Ind. Eng. Chem. Prod. Res. Dev. 1981, 20,599-608
Optical Fiber Coatings. An Overview with Regard to Opto-Acoustic Underwater Detection Systems Rodger N. Capps Neval Research Laboratory, Underwater Sound Reference Detachment, Orlando, Florida 32856
The current state of the art in coatings technology for optical fiber waveguides is reviewed. Problem areas in the use of coatings for optical fibers in sonar systems are discussed, and some approaches to the solution of the problems are suggested. A short review of pertinent topics in the proof testing and stress corrosion of optical fibers is also included.
Introduction The development of low-loss optical fibers in multi-kilometer lengths, coupled with their superior performance characteristics, has led to the implementation of optical fibers in telecommunication and computer data links. More novel applications of optical fibers in other areas, such as underwater cables and acoustic detection, have been slow to develop because currently used coating materials and technology do not offer the necessary protection against a hostile service environment. The use of fiber-optic cables instead of conventional cables as data links for towed sonar arrays, sonar buoys, torpedoes, and other applications offers significant advantages in cases where small size, large bandwidth, low attenuation, freedom from electromagnetic interference and ground loops, and protection against signal interception are important (Sigel, 1976; Wilkens, 1977; Giallorenzi, 1978). Extreme service hazards (such as high tensile stresses, large elongations, high hydrostatic pressures, effects of rapid cable payout, coiling and uncoiling effects, and corrosive effects of seawater) are encountered in most of these applications. There is currently a good deal of interest in the application of optical fibers to underwater acoustic sensing. The use of opto-acoustic interactions in glass fibers to detect acoustic waves has been reported in a number of recently published articles (Bucaro and Carome, 1978; Bucaro et al., 1977; Carome et al., 1977; Cielo, 1979; Cole et al., 1977; Culshaw et al., 1977; Shajenko et al., 1978). A practical fiber-optic hydrophone would alleviate a number of problems commonly encountered with conventional electroacoustic transducers. The elaborate impedance matching techniques used with conventional hydrophones would be unnecessary with a fiber-optic hydrophone. Conventional transducers are susceptible to such factors as electromagnetic interference and to thermal and mechanical shock. They are also relatively limited in area coverage. Fiber-optic hydrophones are relatively unconstrained in size and shape. They can also be interfaced directly as part of an optical data link and are immune to electromagnetic interference. The most frequently considered configuration for a fiber-optic hydrophone is that of an interferometer (Bucaro and Carome, 1978; Carome et al., 1977; Cielo, 1979). The two arms of the interferometer are formed by two singlemode fibers through which light from a laser is beam-split and passed. One fiber is immersed in water and serves as the acoustic sensing element, while the other fiber serves as a reference. The two beams are recombined and allowed to interfere on a photodetector. The detection scheme is based upon either an acoustically induced phase shift or
intensity fluctuations which reproduce acoustic pressure fluctuations incident upon the immersed arm of the interferometer. The performance of present fiber-optic hydrophones is susceptible to factors such as hydrostatic pressure changes, temperature variations, and mechanical vibrations. Alternative configurations have been investigated (Bucaro and Carome, 1978; Cielo, 1979; Davis et al., 1980; Young et al., 1980) in an effort to solve some of these problems. One of the primary objectives of the US.Navy’s Fiber-optic Sensor System Program is the development of a field-operational fiber-optic hydrophone. A comprehensive review of current research to establish the technological feasibility of an opto-acoustic sonar system can be found in the report of Davis et al. (1980). In order to develop a fiber-optic hydrophone in presently considered configurations that will be suitable for longterm use in an aqueous environment, there are a number of problems inherent to the optical fiber and associated coatings and encapsulants that must be solved. These problems include optical losses due to fiber microbends, effects of fiber curvature and axial strain, and fiber contamination and corrosion from water. Additionally, the chemical and mechanical compatibilities of any encapsulant material or acoustic couplant fluid must be considered. The purpose of this paper is to provide a survey of relevant areas of current coating technology, identify problem areas in coatings for optical fibers in sonar systems, and suggest some approaches to the solution on these problems. A short review of pertinent topics in the tensile testing and stress corrosion of optical fibers is also included. Although the emphasis here is more toward fiber-optic hydrophones, much of the discussion is also applicable to the use of optical fibers in towed sonar arrays, sonobuoys, and torpedoes. The bibliography of this article is not intended to be exhaustive. However, it is adequate to provide the reader with a survey of previous and current works in fiber-optic coatings, as well as relevant background material. The Glass Fiber Optical fibers are normally made from glass composed of various oxides, such as Si02, GeOz,P2OS,BzOa, Na20, and others. Those who are interested in the details of optical waveguide fabrication should consult the various reviews available (Pol, 1979; Bendow and Mitra, 1979; Midwinter, 1979). It is interesting to note that although glass has been in use for centuries, no completely satisfactory theory of glass formation exists. A number of theories have been proposed to explain glass formation (Rawson, 1967). One approach emphasizes structural considerations of the glass-forming material, such as the geometrical configuration of constituent atoms or the nature and strength of interatomic bonding. Another approach initially disregards structural considerations and deals with the kinetics of the crystallization of a liquid at temperatures which are below the melting point. Both of these approaches are necessary to obtain an understanding of glass formation. Pristine glass is inherently a very strong structural material. This is important for the service life of optical
This article not subject to US. Copyright. Published 1981 by the American Chemical Society
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Ind. Eng. Chem. Prod. Res. Dev., Vol. 20, No. 4, 1981
fibers, since the two fundamental mechanical properties are static fatigue and strength (Maurer et al., 1974; Justice, 1977; Maurer, 1975). The theoretical tensile strength of fused SiOzis approximately 16 GPa (2.3 X lo6psi) (Lawn and Wilshaw, 1975). Carefully prepared, fused, silica fibers have attained tensile strengths of almost 6 GPa (0.86 X lo6 psi) (Thomas, 1960). In contrast, many glass objects exhibit fracture strengths on the order of 100 MPa. The tensile strength of unprotected glass fibers will degrade rapidly under environmental influences. The degradation may occur under a constant stress (static fatigue), a constant rate of stress (dynamic fatigue), or as a result of zero-stress aging. Surface Flaws in Glass Fibers. Any discrepancy between the theoretical and experimentally observed strengths in glasses is known to be due to the existence of flaws on the surface of the glass. Griffith (1920) considered the effects of stress concentration around microcracks in the surface of a glass. He showed that a flaw in the glass will become unstable when the rate of energy increase that comes from creating new surfaces becomes equal to the rate of release of stored elastic energy. Through the use of Griffith’s analysis, the fracture strength (af)can be related to the flaw size by the relation (Kalish et al., 1978)
where Y is a geometric constant for the given specimenflaw configuration, E is Young’s modulus, A, is the flaw size, and y is the surface free energy of the glass surface. A different approach to the problem was developed by Irwin (1968), who analyzed the stresses near the crack tip instead of throughout the body. For surface flaws under plane stress conditions, he showed that the fracture strength and the flaw size are related by (Kalish et al., 1978) KIC Uf = YAclIz where KI,, the critical stress intensity factor, is a constant that is characteristic of the material under consideration. For a fused silica fiber with a semicircular crack normal to the fiber axis, a flaw of 0.4 pm will reduce the fracture strength to about 1 GPa, or about 7% of the theoretical pristine glass strength (Kalish et ai., 1978). The Weibull Probability Distribution in Glass Fibers. A given length of glass fiber will contain a number of flaws with a distribution of flaw sizes. A study of a large number of glass samples would show statistical variations in the failure strains, resulting from statistical variations in their surfaces. Enough data should be taken to determine the true statistical nature of the flaws. It would be desirable to be able to determine the strengths of long lengths of fiber from measurements on short lengths of fiber. The probability of failure of a given length of glass fiber can be adequately described by the simple power law, which was empirically postulated by Weibull (1951) for strength distributions in engineering materials. Freudenthal and Gumbel(1953), using extreme-value theory, established the validity of the use of the distribution to represent breaking strengths of materials. The cumulative probability of failure, F, for a fiber at a stress level, u, for a duration of time can be expressed as (Weibull, 1951; Gulati et al., 1975) F = 1 - e x ~ [ - ( a / u ~ ) ~ ( t / t o ) ~ ( L / L ~(3) )l
where Lo is the length of the test sample; go is the stress required to obtain 63.2% failures at time to for length Lo. The parameters m and b represent the slope of failure probability vs. failure stress, and failure probability vs. time of applied stress, respectively for a Weibull plot. Numerous examples of the use of Weibull plots can be found in the literature (Maurer et al., 1974; Justice, 1977; Maurer, 1975; Kalish et al., 1978; Gulati et al., 1975; Justice and Gulati, 1978; Rast, 1980; Tariyal and Kalish, 1977). It should be noted that Weibull testing is a destructive test method, and that different fiber lengths will give different results due to variations in surface area and flaw distributions. It is common practice to extrapolate the stress at failure, 02,for a gauge length, L2, from a tested fiber of length L1through the expression uz
=
UI(LZ/JhP
(4)
It has been observed (Kalish et al., 1977) that strength data from fiber test lengths of 1 m and longer allow better extrapolation to kilometer-length fiber strength that do test data from fibers shorter than 1 m. Time-Dependent Failure of Glass Fast fracture and fatigue degradations are dominant phenomena in the failure of glasses. Both conditions depend upon the existence of surface flaws, but the timeto-failure is different in each case. Static fatigue and dynamic fatigue are the two common manifestations of time-dependent fractures in glass. They differ from each other by the manner in which the stress is applied. Static fatigue occurs when the applied stress is maintained at a constant level below the inert strength, so that the glass fractures after a shorter time than it does in an inert atmosphere (one other than a vacuum or an inert gas). Dynamic fatigue results if the applied stress is continuously increasing, so that the strength of the glass in an active environment will be lower than that in an inert environment. Water in the environment is primarily responsible for slow crack growth in glass. Stress-corrosioncracking causes subcritical crack growth that results in delayed failure of the glass. In order for static fatigue to occur, a surface flaw, moisture, and a tensile stress all must be present. In cases where fibers are immersed in water but are not under stress, no flaw growth occurs (Kalish et al., 1978). The majority of the detailed studies of crack growth and stress corrosion in ceramic materials has been carried out on glasses (Charles, 1958a,b,c; Ritter, 1973; Ritter and Sheburne, 1971; Weiderhorn and Bolz, 1974; Charles and Hillig, 1961; Hillig and Charles, 1965; Freiman, 1974,1975; Freiman et al., 1979). Water is the most commonly used immersion solvent, although other materials have also been investigated (Freiman, 1974, 1975; Freiman et al., 1979). Various models have been proposed to account for crack growth in glasses. These include diffusion-controlled crack growth (Hasselman, 1970; Sevens and Dutton, 1971; Gerberich and Stout, 1976) and generation of cracking within the glass controlled by alkali-ion mobility (Cox, 1969). One of the most widely applied models is that of Charles and Hillig (1961,19651, because it fits experimental data over the widest range of experimental conditions. There has been some doubt expressed (Gerberich and Stout, 1976) as to whether or not this model is applicable to stress corrosion cracking of glass in an aqueous environment. The model of Charles and Hillig assumes that the velocity of crack growth is given by the rate of dissolution of the glass, as influenced by the environment. The crack growth rate, V, is given as (Hillig and Charles, 1965)
Ind. Eng. Chem. Prod. Res. Dev., Vol. 20, No. 4, 1981 801
+ V*u,
V = Voexp[-E*
- (V,a/p)]/RT
(5)
where E* is the stress-free activation energy, V* is the activation volume, u, is the molar volume, (Y is the interfacial surface energy between the glass and reaction products, R is the gas constant, and T is the temperature. The same general form of this equation was derived by Bartenev et al. (1955,1958) in terms of the rate of bond breakage. Alternative approaches have been the application of fracture mechanics (Weiderhorn and Bolz, 1974; Irwin, 1966) and the model of Gibbs and Cutler (1951). As stress corrosion occurs, the crack intensity factor, KI, increases, although the stress remains constant. This is due to the fact that the crack length, I , is increasing, as can be seen from the definition of KI (Weiderhorn and Bolz, 1974)
KI = Yu1'/2
(6)
where Y is a constant characteristic of the flow geometry. The stress intensity factor, KI, has been found to be related to the velocity, V , of subcritical crack growth by the relationship (Weiderhorn et al., 1974)
V = AKI"
(7)
where A and n are empirical constants for a given glass composition and test environment. When KI becomes equal to the critical stress intensity factor KI,, eq 7 is no longer obeyed. Instead, there is a rapidly increasing crack velocity approaching the terminal value, and the fiber fails catastrophically. Ritter (1978; Ritter et al., 1978), using the fracturemechanics approach of Evans and Weiderhorn (1974),has shown that the time-to-failure for a glass fiber under load can be expressed as
r r
tf =
A u , ~P ( n - 2)
KIi(n - 2)
1
' I
-
K~c(n- 2)
(8)
where a, is the applied constant tensile stress. Since KIc > KIi and n is a large number, the above expression can be simplified to give 2KIi(2 - n) tf = (9) A U ; Y ~ ( ~- 2) In order to calculate tf, it is necessary to know K I ~A, , and n. KIi depends on the initial flaw size and can be inferred from proof testing. The parameter n can be determined from the Weibull parameters m and b, as n = m /b. The constant Y will depend upon the flaw geometry. Minnear and Bradt (1975) compiled crack growth results for a number of ceramic materials and obtained the following empirical relation for log A and n log A = -8.94 - 5.414n (10) uo, Y,
In the case of fused silica fibers, the correlation between log A and n has been shown to be (Kalish et al., 1978) log A = -11.598 + 3.387n (11) The excellent correlation found in this case supports the stress-corrosion mechanism for time-dependent fracture of glass (Kalish et al., 1978). The relationship between fiber life, service stress (IJ,), and proof stress (uJ, has been given by Ritter (1978) as
where
u
is in MPa, tf is in seconds, and B is an experi-
mentally determined constant. For fused silica fibers, Ritter (1978) gives a B value of 0.37 in metric units. Results of a study of the static and dynamic fatigue behaviors of polymer-coated, fused, silica fibers (Kalish et al., 1978) indicate that they are susceptible to static fatigue and dynamic fatigue, independent of the presence or type of polymer coating. Polymer coatings apparently do not influence the kinetics of static and dynamic fatigue, but simply provide protection against mechanical damage. Dynamic fatigue and static fatigue both appear to be manifestations of stress corrosion. Apparently, the only completely effective way in which a coating can protect the glass fiber from the detrimental effects of moisture is to provide a hermetic seal.
Fiber-optic Coatings Previous efforts in the coating technology of optical fibers have been directed toward optimization of coatings according to the following criteria (Wang et al., 1977; Blyler and Hart, 1979; Vazirani and Kwei, 1979; Suzuki and Osani, 1979; T. J. Miller, 1979; Blyler, 1980): (i) preservation of pristine fiber strength; (ii) protection against microbending losses; (iii) protection against environmental degradation (Factors that may contribute to shortened fiber lifetimes are static fatigue and water corrosion, temperature extremes and rapid thermal excursions, and radiation damage.); (iv) easy removability of coatings for splicing and connecting, and protection against fiber degradation during cabling and installation. Single material coatings have not been particularly effective in meeting the above requirements. Thicker composite coatings have exhibited a number of advantages over single material coatings, particularly in handling during manufacturing and cabling. Currently available commercial coating systems were developed primarily for the purpose of producing low-loss, long-lived fibers for use as data links in telecommunication systems. They were not specifically designed to cope with the additional requirements that the use of optical fibers in underwater acoustic detection systems imposes upon the performance of coatings. Areas in which coatings may be involved in the performance of a fiber-optic acoustic detection system are the following. 1. Protection against Microbending Losses and Effects of Fiber Curvature and Strain (Gloge, 1975; Gardner, 1975; Olshansky, 1975; Nelson et al., 1977). Microbending losses are losses of electromagnetic radiation that result from statistical surface variations and lateral pressures on the surface of the fiber. The losses arise from the mode coupling generated by random bends of the optical fiber and from radiation losses for higher order modes of light. Microbending losses will be influenced by the coating material, the numerical aperture of the fiber, and whether the fiber is single- or multi-mode (Giallorenzi, 1978; Gloge, 1975; Akers, 1978, 1979). The topic of microbending losses has been extensively treated in the literature (Gloge, 1975; Gardner, 1975; 01shansky, 1975; Nelson et al., 1977; Akers, 1978, 1979). Gloge (1975) pointed out that the coating can offer protection from microbending losses by acting either as a soft compliant enclosure to mask the rises and depressions of adjacent surfaces in a mechanical way, or by acting as a hard enclosure that makes the fiber resistant to conforming to environmental surfaces. Through a proper choice of materials, both mechanisms can be incorporated into a single coating by using a sift inner layer and a hard outer covering. 2. Abrasion Protection. It is important to the service life of the fiber that the generation of surface flaws be
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avoided. A freshly drawn fiber has the best surface flaw distribution that it will ever have. It is therefore important that the coating be applied in-line with the fiber drawing process. The coating should serve to protect the fiber from abrasion during transport, take-up, and subsequent handling. It is also important that the process used to apply the coating not be a source of abrasion. 3. Enhancement of Sensitivity. Bucaro and Hickman (1979), using coated, single-mode optical fibers in a hydrophone, observed an amplification of pressure-induced phase changes as compared to bar fibers. This was attributed to an enhanced axial strain on the fiber due to the coating. The effect was theoretically and experimentally investigated by Hughes and Jarzynski (1980). Using a plane-strain model, they examined opto-acoustic coupling for multi-layer, jacketed, single-mode optical fibers in terms of both hydrostatic (uniform pressure over the entire fiber) and radial (no pressure on the ends of the fiber) boundary conditions. The effects of various plastic and rubber jackets, as well as jacket thickness, were examined. The best agreement between theory and experiment was observed with the hydrostatic model. For hydrostatic boundary conditions, this study indicated that the coating material should have both a low Young’s modulus and a low Poisson’s ratio for maximum enhancement of sensitivity. The results of this study indicate that for uniform hydrostatic conditions, the maximum pressure sensitivity can be achieved with Teflon jacketing. For radial boundary conditions, the jacket should have a low Young’s modulus and a high Poisson’s ratio. 4. Cladding. Fluorocarbon and silicone polymers are normally used, due to refractive index requirements. Due to water-permeability requirements, it is questionable whether polymeric claddings would be suited for use in underwater applications. 5. Protection against Environmental Degradation. As mentioned earlier in the discussion of proof-testing, static fatigue and water corrosion are important environmental factors in determining the service life of an optical fiber in underwater applications. The extent of static and/or dynamic fatigue will be influenced by the final design configuration of the acoustic detection system (bend radius of the fiber, pressure and temperature cycling, cyclic and/or acyclic mechanical stresses, etc.). Zero-stress aging of the fiber will also cause a long-term loss in strength that may be important in some applications. Hermetic coatings provide protection against this, but they may have disadvantages for use in acoustic detection systems. A fiber-optic hydrophone deployed in an ocean environment will be subject to a variety of environmental influences. Intelligent design and use require knowledge of the ways the fibers are affected by temperature and pressure. Table I summarizes the requirements for coatings to be used in an underwater acoustic detection system. In addition to the fiber coating, an encapsulation system is needed to help protect the optical fiber from corrosive attack by seawater or acoustic coupling fluids, to provide mechanical reinforcement and prevent abrasion, as well as mechanically fixing the fiber in the desired geometry. The combination of the encapsulation system and fiber coating should also help to minimize any effects of temperature and pressure changes on the sensitivity of the detection device. The basic problem is to provide a coating and encapsulation system that is chemically, mechanically, and acoustically compatible with the fiber-optic element, acoustic couplant fluids, and associated optical connectors. Before proceeding to a discussion of research areas that must be addressed, it is appropriate first to review briefly
Table I. Coating Resuirementsa processing consideration requirements
mater- appliial cator
(1)amenable t o on-draw application (a) applicable a t high rates ( b ) allow a range of coating thicknesses ( c ) dry, cure or solidify to a non-tacky finish ( 2 ) compatible with pre- and post-coating ( 3 ) afford complete surface coverage (4)smooth, longitudinally uniform, and concentric ( 5 ) abrasion resistant ( 6 ) flexible ( 7 ) environmentally durable (8) strippable (9) possess a low coefficient of friction (10) possess a softening point > subsequent processing temperatures (11)not degradable with time or temperature < subsequent processing temperatures (12) not degrade waveguide optical properties ( a ) as applied ( b ) with time or temperature ( c ) on bending o r twisting ( 1 3 ) not degrade waveguide strength (a) during application ( b ) long term ( 1 4 ) provide effective shielding against microdistortion producing forces (15) acoustic compatibility (16) chemical compatibility with associated acoustic couplant fluids and encapsulants ( 1 7 ) thermal isolation (18) resistant t o water permeation a
X X X X X X
X X X X X X
X
X X X
X X X X
X X
Adapted from R. A. Miller (1979).
the current state of coating technology.
Present State of Coatings Organic Coatings. Currently used organic coatings are normally strength-preserving or buffering coatings. That is, they act to prevent surface abrasion and/or microbending losses. The strength-preserving coatings may be single or multilayer coatings, while the buffering coatings have two or more components. Most of the materials used in organic coatings can be divided into the following types (Blyler and Hart, 1979). 1. UV Cured Materials. These materials are commonly epoxy or urethane acrylate materials. The UV cured acrylate systems appear promising as optical waveguide coatings (Schonhorn et al., 1976; Vazirani, 1979; Schlef et al., 1979; Quan and Lynn, 1980). The acrylate systems are versatile because they can be applied to fibers in very thick or relatively thin configurations in a single pass Quan and Lynn, 1980). They also allow easy modification of physical properties and can be cured rapidly. Recent research has been directed toward attaining lower cross-link density and higher chain flexibility in these
Ind. Eng. Chem. Prod. Res. Dev., Vol. 20, No. 4, 1981 803
Table 11. Materials for Organic Coatings" applied from material PYF,-TFE copolymer (KYNAR) acrylics cellulosics epoxies phenoxies phenolics poly amide-imides polyesters polyesterimides silicones urethanes epoxy acrylates urethane acrylates ethylene vinyl acetates fluoroplastics nylons poly olefins polyvinyls thermoplastic rubbers thermoplastic polyesters a
solu- 100% tion solids
X X X X X X X X X
X
X X
X X
X X
X X X X
X
X X X
Table 111. Physical Properties of Some Currently Used Oreanic Coating Materials parameter density, g/cm3 ultimate tensile strength, kg/cm' tensile modulus, kg/cmz flexural modulus, kg/ cm3 ultimate elongation, % shore hardness water absorption, % (ASTM D570) chemical resistance coefficient of linear thermal expansion, cm/(cm/"C) thermal conductivity, cal/(s cmz "C/cm)
Hytrel polyester"
Tefzelb
Sylgard 184c
1.25 400
1.70 455
1.05 63
-_ 5273
8400 14000
350 D7 2 0.3
200 D75 0.029
_._ ---__
excellent excellent 1.98 x 10-4 9 x 10-5
___
5.71
A3 5
-__
good
---
3.50
E. I. Du Pont (1973).
International Plastics (1977). International Plastics (1979).
Adapted from R. A. Miller (1979).
materials. The resultant lowering of the modulus and glass transition temperature provides increased protection against microbending losses. A recently reported acrylate, composite, coating system has shown excellent strength protection and performed well in environmental tests (Quan and Lynn, 1980). 2. Thermally Cured Materials (France et al., 1979; Kumura and Nakahara, 1980). This category is comprised primarily of silicones and polybutadienes. The Dow Sylgard series is used commerically. These are two-part, heat-curing resins. Silicones have a low modulus, low glass transition temperature, and a rapid curing rate. They also are easily amenable to in-line application with fiber drawing. Their abrasion resistance is not as great as that of other materials, and their resistance to water permeation is poor. 3. Hot-Melt Thermoplastic Elastomers (Blyler and Hart, 1979; T. J. Miller, 1979). In general, these exhibit good low-temperature performance but are somewhat limited in applicability at higher temperatures. Du Pont Hytrel polyester is currently used as a part of composite coatings. It has good thermal properties but relatively poor resistance to water permeability. The general classes of materials currently used for organic coatings are summarized in Table 11. Some of these materials, which are currently used commercially, are Kynar (a polyvinyledene-tetrafluoroethylene copolymer), Tefzel (a melt-processible thermoplastic fluoropolymer), Kevlar (a polyaramid commonly added to optical cable jackets as a strength reinforcing material), Teflon PFA, and ethylene-vinyl acetate. The composite coating on single-mode fibers currently used in optical hydrophone research at the Naval Research Laboratory consists of an inner coating of Dow Sylgard resin and an outer coating of Hytrel polyester. The physical properties of some currently used coating materials are summarized in Table 111. The process of applying the coating is an important factor in the selection of a coating material. The first layer of the coating should be applied in tandem with fiber drawing, in order to prevent chemisorption of water onto the fiber surface. I t is also important that mechanical abrasion be avoided during the coating process (to avoid generation of surface flaws that could lead to stress cor-
1
FIBER MOTION
Figure 1. Die application of coating.
rosion in the presence of water). Additionally, the coating should be concentric with the fiber, to avoid optical losses (R. A. Miller, 1979). The exact process of application will depend upon the coating material used. Solution coatings are normally applied either by die application or open-bath withdrawal. It seems intuitively obvious that a die-applied coating should be of a more uniform thickness than one applied from an open bath. This has indeed been shown to be the case (Homsy and Geyling, 1977). The technique of die application may involve the use of a wick, or it may require that the fiber pass through a hole or die somewhere in the coating apparatus. The use of wicks is less satisfactory than use of dies in terms of fiber abrasion and uniformity of fiber coatings. A simplified example of a die application is shown in Figure 1. The fiber passes through a nozzle attached to a fluid reservoir. Upon exiting the nozzle, the fiber is covered by a coating fluid, which later cures to a solid coating. The fiber may be centered in the die by a guide, or the die may be designed in such a way that fluid dynamic forces tend to center the fiber within the die. This requires that the die be in the form of a tapered nozzle. Discussions of centering forces within the die and mathematical modeling of fluid flow to predict centering forces within the die may be found in the literature (France et al., 1979; Torza, 1976; Blyler et al., 1979). Centering of the fiber
604
Ind. Eng. Chem. Prod. Res. Dw.. VoI. 20. NO. 4, 1981
MOLTEN POLYMER
I1ti\
POLYMER TUBE
WLYMER COATING
Figure 2. Pressure die for coating application. (Reproduced with permission from Blyler et al. (1979). Copyright 1979 Academic Press.)
within the coating is apparently desirable, since highly eccentric coatings have been associated with fiber weaknesses and microbending losses (R. A. Miller, 1979). Coatings may he applied to fibers from solid resins, by fluid bed application, or by electrostatic spray techniques. A more commonly used method is one derived from the wire and cable industry: the technique of melt-extrusion coating. In melt extrusion, a polymer is melted and forced through a die that has an opening shaped to produce the desired cross section. The type of die normally used for insulating wire, a pressure die, is shown in Figure 2. The pressure die is unsuitable for primary coating of optical fibers, because the core tube bore is only slightly larger than the fiber diameter in order to prevent molten polymer from leaking back past the fiber. This often results in the fiber being damaged by scraping against the core tube wall. Additionally, distorted coatings due to viscoelastic flow instabilities usually occur due to the generation of high shear rates when applying thin coatings to small-diameter fibers (Blyler et al., 1979). For this reason, the method is more useful for jacketing previously coated fibers. The tubing method of extusion, as illustrated in Figure 3, is more practical for in-line coating of freshly drawn fibers. The fiber is able to pass through the core tube without contact. The extended polymer coating is drawn down to form a thin, tight jacket on the fiber. Melt-extrusion coating requires that the polymer be capable of withstanding large extensional deformations without tearing. It is also important that particulates and other inhomogeneities be removed before the fiber coating is applied. It is most effective when coupled with the initial application of a thin primary coating. In this case, the principal function of the primary coating is the long-term protection of the glass surface from the effects of water. The extent of this protection will depend upon the degree of its interaction with the silica surface, as will be discussed shortly. Polymeric materials are generally unsuited for use as fiber coatings in cases where exposure to water is a service hazard. This is because they are poor water barriers. In general, three types of interactions are possible between these materials and water: swelling, chemical reaction (usually hydrolysis), and permeation of water through the polymer. It is possible (and usually probable) that all three of these will occur simultaneously. Swelling and chemical reactions degrade the polymer itself so that the material becomes unsuitable for use after a time, which may be
Figure 3. Tubing die for coating application. (Reproduced with permission from Blyler et al. (1979). Copyright 1979 Academic Press.)
much shorter than the intended service lifetime. Permeability is objectionable because of effects of moisture on the glass fiber. The permeation of most polymers by liquids takes place by a mechanism of activated diffusion (Lebovits, 1966a,h, 1968). This is a three-step process. The fist step involves absorption of the permeating molecule at ita ingoing surface and dissoluing of the molecule into the polymer. The rate of solution will be govemed by a vapor pressure of the permeant on the high-pressure side of the polymer barrier (Lebovits, 1966a,b, 1968). The second step is diffusion of the permeating molecule within the polymer membrane. Diffusion will occur both with and against the direction of flow but is greater in the direction of flow. Factors affecting the rate of diffusion are concentration gradient with the polymer, activation energy for the process, and temperature. The concentration gradient is responsible for diffusion primarily in the direction of flow, because the probability of a given molecule moving will be proportional to the concentration at the site of the molecule (Lebovits, 1966a,b, 1968). The activation energy is that energy needed to separate the polymer chains and permit passage of the migrating molecule. The percentage of molecules with necessary energy is determined by the temperature. Desorption of the migrating molecule at the outgoing surface of the polymer membrane is the third and final step in this process. The rate of desorption will be determined by the vapor pressure at the outgoing surface, which also influences the rate of permeation by determining the concentration gradient within the polymer membrane. The activated diffusion mechanism explains the experimental observation that hydrostatic pressure has little effect on the rate of permeation for polymers (Lebovita, 1968). The force exerted by the hydrostatic pressure is too small to cause movement of molecules through the polymer. The probability of the molecule having sufficient energy to form a hole between polymer chains determines whether it will move at all. The concentration gradient determines whether the molecule will move in the right direction. This, in turn, is determined by the vaporpressure differential on the two sides of the membrane. A detailed thermodynamic argument has been given by Lebovits (1968) to show that the vapor pressure, and not
Ind. Eng. Chem. Prod. Res. Dev., Vol. 20, No. 4, 1981 605
the hydrostatic pressure, determines concentration of the permeant within the polymer. Additional complications, due to such factors as permeants that swell the membrane, second-order transition points in the polymer, or two membranes in series, are treated by Lebovits (1966a). The rate of permeation through the polymer barrier will be influenced by various aspects of the material. These include the chemical composition of the polymer, chemical similarities between the polymer and permeant, degree of cross-linking, crystallinity of the polymer, degree of plasticization by a foreign plasticizer (or the permeant itself'), and sometimes by the previous history of the polymer (Lebovits, 1966a;Felder and Huvard, 1980). For a polymer to possess good barrier properties, the molecular structure must be such that it hinders the diffusion process, and the polymer should not possess chain structures or functional groups chemically similar to the penetrant molecule. The interaction occurring between the silica surface and water after penetration of the polymer barrier is important to the service life of the fiber. Generally, simple silicate glasses are viewed as consisting of a continuous network containing directed silicon-oxygen bonds with structural discontinuties due to broken silicon-oxygen bonds. Any water trapped in the glass melt will be incorporated into the structure in the form of silanol groups, SiOH. Positive metal ions, such as sodium and calcium, are normally located adjacent to the discontinuities in order to achieve overall electrical neutrality. Hydrolysis of the silicon-oxygen bonds is involved in the nucleation and development of cracks in high-strength glasses that have absorbed moisture. As has been shown for glasses with microcracks (Kalish et al., 1978; Bershtein et al., 1973), activation and acceleration of the hydrolysis will take place under tensile stresses. Proceeding by analogy with mechanisms of reaction of organic substances, Budd (1961) devised a scheme to explain the reactions of silicate glasses with various reagents, including water and alkaline solutions. The scheme of hydrolysis of silicon-oxygen linkage was considered within the framework of electrophilic and nucleophilic mechanisms. Electrophilic reagents (those with a deficiency of electrons, such as H30+) were considered to attack at negatively charged nonbridging oxygen atoms. Nucleophilic attack (reagents with an excess of electrons) was viewed as taking place at network silicon atoms. Since water ionizes to form H30+and OH, it is capable of forming both electrophilic and nucleophilic reagents in small quantities. The neutral water molecule itself may act as a weak nucleophile and attack the acidic silica surface. This leads to the breaking of silicon-oxygen bonds and the formation of microcracks or crack growth in flaws already present. There are several ways in which the fiber may be protected from failure due to water. These include the use of hermetic coatings, the use of thick glass claddings that have a slow penetration rate by water and strengthening of fibers by compressive stuffing (Krohn and Cooper, 1969; Mohr et al., 1979), and modifying the chemistry of the interface between the coating and glass fiber. Poor wetting, or incomplete contact, between the glass fiber and an organic coating will cause void formation at the interface between them. Normally, the rate at which water proceeds along this interface will be much faster than the movement of water through the organic material (Laird and Nelson, 1964). This is because the large amount of void volume present at the interface will lead to convective water movement. The interactions and stability of silico-organic interfaces and the effect of environmental in-
teractions are discussed by Blyler et al. (1979). Silane coupling agents have been successfully used to promote the adhesion of organic polymers to glass (Vanderbilt and Clayton, 1965; Sterman and Marsden, 1966; Cassidy and Yager, 1971; Pleudemann, 1972; Gent and Hsu, 1974). These coupling agents afford the possibility of modifying the interfacial area between the coating and the fiber in order to prevent the reaction of water with the interface. Silane coupling agents are ambifunctional reagents of the form R,SiR4-z, which are capable of reaction with both the organic polymer and the mineral substrate (glass). The coupling mechanism of these reagents at the silicateorgqnic interface is complex but appears to be reasonably well understood (Vanderbilt and Clayton, 1965; Sterman and Marsden, 1966; Cassidy and Yager, 1971; Pleudemann, 1972; Gent and HSU, 1974; Chang, 1976). The process apparently involves more than a simple promotion of adhesion between the glass surface and the organic polymer due to the effect of silanes on wetting of the glass surface. Labile groups in the silane coupling agent hydrolyze in the presence of water to form silanol groups that can condense with OH groups on the surface of the glass to form siloxane bonds. The other functional groups on the silane coupling agent are usually chosen to be compatible or chemically reactive with the organic substrate. Although the evidence is not unequivocal, the results of various spectroscopic investigations (Gent and Hsu, 1974; Koelling and Kolb, 1965; Kaas and Kardos, 1971; Bascom, 1972; Koening and Shih, 1971) indicate that covalent bonding takes place between the glass and coupling agent and between the coupling agent and organic polymer. Pleudemann (1972) has discussed the requirements for, and advantages of, the use of silanes in bonding thermoplastic polymers to mineral surfaces. Organic resins will form a strong, water-resistant bond to a mineral surface if the resin is modified with a silanizing agent to present a silanol surface to the mineral substrate and if the morphology of the interface allows equilibrium bonding of silanols with the surface (Pleudemann, 1972). There are several possible choices by which a suitable silane coupling agent might be applied to protect the glass surface. The silane might first be applied and reacted with the glass surface prior to application of the organic resin. Alternately, it might be an integral part of the resin and migrate to the interface. The alternate approach was chosen in the initial work on silane agents in fiber coatings. A UV-curable epoxy acrylate was used for the coating; it was reasonably successful in preserving fiber strength and providing environmental stability (Schonhorn et al., 1976; Vazirani, 1979). The prepolymer was polymerized by UV radiation after application, using the isobutyl ether derivative of benzoin as a sensitizer. Recent work (Gent and Hsu, 1974) indicates that silanol groups condense on a silica surface to yield Si-0-Si bonds. If the nonhydrolyzable group of the silane coupling agent contains a reactive group, such as a vinyl, then this might be copolymerized with functional groups in the organic resin. This would lead to a bond that should provide good water resistance. Due to the increased degree of bonding between the fiber and coating, it might also be possible to use such coatings to enhance the sensitivity of optoacoustic detection devices. Inorganic Coatings. The only materials to date shown to be capable of providing complete water protection for optical fibers are the inorganic coatings. The earliest reported work in this area was done by Pinnow et al. (1977)
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of Hughes Labs. They used in-line dip coating of fibers with metals such as aluminum. These coatings were not completely satisfactory for a number of reasons. The complex refractive index of a metal coating sometimes led to excessive optical losses if the cladding was insufficiently thick. The metal coatings also exhibited plastic flow, resulting in dynamic fatigue and stress discontinuities in the jacket. This led to microbending losses and premature fiber failure. Later research emphasized the use of different materials. Vapor permeability of water is one of the more important criteria for selection of a hermetic coating material. Metals, various inorganic nitrides, and other substances with low water permeabilities are potential candidates for hermetic coatings. Work currently in progress at Galileo Electro-Optical Corporation (GEOC), under the auspices of the Rome Air Development Center (Gulf and Western, 1980),is focusing upon the use of composite coatings using a base coating of a dielectric, a metal jacketing, and a polymeric overcoat for abrasion protection. The material being used as a dielectric is an ion-deposited carbon film applied under vacuum. The deposition technique involves the use of ion milling to permit the selection formation of stronger carbon-to-carbon bonds (Gulf and Western, 1980; Spencer et al., 1976). The ion-deposition technique is employed to coat optical fibers in-line after drawing. X-ray analyses (Aisenberg and Chabot, 1971; Spencer et al., 1976) indicate that the film possesses a microcrystalline diamond-like structure. In general, the film tends to be polycrystalline with a very tight lattice. Appendix A of the GEOC report (Gulf and Western, 1980) contains a summary of the general characteristics and properties of this material. The metal initially chosen for use in this program was indium. It was selected because it has no allotropic forms in the intended temperature range of application and is sufficiently ductible to allow repeated stress cycling without memory. It also wets glass very well. The strength of its adhesion to glass appears to be greater than the cohesive strength of indium itself (Gulf and Western, 1980). Its primary disadvantage is its softness. For this reason, a UV-curablepolymeric coating is applied after the hermetic coating. This material is a fluoroacrylate, which has been successfully used by GEOC on bare silica fibers. In addition to providing abrasion protection to the indium layer, the organic overcoat is intended to prevent chemical reactions of the metal layer when the fiber is immersed in water. Present results indicate that the ion deposition method can be used to successfully produce thin films on glass fibers. The remainder of the program will involve optical and mechanical testing of fiber performance, as well as characterizations of flaw distributions, fiber diameter, coating eccentricity, and optical attenuation (Gulf and Western, 1980). Evaluation of the ability of the coating to provide increased static-fatigue protection will also be conducted. Another material that has found use as a hermetic coating for optical fibers is silicon nitride (AuCoin et al., 1978). The technique initially used was a chemical vapor that proceeded according to the reaction deposition (0) 4NH3 + 3SiH4
600 O C
Si3N4+ 12H2
The deposition was carried out in-line with fiber drawing. Other techniques for formation of CVD Si3N4involve the vapor-phase interaction of ammonia with silicon halides (Madiyanski and Cooke, 1973) or silane derivatives (Delong et al., 1972). The laser-induced, gas-phase photochemical
reaction of SiF4with NH3 or SiH4with NH3 in a controlled reaction chamber (Merrit, 1979) has also been used to coat optical fibers. The thickness of the coating is proportional to the flow rate of the reactants through the chamber. Silicon nitride would appear to be a good material for use in fiber-optic hermetic coatings, since it is not readily subject to oxidation or chemical corrosion. It is also impervious to moisture, gases, and chemically corrosive environments. It has a relatively low coefficient of thermal expansion (AuCoin et al., 1978; Hirai et al., 1978), which closely matches that of the glass fiber. This results in good thermal shock resistance for the coated fiber. Additionally, the physical and mechanical properties of this substance have been rather extensively investigated (Hirai et al., 1978; Bauer, 1977; Evans and Daridge, 1970; Tanzilli et al., 1977; 1979). The material normally deposited during the thermally activated CVD of silicon nitride is the a-form of this material (Tanzilli et al., 1979). This is actually a silicon oxynitride that may contain varying proportions of oxygen, depending upon the deposition conditions (Tanzilli et al. 1979; Hewlett-Packard, 1980). Work currently in progress at Hewlett-Packard is investigating the use of CVD silicon nitride as a hermetic fiber-optic coating (Hewlett-Packard, 1980), together with a more conventional jacketing for abrasion protection. Factors such as the influence of the NH3/SiH4ratio and absolute flow rate through the reaction chamber upon silicon oxynitride thickness and composition and upon fiber strength and hermeticity are being examined. ongoing the future research will involve measurements of optical absorption in the silicon oxynitride fibers, static fatigue tests, and comparisons of the resistance to cyclic fatigue of silicon oxynitride-coated fibers to conventionally coated fibers. Directions for F u t u r e Research The highly specialized applications of optical fibers, such as in underwater cables and hydrophones, will require further development and evaluation of materials (Sigel, 1976; Wilkins, 1977; Giallorenzi, 1978). In the case of a fiber-optic hydrophone, an evaluation must be made of the influence of such factors as hydrostatic pressure changes, temperature variations, and mechanical vibrations on hydrophone performance and life characteristics. In order to optimize these, it is necessary that materials used for the fiber core, cladding, coating, potting, and pressure seals be chosen judiciously. In the case of a fiber-optic hydrophone, the materials and interfaces of concern are the fiber/fiber coating and the coating/encapsulant/coupling fluid/water. It is reasonable to assume that any coupling fluid used in a normal service environment will be saturated with water. Problem areas that should be considered are: (i) permeabilities of the encapsulant and fiber coating for coupling fluids and water; (ii) byproducts of the curing process for encapsulants; (iii) any potential reactions of coatings or encapsulants, with water or coupling fluids; (iv) effects of chemical interactions on the opto-acoustic and mechanical properties of fibers; and (v) mechanical compatibility of the encapsulation system with the fiber-optic element and connectors including enhanced sensitivity and undesirable effects of temperature and hydrostatic pressure. Experimental evaluations should be conducted on the ability of coating and encapsulation systems to protect an optical fiber and to minimize degradation of performance. Characterization of the effects of pressure and temperature on fibers and fiber configurations should be made. This would involve the fabrication and testing of model elements incorporating the variables of fiber types, coating
Ind. Eng. Chem. Prod. Res. Dev., Vol. 20, No. 4, 1981 607
and encapsulant candidates, and bend radius. Additonally, different mandrel materials to be used for winding fiber coils would need to be evaluated. Questions of long-term reliability and compatibility of coating materials and encapsulants with currently used acoustic couplant fluids should be addressed. This would involve measurements of the permeabilities of encapsulant materials and fiber coatings for water and coupling fluids (Lebovits, 1966a,b, 1968). Another parameter that should be investigated is the Young’s moduli rate-of-change for the coating or encapsulant materials as a function of immersion times and temperatures in water and fill fluids. This could be done through the use of a Nolle-type string apparatus with temperature control (Madigosky and Lee, 1979). Alternatively, a torsional-braid apparatus might provide useful shear modulus information (Gilham, 1981). Young’s modulus is an appropriate parameter to measure, as this quantity appears to strongly affect the enhancement of fiber sensitivity due to jacketing materials (Hughes and Jarzynski, 1980). Initial measurements might involve two commonly used jacketing materials that have different water permeabilities and different hydrolytic stabilities (such as Hytrel and Tefzel). I t should be possible to mathematically model the change in acoustic sensitivity of the fiber with changes in the elastic modulus of the coating and to correlate this with any experimentally observed changes in sensitivity for fibers that have been immersed in fill fluids or water for an identical length of time. In the ideal case, no change at all should occur. These problems will be addressed under a research program currently in progress at NRL. The problem of glass aging and long-term reliability of optical fibers might be approached through a study of the chemistry of the interface between the optical fiber and its coating. This would involve studying the effects of silanization on acoustic sensitivity and aging of fibers. One area of difficulty with this is that much of the research in industry on silanizing glass fibers has not been published. The available results of previous work on glass-silane interactions would be used as a starting point. Appropriate reagents to silanize and coat optical fibers would be used to determine the effect on acoustic sensitivity of silanizing reagents or glass fibers. Model systems to determine the degree of chemical bonding between various silanizing agents and glasses could be investigated spectroscopically. Alternative coating materials, such as the silicon nitride and ion-deposited carbon-coated fibers discussed earlier, should be investigated for use in fiber-optic hydrophones. To the author’s knowledge, this has yet to be done. Fibers coated with the materials would need to be evaluated for acoustic compatibility and other factors, as specified earlier in Table I. Other areas that should be investigated are: (i) measurement of the strength and fatigue characteristics of various fiber and coating combinations under a variety of exposure conditions (Gulati et al., 1975); (ii) fractographic analysis of failed fibers (Mecholsky et al., 1977a,b) to investigate the fracture of optical fibers under various loading conditions. (Chemical surface analysis (ESCA/ Auger) might also be useful in providing information on alteration of fiber characteristics due to production processes or post production processes.) (iii) development of accelerated-lifetesting methods and reliability and lifetime predictions on components and Systems for opto-acoustic detection. These determinations would be common to various fiberlcoating systems. The properties of each new fiber and coating combination will most likely be different and must
be evaluated before any selection of the optimal combination can be made. Acknowledgment The author is grateful to the Acoustic Division’s Physical Acoustics Branch of NRL for financial support of this work and to Dr. George Siege1 of the Optical Sciences Division of NRL, Mr. David Lefebre of Maxlight Optical Waveguides, Inc., Mr. Frederic Quan of Corning Glass Works, and Mr. Frederic Weber of Texas Research Institute for useful discussions. Literature Cited Aisenberg, S.; Chabot. R. J. Appl. Phys., 1971, 42, 7. Akers, F. I. “High N.A. Single Mode Fiber”; Intematlonai Telephone and Telegraph Co.-Electro-Optical Products Division Final Tech. Report undet Contract N00173-784-0196 for Office of Naval Research, Sept 15, 1978 to Mar 15, 1979. Akers, F. I. “Study of the Effects of Bending and Microbending on Glass Fibers”; InternationalTelephone and Telegraph Co.-Electro-Optical Roducts Division Final Tech. Report under Contract N00014-78-GO852 for Office of Naval Research, Oct 1, 1978 to Nov 30, 1979. AuCoin, T. R.; DiVita. S.; Wade, M. J. U.S.Patent No. 4118271, Oct 3, 1978. Bartenev, G. M. Izv. Akad. Nauk, SSSR. Otd. Tekh. Nauk 1955, 9 , 53. Bartenev, G. M.; Yudina, I.V.; Rehbinder. P. A. KollOM. Zh. 1958, 20, 655. Bascom, W. D. Macromoiecules 1972, 5 , 792. Bauer, J. Phys. Stat. Sol. 1977, A39, 411. Bendow, 6.; Mttra, S. S., Ed.; ”Fiber Optics“; Plenum Press: New York, 1979. Bershtein, L. A.; Movchan, Y. N.; Nikitin, V. V. Sov. Phys. SolMState 1973, 14, 2422. Blyler, L. L., Jr. Prepr. Am. Chem. SOC. Dlv. Org. Cast. Plast. Chem. 1980, 42, 246. Blyier, L. L., Jr.; Elchenbaum, B. R.; Schonhorn, H. I n “Optical Fiber Telecommunications”; Miiier, S. E.; Chyonoweth. A. G., Ed.; Academic Press: New York, 1979; pp 299-342. Blyier, L. L., Jr.; Hart, A. C., Jr. Am. Chem. SOC. Dlv. Org. Coat. &st. Chem. 1979, 40, 87. Bucaro, J. A.; Carome, E. F. Appi. Opt. 1978, 17, 330. Bucaro, J. A.; Dardy, H. D.; Carome, J. A. J. Acoust. Soc. Am. 1977, 62, 1302. Bucaro, J. A.; Hickman, T. R. Appl. Opt. 1979, 18, 938. Budd, S. M. Phys. Chem. Glasses 1961, 2 , 111. Carome, J. A.; Dardy, H. D.; Carome, E. F. Appl. Opt. 1977, 16, 761. Cassidy, P. E.; Yager, B. J. J. Macromol. Sci., Rev. Polym. Techno/. 1971. 1, 1. Chang, S. Master’s Thesis, University of Texas at Austin, Aug 1976. Charles, R. J. J. Appl. Phys. 1958a, 29, 1549. Charles, R. J. J. Appi. Phys. 1958b, 1554. Charles, R. J. J. Appl. Phys. 1958c, 1657. Charles, R. J.; Hiiiig, W. B. In “Symposium on Mechanical Strength of Glass and Ways of Improving It”, Fiuorence, Italy, Sept 25-29, 1961; Unkn Sclentifique Continentale du Verre, Charieroi, Belgium, 1962; pp 511-527. Cieio, P. G. Appl. Opt. 1979, 18, 2933. Cole, J. H.; Johnson, R. L.; Bhuta, P. G. J. Acoust. SOC. Am. 1977. 62, 1136. Cox, S. M. Phys. Chem. Glasses 1989, 10, 226. Cuishaw, 6.; Davies, D. E. N.; Kingsiey, S. A. Electron Lett. 1977, 13, 760. Davis, C. M.; Elnzig, R. E.; Bucaro, J. A.; Gaiiorenzi, T. 0. “Fiber Optic Sensor System (FOSS) Technology Assessment (Unclassified Version)”; Dynamic Systems, Inc. Robert DSI-TR-80-01, Jan 1, 1980. Delong, D. J.; Ozias, A. E.; Benzing, W. C. “Operating Conditions for the Use of Dichlorosilaneto Depostt Silicon Nitride Films”; Electrochemical Soclety-Spring Meeting: Washington, D.C., May 7-1 1, 1972. Du Pont, E. I. “DuPont Tefzel Fluoropolymer Design Handbook”; E. I.Du Pont Technical Brochure, 1973. Evans, A. G.; Daridge, R. W. J. Mater. Sci. 1970, 5 , 314. Evans, A. G.; Wiederhorn, S. M. Int. J . Fract. Mech. 1974, 10, 379. Felder, R. M.; Huvard, G. S. In “Methods of Experimental Physics”; Fava, R. A., Ed.; Academic Press: New York, 1981; Vol. 16. Part C, Chapter 17. France, D. W.; Dunn, P. L.; Reeve, M. H. Fiber Integr. Opt. 1979, 2 , 267. Freiman, S. W. J. Am. Ceram. SOC.1974, 57, 350. Frieman, S . W. J. Am. Ceram. SOC. 1975, 58, 339. Freiman, S. W.; Weiderhorn, S. M.; Simmons, C. J.; Fuiier, E. R., Jr.; Krish ran, P. N.; Wolrofen, G. E.; Sanders, D. M. “Stress Corrosion of Ceramic Materials”; Tech. Report for Office of Naval Research Contract NO0014 79F-0030, Dec 1979. Freudenthal, A. M.;Gumbel, E. J. P r m . R. SOC.London, Ser. A 1953, 261, 309. Gardner, W. B. Bell Syst. Tech. J . 1975, 54, 457. Gent, A. N.; Hsu, E. C. Macromolecules 1974, 7 , 933. Gerberich, W. W.; Stout, M. J. Am. Ceram. SOC.1978, 58, 222. Giailorenzi, T. G. Proc. I€€€ 1978, 66, 744. Gibbs, P.; Cutler, I.B. J. Am. Ceram. SOC. 1951, 34. 200. Gilham, J. K. Prepr. Am. Chem. SOC.Div. Org. Coat. f i s t . Chem. 1980, 44, 503. Gloge, D. BeilSyst. Tech. J. 1975, 54, 275. Griffith, A. A. PhiEos. Trans. Roy. SOC. London, 1920, A221, 163. Gulati, S . T.; Justice, 6.; Snowden, W. E.; Jenkins, D. M. “Water Integrity of Optical Fibers”; Final Report for Naval Electronics Laboratory Center un-
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Received for review April 17, 1981 Accepted July 13,1981
