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Synthesis and deposition of coatings in the electron beam plasma
Surface and Coatings Technology 180 – 181 (2004) 132–135
Synthesis and deposition of coatings in the electron beam plasma M.N. Vasiliev*, A.H. Mahir Moscow Institute of Physics and Technology, Institutsky per., 9, Dolgoprudny, 141700 Moscow Region, Russia
Abstract Non-equilibrium electron beam plasma (EBP) was proved to be promising for surface modification of materials, synthesis of protective coatings, and film deposition at sufficiently high plasma pressures (up to 50 Torr). The optimal EBP parameters were selected to produce active plasma particles in desired concentrations and to heat the surface to the required temperature. Synthesis of nitride, carbide oxide and boride surface layers, carbon and ceramics deposition on polymers at low substrate temperatures, powder treatment in both the dusty EBP and the dust-plasma crystals were studied. 䊚 2003 Elsevier B.V. All rights reserved. Keywords: Plasmachemical treatment; Electron-beam plasma
1. Introduction The electron-beam plasma (EBP) is generated by injecting the electron beam (EB) into gaseous media. Fig. 1 illustrates a typical way of the EBP generation. The focused EB 3 generated by the electron-beam gun 1 located in the high vacuum chamber 2 is injected into the working chamber 6 filled with the molecular gas through the injection window (IW) 4. In passing through the gas, the EB is scattered by elastic collisions and the energy of fast electrons gradually diminishes during various inelastic interactions with the medium—ionization, excitation of atoms and molecules resulting in a probable molecule dissotiation. As a result, the plasma cloud 5 is generated. In general, the following particles are available in the EBP: ● fast electrons of the EB; ● secondary electrons of moderate and low energy; ● atoms and molecules of the plasma generating gas in ground and excited states; ● atomic and molecular ions in both ground and excited states; and ● radicals produced due to dissociation of molecules and conversion of plasma particles. Usually the electron-beam plasma particles are present in super-equilibrium concentrations and there are a lot *Corresponding author. Tel.: q7-095-4086-798. E-mail address: [email protected] (M.N. Vasiliev).
of unique heavy particles in the plasma bulk. Thus, the EBP is non-equilibrium and chemically active. With respect to non-equilibrium plasmas generated in conventional ways (for instance, the plasma of gas discharges) the EBP has the following advantages: ● the EB can be injected into any gases or gaseous and gas-vapor mixtures; ● there are no technical problems with introducing dispersed additives and compact bodies into the plasma bulk; ● large EBP bulks can be produced since the EBP does not contract even at high pressure; ● there are no restrictions as to the pressure of the gas used to generate the EBP; ● the EB parameters and gas pressure can be varied separately; ● both sub- and supersonic EBP flows are usually stable; and ● it is possible to inject the EB into gas discharges of various types. The EBP advantages mentioned above are the basis for novel technologies of the material treatment and coating. The equipment realizing these technologies is highly effective and does not pollute the environment. The electron-beam plasmachemical reactors (EBPR) with gas-phase and heterogeneous active zone are considered in terms of the medium composition. In the EBPR of the first type, the synthesis and decomposition of substances occur in the gaseous phase, i.e. all reagents and products are gases or vapors. Low energy beams
0257-8972/04/$ - see front matter 䊚 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2003.10.142
M.N. Vasiliev, A.H. Mahir / Surface and Coatings Technology 180 – 181 (2004) 132–135
Fig. 1. The EBP generation and principle of the plasmachemical treatment by the EBP: (1) electron gun, (2) EB, (3) high-vacuum chamber, (4) IW, (5) plasma cloud, (6) reaction chamber, (7) target, (8) MW-resonator, (9) window, (10) pyrometer, (11) CCD-camera, (12) spectrometer.
(Eb-100 keV) are unlikely to be competitive with conventional plasmachemical reactors for gas treatment both at low (Pm-10y1 Torr) and high (Pm;102 Torr) pressures. The heterogeneous EBPR is studied in more detail since it is the apparatus that demonstrates the advantages of the EBP. Powders, liquid droplets and large solid bodies can be introduced into the reaction bulk. The EBP is proved to be promising for surface modification of materials, synthesis of protective coatings and film deposition. The following EBP-assisted processes were studied w1x: ● synthesis of nitride, carbide oxide and boride layers on the surface of metallic bodies, synthesis of combined compounds and multylayers; ● carbon deposition on substrates of various types: glass, natural and artificial polymers, fibers and clothes (the basalt cloth, for example); ● ceramics deposition on polymers at low temperature of the substrate; and ● powder treatment in both the dusty EBP and the dustplasma crystals. The reaction chamber of the EBPR can be filled with still gas or the gas flow is formed inside the chamber. The dynamic surface material treatment in the EBP flow was proved to be more effective than the treatment in the still plasma.
133
Fig. 2. Treatment of the internal surface of the tube by the EBP flow.
2. Surface layers synthesis in the EBP Fig. 1 illustrates the main principle of the technology. The target 7 to be treated is placed inside the reaction chamber 6 filled with nitrogen at a pressure of approximately 20 Torr. The concentrated EB 2 is injected into the chamber 6 and generates the cloud 5 of the EBP contacting with the target surface. At the same time, the fast electrons of the EB bombard the target and heat it. The chemically active plasma particles interact with the material and produce nitride compounds in the surface layer of the sample. Preliminary experiments carried out with flat targets made of Ti-alloy BT-18Y showed that: 1. On being treated, the target surface facing the IW targets had a golden color. The X-ray photoelectronic and Auger spectroscopy analysis showed the modified layer to consist of TiN in thickness of 2–10 mm (depending on the treatment conditions). 2. Average speed of the TiN synthesis is 5=103 – 3=104 mg cmy2 hy1. The obtained efficiency of the TiN-synthesis is one–two order of magnitude higher than the efficiency of the conventional nitriding process in RF- and glow discharge plasmas. 3. The TiN-layer is homogeneous and the roughness of the treated surface is one–two grade of finish better than that of the target surface before treatment. The boundary between the TiN-layer and the substrate is sharp enough. 4. The optimal process temperatures considered lie within the range 400–450 8C. These are significantly lower temperatures than required for the conventional thermal nitriding in the nitrogen atmosphere. The EB power is sufficient to heat the target to the desired
134
M.N. Vasiliev, A.H. Mahir / Surface and Coatings Technology 180 – 181 (2004) 132–135
temperature, i.e. no additional heaters are required. 5. The atomic nitrogen rather than molecules or ions is responsible for the TiN synthesis.
Fig. 3. Powder treatment by the EBP: (1) reaction chamber, (2) powder layer, (3) rarefied fluidized bed, (4) scattered EB.
Fig. 2 illustrates the procedure of nitride layer synthesis on inner surfaces of long orifices or tubes. The EBP stream is generated by the co-axial injection of the EB and subsonic gas jet, the plasma flowing inside the tube to be treated. Scattered fast electrons heat the tube and the atomic nitrogen produced in the EBP takes part in the nitride-layer synthesis. The treatment procedure considered was used to prolong the lifetime of draw plates for carbon and basalt fiber production and to improve the resistance of honeycombs working in the contact with chemically active media. Similar procedure can be used to form a boride layer on the material surface. The boron-containing EBP is generated by the EB injection into gaseous diborane B2H6, chemically active boron being produced by the diborane dissociation due to the EB. Oxide surface layers can be synthesized in oxygen-containing EBP. 3. The powder treatment in the EBP The typical scheme of the EBPR used for powder treatment is presented in Fig. 3. The plasma generating gas is blown through the powder layer 2 sprinkled over the bottom of the reaction chamber 1 that has porous walls. As a result, the rarefied fluidized bed 3 containing the powder particles is formed. The EB 4 injected into the aerosol generates the chemically active EBP and heats the powder particles to the required temperature. As a result, the plasmachemically-modified layer is formed on the surface of each powder particle. Fig. 4 illustrates the powder cladding, the powder aerosol being in the state of the dust-plasma crystal. The crystal 4 levitates in the RF electric field produced by the electrode 5. Simultaneously, the RF-gas discharge activates the surfaces of the powder particles. The EB 6 is used to: ● evaporate the target 2 that is made of the material to be deposited on the powder particles; ● control the position of the dust-plasma crystal. (The dust-plasma crystal was proved to be displaced in both x- and y-directions by means of the EB scanning). The procedure involved was used to deposit the ferrielectric ceramic layer on the surface of powder particles made of another kind of ceramics or glass. 4. Experimental study of the EBP and EBPR
Fig. 4. Treatment of the dust-plasma crystal: (1) reaction chamber, (2) evaporable target, (3) powder feeder, (4) dust-plasma crystal, (5) RF-electrode, (6) EB.
The principle scheme of experimental EBPR used to study the process of surface layer synthesis is also given in Fig. 1, the figure illustrating the unit that has a still
M.N. Vasiliev, A.H. Mahir / Surface and Coatings Technology 180 – 181 (2004) 132–135
reaction zone. Two-stage gas-dynamic window injects the EB into the reaction chamber of large size to generate the plasma cloud. The adjustable feeder maintains the required pressure Pm in the reaction chamber and controls the shape of the EBP cloud. Special diagnostic system was developed to measure the parameters responsible (depending on the treatment conditions) for the treatment process. The table below lists the techniques of measurements and ranges of values to be measured. Parameters to be measured
Range of the value
Method of measurement
EB current, Ib Electron energy, Eb
1y103 mA 20–100 keV
EB power, Nb Gas pressures, Pm
0.1y10 kW 10y5y102 Torr
3=102y3=103 K
Faraday cup Measurement of the electron gun gun accelerating voltage U Absolute calorimeter Vacuum gages of various types CCD TV-camera, X-ray camera Spectrometer Spectrometer
6=102y2=103 K 108y1013 cmy3
Pyrometer MW-diagnostics
EBP cloud shape EBP composition Gas temperatures, Tm, Tr and Tv Target temperature, Tt Electron and ion densities, nepsnip
135
Valuable information about EBP composition and properties can be gleaned from plasma radiation. The shape and space distribution of radiation intensity of the plasma bulk can be obtained from the TV-camera. The CCD camera provided with the X-ray module was proved to be efficient to study the EBP near the target surface. More detailed information about composition and particles temperatures of the EBP can be obtained from the plasma spectra. Rotating Tr and vibrating Tv temperatures of heavy particles in the nitrogen EBP at various distances from the target were found by analyzing the bands (0, 1) and (1, 2) of molecular ion Nq 2 . The target temperature Tt can be measured by optical pyrometer because the EBP (and the EBP near the surface, in particular) was proved to be optically thin. Microwave methods seem to be the most promising to measure densities of charged particles in the EBP. The open barrel-shaped resonators were used to probe the EBP bulk. References w1x M.N. Vasiliev, The application of the electron beams in the plasmachemistry, in: V.E. Fortov (Ed.), Encyclopedia of the Low-Temperature Plasma, XI, Nauka, Moscow, 2001, pp. 436–445.
Synthesis and deposition of coatings in the electron beam plasma M.N. Vasiliev*, A.H. Mahir Moscow Institute of Physics and Technology, Institutsky per., 9, Dolgoprudny, 141700 Moscow Region, Russia
Abstract Non-equilibrium electron beam plasma (EBP) was proved to be promising for surface modification of materials, synthesis of protective coatings, and film deposition at sufficiently high plasma pressures (up to 50 Torr). The optimal EBP parameters were selected to produce active plasma particles in desired concentrations and to heat the surface to the required temperature. Synthesis of nitride, carbide oxide and boride surface layers, carbon and ceramics deposition on polymers at low substrate temperatures, powder treatment in both the dusty EBP and the dust-plasma crystals were studied. 䊚 2003 Elsevier B.V. All rights reserved. Keywords: Plasmachemical treatment; Electron-beam plasma
1. Introduction The electron-beam plasma (EBP) is generated by injecting the electron beam (EB) into gaseous media. Fig. 1 illustrates a typical way of the EBP generation. The focused EB 3 generated by the electron-beam gun 1 located in the high vacuum chamber 2 is injected into the working chamber 6 filled with the molecular gas through the injection window (IW) 4. In passing through the gas, the EB is scattered by elastic collisions and the energy of fast electrons gradually diminishes during various inelastic interactions with the medium—ionization, excitation of atoms and molecules resulting in a probable molecule dissotiation. As a result, the plasma cloud 5 is generated. In general, the following particles are available in the EBP: ● fast electrons of the EB; ● secondary electrons of moderate and low energy; ● atoms and molecules of the plasma generating gas in ground and excited states; ● atomic and molecular ions in both ground and excited states; and ● radicals produced due to dissociation of molecules and conversion of plasma particles. Usually the electron-beam plasma particles are present in super-equilibrium concentrations and there are a lot *Corresponding author. Tel.: q7-095-4086-798. E-mail address: [email protected] (M.N. Vasiliev).
of unique heavy particles in the plasma bulk. Thus, the EBP is non-equilibrium and chemically active. With respect to non-equilibrium plasmas generated in conventional ways (for instance, the plasma of gas discharges) the EBP has the following advantages: ● the EB can be injected into any gases or gaseous and gas-vapor mixtures; ● there are no technical problems with introducing dispersed additives and compact bodies into the plasma bulk; ● large EBP bulks can be produced since the EBP does not contract even at high pressure; ● there are no restrictions as to the pressure of the gas used to generate the EBP; ● the EB parameters and gas pressure can be varied separately; ● both sub- and supersonic EBP flows are usually stable; and ● it is possible to inject the EB into gas discharges of various types. The EBP advantages mentioned above are the basis for novel technologies of the material treatment and coating. The equipment realizing these technologies is highly effective and does not pollute the environment. The electron-beam plasmachemical reactors (EBPR) with gas-phase and heterogeneous active zone are considered in terms of the medium composition. In the EBPR of the first type, the synthesis and decomposition of substances occur in the gaseous phase, i.e. all reagents and products are gases or vapors. Low energy beams
0257-8972/04/$ - see front matter 䊚 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2003.10.142
M.N. Vasiliev, A.H. Mahir / Surface and Coatings Technology 180 – 181 (2004) 132–135
Fig. 1. The EBP generation and principle of the plasmachemical treatment by the EBP: (1) electron gun, (2) EB, (3) high-vacuum chamber, (4) IW, (5) plasma cloud, (6) reaction chamber, (7) target, (8) MW-resonator, (9) window, (10) pyrometer, (11) CCD-camera, (12) spectrometer.
(Eb-100 keV) are unlikely to be competitive with conventional plasmachemical reactors for gas treatment both at low (Pm-10y1 Torr) and high (Pm;102 Torr) pressures. The heterogeneous EBPR is studied in more detail since it is the apparatus that demonstrates the advantages of the EBP. Powders, liquid droplets and large solid bodies can be introduced into the reaction bulk. The EBP is proved to be promising for surface modification of materials, synthesis of protective coatings and film deposition. The following EBP-assisted processes were studied w1x: ● synthesis of nitride, carbide oxide and boride layers on the surface of metallic bodies, synthesis of combined compounds and multylayers; ● carbon deposition on substrates of various types: glass, natural and artificial polymers, fibers and clothes (the basalt cloth, for example); ● ceramics deposition on polymers at low temperature of the substrate; and ● powder treatment in both the dusty EBP and the dustplasma crystals. The reaction chamber of the EBPR can be filled with still gas or the gas flow is formed inside the chamber. The dynamic surface material treatment in the EBP flow was proved to be more effective than the treatment in the still plasma.
133
Fig. 2. Treatment of the internal surface of the tube by the EBP flow.
2. Surface layers synthesis in the EBP Fig. 1 illustrates the main principle of the technology. The target 7 to be treated is placed inside the reaction chamber 6 filled with nitrogen at a pressure of approximately 20 Torr. The concentrated EB 2 is injected into the chamber 6 and generates the cloud 5 of the EBP contacting with the target surface. At the same time, the fast electrons of the EB bombard the target and heat it. The chemically active plasma particles interact with the material and produce nitride compounds in the surface layer of the sample. Preliminary experiments carried out with flat targets made of Ti-alloy BT-18Y showed that: 1. On being treated, the target surface facing the IW targets had a golden color. The X-ray photoelectronic and Auger spectroscopy analysis showed the modified layer to consist of TiN in thickness of 2–10 mm (depending on the treatment conditions). 2. Average speed of the TiN synthesis is 5=103 – 3=104 mg cmy2 hy1. The obtained efficiency of the TiN-synthesis is one–two order of magnitude higher than the efficiency of the conventional nitriding process in RF- and glow discharge plasmas. 3. The TiN-layer is homogeneous and the roughness of the treated surface is one–two grade of finish better than that of the target surface before treatment. The boundary between the TiN-layer and the substrate is sharp enough. 4. The optimal process temperatures considered lie within the range 400–450 8C. These are significantly lower temperatures than required for the conventional thermal nitriding in the nitrogen atmosphere. The EB power is sufficient to heat the target to the desired
134
M.N. Vasiliev, A.H. Mahir / Surface and Coatings Technology 180 – 181 (2004) 132–135
temperature, i.e. no additional heaters are required. 5. The atomic nitrogen rather than molecules or ions is responsible for the TiN synthesis.
Fig. 3. Powder treatment by the EBP: (1) reaction chamber, (2) powder layer, (3) rarefied fluidized bed, (4) scattered EB.
Fig. 2 illustrates the procedure of nitride layer synthesis on inner surfaces of long orifices or tubes. The EBP stream is generated by the co-axial injection of the EB and subsonic gas jet, the plasma flowing inside the tube to be treated. Scattered fast electrons heat the tube and the atomic nitrogen produced in the EBP takes part in the nitride-layer synthesis. The treatment procedure considered was used to prolong the lifetime of draw plates for carbon and basalt fiber production and to improve the resistance of honeycombs working in the contact with chemically active media. Similar procedure can be used to form a boride layer on the material surface. The boron-containing EBP is generated by the EB injection into gaseous diborane B2H6, chemically active boron being produced by the diborane dissociation due to the EB. Oxide surface layers can be synthesized in oxygen-containing EBP. 3. The powder treatment in the EBP The typical scheme of the EBPR used for powder treatment is presented in Fig. 3. The plasma generating gas is blown through the powder layer 2 sprinkled over the bottom of the reaction chamber 1 that has porous walls. As a result, the rarefied fluidized bed 3 containing the powder particles is formed. The EB 4 injected into the aerosol generates the chemically active EBP and heats the powder particles to the required temperature. As a result, the plasmachemically-modified layer is formed on the surface of each powder particle. Fig. 4 illustrates the powder cladding, the powder aerosol being in the state of the dust-plasma crystal. The crystal 4 levitates in the RF electric field produced by the electrode 5. Simultaneously, the RF-gas discharge activates the surfaces of the powder particles. The EB 6 is used to: ● evaporate the target 2 that is made of the material to be deposited on the powder particles; ● control the position of the dust-plasma crystal. (The dust-plasma crystal was proved to be displaced in both x- and y-directions by means of the EB scanning). The procedure involved was used to deposit the ferrielectric ceramic layer on the surface of powder particles made of another kind of ceramics or glass. 4. Experimental study of the EBP and EBPR
Fig. 4. Treatment of the dust-plasma crystal: (1) reaction chamber, (2) evaporable target, (3) powder feeder, (4) dust-plasma crystal, (5) RF-electrode, (6) EB.
The principle scheme of experimental EBPR used to study the process of surface layer synthesis is also given in Fig. 1, the figure illustrating the unit that has a still
M.N. Vasiliev, A.H. Mahir / Surface and Coatings Technology 180 – 181 (2004) 132–135
reaction zone. Two-stage gas-dynamic window injects the EB into the reaction chamber of large size to generate the plasma cloud. The adjustable feeder maintains the required pressure Pm in the reaction chamber and controls the shape of the EBP cloud. Special diagnostic system was developed to measure the parameters responsible (depending on the treatment conditions) for the treatment process. The table below lists the techniques of measurements and ranges of values to be measured. Parameters to be measured
Range of the value
Method of measurement
EB current, Ib Electron energy, Eb
1y103 mA 20–100 keV
EB power, Nb Gas pressures, Pm
0.1y10 kW 10y5y102 Torr
3=102y3=103 K
Faraday cup Measurement of the electron gun gun accelerating voltage U Absolute calorimeter Vacuum gages of various types CCD TV-camera, X-ray camera Spectrometer Spectrometer
6=102y2=103 K 108y1013 cmy3
Pyrometer MW-diagnostics
EBP cloud shape EBP composition Gas temperatures, Tm, Tr and Tv Target temperature, Tt Electron and ion densities, nepsnip
135
Valuable information about EBP composition and properties can be gleaned from plasma radiation. The shape and space distribution of radiation intensity of the plasma bulk can be obtained from the TV-camera. The CCD camera provided with the X-ray module was proved to be efficient to study the EBP near the target surface. More detailed information about composition and particles temperatures of the EBP can be obtained from the plasma spectra. Rotating Tr and vibrating Tv temperatures of heavy particles in the nitrogen EBP at various distances from the target were found by analyzing the bands (0, 1) and (1, 2) of molecular ion Nq 2 . The target temperature Tt can be measured by optical pyrometer because the EBP (and the EBP near the surface, in particular) was proved to be optically thin. Microwave methods seem to be the most promising to measure densities of charged particles in the EBP. The open barrel-shaped resonators were used to probe the EBP bulk. References w1x M.N. Vasiliev, The application of the electron beams in the plasmachemistry, in: V.E. Fortov (Ed.), Encyclopedia of the Low-Temperature Plasma, XI, Nauka, Moscow, 2001, pp. 436–445.