Physico-mechanical properties and microstructure of multi-phase ceramic composites based on zircon and dolomite mixtures

Abstract

Four composites containing zircon and dolomite were designed by adding dolomite from 5wt% to 35wt% at the expense of zircon content. Densification parameters in terms of bulk density, apparent porosity and linear change were determined at different firing temperatures (1200°C–1400°C). Cold crushing strength of sintered composites, phase composition and microstructure were investigated. Samples contain 35wt% of dolomite and fired at 1200°C for 2 hours exhibited higher porosity (AP ∼ 51.25%) than other samples and can be used as porous ceramics. This is due to CO₂ evaporation through the thermal decomposition of dolomite. Dense ceramics can be obtained by adding 5wt% of dolomite and fired at 1400°C for 2 hours (bulk density ∼3.67 g/cm³ and apparent porosity ∼4.5%). Only zirconia and diopside crystalline phases were detected at composite containing 35wt% of dolomite and fired at 1400°C. Due to the liquid phase sintering process, the densification parameters of the sintered samples have been enhanced by increasing the temperature. The mechanical properties of the sintered samples were improved due to the transformation toughening mechanism of tetragonal zirconia. Microstructure and EDAX analysis of the sintered composites show the presence of sub-prismatic zircon and rounded zirconia crystals as well as irregularly dark diopside crystals.

Keywords

Ceramic matrix composites sintering microstructure characterization zircon dolomite

1 Introduction

Zircon (ZrSiO₄) has excellent thermal shock resistance and chemical stability combined with a very low coefficient of thermal expansion (4.1×10^–6 °C from room temperature to 1400°C) and low heat conductivity coefficient (5.1 W/m.°C at room temperature and 3.5 W/m.°C at 1000°C). High purity sintered zircon can retain its bending strength up to 1200–1400°C [1, 2]. These properties make zircon a potential candidate as a useful structural ceramic, especially in those areas where a sudden temperature change can occur [1, 3]. Zircon is widely used as an acid refractory in furnaces melting low-alkaline aluminoborosilicate glasses for the production of glass spheres and glass fibers. Zircon porcelain possesses outstanding dielectric and mechanical properties, and is used to make electrical insulators, spark plugs, etc [1]. The mechanism of zircon dissociation is influenced by the content of impurity. It has been shown that the temperature of the onset dissociation decreases with the content of impurities. Zircon dissociation products are zirconia and silica at temperatures above 1650°C; however, the presence of impurities such as TiO₂, FeO_x and/or alkali decreases the temperature of dissociation by about 200°C [4 –6]. The tetragonal zirconia resulting from zircon dissociation might be transformed into monoclinical zirconia during the cooling process, which is the stable phase at room temperature [6]. It is difficult to obtain fully dense zircon ceramics because of its refractoriness. The employed sintering aids like TiO₂ [7], SiO₂ [8] and Al₂O₃ [9] contribute to decrease the sintering temperature of zircon. Recently, Yi Ding et al. (2018) studied the preparation of dense zirconia/zircon (ZrO₂/ZrSiO₄) ceramics composite at low temperature 1400°C for 6 hours by a hydrothermal-assisted sol-gel process [10]. Dolomite is a mineral of anhydrous carbonate composed of calcium magnesium carbonate. Like other carbonates, dolomite is completely decomposed into a highly porous and reactive mixture of calcium and magnesium oxides and carbon dioxide when heated above 900°C [11] $CaMg {(C O_{3})}_{2} (s) \to CaO (s) + MgO (s) + 2 C O_{2} (g)$ (1)

The aim of this work is to investigate the effect of dolomite additions on the phase composition, physico-mechanical properties and microstructure of the zircon ceramics at different firing temperatures 1200°C, 1300°C and 1400°C.

2 Materials and experimental procedures

Table 1
Chemical composition of the starting materials, wt%

Oxides Zircon Dolomite

ZrO₂ 65.74 –

SiO₂ 34.01 0.43

CaO – 29.46

Al₂O₃ – 0.01

Fe₂O₃ 0.10 0.39

TiO₂ 0.15 0.01

MgO – 22.95

K₂O – 0.01

MnO – 0.10

Na₂O – <0.01

SO₃^- – <0.01

Cl^- – <0.01

L.O.I. – 46.42

Oxides	Zircon	Dolomite
ZrO₂	65.74	–
SiO₂	34.01	0.43
CaO	–	29.46
Al₂O₃	–	0.01
Fe₂O₃	0.10	0.39
TiO₂	0.15	0.01
MgO	–	22.95
K₂O	–	0.01
MnO	–	0.10
Na₂O	–	<0.01
SO₃^-	–	<0.01
Cl^-	–	<0.01
L.O.I.	–	46.42

Table 2

Batch Design of the investigated samples D1-D4 / wt%

Sample	Zircon	Dolomite
D1	95	5
D2	85	15
D3	75	25
D4	65	35

The starting raw materials used in this work were zircon (ZrSiO₄, D_0.5 = 1.0μm, Kreutzonit Super FF, Helmut Kreutz, Germany) and dolomite (CaMg(CO₃)₂, <90μm, Saudi Arabia). X–ray fluorescence spectrometer (AXIOS, WD-XRF Sequential Spectrometer PAnalytical, Netherlands) was used to know the chemical analysis of the starting materials and the results shown in Table (1). Four composites, D1-D4, containing zircon and dolomite were designed as illustrated in Table (2). Dolomite (5, 15, 25 and 35 wt.%) was added at the expense of zircon content. Each batch was dry mixed for 2 hours, followed by semi-dry uniaxially pressed at 150 MPa using 7 wt% of water. After drying the samples at 120°C for 24 h, samples were sintered at different temperatures 1200°C, 1300°C and 1400°C for 2 hours. Bulk density and apparent porosity of the sintered samples were determined using Archimedes method, ASTM-C20. The total linear change of the sintered samples was studied by length measurement before and after the sintering process. The cold crushing strength, ASTM C133, was evaluated using a hydraulic press model SEIDNR - Riedlinger (Germany). The phase composition of the sintered samples was examined using X - ray diffractometer (Philips 1730 diffractometer with a Ni-filtered Cu-Kα radiation at 1°/min scanning speed) and the diffraction peaks were detected using PCPDFWIN version 2.2, 2001, JCPDS-ICDD program. A scanning electron microscope (SEM; JXA-840A electron microanalyzer, JEOL, Japan) attached to an EDAX unit was used to observe the microstructure and chemical analysis points of the sintered samples. For SEM testing, the samples were coated with gold.

3 Results and discussion

3.1 Chemical analysis of the starting raw materials

Table 1 shows that the main zircon contents are zirconia (65.74 wt%) and silica (34.01 wt%) with traces of Fe₂O₃ and TiO₂. On the other hand, dolomite used in this study is considered pure ore, consisting of CaO and MgO at a percentage of 29.46 wt% and 22.95 wt% (as major oxide) as well as a small percentage of SiO₂ and Fe₂O₃ as impurities. The percentage of CaO and MgO in the calcined base was approximately 56 wt% and 43 wt%.

Fig. 1

XRD patterns of the sample D3 fired at different temperatures 1200°C–1400°C.

Fig. 2

XRD patterns of the samples D1-D4 fired at 1400°C.

3. 2 X-ray diffraction of the sintered samples

Figure 1 shows X-ray diffraction patterns of the sample D3 fired at different temperatures from 1200°C to 1400°C. Zircon (ZrSiO₄, PDF 75-1590) was observed at 1200°C as the main phase beside some of weak diffraction peaks of m- zirconia (baddeleyite: PDF 80-0966), t-zirconia (Tetragonal: PDF 81-1544) and clinoenstatite (MgSiO₃, PDF 84-0652). This is due to the partially dissociation of zircon to zirconia and silica and the thermal decomposition of dolomite to magnesia and calcia. Silica reacted with magnesia forms clinoenstatite phase (MgSiO₃), while calcium oxide and/ or some of magnesia participates in the partially stabilization of zirconia to form tetragonal zirconia. Only zircon, zirconia (baddeleyite and tetragonal phase) and diopside phase (Ca Mg (SiO₃)₂, PDF 75-0945) peaks were detected at 1300°C. Diopside phase resulted from solid state reaction mechanism between magnesia, calcia and silica. At 1400°C, the intensity of zircon peaks was decreased while the intensity of m-zirconia, t-zirconia clinoenstatite and diopside peaks were increased. Zircon dissociation increased from 1200°C to 1400°C by increasing the firing temperature. The intensity of tetragonal zirconia is lower than m-zirconia due to the grinding process of the samples to XRD analysis, which transforms from tetragonal zirconia into a monoclinical form (baddeleyite). Figure 2 shows the XRD patterns of samples D1- D4 fired at 1400°C. It was observed that zircon main peaks were detected in sample D1 alongside some of few peaks of zirconia. The intensity of zirconia peaks was increased while that of zircon was decreased from sample D1 to D4 by increasing of dolomite content. In sample D4, only zirconia (Baddeleyite and tetragonal zirconia) peaks are shown as a main phase in addition to weak diopside diffraction peaks. This is due to the enhancing of zircon dissociation by addition of dolomite. In samples D2 and D3, some weak peaks of diopside and clinoenstatit were also detected.

3.3 Densification parameters

Fig. 3

Bulk density of the samples D1-D4 fired at different temperatures 1200°C–1400°C.

Fig. 4

Apparent porosity of the samples D1-D4 fired at different temperatures 1200°C–1400°C.

Figure 3 shows the bulk density of the samples D1 –D4 fired at different temperatures. It was observed that the bulk density increased by increasing the firing temperature from 1200°C to 1400°C. This is due to enhancing of the zircon dissociation by increasing of the firing temperature and formation of both zirconia and diopside phases, which improves the solid state sintering and liquid phase sintering process. At 1200°C, the bulk density decreased by increasing of the dolomite additions from sample D1 to D4. This is due to CO₂ evaporation through the thermal decomposition of dolomite, which cases a decrease in the bulk density. At 1300°C, the bulk density appears to be steady by increasing of dolomite additions due to the solid state & liquid phase sintering process. Bulk density increased in the samples fired at 1400°C than the samples fired at 1300°C. This is due to the diopside phase beginning to melt (melting temperature = 1391°C) and forming the liquid phase that fills the pore spaces of the ceramic matrix and solidifies in it at cooling leads to the increase of the bulk density. By increasing of the dolomite additions from sample D1 to D4 at 1400°C, the bulk density was decreased. This is may be attributed to the reducing of zircon content from sample D1 (95 wt. %) to sample D4 (65 wt. %), which has a higher density (density of zircon ∼ 4.7 g/cm³) than other formed phases (density of diopside ∼ 3.38 g/cm³ and density of clinoenstatite ∼ 3.2 g/cm³). Sample D1 fired at 1400°C shows a higher bulk density (3.67 g/cm³) and a lower apparent porosity (4.5%) than other samples. Sample D4 was partially deformed at 1400°C. This is may be due to the volume and nature of the liquid phase formation at this sample (D4 fired at 1400°C)

Figure 4 shows the effect of dolomite additions and firing temperature on the apparent porosity of the samples D1-D4. It was observed that the apparent porosity decreased by increasing of the firing temperature from 1200°C to 1400°C due to the effect of solid state & liquid phase sintering process. However, the apparent porosity increases at 1200°C from sample D1 (40.23%) to D4 (51.25%) due to the effect of thermal decomposition of dolomite.

3.4 Linear change

Figure 5 exhibits the linear change of the samples D1-D4 fired at 1200°C up to 1400°C. Sample D1 (5 wt. % dolomite) showed an increase in the linear shrinkage by increasing of the firing temperatures from 1200°C to 1400°C. This is mainly due to the solid state & liquid phase sintering process of this sample which increased by increasing the firing temperature. Samples D2, D3 and D4 show a linear expansion at 1200°C due to the evaporation of CO₂ (These samples have a higher additions of dolomite, 15, 25 and 35 wt. %). At 1300°C and 1400°C, all samples exhibit the similar behavior in which linear shrinkage was observed and increased by increasing of the firing temperature and dolomite additions. This is attributed to the role of diopside and clinoenstatite phases in the sintering behavior of these samples.

Fig.5

Linear change of the samples D1-D4 fired at different temperatures 1200°C–1400°C.

Fig.6

Cold crushing strength of the samples D1-D4 fired at different temperatures 1200°C–1400°C.

3.5 Mechanical properties

Figure 6 shows the effect of firing temperature and dolomite additions on the cold crushing strength (CCS) of the samples D1-D4. It was observed that the CCS values increased by increasing the firing temperature from 1200°C to 1400°C due to an increase in the sintering process and a decrease in the samples porosity. At 1200°C, the cold crushing strength values were decreased from sample D1 to D4 by increasing of dolomite additions due to the increasing of samples porosity. Porosity plays an essentially role in deteriorate the CCS of the samples [12]. At 1300°C, the CCS values were increased from sample D1 (5 wt% dolomite) to sample D3 (25 wt % dolomite) and then decreased at sample D4 (35 wt% dolomite). This is due to the liquid phase sintering process, which enhances the contact between the grains in the ceramic matrix and closes the pore spaces, leading to an increase in CCS values [13 –15]. Also, tetragonal to monoclinic phase transformation of zirconia crystals is associated by a volume expansion (∼ 4%) leads to compressive stresses in the ceramic matrix inhibits further crack propagation by the transformation toughening [6, 11]. The CCS decreased at 1300°C in sample D4 due to the decreasing of zircon content (65 wt.%) and therefore a decrease in zirconia content. At 1400°C, the CCS improved from sample D1 to sample D3 due to the enhancement of the liquid phase sintering process and zircon dissociation by increasing of dolomite additions (up to 25 wt %). Also, the thermal mismatch between the different thermal expansion coefficients of the constituent phases; m-zirconia ∼ 6.5×10^–6 K^–1, zircon ∼ 4.1×10^–6 K^–1 and diopside ∼ from 2.16×10^–5 K^–1 (at 300 K) to 5.91×10^–5 K^–1 (at 1,600 K) in the ceramic matrix improved the mechanical properties by microcrack toughening mechanism [6 , 17]. The cold crushing strength was decreased at the sample D4 fired at 1400°C due to sample deformation.

3.6 Microstructure

Figure 7 and Table 3 show the SEM and EDAX analysis of the sintered samples D1 and D4 fired at different temperatures 1200°C–1400°C. Sample D1 which was fired at 1200°C showed sub- prismatic particles of zircon connected together and many of pore spaces between the particles. According to EDAX analysis, light particles referred to zircon (point 1) connected with few dark particles of Ca, Mg, Zr silicats (point 2) as observed in Table 3. At 1400°C, sample D1 exhibit the presence of zircon (point 3) and rounded zirconia grains (point 4). On the other hand, sample D4 fired at 1300°C show the presence of light grey zircon particles (Point 5) in the ceramic matrix connected with rounded and sub-rounded grains of zirconia (point 6). Also, irregularly dark diopside crystals (Point 7) were detected on the surfaces of zircon and zirconia grains. SEM and EDAX analysis agrees well with XRD data results.

Fig.7

SEM microphotographs of the sample D1 (fired at 1200°C and 1400°C) and sample D4 (fired at 1300°C).

Table 3

Chemical analysis point according to EDAX analysis, wt%

Element	Point 1	Point 2	Point 3	Point 4	Point 5	Point 6	Point 7
O	36.45	48.22	40.74	38.93	47.62	32.3	58.84
Mg	0.4	3.45	0.72	0.55	1.78	1.67	1.66
Si	16.63	4.38	15.55	14.52	12.76	12.37	2.57
Zr	45.46	9.25	42.19	44.79	34.69	43.89	6.51
Ca	0.97	34.69	0.79	1.2	3.16	9.77	30.42

4 Conclusions

XRD analysis of the fired composites shows the presence of zircon and zirconia (baddeleyite and tetragonal) as the main phases in addition to low intensity peaks of diopside and clinoenstatite. Zircon dissociation improved by increasing the sintering temperature and addition of dolomite. At 1400°C, only zirconia and diopside phases were detected in a composite containing 35% wt of dolomite.

Densification parameters and CCS of the fired samples were enhanced by increasing the firing temperature from 1200°C to 1400°C due to solid state & liquid phase sintering process. Samples contain 35wt% of dolomite and fired at 1200°C for 2 hours exhibited higher porosity (AP ∼ 51.25%) than other samples and can be used as porous ceramics. This is due to CO₂ evaporation through the thermal decomposition of dolomite. Sample D1 fired at 1400°C shows a highest bulk density (3.67 g/cm³) and a lowest apparent porosity (4.5%) as well as a good cold crushing strength (177 MPa) compared to other samples. At 1200°C, bulk density was decreased by increasing of dolomite additions from sample D1 to D4 due to the evaporation of CO₂ through thermal decomposition of dolomite. At 1300°C and 1400°C, the CCS was enhanced from sample D1 to sample D3 by increasing of dolomite additions (up to 25 wt %). This is due to the liquid phase sintering process, which enhances the contact between the grains in the ceramic matrix and closes the pore spaces, leading to an increase in CCS values. Also, tetragonal to monoclinic phase transformation of zirconia crystals improves the CCS of the samples by the transformation toughening which leads to compressive stresses in the ceramic matrix inhibits further crack propagation.

Microstructure examination of some sintered samples shows the presence of sub- prismatic zircon crystals in sample D1 at 1200°C and 1400°C. Also, some of rounded and sub- rounded zirconia crystals for the samples D1 (fired at 1400°C) and D4 (fired at 1300°C) were observed in addition to some of irregularly dark diopside crystals specially in the sample D4 (fired at 1300°C).

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Physico-mechanical properties and microstructure of multi-phase ceramic composites based on zircon and dolomite mixtures

Abstract

Keywords

1 Introduction

Table 1 Chemical composition of the starting materials, wt% Oxides Zircon Dolomite ZrO2 65.74 – SiO2 34.01 0.43 CaO – 29.46 Al2O3 – 0.01 Fe2O3 0.10 0.39 TiO2 0.15 0.01 MgO – 22.95 K2O – 0.01 MnO – 0.10 Na2O – <0.01 SO3- – <0.01 Cl- – <0.01 L.O.I. – 46.42

3.1 Chemical analysis of the starting raw materials

3.3 Densification parameters

3.6 Microstructure

References