Electrostatic Spray Assisted Vapour Deposition
Current Position of the Electrostatic Spray Assisted Vapour Deposition (ESAVD)
Based Technology
The technique is incredibly flexible in terms of substrate and coating material(s) to be
applied. Just by adjusting the properties of the reaction zone we can use the same coating
equipment to produce porous or dense coatings, with thickness measured in single
angstroms up to hundreds of µm or more, onto porous or dense substrates. We have
proved the capability of this process to coat a wide range of substances and combinations of
substances onto various commonly used groups of substrates.
The Technology (ESAVD)
 Use of electrostatics
Atomised, charged droplets containing carefully formulated mixtures of coating precursor
material are sprayed through an electric field, in an otherwise ambient environment,
towards a mildly heated substrate. Careful control of process conditions in the spray
reaction zone (ie. zone between spray nozzle and substrate) allows the appropriate chemical
reactions to occur. These include evaporation/decomposition of aerosol droplets and
formation of intermediate reactants that undergo chemical reactions in the vicinity of the
heated surface of the substrate. This leads to the formation of an adherent coating with the
correct chemical and physical characteristics, together with the desired microstructure.
ESAVD enables the use of a simple aerosol type precursor delivery system to be combined
with the vapour phase based coating of CVD. ESAVD is a non-line-of-sight process, and
therefore able to coat complex geometries, the electric field helps to ensure that a very high
proportion of the precursor ends up on the substrate via electrostatic attraction.
 Coating in open atmosphere
The unique spray reaction zone is a distinct environment that enables the chemical vapour
deposition to occur unhindered. This means that coatings normally applied in dedicated
reactors using moderate to high vacuums, and hence expensive vacuum systems, can now
be applied in open atmosphere. The resulting deposition equipment is simple to construct
and can be maintained easily with minimum equipment downtime, requiring a relatively low
capital investment.
 Flat and 3D substrates
The use of electrostatics also ensures that ESAVD is a "non-line-of-sight" process and can
therefore be used to coat either flat surfaces or complex 3D geometries, eg. hip implants,
engine components and curved windscreens.
 Thick and thin films
This is a new method for producing high quality, low cost coatings from less than 1 µm to
over 500 µm thickness. Deposition rates of 1 to 10 µm per minute can be achieved
depending on conditions. Uniformity and microstructure can be precisely controlled to
produce very high standard coatings, whether involving a dense coating onto a porous
substrate or a porous coating onto a dense substrate. One of the reasons for this is that
ESAVD involves constructing the coating layer by layer at the molecular level.
 Superior in many ways to CVD
ESAVD is a faster, more efficient process than CVD, due, in part to the use of electrostatic
attraction. This helps to direct the precursor to the substrate, thereby minimising losses to
the surroundings, unlike the CVD process. CVD often leads to problems with controlling the
stoichiometry of multi-component films due to differences in vapour pressures of CVD
reagents. Moreover the gaseous precursors used in the CVD process tend to be toxic and
require special precursor and effluent gas handling systems. Also a significant proportion of
the precursor can end up coating the insides of the coating chamber leading to high
wastage. CVD coating installations are therefore expensive to build and run, limited by
availability of suitable gaseous forms for precursors, and the size of the substrate is limited
by the size of the coating chamber. When ESAVD is optimised over 90% of the precursor
will end up on the substrate. Our process can also produce complex, multi-component
coatings from relatively simple, inexpensive precursors in simple aqueous or organic
solvents.
 Advantages over PVD, FSD and Sol-gel techniques
Physical Vapour Deposition (PVD) methods, such as Radio Frequency, Reactive Magnetron
and Ion Beam sputtering all produce relatively low deposition rates and yields. In addition
they all require expensive vacuum chambers and equipment, operated and maintained by
highly skilled technicians. Both CVD and PVD tend to be limited to thin film deposition (apart
from Electron Beam PVD). With ESAVD there is virtually no limit to thickness, from a few
angstroms to >500 µm.
Flame Spraying Deposition (FSD) relies on careful control of multiple parameters in an
inherently unstable combustion zone in contact with the substrate. Control is difficult and
the relatively high energies and velocities produced by the "flame" can adversely affect the
substrate. For the production of ceramic films the sol-gel process has two major drawbacks;
the films tend to crack due to the removal of the binder during subsequent sintering stages;
also the thickness of the film is limited to around 1 µm per layer. Films of up to approx. 5
µm can be produced by using sol-gel but for this thickness the process has to be repeated
several times.