Cutting Tool Materials Part 3 pps

‘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by additions of zirconia Zr to greatly increase the toughness somewhat, but such cutting tool material, has been widely superseded today, by ‘mixed

Moreover, ceramic tools at this juncture, were only

re-ally employed for turning operations and in particular,

in ‘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

These cutting inserts consist of:

• ‘Pure’ ceramic – this is the traditional tooling

in-sert material, consisting of aluminium oxide The

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 signifi-cantly decreasing porosity These ceramics, have been known in the past as ‘pure oxide’ , or ‘cold-pressed’ ceramics The 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) These thermal shock

 These consolidated cutting inserts produced in compound,

or ‘floating’ 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 – thereafter the desired mechanical strength occurs.

Figure 11 A typical ‘super-glide’ coating of molybdenum disulfide (MoS 2) 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]

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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has been widely superseded today, by ‘mixed grade’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

effects 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 Therefore 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

ceramics’ , 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 choco-later

in-serts, became irrelevant These ‘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-efficient of thermal expansion This 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 difficult 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 benefits

of: ease of pressing and sintering, with equally as

good thermal shock resistance A notable later

el-emental addition was that of yttria (YO), which

aided sintering performance and during sintering

The silica (SiO) on the surface of the silicon nitride

 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 This 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:

Si.ZAlZOZN.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 These 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 These chemical and mechanical changes, result in a higher ‘hot-hardness’ for the cutting insert when in-cut An additional and probably greater benefit is gained by the significant 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

Today, with the more complex material technology

cermet0 insert grades (Fig 10b), they can easily

ma-chine ferrous-based workpieces at high cutting speeds,

tool lives and excellent surface finishes 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 The 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-finish turned using the

same insert on a 34CrMo4 grade workpiece has been

shown to be:

Cutting data: cutting speed (Vc) 140 m min–, feed

(f) 0.2 mm rev–, depth of cut (DOC) 1.0 mm and with

flood coolant

In interrupted cutting trials with the cutting data

mentioned on this workpiece material (i.e having

4 equally-spaced splines around its periphery), the

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

edge This 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) The latest multi-coatings

for indexable cutting inserts have individual

‘nano-coatings’ and are extremely hard, approaching 4000

0 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.

 One nanometre is equal to 10 – m, or one millionth of a mm.

HV 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

The 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–, this equates to the highest isothermal edge temperature of 1,000°C – cre-ating the potential for localised thermal softening and edge failure While an equivalent multi-coated Cermet, can readily turn alloy carbon steels at a depth of cut (DOC ) of up to 3 mm, with cutting speeds of between

200 to 300 m min–, with feedrates ranging from 0.1 to 0.3 mm rev– Moreover, as less flank wear takes place, the dimensional size of subsequent components in a batch will not significantly ‘statistically-drift’ , produc-ing much less tolerance variation (i.e reliable size-for-size consistency) in the completed turned parts This increased multi-coated cermet tool life, allows for an excellent surface finish 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 offer considerable economical advantages when in particular, turning hardened steel parts

 A comparison of the hardness of different 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.

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

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1..10 Cubic Boron Nitride (CBN) and

Poly-crystalline Diamond (PCD)

Cubic Boron Nitride (CBN)/Synthetic Diamond –

Extraction 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 first

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 different 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 effortlessly 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

HRC, 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

60 GPa and temperatures 1,500°C As this transforma-tion proceeds in the reactransforma-tion volume of a high pres-sure system, the CBN/synthetic diamond grows, being embedded in a portion of reaction mass and extracted afterward 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

of approximately 8 µm, down to sub-micron sizes, for fine-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

 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 2 O 3 + TiC) Cemented carbide (ISO K10 grade) CBN (DBC50) CBN (DBC80) Physical properties:

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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 soft 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 fillers,

this will have a profound effect on the mechanical and

physical properties of the final product – Table 1 lists

 Isotropic materials can be considered to have the same

prop-erties in different 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) These CBN/PCD inserts are either full-size, or smaller tips that are then brazed onto suitably-shaped blanks, to fit 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 differences 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,

after only a few seconds in-cut This reversion, does

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

ele-ment 6]

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Figure 14 A diagram illustrating how Cubin Boron Nitride

(CBN) and Polycrystalline Diamond (PCD) applications are

grouped, by workpiece materials Their effectiveness when

tempera-tures in the cutting vicinity

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

[Courtesy of DeBeers – element 6]

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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 softer hexagonal form and oxidise in air This

means that CBN can machine many ferrous parts and

cast iron grades The 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 finish can be obtained

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

opera-tions, the cutting tool must not only machine widely

differing materials that are situated adjacent to one

another in many passes over such a diverse material

workpiece, but produce an excellent machined surface

finish, which really ‘challenges’ the tool Tool life can

be extended greatly by utilising either CBN, or PCD

tooling, often tool lives can be increased by 50 to 200

times that of the previous cemented carbide

alterna-tives This 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 often 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 These

‘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 micro-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 finish, 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 finish that can be obtained will be in the region

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

a surface finish of better than 0.15 µm If one really wants the ultimate surface finish currently obtainable

by 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-finish’ 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-finishes’ of just a few angströms (i.e 10–0 m) The optical industries in particular find that the latest blemish-free ultra-sharp cutting edges of SCD, means that diamond (paste) polishing after machining has been virtually, if not completely eliminated, this fact in particular being a very big production cost for the fi-nal manufacture of large monolithic astronomical mir-rors A cautionary note, is that to use SCD tooling for anything other than as a finishing cut is totally uneco-nomic, as these precision components to be machined, should have been roughly configured to the desired shape, prior to diamond machining Therefore, SCD tools should be employed for exceedingly light finish 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 coefficient 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:

 Knoop indentors produce a wedge-shaped indentation in the

form of a parallelogram, with one diagonal seven times lon-ger than the adjacent one The Knoop test method is generally considered the optimum technique, for crystalline solids – having crystallographic directionality (i.e anisotropy).

• Natural diamond – has a hardness of 9,000 kg mm –

(ie diamond orientation and test conditions):

Dia-mond (111) surface, <110> direction, 500g load,

• Cubic boron nitride (CBN) – has a hardness of

4,500 kg mm–, (111) surface, <110> direction, 500g

load

One of the main limitations of natural diamond is that

it has distinct cleavage planes (111) This consistent

cleavage plane makes it ideal for jewellery-makers to

cleave the beautiful facets demanded of diamond

jew-ellery, but this means that monolithic diamonds must

be mounted in their respective tool holders in exactly

the correct orientation/plane, so avoiding any

poten-tial cleavage in-cut

SCD tool cost is a draw-back, because these tools

cost in the region of four times more than the

equiva-lent PCD tool However, despite this very high cost

difference, SCD can reduce the overall operating costs

and significantly improve productivity, when applied

to the correct machining process Expensive tooling

such as SCD, must be handled with care, because

al-though it is the hardest material known, it is also very

brittle and subject to thermal shock, the problem being

exacerbated with its very sharp tool edges Therefore,

it is essential that sudden impacts to the tool’s edge

must be avoided, through either inappropriate cutting

applications, or by rough handling

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