Theory and practice of making foils by vacuum deposition

MINISTRY OF EDUCATION
AND SCIENCE OF RUSSIAN FEDERATION
     NATIONAL UNIVERSITY OF SCIENCE AND TECHNOLOGY “MISIS”



                    I.B. Ulanovskiy







        THEORY AND PRACTICE OF MAKING FOILS BY VACUUM DEPOSITION












            MISIS


Moscow 2016

УДК 544.3
      У47







      Ulanovskiy I.B.
У47 Theory and practice of making foils by vacuum deposition / I.B. Ulanovskiy. - M. : Izdatelskiy Dom “MISIS”, 2016. - 232 c.
         ISBN 978-5-87623-975-4




             This book is the first attempt ever to offer a consistent review of the main stages of a new way to produce foil from metals and alloys through vacuum deposition; the author has created physical models adequately describing this process and enabling engineering calculations of its basic parameters. The results of this study have laid a scientific foundation for the development of commercial scale process of making ultrathin foils, including those from difficult-to-form titanium alloys, as well as for the development of vacuum engineering devices working something of semiconductors.
             This book aims to draw the attention of materials science experts, students and postgraduate students of technical universities.

УДК 544.3



ISBN 978-5-87623-975-4

© I.B. Ulanovskiy, 2016

CONTENT

Foreword............................................................6
Introduction........................................................10
Chapter 1.  General regularities of vacuum condensate structure formation and basic requirements to industrial thin foil production through vacuum deposition...........................................14
   1.1. Mechanisms of condensation from a gas phase and “original” structure ofthe condensate.......................................15
     1.1.1. Critical temperature of condensation....................17
     1.1.2. Influence ofthe substrate temperature on the condensation mechanism......................................................19
     1.1.3. Primary structure formation in thin films...............21
   1.2. The structure of“thick” condensates.........................23
     1.2.1. The influence of substrate temperature on the condensate structure......................................................24
     1.2.2. Textural effects in the structure of vacuum condensates.29
   1.2.3. Specifics of structure formation in the case of condensation onto a moving substrate..........................................33
     1.2.4. The influence of vacuum rarefaction, residual gas composition, condensation rate and the deposited layer thickness on condensate structure..............................36
   1.3. Main requirements to the development of foil production process by vacuum deposition.....................................39
Chapter 2.  Developing a specialized equipment complex and mastering the foil production prosess...........................44
   2.1. Development ofYBO -75-1, УВФ -78-1 plants and a foil heat treatment plant..................................................45
   2.2. Development of the first national pilot plant УВ68Л for foil production by vacuum deposition.........................47
   2.3. Development ofhigh-capacity plant УВФ-2,0 for making
   foil from hard-to-deform alloys and auxiliary facilities. Launching foil production process..........................................51
Chapter 3.  Heat and mass transfer at a high-speed electron beam evaporation of metals, and vacuum vapor deposition onto a moving substrate   55
   3.1. Calculation ofheat and mass transfer between vaporizer and substrate....................................................57

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     3.1.1.  Methods of description of spatial distribution of vapor and radiation flows.............................................57
     3.1.2.  Determining local angle coefficients of radiation from the substrate onto the lunule..............................68
     3.1.3.  Calculations of vapor and heat flow densities on the strip surface............................................74
   3.1.4. Determining the condensate depth, the vapor utilization coefficient and the grading screen shape....76
   3.2. Mathematical description ofheattransfer in amoving substrate .78
   3.3. Numerical solution ofthe problem.............................84
     3.3.1.  The algorithm for calculating the temperature field averaged over the strip width and thickness ..............84
     3.3.2.  The algorithm for calculating the temperature field averaged overthe strip width .............86
     3.3.3.  The algorithm for calculating the temperature field averaged overthe strip thickness..........................90
   3.4. Testing the adequacy ofthe developed mathematical simulation.93
   3.5. Application of the developed mathematical simulation to predict heat and mass transfer processes during vacuum deposition.........96
Chapter 4.  Main laws of the evaporation process of multi-component alloys out of a continuously fed molten pool........................104
   4.1. The composition of the equilibrium pool of molten metal with an arbitrary number of alloy components.....................105
   4.2. The evaporation kinetics for multicomponent alloys..........109
   4.3. The evaporation of the melts close to dilute solutions......120
   4.4. Determination of the activity and Henry coefficients. Comparing the calculation methods of the equilibrium pool.........124
Chapter 5.  Developing principles for substrate strip and antiadhesive material selection and a technology of foil manufacturing process....131
Chapter 6.  Through porosity of the foil made by the vacuum condensation technique..............................................139
   6.1. Vacuum tightness and through porosity of the foil...........140
     6.1.1. A quantitative criterion of the foil vacuum tightness....141
     6.1.2. Experimental determination of foil vacuum tightness......143
   6.2. The influence of vacuum deposition parameters on the characteristics of through porosity in the foil...........146
     6.3.  The model of through porosity formation in the foil......151

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   6.4. Calculation of the preset vacuum tightness of the foil......158
   6.5. Calculation ofthe gas permeability of porous foil screens at high temperatures.............................................161
Chapter 7.  Scientific and practical research to develop the process of producing thin foil by vacuum vapor deposition onto a moving substrate strip.....................................................170
   7.1. The choice of alloys concentration range for practical applications.....................................................171
   7.2. Substrate strip and antiadhesive - materials, structures and structural interaction.......................................172
7.3.  The influence of vacuum deposition process parameters on the condensate structure......................................174
     7.3.1. Substrate strip temperature.............................174
     7.3.2. Residual gases pressure and condensation rate...........192
   7.4. The influence of condensation process parameters on condensate mechanical  properties.............................196
   7.5. The use ofheat treatment of condensates.....................200
   7.6. The influence of condensation conditions on the foil surface.206
Chapter 8.  Using the work results to adress challenges of industry..211
Conclusion..........................................................218
References..........................................................219

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                         This work is a tribute to Academician Alexander F Belov - an architect of the Soviet aviation metallurgy, the founder of the National Institute of Light Alloys

Foreword
   This monograph is based on the author’s thesis “The study of principles of formation of the structure of vacuum condensates of titanium-based alloys and the development of the process of titanium foil production” submitted for the Doctor of Science in Process degree and successfully defended in 1991. The study was made in 1973-1990 at the National Institute ofLight Alloys (VILS) - one of major research and process centers of the Soviet aviation industry at the time.
   Due to the secrecy policies enforced at the time, this work was classified “For Official Use Only”, which greatly impeded the possibility to publish its findings. Today such formal barriers have become obsolete, but it is the author’s belief that its consistently structured solutions to all the problems involved may be still useful for a broad scientific community. This is the monograph’s mission.
   Academician Alexander F. Belov, the founder of the National Institute of Light Alloys under the auspices of the Ministry of Aviation Industry of the USSR, was enthusiastic concerning about all kinds of new manufacturing processes, materials and equipment. In early 1970s, at the International Conference on Vacuum Metallurgy, he learned from a report of American experts about allegedly successful development of a breakthrough process of making foil from titanium alloys through vacuum evaporation and deposition.
   This method works as follows. A titanium alloy ingot, from which a foil shall be made, is put into a vacuum chamber. The surface of the ingot is heated by an electron beam gun. It causes the alloy to evaporate and deposit on the surface of substrate strip continuously moving over the vaporizer. Once the coating is formed, it is separated from the substrate strip as finished foil. Before the start of the process, the surface of the substrate strip is coated with a special separating/antiadhesive material to provide separation of the condensate from the substrate strip.
   Leading experts from VILS were quite skeptical concerning this idea as the development of such process obviously involved a number of brand new challenges.


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   It was not without good reason that skeptics would argue that even the traditional process of crystallization of alloys from the liquid phase has not been thoroughly studied over many decades of researches. The processes related to foil structure formation during subsequent thermomechanical processing also were not clear in detail. Moreover, the process of titanium alloy crystallization from vapor phase was not studied at all, to say nothing of the absence of any information on the resulting structure and physical and mechanical properties of the foil obtained.
   Another challenge was making the foil of a given chemical composition with the even distribution of alloying elements along width, thickness and length of the foil. The thing is that titanium alloys contain alloying elements characterized by respective vapor pressures varying hundredfold or thousandfold. Agents with a higher vapor pressure evaporate from the molten metal before other agents. Therefore it was necessary to study kinetics and thermodynamics of the evaporation process for alloys containing components having different vapor pressures.
   Besides, the foil should have a given thickness with minimum deviations throughout its width and length. It is known that the thickest part of the foil forms directly over the center of the vaporizer while away from the center it becomes thinner. Therefore, the challenge was to ensure proper spatial distribution of the vapor flow during highly intensive electron beam evaporation.
   It was difficult to select a proper antiadhesive agent and adjust the parameters of its application on the substrate strip. In fact, if the condensate would be peeled off the substrate strip over the vaporizer too early a part of the foil will melt down due to the lack of heat removal through the substrate strip. Besides, the condensate must not peel off the coiled substrate strip in the process of coiling since the foil peeled too early will be damaged. The condensate must peel off the substrate strip free in the form of finished foil once the process is over.
   However, despite any understandable difficulties, the suggested process looked very promising, because it allowed to produce the finished product - a thin foil in one run, while the traditional rolling process of making foil from hard alloys required many processes, not to say that thin and wide strip of foil cannot be made from titanium alloys is virtually impossible. Therefore, Academician A.F. Belov decided to develop at VILS the process of making titanium foil from hard titanium alloys through metal evaporation and deposition in a vacuum.

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    At the time of the Cold War, we were still unaware that the development of new production processes by our counterparts overseas hinged on their proprietary organizational and economic know-how. A successful invention would be no longer mentioned in any scientific, technical literature and international conference reports. However, a failed invention would be widely publicized in scientific and technical literature and highlighted in reports made at international conferences alleging a success of a new production process.
    What was the reason for this? As the money put into the development of such “dead-end” process was gone, they tried to lure their counterparts into the development of such process thus causing them substantial financial losses as well.

    This monograph was inspired by this know-how. We did succeed in developing a new production process.
    In 1972, the author of this book, a graduate of the Department of Physical Chemistry, Moscow Institute of Steel and Alloys, quite young employee at VILS, successfully defended his PhD thesis on “Hydrogen diffusion and development of porosity in aluminum.” The thesis addressed the most topical scientific issues and offered solutions to a number of practical problems in VILS’ major field of research. Previously, VILS would organize annual conferences on the interaction of hydrogen with aluminum causing porosity in aluminum semi-products. The author’s PhD thesis virtually exhausted

Academician A. F Belov andI. B. Ulanovskiy

this subject, having addressed its main issues, thus making any further conferences on this topic unnecessary.
    A good knowledge of physical chemistry, obtaining fundamentally important scientific results and their successful application to industries, apparently, prompted Academician A.F. Belov to appoint the author as

research manager and administrator of a new unit in VILS aiming to solve the above problem.
   The first specimens of titanium alloy foil were obtained just a few months after the appointment, despite overwhelming skeptical attitude. But that was only the beginning, only the foretaste of the actual complexity of future problems.
   Our main experiments and invention of new production processes aimed to make a foil from titanium alloys. However, process patterns and solutions to technological problems discovered during this work are rather general and can be applied to processes of foil making by vacuum evaporation and deposiyion of other metals and alloys.
   Along with the development of scientific platform of the process, we developed technological parameters of the foil production and created all necessary equipment for its manufacture.
   This monograph also shows a practical application of the resulting foil for making heavy-duty products. Based on our theory of gas-permeable screens consisting of multi-layered titanic foil with a preset through-thickness porosity pattern, we developed a vacuum device resembling electronic semiconductors.
   Such screening device is gas-permeable at a moderate temperature and it ensures the pumping of all remaining gases through pores in the layers of foil, while at a higher temperature it becomes selectively impermeable for gases and prevents the access of “harmful” components of remaining gases to the product placed underthe screens.
   It is noteworthy that during this work the author met outstanding scientists and specialists who greatly contributed to successful implementation of this project. First of all, I would like to acknowledge the input from the Head of SKB “Vacuum Coatings” unit (currently Sidrabe company, Riga), a prominent scientist and designer, PhD in Technical Sciences, Edgar V. Yadin; Director of the E. O. Paton Institute of Electric Welding (Kiev), Academician Boris Paton and Head of the Department of E. O. Paton Institute of Electric Welding, Academician Boris A. Movchan; Chief Engineer of Vekshinsky Research Institute (Moscow) Vladimir Vladimirovich Ivanov; Head of Laboratory of E. O. Paton Institute of Electric Welding Mikhail Ivanovich Vinogradov and Leading Specialist Vladimir Fedorovich Ulyanov; Director of Kharkov Institute of Physics and Process, Academician Viktor Vladimirovich Ivanov; specialists of

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ANTK Tupolev: Head of Department of New Equipment Implementation (one of the main units of the complex) a successful Design Engineer Oleg Nikolayevich Sankov, Chief Technologist Vladimir Vasilyevich Sadkov, Head of Laboratory, Phd in Technical Sciences, Yury Viktorovich Gorshkov, Chief Specialist of the Ministry of Aviation Industry Lev Isidorovich Karseladze, and VILS employees - Deputy Director of the Institute, Prof. Nikolai Fedorovich Anoshkin, Head of Unit, Phd in Technical Sciences, Vyacheslav Petrovich Mitin, Head of Research and Production Complex, Phd in Technical Sciences, Vladimir Mikhailovich Lovtsov, Scientific Secretary, Phd in Technical Sciences, Lev Khaskelevich Roytberg, Senior Fellow, Phd in Technical Sciences, Boris Abramovich Kopeliovich.
   My special acknowledgements go to the specialists who worked under my direct supervision: Evgeny Stepanovich Zhiltsov, Alexander Vladimirovich Bushuev, Valentin Fedorovich Eliseev, Georgy Isaakovich Dubnik, Alexander Fedorovich Sorokin, Alexander Dmitrievich Dolmatov, Viktor Stepanovich Zhukovsky, Tikhon Sergeyevich Tretyakov to name but a few.
   I am especially grateful to my teacher - Head of Department of Physical Chemistry of MISiS, Honored Scientist of the RSFSR, Professor, Doctor of Chemistry, Alexander Abramovich Zhukhovitsky.
   I would also like to thank Mikhail Soloveitchik, PhD, and Vladimir Soloveitchik, PhD, for help in fulfilling this work.
   Finally, I would like to thank Irina Vladimirovna Apykhtina for her comprehensive assistance in preparing this monograph, as well as the Head of the Department of Physical Chemistry of NITU “MISiS”, Doctor of Chemistry, Professor Mikhail Vasilyevich Astakhov, Doctor of Sciences in Physics and Mathematics Yuri Rakhmilyevich Nemirovskiy for their kind attention towards this work.

Introduction
   The development of new material and processes is an integral part of scientific and technological processes. The evolution of aviation and space engineering, modern acceleration devices, vacuum process and a number of other industries needs unique foil materials, including ultra thin ones made from difficult-to-deform metals, alloys of complex chemical composition, as well as multilayer foil, a foil with high strength, ductility, heat resistance, and presetthrough-thickness porosity, etc.


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   The traditional method of producing foil by rolling the original billet has a number of significant limitations.
   The principal limitation of the chemical composition of the foil made by this technique is due to solubility of alloying elements in the initial molten metal. It is virtually impossible to produce a wide ultrathin foil from difficult-to-deform alloys by this method. Fabrication of multilayered foils is only possible within a limited range of the layers thickness ratio and a limited combination of different materials. Furthermore, manufacturing foils from difficult-to-form alloys by gradually reducing the thickness of initial ingot through several steps is a rather time-consuming process.
   Foil production by vacuum deposition is free of such constraints. The process involves evaporation of alloy in a vacuum and vapor deposition onto the surface of a substrate continuously moving over vaporizer followed by the separation of the condensate from the substrate in the form of finished foil. To ensure the separation of the condensate the substrate is pre-coated with a special separating material, i.e. antiadhesive.
   Rolling technique begins with making a thick ingot followed by gradual reduction of its thickness, while the new process allows to make foil consistently increasing its thickness to the preset parameters.
   Potential advantages of this process are obvious because it allows to make a wide ultra-thin foil from various alloys, as deformability limits do not apply in this case. It makes possible to produce multilayer foils by stepwise deposition of various metals and alloys, while freely varying physical and mechanical properties of the foil due to unlimited solubility of the different elements in the vapor phase and the rapid crystallization effect.
   Its other advantages include zero environment pollution as the process takes place in a sealed equipment; the possibility to combine most production stages within one apparatus and the streamlined process, plus no oil traces on the foil surface, unlike in the rolling method.
   In addition, this process provides a unique opportunity to study and develop new alloys. Typically, the study and development of new alloys involves much work to melt ingots with different concentrations of selected alloying elements, fabrication of deformable semi-finished products from the ingots with subsequent heat treatment thereof, production of specimens for mechanical tests; a labor-intensive manufacturing of thin specimens for electronic microscopy and other types of research, etc.
   The new process allows to make, in one run the foil specimens with a smoothly varying content of alloying elements ready for research of the

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foil structure and properties. By varying the substrate temperature, one can study the influence of crystallization rate on the structure and properties of resulting alloys in a wide range of parameters. These specimens also can be used to study the impact of heat treatment regimes on the structure and properties of new alloys made by this process.
   Finally, thanks to unlimited solubility of various materials in a vapor phase it is possible to make and study the specimens of brand new alloys that can not be produced by crystallization from a liquid state due to limited solubility of its components.
   These advantages demonstrate not only the unique potential of the new process, but also its advisability in a wider range of metallurgical and materials science applications. However, a wider use of the new process is largely impeded by a number of unsolved scientific problems that are crucial forthe development ofbasic process parameters.
   To ensure the reqoired thickness of the foil, its minimal deviations and the optimal temperature of the substrate it was necessary to solve the task of mass and heat transfer during a high-speed evaporation and vapor deposition on the moving substrate
   In order to obtain appropriate chemical composition ofthe foil it is necessary to know the basic patterns of the multi-component alloys evaporation process.
   To make sure that the foil separates from the substrate it is necessary to determine the principles for the selection of materials of the substrate and antiadhesive.
   General rules of crystallization from the gas phase, as well as the main features of the formation of structure and phase composition of vacuum condensates of alloys were studied in order to produce the foil with appropriate working properties.
   Significant attention is paid to the problems of the formation of foil through-thickness porosity by vacuum deposition and the application of the process features for making foil with freely adjustable open porosity and the practical use of this foil in vacuum technologies, resembling the role of semiconductors in electronics.
   There are various methods of vacuum evaporation of materials, however, for industrial foil production it is expedient to use the most efficient highspeed electron beam evaporation technique for industrial production of the foil. Therefore, this book considers this particular evaporation method.
   Technical literature has some information on the above issues but without any physical models adequately describing this process and allowing for engineering calculations in orderto determine and optimize its key parameters. 12

   We have pioneered a consistent study of the main stages of the high-rate evaporation process and deposition of metals and alloys on a moving substrate in a vacuum and also we thoroughly described physical processes during the manufacture of foil by vacuum deposition. Therefore the resulting principles and practical advice may generally apply to the manufacture of foil from various metals and alloys.
   This monograph is based on the findings obtained by successful addressing of scientific, industrial and technological challenges involved in foil production from titanium and its alloys by vacuum deposition, as well as a description of some solutions to the problems of practical use of the unique properties of such foil.
   However, the complex nature of the specific problem facing us along with peculiar physical and chemical processes and the conditions of their controlled implementation - all have created objective conditions and incentives for the examination of scientific and technological tasks in a rather general way.
   This monograph presents a comprehensive summary of all issues related to the use of vacuum deposition technique for successful and cost-effective fabrication of a thin foil, and the author believes that these findings are undoubtedly of scientific and practical value. These include:
   -     description, based on literature data, of general physical and chemical patterns of the process of metals vacuum deposition and, specifically, of the results of ourtitanium alloys studies (Chapter 1);
   -     description of all the equipment crea during our work in order to ensure consistent foil quality (Chapter 2);
   -     general theoretical solution of the problem of heat and mass transfer during a vacuum vapor deposition onto a moving substrate (Chapter 3) and the problem of electron beam evaporation of multi-component alloys at a continuous feed of molten metal (Chapter 4);
   -     development of principles for the selection of the substrate and antiadhesive materials (Chapter 5);
   -    formation and use of the through-thickness porosity of foil (Chapter 6).
   The results of our research of the structure and properties of foil made by vacuum deposition of titanium alloys are presentedin Chapter 7, while Chapter 8 gives an example of using titanium foil with adjustable through-thickness porosity.

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