HfO2 Particles Research Articles

Effects of raw materials on microstructure and mechanical properties of second phase particles reinforced W[sbnd]Re composites

Combined strengthening of micron and nanoscale second phase particles is of great significance for improving the comprehensive properties of tungsten (W) based refractory alloys. In this work, with the addition of raw HfC, HfH2 and carbon powders, three types of W-3 wt%Re-0.3 wt%HfC (WRH) composites were fabricated by powder metallurgy and rotary swaging. The microstructure and mechanical properties of three WRH samples were investigated comparatively. Results show that three WRH samples are constituted by W matrix, HfO2 and HfC second phase particles. When HfH2 powder is served as the raw material, the obtained two kinds of composites show the microstructure configuration with a micro-nano combined second phase particles. That is, some larger HfO2 particles are located at the matrix grain boundaries and fine nanoscale HfC particles are inside W matrix grains. Based on it, both composites exhibit the better overall performance. When the carbon powder ratio reaches 0.03 wt%, WRH composite shows the smallest average matrix grain size and a smaller size of second phase particles inside the matrix grains, and thereby gaining the better ultimate tensile strength and hardness. As carbon powder ratio rises to 0.07 wt%, WRH sample presents an excellent elongation. Besides, the formation mechanisms of HfO2 and HfC particles, as well as the correlation between microstructure and mechanical properties, were discussed particularly.

Research on Microstructure, Mechanical Properties, and High-Temperature Stability of Hot-Rolled Tungsten Hafnium Alloy.

W-(0, 0.1, 0.3, 0.5) wt.% Hf (mass fraction, wt.%) materials were fabricated by the powder metallurgy method and hot rolling. The microstructure, mechanical properties, and high-temperature stability of alloys with varying compositions were systematically studied. The active element Hf can react with the impurity O segregated at the grain boundary to form fine dispersed HfO2 particles, refining the grains and purifies and strengthening the grain boundary. The average size of the sub-grains in the W-0.3 wt.% Hf alloy is 4.32 μm, and the number density of the in situ-formed second phase is 6.4 × 1017 m-3. The W-0.3 wt.% Hf alloy has excellent mechanical properties in all compositions of alloys. The ultimate tensile strength (UTS) is 1048 ± 17.02 MPa at 100 °C, the ductile fracture occurs at 150 °C, and the total elongation (TE) is 5.91 ± 0.41%. The UTS of the tensile test at 500 °C is 614 ± 7.55 MPa, and the elongation is as high as 43.77 ± 1.54%. However, more Hf addition will increase the size of the second-phase particles and reduce the number density of the second-phase particles, resulting in a decrease in the mechanical properties of the tungsten alloy. The isochronal annealing test shows that the recrystallization temperature of W-Hf alloy is 1400 °C, which is 200 °C higher than rolling pure tungsten.

Effect of β phase size on the microstructure of thermally grown oxide in water vapor atmosphere at 1100 °C

The arc-melted and vacuum-casted CoNiCrAlHf alloys with a 7YSZ top coat were designed to develop a protective alumina scale at 1100 °C in water vapor content. Arc-melted CoNiCrAlHf alloys provided substantial improvements in oxidation resistance. In this work, the oxidation behavior and void distribution of thermally grown oxide (TGO) formed on CoNiCrAlHf alloys were analyzed. The Al2O3 crystal structure and void distribution were studied as a function of the grain size of the CoNiCrAlHf substrate. Numerous voids developed in the oxide scale, and coarse HfO2 particles segregated at the Al2O3 grain boundaries of vacuum-cast CoNiCrAlHf alloys. The uniformly distributed HfO2 particles prevented void formation in the oxide scale. As a result, the phase transformation of Al2O3 was effectively impeded in water vapor.

The influence of weld fluid flow on the formation mechanism of intermetallics and HfO2 precipitates in dissimilar electron beam welding of C-103 niobium alloy to nickel

In this research, the effect of weld fluid flow in molten pool on the formation and behavior of intermetallic compounds NbNi3 and Nb7Ni6 and HfO2 precipitates in dissimilar electron beam welding of niobium base alloy C-103 to nickel was investigated. It was observed that due to the large difference in the melting temperature of the base metals, nickel contributed 82% by volume and C-103 contributed 18% by volume in weld zone. In the weld zone and attached to the C-103 base metal, a thin and continuous intermetallic zone was formed, including the initial phase of NbNi3 and the eutectic structure of NbNi3+Nb7Ni6. In the areas after the continuous intermetallic layer in welding, the initial phase of NbNi3 and the eutectic structure of NbNi3+γ-Ni are formed. During welding, HfO2 particles in the C-103 base metal enter the molten pool due to the weld fluid flow and remain as white precipitates in the welding zone and close to C-103. In addition to these precipitates, nanometer precipitates of HfO2 are also observed in the weld, which are formed in-situ in the weld due to the reaction of hafnium with dissolved oxygen in the melt. In the central zone of the weld, continuous band-shaped intermetallic layers can be observed which have been removed from the wall of C-103 due to the fluid flow of the molten pool and moved towards the central zone of the weld. These layers act as nucleus in the center of the molten pool and solidification occurs from their surface. At first, solidification begins with the planar growth of the initial phase of NbNi3, and then cellular and columnar dendritic growth occurs. At the end of solidification, the eutectic structure of NbNi3+γ-Ni forms between cells and dendritic branches. Due to the presence of intermetallic compounds NbNi3 and Nb7Ni6 as well as HfO2 nanometer precipitates in the weld, the hardness of the weld zone was higher than that of the base metals.

HfC[sbnd]HfO2[sbnd]SiO2 nanocomposite ceramic coating formed on TiAlNb medium-entropy alloy by LPDS: Microstructure evolution and ablation behavior

HfCHfO2SiO2 nanocomposite ceramic coating was prepared on the TiAlNb medium-entropy alloy by a novel one-step liquid plasma-assisted particle deposition sintering (LPDS) method to improve the ablation resistance. Meanwhile, PEO coatings were prepared as a comparison at different voltages (400 V, 450 V, 500 V). When the PEO coating fabricated at 500 V was ablated for 360 s, the peeling area was the largest at 41.05 %, which was 2.3 times larger than the PEO coating (400 V, 18.03 %). However, the HfCHfO2SiO2 nanocomposite coating exhibits no surface peeling. The surface particles of the coating were sintered together, reducing the presence of pores and enhancing the structure density. Additionally, a large amount of HfO2 and stable HfSiO4 particles were embedded in the amorphous SiO2 layer, which improved the melting point and stability of the glass layer while reducing oxygen permeability. The formed HfSiO4 after ablation acted as “nails”, effectively suppressing the generation of cracks.

Effect of HfO2 Particles on Ceramic Coating Fabricated on Ti6Al4V Alloy via Plasma Electrolytic Oxidation

Hafnium dioxide (HfO2) has a wide bandgap and high dielectric constant. We prepared ceramic coatings on Ti6Al4V alloys via plasma electrolytic oxidation (PEO) in an electrolyte with HfO2 particles. The influence of the HfO2 particles on the microstructure, phase composition, elemental distribution, and corrosion resistance of the PEO coatings was systematically investigated. The results showed that the addition of HfO2 increased the oxidation voltage (from 462 to 472 V) and promoted the microarc sintering reaction so that the thickness and hardness of the resulting PEO coating increased. Moreover, the quantity of the micropores on the coating surface caused by the discharge decreased after adding the HfO2 particles. The X-ray diffraction patterns confirmed that the HfO2 particles were incorporated into the coating by remelting and sintering the microarc. Furthermore, the corrosion resistance of the PEO coating was remarkably increased after introducing HfO2, which was attributed to the increase in the electrode potential and the densification of the coating structure.

Ablation behavior of C/C–Zr1-xHfxC-SiC composites under an oxyacetylene flame at above 2500 °C

In order to enhance ablation resistance of C/C composites, Zr1-xHfxC and SiC were introduced into C/C composites, and ablation behavior of the composites was studied. In the composites (CHZS), HfC and ZrC produced a solid solution, and solid solubility of HfC in ZrC was 0.4, which indicated that the solid solution was defined to be Zr0.6Hf0.4C. Compared with C/C–ZrC–SiC composites (CZS), CHZS possessed a better ablation resistance. During ablation of CHZS, Zr0.6Hf0.4C is oxidized to produce HfO2 and ZrO2, and HfO2 particles played a pinning effect, which fixed molten oxide coating. Moreover, due to low CTE of HfO2 and decreasing content of ZrO2, there was a smaller thermal stress and its distribution area on CHZS ablation surface, thus thermal stress corrosion was weakened to strengthen integrity of oxide coating. In this case, a complete and continuous HfO2-ZrO2 coating was formed on CHZS surface, which could resist oxygen diffusion and reduce oxidation rate of the composites.

Effects of Stepped Heating on the Initial Growth of Oxide Scales on NiCrAlHf Bond Coat Alloy under Air and Water Vapor Atmospheres.

Temperature and atmosphere have a significant effect on the oxidation of MCrAlY (M = Ni, Co) bond coating. The initial growth behavior of the NiCrAlHf bond coat alloy was investigated at 1100 °C under different atmospheric conditions and using heating methods. A thick Al2O3 oxide layer and large HfO2 particles were observed, perhaps due to metastable oxide growth at low temperatures when using stepped heating. However, in air and water vapor atmospheres, the oxide scale was thinner and the HfO2 precipitates were smaller in stepped heating than in constant heating. The size and distribution of the HfO2 particles might have induced different microstructures, particularly voids within the oxide scale.

Effect of HfO2 framework on steam oxidation behavior of HfO2 doped Si coating at high temperatures

HfO2 doped Si is designed as bond coat material in thermal/environmental barrier coatings (TEBCs). In this work, the HfO2-Si composite coatings with different HfO2 contents were prepared by atmospheric plasma spraying (APS). The steam oxidation behavior of the coatings was comparatively studied at 1300 °C and 1400 °C. Volatilization of Si occurred during spraying, leading to the deviation of coating compositions. The sprayed coatings contained different HfO2 structures. During steam oxidation, HfSiO4 phase was formed at the SiO2/HfO2 interface by solid-state reaction between them. The HfSiO4 or HfO2/HfSiO4 mixture particles worked to deflect or pin micro-cracks, thus improving the resistance of the coating to cracking. At 1300 °C, a protective oxide scale was formed on the traditional Si coating or the HfO2-Si coating with isolated HfO2 particles. However, the HfO2-Si coating with inter-connected HfO2 framework revealed poor oxidation-resistance. At 1400 °C, accelerated oxidation degradation, steam corrosion volatilization, interface reaction and sintering occurred. The HfO2 framework structure played a dominating role in determining the steam oxidation resistance of the HfO2-Si coating, and the connected HfO2 framework and TGO network provided a rapid diffusion path for oxidants (H2O, O2− and OH−) and deteriorated the oxidation resistance.

The influence of Hf content on oxide scale microstructure and HfO2 formation mechanisms in two-phase (γ' + β) Ni-Al alloys

Hafnium is often doped into alumina-forming alloys to enhance their oxidation resistance, though the effect varies with the doping amount. In this paper, two-phase (γ' + β) Ni-30Al alloys with different Hf contents were prepared and the cyclic oxidation behaviour at 1100 °C was investigated. The oxidation rate increased remarkably with the increase of Hf content, because excessive Hf leads to severe internal oxidation. And it is interesting to note that oxide scales that formed on the alloys exhibited very different microstructures. On the Ni-30Al-0.1Hf alloy the scale mainly consisted of single α-Al2O3. But on the Hf over-doped alloys (Hf > 0.1 at.%), the scales microstructure varied with locations. The scale in-situ formed on β phase consisted of an outer spinel layer and an inner α-Al2O3 layer; while, the one formed on γ' phase consisted of an outmost NiO layer, a middle spinel layer dispersed with fine HfO2 particles and an inner α-Al2O3 layer. Moreover, Hf-rich oxide pegs were prone to be formed at scale/alloy interface on original γ' phase. However, when Hf content reached to 2 at.%, the pegs were also found at scale/alloy interface on original β phase. The different oxidation behaviour and peg formation mechanisms of the alloys are discussed.

Comparative study on microstructure and performance of sintered, forged and annealed W-3Re-HfC composites

By the powder metallurgy, as well as subsequent forging and annealing processes, a series of W-3Re-HfC (WRH) composites were fabricated successively. Microstructure and performance of WRH composites were investigated comparatively. It is found that the sintered, forged and annealed WRH composites are consistently made up of W-Re matrix, HfO2 and HfC second phase particles. The larger HfO2 particle is derived from the oxidation of HfH2 powder, playing an important role in purifying grain boundary and refining grain of matrix. The smaller HfC particle inside W-Re grains has a dispersion reinforcement effect. The forged WRH composites can obtain the highest relative density (99.9%), the micro-hardness (534.1 HV) and ultimate tensile strength (758 MPa), which is ascribed to the refinement of grains and dispersion strengthening. Although the performance of annealed WRH composites has a slight drop, an excellent interfacial matching between second phase particles and W-Re matrix can be produced, which facilitates to gain a better interfacial strength and homogenized microstructure.

Accelerated sintering of high-performance oxide dispersion strengthened alloy at low temperature

Powder metallurgy generally is a preferred method to consolidate and prepare some refractory metals considering their high melting temperatures. However, sintering such materials to high density while maintaining fine grain structure is challenging, and generally require ultrahigh temperature or need to be assisted by external pressure. Especially, the preparation of nanostructured oxide dispersion strengthened (ODS) refractory metals at low temperature is more difficult because grain coarsening and densification are simultaneously hindered by the common oxide addition. Thus, it is of great sense to explore a kind of oxide that can be used as both strengthening phase and sintering accelerator. In this work, after extensive theoretical calculation and experimental observation, it is found that the accelerated sintering at low temperature can be achieved in W-HfO2 system. In the absence of external pressure or activator, this system can be rapidly densified without undesirable grain coarsening at a relative low temperature of only 1480 ˚C. Through kinetic analysis and density functional theory (DFT) calculation, it was confirmed that the intergranular HfO2 particles provide a rapid diffusion transport pathway for W atoms in support of densification at temperature of interest. As a result, this alloy exhibits a homogeneous microstructure with ∼97% relative density and ∼200 nm grain size, which renders our alloy with an ultrahigh compressive strength of ∼3 GPa, much higher than the reported values. We believe that the accelerated sintering mechanism found in our W-HfO2 system can be applicable to other refractory metals to prepare high-density nanocrystalline alloys with excellent properties.

Interfacial oxidation of hafnium modified NiAl alloys

The high-temperature oxidation of Hf-modified NiAl alloy at 950 °C results in the formation of γ-Al2O3 and θ-Al2O3. Exclusive formation of HfO2 particles is observed at the adherent γ-Al2O3/NiAl interface. By contrast, the θ-Al2O3/NiAl interface oxidation results in multi-layered oxide growth consisting of alternate thin and thick layers of θ-Al2O3 and NiAl2O4 along with dispersed HfO2 particles in the oxide layers. This difference is elucidated by first-principle calculations showing that the θ-Al2O3 lattice is more favorable than γ-Al2O3 to host Hf and Ni atoms, thereby resulting in the growth of mixed oxides at the θ-Al2O3/NiAl interface.

Effects of Hf additions on the microstructures and mechanical properties of Ni-rich Ni55Ti45 alloys

The microstructural characterization and mechanical properties of the oil quenched binary Ni55Ti45 alloy and Hf-modified Ni55Ti45-xHfx (x = 3, 5, 7 at.%) alloys were investigated. The results showed that the microstructure of binary alloy is composed of NiTi phase, nanoscale Ni4Ti3 precipitates, and a small amount of Ti4Ni2O and Ni3Ti2 phases. The Hf additions suppresses the formation of the Ti4Ni2O and Ni3Ti2 phases and promotes the HfO2 particles. When the Hf content is less than 5 at.%, the main precipitated phase is still Ni4Ti3, but which is gradually refined and spheroidized with the increase of Hf content. While the Ni4Ti3 phases disappeared and the fine H-phases precipitated massively in the Ni55Ti38Hf7 alloy. With the gaining content of Hf, the Vickers hardness increases, but the fracture toughness and tensile strength increase first and then decrease. When the addition of Hf is 3 at.%, the alloy possesses the best property with the fracture toughness of 21.02 MPa/m1/2 and the tensile strength of 2.25 GPa.

Mechanical properties and ablation resistance of La2O3-modified HfC-SiC coating for SiC-coated C/C composites

Plasma-sprayed La2O3-modified HfC-SiC (HSL) coating was prepared on SiC-coated carbon/carbon composites to obtain an elevated ablation resistance under a heat flux of 2.38 MW/m2. The elastic modulus of sprayed HSL coating is about 140.97 GPa, 41.48 % greater than HfC-SiC (HS) coating. HSL coating also performed superior ablation resistance evidenced by integrated scale and stable ablation rates. Besides, the elastic modulus and adhesion strength of ablated HSL coating gained 65.9 % and 26.9 % increase compared with HS coating. Its reliable service performance is primarily attributed to the formed scale constituted by HfO2 and La2Hf2O7 particles encapsulated by La-Si-O glass.

Hafnium silicate formation during oxidation of a permeable silicon + HfO2 powder composite system

Silicon bond coats, applied to silicon carbide composites used in gas turbine engines as a part of an environmental barrier coating (EBC) system, arrest the transport of oxidizing species to the SiC composite by forming a SiO2 thermally grown oxide. Above 1200°C, a crystalline ß-cristobalite SiO2 phase is formed that subsequently undergoes a large thermal contraction during its ß→α-cristobalite transformation upon cooling, leading to oxide layer cracking and eventual EBC delamination. A model system of silicon particles in a highly porous HfO2 particle matrix is used to investigate the rate and mechanism by which ß-cristobalite SiO2 is dynamically converted to hafnium orthosilicate (HfSiO4; Hafnon) when oxidizing silicon is in contact with m-HfO2. The study compares the net rate of SiO2 formation in the model silicon + HfO2 system with that on only silicon particles, and finds that the thickness of the SiO2 layer that forms during high temperature (1250–1316°C) oxidation can be greatly reduced by the hafnon formation reaction. The study also investigates the mechanism by which hafnon forms, and reports a SiO2 rumpling process when its outward (radial) growth is locally constrained by HfO2 particles. Since hafnon is phase stable over the temperature range of EBC interest, and has a thermal expansion coefficient similar to silicon, the dynamic conversion of SiO2 to hafnon provides a promising approach for controlling the delamination of EBC systems that use silicon bond coats.

Hafnium silicate formation during the reaction of β‐cristobalite SiO2 and monoclinic HfO2 particles

AbstractHafnium silicate (HfSiO4; hafnon) is under consideration as an environmental barrier coating material for high‐temperature applications. However, its rate of formation from mixtures of monoclinic HfO2 and crystalline (β‐cristobalite) SiO2 powders is unknown. Here it has been synthesized and its formation rate measured during their solid‐state reaction at temperatures from 1250°C to 1400°C. Rietveld refinement of X‐ray diffraction patterns indicates that at 1250°C the hafnon phase fraction increases linearly with time, while at the highest reaction temperature, the hafnon phase fraction exhibited a parabolic dependence upon time. Between these two limiting temperatures, a region of linear behavior preceded a transition to parabolic kinetics, with the transition occurring at an earlier time as the reaction temperature increased. Arrhenius relations fitted the kinetics of hafnium silicate formation in both the linear and parabolic regimes. Scanning electron microscopy indicated that the reaction proceeded by diffusion of SiO2 into HfO2, similar to the mechanism by which zirconium silicate has been formed from vitreous SiO2 and tetragonal ZrO2. The initial linear rate of reaction is consistent with the growth of the contact area between the SiO2 and HfO2 particles combined with rapid permeation of the Si4+ and O2− through the initial, incompletely formed hafnon. After a thin hafnon layer had formed between the reactants, the rate of hafnium silicate growth slowed and further growth was governed by the rate of diffusion of Si4+ and O2− through the reaction product consistent with the observed parabolic dependence of the phase fraction upon time.

Mn-modified HfO2 nanoparticles with enhanced photocatalytic activity

Round shaped Mn-modified HfO2 nanoparticles were prepared by the Pechini type sol-gel method. The effects of Mn ions on the structure, particle growth, composition, optical properties and photocatalytic performance of HfO2 particles were investigated. The structure analysis revealed that the insertion of Mn2+, Mn3+, and Mn4+ ions inhibited the complete stabilization of tetragonal HfO2. Also, the decrease of particle size to values lower than 5 nm and the shift of optical band gap from 5.7 to 2.1 eV was obtained as effect of increasing the Mn content. HfO2 nanoparticles modified with 10 w% Mn exhibited the highest photocatalytic performance, reaching an efficiency of 91.93% in the decomposition of methylene blue, after 120 min of sunlight irradiation. The efficient trapping of photogenerated electrons on the surface of these nanoparticles generated •O2− radicals, which were the main oxidative species involved in the degradation of the dye. The improved photocatalytic performance of these nanoparticles is then attributable to their increased surface area, suitable photoactivation, and effective transport of photoexcited charge carriers. Based in studies of band gap, valence band position and active species, a mechanism of photodegradation for this photocatalyst was also proposed and discussed.

Microstructure and anti-ablation performance of HfC-TaC and HfC-ZrC coatings synthesized by CVD on C/C composites

HfC-TaC and HfC-ZrC biphase coatings were synthesized on C/C composites through CVD technique with the aim of elevating the anti-ablation performance of the HfC monophase coating. The phase composition, microstructure and anti-ablation performance of the biphase coatings were researched, as well as the ablation mechanism of the coated C/C samples. Results manifested that both the HfC-TaC and HfC-ZrC coatings exhibited a compact and particle-stacked structure and combined well with the C/C substrate. Incorporating TaC is capable of enhancing the anti-ablation performance of the HfC coating by reason of forming the stable Hf6Ta2O17 phase during ablation. The evaporation and mechanical denudation of the Hf-Ta-O glassy film leaded to the HfC-TaC coating exfoliation. In comparison to the HfC and HfC-TaC coatings, the HfC-ZrC coating exhibited superior anti-ablation performance on account of the solid solution sintering between the HfO2 and ZrO2 particles and the good anti-scour performance of the oxide layer. After exposure to oxyacetylene torch for 120 s, the mass and linear ablation rates of the HfC-ZrC-coated C/C sample were only 0.63 mg/s and 0.21 μm/s, respectively.

Effect of residual excess carbon on the densification of ultra-fine HfC powder

The residual carbon content of ultra-fine hafnium carbide (HfC) powder was controlled by the optimization of the synthesis process, and the effect of residual carbon on the densification of HfC powder was analyzed. The amount of residual carbon in the HfC powder could be reduced by the de-agglomeration of HfO2 powder before the carbo-thermal reduction (CTR) process. The average particle size of HfO2 powder decreased from 230 to 130 nm after the de-agglomeration treatment. Ultra-fine (d50: 110 nm) and highly pure (metal basis purity: >99.9 % except for Zr) HfC powder was obtained after the CTR at 1600 °C for 1 h using the C/Hf mixing ratio of 3.3. In contrast, the C/Hf ratio increased to 3.6 without the de-agglomeration treatment, indicating that a large amount of excess carbon was required for the complete reduction of the agglomerated HfO2 particles. HfC ceramics with high relative density (>98 %) were obtained after spark plasma sintering at 2000 °C under 80 MPa pressure when using the HfC powder with low excess carbon content. In contrast, the densification did not complete at a higher temperature (2300 °C) and pressure (100 MPa) when the HfC powder contained a large amount of residual carbon. The results clearly indicated that residual carbon suppressed the densification of HfC powder in case the carbide powder had low oxygen content, and the residual carbon content could be controlled by the optimization of the synthesis process. The average grain size and Vickers hardness of the sintered specimen were 6.7(±0.7) μm and 19.6 GPa, respectively.