Solid lubricant composite

Dr. Neil Canter, Contributing Editor | TLT Tech Beat November 2017

Researchers have prepared a new solid lubricant produced by mixing graphene with zinc oxide and polyvinylidene difluoride. 

KEY CONCEPTS
• A new solid lubricant was prepared from graphene, zinc oxide and polyvinylidene difluoride.
• Based on instrumental analysis, the solid lubricant contains a random arrangement of graphene micro-scale particles with a monodispersed mixture of nanoscale particles of zinc oxide and polyvinylidene difluoride.
• This solid lubricant displayed strong friction and wear reduction attributed to the presence of graphene.

THE GOAL FOR MINIMIZING FRICTION AND WEAR through the development of new lubricants is ongoing. This column has discussed various approaches that include surface modification and the use of new solid lubricants.

In a previous TLT article, shot peening was used by researchers to modify a cast iron block (1). A tenfold reduction of coefficient of friction was achieved. 

One of the new solid lubricants under evaluation is graphene—two-dimensional sheets of carbon organized into hexagonal honeycombs. Graphene has shown promise but does have limitations. 

A previous TLT article discussed the characteristics of a key graphene derivative, graphene oxide, which is used because it can easily be chemically modified (2). Graphene oxide is highly flammable due to the presence of byproducts formed during its manufacture. Researchers found that cross-linking graphene oxide with aluminum cations eliminated the flammability problem because one of the flammable by-products, epoxides, is eliminated through reaction with the aluminum cations. 

Vilas Pol, associate professor of chemical engineering at Purdue University in West Lafayette, Ind., discussed the positive and negative characteristics of graphene. He says, “Graphene exhibits superior thermal conductivity, extreme mechanical strength and ultralow friction. This solid lubricant also displays friction and wear reductions under low-contact pressures, but performance deteriorates when contact pressures increase above 0.5 gigapascals.”

This limitation causes friction and wear to rapidly increase and is due to graphene’s poor adhesion with the surface of the substrate under high contact pressure. Two other factors also may limit the effectiveness of graphene. Pol says, “There is concern that graphene may be a human health hazard. The high cost of graphene may reduce commercial applications for this material.”

A new solid lubricant now has been developed that minimizes the performance problems with graphene through the preparation of a solid lubricant composite.

ZINC OXIDE
Pol, Farshid Sadeghi, Cummins Distinguished Professor of Mechanical Engineering at Purdue and an STLE Fellow, and their colleagues prepared a new solid lubricant that is a composite produced by mixing graphene with zinc oxide and polyvinylidene difluoride (PVDF). In ball-on-disk tribometer testing, this solid lubricant composite displays significant friction and wear reduction under high-contact pressures. 

As shown in Figure 2, the composite is prepared by first heating zinc acetate in a furnace at elevated temperature to produce zinc oxide. This material was then blended with graphene and PVDF in the solvent N-Methyl-2-pyrrolidone to form a slurry through the use of ultrasonic homogenization. The slurry is then applied directly to the tribometer disk using a spin-coating technique. Upon drying at 80 C to remove the solvent, a coating with an approximate thickness of 10 microns is formed.


Figure 2. A solid lubricant formed by the process shown in the top equation displays a much lower coefficient of friction and superior wear reduction in testing done versus an unlubricated surface (bottom image). (Figure courtesy of Purdue University.)

Tribometer testing under a load of 10 newtons and a pressure of 0.89 gigapascals produced a 90% friction reduction as compared to an uncoated disk. Graphs showing the coefficient of friction versus sliding distance for both tests are shown in Figure 2.

In testing under similar conditions versus graphene and a combination of graphene and PVDF, the dry lubricant composite exhibits a coefficient of friction of 0.08 while the figure for graphene is 0.10 and is 0.14 for the combination for graphene and PVDF.

Wear reduction was determined through three-dimensional surface scans of the ball specimens as shown in Figure 2. The top image shows an unlubricated surface that exhibits a significant wear scar. The bottom image shows an image of a surface that was prepared with the dry lubricant composite and does not show any evidence of a wear scar. 

Wear scars also were detected with graphene and the combination of graphene and PVDF. 

Pol says, “Zinc oxide was used with graphene because this material is known to provide strong adhesion to metal surfaces, is compatible with graphene and has a low cost. It functioned as a binding agent between the graphene and the surface of the ball and disk steel specimens. We also were aware that zinc derivatives such as zinc dialkyldithiophosphate are well-known antiwear additives used in a variety of lubricant applications.”

PVDF acted as a binder to hold the composite together. 

X-ray powder diffraction and Raman spectroscopy were used to analyze the structure of the solid lubricant composite. The researchers conclude that the composite consists of a monodispersed mixture of nanoscale particles of zinc oxide and PVDF in a random arrangement of graphene micro-scale particles. None of the raw materials used are chemically altered when mixed. 

The main contributor of friction and wear reduction was graphene. This conclusion was reached after evaluating the wear track on the rotating disk specimen and finding that it contained a high graphene content. Raman spectral mapping of the solid film present on the ball and disk specimens after testing shows a high concentration of graphene. This demonstrates that the film exhibits a high degree of durability under the severe test conditions. 

Pol indicates there is potential for the solid lubricant composite in commercial applications. He says, “We are now working on an industrial product application that involves the use of the solid lubricant composite.”

Additional information about the solid lubricant composite can be found in a recent article (3) or by contacting Pol at vpol@purdue.edu. 

REFERENCES
1. Canter, N. (2017), “Low coefficient of friction without formulated lubricants,” TLT, 73 (1), pp. 10-11.
2. Canter, N. (2017), “Non-flammable graphene oxide,” TLT, 73 (7), pp. 12-13.
3. Alazemi, A., Dysart, A., Shaffer, S., Pol, V., Stacke, L. and Sadeghi, F. (2017), “Novel tertiary dry solid lubricant on steel surfaces reduces significant friction and wear under high load conditions,” Carbon, 123, pp. 7-17.


Neil Canter heads his own consulting company, Chemical Solutions, in Willow Grove, Pa. Ideas for Tech Beat items can be submitted to him at neilcanter@comcast.net.