1 Introduction

Fracture toughness is a crucial mechanical property in the realm of advanced ceramics as it measures the material's resistance to crack propagation (Ref 1). Unlike traditional ceramics, advanced ceramics are specifically engineered for extreme environments such as aerospace, biomedical, armor components, and cutting tools (Ref 2,3,4). Given that these materials are often subjected to harsh mechanical conditions, their ability to resist cracking and fracture is of utmost importance (Ref 5). In advanced ceramics, where brittleness is a common challenge, maintaining a relatively high fracture toughness is integral for preventing catastrophic failure (Ref 6). Accordingly, many attempts have been made to enhance the fracture toughness of advanced ceramics. The dispersing of a second phase such as fibers, whiskers, and platelets in the microstructure can increase the toughness (Ref 7,8,9). Nevertheless, this process has some drawbacks, involving complex preparation stages, high expenses, difficulty in achieving a homogeneous dispersion of additives, and possible health hazards connected to the dispersion of fibers and whiskers (Ref 10).

Alumina (Al2O3) ceramic is a basic advanced ceramic material used in many structural applications; therefore, many studies focused on the fracture toughness improvement of Al2O3-based materials are encountered (Ref 11, 12). The fracture toughness of Al2O3-based materials can be enhanced by elongated grains, such as rare earth aluminates. Various aluminate compounds, like LaAl11O18, which has an anisotropic growth habit, are appropriate for toughening reinforcement. The utilization of rare earth oxides such as La2O3, Y2O3, CeO2, Nd2O3, and Eu2O3 can improve the microstructure of Al2O3 ceramics in favor of toughness (Ref 6, 13,14,15). Rani et al. reported the impact of rare earth additives Yb3+, La3+, and Er3+ on the enhancement of mechanical properties of Al2O3 ceramics at ambient temperature. For the samples sintered at high sintering temperatures, a considerable morphologic alteration appeared. In Al2O3 grain boundaries, rare earth ions preferentially segregate to basal planes (0001), which promotes the growth of anisotropic elongated grains and increases toughness (Ref 15). However, there are a few studies focusing on toughness evaluation of Al2O3 by adding rare earth oxide additives (Ref 13,14,15,16). There is a lack of detailed examination of the effects of rare earth oxides on the microstructure and toughness of Al2O3 materials using a reliable measurement method.

Accurate fracture toughness testing of ceramic materials is essential to fully understand the brittleness prevalent in these materials. At present, the most common techniques for evaluating ceramics' fracture toughness include the indentation fracture method, single-edge precracked beam method, and single-edge notched beam method (Ref 1, 17). Among these, the indentation fracture method has gained considerable attention due to its simplicity in sample preparation and ease of technique application. This method often involves creating surface cracks using the Vickers indentation technique. A hardness indenter generates a crack at the surface, and the crack length is used to calculate fracture toughness (Ref 5). However, this approach has significant sources of variance such as changing crack-length measurements, differing indentation-crack shapes, and the relationship between computed formulas and indentation force all of which lead to inconsistent outcomes (Ref 18). In the single-edge notched beam method, a rectangular beam with a single-edge notch is loaded in three-point bending till fracture appears. This method is standardized and commonly approved but needs precise notch preparation, sensible alignment and notch geometry (Ref 1, 18, 19).

The classic configuration of the uniaxial flexural strength method is used in the single-edged precracked beam (SEPB) methodology, which yields toughness results with good precision (Ref 1, 17). SEPB method allows a bending beam with a through-the-edge crack that has a straight-through geometry and precracking is attained using a “bridge indentation” procedure that consists of the compression of a ceramic beam lying on a hardened steel anvil via a flat stiff pusher. On the tensile side of the ceramic beam, which is pre-damaged by indentations that serve as a precrack starter, the anvil's groove causes local tensile stresses. With a rising compression load, the median crack produced underneath a Vickers or Knoop indent extends stably because of the residual stress resulting from the indentation (Ref 1). The precracked ceramic beam is then fractured in a three-point flexure, and the toughness can be evaluated (Ref 1, 20, 21). The precrack created by the SEPB technique is accepted as sharp as inherently grown or fatigue-induced cracks. This meets the fundamental requirement of fracture toughness testing that a crack must be atomically sharp. The SEPB method provides an improvement over the drawbacks of the single-edge notched beam method where razor blade-sharpened notches are created (Ref 1).

The effect of different contents of samarium oxide (Sm2O3) additives on the fracture toughness of Al2O3 ceramics using a reliable measurement technique has not been thoroughly examined. In the present research, Sm2O3 additive in different low amounts (0.1-2 vol.%) on the fracture toughness of Al2O3 was investigated via the SEPB method. Sm2O3, a light rare earth element oxide with a high melting temperature of over 2000 °C, is a potential additive to create ceramic composites with superior mechanical properties because SmAlO3 rod-like phase is observed to elongate in the direction of growth after the reaction between Al2O3 and Sm2O3 at sintering temperature. Mechanical properties can be improved by the SmAlO3 phase that acts as short fibers or whiskers (Ref 14). The relationship between microstructural effects and fracture toughness of Al2O3-Sm2O3 ceramics was examined to demonstrate the potential of Sm2O3 in increasing fracture toughness according to its content. Improving the fracture toughness of Al2O3 ceramics can significantly develop their performance and range of structural applications by enhancing longevity and reliability. Al2O3 ceramics are widely used in personal and vehicle armor, and it is expected that the production of these materials with higher toughness will provide high ballistic performance, especially in multihit conditions (Ref 22). Also, high toughness is expected for biomedical implants like hip or knee joint replacements to reduce the risk of implant failure (Ref 23). The present study has evaluated the applicability of Al2O3-Sm2O3 ceramics for structural applications requiring high toughness such as armor and biomedical technologies using a highly precise toughness measurement technique.

2 Experimental Procedure

In the processing stage of the Al2O3-Sm2O3 ceramics, high-purity α-Al2O3 powder (99.95%, 0.25-0.45 μm, Alfa Aesar, Germany), Sm2O3 powder (99.95%, 50 nm, Nanografi, Türkiye), polyacrylic acid, polyvinyl alcohol, and glycerol were utilized. The high-purity powders were chosen on purpose to be able to see the effects of Sm2O3 addition on both densification and fracture toughness clearly. The processing involved mixing different amounts of Sm2O3 powder (0, 0.1, 0.3, 0.5, 0.8, 1.0, 2.0 vol.%) with Al2O3 powder in distilled water with a dispersant (0.5 wt.%) through ball milling. After adding an aqueous solution consisting of 2.25 wt.% binder and 0.75 wt.% plasticizer into the ball-milled mixture, the resulting mixture was dried, crushed and sieved through a 90 μm sieve.

The prepared Al2O3-Sm2O3 powders were uniaxially dry pressed at 100 MPa in a 57 mm length and 6 mm width rectangular steel mold for the fracture toughness tests. The green specimens were then subjected to pressureless sintering in air at 1550 °C/2 h and binder removal was carried out at 600 °C/1 h utilizing a heating rate of 2 °C per minute while sintering. Following these processes, the samples were ground using a single-sided lapping machine, employing SiC abrasive powder. The aim was to achieve uniform thickness for the specimens with a tolerance of ± 0.05 mm, to ensure compatibility with subsequent testing. After the lapping, the surfaces of the specimens were sanded with 1200 grit SiC sandpaper and polished to 1 μ diamond suspension. Archimedes’ method was used to measure the bulk density of all the ceramics and the relative density was calculated from the attained bulk density and the theoretical density respecting the rule of mixtures. The samples were obtained as rectangular bars with a width of 5 mm, a thickness of 3 mm, and a length of 47.5 mm with minimal variations between samples.

After the grinding and polishing process, the polished 3 mm wide surface of each sample was indented by a Vickers indenter with equally spaced three loads of 98 N as shown in Figure 1(a) using Instron® Wolpert Testor 2100 test machine to prepare for the SEPB toughness test (ASTM C1421). Compressive load was employed to achieve a bridge indentation precrack via an anvil with a 3 mm wide groove as given in Figure 1(b) in Instron® 5569 device. Next, the apparatus was operated in the bending mode, and an attempt was made to propagate cracks on the surface created by the Vickers indentation. These operations were conducted at a loading rate of 0.05 mm/min, and 15 samples were prepared for each Sm2O3 volume content. Since there was no sonic sensor in the setup, sounds coming from the samples were listened to, and when a very faint crack sound was heard, the load was lifted. In fact, in some samples, the cracks were observed when examined even without hearing any sound. A pink dye penetrant was subsequently sprayed on the samples in which a precrack was created and then allowed to dry to better detect the length and location of the precrack. The location where the cracks opened was marked with an arrow using a pencil. The samples containing cracks that extended from one end of the sample to the other were discarded.

Fig. 1
figure 1

Stages of the single-edge-precracked-beam method (a) Vickers indentation, (b) crack initiation, (c) fracturing by three-point bending after the precrack formation in the samples, (d) the surfaces of a fractured sample

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The samples, which were intentionally precracked in a controlled manner, were placed centrally in the three-point bending test setup with the crack facing downward and fractured as in Figure 1(c). During the test, a support span of 30 mm, a loading rate of 0.3 mm/min, and a data acquisition rate of 0.25 N were used. After the tests, using the pictures of fracture surfaces as in Figure 1(d), the crack lengths (a = (a0.25 + a0.50 + a0.75)/3; as defined in ASTM C1421 standard) were measured employing ImageJ software. Using these crack lengths, the fracture toughness values were calculated with the equation provided below (Ref 21):

$${K}_{Ipb}=g\left(\frac{{P}_{\text{max}}{S}_{0}{10}^{-6}}{B{W}^{3/2}}\right)\left(\frac{{3\left(a/w\right)}^{1/2}}{2{(1-\frac{a}{w})}^{3/2}}\right)$$
(1)

where \(g\) is the function of the ratio a/W calculated from the standard, KIpb is the fracture toughness (MPa·m1/2), Pmax represents the maximum fracture load (N), So is the span aperture (m), B is the thickness of sample (m), W is the width of sample (m), and a represents the average crack length (m).

In addition to conducting toughness tests, the x-ray diffraction method (Bruker® D8 Advance) was utilized to determine the phases in the sintered specimens within the 2θ, range of 10°-90°. The specimens underwent thermal etching at 1450 °C for 90 min at air atmosphere. The microstructure was examined, and the mapping analysis was performed using scanning electron microscopy (SEM, TESCAN Mira3 XMU, Czechia). The analysis of the grain size of Al2O3 was carried out using the linear intercept method with over 100 intercepts marked. A set of random lines were superimposed over the microstructure images and the number of times the lines intersect the boundaries of Al2O3 grains were counted. The total length of the lines used for counting the intercepts was measured. The average intercept length was calculated by dividing the total length of the lines by the number of intercepts. The average intercept length is related to the average grain size. The average intercept length correction factor was multiplied by 1.56 for the conversion from the two-dimensional intercept length to the three-dimensional particle size (Ref 24).

3 Results and Discussion

Figure 2 presents the XRD diffraction peaks for Al2O3-Sm2O3 ceramics prepared in varying Sm2O3 volume content. The analysis reveals the presence of two phases: α-Al2O3 and SmAlO3 formed through the reaction between Al2O3 and Sm2O3. “A” corresponds to the peaks associated with Al2O3, while "SA" signifies the peaks attributed to SmAlO3. The SmAlO3 peaks became notably pronounced, particularly with Sm2O3 content exceeding 0.3 vol.%. Based on these results, it is appropriate to state that the ceramics produced after sintering were actually in Al2O3-SmAlO3 composition. These results were also confirmed in the study by Taşdemir et al. (Ref 25). In the XRD graphs and in the subsequent figures, the names of ceramics were referred to as “AXSm” where “X” is the volume ratio of Sm2O3 and ‘‘Sm’’ represents Sm2O3.

Fig. 2
figure 2

XRD results of the sintered Al2O3-Sm2O3 ceramic samples (A: Al2O3; SA: SmAlO3)

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The SEPB method was chosen for this study as a measurement method of fracture toughness owing to the advantages over the other techniques such as SENB and Vickers indentation as mentioned in the previous section. The SEPB method is not only superior precision in measurement of toughness compared to the commonly used Vickers indention technique but also the microstructural influences make the Vickers indentation method impractical. Anstis et al. stated that the grain size of polycrystalline Al2O3 affects cracking so irregular intergranular cracking occurs in coarse-grained Al2O3 ceramics (Ref 26). In this regard, the Vickers indentation technique was not considered a suitable toughness measurement method for such coarse-grained ceramics. A similar situation was encountered with Sm2O3-containing samples since the Vickers indentation was not clearly obtained as given in Figure 3(b) due to large Al2O3 grain sizes. Figure 3 shows the SEM micrographs of the Vickers indentations on the surface of pure Al2O3 and 0.3 vol.% Sm2O3 containing Al2O3 ceramics. Therefore, the SEPB method has been preferred as the toughness measurement method for the coarse-grained Al2O3-Sm2O3 ceramics in this study.

Fig. 3
figure 3

SEM micrographs of the indents generated under a 10 kg load using the Vickers indentation (a) the pure Al2O3, (b) 0.3 vol.% Sm2O3 containing Al2O3

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SEM analyses were carried out to perform the microstructural investigations. The SEM image of pure Al2O3 is given in Figure 4 separately due to significant grain size differences with the Al2O3-Sm2O3 ceramics. After the reaction of Al2O3 and Sm2O3 at the sintering temperature, SmAlO3 phase formation occurred in different morphologies based on the Sm2O3 content. Figure 5 shows the thermally etched SEM micrographs of Al2O3-Sm2O3 ceramics. In the SEM micrographs, gray areas belong to Al2O3, while white phases belong to SmAlO3 (Ref 25). The SmAlO3 phase distributed relatively homogeneous for 0.5, 0.8 vol.% Sm2O3 containing ceramics but its morphology changed up to the Sm2O3 content. For 0.1 vol.%, the SmAlO3 phase is very rare. In certain areas, the SmAlO3 phase appeared relatively spherical, while in other regions, it took on an irregular, short rectangular shape when containing 0.1, 0.3, 0.5, 0.8, and 1.0 vol.% Sm2O3. However, the morphology of SmAlO3 turned highly spherical after the addition of 2.0 vol.% Sm2O3 as shown in Figure 5(f).

Fig. 4
figure 4

Thermally etched SEM micrograph of the pure Al2O3.

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Fig. 5
figure 5

Thermally etched SEM micrographs of Al2O3-Sm2O3 ceramics for different Sm2O3 volume contents (a) A0.1Sm, (b) A0.3Sm, (c) A0.5Sm, (d) A0.8Sm, (e) A1Sm, and (f) A2Sm

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Table 1 illustrates the relative density, Al2O3 grain size and fracture toughness values of Al2O3-Sm2O3 ceramics. Although the lowest relative density and high porosity occurred for the ceramic containing 0.1 vol.% Sm2O3 as shown in Figure 5(a), it was observed that the addition of Sm2O3 did not significantly affect the densification of Al2O3, resulting in close relative density values for Al2O3-Sm2O3 ceramics with the pure Al2O3. However, a notable increase in fracture toughness was observed after the addition of Sm2O3 for all volume ratios. Despite this, the toughness values for the samples with Sm2O3 additive displayed relatively high standard deviations. No standard deviation was provided for the 2.0 vol.% Sm2O3 samples due to limited data. When considering the standard deviations, the highest toughness values were seen in the ceramic with 0.8 vol.% Sm2O3 content, while the samples containing 0.5 vol.% Sm2O3 displayed an average toughness value that was almost equal to the 0.8 vol.% Sm2O3 ceramic, but around 70% higher than the pure Al2O3.

Table 1 The relative density, Al2O3 grain size and fracture toughness values of the Al2O3-Sm2O3 ceramics
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The fracture toughness values obtained point out the importance of considering both the size of Al2O3 grains and the presence of the SmAlO3 phase concerning the characteristics of toughness and microstructure. The sizes of Al2O3 grains in each composition were analyzed using the linear intercept method as explained in the previous section, with detailed results presented in Table 1 and Figure 6. After the addition of Sm2O3, a solid-state reaction occurs between Sm2O3 and Al2O3, causing an increase in aluminum vacancy concentration due to Sm3+ cations replacing Al3+ cations. This higher aluminum vacancy concentration enhances the diffusion of all elements and accelerates solid-state sintering, leading to larger Al2O3 grain sizes (Ref 14, 25). In this study, grain growth became more pronounced since no additive was used to prevent abnormal grain growth in the source powder of Al2O3. Significant grain growth was observed in all Sm2O3 volume ratios compared to the pure Al2O3, with grain sizes reaching up to 35 μm. The presence of randomly distributed SmAlO3 phase at grain boundaries or within grains did not hinder the growth of Al2O3 grains, as evidenced by the clear grain growth observed. Figure 6 depicts the relationship between fracture toughness and Al2O3 grain size as influenced by Sm2O3 content, showing that an increase in Al2O3 grain size leads to an improvement in toughness.

Fig. 6
figure 6

Fracture toughness and Al2O3 grain size relationship of Sm2O3 content. (■: grain size, ★: toughness)

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The fracture toughness of ceramic materials is proportional to the square root of the fracture energy; as can be seen in the relationship of K = (2Eγ)1/2 where γ is the surface energy and E is the elastic modulus. Fracture energy and fracture toughness of anisotropic ceramics like Al2O3 can be improved through grain growth, as crack bridging occurs due to larger grains which create compressional stresses due to thermal expansion anisotropy. Al2O3 ceramics possess residual stresses due to the thermal expansion anisotropy, and as the grain size increases, these stresses become larger and more irregular (Ref 27). Fracture energy increases as the grain size of Al2O3 increases up to a certain level, approximately 100 μm (Ref 28). In this research, the Al2O3 grain size, initially at 2 μm, increased to 35 μm after the addition of Sm2O3, resulting in a significant improvement in fracture toughness from 3.8 MPa·m1/2 to values ranging from 4.7 to 6.6 MPa·m1/2, as illustrated in Figure 6.

Three primary mechanisms that affect the toughness of polycrystalline ceramics have been recognized. These include dispersed microcracking, crack-trapping tough grains, and the dissipation of frictional energy when grains are pulled out during cracking. In the case of a coarse-grained material undergoing frictional fracture bridging, the amount of energy dissipated due to this process increases as the size of the bridging grains grows, resulting in enhanced toughness (Ref 29). This phenomenon was evident when the SEM images of the fracture surfaces of the samples were analyzed. In Figure 7, the SEM image of the fracture surface of pure Al2O3, representative of the unstable crack growth region resulting from the toughness test, is depicted. This fracture mode exhibited characteristics of both intergranular and transgranular cracking. Specifically, intergranular cracking was observed in regions with smaller grain sizes, while transgranular cracking was evident in regions characterized by coarse-grained structures. In monolithic Al2O3 ceramics, coarse grains, elongated grains, and grains with a high aspect ratio can lead to an increase in fracture toughness. When the crack path increases in intergranular crack propagation, it can contribute to fracture toughness by grain bridging and grain pull-out (Ref 30). Al2O3 grains with equiaxed morphology had low toughness, and these grains fractured intergranularly. The current toughness value of Al2O3 (3.8 MPa·m1/2) was provided by the mixture of these mechanisms, but the smaller grain size of the pure Al2O3 compared to the Al2O3-Sm2O3 ceramics resulted in lower fracture toughness.

Fig. 7
figure 7

Unstable crack growth region of the pure Al2O3 sample after the toughness test

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Figure 8 shows the SEM images of the fracture surfaces of unstable crack growth regions for the Al2O3-Sm2O3 ceramics with large Al2O3 grain sizes. Generally, for all Sm2O3-containing ceramics, both transgranular and intergranular fracture behavior can be noticed and the transgranular regions became more pronounced as the Sm2O3 ratio increased. For the 0.1 vol.% Sm2O3, intergranular fracture regions are quite distinct. It is thought that mechanisms such as grain pull-out effect and crack deflection occurring in coarse Al2O3 grains are the main mechanisms that provide increased toughness in Al2O3-Sm2O3 ceramics. The grain pull-out effect was manifested in areas where there was intergranular fracture. The areas where intragranular and intergranular regions were side by side indicated the presence of a crack deflection mechanism.

Fig. 8
figure 8

Fracture surface images of the unstable crack region of the Al2O3-Sm2O3 ceramics after the toughness test, (a) A0.1Sm, (b) A0.3Sm, (c) A0.5Sm, (d) A0.8Sm, (e) A1Sm, and (f) A2Sm

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The effect of coarse Al2O3 grain size on toughness was seen especially when the toughness value of pure Al2O3 was compared with the ceramics containing 0.1 vol.% Sm2O3, where the grain size increased initially. As shown in Table 1 and Figure 6, the increase in Al2O3 grain size for the pure Al2O3 from around 2 to 18 μm even after the addition of 0.1 vol.% Sm2O3 was related to the effect of coarse Al2O3 grains on fracture toughness. However, the volume content, morphology and dispersion of the SmAlO3 phase were the determining factors for the fracture toughness in other additive ratios that have relatively closer Al2O3 grain sizes to each other. This is especially pronounced for 0.5 and 1.0 or 2.0 vol.% Sm2O3 content. Despite having similar Al2O3 grain sizes, these ceramics exhibited differences in toughness values. In the sample containing 0.1 vol.% Sm2O3, the SmAlO3 phase did not make a significant contribution to the toughness since it was insufficient in quantity and so not distributed homogeneously. A similar situation occurred in the sample containing 0.3 vol.% Sm2O3 and almost similar toughness values (5.3-5.6 MPa·m1/2) were obtained with 0.1 vol.% Sm2O3 containing ceramics with high standard deviations. The highest standard deviations for the toughness and Al2O3 grain size belong to the ceramics with 0.8 vol.% Sm2O3 content, and the highest toughness values were also obtained for these ceramics.

Different particle morphologies affect fracture toughness differently for particle-reinforced ceramic composites. Angular particles with sharp edges can promote enhanced fracture toughness by enabling more significant mechanical interlocking with the matrix. This situation increases energy absorption in fracture since more energy is needed to pull these particles out. Nevertheless, spherical particles make it possible to have less mechanical interlocking but can still improve toughness by promoting crack deflection. Their smooth structure can allow for easier pull-out, that can dissipate fracture energy (Ref 14, 31,32,33). The morphology of the SmAlO3 phase in the samples had mostly an irregular and short rectangular shape in various regions but turned mainly into a spherical shape for 2 vol.% Sm2O3 content for this study as shown in Figure 5(f). It is thought that this situation affected the toughness difference between the Al2O3-Sm2O3 ceramics with 2 vol.% Sm2O3 and the samples with 0.5 and 0.8 vol.% Sm2O3 content. Additionally, the presence of a significant amount of brittle SmAlO3 phase could reduce the fracture toughness of Al2O3-Sm2O3 ceramics (Ref 14). Consequently, the toughness decreased after the addition of 1.0 and 2.0% Sm2O3 content which shows that in this case, 0.5 and 0.8 vol.% Sm2O3 contents are more optimum in increasing fracture toughness of Al2O3-Sm2O3 ceramics.

The dispersion of the SmAlO3 phase also affected the toughness values in Al2O3-Sm2O3 ceramics for 1.0 vol.%Sm2O3 as compared to 0.5 and 0.8 vol.% Sm2O3 content. In comparison with the SEM image of the sample with 0.8% Sm2O3, it appeared to have less SmAlO3 phase, but this was due to the non-uniform dispersion of SmAlO3 phase. The dispersion was not homogeneous as shown in Figure 5(e) and as given in SEM elemental mapping analysis in Figure 9 for 1.0 vol.%Sm2O3 content. In these images, the regions where green dots are concentrated indicate the presence of the Sm element. Regions with SmAlO3 phase content, which are often inhomogeneous and where clustering is more pronounced, were identified for the Al2O3-Sm2O3 ceramics with 1 vol.% Sm2O3.

Fig. 9
figure 9

Elemental maps of Al2O3-Sm2O3 ceramics for (a) A0.5Sm, (b) A0.8Sm, (c) A1Sm

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The toughening mechanisms of Al2O3 were influenced by large grain sizes, as well as the existence of cavities within the Al2O3 grains, which suggest that the presence of SmAlO3 particles contributes to toughness through the pull-out of SmAlO3. This is evident for all ratios of Sm2O3, as depicted in Figure 10 for the ceramic containing 0.8 vol.% Sm2O3. The fracture surface in Figure 10 also shows indicators of these toughening mechanisms. The appearance of cavities resulting from the pull-out of SmAlO3 phase particles within the Al2O3 grains during fracture revealed a potential contribution to toughness by grain pull-out. Therefore, it has been observed that adding Sm2O3 rare earth oxide to Al2O3 ceramics without negatively impacting densification or production conditions could enhance fracture toughness at certain volume ratios which were 0.5 and 0.8 vol.% for this study.

Fig. 10
figure 10

Unstable crack growth region SEM images of the Al2O3-0.8 vol.% Sm2O3 sample after the toughness test (secondary electron mode on the left, backscattered electron mode on the right)

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4 Conclusion

The fracture toughness of Al2O3-Sm2O3 ceramics was determined at room temperature using the single-edge-precracked-beam method. The relationship between microstructure and fracture toughness was studied by varying the Sm2O3 content (0, 0.1, 0.3, 0.5, 0.8, 1.0, 2.0 vol.%). The formation of the SmAlO3 phase was observed after the Al2O3-Sm2O3 reaction during sintering, leading to the large Al2O3 grains. The highest toughness values were obtained for Al2O3-Sm2O3 ceramics with 0.8 vol.% Sm2O3 content, while samples with 0.5 vol.% Sm2O3 content exhibited similar average toughness values that were approximately 70% higher than pure Al2O3. The improved fracture toughness in comparison with the pure Al2O3 was attributed to grain bridging by large Al2O3 grains, energy dissipation through the frictional pulling-out of large Al2O3 grains, and the pull-out of SmAlO3 phase particles from within the Al2O3 grains. A significant increase in fracture toughness was achieved by simply adding Sm2O3 without changing the production conditions of the pure Al2O3. Thanks to higher toughness, Al2O3-Sm2O3 ceramics seem to have the potential to find a place in critical applications such as armor components and biomedical implants, and it is recommended to test their performance in these areas under appropriate conditions.