M. John Matthewson

Densification of B ₄ C during sintering can be aided by removing the native B ₂ O ₃ (condensed) (B ₂ O ₃ (c)) layer present on the starting B ₄ C powder. B ₂ O ₃ can be removed by adding excess C and holding the powder compact at an intermediate temperature below the normal sintering temperature. This allows time for CO and minor boron gases to diffuse out from the porous compact before the pores close. This process was examined using a computational model based on codiffusion of multiple gas species, which enables prediction of the gas‐ and condensed‐phase composition as a function of time and position in the specimen. The model, described previously elsewhere, was originally applied to the SiC/SiO ₂ system but has been adapted for the B ₄ C/B ₂ O ₃ system. The results are used to determine the optimum holding time for complete B ₂ O ₃ (c) removal as a function of key parameters, such as specimen thickness, particle size, temperature, etc. The role of gas‐phase transport in residual C and B ₄ C profiles is also examined.

Carbon, which is often used as an additive to silicon carbide powder, is thought to facilitate densification during sintering by aiding the removal of the native SiO ₂ layer, which is present on the starting SiC powder. The mechanism is the reduction of SiO ₂ to SiC with the formation of primarily CO gas, which diffuses out from the porous compact at a temperature below the normal sintering temperature. It has been found beneficial to hold the compact at an intermediate temperature to allow time for the CO and other gases to diffuse out before the pores close. We investigate this process using a computational model based on codiffusion of multiple gas species, which enables prediction of the gas and condensed phase compositions as a function of time and position in the specimen. The results are used to determine the optimum holding time for complete SiO ₂ removal as a function of key parameters, such as specimen thickness, particle size, temperature, etc., as well as the necessary amount of C additive. The results of the modeling are consistent with the experimentally observed spatial variation of density and composition in SiC compacts.

A convenient route to enhancing the thermal degradation on-set temperature of existing commercial optical fiber coatings is presented. UV curable acrylate coatings were modified through the addition of a multi-functional cross-linking agent and are shown to increase their degradation temperature by 65^degC without any degradation in the mechanical or optical properties of the resultant fiber. Such enhanced thermal robustness in coatings is important for optical fiber applications in high energy laser (HEL) systems and selected higher temperature sensing environments.

Strength-Probability-Time (SPT) diagrams provide an intuitively pleasing method for presenting reliability data based on extrapolations from accelerated fatigue testing data. If power-law crack growth kinetics are assumed the calculations required to generate the SPT diagram are particularly simple. However, if exponential or other more complex forms are used, this is not the case. If the accelerated data are for dynamic fatigue measurements (strength as a function of stressing rate) the SPT diagram can only be determined after numerical integration of the crack growth equations, followed by non-linear regression to the fatigue data. However, we have developed software to perform this task. In this paper we describe the methods used and show sample results of lifetime predictions using SPT diagrams. Also, the effect of using different crack growth kinetics models on predicted lifetimes is discussed.

Specialty optical fibers operating in harsh aerospace environments are typically exposed to high temperatures and elevated humidity. This calls for better performing protective coatings. Recently developed sol-gel derived inorganicorganic hybrid materials called hybrid glass offered improved protective performance as compared to standard dual polymer coated fibers [1]. In this paper we examine the effectiveness of online UV curing for the protective ability of hybrid glass coatings. For this purpose two types of UV-curable hybrid glass candidates representing two different concentrations of acrylate groups were applied online to silica fibers as single and dual coats. Samples of fibers were collected and subjected to dynamic fatigue testing by two-point bending. The stress corrosion parameter, n, as well as the strength of the fibers were determined. Both the strength and n were higher for fibers with two layers of coating as compared to single coatings even when the thickness of both one and two layer coatings was the same. This may be caused by the greater degree of cross linking of the inorganic component when the coating is exposed twice to the heat generated in the UV chamber. Coating materials with reduced acrylate group content had higher values of the fatigue parameter n but at the same time reduced strength.

The lifetime of an optical fiber depends on its environment. Previous work extensively measured and characterized the separate effects of humidity and temperature on the fatigue parameters using three different kinetics models, but the combined effect has not been determined in detail. In this work, the details of how the fatigue parameters vary with temperature in a humid environment were investigated. It was found that the kinetics model parameters were different from values obtained elsewhere in liquid water. This may be the result of differences in the apparent activation energy for fatigue in liquid and vapor environments.

Often lightguide fiber processing involves steps that may cause degradation of very high strength or flaw-free, perfect fiber. A very obvious type of degradation is the development of abrasion flaws during handling. Also, heating of a fiber to moderate temperatures (~300-600°C), for instance during the soldering of pigtails, has been shown to result in strength degradation of strong fiber. It has been suggested that the use of HF etching may be a reasonable technique for the elimination of many types of strength-lowering defects. In this paper we discuss early results from the literature on the effects of heating and HF etching on the strength of silica glass and present new results on both.