780 Int. J. Microstructure and Materials Properties, Vol. 3, No. 6, 2008

Copyright © 2008 Inderscience Enterprises Ltd.

Microstructure and indentation fracture toughness of mica glass ceramics

Abdullah Ozturk Metallurgical and Materials Engineering Department Middle East Technical University 06531 Ankara, Turkey Fax: +90 312 210 2518 E-mail: abdullah@metu.edu.tr

Abstract: Mica glass ceramics were produced through controlled crystallisation of a glass in the SiO2-Al2O3-CaO-MgO-K2O-MgF system. TiO2, P2O5 and ZrO2 were added individually as nucleating agents in the amount of 0.5 or 1.0 wt% of the glass. Crystalline phases were identified by X-ray diffraction. The effects of small amounts of various nucleating agents on the microstructure were determined by scanning electron microscopy. Microstructural analyses were supplemented with measurements of the Vickers microhardness and indentation fracture toughness. The amount and kind of nucleating agent did not change the number of crystalline phases present but had an influence on the shape and size of the crystals, and hence on the indentation fracture toughness of the mica glass ceramics. The values for the indentation fracture toughness for the mica glass ceramics containing 0.5 wt% of TiO2, P2O5 and ZrO2 were 1.69±0.09 MPa.m1/2, 1.74 ± 0.10 MPa.m1/2 and 1.77 ± 0.12 MPa.m1/2, respectively.

Keywords: microstructure; microhardness; indentation; fracture toughness; mica; glass ceramic.

Reference to this paper should be made as follows: Ozturk, A. (2008) ‘Microstructure and indentation fracture toughness of mica glass ceramics’, Int. J. Microstructure and Materials Properties, Vol. 3, No. 6, pp.780–790.

Biographical notes: Dr. Abdullah Ozturk is currently a Professor and the Associate Chairman of the Metallurgical and Materials Engineering Department of Middle East Technical University, Turkey. He received his PhD in Ceramic Engineering from the University of Missouri-Rolla, MO, USA. His main areas of interest include the production, characterisation and properties of ceramics, glasses and glass ceramics, and ceramic matrix composites.

1 Introduction

The discovery of glass ceramics containing mica crystals as the predominant crystalline phase in the early 1970s has drawn special attention principally due to their machinability (Beall, 1972; Grossman, 1972; Vogel and Höland, 1990). They can be turned, milled, drilled and tapped with the same tools used for metals, without breaking as ceramics normally would (Vogel et al., 1991). In the 1980s, mica glass ceramics specific to biomedical applications were developed in view of the fact that they provide

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biocompatibility, translucency, improved chemical durability and a hardness similar to natural tooth enamel (Höland et al., 1991; Hockin et al., 1996). These materials also possess the appropriate mechanical, thermal, dielectric and physical properties (Rounan and Peinan, 1986; Uno et al., 1991; Tzeng et al., 1993). The combination of desirable properties that can be tailored through changes in composition and thermal history makes them useful for special biomedical applications, especially in the replacement of natural bone and in dental restoration (Vogel et al., 1991; Bapna and Mueller, 1996). It is possible for practitioners to successfully mimic the aesthetics of natural teeth from mica-based glass ceramics (Denry et al., 1999). The unique properties of mica-based glass ceramics result from the uniform, reproducible, fine-grained microstructure consisting of easily cleavable and interlocking laminar mica crystals dispersed in a glassy matrix (James, 1995; Cheng et al., 1996; Baik et al., 1997). The type of crystallinity and the distribution of the crystalline phase(s) formed during crystallisation also depend on the kind and amount of nucleating agent used. The ability to alter the microstructure by using various additives would allow an excellent control over the machinability and the properties of the product.

In spite of the good unique qualities, the failure of some mica-containing glass ceramics during service, because of insufficient mechanical performance, has been reported (Bapna and Mueller, 1996). Mechanical properties such as hardness and strength have had limited success in predicting the performance of these materials in service. However, the fracture toughness, i.e., the resistance of a material to crack growth from an initiating flaw, is an exception and has become of interest for structural and dental applications. Hence, studies conducted on the fracture behaviour of mica-based glass ceramics have both scientific and practical significance.

The purpose of this study was to evaluate the effects of small amounts of various nucleating agents on the microstructure and indentation fracture toughness of glass ceramics containing mica crystals as the predominant crystalline phase.

2 Experimental procedure

A glass with the batch composition presented in Table 1 was formed by melting appropriate amounts of extra-pure grade powders. Mica (fluorophlogopite) crystals can be formed from this glass upon subjecting it to a suitable heat treatment (Gür and Ozturk, 2005). TiO2, P2O5 and ZrO2 were added individually into the batches as nucleating agents. TiO2 addition was made in the amount of 0.5 or 1 weight percent (wt%) of the glass, while the P2O5 and ZrO2 additions were made in the amount of 0.5 wt% of the glass. Batches of 16 g were placed in a platinum crucible and melted in an electric furnace at 1500°C. Melting took place at normal laboratory conditions without controlling the atmosphere. When melting was complete, the melt was poured onto a stainless steel plate to obtain the glass.

The as-cast glass pieces of different chemistries, in terms of the amount of various nucleating agents, were annealed at 600°C for 1 h. After annealing, the glass pieces were subjected to a two-step heat treatment to obtain mica glass ceramics. The pieces were heated up to the nucleation temperature, at which nuclei formed. They were kept at this temperature for some time and then heated further to the growth temperature, at which nuclei growth occurred. The nucleation and crystal growth temperatures were 620°C and 940°C, respectively. The pieces were held at each temperature for 8 h. The heating and

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cooling rates were 3°C/min. Following the crystallisation, the surfaces of the pieces were ground and polished to achieve a mirror-like surface finish prior to fracture toughness measurements. Polishing was accomplished with the application of a series of SiC emerald papers, alumina powder solution on a cloth and finally 0.25 µm diamond paste onto the surfaces consecutively.

Table 1 Ingredients and batch composition of the glass used for preparation of the glass ceramics studied

Ingredients Percentage (wt%)

SiO2 50.7

K2CO3 23.2

Al(OH)3 0.8

Mg(OH)2 10.4

MgF2 9.0

CaCO3 5.9

A microhardness tester, Zwick T 3212 B, with a square-base pyramidal diamond indenter, was used to measure the microhardness and the indentation fracture toughness. The load was 0.8 kg and the loading time was 15 sec. The values of the Vickers microhardness, Hv, were calculated by the equation (Li et al., 1989):

2463.6 P/a ,vH = (1)

where P is the indentation load in newtons and a is the half-diagonal of the indent in micrometres. The equation used in determining the indentation fracture toughness, KIc, values is (Niihara et al., 1982):

0.4 0.51/ 2

v

E0.018 .

HIc v

lK H a

a

−⎛ ⎞ ⎛ ⎞= ⎜ ⎟ ⎜ ⎟⎝ ⎠⎝ ⎠

(2)

where:

E = Young’s modulus in GPa

Hv = the Vicker’s microhardness in GPa

l = the crack size in micrometres

a = the half-diagonal of the indent in micrometres.

This equation was chosen to compare and evaluate the values obtained in this study with those reported by Baik et al. (1995) for the mica-based glass ceramics of similar chemistry and microstructure. A total of nine perfect indentations were made at different locations of the specimens to obtain the values for the Vickers microhardness and indentation toughness.

Crystalline phases precipitated in the crystallised counterparts of the glasses were identified using an X-ray diffractometer, XRD, Rigaku Geigerflex (DMAX/B). Scans were run from 2θ of 5° to 85° at a speed of 2°/min using Cu-Kα radiation. A scanning electron microscope, Jeol 6400, was used for the microstructural analyses.

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3 Results and discussion

The appearance of glass ceramic samples of different chemistries in terms of the amount and kind of the nucleating agent suggested that some amount of a residual glassy phase was present along with the crystalline phases. The X-ray Powder Diffraction (XRD) analyses revealed that at least two crystalline phases precipitated during crystallisation heat treatment. Fluorophlogopite was the primary crystalline phase in all of the samples independent of their chemistry. Wollastonite and kaliophilite were also observed in the crystallised counterparts of the glasses. A representative XRD pattern of a glass ceramic produced in this study was compared (see Figure 1) with that of the commercial mica-containing glass ceramic (Dicor) designed for use in the dental restoration of occlusal surfaces, as presented by Tzeng et al. (1993). The patterns illustrate identical features, that is, the crystalline peaks corresponding to certain 2θ values for the mica glass ceramic studied completely fit those of Dicor.

Figure 1 Comparison of the XRD patterns of (a) Dicor and (b) the mica glass ceramic produced in this study

The amount and kind of nucleating agent additions did not change the number of crystalline phases present but had an influence on the shape and size of the crystals. This became apparent early in the microstructural examinations by Scanning Electron Microscopy (SEM). Samples of different chemistries resulted in different microstructures. A representative SEM micrograph of the glass ceramic containing no nucleating agent is shown in Figure 2. SEM micrographs of the glass ceramics containing 0.5 and 1 wt% TiO2 as the nucleating agent are shown in Figures 3 and 4, respectively. The images were taken from the freshly fractured surfaces. When a comparison is made between the microstructure of the samples containing no and some amount of TiO2, it can be recognised that additions of as small as 0.5 wt% TiO2 produced a notable change in

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the microstructure of the mica glass ceramics. Grains and grain boundaries could easily be distinguished in the samples containing some amount of TiO2, while that was not the case for the sample containing no TiO2. It is evident that much more crystals were formed and the residual glassy phase was less as TiO2 content was increased from 0.5 wt% to 1 wt%. The shape of the crystals changed from nearly spherical to sharp-edged flake-like crystals exhibiting an interlocking, referred to as ‘house of cards’, structure as seen in Figure 4. The average grain size increased but the aspect ratio of the crystals decreased with increasing TiO2 addition, as shown in Table 2. Measurements of the size and aspect ratio of crystals were made by selecting groups of 30 crystals in SEM micrographs. Then the length and the thickness of the cross-section for each crystal were measured. The grain size was found by averaging the lengths over the group. The aspect ratio was calculated by means of dividing the length by the thickness for each crystal and averaging the result over the group. The decrease in the aspect ratio with increasing TiO2 additions is interpreted to mean that TiO2 plays the role of a nucleating agent in this system and improves the nucleation and growth rates. The increase in nucleation and growth rates yields the formation of more and more nuclei, which cause an increase in the amount of crystallinity. As the amount of crystallinity increases, the growth of the crystals in length is restricted because the crystals can grow only until they reach adjacent crystals, which inhibit further growth. However, the growth of the crystals in width or diameter is likely to continue since they are surrounded with a glassy matrix to be crystallised, instead of crystals.

It is obvious that TiO2 additions promoted the formation of an interlocking microstructure. The microstructure formed in the mica-based glass ceramic containing 1 wt% TiO2 resembled the microstructure of mica-based glass ceramics of similar chemistries (Beall, 1972; Quinn et al., 2003; Baik et al., 1995). It has been reported that the house of cards structure is desirable for the best machinability and mechanical properties (Vogel and Höland, 1990; Chyung et al., 1971; Bapna and Mueller, 1996). When the crystal size is large and anisotropic, a large deflection of the crack path is needed; but when the crystal size is very small and nearly equiaxed, the crack passes more easily between crystals and at the crystal/glass interface without a high degree of deflection.

Figure 2 SEM micrograph of the mica glass ceramic containing no nucleating agent

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Figure 3 SEM micrograph of the mica glass ceramic containing 0.5 wt% TiO2

Figure 4 SEM micrograph of the mica glass ceramic containing 1 wt% TiO2

Table 2 The microstructural factors Vickers microhardness (H) and fracture toughness (KIc) of the mica glass ceramics investigated

Sample no. Additive

kind

Additive percentage

(wt %) Aspect ratio* Grain size*

(µm) H* (GPa) KIc*

(MPa.m1/2)

1 no no Not measurable Not measurable 4.71 ± 0.05 1.37 ± 0.07

2 TiO2 0.5 1.78 0.30 4.64 ± 0.10 1.69 ± 0.09

3 TiO2 1.0 1.72 0.37 4.63 ± 0.05 1.72 ± 0.10

4 P2O5 0.5 1.75 0.55 4.66 ± 0.07 1.74 ± 0.10

5 ZrO2 0.5 2.01 0.45 4.69 ± 0.09 1.77 ± 0.12

Note: *Accurate to nearest significant figure.

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Figures 5 and 6 are representative SEM micrographs of the glass ceramics containing 0.5 wt% ZrO2 and 0.5 wt% P2O5, respectively. It is evident that the addition of as small as 0.5 wt% of ZrO2 or P2O5 to the SiO2-Al2O3-CaO-MgO-K2O-MgF system had a profound influence on the microstructure developed. Grains and grain boundaries became visible. The interlocking platelets of fluorophlogopite crystals are apparent in Figure 6. A comparison of Figure 3 with Figures 4 and 5 reveals that the microstructure of the samples containing 0.5 wt% of either ZrO2 or P2O5 was akin to that of the samples containing 1 wt% TiO2, implying that both ZrO2 and P2O5 have greater influence on the microstructure than TiO2. The average grain size of the sample containing 0.5 wt% ZrO2 was decidedly bigger; but the aspect ratio of the crystals was smaller than that of the sample containing 0.5 wt% TiO2. When P2O5 was added instead of TiO2, the average grain size of the crystals increased from 0.30 µm to 0.55 µm, but the aspect ratio of the crystals decreased from 1.78 to 1.75. As seen in Table 2, the aspect ratio and average grain size of the crystals varied from 1.72 to 2.01, and 0.30 µm to 0.55 µm, respectively, with regard to the amount and kind of nucleation agent used.

Figure 5 SEM micrograph of the mica glass ceramic containing 0.5 wt% ZrO2

Figure 6 SEM micrograph of the mica glass ceramic containing 0.5 wt% P2O5

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The values of the Vickers microhardness and the indentation fracture toughness for the investigated mica glass ceramics are presented in Table 2. The determined values are the mean values of the nine measurements obtained from a single test specimen. The ± signs next to the values indicate the standard deviation of the data from the averages. The values of Young’s modulus (E) used in calculating the indentation fracture toughness values were determined according to ultrasonic velocity measurement (Gür and Ozturk, 2005).

The values for the Vickers microhardness and the indentation fracture toughness of the mica glass ceramic containing no nucleating agent were 4.71 ± 0.05 GPa and 1.37 ± 0.07 MPa.m1/2, respectively. It is apparent that the microhardness decreased but the indentation fracture toughness increased with the addition of a small amount of nucleating agent. The addition of small amounts of various nucleating agents did have a notable effect on the indentation fracture toughness. The values of the indentation fracture toughness for the glass ceramics containing various kinds and amounts of nucleating agent varied from 1.69 ± 0.09 MPa.m1/2 to 1.77 ± 0.12 MPa.m1/2. The highest indentation toughness was achieved with a ZrO2 addition, indicating that ZrO2 is comparatively superior in terms of the indentation fracture toughness, although the difference is not significant. The results are consistent with the findings of Quinn et al. (2003). The values for the indentation fracture toughness of the mica glass ceramics studied fall within the range of the values of the fracture toughness reported for the commercial mica-containing glass ceramics that are similar in chemistry. Baik et al. (1995) reported that the fracture toughness of the mica glass ceramics ranges from 1.2 and to 2.2 MPa.m1/2 depending on the heat treatment temperature. Quinn et al. (2003) measured the fracture toughness by the single-edge-precracked beam method and realised that fracture toughness varies between 1.04 and 1.65 MPa.m1/2 depending on the mica platelet diameter.

The indentation tests revealed that the fluorophlogopite crystals embedded in a glass matrix facilitated the microfracture formation along the weak mica-glass interfaces and mica basal planes. Microcracks emanating from the corners of the indentations propagated along the mica-glass interfaces or the mica cleavage planes, which are the basal planes in the hexagonal close-packed crystallographic structure (Jahanmir and Dong, 1995). The propagation path of the advancing crack was repeatedly deflected, causing crack branching and blunting, which absorb a great deal of energy during fracture (Chyung et al., 1971). The fracture toughness is improved even though the material is brittle as a result of crack branching and deflection.

The microstructural factors such as grain size, grain shape, aspect ratio of the grain and percent crystallinity can influence the propagation of a crack front (Guazzato et al., 2004). Crack path deflection is enhanced by the larger platelets and aspect ratios (Quinn et al., 2003; Baik et al., 1995), and also occurs when a mismatch in the Young’s modulus and/or the thermal expansion exists between the crystalline phases and between the crystalline phase(s) and the glassy matrix (Faber, 1983). A dual ceramic microstructure resulting from the combination of two crystalline phases may result in a substantial increase in fracture toughness (Ruhle and Evans, 1989; Becher, 2001). The presence of two or more crystalline phases could lead to different residual strains in the glass ceramic due to thermal expansion mismatches and/or modulus of elasticity disparities between the crystalline phases, and between the crystalline phases and the glassy matrix. It is commonly known that phlogopite crystals have a lower strength and elastic modulus

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than wollastonite crystals. The formation of two crystalline phases could account for the increase in fracture toughness, as the result of an increase in the fracture surface energy of the crystallised product. In addition, the existence of the wollastonite phase may result in an increase in the fracture resistance due to the whisker-like wollastonite grains that act as a filler-reinforcing matrix (Fu et al., 2002). When an advancing crack front reaches a wollastonite grain, it faces extra resistance to fracturing the wollastonite grains. Instead of fracturing the grain, which requires a higher energy, it deflects or blunts. Kokubo et al. (1986) reported that the presence of wollastonite crystals can effectively increase the fracture surface energy of the crystallised product, even when they are present as fine dendrites rather than long fibres.

4 Conclusions

These are the conclusions of the study:

• TiO2, P2O5 and ZrO2 can be used as nucleating agents for the glass in the SiO2-Al2O3-CaO-MgO-K2O-MgF system. The effect of either ZrO2 or P2O5 as a nucleating agent is better than that of TiO2.

• The amount of nucleating agents did not have a notable effect on the crystalline phases present but produced a profound change in the microstructure developed after a regulated two-step heat treatment. The formation of a microstructure consisting of randomly oriented and highly interlocked fluorophlogopite crystals was promoted with a small addition of a nucleating agent.

• A small addition of a nucleating agent, independent of the kind of nucleating agent, produced a profound increase in the fracture toughness. The microstructure consisting of crystals with large grains and a high aspect ratio produced an increase in the indentation fracture toughness as a result of crack branching and blunting.

• The mica glass ceramics studied exhibited a microstructure and properties analogous to commercial mica-based glass ceramics designed for use in special structural and dental applications.

Acknowledgement

The financial support by The Scientific and Technical Research Council of Turkey, with project number MISAG-106, is greatly appreciated.

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