DEVELOPMENT OF HOT POWDER PREFORM FORGING FOR HEAVY DUTY BRAKE PADS INVOLVING ABRASIVES AND LUBRICANTS

Abstract

Friction materials are very complex multiphase composites. These materials are also used in manufacturing clutches and gear assemblies of automobiles, brake blocks of locomotive trains, brake assemblies of 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 vehicles converts into heat due to friction between friction material on brake pads/discs/linings and rotor, which dissipates through brake assembly to the surrounding. There are three types of friction materials: resin bonded friction materials, sintered friction materials, and carbon-carbon friction materials. Out of these, sintered friction materials are chosen to improve upon the quality of brake pads to meet higher expectations of aircraft industries and to have an alternative simplified manufacturing technology. The. present investigation is aimed to develop Hot Powder Preform Forging technique for manufacturing of (Net shape) brake pads with-built-in powder based backing plate with an appreciably reduced cost. In the present work new sets of Iron and Aluminum based brake friction elements are developed in desired shapes and sizes for different applications. Dr. Peter Filip has reported that the average coefficient of friction (μ) value of the C,Xoq] commercial brake pads used in North America is around 0.35---0.45,E This is .lower than that of of pads (-- 0.45) employed in Europe and Asia. It has been established that the μ value of commercial brake pad formulation available in North America is around 0.357, but wear percentage (Ratio of weight loss to initial weight of brake pad) of the pads is too high ( 19.75 %) and it needs to be reduced for extending its life[109]. 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 the temp. of 600°C whereas Iron based sintered friction materials are suitable up to the temp. of 1100°C. These friction materials have high density, high hardness, low friction coefficient and high cost limiting their brake application in road vehicles. Another limitation is oxidation of material at high temperature, necessitating the use of inert gas/vacuum or special ii environment in manufacturing. Further, they have limited joint strength of back plate with friction material. The present work aims in the development of Iron and Aluminum powder base brake pads; where the back plates are also made of Iron and Aluminum powders respectively. Thus it is aimed to fabricate net-shape Iron and Aluminum powder based• brake pads in a single forming operation with better characteristics (low wear, low temperature rise, stable coefficient of friction) employing a newly developed technology namely "Hot Powder Preform Forging" to replace the existing compacting and sintering technique for manufacturing of metello-ceramic rotor brake pads for heavy duty applications such as for AN-32 and MIG-27 aircrafts. Aluminum brake pads so developed have substantially reduced weight as compared to Iron/Copper based brake pads. These brake pads so developed, consists of two major parts namely friction layer and backing plate which are formed simultaneously, such that these two layers do not separate out during operation. Joining of these two layers by sintering is usually difficult because of widely varying chemistry and processing involved. So the present investigation relies on manufacturing of these two parts simultaneously employing different chemistry of powders for friction layer and backing plate but having similar constituents, which results in better joint quality between them. Fourteen Iron based friction composites which are designated as FA01, FA02 ... to FA 10 and FMO 1, FM02, FM03 and FM04; and three Aluminum based friction composites which are designated as ALM01, ALM02 & ALM03 were formulated. Iron based friction composites consists of metallic powders (Fe = 70-80 wt %, P = 0.3-0.8 wt %, Cu = 5-10 wt %, Sn = 1-2 wt % etc), abrasives (SiC = 3-7 wt %, A1203 = 1-2 wt %), and solid lubricants/fillers (graphite = 8-10 wt %, BaSO4 = 1-6 wt %, Sb2S3 = 1-5 wt %, CaSO4 = 0-4wt.%.). The backing plate of Iron based brake pad consists of Fe = 95-97 wt %, Cu = 1-3 wt %, SiC = I wt %, graphite 0.1-0.5 wt %. The.constituents in Aluminum based friction material are Aluminum from 70 to 83 wt. %, silicon carbide (coarse-200 μm) from 6.5 to 25 wt. % and graphite (fine 50 μm and flake 350 μm) from 4 to 12 wt.%. Other constituents are zinc, antimony tri sulphide, and barium sulphate in varying percentages. Silicon carbide (fine z 50 μm) about 30 wt. % is added in pure Aluminum powder to develop a chemistry for back plate material. iii The metal constituents in both Iron and Aluminum based composites provide strength, high temperature stability, oxidation resistance and high thermal conductivity. The coarse abrasives improves strength, co-efficient of friction, fade resistance, wear resistance and interlocking between friction and backing plate layer at interface whereas fine abrasives improves the strength of back plate material. Coarse and flaky graphite usually acts as solid lubricant, fine graphite coats the SiC particles to improve the wettability between graphite coated SiC particles and matrix. Solid lubricant are meant for smoother application of brakes and improve anti-seizure and anti galling properties. The manufacturing sequence for brake pads is as follows: The `Hot Powder Preform Forging' method (Indian Patent application No. 152/DEL/2010 dated 27-01-2010), employs high rate of forming where the material undergoes severe plastic deformation under application of impact load. This method is a combination of cold compacting and hot forging processes. It involves five steps viz. i) powder blending/mixing; ii) cold compacting, iii) Preform is coated by high temperature oxidation resistant glassy coating as applicable in Iron based pads, iv) hot forging of the preform in open air and v) Annealing. The mixing/blending of powders for preparation of homogeneous powder mixture for backing and friction layer, was achieved by dry mixing of powders using Attritor and Pot mills in a particular sequence. Thereafter homogeneous powder mixture is cold compacted in closed die by screw forging press of 100 ton capacity to yield preforms. These preforms were coated by high temperature oxidation_ resistant glassy coating like paint as applicable in Iron based samples. Then hot forging of the preform at 1050°C for Iron based samples and 450°C for Al. based samples in another closed hot forging die, which is preheated and duly lubricated. The preform fully consolidates to its near theoretical density on forging. The forged component is a net shape product (of actual dimensions) is ejected out of the die, then these Iron based forged products are subjected to annealing operations at a temperature of 710°C for 2 hours to adjust hardness of friction layer to < 100 BHN. This method enables to fabricate highly densified Iron and aluminum powder based brake pads with built in powder based backing plate. It does not required inert/vacuum environment for processing and secondary processes like sintering, machining and homogenization for densification or tribo properties improvement. Such a product has been found to be suitable for medium to heavy duty brake pad applications such as for AN-32 rotor aircraft and MIG-27 fighter iv 0 aircraft where weight reduction in association with low coefficient of thermal expansion, higher thermal conductivity and better thermal capability, stability of friction coefficient at high temperature are of prime considerations. The process as described above largely overcomes the limitations of presently practiced conventional P/M route based on compacting and sintering. Detailed characterization of Iron and Aluminium based friction pads and powder based steel backing plate is carried out & results showed improved physical, mechanical, frictional and metallurgical properties. It has been completed in four stages, i) Physical test, ii) Pin-on-disc test, iii) Subscale dynamometer test, and iv) Metallography examination. The physical tests have been conducted to measure the density and hardness of brake pad materials. Subscale dynamometer test have been performed and compared with results of pads manufactured in H2 atmosphere for heavy duty applications such as AN-32, and MIG-27 military rotor aircrafts. Result and Discussion 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 (medium to heavy duty) by merely controlling these two parameters. The density variation of Iron based friction composites is from 5.2 gm/cc to 6.3 gm/cc, which is in the permissible range (5-6.8 gm/cc) and hardness variation is from 69 to 130 BHN. Some composites like FM02, FM03 and FM04 has high hardness, this is due to the presence of hard constituents like SiC and improper heat treatment. In Aluminum based composites density is in the range of 2.86-3.02 gm/cc, and hardness is 69-74 BHN, which is in the permissible range. It can be inferred that these variations of density and hardness are very nominal and depend upon quality and quantity of different ingredients in the composites. All Iron based composites could have shown even better performance, if they were properly annealed. This is on account of the fact that annealing would lead to coarse grain microstructure which will offer better grip for the brake pads and hence would provide better performance. The pin-on-disc wear tests were carried out at loads; 5 and 8 kg at constant speed: 9 m/s for all the Iron and Aluminum based composites made in the present investigation to Iry asses the tribological characteristics at laboratory level to provide a base for selection of chemistry for standard test. The performance of these was evaluated in terms of cumulative wear (gm), coefficient of friction (μ), temperature rise (°C) and noise level (dB). The significant features recorded were i) cumulative wear, which depends upon the applied load and material related factors. In some composites cumulative wear is gradually increasing from lower 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 like SiC and lubricants, ii) Coefficient of friction of some composites is stable and lies in aeronautic industry standard range; 0.20 to 0.45 at lower as well as higher applied loads, iii) Temperature rise varies within a range from 100 to 300°C. It is noticed that 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 metal content in the -brake elements, which is in our Iron based composites .varies from 75% to 85%. In Al. based composites temperature rise varies within a range from 100°C to 200°C. It is noticed that there is minor affect of changing the applied load on temperature rise. It is also noticed that the temperature rise and noise levels in case of Al. based friction composites is almost equivalent to Iron based friction composites. On the basis of pin-on-disc test, the Iron based composites FA01, FA02, FA06, FA07, FA09, FA10, FM01, FM02, FM03, and FM04, and Aluminum based composites ALM02 and ALM03 were selected for standard level tribo-characterization to develop brake pads for medium to heavy duty aircraft applications. The subscale brake inertia dynamometer tribo-tests of selected Iron and Al. based friction composites against cast Iron inertia wheel were carried out under low and high energy levels to asses and evaluate the tribological characteristics. After evaluation of tribological characteristics at low -and high energy levels, high energy standard test for AN-32 and MIG-27 rotor aircraft is carried out under Rejected Take off (RTO) conditions. The Iron based composites FA06 and FM02 and Aluminium based composites ALM03 is selected to judge their suitability for AN-32 and MIG-27 rotor aircraft brake applications. The performance of samples were estimated in terms of wear, coefficient of friction, run down revolution, run down time, temperature rise. In sub scale dynamometer test, the most significant aspect relate to stable performance with increasing number of cycles. In this context, brake fading tendency in vi the pads is almost absent. It is also noteworthy that for heavy duty applications, 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. The significant features recorded were i) The wear for Iron based composite FA06 for AN-32 aircraft, and FM02 for MIG-27 fighter aircraft is as per the Aeronautic industry standard wear ( 1.25mm for AN-32 and 0.75mm for MIG-27 rotor aircraft as reported by Singaravelu Lenin D), ii) Coefficient of friction for composite FA06 and FM02 lies in standard range (-- 0.18 to 0.45), iii) RD time for composite FA06 and FM02 is 8.6 min and 16.7 min which lies within Aeronautic industry standard. RD time range ( 6 to 12 min and 18 min). Similarly in Al. based composites significant features recorded were: i) The wear in terms of pad thickness (mm) of developed Al based composites is lower than standard wear except for composite ALM01, ii) RD time range for developed Al based composites lies in the standard RD time range for AN-32 aircraft, iii) Coefficient of friction for developed Al. based composites is equal to standard coefficient of friction for AN-32 aircraft applications , resulting in low vibration and judder, and iv) Weight reduction per pad is about 40 %. Powder based steel used as backing plate in Iron based composites have been characterized in terms of porosity, tensile properties such as yield strength, ultimate tensile strength, elongation and hardness. It is found that steel backing plates developed in the present investigation have very good hot workability. Steel backing plate containing 0.5% C, 1% SiC and 2.5% Cu showed higher strength (>539MPa) and higher resilience value with lower ductility under annealed conditions. Optical microscopy, SEM-EDAX analysis and X-Ray mapping have been performed to analyze the distribution of ingredients in the matrix and elements diffusion at the interface. All the microstructures of preform 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. The significant features recorded are: i) Distribution of ingredients in matrix is homogeneous or uniformly distributed and there appears to be no segregation of constituents in the structure. ii) Few .porosities are noticed in back plate materials, and iii) Interface between friction layer and back plate materials are sometimes vii not clearly defined due to the mass diffusion of elements at the interface. This is expected to give strong joint between the layers. Analyses of results have been carried out to optimize the chemistry. To find the suitability of optimized chemistry for different applications, results are evaluated and compared with the standard results. Conclusions Overall conclusions are i) Hot Powder Preform forging (with built-in powder based backing plate), a process, for fabricating Iron and Aluminium based brake pads has been successfully developed for the first time, ii) Sintering, inert/vacuum environment and secondary processes associated with conventional powder metallurgical methods have been completely eliminated, iii) The technology completely eliminates the problem of joining of backing plate to friction element on account of simultaneous processing, iv) It has been possible to reduce the failure rate of brake padsdue to breakage and warpage of the product. This is mainly because of the simplified chemistry with overall greater weightage of metallic constituents, v) Developed an indigenous high temperature oxidation resistants, glassy coating which can be applied on Iron based preforms like paints, vi) Coefficient of Friction (COF) can be maneuvered with chemistry modifications=to suit a vide.nature of application, vii) The results shows a suitable correlation between pin-on-disc tests and sub- scale dynamometer tests, which is likely to encourage further development of novel friction materials, viii) Wear is within the range for AN-32 and MIG-27 rotor aircraft brake element applications, ix) Powder based steel backing plate developed in the present investigation have very good hot workability,- - 2• S ';Io Cu. x) Steel backing plate containing 0.5% E and 1 %o' SiC showed higher strength. viii