Strength and fracture resistance of quartz fibers with polyimide coatings

Abstract

Polyimide coatings currently provide the highest performance properties of quartz fibers. The purpose of this research is to determine the strength, hardness, dynamic fatigue, performance period and crack resistance of optical fibers with polyimide coatings. The strength limit of fibers determined by the method of axial stretching over the distance between capstans of 500 mm was 4.8–6.0 GPa at a loading speed of 10–500 mm/min. W. Weibull distribution curves were plotted in coordinates that relate the probability of failure to the strength, fiber length, and parameter describing the ultimate strength. The dynamic fatigue parameter n was found, which in physical sense corresponds to the slope tgα equal to 1/(1+n) in double logarithmic coordinates. Hardness and crack resistance values of quartz fibers were measured by indentation. Crack resistance K1c was calculated using the A. Niihara semi-empirical dependence, which connects the indentation size, radial crack length, and crack resistance. The initial crack length was calculated and the size of the characteristic defect was determined using scanning electron microscopy. Thermogravimetric analysis demonstrated that polyimide coated fibers maintain thermal stability up to 450 °С. The service life of optical fibers was determined based on the dynamic fatigue data, and it amounted to at least 25 years at a load of 0.2 GPa. The greater the difference between the lower strength level and the upper one in the stretch tests of fiber segments, the higher the distribution parameter m describing the ultimate strength of optical fibers. The values of this parameter are determined by the fiber quality: m = 50÷100 for coated fibers and m = 1÷5 for uncoated ones.