T. E. Bloomer

High temperature environmental attack and mechanical degradation of coatings in gas turbine blades

Abstract This paper examines how in-service and thermal environmental attack influence the mechanical properties (22–950°C) of CoNiCrAlY coatings over Rene 80 substrates in gas turbine blades using a small punch (SP) testing technique in conjunction with scanning Auger microprobe analysis. SP tests have clearly demonstrated strong dependence of mechanical degradation of near surface coatings on the elevated temperature environmental condition. The room temperature (RT) ductility in blade coatings decreased with increasing operating time under combined fuels of kerosene and liquefied natural gas (LNG) despite softening in used coatings. All the coatings depicted lower ductility at 825°C in air than at RT but not in vacuum so that the oxidizing environment would produce deleterious effects. In-service operation under the combined fuels led to a two-fold increase in the ductile—brittle transition temperature (DBTT) over coatings observed under mainly LNG because of more extensive oxidation and grain boundary sulfidation. However, the DBTT of coating did not change during thermal ageing at 870°C in air that produced only oxidation. These findings imply that the grain boundary sulfidation would exert a stronger embrittling effect on the CoNiCiAlY coatings than the oxidation.

Degradation Characteristics of Intermetallic Coating on Nickel Base Superalloy Substrate in Gas Ttairbine Blade

Abstract In-service degradation of the mechanical properties (295-1223 K) and microstructure/chemistry in gas turbine blades made of CoNiCrAlY coatings and Rene 80 substrates has been studied by means of a small punch (SP) testing technique and scanning Auger microprobe (SAM). In SP tests, brittle coating cracks continuously and discretely propagated along the radial and tangential directions at 295 K and elevated temperatures, respectively. The ductility of the coating and substrate at 295 K was lowered during long time operation of the blades. The ductile-brittle transition temperature of used coatings was increased by 90 K, compared with that of unused ones while that of the substrate remained unchanged. From SAM analyses of the unused and used blades, it was found that oxidation and S segregation near the coating surface region profoundly occur in-service. The relationship between the mechanical property degradation and microstructural/chemical evolution near the coating surface is presented which ser...

Mechanical properties of aluminized CoCrAlY coatings in advanced gas turbine blades

The microstructure/composition and mechanical properties (22-950 C) in aluminized CoCrAlY coatings of advanced gas turbine blades have been examined using scanning Auger microprobe and a small punch (SP) testing method. Aluminized coatings were made of layered structure divided into four regimes: (1) Al enriched and Cr depleted region, (2) Al and Cr graded region, (3) fine grained microstructure with a mixture of Al and Cr enriched phases and (4) Ni/Co interdiffusion zone adjacent to the interface SP tests demonstrated strong dependence of the deformation and fracture behavior on the various coatings regimes. Coatings 1 and 2 showed higher microhardness and easier formation of brittle cracks in a wide temperature range, compared to coatings 3 and 4. The coating 3 had lower room temperature ductility and conversely higher elevated temperature ductility than the coating 4 due to a precipitous ductility increase above 730 C. The integrity of aluminized coatings while in-service is discussed in light of the variation in the low cycle fatigue life as well as the ductility in the layered structure.

Oxidation/carbonization/nitridation and in-service mechanical property degradation of CoCrAlY coatings in land-based gas turbine blades

This article describes variations in the microstructure/composition and mechanical properties in plasma sprayed CoCrAlY coatings and a modified René 80 substrate of gas turbine blades operated for 21,000 h under liquefied natural gas fuels. Substantial oxidation/carbonization occurred in the near surface region of concave coatings, but not in the convex coatings. Aluminum and nickel/titanium-rich nitrides formed in near interface coatings and substrates of concave side of blades, respectively. Small punch (SP) specimens were prepared from the different blade location to examine the variation of the mechanical properties in the coatings. In SP tests, brittle cracks in the near surface and interface coatings of the concave side easily initiated up to 950 °C. The convex coatings exhibited higher ductility than the concave coatings and substrate and showed a rapid increase in the ductility above 800 °C. Thus it is apparent that the oxidation/carbonization and nitridation in the concave coatings produced a significant loss of the ductility. The in-service degradation mechanism of the CoCrAlY coatings is discussed in light of the operating temperature distribution and compared to that of CoNiCrAlY coatings induced by grain boundary sulfidation/oxidation.

Examination of In-Service Coating Degradation in Gas Turbine Blades Using a Small Punch Testing Method

This paper describes examination of in-service coating degradation in land based gas turbine blades by means of a small punch testing (SP) method and scanning Auger microprobe (SAM). SP tests on coated specimens with unpolished surfaces indicated large variations of the mechanical properties because of the surface roughness and curvature in gas turbine blades. SP tests on polished specimens better characterized the mechanical degradation of blade coatings. The coated specimens greatly softened and the room temperature ductility of the coatings and substrates tended to decrease with increasing operation time. The ductile-brittle transition temperature of the coatings shifted to higher temperatures during the blade operation. From SAM analyses on fracture surfaces of unused and used blades, it has been shown that oxidation and sulfidation near the coating surface, which control the fracture properties, result from high temperature environmental attack.

Mechanical Property Degradation Induced by Elevated Temperature Environmental Attack in MCrAlY Coatings of Gas Turbine Blades

This paper presents the influence of in-service environmental attack on the mechanical properties and microstructure/composition of plasma sprayed MCrAlY coatings over Ni based superalloy substrates in gas turbine (GT) blades using a small punch (SP) testing technique and scanning Auger microprobe analysis. SP tests on near surface CoNiCrAlY coatings demonstrated strong dependence of mechanical degradation on the elevated temperature environmental condition. In-service operation under the combined fuels of kerosene and liquefied natural gas (LNG) led to a two-fold increase in the ductile-brittle transition temperature over coatings observed mainly under LNG because of more extensive oxidation and grain boundary sulfidation. In CoCrAlY coatings of GT blades operated at higher temperatures than the CoNiCrAlY ones under LNG fuels, substantial oxidation/carbonization and nitridation occurred in near surface and interface regions of concave coatings, respectively, but not in convex coatings. Brittle cracks in the near surface and interface of concave coatings more easily initiated up to 950 °C than in the convex coatings. It was found that the oxidation/carbonization and nitridation in the concave CoCrAlY coatings produced a greater ductility loss than the oxidation/sulfidation in the CoNiCrAlY coatings.

Ductility Loss in Aluminized CoCrAlY Coatings Exposed to In-Service Environment

By applying a small punch testing technique, it has been previously shown that near-surface aluminized CoCrAlY coatings of unused advanced gas turbine blades had very low ductility due to the formation of Al and Cr rich phases, compared to internal and near-interface regions. Thus, it is important to examine how in-service operation affects the mechanical properties of the internal, near-interface coatings and substrates to maintain the integrity of gas turbines. This study attempts to compare the effects of in-service operation for 20,000 h under combustion of liquefied natural gas and thermal ageing in air. The in-service operation led to a larger ductility loss in concave coatings near the tailing edge, although the ductility slightly improved above testing temperature at 950 °C. Substrate used in-service had lower ductility at 950 °C than the used concave coatings. The ductility of used internal coatings depended on the blade location. In convex coatings near the leading edge, in-service degradation was not significant and the ductility was about two-fold greater than in the thermally aged blade. The in-service degradation mechanism of the aluminized CoCrAlY coatings is discussed in light of the operating temperature distribution and microstructural evolution.Copyright

Microstructure/Composition Evolution and Ductility Variation in Thermally Aged Aluminized CoCrAlY Coatings

The effect of thermal ageing at 870 °C for 8000 h in air on the microstructure/composition and mechanical properties (RT and 870 °C) has been studied in aluminized CoCrAlY coatings consisting of four layered structure (region I-IV) of advanced gas turbine blades. Thermal ageing led to a little oxidation/nitridation and a decrease in the Al content in a near surface region I. In a coating region II, coarse Cr rich σ precipitates formed during the thermal ageing. Thermally aged internal (III) and near interface (IV) coating regions showed extensive dispersion of σ and/or Al/Ni rich β/α eutectic precipitates. Small punch tests at RT and 870 °C in air have shown that the coating regions I and II of imaged and aged blades indicated easier formation of brittle cracks regardless of the composition change. The ductility of the regions III and IV at RT and 870 °C, and the low cycle fatigue life of the region III were reduced by the thermal ageing. The mechanical degradation at elevated temperatures in the aged coating regions III and IV is elucidated by taking into account the microstructure/composition evolution and environmental oxidizing effects.Copyright

High Temperature Degradation of Coating and Substrate in Gas Turbine Blade

This paper studied high temperature degradation behavior of gas turbine blades consisting of CoNiCrAlY coatings and Rene 80 substrates using a small punch (SP) testing technique at 295–1223 K and scanning Auger microprobe (SAM). In SP tests, coating cracks continuously propagated along the radial direction at 295 K and many cracks discretely were formed along more random directions at higher temperatures. The ductility of the coating at 295 K was reduced and the ductile-brittle transition temperature was increased during long time exposure of gas turbine blades to high temperature oxidation environments. SAM analyses on cross sections and fracture surfaces of the coatings indicated that oxidation and S segregation near the coating surface are profoundly induced in-service. The relationship between the mechanical properties and microstructural/chemical evolution near the coating surface is presented which serves as a data base for determining the remaining life of gas turbine blades.Copyright