On the Influence of Carbides in Laser-Cladded Coating on Friction, Wear, and Airborne Particle Emissions

Bassent Dridi1,*, Rikard Hjelm1, Samuel Awe2, Yezhe Lyu1, Joakim Pagels3,4, Jens Wahlström1

*Corresponding Author: bassent.dridi@lth.lu.se

1Department of Mechanical Engineering Sciences, Lund University, Sweden.

2Automotive Components Floby AB, Sweden.

3Ergonomics and Aerosol Technology, Lund University, Sweden.

4Nanolund, Lund University, Sweden

Research

Brake wear is recognized as one of the most significant non-exhaust traffic-related sources of particulate matter (PM), contributing up to 55% to non-exhaust traffic-related emissions and 21% to total traffic-related PM10 emissions. A promising approach for reducing PM10 emissions is to add hard coatings to the brake disc surfaces. This study compares friction, wear, and particle emissions from two distinct laser-cladded hard coating materials, i.e., silicon carbide (SiC), and tungsten carbide (WC) reinforcements in stainless steel matrix, together with a non-asbestos organic (NAO) pad. Both combinations are compared to a commercial reference using a grey cast iron (GCI) disc and a low-metallic (LM) pad.

Methodology

A pin-on-disc (POD) tribometer (Nanovea model T50®) was employed within a one-way ventilated enclosure to perform emissions measurement. All tests were conducted at 0.6 MPa contact pressure and a sliding speed of 2 m/s (800 RPM and a mean wear track radius of 24 mm), corresponding to typical city driving conditions. Each test run had a duration of 2 hours, and three repetitions were performed for each disc and pad combination. The coefficient of friction (CoF) was registered online during the tests by the POD software. The wear was determined by weighting the test samples before and after testing with a digital balance (Mettler model AT261 DeltaRange®). Airborne particle emissions generated during the tests were measured using an APS (TSI® aerodynamic particle sizer model 3321) that measures aerodynamic particle size from 0.5 to 20 μm in 52 size channels.

Results

Results showed a reduction of approximately 85% in the wear mass loss of the laser-cladded discs and NAO pins compared to the reference. This indicates that the coating possesses a favourable resistance to wear, which is crucial in terms of pad and disc lifespans. However, the POD tests also indicated a decrease of about 20% of the CoF of the coated discs compared to the reference. Furthermore, the coated discs and NAO pads demonstrated an advantage in reducing the airborne particle number concentration by approximately 70% for NAO+SiC and 80% for NAO+WC. In all cases, the particle size showed a unimodal distribution with a peak at about 1 μm in diameter.

Limitations

The results of this study were derived from experiments conducted in a controlled laboratory environment and chosen test settings. However, outcomes might vary when subjected to different circumstances, such as different temperatures, loads, speeds, and/or environmental conditions. Furthermore, this research focuses on a limited range of coating materials, while the wider spectrum of possible materials remains unexplored. Additionally, this study examined the concentration and size distribution of airborne particles from coated discs but not their composition or toxicity. Acknowledging these limitations is crucial for further analysis of the findings and for further evaluation of the approach of using coatings to reduce PM10 emissions.

Add on

Laser-cladded coatings have gained popularity as an innovative approach to enhance braking systems. However, there is a lack in the literature of studies of particle emissions from brakes using coated discs. The research presented in this study provides an analysis of the frictional behaviour, wear mass loss, and airborne particle emissions from two laser-cladded hard coating materials (SiC, WC) together with NAO pads. Insights from this research can guide the development of future brake materials, contributing to both decreased particle emissions and reduced wear of disc brake systems.

Conclusion

Overall, the novel coatings demonstrated reduced wear and lower emissions with respect to the reference materials. The CoF of the coated brake discs depicted a decrease in comparison with the reference brake discs. The decrease in wear is favourable for reducing airborne particle emissions during braking. Thus, the laser-cladded coating of brake discs together with the NAO pin is a promising solution with a significant potential for emission reductions. Further studies on particle composition and toxicity are necessary to identify their potential effects on the environment and human health.

Dr. Eng. Samuel Awe, Automotive Components Floby AB, SWEDEN

Mr. Adam Thomas, Automotive Components Floby AB, SWEDEN

In the recent years, the automotive industry has been under constant pressure to reduce the vehicle's weight, minimize particulate matter emissions as well as improve corrosion resistance. For instance, several concerted efforts are being put together by the stakeholders to source for alternative brake disc materials that can fulfil these requirements better than the regular grey cast iron. However, this communication discusses some of the developing demands that the current and future automobile disc brake must meet. The paper also highlights some of the characteristics and prospects of lightweight aluminium matrix composite (SICAlight) rotor as a potential alternative to the traditional grey cast iron brake discs