Coating: is a layer of material deposited onto a substrate to enhance the surface properties for corrosion and wear protection. Factors affecting the choice of a coating include service environment, life expectancy, substrate material compatibility, component shape and size, and cost.

Coating of Carbide Grades:

About 80 percent of the inserts used in machining today are coated cemented carbide grades. These tool inserts earned and maintain that growing market share because of their broad application for removing large amounts of material while they sustain a long tool-life.

The high material removal rates and long life for these tools are achieved through an incomparable balance of wear resistance and toughness. Relatively thin insert coatings (0.00004 in. to 0.0007 in.) protect inserts from the heat, corrosion and abrasion that shorten their lives.

Cemented carbide:  is made up of hard particles in a binder, and slight differences to the type and size of hard particles lead to considerable changes in insert performance.

Coating materials:

Most coating-materials used today are classified as ceramic.

  • Titanium carbo-nitride (TiCN) has very good abrasive wear resistance and good adherence to carbide substrate.
  • Aluminum oxide (alumina) (Al2O3) provides very good outer thermal and chemical protection for the insert substrate.
  • Titanium nitride (TiN) mainly used for the golden color which provides clear wear detection.
  • Zirconium oxide (ZrO2) gives very good thermal and chemical protection for the substrate.

Coating Process:

Two principal coating processes are used for indexable inserts to provide cutting edges with fundamentally different properties for machining:

  • Chemical vapor deposition (CVD) which uses a higher temperature and gives thicker coatings.
  • Physical vapor deposition (PVD) uses a lower temperature and gives thinner coatings.

Note: Each of these processes give rise to different tensions in the coating material. Chemical vapor deposition develops tensile stress in the substrate, while physical vapor deposition tends to develop compressive stress. These stresses provide different desirable characteristics for the insert.

Chemical vapor deposition:

CVD-coated inserts work well in turning, milling, and drilling applications involving ferrous materials. In fact, CVD-coated inserts are recommended over PVD-coated inserts for turning, milling, or drilling steels and cast irons.

Characteristics of CVD Coating Types: 

  • TiN Coatings:
    • Excellent build-up edge resistance
    • Excellent on gummy materials
    • Excellent for threading and cutoff operations
    • Makes it easy to identify what insert corners have been used
    • Effective at lower speeds
    • TiC Coatings:
    • Excellent wear resistance
    • Effective at medium speeds
    • Excellent on abrasive materials
    • Al203Coatings:
    • Excellent crater resistance
    • Effective at high speeds and high heat conditions

Physical vapor deposition:

PVD-coated inserts are especially useful for turning, milling, and drilling applications involving high-temperature alloys, titanium alloys and stainless steel. PVD-coated inserts are recommended when turning high-temperature alloys; however, if the alloy is on the softer side and can be machined at higher speeds, a CVD coating is preferred.

Characteristics of PVD Coating Types:

  • TiN Coatings:
  • Excellent built-up edge resistance
  • Broad application range
  • Effective on high-temperature alloys
  • Effective on stainless steels
  • TiCN Coatings:
  • Harder than TiN coatings
  • Effective on end mills
  • Sometimes used in milling applications
    where the work material is abrasive
  • TiAIN Coatings:
  • Harder and more stable than other PVD coating types
  • Becomes harder and more stable with time
  • Effective on high-temperature alloys and stainless steels
  • Effective on abrasive irons at lower speeds

Some common coating types:

  • Anodizing

While anodizing is most common with aluminum, it can also be used on other metals such as titanium and zinc.  However, ferrous metals cannot be anodized because the iron oxide, or rust, flakes off, which causes the anodized layer to also flake off.  When aluminum is exposed to oxygen, a layer of aluminum oxide is formed on the surface of the metal. Anodizing effectively increases the thickness of this aluminum oxide layer, which makes it more resistant to corrosion. To further increase the durability and corrosion resistance, a clear sealant can be used. 

The anodizing process also provides better adhesion for paints and other finishes.  Finally, the anodized aluminum can be dyed prior to sealing to provide a colored aluminum.

  • Galvanizing

Galvanizing is a process by which a layer of zinc is applied to a ferrous metal to prevent corrosion. The process most-commonly refers to hot-dip galvanizing, where a piece of steel is dipped into a bath of molten zinc. The zinc adheres to the steel and immediately reacts with oxygen in the air to form a very strong zinc oxide layer, which prevents corrosion of the steel below. The zinc and steel form a metallurgical bond so the coating will not flake off. The finish is a dull gray and has a crystalline appearance. The galvanized steel can be painted to achieve a specific color. When galvanized metal is welded, the weld and exposed steel must be coated with a special zinc paint to prevent the joint from rusting since the galvanized coating has been removed.

  • Electroplating

Electric current is used to adhere a solution of (generally) cadmium and chromium to a metal.  Nickel plating is a form of electroplating.  One problem with electroplating is that it is difficult to achieve a uniform thickness on a piece. The plating resists corrosion and provides a pleasing appearance.

  • Powder Coatings

A dry powder is electrostatically applied to a metal part.  The part is then cured under heat of about 200 degrees Fahrenheit, which produces a very consistent and pleasing appearance.  Powder coatings are generally more environmentally friendly than paints since solvents are not require.  Powder coatings come in an unlimited range of colors.

Most commercial metal furniture is powder coated since it provides a more durable surface than paint. Powder coatings can also be applied to wood if the moisture content is high enough to provide an electrostatic charge.

  • Porcelain Enamel Coatings

Most seen in cast-iron cookware, enamel coatings provide a smooth and consistent coating that is resistant to stains and scratches.  The coating also prevents corrosion.  Since they are resistant to stains, enamel coatings provide easy cleaning for surfaces prone to graffiti.  Porcelain enamels are used most often on toilet-room partitions.