Frictional and Particle Emission Behavior of Different Brake Disk Concepts Correlated with Optical Pin Surface Characterization

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The authors came up with a new way to look at brake pad surfaces, making it easier to spot and measure different types of wear on GCI brake disks. They compared GCI and HS-LC brake disks under different braking conditions and found some useful differences in how they handle friction, wear, and emissions.

HS-LC seems to cut PM emissions by 70%, which is nice. But what does it cost to actually use it? Has any economic study done? Or overall life cycle analysis.  Things like re-use with coating again ?

Why did the author only compare GCI and HS-LC? There are  other coatings and treatments out there why was it limited to only 2? I just feel it limits the broader applicability and would have helped validate the findings better.

Was an IR camera also used? If so, I didn’t see any results or comments about it. Would be nice to show how temperature varies across the disk surface in real time

Was there any analytical methods to look at the chemical composition of debris? Like a lot of hypothesis around stable patches, oxidation under heat  could have been explored by understanding the chemical nature.

Overall ,the methods presented in this study allow for the generation of extensive and useful data. This can help improve the understanding of friction and wear in braking systems. It also provides a good base for creating better models and even using machine learning in the future as author suggested.

Author Response

Dear reviewer,

thank you for your thorough and insightful review of my manuscript. We appreciate the time and effort you have invested in providing such detailed feedback, specially by pointing out relevant aspects that would enhance the analysis depth, like thermal imaging or material analysis, that shall be considered in future studies.

The authors came up with a new way to look at brake pad surfaces, making it easier to spot and measure different types of wear on GCI brake disks. They compared GCI and HS-LC brake disks under different braking conditions and found some useful differences in how they handle friction, wear, and emissions.

HS-LC seems to cut PM emissions by 70%, which is nice. But what does it cost to actually use it? Has any economic study done? Or overall life cycle analysis.  Things like re-use with coating again ?

The focus of this paper is purely technical.  We limited the scope to a material comparison with the focus of visualizing and quantifying the differences on the surface. An economics perspective is outside of the scope of the current paper.

Why did the author only compare GCI and HS-LC? There are other coatings and treatments out there why was it limited to only 2? I just feel it limits the broader applicability and would have helped validate the findings better.

We mainly focus on the techniques for assessing the disk/pad-pairs. GCI and HS-LC are more examples than they are a comprehensive list of possible or useful tribo-pairs. Nevertheless, several coating technologies result in disks that reduce the particle emission, see review from Aranke et al. (2019) “Coatings for automotive gray cast iron brake discs: A review”. Most of the coating have a higher surface hardness in common, which is the main difference we aimed to compare with the common gray cast iron disk. The applicability of the herein gained insight is therefore primary limited to high-speed laser cladded disks. Further applicability on similar systems (hard surface) might be given but must be proven. 

Was an IR camera also used? If so, I didn’t see any results or comments about it. Would be nice to show how temperature varies across the disk surface in real time

It has been shown with full-scale brakes that the temperature on the surface is not homogeneous and can lead to hot-band [Hesse et al. (2021) “Testing of alternative disc brakes and friction materials regarding brake wear particle emissions and temperature behavior”]. The disk temperature rise in the presented experiments does not show significant levels as those in the mentioned publication. Therefore, no IR-camera was used, as the influence of a possible heterogenous temperature distribution was deemed insignificant.

Was there any analytical methods to look at the chemical composition of debris? Like a lot of hypothesis around stable patches, oxidation under heat could have been explored by understanding the chemical nature.

The particle emission characterization was limited to the particle size distribution. Information on the elemental composition and oxidation state of the debris could provide indeed a further analysis aspect, which is usually achieved through ex-situ measurement. This would limit the benefits of the used at-line approach. Nevertheless, the integration of chemical analysis tools, such as Raman spectroscopy or laser-induced breakdown spectroscopy could expand the analysis possibilities of the at-line system.

Overall, the methods presented in this study allow for the generation of extensive and useful data. This can help improve the understanding of friction and wear in braking systems. It also provides a good base for creating better models and even using machine learning in the future as author suggested.

Reviewer 2 Report

Comments and Suggestions for Authors

This manuscript investigated the wear emissions of a gray cast iron and a laser-cladded brake disk by a pin-on-disk experiment with integrated optical pin surface characterization. A segmentation method was developed for the pin surface images and topographical data to extract and quantify different features on the pin. This is a meaningful work.It's suitable for publication.

Author Response

This manuscript investigated the wear emissions of a gray cast iron and a laser-cladded brake disk by a pin-on-disk experiment with integrated optical pin surface characterization. A segmentation method was developed for the pin surface images and topographical data to extract and quantify different features on the pin. This is a meaningful work. It's suitable for publication.

Dear reviewer,

Thank you very much for your positive feedback and for recognizing the significance of our work. We are pleased to hear that you found our investigation into the wear emissions of gray cast iron and laser-cladded brake disks meaningful and suitable for publication. Your acknowledgment of our segmentation method for pin surface images and topographical data is greatly appreciated.

Reviewer 3 Report

Comments and Suggestions for Authors

Most ground transportation vehicles depend on braking systems with sliding contacts between rotating and stationary parts. In such scenarios, the emission of particles can present human health risks based on the size, number, and composition of particles.

• Experimental methodology is well explained along with the instrumentation and its capabilities.
• Discussion: in general, the outcomes of the manuscript are principally transferable to automotive sector in virtually every country around the globe. The work is interesting and aims at giving a better situation in the proposed sphere of application.
• Conclusions listed in the last section are in line with the objectives of the paper. All references mentioned in the Reference list are cited in the text, and vice versa. Articles of high scientific quality, originality, and significance have been cited in the manuscript.

Minor points:

• The data presented looks like the well-planned and executed laboratory experiment without any description of the novelty of your work. What aspects in this study can be described as unique what makes the whole outcome different from other studies?
• Please do not use Italic for the in-text citations. Example: [35, 36, 42, 44, 45], the list goes on.
• Most times when we repeat experiments we obtain slightly different results. Knowing the uncertainty in the result of a physical measurement greatly enhances the value of that measurement, for then we know how far it can be trusted. Add errors to the determinations and pay attention to the decimals significance.

Author Response

Dear reviewer,

Thank you for your detailed and thoughtful review of my manuscript. We are grateful for the time and effort you have dedicated to providing such comprehensive feedback. Especially the point about the uncertainty is relevant as each braking is unique and depends on non-considered factors, like ambient conditions, pad material homogeneity, amongst others.

Most ground transportation vehicles depend on braking systems with sliding contacts between rotating and stationary parts. In such scenarios, the emission of particles can present human health risks based on the size, number, and composition of particles.

• Experimental methodology is well explained along with the instrumentation and its capabilities.
• Discussion: in general, the outcomes of the manuscript are principally transferable to automotive sector in virtually every country around the globe. The work is interesting and aims at giving a better situation in the proposed sphere of application.
• Conclusions listed in the last section are in line with the objectives of the paper. All references mentioned in the Reference list are cited in the text, and vice versa. Articles of high scientific quality, originality, and significance have been cited in the manuscript.

Minor points:

• The data presented looks like the well-planned and executed laboratory experiment without any description of the novelty of your work. What aspects in this study can be described as unique what makes the whole outcome different from other studies?

The at-line surface characterization system integrated in the Automatic Universal Tribotester (AUT) has been presented and used in prior investigations. Furthermore, prior investigations visualize the brake pad / pin surface and implement a histogram-based binarization threshold for segmentation of max. 3 classes [Neis et al. (2015) “Characterization of surface morphology and its correlation with friction performance of brake pads”].

The novelty of this investigation lays in the analytic capabilities based on the newly developed segmentation method, which allows to extract quantitative data from images (> 5 classes), register them to topographical data (addition of 3 classes), and evaluate how these classes are impacted by the braking conditions. The capabilities are shown using a GCI and HS-LC brake disk.

• Please do not use Italic for the in-text citations. Example: [35, 36, 42, 44, 45], the list goes on.

Italics have been removed from all in-text citations.

• Most times when we repeat experiments, we obtain slightly different results. Knowing the uncertainty in the result of a physical measurement greatly enhances the value of that measurement, for then we know how far it can be trusted. Add errors to the determinations and pay attention to the decimals significance.

The results from the two runs are shown in Figures 2 and 4. Figure 3 shows only the data from a single run, as only in the second run the particle measurement device was integrated.

Previously, Figure 10 only showed the coverage ratios from run 2. The new Figure 10 shows the averaged data from both runs and the standard deviation as error bar. The observations of the HS-LC disk (line 514 and 519) were slightly adapted to match averaged coverage factors. All previously described trends and results are still valid.