Warfighters on the battlefield often rely on machines, vehicles, and other technologies with rotating parts to complete their mission. Researchers have devised a new method of testing for a major factor in equipment failure and breakdown in order to ensure that those tools meet the proper standard of quality.
When mechanical parts slide against each other for long periods of time, the constant grinding may wear down the metal surfaces until the parts are no longer functional. The study of friction, wear, and lubrication as two or more surfaces interact in relative motion is known as tribology and its importance in material science and engineering has led researchers to find new ways to examine dry mechanical contact. The new approach analyzes the tribological response between steel and silicon nitride that takes place as the two metals interact, rather than after the samples have cooled off. This method of studying wear and tear may allow researchers to observe fleeting chemical reactions that occur at the contact site.
The interaction between steel and silicon nitride is one that commonly takes place during the dry machining process of certain cutting tools and in emergency situations with high-speed bearings when they lose their lubrication source, like those in jet engine turbines. Understanding the kinetics behind the high-speed sliding contact between these two metals would be vital in developing better and safer vehicles and equipment for soldiers.
A test using a ball-on-disk tribometer slid a rolling silicon nitride ball against a steel rotating disk that was heated to 120 °C with a hot plate underneath. A stereo-optical microscope with a color charge-coupled device (CCD) camera and an infrared camera obtained thermal imaging data as the rotating speed of the disk sped up from 1 m/s to 16 m/s. Afterwards, an analysis of the wear tracks was done using a backscatter electron detector that mapped the elemental composition of the leftover film residue.
The team discovered that the frictional heating caused at a threshold sliding speed of around 4.5 m/s induced a chemical reaction that left behind a lubricating thin film at the highly loaded contact zone. This slippery thin film was what allowed the mechanical interaction between steel and silicon nitride to demonstrate lower friction and wear as the sliding speed increased. Using the new approach, the team managed to pinpoint the exact time that the chemical reaction occurred from observations of the wear tracks’ color change during the experiment. Additionally, they determined that this phenomenon is fully active when the sliding speed rose above 9 m/s under gear- and bearing-like conditions.