Stress Evolution in Thin film Niti Shape Memory Alloys

Abstract

ABSTRACTAs a potential micro-actuator material thin film shape memory alloys (SMAs) have drawn attention because of their capability of generating strains of a few percent reversibly. In contrast to SMAs the output strain of traditional actuator material, such as quartz and PZT ferroelectrics, is limited to 100 to 1000 ppm. The authors have investigated the transformation properties and microstructures of thin Ni50Ti50 films on Si. From application point of view, the evaluation of the actuator property should come from direct measurements of the strain output of the film/substrate composite. In the film/substrate composite, the martensitic transformation strains in the SMA thin film are coupled to elastically to the substrate through interface load transfer. Interface properties thus play crucial role on the efficiency of the composite actuators. Since the capability of interface load transfer is determined by the substrate stiffness, experiments were designed to measure the strain output of NiTi/Si composite with varying substrate stiffness. The experiments indicated that the output efficiency of the martensitic transformation strain increases by a factor of 2 when the film/substrate thickness ratio increases from 1/345 to 1/70. The result is favorable for micro-actuator application since much larger film/substrate ratios can be achieved by micro-machining. The absolute value of the transformation strain is about 2 percent which can be improved if the film texture is controlled.Another problem related to the nature of the film/substrate composite is the thermal stress arising from the difference of the thermal expansivities of the film and substrate. The as deposited films were amorphous with a growth tensile stress ranging from 0.6 GPa to 1 GPa, increasing with the substrate stiffness. The initial recrystallization of the amorphous NiS0Ti,0 film results in an increase of the tensile stress by 80 MPa; however upon completion of the recrystallization process the tensile stress was relaxed. Young’s moduli of the recrystallized austenitic NiTi films were calculated from the thermal stresses arising in the composite samples. It was found that the moduli apparently depend on the substrate thickness. This effect is tentatively interpreted to indicate a sensitivity of the mechanical properties of the film on the grain structure of the recrystallized state which in turn might depend on the constraint due to the substrate. TEM studies will clarify the situation.It is concluded that the interface stress restrained the efficiency of the martensitic transformation. Since the NiTi film has a high (interface) yield stress, above 1 GPa, it is proposed that the actuation efficiency of the SMA can be improved by reducing the substrate stiffness while maintaining a high load transfer efficiency. If Si is used as a substrate because of its high thermal diffusivity, the actuator design must weight the mechanical actuator output against its speed.