African Fusion March 2015

SAIW: Thermal spray coatings seminar

key disadvantage, however, is that the consumable must be available as wire. Typical uses include: aluminium or zinc coatings for corrosion mitigation; salvage, build-up and repair of pitting damage, casting defects and/or wear damage; the reclamation of steel and cast iron parts, typically using NiCr, NiCrAl alloys; the repair of aluminium and magnesium alloy parts, “although cold spray is fast becoming the preferred process here”; and aluminium-based coatings for anti-skid wear resistance. The twin wire arc spray process is commonly used for onshore wind tow- ers, where a zinc layer is sprayed onto the area around the flange connections andaccessdoors for cathodicprotection. Arc sprayed aluminium is widely used to protect offshore steel structures, such as offshore wind turbine foundations, fromseawater corrosion. This is the only coating option that is truly effective in the highly corrosive splash zone, where the water line meets the air. Plasma spray coating As the heat source, the plasma spray coating process relies on a highly con- trolled plasma arc, struck between a tungsten cathode and a copper anode within the plasma gun. An inert or reduc- ing gasmix is fed into the space between the anode and cathode, where it is ionised and dissociated in the arc, trans- forming the gas into a plasma stream of charged particles. The plasma passes through a nozzle as a high velocity jet and recombines into the gas phase along the way, releasing heat energy that raises the gas temperature to up to > 10 000 °C. A powder consumable is injected into this hot gas/plasma stream, where it is melted or softened and pro- pelled onto the substrate. Ideal for materials with very high melting points, such as oxides, interme- tallics, and refractory metals, Lovelock says, “through a combination of high temperature, high energy, a relatively inert spraying medium and fairly high particle velocities, the process produces high quality coatings”. The gasmixtures used strongly influ- ence energy content and temperature. A mixture of two gases, selected from Ar, He, H 2 and N 2 are typically used, with N 2 and H 2 mixes offering higher enthalpy of melting (heat of fusion) for higher temperaturematerials and Ar and/or He being more suitable for materials that melt or vaporise easily.

“A wide variety of powders are avail- able with the finer powders (5-25  m) offering smoother and denser coatings. There is a need to balance economics of the process, i.e. high deposition rate vs. the ability to melt or soften the powder sufficiently. If the feed rate is too high, unmelted particles and/or a low depo- sition efficiency will result,” she warns, adding that numerous process variables, such as spray distance andangle, surface speeds, heat input, cross-over speedand the plasma arc parameters need to be understood and optimised for success- ful results. Plasma spraying can also be done in various low pressure or soft vacuum atmospheres (LPPS/VPS), for reactive metals such as titanium alloys – and underwater spraying is also possible, though seldom used. In terms of porosity, oxides and bond strength, plasma spray offers bet- ter results than the conventional flame and arc spray processes and “the coating bond strength can be nearly as good as HVOF coatings. For carbides, however, bonding is not as goodasHVOF. Carbides are not aswell retained in thematrix and tungsten and chrome carbides begin to decompose because of the higher temperatures,” Lovelock adds. As well as the high capital cost, the only other disadvantage is that the consumables must be in free-flowing powder form. Uses include: • Aluminium and Al alloys: for build- up and repair of Al/Mg parts (al- though cold spray is now becoming preferred). Copper alloys: In the printing indus- try, for example, copper and CuAl is used and for build-up of copper print rolls. CuNi alloys are applied to resist fretting and cavitation and CuNiIn alloys are used on turbine blade roots. Molybdenum: Mo-NiCrSiB coatings offers a high bond strength, good sliding wear and scuffing resistance and a low friction coefficient, mak- ing these coatings ideal for piston rings in large diesel engines. Titanium, tantalum, tungsten al- loys, which can be used for corro- sion resistance in chemical plants if properly sealed. These alloys are best deposited using VPS/LPPS, however, andcold spray is becoming the preferred process for depositing Ta and Ti alloys. Ti and Ti-6Al-4V is widely used for medical implants. • • •

Other applications include: the ap- plication of CoNiCrAlY coatings for re- sistance to high-temperature oxidation. These are used as bond undercoats for thermal barrier coatings (TBCs) – which are later applied using ZrO 2 stabilised with MgO, Y 2 O 3 , CaO, CeO 2 , and others – and the nickel-aluminium/nickel- chrome (NiAl, NiCrAl, NiCrAlMoFe, NiCr) alloys, for salvage and repair applica- tions and as bond coats under oxide- based layers. Pure alumina (Al 2 O 3 ) has high di- electric strength, is a good electrical insulator and is very hard and abrasion resistant. Adding TiO 2 to Al 2 O 3 makes the surface less electrically insulating and decreases resistance to chemical attack, but adds toughness and makes finish grinding easier. Pure TiO 2 resists static build-up, is abrasion resistant and chemically inert inmany environments. Its sliding wear resistancemakes it ideal for textile guides/rollers, pump plungers and mechanical seals in the chemical industry. “Plasma sprayed chromium oxide (Cr 2 O 3 ) coatings are applied to resist corrosion, sliding wear, abrasion, and low-angle erosion at up to about 600 °C. Thermal conductivity is quite high com- pared to other oxides and it is used for pump impellers and housings and, in the printing industry, because the coating is laser-engravable, it is used for Anilox rolls that areprecisely textured to control ink transfer.” HVOF The HVOF process evolved from Union Carbide’s Detonation flame spraying (D-gun) process, which was a propri- etary technology of Union Carbide from the 1960s, and was the only supersonic flame spraying system available until the early 80s when HVOF was developed as a commercially available alternative.

Courtesy of Kennametal Stellite.

An HVOF WC-Co-Cr coating being applied to a hydraulic cylinder.

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March 2015

AFRICAN FUSION