In situ and real-time optical investigation of junction growth in spherical elastic–plastic contact

The contact area evolution during pre-sliding (junction growth) of copper spheres loaded against a hard sapphire flat is investigated experimentally. Tests are performed with a novel test rig for real-time and in situ direct measurements that provide, for the first time, a new insight of the junction growth mechanism. It is found that junction growth at sliding inception can cause up to 45% increase in the initial contact area that is formed under normal preload alone. Good correlation is found between the present experimental results and a theoretical model for medium and high normal preloads.