Ceramics
Ceramics
Ceramics
Content:
Introduction on engineering materials Classification & grouping of Ceramics Comparison metals vs. ceramics Structural ceramics & its properties Application of advanced ceramics Disadvantages and Its overcome Ductile to brittle transition Toughening mechanism Fabrication & processing of advanced ceramics Microstructure of TiB2 ceramics Characteristics of Sintered ceramics Surface Coating of ceramics
Engineering Materials
There is a general consensus that engineering materials can be classified into three metals and alloys ceramics and glasses and polymers. widespread use of metallic materials is driven by their o high tensile strength and o high toughness (crack growth resistance), most metals have m.p ( < 2000 C) polymers have distinct advantages in terms of their low density, high flexibility, and ability to be molded into different shapes an sizes. low melting point (less than 400C) very low strength and elastic modulus. So we need to find a material that has a capability to withstand high temperatures, retain strength at high temperature, high melting point, and good mechanical properties line hardness, elastic modulus, and compressive strength
Ceramics
ceramics are considered as potential materials for high-temperature material high hardness and wear resistance.
This unique properties leads development of Advanced Ceramics
Ceramics A ceramic is, an inorganic , non-metallic solid prepared by the action of heat and subsequent cooling. Ceramic materials may have a crystalline or may be amorphous (e.g., a glass).
Bonding
Usually a compound or a combination of compounds, between metallic and non-metallic elements To achieve high M.P, B.P,
Glass Structure
Basic Unit:
4Si0 4 tetrahedron
Si 4+ O2 Glass is noncrystalline (amorphous) Fused silica is SiO2 to which no impurities have been added Other common glasses contain impurity ions such as Na+, Ca2+, Al3+, and B3+
Na + Si 4+ O2 -
(soda glass)
Classification of Ceramics
Ceramic Materials
Glasses Clay Refractories products Abrasives Cements Advanced ceramics
-optical -whiteware -bricks for -sandpaper -composites -engine -composite -structural high T -cutting -structural rotors (furnaces) -polishing reinforce valves -containers/ bearings -sensors household
The current use of ceramics extends from pottery to refractories, abrasives, cements, ferroelectrics, glassceramics, magnets, and so on.
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Grouping
They can be grouped according to their major functions Traditional ceramics :Clay or silica based products Engineering ceramics : engineering ceramics are fabricated from high-purity ceramic powders, and their properties can be manipulated by varying process parameters and, thereby, microstructures. Based on the application engineering ceramics are classified into Structural ceramics and Functional ceramics
the performance of functional ceramics is controlled by electric, magnetic, dielectric, optical, and other properties Ceramic insulators, piezoelectric ceramics Ferroelectric ceramics
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Structural ceramics
Structural Ceramics can be class base on chemical composition oxides, carbides, nitrides, sulfides and fluorides. In general, structural ceramics can be further classified into two classes: oxide ceramics (Al2O3, ZrO2, SiO2, etc.) and non-oxide ceramics (SiC, TiC, B4C, TiB2, Si3N4, TiN, etc.).
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Disadvantages
Despite having such potential applications, the widespread use of ceramics has been limited, because of their Brittleness (poor fracture toughness) Low strength reliability ( depends the crack size & orientation) Griffiths Theory
By far, the most widely accepted theory of brittle fracture is Griffiths theory,4 which is based on the total change in potential energy of a brittle solid during crack propagation under external tensile loading. the fracture strength of a brittle material is, in particular, determined by the critical crack length according to Griffiths theory:
where f is the failure or fracture strength, KIC, is the critical stress intensity factor (a measure of fracture toughness)
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The critical fracture stress or corresponding critical crack length can be derived as
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Ceramics fracture
Transgranular fracture: Fracture cracks pass through grains. Fracture surface have faceted texture because of different orientation of cleavage planes in grains. Intergranular fracture : Fracture crack propagation is along grain boundaries (grain boundaries are weakened or embrittled by impurities segregation etc.)
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Ductile-to-brittle transition
As temperature decreases a ductile material can become brittle - ductile-tobrittle transition FCC metals remain ductile down to very low temperatures. ( super alloy) Alloying usually increases the ductile-to-brittle transition temperature. For ceramics, this type of transition occurs at much higher temperatures than for metals. As grain size moves in nanoscale ceramics become ductile for hall-petch relation hardness inversely to square root of grain size ( super plascity is observed in nanocrystalline ceramics also in metals )
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Toughening of ceramics
Once cracking starts, it is rather easy for cracks to grow in ceramics. ( brittleness of material) Toughening mechanisms to improve crack growth resistance, Process Zone Mechanisms -- transformation mechanism for example the transition from tetragonal to monoclinic zirconia, in the crack tip stress field involves volume expansion.
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t-ZrO2 phase undergoes a phase transition to the stable monoclinic symmetry in the tensile stress field around a propagating crack. The volume expansion (45%) involved in such phase transition introduces a net compressive stress in the process zone around the crack tip. This essentially reduces the local crack tip stress intensity factor as well as the driving force for crack propagation. The mechanism of transformation toughening can be described by a change in the stress intensity factor C= Ktip K Where K : applied stress Crack shielding occurs when Ktip< K i.e, C is negative
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Glass: Main ingredient is Silica (SiO2) If cooled very slowly will form crystalline structure. If cooled more quickly will form amorphous structure consisting of disordered and linked chains of Silicon and Oxygen atoms. Glass can be tempered to increase its toughness and resistance to cracking.
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Note : Synthesis of powder is important for the fabrication of advanced ceramics with improved mechanical properties and reliability as well as reproducible behaviour.
In particular, strength and fracture toughness are observed to be strongly dependent on the particle size, the chemistry of the starting powder, and the sintering parameters.
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Sintering of Ceramics
powder blend (powder + binder or sinter additive) this is done either by Ball milling, blending, spray drying of powders using processing additives Compaction: (Green ceramics : ceramics not yet been sintered) Shaping of powders into using pressing, slip casting, tape casting Sintering : It is a technique based on powder metallurgy and produces highdensity materials and components from metal or ceramic powders by applying thermal energy and/or mechanical pressure. Sintering refers to the process of firing and consolidation of powders at T>0.5Tm ( for nanocrystalline T < 0.5Tm because of super plasicity ) Sintering involves a process of transformation from a porous state to a state of dense material and it must involve the process of neck formation and growth.
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Sintering
Sintering occurs during firing of a piece that has been powder pressed
-- powder particles coalesce and reduction of pore size
Sintering must involve the process of neck formation and growth to reduce the porous
Sintering can be broadly classified into two categories: (a) Liquid phase sintering (LPS) (b) Solid-state sintering (SSS)
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Sintering at a temperature much lower than that used to obtain a dense ceramic can also produce a porous material.
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Porosity: Pores represent defects in polycrystalline ceramics and are usually detrimental to the mechanical properties of bulk ceramics. Pores may be either interconnected or closed. The apparent porosity measures the interconnected pores and determine permeability or ease with which gases or fluids seep through ceramic components. Apparent porosity = Ww Wd / Ww Ws x 100 (W = weight either after removal from water (s), dry (d) or suspended in water (s).
True porosity = - B / x 100 (B = bulk density, = true density or specific gravity of the ceramic material.
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