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ВЕСТНИК ЮГОРСКОГО ГОСУДАРСТВЕННОГО УНИВЕРСИТЕТА
2012 г. Выпуск 2 (25). С. 28-33
УДК 533.924; 620.22.8
IN-SITU SELFPROPAGATING-HIGHTEMPERATURE-SYNTHESIS CONTROLLED BY PLASMA
P. Yu. Gulyaev, I. P. Gulyaev, Cui Hongzhi, I. V. Milyukova
Introduction
One of expedients of protection against electromagnetic disturbances, drawing on a surface of a product of express mattings is. It is possible to apply a plasma spraying of a powder for this purpose from ferromagnetic substance on a base of nickel, iron, titanium or a cobaltous. Properties and structure of a magnetic medium should remain invariable after action of high temperature of plasma. Such powder materials are easily available with minimal power inputs by means of SHS. It is not quite clear at which temperature regimes of plasma spraying SHS-materials retain its physical-mechanical properties and composition in the obtained coating. In fact the synthesis of material occurs in two steps: “rapid” - due to the melting and quenching of intermediate products in the SHS
combustion wave, and “slow” - due to the diffusion processes in the afterburning zone and slow cooling down of final synthesis products. Subsequent plasma treatment of SHS powder materials can lead to unpredictable changes in structure and in phase composition.
The combination of plasma spraying and SH-synthesis at the parallel and subsequent application sufficiently broadens capabilities of the both technologies (e. g. [1]) However in such case plasma generators requirements become even tougher and demand technologically necessary process conditions: stable and uniform along the cross-sections plasma jet, variable in wide range thermal power (plasma enthalpy), possible use of different working gases, as well as economical demands of long life time and high electrical and thermal efficiency.
At present time there are two basic types of plasma torches used in the industry:
1) conventional, with self adjusting electrical arc length;
2) cascade, allowing to stabilize arc length with the inter electrode insert (IEI).
o 4—■ i ■ 1 ■ 1 ■ \ ■ T
0 400 800 1200 1600 2000
Reynolds number
Figure 1. Plasma torches operating window
The advantage of the former type is considerably simpler construction, but significant burning arc voltage pulsations (therefore and power) at characteristic dwell time of the particles, lower thermal efficiency, irregularities of the gas flow due to the permanently altering arc attachment spot at the anode, do not allow to produce high quality laminar jet, required for the spraying [2].
Cascade plasma torches, which have been designed by ITAM SB RAS, successfully solve all the mentioned problems; at the same time they have wide operating window which covers similar parameters of widespread commercial analogs (figure 1). It describes such IEI plasma torches as universal and multi-functional devices.
The influence of plasma spraying parameters on adhesion-cohesion strength of the coating was not investigated in present paper, but only the changes in structure and phase characteristics of coating material during the spraying process were studied.
Experimental Procedure
The research was aimed at investigation of possibilities of nickel and titanium aluminide powders obtaining, detection of the most efficient controlling methods of obtained materials structure and phase composition (as parameters which determine overall functional properties) in order to produce industrial samples of materials for plasma spray coating.
Figure 2. The complex technique of structure and phase transformation investigation
Investigation was held for systems Ni - 18 mass % Al, Ni - 31.5 mass % Al, Ti - 31.5 mass % Al and Ti - 66,3 mass % Al. Size of initial nickel and aluminum powder particles was 5-15 p,m, prepared powders mixture apparent density was 2.7 g/cm3. Initial material temperature was varied from 293 to 573 K. Crushing of obtained SHS sintered material was performed with cone inertial mill KID-100, 63-160p,m fraction was prepared for plasma spraying. Coating was sprayed at steel substrates with plasma spraying device “Kiev-7” at the following conditions: working gas - argon, arc voltage U=250V, current I=230A, spraying distance 180 mm. Coating temperature was measured with 2-color pyrometer using IR filter, cutting off plasma radiation.
Investigation of sprayed material was performed using X-ray diffractometer DRON-UM1 at Co wavelength, raster electronic microscope BS-300, metallofgaphic microscope Neophot, microana-lizer MAP-2.
Figure 2 shows the complex technique of structure and phase transformation investigation.
The synthesis in the reacting mixture of two or three components was initiated by application of local thermal impulse. Propagating through the volume of reacting mixture combustion wave was investigated by means of brightness pyrometry and digital video filming: the set of characteristic reaction parameters was obtained as a result.
Using the developed SHS-process diagnosis technique based on the brightness pyrometry we were able to measure SHS parameters: characteristic times (heat emission time, heat sink time, thermal induction time) and temperature from the thermograms and thermal half-width and combustion front velocity from the thermal imaging of the combustion zone (figure 3).
Figure 3. SHS-process characteristic parameters
SHS-process temperature spots, corresponding to the phase transitions, were found comparing thermal profile of combustion wave and phase state diagrams (figure 4).
The mechanism of reacting components interaction was suggested using known information on thermal effect of formation, activation energy of processes, temperature temporal dynamics. Joint course of different physical-chemical processes corresponding to the different areas of the state diagram was considered.
Figure 4. Thermogram of SHS-process at different initial temperatures for Ni - 31.5 mass % Al system
For example, figure 5, a shows Ti particle, surrounded by titanium aluminides of different stoichiometry. This structure was obtained during “quenching” of the sample in the SHS-process in the copper cone setup, where cooling rate was 104-105 K/s. Fig. 5, b shows the thermogram of this process, typical for the quasi-homogeneous regime of combustion wave propagation.
Figure 5. Structure of the reaction product in the blow out zone in the Ti - 39.6 mass % Al (x320) - (a), and thermogram of synthesis process - (b)
Investigation of interaction in the Ti-Al system showed, that structure formation in the Ti-Al system occurs under the reactionary diffusion mechanism, related to initiation of intermetallic layer of TiAl3 at the surface of Ti particle and its subsequent growth and re crystallization.
In order to be able to produce coatings with predictable properties we should analyze the influence of low-temperature plasma on the obtained SHS-powder. For this purpose structure and phase composition of sintered material, powders prepared for plasma spraying and coatings were studied [3, 4].
Results Analysis
According to the X-ray structure and metallographic analysis, Ni3Al phase with micro-hardness of 3.02-3.48 GPa mostly formed during Ni - 18 mass % Al system combustion. Moreover, there was unreacted Ni and NiAl composite with micro-hardness of 4.07-4.24 GPa discovered in the synthesis product (figure 6, a).
Increasing the initial temperature T0 of the SH-synthesis in this mixture up to 473 K leads to more complete transition of components (decrease of residual Ni amount) and appearance of Ni2Al3
phase in the product.
60 50 40 20
Figure 6. Diffraction patterns of Ni - 18 mass % Al: a) sintered(T0=293K); b) sintered(T0=473K); c) coating (T0=293K); d) coating (T0=473K)
t---------'--------1---------■----------r
60 50 40 2©
Figure 7. Diffraction patterns of Ni - 31.5 mass % Al: a) sintered (T0=293 K); b) sintered (T0=473 K); c) coating (T0=293K); d) coating (T0=473K)
Combustion product in the Ni - 31.5 mass % Al system is single-phase NiAl with microhardness 4.24-5.81 GPa. Preliminary heating does not lead to any structural changes of synthesis product in given mixture (figure 7, a, b).
All the investigated systems had similar phase and structure composition of sintered material and corresponding crushed powders, prepared for spraying. Only some differences in microhardness were discovered; e.g. Ni - 31.5 mass % Al system particles have micro-hardness of 3.23-4.48 GPa, which is somewhat lower than one of sintered material. Probably, crushed edges become a drain of structure defects (such as vacancies, dislocations) which leads to the relaxation of internal stresses [5].
The matter of interest is whether plasma sprayed coatings inherit SHS-materials phase properties. X-ray structure and metallographic analysis of SHS sintered material of Ti - 39.6 mass % Al showed, that main part of material is mono-aluminide of titanium; moreover Ti3Al phase was found as inclusions in the center of grains (fig. 8, 9, a). The structure of final SHS product in the Ti - 66.3 mass % Al is formed by homogeneous intermetallide TiAl3; also the structure contain some amount of residual aluminum, which is proved by diffraction phase analysis also (fig. 8).
TiAl+Ti5Al
TiAl3
TiAl3
~55 50 45 40 35"~2©
Figure 8. Diffraction patterns of Ti - 39.6 mass % Al: 1 - sintered material, 1’ - coating;
Diffraction patterns of Ti - 66.3 mass % Al: 2 - sintered material, 2’ - coating
X-ray phase analysis of coatings has shown that process of plasma spraying does not produce formation of new phases, leading only to small changes in phase quantitative ratio in those samples, where material is multi-phase (fig. 6-8).
Additional investigation of sprayed SHS products of Ni-Al, Ti-Al and also some alloyed systems showed, that significant changes in the phase and structure characteristics occur only at spraying conditions, where temperature of particles significantly exceeds temperatures reached in the SHS process of given materials [6, 7, 8].
Conclusion
Metallographic investigations of sprayed coatings have shown features of coated layer construction. The structure of layer is not uniform due to irregularity of particle temperature and veloci-
ty distribution across the jet, which proves strict requirements of plasma generators characteristics. Formation of the coating by consequent deposition of great number of particles with different velocity, temperature and aggregate state leads to the laminar structure with large amount of grain inclusions and micro-opens (fig. 9, b).
(a) (b)
Figure 9. Microstructure of sintered material - a, and coating - b in Ti - 39.6 mass % Al system (x300)
It should be noted, that two groups of coatings with different kind of structure and phase composition heredity were discovered:
• single-phase and heterophase products of refractory compositions which do not experience diffusion rearrangement of components
• materials with eutectic structure which experience quantitative phase changes during plasma spraying.
Application of high-speed pyrometry technique during SH-synthesis as well as plasma spraying allowed to compare temperature regimes of these processes and determine simple and reliable criteria of producing coatings with predictable - “heritable” properties of SHS-materials: critical parameters (gradient and peak temperatures) in the plasma flow should not exceed similar characteristics of SHS combustion wave.
REFERENCES
1. Solonenko, O. P. State of the art of thermophysical fundamentals of plasma spraying, In book : Thermal Plasma and New Materials Technology, Ed. by O. P. Solonenko and M. F. Zhukov, Cambridge, England : Cambridge International Science Publishing, 1995, Vol. 2, P. 7-97.
2. Gulyaev, P. Yu. The Structurally-phase transformation of SHS-materials after plasma jet coating / P. Yu. Gulyaev, V. V. Evstigneev, A. V. Kalachev, I. V. Milyukova // Abstr. book : VIII International Symposium on Self-Propagating High-Temperature Synthesis (SHS 2005). -Quartu S. Elena (CA), Italy 21-24 June, 2005. - P. 43.
3. Wu Jie. Open-celled porous NiAl intermetallics prepared by replication of carbamide space-holders [Text] / Wu Jie, Cui Hong-zhi, Cao Li-li, Gu Zheng-zheng. Trans. Nonferrous Met. Soc. China, 2011. - V. 21. - P. 1750-1754.
4. Evstigneev, V. V., Milyukova, I. V. et al., Integral technologies of self-propagating high-temperature synthesis, Moscow, Vyshaya shkola, 1996. - 274 p.
5. Evstigneyev, V. V. A new procedure of high-rate brightness pyrometry for studying the SHS processes [Text] / V. V. Evstigneyev, P. Yu. Gulyayev, A. B. Mukhachev, D. A. Garko // Combustion, Explosion, and Shock Waves, 1994. - Vol. 30. - № 1. - P. 72-77.
