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Effect of substrate surface modification on alumina composite sol–gel coatings
Surface & Coatings Technology 200 (2006) 2800 – 2804 www.elsevier.com/locate/surfcoat
Effect of substrate surface modification on alumina composite sol–gel coatings Minghua Zhou*, Quanzu Yang, Tom Troczynski Department of Metals and Materials Engineering, University of British Columbia, Vancouver, BC, Canada V6T 1Z4 Received 12 July 2004; accepted in revised form 25 March 2005 Available online 8 June 2005
Abstract Chemically bonded composite sol – gel coatings have been recently proposed as a novel, low-cost method of protection of metals against corrosion and wear. However, the process operates in mildly acidic (pH = 3 – 4) conditions sufficient to initiate corrosion of mild steel during heat treatment of the coating at 300 -C. Therefore, there is a need to protect surface of steel from contact with the coating slurry. To achieve this, a protective film of iron phosphate was produced on mild steel substrate surface using commercial product (Crystcoat 747). The film also improved bonding between the substrate and the ceramic layer. X-ray diffraction (XRD) and scanning electron microscopy (SEM) characterized the microstructure of the iron phosphate coating and the ceramic coating. D 2005 Elsevier B.V. All rights reserved. Keywords: Phosphating; Sol – gel coating; Chemical bonding
1. Introduction The surface properties of metals, such as microhardness, wear and corrosion resistance and electrical insulation, can be improved by using various surface engineering techniques, including depositing protective ceramic coatings by physical vapor deposition (PVD) [1] and plasma spray coating method [2] and by sol – gel method [3]. In comparison with the high cost of using both PVD and plasma spray methods, sol – gel method has been showing its promising aspects in coating ceramic layers on several kinds of metal substrates, such as stainless steel, aluminium, etc. [4]. Recently, a novel technology of chemically bonded composite sol –gel coatings has been successfully developed in UBCeram lab. The coating is conducted using slurry prepared by dispersing nanometer scale a-alumina powder into a water based alumina sol [5,6]. The slurry is sprayed * Corresponding author. Current address: Advanced Materials Group, Materials Technology Laboratory, NRCan/CANMET/MMS/MTL, 405 Rochester Str., Ottawa, ON, Canada K1R 7T9. Tel.: +1 613 992 1397; fax: +1 613 992 9389. E-mail address: [email protected] (M. Zhou). 0257-8972/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2005.03.045
using conventional pressurized air spray gun, followed by cure at relatively low temperatures, e.g. 300 –400 -C to give a low cost ceramic coating. The chemical bonding through phosphating of sol –gel derived oxides or hydrated oxides leads to dense, hard, wear and corrosion resistant ceramics for variety of applications, including coatings and bulk components [5– 8]. In the case of coatings, the bonding is achieved after heat treatment at temperatures as low as 300 -C, without chemically attacking the substrate by the phosphates. The key phenomenon of the disclosed process is the reactivity of the fine sol particles with phosphate precursors. The inorganic products of this reaction polymerize and form complex phosphates, which may crystallize at elevated temperatures [9,10]. However the challenging issue is arising when this technique is implemented on plain steel, which is that the substrate gets rusted during sol –gel coating. As employed in the steel industry for painting of the plain steel, the pre-surface treatment– iron phosphating – was adopted to coat a thin layer of iron phosphates, which could prevent the substrate from getting rusted during sol – gel coating in mild acidic conditions. In present research, iron phosphate thin film was used as interfacial layer to prevent the corrosion of mild steel during composite sol –gel
M. Zhou et al. / Surface & Coatings Technology 200 (2006) 2800 – 2804
2801
N 550 °C N
N
N
N
450 °C 400 °C 300 °C 50 °C
A
A
A A
A Fe
no phosphating
15
25
A
35
A Fe
45
55
65
Fig. 3. Micrograph of the iron phosphated surface on sandblasted sample.
2 Theta Fig. 1. XRD patterns of the iron phosphate coating after thermal treatment at different temperatures (A—corundum, N—Fe1.5Ti0.5O3, iron titanium oxide).
coating processing. The film microstructure of the iron phosphate coating layer and the ceramic coating layer were studied. The corrosion resistance was tested and the bonding strength between the coating layer and the substrate was measured.
2. Experimental procedure A commercial product (Cryscoat 747, Chemetall Oakite, Oakite Product INC., 50 Valley Road, Berkeley Heights, NJ 07922) was used to conduct the iron phosphating of the plain steel. The concentrated Cryscoat 747 solution was diluted in distilled water in the range of 10 to 20 vol.%, and heated up to 50 -C to 70 -C, the plain steel pellets with a sandblasted fresh surface was submerged into the solution
accompanied by strong stirring for 5 to 8 min, then flushed by water. The obtained samples were thermal treated at different temperatures in the range of 20 -C to 550 -C for 20 min; X-ray diffraction analysis (XRD, model XDMAX, Rigaku) was used to identify the phase transitions of the iron phosphate layers. The microstructure of the iron phosphate on the plain steel pellet and its cross section was characterized by scanning electron microscopy (SEM) (Philips XL-30 Scanning Electron Microscopy). The alumina slurry for the spray coating was prepared from boehmite and a-alumina, detailed information about this sol – gel coating has been released by Troczynski and Yang, U. S. Pat. No. 6, 284, 682 B1 [4]. In order to reduce the permeability of the coating layer, which is essential for corrosion resistance, silicone – polymethylphenylsiloxane was sprayed on the coated ceramic layer to seal the pores. Coatings were also conducted on industrial tiles to study the permeability of the coating layer and the influence of phosphating and sealing on the permeability of the coating layer. A tile was cut into small plates of 30 30 3 mm.
5 µm
Fig. 2. Micrograph of the cross section of the iron phosphating layer on the sandblasted mild steel surface.
Fig. 4. Micrograph of the phosphated surface of the polished mild steel surface.
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M. Zhou et al. / Surface & Coatings Technology 200 (2006) 2800 – 2804
Alumina sol-gel composite coating layer
Iron phosphated layer
Mild steel substrate
Fig. 5. Cross section of mild steel with an iron phosphating layer and alumina sol – gel composite coating layer.
The surfaces of the plates on both sides were polished with abrasive papers (down to 1200 mess) and washed in ultrasonic bath. The permeability of the plate without coating and after each of the following steps –(No. 1) alumina sol – gel coating (50 Am), (No. 2) phosphating, (No. 3) alumina sol – gel coating (20 Am), (No. 4) phosphating, (No. 5) sealing (silicone) –was measured respectively by Vacuum Decay Permeameter (Vacuperm) developed by the Univ. of Missouri-Rolla. Pullout adhesion test (bonding strength test) was performed on the coating specimens by attaching two aligned cylindrical holders (25.4 mm in diameter) to both sides of the substrates through high-strength epoxy adhesive (3M Scotch-Weld, catalogue no. 2214). The test specimens were cured at 175 -C for 60 min, and the pullout test performed at a crosshead speed of 1 mm/min, using universal mechanical tester (Instron). Average of at least six test specimens was used for each experimental point. The bonding strength of the epoxy adhesive was about 70 MPa, as determined through a blank test without coating.
3. Results and discussion The thermal treatment of the alumina composite sol – gel coating is 300 -C for 20 min. The phase transitions of iron phosphate coatings at different temperatures were identified by XRD patterns as illustrated in Fig. 1.
It shows that corundum phase exists in the coating layer which implies the commercial product contains corundum powders. The expected iron phosphate phases such as Fe5H2(PO4)4I4H2O or Fe3(PO4)2I8H2O usually formed in temperature range of 50 –450 -C are not obvious in those XRD patterns, instead they may exist in an amorphous phase or at low crystalline degree. A new crystalline phase, iron titanium oxide, occurred at approximately 500 -C, which might originate from the decomposition of the coated iron phosphate layer; further work needs to be done to identify the precise composition of the iron phosphate layer. However, the processing temperature of chemically bonded composite sol – gel coatings is far below the phase transition temperature of the iron phosphate layer. Microcracks in iron phosphate film were observed at the phase transition temperature of the iron phosphate, which is accompanied by volume shrinkage and the generation of cracks. This could cause the rusting of the plain steel during the alumina sol –gel coating due to an exposure of the plain steel surface to the moisture and coating slurry. The SEM micrograph in Fig. 2 shows the cross section of the iron phosphate layers coated on the sandblasted mild steel surface. It shows that the iron phosphate layer is a porous structure. It is also observed that the thickness of the layer increases with the phosphating time. The micrograph of the surface as depicted in Fig. 3 also shows the rough surface with porous structure. In comparison with sandblasted substrates, there are a lot of cracks on the iron phosphate layer formed on the smooth surface of mild steel plate; the areas along the cracks are also covered by the iron phosphate as shown in Fig. 4. It is believed that the rough surface with large and reactive surface area generated by sandblasting could result in a continuous iron phosphating layer with porous structure in Fig. 3, which could eventually increase the bonding strength between the sol –gel alumina coating layer and substrate. Fig. 5 shows the cross section of the mild steel iron phosphated with chemically bonded alumina sol – gel coating; the bonding strength between the iron phosphated mild steel and alumina coating layer coating is around 10– 12 MPa. Since the thermal treatment of the alumina sol –gel composite coating layer is 300 -C, the alumina in composite coating layer originated from alumina sol is still in the form of g-alumina with a very low crystallinity; even after phosphating, this composite coating layer still could not resist acidic corrosion. A submersion test inside a sulfuric
Table 1 Influence of alumina sol – gel coating, further phosphating and sealing on the permeability of the porous industrial tiles Step no.
1
2
3
4
5
6
Treatments
Bare plate
First coating (50 Am)
First phosphating
Second coating (20 Am)
Second Phosphating (light)
Sealing (silicone)
Permeability (millidarcy) (T0.01)
4.03
2.19
1.27
0.52
0.53
0.01
M. Zhou et al. / Surface & Coatings Technology 200 (2006) 2800 – 2804
(a)
2803
(b) Al+Si
Al
Mild-Fe substrate
Alumina sol-gel coating layer
Sealing layer
Si
Sealing layer Alumina coating layer
Aluminium substrate
Fig. 6. SEM micrographs of the cross sections of the alumina sol – gel coating with sealing on (a) mild steel, and (b) aluminium with EDS element mapping of Al (green) and Si (yellow). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
solution with 3.6 mol/L concentration confirmed that the coating layer was etched away by the acid solution within a few hours. The influence of the alumina sol –gel composite coating layer, the further phasphating and sealing treatment on the permeability of the porous industrial tile plate is shown in Table 1. The permeability of the bare plate without coating is 4.03 millidarcy. With one phosphated coating layer (after step 2 and 3 treatments), the permeability of the tile could be reduced to 30% of its initial value. The second phosphated coating layer (after steps 4 and 5) could reduce the permeability to 12% of its initial value. In order to reduce the permeability then to improve the corrosion resistance without losing the advantages of ceramic coating, a sealant (silicone– polymethylphenylsiloxane) was used to seal the ceramic coating layer. After sealing treatment, the permeability is reduced to almost zero. By submerging the mild steel sample treated in the same way as the tile plate (two consecutive coating – phosphating followed by sealing) inside 3.6 M sulfuric acid for 72 h, the coating layer still keeps its good shape. Fig. 6 shows the SEM micrographs of the cross section of the sealed coating layers on both (a) mild steel and (b) aluminium substrate. A sealing layer was coated on the alumina sol – gel coating; the EDS element mappings of Al and Si in Fig. 6(b) reveal that the sealant has already penetrated the ceramic coating layer through the crack in the alumina coating layer, forming a composite coating layer. The corrosion test in 3.6 mol/L sulfuric acid for 72 h shows that the acidic solution cannot go through the sealing layer Table 2 Bonding strength between substrate and coating layer Surface treatment Sandblasted and Sandblasted without Phosphated phosphated phosphating surface smooth surface Bonding strength 10 – 12 MPa
7 – 9 MPa
3 – 4 MPa
to reach the ceramic coating layer; the corrosion resistance of the ceramic coating was improved by the sealing. The bonding tests were performed on the three sets of samples with different surface treatments, sandblasting surfaces with and without phosphating, smooth surface with phosphating. The primary test results listed in Table 2 show that the roughness of the samples caused by sandblasting is one of the important factors to enhance the bonding strength due to the large and reactive surface area, which results in a strong mechanical interlocking and chemical bonding after phosphating. The iron phosphating for the sandblasted creates a porous and crack free iron phosphate on the surface, which also helps to improve the bonding strength due to the penetration of the alumina sol – gel coating into the porous iron phosphate layer. However the iron phosphated smooth surface of the mild steel does not give a good bonding with alumina sol – gel coating. One reason is the weak mechanical interlocking between the coating layer and the substrate because of the smoothness of the surface. Another thing is the non-uniform iron phosphate layer with cracks, which may results in the weak bonding between the alumina sol – gel coating layer and the substrate with iron phosphate layer. Further experiments in improving the mechanical performance of the coating layer are under way.
4. Summary The iron phosphating for the sandblasted creates a porous and crack free iron phosphate on the surface; this phosphating layer can improve the bonding strength due to the penetration of the alumina sol –gel coating into the porous iron phosphate structure. However the iron phosphated smooth surface of the mild steel does not give a good bonding with alumina sol – gel coating. The roughness of the samples caused by sandblasting is one of the important
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M. Zhou et al. / Surface & Coatings Technology 200 (2006) 2800 – 2804
factors to enhance the bonding strength due to the large and reactive surface area, which could result in a strong mechanical interloging and chemical bonding after phosphating. The phosphating of the sol – gel alumina composite coating layer can reduce its permeability; with sealing, this surface treatment technique can isolate the substrate from the moisture in the environment, increasing the anticorrosion ability.
References [1] E. Lugscheider, C. Barimani, S. Guerreiro, K. Robzin, Surf. Coat. Technol. 108 – 109 (1998) 408. [2] A.R. de Arellano-Lopez, K.T. Faber, J. Am. Ceram. Soc. 82 (8) (1999) 2204. [3] A.R. Di Giampaolo Conde, M. Puerta, H. Ruiz, J. Lira Olivares, J. Non-Cryst. Solids 147 (148) (1992) 467.
[4] Tom Troczynski, Quanzu Yang, ‘‘Processing for making chemically bonded sol – gel ceramics’’, U. S. Pat. No. 6, 284, 682 B1. [5] G. Li, H. Kim, M. Chen, Q. Yang, T. Troczynski, Proceedings of the International Symposium on Ecomaterials and Ecoprocesses, 2nd, Vancouver, BC, Canada, Aug. 24 – 27, 2003 (2003), 363 – 372. Editor(s): Mostaghaci, Hamid. Ecomaterials and Ecoprocesses, Publisher: Canadian Institute of Mining, Metallurgy and Petroleum, Montreal, Que CODEN: 69EXVB Conference written in English. CAN 140:289880 AN 2004:12036 CAPLUS. [6] H.M. Hawthorne, A. Neville, T. Troczynski, X. Hu, M. Thammachart, Y. Xie, J. Fu, Q. Yang, Surf. Coat. Technol. 176 (2004) 243. [7] S. Wilson, H.M. Hawthorne, Q. Yang, T. Troczynski, Wear 251 (2001) 1042. [8] S. Wilson, H.M. Hawthorne, Q. Yang, T. Troczynski, Surf. Coat. Technol. 133 – 134 (2000) 389. [9] G. Lorin, Phosphating of Metals, Finishing Publications, Middlesex, 1974. [10] D.B. Freeman, Phosphating and Metal Pre-Treatment, WoodheadFaulkner, Cambridge, 1986.
Effect of substrate surface modification on alumina composite sol–gel coatings Minghua Zhou*, Quanzu Yang, Tom Troczynski Department of Metals and Materials Engineering, University of British Columbia, Vancouver, BC, Canada V6T 1Z4 Received 12 July 2004; accepted in revised form 25 March 2005 Available online 8 June 2005
Abstract Chemically bonded composite sol – gel coatings have been recently proposed as a novel, low-cost method of protection of metals against corrosion and wear. However, the process operates in mildly acidic (pH = 3 – 4) conditions sufficient to initiate corrosion of mild steel during heat treatment of the coating at 300 -C. Therefore, there is a need to protect surface of steel from contact with the coating slurry. To achieve this, a protective film of iron phosphate was produced on mild steel substrate surface using commercial product (Crystcoat 747). The film also improved bonding between the substrate and the ceramic layer. X-ray diffraction (XRD) and scanning electron microscopy (SEM) characterized the microstructure of the iron phosphate coating and the ceramic coating. D 2005 Elsevier B.V. All rights reserved. Keywords: Phosphating; Sol – gel coating; Chemical bonding
1. Introduction The surface properties of metals, such as microhardness, wear and corrosion resistance and electrical insulation, can be improved by using various surface engineering techniques, including depositing protective ceramic coatings by physical vapor deposition (PVD) [1] and plasma spray coating method [2] and by sol – gel method [3]. In comparison with the high cost of using both PVD and plasma spray methods, sol – gel method has been showing its promising aspects in coating ceramic layers on several kinds of metal substrates, such as stainless steel, aluminium, etc. [4]. Recently, a novel technology of chemically bonded composite sol –gel coatings has been successfully developed in UBCeram lab. The coating is conducted using slurry prepared by dispersing nanometer scale a-alumina powder into a water based alumina sol [5,6]. The slurry is sprayed * Corresponding author. Current address: Advanced Materials Group, Materials Technology Laboratory, NRCan/CANMET/MMS/MTL, 405 Rochester Str., Ottawa, ON, Canada K1R 7T9. Tel.: +1 613 992 1397; fax: +1 613 992 9389. E-mail address: [email protected] (M. Zhou). 0257-8972/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2005.03.045
using conventional pressurized air spray gun, followed by cure at relatively low temperatures, e.g. 300 –400 -C to give a low cost ceramic coating. The chemical bonding through phosphating of sol –gel derived oxides or hydrated oxides leads to dense, hard, wear and corrosion resistant ceramics for variety of applications, including coatings and bulk components [5– 8]. In the case of coatings, the bonding is achieved after heat treatment at temperatures as low as 300 -C, without chemically attacking the substrate by the phosphates. The key phenomenon of the disclosed process is the reactivity of the fine sol particles with phosphate precursors. The inorganic products of this reaction polymerize and form complex phosphates, which may crystallize at elevated temperatures [9,10]. However the challenging issue is arising when this technique is implemented on plain steel, which is that the substrate gets rusted during sol –gel coating. As employed in the steel industry for painting of the plain steel, the pre-surface treatment– iron phosphating – was adopted to coat a thin layer of iron phosphates, which could prevent the substrate from getting rusted during sol – gel coating in mild acidic conditions. In present research, iron phosphate thin film was used as interfacial layer to prevent the corrosion of mild steel during composite sol –gel
M. Zhou et al. / Surface & Coatings Technology 200 (2006) 2800 – 2804
2801
N 550 °C N
N
N
N
450 °C 400 °C 300 °C 50 °C
A
A
A A
A Fe
no phosphating
15
25
A
35
A Fe
45
55
65
Fig. 3. Micrograph of the iron phosphated surface on sandblasted sample.
2 Theta Fig. 1. XRD patterns of the iron phosphate coating after thermal treatment at different temperatures (A—corundum, N—Fe1.5Ti0.5O3, iron titanium oxide).
coating processing. The film microstructure of the iron phosphate coating layer and the ceramic coating layer were studied. The corrosion resistance was tested and the bonding strength between the coating layer and the substrate was measured.
2. Experimental procedure A commercial product (Cryscoat 747, Chemetall Oakite, Oakite Product INC., 50 Valley Road, Berkeley Heights, NJ 07922) was used to conduct the iron phosphating of the plain steel. The concentrated Cryscoat 747 solution was diluted in distilled water in the range of 10 to 20 vol.%, and heated up to 50 -C to 70 -C, the plain steel pellets with a sandblasted fresh surface was submerged into the solution
accompanied by strong stirring for 5 to 8 min, then flushed by water. The obtained samples were thermal treated at different temperatures in the range of 20 -C to 550 -C for 20 min; X-ray diffraction analysis (XRD, model XDMAX, Rigaku) was used to identify the phase transitions of the iron phosphate layers. The microstructure of the iron phosphate on the plain steel pellet and its cross section was characterized by scanning electron microscopy (SEM) (Philips XL-30 Scanning Electron Microscopy). The alumina slurry for the spray coating was prepared from boehmite and a-alumina, detailed information about this sol – gel coating has been released by Troczynski and Yang, U. S. Pat. No. 6, 284, 682 B1 [4]. In order to reduce the permeability of the coating layer, which is essential for corrosion resistance, silicone – polymethylphenylsiloxane was sprayed on the coated ceramic layer to seal the pores. Coatings were also conducted on industrial tiles to study the permeability of the coating layer and the influence of phosphating and sealing on the permeability of the coating layer. A tile was cut into small plates of 30 30 3 mm.
5 µm
Fig. 2. Micrograph of the cross section of the iron phosphating layer on the sandblasted mild steel surface.
Fig. 4. Micrograph of the phosphated surface of the polished mild steel surface.
2802
M. Zhou et al. / Surface & Coatings Technology 200 (2006) 2800 – 2804
Alumina sol-gel composite coating layer
Iron phosphated layer
Mild steel substrate
Fig. 5. Cross section of mild steel with an iron phosphating layer and alumina sol – gel composite coating layer.
The surfaces of the plates on both sides were polished with abrasive papers (down to 1200 mess) and washed in ultrasonic bath. The permeability of the plate without coating and after each of the following steps –(No. 1) alumina sol – gel coating (50 Am), (No. 2) phosphating, (No. 3) alumina sol – gel coating (20 Am), (No. 4) phosphating, (No. 5) sealing (silicone) –was measured respectively by Vacuum Decay Permeameter (Vacuperm) developed by the Univ. of Missouri-Rolla. Pullout adhesion test (bonding strength test) was performed on the coating specimens by attaching two aligned cylindrical holders (25.4 mm in diameter) to both sides of the substrates through high-strength epoxy adhesive (3M Scotch-Weld, catalogue no. 2214). The test specimens were cured at 175 -C for 60 min, and the pullout test performed at a crosshead speed of 1 mm/min, using universal mechanical tester (Instron). Average of at least six test specimens was used for each experimental point. The bonding strength of the epoxy adhesive was about 70 MPa, as determined through a blank test without coating.
3. Results and discussion The thermal treatment of the alumina composite sol – gel coating is 300 -C for 20 min. The phase transitions of iron phosphate coatings at different temperatures were identified by XRD patterns as illustrated in Fig. 1.
It shows that corundum phase exists in the coating layer which implies the commercial product contains corundum powders. The expected iron phosphate phases such as Fe5H2(PO4)4I4H2O or Fe3(PO4)2I8H2O usually formed in temperature range of 50 –450 -C are not obvious in those XRD patterns, instead they may exist in an amorphous phase or at low crystalline degree. A new crystalline phase, iron titanium oxide, occurred at approximately 500 -C, which might originate from the decomposition of the coated iron phosphate layer; further work needs to be done to identify the precise composition of the iron phosphate layer. However, the processing temperature of chemically bonded composite sol – gel coatings is far below the phase transition temperature of the iron phosphate layer. Microcracks in iron phosphate film were observed at the phase transition temperature of the iron phosphate, which is accompanied by volume shrinkage and the generation of cracks. This could cause the rusting of the plain steel during the alumina sol –gel coating due to an exposure of the plain steel surface to the moisture and coating slurry. The SEM micrograph in Fig. 2 shows the cross section of the iron phosphate layers coated on the sandblasted mild steel surface. It shows that the iron phosphate layer is a porous structure. It is also observed that the thickness of the layer increases with the phosphating time. The micrograph of the surface as depicted in Fig. 3 also shows the rough surface with porous structure. In comparison with sandblasted substrates, there are a lot of cracks on the iron phosphate layer formed on the smooth surface of mild steel plate; the areas along the cracks are also covered by the iron phosphate as shown in Fig. 4. It is believed that the rough surface with large and reactive surface area generated by sandblasting could result in a continuous iron phosphating layer with porous structure in Fig. 3, which could eventually increase the bonding strength between the sol –gel alumina coating layer and substrate. Fig. 5 shows the cross section of the mild steel iron phosphated with chemically bonded alumina sol – gel coating; the bonding strength between the iron phosphated mild steel and alumina coating layer coating is around 10– 12 MPa. Since the thermal treatment of the alumina sol –gel composite coating layer is 300 -C, the alumina in composite coating layer originated from alumina sol is still in the form of g-alumina with a very low crystallinity; even after phosphating, this composite coating layer still could not resist acidic corrosion. A submersion test inside a sulfuric
Table 1 Influence of alumina sol – gel coating, further phosphating and sealing on the permeability of the porous industrial tiles Step no.
1
2
3
4
5
6
Treatments
Bare plate
First coating (50 Am)
First phosphating
Second coating (20 Am)
Second Phosphating (light)
Sealing (silicone)
Permeability (millidarcy) (T0.01)
4.03
2.19
1.27
0.52
0.53
0.01
M. Zhou et al. / Surface & Coatings Technology 200 (2006) 2800 – 2804
(a)
2803
(b) Al+Si
Al
Mild-Fe substrate
Alumina sol-gel coating layer
Sealing layer
Si
Sealing layer Alumina coating layer
Aluminium substrate
Fig. 6. SEM micrographs of the cross sections of the alumina sol – gel coating with sealing on (a) mild steel, and (b) aluminium with EDS element mapping of Al (green) and Si (yellow). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
solution with 3.6 mol/L concentration confirmed that the coating layer was etched away by the acid solution within a few hours. The influence of the alumina sol –gel composite coating layer, the further phasphating and sealing treatment on the permeability of the porous industrial tile plate is shown in Table 1. The permeability of the bare plate without coating is 4.03 millidarcy. With one phosphated coating layer (after step 2 and 3 treatments), the permeability of the tile could be reduced to 30% of its initial value. The second phosphated coating layer (after steps 4 and 5) could reduce the permeability to 12% of its initial value. In order to reduce the permeability then to improve the corrosion resistance without losing the advantages of ceramic coating, a sealant (silicone– polymethylphenylsiloxane) was used to seal the ceramic coating layer. After sealing treatment, the permeability is reduced to almost zero. By submerging the mild steel sample treated in the same way as the tile plate (two consecutive coating – phosphating followed by sealing) inside 3.6 M sulfuric acid for 72 h, the coating layer still keeps its good shape. Fig. 6 shows the SEM micrographs of the cross section of the sealed coating layers on both (a) mild steel and (b) aluminium substrate. A sealing layer was coated on the alumina sol – gel coating; the EDS element mappings of Al and Si in Fig. 6(b) reveal that the sealant has already penetrated the ceramic coating layer through the crack in the alumina coating layer, forming a composite coating layer. The corrosion test in 3.6 mol/L sulfuric acid for 72 h shows that the acidic solution cannot go through the sealing layer Table 2 Bonding strength between substrate and coating layer Surface treatment Sandblasted and Sandblasted without Phosphated phosphated phosphating surface smooth surface Bonding strength 10 – 12 MPa
7 – 9 MPa
3 – 4 MPa
to reach the ceramic coating layer; the corrosion resistance of the ceramic coating was improved by the sealing. The bonding tests were performed on the three sets of samples with different surface treatments, sandblasting surfaces with and without phosphating, smooth surface with phosphating. The primary test results listed in Table 2 show that the roughness of the samples caused by sandblasting is one of the important factors to enhance the bonding strength due to the large and reactive surface area, which results in a strong mechanical interlocking and chemical bonding after phosphating. The iron phosphating for the sandblasted creates a porous and crack free iron phosphate on the surface, which also helps to improve the bonding strength due to the penetration of the alumina sol – gel coating into the porous iron phosphate layer. However the iron phosphated smooth surface of the mild steel does not give a good bonding with alumina sol – gel coating. One reason is the weak mechanical interlocking between the coating layer and the substrate because of the smoothness of the surface. Another thing is the non-uniform iron phosphate layer with cracks, which may results in the weak bonding between the alumina sol – gel coating layer and the substrate with iron phosphate layer. Further experiments in improving the mechanical performance of the coating layer are under way.
4. Summary The iron phosphating for the sandblasted creates a porous and crack free iron phosphate on the surface; this phosphating layer can improve the bonding strength due to the penetration of the alumina sol –gel coating into the porous iron phosphate structure. However the iron phosphated smooth surface of the mild steel does not give a good bonding with alumina sol – gel coating. The roughness of the samples caused by sandblasting is one of the important
2804
M. Zhou et al. / Surface & Coatings Technology 200 (2006) 2800 – 2804
factors to enhance the bonding strength due to the large and reactive surface area, which could result in a strong mechanical interloging and chemical bonding after phosphating. The phosphating of the sol – gel alumina composite coating layer can reduce its permeability; with sealing, this surface treatment technique can isolate the substrate from the moisture in the environment, increasing the anticorrosion ability.
References [1] E. Lugscheider, C. Barimani, S. Guerreiro, K. Robzin, Surf. Coat. Technol. 108 – 109 (1998) 408. [2] A.R. de Arellano-Lopez, K.T. Faber, J. Am. Ceram. Soc. 82 (8) (1999) 2204. [3] A.R. Di Giampaolo Conde, M. Puerta, H. Ruiz, J. Lira Olivares, J. Non-Cryst. Solids 147 (148) (1992) 467.
[4] Tom Troczynski, Quanzu Yang, ‘‘Processing for making chemically bonded sol – gel ceramics’’, U. S. Pat. No. 6, 284, 682 B1. [5] G. Li, H. Kim, M. Chen, Q. Yang, T. Troczynski, Proceedings of the International Symposium on Ecomaterials and Ecoprocesses, 2nd, Vancouver, BC, Canada, Aug. 24 – 27, 2003 (2003), 363 – 372. Editor(s): Mostaghaci, Hamid. Ecomaterials and Ecoprocesses, Publisher: Canadian Institute of Mining, Metallurgy and Petroleum, Montreal, Que CODEN: 69EXVB Conference written in English. CAN 140:289880 AN 2004:12036 CAPLUS. [6] H.M. Hawthorne, A. Neville, T. Troczynski, X. Hu, M. Thammachart, Y. Xie, J. Fu, Q. Yang, Surf. Coat. Technol. 176 (2004) 243. [7] S. Wilson, H.M. Hawthorne, Q. Yang, T. Troczynski, Wear 251 (2001) 1042. [8] S. Wilson, H.M. Hawthorne, Q. Yang, T. Troczynski, Surf. Coat. Technol. 133 – 134 (2000) 389. [9] G. Lorin, Phosphating of Metals, Finishing Publications, Middlesex, 1974. [10] D.B. Freeman, Phosphating and Metal Pre-Treatment, WoodheadFaulkner, Cambridge, 1986.