English
Menu

Surface treatment and tribology

In the adhesion theory (the view of Bowden and Tabor), the friction force is set by the force needed to shear the adhesive junctions at the real contact spots, and the friction coefficient is μ = s/H (s is the shear strength of the junctions and H is the indentation hardness of the softer material). For s = 0.30 GPa and H = 1.8 GPa, which is closest to μ?

  1. AAbout 0.17
  2. BAbout 6.0
  3. CAbout 0.50
Show answer and explanation

Answer: A. About 0.17

μ = s/H = 0.30/1.8 ≈ 0.17. The friction force is the product F = sAr of the real area of contact Ar = W/H and the junction shear strength s, so F/W = s/H, giving a friction coefficient that depends neither on the load nor on the apparent area. About 6.0 is the value with numerator and denominator swapped, and about 0.50 is the value obtained by dividing by the yield stress estimate H/3 = 0.60 GPa instead of the hardness. Between real metals, the growth of junctions under combined load and shear (junction growth) and ploughing by hard asperities add to this, so μ is often larger than 0.17. The formula also shows that lowering the shear strength s with a lubricant film or an oxide film lowers μ.

Report an error

Practice this topic Back to topic

More Surface treatment and tribology questions

  1. Which of the following descriptions of wear mechanisms corresponds to adhesive wear?
  2. A construction-machine part is rubbed by soil containing quartz sand grains (hardness about 1000 HV). Which is the correct way to choose a material that reduces abrasive wear?
  3. In a pin-on-disk test, a pin of hardness H = 2.0 GPa is loaded with a normal force of 50 N and slid over a distance of 1.0 km. Using the Archard wear equation V = KWL/H with a dimensionless wear coefficient K = 1.0×10⁻⁴, which correctly pairs the wear volume of the pin with the change in wear volume when the load is doubled?
  4. Which statement correctly describes the Stribeck curve, which plots the friction coefficient of a plain bearing against the bearing characteristic number ηN/P formed from the lubricant viscosity η, the rotational speed N and the bearing pressure P?
  5. In a rolling bearing, the minimum oil film thickness is h = 0.40 µm and the root-mean-square roughnesses of the two surfaces are Rq1 = 0.15 µm and Rq2 = 0.20 µm. Find the film thickness ratio λ = h/√(Rq1² + Rq2²) and, taking λ < 1 as boundary lubrication, 1–3 as mixed lubrication and above 3 as hydrodynamic (full-film) lubrication, choose the correct pairing.