C 2006 The American Ceramic Society urna Mechanical Design for Accommodating thermal Expansion Mismatch in Multilayer Coatings for Environmental Protection at Ultrahigh Temperatures Jie Bai, Kurt Maute, Sandeep r Shah, and Rishi raj Ultrahigh Temperature Materials Laboratory, Department of Mechanical Engineering, University of Colorado, Boulder. colorado 8030 The design of coatings is like designing a system. Every coating The next generation gas turbines are slated to contain ceramic or more specific functions that determine the choice of components made from silicon nitrite(Si3 N4). However, silicon Is, and its architecture. In the case of environmental nitride suffers from erosion in the streaming humid environment atmosphere In high-temperature applications the stresses aris- the volatilization of the passivating silica scale. 3- Example of ing from thermal expansion mismatch between the topcoat and such a result from our laboratory given in Fig. l, which show 如出 strate must be ameliorated. In this article we consider the increasing weight loss with higher streaming velocity. These data of an intermediate layer of a multilayer coating system reach up to a relative velocity of 35 cm/s; the velocities en explicit objective of managing thermal expansion dif- ountered in the gas turbine are an order of magnitude higher ween the topcoat and the substrate. The design is which would be clearly intolerable for Si3 N4. The objective ed upon a columnar architecture where the columns serve as environmental barrier coatings(EBCs) is to protect the load cible beams to accommodate relative displacement withou bearing silicon-nitride structure from corrosive weight loss in the racture, The value of the maximum stresses in the beam and high velocity, high temperature, and humid environment of the the topcoat are calculated and used to develop a map with fail gas turbine and safe regimes. The safe region is defined by the prevention of The design of EBCs is constrained by at least three criteria: tracture in the beams, since their fracture would precipitate de- (a) the topcoat of the ebc must be able to survive in the gas- lamination of the topcoat. As a rule of thumb the topcoat thick- turbine environment, (b) the topcoat must be securely bonded to ness should be less than the width of the columns for safe the Si3 N4 substrate, and(c) the coating architecture must have operation(this condition changes somewhat with the aspect good thermal shock resistance atio of the columns). A larger aspect ratio of the columns The choice of the optimum material for the topcoat is often also promotes safe design. We further consider how the tractions juxtaposed against the issue of thermal shock, as matching ther- induced by the thermal stresses on the surface of the substrate mal expansion and, at the same time providing chemical dur- may influence the intrinsic fracture strength of the substrate ability, can pose a challenge to the materials engineer. Howeve The stresses in the coating are predicted to have an insignificant his approach has been successfully used to develop coating effect on the intrinsic fracture strength of the substrate. siliconcarbide-based ceramic structures. In the present work we consider another approach, one where the topcoat is chosen en L. Introduction tirely for its corrosion resistance, and then the thermal shock is managed by adding a compliant intermediate layer which ac- This h ase turbine ep itomines the significance and the need hr commodates the difference in the coefficients of thermal expa highest temperatures and the most severe corrosive environ- ments in the gas turbine are experienced by nozzles, linings, and ing it a good choice as the material for the topcoat. However,its most of all, by the rotating turbine blade. The push for higher combustion temperatures is creating a need for multifunctional thermal expansion is much larger than that of Si3 N4(10 ppm vs coatings that can withstand thermal shock, adhere well to the about 3.5 ppm/k) which would cause it to spall. A coating de- substrate, provide thermal insulation, and protect from envi- ign which can ameliorate the thermal stresses is illustrated in ronmental corrosion. The state-of-the-art blade materials are ig. 2. It consists of a topcoat, a compliant intermediate coat that accommodates thermal strains and a bond coat that se- metallic superalloys. Zirconia-based thermal barrier coatings cures the upper lavers to the substrate. The mechanical design of (TBCs) for superalloys have been in use for over a decade. the compliant intermediate coat is the main subject of this paper The TBCs were developed by intuition and experience, yet they The principal purpose of the above coating architecture is have laid the foundation for the conceptual design of high-tem- to prevent the high velocity humid environment from imping- oatings. As described in Strangman and Schiele- they g directly on to the surface of Si3 N4. The thermal expansio ased upon a reactive metallic bond coat for adherence of the topcoat can be expected to produce"periodically zirconia and the superalloy, a columnar, strain toleran spaced"cracks, which will allow the humid environment to zirconia layer, and a dense zirconia topcoat. seep into the coating. Therefore the function of this EBC is merely to subdue the velocity of the environment. The questio C.H. Hsuch--contnibuting editor hen arises whether oxidation of Si3 N4 under static humid environments can be acceptable. Work to be published in a companion paper by Shah and Raj shows that a special bond Manuscript No. 21601. Received March 17, 2006: approved August 24. 2006. coat that is made from polymer derived siliconcarbonitride is as supported by the MEANS program at the ce Office of Sci- effective against oxidation at high temperatures in static humid ty(but not under high-velocity conditions). Thus a combination CA P experimgtanl part if th ip reseaich wa s supported inner the Power and bnergy of the polymer-derived coating o and the thermal stress Author to whom correspondence should be addressed. e-mail: rishi. raja colorado.edu management approach described in this paper can be usedMechanical Design for Accommodating Thermal Expansion Mismatch in Multilayer Coatings for Environmental Protection at Ultrahigh Temperatures Jie Bai, Kurt Maute, Sandeep R. Shah, and Rishi Rajw Ultrahigh Temperature Materials Laboratory, Department of Mechanical Engineering, University of Colorado, Boulder, Colorado 80302 The design of coatings is like designing a system. Every coating has one or more specific functions that determine the choice of materials, and its architecture. In the case of environmental barrier coatings the topcoat must be chemically inert to the atmosphere. In high-temperature applications the stresses aris￾ing from thermal expansion mismatch between the topcoat and the substrate must be ameliorated. In this article we consider the design of an intermediate layer of a multilayer coating system with the explicit objective of managing thermal expansion dif￾ference between the topcoat and the substrate. The design is based upon a columnar architecture where the columns serve as flexible beams to accommodate relative displacement without fracture. The value of the maximum stresses in the beam and in the topcoat are calculated and used to develop a map with fail and safe regimes. The safe region is defined by the prevention of fracture in the beams, since their fracture would precipitate de￾lamination of the topcoat. As a rule of thumb the topcoat thick￾ness should be less than the width of the columns for safe operation (this condition changes somewhat with the aspect ratio of the columns). A larger aspect ratio of the columns also promotes safe design. We further consider how the tractions induced by the thermal stresses on the surface of the substrate may influence the intrinsic fracture strength of the substrate. The stresses in the coating are predicted to have an insignificant effect on the intrinsic fracture strength of the substrate. I. Introduction THE gas turbine epitomizes the significance and the need for high temperature coatings for structural applications. The highest temperatures and the most severe corrosive environ￾ments in the gas turbine are experienced by nozzles, linings, and most of all, by the rotating turbine blade. The push for higher combustion temperatures is creating a need for multifunctional coatings that can withstand thermal shock, adhere well to the substrate, provide thermal insulation, and protect from envi￾ronmental corrosion. The state-of-the-art blade materials are metallic superalloys. Zirconia-based thermal barrier coatings (TBCs) for superalloys have been in use for over a decade. The TBCs were developed by intuition and experience,1 yet they have laid the foundation for the conceptual design of high-tem￾perature coatings. As described in Strangman and Schienle2 they were based upon a reactive metallic bond coat for adherence between zirconia and the superalloy, a columnar, strain tolerant zirconia layer, and a dense zirconia topcoat. The next generation gas turbines are slated to contain ceramic components made from silicon nitrite (Si3N4). However, silicon nitride suffers from erosion in the streaming humid environment of the gas turbine. The weight loss can be severe and is caused by the volatilization of the passivating silica scale.3–5 Example of such a result from our laboratory given in Fig. 1, which shows increasing weight loss with higher streaming velocity. These data reach up to a relative velocity of 35 cm/s6 ; the velocities en￾countered in the gas turbine are an order of magnitude higher which would be clearly intolerable for Si3N4. The objective of environmental barrier coatings (EBCs) is to protect the load bearing silicon-nitride structure from corrosive weight loss in the high velocity, high temperature, and humid environment of the gas turbine. The design of EBCs is constrained by at least three criteria: (a) the topcoat of the EBC must be able to survive in the gas￾turbine environment, (b) the topcoat must be securely bonded to the Si3N4 substrate, and (c) the coating architecture must have good thermal shock resistance. The choice of the optimum material for the topcoat is often juxtaposed against the issue of thermal shock, as matching ther￾mal expansion and, at the same time providing chemical dur￾ability, can pose a challenge to the materials engineer. However, this approach has been successfully used to develop coatings for siliconcarbide-based ceramic structures.7 In the present work we consider another approach, one where the topcoat is chosen en￾tirely for its corrosion resistance, and then the thermal shock is managed by adding a compliant intermediate layer which ac￾commodates the difference in the coefficients of thermal expan￾sion of the topcoat and the substrate. For example, zirconia has a proven record of durability as thermal barrier coatings, mak￾ing it a good choice as the material for the topcoat. However, its thermal expansion is much larger than that of Si3N4 (10 ppm vs about 3.5 ppm/K) which would cause it to spall. A coating de￾sign which can ameliorate the thermal stresses is illustrated in Fig. 2. It consists of a topcoat, a compliant intermediate coat that accommodates thermal strains, and a bond coat that se￾cures the upper layers to the substrate. The mechanical design of the compliant intermediate coat is the main subject of this paper. The principal purpose of the above coating architecture is to prevent the high velocity humid environment from imping￾ing directly on to the surface of Si3N4. The thermal expansion of the topcoat can be expected to produce ‘‘periodically spaced’’ cracks,8 which will allow the humid environment to seep into the coating. Therefore the function of this EBC is merely to subdue the velocity of the environment. The question then arises whether oxidation of Si3N4 under static humid environments can be acceptable.9 Work to be published in a companion paper by Shah and Raj10 shows that a special bond coat that is made from polymer derived siliconcarbonitride is effective against oxidation at high temperatures in static humid￾ity (but not under high-velocity conditions). Thus a combination of the polymer-derived coating10 and the thermal stress management approach described in this paper can be used to C.-H. Hsueh—contributing editor This research was supported by the MEANS program at the Air Force Office of Sci￾entific Research under the direction of Dr. Joan Fuller. The Grant number is F49620-01-1- 052. The experimental part of this research was supported under the Power and Energy CTA Program at Honeywell Inc., Phoenix, AZ, under the direction of Laura Lindberg. w Author to whom correspondence should be addressed. e-mail: rishi.raj@colorado.edu Manuscript No. 21601. Received March 17, 2006; approved August 24, 2006. Journal J. Am. Ceram. Soc., 90 [1] 170–176 (2007) DOI: 10.1111/j.1551-2916.2006.01354.x r 2006 The American Ceramic Society 170