Study of Indentation Response of Functional Graded Al2TiO5- Based Bioceramics

Zeya Oo1* , Alexander Gorin1 and It-Meng Low2

1School of Engineering and Science, Curtin University, Malaysia 2Centre of Materials Research, Curtin University, GPO Box U1987, WA 6845

Australia,*zeya.oo@curtin.edu.my

Abstract. Aluminium titanate, Al2TiO5 (AT) with the pseudobrookite structure is the only compound in the alumina-titania system. It is an excellent refractory and thermal shock resistant material due to its relatively low thermal expansion coefficient (1 ×10-6 ºC –1) and high melting point (1860ºC). However, its low mechanical strength, hardness and fracture resistance together with susceptibility to decomposition in the temperature range 900–1200ºC has limited its wider application. In this paper, the innovative tailored design of functionally- graded Al2TiO5 – based ceramics system is presented. This involves the use of a vacuum heat-treatment or die-pressing to form hard graded layers of alumina on Al2TiO5. These hard outer layers will provide hardness and wear resistance to protect the softer but damage resistant underlayers. The results will also explore unresolved issues concerning the effect of graded interfaces on their physical and mechanical performance properties.

Keywords: refractory, functionally graded materials, Vickers contacts, bonded interface.

1. Introduction Aluminum titanate (Al2TiO5) (AT) is widely used as a refractory material and as a thermal insulator in

engine components by virtue of its low thermal expansion coefficient (∼1×10-6˚C-1) and excellent thermal shock resistance [1]. AT is thermally unstable in the range ~900-1250˚C where it decomposes into α–Al2O3

and rutile (TiO2) by an eutectoid reaction [2]. However, it was observed that there was no decomposition of Al2TiO5 in atmosphere up to 1100°C. In the temperature range of 1150-1350°C, Al2TiO5 became unstable and decomposed into Al2O3 and TiO2. It was shown that the process of decomposition of Al2TiO5 is reversible at temperatures above 1350°C with the phase abundance of Al2TiO5 restored to 80 wt% [6]. Moreover, its low mechanical strength, hardness and fracture resistance together with susceptibility to decomposition in the temperature range 900–1200ºC has limited its wider application [1, 2]. The inherent weakness and brittleness of AT is due to its susceptibility to undergo microcracking by virtue of its pronounced thermal expansion anisotropy [3]. By judiciously adjusting the composition through addition of stabilizers such as SiO2, Fe2O3 and MgO, the thermal and mechanical strength performance of AT can be greatly enhanced [4]. In this paper, we present the mechanical characteristics and the indentation responses in a functionally graded alumina / aluminium-titanate (A/AT) bilayer. The evolution and accumulation of damage modes below the Vickers contacts are examined using the “bonded-interface” specimens [5].

2. Experimental Procedure 2.1 Sample Preparation

Alumina and rutile powders of commercial purity (~99%) were used for the processing of pure AT samples. Functionally–graded Al2O3/AT bilayers were also prepared by mixing Al2O3 and TiO2 in the appropriate weight ratio, followed by conventional die-pressing and reaction-sintering at 1600°C for 2 hr. In

+ Corresponding author. Tel.: + 6085443826; fax: +6085443837. E-mail address: zeya.oo@curtin.edu.my

2011 International Conference on Biomedical Engineering and Technology IPCBEE vol.11 (2011) © (2011) IACSIT Press, Singapore

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order to improve the thermal stability of AT, 10%wt MgO was used as an additive. Five batches of powder were prepared for the fabrication of layered-graded Al2TiO5/Al2O3 samples using the die pressing method. The 1st batch consisted of 100% AT, the 2nd, 3rd, and 4th batches consisted of a mixture of AT and Al2O3 with a ratio of 25:75, 50:50, 75:25 respectively, and the 5th batch was 100% Al2O3. Bi-layer samples with graded interfaces of varying thickness were prepared to investigate the effect of graded layer thickness on the damage modes and failure processes. A pressure of 150 MPa was used during the die-pressing to obtain the ensuring bi-layer compact with thin graded interfaces of thicknesses 0.1 mm, 0.3 mm, 0.5 mm and 0.7 mm. 2.2 Evaluation of mechanical properties

The Vickers indentation method was used to evaluate the hardness and fracture toughness along the cross section of both graded and non-graded samples. Contact damage tests were conducted on the graded layers of thickness 0.3mm and 0.5 mm by using a bonded-interface t specimen configuration. Loads of 3 kg and 5kg were used for samples sintered at 1500°C and 1550°C respectively. Samples were indented along the cross-sections with an average of 6 indents per sample, starting from the Al2O3 layer to the AT layer. The nature of subsurface contact-induced cracking or damage was observed using the “bonded-interface” specimens [5].

3. Results and discussion Results in Figure 1 show that as the indents progress along the cross section, starting at the

alumina layer, the hardness decreased from > 14 GPa to ~0.4 GPa by the end of the first graded layer of 75%A/25%AT. In contrast, the fracture toughness increased until cracking stopped altogether at approximately the start of the first graded layer of 75% A / 25% AT.

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1500 1hr 0.7mm layers AL~4mm 1500 1hr 0.3mm layers AL~4mm 1500 1hr 0.5mm layers AL~4mm1550 1.5hr 0.3mm layers AL~2mm 1500 1hr 0.1mm layers AL~4mm 1550 1.5hr 0.1mm layers AL~2mm

Fig. 1: Hardness versus distance (Alumina layer taken as starting point)

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1500 1hr 0.7mm layers AL~4mm 1500 1hr 0.3mm layers AL~4mm 1500 1hr 0.5mm layers AL~4mm1550 1.5hr 0.3mm layers AL~2mm 1500 1hr 0.1mm layers AL~4mm 1550 1.5hr 0.1mm layers AL~2mm Fig. 2: Fracture toughness versus distance (Alumina layer was taken as starting point)

In contrast, the fracture toughness (Figure 2) increased until cracking stopped altogether at

approximately the start of the first graded layer of 75% Al / 25% AT. The 5 kg and 10 kg indentation made on these two samples did not create cracks that penetrated far enough through the alumina layer to reach the first graded layer 75% Al / 25% AT. This result shows how the alumina layer is used to protect the soft inner core by resisting surface deformation due to indentation. Hence, the soft AT layer is protected from the indents by a hard wearing surface layer of alumina. This process provides the graded layers damage resistance as all the energy from the indentation process is absorbed or dissipated.

((aa)) ((bb))

Fig. 3: SEM images (a) contact damage of 0.3 mm (b) 0.5 mm FGM layers at the 50%A/50%AT Grade with a 20 kg showing the surface (top) and sub-surface damage (bottom)

(a) (b)

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Fig. 4: SEM images (a) Sub-surface damage of a 0.3 mm graded layers FGM with a 20kg load (b) Cross sectional contact damage at 75%A/25%AT grade.

Scanning electron micrographs of the contact damage of 0.3 mm and 0.5 mm graded layer samples are

shown in Figures 3 and 4. The 5 kg and 10kg indentations made on these two samples did not create cracks that penetrated far enough through the alumina layer to reach the first graded layer 75%A/25%AT. This result shows how the alumina layer is used to protect the soft inner core by resisting surface deformation due to indentation. Hence, the soft AT layer is protected from the indents by a hard wearing surface layer of alumina. The 0.3 mm graded layer sample showed crack propagation, which was deflected at the start of the first graded layer. This indicates the effectiveness of the graded layers in arresting any advancing cracks. It is interesting to note that no cracks were observed when the 0.5 mm graded sample was ground down to approximately before the 50%A/50%AT graded layer and then indented. This observation confirms that damage tolerance was imparted upon the graded layers. The capability of the graded layers in arresting the cracks is clearly shown in Figure 3 (b) where t the indent increased in size and the radial cracks were stopped entirely. This capability of damage control can be attributed to weakly bonded gains being debonded, lifted up and eventually pushed out from their positions. This process provides the graded layers damage resistance as all the energy from the indentation process is absorbed or dissipated.

4. Conclusion After contact damage via Vickers indentation testing, it was found that the graded layers provide the

necessary damage resistance to subdue subsurface cracking at loads of 5 kg, 10 kg and 20 kg by the 75% AL/ 25% AT. Standard Vickers hardness testing across the cross section of various samples showed that hardness decreased across the cross section, starting high at the outer AL layer and whittling down to lower levels at the inner AT layer. This is inversely proportional to the increase in the fracture toughness of the sample which was increasing from the AL layer to the AT layer. Crack energy was generally absorbed by the first graded layer of 25% AT/ 75% AL. This is due to the micro damage that occurs caused by the AT presence.

5. Acknowledgements This work was supported by Curtin Small grant, Curtin Sarawak Research Fund (CSRF), and Curtin

Sarawak Collaborative Research (CSCR). The author would like to thank to Mr N. Chen-Tan, Department of Applied Physics, Curtin University, Bentley campus, Australia for Vickers hardness testing.

6. References [1] P.Stingl, et al., Proc. Of 2nd Int. Symp. On Ceramic Materials and Components for Engines, Lubeck-Travemunde,

FRG, April 1986, ed. W. Bunk and H.Hausner, pp. 369-380, Bad Honnef, DKG (1986)

[2] Kato, et al., “Decomposition temperature of aluminium titanate”, J.Am. Ceram..Soc. 63 (1980) 355. [3] Thomas, et al., “Aluminium titanate-A literature review”, Br. Ceram. Trans.J. 88 (1989) 144. [4] Zografou, et al, “Thermal durability of Aluminium Titanate Ceramics prepared from electrofused

powders”, Sci. Ceram. 1, 757-762. [5] S.Pratapa and I.M. Low., “Infiltration-proceed, functionally graded aluminium titanate /zircona-alumina

composite”, J .Mater. Sci. 33 (1998) 3046-3053. [6] I.M Low and Z. Oo, “In Situ Diffraction Study of Self-Recovery in Aluminum Titanate”, J. Am. Ceram. Soc., 91

[3] (2008) 1027–1029.

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