Surface coating by particle bombardment

9.3.3 Surface coating by particle bombardment

Since the first practical realization of gas-turbine engines in the 1940s, the pace of engineering development has largely been prescribed by the availability of suitable high-temperature materials. Components in the most critical sections of the engine are exposed to hot products of combustion moving at high velocity. In addition, there are destructive agents passing through the engine, such as sea salt and sand. In this hostile environment, it is extremely difficult, if not impossible, to develop an alloy that combines the necessary high-temperature strength with corrosion resistance. Much effort has therefore been devoted to the search for alloy systems that will develop a thin, self-healing, ‘protective’ oxide scale. In practice, this outer layer does not prevent diffusing atoms from reaching and reacting with the alloy substrate, and it may also be subject to thinning by erosion. The difference in thermal expansion between the oxide (ceramic) scale and the metallic substrate can lead to rupture and spalling of the scale if the scale lacks plasticity or is weakly bonded to the alloy. Refractory coatings which resist wear and corrosion provide one possible answer to these problems.

The two established thermal spray methods 2 of coating selected here for brief description are used for gas-turbine components. In thermal spraying, powders are injected into very hot gases and projected at very high velocities onto the component (substrate) surface. On impact, the particles plastically deform and adhere strongly to the substrate and each other. The structure, which often has

a characteristic lenticular appearance in cross-section, typically comprises refractory constituents, such as carbide, oxide and/or aluminide, and a binding alloy phase. Many types of thermally sprayed coatings can operate at temperatures >1000 ◦

C. They range in thickness from microns to millimeters, as required. In the detonation-gun method (Figure 9.17) a mixture of metered quantities of oxygen and acetylene (C 2 H 2 ) is spark-ignited and detonated. Powder of average diameter 45 mm is injected, heated by the hot gases and projected from the 1-m-long barrel of the gun onto the cooled workpiece at a velocity of roughly 750 m s −1 . Between detonations, which occur four to eight times per second, the barrel is purged with nitrogen. Typical applications, and coating compositions, for wear-resistant D-Gun coatings are bearing sealing surfaces (WC–9Co), compressor blades (WC–13Co) and turbine blade

shroud interlocks (Cr 3 C 2 / 80Ni–20Cr).

In the plasma-spray technique, powder is heated by an argon-fed d.c. arc (Figure 9.18) and then projected on to the workpiece at velocities of 125–600 m s −1 . A shielding envelope of inert gas (Ar)

2 The Union Carbide Corporation has been granted patent rights for the D-Gun and plasma-spraying methods.

506 Physical Metallurgy and Advanced Materials

Plasma

Cathode

Anode flame

Gas

Water cooling Powder

Workpiece

Figure 9.18 Coating by plasma-spray torch ( from Weatherill and Gill, 1988; by permission of the Institute of Materials, Minerals and Mining).

is used to prevent oxidation of the depositing material. The process is used to apply MCrAlY-type coatings to turbine components requiring corrosion resistance at high temperatures (e.g. blades, vanes), where M signifies the high-m.p. metals Fe, Ni and/or Co. These coatings can accommodate much more of the scale-forming elements chromium and aluminum than superalloys (e.g. 39Co– 32Ni–21Cr–7.5Al–0.5Y). They provide a reservoir of oxidizable elements and allow the ‘protective’ scale layer to regenerate itself. The small amount of yttrium improves scale adhesion. This particular composition of coating is used for hot gas path seals in locations where a small clearance between the rotating blades and the interior walls of the engine gives greater fuel efficiency. These coatings will withstand occasional rubbing contact.