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.Wear 232 1999 168175rlocaterwearTribological properties of automotive disc brakes with solid lubricantsLuise Gudmand-Hyera,), Allan Bachb, Georg T. Nielsenc, Per MorgenaaPhysics Department, Odense Uniersity, Campusej 55, 5230 Odense M, DenmarkbRoulunds Fabriker, Hestehaen 51, 5260 Odense S, DenmarkcTristan, Kochsej 1, 2600 Glostrup, DenmarkAbstractIn automotive brake systems, high temperatures and pressures are generated at the contacting surfaces. This affects the pad and discmaterials, creating a friction film between the surfaces, which consists of wear particles and volatile reactants from the pad and disc. Toacquire suitable tribological properties, a pad matrix contains up to 20 different ingredients, mainly selected from experience. In thepresent report the effect of solid lubricants has been studied in three different pad matrices with relatively few components as compared tocommercial brake pads. The components are Cu S, PbS and Sb S , which are known to modify and stabilise the friction coefficient. The223friction coefficient and wear rates of the pads are examined on a dynamometer, which simulates series of real-life car brake events. Twodifferent energy levels and two ambient temperatures are included. After these tests, the brake disc surfaces are analysed with energy.dispersive X-ray EDX and Auger electron spectroscopy AES in combination with argon ion sputtering to study the microscopic lateraland in-depth distribution of elements on and below the surface, looking for traces of the friction film. These experiments are used todiscuss the correlation between the tribological properties and the external variables braking temperature, solid lubricants and padmatrix. q1999 Elsevier Science S.A. All rights reserved.Keywords: Tribology; Automotive brakes; Solid lubricants; Friction film; Surface analysis1. IntroductionThe development of new pad materials is a complicatedmatter as the components interact and synergetic effectsthat are hard to disentangle arise. Quite often, the selectionof the different components is based on empirical experi-ence, which means that some components are added forreasons based more on tradition and less on knowledgeabout their influence on the brake performance. In thecontact zone between the disc and pad, energy is trans-ferred and heat is dissipated. As a consequence, a frictionfilm is created in the contact zone. The character of thefilm depends on the friction pad formulation. Metal sul-phides are known to modify and stabilise the frictioncoefficient, though the more specific effects are not wellknown. They are usually added though very expensive and.therefore primarily used for original equipment OE and)Corresponding author. Tel.: q45-65573509; fax: q45-65158760;E-mail: lghfysik.ou.dk.original spare parts OES . Thus a better knowledge of theeffects of adding those components can improve the opti-misation process and limit their use to minimise the price,which is an important issue in the car industry.Other groups have studied the formation of the frictionfilm. They looked for the influence of the film propertieson the friction coefficient. The chemistry of a solid transfer.film was studied with energy-dispersive X-ray EDX andw ximaging XPS by Wirth et al. 1 . A wear-in period hasbeen described as the time during which the friction isfluctuating with decreasing amount of material transfer asstable conditions are reached. The specific role of additiveswith respect to the relative amount of iron and oxygen inw xthe discs was discussed recently by Holinski2 . Theformation of an iron layer on asbestos brake pads slidingw xover steel surfaces has been investigated by Scieszka 3with SEM together with tribo-oxidation.Experience from previous studies on friction film for-mation under tribological testing with samples of hard ballw xbearing steel indicated a very thin film of around 30 A 4 .The references above have inspired us to study theeffects of metal sulphides on the formation of a friction0043-1648r99r$ - see front matter q 1999 Elsevier Science S.A. All rights reserved.PII: S0043-1648 99 00142-8()L. Gudmand-Hyer et al.rWear 232 1999 168175169Table 1The composition of the friction materials is given in vol.%. The numbers in parenthesis indicate the reference friction material without metal sulphide.Main elementCharacteristic namesNmet vol.%Low1 vol.%Low2 vol.%.FibreOrganic aramid5 5.411 125 5.4.Metal9 9.8.Other inorganic18 19.615 16.318 19.6.FillersBarytes, friction dust, and vermiculite41 44.625.5 27.729 31.5.MetalsBrass5 5.49 9.8.AbrasivesZirconium silicate5 5.45 5.4.Aluminium oxide0.5 0.5.BindersResin, rubber23 25.026 28.326 28.3 . . .Metal sulphideCopper, lead or antimony sulphide8 08 08 0Total vol.%100100100film in a brake system and its effect on the surface of thedisc as indication of specific wear and friction behaviour.2. Materials and experimental procedures2.1. Friction pad formulationThree different base matrices are designed especiallyfor this project, and they are simplified and compared to acommercial brake pad. The matrices are: one with no.metal fibre Nmet and two with a low content of metal.fibreLow1 and Low2 . The Nmet is very simple withonly six components while Low1 and Low2 consist of1314 constituents. Three different metal sulphides areadded to the base matrices to examine their effects. Welabelled the different brake pads with the base matrix andthe respective additive, e.g., Low1-Cu is the label of basematrix Low1 with 8 vol.% Cu S added, and Low1-Ref2designates the base matrix Low1 without metal sulphide.In this case the vol.% in Table 1 are adjusted to equal atotal of 100% of the components in the brake pads.2.2. Tribological testsThe samples are tested on a dynamometer equippedwith a car brake caliper. Two test procedures one atlow and one at high duty are designed to examine theeffects of the metal sulphides at two different energy.inputs. The test at low duty 1A consists of 600 brakingcycles performed from 50 kmrh down to rest. When thetemperature of the disc has decreased to 508C, the nextbraking cycle begins. Each braking is performed with aconstant braking pressure. During braking the torque ismeasured, from which the friction coefficient is extractedas the ratio between measured momentum and brakingpressure multiplied by a constant value accounting for thegeometry. A similar procedure is performed at high duty.2A , and the parameters are summarised in Table 2.Before each test a new disc is mounted on the rig. Thediscs used are Volvo standard rear discs, which are unven-tilated. After testing, the discs are labelled according totype of friction material and test procedure, e.g., Low1-Sb-1A corresponds to base matrix Low1 with antimony sul-phide tested at 1A.After the dynamometer testing, the discs are cut intopieces of 14=14 mm2with a saw while cooling with acommercial fluid to prevent heating. The samples arestored in atmospheric air before analysis. The wear of the.pads is measured as change of thickness mm and weight .loss g . Compared to the friction material, the wear of thediscs is estimated to be much lower as it is immeasurableusing a micrometer screw.2.3. Surface analysisFirst, the samples of the disc are analysed in a Jeol 35keV electron microscope with an EDAXe EDX system.The analysis is performed as a spot analysis, using aprimary energy of 15 keV. The composition is calculatedfrom the peaks with the EDAXe computer program thatfits the background of the spectra with a smooth functionand subtracts it from the spectrum. The peaks are com-pared to standard spectra from a database, and evaluatedw xwith the ZAF-method 5 . The surfaces of the discs areexamined with SEM in which an image is created withsecondary electrons ejected from the surface. The combi-nation SEMrEDX is a strong tool to get a quick overviewTable 2Dynamometer tests 1A and 2A, indicating the initial velocity from whichthe braking is performed, the initial temperature of the disc when eachbraking cycle begin, and the applied brake pressure. The cycles arerepeated N timesDynamometerVelocityTemperaturePressureStop no.testkmrh8CbarN1A5070306002A12020040350()L. Gudmand-Hyer et al.rWear 232 1999 168175170of both the topography and the main composition of thesurface of a large number of samples in a short time.Afterwards, a selection of the samples is examined with.Auger electron spectroscopy AES . This is done with 3keV primary electron energy combined with depth profil-ing using 1 keV argon ion sputtering for 1 min betweeneach recorded spectrum. This analysis method is verysurface sensitive and gives the concentration of the compo-nents with depth as spectra are recorded between eachcycle of argon iron sputtering. With AES it is possible tosee other elements than with EDX, e.g., oxygen andcarbon which are believed to play an important role in theformation of a friction film.3. Results3.1. Friction and wearThe average friction levels measured for each brakingcycle for dynamometer tests 1A and 2A are shown in Fig.1. The two graphs show how the friction level develops.from the beginning, during conditioning bedding-in untilthe end. At that stage the friction level is stable and thesurfaces are analysed to study the existence of a frictionFig. 1. The friction levels of different brake couples measured on adynamometer. The shown friction material is base matrix Low1 with .Cu S, PbS or Sb S , and the reference without a metal sulphide.a223 .Dynamometer test 1A. b Dynamometer test 2A.Fig. 2. Dynamometer results. The average friction of the different friction. .materials tested against cast iron brake discs B . The pad wear = . a .Dynamometer test 1A. b Dynamometer test 2A.film. These results are typical for all friction materials,though with some variations in the friction level betweenthe three base matrices.The results of wear and friction are visualised in Fig. 2.The friction value is the average value of the measuredfriction after the bedding-in period when the friction hasstabilised, which is respectively after 200 and 100 stops fordynamometer test 1A and 2A. A comparison of the differ-ent friction materials for dynamometer test 1A shows littlevariation of the wear, around 0.10.3 mm. The frictionvaries from 0.360.53. For all three base matrices thefriction is highest for Sb S , around 0.470.49, and a little23lower for PbS, around 0.400.47. The largest variationsare seen with Cu S, from 0.370.53. For dynamometer2test 2A, the wear varies much more with the frictionmaterial, from 1.56.5 mm. The wear is by far highest forbase matrix Nmet and much lower for Low1 and Low2.The addition of one of the metal sulphides to Low1improves the wear, and the situation is opposite for Low2.Addition of Cu S leads to relatively higher wear and2.sometimes lower friction.The repeated measurements show some variations forboth friction and wear. The variation is 510% on thefriction, and 2030% on the pad wear. Several factorscause the large variations of wear. Before the weight()L. Gudmand-Hyer et al.rWear 232 1999 168175171measurement, wear particles are not removed. The thick-ness is affected by the permanent swell of the pads afterheating. The roughness of the discs varies, which cancause different wear during bedding-in. The result is there-fore to be used merely as indication of the trend.3.2. SEMThe surfaces of the discs are examined with SEM inwhich the image is created with secondary electrons ejectedfrom the surface. The image reflects a dependency both ontopography and composition. Fig. 3 shows an image of anew disc before and after testing scanned with 15 keV andmagnified 400=. The underlying longitudinal lines arefrom the machining of the disc. Apart from that, two .different areas are circled. One area looks smooth S and .the other area is roughR . Both types of areas are foundon all the discs including the new disc. The next paragraphpresents the results of EDX examinations of the rough andsmooth areas to demonstrate the character of the frictionfilm.3.3. EDX analysisAfter testing, all the discs have been analysed withEDX at a smooth and rough area. The overall results arediscussed by comparing two discs, one tested against thereference material without a metal sulphide and the othertested against a friction material with antimony sulphide.The variation from sample to sample is comparable to thedifferences between the individual friction materials. Thescatter from spot to spot on one sample reflected thesurface topography; thus, the smooth areas were similar incomposition.A typical example of the EDX analysis of the discs isshown in Figs. 4 and 5 of Low1-Sb-1A and Low1-Sb-2A, .respectively, giving the composition of both a roughR .and a smooth S part of the discs. The spectra in Fig. 4a .and Fig. 5a are both recorded at a smoothSarea. Theformer shows no other components than those from thedisc: silicon, iron and manganese. The latter shows somesimilarities with Fig. 4b and Fig. 5b, which are recorded at .roughR areas on the discs. These spectra contain peaksfrom components with origin in the pad matrix.The existence of several components with origin in thepad indicates a transfer of friction material. The ratiosbetween antimony and sulphur, and barium and sulphur,which are original compounds in the pads, are especiallyinteresting. If chemical changes have taken place, theirratios in the discs should have changed from the values ofthe original material.The element concentrations for Low1-Sb-1A and 2Aand Low1-Ref-1A and 2A are calculated from the spectra,as shown in Table 3. For all eight measurements shown,except for Low1-Ref-1A, the ratio MnrFe is almost thesame. At the rough areas, the ratio BarS and SbrS are the . .same for Low1-Sb-1AR and Low1-Sb-2AR , but theconcentrations are different. This indicates that the surfacelayers are similar and that the same reactions have takenplace, but the thickness of the layers is different.At the smooth areas no antimony and barium is found .for Low1-Sb-1A S , only sulphur, copper, and zinc. This .is in contrast with the spectrum of Low1-Sb-2A S , which .contains the same elements as Low1-Sb-2ARbut theratios between the elements are different. Compared to . .Low1-Sb-2A S , Low1-Ref-2A S contains 10 times lesssulphur and no barium, indicating that no friction material . .Fig. 3. Typical SEM images of a cast iron disc before and after testing on a dynamometer. The rough R and smooth S areas are indicated with a circle. . .a New disc. b Low1-Sb-2A.()L. Gudmand-Hyer et al.rWear 232 1999 168175172 . .Fig. 4. Typical EDX-spectra measured on the cast iron disc surface tested against friction material Low1-Sb-1A. The spectra are recorded on a a smooth area and b a rough area.()L. Gudmand-Hyer et al.rWear 232 1999 168175173 . .Fig. 5. Typical EDX-spectra measured on the cast iron disc surface tested against friction material Low1-Sb-2A. The spectra are recorded on a a smooth area and b a rough area.()L. Gudmand-Hyer et al.rWear 232 1999 168175174has been transferred at the smooth areas for the referencematerial. The ratio ZnrCu is the same for Low1-Ref-1A . .S and Low1-Ref-2A S .3.4. AES depth profileThe existence of antimony on the surface can be ob-served in Fig. 6 showing a decreasing concentration withdepth; thus, antimony has been transferred and dissolved atthe very upper surface of the disc. The amounts of iron,oxygen and carbon also change with depth. The oxygenconcentration follows the iron concentration closely, in-creasing with depth whereas the carbon concentration de-creases. The ratio between iron and oxygen indicates that ahomogeneous phase of FeO has formed at the surface. Thisiron oxide phase is stable compared to higher oxidationw xlevels and is discussed by Jansson et al. 6 . A furtherdiscussion of the AES analysis of all samples is to bepublished later in relation to a discussion of the generalconcepts of formation of the friction film.4. DiscussionIt takes many small bits of information to create a fullpicture of the effects, e.g., to see which constituents havebeen transferred and which are gone or not active. Also,the atomic stoicheiometric percentages indicate if there hasbeen any chemical change. When one component is pre-sent with a different relative amount than the originalconstituent, as in the case of barium sulphate and antimonysulphide, it indicates a chemical reaction.The present measurements and procedures allow for atleast a partial understanding of the friction film formationprocess. This is definitely demonstrated here with Low1-Sb-2A where a friction film forms at 120 kmrh, 2008C,and a brake pressure of 40 bar at the presence of antimonysulphide in the pad. In this case, even smooth-lookingareas of the disc have undergone compositional modifica-tion, as shown with EDX. The presence of sulphur moni-Table 3Surface concentration measured with EDX of the cast iron discs testedwith the friction materials Low1-Sb and Low1-Ref analysed at smooth . .S and roughR areas on the discsElementLow1-Sb-1ALow1-Sb-2ALow1-Ref-1A Low1-Ref-2A . . . . . . . .SRSRSRSRAt.%At.%At.%At.%SiK3.310.3SK10.00.13.00.112.1SbLBaL2.914.9MnK1.40.8FeK95.475.289.558.795.178.093.953.9CuK0.96.7ZnK0.40.5Fig. 6. A typical AES depth profile measured on the cast iron disc surfacetested against friction material Low1-Sb-1A. Intensity of the signals fromoxygen, carbon, iron, antimony and sulphur is mounted against sputtertime in minutes.tored by EDX shows a much higher concentration of .sulphur in Low1-Sb-2AScompared to the other sam-ples. The presence of oxygen and carbon is monitored byAES, showing an oxidised surface covered or passivatedby a thin carbon layer. The systematic changes of frictionand wear with temperatures are seen from Figs. 1 and 2. Itis noted how a higher brake temperature minimises thebedding-in time, before friction stabilises. The addition ofcopper sulphide causes a gradual loss of friction while leadsulphide and antimony sulphide stabilise the friction, atboth the higher and lower temperatures. For antimonysulphide, the absolute friction level decreases strongly atthe higher temperature while for lead sulphide it stays atthe same level. In the absence of metal sulphides, thefriction varies more between the two temperatures, increas-ing with temperature.The wear generally increases with temperature, and inparticular for pads without metal fibre content. The wear isgenerally much smaller and roughly independent of thepad matrix and the content of metal sulphide for dy-namometer tests at low duty in contrast to high dutytesting. Discs
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