THE EFFECTS OF PAD SURFACE TOPOGRAPHY ON DISC BRAKE SQUEAL

THE EFFECTS OF PAD SURFACE TOPOGRAPHY ON DISC BRAKESQUEALAhmad Razimi Mat Lazim1, a,*, Mohd Kameil Abdul Hamid1, band Abd Rahim Abu Bakar1, c1Department of Automotive Engineering, Faculty of Mechanical Engineering,Universiti Teknologi Malaysia, 81310 UTM Skudai, MALAYSIAa*gemee98@gmail.com, bkameil@fkm.utm.my, carahim@fkm.utm.myKeywords: Squeal, brake pad, wear, EDX, Optical MicroscopeAbstract. Brake squeal has always been a major NVH problem to many car makers due tosignificant number of warranty claims. Brake squeal is a high frequency noise (above 1 kHz)emanating from car disc brakes that get excited due to one or more mechanisms such as modecoupling, stick-slip, hammering and sprag-slip. This paper attempts to investigate the effects ofbrake pad surface topography on squeal generation. Two pairs of a non-asbestos organic (NAO)brake pad will be tested on a brake dynamometer test rig. Surface topography of the brake pad willbe analyzed through microscopic techniques using energy dispersive X-ray analysis (EDX), andoptical microscope.IntroductionBrake squeal is a challenging subject to tackle not only due to its strong dependence on manyparameters but also the mechanical interactions in the brake system are very complicated [1]. It iswell accepted in the brake research community that squealing brake is due to one or more triggeredmechanisms such as stick–slip, sprag–slip, negative damping, mode coupling and hammering [2-4].Despite many investigations over the years to clarify the mechanism causing automotive brakesqueal the phenomenon is not yet fully understood. Therefore, increasing the information on theknowledge that contributes brake squeal is important to the extensive research and developmentwork currently being performed.There is a very limited knowledge on what really happens in the material behavior duringbraking process of a brake system. Earlier studies have shown that the reaction on the slidingsurface has a strong influence on the generation of squeal [5]. Rhee et al. [6] among the earlyresearchers who study the effect of tribological behavior on automotive brakes believes that thesurface changes contribute to a major factor for controlling noise, friction and wear. This is truewhere Eriksson et al. [7] through his PhD thesis of friction and contact phenomena of disc brakesrelated to squeal conclude that the friction behavior on brakes surface is closely related withformation of plateaus which is due to wear resistance of components. Research of the surfacetopography on squeal generation continued by Sheriff [8] where he identified an evident to proveeither squeal are generated or eliminated at the surface topography of pad and disc. Rusli andOkuma [9] study the effect of surface topography of dry sliding surface founded that squeal noisetend to be generated at both smooth and rough surface.Ertan and Yavuz [10] conducted the friction test using chase type friction test with gray cast ironrotor to study the behavior of wear resistance and friction stability and tribological properties on thepad surface. Result showed that the manufacturing parameters of friction and wear play animportant part and contribute to improve the tribological behavior of brake lining system.With the availability of new testing methods and instruments, the detailed study of themicrostructure, nanostructure and composition of the contact surface are possible. This workinvestigates surface topography of the brake pads using Leica DM2500M optical microscope, andenergy dispersive X-ray analysis (EDX).

Experimental ProcedureTest Rig. The test rig used is shown in Fig. 1. The maximum power output for this AC motor is11 kW. The power available from the motor matching with the power absorbed by the brake with ahydraulic pressure 20 bars where the maximum brake-line pressure for squeal occurrence. Themaximum torque is 413 Nm. A number of transducers were used to measure the squeal data and thedata was then recorded through data acquisition system (DAQ) of 16 channel Dewetron FastFourier Transform (FFT) analyzer.Fig. 1 Brake squeal dynamometerBrake Pad Materials. Two pairs of a brake pad from different suppliers were used and tested.Since no composition and exact formulation of the brake pads are available, an energy dispersive Xray spectroscopy (EDX) was used to identify the main constituent present in the brake pads. Thepad constituents are in the mixture of 6 different ingredients including phenolic resin, reinforcingfiber, friction modifiers, abrasive particles, fillers, solid lubricant and metal oxide. This ingredientsare normally weighed in given proportions, blended well, molded in a steel die and heat treated withthe given parameters [10]. Table 1 gives the main constitutes of both brake pads. The disc materialis made of a commercial gray cast iron which contains of graphite flakes in a pearlitic matrix withsmall quantity of free ferrite and homogeneous structure which suggest a typical cast dendriticmicrostructure [11].Table 1 The main constituents of brake pad sampleSampleElementsSample 1CaOSoMgAlSiSuChoFePoCalSample 2CaOMgAlSiSuPoCalFe--Results and DiscussionSqueal Test Result. The output signal from the microphone was received on FFT-signal analyzer tomeasure the amplitude and the frequency of the squeal noise. Squeal event for both pads were identifiedbased on the synchronization of peak value between Sound Pressure Level (SPL) and acceleration inpower spectrum. Fig. 2 is a time domain response describing the squeal generation.

SquealSquealSilentSilentFig. 2 Time domain response during squeal and silent conditionSurface Topography. The analysis of surface topography using optical microscope highlightsthe actual images of contact condition under both before and after the squealing test. The padsurface was analyzed at both piston side and finger side before and after braking with squeal asshown in Fig. 3 and Fig.4. It can be seen that the sliding surface after squeal is distinctly differentfrom that without squeal. For pad sample 1 (Figs. 3c, and 3d) and pad sample 2 (Figs. 4c, and 4d)shows that the surface roughness influences the nature of surface damage where almost all testedpad samples showing a major scratch on both pads surface during squeal events. Further it can beseen that the existence of flat surface of both pads with different size and shape of shallow groovesin the sliding direction. The surface layer also shows lots of lowlands (hole) which is out of slidingcontact as this situation is more likely can be seen at the pad sample 2 (Figs. 4c and 4d). It isdemonstrated that by comparing their profile through the sliding direction both pads shows thedifferent uniform sliding which result in different roughness.Further investigation show that the wear factor is also influence the surface topography of padsarea. Figs. 3c and 3d (pad sample 1) and Figs. 4c and 4d (pad sample 2) shows clear images of weareffect during squealing condition of pad surface. These photo images show a lot of groove structureand wear scars on the pad surface.acbdFig. 3 Surface topography of Sample 1: Top is the piston side and bottom is the finger side, (a) and(b) are surface condition before the test, (c) and (d) are surface condition after the test and squealevent.

acbdFig. 4 Surface topography of Sample 2: Top is the piston side and bottom is the finger side, (a) and(b) are surface condition before the test, (c) and (d) are surface condition after the test and squealevent.Also, under certain circumstances the existence of large amount of small loose wear particles(brake dust) passing through the contact which assumed to form accumulation of wear debris(agglomerates particles) also known as third body entrapped and embedded into pad interface due tothe applied pressure, shear force and friction heat which combines and compact the debris are alsoinfluence the topography of pad surface.In addition, the investigation has shown that factor such as less wear resistance of padcomposition, the effect of ingredients also contribute to the surface characteristic thus resultingnoise generation during braking process. This is true as investigated by Cho et al. [11] whichindicated that the composition of friction material contribute to noise propensity. Eriksson’sinvestigation [7] on the existence of shiny spot against a dark background known as a plateaus, alsofound similar result with the surface condition shown in Fig. 3 and Fig 4.ConclusionsBrake squeal tests were conducted on two pairs of different non asbestos organic (NAO) brakefriction material to study the effect of pad topography during squeal event. Using different speedand hydraulic pressure brake pads were then analyzed through energy dispersive X-rayspectroscopy (EDX) and optical microscope. The main conclusions of the present work aresummarized as follows:1. Squeal sliding event contribute to damage of pad surface.2. The surface topography of pads during squealing condition demonstrates differentcharacteristic compared to a new pair of brake pads.3. The surface condition shows more wear scratch and increase in roughness of the area wheresqueal occurred.4. Pressure, sliding speed and vibration significantly influences the transformation or surfacetopography of pad materials.The existence of third body during sliding process also influences the topography of the pad. Thenature of topography condition in pad surface is manifold and complex. Therefore a deeperunderstanding of topography is needed in the development of a silent braking system.

AcknowledgementThe authors would like to thank Universiti Teknologi Malaysia for supporting this research work(GUP-Tier 2 Vot 00J45)References[1] F. Massi, Y. Berthier, L. Baillet, Contact surface topography and system dynamics of brakesqueal, Wear 265 (11-12) (2008) 1784-1792.[2] N.M. Kinkaid, O.M. O’Reilly, P. Papadopoulos, Automotive disc brake squeal, Journal ofSound and Vibration 267 (2003) 105–166.[3] A. Papinniemi, J.C.S. Lai, J. Zhao, L. Loader, Brake squeal: a literature review, AppliedAcoustics 63 (2002) 391–400.[4] F. Chen, F. Tan, C. Chen, C.A. Tan, R.L. Quaglia, Disc Brake Squeal: Mechanism, Analysis,Evaluation and Reduction/Prevention, SAE-Society of Automotive Engineers, 2006.[5] S.K. Rhee, M.G. Jacko, P.H.S. Tsang: The role of friction film in friction, wear, and noise ofautomotive brakes, Technical Report 900004, SAE, Warrendale, PA, 1990.[6] Eriksson, M., Bergman, P., and Jacobson, S: Surface characterization of brake pads afterrunning under silent and squealing conditions,” Wear (232), 1999, pp. 163–167.[7] M. Eriksson: Friction and Contact Phenomena of Disc Brakes Related to Squeal, Ph.D. Thesis,Uppsala University, Uppsala, Sweden, 2000.[8] Sherif H.A. :Investigation on effect of surface topography of pad/disc assembly on squealgeneration (2004) Wear, 257 (7-8), pp. 687-695.[9] Rusli M., Okuma M. :Effect of surface topography on mode-coupling model of dry contactsliding systems (2007) Journal of Sound and Vibration, 308 (3-5), pp. 721-734.[10] Ertan R., Yavuz N. : An experimental study on the effects of manufacturing parameters on thetribological properties of brake lining materials (2010) Wear, 268 (11-12), pp. 1524-1532.[11] Cho M.H., Kim S.J., Basch R.H., Fash J.W., Jang H. , Tribological study of gray cast iron withautomotive brake linings: The effect of rotor microstructure (2003) TribologyInternational, 36 (7), pp. 537-545.

Keywords: Squeal, brake pad, wear, EDX, Optical Microscope Abstract. Brake squeal has always been a major NVH problem to many car makers due to significant number of warranty claims. Brake squeal is a high frequency noise (above 1 kHz) emanating from car disc brakes that get excited due to one or more mechanisms such as mode