59th Annual Report on Research 2014

The investigation of the effects of tribocharging, friction, wear and mechanical properties of nano-objects, such as Au nanoparticles and nanorods, MoS₂ and WS₂ nanotubes 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. To characterize friction forces and understand the mechanism of friction and wear reduction of nano-objects, studies will be carried out in both single- and multiple-nano-object contact using an AFM. In multiple nano-object contact nano-objects will be aligned in a magnetic field to also investigate rolling and sliding friction. 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, nano-objects will be deposited on silicon substrates either in dry or submerged in water conditions. In the next phase single and multiple nano-object studies of gold nano-objects 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 properties of Au nanoparticles and nanorods, MoS₂ and WS₂ nanotubes and CNHs 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 liquids with enhanced lubrication properties.

Objectives

1) Study of dry nano-objects

• Deposit dry Au nanoparticles and nanorods, MoS₂ and WS₂ nanotubes and carbon nanohorns on silicon substrates.

• Perform single nano-object contact studies on Au nanoparticles and nanorods to compare the effect of shape on the friction mechanism.

• Perform multiple nano-object contact studies on Au nanoparticles and nanorods to simulate contacts nano-objects experience when introduced for the purpose of friction and wear reduction. Determine the role of nano-object size, and shape on friction and wear reduction.

• Perform friction and wear studies on the macroscale using a ball-on-flat tribometer to compare to the nanoscale.

• Perform indentation and compression on Au nanoparticles and nanorods, MoS₂ and WS₂ nanotubes and carbon nanohorns.

• Indent Au nanoparticles and thin film to compare scale effects.

• Determine the role of nano-object size, shape and material on mechanical properties.

• Perform tribocharging by sliding a glass sphere over nano-objects and measure adhesive force on the nanoscale.

• Use EFM to measure the charge density and correlate to change in adhesive force on the nanoscale.

2) Study of nano-object and water hybrid lubricants

• Produce stable dispersions of Au nano-objects in water using sonication

• Perform single nano-object contact studies on Au nanoparticles and nanorods to determine the effect of shape on the friction mechanism and to compare to dry conditions

• Perform multiple-nano-object contact studies on Au nanoparticles and nanorods to simulate contacts nano-objects experience when introduced for the purpose of friction and wear reduction and compare to dry conditions.

• Determine the role of nano-object size and shape on friction and wear reduction.

• Perform friction and wear studies on the macroscale using a ball-on-flat tribometer to compare to the nanoscale.

• Use EFM to measure the charge density and correlate to the change in adhesive force on silicon coated with hybrid nano-object-water lubricants.

Progress to date

Au nanorods were investigated for their effect on friction and wear and compared to spherical Au nanoparticles of the same diameter in dry and submerged in water conditions. Studies were conducted in both single- and multiple-nano-object contacts with the aid of an AFM. In macroscale studies, a ball on flat tribometer was used to compare friction and wear to the nanoscale. Results show that Au nano-objects contribute to friction and wear reduction due to the reduced contact area and possible rolling and sliding on the nanoscale.

The deformation behavior of the Au nano-objects under indentation and compression was also investigated to understand their role in friction and wear reduction. Scale effects were investigated by nanoindentation of Au nanoparticles and thin films of different sizes. Nanoscale hardness of the film was found to be higher than the nanoparticles with both being higher than bulk

MoS₂ and WS₂ nanotubes and CNHs were also investigated to determine their mechanical behavior. MoS₂ nanotubes were investigated to study their mechanical properties and scale effects during local deformation (nanoindentation). Global deformation (compression) studies were performed on MoS₂ and WS₂ nanotubes and CNHs. Nanoscale hardness of the MoS₂ nanotube was found to be similar to bulk and does not follow the smaller is stronger phenomenon. For compression, the highest loads were required for WS₂ nanotubes followed by MoS₂ nanotubes and CNH to achieve the same displacement. Hardening was observed for all nano-objects during repeat compression.

The knowledge gained from these studies will have far reaching effects when designing macro-to nanoscale systems that incorporate materials with nano-dimensions for reducing friction and wear.

Journal Publications

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.