Mechanical property requirements for aero gas turbine materials

Abstract

The outstanding performance of current military and civil aero gas turbine engines is linked closely to the way in which modern design and manufacturing techniques have become totally integrated with materials designed specifically for operation within the hostile environment of a gas turbine. Advanced titanium alloys are used extensively throughout the compressor and nickel-base superalloys dominate materials application in the turbine. In spite of current achievements, the engine designer is still under severe competitive pressure to improve engine performance still further and this will inevitably lead to even more demanding material requirements. The present paper outlines the continuing trends in engine development and describes the impact these are having on materials technology in general and the mechanical property requirements of nickel-base superalloys in particular.

MST/512     

Graded Thermal Barrier Coating Systems for gas turbine applications

Improved thermal barrier coatings can lead to increased engine operating efficiency. A significant problem is that multilayer systems are not thermally stable at elevated temperatures. Phenomena such as interdiffusion, oxidation and sintering take place, limiting coating lifetime. In order to extend lifetimes and/or increase service temperatures several modifications can be made to coating-substrate systems. Some solutions involve the utilization of the functionally graded material concept. In this paper, potential applications for functionally graded materials in thermal barrier coating systems are discussed, and research results are summarized providing an overview of the state-of-the-art.     

High temperature materials in gas turbine engines

The current performance trends in civil, aero and industrial gas turbines are discussed. Future improvements in materials technology are needed for major components — aerofoils, discs and combustors — to achieve improved performances.     

Improved plasma-sprayed Ni-Co-Cr-Al-Y and Co-Cr-Al-Y coatings for aircraft gas turbine applications

18 plasma-sprayed coating systems, nine based on the Ni-Co-Cr-Al-Y composition and nine based on the Co-Cr-Al-Y composition, were evaluated to identify coating systems which will provide equivalent or superior lives to those of coatings of Ni-Co-Cr-Al-Y and Co-Cr-Al-Y respectively produced by electron beam physical vapor deposition. Ni-Co-Cr-Al-Y-type coatings were examined on a single-crystal alloy and the Co-Cr-Al-Y-based coatings were optimized on the B1900-Hf alloy. Cyclic burner rig oxidation and hot corrosion and tensile ductility tests were used to evaluate the coatings. For the single-crystal alloy, a low pressure chamber plasma-sprayed (Ni-Co-Cr-Al-Y)-Si coating exhibited an oxidation life improvement of better than a factor of 2 at 1121°C (2050°F) over the vapor- deposited Ni-Co-Cr-Al-Y material while showing equivalent tensile ductility. A silicon-modified Co-Cr-Al-Y coating prepared by plasma spraying in a low pressure chamber was found to be more durable than the baseline vapor-deposited Co-Cr-Al-Y coating on the B1900-Hf Alloy.     

Modified zirconia abradable seal coating for high temperature gas turbine applications

The results and development of a new full ceramic abradable turbine seal coating material prepared by thermal spraying are presented. The main objective was to achieve high temperature abradability and low mating part wear using an erosion-resistant coating with high temperature stability and thermal shock resistance. The new coating was successfully laboratory tested at temperatures of at least 1100°C (2012°F). Commercial metal-based abradable coatings which are currently available are limited to lower operating temperatures. Typical plasma- sprayed ceramic coatings, because of inherent high particle velocities, are normally to dense to permit abrading without experiencing high turbine blade tip wear damage. In contrast, lower velocity combustion-sprayed ceramic coatings frequently have lower toughness and cohesive particle strength for resistance to abrasive erosion. The new coating material is designed to react exothermically, during combustion spraying, to produce a coating both with high interparticle cohesive strength for resistance to abrasive particle erosion and with controlled porosity for low turbine blade tip wear and effective abradability. Adjustment of spraying conditions gives flexibility to alter the coating hardness and porosity and permits the tailoring of abradability and erosion resistance properties for specific operating requirements.Based on specially developed test methods for high temperature abradability, high temperature particle erosion and thermal cycling, the modified zirconia coating showed superior performance to high performance baseline materials tested in the program. Industrial evaluation of this coating is presently being conducted.     

Clinching for sheet materials

Abstract

Latest developments in the clinching of sheet materials are reviewed in this article. Important issues are discussed, such as tool design, process parameters and joinability of some new lightweight sheet materials. Hybrid and modified clinching processes are introduced to a general reader. Several unaddressed issues in the clinching of sheet materials are identified.     

Design and statistical analysis of plasma coatings on superalloy for gas turbine applications

The Influence of plasma sprayed NiCr–Cr₂O₃ and Al₂O₃–40% TiO₂ (A40T) coatings on Inconel 617 are investigated at 900 and 1000 °C under mixed salt environment for gas turbine applications. The results revealed that the plasma coated alloy exhibits lower mass change than bare alloy in the Na₂SO₄ and NaCl salt mixture with and without V₂O₅. The response to the experimental analysis is statistically evaluated using 2³ factorial design. Further the exposed samples were analyzed by means of optical image, SEM, EDS and XRD results. The EDS and XRD analysis revealed that the oxides are rich in Ni and Mo on the surface of the samples. The coated samples exhibited better corrosion resistance in A40T when compared to NiCr–Cr₂O₃ coatings than bare alloy.     

High-temperature polymers for adhesive and composite applications

Abstract

The aerospace industry is currently making extensive use of both structural adhesives and composite materials in aircraft and space vehicle construction. Within the past decade, there has been a growing requirement for adhesives and composites capable of withstanding temperatures in excess of 150°C for both short- and long-term applications. Although polymers offering such capabilities have been commercially available for many years, in numerous cases adverse processing characteristics have severely restricted their acceptance. As a result, substantial efforts have been made to develop high temperature resistant polymers having considerably improved processing characteristics. In this paper, a critical assessment is given of the use of high-temperature polymers in both adhesive and composite applications with particular emphasis on some of the newer, more easily processed types which are, or will probably be, commercially acceptable.

MST/659     

EMD改进方法研究及其在燃气轮机工频特征提取中的应用

为抑制经验模态分解(empirical mode decomposition,EMD)处理过程中的端点效应,在整理和研究现有方法的基础上,提出一种镜像延拓和极值平移相结合的端点处理方法,在最大程度地融合两种传统方法优点的同时尽可能地还原信号边界特征。该方法通过构造特征平行四边形使延拓极值处于理想区域,从而避免三次样条差值过程中包络线与信号交叉的产生,并引入Blackman窗函数对延拓信号进行边界处理,进而有效地控制延拓误差影响。经过仿真信号验证与实测信号分析,对比镜像延拓、极值平移与加窗边界处理方法的端点抑制效果,证明该改进方法能有效地抑制分解过程中出现的端点效应,并能在高频噪声干扰下较完整地提取低频信息,为燃气轮机工频特征的获取提供可靠的保证。     

Functionally gradient ceramic/metallic coatings for gas turbine components by high-energy beams for high-temperature applications

Failure of turbine blades generally results from high-temperature oxidation, corrosion, erosion, or combinations of these procedures at the tip, and the leading and trailing edges of a turbine blade. To overcome these limitations, functionally gradient ceramic/metallic coatings have been produced by high-energy beams for high-temperature applications in the aerospace and turbine industries to increase the life of turbine components. Thermal spray processes have long been used to apply high-temperature thermal barrier coatings to improve the life of turbine components. However, these processes have not met the increased demand by the aerospace and turbine industries to obtain higher engine temperatures and increased life enhancement as a result of the inhomogeneous microstructure, unmelted particles, voids, and poor bonding with the substrate. High-energy beams, i.e. electron beam-physical vapour deposition (EB-PVD), laser glazing, laser surface alloying, and laser surface cladding, have been explored to enhance the life of turbine components and overcome the limitations of the thermal spray processes. EB-PVD has overcome some of the disadvantages of the thermal spray processes and has increased the life of turbine components by a factor of two as a result of the columnar microstructure in the thermal barrier coating (TBC). Laser glazing has been used to produce metastable phases, amorphous material, and a fine-grained microstructure, resulting in improved surface properties such as fatigue, wear, and corrosion resistance at elevated temperatures without changing the composition of the surface material. Laser surface alloying and laser surface cladding have shown promising results in improving the chemical, physical, and mechanical properties of the substrate's surface. Metal-matrix composite coatings have also been produced by a laser technique which resulted in increased wear and oxidation-resistant properties. The advantages and disadvantages of thermal spray processes, EB-PVD, laser glazing, laser surface alloying, and laser surface cladding will be discussed. Microstructural evolution of thermal barrier coatings, recent advancements in functionally gradient coatings, laser grooving, and multilayered textured coatings will also be discussed.     

Beta zeolite supported sol-gel TiO2 materials for gas phase photocatalytic applications

Beta zeolite supported sol-gel TiO(2) photocatalytic materials were prepared according to a sol-gel route in which high specific surface area Beta zeolite powder was incorporated into the titanium isopropoxide sol during the course of the sol-gel process. This led to an intimate contact between the zeolite surface and the TiO(2) precursors, and resulted in the anchorage of large amounts of dispersed TiO(2) nanoparticles and in the stabilization of TiO(2) in its anatase form, even for high TiO(2) wt. contents and high calcination temperatures. Taking the UV-A photocatalytic oxidation of methanol as gas phase target reaction, high methanol conversions were obtained on the Beta zeolite supported TiO(2) photocatalysts when compared to bulk sol-gel TiO(2), despite lower amounts of TiO(2) within the photoactive materials. The methanol conversion was optimum for about 40 wt.% TiO(2) loading and calcination temperatures of 500-600°C.     

Effect of overloads on the growth of fatigue cracks in materials for gas turbine blades

Institute of Strength Problems, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Problemy Prochnosti, No. 4, pp. 47–52, April, 1988.     

Influence of ash deposits on the heat resistance of materials for gas turbine engine blades

Leningrad. Translated from Problemy Prochnosti, No. 1, pp. 71–76, January, 1989.     

High-temperature rheology of SHS materials

Specific features of SHS materials as the objects of rheology are discussed, various types of viscosimetric flows and the choice of rheological variables are delineated, a technique of constructing isochoric rheological curves for porous bodies is illustrated, and their possible types are reported. Such factors as nonuniformity of the porosity distribution external friction, and nonisothermality of the process are considered. A compression rheometer intended for studying the compaction kinetics and rheological properties of the SHS materials is described, and data on the rheological properties of the material based on molybdenum disilicide are presented.Institute of Structural Macrokinetics, Russian Academy of Sciences, Chernogolovka. Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 63, No. 5, pp. 593–604, November, 1992.     

High-temperature fracture of composite materials

The high-temperature fracture of composite materials containing strong particles is discussed on the basis of micromechanical models in which the recovery effect by diffusion of atoms is taken into account. The results of the calculations are compared with experimental results on 21Cr-4Ni-9Mn steels which consist of equiaxed grains containing rod-like M₂₃ C₆ carbide particles. The internal stresses acting in the particles and the grains are calculated on the basis of the model. The fracture mode of the material changes from transgranular to grain boundary fracture with increasing temperature, and the elongation of the material increases as a result. These experimental observations can be interpreted in terms of the calculation results of the constant stress model in which non-uniform distribution of plastic strain is assumed, but the calculated results of the constant strain model cannot explain the experimental results.     

New hygrothermal balanced moisture meters for sheet materials

Conclusions The investigations carried out have shown that the static characteristic of hygrothermal moisture meters for sheet materials depends strongly on the temperature of the surrounding medium, i.e., on the temperature of the investigated material. In order to eliminate the temperature error in such moisture meters it is possible to recommend a method based on use of two identical sensitive elements-resistance thermometers the first of which is in hygrothermal balance and the second of which is only in thermal balance with the investigated material.Analysis of the static and transient characteristics of hygrothermal balanced moisture meters for cellulose, paper linen, and leather shows that the moisture meters developed can be used for other sheet materials up to their maximum hygroscopic moisture contents.The hygrothermal method can be the basis for creating instruments for automatic monitoring and control of the moisture of solid and friable materials for which the electrical and other measurement methods for moisture measurement are unsuitable for a number of reasons.Translated from Izmeritel'naya Tekhnika, No. 12, pp. 65–67, December, 1971.     

Composite materials for aerospace applications

Fibre-reinforced polymer composite materials are fast gaining ground as preferred materials for construction of aircraft and spacecraft. In particular, their use as primary structural materials in recent years in several technology-demonstrator front-line aerospace projects world-wide has provided confidence leading to their acceptance as prime materials for aerospace vehicles. This paper gives a review of some of these developments with a discussion of the problems with the present generation composites and prospects for further developments. Although several applications in the aerospace sector are mentioned, the emphasis of the review is on applications of composites as structural materials where they have seen a significant growth in usage. The focus of the paper is especially on the developments on the Indian aerospace scene. A brief review of composites usage in aerospace sector is first given. The nature of composite materials behaviour and special problems in designing and working with them are then highlighted. The issues discussed relate to the impact damage and damage tolerance in general, environmental degradation and long-term durability. Current solutions are briefly described and the scope for new developments is outlined. In the end, some directions for future work are given.