Materials for cutting tools.

The cutting edge is the most important part of the tool. It depends on the progress of their own process of machining, machining productivity and efficiency. It is therefore necessary to devote choice of material cutting tools much attention. Properties of the material on the cutting tool, i.e. its hardness (must be min. 6 HRC more than machined material), strength, toughness, abrasion resistance and heat resistance, are called collectively cuttability.

Cuttability is guaranteed by: the chemical composition, method of manufacture, thermal and other processing, e.g. by molding, soldering and the like. At present, cutting tools are used for the following base materials:

  1. Tool steel a) carbon, b) alloyed
  2. Cemented carbides
  3. Ceramic cutting materials
  4. Diamonds
  5. Abrasive materials

Application areas of cutting tool materials are defined by their physical, thermal and mechanical properties. Tool materials with high hardness may be used at higher cutting speeds and small chip cross section (finish machining), where the thermal load prevails over mechanical. Materials with high toughness can be used at higher traverse speeds (rough machining), where due to larger size of chips prevails mechanical load above the heat.

For high performance cutting tools used today mainly cemented carbide (produced by compression of carbide powder with binder metal powder and subsequent sintering at a temperature close to melting binder) generally coated with the enhancing tool. And are increasingly used cutting ceramics, cermets and super-hard cutting materials such as diamond and cubic boron nitride.

Carbide - highly wear-resistant material

Use of tungsten carbide as a material with high wear resistance is due to its properties: high hardness, high compressive strength, high modulus of elasticity, sufficient hardness, low thermal expansion, good machinability and a low tendency to welding cold.

The most important mechanical properties are the hardness, strength and toughness. Their knowledge is an important basis for selecting the correct type of hard metals. For hard metals are generally characterized by a Vickers hardness test (ISO 3878) and tensile bending test (ISO 3327). Characteristics toughness of brittle materials is relatively problematic. Used to test the toughness which can be determined e.g. by loading the sample with a fine slit or crack length in the Vickers test fingerprint. To determine the toughness of tungsten carbide No ISO, but only recommendations union experts for powder metallurgy - carbides field.

Mechanical properties of hard alloys

Mechanical properties of hardmetals may vary widely. For hard metals of WC-Co, which are preferably used when the wear resistance, are most major elements of Co content and WC grain size. Strength in compression carbide is clearly higher than in bending, while the tensile strength is only about 50% of the flexural strength. It basically follows the rule to use tungsten carbide so as to avoid large tensile load. Magnification WC grain usually leads to decrease hardness and increase toughness. Basically it can be stated that the hardness describes the resistance to abrasive wear, while the flexural strength and impact strength characterizes the behavior of the crack and fracture.
The most important feature which distinguished carbide, the wear resistance. This property is a combination of the listed essential characteristics and shows the relationship to the specification of use. Wear is the loss of the surface layer, and preferably it can be checked during a practical test, but are possible in vitro test.

Types of carbides

We distinguish two main groups of hard metals, hard metals secondly WC-Co has the broadest use, partly hard metals containing impurities carbide, consisting essentially of WC-TiC-Co-TaNbC. The last is used for machining. Besides them, there are special hardmetals with different binders, e.g. Ni, Fe, Cr, possibly with different carbides, e.g. Cr3C2, Mo2C, VC. The tvrdokovům also include cermet, e.g. based TiCN-NiMo-Co, which also finds use in machining.
Designation hardmetals according carbide grain size is performed e.g. according to the following categories: nano, very soft, softer, fine, medium, coarse and very coarse. In recent years, the trend of development, especially in the direction of the fine-grained carbides and hard metals with special properties, e.g. corrosion resistance, resistance to thermal and mechanical changes and erosion.


Basic methods

1. Method PVD (Physical Vapor Deposition- physical vapor deposition)

This method is among the most exploited. The main character is low working temperature and below 500 ° C, it is also suitable for coating of tools of high speed steel (HSS), where no thermal influence tool.

The coating is most often produced:
- evaporation (evaporation is a coating material),
- sputtering (physical dedusting process),
-ion implantation (hybrid PVD coating process in which a substrate surface is bombarded with a particle beam of high energy.

2. CVD method (Chemical Vapor deposition- CVD vapor)

It is the main method of coating cemented carbide, which takes place at high temperatures in the range 1000-1200°C.

The coating is most often produced:
- heat-induction,
- plasma activating,
- electron-induction,
- photon-induction

3. CVD (plasma-activated CVD methods) and MTCVD (Middle Temperature Chemical Vapor deposition- CVD as mean values)

From classic CVD methods differs lower operating temperatures from 400 to 600 ° C by CVD and MTCVD 700-850 ° C. Wherein not change the principle (of coating from the gas phase).

Layers properties

The basic and most important layer properties include:

  • abrasion resistance - cutting tools extend their lifespan severalfold,
  • thermal resistance - coatings withstand temperatures up to 800 DEG C. (coatings based on Cr and Al), and also forms a thermal barrier. This property is used in high-speed machining, where 78% of the heat generated during cutting is removed by chipping,
  • corrosion resistance - protection they provide depends on the microporosity and the ability of some elements contained in the coating create a protective layer. Here can be mentioned coatings containing aluminum, e.g. TiAlN and carbon coatings,
  • reduce the frictional resistance - low coefficient of friction are e.g. MoS2, WC/C a DLC (Diamond Like Carbon),
  • thickness - it is most often measured by Kalotest (grinding through the spherical cap and the subsequent reading of an optical microscope), usually in the range 1-4 microns
  • adhesion - adhesion of coatings on the starting material, the simplest measurement is observing the edges of the puncture formed by the Rockwell tip,
  • microhardness - is one of the toughest DLC coatings, microhardness exceeds 30 GPa. For comparison, e.g., prepared by galvanically "tvrdochromu" can be measured max. 9 GPa.

Commonly used coatings

On cutting tools are applied most often below coatings:

- TiN (titanium nitride) - basic and applied longest layer. Microhardness 20 to 25 GPa, color gold. It can be used in almost all applications. Preferably TiN is good elasticity and adhesion. Moreover, the majority of coating centers is precisely with this layer greatest experience,

- TiAlN (titanium aluminum nitride) - microhardness of 25-33 GPa, the color from pink-purple after black and gray. In the field of cutting tools continues to increase market share at the expense of other layers, particularly TiN, despite higher production costs. It has excellent resistance to high temperatures. Currently ideal for high speed machining. Its interesting feature is forming a surface layer of Al2O3 that during cutting helps to reduce friction, increase the diffusion resistance and improved cutting properties,

- DLC - diamond-like carbon coatings with very low friction coefficient and high hardness (up to 60 GPa), black color. They are used mainly in the automotive industry for coating parts (pumps, locks, etc.) Are not suitable for tool steel proces.