HVOF spraying evolves to meet 21st century challenge
TWI Bulletin, January/February 2001
David Harvey joined TWI in 1986 with a Masters degree in Chemical Engineering, working initially on projects related to process control in gas shielded arc welding. He has subsequently been closely involved with the development of TWI's thermal spraying facilities, including the installation of four HVOF spraying systems. He was Chairman of the UK Surface Engineering Society 1994-97 and helped establish the UK Thermal Spraying & Surface Engineering Society in January 1998. His current responsibilities are in business development for the Metallurgy, Corrosion, Arcs and Surfacing Technology Group.
When TWI installed a high velocity oxyfuel (HVOF) system in 1991, it was one of a handful of systems in the UK and exploitation of the technology was in its infancy. Ten years later, the HVOF process has established itself alongside detonation and plasma spraying as a leading industrial coating technology, with almost 100 systems in operation in the UK today. David Harvey reviews the evolution of the technology in the last decade and discusses the developments that keep this exciting process at the forefront of thermal spraying technology.
The first HVOF unit to be installed at TWI was the UTP TopGun system, Fig.1 . At the time of installation this was, and perhaps still remains, the most versatile of the HVOF systems in terms of the coating materials that it can spray. This is the result of both the wide range of fuel gases this system can burn (propylene, propane, hydrogen and acetylene) and the resulting range in combustion temperature that this provides, and the gun design, which allows the powder consumable to be fed directly into the combustion chamber. Consequently, materials with very high melting points, such as ceramics (alumina, chromium oxide and yttria stabilised zirconia) and refractory metals (molybdenum and tantalum) or materials with very low melting points, for example aluminium and copper alloys, can be sprayed through this system.
Fig.1. TopGun HVOF system
The next generation...
Although TopGun meets the supersonic flame velocity criterion of HVOF spraying (as characterised by the shock diamond flame), it was clear that the process relies heavily on heat transfer to the spray particles and their subsequent melting for substrate adhesion and the formation of coatings. In addition, the high particle temperature attained in this system encourages the generally undesirable oxidation of spray particles, characterised in tungsten carbide coatings by the conversion of WC to less wear resistant W 2C and cobalt alloy phases, or in metal alloy coatings by high levels of metal oxide. New process variants, commercially available since the mid-1990s, have addressed this reliance on high particle temperature by reducing heat transfer and increasing flame, and hence particle, velocity. Two systems, typical of this next generation are the Tafa JP5000 and Sulzer Metco Diamond Jet (DJ) hybrid HVOF systems, Fig.2 .
Fig.2. Next generation of HVOF systems: JP500
Fig.2. Next generation of HVOF systems: Diamond Jet
To reflect this new approach TWI installed a JP5000 in 1996 and a DJ hybrid HVOF system some eighteen months later. At this time, the JP5000 represented a significant development in terms of fuel, being the first globally successful, commercial liquid fuel (kerosene or paraffin) system. The use of a liquid fuel generates very high power (over 200kW) and flame velocity during the combustion process, but the relatively low combustion temperature of kerosene (300°C below gaseous fuels) and the post-combustion chamber powder feed results in (the more desirable) faster, cooler particles. Although similar to the TopGun system in the use of gaseous fuels and axial powder feed injection, the DJ hybrid HVOF system has a much smaller combustion chamber (which leads to less heat transfer) and a superior nozzle design (resulting in faster flame and hence particle velocity). Although appreciably different in design, it is TWI's experience that metallic and metal carbide coatings produced by the JP5000 and DJ hybrid systems are remarkably similar to each other in terms of most characteristics and properties ( eg density, adhesion, oxide content, wear resistance and residual stress).
This technical superiority of the coatings, however, has been achieved at a price and many operators have noticed that the low heat input, higher particle velocity approach often results in a lower coating deposition efficiency - particularly with regard to the important market of metal carbides ( ie tungsten carbide, chromium carbide). Manufacturers are addressing the issue by modification of the gun design (to increase heat transfer) and by improving the powder consumables by optimisation of powder manufacturing techniques and tighter control of particle size distribution.
New developments in HVOF spraying
HVOF technology continues to move forwards to what many consider to be the 'Holy Grail' of thermal spraying. This is the production of coatings from very high speed, solid particles, and it has been predicted that this can be achieved by generating parameters that meet those defined in the 'zone of solid impact' as indicated in Fig.3 . This requires the delivery of greater thrust and less heat transfer to optimum-sized spray particles. It is anticipated that this can be achieved in a number of ways:
- High velocity impact forging (HVIF): Very high oxygen supply pressures of up to 40bar ( cf JP5000 at 15bar) have been used in prototype HVIF systems developed by Draco Technologies, with a view to cold welding spray particles onto the substrate. Initial results are promising, but there is appreciable industrial resistance to operating such a high-pressure combustion system. A second approach to HVIF spraying has introduced water injection to the combustion process, typically at 10-18bar. Again, initial results are encouraging, and systems operate at more industrially acceptable combustion pressures.
- Fine particle HVOF: This is basically a variant of the HVIF process, where the average particle mass is of the order of about 10-15% of conventional spray powders. The lighter spray particles can be accelerated to much higher particle velocities with little heat transfer from the combustion process. One of the major achievements of the system manufacturer, Thermico, has been to build a reliable powder feed system for such fine powders.
- Cold spray: This is not in fact a combustion process, and neither is it cold! One of the first commercial systems constructed by Cold Gas Technology delivers argon or nitrogen gas at up to 35bar and heated up to 700°C. Cold spray systems are currently being assessed for depositing materials with relatively low melting points such as copper and aluminium alloys.
It can be seen from Fig.3 that these process innovations are approaching the boundaries of the 'zone of solid impact', and it is anticipated that further developments in the near future will lead to true solid particle impact coatings.
Fig.3. Relationship between current HVOF process developments and the deposition of coatings from solid particles or the 'zone of solid impact'
Whilst some manufacturers strive for the Holy Grail, others are focused on reducing costs and adapting HVOF spraying for on-site use. For example, equipment manufacturer High Velocity Technologies now offers HVOF wire spray and high velocity air fuel (HVAF) systems. The former offers the advantage of lower cost consumables, the latter the cost benefit of running on compressed air.
Staying at the top...
TWI is already taking steps to remain at the forefront of HVOF spraying innovation. High thrust, water injection variants of the current HVOF systems are already available at TWI. In November 2000, TWI installed a high velocity (HV) spraying unit with wire feed,
Fig.4 . The High Velocity Technologies system represents both a significant advance in wire flame spraying technology and a welcome complement to the powder-fed HVOF systems. Wire feed permits the spraying of materials that are difficult to spray in the powder form (
eg low melting point materials such as aluminium, zinc and copper). Significant cost savings can also be made since wire consumables are almost invariably cheaper than their powder equivalents.
Fig.4. High velocity wire spraying system
The HV wire system features a lightweight gas control console and wire feed unit. Coupled with its ability to operate from single cylinders of fuel gas and oxygen and a conventional compressed air supply, the HV wire system is well suited to on-site production of coatings.
As part of the new Core Research Programme for 2001-3 on Novel Surfacing Techniques, TWI plans to build a cold spray system in 2001. Current prototype cold spray systems are based around welding rectifier gas heating, but TWI proposes to build a microwave-heated gas delivery system. This will make a significant contribution to simplifying the design and reducing the cost of cold spray system construction. TWI will investigate the spraying of low melting point materials such as polymers and metals eg Zn, Al and Cu alloys.
In addition, TWI will be assembling a mass flow control system that can be used on all four HVOF systems. This reflects increasing industrial requirements for more accurate control and monitoring of gas flows through HVOF systems. Although a number of commercial mass flow systems are presently available, the objective is to build a unit at a fraction of the price of current systems.
Conclusions
The HVOF process has become a well-established tool for producing wear and corrosion resistant coatings, but industrial demand for higher quality coatings at lower price continues to deliver exciting process and materials developments. The availability of high thrust water injection HVOF, the installation of an HV wire system and future plans to build cold spray facilities and a universal mass flow control console, confirms TWI's position as a leading independent organisation in the field of HVOF research and applications development.
