P-Sialons

and Eu) [79] in interstitial positions (%) of the a-Si3N4 structure. The index, m, which expresses the amount of Si—N bonds replaced by longer Al—N bonds, depends on the valence of the M cation and on the x-value. 

The a-sialon-forming area expands with decreasing size of the M ion in M3cSii2-(m-n)Al(m+n)0 N(i6- )- The maximum w-value is 1.0 for Nd, and approximately 1.2 for the other dopants. The m-value varies from 1.0 in all systems studied to a value of 2.75 in the Yb-a-sialon system, indicating that the substitution of Al-O units for Si-N in a-sialon is more restricted than the substitution of Al-N for Si-N. 

The following empirical relationship between the sizes of the a and c lattice parameters axes and the m- and n-values of the a-sialon phase was obtained [80]  

The P-SiAlON solid solutions are isostructural with P-Si3N4, and formed by the simultaneous equivalent substitution of Al-O for Si-N. The P-SiAlONs are most commonly described by the formula Si6 2Al202Ng 2 (0 z 4.2), retaining the 3 4 metal nonmetal ratio, where z (Si—N) bonds are replaced by z (Al-O) bonds. As the 

The P SiAlON z-values can be determined from the measured lattice parameters using the mean of the following equations [93]  

Carbothermal synthesis and its variant, silicothermal synthesis, have proved attractive routes for preparing sialons from readily available clay mineral raw materials. The clay is mixed with fine carbon and/or silicon powder and reacted in a stream of purified nitrogen at 1400°C. The 3-sialon product carbothermally synthesised from kaolinite has the composition Si3Al303N5 (z = 3), controlled by the Si02 Al203 ratio of the clay 

The complex sequence of reactions preceding the formation of the sialon has been studied by Al and Si NMR (MacKenzie et al. 1994a, MacKenzie et al. 1996) as has sialon formation itself (Neal etal. 1994, MacKenzie et al. 1994). 

The formation of amorphous volatile products during carbothermal reduction processes can cause problems by condensing in the cooler parts of the furnace and altering the composition of the original mixture. Al NMR shows that these amorphous phases all contain Al solely in tetrahedral coordination, with chemical shifts consistent with the potassium feldspars microcline and sanidine (54-58 ppm). Thermodynamic analysis confirmed the feasibility of vapour-phase transport of both Al and alkali metal impurities from the clay which can then condense to form short-range feldspar-like units (Ekstrom et al. 1996). 

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K. J. D. MacKenzie and R. H. Meinhold, Additive-assisted pressureless sintering of carbothermal p -sialon an X-ray and solid-state MAS NMR study. /. Mater. Chem., 1996, 6,821-831. 

P-SiAlONs, which are the first developed group in the SiAlON materials, are formed by substituting up to two-thirds of the Si in the P-Si3N4 by A1 provided that valency compensation is maintained by the replacement of an equivalent concentration of N by O to give a range of P-SiAlONs, Si6 zAI/)zN8 z with 0 < z < 4.2.29 Thus, z(Si-N) bonds are replaced by z (Al-O) bonds, since the difference between the respective bond lengths (1.74 A for Si-N and 1.75 A for Al-O) is small and the extent of replacement is wide. 

With increasing zvalue the density of P-SiAlON decreases linearly, similarly lowering Young s modulus, strength, thermal conductivity, hardness and fracture toughness.30 On the other hand, for low zvalues (z< 1) the hardness and fracture toughness increase and when z > 1 they decrease. It has been also depicted that glass-free microstructures could be obtained in P-SiAlON polycrystals with substitutional level z> 2.31... 

The a-SiA10N has twice the unit-cell parameter in the c-direction compared to P-SiAlON, and the doubling of the Burgers vector for c[0001] dislocations means that dislocation movement is more difficult and the hardness is enhanced. 

O-SiAlON is another crystalline phase of interest. There is a limited solubility of alumina in silicon oxynitride structure to give O-SiAlONs, represented by the formula Si2 A-Aly01+A.N2 where ovaries from zero to 0.2. Formation of O-SiAION occurs in the same mechanism as P-SiAlON i.e. Si + N is replaced by Al + O. The lattice parameters of O-SiAION, Si2 AI/)i+aN2 a, increase in a very typical way with the x value.37... 

For B2-A1 laminate, there is a gradual transition from an a-rich region to P-rich one in XRD analyses. The transition zone is about 400 pm in thickness, and the P-SiAlON proportion in this zone changed from 20% to 70% as expected (Fig. 6.6). The gradual change in the amount of phases was also... 

Fig. 6.9). Hardness values increased gradually from 16 GPa to about 19 GPa, shifting from the P-SiAlON-rich region to the a-SiAlON-rich one (Fig. 6.10). However, the gradient layer was longer in this sample, which may be due to easier diffusion of species in a larger amount of liquid phase in B3. 

Both XRD and SEM studies support the results of hardness measurement, as illustrated in Fig. 6.13. Hardness values decreased gradually from 19 to 15 GPa, over a distance from a-SiAION to P-SiAlON rich sides for each functionally graded SiAION ceramic. 

In another study, Mandal and co-workers produced functionally graded SiAION ceramics using the powder bed method.50 In their study, P-SiAlON compacts were embedded in two different homogeneously mixed powder bed compositions, a-SiA10N (100 wt%) and A1N BN (50 50 wt%). The effects of powder bed composition and pressure on the formation of a-SiAlON on the compact surface were investigated. 

For the powder bed method, a composition rich in P-SiAlON (named B2) was selected as a compact composition to observe compact-powder bed interaction. Two different powder bed compositions, a-SiA10N (100 wt%) and A1N BN mixture (50 50 wt%), were prepared. P-SiAlON-rich pellets were embedded into the powder bed compositions, as schematically shown in Fig. 6.21. Both green and sintered pellets were embedded into the same powder bed composition in order to compare the effect of presintering on the interaction. Sintering of the pellets was carried out under 22 bar nitrogen gas pressure at 1800°C for 1 hour. To understand the effect of pressure on the interaction zone, pressureless sintering was also carried out for comparison. 

Changes in the amount of P-SiAlON from the surface of the samples for green and presintered compacts are shown in Figs 6.22 and 6.23, respectively, after sintering in a-SiAlON and A1N-BN powder beds. Both figures clearly illustrate that the A1N-BN powder bed is more effective for the formation of a-SiA10N at the surface than the a-SiAlON powder bed. Formation of an a-SiAlON-rich layer on the component surface is due to the transfer of a-SiAlON-forming ions from the powder bed. This transfer could be in various... 

In the fast cooling method, two different samples were heated and cooled very rapidly. The first was B2 (same composition in powder lamination method) the second was B4, obtained from B2 by sintering in an A1N BN powder bed. The surface of B2 was ground until 70% P-SiAlON was obtained before fast cooling treatment. Then, the specimens were inserted in a furnace with high-speed cooling. They were heated to 1600°C at a rate of 15°C/min and cooled rapidly by immediate removal from the furnace. 

Zhao, Z., Johnson, M., Shen, Z. (2002), Microstructure and mechanical properties of titanium carbonitride whisker reinforced P-sialon composites , Materials Research Bulletin, 37, 1175-1187. 

Section 18.2 gives a very brief introduction to sialons. Section 18.3 outlines the challenges to be overcome in order to make toughened and strengthened sialon products. Progress in developing sialon composites forms the main part of this chapter and is discussed in section 18.4, which deals with a/p-sialon composites, particle/whisker-reinforced sialons, and fibre-reinforced sialons. In the final section, several conclusions and suggestions for future work are summarised. 

Sialon is the generic name for the large family of silicon nitride solid solutions containing the basic elements Si, Al, O and N. Over the last three decades, the matrix sialon phases (a-, P-, and 0-) have been developed and various excellent reviews are available (Cao and Metselaar, 1991 Ekstrom and Nygren, 1992 Izhevskiy et al., 2000). In the sialon family, a- and P-sialons offer most interest as engineering ceramics because of their excellent combination of mechanical and high-temperature properties. 

As with other ceramic composites, the combination of a- and/or P-sialon with reinforcement agents results in sialon composites. This simple and obvious statement encompasses many factors which must be taken into account for successfully fabricating composites with a designed microstructure and improved properties (Prewo, 1989). For sialon matrix composites, the most important factors are physical compatibility including Young s modulus, elastic strain (Kerans and Parthasarathy, 1991) and thermal expansion coefficient (Sambell etal., 1972a, b), and chemical compatibility between sialon matrix... 

Toughening mechanisms in a/p-sialon composites are similar to those operative in second-phase particle reinforced composites, but, rather than the deliberate addition of a second phase, a/P-sialon composites are fabricated by simultaneous crystallisation of the two solid solutions a- and P-sialon from a eutectic composition liquid. This requires careful design of the starting composition which is usually located within the (a + P)-sialon region of the a-sialon plane as illustrated in Fig. 18.1. 

The a/p phase ratio can be controlled just by changing the overall composition in most sialon systems. P-Sialon grains normally grow in elongated shape with a high aspect ratio (Lange, 1979 Wotting et al., 1986), and this... 

Schematic illustration of a- and P-sialon phase regions on the a-sialon plane (M metal cation with a valence of 3+). 

P-Sialon z-value Fibre Sintering additives Temperature (°C) TD (%) Strength (MPa) Toughness (MPa m1/2)... 

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