Background

Zirconium carbide (ZrC) and its composites are potential candidates for applications such as nose cones for re-entry vehicle, engines, wear resistant parts and in nuclear fuel cladding. Producing fully dense zirconium carbide requires very high temperature which is a direct consequence of its high melting point. Higher processing temperatures increase grain size, thereby also causing a loss in strength along with the increased cost. Therefore, there is always a driving force to produce such a material in fully densified form at as low a temperature as possible.

Reactive Hot Pressing of Zirconium Carbide

Recent work has shown that reactive hot pressing (RHP) is a promising method to produce fully densified zirconium carbide (ZrC) at lower temperatures compared to the conventional hot pressing. But the reason for low temperature densification in RHP is not well understood. In this work densification mechanisms involved during RHP of ZrC have been studied in detail. RHP has been carried out using zirconium (Zr) and graphite (C) powders in the molar ratios 1:0.5, 1:0.67, 1:0.8, and 1:1 at 40 MPa, 800ºC–1200ºC for different durations. The volume fractions of phases formed, including porosity, are determined from the measured density and from Rietveld analysis. Increased densification with an increasing non-stoichiometry in carbon has been observed. Microstructural and X-ray diffraction observations coupled with the predictions of a model based on the constitutive laws governing plastic flow of zirconium suggest that the better densification of non-stoichiometric compositions arise from the higher amount of starting Zr and also the longer duration of its availability for plastic flow during RHP. Volume shrinkage due to reaction between Zr and C and the gradual elimination of the soft metal phase limit the final density. Based on these observations, a two-step RHP carried out at 800 ºC and 1200ºC leads to better densification than a single RHP at 1200ºC.

Modeling of Reactive Hot Pressing

To get an insight and also to optimize the parameters, a model of RHP has been constructed based on four different parts, namely: 1. Densification of zirconium under pressure 2. Reaction of zirconium and carbon 3. The constraint on sintering from a rigid phase and, finally, 4. The volume contraction during reaction. The model uses published data for the 4 steps and shows reasonable qualitative and quantitative agreement with the experimental results. Further experiments are done with the model to optimize the processing parameters. Results from the virtual experiments consolidates our earlier conviction gained from experimental results, by showing zirconium is the principal factor in densification and exhaustion of zirconium coupled with reaction derived de-densification prevent the higher stoichiometric carbide from achieving full densification. It also shows, RHP gives best densification when reaction is 70-80% complete. So two step RHP where the first RHP will only complete the reaction 70-80%, and a final RHP at temperature which will complete the reaction, will possibly be the way to achieve best densification.

Low Temperature Reactive Hot Pressing of Zirconium Carbide

RHP as a low temperature processing route for producing non-stoichiometric carbide has also been explored in this study. Two different compositions namely Zr, ZrC and Ti, ZrC in 1:1 molar ratio are chosen for this study. RHP is carried out at 900oC and 50 MPa for 30 min. Following the RHP, further heat treatments are carried out at higher temperatures to complete the reaction. The results show that both the compositions reach near theoretical relative density. This study highlights that the fully densified non-stoichiometric carbide can be produced much more cost effectively by low temperature RHP followed by higher temperature pressureless reaction sintering.