High Performance Friction Materials Employing Refractory Metal Additions

摘要

Heavy duty friction materials with metal matrices have been manufactured using powder metallurgy technology for decades. The metal matrices are usually copper, iron, or some combination these [1]. For copper-matrix friction materials, other alloying elements such as Sn, Zn, Ni, Al, Sb, Si, Cr, Ti are added as sintering aids and/or solid-solution strengtheners. A class of friction materials for high energy/temperature applications generally has as the metallic portion Cu and Fe in ratios of 3:2 to 4:1 volume percent. The iron and copper remain as discrete metallic phases, where the copper lends good sinterability and good thermal conductivity. The iron provides for better high energy/temperature strength and friction characteristics, but tends to worsen the NVH (noise, vibration, and harshness) characteristics of the friction material. It was observed in prior tests that additions of refractory metals [2,3] and intermetallic compounds [4] in place of iron had beneficial effects on the friction and wear performance of the brake material. In this study the other metal ingredients added to the copper matrix are Fe, Mo, W, Nb, Co, FeMo, and FeCoCu, the latter two ingredients being composite powders. All of these metallic additions were selected for their limited solubility in the copper matrix, with the intent on providing hard particle reinforcement that bonds well to the copper matrix, unlike the silicate additives. This study focuses on the effects of other metallic additions to a copper matrix, so the friction material compositions evaluated in this study all have the same amounts of carbon and nonmetallic ingredients. These nonsinterable ingredients are present in large volume fractions in order to produce the desired friction levels and wear rates demanded by the requirements of high energy brake systems, such as those used in regional, commercial and military aircraft. Because many of these particles (metallic oxides, carbides, nitrides, etc) are essentially incompressible, the metallic powders employed must be relatively fine and have high green strength, so as to lend compactability to the composite material. This somewhat limits our selection of metallic and intermetallic powders to those powders that will lend green and sintered strength to the friction material. In the present study, the friction and wear properties of the materials with different refractory metal and intermetallic additions were evaluated in dynamometer tests that simulate the service environment of the aircraft brake, which in this case is a commercial airliner with capacity greater than 70 passengers. Strength, hardness and microstructures of the composite brake materials were investigated so as to find relationships between intrinsic material properties and friction & wear performance.