Preparation and Mechanical Property of Alumina Fibers Reinforced Thin Architectural Ceramic Plate

The bers reinforced thin architectural ceramic plate of 900 mm×1800 mm×2.5 mm with high mechanical property was prepared by a fast-sintering method with a controllable ber dispersion process. The effects of ball-milling time to the dispersity, average length-diameter ratio and microstructure of alumina bers were investigated respectively. Meanwhile, the alumina ber contents to the volume density, water absorption, phase transformation and microstructure of the thin ceramic plate were researched. It is found that the two-steps ball-milling process can control the average length-diameter ratio of the alumina bers effectively and achieve a well dispersion mixture of bers and ceramic powders, the fast-sintering method is benecial for the protection of ber/matrix interface. The trend of the volume density and bending strength increases with the ber content from 0 wt% to 5 wt% and then decreases with the ber content from 5 wt% to 15 wt%. The bending strength of this composite reaches the maximum value of 146.8 MPa with the ber content is 5 wt%, which is corresponding to the strengthening of alumina bers and the formation of mullite crystallization in ber/matrix interface and matrix during the fast-sintering process.


Introduction
As a family of architectural ceramic products, the large-sized architectural ceramic plate more than 800 mm×800 mm, is attractive in appearance and convenient for decorating, has attracted research interest.
However, the raw materials of the architectural ceramic plate with high energy consumption are nonrenewable mineral resources, which is limited and over-exploited [1,2]. Thus, it is the signi cant tendency of the architectural ceramic plate to reduce energy consumption, emissions in industrial production and decrease the thickness of the plate to improve the utilization rate of mineral raw materials [3,4]. In comparison, the thickness of the large-sized thin architectural ceramic plate is only 3 ~ 5 mm, which is 50%~85% thinner on thickness and 40%~53% lower in energy consumption, compared to the architectural ceramic plate with the thickness of 10 ~ 20 mm [5][6][7]. Unfortunately, the bending strength of the large-sized architectural ceramic plate declines so dramatically with the decrease of thickness that it is unable to mold, corresponding to strong bonding energy of the covalent bond, which has di culty in slipping, deforming and tending to form brittle fracture in it under external loads [8][9][10]. Hence, the largesized thin architectural ceramic plate with the thickness less than 3 mm and enough mechanical property for engineering application is quite di cult to achieve [11].
Various strategies have been proposed to improve the bending strength of the large-sized thin architectural ceramic plate, mainly including increasing the molding pressure and adjusting the composition of ceramic matrix [12,13], which ultimately improves the bending strength of the large-sized architectural ceramic plate of 900 mm×1800 mm×3 mm to 96 MPa, yet remains to be barely satisfactory in further preparation. Besides, introducing the third phase reinforcement with good creep resistance in ceramic matrix, such as particles and bers, has proved to be a valid reinforcement method [14]. However, there are still signi cant challenges to effectively dispersing the bers in matrix, preventing creep deformation, non-oxide bers oxidization and controlling the uncontrollable reaction in ber/matrix interface over 800℃ to t production requirements [15,16]. As a consequence, the above improvements are common in special ceramic and there is too little work has been devoted to the corresponding applications for large-sized thin architectural ceramic plate at present [17,18].
Inspired by above considerations, to improve the mechanical property of the large-sized thin architectural ceramic plate, we rstly propose a fast-sintering method with the two-steps ball-milling process, which dispersed uniform dispersion of bers with appropriate aspect ratio into architectural ceramic matrix. And then brie y dispersing optimized inexpensive alumina bers as reinforcements in matrix and preparing the large-sized thin architectural ceramic plate of 900 mm×1800 mm×2.5 mm. Specially, the bending strength of the reinforced ceramic plate reaches the maximum value of 146.8 MPa with 5 wt% ber content, corresponding to the strengthening of alumina bers and the formation of mullite crystallization via fast-sintering method.

Preparation of ber reinforced thin ceramic plate
The preparation process was consisted of the following four steps. First, the optimized alumina bers (83 wt% θ-Al 2 O 3 , 12 wt% SiO 2 and 5 wt% Y 2 O 3 , the average length-diameter ratio is between 80 ~ 85) were immersed in EtOH (C 2 H 6 O, 99% purity, Sinopharm Chemical Reagent Co., Shanghai, China) via ultrasonic treatment for 40 min.; then the bers with a certain degree of dispersion were in ltrated in 2.8 mol/L KH570 solution (C 10 H 22 O 4 Si, 95% purity, Sinopharm Chemical Reagent Co., Beijing, China) for surface modi cation; after that, the mixed solution was stirred gently for 20 min. In the second step, the preliminarily dispersed alumina bers were weighed based on the mass ratio of 1 wt%, 3 wt%, 5 wt%, 7 wt% and 10 wt% for ceramic powders; then the alumina bers, EtOH and 0.5 ~ 0.8 mm talumina ballstone were evenly mixed in jar mill with the mass ratio of alumina bers: ballstone: EtOH = 1:1.5:1; after that, the rst-step ball-milling process was carried out for 3 min., 5 min., 7 min., 10 min. and so on. In the third step, the alumina bers were optimized via optical microscope and vertical centrifuge, then the chopped bers were evenly mixed up with the ceramic powders whose composition was illustrated in Table 1 via secondstep ball-milling method. In the fourth step, the mixture powders were molded to 900 mm×1800 mm×2.5 mm as forming green body under the pressure of 35 MPa, and then the alumina bers reinforced thin architectural ceramic plate was prepared via fast-sintering at 1200℃ for 30 min.

Characterization
The microstructure of optimized alumina bers was observed by high power optical microscope (model EVOS M7000, China) and a vertical centrifuge (model TG-20W, Germany). The scanning electron microscopy (SEM, JSM-IT200, Japan) was used to observe the morphology of dispersed alumina bers and the cross-section microstructure etched by 1.6 mol·L − 1 hydro uoric acid. The phase compositions were characterized by an X-ray diffraction (XRD, Empyrean Alpha-1, Malvern) under the diffraction conditions of Cu and K α . Meanwhile, the diffraction angle parameters were set as 10°~85°, the scanning speed was 5°/min. The bending strength of the samples was characterized by three point bending method in universal mechanical testing machine (CTM2050, China), the average value of ve parallel samples reduces the errors of measurement. Moreover, the water absorption of the thin plate must be less than 0.5% according to the ISO 10545-3-2018 [19]. The volume density was measured by the Archimedes method, which could be calculated according to the following formula: where W represented the value of water absorption, B denoted the mass value after absorbing water and G represented the mass value after drying.
3 Results And Discussion 3.1 Effects of ball-milling time on microstructure and dispersity of bers  Fig. 1 (a)), which results from electrostatic attraction between bers. The dispersion of alumina bers after the rst-step ball-milling process for 5 minutes is improved obviously ( Fig. 1 (b)), con rming that the short-time friction unties the cross-linked structure and disperses bers effectively. As deteched in Fig. 1 (c), alumina bers treated for 10 minutes lose their original brous structure, which may attribute to the drastic and long-time physical friction, promoting the cleavage of the inner ber grains. Fig. 1 (d, e, f) shows the dispersion of alumina bers in matrix after the second-step ball-milling process in different milling time. It can be found that ceramic powders obviously agglomerated distribute in-homogeneously around dispersed alumina bers ( Fig. 1 (d)), this is because original ceramic powders are too agglomerating to in ltrate evenly into the inner cross-linked space. Ulteriorly, the numerous inter-space between uneven matrix and alumina bers may lead to weak bonding between ber/matrix interface, which can't transfer the internal stress from matrix to bers in time while carrying external loads [20][21].
Besides, the alumina bers treated for 1 minute in the second-step ball-milling process are uniformly distributed in ceramic powders (Fig. 1 (e)), revealing the two-steps ball-milling process can achieve a well dispersion mixture of bers and ceramic powders. Inversely, the brous structure of bers is destroyed absolutely over 3 minutes (Fig. 1 (f)), which is consistent with the analysis in Fig. 1 (c). Fig. 2 (a) shows the average length-diameter ratio of bers after the rst-step ball-milling process in different ball-milling time. The average length-diameter ratio of bers decreases tardily from 83.8 to 58.9 with the ball-milling time from 0 minutes to 5 minutes, and then declines drastically from 58.9 to 4.6 with the time from 5 minutes to 10 min., illustrating that the friction between bers and ballstone has a top priority to disperse the cross-linked structure, and then chie y damages the original brous structure of bers, decreasing the average length-diameter ratio. Notably, the alumina bers with little average lengthdiameter ratio by long-time friction are di cult to de ect micro-crack paths effectively, which is adverse to promoting the mechanical property of the thin architectural ceramic plate [22]. In comparison, the average length-diameter ratio of bers after the second-step ball-milling process decreases sharply from 58.9 to 7.6 with the time from 0 minutes to 3 minutes (Fig. 2 (b)). Besides, the drop rate of it is much faster than that in Fig. 2 (a), which is corresponding to the synergistic drastic friction of matrix, bers and ballstone. Based on above results, the rst-step ball-milling for 5 minutes followed by the second-step ball-milling for 1 minute is proved to be the best two-steps ball-milling process.  [23]. In comparison, the alumina bers of α-Al 2 O 3 have higher tensile strength than the bers of θ-Al 2 O 3 , further con rming that the fast-sintering method with short holding time is bene cial to promoting the mechanical properties of alumina bers as the reinforcement instead of damaging them unilaterally. Besides, the diffraction peaks of 16.4°, 26.3°, 33.3°, 40.8° and 60.9° can be indexed to the mullite (PDF no. 15-0776), attributing to the appropriate phase ratio of alumina and silicon in glass phase. Furthermore, the mass ratio of alumina to silicon of the bers is 7.85 which is much higher than that of the matrix of 1.51, the fused alumina phase in bers may in ltrate into the glass phase in fastsintering process, which collectively increases the alumina content in liquated glass phase and promotes the crystallization of mullite and whiskers [24,25]. Fig. 5 shows the cross-section SEM images of the as-prepared samples with different alumina ber contents. It can be noticed that the particles without obvious coarse grains of the cross-section sintered at 1200℃ for 30 minutes is compact (Fig. 5 (a)), suggesting that the fast-sintering system is suitable.

Effects of alumina ber contents on the thin ceramic plate
Meanwhile, the alumina bers are perpendicular to the cross-section and embed tightly in the matrix (Fig.  5 (b)), this is because bers and surrounding matrix are subjected to tensile and compressive stress respectively during the fast-sintering process, which results from thermal expansion coe cient difference between alumina ber of 9.2×10 -6 /℃ and matrix of 4.8×10 -6 /℃ [26]. Namely, the existence of internal stress is bene cial to offset the external stress loads and improve the bending strength of the thin architectural ceramic plate. However, it is the matrix that mainly bears the loads at this moment limited by only 3 wt% bers contents. As shown in Fig. 5 (c, d), the number of pores in matrix increases obviously with the ber content more than 5 wt% and bers in the cross-section of the thin ceramic plate disperse uniformly. Close observation reveals that there are numerous white substances in the pores of the matrix, which is presumed to be the in-situ brous mullite and whiskers conformed by the analysis in Fig. 4.   Fig. 6 shows the volume density and water absorption of the thin architectural ceramic plate with different alumina ber contents. On the one hand, the volume density of the thin plate increases slowly from 2.722 g·cm -3 to 2.733 g·cm -3 with the ber content from 0 wt% to 5 wt%, which mainly ascribes to the density of alumina bers for 6.22 g·cm -3 is much higher than that of matrix for 2.722 g·cm -3 . On the other hand, the volume density of the thin plate decreases from 2.782 g·cm -3 to 2.587 g·cm -3 with the ber content from 5 wt% to 15 wt%, attributing to the inner pores increase in matrix. Simultaneously, the water absorption of the thin architectural ceramic plate decreases tardily from 0.172% to 0.152% with the ber content from 0 wt% to 5 wt%, further con rming that the in-situ mullite and whiskers during the fastsintering method can ll the inner pores and increase the compactness of ceramic matrix. Moreover, the maximum bending strength of the thin architectural ceramic plate reaches 146.8 MPa that is 70.31% higher than the blank sample of 86.2 MPa with 5 wt% ber contents, which is corresponding to the strengthening of alumina bers and the formation of mullite crystallization of the matrix during the fast-sintering process. Fig. 8 shows the cross-section SEM images of the as-prepared samples with 5 wt% alumina ber content. Apparently in Fig. 8 (a, b), the expanding micro-cracks which are supposed to be single orientation are de ected to be discontinuous or winding orientation by inner particles and bers, indicating that the reinforcements can slow down the prolongation speed of micro-cracks in matrix and retard the stress concentration in the micro-crack tip owing to the Gri th micro-crack theory [27,28]. Combined with the EDS analysis of spot 1 with the mass contents of Al, Si, O and Y are 36.95 wt%, 8.5 wt%, 41.02 wt% and 9.26 wt% (Fig. 8 (e)), illustrating that there are alumina bers embed tightly in the matrix. In addition, the ber/matrix interface has clear boundaries after the fast-sintering method and there is plenty of white substance in it, suggesting that the fast-sintering method is appropriate and it can prevent the ber/matrix interface from excessive thermodynamics damage. To clarify the components of the white substance in ber/matrix interface, the EDS analysis of spot 2 is analyzed in Fig. 8 (f), proving that it is the mullite with the mass contents of Al, Si and O are 41.22 wt%, 11.45 wt% and 42.86 wt%, severally. Fig.  8 (b, c and d) shows the distribution of reinforcements along the fracture micro-cracks. It is found that there are not only thick bers but also ner brous substance at the micro-cracks fracture, both of them are bridge-pulled out of the matrix. Ulteriorly, as shown in Fig. 8 (g, h), the element mass contents of the thick ber at spot 3 are 32.47 wt% Al, 10.58 wt% Si, 39.68 wt% O, 11.21 wt% Y and the ner ber at spot 4 are 38.32 wt% Al, 14.18 wt% Si and 45.17 wt% O, respectively. It can be accurately inferred that the thick ber is alumina ber and the ner ber is brous mullite that is precipitated out by crystallization in matrix. Hence, the reinforcements of alumina bers, particles and in-situ brous mullite in matrix and ber/matrix interface are bene cial to de ecting the paths of the micro-cracks (Fig. 9), which can synergistically slow down the prolongation speed of inner micro-cracks and enhance the mechanical property of the large-sized thin architectural ceramic plate.

Conclusion
In summary, the alumina bers reinforced thin architectural ceramic plate of 900 mm×1800 mm×2.5 mm with high mechanical property is successfully prepared by the fast-sintering method with a controllable ber dispersion method. It is found that the rst-step ball-milling for 5 minutes followed by the secondstep ball-milling for 1 minute is proved to be the best two-steps ball-milling process, which can evenly disperse bers with an appropriate average length-diameter ratio in matrix. The fast-sintering method can prevent the ber/matrix interface from excessive thermodynamics damage and promote the fast crystallization process of mullite, which is bene cial to improve the mechanical property of this thin plate. The maximum bending strength of the thin plate with 5 wt% alumina bers contents is 146.8 MPa that is 70.31% higher than the blank sample of 86.2 MPa, corresponding to the strengthening of alumina bers and the formation of mullite crystallization in ber/matrix interface and ceramic matrix. In addition, the micro-cracks de ection, particle diffusion enhancement and ber bridge-pull out of the matrix by particles, alumina bers and in-situ brous mullite, are proved to be the main reinforcement mechanisms. We expect that our ndings will have a guiding signi cance for industrial production and future research of the large-sized thin architectural ceramic plate. Figure 1 SEM images of the morphology and dispersity of alumina bers in different ball-milling time; after the rst-step ball-milling process (a) 0 min.; (b) 5 min.; (c) 10 min.; and after the second-step ball-milling process (d) 0 min.; (e) 1 min.; (f) 3 min.

Figure 2
The average length-diameter ratio of alumina bers in different ball-milling time; (a) the rst-step ballmilling process; (b) the second-step ball-milling process.

Figure 3
The optical photos of the as-prepared thin architectural ceramic plate; (a) the sample of the prepared product; (b) the sample for characterization.

Figure 5
The cross-section SEM images of the as-prepared samples with different ber contents; (a) blank sample; (b) 3 wt% ber contents; (c) 5 wt% ber contents; (d) 7 wt% ber contents.

Figure 6
The volume density and water absorption of the thin architectural ceramic plate with different alumina ber contents. The cross-section SEM images of the as-prepared samples with 5 wt% alumina bers contents; (a) microcracks de ection and particles diffusion enhancements; (b, c and d) ber bridge-pull out from matrix; (e, f) the EDS analysis corresponding to the spot 1 and spot 2 in (b); (g, h) the EDS analysis corresponding to the spot 3 and spot 4 in (d).