeal noise is a major issue in the automotive industry for many years which can cause the customer dissatisfaction. Thus, more attention should be paid on investigating the airborne wear particles and brake squeal noise emitted from disc brakes. Copper and its alloy have been widely used in the formulation of non-asbestos organic (NAO) friction materials as reinforcements and/or additives. But U.S.A has raised a legislation to reduce the copper content in friction materials due to the toxicity of copper in wear debris. Thus, there is a need to develop copper-free friction materials which also own the desired tribology performance. In the present study, influences of braking conditions on the friction and wear behaviours, characteristics of brake pad surface, airborne wear particle emissions and brake squeal noise were investigated. Low metallic (LM), semi metallic (SM) and non-asbestos organic (NAO) brake pads sliding against an iron disc were tested using a pin-on-disc tribometer. Results show that the friction coefficient and the specific wear rate decrease with increasing contact pressure and sliding velocity. Observation on the morphology of brake pad surface shows that friction layers are large with few cracks on the surfaces of SM and LM brake pads; while for NAO brake pad, the friction layers are small with an irregular shape. From fractal analysis, the fractal dimension of brake pad surface is in the range of 2.38 to 2.84 for SM, LM and NAO brake pads. Regarding the brake squeal noise, the A-weighted SPL of brake squeal noise increases with increasing sliding velocity and contact pressure for all the brake pads. Moreover, sliding velocity can significantly affect the brake squeal noise for all the brake pads, but contact pressure can only significantly affect the brake squeal noise for SM and LM brake pads. Regarding the airborne particle emissions, Particle number-size distribution has a single peak around 100 nm for all the brake pads. Total number concentration (TNC) and particle mass concentration (PMC) increase with increasing sliding velocity and contact pressure in most cases. Both sliding velocity and contact pressure can significantly affect the TNC and PMC results for all the brake pads. These is no special trend that can be observed for the variation of geometric mean diameter (GMD) with sliding velocity and contact pressure. Only sliding velocity can significantly affect the GMD of the emitted particles for SM and LM brake pads. For NAO brake pad, both sliding velocity and contact pressure cannot significantly change the GMD result. Four new copper-free nonasbestos organic (NAO) friction materials were developed by replacing the copper fiber with steel fiber (STFM), ceramic fiber (CEFM), carbon fiber (CAFM) or carbon nanotube (CNFM) respectively. Friction and wear behaviors, airborne wear particle emissions and brake squeal noise of these copper-free friction materials were evaluated and compared with a reference copper containing friction material (RFM) by using a pin-on-disc test rig. Results show that STFM and CEFM show comparable coefficient of frictions (COFs) with RFM at all the test modes. While CAFM and CNFM have lower COFs than RFM at the braking conditions with low and medium contact pressures. The specific wear rates of all the copper-free friction materials are larger than that of RFM. Regarding the morphology of the pin sample surface, the morphology of the friction layers on the STFM and CEFM surfaces are comparable with that on the RFM surface. The friction layers almost fully cover the worn surface of the CAFM. Regarding the CNFM, the number and area of friction layers on the CNFM surface are much smaller than those on the surfaces of the other friction materials. Regarding the brake squeal noise, all friction materials present the comparable A-weighted sound pressure levels. Regarding the airborne particle emissions, STFM, CEFM and CNFM present larger TNCs than RFM, while CAFM presents a smaller value than RFM. Moreover, all copper-free friction materials exhibit larger particle sizes than RFM. Since the brake squeal noise of all the friction materials are comparable, carbon fiber is the best choice among the copper substitutions in the present study because it induces the lowest airborne particle emission among the new developed copper-free friction materials. |
| Rights: | All rights reserved |
| Access: | open access |
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