Inelastic electron tunneling spectroscopy, or lETS, provides an extremely sensitive method for monitoring the chemical and physical state of a molecular substance adsorbed onto an oxide surface. Inelastic tunneling data directly reflect the molecular vibrational frequencies of the first monolayer of adsorbed molecules and changes in the vibrational spectrum can be correlated with changes in the chemical state of the molecule/oxide interface. We have carried out lETS experiments on the components of the commercial adhesive, Hercules 3501. This epoxy system consists ·of two molecular components; diamino diphenyl sulfone (DPS) and tetraglycidycl 4,4\u27 diamino diphenyl methane (DPM). lETS spectra of the individual components and of the epoxy mixture adsorbed on aluminum oxide have been obtained and the vibrational modes and frequencies assigned by comparison with computer calculations and existing infrared optical spectra. Some evidence for an aging effect has been observed for the adsorbed DPS. This effect appears as a dramatic change in the low frequency vibrational modes and may be associated with the formation of hydrogen bonds or the polymerization of the DPS. Further studies of this effect are in progress. The effects of water permeation may be studied using D2O as a tracer. The vibrational modes of D20 are easily distinguished from those of water which may be present as a contaminant. If the exchange reaciton D2O + HCR → DHO + DCR occurs, it would be easily detected in the lETS spectrum. Initial experiments performed by simply immersing the tunnel junction into liquid D2O for several hours were unsuccessful because severe corrosion of the tunnel junction resulted. Experiments employing aluminum/aluminum oxide/adhesive/gold thin film junction for the study of H2O permeation are in progress. Further studies are planned to monitor the effects of heat treatment on the adhesive components and mixture

Godwin, L. M.

White, H. W.

Wolfram, T.

English

Inelastic electron tunneling spectroscopy, or lETS, provides an extremely sensitive method for monitoring the chemical and physical state of a molecular substance adsorbed onto an oxide surface. Inelastic tunneling data directly reflect the molecular vibrational frequencies of the first monolayer of adsorbed molecules and changes in the vibrational spectrum can be correlated with changes in the chemical state of the molecule/oxide interface. We have carried out lETS experiments on the components of the commercial adhesive, Hercules 3501. This epoxy system consists ·of two molecular components; diamino diphenyl sulfone (DPS) and tetraglycidycl 4,4' diamino diphenyl methane (DPM). lETS spectra of the individual components and of the epoxy mixture adsorbed on aluminum oxide have been obtained and the vibrational modes and frequencies assigned by comparison with computer calculations and existing infrared optical spectra. Some evidence for an aging effect has been observed for the adsorbed DPS. This effect appears as a dramatic change in the low frequency vibrational modes and may be associated with the formation of hydrogen bonds or the polymerization of the DPS. Further studies of this effect are in progress. The effects of water permeation may be studied using D2O as a tracer. The vibrational modes of D20 are easily distinguished from those of water which may be present as a contaminant. If the exchange reaciton D2O + HCR → DHO + DCR occurs, it would be easily detected in the lETS spectrum. Initial experiments performed by simply immersing the tunnel junction into liquid D2O for several hours were unsuccessful because severe corrosion of the tunnel junction resulted. Experiments employing aluminum/aluminum oxide/adhesive/gold thin film junction for the study of H2O permeation are in progress. Further studies are planned to monitor the effects of heat treatment on the adhesive components and mixture.</p

White, H

Godwin, L

Wolfram, T

Digital Repository @ Iowa State University (ISU)

MICROSCOPIC FEATURES OF ADHESIVE BONDS FOR NON-DESTRUCTIVE MEASUREMENTS H. W. White, L. M. Godwin, and T. Wolfram University of Missouri Columbia, Missouri 65201 ABSTRACT Inelastic electron tunneling spectroscopy, or lETS, provides an extremely sensitive method for monitoring the chemical and physical state of a molecular substance adsorbed onto an oxide surface. Inelastic tunneling data directly reflect the molecular vibrational frequencies of the first monolayer of adsorbed molecules and changes in the vibrational spectrum can be correlated with changes in the chemical state of the molecule/oxide interface. We have carried out lETS experiments on the components of the commercial adhesive, Hercules 3501. This eooxy system consists ·of two molecular components; diamino diphenyl sulfone (DPS) and tetraglycidycl 4,4' diamino diphen¥1 methane (DPM). lETS spectra of the individual components and of the epoxy mixture adsorbed on olwninum oxide have been obtained and the vibrational modes and frequencies assigned by comparison with computer calculations and existing infrared optical spectra. Some evidence for an aging effect has been observed for the adsorbed DPS. This effect appears as a dramatic change in the low frequency vibrational modes and may be associated with the formation of hydrogen bonds or the polymerization of the DPS. Further studies of this effect are in progress. The effects of water permeation may be studied using D20 as a tracer. The vibrational modes of D20 are easily distinguished from those of water which may be present as a contaminant. If the exchange reaciton D20 + HCR ~ DHO + OCR occurs, it would be easily detected in the lETS spectrum. Initial experiments performed by simply immersing the tunnel junction into liquid D20 for severl hours were unsuccessful because severe corrosion of the tunnel junction resulted. Experiments employing aluminum/aluminum oxide/adhesive/gold thin film junction for the stu4Y of H20 oermeation are in progress. Further studies are planned to monitor the effects of heat treatment on the adhesive components and mixture. Introduction The objective of this program is to assess the feasibility of utilizing Ineslastic Electron Tunneling Spectroscopy,or lETS, to monitor the chemical state of an adhesive/adherend interface of the type encountered in the adhesive bonding of aluminum components. The goal of the project is to determine what changes occur at an adhesive bondline during thermal curing and in bond degrada-t,on resulting from hydrothermal aging. Through such studies we hope to determine some of the important mechanisms of adhesive bond failure. lETS provides an extremely sensitive .ethod for monitoring the chemical and physical state of a molecular substance adsorbed onto an oxide surface. Ineslastic tunneling data directly reflect the molecular vibrational frequencies of the first monolayer of adsorbed molecules. Changes in the vibrational spectrum can be correlated with changes in the chemical state of the molecule/oxide inter-face. Since aluminum and other metals used in adhesive bonds have an oxide covering, the mole-cule/oxide interface geometry in inel1stic tunnel junctions closely approximates that found in real adhesive systems. The method of lETS is not directly applicable as an "on-line" filE technique but specific information concerning ~ mechanisms of bonding and bond degradation is needed in order to devise more meaningful NDE procedures. We have carried out lETS experi.ents on the components of the high performance cORmercial adhesive, Hercules 3501. This epoxy system con-sists of two molecular components; diamino diphenyl sulfone (DPS) and tetraglycidyl 4,4' diamino diphenyl methane (DPM). 186 lETS spectra of the individual components and of the epoxy mixture adsorbed on aluminum oxide have been obtained and the vibrational modes and frequencies assigned by comparison with com-puter calculations and existing infrared optical spectra. Some evidence for an aging effect has been observed for the adsorbed DPS. This effect appears as a dramatic change in low frequency vibrational modes and may be associated with the formation of hydrogen bonding of the NH2 groups to the oxide layer. The effects of water permeation are being studied. D20 is being used as a tag molecule since it has vibrational modes which are easily distin-guished from those of H20. Using a new and exciting technique called external doping, molecules of H20 and D20 can be introduced into or removed from an already fabricated tunnel junction. Experimental Results Figure 1 shows a schematic of a typical adhesive bond between two pieces of aluminum metal. The adhesive is in contact with the aluminum oxide rather than the metal since an oxide quickly forms an aluminum when it is exposed to air or cleaned with etching solutions prior to bonding. In commercial applications the oxide thickness is typically 100 to 200 angstroms. There is a fantastic need for information regarding the micro-scopic interface between the adhesive and the oxide. It is difficult, however, to obtain the information using conventional methods. Ultrasonic techniques give critically needed information about the bulk properties of the adhesive but give little information about the interface region. The wave-lengths in ultrasonic studies are considerably longer than the several angstroms associated with the thickness of the interface layer. ADHESIVE BOND Figure 1. Schematic of an adhesive bond between two pieces of aluminum. The chemical nature of the interface is very important to the integrity and service life of a bond. The dashed line in Fig. 1 shows a typical fracture line. Most of the fracture line surface is not at the interface; however, it often happens that the fracture line oriqinates at the interface region. Black and Blomquist 1 have studied several metal adhesive systems and find that when they are exposed to a heat treatment the shear strength decreases significantly, often by 10re than a factor of two. They concluded that t he primary reason for this decrease in shear strength is due to catalytic action at the bondline. Hydrothermal aging is believed to be one of the most important causes for bond failure in service. Studies have been done on the bulk properties in order to determine how water which permeates a bond contributes to its degradation. The role of the water at the interface is con-sidered to be very important~ but to date ~re have been no good means of studying its effect. The fact that an adhesive interface coasists of a molecular layer bonded to an oxide suggests the possibility of inelastic electron tunneling spectroscopy as a useful tool, Our programs in both lETS and adhesives are relatively new ~nd, as a consequence, most of the information pre-sented is preliminary. The high performance Hercules 3501 epoxy system, distributed by Hercu 1 es, Inc . 2 , was chosen for study during the third year. The two molecules which form the epoxy system are shown in Fig . 2. The upper one is tetraglycidycl 4,4' diamino diphenyl methane (OPH) . It has four identical arms, each with an epoxy ring. The other molecule shown is di~ino diphenyl sulfone (DPS) which is the curative for the DPM epoxy molecule. The molecules are not planar. The atoms in the DPS molecule lie in two planes whose intersection is a line in the plane defined by the two oxygens and the sulphur. The DPH molecules are even more 3-dimensional . They form dimers relatively easily even at room temperature. Spectra must therefore be interpreted with an awareness of the fact that one or more of the anns may be joined to other molecules. 187 Figure 2. Schematics of the two molecules in the Hercules 3501 epoxy system. The cross-link reaction for the DPM and DPS molecules is shown in Fig. 3. In the cross-l ink molecule there is an OH group on each arm. The arms in the cross-linked structure link with other arms in a somewhat disorganized fashion. The result is a very 3-dimensional structure. The methane group and rings are incorporated into the matrix. Crosslink Rea:::t1on Figure 3. The cross-link reaction for the Hercules 3501 epoxy system. Bonding to the alUMinum oxide probably occurs through the OH group, in much the same way in which phenol bonds to an alUQrtnum oxide fil~. namely, as C6HsO-. 3 The OH groups make this molecule very polar and thereby susceptible to water perme-ation. The presence of water on the substrate could adversely affect the bonding properties. Aluminum/aluminum oxide/dopant/lead tunnel junctions were fabricated, where dopant refers to the molecules on which lETS spectra are to be obtained. The aluminu- electrode was first evaporated at lo-6 Torr. A glow discharge was then used to form the oxide layer. A liquid doping technique was used to deposit approximately one monolayer of tne molecules on the oxide. The process involves making a dilute solution of the molecular species to be studied, placing a drop of the solution on the electrode, and spinning off the excess. A lead counter electrode was then evaporated over the molecules. The OPH resin was not easy to dissolve. Methyl ethyl ketone (mekol) and tetrahydrofuran (THF) worked best. These two solvents, and to a lesser extent chloroform, dissolved the OPS curative. Figure 4 is a block diagram of the spectro-meter system. The junction is cooled to 4.2K. A DC bias across the j unction is slowly swept from 50 to 500 millivolts. An AC signal of 1000 Hz and approximately 1 millivolt (rms) is applied continuously. The amplitude of the second harmonic signal as generated by the tunnel junction Is recorded on an XY recorder as a function of the DC bias. The second harmonic signal is proportional to: d2V dl2 Peaks in this signal locate the energies of the vibrational modes as read from the bias voltage (energy) scale. lETS SPECTROtviET E R Figure 4. A block diagram of the inelastic elec-tron tunneling spectrometer. Spectra were first obtained on each of the 10lecules, then for the two molecules placed together on the same junction. fhis work is to be fullowed by studying the effects of heat treatment and hydrothermal aging on the observed spectra. Figure 5 shows two curves , A and B, taken on a junction doped with DPS using mekol as a solvent. The energy range is from about 50 to 500 mllli-electron volts (meV), which corresponds approxl tmately to the wavenumber range 400 to 400 em-The OH stretch peak near 450 meV is observed in most lETS spectra. The CH stretch near 355 meV is found in all hydrocarbon spectra . The lower energy peaks for the DPS molecule were assigned by comparison with IR, other lETS data, and with normal mode frequencies calculated using literature values for the force cons tents. Both atomic motions and frequenctes were obtained from the normal mode calculations. If an observed peak was more than 3 meV from a calculated vibrational energy a question mark was placed after its assignment label in Fig. 5. The measured locations, in meV, of the peaks are shown following the group assign-.ent. The s-o stretch at 134 meY agree well with calculated vibrational energies and other data. The peak at 116 meV could be either a NH bend or an oxide phonon. Many doped junctions and nearly all undoped junctions show the presence of an oxide phonon at 117; however, the calculated vibrational energy for the NH bend is 115 meV. The peak at 102 meV was tentati.ely identified as an S=O bend since this group had a calculated frequency near 9B meV. The assignment at 52 .eV was made in the same manner except that the S=O motion had a calculated frequency of 48 meV. 188 0 Figure 5. Two spectra taken on a junction doped with the curative DPS. Until now the features discussed have been the same for both spectra A and B. In the energy range 55 to 75 meV the spectra are different. Spectrum A was taken on the junction soon after preparation. Spectrum B was taken nine days later, during which time the junction had been stored in a clean, dry atmosphere. The junction resist-ance remained constant at 1500 ohms. Spectrum A shows a small peak at 72 ~v which is within 2 meV of the value of the normal mode energy calculated for an NH2 stretch in DPS. Spectrum B shows no evidence for a peak at 72 meV but shows a new distinct peak at 62 meV. An aging effect has occurred. There is very little evidence in the literature of any polymerization occurring inside of a tunnel junction, but there is evidence for chemisorption. We speculate that the NH2 groups on the ends of the DPS molecule have chemisorbed to the oxide layer. Figure 6 shows the spectra of four separate junctions. The lower curve is the spectrum for a junction doped with only the solvent THF. It serves as a background spectrum. Both THF and mekol spectra show a small peak near 174 meV which is undoubtedly due to CH bend modes and a small peak near 117 meV which is probably due to an oxide phonon. The second curve from the bottom, labeled "DPM, THF" is for the DPM epo)IY molecule dissolved in the THF. Again, there is a large CH stretch peak located at 355 meV. The third curve labeled "DPM, meko 1" shows essen t ia 11 y the same features as the second. The peak at 177 meV is probably a CH bend. The peak at 168 meV is probably due to NH2 stretch modes. CHz rock modes are expected to be observed near 116 meV; however, the peak at that location could be due, at least in part, to an oxide phonon expected near 117 meV. Jv d? 0 .1 Figure 6. Spectra taken on junctions doped with the epoxy DPM. The curve labeled "DPN (D)" is a spectrum for DPM which had been deuterated by an exchange reaction before placing in the junction. One of the goals of this project is to study the role of water at the interface of an adhesive bond. Water can be diffused into a junction and its presence can be monitored by using 020 as tag molecules. The "DPM(D)" spectra was taken to provide information on the size and location of peaks assocfated with a deuterated DPM molecule. The spectrum has peaks not observed in the other spectra. We must be cautious in our interpre-tation of this spectrum since some structural changes may have occurred during the deuteration process. 189 At the First International Conference on Inelastic Electron Tunneling SpectroscosY held at the University of Missouri-Columbia uring May 1977, Jeklevic and Gaerttner 4 reported the observation of lETS spectra of molecules which had been introduced into completely fabricated tunnel junctions. Curve A in Fig. 7 shows a clean junction made in the presence of H20 vapor. Note the OH stretch at 450 meV. Curve B is the spectrum for a clean junction exposed to 020 vapor after fabrication. The peak at 327 meV is an OD stretch mode indicating that 020 penetrated into the junction. This new technique of external doping appears to be a very promising technique for studying the interaction of molecules with surface~ Of particular interest to us is the diffusion of H20 and DzO into and out of tunnel junctions under the proper experimental conditions. A N > u -N-u B 0 eV Figure 7. Curve A shows a clean junction made in the presence of H20 vapor. Curve B is the spectrum for a clean junction ex-posed to 020 vapor after fabrication. From R. C. Jaklevic and M. R. Gaerttner. Figure 8 shows the spectrum for a junction containing both DPS and DPM molecules. The features are similar to those in spectrum A of the DPS molecule. One reason for this similarity may oe due to the fact that the s1gnature for the DPS molecule was always much stronger than that for the DPM molecule. Obtaining good noise-free junctions was considerably more difficult for the DPM molecule than for the smaller DPS. We are currently trying to improve spectral resolution and do some heat treatment and aging studies on the combined system.  0 DPM+DPS mekol Q5 eV Figure B. A spectrum on a junctiun containin~ both epoxy (DPM) and curative (OPS) molecules. Our current activities are as follows: (1) Improve spectral quality. A signal averaging system is needed to eliminate the effects of low frequency noise. The resolution in our spectra could be enhanced considerably. (2) Investigate possible surface bonding mechanisms. The character of the bonding mec~anisms for adhesive systems is not well known but is critical to an understanding of adhesive systems. (3) Effects of heat treatment and hydrothermal aging. The recent advances reported in the exter-nal doping technique are being tried. Conclusions lETS (inelastic electron tunneling spectro-scopy) is not directly applicable as a practical NDE technique and it is not trivial to apply lETS to the determination of adhesive-interface properties. However, at the present time, it is the~ method available which allows an in situ study-of adhesive/oxide interface bonding. The feasibility of applying lETS to study changes in the molecular properties of an adhesive adsorbed on oxide due to time, teMperature and permeating water has been demonstrated. lETS offers a technique which, in the very near future, will become the most important (if not the only) method for determining the inter-face physics and chemistry of adhesive bondlines. Information obtained from lETS studies will add greatly to the understanding of non-mechanical adhesive bond failure. Such knowledge should help in devising appropriate NOE measurements. Initial studies using lETS should be applied to adhesives of simple molecular structure before proceeding to more complex syste.s. Acknowledgements The authors are grateful to C. Holmes and P. Corbin for their contributions to the design and fabrication of experimental ipparatus. This research was sponsored by the Center for Advanced NDE operated by the Science Center, Rockwell International, for the Advanced Research Projects Agency and the Air Force Materials Lab-oratory under contract F33615-74-C-5180. References 1. J. H. Black, R. F. Blomquist, Ind. Eng. Chern., 50, 918 (195B). 2. Hercules, Inc., P.O. Box 9B, Magna, Utah, 84044. 3. B. F. Lewis, W. H. Bowser, J. L. Horn, Jr., T. Luu, and W. H. Weinberg, J. Voc. Sci. Technol., 1J., 262 (1974). 4. R. C. Jaklevic and H. R. Gaerttner, First International Conference on lnelastictlectron Tunneling Spectroscopy, University of Missouri, Missouri 65201, May 25-27, 1977. 190 

Microscopic Features of Adhesive Bonds for Non-Destructive Measurements

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