Raman spectroscopy in coatings research and analysis: Part II. Practical applications

JCT CoatingsTech, Sept, 2005 by Neil J. Everall

Perhaps surprisingly, Raman spectra can be very sensitive to crystal size. (68) The sensitivity is highest for nanophase materials, where small changes in crystal diameter give a large change in the surface/volume ratio. As an example, Werninghaus and colleagues applied micro-Raman spectroscopy to characterize cubic boron-nitride (c-BN) thin films deposited onto Si(100). (69) Prior measurements on c-BN crystals of known size established the relationship between position and asymmetry of the ~1260 [cm.sup.-1] Raman band, and the approximate crystal size. The average crystallite size in the film was ~1.3 nm (although strictly speaking, as the authors pointed out, this is the average distance between lattice defects, rather than a true crystal size). The authors were also able to map damage and stress at the silicon surface by measuring the frequency and shape of the silicon phonon near 520 [cm.sup.-1], and related this to the state of the c-BN overlayer.

Raman spectroscopy is ideal for following phase transitions as a function of temperature. Djaoued et al. deposited brookite-rich titania films onto glass substrates by a sol-gel process, and then monitored the phase composition and crystal size as the temperature was ramped up to 900[degrees]C. (70) At temperatures below 600[degrees]C, nanophase brookite dominated. Anatase was the dominant phase between 700-800[degrees]C, with significant crystal growth occurring in this regime, while at 900[degrees]C the system adopted the pure rutile phase. These changes were irreversible. Surface coatings on biomedical implants are amenable to macro and micro Raman characterization, as exemplified by the study of the hydroxyapatite (HA) coatings which are added to improve the biocompatibility of titanium implants. Darimont et al. used confocal Raman microscopy to map the phase structure in plasma-sprayed implants as one moves from the "bulk" coating towards the titanium substrate. (71) They identified two phases--HA, and [beta]-tricalcium phosphate (TCP)--in the coating, with the TCP/HA ratio increasing towards the metal surface. This was attributed to differential cooling rates, with fast cooling at the metal favoring the TCP structure. Because the TCP is acid-soluble, it is bio-active and can be reabsorbed (an undesirable effect since it weakens the joint). Hence, Raman microscopy allows one to adjust the plasma-deposition to produce a thick-enough layer of HA to prevent reabsorption of the underlying TCP. Raman measurements were ideal for this study since they can differentiate these materials quite easily compared with X-ray diffraction (XRD) or FTIR, and the spatial resolution of confocal Raman is well-matched to the morphology of the system.

We have already discussed how Raman bandshifts can be used to measure stress in organic materials, but the same is true of inorganics. Portinha et al. measured residual stresses in thermal barrier coatings (Zr[O.sub.2] doped with [Y.sub.2][O.sub.3]) on iron, using Raman and XRD. (72) They analyzed samples after spraying, thermal-shock at 1000[degrees]C, and annealing at 1100[degrees]C. The authors mapped the stress distribution through the coat thickness using the shift in position of the 640 [cm.sup.-1] zirconia band. Prior calibration showed a linear relationship between band shift and applied stress, with a gradient of 220 MPa per [cm.sup.-1]. (73) They found a compressive stress at the zirconia/iron interface, and a tensile stress at the air surface. The compressive stresses increased after annealing.