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-Si 3N 4-ALN-AL 2O 3-SiO 2. 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: Si 3N 4, Si, AlN are active, which contributes to SiALON formation at relatively low temperature, at 1250 °C - 1300 °C. Results- β-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. Probably this should be connected with transition of the reactively baked structure of the hot-compressed material into compact structure. Obtained materials are used in protecting jackets of thermo couples used for melted metal temperature measuring (18 - 20 measuring) and for constructions used for placing objects in factory furnaces.
Ceramic that contains various phases in the system Si-Al-O-N is called SiAlON. SiAlONs belong to the simonyite class [
There are several types of SiAlONs: α; β; X; O1; H; R [
Pology clay chemical composition (mass%): SiO2-47.92, Al2O3-35.20, Fe2O3-2.06, CaO-0.40, MgO-0.30, K2O-2.00, Na2O-0.50, loss at heating-12.24; fire-resistance 1710˚C - 1730˚C.
name | chemical formula | structure type |
---|---|---|
α | Mex(SiAl)12(ON)16 X = 0 ÷ 2 | α-Si3N |
β | Si6-X AlxN8-X X = 0 ÷ 4.2 | β-Si3N4 |
O1 | Si2-XAlXO1+XN2-X X = 0.04 ÷ 0.4 | Si2ON2 |
X | Si2-XAl1-XOxN1-X x = 0.04 ÷ 0.2 | 3Al2O3∙2SiO2 |
H | SiAl3O2N3 SiAl5O2N5 | AlN |
R | SiAl4O2N4 SiAl6O2N6 | AlN |
Name | geopolymer-kaoline Prosiannaia (Ukraine) | geopolymer Pology clay fire proof (Ukraine) | Al | SiC | Si | Perlite (Armenia) | MgO | Y2O3 |
---|---|---|---|---|---|---|---|---|
CN-7 | 13.9 | 4.63 | 23.15 | 27.78 | 25.00 | 2.78 | 0.92 | 1.8 |
Chemical composition of kaoline (mass%): SiO2-46.45, TiO2-0.33, Al2O3-38.70, Fe2O3-0.46, MgO-trace, CaO-0.36, Na2O-0.45, K2O-0.60, loss at heating 13.63; fire-resistance-1770˚C.
Optimal phase composition of cutting instrumental SiAlONs is: 75% SiAlON and 25% glassy phase. We added glassy perlite of Aragats (Armenia) to the mix, up to 3 mass%. Glassy perlite consisted of 96% glassy phase and the rest are structural water and gasses. Perlite undergoes dehydration at 860˚C; the structure becomes friable which contributes to intensification of diffusion processes and to the creation of conditions for creation of new phases.
According to the X-Ray diffraction pattern of CH-7 that was obtained by reactive sintering at 1450˚C (
The data of micro-structural study of CH 7 composite (
Materials selected for obtaining composites: kaoline, Pology clay, aluminum nano-powder, SiC, Si, perlite, and additives:Y2O3, MgO, after weighing (batching) were grinded in porcelain ball-mill. To receive the mold the mix was pressed under hydraulic press at 20 MPa. The molded specimens were dried on
air for 24 hr and then in a drier at 110˚C. It was sintered in silite furnace in nitrogen medium at 1400˚C - 1500˚C; hold-time at final temperature was 30 min. 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
We investigated physical-technical characteristics of a sample that was hot- compacted at 1620˚C. The obtained results are given in
The value of computed fragmentation factor of the material is given in
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 [
According to Anstis [
composite index | open porosity w, % | general porosity, P, % | density, ρ, g/cm3 | compression pressure, MPa. | ultimate compressive strength, σc, MPa. | ultimate bending strength, σb, MPa. |
---|---|---|---|---|---|---|
CH-7 (1600˚) | 0.28 | 3.10 | 3.17 | 30 | 1910 | 470 |
composite index | Kic, MPa. | fragmentation factor, B, MPa. | n?factor |
---|---|---|---|
CH-7 (1620˚) | 5.54 | 4.56 | −2.44 |
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 [
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
Indentations on SiAlON matrix given on
Indentation readings taken on the border of a matrix and grain are rather interesting. Length of indent diagonal on the
Test mode | Load-unload | ||
---|---|---|---|
Sample name | SiAlON-zv | Sample No. | #1 |
Test force | 200.000 [gf] | Minimum force | 0.200 [gf] |
Loading speed | 1.0 (7.1448 [gf/sec]) | Hold time at load | 5 [sec] |
Hold time at unload | 3 [sec] | Test count | 21 |
Parameter name | Temp | Parameter | 20 |
Comment | 21.06.17-SiAlon-zv-200; DHV5-3 | ||
Poisson’s ratio | 0.190 | ||
Cf-Ap, As Correction | ON | Indenter type | Vickers |
Read times | 2 | Objective lens | 50 |
Indenter elastic | 1.140e+006 [N/mm2] | Indenter Poisson’s ratio | 0.070 |
SEQ | Fmax | hmax | hp | hr | DHV-1 | DHV-2 | Eit | Length | HV | Data name |
---|---|---|---|---|---|---|---|---|---|---|
[gf] | [um] | [um] | [um] | [N/mm2] | [um] | |||||
1 | 200.710 | 4.7107 | 1.9264 | 3.1017 | 442.157 | 2643.803 | 7.211e+004 | 15.792 | 1492.537 | SiAlON-200(2) |
2 | 200.786 | 4.2612 | 1.6795 | 2.7414 | 540.546 | 3479.868 | 8.707e+004 | 14.621 | 1741.886 | SiAlON-200(4) |
3 | 200.800 | 4.9636 | 1.7638 | 3.3296 | 398.419 | 3155.263 | 6.588e+004 | 16.959 | 1294.659 | SiAlON-200(5) |
4 | 200.674 | 4.5307 | 1.7788 | 3.0421 | 477.884 | 3100.234 | 8.083e+004 | 15.644 | 1520.484 | SiAlON-200(6) |
5 | 200.675 | 4.3294 | 2.1587 | 2.9575 | 523.381 | 2105.199 | 9.024e+004 | 15.498 | 1549.415 | SiAlON-200(7) |
6 | 200.662 | 3.5295 | 1.5855 | 2.1773 | 787.444 | 3902.198 | 1.254e+005 | 16.595 | 1351.275 | SiAlON-200(8) |
7 | 200.661 | 3.6147 | 1.8441 | 2.4494 | 750.723 | 2884.448 | 1.349e+005 | 17.179 | 1260.907 | SiAlON-200(9) |
8 | 200.738 | 3.0333 | 1.1085 | 1.7530 | 1066.516 | 7985.353 | 1.660e+005 | 12.866 | 2248.651 | SiAlON-200(10) |
9 | 200.959 | 2.8595 | 1.0929 | 1.5884 | 1201.396 | 8224.728 | 1.857e+005 | 12.134 | 2531.125 | SiAlON-200(11) |
10 | 200.866 | 3.0653 | 1.3375 | 2.0446 | 1045.024 | 5488.768 | 1.924e+005 | ----- | ----- | SiAlON-200(12) |
11 | 200.737 | 3.1154 | 1.3372 | 2.0317 | 1011.028 | 5488.160 | 1.790e+005 | ----- | ----- | SiAlON-200(13) |
12 | 200.960 | 2.5787 | 1.1425 | 1.5447 | 1477.302 | 7525.888 | 2.536e+005 | 12.135 | 2530.738 | SiAlON-200(14) |
13 | 200.923 | 2.7215 | 1.1113 | 1.5055 | 1326.134 | 7952.513 | 2.077e+005 | 11.989 | 2592.358 | SiAlON-200(16) |
14 | 200.501 | 2.8549 | 1.0966 | 1.5509 | 1202.544 | 8150.998 | 1.824e+005 | 12.135 | 2524.953 | SiAlON-200(17) |
15 | 200.497 | 3.4966 | 1.3136 | 2.2145 | 801.640 | 5679.626 | 1.320e+005 | ----- | ----- | SiAlON-200(18) |
16 | 200.702 | 2.9626 | 1.1801 | 1.6771 | 1117.798 | 7044.719 | 1.729e+005 | 12.428 | 2409.746 | SiAlON-200(19) |
17 | 200.589 | 3.4541 | 1.4444 | 2.0858 | 821.888 | 4700.234 | 1.288e+005 | 14.474 | 1775.634 | SiAlON-200(20) |
18 | 201.195 | 3.0666 | 1.0307 | 1.5932 | 1045.886 | 9257.288 | 1.515e+005 | 11.698 | 2726.384 | SiAlON-200(21) |
Average | 200.757 | 3.5082 | 1.4407 | 2.1882 | 890.984 | 5487.183 | 1.449e+005 | 14.143 | 1970.050 | |
Std. Dev. | 0.174 | 0.738 | 0.346 | 0.611 | 324.195 | 2330.548 | 52250.109 | 2.028 | 548.126 | |
CV | 0.087 | 21.043 | 23.994 | 27.907 | 36.386 | 42.473 | 36.057 | 14.341 | 27.823 |
crack length on the grain is-5.75 µM. Crack spreading in matrix is not observed. Indentation diagonal length on
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 (
after it is detached, is developed by 2000 m/sec speed and at this time material resistance is determined not only by the speed of crack shock on matrix, but also by Kic value.
Dynamic micro-hardness (DH) is determined by indenter load value and depth of indentation in the material in the process of testing. Its significance is computed by the formula: DH = a × F/h2; where “a” is a constant value and depends on indenter form; for Vicker’s indenter it equals to a = 3.8584.
Advantage of the method over the common, static method, that is, measuring of linear sizes of an indentation (diagonal) is that it contains plastic as well as elastic components. Results of measuring don’t depend on indentation sizes, loads and non-homogeneity of elastic restoration.
Dynamic hardness was determined in load-unload regime, till elastic relaxation started, by taking seven readings per each concrete load tested, by discarding two extreme values and by averaging the remaining five values. The value of micro-hardness was determined mechanically. Hold time at maximum load equaled to 5 sec, at the end of unload―3 sec (
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 (
Test mode | Load-unload | ||
---|---|---|---|
Sample name | SiAlON-zv | Sample No. | #1 |
Test force | 100.000 [gf] | Minimum force | 0.200 [gf] |
Loading speed | 1.0 (7.1448 [gf/sec]) | Hold time at load | 5 [sec] |
Hold time at unload | 3 [sec] | Test count | 23 |
Parameter name | Temp | Parameter | 20 |
Comment | 20.06.17-SiAlON-zv-100; DHV5-3 | ||
Poisson’s ratio | 0.190 | ||
Cf-Ap, As Correction | ON | Indenter type | Vickers |
Read times | 2 | Objective lens | 50 |
Indenter elastic | 1.140e+006 [N/mm2] | Indenter Poisson’s ratio 0.070. |
SEQ | Fmax | hmax | hp | hr | DHV-1 | DHV-2 | Eit | Length | HV | Data name |
---|---|---|---|---|---|---|---|---|---|---|
[gf] | [um] | [um] | [um] | [N/mm2] | [um] | |||||
1 | 100.753 | 2.0927 | 1.0353 | 1.3623 | 1124.606 | 4595.143 | 2.023e+005 | 12.133 | 1269.108 | SiAlON-100(1) |
2 | 100.862 | 2.1408 | 1.1973 | 1.4454 | 1075.849 | 3439.729 | 2.028e+005 | 10.673 | 1641.878 | SiAlON-100(2) |
3 | 100.954 | 2.1185 | 1.0085 | 1.3472 | 1099.608 | 4852.203 | 1.911e+005 | 11.989 | 1302.427 | SiAlON-100(3) |
4 | 100.844 | 2.1300 | 0.9980 | 1.3526 | 1086.598 | 4949.256 | 1.881e+005 | 11.623 | 1384.295 | SiAlON-100(4) |
5 | 100.935 | 2.1822 | 1.1183 | 1.4290 | 1036.181 | 3945.265 | 1.855e+005 | 12.721 | 1156.721 | SiAlON-100(5) |
6 | 100.624 | 2.0945 | 1.0240 | 1.3135 | 1121.301 | 4691.482 | 1.921e+005 | 11.843 | 1330.428 | SiAlON-100(6) |
7 | 100.551 | 2.1229 | 1.0193 | 1.3350 | 1090.715 | 4731.042 | 1.868e+005 | 11.551 | 1397.624 | SiAlON-100(7) |
8 | 100.826 | 2.1357 | 1.0016 | 1.3362 | 1080.626 | 4912.610 | 1.834e+005 | 11.550 | 1401.679 | SiAlON-100(8) |
9 | 100.826 | 2.1173 | 0.9846 | 1.2881 | 1099.473 | 5084.458 | 1.815e+005 | 11.404 | 1437.730 | SiAlON-100(9) |
10 | 100.825 | 2.1761 | 1.0974 | 1.4160 | 1040.858 | 4092.733 | 1.848e+005 | 11.697 | 1366.620 | SiAlON-100(10) |
11 | 100.807 | 2.1566 | 1.0491 | 1.3859 | 1059.580 | 4477.130 | 1.857e+005 | ----- | ----- | SiAlON-100(11) |
Average | 100.801 | 2.1334 | 1.0485 | 1.3646 | 1083.218 | 4524.641 | 1.895e+005 | 11.718 | 1368.851 | |
Std. Dev. | 0.120 | 0.029 | 0.064 | 0.049 | 28.966 | 502.835 | 7155.469 | 0.529 | 125.730 | |
CV | 0.119 | 1.372 | 6.141 | 3.617 | 2.674 | 11.113 | 3.777 | 4.518 | 9.185 |
depth at 1 N load (100 g). We can conclude that optimal load for such composition material is 1 N.
In this case, hardness according to Vickers equals to 13.68 GPa. Average indentation depth −2.13 µM; dynamic hardness DHV = 10.83 GPa. E = 189 MPa. Indentation diagonal −11.71 µM. The table and graphical material shows that SiAlON matrix is homogeneous and its properties, irrespective of readings taken from various spots of matrix, are not characterized by fluctuation (
To compute mechanical module of material we used Kovziridze’s module [
M = K v o l ⋅ E ⋅ K i c ⋅ P d K m ⋅ G v o l ⋅ P v o l ⋅ P m MPa / μ M 2
where, Kvol. is crystalline phase volume in the material, in %; E―elasticity module-MPa; Kic―critical stress intensity coefficient; Pd―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
indentation picture № | indentation diagonal length, a, µM | half indentation diagonal a/2. µM | crack mean length ℓ, µM | sizes of SiC grain with indentations, µM | note |
---|---|---|---|---|---|
14 | 12.135 | 6.067 | 10.40 | A-50.8 B-28.8 | indentation on the referred size grain |
16 | 12.428 | 6.214 | 10.40 | A-54.8 B-25.6 | -„- |
17 | 14.474 | 7.237 | 8.20 | A-33.3 B-20.8 | -„- |
average | 9.67 | ||||
3 | 16.959 | 8.479 | 5.75 | A-17.24 B-10.34 | indentation on the interface of the referred size grain and matrix |
12 | 12.135 | 6.067 | 5.22 | A-17.24 B-17.24 | -„- |
13 | 11.989 | 5.994 | 11.71 | A-20.64 B-27.59 | -„- |
average | 7,56 | ||||
2 | 14.621 | 7.310 | cracks are not fixed | indentation on matrix | |
4 | 15.644 | 7.822 | 7.00 | -„- | |
7 | 17.179 | 8.589 | 6.80 | -„- | |
average | 4.50 |
pore size in matrix-µM. Module dimension MPa/ µM2. The formula doesn’t consider Griffith’s [
Electron-microscopy (
secondary (SEI) as well as in reflected (BES) electrons by the use of 20 kW accelerating voltage. In some cases, to decrease surface load, specimens were coated with approximately 10 nm thickness Pt layer by a device for vacuum coating JEC-3000 FC of the Japanese company JEOL.
Electron microscopy morphological figures offer porous phase composition in our material, at various magnifications.
Results of crystalline phase analysis are given in
indentation picture # | vision area S, µM2 | number of counted pores, n | largest pores Dmax. µM | smallest pores Dmin. µM | Average pores Dmid. µM | pores composition, % |
---|---|---|---|---|---|---|
27 | 1100 | 9 | 2.2 | 0.4 | 1.60 | 2.85 |
30 | 1600 | 18 | 2.8 | 0.8 | 1.90 | 2.35 |
average | 27 | 1.75 | 3.10 |
indentation picture № | phase name | vision area S, µM2 | number of counted grains, n | largest grain Dmax. µM | smallest grain Dmin. µM | aver. grain Dmid. µM | phase composition, % |
---|---|---|---|---|---|---|---|
17 | SiC | 1740 | 85 | 33,05 | 2.70 | 4.80 | 26.8 |
13 | SiC | 3225 | 300 | 23.00 | 2.75 | 4.90 | 27.6 |
aver. | 4.85 | 27.2 | |||||
5 | SIALON | 35500 | 250 | 19.60 | 5.50 | 8.60 | 53.0 |
32 | SIALON | 8200 | 200 | 24.30 | 5.80 | 7.50 | 59.8 |
26 | SIALON | 1400 | 220 | 21.70 | 5.30 | 8.50 | 59.3 |
aver. | 8.20 | 56.4 | |||||
5 | Al2O3 | 35500 | 60 | 2.6 | 1.27 | 1.93 | 5.7 |
grain average size, general | 5.00 | Kristal phase composition in matrix, general % 89.3 |
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
M = 89.3 × 145 × 5.54 × 0.9 / 5.0 × 7.6 × 1.75 × 3.1 = 313 MPa / μ M 2
As it was stated above this formula doesn’t provide for Grifiths’s defects [
Electric characteristics of the material obtained by hot compression are given in
Relation “lgρ-Τ” is linear and for materials obtained by A and B versions are presented as parallel lines (
specimen # | method of sample making | electric resistance at 298 К-, lgρ, λ∙m | temperature coefficient of electric resistance, aΤ∙10−2, К−1 | conductivity activation energy, E, ℓV |
---|---|---|---|---|
A | hot-compression | 6.4 | 2.3 | 1.08 |
B | nitro-alum thermal synthesis | 8.4 | 2.3 | 1.08 |
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/µM2. 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 aΤ and E-values are identical, which refers to unalterability of a mechanism of current transmission (
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