EFFECT OF ATMOSPHERIC HUMIDITY ON THE DYNAMIC FRACTURE STRENGTH OF SIC ABRASIVES

Silicon carbide abrasive grits were fractured dynamically in a roller crusher at ambient temperature and under controlled levels of atmosphe ric humidity, covering the full range from 10 to 100% relative humidit y. The fracture stress depends strongly both on the grit size below a certain value and on the partial pressure of the water vapor. The size effect causes an increase in fracture stress as the grit diameter is decreased below a certain size. This is thought to be due to scaling o f the size of pre-existing surface defects with grit diameter. The moi sture effect causes a drop in fracture stress at any moisture level ab ove 0% relative humidity (r.h.) for sufficiently small specimens, whil e the number of fragments created in the crushing process increases. T his effect is thought to be due to moisture-assisted sharpening of the tips of surface defects, which serve as crack initiation sites, durin g the early stages of loading. The tip sharpening facilitates initiati on of brittle fracture. It lowers the measured fracture stress and ena bles smaller defects to initiate secondary fractures when the stored e lastic energy suddenly is released during the primary fracture; theref ore, more fragments form. The results explain the previously observed moisture-assisted self-sharpening of abrasives which may result in inc reased abrasive wear rates for metals as the levels of atmospheric moi sture increase. The results may also have implications for the interpr etation of certain types of ceramic wear.