DEVELOPMENT AND CHARACTERISATION OF BRAKE PADS FOR LIGHT TO HEAVY DUTY APPLICATIONS

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. When the brakes are applied, the kinetic energy of the moving system transforms into heat due to friction between brake pads/discs/linings and rotor, which dissipates through brake assembly to the surroundings. Three main categories of friction materials are normally available namely: organic based friction materials, metallic based friction Materials and carbon-carbon friction materials. Sintered metallic friction materials can be operated in either dry or wet conditions. 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. Al based brake friction element such as brake pad with built-in backing plate has not been develope until now as per letarature review. Present investigation is the first example of such type of friction materials. The present work relates to the development of aluminium powder based brake pads/stators; which comprise of Al powder based friction and back plate materials. It is aimed to fabricate net-shape Al powder based brake pad in a single forming operation, employing a technology namely "Preform Hot Powder Forging". This is likely to provide alternative to resin based brake pads/stators for light/medium/heavy vehicles and Iron/Cu/Resin based brake pads for aircraft with appreciably low wear, low temperature rise, stable coefficient of friction, high recovery, low fade, light weight and low cost. In this invention, thirty different formulations for friction layer designated as FA101 to FAI30 and 31st formulation for back plate layer designated as FA131 were developed. The weight percentages of different constituents in aluminum matrix based friction material were varied in following ranges; Silicon carbide (coarse-200 g) from 6.5 to 25 wt. % and graphite (fine 50g and flake 350g) from 4 to 12 wt.%. Other constituents are zinc, ceramic wool, coconut fiber, antimony tri sulphide, and barium sulphate in varying percentages. Silicon carbide (fine 50g) about 30 wt. % is added in pure aluminium powder to develop a chemistry for back plate material. The metallic constituents provide strength, high temperature stability, oxidation resistance and high thermal conductivity. The coarse abrasive particles improve wear strength, coefficient of friction; fade resistance, whereas fine abrasive particles in back plate improve the strength of back plate material. Fine graphite is usually a solid lubricant and is introduced for ii coating of hard SiC particles to improve their matching with the matrix whereas flaky graphite is a solid lubricant meant for smoother application of brakes and improvement of anti-seizure and anti-galling properties. Their simultaneous presence gives rise to parallel enhancement of tribo-performance of brake material. The 'Preform Hot Powder Forging' method (described in Indian patent filed on Feb. 7, 2009) 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 three steps viz. i) powder blending/mixing; ii) cold compacting and iii) hot forging of the preform in open air. The mixing/blending of powders for preparation of homogeneous powder mixture is achieved by dry mixing of powders using attritor and pot mills in a particular sequence. Thereafter homogeneous powder mixture is cold compacted in a die to yield a green preform followed by hot forging of the preform at 450 °C in another die to obtain a net shaped product as brake pad of actual dimensions. This method enables to fabricate highly densified, net shaped aluminum powder based brake pads/stators with built in backing plate. It does not require inert/vacuum environment for processing and secondary processes like sintering, machining and homogenization. Such a product has been found to be suitable for light/medium/heavy duty automobiles and for AN-32 military transport aircraft where weight reduction in association with low coefficient of thermal expansion, higher thermal conductivity, stability of friction coefficient at high temperature, high recovery and low fade are 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 brake pad materials produced this way exhibit improved physical, tribological, mechanical, thermal and metallurgical properties. It has been completed in seven stages namely i) physical tests, ii) pin-on-disc test, iii) krauss rig test, reduced/full scale test, iv) subscale dynamometer test, v) mechanical tests, vi) thermal tests and vii) metallographic examination. The physical tests have been conducted to measure the density and hardness of brake pad materials. The range of obtained green and forged densities is 2.26 to 2.69 gm/cc and 2.39 to 3.00 gm/cc respectively and hardness variation is from 56 to 80 BHN for composites from FAI01 to FA131 The pin-on-disc wear tests have carried out at loads of 5 and 8 kg at a constant speed of 9 m/s for all the samples made in the present investigation to asses the tribological characteristics at the laboratory level. The chemistry developed are evaluated in terms of specific wear, coefficient of friction, and temperature rise and noise level. Results are compared to corresponding values of Fe based friction materials. Based on pin-on-disc tests, the iii compositions; FAI17, FA121, FA122, FA123, FAI24, FAI25, FA126, FA128, FA129 and FAB() were selected for standard level tribo-characterization. Krauss test: ECR R-90 standard regulation friction tests of selected Al based brake pads against cast iron disc have been carried out on near-to-actual field conditions to judge their suitability for heavy/medium/light automobile vehicles. Parameters evaluated are in terms of i) performance friction coefficient (n. Performance), ii) performance friction fade coefficient (1Fade), iii) performance recovery (paecovery), iv) fade (%), v) recovery (%), vi) disc ternpeiature rise (DTR), vii) wear, and viii) fluctuation in coefficient of friction. This test has been completed in two phases. Reduced scale friction test: a non-standard test, which has been, carried out to asses the tribological characteristics at low brake pressures (0.5 to 0.7 MPa). Full-scale friction test: a standard test carried out at high brake pressures (1.0 to 1.3 MPa) to judge the suitability of developed brake pad materials in heavy/medium/light automobile vehicles. The developed Al based composites namely FA121, FA123 FA124 FA125 and FA126 qualify the standard operating parameters and have been found suitable for light/medium/heavy duty automotive brake applications. Subscale dynamometer tests has been performed for evaluation of brake pad charactefistics on near-to-actual field conditions for AN32 aircraft in three phases. Initial subscale dynamometer tests under low and medium input kinetic energy have been carried out to assess the tribo-performance and develop a base for selection of composites for higher energy test. High kinetic energy subscale dynamometer standard test -TQ 1 has been carried out to assess the tribological characteristics of brake pads materials for AN-32 aircraft. Based oft overall performance and their evaluation in comparison to commercially used sintered iron based friction materials in AN32 aircraft, developed composites namely FAI28, FA129 and FAI30 qualify the standard parameters and have been found suitable for AN32 aircraft brake application. The thermal properties namely thermal diffusivity, specific heat, thermal conductivity, and thermal expansion coefficient at different temperature ranges have been measured using thermal dilatometer, flash thermometer, thermal calorimeter and differential calorimeter respectively. For the study of mechanical properties namely elastic modulus, yield and ultimate tensile strength, compressive strength, shear modulus and shear strength, the mechanical test of samples has been conducted using TPT-426 tensometer. The thermal properties like thermal diffusitivity, specific heat, thermal conductivity and thermal expansion coefficient vary with in a range from 4.10 to 7.23 (10 -5 m 2 s -1), from 830 to 876 (Jkg'K -1), from 167 to 171 (Wm-1K-) and from 7.10 to 10.3 (10-6/ K) respectively and mechanical properties namely longitudinal young's modulus, transverse young's modulus, yield tensile strength, ultimate iv tensile strength, yield compressive strength, shear modulus and shear strength vary with in a range from 228 to 247 GPa, from 100 to 105 GPa, from 310 to 323 MPa, from 347 to 366 MPa, from 253 to 268 MPa, 24 to 32 GPa and from 76 to 88 MPa respectively. Optical microscopic, EDAX, X-ray mapping and SEM micro examinations have been performed to analyze the distribution of ingredients in matrix and their diffusion. The significant features recorded were i) distribution of ingredients in matrix is homogeneous, ii) few voids were noticed, iii) interface between friction layer and back plate material is not clearly defined due to mass diffusion of elements at the interface. Analysis of results has been made to optimize the chemistry of friction materials and investigate their suitability for different applications. Results are evaluated quantitatively and qualitatively with published standards for specific applications. Overall conclusions are i) Al based brake pad compositions are suitable and better than existing brake pad materials for light to heavy duty vehicles and also for AN-32 aircraft, ii) Preform Powder Forging' technology overcomes the limitations of sintering process and has been successfully adopted for brake pad with built-in back plate manufacture, iii) Friction layer with back plate layer (brake pad) can be fabricated in one operation alone, iv) Sintering, inert/vacuum environment and secondary process have been completely eliminated, v) For optimum tribo-performance of Al based friction composites, ingredients should be in the range from 22 to 25 wt.%, vii) New set of Al based friction materials have been prepared with simpler chemistry and superior characteristics for automobiles and aircrafts and viii) Significant mass reduction (40-60%) in friction elements has been achieved in case of aircrafts and automobiles with excellent recyclability and environmental