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Steve Bomford is a Training and Application Support Manager with Oerlikon Metco in the UK. Steve is passionate about thermal spray and have over the years in many different positions gathered an extensive amount of knowledge abou thermal spray.
There are many of you out there that already have a deep understanding of the thermal spray process and equally, I am sure, some of you reading this article have come across this process for the first time. One thing I have learned over the years is that with thermal spray, every day is a school day!
With that in mind, I’ve decided to put virtual pen to paper and pass on some of my learnings from over the past 38 years. These may well be somewhat personalized, so don’t take everything completely to heart. The top three thermal spray processes are, of course, up for debate, but in the interests of available content space, I will go for:
Number 3: Electric arc wire spray
Electric arc wire spray uses two metallic wires as the coating material. They are electrically charged (one wire negatively charged, the other positively charged) and fed into the arc gun. When the wires are brought together at the nozzle, the opposing charges on the wires create an arc and heat to continuously melt the tips of the wires. Compressed air (or an inert gas) is used to atomize the now molten material and accelerate it onto the workpiece surface.
Capable of producing excellently bonded metallic coatings with a wide variety of functional properties, this process is used extensively in a broad spectrum of industries. From anti-corrosion coatings for Offshore applications to high specification dimensional restoration deposits in the aerospace sector, electric arc wire spray has proven itself to be a flexible tool for hand-held and automated processes.
In fact, when used for the demanding requirements specified in aerospace, it has replaced more traditionally used deposition techniques for metallic coatings. Operators see shorter spray times because of the high wire feed rates employed as well as fewer issues with dis-bonding of coatings during machining. A win-win situation!
The basic principle of electric arc wire spray.
Number 2: High velocity oxygen fuel (HVOF)
In fact, I am cheating a little here as we have thrown in two processes for the price of one! HVOF gas-fuel and HVOF liquid-fuel. Typically, with HVOF-GF, an oxygen-fuel gas mixture is ejected from a nozzle and ignited externally of the gun. The ignited gases form a flame. Powder is injected axially into the combustion zone. The powder exits the gun through the flame and is propelled to the workpiece. In the case of the Oerlikon Metco Diamond Jet™ HVOF gun (crosssectioned in the image below), the process also uses compressed air or an inert gas to cool the internal workings of the gun as well as reducing negative reactions of powder particles with the outside atmosphere.
In the case of HVOF-LF, an oxygen-kerosene mixture is injected into a combustion chamber within the gun. Either a pilot hydrogen flame or a spark plug will ignite the flame. Powder is injected radially into the combustion zone. The powder exits the gun through its barrel and is propelled to the workpiece.
A late-comer to the list of thermal spray processes, HVOF was invented in the 1980s by Jim Browning. Interesting fact that Jim’s invention was originally designed to be a rock-splitting tool! It quickly changed application direction and became the forerunner of the HVOF guns we are familiar with today.
The key characteristic of HVOF is the exceptionally high particle velocities (supersonic in most cases). This enables deposits produced to exhibit very high bond strengths together with low porosity levels. The resultant coatings are used widely where properties such as exceptional wear and corrosion resistance are mandated.
The HVOF gas fuel (HVOF-GF) process
The HVOF liquid fuel (HVOF-GF) process
Number 1: Air plasma spray (APS)
Last, but by no means least, is air plasma spray. By generating an ignition between an electrically positive pole (anode/nozzle) and an electrically negative pole (cathode/electrode), an arc is generated. Through the pressure of the gas, the temperature of the arc increases, the gas ionizes and a plasma develops. Once parameters have stabilized, powder is injected into the plasma jet; the powder particles soften and reach the surface of the workpiece at high velocity creating a coating.
The APS process is an enormously flexible one. The high level of enthalpy available together with the wide range of sprayable powder compositions means that it is applicable to a wide range of applications. Coatings can be applied that resist wear (carbide containing materials), offer high temperature insulation (Thermal Barrier Coatings — TBCs), improve corrosion performance in extreme environments (MCrAlYs, e.g., materials containing cobalt and/or nickel with chromium, aluminum and yttrium). The list goes on.
The future is also very bright for plasma spraying. The development of cascaded arc technology has created a more stable and higher enthalpy plasma system. The extended arc produced allows for higher feed rates, more efficient deposition and less process variability. All things the market desires.
The traditional air plasma spray process.
There are in the end of the day many different thermal spray processes that have different characteristics, pros and cons. This is just really a taste of where we are with thermal spray. The world is in constant change and we at Oerlikon Metco strive to keep pace with new developments.
A note from the author
Dear Friends and Colleagues,
I know that many of you reading this are senior technical experts in thermal spray, while some are just starting on the thermal spray learning curve. I hope of providing interest, education and entertainment to all, while I have decided to write my version of the thermal spray process review. There are a handful of short articles, related to the thermal spray. The idea behind this is to give you a subjective overview of the process and the latest developments in the world of thermal spray. This overview may well be wide and varied. It will not focus on too many technicalities, but it should hopefully provide an entertaining summary enabling a better understanding of the technology and its advantages. If you are a more experienced reader, I would like to invite you to join me in the discussions, ask questions, and leave your personal opinion. If there is a related topic you want me to write an overview of, I will be happy to consider that. I hope it will be interesting to read and will be happy to get your feedback.
Steve Bomford
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