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Al2O3 double layer coatings on AISI 440C stainless steel
Surface and Coatings Technology 182 (2004) 242–250
Characterization of electrolytic ZrO2 yAl2O3 double layer coatings on AISI 440C stainless steel C.C. Chang, S.K. Yen* Department of Materials Engineering, National Chung Hsing University, Taichung 40254, Taiwan, ROC Received 18 April 2003; accepted in revised form 11 July 2003
Abstract Electrolytic ZrO2 yAl2 O3 double layer deposition was conducted on AISI 440C stainless steel in ZrO(NO3 )2 and subsequently in Al(NO3)3 aqueous solutions. After drying and annealing, the ZrO2 yAl2 O3 coated specimens were evaluated by X-ray diffraction analysis, SEM, AES component depth profiling, hardness tests, wear tests, scratch tests, and dynamic polarization tests. Using poly-etheretherketone (PEEK) as a pin, weight loss of the coated specimen was only 1y4 of the uncoated materials. The scratch tests indicated that the adhesion strength of the coating was greater than the yield stress of the metal substrate. Electrochemical polarization tests of the coated specimen and optical photograph or SEM observation revealed 10 times more corrosion resistance than the uncoated in 1 M HCOOH and 3.5% NaCl aqueous solutions. 䊚 2003 Elsevier B.V. All rights reserved. Keywords: Electrolytic deposition; ZrO2 yAl2O3 double layer coating; AISI 440C stainless steel
1. Introduction AISI 440C stainless steel, one of the martensitic stainless steels, is capable of being age hardened with the tensile strengths of 1970 MPa. In addition to corrosion resistance, it exhibits the highest hardness of hardenable stainless steels. It has been widely used for ball bearings and molds w1,2x. The nominal chemical compositions in wt.% of AISI 440C stainless steel are 17.0 Cr, 1.0 Si, 1.0 Mn, 0.75 Mo, 1.1 C, 0.03 S and 0.04 P. Die materials used for plastic injection molding applications are generally subjected to severe abrasive wear from fibrous fillers and corrosion from epoxy resin. Due to the high precision requirement on the semiconductor integrated circuit (IC) chip packaging, wear andyor corrosion are always the major problems of die molds. Therefore, some surface modification technologies are used to reduce these problems. Currently, most of packaging dies are coated with hard chrome to reduce wear and corrosion problems. However, the environment contamination is getting serious, and the less contaminative coating technologies are required to replace hard *Corresponding author. Tel.: q886-4-22852953; fax: q886-422857017. E-mail address: [email protected] (S.K. Yen).
chrome. Also, as IC lead frame design becomes more sophisticated, the thickness of hard chrome is too large to be compatible. Recently, electrolytic Al2O3 and ZrO2 coatings on metal have been verified to improve the corrosion resistance w3–7x and wear resistance w6,7x of implant alloys. This method has several potential advantages, including the cheap deposition technology, low preparation temperature, high purity of deposits, the ability to cover complex shapes of various materials and the application of multi-component oxides w8,9x. Especially, the film thickness can be controlled under 3 mm which is required for the sophisticated IC packing molds. Electrolytic deposition is achieved via hydrolysis of metal ions or complexes by electrogenerated base to form hydroxide or peroxide deposits on cathode substrates. Hydroxide and peroxide deposits can be converted to corresponding oxides by thermal treatment w3,5,8x. In this study, the electrolytic ZrO2 yAl2O3 double layers coating has been deposited on AISI 440C in ZrO(NO3)2 and Al(NO3)3 aqueous solutions, respectively. The characteristics of the coating have been examined, including crystal structures, hardness, adhesion, wear and corrosion resistance.
0257-8972/04/$ - see front matter 䊚 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0257-8972(03)00862-4
C.C. Chang, S.K. Yen / Surface and Coatings Technology 182 (2004) 242–250
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Fig. 2. SEM Cross-sectional observation of ZrO2 yAl2 O3 coated specimen annealed at 450 8C for 3 h.
ultrasonically cleaned in deionized water and acetone, then dried by N2 gas. Electrolytic deposition — The electrolytic ZrO2 y Al2O3 double layers coating was deposited on AISI 440C stainless steel in 0.03125 M ZrO(NO3)2 and subsequently in 0.01 M Al(NO3)3 aqueous solution at voltage y1.0 to y1.2 V for 500 to 700 s at room temperature, using an EG&G M273A Potentiostat and M352 software. The sample was the cathode, graphite bars (fs5 mm, ls55 mm) were the anode and saturated AgyAgCl was the reference electrode. The coated specimens were then naturally dried in air and annealed for 3 h at 300, 350, 400, 450, 500 and 550 8C, respectively. The thickness of coatings was measured using surface profilometer (Dek3tak, USA). SEM, XRD and AES — The surface morphology of coated specimens was observed by scanning electron microscopy (SEM, JEOL, JSM-5400, Japan). The crystal structure of ZrO2 yAl2O3 coated specimen was analyzed grazing incident X-ray diffraction (GI-XRD) using a MAC MO3X-HF diffractometer, with Cu Ka radiation
Fig. 1. SEM observations of (a) ZrO2 coated, (b) Al2O3 coated and (c) ZrO2yAl2O3 double layer coated specimens annealed at 450 8C for 3 h.
2. Experimental Sample preparation — AISI 440C stainless steel was cut into sheets with 55=40=1 mm. All specimens were polished to a mirror finish (Ras0.059 mm) with Al2O3 powder, then degreased by detergent and further
Fig. 3. XRD diagrams of ZrO2 as coated specimen, and annealed at 450 8C for 3 h.
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Fig. 4. XRD diagrams of ZrO2yAl2O3 as coated specimen, annealed at 350, 400 and 450 8C, respectively, for 3 h.
(ls15.418 nm), the range of 2u from 10 to 708 and incident angle of 0.58, at a scanning rate of 18 miny1, a voltage of 40 kV, and a current of 30 mA. The component depth profiles were plotted using an auger electron spectroscopy (AES Fison VG Microlab 310D). Specimens were sputtered by 5 keV Arq with a beam current of 0.05 A, and a sputtering rate of 1 nm sy1. Hardness, wear and scratch tests — Hardness measurements were made on all specimens using a Vickers microhardness tester (Mitutoyo MVK-GB) with a load of 25 g. Wear test was carried out by a pin-on-disc system with poly-etheretherketone (PEEK) pin (cs2
mm), and the coated specimen disc, at a load of 8.28 MPa, sliding speed of 0.12 m sy1 and a total sliding distance of 10 000 m. PEEK is a new generation of engineering plastics for plastic packaging in microelectronics that has high Tg value and can stand temperature up to 250 8C for long period of time w10x. PEEK is the high-performance engineering plastic, with high thermal stability ()250 8C), excellent chemical resistance and superior mechanical, such as the hardness of 108 HRB, and the tension strength of 110 MPa at RT, and 48 MPa at 150 8C w10,11x. The wear loss was measured by the disc weight difference before and after each test. To study the adhesion of the coating on substrate, the ZrO2 yAl2O3 coated specimens annealed at 450 8C were tested by scratch (TEER-200, UK), using diamond indenter as the pin with an initial load 2 N, scratch speed 10 mm miny1, loading rate 50 N miny1 and the end load 100 N. The friction force between the specimen and diamond indenter was monitored by a load cell attached to the working table, as shown in the previous paper w7x. Surface energy and work of adhesion — Surface energy measurement were made on the uncoated, ZrO2 coated, and Al2O3 coated specimens using the threeliquid (water, formamide and ethyleneglycol) procedure by the dynamic contact angle analyzer research model ˚ (FTA200, First Ten Angstroms, USA). When two unlike phases i and j are brought together reversibly, the surface free energy change equals to the free energy of adhesion DGaij or the negative of adhesion work w12,13x. In other words, it can be presented by the following equation, DGaijsgijygiygjsyWaij
(1)
where gij is the interface energy of iyj, gi and gj the surface energies of phase i and j.
Fig. 5. AES component depth profiles of ZrO2yAl2O3 coated specimen annealed at 450 8C for 3 h.
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Fig. 6. Vickers hardness of the uncoated and ZrO2yAl2 O3 coated specimens annealed at various temperatures and the hardness increased by coatings.
Polarization test — The corrosion resistance of the ZrO2 yAl2O3 coated were evaluated by an electrochemical dynamic polarization tests in 1 M HCOOH and 3.5% NaCl aqueous solutions, respectively, also by using a EG&G Model 273A, M352. After the open-circuit potential became steady, the cyclic polarization test was from y0.2 V (vs. open-circuit potential) to 1 V (Agy AgCl), then back to open-circuit potential at a scanning rate of 0.167 mV sy1. The first oxidation-reduction
equilibrium potential E01 was derived when current density equaled zero during the applied voltage increased (forward cycle). If oxidation is due to the corrosion of electrode, this potential is also named corrosion potential Ecorr, and the corrosion current density icorr can be calculated at this potential. The second oxidation-reduction equilibrium potential E02 was derived when current density was back to zero again during the applied voltage decreased (backward cycle).
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3.2. Crystal structures and component profiles
Fig. 7. Friction force vs. the normal load of the scratch test on ZrO2yAl2O3 coated specimen annealed at 450 8C for 3 h.
If the oxidation is decreased abruptly at some passivation potential Epass, because of the formation of passivation films and corrosion rate falls to very low, and the beginning current density is named the critical passive current density icrit, and the stable current density is named the passive current density ipass. If the oxidation is increased abruptly at some pitting potential Epit because of the breakdown of passive film and the localized corrosion of the electrode, the crossover of the backward cycle and the forward cycle on the anodic polarization region will be found because of repassivation and defined as protection potential Epp. 3. Results and discussion 3.1. Surface morphology SEM observations of the ZrO2, Al2O3 and ZrO2 y Al2O3 coated specimens with the same film thickness of 0.7 mm are shown in Fig. 1. More and larger cracks are found in Fig. 1a and b. On the other hand, less and smaller cracks are found in Fig. 1c. Obviously, the mud cracks found on the single layer coating, can be reduced by the double layers coating for the same film thickness of 0.7 mm. SEM cross sectional observation of the ZrO2 yAl2O3 coated specimen is shown in Fig. 2. The thickness of ZrO2 coating and Al2O3 coating was approximately 0.3 mm and 0.4 mm, respectively.
The XRD diagrams of ZrO2 coated specimens are shown in Fig. 3. No obvious diffraction peak was found on the patterns of the as coated samples. The (101), (110) and (112) peaks of tetragonal ZrO2 were found on the specimen annealed at 450 8C. The XRD diagrams of the ZrO2 yAl2O3 coated specimens are shown in Fig. 4. The (100), (002) and (200) peaks of aluminum hydroxide hydrate (Al2(OH)6H2O) were found on the as-coated. The (020), (110), (130), (111), (140), (221) and (002) peaks of aluminum oxide hydroxide (AlO(OH)) were found on the coated specimens annealed at 350 and 400 8C. The (102) and (220) peaks of aluminum oxide hydrate (Al10O15H2O) were found at 400 and 4508C, and finally, the (111) and (220) peaks of g-Al2O3 were found at 450, 500 and 550 8C. No obvious ZrO2 peak was found for double layer coating. If the missing of ZrO2 peak was caused by value of X-rays penetration depth at GI-XRD the grazing angle chosen, the peaks of metal substrate should not be found either. However, the (110) and (200) peaks of 440C substrate were found, as shown in Fig. 4. Therefore, the factor of grazing angle chosen should be excluded. However, it is more possible that the condensation of Zr(OH)4 and the crystallization of ZrO2 should be delayed by the existence of H2O in the top layer. AES component depth profiles are plotted in Fig. 5, It is confirmed that the deposited coating is composed of two layers. The inner layer is ZrO2 and the out layer is Al2O3. A part of Fe diffused out to ZrO2 layer and a part of O diffused into metal substrates. This result is similar to the oxidation of AISI 430 stainless steel where the oxidation rate of Fe is greater than that of Cr at 450 8C w14x. The phenomenon of interdiffusion may be also caused by the vacancy which was formed due to the evaporation of H2O during the condensation reaction of Zr(OH)4 w5x. Such an interdiffusion between ZrO2 and substrate also retard the crystallization of ZrO2, so that no obvious diffraction peaks of ZrO2 was found at 450 8C annealing, as shown in Fig. 4. 3.3. Hardness Vickers Hardness of the uncoated and ZrO2 yAl2O3 coated specimens annealed at various temperatures are
Table 1 Weight Loss of the uncoated and ZrO2yAl2O3 coated specimens annealed at various temperatures, using PEEK as the pin Uncoated specimen Weight loss (mg)
0.1833 "18.4%
ZrO2 yAl2O3 coated specimen annealed at various temperatures 300 8C
350 8C
400 8C
450 8C
500 8C
0.1133 "11.9%
0.0767 "7.2%
0.0573 "9.7.%
0.0485 "13.4%
0.0474 "14.2%
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Fig. 8. SEM observations and EDS mapping of ZrO2 yAl2 O3 coated annealed at 450 8C for 3 h at the scratch load around (a) 22 N and (b) 100 N.
shown in Fig. 6a. The temper softening effect during annealing and the secondary hardening effect approximately 450 8C were found on the AISI 440C substrate. Increasing annealing temperature enhanced the hardness increase in the coatings. The increased hardness was more obvious at (or above) the annealing temperature of 450 8C, due to the formation of g-Al2O3 as shown in Figs. 4 and 6b.
3.4. Wear resistance With PEEK as the pin, the weight losses of the uncoated and coated specimens are listed in Table 1. The wear of coated specimens decreased with increase in annealing temperature. The increasing wear resistance was resulted from the increased hardness of the coatings, which was linked to the phase transformation from
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Table 2 Contact angles of water, formamide and ethyleneglycol on the uncoated, ZrO2 coated and Al2O3 coated, respectively
Water Formamide Ethyleneglycol
AISI 440C 8
ZrO2 coated 8
Al2O3 coated 8
87 60 64
70 50 63
83 59 63
Table 3 Surface energies gI, interface energies gij , and adhesion works W a calculated from contact angles listed in Table 2 by Van Oss method w12,13x AISI 440C Surface energy, gI (mJym2)
70.406
Interface energy, gij (mJym2) Adhesion work, Wa (mJym2)
ZrO2 coated 94.408
Al2O3 coated 67.165
ZrO2 y440C 16.416
Al2O y440C 0.041
ZrO2 yAl2O3 13.796
148.398
137.530
147.777
aluminum hydroxide hydrate (Al2(OH)6H2O) to aluminum oxide hydroxide (AlO(OH)), aluminum oxide hydrate (Al10O15H2O) and finally tog-Al2O3, as shown in Figs. 4 and 6. In contrast, the wear resistance seems to be independent of the tempering softening and secondary hardening effects of the substrate. 3.5. Adhesion Fig. 7 shows the friction force vs. the normal load on the stylus during the scratch test. This curve may be divided into two regions. One revealed a slope of 0.13 and the other 0.34. The transition point was around the load 22 N. The increased slope was caused by the deformation of the metal substrate, which was plastically deformed by the indenter at the load beyond 22 N. However, most of the ZrO2 yAl2O3 coating was found at 22 and 100 N of the scratch test, as shown in Fig. 8a and b. This means that the adhesion strength of ZrO2 y Al2O3 and ZrO2 y440C was stronger than the yield stress of the 440C. Contact angles of water, formamide, and ethyleneglycol on specimens are listed in Table 3. Surface energies,
Fig. 9. Polarization curves of the uncoated and ZrO2yAl2O3 coated specimens in 3.5% NaCl aqueous solution.
interface energies and adhesion work have been calculated by Van Oss method w11,12x, as listed in Table 4. The adhesion work of ZrO2 y440C was a little greater than that of Al2O3 y440C. Also, the adhesion work of ZrO2 yAl2O3 was greater than that of Al2O3 y440C. It is concluded that the better choice of the bottom layer is ZrO2 coating and that of the top layer is Al2O3 coating. 3.6. Corrosion resistance Representative polarization curves of the uncoated and coated specimens in 3.5% NaCl and 1 M HCOOH aqueous solutions are shown in Figs. 9 and 10, respectively. From the forward curves, E01 (or Ecorr), i0 (or icorr), Epass, icrit, ipass and Ep were analyzed, and from the backward curves E02 and Epp were measured, as given in Table 4. In 3.5% NaCl aqueous solution, the ZrO2 y Al2O3 coated specimens revealed the higher Ecorr, and lower icorr. No obvious Epit, was found on the coated samples, but it was found on the uncoated ones at y 136 mV. In 1 M HCOOH, the coated sample exhibited a lower icorr and ipass than the uncoated. After polarization tests in 3.5% NaCl aqueous solution, a large area penetrated on the uncoated was observed, as shown in Fig. 11a, but none on the coated, as shown in Fig. 11b. Similar results were found in 1M HCOOH, as shown in Fig. 12. These results indicate that the ZrO2 yAl2O3 film was more corrosion resistant or inert than the natural
Table 4 List of the first redox potential E01 (Ecorr), exchange current density i (icorr), critical passive current density icrit , passive current density ipass, pitting potential Epit, the second redox potential E02 and protection potential Epp of the uncoated and ZrO2 yAl2 O3 -coated specimens, derived form the polarization tests in 3.5% NaCl and 1 M HCOOH aqueous solutions Test solutions
Specimens
E01 (Ecor) (mV)
i (icor) (mAycm2)
icrit (mAycm2)
ipass (mAycm2)
Epit (mV)
E02 (mV)
Epp (mV)
3.5% NaCl 1M HCOOH
Uncoated Al2O3 yZrO2 y440C Uncoated Al2O3 yZrO2 y440C
y534 y341 y486 y457
231 25 96 8
— — 980 9
— — 30 0.9
y136 — — —
— y272 — —
— y96 — —
C.C. Chang, S.K. Yen / Surface and Coatings Technology 182 (2004) 242–250
Fig. 10. Polarization curves of the uncoated and ZrO2yAl2O3 coated specimens in 1 M HCOOH aqueous solution.
passivation film of AISI 440C stainless steel, in both 3.5% NaCl and 1M HCOOH aqueous solutions. Also, the lower ipass of the coated specimen at the anodic polarization region means that the coating film is more stable than the natural passivation film of the substrate. 4. Conclusions A new electrolytic coating method of ZrO2 yAl2O3 double layers has been successfully applied on an AISI 440C stainless steel to investigate its characteristics. The following conclusions are drawn: 1. The single layer deposition of ZrO2 or Al2O3 revealed many large cracks, which can be reduced by the ZrO2 yAl2O3 double layer coating. 2. The annealing resulted in increase of the hardness of the coatings, but was independent of the temper softening and secondary hardening effect on the 440C substrate. The increasing hardness of the coatings was caused by the series of phase transformations from the as-deposited Al2(OH)6H2O to AlO(OH) at 350 8C, to Al10O15H2O at 400 8C, and finally to g-Al2O3
249
Fig. 11. Optical photograph of (a) the uncoated and (b) ZrO2yAl2O3 coated specimens after polarization test in 3.5% NaCl.
at 450 8C. Wear resistance was increased with increasing annealing temperature. Especially, the weight loss of the ZrO2 yAl2O3 coated specimen annealed at 450 8C is only 1y4 of the uncoated. 3. The scratch tests revealed that the adhesion strength of the ZrO2 yAl2O3 coating film on the substrate was stronger than the yield stress of AISI 440C stainless steel. Also, the surface energy analyses suggested that the better choice of the bottom layer is the ZrO2 and that of the top layer is the Al2O3 coating. 4. The electrochemical polarization tests in 1 M HCOOH and 3.5% NaCl aqueous solution revealed that the ZrO2 yAl2O3 coated specimen was 10 times more corrosion resistance than the uncoated. Acknowledgments The authors are grateful for the support of this research by the National Science Council, Taiwan, under Contract No. NSC89-2216-E-005-011.
Fig. 12. SEM observations of (a) the uncoated and (b) ZrO2yAl2O3 coated specimens after polarization test in 1 M HCOOH.
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References w1x J.W. Dini, Surf. Coat. Technol. 78 (1996) 14–18. w2x C.M. Tomasello, J.L. Maloney III, Adv. Mater. Process. 154 (1998) 58–60. w3x S.K. Yen, J. Electrochem. Soc. 146 (1999) 1392–1396. w4x H.C. Hsu, S.K. Yen, Dent. Mater. 14 (1998) 339–346. w5x S.K. Yen, T.Y. Huang, Mater. Chem. Phys. 56 (1998) 214–221. w6x S.K. Yen, M.J. Guo, H.Z. Zhan, Biomaterials 22 (2001) 125–133. w7x S.K. Yen, S.W. Hsu, J. Biomed. Mater. Res. 54 (2001) 412–418.
I. Zhitomirsky, J. Euro. Ceram. Soc. 19 (1999) 2581–2587. I. Zhitomirsky, J. Euro. Ceram. Soc. 18 (1998) 849–856. C.P. Wong, Mater. Chem. Phys. 42 (1995) 25–30. F.J. Medellin-Rodriguez, P.J. Phillips, Polym. Eng. Sci. 30 (1990) 860–869. w12x R.J. Good, in: K.L. Mittal (Ed.), Contact Angle, Wettability and Adhesion, VSP, 1993, pp. 3–36. w13x R.J. Good, C.J. Van Oss, in: M.E. Schrader, G. Loeb (Eds.), Modern Aspexts of Wettability, Plenum, 1992, pp. 1–27. w14x S.K. Yen, Y.C. Tsai, J. Electrochem. Soc. 143 (1996) 2493–2497.
w8 x w9 x w10x w11x
Characterization of electrolytic ZrO2 yAl2O3 double layer coatings on AISI 440C stainless steel C.C. Chang, S.K. Yen* Department of Materials Engineering, National Chung Hsing University, Taichung 40254, Taiwan, ROC Received 18 April 2003; accepted in revised form 11 July 2003
Abstract Electrolytic ZrO2 yAl2 O3 double layer deposition was conducted on AISI 440C stainless steel in ZrO(NO3 )2 and subsequently in Al(NO3)3 aqueous solutions. After drying and annealing, the ZrO2 yAl2 O3 coated specimens were evaluated by X-ray diffraction analysis, SEM, AES component depth profiling, hardness tests, wear tests, scratch tests, and dynamic polarization tests. Using poly-etheretherketone (PEEK) as a pin, weight loss of the coated specimen was only 1y4 of the uncoated materials. The scratch tests indicated that the adhesion strength of the coating was greater than the yield stress of the metal substrate. Electrochemical polarization tests of the coated specimen and optical photograph or SEM observation revealed 10 times more corrosion resistance than the uncoated in 1 M HCOOH and 3.5% NaCl aqueous solutions. 䊚 2003 Elsevier B.V. All rights reserved. Keywords: Electrolytic deposition; ZrO2 yAl2O3 double layer coating; AISI 440C stainless steel
1. Introduction AISI 440C stainless steel, one of the martensitic stainless steels, is capable of being age hardened with the tensile strengths of 1970 MPa. In addition to corrosion resistance, it exhibits the highest hardness of hardenable stainless steels. It has been widely used for ball bearings and molds w1,2x. The nominal chemical compositions in wt.% of AISI 440C stainless steel are 17.0 Cr, 1.0 Si, 1.0 Mn, 0.75 Mo, 1.1 C, 0.03 S and 0.04 P. Die materials used for plastic injection molding applications are generally subjected to severe abrasive wear from fibrous fillers and corrosion from epoxy resin. Due to the high precision requirement on the semiconductor integrated circuit (IC) chip packaging, wear andyor corrosion are always the major problems of die molds. Therefore, some surface modification technologies are used to reduce these problems. Currently, most of packaging dies are coated with hard chrome to reduce wear and corrosion problems. However, the environment contamination is getting serious, and the less contaminative coating technologies are required to replace hard *Corresponding author. Tel.: q886-4-22852953; fax: q886-422857017. E-mail address: [email protected] (S.K. Yen).
chrome. Also, as IC lead frame design becomes more sophisticated, the thickness of hard chrome is too large to be compatible. Recently, electrolytic Al2O3 and ZrO2 coatings on metal have been verified to improve the corrosion resistance w3–7x and wear resistance w6,7x of implant alloys. This method has several potential advantages, including the cheap deposition technology, low preparation temperature, high purity of deposits, the ability to cover complex shapes of various materials and the application of multi-component oxides w8,9x. Especially, the film thickness can be controlled under 3 mm which is required for the sophisticated IC packing molds. Electrolytic deposition is achieved via hydrolysis of metal ions or complexes by electrogenerated base to form hydroxide or peroxide deposits on cathode substrates. Hydroxide and peroxide deposits can be converted to corresponding oxides by thermal treatment w3,5,8x. In this study, the electrolytic ZrO2 yAl2O3 double layers coating has been deposited on AISI 440C in ZrO(NO3)2 and Al(NO3)3 aqueous solutions, respectively. The characteristics of the coating have been examined, including crystal structures, hardness, adhesion, wear and corrosion resistance.
0257-8972/04/$ - see front matter 䊚 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0257-8972(03)00862-4
C.C. Chang, S.K. Yen / Surface and Coatings Technology 182 (2004) 242–250
243
Fig. 2. SEM Cross-sectional observation of ZrO2 yAl2 O3 coated specimen annealed at 450 8C for 3 h.
ultrasonically cleaned in deionized water and acetone, then dried by N2 gas. Electrolytic deposition — The electrolytic ZrO2 y Al2O3 double layers coating was deposited on AISI 440C stainless steel in 0.03125 M ZrO(NO3)2 and subsequently in 0.01 M Al(NO3)3 aqueous solution at voltage y1.0 to y1.2 V for 500 to 700 s at room temperature, using an EG&G M273A Potentiostat and M352 software. The sample was the cathode, graphite bars (fs5 mm, ls55 mm) were the anode and saturated AgyAgCl was the reference electrode. The coated specimens were then naturally dried in air and annealed for 3 h at 300, 350, 400, 450, 500 and 550 8C, respectively. The thickness of coatings was measured using surface profilometer (Dek3tak, USA). SEM, XRD and AES — The surface morphology of coated specimens was observed by scanning electron microscopy (SEM, JEOL, JSM-5400, Japan). The crystal structure of ZrO2 yAl2O3 coated specimen was analyzed grazing incident X-ray diffraction (GI-XRD) using a MAC MO3X-HF diffractometer, with Cu Ka radiation
Fig. 1. SEM observations of (a) ZrO2 coated, (b) Al2O3 coated and (c) ZrO2yAl2O3 double layer coated specimens annealed at 450 8C for 3 h.
2. Experimental Sample preparation — AISI 440C stainless steel was cut into sheets with 55=40=1 mm. All specimens were polished to a mirror finish (Ras0.059 mm) with Al2O3 powder, then degreased by detergent and further
Fig. 3. XRD diagrams of ZrO2 as coated specimen, and annealed at 450 8C for 3 h.
244
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Fig. 4. XRD diagrams of ZrO2yAl2O3 as coated specimen, annealed at 350, 400 and 450 8C, respectively, for 3 h.
(ls15.418 nm), the range of 2u from 10 to 708 and incident angle of 0.58, at a scanning rate of 18 miny1, a voltage of 40 kV, and a current of 30 mA. The component depth profiles were plotted using an auger electron spectroscopy (AES Fison VG Microlab 310D). Specimens were sputtered by 5 keV Arq with a beam current of 0.05 A, and a sputtering rate of 1 nm sy1. Hardness, wear and scratch tests — Hardness measurements were made on all specimens using a Vickers microhardness tester (Mitutoyo MVK-GB) with a load of 25 g. Wear test was carried out by a pin-on-disc system with poly-etheretherketone (PEEK) pin (cs2
mm), and the coated specimen disc, at a load of 8.28 MPa, sliding speed of 0.12 m sy1 and a total sliding distance of 10 000 m. PEEK is a new generation of engineering plastics for plastic packaging in microelectronics that has high Tg value and can stand temperature up to 250 8C for long period of time w10x. PEEK is the high-performance engineering plastic, with high thermal stability ()250 8C), excellent chemical resistance and superior mechanical, such as the hardness of 108 HRB, and the tension strength of 110 MPa at RT, and 48 MPa at 150 8C w10,11x. The wear loss was measured by the disc weight difference before and after each test. To study the adhesion of the coating on substrate, the ZrO2 yAl2O3 coated specimens annealed at 450 8C were tested by scratch (TEER-200, UK), using diamond indenter as the pin with an initial load 2 N, scratch speed 10 mm miny1, loading rate 50 N miny1 and the end load 100 N. The friction force between the specimen and diamond indenter was monitored by a load cell attached to the working table, as shown in the previous paper w7x. Surface energy and work of adhesion — Surface energy measurement were made on the uncoated, ZrO2 coated, and Al2O3 coated specimens using the threeliquid (water, formamide and ethyleneglycol) procedure by the dynamic contact angle analyzer research model ˚ (FTA200, First Ten Angstroms, USA). When two unlike phases i and j are brought together reversibly, the surface free energy change equals to the free energy of adhesion DGaij or the negative of adhesion work w12,13x. In other words, it can be presented by the following equation, DGaijsgijygiygjsyWaij
(1)
where gij is the interface energy of iyj, gi and gj the surface energies of phase i and j.
Fig. 5. AES component depth profiles of ZrO2yAl2O3 coated specimen annealed at 450 8C for 3 h.
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Fig. 6. Vickers hardness of the uncoated and ZrO2yAl2 O3 coated specimens annealed at various temperatures and the hardness increased by coatings.
Polarization test — The corrosion resistance of the ZrO2 yAl2O3 coated were evaluated by an electrochemical dynamic polarization tests in 1 M HCOOH and 3.5% NaCl aqueous solutions, respectively, also by using a EG&G Model 273A, M352. After the open-circuit potential became steady, the cyclic polarization test was from y0.2 V (vs. open-circuit potential) to 1 V (Agy AgCl), then back to open-circuit potential at a scanning rate of 0.167 mV sy1. The first oxidation-reduction
equilibrium potential E01 was derived when current density equaled zero during the applied voltage increased (forward cycle). If oxidation is due to the corrosion of electrode, this potential is also named corrosion potential Ecorr, and the corrosion current density icorr can be calculated at this potential. The second oxidation-reduction equilibrium potential E02 was derived when current density was back to zero again during the applied voltage decreased (backward cycle).
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3.2. Crystal structures and component profiles
Fig. 7. Friction force vs. the normal load of the scratch test on ZrO2yAl2O3 coated specimen annealed at 450 8C for 3 h.
If the oxidation is decreased abruptly at some passivation potential Epass, because of the formation of passivation films and corrosion rate falls to very low, and the beginning current density is named the critical passive current density icrit, and the stable current density is named the passive current density ipass. If the oxidation is increased abruptly at some pitting potential Epit because of the breakdown of passive film and the localized corrosion of the electrode, the crossover of the backward cycle and the forward cycle on the anodic polarization region will be found because of repassivation and defined as protection potential Epp. 3. Results and discussion 3.1. Surface morphology SEM observations of the ZrO2, Al2O3 and ZrO2 y Al2O3 coated specimens with the same film thickness of 0.7 mm are shown in Fig. 1. More and larger cracks are found in Fig. 1a and b. On the other hand, less and smaller cracks are found in Fig. 1c. Obviously, the mud cracks found on the single layer coating, can be reduced by the double layers coating for the same film thickness of 0.7 mm. SEM cross sectional observation of the ZrO2 yAl2O3 coated specimen is shown in Fig. 2. The thickness of ZrO2 coating and Al2O3 coating was approximately 0.3 mm and 0.4 mm, respectively.
The XRD diagrams of ZrO2 coated specimens are shown in Fig. 3. No obvious diffraction peak was found on the patterns of the as coated samples. The (101), (110) and (112) peaks of tetragonal ZrO2 were found on the specimen annealed at 450 8C. The XRD diagrams of the ZrO2 yAl2O3 coated specimens are shown in Fig. 4. The (100), (002) and (200) peaks of aluminum hydroxide hydrate (Al2(OH)6H2O) were found on the as-coated. The (020), (110), (130), (111), (140), (221) and (002) peaks of aluminum oxide hydroxide (AlO(OH)) were found on the coated specimens annealed at 350 and 400 8C. The (102) and (220) peaks of aluminum oxide hydrate (Al10O15H2O) were found at 400 and 4508C, and finally, the (111) and (220) peaks of g-Al2O3 were found at 450, 500 and 550 8C. No obvious ZrO2 peak was found for double layer coating. If the missing of ZrO2 peak was caused by value of X-rays penetration depth at GI-XRD the grazing angle chosen, the peaks of metal substrate should not be found either. However, the (110) and (200) peaks of 440C substrate were found, as shown in Fig. 4. Therefore, the factor of grazing angle chosen should be excluded. However, it is more possible that the condensation of Zr(OH)4 and the crystallization of ZrO2 should be delayed by the existence of H2O in the top layer. AES component depth profiles are plotted in Fig. 5, It is confirmed that the deposited coating is composed of two layers. The inner layer is ZrO2 and the out layer is Al2O3. A part of Fe diffused out to ZrO2 layer and a part of O diffused into metal substrates. This result is similar to the oxidation of AISI 430 stainless steel where the oxidation rate of Fe is greater than that of Cr at 450 8C w14x. The phenomenon of interdiffusion may be also caused by the vacancy which was formed due to the evaporation of H2O during the condensation reaction of Zr(OH)4 w5x. Such an interdiffusion between ZrO2 and substrate also retard the crystallization of ZrO2, so that no obvious diffraction peaks of ZrO2 was found at 450 8C annealing, as shown in Fig. 4. 3.3. Hardness Vickers Hardness of the uncoated and ZrO2 yAl2O3 coated specimens annealed at various temperatures are
Table 1 Weight Loss of the uncoated and ZrO2yAl2O3 coated specimens annealed at various temperatures, using PEEK as the pin Uncoated specimen Weight loss (mg)
0.1833 "18.4%
ZrO2 yAl2O3 coated specimen annealed at various temperatures 300 8C
350 8C
400 8C
450 8C
500 8C
0.1133 "11.9%
0.0767 "7.2%
0.0573 "9.7.%
0.0485 "13.4%
0.0474 "14.2%
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Fig. 8. SEM observations and EDS mapping of ZrO2 yAl2 O3 coated annealed at 450 8C for 3 h at the scratch load around (a) 22 N and (b) 100 N.
shown in Fig. 6a. The temper softening effect during annealing and the secondary hardening effect approximately 450 8C were found on the AISI 440C substrate. Increasing annealing temperature enhanced the hardness increase in the coatings. The increased hardness was more obvious at (or above) the annealing temperature of 450 8C, due to the formation of g-Al2O3 as shown in Figs. 4 and 6b.
3.4. Wear resistance With PEEK as the pin, the weight losses of the uncoated and coated specimens are listed in Table 1. The wear of coated specimens decreased with increase in annealing temperature. The increasing wear resistance was resulted from the increased hardness of the coatings, which was linked to the phase transformation from
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Table 2 Contact angles of water, formamide and ethyleneglycol on the uncoated, ZrO2 coated and Al2O3 coated, respectively
Water Formamide Ethyleneglycol
AISI 440C 8
ZrO2 coated 8
Al2O3 coated 8
87 60 64
70 50 63
83 59 63
Table 3 Surface energies gI, interface energies gij , and adhesion works W a calculated from contact angles listed in Table 2 by Van Oss method w12,13x AISI 440C Surface energy, gI (mJym2)
70.406
Interface energy, gij (mJym2) Adhesion work, Wa (mJym2)
ZrO2 coated 94.408
Al2O3 coated 67.165
ZrO2 y440C 16.416
Al2O y440C 0.041
ZrO2 yAl2O3 13.796
148.398
137.530
147.777
aluminum hydroxide hydrate (Al2(OH)6H2O) to aluminum oxide hydroxide (AlO(OH)), aluminum oxide hydrate (Al10O15H2O) and finally tog-Al2O3, as shown in Figs. 4 and 6. In contrast, the wear resistance seems to be independent of the tempering softening and secondary hardening effects of the substrate. 3.5. Adhesion Fig. 7 shows the friction force vs. the normal load on the stylus during the scratch test. This curve may be divided into two regions. One revealed a slope of 0.13 and the other 0.34. The transition point was around the load 22 N. The increased slope was caused by the deformation of the metal substrate, which was plastically deformed by the indenter at the load beyond 22 N. However, most of the ZrO2 yAl2O3 coating was found at 22 and 100 N of the scratch test, as shown in Fig. 8a and b. This means that the adhesion strength of ZrO2 y Al2O3 and ZrO2 y440C was stronger than the yield stress of the 440C. Contact angles of water, formamide, and ethyleneglycol on specimens are listed in Table 3. Surface energies,
Fig. 9. Polarization curves of the uncoated and ZrO2yAl2O3 coated specimens in 3.5% NaCl aqueous solution.
interface energies and adhesion work have been calculated by Van Oss method w11,12x, as listed in Table 4. The adhesion work of ZrO2 y440C was a little greater than that of Al2O3 y440C. Also, the adhesion work of ZrO2 yAl2O3 was greater than that of Al2O3 y440C. It is concluded that the better choice of the bottom layer is ZrO2 coating and that of the top layer is Al2O3 coating. 3.6. Corrosion resistance Representative polarization curves of the uncoated and coated specimens in 3.5% NaCl and 1 M HCOOH aqueous solutions are shown in Figs. 9 and 10, respectively. From the forward curves, E01 (or Ecorr), i0 (or icorr), Epass, icrit, ipass and Ep were analyzed, and from the backward curves E02 and Epp were measured, as given in Table 4. In 3.5% NaCl aqueous solution, the ZrO2 y Al2O3 coated specimens revealed the higher Ecorr, and lower icorr. No obvious Epit, was found on the coated samples, but it was found on the uncoated ones at y 136 mV. In 1 M HCOOH, the coated sample exhibited a lower icorr and ipass than the uncoated. After polarization tests in 3.5% NaCl aqueous solution, a large area penetrated on the uncoated was observed, as shown in Fig. 11a, but none on the coated, as shown in Fig. 11b. Similar results were found in 1M HCOOH, as shown in Fig. 12. These results indicate that the ZrO2 yAl2O3 film was more corrosion resistant or inert than the natural
Table 4 List of the first redox potential E01 (Ecorr), exchange current density i (icorr), critical passive current density icrit , passive current density ipass, pitting potential Epit, the second redox potential E02 and protection potential Epp of the uncoated and ZrO2 yAl2 O3 -coated specimens, derived form the polarization tests in 3.5% NaCl and 1 M HCOOH aqueous solutions Test solutions
Specimens
E01 (Ecor) (mV)
i (icor) (mAycm2)
icrit (mAycm2)
ipass (mAycm2)
Epit (mV)
E02 (mV)
Epp (mV)
3.5% NaCl 1M HCOOH
Uncoated Al2O3 yZrO2 y440C Uncoated Al2O3 yZrO2 y440C
y534 y341 y486 y457
231 25 96 8
— — 980 9
— — 30 0.9
y136 — — —
— y272 — —
— y96 — —
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Fig. 10. Polarization curves of the uncoated and ZrO2yAl2O3 coated specimens in 1 M HCOOH aqueous solution.
passivation film of AISI 440C stainless steel, in both 3.5% NaCl and 1M HCOOH aqueous solutions. Also, the lower ipass of the coated specimen at the anodic polarization region means that the coating film is more stable than the natural passivation film of the substrate. 4. Conclusions A new electrolytic coating method of ZrO2 yAl2O3 double layers has been successfully applied on an AISI 440C stainless steel to investigate its characteristics. The following conclusions are drawn: 1. The single layer deposition of ZrO2 or Al2O3 revealed many large cracks, which can be reduced by the ZrO2 yAl2O3 double layer coating. 2. The annealing resulted in increase of the hardness of the coatings, but was independent of the temper softening and secondary hardening effect on the 440C substrate. The increasing hardness of the coatings was caused by the series of phase transformations from the as-deposited Al2(OH)6H2O to AlO(OH) at 350 8C, to Al10O15H2O at 400 8C, and finally to g-Al2O3
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Fig. 11. Optical photograph of (a) the uncoated and (b) ZrO2yAl2O3 coated specimens after polarization test in 3.5% NaCl.
at 450 8C. Wear resistance was increased with increasing annealing temperature. Especially, the weight loss of the ZrO2 yAl2O3 coated specimen annealed at 450 8C is only 1y4 of the uncoated. 3. The scratch tests revealed that the adhesion strength of the ZrO2 yAl2O3 coating film on the substrate was stronger than the yield stress of AISI 440C stainless steel. Also, the surface energy analyses suggested that the better choice of the bottom layer is the ZrO2 and that of the top layer is the Al2O3 coating. 4. The electrochemical polarization tests in 1 M HCOOH and 3.5% NaCl aqueous solution revealed that the ZrO2 yAl2O3 coated specimen was 10 times more corrosion resistance than the uncoated. Acknowledgments The authors are grateful for the support of this research by the National Science Council, Taiwan, under Contract No. NSC89-2216-E-005-011.
Fig. 12. SEM observations of (a) the uncoated and (b) ZrO2yAl2O3 coated specimens after polarization test in 1 M HCOOH.
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