The Influence of Surface Roughness and Humidity on Glass-Glass and Glass-Steel Friction

Abstract

Effects of environment, and surface roughness on the frictional behavior of aluminosilicate, borosilicate, and soda-lime glasses in contact with glass and 304 stainless steel substrates were tested. Glass and stainless steel substrates produced distinctly different frictional behavior, particularly in dry and humid environments. All dry interactions except for roughened glass-on-glass exhibited a significant increase in mean friction relative to measurements at ambient humidity; this effect was most prominent on stainless steel substrates, and grew progressively stronger for smoother stainless steel substrates. There was a distinct interactive effect between the roughness of the system and its frictional behavior at elevated humidity For all test conditions where at least one substrate was substantially rough, there was little or no change in friction from ambient humidity to humid conditions, while smooth-on-smooth interactions exhibited substantial increases. This can be attributed to the adhesive contribution of adsorbed surface water films at elevated humidity; the thickness of these adsorbed films relative to the surface topography of the samples is critical to this interaction. Surface roughness was an important factor in frictional response, but conventional surface roughness metrics such as Ra and rms values were insufficient to define the difference between “smooth” and “rough” surfaces, as electropolished stainless steel behaved in a manner more consistent with smooth surfaces, despite its relatively large roughness values over the length scales employed for the surface roughness measurements. This can be attributed to the fact that electropolishing produces a very smooth surface over short length scales that is not completely quantified by conventional roughness measurements, and that this process produces broader, smoother asperities with a larger effective radius, thereby increasing effective contact area and adhesion effects. Results can be explained by the dual roles of water in frictional processes, as both a chemical facilitator of hydrolysis for bonds under mechanical strain, and as a film creating adhesion between two contacting surfaces.