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.     

Effect of sintering temperature on microstructure and mechanical properties of zirconia-toughened alumina machinable dental ceramics

This study investigated the effect of sintering temperature on the microstructure and mechanical properties of dental zirconia-toughened alumina (ZTA) machinable ceramics. Six groups of gelcast ZTA ceramic samples sintered at temperatures between 1100 °C and 1450 °C were prepared. The microstructure was investigated by mercury intrusion porosimetry (MIP), X-ray diffraction (XRD), and scanning electron microscopy (SEM) techniques. The mechanical properties were characterized by flexural strength, fracture toughness, Vickers hardness, and machinability. Overall, with increasing temperature, the relative density, flexural strength, fracture toughness, and Vickers hardness values increased and more tetragonal ZrO₂ transformed into monoclinic ZrO₂; on the other hand, the porosity and pore size decreased. Significantly lower brittleness indexes were observed in groups sintered below 1300 °C, and the lowest values were observed at 1200 °C. The highest flexural strength and fracture toughness of ceramics reached 348.27 MPa and 5.23 MPa m^1/2 when sintered at 1450 °C, respectively. By considering the various properties of gelcast ZTA that varied with the sintering temperature, the optimal temperature for excellent machinability was determined to be approximately 1200–1250 °C, and in this range, a low brittleness index and moderate strength of 0.74–1.19 µm^−1/2 and 46.89–120.15 MPa, respectively, were realized.     

Synthesis and characterization of bioactive zirconia toughened alumina doped with HAp and fluoride compounds

Alumina–zirconia nanostructured composites (ZrO₂ addition by 20 wt%) were prepared using combined gelation–precipitation process. A modified sol–gel process has been developed to prepare nano structured spinel [MgAl₂O₄], Al₂O₃, ZrO₂ and their composite materials. This process is useful in retaining tetragonal phase of zirconia at room temperature, which provides transformation toughening in the nano composites. Dried gels powders were calcined up to 1250 °C. Similarly, hydroxyapatite powders were produced by wet-chemical method and calcined at different temperatures. All the dried gel and calcined powders were characterized by using X-ray diffraction, DTA/TGA and SEM. Samples were prepared by uniaxial pressing the composites powders using ZTA, HAp, MgF₂ and CaF₂ in different ratio. Incorporation of CaF₂ and MgF₂ as a source for fluorine was also done to improve the sinterability of composites. The samples were sintered at 1400 °C for three hours. Densification and mechanical behaviour of sintered samples were observed. Bioactivities of all compositions were tested using SBF solution and then characterizing by FTIR. The main objective of work was to dope ZTA nano composites with HAp and fluoride compounds to obtain better sinterability at lower temperatures. Then evaluate the obtained ZTA based bioactive composite ceramics that have high mechanical strengths. This study verifies the bioactivities of HAp-added ZTA composites.     

Preparation and properties of in-situ mullite whiskers reinforced aluminum chromium phosphate wave-transparent ceramics

In-situ mullite whisker reinforced aluminum chromium phosphate wave-transparent ceramics were designed and prepared. The phase transformation, microstructure, mechanical and electrical properties of the ceramics were investigated, and the mechanisms of in-situ growth and toughening were discussed. Results indicated that the in-situ growth of mullite whisker significantly improved the mechanical properties of the matrix, especially the high temperature flexural strength. The room temperature flexural strength, 1000 °C flexural strength and fracture toughness of the ceramics were 135.60 MPa, 121.71 MPa and 4.52 MPa m^1/2. After sintering at 1500 °C, the optimum properties of ε^'_r, tanδ and microwave transmittance at region 8–12 GHz were <3.6, <0.03 and>80%, respectively. The sinterability of ACP matrix was improved by the in-situ process of high mullization above 1450 °C. Using ACP binder as the raw material can avoid the phase transformation from B-AlPO₄ to T-AlPO₄. The synthesized mullite whiskers played a role in toughening by whiskers fracture, crack deflection and whisker pulling out.     

Thermodynamic calculations of t to m martenstic transformation of ZrO2–CaO binary system

The phase transformation of different polymorphs in zirconia is very important for the processing and mechanical properties of zirconia ceramics. This paper adopts thermodynamic model which is suitable for ceramic system to calculate the Gibbs free energy change of tetragonal and monoclinic phases in ZrO₂–CaO binary system. The difference of the Gibbs free energy between tetragonal and monoclinic phases in ZrO₂–CaO as a function of composition and temperature, namely t → m phase transformation driving force, is thermodynamically calculated from the binary systems. Furthermore, in 8.0 mol% CaO–ZrO₂, the equilibrium temperature between tetragonal and monoclinic phases, T₀, was obtained as 1270.3 K, and martensitic transformation starting temperature (M_s) for t → m transformation of this ceramic with a mean grain size of 2.0 mm was calculated as 805.9 K, which is good agreement with experiment one of 793 K with 12.9 K residual.     

Cold sintering process for 8 mol%Y2O3-stabilized ZrO2 ceramics

In this communication, the cold sintering process was applied to benefit the green body compaction of 8 mol%Y₂O₃-stablized ZrO₂ ceramics (8Y-YSZ). Compared to conventionally processed ceramics, an enhanced densification behavior was demonstrated in cold sintering related ones following a second step conventional sintering process. Dense ceramics up to ∼96% of theoretical density were achieved after sintering at 1200 °C. The resulted ceramics demonstrated a fine microstructure with a grain size ∼200 nm. A mechanical performance with a Vickers hardness of 13.6 GPa and a fracture toughness of 2.85 MPa m^1/2 was also reported.     

Hydrolysis-induced aqueous gelcasting for near-net shape forming of ZTA ceramic composites

A new near-net shape forming process called “hydrolysis-induced aqueous gelcasting” (GCHAS) is reported in this paper for the consolidation of ZTA composites, ZTA-30 (70 wt.% Al₂O₃ + 30 wt.% ZrO₂) and ZTA-60 (40 wt.% Al₂O₃ + 60 wt.% ZrO₂). For comparison purposes, ceramics having the same chemical compositions were also consolidated by hydrolysis-assisted solidification (HAS). All the starting suspensions contained a solids loading of 50 vol.%. In the precursor powder mixtures, 1–5 wt.% of Al₂O₃ was replaced by equivalent amounts of AlN to enhance or promote or co-promote the consolidation of suspensions by HAS or by GCHAS, respectively. The suspensions for GCHAS were prepared by dispersing the ZTA powder precursor mixtures in a premix solution of 20 wt.% MAM (methacrylamide), MBAM (methylenebisacrylamide) and NVP (n-vinylpyrrolidinone) in 3:1:3 ratio in de-ionized water. Ceramics consolidated via GCHAS exhibited superior mechanical properties after consolidation and after sintering for 1 h at 1600 °C in comparison to those consolidated by HAS.     

Fabrication and characterization of highly transparent Er:Y2O3 ceramics with ZrO2 and La2O3 additives

In this study, we report highly transparent Er:Y₂O₃ ceramics (0–10 at% Er) fabricated by a vacuum sintering method using compound sintering additives of ZrO₂ and La₂O₃. The transmittance, microstructure, thermal conductivity and mechanical properties of the Er:Y₂O₃ ceramics were evaluated. The in-line transmittance of all of the Er:Y₂O₃ ceramics (1.2 mm thick) exceeds 83% at 1100 nm and 81% at 600 nm. With an increase in the Er doping concentration from 0 to 10 at%, the average grain size, microhardness and fracture toughness remain nearly unchanged, while the thermal conductivity decreases slightly from 5.55 to 4.89 W/m K. A nearly homogeneous doping level of the laser activator Er up to 10 at% in macro-and nanoscale was measured along the radial direction from the center to the edge of a disk specimen, which is the prominent advantage of polycrystalline over single-crystal materials. Based on the finding of excellent optical and mechanical properties, the compound sintering additives of ZrO₂ and La₂O₃ are demonstrated to be effective for the fabrication of transparent Y₂O₃ ceramics. These results may provide a guideline for the application of transparent Er:Y₂O₃ laser ceramics.     

Preparation and characterization of Si3N4-based composite ceramic tool materials by microwave sintering

Si₃N₄-based composite ceramic tool materials with (W,Ti)C as particle reinforced phase were fabricated by microwave sintering. The effects of the fraction of (W,Ti)C and sintering temperature on the mechanical properties, phase transformation and microstructure of Si₃N₄-based ceramics were investigated. The frictional characteristics of the microwave sintered Si₃N₄-based ceramics were also studied. The results showed that the (W,Ti)C would hinder the densification and phase transformation of Si₃N₄ ceramics, while it enhanced the aspect-ratio of β-Si₃N₄ which promoted the mechanical properties. The Si₃N₄-based composite ceramics reinforced by 15 wt% (W,Ti)C sintered at 1600 °C for 10 min by microwave sintering exhibited the optimum mechanical properties. Its relative density, Vickers hardness and fracture toughness were 95.73 ± 0.21%, 15.92 ± 0.09 GPa and 7.01 ± 0.14 MPa m^1/2, respectively. Compared to the monolithic Si₃N₄ ceramics by microwave sintering, the sintering temperature decreased 100 °C,the Vickers hardness and fracture toughness were enhanced by 6.7% and 8.9%, respectively. The friction coefficient and wear rate of the Si₃N₄/(W,Ti)C sliding against the bearing steel increased initially and then decreased with the increase of the mass fraction of (W,Ti)C., and the friction coefficient and wear rate reached the minimum value while the fraction of (W,Ti)C was 15 wt%.     

Effect of sintering temperature and dopant concentration on microstructure and electrical conductivity of ultra-fine-grained ZrO2–Sc2O3 ceramics

Phase transformations in ZrO₂ + xSc₂O₃ solid solutions (6.5 < x < 11 mol%) at sintering of ceramics obtained from nanopowders produced by laser evaporation of the ceramic targets have been studied. The Sc₂O₃ concentration increasing from 6.5 to 11 mol% is accompanied by the sintering temperature decreasing and the average grain size growth from 130 nm to 760 nm. At concentration of about 7 mol% Sc₂O₃ an abrupt increase of the average grain size and electric conductivity is observed. The sinterability of the ZrO₂ − хSc₂O₃ ceramics is affected by the prehistory of nanopowders preparation. The characteristics of ceramics obtained from nanopowders evaporated from the targets based on (ZrO₂ + xmol% Sc₂O₃) mixture and on the (ZrO₂ − 11mol% Sc₂O₃) solid solution significantly differ, namely, in the latter the sintering temperature is markedly lower and the shrinkage rate is higher. Besides, its average grain size is substantially lower and the conductivity is higher.     

Effects of manganese oxide addition and reductive atmosphere annealing on the phase stability and microstructure of yttria stabilized zirconia

The effects of Mn₃O₄ addition and reductive atmosphere (N₂:H₂ = 97:3) annealing on the microstructure and phase stability of yttria stabilized zirconia (YSZ) ceramics during sintering at 1500 °C for 3 h in air and subsequent annealing in a reductive atmosphere were investigated. Mn₃O₄ added 6 mol% YSZ (6YSZ) and 10 mol% YSZ (10YSZ) ceramics were prepared via the conventional solid-state reaction processes. The X-ray diffraction results showed that a single cubic phase of ZrO₂ was obtained in 1 mol% Mn₃O₄ added 6YSZ ceramic at a sintering temperature of 1500 °C for 3 h. A trace amount of monoclinic ZrO₂ phases were observed for 1 mol% Mn₃O₄ added 6YSZ ceramics after annealing at 1300 °C for 60 cycles in a reductive atmosphere by transmission electron microscopy. Furthermore, a single cubic ZrO₂ phase existed stably as Mn₃O₄ added 10YSZ ceramics was annealed at 1300 °C for 60 cycles in reductive atmosphere.     

Influence of milling time on the synthesis,microstructure and mechanical properties of ZrB2SiCZrO2f ceramic composites

Zirconium diboride toughened by silicon carbide and zirconia fiber (ZrB₂SiCZrO_2f) was prepared by using planetary ball mill and the effect of milling time was investigated. The results showed that both the length of fiber and particle size of ZrB₂SiC-matrix were reduced as the ball milling time increased. When milling time varied from 8 h to 12 h, the accumulated fibers and agglomerated particles were observed. The production of a homogeneous ceramic could be successfully achieved by using a combination of 20 h milling time and hot-pressing at 1850 °C for 60 min under a uniaxial load of 30 MPa. The optimal flexural strength and fracture toughness of the hot-pressed ZrB₂SiCZrO_2f ceramics reached 1084 MPa and 6.8 MPa m^1/2, respectively. The main toughening mechanisms were fiber debonding, fiber pull-out and transformation toughening. The results indicated that the ball milling technique was proposed as a potential and simple method to obtain usable quantities of ZrB₂SiCZrO_2f ceramic.     

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.     

Effects of ZrO2-La2O3 co-addition on the microstructural and optical properties of transparent Y2O3 ceramics

Commercial Y₂O₃ powder was used to fabricate Y₂O₃ ceramics sintered at 1600 °C and 1800 °C with concurrent addition of ZrO₂ and La₂O₃ as sintering aids. One group with different contents of La₂O₃ (0–10 mol%) with a fixed amount of 1 mol% ZrO₂ and another group with various contents of ZrO₂ (0–7 mol%) with a fixed amount of 10 mol% La₂O₃ were compared to investigate the effects of co-doping on the microstructural and optical properties of Y₂O₃ ceramics. At low sintering temperature of 1600 °C, the sample single doped with 10 mol% La₂O₃ exhibits much denser microstructure with a few small intragranular pores while the samples with ZrO₂ and La₂O₃ co-doping features a lot of large intergranular pores leading to lower density. When the sintering temperature increases to 1800 °C, samples using composite sintering aids exhibit finer microstructures and better optical properties than those of both ZrO₂ and La₂O₃ single-doped samples. It was proved that the grain growth suppression caused by ZrO₂ overwhelms the acceleration by La₂O₃. Meanwhile, 1 mol% ZrO₂ acts as a very important inflection point with regard to the influence of additive concentration on the transmittance, pore structure and grain size. The highest in-line transmittance of Y₂O₃ ceramic (1.2 mm in thickness) with 3 mol% of ZrO₂ and 10 mol% of La₂O₃ sintered at 1800 °C for 16 h is 81.9% at a wavelength of 1100 nm, with an average grain size of 11.2 µm.     

Effect of grain growth on the thermal conductivity of liquid-phase sintered silicon carbide ceramics

The effect of grain growth on the thermal conductivity of SiC ceramics sintered with 3 vol% equimolar Gd₂O₃-Y₂O₃ was investigated. During prolonged sintering at 2000 °C in an argon or nitrogen atmosphere, the β → α phase transformation, grain growth, and reduction in lattice oxygen content occurs in the ceramics. The effects of these parameters on the thermal conductivity of liquid-phase sintered SiC ceramics were investigated. The results suggest that (1) grain growth achieved by prolonged sintering at 2000 °C accompanies the decrease of lattice oxygen content and the occurrence of the β → α phase transformation; (2) the reduction of lattice oxygen content plays the most important role in enhancing the thermal conductivity; and (3) the thermal conductivity of the SiC ceramic was insensitive to the occurrence of the β → α phase transformation. The highest thermal conductivity obtained was 225 W(m K)⁻¹ after 12 h sintering at 2000 °C under an applied pressure of 40 MPa in argon.     

Fabrication of micro-scale textured grooves on green ZrO2 ceramics by pulsed laser ablation

The green ZrO₂ ceramics were fabricated by cold isostatic pressing. Pulsed laser ablation with a wavelength of 1064 nm was performed to fabricate micro-scale textured grooves on the surface of green ZrO₂ ceramics. The influence of laser parameters on surface quality was studied. The heat-affected zone around the machined grooves and micromorphology of laser-irradiated surface were investigated. Results showed that micro-scale textured grooves with a width of 30–50 µm and a depth of 15–50 µm on the green ZrO₂ ceramic surfaces were successfully fabricated by pulsed laser ablation. The laser parameters had a profound influence on the surface quality of micro-scale textured grooves. Better surface quality could be obtained with frequency below 40 Hz, power below 6 W, and scanning velocity above 200 mm/s. A sintering layer was found on the laser-irradiated surfaces when frequency was above 60 Hz, power was above 10 W, and scanning velocity was below 150 mm/s. Analysis of this sintering layer revealed clear melting and resolidification of ZrO₂ particles.     

Fabrication and mechanical properties of short ZrO2 fiber reinforced NiFe2O4 matrix composites

Short ZrO₂ fibers (ZrO_2(f)) reinforced NiFe₂O₄ ceramic composites were fabricated by cold pressing process. The phase composition, microstructure, mechanical properties and fiber/matrix interface of the composites were investigated by X-ray diffraction, scanning electron microscopy and mechanical testing machines. ZrO_2(f) show good thermodynamic and chemical compatibility with NiFe₂O₄ ceramic matrix and effectively enhanced the mechanical properties. The toughening mechanisms are fiber bridging, interfacial debonding, fiber pullout, phase transformation and the matrix constraint effect. By incorporation of 3 wt% fibers with the average length of 5～6 mm, the bending strength and fracture toughness of the composites reached 88.92 MPa and 4.62 MPa m^1/2, respectively, while the strength conservation ratio after thermal shock increased from 48.85% to 75.86%. The weak interface bonding built up between ZrO_2(f) and NiFe₂O₄ facilitates the reinforcing effects of the fibers to operate.     

Pressureless sintering and mechanical properties of 3Y-TZP-reinforced LZAS glass-ceramic composites

LZAS glass-ceramic composites toughened by 5, 10, 15 and 20 vol% 3-mol%-Y₂O₃-tetragonal-ZrO₂-polycrystal (3Y-TZP) were prepared via pressureless sintering. Sinterability of composites was investigated in the temperature range of 520–720 °C using soaking time of 30 min. The sintered specimens were characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD) methods. The results revealed that during sintering 3Y-TZP particles agglomerated between the glass powders and were not dissolved by glass-matrix. Mechanical properties of the sintered samples such as bending strength, Vickers micro-hardness and fracture toughness were also investigated. Measurements showed that the relative density of the samples decreased with increasing 3Y-TZP content. The composite containing 15 vol% 3Y-TZP has a best mechanical properties and it would be the optimum composition. It can be confirmed that crack deflection and transformation toughening are the dominant mechanisms for improving mechanical properties of the composites.     

Study on surface quality,precision and mechanical properties of 3D printed ZrO2 ceramic components by laser scanning stereolithography

Zirconia (ZrO₂) ceramic bars with three different printing sizes were fabricated by a stereolithographic (SLA) 3D-printing process and subsequent sintering. An anisotropic character of the ceramics surface quality was observed. The surface roughness of the horizontal surface was below 0.41 µm, whereas it reached 1.07 µm along the fabrication direction on the vertical surface. The warpage and flatness were utilized to measure the dimensional accuracy of the 3D printed ZrO₂. Furthermore, it was evaluated that the warpage and flatness were below 40 µm and 27 µm, respectively, even if the printed size of ceramic bar reached 3 mm × 4 mm × 80 mm. In addition, the flexural strength, the fracture toughness, the hardness and the density of ZrO₂ ceramics can reach to 1154 ± 182 MPa, 6.37 ± 0.25 MPa m^1/2, 13.90 ± 0.62 GPa and up to 99.3%, respectively. Moreover, the effects of scanning paths and printing size on properties of the sintered ZrO₂ samples were analyzed. The anisotropic character of surface quality was related to the various scanning paths. The warpage and flatness of 3D printed ZrO₂ bars were apparently affected by the various printed sizes. Also, the effects of special microstructure on the mechanical properties of sintered ZrO₂ samples were investigated.     

Fabrication and properties of Y2O3 transparent ceramic by sintering aid combinations

Transparent Y₂O₃ ceramics were fabricated by the solid-state reaction and vacuum sintering method using La₂O₃, ZrO₂ and Al₂O₃ as sintering aids. The microstructure of the Y₂O₃ ceramics sintered from 1550 °C to 1800 °C for 8 h were analyzed by SEM. The sintering process of the Y₂O₃ transparent ceramics was optimized. The results showed that when the samples were sintered at 1800 °C for 8 h under vacuum, the average grain sizes of the ceramics were about 3.5 µm. Furthermore, the transmittance of Y₂O₃ ceramic sintered at 1800 °C for 8 h was 82.1% at the wavelength around the 1100 nm (1 mm thickness), which was close to its theoretical value. Moreover, the refractive index of the Y₂O₃ transparent ceramic in the temperature range from 30 °C to 400 °C were measured by the spectroscopic ellipsometry method.