Flame‐assisted flash sintering: A noncontact method to flash sinter coatings on conductive substrates

We present a novel and effective method for sintering ceramic coatings onto metallic substrates. This new technique, called Flame‐assisted flash sintering (FAFS), utilizes a flame as both a heating source and a conformal, current‐carrying top electrode to facilitate flash sintering. Using this method, Yttria‐stabilized Zirconia (8 mol% Y, 8YSZ) coatings are sintered onto stainless steel substrates to controlled degrees of porosity in rapid fashion. Flame‐assisted flash sintering utilizes a dynamic soft electrode for flash sintering and has commercial potential to sinter ceramic coatings on complex‐shaped substrates for a variety of applications including tribological or thermal protection coatings.     

Solid-state sintering of core-shell ceramic powders fabricated by particle atomic layer deposition

The properties of technical ceramics are highly dependent on their microstructure, which evolves during sintering. Sintering is the process by which ceramic parts are subjected to high temperatures to activate chemical diffusion and the consumption of porosity. During the initial stage of sintering, interparticle necks between neighboring particles form and subsequently increase in size, consuming porosity as the particle centers move closer together. To experimentally determine how this process depends on particle surface composition, particle atomic layer deposition (ALD) was used to deposit a thin film of amorphous aluminum oxide (Al₂O₃) onto yttria-stabilized tetragonal zirconia (3YSZ) particles, producing core-shell structured powders. The uniformity of the Al₂O₃ film was confirmed with transmission electron microscopy and energy dispersive spectroscopy. Scanning electron microscopy was used to observe microstructural evolution during sintering, and the dihedral angles of Al₂O₃ and 3YSZ grains were measured to determine the ratio of interfacial energies between the 3YSZ|3YSZ, 3YSZ|Al₂O₃, and Al₂O₃|Al₂O₃ interfaces. Analysis of the densification kinetics revealed that the initial stage of densification is dependent on the material at the surface of the particles (ie, the Al₂O₃ film) and is controlled by the diffusion of Al³⁺ cations through Al₂O₃. Once the Al₂O₃ film has coalesced, the sintering behavior is controlled by the densification of the core material (3YSZ). Thus, core-shell powders fabricated by particle ALD sinter by a two-step process where the kinetics are dependent on the material present at interparticle contacts.     

Onset of selective laser flash sintering of AlN

Flash sintering uses a combination of heating and electric fields to rapidly densify ceramics. Previously, it has been shown that a scanning laser can be used to initiate flash sintering in localized regions on an yttria-stabilized zirconia (YSZ) sample in a process known as selective laser flash sintering (SLFS). In this work, we show using a combination of measurements of electric current flowing through the sample and observations of necks formed between powder particles that aluminum nitride (AlN) can also undergo SLFS. Scan conditions required to initiate SLFS are characterized over a range of laser powers and laser scan speeds in a dry nitrogen environment. It is shown that initiation of SLFS in AlN is governed by both the local input energy density per scan and heat dissipation and a numerical model is developed to predict temperatures during SLFS. Assuming the minimum temperature along the conductive path determines the onset of SLFS, the minimum temperature and time required is 450–670 K in 2–0.25 s for the pressed AlN pellets used in this study for laser scan speeds of 33–300 m/s, laser powers of 10–30 W, and an applied electric field of 3000 V/cm.     

Advances in the control of electrophoretic process parameters to tune the ytterbium disilicate coatings microstructure

Suspensions of ytterbium disilicate in isopropanol were prepared using iodine dispersant. Their zeta potential, electrical conductivity, and pH dependence with iodine concentration is detailed. Electrophoretic deposition was performed on silicon substrates at various voltages (100-200 V) and times (until 10 minutes) and the growth dynamic was investigated. It was observed that the deposited mass reaches a maximum value for [I₂] = 0.2 g/L, and the coating microstructure becomes porous at higher iodine concentrations. Current density and voltage measurements allowed to correlate this behavior to the increase of free protons concentration in the suspension. In these conditions, it was proved that porosity increases with the increase in applied voltage, and a compaction occurs as the deposition time increases. This has been related to the coating resistance increase and subsequent decrease in effective voltage in the suspension. The denser coatings (20% of porosity) were obtained in the case of suspension without iodine, at the minimum applied voltage and for the longest deposition times.     

Experimental and computational study on sintering of ceramic coating layers with complex porous structures

This study investigated the sintering behavior of an yttria-stabilized zirconia coating for thermal barrier coatings (TBCs) with a complicated porous structure via both experiment and simulation using the finite element method for samples with only a coating (free coating) and samples with coating on a substrate (constrained coating). Sintering and grain growth proceeded from the bottom of the coating, and the coating bent convex upward in the free coating. In the constrained coating, sintering and grain growth proceeded in a manner similar to the free coating; however, the degrees of sintering and grain growth were small. Furthermore, sintering and grain growth were delayed because of substrate constraints. As a simulation result, the free coating was bent in a manner similar to the experiment. The experimental results could be reproduced in terms of time dependency and temperature dependency. The decrease in the porosity of the constrained coating was delayed compared with that in the free coating because of substrate constraints. This simulation result was able to reproduce the experimental results. Thus, the sintering behavior for the complex porous structures of TBCs can be predicted by experimental research and simulation, which could aid in the development of a prediction technology for the delamination of coatings (TBC lifetime).     

Spark plasma sintering of silicon nitride/barium aluminum silicate composite

Si₃N₄ based composites were successfully sintered by spark plasma sintering using low cost BaCO₃, SiO₂ and Al₂O₃ as additives. Powder mixtures were sintered at 1600–1800 °C for 5 and 10 min. Displacement-temperature-time (DTT) diagrams were used to evaluate the sintering behavior. Shrinkage curve revealed that densification was performed between 1100 and 1700 °C. The specimen sintered at 1700 °C showed the maximum relative density (99.8±0.1%), flexural strength (352±16 MPa), Vickers harness (11±0.1 GPa) and toughness (5.6±0.05 MPa m^1/2).     

Steam oxidation of ytterbium disilicate environmental barrier coatings with and without a silicon bond coat

The current generation of multilayer Si/Yb₂Si₂O₇ environmental barrier coatings (EBCs) are temperature limited by the melting point of Si, 1414°C. To investigate higher temperature EBCs, the cyclic steam oxidation of EBCs comprised of a single layer of ytterbium disilicate (YbDS) was compared to multilayered Si/YbDS EBCs, both deposited on SiC substrates using atmospheric plasma spray. After 500 1-h cycles at 1300°C in 90 vol%H₂O-10 vol%air with a gas velocity of 1.5 cm/s, both multilayer Si/YbDS and single layer YbDS grew thinner silica scales than bare SiC, with the single layer YbDS forming the thinnest scale. Both coatings remained fully adherent and showed no signs of delamination. Silica scales formed on the single layer coating were significantly more homogeneous and possessed a markedly lower degree of cracking compared to the multilayered EBC. The single layer EBC also was exposed at 1425°C in steam with a gas velocity of 14 cm/s in an alumina reaction tube. The EBC reduced specimen mass loss compared to bare SiC but formed an extensive 2nd phase aluminosilicate reaction product. A similar reaction product was observed to form on some regions of the bare SiC specimen and appeared to partially inhibit silica volatilization. The 1425°C steam exposures were repeated with a SiC reaction tube and no 2nd phase reaction product was observed to form on the single layer EBC or bare SiC.     

Steam oxidation and microstructural evolution of rare earth silicate environmental barrier coatings

A primary failure mode for environmental barrier coatings (EBCs) on SiC ceramic matrix composites (CMCs) is the oxidation of the intermediate Si-bond coating, where the formation of SiO₂ at the bond coating–EBC interface results in debonding and spallation. This work compares the microstructure evolution and steam oxidation kinetics of the Si-bond coating beneath yttrium/ytterbium disilicate ((Y/Yb)DS) and ytterbium disilicate/monosilicate (YbDS/YbMS) EBCs to better understand the impact of EBC composition on oxidation kinetics. After 500 1-h cycles at 1350°C, (Y/Yb)DS displayed a decreasing concentration of the monosilicate minor phase and increasing concentration of porosity as furnace cycling time increased, whereas the YbDS/YbMS EBC displayed negligible microstructural evolution. For both EBC systems, thermally grown oxide growth rates in steam were found to increase by approximately an order magnitude compared to dry air oxidation. The (Y/Yb)DS EBC displayed a reduced steam oxidation rate compared to YbDS/YbMS.     

Combustion synthesis of fine aluminum nitride powders under micropositive N2 pressure with water additive

Fine aluminum nitride (AlN) powders were prepared by a facile and efficient way of combustion synthesis under micropositive N₂ pressure of 0.15 MPa and with 3 wt% water as additive. By this approach, the maximum combustion temperature was well regulated to a low value. The influence of water on the reaction rate, the phase composition, and crystal growth of the products was systematically investigated. The addition of water was crucial to the complete nitridation of Al. Furthermore, H₂O vapor played a two-sided role in the reaction. It could accelerate the reaction by promoting the diffusion of Al vapor and N₂ and restrain the nitridation rate by absorbing heat.     

Effects of two-step sintering on thermal and mechanical properties of aluminum nitride ceramics by impedance spectroscopy analysis

The effects of two-step sintering on the microstructure, mechanical and thermal properties of aluminum nitride ceramics with Yb₂O₃ and YbF₃ additives were investigated. AlN samples prepared using different sintering methods achieved almost full density with the addition of Yb₂O₃–YbF₃. Compared with the one-step sintering, the grain sizes of AlN ceramics prepared by the two-step sintering were limited, and the higher flexural strength and the larger thermal conductivity were obtained. Moreover, the electrochemical impedance spectroscopy of AlN ceramic was associated with thermal conductivity by analyzing the defects and impurities in AlN ceramics. The fitting grain resistance and the activation energy for the grain revealed the lower concentrations of aluminum vacancy in the two-step sintered AlN ceramics, which resulted in the higher thermal conductivity. Thus, mechanical and thermal properties for AlN ceramics were improved with Yb₂O₃ and YbF₃ additives sintered using two-step regimes.     

Gas Pressure Sintering of Silicon Nitride Powder Coated with Al2O3 and TiO2

Gas pressure sintering kinetics of silicon nitride powder coated with 10 wt% (9:1) Al₂O₃ and TiO₂ have been studied at 1850°C with a pressure schedule of 0.3 MPa in the first stage and 1 MPa in the second stage. The rates have been analyzed with a liquid-phase sintering model. Diffusion-controlled intermediate-stage kinetics have been observed. The role of second-step pressurization with nitrogen and argon has been determined by monitoring the kinetics. Pressurization at an earlier stage (∼90% relative density) reduces the densification rate but produces a denser material at the final stage. Although final density is greater, a porous surface layer forms on samples sintered with argon pressurization at the second stage. No such porous layer is formed in the case of pressurization with nitrogen. The mechanism of the intermediate-stage kinetics has been discussed with respect to the nature of the product analyzed by XRD after sintering.     

Nitrate fusion synthesis and two-step sintering of nanocrystalline yttria stabilized hafnia powders

Bulk quantities of nanocrystalline yttria stabilized hafnia (YSH) powders with crystallite size ranging from 8 to 15 nm were successfully prepared for the first time through nitrate fusion synthesis at a temperature as low as 673 K. The yttrium content was varied from 6 to 30 mol%. The dependence of the properties of the final product on the quantity of the dopant was investigated. Microstructural investigations were carried out with scanning electron microscopy and transmission electron microscopy. A maximum relative sintered density of 98.2 ± 0.3% T.D (theoretical density) was obtained for YSH containing 10 mol% yttrium by using “two-step sintering” at a final temperature of 1773 K. Anisotropic shrinkage factor (0.70–0.95) was found to vary linearly with the compaction pressure. SEM investigations reaffirmed that the sintered pellets comprised uniform distribution of faceted grains and elemental mapping revealed that yttrium is distributed uniformly in these sintered YSH monoliths.     

Thin coatings of hafnon abate oxidative recession of SiC fibers

The oxidation behavior of SiC fibers coated with (a) undoped polysilazane and (b) precursors containing a mixture of polysilazane and hafnium butoxide in equal weight fractions, is reported. The coatings were prepared by repetitive cycles of nanolayer depositions, as reported in recent publications. The oxidation experiments were carried out at 1400°C in ambient air (Boulder, CO) for up to 100 hours. The extent of degradation of SiC was measured by the recession in the diameter of the fibers as a function of time. The fibers with undoped polymer precursor recessed significantly, whereas the fibers coated with HfSiCNO remained essentially unchanged. These results are in agreement with earlier work from our laboratory where the resilience of hafnon and zircon, as well as hafnia and zirconia, against high-temperature corrosion in streaming humid environments had been highlighted.     

Thermochemical stability of Y2Si2O7 in high-temperature water vapor

The thermochemical stability of Y₂Si₂O₇ was assessed in a high-temperature high-velocity water vapor environment to improve the understanding of the mechanisms that lead to SiO₂ depletion. Spark plasma sintered Y₂Si₂O₇ specimens were exposed in a steam-jet furnace at 1000°C and 1200°C for 3-250 hours, steam velocities of 131-174 m/s and at 1 atm H₂O pressure. These exposures resulted in the selective volatilization of SiO₂ to form volatile Si(OH)₄ and porous Y₂SiO₅. Microstructural evolution from fine rectangular pores at short times to larger rounded pores at longer times was observed. Mechanisms contributing to the overall depletion reaction kinetics were evaluated and include the interface reaction to form Y₂SiO₅ and Si(OH)₄ (g), Y₂SiO₅ coarsening, development of tortuosity in the pore network and diffusion of H₂O (g) and Si(OH)₄ (g) through pores by molecular diffusion and/or Knudsen diffusion. SiO₂ depletion was found to follow parabolic volatilization kinetics (k_p = 0.38 µm²/h) at 1200°C indicating the reaction is limited by a diffusion process, most likely the outward diffusion of Si(OH)₄ (g) through pores. Results are utilized to assess the viability of Y₂Si₂O₇ and other rare-earth silicates as environmental barrier coating (EBC) materials for SiC ceramic matrix composites (CMCs).     

Sintering of AlN Using CaO-Al2O3 as a Sintering Additive: Chemistry and Microstructural Development

The densification of aluminum nitride using Ca₁₂Al₁₄O₃₃ as a sintering aid has been studied with emphasis on the effect of using coarse or fine powder, the amount of sintering aid, the sintering temperature, and embedding. Both crystalline and amorphous grain boundary phases were observed. Significant weight losses were observed for coarse-grained samples, and if suitable embedding was not used. Porous and coarse-grained ceramics with high contiguity and minor amounts of secondary phases were obtained by enhanced evaporation while dense ceramics were obtained limiting the evaporation. High weight losses in the graphite environment resulted in formation of a dense AlN surface layer.     

Phase stability and tensorial thermal expansion properties of single to high-entropy rare-earth disilicates

Temperature limitations in nickel-base superalloys have resulted in the emergence of SiC-based ceramic matrix composites as a viable replacement for gas turbine components in aviation applications. Higher operating temperatures allow for reduced fuel consumption but present a materials design challenge related to environmental degradation. Rare-earth disilicates (RE₂Si₂O₇) have been identified as coatings that can function as environmental barriers and minimize hot component degradation. In this work, single- and multiple-component rare-earth disilicate powders were synthesized via a sol-gel method with compositions selected to exist in the monoclinic C 2/m phase (β phase). Phase stability in multiple cation compositions was shown to follow a rule of mixtures and the C 2/m phase could be realized for compositions that contained up to 25% dysprosium, which typically only exists in a triclinic, P $\overline{1}$, phase. All compositions exhibited phase stability from room temperature to 1200°C as assessed by X-ray diffraction. The thermal expansion tensors for each composition were determined from high-temperature synchrotron X-ray diffraction and accompanying Rietveld refinements. It was observed that ytterbium-containing compositions had larger changes in the α₃₁ shear component with increasing temperature that led to a rotation of the principal axes. Principal axes rotation of up to 47° were observed for ytterbium disilicate. The results suggest that microstructure design and crystallographic texture may be essential future avenues of investigation to ensure thermo-mechanical robustness of rare-earth disilicate environmental barrier coatings.     

有机羧酸改性氮化铝粉体的抗水解性能

采用有机羧酸和PEG作为表面活性剂对工业AlN粉体进行表面改性处理,研究了改性前后AlN粉体的抗水解特性以及表面改性机制。研究结果表明,AlN表面与水分子发生化学反应,导致溶液的pH值迅速提高,表面包覆有机羧酸可有效地改善AlN粉体的抗水解性能;AlN粉体在水中浸泡48h后,氮含量基本保持不变,除了AlN晶相外,没有其他晶相出现,且改性粉体在高剪切应力的水基球磨介质中也保持较强的稳定性。     

Flash sintering of yttria-stabilized zirconia powders coated with nanoscale films of alumina by atomic layer deposition

The addition of small quantities of aluminum oxide (Al₂O₃) to 8 mol% yttria-stabilized zirconia (8YSZ) benefits conventional sintering by acting as a sintering aid and altering grain growth behavior. However, it is uncertain if these benefits observed during conventional sintering extend to flash sintering. In this work, nanoscale films of Al₂O₃ are deposited on 8YSZ powders by particle atomic layer deposition (ALD). The ALD-coated powders were flash sintered using voltage-to-current control and current rate experiments. The sintering behavior, microstructural evolution, and ionic conductivities were characterized. The addition of Al₂O₃ films changed the conductivity of the starting powder, effectively moving the flash onset temperature. The grain size of the samples flashed with current rate experiments was ~65% smaller than that of conventionally sintered samples. Measurement of grain size and estimates of sample density as a function of temperature during flash sintering showed that small quantities of Al₂O₃ can enhance grain growth and sintering of 8YSZ. This suggests that Al₂O₃ dissolves into the 8YSZ grain boundaries during flash sintering to form complexions that enhance the diffusion of species controlling these processes.     

Tailorable thermal expansion in leucite-pollucite materials derived from geopolymers for environmental barrier coatings

The K[AlSi₂O₆]-Cs[AlSi₂O₆] pseudo-binary system was synthesized by geopolymer crystallization. The thermal expansion properties of these materials were studied by in situ high-temperature X-ray diffraction to characterize thermal expansion behavior for potential application as environmental barrier coatings. Tailorable thermal expansion through changing cation stoichiometry allowed reduced thermal expansion mismatch with SiC_f/SiC composites compared to rare-earth-based coatings.     

Interactions of Na2SO4 and yttrium-silicate environmental barrier coatings at 825°C

Yttrium-silicates (Y₂Si₂O₇ and Y₂SiO₅) are candidate environmental barrier coating (EBC) materials for silicon carbide ceramic matrix composites (SiC-CMCs). These materials’ high-temperature, high-velocity steam, and siliceous debris resistance are well studied. However, Na₂SO₄-induced hot corrosion mechanisms are less understood. Free-standing atmospheric plasma sprayed Y₂Si₂O₇ and Y₂SiO₅ coupons were exposed to 2.5 mg/cm² of Na₂SO₄ at 825°C in 0.1% SO₂-O₂ (g). Scanning electron microscopy, X-ray diffraction, transmission electron microscopy, and inductively coupled plasma-optical emission spectrometry were used to identify a previously unknown damage mechanism. Water-soluble Y and Na-Y sulfates and oxysulfates formed in reaction with Na₂SO₄, causing significant damage to the yttrium-silicate EBCs materials.