Thermal barrier coatings issues in advanced land-based gas turbines

The Department of Energy’s Advanced Turbine Systems (ATS) program is aimed at fostering the devel-opment of a new generation of land-based gas turbine systems with overall efficiencies significantly be-yond those of current state-of-the-art machines, as well as greatly increased times between inspection and refurbishment, improved environmental impact, and decreased cost. The proposed duty cycle of ATS ma-chines will emphasize different criteria in the selection of materials for the critical components. In par-ticular, thermal barrier coatings (TBCs) will be an essential feature of the hot gas path components in these machines. The goals of the ATS will require significant improvements in TBC technology, since these turbines will be totally reliant on TBCs, which will be required to function on critical components such as the first-stage vanes and blades for times considerably longer than those experienced in current applications. Important issues include the mechanical and chemical stability of the ceramic layer and the metallic bond coat, the thermal expansion characteristics and compliance of the ceramic layer, and the thermal conductivity across the thickness of the ceramic layer.     

Thermal shock testing of thermal barrier coating/bondcoat systems

Various methods of thermal shock testing are used by aircraft and industrial gas turbine engine (IGT) manufacturers to characterize new thermal barrier coating systems in the development stage as well as for quality control. The cyclic furnace oxidation test (FCT), widely used in aircraft applications, stresses the ceramic/bondcoat interface, predominantly through thermally grown oxide (TGO) growth stress. The jet engine thermal shock (JETS) test, derived from a burner rig test, creates a large thermal gradient across the thermal barrier coating (TBC), as well as thermomechanical stress at the interface. For IGT applications with long high-temperature exposure times, a combination of isothermal preoxidation and thermal shock testing in a fluidized bed reactor may better represent the actual engine conditions while both types of stress are present. A comparative evaluation of FCT, JETS, and a combined isothermal oxidation and fluidized bed thermal shock test has been conducted for selected ceramic/bondcoat systems. The results and the failure mechanisms as they relate to the TBC system are discussed. A recommendation on the test method of choice providing best discrimination between the thermal shock resistance of the ceramic layer, the ceramic/bondcoat interface, and even substrate related effects, is given. This paper was presented at the 2nd International Surface Engineering Congress sponsored by ASM International, on September 15–17, 2003, in Indianapolis, Indiana, and appeared on pp. 520–29.     

Formation and behavior of thermal barrier coatings on nickel-base superalloys

Plasma-sprayed thermal barrier coatings (TBCs) have been used to extend the life of combustors. Electron beam physical vapor deposited (EB-PVD) ceramic coating has been developed for more demanding rotating as well as stationary turbine components. Here 3 kW RF magnetron sputtering equipment was used to gain zirconia ceramic coatings on hollow turbine blades and vanes, which had been deposited NiCrAIY by cathodic arc deposition. NiCrAlY coating surface was treated by shot peening; the effects of shot peening on the residual stress are presented. The results show that RF sputtered TBCs are columnar ceramics, strongly bonded to metal substrates. NiCrAlY bond coat is made of β, γ‘ and Cr phases, ZrO2 ceramic layer consists of t‘ and c phases. No degradation occurs to RF ceramic coatings after 100 h high temperature oxidation at 1150℃ and 500 thermal cycles at 1150℃ for 2 min, air-cooling.     

The superior durability of chilled cast iron camshafts

The demands of the automotive industry for cost-effective valve-train components that will function well at high stress and offer extremely low wear resistance under marginal lubrication, with improved emissions, requires guaranteed high-performance materials behavior together with a robust and proven manufacturing technology to support high-volume, low-rejection-rate production. Chilled cast iron (CCI) camshafts and followers have long been successfully meeting these requirements in both European-and Japanese-designed gasoline and diesel internal combustion engines; recent studies extend the role of CCI to modern engine configurations employing roller followers and ceramic mating surfaces. Performance in these newer applications is discussed.     

Keramische Beschichtungen im Pumpenbau

Ceramic coatings for pump components The objective of the present paper is to develop a SiC-based ceramic coating for critical tribological pump components. The paper presents the results obtained for different types of coatings tested under pump-specific loads in comparison with conventional materials. The coatings (WC/Co, WC/Ni, Cr₃C₂/NiCr, Ni-SiC/INCONEL) were applied using the vacuum plasma spraying process. Laboratory tests were carried out on model test stands. The materials were tested with regard to their resistance to corrosion, cavitation and erosion as well as their behaviour in a tribologically critical environment. The coatings examined proved to be suitable for pump engineering applications involving tribological and erosive loads; while they do not provide sufficient protection against cavitation and corrosion attacks in highly concentrated acids.     

Ti3 SiC2 复合材料耐磨性能研究

Ti₃SiC₂是一种六方晶体结构的特殊陶瓷材料,兼具金属与陶瓷的优异性能,拥有优良的高温强度、抗氧化性及可加工性等优点,广泛应用于耐磨润滑材料。本文综述其同金属和SiC、金刚石、TiC、Al₂O₃等复合后的优异性能和广阔应用,并展望其在和金属、陶瓷、金刚石等材料复合领域的研究方向。      

Thermal barrier coating experience in gas turbine engines at Pratt & Whitney

Pratt & Whitney has accumulated more than three decades of experience with thermal barrier coatings (TBCs). These coatings were originally developed to reduce surface temperatures of combustors of JT8D gas turbine engines to increase the thermal fatigue life of the components. Continual improvements in de-sign, processing, and properties of TBCs have extended their applications to other turbine components, such as vanes, vane platforms, and blades, with attendant increases in performance and component du-rability. Plasma-spray-based generation I (Gen I) combustor TBCs with 7 wt % yttria partially stabilized zirconia deposited by air plasma spray (APS) on an APS NiCoCrAlY bond coat continues to perform ex-tremely well in all product line engines. Durability of this TBC has been further improved in Gen II TBCs for vanes by incorporating low-pressure chamber plasma-sprayed NiCoCrAl Y as a bond coat. The modi-fication has improved TBC durability by a factor of 2.5 and altered the failure mode from a “black fail-ure” within the bond coat to a “white failure” within the ceramic. Further improvements have been accomplished by instituting a more strain-tolerant ceramic top layer with electron beam/physical vapor deposition (EB-PVD) processing. This Gen III TBC has demonstrated exceptional performance on rotating airfoils in high-thrust-rated engines, improving blade durability by three times through elimination of blade creep, fracture, and rumpling of metallic coatings used for oxi-dation protection of the airfoil surfaces. A TBC durability model for plasma-sprayed as well as EB-PVD systems is proposed that involves the accumulation of compressive stresses during cyclic thermal expo-sure. The model attempts to correlate failure of the various TBCs with elements of their structure and its degradation with thermocyclic exposure.     

玻璃粉对低温共烧用通孔银浆性能的影响

通孔银浆实现了LTCC不同电路层间的电学导通和元件散热,是低温共烧陶瓷元件经常使用的一种浆料。通过研究浆料组分中玻璃粉的软化点、添加量对通孔银浆印刷填孔工艺、电学性能、匹配性以及基板可靠性的影响,制备出一款匹配Ferro A6瓷料使用的通孔银浆,并进一步揭示玻璃粉对浆料印刷填孔和与瓷料共烧影响的一些内在机理,对其他相关类型浆料和瓷料的研制和使用有一些指导意义。     

Developments in investment casting process—A review

Investment casting has been used to manufacture weapons, jewellery and art castings during the ancient civilization. Today, its applications include jewellery/art castings, turbine blades and many more industrial/scientific components. The present paper reviews various investigations made by researchers in different stages of investment casting and highlights their importance. The paper initially highlights the investigations made on pattern wax properties, effects of blending, additives and fillers. Different ways through which pattern properties (like surface finish, dimensional accuracy, etc.) could be enhanced by properly controlling the injection processing parameters are thoroughly discussed. The paper also looks into the investigations made to enhance the strength, surface finish, etc. of ceramic shell for ferrous alloys/non-ferrous alloys as well as superalloys in investment casting. Investigations made on incorporation of nylon fibers and polymer additions confirm that a ceramic shell reinforced with nylon fibers attains additional permeability compared to the one with polymer additions.     

Promising high-thermal-conductivity substrate material for high-power electronic device:silicon nitride ceramics

High-thermal-conductivity silicon nitride ceramic substrates are indispensable components for nextgeneration high-power electronic devices because of their excellent mechanical properties and high thermal conductivities, which make them suitable for applications in complex and extreme environments. Here, we present an overview of the recent developments in the preparation of high-thermal-conductivity silicon nitride ceramics. First,the factors affecting the thermal conductivity of silicon nitride ceramics are described. These include lattice oxygen and grain boundary phases, as well the oxygen content of the crystal lattice, which is the main influencing factor.Then, the methods to prepare high-thermal-conductivity silicon nitride ceramics are presented. Recent work on the preparation of high-thermal-conductivity silicon nitride is described in detail, including the raw materials used and the forming and sintering processes. Although great progress has been made, the development of a high-quality,low-cost fabrication process remains a challenge. Nevertheless, we believe that high-thermal-conductivity silicon nitride substrates are promising for massive practical applications in the next generation of high-power electronic devices.     

Surface protection of light metals by one-step laser cladding with oxide ceramics

Today, intricate problems of surface treatment can be solved through precision cladding using advanced laser technology. Metallic and carbide coatings have been produced with high-power lasers for years, and current investigations show that laser cladding is also a promising technique for the production of dense and precisely localized ceramic layers. In the present work, powders based on Al₂O₃ and ZrO₂ were used to clad aluminum and titanium light alloys. The compact layers are up to 1 mm thick and show a nonporous cast structure as well as a homogeneous network of vertical cracks. The high adhesive strength is due to several chemical and mechanical bonding mechanisms and can exceed that of plasmasprayed coatings. Compared to thermal spray techniques, the material deposition is strictly focused onto small functional areas of the workpiece. Thus, being a precision technique, laser cladding is not recommended for large-area coatings. Examples of applications are turbine components and filigree parts of pump casings.     

Active brazed ceramic cemented carbide compound drills for machining lamellar graphite cast iron

The optimum and efficient way of machining is linked to the choice of the most appropriate cutting tool and cutting tool material. High performance cutting ceramics are characterized by excellent hardness properties at elevated temperatures. For this reason, cutting ceramics meet the requirements for machining with high cutting speeds to increase process productivity. Whereas cutting ceramics are widely used in turning and milling operations, their use in drilling processes, using ceramic insert tipped tools, is limited to larger diameter applications due to design restrictions. Beyond small diameters, solid ceramic tools were of negligible interest for industrial applications owing to their excessive tool manufacturing costs. This paper presents a new tool concept which addresses this challenge and permits a more productive machining process for drilling small diameter holes using ceramics as the cutting material. An active brazed compound drill combines the advantageous properties of ceramics and cemented carbide as the cutting tool material and basic holder material, respectively. The investigations presented here describe the manufacturing chain as well as the application of a compound drill, and compares it to a widely used industrial reference tool.     

Surface integrity and material removal mechanisms associated with the EDM of Al2O3 ceramic composite

Electric discharge machining (EDM) has been proven as an alternate process for machining complex and intricate shapes from the conductive ceramic composites. The performance and reliability of electrical discharge machined ceramic composite components are influenced by strength degradation due to EDM-induced damage. The success of electric discharge machined components in real applications relies on the understanding of material removal mechanisms and the relationship between the EDM parameters and formation of surface and subsurface damages. This paper presents a detailed investigation of machining characteristics, surface integrity and material removal mechanisms of advanced ceramic composite Al₂O₃–SiC_w–TiC with EDM. The surface and subsurface damages have also been assessed and characterized using scanning electron microscopy (SEM). The results provide valuable insight into the dependence of damage and the mechanisms of material removal on EDM conditions.     

Mechanical properties of thermal barrier coatings after isothermal oxidation.: Depth sensing indentation analysis

Thermal barrier coatings (TBC) are extensively used to protect metallic components in applications where the operating conditions include aggressive environment at high temperatures. Isothermal oxidation degrades the performance of these coatings, so this work analyses the mechanical properties (Young's modulus, E, and hardness, H) of TBC and its evolution after thermal exposure in air. ZrO₂(Y₂O₃) top coat and NiCrAlY bond coating were air plasma sprayed onto an Inconel 600 Ni base alloy. The TBC were isothermally oxidized in air at 950 °C and 1050 °C for 72, 144 and 336 h. Depth sensing indentation tests were carried out on the ceramic coating to evaluate E and H in the as-sprayed materials and after isothermal oxidation. An approach based on multiple tests at different loads was used to determine size independent apparent E an H. These mechanical properties, measured perpendicular to the surface, clearly decreased after isothermal oxidation as a consequence of microcracking within the ceramic coating.     

陶瓷/金属异质钎焊连接研究进展

钎焊作为制造业中材料连接较广泛的方法之一,在医疗、电力电子和汽车等领域应用广泛,各国学术界认为钎焊是陶瓷/金属异质连接中最有效、最具有发展潜力的连接方式。本文主要对近20年各国有关陶瓷/金属钎焊异质连接的研究报道进行详细综述。首先综述了陶瓷/金属钎焊的研究概况,其次分别从氧化物陶瓷、碳化物陶瓷、氮化物陶瓷、陶瓷基复合材料4种陶瓷材料方面以及活性金属钎焊、空气反应钎焊、接触反应钎焊、玻璃钎焊和超声波辅助钎焊5种钎焊方法详细评述陶瓷/金属异质钎焊的研究进展。然后介绍了陶瓷/金属异质钎焊在医疗、电力电子和汽车领域的应用,最后指出陶瓷/金属异质钎焊技术研究和发展过程中存在的不足,并展望陶瓷/金属异质钎焊技术未来发展的方向,为陶瓷/金属异质材料连接的相关研究和工程应用提供理论依据和技术支撑。     

Fractography of critical and subcritical cracks in hard materials

In testing hard and brittle materials like hard metals and ceramics for fracture resistance, stable and unstable crack propagation can be observed. The question arises if critical and subcritical cracks proceed in the same way, and, as a corollary, if results from subcritical fracture can be used to describe catastrophic failure. Results from earlier work are re-evaluated to demonstrate the equivalence or difference of stable and unstable crack growth in hard materials. It is shown and statistically assured by χ² tests for the length distributions of the fracture facets that fracture at room temperature produces identical surface geometry in tungsten carbide–10 wt% cobalt alloys as well as in an alumina–3 wt% SiO₂ ceramic while significantly different fracture surfaces are obtained for the ceramic at high-temperature testing. Some general features of fracture mechanisms are discussed. Assessment of fracture surfaces (measurement of distributions of the lengths and the angles of inclination of profiles along the fracture surface) was carried out using an instrumented stereometer. Recent progress in quantitative analysis of rough surfaces is reviewed which appears to be very useful for a large number of applications in the field of hard materials.     

The use of failure analysis as a preventative quality tool

The materials and processes used to fabricate electronic components have significant effects on their ul-timate reliability. By examining a small number of samples, high risk types of materials and poor work-manship can be flagged, either excluding them from production use,or using them to drive corrective actions. The tools of failure analysis are well suited to this approach, even though the sample components have technically not failed at all. Both nondestructive and destructive types of analysis can be used. By focusing on comparative device analysis between available suppliers, it is possible to find the best in class components for use in high- reliability applications such as automotive electronic modules.     

Development of coated ceramic components for the aluminum industry

In general, due to ceramic’s high hardness, which makes machining operations extremely difficult and very expensive, ceramic components are formed in shapes very close to the final ones. Considering this, a manufacturing process, based on a sol-gel reaction that allows rapid production of ceramic components in the final shape with a low level of shrinkage was developed. Although the ceramics obtained presented good behavior in short-term contact with molten aluminum alloys, there was no guarantee that the components produced would have adequate continuous resistance to chemical and erosive wear by liquid metals. To enhance their resistance, the ceramic parts were coated by flame spray. Different powders and conditions were used to determine the degree of coating adhesion to the substrate. The coated specimens were then submerged in a molten aluminum bath, at different temperatures and time settings, to evaluate the interaction between the ceramic components and the molten aluminum alloys.     

Hot chamber diecasting with Si3N4 ceramic injection system: application to aluminium alloy components

Abstract

Although hot chamber diecasting is an appropriate technology for producing high quality cast components, there are material limitations; at the process temperature, aluminium alloy erodes the immersed steel shot sleeve and plunger. Attempts have been made to employ a Si₃N₄ ceramic in place of steel. High quality Al hot chamber diecast components can be manufactured in this manner, but long production runs are not possible because failures occur in the ceramic sleeve even after, for example, a few thousand shots. The fracture in the ceramic sleeve is caused by high stresses arising from the plunger galling the sleeve surface owing to penetration of the Al alloy into the gap between the sleeve and plunger. The infiltrated Al alloy has been found to react chemically with the ceramics; silicon in the ceramic reacts with iron particles present as inclusions in the Al alloy. Hence, the purity of the molten Al alloy in the crucible is a significant factor in the production of high quality diecast components. To prevent penetration of the Al alloy, the injection system is cleaned periodically using hydrochloric acid, which dissolves the infiltrated Al alloy. Since etching by HCl can reduce the strength of the ceramic parts, this cleaning process must also be reconsidered. Recommendations to improve the quality of hot chamber diecast Al components are proposed.     

Plasma Sprayed Coating Using Mullite and Mixed Alumina/Silica Powders

Plasma sprayed ceramic coatings are widely used for thermal barrier coating applications. Commercially available mullite powder particles and a mixture of mechanically alloyed alumina and silica powder particles were used to deposit mullite ceramic coatings by plasma spraying. The coatings were deposited at three different substrate temperatures (room temperature, 300?°C, and 600?°C) on stainless steel substrates. Microstructure and morphology of both powder particles as well as coatings were investigated by using scanning electron microscopy. Phase formation and degree of crystallization of coatings were analyzed by x-ray diffraction. Differential thermal analysis (DTA) was used to study phase transformations in the coatings. Results indicated that the porosity level in the coatings deposited using mullite initial powder particles were lower than those deposited using the mixed initial powder particles. The degree of crystallization of the coatings deposited using the mixed powder particles was higher than that deposited using mullite powder particles at substrate temperatures of 25 and 300?°C. DTA curves of the coatings deposited using the mixed powders showed some transformation of the retained amorphous phase into mullite and alumina. The degree of crystallization of the as sprayed coatings using the mixed powder particles was significantly increased after post deposition heat treatments. The results indicated that the mechanically alloyed mixed powder can be used as initial powder particles for deposition of mullite coatings instead of using mullite powders.