Knowledge

Cutting tools – Which is the right coating?

Around 95 percent of the carbide tool cutting edges are coated these days. The rise in surface hardness increases the wear resistance of the tool, the reduced slide resistance during chip removal thanks to ultra-smooth surfaces reduces the propensity for weld buildup and built-up edge formation, and the insulating effect of the layer increases the elevated-temperature hardness. Significantly longer tool lives should be achieved as a result.

Two methods are essentially used for coating purposes: PVD coating (Physical Vapour Deposition) and CVD coating (Chemical Vapour Deposition). 

Example: AlTiN coating

CVD coatings

Chemical vapour deposition (CVD) is a method used to produce natural low-stress coatings by means of thermally induced chemical reactions.  

The base substances used for the coating are vaporised and supplied to the coating zone as vapour. The vapour then either decomposes or reacts with additional base substances, thus producing a thin film on the substrate. This can be carried out under vacuum or at atmospheric pressure.  

Substrate temperatures up to 1000 degrees Celsius are needed to achieve the surface reactions. These processes can be plasma-assisted, which increases the reaction rate and reduces the temperature of the coating.   

The CVD process is used to deposit 5 to 12 µm, in some cases up to 20 µm thick coatings. Materials used are TiC, TiCN, TiN and aluminium oxide (Al203). The coatings can be applied as single or multi-layers. 

Properties of CVD coating

  • Low natural stress on the coating
  • Exceptional adhesion of the coating 
  • High load bearing capacity
  • Coatings up to 20 µm are possible
  • Excellent coating uniformity
  • Possibility of internal coating and to coat complex geometries
  • With thicker layers, excellent heat shield effect
  • When turning and milling cast iron, cutting speeds can be achieved which would usually only be possible with ceramics.
  • High process temperatures cause greater brittling of the carbide substrate, thus reducing the degree of toughness of the cutting edge.
  • 20 µm thick coating applications lead to rounding and reduce the sharpness of the cutting edge. 

Diamond coating

Diamond coating is a special form of CVD coating. Here, the introduced hydrogen gas is split into hydrogen radicals by either high temperatures (2000 degrees Celsius) or plasma ignitions. These radicals then react together with the carbonic gas which is also introduced (usually methane, CH4), which in turn leads to the accumulation of carbon on the surface of the substrate. If the correct process parameters are observed, the carbon is deposited in the crystalline form of the diamond. Diamond coatings are ideal for machining extremely abrasive materials, such as graphite or CFK components.

CVD applications

CVD coatings are the first choice when it comes to wear resistance, such as for general turning operations of stainless steels and when drilling into steel, where the thick CVD coatings provide resistance to crater wear. They are also used for milling grades in ISO P, ISO M and ISO K. When drilling, the CVD grades are usually used in the outer cutting edge. 

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PVD coatings

PVD processes, unlike CVD processes, are based on purely physical response methods. Here, it is a material vapour, which condenses on the surface of the substrate. To ensure that the vapour particles reach the components and are not lost through the dispersion of gas molecules, the process takes place under vacuum conditions. Since the PVD manufacturing process occurs at lower temperatures of 400 to 600 degrees Celsius, the properties of the base material are less affected than with the CVD process. The degree of toughness of special, fine-grained carbides is therefore mostly preserved.

With the PVD coating, a distinction is made between four types of coating: evaporation, sputtering, cathodic arc deposition and ion plating. Sputtering is most commonly used. The different PVD variants allow virtually all metals and also carbon to be refined in its purest form. If reactive gases such as oxygen, nitrogen or hydrocarbons are added to the process, then oxides, nitrides and carbonides can also be refined.

PVD properties

  • High purity of coatings
  • Low thermal substrate impact – toughness is preserved
  • Any coating materials
  • Low layer thickness tolerance
  • Exceptional adhesive strength (including across additional intermediate layers)
  • Comparatively small layer thicknesses 

PVD applications

PVD-coated grades are recommended for adhesive materials because of their tough, yet sharp cutting edges. The application ranges include all solid carbide milling cutters and drills and most grades for grooving, thread cutting and milling. PVD-coated grades are also used on a large scale for finishing operations and as centre cutting grades when drilling. 

Multilayer

If a high degree of toughness is required, a multilayer coating is ideal. Up to 2000 individual layers are applied, which are only a few nanometres thick. The multilayer structure prevents any cracks from spreading further inwards during machining. Material that is removed is unable to penetrate the cutting edge as quickly to burst it open. Multilayer coatings therefore contribute to a longer tool life. Besides the layer structure, the top layer is also crucial. Non-ferrous metals especially have a disposition to the formation of built-up edges which increases the cutting forces, temperatures and therefore also tool wear. This problem is minimised by low-friction top layers. 

Which cutting material is most suitable for what?

With the CVD coating, the materials used are generally TiC, TiCN, TiN and aluminium oxide (Al203). The different PVD variants allow virtually all metals and also carbon to be refined. For guidance, here is an overview of the properties of the most commonly used compounds:

TiN: titanium nitride coating

  • Most widely used standard coating and universal application
  • Chemical compound of titanium and nitrogen
  • Nano hardness: up to 24 gigapascal (GPa)
  • Layer thickness: 1-7 μm
  • Friction coefficient: 0.55 μ
  • Application temperature: 600 °C
  • Application: Steel (N/mm²) < 900, brass and cast iron
  • In aluminium, only with stationary machine tools and forced liquid cooling
  • Cooling is recommended
  • Three to four times longer tool life compared with uncoated tools

TiAlN: titanium aluminium nitride coating

  • All-round coating
  • Depending on the application, up to ten times longer tool life
  • High degree of elevated-temperature hardness and oxidation resistance
  • High cutting speed
  • Chemical compound of titanium, aluminium and nitrogen
  • Nano hardness: up to 35 gigapascal (GPa)
  • Layer thickness: 1-4 μm
  • Friction coefficient: 0.5 μ
  • Application temperature: 800 °C
  • Application: Steel (N/mm²) < 1.100, stainless steel, titanium alloys, cast iron, aluminium, brass, bronze and plastic
  • Cooling not essential

AlTiN: aluminium titanium nitride coating

  • Depending on the application, up to fourteen times longer tool life
  • Very high degree of elevated-temperature hardness and oxidation resistance
  • Chemical compound of aluminium, titanium and nitrogen
  • Nano hardness: up to 38 gigapascal (GPa)
  • Layer thickness: 1-4 μm
  • Friction coefficient: 0.7 μ
  • Application temperature: 900 °C
  • Application: Steel (N/mm²) < 1.300, stainless steel
  • Cooling not essential

TiCN: Titanium carbon nitride coating

  • Depending on the application, up to four to five times longer tool life
  • Very high degree of hardness and at the same time good level of toughness
  • Chemical compound of titanium, carbon and nitrogen
  • Nano hardness: up to 32 GPa
  • Layer thickness: 1-4 μm
  • Friction coefficient: 0.2 μ
  • Application temperature: 400 °C
  • Application: Steel (N/mm²) < 1.300, stainless steel
  • Cooling is required for higher cutting speeds