Available online at www.sciencedirect.com ScienceDirect Journal of the European Ceramic Society 35 (2015) 1823–1830 Influence of temperature and humidity on the strength of low temperature co-fired ceramics Clemens Krautgasser a,b , Robert Danzer a,b , Peter Supancic a,b , Raul Bermejo a,∗ a Institut für Struktur- und Funktionskeramik, Montanuniversität Leoben, Peter Tunner Straße 5, 8700 Leoben, Austria b Materials Center Leoben Forschung GmbH, Roseggerstraße 12, 8700 Leoben, Austria Received 26 August 2014; received in revised form 12 December 2014; accepted 26 December 2014 Available online 25 January 2015 Abstract Strength degradation in glass and ceramic materials is related to subcritical crack growth mechanisms acting at the crack tip during mechanical loading. In this work the effect of humidity and temperature on the strength of a commercial low temperature co-fired ceramic was investigated using a biaxial testing procedure. Experiments were performed in argon and in air at different stress rates between 25 and 125 ◦ C. The effect of humidity on strength was assessed at room temperature varying only the relative humidity. The sole effect of temperature was evaluated in argon at high stress rates. The combined effect of humidity and temperature was determined in air, testing at different temperatures. Results showed the existence of an inert strength of the material at room temperature. Measurements in ambient air showed a counterbalance effect of temperature and humidity yielding an almost constant strength for this material between 25 and 125 ◦ C. © 2015 Elsevier Ltd. All rights reserved. Keywords: LTCC; Ceramic matrix composites; Biaxial strength; Temperature; Humidity 1. Introduction Low temperature co-fired ceramics (LTCC) consist of ceramic grains (i.e. alumina) embedded in a silicate glass matrix. Due to the glass content, low sintering temperatures (i.e. below 900 ◦ C) can be achieved. 1 This makes the sintering of the LTCC tapes together with high-conductivity metals such as copper, silver or silver–palladium-alloys feasible. The LTCC- technology provides components with improved electrical (e.g. low dielectric loss factor), thermal and geometrical behaviour compared to the widely used polymer laminate based printed circuit board (PCB) technology. To give some examples, lam- inates of LTCC tapes with internal 3D metal structures can be utilised as functional components or used as ceramic circuit boards, for instance in mobile phones or as WLAN-, Bluetooth- , or RADAR-antennas, as well as in biomedical sensors and devices. 2,3 They are often used in relatively harsh environments (e.g. at elevated temperature, heavy mechanical loading, and ∗ Corresponding author. Tel.: +43 3842 4024115; fax: +43 3842 4024102. E-mail address: raul.bermejo@unileoben.ac.at (R. Bermejo). significant vibrations). Therefore, the functionality of LTCC based components depends on their mechanical strength and resistance to crack propagation in a given environment. It is well known that glass-containing materials are suscepti- ble to subcritical crack growth (SCCG) (sometimes called SCG “slow crack growth” and also known in metals as “stress corro- sion”), where cracks can grow under an applied stress intensity factor, K I , well below the toughness, K Ic , of the material. In ceramics, this phenomenon is related to the presence of humid- ity. Polar molecules, such as water, interact with the strained crack tip, thus weakening the strength of the bonds at the crack tip. 4–10 This may cause a decrease in the measured strength of the material (i.e. strength degradation). Such a phenomenon has been also reported in LTCC or in SOFC materials loaded under mechanical stress, especially in humid environments. 11–15 Among the different models proposed to describe the crack growth in these materials, a direct chemical attack of the environ- ment on the crack tip seems to have the strongest experimental support. 7,9 Depending on the crack growth rate and environment, different mechanisms of SCCG exist, which can be recognised by different slopes in a double logarithmic plot of the v–K I curve, 6 where v represents the crack growth rate and K I the http://dx.doi.org/10.1016/j.jeurceramsoc.2014.12.031 0955-2219/© 2015 Elsevier Ltd. All rights reserved.