Printed in Great Britain. All rights reserved PII:S0955-2219(97)00213-6 0955-221998519.00+0.00 Electrophoretic and Electrolytic Deposition of Ceramic Coatings on Carbon Fibers I Zhitomirsky Israel Institute of Metals, Technion-Israel Institute of Technology, Haifa, 32000 Israel (Received 22 September 1997; accepted 24 November 1997) abstract A number of techniques such as laser ablation, 3 sol-gel, 5.sputtering, 2, 14 electrolytic deposition, 7. 18 Alumina, zirconia, titania, lead zirconate titanate, electrophoretic deposition, 20 and others4, 10, II hydroxyapatite and chemically bonded ceramics in have been used for coating formation on fibers the system Ca0-Sior-P2Os were deposited via Electrolytic and electrophoretic deposition electrophoretic or and electrolytic deposition meth- allows important advantages such as rigid control ods on individual carbon fibers, bundles and felts, of coating thickness, uniformity and deposition hich served as cathodic substrates. experimento rate. These methods offer the added attraction of conditions were determined for formation of uniform simple and low cost equipment, low preparation deposits of controlled thickness. After burning out of temperature and high purity of deposits. 7-21 carbon fibers the corresponding ceramic replicas can Electrodeposition can be performed by be obtained. The possibility to form multilayer alu cathodic. 18,22-24 or anodic methods 24-27 mina/zirconia hollow fibers with sharp interfaces However, anodic deposition method ha between alumina and zirconia layers ha imited utility regarding possible materials to be demonstrated. Obtained coatings and fibers were deposited by this method and substrates used studied by SEM. Experimental results obtained by for deposition. Significant interest has focused on electrophoretic and electrolytic deposition methods cathodic electrolytic deposition(CelD)due to were compared. c)1998 Elsevier Science Limited. the possibility of film formation on various All rights reserved conductive substrates. In this method metal ions or complexes are hydrolyzed by generated base to form oxide or hydroxide films on 1 Introduction the cathodic substrate. 7, 18,22-24,28 Different chemical reactions available for the generation Methods of coating formation on fibers-10 have of base were discussed in the literature. 4, 28A attracted attention for development of advanced great deal of current research has been devoted to composites. Ceramic coatings are necessary for demonstration of the feasibility of CELD of optimization of the fiber/matrix bonding ,>oxida various ceramic materials. 17, 18.22-24,28-33 tion protection of non-oxidic fibers and prevention Electrophoretic deposition(EPD)is achieved via of fiber-matrix interfacial degradation at elevated motion of charged particles towards an electrode temperatures. It is known that mechanical under the applied electric field. Cathodic or anodic properties of fiber-reinforced composites could be deposits can be achieved depending on particle improved by the use of coated fibers. 2,5,9 Tough- charge. There are excellent reviews in the literature ening mechanisms related to crack deflection, describing mechanism of EPD and application of rere discussed in litcrature, I-5,9 C debonding and the role of interfacial fiber coatings this technique for processing of various materi als. 34,35 Recent studies have demonstrated that Coating techniques have attained strong interest EPD can be used for consolidation of fiber rein- for development of hollow fibers and microtubes. forced composites making use of fibrous materials Individual carbon fibers, carbon fiber bundles as working electrodes 36-39 and felts, polyester fibers were used as burnable tem- This paper presents results of EPD and CELD plates for formation of hollow ceramic fibers. -14 of various ceramic materials on carbon fibers and Hollow ceramic fibers are of interest for electronic, 2- addresses different factors controlling coating 14 biomedical and other 1, 16 applications thickness, morphology and deposition rate
Journal of the European Ceramic Society 18 (1998) 849-856 0 1998 Elsevier Science Limited PII: SO955-2219(97)00213-6 Printed in Great Britain. All rights reserved 0955-2219/98/$19.00 + 0.00 Electrophoretic and Electrolytic Deposition of Ceramic Coatings on Carbon Fibers I. Zhitomirsky Israel Institute of Metals, Technion-Israel Institute of Technology, Haifa, 32000 Israel (Received 22 September 1997; accepted 24 November 1997) Abstract Alumina, zirconia, titania, lead zirconate titanate, hydroxyapatite and chemically bonded ceramics in the system CaO-SiOtPz05 were deposited via electrophoretic or/and electrolytic deposition methods on individual carbon fibers, bundles and felts, which served as cathodic substrates. Experimental conditions were determined for formation of untform deposits of controlled thickness. After burning out of carbon fibers the corresponding ceramic replicas can be obtained. The possibility to form multilayer aluminalzirconia hollow fibers with sharp interfaces between alumina and zirconia layers has been demonstrated. Obtained coatings and fibers were studied by SEA4. Experimental results obtained by electrophoretic and were compared. 0 All rights reserved 1 Introduction electrolytic deposition methods 1998 Elsevier Science Limited. Methods of coating formation on fibers’-lo have attracted attention for development of advanced composites. Ceramic coatings are necessary for optimization of the fiber/matrix bonding,2T5 oxidation protection of non-oxidic fibers and prevention of fiber-matrix interfacial degradation at elevated temperatures. 5-7 It is known that mechanical properties of fiber-reinforced composites could be improved by the use of coated fibers.2,5,9 Toughening mechanisms related to crack deflection, debonding and the role of interfacial fiber coatings were discussed in literature.‘-5,9 Coating techniques have attained strong interest for development of hollow fibers and microtubes. Individual carbon fibers, carbon fiber bundles and felts, polyester fibers were used as burnable templates for formation of hollow ceramic fibers.’ l-l4 Hollow ceramic fibers are of interest for electronic,12- i4 biomedical15 and other”J6 applications. 849 A number of techniques such as laser ablation3 sol-ge1,5,9 sputtering, 2,14 electrolytic deposition,17,‘8 electrophoretic depositioni9,20 and others4~10~11 have been used for coating formation on fibers. Electrolytic and electrophoretic deposition allows important advantages such as rigid control of coating thickness, uniformity and deposition rate. These methods offer the added attraction of simple and low cost equipment, low preparation temperature, and high purity of deposits.17-21 Electrodeposition can be performed by cathodici7,isZ-24 or anodic methods.2627 However, anodic deposition method has limited utility regarding possible materials to be deposited by this method and substrates used for deposition. Significant interest has focused on cathodic electrolytic deposition (CELD) due to the possibility of film formation on various conductive substrates. In this method metal ions or complexes are hydrolyzed by an electrogenerated base to form oxide or hydroxide films on the cathodic substrate 17,18,22-24,28 Different chemical reactions available for the generation of base were discussed in the literature.24T28 A great deal of current research has been devoted to demonstration of the feasibility of CELD of various ceramic materials.17~18,22~24,28s33 Electrophoretic deposition (EPD) is achieved via motion of charged particles towards an electrode under the applied electric field. Cathodic or anodic deposits can be achieved depending on particle charge. There are excellent reviews in the literature describing mechanism of EPD and application of this technique for processing of various materia1s.34,35 Recent studies have demonstrated that EPD can be used for consolidation of fiber reinforced composites making use of fibrous materials as working electrodes.3”39 This paper presents results of EPD and CELD of various ceramic materials on carbon fibers and addresses different factors controlling coating thickness, morphology and deposition rate
850 1. Zhitomirsky 2 Experimental Procedures 2.3 Substrates Individual fibers, bundles(Tenax)and felts (Lydall 2. 1 Materials for electrophoretic deposition Inc. were used as substrates. The diameter of the Submicron alumina (Fluka Chemie AG), sub- carbon fibers was 7 um Fiber length was up to micron zirconia partially stabilized with CeO2 TZ- several centimeters, and the bundles included up to 12CE,TOSOH Zirconia Process Company) and 3000 individual fibers. The thickncss of carbon felts gular samples used as substrates was up to 5 ctrl? PZT (PE-600, Fuji Titanium Industry Co Ltd, was in the region of 0.1-0.3 mm, area of rectan average particle size of lum)were used Submicron hydroxyapatite Cano(PO4)(OH) (HA) powders were prepared via a chemical pre- 2.4 Electrophoretic deposition cipitation method described in previous work.40 The electrophoretic cell included the cathodic sub- As starting materials commercially guaranteed strate centered between two parallel counter Ca(NO3)24H2O(Aldrich Chemical Company, electrodes. The distance between the cathode and Inc ),(NH4)2HPO4(Merck)and NH4OH (Palacid anode was 1.5 cm. Experiments were performed at LTD)were used. The precipitation was performed a constant voltage regime, applied voltages were in t 70C by slow addition of a 0 6M ammonium region of 50-200V, deposition times were from 10s phosphate solution to a stirred reaction vessel to 5 min. Laminated coatings were fabricated by containing 1-0 M calcium nitrate solution. The pH multiple deposition accompanied by drying at of the solutions was previously adjusted to 1l by room temperature addition of NH4OH. Stirring was performed dur- g 24 h at 70C and 48 h at room temperature. The 2.5 Electrolytic deposition obtained precipitate was filtered, rinsed with water The electrochemical cell for deposition in a galva and finally with isopropyl alcohol, stored in a nostatic regime included the cathodic substrate and moisture free atmosphere for three weeks and mil- two platinum anodes. Electrodeposition experi led in an agate mortar. The stoichiometry of the ments were pcrformcd at 1 C(stock solutions 1 HA obtained was proven by chemical analysis and 3)and at room temperature(stock solution 2) Chemically bonded ceramic(CBC)in the system Cathodic deposits were obtained at current den Cao-SiO2-P2Os with the composition 59. 3% sities ranging from 3 to 30 mA cm. Deposition Cao, 17.2% SiO2 and 23.5% P2O5 was prepared times were in the range of up to 5 min by a gel-precipitation technique similar to that described in 4 Phosphoric acid H3PO4(Frutarom 2.6 Characterization Ltd), calcium nitrate Ca(NO3)24H2O(Aldrich Coating morphology was studied by a scanning Chemical Company, Inc ),and silica sol (30 wt%, electron microscope (Jeol, model JSM-840). Fiber Merck)were mixed, stirred at 80C (10 h), dried at diameter was evaluated by an optical microscope 90C (48 h)fired at 200, 400 and 600C for 2h and and SEM observations. X-ray, TG/ DTA, Auger finally at 700oC for 24 h. Obtained powders were and other data for electrolytic titania, zirconia and milled and stored in a desiccator PZT deposits, obtained from stock solutions 1, 2 Suspensions for electrophoretic experiments were and 3, respectively, were reported in previous prepared by ultrasonic agitation of ceramic pow- papers. 17, 18, 22,31,42,43 ders in isopropyl alcohol. The powder concentra- tion in the suspensions was in the region of 30 3 Experimental Results 2.2 Materials for electrolytic deposition In electrophoretic experiments alumina, zirconia As starting materials, commercially guaranteed PZT, Ha and CBC particles were positively salts of TiCl4(Merck), ZrOCl2 8H]O(Fluka Che- charged and moved towards the cathode under the mie AG), Pb(NO3)2(Riedel-de Haen AG) and applied field. Deposits of various thicknesses were hydrogen peroxide H2O2(30 wt% in water, Carlo obtained on individual carbon fibers, bundles and Erba Reagenti)were used. Stock solution 1 con- felts used as cathodic substrates. Figure 1(a)shows Lained 0.005 M TiCl4 and 0.01 M H2O2 an SEM image of a green PZT deposit on an indi- mixed methyl alcohol-water (3: 1 volume ratio) vidual carbon fiber. EPD can also be performed on solvent. Stock solution 2 contained 0-005 M bundles of several carbon fibers. Figure 1(b)pre- ZrOCl and 0.01 M H2O2 in water. For prepara- sents a green ha deposit obtained via EPD. The tion of stock solution 3 reagents were dissolved in thickness of HA deposits obtained at a deposition water in a ratio Pb(NO3)2: ZrOCl2: TiCl4: voltage of 50V and various deposition durations H2O2=1: 0.52: 0-48: 10, the Pb(NO3)2 concentra- (up to 2 min ) was in the region of 2-30 um tion was 0-005M Figure 1(c)shows a CBC deposit on a carbon felt
850 I. Zhitomirsky 2 Experimental Procedures 2.3 Substrates 2.1 Materials for electrophoretic deposition Submicron alumina (Fluka Chemie AG), submicron zirconia partially stabilized with CeOz (TZ- 12CE,TOSOH Zirconia Process Company) and PZT (PE-600, Fuji Titanium Industry Co.Ltd., average particle size of - 1 pm) were used. Individual fibers, bundles (Tenax) and felts (Lydall, Inc.) were used as substrates. The diameter of the carbon fibers was 7pm. Fiber length was up to several centimeters, and the bundles included up to 3000 individual fibers. The thickness of carbon felts was in the region of 0.1-0.3mm, area of rectangular samples used as substrates was up to 5 cm*. Submicron hydroxyapatite Ca10(P04)6(OH)2 (HA) powders were prepared via a chemical precipitation method described in previous work.40 As starting materials commercially guaranteed Ca(N0&4H20 (Aldrich Chemical Company, Inc.), (NH&HP04 (Merck) and NH40H (Palacid LTD) were used. The precipitation was performed at 70°C by slow addition of a 0.6M ammonium phosphate solution to a stirred reaction vessel containing I.0 M calcium nitrate solution. The pH of the solutions was previously adjusted to 11 by addition of NH40H. Stirring was performed during 24 h at 70°C and 48 h at room temperature. The obtained precipitate was filtered, rinsed with water and finally with isopropyl alcohol, stored in a moisture free atmosphere for three weeks and milled in an agate mortar. The stoichiometry of the HA obtained was proven by chemical analysis. 2.4 Electrophoretic deposition The electrophoretic cell included the cathodic substrate centered between two parallel counterelectrodes. The distance between the cathode and anode was 1.5 cm. Experiments were performed at a constant voltage regime, applied voltages were in region of 5&200 V, deposition times were from 10 s to 5 min. Laminated coatings were fabricated by multiple deposition accompanied by drying at room temperature. 2.5 Electrolytic deposition Chemically bonded ceramic (CBC) in the system CaO-SiO*--P205 with the composition 59.3% CaO, 17.2% Si02 and 23.5% P205 was prepared by a gel-precipitation technique similar to that, described in.41 Phosphoric acid HsP04 (Frutarom Ltd), calcium nitrate Ca(N0&4H20 (Aldrich Chemical Company, Inc.), and silica sol (30wt%, Merck) were mixed, stirred at 80°C (10 h), dried at 90°C (48 h) fired at 200,400 and 600°C for 2 h and finally at 700°C for 24 h. Obtained powders were milled and stored in a desiccator. The electrochemical cell for deposition in a galvanostatic regime included the cathodic substrate and two platinum anodes. Electrodeposition experiments were performed at 1°C (stock solutions 1 and 3) and at room temperature (stock solution 2). Cathodic deposits were obtained at current densities ranging from 3 to 30mAcm2. Deposition times were in the range of up to 5 min. 2.6 Characterization Suspensions for electrophoretic experiments were prepared by ultrasonic agitation of ceramic powders in isopropyl alcohol. The powder concentration in the suspensions was in the region of 3s 100 g litre-’ Coating morphology was studied by a scanning electron microscope (Jeol, model JSM-840). Fiber diameter was evaluated by an optical microscope and SEM observations. X-ray, TG/DTA, Auger and other data for electrolytic titania, zirconia and PZT deposits, obtained from stock solutions 1,2 and 3, respectively, were reported in previous papers. 17,18,22,31,42,43 3 Experimental Results 2.2 Materials for electrolytic deposition As starting materials, commercially guaranteed salts of Tic14 (Merck) , ZrOC12.8H20 (Fluka Chemie AG), Pb(NO& (Riedelde Haen AG) and hydrogen peroxide H202 (30 wt% in water, Carlo Erba Reagenti) were used. Stock solution 1 contained 0.005 M TiC14 and 0.01 M H202 in a mixed methyl alcohol-water (3:l volume ratio) solvent. Stock solution 2 contained 0.005M ZrOClz and 0.01 M H202 in water. For preparation of stock solution 3 reagents were dissolved in water in a ratio Pb(NO&: ZrOC4: TiC14: Hz02 = 1: O-52:0.48: 10, the Pb(NO& concentration was O-005 M. In electrophoretic experiments alumina, zirconia, PZT, HA and CBC particles were positively charged and moved towards the cathode under the applied field. Deposits of various thicknesses were obtained on individual carbon fibers, bundles and felts used as cathodic substrates. Figure l(a) shows an SEM image of a green PZT deposit on an individual carbon fiber. EPD can also be performed on bundles of several carbon fibers. Figure l(b) presents a green HA deposit obtained via EPD. The thickness of HA deposits obtained at a deposition voltage of 50V and various deposition durations (up to 2min.) was in the region of 2-30 pm. Figure l(c) shows a CBC deposit on a carbon felt
Electrophoretic and electrolytic deposition of ceramic coatings 23)2Kx1018y43 5:8mD39 Fig 1. SEM pictures of green electrophoretic deposits: (a) PZT on a carbon fiber;(b) HA on a bundle, consisting of several carbon fibers;(c, d)CBC on a carbon fiber felt [(c), (d)-difterent ma The CBC deposit appears non-uniform at higher seen that fiber diameter increases with deposition magnifications [Fig. I(d) as relatively large CBC time at constant deposition voltage. The slope of particles(up to several um) were used for the the curve is steeper at the beginning of the depos deposition process tion. however it decreases with time Deposit thickness can be controlled by variation decrease of the deposition rate. Much higher values of deposition time and voltage. Figure 2 shows of fiber diameter were obtained at a deposition diameter of a green alumina deposit on an indivi- voltage of 50 v than those obtained at 25V dual carbon fiber versus deposition time at deposi Sintering experiments performed in ai tion voltages of 25 V and 50 V. In Fig. 2 it can be burning out of carbon at temperatures exceeding 00C and formation of hollow fibers. Figure 3 shows hollow alumina and HA fibers sintered at 1550 and 1150C, respectively. Hollow fibers of various length(up to 5cm)and diameters (up to 150 um)were obtained Laminated alumina/zirconia fibers were obtained by multiple electrophoretic deposition and firing of obtained deposits at 1000, 1400 and 1600C. This firing procedure facilitates reduction in micro- porosity, which is suggested to be a result of gas evolution during the burning out of the carbon Multilayer hollow fibers (up to 30 layers)were obtained. The thickness of individual layers was in obey consisting of 2 and 16 layers after firing at 1400C are shown in Fig 4(a)and(b), respectively. SEM Deposition time, min analysis indicates circular cross-sections of the fibers 2. Diameter of electrophoretic alumina deposits versus as well as individual layers and residual micro- position time at deposition voltages of 25v(a) and 50 v(b) porosity formed at this temperature. Figure 4c)
Electrophoretic and electrolytic deposition of ceramic coatings 851 (4 (b) (4 Fig. 1. SEM pictures of green electrophoretic deposits: (a) PZT on a carbon fiber; (b) HA on a bundle, consisting of several carbon fibers; (c, d) CBC on a carbon fiber felt [(c),(d)-different magnifications]. The CBC deposit appears non-uniform at higher magnifications [Fig. l(d)] as relatively large CBC particles (up to several pm) were used for the deposition process. Deposit thickness can be controlled by variation of deposition time and voltage. Figure 2 shows diameter of a green alumina deposit on an individual carbon fiber versus deposition time at deposition voltages of 25 V and 50 V. In Fig. 2 it can be 120 -i looE 2 5 sos E 4 60- 5 2 40- & 0 1 2 3 4 5 Deposition time, min Fig. 2. Diameter of electrophoretic alumina deposits versus deposition time at deposition voltages of 25V(a) and 50 V(b). seen that fiber diameter increases with deposition time at constant deposition voltage. The slope of the curve is steeper at the beginning of the deposition, however it decreases with time, indicating decrease of the deposition rate. Much higher values of fiber diameter were obtained at a deposition voltage of 50 V than those obtained at 25 V. Sintering experiments performed in air led to burning out of carbon at temperatures exceeding 900°C and formation of hollow fibers, Figure 3 shows hollow alumina and HA fibers sintered at 1550 and 115O”C, respectively. Hollow fibers of various length (up to 5cm) and diameters (up to 150 pm) were obtained. Laminated alumina/zirconia fibers were obtained by multiple electrophoretic deposition and firing of obtained deposits at 1000, 1400 and 1600°C. This firing procedure facilitates reduction in microporosity, which is suggested to be a result of gas evolution during the burning out of the carbon. Multilayer hollow fibers (up to 30 layers) were obtained. The thickness of individual layers was in the range of l-10,~m. Hollovj multilayer fibers consisting of 2 and 16 layers after firing at 1400°C are shown in Fig. 4(a) and (b), respectively. SEM analysis indicates circular cross-sections of the fibers as well as individual layers and residual microporosity formed at this temperature. Figure 4(c)
shows SEM picture of cross-section of laminated thickness higher than 3 um, crack formation was fiber sintered at 1600oC. The dark layer is alumina observed during the drying stay while the brighter layers are zirconia. SEM obser Figure 6 presents SEM pictures of PZT deposits ations revealed very sharp interfaces between alu- formed via CELD and Epd on carbon fibers mina and zirconia layers. It is important to note hese pictures obtained at the same magnification that surface roughness of the interface is influenced make it possible to compare the microstructures of by the particle size of the powders used in the CELd and EPd deposits. Scanning electron deposition process. Fibers fired at 1600 C exhibited micrographs of the green deposits obtained via very little porosity CELD [Fig. 6(a)]reveal a smooth, dense and con In previous CELD experiments the electro- tinuous morphology. The size of deposit particles is deposition yield was studied under various experi mental conditions. 17, 18, 29,31, 42 Deposit thickness can be controlled by variation of deposition time and current density. These results were utilized for formation of titania, zirconia and PzT deposits of controlled thickness on fibrous carbon substrates Figure 5(a) shows a green titania deposit obtained via CELD. The deposit thickness is about I um The ultrafine size of the deposit particles is note- worthy, and far below the micrometer scale. Figure 5(b)shows a zirconia deposit on the carbon felt after thermal treatment at 150C An enlarged view IFig. 5(c)] indicates very fine particle size. By var ation of deposition time at constant current den- sity of 20 mA cm-2, deposits of different thickness up to 5 um were obtained. However, at a deposit Fig. 4. SEM pictures of multilayer alumina/zirconia fibers Fig 3. SEM pictures of alumina (a) and HA (b) hollow fibers tic deposition [(a)2 layers, (b)-16 obtained via electrophoretic deposition and sintered at 1550 layers, thermal treatment at 1400 C]and a cross-section(c)of and 1150%C, respectively a multilayer fiber sintered at 1600.C
852 I. Zhitomirsky shows SEM picture of cross-section of laminated fiber sintered at 1600°C. The dark layer is alumina while the brighter layers are zirconia. SEM observations revealed very sharp interfaces between alumina and zirconia layers. It is important to note that surface roughness of the interface is influenced by the particle size of the powders used in the deposition process. Fibers fired at 1600°C exhibited very little porosity. In previous CELD experiments the electrodeposition yield was studied under various experimental conditions.‘7,18,29,31,42 Deposit thickness can be controlled by variation of deposition time and current density. These results were utilized for formation of titania, zirconia and PZT deposits of controlled thickness on fibrous carbon substrates. Figure 5(a) shows a green titania deposit obtained via CELD. The deposit thickness is about 1 pm. The ultrafine size of the deposit particles is noteworthy, and far below the micrometer scale. Figure 5(b) shows a zirconia deposit on the carbon felt after thermal treatment at 150°C. An enlarged view [Fig. 5(c)] indicates very fine particle size. By variation of deposition time at constant current density of 20mAcm- *, deposits of different thickness up to 5pm were obtained. flowever, at a deposit (b) Fig. 3. SEM pictures of alumina (a) and HA (b) hollow fibers obtained via electrophoretic deposition and sintered at 1550 and 115O”C, respectively. thickness higher than 3bm, crack formation was observed during the drying stage. Figure 6 presents SEM pictures of PZT deposits formed via CELD and EPD on carbon fibers. These pictures obtained at the same magnification make it possible to compare the microstructures of CELD and EPD deposits. Scanning electron micrographs of the green deposits obtained via CELD [Fig. 6(a)] reveal a smooth, dense and continuous morphology. The size of deposit particles is (4 0) (cl Fig. 4. SEM pictures of multilayer alumina/zirconia fibers obtained via electrophoretic deposition [(at2 layers, (b) -16 layers, thermal treatment at 14Oo”C] and a cross-section (c) of a multilayer fiber sintered at 1600°C
Electrophoretic and electrolytic deposition of ceramic coating on nanometric scale. The green deposits obtained EPD and CELD. It is important to note that du CELD [Fig. 6(a)] are tightly attached to the to the use of an electric field EPD and CELD substrate. As seen in Fig. 6(b) the maximum parti- particularly suited for the formation of uniform cle size in a deposit obtained via EPD is about deposits on substrates of complicated shape. The i um and the deposit includes a significant amount uniformity of ceramic coatings obtained via these of very fine particles. As seen in Fig. 6(b)pores up methods results from the insulating propcrtics of to l um were observed in the deposits the deposits and electric field dependence of the deposition rate. 40, 42 44.45 However, as pointed out n, 4 deposit uniformity is limited by the particle 4 Discussion size of the powders used for the deposition process In order to obtain deposits of adequate diameter The reported results indicate that various ceramic on carbon fibers, the size of individual particles materials can be deposited on fibrous substrates via should be significantly lower than the carbon fiber diameter Electrophoretic deposition is based on the use of ceramic particles. Electron microscopic investig tions have shown that the microstructures of deposits were influenced by powder characteristics Indeed, non-uniformity of CBC deposits observed at high magnifications [Fig. I(d)] is attributed to relatively large CBC particles. It should be noted that pZt deposits obtained in EPD experiments consequence of packing of relatively large PZT Pal ders offers advantages in fabrication of Eekt U483, on a carbon fiber;(b), (c)zirconia dep fiber felt after drying at 150C [(b), (c)-difte ig. 6. SEM pictures of green PZT deposits on carbon fibers
Electrophoretic and electrolytic deposition of ceramic coatings 853 on nanometric scale. The green deposits obtained via CELD [Fig. 6(a)] are tightly attached to the substrate. As seen in Fig. 6(b) the maximum particle size in a deposit obtained via EPD is about 1 pm and the deposit includes a significant amount of very fine particles. As seen in Fig. 6(b) pores up to 1 pm were observed in the deposits. 4 Discussion The reported results indicate that various ceramic materials can be deposited on fibrous substrates via (4 EPD and CELD. It is important to note that due to the use of an electric field EPD and CELD are particularly suited for the formation of uniform deposits on substrates of complicated shape. The uniformity of ceramic coatings obtained via these methods results from the insulating properties of the deposits and electric field dependence of the deposition rate. 40,42,44,45 However, as pointed out in > 44 deposit uniformity is limited by the particle size of the powders used for the deposition process. In order to obtain deposits of adequate diameter on carbon fibers, the size of individual particles should be significantly lower than the carbon fiber diameter. Electrophoretic deposition is based on the use of ceramic particles. Electron microscopic investigations have shown that the microstructures of deposits were influenced by powder characteristics. Indeed, non-uniformity of CBC deposits observed at high magnifications [Fig. l(d)] is attributed to relatively large CBC particles. It should be noted that PZT deposits obtained in EPD experiments were porous. This porosity is speculated to be a consequence of packing of relatively large PZT particles. Electrophoretic deposition of submicrometre powders offers advantages in fabrication of monolayer and multilayer coatings on carbon (b) (4 Fig. 5. SEM pictures of deposits obtained via CELD: (a) green titania deposit on a carbon fiber; (b),(c) zirconia deposit on a carbon fiber felt after drying at 150°C [(b),(c)-different magnifications]. (b) Fig. 6. SEM pictures of green PZT deposits on carbon fibers obtained via CELD (a) and EPD (b)
1. Zhitomirsky fibers. The particle size and pore size distributions in isopropyl alcohol. It should be mentioned that are determining factors for the packing situation the adsorbed water in green deposits obtained via within a deposit. It is important to note that pow- CELD leads to cementation of small particles der systems made up from close-packed, fine par- form aggregates and thus diminishes the advantage ticles are highly sinterreactive. Indeed, according of electrodeposition in producing films with nano to4647 the densification rate depends inversely on size particles. 29 However the deposition process the fourth power of particle size. As pointed out, 4/ needs certain amount of water for base generation reduction of particle size from micrometric to and prevention of formation of non-stoichiometric nanometric scale increased densification rate by oxides 29.42 It is clear that CEld can be performed several orders of magnitude from aqueous or mixed solutions CELD produces ceramic materials and provides EPD offers advantage in deposition of complex their deposition. It is important to see that electro- compounds. Indeed, the degree of stoichiometry of deposition allows coating formation on atomic the electrophoretic deposit is controlled by the scale and is now an important tool in formation of degree of stoichiometry of the powder used.45Pro nanostructured materials. 18, 21-23 Owing to use of blems are associated with control of stoichiometry ionic species instead of ceramic particles the of electrolytic deposits due to different deposition deposits obtained via CELd have lower particle rates of individual components. However, the pos- sizes. more uniform and continuous micro ibility of control of the deposit composition by use structures than those prepared via EPD. Therefore of specifically designed precursors, 22, 30, 31 opens higher sintering activity of CElD deposits can be new perspectives in deposition of important cera expected. CELD results in formation of oxide or mic complex oxide compounds of exact stoichio- hydroxide films. Oxide films obtained by thermal metry dehydration of hydroxides exhibited23, 32, 48 crack The above experimental data coupled with ing attributed to drying shrinkage. However, it was results of previous works,42, 4,45 suggest that established73149 that the use of additives and EPD and CELD cable formation of thick and ultiple deposition allows problems related to thin ceramic deposits on fibrous carbon materials Deposition rate Moreover, EPD and CELD can be used to repli and maximal deposit thickness obtained in EPD cate the carbon structures with ceramic materials and CELD experiments depend on various factors to form hollow fibers and microtubes. Obtained discussed in. 18,28, 34,35,, 45 Coating thickness can results have a significant potential for design be controlled by variation of deposition time, vol- composites. HA, alumina, zirconia and titania are tage or current density. The experimental data important materials for biomedical applications. 50 presented in Fig. 2 demonstrate a manner in which The possibility of formation of hollow fibers of the deposit diameter can be controlled. The various diameters is promising for the develop- decrease of deposition rate with time is attributed ment of biomedical implants of controlled poros to the increase in voltage drop across the deposited ity the deposition of pzt via Celd and epd or ayer. 40 Turning to experiments on deposition of fibrous substrates can be utilized for development various materials via EPD and CELD it is seen of advanced piezocomposites of desired phase that deposition rate in electrophoretic process is connectivity 51-53 The possibility of formation of much faster(by 2-3 orders of magnitude) than that multilayer coatings on fibrous substrates can be in electrodeposition process, resulting in sig- used for design of composites based on coated and to particle/electrode reactions are not involved in EPD and ceramic particles do not lose their charge on being deposited. In contrast, CELD is based on 5 Conclusions base generation in clectrode rcactions and ncu- tralization of ionic species to form deposits. As the This investigation has shown that various ceramic coating process progresses, an insulating layer materials can be deposited via EPD and CELD oI forms, which in turn prevents OH- generation. It fibrous carbon substrates. Ceramic coatings can be vas pointed out2,49 that coating resistivity is obtained as monolayers or multilayers, coating major limiting factor of the electrodeposition thickness can be controlled by variation of deposi method for development of thick coatings tion time, voltage or current density. The feasibility The choice of suitable solvent is important for of formation of hollow fibers via EPD has been EPD34,35 and CELD. 33, 42 It should be noted that demonstrated. Obtained results pave the way for non-aqueous solutions enable to decrease porosity formation of coated fibers, hollow fibers and of films, resulting from gas bubble evolution. With microtubes for development of composites for bio- his fact in mind, EPD experiments were performed medical, piezoelectric and other applications
854 I. Zhitomirsky fibers. The particle size and pore size distributions in isopropyl alcohol. It should be mentioned that are determining factors for the packing situation the adsorbed water in green deposits obtained via within a deposit. It is important to note that pow- CELD leads to cementation of small particles to der systems made up from close-packed, fine par- form aggregates and thus diminishes the advantage ticles are highly sinterreactive. Indeed, according of electrodeposition in producing films with nanoto46,47 the densification rate depends inversely on size particles. 29 However the deposition process the fourth power of particle size. As pointed out,47 needs certain amount of water for base generation reduction of particle size from micrometric to and prevention of formation of non-stoichiometric nanometric scale increased densification rate by oxides.29,42 It is clear that CELD can be performed several orders of magnitude. from aqueous or mixed solutions. CELD produces ceramic materials and provides their deposition. It is important to see that electrodeposition allows coating formation on atomic scale and is now an important tool in formation of nanostructured materials.18,21-23 Owing to use of ionic species instead of ceramic particles the deposits obtained via CELD have lower particle sizes, more uniform and continuous microstructures than those prepared via EPD. Therefore higher sintering activity of CELD deposits can be expected. CELD results in formation of oxide or hydroxide films. Oxide films obtained by thermal dehydration of hydroxides exhibited23$32>48 cracking attributed to drying shrinkage. However, it was established17,31T49 that the use of additives and multiple deposition allows problems related to drying shrinkage to be diminished. Deposition rate and maximal deposit thickness obtained in EPD and CELD experiments depend on various factors discussed in.18,28~34,35,40~42,45 Coating thickness can be controlled by variation of deposition time, voltage or current density. The experimental data presented in Fig. 2 demonstrate a manner in which the deposit diameter can be controlled. The decrease of deposition rate with time is attributed to the increase in voltage drop across the deposited layer. 4o Turning to experiments on deposition of various materials via EPD and CELD it is seen that deposition rate in electrophoretic process is much faster (by 2-3 orders of magnitude) than that in electrodeposition process, resulting in significantly higher deposit thicknesses. According to35 particle/electrode reactions are not involved in EPD and ceramic particles do not lose their charge on being deposited. In contrast, CELD is based on base generation in electrode reactions and neutralization of ionic species to form deposits. As the coating process progresses, an insulating layer forms, which in turn prevents OH- generation. It was pointed out 32*49 that coating resistivity is a major limiting factor of the electrodeposition method for development of thick coatings. EPD offers advantage in deposition of complex compounds. Indeed, the degree of stoichiometry of the electrophoretic deposit is controlled by the degree of stoichiometry of the powder used.45 Problems are associated with control of stoichiometry of electrolytic deposits due to different deposition rates of individual components. However, the possibility of control of the deposit composition by use of specifically designed precursors17~22,30,31 opens new perspectives in deposition of important ceramic complex oxide compounds of exact stoichiometry. The above experimental data coupled with results of previous works18,42,44*45 suggest that EPD and CELD enable formation of thick and thin ceramic deposits on fibrous carbon materials. Moreover, EPD and CELD can be used to replicate the carbon structures with ceramic materials to form hollow fibers and microtubes. Obtained results have a significant potential for design of composites. HA, alumina, zirconia and titania are important materials for biomedical applications.50 The possibility of formation of hollow fibers of various diameters45 is promising for the development of biomedical implants of controlled porosity. The deposition of PZT via CELD and EPD on fibrous substrates can be utilized for development of advanced piezocomposites of desired phase connectivity.51-53 The possibility of formation of multilayer coatings on fibrous substrates can be used for design of composites based on coated and laminated fibers. 5 Conclusions The choice of suitable solvent is important for EPD34,35 and CELD.33,42 It should be noted that non-aqueous solutions enable to decrease porosity of films, resulting from gas bubble evolution. With this fact in mind, EPD experiments were performed This investigation has shown that various ceramic materials can be deposited via EPD and CELD on fibrous carbon substrates. Ceramic coatings can be obtained as monolayers or multilayers, coating thickness can be controlled by variation of deposition time, voltage or current density. The feasibility of formation of hollow fibers via EPD has been demonstrated. Obtained results pave the way for formation of coated fibers, hollow fibers and microtubes for development of composites for biomedical, piezoelectric and other applications
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