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|Title: ||Viability and characterization of the laser surface treatment of engineering ceramics|
|Authors: ||Shukla, Pratik P.|
|Keywords: ||Industrial lasers|
Fracture toughness parameter (K1c)
Finite element modelling
|Issue Date: ||2011|
|Publisher: ||© Pratik Shukla|
|Abstract: ||Laser surface treatment of engineering ceramics offers various advantages in comparison with
conventional processing techniques and much research has been conducted to develop applications.
Even so, there still remains a considerable gap in knowledge that needs to be filled to establish the
process. By employing a fibre laser for the first time to process silicon nitride (Si3N4) and zirconia
(ZrO2) engineering ceramics, a comparison with the CO2 and a Nd:YAG lasers was conducted to
provide fundamental understanding of various aspects of the laser beam-material interaction.
Changes in the morphology, microstructure, surface finish, fracture toughness parameter (K1c) were
investigated, followed by thermal finite element modelling (FEM) of the laser surface treatment and
the phase transformation of the two ceramics, as well as the effects of the fibre laser beam parameter
- brightness (radiance).
Fibre and CO2 laser surface treatment of both Si3N4 and ZrO2 engineering ceramics was performed
by using various processing gases. Changes in the surface roughness, material removal, surface
morphology and microstructure were observed. But the effect was particularly more remarkable
when applying the reactive gases with both lasers and less significant when using the inert gases.
Microcracking was also observed when the reactive gases were applied. This was due to an
exothermic reaction produced during the laser-ceramic interaction which would have resulted to an
increased surface temperature leading to thermal shocks. Moreover, the composition of the ceramics
was modified with both laser irradiated surfaces as the ZrO2 transformed to zirconia carbides (ZrC)
and Si3N4 to silicon dioxide (SiO2) respectively.
The most appropriate equation identified for the determination of the fracture toughness parameter
K1c of the as-received, CO2 and the fibre laser surface treated Si3N4 and ZrO2 was K1c=0.016 (E/Hv)
1/2 (P/c3/2). Surfaces of both ceramics treated with CO2 and the fibre laser irradiation produced an
increased K1c under the measured conditions, but with different effects. The CO2 laser surface
treatment produced a thicker and softer layer whereas the fibre laser surface treatment increased the
hardness by only 4%. This is inconsiderable but a reduction in the crack lengths increased the K1c
value under the applied conditions. This was through a possible transformation hardening which
occurred within both engineering ceramics.
Experimental findings validated the generated thermal FEM of the CO2 and the fibre laser surface
treatment and showed good agreement. However, a temperature difference was found between the
CO2 and fibre laser surface treatment due to the difference in absorption of the near infra-red (NIR)
wavelength of the fibre laser being higher than the mid infra-red (MIR) wavelength of the CO2 laser.
This in turn, generated a larger interaction zone on the surface that was not induced further into the
bulk, as was the case with the fibre laser irradiation. The MIR wavelength is therefore suitable for
Viability and Characterization of the Laser Surface Treatment of Engineering Ceramics
the surface processing of mainly oxide ceramics and surface treatments which do not require deep
penetration. Phase transformation of the two ceramics occurred at various stages during the fibre
laser surface treatment. The ZrO2 was transformed from the monoclinic (M) state to a mixture of
tetragonal + cubic (T+C) during fibre laser irradiation and from T+C to T and then a partially liquid
(L) phase followed by a possible reverse transformation to the M state during solidification. The
Si3N4 transformed to a mixture of α-phase and β-phase (α→ α+β) followed by α+β and fully
transforms from α+β →β-phase. What is more, is a comparison of the fibre laser-beam brightness
parameter with that of the Nd:YAG laser. In particular, physical and microstructural changes due to
the difference in the laser-beam brightness were observed.
This research has identified the broader effects of various laser processing conditions, as well as
characterization techniques, assessment and identification of a method to determine the K1c and the
thermal FEM of laser surface treated engineering ceramics. Also, the contributions of laser-beam
brightness as a parameter of laser processing and the influence thereof on the engineering ceramics
have been identified from a fundamental viewpoint. The findings of this research can now be
adopted to develop ceramic fuel cell joining techniques and applications where laser beam surface
modification and characterization of engineering ceramics are necessary.|
|Description: ||A Doctoral Thesis. Submitted in partial fulfillment of the requirements for the award of Doctor of Philosophy of Loughborough University.|
|Appears in Collections:||PhD Theses (Mechanical, Electrical and Manufacturing Engineering)|
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