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All rights reserved.Keywords:CO2; Laser cutting; Ceramic materials; Advanced manufacturing processes1. Introduction and backgroundManual methods of cutting ceramic tiles are verysimilar to that for glass, i.e. scribing the materials withtungsten-carbide tipped cutter, followed by the applica-tion of a bending moment along the scribed line toinitiate controlled fracture. However, manual tech-niques are limited to straight-line cutting and relativelylarge-radius cuts. Internal and undercut profiles arenearly impossible to produce with scoring alone (withthe possible exception of internal circles); more sophis-ticated methods having to be applied to achieve theseprofiles.Traditionally,diamond-saw,hydrodynamic(water jet) or ultrasonic machining are used to createcomplex geometries in ceramic tiles, but these processesare very time consuming and expensive. For example,typical diamond-saw cutting speeds are in the order of20 mm min11, while ultrasonic drilling of Al2O3takes over 30 s per hole 2.The most critical factor arising from use of a CO2laser to cut ceramic tiles is crack damage, which isessentially caused by a high temperature gradientwithin the ceramic substrate during the cutting process.These cracks reduce the strength and are sources forcritical crack growth, which may result in partial orcomplete failure of the tile substrate 3. Thus a reduc-tion of process-induced crack formation is paramountfor the realistic commercial use of lasers to cut ceramictiles.2. Laser cutting parametersLaser machining of any material is a complex processinvolving many different parameters that which all needto work in consort to produce a quality machiningoperation 4, parameters such as: (i) laser power input;(ii) focal setting; (iii) assist gas type and pressure;(iv) nozzle configuration; (v) workpiece thickness; and(vi) optophysical properties.Previous research within the authors department1,5,6 has also demonstrated the criticality of the aboveparameters in efficient laser cutting.2.1.Laser powerLaser power depends on the type of laser used. Forthe work reported in this paper, a Ferranti MF400CNC laser cutter was employed, rated at a poweroutput of 400 W. However, due to upgrading, themaximum beam power achievable was between 520 and* Correspondingauthor.Fax:+441314513129;e-mail:i.blackhw.ac.uk0924-0136/98/$ - see front matter 1998 Elsevier Science S.A. All rights reserved.PIIS0924-0136(98)00078-8I. Black et al./Journal of Materials Processing Technology84 (1998) 475548530 W in continuous wave (CW) cutting mode. Thelaser also had the ability to work in pulse mode (PM)and super-pulse mode (SPM; Fig. 1). To determine theequivalent power output during pulsing operation, apower verses pulsing chart was used in conjunctionwith the following basic equation 9:Pr=Pl/Psf=1/(Pl+Pr)Although the laser cutter could operate between fre-quencies of 50 and 5000 Hz, a value of 500 Hz wasrecommended in previous work 1,5. Since this settingproved to be successful, only limited investigation intoother frequencies was carried out (at 250 Hz, 750 and100 Hz).2.2.Cutting speedThe CNC table used with the Ferranti MF400 lasercutter had a maximum feed rate of 10000 mm min1.Previous work 6 indicated that feed rates above 6000mm min1proved to be unstable for any standardisedtesting. The optimum cutting speed varied with thepower setting and, more importantly, with the thicknessof the workpiece.2.3.Shield gas type and pressureCompressed air, argon, nitrogen and oxygen wereused as shield gases during cutting, with pmax:4 bar.Different shield gases were used to examined their effecton cut quality after processing, since the shield gas notonly cools and cut edges and removes molten material,but also generates a chemical reaction with the sub-strate material 7. The results of this chemical reactiondiffer for each type of shield gas used. For test purposesp was varied in steps of 0.5 bar from 1 to 2.5 bar, thenin steps of 0.2 bar from 2.6 bar to the maximumattainable gas pressure.2.4.Nozzle configurationThe nozzle diameter contributes directly to the maxi-mum achievable gas pressure and hence to the massflow rate of the gas was important for the economics ofcutting, especially when using cylinders of argon andnitrogen. Only circular profiles for the nozzle exits wereavailable (0.6 mm5Ns520 mm), but this uniformnozzle exit geometry allowed cutting in any direction.2.5.Nozzle height and focal positioningThe height at which the nozzle was set was governedby the position of the focal point. The Ferranti MF400laser cutter only possessed a long focal length of 110Fig. 1. Cutting modes.mm (originally a short focal length of 46 mm wasavailable before upgrading) and this length could bealtered by 95 mm. If the nozzle height was incorrectlyset the beam would clip the nozzle and reduce theequivalent power output to the workpiece 6. For thebulk of the testing the focal height was set so the focalpoint was on the job, i.e. on the top surface of theworkpiece. This condition obviously governed the posi-tion of the nozzle above the workpiece.3. Experimental procedureSix types of Si/Al2O3-based ceramic tiles were exam-ined (Table 1), originating from different countries.Note that the composition of the tiles varied, as did thethickness, but all possessed a surface glaze and in thecase of the 7.5, 8.6 and 9.2 mm Spanish tiles the glazewas double layered.3.1.Set-up procedureSince there was a need for standard testing condi-tions, the following procedure was implemented beforethe start of testing: (i) the beam power was validated tospecification, i.e. 520530 W developed at full power(CW), although this dropped to around 50 W afterTable 1Types of ceramic tile usedts(mm)Tile typeBody colour3.7BrazilianWhite4.7WhitePeruvianLight redItalian5.2SpanishRed5.74Spanish7.5RedRedSpanish8.69.2RedSpanishI. Black et al./Journal of Materials Processing Technology84 (1998) 475549about 1 h of testing; (ii) the nozzle and the focal lenswere checked to ensure that they were in good condi-tion, i.e. clean and undamaged; (iii) the shield gaspressure regulator and shield gas tanks were turned onto prevent damage to the focal lens; (iv) the laser beamwas centred within the nozzle using a square test, alower energy input in PM being used to cut a square ona mild steel, the sparking density that resulted fromcutting being checked to see if it was equally distributedabout the cut line; and (v) the focal point was set for itsdesired positioning, i.e. on the job.3.2.TestingA straight-line test (SLT) was used to evaluate thevariablelaserparametersforfullthrough-cutting(FTC). Angular cutting was configured to investigatehow the material reacted during cutting of tight geome-try. Circular testing and square testing were devised todetermine the effects resulting from cutting variousgeometries.The SLT allowed for the combined testing of twoseparate parameters on one testpiece, upon completionthe results being present automatically in a cuttingmatrix in the form of the resulting cuts. P and V arethe most important laser parameters, as they dictate theamount of energy input per unit length of cut, thereforethey were paired for the SLT, as were p and NSwhichgovern the mass flow rate of the shield gas.For the P/V test runs, the power was held constantwhile the cutting speed was increased along the cut(Fig. 2(a). The length of cut at constant cutting speedhad to be of sufficient magnitude to accommodate theacceleration or deceleration of the CNC table betweenfeed changes: previous work 6 indicated that 50 mmwas adequate. Interpreting the results was made easierdue to their tabular format, with the cutting matrixshowing clearly any trends or patterns occurring due tothe changes in parameter settings. The SLT also al-lowed a large number of cuts to be carried out over ashort time-frame. This proved advantageous, as thelaser tended to drift from its initial settings with time.Precautions had to taken to avoid localised heating inthe tile from continuous close proximity cutting, as achange in tile body temperature would invalidate anyresulting data. Initially, a 20 mm separation betweencuts was used and this proved sufficient. In order tostudy how close the cuts could be made to each other,the separation between cuts was reduced by incrementsof 2 mm from an initial 20 mm spacing.During the SLT the other laser parameters had to beheld constant 6. For P versus V, f was held at 500 Hzwith NS=1.2 mm and p=3 bar. The beam focal pointremained on the job. The results from the P/V cuttingmatrix determined the fixed values for the cutting speedand pulse settings for the succeeding SLT. For the NS/pFig. 2. Testing configuration: (a) straight-line testing; (b) angulartesting; (c) circular testing; (d) square testing.cutting matrix, the nozzle size remained constant alongthe x-axis (refer to Fig. 2(a) while p was increased insteps of 0.2 bar from 2 bar in the y-axis (the cutseparation remained constant at 20 mm). A new matrixwas created subsequently for each nozzle size.Angular testing (Fig. 2(b) was used to investigatehow the cut material reacted to sustained exposurefrom the laser beam during the machining of tightgeometries (i.e. where several cuts are made in closeproximity to each other). The proximity test mentionedfor SLT determines how close parallel lines can be cutto each other, whereas angular testing is used to deter-mine how the cutting of acute angles effects the cutquality. The angles cut from a workpiece were reducedfrom 45 to 10 and the corresponding surface finishquality (SFQ) was noted.I. Black et al./Journal of Materials Processing Technology84 (1998) 475550Table 2Multi-pass cutting parametersPlCutting modePsNo. of passesLast cutCW60FTC9000100SPMFTC100Table 3Grading of SFQGrading1No cracking in surface glaze, solid sharp cut edgeMinimal glaze cracking (WcB2 mm) with slight2loss of sharpness in cut edgeMedium cracking (2 mmBWcB4 mm) and slight3damage to unglazed tile substrateSignificant damage to glaze coating (Wc6 mm),4heavy damage to unglazed substrate causing flakingin the glazed surface5Same as 4 but with the formation of cracks in thetiles main body leading to structural failure in apart of the tile (usually at the end of a cut orwithin 8 mm of the tile edge).There are two reasons for conducting square andcircular testing (Fig. 2(c) and (d): first, to determinethe optimum method of laser-beam introduction tointernal cut profiles; and secondly, to determine if therewas any limitation in the dimension of the size ofsquare or hole cut. If not correctly introduced, the laserbeam would cause an internally-cut profile to fail at thepoint of introduction, due to the brief but excessivethermal gradient induced from cutting (i.e. thermalshock). Therefore, utilising methods of beam introduc-tion, such as trepanning, onto a profile enabled com-plex geometries to be investigated. What also becameapparent during testing was the importance of theposition of beam extraction from the cut profile and theposition of the beam starting point relative to thegeometry, i.e. whether it was at a corner or on astraight edge.3.3.Multi-pass and underwater cuttingMulti-pass cutting was begun with a low power(P=100 W) laser beam. The first pass produced a welldefined blind kerf in the substrate, followed by a secondpass to cut deeper and so on. The process was repeateduntil the kerf was about 20 mm deep and then the laserpower was switched to 500 W and do the final FTC.The objective of multi-pass cutting was to reduce ther-mal overload by use of less input energy per unitlength. The parameters used in this test are given inTable 2.Underwater cutting was conducted with the objectiveof reducing the influence of heat around the cut areaand also to examined the effect on cut quality throughaccelerated heat dissipation using water 8. The ce-ramic tile was placed under water and the nozzle wasalso dipped in water, the shield gas pressure preventingany water from entering the nozzle jet chamber.4. Cut qualityMaterial properties, laser parameters and workpiecegeometry have a significant effect on the final result ofthe laser cutting process. Cut quality is essentially char-acterisedbysurfaceroughnessanddrossheight,whereas crack length dictates the strength reduction inthe substrate (Fig. 3). The overall SFQ at the glazesurface was classified according to the grading scalegiven in Table 3. Therefore, the quality of the cutsurface and edge were measured with respected to:(i) surface roughness; (ii) surface finish and; (iii) drossadherence.Fig. 3. Quality criteria for the laser cutting of ceramic tiles.I. Black et al./Journal of Materials Processing Technology84 (1998) 475551Fig. 4. Measurement of Rafor the cut surface.4.1.Surface roughnessIt was important to measure surface roughness asthis allowed the cut quality to be gauged alongsidevalues obtained from previous work 1 and valuesrecorded for other manufacturing processes. Due to thelarge number of cuts being made it was necessary toreduce the number of cuts to be analysed. Thereforecuts with SFQB2 were not measured.The surface roughness of the cut edge was character-ised by the formation of striation lines left by thecutting process Ravalues were measured from thecentre-line of the cut edge (Fig. 4). Measurements weretaken over a 12.5 mm traverse of the stylus with acut-off value of 2.5 mm, i.e. five readings were takenover the traverse, which ensured that the stylus trav-elled over a reasonable number of striation lines.4.2.Dross adherenceDross adherence directly effected the Ravalue of thecut and the ability to remove internally-cut geometries.A micrometer was used to measure the dross height atthree intervals along the cut section. The dross heightremained fairly constant (approximately 1 mm) with alltypes of cutting. Since this value was deemed to be ofno practical importance, it was not recorded in thedatabase.5. ResultsTable 4 contains the current LBM database for cut-ting ceramic tiles that was compiled from the results ofthe work reported in this paper. The first part of thetable contains the parameters and results for substratescut in atmosphere, while the results for underwatercutting are shown in the second part.5.1.Parameter effects5.1.1.Cutting speedFor the thinner tiles (tsB7 mm) the P/V cuttingmatrix showed a wide region of FTC with SFQ=1. Inthe case of the Brazilian tile (ts=3.7 mm) FTC wasobtained with cutting speeds of up to 2200 mm min1and down to Pr=0.5 (with reduced speeds) at f=500Hz. This region diminished with the increase in tilethickness and also with the redness of the body colour(generally the thicker tiles are darker in body colour).Fig. 5 shows how the maximum cutting speed for FTCvaries with ts. The exponential relationship obtainedconcurs with previous work 6 for different materialssuch as steel, wood and perspex. The cutting matrixalso showed that once the cutting speed exceeded valuesfor attainable FTC, scribing or blind cutting results.5.1.2.PulsingPulsing the laser for all but the thick Spanish tileswas not required, as the CW setting produced cuts witha good SFQ grading. Successful FTC was obtained atPr0.4 in the Brazilian tile, but Vmaxwas so low that,in a practical sense, the settings were not viable. On thethick Spanish tiles pulsing of the beam was required, asCW caused cracking in the glaze. This was probablydue to an excess of energy input per unit length of cutcausing thermal shock as the thermal expansion rate ofthe glaze differed sufficiently from that of the parenttile. Since pulsing the laser reduced the energy input byapproximately 25 W for every 0.1 drop in Pr at f=500Hz, the surface glaze cracking virtually disappeared atPr=0.6 and optimum cutting speed, although tinycracks (of the order of 0.5 mm wide) at the cut edge stillremained.5.1.3.Gas pressureThis parameter has a great effect on the quality andthe rate at which cuts could be made successfully.Previous work 2 had shown that high gas pressureswere required to achieve FTC on thick substrates (ts7 mm). This was borne out by the results obtained fromthep/Nscuttingmatrix.Highqualitycutswereachieved in the thinner tiles (tsB6 mm) at gas pressuresof 2 bar but in the double-glazed, thicker tiles values ofSFQB3 were not achieved unless p3 bar. At lowpressures (pB2.5 bar), the maximum cutting speeds forFTC dropped drastically, as the gas failed in its role ofdross clearer. Vmaxfor Brazilian tile in CW droppedfrom 2200 mm min1at p=3.8 bar to 1500 mm min1at p=3 bar. An increase in surface-glaze cracking alsobecame apparent at low gas pressures. This led to theconclusion that the shield gas was acting as a coolantand thus helping to minimise the large thermal gradientcreated by the beam.I. Black et al./Journal of Materials Processing Technology84 (1998) 475552Table 44 LBM database for ceramic tilesAtmospheric cuttingRa(mm)V (mm/min)Tile typets(mm)BodyShield gasSFQGlaze typeNs(mm)Geometricp (bar)Pl-Pscutcolour1.21.51Brazilian3.7WhiteWhiteStraightCW5001000C. air32535253511.21.53180205001000C. air31.21.51253516040500900C. air253511.21.53Internal18020400600C. air1.21.515Angular16040300500C. air32535253511.21.53Radial18020300500C. air31.21.51Peruvian4.72535WhiteWhiteStraightCW500700C. air125351.21.5318020500700C. air31.21.51253516040500700C. air253511.21.53Internal18020300500C. air31.21.5152535Angular16040250450C. air125351.21.53Radial18020250450C. air31.21.51Italian5.21725Light redWhiteStraightCW500700C. air1.21.5118020500700C. air3172511.21.51725316040500700C. air1.21.51Internal18020300500C. air317251725151.21.53Angular16040200400C. air31.21.511725Radial18020200400C. air203011.21.5Spanish5.74Red3WhiteStraightCW300550C. air1.21.5118020300400C. air32030203011.21.5316040300450C. air31.21.512030Internal18020200350C. air2520301.21.53Angular16040200300C. air31.21.512030Radial18020200350C. air163721.21.5Spanish7.5Red3Clear/whiteStraightCW200370C. air31.21.52163718020200350C. air1637121.21.5316040200350C. air31.21.52163714060200350C. air1.21.512Internal14060200300C. air31637351.21.516373Angular14060200250C. air1.21.513Radial16040200300C. air3163791211.2Spanish8.5Red2.8WhiteStraight1002503070Argon2.81.219111002003080Argon121311.22.81003003040Argon1.211502503080Argon2.8101411.22.8Internal1002007080Argon2.81.211502505070Argon31.21.52.2StraightCW150250Argon2.21.21.534618020150250Argon48241.21.52.218030150250Argon2.21.21.5246918040150250Argon247111.21.52.218050150250Argon2.21.21.5391116020150250Argon1.21.5316030150250Argon2.2101131.21.511132.216040150250Argon1.21.5214020150200Argon2.21012101431.21.52.214040150200Argon2.21.21.53101414060150200Argon7911.2Spanish8.5Red3.5WhiteStraight15025050C. air3.51.21100250783050C. air101211.23.51002005070C. air3.51.212Angular1002006080C. air315181.23.5StraightCW70160Nitrogen3.51.219121502504060Nitrogen81011.23.51002503050NitrogenNitrogen3.51.219111002002050I. Black et al./Journal of Materials Processing Technology84 (1998) 475553Table 4 (continued).Atmospheric cuttingts(mm)Tile typeBodyGlaze typeGeometricRa(mm)Pl-PsV (mm/min)Shield gasp (bar)Ns(mm)SFQcutcolourAngular1502504060Nitrogen3.51.2341002503040Nitrogen3.51.234StraightCW80100Oxygen3.51.231002005060Oxygen3.51.211011100