Smith G.T. Cutting Tool Technology: Industrial Handbook

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Figure 10. Multi-coatings applied to

cemented carbides and cermets, together

with tool geometries of cermet cutting

inserts. [Courtesy of Sandvik Coromant]

0 Chapter 1

Moreover, ceramic tools at this juncture, were only re-

ally employed for turning operations and in particular,

n ‘stable machining’ , where interrupted/intermittent

cutting operations did not occur.

With the recent advances in powerful and very

rigid CNC machine tools, this has opened-up the pos-

sibility of utilising ceramic tooling, either in a purely

sintered monolithic tooling insert, or more recently as

a multi-coated variant – more will be said on this topic

shortly. Returning to the monolithic ceramic cutting

tool materials, they have normally been available in

three distinct grades, which will now be mentioned.

ese cutting inserts consist of:

•

‘Pure’ ceramic – this is the traditional tooling in-

sert material, consisting of aluminium oxide. e

alumina is white in colour and is produced by cold

pressing powder in the desired insert geometry

dies

, with subsequent sintering, the fused alumina

particulates are sintered together, thereby signi-

cantly decreasing porosity. ese ceramics, have

een known in the past as ‘pure oxide’ , or ‘cold-

pressed’ ceramics. e major disadvantage of such

ceramics is their low thermal conductivity, making

them highly susceptible to thermal shock (i.e. the

hot and cold thermal cycles that can occur when in-

terrupted cutting takes place). ese thermal shock

8 ese consolidated cutting inserts produced in compound,

or ‘oating’ die sets from the admixed powders, are termed

‘green compacts’ and are friable, that is having very limited

mechanical strength and must be gently handled, prior to sin-

tering – thereaer the desired mechanical strength occurs.

Figure 11. A typical ‘super-glide’ coating of molybdenum disulde (MoS

) applied to a hard-coating on a tool’s sub-

strate – weak bonds between crystal layers allow easy movement of the planes. [Courtesy of Guhring]

Cutting Tool Materials 1

problems are exacerbated by short machining cycle

times, variable depths of cut and higher machining

speeds. ‘Pure’ alumina inserts can be improved by

additions of zirconia (Zr) to greatly increase the

toughness somewhat, but such cutting tool material,

has been widely superseded today, by ‘mixed grade’

ceramics, or cermets – to shortly be discussed,

•

Black, or mixed ceramics – tend to minimise the

eects of thermal shock on the cutting insert, by

having additions of titanium carbide added to the

alumina, this causes the insert to turn black. A

problem with these earlier ‘black ceramics’ was that

they did not sinter as readily as the former ‘pure’

ceramic inserts. erefore they usually had an ad-

ditional ‘hot pressing’ operation to achieve the de-

sired densities, which tended to limit the geomet-

ric shapes for such inserts. A later development

of these cutting tool materials was termed ‘mixed

eramics’ , these had additions of titanium nitride,

which improved thermal shock still further, with

the sintered inserts tending to be brown, or choco-

late in colour – the term ‘black’ for these later in-

serts, became irrelevant. ese ‘mixed ceramics’

had good hot hardness, enabling them to machine

harder steel components, or chilled cast irons and

at greater temperatures, where the combinations of

higher cutting forces and greater chip/tool interface

temperatures would have induced cutting insert

plastic deformation in their previous counterparts.

•

Cermets – the original cermet was developed by

Lucas under the trade name ‘Sialon’ which was a

silicon nitride based material, having a very low co-

ecient of thermal expansion. is low expansion

rate when in-cut, tends to reduce the stresses be-

tween the hotter and cooler isothermal zones of the

insert, giving very high thermal shock resistance.

Originally, it was dicult to sinter these inserts to

full density, although by substituting some of the

silicon and nitrogen with aluminium and oxygen,

the new material ‘Sialon’

it had the added benets

of: ease of pressing and sintering, with equally as

good thermal shock resistance. A notable later el-

emental addition was that of yttria (Y

), which

aided sintering performance and during sintering.

e silica (SiO

) on the surface of the silicon nitride

9 Sialon, this name was coined for the insert, as it represented

the chemical symbols for the constituent elements: Si, Al, O

and N.

particles will react with the yttria forming a liquid.

is chemical reaction forms a ‘glass’ on cooling,

so depending upon the relative proportions of the

reactants, the resultant ‘Sialon’ formed may have

either of the following atomic arrangements: beta

silicon nitride, or alpha silicon nitride. It is possible

to produce a very complex cutting insert material,

having both ‘beta-’ and ‘alpha-Sialons’ in atten-

dance.

A typical ‘beta-Sialon’ might be composed of:

6.Z

8.Z

Where: ‘Z’ represents the degree of substitution of sili-

con and nitrogen by aluminium and oxygen.

Conversely, an ‘alpha-Sialon’ can consist of:

Mx(Si, Al)

(O,N)

Where: ‘M’ is the metal atom, such as yttrium.

All this sounds quite confusing, but basically the ‘Si-

alon’ microstructure consists of a crystalline nitride

phase, held in a glassy, or partially crystallised matrix.



ese crystalline grains can be either ‘beta-Sialon’ , or

a mixture of ‘alpha’ and ‘beta’ , but generally it can be

said that as the ‘alpha’ phase increases, the hardness

of the ‘Sialon’ becomes greater. ese chemical and

mechanical changes, result in a higher ‘hot-hardness’

for the cutting insert when in-cut. An additional and

probably greater benet is gained by the signicant

improvement in insert toughness, which can rival that

of cemented carbide of equal hardness. One limitation

in the past to such cermets, was that they could not

satisfactorily machine steels, owing to their poor per-

formance in resisting solution wear. However, these

earlier cermets when machining nickel-based alloys,

or cast irons they performed very well, but even the

‘mixed’ ceramics based on alumina, having 25% addi-

tions of carbide (i.e. ‘whisker-reinforcement’) within

the insert’s substrate are a direct competitor to such

cermets.

 Chapter 1

Today, with the more complex material technology

cermet

insert grades (Fig. 10b), they can easily ma-

chine ferrous-based workpieces at high cutting speeds,

tool lives and excellent surface nishes. Complex pow-

der particulates are utilised for the current turning

inserts, such powders may have a large core of TiCN,

surrounded by TiN – for superior hardness, adjacent

particulates having a small core of niobium (Nb), sur-

rounded by tungsten (W) and titanium (Ti) – for supe-

rior toughness. e sintered cutting insert product has

a very complex substrate, which is further enhanced

by subsequent multi-coating.

Typical turning data for a high-performance steel

product that can be rough-to-nish turned using the

same insert on a 34CrMo4 grade workpiece has been

shown to be:

Cutting data: cutting speed (V

) 140 m min

–1

, feed

(f) 0.2 mm rev

–1

, depth of cut (D

) 1.0 mm and with

ood coolant.

In interrupted cutting trials with the cutting data

mentioned on this workpiece material (i.e. having

4 e

qually-spaced splines around its periphery), the

cermet insert’s edge withstood over 7,000 impacts per

edge. is can be considered as a ‘true’ testament to

the hardness, shock resistance and life of the latest

such cermet tooling materials.

1.. Cermets – Coated

To enable a wider range of machining applications

while improving still further the original cermet

grades available, tool coatings were introduced and

with sophisticated high-technology cutting insert ge-

ometries (see Fig. 10b and c). e latest multi-coatings

for indexable cutting inserts have individual ‘nano-

coatings’

and are extremely hard, approaching 4000

10 Cermet is derived from the two words ceramic and metallic

and, the clear distinction between this and other cutting tool

materials, such as cemented carbide and ceramic tooling has

become somewhat ‘blurred’ , with one tooling manufacturer

claiming it was developed in 1929, which is ‘at odds’ with the

patented ‘Sialon’ product developed by the Lucas company

– previously discussed.

11 O

ne nanometre is equal to 10

–9

m, or one millionth of a mm.

and the surfaces of such coatings tend to be very

smooth and having a total thickness of less than 3 µm

thick – allowing around 2,000 durable layers. One of

the key factors in successfully applying these com-

plex metallurgy multiple coatings, has been the de-

velopment of ‘super-lattice technologies’ at medium

temperatures, which do not compromise the thermal

properties of the substrate.

e unit cost of the cermet substrate tends to be

lower than its equivalent cemented carbide grade, this

accounts for the fact that at present, in turning opera-

tions in Japan 35% of all the inserts utilised for a range

machining steel grades tend to be cermets, whereas, in

Europe less than 5% of cermets are employed. Cermets

are considerably more wear and heat resistant than

tungsten carbide-based cutting materials. By way of il-

lustration for the reason for edge failure of tungsten

carbide inserts, is the heat generated at the tool/chip

interface – at high cutting speeds. For example, if one

considers the pre-sintering temperature for a typical

tungsten carbide material it is in the region of 1,150°C

and, if turning a: 0.48% C, 0.8% Mn medium carbon

steel workpiece at 200 m min

–1

, this equates to the

highest isothermal edge temperature of 1,000°C – cre-

ating the potential for localised thermal soening and

edge failure. While an equivalent multi-coated Cermet,

can readily turn alloy carbon steels at a depth of cut

) of up to 3 mm, with cutting speeds of between

200 to 300 m min

–1

, with feedrates ranging from 0.1 to

0.3 mm rev

–1

. Moreover, as less ank wear takes place,

the dimensional size of subsequent components in a

atch will not signicantly ‘statistically-dri’ , produc-

ing much less tolerance variation (i.e. reliable size-for-

size consistency) in the completed turned parts. is

increased multi-coated cermet tool life, allows for an

excellent surface nish and dimensional consistency,

whether cut wet, or dry.

In general, the multi-coated cermet cutting tool

materials, can be consolidated (i.e. pressed) in com-

pound die-sets with very complex tool geometries and

have integrated chip-breakers present – as illustrated

in Fig. 10c. Such inserts, have seen a slow take-up in

Europe and oer considerable economical advantages

when in particular, turning hardened steel parts.

12 A comparison of the hardness of dierent popular coatings

may be applicable here, as TiCN coating has a hardness of

around 2,700 HV and TiAlN coating has a hardness of ap-

proximately 3,200 HV.

Cutting Tool Materials 

Figure 12. Ultra-hard cutting tool materials – cubic boron nitride (CBN). [Courtesy of DeBeers – element 6].

 Chapter 1

1..10 Cubic Boron Nitride (CBN) and

Poly-crystalline Diamond (PCD)

Cubic Boron Nitride (CBN)/Synthetic Diamond –

Ex traction and Sintering

Cubic boron nitride (CBN) is one of the hardest ma-

terials available and for machining operations it can

be considered as a ultra-hard cutting tool, it was rst

synthesised in the late 1950’s. In many ways, CBN and

natural diamond are very similar materials, as they

both share the same atomic cubic crystallographic

structure (see Fig. 12a and b). Both materials exhibit

a high thermal conductivity, although they have pro-

foundly dierent properties. For example, diamond is

prone to graphitisation and will readily oxidise in air,

reacting to ferrous workpieces at high temperatures,

conversely, CBN is stable to higher temperatures and

can eortlessly machine ferrous components. CBN

can therefore machine ferrous materials, such as: tool

steels, hard white irons, surface hardened steels, grey

cast irons, (some) austempered ductile irons and hard-

facing alloys. Normally, CBN tools should be used on

workpiece materials with hardnesses greater than 48

, because if workpieces are less hard than this, the

cutting edge will result in excessive tool wear.

In graphite, the carbon atoms are arranged in a hex-

agonal layered structure (Fig. 12ai) and, by the appli-

cation of very high temperatures and pressures

, it can

be transformed into the cubic structure of diamond

(Fig. 12aii) – this transformation does not occur easily.

As boron and nitrogen are two elements on either side

of carbon in the Periodic Table, it is possible to form a

compound of boron nitride, that exhibit’s a hexagonal

boron nitride (HBN) as depicted in Fig. 12bi, having

the characteristics of being both slippery and friable.

HBN can be transformed in a similar fashion to that

of CBN (Fig. 12 bii). In practice, to facilitate the rate

of transformation in the reaction chamber, additions

of solvents/catalysts are utilised for synthesis at more

easily obtainable levels: pressures of approximately

0 GPa and temperatures 1,500°C. As this transforma-

tion proceeds in the reaction volume of a high pres-

sure system, the CBN/synthetic diamond grows, being

embedded in a portion of reaction mass and extracted

aerward from this special-purpose press. By dissolv-

ing away the unwanted matrix, the CBN/synthetic dia-

mond can be liberated and recovered for subsequent

processing. Grain sizes vary from large dimensions

f approximately 8 µm, down to sub-micron sizes, for

ne-grain tooling.

Once the synthesised CBN/diamond has been ex-

tracted, it is possible to sinter together these crystals

of CBN, or diamond, with the aid of a ceramic binder,

to produce polycrystalline masses. Commercially, in

13 To transform hexagonal graphite into the cubic diamond

structure, requires exceedingly high temperatures > 2000°C

and applied pressures > 60 GPa, to enable the conversion to

take place.

Table 1. Cutting tool materials – with some important physical properties

Cutting tool material:

Black ceramic

(Al

+ TiC)

Cemented carbide

(ISO K10 grade)

CBN

(DBC50)

CBN

(DBC80)

Physical properties:

Density [g cm

–3

] 4.28 14.7 4.28 3.52

Knoop hardness [GPa] 1

7 17 27.5 30

Young’s modulus [GPa] 3

90 593 587 649

Fracture toughness [MPam

] 2.94 10.48 3.7 5.90

Thermal expansion [10

–6

–1

] 7.8 5.4 4.7 4.6

Thermal conductivity [Wm

–1

] 9.0 100 44 85

Cutting Tool Materials 

order to speed-up the rate of sintering, additions of a

solvent/catalyst are utilised (i.e. normally metals, or

metal nitrides), but during sintering the whole mass

must be held in the ‘cubic region’ of the respective pres-

sure/phase diagram – to prevent these hard crystals

reverting back to their original so hexagonal form.

By sintering these hard particles together, it is possible

to form a conglomerate of CBN/diamond, in which

randomly orientated crystals are combined to produce

a large isotropic

mass. A very wide range of poly-

crystalline products can be produced, utilising either

CBN, or synthetic diamond as a base. For example, by

changing the: grain size (see Figs. 12 c and d), solvent/

catalyst employed, degree of sintering and particle size

distribution and, the presence/absence of inert llers,

this will have a profound eect on the mechanical and

physical properties of the nal product – Table 1 lists

14 Isotropic materials can be considered to have the same prop-

erties in dierent directions.

the physical properties of various comparable cutting

tool materials.

In order to produce the required tool geometries,

both the polycrystalline layer of CBN and polycrystal-

line diamond (PCD) are bonded to a thick tungsten

carbide backing layer, then cutting inserts are wire-

cut out of this large blank – obtaining the maximum

number of insert shapes per blank (see Fig. 13a). ese

CBN/PCD inserts are either full-size, or smaller tips

that are then brazed onto suitably-shaped blanks, to t

the desired tool holder (as illustrated in Fig. 13b).

Both CBN and PCD cutting tools can successfully

machine: super-alloys (ie with low iron content), grey

cast iron and non-ferrous metals, but show distinct

dierences when other workpiece materials are to be

productively machined – as depicted in Fig. 14.

Polycrystalline diamond cutting tools are not

utilised for machining ferrous workpieces, this is be-

cause when machining under the high temperatures

and sustained pressures that occur during cutting, the

diamond has a tendency to revert back to graphite,

aer only a few seconds in-cut. is reversion, does

Figure 13. Cutting tool materials: Cubic Boron Nitride (CBN) and Polycrystalline Diamond (PCD) . [Courtesy of DeBeers – ele-

ment 6]

 Chapter 1

Figure 14. A diagram illustrating how Cubin Boron Nitride

(CBN) and Polycrystalline Diamond (PCD) applications are

grouped, by workpiece materials. Their eectiveness when ei-

. ther machining highly abrasive components, or high tempera-

tures in the cutting vicinity

Cutting Tool Materials 

Figure 15. Turning operations with Cubic Boron Nitride (CBN) and Polycrystalline Diamond (PCD).

[Courtesy of DeBeers – element 6]

 Chapter 1

not take place when machining many non- ferrous and

non-metallic workpiece materials. Although CBN is

synthesised in a similar fashion to that of PCD cutting

tool products, it is not as hard as PCD and is therefore

less reactive with ferrous metals, as long as the cut-

ting temperature is less than 1,000°C, it will not revert

to its soer hexagonal form and oxidise in air. is

means that CBN can machine many ferrous parts and

cast iron grades. e complementary nature of both

CBN and PCD is clearly depicted in Fig. 14, where the

‘cross-over’ between these ultra-hard cutting tool ma-

terials is shown.

In both CBN and PCD machining applications,

an excellent machined surface nish can be obtained

(see Figs. 15a and b). In the case of many PCD opera-

tions, the cutting tool must not only machine widely

diering materials that are situated adjacent to one

another in many passes over such a diverse material

workpiece, but produce an excellent machined surface

nish, which really ‘challenges’ the tool. Tool life can

be extended greatly by utilising either CBN, or PCD

tooling, oen tool lives can be increased by 50 to 200

times that of the previous cemented carbide alterna-

tives. is boost in output, makes their additional

purchase price irrelevant, when considered against the

massive productive gains that are to be made by their

adoption.

Today, both CBN and PCD can oen be found

as either thin-coated layers on tooling (see Fig. 3 for

their relative tool insert hardnesses/toughnesses), or

as a ‘sandwich’ between metallic backing layers. ese

‘sandwiched’ tool edges, permit brazing on both sides

of the hardened product, which are then accurately

positioned and held onto a tungsten carbide shank,

making them an ideal alternative for many micro-

drilling operations. Such compound drilling edge

technology, gives considerably improved edge reten-

tion and resistance to any abrasive particles present in

the workpiece and its severely work-hardened swarf,

typically found with the latest metal matrix composites

(MMC’s). Such ultra-hard tooling, can be readily used

on high-silicon aluminium alloys used in the automo-

tive industries, while not discounting the wide range

of workpiece composites employed by the aerospace

industries and the resin-based components utilised in

the furniture industry.

1..11 Natural Diamond

Monolithic, or single-crystal diamond (SCD), is the

hardest material available today. If such natural dia-

mond is used correctly in a very rigid machine-tool-

workpiece setup for materials that require the best

possible surface nish, then there is simply no alterna-

tive. By way of illustration of this fact, if production

turning high-silicon content aluminium pistons with

polycrystalline diamond (PCD) tooling, the best sur-

face nish that can be obtained will be in the region

f 0.4 µm, conversely using an SCD tool this will give

a surface nish of better than 0.15 µm. If one really

wants the ultimate surface nish currently obtainable

y machining – in the ‘nano-range’ , then a monolithic

diamond tool, mounted in a special-purpose diamond

turning lathe is the only manner in achieving such su-

perb ‘mirror-nish’ surfaces. SCD tool edges are pro-

duced as either razor sharp edges, or are made with

a perfect radius being chip-free, imparting machined

‘mirror-nishes’ of just a few angströms (i.e. 10

–10

m).

e optical industries in particular nd that the latest

blemish-free ultra-sharp cutting edges of SCD, means

that diamond (paste) polishing aer machining has

been virtually, if not completely eliminated, this fact in

particular being a very big production cost for the -

nal manufacture of large monolithic astronomical mir-

rors. A cautionary note, is that to use SCD tooling for

anything other than as a nishing cut is totally uneco-

nomic, as these precision components to be machined,

should have been roughly congured to the desired

shape, prior to diamond machining. erefore, SCD

tools should be employed for exceedingly light nish

cuts of no deeper than 0.0008 m.

Natural diamond is a truly remarkable material,

that exhibit’s a diverse range of mechanical and physi-

cal properties. For example diamond has the highest

known: bulk hardness, thermal conductivity, while

having a very low coecient of friction and will not

corrode, these properties make it an ideal tool material

for the highest precision and accuracy machined com-

ponents. Of these properties, hardness is probably the

most important characteristic in machining operations

and, when measured by the Knoop indentor

. By way

of comparison of ultra-hard cutting tool materials, the

following two examples may prove informative:

15 Knoop indentors produce a wedge-shaped indentation in the

form of a parallelogram, with one diagonal seven times lon-

ger than the adjacent one. e Knoop test method is generally

considered the optimum technique, for crystalline solids –

having crystallographic directionality (i.e. anisotropy).

Cutting Tool Materials 