ENGINEERING MATERIALS

1. ENGINEERING MATERIALS

2. CERAMIC MATERIALS

3. WHAT ARE CERAMIC MATERIALS? CERAMIC MATERIALS ARE INORGANIC, NONMETALLIC MATERIALS THAT CONSIST OF METALLIC AND NONMETALLIC ELEMENTS BONDED TOGETHER PRIMARILIY BY IONIC AND/OR COVALENT BONDS. THE WORD “CERAMIC” IS DERIVED FROM THE GREEK WORD “KERAMIKOS” MEANING “BURNT STUFF”. IT MEANS THAT DESIRABLE PROPERTIES OF THESE MATERIALS ARE NORMALLY ACHIEVED THROUGH A HIGH-TEMPERATURE HEAT TREATMENT PROCESS OF FIRING. THE WORD CERAMIC BRINGS UP DIFFERENT IMAGES FOR DIFFERENT PEOPLE. FOR MANY, THE WORD SIGNIFIES FINE PORCELAIN AND POTTERY, BUT THE ENGINEER TENDS TO THINK OF THE NEWER INDUSTRIAL CERAMICS SUCH AS OXIDES, CARBIDES AND NITRIDES OF METALS.

4. HOWEVER, THE WORD CERAMICS ALSO INCLUDES STONE, CEMENT, CONCRETE, BRICKS AND OTHER CLAY PRODUCTS, AND A RANGE OF NON-CRYSTALLINE MATERIALS, THE COMMERCIAL GLASSES. THE CHEMICAL COMPOSITIONS OF CERAMIC MATERIALS VARY CONSIDERABLY, FROM SIMPLE COMPOUNDS TO MIXTURES OF MANY COMPLEX PHASES BONDED TOGETHER. HENCE PROPERTIES OF CERAMIC MATERIALS ALSO VARY GREATLY DUE TO DIFFERENCES IN BONDING. IN GENERAL, CERAMIC MATERIALS ARE TYPICALLY HARD AND BRITTLE WITH LOW TOUGHNESS AND DUCTILITY. CERAMICS ARE GOOD ELECTRICAL AND THERMAL INSULATORS BECAUSE OF ABSENCE OF FREE ELECTRONS.

5. CERAMICS HAVE HIGH ELASTIC MODULI, ARE STRONG IN COMPRESSION BUT ARE RELATIVELY WEAK IN TENSION. THESE MATERIALS NORMALLY HAVE RELATIVELY HIGHT MELTING TEMPERATURES AHD HIGH CHEMICAL STABILITY IN MANY HOSTILE AND AGGRESSIVE ENVIRONMENTS DUE TO THE STABILITY OF THEIR STRONG BONDS. BECAUSE OF THESE PROPERTIES, CERAMIC MATERIALS ARE USED IN VARIED ENGINEERING APPLICATIONS SUCH TOOLS FOR GRINDING AND CUTTING PROCESSES, SEALS, BEARINGS AND OTHER COMPONENTS OF ENGINES AND PUMPS. THESE MATERIALS ALSO HAVE MANY APPLICATIONS IN THE ELECTRICAL AND ELECTRONIC INDUSTRIES. HENCE CERAMICS MATERIALS ARE INDISPENSABLE FOR MANY ENGINEERING DESIGNS.

6. THEREFORE, CERAMIC MATERIALS FOR ENGINEERING APPLICATIONS, IN GENERAL, CAN BE DIVIDED INTO FOLLOWING TWO MAIN GROUPS: TRADITIONAL CERAMIC MATERIALS ENGINEERING CERAMIC MATERIALS TYPICALLY TRADITIONAL CERAMIC MATERIALS ARE MADE FROM THREE BASIC COMPOUNDS; CLAY, SILICA AND FELDSPAR. CLAY CONSISTS MAINLY OF HYDRATED ALUMINUM SILICATES WITH SMALL AMOUNTS OF OTHER OXIDES. THE CLAY IN TRADITIONAL CERAMICS PROVIDE WORKABILITY OF THE MATERIAL BEFORE FIRING HARDENS IT AND CONSTITUTES THE MAJOR MATERIAL.

7. STRUCTURAL CLAY PRODUCTS SUCH AS BUILDING BRICK, SEWER PIPE, DRAIN TILE, ROOFING TILE, FLOOR TILE ARE MADE OF NATURAL CLAY WHICH CONTAINS ALL THREE BASIC COMPONENTS. WHITEWARE PRODUCTS SUCH AS ELECTRICAL PORCELAIN, DINNER CHINA, AND SANITARY WARE ARE MADE FROM COMPONENTS OF CLAY, SILICA AND FELDSPAR IN CONTROLLED COMPOSITION. TRADITIOANL CERAMICS , THEREFORE, INCLUDE GLASSES, BRICKS, AND TILES MAINLY USED IN CONSTRUCTION INDUSTRIES AND ELECTRICAL PORCELAIN IN THE ELECTICAL INDUSTRY. IN CONTRAST ENGINEERING CERAMICS TYPICALLY CONSIST OF PURE COMPOUNDS OR NEARLY PURE COMPOUNDS OF CHIEFLY CARBIDES, OXIDES, AND NITRIDES.

8. SOME OF THE IMPORTANT ENGINEERING CERAMICS ARE ALUMINA, SILICON NITRIDE, SILICON CARBIDE AND ZIRCONIA. EXAMPLES OF THEIR USES INCLUDE THE USE OF SILICON CARBIDE IN HIGH-TEMPERATURE AUTOMOTIVE GAS TURBINE ENGINE AND USE OF ALUMINUM OXIDE IN THE SUPPORT BASE FOR IC CHIPS.

9. TRADITIONAL CERAMIC MATERIALS THE ROCKS THAT MAKE UP THE EARTH’S CRUST VARY CONSIDERABLY, BOTH IN COMPOSITION AND STRUCTURE. THE VARIOUS CONSTITUENT MINERALS CRYSTALLISED BY A PROCESS OF NUCLEATION AND GROWTH FROM LIQUID TO GIVE A COMPACT MASS OF IRREGULAR SHAPED CRYSTALS. IGNEOUS ROCKS WHICH INCLUDE GRANITE AND DOLERITE, ARE HARD AND WEATHER-RESISTANT MATERIALS. QUARTZ, WHICH IS A VERY HARD MINERAL, IS A MAJOR CONSTITUENT IN GRANITES. GRANITES AND OTHER IGNEOUS MATERIAL ARE WIDELY USED IN THE FORM OF CRUSHED CHIPS FOR ROAD SURFACING, AS RAILWAY TRACK BALLAST, AND AS AGGREGATE FOR CONCRETE.

10. SANDSTONES, FORMED BY THE CONSOLIDATION OF SAND DEPOSITS, CONSISTS PRINCIPALLY OF SMALL ROUNDED GRAINS OF SILICA. IN SOME SANDSTONES THE SILICA GRAINS ARE BONDED TOGETHER WITH A CLAY OR OTHER SOFT / HARD MINERAL. LIMESTONES AND DOLOMITES ARE FORMED BY THE PRECIPITATION OF CALCIUM CARBONATE, OR CALCIUM AND MAGNESIUM CARBONATE SOLUTION. SOME LIMESTONES AND DOLOMITES ARE VERY HARD WHILE OTHERS ARE VERY SOFT. CEMENT AND CONCRETE PORTLAND CEMENT POWDER IS MADE BY FIRING A MIXTURE OF LIMESTONE WITH SHALE OR CLAY IN A ROTATING KILN. AT HIGH TEMPERATURE IN KILN THE LIME AND CLAY ARE FUSED TOGETHER AS A HARD CLINKER. CEMENT CLINKER IS GOUND INTO POWEDER AND MIXED WITH A SMALL AMOUNT OF GYPSUM TO PRODUCE DRY CEMENT POWDER.

11. WHEN CEMENT POWDER IS MIXED WITH WATER A SERIES OF COMPLEX CHEMICAL REATIONS OCCUR, FORMING HYDRATED SILICATES AND ALUMINATES OF CALCIUM. THESE REACTIONS CAUSE THE WET CEMENT TO HARDEN AND SET AS A RIGID MATERIAL. CONCRETE CONSISTS OF A MIXTURE OF CEMENT, SAND, AND AN AGGREGATE OF SMALL STONES. WHEN WATER IS ADDED TO A DRY CONCRETE MIX, THE CEMENT PASTE FORMED FULLY COAT ALL SAND AND AGGREGATE PARTICLES AND FILL IN THE VOID SPACES BETWEEN AGGREGATE PARTICLES. THE TENSILE STRENGTH OF CONCRETE IS VERY LOW AND TO OVERCOME THIS DISADVANTAGE, CONCRETE FABRICATIONS ARE VERY OFTEN REINFORCED WITH STEEL. IN PLAIN REINFOCED CONCRETE A NETWORK OF STEEL RODS OR BARS IS ASSEMBLED AND THE CONCRETE IS ALLOWED TO SET AROUND THIS FRAMEWORK.

12. ANOTHER FORM OF REINFORCED CONCRETE IS KNOWN AS PRE-STRESSED CONCRETE. IN THIS CASE THE CONCRETE IS PUT INTO A STATE OF COMPRESSION BY MEANS OF HIGHLY STRESSED STEEL WIRES. CLAY AND CLAY PRODUCTS MUD AND CLAY HAVE BEEN USED AS THE BASIS FOR ARTEFACTS THROUGHOUT THE HUMAN HISTORY. CLAY IS A GENERAL NAME FOR MINERALS CONTAINING SMALL PLATE-LIKE CRYSTALS. MANY TYPES OF CLAY OCCUR IN NATURE AND THEY ARE MIXTURES OF THE CLAY MINERALS WITH NON-CLAY MINERALS. THERE ARE CHINA CLAY, BALL CLAY AND FIRECLAY.

13. THE VARIOUS TYPES OF CLAY PRODUCTS CAN BE BROADLY CLASSIFIED INTO WHITEWARE, HEAVY CLAYWARE AND CLAY REFRCTORIES. WET CLAY ARE PLASTIC AND CAN BE MOULDED OR EXTRUDED TO FORM CLAY PRODUCTS SUCH AS BRICKS, TILES AND EARTHWARE ARTICLES. AFTER THE WET CLAY OR PLASTIC CLAY HAS BEEN FORMED INTO THE REQUIRED SHAPES, FIRST THESE ARE ALLOWED TO DRY OUT AND THEN THE FIRING PROCESS IS FOLLOWEDIN A KILN WITH A TEMPERATURE RANGE OF 800 – 1500 C. MOST OF THE CLAY PRODUCTS ARE POROUS, SOME ARE OPEN AND SOME CLOSED. GLLAZING A FIRED CLAY BODY WILL REDUCE THE POROSITY SUCH AS IN CASE OF CHINA CUPS.

14. REFRACTORIES REFRACTORIES AR MINERAL PRODUCTS THAT ARE STABLE AT HIGH TEMPERATURES AND THAT ARE USED IN THE CONSTRUCTION OF FURNACES AND ALLIED EQUIPMENT. REFRACTORY MATERIALS ALSO RESIST THE ACTION OF MOLTEN METALS, SLAGS, GLASSES, ABRASIVE PARTICLES AND HOT GASES. REFRACTORY MATERIALS ARE CLASSIFIED AS ACIDIC, BASIC AND NEUTRAL. ACIDIC REFRACTORIES ARE RICH IN SILICA, SILICA OXIDE, AND ACIDIC OXIDE. BASIC REFRACTORIES ARE COMPOSED MAINLY OF MAGNESITE, DOLOMITE AND LIME. NEUTRAL REFRACTORIES INCLUDE ALUMINA FIRECLAYS, CHROME, CARBON AND OTHER PURE METAL OXIDES SUCH AS ZIRCONIA. ACID REFRACTORIES INCLUDE SILICA AND SILICA OXIDE WHICH HAVE MELTING POINT IN EXCESS OF 1700 C. THESE MATERIALS WITH A SILICA CONTENT OF 93% ARE USED IN METALLURGICAL FURNACES TO RESIST HIGH TEMPERATURES AND ATTACK BY ACID SLAGS. SILICA REFRACTORIES ARE ALSO USED AS REGENENATOR BRICKS AND FOR THE ROOFS OF FURNACES IN THE STEEL AND GLASS INDUSTRIES.

15. REFRACTORIES ACID REFRACTORIES INCLUDE SILICA AND SILICA OXIDE WHICH HAVE MELTING POINT IN EXCESS OF 1700 C. THESE MATERIALS WITH A SILICA CONTENT OF 93% ARE USED IN METALLURGICAL FURNACES TO RESIST HIGH TEMPERATURES AND ATTACK BY ACID SLAGS. SILICA REFRACTORIES ARE ALSO USED AS REGENENATOR BRICKS AND FOR THE ROOFS OF FURNACES IN THE STEEL AND GLASS INDUSTRIES. BASIC REFRACTORIES ARE CONSISTED OF MAINLY BASIC OXIDES SUCH AS MAGNESIA AND LIME. THESE ARE ALSO USED IN METALLURGICAL AND OTHER FURNACES TO RESIST HIGH TEMPERATURES AND ATTACK BY BASIC OXIDES AND SLAGS.

16. THE MOST WIDELY USED RAW MATERIAL FOR BASIC REFRACTORIES IS DOLOMITE, WHICH IS A MIXTURE OF CARBONATES OF MAGNESIUM AND CALCIUM. LIME, CALCIUM OXIDE PRODUCED FROM LIMESTONE HAS BEEN USED AS A BASIC REFRACTORY. THE TERM OF NEUTRAL REFRACTORY COVERS A RANGE OF REFRACTORY MATERIALS MADE FROM CHROME ORE AND MAGNESITE. CHROME-MAGNESITE REFRACTORIES ARE MORE RESISTANT TO THERMAL SHOCK THAN ONLY MAGNESITE. COURSES OF CHROME-MAGNESITE REFRACTORIES ARE USED AS A SEPARATING LAYER BETWEEN BASIC AND ACID REFRACTORIES USED IN SOME TYPES OF HIGH TEMPERATURE FURNACE.

17. ENGINEERING CERAMIC MATERIALS ALTHOUGH THE TRADITIONAL CERAMICS ACCOUNT FOR THE BULK OF THE PRODUCTION, THE DEVELOPMENT OF NEW AND ENGINEERING CERAMICS HAS BEGUN AND IT WILL CONTINUE WITH THE PASSAGE OF TIME. CONSIDERABLE ATTENTION HAS BEEN FOCUSSED IN RECENT YEARS ON THE OXIDES, CARBIDES, BORIDES AND NITRIDES OF METALS. THESE COMPOUNDS ARE GENERALLY OF HIGH HARDNESS, POSSESS HIGH STABILITY AND HIGH MELTING POINTS. ENGINEERING CERAMIC MATERIALS ARE CONSIDERED WHEN THE REQUIREMENTS ARE ONE OR MORE OF THE FOLLOWING: HIGH RESISTANCE TO ABRASION AND WEAR HIGH STRENGTH AT HIGH TEMPERATURE GOOD CHEMICAL STABILITY GOOD ELECTRICAL INSULATION CHARACTERISTICS

18. IN CONTRAST TO THE TRADITIONAL CERAMICS, WHICH ARE MAINLY BASED ON CLAY, ENGINEERING OR INDUSTRIAL CERAMICS ARE MAINLY PURE COMPOUNDS OF CHIEFLY OXIDES, CARBIDES, OR NITRIDES. SOME OF THE AREAS IN WHICH CERAMIC MATERIALS ARE USED IS AS ABRASIVE, CUTTING TOOLS, BEARINGS, INSULATORS, MAGNETIC MATERIALS, SUPERCONDUCTORS, CRUCIBLES FOR METALS ETC. MANY CERAMICS COMPOUNDS ARE MADE BY THE PROCESS OF PRESSING A POWDER INTO A COMPACT, FOLLOWED BY SINTERING. THE POWDER COMPACT WILL HAVE A HIGH DEGREE OF POROSITY BUT A DENSIFICATION OF THE MATERIAL WILL TAKE PLACE DURING SINTERING AND THE AMOUNT OF POROSITY WOULD BE VERY SMALL. SOME OF THE IMPORTANT ENGINEERING CERAMICS ARE ALUMINA (Al2O3) SILICON NITRIDE (Si3N4) SILICON CARBIDE (SiC) ZIRCONIA (ZrO2)

19. ALUMINA ALUMINA, ALUMINUM OXIDE, WAS ORIGINALLY DEVELOPED FOR REFRACTORY TUBING AND HIGH-PURITY CRUCIBLES FOR HIGH-TEMPERATURE USE AND NOW HAS WIDE APPLICATION. A PRECISE CONTROL OF CRYSTAL SIZE, DEGREE OF CALCINATION AND LEVELS OF IMPURITIES IS REQUIRED IN THE MANUFACTURE OF COMMERCIAL ALUMINA AND A VARIETY OF CERAMICS BASED ON ALUMINA ARE AVAILABLE. ALUMINA CERAMICS ARE USED FOR THE MANUFACTURE OF SPARK-PLUG INSULATORS, CERAMIC/METAL ASSEMBLIES IN VACUUM TUBES, SUBSTRATES FOR THE DEPOSITION OF ELECTRONIC MICROCIRCUIT, AND METAL CUTTING TOOLS TIPS.

20. SILICON NITRIDE OF ALL THE ENGINEERING CERAMICS, SILICON NITRIDE HAS PROBABLY THE MOST USEFUL COMBINATION OF ENGINEERING PROPERTIES. THIS IS FULLY RESISTANT TO MOST STRONG ACIDS AND TO MOLTEN ALUMINUM AND OTHER LOW MELTING POINT METALS. SILICON NITRIDE EXISTS IN TWO HEXAGONAL CUBIC CRYSTALLINE STRUCTURES AND SILICON NITRIDE POWDER. THERE ARE DIFFERENT TYPES OF SILICON NITRIDE CERAMICS SUCH AS REACTION BONDED SILICON NITRIDE (RBSN), HOT PRESSED SILICON NITRIDE (HPSN) AND SINTERED SILICON NITRIDE (SSN). SILICON NITRIDE IS USED FOR A RANGE OF ENGINEERING APPLICATIONS AT HIGH TEMPERATURE AND AS WEAR PARTS SUCH AS USED AS POURING TUBES FOR ALUMINIUM, AND ALSO BEING EXPLORED FOR USE OF PARTS OF ADVANCED ENGINE PARTS.

21. SIALONS THE SIALONS ARE DERIVATIES OF SILICON NITRIDE AND THE NAME ITSELF INDICATES THE INVOLVEMENT OF SILICON, ALUMINUM, OXYGEN AND NITROGEN. MUCH RESEARCH AND DEVELOPMENT WORK IS BEING DONE ON THESE MATERIALS. PRESENTLY TWO MAJOR GROUPS OF SIALON CERAMICS ARE BEING MANUFACTURED COMMERCIALLY. THESE ARE KNOWN AS LOW-SUBSTITUTION AND HIGH-SUBSTITUTION SIALONS. THE PRINCIPAL MATERIAL IS A SOLID SOLUTION OF BETA SILICON NITRIDE IN WHICH SILICON IS PARTIALLY SUBSTITUTED BY ALUMINUM AND NITROGEN. THE SIALONS POSSESS HIGHER STRENGTH AND HARDNESS THAN SILICON NITRIDE AND AE USED AS CUTTING TOOL MATERIALS AND FOR ENGINE COMPONENTS AND BEARINGS.

22. SILICON CARBIDE SILICON CARBIDE HAS BEEN USED AS AN ABRASIVE FOR MANY YEARS BUT IS NOW ALSO AVAILABLE AS AN ENGINEERING CERAMIC MATERIAL. IT IS A HARD, REFRACTORY CARBIDE WITH OUTSTANDING RESISTANCE TO OXIDATION AT HIGH TEMPERATURES. THERE ARE A NUMBER OF DIFFERENT TYPES OF SILICON CARBIDE CERAMICS AVAILABLE SUCH AS REATION BONDED SILICON CARBIDE, CLAY-BONDED SILICON CARBIDE, HOT-PRESSED SILICON CARBIDE, SINTERED SILICON CARBIDE, RECRYSTALLISED SILICON CARBIDE, AND NITRIDE-BONDED SILICON CARBIDE. ALL THESE TYPES OF SILICON CARBIDES ARE USED FOR DIFFERENT APPLICATIONS IN ENGINEERING SUCH AS USED AS REFRACTORY FOR THERMAL SHOCK APPLICATIONS. SILICON CARBIDE IS ALSO FORMED AS FIBRES FOR USE AS REINFORCEMENT IN COMPOSITE MATERIALS.

23. BORON NITRIDE AND OTHER CERAMICS BORON NITRIDE IS A SYNTHETIC NITRIDE OF BORON PRODUCED WITH THE ADDITION OF CARBON WHICH EXISTS IN TWO STRUCTURAL MODIFICATIONS SIMILAR TO GRAPHITE (HEXAGONAL) AND DIAMOND (CUBIC). THE HEXAGONAL FORM IS A SOFT AND PLATELIKE MATERIAL AND CAN BE MANUFACTURED AS A CERAMIC BY HOT-PRESSING. THIN WALLED SHAPES, SUCH AS CRUCIBLES FOR SPECIAL GASSES, MAY BE PRODUCED BY A SPECIAL CHEMICAL PROCESS. THE CUBIC FORM IS THE HARDEST SUBSTANCE NEXT TO DIAMOND. IT IS PRODUCED FROM THE HEXAGONAL FORM AT HIGH TEMPERATURE & PRESSURE. IT IS USED AS GRINDING MATERIAL IN GRIT FORM, OR AS A SOLID CERAMIC FOR THE MANUFACTURE OF METAL CUTTING TOOLS.

24. ALUMINUM NITRIDE & ALUMINUM TITANATE ALUMINUM NITRIDE IS A CERAMIC MANUFACTURED BY THE METHOD OF CASTING MIXED WITH THE NITRIDE. SUITABLE SHAPES FOR FORMING SUBSTANCES ARE BLANKED OUT FROM CAST PRODUCT AND SINTERED AT 1700 – 1800 C. ALUMINUM NITRIDE IS USED FOR CHIP-CARRIERS WHERE ITS HIGH THERMAL CONDUCTIVITY ALLOWS GOOD HEAT DISSIPATION. IT IS ALSO USED AS A RAW MATERIAL FOR SIALONS. ALUMINUM TITANATE IS A MIXED OXIDE OF ALUMINA AND TITANIA. THIS TYPE OF CERAMIC IS FORMED AT HIGH TEMPERATURE AND GIVES A SPECIAL STRUCTURE. THE RESULTING STRUCTURE GIVES THE MATERIAL AN EFFECTIVE THERMAL EXPANSION COEFFICIENT OF ZERO UP TO 700 C, AND THEREFORE IS USED IN APPLICATIONS WHERE A HIGH RESISTANCE TO THERMAL SHOCK IS REQUIRED.

25. TITANIA, TITANIUM CARBIDE & ZIRCONIA TITANIA IS THE CERAMIC DIOXIDE OF TITANIUM. TITANIA IS USED AS SILICATE-BONDED CERAMIC FOR MECHANICAL APPLICATIONS SUCH AS THREAD GUIDES, AND IS MAUFACTURED FROM TITANIA WITH A SMALL AMOUNT OF CLAY AND A CALCIUM BASE FLUX. TITANIUM CARBIDE IS HARD CERAMIC MATERIAL AND IS USUALLY USED IN COMBINATION WITH ALUMINA TO GIVE ADDED STRENGTH AND A HIGHER ELASTIC MODULUS. IT IS USED AS TOOL INSERTS FOR METAL CUTTING. ZIRCONIA IS THE OXIDE OF ZIRCONIUM EXISTING IN THREE DIFFERENT CRYSTALLINE MODIFICATIONS. DIFFERENT OXIDES OF MAGNESIUM AND CALCIUM ARE ADDED TO GIVE IT STABILITY. DIFFERENT TYPES OF ALLOYED ZIRCONIA CERAMICS ARE AS FOLLOWS:

26. DIFFERENT TYPES OF ALLOYED ZIRCONIA CERAMICS FULLY STABILIZED ZIRCONIA, CONTAINING CaO AND HAVING CUBIC STRUCTURE PARTIALLY STABILIZED ZIRCONIA, CONTAINING MgO AND HAVING DUPLEX CUBIC AND HEXAGONAL STRUCTURES TETRAGONAL ZIRCONIA POLYCRYSTALS ZIRCONIA TOUGHENED ALUMINA

27. GLASS CERAMICS A GLASS IS A CERAMIC MATERIAL IN THAT IT IS MADE FROM INORGANIC MATERIALS AT HIGH TEMPERATURE. HOWEVER, IT IS DISTINGUISHED FROM OTHER CERAMICS IN THAT ITS CONSTITUENTS ARE HEATED TO FUSION THEN COOLED TO A RIGID STATE WITHOUT CRYSTALLIZATION. THE CHARACTERISTIC OF GLASS IS THAT IT HAS A NONCRYSTALLINE OR AMORPHOUS STRUCTURE. THE MOLECULES IN A GLASS ARE NOT ARRANGED IN A REGULAR REETITIVE LONG-RANGE ORDERS AS EXISTS IN A CRYSTALLINE SOLID. GLASS CERAMICS ARE COMPLEX SILICATES THAT CAN BE READILY FABRICATED BY THE CONVENTIONAL FORMING TECHNIQUES FOR GLASSES BUT WHICH ARE THEN CONVERTED INTO FINE-GRAINED POLYCRYSTALLINE MATERIALS BY CONTROLLED CRSYALLIZATION BY USING HEAT TREATMENT PROCESS.

28. THE RESULTING MATERIALS HAVE CONSIDERABLY HIGHER STRENGTHS THAN MOST GLASSES, AND RETAIN THEIR STRENGTHS TO MUCH HIGHER TEMPERATURES THAN GLASSES BEING CAPABLE OF USE IN STRESSED CONDITIONS AT TEMPERATURES UP TO 900 C. CERAMIC GLASSES ALSO POSSESS EXCELLENT RESISTANCE TO THERMAL SHOCK. SILICA IS THE PRINCIPAL GLASS FORMING OXIDE BUT IT HAS A HIGH MELTING POINT. HOWEVER, OTHER OXIDES LOWER THE SOFTENING POINT SO THAT LOWER MELTING TEMPERATURES CAN BE EMPLOYED.

29. FOLLOWINGS ARE THE MAJOR COMMERCIAL GLASSES SILICA GLASS SODA-LIME SILICA GLASS LEAD GLASSES BOROSILICATE GLASSES ALUMINOSILICATE GLASSES

30. SILICA GLASS THIS TYPE OF GLASS IS ENTIRELY CONSISTED OF SILICA, SILICA OXIDE, WITH GOOD CHEMICAL STABILITY, A VERY LOW THERMAL EXPANSION COEFFICIENT, AND USABLE UP TO 900 C. THIS GLASS IS PRODUCED IN TWO FORMS OF TRANSLUCENT AND TRANSPARENT. THIS TYPE OF GLASS FINDS APPLICATION IN SPACE AND MISSILE TECHNOLOGY BECAUSE OF GOOD THERMAL PROPERTIES. SODA-LIME-SILICA GLASS THIS IS THE MOST COMMON TYPE OF GLASS. THIS TYPE OF GLASS IS USED IN CONTAINERS AND LIGHT BULBS. THE ELECTRICAL PROPERTIES OF GLASS VARY WITH COMPOSITIONS OF SODA, LIME AND SILICA.

31. QUESTIONS AND QUERIES IF ANY! IF NOT THEN GOOD BYE NEXT LECTURE WOULD BE ABOUT COMPOSITE MATERIALS