60th Annual Report on Research 2015

Participants

PI: Prof. Bharat Bhushan

PhD student: Dave Maharaj

Description of Scientific Research Goals

The investigation of the effects of tribocharging, friction, wear and mechanical behavior of nano-objects, such as Au nanoparticles (NPs) and nanorods (NRs), MoS₂ and WS₂ multi- walled nanotubes (MWNTs) and carbon nanohorns (CNHs) is proposed in this research.

Incorporating these nano-objects into liquids may lead to enhanced lubricity. As sliding progresses over time, an increase in attractive electrostatic forces could lead to greater adhesion of nano-objects which can affect the friction and wear mechanism. Studies examined mechanisms in nano-object(s) contact sliding on dry and submerged-in-liquid surfaces, sliding on a single nano-object, and mechanical behavior. Nano-object friction studies used an atomic force microscope. Single nano-object contact studies (lateral-push) provide understanding of friction mechanisms, showing friction is influenced by contact area, work of adhesion, and viscosity of liquids. Friction is lower in liquid environments versus dry environments. Contact studies of multiple nano-objects investigate whether several nano-objects reduce friction and wear between sliding surfaces as a result of lower contact area. Further studies on single nano-object friction reveal dependence on topography, scale, and material. The charge density due to these forces can be characterized by electrostatic force microscopy (EFM) and related to friction and adhesion.

In determining the suitability of nano-objects for tribology from the macro-to-nanoscale, it is also important to study its nanomechanical behavior when subjected to an applied load. Deformation studies provide the opportunity to compare local deformation (nanoindentation) with a sharp tip and global deformation (compression) with a flat punch using a depth sensing nanoindenter. This simulates the type of loading nano-objects experience in tribological applications on the macro-to nanoscale.  

Initially, Au NPs and NRs, MoS₂ and WS₂ MWNTs and CNHs will be deposited on silicon substrates either in dry or submerged in liquid conditions. In the next phase nano-object contact studies will be conducted. Ball-on-flat tribometer studies will be performed on the macroscale to relate macroscale friction and wear to that observed on the nanoscale. In the next phase mechanical behavior of the nano-objects will be evaluated using the Hysitron nanoindenter. Finally tribocharging studies will be performed using AFM and EFM to correlate friction, adhesion and electrostatic attraction. This research will lead to an enhanced understanding of the properties of nano-objects and will lead to the creation of next generation nano-object-liquid hybrid lubricants.

Progress to date

Studies on Au nanoparticles and nanorods, MoS₂ and WS₂ MWNTs and CNHs were performed to understand friction, wear and mechanical behavior. Studies on single and multiple nano-object contact in dry and submerged in liquid environments, on friction on single nano-objects and on mechanical behavior during indentation and compression were examined.

In single nano-object contact, friction at the initiation of sliding on surfaces was influenced by real area of contact, roughness, work of adhesion and liquid viscosity. In submerged in liquid environments, elimination of meniscus forces and the presence of a low shear strength film, reduces friction compared to dry environments. Lower friction occurs in lower viscosity liquids. In multiple nano-object contact friction and wear reduction occurs due to the presence of nano-objects which lowers the real area of contact between sliding surfaces. Exfoliation and tribofilm formation contributes to friction and wear reduction on the macroscale in layered materials. Further friction studies performed by sliding on single nano-objects showed the influence of topography on friction. Lower friction was observed on the nanoscale compared to the macroscale. The presence of moisture results in changes in material composition leading to higher friction.

Deformation studies were performed, which aids in understanding how nano-objects behave during loading and unloading in friction and wear reduction. Studies examined scale effects of Hardness and Young’s modulus and compared mechanisms between local (indentation) and global (compression) deformation. For some nano-objects, studies show hardness can be enhanced which is dependent on material grain size. In other cases where hardness showed no improvement, it was found that structural defects were responsible for lower resistance to deformation. Reverse plasticity was also observed due to high internal stresses during compression. Resistance to deformation during repeat loading was found for all nano-objects.

The knowledge gained from these studies will have far reaching effects when designing macro-to nanoscale systems that incorporate nano-object-liquid hybrid lubricants for reducing friction and wear.

Journal Publications

Maharaj, D. and Bhushan, B. (2012), “Effect of spherical Au nanoparticles on nanofriction and wear reduction in dry and liquid environments,” Beilstein J. Nanotechnol. 3, 759-772.

Maharaj, D. and Bhushan, B. (2013), “Effect of MoS₂ and WS₂ nanotubes on nanofriction and wear reduction in dry and liquid environments,” Tribol. Lett. 49, 323-339.

Maharaj, D., Bhushan, B. and Iijima, S. (2013), “Effect of Carbon Nanohorns on nanofriction and wear reduction in dry and liquid environments,” J. Colloid Interface Sci. 400, 147-160

Maharaj, D. and Bhushan, B. (2014), “Scale effects of nanomechanical properties and deformation behavior of Au nanoparticle and thin film using depth sensing nanoindentation,” Beilstein J. Nanotechnol. 5, 822-836.

Maharaj, D. and Bhushan, B. (2014), “Nanomanipulation, nanotribology and nanomechanics of Au nanorods in dry and liquid environments using an AFM and depth sensing nanoindenter,” Nanoscale, 6, 5838-5852.

Maharaj, D. and Bhushan, B. (2015), “Characterization of nanofriction of MoS₂ and WS₂ nanotubes,” Mater. Lett. (In press).

Maharaj, D. and Bhushan, B. (2015), “Nanomechanical behavior of MoS₂ and WS₂ multi-walled nanotubes and Carbon nanohorns,” Nature – Scientific Reports.

Maharaj, D. and Bhushan, B. (2015), “Friction, wear and mechanical behavior on the nanoscale,” In review