FIZIKALNO HEMIJSKE OSNOVE KERAMIKE

FIZIKALNO HEMIJSKE OSNOVE KERAMIKE1. PREDAVANJE

DEFINITIONInorganic, nonmetallic materials processed or consolidated at high temperature.

Ceramics (ceramic materials) are non-metallic inorganic compounds formed from metallic (Al, Mg, Na, Ti, W) or semi-metallic (Si, B) and non-metallic (O, N, C) elements.

Atoms of the elements are held together in a ceramic structure by one of the following bonding mechanism: Ionic Bonding, Covalent Bonding, Mixed Bonding (Ionic-Covalent).

Most of ceramic materials have a mixed bonding structure with various ratios between Ionic and Covalent components. This ratio is dependent on the difference in the electronegativities of the elements and determines which of the bonding mechanisms is dominating ionic or covalent. ELECTRONEGATIVITY

Electronegativity is an ability of atoms of the element to attract electrons of atoms of another element.

Electronegativity is measured in a relative dimensionless unit (Pauling scale) varying in a range between 0.7 (francium) to 3.98 (fluorine).

Non-metallic elements are strongly electronegative. Metallic elements are characterized by low electronegativity or high electropositivity ability of the element to lose electrons.Ionic bonding occurs between two elements with a large difference in their electronegativities (metallic and non-metallic), which become ions (negative and positive) as a result of transfer of the valence electron from the element with low electronegativity to the element with high electronegativity. The typical example of a material with Ionic Bonding is sodium chloride (NaCl). Electropositive sodium atom donates its valence electron to the electronegative chlorine atom, completing its outer electron level (eight electrons): As a result of the electron transfer the sodium atom becomes a positively charged ion (cation) and the chlorine atom becomes a negatively charged ion (anion). The two ions attract to each other by Coulomb force, forming a compound (sodium chloride) with ionic bonding. Ionic bonding is non-directional.IONIC BONDING

Covalent bonding occurs between two elements with low difference in their electronegativities (usually non-metallics), outer electrons of which are shared between the four neighboring atoms. Covalent Bonding is strongly directional.COVALENT BONDING

IONIC- COVALENT (MIXED) BONDINGIonic-covalent (mixed) bonding with various ratios of the two fractions (ionic and covalent) occurs in most of ceramic materials. Degree of Ionic Bonding can be estimated from the following formula: I.F. = exp(-0.25*E) Where I.F. fraction of ionic bonding; E difference in the electronegativities of the elements.Ceramic materials are inorganic and non-metallic. They are generally moulded from a mass of raw material at room temperature, and gain their typical physical properties through a high temperature firing process. In contrast, the Anglo-Saxon term "ceramics" also often includes glass, enamel, glass-ceramic, and inorganic cementitious materials (cement, plaster and lime).The German ceramics industry also distinguishes between coarse and fine ceramics, depending on the particle size in the raw material (0,1 mm grain grobkeramik).Technical ceramics, tableware, decorative ceramics, ceramic sanitary ware, wall and floor tiles andceramic abrasives belong to the fine ceramics category.The category of coarse ceramics includes, for example, brick or conventional refractory materials.DEFINITION

CLASSIFICATIONCLASSIFICATION

TRADITIONAL CERAMICSADVANCED CERAMICS

CLASSIFICATION

CLASSIFICATIONStructure of ceramic materialsThe following factors affect structure of ceramics: Balance of electrical charges of anions and cationsRadius Ratio (rc/ra)Where rc radius of cation; ra radius of anion. Radius Ratio determines Coordination Number (CN) the maximum number of anion nearest neighbors for a cation.The anion neighbors do not touch each other. rc/ra = 0.2250.414(SiO2) CN = 4 rc/ra = 0.4140.732(SnO2, PbO2) CN = 6 rc/ra = 0.7321.0(ThO2) CN = 8 Covalent bonding component, which tends to form tetrahedral coordination, may affect the Coordination Number. Ions are packed with maximum density, providing minimum energy of the structure.Structure of ceramic materialsMineral Name Formula Coordination Number Structure Characterization Rocksalt NaCl 6 Octahedral unit cell, cubic appearance Zincblende ZnS 4 FCC unit cell with S2- anions at 4 tetrahedral sites

Fluorite CaF2 8-cation CN4-anion CN FCC unit cell with F- anions at 8 tetrahedral sites

Corundum Al2O3 6-cation CN4-anion CN HCP unit cell with O2- anions at the lattice sites and Al3+ at interstitial sites Perovskite CaTiO3 6-cation(Ti) CN2-anion(O) CN Cubic unit cell with Ti4+ cations coordinated octahedrally among six oxygen anions Silicate Combination of SiO44- blocks 4 Tetrahedral arrangement with Si4+ cations at the center bonded to O2- anions at the apices of the tetrahedron Ceramic structures are classified and designated according to the pattern structures of several natural minerals: Examples of some ceramic structures

Examples of some ceramic structuresTetrahedral silica block (SiO4-4) may form various silicate structures: Island and DoubleIsland SilicatesSingle or two silica blocks, containing other cations, form Island (olivine) or Double Island Silicates (hemimorphite). Ring and Chain StructuresSeveral (3,4,5,6) silica units join each other, forming a chain (orthopyroxenes, clinopyroxenes, asbestos) or closed ring (beryl). Sheet (layer) structureSilica units connect to each other, forming infinite sheet (micas, serpentine, chlorite, talc). Framework silicateQuartz, cristobalite, and tridymite minerals are based on the framework silicate structure. Silicates exist in two forms: crystalline and amorphous (glasses).Examples of some ceramic structures

Key Microstructural Features of Engineering CeramicsThe grains are formed as the compacted grains of ceramic powder try to reduce their surface area by coalescing together during sintering. The grain boundaries are formed at the intersection of the grains and often contain impurities that form glassy phases. The glassy phases aiding in the sliding process that occurs as the grains coalesce.Pores are formed due to the inability of the powder compact to sinter to full density i.e. the theoretical density that can be obtained by the individual grains eradicating all the pre-existing porosity of the as-pressed powder compact.How Does the Microstructure of a Ceramic Affect Its Performance?StrengthThe strength of an engineering ceramic is typically measured by machining a bar of material into a solid bar 3x4x45 mm which is the subject to a 3 point bending strength test.ToughnessThe toughness of engineering ceramics in simple terms describes the ability of the material to withstand the propagation or movement of a crack throughout the body of the material. It is often measure by using the type of diamond indenter used in conventional hardness tests.HardnessHardness is perhaps the simplest of all property measurements as it is generally measured by the depth of penetration of an indenter into the surface of the material, either a pyramidal indentation in Vickers hardness or a conical indenter in Rockwell hardness testing.

A typical fracture surface of an alumina ceramic subject to a strength test is shown in Figure 3. It is easily discernible that the fracture has proceeded along the weakest points namely the grain boundaries.The Effect of Machining and Microstructure

The grinding stresses leads to both surface and subsurface defects, both of which may generate the critical defect at failure.Equally, such defects can also reduce the measured values of toughness and hardness.