- Email: [email protected]
(W, Ti)C graded nano-composite ceramic tool materials
Ceramics International ∎ (∎∎∎∎) ∎∎∎–∎∎∎
Contents lists available at ScienceDirect
Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Design and fabrication of Si3N4/(W, Ti)C graded nano-composite ceramic tool materials Xianhua Tian a, Jun Zhao b,n, Zhongbin Wang a, Xinhua Liu a a
School of Mechatronic Engineering, China University of Mining and Technology, Xuzhou 221116, PR China Key Laboratory of High Efficiency and Clean Mechanical Manufacture of MOE, School of Mechanical Engineering, Shandong University, Jinan 250061, PR China
b
art ic l e i nf o
a b s t r a c t
Article history: Received 18 April 2016 Received in revised form 17 May 2016 Accepted 23 May 2016
Si3N4/(W, Ti)C graded nano-composite ceramic tool materials with different thickness ratios and number of layers were fabricated by hot pressing technology. The flexural strength, fracture toughness and hardness of the sintered composites were tested and the microstructure and indention cracks were observed. The experiment results showed that the five-layer graded nano-composites with a thickness ratio of 0.2, which were sintered under a pressure of 30 MPa at 1700 °C in vacuum condition for 45 min, had the optimum comprehensive mechanical properties with a flexural strength of 1080.3 MPa, a hardness of 17.64 GPa, and a fracture toughness of 10.87 MPa m1/2. The formation of elongated β-Si3N4 grains contributes to the favorable mechanical properties. The graded structure can induce residual compressive stress in the surface layer and enhance the mechanical properties. The strengthening and toughening mechanisms are a synergistic effect of intergranular and transgranular fracture, crack bridging and deflection. & 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: Graded ceramic tool material Si3N4/(W, Ti)C Mechanical properties Microstructure Toughening and strengthening mechanisms
1. Introduction Si3N4 ceramics have been extensively studied, because of their outstanding mechanical properties, higher wear resistance, thermal shock resistance and self-lubrication behavior [1–6]. They can be used as components in heat engines and gas turbines, cutting tools materials and some functional materials [1,7–9]. However, the relatively low hardness and fracture toughness, and the strong chemical attraction between Si and Fe elements [10], limit their applications. The most common way to improve the mechanical properties of Si3N4 ceramic materials is by adding various secondary phases and nanophases, such as SiC [11,12], TiC [10], TiN [13], Ti(C, N) [14], (W, Ti)C [15], etc. Functional graded materials (FGMs), made of two or more constituent phases with continuous and smoothly varying composition, were introduced in 1984 [16]. Worldwide attention has been paid to FGMs for their innovative design ideas and outstanding properties in areas of aerospace, medicine, defense, energy, optoelectronics and so on [17]. The introduction of FGM into the design of ceramic cutting tool materials provides a new approach to improve their mechanical properties and cutting performance [18–20]. In previous studies, Si3N4-based composite ceramic tool materials reinforced by nanoscale Si3N4 and microscale (W, Ti)C were n
Corresponding author. E-mail address: [email protected] (J. Zhao).
fabricated [15]. The appropriate addition of 20 vol% of (W, Ti)C and 18 vol% of nanoscale Si3N4, indeed increased the mechanical properties of the composites to a flexural strength of 979 MPa, a fracture toughness of 8.5 MPa m1/2 and a Vicker's hardness of 17.72 GPa. Obviously, the fracture toughness was still limited. Then, Si3N4/(W, Ti) C/Co graded ceramic tool materials were fabricated to improve the fracture toughness to 10.5 MPa m1/2 [21]. However, the flexural strength was improved limited only to 992 MPa. In this paper, to further increase the mechanical properties, Si3N4/(W, Ti)C nano-composite ceramic tool materials with graded structure were fabricated. Combined with the mechanical properties, the optimum thickness ratio and number of layers were determined. The effect of sintering temperature and holding time on mechanical properties and microstructure of the composites was discussed as well. Furthermore, strengthening and toughening mechanisms were investigated by the observation of microstructure of the fracture surfaces and indented cracks on the polished surfaces.
2. Experimental procedure 2.1. Design of graded nano-composite ceramic tool materials Fig. 1 shows a cross-sectional optical graph of the five-layer Si3N4/(W, Ti)C graded ceramic with a thickness ratio of 0.2.
http://dx.doi.org/10.1016/j.ceramint.2016.05.142 0272-8842/& 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: X. Tian, et al., Design and fabrication of Si3N4/(W, Ti)C graded nano-composite ceramic tool materials, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.05.142i
2.2. Material preparation The details of the starting materials used in this paper are shown in Table 3 and α-Al2O3 and Y2O3 are used as sintering additives to promote liquid phase sintering because covalently bonded crystalline silicon nitride is hard to sinter [22]. The preparation process of mixed powders for the composites in Table 1 was as follows. The (W, Ti)C powder was firstly ballmilled in ethanol medium with cemented carbide balls for 10 days to decrease the average grain size to about 1 mm (see Fig. 2) for further use. The dispersion process of the Si3N4 nano-powder was assisted by mechanical stirring and ultrasonic vibration (with HS10260D ultrasonic instrument, China). The Si3N4 nano-powder was prepared into suspensions using ethanol as dispersing medium, then the 0.5 wt% (relative weight ratio to the nano particles to be dispersed) dispersant polyethylene glycol (PEG Sinopharm Chemical Reagent Co., Ltd., Shanghai, China) was added into the suspensions and the pH value was adjusted to about 9.5 with NH3 (Sinopharm Chemical Reagent Co., Ltd., Shanghai, China), and then ultrasonic stirred for 30 min to get the best dispersing effect. After that, according to Table 1, a specified amount of microscale Si3N4, (W, Ti)C, and the sintering aids were added into the well reagglomerated and uniform suspension of Si3N4 nano-particles and ultrasonic stirred for another 15 min. The mixed slurries were ballmilled with alumina balls for 48 h and then dried at 120 °C in a vacuum desiccator (Model DZ-2AⅡ, China). After drying, the mixture powders were sieved through a 120 mesh sieve. At last, according to Table 2, the dried powders were put into the cylindrical graphite mold layer by layer with a predetermined thickness ratio and hot-pressed under 30 MPa at temperature in the range of 1600–1750 °C for 30–75 min in the vacuum condition by a multifunctional hot pressing sintering furnace (Model ZRC85-25T, China), and then furnace cooling to room temperature.
h2
Composite 1 Composite 2
h3
z
H
Considering the ease of fabrication process, a design model with changing compositional distribution only along z direction is presented. Besides, different from traditional FGMs with composition changing from one surface to another, a symmetric structure is modeled, and thus both of the two opposite surfaces of an insert could be used as the rake face. Two parameters are used here to build the graded structure and name the graded composites, that is, the number of layers n and the thickness ratio e. For example, the graded Si3N4/(W, Ti)C ceramic with n ¼5 and e¼0.2 is denoted as GSWT52. As shown in Fig. 1, the thickness of the surface layer, the second layer, the middle layer and the total thickness of the material are set to be h1, h2, h3 and H, respectively. Then, H ¼2h1 þ2h2 þh3, and the thickness ratio e¼ h1/h2 ¼h2/h3 is used to determine the thickness of each layer. The formation of compressive residual stress in the surface layer can alleviate stresses resulting from external loadings in the cutting process and improve the cutting performance of ceramic cutting tools [14]. For this purpose, the thermal expansion coefficient should reduce gradually from the inner layer to the surface layer. Then, compressive residual stress can form in the surface layer of the graded composites during the cooling down process from the sintering temperature. Composites with different composition ratio were designed according to former study [15] as shown in Table 1. The thermal expansion coefficients of each composite were obtained using Kerner's equation [18]. Obviously, the layer with the highest volume fraction of (W, Ti)C should be put in the middle. As shown in Table 2, several kinds of graded composites with different number of layers and thickness ratios were designed and fabricated.
h1
X. Tian et al. / Ceramics International ∎ (∎∎∎∎) ∎∎∎–∎∎∎
2
x y
Composite 3
Composite 2 Composite 1 Fig. 1. Cross-sectional graph of the five-layer graded ceramic material with a thickness ratio of 0.2.
2.3. Specimen processing The hot-pressed disks, with a diameter of 42 mm, were cut (with J5060C-1-type inner-circular cutter), ground and polished into bars with dimensions about of 3 mm 4 mm 40 mm. The specimens’ edges were chamfered to eliminate machining flaws which may act as fracture origins. The flexural strength was measured using a three-point bending tester (Model WDW-50E, China) with a 20 mm span at a loading velocity of 0.5 mm/min. Hardness was measured by Vickers indention method using a diamond pyramid indenter (Model MHVD-30AP, China) with a load of 196 N and a duration time of 15 s and the fracture toughness was determined by the indentation crack length [23]. The density of each specimen was measured by the Archimedes’ method with an analytical balance (Model AUY120, Japan), and the relative density was calculated from the theoretical densities of each raw powder, which was got using the linear mixed estimation method. For each experimental condition, 5 specimens were tested. Cross-sectional optical graph and macro cracks propagation path was observed using a Keyence digital microscope (Model VHX-600E, Japan). The micro cracks on the polished surfaces and the fractured surfaces, etched in molten NaOH for about 1 min, were observed by scanning electron microscopy (SEM, SUPRA-55, ZEISS, Germany). Phase identification of the composites was performed by X-ray diffraction analysis (XRD, D8 Advance, Bruker, Germany) with copper Kα radiation. Energy dispersive spectroscopy (EDS, INCAx-act, UK) was used to distinguish the Si3N4 and (W, Ti)C grains.
3. Results and discussions 3.1. Mechanical properties and microstructure 3.1.1. Effect of the thickness ratio and the number of layers Fig. 3 shows the effect of thickness ratio on mechanical properties of the five layered composites sintered at 1700 °C for 45 min When the thickness ratio is between 0.1 and 0.5, the flexural strength and fracture toughness of the composites increase firstly and then decrease, and both arrive at a maximum at e¼0.2. The hardness increases firstly and then decreases until e¼0.3, and then increases again. Further increasing the thickness ratio to more than 0.6, the flexural strength appears to remain a constant value. The hardness has a tendency to increase while the fracture toughness tends to decrease. Obviously, the composites with a thickness ratio of 0.2, have the comprehensive optimum
Please cite this article as: X. Tian, et al., Design and fabrication of Si3N4/(W, Ti)C graded nano-composite ceramic tool materials, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.05.142i
X. Tian et al. / Ceramics International ∎ (∎∎∎∎) ∎∎∎–∎∎∎
3
Table 1 Composition of different composites (vol%). Composites
Weight ratio of nano-Si3N4 (25 nm) to micro-Si3N4 (0.5 mm)
(W, Ti)C (1 mm)
Y2O3 þ Al2O3 (0.5 mm)
Thermal expansion coefficient α ( 10 6 K 1)
SWT10 SWT15 SWT20 SWT25
1/3 1/3 1/3 1/3
10 15 20 25
8 8 8 8
3.490 3.630 3.767 3.900
Table 2 Composites of the FGM. Number of layers
Composites of each layer
3 5 7
SWT15/ SWT20/ SWT15 SWT15/ SWT20/ SWT25/ SWT20/ SWT15 SWT10/ SWT15/ SWT20/ SWT25/ SWT20/ SWT15/ SWT10
Table 3 Details of the starting materials. Powders Average particle size (μm)
Purity (%)
Manufacturer
α-Si3N4
0.5/0.025
499
(W, Ti)C
3
499.7
α-Al2O3
0.5
499.9
Y2O3
–
Highpurity
Hefei Kaier Nanometer Energy and Technology Co., Ltd., China Changsha Langfeng Metallic Material Co., Ltd., China Shanghai St-nano Science and Technology Co., Ltd., China Sinopharm Chemical Reagent Co., Ltd., Shanghai, China
mechanical properties. Fig. 4 shows the effect of number of layers on mechanical properties of the composites with a fix thickness ratio of 0.2 sintered at 1700 °C for 45 min And abscissa “1” represents the homogenous composites SWT20, which owns the best comprehensive mechanical properties among the four composites in Table 1 [15]. From Fig. 4, it can be seen that the number of layers has a considerable effect on the mechanical properties of the composites. Both the flexural strength and fracture toughness of the composites increase firstly and then decrease with the increase of number of layers. When n¼ 5, the flexural strength and fracture toughness of the composites can reach 1080.3 MPa and 10.87 MPa m1/2. Compared with the homogenous composite SWT20 [15], with a flexural strength of 978.53 MPa and fracture toughness of 8.54 MPa m1/2, the two mechanical properties of the graded composites are increased by 10.4% and 27.3%, respectively. The improved properties are attributed to the combined effect of elastic modulus gradient and the compressive residual stress
4 μm
presented at the surface layer of the graded structure, which is caused by the thermal-mismatch effect. The residual stress is also benefit for the increase in hardness of the graded materials [24]. Since the hardness of the graded composites GSWT52, surface layer of which is SWT15, is 17.64 GPa, and is higher than the homogeneous SWT15 with a hardness of 17.25 GPa [15]. The higher hardness of SWT20, is attributed to the higher condition of (W, Ti)C, which has better hardness than the Si3N4 matrix. It can be concluded from the above analysis that the graded composites GSWT52, with n¼ 5 and e¼0.2, have the comprehensive optimum mechanical properties and are used to optimize the sintering process in the next part. The cross-sectional graph of sintered GSWT52 are shown in Fig. 5. It can be seen that the graded structure is made of five layers and consistent with the design model. 3.1.2. Effect of sintering temperature and holding time The effect of sintering temperature on the mechanical properties of GSWT52 sintered for 45 min is illustrated in Fig. 6. It can be seen that the flexural strength, hardness, fracture toughness and relative density of GSWT52 follow a similar change to increase firstly and then decrease and reach a maximum value at 1700 °C as the sintering temperature increases. It also can be confirmed that the relative density affects the mechanical properties to a large extent. Fig. 7 illustrates the SEM micrographs of etched fracture surfaces of GSWT52 sintering at 1600 °C, 1650 °C, 1700 °C and 1750 °C for 45 min. Generally, the microstructure is characterized by rodlike and some irregular grains. The EDS analysis of point 1 and 2 in Fig. 7(c), as shown in Fig. 8(a) and (b), illustrates that the elongated grains are Si3N4, and the bigger and irregular grains are (W, Ti)C. The XRD patterns of the surface layer of GSWT52, as shown in Fig. 8(d), indicates that the major phases are β-Si3N4 and (W, Ti)C. No α-Si3N4 is identified, revealing a complete phase transformation from α-Si3N4 to β-Si3N4 and some secondary phase Si3Al3O3N5 are formed. Besides, the different phases show a good chemical compatibility. Obviously, the elongated β-Si3N4 grains, which have a pronounced effect on the mechanical properties of the composites, have a tendency to grow bigger with the increased sintering
4 μm Fig. 2. SEM micrographs of (W, Ti)C powders (a) Starting powders, (b) Ball milled powders.
Please cite this article as: X. Tian, et al., Design and fabrication of Si3N4/(W, Ti)C graded nano-composite ceramic tool materials, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.05.142i
4
X. Tian et al. / Ceramics International ∎ (∎∎∎∎) ∎∎∎–∎∎∎
Fig. 3. The effect of thickness ratio on (a) flexural strength, (b) hardness and (c) fracture toughness of the composites with n ¼5.
temperature. The addition of nanoscale Si3N4 particles increases the surface energy of the composites powders and enhances the sintering driven force and promote the formation of the duplex distribution characteristics in the solution-diffusion-precipitation process [10]. Under a relatively lower sintering temperature of 1600 °C, the sintering driven force is not enough and the effect is not obvious, so the elongated grains are tiny. At 1750 °C, the much higher sintering temperature will make the movement of grain boundaries fast, leading to excessive grain growth and some coarsened elongated grains. More pores will appear under these conditions, which reduce the density of the composites and restrains the mechanical properties as shown in Fig. 6. When the sintering temperature is at 1650 and 1700 °C, especially 1700 °C, the α-Si3N4 grains transform completely to rod-like grains and show duplex distribution characteristic. The larger grains own an aspect ratio about 5:1, while some much tinier β-Si3N4 grains with an aspect ratio about 10:1 are filled in larger Si3N4 and (W, Ti)C grains. This increases the compactness of the composites as shown in Fig. 6(d). The larger and smaller β-Si3N4 grains were transformed from the α-Si3N4 microparticles and nanoparticles, respectively. The crossed rod-like grains tend to increase the strength of the composites by a crack deflection mechanism. The relative density of the composites sintered at 1700 °C can reach 98.87%. The effect of holding time on the mechanical properties of GSWT52 sintered at 1700 °C is shown in Fig. 9. The composites sintered for 45 min owe the best flexural strength, hardness and relative density, while the best fracture toughness can be got with
a holding time of 60 min. Fig. 10 illustrates the SEM micrographs of the etched fracture surfaces of GSWT52 sintering at 1700 °C for 30 min, 45 min, 60 min and 75 min. It can be observed that, the growth of β-Si3N4 grains does not follow a fixed trend. With a less holding time of 30 min, the inhibit effect of the growing (W, Ti)C grains on the growth of β-Si3N4 grains is not significant, so some bigger β-Si3N4 grains are formed. However, the undense defects caused by the insufficient sintering, such as pores, are detrimental for the compactness, as shown in Fig. 9(d), and reduces the mechanical properties. Increasing the holding time to 45 min, the β-Si3N4 grains becomes more subtle. The growth of (W, Ti)C grains inhabits the growth of β-Si3N4 grains and makes a finer structure and increases the densification of the composites. With a holding time of 60 min, most of the β-Si3N4 grains are coarsened, leading to the reduction of mechanical properties. Coarsened grains bring more pores and reduce the density and the flexural strength largely. On the other hand, the fracture toughness was increased by the bigger grains. With a longer holding time of 75 min, the βSi3N4 grains becomes smaller again. Some nanoscale grains are agglomerated along the bigger (W, Ti)C and β-Si3N4 grains as marked in Fig. 10(d). EDS analysis of point 3, as shown in Fig. 8(d), declare that the agglomerated grains are nanoscale Si3N4. The steric hindrance effect caused by the agglomerated nanoscale Si3N4 grains inhabits the growth of β-Si3N4 grains and restrains the mechanical properties as well. Through optimizing the composition and sintering process, the five-layer composites GSWT52 with a thickness ratio of 0.2,
Please cite this article as: X. Tian, et al., Design and fabrication of Si3N4/(W, Ti)C graded nano-composite ceramic tool materials, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.05.142i
X. Tian et al. / Ceramics International ∎ (∎∎∎∎) ∎∎∎–∎∎∎
5
Fig. 4. The effect of number of layers on (a) flexural strength, (b) hardness and (c) fracture toughness of the composites with e¼ 0.2.
3.2. Toughening and strengthening mechanisms 3.2.1. Effect of graded structure Macroscopical cracks propagation paths of the homogeneous composite SWT20 and graded GSWT52 from the Vickers indentation test under the same load of 196 N are shown in Fig. 11. The arrows present the end of the cracks. Evidently, the indentation area is almost the same, however the propagation length of the crack on GSWT52 is much shorter than SWT20, which means a higher fracture toughness as shown in Fig. 4. This is due to the elastic modulus gradient and the thermal mismatch effect [11], which induces residual compressive stress on the surface layer of the graded structure. Much more fracture energy will be consumed to resist the impedance effect of the residual compressive stress during the crack propagation process.
Fig. 5. The cross-sectional graph of sintered GSWT52.
sintered under 30 MPa at 1700 °C for 45 min have the optimum mechanical properties, which is a flexural strength of 1080.3 MPa, a hardness of 17.64 GPa, and a fracture toughness of 10.87 MPa m1/2.
3.2.2. Effect of β-Si3N4 and (W, Ti)C grains The fracture surface of the composites without etch, as shown in Fig. 12, shows a mix of transgranular and intergranular fracture, which contributes to the improved mechanical properties largely. The relatively smooth surfaces and cleavage surfaces of the grains, as shown in Fig. 12(a), reflect the occurrence of intergranular and transgranular fracture. According to the above analysis, during the sintering process, α-Si3N4 will completely transform into β-Si3N4 grains. The elongated grains crisscross, which is helpful to improve the strength of the ceramic tool materials. Besides, when fracture meets with inclined elongated β-Si3N4 grains, the interface debonding, as shown in Fig. 12(b), tends to occur. This means some elongated
Please cite this article as: X. Tian, et al., Design and fabrication of Si3N4/(W, Ti)C graded nano-composite ceramic tool materials, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.05.142i
6
X. Tian et al. / Ceramics International ∎ (∎∎∎∎) ∎∎∎–∎∎∎
Fig. 6. The effect of sintering temperature on (a) flexural strength, (b) hardness, (c) fracture toughness and (d) relatively density of GSWT52 sintered for 45 min.
Fig. 7. SEM micrographs of the etched fracture surfaces of GSWT52 sintering at (a) 1600 °C, (b) 1650 °C, (c) 1700 °C and (d) 1750 °C for 45 min.
Please cite this article as: X. Tian, et al., Design and fabrication of Si3N4/(W, Ti)C graded nano-composite ceramic tool materials, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.05.142i
X. Tian et al. / Ceramics International ∎ (∎∎∎∎) ∎∎∎–∎∎∎
Fig. 8. EDS analysis of (a) point 1, (b) point 2 in Fig. 7(c) and (c) point 3 in Fig. 10(d) and (d) XRD patterns of the surface layer of GSWT52.
Fig. 9. The effect of holding time on (a) flexural strength, (b) hardness, (c) fracture toughness and (d) relatively density of GSWT52 sintered at 1700 °C.
Please cite this article as: X. Tian, et al., Design and fabrication of Si3N4/(W, Ti)C graded nano-composite ceramic tool materials, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.05.142i
7
8
X. Tian et al. / Ceramics International ∎ (∎∎∎∎) ∎∎∎–∎∎∎
b
Transgranular fracture
2 μm
Agglomeration
Fig. 10. SEM micrographs of the etched fracture surfaces of GSWT52 sintering at 1700 °C for (a) 30 min, (b) 45 min, (c) 60 min and (d) 75 min.
grains are pulled out from the fracture surface, which is similar to the whisker toughening mechanisms [25,26]. The friction between interfaces will consume fracture energy and improve the flexural strength and fracture toughness. Some pores are also distinguished. Pores will decrease the loading cross sectional area and lead to stress concentration, and thus decrease the strength of composites. The addition of (W, Ti)C grains will induce much transgranular fracture. Transgranular fracture will increase the energy required for rupture since the interfacial toughness is typically much smaller than the toughness of the grains [27]. From Fig. 10(b) and Fig. 12(a), it can be inferred that the transgranular
fracture, which increases the mechanical properties of the composites a lot, mainly among irregular (W, Ti)C grains. SEM micrographs of crack extension paths of GSWT52 generated by Vicker's indentation are shown in Fig. 13. The irregular white grains are (W, Ti)C and the grey grains are β-Si3N4. The inhibition effect of (W, Ti)C grains on the crack propagation at the end of the crack is clearly shown in region A in Fig. 13(a). Under the quasi-static loading conditions, the main fracture mode is intergranular fracture [27] as shown in region C. Transgranular fracture happens simultaneously. Combined with the above analysis, it can be concluded that transgranular fracture is mainly
Fig. 11. Macroscopical cracks propagation path of (a) graded GSWT52 and (b) the homogeneous SWT20.
Please cite this article as: X. Tian, et al., Design and fabrication of Si3N4/(W, Ti)C graded nano-composite ceramic tool materials, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.05.142i
X. Tian et al. / Ceramics International ∎ (∎∎∎∎) ∎∎∎–∎∎∎
9
Fig. 12. The fracture surfaces of GSWT52 without etch sintering at 1700 °C for 45 min.
across the bigger (W, Ti)C grains as shown in regions B. At microscale level, the thermal mismatch effect between the matrix Si3N4 and (W, Ti)C will induce radial residual tensile stress in the matrix around (W, Ti)C particles, which contributes to the forming of transgranular fracture. Intergranular fracture makes the cracks deflect and increases the crack propagating length, while transgranular fracture increases the energy required for rupture. Crack bridging and deflection are also observed. The cracks can be bridged by β-Si3N4 grains as shown in regions D in Fig. 13(a), as well as by the (W, Ti)C grains as shown in region D in Fig. 13(b). Crack bridging can provide a force for crack closure and impedes crack propagation. Crack deflection can make the cracks more tortuous and hinder the propagation of the cracks. All these phenomena will consume a greater amount of rupture energy and improve the fracture toughness of the composite greatly. 3.2.3. Effect of nanoscale grains Fig. 14 illustrates the TEM micrographs of the surface layers of GSWT52. The white grains with a hexagonal prisms structure are β-Si3N4, and the black grains with amorphous shape are (W, Ti)C. The addition of nano-Si3N4 and some nanoscale (W, Ti)C grains contributed to the improved mechanical properties greatly. The nanoscale grains embed into microscale ones and form the intragranular structure as shown in Fig. 14(b), which introduces lots of sub-grain boundaries. Due to the thermal mismatch effect between the matrix Si3N4 and the reinforcement phase (W, Ti)C, some dislocations exist along the sub-grain boundaries and induce stress interference fringes. The dislocations can consume more fracture energy and restrain the propagation of the cracks. Therefore, the toughing and strengthening mechanisms are synergistic effects of intergranular and transgranular fracture,
crack bridging and deflection caused by the addition of reinforcing phases and nanoscale grains and the use of graded structure.
4. Conclusions This work aims to develop a new kind of graded Si3N4/(W, Ti)C ceramic cutting tools materials. A systematic experiments have been conducted to optimize the composition and the sintering process. The following conclusions can be achieved. (1) The five-layer composites GSWT52 with a thickness ratio of 0.2, which are sintered under a pressure of 30 MPa at 1700 °C for 45 min, have the optimum mechanical properties. The optimum mechanical properties are a flexural strength of 1080.3 MPa, a hardness of 17.64 GPa, and a fracture toughness of 10.87 MPa m1/2, which meets the requirements for ceramic cutting tools. (2) The graded structure improves the mechanical properties of the ceramic cutting tools material, especially the flexural strength and fracture toughness. Compared with homogenous SWT20, the flexural strength and fracture toughness of graded GSWT52 are increased by 10.4% and 27.3%, respectively. This is mainly due to the induced compressive residual stress on the surface layer during the fabrication process. (3) The formation of elongated β-Si3N4 grains can introduce toughening mechanisms similar to whisker and the addition of (W, Ti)C grains can induce more transgranular fracture. (4) The toughing and strengthening mechanisms of the composites are synergistic effects of intergranular fracture, transgranular fracture, crack bridging and deflection. These effects
Fig. 13. SEM micrographs of crack extension paths of surface layer of GSWT52.
Please cite this article as: X. Tian, et al., Design and fabrication of Si3N4/(W, Ti)C graded nano-composite ceramic tool materials, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.05.142i
X. Tian et al. / Ceramics International ∎ (∎∎∎∎) ∎∎∎–∎∎∎
10
a
(W, Ti)C
b Intragranular structure
Stress interference fringes β-Si3N4 Fig. 14. TEM micrographs of GSWT52 (surface layers) (a) β-Si3N4 and (W, Ti)C grains, (b) Stress interference fringes.
can improve the mechanical properties of the composites significantly.
Acknowledgement This work is supported by the National Natural Science Foundation of China (51175310 and 51475273), and the Priority Academic Program Development of Jiangsu Higher Education Institution.
[13]
[14]
[15]
[16] [17] [18]
References [19] [1] X. Pan, J. Mayer, M. Rühle, K. Niihara, Silicon nitride based ceramic nanocomposites, J. Am. Ceram. Soc. 79 (1996) 585–590. [2] F.L. Riley, Silicon nitride and related materials, J. Am. Ceram. Soc. 83 (2000) 245–265. [3] I. Schulz, M. Herrmann, I. Endler, I. Zalite, B. Speisser, J. Kreusser, Nano Si3N4 composites with improved tribological properties, Lubr. Sci. 21 (2009) 69–81. [4] P. Rutkowski, L. Stobierski, D. Zientara, L. Jaworskab, P. Klimczykb, M. Urbanik, The influence of the graphene additive on mechanical properties and wear of hot-pressed Si3N4 matrix composites, J. Eur. Ceram. Soc. 35 (2015) 87–94. [5] W.M. Guo, J.J. Yu, M. Xiong, S.H. Wu, H.T. Lin, High-toughness Lu2O3-doped Si3N4 ceramics by seeding, Ceram. Int. 42 (2016) 6495–6499. [6] L. Li, L. Cheng, S. Fan, X. Gao, Y. Xie, L. Zhang, Fabrication and dynamic compressive response of laminated ZrO–Zr2CN/Si3N4 ceramics, Ceram. Int. 41 (2015) 8584–8591. [7] I.A. Choudhury, M.A. El-Baradie, Machinability of nickel-base super alloys: a general review, J. Mater. Process. Technol. 77 (1998) 278–284. [8] K. Zuo, Y. Zeng, D. Jiang, The mechanical and dielectric properties of Si3N4based sandwich ceramics, Mater. Des. 35 (2012) 770–773. [9] S.J. Lee, S. Baek, Effect of SiO2 content on the microstructure, mechanical and dielectric properties of Si3N4 ceramics, Ceram. Int. 42 (2016) 9921–9925. [10] J. Zhao, X. Ai, Z.J. Lü, Preparation and characterization of Si3N4/TiC nanocomposite ceramics, Mater. Lett. 60 (2006) 2810–2813. [11] S.C. Thompson, A. Pandit, N.P. Padture, S. Suresh, Stepwise-graded Si3N4-SiC ceramics with improved wear properties, J. Am. Ceram. Soc. 85 (2002) 2059–2064. [12] M. Poorteman, P. Descamps, F. Cambier, M. Plisnierb, V. Canonneb, J. C. Descamps, Silicon nitride/silicon carbide nanocomposite obtained by
[20]
[21]
[22]
[23] [24]
[25]
[26]
[27]
nitridation of SiC: fabrication and high temperature mechanical properties, J. Eur. Ceram. Soc. 23 (2003) 2361–2366. B. Zou, C.Z. Huang, M. Chen, M.L. Gu, H.L. Liu, Study of the mechanical properties, toughening and strengthening mechanisms of Si3N4/Si3N4w/TiN nanocomposite ceramic tool materials, Acta Mater. 55 (2007) 4193–4202. G.M. Zheng, J. Zhao, Y.H. Zhou, Z.J. Gao, X.B. Cui, A.H. Li, Fabrication and characterization of Sialon-Si3N4 graded nano-composite ceramic tool materials, Compos. Part B-Eng. 42 (2011) 1813–1820. X.H. Tian, J. Zhao, N.B. Zhu, Y. Dong, J.B. Zhao, Preparation and characterization of Si3N4/(W, Ti)C nano-composite ceramic tool materials, Mater. Sci. Eng. A-Struct. 596 (2014) 255–263. I. Shiota, Y. Miyamoto, Functionally Graded Materials 1996, Elsevier,, 1997. R.M. Mahamood, E.T. Akinlabi, M. Shukla, S. Pityana, Functionally graded material: an overview, Lect. Not. Eng. Compu. Sci. 2199 (2012) 1593–1597. X. Ai, J. Zhao, C.Z. Huang, J.H. Zhang, Development of an advanced ceramic tool material functionally gradient cutting ceramics, Mater. Sci. Eng. A-Struct. 248 (1998) 125–131. J. Zhao, X. Ai, X.P. Huang, Relationship between the thermal shock behavior and the cutting performance of a functionally gradient ceramic tool, Mater. Process. Technol. 129 (2002) 161–166. G.M. Zheng, J. Zhao, Z.J. Gao, Q.Y. Cao, Cutting performance and wear mechanisms of Sialon-Si3N4 graded nano-composite ceramic cutting tools, Int. J. Adv. Manuf. Technol. 58 (2012) 19–28. X. Tian, J. Zhao, Y. Wang, F. Gong, W. Qin, H. Pan, Fabrication and mechanical properties of Si3N4/(W, Ti)C/Co graded nano-composite ceramic tool materials, Ceram. Int. 41 (2015) 3381–3389. M. Herrmann, Z.J. Shen, I. Schulz, J.F. Hu, B. Jancar, Silicon nitride nanoceramics densified by dynamic grain sliding, J. Mater. Res. 25 (2010) 2354–2361. A.G. Evans, E.A. Charles, Fracture toughness determinations by indentation, J. Am. Ceram. Soc. 59 (1976) 371–372. J.X. Deng, Z.X. Duan, D.L. Yun, H. Zhang, X. Ai, J. Zhao, Fabrication and performance of Al2O3/(W,Ti)C þ Al2O3/TiC multilayered ceramic cutting tools, Mater. Sci. Eng. A-Struct. 527 (2010) 1039–1047. L. Silvestroni, D. Sciti, C. Melandri, S. Guicciardi, Toughened ZrB2-based ceramics through SiC whisker or SiC chopped fiber additions, J. Eur. Ceram. Soc. 30 (2010) 2155–2164. P.F. Becher, K. Hirao, M.E. Brito, E.Y. Sun, K.P. Plucknett, K.B. Alexander, C. Hsueh, H. Lin, S.B. Waters, C.G. Westmoreland, E. Kang, Microstructural design of silicon nitride with improved fracture toughness: I, effects of grain shape and size, J. Am. Ceram. Soc. 81 (1998) 2821–2830. G. Hu, C.Q. Chen, K.T. Ramesh, J.W. Mccauley, Mechanisms of dynamic deformation and dynamic failure in aluminum nitride, Acta Mater. 60 (2012) 3480–3490.
Please cite this article as: X. Tian, et al., Design and fabrication of Si3N4/(W, Ti)C graded nano-composite ceramic tool materials, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.05.142i
Contents lists available at ScienceDirect
Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Design and fabrication of Si3N4/(W, Ti)C graded nano-composite ceramic tool materials Xianhua Tian a, Jun Zhao b,n, Zhongbin Wang a, Xinhua Liu a a
School of Mechatronic Engineering, China University of Mining and Technology, Xuzhou 221116, PR China Key Laboratory of High Efficiency and Clean Mechanical Manufacture of MOE, School of Mechanical Engineering, Shandong University, Jinan 250061, PR China
b
art ic l e i nf o
a b s t r a c t
Article history: Received 18 April 2016 Received in revised form 17 May 2016 Accepted 23 May 2016
Si3N4/(W, Ti)C graded nano-composite ceramic tool materials with different thickness ratios and number of layers were fabricated by hot pressing technology. The flexural strength, fracture toughness and hardness of the sintered composites were tested and the microstructure and indention cracks were observed. The experiment results showed that the five-layer graded nano-composites with a thickness ratio of 0.2, which were sintered under a pressure of 30 MPa at 1700 °C in vacuum condition for 45 min, had the optimum comprehensive mechanical properties with a flexural strength of 1080.3 MPa, a hardness of 17.64 GPa, and a fracture toughness of 10.87 MPa m1/2. The formation of elongated β-Si3N4 grains contributes to the favorable mechanical properties. The graded structure can induce residual compressive stress in the surface layer and enhance the mechanical properties. The strengthening and toughening mechanisms are a synergistic effect of intergranular and transgranular fracture, crack bridging and deflection. & 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: Graded ceramic tool material Si3N4/(W, Ti)C Mechanical properties Microstructure Toughening and strengthening mechanisms
1. Introduction Si3N4 ceramics have been extensively studied, because of their outstanding mechanical properties, higher wear resistance, thermal shock resistance and self-lubrication behavior [1–6]. They can be used as components in heat engines and gas turbines, cutting tools materials and some functional materials [1,7–9]. However, the relatively low hardness and fracture toughness, and the strong chemical attraction between Si and Fe elements [10], limit their applications. The most common way to improve the mechanical properties of Si3N4 ceramic materials is by adding various secondary phases and nanophases, such as SiC [11,12], TiC [10], TiN [13], Ti(C, N) [14], (W, Ti)C [15], etc. Functional graded materials (FGMs), made of two or more constituent phases with continuous and smoothly varying composition, were introduced in 1984 [16]. Worldwide attention has been paid to FGMs for their innovative design ideas and outstanding properties in areas of aerospace, medicine, defense, energy, optoelectronics and so on [17]. The introduction of FGM into the design of ceramic cutting tool materials provides a new approach to improve their mechanical properties and cutting performance [18–20]. In previous studies, Si3N4-based composite ceramic tool materials reinforced by nanoscale Si3N4 and microscale (W, Ti)C were n
Corresponding author. E-mail address: [email protected] (J. Zhao).
fabricated [15]. The appropriate addition of 20 vol% of (W, Ti)C and 18 vol% of nanoscale Si3N4, indeed increased the mechanical properties of the composites to a flexural strength of 979 MPa, a fracture toughness of 8.5 MPa m1/2 and a Vicker's hardness of 17.72 GPa. Obviously, the fracture toughness was still limited. Then, Si3N4/(W, Ti) C/Co graded ceramic tool materials were fabricated to improve the fracture toughness to 10.5 MPa m1/2 [21]. However, the flexural strength was improved limited only to 992 MPa. In this paper, to further increase the mechanical properties, Si3N4/(W, Ti)C nano-composite ceramic tool materials with graded structure were fabricated. Combined with the mechanical properties, the optimum thickness ratio and number of layers were determined. The effect of sintering temperature and holding time on mechanical properties and microstructure of the composites was discussed as well. Furthermore, strengthening and toughening mechanisms were investigated by the observation of microstructure of the fracture surfaces and indented cracks on the polished surfaces.
2. Experimental procedure 2.1. Design of graded nano-composite ceramic tool materials Fig. 1 shows a cross-sectional optical graph of the five-layer Si3N4/(W, Ti)C graded ceramic with a thickness ratio of 0.2.
http://dx.doi.org/10.1016/j.ceramint.2016.05.142 0272-8842/& 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: X. Tian, et al., Design and fabrication of Si3N4/(W, Ti)C graded nano-composite ceramic tool materials, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.05.142i
2.2. Material preparation The details of the starting materials used in this paper are shown in Table 3 and α-Al2O3 and Y2O3 are used as sintering additives to promote liquid phase sintering because covalently bonded crystalline silicon nitride is hard to sinter [22]. The preparation process of mixed powders for the composites in Table 1 was as follows. The (W, Ti)C powder was firstly ballmilled in ethanol medium with cemented carbide balls for 10 days to decrease the average grain size to about 1 mm (see Fig. 2) for further use. The dispersion process of the Si3N4 nano-powder was assisted by mechanical stirring and ultrasonic vibration (with HS10260D ultrasonic instrument, China). The Si3N4 nano-powder was prepared into suspensions using ethanol as dispersing medium, then the 0.5 wt% (relative weight ratio to the nano particles to be dispersed) dispersant polyethylene glycol (PEG Sinopharm Chemical Reagent Co., Ltd., Shanghai, China) was added into the suspensions and the pH value was adjusted to about 9.5 with NH3 (Sinopharm Chemical Reagent Co., Ltd., Shanghai, China), and then ultrasonic stirred for 30 min to get the best dispersing effect. After that, according to Table 1, a specified amount of microscale Si3N4, (W, Ti)C, and the sintering aids were added into the well reagglomerated and uniform suspension of Si3N4 nano-particles and ultrasonic stirred for another 15 min. The mixed slurries were ballmilled with alumina balls for 48 h and then dried at 120 °C in a vacuum desiccator (Model DZ-2AⅡ, China). After drying, the mixture powders were sieved through a 120 mesh sieve. At last, according to Table 2, the dried powders were put into the cylindrical graphite mold layer by layer with a predetermined thickness ratio and hot-pressed under 30 MPa at temperature in the range of 1600–1750 °C for 30–75 min in the vacuum condition by a multifunctional hot pressing sintering furnace (Model ZRC85-25T, China), and then furnace cooling to room temperature.
h2
Composite 1 Composite 2
h3
z
H
Considering the ease of fabrication process, a design model with changing compositional distribution only along z direction is presented. Besides, different from traditional FGMs with composition changing from one surface to another, a symmetric structure is modeled, and thus both of the two opposite surfaces of an insert could be used as the rake face. Two parameters are used here to build the graded structure and name the graded composites, that is, the number of layers n and the thickness ratio e. For example, the graded Si3N4/(W, Ti)C ceramic with n ¼5 and e¼0.2 is denoted as GSWT52. As shown in Fig. 1, the thickness of the surface layer, the second layer, the middle layer and the total thickness of the material are set to be h1, h2, h3 and H, respectively. Then, H ¼2h1 þ2h2 þh3, and the thickness ratio e¼ h1/h2 ¼h2/h3 is used to determine the thickness of each layer. The formation of compressive residual stress in the surface layer can alleviate stresses resulting from external loadings in the cutting process and improve the cutting performance of ceramic cutting tools [14]. For this purpose, the thermal expansion coefficient should reduce gradually from the inner layer to the surface layer. Then, compressive residual stress can form in the surface layer of the graded composites during the cooling down process from the sintering temperature. Composites with different composition ratio were designed according to former study [15] as shown in Table 1. The thermal expansion coefficients of each composite were obtained using Kerner's equation [18]. Obviously, the layer with the highest volume fraction of (W, Ti)C should be put in the middle. As shown in Table 2, several kinds of graded composites with different number of layers and thickness ratios were designed and fabricated.
h1
X. Tian et al. / Ceramics International ∎ (∎∎∎∎) ∎∎∎–∎∎∎
2
x y
Composite 3
Composite 2 Composite 1 Fig. 1. Cross-sectional graph of the five-layer graded ceramic material with a thickness ratio of 0.2.
2.3. Specimen processing The hot-pressed disks, with a diameter of 42 mm, were cut (with J5060C-1-type inner-circular cutter), ground and polished into bars with dimensions about of 3 mm 4 mm 40 mm. The specimens’ edges were chamfered to eliminate machining flaws which may act as fracture origins. The flexural strength was measured using a three-point bending tester (Model WDW-50E, China) with a 20 mm span at a loading velocity of 0.5 mm/min. Hardness was measured by Vickers indention method using a diamond pyramid indenter (Model MHVD-30AP, China) with a load of 196 N and a duration time of 15 s and the fracture toughness was determined by the indentation crack length [23]. The density of each specimen was measured by the Archimedes’ method with an analytical balance (Model AUY120, Japan), and the relative density was calculated from the theoretical densities of each raw powder, which was got using the linear mixed estimation method. For each experimental condition, 5 specimens were tested. Cross-sectional optical graph and macro cracks propagation path was observed using a Keyence digital microscope (Model VHX-600E, Japan). The micro cracks on the polished surfaces and the fractured surfaces, etched in molten NaOH for about 1 min, were observed by scanning electron microscopy (SEM, SUPRA-55, ZEISS, Germany). Phase identification of the composites was performed by X-ray diffraction analysis (XRD, D8 Advance, Bruker, Germany) with copper Kα radiation. Energy dispersive spectroscopy (EDS, INCAx-act, UK) was used to distinguish the Si3N4 and (W, Ti)C grains.
3. Results and discussions 3.1. Mechanical properties and microstructure 3.1.1. Effect of the thickness ratio and the number of layers Fig. 3 shows the effect of thickness ratio on mechanical properties of the five layered composites sintered at 1700 °C for 45 min When the thickness ratio is between 0.1 and 0.5, the flexural strength and fracture toughness of the composites increase firstly and then decrease, and both arrive at a maximum at e¼0.2. The hardness increases firstly and then decreases until e¼0.3, and then increases again. Further increasing the thickness ratio to more than 0.6, the flexural strength appears to remain a constant value. The hardness has a tendency to increase while the fracture toughness tends to decrease. Obviously, the composites with a thickness ratio of 0.2, have the comprehensive optimum
Please cite this article as: X. Tian, et al., Design and fabrication of Si3N4/(W, Ti)C graded nano-composite ceramic tool materials, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.05.142i
X. Tian et al. / Ceramics International ∎ (∎∎∎∎) ∎∎∎–∎∎∎
3
Table 1 Composition of different composites (vol%). Composites
Weight ratio of nano-Si3N4 (25 nm) to micro-Si3N4 (0.5 mm)
(W, Ti)C (1 mm)
Y2O3 þ Al2O3 (0.5 mm)
Thermal expansion coefficient α ( 10 6 K 1)
SWT10 SWT15 SWT20 SWT25
1/3 1/3 1/3 1/3
10 15 20 25
8 8 8 8
3.490 3.630 3.767 3.900
Table 2 Composites of the FGM. Number of layers
Composites of each layer
3 5 7
SWT15/ SWT20/ SWT15 SWT15/ SWT20/ SWT25/ SWT20/ SWT15 SWT10/ SWT15/ SWT20/ SWT25/ SWT20/ SWT15/ SWT10
Table 3 Details of the starting materials. Powders Average particle size (μm)
Purity (%)
Manufacturer
α-Si3N4
0.5/0.025
499
(W, Ti)C
3
499.7
α-Al2O3
0.5
499.9
Y2O3
–
Highpurity
Hefei Kaier Nanometer Energy and Technology Co., Ltd., China Changsha Langfeng Metallic Material Co., Ltd., China Shanghai St-nano Science and Technology Co., Ltd., China Sinopharm Chemical Reagent Co., Ltd., Shanghai, China
mechanical properties. Fig. 4 shows the effect of number of layers on mechanical properties of the composites with a fix thickness ratio of 0.2 sintered at 1700 °C for 45 min And abscissa “1” represents the homogenous composites SWT20, which owns the best comprehensive mechanical properties among the four composites in Table 1 [15]. From Fig. 4, it can be seen that the number of layers has a considerable effect on the mechanical properties of the composites. Both the flexural strength and fracture toughness of the composites increase firstly and then decrease with the increase of number of layers. When n¼ 5, the flexural strength and fracture toughness of the composites can reach 1080.3 MPa and 10.87 MPa m1/2. Compared with the homogenous composite SWT20 [15], with a flexural strength of 978.53 MPa and fracture toughness of 8.54 MPa m1/2, the two mechanical properties of the graded composites are increased by 10.4% and 27.3%, respectively. The improved properties are attributed to the combined effect of elastic modulus gradient and the compressive residual stress
4 μm
presented at the surface layer of the graded structure, which is caused by the thermal-mismatch effect. The residual stress is also benefit for the increase in hardness of the graded materials [24]. Since the hardness of the graded composites GSWT52, surface layer of which is SWT15, is 17.64 GPa, and is higher than the homogeneous SWT15 with a hardness of 17.25 GPa [15]. The higher hardness of SWT20, is attributed to the higher condition of (W, Ti)C, which has better hardness than the Si3N4 matrix. It can be concluded from the above analysis that the graded composites GSWT52, with n¼ 5 and e¼0.2, have the comprehensive optimum mechanical properties and are used to optimize the sintering process in the next part. The cross-sectional graph of sintered GSWT52 are shown in Fig. 5. It can be seen that the graded structure is made of five layers and consistent with the design model. 3.1.2. Effect of sintering temperature and holding time The effect of sintering temperature on the mechanical properties of GSWT52 sintered for 45 min is illustrated in Fig. 6. It can be seen that the flexural strength, hardness, fracture toughness and relative density of GSWT52 follow a similar change to increase firstly and then decrease and reach a maximum value at 1700 °C as the sintering temperature increases. It also can be confirmed that the relative density affects the mechanical properties to a large extent. Fig. 7 illustrates the SEM micrographs of etched fracture surfaces of GSWT52 sintering at 1600 °C, 1650 °C, 1700 °C and 1750 °C for 45 min. Generally, the microstructure is characterized by rodlike and some irregular grains. The EDS analysis of point 1 and 2 in Fig. 7(c), as shown in Fig. 8(a) and (b), illustrates that the elongated grains are Si3N4, and the bigger and irregular grains are (W, Ti)C. The XRD patterns of the surface layer of GSWT52, as shown in Fig. 8(d), indicates that the major phases are β-Si3N4 and (W, Ti)C. No α-Si3N4 is identified, revealing a complete phase transformation from α-Si3N4 to β-Si3N4 and some secondary phase Si3Al3O3N5 are formed. Besides, the different phases show a good chemical compatibility. Obviously, the elongated β-Si3N4 grains, which have a pronounced effect on the mechanical properties of the composites, have a tendency to grow bigger with the increased sintering
4 μm Fig. 2. SEM micrographs of (W, Ti)C powders (a) Starting powders, (b) Ball milled powders.
Please cite this article as: X. Tian, et al., Design and fabrication of Si3N4/(W, Ti)C graded nano-composite ceramic tool materials, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.05.142i
4
X. Tian et al. / Ceramics International ∎ (∎∎∎∎) ∎∎∎–∎∎∎
Fig. 3. The effect of thickness ratio on (a) flexural strength, (b) hardness and (c) fracture toughness of the composites with n ¼5.
temperature. The addition of nanoscale Si3N4 particles increases the surface energy of the composites powders and enhances the sintering driven force and promote the formation of the duplex distribution characteristics in the solution-diffusion-precipitation process [10]. Under a relatively lower sintering temperature of 1600 °C, the sintering driven force is not enough and the effect is not obvious, so the elongated grains are tiny. At 1750 °C, the much higher sintering temperature will make the movement of grain boundaries fast, leading to excessive grain growth and some coarsened elongated grains. More pores will appear under these conditions, which reduce the density of the composites and restrains the mechanical properties as shown in Fig. 6. When the sintering temperature is at 1650 and 1700 °C, especially 1700 °C, the α-Si3N4 grains transform completely to rod-like grains and show duplex distribution characteristic. The larger grains own an aspect ratio about 5:1, while some much tinier β-Si3N4 grains with an aspect ratio about 10:1 are filled in larger Si3N4 and (W, Ti)C grains. This increases the compactness of the composites as shown in Fig. 6(d). The larger and smaller β-Si3N4 grains were transformed from the α-Si3N4 microparticles and nanoparticles, respectively. The crossed rod-like grains tend to increase the strength of the composites by a crack deflection mechanism. The relative density of the composites sintered at 1700 °C can reach 98.87%. The effect of holding time on the mechanical properties of GSWT52 sintered at 1700 °C is shown in Fig. 9. The composites sintered for 45 min owe the best flexural strength, hardness and relative density, while the best fracture toughness can be got with
a holding time of 60 min. Fig. 10 illustrates the SEM micrographs of the etched fracture surfaces of GSWT52 sintering at 1700 °C for 30 min, 45 min, 60 min and 75 min. It can be observed that, the growth of β-Si3N4 grains does not follow a fixed trend. With a less holding time of 30 min, the inhibit effect of the growing (W, Ti)C grains on the growth of β-Si3N4 grains is not significant, so some bigger β-Si3N4 grains are formed. However, the undense defects caused by the insufficient sintering, such as pores, are detrimental for the compactness, as shown in Fig. 9(d), and reduces the mechanical properties. Increasing the holding time to 45 min, the β-Si3N4 grains becomes more subtle. The growth of (W, Ti)C grains inhabits the growth of β-Si3N4 grains and makes a finer structure and increases the densification of the composites. With a holding time of 60 min, most of the β-Si3N4 grains are coarsened, leading to the reduction of mechanical properties. Coarsened grains bring more pores and reduce the density and the flexural strength largely. On the other hand, the fracture toughness was increased by the bigger grains. With a longer holding time of 75 min, the βSi3N4 grains becomes smaller again. Some nanoscale grains are agglomerated along the bigger (W, Ti)C and β-Si3N4 grains as marked in Fig. 10(d). EDS analysis of point 3, as shown in Fig. 8(d), declare that the agglomerated grains are nanoscale Si3N4. The steric hindrance effect caused by the agglomerated nanoscale Si3N4 grains inhabits the growth of β-Si3N4 grains and restrains the mechanical properties as well. Through optimizing the composition and sintering process, the five-layer composites GSWT52 with a thickness ratio of 0.2,
Please cite this article as: X. Tian, et al., Design and fabrication of Si3N4/(W, Ti)C graded nano-composite ceramic tool materials, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.05.142i
X. Tian et al. / Ceramics International ∎ (∎∎∎∎) ∎∎∎–∎∎∎
5
Fig. 4. The effect of number of layers on (a) flexural strength, (b) hardness and (c) fracture toughness of the composites with e¼ 0.2.
3.2. Toughening and strengthening mechanisms 3.2.1. Effect of graded structure Macroscopical cracks propagation paths of the homogeneous composite SWT20 and graded GSWT52 from the Vickers indentation test under the same load of 196 N are shown in Fig. 11. The arrows present the end of the cracks. Evidently, the indentation area is almost the same, however the propagation length of the crack on GSWT52 is much shorter than SWT20, which means a higher fracture toughness as shown in Fig. 4. This is due to the elastic modulus gradient and the thermal mismatch effect [11], which induces residual compressive stress on the surface layer of the graded structure. Much more fracture energy will be consumed to resist the impedance effect of the residual compressive stress during the crack propagation process.
Fig. 5. The cross-sectional graph of sintered GSWT52.
sintered under 30 MPa at 1700 °C for 45 min have the optimum mechanical properties, which is a flexural strength of 1080.3 MPa, a hardness of 17.64 GPa, and a fracture toughness of 10.87 MPa m1/2.
3.2.2. Effect of β-Si3N4 and (W, Ti)C grains The fracture surface of the composites without etch, as shown in Fig. 12, shows a mix of transgranular and intergranular fracture, which contributes to the improved mechanical properties largely. The relatively smooth surfaces and cleavage surfaces of the grains, as shown in Fig. 12(a), reflect the occurrence of intergranular and transgranular fracture. According to the above analysis, during the sintering process, α-Si3N4 will completely transform into β-Si3N4 grains. The elongated grains crisscross, which is helpful to improve the strength of the ceramic tool materials. Besides, when fracture meets with inclined elongated β-Si3N4 grains, the interface debonding, as shown in Fig. 12(b), tends to occur. This means some elongated
Please cite this article as: X. Tian, et al., Design and fabrication of Si3N4/(W, Ti)C graded nano-composite ceramic tool materials, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.05.142i
6
X. Tian et al. / Ceramics International ∎ (∎∎∎∎) ∎∎∎–∎∎∎
Fig. 6. The effect of sintering temperature on (a) flexural strength, (b) hardness, (c) fracture toughness and (d) relatively density of GSWT52 sintered for 45 min.
Fig. 7. SEM micrographs of the etched fracture surfaces of GSWT52 sintering at (a) 1600 °C, (b) 1650 °C, (c) 1700 °C and (d) 1750 °C for 45 min.
Please cite this article as: X. Tian, et al., Design and fabrication of Si3N4/(W, Ti)C graded nano-composite ceramic tool materials, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.05.142i
X. Tian et al. / Ceramics International ∎ (∎∎∎∎) ∎∎∎–∎∎∎
Fig. 8. EDS analysis of (a) point 1, (b) point 2 in Fig. 7(c) and (c) point 3 in Fig. 10(d) and (d) XRD patterns of the surface layer of GSWT52.
Fig. 9. The effect of holding time on (a) flexural strength, (b) hardness, (c) fracture toughness and (d) relatively density of GSWT52 sintered at 1700 °C.
Please cite this article as: X. Tian, et al., Design and fabrication of Si3N4/(W, Ti)C graded nano-composite ceramic tool materials, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.05.142i
7
8
X. Tian et al. / Ceramics International ∎ (∎∎∎∎) ∎∎∎–∎∎∎
b
Transgranular fracture
2 μm
Agglomeration
Fig. 10. SEM micrographs of the etched fracture surfaces of GSWT52 sintering at 1700 °C for (a) 30 min, (b) 45 min, (c) 60 min and (d) 75 min.
grains are pulled out from the fracture surface, which is similar to the whisker toughening mechanisms [25,26]. The friction between interfaces will consume fracture energy and improve the flexural strength and fracture toughness. Some pores are also distinguished. Pores will decrease the loading cross sectional area and lead to stress concentration, and thus decrease the strength of composites. The addition of (W, Ti)C grains will induce much transgranular fracture. Transgranular fracture will increase the energy required for rupture since the interfacial toughness is typically much smaller than the toughness of the grains [27]. From Fig. 10(b) and Fig. 12(a), it can be inferred that the transgranular
fracture, which increases the mechanical properties of the composites a lot, mainly among irregular (W, Ti)C grains. SEM micrographs of crack extension paths of GSWT52 generated by Vicker's indentation are shown in Fig. 13. The irregular white grains are (W, Ti)C and the grey grains are β-Si3N4. The inhibition effect of (W, Ti)C grains on the crack propagation at the end of the crack is clearly shown in region A in Fig. 13(a). Under the quasi-static loading conditions, the main fracture mode is intergranular fracture [27] as shown in region C. Transgranular fracture happens simultaneously. Combined with the above analysis, it can be concluded that transgranular fracture is mainly
Fig. 11. Macroscopical cracks propagation path of (a) graded GSWT52 and (b) the homogeneous SWT20.
Please cite this article as: X. Tian, et al., Design and fabrication of Si3N4/(W, Ti)C graded nano-composite ceramic tool materials, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.05.142i
X. Tian et al. / Ceramics International ∎ (∎∎∎∎) ∎∎∎–∎∎∎
9
Fig. 12. The fracture surfaces of GSWT52 without etch sintering at 1700 °C for 45 min.
across the bigger (W, Ti)C grains as shown in regions B. At microscale level, the thermal mismatch effect between the matrix Si3N4 and (W, Ti)C will induce radial residual tensile stress in the matrix around (W, Ti)C particles, which contributes to the forming of transgranular fracture. Intergranular fracture makes the cracks deflect and increases the crack propagating length, while transgranular fracture increases the energy required for rupture. Crack bridging and deflection are also observed. The cracks can be bridged by β-Si3N4 grains as shown in regions D in Fig. 13(a), as well as by the (W, Ti)C grains as shown in region D in Fig. 13(b). Crack bridging can provide a force for crack closure and impedes crack propagation. Crack deflection can make the cracks more tortuous and hinder the propagation of the cracks. All these phenomena will consume a greater amount of rupture energy and improve the fracture toughness of the composite greatly. 3.2.3. Effect of nanoscale grains Fig. 14 illustrates the TEM micrographs of the surface layers of GSWT52. The white grains with a hexagonal prisms structure are β-Si3N4, and the black grains with amorphous shape are (W, Ti)C. The addition of nano-Si3N4 and some nanoscale (W, Ti)C grains contributed to the improved mechanical properties greatly. The nanoscale grains embed into microscale ones and form the intragranular structure as shown in Fig. 14(b), which introduces lots of sub-grain boundaries. Due to the thermal mismatch effect between the matrix Si3N4 and the reinforcement phase (W, Ti)C, some dislocations exist along the sub-grain boundaries and induce stress interference fringes. The dislocations can consume more fracture energy and restrain the propagation of the cracks. Therefore, the toughing and strengthening mechanisms are synergistic effects of intergranular and transgranular fracture,
crack bridging and deflection caused by the addition of reinforcing phases and nanoscale grains and the use of graded structure.
4. Conclusions This work aims to develop a new kind of graded Si3N4/(W, Ti)C ceramic cutting tools materials. A systematic experiments have been conducted to optimize the composition and the sintering process. The following conclusions can be achieved. (1) The five-layer composites GSWT52 with a thickness ratio of 0.2, which are sintered under a pressure of 30 MPa at 1700 °C for 45 min, have the optimum mechanical properties. The optimum mechanical properties are a flexural strength of 1080.3 MPa, a hardness of 17.64 GPa, and a fracture toughness of 10.87 MPa m1/2, which meets the requirements for ceramic cutting tools. (2) The graded structure improves the mechanical properties of the ceramic cutting tools material, especially the flexural strength and fracture toughness. Compared with homogenous SWT20, the flexural strength and fracture toughness of graded GSWT52 are increased by 10.4% and 27.3%, respectively. This is mainly due to the induced compressive residual stress on the surface layer during the fabrication process. (3) The formation of elongated β-Si3N4 grains can introduce toughening mechanisms similar to whisker and the addition of (W, Ti)C grains can induce more transgranular fracture. (4) The toughing and strengthening mechanisms of the composites are synergistic effects of intergranular fracture, transgranular fracture, crack bridging and deflection. These effects
Fig. 13. SEM micrographs of crack extension paths of surface layer of GSWT52.
Please cite this article as: X. Tian, et al., Design and fabrication of Si3N4/(W, Ti)C graded nano-composite ceramic tool materials, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.05.142i
X. Tian et al. / Ceramics International ∎ (∎∎∎∎) ∎∎∎–∎∎∎
10
a
(W, Ti)C
b Intragranular structure
Stress interference fringes β-Si3N4 Fig. 14. TEM micrographs of GSWT52 (surface layers) (a) β-Si3N4 and (W, Ti)C grains, (b) Stress interference fringes.
can improve the mechanical properties of the composites significantly.
Acknowledgement This work is supported by the National Natural Science Foundation of China (51175310 and 51475273), and the Priority Academic Program Development of Jiangsu Higher Education Institution.
[13]
[14]
[15]
[16] [17] [18]
References [19] [1] X. Pan, J. Mayer, M. Rühle, K. Niihara, Silicon nitride based ceramic nanocomposites, J. Am. Ceram. Soc. 79 (1996) 585–590. [2] F.L. Riley, Silicon nitride and related materials, J. Am. Ceram. Soc. 83 (2000) 245–265. [3] I. Schulz, M. Herrmann, I. Endler, I. Zalite, B. Speisser, J. Kreusser, Nano Si3N4 composites with improved tribological properties, Lubr. Sci. 21 (2009) 69–81. [4] P. Rutkowski, L. Stobierski, D. Zientara, L. Jaworskab, P. Klimczykb, M. Urbanik, The influence of the graphene additive on mechanical properties and wear of hot-pressed Si3N4 matrix composites, J. Eur. Ceram. Soc. 35 (2015) 87–94. [5] W.M. Guo, J.J. Yu, M. Xiong, S.H. Wu, H.T. Lin, High-toughness Lu2O3-doped Si3N4 ceramics by seeding, Ceram. Int. 42 (2016) 6495–6499. [6] L. Li, L. Cheng, S. Fan, X. Gao, Y. Xie, L. Zhang, Fabrication and dynamic compressive response of laminated ZrO–Zr2CN/Si3N4 ceramics, Ceram. Int. 41 (2015) 8584–8591. [7] I.A. Choudhury, M.A. El-Baradie, Machinability of nickel-base super alloys: a general review, J. Mater. Process. Technol. 77 (1998) 278–284. [8] K. Zuo, Y. Zeng, D. Jiang, The mechanical and dielectric properties of Si3N4based sandwich ceramics, Mater. Des. 35 (2012) 770–773. [9] S.J. Lee, S. Baek, Effect of SiO2 content on the microstructure, mechanical and dielectric properties of Si3N4 ceramics, Ceram. Int. 42 (2016) 9921–9925. [10] J. Zhao, X. Ai, Z.J. Lü, Preparation and characterization of Si3N4/TiC nanocomposite ceramics, Mater. Lett. 60 (2006) 2810–2813. [11] S.C. Thompson, A. Pandit, N.P. Padture, S. Suresh, Stepwise-graded Si3N4-SiC ceramics with improved wear properties, J. Am. Ceram. Soc. 85 (2002) 2059–2064. [12] M. Poorteman, P. Descamps, F. Cambier, M. Plisnierb, V. Canonneb, J. C. Descamps, Silicon nitride/silicon carbide nanocomposite obtained by
[20]
[21]
[22]
[23] [24]
[25]
[26]
[27]
nitridation of SiC: fabrication and high temperature mechanical properties, J. Eur. Ceram. Soc. 23 (2003) 2361–2366. B. Zou, C.Z. Huang, M. Chen, M.L. Gu, H.L. Liu, Study of the mechanical properties, toughening and strengthening mechanisms of Si3N4/Si3N4w/TiN nanocomposite ceramic tool materials, Acta Mater. 55 (2007) 4193–4202. G.M. Zheng, J. Zhao, Y.H. Zhou, Z.J. Gao, X.B. Cui, A.H. Li, Fabrication and characterization of Sialon-Si3N4 graded nano-composite ceramic tool materials, Compos. Part B-Eng. 42 (2011) 1813–1820. X.H. Tian, J. Zhao, N.B. Zhu, Y. Dong, J.B. Zhao, Preparation and characterization of Si3N4/(W, Ti)C nano-composite ceramic tool materials, Mater. Sci. Eng. A-Struct. 596 (2014) 255–263. I. Shiota, Y. Miyamoto, Functionally Graded Materials 1996, Elsevier,, 1997. R.M. Mahamood, E.T. Akinlabi, M. Shukla, S. Pityana, Functionally graded material: an overview, Lect. Not. Eng. Compu. Sci. 2199 (2012) 1593–1597. X. Ai, J. Zhao, C.Z. Huang, J.H. Zhang, Development of an advanced ceramic tool material functionally gradient cutting ceramics, Mater. Sci. Eng. A-Struct. 248 (1998) 125–131. J. Zhao, X. Ai, X.P. Huang, Relationship between the thermal shock behavior and the cutting performance of a functionally gradient ceramic tool, Mater. Process. Technol. 129 (2002) 161–166. G.M. Zheng, J. Zhao, Z.J. Gao, Q.Y. Cao, Cutting performance and wear mechanisms of Sialon-Si3N4 graded nano-composite ceramic cutting tools, Int. J. Adv. Manuf. Technol. 58 (2012) 19–28. X. Tian, J. Zhao, Y. Wang, F. Gong, W. Qin, H. Pan, Fabrication and mechanical properties of Si3N4/(W, Ti)C/Co graded nano-composite ceramic tool materials, Ceram. Int. 41 (2015) 3381–3389. M. Herrmann, Z.J. Shen, I. Schulz, J.F. Hu, B. Jancar, Silicon nitride nanoceramics densified by dynamic grain sliding, J. Mater. Res. 25 (2010) 2354–2361. A.G. Evans, E.A. Charles, Fracture toughness determinations by indentation, J. Am. Ceram. Soc. 59 (1976) 371–372. J.X. Deng, Z.X. Duan, D.L. Yun, H. Zhang, X. Ai, J. Zhao, Fabrication and performance of Al2O3/(W,Ti)C þ Al2O3/TiC multilayered ceramic cutting tools, Mater. Sci. Eng. A-Struct. 527 (2010) 1039–1047. L. Silvestroni, D. Sciti, C. Melandri, S. Guicciardi, Toughened ZrB2-based ceramics through SiC whisker or SiC chopped fiber additions, J. Eur. Ceram. Soc. 30 (2010) 2155–2164. P.F. Becher, K. Hirao, M.E. Brito, E.Y. Sun, K.P. Plucknett, K.B. Alexander, C. Hsueh, H. Lin, S.B. Waters, C.G. Westmoreland, E. Kang, Microstructural design of silicon nitride with improved fracture toughness: I, effects of grain shape and size, J. Am. Ceram. Soc. 81 (1998) 2821–2830. G. Hu, C.Q. Chen, K.T. Ramesh, J.W. Mccauley, Mechanisms of dynamic deformation and dynamic failure in aluminum nitride, Acta Mater. 60 (2012) 3480–3490.
Please cite this article as: X. Tian, et al., Design and fabrication of Si3N4/(W, Ti)C graded nano-composite ceramic tool materials, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.05.142i