Thermal Spray Processing of FGMs
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MRS BULLETIN/JANUARY 1995
ing of dense, high performance deposits of virtually any refractory material. The vast majority of plasma-spray torches are gas-stabilized, i.e., the plasma originates within a gas which acts to both form and sustain a flame. A brief description of plasma spraying will help readers in understanding the versatility offered by this technique for the production of FGMs. As depicted in Figure 1, a dc arc is struck between a thoriated tungsten cathode and an internally water-cooled copper anode. The arc in this so-called "nontransferred" plasma torch is stabilized through a balance of gas flow and power. There is considerable fundamental and technological literature on plasma spraying.3 Suffice it to say that the standard plasma torch can operate for extended periods of time without excessive maintenance. The torches normally operate at 30-40 kW and yield a Cu-ANODE
material throughput of 2-5 kg/h. More recently, a new generation of high-powered, high throughput plasma torches have been introduced. For example, the Institute of Plasma Physics, Czech Republic, has developed a 160 kW water-stabilized plasma torch which has a material throughput capability that is approximately 30 times that of conventional gas-stabilized torches.4 This enhanced capacity can have great potential for the fabrication of functionally graded freestanding parts. Controlled-atmosphere plasma spraying, such as low pressure plasma spraying (LPPS), has expanded the capabilities of the deposition process for reactive metals and intermetallics. LPPS processing is usually conducted in a low-pressure inert gas-filled chamber and has proven to be a highly reliable method for depositing superalloy-type coatings on turbine blades and other aircraft engine components. In addition, LPPS yields enhanced particle velocities. This results in virtually poreand oxide-free metallic deposits.5 The high solidification rates associated with plasma spraying (106K/s)6 result in fine-grain sizes and, in many systems, metastable phases. However, during the process, the deposit is exposed to the hightemperature flame, as well as to adiabatic recalescence associated with successive solidification of one droplet upon another. These factors can lead to phase transformations within the deposit.7 The effect of the flame-deposit interaction is particularly pronounced in LPPS, resulting in deposit-substrate temperatures in excess
W-CATHODE
COOLING WATER FOR CATHODE AND NEGATIVE POWER CABLE
PLASMA FLAME
POWDER INJECTION — I PORT COOLING WATER FOR ANODE AND POSITIVE POWER CABLE
1
WORKING GAS
Figure 1. Schematic of a typical dc plasma-spray torch.
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Thermal Spray Processing of FGMs
Step I
Component A Blend of Components A + B Blend of Components B + C
III
IV
HI
Component D
Substrate Figure 2. Schematic illustrating FGM processing with a single torch and single feeder, along with incremental grading of discrete layers from blended materials.
of 800°C. This leads to "self-annealing" of the deposit, recovery, and recrystallization, which
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