World PM2016 – Iron Based MMCs Manuscript refereed by Prof Alberto Molinari (Trento University, Italy)
Development of Iron-Based Alloys for Copper-Free Organic Brake Pads Matteo Zanon, Ilaria Rampin (Pometon S.p.A., via Circonvallazione 62, I-30030 Maerne -Venice – Italy)
[email protected];
[email protected]; Ane Maite Martinez, Jon Echeberria (CEIT and TECNUN, Paseo de Manuel Lardizabal, Nº 15 20018, Donostia - San Sebastián, Spain)
[email protected],
[email protected]; Antonio di Loreto (Gama S.p.A., Strada Provinciale Bonifica – Zona Ind. 64010 Ancarano, Italy)
[email protected]; Abstract Organic brake pads for automotive are extremely complex and sophisticated composite materials, in which their numerous constituents interact together during braking action. A combination of partly conflicting friction, wear and NVH properties is achieved by careful ingredients selection and mixture formulation. Recently enacted US legislation has pushed brake pads manufacturers to develop copperfree alternatives. Being copper a key ingredient in many high performance formulations, thanks to its attractive friction and thermal properties, considerable effort is being put into newly designed formulations and materials. The work presented here is part of a successful development program that led to the creation of a new family of iron-based powders, specifically designed for copper-free organic formulations. A portion of the experimental study is presented here, comprising both macroscopic friction behaviour and microscopic reaction mechanisms, in traditional and new copper-free formulations. Relationships between these two aspects are analysed, stressing how the unique chemical behaviour of newly-designed alloys is linked to their friction performance. Introduction Organic brake pads for automotive are sophisticated composite materials, where a thermoset resin matrix holds together the other ingredients. The components of the brake pads can be divided into four categories [1]: 1) abrasives that increase coefficient of friction µ and regulate the film thickness on the friction surface (Al 2O3, SiO2, Fe2O3, Fe3O4, ZrSiO4 ), 2) friction modifiers that increase µ, act as lubricants or react with the oxygen to avoid oxidation of surface films (sulphur and graphite as lubricants, metallic powders as µ modifiers and heat conductors, Mo/Sb/Bi as oxygen reagents), 3) reinforcements that increase the wear resistance (steel/brass fibers, organic fibers), 4) filler to decrease costs (barite/CaCO3). Three main product families are established in the market, each one with different properties and formulation rules [2]: semi-metallic materials, which have more than 50% of ferrous metals (iron powder, steel fibers); low metallic materials or low steel, with 5-35% of ferrous metals, and Non-Asbestos Organic Materials (NAO), which do not contain ferrous materials. Low metallic materials (low-met) are prevalent in Europe, while NAO formulations are typical of Far East and in US, where they have progressively replaced semi-metallics [cit. pred. FM-tesi Ane]. Each family has its own strength and weaknesses, with low-met emphasizing friction coefficient stability while NAO privileges braking comfort and noise reduction [2]. Pometon Powders S.p.A. promoted a R&D project, in collaboration with Gama S.p.A. and CEIT, to define new solutions for the replacement of copper in organic-based friction materials. Scientific investigations were focused on the so called “transfer layer” or “third body layer”, the surface film created during braking from wear debris transferred from brake pad to disc and also due to the tribo-oxidation of the disc surface. The characteristics of this intermediate layer are critical for friction control, and are the result of complex chemical/metallurgical interactions among disc and brake pad materials. This resulted in the development of Brakealloys, a range of innovative, environmentally friendly, ironbased metal powders. These products share a common basic concept, but different thermo-mechanical properties. This translates into distinct behaviours during braking action, according to their active contribution to transfer layer formation. Diverse thermo-chemical and microstructural characteristics allow the selection of the most suitable grade, depending on brake pad formulation and user requirements. Examples of DSC curves for grades Z and S in Figure 1 show the absence of endo/exothermic peaks connected with phase transformations up to 700 °C. This means that, despite the presence of low melting alloying elements, the materials are thermally stable in the range of average temperatures reached during passenger cars braking, which is below 600 °C during hot fade test. The presence of bismuth lowers instead the melting point of the alloy, as can be inferred from the small endothermic peak around 150 °C
World PM2016 – Iron Based MMCs on the DSC curve; this translates into a quicker and more intense transfer layer formation during both cold and hot braking action.
Figure 1: DSC curves of Brakealloy grades, showing endo– and exothermic peaks in correspondence to phase transformations. Heating rate 10ºC/min up to 1000ºC, argon atmosphere. Left hand: Brakealloy Z (Fe-Zn alloy); center: Brakealloy S (Fe-Sn alloy); right hand: Brakealloy SB (Fe-Sn-Bi alloy). Solid lubricants play crucial roles reducing wear on both brake pad and disc, optimizing friction level and controlling brake induced vibrations [3]. The effectiveness of the solid lubricant is affected by different braking conditions (temperature, pressure, speed). Therefore often two or more lubricants are used to play complementary roles. Typical solid lubricant classes are: graphites, metal sulphides and soft metals. Among the metal sulphides, two of the most commons are antimonium sulphide Sb2S3 and tin sulphides SnS/Sn2S3. Sb2S3 is a cheap alternative, but due to concerns with toxicity of Sb compounds, US and EU brake pads manufacturers are beginning to be forced towards Sb-free formulations [4]. SnS/Sn2S3 is more expensive, but greener. The effect of the sulphides as lubricant depends not only on their own physical properties, but also on their reactivity with the other ingredients of the brake pad formulation, in particular with the metals. This work gives an overview of the effect of solid lubricant change for a copper-based and for a newly designed copper-free formulation, and how the metal-sulphides reactions at the microscopic level reflect different macroscopic friction behaviours. Experimental The fabrication of the friction material was based on keeping the weight fraction of parent composition constant at 84 wt% and varying metal powders ingredients (copper, iron and specially designed iron alloy), lubricants and the addition of special fibres (SF) in each composition. Full design of formulations is shown Table II. In order to evaluate the influence of the different constituents, these formulations were specifically designed for obtaining brake pads with low porosity content (< 10 vol%). Ingredients (wt%) / Designation
M1
M2
M3
M4
Parent composition*
84
84
84
84
Sb2S3
6
6 6
6
4
4
6
6
SnS Copper powder Non metallic fibers Brakealloy Z (iron-zinc alloy)
10
10
*Binder (Phenolic resin) 6.5wt.%; friction modifiers (alumina, magnesia, graphite, coke, iron oxide) 28wt.%; reinforcements (steel fibre, aramid fibre) 26wt.%; fillers (NBR powder, barite, Zn powder) 23.5wt.%.
Table II: Design and designations of the different formulations. The brake pads from the formulations studied in this work have been manufactured at laboratory scale at CEIT according to the process described in a previously published work [5]. Brake pads were characterised for physical (apparent density and porosity by water immersion and picnometry), thermo -
World PM2016 – Iron Based MMCs physical (thermal conductivity, diffusivity, specific heat, etc.) and mechanical (TRS by Three Point Bending and hardness, HRS) properties as per standard practice. Thermo-physical properties were measured on Fl-3000 Laser Flash instrument using samples of square size (10mmx10mm) and thickness 2-3mm, cut perpendicular or parallel to the direction of compression, from room temperature to 150ºC. From these samples the thermal conductivity was measured parallel to the brake pad surface (“in plane”) and perpendicular to this direction (“out of plane”). The friction performance was evaluated according to SAE J2522 procedure in GAMA using a singleended inertia full scale brake dynamometer (dyno) [5]. Friction surfaces and cross-section of the brake pads and discs after friction test were studied using Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray Spectroscopy (EDS) techniques. Cross sections of the worn brake pads and discs were prepared by Focused Ion Beam (FIB). The microstructures of the different brake pads were analysed using a Scanning Electron Microscope (SEM) XL30CP with EDS. Results and discussion Table III gives an overview of the thermo-physical-mechanical properties of the brake pads prepared for this study. The new Brakealloy formulations show on average a higher hardness (+6 HRS), the same TRS and a slightly higher thermal conductivity, both “in plane” and “out of plane “. This last effect was shown [5] to be related to fiber addition, that help to compensate for the removal of copper powder. Switching to SnS leads to higher porosity, lower hardness and TRS, and also to worse thermal conductivity. this does not necessarily translates into better hot friction performance, as will be seen later on. As a matter of fact, this measurement relates to virgin material near room temperature, and not to a worn material during braking action.
Sample
dap 3 (g/cm )
Porosity (%)
Hardness (HRS)
TRS (MPa)
Thermal Conductivity (W/(m K)) out of plane
in plane
M1 (Cu, Sb2S3)
3,21
6,2
94
67
4,2
7,1
M2 (Cu, SnS)
3,15
7,6
87
31
2,4
6,3
M3 (Br. Z + F + Sb2S3)
3,01
6,7
101
55
3,5
7,9
M4 (Br. Z + F + SnS)
2,90
11,3
92
41
3,4
6,5
Table III – Physical, thermo-physical and mechanical properties of the brake pads AK-Master friction tests results are show in Fig. 2 and 3. The charts highlight the effect of lubricant change on the baseline 10% Cu formulation and on the Cu-free product. Switching to SnS with the Cu material led to quite drastic worsening of the overall friction performance, both for cold as well as for hot performance. With an overall higher µ, speed sensitivity is augmented, a more erratic behaviour is observed in the third part and even a worse fade performance. A different behaviour is observed with Brakealloy mixes, where instead a lower average µ is registered, with a slightly less stable behaviour in the cold phase but a better fade performance, even superior to copper formulation, as can be more clearly visualized in fig. 4.
Left: Figure 2 - AK-Master test on Cu-based formulations Right: Figure 3 – AK-Master test on newly designed Cu-free formulations
World PM2016 – Iron Based MMCs
As for friction coefficient, also for wear a strong metal – lubricant interaction seems at work, if one looks carefully at Fig. 5. Pad wear is slightly increased in copper mix when SnS is used, while disc wear is driven down to almost null. On Brakealloy mix, pad wear instead is reduced, while disc wear remains constant, and at a higher level with respect to Cu-containing baseline.
Left: Figure 4 – Coefficient of friction during “Fade 1 “ session of AK-Master test. Right: Figure 5 – Pad and disc wear measured after AK-Master Disc rugosimetry profiles show in general a wavy profile, especially for Cu-based formulations, which can be related to transfer layer deposition on disc surface. Brakealloy materials display a somewhat less undulated pattern, with SnS yielding the smoothest surface. SEM analysis on FIB sections of cast iron disc revealed a transfer layer of variable thickness, around 3 – 10 µm, showing a complex and composite microstructure. Although some authors [6] showed it to be composed mainly by iron oxides nanocrystals, it can be seen in Fig. 7 that various other elements coming from BP material are present, like copper, barite, alumina, magnesia and carbon. STEM analysis [7] on Brakealloy Z formulation (without fiber addition) proved also the presence of sulphide lubricants, and in particular of reaction products between metal powders and Sb2S3. Sb2S3 has a strong tendency to decompose and to form antimonides and sulphides with other metals, thus changing the chemical nature of the active friction constituents. AKM05712 (CEIT0) cara superior
FRONT AKM01013
2,50E-02
2,50E-02
2,00E-02
2,00E-02
1,50E-02 1,50E-02
1,00E-02
0,00E+00 0,00E+00
5,00E+00
1,00E+01
1,50E+01
2,00E+01
2,50E+01
3,00E+01
3,50E+01
4,00E+01
4,50E+01
5,00E+01
Perfil (mm)
perfil (mm)
1,00E-02
5,00E-03
Perfil 1
5,00E-03
Perfil 2
Perfil 3
0,00E+00 0
5
10
15
20
25
30
35
40
-5,00E-03 -5,00E-03
-1,00E-02 -1,00E-02
-1,50E-02 -1,50E-02
-2,00E-02
Distancia (mm) perfil 1
perfil 2
perfil 3
-2,00E-02 perfil 4
a) M1 (10% Cu + 6% Sb2S3);
Distance (mm)
b) M2 (Cu + SnS)
c) M3 (6% BrZ + 4% F + 6% Sb2S3); d) M4 (6% BrZ + 4% F + 6% SnS) Figure 6 – Rugosimetry profiles on worn brake discs surface.
45
50
Perfil 4
World PM2016 – Iron Based MMCs
Figure 7 – SEM pictures of worn brake disc FIB section, showing transfer layer microstructure. From left to right: a) M1 (10% Cu + 6% Sb2S3); b) M3 (6% BrZ + 4% F + 6% Sb2S3); d) M4 (6% BrZ + 4% F + 6% SnS) This tendency was proven in controlled DSC experiments on binary metal powder – metal lubricant mixtures [8], showing that these intermetallic reactions take place even under oxidising conditions. SnS lubricant was shown to be less prone to metal alloys/intermetallics formations, which take place with much more difficulty under oxygen-rich atmosphere. The friction-generated heat triggers metal – lubricant reactions also inside the brake pad, where it was found they take place in a macroscopic area. The absence of air creates a non oxidising environment, in which intermetallic reactions are unhindered by competing oxidation. Figure 8 presents evidence of such reactions for each one of the tested formulations. Fig. 8a and 8b show details of reacted copper particles that have formed respectively copper antimonides and bronze (CuSn) alloy. Brakealloy particles in Fig. 8c and 8d formed instead iron antimonides (white spots) and sulphides (grey phases). Table IV summarizes the reaction products observed within the worn brake pads, together with the maximum depth at which such compounds could be observed. As a matter of fact, the extent of these reactions was found to be different according to metal and lubricant powders, with copper-containing materials experiencing a deeper alteration.
Figure 8 – SEM pictures taken on worn brak pads cross section, highlighting reacted metal particles. From left to right: a) M1 (Cu-Sb2S3); b) M2 (Cu-SnS); c) M3 (BrZ-F-Sb2S3); d) M4 (BrZ-F-SnS) This was found to be related to a higher affinity of Cu powder to react with metallic sulphides. DSC experiments on binary mixes under argon atmosphere [9] proved that copper antimonides already form at 200 °C, while with other metals such as Fe and Zn a minimum temperature around 350-400 °C is necessary for such reactions to take place. This means that, for copper powder, the conditions for such chemical reactions exist in a wider brake pad volume. At this point, the different effect on friction properties given by lubricant change for copper and Brakealloy Z materials can be traced back to the different chemical reactions they experience. The superior friction behaviour of Cu – Sb2S3 mix with respect to Cu – SnS seems to be related to the formation of copper antimonides, being copper sulphide is present for both lubricants. Being intermetallic compounds harder than base metals, it may be hypothesized they confer anti-fade properties just as local temperatures increase, reducing the friction coefficient loss. A similar mechanism is thought to be in place for copper oxides [rif.], although specific characterization of the physical properties of Cu-Sb compounds is needed to prove this. With Brakealloy materials we observe less intense metal-lubricant reactions, extending to depth that is only one third of the one observed for copper materials. This lower reactivity may then prevent the alteration of SnS, so that it can more fully express its well-known benefits [10] especially during hot friction tests, where chemical
World PM2016 – Iron Based MMCs reactions are otherwise enhanced. The new Cu-free formulation is then in better position for successful antimony-based lubricant replacement with a greener alternative.
METAL POWDER
REACTION PRODUCTS BETWEEN METAL POWDERS AND LUBRICANT DURING BRAKING* Sb2S3
SnS
DEEPNESS OF REACTION FOUND IN BRAKE PADS [mm] Sb2S3
SnS
Cu
even at 9 9 Cu2Sb, Cu2S Cu2S, bronze (CuSn) ZnS, FeS, ZnFeS, Brakealloy Z + F. ZnS, FeS, ZnFeS, Sn 3
