Abstract
Mercury porosimetry (MP) is commonly used to characterize the surface of porous materials, it is a complete and not complex technique and it can be used to measure the total porosity of a bulk body, but specially to understand the characteristics of its porosity: dimension of pores, their distribution, the compressibility of the material, its permeability, its surface area, etc. When talking about composites-like materials that base its functionality on their surface characteristics, MP becomes an important and useful instrument.
Friction materials behaviour depends on the interaction that their surface and matrix may have with the surface of a metallic disk, they exercise friction to stop the car by transforming dynamic energy into heat and particle deformation or wear. From this point of view, it is very important to know the FM surface properties and to understand the possible changes that it may have during the braking action. MP would become an interesting instrument that may help formulators to identify the surfaces’ characteristics –porosity, compressibility, permeability– that may be related to efficiency.
MP has been applied to characterize porosity, permeability and pores distribution on FM surfaces. The mechanism of measurement and the calculation of porosity by means of MP technique depends on two different variable values: applied pressure and absorbed mercury. Pressure is measured directly on the MP apparatus but, on the other hand, absorbed mercury is calculated with the use of an indirect capacity value that takes into account the variation on mercury volume without considering the nature of the absorption. The formulation of FM includes an important amount of metals that may change and false the real porosity values (by changing the absorption of mercury). We demonstrate through experimental data and different techniques comparisons, that MP is not the best option to study FM surface characteristics.
This paper discuses the effectiveness, feasibility and economy of using this technique as a surface porosity control for FM. The study is focused on materials with a normal metals content and the obtained values are compared with those measured on a non metallic material. The analysis takes into account the repeatability and precision of the obtained data when the analysis is done on different samples of a single brake pad and the feasibility of compare data from different production batches.
KEYWORDS – Mercury porosimetry, alpha alumina, friction material, hysteresis, amalgams.