INTRODUCTION
2.01 Fundamental Aspects of Hard Ceramics
2.01.6 Some Examples of Hard Ceramics .1 Silicon Nitride
Besides toughness and hardness, there are several mechanical properties of ceramic materials which are relevant to their applications, including wear resistance, hot hardness, creep resistance, tribochemical stability, and thermal expansion coefficient. Wear behavior of ceramic materials is complex and is dependent upon many variables, of which hardness is an important variable but not the only significant one (De Portu & Guicciardi, 2013). For example, in many wear environments, such as the erosive wear behavior of oxide engineering ceramics, it is the ratio of fracture toughness to hardness (related to brittleness index) which is found to be significant in determining wear behavior. Chemical changes, especially corrosion and oxidation, also influence wear behavior, especially at high temperatures which may be operating at the surface of the materials under consideration.
2.01.6 Some Examples of Hard Ceramics
The use of mixed oxide additives, such as alumina and yttria, or various rare earth oxides allows the development of specific microstructures by modifying the chemistry of the GB phase which has a significant effect on the grain size and aspect ratios of theb-Si3N4grains. The microstructural development results from the fact that the additives (alumina and yttria) combine with the silica layer which is present on the surface of Si3N4 powder particles to provide a liquid phase to densify the ceramics by rearrangement of the a-Si3N4 particles followed by solution–diffusion–reprecipitation (Hampshire, 1994, 2009). Thea-Si3N4dissolves in the liquid and is precipitated asb-Si3N4which grows in the longitudinal direction as prismatic hexagonal rod-like crystals that eventually impinge on each other forming an interlocked microstructure. The liquid cools as an inter- granular phase, usually a glass, according to the following:
aSi3N4þSiO2þMxOy/bSi3N4þMSiON phase (23) A typical scanning electron micrograph of Si3N4sintered with 6 wt.% yttria and 2 wt.% alumina is shown in Figure 12 in which can be seen high aspect ratio hexagonal rod-like b-Si3N4 grains surrounded by an inter- granular oxynitride glass (Hampshire & Pomeroy, 2012).
A systematic study of pressureless sintering kinetics for Si3N4ceramics (Hampshire & Jack, 1981) applied the Kingery liquid-phase sintering model in which three stages are identified, as shown schematically by the log shrinkage–log time plot ofFigure 13. The stages are:
1. Particle Rearrangement following formation of the initial liquid phase, where the rate and the extent of shrinkage depend on both the volume and viscosity of the liquid; this is also the incubation period for the a/btransformation;
2. Solution-diffusion-reprecipitation, where, according to Kingery, shrinkage is given by:
DV=Voatl=n (24)
wheretis time andn¼3 if solution into or precipitation from the liquid is rate controlling, as was found in the case of MgO as additive, andn¼5 if diffusion through the liquid is rate-controlling, as was found for the Y2O3 additive, where diffusion through a more viscous oxynitride liquid is much slower and densification without pressure proves difficult; thea/btransformation begins in this stage and is more rapid for Y2O3;
3. Coalescenceorfinal elimination of closed porosity during which the liquid acts to further grow the elongated bgrains but this critically depends on GB glass chemistry;final density is greater than 95% of the theoretical value and normally>99%.
The types and amounts of additives used for sintering determine the nature and quantity of the resulting GB phase. The term“GB engineering”was coined (Gazza, 1975) and this aimed to understand the structure of the GBs in Si3N4based materials and the reactions occurring at them during sintering in order to achieve significant Figure 12 Scanning electron micrograph of silicon nitride (6 wt.% Y2O3þ2 wt.% Al2O3) showing darkb-Si3N4grains and bright YSiAlON glass.
Fundamental Aspects of Hard Ceramics 23
advances in materials properties.Clarke and Thomas (1977)subsequently found that the amorphous phase extended and formed thinfilms between most grains. A typical transmission electron micrograph of a glass triple point and the thin intergranular oxynitridefilm between Si3N4grains is shown inFigure 14(Hampshire &
Pomeroy, 2012). The thickness of the IGF is very sensitive to the type of oxide additive used and its concen- tration andfilm thickness (in the range 0.5–1.5 nm) depends strongly on chemical composition but not on the amount of glass present (Wang, Pan, Hoffmann, Cannon, & Rühle, 1996).
It has been shown that nitrogen increases Tg, viscosities, elastic moduli and microhardness of oxynitride glasses (Becher et al., 2011; Hampshire, 2008). These property changes can be compared with known effects of GB glass chemistry in Si3N4 ceramics where significant improvements in fracture resistance of Si3N4 can be achieved by tailoring the intergranular glass chemistry (Becher et al., 2010; Hampshire & Pomeroy, 2012). The impact of various rare-earth and related additive elements (RE¼Lu, Sc, Yb, Y, Sm, La) on grain growth anisotropy and mechanical properties of Si3N4ceramics has been studied (Becher et al., 2006). In this case, an RE-Si-Mg-O-N glass matrix is formed and with increasing ionic radius of the RE, grain anisotropy increases and ceramics with equivalent grain sizes and morphologies exhibit increasing toughness with increasing ion size of the RE3þ, reflecting an increasingly intergranular crack mode. Afirst-principles model, the differential binding energy (DBE), was developed to characterize the competition between RE and Si as they migrate to theb-Si3N4 grain surfaces. The theory predicts that La should have the strongest and Lu the weakest preferential segregation to the grain surfaces and this was confirmed by unique atomic-resolution images obtained by aberration- corrected Z-contrast scanning transmission electron microscopy (STEM) (Shibata et al., 2004).
The viscosity of these glassy phases decreases with increasing temperature, which can lead to GB sliding and cavitation when the ceramic is subjected to stress at elevated temperatures and thus these oxynitride glassy
Solution - precipitation
Nucleation/growth Kingery
0
–1
–2
–3
n = 3, rate-controlling step is solution /
precipitation n = 5, rate control by
diffusion
Log time
Vo
Particle rearrangement Influenced by volume, viscosity of liquid formed Incubation period for α→β transformation
Log shrinkage
Elimination of closed porosity
Density > 95%
theoretical Grain coarsening α→β transformation
ΔV
∝
t1nFigure 13 Schematic log shrinkage–log time plot showing three stages of liquid phase sintering of silicon nitride ceramics according to model ofKingery (1959).
Figure 14 TEM micrograph of silicon nitride showing twob-Si3N4grains, a triple point (TP) glass pocket and intergranular glassfilm (IGF).
24 Fundamental Aspects of Hard Ceramics
phases control many high-temperature properties such as creep (Wiederhorn, Krause, Lofaj, & Täffner, 2005) and high-temperature strength as well as oxidation resistance.
One significant milestone in improvements of properties through reductions in GB phases was the discovery of the“SiAlONs”(Jack, 1975).
2.01.6.1.3 SiAlONs
SiAlONs are solid solutions based on the Si3N4structure.b0-SiAlON is formed when oxygen replaces nitrogen in the b-Si6N8structure while, at the same time, silicon is replaced by aluminum to maintain charge neutrality (Oyama & Kamigaito, 1971; Jack & Wilson, 1972). The solid solution composition is:
Si6zAlzOzN8z
retaining the 6:8 metal:non-metal ratio ofb-Si6N8, withzvalues in the range 0–4.2. The single-phaseb0-sialon still requires a sintering additive such as Y2O3 in order to densify the ceramic (Ekstrøm & Nygren, 1992).
Microstructures are similar tob-Si3N4with elongated hexagonal grains ofb0-sialon but a reduced amount of intergranular glass phase.
a-sialons (a0) are based on thea-Si12N16unit cell with general composition (Hampshire et al., 1978):
MxSi12ðmþnÞAlðmþnÞOnNð16nÞ
Ina0-SiAlON, partial replacement of Si4þby Al3þoccurs if, at the same time, charge compensation is effected by the accommodation of other ions, M¼Liþ, Ca2þ, Y3þor other rare earth lanthanide ions (Ln3þ), in the two interstitial sites (x) in the unit cell.x(<2) is determined by the valence (v) of the Mvþion. The structural principle is similar to that in the formation of the“stuffed”quartz derivatives in which A13þreplaces Si4þand valency charge balance is maintained by“stuffing”Liþor Mg2þinto the interstitial sites. As withb0, Y2O3is used as a densification aid but also provides Y3þions for stabilization ofa0. Other oxides are also used for sintering (Mandal, 1999).
Unlike Si3N4, whereb-Si3N4is the main stable phase after sintering, the two SiAlON phases,a0andb0 can coexist depending on the sintering additives used. Therefore, there are more variables to be used as design parameters for SiAlON ceramics and the amount ofa0-SiAlON can be varied from 0 to 100% continuously and the type and amount of intergranular phase can also be modified by using mixed additives (Mandal, Oberacker, Hoffmann, & Thompson, 2000). These are usually combinations of CaO and two rare earth lanthanide oxides which allow more easier densification but on subsequent heat treatment, thefinal properties of the SiAlON ceramics can be tailored so that when high hardness is needed,a0-SiAlON content is increased and when higher toughness is required, then additive chemistry is changed to give predominantlyb0-SiAlON. These ceramics are already available as cutting tools or refractories and are being developed as wear parts.
2.01.6.2 Silicon Carbide
Silicon carbide has excellent high-temperature strength, good oxidation and thermal shock resistance, high hardness, and low specific weight. Because of its strong covalent bonding character, however, SiC proved difficult to densify without sintering additives and external pressure. In order to obtain dense SiC ceramics by conventional sintering techniques, different sintering additives were investigated.Prochazka (1975)found that b-SiC powders could be sintered to high densities at very high temperatures ofw2100C by using additions of both boron and carbon (see alsoCao et al., 1996).Coppola, Hawler, and McMurtry (1978)demonstrated that a-SiC powders are sinterable under similar conditions. Several authors (Lee & Kim, 1994; She & Ueno, 1999) have shown that botha- andb-SiC powders can be densified at lower temperatures of 1850–2000C with the addition of Al2O3and Y2O3which involves liquid-phase-sintering.Omori and Takei (1982)showed that oxide additives, including a wide variety of rare-earth oxides, usually in combination with alumina and/or boron compounds, promote densification of SiC via liquid phase sintering.Sigl and Kleebe (1993) used an yttrium–aluminium garnet (YAG: 3Y2O3.5Al2O3) powder as the densification additive and showed that Ost- wald ripening by solution and re-precipitation controls the sintering mechanism.Lee and Kim (1994)densified SiC ceramics by pressureless sintering of botha- andb-SiC powders using Y2O3and Al2O3additives. Witha- SiC, the microstructure consisted of equiaxed SiC grains, whereas withb-SiC, a plate-like grain structure resulted from the grain growth associated with the b/a-SiC phase transformation during sintering. As grain size increased for the sintered SiC froma-SiC powder, fracture toughness increased slightly whereas for the sintered SiC fromb-SiC powder, fracture toughness increased significantly which was attributed to crack bridging and Fundamental Aspects of Hard Ceramics 25
crack deflection by the plate-likeb-SiC grains. A detailed review on processing of silicon carbides is given in a companion paper of this book byKriegesmann (2013).
2.01.6.3 Borides
Transition metal borides are widely recognized as an attractive class of materials for a broad range of mechanical applications in abrasive, erosive, corrosive and high-temperature environments, owing to their high melting points and hardness, thermodynamic stability and excellent electrical conductivity. Most studies (Anisimov, Iavnovskii, Gubanov, & Kurmaev, 1986) have focused on the borides of Ti, Zr and Hf for application as cutting tools, for molten metals processing or as sharp components of new generation space vehicles in ultrahigh-temperature environments. The densification of ZrB2 powder is achieved using hot-pressing and generally requires very high temperatures, 2100C or above and moderate pressure (20–30 MPa), because of the covalent character of the bonding and its low volume and GB diffusion rates.Sciti, Silvestroni, Medri, and Guicciardi (2011)studied pressureless sintering as an in situ toughening method for ZrB2–SiC composite ce- ramics with addition of Si3N4or MoSi2at temperatures of 2100–2150C that induced SiC anisotropic growth from particles to platelets, within a ZrB2matrix, which consisted of more equiaxed rounded grains. The method has promise in terms of producing near-net shaped or large-sized components using atmospheric process sintering with the possibility of increasing the volume of the reinforcing phase.