Rolling Contact Performance of a Ti-Containing MoS2 Coating Operating Under Ambient, Vacuum, and Oil-Lubricated Conditions

Harpal Singh,Gary L Doll,Kalyan C Mutyala

doi:10.3390/coatings9110752

Abstract

Solid lubricant molybdenum disulfide (MoS2) coatings have been frequently used to lubricate mechanisms operating in environments where oil and grease lubrication are ineffective. This work evaluated the rolling contact performance of a Titanium-containing MoS2 coating under humid ambient, vacuum, and oil-lubricated conditions. Weibull analyses of L50 lifetimes of AISI 52100 steel balls coated with a Ti-MoS2 coating paired with uncoated M50 steel rods were determined to be 3.7, 14.5, and 158.6 million cycles in ambient, vacuum, and oil-lubricated environments, respectively. In the ambient and vacuum tests, failures were determined to be associated with the onset of abrasive wear rather than fatigue or spalling. The L50 lifetimes of tests performed in those environments were found to depend upon the wear rate of the coatings on the balls. That is, the Ti-MoS2 functioned as a barrier to the onset of abrasive wear between the steel alloys until the coating was sufficiently worn away. Under oil-lubricated (boundary lubrication) conditions, L50 was found to depend on the durability and composition of tribofilms formed in-situ on the surfaces of the uncoated M50 rods. The tribofilms were comprised of mixtures of MoS2 crystallites and amorphous hydrocarbon (a-C:H). The crystalline MoS2 in the tribofilm originated from the amorphous Ti-MoS2 coating and likely underwent a thermodynamic phase transition as a result of the applied Hertz stress and frictional heating in the contact. The a-C:H in the tribofilm probably originated from a catalytic scission of the polyalphaolefin (PAO) molecules caused by the d-band character of the Mo or Ti in the coating. Overall, the Ti-MoS2-coated balls were effective at extending the operational lifetimes of M50 rods under ambient, vacuum, and oil-lubricated conditions by an order of magnitude.

Highlights

Holmberg and Erdemir estimated that about 23% of global energy is consumed to overcome friction and associated wear losses [1]
Figure display the aarchitecture of the Ti-MoS2 coating
It was demonstrated that Ti (~16 at %)-containing MoS2 coatings with a dense, featureless, and amorphous microstructure improved rolling contact life of M50 bearing grade steels in ambient, vacuum, and oil-lubricated environments

Summary

Introduction

Holmberg and Erdemir estimated that about 23% of global energy is consumed to overcome friction and associated wear losses [1]. Whereas oil and grease lubrication are used to minimize friction and wear losses in most situations, in challenging environments such as high vacuum, solid lubricants such as molybdenum disulfide (MoS2 ) are often employed [2]. Numerous studies established that interfilm sliding and intrafilm flow control the lubricity and the durability of MoS2. MoS2 crystallites in coatings were observed to have a tendency to reorient from an initially random orientation to a state where the (002) basal planes were parallel to the surface [3,4].

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Conclusion