Stress corrosion cracking of NiTi in artificial saliva.
OBJECTIVE
This paper aimed to study the mechanism of the cracking of orthodontic NiTi wire.


METHODS
Two orthodontic NiTi wires were subjected: (1) optical and scanning electron microscopy (SEM) to observe the fracture surface; (2) energy dispersive X-ray spectroscopy to determine the composition of the surface product; (3) anodic polarization to remove the surface product. Samples of NiTi alloy were subjected to the constant loading test to study the stress corrosion cracking (SCC) behavior of NiTi shape memory alloy in artificial saliva.


RESULTS
The results showed that there were three typical areas at the fracture surface of NiTi orthodontic wire. Area '1' was a tool-made notch. Crack initiated from the root of this notch and propagated to form Area '2', which was perpendicular to the wire axis and covered by surface film. This film consisted of Na, K, Cl, P, S and O except Ni and Ti. The cracking process of NiTi alloy under the constant loading test depended on the pH of saliva and applied stress. The crack length was about 262microm, the longest at 300MPa and pH 3.0.


CONCLUSIONS
A tool-made notch in orthodontic NiTi wires can cause SCC. At high stress and low pH, this NiTi alloy was most sensitive to cracking.
Wang
Jianqiu
J
Environmental Corrosion Center, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China. jiqwang@imr.ac.cn
Li
Nianxing
N
Rao
Guangbin
G
Han
En-Hou
EH
Ke
Wei
W
D002713:Chlorine / Q000737:chemistry; D003343:Corrosion; D003722:Dental Alloys / Q000737:chemistry*; D004577:Electron Probe Microanalysis; D004868:Equipment Failure; D006801:Humans; D006863:Hydrogen-Ion Concentration; D008422:Materials Testing; D008855:Microscopy, Electron, Scanning; D009532:Nickel / Q000737:chemistry*; D009969:Orthodontic Wires; D010100:Oxygen / Q000737:chemistry; D010758:Phosphorus / Q000737:chemistry; D011188:Potassium / Q000737:chemistry; D012464:Saliva, Artificial / Q000737:chemistry*; D012964:Sodium / Q000737:chemistry; D013314:Stress, Mechanical; D013455:Sulfur / Q000737:chemistry; D013499:Surface Properties; D014025:Titanium / Q000737:chemistry*
D003722:Dental Alloys; D012464:Saliva, Artificial; C040654:titanium nickelide; D010758:Phosphorus; D002713:Chlorine; D013455:Sulfur; D009532:Nickel; D012964:Sodium; D014025:Titanium; D011188:Potassium; D010100:Oxygen