Improving the Surface Roughness of a CVD Coated Silicon Carbide Disk By Performing

Ductile Regime Single Point Diamond Turning

Deepak Ravindra(Department of Mechanical & Aeronautical Engineering)

&John Patten

(Department of Manufacturing Engineering)

Manufacturing Research Center

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Presentation Overview

Introduction

Research Background

Preliminary Machining on CVD-SiC

Final Machining on CVD-SiC

Past, Ongoing & Upcoming work

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Ductile Regime Machining of Ceramics

Plastic flow of material in the form of severely sheared machining chips occur

Possible due to High Pressure Phase Transformation (HPPT) or direct amorphization

Plastic deformation caused from highly localized contact pressure and shear stresses.

High pressure (metallic) phase could be used to improve manufacturing processes and ductile response during machining.

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Ductile Regime Machining of Ceramics

DBT is calculated based on material properties

Depth exceeding critical depth will result in brittle machining

Micro-cracks / surface damage depth, yc

should not extend beyond the cut surface plane

Model proposed by Blake & Scattergood

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Critical Depth of Cut (dc

)

Griffith fracture propagation criteria:dc

~ 0.15 .

(E / H) .

(Kc

/ H)2

where: 0.15 = estimated constant of proportionality E = elastic modulusH = hardnessKc

= fracture toughness

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Why Use Silicon Carbide?Extreme hardness (~27GPa for 3-C

Polycrystalline CVD coated)

High wear resistance

High thermal conductivity

High Temperature Operation

Wide energy bandgap

High electric field breakdown strength

High maximum current density

High saturated electron drift velocity

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Project Goals

Improve surface finish (surface roughness) via ductile mode machining

Increase material removal rate (MRR) by altering:

Feed

Depth of Cut

Cutting Speed

Minimize diamond tool wear

Minimize/Eliminate sub-surface damage

*Establish machining parameters to meet all three criteria's (project goals)

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Concept of improving surface roughness1

2

3

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Improving the Surface Roughness of a CVD-SiC by Performing Ductile Regime SPDT

6

disk from POCO Graphite Inc. was used (3-C ,

Polycrystalline )

Mirror finish surface required

To be used as optic mirrors in an Airborne Laser (ABL) device

CVD coated SiC is preferred because:

High Purity (>99.9995%)

High Density (99.9%)

Homogeneity

Chemical & Oxidation Resistance

Good Cleanability

& Polishability

Good Thermal & Dimensional Stability

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Experimental Procedure

Initial matrix designed (with varying depths of cuts and feeds)

Preliminary machining on 6

CVD-SiC (as- received)

Final experimental matrix is designed based on preliminary machining results

Final machining on 6

CVD-SiC

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Experimental Setup for SPDT

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SPDT of SiC

Preliminary Machining Results (surface roughness & feed correlation)

Ra improved from 1.16m to 83nm

Feed is more dominant than depth of cut when improving surface roughness

Feed is reduced when surface roughness does not improve

Surface Roughness (Ra) vs. Feed

Pass

#16

Pass

#15

Pass

#14

Pass

#13

Pass

#12

Pass

#11

Pass

#10

Pass

#9

Pass

#8

Pass

#7

Pass

#6

Pass

#5

Pass

#4

Pass

#3

Pass

#2Pa

ss #

1

0

200

400

600

800

1000

1200

1400

AsReceived

30 2m

30 2m

30 2m

30 2m

30 2m

5 2m

5 2m

5 2m

5 0.5m

5 0.25m

1 1m

1 0.5m

1 0.5m

1 0.5m

1 0.5m

1 0.25m

Feed (m/rev)

Ra(

nm)

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Preliminary Machining Results

(Rz for all machining passes)

Rz improved from 8.49m to 0.51m

The Rz value was used to determine the required depth of cut

The feed and/or depth of cut is reduced when the Rz value does not improve.

Peak-to-valley (Rz) vs. Pass Numbers

0

1

2

3

4

5

6

7

8

9

AsRecd.

1 2m

2 2m

3 2m

4 2m

5 2m

6 2m

7 2m

8 2m

9 0.5m

10 0.25m

11 1m

120.5m

130.5m

140.5m

150.5m

160.25m

Pass Numbers

Rz

(m

)

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Preliminary Machining Results (Cutting force vs. depth of cut)

Depth of cut, feed and surface roughness influence the cutting forces

Depth of cut is the dominant parameter

Cutting forces vs. depth of cut and the corresponding feed

Pass

#10

Pass

#16

Pass

#9

Pass

#15

Pass

#14

Pass

#13

Pass

#12P

ass

#11

Pass

#8

Pass

#7

Pass

#6

Pass

#5

Pass

#4

Pass

#3

Pass

#2

Pass

#1

0

100

200

300

400

500

600

2 (30

m/re

v)2 (

30m

/rev)

2 (30

m/re

v)2 (

30m

/rev)

2 (30

m/re

v)2 (

5m/

rev)

2 (5

m/rev

)2 (

5m/

rev)

1 (1

m/rev

)0.5

(1m

/rev)

0.5 (1

m/re

v)0.5

(1m

/rev)

0.5 (1

m/re

v)0.5

(5m

/rev)

0.25 (

1m/

rev)

0.25 (

5m/

rev)

Depth of cut (m) with corresponding feed

Cut

ting

Forc

es (m

N)

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Results (Surface image of 6 passes)

Image was taken at 50x magnification

Surface continuously improved after each pass

Band between pass 4 & 5 was due to tool chatter

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Results (Surface image comparison)

Images comparing the surface before (left) and after (right) SPDT

Images were taken at a 1000x magnification

The sharp/uneven peaks on the surface disappeared after the SPDT operation

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Preliminary Machining Results (Cutting force vs. tool wear across cutting edge)

Wear across cutting edge radius is a function of depth of cut

Wear length and cutting forces increase as depth of cut increases

Cuting Force vs. Measured Tool Wear Across Cutting Radius

Pass

#10

(0.2

5m

)

Pass

#16

(0.2

5m

)

Pass

#1(

2m

)

Pass

#2

(2m

)

Pass

#3

(2m

)

Pass

#4

(2m

)

Pass

#5

(2m

)

Pass

#6

(2m

)

Pass

#7

(2m

)

Pass

#8

(2m

)

Pass

#11

(1m

)

Pass

#12

(0.5m

)

Pass

#14

(0.5m

)

Pass

#13

(0.5m

)

Pass

#15

(0.5m

)

Pass

#9

(0.5

m)

0

50

100

150

200

250

300

350

400

450

500

488360355355346345340340268217208214205147145122

Measured Wear (m)

Cut

ting

Forc

e (m

N)

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Choosing Machining Parameters for Final Pass

Surface Roughness (Ra) vs. Feed

Pass

#16

Pass

#15

Pass

#14

Pass

#13

Pass

#12

Pass

#11

Pass

#10

Pass

#9

Pass

#8

Pass

#7

Pass

#6

Pass

#5

Pass

#4

Pass

#3

Pass

#2Pa

ss #

1

0

200

400

600

800

1000

1200

1400

AsReceived

30 2m

30 2m

30 2m

30 2m

30 2m

5 2m

5 2m

5 2m

5 0.5m

5 0.25m

1 1m

1 0.5m

1 0.5m

1 0.5m

1 0.5m

1 0.25m

Feed (m/rev)

Ra(

nm)

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Final Machining Parameters

Recommended machining parameters for commercial manufacturing of CVD-SiC

The actual depths are usual about 50% of the programmed depths of cuts (due to elasticity)

Best surface finish is achieved from the lowest feed

Pass # Depth of Cut Feed (m/rev)

1 2m 302 2m 303 2m 54 500nm 15 500nm 16 250nm 1

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Results for final machining (surface roughness)Surface roughness vs. depth of cut and the corresponding feed

Pass 6Pass 5Pass 4

Pass 3

Pass 2

Pass 1

0

1000

2000

3000

4000

5000

6000

7000

8000

9000

10000

As Received 2 (30m/rev) 2 (30m/rev) 2 (5m/rev) 0.5 (1m/rev) 0.5 (1m/rev) 0.25 (1m/rev)

Depth of cut (m) with corresponding feed

Surf

ace

Rou

ghne

ss (n

m)

RaRz

Ra improved from 1.23m to 88nm

in six passesRz improved from 8.9m to 0.27mWorkpiece in general had lesser run-out due to flatter back

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Results for final machining (cutting forces)

Cutting forces vs. depth of cut and the corresponding feed

Pass 6Pass 5Pass 4

Pass 3

Pass 2Pass 1

0

50

100

150

200

250

300

350

400

450

2 (30m/rev) 2 (30m/rev) 2 (5m/rev) 0.5 (1m/rev) 0.5 (1m/rev) 0.25 (1m/rev)

Depth of cut (m) with corresponding feed)

Cut

ting

Forc

e, m

N

Larger depths of cuts result in larger cutting forcesForces can also increase due to rough surfaces, machining debris, tool vibration, workpiece run-out, etcCutting fluid helps several problems such as machining debris

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Results (SEM images of tool wear)

Tool was used for pass 1 (2m depth & 30m/rev feed)

SEM images are used to measure tool wear

Wear length across cutting edge

Rake wear

Flank wear Dee

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Results (6 disk before & after SPDT)

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Ductile Machining vs. Brittle MachiningDuctile Brittle

Well defined & straight edges

Jagged edges & chipped material

Controlled material removal process

Hard to control as microcracks

extend below

the machined surfaceFinal depth of cut can be predicted below the DBT depth

No direct control of the resultant depth beyond the DBT depth

Good surface finish and mechanical properties

Poor surface finish and could end in a catastrophic failure at times

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Other projects accomplished

Ductile to Brittle Transition (DBT) of single crystal 4H SiC

Nanometric cuts are performed on wafer

Cuts were imaged using AFM

Developing a hybrid laser-SPDT machining process for smoothing ceramics

CVD-SiC is laser ablated with various parameters

SPDT is carried out on the ablated plateaus

Best combination is chosen based on smoothest surface achieved, longest tool life and minimum cutting forces obtained.

SPDT of Quartz (Spectrosil 2000)

Establish machining parameters to SPDT a 14

piece

Subsurface damage analysis on all SPDT work pieces

Scanning acoustic microscopy (ORNL)

Raman spectroscopy (WMU)

No subsurface damage was identified

Ductile to Brittle Transition depth of AlTiC

Scratch tests using variable loads

Cuts were imaged using AFM, while light Interferometer & profilometer

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Use of final SiC & Quartz disk

To be used as ABL device nose cover

Mirror finish surface required for the above use

Image courtesy of Boeing Corporation Dee

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Ongoing Work

Micro Laser Assisted Machining (LAM) on single crystal SiC

Study the effect of heating (from laser)/softening of material and combining it with SPDT

Identifying crystal orientation & preferred machining direction (if any) for CVD-SiC

X-ray Diffraction and Orientation Imaging Microscopy (OIM) to identify crystal orientation

Scratch tests on single crystal SiC wafers to observe ductile to

brittle transition (DBT)

SPDT of Spinel

Establishing the DBT of the material

Developing machining parameters

Attempting to improve the surface roughness via ductile regime machining

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Future Developments

Transmission Electron Microscopy on SiC & quartz machined chips

Experiment other tool & machining parameters (i.e. rake angle, tool nose radius, depth of cut and feed)

Attempt to design a more efficient/accurate ductile machining model

Experiment on simultaneously machining brittle materials with the assistance of thermal softening (laser).

Analyzing Acoustic Emission signal to identify fracture of the material

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Thank you

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Improving the Surface Roughness of a CVD Coated Silicon Carbide Disk By Performing Ductile Regime Single Point Diamond Turning Presentation OverviewDuctile Regime Machining of Ceramics Ductile Regime Machining of CeramicsCritical Depth of Cut (dc)Why Use Silicon Carbide?Project GoalsConcept of improving surface roughnessImproving the Surface Roughness of a CVD-SiC by Performing Ductile Regime SPDTExperimental ProcedureExperimental Setup for SPDTSPDT of SiCPreliminary Machining Results (surface roughness & feed correlation)Preliminary Machining Results (Rz for all machining passes)Preliminary Machining Results (Cutting force vs. depth of cut)Results (Surface image of 6 passes)Results (Surface image comparison)Preliminary Machining Results (Cutting force vs. tool wear across cutting edge)Choosing Machining Parameters for Final PassFinal Machining ParametersResults for final machining (surface roughness)Results for final machining (cutting forces)Results (SEM images of tool wear)Results (6 disk before & after SPDT)Ductile Machining vs. Brittle MachiningOther projects accomplishedUse of final SiC & Quartz diskOngoing WorkFuture DevelopmentsThank you