WO2021133226A1

WO2021133226A1 - Method for fabrication of ultra-high-temperature ceramic material based on hafnium carbide and carbonitride

Info

Publication number: WO2021133226A1
Application number: PCT/RU2020/050295
Authority: WO
Inventors: Veronika Sergeevna BUINEVICH; Andrey Aleksandrovich NEPAPUSHEV; Dmitry Olegovich MOSKOVSKIKH; Aleksandr Sergeevich ROGACHEV; Alexander Sergeevich MUKASYAN
Original assignee: National University of Science and Technology “MISIS”
Priority date: 2019-12-24
Filing date: 2020-10-26
Publication date: 2021-07-01
Also published as: RU2729277C1; CN115151358A

Abstract

This invention relates to space engineering and materials science. The technical result of the invention disclosed herein regarding the method is facilitation and significant reduction of power and time consumption for the material synthesis, and regarding the material is providing a compact ceramic material of a nonstoichiometric composition. Said technical result is achieved by providing the method of synthesizing ultrahigh-temperature materials on the basis of hafnium carbide and carbonitride comprising preliminary mechanical activation of the initial hafnium and carbon powders in a high-energy planetary ball mill, self-propagating high-temperature synthesis in argon or nitrogen atmosphere and subsequent sintering of the synthesized mixture. Consolidation of the synthesized powder is achieved using the spark plasma sintering method.

Description

Method for Fabrication of Ultra-High-Temperature Ceramic material based on Hafnium Carbide and Carbonitride

Field of the Invention. This invention relates to missile and space engineering and materials science, more specifically, to the development of ultra- high-temperature materials (T_m > 3000 °C) on the basis of hafnium (Hf) which can be used for the protection of the most heat-affected units (air-breathing engines, nose cones and sharp leading edges) of hypersonic aircrafts, and can be furthermore used in the nuclear industry.

Prior Art. Fabrication of this type of the materials for operation at ultrahigh temperatures (above 2000 °C) requires providing a combination of the following key properties: efficient heat removal, high oxidation resistance and refractory quality. Of greatest interest from this viewpoint are binary and ternary non- stoichiometric compounds, more specifically, carbides, nitrides and carbonitrides of transition metals of groups 4B and 5B of the Periodic Table including hafnium. Fabrication of these non- stoichiometric compounds is a complex task due to the difficulties associated with the nitrogen and/or carbon quantity adjustment in the system and the high melting points of the main components.

Transition metal nitrides fabrication method is known, comprised of a mixture preparation, containing oxide of the metal to be nitridized, powder of the metal to be nitridized (Hf, Ti, Nb, Zr) and azide of an alkaline metal (e.g. sodium azide), pressing of the billets from mixed powders and their ignition using a tungsten wire in a nitrogen atmosphere. This method allows fabricating metal nitride powders with a nitride yield of at least 96% and a nitrogen content of at least 7.17% (RU 2256604, C01B21/076, 20.07.2005). Failure to deliver a 100% reaction product yield and the necessity to use powders along with their oxides, which leads to a cost increase of the method, as well as the impossibility to vary the nitrogen content in the compound are disadvantages of this fabrication method.

Titanium carbonitride producing method is known, comprising high- temperature synthesis of titanium-containing compounds in a nitrogen atmosphere during magnesium-thermic reduction of a titanium tetrachloride and tetrachloroethylene mixture in a ratio of 4.5 - 5.1 and the temperature range of 1010-1080 °C. Above-mentioned method allows fabricating titanium-containing compounds including titanium carbonitride with a minimal quantity of impurities (RU 2175021, C22B34/12, C22B5/04, C01B31/30, C01B21/076, 20.10.2001).

Disadvantages of this method are the impossibility of varying the C/N ratio over the entire composition range, the presence of free carbon in the synthesized compound, the necessity of using a reducing metal and the high power consumption of the method caused by the necessity of maintaining high temperature for the synthesis.

The closest counterpart of the material and method of its fabrication disclosed herein is the method of refractory metal carbides, nitrides and carbonitrides fabrication, comprising mixing of a refractory metal oxide, e.g. HfO₂, ZrO₂ or TiO₂, with a non-metallic material, e.g. soot or calcium-containing compound CaC₂, Ca₃N₂ or CaCN₂, addition of a reducing metal (calcium), synthesis in a pipe-shaped reactor in an argon atmosphere at 450 to 800 °C and removal of the forming calcium oxide CaO by dissolving it in hydrochloric or acetic acid (RU 2225837, C01B31/30, C01B21/06, B22F3/23, 20.03.2004).

Disadvantages of this method are the necessity to use a reducing metal oxide which should be removed from the product compound and the high power consumption of the method caused by the necessity to heat the pipe-shaped reactor to 450 - 800 °C.

Disclosure of the Invention. The technical result of the invention disclosed herein is simplification and significant power and time consumption reduction of the material synthesis method and the possibility of fabricating compact ceramic material of a non- stoichiometric composition.

Said technical result is achieved by exposing the mixture of raw Hf and C components to preliminary mechanical activation in a high-energy planetary ball mill, subsequent self-propagating high-temperature synthesis of the prepared Hf and C mixture and consolidation of the synthesized powders. Said preliminary mechanical activation is carried out for 5 - 10 min, the ball-to-mixture weight ratio being 20:1 - 40:1 and the planetary disc speed being 694 - 900 rpm, said subsequent self-propagating high-temperature synthesis is carried out in a reactor containing an argon or nitrogen atmosphere at a pressure of 0.1 - 0.8 MPa, initiation of a self-sustaining exothermic reaction is performed by incandescent tungsten spiral and the consolidation of synthesized hafnium carbide (argon) or hafnium carbonitride (nitrogen) powder is carried out by means of spark plasma sintering, wherein an argon atmosphere is produced in the chamber and a pulsed current of 1000 - 5000 A is passed through the specimen being sintered at a load of 30 - 70 MPa, the consolidation temperature and exposure time being 1900 - 2200°C and 2 - 10 min, respectively.

Embodiments of the Invention. Consolidated ultra-high-temperature materials fabricated using the method disclosed hereinabove are ceramics having the following properties: a) hafnium carbonitride of the HfC_0.5N_0.2 composition with a relative density of 98.7 %, a Vickers hardness of 21.3 GPa and a fracture toughness of 4.7 MPa m^1/2; b) hafnium carbide of the HfC_0.5 composition with a relative density of 98.5

%, a Vickers hardness of 10.2 GPa and a fracture toughness of 3.6 MPa m^1/2; c) hafnium carbide of the HfC composition with a relative density of

99.3 %, a Vickers hardness of 20.5 GPa and a fracture toughness of 4.1 MPa m^1/2.

The raw components for the fabrication of the ultra-high-temperature hafnium nitride, carbide and carbonitride based ceramics used for the protection of the most heat-affected units of hypersonic aircrafts are GFM-1 Grade Hf metal powder (hafnium powder) (TU Standard 48-4-176-85 (97)) and P804T Grade C powder (powdered soot) (TU Standard 38-1154-88), as well as nitrogen (GOST 9293-74) and argon (GOST 10157-79) gases.

Preliminary mechanical activation comprising grinding and mixing of the raw hafnium powder and soot is performed in an “Activator-2S” high-energy planetary ball mill. Mechanical activation is carried out in steel vials with steel balls for 5 - 10 min at a 694 - 900 rpm main disk speed and a ball-to-powder weight ratio of 20:1 - 40:1. Mechanical activation of the raw mixtures is implemented in an argon atmosphere at an in-vial pressure of 0.4 MPa. Mechanical activation leads to the formation of new non-oxidized surfaces, uniform distribution of the particles and an increase in the contact area between the reactants which in turn accelerates the reaction between them. Preliminary mechanical activation constitutes the first stage of the method described herein.

The second stage of the method described herein which follows the preliminary mechanical activation comprises the self-propagating high-temperature synthesis of the activated powder or Hf + xC powder mixture, where x is the quantity of carbon varying from 0.5 to 1, in order to synthesize hafnium carbide HfC_x if an argon atmosphere in a laboratory reactor or hafnium carbonitride HfC_xN_y if a nitrogen atmosphere in a laboratory reactor. The gas pressure in the laboratory reactor during the process is 0.1 - 0.8 MPa.

The composition of the synthesized compounds, i.e., the x and y parameters, varies depending on the quantity of carbon in the raw mixture and the nitrogen pressure in the reactor.

The third stage of the method described herein comprises the consolidation of the non- stoichiometric HfC_x and HfC_xN_y powders in a spark plasma sintering unit (Spark Plasma Sintering - Labox 650, SinterLand, Japan).

The method of spark plasma sintering is based on the combined exposure to a high temperature and an axial pressure coupled with the passage of pulsed direct electric current with a high amplitude (to 5000 A) through the material being sintered and the graphite matrix in which it is contained. The pulsed current favors uniform heating of the specimen with a minimum impact on its microstructure. The consolidation load is 30 - 70 MPa, the exposure time and sintering temperature being 2 - 10 min and 1900 - 2200 °C, respectively.

A necessary part of each stage is quality control of the as -treated specimens which is implemented either through visual inspection or with instrumental methods.

For the microstructure and phase composition investigations of the as- synthesized and as-consolidated powders we used scanning electron microscopy (SEM) and X-ray diffraction (X-ray phase analysis) methods. For consolidated ultra-high-temperature non- stoichiometric hafnium carbide, nitride and carbonitride based ceramics we further controlled the porosity, hardness, fracture toughness and microstructure. The subject matter of the method disclosed herein will be further supported with examples.

Example 1.

Fabrication of ultra-high-temperature HfC_0.5N_0.2 ceramic.

The raw components Hf and C were mixed in a molar ratio of 2:1 (96.7wt.% Hf and 3.3 wt.% C). The prepared mixture of the raw components was exposed to preliminary mechanical activation in a planetary ball mill in an argon atmosphere at a pressure of 0.4 MPa and a mill speed of 900 rpm with a ball-to- powder weight ratio of 20:1 in order to mix, grind and clean powder surface from oxides. The ball diameter was 6 mm. The time of preliminary mechanical activation was 10 min.

The resultant reaction powder mixture was exposed to self-propagating high-temperature synthesis in a reactor at a nitrogen pressure of 0.8 MPa and a self- sustaining exothermic reaction was initiated with an incandescent tungsten spiral. The resultant hafnium carbonitride powder had the HfC_0.5N_0.2 composition.

The as-synthesized HfC_0.5N_0.2 powder was consolidated using the method of spark plasma sintering. For this purpose the powder was placed in a cylindrical graphite matrix and clamped between two punches which simultaneously acted as electrodes, the matrix was placed into the working space of the spark plasma sintering unit, a argon atmosphere was produced in the chamber and pulsed electric current was passed through the specimen being sintered at a load of 50 MPa applied to the specimen. The consolidation temperature was 2000 °C and the exposure time was 10 min. The heating rate to the sintering temperature was 100 °C/min. As a result, the specimens had disc shapes and were 15 - 50 mm in diameter and 2 - 10 mm in thickness. The ultra-high-temperature material has the following parameters: relative density 98.7 %, Vickers hardness 21.3 GPa and fracture toughness 4.7 MPa m^1/2. Figure 1 and 2 shows a diffraction pattern and the microstructure of HfC_0.5N_0.2-

Example 2.

Fabrication of ultra-high-temperature HfC_0.5 ceramic.

The raw components Hf and C were mixed in a molar ratio of 2:1 (96.7wt.% Hf and 3.3 wt.% C). The prepared mixture of the raw components was exposed to preliminary mechanical activation in a planetary ball mill in an argon atmosphere at a pressure of 0.4 MPa and a mill speed of 694 rpm with a ball-to- powder weight ratio of 20:1 in order to mix, grind and clean powder surface from oxides. The ball diameter was 6 mm. The time of preliminary mechanical activation was 5 min.

The resultant reaction powder mixture was exposed to self-propagating high-temperature synthesis in a reactor at a argon pressure of 0.8 MPa and a self- sustaining exothermic reaction was initiated with an incandescent tungsten spiral.

The as-synthesized HfC_0.5 powder was consolidated using the method of spark plasma sintering. For this purpose the powder was placed in a cylindrical graphite matrix and clamped between two punches which simultaneously acted as electrodes, the matrix was placed into the working space of the spark plasma sintering unit, a argon atmosphere was produced in the chamber and pulsed electric current of 1000 - 5000 A was passed through the specimen being sintered at a load of 70 MPa applied to the specimen. The consolidation temperature was 2200 °C and the exposure time was 10 min. The heating rate to the sintering temperature was 100 °C/min. As a result, the specimens had disc shapes and were 15 - 50 mm in diameter and 2 - 10 mm in thickness. The ultra-high-temperature HfC_0.5 ceramic has the following parameters: relative density 98.5 %, Vickers hardness 16.2 GPa and fracture toughness 3.6 MPa m^1/2. Figure 3 and 4 shows a diffraction pattern and the microstructure of HfC_0.5

Example 3.

Fabrication of ultra-high-temperature HfC ceramic.

The raw components Hf and C were mixed in a molar ratio of 1:1 (93.7wt.% Hf and 6.3 wt.% C). The prepared mixture of the raw components was exposed to preliminary mechanical activation in a planetary ball mill in an argon atmosphere at a pressure of 0.4 MPa and a mill speed of 900 rpm with a ball-to- powder weight ratio of 20:1 in order to mix, grind and clean powder surface from oxides. The ball diameter was 6 mm. The time of preliminary mechanical activation was 5 min.

The resultant reaction powder mixture was exposed to self-propagating high-temperature synthesis in a reactor at a argon pressure of 0.4 MPa and a self- sustaining exothermic reaction was initiated with an incandescent tungsten spiral.

The as- synthesized HfC powder was consolidated using the method of spark plasma sintering. For this purpose the powder was placed in a cylindrical graphite matrix and clamped between two punches which simultaneously acted as electrodes, the matrix was placed into the working space of the spark plasma sintering unit, a argon atmosphere was produced in the chamber and pulsed electric current was passed through the specimen being sintered at a load of 30 MPa applied to the specimen. The consolidation temperature was 1900 °C and the exposure time was 10 min. The heating rate to the sintering temperature was 100 °C/min. As a result, the specimens had disc shapes and were 15 - 50 mm in diameter and 2 - 10 mm in thickness.

The ultra-high-temperature ceramic has the following parameters: relative density 99.3 %, Vickers hardness 20.5 GPa and fracture toughness 4.1 MPa m^1/2. Figure 5 and 6 shows a diffraction pattern and the microstructure of HfC.

Claims

What is claimed is a

Method of fabrication of ultra-high-temperature ceramics on the basis of hafnium carbide or carbonitride ultra-high-temperature ceramics fabrication method, comprising preliminary mechanical activation of the Hf and C raw components mixture in a high-energy planetary ball mill, subsequent self- propagating high-temperature synthesis of the prepared Hf and C mixture and consolidation of the synthesized powders, wherein said preliminary mechanical activation is implemented for 5 - 10 min at a ball-to-powder weight ratio of 20:1 - 40:1 and a main disc speed of 694 - 900 rpm, said subsequent self-propagating high-temperature synthesis is carried out in a reactor with an argon or nitrogen atmosphere at a pressure of 0.1 - 0.8 MPa, a self-sustaining exothermic reaction is initiated with an incandescent tungsten spiral and the synthesized hafnium carbide or carbonitride powder is consolidated by means of spark plasma sintering, further wherein a argon atmosphere is produced in the reaction chamber and a pulsed current of 1000 - 5000 A is passed through the specimen being sintered at a load of 30 - 70 MPa, the consolidation temperature and exposure time being 1900 - 2200°C and 2 - 10 min, respectively.