Abstract
Thermal barrier coatings are widely used in all turbine engines, typically using a 7 wt.% Y2O3–ZrO2 formulation. Extensive research and development over many decades have refined the processing and structure of these coatings for increased durability and reliability. New compositions demonstrate some unique advantages and are gaining in application. However, the “7YSZ” (7 wt.% yttria stabilized zirconia) formulation predominates and is still in widespread use. This special composition has been universally found to produce nanoscale precipitates of metastable t’ tetragonal phase, giving rise to a unique toughening mechanism via ferro-elastic switching under stress. This note recalls the original study that identified superior properties of 6–8 wt.% yttria stabilized zirconia (YSZ) plasma sprayed thermal barrier coatings, published in 1978. The impact of this discovery, arguably, continues in some form to this day. At one point, 7YSZ thermal barrier coatings were used in every new aircraft and ground power turbine engine produced worldwide. 7YSZ is a tribute to its inventor, Dr. Stephan Stecura, NASA retiree.
Highlights
Yttria stabilized zirconia (YSZ) thermal barrier coatings (TBC) are widely used in turbine engines to protect underlying metal structures from the intense heat of combusted jet fuel
7YSZ thermal barrier coatings were used in every new aircraft and ground power turbine engine produced worldwide. 7YSZ is a tribute to its inventor, Dr Stephan Stecura, NASA retiree
It has been estimated that industry capital expenditures for finished yttria stabilized zirconia (YSZ) thermal barrier aerospace coatings reached $1500 M, just for 2015 alone [1], with 1–2 Mkg of YSZ used for plasma sprayed coatings [2]
Summary
Yttria stabilized zirconia (YSZ) thermal barrier coatings (TBC) are widely used in turbine engines to protect underlying metal structures from the intense heat of combusted jet fuel. It has been estimated that industry capital expenditures for finished YSZ thermal barrier aerospace coatings reached $1500 M, just for 2015 alone [1], with 1–2 Mkg of YSZ used for plasma sprayed coatings [2]. The early years, around 1970 and before, saw increasing interest and trials of air plasma sprayed (APS) oxides and fully stabilized zirconia (~12 wt.% Y2 O3 ) as thermal barriers for hot components in general [3,4,5]. Use in critical aircraft components was restricted because of limited cyclic lifetime: the coatings had a propensity to spall from compressive thermal stress, aggravated by the oxidation of underlying substrates or bond coats
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