Carbide Particle-Reinforced Tungsten Composites in Extreme Hazard Environments: Ablation, Thermal Shock, and Finite Element Calculation

Introduction

Tungsten and tungsten composites (such as copper-infiltrated tungsten materials) are widely used in ultra-high temperature environments due to their high melting points, high thermal shock resistance and high ablation resistance (Olcott et al., 1964; Song et al., 1998a). For instance, some rudders and nozzles of rocket motors, nose caps of missiles are made of tungsten alloys or tungsten composites, where ablation and the associated thermal shock induced damage usually are the main sources for failure (De Morton 1977). For example, the work temperature of a nozzle increases drastically from room temperature to above 2000 ºC in one second at the retrofire time of a rocket motor, so the nozzle has to experience a very severe thermal shock. Another example, the temperature of the nose cap of a missile is often as high as 2500-5000 ºC as the missile re-enters the atmosphere (Yu, 1991; Dickerson et al., 2004). Therefore, it is highly demanded to improve the ablation resistance of materials in order to keep a good pneumatic shape of these components during their lifetime, as well as the thermal shock resistance to avoid any disaster. The rapidly increasing demand for high temperature materials with super ablation resistance as well as thermal shock resistance in the last 30 years has placed no single material in the position to fulfill the rising requirements.

The ablation of ultra-high temperature materials is often associated severe thermal shock due to their particular work environments (like rocket nozzle, nose caps and rudders). The evaluation of the ablation/thermal shock properties of these materials are usually tested with rocket motors (Yu, 1991). Since the rocket experiments are so expensive, some alternative methods are employed to study the ablation/thermal shock in order to reduce the cost and obtain primary evaluation of materials. The oxyacetylene method is cheap and easily performed, consequently is widely used.

A failure of a thermal-structural component is not only governed by the transient macro-scale thermal stress field induced by a transient temperature field, but also by its micro-scale stress field if the component material consists of several phases (such as a composite component, which often contains a reinforcement and a matrix). The experimental investigation of the microstructural factors, which control the thermal shock resistance of a composite component, is very difficult. Some theoretical calculation of the thermal stress of various components upon heating/cooling has been well documented, but these works mainly focus on the macro-scale thermal stress field. The micro-scale stress existing in the microstructure has not yet been well understood.

The currently developed TiC and ZrC particle-reinforced tungsten-matrix composites (TiC/W and ZrC/W) show unusual high temperature strength: the two series of composites possessed better high temperature strength compared with monolithic tungsten in the temperature region of 20-1600 ºC (Song et al., 1998b; Song et al., 2002; Song et al., 2003; Wang et al., 2011). In this chapter, the effect of carbide particles on the ablation/thermal shock behaviors of TiC/W and ZrC/W composites under oxyacetylene firing are presented. The macro-/micro-scale thermal stress fields of the composites induced by the transient temperature field are analyzed with finite element method. The calculated results of the failure behavior of the composites under thermal shock are compared with the experimental results.