| Summary: | 碩士 === 國立東華大學 === 材料科學與工程學系 === 106 === This study was carried out in 3 series experiments. In the first series, thick 4-layered ZrB2/TiB2 multilayer was deposited on Si substrate by sputtering TiB2 and ZrB2 ceramic targets in different bias so as to choose a suitable bias voltage for rest of the experiments. Results from this series experiment showed at -90 V substrate bias, both TiB2 and ZrB2 layers gained preferred orientation with highly crystalline diffraction peaks. Ion bombardment as a result of -90 volt substrate bias induced appropriate amount of defects and increased the compressive stress of the film which resulted in hardness enhancement and shorter crack length upon indentation fracture.
In the second series, using the -90V substrate bias as seen best from 1st series, multilayers of ZrB2/TiB2 with bilayer thickness varying from 200 nm to 25 nm were synthesized. Results from this series experiment showed hardness increment, as the bilayer thickness decreased the crystallinity of individual layers decreased due to insufficient crystal growth time in thinner layers. The hardness increased from 31 GPa to 36.6 GPa as the bilayer thickness varied from 200 nm to 25 nm. All the films showed enhanced toughness when compared to similar thickness single layer TiB2 or ZrB2. As long as the films were crystalline, increase in number of bilayers caused increase in residual stress most probably due to interface induced stress. Cross sectional FESEM images reveals clear intact layers. However, by this deposition technique, bilayer thickness lower than 25 nm was not possible to synthesize.
Finally, third series refers to deposition of ZrB2/TiB2 multilayers with bilayer thickness varying from 19 nm to 1 nm. Films with bilayer thickness 19 nm to 7 nm were very less crystalline or amorphous. However, for samples with bilayer thickness 4 nm to 2 nm, the crystallinity enhanced with the peaks lying between TiB2 001 and ZrB2 001. These samples also possess additional unique peaks at angles lower than ZrB2 001 peak. The diffraction peak of sample at 1 nm bilayer thickness corresponds to ZrB2 001. Samples from this series were the toughest with the highest Kc value of 2.6 MPa m½ for 2 nm bilayer sample. The maximum hardness reached upto 44 GPa for this same sample which was harder than single layer ZrB2 or TiB2.
|
|---|
