By Saurabh R Virkar Under guidance of Dr. John A Patten ICOMM 2010 Venue: University of Wisconsin, Madison

 Silicon Carbide (SiC) is an advanced engineered ceramic and an alternative to semiconducting Silicon (Si) for operation at elevated temperatures and high power applications. Some of SiC’s beneficial properties include: chemical resistance, high temperature resistance, extreme hardness and high stiffness  Hardness of SiC: 26 GPa  The machining of SiC is difficult due to its high hardness and brittle nature.  Ductile mode µ-LAM has been studied to replace grinding and polishing processes and to increase the material removal rates and maintaining the workpiece surface quality

 In µ-LAM, the laser beam passes through the diamond tool, thus heating the surface just below the tool tip in the chip formation zone SiC Schematic of µ-LAM Diamond tool

 The ductile material removal can be attributed to a High Pressure Phase Transformation (HPPT) at the tool-chip interface and the resultant phase is metallic or amorphous  The HPPT occurs due to contact between the sharp tool and workpiece at or below critical depth of cut, i.e., below the ductile to brittle transition

 Silicon Carbide (SiC) is very expensive semiconductor  Measurement of temperatures at nano-scale is practically not possible  Also the rate of heat transfer and pressures on tool and workpiece can be studied  There is a metallic phase at tool chip interface due to high pressure phase transformation Software used: AdvantEdge version 5.4  Commercial software for machining solutions in metals developed by Third Wave Systems Inc.

 To simulate different heating conditions over a temperature range for studying the laser heating effect  To study the change in chip formation, cutting forces and pressures with changes in heating/temperature conditions

Drucker Prager Yield Criterion: …(1) …(2) Where I 1 is first invariant of stress tensor …(3) Where J 2 is second invariant of deviatoric stress tensor Hence initial yield stress is given by: …(4) For uniaxial stress, σ 2 = σ 3 = 0 and also σ c = σ 1 = H= 26 GPa σ t = H/2.2 = GPa (for ceramics) Hence, К= GPa and Drucker-Prager coefficient (α) = 0.375

Material propertiesValueUnits Elastic Modulus, E330GPa Poisson’s ratio Hardness, H26GPa Initial yield stress, σ GPa Reference plastic strain, ε 0 p Accumulated plastic strain, ε p 1- Strain hardening exponent, n50- Low strain rate sensitivity exponent, m High strain rate sensitivity exponent, m Threshold strain rate, ε t p 1E7sec -1 Drucker-Prager coefficient (DPO)0.375 Workpiece Material properties:

PropertiesValue Thermal Conductivity (W/cm K) 3.21 *Thermal Cutoff temperature ( ⁰ C) 1500 *Melting temperature ( ⁰ C) 2830 Initial reference temperature ( ⁰ C)20 * Note: The values for temperature from 20° C to 1500° C which is thermal cutoff temperature are estimated based on various references. The value for melting temperature of SiC is also estimated from a reference.

Cutting Edge Radius, r, (nm)100 Rake angle, α- 45º Relief angle, β5º Width of tool (µm)20 Tool geometry: Thermal Conductivity, W/m ⁰ C 1500 Heat Capacity, J/kg ⁰ C Density, kg/m³3520 Elastic Modulus, GPa1050 Poisson's ratio0.2 Tool Properties: The -45 ⁰ rake angle creates a high pressure sufficient to accommodate the HPPT, thus the chip formation zone is conducive for ductile deformation

 Tooltip Boundary Condition  Rake and Clearance face Heated Boundary Condition  Workpiece Boundary Condition

A thermal boundary condition was provided on the tool tip about 2µm on rake and clearance face from cutting edge

A thermal boundary was provided on the workpiece top surface

Parameters Values Feed (nm)500 Cutting speed (m/s)1 Width of cut (mm)0.02 Co-efficient of friction0.3 Temperature range of the simulation work: 20° C, 700° C, 1500° C, 2200° C and 2700° C where 1500° C is the thermal cutoff point in the material model. From 20° C till 1500° C, the thermal softening curve has a 3 rd order polynomial fit in the material model From the thermal cutoff point (1500° C) till melting point (2830° C) the curve is linear

 AdvantEdge does not provide for the direct incorporation of the laser heat source, thus the heating effect is modeled with these thermal conditions  For this study, the crystalline dependency of the brittle behavior of SiC is not included in the model Note: The temperature scale changes in each figure, as the minimum temperature is set slightly above and below the boundary condition temperature

Temperatures (° C) Cutting Force (mN) Thrust Force (mN) Chip formationPressure (GPa) Tooltip Boundary Condition simulation Yes No No No Yes8 Workpiece Boundary Condition simulation Yes No No No No3 Toolface Boundary Condition Yes No No No Yes6

 The thermal effects successfully simulated the laser heating effect  Decrease in cutting forces and pressures is studied with increase in temperature  The change in chip formation due to change in temperature above and below the thermal cutoff point is studied is studied

 To determine the effect of interaction between temperature and compressive stress on the cutting forces and pressures from room temperature till melting point of SiC  3D scratch test simulations for comparison with experiments

Support from NSF (CMMI ) Support from ThirdWave Systems