Continuous alumina fiber-reinforced yttria-stabilized zirconia composites with high density and toughness

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Original Article

Continuous alumina fiber-reinforced yttria-stabilized zirconia composites with high density and toughness Xuewu Lia, Xiaohui Fana,*, Na Nib,c,*, Xiaofeng Zhaoa, Chuanwei Lid, Ping Xiaoe a Shanghai Key Laboratory of Advanced High Temperature Materials and Precision Forming, School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai, 200240, China b Key Lab of Education Ministry for Power Machinery and Engineering, School of Mechanical Engineering, Shanghai Jiao Tong University, Shanghai, 200240, China c Gas Turbine Research Institute, Shanghai Jiao Tong University, Shanghai, 200240, China d Institute of Materials Modification and Modeling, School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai, 200240, China e School of Materials, University of Manchester, MSS Tower, Manchester, M13 9PL, UK

A R T I C LE I N FO

A B S T R A C T

Keywords: Fiber-reinforced composites Alumina fibers Yttria-stabilized zirconia Mechanical properties Residual stress

Continuous alumina fiber-reinforced yttria-stabilized zirconia (YSZ) composites with a LaPO4 fiber coating were fabricated by slurry infiltration and spark plasma sintering (SPS). The LaPO4 coating was deposited on the reinforcement alumina fabrics by a modified sol-gel method. The YSZ slurry with good dispersion and stability was prepared by optimizing the pH value, dispersant addition and ball milling time. The fabricated composite with a high density of ∼ 92 % has a good flexural strength of 277 ± 43 MPa, and a superior fracture toughness of 15.93 ± 0.75 MPa·m1/2 exhibiting a non-brittle failure behavior. It was found that the LaPO4 coating reduced the residual stress near the fiber/matrix interface to 131 ± 41 MPa, which was 369 ± 63 MPa in the composite without the fiber coating. The LaPO4 coating renders a weak interphase to improve the composite toughness by activating several toughening mechanisms including crack deflection, fiber debonding and pullout, and delamination behavior.

1. Introduction Continuous fiber-reinforced ceramic matrix composites (CFCCs) are promising candidates for structural materials due to their outstanding mechanical properties and chemical stability at high temperatures. Particularly, the materials with a low density and high damage tolerance are required in aerospace and power generation industries [1–3]. In the past three decades, much attention was paid to the robust SiCbased composites with superior strength, good creep resistance, and high thermal conductivity [4–7]. However, the application of SiC-based composites is limited in oxidation and combustion environments because of the oxidation embrittlement and performance degradation [8–10]. Consequently, oxide/oxide composites are of great interest for high-temperature applications owing to their inherently excellent stability in an oxidizing environment [11,12]. In addition, oxide/oxide composites have a significant cost advantage over SiC-based composites [13]. Alumina fibers, commercially available oxide fibers, have been widely used as the reinforcement of oxide/oxide composites due to their high elastic modulus (370 – 380 GPa) and tensile strength

⁎

(2.5–3.3 GPa) [3]. Many previous studies have focused on the fabrication and properties of alumina fiber-reinforced alumina matrix composites [14–16]. Unfortunately, a commonly high porosity (25–30 %) of the alumina matrix results in low composite strengths (< 200 MPa) [14], and the composites generally suffer a substantial strength loss due to grain coarsening at relatively high temperatures (> 1000 °C) [16]. Another oxide ceramic matrix, mullite (Al6Si2O13) with good creep resistance and excellent oxidation resistance [17], was reinforced with alumina fibers displaying a non-brittle fracture behavior and strength retention up to 1200 °C [18]. However, the flexural strength and elastic modulus of the Al2O3,f/mullite composites were low. The poor sinterability of mullite and alumina matrix, mainly caused by the low diffusion rates of aluminum ions [19,20], results in the high matrix porosity, which is detrimental to the mechanical performance of the composites. Moreover, the low fracture toughness (2–3 MPa m1/2) of alumina and mullite can limit the optimization of the composite toughness [21]. Zirconia is a competitive matrix material for ceramic matrix composites, since it has a pack of attributes for structural applications, including a high melting point (Tm ≈ 2700 °C) and chemical stability,

Corresponding authors. E-mail addresses: [email protected] (X. Fan), [email protected] (N. Ni).

https://doi.org/10.1016/j.jeurceramsoc.2019.12.041 Received 6 June 2019; Received in revised form 20 October 2019; Accepted 19 December 2019 0955-2219/ © 2019 Published by Elsevier Ltd.

Please cite this article as: Xuewu Li, et al., Journal of the European Ceramic Society, https://doi.org/10.1016/j.jeurceramsoc.2019.12.041

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high strength and toughness, unique wear resistance, and low thermal conductivity [22–24]. The relative high fracture toughness of zirconiabased or zirconia-related ceramics, contributed by transformation toughening along with microcracking and deflection mechanisms [25], could give a good mechanical support in zirconia matrix composites. However, research on ZrO2-matrix composites reinforced with continuous fibers is very limited to date. A matrix of ZrO2-SiO2 or ZrO2TiO2 reinforced with continuous SiC fibers was prepared by organometallic precursor impregnation and pyrolysis process [26]. It was noted that the strength degradation of the composites due to the reaction of the fibers and matrix was inevitable without fiber coating. Similarly, in carbon fiber reinforced calcium stabilized zirconia composites prepared by slurry infiltration and hot-pressing method, the carbon fiber reacted with zirconia to form a ZrC phase, resulting in the brittleness and the degradation of mechanical properties of the composites at hot-pressed temperatures [27]. Interfacial reaction was avoided in chopped alumina fiber-reinforced tetragonal zirconia polycrystal (TZP) composites [28]. The composites with an addition of 10 % by weight of alumina fibers obtained a two-fold increase in the fracture toughness over the monolithic TZP, but still failed in a brittle manner, presumably due to the use of short fibers and non-optimized fiber/ matrix interface. It has been well established that the fiber/matrix interface plays a critical role in CFCCs toughening behavior [3,29,30]. A tough CFCC requires that crack propagates along the fiber/matrix interface rather than penetrates the fibers. The interface should be weak enough to release the stress concentration at the crack tip, facilitating the crack deflection. In general, a fiber coating is introduced as a weak interphase that allows crack deflection and fiber/matrix debonding [31]. Initially, layered oxides such as magnetoplumbite (CaAl12O19) and β-alumina (βAl2O3) were used as fiber coatings for oxide/oxide composites [32,33]. Even though the layered structure promotes crack deflection and subsequently fiber sliding and pullout, CaAl12O19 has chemical stability issues with alumina or alumina-based fibers [32], and β-Al2O3 requires a high processing temperature (> 1000 °C), which results in fiber strength degradation due to the fiber grain coarsening [33]. A prevailing oxide, monazite (LaPO4), is chemically stable and weakly bonded to other oxides, and can provide a weak interface [14,34–37]. Its low hardness compared with other oxides could also promote plastic deformation during fiber sliding [38]. These intrinsic features make it a superior coating for oxide CFCCs. In this study, continuous alumina fiber-reinforced yttria-stabilized zirconia (Al2O3,f/YSZ) composites were fabricated by the slurry infiltration and spark plasma sintering (SPS) process. The stabilization of YSZ slurry was investigated by optimizing the pH value, dispersant addition and ball milling time. To provide an appropriate fiber/matrix interface, a LaPO4 coating on the alumina fabrics was prepared by a modified sol-gel method [39,40]. The rapid heating rate and short sintering time of SPS processing can minimize the thermal degradation of the alumina fibers and improve densification efficiency and therefore SPS was applied to densify the composites instead of hot pressing used in previous CFCC research [35,41–43]. The microstructure, mechanical properties of the fibers and the composites were studied, and the related toughening mechanisms were discussed. The effects of the LaPO4 coating on the residual stress and the fracture behavior of the composites were investigated.

Table 1 Properties of Nextel™ 610 fibers. Fiber diameter (μm)

Density (g/cm3)

CTEa (ppm/°C)

Tensile strength (GPa)

Elastic modulus (GPa)

10–12

3.9

8.0

3.1

370

a

Coefficient of thermal expansion (20–1100°C).

2.2. Preparation of the fiber coating The LaPO4 coating on the woven Al2O3 fabrics was prepared by the modified sol-gel method based on the previous work by Fair et al. [39,40]. Solutions of phosphoric acid and lanthanum citrate were prepared by dissolving concentrated phosphoric acid or a mixture of lanthanum nitrate and citric acid in deionized water. The precursor solution for the coating process was then prepared by mixing equal volumes of each above solution at ∼ 5 °C to yield 100 g monazite/L. The fabrics were desized at 800 °C for 1 h in air, and then the desized fabrics were submerged in the chilled precursor solution. The precursor solution was then heated to 90 °C quickly. After 5 min, the fabrics were removed and washed in the deionized water. The fabrics were subsequently dried in air at 100 °C for 1 h and finally fired at 900 °C in air for 5 min. The coating process was repeated for 10 times. 2.3. Preparation and characterization of YSZ aqueous slurries YSZ aqueous slurries were prepared using high-energy ball milling by dispersing YSZ powders in deionized water mixed with ammonium polyacrylate (APAA) as a dispersant and polyvinyl alcohol (PVA) as a binder. A series of slurries with various pH were prepared for the zeta (ζ) potential test using a zeta potential analyzer (Zetasizer Nano ZSE, Malvern Instruments Ltd., Malvern, England). The viscosity of the slurries was monitored using a LVDV-1 viscometer (Fangrui Instrument CO., LTD., Shanghai, China). The suspension stability of the YSZ slurries was investigated by the static settlement experiment for 3 days. The particle size of YSZ was monitored during the ball milling using laser particle analyzer (Mastersizer 2000, Malvern Instruments Ltd., Malvern, England). 2.4. Preparation and characterization of Al2O3,f/YSZ composites The Al2O3,f/YSZ composites with and without the LaPO4 coating were prepared by slurry infiltration and SPS. Fig. 1 shows the schematic diagram of the Al2O3,f/YSZ composites preparation. A simple apparatus was set up utilizing the pressure difference to facilitate the slurry penetration through the fabrics and improve infiltration efficiency. The infiltration process was repeated for five times. The infiltrated fabrics were dried at 80 °C for 2 h, and then heated at 750 °C for 2 h to burn out organics in a ventilated furnace. Subsequently, the green body obtained by stacking ten layers infiltrated fabrics was densified in the spark plasma sintering furnace (SPS, KCE-FCT-HP D25/4-SD, Germany) at 1200 °C for 10 min under a pressure of 50 MPa. The density of the composites was measured by the Archimedes’ method. The Al2O3,f/YSZ composites were cut into 2 × 4 × 40 mm bars for three point bending tests with a span of 32 mm and a cross head speed of 0.09 mm/s according to the standards in ASTM C1341-16. The fiber volume fraction for composites with and without the LaPO4 coating was estimated by Eq. (1) and Eq. (2), respectively:

2. Experimental procedure 2.1. Materials

1

Vf = 1+

YSZ powder (3 mol.% Y2O3-ZrO2, 99 % pure, mean particle size: 50 nm; Fanmeiya, China) and 2-D woven Al2O3 fabrics (Nextel™ 610 fabrics, Al2O3 > 99 wt.%, 0°/90° woven with 400 filaments per roving; 3 M Corp., USA), were used as starting materials. The properties of the Nextel™610 fibers are given by the supplier and listed in Table 1.

(1)

mf ρm

1

Vf = 1+ 2

(ms − mf ) ρf

mc ρf mf ρc

+

(ms − mf ) ρf mf ρm

(2)

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Fig. 1. Schematic of Al2O3,f/YSZ composites preparation.

pH = 11.3, where the mutual repulsive force among YSZ particles is the strongest due to the largest net charge on the surface of YSZ particles with the maximal ζ potential. After adding the dispersant APAA, the isoelectric point of the slurry shifts to point P' (pH = 1.8). The absolute value of ζ potential increases at the same pH in the full test range compared to that of the slurry without APAA. For pH > 6, the ζ potential increases rapidly and reaches the peak value of – 40.3 mV at pH = 10.6. It suggests that APAA can effectively enhance the surface potential and increase the repulsion of YSZ particles in the slurry. Fig. 5 shows the dependence of the viscosity and the sedimentary volume percentage (respect to the total slurry volume) of the YSZ slurries on the pH value and the APAA addition. Both the viscosity and sedimentary volume percentage of the YSZ slurries decrease with the increase of the pH value and reach a minimum at pH = ∼ 9. Similarly, the viscosity and the sedimentary volume percentage of the YSZ slurries reduce as the APAA adds, and reach a minimum when the addition of APAA is around 3 wt.%, as shown in Fig. 5(c) and (d). Excessive APAA would result in the increase of the viscosity and the rise of sedimentary volume percentage since the entanglement of the APAA molecules reduce the stability of the slurries [44]. Fig. 5(a) and (c) also illustrate that the viscosity increases with the increase of YSZ addition. To prepare YSZ slurries with good dispersion and stability, therefore, the pH value of the YSZ slurries should be adjusted to ∼ 9, and the APAA addition of 3 wt.% is appropriate. Fig. 6 shows the effects of the ball-milling time on the particle size and the viscosity of 50 vol.% YSZ slurry. The particle size and the viscosity decrease simultaneously with the prolongation of the ballmilling time and both values stabilize after 9 -h ball milling. This is because the large YSZ agglomerates are broken down and YSZ particles are dispersed increasingly better during the ball milling, which improves the dispersity and fluidity of the slurry. Fig. 7(a) and (b) show the SEM images of the as-received and the 9 -h ball-milled YSZ powder. Obviously, the as-received YSZ powder is severely agglomerated. After 9 -h milling, the agglomerates disappear, and the powder is well dispersed with a mean particle size of ∼ 60 nm. Therefore, 9 h for ball milling is enough to prepare slurries with a good dispersion of YSZ particles. Based on above results, high solid loading of 50 vol.% YSZ aqueous slurries were prepared for the infiltration process. The properties of the slurries are summarized in Table 2.

Where mf , ms , mc , ρf , ρm , ρc represent the mass of the fabrics before infiltration, the mass of the infiltrated and fired fabrics, the mass of the LaPO4 coating, the fiber theoretical density, the matrix theoretical density and the LaPO4 theoretical density, respectively. It is assumed that the fiber and matrix are fully dense in the calculation. Therefore, it should be noted that this assumption will overestimate the fiber volume in the composites using Eqs. (1) and (2). Ten samples were tested for both density and fiber volume fraction measurements. The fracture toughness was determined by the single edge notched beam (SENB) method with a span of 20 mm and a crosshead speed of 0.05 mm/min under three point bending. The SENB samples were 2.5 × 5 × 36 mm in dimensions with a notch depth of around 2.50 mm. The standards for sample preparation and calculation formula for fracture toughness are summarized in ASTM E399-74. The three-point bending test was carried out using an universal testing machine (Zwick/ Roell Z020, Zwick/Roell, Ulm, Germany). The polished cross section and fracture morphology of the composites were examined by scanning electron microscope (SEM, FEI Quanta 200, Eindhoven, Netherlands). The residual stress at the fiber edge adjacent to the matrix or LaPO4 coating in the composites was evaluated by photoluminescence piezospectroscopy using a Raman microprobe (LabRAM HR Evolution, Horiba, France) coupled with a 532 nm laser. 3. Results 3.1. Microstructure of fibers Fig. 2 shows the microstructure of the surface and the interior of the uncoated Al2O3 fiber. The average grain size of the fibers is 104 ± 25 nm. Fig. 3(a)-(b) present the morphology of the LaPO4 coated Al2O3 fibers. Fig. 3(a) reveals that fiber coating bridging between the fibers was hardly observed after the coating process. The LaPO4 coating was found to be of a thickness around 0.8–1 μm and uniformly bonded with the fiber as shown in Fig. 3(b). 3.2. Design and characterization of YSZ slurries Fig. 4 shows the change of zeta (ζ) potential versus pH for the YSZ slurries with and without APAA. The isoelectric point of the slurry without APAA is obtained at point P (pH = 2.3), where the net surface charge of the particles is zero. With the increase of the pH, the ζ potential increases negatively and reaches a peak value of – 25.6 mV at 3

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Fig. 2. SEM micrographs of the uncoated Al2O3 fiber with an average grain size of ∼100 nm.

3.3. Microstructure of Al2O3,f/YSZ composites Fig. 8 shows the microstructure of the polished cross-section of the Al2O3,f/YSZ composites. The total porosity is ∼ 8 % and the fiber volume fraction is ∼ 48 %. The interfiber and interbundle gaps are sufficiently filled with the dense YSZ matrix, confirming the effective infiltration and densification process. A few micropores and cracks in the vicinity of the Al2O3 fibers are observed in Fig. 8(b) and (c). The occurrence of the micropores was likely caused by the disruption in YSZ powder packing around the fibers referred to the “wall effect” during the matrix densification [45,46]. The interfiber cracks mostly perpendicular to the axis of fibers were caused by the shrinkage during drying process or the thermal stress caused by the difference in the coefficients of thermal expansion (CTE) between the fiber (8.0 × 10−6 /K) and matrix (10.8 × 10-6 /K) [47]). It is observed that the fibers in the composites with and without the LaPO4 coating are deformed after SPS process, which is attributed to the low creep resistance of alumina fibers [48,49]. Fig. 8(d) illustrates that the LaPO4 coating is well bonded with the fiber and matrix as an interphase after sintering.

Fig. 4. Zeta (ζ) potential versus pH of the YSZ slurries with and without APAA.

3.4. Mechanical properties of Al2O3,f/YSZ composites

277 ± 43 MPa, respectively, which are higher than that of alumina or mullite matrix composites [13] due to the much denser matrix in this study. The strength reduction compared to monolithic YSZ is consistent with the common behavior for CFCCs reported in literature where the mechanical strength of the composites is much lower than that of monolithic matrix materials [12,14,15]. The fracture toughness of

The mechanical properties of the Al2O3,f/YSZ composites with and without the LaPO4 coating are summarized in Table 3. The monolithic YSZ sintered by SPS using the same processing condition with the composites is included for comparison. The flexural strengths of the composites with and without the LaPO4 coating are 296 ± 31 MPa and

Fig. 3. SEM micrographs of the LaPO4 coated fiber. 4

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Fig. 5. The viscosity and the sedimentary volume percentage of YSZ slurries versus pH value ((a) and (b)) and APAA addition ((c) and (d)).

and the crack propagation paths of the Al2O3,f/YSZ composites with and without the LaPO4 coating under three-point bending tests. In Fig. 9(a), the Al2O3,f/YSZ composite without the LaPO4 coating shows a brittle fracture with a catastrophic failure at the maximum stress as most monolithic ceramic materials do. In contrast, the composite with the LaPO4 coating reveals significant damage tolerance and a non-brittle fracture mode. Its flexural stress versus displacement curve shows an initial stage with the elastic response followed by a deviation at some load, which indicates the YSZ matrix microcracking. After a non-linear stage up to the maximum stress, the stress drops due to fiber bundle failure near the tensile surface. Particularly, a non-brittle stage is observed, indicating energy dissipation during the crack propagation through toughening mechanisms such as crack deflection, fiber sliding and pullout before the composite ultimately ruptures [51]. Fig. 9(b) shows that in the composites with the LaPO4 coating, the crack deflects along the fiber-matrix interface and impedes the crack penetrating the fabrics. In comparison, the composite without the LaPO4 coating presents penetrating crack through fibers and fabrics (Fig. 9(c)), which is associated with the low fracture toughness. From the fractography shown in Fig. 10, it is clear that the composite LaPO4 without the coating is cleaved catastrophically, while the coating containing composite exhibits evident fiber debonding and pullout with delaminating cracks. The delamination behavior is likely to be caused by the incorporation of LaPO4 coating resulting in low interlaminar shear strength between the matrix and fabrics layer, which is usually beneficial to toughness improvement, even though at a cost of the mechanical strength. Therefore, the LaPO4 coating renders a weak interphase and results in a high damage tolerant composite, which is consistent with the results in previous work [34–37]. It should be noted that the fiber volume of the composite with the LaPO4 coating in Fig. 10(c) seems lower than the calculated value (44 %) in Table 3. This

Fig. 6. Median diameter of particles in 50 vol.% YSZ slurry and the viscosity of the slurry versus ball-milling time.

monolithic YSZ sintered by SPS is consistent with that reported in the previous study [50]. The sample with the LaPO4 coating exhibits a toughness of 15.93 ± 0.75 MPa·m1/2 which is significantly higher than that of the sample without the LaPO4 coating. As noted above, deformation of the fibers were observed in both composites with and without the coating. This might have affected the mechanical behavior of the composites due to the change of interfacial geometry leading to varied stress distribution and load transfer across the fiber/matrix interface. Nevertheless, the degree of deformation for the fibers in both composites are similar, which allows the comparison of the mechanical behavior between the two types of samples. Fig. 9 shows the typical flexural stress versus displacement curves 5

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Fig. 7. SEM images of (a) as-received and (b) after 9 -h ball-milled YSZ powder.

volume of ∼63 %. The inhomogeneity of the fiber distribution might have a negative impact on the mechanical strength.

Table 2 Properties of YSZ aqueous slurries for infiltration. Composition

50 vol.% YSZ +3 wt.% APAA

pH

∼9

Particle size (nm)

ζpotential (mV)

Viscosity (mPa·s)

∼ 60

38 – 40

160 – 180

4. Discussion As aforementioned, the Al2O3,f/YSZ composite without the LaPO4 coating shows the brittle failure behavior without fiber debonding and pullout due to the strong interfacial bonding between the fiber and matrix, as shown in Fig. 12(a) and (b). According to the Al2O3-ZrO2-Y2O3 ternary phase diagram [53], there is no expected chemical reaction between Al2O3 and YSZ at the sintering temperature used in this study, so that the fiber/matrix interface is frictionally bonded in the composites. The resistance to fiber sliding on the interface are determined by the sliding friction stress (τ) against fiber pullout as described in the following equation:

is a result of the inhomogeneous distribution of the fiber in the composite, as evidenced in Fig. 11 showing two typical areas of the sample with different fiber contents. The local fiber volume content was estimated by image analysis method using ImageJ software [52]. The fibersparse area with a thick matrix between fabrics has a fiber volume of ∼32 %. In contrast, the fiber-rich area has an obviously higher fiber

Fig. 8. Cross-section SEM micrographs of the Al2O3,f/YSZ composites (a)-(b) without LaPO4 coating, and (c)-(d) with LaPO4 coating. The inset in (d) shows the intact LaPO4 coating bonded to fiber. 6

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Table 3 Properties of Al2O3,f/YSZ composites and monolithic YSZ. Sample

Density (g/cm3)

Porosity (%)

Fiber fraction (vol.%)

Flexural strength (MPa)

Fracture toughness (MPa·m1/2)

YSZ Al2O3,f/YSZ Al2O3,f/YSZ (LaPO4 coated)

6.02 ± 0.05 4.55 ± 0.12 4.63 ± 0.19

∼ 0.05 ∼8 ∼8

– ∼ 48 ∼ 44

1047 ± 124 296 ± 31 277 ± 43

3.72 ± 0.30 6.89 ± 0.44 15.93 ± 0.75

τ=μσr

obtained under the same experimental conditions, is also shown for comparison. In a polycrystalline system, like the alumina fiber (Nextel™ 610) used in this study, the effect of polarization on the position of the R line is considered to be negligible [61], and the probing depth is much larger than the grain size of the fiber [62], so that the residual stress in polycrystalline alumina fiber can be assumed to be uniform over the probed volume. The peak shift is given by [59]

(3)

Where μ is the interfacial coefficient of friction between the Al2O3 fiber and the YSZ matrix, and σr the residual radial compressive stress. Both the fiber/matrix and fiber/LaPO4 coating/matrix interfacial coefficient of friction μ could be regarded to be a similar constant (0.2 – 0.3) in this study [54,55]. Therefore, the fibers pullout activity depends on the residual radial compressive stress at the interface. The residual radial stress for the composites without the fiber coating is given by [56]

σr =

Em Ef Ef (1 + vm) + Em (1 − vf )

(αm − αf )ΔT

Δv = (2Πa + Πc ) P

(5)

where Πa and Πc are the piezospectroscopy coefficients of the crystallographic a and c axis of chromium-doped alumina, and P is the stress. The sum of the piezospectroscopy coefficients (2Πa + Πc ), determined by He and Clarke [63], has the values of 7.59 and 7.61 cm–1 GPa–1 for the R1 and R2 lines, respectively. The R2 peak only depends on the density compression regardless of nonhydrostatic stresses and crystal orientation [64], so the R2-line shift (Δv2 ) is preferentially used to estimate the residual stress in alumina. From Eq. (4), another relation follows:

(4)

where Em and Ef represent the elastic modulus of the matrix and the fiber, αm and αf represent the coefficients of the thermal expansion of the matrix and the fiber, vm and vf represent the Poisson’s ratio of the matrix and the fiber, ΔT is the temperature range of cooling. For Al2O3,f/YSZ composites in this study, αf = 8.0 × 10−6 /K and αm = 10.8 × 10−6 /K at the temperature range of 20 ∼ 1000 °C [47], thus αm > αf , and the residual radial stress is compressive. Using Em = 212 ± 8 GPa and Ef = 354 ± 11 GPa determined from microindentation experiments, and literature data of vm = 0.3 [57] and vf = 0.2 [58], the radial compressive stress σr is estimated to be 422 ± 6 MPa. Further, the residual stresses in the Al2O3 fibers of the composites with and without the LaPO4 coating are determined by photostimulated Cr3+ luminescence based on piezospectroscopy. The luminescence of Cr3+ R-lines in Al2O3 exhibits a remarkable shift under stress, so an effective method has been developed to measure the residual stress in alumina containing materials [59,60]. Fig. 13 presents the typical luminescence of Cr3+ R-lines from the Al2O3 fiber at the fiber/matrix interface in the composites with and without the LaPO4 coating. The insert shows one location of the interfacial point used for luminescence collection. Ten points were collected for each fiber and thirty fibers were measured for each sample. The luminescence of unstressed fiber,

P=

Δv2 (GPa) 7.61

(6)

The residual stress of the fibers in the composite without the LaPO4 coating was therefore estimated from the shift of R2-line and Eq. (6) to be – 369 ± 63 MPa. The negative sign indicates a compressive stress. The experimentally average compressive stress appears to be lower than the theoretical average values calculated by Eq. (4), which may be attributed to stress release in the composite with the low porosity density. However, when the associated errors are considered, the experimental and theoretically calculated values agree with each other fairly well. On the other hand, as LaPO4 is coated on fibers in the composite, the measured residual compressive stress dramatically reduces to 131 ± 41 MPa, even that the CTE of LaPO4 (10.8 × 10−6 /K [35]) is similar to that of YSZ. The much reduced stress is likely caused by the smaller elastic modulus of LaPO4 (133 GPa [35]) and possibly stress release due to a relatively looser structure of the coating

Fig. 9. (a) Typical flexural stress versus displacement curves, and crack propagation in the Al2O3,f/YSZ composites (b) with the LaPO4 coating and (c) without the LaPO4 coating. 7

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Fig. 10. SEM fractography of the Al2O3,f/YSZ composites (a) and (b) without the LaPO4 coating, (c) and (d) with the LaPO4 coating.

Fig. 11. Two typical areas of the sample in Fig. 10(c). (a) a fiber-sparse area with a fiber volume of ∼32 %; (b) a fiber-rich area with a fiber volume of ∼63 %.

Fig. 12. SEM photographs of the interface of Al2O3,f/YSZ composite without the LaPO4 coating. 8

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Fig. 13. Luminescence of the R-lines of Cr3+ from Al2O3 fiber in the composites.

compared to the fiber and matrix. 5. Conclusions Continuous alumina fiber-reinforced yttria-stabilized zirconia (Al2O3,f/YSZ) composites with a high density and superior fracture toughness were successfully prepared by the optimized slurry infiltration and spark plasma sintering process. The microstructure and mechanical properties of the composites were characterized and the effect of LaPO4 fiber coating in the composite was discussed. It can be concluded that: i The pH value, APAA addition and ball milling time have a significant influence on the dispersity and stability of YSZ powders in the slurries. The optimized pH value, APAA addition and ball milling time are 9, 3 wt.% and 9 h, respectively. ii The composite with the fiber coating of LaPO4 achieves a high density of ∼ 92 % and high flexural strength of 277 ± 43 MPa and exhibits more damage-tolerance with a high fracture toughness (15.93 ± 0.75 MPa·m1/2) compared to the composite without the LaPO4 coating. iii The fiber pullout is restricted in the composite without the LaPO4 coating because of the large residual compressive stress (–369 ± 63 MPa) caused by the thermal mismatch between the fiber and matrix. The LaPO4 fiber coating works as a weak interphase and reduces the thermal stress at the interface effectively. As a result, the composite with the LaPO4 coating displays the non-brittle failure mode associated with the activation of various toughening mechanisms including crack deflection, fiber debonding and pullout, and delamination behavior. The current work shows that the Al2O3,f/YSZ composites with a low porosity (∼ 8 %) can possess not only higher strength but also improved fracture toughness compared with those conventional porous alumina or mullite matrix composites with a high porosity (25 ∼ 30 %) [14,65]. A dense and tough ceramic matrix is expected to be more resistant to environmental degradation at elevated temperatures, making Al2O3,f/YSZ a desirable material for applications at high temperature in extreme environments. Acknowledgements This research was supported by the National Natural Science Foundation of China (No. 51902197), the Shanghai Pujiang Program (No. 18PJ1406500), and the Start-up Foundation for the Youth Scholars of Shanghai Jiao Tong University (18X100040024). 9

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