Ceramic material is an important field to be developed in the future.
Ceramic cutting tools are widely used in high-speed cutting, dry cutting, hard cutting and machining of difficult to machine materials.
（1） Characteristics of ceramic cutting tools
1. High hardness and good wear resistance
Although the hardness of ceramic tools is not as high as that of PCD and PCBN, it is much higher than that of cemented carbide and high-speed steel tools, reaching 93~95HRA.
The optimum cutting speed of ceramic tools can be 2~10 times higher than that of cemented carbide tools, and the tool life is long, which can reduce the number of tool changes, thus greatly improving the cutting efficiency.
Therefore, ceramic cutting tools can process high hard materials that are difficult to be processed by traditional cutting tools, so as to realize “turning instead of grinding”.
Ceramic cutting tools are suitable for high-speed cutting and hard cutting.
2. Good high temperature resistance and heat resistance
Ceramic cutting tools can still cut at high temperatures above 1200 ℃.
Ceramic cutting tools have good high-temperature mechanical properties. The hardness is 87hra at 800 ℃ and 80hra at 1200 ℃.
With the increase of temperature, the high-temperature mechanical properties of ceramic tools decrease very slowly.
The oxidation resistance of alumina ceramic cutting tools is particularly good, and the cutting edge can be used continuously even in the red hot state.
Therefore, ceramic cutting tools can realize dry cutting, thus cutting fluid can be omitted.
3. Good chemical stability
Ceramic cutting tools are not easy to bond with metal, and have good corrosion resistance and chemical stability, which can reduce the bonding wear of cutting tools.
4. Low friction coefficient
Ceramic tools have low affinity with metal and low friction coefficient, which can reduce cutting force and cutting temperature.
5. Abundant raw materials
Tungsten, cobalt and other resources contained in cemented carbide are scarce and expensive. Alumina, silicon oxide and carbide, the main raw materials used in ceramic tool materials, are the most abundant elements in the crust, which is very beneficial to the development of ceramic tool materials.
Therefore, the development and use of ceramic cutting tools is of great significance to save strategic precious metals.
According to the above characteristics, ceramic cutting tools are mostly used for finishing and semi finishing of steel, cast iron and non-ferrous metal materials, or machining of difficult to machine materials without impact and vibration and high-precision large workpieces.
The ceramic tool is shown in Fig. 2-29.
Fig. 2-29 ceramic tool (piece)
The development history of ceramic tool materials is shown in table 2-31.
Table 2-31 development of ceramic tool materials
| Ceramic tool materials | Age of appearance | Bending strength / (N/mm2) |
| Al2O3 for moulds and Cutters Sintering Al2O3 Al2O3 +Gr2O3 Thermocompression Al2O3 Al2O3 +(O.5-1)%MgO Al2O3 +Mo2C+(Mo) Al2O3 +Ti, TiC, TiC/WC Superfine Al2O3 Thermocompression Al2O3 +TiC/Ni Al2O3 +ZrO2 Al2O3 +TiN Al2O3 +TiB2 Si3N4 Si3N4+Y2O3+Al2O3 +TiC Si3N4+TiC+Co Si3N4+Al2O3(Sialon) | 1912-1913 1930-1931 1937-1938 1944-1955 1948-1951 1951-1959 1955-1958 1968-1970 1968-1970 1976-1977 1976-1977 1979-1980 1973-1975 1976-1978 1980-1981 1978-1980 | 150-230 200-350 300-400 500-700 400-500 350-450 400-550 700-900 800-1000 Thermocompression900~1500 Cold compression700~1000 800-840 700-950 600-800 850-950 700-800 800-900 |
（2） Classification of ceramic cutting tools
At present, most of the most widely used ceramic tool materials at home and abroad are multiphase ceramics, including alumina based ceramics and silicon nitride based ceramics.
Alumina based ceramics can be divided into pure ceramics (“white” ceramics) and alumina + carbide mixed ceramics (“black” ceramics).
“White” ceramics are mainly composed of alumina (Al2O3) and alloying additives (MgO, ZrO2, etc.).
Its main advantage is that it has high hardness and thermal hardness, and its main disadvantage is that its strength is relatively low.
“Black” ceramics are composed of Al2O3, TiC, ZrO2 and other refractory metal carbides and additives.
It can process cast iron, Quenched and tempered steel, carburized steel and quenched and hardened steel (30~50HRC).
Silicon nitride based ceramics are based on silicon nitride and alloyed with yttrium oxide, zirconium oxide and hafnium oxide.
They are usually produced by hot pressing.
Ceramic tool materials can be roughly divided into three categories: alumina based ceramics as shown in Fig. 2-30, silicon nitride based ceramics as shown in Fig. 2-31, and composite silicon nitride alumina based ceramics.
Their properties are shown in table 2-32.
Fig. 2-30 alumina ceramics
Fig. 2-31 silicon nitride ceramics
Table 2-32 characteristics of composite silicon nitride alumina ceramics
| Hardness /HRA | Bending strength /GPa | Failure toughness value () | Thermal conductivity / (w/m-K) | Purpose | |
| Al2O3 series | 92.0~93.0 | 0.3~0.4 | 3-5 | 17 | Cast iron turning finishing |
| Si3N4 series | 91.5~92.5 | 1.0~1.3 | 7~9 | 75 | Intermittent milling of cast iron |
1. Alumina ceramic cutting tools
It is a ceramic material with Al2O3 as the main body, including pure alumina ceramics, composite ceramics with various carbides, oxides, nitrides and borides added to alumina, and composite ceramics with compounds and bonding metals added to alumina at the same time.
Pure alumina ceramic, which is the earliest developed ceramic tool material, is mostly white.
It is made of aluminum oxide with purity over 99.9% as the main component and a small amount (0.1%~0.5%) of MgO or other glass oxides such as Cr2O3, TiO2, etc. mixed into powder by cold pressing or hot pressing.
The grain size of alumina has a great impact on the mechanical properties of ceramic tools. Now the grain size has developed from a few microns to 1~2μm or even less than 1μm, so the strength and hardness have been greatly improved.
The purity of alumina has a great influence on the strength of ceramics.
The higher the purity, the greater the strength.
Alumina ceramic tools are most suitable for high-speed cutting of hard and brittle metal materials, such as chilled cast iron or hardened steel;
It is used for cutting large mechanical parts and high-precision parts.
The effect of alumina ceramic cutting tools for short, small parts, steel parts and Mg, Al, Ti, be and other simple materials and their alloy materials is poor, which is easy to cause defects such as diffusion wear, peeling and edge collapse.
The brief introduction of several alumina based ceramic tools is as follows.
(1) Al2O3 TiC ceramic tool.
This is the most attractive oxide carbide hybrid ceramic at present.
Adding a certain amount of TiC (generally 15%~30%) into Al2O3 can improve the bending strength and fracture toughness of ceramics, improve the mechanical impact resistance, and improve the hardness.
The applicable cutting speeds of alumina based ceramic tools are shown in table 2-33.
Table 2-33 applicable cutting speeds of alumina based ceramic tools
|
Use classification |
Recommended cutting speed / (m/min) |
Ceramic cutting tool |
Representative machined parts |
||
|
Baise (pure alumina) |
Black (composite ceramic) |
||||
|
turning |
High speed finishing of general cast iron. |
350~800 |
 |
|
Brake drum, flywheel |
|
Semi finish and rough machining of general cast iron. |
250~400 |
|
|
Cylinder liner, pressure plate |
|
|
High speed finishing of general steel. |
200~400 |
|
|
Shaft and transmission parts |
|
|
Medium and high speed machining of high hardness materials. |
50~200 |
— |
|
Roll, die steel, hardened steel |
|
|
High speed finish milling of general cast iron. |
300-500 |
— |
|
Cylinder block and bed |
|
|
High speed finish milling of general steel. |
200-400 |
— |
|
Steel base |
|
Note: □ – not recommended: ○ – available
(2) Al2O3-ZrO2 ceramic tool.
When ZrO2 was added to Al2O3, the dispersed phase of ZrO2 played a positive role in improving the toughness of the ceramic matrix.
The fracture toughness can be improved by adding about 1% ZrO2 to Al2O3.
The test shows that the wear resistance of Al2O3-ZrO2 ceramics is the highest when the content of ZrO2 is 20%, which means that the strength and fracture toughness of ceramics are also the largest, which further indicates that improving the strength characteristics of ceramics can improve its wear resistance.
(3) Al2O3 SiC whisker reinforced ceramic tool
The reinforced ceramic with 15%~40%sic whiskers added to Al2O3 matrix is a new developed composite tool material in recent years.
SiC whisker is a single crystal with a certain fiber structure.
Its diameter is less than 1μm and its length is 10~300 μ m.
The tensile strength is 7000mpa, the tensile elastic modulus is more than 700GPa, and the thermal stability is 1760 ℃.
The distribution of SiC whiskers in this reinforced ceramic is very regular, and they are firmly integrated with Al2O3 crystal.
SiC whiskers can enhance the strength and toughness of ceramics, increase the thermal conductivity by 40% and reduce the cutting temperature.
The comparison of physical and mechanical properties between SiC whisker reinforced ceramics and other ceramics is shown in table 2-34.
Table 2-34 comparison of physical and mechanical properties between SiC whisker reinforced ceramics and other ceramics
| Physical and mechanical properties ceramic material types | Al2O3-ZrO2 ceramic AC5 | Al2O3-TiC mixed ceramic MC2 | Silicon nitride ceramic NC1 | SiC whisker reinforced ceramic MC3 |
| Density / (g/cm3) | 4.0 | 4.25 | 3.3 | 3.75 |
| Average grain size /um | 1.5 | 1.0 | —— | 1.5 |
| Hardness /GPa | 17.0 | 20.0 | 15.0 | 16.5 |
| Compressive strength /MPa | 4000 | 4300 | 2500 | 4100 |
| Bending strength /MPa | 500 | 600 | 800 | 650 |
| Breaking strength /MPa | 190 | 200 | 250 | 230 |
| Thermal conductivity / (W/mk) | 23 | 28 | 26 | 30 |
2. Silicon nitride ceramic tools
It includes silicon nitride (Si3N4) ceramics and composite silicon nitride ceramics based on silicon nitride and added with other carbides.
This kind of ceramic cutting tools mainly use MgO as additive to hot press ceramics.
Because Si3N4 ceramics are covalently bonded and have long columnar grains, it has high hardness, strength and fracture toughness.
Its hardness is 91 ~ 93HRA, bending strength is 0.7 ~ 0.85GPa, heat resistance can reach 1300 ~ 1400 ℃, and has good oxidation resistance.
At the same time, it has a small coefficient of thermal expansion (3 × 10-6/ ℃), so it has good mechanical shock resistance and thermal shock resistance.
Sin tools are suitable for rough and finish machining of cast iron and high-temperature alloy, high-speed cutting and heavy cutting.
Their cutting life is several times to ten times higher than that of cemented carbide tools.
In addition, Si3N4 ceramics have self-lubricating property, small friction coefficient, strong anti bonding ability, not easy to produce chip nodules, and the cutting edge can be sharpened.
It can produce good surface quality, especially suitable for turning workpiece materials that are easy to form chip nodules, such as cast silicon aluminum alloy, and cast iron cylinder block of automobile engine.
3. Composite silicon nitride alumina ceramics
Due to the large difference of thermal expansion coefficients between titanium carbide and silicon nitride, the sharp increase of tool tip temperature during high-speed machining will produce large thermal stress and reduce the service life of the tool.
For this reason, many countries have developed （α′＋β′）-Sialon-composite ceramic cutting tools (see fig. 2-32).
Fig. 2-32 Sialon ceramic tools
Sialon ceramic cutting tools are materials obtained by hot pressing and sintering the mixture of aluminum nitride, aluminum oxide and silicon nitride at high temperature.
The addition of Y2O3 can densify the microstructure.
Sialon ceramic tools have high strength and toughness, and are good tool materials for high-speed rough machining of cast iron and nickel base alloy.
However, the thermal expansion coefficient of Sialon ceramic tool is low, and the dissolution wear rate with steel is much higher than that of Al2O3 based ceramic tool.
The diffusion of Fe into the tool will cause very serious crescent wear, so it is not suitable for processing steel.
The physical and mechanical properties of the above alumina, silicon nitride and composite silicon nitride alumina ceramics can be summarized as shown in table 2-35.
Table 2-35 physical and mechanical properties of alumina, silicon nitride and composite silicon nitride alumina ceramics
|
Performance |
Density/（g/cm3） |
Hardness/HRA |
Bending strength/（N/mm2） |
Fracture toughness/（MN/m3/2） |
|
|
Alumina system |
Al2O3（pure） |
3.9-4.0 |
93~94.5 |
400-700 |
22-3.2 |
|
Al2O3-carbide |
4.2~6.6 |
93.5~95.3 |
700~1000 |
3.1~4.4 |
|
|
Al2O3-ZrO2 |
4.2~4.3 |
91~92 |
700 |
4.5~5.0 |
|
|
Compound Al2O3-Zr |
4.3 |
93.2 |
800 |
|
|
|
Al2O3，-TiB2 |
4.1 |
94 |
750 |
|
|
|
Silicon nitride system |
Si3N4 |
32~3.4 |
91-92 |
700~900 |
4.2-5.2 |
|
Combination Si3N4 |
3.14~3.19 |
93.5~93.6 |
740~950 |
4.3~7.21 |
|
|
Silicon nitride alumina system |
3.2-3.3 |
91-94 |
800~1000 |
3.8~4.0 |
|