Characterization of a Ti(Mo)C-Ni Cermet for Use in Impact Resistant Sandwich Panels

The deflection of edge-clamped plates during out of plane impulsive loading is proportional to a merit index given by the square root of the ratio of the plate material strength to its density [1]. Even smaller deflections can be achieved if sandwich panels with cellular cores can be made from these high specific strength materials. Ti(Mo)C-Ni based cermets have been identified as a potentially superior material for this application since their compressive strength is typically 2-3 GPa and their density is in the 5,500 kg/m3 range. The merit index of this cermet exceeds that of all known metallic systems. Although the toughness of cermets generally exceeds that of ceramics, they have low fracture toughness relative to many metals and this will inhibit their suitability for some applications.

A comprehensive characterization of the mechanical properties of Ti(Mo)C-Ni cermets has been conducted in order to determine if it is a viable candidate for use in truss core sandwich panels. Additionally, the lack of well-established fabrication and joining techniques needed to fabricate sandwich panels is also a concern. The feasibility of using a transient liquid phase bonding (brazing) technique to fabricate a cellular structure was also therefore investigated.
The cermet system investigated here is found to have a density of 5,520 kg/m3 in its as-received condition. Using image analysis and quantitative x-ray diffraction-based phase identification, it was determined to consist of approximately 83 vol % TiC ceramic with 15 vol% of the nickel binder, and an average porosity of 1.6 vol%. Chemical analysis via inductively coupled optical emission and energy dispersive x-ray spectroscopy indicate the ceramic phase has a metal composition of 88% Ti, 11% Mo, and <1% Ni. Similar analysis of the binder gives a composition of 94% Ni, 5% Ti, and <1% Mo, respectively. The elastic modulus of the system was determined to be 380 GPa via pulse-echo ultrasonic technique. The cermets measured compressive strength was 2.7 GPa but its bending strength was only 520 MPa, which is likely reconciled in terms of the characteristic flaws within the material.

The density and elastic moduli are well estimated by a rule of mixtures of the ceramic hard phase and the metallic binder properties. A computation of the theoretical density of the individual phases permits a calculation of the aggregate porosity level from the independently measured phase fractions. The elastic modulus of the cermet is nearly equivalent to that upper bound prediction computed based upon the moduli of pure TiC and Ni phases, but this may be a consequence of force chain formation.

The as-received cermet was discovered to undergo cracking during electro-discharge machining (EDM) of truss core shapes. X-ray diffraction measurements showed that a significant level of residual stress of 165 ± 15 MPa was present in the TiC phase. Stress-relief annealing at 900˚ C with a subsequent cooling rate of 15˚ C/min was observed to relieve the stress to a value lower than the experimental uncertainty. Hot-isostatic-pressing at 1270˚C (with a cooling rate of 50˚C/min) was found to increase residual stress, and it failed to reduce the porosity in the samples. No major differences between the microstructure and mechanical properties were observed following either the stress-relief or hot-isostatic pressed treatments.
Brazing with a Ni-Cr-P braze alloy was found to be an adequate cermet bonding method for assembly of a sandwich panel structure with a cermet truss core. Preliminary testing of brazed cellular unit cells revealed that failure occurs entirely in the base material, indicating that the brazed joint was not a limiting factor.