TBC experience in land- based gas turbines

Abstract

This paper summarizes prior and on-going machine evaluations of TBC coatings for power generation applications.Rainbow testing of various TBCs on turbine nozzles, shrouds and buckets are described along with one test on combustor liners. GEPG has conducted over 15 machine tests with TBC coated turbine nozzles of various coatings. Rainbow test times generally range between 10,000 to 24,000 hours. TBC performance has been quite good and additional testing, including TBC's on shrouds and buckets, is continuing.The results show that TBCs have the capability of surviving in power generation machines for the times required. The earlier rainbow tests which evaluated various top coat compositions resulted in confu'mation of the superiority of YSZ and especially the 6-8 YSZ composition.On-going tests are more focused on TBC process and property variations. The prevalent failure modes seen thusfar in the various rainbow tests are erosion, foreign object damage and buildup of deposits. Additional post test analysis is required to investigate bond coat oxidation and other time/temperature dependent changes to the system.

Erosion and Foreign Object Damage of Thermal Barrier Coatings

“…Large particles (>40µm) move relatively slowly in the gas stream and can be hit by the rotating turbine hardware damaging the leading edges [14,[36][37]. In this latter respect, particulates generated within the engine, such as carbon particles formed within the combustion process [37], and engine wear residues, such as thermally spalled TBC from the combustor [14], can contribute significantly to the observed erosion damage.…”

“…For life-critical applications, erosion of the ceramic top coat is perceived as a potential problem, whether for aero- [5,[10][11][12][13]40] or industrial applications [5,14]. Thermal barrier coatings are more susceptible to erosion than fully dense ceramics [41] because the coating microstructures contain many crack-like featuresdiscussed later in section 3.…”

“…Experimental studies are roughly evenly split between studies of plasma sprayed systems [1,9,38,40,41,43,[46][47][48][49][50] and EB-PVD systems [1,10,14,15,40,[43][44][45], although data on the latter, to a significant degree, have only recently become available. Both laboratories (Cincinnati and Cranfield) have the ability to test at room temperature and elevated temperatures.…”

“…The EB-PVD TBC [1][2][3][4][5][6] and the segmented, thermally sprayed TBCs [7,8] were outcomes of this research. The EB-PVD has proved particularly successful, resulting in significant improvements to coating durability on heavily loaded components as a result of the columnar strain-tolerant microstructure that is developed [5,[9][10][11][12][13] EB-PVD TBCs are now specified for aerofoil components within aero-gas turbines and their derived industrial counterparts [5,14]. Flight service evaluation -published in the open literature [9,15] has shown that this class of coating can survive up to 16,000h in a turbine environment.…”

Lanthanum zirconate based thermal barrier coatings: A review

“…The bond coat consists of a MCrAlM' intermetallic alloy with a thickness of 100 ~ 300 μm, wherein M is an element selected from nickel, cobalt, iron and their mixture, and M' is an element selected from yttrium, zirconium, hafnium, ytterbium and mixture thereof [4]. Typically, TBCs can be deposited directly on the substrate using various techniques, such as air plasma spraying (APS), electron-beam physical vapor deposition (EB-PVD), high velocity oxygen-fuel (HVOF) spraying, vacuum plasma spraying, low-pressure plasma spraying and diffusion bond method [5][6][7][8]. In high-temperature operation conditions, a TGO layer is formed at the interface between the bond and top coats with a thickness of 1~10 μm.…”