Thesis Type: Doctorate
Institution Of The Thesis: Gazi Üniversitesi, Fen Bilimleri Enstitüsü, Turkey
Approval Date: 2014
Student: TUNCAY ŞİMŞEK
Supervisor: ADNAN AKKURTAbstract:
In this study, ZrB2 nanoparticles that has high melting point and high hardness of transition metal borides, were synthesized via mechanochemical method. In the experiments as a zirconium source, ZrO2 and elemental Zr, as a boron source B2O3 and elemental Mg were used as a reductant. The effect of different ball-to powder ratio and rotating speed on the microstructures of sythesized nanoparticles and reaction completion time with planetary ball milling were investigated. Nanoparticles leached with acetic acid were characterized with XRD, SEM, TEM, VSM, BET and helium pycnometer. Afterwards synthesized ZrB2 nanoparticles were coated on the surface of S355JR carbon steel and AA7075 aluminum alloys by pre-placed laser cladding method with CO2 laser under N2 atmosphere. For the precoating process ZrB2 nanoparticles were mixed with phenolic resin and heat treated under vacuum to make a strong bonding to substrate. Laser experiments were conducted with different laser power, different feed rate and effect of parameters on the coating microstructure, phase content, hardness and wear properties were investigated. When laser coated surfaces were examined, the hardest coating layers including crack and cavity were obtained at 75 W and 125 W laser power, despite that the decline of coating layers hardness, crack free and homogen coating layers were obtained experiments conducted at 160 W and 200 W laser power on the surface of S355JR carbon steel. However there were microcrack and porosity on the surface of aluminum alloy because of having high melting difference between ceramic nanoparticles and aluminum alloy. According to wear and micro scratch tests results, coated layers wear strength had been improved and coating layers had a good metallurgical strentgh to substrate. After that coated layers were cut by important advanced cutting methods such as abrasive water jut, wire erosion, laser and abrasive cutting disk then the effect of these methods on the layers were investigated. The results of this study showed that the abrasive cutting methods are the best method.