Fretting damage of TiN coated zircaloy-4 tube

Fretting has been reported and investigated for over 50 years. However, it is still one of the modern plagues for several industrial machineries. Especially, fretting of fuel rod cladding material, zircaloy-4 tube, in pressurized water reactor (PWR) must be reduced and avoided. Thin hard coatings are employed to improve the tribological properties such as friction and wear of conventional engineering materials. Among these coatings, physical vapor deposition (PVD) TiN coating is probably one of the most frequently and successfully used PVD coatings for the mitigation of fretting wear. Therefore, in this study a fretting wear experiment was performed using TiN coated zircaloy-4 tube as the fuel rod cladding material and uncoated zircaloy-4 tube as one of the grids. The fretting tester was designed and manufactured for this experiment. The number of cycles, slip amplitude and normal load were selected as main factors of fretting. The type of contact was cylinder-to-cylinder contact. The worn surface was observed by optical microscope, 3-D surface measuring instrument and scanning electron microscope (SEM). The results of this research showed that the wear volume of TiN coated zircaloy-4 tube decreased about 1.2–3 times more than uncoated tube and wear mechanisms were brittle fracture, fatigue fracture, adhesion, abrasion and oxidation.