Comparison of spark plasma and microwave sintering of mullite based composite: Mullite/Ta2O5 reaction (2023)

The present work investigates the effect of high praseodymium oxide (Pr₆O₁₁) content on the microstructure and phase formation of mullite (3Al₂O₃·2SiO₂) precursor by means of the spark plasma sintering process. 30wt% Pr₆O₁₁ was added to a mullite precursor consisting of aluminum nitrate nonahydrate and tetraethyl orthosilicate through a high energy mixer mill in ethanol media. The spark plasma sintering was performed at a temperature of 1200°C under 23Pa vacuum conditions by applying initial and final pressure of 10 and 30MPa, respectively. XRD analysis confirms the existence of mullite, alumina, Pr₆O₁₁, Pr₂O₃ and quartz as the only crystalline phases. FESEM images reveal an interesting deposition of hexagonal-shaped Al₂O₃ particles on polished surfaces and complex oxide phases of the fiber network adhering to alumina particles in the form of tails seen on the fracture surfaces. Moreover, the bending strength of 213±21MPa, Vickers hardness of 9.3±0.1MPa and fracture toughness of 6.21±0.12MPa·m^1/2 are obtained for the prepared composite.

Fabrication feasibility of NiCoCrAlSi/IN-738 (HEA-15), NiCoCrAlSi/Al/IN-738 (HEA-15F) joints via plasma spark sintering (SPS) method was investigated in this study. The bonding strength and hot corrosion behavior of these systems are compared with NiCrAlY/IN-738 (NiCrAlY) joint. The mentioned high entropy alloys (HEAs) were fabricated by mechanical alloying method for 40h in Ar atmosphere. The milled powders were then applied on IN-738 superalloy by SPS method at 1170°C and soaking time of 15min with an average thickness of 1.1mm. The microstructural/phase characterization of these samples was evaluated by field emission scanning electron microscopy (FESEM), energy-dispersive X-ray spectroscopy (EDS) and X-ray diffraction (XRD) analysis. The hot corrosion behavior of these joints in 90wt% Na₂SO₄ −10% wt. NaCl at 800°C for 100h were studied. Bonding strengths were evaluated using the pull-off adhesion test. The lowest amount of hot corrosion was obtained for the HEA-15 joint. The highest thickness of oxide layers after the hot corrosion test was obtained for the NiCrAlY sample (366µm), which was higher than the HEA-15 sample (194µm). High bonding strength of 155MPa was obtained for HEA-15 sample which was higher than that of the other joints. This value was 114MPa for NiCrAlY joint. Contrary to expectations, HEA-15F sample did not show appropriate bonding strength due to the presence of surface oxides in aluminum intermediate layer.

The main objective of this article is to obtain reaction sintered mullite based ceramics through spark plasma sintering process. 5, 10 and 15wt% NbN particles were mixed with kaolinite and alumina powders as mullite precursor through a wet milling process. The mullite ceramic and composites were fabricated by spark plasma sintering (SPS) method at 1350°C with initial and final applied pressure of 10 and 50MPa, respectively. All of prepared composites showed relative density higher than 99% of theoretical density as well as mullite ceramic. The XRD patterns obtained from sintered composites demonstrated dominant crystalline phase of mullite and NbN and also some low-intensity of alumina peaks were observed. The FESEM images of sintered composites illustrated uniform distribution of reinforcement particles for 5 and 10wt% addition of NbN. While using 15wt% NbN reinforcement phase lead to form some agglomerates areas. The optimum mechanical properties were obtained for composites containing 10wt% NbN as bending strength of 441±36MPa, Vickers hardness of 17.89±0.61GPa and fracture toughness of 3.79±0.12MPam^1/2.

In this paper, amorphous coatings on tantalum obtained by plasma electrolytic oxidation (PEO) in alkaline sodium aluminate electrolyte (2–10g/l NaAlO₂+2g/l KOH) are investigated. It was found that the amorphous component of the oxide coatings increased with NaAlO₂ concentration in the electrolyte. The coatings obtained in the electrolyte of 10g/l NaAlO₂+2g/l KOH are completely amorphous. Subsequently, the amorphous coatings were subjected to vacuum heat treatment at 600, 800, 900 and 1300°C to study their crystallization behavior. The results show that crystallization does not occur at 600°C. However, crystallization occurred partially at 800°C and completed at 1300°C, with orthorhombic AlTaO₄ as the main phase structure. Nanoindentation tests show that hardness of the coating at dense regions increases after crystallization, but defects and cracks in the coating are also increased after heat treatment. The amorphous coatings have excellent corrosion resistance, but the defects generated in the crystallized coatings are detrimental to the corrosion performance. The reason for the formation of amorphous coatings may be derived from rapid cooling and the glass-forming ability of the binary system of Ta₂O₅ and Al₂O₃.

In this study, microstructural and mechanical properties of (Zr_xTi_1−x)N-(Ti_xZr_1−x)B₂ composite fabricated by reactive spark plasma sintering (SPS) of ZrB₂ and TiN was investigated. ZrB₂ and TiN starting materials with equimolar ratio were sintered at 2000°C for 8min under a pressure of 30MPa. An expansion (about 4%) was observed during the sintering process of the composite in the temperature range between 1290°C and 1625°C, which was attributed to the reaction of the starting materials. XRD pattern of the fabricated composite revealed the formation of TiB₂ and ZrN phases after the sintering process. It was found that (Ti_0.91Zr_0.09)B₂ and (Zr_0.92Ti_0.08)N phases are formed after sintering. High values of Vickers microhardness (28±0.2GPa), flexural strength (706±51MPa) and fracture toughness (8.49±0.92MPam^1/2) were achieved for this composite. Fracture of the (Zr_xTi_1−x)N and (Ti_xZr_1−x)B₂ phases and crack path deflection mechanisms was founded as dominant toughening mechanisms in this composite.

The present study was conducted to investigate the microstructural and mechanical properties of SiC-45vol%B₄C-10vol%Ni and SiC-45vol% B₄C-10vol%Ni-5vol% CNT in-situ composites fabricated through reactive spark plasma sintering (SPS) method at 1650°C for 5min. The phase and microstructural characterization of the composites were evaluated utilizing x-ray diffraction (XRD) and field emission scanning electron microscopy (FESEM). Ultimately, microhardness, flexural strength, and fracture toughness of the composites were measured. Densification behavior of the samples revealed that the initial shrinkage of the composites took place at about 750°C, which is related to the reaction between Ni and SiC phases. XRD results confirmed the formation of Ni₂Si phase. The relative density of 98.3±0.32% was achieved for the SiC-B₄C-10Ni sample; this value was 97.8±0.62% for the SiC-B₄C-10Ni-5CNT sample. The flexural strength of 362±23MPa was achieved for the SiC-B₄C-10Ni sample; this value was 369±15MPa for the SiC-B₄C-10Ni-5CNT sample. The comparison between the fabricated composites concerning the fracture toughness indicated further fracture toughness of the SiC-B₄C-10Ni-5CNT sample (5.82±0.32MPam^1/2) than that of the SiC-B₄C-10Ni sample (3.35±0.56MPam^1/2). Formation of micro-cracks, crack path deflection, and CNT bridging was the main toughening mechanisms in the SiC-B₄C-10Ni-5CNT sample.

Ceramics International, Volume 45, Issue 6, 2019, pp. 6616-6623

The vital thermo-mechanical properties of thermal and environment barrier coatings (TBCs/EBCs) include high hardness, low Young's modulus, matching thermal expansion coefficients (TECs) with substrate and low thermal conductivity. The effect of distortion degree of crystal structure on thermo-mechanical properties of AlMO₄ (M=Ta, Nb) ceramics are assessed in this work. AlMO₄ ceramics display modest TECs and no phase transformation is detected from room temperature to 1200 ℃. The experiment thermal conductivity can be dropped further as the theoretical minimum thermal conductivity of AlTaO₄ and AlNbO₄ is 1.48 W m⁻¹ K⁻¹ and 1.05 W m⁻¹ K⁻¹, respectively. The temperature dependent phonon thermal diffusivity of AlMO₄ ceramics has been confirmed; the intrinsic lattice thermal conductivity is determined. The extraordinary thermo-mechanical properties make it clear that AlMO₄ ceramics are suitable for high-temperature applications.

Ceramics International, Volume 45, Issue 16, 2019, pp. 20844-20854

In this study, the microstructural and mechanical properties of three different mullite matrix composites were investigated. Accordingly, Mullite-TiN-CNT, Mullite-TiN-TiB₂-CNT and Mullite-TiN-TiB₂-ZrB₂-CNT composites with 10 wt % of each TiN, TiB₂ and ZrB₂ reinforcement particles as well as 1 wt % CNT were prepared by the spark plasma sintering (SPS) technique. The sintering processes were conducted at 1350 °C for 5 min with a mean heating rate of 60 °C/min. The relative densities of the composites achieved were higher than 97% of theoretical density. The results of mechanical properties of the fabricated composites showed that Mul-TiN-TiB₂-ZrB₂-CNT composites obtained the highest hardness and fracture toughness values. XRD results confirmed the presence of mullite, TiN, TiB₂ and ZrB₂ phases without any additional reactions between reinforcement and matrix phases. Microstructural studies demonstrated the uniform distribution of reinforcing phases in the matrix for all composites. The fracture surface micrographs revealed the dominating transgranular fracture mode indicated the strong adhesion of the reinforcing particles to the matrix. Pulling out of CNTs was also observed in the fracture surface of composites, which plays a vital role in the fracture energy absorption and joining of interfaces.

Materials Science and Engineering: A, Volume 627, 2015, pp. 27-30

In the present work, aluminum-10wt% ZrB₂-1wt% Co composite was prepared by two different sintering methods: microwave and spark plasma, and the relative density of 97±0.3% TD (at 640°C) and 98±0.3% TD (at 450°C) was obtained, respectively and XRD analysis revealed Al and ZrB₂ as the only crystalline phases. Mechanical investigations revealed that the bending and compressive strength of SPS samples were 275±10 and 381±16MPa, respectively, which were higher than those of microwave samples. The microhardness values of SPS and microwave sintered specimens were obtained 160 and 110±12 Vickers, respectively. Microstructural investigations revealed a homogeneous distribution of ZrB₂ particles in the aluminum matrix in both methods.

Archives of Civil and Mechanical Engineering, Volume 18, Issue 4, 2018, pp. 1042-1054

Graphene and carbon nanotube due to their outstanding mechanical performance were used as reinforcement in aluminum (Al) based composite through spark plasma sintering (SPS), microwave (MW) and conventional techniques. The initial compositions of Al-1wt% CNT, Al-1wt% GNP and Al-1wt% CNT–1wt% GNP were mixed by a high energy ultrasonic device and mixer mill to achieve homogenous dispersion. The SPS, MW and conventional processes were conducted at almost 450, 600 and 700°C, respectively. The maximum relative density (99.7±0.2% of theoretical density) and bending strength (337±11MPa) obtained by SPS, while maximum microhardness of 221±11 Vickers achieved by microwave for Al-1wt% CNT–1wt% GNP hybrid composite. X-ray diffraction (XRD) examinations identified Al as the only dominant phase accompanied by very low intensity peaks of Al₄C₃. Field emission scanning electron microscopy (FESEM) micrographs demonstrated uniform distribution of GNP as well as CNT reinforcement in spark plasma sintered samples.

Surface and Coatings Technology, Volume 288, 2016, pp. 36-45

In the present work, the hot corrosion behavior of two types of multilayer plasma sprayed TBC were investigated and compared with functionally graded and conventional TBCs. These kinds of multilayer coatings consisted of nano/μ alumina as a top coat on YSZ layer, a metallic bond coat and a functionally graded intermediate layer deposited between YSZ and bond coat layers. All the layers were sprayed on the Ni-base super alloy substrate. The hot corrosion resistance of the plasma sprayed coatings was examined at 1050°C for 40h, using a fused mixture of 45wt% Na₂SO₄ + 55wt%V₂O₅. Before and after hot corrosion, the microstructure and phase analysis of the coating were studied using scanning electron microscope and X-ray diffractometer. The results showed that, the Al₂O₃ top layer acted as a barrier against the infiltration of the molten salt into the YSZ layer during exposure to the molten salt mixture at the high temperature and the multilayer coatings of zirconia/alumina with the nanostructured alumina as a top coat showed higher hot corrosion resistance. Also, the failure mechanisms of the functionally graded coating and duplex TBC were investigated. The spallation occurred between the graded layer and the bond coat/top coat in functionally graded TBC and duplex TBC, respectively.

Ceramics International, Volume 45, Issue 13, 2019, pp. 16015-16021

A high density 10 wt% ZrB₂ and 1 wt%. CNT-reinforced mullite-based composite was prepared by spark plasma sintering (SPS) at temperature of 1350 °C, average heating rate of 60 °C/min and a soaking time of 5 min. Under these conditions, the sintered composite obtained a high hardness (16.24 ± 0.12 GPa), fracture toughness (4.18 ± 0.51 MPa m^1/2) and flexural strength (488 ± 21 MPa). The results of FESEM images showed a uniform distribution of the reinforcement particles in the composite. The resulting CNTs survived the sintering process and did not undergo any transformation. A transgranular fracture was predominantly observed in the fractured surface micrographs of the composite. The CNTs pullout and crack-bridging toughening mechanisms were also observed in the composite, which could have a significant contribution to the fracture energy and the interfacial adhesion.