Fundamentals of Ceramic Materials 2024-2025 PDF

FUNDAMENTALS OF CERAMIC MATERIALS Prof.Dr. Filiz Şahin 2024-2025 Fall Powder Processing  Ceramic Raw Materials The nature of the raw material has a major effect on the final properties of a ceramic component. Purity, particle size distribution, r...

FUNDAMENTALS OF CERAMIC MATERIALS Prof.Dr. Filiz Şahin 2024-2025 Fall Powder Processing  Ceramic Raw Materials The nature of the raw material has a major effect on the final properties of a ceramic component. Purity, particle size distribution, reactivity, polymorphic form, availability, and cost must all be considered and carefully controlled. 1. Traditional ceramic raw materials; Derived from common, naturally occurring raw materials such as clay minerals and quartz sand. Used for traditional ceramics such as wall and floor tiles, sanitary ware, white ware, refractories, bricks…. 2. Advanced ceramic raw materials; Mostly derived from synthetic(not naturally occurred) powders obtained by special synthesis processes. High purity materials and precise methods of production must be employed to ensure that the desired properties of these advanced materials are achieved in the final product. Used for special purpose such as space shuttle tile, engine components, artificial bones and teeth, computers and other electronic components, and cutting tools,. 3  1.Traditional Ceramics Raw Materials Ceramics have been produced for centuries. The earliest ceramic articles were made from naturally occurring raw materials. Early civilizations found that clay minerals became plastic when water was added and could be molded into shapes. The shape could then be dried in the sun and hardened in a high-temperature fire. Many of the raw materials used by the ancient civilizations are still utilized today and form the basis of a sizable segment of the ceramic industry. These ceramic products are often referred to as traditional ceramics. 4 Ceramic Tile Production Process 5 6 7 8  Traditional Ceramics Raw Materials Clay Kaolin Quartz Felspar Wollastanite Talc 9  CLAY MINERALS They form the basis of pottery and building bricks. They are layer materials and a subgroup of the sheet layer silicates. The clay minerals are hydrated aluminum silicates based on (Si 2O5)n sheets. They contain chemically bound water. Clays occure by the weathering of aluminosilicate rocks and sedimentation. 10  In terms of formation, kaolins are formed as a result of the alteration of feldspars in granitic or volcanic rocks into the mineral kaolinite. Primary Formation Secondary Formation Formation Formation through resulting from the the transportation of decomposition of decomposed feldspathic rocks in kaolinite by nature. floodwaters, erosion, etc Higher purity, Due to impurities whiter raw color and firing in the structure, color, darker raw and coarse-grained firing colors, Less plasticity. fine-grained. 11  CLAY MINERALS They have a broad range of physical characteristics, chemical compositions, and structures. Common impurities include compounds (usually oxides) of Ba, Ca, Na, K and Fe, and also some organic matter. Average particle size 2 µm 4 µm 12  CLAY The plate like morphology of clay important; 1. Gives easy cleavage which leads to a fine particle size and a narrow particle size distribution. 2. Allows the particles to easily move over one another. TEM image of single hexagonal plates of Planar and hexagonal plates kaolinite 13  Functions of Clay in Ceramic Body 1. Develop plasticity when mixed with water (hydroplasticity); the plasticity of clay suspensions is basic to many of the forming processes commonly used to fabricate ceramic bodies. 2. Clays melt over a temperature range (depending on composition) thus, a dense and strong ceramic piece can be produced without losing their shape or without complete melting. Desired shape can be maintained. The formed piece green body is dried to remove some of the moisture, and then fired at an elevated temperature to improve its mechanical strength 14  CLAY MINERALS Most of the clay based products fall within 2 broad classifications; 1. Structural clay products (building bricks, tiles, etc applications in which structural integrity is important). 2. Whitewares (become white after the high temperature firing Porcelain, pottery, tableware, sanitary ware etc.) 15  CLAY MINERALS Plasticity: When mixed with limited amounts of water, clays become plastic, and are able to be molded and formed easily. Example: Flour mixed with water = Easy form (plastic), Sand mixed with water = not plasticity Water have to add to clay to get plasticity, nothings give plasticity to clay except water. Cohesion: This property proves the clay to keep its form when it dried. Since sand doesn’t have this property, it changed its form when it dried. 16  CLAY MINERALS Color: Since clays contain some mixed oxides in their body, they are naturally colored. If the clay is pure, its color is white and is called as kaolin. Their color can be yellow, pink, reddish, blue-gray, green or black. Their colors give some information about their compositions; · If the clay contains limonite, its color is dark. · If it contains iron peroxide, its color is red. · If it contains manganese oxide, its color is black. · Organic materials change the clay color to violet. However its color can change after firing due to change in color of oxides at high temperatures. Shrinkage: The dimensions of clay after forming with water reduce during drying. This volume shrinkage continues during the firing step of ceramic material processing. This shrinkage is related with plasticity of the clay. 17  CLAY MINERALS There are 6 types of commercial clays. Their composition, plasticity, color firing characteristics are different. 18  CLAY -Kaolinite Al2(Si2O5)(OH)4 The most common clay mineral. The bonding within this two layered sheet is strong and Electron micrograph of kaolinite crystals. They intermediate ionic covalent, are in the form of hexagonal plates, some of adjacent sheets are only loosely which are stacked on top of one another (x21k) bound to one another by weak 19 van der Waals forces  CLAY -Kaolinite Al2(Si2O5)(OH)4 The general structure of the kaolinite group is composed of silicate sheets (Si2O5) bonded to aluminum oxide/hydroxide layers (Al2(OH)4) called gibbsite layers. The silicate and gibbsite layers are tightly bonded together with only weak bonding existing between the s-g paired layers It also known as China clay. It contains coarse grains (10-0.1µm). Fired color is white. Uses: In ceramics, as a filler for paint, rubber and plastics and the largest use is in the paper industry that uses kaolinite to produce a glossy paper such as is used in most magazines. In ceramic industry, kaolinite mostly used for sanitary ware, porcelain and insulator materials. For tiles processing it uses max.20 % of kaolinite. The most famous kaolinite reserves are situated in southwest of England, the Ukrain China. In Turkey; Balıkesir(M.Kemal Paşa), Çanakkale (Biga, Çan), Eskişehir (Bigadiç), Uşak, Bursa, İstanbul 20  CLAY -Kaolinite Al2(Si2O5)(OH)4 Chemical formula of kaolinite; Al2O3.2SiO2.2H2O Chemical composition of kaolinite; Al2O3 46.54 % SiO2 39.50 % H2O 12.96 % When kaolinite heated upto 200°C, hygroscopic water removes. At 500-600°C;crystal water removes and it transforms metakaolinite as follows; 3(Al2O3.2SiO2.2H2O) 3(Al2O3.2SiO2) + 6H2O At 1000°C metakaolinite transforms to mullite and silicate (Crystobalite); 3(Al2O3.2SiO2) 3Al2O3.2SiO2 + 4SiO2 Mullite forms at 1000°C from metakaolinite, is in the nidle form crystall structure rather than layers and causes very high hardness, chemical resistance, high strength and electrically insulator structure. 21  CLAY -Kaolinite Al2(Si2O5)(OH)4 Natural impurities (i e MgO, CaO, Na2O K2O in kaolinite act as mineralizers, which promote the crystallization of different mineral phases and enhance strength at 1000°C 22  CLAY –other Clay Minerals ü Halloysite; Al2(Si2O5)(OH)4·2H2O ü Pyrophyllite; Al2(Si2O5)2(OH)2 ü Montmorillonite; Al1.67Na0.33Mg0.33(Si2O5)2(OH)2 ü Mica; Al2K(Si1.5Al0.5O5)2(OH)2 ü Illite; Al2-xMgxK1-x-y (Si1.5y Al0.5+y O5)2 (OH)2 These minerals have different stacking of the silica and alumina layers as well as, incorporating metal hydrates of Na, K, Mg, Al or Fe between the silica and alumina layers 23 24  In Turkey there are 2 different clay rich areas. These are; Bilecik-Söğüt Istanbul-Şile-Kemerburgaz 1.500.000 ton clay produces in a year and whole clay is consumed in Turkish ceramic production industry. Main clay producers are; Matel A.Ş., Kalemaden A.Ş., Toprak A.Ş., Sörhaz A.Ş., -in Istanbul Söğüt A.Ş., Essan A.Ş., etc.-in Söğüt 25  Quartz Quartz is the most common mineral found on the surface of the Earth. A significant component of many igneous, metamorphic and sedimentary rocks, this natural form of silicon dioxide is found in an impressive range of varieties and colors. Silica (Si02) is a major ingredient in glass, glazes, enamels, refractories, abrasives, and white ware. Its major sources are in the polymorphic form quartz, which is the primary constituent of sand, sandstone, and quartzite. 26  Silica does not have plasticity, improves strength at high temperature in ceramic body. Quartz; reduces drying shrinkage of ceramic structure helps to regulate plasticity allow gas outlet during firing without deformation. Quartz is available in different minerals in the nature. Quartz sand: The most common minerals of quartz. It generally contains iron compounds. Quartz sand without iron use in the ceramic industry. Quartzit: It is pure quartz stone. It is available as large rocks. It is commonly used in the ceramic industry. Flint stone: It is used as milling media in mills. Diatomite: It has porous structure and it is amorphous. It is used for high temperature furnace bricks. 27 28  The silica has many different polymorphic forms. Increasing the temperature, forms are stable; – α-quartz – β-quartz – β-trydimite – β -crystobalite The transformation phases between α and β is displacive type The transformations between quartz, tridimite and crystobalite are reconstructive. 29  Feldspar In the manufacture of ceramics, feldspar is the second most important ingredient after clay. Feldspar is the name of a group of rock-forming minerals which make up as much as 60% of the Earth's crust. Feldspar minerals range in composition from KAlSi3O8 (Orthoclase) to NaAISi3O8 (Albite) to CaAl2Si2O8 (Anorthite). Feldspar does not have a strict melting point, since it melts gradually over a range of temperatures. This greatly facilitates the melting of quartz and clays and, through appropriate mixing, allows modulations of this important step of ceramic making. Feldspars are used as fluxing agents to form a glassy phase at low temperatures and as a source of alkalies and alumina in glazes. They improve the strength, toughness, and durability of the ceramic body. Feldspar is a source of Al2O3 which improves the mechanical properties of glass such as its scratch resistance and its ability to withstand thermal shock. 30  Melting Range of Feldspar Na-Feldspar :1150-1225°C K-Feldspar :1200-1250°C Ca-Feldspar :1500-1550°C Chemical Composition, (%) 31 32  Wollastonite Wollastonite forms from the interaction of limestones, that contain calcite, CaCO3, with the silica, SiO2, in hot magmas( high temperature and pressure). It forms by the following formula: CaCO3 + SiO2 ----> CaSiO3 + CO2 It contains 48.3 % CaO and 51.7 % SiO2. It is an important constituent in refractory ceramics (those ceramics that are resistant to heat) such as refractory tile and as a filler for paints. Some of the properties that make wollastonite so useful are its high brightness and whiteness, low moisture and oil absorption, and low volatile content. Wollastonite is used primarily in ceramics, friction products (brakes and clutches), metal making, paint filler, and plastics. It improves strength of ceramic products and reduces firing time of tiles. 33  Wollastonite Source of water insoluble calcium is wollastonite (CaO·SiO2). Wollastonite is found in either a pure form or in association with garnet or calcite and dolomite. In glazes, wollastonite may be used as a substitute for calcite, which reduces the volatiles and increases the gloss and texture of the glaze Wollastonite deposits are known for their low iron content which gives a glaze with an excellent fired color In enamels, wollastonite acts as a natural frit to reduce gas evolution It is used in ceramic insulating bodies and as an auxiliary flux in electrical insulators 34  Talc Talc is a mineral composed of hydrated magnesium silicate with the chemical formula H2Mg3(SiO3)4 or 3MgO.4SiO2.H2O. Natural raw material for producing tile, dinnerware, and electronic components Reasonable sintered strengths can be obtained when talc is sintered at 1000°C. 35  Talc In loose form, it is the widely used substance known as talcum powder. It has a perfect basal cleavage, and the folia are non-elastic, although slightly flexible. It is soft, with a hardness of 1 (Talc is the softest of the Mohs' scale of mineral hardness). It has a specific gravity of 2.5–2.8, a clear or dusty luster, and is translucent to opaque. Its colour ranges from white to grey or green and it has a distinctly greasy feel. Its streak is white. 36  Talc Talc is primarily formed via hydration and carbonation of serpentine, via the following reaction; Serpentine + Carbon Dioxide → Talc + Magnesite + Water 2Mg3Si2O5(OH)4 + 3CO2 → Mg3Si4O10(OH)2 + 3MgCO3 + 3H2O Talc can also be formed via a reaction between dolomite and silica, which is typical of skarnification of dolomites via silica-flooding in contact metamorphic aureoles; Dolomite + Silica + Water → Talc + Calcite + Carbon Dioxide CaMg(CO3)2 + 4SiO2 + H2O → Mg3Si4O10(OH)2 + 3CaCO3 + 3CO2 Talc can also be formed from magnesian chlorite and quartz in blueschist and eclogite metamorphism via the following metamorphic reaction: Chlorite + Quartz → Kyanite + Talc + H2O In this reaction, the ratio of talc and kyanite is dependent on aluminium content with more aluminous rocks favoring production of kyanite. 37  Usage of Talc A coarse grayish-green high-talc rock is soapstone or steatite and has been used for stoves, sinks, electrical switchboards, etc. Talc finds use as a cosmetic (talcum powder), as a lubricant, as a filler in paper manufacture. Talc is used in baby powder, an astringent powder used for preventing rashes on the area covered by a diaper (see diaper rash). In the ceramic industry, due to its low volume expansion, it is used in bathroom and kitchen ceramics and even in electrical stoves plates. 38  TRADITIONAL RAW MATERIALS Clay Plasticity à formability Must be clean and pure enough for Feldspar (Flux) products Reduction for vitrification Sand temperature Increasing strength Additives may be added for improving particular properties 39  The increase of clay makes easy molding processes The increase of feldspar lowering the temperature of sintering and introduces in a liquid phase sintering The increase in silica limit shrinkages and deformations in sintering. 40  2.Advanced Ceramics Raw Materials During the past 50 years scientists and engineers have acquired a much better understanding of ceramic materials and their processing and have found that naturally occurring minerals could be refined or new compositions synthesized to achieve unique properties. These refined or new ceramics are often referred to as modern ceramics or advanced ceramics. They typically are of highly controlled composition and structure and have been engineered to fill the needs of applications too demanding for traditional ceramics. 41  General Comparison The wide variety of application possibilities for advanced ceramic materials arise from their specific properties which in many respects cannot be achieved by other materials. To highlight some positive properties: low density, high hardness, high mechanical strength, dimensional stability (specific stiffness), resistance to wear, resistance to corrosion (resistance to chemical attack), weathering resistance, high working temperature, low or high thermal conductivity, good electrical insulation and dielectric and ferroelectric properties 42  In accordance with their chemical composition the advanced ceramic materials can be divided into main groups SILICATE-CERAMICS technical porcelain-The natural starting materials of technical porcelain are quartz, feldspar and kaolin. steatite -major component: soapstone, additives: clay and flux, Mg 3 Si4 O10 (OH)2 cordierite -These magnesium silicates occur during the sintering of soapstone with added clay, kaolin, corundium and mullite. Mg2Al4Si5O18 mullite-ceramic -3 Al2O3.2 SiO2 OXIDE CERAMICS aluminium oxide-Al2O3 magnesium oxide-MgO zirconium oxide-ZrO2 aluminium titanate-Al2TiO5 or Al2O3.TiO2 piezo ceramic-lead zirconate to lead titanate NONOXIDE CERAMICS CARBIDES NITRIDES silicon carbide-SiC silicon nitride-Si3N4 tungsten carbide-WC aluminium nitride-AlN boron carbide-B4C boron nitride-BN carbide-TiC SIALON Titanium nitride-TiN BORIDES ELEMENTEL Titanium Borides- TiB2 Boron Zirconiumdiborides-ZrB2 Graphide Diamond 43  Aluminum Oxide Aluminium oxide is the most These synthetically manufactured important advanced oxide materials with aluminium oxide ceramic material and has the contents ranging from 80 % to widest range of applications. more than 99 %, have been Densely sintered aluminium oxide proven in practice. is characterized by; The choice of this material is high strength determined by technical and high hardness, economic criteria. A material with temperature stability, higher aluminium oxide content excellent dielectric properties does not necessarily fulfill the high wear resistance and needs of an application best. corrosion resistance even at high temperatures. 44  Table 1. Typical physical properties of Alumina PropertyPROPERTIES Melting point (°C) 2015±15 Refractive index 1.765 Molecular wt (g.mol-1) 101.96 DGf° Free Energy of Formation (kJ.mol-1) -1582 Table 2. Typical physical and mechanical properties of 86% to 99.9% Alumina PROPERTIES Alumina Grade 86% 94% 97.5% 99.5% 99.9% 99% Saph** recry.* Density (.gcm-3) 3.5 3.7 3.78 3.89 3.9 3.9 3.985 Dielectric Constant 8.5 9.2 9.5 9 – 10.1 9 – 10.1 7.5 – 10.5 Dielectric Strength (kVmm-1) 28 30 - 43 10 - 35 10 - 35 17 Volume Resistivity Ohm.cm >1014 >1014 >1014 >1014 >1014 >1014 >1016 Thermal Cond. (Wm-1K-1) 15 20 24 26 28-35 28-35 41.9 Thermal Exp. Coefficient 7 7.6 8.1 8.3 8 8 5.8 (20-1000°C x10-6K-1) Specific Heat (JK-1kg-1) 920 900 850 753 Compressive Strength (MPa) 1800 2000 1750 - 2200 - 2200 - 2100 2500 2600 2600 Modulus of Rupture (MPa) 250 330 262 320 - 400 260 Hardness (Vickers kgf.mm-2) 1500 - 1500 - 1500 – 2500 - 3000 45 1600 1650 1650  Typical Applications of Al2O3; High voltage insulators High temperature electrical insulators Electronic substrates Furnace liners, tubes Thread and wire guides Grinding media Ballistic armor Wear pads Seal rings Thermometry sensors Biomedical purpose; orthopaedic implants particularly in hip replacement surgery 46  Processing of Alumina Alumina occurs abundantly in nature, most often as impure hydoxides which are the essential constituents of bauxite ores. Bauxite is an impure mixture of gibbsite, boehmite and diaspore. The most common process for the extraction and purification of alumina is the ‘Bayer’ process. 47  BAYER PROCESS STEPS Bayer process involves the following operations: (1) Digestion (2) Clarification (3) Precipitation (4) Calcination 48 BAYER PROCESS FLOW CHART 49  Digestion The exact procedure required for digestion, depends on the nature of the ore deposits. In order to remove the iron oxides and most of the silicon oxides present, the ore is first treated with sodium hydroxide. The digestion process takes advantage of the solubility of amphoteric aluminum oxides to form a solution of aluminate ions, whilst the basic iron oxides which form do not dissolve and are separated by filtration. Gibbsite Al2O3.3H2O + 2NaOH → 2 NaAlO2 + 4 H2O (135-150 °C) Boehmiite Al2O3.H2O + 2NaOH → 2 NaAlO2 + 2 H2O (205-245 °C) Diaspore Al2O3.H2O + 2NaOH → 2 NaAlO2 + 2 H2O (high T and P) 50  Clarification of the liquor stream The alumina-bearing solution is separated from the insoluble impurities that were part of the original bauxite. With all solids removed, the pregnant liquor leaving the filter area, contains alumina in clear supersaturated solution. It is cooled by flash evaporation, the steam given off being used to heat spent liquor returning to digestion. 51  Precipitation The alumina is precipitated or crystallised from the solution as crystals of alumina trihydrate. The solution is mixed in tall vessels with recycled seed crystals. When completed the solid alumina hydrate is passed on to the next stage and the remaining liquor, which contains caustic soda and some alumina, goes back to the digesters. Dissolved alumina is recovered from the liquor by precipitation of crystals. Alumina precipitates as the trihydrate Al2O3.3H2O in a reaction which is the reverse of the digestion of trihydrate 2NaAlO2 + 4H2O → Al2O3.3H2O + 2 NaOH 52  Calcination In the final stage the alumina trihydrate is washed to remove any remaining caustic. It is heated to about 1050°C in special calciners or kilns to drive off the water of crystallisation, leaving the alumina, which is a dry, white, sandy material. In the calcination process water is driven off to form alumina: 2Al(OH)3 ---> Al2O3 + 3H2O 53  Bayer process produces highly aggregated powders which must be milled to release the ultimate particles and so enable high packing densities and reduced porosity in the green formed state. The evolution of powder morphology can be controlled during precipitation and calcination so that the textures can be used to facilitate break up of the aggregates upon milling. The most common preparation route for a powder alumina ceramic fabrication is wet ball milling followed by sprey drying of high solid content slip. 54  Normal Bayer alumina grades contain 0.5 % Na2O which degrades many properties. – The Na ion is mobile in an electric field causing deterioration of electrical properties. – Na β-alumina can be form on sintering to reduced density, strength, thermal shock and corrosion resistance. Consequently, there is significant demand for low soda alumina. Table 5.1 indicates typical chemical analyses of the main forms of calcined alumina. Table.5.1 Composition of calcined alumina Normal Na2O Low Na2O Reactive Composition Al2O3 98.8-99.7 99.5-99.8 >99.5 SiO2 0.02-0.05 0.07-0.12 0.04-0.08 Fe2O3 0.04-0.05 0.04-0.06 0.01-0.02 Na2O 0.3-0.6 < 0.1 0.08 55  Table 5.2 indicates the Na2O contents required for common application of calcined Bayer alumina powders. Table.5.2 Soda contents required of calcined aluminas in commericial application. Application Median Crystal Na2O Content range Size(µm) (%) Electronic Ceramics