Mica-containing Glass-ceramics Research Articles

Crystallization behavior and mechanical properties of high strength mica-containing glass-ceramics from granite wastes

The high-strength mica-containing glass-ceramics were prepared from granite wastes by bulk crystallization. The influences of SiO2/Al2O3 molar ratio (S/A = 7.72, 9.62, 12.58, 17.82 and 29.67) on the crystallization behavior, microstructure, mechanical properties and machinability of glass-ceramics were investigated. The results demonstrated that the polymerization degree of the glass network decreased with the S/A ratio increasing, which further caused the decrease in glass transition temperature and crystallization temperatures. The increase in the S/A ratio promoted the precipitation of diopside, hectorite, kalsilite and tainiolite in glass-ceramics when the samples were heated at 750 °C, while inhibiting the precipitation of forsterite. For the glass-ceramics crystallized at 800 and 900 °C, the main crystalline phases transformed from diopside, forsterite, and nepheline to diopside, kalsilite, and tainiolite, with the S/A ratio increasing. As the SiO2 gradually replaced Al2O3, the morphology of crystals changed from lamellar to granular, while the mean size of crystals reduced. The Vickers-Hardness values of glass-ceramics crystallized at 800 and 900 °C ascended with S/A ratio rising, and the values were above 6.30 GPa. The bending strength of most glass-ceramics is stable between 90 and 140 MPa, among which the maximum bending strength is 133.28 ± 14.81 MPa. The fracture toughness of the glass-ceramic crystallized at 800 and 900 °C declined, while that at 700 °C increased with a larger S/A ratio. Glass-ceramics after heat-treated at 900 °C with S/A ratio of 9.62 had the largest fracture toughness of 3.28 ± 0.15 MPa m1/2. In preliminary tests of machinability, glass-ceramic after heat-treated at 900 °C with S/A ratio of 9.62 showed better results.

Structural effects of Zn+2/Mg+2 ratios on crystallization characteristics and microstructure of fluorophlogopite mica-containing glass-ceramics

We report the effect of Mg+2 substitution (by Zn+2) on crystallization kinetics, microstructure, thermal and mechanical properties of boroaluminosilicate glass. Zn2+ was selected for Mg2+ on the basis of similar ionic radius in six coordination system (Mg2+∼0.72 Å, Zn2+∼0.75 Å). The melt-quenched glasses with SiO2–(1 − x) MgO–Al2O3–K2O–B2O3–MgF2 (BPAS)/x ZnO system, have been investigated to establish the effect of Zn+2/Mg+2 ratios. It is found that the density of BPAS glass without zinc content is 2.52 g/cm3 and increased linearly on substitution of Mg2+ by 5–32 mol% ZnO. Tg and Td of BPAS glass initially increased on adding 5 mol% ZnO and then decreased on further addition. From DSC study, it is found that the crystallization exotherm changes significantly in the temperature range 750–1000 °C, where different crystalline phases are formed, and the activation energy of crystallization (EC) varies in the range of 254–388 kJ/mol. The crystalline phases formed in opaque BPAS glass-ceramic, derived by controlled heat treatment at 800 and 1050 °C (4 h), are identified as fluorophlogopite [KMg3(AlSi3O10)F2] mica and willemite (Zn2SiO4) by XRD technique, and confirmed by FTIR spectroscopy. The change of crystallization phenomena varying Zn+2/Mg+2 ratios correspond to significant microstructural change. A wide range of thermal expansion (CTE) values are obtained for the BPAS glasses and corresponding glass-ceramics. CTE (50–500 °C) of BPAS glass without zinc content is 7.76 × 10−6/K, and decreased sequentially on increasing Zn+2/Mg+2 ratio. The density of glass-ceramics after heating at 800 and 1050 °C increased linearly with increasing Zn+2 substitution for Mg+2. Microhardness of the BPAS glasses is in the range of 4.26–6.15 GPa and found to be increased to 4.58–6.78 GPa after crystallized at 1050 °C.

Effects of nucleating agents on crystallization and microstructure of fluorophlogopite mica-containing glass–ceramics

The alteration of crystallization behavior, microstructure, and thermal properties of fluorophlogopite mica-containing glass–ceramics by nucleating agent is systematically studied. TiO2, TiO2 + ZrO2, and ZrO2 have been doped as the nucleating agents in the SiO2–MgO–Al2O3–B2O3–K2O–MgF2 (BMAPS) glass system and prepared by the melt-quench technique. The glass without nucleating agent is also prepared to ascertain the influence of nucleating agent. Addition of nucleating agents effectively increases the softening as well as glass transition temperatures. From the DSC study, it is found that the fluorophlogopite mica crystallization exotherm exhibited in the temperature range 800–850 °C and the activation energy varies in the range 167–182 kJ/mol. The opaque mica glass–ceramics are derived from these BMAPS glasses by a controlled heat treatment process and heat treatment at 1050 °C is found to be optimum. The mica crystals were identified as fluorophlogopite for all the four BMAPS glasses by X-ray powder diffraction (XRD) and subsequently confirmed by FTIR spectroscopy. Excellent matching with fluorophlogopite crystal was obtained in Zirconia-containing glass–ceramic as perceived from the XRD and FTIR studies. The microstructure of interlocked card-like mica flake crystals is found to form as seen from scanning electron microscopy, and such microstructure is obtained when ZrO2 has been used as nucleating agent. Glass–ceramic without nucleating agent possesses Vickers hardness value 4.58 Gpa and it is increased with addition of the nucleating agent (5.67–6.56 GPa), ZrO2-containing glass–ceramic possess lower hardness (5.67 GPa) and better machinability. Therefore, ZrO2 is the most efficient nucleating agent to generate fluorophlogopite mica in these glass–ceramics useable for SOFC applications.

Property–process relations in simulated clinical abrasive adjusting of dental ceramics

This paper reports on property–process correlations in simulated clinical abrasive adjusting of a wide range of dental restorative ceramics using a dental handpiece and diamond burs. The seven materials studied included four mica-containing glass ceramics, a feldspathic porcelain, a glass-infiltrated alumina, and a yttria-stabilized tetragonal zirconia. The abrasive adjusting process was conducted under simulated clinical conditions using diamond burs and a clinical dental handpiece. An attempt was made to establish correlations between process characteristics in terms of removal rate, chipping damage, and surface finish and material mechanical properties of hardness, fracture toughness and Young's modulus. The results show that the removal rate is mainly a function of hardness, which decreases nonlinearly with hardness. No correlations were noted between the removal rates and the complex relations of hardness, Young's modulus and fracture toughness. Surface roughness was primarily a linear function of diamond grit size and was relatively independent of materials. Chipping damage in terms of the average chipping width decreased with fracture toughness except for glass-infiltrated alumina. It also had higher linear correlations with critical strain energy release rates (R2=0.66) and brittleness (R2=0.62) and a lower linear correlation with indices of brittleness (R2=0.32). Implications of these results can provide guidance for the microstructural design of dental ceramics, optimize performance, and guide the proper selection of technical parameters in clinical abrasive adjusting conducted by dental practitioners.

Crystallization of R 2O–MgO–Al 2O 3–B 2O 3–SiO 2–F (R=K +, Na +) glasses with different fluorine source

The effect of fluorine source on the crystallization behaviors of the R 2O–MgO–Al 2O 3–B 2O 3–SiO 2–F (R=K +, Na +) glasses is studied by differential thermal analysis (DTA), X-ray diffraction (XRD), scanning electron microscopy (SEM) and Raman spectrum. Two glasses with the same composition were prepared by using NaF and MgF 2 as the source of fluorine, respectively. The glass with the fluorine source of NaF is quite stable during heat-treatment. Only large dendrite crystals form on the surface of this glass, while the interior of the glass still remains amorphous. In contrast, the glass with the fluorine source of MgF 2 exhibits uniform bulk crystallization and forms mica-containing glass-ceramics after heat-treatment. The structure difference between these two glasses was investigated by Raman spectrum. The glass with the fluorine source of NaF shows a simple Raman spectrum. Boroxol rings are found to be the dominant unit in this glass. In contrast, multiple peaks appear on the Raman spectrum of the glass with the fluorine source of MgF 2, and these peaks are resolved to the vibrations of three- or four-coordinated boron units, such as chain-type metaborate groups, pentaborate groups, diborate groups, highly changed orthoborate groups, pyroborate groups, as well as boroxol rings. The stability of the glass with the NaF fluorine source is attributed to the high stability of the boroxol rings in this glass.

Effect of fluorine source on crystallization of R2O-MgO-Al2O3-B2O3-SiO2-F (R=K+, Na+) glasses

Two sets of R 2 O-MgO-Al 2 O 3 -B 2 O 3 -SiO 2 -F (R=K + , Na + ) glasses were prepared by using NaF and MgF 2 as the source of fluorine, respectively. The crystallization behaviors of these glasses were studied by differential thermal analysis, X-ray diffraction, scanning electron spectroscopy, and wave-dispersive spectroscopy. Large dendrite norbergite crystals were found on the surface of the glasses with the fluorine source of NaF due to the strong surface crystallization, but no crystalline phase precipitates in the interior of these glasses were found. On the contrary, the glasses with the fluorine source of MgF 2 exhibited a uniform bulk crystallization and, thus, mica-containing glass-ceramic was formed. It is concluded that the presence of fluorine in the R 2 O-MgO-Al 2 O 3 -B 2 O 3 -SiO 2 -F (R=K + , Na + ) glasses does not always act as an effective nucleating agent to form mica-containing glass-ceramics. The primary crystalline phase and the crystallization mechanism are strongly dependent on the source of fluorine.

Enhanced mechanical properties of oriented mica glass-ceramics

Oriented mica-containing glass-ceramics have been fabricated by hot-pressing the glasses after they were crystallized. X-ray diffraction analysis and scanning electron microscopy show that mica plates were aligned in the hot-pressed glass-ceramics, forming a (001) preferential texture. The formation of the oriented structure was attributed to the combination of viscous flow of residual glass and rolling of mica plates when hot-pressure was applied. By hot-pressing technique, the flexural strength and fracture toughness of the glass-ceramics were improved up to 380 MPa and 2.8 MPa m 1/2, respectively, two to three times higher than traditional mica glass-ceramics.

Investigation on phase separation, nucleation and crystallization in bioactive glass-ceramics containing fluorophlogopite and fluorapatite

Apatite and mica-containing glass-ceramics in the system K 2OMgOCaOAl 2O 3B 2O 3 SiO 2P 2O 5F can be used to repair and reconstruct diseased or damaged bones and teeth, due to their biocompatibility and bioactivity. Like other glass-ceramics, the properties of these machinable glass-ceramics depend on composition, size and volumetric ratios of the crystalline phases in the materials. Phase separation, nucleation and crystallisation of these multi-phase glass-ceramics were investigated with DTA, SEM/EDAX, XRD and FTIR techniques. The phase relationships are discussed with regard to crystal chemistry, thermodynamics and kinetics.

Machinability and brittleness of glass-ceramics

The relationship between the machinability and the brittleness of glass-ceramic materials is investigated. A brittleness index ( B), given by the ratio of the hardness to the fracture toughness, is proposed as a parameter for estimating the machinability. This approach is confirmed by considering experimental data from the literature on turning operations of mica-containing glass-ceramics. It is shown that machinability parameters, such as the slope of the log-log plot of the specific cutting energy versus the cutting rate, or the specific cutting energy at low cutting rates, are in good agreement with the brittleness indices for seven different glass-ceramics. In order for a glass-ceramic to be machinable, it was found that the brittleness index of the material should be lower than B ≈ 4.3 μm −12.

The relationship between microstructure and wear of mica-containing glass-ceramics

A series of mica-containing glass-ceramics with three different mica platelet sizes were tested for wear against a harder alumina counterface in a pin-on-disk tribometer under distilled water lubrication. Under all test conditions (i.e. different loads and number of passes), the wear rate increased with the increase in mica platelet diameter. Examination of the wear scars on the glass-ceramic samples using a scanning electron microscope indicated that the wear process was dominated by a microfracture mechanism either along the mica cleavage planes or along the weak mica-glass interfaces. Three wear modes were observed. (1) At low loads and small number of passes, wear occurred by formation of isolated fracture sites on the wear track. No measurable wear took place during this ‘localized fracture’ wear mode. (2) The number of fracture sites increased and covered the entire contact width as the load or number of passes increased. This ‘contact fracture’ mode resulted in wear factors on the order of 10 −4 mm 3 Nm −1. (3) At higher loads or after a large number of passes, accumulation of subsurface microfracture damage led to ‘spallation’ wear mode which resulted in wear factors on the order of 10 −2 mm 3 Nm −1. The mica platelet diameter was found to be an important factor in controlling the wear modes and wear transfusion. The transitions from ‘localized fracture’ to ‘contact fracture’ and to ‘spallation’ occurred at lower loads and smaller number of passes as the mica plate diameter was increased.

Indentation stress-strain curves for “quasi-ductile” ceramics

A micromechanical model describing “quasi-ductile” Hertzian contacts in otherwise brittle ceramics is developed. The elemental basis of the model is a discrete “fault” along an internal weak interface, constrained at its ends by an elastic matrix and subject to frictional sliding, in the subsurface zone of high shear stress in the Hertzian field. By summation over a prescribed density of shear faults within the active plastic zone, the analysis leads to a constitutive identation stress-strain function, with special provision for the incorporation of microstructural variables. Experimental data from a series of mica-containing glass-ceramics with contiguous platelet microstructures are used to confirm the essential predictions of the model. It is demonstrated that plasticity increases with volume fraction and aspect ratio, but not size, of the platelets. Parametric evaluations by curve fitting the indentation stress-strain data allow for predictions of intrinsic stress-strain responses for the glass-ceramics in conventional uniform stressing states.

Deformation and fracture of mica-containing glass-ceramics in Hertzian contacts

The Hertzian indentation response of a machinable mica-containing glass-ceramic is studied. Relative to the highly brittle base glass from which it is formed, the glass-ceramic shows evidence of considerable “ductility” in its indentation stress-strain response. Section views through the indentation sites reveal a transition from classical cone fracture outside the contact area in the base glass to accumulated subsurface deformation-microfracture in the glass-ceramic. The deformation is attributed to shear-driven sliding at the weak interfaces between the mica flakes and glass matrix. Extensile microcracks initiate at the shear-fault interfaces and propagate into the matrix, ultimately coalescing with neighbors at adjacent mica flakes to effect easy material removal. The faults are subject to strong compressive stresses in the Hertzian field, suggesting that frictional tractions are an important element in the micromechanics. Bend-test measurements on indented specimens show that the glass-ceramic, although weaker than its base glass counterpart, has superior resistance to strength degradation at high contact loads. Implications of the results in relation to microstructural design of glass-ceramics for optimal toughness, strength, and wear and fatigue properties are discussed.

Mechanical Properties of Barium-Mica-Containing Glass-Ceramics

Fine mica crystals (1-2μm) were precipitated by reheating glasses in the system Ba0.5Mg3(Si3AlO10)F2-Mg2Al4Si5O18-Ca3(PO4)2. This glass-ceramics were two times stronger (≈350MPa in bending) than conventional mica-containing glass-ceramics. The glass-ceramics contained two types of mica crystals, i.e., calcium-rich type and barium-rich type. MgF2 in the glass compositions was effective to increase the content of the barium-mica crystal. It was found that barium-rich mica crystal effectively deflected cracks to improve mechanical strength, and that calcium-rich mica crystal generated the ease of machining.

High‐Strength Mica‐Containing Glass‐Ceramics

Glass‐ceramics containing barium–mica in the system Ba0.5 Mg3 (Si3AIO10)F2–2MgO · 2Al2O3· 5SiO2–Ca3 (PO4)2 are two to three times stronger than conventional mica‐containing glass‐ceramics. Moreover, the barium‐mica glass‐ceramics are easier to machine, as confirmed by a drilling test using conventional steel tools. Such mechanical properties are attributable to the microstructure of the barium–mica glass‐ceramics. Very fine, interlocking mica crystals are precipitated in the glass, and a crack‐deflection mechanism is observed by scanning electron microscopy.