| Summary: | 碩士 === 國立臺灣科技大學 === 機械工程系 === 89 === Diamond grits were brazed onto a steel substrate using a pre-alloyed Cu-10Sn-15Ti (wt pct.) brazing alloy, operated at temperatures ranging from 900℃ to 1050℃ with isothermal holds from 5 to 300 minutes in a vacuum (<10-5 torr) environment. The Sn-Ti intermetallic compounds of high melting points, including SnTi3, SnTi2, and Sn5Ti6, were formed during brazing while compounds of lower melting points, including CuSn and CuTi developed during cooling. The reaction of Ti with diamond yielded a continuous TiC layer on the surfaces of the diamond grits, and there was a thin layer rich in Ti and Sn formed when the brazing operation was carried out at temperatures lower than 1000℃. When increased the brazing temperature to 1050℃, the solubility of Ti in the liquid phase increased and this resulted in a formation of abundant CuTi intermetallic compound during cooling. The thickness of this Ti and Sn segregation layer increased linearly with the square root of the isothermal holding time, indicating a diffusion-controlled process was operative. The activation energy of the growth of Sn and Ti segregation layer is about 335.7 kJ/mol.
On structural analyses of the interfacial reaction layer, the heteroepitaxial nanocrystalline TiC films were successfully nucleated and grew on a single crystalline diamond substrate with the Cu-10Sn-15Ti alloy. The thickness of the TiC layer ranged from 500 to 600nm, and no interlayers of amorphous carbon or other transitional phases were observed at the diamond-TiC interface. The lattice volumes of the TiC unit cell and carbon content in the TiC phase were decreased as the distance between the TiC-diamond interface and the detecting position increased. Linear expansion of TiC lattice parameter along [100] direction was also decreased with a similar fashion as the lattice volume and carbon content of the TiC crystal were changed in the case that a TiC film was grown on diamond (100) substrate. Besides, the interfacial stress arising fron the lattice mismatch resulted in the ‘five-to-six’ registry occurring at the interfaces. The constrained lattice of a heteroepitaxial TiC film was tilted and twisted along the [110] axis, which developed on the diamond (110) substrate. The nucleation mechanism of the TiC grains was determined as a type of the ‘intrinsic heterogeneous nucleation’. The TiC crystal grew to form a columnar grain structure with a uniform morphology and the grain size ranging from 40 to 60nm. The TiC grains were similarly developed in a spiral growth along the [110] axis on the diamond (110) plane.
The corrosive behavior and the associated characteristics of the brazed specimens were investigated by immersing the samples in HNO3, HCl, and KOH solutions, respectively. Result indicated that only the Cu phase was removed from the surface of the brazing alloy, and the corrosion passivity of all the Sn-Ti intermetallic compounds was comparatively better than that of the Cu phase in all specimens. A mean particle size of the Sn-Ti intermetallic compounds was determined to range from 2 to 8μm in a powdered form. Judging from the corrosion rate and the corrosion potential of the specimens, which were immersed in the solutions, indicates that the KOH is a powerful environmental solution to severely corrode the surface and the bonding interface of the Cu-based alloy. A pitting corrosion phenomenon occurred apparently as immersing the specimens in HCl and KOH solutions.
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