Mechanical performance and thermal shock resistance of alumina composites fabricated by temperature gradient spark plasma sintering

Temperature gradient spark plasma sintering technology (TGSPS) was used to fabricate alumina composites with addition of tremolite. The mathematics model of TGSPS was established using piecewise interpolation spline function. Liquid phase sintering took place in the sintering process. Vickers hardness, fracture toughness and exural strength of the composites were measured. Microstructure observations on fracture surfaces of alumina composites were analyzed. Thermal shock resistance of alumina composites was investigated by measuring the strength retention after varying the temperature difference through the water-quench method. Results showed that the addition of tremolite could promote the liquid phases sintering of alumina composites, decrease the porosity, strength the grain boundaries, improve the mechanical properties, and thus contribute to the improvement in TSR of alumina composites.


Introduction
Recently, alumina ceramic composites have been receiving growing attention owing to their sound mechanical performances such as high hardness, good chemical inertness, high wear resistance, and so on. High-quality alumina ceramic bodies are widely used for casting hollow turbine blades in aircraft engines and gas turbines [1], ceramic cutting tools for machining hard materials [2,3], ceramic nozzles in sand blasting treatments [4,5], Spark plugs [6], coatings [7][8][9], bearings [10,11] and dental materials [12]. It is well known that the addition of CaO could cause alumina to crystallize at low temperature and promote the sintering process [13]. Meanwhile, the addition of small amounts of MgO suppresses abnormal grain growth of alumina, induce liquid-phase sintering and promote enhanced densi cation rate [14][15][16][17][18][19][20], so ne microstructures and good mechanical properties of alumina ceramic composites can be obtained if MgO is introduced with appropriate content. It has been reported in reference [21] that, the smaller the sizes of particles are, the lower the temperature to sinter alumina will be. Being composed of SiO 2 , CaO and MgO phases, tremolite (Ca 2 Mg 5 Si 8 O 22 (OH) 2 ), which is a member in amphibole groupwhich and can be formed by the conversion of dolomite, silica and water together with calcite and carbon dioxide [22], is expected to decrease the sintering temperature of alumina ceramic materials.
Spark plasma sintering (SPS) has the characteristics of fast rising and cooling, low sintering temperature and short holding time, which can reduce the abnormal growth of particles [23][24][25]. It is possible to achieve high densi cation Al 2 O 3 composite prepared by SPS at low temperature, which suppresses its grain growth [26][27][28]. The densi cation of Al 2 O 3 -YSZ-TiO 2 composites sintered by SPS at temperature of 1350-1400 °C was higher than 99.2% [28]. The average grain size of Al 2 O 3 /WC composite fabricated by SPS was 1-1.5 µm [29]. The hardness and exural strength of Al 2 O 3 -Ni nano composites fabricated by SPS at 1573 K reached 20.5 GPa and 170 MPa [27]. The effect of grain re ning can be achieved with the help of SPS approach. Nano-structured alumina composite was prepared from micro-sized powders sintered by SPS at 1200 and 1300 °C when the starting powder was 3 µm [30].
Thermal shock resistance (TSR) is one of a major topic for the application of alumina composites in the high-temperature environment. The TSR has been investigated by various researchers for better understanding the mechanisms of improving TSR of Al 2 O 3 composites [31][32][33][34][35]. The improved TSR of Al 2 O 3 -SiC w composites were achieved by enhancing its densi cation and the content of yttrium aluminum garner, producing of tabular alumina [31]. The TSR of laminated Al 2 O 3 /Mo-Al 2 O 3 composites was improved by introducing heat-resistant particles into weak layers and improving the interfacial bonding strength of the two layers [32]. The effect of different additives on TSR of Al 2 O 3 composites is various, and some additives can decrease the TSR of Al 2 O 3 composites. The TSR of porous Al 2 O 3 composites was enhanced by introducing SiC nano powder, while the TSR of the samples with microsized SiO 2 was slightly lower than that of unmodi ed porous Al 2 O 3 ceramics [33]. The TSR of Al 2 O 3 composites was sensitive to quenching treatment, however the loss of strength was gradual when Al 2 O 3 -Er 3 Al 5 O 12 composite was quenched in boiling water [34]. There are many factors in uencing the TSR of ceramic composite, including mechanical properties and thermal physical parameters, thermal expansion coe cient, etc. [36]. The TSR of laminated Al 2 O 3 composites could be characterised by the method of macro features (displacement and residual strength) and micro features (crack growth resistance and fracture mode) [35], which provided the comprehensive understanding of the TSR of Al 2 O 3 composites.
Accordingly, much effort has been made to prepare alumina composites using various technologies mentioned from the above, however, so far the effect of tremolite on the thermal shock behavior of  Table 1, and milled with alumina balls for 90-100 h in an alcohol medium to obtain a homogeneous mixture. The ball to powder mass ratio is 3 to 1 and that of alcohol to powder is 2 to 1.
The mixture was dried in vacuum and screened. And then placed in a graphite die using the technology of TGSPS at temperature of 1350 ℃ with pressure of 35 MPa for 5 min.

Characterization
Rectangular bar specimens of size 3 × 4 × 36 mm 3 were used for mechanical properties and TSR tests, as shown in Fig. 1. The bars were ground with a diamond wheel and polished using diamond pastes to a Ra of ~ 0.1 µm. Three-point-bending was used to measure the exural strength on an electronic universal experimental instrument (WD-10, produced by Jinan TEST Co., Ltd. P.R. China) with a span of 20 mm at a crosshead speed of 0.5 mm min − 1 . Six specimens with the same compositions were used for measuring the exural strength in air at room temperature.
The exural strength was calculated by the following formula [37]: where σ f was exural strength (MPa) and P was load (N) under which the samples broke, b and h were width and height (mm), respectively, and L was span (mm).
Vickers hardness was measured on polished surfaces with a load of 9.8 N for 5 s with a micro-hardness tester (MH-6). Indentation fracture resistance measurement was performed using the indentation method with a hardness tester (Hv-120, produced by Shanghai Hengyi electronic testing instrument corporation, P.R. China), and results were obtained by the formula proposed by Cook and Lawn [38]. Five indents were made in a row at the middle of each specimen to obtain an average value. XRD (D/max-2400) analysis was adopted to identify the phases after sintering. Microstructures of the samples were studied on fracture surfaces by scanning electron microscopy (SEM, HITACHI S-570, produced by Japan Hitachi Co., Ltd.).
Ground and polished rectangular bars for thermal shock testing were placed into a furnace and heated to the desired temperatures at a heating rate of 20 ℃/min. After the specimens were heated for 30 min in the furnace to induce the homogeneous temperature distribution, they were dropped into a bath of water at room temperature. ΔT, the temperature differences for all composites were chosen referring to literature [35,[38][39][40][41]. The experiment temperature differences are 200, 300, 400, 500 and 600℃, respectively. The exural strengths of the specimens after thermal shock at each temperature were also measured by three-point-bending method. The results were the average value of a minimum number of six specimens tested in the same condition. The surface of the specimens after thermal shock examined by the same SEM equipment with that of before thermal shock tests.

Mathematics model of temperature gradient spark plasma sintering
The spline interpolation feature points, as listed in Table 3, were obtained according to the given conditions listed in Table 2. Table 3 The spline interpolation feature points during the sintering process where T represented the sintering temperature and t was the sintering time. The sintering temperature changed evenly as time went in each temperature interval. The functional relation of temperature and time was as follows: where T i and T i+1 represented the sintering temperature at i and i + 1. t i and t i+1 represented the sintering time at i and i + 1. It was indicated that the relationship between sintering temperature and time was a linear interpolating function of subsection splines, and it was a line chart. The sintering temperature was raised or reduced constantly in every temperature interval and the rising of whole temperature was decelerated. It spent approximately 30 min in the whole SPS process, in which 5 min holding time was used.

X-ray diffraction phase analysis
The X-ray diffraction phase analysis of sample AOT4 is shown in Fig. 2 (3) The Gibbs free energy, of reaction (3) was − 162KJ at the temperature of 1350℃, which indicated that reaction (3) was possible to take place based on thermodynamic analysis. X-ray diffraction phase analysis of sample AOT4 con rmed reaction (3).

Mechanical properties
The mechanical properties of TGSPS sintered alumina composites are listed in Table 1 Figure 3 shows a plot of residual exural strengths versus temperature difference (ΔT) for alumina composites after the water-quenching tests. A three-stage behavior of exural strength, which is coincidence with the Hasselman's theory, was presented, and all the samples exhibited a drastic reduction in exural strength compared with their initial strengths. The residual strength of sample PAO was lower than that of sample AOT4 and AOT6 through the entire range. The curve for sample PAO showed a critical temperature difference (ΔT c ) of 320 ℃ and the characteristic sharp loss of exural strength at the point, followed by a gradual decrease for further increasing. ΔT c of sample AOT4 and AOT6 was 372 and 360 ℃, respectively. Sample of AOT4 and AOT6 showed appreciably greater ΔT c and residual strength after ΔT c than sample PAO. The results demonstrated that the addition of tremolite in alumina modi ed the composites mechanical properties and, hence improved the thermal shock behaviours.

Analysis of microstructures
SEM photomicrographs of fracture surface of samples PAO, AOT4 and AOT6 are shown in Fig. 4. The grain boundaries of sample PAO were unobservable and the fracture mode was mainly transgranular. There existed apparent pores (marked with arrow in Fig. 4a) on the fracture surface of sample PAO. Pores in samples AOT4 and AOT6, however, were much fewer than that in sample PAO, which showed that the addition of tremolite decreased the amount of pores in TGSPS sintered alumina composites. The eliminating of porosities contributed to a certain extent enhanced exural strength of samples AOT4 and AOT6. The sintering temperature was 1350 ℃, which was higher than the melting point of tremolite, liquid phase might appeare in the TGSPS process. The eliminating of porosities might be mainly due to the smooth owing of liquid phases, which was formed by tremolite. There were some smaller grains formed in the crystal boundaries for sample AOT4 (circled in Fig. 4b). These small particles formed in a typical intergranular fracture mode, extending the path of crack growth, and contributed to the improvement of fracture toughness for sample AOT4. The fracture surfaces of sample AOT6 (Fig. 4c) were relatively rough and presented brittle character. As was previously discussed in Sect. 3.2, during the TGSPS process, interface reaction took place among CaO and SiO 2 to yield Ca 3 SiO 5 , existed on the crystal boundaries of alumina composites, strengthened the grain boundaries of the composites, and as a result, contributed to the improvement in exural strength of alumina composites.
The effect of porosity on mechanical properties is as follows: [42,43] M p = M p0 exp (-bV fp ) (4) Where M p was the mechanical property, V fp was the volume fraction of porosity, b was an empirical constant and the subscript 0 indicated zero porosity. The following Eq. (5) could be concluded from Eq. (4): Where R represented the thermal shock parameters, b σ and b E was empirical constant for the exural strength and Young modulus of alumina composites, respectively. Usually b E < b σ [44][45][46], so the higher the V fp is, the lower the R is, and the poorer the TSR of composites will be. There were some pores existed in the fracture surface of sample PAO (Fig. 4a), fewer pores were found in the fracture surfaces of sample AOT4 and AOT6 (Fig. 4b and c), indicating that the improved TSR was obtained for sample AOT4 and AOT6 comparing with sample PAO. It was concluded that the addition of tremolite could promote the liquid phases sintering of alumina composites, decrease the porosity, strength the grain boundaries, improve the mechanical properties, and thus contribute to the improvement in TSR of alumina composites. Figure 5 indicates the schematic of the mechanisms of interactions between the crack and particles. Comparing Fig. 5a with b, one could nd that the difference in the opening displacement of crack tip between different sizes of grains when the crack propagated the interface between different sizes of grains (Fig. 5b), which would result in the fact that the crack propagated along the big grain. This meant that the crack was locally blunted, and the propagation length of the crack was extended, which contributed to the improvement of fracture toughness for sample AOT4.
The fracture surface morphologies of alumina composites at ΔT = 400 °C are presented in Fig. 6. A at fracture surface was found for sample PAO at ΔT = 400 °C, and the sample remained the transgranular fracture type. The atter and large size cleavages extended over the entire fracture surfaces, indicating that the cracks propagated fairly easy across sample PAO without crack de ections like Fig. 5a, and the cracks penetrated the alumina particles at ΔT = 400 °C (Fig. 6a) without appreciable resistance, giving rise to great drops in its residual strengths, which could explain the poor TSR of sample PAO. The enhancement of the residual strength of the alumina composites was attributed to the microstructure evolution of composites after thermal shock, and the microstructure of the samples after the quenching test provided an insight on the important details of thermal shock behavior. The fracture surface of sample AOT4 and AOT6 at ΔT = 400 °C (Fig. 6b and c) showed a relatively rough surface with a fracture mode of the combination of intergranular and transgranular failure. Dimple with a characteristic of ductile fracture could be obviously observed in Fig. 6b (marked with arrow), showing the pullout of alumina grains and indicating the high residual exural strength of sample AOT4 at ΔT = 400 °C. Transgranular cleavage was found in the fracture surface of sample AOT6 (marked with arrow in Fig. 6c), indicating a relative lower residual exural strength of sample AOT6 than that of AOT4.
The alumina composites were sintered to a high densi cation rate (Table 1) and there remained few pores in alumina composite, which enhanced the resistance to the crack initiation and load-bearing capacity of alumina composites. The compact structure of alumina composites led to a better exural strength of alumina composite. Sample AOT6 has the higher initial exural strength (538.2 MPa) than sample AOT4 (503.5 MPa), According to the theory of thermal shock fracture, sample AOT6 was more resistant to crack initiation. However, its ΔT c was lower than that of sample AOT4. Sample AOT4 has the higher initial fracture toughness (4.77 MPa·m 1/2 ) than sample AOT6 (4.15 MPa·m 1/2 ), which meant sample AOT4 possessed higher resistance to crack propagation. It was con rmed that the crack propagation rather than crack initiation dominated TSR in current composite ceramic system. It is well known that the exural strength and fracture toughness are the most important factors to in uence the TSR of ceramics [47][48][49][50]. The TSR of composites could be assessed by using the critical crack length, L c , as follows [49]: where K IC was the fracture toughness of the composites. The improvement of L C contributed to the enhanced TSR of alumina composites. Sample AOT4 had the higher L C value (0.014) than that of sample AOT6 (0.01), which could explain the improvement in the TSR of sample AOT4.