Abstract
Friction material properties depend on the raw materials and production processes used during their fabrication as any other composite-like material. Efficiency, wear, vibrations and physical-chemical characteristics are tested in order to evaluate if the developed material has the correct composition for the related application. Brake pads industry puts many resources to understand and manipulate the real impact and influence of each raw material. The increasing number of quality controlled characteristics and the higher attention on ecology and health have pushed product developers to understand which kind of compound they can use and why.
Chemical composition is very important when talking about formulation, but when it is about composites behaviour, it is not enough to know the forming elements of our formula. It is interesting to observe that a group of ingredients may have the same chemical composition but that it may behave completely different under same testing conditions. There is more information to find about each raw material: morphology, surface area, size, density, porosity, hardness, etc.
The use of potassium titanates is very common in the production of brake pads, a very wide range of those compounds is available on the market and their quantity on the formula can vary depending on the desired characteristics of the final product. The selection of the right one and its quantity, according to the pointed market, is very important when speaking about efficiency and in particular about fading. A deep study on the characteristics of different potassium titanates has been done to understand and correlate the raw materials properties with the performance of the final friction materials. Mercury porosimetry, laser granulometry, surface absorption studies (N2 adsorption at 77K), x-ray diffraction (XRD), x-ray fluorescence (XRF) and scanning electron microscopy (SEM) have been carried out in order to characterize the materials in terms of surface area, morphology, density, purity and grain size. An interesting aspect evidenced in this study concerns the comparison between the data of specific surface areas and porosity obtained through mercury porosimetry and gas-volumetric nitrogen adsorption at 77K. Mercury porosimetry (MP), in fact, is the most widely used technique for porosity determination of this kind of materials, but it was found to have an useful complementary technique in gas-volumetric nitrogen adsorption, since nitrogen is able in determining the real porosity of samples, even the presence of small pores, not accessible (and thus not visible) to mercury.
Results were then related to efficiency tests (AK Master), and wear tests. This way, it was possible to find the correlations of morphological and structural behaviours vs. efficiency and stability given by potassium titanates to friction materials. This should help brake producers to achieve materials desired characteristics without passing by a long and costly trial and error process.
KEYWORDS: friction material, potassium titanates, surface area, morphology, granulometry.