rus
Issue #2/2022
M. A. Sheindlin, M. V. Brykin, T. V. Bgasheva, A. A. Vasin, P. S. Vervikishko, S. V. Petukhov, A. M. Frolov
Ultra-High Temperature Carbides Under Irradiation of the Power Industrial Lasers
Ultra-High Temperature Carbides Under Irradiation of the Power Industrial Lasers
DOI: 10.22184/1993-7296.FRos.2022.16.2.142.154
The paper presents an experimental study of the behavior of high-density zirconium carbide in oxidizing atmosphere (in air flow). The study has been conducted with a specially developed experimental setup with a high-power technological laser providing heating of zirconium carbide in a wide range of heating rates, which make it possible to study the formation of an oxide layer at different regimes. High-speed video recording of the heated surface in the conditions of external illumination in combination with the multichannel pyrometry enabled to analyze the main features of the formation of the oxide layer at the surface of zirconium carbide. A detailed study of the zone of laser radiation exposure in an oxidizing atmosphere was performed using electron microscopy, X-ray diffraction and Raman spectroscopy.
The paper presents an experimental study of the behavior of high-density zirconium carbide in oxidizing atmosphere (in air flow). The study has been conducted with a specially developed experimental setup with a high-power technological laser providing heating of zirconium carbide in a wide range of heating rates, which make it possible to study the formation of an oxide layer at different regimes. High-speed video recording of the heated surface in the conditions of external illumination in combination with the multichannel pyrometry enabled to analyze the main features of the formation of the oxide layer at the surface of zirconium carbide. A detailed study of the zone of laser radiation exposure in an oxidizing atmosphere was performed using electron microscopy, X-ray diffraction and Raman spectroscopy.
Теги: high temperatures laser heating optical pyrometry zirconium carbide zirconium dioxide высокие температуры диоксид циркония карбид циркония лазерный нагрев оптическая пирометрия
Ultra-High Temperature Carbides Under Irradiation of the Power Industrial Lasers
M. A. Sheindlin, M. V. Brykin, T. V. Bgasheva, A. A. Vasin, P. S. Vervikishko, S. V. Petukhov, A. M. Frolov
Joint Institute for High Temperatures of the Russian Academy of Sciences, Moscow, Russia
The paper presents an experimental study of the behavior of high-density zirconium carbide in oxidizing atmosphere (in air flow). The study has been conducted with a specially developed experimental setup with a high-power technological laser providing heating of zirconium carbide in a wide range of heating rates, which make it possible to study the formation of an oxide layer at different regimes. High-speed video recording of the heated surface in the conditions of external illumination in combination with the multichannel pyrometry enabled to analyze the main features of the formation of the oxide layer at the surface of zirconium carbide. A detailed study of the zone of laser radiation exposure in an oxidizing atmosphere was performed using electron microscopy, X-ray diffraction and Raman spectroscopy.
Keywords: zirconium carbide, zirconium dioxide, laser heating, optical pyrometry, high temperatures.
Article received on: 24.01.2022
Article received on: 01.02.2022
Introduction
At present, interest in studying the behavior of so-called ultra-refractory compounds or UHTC (Ultra High Temperature Ceramics) when exposed to high energy flows. This study is aimed at investigation of the resistance of materials and coatings made from super-refractory transition metal carbides when exposed to high energy densities and, development of the corresponding methods for their testing under extreme conditions.
Transition metal carbides are the most refractory UHTC materials (they have melting points in the vicinity of 4000 K and above). They are considered as one of the most promising classes of materials capable of withstanding high thermal loads in a “non-ablative” regimes [1, 2]. Most of the recent studies (e. g. [2–3]) mainly concern the chemical – and technological aspects of the synthesis of powders of the required composition and their further high-temperature sintering.
However, in order to create materials and coatings that can withstand extreme energy fluxes, one have to solve a number of interrelated tasks: the manufacturing of samples of a given composition → studying their properties → testing under the influence of a high-energy fluxes.
At present, the results of studying the dependence of the resistance of materials on the technology of their manufacture are fragmentary. As a rule, we are talking about studies in which the testing of a sample is carried out in a plasmatron gas jet [4]. At the same time, in most studies there is no measurement of the surface temperature, which is the most important parameter that determines the physicochemical processes on the surface. The same is related to measurements of the characteristics of the applied heat flux. Therefore, the latter makes it difficult to compare data from different experiments. Even in the experiment where the surface temperature of the test material was directly measured [5], the heat flux on the surface was estimated very roughly.
An alternative approach is the use of the power multi-kilowatt disk or the fiber technological lasers as a source of high-energy fluxes. Here one can relatively easy and quite reliably determine the laser power density at the surface of the testing sample. It is important that the use of laser heating makes it possible to test materials at heat flows inaccessible to electric arc heaters or gas burners and, most importantly, in conditions of an almost uniform distribution of power density over the heating spot, which is achieved by using optical fibers for delivery of laser radiation to the focusing optics. In addition, that makes it possible to program the time power shape arbitrary, which broaden the possibilities for using laser radiation for the high-temperature testing of the UHTS materials.
It is important to note that such tests can be carried out in various gas environments, as well as in vacuum. In the latter case, it is possible to obtain important information about both the high-temperature release of molecular products and to study the molecular composition of the vapor during high-temperature evaporation of the material [6, 7]. The use of high-power laser radiation to study the behavior of the UHTS ceramics under high energy densities can be considered as one of the most obvious ways. However, presently, there are very limited number of publication concerning the above issue (see, e. g. [8]).
When testing materials using for example the setup in a ref [9], the use of one or two standard pyrometers cannot be sufficient. Indeed, the issue of reliable measurement of the surface temperature in this kind of experiments is of paramount importance – here the use of brightness (monochromatic) pyrometry leads to large errors, mainly due to the unknown value of the emissivity of the material at high temperature under conditions of complex processes on the surface as, for example, melting or possible chemical reactions. The use of a bichromatic (color) pyrometer, which, according to some authors [6], allows for more accuratedetermination of the true temperature (which, at least, is insufficiently justified), leads to significant and unpredictable errors. While bichromatic pyrometry does not lead to fatal errors in the study of carbon materials, which are sufficiently “gray” emitters, for carbide and nitride materials, the error at temperatures close to their melting points can reach hundreds of Kelvins.
An alternative method for determining the temperature is a polychromatic (or multichannel) pyrometry, which was used by the authors of the present study to investigate the phase diagrams of UHTC carbides [9] and the melting of refractory oxide ceramics [10]. It is important to note here that with the help of polychromatic pyrometry, the true temperature and emissivity are simultaneously defined in a wide range of wavelengths from about 450 to 900 nm. The latter makes it possible to reliably extrapolate its value to the domain of about 1000 nm, which corresponds to the wavelengths of the modern high-power technologycal” disk and fiber lasers and, thus, to monitor the variation in the absorbance of the material at the laser wavelength, which latter is important for the correct interpretation of the measurement results.
Obviously, the main process of degradation of refractory carbides at high temperatures in an oxidizing environment is the formation of an oxide scale on the surface, the thickness and structure of which is determined by many factors. Among them: temperature, duration and the regime of heating, structure and stoichiometry of the particular carbide samples. As for the zirconium carbide, which is the subject of this work, it should be noted that a large number of works have been devoted to the oxidation process, i. e., the formation of zirconium oxide scale on the surface of zirconium carbide (see [11]). As a rule, they are performed under “ideal”, well-controlled oxidation conditions: the sample is placed in a chamber in which a certain temperature and a gas mixture with a given oxygen partial pressure are maintained. After the formation of oxide scale on the surface of the ZrC sample, a detailed analysis of the zone of chemical reaction is carried out using various analytical methods. Here, special attention should be paid to the recent work [12], carried out using the above methodology. The temperature in such experiments usually does not exceed 1500 K (i. e., being only a few hundred degrees higher than the temperature at which active oxidation of ZrC begins), and the duration of the oxidation ranges from tens of minutes to hours. The thickness of the oxide scale in this case exceeds hundreds of microns. With this kind of experiments one can perform rather deep study of the kinetics of oxidation and study some basic processes occurring near the carbide-oxide interface. However, the possibility of their application to predicting the behavior of zirconium carbide under high-energy fluxes in an oxidizing environment needs additional confirmation.
Thus, the purpose of the present work was to study the behavior of zirconium carbide of various compositions within the homogeneity domain (solid solution) under exposure of laser radiation in an oxidizing medium (air). In this case, the experiments were carried out with the different duration of heating, while the maximum heating temperature remained constant.
Method and apparatus
The scheme of the experimental setup is shown in Fig. 1. A sample of zirconium carbide with a diameter of about 5 mm and a thickness of 1.5 mm, manufactured as described below, was in an air flow directed perpendicular to the surface at a speed of about 1 m / s for elimination of the influence of the natural convection on the oxide formation on the sample surface. Heating was carried out using a disk laser; in this case, three heating regimes were used with the different durations and the corresponding temperature excursions (Fig. 2).
The exact duration of the individual laser shots was limited by the time for reaching a temperature of 2530 K, which was controlled with a brightness pyrometer. The sighting spot of the pyrometers was about 0.3 mm. This ensured that the temperature was measured at the center of the isothermal zone of the sample. The brightness pyrometer measured the brightness temperature starting from about 1200 K, while the spectropyrometer, operating in the spectral range 400–900 nm with a time constant of 1.5 ms, measured the true temperature starting from 2000 K. Both the pyrometers were specially designed for experiments with laser heating and adapted to the given heating rates and the expected temperature ranges.
The spectropyrometer was calibrated up to a temperature of 3300 K using a high-temperature black body model, which temperature was measured by a CHINO IR-RST 65H reference pyrometer. The basic principles of the true temperature evaluation and spectral emissivity using the spectropyrometer are described in [11, 13].
During the experiment the processes on the surface were controlled with the high-speed video camera at frequency of 2000 frames per second at a spatial resolution of about 900 × 900 pixels. Due to the fact that the intensity of the sample’s self-radiation during heating varied by several orders of magnitude, the use of video recording of the surface in the self-radiation was inappropriate. Therefore, the sample surface was uniformly illuminated by the radiation of a diode laser at a wavelength of 808 nm, and an appropriate filter was installed in front of the camera, which enabled video recording in reflected light with almost complete blocking of the surface’s own radiation up to the maximum temperature. Reflected diode-laser radiation at a wavelength of 808 nm, which significantly exceeded the background of thermal radiation, was reliably recorded by a spectropyrometer (see the characteristic spectrum in Fig. 3). The reflected signal recorded in this way, helped to obtain additional qualitative information about the processes occurring on the heated surface, although this signal was not directly related to the directional hemispherical reflectivity due to the change in the angular distribution of the reflected radiation during the experiment.
Preparation of samples of
non-stoichiometric zirconium carbide
Due to the fact that zirconium carbide ZrCx (x < 1) is a substance with a wide homogeneity domain with vacancies in the non-metallic sublattice, it seems interesting to study the process of high-energy impact on this carbide with different stoichiometry parameters x. For this purpose, samples of three compositions with x = 0.98, 0.9, and 0.77 were prepared having insignificant amount of impurities in the material, high homogeneity and low porosity. The initial powder of zirconium carbide was obtained by the method of self-propagating high-temperature synthesis (SHS). This technology was chosen due to the possibility of obtaining cleaner samples compared to the methods of carbothermal reduction of oxides and some others. For the synthesis of zirconium carbide, weighed portions of zirconium powders (PTsRK‑1 grade) and acetylene soot were mixed in a ball mill, after which a temporary technological binder, cetane, was introduced into the resulting mixture. Cylindrical green pellets for synthesis were pressed from the resulting mixture. Then the temporary technological bond was removed by heating in a high-temperature vacuum furnace. Next, the furnace was filled with argon, and the temperature was raised to 1673 K to carry out the carbide synthesis reaction.
To obtain samples with low porosity, the synthesized zirconium carbide was dispersed in a planetary mill. The crushed powder after washing away from the grinding media material was sintered in a hot pressing unit with induction heating at a temperature of 2130 K and a pressure of 60 MPa. The thickness of the sintered specimens was typically about 2 mm with a diameter of about 12 mm, which was determined by the internal diameter of the hot press graphite mold. The sintered sample is processed on a grinding machine to remove graphite and reduce the thickness of the sample to 1.5 mm. Then the zirconium carbide sample was subjected to laser-erosion treatment to cut it to three reduced samples with a diameter of 5 mm (Fig. 3), after which these samples were re-ground. As a result, three samples of zirconium carbide with a diameter of 5 mm and a thickness of 1.5 mm were obtained having the same characteristics (density, composition) necessary for conducting a series of tests in different heating regimes.
The carbon content in zirconium carbide samples was determined by the dynamic flash method with a Combustion elemental analyzer Master CS (NCS, Germany); the content of oxygen and nitrogen with the method of reduction-melting in the elemental analyzer Fusion Master ON (NCS, Germany). XRD phase analyses of zirconium carbide samples was carried out with a diffractometer SmartLab SE (Rigaku, Japan) using Cu Kα radiation. The diffraction patterns (Fig. 4) contain only reflections corresponding to the zirconium carbide phase. General characteristics of the samples are presented in Table 1.
Experimental results
The experiments were carried out on samples of zirconium carbide of three different compositions: ZrC0.77, ZrC0.9, ZrC0.98. A sample of 5 mm in diameter was placed in a graphite holder. The sample surface was blown with a weak air flow. Radiation from the high-power disk laser and the diode laser was focused onto the surface of the sample, with the diameters of the focusing spots being close to the diameter of the sample. Experiments with samples of each composition were carried out with different heating rates shown in Fig. 2. In this case, the temperature increase rate on the surface was 700–1000 K / s, 2000–3800 K / s, and 8500–12000 K / s, respectively. By approximating the thermal emission spectra of the sample surface (Fig. 5) by the product of the Planck function and emissivity, the values of the true surface temperature and spectral emissivity were calculated under the assumption that in a relatively narrow wavelength range, the emissivity does not depend on the wavelength (graybody approximation). The approximation was carried out in two wavelength ranges (730–790 nm and 820–870 nm) in order to exclude from consideration the effect of diode laser radiation. Taking into account the fact that, as will be shown below, a significant layer of zirconium oxide is already formed on the surface by the time the lower temperature limit for the spectropyrometer is reached (which is approximately 2000 K), the gray body approximation for zirconium oxide looks quite adequate [14].
Fig. 6 shows the results of a typical experiment with a heating time of about 1.5 s for a sample with a minimum carbon content. The spectral emissivity in the wavelength range used for the non-linear fitting (730–870 nm) in the temperature range recorded by the spectropyrometer lies in the range of about 0.6–0.75. This is in a good agreement with the measurements performed in [16] under conditions of laser heating of a bulk sample of with the yttria stabilized zirconium dioxide. At the initial stage of heating, a sharp decrease in reflection by the sample surface is observed, which begins at a temperature in the vicinity of 1000 K. According to [12, 13], this temperature is in a good agreement with the temperature at which an active oxidation of zirconium carbide begins. The reflectivity of zirconia at this temperature is very high [17] and is comparable to the reflectivity of zirconium carbide. Despite this, the thin oxide scale that appears on the surface is most likely a substoichiometric oxide, which has a very low reflectivity.
Registration of the heating process using a high-speed video camera also confirms the sharp darkening of the surface irradiated by the laser. Near time point A (see Fig. 6), the emissivity and reflection signal show a feature that is clearly visible in the heating thermogram. The temperature corresponding to it, about 2600 K, is close to the well-known transition of zirconium dioxide from the monoclinic phase to the cubic one.
This behavior of ZrCx samples was qualitatively observed for all three initial stoichiometric compositions and manifested itself in experiments performed with different heating rates. During heating at a lower rate and, consequently, with a slower rate of temperature increase, the thickness of the zirconium dioxide scale reached the maximum value of about 20 μm. During heating at an medium rate (see Fig. 2), the scale thickness was about 10 µm, and during the fastest heating, about 5 µm.
After the experiment, the samples were studied by various methods. Figure 7 shows the X-ray diffraction pattern of the sample surface after the experiment, taken with a diffractometer SmartLab SE. It turned out that the white scale formed on the surface corresponds to the monoclinic ZrO2.
Raman scattering spectra of the modified surface of the initial carbide samples were obtained using an M 532 Raman microscope. Fig. 8 shows the Raman spectrum of an oxide scale compared to high purity zirconia powder. The obtained Raman spectra did not depend on the initial stoichiometric composition of the zirconium carbide samples and were in a good agreement with a pure ZrO2 powder. The resulting Raman spectrum corresponded to the monoclinic phase of ZrO2 being in accordance with the data of [18].
Thus, it turned out that only the monoclinic phase was found in zirconium dioxide formed on the surface of zirconium carbide during intense laser heating, although in [13] both the tetragonal and cubic phases were found along with the monoclinic phase. This issue is expected to be clarified in the course of further research.
To determine the thickness and the structure of the region of thermochemical transformation of the surface of zirconium carbide into zirconium dioxide, the SEM analysis of the surface area subjected to heating was performed. Fig. 9 shows the SEM image of a cleavage perpendicular to the sample surface. In the left image, the colored lines show different zones: the surface of the sample; the cleavage zone is the initial ZrC0.77 sample; 1 – transition zone of carbide material; 2 – formed oxide. On the right, the zones 1 and 2 shown at higher magnification.
The distribution of elements in the vicinity of the oxidation zone was studied using SEM-EDS analysis of the thin section, made perpendicular to the sample surface. The results of the analysis are presented in Fig. 10. It turned out that the oxidized zone has a very clear interface with the original carbide. Fig. 10a shows a section made perpendicular to the sample surface with an indication of the area of elemental EDS mapping – A, and the direction of scanning the distribution of elements – B. From Fig. 10 b and 10 d, it follows that the distribution of oxygen in the near-surface layer with a thickness of about 20 μm is very uniform. It corresponds to a lower density of zirconium in this layer compared to the original sample (see Fig. 10b and 10c). In general, the distribution of both zirconium and oxygen is very uniform both in the initial zone of the carbide and in the formed oxide scale. In all regimes of laser exposure and for all the compositions of the samples considered, a tight fit of the oxide layer to the initial carbide is noted. Thus, it is possible that the forming oxide layer will effectively prevent the oxidative degradation of the dense zirconium carbide under a certain heating conditions.
Acknowledgments
The study was supported by the Russian Foundation for Basic Research and Rosatom State Corporation within the framework of the scientific project No. 20-21-00115
ABOUT AUTHORS
Sheindlin Mikhail Aleksandrovich, head of laboratory, laboratory for extreme energy impacts, JIHT RAS, Moscow, Russia.
ORCID: 0000-0002-4960-7757
Brykin Mikhaik Vladimirovich senior researcher, laboratory for extreme energy impacts, JIHT RAS, Moscow. Russia.
ORCID: 0000-0002-8046-888Х
Bgasheva Tatiana Vladimirovna, junior researcher, laboratory for extreme energy impacts, JIHT RAS, Moscow, Russia.
ORCID: 0000-0002-5258-0153
Vasin Andrey Andreevich, leading engineer, laboratory for extreme energy impacts, JIHT RAS, Moscow, Russia.
ORCID:0000-0002-2384-7090
Vervikishko Pavel Sergeevich, researcher, laboratory for extreme energy impacts, JIHT RAS, Moscow, Russia. E-mail: pvervikishko@gmail.com.
ORCID: 0000-0002-4527-6524
Petukhov Sergei Vladimirovich, leading engineer, laboratory for extreme energy impacts, JIHT RAS, Moscow, Russia.
ORCID: 0000-0003-3852-1314
Frolov Aleksander Michailovich, researcher, laboratory for extreme energy impacts, JIHT RAS, Moscow, Russia. E-mail: matotz@gmail.com.
0000-0002-3091-9451
M. A. Sheindlin, M. V. Brykin, T. V. Bgasheva, A. A. Vasin, P. S. Vervikishko, S. V. Petukhov, A. M. Frolov
Joint Institute for High Temperatures of the Russian Academy of Sciences, Moscow, Russia
The paper presents an experimental study of the behavior of high-density zirconium carbide in oxidizing atmosphere (in air flow). The study has been conducted with a specially developed experimental setup with a high-power technological laser providing heating of zirconium carbide in a wide range of heating rates, which make it possible to study the formation of an oxide layer at different regimes. High-speed video recording of the heated surface in the conditions of external illumination in combination with the multichannel pyrometry enabled to analyze the main features of the formation of the oxide layer at the surface of zirconium carbide. A detailed study of the zone of laser radiation exposure in an oxidizing atmosphere was performed using electron microscopy, X-ray diffraction and Raman spectroscopy.
Keywords: zirconium carbide, zirconium dioxide, laser heating, optical pyrometry, high temperatures.
Article received on: 24.01.2022
Article received on: 01.02.2022
Introduction
At present, interest in studying the behavior of so-called ultra-refractory compounds or UHTC (Ultra High Temperature Ceramics) when exposed to high energy flows. This study is aimed at investigation of the resistance of materials and coatings made from super-refractory transition metal carbides when exposed to high energy densities and, development of the corresponding methods for their testing under extreme conditions.
Transition metal carbides are the most refractory UHTC materials (they have melting points in the vicinity of 4000 K and above). They are considered as one of the most promising classes of materials capable of withstanding high thermal loads in a “non-ablative” regimes [1, 2]. Most of the recent studies (e. g. [2–3]) mainly concern the chemical – and technological aspects of the synthesis of powders of the required composition and their further high-temperature sintering.
However, in order to create materials and coatings that can withstand extreme energy fluxes, one have to solve a number of interrelated tasks: the manufacturing of samples of a given composition → studying their properties → testing under the influence of a high-energy fluxes.
At present, the results of studying the dependence of the resistance of materials on the technology of their manufacture are fragmentary. As a rule, we are talking about studies in which the testing of a sample is carried out in a plasmatron gas jet [4]. At the same time, in most studies there is no measurement of the surface temperature, which is the most important parameter that determines the physicochemical processes on the surface. The same is related to measurements of the characteristics of the applied heat flux. Therefore, the latter makes it difficult to compare data from different experiments. Even in the experiment where the surface temperature of the test material was directly measured [5], the heat flux on the surface was estimated very roughly.
An alternative approach is the use of the power multi-kilowatt disk or the fiber technological lasers as a source of high-energy fluxes. Here one can relatively easy and quite reliably determine the laser power density at the surface of the testing sample. It is important that the use of laser heating makes it possible to test materials at heat flows inaccessible to electric arc heaters or gas burners and, most importantly, in conditions of an almost uniform distribution of power density over the heating spot, which is achieved by using optical fibers for delivery of laser radiation to the focusing optics. In addition, that makes it possible to program the time power shape arbitrary, which broaden the possibilities for using laser radiation for the high-temperature testing of the UHTS materials.
It is important to note that such tests can be carried out in various gas environments, as well as in vacuum. In the latter case, it is possible to obtain important information about both the high-temperature release of molecular products and to study the molecular composition of the vapor during high-temperature evaporation of the material [6, 7]. The use of high-power laser radiation to study the behavior of the UHTS ceramics under high energy densities can be considered as one of the most obvious ways. However, presently, there are very limited number of publication concerning the above issue (see, e. g. [8]).
When testing materials using for example the setup in a ref [9], the use of one or two standard pyrometers cannot be sufficient. Indeed, the issue of reliable measurement of the surface temperature in this kind of experiments is of paramount importance – here the use of brightness (monochromatic) pyrometry leads to large errors, mainly due to the unknown value of the emissivity of the material at high temperature under conditions of complex processes on the surface as, for example, melting or possible chemical reactions. The use of a bichromatic (color) pyrometer, which, according to some authors [6], allows for more accuratedetermination of the true temperature (which, at least, is insufficiently justified), leads to significant and unpredictable errors. While bichromatic pyrometry does not lead to fatal errors in the study of carbon materials, which are sufficiently “gray” emitters, for carbide and nitride materials, the error at temperatures close to their melting points can reach hundreds of Kelvins.
An alternative method for determining the temperature is a polychromatic (or multichannel) pyrometry, which was used by the authors of the present study to investigate the phase diagrams of UHTC carbides [9] and the melting of refractory oxide ceramics [10]. It is important to note here that with the help of polychromatic pyrometry, the true temperature and emissivity are simultaneously defined in a wide range of wavelengths from about 450 to 900 nm. The latter makes it possible to reliably extrapolate its value to the domain of about 1000 nm, which corresponds to the wavelengths of the modern high-power technologycal” disk and fiber lasers and, thus, to monitor the variation in the absorbance of the material at the laser wavelength, which latter is important for the correct interpretation of the measurement results.
Obviously, the main process of degradation of refractory carbides at high temperatures in an oxidizing environment is the formation of an oxide scale on the surface, the thickness and structure of which is determined by many factors. Among them: temperature, duration and the regime of heating, structure and stoichiometry of the particular carbide samples. As for the zirconium carbide, which is the subject of this work, it should be noted that a large number of works have been devoted to the oxidation process, i. e., the formation of zirconium oxide scale on the surface of zirconium carbide (see [11]). As a rule, they are performed under “ideal”, well-controlled oxidation conditions: the sample is placed in a chamber in which a certain temperature and a gas mixture with a given oxygen partial pressure are maintained. After the formation of oxide scale on the surface of the ZrC sample, a detailed analysis of the zone of chemical reaction is carried out using various analytical methods. Here, special attention should be paid to the recent work [12], carried out using the above methodology. The temperature in such experiments usually does not exceed 1500 K (i. e., being only a few hundred degrees higher than the temperature at which active oxidation of ZrC begins), and the duration of the oxidation ranges from tens of minutes to hours. The thickness of the oxide scale in this case exceeds hundreds of microns. With this kind of experiments one can perform rather deep study of the kinetics of oxidation and study some basic processes occurring near the carbide-oxide interface. However, the possibility of their application to predicting the behavior of zirconium carbide under high-energy fluxes in an oxidizing environment needs additional confirmation.
Thus, the purpose of the present work was to study the behavior of zirconium carbide of various compositions within the homogeneity domain (solid solution) under exposure of laser radiation in an oxidizing medium (air). In this case, the experiments were carried out with the different duration of heating, while the maximum heating temperature remained constant.
Method and apparatus
The scheme of the experimental setup is shown in Fig. 1. A sample of zirconium carbide with a diameter of about 5 mm and a thickness of 1.5 mm, manufactured as described below, was in an air flow directed perpendicular to the surface at a speed of about 1 m / s for elimination of the influence of the natural convection on the oxide formation on the sample surface. Heating was carried out using a disk laser; in this case, three heating regimes were used with the different durations and the corresponding temperature excursions (Fig. 2).
The exact duration of the individual laser shots was limited by the time for reaching a temperature of 2530 K, which was controlled with a brightness pyrometer. The sighting spot of the pyrometers was about 0.3 mm. This ensured that the temperature was measured at the center of the isothermal zone of the sample. The brightness pyrometer measured the brightness temperature starting from about 1200 K, while the spectropyrometer, operating in the spectral range 400–900 nm with a time constant of 1.5 ms, measured the true temperature starting from 2000 K. Both the pyrometers were specially designed for experiments with laser heating and adapted to the given heating rates and the expected temperature ranges.
The spectropyrometer was calibrated up to a temperature of 3300 K using a high-temperature black body model, which temperature was measured by a CHINO IR-RST 65H reference pyrometer. The basic principles of the true temperature evaluation and spectral emissivity using the spectropyrometer are described in [11, 13].
During the experiment the processes on the surface were controlled with the high-speed video camera at frequency of 2000 frames per second at a spatial resolution of about 900 × 900 pixels. Due to the fact that the intensity of the sample’s self-radiation during heating varied by several orders of magnitude, the use of video recording of the surface in the self-radiation was inappropriate. Therefore, the sample surface was uniformly illuminated by the radiation of a diode laser at a wavelength of 808 nm, and an appropriate filter was installed in front of the camera, which enabled video recording in reflected light with almost complete blocking of the surface’s own radiation up to the maximum temperature. Reflected diode-laser radiation at a wavelength of 808 nm, which significantly exceeded the background of thermal radiation, was reliably recorded by a spectropyrometer (see the characteristic spectrum in Fig. 3). The reflected signal recorded in this way, helped to obtain additional qualitative information about the processes occurring on the heated surface, although this signal was not directly related to the directional hemispherical reflectivity due to the change in the angular distribution of the reflected radiation during the experiment.
Preparation of samples of
non-stoichiometric zirconium carbide
Due to the fact that zirconium carbide ZrCx (x < 1) is a substance with a wide homogeneity domain with vacancies in the non-metallic sublattice, it seems interesting to study the process of high-energy impact on this carbide with different stoichiometry parameters x. For this purpose, samples of three compositions with x = 0.98, 0.9, and 0.77 were prepared having insignificant amount of impurities in the material, high homogeneity and low porosity. The initial powder of zirconium carbide was obtained by the method of self-propagating high-temperature synthesis (SHS). This technology was chosen due to the possibility of obtaining cleaner samples compared to the methods of carbothermal reduction of oxides and some others. For the synthesis of zirconium carbide, weighed portions of zirconium powders (PTsRK‑1 grade) and acetylene soot were mixed in a ball mill, after which a temporary technological binder, cetane, was introduced into the resulting mixture. Cylindrical green pellets for synthesis were pressed from the resulting mixture. Then the temporary technological bond was removed by heating in a high-temperature vacuum furnace. Next, the furnace was filled with argon, and the temperature was raised to 1673 K to carry out the carbide synthesis reaction.
To obtain samples with low porosity, the synthesized zirconium carbide was dispersed in a planetary mill. The crushed powder after washing away from the grinding media material was sintered in a hot pressing unit with induction heating at a temperature of 2130 K and a pressure of 60 MPa. The thickness of the sintered specimens was typically about 2 mm with a diameter of about 12 mm, which was determined by the internal diameter of the hot press graphite mold. The sintered sample is processed on a grinding machine to remove graphite and reduce the thickness of the sample to 1.5 mm. Then the zirconium carbide sample was subjected to laser-erosion treatment to cut it to three reduced samples with a diameter of 5 mm (Fig. 3), after which these samples were re-ground. As a result, three samples of zirconium carbide with a diameter of 5 mm and a thickness of 1.5 mm were obtained having the same characteristics (density, composition) necessary for conducting a series of tests in different heating regimes.
The carbon content in zirconium carbide samples was determined by the dynamic flash method with a Combustion elemental analyzer Master CS (NCS, Germany); the content of oxygen and nitrogen with the method of reduction-melting in the elemental analyzer Fusion Master ON (NCS, Germany). XRD phase analyses of zirconium carbide samples was carried out with a diffractometer SmartLab SE (Rigaku, Japan) using Cu Kα radiation. The diffraction patterns (Fig. 4) contain only reflections corresponding to the zirconium carbide phase. General characteristics of the samples are presented in Table 1.
Experimental results
The experiments were carried out on samples of zirconium carbide of three different compositions: ZrC0.77, ZrC0.9, ZrC0.98. A sample of 5 mm in diameter was placed in a graphite holder. The sample surface was blown with a weak air flow. Radiation from the high-power disk laser and the diode laser was focused onto the surface of the sample, with the diameters of the focusing spots being close to the diameter of the sample. Experiments with samples of each composition were carried out with different heating rates shown in Fig. 2. In this case, the temperature increase rate on the surface was 700–1000 K / s, 2000–3800 K / s, and 8500–12000 K / s, respectively. By approximating the thermal emission spectra of the sample surface (Fig. 5) by the product of the Planck function and emissivity, the values of the true surface temperature and spectral emissivity were calculated under the assumption that in a relatively narrow wavelength range, the emissivity does not depend on the wavelength (graybody approximation). The approximation was carried out in two wavelength ranges (730–790 nm and 820–870 nm) in order to exclude from consideration the effect of diode laser radiation. Taking into account the fact that, as will be shown below, a significant layer of zirconium oxide is already formed on the surface by the time the lower temperature limit for the spectropyrometer is reached (which is approximately 2000 K), the gray body approximation for zirconium oxide looks quite adequate [14].
Fig. 6 shows the results of a typical experiment with a heating time of about 1.5 s for a sample with a minimum carbon content. The spectral emissivity in the wavelength range used for the non-linear fitting (730–870 nm) in the temperature range recorded by the spectropyrometer lies in the range of about 0.6–0.75. This is in a good agreement with the measurements performed in [16] under conditions of laser heating of a bulk sample of with the yttria stabilized zirconium dioxide. At the initial stage of heating, a sharp decrease in reflection by the sample surface is observed, which begins at a temperature in the vicinity of 1000 K. According to [12, 13], this temperature is in a good agreement with the temperature at which an active oxidation of zirconium carbide begins. The reflectivity of zirconia at this temperature is very high [17] and is comparable to the reflectivity of zirconium carbide. Despite this, the thin oxide scale that appears on the surface is most likely a substoichiometric oxide, which has a very low reflectivity.
Registration of the heating process using a high-speed video camera also confirms the sharp darkening of the surface irradiated by the laser. Near time point A (see Fig. 6), the emissivity and reflection signal show a feature that is clearly visible in the heating thermogram. The temperature corresponding to it, about 2600 K, is close to the well-known transition of zirconium dioxide from the monoclinic phase to the cubic one.
This behavior of ZrCx samples was qualitatively observed for all three initial stoichiometric compositions and manifested itself in experiments performed with different heating rates. During heating at a lower rate and, consequently, with a slower rate of temperature increase, the thickness of the zirconium dioxide scale reached the maximum value of about 20 μm. During heating at an medium rate (see Fig. 2), the scale thickness was about 10 µm, and during the fastest heating, about 5 µm.
After the experiment, the samples were studied by various methods. Figure 7 shows the X-ray diffraction pattern of the sample surface after the experiment, taken with a diffractometer SmartLab SE. It turned out that the white scale formed on the surface corresponds to the monoclinic ZrO2.
Raman scattering spectra of the modified surface of the initial carbide samples were obtained using an M 532 Raman microscope. Fig. 8 shows the Raman spectrum of an oxide scale compared to high purity zirconia powder. The obtained Raman spectra did not depend on the initial stoichiometric composition of the zirconium carbide samples and were in a good agreement with a pure ZrO2 powder. The resulting Raman spectrum corresponded to the monoclinic phase of ZrO2 being in accordance with the data of [18].
Thus, it turned out that only the monoclinic phase was found in zirconium dioxide formed on the surface of zirconium carbide during intense laser heating, although in [13] both the tetragonal and cubic phases were found along with the monoclinic phase. This issue is expected to be clarified in the course of further research.
To determine the thickness and the structure of the region of thermochemical transformation of the surface of zirconium carbide into zirconium dioxide, the SEM analysis of the surface area subjected to heating was performed. Fig. 9 shows the SEM image of a cleavage perpendicular to the sample surface. In the left image, the colored lines show different zones: the surface of the sample; the cleavage zone is the initial ZrC0.77 sample; 1 – transition zone of carbide material; 2 – formed oxide. On the right, the zones 1 and 2 shown at higher magnification.
The distribution of elements in the vicinity of the oxidation zone was studied using SEM-EDS analysis of the thin section, made perpendicular to the sample surface. The results of the analysis are presented in Fig. 10. It turned out that the oxidized zone has a very clear interface with the original carbide. Fig. 10a shows a section made perpendicular to the sample surface with an indication of the area of elemental EDS mapping – A, and the direction of scanning the distribution of elements – B. From Fig. 10 b and 10 d, it follows that the distribution of oxygen in the near-surface layer with a thickness of about 20 μm is very uniform. It corresponds to a lower density of zirconium in this layer compared to the original sample (see Fig. 10b and 10c). In general, the distribution of both zirconium and oxygen is very uniform both in the initial zone of the carbide and in the formed oxide scale. In all regimes of laser exposure and for all the compositions of the samples considered, a tight fit of the oxide layer to the initial carbide is noted. Thus, it is possible that the forming oxide layer will effectively prevent the oxidative degradation of the dense zirconium carbide under a certain heating conditions.
Acknowledgments
The study was supported by the Russian Foundation for Basic Research and Rosatom State Corporation within the framework of the scientific project No. 20-21-00115
ABOUT AUTHORS
Sheindlin Mikhail Aleksandrovich, head of laboratory, laboratory for extreme energy impacts, JIHT RAS, Moscow, Russia.
ORCID: 0000-0002-4960-7757
Brykin Mikhaik Vladimirovich senior researcher, laboratory for extreme energy impacts, JIHT RAS, Moscow. Russia.
ORCID: 0000-0002-8046-888Х
Bgasheva Tatiana Vladimirovna, junior researcher, laboratory for extreme energy impacts, JIHT RAS, Moscow, Russia.
ORCID: 0000-0002-5258-0153
Vasin Andrey Andreevich, leading engineer, laboratory for extreme energy impacts, JIHT RAS, Moscow, Russia.
ORCID:0000-0002-2384-7090
Vervikishko Pavel Sergeevich, researcher, laboratory for extreme energy impacts, JIHT RAS, Moscow, Russia. E-mail: pvervikishko@gmail.com.
ORCID: 0000-0002-4527-6524
Petukhov Sergei Vladimirovich, leading engineer, laboratory for extreme energy impacts, JIHT RAS, Moscow, Russia.
ORCID: 0000-0003-3852-1314
Frolov Aleksander Michailovich, researcher, laboratory for extreme energy impacts, JIHT RAS, Moscow, Russia. E-mail: matotz@gmail.com.
0000-0002-3091-9451
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