Investigation of diffusion processes in multilayer composite thermal coatings.
In addition to practical interest, CHT of the plasma coatings also represents scientific interest in connection with peculiarities of the sprayed coating structure --laminar, discrete, inhomogeneous character of structure of the coatings, presence of pores and oxide inclusions in them, and presence in the structure, in addition to conventional intergranular boundaries, of different additional boundaries (between particles located in the same layer; interlayer boundaries parallel to the base plane, and, finally, boundaries between the base and the coating), which represent a barrier for progress of the diffusion process. In addition, specificity of the spraying process creates premises for formation of non-equilibrium structures and occurrence of a complex stressed state. Such difference in structures of a coating and a cast material affects processes of the diffusion interaction when CHT of thermal coatings is performed.
Available in the literature data relate mainly to CHT of the coatings, produced by the method of thermal spraying (TS) of powders of metals and metal alloys [2-10] and composite powders [11-16]. Main technological version of CHT is diffusion saturation in powder mixtures in a container with a melting gate or in the shielding environment.
In all kinds of CHT, implemented by application of different technological versions, the goal of this treatment is improvement of certain characteristics of the coating, for example, density, hardness, and strength of cohesion with the base.
In a number of works combination of TS and CHT was performed by application on surface of an item by the metallization method of chromium , NiCr  and AlSn [19, 20] alloys, or by plasma spraying of silicon  with subsequent annealing that caused change of phase composition of the surface layer.
Main peculiarity of the CHT process of the TS coating consists in the need to select CHT conditions, which allow obtaining required characteristics of the coating without reduction of strength of its adhesion to the base and strength properties of the base metal. It is important to avoid in this case Kirkendall's effect, i.e. formation of porosity on the boundary of layers because of different diffusions rates of chemical elements. For forecasting working life of the coatings (taking into account losses of protection properties because of the diffusion resolving) it is necessary to study the diffusion interaction processes, which proceed in a multilayer coating at the temperatures of possible operation.
In this work object of the investigation are composite coatings, containing nickel-bound oxygen and oxygen-free compounds. The most acceptable type of CHT for them, which would cause increase of wear, heat and corrosion resistance, may be alitizing and alumosiliconizing, due to which formation of heat-resistant intermetallics of the Ni--Al system and [Ni.sub.16][Ti.sub.6][Si.sub.7], [Ti.sub.5][Si.sub.4] silicides should take place. That's why layer of the Al-Si alloy was selected as second upper layer.
Material and methodology of the experiment.
For plasma spaying composite powder with size of particles 40-80 mm, produced by the method of self-propagating high-temperature synthesis, of the following composition was used, wt.%: Ti[Si.sub.2]--30.5; [Al.sub.2][O.sub.3]--17; Ni--42.5; [Cr.sub.3][C.sub.2]--10. For electric arc metallization wire of 95Al--5Si (wt.%) alloy of 2 mm diameter was used.
On the layer, produced by plasma spraying of Ti[Si.sub.2]-[Al.sub.2][O.sub.3]-Nix[Cr.sub.3][C.sub.2] powder on the base from carbon steel (first layer), AlSi coating (second layer) was applied by arc metallization. Conditions of spraying are presented in Table 1. Then annealing at temperature values 700 and 750 [degrees]C within 1.5 h in the flow of high-purity argon was performed (GOST 1057-79).
Metallographic investigations of the specimens (polished and after chemical etching in 0.5 % water solution of hydrofluoric acid) were performed on the <<Neophot-32>> metallographic microscope.
Microhardness of the coatings was determined on the LECO instrument M-400 at the load 0.249 and 0.490 N. Strength of cohesion between the coating and the base was estimated by the glue method.
X-ray diffraction phase analysis (XDPA) was performed on the DRON-UM1 diffractometer in Cu[K.sub.[alpha]] copper radiation. Chemical composition of separate areas of the coating was determined by means of X-ray spectral microanalysis (XSMA) on the Camebax SX50 instrument.
Wear resistance of the coatings was estimated under conditions of the gas-abrasive wear at high temperature values on a specially designed installation at the following parameters: temperature of the specimen (600 [+ or -] 10) [degrees]C; pressure of heated air supplied by a compressor through the nozzle equaled 0.5 MPa, which ensured velocity of air flow with the abrasive of 140 m/s; distance from the nozzle edge to the specimen was 50 mm. Powder of [Al.sub.2][O.sub.3] synthetic corundum of 0.6-0.8 mm dispersity was used as the abrasive. Angle of the air-abrasive flow attack was 90[degrees].
Wear of the coating was established by loss of the mass in relation to a unit of the specimen area and 1 kg of the abrasive mass, getting on the specimen, i.e. intensity of the mass wear (mg/([cm.sup.2] x kg)) was determined.
Results and discussion thereof. As a result of TS dense two-layer coating without cracks and lamination in the zone of connection with the base, having strength of cohesion with the latter 40 MPa, has formed. Porosity of the layer produced by the plasma spraying was 10-12 vol.%; of the layer produced by the electric arc metallization--4-5 vol.%.
In Figure 1 and Table 2 microstructure and characteristics of the two-layer coating are presented. Structure of the internal layer is of lamellar-granular character. White lamellas of nickel, light orbicular particles, lamellas of titanium silicides of complex composition, fine-dispersed particles of chromium carbide of up to 5 mm size, and dark coarse particles and lamellas of aluminium oxide prevail in it.
External layer of the coating, produced by the method of electric arc metallization, is formed from lamellas of the Al--Si alloy, containing dispersed (up to 1 mm) particles of silicon (Si[O.sub.2]) and aluminium ([alpha]-, [gamma]-[Al.sub.2][O.sub.3]) oxides.
[FIGURE 1 OMITTED]
On X-ray photograph of the surface layer diffusion halo within the range of angles 28 [less than or equal to] 29 [less than or equal to] 52 [degrees] was detected, which proves presence of the amorphous component in the structure.
As a result of heat treatment at the temperature 700 [degrees] C within 1.5 h processes of intensive diffusion interaction took place between layers of the coating (Figures 1, b and 2, Table 2). Diffusion of nickel, chromium and titanium from the plasma-sprayed layer into external aluminium-base layer with formation of nickel and titanium aluminides and complex compounds of aluminium, chromium and silicon was registered.
[FIGURE 2 OMITTED]
Sources of nickel and titanium are nickel bonds of the plasma coating and complex silicide [Ni.sub.16][Ti.sub.6][Si.sub.7], which forms during heating thermodynamic more stable low silicide [Ni.sub.3][Ti.sub.2]Si. Chromium carbides [Cr.sub.3][C.sub.2] and [Cr.sub.7][C.sub.3] transformed as a result of heating into the low carbides [Cr.sub.7][C.sub.3]-[Cr.sub.23][C.sub.6] with reduction of their general amount. By this fact formation of compound [(Si, Al).sub.2]Cr in the zone adjacent to the plasma layer (Figure 2, c) can be explained. In the zone of connection of two layers microporosity, caused by diffusion of nickel, titanium and chromium, was detected. As a result of diffusion of these elements compaction of the upper metallization layer occurs. At the same time formed microporosity does not cause lamination of the two-layer coatings and does not reduce strength of cohesion with the base.
[FIGURE 3 OMITTED]
Cross diffusion of aluminium from external layer into the internal composite one, which is characteristic of aluminide coatings on the bases of nickel and its alloys , was not registered. It is, evidently, connected with complex composition and presence of additional boundaries in the composite thermal layer, which acts as a barrier for progress of the diffusion processes.
Microstructure of the external (second) layer, in contrast to the internal (first) one, underwent significant changes: it got compacted and became practically pore-free; three diffusion zones, differed by structure, phase composition and hardness, may be singled out in it (see Figure 1, b, Table 2).
In the surface layer 2a content of the dispersed strengthening particles has increased in comparison with non-annealed state mainly due to newly formed NiAl nickel aluminides and presence of oxides.
On X-ray photograph of the surface layer, as well as before the annealing, diffusion halo was detected within the same range of angles that proves presence of the amorphous component of the structure. Microhardness of this layer has increased two-fold after the annealing.
Lower located zone 2b consists of a light matrix with elongated and columnar crystals of dark color. Direction of growth of the crystals coincides with directed diffusion of nickel, chromium and titanium from first layer into second one. According to the data of layer-by-layer XDPA, nickel and titanium aluminides and titanium silicides are formed as a result of the diffusion interaction between these elements and Al--Si alloy, which causes significant strengthening of this zone. Microhardness of this area has increased by one order (Table 2).
In direct proximity to the plasma layer the hardest dark zone 2c is located, which mainly formed from particles of nickel (AlNi, [Al.sub.3]Ni) and titanium ([Al.sub.3]Ti) aluminides, titanium silicides ([Al.sub.5][Si.sub.4]), and complex compounds--chromium alumosilicides [(Si, Al).sub.2]Cr. In this zone of second layer the least amount of the aluminium-containing component was noted (see Table 2).
Phase composition of the products of solid-phase chemical reaction between two components is predetermined by correspondence of thermodynamic conditions of equilibrium of phases of a certain composition and the conditions, established in a certain area of the reaction zone. Most frequently sequence of arrangement of the single-phase layers qualitatively corresponds to that of the phases along isothermal horizontal on the constitutional diagram. Such dependence is observed in diffusion of nickel, weight share of which prevails in the plasma coating (42.5 wt.%), into the metallized aluminium-base layer with formation of aluminides.
It is also necessary to take into account in the multicomponent systems ratio of the diffusion rate of different components, which jointly diffuse in the same direction, and chemical activity thereof. Both these factors may stipulate non-uniformity of distribution of concentration of components of a complex system in layers of the formed reaction products. This, in its turn, enables formation of the compounds, which do not correspond to sequence of arrangement of the phases along isothermal horizontal on the constitutional diagram. Exactly such effect was registered in formation of [Al.sub.23][Ti.sub.9] and [Al.sub.3]Ti titanium aluminides in the areas 2b and 2c.
According to the results of metallographic analysis and layer-by-layer XDPA and XSMA, principal differences in phase composition and structure of the coatings after their heat treatment at temperatures 700 and 750 [degrees] C, as well as in character of diffusion of nickel, titanium, chromium and similarly located layers and diffusion zones of these complex coatings, were not detected. But the tendency to microhardness decrease was observed at annealing temperature of 750 [degrees] C. Evidently further increase of the heat treatment temperature may enable reduction of microhardness of the layer, produced by metallization, due to coagulation of the dispersed strengthening particles formed due to the diffusion interaction between layers of the coating.
The character of kinetic dependence of the gas-abrasive wear intensity at 600 [degrees] C (Figure 3) is affected by the fact that this two-layer coating has after heat treatment phase composition that changes over the depth. Fractography investigations of the surface (Figure 4) and phase composition of the coatings at different stages of the tests showed that external dense zone 2 a, consisting mainly from Al--Si alloy strengthened by dispersed inclusions of nickel aluminides and aluminium and silicon oxides, is most wear-resistant. Intensity of its wear does not exceed 1.5 mg/([cm.sup.2]-kg), whereby formation of cracks and shelling-out on surface in the process of tests was not detected (Figure 4, b).
After removal of surface zone 2 a of about 100 mm thickness intensity of wear increases up to 3.0-3.5 mg/([cm.sup.2]-kg) and remains constant up to full attrition of the metallization layer. At transition into the plasma-sprayed layer intensity of the gas-abrasive wear sharply increases up to 12.5 mg/([cm.sup.2] x kg).
[FIGURE 4 OMITTED]
Structure of zones 2b and 2c is less homogeneous; columnar crystals, containing according to XSMA data aluminium, titanium, nickel and silicon, which are nickel and titanium aluminides and titanium silicides, have formed in them (see Table 2). Wear of mentioned zones is accompanied by formation of cracks and cohesion destruction (Figure 4, c, d).
So, mechanical properties of the diffusion zones depend upon the shape and distribution of the strengthening particles. The most orbicular, uniformly distributed fine particles enable high ductility and relatively high strength of the layer; acicular (columnar) crystals significantly increase its hardness, but essentially reduce ductility. Wear resistance of zones 2a and 2b is higher than that of the plasmasprayed layer 8.0 and 3.5 times, respectively.
1. Processes of the diffusion interaction between layers of the two-layer thermal coating, lower layer of which is produced by the plasma spraying of composite powder Ti[Si.sub.2] x [Al.sub.2][O.sub.3] x Ni x Cr[C.sub.2] and upper one--by electric arc metallization of AlSi alloy at annealing in flow of argon at 700 [degrees] C within 1.5 h, were investigated.
2. It was established that annealing enabled significant changes in phase composition of the layers. In the compacted upper layer nickel and titanium aluminides, chromium alumosilicides and titanium silicides, formed in the process of diffusion of nickel, chromium and titanium of the plasma coating, were detected, whereby cross diffusion of aluminium was not registered, i.e. alitizing of the plasma coating did not occur.
3. It was determined that changes in morphology and phase composition of the coatings as a result of the diffusion interaction caused sharp increase (more than 3 times) of wear resistance at temperature 600 [degrees] C.
The authors express their gratitude to ScientificTechnical Center of Ukraine for financial support of this work, carried out within framework of project G 046.
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M.L. ZHADKEVICH, V.A. SHAPOVALOV, D.M. ZHIROV, G.A. MELNIK, M.S. PRIKHODKO and A.A. ZHDANOVSKY
E.O. Paton Electric Welding Institute, NASU, Kiev, Ukraine
Table 1. Conditions of spraying of two-layer coating Type of Voltage, Current, Plasma installation V A gas Plasma spraying--first layer UPU-8M 55 500 Ar + [N.sub.2] Electric arc metallization--second layer KDM-2 35 200 - Type of Spraying Powder MUF * installation distance, consumption, mm kg/h Plasma spraying--first layer UPU-8M 120 3.5 0.80 Electric arc metallization--second layer KDM-2 200--210 N/D 0.75 * Material utilization factor. Table 2. Characteristic of multilayer coating State of coating Layer of coating Thickness of layer, [mu]m After spraying First 300 Second 350 After annealing at First 300 700 [degrees]C, 1.5 h Second 350 After annealing at First 300 750 [degrees]C, 1.5 h Second 350 State of coating Phase composition After spraying Ni, [Ni.sub.16][Ti.sub.6][Si.sub.7], [gamma]-[Al.sub.2][I.sub.3], [alpha]-[Al.sub.2][I.sub.3], Ti[Si.sub.2], [Cr.sub.3][C.sub.2], [Nr.sub.7][N.sub.3] Al, Si, [alpha]-[Al.sub.2][I.sub.3], [gamma]-[Al.sub.2][I.sub.3], Si[O.sub.2], halo of 28 [less than or equal to] 20 [less than or equal to] 52[degrees] After annealing at Ni, [Ni.sub.3][Ti.sub.2]Si 700 [degrees]C, 1.5 h [alpha]-[Al.sub.2][I.sub.3] TiSi [Ti.sub.5][Si.sub.4], [Nr.sub.23] [N.sub.6], [Nr.sub.7][N.sub.3], [gamma]-A[chi][I.sub.3] a--Al, [Al.sub.3]Ni, [alpha]-[Al.sub.2][I.sub.3], [gamma]-[Al.sub.2][I.sub.3], [Al.sub.2]Si[O.sub.3], Si[O.sub.2], halo of 28 [less than or equal to] 20 [less than or equal to] 54[degrees] b--Al, AlNi, [Al.sub.3]Ni, [Al.sub.23][Ti.sub.9], [Ti.sub.5][Si.sub.4], [gamma]-[Al.sub.2][I.sub.3], [alpha]-[Al.sub.2][I.sub.3] c--AlNi, [Al.sub.3]Ni, Al, [Al.sub.3]Ti, [Ti.sub.5][Si.sub.4], (Si, Al)[sub.2.Cr] After annealing at Ni, [Ni.sub.3][Ti.sub.2]Si, 750 [degrees]C, 1.5 h [alpha]-[Al.sub.2][O.sub.3], TiSi, [Ti.sub.5][Si.sub.4], [Cr.sub.23] [C.sub.6], [Cr.sub.7][C.sub.3], [gamma]-[Al.sub.2][I.sub.3] a--Al, [Al.sub.3]Ni, [alpha]-[Al.sub.2][I.sub.3], [gamma]-[Al.sub.2][I.sub.3], [Al.sub.2]Si[O.sub.3], Si[O.sub.2], halo of 28 [less than or equal to] 29 [less than or equal to] 54[degrees] b--Al, AlNi, [Al.sub.3]Ni, [Al.sub.23][Ti.sub.9], [Ti.sub.5][Si.sub.4], [gamma]-[Al.sub.2][I.sub.3], [alpha]-[Al.sub.2][I.sub.3] c--AlNi, [Al.sub.3]Ni, Al, [Al.sub.3]Ti, [Ti.sub.5][Si.sub.4], (Si, Al)[sub.2.Cr] State of coating HV0.05, GPa After spraying 8.66 [+ or -] 1.50 0.76 [+ or -] 0.10 After annealing at 8.20 [+ or -] 1.00 700 [degrees]C, 1.5 h 1.60 [+ or -] 0.50 7.86 [+ or -] 0.65 8.69 [+ or -] 1.00 After annealing at 8.10 [+ or -] 1.00 750 [degrees]C, 1.5 h 1.76 [+ or -] 0.50 6.69 [+ or -] 0.70 8.34 [+ or -] 1.00 Note. First layer is produced by plasma spraying of Ti[Si.sub.2][Al.sub.2][O.sub.3]-Ni-&3[C.sub.2], second layer--by electric arc metallization of [Al.sub.5]Si.
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|Title Annotation:||GENERAL PROBLEMS OF METALLURGY|
|Author:||Borisov, Yu.S.; Adeeva, L.I.; Kaplina, G.S.; Tunik, A.Yu.; Gordan, G.N.; Demianov, I.A.; Revo, S.L.|
|Publication:||Advances in Electrometallurgy|
|Date:||Jul 1, 2007|
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