Automotive

The undeniable importance of braking power to the safe operation of an automobile has led to on-going research into the optimal properties of materials for modern braking systems. This note shows how the C-Therm Trident Thermal Conductivity Analyzer system is employed to effectively measure the thermal conductivity (k) of a wide range of sample types using its multi-sensor options.

In its simplest form, brakes work by first converting the kinetic energy of your vehicles motion to heat and then dissipating that heat to the atmosphere. The major component of importance in modern braking systems are the brake pads. Brake pads were traditionally made from pure metals due to their high thermal conductivities resulting in their uncanny ability to resist thermal stress and dissipate generated heat. Unfortunately, metal-based components can carry a significant weight and in-turn lower overall efficiency. Today, most brake pads have since been replaced by semi-metallic, ceramic or organic based composites to overcome the drawbacks of pure metals. A brief overview of the various brake pad types can be seen in Table 1, below.

Table 1: Brake Pad Types and Considerations

While there are many considerations to take into account (see above), ultimately stopping power can be most strongly related to thermal conductivity. To illustrate it’s ability to accurately and precisely test various brake pad materials, a C-Therm Trident Thermal Conductivity Analyzer was utilized with the Transient Plane Source  (TPS)  configuration operating in accordance to ISO 22007-2 to compare semi-metallic to ceramic brake pads (see Figure 1 & 2). Both sets of pads are base level units (<100$), however it should be noted the ceramic pads were ~20% more expensive. For testing of these samples the reported results are the average of five tests.

Figure 1: Semi-Metallic Brake Pad

Figure 2: Ceramic Brake Pad

Figure 3: Thermal Conductivity of Semi-Metallic and Ceramic Brake Pads

As can be seen from figure 3 above, the semi-metallic pads resulted in thermal conductivity of 4.36 W/mK, whereas the ceramic pads were only about 2.79 W/mK (44% difference). Trident’s TPS configuration was the optimal way to test these brake pads due to the hard and rigid nature of the samples. While not reported herein, it should be noted that C-Therm’s TPS module also simultaneously collects and displays diffusivity data for applications where both are required. All testing had a precision of better then 1.7%. The above results clearly demonstrate the superior thermal properties of semi-metallics over ceramics as well as coming in at a lower price point. Ultimately the higher thermal conductivity will result in an improved stopping performance as it can better dissipate the generate heat and lower the net thermal stress on the braking system.

With much of the focus on the structural and mechanical aspects in automotive manufacturing, one can’t forget the measure of a quality vehicle also encompasses the individual’s comfortability. Simple aspects such as the “touch” sensation of the interior fabrics or panelling can all add or takeaway from the customers experience. This note shows how the C-Therm Trident system can be used to not only obtain important thermal conductivity (k), but also simultaneously obtain thermal effusivity data in the range of 5 to 40,000 Ws^1/2/m²K using the Modified Transient Plane Source (MTPS)  sensor.

Thermal effusivity plays a huge role in an individual’s perception of touch sensation and is most commonly associated with textiles (see Figure 1). While textiles have massive applications in the fashion and apparel sector, automotive interiors are also very demanding with regards to textile R&D. Aspects such as upholstery fabric, interior panelling and more are all textile heavy areas with the need for robust and precise testing of thermal effusivity properties. The ability to compare the quality perception of synthetic vs. natural materials is also of great importance to product development and is yet another area where C-Therm’s line of Thermal Analyzers can provide fast and precise testing results on a wide range of material types.

Figure 1: Honeycomb Paneling

The following is a study of various automotive companies’ overhead paneling which help to not only insulate heat, but also sound. Tests were performed on both the front (interior) and back (exterior) side using C-Therm’s Thermal Analyzer in MTPS configuration. For the testing of these samples, the panel layers consisted of at minimum an interior facing fabric, board or foam core and a backing material facing the exterior. The reported test values are the average of 3 tests per side. The effusivity test results can be seen below in Figure 2.

Figure 2: MTPS Thermal Effusivity Analysis of Automotive Composite Paneling

As can be seen from the figure above, in all cases the interior facing fabric was lower in thermal effusivity then the exterior facing backing material. This is a purposeful design as the lower thermal effusivity side, which is directly visible as the car interior, has a better ability to maintain the internal temperature. Conversely, the back-side material which has a larger thermal effusivity would be better at dissipating any external heat from the external environment. It should be noted that in this study the Hyundai panelling material resulted in the highest thermal effusivity for both the front and rear facing side.

Today’s tires aren’t made of pure rubber, but composite materials of elastomers, reinforcement fibers, plasticizers and much more which help improve performance and extend lifetime. Composite manufacturing can be complex in nature due to issues such as non-homogenous dispersion of additives which can lead to weak points. It is therefore crucial to be able to not only measure, but “map” results of interest. This note will help demonstrate the importance of thermal conductivity (k) in composite tire manufacturing and how C-Therm’s Trident system can be used to obtain that critical information using the Modified Transient Plane Source (MTPS)  sensor conforming to ASTM D7984. This proprietary, single-sided sensor is ideal for fast paced QA/QC and R&D environments with test times ranging from 0.8 to 3 seconds.

The frictional resistance between your tires and the road determine exactly how fast you can accelerate, take turns and most importantly how fast you can stop. On a very basic level, friction can be thought to come from the “surface roughness” between the two objects that are pressed in contact.  The frictional force is also proportional to the coefficient of friction (µ). As is with everything involving friction, generated heat plays a major role in overall performance. It is therefore vital to ensure a strong understanding of thermal conductivity characteristics.  For a closer look at the physics behind car tires and friction here is a detailed video from Michel Van Biezan, University of Los Angels.

C Therm’s MTPS method was employed by P. Vizureanu et al., Faculty of Material Science and Engineering at the Technical University of Iaşi (2015), with the goal of investigating the properties of reinforced rubber composites with different carbon nanotube (CNT) concentrations. The single-sided MTPS module was an excellent fit for analysis of these sample types as both thermal conductivity and effusivity are measured directly providing a detailed overview of the thermal characteristics of the samples. Various CNT loadings from 5-30% were investigated on two different substrate thicknesses (10 and 20mm). Results from this study can be seen in Figure 1, below. Data represents the average result of 10 runs per sample.

Figure 1. MTPS Thermal Conductivity Analysis of Rubber Composite Samples

Figure 2. SEM Imaging of Rubber Composite at Different CNT Concentrations¹

P. Vizureanu et al. (2015) found that an increase to CNT concentration lead to an overall increase in thermal conductivity (see Figure 1). Most notably the 30% CNT loading had the largest thermal conductivity increase of ~26% for both the 10 and 20mm samples. Further analysis found that the effective increase was not only dependant on CNT concentration, but also the geometry and distribution of the additives. It was concluded that the large increase to thermal conductivity represented a success for all applications with thermal input. Although done qualitatively (see Figure 2), this paper is an excellent example of how the non-destructive MTPS method would allow for high precision (better then 1%) thermal mapping of the samples to gain quantitative insight into the overall distribution of the CNTs.