One of the best ways to increase fuel efficiency in today's vehicles is to reduce their overall weight by using lightweight aluminum or magnesium-based alloys. But forming these alloys into automotive body parts is very difficult, mainly because of the high friction of the alloy surfaces, the extreme pressures required for metalworking operations, and the inability of conventional lubricants to prevent wear under the extreme conditions encountered during metalworking. Also, most conventional lubricants are flammable and contain chlorine, phosphorus, and sulfur-bearing additives that are potentially hazardous: removing these lubricants from finished products and treating them for disposal is difficult and costly.

Several years ago scientists at Argonne National Laboratory discovered that boric acid, used as a lubricant, is one of the most slippery solids around. Tests show that boric acid can provide friction coefficients as low as 0.02 to 0.05 - one fourth to one-sixth the value of other, more expensive solid lubricants. Its exceptionally low friction coefficient prevents the aluminum and magnesium-based alloys from sticking or transferring to the die or roll surfaces.

The lubricating mechanism of boric acid is controlled by its special structure. The compound is crystallized in layers in which the atoms are tightly bonded to each other. The layers themselves are weakly bonded; when stressed, they shear and slide over one another easily, so friction is low. The strong bonding between the layers prevents direct contact between sliding parts, thereby minimizing wear.

"Boric acid is a cheap, abundant, and environmentally friendly substance that greatly reduces friction and wear of dies and molds and at the same time provides an ultra smooth surface finish on final products. After metal-forming operations, parts can be rinsed in water to remove the excess lubricants - no toxic or flammable solvents are necessary," said Argonne principal investigator Ali Erdemir. He also pointed out that use of boric acid decreases the unit cost for automotive parts because the near-perfect finished products don't require secondary machining or grinding.

Other potential automotive applications for boric acid include its use as a lubricant for gears and bearings. Boric acid may also be mixed with existing liquid and solid lubricants; if the conventional lubricant is inadequate, boric acid becomes active and acts as a backup.

The discovery of boric acid as a solid lubricant was an R&D 100 award winner in 1991. The awards recognize "the most significant technical products of the year" as selected by R&D magazine on the basis of importance, uniqueness, and usefulness.

Boric acid is an effective solid lubricant providing friction coefficients of 0.02 to 0.1 to the sliding surfaces of metallic & ceramic materials. Its lubricity, self-lubrication and self-replenishment mechanisms and its potential importance for tribological applications demand serious consideration.

Self-lubricating H3BO3 films result Hertzian contact pressure induced by applied load. Shear strength of H3BO3 is about 23 MPa. In other words, the higher the applied normal loads, the lower the H3BO3 coefficient of friction.

In this study, an atomic force microscopy (AFM) was employed to explore the surface morphology, frictional characteristics (i.e., friction mapping), & surface adhesive forces of boric acid-treated aluminum surfaces though the friction & wear behavior of cleaved single crystals of boric acid was the main focus. Tests were performed on uncoated & boric acid coated surfaces of aluminum substrates as well as for the bulk boric acid material.

The nanoscale wear of the boric acid crystals manifested itself in the displacement of atomic planes which in turn resulted in the formation of the worn area grown by a hexagonal spiral growth mechanism. In addition, new crystals in the vicinity of the sliding contact areas were formed. These crystals formed by a spiral dislocation mechanism.

The nanoscale friction measurements showed that the friction coefficient of AFM's Si3N4 tip against the boric acid single crystal was in the range of 0.07 to 0.13, depending on the sliding direction with respect to the crystallographic orientation. The friction coefficients of the boric acid coated aluminum substrates varied between 0.11 & 0.19.

While these values were consistent with earlier findings & further demonstrated the lubricity of boric acid, they were higher than the previous measurements (i.e., 0.01-0.1) obtained on a pin-on-disk machine using steel or ceramic pins. An explanation is provided for this phenomenon.

Velocity-dependence study of friction force showed that the friction force decreased with an increase in AFM scanning velocity. This dependence of the friction on the velocity can be plausibly explicated by a change of the energy dissipation regime from the nonlinear dynamics of a sliding system to phonon excitation.