Experimental and DFT modelling of the frictinal behavior of sof Bi2S3 coatigs

B.A.PILOTTI; MARIANA DENNEHY,; A. JUAN; C. MÜLLER; E. BROITMAN; W.R. TUCKART; G. PRIETO; G. ZORZI RUGGIERO; R. FACCIO

San Luis

Congreso; VIII ENCUENTRO DE FÍSICA Y QUÍMICA DE SUPERFICIES; 2018

INFAP-CONICET-UNSL

In high demanding environments, such as in aerospace, mining and heavy-duty applications, solid lubricants are the best choice in order to reduce friction and wear, thanks to their resistance to high temperatures and contact pressures.For this purpose, traditional solid lubricants, such as graphite, molybdenum disulfide (MoS2) and tungsten disulfide (WS2) have been extensively used. However, all of them show weaknesses in some aspects. For example, graphite needs moist air, while MoS2 requires high contact pressures in order to properly lubricate1.The aim of this study was to analyze the frictional behavior of a Bi2S3 based soft coating2 under variable relative humidity (RH) atmospheres and contact pressures. In addition, its performance was compared to a MoS2 coating produced by the same method. Bi2S3 particles were provided by Sigma Aldrich and MoS2 particles were purchased from Climax Molybdenum Co. The coatings were produced by rubbing a rotating polishing cloth against polished SAE 1045 steel disks at 20 rpm and 30 MPa of contact pressure, during 1 h. 300 mg of nanoparticles were deposited at the beginning of the procedure and the remaining loose particles were blown with dry compressed air.The frictional response of the soft coating was evaluated by a reciprocal test using an Oscillating TRIBOtester (TRIBOtechnic, France). A ϕ 6 mm AISI 52100 ball was used as the counterpart, while the total sliding distance was 10 m. Tests were performed under different relative humidity contents, contact pressure levels and sliding speeds.We also used Density Functional Theory (DFT-VASP) to represent a simplified model of the Bi2S3 or MoS2/Steel interface. Calculations were performed with the PAW method using the GGA (PBE) functionals. The SAE 52100 was modeled from a bulk BCT unit cell, choosing the (110) plane for the calculations. The coatings were modelled as a perfect hexagonal monolayer in the case of MoS2 or a (010) orthorhombic monolayer for the Bi2S33. The interface was built by stacking 7 layer slab of Fe BCT (110) face putting on top both the MoS2 (001) or the Bi2S3 (010) monolayers. Although the interfacial energy is slightly better for MoS2, the most important feature is the difference in bonding connections/unit area on both surfaces. For MoS2, there are more S species close to the surface, with Fe-S bond distances from 2.19 to 2.36 Å and the number of effective Fe-S bond contacts favors MoS2. Bi2S3 presents 7 bonds in a cell of 48.2 Å2 (0.15 bonds/Å2), while MoS2 shows 6 bonds in cell of 24.1Å2 (0.25 bonds/Å2). This can be related to the obtained tribological results. We found that Bi2S3 is less influenced by variations in load and RH, with a CoF of ~0.15. However, we observed adhesion problems at both high load and high RH. We associate this phenomenon to the crystal structure of Bi2S3 and to the lower effective Fe-S bonds between the coating and the substrate.