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Surface treatment and tribology

Which statement correctly describes electroless nickel plating (Ni-P plating)?

  1. AAs in electroplating, it becomes thick at corners and edges where current concentrates and hardly deposits deep inside holes, so it is unsuited to parts of complex shape
  2. BBecause it contains phosphorus it never becomes amorphous, and heat treatment at about 400 °C drives out the phosphorus and softens it
  3. CIt deposits autocatalytically via a reducing agent without external power, so it coats complex shapes almost uniformly and hardens on heat treatment
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Answer: C. It deposits autocatalytically via a reducing agent without external power, so it coats complex shapes almost uniformly and hardens on heat treatment

In electroless nickel plating, hypophosphite in the bath reduces nickel ions, and the deposited nickel itself catalyzes the reaction so that growth continues. Because it is not governed by current distribution, it deposits with uniform thickness inside holes and on parts of complex shape, and with suitable pretreatment it can even plate nonconductors such as plastics. The deposit contains a few to over ten percent phosphorus, and high-phosphorus deposits are amorphous. Heat treatment at about 400 °C precipitates Ni₃P and raises the hardness from about 500 HV to about 900–1000 HV. Thickening at corners and edges and poor coverage deep in holes are properties of electroplating, which passes a current. Heat treatment does not drive out the phosphorus and soften the deposit.

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More Surface treatment and tribology questions

  1. 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?
  2. 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?
  3. Which statement correctly describes the empirical law of friction (the Amontons–Coulomb law) that often holds for dry sliding friction between metals?
  4. 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?
  5. 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?