| Automotive brake friction material is a multi-phase polymer matrix composite, usually containing four classes of ingreidients:binders, reinforcements, friction modifiers, and space fillers. Commercially available automotive brake friction materials can be roughly categorized as semi-metallic (SM) and non-asbestos organic (NAO) materials. Repacing asbestos in friction materials and improving friction performace have been the main drive powers for the development of the friction material industry. The future trends of brake friction materials are high performance, high load, ecological friendliness, and economic effectiveness.Effects of several raw materials on friction and wear performance were studied in this study, including abrasives (alumina and silicon carbide), reinforcements (aramind pulp and natural fiber), and a filler (vermiculite). Based on the ingredients mentioned above, different series of tailored friction formulations were designed through the combinational friction materials research (CFMR) strategy. The prepared friction samples were tested their tribological performance by using the constant-speed friction test mechine, the friction assessment and screening test (FAST) mechine, and the automotive single-ended full-scale brake dynamometer. Several analytical techniques were combined to study the topography, microstructure, chemical composition and thermal stability of the friction materials, including SEM, EDX, XRD, TGA, EMA, XRF and profilometry. The friction and wear mechanisms were discussed, and the "V" shaped friction mechanism was addressed in detail. The relationship between the emissivity of disk surface and the friction temperature was determined, while the correction functions for infrared radiation (IR) thermometer were constructed and confirmed by comparing with the test results from a sliding thermocouple. Based on the merit of extension theory, the extension evaluation model was established to rank the friction materials. In the extension evaluation, different dependent functions were used for different parameters. The weighted average dependent degree was used to describe the overall quality of a friction material, by considering several aspects.The main research results were summarized as follows.(a) The study on friction performance and tribological mechanism of friction materials:The poor thermal fade and negative wear rate of the sample without abrasive can be impoved by adding alumina or silicon carbide, but the improvement didn't continuously increase with the increase of abrasive usage. Due to the high hardness of abrasives, the volume contents of alumina and silicon carbide were suggested as 5.6%and 3.4%, respectively. The aramid pulp as an organic reinforcement can efficiently improve the friction stability and wear resistance. Considering its relatively high price, the aramid pulp was suggested using 3.4 vol.% in the formulation of friciton materials. The friction coefficient and wear resistance increased when increasing the usage of vermiculite. The iron-riched or barite-riched friction layer was formed through the competition between the abrasive friction and adhesive friction, which is the so-called "V" shaped friction mechanism. And this friction mechanism can be confirmed when performing EMA and XRF analysis.(b) The study on measurement and correction of friction temperature: The experimental appratus was designed to determine the relationship between the emissivity of disk surface covered with a friction layer and the friction temperature. The emissivity of disk surface wad found decrease with the elevating temperature in the exponential decay. The linear function, y=Ax+B, was used to correct the temperature measured by an IR thermometer. And the constants A and B can be calculated though their relationship with the emissivity of disk surface. The temperarure measured by the IR thermometer with any preset emissivity, can be corrected by this method. The correction method was confirmed using temperature data recorded by a thermocouple in the FAST test.(c) The development of eco-friendl y brak e friction materials:The concerns on environmental impacts and human health problems related to friction materials call for developing eco-friendly brake friction materials. Based on a typical low-metallic brake model formulation, the eco-friendly brake friction materials (eco-pad) were designed and prepared by partly or fully replacing the metals, antimony trisulfide, and aramid pulp in the model formulation. The newly developed eco-pad is a non-asbestos, non-metal, and non-aramid friciton materials, which meets all the developing trends of the modern friction industry. The eco-pad successfully passed the SAE J2430/BEEP dynamometer test, and exhibited a comparable brake performance with the original equipment (OE) brake pad. Considering the low cost and low envirnmntal impact of the eco-pad, it can be potentially commercialized in future.(4) The extension evaluation of brake friction materials:The extension evaluation method represents an effective tool for the ranking/selection of friction materials, based on user defined criteria (performance, physical properties, costs, and others). The single-criterion extension evaluation was successfully applied to access the friction stability or wear resistance of the friction materials, reinforced by alumina, silicon carbide, aramid pulp, and vermiculite. For the eco-pad, a multi-criteria extension evaluation was developed by considering several aspects. The brake effectiveness, wear, thermal stability, cost of raw materials, and output parameters from the BEEP assessment were selected for the extension evaluation, which used four different kinds of dependent functions. Based on the extension evaluation results, the ranking of the friction materials from the best to the worst is in the order of A, E, D, B, and C. Compared with the OE sample A, the eco-friendly sample E was evaluated as the best among the samples developed in this study.(5) The summarized assessment of the brake friction materials:The overall quality was used to rank friction materials, considering a stable friction coefficient, a low wear rate, a low cost, and low environmental impacts. The weighted average dependent degree in an extension evaluation can be used to describe the overall quality of a friciton material. In ths study, brake friction materials with the best overall quality are:alumina contained samples (5.6 vol.%), silicon carbide contained samples (3.4 vol.%), aramid pulp contained samples (3.4 vol.%), vermiculite contained samples (14.6 vol.%), and the eco-pad without metals, aramid pulp and antimony trisulfide.
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