Ceramics

Definitions Inorganic (metal) & non-metallic Bonds: ionic or covalent Simply: Clay, SiO 2 and feldspar Complex: Al 2 O 3 SiO 2 H 2 O clay with small amounts of other oxide such as TiO 2, Fe 2 O 3, MgO, CaO, Na 2 O and K 2 O. Feldspar: K 2 OAl 2 O 3 6SiO 2 Many are a mixture of compounds, present in a variety of phases Compounds: homogeneous substance consisting of atoms/ ions of 2 or more different elements in definite proportions

Classification of ceramic materials

Traditional ceramics Clay-workability Silica/fint/quarts-high temperature resistance Feldspar/potash-low melting temp-firing Examples: Building bricks, sewer pipe, drain tile,roofing tile and floor tile, white ware products such as electrical porcelain, table ware, and sanitary ware

Engineering/Technical ceramics Pure compounds Oxides: Al 2 O 3 (electrical applications, refractory components, integrated circuit chips, automotive gas turbine engine), ZrO 2 (heat engine components) Carbides: WC (dies and cutting tools), SiC (reinforcements) Nitrides: Si 3 N 4 (structural components in heat exchangers) TiN (coating), CBN (hard cutting tools) Cermets: ceramics + metal Diamond, graphite and asbestos Use: Machine tools and agricultural machinery, increasing life by up to 10 times. inert and biocompatible so they are potentially suitable for artificial joints where wear is a large consideration

Structure Most ceramics phases are crystalline Material is made of a collection of phases Crystal of bulk phase important Extent of crystallinity of other phases Most ceramics are composed of light atoms O, C, N, Al and Si therefore have low densities. Because of this, their specific moduli (E/  ) are high. Properties depends on Bonding in each phase Phase distribution and size Phase boundary properties Glasses Based on SiO 2, heated to fusion (about 1200 o C), high viscosity, network modifiers (Na 2 O and CaO) to reduce viscosity.

Ceramics processing Additives can act as - binder - lubricant - wetting agent - plasticizer - deflocculant 1 Problems - high costs - dimensional stability - control of porosity - 20 % shrinkage 1. changes the electrical charges on the particles of ceramic so that they repel instead of attract each other while in a liquid suspension. Typical deflocculants are Na 2 CO 3 and Na 2 SiO 3 in amounts of <1%

Classification scheme for ceramics-forming techniques

Fig: Basic PM processing route

Fig: Steps in (a) solid and (b) drain slip casting

Ceramics processing route Mixing (powder + additives) Sintering Hot isostatic Pressing

Fig: Batch attritor Fig: Irregular movement

Fig: Several stages in the removal of water from between clay particles during (a) Wet body (b) Partially dry body ( C) completely dry body

Fig: Liquid phase sintering

Ceramics - parts

Properties of ceramics - mechanical In general -hard and brittle, low toughness and ductility (strain 0.1% - 1%) -good electrical and thermal insulators -high melting points -high chemical stability But there is a wide variation

Strength Strength: in tension - one order of magnitude less than in compression UTS  UTS 0 e- nP where P is the volume fraction of porosity (50%), e.g.; Common earthware has porosity in the range of 10 and 15 %, with porcelain having approx. 3% UTS 0 is the ultimate tensile strength at zero porosity, and exponent n ranges from 4 to 7.

Young’s modulus Usually linear elastic behaviour Specific Stiffness is high Identifiable Young’s modulus depends on temperature > metal E depends on porosity and temperature E  E 0 (1-1.9P ) where E 0 is Young's Modulus at absolute zero, and b and T 0 are empirical constants, 1% E change per 100  K.

Density & hardness Density: Most ceramics are composed of light atoms O, C, N, Al and Si therefore have low densities. Because of this, their specific moduli (E/  ) are high Hardness: they are the hardest of solids

Ductility and toughness Typically on 0.1% strain before failure Processing results in many initiations sites for cracking Surface flows often key Aim is to detect, branch or arrest crack growth Scatter is important in analysis data – Weibull statistics

Thermal shock Need low thermal expansion Good thermal conductivity Conductivity depends on porosity K = K 0 (1-P) Where K 0 is the conductivity at zero porosity and P is the fraction porosity There is a wide range

Research in ceramics Improve toughness - tailor grain boundary phases and bulk phases - use of reinforcements Improve processability - lower pressure, lower temperature, consolidation, near- net shape processing, - improve machinability - joining Understand structure/property and relationship

Ceramic die materials Non-oxide ceramics Metal carbides: B 4 C, SiC, TiC Metal nitrates: Si 3 N 4, BN, TiN –High strength and thermal shock resistance but can corrode due to excessive oxidation. Oxide ceramics Metal oxides: Al 2 O 3, ZrO 2 –Usable up to 1200 deg C and corrosion resistant against oxidisation but lower mechanical properties and thermal strength. Dense versus porous ceramics Dense - high temp strength, corrosion and wear resistance - poor thermal fatigue Porous – better thermal fatigue and worse strength 1. Munstermann, S. and Telle, R., Ceramic tool concepts for semi-solid processing of steel, Mat. – wiss, u. Werkstofftech, 37, 4, 2006

Case Study What die materials and fabricated structures could be investigated to allow for semi- solid steel forming?

Ceramic die materials Characteristics required Mechanical – Strength, Wear, Fatigue Thermal shock Thermal fatigue Corrosion Oxidation 1. She, J., and Ohji, T., Thermal shock behavior of porous silicon carbide ceramics, J. Am. Ceram. Soc., 85, 8, Meyer-Rau, S. and Telle, R., Testing strategies for corrosive interaction of ceramics with semi-solid and molten metal alloys, J. of European Ceramic Society, 25, Evans, H., and Taylor, M., Oxidation of high temperature coatings, J. Aerospace Engineering, IMechE, 220, Part G, Velay, V., et al., A continuum damage model applied to high temperature fatigue lifetime prediction of a martensitic tool steel, Fatigue Fract Engng Struct, 28, 2005.

Ceramic die materials Partially stabilised zirconia – US (’81) and by Mg, Ca or Y additions CA (’79) Cermet casting mould – FR (Renault) –alumina in Fe, Cr, Al mix; pressed between 7000 and 70,000 psi and sintered between 2190 and 2640 deg F; TBC / lubricants applied: alumina, silica, silico-aluminate, graphite, lamp or acetylene black

Metal die materials H13 tool steel –reduce heat checking by substituting with material with higher elevated mechanical properties –reduce magnitude of thermal stress, for example, by increasing the bulk metal die temperature Ni based super alloys –e.g. INCONEL, Cr-Ni-Mo Electroslag cast steel –good ductility and toughness as well as high tensile strength at elevated temperatures - important for high thermal shock resistance 1.Sakhuja, A., and Brevick, J., Prediction of thermal fatigue life in tooling for die-casting copper via finite element analysis, Am. Inst. Phys., Fang, J., et al., The characteristics of fatigue under isothermal and thermo-mechanical load in Cr-Ni-Mo cast hot die steel, Fatigue Fract Engng Struct, 25, Moon, Y., Kim, J., and Tyne, C., Thermal shock resistance of electroslag cast steel for hot working tools, J. Mat. Proc. Tech., , 2004.

Die fabtrication methods Machining from bulk feedstock – conventional Sintering – metal and ceramic feedstock Cladding by shrink fitting – GKN Patent US (’73) Multiple layer ceramic – similar to LOM principle – EP (2000) –glass & alumina powder sintered into 0.2 mm thick sheets; applicable to metal carbides: B 4 C, SiC, & TiC and metal nitrates: Si 3 N 4, BN, & TiN Sialon ceramic with metal coating – JP (’89) –  and  sialon with metal compound coating film e.g. super hardened Ti, Si, … film, by ion planting Clamped ceramic columns of polygon cross section – JP (’88) –applicable with metal oxide or metal nitride (e.g. Si 3 N 4 and ZrO 2 ) Mould insert used as cladding – WO (’06) –from developer of melt away core process

Fabricated die structure Bulk metal Bulk ceramic Cermet Layered structure / Hybrid structure Channels for heating/cooling cartridges / heat transfer fluid Inserts / shapes that reduce heat build up