Brazing in SiH 4 -Doped Inert Gases: A New Approach to an Environment Friendly Production Process
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Brazing in SiH4‑Doped Inert Gases: A New Approach to an Environment Friendly Production Process Ulrich Holländer1 · Daniel Wulff1 · André Langohr1 · Kai Möhwald1 · Hans Jürgen Maier1 Received: 21 June 2018 / Revised: 2 April 2019 / Accepted: 8 April 2019 © The Author(s) 2019
Abstract Engineering under protective atmospheres or in vacuum allows the production of materials and components, where the absence of oxygen is an essential requirement for a successful processing. Ideally, joining or coating of (and with) metallic materials needs oxide free material surfaces, in order to achieve durable joints or coatings. Using the established technology of brazing in controlled atmosphere, fundamental physical mechanisms for deoxidation of metal surfaces are presented and the role of oxygen and water residue in the process atmosphere is analyzed. Furthermore, the doping of gases with monosilane for generating virtually oxygen-free process atmospheres is introduced and its advantages for an oxygen-free production are discussed. Keywords Brazing · Inert gas · Monosilane · Deoxidation · Physical model · Production Abbreviations MxO2y Metal oxide (stoichiometric indices x and 2y) ΔfG0 Standard free enthalpy of formation referred to 2 mol O R Gas constant (8.3144 mol J−1 K−1) ai Activity of component i Mi Molar mass of component i ρi Density of component i DO Diffusion coefficient of oxygen Xi Substance concentration of component i wi Mass fraction of component i Ka Equilibrium constant referred to activities of reactants 1 NO Number of oxygen atoms per unit volume NA Avogadro constant (6.022 × 1023 mol−1) JO Mass flow of oxygen d Thickness of oxide layer
1 Introduction The vast majority of processes in the metal-working is performed in the presence of oxygen and humidity. As a result, a rapid oxidation of the metal surfaces takes place. The * Ulrich Holländer [email protected]‑hannover.de 1
Institut für Werkstoffkunde (Materials Science), Leibniz Universität Hannover, 30823 Garbsen, Germany
actual thickness of the oxide layer formed depends on the process temperature and the reactivity of the metal. While passivating oxide layers usually have a positive effect on the performance of the finished work pieces due to the resulting corrosion protection, they are a disruptive factor in numerous important production processes like welding, brazing or coating in all their facets. Therefore, extensive efforts are necessary to remove the oxide layers directly before or during these processes. Depending on the actual process, this is typically performed by a mechanical (grinding, grit blasting etc.) and/or chemical treatment (acid cleaning, use of flux etc.). In terms of both environmental impact and work safety both approaches are critical. Especially the use of chemicals for such cleaning procedures is increasingly restricted by stringent rules and regulations. In demanding (elevated temperature) processes, the oxygen (and humidity) concentration in the su
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