Contents lists available at ScienceDirect Journal of the European Ceramic Society journal homepage: www.elsevier.com/locate/jeurceramsoc Characterization of a silicon nitride ceramic material for ceramic springs Iyas Khader a,b, *, Christof Koplin a , Christian Schröder a , Jens Stockmann c , Wieland Beckert c , Willy Kunz c , Andreas Kailer a a Fraunhofer Institute for Mechanics of Materials IWM, Wöhlerstraße 11, 79108 Freiburg, Germany b Department of Industrial Engineering, German-Jordanian University, P.O. Box 35247, 11180 Amman, Jordan c Fraunhofer Institute for Ceramic Technologies and Systems IKTS, Winterbergstraße 28, 01277 Dresden, Germany ARTICLE INFO Keywords: Ceramic springs Helical springs Disk springs High temperature fatigue Silicon nitride ABSTRACT Under extreme working conditions such as high temperature, strong electric and magnetic fields and acidic or basic environments, ceramic springs offer a clear advantage over conventional steel springs. In this study, a tailored grade of silicon nitride ceramic was characterized as spring material. The basic characterization was complemented with component tests. Bend bars, helical springs and conical disk springs were manufactured and tested under various loading scenarios. Manifested by the smallest effective volume of the three tested geometries, helical springs showed the highest fatigue strength. Nevertheless, the complexity involved in manufacturing helical springs pertaining to their geometrical features resulted in a relatively large scatter in fatigue data. The results pointed out the importance of proper design and machining of the contact surface edges in disk springs, which bear the highest stresses. This work demonstrates the potential of producing ceramic springs with broad applicability and sufficient strength and fatigue resistance. 1. Introduction Advanced engineering ceramic materials have proven their superior mechanical, thermophysical and chemical properties in a wide range of industrial applications. Although highly dependent on the material system under consideration, engineering ceramics generally possess high stiffness and high strength and hardness over a wide range of temperatures. They also exhibit low thermal expansion coefficients, exceptionally low densities, high resistance to wear and contact fatigue and low adhesion affinity to metals. Nowadays, advanced ceramics are standard materials for components and tools that are exposed to severe mechanical, thermal and tribological loads [1–10]. The application of advanced ceramics is widely found in biomaterials [11–16], metal cutting [17–24], metalforming [25–29] and other manufacturing tools [30] and machine components such as valves, pistons, screws, gears and bearings. At high temperatures and in corrosive environments ceramics offer a viable alternative to metals. For instance, silicon ni- tride ceramics have been successfully applied up to their glass transition temperature of approx. 950 °C [31]. Ceramic springs show further ad- vantages of being electrically insulating and non-magnetic; thus, sig- nificantly expand their field of applications. Notwithstanding their advantages, a thorough knowledge of their fatigue behavior is crucial for designing reliable components. Standardized fatigue tests on ceramic samples are typically carried out at a positive load ratio R (where R = σ min /σ max ), which means that on a particular surface, the material will be subjected to either fluctu- ating tensile or compressive stress. This approach is followed due to the difficulty of loading a bend bar in a fully reversible mode. Previous studies [32–37] reported that fatigue starts much earlier with a de- creasing stress ratio. Few exceptions are found in the literature [38,39,34], in which fatigue tests on bend bars were carried out under fully reversible load. The results obtained by Lube et al. [38] manifested a distinct failure behavior for each loading regime. Under fluctuating tensile loading (R = 0.1), the microstructure showed transgranular fracture in addition to β-Si 3 N 4 grain pullout suggesting intergranular fracture. On the other hand, the samples loaded with fully reversible load (R = -1) clearly showed signs of localized wear and debris for- mation (typical for fatigue crack propagation in ceramics as observed by Reece et al. [34] and Jacobs and Chen [33]), which may have caused wedging effect at the crack faces, thus, leading to further crack pro- pagation during the compression component of the loading cycle. In general, fatigue lifetime predictions in ceramics are based on the assumption that macroscopic cracks develop from natural flaws asso- ciated with preexisting defects under cyclic loading. It was shown by https://doi.org/10.1016/j.jeurceramsoc.2020.03.046 Received 17 December 2019; Received in revised form 13 March 2020; Accepted 22 March 2020 ⁎ Corresponding author at: Fraunhofer Institute for Mechanics of Materials IWM and German-Jordanian University. E-mail address: iyas.khader@gju.edu.jo (I. Khader). Journal of the European Ceramic Society xxx (xxxx) xxx–xxx 0955-2219/ © 2020 Elsevier Ltd. All rights reserved. Please cite this article as: Iyas Khader, et al., Journal of the European Ceramic Society, https://doi.org/10.1016/j.jeurceramsoc.2020.03.046