Doctor of Philosophy
School of Mechanical, Materials and Mechatronic
Yang, Limei, Cavitation erosion resistance of NiTi thin films produced by Filtered Arc Deposition, Doctor of Philosophy thesis, School of Mechanical, Materials and Mechatronic, University of Wollongong, 2010. https://ro.uow.edu.au/theses/3220
The objective of this work was to produce near-equiatomic NiTi thin films by filtered arc deposition system (FADS) to improve the cavitation erosion resistance of a steel base working in liquid. First, deposition conditions were optimized in order to obtain near-equiatomic NiTi films. Energy dispersive spectroscopy (EDS) was used to measure the composition of films. It was found that both the substrate bias voltage and arc current affect the compositions of as-deposited NiTi films. X-ray diffractometry (XRD) and transmission electron microscopy (TEM) were used to investigate the microstructures of Ni-rich, Ti-rich and near-equiatomic films. Their thermal behaviours were tested using differential scanning calorimetry (DSC). Atomic force microscopy (AFM), ultra micro-indentation system (UMIS) and Romulus scratch testing were used to measure the roughness, hardness and adhesive force of a near-equiatomic thin film. The cavitation erosion resistance of the near-equiatomic NiTi film was assessed using ASTM Test Method G32. It was found that NiTi thin films showed significantly higher cavitation erosion resistance than 316 austenitic stainless steel. The improved cavitation erosion resistance was attributed to the recoverable deformation associated with pseudoelasticity of the NiTi thin film.
The effect of substrate temperature on the properties of NiTi thin films was investigated by preheating substrates to different temperatures (130oC, 300oC, 430oC and 600oC).The films deposited at low temperatures (130C and 300C) were amorphous and at high temperatures (430oC and 600oC) were crystalline with a preferred orientation. The crystalline films were sectioned using the focused ion beam (FIB) milling technique and the cross-sections were viewed by TEM. The crystalline NiTi film deposited at the highest temperature (600oC) is dominated by B2 parent phase. The retention of B2 phase probably arises from increased solution of O and N from the chamber atomosphere at the higher deposition temperature. The crystalline films deposited at 430oC contained a mixture of B2 parent phase, B19’ product phase and R intermediate phase. The crystalline films were significantly more resistant to cavitation erosion than the amorphous films because of their capacity to accommodate stress by pseudoelastic strain. The film deposited at 600oC showed less cavitation erosion resistance than thin films deposited at 430oC because of its larger grain size and precipitates formed at high temperature. The mechanical properties of the deposited NiTi films were also dependent on the substrate temperature. The surface roughness increased with increasing substrate temperature due to an increase in grain size. The adhesive force between the film and substrate was strengthened by atomic interdiffusion at higher temperatures.
The effect of deposition bias voltage on the properties of NiTi thin films was investigated under different bias voltages (50 V, 80 V, 120 V and 150 V). A higher bias voltage led to a high deposition temperature likely due to the higher kinetic energy of atoms and ions. The thin film deposited at the highest bias voltage (150 V) was dominated by B2 parent phase. The bonding force of the films increased with increasing bias voltage because of the enhanced interdiffusion occurring at high temperature. Due to improved pseudoelastic behaviour, the thin film obtained at 80 V with a substrate temperature of 600oC was more resistant to cavitation erosion than films deposited at other bias voltages with a similar substrate temperature.
The effect of substrate material on the properties of NiTi thin films was investigated by comparing films on mild steel and on austenitic stainless steel. For similar deposition conditions, the NiTi film deposited on mild steel consisted of B2, B19’ and R phases, while the NiTi film on stainless steel was dominated by B2 parent phase. This is probably because the lower thermal conductivity of austenitic stainless steel resulted in a higher deposition temperature.
The dominance of the B2 phase for deposition with a high substrate temperature, a high bias voltage and the use of stainless steel substrate was not found to be due to Ni-enrichment of the films, but is consistent with stabilization of the parent phase by possible pick-up oxygen and nitrogen from the chamber atmosphere when the deposition temperature is high.
The research program successfully produced homogeneous near-equiatomic NiTi thin films by using an advanced deposition technique, FADS. Although only two micron thick, NiTi films exhibited markedly superior cavitation erosion resistance, compared with an austenitic stainless steel reference material, and this is ascribed to their pseudoelastic behaviour under cavitation stress.