DEVELOPMENT OF FRICTION MATERIALS THROUGH POWDER METALLURGY

Abstract

Friction materials are mainly employed in brakes, clutches and gear assemblies of automobiles, locomotive trains, commercial/fighter aircrafts, earth moving equipments, agricultural equipments, cranes, heavy presses, and excavators etc. When the brakes are applied, the kinetic energy of the moving system converts into heat due to friction between friction material on brake pads/discs/linings and rotor, which dissipates through brake assembly to the surrounding. Three categories of friction materials are normally available. First, resin bonded friction materials, second, sintered friction materials, and third, carbon-carbon friction materials. Out of these categories, sintered friction materials segment has selected to improve upon its quality to meet higher expectations of automobile and aircraft industries. It is aimed to produce them with better characteristics employing newly developed technology, namely, Hot Powder Forging with appreciably reduced cost. Metal based friction materials contain different constituents such as lubricants, abrasives along with other alloying elements. The metallic constituents provide strength, high temperature stability, oxidation resistance, and high thermal conductivity, whereas abrasives improve co-efficient of friction, fade resistance and wear resistance. Lubricants in these are usually solid type which are meant for smoother application of brakes and anti-seizure. Their simultaneous presence gives rise to parallel enhancement of certain properties of brake material. Sintered metallic friction materials can be operated under two conditions either dry or wet. Usually, copper based friction materials are used in wet conditions i.e. immersed in oil whereas iron based friction materials are operated in dry conditions only. The sintered copper based friction materials are suitable for temperatures up to 600°C whereas iron based sintered friction materials are suitable up to 1100°C. In the present investigation, the new iron-based friction material pads with improved performance and with built-in steel backing plate are produced by a novel P/M technique i.e. 'Hot Powder Forging' (described in Indian patent filed on Nov. 7, 2006). It employs high rate of forming where the material undergoes severe plastic deformation under application of impact load; purpose of compacting and sintering both are thus ii fulfilled simultaneously in a single step. The powder mixture is encapsulated in steel capsule and is heated to temperature range of 1000-1100°C and then hot forged in a forging press followed by homogenization treatment. The steel capsule cover later on serves as backing plate since it joins securely with the forged slab. The process as described above largely overcomes the limitations of presently practiced conventional P/M route based on compacting and siintering. Detailed characterization of friction pads produced this way exhibits improved physical, mechanical, frictional and metallurgical properties. Subscale dynamometer tests have been performed and compared with results of existing sintered materials for military aircraft applications such as AN-32, MIG 21 and MIG 27 aircrafts. The technology developed has capability to control density and hardness parameters in very precise manner and therefore it is possible to develop friction layers for different ranges of application (light to very heavy duty) by merely controlling these two parameters. In Pin-on-disc wear test, wear is gradually increasing from very low value to higher values thereafter it shows a significant drop in the amount of wear. This is possible because there is simultaneous role of wear resistant constituents and lubricants. The noise level during the test is not significantly altered in all the cases. This is because of the fact that the noise level is related to met metal content of the brake elements which in our case varies from 82% to 90%. In sub scale dynamometer tests, the most significant aspect of the samples relate to stable performance with large number of braking cycles. In this context, brake fading tendency in our pads is completely absent. It is also noteworthy to mention that for application like AN 32 aircraft, a high dose of abrasives in the chemistry formulation is not necessary. Mere addition of 0.8% Phosphorous takes care of abrasive resistance of the pads, and, accordingly, a very simple chemistry like FMO8N has qualified for such an application. This aspect has emerged out from the present investigation only and has not been reported sofar. Further, it is noteworthy to mention, that MIG 27 stator application is very specialized high energy condition for which chemistry of sintered pads is very complex and costly. In comparison to this, chemistry developed in the present investigation is much simpler and offer wide variety of choices to suit this challenging application. iii Based on the results described in these tests, it can be mentioned that higher density, greater than 5.4 gm/cc of brake pad friction element, is desirable for low energy application such as AN 32. But contents of the lubrication (such as Ba SO4, Sb2S3, Sn, and graphite) should not exceed 13%. It may be therefore inferred that density for such a range of application is significant parameter and higher density is desirable for improved wear performance. For MIG 27 aircraft, stator application is very specialized high energy condition for which chemistry of sintered pads is very complex and costly. In comparison to this, chemistry developed in the present investigation is much simpler and offer wide variety of choices to suit this challenging application. Quality of lubricants such as Sb2S3 would be more suited in comparison BaSO4 and BN. It is established form the present investigation that apart from chemistry, annealing treatment and density adjustments are equally important for different energy ranges of application All the microstructures of forged samples at the interface indicate strong bonding between backing plate and friction element because of enhanced diffusion of carbon from friction element side to backing plate side and absence of any layered structure at the interface. It is clear from SEM with EDAX pictures that the constituents present in the sample are uniformly distributed. There appears to be no segregation of constituents in the structure. Accordingly, the performance of these samples in actual service conditions is expected to be more consistent in comparison to sintered samples. Bend test is performed to confirm the joint quality of friction material and backing plate. The bending behavior of forged brake pad samples is significantly superior to sintered sample for equivalent applications. It may also be mentioned that the separation of backing plate with friction element is not observed after bending in case of our forged samples where as for sintered sample there is clear cut separation along the interface. This establishes that joining between backing plate and friction element is superior owing to hot forging technique in comparison to pressure sintering technique employed in MIG 21 sintered pads. Ultrasonic testing and metallographic examination further confirm that the forged brake pads developed in the present investigation possess sound, defect free interface between backing plate and friction layer unlike sintered pads. iv