Microstructure and Interface Characteristics of 17-4PH/YSZ Components after Co-Sintering and Hydrothermal Corrosion

ceramics Article Microstructure and Interface Characteristics of 17-4PH/YSZ Components after Co-Sintering and Hydrothermal Corrosion Anne Günther 1, *, Tassilo Moritz 1 1 2 * and Uwe Mühle 2 Fraunhofer Institute for Ceramic Technologies and Systems, Winterbergstraße 28, 01277 Dresden, Germany; Robert Bosch Semiconductor Manufacturing Dresden GmbH, Knappsdorfer Str. 12, 01109 Dresden, Germany; Correspondence: ; Tel.: +49-(0)351-2553-7397 Received: 6 March 2020; Accepted: 13 April 2020; Published: 21 May 2020



 Abstract: Combining stainless steel with zirconia components by powder technological shaping routes for manufacturing of multifunctional parts is an advantageous and promising one-step method making expensive and time-consuming additional joining steps redundant. However, several requirements for co-shaping and co-sintering of the very different compound partners have to be met. The microstructural and chemical constitution of the interface between both materials plays an important role for the mechanical properties, durability and corrosion resistance of the manufactured parts. In the present study, different shaping techniques for co-shaping of stainless steel and zirconia are introduced. The microstructure and the interphase properties of metal/ceramic hybrid parts have been investigated for samples made by tape casting, subsequent lamination and co-sintering. Nevertheless, the results of this study are valid for components made by other hybrid shaping processes as well. The interfaces were characterized by TEM, FESEM, EDX, and X-ray diffraction. Furthermore, the hydrothermal stability of the material compound was investigated. Keywords: metal/ceramic material compound; hybrid materials; stainless steel; zirconia; co-sintering; co-shaping; multimaterial compound; hydrothermal corrosion; tape casting; interface 1. Introduction Combining different ceramics, glasses or metals in one component for attaining multifunctional properties have been reported in several publications for at least one decade. The authors used powder technological co-shaping routes for property combinations like ductility and wear resistance [1,2], electrical conductivity and insulation [3,4], dense and porous components [5], different coloring [6,7] or magnetic and non-magnetic properties [8]. Co-shaping thereby allows for a combination of different materials in only one processing step without any additional time-consuming joining steps. However, powder technological shaping routes always require debinding and sintering steps for removal of organic additives or binders and for attaining a complete densification of the sintered part with final properties. For multimaterial approaches the co-sintering of the components is even more demanding. In this case both materials which shall be combined durably in one part must fulfill a number of requirements: 1. The coefficient of thermal extension (CTE) must be comparable over the whole range from sintering temperature down to ambient temperature. Without fulfilling this requirement the co-sintered materials compound definitely fails during cooling down from sintering temperature or, at latest during thermal cycling in the application of the part. Ceramics 2020, 3, 245–257; doi:10.3390/ceramics3020022 www.mdpi.com/journal/ceramics Ceramics 2020, 3 2. 3. 4. 246 Both materials must be sinterable at the same temperature, under the same gas atmosphere, and under the same gas pressure. None of the partners must melt before reaching the sintering temperature. The compound partners must not tend to undesired solid state chemical reactions during sintering. Both components must show identical sintering behavior, i.e., a. b. c. comparable onset of shrinkage comparable shrinking rate and the same total shrinkage. Whereas the first three requirements are material inherent properties which have to be taken into account already for choosing the fitting compound partners, the last point—the sintering behavior can be adjusted by choosing powders with suited particle size distributions and by equalizing the solid content of both components in the green state. Beside the above mentioned requirements mixing up the material classes for multicomponent parts, e.g., combining ceramics with metals by powder technological routes has a further challenge in store. The chemical bonding mechanisms in ceramics are ionic or covalent bonding, whereas metallic bonding is characteristic for metals. For that reason the question arises which bonding can be achieved by co-sintering of both materials forming one material compound. The development work in this article pursues this questions for the material combination stainless steel and zirconia. This material combination is worth of investigation due to its excellent property combinations like hardness and ductility, electrical conductivity and insulation, white or black color and metallic gloss and the consequent applications for micro surgical instruments, heating elements, design parts or metal supported membranes. Several co-shaping technologies have been successfully investigated so far for this material combination, e.g., 2-component injection molding [9], Fused Filament Fabrication [10], Thermoplastic 3D Printing [11] or tape casting [12]. The last mentioned method had been chosen for describing the interphase formation in this article due to the simplicity of the shaping route and the large interphase area which can be produced with a relatively low amount of both materials. Co-sintering generally describes the common heat treatment of the materials in a composite powder compact or in a green composite part. However, the step before is co-debinding for complete removal of any organic additives necessary for the shaping process in a powder technological route, which can have a significant effect on the corrosion behavior of the composite. The particle packing density and the interface formation between the material partners control the properties of the composite after the heat treatment. For a homogeneous structure and a high sinter density, the theoretical green density is crucial. In the case of spherical particles, they occupy an orthorhombic structure and thus achieve a maximum density of 62.5%. Multimodal particle distributions can obtain up to 97.5% [13]. For the formation of interfaces, [14] considers the wettability during the metal–ceramic composite formation. This is influenced by the presence of atomic oxygen, the predominant surface orientation of the material particles and their electrical as well as ionic conductivity. These influences become especially clear when a partial melting of the metal occurs at the maximum sintering temperature [14]. Regardless of whether a couple of particles or a layered composite is considered, local chemical processes, the bonding mechanisms and crystallographic orientation play a very important role in the formation of the interface. In addi (...truncated)