Abstract:
The work presented in this thesis addresses the parametric study of ion beams emitted
from Mather type plasma focus devices and their flourishing utilization in materials
processing. Experiments have been performed by using two different plasma focus
devices; a conventional 2.3 kJ plasma focus device developed under the joint venture of
the United Nations University (UNU) and the Abdus Salam International Centre for
Theoretical Physics (ICTP) designated as the UNU/ICTP device operational at the GC
University Lahore and a modified version called the Nanyang X-ray source-2 designated
as the NX2 device (a repetitive plasma focus) operational at the National Institute of
Education (NIE),
Nanyang Technological
University (NTU),
Singapore.
The
measurements of ion parameters such as energy, energy distribution, number density and
current density are carried out in the ambient gas pressure by employing a BPX65
photodiode and a Faraday cup (FC) using time of flight technique.
A major motivation is to establish the optimum processing conditions for ion nitriding,
surface modification, phase changes and carburizing of materials of industrial interest
like Ti, AlFe 1.8 Zn 0.8 alloy and SS-321 in plasma environment. The processed samples are
characterized for structural and morphological changes, compositional profile and surface
hardness by employing X-ray diffraction (XRD) at GC University Lahore, scanning
electron microscopy (SEM) at University of Peshawar, field emission SEM (FESEM) and
energy dispersive X-ray spectroscopy (EDX) at the NIE NTU Singapore, X-ray
photoelectron spectroscopy (XPS) at the National University of Singapore (NUS)
Singapore, Raman spectroscopy and Vickers microhardness test at Quaid-i-Azam
University Islamabad, Pakistan. The SRIM code and microindentation measurements are
used to estimate the depth profile of the modified layers.
Nanocrystalline spatially uniform TiN thin films with petal like features are developed on
Ti substrates exposed to 30 focus shots at various axial positions. The surface roughness
and the relative proportion of the TiN films are strongly influenced by the ion beam
energy flux. The film acquires eminent appearance with maximum relative proportion of
nitrogen at 7 cm axial position. The probable energy of the ions reaching this position is
64 keV with the maximum ion number density of 5.9 ́10 13 cm -3 . The corresponding
energy flux and current density are 2.69 ́10 13 keV cm -3 nsec -1 and 1142 A cm -2
viiirespectively. The grain size of the film is estimated to be about 90 nm while the
compound layer thickness is about 0.66 μm. The surface microhardness is also maximum
at this axial position with typical value of 7650±10 MPa.
The SEM images of a typical microcracked TiN thin film and the SRIM code estimations
of ion penetration help in understanding the growth mechanism of the film in terms of ion
dose. The granular nanostructures appearing on the substrate surface are grown from
nucleates of a few nm size developed by the energetic ions induced collision cascades.
The predeposited nitride layer or nitrogen ions interstitially implanted into the substrate
surface are also redistributed by the successive pulses of the ion beams leading to layer
densification along with possible resputtering. Moreover, the temperature evolution
during the DPF ions irradiation also enhances the reactivity of the nitrogen already
introduced during the preceding pulses. The residual tensile stresses on the sample
surface are transformed to the compressive stresses after DPF ion irradiation.
Nitrogen ions induced surface changes in AlFe 1.8 Zn 0.8 alloy are investigated as functions
of axial and angular positions for 30 shots. The expanded fcc phase of Al is evolved
owing to the incorporation of nitrogen along with Fe and Zn into the Al lattice. A
comparatively smooth and crack free nitride layer is formed on the sample treated at 7 cm
axial and 10 0 angular position with 4- to 5-fold increase in Vickers hardness.
TiN 0.9 and (Fe,Cr) 2 N are deposited on SS-321 along with formation of non-stoichiometric
(Fe,Cr) x N phase by exposing the samples to multiple focus shots in nitrogen plasma at
different axial and radial positions. The transformation from (Fe,Cr) x N to (Fe,Cr) 2 N is
attributed to an increased nitrogen ion dose. The point-like structures of flakes reveal the
nucleation of crystal growth with the increased ion doses. The nitride layer is golden in
colour and is spatially uniform with improved surface hardness.
Multiphase nanocrystalline titanium oxycarbide TiC x O y thin films composed of TiC 2 ,
TiO 0.325 , Ti 2 O 3 and carbon phases are deposited on titanium substrate in CH 4 discharges
by the UNU/ICTP and the NX2 devices. The nanocomposite films are non-porous and
microcrack-free with grain-like surface morphology having spatially uniform carbon
distribution. XRD, Raman and XPS results reveal the favorable evolution of multiphase
coatings having a stoichiometric TiC 2 phase and graphitic carbon adsorbates along with
ixthe residual oxide (TiO 0.325 , Ti 2 O 3 ) phases with the lower energy flux and lower repetition
rate in the UNU/ICTP treatment. Whereas, the deposition of carbon and a non-
stoichiometric TiO 0.325 phase is favored due to the improved oxide removal and enhanced
disorder in the substrate surface during the NX2 treatment. In addition, TiC 2 phase is also
suppressed, possibly due to the enhanced substrate temperature caused by the higher
energy flux of the ion beams and the higher repetition rate. The granular profile of the
films attains a definite coagulation pattern. The energy flux of the ion beam and the
repetition rate are found to be critical parameters which influence the preferred evolution
of a particular phase during the restructuring of various phases.