Sintered compacts (CBN and Diamond)
The advancement of ultra high pressure technology has allowed for the manufacture of an artificial composition of diamond and CBN (Cubic Boron Nitride). Both the diamond and CBN tool grades have a similar crystal structure, and as a result both posses excellent properties as tool materials.
CBN is not a naturally occurring material. CBN has hardness properties second only to diamond and is manufactured by transforming cubic system BN into CBN using an alkali metal or alkali metal nitride as a catalyst. For tool materials an abrasive type grain or sintered CBN is used. Sintered compacts are a tool grade material that consist of microcrystal synthetic diamond or CBN. The tool grade is formed by the addition of a number of binder metals under high pressure.
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The manufacturing process of sintered tools can be broken into two phases. The first process is to produce the actual sintered compact, the next process is to make a finished tool from it.
Diamond is the hardest (wear resistant) material known on earth. It has a high Young’s modulus* and a low thermal expansion coefficient (thermal shock resistance). The overall mechanical properties of a sintered diamond tool grade depend heavily on the diamond content and the particle size during sintering.
CBN has properties similar to that of diamond. In addition, unlike diamond, CBN has the added advantages of low affinity with ferrous materials (Fe, Ni, Co), and that when under high temperature conditions it maintains its high heat hardness properties.
Manufacturing process of sintered compacts
Sintered compacts are manufactured by sintering diamond or CBN powder with a binder powder. The powder mix is placed in a carbide capsule and is subject to high heat and pressure.
The diamond content of sintered diamond currently available for cutting tools is approximately 85vol%~95vol%. These tool grades also contain Co as a binder material. Sintered CBN tool grades also contain Co as a binding material (CBN-metal), there are however CBN tool grades that are combined with TiC, TiN, Al2O3 and AlN (CBN-ceramic).
Manufacturing process of tools
Once the sintered material is manufactured, it is then cut into segments. These segments are then brazed onto a cemented carbide base to make the finished tools. The sintered segments can also be brazed onto a steel tool body and in both cases the cutting edge is finish ground to produce an effective cutting edge.
Use and machining examples
Sintered diamond is used for high-speed finishing operations of soft non-ferrous materials such as aluminium and copper alloys, and for non-metallic materials such as plastics and carbon.
One of the main features of this tool grade material is the high wear resistance property when machining non-ferrous and non-metallic materials. An example of which is shown in the graph below. This example shows a comparison between the flank wear when machining an aluminium alloy AC8A-T6 (11%13% Si content) when using cemented carbide and a sintered diamond.
Tool: SPGN 120304
Cutting conditions: vc=300m / min, ap=0.2mm, f=0.1mm / rev
Sintered diamond tools also show superior wear resistance when machining ceramics, cemented carbide, semi-sintered ceramics, semi-sintered cemented carbide and non-metallic materials such as reinforced plastic and building material boards.
However, when machining materials such as titanium alloys, molybdenum etc, with carbides particles (such as TiC, MoC) a chemical reaction takes place easily and can accelerate tool wear. Additionally if sintered diamonds are used to machine ferrous materials then the carbon within the diamond chemically reacts iron in the workpiece, this too leads to quickened tool wear.
Sintered CBN does not react easily with Fe and as such it can be effectively used for the finish machining of hardened steels, cast iron and ferrous sintered alloys. One other advantage is that when machining hardened steels, the surface finish attained can make the grinding process redundant and therefore save time.
Microstructure of sintered CBN.
Microstructure of sintered diamond.