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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. Lu¨scher a, M. Krack a, S. Bruneau b, J. Hermann b, Y. Gerbig
c
a
University of Applied Sciences Aargau, Steinackerstrasse 5, 5210 Windisch, Switzerland LP3-UMR 6285 CNRS/Aix-Marseille II University, Luminy, Case 917, 13288 Marseille, France CSEM Swiss Centre for Electronics and Microtechnology, Jaquet Droz 1, 2007 Neuchaˆtel, Switzerland b
c
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 techniques. 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 microstructuring 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 vaporization of the work piece material is an effective technique 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, tribological surfaces. The fine laser machining of hardmetal parts came into prominence at the end of the Õ90s, connected with the development of rugged laser sources that met both physical and industrial requirements. Details on the laser ablation of WC–Co with short laser pulses (nanoseconds, excimer lasers) are reported in literature [3–5] and basic features of the WC–Co laser ablation in femtosecond regime are mentioned in [6,7].
*
Corresponding author. Tel.: +41 56 462 4154; fax: +41 56 462 4151. E-mail address: [email protected] (G. Dumitru).
0263-4368/$ - see front matter 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijrmhm.2005.04.020
Although accurate laser processing of WC–Co is representing a high potential emerging technique, the electrical 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 channel between the work piece and a shaped electrode. Extremely high temperatures are subsequently reached and work piece material is vaporized; electrical energy is converted into thermal energy. Since the area, in which the spark erosion occurs, is given by the shaped electrodes that are utilized, EDM accuracy is fairly high [8]. The present work commences with theoretical considerations regarding the influence of laser parameters like: wavelength, energy density (fluence), and pulse duration on the WC–Co material removal. The subsequent section contains the description of the experimental conditions used and presents results regarding the 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
G. Dumitru et al. / International Journal of Refractory Metals & Hard Materials 23 (2005) 278–286
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 technique, 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 therefore its reflectivity plays an important role in the coupling efficiency of the laser energy into the work piece. The locally absorbed energy leads to a confined temperature increase, followed by phase changes (melting, vaporization), which yield material removal by molten material ejection and by vaporization [9]. Hence, the effects 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 correlated with the incident fluence; more intense surface energy sources yield increased 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), accurate material 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 suitable for a selective removal of the Co binder, which increases 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 collateral affected zones [7]. It is important to note that the laser energy density on the 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 laser energy that is actually absorbed by the machined material. Depending on their electronic structure (e.g.,
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band gap, position of Fermi level), materials exhibit specific absorption behaviors at different incident wavelengths. 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 coefficient from 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 proportional 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 diameter 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 specific times of laser–matter interactions should be considered [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 gain kinetic 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 their particularities 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 different 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 reaction. 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 melting. 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 occurs mainly by selective binder removal. This allows WC grains to be removed either by the ejected Co melt or by Co vapors. At high energy densities, temperatures surpassing the melting point of WC can be reached and
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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 sffiffiffiffiffiffiffiffiffiffi k sp ð1Þ l/ qc is a very useful parameter for a first approximation. It depends on the pulse duration sp, on the thermal conductivity k, on the mass density q, and on the heat capacity c of the considered material. For 100 ns laser pulses, 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 mechanisms (e.g., induced local stresses, Coulomb explosion, material breakdown). Succeeding to the laser pulse, if there is still energy deposited in the hot electrons, this energy will be transferred to the lattice within the thermalization 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 driven 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
are depicted as grey rectangles, the derivate parameters are indicated as white rectangles, and the arrows mark causal dependencies. For example, the primary 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 precision and affected zones, the overall material removal efficiency must also be taken into account. Melt-driven processes at long pulses (microseconds) yield a more efficient 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 the material 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 nonthermal processes can be also responsible for the occurrence 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 prepared identically; they were cut with a diamond saw and the resulting surfaces were ground and polished. The
Fig. 1. Laser parameters and material properties that are significant in laser machining; their influences on the end results are indicated by arrows.
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Table 1 Parameters of the laser experiments
1 2 3
Laser type
Pulse duration
Wavelength (nm)
Fluence (J/cm2)
Spot diameter (lm)
Nd:YAG, Q-switched Nd:YVO4, Q-switched, 3· freq. Ti:sapphire
80 ns 30 ns 100 fs
1064 355 800
10–50 15–45 2, 10
40 20 25
cross-sections were analyzed by optical microscopy and by scanning electron microscopy (SEM). Element mapping and line scans (W, Co) by EDX were also carried out. 3.2. Nanosecond pulses SEM investigations showed collateral zones with structures that differed from the initial WC–Co grain structure for almost all fluence conditions. However, these zones were not uniformly distributed over the laser-machined area margins and for some conditions (Fig. 2) locations with collateral affected zones of negligible extension could be found. As mentioned, zones with different thicknesses were found and the broadest is depicted in Fig. 3; its thickness can 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. 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 ablated grains indicate that the material removal under these conditions had no thermal component. However, confined zones (Fig. 5) of material with modified structure were found at 10 J/cm2. 3.4. Affected zones: particularities in ns and fs cases
Fig. 2. Cross-section cut through a laser-machined WC–Co piece (ns, 355 nm, 10 J/cm2).
Although limited collateral zones were found in both cases, the W and Co EDX line scans revealed some differences that may indicate different occurrence mechanisms.
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Fig. 4. SEM images from a margins of a laser-machined zone (fs, 2 J/cm2).
Fig. 6. Detail from an affected zone: SEM image and path of EDX analysis, and results of EDX line scan (ns case).
Fig. 5. SEM image from a pore induced in WC–Co (fs, 10 J/cm2).
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 arrows in bottom part of Fig. 6) and then ends in the zone with modified grain structure. In this zone, the ratio between W and Co contents is similar to that inside unaffected WC grains (start of white line). This, together with its homogenous structure, suggests that the modified 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 structure, with pores smaller than the normal grain interstices. As the scanning line enters the modified layer, a sudden composition change occurs; the W content
drops, the Co content increases and both exhibit quasi-identical element counts. Such occurrence characterizes a pulsed laser deposition (PLD) process and may suggest that the collateral zones of affected material resulted 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 intensity is a very important parameter in laser machining. In the particular case of surface engineering by laser engraving, an appropriate parameter choice allows the ablation of small surface features from the bulk material. 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 are mainly improved by functional coatings and for that reason the combination between coating techniques
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• 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 processing approach can be chosen and according to this choice, suitable laser parameters must be selected. For instance, in the case of direct processing these parameters 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
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:
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 laser fluence 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.
Fig. 8. Direct and indirect laser processing of WC–Co coated/uncoated parts.
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Fig. 9. Laser ablated pore in coated WC–Co (100 pulses of 2 J/cm2).
Fig. 10. Indirectly processed WC–Co surface (coating: TiCN).
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 treatment; 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 any spikes or sharp rims (that could eventually initiate film delamination) at the film–substrate interface. These outcomes support the idea that direct laser patterning (ultrashort pulses and low incident fluences) enables the gentle engineering of coated surfaces, in order to optimize some of their properties and without affecting them in any negative way.
used to pattern WC–Co substrates, which can be subsequently coated. The coating films do not fill up the laserengraved 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.
4.3. Indirect processing The first step was the laser engraving of pores in uncoated 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. For each pore, six laser pulses were used and this led to depths of 10 lm. Through optimization [17] of laser processing 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 lm thin film of TiCN was deposited by CVD on the laserpatterned 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 slight diameter decrease (less than 20%) and a small depth increase (less than 10%) were noticed. These results demonstrate that industrial laser sources (Q-switched Nd:YAG lasers) can be efficiently
5. Applications 5.1. Laser-structured WC–Co tribological surfaces It is generally acknowledged that the use of hard surfaces, 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 hardness properties [1,2], hardmetals are chosen for various tribological systems and the machining of microstructures 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 distributed over the tribocontact surface act as collection of lubricant reservoirs distributed over the critical points. Simultaneously, structured WC–Co surface can perform similar to a soft gliding surface (it allows the ‘‘burying’’
Fig. 11. Laser microstructured WC–Co surface (UV laser, 355 nm, 20 J/cm2).
G. Dumitru et al. / International Journal of Refractory Metals & Hard Materials 23 (2005) 278–286
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 surfaces are shown. In these tests a hard steel ball oscillated over the tested surfaces under lubricated conditions, the evolution 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 patterned 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.
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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 element on the WC–Co surface, with accumulation maxima coinciding with the filled-up pores. 5.2. Laser-machined integral chipbreakers Depending on the strength, strain, hardness and ductility 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 disadvantageous 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 improve the wear and fracture resistance. They are afterwards 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
Fig. 13. SEM image of a microstructured WC–Co surface after a tribo-test (left), Fe-mapping by EDX of the same zone (right).
Fig. 14. Integral chipbreaker (general view and detail) fabricated by laser machining out of a standard sintered cutting plate (1064 nm, ns pulses).
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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, without any specific surface features, one can use the versatility 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 processing 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 laser machining did not alter the any cutting or wear resistance properties.
6. Conclusions The laser processing of WC–Co hardmetals was analyzed 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) and WC–Co thermal properties 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 fluence 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 ablated material in the later. Besides EDX, further materials analyzes are necessary, in order to investigate all the phenomena which can occur in a possible laser deterioration 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, optical structures, medicine, tribology). Experiments with ns- and fs-pulses were carried out and no film delamination 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: microstructuring of WC–Co tribological surfaces and laser machining of WC–Co integral chipbreakers.
References [1] Upadhyaya GS. Nature and properties of refractory carbides. New York: Nova Science Publishers; 1996. [2] Schedler W. Hartmetall fu¨r den Praktiker. Du¨sseldorf: VDI Verlag; 1988. [3] Yeh LY, Hellrung D, Gillner A, Poprawe R. Development in the model-industry: micro-machining of hardmetal (WC–10%Co) by Nd:YAG-laser. In: Proceedings of ICALEO 1998. p. 144– 52. [4] Bleiner D, Plotnikov A, Vogt C, Wetzig K, Gu¨nther D. Depth profile analysis of various titanium based coatings on steel and tungsten carbide using laser ablation inductively coupled plasma—‘‘time of flight’’ mass spectrometry. Fresnius J Anal Chem 2000;368:221–6. [5] Li T, Lou Q, Dong J, Wie Y, Liu J. Phase transformation during surface ablation of cobalt-cemented tungsten carbide with pulsed UV laser. Appl Phys A 2001;73:391–6. [6] Dumitru G, Romano V, Weber HP, Sentis M, Marine W. Femtosecond ablation of ultrahard materials. Appl Phys A 2002;74:729–39. [7] Dumitru G, Romano V, Weber HP, Sentis M, Hermann J, Bruneau S, et al. Metallographic analysis of steel and hardmetal substrates after deep drilling with femtosecond laser pulses. Appl Surf Sci 2003;208:181–8. [8] Ho KH, Newmann ST. State of the art electrical discharge machining (EDM). Int J Mach Tools Manuf 2003;43:1287–300. [9] von Allmen M. Laser-beam interactions with materials. Berlin: Springer; 1987. [10] Li T, Lou Q, Dong J, Wei Y, Liu J. Surface removal of cobalt binder in surface ablation of tungsten carbide hardmetal with pulsed UV laser. Surf Coat Technol 2001;145:16–23. [11] Heyl P, Olschewski T, Wijnaendts R. Manufacturing of 3D structures for micro-tools using laser ablation. Microelectron Eng 2001;57:775–80. [12] Ba¨uerle D. Laser processing and chemistry. Berlin: Springer; 2000. [13] Chichkov BN, Momma C, Nolte S, von Alvensleben F, Tu¨nnermann A. Femtosecond, picosecond and nanosecond laser ablation of solids. Appl Phys A 1996;63:109–15. [14] Bauccio M. ASM engineered materials reference book. 2nd ed. Materials Park, OH: ASM International; 1994. [15] Weast RC. CRC handbook of chemistry and physics. 62nd ed. Boca Raton, FL: CRC Press; 1981. [16] Lassner E, Schubert WD. Tungsten—properties, chemistry, technology of the element, alloys, and chemical compounds. London: Kluwer Academic/Plenum Publishers; 2000. [17] Dumitru G, Romano V, Weber HP, Haefke H, Gerbig Y, Pflu¨ger E. Laser microstructuring of steel surfaces for tribological applications. Appl Phys A 2000;70:485–7.
Laser processing of hardmetals: Physical basics and applications G. Dumitru
a,*
, B. Lu¨scher a, M. Krack a, S. Bruneau b, J. Hermann b, Y. Gerbig
c
a
University of Applied Sciences Aargau, Steinackerstrasse 5, 5210 Windisch, Switzerland LP3-UMR 6285 CNRS/Aix-Marseille II University, Luminy, Case 917, 13288 Marseille, France CSEM Swiss Centre for Electronics and Microtechnology, Jaquet Droz 1, 2007 Neuchaˆtel, Switzerland b
c
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 techniques. 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 microstructuring 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 vaporization of the work piece material is an effective technique 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, tribological surfaces. The fine laser machining of hardmetal parts came into prominence at the end of the Õ90s, connected with the development of rugged laser sources that met both physical and industrial requirements. Details on the laser ablation of WC–Co with short laser pulses (nanoseconds, excimer lasers) are reported in literature [3–5] and basic features of the WC–Co laser ablation in femtosecond regime are mentioned in [6,7].
*
Corresponding author. Tel.: +41 56 462 4154; fax: +41 56 462 4151. E-mail address: [email protected] (G. Dumitru).
0263-4368/$ - see front matter 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijrmhm.2005.04.020
Although accurate laser processing of WC–Co is representing a high potential emerging technique, the electrical 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 channel between the work piece and a shaped electrode. Extremely high temperatures are subsequently reached and work piece material is vaporized; electrical energy is converted into thermal energy. Since the area, in which the spark erosion occurs, is given by the shaped electrodes that are utilized, EDM accuracy is fairly high [8]. The present work commences with theoretical considerations regarding the influence of laser parameters like: wavelength, energy density (fluence), and pulse duration on the WC–Co material removal. The subsequent section contains the description of the experimental conditions used and presents results regarding the 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
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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 technique, 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 therefore its reflectivity plays an important role in the coupling efficiency of the laser energy into the work piece. The locally absorbed energy leads to a confined temperature increase, followed by phase changes (melting, vaporization), which yield material removal by molten material ejection and by vaporization [9]. Hence, the effects 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 correlated with the incident fluence; more intense surface energy sources yield increased 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), accurate material 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 suitable for a selective removal of the Co binder, which increases 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 collateral affected zones [7]. It is important to note that the laser energy density on the 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 laser energy that is actually absorbed by the machined material. Depending on their electronic structure (e.g.,
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band gap, position of Fermi level), materials exhibit specific absorption behaviors at different incident wavelengths. 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 coefficient from 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 proportional 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 diameter 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 specific times of laser–matter interactions should be considered [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 gain kinetic 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 their particularities 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 different 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 reaction. 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 melting. 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 occurs mainly by selective binder removal. This allows WC grains to be removed either by the ejected Co melt or by Co vapors. At high energy densities, temperatures surpassing the melting point of WC can be reached and
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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 sffiffiffiffiffiffiffiffiffiffi k sp ð1Þ l/ qc is a very useful parameter for a first approximation. It depends on the pulse duration sp, on the thermal conductivity k, on the mass density q, and on the heat capacity c of the considered material. For 100 ns laser pulses, 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 mechanisms (e.g., induced local stresses, Coulomb explosion, material breakdown). Succeeding to the laser pulse, if there is still energy deposited in the hot electrons, this energy will be transferred to the lattice within the thermalization 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 driven 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
are depicted as grey rectangles, the derivate parameters are indicated as white rectangles, and the arrows mark causal dependencies. For example, the primary 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 precision and affected zones, the overall material removal efficiency must also be taken into account. Melt-driven processes at long pulses (microseconds) yield a more efficient 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 the material 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 nonthermal processes can be also responsible for the occurrence 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 prepared identically; they were cut with a diamond saw and the resulting surfaces were ground and polished. The
Fig. 1. Laser parameters and material properties that are significant in laser machining; their influences on the end results are indicated by arrows.
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Table 1 Parameters of the laser experiments
1 2 3
Laser type
Pulse duration
Wavelength (nm)
Fluence (J/cm2)
Spot diameter (lm)
Nd:YAG, Q-switched Nd:YVO4, Q-switched, 3· freq. Ti:sapphire
80 ns 30 ns 100 fs
1064 355 800
10–50 15–45 2, 10
40 20 25
cross-sections were analyzed by optical microscopy and by scanning electron microscopy (SEM). Element mapping and line scans (W, Co) by EDX were also carried out. 3.2. Nanosecond pulses SEM investigations showed collateral zones with structures that differed from the initial WC–Co grain structure for almost all fluence conditions. However, these zones were not uniformly distributed over the laser-machined area margins and for some conditions (Fig. 2) locations with collateral affected zones of negligible extension could be found. As mentioned, zones with different thicknesses were found and the broadest is depicted in Fig. 3; its thickness can 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. 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 ablated grains indicate that the material removal under these conditions had no thermal component. However, confined zones (Fig. 5) of material with modified structure were found at 10 J/cm2. 3.4. Affected zones: particularities in ns and fs cases
Fig. 2. Cross-section cut through a laser-machined WC–Co piece (ns, 355 nm, 10 J/cm2).
Although limited collateral zones were found in both cases, the W and Co EDX line scans revealed some differences that may indicate different occurrence mechanisms.
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Fig. 4. SEM images from a margins of a laser-machined zone (fs, 2 J/cm2).
Fig. 6. Detail from an affected zone: SEM image and path of EDX analysis, and results of EDX line scan (ns case).
Fig. 5. SEM image from a pore induced in WC–Co (fs, 10 J/cm2).
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 arrows in bottom part of Fig. 6) and then ends in the zone with modified grain structure. In this zone, the ratio between W and Co contents is similar to that inside unaffected WC grains (start of white line). This, together with its homogenous structure, suggests that the modified 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 structure, with pores smaller than the normal grain interstices. As the scanning line enters the modified layer, a sudden composition change occurs; the W content
drops, the Co content increases and both exhibit quasi-identical element counts. Such occurrence characterizes a pulsed laser deposition (PLD) process and may suggest that the collateral zones of affected material resulted 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 intensity is a very important parameter in laser machining. In the particular case of surface engineering by laser engraving, an appropriate parameter choice allows the ablation of small surface features from the bulk material. 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 are mainly improved by functional coatings and for that reason the combination between coating techniques
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• 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 processing approach can be chosen and according to this choice, suitable laser parameters must be selected. For instance, in the case of direct processing these parameters 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
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:
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 laser fluence 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.
Fig. 8. Direct and indirect laser processing of WC–Co coated/uncoated parts.
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Fig. 9. Laser ablated pore in coated WC–Co (100 pulses of 2 J/cm2).
Fig. 10. Indirectly processed WC–Co surface (coating: TiCN).
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 treatment; 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 any spikes or sharp rims (that could eventually initiate film delamination) at the film–substrate interface. These outcomes support the idea that direct laser patterning (ultrashort pulses and low incident fluences) enables the gentle engineering of coated surfaces, in order to optimize some of their properties and without affecting them in any negative way.
used to pattern WC–Co substrates, which can be subsequently coated. The coating films do not fill up the laserengraved 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.
4.3. Indirect processing The first step was the laser engraving of pores in uncoated 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. For each pore, six laser pulses were used and this led to depths of 10 lm. Through optimization [17] of laser processing 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 lm thin film of TiCN was deposited by CVD on the laserpatterned 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 slight diameter decrease (less than 20%) and a small depth increase (less than 10%) were noticed. These results demonstrate that industrial laser sources (Q-switched Nd:YAG lasers) can be efficiently
5. Applications 5.1. Laser-structured WC–Co tribological surfaces It is generally acknowledged that the use of hard surfaces, 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 hardness properties [1,2], hardmetals are chosen for various tribological systems and the machining of microstructures 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 distributed over the tribocontact surface act as collection of lubricant reservoirs distributed over the critical points. Simultaneously, structured WC–Co surface can perform similar to a soft gliding surface (it allows the ‘‘burying’’
Fig. 11. Laser microstructured WC–Co surface (UV laser, 355 nm, 20 J/cm2).
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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 surfaces are shown. In these tests a hard steel ball oscillated over the tested surfaces under lubricated conditions, the evolution 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 patterned 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.
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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 element on the WC–Co surface, with accumulation maxima coinciding with the filled-up pores. 5.2. Laser-machined integral chipbreakers Depending on the strength, strain, hardness and ductility 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 disadvantageous 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 improve the wear and fracture resistance. They are afterwards 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
Fig. 13. SEM image of a microstructured WC–Co surface after a tribo-test (left), Fe-mapping by EDX of the same zone (right).
Fig. 14. Integral chipbreaker (general view and detail) fabricated by laser machining out of a standard sintered cutting plate (1064 nm, ns pulses).
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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, without any specific surface features, one can use the versatility 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 processing 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 laser machining did not alter the any cutting or wear resistance properties.
6. Conclusions The laser processing of WC–Co hardmetals was analyzed 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) and WC–Co thermal properties 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 fluence 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 ablated material in the later. Besides EDX, further materials analyzes are necessary, in order to investigate all the phenomena which can occur in a possible laser deterioration 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, optical structures, medicine, tribology). Experiments with ns- and fs-pulses were carried out and no film delamination 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: microstructuring of WC–Co tribological surfaces and laser machining of WC–Co integral chipbreakers.
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