Plasma spray deposition is one of the most important technologies available for producing the high–performance surfaces required by modern industry. Over the past 25 years, there have been significant advances in the understanding of plasma physics and in the development of spraying equipment and techniques. This has enabled a range of materials including metals, alloys, ceramics, and cermets to be plasma sprayed on to a great variety of substrate types and geometries. During this period, the uniquely aggressive environment within the gas turbine engine has provided not only some of the greatest challenges to plasma spraying technology, but also some of its most successful applications. In this paper, the nature of plasma and plasma spray devices are discussed and factors affecting coating quality are considered. Practical aspects of plasma spraying are considered and finally the application of plasma spray coating processes to the protection of high–temperature gas turbine components is discussed...

Plasma spray coating processes

A turbine shroud includes a ceramic sealing layer secured to a metal substrate. In one form, the metal substrate includes a plurality of pegs extending therefrom. Intermediate bonding layers are disposed on the peg-metal substrate structure. A ceramic sealing layer of zirconium oxide with about 20 weight percentage magnesium oxide is disposed, e.g., plasma sprayed, on the intermediate bonding layers. The ceramic sealing layer includes an ordered pattern of very fine cracks therein which reduce the thermal stress in the ceramic sealing layer. A method of constructing a turbine shroud structure is also disclosed.

Metal-ceramic turbine shroud and method of making the same

A survey of gas path seals is presented with particular attention given to sealing clearance effects on engine component efficiency The effects on compressor-pressure ratio and stall margin are pointed out Various case-rotor relative displacements, which affect gas path seal clearances, are identified Forces produced by nonuniform sealing clearances and their effect on rotor stability are discussed qualitatively, and recent work on turbine-blade-tip sealing for high temperatures is described The need for active clearance control and for engine structural analysis is discussed The functions of the internal-flow system and its seals are reviewed Presented as an American Society of Lubrication Engineers paper at the ASLE/ASME Lubrication Conference in Minneapolis, Minnesota, October 24–26, 1978

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Gas Path Sealing in Turbine Engines

Modified zirconia abradable seal coating for high temperature gas turbine applications

Factors in the degradation of ceramic coatings for turbine alloys

Development of the plasma sprayed graded, layered ZRO2/CoCrAlY seal system for gas turbine engine blade tip seal applications up to 1589 K (2400 F) surface temperature was continued. The effect of changing ZRO2/CoCrAlY ratios in the intermediate layers on thermal stresses was evaluated analytically with the goal of identifying the materials combinations which would minimize thermal stresses in the seal system. Three methods of inducing compressive residual stresses in the sprayed seal materials to offset tensile thermal stresses were analyzed. The most promising method, thermal prestraining, was selected based upon potential, feasibility and complexity considerations. The plasma spray equipment was modified to heat, control and monitor the substrate temperature during spraying. Specimens were fabricated and experimentally evaluated to: (1) substantiate the capability of the thermal prestrain method to develop compressive residual stresses in the sprayed structure and (2) define the effect of spraying on a heated substate on abradability, erosion and thermal shock characteristics of the seal system. Thermal stress analysis, including residual stresses and material properties variations, was performed and correlated with thermal shock test results. Seal system performance was assessed and recommendations for further development were made.

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L. T. Shiembob

Papers

Development of a plasma sprayed ceramic gas path seal for high pressure turbine applications