The ultimate coating - thermal spraying at Abington

TWI Bulletin, March/April 1994

With a degree in chemical engineering, David Harvey joined TWI's Arc Welding Department in 1986. His initial activities covered process and materials problems related to MIG and TIG welding, followed by development and application of process control systems, such as magnetic arc and backface penetration control Since 1990, David has overseen the expansion of TWI's thermal spraying and surface engineering activities, in particular the installation of high velocity oxyfuel, air plasma and flame spraying facilities. His current priorities include promotion of industrial applications for the HVOF process, process development and technology transfer.

TWI's thermal spraying activities were established in 1986 with completion of the first spraying booth in AWD, and subsequently installation of high velocity flame and plasma spraying systems has established TWI as the leading independent UK thermal spraying R&D centre. Surfacing Section leader David Harvey reports on current facilities available to TWI Member Companies.

Thermally sprayed coatings are generated by propelling fine, molten or softened, particulate materials on to a prepared substrate. These coatings are characterised principally in terms of adhesion to the substrate, porosity levels and oxide content. Generally speaking, the higher the particle velocity the better the adhesion, the higher the density and the lower the oxide content of the coating ( Table 1).

Table 1 Principal thermal spray processes and coating characteristics

The bond between the coating (as sprayed) and the substrate is primarily a mechanical bond, although some coating materials are fused after spraying, and a proportion of metallurgical bonding may occur with the higher velocity processes. Adhesion is generally enhanced by roughening the cleaned substrate surface by grit blasting or rough machining. Unlike welded coatings, there is no dilution of either coating or substrate by fusion, and the melting point of the coating material can be higher than that of the substrate. Other advantages in spraying include little or no pre- and post-heat treatment and minimal distortion of the substrate.

There are four thermal spraying processes in general use for depositing a wide range of coating materials, listed here in order of decreasing particle velocity:

• High velocity oxyfuel (HVOF);
• Air plasma;
• Arc;
• Flame.

Examples of each type of equipment are currently used at TWI and are described below.

High velocity oxyfuel spraying

The most recent addition to the thermal spraying family, high velocity oxyfuel spraying, has become established as an alternative to the proprietary Praxair detonation (D-Gun ^TM ) flame spraying and lower velocity, air plasma spraying processes for depositing wear resistant tungsten carbide-cobalt coatings. Volume applications include aeroengine parts, gate valves, and engineering components such as wear plates, journals and bearing surfaces.

TWI's HVOF facilities comprise a UTP Top Gun ^TM pistol, two powder feed hoppers, a gas flow control cabinet, a heat exchanger and six high pressure high flow gas lines. A combination of high fuel gas and oxygen flow rates and high pressure in the combustion chamber leads to generation of a high velocity flame, displaying the characteristic shock diamonds of a supersonic flame. Flame speeds of 2000 m/sec and particle velocities of 600-800 m/sec are claimed by HVOF equipment suppliers. A range of gaseous fuels is currently used, including propylene, propane, hydrogen and acetylene.

In addition to the normal range of materials sprayed through HVOF systems metals, alloys and carbides), the Top Gun ^TM system can spray a number of industrially important ceramics (alumina, alumina-titania and chromia), ( Fig.1). The wide range of materials melting points is accommodated by control of powder size and combustion chamber length.

In general, high melting point ceramic materials have a fine powder size (5-15µm) and require a long combustion chamber to soften the particles. Lower melting point materials (e.g. aluminium) have coarser particle size (15-45µm) and use the shortest combustion chamber. An inert gas, usually argon (but occasionally nitrogen), carries the powder co-axially through the centre of the combustion chamber. High powder carrier gas flow rates are often used to reduce heat transfer to lower melting point materials. HVOF coatings are generally characterised by the highest bond strength (>70MPa), lowest porosity (<1%) and lowest oxide (1-5%) content of the conventional spraying processes.

Air plasma spraying

Air plasma spraying (APS) has been extensively used for over 40 years in the aerospace industry for coating turbine components with a range of wear and heat resistant coatings. The lower particle velocities (than HVOF spraying) generally result in coatings characterised by higher porosity and lower bond strength. However, two process developments have enabled plasma spraying to keep pace with HVOF and D-Gun ^TM . The first of these is the application of plasma spraying in either a low pressure or a vacuum chamber.

The higher cost of low pressure plasma spraying (LPPS) or vacuum plasma spraying (VPS) equipment (vacuum chamber, automatic manipulation and computer control) generally limits these processes to very specialised coatings (e.g. MCrAlY, Ti and Ta) requiring very low ppm levels of oxide. The second advancement in plasma spraying has been an increase in available power. Plasma spraying equipment now ranges from less than 20kW for manual systems lo over 80kW for high energy mechanised systems.

For example, the Miller Thermal plasma spraying unit at TWI incorporates a 100kW power supply, capable of delivering over 80kW to the SG100 plasma gun at a 100% duty cycle ( Fig.2). The combination of higher energy and increased gas flow through the control console can be used to generate a high velocity, Mach 2 plasma. This system has a number of advantages over sub-sonic plasma spraying equipment. The high velocity plasma significantly increases particle speed, resulting in higher density coatings with greater adhesion. The additional energy also increases potential deposition rate and deposition efficiency.

One further feature of plasma spraying offers an important advantage over both high velocity oxyfuel and detonation flame spraying. The very high temperature of the plasma arc enables refractory materials, such as zirconia or tungsten, to be sprayed.

The principal gases used in plasma spraying are argon, hydrogen and helium (and mixtures). The gases are either inert or reducing to prevent oxidation of the plasma electrodes. The time during which the powder is exposed to the plasma energy source is longer than HVOF spraying (since the particle velocity is much Lower). As a consequence, although the range of consumables is similar to HVOF, the powder particle sizes are generally larger, typically 30-70µm.

Arc spraying

Arc spraying is well established as a technique for depositing metal coatings at a high rate. The equipment comprises a power supply and two reels of wire fed simultaneously into the arc spraying pistol. An arc is struck between the ends of the wires, generating molten droplets, which are propelled at the substrate by a gas jet usually dry, compressed air.

The principal application for are spraying is protection of steel structures, e.g. bridges and offshore platforms, from atmospheric and marine corrosion. For many years, zinc has been the primary coating, but aluminium is being increasingly used, particularly when the component must be protected against polluted, acidic industrial atmospheres of sulphur and nitrogen oxides. It is common practice to seal these relatively porous coatings after spraying with epoxy resin-based or silicate sealants.

Arc spraying is also used for reclamation of worn shafts, bearings and journals, with materials such as low carbon steel, 13%Cr steel and aluminium bronzes. The porous characteristic of the deposit is often very useful for absorbing lubricant into the surface, and can often extend the useful life of the coating.

The unit installed at TWI comprises a Metallisation 300A Energizer ^TM power source and a 528 pistol. This pistol is designed for long periods of continuous mechanised operation, and small portable arc spraying guns are widely available for manual use.

Flame spraying

The simplest and cheapest of the thermal spraying processes, flame spraying can be carried out using both powder and wire spraying based systems. A range of fuel gases is used, including acetylene and propane, but the relatively low pressure and low flow rate lead to low velocity particles (40 m/sec). Consequently, the coatings are more porous and the bond strength low. Many applications are similar to arc spraying, i.e. zinc and aluminium for corrosion protection, steels and bronzes for reclamation. Systems are mostly for manual use and are very simple to operate.

The system used at TWI is a UTP Unispray-Jet ^TM powder spraying pistol, which can spray ceramic coatings, as well as the standard zinc and aluminium consumables ( Fig.3).

Spraying booths

TWI has installed two spraying booths ( Fig.4), both with computer-controlled high speed traverses, one built at TWI, specifically designed for thermal spraying ( Fig.5). High speed is required because, in general, the best sprayed coatings are produced by depositing multiple thin layers. This minimises the stresses in the coatings and maintains an even temperature throughout the component. Often, the component itself may be moved at high velocity, e.g. on a high speed turntable ( Fig.6). Overspray is collected by an efficient extraction system incorporating water wash curtains.

Testing

TWI has comprehensive facilities for testing thermally sprayed coatings. Testing is divided into two types - performance and characterisation. A major application area for thermally sprayed coatings is for abrasive wear resistance. The two principal wear tests used at TWI are the rubber wheel abrasion test (ASTM G65) and a pin-on-disc test (to a TWI procedure). Both of these tests measure high stress abrasion resistance, although the ASTM version of the pin-on-disc test measures sliding wear rate. For certain materials, there is a ranking correlation between the results of the rubber wheel and pin-on-disc tests, but as with all wear testing, care is needed in interpreting the results.

Two corrosion tests are commonly used. First, electrochemical testing to measure both the rest potential and corrosion current density of HVOF coatings of materials such as stainless steel, nickel and cobalt-based alloys. Salt spray corrosion testing has also been used to test the more porous aluminium and zinc-based coatings produced by arc spraying. Although these tests are principally qualitative (ranking), measurement of corrosion current density allows calculation of corrosion rates.

Standard chemical analytical techniques, such as inert gas fusion and combustion in oxygen are used to determine the chemical composition of coatings. In addition, scanning electron microscopy is used, not only for quantitative analysis, but also for determining element distribution ( Fig.7).

Metallographic examination is, of course, widely used to determine microstructural details and microhardness. X-ray diffraction techniques assist with identification and semi-quantitative measurement of the crystalline phases present in the sprayed coatings. Other characteristics evaluated at TWI include density by an immersion technique and coating surface roughness using a diamond stylus.

In conclusion

A combination of up-to-date thermal spraying processes, and facilities for coatings characterisation and performance testing ( Table 2) has established TWl's unique position as the UK's leading independent authority in thermal spraying research, applications development and consultancy. Collaboration with equipment manufacturers and industrial end users is further developing insights into the thermal spraying processes and extending the range of coating materials applied.

Table 2 Summary of coatings performance and characterisation tests