Obtaining of SiAlON Composite via Metal-Thermal and Nitrogen Processes in the SiC-SiAl-Geopolymer System

Goal: obtaining of composite in the SiC-SiAlON system with the metal thermal method in the nitrogen medium. Method: SiALON-s are solid metal oxide solutions in nitrides. Area of their presence is considered in four-component system-Si3N4-ALN-AL2O3-SiO2. In the present paper SiALON-containing composite was obtained through alum-thermal process in the nitrogen medium on the base of Geopolymer (kaolin and pologycley-Ukraine), SiC, aluminum nano-powder and Si powder with small additives of perlite (Aragatz, Armenia) by the reactive baking method. The advantage of this method is that compounds, which are newly formed thanks to interaction going on at thermal treatment: Si3N4, Si, AlN are active, which contributes to SiALON formation at relatively low temperature, at 1250 ̊C 1300 ̊C. Results-ß-SiAlON was formed at the sintering of SiC-aluminium and silicium powder, geopolymer at 1450 ̊C. Porosity of carbide SiAlON composite obtained by reactive sintering, according to water absorption, equals to 13% 15%. The samples were fragmented in a jaw-crusher and were powdered in attrition mill till micro-powder dispersion was obtained. Then samples were hot-pressed at 1620 ̊C under 30 MPa pressure. Hold-time at the final temperature was 8 min. Sample water absorption, according to porosity, was less than 0.4%. Further studies were continued on these samples. Conclusion: the paper offers processes of formation of SiC-SiAlON composites and their physical and technical properties. Phase composition of the composites was studied by X-ray diffraction method, while the structure was studied by the use of optic and electron microscope. Electric properties showed that the specimen A obtained by hot-compression is characterized by 2 signs lower resistance than the porous material B, which was used to receive this specimen. How to cite this paper: Kovziridze, Z., Nijaradze, N., Tabatadze, G., Cheishvili, T., Mshvildadze, M., Mestvirishvili, Z., Kinkladze, V. and Daraxvelidze, N. (2017) Obtaining of SiAlON Composite via Metal-Thermal and Nitrogen Processes in the SiC-Si-Al-Geopolymer System. Journal of Electronics Cooling and Thermal Control, 7, 103-122. https://doi.org/10.4236/jectc.2017.74009 Received: November 27, 2017 Accepted: December 24, 2017 Published: December 27, 2017 Copyright © 2017 by authors and Scientific Research Publishing Inc. This work is licensed under the Creative Commons Attribution International License (CC BY 4.0). http://creativecommons.org/licenses/by/4.0/ Open Access


Introduction
Ceramic that contains various phases in the system Si-Al-O-N is called SiAlON.
SiAlONs belong to the simonyite class [1].Structural unit of SiAlONs is a tetrahedron (Si, Al) (ON) 4 similar to tetrahedron SiN 4 in silicon nitride and silicon oxynitride-SN 2 O. SiAlONs contain the structural types and phases, which are based on: aluminum nitride, apatites, silicon α and β nitride, silicon oxynitride, spinels and others.SiAlON-s can be obtained in neutral atmosphere at 1600˚C, by reactive sintering at 2000˚C or by hot-compression at 1750˚C or higher temperatures in the mixes: aluminum nitride and silicon, aluminum oxide and silicon oxide, silicon oxynitride as well as by lithium-aluminum or magnesium-aluminum spinels.Single phase SiAlONs can exist in relatively narrow region with the formula Si (6-X) Al X O X N (8-X) where X varies from 0 to 5. In the system of SiAlONs the Si 3 Al 3 O 3 N 5 , which is structurally close to silicon nitride and by its chemical properties close to aluminum oxide has been studied better than other SiAlONs.

Major Part
According to the X-Ray diffraction pattern of CH-7 that was obtained by reactive sintering at 1450˚C (Figure 5) by the use of by metalothermal and nitriding method, the main phase is ß-SiAlON.The composite, alongside with ß-SiAlON contains X-SiAlON, in insignificant amount.In the CH 7 composite matrix we observe carbide grains, which by their size exceed that of the just-formed silicon nitride grains [23].
The data of micro-structural study of CH 7 composite (Figure 6) are in conformity with the results of X-ray structural analysis, which shows that the matrix of this composite is ß-SiAlON.Ready product, after furnace cutoff was cooled together with a furnace in free regime.
To receive a hard product the composite CH-7 obtained by reactive sintering and nitro-alumino-therman methods was fragmented in a jaw crusher, grinded in a ball-mill for 8 hours and then in an attritor mill for 8 -10 minutes.
At hot compression, at low temperature, active process of crystallization, that is, growth of sintered substances doesn't start yet.This means that the sintered product will have fine-grain structure and high specific density.
Precursor for hot compression was prepared in a thermostat at 150˚C, it was cold-pressed twice under 12 -15 MPa and 20 -25 MPa; was hot-compressed at 1620˚C under 30 MPa, vacuum equaled to 10 −3 Pa, hold-time at final temperature 10 -12 min; sintering regime was as follows: 20˚C -500˚C 7˚C/min, 500˚C -1400˚C 150˚C/min, 1400˚C -1620˚C 10˚C/min; cooling 10˚C/min.Temperature regime of sintering is given on Figure 7.
We investigated physical-technical characteristics of a sample that was hotcompacted at 1620˚C.The obtained results are given in Table 3. Water absorption value of hot-compressed sample according to porosity is less than 0.3%, while the ultimate strength at compaction 1910 MPa.hardness = HV -19.70.
The data given by us in the table enable us to conclude that 1620˚C is a sufficient temperature for complete hardening of specimens.
The value of computed fragmentation factor of the material is given in Table 4.
Quantitative factor of fragmentation (B) was determined which was obtained on the basis of the value of experimentally defined micro-hardness and tension intensity critical coefficient (Kic): B = Hv/Kic.Low value of fragmentation implies low chances of catastrophic spread of cracks [24].
According to Anstis [25]   top in meters; E-Young's modul in GPa; V-micro-hardness according to Vickers, in-GPa.n-factor is an important parameter at mechanical processing of materials.This factor enables us to speak of easiness of machine processing of the material.N = 0.643 -0.122 Hv; ceramic and ceramic composite will be processed easily if it has positive n-value [24].In case of our material, cutting as well as mechanical processing for grinding is complex due to high hardness of material.It should be stated that while cutting by diamond abrasive discs we encountered resistance, which damaged some diamond grinders and this, quite naturally contributed to developing cracks in the material.It decreased mechanical properties of our material.It would be better to have the possibility to process by laser cutting.Dynamic micro-hardness and elasticity modulus of the obtained materials were determined according to the demands of International standard ISO-14577 by the dynamic ultra-micro-hardness tester DUH 211S, which is used for determination of mechanical characteristics (micro-hardness, elasticity module) of solid bodies.Results are given in Table 5.
Figure 8 offers composite's micro-mechanical characteristics.Images of indentations in matrix, at the interface of matrix and grains and on grains are given in Figures 9-11.
Figures 8-11 should be considered in one context.Results are offered in Table 5.Table 5 gives the results of tests carried out on the CH-7 specimen hot-compacted at 1620˚C, indentation reading is taken from matrix (Figure 9); indentation readings were taken several times and results are given in the Table according to which average hardness is HV: 19.70 GPa.Dynamic hardness DH-8.9 GPa, elasticity module E-145 MPa.
Indentations on SiAlON matrix given on Figure 9 shows that indentation form is sharp, with clearly cut edges.We don't see a crack along the edges on the Figure 9(a).On Figure 9(b) we observe a small, 6.6 µM size crack along the right edge of the indentation, while on Figure 9(c) we again observe a small 7.0 µM size crack along the right edge of the indentation, which speaks of homogeneity of SiAlON matrix and of its high relative density.
Indentation readings taken on the border of a matrix and grain are rather interesting.Length of indent diagonal on the Figure 10    Grain has just one crack.Figure 10(c)) shows that indentation length is 12 µM, while average length of a crack-11.71µM.Table 5 offers test results of SiC grains of the composite CN-7 microstructure.Indentation was made at 2 N loading on SiC grains.
Borders of indentations on carbide grains are sharp (Figures 11(a)-(c)); a crack which is formed at the indenter load on the grain doesn't spread beyond the grain limit.Matrix, because of its high mechanical properties and energy dissipation, subdues crack spreading and the composite strength is preserved.Such big size grains are rare (Table 8) and discussion about material mechanical properties should not be relevant, since increase of their dispersion grade is not a problem, while it gives interesting picture for the research of the issue.Especially interesting is a Figure 11(b).In this case a crack on the right side of the indentation is developing so intensely that it reaches matrix and colliding with the strong matrix, goes back, transects the grain diagonally and collides with the matrix from the other side of a grain, but fails to destruct it.It should be stated that the crack keeps its high energy and develops diagonally on the other side of SiC grain, but having lost its energy is unable to reach matrix.Crack which spreads from the lower edge reaches matrix, but energy dissipation in a grain and matrix Kic is so high, that a crack disappears at the matrix.Figure 11(a Indentation was made in the matrix of a specimen consisting of β-SiAlON.As a result of testing its mean hardness equaled to DHV = 8, 9 GPa which is a rather high value. From the load-unload relation diagram (Figure 8) we define the value of elasticity module by determination of hardness S = (dF/dh) h-h max -.It is a tangent of load-unload diagram at the starting point of unload.A device defines elasticity module of the material under the study and its mean value in case of our materi-al E = 145 GPa.(Figure 5).Indentation pictures are in full conformity with graphical data on Figure 8.As is seen from Figure 8(a) indentation reading taking lasted 78 seconds and 18 indentation readings were taken.Depth of every indentation varies and it varies from 2.5 to 55 µM.As we see 2N load is somewhat high than optimal load for this material.The same is evidenced at the application of test force (200 g approximately = 2 N) when we define indentation depth (Figure 8(b).In this case, again indentation depths for all 18 tests vary and they vary from 2.5 to 5 µM.For comparison we take Table 6 and Figure 12, which show that all indentations in SiALON matrix are almost of the same that SiAlON matrix is homogeneous and its properties, irrespective of readings taken from various spots of matrix, are not characterized by fluctuation (Table 7).

Mechanical Module of Materials
To compute mechanical module of material we used Kovziridze's module [26] [27]: where, Kvol. is crystalline phase volume in the material, in %; E-elasticity module-MPa; Kic-critical stress intensity coefficient; P d -pore dislocation factor in matrix, which was considered equal to 1 in case of homogeneous redistribution, 0.9-in case of non-homogeneous redistribution and 0.8-in case of pores coalescence.Km-average size of crystals in matrix-µM; Gvol-glassy phase composition in matrix, in %; Pvol.-volume of pores in matrix-in %; Pm-average pore size in matrix-µM.Module dimension MPa/ µM 2 .The formula doesn't consider Griffith's [28] cracks, dislocations in crystals, nano-defects in glass, but the formula gives us thorough impression about resistance of materials to external loads, which is approximated to the values computed for strength of bonds between atoms.This is namely why the elasticity module was inserted in the formula.
Electron-microscopy (Figure 13, Figure 14) and X-Ray structural analyses (Figure 15) were performed for phase analysis, while for computation of Kic, micro-mechanical properties were investigated, the results of which are   Electron microscopy morphological figures offer porous phase composition in our material, at various magnifications.Table 8 gives the results of pore analysis.
Pores are mainly rounded, but if there are no such pores, latitudinal and longitudinal data of pores are taken and the average diametric result is calculated.Total volume of closed pores reaches 3.1%.Through and halfthrough pores are not observed in the matix.According to morphological picture we can state that pore distribution in the material is between homogeneous and non-homogeneous.Therefore we considered that pore distribution factor in the matrix = 0.9.

Crystalline Phase Composition and Average Sizes
Results of crystalline phase analysis are given in Table 9, which shows that a rather big number of grains were counted of silicon carbide, as awell as SiAlON and aluminum oxide.It enables us to judge of compound elements of the crystalline phase of the matrix.SiAlON is approximately 57% of matrix, silicon carbide-approximately 27%, pores-approximately 3%.X-ray structural analysis fixed aluminum oxide reflexes, which probably were emitted mainly from the geopolymer; perhaps its small concentration was due to aluminum nano-powder, since nitrogen was not purified.We considered that its concentration was 6%.As to the glassy phase, perlite is completely glassy mass that undergoes melting at 1240˚C.
Evidently 3% perlite added to the composite forms eutectic melts with geopolymer ingredients, especially with alkali oxides, which contributes to the increase of concentration of glassy phase in the material.We considered that its concentration equals to 7.4%.
It is seen both from X-ray and electron-microscopy figures.Elasticity module, average, according to the Table 5 was defined-145 MPa.Thus formula of Kovziridze's module, in numerical expression will acquire the following form: As it was stated above this formula doesn't provide for Grifiths's defects [28] in the matrix, dislocations and other flaws in crystalline phases and nano-derangements in glassy phase.For the porcelain with 50% glassy phase, 25% mullite and 25% quartz crystalline phase, if we admit that elasticity module is approximately 75 MPa, Kic-3.5, and porous phase data and crystals sizes are the same as in our tables, the value of the given module will equal to 8.7 Mpa/ µM 2 .SiAlON ceramics is far stronger than the porcelain.

Electric Properties of the Material
Electric characteristics of the material obtained by hot compression are given in Table 10, while the resistance-temperature relation is given on diagram 16, where the studied material was marked by the index "A".The same table and diagram offer electric characteristics (marked by "B") of starting material obtained by nitro-alum thermal method; these data are taken from the reference [29].
Relation "lgρ-Τ" is linear and for materials obtained by A and B versions are presented as parallel lines (Figure 16); besides, a Τ and E values are identical, which refers to similar mechanism of current transfer (Table 10).Difference is fixed only in resistance values with the peculiarity that the hot-compressed sample "A" is characterized by 2 signs lower resistance than the material B, which was used to receive it.This should be associated with the transition of reactively baked structure of the hot-compressed material-to a dense, compact structure.

Conclusion
The composite was synthesized in the SiC-SiAlON system by the method of reactive sintering using metal-thermal and nitriding processes.According to the results SiAlON's creation commences at 1200˚C and the process progresses intensely in the 1350˚C -1450˚C range.Thus, we significantly reduced temperature of SiAlON synthesis; for reactive sintering-by approximately 550˚C and for hot-compression by approximately 130˚C, which was contributed greatly by glassy perlite additive.It is thanks to just formed imperfect crystalline lattice of silicon nitride, created at such low temperatures, which due to its relatively large hollow spaces receives alum oxide, aluminum nitride and silicon oxide.Then, at relatively high temperature, at 1350˚C -1450˚C it takes a form of ß-SiAlON structure.Mechanics at bending equals to 470 MPa, while at compaction 1910 MPa.Micro-mechanical analysis showed that in most cases a crack in the SiAlON matrix is not created and if it is created it is of small size.
Similar result was observed at the interface of matrix-SiC grain.We computed the brittleness factor "B" of the material, which is not high.It shows low chances of swift spreading of a crack, while negative value of "n" factor refers to high hardness of the material to resist external mechanical loads at mechanical processing.The result was proved in the process of cutting specimens with diamond disks, when some disks were broken.High properties of the composite were confirmed when computing Kovziridze's mechanical module −313 MPa/µM 2 .The obtained results exceeded our expectations.The material was obtained through solid phase sintering.It is proved by relatively small concentration of glassy phase, which is less than 12%.Study of electric properties showed that a Τ and E-values are identical, which refers to unalterability of a mechanism of current transmission (Table 9).Difference is fixed only in resistance values with the peculiarity that the hot-compressed specimen A is characterized

Figure 1 .
Figure 1.Scheme of distribution of components in Si-Al-O-N system.

Figure 2 3 and
offers phase equilibrium graph, where phases are considered SiAlONs.Single phase area (β 1 SiAlONs) is spread along X composition (Al 2 O 3 AlN) + (1 − X) Si 3 N 4, where X = 0 ÷ 0.8, but the lines with the cation/anion ratio = 3/4.Creation of solid liquid of Al 2 O AlN in β-Si 3 N 4 doesn't need the presence of vacancies of inculcated cations and anions in crystalline lattice of β-Si 3 N 4 .At the increase of Al 3+ and O 2− concentrations we observe linear growth of β-Si 3 N 4 -lattice in solid liquid [14]-[21].Journal of Electronics Cooling and Thermal Control

Figure 10 .
Figure 10.(a)-(c) Indentation on the border of silicon carbide and matrix.

Figure 16 .
Figure 16.Changes in electric resistance in materials obtained by hot compression (a) and nitro-alum thermal synthesis (b) according to temperature.

Table 1 .
Names and structures of SiAlONs phases.

Table 7 .
Average sizes of indentations and cracks.

Table 9 .
SiC, Al 2 O 3 and SiAlON grain sizes and composition in matrix.Journal of Electronics Cooling and Thermal Control

Table 10 .
Effect of terms for material obtaining on its electric characteristics.