Piezospectroscopic measurements on alumina-epoxy

composites

Amanda Wright, Christopher Gonzalez, Ashley Jones and Seetha Raghavan�

Mechanical, Materials and Aerospace Engineering, University of Central Florida, Orlando, Florida 32816

Alumina-epoxy composites, under laser excitation, are investigated in this work to es-tablish shifts of spectral peak positions with stress known as the piezospectroscopic e�ect.The measurements reveal the well-known R-lines, seen in polycrystalline alumina, are dis-tinct features in the spectral emission of these composites as well. Results indicate thatthe volume fraction of alumina in the composite has an e�ect on the intensity and peakpositions of the R-lines, revealing the potential for these spectral characteristics to yield in-formation on material characteristics and response. Based on these results, con�gurationsof embedded alumina nanoparticles can be established for testing and future developmentas stress sensitive adhesives and coatings in typical aircraft structural con�gurations.

I. Introduction

The spectral properties of luminescent materials such as chromium-doped alumina have been used inearly applications to monitor stresses in diamond anvil cells1 and later to measure stresses and therefore

predict life in the thermally grown oxide (TGO) of thermal barrier coatings used on gas turbine engineblades.2,3 The non-destructive technique used, known as piezospectroscopy (PS), measures stress-inducedshifts of the well-known R-line peaks present in the emission spectrum of chromium-doped alumina. Thesecharacteristic R-lines occur when laser excitation of the material causes the Cr3+ ions within �-alumina totransition from an excited state back to ground state, and can easily be identi�ed as two sharp and closely-spaced peaks. Recent work has extended the capabilities of the photo-stimulated luminescence spectroscopy(PSLS) technique through measurement and deconvolution of the less-studied vibronic band region alongwith the R-lines in the emission spectra of polycrystalline alumina.4 In this study, we develop and applythis technique to �-alumina �lled epoxy composites with the aim of determining appropriate con�gurationsof polymer-based adhesives and coatings that can be used for stress-sensing applications.

Figure 1. Potential applications of alumina particles forstress sensing applications using piezospectroscopy

Epoxy-based �lm adhesives are currently imple-mented for the joining of aluminum and polymericcomposite parts in the aerospace industry. Theyserve to alleviate stress concentrations associatedwith fastener holes, reduce damage to the compositelaminates due to machining di�culties, and reduceweight from metallic fasteners. There is a signi�-cant need to have a means of assessing the integrityof these materials through stress measurements. Byembedding luminescent particles in the adhesives,PSLS has the potential to address this need. It hasbeen shown that the addition of ‘modi�ers’ such as�-alumina to these epoxy resins not only increasesthe adhesion strength, but also provides a positivee�ect on the toughness and the peel strength of thematerial.5 These modi�ers are typically on the nanoscale range and must be homogeneously distributed intothe epoxies in order to obtain accurate stress measurements within the resin. While several studies focus

�Assistant Professor, AIAA Senior Member, sraghava@mail.ucf.edu

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on the e�ect of alumina particles on the strength of the matrix material, very little work has been done todevelop the potential of these particles to provide stress information through piezospectroscopy. In one otherknown study, Muraki et al.,6 devised a system in which polymeric materials were �lled with alumina particlesfor thermal stress measurement using PSLS in microelectronic devices. The results of the work showed theability of peakshifts from the alumina �ller particles to indicate the thermal residual stresses in the polymermatrix. The goal of the present study is to investigate the spectral peaks of alumina-�lled epoxy compositeswith the aim of establishing a method of non-invasive integrity assessment or stress-sensing adhesives. Themethod, once established, can be applied to studies of adhesive stress distribution under load and to typicalaircraft structural con�gurations as shown schematically in Fig. 1.

II. Piezospectroscopy

Figure 2. Stress e�ects cause the R1 and R2 spec-tral peaks to shift. These shifts have been calibratedfor polycrystalline alumina to provide the piezospectro-scopic coe�cients.

The piezospectroscopic e�ect relates frequencyshifts of the R-lines due to the applied stress asshown in Fig. 2. The relationship between stressand an observable shift in spectral peaks, known asthe piezospectroscopic (PS) e�ect, was introducedby Grabner7 and is governed by the following equa-tion:

�� = �ij�ij (1)

where �� is the frequency shift, �ij representsthe PS coe�cients and �ij is the stress tensor.

The shift of the characteristic R1 and R2 lines of �-alumina depend on the crystallographic axes and thePS coe�cients along each direction are di�erent. Byneglecting the o�-diagonal components of �ij ,8 thePS coe�cients can be expressed using the followingequations:

��(R1) = 2:56�11 + 3:50�22 + 1:53�33 (2)

��(R2) = 2:66�11 + 2:80�22 + 2:16�33 (3)

According to Ma and Clarke,9 when uniform stress is applied to polycrystalline alumina, the peak shiftsshown in equations 2 and 3 can be reduced by using the hydrostatic pressure dependence of peak shifts forthe R1 and R2 lines. The reduction of equations 2 and 3 is expressed by the following equation:

�� = �ave�uni (4)

where �� is the frequency shift, �ave represents the averaged PS coe�cient which is 2.53 for R1 and 2.54for R2.

III. Experimental Procedure

For the fabrication of the alumina-epoxy samples, �-alumina nanopowder with an average particle sizeof 150nm and 99.85% purity was the �ller material used. The epoxy resin and curing agent used was Epon862 (Bisphenol-F type) and Epikure-W, respectively. Three di�erent alumina �lled composites were created,having volume fractions of 5%, 25% and 43% �ller material. It has been shown that 43% �-alumina is themaximum amount that can be conveniently added to an epoxy, without reducing the overall mechanicalproperties of the epoxy itself.10 The calculated ratios of �-alumina nanoparticles, epoxy resin and curingagent for each specimen was measured, and initially mixed using a high shear mixer for a duration of �fteenminutes, in order to remove any agglomerates if present. To further ensure homogeneity, the mixture wasthen placed in a sonicator for approximately twenty minutes, which also served to cause entrapped air bubblesto surface. Next, a low-pressure vacuum system was utilized to completely remove remaining air bubblespresent, the duration of this step lasting approximately fourty-�ve minutes, or until all air bubbles were no

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longer visible. The material was then collected and transferred to an aluminum mold of dimensions 10in x6in x 3.5in, which was prepared in advance with Partall Hi-Temp Wax, a mold release. The specimens thenunderwent a two-step curing process of 6 hours at 54�C and then 16 hours at 93�C, and were allowed to coolin the oven upon completion.

A Renishaw Raman spectrometer was used to obtain PSLS readings. This system consists of a spec-trometer, a microscope and a laser at an excitation wavelength of 532 nm. The laser beam was focused onthe sample and the photoluminescence data was captured at 0.05 second intervals. The complete spectra ofthe alumina-epoxy samples were taken. This information was received via the backscattering collected bythe attached charged couple device (CCD). Multiple data readings were collected at di�erent locations onthe sample to be averaged for analysis. The results were then analyzed using a deconvolution and �ttingmethodology11 to establish the spectral peak positions.

IV. Results & Discussion

Deconvolution of the data allowed for the analysis of the spectral characteristics from the photolumines-cence readings taken from the samples at each volume fraction. The data points in the �gures shown areaverages from each peak as several collections of data were recorded to ensure accuracy. In the �rst twoplots shown in Fig. 3, the depiction of the linear relationship between peak position and volume fractionis shown for peaks R1 and R2, respectively. Since the shifts in the peak positions relate to stress in thealumina, the results show that the percentage of alumina is directly proportional to the residual stress ineach sample. As the percentage of alumina increases, the peak position shifts more to the right indicating achange in the stress of the �ller particles in the probed area. It is expected that the residual stress in thealumina-epoxy composite will be concentrated in the alumina �ller particles since the Young’s modulus ofthe �ller is much larger than the matrix resin. The results indicate that the increase in volume fraction ofalumina may alleviate the stress taken by the particles, reducing this stress over the probed region.

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Figure 3. R1 and R2 peak positions shift as volume fraction increases

By conducting calibration load experiments in which the spectral characteristics were monitored, Muraki’s6

study of alumina �lled polymeric matrix revealed the PS coe�cients were almost twice the value for thesematerials as compared to monolithic alumina. The coe�cients established by Muraki are speci�c to thecon�guration of the specimen tested, with regards to the volume fraction of alumina and the properties ofthe matrix material. A calibration for alumina-epoxy specimens of varying volume fraction under load willbe carried out to detail the e�ects of volume fraction on the PS coe�cient. In addition, matrix materialsof varying elastic modulus can be tested to establish the e�ect of the properties of the matrix on the PScoe�cient. Such experiments are aimed at establishing the appropriate calibration data for stress sensitivecoatings and adhesives. In performing the calibration, it is critical that methods to ensure accuracy of volumefraction are e�ective during the manufacturing and this volume fraction is veri�ed after the manufacturingprocess. In the experiments conducted, the average of multiple readings over the manufactured sample weretaken to address these issues. It is expected that the challenge in achieving homogenous samples duringmanufacture will lead to deviations caused by the variation of concentration of alumina �ller in the epoxy

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Figure 4. The area under the R1 and R2 curves increase as volume fraction increases

as well as the inhomogeneity of the alumina �ller itself. Further imaging and analysis will be conducted toestablish the dispersion and absence of agglomeration in the specimens.

Another important variable that was analyzed was the area under the curve. This variable representsthe integrated intensity or energy of the emission. The plots were made for each peak (R1 and R2) foreach volume fraction. The data represents average values collected from the deconvoluted data and thenplotted to show a trend. A display of the linear relationship between areas vs. volume fraction can beseen for peaks R1 and R2 in Fig 4, respectively. As shown, the area increases as the volume percentageincreases. We can deduce that this is due to the higher energy levels emitted by the higher volumes ofalumina particles. This measure when calibrated and recorded for each volume fraction can also be used toidentify the composition of alumina in an epoxy matrix. Another important application for this result is thatit can be used as a quality control to establish the homogenous distribution of particles within the sample asissues of agglomeration, discussed earlier, are of signi�cance in the manufacturing of these composites. Thisdata is useful in the further study of relationships between stress and the spectral response of the aluminaparticles.

V. Conclusions

Alumina epoxy composites of varying volume fraction were manufactured and studied using PSLS re-vealing signi�cant spectral R-lines in this work. A comparison of the peak positions for each volume fractionshows a linear shift indicating that the residual stress in the particles are a function of the volume fractionof alumina nanoparticles. This data when calibrated can be used for the development of stress sensitive ad-hesives and coatings for the study of stress distributions in these materials under load as well as for integrityassessment. As anticipated, the integrated intensity or areas under the curve increased for each sample ofincreasing volume fraction. This information can be used as a quality control method to ensure accuratedispersion and identify issues of agglomeration in the material. Future studies will focus on the calibrationof the spectral peaks with stress for varying volume fraction samples. In addition, studies on alumina �lledepoxy coatings and adhesives on a metallic/composite substrate will be performed to assess the resolutionof the method for stress sensing applications.

VI. Acknowledgements

We would like to thank Dr. Raj Vaidyanathan, Dr. Nina Orlovskaya and Dr Gou Jihua for use of facilitiesand equipment, as well as Christopher Penny for assisting with the fabrication of the alumina-�lled epoxycomposites. This research was funded by the UCF ORC Inhouse Research Grant.

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