Sialon from synthesis to applications: an overview

ABSTRACT Sialons are the spotlight of current investigation as low-cost and outstanding alternatives to the currently used metal alloys in various applications. The excellent high-temperature properties, high mechanical properties, structural reliability, good sinterability, easy densification with its low-cost processing make them superior candidates in many applications such as automotive engines, high-performance bearings, wear components, and gas turbine blades. In addition, the existence of two interstitial crystallographic sites in their crystal structure gives the opportunity to accommodate some rare earth element activators such as Eu2+, Yb2+, Ce3+, and Pr3+. These types of sialon-based materials have recently found a new promising application as a luminescent material for white light-emitting diodes. The aim of this review is to survey and provide a comprehensive look at the most relevant and significant publications regarding the development of sialons and their processing into both structural and luminescent materials. Such information forms a database that could enable scholars and engineers to tailor a final product derived from sialons with specific characteristics for a certain application. This review article should be of concern to engineers and scientists interested in the development and utilization of sialons for structural and wLEDs applications.


Introduction
Alumino-silicate oxynitrides (sialons) are solid solutions of Al 2 O 3 and Si 3 N 4 . There are two main representative members for sialon materials, βand α-phases. The former is the low-temperature phase that has elongated grains and exhibits good thermal conductivity and high fracture toughness but relatively low hardness [1]. The latter is the high-temperature phase that reveals equiaxed grains with good wear resistance and high hardness but low fracture toughness and thermal conductivity. In general, sialon ceramics are superior hard materials with comparable or even better high-temperature properties than silicon nitride ceramics. In addition, the other properties of sialon are comparable with that of silicon nitride ceramic from mechanical, thermal, and electrical points of view (as shown in Table 1). Nevertheless, they have better sinterability and offer advantages of low-cost fabrication and easier densification compared to silicon nitride ceramics. Moreover, the machining of sialon-based ceramics into complex industrial components is costeffective and much easier compared to the machining of silicon nitride ceramics. Accordingly, sialon-based ceramics are the spotlight of current investigations as low-cost and outstanding alternatives to the currently used silicon nitride ceramics and metal alloys for several harshenvironment industrial applications. On the other hand, rare-earth (Re) doped sialon materials have recently attracted considerable attention as good phosphors for wLEDs application due to the multiple characteristics provided by sialon host materials over the conventional oxide-based host materials. These characteristics originated from the rigid structure of sialon hosts and the covalent bonding nature between the activators (rare-earth element) with the ligands in sialon host [2]. Accordingly, this review article provides an overview for the most relevant and significant publications regarding development of sialon-based materials and their processing into structural and luminescent materials. Information reported about the crystal structure, properties, and synthesis of different sialon phases and their respective applications have been reviewed and discussed.

Definition
Alumino-silicate oxynitrides (sialons) are solid solutions of Al 2 O 3 and Si 3 N 4 , which are typically formed by partial substitution of Si-N bonds by Al-O bonds in a Si 3 N 4 hexagonal crystal structure. There are four representative members for sialon materials, β-, α-, X-, and O-phases.
Gunchenko et al. [12] have made-up single-phase βsialon sintered ceramics with optimum physicochemical and mechanical properties using solid-state process starting from powder mixtures prepared from Si 3 N 4 -AlN-Al 2 O 3 system with compositions of Si 6-Z Al Z O Z N 8-Z (Z = 0-4.2). In [13], crystal structure and lattice parameters of the four β-sialons synthesized using selfsustained high-temperature synthesis were precisely studied using a neutron diffractometer; the results of neutron diffraction patterns agreed well with the model structure of β-Si 3 N 4 material with unit cell values in the range of a = 0.76072-0.77004 nm and c = 0.29274-0.30003 nm based on the value of Z = 1-4, matching with P63 space group. Early studies on resistance of two β-sialon materials to oxidation in the temperature range of 800-1300 °C showed that residual β-Si 3 N 4 as well as a glassy phase affects negatively on oxidation resistance of β-sialons, and the oxidation of sialons is a diffusion limited process [14]. In another study, it was shown that the optimal characteristics of β-sialon ceramics were displayed by a single-phase structure; this study also verified that mechanical properties of β-sialon ceramics are highly dependent on their structure, phase composition, and the existence of the intergranular glassy phase. β-Sialon ceramics prepared by slip casting process in reference [15] demonstrated about 600 MPa bending strength, 4.1 MPa.m 0.5 fracture toughness for Z = 0.5 to 1; and the mechanical properties have significantly degraded for Z values within the range from 2 to 4. β-Sialon-based ceramics have recently received an increasing attention from materials scientific community for radomes utilized for high-speed missile applications. β-Sialon-based ceramics with a wide-range composition (Si 6-z Al z O z N 8-z ; z = 0-4.1) provide identical characteristics to the typical radome materials (fused silica, Si 3 N 4 , Si 3 N 4 /BAS and Si 2 N 2 O) (see Table 1), together with their high capability to be sintered in the full-dense structure using pressureless sintering preserving the nearnet-shaped radome structure [16][17][18][19][20]. Conversely, Si 3 N 4 and Si 3 N 4 /BAS display poor sinterability and their densification using pressureless sintering is impossible without using huge amount of certain sintering aids that deteriorate their characteristics and lead to structure failure. Therefore, the latter materials are generally densified using hot isostatic pressing that disturbs and destroys the near-netshaped radome structure. In addition, fused silica is the ideal material for radome applications; however, its inferior strength (40)(41)(42)(43)(44)(45)(46)(47)(48)(49)(50) and limited temperature withstanding capability (>800 °C) restrict its use for only certain low-speed radome applications. Besides, synthesis of Si 2 N 2 O with consistent properties is not an easy task because it is typically synthesized via calcination of α-Si 3 N 4 in atmospheric air; this reaction is quite sensitive to humidity, powder particle size, and temperature fluctuation of the furnace. In this sense, β-sialonbased ceramics are still the superb materials that can be easily consolidated into the near-net-shape dradome structure for high-speed missile applications. Photograph of β-Si 4 Al 2 O 2 N 6 proto-type Table 1. Radome materials and their main properties [16]. radome structure sintered by pressureless sintering process at 1675 °C for 4 h under N 2 atm is shown in Figure 1.

α-Sialon
On the other hand, high-temperature α-phase reveals equiaxed microstructure with good wear resistance and high Vickers hardness (1900-2100 kg/mm 2 ), but low fracture toughness (3)(4) MPa m 0.5 ), flexural strength (350-500 MPa), and thermal conductivity (8.16-8.22 W/mK). α-Sialon is synthesized by the addition of Al 2 O 3 into α-Si 3 N 4 with small amounts of yttria or rare-earth oxides. Numerous reports have shown that α-sialon powders can be prepared by heating carbon-containing powder mixtures of SiO 2 -Al 2 O 3 -CaCO 3 [21], SiO 2 -Al 2 O 3 -metal (Ca or Y) [22,23], clay metal compounds [24], and talc (Mg 3 Si 4 O 10 (OH) 2 )halloysite clay minerals [25,26]. α-Sialon has the general formula M x Si 12-n Al n O n N 16-n , where M is the metal cation present in the sintering aid, which is used as a stabilizer of α-phase structure that can transform at high temperatures into β-phase in the presence of liquid-phase oxynitride glass [1], which forms typically at the grain boundaries due to reaction of alumina and sintering aids with SiO 2 formed on the Si 3 N 4 powder surface [27]. Sintering aids used for α-phase stabilization are typically oxides of Li, Ca, Mg, Y [28] and most rare earths such as Nd, Sm [29], Dy, and Yb [30] (excluding La, Pr, Ce, and Eu), x = n divided by the valency of the M cation. The metal cations present in the sintering aid mainly control the stability of α-phase; hence, selecting a sintering aid material is related to the capability to stabilize the sintering aid. Solubility limit of stabilizing cations in α-sialons and also thermal stability of the latter materials increase with the decrease of the cation radius following the sequence of Nd ≈Sm> Y > Yb. According to this study, M-α-sialon ceramics (M = Nd, Sm, Y, Yb) were heated at 1250 °C for 20 h; Ybdoped-α-sialons showed the best oxidation behavior, and different stabilizing cations were found to have a negligible effect on mechanical properties of α-sialon ceramics.

X-sialon and O-sialon
, and they belong to the orthorhombic space group (CTC21) with lattice parameters varying based on the prepared composition: a = 0.88807(13) -0.89254(5) nm, b = 0.54965(4) -0.54988(7) nm, and c = 0.48550(6) -0.48596(2) nm. The latter phase exhibits the highest oxidation resistance among all sialon phases; more importantly, low-dense O-sialon ceramics are typically applied as electromagnetic transparent materials due to their light weight, low dielectric constant, and dielectric loss [33]. More information about the physicomechanical properties of O-sialons can be found in reference [34].

Crystal structure of α-and β-sialons
According to a study of the literature, the two sialon polymorphs have a hexagonal structure with (Si,Al)(O,N) 4 tetrahedra of different orientation along the C axis. α-Sialons have the same α-Si 3 N 4 structure that is built upon [(Si, A1)(N, O) 4 ] tetrahedra, and the modifying cations are partially accommodated in the interstitial positions in the (Si, AI)-(N, O) network [35] ( Figure 2). Each (A1, Si) atom in α-structure is tetrahedrally coordinated by (O, N) atoms, and every (N, O) atom has three (A1,Si) atoms as closest neighbors. Each modifying cation is bound to seven (N/O) atoms, and the average M-(N, O) bond lengths are almost the same, about ~0.26 nm, when M = Y or Ca. The (Si, A1)-(N, O) bond lengths lie between 0.17 and 0.18 nm with a mean at 0.176 nm, which is a bit larger than the average Si-N bond length in α-Si 3 N 4 (0.174 nm). The pretty small difference in the bond length values among Si 3 N 4 and sialons proposes that Al and O atoms do not separate appreciably in any of the Si and N sites, respectively. Besides, the cell volume of α-sialons is always a bit greater than that of α-Si 3 N 4 due to the replacement of a small number of Si-N bonds by A1-N bonds [36]. In the meanwhile, the proposed crystal structure of M 2 SiAlO 5 N β-sialon is shown in Figure 3 [37]. β-Sialon is a framework of corner-sharing (Si,Al)(O,N) 4 tetrahedra

Sialon properties dependence on its microstructure and phase composition
So far, the vast majority of commercially available sialons is the β-phase [38]. In β-sialons, lower viscosities of oxynitride glass formed occasionally during its sintering of β-sialons allows easier densification but deteriorate its mechanical and chemical properties at high temperatures [1], reducing high-temperature strength and creep resistance. This consequently limits the practical maximum operating temperature of β-sialon ceramics to approximately 1000 °C. In comparable, the cations of sintering aids used to stabilize α-structure is often interstitially incorporated into the solid solution, leading to reducing volume of intergranular glassy phase [1]. Besides, oxynitride glasses with improved creep resistance and higher viscosity are formed by rare-earth cations with small ionic radii such as Dy, Er, Lu, and Ho [27]. Accordingly, the mechanical properties of α-sialons at elevated temperatures retain better, and correspondingly the maximum operating temperature of these compounds is high (up to 1400 °C). Further, improving the fracture toughness and flexural strength of α-sialon could be achieved by producing a bimodal equiaxed-acicular microstructure by using elongated seed crystals in the starting powder or carefully selecting the starting composition and control the sintering conditions [39,40]. Ultimately, α-sialon materials can be applied in corrosive environments and for applications requiring high temperature and strength since they retain desirable properties at elevated temperatures such as hardness, creep resistance, strength, corrosion and oxidation resistance, wear resistance, thermal expansion with low coefficient, and good thermal shock resistance. On the other hand, the thermal conductivity of single-phase β-sialon was found to have value in the range from 13.5 to 19.7 W/mK. These values decline linearly with rising α-sialon content in the douphase sialon composite. This decline follows the following equation: K = 12.46-0.043 f. This equation only applied for douphase sialon composite, where f is wt.% of α-sialon phase; K is the thermal conductivity of the douphase composite. The thermal conductivity (K) value of the single-phase αsialon is typically ~8.16 W/mK, which is almost half that of β-sialon phase. The low K value of α-sialons can be primarily ascribed to the high degree of defects and lattice asymmetry of the latter sialons caused by incorporation of stabilizing cation and further substitution of Al-O bonds by Si-N ones. This in turn makes a significant decline in the mean free path length of the phonons and result in reducing the K value of αsialon compared to that of β-polymorph [38].

Disadvantages of sialons
Currently, sialons are one of the most important advanced structural materials since they have a good combination of properties including high dimensional stability, high-strength at elevated temperatures, good wear and corrosion resistance, low weight density, high hardness, good resistance to thermal shock resistance, and high elastic modulus. In spite of that, they suffer indelibly from defects of a rather weak fracture toughness (3-8 MPa m 1/2 ) [1,[41][42][43]. Because of their low fracture toughness, reliability of sialons is not yet adapted to production commitment. Several  techniques have been developed and many challenges have been overcome to manufacture strengthened and toughened sialon-based products. These techniques included fabrication of α/β-sialon composites [41,[44][45][46][47], fiber-reinforced sialons using high quality carbon fiber and/or SiC fiber [48][49][50][51][52], particle/whiskerreinforced sialons [53]. The best method to enhance the fracture toughness of sialons was the preparation of a sialon matrix composite using a reinforcement material. Sialon-reinforced composites exhibited improved toughness with better reliability, reduced crack propagation, and even more resistance to failure.

Solid state reaction
The solid-state reaction method is applied by mixing and blending of activators, host crystal with highpurity materials, and fluxes. The mixture is then fired under an appropriate atmosphere. Afterward, the fired mixture is crushed, milled, and sieved to remove the excessive crushed and large particles. The surface of the product might undergo treatments in some cases. Reactants used for the synthesis of sialons by the solidstate process included carbonates (e.g. CaCO 3 , Li 2 CO 3 ), oxides (e.g. Al 2 O 3 , Y 2 O 3 ), nitrides (e.g. Si 3 N 4 . AlN), metals (e.g. Al, Si), and carbides (e.g. SiC) [54][55][56][57][58][59][60][61][62][63][64][65][66]. Among the synthetic methods used for sialon production, the solid-state method is the most suitable and cost-effective approach for mass-producing sialon powders at large scale. This method was also found to be the most adequate technique for producing near-net shaped sialon-based ceramics with high mechanical properties [54]. On the other side, the main drawbacks of this process are related to the formation of coarse particles with low surface area, the inhomogeneity of the product, and the appearance of some defects that are harmful to luminescence [67]. The use of solid-state method for the synthesis of sialon in an atmosphere of N 2 or N 2 /H 2 mixture requires extreme temperature conditions, sometimes exceeding the thermal stability limit of nitrides. This synthesis method is a multistep process, which is limited by many flaws such as long synthesis time, high sintering temperatures, and contamination of the final product during pulverization of the strong sintered briquettes.

Microwave synthesis method
The microwave synthesis method is depending on the quick and uniform heating over the whole sample since the energy of the microwave is absorbed immediately by the sample. There are two microwave apparatus available; the conventional microwave oven that has a multimode microwave such as the ceramicssintering furnaces and domestic microwave ovens, and the other apparatus is the custom-tailored one that has a single-mode microwave and its energy focused to a small area. Reactants used for preparation of sialon-based materials by microwave technique include (Clay-carbon-Nitrogen Montmorillonite), (Boehmite-silica-SiC-carbon-Nitrogen) and (Si 3 N 4 + Al 2 O 3 ) [68]. Microwave synthesis of ceramic materials is an easy, facile technique for processing broad range of new ceramic materials of wide applications for the future. The advantage of the microwave heating techniques is represented in the saving of energy and the short time of processing, which reduce the manufacturing cost. Microwave synthesis requires short sintering time because the microwave heat is deposited in the sample core; that is why shorter time is demanded to sinter the entire sample than would be demanded to diffuse the energy from the exterior. In addition, a specific component in the mixture can be heated, and hence, the microwave method can develop a new synthetic material [56]. That is why, the prospective advantages of microwave approach over conventional solid-state methods for ceramic materials processing include more-uniform heating, high product yield and uniformity, unique or enhanced texture and microstructure, better characteristics of the product, greater energy efficiency, large-scale production using smaller plant size, and the capability to prepare new functional materials. What is more important, the rapid sintering process leads to final product with smaller particle size and enhanced mechanical properties. In spite of the advantageous application of microwave synthesis in ceramic sintering, the perceived output of this technology has not been largely realized at the production scale.

Combustion method
Combustion process or self-propagating hightemperature synthesis (SHS) is a wet-chemical synthesis method in which repeated heating and further calcinations of materials are not required. This method is an exothermic reaction that occurs by evolution of light and heat, which leads to crystallization and formation of phosphor materials. This method is timesaving and energy-efficient process for synthesizing various industrial ceramic materials. It is a selfsustaining process that exploits the heat evolved from the continuous exothermic reaction. That is why, no external energy or heat is demanded, apart from the ignition energy. This combustion technique required a mixture of fuel and oxidizers that ignited to start the combustion [56]. This method is a good alternative to the conventional sintering methods, owing to its potential advantages including (1) short sintering time, (2) high purity of the final sialon products, (3) simplicity of the procedure, and (4) lowenergy consumption. For the SHS to be self-sustaining, the combustion process should be associated with high-temperature exothermic reaction with adiabatic temperature T ad ≥ 1800. T ad is a thermodynamic parameter that expresses the product temperature under adiabatic conditions as a result of the heat evolution from the reaction. T ad is calculated by the following equation: ò where n i is the number of moles of the product (i), C ip expresses the specific heat capacity of the product (i), and ΔH o r denotes the reaction enthalpy. Actually, the adiabatic temperature is commonly higher than the combustion temperature because of the heat losses that occurred during the combustion process to the surroundings. T ad gives a good estimate to the reaction temperature, and it gives an indication whether or not combustion process can continue via the self-propagating regime [69][70][71][72][73][74][75]. Reactants utilized for the synthesis of sialons by the combustion process included Si, Al, SiO 2 , Si 3 N 4 , AlN, Y 2 O 3 , kaolin, Al dross, CaCO 3 , CaO, MgCO 3 , and SrCO 3 [76][77][78][79][80][81][82][83][84][85][86].

Gas reduction and nitridation
Nitride and oxynitride phosphors can be synthesized with gas reduction and nitridation (GRN) process, which is a cheap and an effective method that involves commercially available oxides as the starting powders. The reaction of this method is proceeding in an alumina or quartz tube furnace and with NH 3 -CH 4 or NH 3 gas flows, which acts as nitriding and reducing agents. There are many parameters that control the purity of nitride phosphors phase after the GRN reaction, such as the firing temperature, gas flowing rate, heating rate, holding time, and post-annealing. Hence, controlling these processing parameters can produce highly efficient phosphors with a narrow particle size distribution and small particle size (1 ~ 2 μm) [57].

Carbothermal reduction and nitridation
Carbothermal reduction and nitridation (CRN) process has been used extensively to produce nitride ceramic powders, including aluminum nitride, silicon nitride and β-sialons. Such a method was also used to produce multinary nitride and oxynitride luminescent materials, such as Ca-α-sialon: Eu 2 . Synthesis of Ca-α-sialon: Eu 2+ includes reactants as Al 2 O 3, Si 3 N 4 , CaCO 3 , Eu 2 O 3 , and carbon (graphite), and the purity of the phase is depending on the firing temperature, holding time, heating rate, post-annealing, and the starting composition (e.g. carbon/oxides ratio). The stoichiometric excess of carbon has a role in the success of the CRN process since it can enhance the dispersion of the powder, raise the reaction rate, manage the aggregation of the powder, and allow the transformation to complete easily. The rate-determining step in the CRN process includes the solid-solid reaction between oxides and carbon, and the rate-controlling mechanism includes the availability of carbon. It is important to remove the excess carbon in the prepared nitride phosphors after CRN process because it reduces the luminescence and absorption of the phosphor powder itself. One of the common ways to remove carbon is firing the powder at temperatures above 600 °C in an oxidizing atmosphere. However, the oxidation of phosphor powders that occurred by this method decreases the phosphors luminescence. Another method to remove residual carbon is the annealing of the phosphor powders at high temperature in a carbon-free furnace and under NH 3 or N 2 atmosphere [57]. CRN process has many disadvantages; for example, it may cause concurrent formation of SiC and contamination of carbon [67,87]. In addition, the precise control of the product phase composition is difficult because the process is associated with the emissions of greenhouse gas CO 2(g) and CO (g) , which have different partial pressures. Furthermore, the large linear shrinkage complicates the control of the product dimension and shape. Hence, the products from CRN process are generally very fragile, and as a result, their compaction is very low.

Aluminothermic nitridation reaction
Aluminothermic nitridation reaction (ARN) process can be achieved by wet blending and calcining of the calculated amounts of aluminosilicate and Al under N 2 gas flow. The equations that explain the reactions are as follows: First, aluminum melts according to Eq. [1]; thereafter, Eqs. [2, 3 & 4] is a generalization of the reaction, where m, n, l, and x are stoichiometric coefficients, as well as "A" is a mixture of oxygenated products, which is totally different from the dehydrated phase [88]. Nitrogenous phases (Si-Al-O-N) and α-Al 2 O 3 are formed by the reaction of aluminum nitride, transition alumina, and silicon according to the simplified Eq. [5& 6]. The aluminothermic reduction and nitridation process is a preferable choice to obtain high pure sialon, in which the reducing agent used in the process is aluminum, which is reactive much more than C or SiC (that used in CRN) and can be cost-effectively extracted from many waste resources such as coal fly ash [89] and aluminum dross [88]. Compared to carbon coke or carbon black, aluminum is more expensive, but the free-carbon removal process and the release of heat in the nitridation process of aluminum make it more attractive to synthesize sialon composites [90]. Different starting materials used in the preparation of different types of sialons via various synthesis methods together with the most important results were concluded in Table 2 [7,15,. Such information could be used as a database for the material scientists who are interested in dealing with sialon-based materials.

Limitations of metal alloys
Nowadays, the demands for materials that can work in more aggressive environments than ever before with stronger, harder, more heat-and wear resistant features and with reasonable cost, have been escalating in various industries. For example, current aircraft engines require increasing the ratios of thrust-to-weight, which is normally accomplished by both weight lessening and turbine inlet temperatures enhancement, where the required operating temperatures of modern jet engines exceed the temperature limitations imposed by metallic turbine components. Besides, the reduction of energy consumption and the greenhouse gas emissions are the greatest challenges for the transport sector over the next decades. In this context, it has therefore become essential to seek for super-hard lightweight inert materials as alternatives to the currently available metal alloys. In fact, non-oxide engineering ceramics (like silicon nitride ceramics) have recently attracted the attention, and they have become accepted in some industries.

Si 3 N 4 ceramics and their limitations
Silicon nitride (Si 3 N 4 ) is a superior non-oxide engineering ceramic that reveals extreme strength and toughness at elevated temperatures. Early orientation to Si 3 N 4 for high-temperature structural applications is related to its covalently bonded structure and the adamantine characteristic of two electrons per Si-N bond. These bonds with their 3-D linked network gave rise to materials with outstanding thermal shock resistance, superb hardness, high Young's modulus, and excellent refractoriness in addition to a relatively low weight of the final product [192]. Si 3 N 4 material is twice as hard as tool steel and 60% less in weight than steel. It does not deteriorate at high temperatures due to its superior thermal shock resistance and good resistance to creep, wear as well as oxidation. Accordingly, it has been widely used in the applications that require a unique material with elevated strength, high fracture toughness, and low wear properties such as automotive engines, gas turbine blades, high-performance bearings, and glow plugs [193][194][195][196][197] ( Figure 4). Nevertheless, some disadvantages of silicon nitride ceramics deteriorate their market expansion and motivate the search for alternatives. One of these disadvantages is the superplastic machining of the silicon nitride ceramics into complex-shaped components. This process is time-consuming and cost-ineffective because of the low strain rate (10 −4 S −1 ) and high superplastic deformation temperature (1600°C) of the silicon nitride ceramics [198]. In addition, the poor sinterability of the silicon nitride makes its densification an energy consuming process that requires harsh temperature conditions (1700-1900°C) and huge amounts of sintering additives, such as rare earth oxides or yttria [199,200]. These additives, on the other hand, react with part of the nitride itself forming M-Si-O-N liquid phase (M is yttrium or rare-earth metal), which solidifies into a glass at the grain boundaries. This glassy grain-boundary phase significantly affects the developed silicon nitride-based ceramics as it deteriorates its high-temperature mechanical and chemical properties. As a result, subcritical crack growth, creep, oxidation, and other lifetimedetermining properties are significantly affected, which is leading to accelerate the material destruction [201] (see Figure 5).

How to solve the limitations of Si 3 N 4 ceramics
The promising way to solve the aforementioned disadvantages of silicon nitride ceramics is to partially substitute Si-N bonds by Al-O bonds in a Si 3 N 4 hexagonal crystal structure forming solid solutions of Al 2 O 3 and Si 3 N 4 that are so-called alumino-silicate oxynitrides (sialons). Beside the fact that sialons have properties comparable or even better (in some properties) than silicon nitride, they have better sinteriability, low-cost fabrication, and easier densification compared to silicon nitride ceramics. The appearance of alumina in the starting mixture facilitates formation of Si-Al-O-N liquid phase having lower viscosity, which eventually enhances material densification [192]. Moreover, the lower superplastic deformation temperature of sialon (1200 °C) with its higher strain rate (10 −2 S −1 ) compared to that of silicon nitride (1600°C and 10 −4 S −1 , respectively) make their machining into complex shape much easier and cost-effective [198]. These advantages of sialon-based ceramics with the fact of its structure reliability and stability under harsh environmental conditions make them outstanding alternatives to the currently used silicon nitride ceramics in various industrial applications.  [55] Temp. effect on the mechanism of α-sialon formation The main factor that considerably affected the rate of αsialon formation was the sintering temperature. Oxygen conc. and small variation in the composition of precursors did not affect the reaction rate ‡   [112] Effect of hot pressing on characteristics of produced sialon ceramics Two-step hot pressing process gave rise to highly dense Nd-α-sialon ceramics consisting of elongated grains with high aspect ratio embedded in finer equiaxed grains ⁑ (Continued)      [182] Effect of corrosion on the prepared β-sialons β-Sialon prepared by combustion synthesis followed by spark plasma sintering could be severely corroded in supercritical water than in air and steam atmosphere Combustion-synthesized β-sialon demonstrated more than 86% purity, whereas SPSed powders attained more than 99% of the theoretical density and higher purity

⁑ &
Si + Al + CaCO 3 + Y 2 O 3 + NaCl + MgCl 2 [187] Effect of metal chlorides on the α/β ratio and morphology of sialon products NaCl additive gave rise to high α/β ratios and equiaxed grains, whereas MgCl 2 resulted in low α/β ratios and elongated structure ⁑ Si + AlN + Al 2 O 3 + ZrO 2 [188] Effect of ZrO 2 addition on produced sialons Highly-dense β-sialon ceramics have been produced with addition of ZrO 2 that promoted nitridation effect and acted as sintering aid for encouraging densification of β-sialon composites ‡   [237] Effect of mechanical activation on the reaction activity of raw materials Yttrium-stabilized α-sialon has been prepared where the reaction activity of metallic particles has elevated after being mechanically activated ⁑ Si + Al + SiO 2 [238] Effect of mechanical activation using ball mill on β-sialon synthesis Fine-grained β-sialon was combustion synthesized at mild conditions using waste Si material for costeffective procedures ⁑ Si + Al + Al 2 O 3 [239] Effect of temperature on oxidation of elongated β-sialon material Elongated β-sialon materials have displayed outstanding resistance to oxidation and the crystal shape was well retained up to 1373 K. † Si + Al + SiO 2 + KCl + MgCl 2 + CaCl 2 [240] Effect of metal chlorides (KCl, MgCl 2 , and CaCl 2 ) on β-sialon synthesis Single-phase composition of β-sialon has been obtained with different morphologies using different chlorides; in addition, the size of β-sialon rod-like crystals became smaller with addition of KCl and they became coarser in presence of MgCl 2 or CaCl 2 ⁑ SiO 2 + AlN [241] Effect of milling time on the formation of βsialon Nanocrystalline β-sialon was prepared with high-energy ball milling after sintering at 1450 °C while and milled-powders for a short time required much higher sintering temperature † Si + Al + SiO 2 [242] Effect of starting composition on the oxidation kinetics of β-Si 6−z Al z O z N 8−z s (z = 1, 2, and 3) materials Oxidation kinetics have followed a parabolic behavior, and the variation from that was found to increase with the decrease of the z-value. The oxide products formed on β-sialon contained silica and mullite.  [248] Effect of β-sialon addition as crystal seed on the formation of sialon powder Ultrafine β-sialon powder was formed and enhanced by adding small amounts of β-sialon as crystal seed, which had accelerated the formation of final β-sialon product and lower its formation temperature ⁞ Si + Al + SiO 2 [249] Effect of starting composition on combustion-synthesized sialon powders β-Sialon has been prepared via combustion synthesis under a low N 2 pressure ⁑ FeSi 75 alloy + α-Al 2 O 3 [250] Effect of alumina addition on physicomechanical properties of Fe-sialon ceramics Fe-sialon ceramic matrix composite has been developed with better resistance to erosion wear compared to Al 2 O 3 ceramic † Si + Al + SiO 2 + NaCl [251] Effect of the amount of NaCl on sialon synthesis NaCl diluent acted as a diffusion barrier and prevented grain growth of β-sialon particles, resulting in formation of submicron-sized β-sialon crystals ⁑ †: Gas pressure sintering; ‡: Pressure-less sintering; ⁑: combustion synthesis method; ⁂: Self-propagation high temperature; microwave sintering method; ⁞: carbothermal-reduction-nitridation; Spark plasma sintering; ⁑: Hot-pressing sintering; ⁞: Gas-reduction nitridation; †⁑: Aluminothermic nitridation method; ‡⁞ ‡⁞: Silicothermic nitridation method; : Slip casting; Reaction sintering; ⁎⁑⁎: Freeze casting; ⁑⁑⁑: Mechanical milling; ⁞: Ammonolysis.

Recent trends and future prospect in sialon usage for structural applications
Duplex α + β sialons-derived ceramics have achieved the utmost collection of mechanical properties, in between those of α-and β-monolithic sialons [29], where hardness and thermal shock resistance escalate and toughness as well as thermal conductivity diminish with the increase of α-phase percentage in the final sialon composite [1]. Also, fully crystalline ceramic or glass ceramic can be formed through crystallization of the oxynitride grain boundary glass achieved by postannealing of sialon ceramics, which in turn improve elevated temperature properties. Complete devitrification of oxynitride grain boundary glass have raised the maximum operating temperature of β-sialon from ~1000 °C to 1400 °C [27]. Moreover, N content enhancing in the oxynitride glass increases its viscosity, glass transition temperature, elastic moduli, and hardness via increasing the crosslinking in this glass, leading to service temperature improvement [27]. Further, fabrication of graded sialon ceramics enriched at their surfaces by α-sialon would, respectively, take the advantage of high hardness of α-sialon and outstanding toughness of βsialon at the surface and in the core of the fabricated ceramics (See Figure 6). This graded sialon ceramics will open the door wide for their utilization in the harshenvironment applications that require superior hardness and toughness such as automotive engines, highperformance bearings, wear components, cutting tools, gas turbine blades, and glow plugs. Physical lamination approach was found to be the best way to manage the thickness of α-sialon layer at the surface of the graded sialon ceramics depending on the targeted application [4,202]. Accordingly, sialon-based ceramics have recently attracted an increasing attention as low-cost and outstanding alternatives to the currently used silicon nitride ceramics and metal alloys for the harshenvironment applications owing to their fast-growing commercial and technological significance.

Phosphor converted white LEDs
The major interest of ecofriendly products has increased utilization of light emitting diodes (LEDs) as light sources in communications, medical services, display backlighting, signage, and general lighting. Regarding general lighting, phosphor converted white LEDs (pc-WLEDs) Figure 4. The most important Si 3 N 4 products: (a) bearings [194]; (b) glow plugs [195]; (c) micro-gas turbine blades [197]; (d) blades for Aerospace industry [196] (reproduced from references [5][6][7][8]). have surpassed conventional lighting sources (e.g. fluorescent lamps and incandescent lamps) due to its low power consumption with high energy efficiency, long life up to 50,000 h, high thermal management, high mechanical impact resistance, wide range of controllable color temperatures (4500 K-12,000 K), ultra-highspeed response time (micro-second-level on-off switching), and wide operating temperature range (−20°C to 85°C) [87,88]. The pc-wLEDs can be generated by three different methods ( Figure 7): (1) by using a yellow emitting phosphor that can be excited through a blue emitting diode (di-chromatic approach), (2) by using RGB (red, green, blue) phosphor stimulated by an ultraviolet (UV) LED or blue LED chip (tri-chromatic approach), (3) by mixing three colors of phosphor (red, yellow, green) (RYG) in the use of blue LED chip (tetra-chromatic approach). Despite of the high luminous efficiency of dichromatic pc-wLED, it has a low color rendering index (CRI) (<80) [54], which poorly render the colors of the objects. On the other hand, the tetra-chromatic pc-wLED with its excellent CRI (>90) has a lower luminous efficiency than that of the di-and tri-chromatic white light sources. In comparable, the high luminous efficiency and good color rendition can present in the trichromatic pc-wLED.

Phosphors
Phosphors are materials that emit light as a result of absorbing energy. They are composed of the host lattice such as CaWO 4 , YTaO 4 , Y 2 O 3 , and Y 3 Al 5 O 12 and the activator such as Eu 3+ , Tb 3+ , Ce 3+ , or others. The activator is a dopant that makes substitutional defects in the host structure. They are commonly transition metals, main group metals, which are actually rare earth (RE) ions from lanthanides or a collection of alkaline earth metals, alkali metals, and otherwise unclassified metals. Ions activator with f-d or d-d electron configurations are the best choices for phosphors as they have the ability to emit a broad-band and visible light under the effect of crystal field and nephelauxetic effect, which is a phenomenon that occurs when a decrease of the electronic transition energy meets an increase of the covalency [55,56]. Classification of phosphors can be according to either their emission colors (i.e. red-, blue-, and green-emitting phosphors) or according to their chemical components [55]. In addition, some of phosphors are based on borates, nitrides, silicates, and phosphates.

Nitride phosphors
Among various types of phosphors, nitride compounds are characterized with their great diversity of crystal structures and variation of local structures that surrounding the activator ions, which can change the centroid shift and the splitting of crystal field of 5d energy levels of rare earth ions and lead to numerous releases of colors from nitride phosphors. Nitride phosphors can withstand against chemical or thermal attacks due to the structure stability of nitride host lattices against these conditions, which results in small degradation or thermal quenching of nitride phosphors. Nitride phosphors with their promising photoluminescent properties can be used in white LEDs as down conversion luminescent materials and form a new and important family of phosphor materials for lighting [57]. Among various kinds of nitrides, rare-earth (RE) doped-sialon materials have attracted significant attention as good phosphors for wLEDs due to the presence of two interstitial crystallographic sites in the crystal structure of sialons that could accommodate the rare earth activators such as Eu 2+ , Yb 2+ , Ce 3+ and Pr 3+ . In the meanwhile, sialon host materials provide two extra merits over the conventional oxidebased host materials: (1) low excitation energy of the 5d electrons of activators due to the strong nephelauxetic effect (i.e. electron cloud expansion) originated from the covalent bonding nature between the activators with the ligands in sialon host; (2) low thermal quenching probability due to the rigid structure of sialon hosts with high Debye temperature, where the formal charge of N (−3) is higher than that of O (−2) and the covalent bond of Re-N is more stiff than the of Re-O. Furthermore, the high chemical and thermal stabilities of sialon phosphors is owing to their host lattice, which built on stiff frameworks consisting of Si-N or Al-O tetrahedra. In addition, a small thermal quenching and high conversion efficiency of  phosphors occurs as the Stokes shift in a strong lattice becomes smaller [2]. Therefore, many nitride phosphors have been commercialized such as Eu 2+activated β-sialon, α-sialon, and CaAlSiN 3 and played an important role in the production of reliable and highly efficient white LED product.

Sialon-based Yellow Phosphors
Ce 3+ or Eu 2+ activator ions with α-sialon host lattice primarily generate yellow radiations. The luminescence of Ce 3+ and Eu 2+ derives from 5d-4f transitions and is red-shifted from the blue/green to the yellow region due to the shift of 5d states to lower energy levels because of the stronger interactions between host materials and activator ions. In addition, electric dipole transitions are allowed by 5d-4f transitions, and hence, the emission from Eu 2+ and Ce 3+ is relatively bright, and these strong transitions are suitable for pc-WLED applications [5]. The large ionic radius of europium eliminated the formation of pure europium α-sialon, but the co-doping of yttrium or calcium allows the incorporation of europium into the α-sialon structure. Ca-Eu-sialon phosphor chromaticity can be managed by altering the composition. Combining blue LED chip with various Ca-Eu-sialon phosphors can produce white LEDs of color temperatures ranging from 1900 to 1300 K [59]. Fine yellow α-sialon: Eu 2+ phosphors have been prepared by Li et al. [203] using gas-reduction-nitridation method. The mentioned study has involved CH 4 -NH 3 mixture gas as a reduction-nitridation agent and homogeneous mixture of the Ca-Eu-Al-Si-O system to obtain Eu 2+ -doped Ca-α-sialon phosphors with a target composition of Ca 0.925 Eu 0.075 Si 9 Al 3 ON 15 prepared by a sol-gel process. According to Sakuma et al. [204], a yellowish orange high-efficiency Ca-Eusialon ceramic phosphor has been developed with composition of Ca 0.875 Si 9 h. Nitrogen-rich Ca-α-sialon: Eu 2+ phosphors were produced by Jian-Jun [206] using CaAl alloy, AlN and Si 3 N 4 powders as starting materials using solid state reaction. The obtained phosphors characterized by their intensive orange emission and the emission wavelengths tend to shift toward the red region with increasing concentration of Eu. Michalik et al. [207] synthesized yellowish phosphor of Ca,Eu-α-sialon through solid-state reaction in a carbon-resistant furnace with flowing nitrogen and discussed the influence of Eu 2+ concentration on the quantum efficiency and emission spectrum of the phosphor. It was indicated that doping with europium has affected the crystal lattice parameters and that Eu with up to 6 mol% has increased the unit cell volume. Sialon phosphors dispersed variously according to the glass matrix type, and thus Segawa et al. [208] have investigated the reaction between sodium borosilicate glasses with a composition of xNa 2 O-(60 − x) B 2 O 3 -40SiO 2 (x = 2-30 mol%) and a Ca-α-sialon:Eu 2+ phosphor powder prepared by gas-pressure sintering based on the glass structures. Eu 2+ -doped Ca-α-sialon yellow phosphor with molecular formula of Eu 0.05 Ca 0.75 Si 9.6 Al 2.4 O 0.8 N 15.2 was prepared by Sopicka-Lizer et al. [209] through a pressure-less solid state reaction route, where an improvment in the quantum yield of these phosphors by 10% was achieved by applying Al 2 O 3 or AlF 3 as the additional source of the liquid phase at crystallization temperature. The optical properties of Ca-α-sialon: Eu phosphors have been modified by Sakuma et al [210] by partial substitution of Ca by other elements as yttrium, where the correlated color temperature of white LED lamps can be shifted to the lower temperature region when the (Ca,Y)-α-sialon: Eu is combined with a blue LED chip. The sintering behavior of yellow-emitting Eu 2+ -doped Ca-αsialon and its photoluminescence (PL) characteristics were studied by Young-Jo Park et al. [211], and they deduced that the effect of the sintering conditions on the PL intensity of the synthesized sialon is related to Eu concentration in both the sialon crystal and the interfacial glass phases. The microwave sintering method was applied to prepare Ca-α-Sialon: Eu 2+ phosphor, LiHong et al. [212]. This method enabled to reduce the synthesis temperature and avoided the sintering-like agglomeration phenomenon that is usually observed in gas-pressure sintering preparation method. The salt-assisted combustion synthesis method was used by Genkiet al. [213] to prepare Ca-α-sialon: Eu 2+ , which had showed a superior thermal stability by using Eu 2 O 3 , CaO, Al, and Si as raw materials, while Si 3 N 4 and NaCl were applied as diluents to modulate the temperature of combustion.

Sialon-based Green Phosphor
Green emitting phosphors can be applied for solid state lighting (SSL) with the excitation of a near ultraviolet (NUV) or a blue LED so that they turn radiation in the ranges 380-410 or 450-480 nm to light centered in the green region (520-565 nm  [216] to fabricate green-emitting phosphor in glass materials for high-power blue laser lighting. Using the solid-state reaction, Ce 3+ , Tb 3+ -codoped β-sialon phosphors were prepared by Yang et al. [217] who indicated that the obtained phosphor have a broad excitation band that matches well with that of a UV LED chip, and adjusting the amount of Tb 3+ ions can modulate the emission color of the obtained phosphor from blue to green due to the energytransfer from Ce 3+ and Tb 3+ ions. Mixing and firing of Si 3 N 4 , AlN, CaCO 3 and Yb 2 O 3 in a N 2 atmosphere has also been applied by Dierre et al. [218] to obtain several Ca-and Yb-doped sialon green phosphors. Jing Niu et al. [219] have applied a new method for preparation of Eu-doped β-Sialon green phosphors in which inexpensive raw materials as SiO 2 , Al and Si were used in addition to a small amount of NaCl as a diluent. The process has been applied under a relatively low nitrogen pressure and produced a single-phase, uniform, and rod-like particles with smallest thermal quenching of PL emission efficiency for high-power or white LEDs. Lihong Liu et al. [220] have used the solid-state reaction method to obtain Yb 2+ -activated Si 6−z Al z O z N 8−z (0.05 ≤ z ≤ 2.3, 0.03 mol% ≤ Yb 2+ ≤ 0.7 mol%) green phosphors, at 1900°C for 2 h under a nitrogen pressure of 1.0 MPa, indicating that βsialon:Yb 2+ phosphor exhibits green emission with an intensity controlled by the chemical composition of the host lattice and the concentration of Yb 2+ . Another green oxynitride phosphor, with a nominal composition of Eu 0.00296 Si 0.41395 Al 0.01334 O 0.0044 N 0.56528 (β-Sialon:Eu 2+ ), has been studied by Naoto Hirosaki et al. [221] who affirm that the prepared phosphor at 1900°C in a nitrogen atmosphere is superior to the commercially available green phosphors YAG:Ce 3+ and ZnS:Cu,Al. This phosphor revealed, respectively, the internal and external quantum efficiencies of 70% and 61% at λ ex = 303 nm. The same green oxynitride phosphor was successfully prepared by Ryu et al. [222] using gas pressure sintering at 2000°C under a nitrogen pressure of 0.92 MPa, concluding that enhancing the Eu 2+ concentration could control the emission bandwidths of the synthesized phosphor. A new route was used by Qi Wang et al. [223] to synthesize Eu-doped β-Sialon green phosphors through a self-propagation high-temperature synthesis (SHS) method that is a highly efficient and resulted in obtaining strong green-light emission that could be excited using UV or blue LED.

Sialon-based red phosphor
Red-emitting materials can be also applied for SSL upon excitation of a NUV or a blue LED; therefore, they must be able to turn efficiently radiation in the ranges 380-410 or 450-480 nm to light centered in the red region (625-740 nm). Such a conversion is not easy to achieve in an efficient way since the most common activator ions in many hosts are Eu 2+ and Ce 3+ , which weakly emit in the red region. Host materials that can achieve efficient red emission based on 5d-4 f excitation and emission transitions should have (1) a strong ligand field, lowering the energy of the lowest (emitting) level belonging to the 4 fn-15d1 configuration and/or (2) the centroid of the 4 fn-15d1 configuration shifted to low energy (nephelauxetic effect, due to increased covalency). The aforementioned points occur only in a few hosts, as referred hereafter. The substitute is using activator ions that emit in the red region, exploiting intraconfigurational transitions. For instance, Mn 4+ and Eu 3+ provide luminescence through 4 f-4 f and 3d-3d transitions, respectively. Such transitions are intraconfigurational and parityforbidden through the electric dipole mechanism; therefore they are weak, which is not suitable for phosphor performance [58]. Translucent Mg-α/βsialon doped with different rare earth oxides (REOs), i.e. Pr 2 O 3 , Gd 2 O 3 , and Eu 2 O 3 , were fabricated by Joshi et al. [224] by hot press sintering, where Mg 2+ cation acts as  [225]. Pr 3+ -doped β-sialon red phosphor was also prepared by gas pressure sintering under 1950°C for 2 h by Tzu-Chen Liu et al. [226] where an excitation with 460 nm blue light has achieved red luminescence in the range 600-650 nm, showing that the phosphor can be excited by blue InGaN light-emitting diodes (LED). Table 3 summarizes the previous work on sialonbased phosphors with all data about activator mole ratio, phosphor color, synthesis method and excitation as well as emission ranges.

Future prospect and conclusion
Based on this review, high-quality α-and β-sialon powders could be easily produced by carbothermal reduction-nitridation (CRN) using low-cost secondary resources, kaolin deposits and kaolinite. In addition, functionally graded sialon ceramics reinforced at their surfaces by α-sialon in order to take advantage of high hardness of α-sialon at the surface could be promising alternative candidates to the currently available silicon nitride-based blades that are used in the steam turbines used for power production from combined heat. What's more, the remarkable resistance to thermal shock and oxidation at elevated temperatures has made composite sialon a promising precursor for solar receiver applications [227]. More importantly, there are three RYG-sialon-based blue-excited phosphors with superior thermal stability and high quantum efficiency; Sr-containing sialon: Eu 2+ red-emitting phosphor; Ca-α-Sialon: Eu 2+ yellow-emitting phosphor [204,228] and β-Sialon: Eu 2+ green-emitting phosphor [203]. Therefore, it is strongly recommended to combine these thermochemically stable RYG-sialon-based phosphors with the commercial blue LED chip to realize a warm wLED that will find a suitable application for general lighting and as a backlight in large-scale LCDs for flat-panel television sets or for in-vehicle use. Based on this review, the suggested packaging structure for a highly efficient sialon-based RYG-blue-pumped wLED could be plotted as shown in Figure 8, where blue-LED die could be stuck on the bottom of the metal cup. The RYG-sialon-based powder phosphors will be well-distributed in the wax and put inside the cup to transmit part of blue photons to red, yellow and green photons. A hemi-sphere silicon lens will be attached to the top surface of the phosphors in order to boost the light extraction. Information about sialon phase properties and their potential applications, have been reviewed. Crystal structure, synthesis methods and final product properties and applications have been discussed to a greater extent. From the above study, the following concluded remarks can be drawn: (1) The incorporation of both alumina and alumina plus metal cation into silicon nitride to produce α and β sialons do not deteriorate the superior mechanical properties of the parent silicon nitrides because basically the same structure was reserved. Besides, the enhanced densification that resulted from the lower viscosities and eutectic temperatures of M-Si-Al-O-N liquids compared with  Table 4 [4,28,29,38,229,230]. The best bending strength and fracture toughness were exhibited by single β-phase Z = 0.5, and the highest oxidation resistance in the range of 800-1300°C was demonstrated in a single-β-sialon phase with a stoichiometric Si 5.5 Al 0.5 O 0.5 N 7.5 .
(3) Single-phased α-sialon-derived-ceramics revealed 3.5-4.8 MPa.m 0.5 fracture toughness, 19-21 HV, and their resistance to oxidation depends on the stabilizing cation and its ionic size (Yb-α-sialon revealed the highest oxidation resistance). (4) β-Sialon ceramics exhibit a quite high thermal conductivity value, and such value decreases linearly with rising α-sialon content in the double-phase sialon composite, following K = 12.46-0.043 f equation. (5) In spite of the vast majority of commercially available structural sialon ceramics that are β-phase-derived products, α-sialons provide several desirable properties over β-sialons at elevated temperatures including higher maximum service temperature (1400 °C), higher creep resistance, and better thermal shock resistance. These valuable hightemperature characteristics of α-sialons will open the door wide for their potential use in a wide range of high-temperature, highstress structural applications in corrosive environment. (6) Sialon ceramics with specific characteristics can be potentially tailored by controlling both microstructure and phase composition of the final product. Table 4. Properties of α-and β-sialon-based ceramics.