Effect of surface finishing and thickness on the translucency of zirconia dental ceramics

The effects of different surface finishing procedures on the translucency of yttria stabilized zirconia ceramics (ZrO₂) were evaluated. ZrO₂-3mol.%Y₂O₃ (designed Zr3) or ZrO₂-5mol.%Y₂O₃(Zr5) specimens with thickness varying between 0.5 and 1.5 mm were obtained by sintering at 1500 °C for 2 h. Surface finishing of the sintered specimens was done in three distinct manners: polishing with diamond pastes, blasting with Al₂O₃ particles and blasting with Al₂O₃–SiO₂ particles, according to the preparation protocol for dental prostheses. These sample groups were characterized by X-Ray diffraction analysis, scanning electron microscopy, surface roughness and spectrophotometry. The sintered ceramics exhibited relative densities higher than 99% and were composed of 68.5% and 31.5%of tetragonal (t)-ZrO₂ and cubic(c)-ZrO₂ for composition Zr3 and 25.2% and 74.8% for composition Zr5, respectively. Furthermore, 1.5 to 1.8% of monoclinic (m)-ZrO₂, has been found in the blasted surfaces. The average grain size varied from 0.5 μm for Zr3 to 1.45 μm for the Zr5 samples. After polishing, all samples presented a surface roughness R_aof about 0.05 μm, while blasting resulted in an increased roughness ranging between 1.24 μm and 1.49 μm.Samples rich in (c)-ZrO₂ phase (Zr5) are more translucent than samples rich in (t)-ZrO₂ (Zr3) because of their larger grain size and because the cubic phase is less anisotropic than (t)-ZrO₂. Furthermore, the translucency of thinner samples is more affected by abrasive blasting because they also present the highly anisotropic monoclinic(m)-ZrO₂ phase and, therefore, the reduction of translucency is more pronounced. Parameters such as grain size, crystalline phase composition, porosity and grain boundary density are used to explain the phenomena involved in the differences of translucency of these materials.     

Simultaneous enhancement of mechanical and microwave absorption properties with a novel in-situ synthesis α-Al2O3 fillers for SiCf/SiC composites

The Al and H₃BO₃ mixed powder was introduced into the PCS/Xylene precursor solution as in-situ synthesis α-Al₂O₃ filler by precursor infiltration and pyrolysis (PIP) method. The in-situ synthesis filler can effectively decrease the open porosity of SiC_f/SiC composites and give rise to multiple scattering of microwave and dipolar polarization. Therefore, the mechanical and microwave absorption properties of SiC_f/SiC composites can be simultaneously enhanced. The effects of in-situ synthesis filler on the morphologies, flexure strength and reflection loss values of SiC_f/SiC composites were investigated. With 2 wt% in-situ synthesis filler, the flexure strength of SiC_f/SiC composite was 305 MPa and the maximum reflection loss (RL_m) can reach ? 54.68 dB with the effective absorption band (EAB) of 3.51 GHz in the X band. With 5 wt% in-situ synthesis filler, the flexure strength of SiC_f/SiC composite was 207 MPa and the RL_m was ? 30.91 dB. Due to the inefficient infiltration process, the RL_m of SiC_f/SiC composites with 10 wt% in-situ synthesis filler was only ? 27.36 dB. Nevertheless, the flexure strength of that composite was 259 MPa, owing to the dense matrix. Additionally, the flexure strength of SiC_f/SiC composite without filler was 148 MPa and the RL_m was ? 26.40 dB.     

Abrasive wear of Al2O3–SiC and Al2O3–(SiC)–C composites with micrometer- and submicrometer-sized alumina matrix grains

The response of Al₂O₃, Al₂O₃–SiC–(C) and Al₂O₃–C nanocomposites to grinding was investigated in terms of changes of quality of ground surfaces and of the weight losses with time. The study used monolithic polycrystalline aluminas as references, and alumina-based composites with nanosized SiC and C inclusions and with alumina matrix grain size varying from submicrometer to approximately 4 μm. The studied materials can be roughly divided into two groups. Materials with submicrometer alumina matrix grains (Group 1) wear predominantly by plastic deformation and grooving. Coarse-grained materials (Group 2) wear by mixed wear mechanism involving crack initiation and interlinking accompanied by grain pull-out, plastic deformation and grooving. The wear rate of composites increases with increasing volume fraction of SiC. The Group 2 materials wear much faster then those with submicron microstructure. In all cases (with one exception) the wear resistance of composites was higher than that of pure aluminas of comparable grain sizes used as reference materials.     

Tribo-mechanical characterization of spark plasma sintered chopped carbon fibre reinforced silicon carbide composites

Short carbon fibre (C_f) reinforced silicon carbide (SiC) composites with 7.5 wt% alumina (Al₂O₃) as sintering additive were fabricated using spark plasma sintering (SPS). Three different C_f concentrations i.e. 10, 20 and 30 wt% were used to fabricate the composites. With increasing C_f content from 0 to 20 wt%, micro-hardness of the composites decreased ~28% and fracture toughness (K_IC) increased significantly. The short C_f in the matrix facilitated enhanced fracture energy dissipation by the processes of crack deflection and bridging at C_f/SiC interface, fibre debonding and pullout. Thus, 20 wt% C_f/SiC composite showed >40% higher K_IC over monolithic SiC (K_IC≈4.51 MPa m^0.5). Tribological tests in dry condition against Al₂O₃ ball showed slight improvement in wear resistance but significantly reduced friction coefficient (COF, μ) with increasing C_f content in the composites. The composite containing 30 wt% C_f showed the lowest COF.     

Alumina/zirconia ceramic membranes fabricated by temperature induced forming technique

It is well known that the fabrication technique of porous ceramic composites has a significant effect on their microstructure and properties. In the present study, alumina/zirconia ceramic composites doped with magnesia were fabricated by temperature induced forming technique using tri-ammonium citrate and polyacrylic acid (PAA) as dispersant and gelling agents, respectively. The zirconia content was up to 20 wt% and added at the expense of alumina while the magnesia content was up to 2 wt% over the total mass. The optimum amount of ammonium citrate tribasic needed for dispersing the ceramic slurry was determined by measuring zeta potential of slurries. The prepared green alumina/zirconia composites were subjected for solid state sintering at 1550 °C for 1 h. The densification parameters, phase composition, average pore diameter, microstructure and cold crushing strength of sintered alumina/zirconia ceramics were investigated by the suitable techniques. The results revealed that the addition of tri-ammonium citrate to ceramic slurries enhanced the zeta potential which reached ?28.3 mV by adding 1 wt.-%. The bulk density was decreased while the apparent porosity was increased with the increase of zirconia content. The apparent porosities of sintered porous composites were in the range of 38.8–48.5%. The average pore diameter for the composite containing 15% zirconia was 1.79 μm and pore volume was 0.11 ml/g. The obtained microstructure exhibited zirconia grains located on the grain boundaries of Al₂O₃ matrix. The existence of zirconia in addition to magnesia hindered the growth and deformation of the matrix. The cold crushing strength of porous composites was decreased from 16.0 to 8.5 to MPa by increasing the zirconia content from 5 to 20 wt.-%.     

The influence of different SiC amounts on the microstructure,densification, and mechanical properties of hot-pressed Al2O3-SiC composites

The hot pressing process of monolithic Al₂O₃ and Al₂O₃-SiC composites with 0-25 wt% of submicrometer silicon carbide was done in this paper. The presence of SiC particles prohibited the grain growth of the Al₂O₃ matrix during sintering at the temperatures of 1450°C and 1550°C for 1 h and under the pressure of 30 MPa in vacuum. The effect of SiC reinforcement on the mechanical properties of composite specimens like fracture toughness, flexural strength, and hardness was discussed. The results showed that the maximum values of fracture toughness (5.9 ± 0.5 MPa.m^1/2) and hardness (20.8 ± 0.4 GPa) were obtained for the Al₂O₃-5 wt% SiC composite specimens. The significant improvement in fracture toughness of composite specimens in comparison with the monolithic alumina (3.1 ± 0.4 MPa.m^1/2) could be attributed to crack deflection as one of the toughening mechanisms with regard to the presence of SiC particles. In addition, the flexural strength was improved by increasing SiC value up to 25 wt% and reached 395 ± 1.4 MPa. The scanning electron microscopy (SEM) observations verified that the increasing of flexural strength was related to the fine-grained microstructure.     

Preparation of a novel bi-layer modified alumina-based hybrid material and its effect on the thermal conductivity enhancement of polymer composites

In this work, a new kind of double layers modified alumina-based hybrid (silver@copper@alumina (Ag@Cu@Al₂O₃) hybrid) was successfully synthesized through the two-step layer-by-layer process. First, copper (Cu) nanoparticles were assembled onto alumina (Al₂O₃) particles by reduction of Cu²⁺. Second, Ag@Cu@Al₂O₃ hybrids were assembled via Ag deposition on the surface of Cu@Al₂O₃ particles. The obtained Ag@Cu@Al₂O₃ hybrids served as thermally conductive fillers to greatly boost the thermal conductivity of poly (dimethylsiloxane) (PDMS). The thermal conductivity reached 1.465 W m^?1 K^?1 at 85 wt% filler loading. The thermal conductivity of PDMS matrix was increased more than 7 times by the addition of Ag@Cu@Al₂O₃ hybrid, which was much higher than single layer modified alumina-based hybrids (Ag@Al₂O₃ and Cu@Al₂O₃ hybrids) and virgin Al₂O₃ particle. The effect of double layers modified filler, single layer modified filler and virgin filler on the thermal conductivity of PDMS matrix was discussed in detail and the mechanism of these fillers for improving thermal conductivity was studied through Foygel's thermal conduction model. Otherwise, electric, mechanical and thermal properties of Ag@Cu@Al₂O₃/PDMS composites were also further tested and analyzed.     

Effect of second phase addition of zirconia on the mechanical response of textured alumina ceramics

The effect of second phase addition of zirconia on the mechanical response of textured alumina was analysed. Highly textured monolithic tape-casted alumina was obtained through templated grain growth. Compositions containing 1, 2, 5 and 10 vol% of (i) non-stabilised and (ii) 3 mol% yttria-stabilised zirconia, respectively, were investigated. XRD analyses revealed that the texture degree decreased with increasing second phase content. Microstructural analysis showed zirconia grains inside the textured alumina grains for contents ≤ 5 vol%, affecting the mode of fracture. Fracture toughness of textured alumina significantly decreased with the addition of a second phase. In the case of non-stabilised zirconia, the constraint of the alumina matrix and the small grain size led to a lower fracture toughness in comparison to monolithic textured alumina (K_Ic = 5.1 MPa m^1/2). The fracture toughness of textured alumina with 3 mol% yttria-stabilised zirconia was comparable to equiaxed alumina, independent of the content ratio (K_Ic = 3.5 MPa m^1/2).     

A novel method for fabricating brick-mortar structured alumina-zirconia ceramics with high toughness

The present work reports a novel and simple approach to prepare alumina-zirconia composites with superior toughness. Alumina microspheres were innovatively used as the raw materials, followed by coating zirconia and hot-pressing sintering to fabricate alumina-zirconia ceramics. The resultant ceramics are given a unique brick-mortar microstructure, in which the zirconia “mortar” layers continuously distribute around the alumina “brick” matrix, leading to outstanding fracture toughness of 7.34 MPa·m^1/2 and high strength of 635.84 MPa when prepared with zirconia contents of 10 wt%. The major explanation could be ascribed to that crack tips in sintered samples tend to propagate along the zirconia “mortar” layer, accompanied by deflection and branching, which effectively improve the fracture toughness of composites. The uniformity and integrity of the brick-mortar structure could be well tuned by varying the amount of zirconia. This method has reference significance for the preparation of high toughness alumina-based multiphase ceramics.     

Mechanical and thermal properties of spark plasma sintered Al2O3-graphene-SiC hybrid composites

Monolithic Al₂O₃ and Al₂O₃-graphene-SiC hybrid composites were prepared by spark plasma sintering (SPS) under vacuum atmosphere. The results show that the hybrid composites were almost completely dense (>97%). SiC content has a significant effect on the microstructure of the composites. With the increase of SiC content, the average grain size of alumina decreased gradually. The addition of SiC to alumina changed fracture mode from inter-granular fracture to mixed fracture mode of inter-granular fracture and trans-granular fracture. The Al₂O₃-0.4 wt%graphene-5 wt% SiC hybrid composite has the highest bending strength and hardness, which were 57% and 19.22% higher than those of the monolithic alumina, respectively. The room temperature (RT) thermal conductivity of the monolithic Al₂O₃ (25.5 W/m·K) was the highest. The thermal conductivity and thermal diffusivity coefficient of the composites decreased with the increase in temperature, while the specific heat of monolithic alumina and composites increased with the increase in temperature and additives. These properties were related to the microstructure of materials and the possible transport mechanisms were discussed.     

Alumina toughened zirconia reinforced with equiaxed and elongated lanthanum hexa-aluminate precipitates

Three different alumina sources (boehmite, aluminium nitrate and α-alumina particles) and 12Ce-TZP powder containing 1 wt% lanthanum oxide were used to prepare 12Ce-TZP-based alumina-toughened-zirconia (ATZ) composites. The obtained ATZs had similar density and phase composition, whereas the microstructures were significantly different. Alumina-particle addition gave rise to a typical ATZ microstructure consisting of equiaxial sub-micrometer zirconia and alumina phases, while the lanthanum hexa-aluminate phase was formed in large and non-homogenously distributed precipitates (∼3.5 μm in length). The boehmite and aluminium nitrate-based composites contained not only sub-micrometer equiaxial alumina and zirconia grains but also small-sized lanthanum hexa-aluminate precipitates (∼1.2 μm in length) that were inter- and transgranularly positioned in the zirconia matrix and effectively promoted crack deflection and toughening. In combination with a higher t-ZrO₂ transformability, the boehmite-based composites had a higher indentation fracture resistance, strength and reliability compared to the aluminium-nitrate and alumina-particle based equivalents.     

Effects of ZrO2 nanoparticles on the microstructure and mechanical properties of ZrO2/AlSi10Mg composites manufactured by laser powder bed fusion

In this work, the nano-ZrO₂ particles were mixed into AlSi10Mg alloy to prepare ZrO₂/AlSi10Mg composites with different x wt.% ZrO₂ (x = 0, 0.15, 0.3, 0.45, 0.6, 0.75). The microstructure, mechanical properties and the anisotropy of the ZrO₂/AlSi10Mg composites fabricated by laser powder bed fusion (LPBF) were studied. The results show that nano-ZrO₂ particles can be uniformly dispersed on the AlSi10Mg powder by the method of pre-dispersion and mechanical mixing. When the mass ratio of ZrO₂ in ZrO₂/AlSi10Mg composites is 0.3 wt%, the values of the tensile strength, yield strength and elongation are 493.64 MPa, 321.30 MPa and 11.74%, respectively. Compared with AlSi10Mg alloy, the tensile strength of ZrO₂/AlSi10Mg composites with 0.3 wt% is increased by 30–55 MPa and the elongation is increased by 3–5%. In addition, the mechanical properties of AlSi10Mg alloy and ZrO₂/AlSi10Mg composites of 0.3 wt% exhibit antistrophic behavior in different direction, which is due to the differences of microstructure, texture and stress distribution between transverse direction (TD) and build direction (BD). Compared with other AlSi10Mg matrix composites, ZrO₂/AlSi10Mg composites of this work show excellent strength and plasticity matching.     

Effect of manganese addition on sintering behavior and mechanical properties of alumina toughened zirconia

The effect of MnO₂ additives on the sintering behavior and mechanical properties of alumina-toughened zirconia (ATZ, with 10 vol% alumina) composites was investigated by incorporating different amounts of MnO₂ (0, 0.5, 1.0, and 1.5 wt%) and sintering at various temperatures ranging from 1300 to 1450 °C. The addition of MnO₂ up to 1.0 wt% improved the sintered density, hardness, flexural strength, and fracture toughness of the composite. However, the addition of 1.5 wt% MnO₂ degraded the relative density, hardness, and flexural strength of the composite due to the transformation of the ZrO₂ phase from tetragonal to monoclinic and grain coarsening. Optimal results were obtained with 1.0 wt% MnO₂ and sintering at 1450 °C, which improved the mechanical properties (hardness: 13.5 GPa, flexural strength: 1.2 GPa, fracture toughness: 8.5 MPa m^1/2) and lowered the sintering temperature compared to the conventional sintering temperature of ATZ composites (1550 °C). Thus, the ATZ composite doped with MnO₂ is a promising material for structural engineering ceramics owing to its improved mechanical properties and lower sintering temperature.     

The effect of gamma irradiation on mechanical,thermal and morphological properties of glass fiber reinforced polyethylene waste/reclaim rubber composites

Composites of waste polyethylene (WPE), collected from municipal solid waste/recycled waste rubber powder (RWRP) reactive compatibilizing agent, maleic anhydride (MA) and glass fiber (GF) up to 20 wt%, prepared by melting and irradiated with gamma-rays up to 150 kGy have been studied. Tensile strength (T_S), elongation at break (E_b), elastic modulus, hardness, thermal and morphological parameters of the irradiated composites were investigated. The examined mechanical properties have been found to improve largely with filler content. Interesting E_b behavior has been detected for the irradiated composites loaded up to ∼10 wt% GF and has been basically discussed in view of matrix crystallinity and morphology. TGA thermograms of unirradiated composites revealed enhanced thermal stability than that reported for the blend whereas comparatively slight improvement has been demonstrated by irradiation. Whereby insignificant alteration in T_m values was observed by loading or irradiation, yet ΔH_m maximum of 3.41 J/g, indicated for the 5 wt% GF irradiated composite with an integral dose of 75 kGy, emphasizes the influence of the relatively moderate load and dose levels on matrix stability. The phenomenon has been confirmed by the respective SEM micrographs.     

Influence of processing on fracture toughness of Si3N4 + graphene platelet composites

Silicon nitride + 1 wt% graphene platelet composites were prepared using various graphene platelets (GPL) and two processing routes; hot isostatic pressing (HIP) and gas pressure sintering (GPS). The influence of the processing route and graphene platelets’ addition on the fracture toughness has been investigated. The matrix of the composites prepared by GPS consists of Si₃N₄ grains with smaller diameter in comparison to the composites prepared by HIP. The indentation fracture toughness of the composites was in the range 6.1–9.9 MPa m^0.5, which is significantly higher compared to the monolithic silicon nitride 6.5 and 6.3 MPa m^0.5. The highest value of K_IC was 9.9 MPa m^0.5 in the case of composite reinforced by the smallest multilayer graphene nanosheets, prepared by HIP. The composites prepared by GPS exhibit lower fracture toughness, from 6.1 to 8.5 MPa m^0.5. The toughening mechanisms were similar in all composites in the form of crack deflection, crack branching and crack bridging.     

Al2O3/Cu-O composites fabricated by pressureless infiltration of paper-derived Al2O3 porous preforms

Al₂O₃/Cu-O composites were fabricated from the paper-derived alumina matrix infiltrated with a Cu-3.2?wt% O alloy. Paper-derived alumina preforms with an open porosity ranging from ～ 14 to ～ 25?vol% were prepared by sintering of alumina-loaded preceramic papers at 1600?°C for 4?h. Pressureless infiltration at 1320?°C for 4?h of the preforms with Cu–O alloy resulted in the nearly dense materials with good mechanical and electrical properties, e.g. fracture toughness up to 6?MPa?m^0.5, four-point-bending strength up to 342?MPa, Young's modulus up to 281?GPa and electrical conductivity up to 2?MS/m depending on the volume fraction of copper alloy in the composites. The technological capability of this approach was demonstrated using prototypes in various engineering fields fabricated by lamination, corrugating and Laminated Object Manufacturing (LOM) methods.     

Effect of Dy2O3 content on microstructure and mechanical properties of zirconia-toughened alumina

In this study, the effect of adding Dy₂O₃ on the microstructure and mechanical properties of zirconia-toughened alumina (ZTA) stabilised by yttrium oxide was investigated. ZTA-Dy₂O₃ composites with different Dy₂O₃ contents (0 wt%, 1 wt%, 2 wt%, 3 wt%, and 4 wt%) were prepared by sintering at 1600 °C for 4 h. The phases and structures of the samples were characterised through X-ray diffraction (XRD) and scanning electron microscopy (SEM). Dy₂O₃ formed a solid solution c-DYZ with YSZ, and an appropriate amount of Dy₂O₃ could refine the grains and contribute to densification. The densities, hardness, flexural strength and toughness all increased and then decreased with increasing Dy₂O₃ content, reaching maximum values of 99.2%, 1741 ± 19 HV, 449 ± 10 MPa and 5.87 ± 0.42 MPa?m^1/2, respectively, at 3 wt% Dy₂O₃ content.     

The wear resistance of Ni alumina and NiAl2O4 alumina nanocomposites

The influence of metallic Ni or NiAl₂O₄ as a reinforcing particle on grain growth and wear resistance in alumina matrix composites was evaluated. Alumina composites with various Ni or NiAl₂O₄ concentrations were prepared by multiple-infiltrations of Ni-nitrate into bisque-fired (necked) alumina green bodies followed by heat treatment and sintering at 1600 °C for 2 h. Sintering in a reducing environment resulted in composites with metallic Ni nanoparticles, while NiAl₂O₄ alumina composites were formed when sintering in air. The addition of Ni or NiAl₂O₄ resulted in a reduction in alumina grain size after sintering. The material response to abrasive wear was estimated by measuring the time to section samples of a defined area using a diamond wafering saw and was compared to the wear resistance of undoped alumina. In both cases, reinforcing alumina with Ni or NiAl₂O₄ particles resulted in a significant increase in wear resistance, correlated to the reduced grain size.     

BaTiO3 nanocubes-Gelatin composites for piezoelectric harvesting: Modeling and experimental study

Flexible composites containing BaTiO₃ nanoparticles into Gelatin bio-polymer matrix were designed and investigated. Following the idea that the electric field concentration in corners/edges at the interfaces between dissimilar materials give rise to enhanced effective permittivity in composites, cuboid-like BaTiO₃ nanoparticles have been employed as nanofillers into Gelatin matrix by using an inexpensive solution-based processing method. As predicted by finite element method simulations developed for cubic-like inclusions into a homogeneous polymer matrix, the experimental permittivity of xBT-(1-x)Gelatin composites increases when increasing the high-permittivity filler addition. For the composition x = 40 wt% (corresponding to 12 vol% BaTiO₃ addition), permittivity reaches ε_r ～15.7 with respect to ε_r ～9.8 of pure Gelatine (measured at 10⁵ Hz), while the average piezoelectric coefficient d₃₃ as determined by piezoelectric force microscopy shows a remarkable increase up to 21 pm/V in composites with x = 40 wt%, in comparison to ～7 pm/V in pure Gelatin. By using the experimentally determined material constants, the simulated piezoelectric voltage output vs. time has shown a similar increase (about a doubling of its amplitude) of the harvesting signal in the composite with x = 40 wt% BT, with respect to one of the polymer matrix, thus demonstrating the beneficial role of embedding BT nanoparticles into the biopolymer for increasing the mechanical harvesting response.     

Investigation of the structural and mechanical properties of micro-/nano-sized Al2O3 and cBN composites prepared by spark plasma sintering

Alumina-cubic boron nitride (cBN) composites were prepared using the spark plasma sintering (SPS) technique. Alpha-alumina powders with particle sizes of ∼15 µm and ∼150 nm were used as the matrix while cBN particles with and without nickel coating were used as reinforcement agents. The amount of both coated and uncoated cBN reinforcements for each type of matrix was varied between 10 to 30 wt%. The powder materials were sintered at a temperature of 1400 °C under a constant uniaxial pressure of 50 MPa. We studied the effect of the size of the starting alumina powder particles, as well as the effect of the nickel coating, on the phase transformation from cBN to hBN (hexagonal boron nitride) and on the thermo-mechanical properties of the composites. In contrast to micro-sized alumina, utilization of nano-sized alumina as the starting powder was observed to have played a pivotal role in preventing the cBN-to-hBN transformation. The composites prepared using nano-sized alumina reinforced with nickel-coated 30 wt% cBN showed the highest relative density of 99% along with the highest Vickers hardness (H_v2) value of 29 GPa. Because the compositions made with micro-sized alumina underwent the phase transformation from cBN to hBN, their relative densification as well as hardness values were relatively low (20.9–22.8 GPa). However, the nickel coating on the cBN reinforcement particles hindered the cBN-to-hBN transformation in the micro-sized alumina matrix, resulting in improved hardness values of up to 24.64 GPa.