Evaluation of alumina reinforced oil fly ash composites prepared by spark plasma sintering

The thermomechanical behavior of micro/nano-alumina (Al₂O₃) ceramics reinforced with 1-5 wt.% of acid-treated oil fly ash (OFA) was investigated. Composites were sintered using spark plasma sintering (SPS) technique at a temperature of 1400°C by applying a constant uniaxial pressure of 50 MPa. It was evaluated that the fracture toughness of micro- and nanosized composites improved in contrast with the monolithic alumina. Highest fracture toughness value of 4.85 MPam^1/2 was measured for the nanosized composite reinforced with 5 wt.% OFA. The thermal conductivity of the composites (nano-/microsized) decreased with the increase in temperature. However, the addition of OFA (1-5 wt.%) in nanosized alumina enhanced the thermal conductivity at an evaluated temperature. Furthermore, a minimum thermal expansion value of 6.17 ppm*K⁻¹ was measured for nanosized Al₂O₃/5 wt.% OFA composite. Microstructural characterization of Al₂O₃-OFA composites was done by x-ray diffraction and Raman spectroscopy. Oil fly ash particles were seen to be well dispersed within the alumina matrix. Moreover, the comparative analysis of the nano-/microsized Al₂O₃/OFA composites shows that the mechanical and thermal properties were improved in nanosized alumina composites.     

Enhanced thermal shock response of Al2O3–graphite composites through a layered architectural design

The use of ceramics such as alumina in moving components often requires the addition of low friction materials such as graphite. A new strategy for improving toughness, strength, and thermal-shock resistance of Al₂O₃–graphite self-lubricating composites was proposed in this study. Alumina layers embedded between Al₂O₃–graphite layers were fabricated and tested after thermal shock conditions ranging between 500 °C and 800 °C maximum temperature. Retained strength and apparent fracture toughness after the tests were compared to room temperature values. Results show that compressive residual stresses generated in the outer Al₂O₃–graphite layers during cooling down from sintering improve the failure resistance of the materials. The introduction of heat-resistant particles (Al₂O₃ particles) into graphite layers combined with a layered architecture can greatly decrease the oxidation degradation of the materials below 500 °C. In addition, the retained strength and toughness in the layered architectures after thermal shock between 550 °C and 800 °C remains constant, thus indicating that the new-developed Al₂O₃/Al₂O₃–graphite laminated composites may be reliable candidates for self–lubricating applications also for elevated temperatures.     

A comparison of the effects of multi-wall and single-wall carbon nanotube additions on the properties of zirconia toughened alumina composites

The use of multi-wall carbon nanotubes (MWCNTs) or single-wall carbon nanotubes (SWCNTs) as filler in ceramic matrices could create composites with exceptional mechanical properties. We have prepared dense monolithic alumina (Al₂O₃) and zirconia-toughened alumina (ZTA) composites with additions of 0.01 wt% of MWCNTs or 0.01 wt% of SWCNTs by conventional sintering and have demonstrated that the mechanical properties depend on (a) the distribution of CNTs in the matrix and (b) the interaction between the ceramic phases and CNTs. The fracture toughness of Al₂O₃ ceramics reinforced with SWCNTs was significantly better than those reinforced with MWCNTs. However, fracture toughness in MWCNT-reinforced ZTA increased 41% over ZTA free of the toughening agent and 44% over ZTA reinforced with SWCNTs. A well dispersed and small amount of MWCNTs was enough to produce an increase of fracture toughness in ZTA composites.     

Processing and characterization of fiber-reinforced and layered alumina - graphene composites

The aim of the present contribution is the processing and characterization of fiber-reinforced and layered alumina - graphene composites, prepared by the combination of electrospinning, calcination, chemical vapor deposition (CVD) and spark plasma sintering (SPS). The fiber-reinforced composite contains homogenously distributed graphene-coated polycrystalline alumina microfibers in the Al₂O₃ matrix. The layered composites contain Al₂O₃ layers and layers of graphene-coated alumina microfibers or layers of graphene-coated alumina grains of submicron size. The systems with high density, 99.5–99.9 %, show different grain sizes of Al₂O₃ in their constituents, changing from 0.08 to 1.9 μm in comparison to the monolithic alumina with the average grain size of 2.6 μm. The composites and their layers show increased electrical conductivity, hardness, and fracture toughness by approximately five orders of magnitude, 31 %, and 8%, respectively, in comparison to the monolithic alumina due to the presence of graphene layers, small grain-sized alumina, and microfibers in the composites.     

Ultrafine-grained β-Sialon fabricated by two-stage sintering and study on diffusion toughening of Ti–22Al–25Nb

The ultrafine-grained β-Sialon ceramics were fabricated by spark plasma sintering at different temperatures with inorganic Al₂O₃–Y₂O₃ and Ti–22Al–25Nb intermetallic powder as composite additives. The research showed that β-Sialon ceramics achieve two-stage sintering densification. Al₂O₃–Y₂O₃ inorganic additives promoted the synthesis and densification of β-Sialon ceramics at 1125–1215°C. Ti–22Al–25Nb intermetallic powder diffused Ti and Nb elements at 1240–1425°C, thereby improving the fracture toughness of β-Sialon ceramics. The maximum fracture toughness (∼9.69 MPa m^1/2) under 19.6 N was obtained for β-Sialon ceramics sintered at 1600°C.     

The effects of microstructure on mechanical and electrical properties of W dispersed Al2O3 ceramics

This work aims to enhance the fracture toughness of brittle Al₂O₃ ceramics and apply insulated Al₂O₃ ceramics with electrical conductivity by dispersing second tungsten (W) metal particles. In order to investigate the effects of W dispersion on mechanical and electrical properties, Al₂O₃–W composites with various amounts of W (ranging from 5 vol% to 20 vol%) were fabricated by the hot-press sintering method at various sintering temperatures. Microstructure analysis revealed submicron Al₂O₃ matrix grains and W particles. The existence of three phases of Al₂O₃, W, and AlWO₄ was confirmed by X-ray diffraction patterns. All Al₂O₃–W composites showed higher fracture toughness than monolithic Al₂O₃. The toughening mechanism was attributed to crack deflection and crack bridging. Transgranular fracture was visible in all composites. Electrical resistivity dramatically lowered from 2.9 × 10¹² Ω cm of monolithic Al₂O₃ to 4.1 × 10² Ω cm of the composite with 20 vol% W addition. The percolation threshold is calculated as 18.5%. With the increase in sintering temperature, the amount of W particles was decreased and Al₂O₃ grains became large, leading to the reduced number of conductive pathways formed by the dispersed W particles. As a result, electrical conductivity was decreased.     

Mechanical properties of SiCp/Al2O3 ceramic matrix composites prepared by directed oxidation of an aluminum alloy

Silicon carbide particulate reinforced alumina matrix composites were fabricated using DIrected Metal OXidation (DIMOX) process. Continuous oxidation of an Al-Si-Mg-Zn alloy with appropriate dopants along with a preform of silicon carbide has led to the formation of alumina matrix surrounding silicon carbide particulates. SiC_p/Al₂O₃ ceramic matrix composites fabricated by the DIMOX process, possess enhanced mechanical properties such as flexural strength, fracture toughness and wear resistance, all at an affordable cost of fabrication. SiC_p/Al₂O₃ matrix composites were investigated for mechanical properties such as flexural strength, fracture toughness and hardness; the composite specimens were evaluated using standard procedures recommended by the ASTM. The SiC_p/Al₂O₃ ceramic matrix composites with SiC volume fractions from 0.35 to 0.43 were found to possess average bend strength in range 158-230 MPa and fracture toughness was found to be in range of 5.61-4.01 MPa√m. The specimen fractured under three-point loading as observed under scanning electron microscope was found to fail in brittle manner being the dominant mode. Further the composites were found to possess lower levels of porosity, among those prepared by DIMOX process.     

Evaluation of mechanical reliability of zirconia-toughened alumina composites for dental implants

Zirconia-toughened alumina composites containing 0–30 vol% of 3Y-TZP were fabricated by sintering at 1600 °C for 2 h in air. The effect of the 3Y-TZP content on the mechanical properties and microstructure of the alumina ceramics was investigated. The fracture toughness and biaxial flexural strength increased as the 3Y-TZP content increased. The Young's modulus decreased with 3Y-TZP content according to the rule of mixture, while the hardness showed the contrary tendency. The Weibull modulus of the Al₂O₃ with 20 vol% 3Y-TZP composite is higher than that of alumina. The residual hoop compressive stress developed in ZTA ceramic composites probably accounts for the enhancement of strength and fracture toughness, as well as for the higher tendency of crack deflection. No monoclinic phase and strength degradation were found after low temperature degradation (LTD) testing. The excellent LTD resistance can be explained by the increased constraining force on zirconia embedded in alumina matrix.     

Joining of Al2O3 to ZTA using a B2O3–Al2O3–SiO2 glass with in-situ precipitated whiskers

A novel B₂O₃–Al₂O₃–SiO₂ (BAS) glass filler was first developed to join Al₂O₃ and zirconia toughened alumina (ZTA) ceramics. The microstructure, crystallization products, and interfacial reaction layer of the joint were all studied. Detailed growth process and the microstructural evolution mechanism of aluminum borate (Al₁₈B₄O₃₃ and Al₄B₂O₉) crystal whiskers were revealed through controlling the joining temperature and the holding time. The results showed that the Al₁₈B₄O₃₃ and Al₄B₂O₉ whiskers formed at the interfaces and in the joining seam, owing to the reaction between the substrates and the BAS glass system, and the precipitation out of the glass, respectively. Finally, bonded with this BAS glass filler at 1400 °C for one hour, the joints exhibited a maximum shear strength of 42 MPa at room temperature and good mechanical performance after thermal cycling.     

Toughening of zirconia/alumina composites by the addition of graphene platelets

A study on graphene platelet/zirconia-toughened alumina (GPL/ZTA) composites was carried out to evaluate the potential of the new structural materials. GPL–ZrO₂–Al₂O₃ powders were obtained by ball milling of graphene platelets and alumina powders using yttria stabilized ZrO₂ balls. Samples were sintered at different temperatures using spark plasma sintering. Fracture toughness was determined by the single-edge notched beam method. The results show that the GPLs are uniformly distributed in the ceramic matrix and have survived high temperature sintering processes. Several sintering experiments were carried out. It is found that at 1550 °C, GPL/ZTA composites were obtained with nearly full density, maximum hardness and fracture toughness. A 40% increase in fracture toughness in the ZTA composite has been achieved by adding graphene platelets. The toughening mechanisms, such as pull out, bridging and crack deflection, were observed and are discussed.     

Thermal stability and oxidation resistance of C/Al2O3 composites fabricated from a sol with high solid content

To improve fracture toughness of monolithic Al₂O₃ ceramics, three-dimensional carbon fiber preform was used as reinforcement, and the C/Al₂O₃ composites without interfacial coating were fabricated through vacuum impregnation-drying-heat treatment route with an Al₂O₃ sol as starting material. Characteristics of the Al₂O₃ sol with high solid content were firstly analyzed. Then thermal stability and oxidation resistance of the C/Al₂O₃ composites were investigated. It is found that the Al₂O₃ sol is an appropriate raw material for the fabrication of C/Al₂O₃ composites. The C/Al₂O₃ composites with a total porosity of 15.5% show a flexural strength of 208.5 MPa and a fracture toughness of 8.1 MPa m^1/2. Strength loss is observed after the composites were annealed at 1400 °C and 1600 °C under inert atmosphere. Oxidation resistance of the C/Al₂O₃ composites is unsatisfactory because of the existence of open pores and microcracks. When Al₂O₃ matrix was modified with SiO₂, the oxidation resistance is remarkably improved due to the viscous flow effect of SiO₂.     

Tensile strength and corrosion resistance properties of porous Al2O3/Ni composites prepared with rice husk pore-forming agent

The mechanical performance and chemical stability of porous alumina materials operating under harsh service conditions are of utmost importance in understanding their operational behavior if they are to stand the test of time. In the present study, the joint effect of nickel (Ni) reinforcement and rice husk (RH) pore-forming agent (PFA) on the tensile strength and the corrosion resistance properties of composite porous alumina ceramics was studied. To exploit the potential of this new porous alumina system, plain and Ni-reinforced porous alumina samples (Al₂O₃-xNi-RH; x?=?2, 4, 6 and 8?wt%) were developed through the powder metallurgy technique. Comprehensive investigation on the tensile strength properties of the developed porous alumina ceramics showed that relative to the plain sample (tensile strength and elastic modulus; 6.1?MPa and 1201?MPa), the presence of highly stable Ni₃Al₂SiO₈ spinelloid promoted the tensile strength enhancement (12.6–6.4?MPa) and the elastic modulus decline (897–627?MPa) of the composite samples. Similarly, corrosion resistance test was performed on the composite porous alumina samples in both 10?wt% NaOH and 20?wt% H₂SO₄ hot aqueous solutions. Overall, the composite samples demonstrated superior chemical stability in NaOH solution as compared with the plain sample. On the other hand, the composites were more prone to attack in H₂SO₄ solution, except for the Al₂O₃-2Ni-10RH composite sample which maintained its superiority over the plain counterpart.     

Microwave hybrid sintering of Al2O3 and Al2O3–ZrO2 composites,and effects of ZrO2 crystal structure on mechanical properties,thermal properties,and sintering kinetics

Structural ceramics such as Al₂O₃ and Al₂O₃–ZrO₂ composites are widely used in harsh environment applications. The conventional sintering process for fabrication of these ceramics is time-consuming method that requires large amount of energy. Microwave sintering is a novel way to resolve this problem. However, to date, very limited research has been carried out to study the effects of different ZrO₂ crystal structures on Al₂O₃–ZrO₂ composites, especially on the sintering kinetics, when fabricated by microwave sintering.The microwave hybrid sintering of Al₂O₃ and Al₂O₃–ZrO₂ composites was performed in this study. Tetragonal zirconia and cubic zirconia were used as two different reinforcements for an α–alumina matrix, and the mechanical and thermal properties were studied. It was found that Al₂O₃ experienced a remarkable increase in fracture toughness of up to 42% when t-ZrO₂ was added. Al₂O₃–c-ZrO₂ also showed increased fracture toughness. The sintering kinetics were also thoroughly investigated, and the average activation energy values for the intermediate stage of sintering were estimated to be 246 ± 11 kJ/mol for pure Al₂O₃, 319 ± 71 kJ/mol for Al₂O₃–c-ZrO₂, and 342 ± 77 kJ/mol for Al₂O₃–t-ZrO₂. These values indicated that the activation energy was increased by the addition of either type of ZrO₂, with the highest value shown by Al₂O₃–t-ZrO₂.     

High strength and hardness BNNSs/Al2O3 composite ceramics prepared by hot-press sintering of in-situ composite BNNSs/Al2O3 powder

In this paper, BNNSs/Al₂O₃ composite powder was prepared by in-situ reaction using borate nitridation method and BNNSs/Al₂O₃ composite ceramics were prepared by hot-pressing sintering. This method achieves uniform mixing of BNNSs and Al₂O₃ ceramic matrix and reduces the introduction of impurities in the processing process. The BNNSs/Al₂O₃ composite ceramics have excellent bending strength (549.4 MPa), fracture toughness (5.18 MPa m^1/2) and hardness (21.3 GPa). The high hardness of composite ceramics is attributed to high grain boundary strength and density. The reinforcing mechanisms of ceramics include BNNSs pull-out, BNNSs bridging, crack deflection as well as the transgranular fracture and intergranular fracture of Al₂O₃ matrix.     

High toughness TiB2–Al2O3 composite ceramics produced by reactive hot pressing with fusible components

A manufacturing method of high toughness TiB₂–Al₂O₃ ceramic composites by reactive hot pressing via exothermic reactions with Al, B₂O₃, C and TiB₂ has been developed. The usage of fusible initial components (Al and B₂O₃) in the hot pressing procedure allowed forming dense ceramics with homogeneous fine structure at 1900°C and 20 MPa (after 8 min exposure) without any grinding of the initial powders. The hot pressed TiB₂–Al₂O₃ composites exhibit the fracture toughness of 9 MPa m^1/2, which is much higher than that of both single phase TiB₂ (6 MPa m^1/2) and Al₂O₃ (4 MPa m^1/2) ceramics.     

Radical reaction-induced Turing pattern corrosion of alumina refractory ceramics with CaO–Al2O3–SiO2–MgO slags

Quasi-volcanic corrosion occurs at the triple-phase interface of alumina refractory ceramics and MgO-containing CaO–Al₂O₃–SiO₂ slags in the air, causing severe damage to ceramics. To address this limitation, in this study, a slag corrosion experiment is performed on alumina refractory ceramics using CaO–Al₂O₃–SiO₂–MgO slags. Various spectroscopic techniques, including electron paramagnetic resonance spectroscopy, are used to investigate the influence of slag structures with varied MgO contents on the corrosion peaks and mechanism. The results show large quantities of reactive radicals, including superoxide radicals, in the slags. Free-radical reactions between refractory ceramics and slags lead to Turing pattern corrosion. An increase in the amount of non-bridged oxygen in the slag structure decreases the amount of original superoxide radicals. Consequently, the intensity of the free-radical reactions of alumina dissolution increases, thereby increasing the height of the corrosion peaks.     

Mechanical properties of graphene platelet-reinforced alumina ceramic composites

Alumina (Al₂O₃) ceramic composites reinforced with graphene platelets (GPLs) were prepared using Spark Plasma Sintering. The effects of GPLs on the microstructure and mechanical properties of the Al₂O₃ based ceramic composites were investigated. The results show that GPLs are well dispersed in the ceramic matrix. However, overlapping of GPLs and porosity within ceramics are observed. The flexural strength and fracture toughness of the GPL-reinforced Al₂O₃ ceramic composites are significantly higher than that of monolithic Al₂O₃ samples. A 30.75% increase in flexural strength and a 27.20% increase in fracture toughness for the Al₂O₃ceramic composites have been achieved by adding GPLs. The toughening mechanisms, such as pull-out and crack deflection induced by GPLs are observed and discussed.     

High damage tolerant Al2O3 composite ceramics constructed with short Al2O3 fibers

The catastrophic fracture characteristics of ceramic materials have become one of the most serious factors limiting their application in critical areas, as a result, it is urgent to overcome the brittleness and improve the damage tolerance of ceramic materials. Herein, a series of Al₂O₃ composite ceramics developed with short Al₂O₃ fibers and a compound interface phase composed of Al₂O₃ and h-BN powders, followed by investigating their fracture behaviors and damage tolerance. Results show that these composites present progressive fracture manners with the rising resistance curve (R-curve) behaviors, and the maximum crack growth toughness of the sample with 15% compound interface phase reaches above 10 MPa·m^1/2 (135% increase with respect to the reference alumina). Meanwhile, the composite ceramic exhibits an excellent ability to resist catastrophic failure with a large critical crack size (105.47 ± 19.11 μm) and high damage tolerance parameter (0.71 ± 0.06 m^1/2), which are close to 14.57 times and 5.92 times higher than those of the reference alumina. The superior performances are mainly attributed to the precise combination of compound interface phase for inducing crack and interlocking Al₂O₃ fibers for load capacity.     

Determination of mechanical properties of Al2O3/Y-TZP ceramic composites: Influence of testing method and residual stresses

A series of Al₂O₃/Y₂O₃-stabilized zirconia (Y-TZP) ceramic composites with different zirconia contents (5 and 40 vol% Y-TZP) and fabricated by different green processing techniques (a novel tape casting and conventional slip casting) were studied. The microstructure and mechanical properties of the composites were investigated systematically, by means of scanning electron microscopy, Vickers indentation, depth-sensing nanoindentation, and single-edge laser-notched beam (SELNB) techniques. The indentation fracture method was found to be unsuitable for fracture toughness determination in this work. Reliable values of fracture toughness were obtained by the SELNB method with an almost atomically sharp laser-machined initial notch. The microstructure and mechanical properties of the ceramic composites mainly depended on the Y-TZP content. No significant differences were induced by the choice of green processing technique. The contribution of residual stresses to fracture toughness in Al₂O₃/Y-TZP ceramic composites was investigated. To this end, a theoretical model was applied to estimate the increase in fracture toughness due to the measured residual stresses in the samples. It was found that in this case, residual stresses were not the main factor responsible for the toughening in Al₂O₃/Y-TZP composites.     

Processing and mechanical properties of Al2O3–5 vol.% Cr nanocomposites

The use of chromium (III) acetylacetonate as a source of nanometre sized chromium particles for the production of Al₂O₃–5 vol.% Cr nanocomposites has been investigated. The details of the processing procedure are crucial in determining the mechanical properties of the composite. The highest strength and fracture toughness, 736±29 MPa and 4.0±0.2 MPa m^1/2, respectively, were obtained for the nanocomposite hot pressed at 1450 °C. It is shown that the strengthening in Al₂O₃–5% Cr nanocomposites mainly results from microstructure refinement in that the mean alumina matrix grain size in the optimum composite was 0.68 μm compared with a grain size of 3.6 μm in the monolithic alumina hot pressed under identical conditions. Crack bridging and crack deflection by the nano-sized Cr particles did not occur to any significant extent. The slight improvement in fracture toughness may result from the observed change in fracture mode from intergranular fracture for monolithic alumina to transgranular failure for the nanocomposites.