Combining Benchtop Testing with 3D Surface Metrology for Non-Destructive Analysis of Brake Pad Material

New compositions for brake pad materials have been driven by environmental regulations and the need for enhanced braking systems. Studies on the impact of different materials on the durability, integrity, and strength is integral to the design and improvement of brake pad.

These studies are generally performed by means of a tedious, back-and-forth period of testing as well as detailed metrology analysis. Propitious brake materials are, in general, assessed before on-vehicle stopping tests through dynamometer tests, which enable testing of a real rotor or pad under protocols that simulate the conditions required to stop a vehicle.

Dynamometer testing is, however, a complicated and cost-intensive technique that demands the brake pads and rotor to be in their final form. A cost-effective and faster means for testing and complete analysis of prospective materials for automotive brake applications has been developed by Bruker, which reduces the required extent of dynamometer testing.

This article discusses the pros of combining benchtop testing of smaller samples with fast and accurate 3D surface metrology for performing several, pre- and post-test non-destructive analyses more rapidly than ever before. Although the data furnished in this article are particularly related to brake pad materials, the process can be adopted for any material used in high-friction environments.

Testing Brake Materials

In Bruker’s UMT TriboLab™ Brake Material Screening Tester (see Figure 1), small sample coupons made of friction materials are tested in real time under real brake operating conditions with respect to industry-standard dynamometer standards and protocols. If the brake system has to be scaled down and properly simulate, critical physical parameters have to be consistently matched with the help of dynamometers for protocols such as the SAE J2522, regulating the sliding speed, contact pressure between the rotor and pad, initial temperature, and deceleration.

Figure 1. UMT TriboLab Brake Material Screening Tester

Due to small inertia of benchtop systems when compared to a dynamometer or vehicle, simulation of deceleration is performed by controlling the velocity of the motor as a function of time. Temperature of the rotor, torque, temperature of the pad, CoF, and sliding speed are the important parameters monitored as part of the various steps of the simulation.

Another important benefit of performing small-scale tests is not only the ability to easily control environmental parameters, such as temperature and relative humidity, but also the potential to collect the debris discharged from the sliding contact interfaces at the time of the test. These brake wear particles can subsequently be subjected to post-test physical and chemical characterization.

During benchtop friction material testing, the minimum contact size of the coupons plays a vital role. Although it is possible to select the sliding speed and contact pressure from the real application or test protocol, there exists a minimum sample size that can represent the non-homogeneous morphology and composition of the brake pad (see Figure 2).

Figure 2. Samples that simulate the brake pad (with smaller coupons) and rotor for testing with the TriboLab Brake Material Screening Tester

Bruker performed a sequence of experiments with the help of the new tester and small coupons and compared the outcomes to full-scale dynanometer tests. The data gathered from the tests revealed good correlation between the benchtop and full-scale tests, from the calculated average CoF as well as the function of the torque with similar shape and trend (see Figure 3). The excellent correlation between the methods indicates that the Brake Material Screening Tester is, in fact, a reliable and effective system for the assessment and screening of friction materials for brake applications.

Figure 3. Comparison of the TriboLab test versus a dynanometer test (the SAE J2522—6.6 cold application) shows good correlation

LightSpeed Focus Variation Technology

Yet, physical testing is only one aspect of the process for characterizing brake pad materials. Further necessary information can be obtained through complete analysis with a reliable metrology system. Bruker’s Contour LS-K 3D optical profiler with LightSpeed™ Focus Variation technology allows between easy-to-acquire high-resolution images as well as reliable metrology data (see Figure 4).

Figure 4. Bruker Contour LS-K 3D Optical Profiler

The system has the ability to quickly capture surface data with a large field of view (FOV) at vertical scanning speeds of up to 5 mm per second. Display of high-resolution, data-rich images in real color can be achieved within seconds. In contrast to comparable solutions, contour LS-K also allows access to raw measurement data, thereby enabling the operator to precisely observe what is on the surface without data modification or filtering.

Contour LS-K has the ability to continuously capture intensity images when the objective is moved toward the sample. After completing the scan, the software analyzes the data for each pixel from every frame to choose the frame in which that pixel exhibits the optimum focus. Recognizing the frame in which this pixel exists, and distance of the objective from the surface when that frame was captured, the surface feature height at that pixel can be estimated. This process is performed for every pixel in the image (more than 1,000,000 points) to develop a 3D map, and also an all-in-focus color image (see Figure 5). As soon as it is captured, the data can be completely analyzed using industry-standard Vision64^® software.

Figure 5. 3D view of used test pad after tribology test, with color map overlay. The color overlay reveals differences of reflectivity linked to the material components.

The image of a brake test pad after testing is shown in Figure 5. The sliding direction is evident from the image surface texture, and it is easy to observe the damage. Metallic debris from the brake pad composite material can also be observed from the color image, thereby allowing a correlation to be established between material properties and the damage.

More specifically, it is possible to further enhance the accuracy of the brake test by comparing the surface of the pad before and after the test (see Figure 6).

Figure 6. Comparison of brake pad before (left) and after (right) wear testing utilizing exact same lateral and vertical scale reveals measurable effects of tribology

3D surface metrology offers a clear proof of correlation between theoretical assumptions of flat-on-flat conformation and reality. For example, a new part’s potential curvature could prospectively result in an underestimation of the pressure and contact area, which impedes the accurate calculation of brake efficiency or wear rate. Similarly, 3D metrology can be used to evaluate the requirement of longer run-ins before gathering tribology data to guarantee flat-on-flat geometry and the elimination of matter peaks (see Figure 6 left). Apart from the qualitative observation, the software can rapidly and easily calculate high-level analyses, for instance, the Abbott-Firestone curve (see Figure 7). This robust bearing ratio analysis greatly reiterates the difference between pre- and post-test topographies, and quantifies the fall in the highest protrusion points with the increase in contact area.

Figure 7. Comparison of Abbott curves from pad topography before (blue) and after (red) the tribology brake test

Ultimately, the quantification of damage as a result of galling and tearing on used brake test pads can be readily performed with the help of default advanced islands analysis (see Figure 8). With this method, vital information — such as number of pits, average and maximum depth — can be automatically obtained to rank severity of damages for various materials being tested.

Figure 8. Automatic characterization of parameters such as average diameter, area, deepest point (Rv%), and volume can be calculated for each pit with summary results for complete assessment of galling/tearing damages. Data are ranked from lowest to highest volume.

Conclusion

These are only a handful among various analyses that can be employed by manufacturers and engineers for brake pad material applications, or other materials in high-friction applications. The combination of the Contour LS-K’s speed, metrology capability, ease of use, and simplicity and the UMT TriboLab Brake Material Screening Tester allows brake pad manufacturers to perform in-depth analysis and testing of the performance of brake pad materials. Eventually, engineers can make the most of the complementary nature of these technologies to enhance pad quality at reduced cost, thereby reducing the time taken for the final new product to hit the market.

This information has been sourced, reviewed and adapted from materials provided by Bruker Nano Surfaces.

For more information on this source, please visit Bruker Nano Surfaces.