Question

LAB 1G Sometimes tools such as drill bits and saw blades are coated with very hard compounds to increase their durability and cutting performance. Based on your extensive understanding of electro-negativity and bonding, please rank the following coatings in terms can all be deposited on the steel tool equally well). Additionally please justify your assertions and please comment on which properties would be important for the ideal tool coating of suitability for use as a tool coating (assume they Silicon Nitride (SiN) Zirconium Oxide (ZrO2) (CZ for the thugs in the audience) Titanium Nitride (TiN) Zirconium Nitride (ZrN) Silicon Carbide (SiC)

Answer 1

• Coated tools are becoming the norm in metal working industry because coatings can consistantky improve tool life 200 or 300% or more.
• In cutting tools, material requirements at the surface need to be
  • Abrasion resistance
  • Hardness
  • Chemical inertness to prevent work tool chemical interaction
• A thin, chemically stable, hard refractory coating acomplishes this objective.
• The properties for bulk of the tool are
  • Toughness
  • Shock resistance
  • Resistance to high temperature deformation
  • Breakage or fracture resistance
• Coating properties include
  • Fine grained
  • Free from binders and porosity
  • Metallurgically bonded to substrate
  • Thick enough to prolong tool life
  • Thin enough to prevent brittleness
  • Liw coefficient of friction to prevent chip adhesion on rake surface
• Multiple coatings may be used, with each layer imparting its own properties
• The most popular process for coating is Chemical Vapour Deposition(CVD)

Bonding of coatings

Hardness of intrinsic hard coatings comes from the strong covalent bond with short bond length and high coordination number. Nitrides and carbides both exhibit great mechanical performance. Intrinsic nitride coatings are divided into interstitial nitrides and covalent nitrides.

• Interstitial nitrides are nitride compounds of early transition metals, namely, that of group IV (Ti, Zr, and Hf), group V (V, Nb, and Ta), and group VI (Cr, Mo, and W).
• Among these, nitrides of group IV and V have higher melting points (> 1800°C) and better chemical stability, but those based on group VI (Cr2N, Mo2N, and W2N) have relatively low melting points and easily decompose into pure metals and N2 at above 1000°C.
• Stoichiometric nitrides such as TiN, TaN, CrN, and ZrN are in the rock salt (NaCl) structure.
• Covalent nitrides form from elements in Group 3A (Al, B, and Ga) and Group 4A (Si and C).
• Their electronic bondings are mostly covalent because of the difference in both electronegativity and atomic size between nitrogen and metals.
• Covalent nitrides usually have HCP structure, they exhibit similar properties as ceramics, such as high melting point, low density, and good thermal/electric insulation.
• Hard coatings with enhanced resistance to cracking are usually the oxide–oxide composite coatings such as ZrO₂ are employed.

Ranking tool material according to suitability of coating

TiN > ZrN > ZrO₂ > SiN > SiC

As far as ideal tool coating is concerned, none of the coatings can actually perform all tge operations required for the properties of ideal tool material so a compromise of pros and cons can be described in case of TiN due to high toughness, hardness, low friction coefficient, temperature resistance and crack resistance.