Abstract
Cold Gas dynamic spray (CGDS) is an additive manufacturing process that involves particle
impaction on underlying substrate at supersonic velocity. Upon impact a particle will either bond to
the substrate or rebound. This doctoral thesis reports an investigation of the CGDS Technique for the
Fabrication of a Thin Film for a Plasmonic Biosensor. This doctoral study involved an investigation
on the structure of the coatings, through numerical modeling and simulations and analytical modeling,
of the CGDS deposition method. The motivation of this study was on the basic understanding of the
CGDS with a view to understanding the deposition parameters and the resulting functional properties
in the coatings – where these properties are relevant in thin film applications for instance to plasmonic
biosensors. The study was further motivated by the need to development of biosensors to solve a
number of biomedical problems through thin film based sensor, yet the fabrication methods for these
sensors still remain costly. A need for cost effective fabrication method still remains even though a lot
of progress has been made in existing standard manufacturing methods. CGDS is a fast and low
temperature thermal deposition technique with proven performance in thick films. The primary
question was on the fundamental characteristics of the coatings and whether it is possible that this
method can provide a cost effective fabrication method for devices that operate on nano thin films. It
was therefore important to understand control parameters that govern the functional properties in the
CGDS film structure. This required a better understanding of the CGDS deposition process.
A numerical model for CGDS analysis was developed using finite element simulations. A series of
numerical simulations were performed to get a better understanding of the governing parameters in
CGDS generated coatings on interfaces, surfaces, and residual stresses. Further, connections of the
film structure to the off nozzle process variables in particular to the flux density, were established.
Non dimensional analysis gave understanding of the fundamental properties in the film structure.
These properties could give connections to applicability in plasmonics thin films. The impact
adhesion process was studied which led to development of analytical equations.
CGDS coatings are made by particle bonding upon impact on a substrate. Implementation of the
adhesion mechanism in the numerical model was challenging and therefore a simple criterion was
used. The results were compared to experimental evidence for a validation of the concepts. Numerical
results were found to have correlations to experimental data. The microstructure of the coatings,
which is the grain properties in relationship to the deposition variables, was not studied. However the
microstructure of the particles was studied through analytical equations. Further there was limited
analysis given between residual stress and deposition variables, though links to the particle diameter
were observable in the stress profiles and also the depth of plastic zone. The links of residual stress to
particle diameter were also given in a previous analytical equation. The interaction of particles during the deformation process was found to have effects to the levels of
plastic strains and transient temperatures in higher flux densities. The plastic strains and transient
temperatures increased with multiple particle interaction compared to single particle impacts.
On roughness characteristics, surface roughness was found to be highly dependent on deposition
efficiency, the surface roughness increased exponentially with decrease in deposition efficiency while
interface roughness was observed to be independent of the deposition efficiency. However both the
size of surface roughness heights and interface roughness heights were directly dependent on particle
size. Therefore the particle size gave connections to the order of magnitude of functional properties.
The modeling concept on residual stress was performed. From this study stress birefringence could be
understood in the context of film thickness being optically thin, while stress calculated based on
membrane deposition gives the residual stress, which is mainly compressive in CGDS, however no
detailed analysis was given on stress birefringence for the application case study. It was however
noted that particle size had effects on the resulting residual stress profiles.
It was found that fundamentally the deposition efficiency, and the roughness amplitudes and residual
stress amplitudes were dependent on severity of impact parameter and the elastic properties of the
materials.
It was concluded that the structure and properties of CGDS deposited films can be controllable to
desired functional properties of the thin film – substrate structure such as is required in biosensor
applications. For a chosen substrate material combination, some of the control variables are the
deposition efficiency, the particle diameter, and the impact energy (i.e impact velocity for a given
materials). Some of the functional variables that can be controlled are the interface roughness, the
surface roughness, the thickness of the coating, the residual stresses. It could be possible to control the
grain sizes, however this was outside the scope.
In future the interface can be examined for properties such as the alloying properties of the interface
and epitaxy which may be a functional variable for semiconductors or solar panels. It is required to
carry further experimental study on correlation functions or correlation variables for the surface and
interface roughness using the results of this work as a foundation. Working with nano size particles
will give a better description of size dependent CGDS deposition and nano thin film properties.