Application of Engineering Ceramics

Transcription

1 MME 467: Lecture 15 Application of Engineering Ceramics The use of ceramic materials in science and industry is becoming increasingly widespread. This chapter will briefly review structural, refractory, and energy production applications of ceramic materials. Research and development efforts will continue to expand the applications of ceramic materials. Thus, it should be noted that many applications that are termed "potential" here may soon become actual applications, and many new ones will be added to the current list of applications. 1. Structural Applications Due to their brittle nature, monolithic ceramics are sensitive to defects that act as stress concentrators. Therefore, structural applications of monolithic ceramics are limited to parts that are subjected to compressive loading or limited tensile or multiaxialloading. Another important factor to consider is conditions that can lead to impact loading and thermal shock. Most monolithic ceramics are not suitable for these conditions. A well-known example is glass and porcelain ware commonly used in our 'daily lives. Some special glasses with low thermal expansion have been developed for improved thermal shock resistance. Porous ceramics typically used as refractories and some dense, monolithic ceramics such as Si 3 N 4 and SiC have good thermal shock properties. Ceramic matrix composites provide some tolerance to defects and dynamic or thermal loading. Some examples of structural applications of ceramic materials are bearings, seals, armours, liners, nozzles, and cutting tools. Bearings Due to their current high cost, ceramic bearings and journals are used only for precision systems. Si 3 N 4 balls are used in spindle bearings for cutting tools, turbomolecular pumps, dental drills, and specialty instrumentation bearings. Boron carbide and single-crystal sapphire are used in bearings and seals. Silicon nitride and sialons are being considered for gasturbine bearings. Advantages of such ceramics over conventional materials, e.g., steel, are their lower densities, which reduce the centrifugal load on the balls, higher resistance to wear, and superior hightemperature properties. Slide bearings made from siliconized SiC have been mass-produced since the 1980s [6.5]. Washers and seals Ceramic materials such as Al 2 O 3 and SiC are employed as faucet washers and counterfaces in mechanical seals for pumps and blowers. The advantages of ceramic materials in such 1

2 applications are their corrosion resistance, high hardness, low coefficients of friction, and minimum deposition. The seal is formed by a pair of rings, one stationary and one rotating, which work at a certain gap. The rings are elastically supported by the casing and the shaft. Sb- or synthetic resin-impregnated carbon is used as the wearing partner in hard/soft sliding material combinations. Mechanical seals for boiler feed pumps originally employed Ni-bonded tungsten carbide and Sb-impregnated carbon tribocouples. However, selective corrosion and accelerated abrasion resulted in high leakage rates. To solve this problem, the tungsten carbide rings were replaced with SiC-Si rings and Sb- impregnated carbon with synthetic-resin-impregnated carbographite that contains a high-strength base carbon. This combination enabled sliding speeds up to 52 m/s. In some HR-type seals, the steel casing ring was substituted by ZrO 2 to prevent corrosion and erosion. Ceramic materials are preferred in high-performance valve applications for handling corrosive and erosive media such as brine, coal slurries, and drilling muds. Dies Hot pressing and forming dies are well-developed applications. Graphite dies are used almost exclusively under inert conditions because they are easy to machine and they have excellent properties including structural-mechanical stability up to 2000 C, very low sliding resistance that provides ease in part-die separation, and inductivecoupling capability. Al 2 O 3, BN, and SiC are used as rolling components in steel plants. Tube- and wire-drawing dies are made from Al 2 O 3 and PSZ. Al 2 O 3 guides are used successfully in the textile industry. Corrosive applications Corrosive and abrasive environments occur in the pulp and paper industry, and during molten metal handling. Graphite and carbon are very resistant to wetting and corrosion, except with metals that form carbides. Al 2 O 3, BN, Si 3 N 4, and MgO are commonly employed for handling molten metal and glass. Relatively new application in this category is in beverage dispensers used by major soft drink companies and fast food restaurants. Ceramic materials are used in pistons and cylinders of soft drink beverage valves. These valves, which have to withstand both corrosion and erosion, mix CO 2 and syrup. Nozzles Nozzles used for welding and oil burners have to withstand high temperatures and thermal shock. Such nozzles have been produced from reaction-bonded silicon nitride. Ceramic membrane Ceramic membranes are used in the field of separation. Examples of such areas are the dairy industries; fruit juice, beer, and wine processing; and chemical and biotechnical applications. 2

3 Ceramic membranes are more expensive and less fracture resistant than their organic counterparts, but they have advantages when chemical and physical demands prevail. To separate oil from water, especially when contaminated with corrosive chemicals or abrasive materials, the best choice is ceramic membranes. Usually, alumina membranes are used for these microfiltration applications. 2. Military Applications High-performance ceramics are an integral part of modem weapons and defence systems. Lightweight ceramics are used as armors in many modem military attack helicopters [6.12]. C-C composite applications include components for missiles and military aircraft. Important applications are reentry bodies, rocket nozzles, and exit cones for strategic missiles; and brake disks for military and commercial aircraft [6.13]. Advantages of C C composites are high specific strength, high-temperature strength, high toughness, superior thermal shock, ablation, and high-speed friction properties. Modem day experimental or manufactured ceramic armour usually contains ceramic matrix composites such as Al 2 O 3 -SiC w, borosilicate glass reinforced with SiC or B 4 C particles, and improved monolithic ceramics including Al 2 O 3, B 4 C, SiC, TiB 2, or AlN. Ceramic materials are used in armour applications mainly because of their superior ballistic performance and low weight. Typically, armour ceramics are used in combination with metal or polymer composites as backing plates. 3. Cutting Tools Widespread application of ceramic materials is in cutting tools. Ceramic cutting tools have high hot-hardness and do not react with the work piece materials. Advantages include long tool lives, machining at very high speeds, and very high metal removal rates. Some applications use ceramics bonded with a suitable binder, but many cutting tools consist of bulk ceramics. Examples of the former type are resin-bonded diamond and SiC cutting and grinding wheels and cutting tips made from cemented carbides. Cemented carbides are hard carbides such as WC, TiC, TaC, and NbC, cemented together by a binder, typically Co. Additional coating by TiN, TiC, Al 2 O 3 or multiple coatings may be used to improve cutting performance. Cutting tool inserts made from Al 2 O 3 and Al 2 O 3 -TiC have been in industrial use for many years. In recent years, many additional ceramic cutting tools have been developed. Among these are ZTA, SiC whisker-reinforced Al 2 O 3, Si 3 N 4, sialons, and PSZ. Silicon nitride is the number one choice for machining grey cast iron. It has higher thermal shock resistance and toughness than un-reinforced oxide ceramics. Polycrystalline diamond and cubic boron nitride (c-bn) cutting tools prepared by cladding a thin layer of ceramic onto a WC-Co substrate are the hardest tools available, but they are relatively expensive. 4. Abrasives Abrasives are hard materials used for cutting or abrading other materials. They are usually employed in grinding wheels and grinding tools with special shapes. 3

4 Abrasive grits are commonly bonded by phenolic resins, sodium silicates, shellac, rubber, vitrified bonding, and metallic bonding. Cloth, paper, or polymer substrates coated with abrasives are also available in belt or disk form. Some abrasive processes employ abrasive slurries. Lapping or polishing operations of various materials such as glass, marble, metals, and ceramics are performed with slurries of free abrasive grains such as SiC, Al 2 O 3, or diamond. The slurry is used on a suitable cloth attached to a revolving or vibrating plate. General requirements of abrasive materials are high hardness, wear resistance, toughness of individual grains, and friability (the ability to crumble) of the abrasive tool. Abrasives of unusual hardness are called superabrasives. Cubic BN and diamond are two ceramics that fall into this category. Other abrasives have much lower hardness and wear resistance. The first abrasives used in industry were natural hard materials such as emery (about 50% Al 2 O 3 with other oxides, principally magnetite-fe 3 O 4 ), corundum (natural Al 2 O 3 ), and diamond. Among the limited natural abrasives still employed are diamond, quartz sand, and garnet. The commercial importance of natural abrasives significantly decreased after the introduction of various synthetic abrasives. Common synthetic ceramic materials used for various abrasive machining processes include Ah03, SiC, c-bn, and diamond. The surface morphology and grain size of an abrasive particle (grit) are important parameters in abrasive machining. Grits interact with the surface by cutting, ploughing, and rubbing. Grits with large negative rake angles or rounded cutting edges interact by ploughing instead of cutting. Controlled breakdown of the abrasive or the matrix is a requirement in abrasive wheels to provide fresh, sharp cutting surfaces. Dressing the wheel with porous SiC aids the formation of fresh cutting surfaces. The grain size largely determines surface finish and material removal rates. Small grit sizes provide a better surface finish whereas large grit sizes produce higher material removal rates [6.22]. Hard workpieces require smaller grain sizes for efficient cutting and vice versa. Grain size ranges from 8 (very coarse) to 250 (very fine). 5. Automobile and Aerospace Applications Automotive applications of ceramics include turbocharger rotors, exhaust port liners, honeycombs for catalytic converters, spark plugs, glow plugs, and sensors. Turbocharger rotors made of Si 3 N 4 have been used in some Nissan models since 1987 and by Toyota since Various types of ceramic filters exist for emission control on diesel engines. These filters are required to filter solid particles, which are composed of solid carbon on which uncombusted hydrocarbons and inorganic substances such as Na, Ca, Zn, Fe, and V are adsorbed. Available ceramic filter types include extruded monolithic filters, fibre mats, and ceramic foams. Ceramic materials usually employed in diesel filters include SiC, mullite, cordierite, and titanium aluminate (TiAl 2 O 5 ). Catalytic converters, together with lambda sensors have been used for emission control in passenger cars since the late 70s. Heated by exhaust gases and auxiliary devices when necessary, the three-way catalytic system aids the reaction of CO and various hydrocarbons to produce harmless CO 2 and H 2 O and the simultaneous reduction of NO 2 to N 2 and O 2.The ceramic material usually employed is cordierite (2MgO.2Al 2 O 3.5SiO 2 ). Mullite-aluminum titanate ceramics have been developed for temperatures higher than 200 C. 4

5 The lambda sensor is an oxygen sensor, which provides information for the electronic fuel injection system or electronic carburettor. A typical lambda sensor used in emission control has a ceramic body and a porous ZrO 2 electrode. Other automotive parts employing ceramics include spark plug insulation and riser heaters made from barium titanate, exhaust port liners, valve guides, headface plates, wear surface inserts, piston caps, bearings, bushings, and intake manifold liners. Ceramic glass matrices of borosilicate, lithium aluminosilicate, or calcium aluminosilicate glass reinforced with C or SiC fibers are used in various aerospace applications. Including compressor cylinders, valves, brake systems, turbine blades, and supports for satellite mirrors. 6. Refractory Applications Ceramic materials are well known for their refractory properties. Refractory ceramics have been used for lining boilers, ladles, kilns, and other vessels. Several layers of refractory are used in these applications. The inner layers, where contact with the most severe environment occurs, employ refractories with higher density. These layers must endure corrosive and erosive media such as molten metal, slag, fluidized particles, high velocity corrosive gases, and corrosive waste. The outer layers must provide thermal insulation. Low thermal conductivity, high melting or decomposition points, and low thermal expansion are critical properties. The outer linings usually do not carry high stresses, nor are they exposed to erosive and corrosive media. Porous refractories are better fit for this purpose because they are less costly, lighter, and have better insulation capability. Ceramic linings are also employed in waste incinerators. Incineration is the controlled burning of municipal waste. Linings for combustion chambers have to resist wear caused by abrasion and thermal cycling at temperatures up to 1300 C. Some refractory ceramics include compact heat exchangers, heat exchanger tubes, immersion tubes, radiant tubes, furnace components, insulators, thermocouple protection tubes, burner nozzles, atmosphere inlet tubes, muffle tubes, high temperature exhaust stream filters, and kiln furniture. Other examples of refractory applications that use dense ceramics are crucibles, tubes, thermocouple sleeves, and igniters. Many refractories are produced from naturally occurring minerals, and they contain relatively high amounts of impurities. Examples of conventional refractories include silica, aluminasilica, and basic refractories such as dolomite, magnesite, calcite, and forsterite, zirconia, zircon, and spinel. These products are commercially available in a variety of forms such as brick, castables, sheet, cloth, tape, and formable shapes. Most refractories are porous and relatively impure, but many refractory applications involve dense, high technology ceramics, including SiC, Si 3 N 4, BN, AlN, and pure oxides. Ceramic foams produced from alumina, silicates, mullite, cordierite, or zirconia, are used for molten metal filtration. These filters remove dross, slag, and nonmetallic inclusions, minimize turbulence, and decrease erosion due to hard particles. Ceramic membranes are preferred over metallic and polymeric membranes when high temperatures, abrasion, and corrosion resistance become important. Applications include food processing, pharmaceutical and petrochemical production, gas separation, and molten metal purification. 5

6 7. Ceramics for Energy Production Ceramic materials are currently used in various nuclear applications, and many additional ones are foreseen in the future. In fission reactors, B4C is used as control rods and neutron absorbers. Control rods are used in reactor cores to vary the power, depending on demand, and if necessary, to shut down the reactor. The fuel itself, (U 0.8 Pu 0.2 )O 2 (-x), is also a ceramic material. Other ceramic fuels are also considered for fast breeder reactors, including Th O 2 and the so-called advanced nuclear fuels, carbides, nitrides, and carbonitrides of U and Pu. A variety of applications are predicted for ceramics in fusion reactors. Some of the potential applications are insulators for magnetic coils, active coils, and divertor coils; windows and dielectrics for radio frequency (RF) heating; structural components; current breaks; and insulators for neutral beam injectors, magnets, and direct converters. Candidate materials for lightly shielded magnetic coil insulators are magnesium aluminate spinel (MgAl 2 O 4 ), MgO, and Al 2 O 3. Tritium breeding materials are also an important field of application for ceramics. These are Li-based ceramics for solid blankets, which serve to breed and release tritium as fuel to maintain the fusion reaction and at the same time to convert the energy into useful heat. Some examples of these materials are Li 2 O, -LiAlO 2, (LiAl 5 O 8. Li 2 SiO 3. Li 4 SiO 4 ), Li 2 TiO 3, and Li 2 ZrO 3. These materials are attractive because of their inherent safety advantages. Ceramic materials are also being considered as nuclear waste storage materials, especially for corrosive environments. One container concept is composed of two dense Al203 sections fabricated by HIP and sealed after insertion of the radioactive waste. Borosilicate glasses, along with other materials such as sphene (CaTiSiO 5 ), monozite orthophosphates (e.g. (Ce,La)PO 4 ), and SYNROC (synthetic rock) are also used. Si 3 N 4 and SiC are frequently used in conventional energy production since they have higher strength than superalloys above l100 C (the typical strength of hot pressed Si 3 N 4 is 800 MPa at 1100 C and that of a superalloy is 200 MPa). The lower densities of these ceramics (roughly one-third that of superalloys) are also an advantage for rotating and oscillating parts such as turbine blades. Ceramics are used as insulators and heat exchangers in magnetohydrodynamic (MHD) power generation. Si 3 N 4 and reaction bonded SiC are used in advanced heat exchangers necessary in energy production. These applications require resistance to temperatures up to 1000 C, high corrosion resistance to aggressive liquid or gas media, and thermal shock resistance. Oxygen ion conductor ceramic materials are employed in high-temperature fuel cells still under development. Stabilized zirconia is the material of choice due to its high oxygen ion conductivity. 6