Umgebungsbedingtes unterkritisches Risswachstum in kaltgesinterten ZnO Keramiken

Sintering of ceramics is a crucial step in ceramic manufacturing, requiring high temperatures above 1000 °C to achieve dense materials. However, manufacturing routes with reduced energy consumption and lower processing temperatures are gaining increasing importance due to the rising demand for advanced multifunctional materials and the necessity to support global sustainability efforts. The Cold Sintering Process (CSP) represents one of the most recent developments in sintering technology, enabling the densification of ceramic materials at temperatures below 350 °C. This process involves the addition of a chemically active liquid phase to the ceramic powder, which is subsequently compacted under elevated pressure at moderate temperatures. Recent research has demonstrated that various ceramic systems can successfully be densified at low temperatures. Nevertheless, the long-term mechanical behavior of cold-sintered materials has so far remained largely unexplored. Of relevance in this context is the phenomenon of subcritical or slow crack growth (SCG), which leads to a time-dependent reduction in strength and thus limits the lifetime of ceramic components. SCG can cause ceramics to fail even when stressed below their characteristic strength limit.
The objective of this work is to investigate subcritical crack growth in cold-sintered (140 °C) ZnO samples and to analyze the influence of external factors on this mechanism. Furthermore, the crack growth behavior was compared with that of high temperature sintered (1000 °C) ZnO samples, and their lifetime was assessed.
In cold sintering (CS), densification was achieved by adding formic acid and applying a pressure of 200 MPa at a low temperature of 140 °C, whereas rapid sintering (RS) employed a fast heating rate (500 °C/min) to achieve densification at 1000 °C. Disc-shaped test specimens were subjected to biaxial loading under different environmental conditions (air and water) as well as various loading rates. Subsequently, fractographic analyses were conducted to better understand crack initiation and propagation in CS and RS samples. Complementary long-term static loading tests were performed, in which the specimens were loaded to approximately 70% of their characteristic strength until failure occurred.
The results show that CS samples—unlike RS samples—exhibit a threshold for subcritical crack growth of approximately 0.7 K_Ic, below which no crack propagation occurs. It can also be demonstrated that the strength of the samples decreases by around 50 % in water at loading rates of approximately 0.01 and 1000 MPa/s, while a reduction of about 30 % occurs in air. This finding has a significant impact on the lifetime of ceramic components. A deeper understanding of the mechanical behavior of cold-sintered (CS) and rapid-sintered (RS) ceramics allows for targeted optimization of material properties and extends potential fields of application.