The Fracture Toughness of the Casted Hydraulically Yttria Zirconia-Mullite-Magnesia System

Current Advances in Materials Sciences Research (CAMSR)

CAMSR Volume 1, Issue 1 Jan. 2014 PP. 12-16 www.vkingpub.com © American V-King Scientific Publishing

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The fracture toughness of the casted hydraulically

Yttria Zirconia-Mullite-Magnesia system Budi L. Hakim

*1, Syoni Soepriyanto

2, Akhmad A. Korda

3, Bambang Sunendar

4

*1, 2, 3Department of Metallurgical Engineering-Faculty of Mining and Petroleum, Indonesia

4Laboratory of Material Science and Engineering- Faculty of Industrial Technology, Indonesia

Bandung Institute of Technology, Jl. Ganesha 10 Bandung 40132, Indonesia *1

[email protected]; [email protected];

[email protected];

[email protected]

Abstract-This experiment intended to study the properties of

the casted hydraulically Yttria Zirconia-Mullite-Magnesia system. The specimens were prepared containing 3% Yttria

into Tetragonal Zirconia Polycrystalline compound or 3Y-TZP.

The additional Mullite was synthesized from alumina and silica with excessive silica content at about 15 to 25%. It can be said

that the Mullite or likely Mullite content tend to reduce the

values of hardness and fracture toughness parameters. High

concentration of Mullite may decrease the mechanical properties of the ternary system. However, the optimization of

magnesia concentration has not been reached completely and

may influence directly to Mullite-Magnesia performance and

also whole mechanical properties of the 3Y-TZP-Mullite-Magnesia system.

Keywords-Yttria Zirconia; Mullite; Magnesia; Vikers Hard-

ness; Fracture toughness.

I. INTRODUCTION

Cracking is a detectable response of the ceramic to indentation even at small loads, may either be localized or, at higher loads, massive to the extent that crushing follows. The ceramics product mostly specifies the minimum hardness requirement. ASTM F1873-98, the zirconia specification for surgical implants stipulates that Vickers hardness (HV) shall be no less than 11.8 GPa (1200 kgf/ mm

2) at a load of 9.8 N (1 kgf). Vickers hardness covers

approximately 60% of worldwide published ceramic hardness values, with loads typically in the range of a few newtons to 9.8 N (1 kgf) and occasional data for soft or high-toughness ceramics as high as 98 N (10 kgf). Whereas, Knoop hardness involves about 35% with loads from as low as 0.98 N (100 gf) to 19.6 N (2 kgf) [1].

Many contemporary structural ceramics have hardnesses in the range from 10 to 30 GPa. For the higher hardness, Vickers indentations made at 9.8 N (1 kgf) load are 25 microns in size and Knoop indentations are 68 microns long. The ASTM C1327 is a new standard for Vickers hardness of advanced ceramics and recommends a load of 9.8 N (1 kgf).

Hardness was calculated from the standard formula for force divided by contact area [2]:

HV = 1.8544 P / d2 (1)

where HV is Vickers hardness, P is the applied indenter load, and d is the average diagonal length for an individual indentation.

An indentation fracture mechanics was applied as a

simple technique for evaluating mechanical properties of

ceramic materials. Evans and Charles uniquely

characterized the surface radial cracks in brittle materials

generated by Vickers indentation. Evans-Charles was

modified by Lawn et.al [3-6], based on approach for median

or radial cracks. Recently Niihara et al modified the analysis

of Evans and Charles and Lawn by incorporating Palmqvist

cracks, rather than median cracks, at low crack to indent

ratio size (0.25<l/a<2.50), in which for the Palmqvist cracks

is expressed as [7]:

(KICØ/Ha1/2

)(H/EØ)2/5

= 0.035(l/a)-1/2

(2)

and for the median cracks is applied for (c/a)>2.50, as the

following:

(KICØ/Ha1/2

)(H/EØ)2/5

= 0.129(c/a)-3/2

(3)

where H is the hardness, a is the indent half diagonal, E is

Young’s modulus and c is the radial crack size (Fig.1.).

Whereas Ø is the constraint factor as H/σy or nearly equal to

3, in which σy is the yield stress, and l is the Palmqvist crack

length.

Fig.1. Palmqvist and median cracks around Vickers Indentation

Source: Nihaara (1983) [7]

A fracture mechanics model for the median cracks has

been provided by Lawn and Fuller, and Evans and Charles;

however there is no corresponding analysis for Palmqvist

cracks. Recently, numerous studies about the indentation

fracture on brittle materials have shown that the initially

cracks are the Palmqvist cracks. They extend radially along

median planes of indentation and remain close to the

specimen surface.

mailto:[email protected]



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This study analyzed the transition point and related it to

a new index of ceramic brittleness, which is defined as

B=HE/(KIC)2. The brittleness index will be important for

predicting mechanical properties such as for the

characterization of machinability, wear or erosion resistance.

The additive application in the ceramic system has being

examined. For instance, additional titania in the zirconia-

mullite-alumina system showed the increase in hardness and

density and the decrease of abrasion loss properties [8]. As

3Y-TZP presented the higher relative densities which

contributed to a better mechanical and thermal behaviour

rather than 8Y-TZP [9], this experiment preferably focused

on application 3Y-TZP as the basic substance of ternary

system.

The synthesis of Mullite compound should be based on

the reaction molarity composition, instead of the weight

basis. The difference between two perspectives implied an

occurrence of an excess of unreacted silica substances.

However the physical mechanical testing may still obtain a

reasonable trend of data to explain the parameter of Vikers

hardness and fracture thickness of metal ceramics composite

system.

II. EXPERIMENTAL PROCEDURE

This experiment employed local source of Zirconia

(ZrO2) which was obtained through caustic fusion and

calcination process of zircon opacifier raw material (ZrSiO4),

Yttria (Y2O3). The other materials were Alumina (Al2O3),

Silica (SiO2), and Magnesia (MgO). The Zirconia synthesis

was performed in the Ceramic Technology laboratory of

Metallurgy Department - Bandung Institute of Technology.

The 3Y-TZP powder was synthesized through mixing of

local Zirconia (ZrO2) powder with 3% mole of Yttria (Y2O3)

and poly vinyl butyrate. The blend was mixed

homogeneously, calcinated, dried and finally grinded to

obtain 3Y-TZP powder. The 3Y-TZP powder was then

casted hydraulically with applied maximum load at about 4

ton/cm2, and then sintered at temperature of 1500

oC during

4 hours. All other powders of Al2O3, SiO2, and MgO were

commercially available.

This experiment prepared vary of samples, included: (a)

Mullite system with more silica proportion to study the

excess impact of this substance, (b) Synthesis of local

Zirconia from raw material, and (c) Binary and ternary

system involved 3Y-TZP-Mullite-Magnesia.The specimens

were prepared in pellet and plate shapes. The pellet

specimen has diameter of 8 mm, and 3 mm thickness.

Whereas the plate specimens referred to ASTM C1161

requirement of size with 4 mm width, 3 mm thickness, and

45 mm length as a three point bending test specimen.

In this experiment, the specimens were analyzed using

X-ray diffraction (XRD) to examine the crystal structure

and phase of ceramic 3Y-TZP- Mullite - MgO. Whereas the

ceramics morphology was analyzed using SEM. The

fracture toughness (KIC) was calculated based on Vikers

indentation.

III. RESULT AND DISCUSSION

A. Silica Rich Mullite

Application of excessive silica in mullite synthesis was evaluated in this experiment (Table.1.). Thermodynamically each 72 grams of alumina should thermally combine with 28 grams of silica to form alumina silica (mullite) compound system. The benefit of Mullite addition was relatively complex to be evaluated independently due to silica rich impact in the system, in which not all of silica molecule can be absorbed to form new complex molecule name YTZP-Mullite system.

TABLE I SILICA RICH MULLITE INTO 3Y-TZP SYSTEM

Specimen

Code

Alumina,

gram

Silica,

gram

Silica excess,

gram

Silica

excess, (%)

YZ00OS 72.0 28.0 0.0 0.0%

YZ00ES 66.0 25.9 6.1 24%

YZ10ES 60.0 23.5 4.5 19%

YZ15ES 57.0 22.4 3.6 16%

YZ20ES 53.0 20.8 4.2 20%

The excess of alumina was studied by Obal et.al (2012) that it effects on the electrical conductivity of 3Y-TZP [10]. They assumed that the low grain boundary conductivity of 3Y-TZP is attributed to inter granular siliceous phases, which wet the grain boundaries. The added alumina reacts with silica, forming a stable Mullite:

3 Al2O3 + 2 SiO2 3Al2O3.2SiO2 (4)

According to thermodynamic calculations, reaction (4) may occur when the activity of silica is at least 0.03 at 1600

oC. On the other hand, the reaction between zirconia

and silica yields the unstable silicate ZrSiO4, which dissociates above 1540

oC. However, this compound is not

stable in the presence of alumina. So the occurrence of Mullite may contribute to reduce possible formation of unstable silicate ZrSiO4.

In this experiment, the remaining silica was occurred caused from silica rich mullite composition. Fig.2 and 3 show examples of the XRD patterns of silica rich mullite without 3Y-TZP, and 3Y-TZP-Mullite-Magnesia system. In the 3Y-TZP system, the unstable ZrSiO4 may be formed at the interfacial region via a reaction between the excess SiO2 and the ZrO2 substrate.

Fig.2. XRD pattern of the Mullite (3Al2O3.2SiO2)

Al

Al

Al

Al

Al

Intensity (%)

Position (2θ)

Al Al Al

Al Si

Si

Si Si

Si Si

Si



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Fig.3. XRD pattern of the (10% 3Y-TZP-19% silica rich Mullite- 10%MgO)

The XRD pattern of the silica rich Mullite indicates

alumina and silica compounds. The mullite reacted with the

zirconia to form 3YTZP-Mullite. The occurrence of

magnesia promotes to form 3YTZP-Mullite-Magnesia. It

indicated that there were some shifts of the peak positions

for both the zirconia phase and the mullite phase. The peak

shifts indicated some solubilities between mullite, zirconia,

and Yttria present in the metal composites.

As the 3Y-TZP specimens were placed in the excess

SiO2 concentration, some of the SiO2 may deposit on the

substrate. Initially, the SiO2 that was involved in the 3Y-

TZP apparently reacted with the substrate to form combined

molecule such as ZrSiO4. However, as the reaction

continued, the diffusion path for zirconium or silicon

increased such that the formation of ZrSiO4 was restricted,

and crystallized into the other molecule e.g. cristobalite [11].

The additional data of XRD analysis of other content of

3Y-TZP shows similar trend of peak location with slightly

different of magnitudes. It implied that both of Mullite or

likely Mullite components of alumina and silica may

thermally combine rather chemically combines into the new

ceramic composite system.

B. Grain Sizes

Through a controlled crystallization, it was reported that

zirconia co-reinforced mullite composite micro structure can

be synthesized [12]. The tetragonal zirconia (t-ZrO2) grains

with the average diameter at about 0.1 to 2 μm, were

observed in spherical and bar-like shape that were observed

in the mullite lattice (Fig.4.a). Further, it is shown in Fig.4.b

that the spherical objects of zirconia with the average

diameter at about 0.1 to 2 μm were embedded in the mullite-

magnesia system. In this system, mullite incorporated with

or without magnesia, roles as a matrix medium for the

zirconia. The mechanical strength depends on the proportion

of zirconia, whereas mullite penetrates into the available

space between the grains mixture. The benefit of magnesia

occurrence may correlate with the possible crack resistance

during sintering cycle.

The influence of additives may correlate with the grain

sizes. X.Miao, et.al (2004) observed the SEM micrographs

of the thermally etched surfaces of the Y–ZrO2–TiO2

composites sintered at 1300oC with TiO2 content varied

from 0 vol.% to 30 vol.% [13]. They found that the increase

of an additive content in Y-TZP system tends to result the

bigger grain sizes.

(a) 3Y-TZP-Mullite

(b) 3Y-TZP-Mullite-Magnesia

Fig.4. The 3Y-TZP-Mullite with and without magnesia

The modified 3Y-TZP by additional Mullite will reduce

its hardness, modulus of elasticity, and stress concentration.

Practically, it was recommended that an additional Mullite

should be no more than 10% to 20%, due to reduction of

mechanical properties tendency, although it may be

compensated through smoother grain and better sintering

technique [14].

C. Vikers Hardness

In indentation testing, it is usually desired to avoid

cracking that interferes with the hardness measurement.

However, a simple method to estimate fracture toughness

(KIC) of ceramic employs the length of the cracks that

propagates from the corners of a Vickers indentation. The

lengths of the cracks and the indentation half-diagonal size

are related to the hardness, elastic modulus, and fracture

toughness by an analytical expression.

An example of indentation testing was shown in the

following figure (Fig.5.). This figure identifies crack length

(L) as 0.015mm (15µm) both on the left and the right side of

the Vickers indentation, whereas the blue line, at the

bottom-right side, represents 0.050 mm length as a reference

of cracks length (l).

Position (2θ)

Intensity (%) Al

Al

Al Al Al

Al

Al Zr Al Al

Zr Al Al Zr Zr Y Zr Zr

Zr

Zr



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Fig.5. Vikers micro hardness and crack measurement

It was revealed that the correlation between the crack

lengths (l) to the indentation half-diagonal size (a) is related

to the hardness. Fig.6 configures the Palmqvist cracks

criterion that prevails if the value of indent ratio (l/a) is in

the range of 0.25 to 2.50. However, if the value of (c/a) ratio

may exceed 2.5, the harness value should be considered as

the Median crack. The variable of c is defined as the crack

lengths (l) plus the indentation half-diagonal size (a).

Through the Vickers hardness testing, it was obtained that

the shorter the crack, the harder the ceramic material.

Fig.6. Vikers hardness curve of (Y-TZP + Mullite + MgO) system

It is shown that Mullite or likely Mullite shows lower

hardness than pure 3Y-TZP system, in which gradually

additional of Y-TZP into the Mullite system tend to increase

its hardness properties. In this case, the experiment tended

to comply with the Palmqvist crack criteria, in which the

length of crack conformed to Palmqvist curve in the range

of 19 microns to 31 microns.

In this experiment, the laboratory testing of rich silica

mullite (specimen A) achieved the mean hardness value

(HV) of 27.78 GPa. Actually, the sample contains 24%

silica rich in mullite compound system. The previous study

(Bodhak, et.al. 2011) measured the hardness value of 92%

densified mullite with 1 wt% MgO and 10 wt%

ZrO2 composites and sintered at 1500°C in microwave

furnace, exhibited a hardness value of 10.24 GPa. They

observed additional 1% weight of magnesia as a sintering

aid for Mullite proved an increase of compressive strength

typically from 128 to become 387 MPa [15].

The pure Mullite system has the lowest hardness while

compared among the other experiment data. Meanwhile,

Mullite performance may be influenced by occurrence of

magnesia (MgO) additive. S.Prutty, et.al (2012) observed

that mullite Zirconia not only stabilizes the cubic zirconia

phase but also acts as a sintering aid for the formation of

cross-linked mullite grains [16]. However it requires

further study about the role of magnesia into the ternary

system characteristics.

The specimen B represented the 3Y-TZP containing

10% Mullite and MgO having Vikers hardness (HV) value

of 35.27 GPa. By additional Mullite composition at about

15% (specimen C), it increased the hardness up to 42.73

GPa. As comparison, the pure 3Y-TZP in the similar testing

condition resulted the the average hardness (HV) value up to

60.36 GPa.

D. Fracture Toughness

The fracture toughness parameter was expressed

employing Vickers hardness indentation measurement.

Fracture toughness of silica rich Mullite shows the lowest

fracture toughness among data of Yttria–Zirconia-Mullite

system. In this experiment, the Mullite (specimen A)

showed the fracture toughness value of 3.69, whereas the

3Y-TZP reached the highest fracture toughness value at 4.32.

It seems that mullite or likely-mullite occurrence in the 3Y-

TZP-Mullite system may reduce fracture toughness

properties. The rest of silica was mixed in the ceramic

composite system to cause heterogeneous system.

The Yttria-TZP content tends to increase HV and

fracture toughness of the Mullite system. As a comparison,

the specific density of tetragonal zirconia is 6.13, whereas

the Mullite is 3.05 and zircon (ZrSiO4) is 4.65. Meanwhile

the hardness of tetragonal zirconia, in Mohs unit, is 6.5,

whereas the Mullite is 6 to 7 and zircon (ZrSiO4) is 7.5 [17].

In similar testing condition with the equal load of

indentation, the additional 10% 3Y-TZP in the Mullite

system increased the fracture toughness up to 4.26 MPa.m1/2

.

At the higher 3Y-TZP content as much as 15% in the

Mullite and Magnesia system, the fracture toughness value

increased at 4.56 MPa.m1/2

. In the case of the magnesia

occurrence, it may positively contribute in the increase of

this metal ceramic properties.



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Fig.7 represented the correlation between fracture toughness and the Vikers hardness parameters. The brown line in the figure acts as a reference line that represents fracture toughness values based on Vikers hardness. With an average crack length of 3Y-TZP equal to 22 microns, it complies with Palmqvist crack hardness curve criterion, in which the parameter of indent ratio (l/a) was equal to 1.87.

Fig.7. Fracture toughness of Mullite into 3Y-TZP system

The excessive content of silica in a Mullite compound may increase gap of coefficient of thermal expansion between Y-TZP, Mullite and the rest of silica. This case can be identified by crack phenomenon of the system during cooling after ternary system formation. Beside this reduce the KIC and hardness of the ceramic system, it can be visually observed through SEM analysis that the unreacted silica component seems unable to completely mix into the alumina-silica system.

IV. CONCLUSION

The influence of additional Mullite (or likely Mullite) content into casted hydraulically Y-TZP ternary system was reviewed. This experiment employed 3% Yttria content into Tetragonal Zirconia Polycrystalline compound or 3Y-TZP. The experiment utilized Mullite that was synthesized from alumina and silica in high content at about 15 to 25%. Although the test analysis unable not focused on pure mullite due to an excessive silica content, however it can be concluded that the Mullite or likely Mullite occurrence tended to reduce the values of hardness and fracture toughness parameters. High concentration of Mullite tends to decrease the mechanical properties of the ternary system. The optimization of magnesia concentration may correlate directly to the true Mullite performance in the ceramic composite system.

ACKNOWLEDGMENTS

The authors would like to thank Indonesian Aerospace-Bandung, for contributing the silica and alumina materials. The authors are grateful to H.Subawi from Indonesian Aerospace for discussion and helpful suggestions. The authors also appreciate National Atomic Power Council-Bandung for facilitating specimen preparation.

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The Fracture Toughness of the Casted Hydraulically Yttria Zirconia-Mullite-Magnesia System

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