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PRESS RELEASE (January 2007)
Please send copy to:
MM Mertig Marktkommunikation
Tassilostr. 13 - 82131 Gauting, Phone: 089/1266690 Fax: 089/12666915
E-mail: ursula.mertig@mertig.de,
www.mertig.de
7th Colloquium High Velocity Oxy-Fuel Flame Spraying
November
2006 in Erding (near Munich), Germany
HVOF: Latest developments improve economic efficiency in surface
treatment
Cold gas spraying: developments for more kinetic energy
The special properties of HVOF – high-velocity flame spraying, in
which the accelerated process reaches supersonic speeds – in comparison
with other thermal coating processes lie in the greater speed reached
combined with the lower temperature of the spraying particles. The
higher kinetic energy achieved produces coatings that are thicker and
adhere better. The progress from thermal to kinetic energy in surface
treatment is reflected in new developments of cold gas spraying, which
was the main focus of the Colloquium.
Like a snowball
At the start of the Colloquium, Professor Dr Heinrich Kreye from the
German Armed Forces University in Hamburg reported on the current state
of research. In cold gas spraying, the particles adhere solely because
of their high kinetic energy and the deformation on impact. The
particles are accelerated in a Laval nozzle with the help of a preheated
gas under high pressure. In contrast to conventional thermal spraying,
the particles are just slightly heated and impact the substrate in a
solid state. Through the expansion of the gas in the divergent area of
the nozzle, the gas and particles accelerate to ultrasonic speed and
cool down. Professor Kreye vividly compared this process with a snowball
that is thrown against a wall. This too will only stick if the speed and
the consistency of the snow are right. The spray rate for cold gas
spraying – which is comparable with other thermal spraying methods – is
between three to six kilograms of powder per hour. Impact speeds of 200
– 1,200 m/s can be reached, depending on the nozzle shape, the shape and
size of the particles, the type of process gas and the state of the
process gas in terms of pressure and temperature. Heating the gas
increases the flow speed. Through the warming of the particles, this
encourages their deformation on impact, thus reducing the critical speed
necessary for adhesion.
Until now, in cold gas spraying for the production of high-quality
coatings, either expensive helium needed to be used as process gas or,
if far cheaper nitrogen was used, the grain size of a special fine
powder had to be carefully matched to the spraying process. In order to
achieve efficiency levels of around 60% with nitrogen, a grain size of 5
to -25 µm for copper was necessary. As tests have shown, in comparison
with traditionally used nitrogen, far higher flow speeds can be achieved
with helium as the process gas (the snowball flies faster). The
temperature of the particles, however, is much lower. In standard
conditions, i.e. if nitrogen is used, a 20 µm copper particle hits the
substrate with a speed of 490 m/s and at 70°C. With helium, the particle
reaches 690 m/s, but the temperature is below room temperature. However,
there is a disadvantage in the cost of helium, which is about 10 times
more expensive than nitrogen.
In order to optimise the process, nozzles with a slight bell shape were
developed and the MOC nozzle was made as a tungsten carbide cobalt hard
metal, which means that the process gas temperature and particle speed
can be increased and the baking of the powder on the nozzle wall can be
reduced. The next step was to introduce a pre-chamber, in which the
powder is injected at a greater distance from the nozzle neck. In this
way, the particles are warmed before entering the MOC nozzle, and the
speed needed for adhesion of the particles is lower. Since larger
particles cool down less in the expanding gas jet, their impact
temperature is greater than that of the smaller particles.
New systems increase economic efficiency
The latest new development aimed at optimising the process was the
integration of additional heating into the spray gun, with which the
already preheated gas could be heated up 900°C in the event of nitrogen
or 700°C in the event of helium. For both processes, the impact speed is
more than 200 m/s above the critical speed. Under these conditions, with
nitrogen as the process gas, it is possible to achieve mechanical
properties which are comparable with those of solid material. If only
the nozzle design were improved, helium would be necessary for this.
Through optimising the spraying process, it proved possible to improve
conditions with nitrogen by 250m/s and 400°C. As a result, similar
conditions can be achieved with nitrogen as with helium. The resultant
increase in economic efficiency makes cold gas spraying cost-effective
for new applications. Cold gas spraying is suitable for a wide range of
metals if the speeds necessary for the adhesion of the particles can be
achieved with the available system. With most metals and alloys, this is
possible with nitrogen as the process gas.
The greatest progress in cold gas spraying can be seen in a comparison
of the coatings which are produced under standard conditions and with
use of the latest developments. If the extended pre-chamber and
additional heating in the pistol are used, it is then possible, with
nitrogen, at a pressure of 30 bar and temperature of 900°C, to achieve
coatings with an application efficiency of over 90% even with powders of
grain size -70+30 µm, with an adhesion strength of around 80 MPa. The
lower critical speed and higher impact temperature of the larger
particles improve the quality of the coatings. Because coarser powders
cost less than fine ones, this also improves the economic efficiency of
cold gas spraying.
With the development of new nozzles for better acceleration of the
particles and using suitable nozzle material to prevent the powder from
baking onto the nozzle wall, the speed and temperature of the particles
at the moment of impact was increased. This means coarser powders at a
lower price can also be used.
The CGT (Cold Gas Technology) system available at the University's
laboratory was refined in order to provide more efficient heating of the
particles and higher particle speeds. Both measures increased the
distance between the achievable impact speeds and the critical speed
necessary for adhesion. With copper as the spray material and grain
sizes of 10 – 70 µm, it was thus possible to achieve adhesion strengths
comparable with HVOF coatings and mechanical properties comparable with
cold formed solid material. This result can be transferred to other
spray materials.
High pressure for better characteristics
Until now, process optimisation measures in cold gas spraying have
concentrated mainly on the design of new nozzles to achieve a more
effective acceleration of the particles. Spray powders were kept very
fine, since only fine particles reached the speed necessary for
adhesion. As Tobias Schmidt from the German Armed Forces University in
Hamburg explained, more recent studies show that very fine spraying
powders do not always produce an optimum coating quality, since larger
particles deform more easily and bond better. Optimum impact conditions
have been achieved with a newly developed spray gun which is integrated
into a conventional spraying system. The result: the qualities of cold
gas sprayed copper coatings were improved through higher process gas
temperatures, an improved particle injection and optimised particle size
distribution. The spray gun newly developed for this, with a 17 kW
heater and extended pre-chamber, was integrated into the Kinetiks® 3000
CGT spray system, which was further developed into the Kinetiks® 4000,
which Helmut Paul Höll from Cold Gas Technology GmbH presented in his
speech. With this, using nitrogen, process gas temperatures of up to
900°C and process gas pressures of up to 45 bar can be achieved. The
spraying powder can be specifically warmed, to over 500°C in some cases.
The result of this, for example with niobium, is a far better adhesion.
The maximum pressure was increased to 44 bar. Now, temperature
differences of 550°C are possible, giving considerable savings on gas
consumption. Without preheating, the ActiveJet gun can reach
temperatures of 550°C for nitrogen, or 800°C with preheating in front of
the nozzle. Two powder conveyors can work in parallel and coatings can
be made from powder mixtures. The new Kinetiks® 4000 system is designed
on a modular basis and can be modified to suit all sorts of
applications.
Application efficiency greatly improved
With cold gas spraying, various spray materials can be processed
without exposing them or the substrate to high thermal loads. In
comparison with other thermal spraying processes such as plasma spraying
or flame spraying, cold gas spraying allows coatings to be made with an
extremely low porosity and extremely low oxygen content. The application
efficiency is very high and can reach up to 90%.
Because of their low melt temperature and high level of toughness,
materials such as copper, aluminium or zinc have become well-established
coating materials for cold gas spraying. They are used, for example, in
the aerospace, automobile and electronics industries. Using the example
of tantalum, Dr Stefan Zimmermann from H.C. Starck explained how,
through optimisation of the powder properties, it was possible to
improve both application efficiency and coating qualities specifically.
With tantalum, niobium or nickel, extremely dense coatings can be
produced which ensure effective corrosion protection. Because, with cold
gas spraying, the composition of the material is not changed by
oxidation or metallurgical reactions, these coatings should also be used
in cases in which high levels of purity and specific phases are
important factors.
The optimisation of the powders is also economically interesting, partly
through and considerable increase in application efficiency and also
through the improvement of process reliability because there is little
or no nozzle caking. The costs of producing coatings from tantalum,
niobium or materials containing nickel in which the application
efficiency was increased to well over 50% through further development of
the powder thus falls to a competitive level.
Convincing successes in practical application
Cold gas spraying is now an integral part of the aerospace industry.
The approval of the use of the process in this demanding industry proves
that, for the processing and finishing of all different types of
surfaces, there is a new, convincing aspect which has already proved
successful in practice. Dr Thorsten Stoltenhoff from Praxair Surface
Technologies GmbH was one of the few speakers prepared to talk openly
about their applications. He talked about the coating of dynamically
highly stressed aerospace components made from aluminium using cold gas
spraying, used, for example, in engine technology or for the coating of
rotor blades. The process has produced considerable improvements in
performance in engine technology in particular.
The first steps have already been taken for 18 further applications,
although nobody was allowed to speak about products and details as yet,
since the clients had requested absolute confidentiality in this
respect.
Excellent development potential in the USA and Australia
There are very promising, up-and-coming markets for cold gas
spraying in the USA and Australia. Both countries have a wide degree of
experience with HVOF processes and it is now possible to build upon it
with the new cold gas spraying process. In the USA, studies on cold gas
spraying are being carried out in many research facilities, mainly led
by ASB Industries. It is one of the goals of the CISRO Institute of
Management to offer, at the US research facilities, innovative and
flexible training programmes which provide a link between traditional
teaching institutions and the demands of the market. HVOF spraying has
become established in particular in the metal and steel industry, in
which it is used mainly to proven protection against wear, according to
Charles Kay from ASB Industries, USA. In Australia, there has been a
cold gas spraying working group since 2003. Dr Mahnaz Jahedi from CSIRO
(Commonwealth Scientific and Industrial Research Organisation)
Manufacturing and Materials Technology, Australia, – the only woman
amongst the speakers – explained that cold gas processes are
increasingly replacing traditional processes for coating, since they
cause less environmental pollution and often offer better protection
against oxidation. Research is focusing on titanium, which is to be
found in high volumes in the fifth continent and which is regarded as a
very promising alternative for the coating of parts.
Thermal spraying –ultrasonic waxing
Dr Frank Gärtner from the German Armed Forces University in Hamburg
referred in his presentation to the approaching 100th birthday of
thermal spraying. This is a very successful sector with great promise
for the future, with annual growth rates of 10% and more. High-velocity
flame spraying is the fastest growing area of thermal spray
applications. Various HVOF spray systems are competing in the market,
depending on the coating properties required. The latest developments in
HVOF spray systems are aimed primarily at optimising combustion and at
improving particle acceleration (e.g. WOKA®-Star 600 from Sulzer Metco
or JP 8000 from Praxair) or HVOF nozzles that operate at higher pressure
(CJS HVOF from Thermico).
Wire as an economic alternative
The latest developments also include high-velocity combustion wire
(HVCW) systems such as the W1000 gun made by Metatherm. In the HVCW
spray process, ultrasonic speeds are achieved through high process gas
pressures of 6 – 8 bar. The hot acceleration gas enclosures the tip of
the wire, cuts of droplets and accelerates these further towards the
substrate. The melting temperature of the spray material must be
exceeded. The speed of the droplets is only about half of the speed with
HVOF powder flame spraying. With steel wire, spray rates of five to nine
kilograms per hour are achieved.
In comparison with HVOF spraying with powder, HVOF combustion wire
spraying is an economic alternative because of the lower investment and
running costs for the necessary plant technology, according to Professor
Dr. Bernhard Wielage from the Technical University in Chemnitz. With
combustion wire spraying, it is mainly simple systems using combustion
gas and compressed air cooling that are used. The uncomplicated handling
is an advantage, but the limited choice of spray materials is a
disadvantage. The precondition for high coating qualities is the
homogenous melting of the wire tips. This requires precise coordination
of the flame parameters, speed of the forward movement of the wire and
the wire design.
Mobile computer tomography for thermal coating
How can the properties of a sprayed coating be determined?
Diagnostic processes for the optimising and monitoring of thermal
spraying systems was the subject of a presentation by Professor Dr.
Klaus Landes from the German Armed Forces University in Neubiberg. Two
new procedures have been developed to carry out control measurements and
monitoring of the spraying processes as it is happening: using computer
tomography, known from the world of medicine, data can be obtained which
had been so far unavailable because of their three-dimensional nature.
One element that is particularly interesting in practice is the
development of a mobile tomography system which, because of its simple
assembly and low weight, can be used on site. With Axial Particle
Velocimetry (APV) data can be obtained on particles throughout the spray
jet.
In addition to the required coating properties, economic efficiency is
also a criterion for particular applications using HVOF spraying. As
Rainer Schwetzke from KVT Kurlbaum GmbH explained, it is not only the
cost of operation which determines economic efficiency calculations.
Cost effectiveness can also be enhanced by improving the powder
conveyance rate and increasing the application efficiency.
GTS – A strong community
The success of thermal spraying is reflected, not least, in the fact
that there are so many members of the GTS Gemeinschaft Thermisches
Spritzen e.V. (=thermal spraying federation). The work of the Federation
was discussed by Peter Heinrich, Managing Director. Three elements are
responsible for the long-term success of the technology – "…having a
solid basis, adopting fundamental goals and being independent", he
stated. The basis for the GTS was formed by the active member companies
with their highly qualified employees. The aim of the GTS is to ensure
the quality of thermal spraying and to demonstrate it by means of the
GTS Certificate. Independence, as the third pillar, was ensured with the
founding of the GTS in 1992.
Out of the 75 full members today, 53% are companies employing fewer than
10 people. This shows that smaller and medium-sized companies in
particular have found a sound body to represent their interests. The
only companies who are granted full membership are those which apply the
technology and process of thermal spraying and which are certified in
accordance with the tough GTS guidelines. The area of work of the GTS
covers, for example, the issue of certificates for high-quality
products, ensuring that thermal spraying remains a sophisticated
technology through certification, the provision of training and
continuing professional development for employees, publicity work and
the promotion of an exchange of information amongst members. The lively
cooperation within the GTS is also reflected in its many small
communication groups. The Newcomer@GTS circle, representing the younger
generation, is particularly promising for the future.
Great expectations for the future
Many other topics were also presented and discussed in Erding, such
as arc jet spraying, certification and training. There was also a lively
exchange of information at the exhibitors' stands. Visitors were able to
find out about the latest state of technology and the most recent
developments in systems, applications and materials. However, there was
one sad moment: at the Colloquium, Professor Kreye announced that he
would be retiring after many years of active service for this forum.
The extremely lively event, with its presentations, exhibitors,
discussions and talks showed that thermal spraying is now a well
established process for applying coatings to metal surfaces to provide
protection against wear, corrosion or heat. High-velocity flame spraying
has become an increasingly cost-effective process for which many new
applications are set to open up in the future.
Colloquium chairman Peter Heinrich closed the event with a call to
develop new systems over the coming three years so that the standard of
the next Colloquium in 2009 could be just as high as this year's.
Further information online: www.gts-ev.de
(Text: MM Mertig Marktkommunikation, Munich-Gauting)
Contact:
Werner Krömmer
Managing Director, Gemeinschaft Thermisches Spritzen e.V. (GTS)
c/o Linde AG, Linde Gas Division
Carl-von-Linde-Str. 25, 85716 Unterschleissheim, Germany
tel: +49 89-31001-5546, fax: +49 89-31001-5364
e-mail: info@gts-ev.de
Internet: www.gts-ev.de
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