Indentation and laser annealing of NiTi shape memory alloys
Su, Jun Feng
Date of Issue2005
School of Mechanical and Aerospace Engineering
A shape memory alloy (SMA) is able to memorize and recover its original shape, after it has been deformed, by heating it over its transformation temperature. The recoverable strain can be as much as around 7% and actuation stress up to 500 MPa in NiTi SMAs. Because of this unique thermomechanical characteristic, SMAs, in particular, NiTi, have been used in a variety of applications, from medical equipment to Micro-Electro-Mechanical Systems (MEMS). This thesis includes two independent parts: one is on small scale shape memory behavior of NiTi using instrumented indentation technique; the other is about laser annealing of NiTi thin films for MEMS applications. For a better design of SMA based MEMS, a good understanding of small-scale mechanical properties and shape memory behavior of SMAs is necessary. At present, this is still under investigation. In this thesis, the microscopic shape memory effect and microindentation behavior of a NiTi alloy are studied by instrumented indentation technique and temperature controllable atomic force microscopy/3-D surface interferometer. Two-way shape memory effect (TWSME) induced by indentation is investigated at micro-scale. The TWSME produced by indentation may provide an alternative as a new actuation mechanism in micro actuators. A schematic explanation of the pile-up/sink-in phenomenon in SMAs upon indentation is presented. Some interesting findings include: 1) all indentations are pile-up type after thermal cycling; 2) the size of indentation is seemingly independent on the indentation temperature after one thermal cycle, but almost linearly proportional to the indentation load. CO2 laser annealing of NiTi thin films from Si wafer side has been developed in our group by He (2004). The films are locally re-crystallized and with shape memory function. However, this technique is not repeatable every time, so that the reliability is a problem to be solved. Previous theoretical investigation in He (2004) is not fruitful. In this thesis, a further study on annealing temperature is presented. First, laser energy absorption ratio is determined and the exact thickness of Si wafer is found to be a dominant factor. Then the temperature distribution in the laser annealing process is investigated by finite element analysis. It confirms that CO2 laser can effectively anneal NiTi films if the laser beam impinges from backside of Si wafer, and the high temperature zone is highly localized, almost within a circle with a radius of the laser beam spot diameter. The effective annealing zone is about the spot size of laser beam, which agrees well with previous experimental observation. Furthermore, effects of oxidation and interfacial layer are discussed. In addition, a theoretical analysis is carried out on direct laser annealing of NiTi thin films by different lasers.
Nanyang Technological University