Micro powder injection moulding of alumina micro-channel part

A feedstock consisting of submicron alumina powder and a formulated binder, was developed to fabricate alumina micro-channel part by micro powder injection moulding. During small scale-mixing, the mixing torques of feedstocks with four different powder loadings were used to establish a suitable powder loading. The thermal and rheological properties of the selected feedstock were examined and used to establish conditions for large scale mixing, debinding and injection moulding. The micro-channel parts were pressureless sintered at different temperatures. The results showed that the moulded, debound and sintered micro-channel parts had good shape retention. The dimensions of the micro-channel part changed with the different processing steps. High densification of the micro-channel parts was achieved at sintering temperatures of 1350 °C and above. Above 1350 °C, the grain grew significantly with increasing the sintering temperatures and thus it led to a decrease in the microhardness.     

Binder removal via a two-stage debinding process for ceramic injection molding parts

Debinding binders in two stages is critical to maintaining the shape of injected parts; the resulting decomposition affects the strength and rigidity of a structure. This study determines the optimal debinding process on the basis of a higher binder removal rate and the production of defect-free parts. The feedstock used was a combination of alumina–zirconia powder with a binder that consists of high-density polyethylene (HDPE), paraffin wax (PW), and stearic acid (SA). During the first stage, the injected parts were immersed in an n-heptane solution at 50 °C, 60 °C, 65 °C, and 70 °C to remove PW and SA. Binder weight loss was evaluated as a function of time. In the second stage, HDPE was removed by using thermal debinding. The results show that the optimum solvent debinding process runs for 16 h at 60 °C. The weight loss of the binder reaches 41.1% and results in the formation of defect-free parts. The binders are degraded at approximately 550 °C during thermal debinding. This degradation resulted in decomposition of nearly 96.9% of the binders. Low heating rates (1 °C/min to 2 °C/min) prevent defects from forming in the injected parts.     

Effects of oxidation on the strength of debound SiC parts by powder injection moulding

In order to increase the production efficiency of powder injection moulding for SiC parts, thermal debinding was performed in air furnace without gas shield. Bending tests were performed to evaluate the strength of samples debound under different temperatures. The effects of oxidation on debinding process were also analyzed. Analysis indicates that air could accelerate the debinding rate of green parts without defects occurring. The bending strength of debound samples increases from 6.55 MPa to 11.58 MPa as the pre-sintering temperature increases from 550 °C to 850 °C. On the other hand, the bending strength of the samples pre-sintered at 1200 °C in argon atmosphere is only 11.52 MPa. It was found that, blanks have enough strength for transport after being pre-sintered in air atmosphere at 850 °C. The technology could reduce the requirement for heating equipment and enhance the efficiency of debinding for SiC parts.     

Surface modification of ceramic powders by titanate coupling agent for injection molding using partially water soluble binder system

In the conventional process of ceramic injection molding (CIM), wax-based binders could only be removed by thermal or organic solvent debinding. Recently, water solvent debinding, with its high efficiency and environmental acceptability, has appeared as a good alternative. In this study, zirconia powder modified by titanate coupling agent was applied in partially water soluble binder system for injection molding. In contrast to previous researches about titanate modification mainly focusing on rheological behavior and modification mechanism, investigations on the sintering behavior and densification process were also made in this study. Experimental results reveal that titanate modified powder exhibits densification temperature almost 100 °C lower than that required for the pure, original powder, giving rise to finer microstructure and therefore hopefully improved mechanically properties. It suggests a novel modification route to fabricate injection molded ceramic components using partially water soluble binder system.     

Synthesis of nanocrystalline 8 mol% yttria stabilized zirconia by the oleate complex route

Nanocrystalline 8 mol% yttria stabilized zirconia (YSZ) powder has been synthesized by the oleate complex route. Oleate complexes of zirconium and yttrium were formed in the hexane rich layer by the reaction of sodium oleate with zirconyl chloride and yttrium chloride at the interface of the two ternary solutions in water–ethanol–hexane system. The zirconyl oletae and yttrium oleate complexes on heating decomposed to oxide through the formation of carbonate intermediates. The powder obtained by calcination at 600 °C for 2 h was cubic YSZ with surface area of 42 m²/g. The YSZ powder contained primary particles of ∼300 nm size and the primary particles were aggregate of crystallites of 5–10 nm. The compacts prepared from the YSZ powder were sintered to ∼99% TD (theoretical density) at 1400 °C. The sintered YSZ had a low average grain size of 0.73 μm.     

Eight mole percent yttria stabilized zirconia powders by organic precursor route

An organic precursor-mixing route has been developed for preparation of 8 mol% yttria stabilized zirconia (8YSZ) ceramics. Polymeric salt of succinic acid with yttrium and zirconium has been prepared separately by treating sodium succinate with yttrium chloride and zirconyl chloride followed by washing with water and drying at 120 °C. Thorough mixing of the two salts in stoichiometric proportions by planetary ball milling followed by calcination at 850 °C resulted in a precursor powder containing nanocrystalline (∼40 nm) monoclinic zirconia, tetragonal YSZ, cubic YSZ and yttria. Compacts prepared after deagglomeration of powder by planetary ball milling produce 8YSZ ceramics having density 99.3% TD on sintering at 1550 °C for 2 h.     

Suspension stability and sintering influence on yttria-stabilized zirconia fabricated by colloidal processing

The stability of nano-zirconia 3YSZ powder in suspension was extensively studied by the colloidal method, and the optimum sintering temperature of the green sample fabricated through slip casting was determined. Zirconia suspensions with 10 vol% powder loading were prepared with distilled water, and HNO₃ was used to adjust the pH of the suspension to pH 1–6. All of the suspensions were subjected to sedimentation test, and the results showed that the suspensions adjusted to pH 2 had the lowest sediment volume. This finding indicates that a suspension with pH 2 produces higher packing density. Viscosity test was carried out for the suspensions added with dispersant ranging from 0.3 wt% to 0.7 wt% polyethyleneimine (PEI) with and without pH adjustment. The suspension containing 0.5 wt% PEI with pH 2 adjustment produced the lowest viscosity because of interparticle bond breakage in the aggregates, thus forming colloidally stable suspensions. The zirconia suspension containing 0.5 wt% PEI and whose pH was adjusted to pH 2 was chosen to be slip casted into cylindrical shape. Green samples were sintered at various sintering temperatures that ranged from 1100 °C to 1500 °C through a two-step sintering method. The sample sintered at 1500 °C was found to be porosite-free, and its highest relative density was 99.6% of the theoretical density. Morphological studies detected pores in the microstructure of the samples sintered at low sintering temperatures (1100 and 1200 °C). By contrast, the samples sintered at 1400 and 1500 °C were fully densified. However, the grain size of the sample sintered at 1500 °C was 230 nm, which indicated excessive grain growth. The Vickers hardness of the sample sintered at 1400 °C was found to be highest (12.9 GPa) and comparable to results found in the literature.     

Fabrication of 8-YSZ thin-wall tubes by powder extrusion moulding for SOFC electrolytes

In this work the processing steps for producing YSZ thin tubes by means of powder extrusion moulding (PEM) technique are investigated. Different feedstocks were prepared from a commercial YSZ powder and a multicomponent thermoplastic binder system based in polypropylene and paraffin wax. The surface coating of YSZ powder with stearic acid in a high-performance dispersing instrument reduces the viscosity of the feedstock one order of magnitude respect to a feedstock with a same composition and un-coated powder. This fact allows increasing the solid loading up to 58 vol.% to obtain sintering tubes with densities higher than 97% and with wall thickness lower than 200 μm.     

Effect of CeO2 addition on densification and microstructure of Al2O3-YSZ composites

Alumina (Al₂O₃) and alumina-yttria stabilized zirconia (YSZ) composites containing 3 and 5 mass% ceria (CeO₂) were prepared by spark plasma sintering (SPS) at temperatures of 1350-1400 °C for 300 s under a pressure of 40 MPa. Densification, microstructure and mechanical properties of the Al₂O₃ based composites were investigated. Fully dense composites with a relative density of approximately 99% were obtained. The grain growth of alumina was inhibited significantly by the addition of 10 vol% zirconia, and formation of elongated CeAl₁₁O₁₈ grains was observed in the ceria containing composites sintered at 1400 °C. Al₂O₃-YSZ composites without CeO₂ had higher hardness than monolithic Al₂O₃ sintered body and the hardness of Al₂O₃-YSZ composites decreased from 20.3 GPa to 18.5 GPa when the content of ZrO₂ increased from 10 to 30 vol%. The fracture toughness of Al₂O₃ increased from 2.8 MPa m^1/2 to 5.6 MPa m^1/2 with the addition of 10 vol% YSZ, and further addition resulted in higher fracture toughness values. The highest value of fracture toughness, 6.2 MPa m^1/2, was achieved with the addition of 30 vol% YSZ.     

Electrochemical decomposition of CO2 and CO gases using porous yttria-stabilized zirconia cell

Electrochemical decomposition of CO₂ and CO gases using a porous cell of Ru-8 mol% yttria-stabilized zirconia (YSZ) anode/porous YSZ electrolyte/Ni–YSZ cathode system at 400–800 °C was studied by analyzing the flow rate and composition of outlet gas, current density, and phases and elementary distribution of the electrodes and electrolyte. A part of CO₂ gas supplied at 50 ml/min was decomposed to solid carbon and O₂ gas through the cell at the electric field strengths of 0.9–1.0 V/cm. The outlet gas at a flow rate of 3 ml/min included 61–63% CO₂ and 37–39% O₂ at 700–800 °C and the outlet gas at a flow rate of 50 ml/min included 73–96% (average 85%) CO₂ and 4–27% (average 15%) O₂ at 800 °C. On the other hand, the supplied CO gas was also decomposed to solid carbon, O₂ and CO₂ gases at 800 °C. The fraction of outlet gas at a flow rate of 50 ml/min during the CO decomposition at 800 °C for 5 h was 11–36% CO, 59–81% O₂ and 2–9% CO₂. The detailed decomposition mechanisms of CO₂ and CO gases are discussed. Both Ni metal in the cathode and porous YSZ grains under the DC electric field have the ability to decompose CO gas into solid carbon and O²⁻ ions or O₂ gas.     

Direct coagulation casting of YSZ powder suspensions using MgO as coagulating agent

Direct coagulation casting (DCC) of aqueous 8 wt% yttria stabilized zirconia (YSZ) powder suspensions prepared using ammonium poly(acrylate) dispersant has been studied using MgO as coagulating agent. Small amount (<0.1 wt% based on YSZ) of MgO powder dispersed in the YSZ powder suspension at ∼5 °C set the suspension in to stiff wet-coagulated body when exposed to room temperature (30 °C) due to the reaction between ammonium poly(acrylate) and MgO. MgO concentration equivalent to react with dispersant did not coagulate the YSZ powder suspension though it precipitate the whole ammonium poly(acrylate) dispersant as Mg-poly(acrylate). This is because of the ability of the YSZ powder to disperse in water at alkaline pH (∼9.5) without any dispersant by electrostatic mechanism. The YSZ powder suspensions form stiff coagulated bodies at MgO concentration double or more of the equivalent amount required for reacting with the dispersant. Setting of the YSZ powder suspension is due to the heterocoagulation of the YSZ particles and MgO particles added in excess of the equivalent amount to react with the dispersant, having opposite surface charges. The wet-coagulated body showed relatively high compressive yield strength (155 kPa) and Young’s modulus (3.1 MPa). The green bodies prepared by humidity controlled drying of the wet-coagulated bodies sintered to >98% TD at 1550 °C.     

Microstructure evaluation of sintered combustion-derived fine powder NiO–YSZ

Citrate–nitrate combustion synthesis was used for the preparation of NiO–YSZ. The main advantage of the preparation method used was reflected in the fact that after the synthesis both phases NiO and YSZ were randomly distributed on a nanometre level. The prepared NiO–YSZ powder composites were shaped, sintered and reduced to Ni–YSZ and subsequently submitted to microstructure investigations. Relative sintered densities higher than 90% were obtained at sintering temperatures as low as 1200 °C. A sintering temperature 1200 °C was also recognized as the preparation temperature that provided the smallest Ni grains in the final Ni–YSZ cermet with an average Ni-particle diameter as low as 0.27 μm.     

3D printing of high solid loading zirconia feedstock via screw-based material extrusion

Zirconia ceramic (3Y-TZP) feedstocks with solid loadings from 50 vol% to 68 vol%, in a 60:40 paraffin wax to LDPE ratio binder system, were prepared and printed using a screw-based material extrusion printer. A two-step debinding process involving solvent debinding (cyclohexane + ethanol) and thermal debinding (140 °C–600 °C at 0.2 °C/min) followed by sintering at 1500 °C for 2 h was employed. Tests performed include TGA, density test, Vickers hardness and fracture toughness, XRD, and SEM. The TGA result showed two significant drops in weight starting at 180 °C and 380 °C, which corresponds to the decomposition of paraffin wax and LDPE, respectively. A minimum of 40 wt% of soluble binder was removed from the green sample after solvent immersion for 3 h at 40 °C for solid loadings ≥55 vol%. High solid loading feedstocks produced samples with comparable density, Vickers hardness and fracture toughness, which are 97.5%, ∼12.3 GPa, and ∼5.5 MPa m^1/2, respectively; while XRD and SEM shows no adverse tetragonal to monoclinic phase transformation and grain growth, respectively. This study demonstrates that 3D printing of granular 3Y-TZP ceramic feedstock via screw-based material extrusion technique is feasible even with high solid loadings, which is usually difficult to fabricate into flexible filaments and print due to high viscosity.     

Multi-material ceramic material extrusion 3D printing with granulated injection molding feedstocks

Material Extrusion (MEX) is an advanced technology for polymer 3D printing and countless printers are commercially available. MEX has also been demonstrated for ceramics. For that purpose, thermoplastic binders are filled with high loads (>40 vol%) of a ceramic powder. The printed parts are subsequently debound and sintered. In contrast to most MEX printers, the ceramic printer presented herein works with granulated feedstock instead of filaments. Therefore, the development of novel feedstocks is faster and more straightforward since the challenges associated with filament production are omitted. Furthermore, commercial ceramic injection molding (CIM) feedstocks can be used which allows fast prototyping with the same material that is later used in high-quantity industrial production by CIM.In this study, a method to fabricate multi-material ceramic parts using a granulate-fed printer is presented. Examples of multi-material printing include colored ZrO₂ parts as well as ceramic high-temperature heating elements in various shapes consisting of an electrically conductive and a non-conductive component. Light- and electron microscopy confirms that the layer adhesion before and after sintering is flawless, even between different materials if the material combination is chosen carefully. All feedstocks are based on a commercially available CIM binder filled with the desired ceramic powder. Consequently, the feedstock preparation as well as optimizing of debinding and sintering conditions are simple and reproducible.     

Glass–ceramics as oxidation resistant bond coat in thermal barrier coating system

Thermal barrier coating (TBC) system consisting of yttria stabilized zirconia (YSZ) top coat, glass–ceramic bond coat and nickel base superalloy substrate was subjected to static oxidation test at 1200 °C for 500 h in air. Oxidation resistance of this TBC system was compared with the conventional TBC system under identical heat treatment condition. Both the TBC systems were characterized by SEM as well as EDX analysis. No TGO layer was found between the bond coat and the top coat in the case of glass–ceramic bonded TBC system while the conventional TBC system exhibited a TGO layer of about 16 μm thickness at the bond coat-top coat interface region.     

The effects of nanoparticle addition on the sintering and properties of bimodal AlN

The effects of nanoparticle addition on the pressureless sintering of injection molded and debound aluminum nitride (AlN) samples were studied. Variations in the densification, microstructure, and properties owing to the increased powder content and reduced particle size are discussed. The results indicate the formation of liquid phase at 1500 °C in the bimodal micro (μ)–nano (n) AlN samples, a temperature that is at least 100 °C lower than typically reported values in the literature. Consequently, a densification ≥ 99% was achieved by pressureless sintering at a relatively lower temperature of 1650 °C with ∼14% isometric shrinkage. Additionally, thermal and mechanical properties of the sintered bimodal AlN samples are presented and compared with sintering studies on conventional monomodal μ-AlN systems reported in the literature.     

Synthesis and characteristics of nanocrystalline YSZ powder by polyethylene glycol assisted coprecipitation combined with azeotropic-distillation process and its electrical conductivity

Nanoscale 8 mol% yttria stabilized zirconia (YSZ) powders were prepared by polyethylene glycol (PEG-1540) assisted coprecipitation coupling with azeotropic distillation drying process. The role of PEG and azeotropic-distillation on the morphology and particle size of YSZ was studied. Thermogravimetry and X-ray diffraction results showed that azeotropic-distillation could reduce the formation temperature of YSZ phase. X-ray patterns of the YSZ powders revealed that the crystallite size of the powders increases with increasing calcination temperature, which is consistent with transmission electron microscopy observations. The sintering behavior and the ionic conductivity of the pellets prepared from YSZ powders calcined at 800 °C were also studied. At sintering temperatures ≥1400 °C, more than 99% of the relative density was obtained. The alternating-current impedance spectroscopy results showed that the YSZ pellet sintered at 1450 °C has ionic conductivity of 0.0726 S cm⁻¹ at 800 °C in air. The present work results have indicated that the PEG assisted coprecipitation combined with azeotropic-distillation drying process is an alternative method to synthesize yttria stabilized zirconia powders with a high sinterability and a good ionic conductivity.     

Lanthanum-titanium-aluminum oxide: A novel thermal barrier coating material for applications at 1300 °C

Considerable efforts are being invested to explore new thermal barrier coating (TBC) materials with higher temperature capability to meet the demand of advanced turbine engines. In this work, LaTi₂Al₉O₁₉ (LTA) is proposed and investigated as a novel TBC material for application at 1300 °C. LTA showed excellent phase stability up to 1600 °C. The thermal conductivities for LTA coating are in a range of 1.0-1.3 W m⁻¹ K⁻¹ (300-1500 °C) and the values of thermal expansion coefficients increase from 8.0 to 11.2 × 10⁻⁶ K⁻¹ (200-1400 °C), which are comparable to those of yttria stabilized zirconia (YSZ). The microhardness of LTA and YSZ coatings were in the similar level of ∼7 GPa, however, the fracture toughness value was relatively lower than that of YSZ. The lower fracture toughness was compensated by the double-ceramic LTA/YSZ layer design, and the LTA/YSZ TBC exhibited desirable thermal cycling life of nearly 700 h at 1300 °C.     

A new direct coagulation casting process for alumina slurries prepared using poly(acrylate) dispersant

Direct coagulation casting (DCC) of concentrated aqueous alumina slurries prepared using ammonium poly(acrylate) dispersant has been studied using MgO as coagulating agent. Addition of small amounts of MgO increased the viscosity of the concentrated alumina slurries with time and finally transformed it in to a stiff gel. Sufficient working time for degassing and casting could be achieved by cooling the slurries to a temperature of ∼5 °C after proper homogenization after the addition of MgO. The DCC slip with alumina loading in the range of 50–55 vol% showed relatively low viscosity (0.12–0.36 Pa s at shear rate of 93 s⁻¹) and yield stress (1.96–10.56 Pa) values. The wet coagulated bodies prepared from slurries of alumina loading in the range of 50–55 vol% had enough compressive strength (45–211 kPa) for handling during mould removal and further drying. The coagulated bodies prepared from slurries of alumina loading in the range of 50–55 vol% showed linear shrinkage in the range of 4.8–2.3 during drying and 17.1–16.2 during sintering respectively. Near-net-shape alumina components with density >98% TD could be prepared by the DCC process.     

Influence of powder characteristics on the behaviour of PIM feedstock

This work aims to analyse the impact of powders which are not conventionally intended for powder injection moulding (PIM) and how their characteristics influence the behaviour of the feedstock during mixing. Tests were performed with different alumina powders using the same binder system. The results show that mixing has a strong impact on the packing density of powders inside the feedstock, while the deagglomeration of powders makes it possible to achieve high critical powder volume concentrations (CPVCs) equal to or greater than 58 vol%. The CPVC depends on the deagglomeration efficiency. The agglomeration state – especially cohesion of the agglomerates – has an influence on the CPVC. The comparative study of mixing torques shows that the grain size and surface area of powders have a major impact on the mixing behaviour of the feedstock. During the implementation of powders, variabilities in the homogenisation of the powder/binder system and in deagglomeration are achieved as a result of powder agglomeration. It was demonstrated that the powders in this study perfectly satisfy the criteria imposed by the mixing process although they are not intended for PIM.