laser processing of hardmetals: physical basics and applications

International Journal of Refractory Metals & Hard Materials 23 (2005) 278–286

www.elsevier.com/locate/ijrmhm

Laser processing of hardmetals: Physical basics and applications

G. Dumitru a,*, B. Luscher a, M. Krack a, S. Bruneau b, J. Hermann b, Y. Gerbig c

a University of Applied Sciences Aargau, Steinackerstrasse 5, 5210 Windisch, Switzerlandb LP3-UMR 6285 CNRS/Aix-Marseille II University, Luminy, Case 917, 13288 Marseille, France

c CSEM Swiss Centre for Electronics and Microtechnology, Jaquet Droz 1, 2007 Neuchatel, Switzerland

Received 11 November 2004; accepted 13 April 2005

Abstract

Laser material removal is an effective processing technique for hardmetals, which cannot be machined by chip-removal tech-

niques. The basic physics of the laser–matter interactions and the influence of different laser parameters are discussed, with emphasis

on sintered WC–Co specific features. The collateral affected zones and their occurrence mechanisms for laser machining with both

nanosecond and femtosecond pulses are discussed. Experiments were carried out with pulsed laser systems operating in IR and UV,

with ns and fs pulses and their results endorse the theoretical considerations. The use of direct or indirect laser processing (ns and fs

pulses) in the surface engineering of coated/uncoated WC–Co parts is also presented. Subsequently, applications like laser micro-

structuring of tribological WC–Co surfaces and laser machining of integral chipbreakers are discussed.

� 2005 Elsevier Ltd. All rights reserved.

Keywords: Cemented tungsten carbide; Laser–matter interactions; Material removal; Tribology; Net shape laser engraving

1. Introduction

Precise laser machining by local melting and vapori-

zation of the work piece material is an effective tech-

nique for hardmetals, which are not easily machinable

by classical chip-removal techniques [1,2]. Among the

application fields of laser-machined WC–Co parts, one

can enumerate: cutting tools, drills, injection molds, tri-

bological surfaces.

The fine laser machining of hardmetal parts cameinto prominence at the end of the �90s, connected with

the development of rugged laser sources that met both

physical and industrial requirements. Details on the la-

ser ablation of WC–Co with short laser pulses (nanosec-

onds, excimer lasers) are reported in literature [3–5] and

basic features of the WC–Co laser ablation in femtosec-

ond regime are mentioned in [6,7].

0263-4368/$ - see front matter � 2005 Elsevier Ltd. All rights reserved.

doi:10.1016/j.ijrmhm.2005.04.020

* Corresponding author. Tel.: +41 56 462 4154; fax: +41 56 462 4151.

E-mail address: [email protected] (G. Dumitru).

Although accurate laser processing of WC–Co is rep-

resenting a high potential emerging technique, the elec-trical discharge machining (EDM) is nowadays still the

most widespread technology to process WC–Co parts.

Briefly, EDM material removal mechanisms make use

[8] of electrical energy, which generates a plasma chan-

nel between the work piece and a shaped electrode. Ex-

tremely high temperatures are subsequently reached and

work piece material is vaporized; electrical energy is

converted into thermal energy. Since the area, in whichthe spark erosion occurs, is given by the shaped elec-

trodes that are utilized, EDM accuracy is fairly high [8].

The present work commences with theoretical

considerations regarding the influence of laser parame-

ters like: wavelength, energy density (fluence), and pulse

duration on the WC–Co material removal. The

subsequent section contains the description of the exper-

imental conditions used and presents results regardingthe extension of the collateral affected zones in the laser

machining using ns and fs laser pulses. Next, two laser

processing approaches in the surface engineering of

mailto:[email protected]

G. Dumitru et al. / International Journal of Refractory Metals & Hard Materials 23 (2005) 278–286 279

coated/uncoated WC–Co are discussed. Finally, two

application fields are presented in order to illustrate

the potential of WC–Co laser machining.

2. Basics physics of laser–matter interactions

Laser processing of WC–Co is a non-contact tech-

nique, wherein a focused laser beam transfers a part of

its energy to the work piece. The absorption processes

take place at the surface of the machined part and there-

fore its reflectivity plays an important role in the cou-

pling efficiency of the laser energy into the work piece.

The locally absorbed energy leads to a confined temper-ature increase, followed by phase changes (melting,

vaporization), which yield material removal by molten

material ejection and by vaporization [9]. Hence, the ef-

fects of the machining beam depend on laser parameters

(fluence, wavelength, temporal features), on thermal

properties and also on the surface condition of the

WC–Co target material.

2.1. Energy density

The energy source of the thermal processes occurring in

the machined WC–Co part is the absorbed laser energy

and the extent of these processes is correlatedwith the inci-

dent fluence; more intense surface energy sources yield in-

creased enthalpies, i.e., larger temperature increases and

enough energy to induce phase changes. For instance,depending on the incident fluence, surface properties

changes (surface hardening, quenching), accuratematerial

removal (micropatterning, engraving) or substantial

material removal (cutting, drilling) can be induced.

Beams from excimer or Q-switched industrial lasers

(UV, ns pulses) with fluences around 2.5 J/cm2 (i.e.,

intensities of �108 W/cm2) are reported [10] to be suit-

able for a selective removal of the Co binder, which in-creases the adhesion of subsequent diamond films. For

effective machining of forms for microembossing, values

in the range of 10–20 J/cm2 (i.e., 0.5–1 · 109 W/cm2) are

reported to yield [3,11] good results. WC–Co machining

with a fs laser system at fluences of 2 J/cm2 (i.e.,

�1.5 · 1013 W/cm2) led to a complete absence of collat-

eral affected zones [7].

It is important to note that the laser energy density onthe machined work piece can be adjusted by means of

properly chosen delivery optics (e.g., filters, beam

expanders, focusing objectives).

2.2. Wavelength

In general, there is a strong connection between the

laser processing wavelength and the part of incident la-ser energy that is actually absorbed by the machined

material. Depending on their electronic structure (e.g.,

band gap, position of Fermi level), materials exhibit

specific absorption behaviors at different incident wave-

lengths. A common rule is frequently valid for metallic

and ceramic surfaces: the shorter the wavelength (from

IR to UV), the higher the absorption. However, the

aforementioned increase of the absorption coefficientfrom IR to UV is rather small in the particular case of

WC–Co [1]: from 76–77% at 1064 nm to 85% at 355 nm.

Due to the fact that the focused spot diameter is pro-

portional to the laser wavelength [9], the later has also

an impact on the machining precision. For instance, a

three times smaller wavelength (e.g., from 1064 nm to

355 nm) yields a three times smaller focused spot dia-

meter and therefore a nine times larger energy density.

2.3. Pulse durations and WC–Co thermal properties

In discussing the influence of pulse durations, the spe-

cific times of laser–matter interactions should be consid-

ered [12]. A laser beam incident on a surface generates

an intense electric field, localized under the irradiated

surface. Electrons are accelerated by this field and gainkinetic energy; due to their mobility, they collide with

lattice atoms and transfer them energy. The vibration

energy of lattice atoms is macroscopically mirrored in

material heating and in phase changes. This energy

transfer chain: photons–electrons–phonons needs about

1 ps in metals [13] and slightly more in ceramics [9]. This

thermalization time is critical in determining the effects

of different laser pulse durations and in discussing theirparticularities in materials processing.

For ns pulses (e.g., excimer or Q-switched lasers), the

energy transfer occurs during the pulse, under thermal

equilibrium conditions. Material removal takes place

mostly through melting and vaporization and in this

case the thermal properties [14,15] of the WC grains

and of the Co binder play an important role. These

phases exhibit comparable thermal conductivities (WC:60–80 W/m K, Co: 70–75 W/m K), but they show differ-

ent melting behaviors. The melting point of pure Co is

situated at 1495 �C and its boiling temperature

(2927 �C) lies in the vicinity of WC melting temperature

(2870 �C). Still, the partial melting of the binder phase

begins already at 1250–1300 �C, due to an eutectic reac-

tion. The binary eutectic temperature WC–Co lies at

1310 �C, whereas the ternary eutectic temperature W–C–Co is 1280 �C [16]. Further temperature increase

yields additional WC dissolution and complete Co melt-

ing. At the same time, smaller WC grains dissolve in the

liquid and reprecipitate to form larger WC grains.

According to these WC–Co specific features, the Co

phase melts and vaporizes first and material ablation oc-

curs mainly by selective binder removal. This allows WC

grains to be removed either by the ejected Co melt or byCo vapors. At high energy densities, temperatures sur-

passing the melting point of WC can be reached and

280 G. Dumitru et al. / International Journal of Refractory Metals & Hard Materials 23 (2005) 278–286

in this case larger WC grains can grow (locally, over a

few microns) from the melt [5].

In describing laser-induced thermal phenomena, the

thermal diffusion length l

l /ffiffiffiffiffiffiffiffiffiffikqc

sp

sð1Þ

is a very useful parameter for a first approximation. It

depends on the pulse duration sp, on the thermal con-

ductivity k, on the mass density q, and on the heat

capacity c of the considered material. For 100 ns laserpulses, this length can be calculated at 1.3 lm for the

WC grains (60 W/m K, 15.8 · 103 kg/m3, 200 J/kg K)

and at 1.4 lm for the Co binder (75 W/m K,

8.9 · 103 kg/m3, 430 J/kg K) [12,14,15]. The thermal

penetration depths are therefore comparable, and only

the large difference between the melting points of Co

and WC dictates their different laser ablation behavior

in ns regime.For pulses shorter than the thermalization time (fs

pulses), the energy transfer occurs firstly in a superficial

layer under non-equilibrium conditions. Hot electrons

are generated, whereas lattice atoms still have

undisturbed energies (cold lattice). In this phase material

removal occurs through different non-thermal mecha-

nisms (e.g., induced local stresses, Coulomb explosion,

material breakdown). Succeeding to the laser pulse, ifthere is still energy deposited in the hot electrons, this

energy will be transferred to the lattice within the ther-

malization time; at high incident fluences, a large part

of the energy remains saved in the hot electrons and it

is transferred to the lattice after the pulse; heat flow dri-

ven processes may prevail.

2.4. Synopsis

The influence and the simultaneous interdependences

of the laser- and the material-related parameters are

synthesized in Fig. 1, where the primary parameters

Fig. 1. Laser parameters and material properties that are significant in laser

are depicted as grey rectangles, the derivate para-

meters are indicated as white rectangles, and the

arrows mark causal dependencies. For example, the pri-

mary laser parameter ‘‘wavelength’’ has an impact on

the chain: wavelength ! beam diameter ! processing

width ! machining precision, but also on the chain:wavelength ! surface reflectivity ! processing energy

density ! machining efficiency.

In addition to these considerations on machining pre-

cision and affected zones, the overall material removal

efficiency must also be taken into account. Melt-driven

processes at long pulses (microseconds) yield a more effi-

cient material removal than short and ultrashort pulses

and may be more effective in patterning bulk substrates.Nevertheless, if thin films are to be processed or high

material removal accuracy is requested, the latter should

be the tool of choice.

3. Extension of collateral affected zones

Zones adjacent to laser-machined areas, where thematerial structure differs from the initial WC–Co grain

morphology, were observed not only in fine machining,

but also in laser welding or cutting [9,12]. The related

material changes are induced in most cases by heat

diffusion processes (‘‘heat affected zones’’), but non-

thermal processes can be also responsible for the occur-

rence of such morphology changes.

3.1. Experiment

The lateral extension of the collateral affected zones

was studied for both ns and fs pulses. The experimental

conditions are listed in Table 1; the laser fluences were

chosen within values domains, which are characteristic

for fine and accurate laser machining.

After laser processing, all WC–Co samples were pre-pared identically; they were cut with a diamond saw and

the resulting surfaces were ground and polished. The

machining; their influences on the end results are indicated by arrows.

Table 1

Parameters of the laser experiments

Laser type Pulse duration Wavelength (nm) Fluence (J/cm2) Spot diameter (lm)

1 Nd:YAG, Q-switched 80 ns 1064 10–50 40

2 Nd:YVO4, Q-switched, 3· freq. 30 ns 355 15–45 20

3 Ti:sapphire 100 fs 800 2, 10 25


cross-sections were analyzed by optical microscopy and

by scanning electron microscopy (SEM). Element map-

ping and line scans (W, Co) by EDX were also carried

out.

3.2. Nanosecond pulses

SEM investigations showed collateral zones withstructures that differed from the initial WC–Co grain

structure for almost all fluence conditions. However,

these zones were not uniformly distributed over the la-

ser-machined area margins and for some conditions

(Fig. 2) locations with collateral affected zones of negli-

gible extension could be found.

As mentioned, zones with different thicknesses were

found and the broadest is depicted in Fig. 3; its thicknesscan be estimated at 3 lm. The EDX analyses performed

on this zone revealed a small Co content, indicating that

the Co vaporization temperature was surpassed; the W

content was found similar to that corresponding to

WC grains from the unmodified areas.

Fig. 2. Cross-section cut through a laser-machined WC–Co piece (ns,

355 nm, 10 J/cm2).

Fig. 3. SEM images from margins of laser-machined areas (ns,

1064 nm, 40 J/cm2).

3.3. Femtosecond pulses

In the case of fs pulses, no collateral affected zone

were found at 2 J/cm2, and even partially ablated WC

grains were noticed (Fig. 4). These partially ablatedgrains indicate that the material removal under these

conditions had no thermal component. However, con-

fined zones (Fig. 5) of material with modified structure

were found at 10 J/cm2.

3.4. Affected zones: particularities in ns and fs cases

Although limited collateral zones were found in bothcases, the W and Co EDX line scans revealed some

differences that may indicate different occurrence

mechanisms.

Fig. 4. SEM images from a margins of a laser-machined zone (fs,

2 J/cm2).

Fig. 5. SEM image from a pore induced in WC–Co (fs, 10 J/cm2).

Fig. 6. Detail from an affected zone: SEM image and path of EDX

analysis, and results of EDX line scan (ns case).


In the ns case, the EDX line scan (white line in Fig. 6)

begins from a point situated inside the unaffected WC–

Co. The line crosses several grain interstices (gray ar-rows in bottom part of Fig. 6) and then ends in the zone

with modified grain structure. In this zone, the ratio be-

tween W and Co contents is similar to that inside unaf-

fected WC grains (start of white line). This, together

with its homogenous structure, suggests that the modi-

fied zone is actually a large WC grain that was formed

due to occurrence of high temperatures (around or

above WC melting point).In the fs case (Fig. 7), the EDX line scan starts from a

point inside the unaffected WC–Co, crosses two grain

interstices (gray arrows) and ends in the modified zone.

The modified zone exhibits a significant porous struc-

ture, with pores smaller than the normal grain inter-

stices. As the scanning line enters the modified layer, a

sudden composition change occurs; the W content

drops, the Co content increases and both exhibit qua-

si-identical element counts. Such occurrence character-

izes a pulsed laser deposition (PLD) process and may

suggest that the collateral zones of affected material re-

sulted as a secondary redeposition of laser ablated

material.

4. Laser surface engineering of uncoated/coated

WC–Co parts

4.1. General considerations

As mentioned in Section 2, the processing laser inten-

sity is a very important parameter in laser machining. Inthe particular case of surface engineering by laser

engraving, an appropriate parameter choice allows the

ablation of small surface features from the bulk mate-

rial. This does not influence the mechanical properties

of the processed part, but can improve significantly its

surface properties. Such surface shape changes aim to

enhance specific properties of the processed part or to

create new ones. Currently, surface properties aremainly improved by functional coatings and for that

reason the combination between coating techniques

Fig. 7. Detail from an affected zone: SEM image and path of EDX

analysis, and results of EDX line scan (fs case).


and laser processing has to be considered. Two such

approaches (Fig. 8) are:

Fig. 8. Direct and indirect laser processin

• direct processing, where a previously coated WC–Co

part is laser processed;

• indirect processing, where the work piece is firstly

laser processed and then coated.

Depending on application requirements, the process-ing approach can be chosen and according to this

choice, suitable laser parameters must be selected. For

instance, in the case of direct processing these parame-

ters must be chosen to minimize the collateral affected

zones, whereas in the indirect processing the parameters

must be chosen to optimize the efficiency of the ablation

process.

4.2. Direct processing

Experiments were carried out on TiCN coated WC–

Co samples; the thickness of the TiCN film was

3.5 lm. The laser processing took place in air using a

Ti:sapphire laser delivering 100 fs pulses. The laser beam

was attenuated and then focused, in order to obtain pore

diameters of approximately 30 lm at an incident laserfluence of 2 J/cm2. This fluence was chosen according

to results presented in Section 3, in order to avoid any

collateral affected zones. A number of 100 laser pulses

were incident on each irradiation spot, yielding a pore

depth of approximately 15 lm. After laser irradiation

the samples were prepared as described in Section 3

and investigated by optical microscopy and SEM.

g of WC–Co coated/uncoated parts.

Fig. 9. Laser ablated pore in coated WC–Co (100 pulses of 2 J/cm2). Fig. 10. Indirectly processed WC–Co surface (coating: TiCN).

Fig. 11. Laser microstructured WC–Co surface (UV laser, 355 nm,

�20 J/cm2).


The optical investigations of crater boundary zones

did not reveal any film cracks, delamination or other

surface modifications. They also showed that the

remaining surfaces were not affected by the laser treat-

ment; their roughness values did not change and they

could be used as tribological surfaces directly after the

fs-laser processing. The SEM investigations (Fig. 9)

did not reveal significant rims at crater borders or anyspikes or sharp rims (that could eventually initiate film

delamination) at the film–substrate interface.

These outcomes support the idea that direct laser pat-

terning (ultrashort pulses and low incident fluences) en-

ables the gentle engineering of coated surfaces, in order

to optimize some of their properties and without affect-

ing them in any negative way.

4.3. Indirect processing

The first step was the laser engraving of pores in un-

coated hardmetal. Experiments were performed in air

using WC–Co samples, that were machined using

the beam delivered by a Q-switched Nd:YAG laser

(1064 nm, pulses of 80 ns). The laser fluence was slightly

above 10 J/cm2 and the pore diameter was 25 lm. Foreach pore, six laser pulses were used and this led to

depths of 10 lm. Through optimization [17] of laser pro-

cessing parameters, the occurrence of melt rims around

the laser-induced pores was minimized and these rims

could be removed by gentle polishing. By ultrasound

and electrochemical cleaning, all remaining particles

(debris or polishing rests) were removed and a 3.5 lmthin film of TiCN was deposited by CVD on the laser-patterned and polished WC–Co substrates.

After coating, the samples were prepared as described

in Section 3 and analyzed by optical microscopy and

SEM. They showed that the laser-induced pattern was

not affected by the coating procedure (Fig. 10). The

deposited films followed the geometry of the patterned

WC–Co substrates and no film delaminations (potential

sources of adhesion problems) were observed. A slightdiameter decrease (less than 20%) and a small depth in-

crease (less than 10%) were noticed.

These results demonstrate that industrial laser

sources (Q-switched Nd:YAG lasers) can be efficiently

used to pattern WC–Co substrates, which can be subse-

quently coated. The coating films do not fill up the laser-engraved pores, but a decreasing of the microholes

diameters (after coating) must be taken into account

by designing the parameters of the initial laser-induced

micropattern.

5. Applications

5.1. Laser-structured WC–Co tribological surfaces

It is generally acknowledged that the use of hard sur-

faces, the prolonged existence of a lubricant film, and

the constant removal of the abrasive particles from the

tribocontact zones are mechanisms to slow down the

breakdown of a tribological system. Due to their hard-

ness properties [1,2], hardmetals are chosen for varioustribological systems and the machining of microstruc-

tures in the contact surfaces can influence the later two

aforementioned mechanisms. In achieving such precise

surface modifications, the laser material removal has

proved itself as a versatile and reliable tool.

Array of pores (Fig. 11) that are uniformly distrib-

uted over the tribocontact surface act as collection of lu-

bricant reservoirs distributed over the critical points.Simultaneously, structured WC–Co surface can perform

similar to a soft gliding surface (it allows the ‘‘burying’’


of the debris particles), but without compromising its

hardness. The positive effects of these mechanisms are

illustrated in Fig. 12, where the results of tribological

tests for unpatterned and laser pattened WC–Co sur-

faces are shown. In these tests a hard steel ball oscillated

over the tested surfaces under lubricated conditions, theevolution of the friction coefficient was recorded and it is

depicted in Fig. 12.

The aforementioned particle trap role is illustrated in

Fig. 13, where SEM image and EDX-mappings of pat-

terned WC–Co surfaces after tribo-tests are depicted.

Fig. 12. Evolution of friction coefficient during a tribo-test with hard

steel ball moved against structured and unstructured WC–Co surfaces.

Fig. 13. SEM image of a microstructured WC–Co surface after a t

Fig. 14. Integral chipbreaker (general view and detail) fabricated by laser m

These images show that the microstructure withstood

the tribological breakdown; however some pores were

filled up during the test. The EDX-mapping after Fe

(test ball made of steel) indicates the presence of this ele-

ment on the WC–Co surface, with accumulation max-

ima coinciding with the filled-up pores.

5.2. Laser-machined integral chipbreakers

Depending on the strength, strain, hardness and duc-

tility of the work piece, various chip types may occur

during machining. These include continuous chips, as

well as segmented or discontinuous chips. For machine

life and automation, continuous chips are disadvanta-geous because they can yield nesting and entangling of

machine parts. Therefore chipbreakers (i.e., cutting

parts with complicated face geometries) are used to get

discontinuous chips over the largest possible domain

of cutting depths and speeds.

The geometries of such integral chipbreakers are

thoroughly computed and simulated, in order to im-

prove the wear and fracture resistance. They are after-wards fabricated by direct sintering into required

shapes [2]. This is suitable for mass production, but less

efficient for small production quantities. In this case and

also during product developing, the laser engraving of

ribo-test (left), Fe-mapping by EDX of the same zone (right).

achining out of a standard sintered cutting plate (1064 nm, ns pulses).


surface features in range of 0.1–1 mm in ‘‘raw’’ cutting

plates (Fig. 14) may open new dimensions.

Starting from standard sintered cutting plates, with-

out any specific surface features, one can use the versa-

tility of laser techniques in order to fabricate integral

chipbreakers for a small series of tests.The volumes to be removed by laser machining are

divided by means of a dedicated software into process-

ing layers (e.g., with thicknesses down to 0.5 lm) that

are to be removed sequentially, until the designed shape

is obtained. This occurs by deflecting the laser beam

with a scanner head, whose driving software is able to

interact directly the CAD program used to design the

wanted shape.This is illustrated in Fig. 14, where a laser-machined

integral chipbreaker is depicted. In this case, the sintered

chipbreaker (with plane surfaces) was processed by

means of a Q-switched Nd:YAG laser (1064 nm); the la-

ser machining did not alter the any cutting or wear resis-

tance properties.

6. Conclusions

The laser processing of WC–Co hardmetals was ana-

lyzed in this paper and different aspects, starting with

the theory of laser–matter interactions and ending with

specific applications were presented. The relationships

between laser processing parameters (e.g., intensity,

wavelength, pulse duration) andWC–Co thermal proper-ties were discussed and summarized in a processing chart.

Regarding the collateral influences of the processing

laser beams, thin (<3 lm) adjacent zones with modified

structures were found for both ns pulses and high flu-

ence fs pulses. Results of EDX analyses suggested these

modifications occurred due to thermal recrystallization

of small WC grains into larger ones in the former case,

and due to a secondary redeposition of the laser ablatedmaterial in the later. Besides EDX, further materials

analyzes are necessary, in order to investigate all the

phenomena which can occur in a possible laser deterio-

ration of the compact.

It was demonstrated that direct and indirect laser

processing of coated/uncoated WC–Co parts increase

the functionality of their surfaces (e.g., replication, opti-

cal structures, medicine, tribology). Experiments withns- and fs-pulses were carried out and no film delamina-

tion or other coating failures were induced by the laser

treatment.

The potential of laser processing of WC–Co parts

was eventually illustrated with two applications: micro-

structuring of WC–Co tribological surfaces and laser

machining of WC–Co integral chipbreakers.

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