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A Brief History of Ceramic Materials and Introduction to Their Dynamic Behavior

1.1 Introduction

The word "ceramic" has its origins from Greek word "Keramos" which refers to "pottery" or "potter's clay" (Rice 2015). The art of pottery making may have originated independently in various locations during prehistoric times, but the first use of clay to sculpt figures was traced to over 25 000 years ago (Vandiver et al. 1989). Making ceramic figurines or pottery involves mixing moist clay with temper (e.g. sand, small rocks, or crushed pottery), forming the desired shape, and firing the mixture over an open fire or in a kiln (Rice 2015). In ancient times, ceramics were used for storage (e.g. Greek and Roman storage vessels, called amphorae), as building materials (e.g. Roman concrete and clay bricks), and for decoration (e.g. beads, porcelain, and salt-glazed tiles). Ceramics were perhaps the earliest human-made materials from processed raw materials. Today, the word "ceramics" encompasses many materials beyond pottery. Ceramics are nonmetallic, inorganic solids with ionic and covalent bonds (Callister 2013). Most ceramics are metallic oxides or nonoxide compounds. They have high melting point, high hardness, and superior strength compared to metals. A ubiquitous, naturally occurring ceramic is sand, which is composed of silicon dioxide (SiO2) or silica, and it is the basis for glass, an amorphous solid that has many features common to a ceramic. Diamond, the hardest material known to humankind, is another naturally occurring, highly desired ceramic composed entirely of carbon. Cubic zirconia (zirconium oxide or ZrO2), an inexpensive substitute for diamond, is also a ceramic. The high melting point of some ceramics such as aluminum oxide (Al2O3 or alumina) allows them to be used in kilns and high temperature furnaces. Transitional metal diborides such as zirconium diboride (ZrB2) and hafnium diboride (HfB2) are used as ultrahigh temperature ceramics for applications in leading edges and external tiles for hypersonic vehicles. High hardness and wear resistance allow ceramics such as tungsten carbide and silicon carbide to be used as cutting tools, grinding wheels, abrasives, and in brake pads. The high hardness (more than two to five times that of hardened steel) of alumina, boron carbide (B4C), and silicon carbide (SiC) makes them preferred candidates for armor applications. The same properties also make silicon nitride (Si3N4) a good candidate for aerospace bearings while the high resistance to radiation and temperature of uranium oxide (UO2) allows it to be used as a nuclear fuel.

Compared to metals, most structural ceramics also typically exhibit excellent chemical resistance, high elastic moduli, high hardness, but lower densities, as can be seen in Figure 1.1. Ceramics also tend to have high compressive strength, but low tensile strength and low fracture toughness. Upon application of load, ceramics mostly undergo elastic deformation followed by brittle fracture. Although dislocations have been observed in alumina (Staehler et al. 2000), zirconium diboride (Ghosh et al. 2009, 2010), and boron carbide (Reddy et al. 2013), brittle fracture is still the dominant macroscopic deformation mechanism when the applied stress exceeds the ultimate strength of the material. Other inelastic deformation mechanisms are also observed such as transformation toughening in partially stabilized zirconia (Subhash and Nemat-Nasser 1993) and phase transformation in aluminum nitride (AlN) (Grady 1998; Heard and Cline 1980). Though most ceramics exhibit poor electrical and thermal conductivities, several others also serve as semiconductors (e.g. SiC and B4C), and a few (e.g. ZrB2) have thermal conductivities approaching those of metals (Callister 2013). Diamond even has a thermal conductivity greater than copper (Callister 2013). Several ceramics such as ZrB2 have excellent resistance to thermal shock, in addition to exhibiting thermal conductivity close to that of copper (Ghosh et al. 2009, 2010; Subhash et al. 2008). Depending on the properties of interest, ceramics are attractive candidates for a variety of applications including structural uses (construction materials such as bricks, roof and floor tiles, cookware, and pottery), aesthetic purposes (gemstones such as sapphires and rubies and paint pigments), industrial components (toll bits, high-temperature crucibles, tiles for space shuttles, nozzles, disk brakes, biomedical implants, nuclear fuel and cladding, and bearings), and military applications (body armor, ceramic bullets, nose cones for missiles, and vehicle windows).

Figure 1.1 Distribution of density versus hardness for various structural ceramics. Note low hardness and high density of metals compared to ceramics.

Source: Reproduced with permission from Awasthi and Subhash (2020). © 2020, Elsevier.

While the use of ceramics dates to prehistoric times, efforts have only recently been underway to better understand their behavior, particularly for complex, modern applications such as machine tools, nuclear cladding, and protective armor. Early studies from the 1950s to the 1980s have shown that ceramics are highly sensitive to defects and susceptible to unexpected fracture (Griffith 1921; Irwin 1957; Zhou and Molinari 2004). Probabilistic approaches based on defect population and distribution (Weibull 1939) were developed for predicting ceramic strength and to establish the reliability of ceramics for industrial use. In recent years, advancements in processing technology have resulted in ceramics with fewer inhomogeneities and more uniformity in microstructure, properties, and composition. Most of the initial studies conducted on processed ceramics obtained a quasi-static mechanical response, where the load is applied slowly, over several seconds to hours. Such slow loading rates lead to a deformation rate in the range of 10-5 s-1 to 10-2 s-1. Note that the rate of deformation is defined as "strain rate," which is the magnitude of strain divided by duration of loading in seconds. However, ceramic materials are also used extensively in applications where dynamic loading conditions persist such as those encountered during projectile impact on an armor plate, grinding grit interaction with a ceramic workpiece, meteor impact on celestial objects, and explosive loading on structures. In these applications, the load is applied rapidly in only a few microseconds, resulting in a deformation (strain) rate well in excess of 102 s-1. Thus, in the last few decades, research focus has shifted from determining quasistatic mechanical response to identifying dynamic deformation mechanisms, especially dynamic fracture and fragmentation characteristics of ceramics as well as understanding the influence of defects on dynamic behavior.

1.2 Examples of Material Response to High-strain-rate Loading

The rate of deformation is an important mechanistic measure that dictates the deformation response of materials. Many engineering materials exhibit strain-rate-dependent mechanical responses in their deformation and failure behaviors. In the following, the influence of high-strain-rate loading on material behavior is illustrated with several examples: Silly PuttyT, a mixture of corn starch and water, sandbags as protection against bullets, cannon ball impact on masonry structures, fracture of glass due to impact of a sharp projectile, and spallation due to axial impact of a long cylindrical glass rod.

Figure 1.2 Deformation behavior of Silly Putty when subjected to different rates of loading. When pulled slowly, it elongates over a large strain (bottom left) and when pulled rapidly, it snaps (bottom right).

Figure 1.3 (a) Schematic illustration of a bullet (dart) impacting a confined sand mass. (b-d) Time resolved images of sand particles being pushed away to allow penetration. The speed of the bullet is 100 m/s. This process involves inertial movement of particles at high velocities, friction among sand particles and friction between the bullet and the particles, all of which dissipate KE rapidly.

Source: Reproduced with permission from Van Vooren et al. (2013).

Figure 1.4 illustrates the fracture pattern induced by a bullet penetrating a large transparent glass plate (McCauley et al. 2013). The region near the bullet hole appears misty due to a high concentration of cracks and numerous fine fragments. This misty region is surrounded by material with a few larger cracks radiating away from the point of impact. In contrast, if the same material is pushed slowly with a sharp object, no dense cracking or fine fragmentation will be seen; only a few large cracks extending farther into the glass will be noted. Thus, high-rate loading often causes severe fragmentation in brittle materials.

Figure 1.4 Incipient damage in a transparent ceramic (AlON) impacted by a high-velocity projectile. Note the high...