CC BY
23iSHS 2019
Moscow, Russia
SELF-PROPAGATING HIGH-TEMPERATURE SYNTHESIS OF HEAT-RESISTANT ELECTRICALLY CONDUCTIVE COATINGS BASED ON Ni-Al AND Ti-Al-C SYSTEM COMPOUNDS
A. M. Shulpekov*", R. M. Gabbasov", and O. K. Lepakova"
Tomsk Scientific center SB RAS, Tomsk, 634055 Russia
*e-mail: [email protected]
DOI: 10.24411/9999-0014A-2019-10162
Compounds formed in the Ni-Al and Ti-Al-C systems have high heat resistance, oxidation resistance, hardness and electrical conductivity [1, 2]. Coatings of these materials are used as heat-resistant protective layers [3]. Traditionally, such coatings are produced by the method of air-plasma spraying [4]. The method requires the use of a large amount of energy to create a plasma and to provide a high temperature for melting the coating components. In this paper, we have proposed a method for obtaining a coating, which consists in applying a mixture of powders of the initial components to the substrate. Heating a portion of the powder layer leads to an exothermic synthesis reaction of compounds with increasing temperature. Further, the reaction spreads over the surface in the form of a combustion wave. Thus, the energy costs are insignificant and are only needed to initiate the process. This process is the essence of obtaining materials by the method of self-propagating high-temperature synthesis (SHS). The aim of the work is to study the effect of the layer thickness on the temperature and speed of propagation of the combustion wave, the structure and electrical conductivity of the coating. To obtain a coating based on aluminum nickelides, a mixture of powders of nickel PNC1-UT1 and aluminum grade ASD-4 was prepared in a ratio of 31.5 wt % Al and 68.5 wt % Ni. And to obtain coatings based on the MAX phases of Ti2AlC, Ti3AlC2, powders of titanium PTX with a particle size of less than 40 p,m, aluminum (ASD-4), carbon black (PM-15) were used in a ratio of 70 wt % Ti, 20 wt % Al and 10 wt % carbon black. The powders were previously annealed in vacuum at 200°C. A mixture of powders in the form of a suspension in isopropyl alcohol was applied onto a ceramic plate VK-1 through a stencil with a thickness of 0.3 to 2 mm and a width of 20 mm. The coating was dried in air at room temperature for 24 h. Previously, thermocouples of the T-type or WRe 5/20 brand were fixed on the ceramic plate at a distance of 10-35 mm from each other. To record thermograms, thermocouples were connected to an ADC and a personal computer. The propagation velocity was determined from the delay of the signal from two thermocouples (Fig. 1). The initiation of the reaction was carried out using an electric helix heated by an electric current. The phase composition of the synthesis products was determined on a portable tabletop X-ray device RIKOR (CoKa radiation) provided by Tomsk common use center SB RAS. Microstructural studies were performed on an optical microscope (Axiovert 200M, Karl Zeiss). The electrical resistance of the coatings was measured using a F-410 milliohm meter with a measurement range of 1 x 10-2-1 x 107 Q. In separate experiments, the temperature of the combustion wave was measured by spectral pyrometry using a CCD spectrometer (HR 4000, Ocean Optics) (200^1100 nm, frequency 220 Hz, the duration of signal accumulation in a single spectrum 4.5 10-3 s) [5]. The experimental results show that after initiating the reaction, a burning wave with a front about 5 mm wide runs through the sample. The burning wave profile is shown in Fig. 2a. The temperature in the wave of combustion is much lower than that of cylindrical samples of similar composition. This is due to the large heat sink for flat specimens. The wave propagation velocity (5-10 mm/s) is close to the velocity of the combustion wave front of cylindrical samples of similar composition.
XV International Symposium on Self-Propagating High-Temperature Synthesis
Ceramic plate Thermocouples
Fig. 1. Experimental setup.
x-x A, nm
(a) (b)
Fig. 2. (a) Combustion wave profile and (b) the emission spectrum from the combustion wave (for Ni-Al): 1 Ti-Al-C system, 2 Ni-Al.
For samples of composition 2Ti-Al-C, the temperature of the combustion wave front is higher than for samples of Ni-Al. This is also consistent with the data obtained for cylindrical samples. The emission spectrum from the combustion wave, shown in Fig. 2b, is close in shape to the spectrum of the thermal radiation of a heated body; the characteristic emission bands of the ions of the components of the initial mixture are not observed. The brightness temperature calculated from the emission spectra at 200-300°C above the temperature measured by a thermocouple. This may be due to the effect of radiation from the oxidation products of the powders, since the experiment was conducted in air.
With a coating thickness of less than 200 pm, the front does not spread, and with a thickness of more than 1500 pm, the ceramic substrate is destroyed due to thermal shock. With an increase in the thickness of the coating as NiAl, the velocity of the combustion wave front and the maximum temperature increase (Fig. 3a). This is due to the fact that the amount of heat released in the combustion wave increases. Heat losses are proportional to the sample area and, when its diameter changes, increase to a lesser degree than heat generation. Consequently, the energy of the system increases and, as a result, the temperature and speed of propagation of the front. In layered systems, the sample thickness is analogous to the diameter for cylindrical samples and the observed patterns are similar.
h, mm h, mm
(a) (b)
Fig. 3. The velocity of propagation of the front (curve 1) and the maximum temperature (curve 2) of Ni-Al (a) and Ti-Al-C (b) coatings of various thickness.
_SI IS 2019_Moscow, Russia
It can be seen in Fig. 3a that a similar regularity is observed for the Ti-Al-C system, but with a large layer thickness, the curve tends to saturation. This pattern is consistent with theoretical ideas about the processes of the combustion wave.
In addition, the propagation velocity of the front and its temperature largely depend on the relative density of the samples and the initial temperature. The use of metal substrates (Cu, Al, Fe, Ti) leads to an increase in the velocity of propagation of the front.
(a) (b) (c)
Fig. 4. (a) XRD patterns: I NiAl, II Ti2AlC and the microstructure of the coating Ti2AlC (b), and Ni-Al (c). Phases are designated: 1 NiAl, 2 Ni3Al, 3 Ti2AlC, 4 Ti3AlC2, 5 TiC.
The Ni-Al coating contains NiAl and Ni3Al phases (Fig. 4a). The predominant phase is NiAl. Nevertheless, the presence of the Ni3Al phase indicates a significant lack of response in the combustion wave. This is due to the fact that the temperature in the combustion wave is much lower than for samples in the form of a cylinder. Increasing the thickness of the coating increases the concentration of the target phase NiAl. This is due to the increase in temperature in the combustion wave and the intensification of processes. The coating based on the Ti-Al-C system consists of the phases Ti2AlC, Ti3AlC2, TiC. With an increase in the thickness of the powder mixture, the content of Ti2AlC relative to Ti3AlC2 increases. The high content of titanium carbide indicates a significant failure in the system. Figure 4b shows a micrograph of the Ti2AlC coating site. It can be seen that the coating consists of small rounded particles, often hollow, fused together. With a larger magnification, it can be seen that the particles consist mainly of needle-like crystals of MAX phases. Ni-Al coating consists of rounded solid particles fused with each other. At high magnification, it can be seen that the particles consist of two phases NiAl and Ni3Al distributed in each other. The electrical resistance of the coating is 2-8 x 10-2 Q. This coincides with the resistance value for cylindrical samples obtained by the SHS method. This is due to the fact that the coating consists of many small particles fused with each other. It should be noted that the electrical resistance decreases with increasing coating thickness, which indicates that the resistivity of the coating material remains constant.
Thus, the effect of the thickness of the powder layer, its relative density and the initial temperature of the mixture on the front propagation velocity and the maximum temperature of the reaction wave for the layered powder mixtures of Ni-Al and Ti-Al-C during the SHS process is studied. A coating was obtained on the basis of the target heat-resistant phases — NiAl, Ni3Al or Ti2AlC, Ti3AlC2 with an optimum thickness of 200-1200 pm and an electrical resistance of 2-8 x 10-2 Q. The coating can be used as protective layers or film electric heaters.
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