1. Laboratory for Laser Materials Synthesis & Fabrication Department of Materials Science & Engineering UNIVERSITY OF TENNESSEE, Knoxville, TN Ab-initio Computational Approach to Laser Micro-machining of Structural Ceramics Anoop N. Samant, Narendra B. Dahotre Laboratory for Laser Materials Synthesis & Fabrication Department of Materials Science & Engineering, University of Tennessee, Knoxville, TN

2. Laboratory for Laser Materials Synthesis & Fabrication Department of Materials Science & Engineering UNIVERSITY OF TENNESSEE, Knoxville, TN OUTLINE  Objectives  Structural Ceramics  Methodology  Laser Machining of Structural Ceramics  Physical Phenomena in Machining  Data Analysis  Contribution of current work  Significance of research  Future Work  Conclusions

3. Laboratory for Laser Materials Synthesis & Fabrication Department of Materials Science & Engineering UNIVERSITY OF TENNESSEE, Knoxville, TN OBJECTIVES  Demonstrate feasibility of laser machining of structural ceramics.  Understand material removal mechanisms (MRM)  Develop an ab-initio computational model based on MRM.  Use model for advance predictions of laser processing conditions to attain desired attributes.  Save considerable energy and time.

4. Laboratory for Laser Materials Synthesis & Fabrication Department of Materials Science & Engineering UNIVERSITY OF TENNESSEE, Knoxville, TN  Properties  Low thermal and electrical conductivity  High hardness  Chemical stability  High thermal resistance  Applications  Machine tools, valves, bearings, rotors  Optical and Electronic devices  Hazardous Waste Disposal  Examples  Silicon Carbide, Alumina, Silicon Nitride, Magnesium Oxide, Zirconia STRUCTURAL CERAMICS

5. Laboratory for Laser Materials Synthesis & Fabrication Department of Materials Science & Engineering UNIVERSITY OF TENNESSEE, Knoxville, TN LASER MACHINING  An operation similar to laser drilling subsequently conducted on neighboring locations.  Advantages :  Non contact processing  Capability of automation  Reduced manufacturing costs  Efficient material utilization  Reduced heat-affected zone (HAZ)  High productivity

6. Laboratory for Laser Materials Synthesis & Fabrication Department of Materials Science & Engineering UNIVERSITY OF TENNESSEE, Knoxville, TN LASER MACHINING Fig.3 Types of Laser Machining [2] Fig.2 Laser Machining [1] [1]Kalpakjian, Serope and Steven R. Schmid, Manufacturing Engineering and Technology, Upper Saddle River, New Jersey: Prentice Hall, Inc, 2001. [2]Samant and Dahotre, Journal of European Ceramic Society, 29(2009) 969.

7. Laboratory for Laser Materials Synthesis & Fabrication Department of Materials Science & Engineering UNIVERSITY OF TENNESSEE, Knoxville, TN METHODOLOGY  Come up with optimum pulse width, pulse energy, and pulse repetition rate to develop sufficient laser-ceramic interaction.  Vary processing parameters to machine cavities of different dimensions.  Develop 3D-thermal model to generate temperature profiles.  Incorporate different physical phenomena into the developed model.  Correlate predicted attributes of machined cavities with observed features.

8. Laboratory for Laser Materials Synthesis & Fabrication Department of Materials Science & Engineering UNIVERSITY OF TENNESSEE, Knoxville, TN SymbolPropertyValue dOut of focus beam diameter0.5 mm λLaser Wavelength1,064 nm δfδf Focal Length120 mm SILICON CARBIDE MACHINING Table 1. Laser Parameters (JK 701 pulsed Nd:YAG laser ) Fig.4 Through holes in 2mm and 3mm SiC plates [1].  25 and 125 pulses machined 2 and 3 mm plates at 6 J, 0.5 ms and 50 Hz. [1] Samant et. al., International Journal of Advanced Manufacturing Technology, in press.

9. Laboratory for Laser Materials Synthesis & Fabrication Department of Materials Science & Engineering UNIVERSITY OF TENNESSEE, Knoxville, TN  Fourier’s Second Law for maximum surface temperature:  Radiation losses at the surface : (1) (2) (3) ε - emissivity, k(T) - thermal conductivity, T 0 - initial temperature, h(T) - heat transfer coefficient, H – plate thickness, a – absorptivity (1 due to multiple reflections. [1] ) [1] Mazumdar et. al., J.Appl.Phys., 51(2), 1980  Convection taking place at the bottom: TEMPORAL EVOLUTION

10. Laboratory for Laser Materials Synthesis & Fabrication Department of Materials Science & Engineering UNIVERSITY OF TENNESSEE, Knoxville, TN TEMPORAL EVOLUTION  Temperature during pulse OFF time [1] : (4) (5) T i - Temperature during heating of pulse i, t off - OFF period between successive pulses, erf - error function, α(T) - thermal diffusivity T’ i-1 - temperature during cooling of the earlier pulse, t on - pulse duration, P – incident beam power [1] Konstantinos et.al., Journal of Materials Processing Technology 183 (2007), 96.  Temperature during pulse ON time [1] :

11. Laboratory for Laser Materials Synthesis & Fabrication Department of Materials Science & Engineering UNIVERSITY OF TENNESSEE, Knoxville, TN TEMPORAL EVOLUTION Fig.5 Heating curves for a) 2mm and b) 3mm thick SiC plate  Temperature drops during the OFF time and rises during the ON time of the laser giving the heating curve a meandering nature.  Maximum surface temperature reached while processing 3mm thick plate is higher than that reached while processing 2 mm plate.  High temperatures exist for extremely short time and rapidly drops due to self quenching.

12. Laboratory for Laser Materials Synthesis & Fabrication Department of Materials Science & Engineering UNIVERSITY OF TENNESSEE, Knoxville, TN EVAPORATION LOSSES  Rate of evaporation : m v - mass of vapor molecule, T s -surface temperature, k - Boltzmann Constant, p(Ts) - saturation pressure, p 0 – ambient pressure  Material loss :  Corresponding drop in temperature : (6) (7) (8) L v - latent heat of evaporation

13. Laboratory for Laser Materials Synthesis & Fabrication Department of Materials Science & Engineering UNIVERSITY OF TENNESSEE, Knoxville, TN DISSOCIATION ENERGY LOSSES  Possible dissociation species : Si(l), C(s), C(g), Si(g), Si 3 (g), Si 2 (g), SiC 2 (g), Si 2 C(g) and Si(s)  Most likely reaction at 3103 K: (9)  Volume of hole: (10) (assuming cylinder of diameter z ava = available melt depth = z total - z eva = melt depth from surface - evaporated depth  Energy Loss : (11) ≈ (Volume/ 22.4 x 10 -3 ) moles)

14. Laboratory for Laser Materials Synthesis & Fabrication Department of Materials Science & Engineering UNIVERSITY OF TENNESSEE, Knoxville, TN RECOIL PRESSURE  Cause : Evaporation of the melt surface exposed to laser.  Effective melt depth z eff : Portion remaining after expelling a fraction of the total melt depth.  Recoil pressure p r : I s a function of the material properties, maximum surface temperature and input energy [1] : (12) [1] Anisimov, Sov. Phys. JETP 27 (1968), 182

15. Laboratory for Laser Materials Synthesis & Fabrication Department of Materials Science & Engineering UNIVERSITY OF TENNESSEE, Knoxville, TN  Surface tension effect : Modifies pressure on melt, thus affecting depth of machined cavity [1].  Effective beam radius: Beam radius changes with change in machined depth [2].  Velocity of expulsion [3] : SURFACE TENSION (13) (14) τ - surface tension coefficient of liquid Si, t – time, r eff – effective beam radius, ρ - density [1] Han et.al., Journal of Heat Transfer, 127 (2005),1005. [2] Salonitis et.al., Journal of Materials Processing Technology 183 (2007), 96 [3] Matsunawa et.al., J. Phys. D: Appl. Phys. 30(1997),798. (13) (14)

16. Laboratory for Laser Materials Synthesis & Fabrication Department of Materials Science & Engineering UNIVERSITY OF TENNESSEE, Knoxville, TN MACHINED CAVITY DEPTH  Machined cavity: Depth of cavity formed due to expulsion of available melt depth [1] : Fig.6 Cavity Evolution (15) (16) z expelled – Depth expelled at different time instants z t – Total cavity depth formed at a certain time instant [1] Semak et. al., J. Phys. D: Appl. Phys. 39 (2006),590.  Predicted pulses: 21 and 103 pulses for machining through a 2 and 3 mm plate.

17. Laboratory for Laser Materials Synthesis & Fabrication Department of Materials Science & Engineering UNIVERSITY OF TENNESSEE, Knoxville, TN MACHINED CAVITY DEPTH Fig.7 Stages of Cavity Evolution  Till t 2, material expelled in upward direction.  Around t 3, direction of material expulsion reversed due to least resistance to the recoil pressure by small mass of supporting material at the bottom.  At t 4, all the rest of molten material expelled and a clean through cavity formed.

18. Laboratory for Laser Materials Synthesis & Fabrication Department of Materials Science & Engineering UNIVERSITY OF TENNESSEE, Knoxville, TN ALUMINA MACHINING  Applications: Substrate in hybrid circuits, aerospace industry.  Dissociation at 3250K:  Machining mechanism: Dissociation, melt expulsion by recoil pressure and evaporation.  Predicted pulses: 3, 7, 16 and 19 pulses for machining 0.26, 0.56, 3.23 and 4.0 mm respectively at 0.5ms, 4J and 20Hz. (17)

19. Laboratory for Laser Materials Synthesis & Fabrication Department of Materials Science & Engineering UNIVERSITY OF TENNESSEE, Knoxville, TN ALUMINA MACHINING Fig.9 Cavities in alumina [1] Fig.10 Evolution of cavities [1] [1] Samant and Dahotre, Int. Journal of Machine Tools and Manufacture, 48 (2008), 1345.

20. Laboratory for Laser Materials Synthesis & Fabrication Department of Materials Science & Engineering UNIVERSITY OF TENNESSEE, Knoxville, TN MAGNESIUM OXIDE MACHINING  Applications: Refractory and brake linings, thin film semi-conductors.  Dissociation at 3123K:  Machining mechanism: Dissociation followed by evaporation.  Predicted machining times: 0.11, 0.2, 0.25 and 0.8 s for machining 0.25, 0.86, 1.54 and 3mm respectively at 0.5ms, 4 J and 20 Hz. (19)

21. Laboratory for Laser Materials Synthesis & Fabrication Department of Materials Science & Engineering UNIVERSITY OF TENNESSEE, Knoxville, TN MAGNESIUM OXIDE MACHINING Fig.13 Cavities in MgO [1] Fig.14 Evolution of cavities with time [1] [1] Samant and Dahotre, Optics and Lasers in Engineering, 47(2009),570.

22. Laboratory for Laser Materials Synthesis & Fabrication Department of Materials Science & Engineering UNIVERSITY OF TENNESSEE, Knoxville, TN PHYSICAL PHENOMENA IN DIFFERENT CERAMICS MeltingDissociationEvaporation Silicon Carbide (SiC) Alumina (Al 2 O 3 ) Magnesium Oxide (MgO)  Physical Process Material Table 2. Physical Phenomena Governing Machining in Different Ceramics [1] [1] Samant and Dahotre, Ceramics International, in press. MeltingDissociationEvaporation Silicon Carbide (SiC) Alumina (Al 2 O 3 ) Magnesium Oxide (MgO) 

23. Laboratory for Laser Materials Synthesis & Fabrication Department of Materials Science & Engineering UNIVERSITY OF TENNESSEE, Knoxville, TN Fig.8 Flowchart for computations FLOW CHART  Stepwise procedure to achieve final machining parameters starting with material properties and process parameters.

24. Laboratory for Laser Materials Synthesis & Fabrication Department of Materials Science & Engineering UNIVERSITY OF TENNESSEE, Knoxville, TN DATA ANALYSIS Table 3. Comparison between experimental and predicted attributes of machined cavities Ceramic Depth of machined cavity (mm) Pulses experimental (Time, sec) Pulses predicted (Time, sec) Al 2 O 3 0.265 (0.25)3 (0.15) 0.5610 (0.5)7 (0.35) 3.2320 (1.0)16 (0.8) 430 (1.5)19 (0.94) SiC 225(0.5)21(0.41) 3125(2.5)103(2.05) MgO 0.253 (0.15)2 (0.11) 0.866 (0.3)4 (0.2) 1.549 (0.45)5 (0.25) 320 (1)16 (0.8)

25. Laboratory for Laser Materials Synthesis & Fabrication Department of Materials Science & Engineering UNIVERSITY OF TENNESSEE, Knoxville, TN CONTRIBUTION OF CURRENT WORK  Prior work conclusions: a) Machining comprises of melting and material removal by expulsion [1]. b) Machining takes place by single step evaporation without melting [2]. c) Effect of multiple reflections neglected.  Current work conclusions : a) Material removal occurs by a combination of melt expulsion, dissociation and evaporation. b) Multiple reflections affect the amount of absorbed energy. [1] Salonitis et.al, Journal of Materials Processing Technology, 183(2007) 96. [2] Atanasov et. al, Journal of Applied Physics, 89(2001) DOI: 10.1063/1.1334367

26. Laboratory for Laser Materials Synthesis & Fabrication Department of Materials Science & Engineering UNIVERSITY OF TENNESSEE, Knoxville, TN SIGNIFICANCE OF RESEARCH  Proposed systematic approach is an advancement of existing computational approach to ceramic machining.  Advance prediction of number of pulses/ pulse duration/ pulse energy possible.  Developed model can be extended to two and three dimensional laser machining.  A system with optimum machining rate can be generated.

27. Laboratory for Laser Materials Synthesis & Fabrication Department of Materials Science & Engineering UNIVERSITY OF TENNESSEE, Knoxville, TN FUTURE WORK  Laser Machining in 2 D (Laser Cutting) and 3D (engraving complex shapes).  Effect of multiple passes on machined depth by considering preheating effect.  In-situ temperature measurements and absorptivity predictions using thermocouples.

28. Laboratory for Laser Materials Synthesis & Fabrication Department of Materials Science & Engineering UNIVERSITY OF TENNESSEE, Knoxville, TN SUMMARY  Structural ceramics were successfully machined.  Theoretical model incorporating several vital effects encountered during machining was developed.  Model predictions compared well with experimental observations for machining.  Model aids to provide an advance estimate of number of pulses required for machining required depth or the depth machined after applying certain number of pulses.  Laser Fluence and machining time could also be predicted.

29. Laboratory for Laser Materials Synthesis & Fabrication Department of Materials Science & Engineering UNIVERSITY OF TENNESSEE, Knoxville, TN THANK YOU