Cutting ceramics

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Cutting ceramics are ceramic cutting materials , i.e. ceramic materials for milling tools , turning tools and drills . Indexable inserts are usually made from cutting ceramics and function as cutting edges in the tools. Cutting ceramics are harder and more wear-resistant than hard metals , even at high temperatures , but they are more fragile and more expensive. Higher cutting speeds are therefore possible with cutting ceramics, but the feed rate and cutting depth must be reduced. Boron nitride tools are even harder, more wear-resistant and more expensive . Cutting ceramics are used primarily in the aviation and aerospace and automotive industry in the mass production of workpieces made of cast iron and difficult to machine materials such as high-strength steel and nickel .

Cutting ceramics are sintered from ceramic powders . In contrast to hard metals, no binding metal is used, which causes the higher hot hardness and lower strength. Attempts to sinter ceramic powders with a binder metal were unsuccessful.

There are two groups based on their composition:

  • Aluminum oxide cutting ceramics. They are harder, more wear-resistant and more fragile than silicon nitride ceramics. They are also susceptible to changing temperatures. Aluminum oxide is also used for grinding wheels and is referred to there as corundum .
    • Pure oxide cutting ceramics ( pure ceramics ). It rarely consists exclusively of aluminum oxide; it usually still contains up to 15% zirconium oxide, which improves the strength.
    • Mixed ceramics . It also contains non-oxidic hard materials such as titanium carbide . This increases the hardness and wear resistance, which is why they are suitable for machining hardened steel ( hard cutting ). Because of their higher thermal conductivity, they are less sensitive to changes in temperature. They are more complex to manufacture, more expensive and the tool shapes that can be produced are limited.
    • Whisker reinforced cutting ceramic. Contains whiskers - tiny crystals - made of silicon carbide that increase strength and thermal conductivity.
  • Silicon nitride cutting ceramics. They are stronger, less fragile and less susceptible to temperature changes. They are also suitable for milling and the use of cooling lubricants. They are not suitable for machining steel because they enter into chemical reactions.
    • Pure silicon nitride cutting ceramic
    • Sialon ( SiAlON ). Contains aluminum oxide additives.
    • Coated (silicon nitride) cutting ceramic

According to ISO 513, the following groups are distinguished. The abbreviation always begins with a 'C' (Ceramics):

  • CA: A luminiumoxid-cutting ceramics
  • CM: M ischkeramik
  • CR: whisker reinforced ceramic ( r enforced = reinforced)
  • CN: Siliciumnidrid-cutting ceramics ( N itrid)
  • CC: coated ( c oatet = coated)

history

Alumina ceramics were first introduced in the 1930s. Since they did not contain any zirconia additives, they were very fragile and did not prevail. The decisive developments for all cutting ceramics date back to the 1980s. Compared to the previously common hard metals, this enabled significantly higher cutting speeds. Together with the boron nitride cutting materials that were created around the same time, it was also possible for the first time to machine hardened steel by turning, drilling or milling. Previously, this was only possible by grinding.

Manufacturing

All cutting ceramics are sintered from powders . The details differ mainly between oxide ceramics and nitride ceramics. The starting material is always powder. The last step is always grinding with diamond tools, during which the final shape and surface quality are determined.

In the case of cutting ceramics made from aluminum oxide, an aluminum oxide powder is used as the starting material, which is mixed with powders containing zirconium oxide , titanium carbide or whiskers . The powder is then poured into molds and either cold pressed and then sintered or pressed and sintered in a single step at high pressure. After cold pressing , the tools can still be machined by drilling .

For cutting ceramics made of silicon nitride, powder made of silicon can be used. It then reacts with the nitrogen from the air during sintering to form silicon nitride. Instead, silicon nitride powder can also be used. They are pressed into large plates, from which the tools are separated by laser cutting .

Manufacture of aluminum oxide cutting ceramics

The starting material is powder made of aluminum oxide, to which small amounts of sintering aids such as magnesium oxide are added, which closes the gaps between the ceramic particles. It is mixed with the desired proportions of zirconium oxide (for the pure oxide ceramic), titanium carbide and titanium nitride for the mixed ceramic or whiskers for the whisker-reinforced ceramic. They are ground together. In this process, known as attrition , the flowability is increased and agglomerates of several particles are dissolved. The grain size is determined when the powder is manufactured and is no longer influenced by grinding. The powder is then poured into molds.

There are several alternatives for further processing: hot pressing, in which it is compressed at high temperature and the tool is completed in a single step, as well as spray drying, cold pressing, sintering and, if necessary, hot isostatic pressing.

In the older variant, the powder is first processed by spray drying , then poured into the molds and cold-pressed at room temperature and pressures of a few hundred bar. The resulting green body already has a fixed shape, but can still be processed by drilling and other cutting processes. In this way, for example, bores can be incorporated with which the indexable inserts can be screwed tight, but this does not make sense with the pure oxide ceramics because of the high brittleness. After cold pressing, sintering takes place at room pressure and temperatures of 1500 ° C to 1800 ° C. The individual particles of the powder grow together and are then firmly connected to one another. In order to further reduce the porosity, they can be further processed by hot isostatic pressing. For this, however, a pore-free structure must be present at least on the surface.

Manufacture of silicon nitride ceramics

In the case of silicon nitride ceramics, pure silicon powder can serve as the starting material. It is sintered at high temperatures and pressures and reacts with the nitrogen from the air to form silicon nitride. The result is known as reaction-bonded silicon nitride. The use of silicon nitride powders is also possible.

The silicon nitride ceramics must always be sintered with overpressure, otherwise they will break down into nitrogen and silicon. Since silicon nitride forms needle-shaped crystals that never completely fill the material but always leave pores free, the nitride ceramics must always be sintered with a sintering aid. Yttrium, magnesium and silicon oxide are used, which form a silicate glass phase and act as a binding phase. These substances do not have a melting point but gradually soften when heated, thus determining the high temperature properties. If the ceramics are still treated by tempering, the glass phases can be rearranged into crystals, which leads to increased strength and hardness at high temperatures.

The silicon nitride ceramics are pressed into large plates from which the individual indexable inserts are cut out with a laser. This is possible because silicon nitride does not melt, but rather sublimes at 1900 ° C.

properties

The cutting ceramics have the following properties. For comparison, a type of hard metal suitable for machining steel is also given.

Hard metal (WC-Co, P10) Oxide ceramic (3.5% ZrO 2 ) Oxide ceramic (15% ZrO 2 ) Whisker-reinforced oxide ceramic (15% ZrO 2 , 20% SiC whisker) Mixed ceramic (10% ZrO 2 , 5% TiC) Mixed ceramic (30% Ti (C, N)) Silicon nitride ceramic (10% Y 2 O 3 ) Sialon (sintered)
Density [g / cm³] 10.6 4th 4.2 3.7 4.1 4.3 3.3 -
Vickers hardness 1560 1730 1750 1900 1730 1930 1750 1870
Flexural strength [N / mm²] 1700 700 800 900 650 620 800 800
Compressive strength [N / mm²] 4500 5000 4700 - 4800 4800 2500-5500 3500
Modulus of elasticity [1000 N / mm²] 520 380 410 390 390 400 280-320 -
Fracture toughness K IC [Nm 1/2 / mm²] 8.1 4.5 5.1 8th 4.2 4.5 7th 6.5
Thermal conductivity [1 / (1,000,000 K)] 25th 8th 8th - 8th 8th 3.4 -
colour metallic White White black Gray

Oxidic cutting ceramics (aluminum oxide base)

Pure oxide ceramics

Oxide ceramics are white to light pink in color and consist mainly of aluminum oxide . This material is very hard and chemically resistant even at high temperatures. While the hardness of hard metals decreases sharply and wear increases from temperatures of 900 ° C, aluminum oxide cutting ceramics remain hard and wear-resistant even at over 1200 ° C. They are used for machining steel and cast iron when turning. Because of their high brittleness and low breaking strength, they tend to break under shock loads. In addition, the thermal conductivity is relatively low and the thermal expansion is high, so that internal stresses occur when the temperature changes, which destroy the tool. The thermal shock resistance is low. Typical forms of wear with changing temperatures are comb cracks . Because of their sensitivity to changing temperatures and fluctuating forces, they are not used for milling and also not with cooling lubricants. Because of their high hot hardness and strength, they can also be used for dry machining without cooling lubricants .

Technically pure aluminum oxide is no longer used because of its low strength and high fragility. Modern ceramics contain up to 15% zirconium dioxide, which is distributed in fine particles in the material. These particles change, depending on the particle size, at temperatures of 1400 ° C to 1600 ° C (i.e. at the sintering temperature). Above they are in a tetragonal lattice modification with a relatively low density, below in a monoclinic one with a greater density. Microcracks, which would spread quickly without the zirconium dioxide particles and thus lead to breakage, hit the zirconium dioxide particles after short distances and end there.

Mixed ceramics

Mixed ceramics are black and contain between 5 and 40% non-oxidic components, mostly titanium carbide (TiC) or titanium carbonitride (TiCN). Since the hard materials limit the grain growth of the aluminum oxide, they have a very fine structure with mean grain sizes of less than 2 µm, sometimes even less than 1 µm. This fine structure leads to higher strengths and higher wear resistance. The additives also increase the thermal conductivity so that mixed ceramics with cooling lubricant can be used. They are suitable for the hard machining of hardened steels, which is used, among other things, for machining roller bearings , as well as for gears, drive gears and ring gears and for machining cast iron at high cutting speeds. When it comes to hard machining, mixed ceramics compete with boron nitride tools.

Whisker reinforced ceramics

In addition to aluminum oxide and zirconium dioxide, whisker- reinforced ceramics also contain whiskers made from silicon carbide . These are tiny rod-shaped single crystals that have a very high strength. Whisker-reinforced ceramics have a fracture toughness that is around 60% higher than that of mixed ceramics. They also increase the thermal conductivity so that cooling lubricant can be used. Silicon nitride forms chemical compounds with steel, which limits its application.

Non-oxide cutting ceramics (silicon nitride base)

Cutting ceramics based on silicon nitride have higher strengths than oxide ceramics, are tougher, less fragile and less sensitive to thermal shock. They can therefore be used for milling, with cooling lubricants and generally for changing loads. The disadvantage is that they react chemically with steel and wear out very quickly. The pure silicon nitride ceramics can therefore not be used for steel processing. Further developments, the sialons and the coated ceramics, try to compensate for this disadvantage.

Silicon nitride cutting ceramics

Silicon nitride cutting ceramics contain even larger amounts of sintering aids in order to close the gaps between the rod-shaped silicon nitride crystals. Mostly yttrium oxide and magnesium oxide are used . The silicon nitride crystals solidify in a hexagonal modification and, because of their rod shape, lead to higher strengths than the spherical aluminum oxide ceramics. The thermal expansion is lower, which leads to a lower susceptibility to thermal shock. The binding phase (yttrium oxide) forms glass phases that soften at high temperatures, which limits the usable cutting speeds.

They are used for machining gray cast iron , nickel-based materials.

Sialon

Sialon (Si-Al-ON, SIALON) consists mainly of silicon nitride and contains additions of aluminum oxide and other sintering aids. They are chemically more resistant to ferrous materials and are used to process them. With gray cast iron, cutting speeds of up to 600 m / min can be achieved.

Coated cutting ceramics

In order to be able to use the good mechanical properties of silicon nitride ceramics for machining steel, some ceramics are coated. Oxide ceramics are not coated. Titanium nitride (TiN), titanium carbide (TiC), titanium carbonitride (TiCN) and aluminum oxide and combinations thereof are used as laminates .

literature

  • Uwe Heisel, Fritz Klocke, Eckart Uhlmann, Günter Spur: manual cutting. 2nd edition, Hanser, Munich 2014, pp. 101f.
  • Wilfried König, Fritz Klocke: Manufacturing process 1: turning, milling, drilling. 8th edition. Springer, Berlin 2008, pp. 177-189.
  • Eberhard Pauksch: Machining technology. 12th edition, Springer Vieweg, Wiesbaden 2008, pp. 62-65.
  • Herbert Schönherr: Machining production. Oldenbourg, Berlin 2002, pp. 35-38.
  • Ralf Riedel (Ed.): Handbook of Ceramic Hard Materials , Wiley-VCH, 2000:
    • A. Krell, Ceramics Based on Alumina: Increasing the Hardness for Tool Applications , pp. 648-682. [Aluminum oxide cutting ceramics and abrasives].
    • M. Herrmann, H. Klemm, Chr. Schubert: Silicon Nitride Based Hard Materials , [Siliciumnitrid-Schneidkeramiken], pp. 749-801.

Individual evidence

  1. Berend Denkena, Hans Kurt Tönshoff : Spanen - basic. 3rd edition, Springer, Berlin 2011. ISBN 978-3-642-19771-0 , pp. 188f
  2. Berend Denkena, Hans Kurt Tönshoff : Spanen - basic. 3rd edition, Springer, Berlin 2011. ISBN 978-3-642-19771-0 , pp. 188fS. 189 f.
  3. Wilfried König, Fritz Klocke: Manufacturing process 1: turning, milling, drilling. 8th edition. Springer, Berlin 2008, p. 179.
  4. Wilfried König, Fritz Klocke: Manufacturing process 1: turning, milling, drilling. 8th edition. Springer, Berlin 2008, p. 130.
  5. ^ Trent, Wright: Metal Cutting . Butterworth Heinemann, 2000, 4th edition, p. 234.
  6. Pauksch: S. 61
  7. Pauksch :? , P. 61
  8. Wilfried König, Fritz Klocke: Manufacturing process 1: turning, milling, drilling. 8th edition. Springer, Berlin 2008, p. 179 f.
  9. Wilfried König, Fritz Klocke: Manufacturing process 1: turning, milling, drilling. 8th edition. Springer, Berlin 2008, p. 180 f.
  10. Wilfried König, Fritz Klocke: Manufacturing process 1: turning, milling, drilling. 8th edition. Springer, Berlin 2008, p. 181.
  11. Wilfried König, Fritz Klocke: Manufacturing process 1: turning, milling, drilling. 8th edition. Springer, Berlin 2008, p. 186 f.
  12. Wilfried König, Fritz Klocke: Manufacturing process 1: turning, milling, drilling. 8th edition. Springer, Berlin 2008, p. 188.
  13. ^ Trent, Wright: Metal Cutting . Butterworth Heinemann, 2000, 4th edition, pp. 321-235.
  14. Wilfried König, Fritz Klocke: Manufacturing process 1: turning, milling, drilling. 8th edition. Springer, Berlin 2008, p. 188.