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Surface treatment and tribology

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?

  1. AHSS gains secondary hardening on tempering and so resists heat better than cemented carbide, so even thick coating by thermal CVD at about 1000 °C does not lower its hardness
  2. BThermal CVD at about 1000 °C suits cemented carbide, but HSS is coated by PVD at about 500 °C or below so as not to exceed its tempering temperature (about 550 °C)
  3. CPVD only evaporates the source physically and uses no chemical reaction, so it cannot make compound films such as TiN and can only deposit metal films
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Answer: B. Thermal CVD at about 1000 °C suits cemented carbide, but HSS is coated by PVD at about 500 °C or below so as not to exceed its tempering temperature (about 550 °C)

Thermal CVD forms films by reacting source gases such as TiCl₄, N₂ and H₂ at about 1000 °C. It gives thick, well-adhering films and is widely used for cemented carbide tools, which do not soften at high temperature. HSS gets its hardness from quenching and tempering, so heating above the tempering temperature softens it, and requenching also raises problems of dimensional change. PVD methods such as sputtering and arc ion plating are therefore used at about 500 °C or below. Even with PVD, compound films such as TiN, TiAlN and CrN can be made by reactive deposition, in which Ti is evaporated or sputtered in a nitrogen-containing atmosphere. The secondary hardening of HSS occurs in the tempering range; its heat resistance is lower than that of cemented carbide, and it cannot withstand thermal CVD temperatures.

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More Surface treatment and tribology questions

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