New topics discuss the definition of ceramics, the combinations of properties fulfilled by ceramics, the evolution of ceramics applications, and their importance in modern civilization. A new chapter provides a well-illustrated review of the latest applications using ceramics and discusses the design requirements that the ceramics must satisfy for each application.
The book also updates its chapter on ceramic matrix composites and adds a new section on statistical process control to the chapter on quality assurance.
Example 1. Th e I and 2 preceding the sa nd p designate the qu antum shell. For oxygen the Is and 2s subshells are both full. The strength of the hot-pressed Si,N, exposed to the dynamic oxidation with sea salt additions decreased to an average of MPa 71, psi from a baseline of MPa 97, psi.
The reaction-bonded material decreased to MPa 17, psi from a baseline of MPa 36, psi. The examples presented so far for oxidation and corrosion have had distinct features that help distinguish the cause of fracture from other mechanisms, such as impact or machining damage. Some corrosion-initi- ated fractures are more subtle.
The corrosion or oxidation may only follow the grain boundaries and be so thin that it is not visible on the fracture surface. Its effects may not even show up in room-temperature strength testing since its degradation mechanism may only be active at high temperature.
How do we recognize this type of corrosion? The following sug- Failure Analysis Figure Specimen size 0. From Ref. The EDX graph shows the relative concentralion of chemical elements in the glassy region at the base of the pi!.
Courtesy Garrett Turbine Engine Company. An electric signal that is to be delayed is input to the first transducer. The signal is converted to a sonic wave impulse that travels along the sound-transmitting "waveguide. The delay results because a sonic wave travels much more slowly than electrons passing through a wire. The time. Delay lines are used extensively in military electronics gear and in color te levision sets. One example is radar systems to compare informatio n from o ne echo with the.
The wave-generation applications discussed so far involve acoustic. Courtesy EDO Corporat io n. Crack growth is relatively easy if the grain boundaries of the material are coated with a glass phase. At high temperature, localized creep of this glass can occur, resulting in grain boundary sliding. Figure 8. The initial flaw was probably a shallow 20 10 40 pm machining crack. It linked up with cracks formed by grain boundary sliding and separation and pores formed by triple-point cavitation to produce the new Haw or structurally weakened region seen in Fig.
This was the effective flaw size at fracture. From Ref. C ASM International. All three of these factors can increase the life of a component. However, a thick buildup reduces the airflow through the engine and decreases efficiency.
Fouling is an inherent problem in the direct burning of coal. A variety of approaches have been or arc being studied to resolve this problem:. Intermittent removal of buildup by thermal shock, melt-off, or passing abrasive material such as nutshells through the system. Figure 9. Spheres are also commonl y used. Court esy Ke maNord. Very hard grinding media can reduce contamination because they wear mo re slowly.
If contamination from the media is a n especially critical consideration, milling can be conducted with medi a made of th e sa me compos iti o n as the powder being milled. Another approach is to mill wit h steel media and remove the contamination by acid leaching.
Milling can be conducted either dry o r wet. The advantages and disadva ntages are listed in Table 9. Dry milling has the advantage that the resulting powde r does not have to be separa ted from a liquid. The major concern in dry milling is that the powde r does not pack in th e corners of. Courtesy of Larry Chick.
Battelle Northwest Laboratories, Richland. Courtesy Cerarnalec, Inc. Let us discuss these in more detail and examine some examples. Binders and Plasticiz ers Table 9. Most binders and plasticizers are organic. They coat the ceramic particles and provide lubrication during pressing and a tem-.
The amount of organic binder required for pressing is quite low, typically ranging from 0. Organic binders normally are decomposed during the high-temperature densification step and evolved as gases. Some binders leave a carbon residue, especialty if fired under reducing conditions. Inorganic binders also exist. The clay minerals such as kaolinite are a good example.
Kaolinite has a layered structure and interacts with water. Based on a powder wi th a compaction ratio of 2: 1. The pressed shape usually contains flash thin sheets of material at edges where the material extruded between the die parts and can deform after pressing if not handled carefully.
For these reasons, wet pressing is not well-suited to automation. Uniaxial Pressing Problems. The following are some of the problems that can be encountered with uniaxial pressing.
The first two are easy to detect by simple measurements on the green compact immediately afte r pressing. Improper density or size are often associated with off-specification powder batches and are therefore relatively easy to resolve. Die wear shows up as progressive change in dimensions. It should also be routinely handled by the process specification and quality control. The source of cracking may be more difficult to locate.
It may be due to improper die design. Often a crack initiates at the top edge of the part during pressure release or ejection of the part. Two mechanisms of this type cracking are illustrated in Fig. The first, shown in Fig. Figure ASM International. Whe n most people hea r the term "casting. A limited amount of casting of molten ceramics is done in the preparation of high-density Alp a nd AI,O.
In the latter case, casting from a melt into cooled metal plates produces rapid quenching, which results in very fine crystal size that imparts high tough ness to th e materia l.
The technique of cas ting molten ceramic refractori es is called fusion cas ing. More frequently. T here are a num be r of va ri ati o ns to thi s process. The most common is refe rred to as slip casling. The principles and co ntro ls for slip cas tin g are similar to those of the other particulate ceramic.
Adapted from Ref. These are referred to as nonaqueous nonwater-based. Another nonaqueous system utilizes trichloroethylene plus ethanol. Nonaqueous systems work well with steric hindrance because they are adequate solvents for the chain polymers. Some of the polymers also provide steric hindrance in an aqueous water-based system, for example, phosphate esters. Aqueous slips utilizing electrostatic repulsion are commonly used for slip casting. Techniques of slip preparation and slip casting are discussed in the following sections.
Nonaqueous slips utilizing steric hindrance are commonly used for tape casting. Tape casting is discussed later in this chapter. The actual physical preparation of the slip can be done by a variety of techniques. Perhaps the most common is wet ball milling or mixing. The ingredients. The slip is then allowed to age until. Below this is the mold produced by dipping and dissolving the pattern. Below the mold is the gree n casting after dissolving the mold and trimming off any material re maining in the reservoir or gating area.
The stator vane discussed above required less than 1-hr casting time. Some solid castings require much. It required over 12 hr. T he slip must be very stable for such long casting time to avoid settling of large particles or adverse changes in viscosity. Other fugitive mold techniques have been developed to fabricate special shapes. One technique produces low weight, but strong ceramic foam Reticulated foam similar to a dishwashing sponge is used as the mold interior.
Ret icu lated polymer foam of the desired pore size is cut to the desired shape and placed in a container in a vacuum chamber. A ceramic slip in poured into the container and under vacu um complete ly infiltrates the pores in the reticulated foam. The slip is dried and fired to burn off the polymer foam and densify the ceramic.
The resulting part consists of an intern al cast of the spongelike foam. Its major characteristic is continuous interconnected links of ceramic and continuous pore channels. Such a cellular structure can be very lightweight and surprisingly strong. Courtesy AiResearch Casting Company. The materials are successfully used for molten metal filtration and kiln furniture and are being evaluated for removing particles from the exraust of diesel engines.
Some components are too complex to be fabricated in one piece. An example is the turbine scroll shown in Fig. The turbine scroll is an important component in many gas-turbine designs. It changes the direction of the hot gases coming out of the combustor to allow them to pass through the rotor.
The scroll in Fig. It was fabricated. The shroud, sleeve , and ring were formed by isostatic pressing and green machining. The body and duct were fabricated by slip casting. A final casting technique is electrophoretic deposition EPD. It utilizes an electrostatic charge to consolidate ceramic particles from a suspension. Division of the Garrett Corporation. An electrical polarity is applied to the mold that is opposite to the polarity at the surface of the ceramic particles.
The ceramic particles are electrically attracted to the mold surface and deposit as a uniform compact. When the desired thickness of deposit is achieved , either the mold is removed from.
Electrophoretic deposition is generally used to deposit a thin coating or to produce a thinwalled body such as a tube. It is also used to achieve very uniform deposition of spray paint onto a conductive surface. All of the casting techniques discussed above result in a relatively weak. A technique recently developed at ORNL results in a much stronger compact. This technique is referred to as gel casting.
The ceramic powder is mixed with a liquid and a polymerizable additive to form a fluid slurry similar to a casting slip. The slip is poured into a container of the desired shape. Polymerization is caused to occur before. The resulting powder compact is quite uniform and strong.
Furthermore, monomers are generally. Photo courtesy of HiTech Ceramics. Alfred, New York. Casting Process Control As was illustrated in Fig. Some of the critical factors include: constancy of properties. Other Tape-Casting Processes A second tapecasting process is the "waterfall" technique. It is iI1ustrated. The slurry is pumped in a recirculating system to form a continuous curtain.
A conveyor belt carries a flat surface through the slurry. The uniform, thin layer of slurry on the carrier is then transferred by Slip. From J, Adair , University of Florida. Albans, Vermont. The acrylic binders are an exception. They burn out cleanly in inert and reducing atmospheres as well as oxidizing atmospheres. Common Extrusion Defects Extrusion is often more of an art than a science. Quality is controlled by careful inspection of extruded compacts for defects.
Defects that can occur. Warpage or distortion can occur during drying or firing due to density variations or during extrusion due to improper die alignment or die design. If the alignment or balance of the die is not correct, greater pressure on one side of the die will occur.
This will cause more material to extrude from this side and result in bending of the extruded column as it exits the die. Laminations are cracks that generally form a pattern or orientation.
Examples are shown in Fig. A common cause is incomplete reknitting as the plastic mix is cut by the auger or flows past the spider portion of the die. The spider is the portion of the die that supports any shaped channels in the die. For example, to extrude a circular tube, a solid rod of the inner diameter of the tube must be supported at the center of the die.
It is generally supported by three prongs at " to each other that run parallel to the length of the die and are attached to the inside of the die. The material being extruded must squeeze around these prongs and reunite into a continuous hollow cylinder before leaving the die. Laminations occur if the material does not completely reknit.
Tearing consists of surface cracks that form as the material exits the extruder. This is illustrated in Fig. The cracks extending from the surface inward result from the contact stresses and friction that are discussed earlier in this chapter.
Too dry a mix with inadequate cohesiveness will tear. A mix with high rebound may also tend to tear. Die design involving a slight divergent taper at the die exit can help prevent tearing. Lamination and tearing are two sources of cracking. Other cracks can occur due to poor mixing, shrinkage variation, and partially dried debris from a prior extrusion run.
Segregation involves a separation of the liquid and solid portions of the mix during extrusion. This can result in cracking or distortion during extrusion or during subsequent drying or firing. Courtesy NGK Insulators. In Fig. Plug flow resulted in both cases and knit line formation was minimized. This is further illustrated in Fig. The "short shot" technique was used whereby injection was interrupted before the cavity was full.
By conducting a sequence of short shots, a good image of the nature of mold fill for each gate configuration could be obtained. After binder removal and densification, knit lines remain as large. The short shot approach has been successfully used at Carborundum Company in developing integral radial rotors for an experimental auto motive gas turbine [37J. Initial rotors were injected from the nose end. Short shots indicated a tendency for folds and knit lines to form in the thick region of the hub near the backface.
This region is exposed to the highest stresses during engine operation, so major iterative efforts. Sixteen resulting rotors were spintested and failed at an average speed of 80, rpm , significantly below. Note the knit Jines in the hub and backface regions. For example , the volume change for one polypropylene system due to thermal contraction was about 2. If the outer shell is rigid and cannot shrink, while the inner material is more fluid and can reposition during further cooling, 4.
Such a void or crack is typically not visible by examining the surface of the injection-molded part and may not eve n be visible after densificiation. Applicatiolls of Illjectioll Moldillg Injection molding is usually selected for ceramics only after other processes have been rejected.
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