The effect of variation of pack activators, compositions, temperature, and time on the thickness and structure of aluminide coatings formed on the nickel‐base superalloy IN‐100 was studied in one‐step packs containing aluminum at unit activity. Times were varied from 4 to 24 hr and temperatures were varied from 982° to 1149°C in normalNaCl ‐activated packs. The other halides of sodium and the ammonium halides were primarily used to activate 1093°C, 16‐hr packs. In addition, an analysis of the thermodynamics and kinetics of aluminizing was carried out. The mechanism of coating formation in each pack was established from agreement between observed coating weights and predictions based on a gaseous diffusion model and solid‐state diffusion considerations which used published diffusion data for the Ni‐Al system. Pack temperature rather than pack aluminum activity controls the principal coating phase formed. The halides ranked according to aluminum weight gain in 1 normalw/normalo (weight per cent) Al packs are F≅normalCl false〉normalBr false〉I . Solid‐state nickel diffusion controlled the rate of coating formation in fluoride‐activated packs. Gaseous diffusion controlled the rate of coating formation in bromide‐, iodide‐, and NH4normalCl ‐activated packs containing 1 w/o Al. In normalNaCl ‐activated packs containing 1 normalw/normalo Al the ability of the substrate to supply nickel appeared to be in balance with the ability of the pack to supply aluminum. However, the observed rate constant and activation energy indicated that solid‐state diffusion controlled coating growth. Increasing pack aluminum content from 1 to 5 w/o shifted control of coating formation from the gas phase to the solid state in the 16‐hr, 1093°C, normalNaBr ‐activated pack. Regardless of the rate‐controlling step, the kinetics of coating formation were parabolic. The activation energy for coating formation controlled by solid‐state diffusion was 88 normalkcal/normalmole . Similar coating microstructures and weight gains were obtained for each halogen regardless of whether its source was a sodium or ammonium halide.
SUMMARYA Mach 0. 3 burner rig test program was conducted to examine the sensitivity of yttria-stabilized zirconia coatings to the combustion products of Na-and V-contaminated fuels and to identify alternate coatings with improved resistance to potential utility gas turbine environments. Coatings were evaluated on aircooled, hollow superalloy erosion bar specimens of nickel-base alloy IN-792 and cobalt-base alloy MM-509. Operating conditions for both single specimen impurity sensitivity and multiple specimen alternate coatings tests were: 1370° C calculated adiabatic flame temperature, 982° C ceramic surface temperature, and 843° C substrate metal temperature.In the single specimen fuel impurity sensitivity studies of the NASAdeveloped duplex thermal barrier coating system,-ZrO 2 -12Y 2 Oo/Ni-16.2Cr-5.6A1-0.6Y -(all in weight percent unless stated otherwise), tests were conducted in combustion gases doped to equivalent fuel impurity levels of 5 ppm Na, 0. 5 ppm Na, 2 ppm V, 0.2 ppm V, and 5 ppm Na + 2 ppm V. The numbers of 1-hour cycles to failure (spalling of the coating over approximately one-quarter of the hot zone of the leading edge) were as follows; 5 ppm Na plus 2 ppm V, 43 cycles; 2 ppm V, 25 cycles; 0.2V, 200 cycles; and 5 ppm Na, 92 cycles. In the 0.5 ppm Na test after 1300 cycles, there was no spalling but the thickness of ZrO 0 -12Y 0 O <3 was reduced by 50 percent due to carbon particle erosion.In the alternate coatings screening tests, two thermal barrier coating systems and one cermet coating system were identified as being significantly more resistant to spalling than the standard ZrO 2 -12Y 2 O 3 /Ni-16. 2Cr-5.6Al-0. 6Y system which spalled before 80 one-hour cycles. In these tests eight coated specimens were tested simultaneously in a rotating air-cooled fixture and the equivalent fuel impurity level was 5 ppm Na plus 2 ppm V.The two promising thermal barrier coating systems and the number of 1-hour cycles they endured before spalling are as follows: Ca 2 SiO 4 /Ni-16. 2Cr-5.6A1-0.6Y, 675 cycles; and ZrO 2 -8Y 2 O 3 /Ni-16.4Cr-5.1A1-0.15Y, 384 cycles. The cermet coating system, 50 volume percent MgO -50 volume percent Nil9 6Cr~lT lAl-0.97Y/Ni-16.2Cr-5.6Al-0.6Y, which was removed from testing alter 1000 1-hour cycles, did not spall but was eroded to approximately 50 percent uf us original thickness by carbon particles in the combustion stream.Cracking and subsequent massive spalling of coatings in both the fuel impurity sensitivity and coatings screening tests occurred within the thermal barrier coating. Generally such cracking occurred from 0.005 to 0.015 cm above the bor
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