Lesson

Lesson

Our goal is, that after this lesson, students are able to recognize the key criteria for selecting ceramics and are able to use this knowledge to support the systematic material selection process.

What are ceramics for engineering applications? ?

Typical ceramics used in engineering applications Aluminium oxide (Alumina) Al 2 O 3 Aluminium nitride AlN Silicon carbide SiC Silicon nitride Si 3 N 4 SiAlON (Si-Al-O-N) Zirconium oxide (Zirconia) ZrO 2 Boron carbide B 4 C Boron nitride BN

Some general properties of ceramics Density In general the density of ceramics is between metals and polymers. Most light weight ceramics are Boron compounds and Silicon compounds. For ceramics two types of density values are used: Density to describe weight Density to discribe the functional porosity (e.g. in filters porosity could be 40-80% of total volume) Functional density can be tuned according to requirements.

Melting point The melting point of ceramics is remarkably higher compared to metals. Heat conductivity Heat conductivity of ceramics is between metals and polymers. Can be tuned according to requirements. Heat expansion Depends a lot about the compound. Can be tuned according to requirements.

Modulus of elasticity The modulus of elasticity is near or above the values of metals. Modulus of elasticity can be tuned by utilizing composite compounds: E.g. WC+Co: E= 600GN/ mm 2, Al 2 O 3 +SiO 2 -particles added + Al: E= 200 GN/mm 2 Al: E=70 GN/mm 2

Strength Brittle behaviour is typical for ceramics. Instead of yeld strength only ultimate tensile/compression strengths are given. Compression strength is even 10-times higher than tensile strength. Porosity affects greatly the strength. Ceramics have higher strength in extreme (high) temperatures compared to metals, but due to so-called “glass deformation phase” also the strength of ceramics usually decreases in elevated temperatures (could be avoided with reaction sintering).

Hardness The highest values of hardness are found among the ceramics (Boron compounds). High hardness remains also in high temperatures (even at 1000 °C). The final hardness is achieved during the sintering process.

Electrical properties Ceramics can function as electric insulators (the most typical option), semi-conductors, conductors or even superconducters Also piezo-electric properties can be produced Some ceramics are good electric insulators and have also good heat and corrosion resistance. Magnetic properties Possible to produce permanent magnets.

DETAILED SELECTION OF CERAMICS SINTERING METHOD LEVEL OF PURITY POROSITY ALLOYING GRAIN SIZE DIRECTION OF COMPRESSION DUCTILITY EFFECT (ZIRCONIUM) PROPETIES OF THE RAW MATERIAL PROPETIES OF THE COMPOUND ASPECTS OF THE POWDER METALLURGICAL PROCESS

MANUFACTURE OF THE MOLD AND THE COMPRESSION TOOLS MANUFACTURING (GRINDING) PROCESSES OF THE POWDER MIXING PROCESSES OF THE POWDER SHAPING OF THE POWDER SINTERING PROCESS FINISHING PROCESSES ATOMIZATION REDUCTION PROCESSES ELECTROLYSIS MgO, CeO 2, Al 2 O 3, SiO 2, Y 2 O 3, ZrO 2, CaO, MoSi 2 Pressureless sintering Nitride bonding Reaction bonding Liquid-phase sintering Recrystallization Hot isostatic pressing Hot pressing Pressureless sintering Nitride bonding Reaction bonding Liquid-phase sintering Recrystallization Hot isostatic pressing Hot pressing POWDER METALLURGIC MANUFACTURING PROCESS

During the properly made systematic material selection process it is necessary to recognize:: Limitations and possibilities of powder metallurgical manufacturing process Guidelines of designing the suitable geometry of the product for the powder metallurgical manufacturing process The optional materials (typically constructional ceramics) which can be applied to powder metallurgical manufacturing process

Temperature T Pressure p Time t Density Compression pressure The most important process parameters in powder metallurgical process

OPTIONAL MANUFACTURING TECHNOLOGY 1 Component made of Silicon carbide and nitride with Gelcasting.

Powder layer to be sintered Motion path of the sintering laser beam Product made of sintered powder layers Utilized space for powder mass Laser beam OPTIONAL MANUFACTURING TECHNOLOGY 2

Example of expressing the purity grade

Ceramics (alloying) Bending strength (N/mm²) Al 2 O Al 2 O 3 + ZrO ZrO 2 + MgO ZrO 2 + Y 2 O How alloying affects the bending strength?

EFFECT OF ALLOYING ON HEAT CONDUCTIVITY SiC (BeO alloying) SiC (B 4 C alloying) TEMPERATURE [°C] HEAT CONDUCTIVITY [W/mK]

Ceramics E [GPa] Hot pressed silicon nitride Alloying with 8% Y 2 O Alloying with 1% MgO325 Alloying with 10% CeO Alloying with 4% Y 2 O 3 + 3% Al 2 O Alloying with 4% Y 2 O 3 + SiO How alloying affects the modulus of elasticity? 10% improvement

Zirconium oxide itself suffers from unbalanced change of length and volume expansion between the tetragonal and monoclinic phase. This causes huge internal stresses and pure Zirconium oxide can not be used for constructional purposes. The phase change can be fully stabilized by Calcium oxide (CaO) alloying, but the strength and heat resistance properties will be so poor that neither the fully stabilized Zirconium oxide can’t be used in mechanical constructions. How Zirconium oxide (ZrO 2 ) alloying affects the mechanical properties of other ceramic compounds?

MOLTEN CUBIC PHASE TETRAGONAL PHASE MONOCLINIC PHASE CRYSTAL STRUCTURE

Partially stabilized Zirconium oxide (PSZ =Partially Stabilized Zirkonia) can be manufactured by alloying Yttrium oxide (Y 2 O 3 ) or cerium oxide (CeO 2 ). This decreases the risk of failure due to internal stresses and increases the strength and ductility. Partially stabilized Zirconium oxide can be utilized as an alloying component in other ceramic compounds to improve their ductility (e.g Zirkonia Toughened Alumina, ZTA).

Different powder particles (Compound) Different sizes Same powder particles Different sizes mixed Same powder particles Optional (same) sizes selected Tuning options of the grain size

Ceramics (commercial grades) StrengthGrain size Grain size ratio Density Modulus of elasticity MPamin μmmax μm max/min g/cm³GPa Sintered Silicon carbide General Electric β-SiC 4390, ,04376 Carborundum α-SiC – 93,09428 Kyocera α-SiC 3861, – 73,14403 When the grain size ratio decreases, the strength decreases and the modulus of elasticity increases. However, the selected sintering process gives a new viewpoint to this conclusion! How grain size affects strength and modulus of elasticity? Same powder particles Different sizes mixed

MPa STRENGTH STRENGTH OF SILICON CARBIDES / GRAIN SIZE Note! Selected sintering process affects together with the grain size! Note! Smaller grain size referres to higher strength Same powder particles Optional (same) sizes selected

GPa MODULUS OF ELASTICITY MODULUS OF ELASTICITY OF SILICON CARBIDES / GRAIN SIZE Note! Smaller grain size referres to lower modulus of elasticity Note! Selected sintering process affects together with the grain size! Same powder particles Optional (same) sizes selected

Powder metallurgy/Focus of review articles

Changes of the grain structure due to sintering and compression. What happens during the compression and sintering?

Examples of the overall shrinkage of some ceramics during the powder metallurgical manufacturing process. CeramicsShrinkage % Silicon carbide18-20 Aluminium oxide17-20 Zirconium oxide25-32

SILICON CARBIDE SiC LPSIC (Liquid-phase sintered) SSIC (Pressureless sintered) HPSIC (Hot pressed) HIPSIC (Hot isostatic pressed) SSIC (Pressure sintered) NSIC (Nitride bonded) RSSIC (Reaction-bonded) RCSIC (Recrystallized) SINTERING PROCESSES / SILICON CARBIDE BONDING REACTION PRESSING TECHNOLOGY

SILICON NITRIDE Si 3 N 4 HPSN (Hot pressed) RBSN (Reaction-bonded) HIPSN (Hot isostatic pressed) SSN (Pressure sintered) SINTERING PROCESSES / SILICON NITRIDE

Examples of typical sintering temperatures CeramicsSintering temperature [°C] Al 2 O SiC2500 Si 3 N

Ceramics and sintering process Bending strength (N/mm²) Reaction sintered SiC Sintered SiC Reaction sintered Si 3 N Sintered Si 3 N How sintering process affects bending strength? Note! If reaction sintering is used, the strength of ceramics will NOT decrease in elevated temperatures!

Ceramics and Sintering process Modulus of elasticity GPa Hot pressed Silicon carbide 450 Sintered Silicon carbide 400 Reaction sintered Silicon Carbide 360 How sintering process affects modulus of elasticity?

Sintering process of Silicon nitride Hardness Pressure sintered HV Hot pressed HV Reaction bonded HV How sintering process affects hardness of ceramics?

MPa STRENGTH STRENGTH OF SILICON CARBIDES / SINTERING PROCESSES Note! Selected sintering process affects together with the grain size!

GPa MODULUS OF ELASTICITY MODULUS OF ELASTICITY OF SILICON CARBIDES / SINTERING PROCESSES Note! Selected sintering process affects together with the grain size!

COMPRESSION STRENGTH / COMPRESSION DIRECTION Note! Bonding reaction affects together with the compression angle!

BENDING STRENGTH / COMPRESSION DIRECTION Note! Bonding reaction affects together with the compression angle!

THERMAL EXPANDING / COMPRESSION DIRECTION Note! Bonding reaction affects together with the compression angle!

Strength! Heat resistance! Compression angle BO, CA, XP!

Justification of ceramic applications CeramicsThe most important property for industrial applications ALUMINIUM OXIDESCost-effectiveness compared to other ceramics with Good chemical resistance. ALUMINIUM NITRIDESExcellent thermal conductor but at the same time excellent electric insulator. SILICON CARBIDESGood heat resistance. SILICON NITRIDESGood heat resistance combined with excellent resistance against heat shocks. Si-Al-O-N (one type of silicon nittide) Mechanical properties close Silicon nitride combined with chemical resistance close to properties of Aluminium oxide. ZIRCONIUM OXIDECould be utilized to improve the toughness/ ductility of other ceramic materials. Use in oxygen sensors. BORON CARBIDEExtremely hard (place 4. in the list of constructional materials) BORON NITRIDEExtremely hard (place 3. in the list of constructional materials)

A ball valve made of aluminium oxide. Applications of Aluminium oxide

Aluminium nitride is used in waveguide amplifiers and angular waveguides (MW-mechanics or high power electronics applications ). Applications of Aluminium nitride

Turbine (blades) made of Silicon carbide Applications of Silicon carbide

Chemically resistant seals made of Silicon carbide

Applications of Silicon nitride

Applications of Si-Al-O-N Si-Al-O-N based cutting tools Si-Al-O-N based seals and sliding bearings

Ceramic foam filters Unit porosity(percentage ): 80…90 % Density (g /cm 3 ): 1.0 Approximate use temperature 1700 °C. Thermal shock resistance: in 1110° C above 7 times Applications of Zirconium oxide

Ceramic Zirconia based oxygen sensors

Applications of Boron Carbide

Applications of Silicon nitride The boron-nitride coatings combine the strength and durability with the lubrication and anti-frictional properties e.g. of pistons and cylinders in an engine.

HARDNESS HEAT RESISTANCE (COMPRESSION) STRENGTH ELECTRICAL OR HEAT CONDUCTIVITY FOUR-FIELD ANALYSIS FOR MECHANICAL/ELECTRICAL ENGINEERING

HEAT RESISTANCE COSTS FUNCTIONAL POROSITY CHEMICAL STABILITY FOUR-FIELD ANALYSIS FOR PROCESS ENGINEERING

How to name properly the selected ceramics? In addition to the ceramics type (e.g. Al 2 O 3, SiC…) the following data is required: Purity level [%] Alloying [%], at least Zirconium content if it used Grain size [either grain size ratio or grain size limit] Porosity level [vol-% and/or density] Sintering method [HPSN, RSSC…] Direction of compression Remember the ”problems” with commercial names of different grades Hot pressed Silicon carbides Norton NC-203 Ceradyne 146A Ceradyne 146I Sintered Silicon carbides General Electric β- SiC Carborundum α-SiC Kyocera α-SiC Reaction sintered Silicon carbides Norton NC-435 Norton NC-430 UKAEA BNF Refel Coors SC-1