Hagen Klemm

Corrosion of silicon nitride materials in gas turbine environment

Abstract In the present study the corrosion behavior of various silicon nitride materials will be presented. Hot gas tests were conducted in an atmosphere similar to that in a gas turbine (high temperature, pressure and flow rate and the water vapor pressure as the corrosive component). While some materials displayed a high degree of microstructural stability, all materials suffered surface degradation during the rig test. The oxidation surface layer of mainly SiO 2 , which is essential for the oxidation protection of nonoxide materials because it induces a passive, diffusion-controlled oxidation mechanism, was found to be degraded by evaporation processes involving volatile silicon hydroxides. As the mechanism of these processes a combination of linear oxidation and corrosion was proposed. The stabilization of the protective oxidation layer of silica should be considered as the main factor in stabilizing these materials in hot gas environments. This can be achieved by environmental barrier coatings (EBC), however their long-term stability was found to be still insufficient.

Effect of long-term oxidation on creep and failure of Si3N4 and Si3N4/SiC nanocomposites

Abstract The high-temperature mechanical behaviour of an Si 3 N 4 /SiC nanocomposite and its monolithic Si 3 N 4 reference material was studied after long-term oxidation treatments intended to simulate future operating conditions in a severe environment. Creep and failure at elevated temperature were significantly affected, in the direction of increased brittleness. The transition stress between the ductile range present at low stresses and the brittle range existing at high stresses was shifted to distinctly lower values. The creep resistance in the low-stress range was increased by the oxidation treatment. The failure time under a given stress was drastically reduced; this was attributed to an increased sensitivity to subcritical crack growth. The failure stress for a given failure time was decreased by about half. The phenomena are explained in terms of a purification of the intergranular phase and by the formation of surface defects and of a uniformly distributed pore population.

High Temperature Oxidation and Corrosion of Silicon-Based Non-Oxide Ceramics

The present study is focused on the oxidation behavior of nonoxide silicon-based ceramics. Various Si{sub 3}N{sub 4} and SiC ceramics were examined after long term oxidation tests (up to 5,000 h) at 1,500 C in ambient air. The damage mechanisms were discussed on the basis of a comprehensive chemical and microstructural analysis of the materials after the oxidation tests. The diffusion of oxygen into the material and its further reaction in the bulk of the material were found to be the most critical factors during long term oxidation treatment at elevated temperatures. However, the resulting damage in the microstructure of the materials can be significantly reduced by purposeful microstructural engineering. Using Si{sub 3}N{sub 4}/SiC and Si{sub 3}N{sub 4}/MoSi{sub 2} composite materials provides the possibility to improve the high temperature stability.

High temperature properties of Si3N4 materials

Abstract The gas pressure sintering of Si3N4 is strongly influenced by the gas atmosphere in the sintering aggregate. During sintering a weight loss takes place. This is caused by the interaction of silicon nitride and the glassy phase with the sintering atmosphere. Carbon compounds such as CO promote these reactions. This leads to a reduction of the oxygen content. Additionally a decomposition process of Si3N4 takes place with formation of finely distributed free silicon. This process is accompanied by a crystallization of grain boundary phases during cooling from sintering temperature. This fact causes an increase of the bending strength of samples with high weight losses at high temperatures.

Environmental Barrier Coatings for Silicon Nitride

Ytterbium silicate layers were deposited on Si3N4 ceramics as environmental barrier coatings (EBCs) by a dip coating-sintering method. Coated samples were tested in an atmosphere simulating the practical conditions of a gas turbine to investigate water vapor corrosion and recession mechanisms of ytterbium silicate coatings. Prior and after tests, phase compositions and morphologies of the coatings varied as the consequence of the formation of silica at the coating/substrate interface. Due to the evaporation and diffusion of silica into the upper layer, a porous interface was finally found, which led to the spallation of coating.

Series Production of Thixoformed Steel Parts

In recent years several attempts were made to transfer the thixoforming technology of steel parts into industrial applicable processes. This paper gives an overview about the progress of a European consortium that established a fully automated thixoforming process for the series production of automotive steel parts. Due to the multi-faceted nature of this technology, problems concerning the development of suitable steel grades and tool materials as well as the development and application of an inductive heating system, a handling unit and of a complex forming tool had to be solved. Besides the development of adapted steel grades and the inductive heating, the handling of the semi solid billets plays a special role because during the manipulation of the parts from the heating station into the tool a loss of heat is unavoidable. Furthermore, scaling of the parts must be prevented. By means of a fully automated process line existing constraints were reduced and the forming process is kept reproducible. Improved silicon nitride composites have been developed as a tool material, which show good mechanical properties in combination with an acceptable chemical stability at the occurring process temperatures as well as in contact with semi solid steel. Basing on the practical experience a comparison of the thixoforming technology to existing processes and an outlook for the future are given.

Corrosion of Ceramic Materials

Ceramic materials exhibit high-corrosion stability at room and elevated temperatures; they are therefore widely used in applications where high corrosion and wear resistance are necessary. Since the corrosion resistance strongly depends on the one hand on the corrosion conditions and on the other hand on the composition and microstructure of the materials, a careful selection of the materials for the application under different conditions is necessary.

Oxidation Behavior of Prospective Silicon Nitride Materials for Advanced Microturbine Applications

Various laboratory tests have shown that high-pressure water vapor environments combined with elevated temperatures and intermediate gas velocities (current facilities limited to about 50 m/s) can cause grain boundary degradation and material recession in silica formers. Recent tests include burner rig testing conducted by NASA [1], Honeywell Engines & Systems [2], Siemens Power Generation [3], CRIEPI in Japan [4, 5], “Keiser rig” testing at Oak Ridge National Laboratory (ORNL) [6], and engine testing in the Allison 501K industrial gas turbine [7]. This paper presents a summary of oxidation test data of candidate silicon nitride materials for advanced microturbine applications. These data are of interest to microturbine component designers in order to determine the limits of safe unprotected component operation with respect to the given turbine environment, as well as to understand the behavior of ceramic microturbine components once local spallation of the protective environmental barrier coating has occurred.This paper intends to give materials and engine development engineers some guidance with respect to the different test facility capabilities and the prevailing oxidation/recession mechanisms to better understand/interprete the oxidation test results when developing new ceramic material compositions and environmental barrier coating systems.Copyright

Advancement of Cellular Ceramics Made of Silicon Carbide for Burner Applications

Lower emissions of CO and NOx as well as a higher power density were observed in combustion processes performed in porous media like ceramic foams. Only a few materials are applicable for porous burners. Open-celled ceramic foams made of silicon carbide are of particular interest because of their outstanding properties. Two different SiC materials have been investigated, silicon-infiltrated silicon carbide (SiSiC) and pressureless sintered silicon carbide (SSiC). The oxidation behaviour of both has been characterized by furnace oxidation and burner tests up to 500 h operating time. Up to a temperature of 1200 °C SiSiC exhibited a good oxidation resistance in combustion gases by forming a protective layer of silica. High inner porosity up to 30% in the ceramic struts was found in the SSiC material. Caused by inner oxidation processes the pure material SSiC allows only short time applications with a temperature limit of 1550 °C in combustion gases. An increase of the lifetime of the SSiC foams was obtained by development of a new SSiC with an inner porosity of less than 12%. The result was a considerable reduction of the inner oxidation processes in the SSiC struts.

High Temperature Oxidation and Corrosion of Silicon-Based Nonoxide Ceramics

The present study is focussed on the oxidation behavior of nonoxide silicon-based ceramics. Various Si3N4 and SiC ceramics were examined after long term oxidation tests (up to 5000 h) at 1500°C in ambient air. The damage mechanisms were discussed on the basis of a comprehensive chemical and microstructural analysis of the materials after the oxidation tests. The diffusion of oxygen into the material and its further reaction in the bulk of the material were found to be the most critical factors during long term oxidation treatment at elevated temperatures. However, the resulting damage in the microstructure of the materials can be significantly reduced by purposeful microstructural engineering. Using Si3N4/SiC and Si3N4/MoSi2 composite materials provides the possibility to improve the high temperature stability.Copyright