- freely available
Coatings 2012, 2(3), 195-209; doi:10.3390/coatings2030195
Abstract: This paper presents nano-impact (low cycle fatigue) behavior of as-deposited amorphous nitinol (TiNi) thin film deposited on Si wafer. The nitinol film was 3.5 µm thick and was deposited by the sputtering process. Nano-impact tests were conducted to comprehend the localized fatigue performance and failure modes of thin film using a calibrated nano-indenter NanoTest™, equipped with standard diamond Berkovich and conical indenter in the load range of 0.5 mN to 100 mN. Each nano-impact test was conducted for a total of 1000 fatigue cycles. Depth sensing approach was adapted to understand the mechanisms of film failure. Based on the depth-time data and surface observations of films using atomic force microscope, it is concluded that the shape of the indenter test probe is critical in inducing the localized indentation stress and film failure. The measurement technique proposed in this paper can be used to optimize the design of nitinol thin films.
2. Experimental Section
2.1. Test Specimen
2.2. Nanoindentation Testing and Measurements
2.3. Nano-Impact Testing and Measurements
|Sl. No.||Impact load, P (mN)||No. of impacts||Total test time (s)||Indenter shape (number of repeats)|
3.1. Film Characterization
3.2. Nano-Impact Testing of TiNi Thin Film
|Impact load, P (mN)||Berkovich||Conical|
|FD out of 5||BD out of 5||FD out of 5||BD out of 5|
|1||0||5; *(3−, 2+)||0||5; *(3−, 2+)|
|10||0||5; *(2−, 3+)||0||5; *(1−, 4+)|
|100||1||4; *(0−, 4+)||5||0|
3.3. Features in Nano-Impact Depth vs. Time Curve
4.1. Impact Residual Impression
4.2. Depth vs. Time Curve and Film-Substrate Failure Behavior
4.3. Fatigue Performance of TiNi Thin Film and Future Prospects
- (a) Based on the indenter contact depth deviation data, two types of film failure mechanisms are proposed. The failure of films starts from delamination at the film-substrate interface, resulting in a decrease in contact depth (backward depth deviation), whereas, a typical increase in contact depth (forward depth deviation) with number of fatigue cycles was also recorded.
- (b) Two types of backward depth deviation were observed for Berkovich and conical indenter. Type I indicates that the final impact depth is below zero, corresponding to a large film delamination gap; whereas, Type II indicates that the final impact depth is above zero, indicating a low film delamination gap.
- (c) The delamination failure indicated release of elastic stored energy (residual stress), and can be indicated through peculiar decrease in contact depth for Berkovich and conical indenters.
- (d) Indenter shape and nano-impact load had little influence on the type (forward or backward) of depth deviations.
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