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 recently developed test rig for real-time and in situ direct measurements. The results provide a new insight of the junction growth mechanism showing new points of the sphere surface that are coming into contact with the flat. It is found that junction growth at sliding inception can cause up to 45 percent 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.

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