Characterization By X-ray Diffraction of Polycrystalline

Diamond Samples from Different Origins

Tshegofatso MoipolaiEnergy Postgraduate

Conference 2013

IntroductionDiamonds • Investigation of natural polycrystalline diamonds remains of

interest to both scientists and researchers as their mechanisms of formation are still under debate

• These mechanisms of formation depend on different pressures and temperatures:o HP and HT - 1 &2o LP and HT - 3o Ultra HT & HP - 4

• Allotrope of carbon • Tetrahedral network of

carbon atoms

Fig. 1. Mechanisms of formation of diamonds found at or near the earth’s surface

Fig.2.Cubic structure of Diamond

Introduction cont..

• Ballaso Occurs in Brazil and Africao Globular aggregates, grey in colour

with negligible poreso Single phase with very few inclusions

Fig.3 The South African Ballas 10 X 10 X 8 mm3

• Carbonadoo Occurs in Brazil ,Central Africa and Soviet Union o Pebble shaped, greyish –black in colour and

porouso Contains secondary phases

Fig 4. The Brazilian Carbonado 14 X 10 X 4 mm3

• Why study this type of diamondo Contribute to the debate around its formation

mechanisms Determination of the near surface

residual stress using x-ray diffraction technique

• Residual stress o Stress that remains in the material when

there is no externally applied force Tensile Compressive

• Residual stress in diamondo Bulk

Extended strain fields having defects dislocations ,bundles of

dislocations stacking faults inclusions plastic deformation surface defects where surface

strain extends to the bulk Inhomogeneous spatial distribution of

point defects concentration gradient of

impuritieso Surface

Termination species Surface processing

chemical and mechanical

Objectives• Characterization investigations:

o SEM: Imaging and EDS of the morphology and elemental composition

o XRD: Chemical phase studies Residual stress of near surface regions

• Motivationo This study presents the new work on near surface residual stress on

naturally occurring polycrystalline diamondo Due to scientific value of samples, no sample preparations could be

applied. Non-destructive investigations essential

o We have a batch of samples from different origins thus enabling comparative studies

• Methodology o SEM

30 kVo XRD

Cu 40kV,40mA

Phase ID

Angle range 15 °-150° Residual Stress

ɸ: 0°,180°,90°,270°,45°,225°Ψ range: 0°-70°

Peak chosen for measurement:311, 92.093°

Mag. 500X

X-Ray Diffraction

2

)(hklF -Structure Factor

-Multiplicity factorhklM

Fig.5 Geometrical illustration of the Bragg’s lawFig.6 A powder X-ray diffraction pattern

sin2

0

211

Y = 0ºY = 10º

Y = 20ºY = 30ºY = 40º

Y = 50ºY = 60ºY = 70ºY = 80ºY = 89.7º

Q211

2211

I

222 22 2222 2

d211

C

2

1 11 22

1s

2sC

Q211Q211

Q211Q211

Q211Q211

Q211Q211

Q211Q211

Q211Q211

Q211Q211

Q211Q211

Q211Q211

Q211Q211 Scattering plane

compressive stress

cot

0

0

d

dd

Results: SEM

Fig. 7 SE& EDS analysis of Carbonado

Fig. 8 SE & EDS analysis of Ballas

Results : XRD Phase ID

Fig.9 The Diffraction pattern of BallasFig.10 The diffraction pattern of Carbonado

Results: Residual Stress

Fig.11 Debye diffraction ring collected using an area detector

• Ballas

Investigations impossible due to:• Large grain size

Uneven intensity distribution around the Debye Scherer ring

Fig.12 Strain vs. sin2Ψ curve of Brazillian Carbonado sample

• Carbonado

Discussions & Results

• EDS: Chemical elements present identified• XRD: Cubic diamond with low content of minority phase . No presence of hexagonal phase• XRD stress: Large spread of values ascribed to:

Irregularities of sample Termination species

• Residual stress on Ballas was not possible due to the presence of large grainsProposed further work

o Detailed neutron diffractiono Detailed synchrotron-XRD

Measurement Position

Elemental composition

Major Minor

Chemical Phase Composition

Major Minor(98%) (< 2%)

Stress σ11

[MPaStress σ22

[MPa]

1 C (89%) O (5%) N (2%) Na (1%) Si (0.5%) Al (0.5%)

Diamond: Cubic Pyrite - FeS Silicon Oxide SiO2

-110 ± 77 -52 ± 84

2 C (84%) Ca (5%) O (6%) Si (2%) Al (1%) Ti (1%)

Diamond: Cubic -285 ± 71 -769 ± 70

3 C (94%) O (3%) Si (1%) Al (1%) N (1%)

Diamond: Cubic-252 ± 62 -90 ± 68

4 C(92%) O (4%) Al (1%) N (1%) Si (1%)

Diamond: Cubic-493 ± 82 218 ± 83

Conclusions

• These results represent amongst the first results of surface residual stress in naturally occurring polycrystalline diamonds

• Despite the irregular surface roughness of these samples, it was possible to get results by careful area selection

• Both tensile and compressive stress could be observed• Theoretical calculations indicate this as expected and can be

ascribed to surface defects and surface chemistry• Surface stress is an interesting new observable for the

characterization of natural diamond samples• The work opens the door for further work to understand the

diamond surface in greater detail in order to develop surface stress analysis as a tool for these studies

Acknowledgements

• Scientific collaborators• Dr. Marco Andreoli for samples• Dr. Remi Bucher (iThemba LABS South for

animations)

References

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2. www.geology.com Accessed on 08 March 20133. S. De, P.J Hearney, E.P. Vicenzi, J. Wang, Earth and Planetary

Science Letters 185 (2001)4. J.E Butler, Y.A Mankelevich, A. Cheesman, J Ma and M.N.R Ashfold,

J. Phys.: Condens. Matter 21 (2009) 364201 205. Krawitz, A.D., “Introduction to Diffraction in Materials Science and

Engineering”, John Wiley and Sons Inc., (2001), ISBN 0-471-24724-3

6. Hauk, V., ed., “Structural and Residual Stress Analysis by Non-destructive Methods Evaluation, Application, Assessment”, Elsevier, Amsterdam (1997), ISBN 0-444-82476-6