Toughening effect of multi-walled boron nitride nanotubes and their influence on the sintering behaviour of 3Y-TZP zirconia ceramics

Different amounts (0.5, 1, 2.5 and 5 wt%) of hollow “cylindrical” and “bamboo-like” boron nitride nanotubes (BNNTs) have been used to reinforce 3Y-TZP zirconia ceramics via spark plasma sintering. No significant influence of different morphologies of BNNTs on the mechanical properties at the macro-scale (elastic modulus, hardness, and fracture toughness) has been observed. The fracture toughness increased continuously with the increasing amount of the BN nanotubes up to 2.5%, resulted in the improvement of ∼100% compared to the reference ZrO₂. A direct influence of BNNTs on the toughening of ZrO₂ has been recognized. The BNNTs strengthen the zirconia grain boundaries resulting in the alteration in fracture mode from inter- to trans-granular. The BNNTs also promoted the transformation toughening of zirconia. Their influence on the bridging and pull out has been confirmed by the investigation of the composites with the amorphous borosilicate matrix.     

Evaluating the role of uniformity on the properties of B4C–SiC composites

Fully dense boron carbide-silicon carbide composites were successfully produced by spark plasma sintering method at 1950 °C under 50 MPa applied pressure. The effect of dry and wet mixing methods on uniformity was observed. Density, elastic modulus, microstructure, Vickers hardness and fracture toughness were evaluated. The results showed that dry mixing did not provide uniformity on composites properties. On the other hand wet mixing provided uniformity in microstructure and consistency in material properties. The hardness of the sample containing 50 wt% B₄C was measured to be 30.34 GPa hardness value was found at 50 wt% B₄C content sample. The increase in the B₄C content of the composites decreased the Young's modulus, shear modulus, bulk modulus and fracture toughness. The highest values were found at 10 wt% B₄C sample which were 415 GPa (E), 177 GPa (G), 209 GPa (K), and 2.89 MPa m^1/2 fracture toughness (K_Ic).     

Zirconium nitride nano-particulate reinforced Alon composites: Fabrication, mechanical properties and toughening mechanisms

Aluminium oxynitride (Alon) exhibits excellent stability, high rigidity and good thermal shock resistance, but it has relatively low strength and poor fracture toughness. The aim of this investigation was to develop a new type of zirconium nitride (ZrN) nano-particulate reinforced Alon composites via a change of ZrO₂ nano-particles during sintering. A reduction of porosity and grain size was observed in the composite. With increasing amount of ZrN nano-particles up to 2.7%, the relative density, hardness, Young's modulus, flexural strength, and fracture toughness all increased. When the ZrN nano-particles exceeded 2.7%, while the flexural strength and fracture toughness decreased slightly, the density, hardness and Young's modulus continued to increase. Different toughening mechanisms including crack bridging, crack branching and crack deflection were observed, thus effectively increasing the crack propagation resistance and leading to a considerable improvement in the flexural strength and fracture toughness of the composites.     

Rare-earth high-entropy aluminate-toughened-zirconate dual-phase composite ceramics for advanced thermal barrier coatings

Superb toughening is achieved by incorporating a secondary ferroelastic phase in high-entropy rare-earth zirconate 5RE₂Zr₂O₇ (HZ). Here, we report an enhancement of 64% in fracture toughness through the addition of 30mol% high-entropy rare-earth aluminate 5REAlO₃ (HA) to the HZ matrix (30HA). The aforementioned rare-earth elements RE are La, Sm, Eu, Gd, and Yb. The present dual-phase composite ceramic 30HA has a large fracture toughness of 2.77 ± 0.14 MPa m^1/2, along with excellent high-temperature phase stability, resulting in good usage for potential thermal barrier coating applications. Particularly, the fracture toughness of the dual-phase composite ceramics at first increases to a maximum and then drops suddenly, as the mole fraction of HA increases from 0 to 50%. A clear definition of fitting parameters and their physical significance is provided for a better interpretation of the experimental data. The present toughening mechanism sheds light on microstructure engineering in high-entropy ceramics for excellent mechanical properties.     

Effect of graphene and CNT reinforcement on mechanical and thermomechanical behavior of epoxy—A comparative study

Graphene‐nanoplateles (Gr) and multiwalled carbon nanotubes (CNTs) reinforced epoxy based composites were fabricated using ultrasonication, a strong tool for effective dispersion of Gr/CNTs in epoxy. The effect of individual addition of two different nanofillers (Gr and CNT) in epoxy matrix, for a range of nanofiller content (0.1–1 wt %), has been investigated in this study. This study compares mechanical and thermomechanical behavior of Gr and CNT reinforced epoxy. Gr reinforcement offers higher improvement in strength, Young's modulus, and hardness than CNT, at ≤0.2 wt %. However, mode‐I fracture toughness shows different trend. The maximum improvement in fracture toughness observed for epoxy‐Gr composite was 102% (with 0.3 wt % loading of Gr) and the same for epoxy‐CNT composite was 152% (with 0.5 wt % loading of CNT). Thorough microstructural studies are performed to evaluate dispersion, strengthening, and toughening mechanisms, active with different nanofillers. The results obtained from all the studies are thoroughly analyzed to comprehend the effect of nanofillers, individually, on the performance of the composites in structural applications. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 46101.     

Short crack fracture toughness in (1−x)(Na1/2Bi1/2)TiO3–xBaTiO3 relaxor ferroelectrics

The fracture toughness in the lead‐free relaxor ferroelectric (1?x)(Na_1/2Bi_1/2)TiO₃–xBaTiO₃ was investigated utilizing the surface crack in flexure method. To allow a comprehensive assessment, unpoled, and poled samples from the rhombohedral, the tetragonal, and the morphotropic phase regime were considered. It was found that the fracture toughness is up to 23% higher for the poled state. In order to cover the transition from ferroelectric to relaxor phase, the temperature dependence of 0.97(Na_1/2Bi_1/2)TiO₃–0.03BaTiO₃ was studied as well. Fracture toughness values of up to 2 MPam^1/2 were determined, which are considerably above data for lead zirconate titanate materials. The results are rationalized using a simple transformation toughening‐type model in conjunction with investigations into the ferroelastic behavior. The presented model can be applied without fitting parameters but utilizes measurements of the coercive stress and remanent strain as well as the elastic modulus.     

Plasma sprayed graphene/carbon nanotube reinforced lanthanum-cerate hybrid composite coating

Lanthanum cerate (LC: La₂Ce₂O₇) is a potential material for thermal barrier coating, whose improved toughness is a crucial necessity for the pathway of its industrialization. Herein, we demonstrated a promising approach to develop graphene/carbon nanotube hybrid composite coating using a large throughput and atmospheric plasma spraying method. Graphene nanoplatelets (GNP: 1 wt %) and carbon nanotube (CNT: 0.5 wt %) reinforced lanthanum cerate (LCGC) hybrid composite coatings were deposited on the Inconel substrate. Addition of 1 wt % GNP and 0.5 wt % CNT in LC matrix has significantly increased its relative density, hardness, and elastic modulus up to 97.2%, 2–3 folds, 3–4 folds, respectively. An impressive improvement of indentation toughness (8.04 ± 0.2 MPa m^0.5) was observed on LCGC coating, which is ～8 times higher comparing the LC coating. The toughening was attributed to the factors: such as the distribution of GNPs and CNTs in the LC matrix, synergistic toughening offered by the GNPs and CNTs; (i) GNP/CNT pull-out, (ii) crack bridging and arresting, (iii) splat sandwiching, mechanical interlocking, etc. Finally, this improved toughness offered an exceptional thermal shock performance up to 1721 cycles at 1800 °C, without any major failure on the coating. Therefore, the GNP and CNT-reinforced LC hybrid composite coating can be recommended to open a path for turbine industries.     

Performance evaluation of basalt fiber-reinforced geopolymer composites with various contents of nano CaCO3

High carbon footprint of cement production is the major drawback of plain cement concrete resulting in environmental pollution. Geopolymer composites paste can be effectively used as an alternative to Portland cement in the construction industry for a sustainable environment. The demand for high-performance composites and sustainable construction is increasing day by day. Therefore, the present experimental program has endeavored to investigate the mechanical performance of basalt fiber-reinforced fly ash-based geopolymer pastes with various contents of nano CaCO₃. The content of basalt fibers was fixed at 2% by weight for all specimens while the studied contents of nano CaCO₃ were 0%, 1%, 2%, and 3%, respectively. The compressive strength, compressive stress-strain response, flexural strength, bending stress-strain response, elastic modulus, toughness modulus, toughness indices, fracture toughness, impact strength, hardness, and microstructural analysis of all four geopolymer composite pastes with varying contents of nano CaCO₃ using scanning electron microscopy (SEM) were evaluated. The results revealed that the use of 3% nano CaCO₃ in basalt fiber-reinforced geopolymer paste presented the highest values of compressive strength and hardness while the use of 2% nano CaCO₃ showed the highest values of flexural strength, impact strength, and fracture toughness of composite paste. The SEM results indicated that the addition of nano CaCO₃ improved the microstructure and provided a denser geopolymer paste by refining the interfacial zones and accelerating the geopolymerization reaction.     

Microstructure,mechanical properties and thermal conductivity of (Ti0.5Nb0.5)C–SiC composites

A series of (Ti_0.5Nb_0.5)C-x wt.% SiC (x = 0, 5, 10, 20) composites were prepared by spark plasma sintering. Dense microstructures with well‐dispersed SiC particles were obtained for all composites. With the increment of SiC content, the Vickers hardness, Young's modulus and fracture toughness increase monotonically. An optimized flexural strength of 706 MPa was achieved in (Ti_0.5Nb_0.5)C-5 wt.%SiC composite. (Ti_0.5Nb_0.5)C-20 wt%SiC composite exhibits the highest fracture toughness of 6.8 MPa m^1/2. The crack deflections and the suppression of grain growth were the main strengthening and toughening mechanisms. Besides, (Ti_0.5Nb_0.5)C-20 wt%SiC composite exhibit the highest thermal conductivity of 45 W/m·K at 800 °C.     

Toughening of epoxy hybrid nanocomposites modified with silica nanoparticles and epoxidized natural rubber

Silica nanoparticles (SN) and epoxidized natural rubber (ENR) were used as binary component fillers in toughening diglycidyl ether of bisphenol A (DGEBA) cured cycloaliphatic polyamine. For a single component filler system, the addition of ENR resulted in significantly improved fracture toughness (K_IC) but reduction of glass transition temperature (T_g) and modulus of epoxy resins. On the other hand, the addition of SN resulted in a modest increase in toughness and T_g but significant improvement in modulus. Combining and balancing both fillers in hybrid ENR/SN/epoxy systems exhibited improvements in the Young’s modulus and T_g, and most importantly the K_IC, which can be explained by synergistic impact from the inherent characteristics associated with each filler. The highest K_IC was achieved with addition of small amounts of SN (5 wt.%) to the epoxy containing 5–7.5 wt.% ENR, where the K_IC was distinctly higher than with the epoxy containing ENR alone at the same total filler content. Evidence through scanning electron microscopy (SEM) and transmission optical microscopy (TOM) revealed that cavitation of rubber particles with matrix shear yielding and particle debonding with subsequent void growth of silica nanoparticles were the main toughening mechanisms for the toughness improvements for epoxy. The fracture toughness enhancement for hybrid nanocomposites involved an increase in damage zone size in epoxy matrix due to the presence of ENR and SN, which led to dissipating more energy near the crack-tip region.     

Fracture characteristics of SiC/graphene platelet composites

Silicon carbide/graphene platelet (SiC/GPLs) composites were prepared using different weight percent of GPLs filler by hot pressing (HP) technology at 2100 °C in argon. The influence of the GPLs addition on bending strength, fracture toughness and related fracture characteristics was investigated. Both the bending strength and fracture toughness increased with increasing GPLs additives. The main fracture origins – strength degrading defects were pores at the low content of platelets and combination of pores and GPLs or clusters of GPLs particles in systems with a higher content of platelets. The fracture toughness increased due to the activated toughening mechanisms mainly in the form of crack bridging and crack branching, while the crack deflection was limited. The highest fracture toughness of 4.4 MPa m^1/2 was achieved at 6 wt.% of GPLs addition, which was ∼30% higher than the K_IC value of the reference material.     

Porosity–Property Relationships of Plasma‐Sprayed Gd2Zr2O7/YSZ Thermal Barrier Coatings

During the past decade, gadolinium zirconate (Gd₂Zr₂O₇, GZO) has attracted interest as an alternative material to partially yttria‐stabilized zirconia (YSZ) for thermal barrier coatings (TBCs). Despite the well‐known benefits of GZO, such as lower thermal conductivity and superior temperature capability compared to YSZ, processing of GZO via atmospheric plasma spraying (APS) still remains a challenge. Here, we report on APS experiments which were performed to investigate the influence of processing on GZO microstructure and lifetime of GZO/YSZ double‐layer TBCs. Different microstructures of GZO were produced and characterized in terms of porosity, stoichiometry, Young′s modulus, and their effects on the lifetime of YSZ/GZO double‐layer TBCs were discussed. Particle diagnostics were utilized for the optimization of the process parameters with respect to different microstructures of GZO and stoichiometry. It was found that both cumulative porosity of GZO and pore size distribution, which alter the Young′s modulus significantly, govern the lifetime of double layers. In addition, it was shown that the deviation in GZO stoichiometry due to gadolinia evaporation in the investigated range does not display any critical effect on lifetime.     

Hard and tough carbon nanotube-reinforced zirconia-toughened alumina composites prepared by spark plasma sintering

It is demonstrated that 0.1 wt% of multi-walled carbon nanotubes (MWCNTs) or single-walled carbon nanotubes (SWCNTs) added to zirconia toughened alumina (ZTA) composites is enough to obtain high hardness and fracture toughness at indentation loads of 1, 5, and 10 kg. ZTA composites with 0.01 and 0.1 wt% of MWCNTs or SWCNTs were densified by spark plasma sintering (SPS) at 1520 °C resulting in a higher hardness and comparable fracture toughness to the ZTA matrix material. The observed toughening mechanisms include crack deflection, pullout of CNTs as well as bridged cracks leading to improved fracture toughness without evidence of transformation toughening of the ZrO₂ phase. Scanning electron microscopy showed that MWCNTs rupture by a sword-in-sheath mechanism in the tensile direction contributing to an additional increase in fracture toughness.     

Enhanced calcium–magnesium–aluminosilicate (CMAS) resistance of GdAlO3 (GAP) for composite thermal barrier coatings

The impact of calcium–magnesium–alumino-silicate (CMAS) degradation is a critical factor for development of new thermal and environmental barrier coatings. Several methods of preventing damage have been explored in the literature, with formation of an infiltration inhibiting reaction layer generally given the most attention. Gd₂Zr₂O₇ (GZO) exemplifies this reaction with the rapid precipitation of apatite when in contact with CMAS. The present study compares the CMAS behavior of GZO to an alternative thermal barrier coating (TBC) material, GdAlO₃ (GAP), which possesses high temperature phase stability through its melting point as well as a significantly higher toughness compared with GZO. The UCSB laboratory CMAS (35CaO–10MgO–7Al₂O₃–48SiO₂) was utilized to explore equilibrium behavior with 50:50 mol% TBC:CMAS ratios at 1200, 1300, and 1400°C for various times. In addition, 8 and 35 mg/cm² CMAS surface exposures were performed at 1425°C on dense pellets of each material to evaluate the infiltration and reaction in a more dynamic test. In the equilibrium tests, it was found that GAP appears to dissolve slower than GZO while producing an equivalent or higher amount of pore blocking apatite. In addition, GAP induces the intrinsic crystallization of the CMAS into a gehlenite phase, due in part to the participation of the Al₂O₃ from GAP. In surface exposures, GAP experienced a substantially thinner reaction zone compared with GZO after 10 h (87 ± 10 vs. 138 ± 4 μm) and a lack of strong sensitivity to CMAS loading when tested at 35 mg/cm² after 10 h (85 ± 13 versus 246 ± 10 μm). The smaller reaction zone, loading agnostic behavior, and intrinsic crystallization of the glass suggest this material warrants further evaluation as a potential CMAS barrier and inclusion into composite TBCs.     

Hard and tough ultrafine-grained B4C-cBN composites prepared by high-pressure sintering

Tough and hard ultrafine-grained B₄C-cBN composites were firstly fabricated by high-pressure sintering mixed B₄C and cBN nanopowders at 6 GPa and 1700 °C. The phase transition from cBN to hBN is avoided by high pressure during the sintering process. The effects of the cBN content on the densification and mechanical properties of B₄C-cBN composites were evaluated. The results indicated that the hardness of the as-fabricated composites increased gradually with the increase of cBN content. The composite composed of 50 wt.% cBN exhibited excellent comprehensive mechanical properties with relative density of 98.6 %, density of 2.9 g/cm³, Vickers hardness of 36.2 GPa and fracture toughness of 6.7 MPa·m^1/2. The introduction of superhard cBN maintained the lightweight and high hardness while enhancing the fracture toughness of the B₄C. The main toughening mechanisms were crack bridging, crack deflection and pull-out of homogeneously dispersed cBN grains.     

Mechanical characterization of porous Ba0.5Sr0.5Co0.8Fe0.2O3−d

The mechanical stability of porous Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−d (BSCF) material was investigated using depth-sensitive microindentation and ring-on-ring biaxial bending tests. The porous BSCF was characterized as potential substrate material for the deposition of a dense membrane layer. Indentation tests yielded values for hardness and fracture toughness up to a temperature of 400 °C, while bending tests permitted an assessment of elastic modulus and fracture stress up to 800 °C. In addition the fracture toughness was evaluated up to 800 °C measuring in bending tests the fracture stress of pre-indented specimens. The results proof that the indentation-strength method can be applied for the determination of the fracture toughness of this porous material. In comparison to dense material the values of the mechanical parameters were as expected lower but the temperature dependences of elastic modulus, fracture strength and toughness were similar to those reported for dense BSCF.     

Correlations Between Microstructure and Mechanical Properties of Air Plasma‐Sprayed Thermal Barrier Coatings Exposed to a High Temperature

An indentation method is used to study the variations in Young's modulus, hardness and fracture toughness of air plasma‐sprayed thermal barrier coatings at a high temperature. The coatings were exposed to 1100°C during 1700 h. A sudden increase in Young's modulus for the first 600 h was observed, while the hardness increased after 800 h as a consequence of sintering. Conversely, there was a reduction of 25% in fracture toughness after 1700 h, evidencing the thermal barrier coating degradation. The evolution of these mechanical properties was correlated with microstructural changes. After 1700 h, the thermally grown oxide thickness reached 6.8 μm, the volumetric percentage of porosity was reduced from 6.8% to 4.7% and the amount of monoclinic phase increased to 23.4 wt%. These characteristics are closely related to the stress distribution in the top coat, which promotes cracks nucleation and propagation, compromising the coating durability.     

Nanoindentation and Morphological Studies of Epoxy Nanocomposites

Summary: This paper investigates the mechanical properties of the epoxy–organoclay nanocomposites by the nanoindentation technique. The nanocomposites were prepared by in situ polymerization and a mixture of exfoliated and intercalated composites structure was obtained as evidenced by X‐ray diffraction (XRD) and transmission electron microscopy (TEM). The hardness, elastic modulus, and the creep behavior of the nanocomposites have been evaluated as a function of clay concentration. It has been found that incorporation of 7.5 wt.‐% of clay nanofiller enhances the elastic modulus and hardness of the epoxy matrix by about 20 and 6%, respectively. The elastic modulus data calculated from indentation experiments are comparable with those obtained from a tensile test. An optimum clay loading level was found to be 2.5 wt.‐% to maximum enhance the creep resistance of the epoxy matrix. The lowered creep resistance with higher clay loading could be due to the reduced crosslinking density near the clay surface caused by the plasticizing effect from the pending of alkyl ammonium chains on the clay surface. An attempt has been made to correlate the fracture toughness of the nanocomposites with the ratio of modulus to hardness obtained from nanoindentation experiments.

Ratio of modulus to hardness (E/H) and the fracture toughness (K_IC) versus clay loading for the epoxy nanocomposites.     

Effect of Lattice Distortion and Disordering on the Mechanical Properties of Titania‐Doped Yttria‐Stabilized Zirconia

TiO₂‐doped Y₂O₃‐stabilized ZrO₂ compounds with low thermal conductivity have been considered as a promising thermal barrier coating material. In the present research, a series of TiO₂‐doped Y₂O₃‐stabilized ZrO₂ compounds have been synthesized and investigated. Lattice distortion and disordering caused by TiO₂ doping were observed and their effects on mechanical properties, such as fracture toughness, elastic modulus, and coefficient of thermal expansion (CTE), were also investigated. Lattice distortion enhanced the ferroelastic toughening and the fracture toughness, whereas the variation in elastic modulus and CTE is due to the lattice disordering. The combination of thermal and mechanical properties bodes well for the potential application as thermal barrier coating materials.     

Thermal cycling performances of multilayered yttria-stabilized zirconia/gadolinium zirconate thermal barrier coatings

Gadolinium zirconate (Gd₂Zr₂O₇, GZO) as an advanced thermal barrier coating (TBC) material, has lower thermal conductivity, better phase stability, sintering resistance, and calcium-magnesium-alumino-silicates (CMAS) attack resistance than yttria-stabilized zirconia (YSZ, 6-8 wt%) at temperatures above 1200°C. However, the drawbacks of GZO, such as the low fracture toughness and the formation of deleterious interphases with thermally grown alumina have to be considered for the application as TBC. Using atmospheric plasma spraying (APS) and suspension plasma spraying (SPS), double-layered YSZ/GZO TBCs, and triple-layered YSZ/GZO TBCs were manufactured. In thermal cycling tests, both multilayered TBCs showed a significant longer lifetime than conventional single-layered APS YSZ TBCs. The failure mechanism of TBCs in thermal cycling test was investigated. In addition, the CMAS attack resistance of both TBCs was also investigated in a modified burner rig facility. The triple-layered TBCs had an extremely long lifetime under CMAS attack. The failure mechanism of TBCs under CMAS attack and the CMAS infiltration mechanism were investigated and discussed.