A Fiber-Laser Process for Cutting Thick Yttria-Stabilized Zirconia: Application and Modeling

A Fiber-Laser Process for Cutting Thick Yttria-StabilizedZirconia: Application and Modeling

Jonathan Parry,* Rehan Ahmed, Fraser Dear, and Jon Shephard

School of EPS, Heriot-Watt University, Riccarton, Edinburgh EH14 4AS, United Kingdom

Marc Schmidt and Lin Li

School of Mechanical, Aerospace and Civil Engineering, Heriot-Watt University, Manchester M60 1QD,United Kingdom

Duncan Hand

School of EPS, Heriot-Watt University, Riccarton, Edinburgh EH14 4AS, United Kingdom

A novel laser processing technique is demonstrated for cutting up to 13-mm-thick sections of Yttria-stabilized tetragonal zirconiapolycrystal (Y-TZP) ceramic. Y-TZP is a high toughness engineering ceramic, which is extremely difficult to machine in its final state.However for some applications, this is nevertheless desirable. As an example, Y-TZP-based dentures represent an example of mass-customization in which it is essential to machine the required shape from solid billets rather than to produce customized molds.Typically this is done mechanically and necessitates the use of diamond-tipped grinding and drilling tools;, however, this process isvery slow and suffers from problems of high tool wear. In this work, an Ytterbium fiber laser system has been used to achieve fullthickness cuts via a novel controlled crack propagation technique at substantially greater thicknesses than previously possible. Thebuild up of heat and stress in the material during the cutting process is analyzed in a finite-element model and compared withexperimental data. This illustrates the mechanism by which cracks are driven in a controlled way to generate a cut. Processing rates aresubstantially faster than alternative cutting techniques available for this material with feed rates of up to 1.8 mm/s demonstrated.

Int. J. Appl. Ceram. Technol., 8 [6] 1277–1288 (2011)DOI:10.1111/j.1744-7402.2010.02559.x

Ceramic Product Development and Commercialization

r 2010 The American Ceramic Society

*[email protected]

mailto:[email protected]

Introduction

Yttria-stabilized tetragonal zirconia polycrystal(Y-TZP) is a particularly tough engineering ceramic.The high toughness arises from a crack arresting processthat occurs due to a stress-induced phase transition (te-tragonal to monoclinic) at the crack tip.1 This propertymakes it desirable for a wide range of industrial andmedical applications including bone, tooth, and jointimplants; however, it also makes the material very diffi-cult to machine in its final state. For applications requir-ing customized and/or high precision components, finalstate machining is often necessary. One example of this isfor the manufacture of dental restorations such as dentalcrowns and bridges. Such items provide a perfect exam-ple of mass customization where large quantities are to beproduced but each item is unique and subject to tightdesign tolerances. Typically, Y-TZP dental restorationsare machined from a sintered block of Y-TZP by grind-ing, using diamond-tipped tools. The process gives agood end result but is very time and tool intensive. Lasermachining represents a possible alternative process formachining this material, which could potentially providemuch more rapid processing times and avoid problems oftool wear. The material properties of Y-TZP, specificallyits high thermal expansion coefficient (10.3� 10�6/1C)and low thermal conductivity (2.2 W/(m K)),2 make it adifficult material to machine using laser processes (typicalcomparative values for alumina are 8� 10�6/1C and20 W/(m K), respectively3).

In previous work, our group has successfullydemonstrated laser machining of this material in themillisecond (ms) regime,4 nanosecond (ns) regime,5

and picosecond (ps) regime.6 Laser machining in thefemtosecond (fs) regime has also been investigated byother authors.7 In general, laser processes at longer pulsedurations can provide (relatively) high material removalrates (2.5 mm3/s for cutting in the ms regime4,8–10) andare suitable for cutting and drilling operations; however,the laser parameters need to be carefully chosen to avoiduncontrolled bulk fracture of the sample and the surfacequality is generally poor as the predominant material re-moval mechanism is melt-eject resulting in a layer of re-cast material on the surface and a significant heat-affectedzone (HAZ). By moving to shorter pulse durations, bettersurface quality and smaller feature size may be achievedbut at the expense of processing speed (2 mm3/min for nsprocessing5). Different laser processes may be combinedto optimize the benefits of each process.11

The ms-laser cutting process developed previously islimited to a maximum material thickness of 4.5 mm with amaterial removal rate of 2.5 mm3/s equating to a feed rate ofapproximately 0.8 mm/s for this thickness. Slightly higherremoval rates may be achieved with mechanical tooling12;however, such techniques are limited to straight cuts only.In this work, we demonstrate a high average power fiberlaser as a tool for bulk processing of Y-TZP blocks using anovel process relying on the controlled propagation ofcracks. Relatively high feed rates are demonstrated withthe ability to make contoured cuts that would not be pos-sible with traditional cutting techniques and in muchthicker sections of this material than has previously beenpossible with a laser. A finite-element model (FEM) is thenpresented to demonstrate the temperature and stress distri-bution within the bulk material during the cutting processand to provide an insight into the mechanics of the process.

Experimental Work

Laser Cutting

The laser used in this work was a YLR-1000-SMYtterbium fiber laser system manufactured by IPG. Thisis a continuous wave (CW) laser capable of deliveringup to 1 kW at 1075 nm. The output may be modulatedat up to 50 kHz with an arbitrary duty cycle. The fibercore diameter was 14 mm and a focusing optic of 7.5 in.(190.5 mm) focal length was used to give a beam waistdiameter of approximately 45 mm.

Figure 1 illustrates the experimental setup used formachining. Fiber-optic beam delivery is incorporated intothe laser system and this is mounted onto a fixed assemblycontaining focusing optics and a gas flow system. The gasflow serves both to protect the optics from debris and toassist with material removal from the work piece. Oxygenwas used as an assist gas at a pressure of 6 bar with a standoff of 2 mm between the nozzle tip and the work piece.Oxygen was chosen to reduce the oxygen depletion thatoccurs at high temperatures, which results in discolor-ation.13 The sample was mounted on precision translationstages synchronized with the laser to enable the sample tobe located and moved under the beam. This work focusedon processing blocks of dental-grade sintered Y-TZP. Bil-lets of dimensions 8� 18� 44 mm are the standard sizefrom which a dental bridge would be manufactured; thelargest size billet generally used is of dimensions13 mm� 18 mm� 44 mm. These two sizes of billetswere used in this work.

1278 International Journal of Applied Ceramic Technology—Parry, et al. Vol. 8, No. 6, 2011

Low initial absorption of Y-TZP at 1075 nm andthe granular nature of the material created an intermit-tent problem with back reflections to the laser (trigger-ing a safety cut-off switch). Coating billets with a thinlayer of graphite (colloidal spray) provided a consis-tently absorbing surface, which both solved this issueand significantly improved the consistency of results dueto improved initial absorption.

To start with, the capability of the laser to drillholes in the Y-TZP blocks was assessed in single pulseand later multiple pulse operation. The main aim, how-ever, was to produce consistent full thickness cuts.

Results and Discussion for Experimental Work

Drilling

The laser was found to easily drill through 13 mmthickness of material. The laser on-time was modulatedto find the shortest time required to penetrate this thick-ness. It is desirable to find the lowest parameters interms of power and pulse length to minimize the ther-mal impact on the material and reduce the formation ofcracks. By stepping the output directly (rather thanramping the intensity) to 400 W, it was found that aminimum laser-on-time of 26 ms was sufficient to con-sistently penetrate a thickness of 13 mm.

Holes drilled in the Y-TZP blocks were typically150mm in diameter, approximately three times largerthan the spot diameter at focus, with aspect ratios ofB95:1. An HAZ extended to approximately 150mminto the bulk material; this is apparent in Fig. 2A in whichradial cracks can be observed extending from the hole. InFig. 2B a layer of redeposited molten material is apparentas well as a layer of material that has suffered thermaldamage. The material removal process is melt-eject and isvery similar to that investigated previously for a ms-pulseduration laser process and a closer examination of the HAZis available in that work.4 Material removal rates comparewell to the Q-switched ms-pulse duration used in previouswork (of B13 mm3/s). Approximately 0.41 mm3 ofmaterial was removed in a 26 ms pulse creating an effec-tive material removal rate of 15.71 mm3/s.

Thicker sections of material were used to investigatethe laser penetration at greater depths by increasingthe laser-on-time in steps. As deeper holes are drilled,the efficiency of the process reduces, meaning that the

Fig. 2. ESEM images of fiber-laser-drilled hole in Y-TZP. (A) Cross-section taken at middepth. (B) Side view from fractured surface, note therecast material on the hole walls.

Fig. 1. Diagram of the experimental setup incorporating fiberlaser, focusing optics, gas flow apparatus, and precision stages.

www.ceramics.org/ACT Fiber-Laser Cutting of Yttria-Stabilized Zirconia 1279

relationship between pulse duration and hole depth isnot linear. The maximum depth achieved for a singlepulse was 15.4 mm by operating the laser at 1 kW for56 ms. Pulse durations longer than this did not producedeeper holes. It is believed that this is due to limitationsin expelling the molten material from the top of the holeand the interaction of the laser with this material. Forlong laser on-times, a cavity is formed around the holeas the laser energy is absorbed by melt in this regionwithout penetration to greater depths.

In general, minimizing crack formation depends onreducing the thermal impact on the material and thiscan be carried out by optimizing the laser process4,14 orby raising the temperature of the bulk material beforeprocessing,15 which is often impractical. One way tominimize the thermal impact is to drill with multiplepulses, which gives time for the material to be removedfrom the hole between pulses and reduces the averagepower used. This approach enabled deeper holes tobe machined without bulk cracking. Best results wereachieved for four laser pulses of 25 ms at 1 kW with a250 ms delay between pulses. These parameters consis-tently produced a through-hole in 18-mm-thick billets.Increasing the pulse duration or the number of pulsesfor thicker material sections produced only limitedincreases in depth and soon resulted in bulk fracturedue to excessive thermal shock.

Cutting

Initial attempts at cutting were made in the CWregime but proved unsuccessful due to the narrow cutwidth produced, which inhibited the removal of meltfrom the cut. The melt trapped in the cut tended toabsorb the laser light preventing full penetration of thebeam and creating a build up of heat within the cut thattypically caused the sample to shatter. However, movingto pulsed operation proved more successful and has leadto the development of a novel cutting process. Thisprocess involves drilling a series of through-holes in aline across the billet, where we found that for appropri-ately chosen parameters, cracks or fractures can be madeto propagate in a controlled way between holes. Thesefractures extend to the full thickness of the material andjoin adjacent holes, enabling full thickness cuts to bemade in Y-TZP of up to 13 mm thickness. This pro-vides a rapid and efficient cutting technique with min-imal heat build up in the bulk material, thus avoidinguncontrolled fracture.

In this novel process, holes were drilled using 400 Wfor 26 ms, as discussed previously, to give consistentthrough-holes. The off time between pulses and thehole separation within the material are critical to the suc-cess of the process. A 3.6% duty cycle (26 ms on, 694 msoff) was found to be most effective. This gives sufficienttime for the material to cool between laser pulses so as toavoid uncontrolled fracture, while maintaining sufficientpreheating to drive the fracture process on the followingpulse. Using these laser parameters, a maximum feed-ratefor successful cutting was shown to be 1.8 mm/s corre-sponding to a hole separation of 1.3 mm.

Directional variation is possible with this processas demonstrated by the shaped cut in Fig. 3. This isimportant as it means that profiled cuts are feasible.Limitations in term of maximum angular variation andcurvature have not currently been investigated fully.In terms of dimensional tolerances, the cut path iscontrollable to within 1 mm, including round bends;however, toward the end of the cut there is a tendencyfor the fracture to deviate slightly from the intendedpath by approximately 71 mm.

However, there does appear to be a minimum limiton the width of cut sections of approximately 3 mm.When machining within 3 mm of a surface parallel tothe cutting direction, there is a tendency for fractures topropagate toward the nearest edge rather than towardthe next laser-drilled hole. With the Y-TZP materialavailable for testing the choice of thicknesses was limitedto 8, 13, or 18 mm. As discussed previously, it waspossible to drill through holes in the 18-mm-thickmaterial by using multiple pulses (four pulses at 1 kW,25 ms on, 250 ms off). Using multiple pulses bothimproves material removal at large depths and reducesthe thermal impact of drilling; however, it was not

Fig. 3. Profile cut of 13-mm-thick Y-TZP using a 1 kW fiber laser.


possible to achieve controlled crack propagation anddevelop cuts by this technique.

FEM

To gain a better understanding of the mechanisms atwork in the crack propagation process, a thermally cou-pled transient FEM was developed using commerciallyavailable software (ABAQUS v6.8) to model the temper-ature and stress distribution within the bulk material dur-ing the laser process. This model aids an understanding ofthe minimum distribution of forces in the material viaa lower bound solution approach, and by comparingthese results to experimental observations, we can developa sensible hypothesis to explain the cutting by crack prop-agation mechanism. In this technique, the implementa-tion of Newton’s method involves a nonsymmetricJacobian matrix as is illustrated in the following matrixrepresentation of the coupled equations

Kuu Kuy

Kyu Kyy

� �DuDy

� �¼ Ru

Ry

� �ð1Þ

where Du and Dy are the respective corrections to the in-cremental displacement and temperature, Kij are submatri-ces of the fully coupled Jacobian matrix, and Ru and Ry arethe mechanical and thermal residual vectors, respectively.

The nonsymmetric Jacobian matrix originates fromthe basic constitutive equations of force (f) and heat flux(q) equilibrium at a given time increment in a coupledanalysis (Eqs. (2) and (3)), where both temperature andmechanical strain are dependent upon each other.

ff g ¼ fKuu;uygfDug ð2Þ

fqg ¼ fK ;yy;yu gfDyg ð3Þ

The basic energy balance is calculated via the fol-lowing equation:Z

Vr _UdV ¼

ZS

qdS þZ

VrdV ð4Þ

where V is a volume of solid material, with surface areaS; r is the density of the material; _U is the material timerate of the internal energy; q is the heat flux per unit areaof the body, flowing into the body; and r is the heatsupplied externally into the body per unit volume.

It is assumed that the thermal and mechanicalproblems are coupled in the sense that U is a functionof Du and Dy, where y is the temperature of the material,

and q and r depend on the strains or displacements ofthe body. ABAQUS solves this dependency integrationof temperature using a backward-difference scheme, andthe nonlinear coupled system is solved using Newton’smethod. Finally, the resulting temperatures and dis-placements are solved to calculate strains and stresses viatemperature-dependent material parameters.

Boundary conditions are specified as prescribed tem-perature y5 y(x, � t). A variational statement of the energybalance, Eq. (4), together with the Fourier law is obtaineddirectly by the standard Galerkin approach as:Z

Vr _UdydV þ

ZV

›dy›X

k›dy›X

dV

¼Z

VdyrdV þ

ZSqdyqdS ð5Þ

where dy is an arbitrary variational field satisfying the es-sential boundary conditions. The body is approximatedgeometrically with finite elements, and hence the temper-ature is interpolated as

y ¼ NNðX ÞyNN ¼ 1; 2; . . . ð6Þ

where yN are nodal temperatures. The Galerkin approachassumes that dy, the variational field, is interpolated bythe same functions

dy ¼ NNdyN ð7Þ

Additional boundary conditions of surface film torepresent cooling caused by the assist gas and mechan-ical conditions of symmetry are also added to the modelusing similar techniques of force and energy balance.

The heat flux into the bulk material during the laserpulse is not trivial to determine. Y-TZP is normally trans-parent at the wavelength used (1064 nm), however,becomes strongly absorbing once the intensity becomessufficiently high to cause optical breakdown. Of theoptical energy that is absorbed the majority contributesto localized heating, leading to melting and ejection of thematerial, which carries away much of the absorbed energy.

Here, it is useful to consider what is occurring asthe hole is drilled. By considering work published onlaser drilling of metals,16,17 we can infer that the pro-cesses that occur during the drilling of the hole are asfollows: (i) After initiation of the drilling process, thelaser beam interacts with molten material at the bottomof the hole, and the resultant heat flux causes furthermelting of the surrounding material. The hole diametersobserved in ‘‘Drilling’’ (Fig. 2) are approximately three


times the diameter of the laser beam, demonstrating thatsufficient heat is conducted to the surrounding materialto cause melting to this depth. (ii) Given a Gaussianbeam profile the intensity is higher in the center of thebeam, and as a result the heating effect in the center ofthe hole is greater resulting in material vaporization.This vapor exerts a force on the melt causing it to ejectin a planar fashion up the sides of the hole, and thethickness of this melt is essentially determined by thespeed at which it is ejected (which decreases as the depthincreases and the drilling rate reduces). When the laserpulse stops so does the pressure driving the melt out ofthe hole and the melt solidifies on the walls in a shorttime scale (� 1 ms in the case of a metal).

As there are too many unknown variables, it is notpractical to make a reliable calculation of the proportionof energy within the laser beam that contributes to aheat flux into the bulk material. In particular, a largeand not well-defined proportion of the beam energyprovides melting and vaporization of material within thehole and causes this material to be ejected from the sys-tem, the ejection velocity, temperature distribution, andproportion of melt to vapor cannot easily be quantified.It was therefore decided to instead work with a lowerbound solution based on the heat transfer between themelt leaving the hole and the bulk material. From theimage shown in Fig. 2B, it is apparent that the recastmolten layer has a thickness of approximately 25 mm. Byassuming that the melt has a minimum temperatureequal to the melting temperature of Y-TZP (27001C2),we can estimate a lower limit for the rate of heat transfer

into the bulk material. For simplicity the motion of thismolten layer is not considered but it is assumed thatthere is a layer of molten material on the surface of thehole continuously throughout the 26 ms period inwhich the laser beam is incident on the material.

To summarize, the boundary conditions for themodel a block of Y-TZP was considered, initially atroom temperature and surrounded by air also at roomtemperature. The laser-drilled holes were already in placein this structure; this is a simplification that greatlyreduces computation time and avoids unnecessarycomplications related to modeling the interaction ofthe laser with the material and the associated materialremoval mechanisms; these interactions are not essentialfor a lower bound solution. It is the temperature andassociated stress field in the bulk material that is evaluatedfrom the temperature of molten material lining the wallsof drilled holes that is of interest and not the drillingprocess itself,. Heat was applied by setting the tempera-ture of a 25mm region around the circumference of thehole in 26 steps at 1 ms time intervals from top to bot-tom to represent the molten material leaving the hole.The height of each step was shorter than the last in ac-cordance with experimental observations of the depth ofholes drilled for different pulse durations. Illustrations ofthe starting geometries are shown in Fig. 4.

Temperature Measurements

To validate the boundary conditions chosen for themodel, experiments were conducted to measure the

Fig. 4. Starting point for model, not drawn to scale. (A) Axisymmetric model with inset showing 25 mm layer, which was set to 27001C inprogressive steps down to full depth at 26 ms. The heating depth at 1 and 10 ms is illustrated. (B) Three-dimensional model showing first andsecond drilled holes.


temperature distribution within the material after thelaser process. Holes were drilled with diamond-tippedtooling to approximately 2 mm diameter at middepth inthe samples; this is the smallest practical hole diameterthat can be achieved in this material with conventionaltooling. K-type thermocouples where chosen as theyoffered a good compromise between speed of response,temperature range, and price (most were destroyed duringthe experiment). These were inserted into the holes andheld in place using an acrylic resin, see Fig. 5. Thermalcontact was tested by heating the sample in a water bathand monitoring the thermocouple response, the thermo-couple time constant is expected to be around 3 ms.

The temperatures measured by the thermocoupleswere recorded as holes were drilled in the blocks. Thelocations of the thermocouples were only known approx-imately at this stage. Holes were drilled starting at ap-proximately 5 mm from the thermocouple and graduallymoved closer reducing the step size according to the mag-nitude of the response recorded on the thermocouple.Sufficient time was allowed between shots for the materialto cool. In all cases, these temperatures were recorded720 ms after the start of the pulse. This reduces the impactof errors due to the response time of the thermocoupleand the sampling rate of the data logging, as the measuredtemperature is both lower and varies slowly at this timescale compared with during or shortly after the pulse. Thesamples were sectioned after the experiment with thethermocouples in place. To measure the radial distancebetween the hole and the thermocouple tip the cross-sec-tions were imaged with a thermal source and camera usinga novel infra-red imaging technique described in Matysiaket al.18 The measured temperature data is shown in Fig. 6.The scatter of data becomes quite large at small radialdistances from the thermocouple. This is likely, at least inpart, to result from the large size of the hole, which had tobe drilled to accommodate the thermocouple, this may

cause a distortion in the flow of heat within the block atsmall distances. There may also be some variability in thethermal contact between the thermocouple and the ce-ramic. Although this was tested for each thermocouple,any small variation in contact will become more signifi-cant when measuring fast transient temperature changesand indeed the B3 ms response time may not be suffi-cient to track the rapid and large magnitude temperaturechanges that occur at very short distances. A thermalpaste that might have improved the thermal contact wasnot used due to concerns of this boiling or vaporizingat high temperatures. In Fig. 6 these data are comparedwith the FEM data, which are described in the nextsection. While these data are limited in precision, it canbe used to give a general indication of the temperaturerange used in the model, and any variation in the actualmagnitude of the temperatures involved will likely leadto an error in the magnitude of associated stresses withinthe material; however, the error in the shape of thepredicted stress field will be less significant and this isthe property of most interest.

Axisymmetric Temperature Distribution Model

Initially, an axisymmetric model was used to vali-date the temperature distribution resulting from a singlehole. Here, a cylindrical Y-TZP specimen was consid-ered with a height of 13 mm and a diameter of 10 mmand containing a full depth hole of radius 100 mm in thecenter as illustrated in Fig. 4A. For simplicity, the hole

Fig. 5. Location of thermocouple within sample with respect tolocation of drilled holes.

Fig. 6. Temperature of bulk material at middepth as a function ofdistance from the drilled hole, 722 ms after start of laser pulse.Measured and modeled data are shown.


was included from the start as discussed above, as it isthe thermal input into the bulk material that is ofinterest, not the drilling process itself. Heat was intro-duced by setting the temperature of a 25 mm region (seeinset in Fig. 4A) around the circumference of the hole to27001C in steps of height at 1 ms intervals to representthe molten material leaving the hole and to give a lowerbound solution, as discussed previously. For the timescale associated with the drilling process, and the heatinput at the drilled hole for the lower bound solution,the diameter of 10 mm was found to be sufficient tomodel this process without modifying the temperatureboundary conditions on the outside surface of the spec-imen. The mesh density for the four-noded quadrilat-eral axisymmetric elements varied from 13 mm near thehole to 50 mm at the outside surface to ensure conver-gence. The depth at 1 ms and 10 ms are illustrated inFig. 4A; at 26 ms, the full depth was achieved. Thetemperature-dependent material properties data2,19–23

that were used in the model are shown in Fig. 7. Latentheats associated with phase changes at 12001C and27001C were included to mimic the associated thermalloading, however, the phase changes themselves werenot modeled. After the heating period, a second coolingstage was modeled lasting 696 ms to give the tempera-ture distribution 722 ms after the start of the laser pulse,in the instant before the second laser pulse.

The resultant temperature profile at middepth fromthis model is compared with the experimental temper-

ature data in Fig. 6. Despite the scatter of the experi-mental data, there is a reasonable agreement between theexperiment and the temperature profile predicted in theaxisymmetric model, which suggests that the assump-tions made regarding heat transfer into the bulk materialare reasonable.

Three-Dimensional Model

Having validated the temperature input into thesystem, the same approach was used to apply heat to athree-dimensional model to predict the thermal-in-duced stress fields that evolve as adjacent holes aredrilled to mimic the cutting procedure. By consideringthe geometric symmetry only a section of the model wasrequired, thus reducing computation time. For the samereason the laser-drilled holes were included at the startof the simulation as illustrated in Fig. 4B. Owing tosymmetry, only a quarter section was modeled and thetemperature and displacement boundary conditionswere applied to mimic the full model. The temperatureboundary conditions were varied in each time step onthe basis of the axisymmetric model as described above.The mesh density for three-dimensional elements variedfrom 12.5 to 80 mm between the two drilled holes dueto the use of eight-noded brick elements (instead of thefour-noded elements used in the axisymmetric model).

The progression of the 3D model was divided intofour time steps. The first step involved a thermal inputconsistent with drilling of the first hole. This was carriedout in exactly the same manner as described for the axi-symmetric model involving setting the temperature of themelt within the hole. In the second step, the system wasallowed to cool for a period of 696 ms. The temperatureprofile generated at the end of this step is plotted in Fig. 6for comparison to the axisymmetric model, and it can beseen that the temperature distribution in each case is verysimilar. Thirdly, the thermal input representing the drill-ing of the second hole was introduced, while allowing thefirst hole to continue cooling. In the final step, the systemwas allowed to cool over an extended time period.

Results and Discussion for FEM and CuttingMechanism

Results showing the thermally induced stress dis-tribution during the drilling of the second hole areshown in Fig. 8. For consistency in this section, the laseris considered to move with respect to the sample, always

Fig. 7. Temperature-dependent data used in model.


drilling the first hole on the left then moving to the rightto drill subsequent holes. The thermal input represent-ing the second laser pulse has reached approximatelyhalf depth at the stage depicted in Fig. 8, although forsimplicity the holes are already in place. We can con-sider that the second hole has been drilled to half depth.Figure 8A shows the thermally induced shear stress dis-tribution (S12) in the plane of the page. There is a regionof high shear stress surrounding the hole at the limit ofthe thermal input (i.e., the bottom of the hole) with apeak magnitude in the region of 420 MPa. As themodel, is run this high-stress region propagates downthe hole following the drilling front. Figure 8B showstensile stress (S33) in the plane perpendicular to the pageat the same time. In this plot, any regions that are incompression are shown in black. There is a region oftensile stress of around 200–300 MPa extending to theright, away from the second hole. A region of compres-sive stress of up to 125 MPa in magnitude is presentextending to the left toward the first hole and this movesdown as the hole is being drilled. The full-length sectionof the model is shown in Fig. 8B and peaks in the stressdistribution can be seen at the entrance to the hole. The

shear and tensile stress fields are generated due to theresidual stress caused by the fast quenching of the materialand by the constraint of material expansion by the neigh-boring material. These stress fields are therefore depen-dent upon the temperature history at a given time, forexample in Fig. 8B the temperature history from the firsthole influences the stress distribution when the neighbor-ing (second) hole is drilled leading to compressive andtensile stresses on either sides of the hole.

Figure 9 shows a close up of a cut surface, againwith the laser drilling holes from left to right. An HAZis apparent at the edges of each hole. Cracks are visibleextending away from the holes but always down and tothe right, toward the next hole to be drilled. Thesecracks generally do not extend the full distance betweenthe holes and the remainder of the space between holesis characterized by a smooth cleaved region.

An HAZ of approximately 150 mm depth sur-rounds the hole. This is typical of laser processes ingeneral and within this region are a large number offractures, which provide initiation points for largercracks. To obtain an indication for the forces requiredto expand a crack, we can consider the fracture strength

Fig. 8. Calculated temperature-induced stress fields during drilling of second hole. These pictures are zoomed in to the drill front, which at thispoint is halfway through the block. (A) Shows the shear stress distribution (S12) in the plane of the page. (B) Shows the tensile stress distribution(S33) in the plane normal to the page. Solid white arrows indicate the direction in which cracks are driven by tensile stress. The broken arrowindicates that crack propagation is inhibited in that direction due to compressive stress. The full length of the model is also shown here for perspective.


of the material. In general, for a ceramic the fracturestrength is inversely proportional to the root of theincipient flaw size.24 The flexural strength for thismaterial is published as 1240 MPa,2 however, as anexample in Parry et al.6 samples of a similar materialthat had been laser machined with an ns laser and wereconsistently found to have surface defects on the scale ofa few mm were found to have a flexural strength value,so of 892 MPa. The defects found within the HAZ aretypically several tens of microns in length and henceforce required to propagate a crack will be significantlylower than the published material strength.

In the model simulations, the large shear stressesobserved to follow the drilling front (Fig. 8A) were seento peak at forces around 420 MPa; this stress field prop-agates the full depth of the material within the 26 mstimeframe and consequently there will be a very highrate of strain around the hole. The actual forces gener-ated in the bulk material may in fact be higher than thisas the assumptions made regarding the heat input werebased on a lower bound solution. This stress is sufficientto extend a small proportion of the micro cracks presentin the HAZ into macro cracks. The location of this peakshear force, at the bottom of the hole and following thedrilling front is consistent with the observation thatthese macro cracks surrounding the hole are drivendownward. However, to explain why the macro cracksare only observed to propagate in the forward direction(Fig. 9), it is necessary to also consider the role of tensileand compressive stress fields on both sides of the hole ata given depth of drill. Considering the tensile stressdistribution illustrated in Fig. 8B the low-magnitudetensile region to the right of the hole is possibly not ofsufficient magnitude to drive cracks in itself but would

support the propagation into this region of cracksgenerated in shear. The compressive stress field to theleft of the hole will impede the propagation of cracks inthe direction of the first hole. Although the forces to theleft of the hole later become tensile as the drilling frontprogresses, this occurs after the high shear forces havesubsided in this region.

As seen in Fig. 9A, the cracks do not typically prop-agate the full distance between holes and the material re-mains in one piece throughout the drilling process. Whilethese cracks form during the individual drills, the materialis not able to fully separate, despite the region being intension, as it is held in place by the larger bulk of thesample. Toward the end of, or shortly after, the drillingprocess the part separates completely as structural integrityis lost across the cut line and the fractures join up. Figure9B shows a part of a sample toward the end of the cut.The part is starting to separate as the last holes are beingdrilled and the separation is only loosely guided by the lasttwo holes, which cannot be seen in this image as they werebypassed by the fracture. It is possible to see some bleed-ing of melt out near the bottom of the last visible holes,circled in the figure. In order for this leakage to occur, thelarge-scale bulk fracture must have been present at an ini-tial but incomplete stage at the base of these holes whilethey were drilled. Complete separation occurred at somestage during the drilling of the last two (subsurface) holes.

As shown in Fig. 3, it is possible to guide thesecracks around curves or corners. Another example isshown in Fig. 10, where again the laser was movingfrom left to right. It is clear in this case that while thedriving cracks that occur during the drilling of the holepropagate forward, the final bulk fracture is guidedalong the path of the line of holes and must occur on

Fig. 9. Sections of fractured surface after laser processing; in each case, the laser was travelling from left to right. (A) Shows an area at bothmiddepth and midlength of the cut, the arrow indicating the crack propagation direction. (B) Shows an area toward the end of the cut, tworegions where melt has leaked from the holes before fracture are circled.


a longer time scale after the holes are drilled. Betweenholes 1 and 3, the line of the fracture deviates slightlyfrom the line through the hole centers, passing the edge ofhole 2 then turning a sharp corner close to hole 3, essen-tially following the line of least resistance to join the twofracture lines preceding hole 2 and proceeding hole 3. Atthis later stage, the joining up of the fracture may in somecases be a bidirectional process, which would help to ex-plain this sharp angle between holes 2 and 3.

It is apparent from this work that, for a successfulcut, the parameter set used is of critical importance.

Firstly, it is essential to drill full depth holes with singlepulses, something that is made possible due to the highbrightness and good beam quality of the fiber laser. Sec-ondly, appropriate separation of holes in both space andtime is important. For a separation in time or distancebetween holes that is too small, uncontrolled fracture willoccur due to excessive heat build up. If the separationbetween the holes is too large, then the stresses inducedwithin the bulk material will be insufficient to cause sep-aration and the influence of the second hole will be tooremote to successfully steer or guide the crack.

Clearly, the surface quality and the limit of precisionof the cuts achieved using this technique means that somepost processing is required. In particular, some undesiredcracking is present surrounding the drilled holes, while itmay be possible to further reduce the crack depth aroundthe holes, as discussed in ‘‘Drilling’’; this will likely reducethe effectiveness of the process of cutting by crack prop-agation. This was found to be the case when attempting tocut the 18-mm-thick section using multiple pulses perhole. However, the value of the process lies in the speed atwhich shaped parts can be produced from a relatively large(thick) billet of material to achieve a rough approximationto the final desired shape. A finer scale but slowermachining process, for example using a ps laser,6 canthen be used to achieve the final shape.

Conclusion

The use of a fiber laser has been evaluated for pro-cessing Y-TZP in its final state. Laser drilled throughholes have been demonstrated for a thickness of 18 mmand a novel cutting process has been developed relyingon controlled crack propagation between laser drilledholes. Profile cuts have been demonstrated through 13-mm-thick Y-TZP (compared with 4.5 mm reportedpreviously4). Both empirical evidence and the result ofa FEM indicate that cracks propagate forward from alaser-drilled hole in the direction of the cut. Thesecracks arise in the HAZ surrounding the hole and aredriven by the large shear stress occurring near the bot-tom of the hole as it is drilled. The distribution ofcompressive and tensile stress fields in the surroundingmaterial determines that the driving cracks only prop-agate in a forward direction. During the drilling process,the ceramic is held together due to the bulk of materialahead of the cut line. As the line of drilled holes reachesthe end of the sample, structural integrity is lost and thesample separates along the line of the drilled holes.

Fig. 10. Machining round a corner. (A) Image of the cut surface.(B) Top view of the cut. Both images are at the same scale and thecorresponding holes are numbered.


Compared with other techniques for cutting thismaterial in its final state, this laser process is significantlyfaster than any mechanical technique available whilegiving directional control and cuts through substantiallygreater thicknesses than alternative laser processes. Thelarge cut thickness is made possible due to the minimalthermal input involved in drilling and propagatingcracks. A conventional approach of removing materialacross the whole cut line would deliver substantiallymore heat to the bulk material and for such thicknesseswould lead to catastrophic bulk fracture. While the sur-face finish is of limited quality and undesired crackingstill exists round the drill holes, a two-stage processcould be envisaged in which the fiber laser is used forrapid profile cutting and a secondary process (short-pulsed laser6 or mechanical) is used for finishing andfine detail. Advantages in machining time comparedwith an all-mechanical process would still be substantial.

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http://www.coorstek.com/materials/ceramics/zirconia/ytzp.asp

http://www.coorstek.com/materials/ceramics/zirconia/ytzp.asp

10.1111/j.1744-7402.2009.02420.x

10.1111/j.1744-7402.2009.02418.x

10.1111/j.1744-7402.2009.02418.x

Documents

A Fiber-Laser Process for Cutting Thick Yttria-Stabilized Zirconia: Application and Modeling