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High Velocity Air Fuel (HVAF) Spraying

Introduction

High velocity oxy-fuel (HVOF) spraying has been used as a hard chrome plating alternative for many years. However, new developments in thermal spraying processes potentially offer higher quality alternatives to hard chrome plating and hard chrome plating alternatives, in a wide range of coating applications. These include updated HVOF guns and high velocity air fuelled (HVAF) spraying, as well as new spray consumables. These are being developed as a result of REACH legislation becoming increasingly restrictive for coatings based on Cr, Co and Ni alloys and, in particular, plating technologies.

HVAF spraying is a ‘warm spray’ process that is cooler than HVOF, but hotter than cold spraying, enabling a wider range of materials to be sprayed. As with the other thermal spray processes, with the exception of cold spraying, the feedstock powder is melted but, in the case of HVAF, this occurs just above the melting temperature of the material, and lower than the vaporisation temperature (HVOF operates above the vaporisation temperature).

Development and Benefits

The development of HVAF spraying systems has been aimed primarily at improving coating quality, while reducing the operating costs versus HVOF spraying via increased spray rate and deposit efficiency as a result of the increased particle velocities and a more efficient mechanism to heat the feedstock powder. Manufacturers of the equipment claim these process developments offer significant benefits for depositing high quality, dense, protective coatings for demanding industrial applications where combined corrosion and wear mitigation is required.

The application of thinner/denser HVAF ‘flash carbides’, already in limited use in industry with thicknesses ranging from 10-50µm, opens up cost/performance improvements over coatings produced using conventional HVOF spraying systems which are typically ~300µm thick. WC-CoCr HVAF coatings are claimed to offer comparable performance to electrolytic hard chrome plating. The deposited coatings are ~1400HV300 with an as-sprayed finish of ~1.5µm Ra, which is comparable to a typical ground surface finish, thus eliminating some of the post-spraying finishing costs. The use of thinner coatings also offers improvements in the fatigue debit over conventional HVOF hard chrome replacement coatings in demanding applications such as aircraft landing gear.

Cost savings are made with HVAF compared to HVOF WC-CoCr and Cr3C2-NiCr coatings, due to the use of less material and process gases, and the elimination of process steps, such as grit blasting and dimensional grinding post spraying. As with all thermal spray processes, HVAF coatings are applied directly to substrates without a heat affected zone, which offers significant benefits over fusion processes such as traditional weld overlays, plasma transferred arc (PTA) coatings, laser additive manufacturing processes and the extreme high-speed laser application (EHLA) process.

The use of air for HVAF spraying, rather than pure oxygen, reduces the oxide content of coatings compared with their HVOF equivalent. The lower flame temperature, typically ~1000°C lower than oxy-fuel combustion, also results in less thermal degradation of feedstock materials as particles are heated to just above their melting point. The combination of lower temperature and lower oxygen content in the combustion gas mixture helps to minimise oxidation of metal alloys and the decomposition or dissolution of carbides into metal binders in cermet coatings. As a result, the coatings can retain higher levels of ductility while maintaining high hardness levels. However, suppliers do offer combined HVAF/HVOF systems, which can operate in HVAF + HVOF mode to achieve a wide parameter window. For HVAF only systems, the requirement for bottled oxygen storage can be eliminated with the systems running entirely off of compressed air supplies.

Figure 1. Temperature / Particle Velocity for spray processes
Figure 1. Temperature / Particle Velocity for spray processes
Figure 2. Kermetico C7 Convertible HVAF+HVOF Gun
Figure 2. Kermetico C7 Convertible HVAF+HVOF Gun
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Gun Design and Application

The gun design is also an important factor in the coating quality. The size of the combustion chamber allows a long residence time for heating of the spray particles. The gun design also enables particles to be accelerated to the gas velocity at 800-1000m.s-1. In this regard, the HVAF process offers some benefits similar to cold spraying technology, with high particle velocities and dense coatings, combined with minimal melting of the feedstock due lower combustion temperatures. As a result HVAF can deposit coatings that would be challenging to achieve with using cold spraying, such as materials with limited ductility or high volume fractions of ceramics.

As with HVOF, HVAF spraying technology offers opportunity for coating large size components at high throughputs / spray rates of up to 33kg.hr-1, making it a candidate for many industrial applications. Deposit efficiencies vary from 40-70% depending on the gun selection, coating type and processing parameters.

Two equipment suppliers offer HVAF spraying systems: Kermetico and UniqueCoat. Their product portfolios include a number of HVAF coating variants, including internal diameter spraying systems, which also open up opportunities for coating internal bores with a hard chrome plating alternative. This has been something that has proved to be challenging to date with traditional HVOF systems.

Applications for HVAF technology include those where hard chrome plating or HVOF coatings have traditionally been applied, including hydraulic rods and cylinders, aircraft landing gear and actuation systems, piston rings and valves, machinery parts, sliding and rotating shafts, automotive drive trains and suspension, tools and dies, and internal bores. HVAF coatings are also finding applications for protecting vessels against corrosion including in H2S environments, cavitation and erosion resistant coatings for pipework, fittings and valves, rollers for the steel and paper industries, along with hydroelectric components.

In addition to coating applications, HVAF has also shown promise in repair and reclamation. Deposits of Al, Cu and bronze alloys can be built up to several millimetres thick with high adhesion and cohesion strength. The deposition of Al alloys potentially offers significant benefits for repair of aluminium and magnesium components, whilst HVAF copper coatings can be machined and engraved and offer an alternative to electroplated copper coatings. High quality copper coatings are increasingly needed for demanding big science projects, such as ITER, where properties such as high thermal and electrical conductivity are required.

 

HVAF and TWI

TWI is planning to install Kermetico HVAF equipment at our Cambridge facility to provide independent assessment of the HVAF technology against competing technologies, such as traditional hard chrome plating replacement alternatives, across a wide range of industrial applications. This facility will be unique in the UK and available for the benefit of TWI Members.

Michael Breitsameter, Director of Sales and Marketing at Kermetico, with over 30 years’ experience in the thermal spraying industry, had this to say on this new collaboration:

“Kermetico is very much looking forward to working with TWI, largely based on its outstanding reputation for supporting a range of industries, the team of experienced individuals who we have been interacting with, but also some personal memories of working with this group back in the 1990s. This was a formative time for HVOF with the Top Gun, the Diamond Jet and the JP5000 HVOF systems being adopted by TWI, two of which I was personally involved with. TWI established a position as the UK's leading independent surface technology research house, utilising cutting edge equipment and resources to push the boundaries, to outreach to new industry spaces, along with conducting high quality research. My role was always to promote and sell the technology, usually to update existing technology, adding capability or capacity at commercial companies selling coatings to a range of industries.

“On one occasion I sent a potential equipment client (the owner of a highly respected spray shop) to TWI to have the JP5000 demonstrated, based on their level of comfort in the fact that TWI had no specific agenda regarding one piece of equipment or another, one technology or another. This type of outreach to industry is a role that TWI fulfils for the good of its Membership and to ensure the latest technologies are available to industry, without bias, leveraging the many years of experience accumulated in this area.”

Dave Harvey, TWI’s Technology Fellow in Surface Engineering, also spoke on the process, saying:

“TWI has been at the forefront of the evaluation of novel thermal spraying processes including HVOF and cold spraying for 30 years. HVAF spraying is one of the most important technology developments in this field in recent years, bridging the gap between the HVOF and cold spray processes. HVAF has the potential to deliver benefits to coating vendors and end-users in terms of reduced coating material costs, higher productivity and longer service life in demanding service conditions, whilst complying with environmental legislation. We really look forward to working with the Kermetico HVAF system and providing TWI Members with an independent evaluation of the characteristics and performance of coatings produced by this exciting technology.”

To support this activity, TWI is launching a Joint Industry Project on New Surface Engineering topics, comparing HVAF technology with the extreme high-speed laser application (EHLA) technology, and conventional HVOF hard chrome plating replacement technologies.

Further R&D support may be accessed by Members through TWI’s core research programme, single client projects and collaborative programmes. To discuss your requirements in more detail please contact us, below.

 

(HVAF process images courtesy of Kermetico)

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Avatar Melissa Riley Principal Project Leader - Surface Engineering

Dr Melissa Riley has a B.Med.Sc degree and PhD in Metallurgy and Materials from the University of Birmingham and is Principal Project Leader within TWI’s Surface Engineering team. She has worked at TWI for over 17 years on a wide range of projects for TWI’s Industrial Members relating to the development and application of thermal spray coatings, including HVOF spraying. Her work has included application and performance evaluation of wear and corrosion resistant coatings (including for biomass and waste to energy plants), ceramic coatings, bearing materials and more novel coatings and applications, including leading TWI’s work on the development of coatings on composites. She has also been instrumental in setting up a number of new facilities to assess coating performance and to support TWI Members in the upscaling and manufacturing development of surface engineering technologies for industrial applications. Melissa is a Chartered Engineer and Fellow of the Institute of Materials, Minerals and Mining.

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