Fabrication and creep properties of eutectic-composition Al2O3/YAG/YSZ sintered composites

https://doi.org/10.1016/j.jeurceramsoc.2021.07.055Get rights and content

Abstract

Three-phase alumina/YAG/yttria-stabilized cubic zirconia (YSZ) composites were fabricated by a solid-state reaction route starting from commercial powders of Al2O3, Y2O3 and monoclinic ZrO2. The final phases Al2O3, YAG and YSZ were obtained after calcination of the powder mixtures at 1400 °C. Dense bulk composites were obtained after sintering, with a homogeneous microstructure of fine and equiaxed grains with sizes of 1 μm. Compressive mechanical tests were performed at 1300–1450 °C in air at constant load and at constant initial strain rate. A brittle-to-ductile transition was found with increasing temperature. Grain boundary sliding is the main deformation mechanism in the ductile regime, characterized by a stress exponent of 2 and by the absence of dislocation activity and changes in grain morphology. Alumina seems to be the rate-controlling phase owing to the improvement in creep resistance by the presence of yttrium and zirconium of the other two phases.

Introduction

Alumina-based composites are probably the most used engineering oxide ceramics nowadays in structural applications, particularly at high temperatures, because of their excellent chemical stability and mechanical properties. Although pure monolithic alumina is inherently brittle, with a low fracture toughness at low and high temperatures [1,2], its mechanical properties can be considerably enhanced by the incorporation of other phases in composite structures, such as fibers, particulates, layers, etc. In particular, particulate composites formed by equiaxed and approximately similar-sized particles exhibit improved thermal and chemical stability relative to their single-phase constituents, thus maintaining optimal mechanical properties of strength, toughness, and thermal shock resistance at elevated temperatures. In this way, dual phase Al2O3/ZrO2 [[3], [4], [5], [6], [7], [8]] and Al2O3/YAG [4,[9], [10], [11]] composites have been produced with higher strength, fracture toughness, and creep resistance than their single-phase counterparts. Remarkably, these fine-grained composites exhibit metal-like superplasticity, which contrasts with the premature failure of undoped monolithic alumina owing to the extensive grain growth and cavitation during high-temperature deformation [2,12].

Directionally-solidified alumina-based eutectic ceramics grown from the melt have received much attention in the last years because of their excellent mechanical properties even at temperatures close to the eutectic temperature [[13], [14], [15], [16], [17], [18], [19]], which derive from the special lamellar microstructures obtained during solidification. In particular, directionally-solidified ternary Al2O3/YAG/ZrO2 eutectic melts have been shown to have a fracture toughness as high as 8 MPa.m1/2 at room temperature [16], and the retention of large flexural, tensile and compressive strengths up to temperatures close to the eutectic temperature [13,14]. The fabrication routes necessary to produce directionally-solidified eutectic ceramics are, however, economically expensive, no suitable for mass production and impose serious restrictions for obtaining bulk components with custom shapes and sizes. Furthermore, the physical properties of these eutectic ceramics are very anisotropic, depending strongly on the solidification direction. These drawbacks can be mitigated, at least partially, by using sintered particulate composites produced by conventional solid-state reaction routes.

Very few studies are, however, concerned with Al2O3/YAG/ZrO2 (thereinafter AYZ) sintered composites, which have been fabricated with different phase contents: equivolumetric (33 vol%) [20], eutectic-composition [21] and 5 vol% YAG + 5 vol% ZrO2 particulate-reinforced Al2O3 [22]. Precisely on this last material, fabricated by surface modification of the alumina powder with inorganic precursors of the second phases, Palmero et al. [23] have reported the only study addressing the high-temperature mechanical behavior of AYZ sintered ceramics; by means of four-point bending tests from room temperature up to 1500 °C, the authors found an enhanced deformability at temperatures above 1400 °C (though the maximum strain was limited to 4% due to restrictions of the experimental setup). The aim of the present study is therefore two-fold: first, to fabricate and characterize the crystalline phases and microstructure of three-phase AYZ composites with the eutectic composition produced by a conventional ceramic processing route; and second, to investigate their creep response at high temperatures by constant cross-head speed and constant load tests in correlation with microstructural observations, in order to assess the microscopic mechanisms involved in the plastic deformation. The creep of the three-phase composite is compared to that of the single-phase constituents.

Section snippets

Starting materials

Samples of polycrystalline AYZ composites with the eutectic composition (65.8 mol% Al2O3, 15.6 mol% Y2O3, 18.6 mol% monoclinic ZrO2 [24]) were produced via a conventional solid-state reaction route. Appropriate amounts of high-purity commercial powders ( 99.99 %, Sigma-Aldrich) of Al2O3, Y2O3 and monoclinic ZrO2 were dry ball-milled in agate media for 1 h at 150 rpm using a planetary ball mill (Pulverisette 6, Fritsch, Germany). The resulting powders were calcined at different temperatures

Phase composition of the calcined powders

Fig. 1 shows the X-ray diffractograms of the as-milled and calcined powders at temperatures between 1200 and 1600 °C. The patterns are rather complex because up to seven different phases could be identified at intermediate temperatures. Every peak in the diffractograms was indexed according to one of the following PDF-2002 database reference patterns (International Centre for Diffraction Data, ICDD): α-Al2O3 (No. 05-0712), Y2O3 (No. 89-5591), monoclinic ZrO2 (No. 36-0420), Y4Al2O9 (YAM, No.

Conclusions

Al2O3/YAG/cubic ZrO2 sintered composites with the ternary eutectic composition have been successfully fabricated by solid-state reaction. Starting with Al2O3, Y2O3 and monoclinic zirconia powders as precursors, X-ray diffraction analysis indicates that the final phases of Al2O3, YAG and Y2O3 fully-stabilized cubic zirconia, without other intermediate phases, are obtained at a calcination temperature of 1400 °C. Bulk composites with a relative density of 98.5 % were obtained after sintering at

Declaration of Competing Interest

The authors report no declarations of interest.

Acknowledgment

This work was supported by the Project no. MAT2009-13979-C03-01, Ministerio de Ciencia e Innovación, Spain.

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