1. Characterisation of the Chemical Reactions between Solid Lubricants and Metal Powders in Low-Metallic Brake Pads during Braking
Ane Maite Martinez, Jon Echeberria (CEIT)
Antonio Di Loreto (Gama S.p.A.)
Matteo Zanon, Ilaria Rampin (Pometon Powders S.p.A.)
Eurobrake 2015, Dresden

2. Outline Introduction: Solid lubricants Objective Methodology Results
Summary
Eurobrake 2015, Dresden
5/05/2015

3. Introduction
Solid lubricants play crucial roles reducing wear (on brake pad and disc), optimizing friction level and controlling brake induced vibrations.
The effectiveness of the solid lubricant is affected by different braking conditions (T, P, v). Therefore often two or more lubricants are used to play complementary roles.
Typical solid lubricants used are: graphite, metal sulfides or soft metals.
Sb2S3 (Tmelting=550ºC, 2Mohs): improves fade and wear resistance. Due to the concern with toxicity of Sb compounds, in US and Europe brake pad manufactures required to used Sb-free compounds.
SnS/Sn2S3: possible substitute of Sb2S3.
Eurobrake 2015, Dresden
5/05/2015

4. Objective
Study of 2 different metal sulfides lubricants: Sb2S3 and a SnS/Sn2S3.
Study of chemical reaction between solid lubricants and metal powders present in the formulation (low density iron powder, Zn powder and Brake alloy Z powder (Pometon S.p.A.)).
Collaborative project: CEIT – POMETON – GAMA
(Cu substitution in a low-metallic brake pad)
Eurobrake 2015, Dresden
5/05/2015

5. Methodology WORN PAD SOLID LUBRICANT BINARY MIXTURES
Section and surface microstructural characterization:
SEM-EDS, FIB
X-ray diffraction
DSC-TGA
Microstructural characterisation:
SEM-EDS
X-ray diffraction
DSC-TGA
Thermal treatments
Microstructural characterisation:
SEM-EDS
X-ray diffraction
Eurobrake 2015, Dresden
5/05/2015

6. Methodology F2 (Zn) F1 F2 F3 84 6 10 F1 (Low density Fe)
Ingredients (wt.%) / Designation F1 F2 F3
Parent composition* 84
Lubricant (Sb2S3 or SnS) 6
Low density Fe 10 Zn
Brake alloy Z
F1 (Low density Fe)
F2 (Zn)
F3 (Brake alloy Z)
Eurobrake 2015, Dresden
5/05/2015

7. Results Lubricant characterisation in air: Sb2S3
Sb2S β-Sb2O α- Sb2O3
Initial composition: Sb2S3
Sb2S3 α-Sb2O4
The oxidation of Sb2S3 to Sb2O3 is completed at 400ºC.
The oxidation of Sb2O3 to Sb2O4 at >550ºC.
Eurobrake 2015, Dresden
5/05/2015

8. Lubricant characterisation in air: SnS
Initial composition: SnS, Sn2S3, Sn, S
SnS Sn2S3 S SnO2
The oxidation of Sn2S3 to SnO2 happened at T= ºC:
Sn2S3 +5O SnO2 + 3SO2
At T>480ºC SnS oxidized to SnO2.
Eurobrake 2015, Dresden
5/05/2015

9. Evidence of reaction: surface worn brake pad
Sb2S3 Sb Fe
White contrast in steel fibers possible reaction with solid lubricant Sb2S3.
Eurobrake 2015, Dresden
5/05/2015

10. Evidence of reaction: surface worn brake pad
SnS/Sn2S3
FeSn
Chemical reaction between steel fibers and SnS lubricant on the worn brake pads.
ZnS
Eurobrake 2015, Dresden
5/05/2015

11. Evidence of reaction: surface worn brake pad
Zn+Sb2S3
Fe+Sb2S3
Barite and Fe3O4
FeSb indicating reaction between Sb2S3 and low density iron powder or steel fibers on the surface.
ZnS on the surface indicating possible reaction between Zn and Sb2S3.
Eurobrake 2015, Dresden
5/05/2015

12. Evidence of reaction: section worn brake pad
Low density Fe powder (Pometon)
FeS
FeSn Fe
FeSb
FeS
SnS
Sb2S3
Reaction between solid lubricants and low density Fe powder in the cross-section of the worn pads.
Eurobrake 2015, Dresden
5/05/2015

13. Evidence of reaction: section worn brake pad
Sb2S3
Brake alloy Z (Pometon)
FeSb
FeSb Fe Fe
FeS
ZnS
Fe-Zn
Reaction between Sb2S3 and Brake alloy Z (Pometon) powder in the cross-section of the worn pads.
Eurobrake 2015, Dresden
5/05/2015

14. Evidence of reaction: section worn brake pad
Sb2S3
SnS
SnS
ZnS
ZnSb
Zn ultrafine particles reacted with Sb2S3 lubricant but not with SnS.
Eurobrake 2015, Dresden
5/05/2015

15. cross-section worn pad
Reaction products in worn pads
QUITAR
METAL POWDER
REACTION PRODUCTS
cross-section worn pad
DEEPNESS
of the reaction [mm]
Sb2S3
SnS
Low density Fe
FeSb, FeS
FeSn, FeS 6 Zn
ZnS, ZnSb, ZnO
No reaction (oxidised)
no reaction
Brake alloy Z
ZnS, FeS, FeSb
ZnS, FeS, FeSn
Reaction products observed during cross-section characterization in worn pads.
Eurobrake 2015, Dresden
5/05/2015

16. Binary mixtures in air Zn + Sb2S3 Mixture SnS Sn2S3 Zn Zn + SnS
Sb2S3 Zn ZnSb ZnS ZnO
Zn + Sb2S3
Mixture
SnS Sn2S Zn
Zn + SnS
Mixture
Eurobrake 2015, Dresden
5/05/2015

17. Binary mixtures in Argon
Zn + Sb2S3
Zn + SnS/Sn2S3
400ºC
400ºC
Sb2S Zn Zn4Sb ZnS
SnS Sn2S Zn Sn ZnS ZnO
In Ar both lubricant reacted with Zn at 400ºC:
Zn +Sb2S ZnS + Zn4Sb3
Zn + SnS/Sn2S ZnS + Sn
Eurobrake 2015, Dresden
5/05/2015

18. Summary
It is possible to explain the chemical reaction into the brake pad formulation during braking with binary mixture using DSC-TGA and X-ray diffraction after thermal treatments in argon and air atmospheres.
A chemical reaction between both lubricants (Sb2S3 and SnS/Sn2S3) and the metal powders and/or steel fibres happened during braking affecting into the transfer layer formation and therefore to the friction behaviour of the brake pads.
The reaction between the metal powders and the solid lubricants form an intermetallic and a metal sulfide phases in each case except for Zn powder when SnS/Sn2S3 lubricant was used.
Eurobrake 2015, Dresden
5/05/2015

19. Thank you! Ane Maite Martinez, Jon Echeberria (CEIT)
Antonio Di Loreto (Gama S.p.A.)
Matteo Zanon, Ilaria Rampin (Pometon Powders S.p.A.)