Hafnium Carbide Powders Research Articles

Синтез порошка карбида гафния в атмосферной дуговой плазме

The paper presents a method, implemented for the first time, for the thermal synthesis of hafnium carbide powder using a DC arc discharge initiated in the open-air atmosphere. Based on the results of the series of experiments, the dependences of the current strength of the power source and the time of thermal treatment on the phase composition of the resulting powder product were established. Required parameters have been determined to ensure the synthesis of a powder containing ~ 98 wt. % of the cubic phase of hafnium carbide: heat treatment of the initial mixture containing the stoichiometric ratio of hafnium to carbon for 60 s at a current of 220 A. The size, shape, and substructure of particles of the synthesized carbide are characterized. The differential thermal analysis carried out in an oxidizing medium showed that the obtained hafnium carbide powder is oxidized most intensively at a temperature of ~700°C.

Improvement of Ti6Al4V-Alloy Wear Resistance by Electric-Spark Hafnium Carbide Coatings

In this paper, we obtain cermet coatings with a thickness of 19 to 22 μm by the method of electrospark treatment of titanium alloy Ti6Al4V in a mixture of titanium granules with hafnium carbide powder. An increase in the volume fraction of hafnium carbide powder in the mixture of granules from 2.4 to 6.7 vol % led to an increase in the concentration of the hafnium carbide phase HfC in the coatings. The coating structure is represented by a metallic Ti–Hf–C binder with large inclusions of hafnium carbide. The average microhardness of Ti–HfC coatings was in the range of 7.1–8.3 GPa, which is 2–2.5 times higher than that of the Ti6Al4V alloy. Wear tests in dry sliding mode showed that Ti–HfC coatings had a wear rate in the range from 3.18 × 10–6 to 1.5 × 10–5 mm3/(N m), which is 25 to 250 times higher than that of Ti6Al4V titanium alloy.

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.

Low-temperature Rapid Synthesis of Ultrafine Hafnium Carbide Ceramic Powders

Nano hafnium carbide (HfC) powders were synthesized by sol-gel combining high-temperature rapid heat treatment process using citric acid and hafnium tetrachloride as the raw materials. The effects of ball milling treatment on the phase and morphology of pyrolysis products (HfO2-C) and final HfC product were investigated and the influences of heat treatment temperature and holding time on the structure and properties of the synthesized hafnium carbide powders were also studied. The experimental results showed that the HfO2-C powders with good uniformity and small particle size were prepared by controlling the milling time. Pure HfC powders with an average particle size of 500 nm were obtained at 1 700 °C with a holding time of 3 min, and the oxygen content was about 0.69wt%, lower than that of the hafnium carbide powders prepared by SPS (0.97%).

Nitrogenation of hafnium carbide powders in AC and DC plasma by Electrical Discharge Assisted Mechanical Milling

Metal carbonitride powders and coatings are well known as superhard materials which are widely applied in many engineering products. Of this group of materials hafnium carbonitride properties are one of the least known due to difficulties with preparation. In this study, the electric discharge assisted mechanical milling (EDAMM) method involving a nitrogen plasma together with mechanical milling was used to synthetise hafnium carbonitride powders within minutes. It was found that difficulties in conventional manufacturing methods, resulting in the formation of only thin layers instead of bulk materials can be overcome by applying the new technique, and HfCxNy powder could be successfully engineered with controlled amount of nitrogen uptake.In this investigation we studied the effects of AC and DC discharges on nitrogen solubility in HfC powder. Microstructure and phase evolution were determined using XRD and SEM, while the nitrogen content in the grains was evaluated by EDS and bulk nitrogen content by CHN analysis. Maximum solubilities of 3.42 wt% and 2.95 wt%N under DC an AC discharge currents, respectively, were obtained. Reaction paths varied depending on processing conditions with HfCxNy formed under DC discharge processing and mixed products containing HfC, several HfCxNy phases and HfN like phase formed after AC processing. After nitrogenation the most highly nitrided powders were subjected to 1 min additional EDAMM processing using high power discharges to allow the formation of large, sintered particles of sufficient size to perform hardness measurements on. Vickers harnesses of fully sintered large particles after 5 and 10 min of EDAMM were 1808 and 2169HV, respectively.

Synthesis of a Fine (Ta0.8,Hf0.2)C Powder from Carbide or Oxide Powder Mixtures

Tantalum hafnium carbide ((Ta0.8,Hf0.2)C) powders were successfully synthesized using a modified spark plasma sintering (SPS) apparatus with TaC/HfC or Ta2O5/HfO2/C starting materials. The (Ta0.8,Hf0.2)C obtained from the carbides had a finer particle size of 220 nm, whereas those obtained from the oxides had less contamination during the milling process (0.35 wt%) than the other case. Particle coarsening of the solid‐solution phase was effectively suppressed by using a modified SPS apparatus because of the fast heating/cooling rate. High‐energy ball milling promoted a solid‐solution reaction for the formation of (Ta0.8,Hf0.2)C by refining the size and inducing the homogeneous mixing of the starting materials. By the combination of the fast heating and high‐energy ball milling, fine tantalum hafnium carbide powders with low contamination were successfully synthesized.

Synthesis of ultra-fine hafnium carbide powders combining the methods of liquid precursor conversion and plasma activated sintering

Ultra-fine hafnium carbide (HfC) powders were synthesized using a novel method combining liquid precursor conversion and plasma activated sintering (PAS). Solution-based processing was used to achieve a fine-scale mixing of the reactants, and further treatment by PAS allowed fast formation of HfC. We investigated the effect of the type of acid used during the liquid precursor conversion on the synthesized powders, where mixtures were prepared using salicylic acid, citric acid, or a combination of these. The results show that pure HfC powders (with an average particle sizes of 350nm) were obtained at a relatively low temperature (1550°C) using a HfOCl2·8H2O precursor with the mixed acids. The oxygen content of the synthesized powders was only 0.97wt%. The type of acid had a significant effect on the synthesis product. When using only citric acid, the temperature required to produce pure hafnium carbide increased to 1700°C. In the case of a salicylic acid precursor, pure HfC was not obtained, even at a synthesis temperature of 1700°C.

Synthesis and densification of nano-crystalline hafnium carbide powder

Ultra-fine hafnium carbide powder was synthesized by the carbothermal reduction of HfO2 at 1600°C using the modified spark plasma sintering (SPS) apparatus. The synthesized powder had a fine particle size of about 125nm and a low oxygen content of below 0.5wt%. The purity of the powder was >99.9% excluding Zr, and 99.6% when including Zr impurity. The intermediate reactions and particle growth were minimized due to the low synthesis temperature, fast heating/cooling rate and the effect of current during the modified SPS process. A relatively large amount of HfC powder could be synthesized by the new synthesis method (>175g/day). HfC ceramics with high relative density (>96%) were obtained after sintering the synthesized powder without sintering additives at 2300°C for 30min under 100MPa pressure by SPS. The average grain size of the obtained specimen was about 6μm.

Synthesis and Characterization of Hafnium Carbide Based Ceramics

Hafnium carbide powder was synthesized by sol-gel polycondensation of hafnium chloride with citric acid. The starting materials were dissolved in water and mixed homogeneously on a hot plate until a precomposite gel was formed. Pyrolysis of the obtained gel resulted in formation of monoclinic hafnia and amorphous carbon, which after subsequent heat treatment transformed into hafnium carbide. Materials were analyzed by means of X-ray diffraction and electron microscopy investigations. The results showed that the obtained carbide powder was composed of nearly equiaxed particles of narrow size distribution. The obtained hafnium carbide powder was densified via spark plasma sintering (SPS) at 1950 oC using molybdenun silicide as sintering additive. Microstructure and mechanical properties of the obtained hafnium carbide ceramics were investigated.

Electrochemical synthesis of hafnium carbide powder in molten chloride bath and its densification

Abstract Nanocrystalline powder of hafnium-rich-HfC has been successfully synthesised by the electro-deoxidation of HfO2–carbon precursors in molten chloride. The progress of the solid state reduction was monitored ex situ by analysing partially reduced samples using X-ray diffraction (XRD) and scanning electron microscopy (SEM). It has been shown that the reduction started by converting HfO2 to CaHfO3 and an oxycarbide phase of the form HfCxO2(1−x). The CaHfO3 phase then also reduced to give HfCxO2(1−x), which subsequently reduced to HfC by ionising oxygen. The morphological analysis indicated almost no growth in the grain size occurred during the course of the electro-deoxidation. This investigation showed some loss of carbon during the electro-deoxidation resulted in metallic rich HfC. The synthesised powder exhibited better sinterability than the commercial HfC powder. Using the synthesised powder, fully dense monolithic HfC ceramics were produced by pressureless sintering at 1973 K with average grain size of about 3 μm.

Synthesis and characterization of hafnium carbide fine powders

Hafnium carbide powder was synthesized by sol–gel polycondensation of hafnium chloride with citric acid. The starting materials were dissolved in water and mixed homogeneously on a hot plate until precursor gel was formed. Pyrolysis of this gel resulted in monoclinic hafnia which after subsequent heat treatment transformed into hafnium carbide. The obtained materials were analyzed by means of X-ray diffraction and scanning electron microscopy. The results showed that the obtained carbide powders were composed of nearly equiaxed particles with narrow size distribution. Characterization by nitrogen adsorption showed that the obtained powders were micro- and mesoporous materials with high specific surface area.

Synthesis of Ultra‐Fine Hafnium Carbide Powder and its Pressureless Sintering

Ultra‐fine HfC powders with particle sizes of 380 or 225 nm were synthesized by carbothermal reduction of HfO2 with graphite or carbon black, respectively. The investigation of the process indicated that a vacuum level as high as 5 Pa was appropriate for decreasing the reaction temperature and consequently limiting the particle growth of HfO2 reactant. Type and size distribution of carbon sources demonstrated an obvious effect on the particle size of the HfC powders obtained. A model was proposed to explain this phenomenon. Carbon black with a characteristic particle size of 42 nm resulted in a HfC powder with a median particle size of 225 nm. Compared with the reported values in the literature, the synthesized HfC powder exhibited better sinterability than the commercial HfC powder. Using the synthesized powder, monolithic HfC ceramics pressureless sintered at 2400°C reached a relative density of 98.4% and an average grain size of about 4 μm.

Synthesis and Fabrication of High Purity Hafnium Nitride and Hafnium Carbide

The procedures used to prepare kilogram quanties of hafnium nitride and hafnium carbide powders and hot-pressed billets are described. In order to maintain high purity, all steps of the process were conducted in either a vacuum or a high purity atmosphere. Analyses conducted on the starting materials and at various stages of processing indicated that target impurity levels, i.e., less than 100 ppm oxygen, could not be attained but that high levels of contamination could be avoided.