Heinrich J.G., Aldinger F. (Eds.) Ceramic Materials and Components for Engines

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CERAMIC MATRIX COMPOSITES FOR DISK BRAKES AND THEIR

MANUFACTURING TECHNOLOGIES

Gadow

University of Stuttgart

Institute for Manufacturing Technologies

Ceramic Components and Composites

Allmandring

GERMANY

D-70569

Stuttgart

ABSTRACT

Thermally stable and corrosion resistant light

weight components are a central challenge in modem

automotive engineering. The competition in the

automotive industry, especially for heavy, high

performance, luxury and sports cars, demands retardation

performance, comfort and all weather braking ability for

new disk brake systems. The required mass reduction

with simultaneously improved performance and

durability in modem trucks and high speed trains

requires disk materials with life time corrosion and wear

resistance. The reinforcement by short, chopped and

endless carbon fibers results in fracture toughened

ceramic matrix composite (CMC) properties with

appropriate friction and reliable mechanical properties in

comparison with conventional materials. Reaction

bonding is a competitive technology for manufacturing

of near net shape formed components like brake disks.

review is given on chemical processing, manufacturing

and design as well as first application results of these

new refractory CMC components in brake technology

are shown.

TECHNICAL AND ECONOMIC

REQUIREMENTS FOR CMC

BRAKE SYSTEM COMPONENTS

Due to their limited corrosion resistance CFC

materials are not suitable for long term operation

temperatures above

500

"C in atmosphere('.*',

that

their application will

limited to aircrafts and racing

cars, where pure performance overcomes cost. Low

melting and softening temperatures limit the application

of coated light metal alloy components and MMCs to

temperatures below

loo0

OC. Only CMC provide

sufficient strength and corrosion resistance up

higher

temperatures. The application of CMC materials, e.g. as

friction materials in brake systems of passenger cars and

trucks, depends on the fulfillment of the following

industrial system requirements

(3):

low cost raw materials and additives

fast, reliable and reproducible compound

manufacturing methods

fast forming process with serial manufacturing

capacity

net shape sinteringkhemical transformation to

CMC

minimized grinding and finishing effort

short production cycles

high temperature and corrosion resistance combined

with CMC fracture toughness

steady friction coefficient for antilocking brake

system under severe road vehicle conditions.

Different types of chemical processing and

manufacturing of components for brake disks can be

used. With a background in aircraft systems chemical

vapour impregnated carbodcarbon composites (CFC)

were the first successful materials, followed by pitch and

precursor impregnated cheaper CFC disks and rotors.

they cannot fulfil the main requirements for low cost

passenger car application, the recent development is

focussed on silicon ceramic composite with SiSiC or

other reaction bonded matrices.

Xomal

hnnc

PCS!PLI"

Fiber

Preform.,

GrcmCompoct

clcansurface,

Final

Statc

CMC,

nur

ncr

shop

RRSC

RB-S6

ZC-Lonp

Fih

Shon

Fiber

Tecbnksl

Process

Variant

fig.

manufacturing cycle times for CFC and CMC

composites('

For

the production of carbon fiber reinforced Sic

ceramics different methods are used. Usually

C-fibre

woven fabrics with phenolic resin as C-precursor or

temporary binder are used for reinforcement. Prepregs

are laminated to a fibrous preform and the Sic-matrix is

formed by chemical vapour infiltration

(CVI)(u6'

or by

infiltration of organosilicon polymers which are

pyrolized to ceramics up to about 1600°C under inert gas

atmosphere (LPI)(4*7*8'. Due to its relatively short

production cycles (fig.

and consequently cost

effectiveness the reaction bonding by silicon melt

impregnation (LSI) and reaction in porous carbonaceous

preforms is a promising technology for industrial

applications

(%I2).

PHYSICOCHEMICAL BASICS FOR

REACTION BONDED SILICON

CERAMIC COMPOSITES

Reaction bonding is a process where a strong

ceramic body is formed from a porous compact or

prepreg by an in-situ chemical rea~tion"~'. For the

fabrication of Sic-composites the liquid metal

impregnation method is used for reaction bonding of

fiber skeletons in originally porous carbon containing

matrices. The use of the heterogeneous chemical reaction

between silicon powder containing compacts and

nitrogen spender phases results in a fine porous or nearly

dense reaction bonded silicon nitride matrix composite.

Reaction bonding is a cost effective and technically

advantageous method to obtain dimensionally stable and

precise CMC components in manufacturing of serial

products as they are used in the automotive industry. The

advantages of the Sic route are fast impregnation and

chemical transformation, but a quite sophisticated metal

melt technology is needed. The Si3N4 route allows

convenient reacting with gaseous species but is more

time consuming and leads to less dense products.

The silicon penetration rate in carbonaceous

preforms is high due to the strong capillary flow effect

and the low wetting angle of liquid silicon on solid

carbon(I4'. The initial wetting angle is changed to values

in the range of

"...40"

during melt infiltration with

excess silicon mainly by two processes: The dissolution

of solid carbon in liquid metal and the formation of a

continuous solid Sic-layer at the interface.

The capillary flow effect of silicon is given by

Hagen-Poiseuille's equation. For more complex models

used by Fitzer and Gadow the pore utilization is taken

In pore systems and networks there are various

pore radii and a pore shrinkage effect is observed due to

the chemical reaction between carbon and silicon during

melt infiltration. The combined effect of mass transfer

from the liquid reactant Si to the solid surface, its

diffusion through the primary formed solid Sic layer and

its chemical reaction (first order) with the solid reactant

carbon was studied by Fitzer and Gadow using reaction

models for non catalyzed heterogeneous chemical

reactions between fluid metal and a shrinking core of

carbon containing ~olid"~'.

It was shown that the process is controlled by

silicon diffusion through the Sic-layer resulting in the

known parabolic correlation between reaction time and

layer thickness. In any case of infiltration by silicon in

carbon-carbon preforms the open porosity content and

into

account.(4.1

2.15.I6.I7)

consequently the pore radii distribution of the preform

are

the critical value'"' to control and optimize the

reaction.

non optimized porosity and non optimized

carbon filler/fiber/SiC powder ratio cause problems like

incomplete infiltration or inhibition of reaction, e.g.

the presence of non desired byproducts like SiO which

effect the wetting properties.

For RBSN-matrices the fiber damage by

mechanical degradation in contact with sharpedged Si

powders and the drawback of mandatory post

impregnation by metals or liquid precursors must be

mentioned.

FUNDAMENTALS FOR CERAMIC

COMPOSITES

fig.

interacting factors determining the composite

proper tie^"^'

The general aim in CMC development is the

improvement of the thermomechanical behavior of the

matrix ceramic by incorporation of high tenacity

refractory fibers'24*

25'.

The performance of such

composites strongly depends on the fibedmatrix

interaction like adhesion, chemical and mechanical

bonding at the interface as well as on the fiber volume

content (fig.

and the fiber orientation, distribution and

geometrical arrangement (fig.

3).

fig.

fiber arrangement variations

For

short

and chopped fiber CMC the critical

fiber length has to

reached or exceeded to provide the

desired stress transfer from matrix to fiber'**'. The

reinforcement mechanism of brittle fibers

brittle

matrices is also dependent on the fiber content. If this

content is lower than the critical one the composite fails

by spontaneous brittle fracture (fig

4).

The fiber volume content to obtain a damage

tolerant mechanical behavior under load must exceed a

critical value

20 ...

vol.

%).

Short fiber structures are

used for cost effective products and need special

compounding technologies. Unidirectionally and

multidirectionally designed CMC show improved

mechanical properties, but are not gas and fluid tight or

fully oxidation resistant. A recently developed novel

concept consists of a short fiber reinforced layer with a

high ceramic matrix content at the surface of the

component and strong endless fibers in the core (fig.

and fig.

10).

singulary

multiple

oBF

fnctun

....................

............................................

.......

.....

...................

.......

....

,.-”’

........

‘BC=‘BM

‘BF

4F=o

4‘F

+F=l

bending strain

fiber volume content

c$F

fig.

stress strain behavior of composites with brittle

matrix(26’

To control the relatively harsh conditions during

reaction bonding, protective coatings for the carbon

fibers are necessary in most cases. These coatings can be

produced e.g. by chemical vapor deposition. Refractories

like TIC, TiN, Sic and pyrolytic carbon can form a

temporary diffusion barrier“3’, but because of cost

arguments other processes like the cheaper liquid phase

fiber coating”’ or the completely different approach by

reducing the chemical activity of the alloyed silicon melt

are to be preferred(”,

23’.

MANUFACTURING

TECHNOLOGIES FOR REACTION

BONDED CERAMICS

The manufacture of fiber reinforced preforms in

general involves two stages. Incorporation of fibers into

the unconsolidated matrix material, followed by a

consolidation of the matrix.(”’. The highest priority for

reaction bonding processes is the manufacturing of a

well designed preform porosity, which is obtained during

pyrolysis of carbon containing resins, and an

advantageous fiber arrangement.

In general two kinds of preform manufacturing

technologies are used which

are

followed by the high

temperature treatment: conventional ceramic

manufacturing methods and modified compounding

techniques of the plastics industry.

Conventional ceramic manufacturing

methods (Slip Casting, Mixing,

Granulation):

In the case of slip casting a slurry is poured into a

porous mould which is covered by the fiber skeleton.

The mould absorbs the liquid carrier, but the

densification is hindered by the fibers and a high

porosity is the result. The matrix deposition can also

occur on the fiber skeleton which acts as a porous

medium itself resulting in a porous preform, which is

compressed by hot-pressing.

Another possibility to obtain a fibrous preform

to fill pellets consisting of fibers, powders and binder in

a mould. For manufacturing of these pellets, either

mixing technologies (e.g. kneader with extrusion screw)

or build up granulation processes

are

used(20’. Depending

on the binder system

the

pellets are either cold- or warm-

pressed to manufacture the preform. If carbonaceous

precursors are used as binder a reaction bonding (RB)

takes place after pyrolysis converting the carbon preform

by a chemical reaction with liquid silicon into a SiC-

ceramic (LSI-liquid silicon infiltration). In order to

obtain a high matrix content n

reinfiltratioddensification

cycles can

used

(21).

In the case of using

organometallic precursors as binder, the pyrolysis is

followed by a crystallization of the glassy matrix without

RB-process (LPI-liquid polymer infiltration). By using

silicon powder based porous preforms a reaction bonding

to RBSN under nitrogen atmosphere is possible.

Modified compounding techniques adapted

from polymer manufacturing:

Long fiber and woven fabrics reinforced preforms

are usually manufactured with these technologies mainly

used for thermoplastics and thermosetting resins. The

matrix consolidation is either performed in one step with

incorporation of fibers into the matrix (Autoclave

Process, RTM) or in a separate step.

Autoclave and

RTM

pressure

forming

processes:

These processes allow the production of large sized, thin

walled articles in medium production quantities. For an

autoclave process heated and pressurized forms are used.

After coating the form with resin mixture, the fiber

reinforcement and, if appropriate, the core material is

applied as an insert. The counterform, which is also

coated with resin mixture, is used to form an airtight

seal. Alternatively the counterform can be a removable

foil.

In the resin transfer moulding (RTM) process,

two-component or multicomponent rigid moulds are

employed, which are opened and closed by hydraulic

mechanisms. The fiber reinforcement is cut

shape and

placed in the mould. The mold is closed and the resin

mixture is injected from the bottom under pressure. The

application of vacuum can assist the impregnation state.

The injected resin must

of low viscosity and highly

reactive.

Filament winding

and

slurry infiltration

fiber

rovings:

The equipment for filament winding consists of

a rotating winding mandrel and an oscillating

fiber

guide,

the

drives of which can

both regulated. Rovings

are

generally used for reinforcement, although woven tapes

can also be employed.

The

rovings

are

fed through

impregnation bath, containing a mixture of resin and

powder fillers. The impregnated fibers are then wound

onto a core under constant tension and with exact

geometrical pattern, governed by the ratio of rotation

speed of the winding mandrel and the desired winding

pattern. This technology is a highly reproducible process,

but only suitable for the manufacture of hollow articles.

For the production of long fiber reinforced sheets, the

laminates on the winding mandrel are cut into pieces,

laminated to two-axial structures (mostly

O',

90')

and

reimpregnated with a resin paste in an autoclave.

Another possibility is to use a solution, containing metal

compounds

(eg

metal alkoxide, acetate or halide),

which is reacted to a sol. This sol-gel technique has been

used mainly to produce oxide and glass ceramic

matrices. The modified filament winding process is

theprecursor route for manufacturing long fiber

reinforced structures'22'

Sheet

Moulding

Compound

(SMC)

Technology:

The SMC (Sheet Moulding Compound) is transformed

from its liquid, fiber and powder filler ingredients into a

sheet product that is usually about

mm thick and

SO0

mm wide. The SMC process is illustrated in fig

compaction

roller

fig.

manufacturing process for SMC -prepregs"O'

The SMC process creates a resin paste and carbon

fiber 'sandwich' which is subsequently sent through a

series of compaction rollers where the carbon fibers are

wetted with the resin paste and trapped excess air

squeezed out

the sheet. In contrast to e.g. the

precursor route the amount of resin is very high, and

there is no reimpregnation necessary.

Endless fibers can

obtained as well by SMC

technology if

cutter blade is used. Consequently SMC

is the only compounding technology which allows a

simple and cost effective incorporation of short and long

fibers in one combined process. Before molding the

SMC sheets have to

cut into pieces of predetermined

size and shape. The cut pieces are subsequently

laminated and assembled into a charge pattern. The

combination of short and long fiber reinforced sheets

opens up the possibility to realize a great spectrum of

suitable structures for different applications. The charge

is finally placed in a preheated mold and compressed. A

near-net-shape forming process is possible due to the

high mold filling capacity of the prepregs and the

precision of

the

mold.

High temperature treatment

and

thermal

processing

The fiber-matrix incorporation and consolidation

step is always followed by a pyrolysis resulting in a

porous preform due to mass loss and the resin shrinkage.

mentioned before, it depends on the used binder and

resin system, if the ceramic matrix is formed by a

ceramizatiodcrystallization

or a reaction bonding

process. An overview of different manufacturing routes

is shown in fig 6.

Raw

Materials

Compoundirg

lncorporahon

and

Alament

Fibers

Consolnlabon

the Matm

(Polymemabon,

Curing,

Thermosethng)

High

Temperature

Treatment

fig.

manufacturing scheme for reinforced carbon-

and Sic ceramics

CMC

BRAKE

MATERIALS

CHARACTERIZATION

The highest mechanical strength values for

RB-

Sic

composites (UD: flexural strength

600

MPa) were

obtained by use of CVBSiC fibers and Sic coated

carbon fibers(''). 2D-composites with woven fabrics are

reported to reach

150-250

The mechanical

properties of most industrial brake disks must exceed the

flexural strength of high quality cast iron (-100 MPa).

comparative summary of friction materials

properties is shown in table

The key properties for

these components

are

stable and predictable friction

behavior and wear resistance. Surface modified CFC

disk materials show a steady increase of the friction

coefficient

with relatively low initial values in

comparison to Sic matrix CMC (fig

7).

The carbon and

graphite dominated friction values are even inferior to

cast iron

(GG25)

at room temperature. Ferrosilicon

alloyed CMC show the most promising friction behavior.

The friction behavior depends on the obtained surface

roughness and homogeneity, which have been optimized

by appropriate machining and finishing operations

during manufact~ring‘~~’.

law)I~2[m25033m)

Mmerelstmkes

fig. 7 comparison of SiSiC-, FeSi75 alloyed-Sic-

CMC with other brake disk materials(23’

tab

friction materials data(23’

mechanical properties are limited. Examples for short

fiber reinforced components are shown in fig

fig.

C/C materials for racing cars (courtesy of

SEP

and Ferrari)

As mentioned earlier, the SMC technology offers

a wide range

possibilities to combine various fiber

length and filler contents. Short and reproducible

production cycles as well as a net shape forming process,

due to the high mould filling capacity of the prepreg

sheets, can be realized by the SMC compounding

process. The fulfillment of the automotive industry

requirements concerning cost effectiveness and

reproducibility opens up the possibility for a serial

manufacturing method for ceramic brake disks.

COMPONENT DESIGN IN

AUTOMOTIVE AND AIRCRAFT

BRAKE SYSTEMS

The first introduced Sic CMC brake disks were

solid bulk components with

280

mm diameter‘3’. High

standard CFC disks and rotors for aircraft and racing car

use (fig

were made

joined elements up to diameters

600

mm.

The actual commercially available RB-Sic- and

carbon composites especially show excellent mechanical

and thermophysical properties due to the use of long

fibers (tab

1).

But on the other side long carbon fibers

reinforcements show limited corrosion resistance,

because the fibers are attacked not only

the

component surface but the corrosion proceeds also inside

of the component due to the high fiber length. By using

short fibers the isolated fiber strands provide improved

overall oxidation behavior, but on the other hand the

fig.

short fiber reinforced brake disks for passenger

cars

Realizing combined short- and UD-fiber

structures with short fibers at the functional surface of

the component could be a promising way to combine

both the damage tolerance of long fibers and the

corrosion resistance

insulated short fibers with

relatively high ceramic matrix volume content (fig

10).

technical design for a

330

mm diameter SIC

CMC brake disk with internal ventilation was realized by

joining two equishaped semidisks by metal melt

infiltration with subsequent reaction bonding (fig

11).

The granulate or SMC based semi-monocoques were

formed by axial warm pressing in a precision tool. They

feature torsional locking by a set of radial keys and slots.

The reinforcement design is fitted to the orientation of

the mechanically and thermally induced main stresses.

The geometry with joined key and slot with a cone

shaped design in radial orientation performs an excellent

distorsion safety during manufacturing and practical use.

The web distance is optimized with reference to the

'/z

brake pad size providing an advantageous mechanical

brake force distribution.

ndomly

distributed

short

fibers

fig. 10 structural design of high performance

multilayer CMC friction material

fig. 11 technical design for brake disks with internal

ventilation, silicon joined

REFERENCES

E. Fitzer: Potentialstudie C/C-Verbundkorper als

Werkstoff im Primkkeislauf einer HTR-Anlage,

Institut fur chemische Technik, Universitat

Karlsruhe

W. McKee et. al.: Carbon

22,285

(1984)

Gadow, R.; Kienzle, A.,,Processing and

Manufacturing of C-Fibre Reinforced SiC-

Composites for Disk Brakes",Proc. 61h Int. Symp.

Ceramic Mat. And Components for Engines,

Arita, Japan, K. Niihara et al. eds., ISBN

Fitzer, W. Fritz and R. Gadow, Chem. Ing. Tech.

9980630-0-6, pp. 41 2-41

(1997)

57, pp. 737-746 (1 985)

R. Naslain,

Hagenmuller, F. Christin et al.,J.

Adv. Comp. Mat. 2, pp.1084-1097 (1980)

6) R. Gadow, Fortschrittsber. d. Dtsch. Keram. Ges.,

7) R. Gadow, A. Kienzle; Proc. TAE/TAW coll.

,,Modeme Werkstoffe". pp. 23-24.1997 Esslingen

8) T. Haug and R. Ostertag, in ,,techn. keram.

Werkst." chapt. 4.4.1.1, DKG ed. (1995)

9) C. W. Forrest et al.: Special Chemics

pp. 99-123

(1 970)

10) W. B. Hillig et al.: GEC Tech. Inform. Ser. 74 RD

182 (1974)

11) E. Fitzer and R. Gadow, Am. Ceram. SOC. Bull., 65,

12) Fitzer,

Fritz, W.; Gadow, R.: Proc. Int. Symp.

Ceram. Comp. Engine, (1983), KTK Scientific

Publishers, Tokyo (1983), pp.

505-5

13) E. Fitzer, W. Fritz, R. Gadow: Possibilities for fiber

reinforcement of silicon carbide, Advanced

Ceramics,

Somiya ed., KTK Scientific

Publishing Company, Tokyo, pp. 81-129 (1987)

14) Li, J.-G.; Hausner, H.: Reactive wetting in the

liquidkolid-carbon system,

Am. Soc.79 [4], pp.

15) P. Godard et. al.: J. Appl. Polym. Sci., Vol. 18, pp.

16) H. Strohmeier: Dissertation, University of

Karlsruhe

98 1)

17) R. Gadow: Dissertation, University of Karlsruhe

986)

18) Singh, M.; Berendt D. R.: Reactive melt infiltration

of silicon-molybdenum alloys in microporous

carbon preforms, Mat. Sci. Eng., A194, pp. 193-

200 (1 995)

19) D.C. Phillips: Fiber reinforced ceramics, Handbook

of Composites, Vol. 4, ed. by A. Kelly and

S.T.

Mileiko, Elsevier Science Publishers B.V., 1993,

ISBN

444

864474

20) Gadow, R; Speicher, M.: Herstellung

faserverstiirkter, reaktionsgebundener

Siliziumkarbid-Keramiken

unter Verwendung

intermetallischer Siliziumlegierungen, Mat.-wiss.

u. Werkstfftech.

30,

No. 8, WILEY-VCH Verlag,

pp.

480-486

(1 999)

21) M. Nader, et. al.: Herstellung von endlos- sowie

schni ttfaserverstiirkten C/SiC-Keramiken,

Verbundwerkstoffe und Werkstoffverbunde, K.

Friedrich ed., pp. 179-184 (1997), ISBN 3-88355-

22)

Greil,

Seibold, Advanced Composite

Material, M.D. Sachs ed., Ceramic Transactions,

Vol. 19 (1990)

23) R. Gadow, M. Speicher: Manufacturing and CMC

component development for Brake Disk in

Automotive Applications, 23d Annual Cocoa

Beach Conference

Exposition: B, USA, pp.

558, Transactions of the ACerS, (1999), ISSN

Bd. 4 (1989)

5-40

pp.326-335 (1986)

873-880

(

1996)

1477-1491 (1974)

250-X

196-62 19

24) K. M. Prewo: Fiber-Reinforced Ceramics: New

Opportunities for Composite Ceramics, Ceramic

Bulletin, Vol. 68,

No.

pp.395-442 (1989

25) R.

Kerans: The Role of Fiber-Matrix Interface in

Ceramic Composites, Ceramic Bulletin, Vol.

68,

26)

Schlichting, Verbundwerkstoffe, Lexika-Verlag

Grafenau 1978,

ISBN

3-88146-149-3

27)

Van de Voorde, M. R. Nedele: CMCs

Research and the Future Potential of CMCc in

Industry, 20th Annual Conference on Composites

Advanced Ceramics, Materials and Structures: B,

Ceramic Engineering and Science Proceedings

NO.

pp.429-442 (1989)

pp. 3-21 (1996)

28) A. Kelly: Strong Solids, Clarendon Press, Oxford

(1966), ISBN 3 528 07703

29)

Papenburg: Faserversmkte keramische

Werkstoffe (CMC), Keramische Werkstoffe, DKG

ed. (1994)

30) SMC/BMC

Design for Success!, European

Alliance for SMC, WDW Werbedruck Winter

(1997)

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CERAMIC CUTTING TOOLS

G. Brandt

ABSTRACT

Sandvik Coromant,

R&D,

Materials and Processes,

SE-126

Stockholm, Sweden

Ceramic cutting tools can, successhlly applied,

increase the metal removal rate by several times over

that obtained with conventional tool materials. Ceramic

tools are either based on alumina or silicon nitride and

applications in metal cutting are determined by the

specific material properties of the tool material in

question.

understanding of the wear mechanisms

prerequisite for further development of these materials.

Applications and development trends are discussed.

INTRODUCTION

Ceramic materials have always

had

a potential for

being used

cutting tool materials in terms of their high

hot hardness and chemical stability.

spite of these

advantages they only represent up to

of the

indexable cutting tool market. This is mainly dependent

on their inferior strength, fracture toughness and thermal

shock resistance compared to high-speed steels and

cemented carbides.

During this century there has been a rapid development

of new tool materials and machines for chip-forming

within metal cutting, which in case of steel machining

has led to, on average, a doubling of productivity every

tenth year. Ceramic inserts are not suited for steel

machining to any large extent, but ceramic coatings on

cemented carbide inserts have no doubt been the major

prerequisite for the dramatically increased cutting speed

capability of

this

category of materials.

Nevertheless, ceramic inserts based on alumina and

silicon nitride, are to-day successfully applied in

machining a variety of metals within the engineering

industry, including cast iron, hardened steel and heat

resistant alloys. Metal removal rates

are

significantly

higher than when using conventional coated or uncoated

cemented carbide tools.

METAL CUTTING

Metal cutting is an operation where a tool produces

thin layers

metal, chips, during the relative motion

between the workpiece and the tool. The most common

operation is turning, which means that the workpiece is

rotating against a stationary tool.

This

also the most

frequent operation for ceramic cutting tools although

they have in some cases also been successfully applied

in milling of cast-iron and heat resistant alloys.

Fig 1. Wear types in metal cutting

During the cutting process the geometry of the tool

will change

a result of wear to the cutting edge. Tool-

life, to day is often counted in minutes, and ending when

the cutting edge no longer produces acceptable

components. Tool life determinant parameters can vary,

especially for ceramics it is important to avoid excessive

wear, which can cause fracture and unpredictable tool-

lives. Tool wear is the result of different load factors

acting on

the

cutting edge. Metal cutting generates a lot

of heat and the thermal load is considerable. Cutting

edge temperatures for ceramic tools often exceed

1000

C. Mechanical loads include both static and dynamic

components. Due to the high pressures and temperatures

along the interface between workpiece material and tool

material, diffusion and chemical reactions normally have

a significant role in the wear process. Inclusions in the

workpiece material can also influence the wear process

considerably, hard particles cause abrasive wear where

softer particles can built up protecting layers

decreasing chemical interactions. The exposed area of

the workpiece material is large, depending on the high

cutting speed, even if the inclusion content is very low,

millions of particles will pass the cutting edge every

minute.

Flank wear, which develops on the clearance face

and crater, wear on the rake chip face. Notch wear at the

extremities of cut is normally observed when machining

heat resistant alloys.

The mechanisms for a certain type of wear may vary

depending on the actual tool and workpiece material

combination; these mechanisms will be discussed

more detail later on within this paper.