J. Ant Cerum. Soc., 85 [3]595-602(2002) ournal Effects of Thermal Aging on the Mechanical Properties of a Porous-Matrix Ceramic Composite Eric A.v. Carelli* Science and Technology Center, Siemens-Westinghouse Power Corporation, Pittsburgh, Pennsylvania 15235 Hiroki Fujita, James Y. Yang, and Frank W. Zok Materials Department, University of California, Santa Barbara, California 93106 properties of an all-oxide fiber-reinforced composite follow a The present article focuses on changes in the mechanical gas turbine and at the burner outlet. In current long-term exposure(1000 h) at temperatures of 1000-1200 C or near their upper use temperature, even with in air. The composite of interest derives its damage tolerance imparted by the use of thermal barrier coatings, thereby from a highly porous matrix, precluding the need for an significant temperature elevations with these alloys interphase at the fiber-matrix boundary. The key issue in- future environmental and performance standards, it is anticipated olves the stability of the porosity against densification and the that the targeted temperature elevations in turbine components will associated implications for long-term durability of the compos- be accomplished through the use of continuous-fiber-reinforced ite at elevated temperatures. For this purpose, comparisons ceramic composites(CFCCs). Among the various ceramic com- are made in the tensile properties and fracture characteristics of a 2D woven fiber composite both along the fiber direction posites that have been developed to date, the ones that have and at 45 to the fiber axes before and after the aging try in the past few years are those made from all-oxide constitu- indentation and through the determination of the matrix non-oxide ones(e.g, SiC/SiC)is their superior resistance to oxidation under typical turbine engine conditions and hence their pled with classical laminate theory. The study reveals that, potential for long-term durability despite evidence of some strengthening of the matrix and the As part of a broad activity aimed at developing and assessing fber-matrix interfaces during aging, the key tensile properties oxide composites for use in future generations of gas turbin in the 0o clud engines, the present study focuses on changes in the mechanical are unchanged. This strengthening is manifested to a more properties of a candidate all-oxide CFCC following long-term exposure(1000 h)at temperatures of 1000-1200C. The compos- orientation, wherein the modulus and the tensile strength each ite material of interest derives its damage tolerance from a highly exhibit a twofold increase after the 1200%C aging treatment. It also results in a change in the failure mechanism, from one fiber-matrix boundary. Although the efficacy of this material involving predominantly matrix damage and interply delami- concept in enabling damage tolerance has been demonstrated, -o it nation to one which is dominated by fiber fracture. Addition- remains to be established whether the matrix pore structure is ally, salient changes in the mechanical response beyond the stable against densification and whether the desirable damage maximum load suggest the existence of an optimum matrix tolerant characteristics can be retained for extended time periods at attains a maximum. The implications for long-term durability the porous-matrix concept have been shown to exhibit severe of this class of composite are discussed. degradation in composite properties once the matrix densifies appreciably. The main matrix constituent in the present composite is mullite, L. Introduction in the form of a weakly bonded particulate network. This selection n industry has been under increased pres- is based on the sluggish sintering kinetics of mullite'at the upper keeping up with market demands for increased power output and s intended to form a contiguous particulate network that should be to red efficiency. These goals can be achieved in part through reductions under subsequent service conditions. The minor matrix constituen airfoils with attendant increases in the temperatures both within the alumina, present both in the form of particulates from a slurr and as a product of pyrolysis of an aqueous precursor solution Because of its more rapid sintering kinetics, alumina serves to bond the mullite particulates and the fibers together, thereby E. Lara-Curzio--contributing editor enhancing the matrix-dominated composite properties, e.g., inter laminar strength and off-axis in-plane strength. However, if the degree of sintering becomes excessive, the damage-tolerant char acteristics may be compromised. The challenge involves selection cript No. 187723 Received May 4, 2001; approved December 21, 2001 of the relative fractions and topologies of the two phases such that under the network of mullite particles remains contiguous and hence nrough both internal research funds at the Science and Technology Center and a prevents global nkage, yet the extent of bonding within this ubcontract to the University of California at Santa Barbara, and by a gift from NGK network is sufficient to impart the requisite matrix integrity for Member. American Ceramic Society. acceptable off-axis properties. These opposing requirements or 595Effects of Thermal Aging on the Mechanical Properties of a Porous-Matrix Ceramic Composite Eric A. V. Carelli* Science and Technology Center, Siemens-Westinghouse Power Corporation, Pittsburgh, Pennsylvania 15235 Hiroki Fujita,* James Y. Yang, and Frank W. Zok* Materials Department, University of California, Santa Barbara, California 93106 The present article focuses on changes in the mechanical properties of an all-oxide fiber-reinforced composite following long-term exposure (1000 h) at temperatures of 1000–1200°C in air. The composite of interest derives its damage tolerance from a highly porous matrix, precluding the need for an interphase at the fiber–matrix boundary. The key issue in￾volves the stability of the porosity against densification and the associated implications for long-term durability of the compos￾ite at elevated temperatures. For this purpose, comparisons are made in the tensile properties and fracture characteristics of a 2D woven fiber composite both along the fiber direction and at 45° to the fiber axes before and after the aging treatments. Additionally, changes in the state of the matrix are probed through measurements of matrix hardness by Vickers indentation and through the determination of the matrix Young’s modulus, using the measured composite moduli cou￾pled with classical laminate theory. The study reveals that, despite evidence of some strengthening of the matrix and the fiber–matrix interfaces during aging, the key tensile properties in the 0°/90° orientation, including strength and failure strain, are unchanged. This strengthening is manifested to a more significant extent in the composite properties in the
45° orientation, wherein the modulus and the tensile strength each exhibit a twofold increase after the 1200°C aging treatment. It also results in a change in the failure mechanism, from one involving predominantly matrix damage and interply delami￾nation to one which is dominated by fiber fracture. Addition￾ally, salient changes in the mechanical response beyond the maximum load suggest the existence of an optimum matrix strength at which the fracture energy in the
45° orientation attains a maximum. The implications for long-term durability of this class of composite are discussed. I. Introduction THE power generation industry has been under increased pres￾sure to reduce NOx emissions from gas turbine engines while keeping up with market demands for increased power output and efficiency. These goals can be achieved in part through reductions in the amount of film cooling of combustor liners and turbine airfoils with attendant increases in the temperatures both within the gas turbine and at the burner outlet.1,2 In current gas turbine engines, many of the superalloy-based components are operating at or near their upper use temperature, even with the benefits imparted by the use of thermal barrier coatings, thereby precluding significant temperature elevations with these alloys. To meet future environmental and performance standards, it is anticipated that the targeted temperature elevations in turbine components will be accomplished through the use of continuous-fiber-reinforced ceramic composites (CFCCs). Among the various ceramic com￾posites that have been developed to date, the ones that have attracted the greatest attention within the power generation indus￾try in the past few years are those made from all-oxide constitu￾ents. The main advantage of the oxide-based composites over non-oxide ones (e.g., SiC/SiC) is their superior resistance to oxidation under typical turbine engine conditions and hence their potential for long-term durability. As part of a broad activity aimed at developing and assessing oxide composites for use in future generations of gas turbine engines, the present study focuses on changes in the mechanical properties of a candidate all-oxide CFCC following long-term exposure (1000 h) at temperatures of 1000–1200°C. The compos￾ite material of interest derives its damage tolerance from a highly porous matrix, precluding the need for an interphase at the fiber–matrix boundary. Although the efficacy of this material concept in enabling damage tolerance has been demonstrated,3–6 it remains to be established whether the matrix pore structure is stable against densification and whether the desirable damage￾tolerant characteristics can be retained for extended time periods at the targeted service temperatures. Indeed, other CFCCs based on the porous-matrix concept have been shown to exhibit severe degradation in composite properties once the matrix densifies appreciably.7 The main matrix constituent in the present composite is mullite, in the form of a weakly bonded particulate network. This selection is based on the sluggish sintering kinetics of mullite3 at the upper use temperature for the fibers (1200°C for Nextel 720). This phase is intended to form a contiguous particulate network that should be immune from appreciable densification both during processing and under subsequent service conditions. The minor matrix constituent is alumina, present both in the form of particulates from a slurry and as a product of pyrolysis of an aqueous precursor solution.3,5 Because of its more rapid sintering kinetics, alumina serves to bond the mullite particulates and the fibers together, thereby enhancing the matrix-dominated composite properties, e.g., inter￾laminar strength and off-axis in-plane strength. However, if the degree of sintering becomes excessive, the damage-tolerant char￾acteristics may be compromised. The challenge involves selection of the relative fractions and topologies of the two phases such that the network of mullite particles remains contiguous and hence prevents global shrinkage, yet the extent of bonding within this network is sufficient to impart the requisite matrix integrity for acceptable off-axis properties. These opposing requirements on the E. Lara-Curzio—-contributing editor Manuscript No. 187723. Received May 4, 2001; approved December 21, 2001. Funding for this work was provided by the Air Force Office of Scientific Research under Contract No. F49620-99-1-0259, by Siemens Westinghouse Power Corp. through both internal research funds at the Science and Technology Center and a subcontract to the University of California at Santa Barbara, and by a gift from NGK Corp. *Member, American Ceramic Society. J. Am. Ceram. Soc., 85 [3] 595–602 (2002) 595 journal