HVOF takes off

TWI Bulletin, September/October 1991

With a degree in chemical engineering and after two years teaching experience, David Harvey joined TWI in 1986. He is now Head of the Surfacing and Cutting Section in the Arc Welding Department with responsibility for research in thermally sprayed and welded coatings.

Since joining TWI, David has looked at a number of difficult areas relating to MIG/MAG welding ranging from materials problems such as porosity in aluminium alloys and cast to cast variation in weld penetration to process problems like contact tip performance, welding wire condition and arc stability. More recently, he has been involved in application of magnetic arc oscillation to the orbital TIG process and development of a backface penetration control system.

High velocity oxyfuel (HVOF) spraying is the most significant development in thermal spraying since vacuum plasma. TWI has recently installed a HVOF spraying system. David Harvey explains the significance of HVOF spraying, describes TWI's latest facility and looks to the future of the process.

The HVOF process involves combustion of a fuel gas with oxygen at high pressure and/or flow rates and generation of a high velocity gas flame which propels powdered coating materials on to a workpiece surface. Much early development work involved applying carbide coatings to aeroengine components using the D-gun process in the early 1950s. The process provided high quality coatings, but concerns about costs and single sourcing have led industry to evaluate other coating methods.

The breakthrough in HVOF spraying arrived with the introduction of the Jet Kote spraying process in 1982. Since then applications have expanded from the initial use of tungsten carbide to include hundreds of different coatings which provide wear, erosion and corrosion resistance, thermal barriers, reclamation and clearance control in a variety of industries. A recent survey estimated that HVOF coating services are now worth £100M a year worldwide.

Since Jet Kote, a number of HVOF systems have been developed, such as Diamond Jet, CDS and Top Gun. TWI has recently installed the Top Gun high velocity oxyfuel spraying system in the Arc Welding Department.

Spraying system

The main components of the high velocity spraying system at TWI are a spraying gun ( Fig. 1), a gas flow control console, a controlled temperature heat exchanger and two roto-feed powder hoppers ( Fig.2). The system also has an automatic ignition system and a remote control pendant which gives the operator the flexibility to work either inside or outside the booth whilst spraying. The gun can be operated manually or can be mounted on a fully programmable computer-controlled, high speed traverse (with a maximum scanning velocity of greater than 5 m/sec). 

The fundamentals of the process are simple. Fuel gas and oxygen are mixed in the gun before the combustion chamber, where the gases are burnt ( Fig.3). Powder is fed using an inert carrier gas (argon or nitrogen) directly into the centre of the combustion chamber, where rapid heating and acceleration of the particles take place. The combination of a fuel gas pressure over 10 bar and gas flow rates of several hundred litres per minute can produce a flame speed of 2000 m/sec and a particle velocity of 800 m/sec. The stand-off distance between torch and substrate is generally in the range 120 - 350mm and is far less critical in HVOF spraying than in plasma spraying.

Choice of fuel gas depends on application, but includes hydrogen, propylene, propane, MAPP and acetylene. The system at TWI is unique in its capacity to burn acetylene. Although restricted to operation below 1.5bar for safety reasons, the higher flame temperature is useful for applications involving ceramic coatings. Particle velocities over 500 m/sec have been reported with acetylene fuel gas.

Coatings

Adhesion to the substrate is excellent for a thermally sprayed coating, with bond strengths over 70MPa being achieved. Whilst coating thicknesses over 0.5mm are unusual from plasma spraying or the D-gun process, the limit for HVOF spraying is over 1.5mm. This is because HVOF coatings have lower residual stresses, often achieved in small components or thick coatings by careful cooling of the substrate by air or liquid carbon dioxide.

At spraying rates of 1-5 kg/hr and a deposition efficiency of over 60% (more for many powders), a layer of 10µm is deposited in each pass. With careful application, a surface finish of less than 3µm can be produced.

Materials

As well as the tungsten carbide cermet type materials, for which the D-gun, plasma and HVOF spraying processes were originally developed, TWI's equipment can be used for spraying metal, alloy and ceramic coatings. Other HVOF systems cannot produce coatings in the highest melting point materials, i.e. ceramics, because the maximum combustion temperature depends on the fuel gases used and the chamber design. Systems for spraying ceramics such as alumina, chromia, zirconia and titania must be able to burn acetylene. The combustion temperature of acetylene of around 3160°C is above the melting point of zirconia, 2800°C. Examples of materials which can be sprayed by TWI's system include:

• metals - molybdenum, copper, nickel, tungsten;
• alloys-stainless steel, Inconel 625, Inconel 718;
• cermets - tungsten carbide in cobalt, chromium or nickel;
• ceramics - alumina, titania, chromia, zirconia and mixtures.

The considerable range in melting points of these materials, from less than 1000°C for some metals to over 2500°C for ceramics, is accommodated primarily by using combustion chambers of different lengths. Higher melting point materials require the longest chamber size to maximise exposure time to the hottest part of the flame in the chamber, and conversely for lower melting point materials.

Applications

The original force driving development of the D-gun, plasma spraying and HVOF spraying was the need for high temperature wear resistant coatings for many aeroengine components such as the cobalt-tungsten carbide materials, ( Fig.4). However, applications have expanded from this somewhat specialised field into many new areas such as the petrochemical and offshore industries, automotive components and general engineering applications, including printing, textiles, and mining ( Fig.5 and Table).

Future work

• biocompatible materials for prostheses;
• graded coatings using two microprocessor controlled powder hoppers;
• unsealed coatings in corrosion resistant alloys.
  

HVOF spraying is a relatively new technology and there is an urgent need to standardise on coating analysis methods and to update thermally sprayed coatings testing techniques. For example, ASTM C633, the standard test for adhesion of flame-sprayed coatings, does not take into account the wear mechanisms and shear stresses often imposed on these coatings. Collaborative work with TWI Industrial Members is planned to determine meaningful testing and analysis techniques.

Application areas for HVOF spraying