Characterization of Dynamic Failure Process of Si 3 N 4 Ceramics, Part II: Dynamic Fracture Toughness

Abstract

A comprehensive experimental and theoretical work aiming at studying the dynamic failure process of silicon nitride ceramics has recently been started. The main goal is characterizing the material behavior by mechanical and microstructural features. The basic force-, strengthand energy type characteristics completed with some fractographic analysis of the damage process were discussed and presented in a companion paper. The current paper deals with the problem of determining the dynamic fracture toughness of the investigated ceramic, especially at high loading rates. A solution is provided by applying the dynamic key curve (DKC) method, that utilizes a recently observed important peculiarity of silicon nitride: it shows electric-emission activity during dynamic failure. This finding makes it possible to determine the fracture time even in cases of impact tests at high loading rates, as well as during investigating notched specimens. The analysis of the mechanical behavior is completed by qualitative and quantitative fractography. Combining the measuring results gained from the instrumented impact tests with the fractographic findings, the dynamic fracture toughness KId is estimated based on key features of the fracture surface of the dynamically broken specimens. The mirror size constant A0 is also determined. INTRODUCTION The wide choice of testing methods relating to the determination of quasi-static KIc fracture toughness of engineering ceramics indicates the efforts and difficulties to satisfy simultaneously the demand of determining reliable material data as well as applying economical and technically simple procedures. Advanced, accurate, and sensitive testing equipment are required for the reliable determination of the force and energy type characteristics for small size specimens and for tracing and detecting extremely short time failure processes during impact loading. In case of instrumented impact tests, a special problem during determination of KId or JId fracture toughness values is that the force Fi,u, corresponding to the unstable crack propagation that is necessary for the calculations based on quasistatic relationships, cannot be taken from the load-time diagrams of the notched specimens, due to the undamped force oscillations. This problem was discussed in detail in Part I and related F-t diagrams were shown. In the current paper, determination of the dynamic fracture toughness will be discussed. Alternatives for solution of the above problem are shown. Three different methods, namely calculations based on the quasi-static relationships, the procedure of the impact response curve, as well as dynamic-key-curve method are reviewed and compared from the aspect of applicability for the given material and testing circumstances. An important observation contributing to find the appropriate solution is that Si3N4 ceramics show electric emission activity which makes it possible to determine the fracture time for this material in an alternative way. Failure analysis of the investigated material has been successfully completed with macroand micro-fractographic investigations. Determining various qualitative and quantitative characteristics by fractography, the nature and peculiarities of the damage process can be analysed in a more complex way. Furthermore, mechanical properties like the critical stress intensity factor and the fracture stress are also compared. They may be determined both by mechanical testing and by fractographic analysis. EXPERIMENTAL WORK Investigated material and test series The testing equipment, similar to that introduced in the companion paper, was a CEAST 25/15/2 type impact testing machine with the same instrumentation. Tested material was similar to that, described in the preceding paper, so Si3N4 based ceramics, having composition of 90% Si3N4, 4% Al2O3 and 6% Y2O3 were investigated, produced by the Research Institute for Technical Physics and Materials Science of Hungarian Academy of Sciences (RITPMS, HAS). All the specimens were produced by uniaxial pressing using polyethylene-glycol additive. The additive was extracted by heating in air at 600°C after pressing. The firing of the pieces occurred in two-stage gas-pressure sintering in a HIP equipment. The specimens were rectangular prisms with dimensions of 4.9 mm x 3.2 mm x 49 mm. The investigations were executed in two test series, with different specimen positions and geometry (see Figure 1.) according to the different aim of the measurement. Figure 1. Specimen geometry and different test arrangements for the instrumented impact tests. Direction of impact L