Two-Stage Sintering of Alumina-Y-TZP (Al2O3/Y-TZP) Composites Sivakumar Sivanesan1,a*, Teow Hsien Loong1,b , Satesh Namasivayam1,c

and Mohammad Hosseini Fouladi1,d 1School of Engineering, Taylor’s University, Taylor’s Lakeside Campus, No. 1 Jalan Taylor’s,

47500, Subang Jaya, Selangor DE, Malaysia asivakumar.sivanesan@taylors.edu.my, bteowhsienloong@sd.taylors.edu.my, csateshnarayana.namasivayam@taylors.edu.my, dhosseini@taylors.edu.my

Keywords: Alumina, Zirconia, Y-TZP, Composite, Densification, Mechanical Properties

Abstract. Alumina-Y-TZP composites between 0 to 25 vol% Y-TZP content produced via conventional two-stage sintering with T1 ranging between 1400°C and 1550°C, heating rate of 20°C/min, followed by T2 of 1350°C and 12 hours dwelling time. The microstructure, density, Vickers hardness (HV), Young’s modulus (E) and fracture toughness (KIC) of the sintered samples were then evaluated. It is observed that all samples up to 10 vol% Y-TZP achieved > 98% T.D. as the T1 increases. Samples with Y-TZP content above 10 vol% resulted in a significant decrease in density and hardness. Samples with ≤ 10 vol% Y-TZP sintered at T1 of 1450°C was able to achieve density > 98% T.D., Vickers hardness > 18 GPa and Young’s modulus > 380 GPa and fracture toughness > 6 MPam1/2 when compared to pure Al2O3 ceramics.

Introduction Attributed to its excellent mechanical properties, Yttria-Tetragonal Zirconia Polycrystals (Y-TZP)

has been widely used in various applications across many industries due to its’ excellent mechanical properties [1-5]. As alumina exhibits low toughness and zirconia is prone to ageing when exposed to humid environment primarily in biomedical applications, a new composite known as alumina-zirconia composites have been proposed to overcome the weaknesses of these two materials [6-9]. The mechanical properties of alumina-zirconia composites has been analyzed by Kurtz et al. [10] and the authors concluded these composites exhibited excellent mechanical properties and enhanced ability to retard ageing effects within the ceramic. The main underlying theory that may be attributed to the success of alumina-zirconia composites in the biomedical field is that these composites shows good mechanical properties of alumina especially hardness and at the same time is able to exhibit enhanced toughness due to the stress induced transformation toughening of zirconia’s tetragonal (t) grains to monoclinic (m) grains without compromising on ageing resistivity under steam or body fluid conditions. However, careful tailoring of the sintering route needs to be taken as many researchers have also reported that the incorporation of Y-TZP into the alumina matrix might improve fracture toughness (KIC) but may cause undesirable effects on the Vickers hardness (HV) and Young’s modulus (E) [11-14]. These effects were caused by high porosity in the microstructure composites due to lower amount of the tetragonal grains present in the composite structure and also different thermal expansion coefficient of the alumina and zirconia matrix [13-15]. Ultra-fine Y2O3 powder has been successfully produced by Chen and Wang [16] using a method known as two-stage sintering (TSS) where samples were sintered at high temperature T1 followed by relatively lower temperature T2 for a long period of time. Through this sintering method, the authors were able to retarding grain growth at final sintering stage and at the same time enhancing its densification [16]. Furthermore, two-stage sintering has also been successfully applied to various types of ceramics materials besides Y2O3 [17-21]. In this study, the effect of Y-TZP content on the grain size, densification and mechanical properties of alumina-zirconia composites sintered via two-stage sintering are evaluated.

Key Engineering Materials Submitted: 2019-03-31ISSN: 1662-9795, Vol. 814, pp 12-18 Revised: 2019-04-29doi:10.4028/www.scientific.net/KEM.814.12 Accepted: 2019-04-30© 2019 Trans Tech Publications Ltd, Switzerland Online: 2019-07-29

All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TransTech Publications Ltd, www.scientific.net. (#506985695-26/07/19,08:08:51)

Methodology Sample Preparation

The as-received powder of alumina Al2O3 and 3 mol% Y-TZP (Kyoritsu, Japan) has a similar mean particle size approximately 200 nm, specific surface area (SSA) of 13 m2/g. The alumina-zirconia composite samples with different Y-TZP contents (0,5,10,15,20 vol%) were prepared through alumina ball milling method and the samples were named in the form of AXYTZP where X represents the vol % of Y-TZP content in the composite and pure alumina were represented by A. Cold Isostatic Pressing (CIP) were subsequently employed to compact the powders into discs of 20 mm diameter and rectangular bar with dimensions of 5 mm width, 5 mm height and 15 mm length. A rapid heating furnace (ModuTemp, Australia) were then used to sinter the samples in air which two-stage sintering method were employed. Firstly, the samples were first heated to temperature T1 (1400°C to 1550°C) at heating rate of 20°C/min with 1 minute dwelling time followed by rapid cooling to T2 (1350°C) at 50°C/min and dwelling time of 12 hours. Silicon carbide papers ranging from 120 to 1200, the sintered samples were grinded on one face followed by polishing after sintering in order to obtain clearer surface for SEM purposes. The sintered samples were polished with diamond paste of 6 µm followed 1 µm.

Using the Archimedes method (ASTM C373-00), the density of the sintered samples were determined. Based on Eq. 1., the theoretical densities (T.D.) of the samples were calculated. The T.D. for pure Al2O3 and Y-TZP is taken to be 3.99 g/cm3 and 6.1 g/cm3 respectively according to ASTM 42-1468 and ASTM 83-113.

𝑇𝑇.𝐷𝐷. = �vol% Al2O3100

× 3.99� + �vol% Y−TZP100

× 6.1� (1)

To determine the hardness of the samples, Vicker’s indentation method were used. The load were applied for 10 N for 15 seconds. Ten measurements were obtained and the average values of HV are calculated. Using the standard ASTM C769 and also Eq.2, the Young’s modulus for each samples were calculated where ρ is the sample’s density and V is the ultrasonic velocity. E = ρV2 (2) The KIC of the samples were calculated using Eq. 3 which was proposed by Niihara et al., [22].

� KIC∅Hva1/2� �

HvE∅�25 = 0.035 �L

a�−12 (3)

Based on the SEM micrograph obtained, the grain size were evaluated using scanning line intercept method.

Results and Discussions Based on Fig. 1, when T1 is between 1400°C to 1550°C, all samples achieved more than 98% T.D.

The density increases as the T1 increases and at T1 of 1550°C, the samples achieved >99% T.D. for pure Al2O3 and up to 10 vol% Y-TZP content.

This phenomena can be attributed to the effect of TSS as this sintering method improves

densification and also limiting grain growth simultaneously [16,17,18,23]. Slight reduction in density were observed across all T1 as the Y-TZP content increases up to 10 vol%. When the Y-TZP content increases beyond 10 vol%, significant decrease in density were apparent across all T1. Significant decrease in density could be caused by the reduction in the amount zirconia’s (t) phase present in the composite matrix due to large amount of zirconia grains start to grow larger than its critical size. In addition, porosity formed in the composite structure might also be responsible for the significant reduction in density due to the differences in sinterability and also coefficient of thermal expansion for Al2O3 and Y-TZP [11,12].

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Figure 1: Densities of pure Al2O3 and alumina-Y-TZP composites sintered via two-stage sintering

at various T1. Based on Fig. 2, regardless of T1, the HV of pure Al2O3 is the highest where it achieved more than

18 GPa and was able to achieve 19.6 GPa when sintered at high T1 of 1550°C. Although based on the rule of mixture, as the Y-TZP content increases, the HV should decrease but up to 10 vol% Y-TZP, the reduction in HV was not significant.

Figure 2: Vickers Hardness of pure Al2O3 and alumina-Y-TZP composites sintered via two-stage

sintering at various T1. The HV of the samples reduced significantly as Y-TZP content increases especially in sample

sintered in T1 of 1400°C and contains 20 vol% Y-TZP where it reduced more than 30% to 12.4 GPa. Above 10 vol% Y-TZP content, the sharp decrease in HV might be effects of significant reduction in density. These findings are in agreement with the works of [13,15]. The reduction of HV in the alumina-Y-TZP composite could also be the effect of porosity formation which caused reduction in density hence leading to reduction in HV.

Based on Fig. 3, the highest Young’s modulus (406 GPa) were recorded for pure Al2O3 sintered at T1 of 1550°C. The lowest Young’s modulus was recorded for A20YTZP sintered at T1 of 1550°C where the Young’s modulus reduced by almost 50 GPa from 398 GPA to 348 GPa compared to

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A10YTZP sintered at similar T1. It is expected that the Young’s modulus would decrease as Y-TZP content increases as the Young’s modulus of Y-TZP is relatively low (~200 GPa) when compared to Al2O3. Formation of porosity and also microcracks might also be responsible for the decrease in Young’s modulus in the composite microstructure. Furthermore, according to Eq.2, the Young’s modulus is directly proportional to density hence reduction in density would lead to reduction in Young’s modulus of the composite.

Figure 3: Young’s modulus of pure Al2O3 and alumina-Y-TZP composites sintered via two-stage

sintering at various T1. Fig. 4 shows the KIC of alumina and alumina-Y-TZP composite sintered at different T1. The KIC

of all samples is recorded to be approximately 4 MPam1/2 for pure Al2O3 regardless of T1. As the Y-TZP content increases up to 15 vol%, there were increase in KIC observed. However, a decrease in KIC values were observed when Y-TZP content increases beyond 15 vol%. Sample A10YTZP sintered at T1 of 1450°C reported to have the maximum KIC of 10.2 MPam1/2. According to previous literature data [13,15], the most ideal amount of ZrO2 content is found to be 10 vol% hence these finding are in agreement with previous literature. Beyond 15 vol% Y-TZP content, tetragonal grains in the composite matrix have been excessively stabilised upon cooling to room temperature might be responsible for the reduction in KIC. The significantly higher KIC reported for A10YTZP sintered at T1 of 1450°C might be the result of residual stress influences on the matrix which effects the critical grain size of zirconia. As a result of increase in zirconia content, the matrix constrain which restrain the (t) to (m) transformation would also decrease. On the other hand, works of Bercher et al., [24] has shown that higher Al2O3 content in the composite matrix would be more beneficial as it increase the effectiveness of the residual stress.

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Figure 4: Fracture Toughness of pure Al2O3 and alumina-Y-TZP composites sintered via two-stage

sintering at various T1. Based on the works of Bercher et al. [24], for alumina-zirconia composite with less than

15 vol% ZrO2 content, the residual stress will be the most influential factor that effects the critical grain size. Hence, for A10YTZP, the critical grain size is much larger than other alumina-Y-TZP composite such as A5YTZP. Above 15 vol%, the effectiveness of matrix stiffness reduces leading to a reduction in critical grain size caused by increase in the (t) to (m) transformation. Table 1 shows of the sintered samples’ grain size and in agreement with works of other researchers, Y-TZP has shown to be effective in restraining the grain growth of alumina due to the constrain known as “pinning effect” exerted by the zirconia grain on the alumina grain size together with the effect of two-stage sintering schedule.

Table 1: The grain size of pure Al2O3 and alumina-Y-TZP composite sintered via two-stage sintering of T1 = 1450°C and T2 = 1350°C and holding time of 12 hours.

Materials Y-TZP Content [Vol %] Grain size [µm]

Al2O3 0 0.86

A5YTZP 5 0.57

A10YTZP 10 0.52

A15YTZP 15 0.45

A20YTZP 20 0.39 Based on the SEM micrographs, there were significant grain growth observed in few grains and also pores formed in the alumina-Y-TZP composite which detrimental to the density, Vickers hardness and also Young’s modulus. The microstructures of the alumina-Y-TZP composites are as shown in Fig. 5.

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Figure 5: SEM micrographs from thermally etched samples which a) A10YTZP sintered at T1 of

1450°C, (b) A20YTZP sintered at T1 of 1550°C where pores are clearly visible.

Conclusion The incorporation of Y-TZP together with ultilisation of TSS have shown to be beneficial in further limiting grain growth. The conditions to improve its KIC up to 10.2 MPam1/2 were found to be addition of Y-TZP content up to 10 vol% combined with sintering temperature of 1450°C for T1. This shows with proper amount of Y-TZP addition and sintering method selection, it is possible to produce alumina-Y-TZP composites with fine grain size which meets the industrial standards for mechanical properties compared to pure Al2O3 ceramics.

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