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Surface treatment and tribology

Check the laws of friction and the real area of contact, adhesive and abrasive wear, the Archard wear equation, the Stribeck curve and lubrication regimes, solid lubrication, and carburizing, nitriding, electroless plating and hard coatings through three-choice questions.

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Surface treatment and tribology: key points

In tribology, the study of friction and wear, the starting point is the real area of contact where tiny asperities on the surfaces touch. The real area of contact equals the load divided by the hardness and is far smaller than the apparent area, which explains why friction force is proportional to load and independent of contact area. Wear includes adhesive wear from adhesion at contact spots and abrasive wear from cutting by hard particles, and its amount is estimated from load and sliding distance with the Archard equation.

The lubrication regime shifts among boundary, mixed and hydrodynamic lubrication with oil viscosity, speed and pressure, as organized by the Stribeck curve. In surface treatment, carburizing and quenching or nitriding hardens the surface layer of steel, and electroless nickel plating and hard coatings such as TiN raise wear resistance. The key is to choose a method while considering how the treatment temperature affects the microstructure and dimensions of the base material.

All Surface treatment and tribology questions (15)
  1. Which statement correctly describes the empirical law of friction (the Amontons–Coulomb law) that often holds for dry sliding friction between metals?
  2. A flat steel block (indentation hardness H = 2.0 GPa) rests on a plate of the same steel, and a normal load of 1.0 kN is applied over an apparent contact area of 10 cm². Assuming every contacting asperity deforms plastically and estimating the real area of contact as Ar = W/H, which is closest to its fraction of the apparent contact area?
  3. 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?
  4. Which of the following descriptions of wear mechanisms corresponds to adhesive wear?
  5. 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?
  6. 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?
  7. 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?
  8. 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.
  9. A plain bearing with an inner diameter of 40 mm and a width of 30 mm carries a radial load of 1.2 kN, and the shaft turns at 600 rpm. Taking the bearing pressure P as the load divided by the projected area (inner diameter × width) and the sliding speed v as the surface speed of the shaft, which is closest to the PV value?
  10. Why is molybdenum disulfide (MoS₂) better suited than graphite as a solid lubricant for mechanisms used in vacuum, such as on satellites?
  11. A gear made of low-carbon steel (about 0.2%C) is carburized and quenched. Which statement about this treatment is correct?
  12. Which statement about nitriding, such as gas nitriding, applied to a nitriding steel containing Al, Cr and Mo is correct?
  13. Which statement correctly describes electroless nickel plating (Ni-P plating)?
  14. Cutting tools are coated with hard films such as TiN. Which is an appropriate way to choose the coating method for cemented carbide tools and for high-speed tool steel (HSS) tools?
  15. The hardness of a TiN film 2.0 µm thick is measured by indentation with a Vickers indenter. To avoid influence from the substrate, the empirical guideline of keeping the indentation depth to one tenth of the film thickness or less is used. The depth of a Vickers indent is about one seventh of its diagonal length. Which is closest to the largest allowable diagonal length?

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