Degradation of structural ceramics by erosion
Materials wastage by solid-particle erosion can be severe and limit lifetimes. This paper will review the theoretical description of solid-particle erosion in brittle materials, which is well-developed for monolithic ceramics. The models can usually account for effects from the principal projectile properties: size, impact velocity, and impact angle. Materials parameters such as fracture toughness and hardness can be included. Steady-state erosion measurements on a wide variety of ceramics, ranging from Si single crystals to SiC-whisker-reinforced A12O3 are reviewed and compared to the models. It is believed thatR-curve behavior and/or particle fragmentation is responsible for discrepancies between the theory and experimental results for composite ceramics. Additionally, the theories make no attempt to describe threshold or incubation effects.
Strength Degradation of Glass Impacted with Sharp Particles: I, Annealed Surfaces
The strength characteristics of annealed brittle surfaces impacted with sharp particles were studied. A theory of the degradation process is constructed in three steps: (1) A sharp particle delivers an impulsive load to the target surface via a plastic contact; (2) the contact loading initiates and propagates median cracks in the surface; (3) the cracks thus induced reduce the strength of the material. Static indentation tests provide essential contact parameters for the degradation equations, thereby allowing for prediction of strengths under in-service conditions. Strength tests on soda-lime glass laths impacted with SIC grit confirm basic predictions of the theory. Higher toughness and lower hardness are the main material requirements for improved resistance to degradation. Initial flaw population in the target surface and projectile geometry are not important factors in the damage process. The study shows that impact energy is the important service variable in determining the extent of strength loss.
Erosion of Silicon Single Crystals
The erosion of Si single crystals by angular Al2O3 particles was investigated over a range of particle diametersD (23 to 270 μm), velocities v (32 to 134 m/s), and impact angles α (22° to 90°). For α≥45°, the steady-state erosion rate can be described within the framework of existing models by the modified relation ΔW∞(v sin α-v0)2.6(D-D0)0.6. The results are consistent with the relation D0v=Dv02200 × 10−6 m2/s for the threshold values D0 and v0. For α=22°, another term must be added, and could represent a contribution of plasticity. The results also indicate that surface morphology and/or subsurface-damage structures are important.