Transactions on Engineering Sciences vol 2, 1993 WIT Press, ISSN

Transcription

1 A study of thin-film continuous coating process by vapour deposition P. Gimondo," F. Arezzo,* B. Grifoni,* G. Jasch& "Centra Sviluppo Materiali SpA, Via di Castel & Von Ardenne Anlagentchnik GmbH, Plattleite Dresden, Germany ABSTRACT Various innovative surface coating technologies have gained wide recognition over the past several years. Physical vapour deposition (PVD) with its many variants is one of the most advanced of these technologies. Recently the emphasis has been on the conversion of discontinuous PVD processes into continuous ones, aiming to replace conventional, electrolytic methods for the coating of steel strips. In this study a special apparatus capable of simulating the PVD process in a continuous mode was utilized. Pretreatment and deposition processes were performed under a variety of experimental conditions, e.g. electron beam or electric resistance evaporation, ion beam assistance and mechanical brushing. The results of zinc and aluminium coating of steel strips are discussed. INTRODUCTION Worldwide, an important portion of coated steel strips, approximately 30 per cent of the cold rolled products, is produced by the traditional techniques of hot dipping and electrodeposition. There are indications that the coating of steel strips could increase up to about 60 percent in the next 15 years. This expectation is strictly related with the use of new dry technologies such as physical vapour deposition (PVD) and chemical vapour deposition (CVD).

2 380 Surface Treatment Effects Spectacular advances in processing and equipment have taken place in the last few years, and PVD is the technology in which the greatest progress has been made. It encompasses several categories such as evaporation, sputtering, ion plating, and ion beam deposition. These categories include many variants and they are well consolidated as batch type processes. In fact, they have made possible the development of completely new material, often possessing surface properties significantly different from those of the bulk. Recently, the emphasis has been on the conversion of discontinuous processes into continuous ones to extend the applicability of the PVD technology to the coating of plastic and metal strips. Continuous lines, mostly used in Japan[l], utilizing one category of the PVD technology or a series of differrent ones, have been built. However, while the plant engineering is generally well advanced, there are, as far as the processing is concerned, many problems, inherent in the continuous mode of operation, which need to be solved. In particular, current pretreatment and deposition processes have to be improved and new ones need to be developed to make the continuous operation more economical and to enlarge the number of functional coatings applicable to steel strips. A special apparatus, based on the electron beam (EB)-PVD technology and capable of simulating a variety of pretreatment and deposition processes in a continuous mode, has been developed. It was used to study the coating processes of zinc and aluminum on steel strips. This paper gives a description of this new apparatus and discusses some preliminary results of the coating processes. EXPERIMENTAL EB-PVD apparatus and technology A schematic drawing of the apparatus is shown in figure l,a. It consists of a horizontally arranged cylindrical chamber which is used for both pretreatment and deposition processes. The interior of the chamber is shown in figure l,b. The strip shaped metal sample (1800 mm long and 300 mm wide) is clamped on the drumtype substrate holder and, after evacuation of the chamber, it can be exposed to single or multiple pretreatments followed by single or multiple coating procedures.

3 Surface Treatment Effects 381 pretreattnent and deposition Chamber Figure l,a. Schematic drawing of the EB-PVD apparatus. Figure l,b. Interior of the pretreatment and deposition chamber.

4 38 Surface Treatment Effects The changeable speed of the drum permits the study of each process as a function of the steel strip motion, whose peripheral speed can be adjusted in the range from 0.4 to 0 m/min. The design of this apparatus is such that it can be easily integrated as a deposition station into a pilot plant with continuous strip travel. Prior to vacuum coating, the metal substrate must be exposed to three processing steps: a) surface degassing, b) surface activation and c) substrate heating. These fundamental steps comprise one or more pretreatment processes. Degassing is obtained usually during the heat treatment of the substrate which contributes also to the activation of the surface. Heat treatment can be performed by means of radiation heating which allows to reach a temperature of the substrate up to 00 C. For higher temperatures of the substrate, either conductive heating by electrical resistance or electron bombardment in vacuum is used. The conductive heating can also be performed under a reducing atmosphere, utilizing an appropriate gas. In addition to the heat treatment, further activation of the substrate surface can be obtained by mechanical brushing under vacuum, by ion etching via a plasmatron discharge[] or by depositing intermediate layers utilizing the electron beam evaporation. The EB-PVD apparatus is equipped with an 80-KW electron beam to vaporize from one or two crucibles metals or alloys. The electron gun is equipped with a separate vacuum system so that work in the deposition chamber can be performed at pressures of less than 0.05 mbar. It is possible also to perform reactive ionassisted coating processes via evaporation by utilizing the ion etching device in the region of the coating zone. Non reactive or reactive coating processes can be performed by means of a magnetron sputter source. In this case the evaporator crucible is replaced with a magnetron of adequate power and suitable targets (dimensions and material: 400 x 10 mm ; Ti, Cr, CrNi, stainless steel, Ni, Pt). Pretreatment and deposition processes Zn and Al are the most studied materials for coating steel strips by the PVD technology[3]. Therefore they represent a good stepping-stone for developing

5 Surface Treatment Effects 383 pretreatment and deposition processes and for gathering technical knowledge that could be eventually extended to other materials. In this work carbon steel strips for deep drawing, 1800 mm long and 300 mm wide, were used for both Zn and Al coating experiments. The strips were precleaned electrolytically; the experimental conditions of pretreatment and deposition are listed in table 1. Table 1. Pretreatment and deposition parameters. Sample Nr Substrate thickness (mm) 0,65 0,50 0,50 0,50 Substrate pretreatment before coating thermal treatment (600*C) brushing (00*C, 7 min) brushing/ion etching (90*C, 7 min) thermal treatment (50 C) Strip speed during pretreatment (m/min) 6 Strip speed during deposition (m/min) 6 Vacuum (mbar) Film material Zn Zn Zn Al The same level of vacuum was used during both the pretreatment and the evaporation processes. The ion etching experiment was performed with Ar ions at a power of KW. A radiation heated graphite crucible was sufficient to evaporate the Zn, due to its low surface tension. The Al was evaporated using the electron gun from a ceramic crucible. Characterization Scanning electron microscopy (S.E.M.) was utilized to evaluate the morphology of the coatings and to determine their thickness. In the case of aluminum, electron spectroscopy for chemical analysis (ESCA) also was used to identify the coating

6 384 Surface Treatment Effects (presence of A^Og and metallic Al). The surface was gradually removed by sputtering with KeV argon ions to estimate the depth of the oxide layer. RESULTS AND DISCUSSION Zn Coating Coatings obtained on steel strip samples, whose surfaces were activited by a thermal treatment only, showed a complete lack of adhesion. On the contrary, the sample surfaces activated under vacuum by mechanical brushing, alone or in combination with ion etching, showed a very good adhesion. These results confirm that the removal of the oxide layer is crucial for improving adhesion in the case of Zn. On this regard, mechanical brushing under vacuum has proved to be a good method. However, the combination of brushing and ion etching is as much efficient, and it has the advantage of allowing a lower temperature of the substrate. Coatings related with both these methods of pretreatment did not show any substantial differences, as it was determined by the SEM evaluation. A typical surface morphology of a Zn coating is shown in figure. An SEM crosssection is shown in figure 3. The thickness was determined to be around 5-6 ^m. The lack of any discontinuity at the interface implies a good adhesion, while the dark spots in the coating probably are due to some contamination. Figure. SEM EB-PVD. micrograph of Zn coating by

7 Surface Treatment Effects 385 Considering that the strip portion exposed to the zinc vapours is 60 mm long and taking into account the tickness of the coating (6 /im) and the speed of the drum (1 revolution per minute) the growth rate of the zinc layer was calculated to be around A/sec. Figure 3. SEM micrograph of cross section Zn coating Al coating A typical surface morphology of an Al coating and its cross section are shown in figures 4 and 5, respectively. In contrast with Zn, it is not necessary for Al coating Figure 4. SEM micrograph of Al coating by EB-PVD. Figure 5. SEM micrograph of crosssection Al coating.

8 386 Surface Treatment Effects to remove the oxide layer from the steel strip surface. A simple preheating operation around 00-50*C is enough to activate the strip surface and to promote a good adhesion. The SEM examination revealed a good uniformity of the coating with a thickness of about pm. The regularity of the interface is an indication of good adhesion. ESCA analysis of this coating showed, as espected, the presence of both a very thin A1O3 film and metallic AL The ESCA spectrum for Aluminium is shown in figure 6 and the atomic concentration data for the surface of the coating and after sputtering are shown in table. + A1 (OX) i bind, energy [ev] Figure 6. Al p ESCA spectrum from an aluminum coated steel strip surface.

9 Surface Treatment Effects 387 Table. ESCA results of Al coating. Surface Atomic concentration Analyzed O C Al<«Al«Surface I (10 A below) H (0 A below) III (30 A below) IV (40 A below) V (60 A below) VI (160 A below) The coating (figure 4) consists mostly of spherical particles which may contribute to some porosity. On this regard, it would be expected a significant improvement (less porosity) on the quality of the coating by using a water cooled copper crucible rather than a ceramic one. The copper crucible, in fact, reduces the surface turbulence of the evaporating material and allows a smoother energetic transfer of the coating mass toward the surface. However, the rate of deposition would be reduced. The Al deposition rate is much lower than that of the Zn, approximately by a factor of 10. That indicates that there is still much work to do for increasing the growth rate of Al in a continuous process which would make Al coating more attractive from an economic point of view. CONCLUSIONS The examples concerning the Zn and Al coating experiments illustrate the potentiality of the EB-PVD apparatus for developing new continuous processes and coating materials. Their success will strongly depend, in addition to the technological aspects, on the cost competitivity of the products.

10 388 Surface Treatment Effects ACKNOWLEDGEMENT The EB-PVD apparatus was developed at Von Ardenne Anlagentechnik, where the coating experiments were also performed. REFERENCES 1. Masayasu, M. et al. "Continuous zinc vapor deposition line" transaction ISIJ, Vol 7 pp 81-83, Schiller, S., Beister, G. Heisig, U. and Foerster, H. "High rate vapor deposition and large systems for coating processes" The Journal of Vacuum Science and Technology, pp 39-45, Jul/Aug Butler, J.F. "Vapour Generation and Deposition of Zinc at High Rates" The Journal of Vacuum Science and Technology, Vol. 7 pp 5-56, 1970.