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Interface interaction and structural transformation in particles of ceramic and cermet composite powders in flame spraying
138
CERAMICS
INTERNATIONAL.
Vol. 9. n. 4. 1983
Interface Interaction and Structural Transformation in Particles of Ceramic and Cermet Composite Powders in Flame Spraying YU. BORISOV and A.L. BORISOVA Institute for Problems
of Materials
Science, Academy
of Sciences of the Ukrainian
SSR, Kiev (USSR)
The behavior of particles of Aiz03, TiOz, Ai203 - TiOz, Al203 Ni, ZrOz - Ni, AlzOs - Ni - Ti powders in flame spraying has been studied. The interaction between the surface tension forces and the wetting forces has been found to produce either a melt shell around the particles core or drops of the second component on the surface of the core. Subsequently a total or partial capture of drops of the second component melt by the core melt or their separation during the movement within the plasma jet volume are possible. At high particle heating and cooling rates, polymorphous transformations, higher - to - lower oxide transformations and metastable phase fixations in the coating take place. These effects influence the conditions of ceramic powder plasma coating formation and also the properties of the coatings.
Moreover, the coating formation process itself proceeds at a particle material cooling rate of the order of 10’ to 1O6 K/s. Of significance also is the variety of the gaseous medium composition, which can be either inert or reducing with a changeover to an oxidizing one. Generally, when using powder composition containing components with various physico-chemical and thermal properties for flame spraying, the associated posssible structural and energy state changes in the particles should be taken into consideration. This eventually affects the structure, composition and properties of the coatings obtained.
1 - INTRODUCTION
2 - OBJECTS OFSTUDY
Ceramic powders are among the most widely used types of materials for flame-sprayed coatings. The Al, Cr, Ti oxide powders are used for wear-resistant coatings, a.nd the Zr oxide powder, for heat-shielding ones. Addition of metals or alloys to ceramics gives cermet coatings which further extends the applications of flame-sprayed coatings based on ceramic powders (ref. 1). One of the modern trends in the field of specialized powders for flame spraying is the development of composite powders. The particles of these powders are heterogeneous structures containing several components (oxides, metals, alloys, carbides, etc.). This gives rise to new interphase boundaries within the particle and complicates the processes occurring inside the particles when it is heated during its movement in the gas jet volume. For the ceramic-metal type compositions the following interphase interactions are feasible: 1. Wetting the surface of the high-melting solid component by the low-melting component melt and spreading (or convolution) of the latter; 2. Interaction between the melts of the components of the composite powder particle (mutual wetting, chemical interaction); 3. Polymorphous transformations in the course of heating, melting and crystallization of the particle; 4. Changes of the chemical composition of particles as a result of effects of the ambient gaseous medium.
The behavior of composite powders based on alumina of different compositons and structures in flame spraying was studied. The initial structures of particles are schematically shown in Fig. 1. A continuous metal envelope was applied to the surface of the oxide particle (type 1) by a chemical deposition method (ref. 1). Conglomerate-type particles (type II), where more disperse (1 to 10 a) particles of the second component were applied to the surface of the core measuring 40 to 100 pI were obtained with the use of organic binders. A mixed structure (type Ill) was obtained by a combination of the two above methods. The composition of the powders studied is given in Table 1.
A specific nature of these processes lies, on the one had, in a combination of a high temperature (3*103 to 3,witiK) and a short duration (usually 10m3 to 10m4 s) of the heating gas jet action and, on the other hand, in a smallness of tfie volume of interacting components (1 to 100 p).
Composition Al203 - Ni ZrOz - Ni AhOt - Ni - Ti ~1~0~ - TiO2
Ni 35 35 33 -
Components, mass % Ti TiO2 ZrO2 10 -
65 -
13
FIGURE 1 - Diagram of composite powder particle structures.
The transformations occurring in particles of Al and Ti oxides during the flame spraying were also studied. To investigate the morphology of partrcies, powder samples from the plasma jet were taken either into a water bath or by means of a cooled wax target.
Al203 65 57 87
Structure type 1 : n
TABLE 1 - Composition and Structure of Particles of Composite Powders.
INTERFACE
INTERACTION
AND STRUCTURAL
3 - BEHAVIOR OF PRIMARY SOLID OXIDE PARTICLE
TRANSFORMATIONS
IN PARTICLES
MELT ON SURFACE
OF CERAMIC
AND CERMET
COMPOSITE
POWDERS
139
IN FLAME SPRAYING
OF
The differences in the melting points of particle components, surface tensions and other physico-chemical properties result in the development of interaction processes, first in the usolid-meltm system (with low-melting shell or lowmelting core versions), and then between the two melts. In the process of heating a composite powder particle consisting of an oxide core with a uniformly distributed metal shell, as the metal melting stage is reacher, a primary (metal) melt film is first formed over the solid (oxide) core. The film is subject, on the one hand, to the effect of the force of adhesion between the solid and the liquid phase which acts towards wetting of the core by the melt, and, on the other hand, to the effect of a convolving force arising from the surface tension of the melt. Their interrelation determines the nature of the developing process: either wetting and spreading of melt over the core surface or convolution into a drop with a certain wetting angle.
Au = _k[WA - UL~(1 + cos f3)]
ill
where AU = coefficient of spreading; WA = work of the adhesive force at the umelt-core* interface; k = coefficient of the solid surface roughness; uLb = surface tension of the shell melt, and case = instantaneous value of the varying contact angle. For plated particles the initial value of 0 is zero, i.e. Au = K (WA - 2~) PI When the AU value is positive, the melt film si retained on the whole surface of the particle core. At WA<2uLg a convolution takes place with formation of a drop with contact angle with the surface. In the absence of a chemical interaction between the oxide and the metal, WA is of 250 to 350 erg/cm’ (ref. 2) and, since the uLg value for metals is usually of 1000 to 2000 erg/cm’, AU<@ and a convolution effect is to be observed. However, along with the thermodynamical estimation of the metal melt film stability on the ceramic particle surface, it is important to compare the convolution process duration with the particle time of the residence in the gas jet. This estimation can be done of the basis of the available data on the process of melt spreading on the surface (ref. 2-4) with respect to which the plating shell convolution problem is the reverse. Disregarding the effect of the roughness and the curvature of the surface and assuming the liberated surface energy of the melt as fully converted into the kinetic energy of the convolving drop, the approximate average rate (W,,) and the duration time (T=~),of the convolution of the melt film over the particle, are W2,, = 5.998uLg* R,
-I e -I .F(A)-“.333. ~ ??
?? [(l -co~&,)~F(A)-~.~‘-l]
131
?as=1.664u-‘L,*RF3*e*F(~)-0~333* .[(I -cosf3,).F(A)-“~667-l]-’ where R, = core radius; e = shell melt density; L\ =relative thickness of the shell (A = A/R,); F(A) = a3 + 3A2+ 3A; 8, = wetting angle. From [3] it follows that the rate of convolution of the plating melt shell of a composite particle depends on the particle size, on the shell thickness and on the physico-chemical properties of the composition. The estimation of the order of magnitude of 7asshows that in the absence of chemical interaction between the shell material and that of the core it takes from 0.01 to 0.1 of the particle total time of residence in the high temperature region. Thus the convolution of outer layer on the composite cermet powder particles at its melting stage is possible also from the
FIGURE 2 - Convolution of nickel film on surface of solld alumina core. standpoint of process kinetics. The investigation of the structure of different metalcoated ceramic powder particles, extracted from the plasma jet at different heating stages confirmed the convolution of metal melt film into drops attached to the ceramic core surface (see Fig. 2) (ref. 5). The introduction of Ti usually results in a considerable increase of the metal-to-oxide adhesion work because of chemical interaction forces (ref. 2). This lead us to investigate the behavior of Al203 - Ni - Ti composite powder on heating in the plasma jet (see Table 1). However, no difference from Al203 - Ni composition has been found and similar convolved melt drops on the surface of Al203 particles have been observed. Though the melt itself is now a Ni-Ti alloy, the rate of alloy formation through dissolution of Ti particles in the Ni shell seems to be lower than rate of shell convolution. Therefore there is not enough time for the composition of the metal near the interphase boundary to change enough to affect the wetting characteristics.
4 - MUTUAL MELTS
WETTING
OF
PARTICLE
COMPONENT
The further interphase interaction process is associated with the characteristics of the mutual wetting of melts of the composite powder particle components. From thermodynamical relations for surface effects (ref. 6, 7) in the absence of the chemical interaction, the following expression for the free energy of interaction of two melt drops L1 and L2 can be obtained
AF = 1/~uL,,SL,,
S’Ls - case I SL&g
= s2LZg-
151
where u is the surface tension, S is the surface area, and the superscripts 1 and 2 correspond to the initial and final states of the system. At cos&,>3 (SzL,, - SL 91) we have AF
YU. BORISOV
and A.L. BORISOVA
b
FIGURE 3 - Ptating nickel film mete&l cepture by aiumlne core m,elt.
at its convolution (the number of drops formed -n). The thermodynamical condition for the shell drop to sink into the core in this case is
It follows from the formula that the probability of the capture of a melt drop by the core is the greater, the smaller is the relative thickness of the plating layer and the greater is the number and hence the smaller is the size of the formed drops. This is also confirmed by studies of the structure of particles having been processed in the plasma jet (see Fig. 3). Relatively small metal inclusions are situated inside the core, while larger metal formations turn out to be captured only partly by the oxide melt. Thus, the interphase effects taking place in the bulk of particles during their heating under the flame-spraying conditions can change drastically the particle structure. The plating metal melt capture by the core melt during cermet composition powder spraying also favours the formation of the coating through the contacts of ceramic components of the particles. This should influence the coating-to-backing adhesion strength and other properties of the coating, such as thermal conductivity, electric conductivity, thermal expansion coefficient etc.
Tf
7
ms
FIGURE 4 - Effect of degree of barrel filling with explodve 1, (a) and of advance time of powder feed Into the barrel with reepect to firing moment it (b) upon a-phaee content in detonatlon Al203 coatings.
The physico-mechanical properties of any polymorphous material as well as its service characteristics depend on the quantitative content of different modifications therein. The phase composition of a material is determined mainly by its preliminary thermal treatment. The materials constituting the fla -spayed coatings are subject to an intense thermomech z@ nical treatment in the course of their spraying, with is the cause of a nonuniformity of the phase composition of such coatings.
and accordingly increases its residence time within the heating zone. The 7r value determines the powder positions by the firing moment, and its increase is associated with shortening of the time of particle interaction with the high-temperature flow of explosion products. The X-ray studies of the coating material and the powder samples taken out of a two-phase flow showed the presence therein, along with a- and r-phases, of transitional structural C- and 6- states, the relative fraction.of a- Al203 in the coating material being higher than in powder samples under otherwise equal conditions. This is likely to item from higher material hardening rates in powder sampling. The change of detonation spraying parameters (f, and or) in the direction, favourable for a higher particle heating leads to an increase of the amount of the a-phase. This suggests that the AlzOs a-phase content in the flame spraying depends mainly upon the degree of development of the phase transition process during the period of the coating formation on the backing, rather than upon the amount KZOOID unmelted particles of the initial corundum.
It is well known, in particular, that alumina coatings sprayed by gase-flame, plasma or detonation methods (using corundum as initial material), along with a-phase, contain also other AlrOJ modifications (ref. 8-10). For a coating, applied by detonation spraying, Fig. 4 shows the dependence of the integral intensity of the strongest X-ray diffraction line c-x-Alz03 (113) upon the degree f, of barrel filling of the explosive, and the advance time rr of the powder feed into the barrel (with respect to the firing moment) (ref. 11). The increase of fa lowers the velocity of the particles of the powder
Among ceramic compositions, AlzOj - Ti02 is of considerable interest for plasma coatings. The constitution diagram of this composition contains two eutectic points at 40 and 80 mass% of titanium dioxide with melting points of 1840° and Of 1705% respectively. The presence of Ti02 on the surface of Al203 particles indicates a possibility of the contact melting effect with the formation of a shell with a lower melting point. This provides for a more dense structure of the coating and an increase of bond strenght betw’een individual particles which form the coating.
5 - POLYMORPHOUS
TRANSFORMATIONS
INTERFACE
INTERACTION
AND STRUCTURAL
TRANSFORMATIONS
IN PARTICLES
OF CERAMIC
60 -
AND CERMET
COMPOSITE
POWDERS
IN FLAME SPRAYING
141
sult, substantial changes may occur both in the morphology of the particles and their chemical and phase composition. The flame spraying conditions affect also the nature of polymorphous transformations in oxides. All these phenomena can greatly affect the properties of sprayed coatings.
REFERENCES
FIGURE 5 - Wear reeirtance of A1203detonation coetlngr of dlfferent epreyfng pammetere: f, = 10096, 7f=250 ms (1); f, = 60%, 7f=25Oms(2);f,=10016,7f=100ms(3). The X-ray phase analysis of plasma coatings from the Alr03 - TiOz composite powder has shown the (Y- Al203 content in those coatings to be higher than in the pure alumina powder coatings. A similar fact of a stronger tendency to form the a-phase of AhO was also revealed in A1203 - CrtOs powder spraying (ref. 8). The significance of polymorphous transformations in the flame-spraying of ceramic powder coatings can be demonstrated by an example of this wear-resistance dependence on the CPAI~O~content (see Fig. 5).
6 - CONCLUSIONS Ceramic and cermet composition powders are among the modern materials for flame spraying of coatings. The structural heterogeneity of the particles gives rise to a heat-induced interphase interaction of the composition components. The possibility of development of processes of convolution of the metal shell in its melting on the ceramic particle surface and a subsequent capture of the metal melt by the oxide melt has been shown both theoretically and experimentally by way of examples of the A1203-Ni and ZrOz-Ni compositions. As a re-
1. Ju.S. BORISOV, S.L. FISHMAN, V.I. JUSHKOV et al., Kermetookrvtiia. V kn. Neoraanicheskie i nve olazmennve o~gandsilikatnye~pokr$tija.~Leningrad, UNaukamlg75, s. 87-96. 2. Ju.V. NAIDICH. Kontaktnve iavlenia v metallicheskikh rasplavakh. Kiev, *Naukovadumk& 1972, s. 196 3. A.D. ZIMON, Adgezija zhidkosti i smachivanie. Moskva, #Khimijam1964. s. 416. 4. S.L. POPEL, V.V. PAPLOV, T.V. ZAKHAROV, Kineticheskie osobennosti rastekanija zhidkikh metallov po poverkhnosti tverdykh. V kn. UAdgezija rasplavovm. Kiev, #Naukova dumkas 1974, s. 7-11. osobennosti mezhfaznogo 5. Ju.S. BORISOV, Nekotorye vzaimodejstvija v chastitsakh pri plazmennonm napylenii komoozitsionnvch ooroshkov. V kn. WZharostoikie ookrvtiia dlia konstruktsionnykh materialovm. Leningrad, .Nauka;, s. i47-15i. 6. A.I. RUSSANOV. Fazowe ravnovesija i ooverkhnostnve _ .iavlenija. Leningrad, aKhimijam;1967, s. 338. . 7. Ju.Ja.GOLGER, V.I. KLASSEN, A.I. RUSANOV, Ob energetike poverkhnostnykh protsessov. V kn. SFizicheskaja khimija poverkhnostnykh javlenij v rasplavakh. Kiev, *Naukova dumkam, 1971, s. 9-14. 6. G.D. KOVALENKO, I.V. DYSHEVSKIJ, Ju.P. PETROV, Fazovye perekhody v pokrytijakh pri plazmennom napylenij okisi aljuminija. Elektronnaja obrabotka materialov, 1974, N. 1, s. 35-39. 9. F. EICHHORN, I. METZLER, W. EYSEL, Rontgenbeugungs Untersuchungen an Plasmagespritzten Aluminiumoxidschichten, Metalloberflache, 1972,26, N. 6,212-213. 10. V. WILMS. H. HERMAN Crvstalloaraohv and microstructure of equilibrium and metastable Ophase-resul~ngfrom plasma spraying of ceramic coatinos. In: 81hInt. Metal Sprav. . Conf., Majami, 1978236-243. 11. A.L. BORISOVA, VS. KLIMENKO, I.E. SHIJANOVSKAJA et al., Fazovye prevrashchenija pri detonatsionnom napylenii i ikh vlijanie na iznosostoikost pokrytij iz okisi aljuminija. V kn. UVysokotemperaturnaja zashchita materialow. Leningrad, UNaukaw1981, s. 112-l 15. Received March 3, 1983; accepted May 17, 1983.
CERAMICS
INTERNATIONAL.
Vol. 9. n. 4. 1983
Interface Interaction and Structural Transformation in Particles of Ceramic and Cermet Composite Powders in Flame Spraying YU. BORISOV and A.L. BORISOVA Institute for Problems
of Materials
Science, Academy
of Sciences of the Ukrainian
SSR, Kiev (USSR)
The behavior of particles of Aiz03, TiOz, Ai203 - TiOz, Al203 Ni, ZrOz - Ni, AlzOs - Ni - Ti powders in flame spraying has been studied. The interaction between the surface tension forces and the wetting forces has been found to produce either a melt shell around the particles core or drops of the second component on the surface of the core. Subsequently a total or partial capture of drops of the second component melt by the core melt or their separation during the movement within the plasma jet volume are possible. At high particle heating and cooling rates, polymorphous transformations, higher - to - lower oxide transformations and metastable phase fixations in the coating take place. These effects influence the conditions of ceramic powder plasma coating formation and also the properties of the coatings.
Moreover, the coating formation process itself proceeds at a particle material cooling rate of the order of 10’ to 1O6 K/s. Of significance also is the variety of the gaseous medium composition, which can be either inert or reducing with a changeover to an oxidizing one. Generally, when using powder composition containing components with various physico-chemical and thermal properties for flame spraying, the associated posssible structural and energy state changes in the particles should be taken into consideration. This eventually affects the structure, composition and properties of the coatings obtained.
1 - INTRODUCTION
2 - OBJECTS OFSTUDY
Ceramic powders are among the most widely used types of materials for flame-sprayed coatings. The Al, Cr, Ti oxide powders are used for wear-resistant coatings, a.nd the Zr oxide powder, for heat-shielding ones. Addition of metals or alloys to ceramics gives cermet coatings which further extends the applications of flame-sprayed coatings based on ceramic powders (ref. 1). One of the modern trends in the field of specialized powders for flame spraying is the development of composite powders. The particles of these powders are heterogeneous structures containing several components (oxides, metals, alloys, carbides, etc.). This gives rise to new interphase boundaries within the particle and complicates the processes occurring inside the particles when it is heated during its movement in the gas jet volume. For the ceramic-metal type compositions the following interphase interactions are feasible: 1. Wetting the surface of the high-melting solid component by the low-melting component melt and spreading (or convolution) of the latter; 2. Interaction between the melts of the components of the composite powder particle (mutual wetting, chemical interaction); 3. Polymorphous transformations in the course of heating, melting and crystallization of the particle; 4. Changes of the chemical composition of particles as a result of effects of the ambient gaseous medium.
The behavior of composite powders based on alumina of different compositons and structures in flame spraying was studied. The initial structures of particles are schematically shown in Fig. 1. A continuous metal envelope was applied to the surface of the oxide particle (type 1) by a chemical deposition method (ref. 1). Conglomerate-type particles (type II), where more disperse (1 to 10 a) particles of the second component were applied to the surface of the core measuring 40 to 100 pI were obtained with the use of organic binders. A mixed structure (type Ill) was obtained by a combination of the two above methods. The composition of the powders studied is given in Table 1.
A specific nature of these processes lies, on the one had, in a combination of a high temperature (3*103 to 3,witiK) and a short duration (usually 10m3 to 10m4 s) of the heating gas jet action and, on the other hand, in a smallness of tfie volume of interacting components (1 to 100 p).
Composition Al203 - Ni ZrOz - Ni AhOt - Ni - Ti ~1~0~ - TiO2
Ni 35 35 33 -
Components, mass % Ti TiO2 ZrO2 10 -
65 -
13
FIGURE 1 - Diagram of composite powder particle structures.
The transformations occurring in particles of Al and Ti oxides during the flame spraying were also studied. To investigate the morphology of partrcies, powder samples from the plasma jet were taken either into a water bath or by means of a cooled wax target.
Al203 65 57 87
Structure type 1 : n
TABLE 1 - Composition and Structure of Particles of Composite Powders.
INTERFACE
INTERACTION
AND STRUCTURAL
3 - BEHAVIOR OF PRIMARY SOLID OXIDE PARTICLE
TRANSFORMATIONS
IN PARTICLES
MELT ON SURFACE
OF CERAMIC
AND CERMET
COMPOSITE
POWDERS
139
IN FLAME SPRAYING
OF
The differences in the melting points of particle components, surface tensions and other physico-chemical properties result in the development of interaction processes, first in the usolid-meltm system (with low-melting shell or lowmelting core versions), and then between the two melts. In the process of heating a composite powder particle consisting of an oxide core with a uniformly distributed metal shell, as the metal melting stage is reacher, a primary (metal) melt film is first formed over the solid (oxide) core. The film is subject, on the one hand, to the effect of the force of adhesion between the solid and the liquid phase which acts towards wetting of the core by the melt, and, on the other hand, to the effect of a convolving force arising from the surface tension of the melt. Their interrelation determines the nature of the developing process: either wetting and spreading of melt over the core surface or convolution into a drop with a certain wetting angle.
Au = _k[WA - UL~(1 + cos f3)]
ill
where AU = coefficient of spreading; WA = work of the adhesive force at the umelt-core* interface; k = coefficient of the solid surface roughness; uLb = surface tension of the shell melt, and case = instantaneous value of the varying contact angle. For plated particles the initial value of 0 is zero, i.e. Au = K (WA - 2~) PI When the AU value is positive, the melt film si retained on the whole surface of the particle core. At WA<2uLg a convolution takes place with formation of a drop with contact angle with the surface. In the absence of a chemical interaction between the oxide and the metal, WA is of 250 to 350 erg/cm’ (ref. 2) and, since the uLg value for metals is usually of 1000 to 2000 erg/cm’, AU<@ and a convolution effect is to be observed. However, along with the thermodynamical estimation of the metal melt film stability on the ceramic particle surface, it is important to compare the convolution process duration with the particle time of the residence in the gas jet. This estimation can be done of the basis of the available data on the process of melt spreading on the surface (ref. 2-4) with respect to which the plating shell convolution problem is the reverse. Disregarding the effect of the roughness and the curvature of the surface and assuming the liberated surface energy of the melt as fully converted into the kinetic energy of the convolving drop, the approximate average rate (W,,) and the duration time (T=~),of the convolution of the melt film over the particle, are W2,, = 5.998uLg* R,
-I e -I .F(A)-“.333. ~ ??
?? [(l -co~&,)~F(A)-~.~‘-l]
131
?as=1.664u-‘L,*RF3*e*F(~)-0~333* .[(I -cosf3,).F(A)-“~667-l]-’ where R, = core radius; e = shell melt density; L\ =relative thickness of the shell (A = A/R,); F(A) = a3 + 3A2+ 3A; 8, = wetting angle. From [3] it follows that the rate of convolution of the plating melt shell of a composite particle depends on the particle size, on the shell thickness and on the physico-chemical properties of the composition. The estimation of the order of magnitude of 7asshows that in the absence of chemical interaction between the shell material and that of the core it takes from 0.01 to 0.1 of the particle total time of residence in the high temperature region. Thus the convolution of outer layer on the composite cermet powder particles at its melting stage is possible also from the
FIGURE 2 - Convolution of nickel film on surface of solld alumina core. standpoint of process kinetics. The investigation of the structure of different metalcoated ceramic powder particles, extracted from the plasma jet at different heating stages confirmed the convolution of metal melt film into drops attached to the ceramic core surface (see Fig. 2) (ref. 5). The introduction of Ti usually results in a considerable increase of the metal-to-oxide adhesion work because of chemical interaction forces (ref. 2). This lead us to investigate the behavior of Al203 - Ni - Ti composite powder on heating in the plasma jet (see Table 1). However, no difference from Al203 - Ni composition has been found and similar convolved melt drops on the surface of Al203 particles have been observed. Though the melt itself is now a Ni-Ti alloy, the rate of alloy formation through dissolution of Ti particles in the Ni shell seems to be lower than rate of shell convolution. Therefore there is not enough time for the composition of the metal near the interphase boundary to change enough to affect the wetting characteristics.
4 - MUTUAL MELTS
WETTING
OF
PARTICLE
COMPONENT
The further interphase interaction process is associated with the characteristics of the mutual wetting of melts of the composite powder particle components. From thermodynamical relations for surface effects (ref. 6, 7) in the absence of the chemical interaction, the following expression for the free energy of interaction of two melt drops L1 and L2 can be obtained
AF = 1/~uL,,SL,,
S’Ls - case I SL&g
= s2LZg-
151
where u is the surface tension, S is the surface area, and the superscripts 1 and 2 correspond to the initial and final states of the system. At cos&,>3 (SzL,, - SL 91) we have AF
YU. BORISOV
and A.L. BORISOVA
b
FIGURE 3 - Ptating nickel film mete&l cepture by aiumlne core m,elt.
at its convolution (the number of drops formed -n). The thermodynamical condition for the shell drop to sink into the core in this case is
It follows from the formula that the probability of the capture of a melt drop by the core is the greater, the smaller is the relative thickness of the plating layer and the greater is the number and hence the smaller is the size of the formed drops. This is also confirmed by studies of the structure of particles having been processed in the plasma jet (see Fig. 3). Relatively small metal inclusions are situated inside the core, while larger metal formations turn out to be captured only partly by the oxide melt. Thus, the interphase effects taking place in the bulk of particles during their heating under the flame-spraying conditions can change drastically the particle structure. The plating metal melt capture by the core melt during cermet composition powder spraying also favours the formation of the coating through the contacts of ceramic components of the particles. This should influence the coating-to-backing adhesion strength and other properties of the coating, such as thermal conductivity, electric conductivity, thermal expansion coefficient etc.
Tf
7
ms
FIGURE 4 - Effect of degree of barrel filling with explodve 1, (a) and of advance time of powder feed Into the barrel with reepect to firing moment it (b) upon a-phaee content in detonatlon Al203 coatings.
The physico-mechanical properties of any polymorphous material as well as its service characteristics depend on the quantitative content of different modifications therein. The phase composition of a material is determined mainly by its preliminary thermal treatment. The materials constituting the fla -spayed coatings are subject to an intense thermomech z@ nical treatment in the course of their spraying, with is the cause of a nonuniformity of the phase composition of such coatings.
and accordingly increases its residence time within the heating zone. The 7r value determines the powder positions by the firing moment, and its increase is associated with shortening of the time of particle interaction with the high-temperature flow of explosion products. The X-ray studies of the coating material and the powder samples taken out of a two-phase flow showed the presence therein, along with a- and r-phases, of transitional structural C- and 6- states, the relative fraction.of a- Al203 in the coating material being higher than in powder samples under otherwise equal conditions. This is likely to item from higher material hardening rates in powder sampling. The change of detonation spraying parameters (f, and or) in the direction, favourable for a higher particle heating leads to an increase of the amount of the a-phase. This suggests that the AlzOs a-phase content in the flame spraying depends mainly upon the degree of development of the phase transition process during the period of the coating formation on the backing, rather than upon the amount KZOOID unmelted particles of the initial corundum.
It is well known, in particular, that alumina coatings sprayed by gase-flame, plasma or detonation methods (using corundum as initial material), along with a-phase, contain also other AlrOJ modifications (ref. 8-10). For a coating, applied by detonation spraying, Fig. 4 shows the dependence of the integral intensity of the strongest X-ray diffraction line c-x-Alz03 (113) upon the degree f, of barrel filling of the explosive, and the advance time rr of the powder feed into the barrel (with respect to the firing moment) (ref. 11). The increase of fa lowers the velocity of the particles of the powder
Among ceramic compositions, AlzOj - Ti02 is of considerable interest for plasma coatings. The constitution diagram of this composition contains two eutectic points at 40 and 80 mass% of titanium dioxide with melting points of 1840° and Of 1705% respectively. The presence of Ti02 on the surface of Al203 particles indicates a possibility of the contact melting effect with the formation of a shell with a lower melting point. This provides for a more dense structure of the coating and an increase of bond strenght betw’een individual particles which form the coating.
5 - POLYMORPHOUS
TRANSFORMATIONS
INTERFACE
INTERACTION
AND STRUCTURAL
TRANSFORMATIONS
IN PARTICLES
OF CERAMIC
60 -
AND CERMET
COMPOSITE
POWDERS
IN FLAME SPRAYING
141
sult, substantial changes may occur both in the morphology of the particles and their chemical and phase composition. The flame spraying conditions affect also the nature of polymorphous transformations in oxides. All these phenomena can greatly affect the properties of sprayed coatings.
REFERENCES
FIGURE 5 - Wear reeirtance of A1203detonation coetlngr of dlfferent epreyfng pammetere: f, = 10096, 7f=250 ms (1); f, = 60%, 7f=25Oms(2);f,=10016,7f=100ms(3). The X-ray phase analysis of plasma coatings from the Alr03 - TiOz composite powder has shown the (Y- Al203 content in those coatings to be higher than in the pure alumina powder coatings. A similar fact of a stronger tendency to form the a-phase of AhO was also revealed in A1203 - CrtOs powder spraying (ref. 8). The significance of polymorphous transformations in the flame-spraying of ceramic powder coatings can be demonstrated by an example of this wear-resistance dependence on the CPAI~O~content (see Fig. 5).
6 - CONCLUSIONS Ceramic and cermet composition powders are among the modern materials for flame spraying of coatings. The structural heterogeneity of the particles gives rise to a heat-induced interphase interaction of the composition components. The possibility of development of processes of convolution of the metal shell in its melting on the ceramic particle surface and a subsequent capture of the metal melt by the oxide melt has been shown both theoretically and experimentally by way of examples of the A1203-Ni and ZrOz-Ni compositions. As a re-
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