Technology Advanced Ceramics The word ceramic appears to have been derived from "keramos", the Greek word for clay. This reflects the fact that the earliest ceramics all had clay as their main constituent and were made by firing the clay body. According to some, the word keramos itself has Indo-European roots and appears to be related to Sanskrit Cra or Car, which means to cook. The word "Shrapika", meaning a potter in Sanskrit, may, according to another view, be the origin of the word ceramic. Even today the more familiar ceramics such as tiles, porcelain, sanitaryware, bricks, etc are all based on the potter's clay. However, in addition to these so called "traditional ceramics", we have a new class of ceramics, called the "advanced ceramics" which came on to the scene in the 20th century as the materials systems became more refined and special compounds and processes were developed for structural and electronic applications. These advanced ceramics are distinguished by their high chemical purity, careful processing and high values of the useful properties. About ninety percent of the advanced ceramics today are used for electronic or related applications and are called electronic ceramics or electroceramics. The other ten percent constitute the structural ceramics in which the mechanical properties such as strength, modulus, toughness, wear resistance, hardness, etc. are of primary interest. A niche market has also developed for ceramics called "bioceramics" which are used inside the human body as hip and bone transplants, as supports for directed delivery of just enough doses of medicines to the affected areas and as components in implant devices such as pace makers. Their inertness to body fluids and adequate mechanical strength makes some ceramics the ideal materials for these applications. Structural Ceramics The current share of the structural ceramics is only 10%; however, this share is expected to grow, especially when the efforts to use them in engines become successful. Due to their high modulus and hardness, low density and resistance to high temperature and corrosive environment, there is a great interest in using ceramics in demanding structural applications such as heat engines, turbines and automotive components where their use would result in long life, operation at high temperatures and weight saving. Table 1 compares the properties of some metals and ceramics used for structural applications. Despite possessing the strength and modulus values which are equal to or better than metals, with the added advantage of low density and chemical inertness, the ceramics are yet to be used on a large scale for demanding structural applications. This is mainly due to their brittle fracture behavior (low fracture toughness) and low reliability. Silicon nitride, silicon carbide and zirconia toughened alumina are currently three of the most widely studied structural ceramics. They are already being used in significant amounts in applications such as bearing components (balls, rollers, raceway blanks), wear plates, sandblast nozzles, acid pump seals, extrusion dies, oil field components, tool bits, liquid metal filters, precombustion chambers, grinding media, etc. Silicon carbide heating elements and refractory products (crucibles, refractory bricks for metallurgical furnaces, etc.) have widely been used for many years. A binder such as clay is used to produce this type of silicon carbide, which has adequate density and strength for the above mentioned applications but not for the more demanding applications. Since the late 1970s, pure SiC has become commercially available due to advances in preparation of fine powders and understanding of the sintering of this material. Silicon nitride has properties comparable to or better than silicon carbide. However, it is much more expensive and fewer companies today are actively pursuing development and application of this ceramic than they did ten years ago. Nevertheless, a few companies are vigorously carrying out development and production of silicon nitride and it may very well become the ceramic of choice in the area of high temperature structural applications. Amongst the oxide ceramics, alumina, Al2O3, is the most commonly used ceramic because of its high hardness, wear resistance, high modulus, inertness, refractoriness and adequate strength. Paradoxically, the development of high purity all alumina ceramics, was driven not by their structural applications but by the need for low electrical conductivity insulators, for spark plugs in automotive and aircraft engines during World War II. The traditional clay _ feldspar _ flint porcelain body, as used widely in houses for low voltage insulation, was progressively developed to yield the fine alumina ceramic of today. Alumina has low fracture toughness but it can be considerably improved by making a composite in which fine particles of zirconia (ZrO2) are uniformly dispersed. Zirconia dispersed in alumina (or other ceramic matrices) acts as a smart material in that just as a crack starts propagating in alumina, it transforms itself crystallographically and expands. The result is that a compressive or crack closure stress is produced which slows down or stops the crack. The toughness of alumina can be easily increased by a factor of 2 and can be even increased to as high as nearly four times with simultaneous increase in strength by a factor of more than 2. A major concern in the processing of the Al2O3-ZrO2 composites is to achieve a uniform dispersion of fine zirconia in Al2O3 matrix. The conventional method of mixing the powders in a ball mill or other device usually leads to formation of clusters (called "agglomerates") of ZrO2, which do not retain the tetragonal structure after processing and are not effective in enhancing the mechanical properties. To achieve a uniform dispersion of ZrO2 in the Al2O3, several techniques have been used. At IIT Kanpur, a hybrid sol-gel process is being used in which alumina powder is mixed with a solution of zirconium alkoxide. Proper processing leads to a microstructure in which nanosized zirconia particles are uniformly dispersed in alumina. The size of zirconia particles can be increased by controlling the sintering cycle. Excellent strength and toughness are obtained. Alumina ceramics are finding increasing use in applications such as pump seals, wear plates for industrial components, sand blast nozzles, extrusion dies, etc. Alumina is also being widely used as a bio-inert material in various orthopedic devices such as hip joint prosthesis, knee joint, etc. Another type of ceramic composites being investigated in the department of materials and metallurgical engineering at IIT Kanpur are the ones in which a metal or intermetallic phase is dispersed as an interconnected and interpenetrating network in a ceramic matrix. For example, aluminum (Al) / aluminum oxide (Al2O3) and Al / aluminum nitride (AlN) composites are being produced by oxidizing or nitriding liquid aluminum melts. Glass, a brittle material becomes very strong when shaped into microns thick fibres. This magical transformation comes about due to the avoidance of flaws on the surface of the fibre by protecting it in a thin polymer coating called sizing just after the fibre is drawn. The fibres when incorporated in polymer matrix produce the glass fibre reinforced plastic (GFRP), a strong, tough, light material finding wide applications ranging from household furniture to aircraft furnishings. While the use of ceramic fibres in composites is yet not significant, the fibre reinforced ceramic composites when developed would probably be the ultimate in strength, high toughness, and high temperature structural materials. Silicon carbide and silicon nitride fibres are being actively looked at for use in such components. These fibres are not available in the country and are available only with great difficulty and at a great expense from abroad. With this in view, work has been initiated to develop different precursors for SiC fibres at IIT Kanpur. Simultaneously, composites made of discontinuous reinforcements such as SiC platelets in silicate matrices are being investigated. Very significant improvements in tensile elongation and strength have been already achieved. Electroceramics As mentioned earlier, the electroceramics today constitute about ninety percent of the value of the total ceramics manufactured. Although we hardly notice them, they are ubiquitous and touch our daily lives at every step. Essential, and often critical, components in devices ranging from the humble gas lighter to the sophisticated cellular phone are made of these ceramics. The insulating, dielectric, piezoelectric, magnetic, optical, and lately, superconducting properties of these ceramics have led to their widespread use in electrical and electronic devices. These include the piezoelectric lead zirconate titanate (PZT) elements in the gas lighter, telephones and autofocus cameras, capacitors made of barium titanate ceramic in televisions, radios and almost all the electronic equipment, the microwave dielectric ceramics used in highly selective filters for cellular phones and satellite communication systems. Soft ferrites are used in sensitive radio antennas and transformers; hard ferrites are used in many small electric motors - as many as twenty such motors are used in some models of automobiles for functions such as power seats and door locks. Fuel cells, which may replace the internal combustion engine in the cars one day, have ceramics such as zirconia working as solid electrolytes to generate power from fuels without combustion. A class of electroceramics which show diverse phenomena and have wide applications are the ferroelectrtic ceramics. Similar to ferromagnets, these ceramics can be electrically polarized permanently and the direction of polarization can also be reversed. Extensive research is at present underway in many laboratories to use the thin films of these ceramics as "nonvolatile memories" in computers. In addition to the potential applications in memories, these materials also find applications in mechanical sensors and transducers and room temperature infrared sensors. At IIT Kanpur, a simple but very effective technique, called the sol-gel process is being used to prepare thin films of lead zirconate titanate (PZT) and related ceramics. Suitable chemical and process modifications have been found to drastically reduce the problems of fatigue (degradation in properties due to repeated electrical cycling). PZT elements, embedded or surface mounted on loaded structural members can be used to sense and control excessive vibrations. This area is being studied in the departments of mechanical and aerospace engineering at IIT Kanpur. In these studies, piezoelectric elements made of pellets or tape cast sheets are embedded in woven fabric epoxy composite flexure beams and their open circuit response is measured and compared with the theoretical models. Bulk of the modern advanced ceramics is made starting from powders that have carefully controlled chemical composition and particle size distribution. The preparation of such powders has emerged as an important area of research and development in which chemistry has played a very important role. At IIT Kanpur nanosized powders of several ceramics are being prepared using solution chemistry and sol-gel techniques. These ultrafine particles yield ceramics and composites having superior properties. Ceramic Processing In ceramic processing the powders are first consolidated in the desired shape to produce what is called a "green body". This consolidation and shaping step is most widely done by pressing the loose mass of powder in a die at pressures of over 200 MPa. The resulting green body has large porosity (up to 40 %). The powder particles are held together by mechanical interlocking and surface forces, aided by small amounts (~ 1 %) of some organic binder such as polyvinyl alcohol (PVA). The green body at this stage is very fragile and needs to be handled carefully . (The medicine tablets are also made by a similar process of pressing the powders; however, they contain large amount of binder which gives them greater strength). To convert the green body into a useful ceramic body having low porosity and adequate strength, the green body is fired at temperatures ranging from ~700o C to ~1700o C depending on the ceramic. During firing several processes occur, the most important of them being "sintering" which causes a drastic reduction in porosity. In addition, the chemical composition becomes more uniform throughout the ceramic, the grain size increases and has to be controlled. The ceramic body after firing is strong and should have desired properties. Often a further finishing step such as grinding to final dimensions is needed. In case of electronic ceramics, the application of metallic coating to act as electrodes is the most common finishing step. Several innovations such as pressure assisted slip casting, isostatic pressing, injection moulding, gel casting, enzyme catalyzed ceramic forming have made it possible to prepare improved green bodies of complex shapes not achievable by powder pressing. Tape casting of ceramics has led to miniaturization of electronic components. By tape casting a thin sheet of any ceramic can be made by a continuous process at rates exceeding several meters per hour. Pieces of desired size can be punched out of the "green" tape . Metallic paste is applied to the tape and the metallic coat and the ceramic are cofired. An intricate, multilevel, three-dimensional component of insulating ceramic andconducting metal can thus be produced giving a high component density and greater functionality. Sol _ gel processing, which totally bypasses the powder stage has emerged as a powerful technique for making ceramic films , powders, fibres, abrasives and glass spheres. Conclusion Starting from humble origins in earthenware and bricks, the ceramics have come a long way where today they seem to be poised to virtually revolutionize the modern living. High temperature superconductors, structural ceramics for engines and electrolyte ceramics for storage batteries and fuel cells _ to name only a few- have the ability to cause a step change in the world around us if their promised potential is realized. The next two decades should see this happen. Then we will have truly entered the "New Stone Age".
D.C. Agarwal Advanced Centre for Material Sciences Indian Institute of Technology, Kanpur Kanpur-208016 E.mail: agrwald@iitk.ac.in |
