Evaluation of Diamond-Like Carbon as a Protective Coating for Holographic Filters
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major disadvantages of the holographic filters is that they are highly hygroscopic. The absorbed moisture can broaden the bandwidth and shift the rejection frequency [2]. DLC is an optically transparent, amorphous hydrocarbon film. It has been reported to be a good moisture barrier for optical materials [3]. Our earlier study of the film showed that it is very good as an abrasion and chemical resistant coating for polymeric substrates [4]. We intend to make use of the DLC barrier properties to provide a hermetic seal to protect the holographic filters. It is difficult to measure the moisture absorption through the DLC film. Thus we attempted to monitor the effect of the coating process and how well DLC acted as a moisture barrier by determining the changes in the rejection bandwidth of the filters using UV/visible spectroscopy (UVNis). The results of the UVIVis spectra of the filters as-received, after coating, and after environmental testing, will be discussed. The hardness measurement, Raman spectra of DLC coating on polycarbonate (PC) as well as the scanning electron micrographs (SEM) of the DLC coating will also be reported. EXPERIMENTAL a) Materials In this experiment, three sets of six holographic filters were provided to us by Kaiser Optical System Inc.(KOSI). Two types of photosensitive materials were used; a water soluble biopolymer known as dichromated gelatin (DCG), and HRF-700 photopolymer, a synthetic, water insoluble, acrylic polymer from Du Pont [2]. The holographic filters were applied to glass 887
Mat. Res. Soc. Symp. Proc. Vol. 356 * 1995 Materials Research Society
substrates instead of PC to ensure that moisture absorption would only occur from the coated side. One set of the photopolymer samples was protected with a siloxane hardcoat applied by a dip coating process that was determined by KOSI to be compatible with their photopolymer material and similar to the undercoat used by Diamonex before coating their PC. The holographic filters were shipped and stored in a controlled low humidity environment at all times except during coating and analyses. Five samples from each set were sent to Diamonex Inc. for DLC coating and one was kept as a control. b) Characterization The samples were inspected visually as received, after DLC coating, and environmental testing. Optical microscopy and SEM were used to examine the DLC coated and environmentally exposed samples. The UV/Vis spectral analysis was carried out using two
Perkin Elmer UV/Vis
spectrometers the Lambda 2, and the Lambda 9 models. The analyses were obtained for samples before and after the DLC coating process, as well as after the environmental testing. The hardness of the DLC coating was measured on the PC substrates using a Nano Instruments Inc. Nano IndenterTM II. This was to avoid the 'thin-ice' effect associated with a hardness measurements made on thin films deposited on rubbery substrate. The hardness of the DLC thin film and the PC substrate with and without siloxane hardcoat were recorded at three different depths: 20, 40 and 80
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