passages is currently of commercial interest (Smith and Boone, 1990).

A plasma gun functions through the operation of a stable nontransferred direct-current electric arc between a watercooled thoriated tungsten cathode and an annular watercooled copper anode. An inert plasma gas, which is generally argon with a few percent of an enthalpy-enhancing gas (e.g., hydrogen), is introduced as a vortex within the interior of the gun. The electric arc between the cathode and the anode creates a plasma arc within this gas. This ionized gas exits from the nozzle, where ionic recombination occurs, releasing enthalpy and yielding an effective temperature of the order of 15,000 K for the typical torch operating at 40 kW. The plasma temperature drops off rapidly from the exit of the anode. The feedstock powder is injected either internally or externally into the exiting plasma flame. The powder particles, approximately 40 microns in diameter, are accelerated and melted in the flame on their path to the target substrate, where they impact and undergo rapid cooling (106 K/sec) and solidification. The particle velocity can range from 100 to several hundred meters per second depending on spray parameters and the ambient atmosphere (e.g., low-pressure plasma spray; see below).

Plasma spray is generally used to form deposits of greater than 50-microns thickness of numerous industrial materials, including nickel-base and ferrous alloys and refractory ceramics (e.g., aluminum oxide and zirconia based ceramics). To approach theoretical bulk density and extremely high adhesion strength for high-performance applications, the plasma spray of metallic coatings is carried out in a reduced-pressure inert gas chamber. This vacuum plasma or low-pressure plasma spray (LPPS) operates at pressures of between 50 and 200 mbar. Shrouded flames can also be used (e.g., as developed by Praxair), where argon or nitrogen excludes oxygen from the vicinity of the flame and the work piece.

Although traditional plasma spray guns are gas-vortex-stabilized and operate in the 40- to 80-kW power range, it is possible to operate at considerably higher power levels (i.e., in the range of 160 kW and beyond) by using water stabilization. With a material throughput of about 30 times that of gas-stabilized torches, these high-production rates allow the manufacture of thick TBCs, such as those required in abradable seal applications (Chraska and Hrabovsky, 1992).

The key features of plasma spraying include the following:

• deposition of metals, ceramics, or any combinations of these materials

• formation of microstructures with fine, noncolumnar, equiaxed grains

• ability to produce homogeneous coatings that do not change in composition with thickness (length of deposition time)

• ability to change from depositing a metal to a continuously varying mixture of metal and ceramic (i.e., functionally graded materials)

• ability to achieve high deposition rates (>4 kg/hr)

• ability to process materials in virtually any environment (e.g., air, reduced-pressure inert gas, high pressure, under water; see the following section)

New and improved powder-processing methods have led to powders having predictable and controllable compositions and well-delineated particle-size distributions, an important parameter in the plasma-spray process. Ceramic powders for plasma spray are processed in diverse ways. Both particle size and shape are important controlling variables. In particular, the particle-size distribution has a great influence on the velocity and melt behavior in the plasma flame. These issues are discussed extensively in the literature (Herman, 1991). The deposition efficiency (i.e., the percentage of powder that actually becomes part of the target body) is of obvious economical importance and arguably represents a measure of deposit quality. Extensive literature exists on feedstock alloys and ceramic materials for plasma spray.

In addition to the starting material and its particle-size distribution, the microstructure of a plasma-sprayed coating also depends on the processing parameters, including plasma power, plasma gas composition, pressures and flow rates, powder injection details and carrier flow, torch/substrate distance, as well as other subtle factors. These parameters are sometimes interconnected in complex ways, leading to cross-terms in the process of parameterization. Statistical process control analysis is used extensively in thermal spray technology. This subject has been covered extensively in papers in the annual proceedings of the National Thermal Spray Conference (ASM International). Process control is becoming more common in plasma-spray processing of high-performance coatings. A clear goal is to achieve on-line feedback control of the process (i.e., intelligent processing of materials), which requires a much more detailed understanding of the process.

The LPPS process was developed by Muehlberger (1973) in the early 1970s and gained widespread commercial use in the mid-1980s. It is competitive with electron-beam physical vapor deposition (EB-PVD) for the production of high-quality metallic (MCrAlY) coatings for certain applications because of the compositional flexibility afforded and the high coating rates achieved through molten droplet transfer.

FRONT MATTER (R1-R16)

SUMMARY (1-7)

1 INTRODUCTION (8-9)

2 APPLICATION NEEDS AND TRENDS (10-18)

3 MATERIALS AND PROCESSES (19-25)

4 FAILURE MODES (26-33)

5 ENGINEERING CONSIDERATIONS (34-38)

6 REFURBISHMENT OF COATED STRUCTURE (39-42)

7 NEAR-TERM TRENDS AND OPPORTUNITIES (43-45)

8 LONG-TERM OPPORTUNITIES AND INNOVATIVE SYSTEMS (46-50)

REFERENCES (51-56)

A TESTING AND STANDARDS (57-64)

B RADIATION TRANSPORT IN THERMAL BARRIER COATINGS (65-66)

C SURVEY OF NONDESTRUCTIVE EVALUATION METHODS (67-71)

D MODELING OF COATING DEGRADATION (72-77)

E MANUFACTURING TECHNOLOGIES OF COATING PROCESSES (78-82)

F EXAMPLE OF A COATING DESIGNATION SYSTEM (83-83)

G BIOGRAPHICAL SKETCHES OF COMMITTEE MEMBERS (84-85)