efficient processing of hard, brittle materials · 2018-04-26 · when hard and brittle ceramic...

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Successfully competing in a

global market requires a

combination of having a range

of unique advantages and ways

of standing out from the crowd.

Precision manufacturing offers

this opportunity, but at the same

time it poses challenges in terms

of machinery, control and tooling.

Six domains were identified in

which a company can make the

difference.

EFFICIENT PROCESSING OF HARD, BRITTLE MATERIALS

SIX DOMAINS PROVIDING OPPORTUNITIES TO EXCEL

Hard materials such as ceramics and cemented carbide are

very useful in numerous components that are exposed to high

mechanical, chemical and/or thermal stresses. However,

these materials are not used as often as they could be as they are

not only hard but also brittle. That means that they can break

unpredictably. They are therefore slow and expensive to machine.

But there are innovative solutions in the pipeline. Scientists are

actively looking for ways to process brittle materials quickly and

cost-effectively. Such as milling with very high precision, thanks to

diamond-coated cutting equipment. Or by removing material with

electrical discharges or vibrations.

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factory of the future

product of the future



businessof the future



MARKET NEEDA higher wear resistance, a fatigue strength, a longer lifetime and a

light weight are sought-after properties when examining hard mate-

rials, such as cemented carbide and ceramics, in product design. Al-

though these materials would be ideally suited for this purpose, they

are not used because of the difficulties in machining, which would

create a high-cost, slow production process. To establish break-

through cost-effective machining, technologies for both small and

large series need to be introduced.

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POTENTIAL & OPPORTUNITYThe challenges when machining carbides and ceramics are, on the

one hand, ensuring the required level of precision and, on the oth-

er, keeping down machining costs. When hard and brittle ceramic

material gets chipped, this results in unpredictable dimensions, and

a high level of tool wear and a low material removal rate add sub-

stantially to the price tag.

However, the high levels of hardness and melting temperatures of

hard and brittle materials make them ideal for a wide range of appli-

cations. In the automotive industry, high-quality technical ceramics

are used so as to consistently comply with requirements that gener-

ally cannot be met by metal- or plastic-based materials. The l-sensor

with doped ZrO2 as an electric conductor is a perfect example of

this. Power stations use ceramic components in any machinery that

is subject to high levels of mechanical, chemical and thermal stress.

In the plastic industry too, components that face high levels of stress

are made of high-quality technical ceramics. In other sectors as well,

these materials can be found being used for various applications:

mechatronics and semiconductors (bearings, precision compo-

nents), the pump industry (valves, bearings, plungers), the food in-

dustry (valves, cutters), the chemicals industry (nozzles), the medical

industry (hip balls, teeth, knives), aerospace (valves, sensors) and

offshore navigation (guidance, bearings).

Grinding is still the standard technology used for machining of hard

and hardened materials. For ceramic components, an NNS will be

produced in soft state (green part), followed by a sintering process

which will lead to the shrinkage – to an unpredictable extent – of

the dimensions of the relevant part, thus making a finishing grinding

operation inevitable. For hard materials that are electrically conduc-

tive, EDM technology is also frequently used. However, nowadays

new technologies are also available for machining free-form shapes








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out of hard, brittle materials. The challenge is to incorporate these

technologies into the production chain and achieving a cost-effec-

tive solution.

RESEARCH RESULTSThe paragraphs that follow will discuss various existing and prom-

ising prospective alternatives to grinding and EDM for machining

cemented carbides and ceramics.

High-precision milling

High-precision milling allows machining of cemented carbide parts

to take place without there being the drawbacks of both grinding

and/or EDM. It thus improves the level of geometrical freedom (in

the form of ensuring, within feasible limits, high surface quality and

short lead times as a result of direct milling). To enable milling, a

very hard diamond coating is required at the cutting edge (HV10 >

9,000 kg/mm2), combined with new milling strategies. Other consid-

erations that need to be taken into account when milling cemented

carbide are a high level of machine stability; avoiding spindle ex-

pansion; minimising tool overhang; cooling due to compressed air;

and high spindle speeds (30,000 rpm for a 2-mm diameter tool).

However technological feasible it is though, the economics of mill-

ing cemented carbides mean that it not suited to all applications:

tool cost (approx.. €250/tool diameter of 2 mm), dimensional limits

(only small sizes) and a limited Material Removal Rate (MRR) of 2.2

mm3/min).

High-precision (micro-)milling of ceramics is even more difficult. Ex-

periments with a 1-mm end mill with different hard coatings and CBN

showed that a nanograin diamond coating is most appropriate. With

cutting parameters (ap = 4 µm, fz = 3 µm and vc = 120 m/min), a

roughness of less than 60 nm and an MRR of 0. 912 mm³/min over a

distance of 2 m are achieved. Delamination of the coating limits tool

performance. The hard carbon coating in the tool breaks up above








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66 mm. The micrograin diamond coating has an intermediate per-

formance, which can be explained by the larger size of the diamond

crystals. With CBN tools and cutting parameters (ap = 10 µm, fz =

2-10 µm and vc = 60 m/min), a roughness of less than 150 nm and

a maximum cumulative milling length of 341 mm was achieved. This

process is mainly restricted in practice to finishing operations and

micro applications.

Electrical discharge machining (EDM)

Electrical discharge machining is a machining technique in which

material is removed by controlled electrical discharges (sparks) be-

tween an electrode (a wire or die in most cases) and a workpiece,

both of which are submerged in a dielectric fluid. The EDM process

imposes one big limitation on the workpiece material: it has to be

sufficiently conductive. By adding a secondary conductive phase

(e.g. WC, TiB2, NbC and TiN) to a non-conductive matrix (ZrO2,

Al2O3, Si3N4), EDM can be enabled by creating a percolating con-

ductive network inside the composite material. This network can

form between 30 and 40 vol.% of conductive secondary phase and

is dependent on the grain size, with smaller grains giving better re-

sults. As well as allowing EDM, the creation of a composite material

has a positive effect on the mechanical properties, such as strength

and fracture toughness.

Different coatings for milling ceramics

Micrograin diamond (DM) Nanograin diamond (DN) Hard Carbon (HC)

Coating material CVD diamond crystalline CVD diamond nanocrystalline Ta-C (>80% sp³)

Coating technology CVD CVD PVD-Arc

Coating morphology Crystals of 1-2 µm Crystals of 30-40 nm -

Hardness (HV 0.05) 10.000 10.000 7.000

Max. service temp. (°C) 600 600 500

Coating thickness 6-10 µm 6-12 µm 0,3-1,2 µm

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Unlike metals, in which melting and evaporation are the main material

removal mechanisms (MRMs), in ceramics and ceramic composites,

other phenomena can play an important role in determining the ma-

terial removal rate, electrode wear and surface quality. Grains can be

dislodged during EDM due to differences in thermal expansion co-

efficients between the two phases, a process called thermal shock.

This in general leads to a high level of surface roughness and a low

level of tensile strength. Another similar MRM is spalling, in which a

newly formed recast layer (a layer of molten material) breaks loose

during the cooling process following a spark due to thermal stresses.

In general this also leads to higher levels of surface roughness. A

number of materials also undergo chemical reactions at the elevat-

ed temperatures occurring during EDM. Si3N4, a popular technical

ceramic, does not melt but decomposes above 1,800°C into Si metal

and N2 gas. The main MRM in WC is oxidation, leaving a nanometric

WO3 layer behind which can be easily removed by warm water.

In general these MRMs occur simultaneously, but most of the time

one is dominant. Thermal conductivity is the main material property

behind the material removal rate (MRR) when melting and evapora-

tion are the dominant MRMs, due to the fact that a lower thermal con-

ductivity ensures that the heat is concentrated at the surface during

EDM. However, fracture toughness and grain boundary strength be-

come important when thermal shock and spalling occur, and grain

size appears to exert a strong influence on the oxidation behaviour.

Therefore, when machining ceramics by means of EDM, knowledge

of the interactions between material properties and machining be-

haviour is crucial when trying to achieve optimal MRRs and surface

quality.

Electro-chemical machining (ECM)

ECM is similar to EDM, but there are a few major differences: no

melting or evaporation takes place; no stress deformations are intro-

duced, material removal occurs by means of dissolution of the anode

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(workpiece) (MRR = 1.5 cm³/min), an electrolyte is used to transport

ions between the cathode and the anode and often the electrode

vibrates to improve evacuation of the electrolyte.

An electrically conductive part can be machined regardless of its

hardness down to a very low surface roughness. Due to the very

narrow gap between the cathode and the anode, parts can be ma-

chined with very high precision and very low Ra values (approx. 20

nm).

Vibration-assisted machining

In vibration-assisted machining, a vibration is added to the cutting

tool. This can be a turning tool, a drilling tool or a grinding/milling

tool. Typically, the amplitude ranges between 1 and 40 µm, and the

frequency can be anything up to 80 kHz. At frequencies between

18 and 25 kHz, the process is known as ultra-assisted machining.

The vibration significantly increases the ability to machine hard and

brittle materials (SiC, Al2O3, ZrO2, etc.). The vibrating movement has

the advantages of lower cutting forces which allow higher material

removal rates, and the machining of small details. The vibration also

results in better cooling conditions, and the different wear mecha-

nisms have a tool sharpening/dressing effect.

Electrolytic in-process dressing (ELID) grinding

ELID grinding enables the use of metal-bonded grinding wheels

which are very durable and difficult to deal with in comparison with

resinous and vitrified wheels. Furthermore, with ELID grinding, it

is possible to obtain both good geometrical accuracy and a very

smooth surface on a workpiece (because of very small abrasives of

1 µm).

Several types of cemented carbide (WC-Co) and ceramics (ZrO2,

Si3N4, etc.) have been ground with ELID to a surface roughness Ra

of a few nanometres. This results in mirror-like surfaces of very pol-

ished quality. You will find below two images of ground workpieces:

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the WC-Co piece on the left measures 40 x 50 mm, has a surface

roughness Ra of 0.007 µm (Rz of 0.063 µm) and a straightness error

of 1 µm over both its width and its length. On the right-hand side is a

piece of ZrO2 of 50 mm in length and 20 mm in width which has a

roughness Ra of 0.006 µm (Rz of 0.050 µm) and a straightness error

of 0.6 µm over its length.

Laser-assisted turning

In laser-assisted turning, a laser beam locally heats up the work-

piece material just before cutting. Locally heating this material im-

proves the machinability of high-strength materials like ceramics.

Besides better machinability, laser-assisted machining holds out

several benefits: higher cutting volumes and longer tool life (MRR of

up to 10 mm3/s), a shorter manufacturing time and lower costs, the

elimination of cooling lubricants (dry machining), geometrical flexi-

bility, affordable manufacturing of complex components made from

technical ceramics, and a highly reproducible manufacturing quality

due to a very good level of control of the laser source.

ELID ground WC-Co ELID ground ZrO2

Laser-assisted turning on Monforts RNC LaserTurn

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A comparison between turning and laser assisted turning of ZrO2

Turning Laser assisted turning

Parameters

vc [m/min] 30 60

F [mm] 0.01 0.01

Ap [mm] 0.17 0.42

Laser power [W] 0 2000

Material removal rate [mm3/s] 0.86 4.31

Results

Ra 0.08 0.10

Rz 0.60 0.67

Tool wear VBmax [µm] 40 46

An example providing an overview of the techniques above

Material E [GPa] Hv [kg/mm³]KIC 10kg

[MPam0.5]K [W/m°K] ρ [10-5Ωm]

ZrO2 - TiN 280 1350 9.7 6.41 2.94

Techno-logy

ELID-grind-ing

Turning

Laser

assisted

turning

UAG

Die-

sinking

EDM

Micro

EDM

Micro

millingMilling

Material removal rate [mm3/s]

1.18 0.86 4.31 1.66 0.15 0.0002 0.0152 2.2

Surface roughness (Ra) [µm]

0.020 0.08 0.10 0.2 0.65 0.5 0.03 0,06

Shape

flexibilityLow Medium Medium High High Medium High High

Milling

DIe sinking EDM

UAG

ELID-Grinding

Micro-EDM

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Laser ablation

Laser ablation is a mass removal technique involving coupling laser

energy to a target material. It is already widely applied in metal-cut-

ting processes. However, for ceramics, due to their extremely high

melting and boiling points and high thermal conductivities, the effi-

ciency of conventional lasers is limited. The development of fem-

tosecond lasers holds out new opportunities in terms of the laser

machining of ceramics with an extreme high melting point. Femto-

second lasers (10-15 s) allow for a non-thermal ablation regime (τpulse

< Tthermal) yielding a reduced dependency on the thermal properties

of the target material. Instead of melting/evaporation, the material

removal mechanism is adsorption/excitation of the target material at

a rate faster than heat is conducted inside the material. In addition

to enabling ceramics to be machined, the non-thermal machining

mechanism of femtosecond lasers considerably increases the sur-

face quality and precision of laser-machined parts.

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INDUSTRIAL EXAMPLETo demonstrate the milling of carbides, a hexalobular-shaped punch

die has been produced (see the image below). The pocket, which

has a diameter of 9 mm and a depth of 6 mm, is machined in ce-

mented carbide WC/Co 90/10 to a precision level of 0.02 mm. Two

diamond-coated micro ball-end mills were used, one for roughing

and one for finishing, both of them 1 mm in diameter. The rough-

ing operation took about 156 minutes, and the finishing oper-

ation 44 minutes at 20,000 rpm and a feed rate of 200 mm/min.

The cutting parameters can be set even higher, since while there

was tool wear, this was at a rather low level, but in this case the limits

of the machinery were reached.

The table above shows that it is possible to machine the punch die

but the economics of the process need to be looked at. Tool costs,

at about €250/tool, are rather high. Alternative technologies, like

wire EDM, can combine multiple parts in one operation, if the shape

of the product allows this. However, as the number of applications

increases, surface tool costs will inevitably go down and the milling

of carbides will become competitive for a wide range of such appli-

cations.








Carbide hexalobular-shaped punch

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SEIZING THE OPPORTUNITYMachining hard and brittle materials is no longer the exclusive pre-

serve of grinding. When looking to increase cost-effectiveness and

geometrical flexibility, a variety of novel technologies are available.

However, it is important to set up the correct combination of tech-

nologies to achieve a cost-effective production chain. The key here

is to remove as much material as possible as fast as possible in the

roughing stage and so reach an optimal starting point for the more

expensive finishing operation. For each product the make-up of this

production chain has to be examined. factory of the future







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EXPERTISE AND FACILITIES AT YOUR DISPOSALThe Precision Machining Lab at Sirris:

• the Fehlmann Versa 825 five-axis high-precision milling centre;

• the high-precision Erowa clamping system;

• the Mitutoyo Apex-S 3D coordinate measuring machine;

• a laser texturing machine for surface functionalization

• an acclimatised chamber.

Various specifications:

• milling of precision components to an accuracy of 3 μm;

• machine travel range: X: 820 mm; Y: 700 mm; Z: 450 mm;

• spindle: 20,000 rpm, 24 kW and 120 Nm at 50-1,920 rpm;

• clamping with micrometric repeatability;

• CNC-controlled (scanning) measurements from CAD;

• measurement accuracy of 1.7 μm + 0.3 L/100 μm (L in mm).

The precision machining lab, its infrastructure and engineers,

are at your service to:

• realise your prototype precision components for new applications;

• become conversant with precision machining before investing

yourself;

• provide you with support with regard to the machinability and

cost-effective manufacturing of precision components.








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THE AUTHORSSirris is the collective centre for the Belgian technology industry. The

Advanced Manufacturing Department boasts more than 60 years of

experience in the field of machining technology. Sirris was the first or-

ganisation in Belgium to introduce NC programming, damped-boring

bars, tool management, high-speed milling, five-axis simultaneous mill-

ing, hard turning and laser ablation. Over the last four years the focus

has been on achieving micrometric precision levels on five-axis milling

machines that, while high-end, is within the reach of SMEs. Working

with industry, our applied research has led to game-changing results.

Peter ten Haaf

Program Manager - Precision Manufacturing

As responsible for the Precision Manufacturing department Peter

defines the research strategy and supports industry in detecting their

own opportunities.

Olivier Malek

Expert Machining Advanced Materials and Surface Functionality

Olivier is responsible for research and industrial projects on high

precision machining. His interests lay in non-traditional machining

technologies and advanced materials in particular.

Krist Mielnik

Expert High-precision Milling

Krist focuses on the finishing process optimisation of the gear

prototype, realignment problems and precision finishing of additive

manufactured parts and methods to evaluate and improve machine

precision.

Tom Jacobs

Expert Machining Advanced Materials and Monitoring Solutions

As a senior engineer, Tom is helping companies with research on

methods to control precision during production with the help of

sensors and real-time data.








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PARTNERSThe research descripted within this publication was a collaboration

between

This publication has been made within the framework of “VIS” and

supported by Agentschap voor Innovatie door Wetenschap en

Technologie (IWT).

DIAMANT BUILDINGBoulevard A. Reyerslaan 80 B–1030 Brussel +32 2 706 79 44 www.sirris.be [email protected] blog.sirris.be








efficient processing of hard, brittle materials · 2018-04-26 · when hard and brittle ceramic...

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