Microstructural characteristics of TZP/Ni cermets plastically deformed at high temperature

The microstructure of 3 mol% Y₂O₃-stabilized tetragonal ZrO₂/Ni (TZP/Ni) composites has been investigated for nickel concentrations of 20, 30 and 40 vol.% before and after creep tests conducted in argon atmosphere at 1250 °C under stresses between 9 and 14 MPa. The microstructures have been characterized using scanning, conventional and high-resolution transmission and analytical electron microscopy. Two types of microstructure were found depending on the presence/absence of percolation in the metal phase. The deformation process strongly affects the percolation threshold due to the elongation of metallic grains and subsequently formation of aggregates. TEM and HRTEM studies in as-received and deformed cermets show the lack of dislocation activity and the absence of secondary phases in both zirconia/zirconia and zirconia/nickel interfaces and triple junctions.     

Correlation between microstructure and mechanical properties of Al2O3/ZrO2 nanocomposites

The aim of this work was to correlate the microstructure of alumina matrix nanocomposites containing 1, 3, and 5 vol% of monoclinic zirconia nanometric particles with the mechanical properties and wear resistance of these composites. The microstructural analysis showed the beneficial effect of the zirconia particles in the alumina matrix regarding grain growth and improvement of the properties: up to 8% for the microhardness, 11% for the flexural strength and 23% for the wear resistance for nanocomposites containing 5 vol% of particles when compared to inclusion-free alumina.     

Pressureless Sintering of Al2O3/Ni Nanocomposites

In the present study, an Al₂O₃/Ni nanocomposite containing 5 vol% Ni is prepared by pressureless sintering at 1400°C for 2 h. Most nickel inclusions, around 70% in the sintered nanocomposite, locate at the intergranular sites, the triple junctions and Al₂O₃/Al₂O₃ grain boundaries. The average size of the nickel inclusions at the triple junctions, grain boundaries and intragranular locations is 145, 131 and 73 nm, respectively. The average size of all nickel inclusions is 118 nm. The presence of nickel inclusions can prohibit the grain growth of matrix grains. The size of Al₂O₃ grains in the sintered nanocomposite is only 490 nm. The strength of the nanocomposite is thus high for the refined microstructure. The matrix Al₂O₃ grains and Ni inclusions at triple junctions underwent considerable coarsening during a post-annealing treatment at 1300°C for 2 h. The strength of the annealed composites is thus reduced significantly after annealing.     

Microstructure and mechanical properties of Ti (C,N) based cermets reinforced with different ceramic particles processed by spark plasma sintering

The addition effect of different ceramic particles such as TiB₂, TiN and nano-Si₃N₄ on the microstructure and mechanical properties of TiCN-WC-Co-Cr₃C₂ based cermets, which are prepared by spark plasma sintering, was studied. Microstructural characterization of the cermets was done by scanning electron microscope. X-ray diffraction was performed to study the crystal structures. Mechanical properties such as hardness and fracture toughness were measured for the different developed cermets. The hardness and fracture toughness of the TiCN-WC-Co-Cr₃C₂ cermets without TiN, TiB₂, and nano-Si₃N₄ were 8.4 GPa and 3.4 MPa m^1/2, respectively. It was found that 5 wt% TiB₂ addition alone improved the corresponding hardness and fracture toughness to 19.2 GPa and 6.9 MPa m^1/2, respectively. The addition of 5 wt% TiN, improved the hardness and fracture toughness to 16.7 GPa and 6.9 MPa m^1/2, respectively. With the combination of 5 wt% TiN and 5 wt% TiB₂, the hardness and fracture toughness were improved to 15.5 GPa and 6.6 MPa m^1/2, respectively. But, the addition of 5 wt% Si₃N₄ showed a balanced improvement in both hardness (17.6 GPa) and toughness (6.9 MPa m^1/2). Fracture toughness did not change much for all the above cermets with different ceramic inclusions.     

Effect of oxide nanofillers on fabrication,structure, and properties of zirconia-based composites

We have studied the effect of NiO on the sintering of yttria-stabilized zirconia at temperatures ranging from 1300 °C to 1500 °C in air and argon environments. It was found that the addition of NiO stabilized the cubic phase of ZrO₂ independently from the sintering atmosphere. The monoclinic phase of ZrO₂ formed only during sintering within the air environment at temperatures higher than 1450 °C. The transformation of NiO to Ni by reversible decomposition depends on the sintering atmosphere, and this can lead to variations in the nature of inclusions and in changes of the structure and properties of nanocomposite materials in the system ZrO₂–NiO(Ni). NiO and Ni inclusions can increase the indentation fracture toughness of zirconia–nickel oxide composite material more than 50%, which can be compared with zirconia ceramics during sintering in a neutral atmosphere alone.     

Porous Cu/YSZ anodes processed by aqueous tape casting for IT-SOFC

Porous copper/yttria-stabilized zirconia (Cu/YSZ) thin tapes were produced to be used as anodes in intermediate temperature solid oxide fuel cells (IT-SOFC). The use of copper in these cermets promises a better chemical resistance and a lower operating temperature for SOFC. Graphite and cassava starch were used as pore forming agents, which led to porosity values between 30 and 40%. Tapes were obtained by aqueous tape casting with controlled thickness (300–2000 μm). Before casting, CuO/YSZ aqueous suspensions were formulated based on rheological characterization. Sintering was carried out at 1150 °C during 1 h. Cu/YSZ tapes were obtained after redox reaction, performed at 400, 600 and 800 °C.     

Mullitization kinetics from kaolinite incorporating nanometric Ba and Ca fluorides

The work presents the kinetic effect of nanometric BaF₂ and CaF₂ particles on kaolinite to mullite transformation. The kinetics were evaluated from dilatometric data using two different non-isothermal procedures: conversional model-fitting method and diffusional sintering analysis. From experimental data, the activation energy of mullite formation calculated from sintering (942 kJ/mol) and from conversional method (910 kJ/mol) were in good agreement with those values reported by other authors (mean value 1030 kJ/mol). After incorporation of 3 mol% of nanometric BaF₂ and CaF₂ in kaolinite and applying both analytical procedures, lower activation energies for mullite formation were obtained, assigning to the transformation the value of 635 kJ/mol for kaolinite/BaF₂ and 428 kJ/mol for kaolinite/CaF₂ composites.     

High-Temperature Plastic Behavior of TZP–Ni Cermets

The creep behavior of tetragonal zirconia TZP–Ni cermets with metal contents below, close to, and above the percolation limit has been studied. Compressive creep tests were performed on as-received materials and samples in which the metal phase was chemically removed (ceramic skeletons). The stress exponent and the activation energy for plastic flow are independent of the nickel content and decrease continuously on increasing the stress and/or the temperature; skeleton structures display the same trend, suggesting that creep is controlled by the zirconia matrix. The steady-state constitutive equation for high-purity monolithic zirconia applies to the cermets when the stress is corrected with the porosity and volume fraction of percolated nickel.     

High-temperature hot-pressing of titanium carbide–graphite hetero-modulus ceramics

Hetero-modulus ceramic–ceramic composite materials (HMC) present the combination of ceramic matrix with high Young's modulus and the inclusions of a dispersed phase with low Young's modulus. The densification of 60–100 vol% TiC–0–40 vol% C (graphite) HMC during hot-pressing of powder compositions at 2200–2400 °C and applied pressures of 8–16 MPa is studied. The general theory of bulk-viscous flow for porous body is developed to describe obtained experimental data. This relation as well as Kovalchenko's equation for the evolution of relative density for non-uniform deformation of porous body, incorporating different stages of creep, are used to determine the activation energy and the exponent constant of hot-pressing process, which values are 270 ± 30 kJ mol⁻¹ and 3 ± 0.3, respectively. The obtained experimental data are discussed on the basis of extensive and detailed analysis of the previous studies on diffusion and diffusion-related phenomena in the Ti–C system.     

Tensile creep behavior of three-dimensional four-step braided SiC/SiC composite at elevated temperature

This article presents experimental results for tensile creep deformation and rupture behavior of three-dimensional four-step braided SiC/SiC composites at 1100 °C and 1300 °C in air. The creep behavior at 1300 °C exhibited a long transient creep regime and the creep rate decreased continuously with time. The creep behavior at 1100 °C exhibited an apparent steady-rate regime and the creep deformation was smaller than that at 1300 °C. However, the creep rupture time at both temperatures showed little difference. The mechanisms controlling creep deformation and rupture behavior were analyzed.     

Slow crack growth and hydrothermal aging stability of an alumina-toughened zirconia composite made from La2O3-doped 2Y-TZP

A very tough zirconia matrix is interesting to fabricate alumina-toughened zirconia (ATZ) and composites generally processed from 3Y-TZP do not exhibit very high toughness. The strategy of lowering the yttria content to increase toughness however is normally associated with an increased hydrothermal aging susceptibility. In this work, a 0.4 mol% La₂O₃ doped 2Y-TZP matrix was investigated to realize a 20 wt.% alumina toughened zirconia composite with a substantially high aging resistance. The higher transformation toughening in the composite shifted the V-K_I towards higher K_I values, while preserving the slope of the curve, resulting in a threshold K_I0 of 4.0 MPa m^1/2 and fracture toughness (K_IC) of 7.1 MPa m^1/2. These composites can offer a better compromise between aging and crack resistance than traditional 3Y-TZPs and plain ATZ composites without La₂O₃ doping.     

Effect of WC content on microstructure and mechanical properties of Ni3Al-bonded cermets

The effect of WC content on microstructure and mechanical properties of the TiC–Ni₃Al system cermets was investigated. Ni₃Al-bonded cermets showed a core–rim structure with carbide particle coupled with rim embedded in Ni₃Al binder. With WC content increasing, TiC grains were refined and the white rim became complete and got thicker gradually. Interface between core and rim showed a completely coherent relationship. The rim enriched in W constituted an ideal coherence between hard phase and Ni₃Al binder phase. With WC content increasing, the densification of cermets was enhanced, and hardness and TRS were increased firstly and then reduced, reaching peak values 90.9 HRA (HV₃₀ 15 GPa) and 1629 MPa, respectively in cermet N5 (25 wt% WC). Similarly, fracture toughness got a peak value (11.6 MPa m^1/2), at the composition with 20 wt% WC.     

Grain-boundary cation diffusion in ceria tetragonal zirconia determined by constant-strain-rate deformation tests

Ceria tetragonal zirconia polycrystals with a content of 12 mol% ceria (CeTZP) have been tested in compression at constant strain rate between 1150 °C and 1300 °C. An accurate analysis of the stress–strain curves has permitted to determine the value of the grain boundary cation diffusion. The results are compared with those reported in literature for this alloy and yttria tetragonal zirconia polycrystals (YTZP). An isotopic effect is found to correlate both grain boundary diffusion coefficients.     

Creep of directionally solidified alumina/YAG eutectic monofilaments

Multi-phase—single crystal oxide fibers offer the best choice for reinforcing oxide matrix composites because they have superior creep resistance up to 1700 °C without significant strength loss at moderate temperatures due to growth of processing flaws. In this work, Directionally Solidified Al₂O₃–YAG eutectic fibers were grown at various rates by the Edge-defined, Film-fed Growth (EFG) method and their microstructure, microstructural stability and creep properties were studied. A methodology was developed in order to determine if the creep behavior of a fiber was affected by any heterogeneous coarsening defects. The creep behavior could be rationalized using a threshold stress concept with activation energy of 1100 kJ/mol K. TEM analysis of the crept fibers suggested that the Sapphire phase was deforming by a dislocation mechanism, while the YAG phase deformed by a diffusional mechanism. A creep model was developed which contained geometrical factors for describing the microstructure. Analysis of the data showed that the creep resistance would increase to single crystal values as the phase aspect ratio increased. Further, these two phases—single crystal structures exhibit a flaw-independent strength and are suggested to have a decrease in slow crack growth rate as the transverse phase size decreases.     

High temperature creep behaviour of 4 mol% yttria tetragonal zirconia polycrystals (4-YTZP) with grain sizes between 0.38 and 1.15 μm

The high temperature mechanical behaviour of 4% mol yttria tetragonal zirconia polycrystals (4-YTZP) with different grain sizes (0.38 < d < 1.15 μm) has been analyzed by means of compression creep tests. The working temperature was 1350 °C and the strain rates ranged between 5 × 10⁻⁷ and 2 × 10⁻⁴ s⁻¹. Experimental results have been fitted to the conventionally accepted creep law for superplastic ceramics. Thus, stress exponents and activation energies have been measured as a function of the grain size. The dependence of strain rate on grain size has also been determined. The experimental data are discussed with respect to the existing theoretical models for these materials.     

High temperature compressive strength and creep behavior of SiTiCO fiber-bonded ceramics

Fiber bonded silicon carbide ceramic materials provide cost-advantage over traditional ceramic matrix composites and require fewer processing steps. Despite their interest in extreme environment thermostructural applications no data on long term mechanical reliability other than static fatigue is available for them. We studied the high temperature compressive strength and creep behavior of a fiber bonded SiC material obtained by hot-pressing of SiTiCO fibers. The deformation mechanism and onset of plasticity was evaluated and compared with other commercial SiC materials. Up to 1400 °C, plasticity is very limited and any macroscopic deformation proceeds by crack formation and damage propagation. A transient viscous creep stage is observed due to flow in the silica matrix and once steady state is established, a stress exponent n ∼ 4 and an activation energy Q ∼ 700 kJ mol⁻¹ are found. These results are consistent with previous data on creep of polymer derived SiC fibers and polycrystals.     

Creep mechanisms of laminated alumina/zirconia-toughened alumina composites

High-temperature plastic deformation of laminar composites containing alternate layers of Al₂O₃ and a mixture of 60 vol.% Al₂O₃ + 40 vol.% 3 mol% Y₂O₃-stabilized tetragonal ZrO₂ (ZTA) produced by tape casting is investigated in isostrain compression testing at temperatures between 1400 and 1500 °C. The stress exponent n and the creep activation energy Q are close to 1 and 700 kJ/mol, respectively. Microstructual observations reveal the lack of differential features in the ZTA layers and a general creep damage of the Al₂O₃ layers, with little microcracking by cavity coalescence even up to strains of 30%. The layer interfaces maintain their initial structural integrity after testing. An isostrain composite creep model predicts correctly the overall mechanical behavior of the laminates, which is dictated by the alumina phase via diffusional creep controlled by oxygen grain boundary diffusion.     

High‐Temperature Deformation Mechanisms in Monolithic 3YTZP and 3YTZP Containing Single‐Walled Carbon Nanotubes

Monolithic 3YTZP and 3YTZP containing 2.5 vol% of single‐walled carbon nanotubes (SWCNT) were fabricated by Spark Plasma Sintering (SPS) at 1250°C. Microstructural characterization of the as‐fabricated 3YTZP/SWCNTs composite shows a homogeneous CNTs dispersion throughout the ceramic matrix. The specimens have been crept at temperatures between 1100°C and 1200°C in order to investigate the influence of the SWCNTs addition on high‐temperature deformation mechanisms in zirconia. Slightly higher stress exponent values are found for 3YTZP/SWCNTs nanocomposites (n~2.5) compared to monolithic 3YTZP (n~2.0). However, the activation energy in 3YTZP (Q = 715 ± 60 kJ/mol) experiences a reduction of about 25% by the addition of 2.5 vol% of SWCNTs (Q = 540 ± 40 kJ/mol). Scanning electron microscopy studies indicate that there is no microstructural evolution in crept specimens, and Raman spectroscopy measurements show that SWCNTs preserved their integrity during the creep tests. All these results seem to indicate that the high‐temperature deformation mechanism is grain‐boundary sliding (GBS) accommodated by grain‐boundary diffusion, which is influenced by yttrium segregation and the presence of SWCNTs at the grain boundary.     

Structured porous Ni- and Co-YSZ cermets fabricated from directionally solidified eutectic composites

NiO-YSZ and CoO-YSZ eutectic rods were produced by directional solidification using the laser floating zone method (LFZ). This technique produces highly structured material consisting of alternate lamellae of transition metal oxide and zirconia with variable interlamellar spacing depending on growth conditions. We have chosen conditions for interlamellar spacing of about 1 μm. The microstructure is homogeneous and mechanically stable during thermochemical reduction. Complete reduction of the transition metal oxide produces a lamellar porous cermet with porous metallic lamellae alternated with the YSZ phase. The thermal expansion coefficients of the cermets are those of the YSZ skeleton. Reaction kinetics at different temperatures during the reduction process were studied by gravimetric methods. The reduction process within the complete temperature range studied for NiO-YSZ, and at high temperatures for CoO-YSZ seems to be controlled by the O²⁻ diffusion through the YSZ phase. The amount of Ni²⁺ and Co²⁺ ions dissolved in the YSZ phase is 2 and 5 mol%. Resistivity values for the cermets along the solidification axis are 50 μΩ cm for Co-YSZ and 130 μΩ cm for Ni-YSZ. These materials are porous, ionic and electronic conductors and could be used as textured anodes for solid-oxide fuel cells (SOFC).