Ceramics in Aerospace Applications PDF

4. Ceramics and Composites Ceramic Materials The word “ceramic” comes from Greek word “keramos” means pottery. The ceramics are inorganic non metallic solid materials with varying properties due to their difference in bounding and structure. Ceramics usually consists of metallic and non me...

4. Ceramics and Composites Ceramic Materials The word “ceramic” comes from Greek word “keramos” means pottery. The ceramics are inorganic non metallic solid materials with varying properties due to their difference in bounding and structure. Ceramics usually consists of metallic and non metallic elements bounded by ionic and covalent bonds. The ceramics are generally hard, brittle, poor conductivity, high melting point, resistant to creep, low toughness, low ductility etc. They are good electrical and thermal insulators due to absence of free electrons. Ceramics includes clay articles, silicate, metallic oxides and their combinations, pottery objects. New ceramic materials were developed for use in advanced ceramic engineering, such as in semiconductors. 1 Spectrum of Ceramics Uses 2  CLASSIFICATIONS OF CERAMICS Ceramic materials can be classified into following basic groups, according to their fields of use. 1) Structural Ceramics : The ceramic materials that are used for constructing buildings and other various structures are called structural ceramics. e.g. Bricks, floors, pipes, roof tiles etc. 2) Facing ceramic materials: Such ceramic materials are used for internal and external facing of building and structures. e.g. Facing bricks, tiles, slabs etc. 3) Refractorie’s ceramics: These are ceramic materials whose mechanical properties at high temperatures do not changes. These materials are used in ovens for making various parts in industries. Also used for lab furnaces, ovens and apparatus for operating at high temperature. 4) Fine ceramics: They are used domestically in electrical appliances and in laboratories. e.g. Dishes, wash basins, porcelain wares, chemical wares glassed pottery sanitary wares etc. 3  Classifications of Ceramics 5) Special ceramics: Ceramics belonging to this class have specific properties hence they are used in instrument manufacture, radio industries etc. Ceramics are also classified in two categories on the basis of characteristic properties and applications of ceramic materials in many fields they are A) Traditional ceramics B) Advanced / Modern or technical ceramics. Traditional ceramics Traditional ceramics are composed of three naturally occurring basic components clay, silica and feldspar. The structure of clay is plate like. It is hydrated compound of alumina, silicate minerals. This clay like structure of clay provides strength after converting into ceramic products such as bricks, tiles, porcelain, sanitary wares etc. 4  Traditional ceramics At a temperature about 1000oC, alumina and silica mixture forms mullite ( 3Al2O3.2SiO2) which is stable compound. Second compound silica is used in ceramic industries in the form of sand, sand stone, quartz. Third compound feldspar is basically potash ( K2OAl2O3.6SiO2) and soda ( Na2OAl2O3.6SiO2) which is common minerals. In traditional ceramics, the mined raw materials are converted into small particles either by milling or grinding. Then powder of desired size of ceramics is obtained by sizing or screening. The powders are then well mixed usually with water and additives to import flow characterizations before melting. 5  Modern Ceramics  Modern ceramics widely used for industrial applications such as in the field of electronics, communications, medicines, transportations, optics, energy conversion and construction. Ceramic material is made by objects from inorganic, non-metallic materials by the action of heat. The modern ceramics are pure compounds such as magnesium oxide, aluminium oxide, barium titanate, silicon carbide and silicon nitrate. Thus starting materials for the modern ceramics are synthesized by chemical reactions. The examples of modern ceramics are Al2O3, MgO,ZrO2, BeO, SiO2, MgAl2O4, BaTiO3 etc. 6  Classification of Ceramics Ceramic Materials Glasses Clay Refractories Abrasives Cements Advanced products ceramics -optical -whiteware -bricks for -sandpaper -composites -engine -composite -structural high T -cutting -structural rotors reinforce (furnaces) -polishing valves -containers/ bearings household -sensors Adapted from Fig. 13.1 and discussion in Section 13.2-8, Callister & Rethwisch 8e. 7  Classification of Ceramics 8 1.10.2024  Classification of Ceramics Traditional Ceramics  the older and more generally known types (porcelain, brick, earthenware, etc.)  Based primarily on natural raw materials of clay and silicates  Applications; building materials (brick, clay pipe, glass) household goods (pottery, cooking ware) manufacturing ( abbrasives, electrical devices, fibers) Traditional Ceramics 9  Classification of Ceramics Advanced Ceramics  have been developed over the past half century  Include artificial raw materials, exhibit specialized properties, require more sophisticated processing  Applied as thermal barrier coatings to protect metal structures, wearing surfaces,  Engine applications (silicon nitride (Si3N4), silicon carbide (SiC), Zirconia (ZrO2), Alumina (Al2O3)) bioceramic implants 10  Classification of Ceramics Oxides CERAMICS Nonoxides Composite Oxides: Alumina, zirconia Non-oxides: Carbides, borides, nitrides, silicides Composites: Particulate reinforced, combinations of oxides and non-oxides 11  Classification of Ceramics Oxide Ceramics:  Oxidation resistant  chemically inert  electrically insulating  generally low thermal conductivity  slightly complex manufacturing  low cost for alumina  more complex manufacturing  higher cost for zirconia. zirconia 12  Classification of Ceramics Non-Oxide Ceramics:  Low oxidation resistance  extreme hardness  chemically inert  high thermal conductivity  electrically conducting  difficult energy dependent manufacturing and high cost. Silicon carbide ceramic foam filter (CFS) http://images.google.com.tr/imgres?imgurl=http://www.made-in- china.com/image/2f0j00avNtpdFnLThyM/Silicon-Carbide-Ceramic-Foam-Filter-CFS-.jpg&imgrefurl 13  Classification of Ceramics Ceramic-Based Composites:  Toughness  low and high oxidation resistance (type related)  variable thermal and electrical conductivity  complex manufacturing processes  high cost. Ceramic Matrix Composite (CMC) rotor http://images.google.com.tr/imgres?imgurl=http://www.oppracing.com/images/cm suploads/Large_Images/braketech%2520cmc%2520rotor%2520oppracing%2520cbr 1000rr.jpg&imgrefurl 14  Structures of Ceramics Most of ceramics have crystalline structures. They are made by two or more elements. Structure of ceramics is complex than metals. The ceramic material is formed due to ionic bonding between two elements resulting Columbic force of attraction between negatively charged ( non-metal) ions called anions and positively charged (metal) ions called cations. The cations and anions are formed respectively due to loss of valence electrons from the metallic elements and conversions of non-metallic elements. Following are the structures of the technical ceramic compounds. 1. Rock Salt ( NaCl) Structure. 2. Cesium Chloride Structure. 3. Zinc blend Structure. 4. Perovskite Structure. 15  Rock Salt ( NaCl) Structure In Rock salt, Sodium atom losses its valance electron and acquire positive charge. While chloride atom has acquired the electron lost by Sodium atom resulting chlorine ion. Both Na+ and Cl- ions attract each other because of electrostatic force and forms NaCl crystal. Unit cell of NaCl is as shown below. 16  Rock Salt Structure( NaCl) rNa = 0.102 nm rCl = 0.181 nm rNa/rCl = 0.564  cations (Na+) prefer octahedral sites Adapted from Fig. 12.2, Callister & Rethwisch 8e. Sodium chloride 17  Rock Salt ( NaCl) Structure  Each cation and anion has six neighbours hence it has six co- ordination number. The unit cell of NaCl crystal has FCC arrangement of anions with one cation at centre of each 12 cube edges. The compounds that are crystallize with NaCl structure includes refractory carbides and nitrates of titanium zirconium. The important ceramic compounds displaying the NaCl structure is MgO. The Na+ ions are replacing by Mg2+ ions and Cl ions are replaced by O2- ions. As the result the properties of MgO in NaCl structure such as melting point, hardness increases which leads to many industrial applications. E.g. MgO is used as good insulating material at elevated temperatures in electric stoves and ovens. It is also used in steel plant furnaces as a refractory material. It has zero porosity and high optical transparency hence it is used in infrared transmission. 18  MgO and FeO MgO and FeO also have the NaCl structure O2- rO = 0.140 nm Mg2+ rMg = 0.072 nm rMg/rO = 0.514  cations prefer octahedral sites So each Mg2+ (or Fe2+) has 6 neighbor oxygen atoms 19  Cesium Chloride (CsCl) structure The unit cell of CsCl crystal has two interpenetrating simple cubic lattices. The corners of one sub-lattice is the body centre of the another one sub-lattice is occupied by Cs ions while the other by Cl ions. The resultant structure of Cesium Chloride is shown below. There are eight ions at cube corners and one cation at centre of cube which forms simple cubic structure. The co-ordination number is eight. 20 High-purity cesium-133 stored in argon 21  Zinc Blend ( ZnS) Structure Zinc Sulphide crystal structure is formed when two face centred cubic sub-lattices are occupied by different elements. It has FCC structure of S with Zn at interior tetrahydral positions. The co-ordination number is four. Unit cell of Zinc Blend is as shown below. 22  Perovskite ( CaTiO3) Structure The structure of CaTiO3 is known as perovskite structure as shown below. The substitution of barium in place of calcium forms very important technical ceramics BaTiO3 which has applications in radios, televisions because it increases dielectric constants due to the large dipole moments. complex oxide BaTiO3 23 Traditional Ceramics 24 Preparation of raw materials 25  Powder Processing Ceramic powder is converted into a useful shape at this step. Processing techniques: – Tape casting – Slip casting – Injection molding 26  Slip Casting A suspension of seramic powders in water , slip, is poured into a porous plaster mold Water from the mix is absorbed into the plaster to form a firm layer of clay at the mold surface 27  Injection Molding http://global.kyocera.com/fcworld/first/process06.html Raw materials are mixed with resin to provide the necessary fluidity degree. Then injected into the molding die The mold is then cooled to harden the binder and produce a "green" compact part (also known as an unsintered powder compact). 28 1.10.2024  Difference Between Casting and Molding  Slip Casting Mixed raw materials are combined with solvating media and a dispersant Then fed into an absorbent die. The materials are dehydrated  Injection molding and solidified raw materials are mixed with resin. Then fed injected into the molding die The mold is then cooled to harden the binder. 29  DRYING PROCESS Water must be removed from clay piece before firing Shrinkage is a problem during drying. Because water contributes volume to the piece, and the volume is reduced when it is removed. 30 1.10.2024  PROPERTIES OF CERAMICS Extreme hardness – High wear resistance – Extreme hardness can reduce wear caused by friction Corrosion resistance Heat resistance – Low electrical conductivity – Low thermal conductivity – Low thermal expansion – Poor thermal shock resistance 31 1.10.2024  Properties of Ceramics  Low ductility – Very brittle – High elastic modulus  Low toughness – Low fracture toughness – Indicates the ability of a crack or flaw to produce a catastrophic failure  Low density – Porosity affects properties  High strength at elevated temperatures 32 1.10.2024  General Comparison of Materials Property Ceramic Metal Polymer Hardness Very High Low Very Low Elastic modulus Very High High Low Thermal expansion High Low Very Low Wear resistance High Low Low Corrosion resistance High Low Low 33 1.10.2024  General Comparison of Materials Property Ceramic Metal Polymer Ductility Low High High Density Low High Very Low Electrical Depends High Low Conductivity on material Thermal Depends High Low Conductivity on material Magnetic Depends High Very Low on material 34 1.10.2024  Ceramics Thermal Properties Most important thermal properties of ceramic materials:  Heat capacity : amount of heat required to raise material temperature by one unit (ceramics > metals)  Thermal expansion coefficient: the ratio that a material expands in accordance with changes in temperature  Thermal conductivity : the property of a material that indicates its ability to conduct heat  Thermal shock resistance: the name given to cracking as a result of rapid temperature change 35 1.10.2024  Thermal Properties  Thermal expansion Comparison of thermal expansion coefficient between metals and fine ceramics The coefficients of thermal expansion depend on the bond strength between the atoms that make up the materials. Strong bonding (diamond, silicon carbide, silicon nitrite) → low thermal expansion coefficient Weak bonding ( stainless steel) → higher thermal expansion coefficient in comparison with fine ceramics 36  Thermal Properties  Thermal conductivity Generally less than that of metals such as steel or copper Ceramic materials, in contrast, are used for thermal insulation due to their low thermal conductivity (except silicon carbide, aluminium nitride) http://global.kyocera.com/fcworld/charact/heat/images/thermalcond_zu.gif 37  Thermal Properties  Thermal shock resistance A large number of ceramic materials are sensitive to thermal shock Some ceramic materials → very high resistance to thermal shock is despite of low ductility (e.g. fused silica, Aluminium titanate ) Result of rapid cooling → tensile stress (thermal stress)→cracks and consequent failure The thermal stresses responsible for the response to temperature stress depend on: -geometrical boundary conditions -thermal boundary conditions -physical parameters (modulus of elasticity, strength…) 38  Electrical Properties of Ceramic Electrical conductivity of ceramics varies with The Frequency of field applied effect Charge transport mechanisms are frequency dependent. The temperature effect The activation energy needed for charge migration is achieved through thermal energy and immobile charge career becomes mobile. 39 1.10.2024  Electrical Properties of Ceramic Most of ceramic materials are dielectric. (materials, having very low electric conductivity, but supporting electrostatic field). Dielectric ceramics are used for manufacturing capacitors, insulators and resistors. 40 1.10.2024  Properties of Ceramics 1) Ceramics are Very brittle ,Low ductility and High elastic modulus 2) Ceramics are Low electrical conductivity, Low thermal conductivity and Low thermal expansion. 3) They have High strength at elevated temperatures 4) Most of ceramic materials are dielectric 5) Hardness of ceramics isVery High 6) Wear resistance is High 7) Elastic modulus is Very High 8) Heat capacity of Ceramics is greater than metals 9) A large number of ceramic materials are sensitive to thermal shock 10) Thermal conductivity of ceramics is generally less than that of metals 11) Oxide ceramics are generally bad conductors 41  Applications of Ceramics Ceramic materials have several industrial and technical applications due to their wide range of properties. Following are some applications of the advanced ceramics in different fields. 1. Dielectric ceramics are used for manufacturing capacitors, insulators and resistors, semiconductors, Lasers magnets etc. 2. The blade of a ceramic knife will stay sharp for much longer than that of a steel knife, although it is more brittle. 3. Advanced ceramics are largely used in aircraft, thermal protection system in rockets, insulating tiles for space shuttle etc. 4. Advanced ceramics has low expansion which finds applications in automotive field for catalytic converters, oxygen sensors, turbocharger rotors etc. 5. The high temperature structural property of ceramics finds applications for cutting tools, dies, molding materials etc. 6. Ceramics are used in nuclear energy production, nuclear core, control rods in nuclear reactors etc. 7. Ceramics are commonly used in biomedical applications for bone repairing, tooth replacement, hearing devices etc. 42  Applications of Ceramics 8. Ceramic brake disks for vehicles are resistant to abrasion at high temperatures. 9. Ceramic material is used to protect the cockpits of some military airplanes, because of the low weight of the material. 10. Ceramics can be used in place of steel for ball bearings due to their higher hardness. 11. High-tech ceramic is used in watch making for producing watch cases due to its light weight, scratch resistance, durability and smooth touch. 12. Turbine engines made with ceramics could operate more efficiently, giving aircraft greater range. 13. In the early 1980s, Toyota researched production of an adiabatic engine using ceramic components in the hot gas area. The ceramics would have allowed temperatures of over 3000 °F (1650 °C). 43  3. Cermets Cermets are composite materials made from ceramic and metallic components. Properties High wear resistance and hardness from ceramics Ductility and toughness from the metal Applications: Cutting tools, wear-resistant coatings, and aerospace components due to their ability to withstand extreme conditions. Example: Tungsten carbide (WC) particles in a cobalt matrix are common in cutting tools. 44  4. Glass Ceramics Glass ceramics are materials that start as glass and are transformed into polycrystalline ceramics through controlled crystallization. Properties: Low thermal expansion Good mechanical properties High resistance to thermal shock Applications: Used in cooking wares, telescope mirrors, and aerospace components that require precise thermal stability. 45  5. Production of Semi-Fabricated Forms Sintering: A process where ceramic powders are pressed and heated below their melting point to bond the particles together. Hot Pressing: Combines pressure and heat to produce denser ceramic components. Extrusion: Used to produce continuous ceramic shapes like tubes and rods by forcing the material through a die. Slip Casting: A method where a liquid ceramic slurry is poured into a mold, dried, and fired to create complex shapes. Powder Metallurgy: Utilized for the production of cermets and composite materials by compacting and sintering powders. 46  6. Carbon/Carbon Composites Carbon/Carbon composites consist of a carbon matrix reinforced with carbon fibers, known for their high-temperature stability and lightweight properties. Fabrication Processes Chemical Vapor Infiltration (CVI): Gaseous precursors are decomposed to deposit carbon onto fiber preforms. Resin Infiltration: Carbon fibers are impregnated with resin, then pyrolyzed to create the carbon matrix. Properties: Excellent thermal and chemical resistance High strength-to-weight ratio Superior thermal shock resistance Applications in Aerospace: Used in thermal protection systems, nose cones, and heat shields of spacecraft due to their ability to withstand extreme temperatures. 47  7. Fabrication Processes of Metal Matrix Composites (MMCs) Stir Casting: Reinforcement particles are dispersed into molten metal through stirring. Powder Metallurgy: Metal and reinforcement powders are compacted and sintered. Infiltration: Molten metal is infiltrated into a preform of ceramic or carbon fibers. Properties: High strength and stiffness Enhanced wear resistance and creep resistance Applications in Aerospace: MMCs are used in components such as landing gear, turbine blades, and structural components requiring lightweight and high-strength properties. 48  8. Polymer Matrix Composites (PMCs) PMCs consist of a polymer matrix reinforced with fibers (usually carbon, glass, or aramid). Fabrication Processes Hand Lay-Up: Layers of fabric reinforcement are manually applied to a mold and impregnated with resin. Autoclave Molding: Composite preforms are placed in a mold, heated, and pressurized in an autoclave to consolidate the layers. Filament Winding: Continuous fibers are wound onto a rotating mandrel and impregnated with resin, forming cylindrical shapes. Properties High strength-to-weight ratio Corrosion resistance Tailored mechanical properties Applications in Aerospace: PMCs are widely used in airframes, fuselage skins, wings, and tail sections, offering weight savings and high performance. 49  9. Aerospace Applications of Composites Lightweight and High Strength: Composites are essential for reducing weight while maintaining structural integrity in aircraft and spacecraft. Thermal and Wear Resistance: Materials like Carbon/Carbon composites and MMCs are used in high-temperature environments such as engine components and heat shields. Cost Efficiency: With advancements in production techniques, composites offer cost savings in fuel and material usage over time. Ceramics, cermets, glass ceramics, and composites each play critical roles in modern aerospace applications due to their unique properties like thermal stability, lightweight, and mechanical strength. Understanding the production and fabrication methods of these materials is essential to exploiting their full potential in aerospace engineering. 50  Ceramic Material Aerospace Structure Construction 1. Introduction to Ceramics in Aerospace Construction Ceramics are inorganic, non-metallic materials that are generally formed by the action of heat and subsequent cooling. They are known for their high melting points, thermal stability, and resistance to oxidation. Importance in Aerospace: Due to their unique properties such as low density, high temperature stability, and resistance to wear and corrosion, ceramics are becoming crucial in the design and construction of aerospace structures. 51 4.2 Properties of Ceramics Relevant to Aerospace Applications High Melting Point: Most ceramics can withstand extreme temperatures without melting, making them suitable for applications like thermal protection systems. Low Density: Ceramics are lighter compared to metals, which helps in reducing the overall weight of the aircraft or spacecraft. High Strength and Hardness: Ceramics offer excellent mechanical strength, especially at high temperatures. Resistance to Oxidation and Corrosion: This property is critical for components exposed to oxidizing environments. Thermal Insulation: Ceramics are good thermal insulators, which protects internal components from extreme external temperatures. Brittleness: While ceramics are strong, they tend to be brittle, which limits their use in load-bearing applications. 52 4.3 Types of Ceramics Used in Aerospace Structures Oxide Ceramics: Alumina (Al₂O₃), zirconia (ZrO₂), and yttria- stabilized zirconia (YSZ) are widely used due to their high thermal stability and resistance to thermal shock. Non-Oxide Ceramics: Silicon carbide (SiC) and silicon nitride (Si₃N₄) are known for their high mechanical strength and thermal conductivity. Glass Ceramics: These are used in applications that require both high strength and good thermal properties, such as optical components and radomes. Ceramic Matrix Composites (CMCs): CMCs, such as SiC/SiC or alumina-based composites, are designed to improve toughness and are used in high-stress environments. 53 4.4 Construction Techniques for Ceramic Aerospace Structures Hot Pressing and Sintering: Ceramic powders are compacted and heated below their melting point to form solid structures. Hot pressing can improve the density and mechanical properties of ceramics. Chemical Vapor Deposition (CVD): Used to deposit ceramic coatings or form ceramic components by chemical reactions at high temperatures. Hot Isostatic Pressing (HIP): This process involves applying pressure and temperature uniformly in all directions to eliminate voids and improve mechanical properties. Ceramic Coating Techniques: Thermal barrier coatings (TBCs) are applied on metal components like turbine blades to protect against high temperatures and oxidation. Powder Injection Molding: A technique for producing complex-shaped ceramic components that are difficult to achieve with conventional methods. Additive Manufacturing (AM): Emerging as a technique for creating complex ceramic structures layer-by-layer with high precision. 54  4.5 Applications of Ceramics in Aerospace Structures Thermal Protection Systems (TPS): Ceramics like reinforced carbon-carbon (RCC) are used in the nose cone and leading edges of space shuttles, protecting them during re-entry. Turbine and Engine Components: Silicon nitride and other ceramics are used in turbine blades, combustors, and nozzles to withstand high temperatures and reduce thermal fatigue. Ablative Materials: Ceramics are used in ablative heat shields that gradually burn away, absorbing and dissipating heat during atmospheric re-entry. Spacecraft and Satellite Structures: Lightweight ceramic composites are used in satellite structures and antenna systems for weight savings and thermal stability. Missile Radomes: Ceramics like alumina are used in radomes to protect electronic systems from aerodynamic heating while allowing transmission of radar signals. Optical and Sensor Components: Glass ceramics and other optical ceramics are used in windows, mirrors, and lenses of optical and infrared sensors. 55 4.6 Challenges in Using Ceramics for Aerospace Structures Brittleness and Fracture Toughness: Ceramics have a tendency to fracture under mechanical stress. This limits their use in structural components unless reinforced with fibers or used in composites. Manufacturing Complexity: Producing high-quality ceramic components is a complex and costly process due to the need for precise control over temperature and pressure. Joining Techniques: Joining ceramics to metals or other materials is challenging due to differences in thermal expansion coefficients and bonding characteristics. Quality Control: Defects like porosity and cracks can lead to catastrophic failures. Non-destructive evaluation (NDE) techniques like ultrasonic testing are essential for ensuring component integrity. 56 4.7. Future Directions in Ceramic Aerospace Construction Development of Advanced CMCs: Researchers are developing new ceramic matrix composites with improved toughness, thermal stability, and oxidation resistance. Integration with Other Materials: Hybrid structures combining ceramics with metals or polymers can offer improved performance in multifunctional applications. Additive Manufacturing: Continued advancements in additive manufacturing techniques for ceramics will enable more complex geometries and integrated structures. Functionally Graded Materials (FGMs): These materials have varying compositions and properties across their volume, which can optimize performance under different loading and environmental conditions. Ceramics play an essential role in aerospace structure construction, offering unique properties like high temperature stability, lightweight, and resistance to oxidation. Despite challenges in manufacturing and brittleness, advancements in ceramic matrix composites and additive manufacturing are paving the way for broader use in high-performance aerospace applications. 57  5. High Temperature Materials Characterization 5.1 Introduction to High Temperature Materials Mechanical properties of regular engineering materials are affected by change in temperature. High temperature materials are those that retain their structural integrity, mechanical properties, and thermal stability when exposed to elevated temperatures, typically above 500°C. Importance in Aerospace: These materials are essential for components that operate in extreme environments, such as engines, thermal protection systems, and hypersonic vehicles, where temperatures can exceed 1,500°C. 58  5.2 Classification of High Temperature Materials Metals and Alloys Superalloys: Nickel-based, cobalt-based, and iron-based superalloys used in turbine blades and engine components. Refractory Metals: Tungsten, molybdenum, tantalum, and niobium, known for their high melting points and stability. Ceramics Oxide ceramics like alumina (Al₂O₃) and zirconia (ZrO₂). Non-oxide ceramics like silicon carbide (SiC) and silicon nitride (Si₃N₄). Composites Ceramic Matrix Composites (CMCs): SiC/SiC, alumina-based CMCs for improved toughness. Carbon/Carbon Composites: Used in thermal protection systems. Coatings Thermal barrier coatings (TBCs) such as Yttria-stabilized zirconia (YSZ) used for engine components. 59 5.3 Production and Characteristics of High Temperature Materials Metals and Alloys Vacuum Induction Melting: Used for superalloys to prevent oxidation. Directional Solidification: Produces single-crystal structures for improved creep resistance. Ceramics Sintering: Ceramic powders are compacted and heated to form dense components. Chemical Vapor Deposition (CVD): Used for coating applications or to produce dense ceramic parts. Composites Hot Pressing: Used for ceramic composites to achieve high density and improved mechanical properties. Resin Infiltration: Used for Carbon/Carbon composites, where resin is impregnated and pyrolyzed to form a carbon matrix. 60 5.4. Mechanical and Thermal Properties of High Temperature Materials Mechanical Properties Creep Resistance: Ability to resist deformation under sustained high temperatures. Tensile Strength: Measurement of the material’s ability to withstand tension at elevated temperatures. Fracture Toughness: Resistance to crack propagation under high-temperature conditions. Thermal Properties Thermal Conductivity: Rate at which heat is transferred through the material. Thermal Expansion: Change in dimensions with temperature variation. Heat Capacity: Amount of heat required to raise the temperature of a material by one degree. 61 5.5 Methods and Testing for High Temperature Materials Mechanical Testing Creep Testing: Measures the time-dependent deformation under constant stress and temperature. Tensile Testing: Performed at elevated temperatures to determine ultimate tensile strength and ductility. Fatigue Testing: Evaluates the material’s resistance to cyclic loading at high temperatures. Thermal Testing Thermal Gravimetric Analysis (TGA): Measures weight changes as a function of temperature. Differential Scanning Calorimetry (DSC): Determines heat flow associated with transitions in materials. 62  Cont… Thermal Expansion Testing: Measures dimensional changes with temperature. Microstructural Analysis Scanning Electron Microscopy (SEM): Examines surface features and microstructure of the material. Transmission Electron Microscopy (TEM): Provides high- resolution images of defects and grain boundaries. X-Ray Diffraction (XRD): Determines crystalline structure and phase transformations. 63 5.6 Determination of Mechanical and Thermal Properties at Elevated Temperatures High Temperature Tensile Testing Carried out using furnaces that can heat samples to desired temperatures while applying tensile loads. Provides data on yield strength, ultimate tensile strength, and elongation at elevated temperatures. Creep Testing Measures deformation over time under a constant load and temperature. Critical for assessing materials used in turbine blades and other high-stress components. Thermal Conductivity and Expansion Measured using methods like laser flash analysis for thermal conductivity and dilatometry for thermal expansion. Hardness Testing Indentation methods like Vickers or Rockwell hardness tests are adapted for high temperatures to determine material hardness. 64 5.7. Application of High Temperature Materials in Thermal Protection Systems of Aerospace Vehicles Thermal Protection Systems (TPS) Ablative Materials: Materials that dissipate heat by ablating, such as phenolic resins and carbon-based composites, are used for spacecraft re-entry. Insulative Ceramics: Silica and alumina ceramics are used in space shuttle tiles. Reinforced Carbon-Carbon (RCC): Used in nose cones and leading edges of space shuttles due to high-temperature stability. Engine and Turbine Components Superalloys and ceramic composites are used in turbine blades, combustors, and nozzles due to their ability to withstand high temperatures and oxidation. Hypersonic Vehicle Components Ultra-high temperature ceramics (UHTCs) such as zirconium diboride (ZrB₂) and hafnium carbide (HfC) are used for leading edges and control surfaces of hypersonic vehicles. 65 5.8. High Temperature Material Characterization Techniques Thermal Shock Testing: Evaluates the ability of materials to withstand rapid temperature changes. Thermal Fatigue Testing: Measures material response to cyclic thermal loading. High Temperature Oxidation Testing: Assesses oxidation resistance of materials in high-temperature environments. Non-Destructive Evaluation (NDE): Ultrasonic testing, X-ray tomography, and acoustic emission techniques are used to detect internal flaws without damaging the material. 66  5.9. Challenges and Future Trends Challenges Development of materials with a combination of high strength, toughness, and resistance to oxidation. Manufacturing complexity and cost of high-temperature materials. Future Trends Use of functionally graded materials (FGMs) to tailor properties across the material. Development of new CMCs and superalloys for next-generation aerospace vehicles. Advancements in additive manufacturing for creating complex high-temperature structures. High temperature materials are essential for aerospace applications, where extreme environments demand materials that can retain their properties at elevated temperatures. Understanding the characterization and testing of these materials is critical for optimizing their performance and ensuring the reliability of aerospace systems. 67