Effect of morphology of reinforcement phase on densification and mechanical properties of zirconium diboride based ceramic composites

By Mehdi Shahedi Asl

Supervisor: Dr. Mahdi Ghassemi Kakroudi

Advisors: Prof. Farhad Golestani-Fard, Prof. Mohammad Rezvani

University of Tabriz

April 2015

Abstract: In this research, monolithic ZrB₂ ceramics, ZrB₂-based composites reinforced with SiC (different content/particle size) as well as nano carbon (with different morphologies) reinforced ZrB₂-SiC composites were fabricated by hot pressing method under different processing conditions (sintering temperatures, dwell times and applied pressures). The project was conducted in five steps. The Taguchi methodology was employed (in steps 2-4) in order to reduce the number of experiments as well as to perform the statistical analyses and to determine the significance of each parameter of the process.

   Step 1: Effects of hot pressing temperate and SiC content on ZrB₂-based composites

   Step 2: Effects of processing parameters on monolithic ZrB₂ ceramics

   Step 3: Effects of processing parameters and SiC content on ZrB₂-based composites

   Step 4: Effects of processing parameters and SiC particle size on ZrB₂-based composites

   Step 5: Effects of morphology of nano carbon on ZrB₂-SiC composites

The results of step 1 showed that achieving a fully-dense monolithic ZrB₂ ceramic was impossible, even at 2000 °C as a relative density of 91% was obtained. This is due to the fact that the presence of oxide impurities leads to the abnormal grain growth of zirconium diboride, which inhibits the complete removal of the porosities. The addition of SiC with an efficient mixing of the starting powders had an important role as the grain growth inhibitor. By adding 30 vol% SiC, a relative density of ~100% was achieved at 2000 °C with the maximum hardness  of  21.3 GPa and the highest fracture toughness  of 4.7 MPa m^1/2. The dominant densification mechanism strongly depended on the hot pressing temperature. In addition, the various toughening mechanisms were investigated in the SiC reinforced ZrB₂-based composites.

In the steps 2-4, the maximum levels of parameters of the hot pressing process (temperature of 1850 °C, dwell time of 90 min and applied pressure of 16 MPa) were identified as the optimal hot pressing conditions. The sintering temperature was recognized as the most important parameter (with a significance of 54% on the density and a significance of 67% on the hardness) in the processing of monolithic ZrB₂ ceramics. On the other hand, the applied pressure was found to be the most influential parameter (with a significance of 62% on the density and a significance of 61% on the hardness) in the hot pressed ZrB₂-SiC composites. In addition, the optimum SiC content was determined to be 25 vol%. In ZrB₂-based composites, which were reinforced with different nano/micro-sized SiC, both the temperature (with a significance of 41% on the density and hardness) and the pressure (with a significance of 43% on the density and hardness) were identified as the important parameters. The optimum SiC particle size was 200 nm. The Taguchi predictions for the results of the optimal conditions were in a good harmony with the outcomes of the verification tests in the steps 2-4.

Adding nano carbons with different morphologies, except carbon nanotube, led to the fabrication of near fully-dense composites. The highest value of fracture toughness (7.1 MPa m^1/2), obtained in this research, belonged to the nano-graphite reinforced ZrB₂-SiC composite.

SEM micrographs of the polished surfaces with indentation crack paths of (a) ZrB₂, (b) ZrB₂-SiC, (c) ZrB₂-SiC-graphene, (d) ZrB₂-SiC-graphite, (e) ZrB₂-SiC-carbon nanotube and (f) ZrB₂-SiC-carbon black