Development And Application Of Advanced Tool Materials, coated carbide, cermets, coronite, high performance ceramics

 Development And
Application Of Advanced Tool  Materials: 
  •    Coated
    carbides 
The properties and
performance of carbide tools could be substantially improved by
1.       Refining
microstructure 
2.      Manufacturing by
casting – expensive and uncommon 
3.      Surface coating –
made remarkable contribution. 
 Thin but hard
coating of single or multilayers of more stable and heat and wear resistive
materials like TiC, TiCN, TiOCN, TiN, Al2O3 etc on the tough carbide inserts
(substrate) by processes like chemical Vapour Deposition (CVD), Physical Vapour
Deposition (PVD) etc at controlled pressure and temperature enhanced material
removal rate (MRR) and overall machining economy remarkably enabling,
  • reduction of cutting forces and
    power consumption 
  •  increase in tool life (by
    200 to 500%) for same VC or increase in VC (by 50 to 150%) for same tool
    life 
  •  improvement in product
    quality 
  •  effective and efficient
    machining of wide range of work materials 
  •  pollution control by less
    or no use of cutting fluid through 
Ø reduction of abrasion,
adhesion and diffusion wear 
Ø  reduction of
friction and BUE formation 
Ø  heat resistance
and reduction of thermal cracking and plastic deformation.
The cutting velocity
range in machining mild steel could be enhanced from 120 ~ 150 m/min to 300 ~
350 m/min by properly coating the suitable carbide inserts. About 50% of the
carbide tools being used at present are coated carbides which are obviously to
some extent costlier than the uncoated tools. Different varieties of coated
tools are available. The appropriate one is selected depending upon the type of
the cutting tool, work material and the desired productivity and product
quality. The properties and performances of coated inserts and tools are
getting further improved by;
  • Refining the microstructure of
    the coating
  • Multilayering (already upto 13
    layers within 12 ~ 16μm)             
                       
        
  •  Direct coating by TiN
    instead of TiC, if feasible 
  •  Using better coating
    materials. 
  • Cermets 
These sintered hard
inserts are made by combining ‘cer’ from ceramics like TiC, TiN orn ( or )TiCN
and ‘met’ from metal (binder) like Ni, Ni-Co, Fe etc. Since around 1980, the
modern cermets providing much better performance are being made by TiCN which
is consistently more wear resistant, less porous and easier to make. The
characteristic features of such cermets, in contrast to sintered tungsten
carbides, are :
  •   The grains are made of
    TiCN (in place of WC) and Ni or Ni-Co and Fe as binder (in place of Co)
  • Harder, more chemically stable
    and hence more wear resistant 
  • More brittle and less thermal
    shock resistant
  •  Wt% of binder metal
    varies from 10 to 20% 
  • Cutting edge sharpness is
    retained unlike in coated carbide inserts
  •  Can machine steels at
    higher cutting velocity than that used for tungsten carbide, even coated
    carbides in case of light cuts. 
Application wise, the
modern TiCN based cermets with bevelled or slightly rounded cutting edges are
suitable for finishing and semi-finishing of steels at higher speeds, stainless
steels but are not suitable for jerky interrupted machining and machining of
aluminium and similar materials. Research and development are still going on
for further improvement in the properties and performance of cermets. 
  •   Coronite 
It is already mentioned
earlier that the properties and performance of HSS tools could have been
sizeably improved by refinement of microstructure, powder metallurgical process
of making and surface coating. Recently a unique tool material, namely Coronite
has been developed for making the tools like small and medium size drills and
milling cutters etc. which were earlier essentially made of HSS. Coronite is
made basically by combining HSS for strength and toughness and tungsten
carbides for heat and wear resistance. Microfine TiCN particles are uniformly
dispersed into the matrix. Unlike a solid carbide, the coronite based tool is
made of three layers; 
  •  the central HSS or spring
    steel core 
  •  a layer of coronite of
    thickness around 15% of the tool diameter 
  •  a thin (2 to 5 μm) PVD
    coating of TiCN.               
                       
                       
                       
                       
                       
                       
                       
                       
                       
                         
                  Such tools are not
    only more productive but also provides better product quality. The
    coronite tools made by hot extrusion followed by PVD-coatring of TiN or
    TiCN outperformed HSS tools in respect of cutting forces, tool life and
    surface finish. 
  •   High Performance
    ceramics (HPC) 
Ceramic tools as such
are much superior to sintered carbides in respect of hot hardness, chemical
stability and resistance to heat and wear but lack in fracture toughness and
strength.
   
 Through last few years remarkable improvements in strength and toughness
and hence overall performance of ceramic tools could have been possible by
several means which include;
  •   Sinterability,
    microstructure, strength and toughness of Al2O3 ceramics were improved to
    some extent by adding TiO2 and MgO 
  •  Transformation toughening
    by adding appropriate amount of partially or fully stabilised zirconia in
    Al2O3 powder 
  •  Isostatic and hot
    isostatic pressing (HIP) – these are very effective but expensive route
  • Introducing nitride ceramic
    (Si3N4) with proper sintering technique – this material is very tough but
    prone to built-up-edge formation in machining steels 
  •  Developing SIALON –
    deriving beneficial effects of Al2O3 and Si3N4 
  •  Adding carbide like TiC
    (5 ~ 15%) in Al2O3 powder – to impart toughness and thermal
    conductivity 
  •  Reinforcing oxide or
    nitride ceramics by SiC whiskers, which enhanced strength, toughness and
    life of the tool and thus productivity spectacularly. But manufacture and
    use of this unique tool need specially careful handling 
  •  Toughening Al2O3 ceramic
    by adding suitable metal like silver which also impart thermal
    conductivity and self lubricating property; this novel and inexpensive
    tool is still in experimental stage.         
                       
                       
                       
                       
                       
                      The
    enhanced qualities of the unique high performance ceramic tools, specially
    the whisker and zirconia based types enabled them machine structural
    steels at speed even beyond 500 m/min and also intermittent cutting at
    reasonably high speeds, feeds and depth of cut. Such tools are also found
    to machine relatively harder and stronger steels quite effectively and
    economically. 



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