Milling of hard metals
 One of the major components of hard metals is tungsten carbide (WC). Tungsten carbide is one of the hardest ceramic material known nowadays (its HRC is about 90) and its hardness can be compared with the hardness of diamond. Besides, tungsten carbide is notable for its very high melting point and wear resistance. Its density is about 15,8 g/cm3, melting point is 2870°C, Rockwell hardness ranges from 87 to 92, and elasticity modulus is 608100 MPa.
Tungsten carbide is extensively used in some technological fields for production of tools having high hardness and corrosion stability. This substance is used in manufacturing of diverse cutting tools such as various gravers, abrasive discs, screw drills, milling tools, drill bits, etc.
The comminution capabilities of planetary mills are characterized by the results of milling of tungsten carbide (the material which is very difficult to mill)
Hard metals contain tungsten carbide and cobalt as a binding component. Cobalt content can be for instance 6, 8, 12 % wt. Technological progress in hard metals production is connected with possibility to obtain extremely fine tungsten carbide powder with average particle size less than 500 nm. . At present, finest commercial tungsten carbide powders are characterized by specific surface area in the range from 0.95 to 1.9 m2/g. This corresponds to the average particle size of 200-400 nm. Conventional methods for production of such powders are time consuming and resource-intensive.
During “Activation” project we undertook grinding of WC to compare the capabilities of planetary mills available from different manufacturers. The initial powder represented a mixture of fine (2,25 m2/g) and submicron (0,85 m2/g) tungsten carbide powders.
Millings were held in MPP-1 planetary mill in the TTD company at an acceleration of 28 g and in a Retsch mill (Germany) at the Institute of Non-Ferrous Metals in Poland at an acceleration of 26.8 g. The mills had different geometry of the jars and different ratios of rotation speed of jars around the central axis and around their own axes. Results obtained by our partners from INM, Poland, are represented in the following Table:
Milling time for tungsten carbide in the laboratory MPP-1 (TTD) and Retsch planetary mills. Specified periods are required to reach comparable specific surface area and equivalent particle size (calculated according to the BET method). Data of INM, Poland [1].
| Planetary mill |
Specific surface area, m2/g
|
Milling time, min.
|
Equivalent particle size, nm
|
| Retsch |
180 |
2,1 |
180 |
| МПП-1 |
10 |
2,1 |
180 |
| Retsch |
900 |
3,1 |
120 |
| МПП-1 |
30 |
3,3 |
115 |
more than 3 m2/g) and average particle size of 120-180 nm 18-30 times faster than analogous laboratory equipment available at the market.
Crushing and milling of hard metal scrap metal (hard metal inserts available from our partner “PyroGenesis” company, Greece) was performed in the frame of the “Activation” project.
Powder containing 40% of recycled material WC/Co was used by Pyrogenesis to apply a coating to an engine part by thermal spraying. The recycled WC/Co material represented metal scrap crushed in a hammer-type crusher and milled in a planetary mill in TTD company.
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An engine part with a coating produced /span>
with the use of recycled
hard metal (“PyroGenesis”, Greece).
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Literature
1. Missol V., Kurylyak V.; Report on “Activation” project of FP7, Institute of Non-ferrous Metals, Poland, 2006..
At the moment, starting from initial tungsten carbide powder with specific surface area 0.8-0.9 m2/g we can obtain powders with specific surface area of 4 m2/g, by milling in the MPP-1 planetary mill for 16 minutes. The latter specific area corresponds to the average particle size of 95 nm according to calculation by the BET method.
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