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Al2O3
Ceramics International 45 (2019) 11099–11104
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Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Short communication
Microstructure and mechanical properties of hot pressed Ti3SiC2/Al2O3 a,1
a,1
c,1
d
a,∗
a,∗∗
Fangfang Qi , Guopu Shi , Kun Xu , Tao Su , Zhi Wang , Junyan Wu Hao Wua, Liu Zhanga, Rongchang Zhue, Binze Yanga
, Qinggang Li
a,b
,
T
a
School of Material Science and Engineering, University of Jinan, Jinan 250002, China Shandong Provincial Key Laboratory of Preparation and Measurement of Building Materials, Jinan 250022, China Anhui Dexinjia Biopharm Co., Ltd, Taihe, 236600, China d AVIC Research Institute of Structures of Aeronautical Composites, Jinan 250023, China e China Building Materials Academy Co., Ltd, Beijing 100024, China b c
ARTICLE INFO
ABSTRACT
Keywords: Ti3SiC2/Al2O3 composites Mechanical properties Orthogonal test Hot pressing sintering
In this work, Ti3SiC2/Al2O3 composites with various volume contents of Ti3SiC2 were fabricated under different orthogonal processes by vacuum hot pressing sintering. optimized comprehensive mechanical properties and relative density (＞97%) were obtained (the microhardness, flexural strength and fracture toughness reached the values of 16.25 GPa, 536.31 MPa and 9.3 MPa m1/2, respectively.) when the Al2O3 (60.0 vol %) and Ti3SiC2 (40.0 vol %) were sintered at 1500 °C for 0.5 h at 30 MPa, which basically correspond to the best level of after orthogonal calculation. The orientational growth behavior of Ti3SiC2 was further confirmed by EBSD, and temperature was found to play a prominent role in the growth of Ti3SiC2 to some extents, fewer columnar Ti3SiC2 grains were observed at lower temperature and weak decomposition of Ti3SiC2 was observed at 1500 °C. Meanwhile, high volume Ti3SiC2 was beneficial to the improvement of mechanical properties for the intrinsical layered structure.
1. Introduction Presently, the development of multiphase ceramics is one key to design the engineering materials because single component cannot meet mechanical property requirements [1–3]. Alumina ceramic possesses strong bonds resulting in significant hardness and strength, however, brittle fracture with scarcely slip in this materials is unavoidable [4,5]. A novel method to improve the overall mechanical properties of composites may be the mixed sintering of alumina composites with Ti3SiC2 which belongs to Mn+1AXn (MAX) phases and possess superb toughness like metal as a ceramic [6,7]. Composites based on Ti3SiC2 and Al2O3 have a range of advantages and feasibility in terms of their physicochemical properties and coefficient of thermal expansion [8,9], which means Ti3SiC2/Al2O3 composites are expected to find further potential applications in structural material fields [8,10–12]. Al2O3 has been proposed for combining with various metals [13,14] and whiskers [15,16] to produce an expected toughness improvement. However, issues of interfacial reaction and wettability mismatch still restrict its controllable preparation and performance improvement
[17–19]. When preparing Ti3SiC2-based materials, the Ti3SiC2 generally acts as the main phase and a small amount of enhancing phases such as SiC, TiC and Al2O3 were introduced into the matrix to increase the hardness (2–5 GPa) [7,20–22]. Ti3SiC2/TiC-Al2O3 composites with different amounts of Al2O3 added from 5 to 30 wt% were prepared by in situ reactions and different fracture modes were drawn [23]. Toughening alumina with 10–20 vol % layered Ti3SiC2 was investigated, with results showing that the flexural strength and fracture toughness enhanced with the increasing of Ti3SiC2 [24]. From previous studies, it can be concluded that Al2O3 has been used as an enhancement phase mostly and the volume percentage of Ti3SiC2 could be improved greatly in the Ti3SiC2/Al2O3 system to achieve better mechanical properties. Put another way, when using alumina as the matrix, the mechanical properties of the material have a maximized latent capacity for further improvement, and in-depth exploration of the appropriate proportions between the two phases is needed to endow the composites with the ability to resist external stress and oxidation. In our previous work, Ti3SiC2/TiC/Al2O3 composites with acceptable mechanical properties were prepared fromAl2O3, Ti and SiC [25], related work needs to be carried out usingTi3SiC2 and Al2O3 as the raw materials.
Corresponding author. Corresponding author. E-mail addresses: [email protected], [email protected] (Z. Wang), [email protected] (J. Wu). 1 These authors contributed equally to this work and should be considered co-first authors. ∗
∗∗
https://doi.org/10.1016/j.ceramint.2019.01.241 Received 21 November 2018; Received in revised form 9 January 2019; Accepted 29 January 2019 Available online 30 January 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Ceramics International 45 (2019) 11099–11104
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Fig. 1. The SEM image (a) and XRD pattern (b) of Ti3SiC2 raw material.
In this work, Ti3SiC2/Al2O3 composites (≥60 vol % Al2O3) with different proportions of the raw materials (Ti3SiC2 and Al2O3) were synthesized by hot-pressing sintering using an orthogonal design process. This research is a foundation for the next step of in situ preparation of Ti3SiC2/Al2O3 composites. The main focus of this work is aim at obtaining some basic process parameters and representative attributes of high alumina content-based Ti3SiC2/Al2O3 composites. The microstructure and mechanical properties of each sample were investigated, and the optimum process and volume ratio for the raw materials was analyzed by orthogonal experimentation.
average of ten measurements. The bulk density of those samples was recorded using a density balance via the Archimedes method at room temperature. In a range analysis, Tij is the sum of the experimental results corresponding to the level i in column j and Rj = maxTij−minTij is the range of dates in column j. The most important factor affecting the experimental results is the one possess the greatest difference. If the index requires that bigger is better, the level that maximizes the index should be selected. To perform an analysis of variance, we use the formula Sj =
1 r
3 T2 i= 1 ij
T2 , p
where p is the sample quantity of 9, n is the
number of levels (n = 3), and T =
2. Experimental The proportions of each group with different volume fractions of Ti3SiC2 (＞98%, average particle size: 2 μm, raw powder test: Fig. 1) and Al2O3 (99.9%, average particle size: 1 μm) are shown in Table 1. During the running period, the weight ratio of powders-Al2O3 ballanhydrous ethanol was 1:2:1 and the jar rotation speed was 200 rpm for 4 h (XQM-2, China). The slurry was dried in a drying oven at 60°C and sifted by a 200 mesh sifter, then, those powers were placed in a graphite mold with a diameter of 45 mm and sintered in a vacuum hot pressing furnace (VVPgr-80-2300, China) under different orthogonally designed sintering processes (temperature, pressure and soaking time, as shown in Table 1). The sintered compacts were cut into smooth strips with a size of 36 mm × 4 mm × 3 mm by an inner circle cutting machine after the pits on the surfaces were gradually removed by SiC power and polished with abrasive paper. The flexural strength and fracture toughness were measured using three-point bending method by electromechanical universal testing machine (CMT5504, MTSSYSTEMS, China) with an effective span and step rate of 30 mm and 0.1 mm/s, respectively. A Vickers hardness tester (HV-1000IS, China) was used to determine the microhardness for a load of 1000 N applied for 20 s, the final reported value was the Table 1 Influence factors and levels in this orthogonal test of Ti3SiC2/Al2O3 composites. Sample
Ti3SiC2:Al2O3/vol. %
Soaking time/ h
Temperature/°C
Pressure/MPa
1 2 3 4 5 6 7 8 9
2:8 2:8 2:8 3:7 3:7 3:7 4:6 4:6 4:6
0.5 1 1.5 0.5 1 1.5 0.5 1 1.5
1400 1450 1500 1450 1500 1400 1500 1400 1450
20 30 40 40 20 30 30 40 20
T i ij,
Fj =
Sj / f j Se / f e
. The value of each
quantile is set to ɑ = 0.25 and is determined from the quantile table based on the F distribution, F0.25 (2, 2) = 3 [26]. The phase composition analysis of those samples was judged by an X-ray diffractometer (XRD, D8 ADVANCE, Bruker). Microstructure investigations and morphology analyses were performed by scanning electron microscopy (SEM, JSM-7610F, United States) equipped with an energy dispersive spectroscopy (EDS) after spotter coating gold on measurement surface for 30 s. The determination of grain orientation behavior was presented by electron backscatter diffraction (EBSD) after fine ion beam polishing technology and carbon coating in parallel to the compression direction. 3. Results and discussion 3.1. Phase composition of the Ti3SiC2/Al2O3 composites XRD analysis for the samples with different volume ratio of Ti3SiC2 and Al2O3 as well as different sintering processes can be seen from Fig. 2. A distinct characteristic diffraction peak with high intensity at 40.93° is confirmed as (008) lattice planes of Ti3SiC2 in the case of the same raw material ratio. Obvious intensity changes of this peaks are observed in Fig. 2a from Samples 1, 2 and 3 that for the more temperature, pressure and holding time the more strong and narrow it become, whereas the diffraction peak of Al2O3 does not fluctuate greatly under the same sintering conditions. It is worth noting that one affirmatory diffraction peak situated at 36.04° is shown to relate to TiC when the Sample 3 was subjected to maximum driving force during sintering process (1500 °C, 40 MPa, and 1.5 h), in which an unavoidable heterozygous phase occurs during the in situ reaction, which has been observed in other works [6,10,20]. The production of TiC can also be observed in other samples despite the intensity of diffraction peaks being generally low. This phenomenon can be attributed to the factor that the gradually increased external field effect, which may facilitate phase decomposition of non absolute pure Ti3SiC2 to some degrees, on the basic of weak Ti-Si bond, unstable
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Sample 7. As shown in Fig. 3, the maximum intensity of Exp. densities (mud) achieves a high value of 85.32 which indicates that the prominent growth orientation was along the (0001) crystal surface direction of Ti3SiC2 grains in this sample. Considering the symmetry and homogeneity of crystals, the EBSD orientation result of (0001) plane are highly correlated with the (008) characteristic peak at 40.93° of XRD patterns. 3.2. Microstructures and relative density of the Ti3SiC2/Al2O3 composites
Fig. 2. The XRD patterns of Ti3SiC2/Al2O3 composites with different volume ratio of raw materials in orthogonal process.
Si connection points among TiC are disconnected and accumulate in the form of liquid phase. Moreover, the patterns of Samples 5 and 6 show that higher pressure does not exhibit a direct relationship with strong diffraction peak (Fig. 2b). Surprisingly, the temperature shows a certain correlation over the pressure, the evolution of the diffraction peak shows that it increases with the temperature. Therefore, temperature is preliminarily considered to be the main factor affecting the preferred orientation growth and high temperature stability of grains, the prominent influence of temperature is also confirmed by an orthogonal calculation later, the role of pressure is not explained here. Sample 7 shows better crystallinity and diffraction peak strength than the others (the volume ratio of Ti3SiC2:Al2O3 is 4:6), which was sintered under the conditions of 1500 °C, 30 MPa for 0.5 h, and the content of TiC is also lower. In order to further determine the level of orientation behavior, EBSD testing was used to obtain pole pictures for
Fig. 4 shows the uniform grain distribution in the fracture surfaces of Ti3SiC2/Al2O3 composite by SEM images. There is no abnormal growth of Al2O3 grains, and the grain orientation of Ti3SiC2 in Sample 1 (Fig. 3a) is not obvious, some spherical pores are found in the sample section. Meanwhile, irregularly sharp particles of Ti3SiC2 raw materials exhibit only a small amount of orientation changes when the temperature is kept at 1400 °C, as seen in Sample 6 (Fig. 3a and f). There are some distinct differences, where Ti3SiC2 grains with higher length to diameter ratio tend to be produced, which is associated with the orientation elongation of Ti3SiC2 for the relatively high temperatures in Samples 3 and 7 (Fig. 4c and f). In this case, the grain size of Ti3SiC2 changes from a length of 2 μm to approximately 5 μm, and Ti3SiC2 grains compact with Al2O3 greatly with the decrease of gaps and pores due to the for higher sintering driving force. Moreover, the Sample 5 which was sintered at a lower pressure and higher temperature shows rich layered structure of Ti3SiC2 (Fig. 4d) than seen in Simple 6 (Fig. 4f). This situation is in accordance with the peak intensity results shown in the XRD, and the effect of temperature on grain orientation is verified in terms of morphology. The micrographs and elements distribution of fracture surface of Sample 7 are shown in Fig. 5 to further determine the composition and microstructure of Ti3SiC2 and Al2O3. On the one hand, the results of point analysis show that the columnar grains with a rich lamellar structure are Ti3SiC2 and the normal grains with a regular morphology is Al2O3. Meanwhile, the constituent elemental distribution of Ti3SiC2 and Al2O3 also show obvious consistency with morphology. On the other hands, there are some small differences where the Si exhibits local aggregation behavior, this may be from a small amount of Si could be decomposed from Ti3SiC2 and aggregate into pores among grains before densification. The broken Ti-Si bonds would cause the surplus of TiC which corresponds to the appearance of TiC in the XRD pattern. It is quite clear from Fig. 6 that the relative density changes over a relatively small range (from 97.32% to 99.58%). Combined with the laws of phase formation and the fracture surfaces from each of the samples, some explanations can be determined. Ti3SiC2 grains with relatively small length to diameter ratio are more easily compacted with Al2O3 at first, as the temperature, pressure and soaking time increase, several tabular grains get in touch with each other which is seem to play a role in enlarging the porosity. This situation only moderates as the sintering drive power in Sample 3 increases (Fig. 4c), Here, although the trend for the columnar growth is more significant, those gaps can be overcome by external mechanical forces to reduce the negative effect of grain morphology, additionally, the soft phase is easier to densify via sintering as theTi3SiC2 volume fraction increases. Range analysis of orthogonal test data (the detailed process of which is shown in Table 2) was used to determine the most influential factors affecting the sintering and mechanical properties of Ti3SiC2/Al2O3 composite materials. From the calculation results, the effects of the volume ratio and temperature are remarkable for the relative density of Ti3SiC2/Al2O3 composites, the best factor level collocation is identified as the volume ratio of Al2O3 and Ti3SiC2, temperature, pressure and soaking time are related to the level of 4:6, 1500 °C, 40 MPa and 0.5 h, respectively. The primary and secondary order of the factors is: Proportion＞Temperature＞Pressure＞Time. The subsequent variance
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Fig. 3. The polar diagrams of Sample 7 under different crystal systems.
Fig. 4. The SEM images of Ti3SiC2/Al2O3 composites under orthogonal process, a: Sample 1 b: Sample 2, c: Sample 3, d: Sample 5, e: Sample 6 and f: Sample 7.
analysis of the orthogonal test (a detailed analysis of which is shown in Table .3) revealed that the effects of proportion and temperature are remarkable, which is consistent with the result of the range analysis. In a similar manner, the theoretical results for the bending strength, fracture toughness and hardness can be obtained in this way. 3.3. Mechanical properties of the Ti3SiC2/Al2O3 composite Fig. 7 shows the test results of mechanical properties, including Vickers' hardness, flexural strength, and fracture toughness of Ti3SiC2/ Al2O3 composites, which displays that the Sample 7 exhibits excellent mechanical properties, with the microhardness, flexural strength and fracture toughness reaching 16.25 GPa, 536.31 MPa and 9.3 MPa m1/2, respectively. The hidden rule in these changes is that well developed long columnar and lamellar Ti3SiC2 grains play an important role in regulating the mechanical properties. From the point of view of harness, higher temperature is beneficial to the growth of layered materials, compared with dispersed raw material grains, mature grains have stronger bond energy and fewer fragile interfaces. Hardness also shows a strong correlation with microdensification of materials, which increases with the decrease of porosity, compact samples are more resistant to compressive stresses from indenters. When the volume
content of Ti3SiC2 is only 20%, the high hardness of Al2O3 also supports the overall hardness of the material. In theory, the hardness of the Ti3SiC2/Al2O3 composites should decrease with the reduced Al2O3, however, due to the effect of compact structure, the material hardness still shows obvious superiority at this time. Based on the semiempirical formula: = 0exp( bp) , where σ and p are strength and porosity, respectively. A greater number of gaps in the interlaced Ti3SiC2 particles have negative effect on the flexural strength of Ti3SiC2/Al2O3 composites. The development of mechanical properties is limited by smaller sintering driving force in the Sample 1, and the content of Ti3SiC in this sample is low at 20 vol %. At this time, apparent grain boundaries and intergranular microcracks exist among the particles, which are associated with a characteristic mode of brittle intergranular fracture in which Al2O3 dominates (Fig. 8a), a small amount of Ti3SiC only plays a toughening role. Ti3SiC2 with a special layered structure absorbs the majority of the crack propagating energy and the crack propagation path would experience significant deflection and prolongation by delamination. In later periods, high densification can reduce defects in the samples and enhance the ability to resist external stresses. More Ti3SiC2 (40 vol %) without excessive decomposition plays a vital role in increasing the bending strength and fracture toughness of the Ti3SiC2/Al2O3
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Table 3 The variance analysis of orthogonal test of relative density. Source of variance
Sum of squares Sj
Freedom fj
Sj/fj
Fj =
Saliency
Proportion Time Temperature Pressure
4.51 0.05 0.35 0.17
2 2 2 2
2.26 0.03 0.18 0.08
87.67 1.00 6.87 3.21
remarkable – remarkable remarkable
Fig. 7. The hardness, flexural strength and fracture toughness of Sample 1–9 under the orthogonal design.
Fig. 5. The EDS analysis of Sample 7 in point scanning and surface scanning.
layer with bending and twisting. Therefore, the change of mechanical properties mainly depends on two aspects: the voids formed by the columnar grains and the stress absorption of layered columnar grains. The final result is mainly supported by the sintering driving force on one side. Based on the orthogonal analysis of the aforementioned experimental results of Vickers' hardness, flexural strength, and fracture toughness (shown in Table 4), considering the overall performance coordination and determining the best level of a factor, the best process parameters are 4:6 content ratio (Ti3SiC2:Al2O3), 0.5 h (soaking time), 1500 °C (temperature), and 30 MPa (pressure). Consequently, strong representativeness in process and component selection can be shown in Sample 7. Temperature and proportion have a strong influence on the mechanical properties of Ti3SiC2/Al2O3 composites. 4. Conclusion
Fig. 6. The relative density of Ti3SiC2/Al2O3 composites under the orthogonal design. Table 2 The range analysis of orthogonal test of relative density.
T1j T2j T3j Rj
Ti3SiC2:Al2O3/vol. %
Soaking time/h
Temperature/°C
Pressure/MPa
293.34 297.48 298.14 4.80
296.61 296.06 296.28 0.55
296.48 295.52 296.95 1.43
296.06 296.00 296.90 0.83
composites, wherein particle pull-out and tear zones of metalloid are distinctly visible. Afterwards, the better bending strength and fracture toughness of Sample 7 benefits from the content and morphology (Fig. 8b). The intrinsic toughness of Ti3SiC2 is bound to improve the toughness of Ti3SiC2/Al2O3 composites, more directly, applied external forces could be dispersed by Ti3SiC2 for the stress propagating along the
In this study, the feasibility about Ti3SiC2/Al2O3 composites with high volumes Al2O3 possess good comprehensive properties and produced via hot pressing sintering using an orthogonal test matrix was verified. The important conclusions obtained in this study are listed below. (1) Good compatibility and complementary performance were identified between Ti3SiC2 and Al2O3, We can get the optimal comprehensive performances when Ti3SiC2 and Al2O3 (a volume ratio of 4:6) were sintered at 1500 °C for 0.5 h under 30 MPa. (2) Better proportions are obtained in high alumina-based Ti3SiC2/ Al2O3 composites. The relative density, microhardness, flexural strength and fracture toughness can reach the maximum values of 99.58%, 16.25 GPa, 536.31 MPa and 9.3 MPa m1/2, respectively. (3) Ti3SiC2 has obvious orientation growth behavior along the (0001) crystal surface direction under the influence of temperature, slight decomposition of it occurs at high temperature, meanwhile, the content and morphology of Ti3SiC2 have prominent influence on the mechanical properties of Ti3SiC2/Al2O3 composites.
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Fig. 8. The SEM images of cross section of Sample 1 and Sample 7. a: the intergranular fracture morphology of Al2O3, b: the transgranular fracture and delamination of Ti3SiC2. Table 4 The best levels of each factors in Ti3SiC2/Al2O3 composites under the orthogonal design. Factor
Ti3SiC2:Al2O3/ vol. %
Soaking time/h
Temperature/°C
Pressure/MPa
relative density hardness flexural strength fracture toughness
4:6 4:6 4:6 4:6
0.5 0.5 0.5 1.5
1500 1500 1500 1500
40 30 30 30
Acknowledgements This work was financially supported by the National Natural Science Foundation of China (Grant No. 51872118), the Major Program of Shandong Province Natural Science Foundation (Grant No. ZR2017ZC0736), the Opening Project of State Key Laboratory of High Performance Ceramics and Superfine Microstructure (SKL201601SIC), the Natural Foundation of Shandong Province, China (Grant No. ZR2018PEM008), the young-aged talents lifting project from Shandong Association for Science & Technology (Grant No. 301-1505001, recoded by University of Jinan), the Scientific and Technological Project (Grant No. XKY1728, XKY1712) and the 111 Project of International Corporation on Advanced Cement-based Materials (No. D17001). References [1] Qian Liu, Fangfang Qi, Junyan Wu, Liu Zhang, Guopu Shi, Zhi Wang, Effects of TiN addition on the properties of hot pressed TiN-Ti/Al2O3 composites, Ceram. Int. 44 (2018) 11136–11142. [2] H.X. Qin, Y. Li, M.L. Long, X. Nie, P. Jiang, W.D. Xue, In situ synthesis mechanism of 15R-SiAlON reinforced Al2O3 refractories by Fe-Si liquid phase sintering, J. Am. Ceram. Soc. 101 (2018) 1870–1879. [3] S. Wang, J. Cheng, S.Y. Zhu, Z.H. Qiao, J. Yang, W.M. Liu, Effect of counterface on the tribological behavior of Ti3AlC2 at ambient, J. Eur. Ceram. Soc. 38 (2018) 2502–2510. [4] A. Centeno, V.G. Rocha, B. Alonso, et al., Graphene for tough and electroconductive alumina ceramics, J. Eur. Ceram. 33 (2013) 3201–3210. [5] M.M. Zhang, Y.F. Chang, R. Bermejo, et al., Improved fracture behavior and mechanical properties of alumina textured ceramics, Mater. Lett. 221 (2018) 252–255. [6] M.W. Barsoum, M. Radovic, Elastic and mechanical properties of the MAX phases, Annu. Rev. Mater. Sci. 41 (2011) 195–227. [7] J. Wang, Y. Zhou, M. Inouye, Recent progress in theoretical prediction, preparation and characterization of layered ternary transition-metal carbides, Annu. Rev. Mater.
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Short communication
Microstructure and mechanical properties of hot pressed Ti3SiC2/Al2O3 a,1
a,1
c,1
d
a,∗
a,∗∗
Fangfang Qi , Guopu Shi , Kun Xu , Tao Su , Zhi Wang , Junyan Wu Hao Wua, Liu Zhanga, Rongchang Zhue, Binze Yanga
, Qinggang Li
a,b
,
T
a
School of Material Science and Engineering, University of Jinan, Jinan 250002, China Shandong Provincial Key Laboratory of Preparation and Measurement of Building Materials, Jinan 250022, China Anhui Dexinjia Biopharm Co., Ltd, Taihe, 236600, China d AVIC Research Institute of Structures of Aeronautical Composites, Jinan 250023, China e China Building Materials Academy Co., Ltd, Beijing 100024, China b c
ARTICLE INFO
ABSTRACT
Keywords: Ti3SiC2/Al2O3 composites Mechanical properties Orthogonal test Hot pressing sintering
In this work, Ti3SiC2/Al2O3 composites with various volume contents of Ti3SiC2 were fabricated under different orthogonal processes by vacuum hot pressing sintering. optimized comprehensive mechanical properties and relative density (＞97%) were obtained (the microhardness, flexural strength and fracture toughness reached the values of 16.25 GPa, 536.31 MPa and 9.3 MPa m1/2, respectively.) when the Al2O3 (60.0 vol %) and Ti3SiC2 (40.0 vol %) were sintered at 1500 °C for 0.5 h at 30 MPa, which basically correspond to the best level of after orthogonal calculation. The orientational growth behavior of Ti3SiC2 was further confirmed by EBSD, and temperature was found to play a prominent role in the growth of Ti3SiC2 to some extents, fewer columnar Ti3SiC2 grains were observed at lower temperature and weak decomposition of Ti3SiC2 was observed at 1500 °C. Meanwhile, high volume Ti3SiC2 was beneficial to the improvement of mechanical properties for the intrinsical layered structure.
1. Introduction Presently, the development of multiphase ceramics is one key to design the engineering materials because single component cannot meet mechanical property requirements [1–3]. Alumina ceramic possesses strong bonds resulting in significant hardness and strength, however, brittle fracture with scarcely slip in this materials is unavoidable [4,5]. A novel method to improve the overall mechanical properties of composites may be the mixed sintering of alumina composites with Ti3SiC2 which belongs to Mn+1AXn (MAX) phases and possess superb toughness like metal as a ceramic [6,7]. Composites based on Ti3SiC2 and Al2O3 have a range of advantages and feasibility in terms of their physicochemical properties and coefficient of thermal expansion [8,9], which means Ti3SiC2/Al2O3 composites are expected to find further potential applications in structural material fields [8,10–12]. Al2O3 has been proposed for combining with various metals [13,14] and whiskers [15,16] to produce an expected toughness improvement. However, issues of interfacial reaction and wettability mismatch still restrict its controllable preparation and performance improvement
[17–19]. When preparing Ti3SiC2-based materials, the Ti3SiC2 generally acts as the main phase and a small amount of enhancing phases such as SiC, TiC and Al2O3 were introduced into the matrix to increase the hardness (2–5 GPa) [7,20–22]. Ti3SiC2/TiC-Al2O3 composites with different amounts of Al2O3 added from 5 to 30 wt% were prepared by in situ reactions and different fracture modes were drawn [23]. Toughening alumina with 10–20 vol % layered Ti3SiC2 was investigated, with results showing that the flexural strength and fracture toughness enhanced with the increasing of Ti3SiC2 [24]. From previous studies, it can be concluded that Al2O3 has been used as an enhancement phase mostly and the volume percentage of Ti3SiC2 could be improved greatly in the Ti3SiC2/Al2O3 system to achieve better mechanical properties. Put another way, when using alumina as the matrix, the mechanical properties of the material have a maximized latent capacity for further improvement, and in-depth exploration of the appropriate proportions between the two phases is needed to endow the composites with the ability to resist external stress and oxidation. In our previous work, Ti3SiC2/TiC/Al2O3 composites with acceptable mechanical properties were prepared fromAl2O3, Ti and SiC [25], related work needs to be carried out usingTi3SiC2 and Al2O3 as the raw materials.
Corresponding author. Corresponding author. E-mail addresses: [email protected], [email protected] (Z. Wang), [email protected] (J. Wu). 1 These authors contributed equally to this work and should be considered co-first authors. ∗
∗∗
https://doi.org/10.1016/j.ceramint.2019.01.241 Received 21 November 2018; Received in revised form 9 January 2019; Accepted 29 January 2019 Available online 30 January 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
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Fig. 1. The SEM image (a) and XRD pattern (b) of Ti3SiC2 raw material.
In this work, Ti3SiC2/Al2O3 composites (≥60 vol % Al2O3) with different proportions of the raw materials (Ti3SiC2 and Al2O3) were synthesized by hot-pressing sintering using an orthogonal design process. This research is a foundation for the next step of in situ preparation of Ti3SiC2/Al2O3 composites. The main focus of this work is aim at obtaining some basic process parameters and representative attributes of high alumina content-based Ti3SiC2/Al2O3 composites. The microstructure and mechanical properties of each sample were investigated, and the optimum process and volume ratio for the raw materials was analyzed by orthogonal experimentation.
average of ten measurements. The bulk density of those samples was recorded using a density balance via the Archimedes method at room temperature. In a range analysis, Tij is the sum of the experimental results corresponding to the level i in column j and Rj = maxTij−minTij is the range of dates in column j. The most important factor affecting the experimental results is the one possess the greatest difference. If the index requires that bigger is better, the level that maximizes the index should be selected. To perform an analysis of variance, we use the formula Sj =
1 r
3 T2 i= 1 ij
T2 , p
where p is the sample quantity of 9, n is the
number of levels (n = 3), and T =
2. Experimental The proportions of each group with different volume fractions of Ti3SiC2 (＞98%, average particle size: 2 μm, raw powder test: Fig. 1) and Al2O3 (99.9%, average particle size: 1 μm) are shown in Table 1. During the running period, the weight ratio of powders-Al2O3 ballanhydrous ethanol was 1:2:1 and the jar rotation speed was 200 rpm for 4 h (XQM-2, China). The slurry was dried in a drying oven at 60°C and sifted by a 200 mesh sifter, then, those powers were placed in a graphite mold with a diameter of 45 mm and sintered in a vacuum hot pressing furnace (VVPgr-80-2300, China) under different orthogonally designed sintering processes (temperature, pressure and soaking time, as shown in Table 1). The sintered compacts were cut into smooth strips with a size of 36 mm × 4 mm × 3 mm by an inner circle cutting machine after the pits on the surfaces were gradually removed by SiC power and polished with abrasive paper. The flexural strength and fracture toughness were measured using three-point bending method by electromechanical universal testing machine (CMT5504, MTSSYSTEMS, China) with an effective span and step rate of 30 mm and 0.1 mm/s, respectively. A Vickers hardness tester (HV-1000IS, China) was used to determine the microhardness for a load of 1000 N applied for 20 s, the final reported value was the Table 1 Influence factors and levels in this orthogonal test of Ti3SiC2/Al2O3 composites. Sample
Ti3SiC2:Al2O3/vol. %
Soaking time/ h
Temperature/°C
Pressure/MPa
1 2 3 4 5 6 7 8 9
2:8 2:8 2:8 3:7 3:7 3:7 4:6 4:6 4:6
0.5 1 1.5 0.5 1 1.5 0.5 1 1.5
1400 1450 1500 1450 1500 1400 1500 1400 1450
20 30 40 40 20 30 30 40 20
T i ij,
Fj =
Sj / f j Se / f e
. The value of each
quantile is set to ɑ = 0.25 and is determined from the quantile table based on the F distribution, F0.25 (2, 2) = 3 [26]. The phase composition analysis of those samples was judged by an X-ray diffractometer (XRD, D8 ADVANCE, Bruker). Microstructure investigations and morphology analyses were performed by scanning electron microscopy (SEM, JSM-7610F, United States) equipped with an energy dispersive spectroscopy (EDS) after spotter coating gold on measurement surface for 30 s. The determination of grain orientation behavior was presented by electron backscatter diffraction (EBSD) after fine ion beam polishing technology and carbon coating in parallel to the compression direction. 3. Results and discussion 3.1. Phase composition of the Ti3SiC2/Al2O3 composites XRD analysis for the samples with different volume ratio of Ti3SiC2 and Al2O3 as well as different sintering processes can be seen from Fig. 2. A distinct characteristic diffraction peak with high intensity at 40.93° is confirmed as (008) lattice planes of Ti3SiC2 in the case of the same raw material ratio. Obvious intensity changes of this peaks are observed in Fig. 2a from Samples 1, 2 and 3 that for the more temperature, pressure and holding time the more strong and narrow it become, whereas the diffraction peak of Al2O3 does not fluctuate greatly under the same sintering conditions. It is worth noting that one affirmatory diffraction peak situated at 36.04° is shown to relate to TiC when the Sample 3 was subjected to maximum driving force during sintering process (1500 °C, 40 MPa, and 1.5 h), in which an unavoidable heterozygous phase occurs during the in situ reaction, which has been observed in other works [6,10,20]. The production of TiC can also be observed in other samples despite the intensity of diffraction peaks being generally low. This phenomenon can be attributed to the factor that the gradually increased external field effect, which may facilitate phase decomposition of non absolute pure Ti3SiC2 to some degrees, on the basic of weak Ti-Si bond, unstable
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Sample 7. As shown in Fig. 3, the maximum intensity of Exp. densities (mud) achieves a high value of 85.32 which indicates that the prominent growth orientation was along the (0001) crystal surface direction of Ti3SiC2 grains in this sample. Considering the symmetry and homogeneity of crystals, the EBSD orientation result of (0001) plane are highly correlated with the (008) characteristic peak at 40.93° of XRD patterns. 3.2. Microstructures and relative density of the Ti3SiC2/Al2O3 composites
Fig. 2. The XRD patterns of Ti3SiC2/Al2O3 composites with different volume ratio of raw materials in orthogonal process.
Si connection points among TiC are disconnected and accumulate in the form of liquid phase. Moreover, the patterns of Samples 5 and 6 show that higher pressure does not exhibit a direct relationship with strong diffraction peak (Fig. 2b). Surprisingly, the temperature shows a certain correlation over the pressure, the evolution of the diffraction peak shows that it increases with the temperature. Therefore, temperature is preliminarily considered to be the main factor affecting the preferred orientation growth and high temperature stability of grains, the prominent influence of temperature is also confirmed by an orthogonal calculation later, the role of pressure is not explained here. Sample 7 shows better crystallinity and diffraction peak strength than the others (the volume ratio of Ti3SiC2:Al2O3 is 4:6), which was sintered under the conditions of 1500 °C, 30 MPa for 0.5 h, and the content of TiC is also lower. In order to further determine the level of orientation behavior, EBSD testing was used to obtain pole pictures for
Fig. 4 shows the uniform grain distribution in the fracture surfaces of Ti3SiC2/Al2O3 composite by SEM images. There is no abnormal growth of Al2O3 grains, and the grain orientation of Ti3SiC2 in Sample 1 (Fig. 3a) is not obvious, some spherical pores are found in the sample section. Meanwhile, irregularly sharp particles of Ti3SiC2 raw materials exhibit only a small amount of orientation changes when the temperature is kept at 1400 °C, as seen in Sample 6 (Fig. 3a and f). There are some distinct differences, where Ti3SiC2 grains with higher length to diameter ratio tend to be produced, which is associated with the orientation elongation of Ti3SiC2 for the relatively high temperatures in Samples 3 and 7 (Fig. 4c and f). In this case, the grain size of Ti3SiC2 changes from a length of 2 μm to approximately 5 μm, and Ti3SiC2 grains compact with Al2O3 greatly with the decrease of gaps and pores due to the for higher sintering driving force. Moreover, the Sample 5 which was sintered at a lower pressure and higher temperature shows rich layered structure of Ti3SiC2 (Fig. 4d) than seen in Simple 6 (Fig. 4f). This situation is in accordance with the peak intensity results shown in the XRD, and the effect of temperature on grain orientation is verified in terms of morphology. The micrographs and elements distribution of fracture surface of Sample 7 are shown in Fig. 5 to further determine the composition and microstructure of Ti3SiC2 and Al2O3. On the one hand, the results of point analysis show that the columnar grains with a rich lamellar structure are Ti3SiC2 and the normal grains with a regular morphology is Al2O3. Meanwhile, the constituent elemental distribution of Ti3SiC2 and Al2O3 also show obvious consistency with morphology. On the other hands, there are some small differences where the Si exhibits local aggregation behavior, this may be from a small amount of Si could be decomposed from Ti3SiC2 and aggregate into pores among grains before densification. The broken Ti-Si bonds would cause the surplus of TiC which corresponds to the appearance of TiC in the XRD pattern. It is quite clear from Fig. 6 that the relative density changes over a relatively small range (from 97.32% to 99.58%). Combined with the laws of phase formation and the fracture surfaces from each of the samples, some explanations can be determined. Ti3SiC2 grains with relatively small length to diameter ratio are more easily compacted with Al2O3 at first, as the temperature, pressure and soaking time increase, several tabular grains get in touch with each other which is seem to play a role in enlarging the porosity. This situation only moderates as the sintering drive power in Sample 3 increases (Fig. 4c), Here, although the trend for the columnar growth is more significant, those gaps can be overcome by external mechanical forces to reduce the negative effect of grain morphology, additionally, the soft phase is easier to densify via sintering as theTi3SiC2 volume fraction increases. Range analysis of orthogonal test data (the detailed process of which is shown in Table 2) was used to determine the most influential factors affecting the sintering and mechanical properties of Ti3SiC2/Al2O3 composite materials. From the calculation results, the effects of the volume ratio and temperature are remarkable for the relative density of Ti3SiC2/Al2O3 composites, the best factor level collocation is identified as the volume ratio of Al2O3 and Ti3SiC2, temperature, pressure and soaking time are related to the level of 4:6, 1500 °C, 40 MPa and 0.5 h, respectively. The primary and secondary order of the factors is: Proportion＞Temperature＞Pressure＞Time. The subsequent variance
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Fig. 3. The polar diagrams of Sample 7 under different crystal systems.
Fig. 4. The SEM images of Ti3SiC2/Al2O3 composites under orthogonal process, a: Sample 1 b: Sample 2, c: Sample 3, d: Sample 5, e: Sample 6 and f: Sample 7.
analysis of the orthogonal test (a detailed analysis of which is shown in Table .3) revealed that the effects of proportion and temperature are remarkable, which is consistent with the result of the range analysis. In a similar manner, the theoretical results for the bending strength, fracture toughness and hardness can be obtained in this way. 3.3. Mechanical properties of the Ti3SiC2/Al2O3 composite Fig. 7 shows the test results of mechanical properties, including Vickers' hardness, flexural strength, and fracture toughness of Ti3SiC2/ Al2O3 composites, which displays that the Sample 7 exhibits excellent mechanical properties, with the microhardness, flexural strength and fracture toughness reaching 16.25 GPa, 536.31 MPa and 9.3 MPa m1/2, respectively. The hidden rule in these changes is that well developed long columnar and lamellar Ti3SiC2 grains play an important role in regulating the mechanical properties. From the point of view of harness, higher temperature is beneficial to the growth of layered materials, compared with dispersed raw material grains, mature grains have stronger bond energy and fewer fragile interfaces. Hardness also shows a strong correlation with microdensification of materials, which increases with the decrease of porosity, compact samples are more resistant to compressive stresses from indenters. When the volume
content of Ti3SiC2 is only 20%, the high hardness of Al2O3 also supports the overall hardness of the material. In theory, the hardness of the Ti3SiC2/Al2O3 composites should decrease with the reduced Al2O3, however, due to the effect of compact structure, the material hardness still shows obvious superiority at this time. Based on the semiempirical formula: = 0exp( bp) , where σ and p are strength and porosity, respectively. A greater number of gaps in the interlaced Ti3SiC2 particles have negative effect on the flexural strength of Ti3SiC2/Al2O3 composites. The development of mechanical properties is limited by smaller sintering driving force in the Sample 1, and the content of Ti3SiC in this sample is low at 20 vol %. At this time, apparent grain boundaries and intergranular microcracks exist among the particles, which are associated with a characteristic mode of brittle intergranular fracture in which Al2O3 dominates (Fig. 8a), a small amount of Ti3SiC only plays a toughening role. Ti3SiC2 with a special layered structure absorbs the majority of the crack propagating energy and the crack propagation path would experience significant deflection and prolongation by delamination. In later periods, high densification can reduce defects in the samples and enhance the ability to resist external stresses. More Ti3SiC2 (40 vol %) without excessive decomposition plays a vital role in increasing the bending strength and fracture toughness of the Ti3SiC2/Al2O3
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Table 3 The variance analysis of orthogonal test of relative density. Source of variance
Sum of squares Sj
Freedom fj
Sj/fj
Fj =
Saliency
Proportion Time Temperature Pressure
4.51 0.05 0.35 0.17
2 2 2 2
2.26 0.03 0.18 0.08
87.67 1.00 6.87 3.21
remarkable – remarkable remarkable
Fig. 7. The hardness, flexural strength and fracture toughness of Sample 1–9 under the orthogonal design.
Fig. 5. The EDS analysis of Sample 7 in point scanning and surface scanning.
layer with bending and twisting. Therefore, the change of mechanical properties mainly depends on two aspects: the voids formed by the columnar grains and the stress absorption of layered columnar grains. The final result is mainly supported by the sintering driving force on one side. Based on the orthogonal analysis of the aforementioned experimental results of Vickers' hardness, flexural strength, and fracture toughness (shown in Table 4), considering the overall performance coordination and determining the best level of a factor, the best process parameters are 4:6 content ratio (Ti3SiC2:Al2O3), 0.5 h (soaking time), 1500 °C (temperature), and 30 MPa (pressure). Consequently, strong representativeness in process and component selection can be shown in Sample 7. Temperature and proportion have a strong influence on the mechanical properties of Ti3SiC2/Al2O3 composites. 4. Conclusion
Fig. 6. The relative density of Ti3SiC2/Al2O3 composites under the orthogonal design. Table 2 The range analysis of orthogonal test of relative density.
T1j T2j T3j Rj
Ti3SiC2:Al2O3/vol. %
Soaking time/h
Temperature/°C
Pressure/MPa
293.34 297.48 298.14 4.80
296.61 296.06 296.28 0.55
296.48 295.52 296.95 1.43
296.06 296.00 296.90 0.83
composites, wherein particle pull-out and tear zones of metalloid are distinctly visible. Afterwards, the better bending strength and fracture toughness of Sample 7 benefits from the content and morphology (Fig. 8b). The intrinsic toughness of Ti3SiC2 is bound to improve the toughness of Ti3SiC2/Al2O3 composites, more directly, applied external forces could be dispersed by Ti3SiC2 for the stress propagating along the
In this study, the feasibility about Ti3SiC2/Al2O3 composites with high volumes Al2O3 possess good comprehensive properties and produced via hot pressing sintering using an orthogonal test matrix was verified. The important conclusions obtained in this study are listed below. (1) Good compatibility and complementary performance were identified between Ti3SiC2 and Al2O3, We can get the optimal comprehensive performances when Ti3SiC2 and Al2O3 (a volume ratio of 4:6) were sintered at 1500 °C for 0.5 h under 30 MPa. (2) Better proportions are obtained in high alumina-based Ti3SiC2/ Al2O3 composites. The relative density, microhardness, flexural strength and fracture toughness can reach the maximum values of 99.58%, 16.25 GPa, 536.31 MPa and 9.3 MPa m1/2, respectively. (3) Ti3SiC2 has obvious orientation growth behavior along the (0001) crystal surface direction under the influence of temperature, slight decomposition of it occurs at high temperature, meanwhile, the content and morphology of Ti3SiC2 have prominent influence on the mechanical properties of Ti3SiC2/Al2O3 composites.
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Fig. 8. The SEM images of cross section of Sample 1 and Sample 7. a: the intergranular fracture morphology of Al2O3, b: the transgranular fracture and delamination of Ti3SiC2. Table 4 The best levels of each factors in Ti3SiC2/Al2O3 composites under the orthogonal design. Factor
Ti3SiC2:Al2O3/ vol. %
Soaking time/h
Temperature/°C
Pressure/MPa
relative density hardness flexural strength fracture toughness
4:6 4:6 4:6 4:6
0.5 0.5 0.5 1.5
1500 1500 1500 1500
40 30 30 30
Acknowledgements This work was financially supported by the National Natural Science Foundation of China (Grant No. 51872118), the Major Program of Shandong Province Natural Science Foundation (Grant No. ZR2017ZC0736), the Opening Project of State Key Laboratory of High Performance Ceramics and Superfine Microstructure (SKL201601SIC), the Natural Foundation of Shandong Province, China (Grant No. ZR2018PEM008), the young-aged talents lifting project from Shandong Association for Science & Technology (Grant No. 301-1505001, recoded by University of Jinan), the Scientific and Technological Project (Grant No. XKY1728, XKY1712) and the 111 Project of International Corporation on Advanced Cement-based Materials (No. D17001). References [1] Qian Liu, Fangfang Qi, Junyan Wu, Liu Zhang, Guopu Shi, Zhi Wang, Effects of TiN addition on the properties of hot pressed TiN-Ti/Al2O3 composites, Ceram. Int. 44 (2018) 11136–11142. [2] H.X. Qin, Y. Li, M.L. Long, X. Nie, P. Jiang, W.D. Xue, In situ synthesis mechanism of 15R-SiAlON reinforced Al2O3 refractories by Fe-Si liquid phase sintering, J. Am. Ceram. Soc. 101 (2018) 1870–1879. [3] S. Wang, J. Cheng, S.Y. Zhu, Z.H. Qiao, J. Yang, W.M. Liu, Effect of counterface on the tribological behavior of Ti3AlC2 at ambient, J. Eur. Ceram. Soc. 38 (2018) 2502–2510. [4] A. Centeno, V.G. Rocha, B. Alonso, et al., Graphene for tough and electroconductive alumina ceramics, J. Eur. Ceram. 33 (2013) 3201–3210. [5] M.M. Zhang, Y.F. Chang, R. Bermejo, et al., Improved fracture behavior and mechanical properties of alumina textured ceramics, Mater. Lett. 221 (2018) 252–255. [6] M.W. Barsoum, M. Radovic, Elastic and mechanical properties of the MAX phases, Annu. Rev. Mater. Sci. 41 (2011) 195–227. [7] J. Wang, Y. Zhou, M. Inouye, Recent progress in theoretical prediction, preparation and characterization of layered ternary transition-metal carbides, Annu. Rev. Mater.
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