Yanjing Su

Direct observation of two 90° steps of 180° domain switching in BaTiO3 single crystal under an antiparallel electric field

Domain-switching in BaTiO3 single crystal under an antiparallel electric field was investigated in this paper by polarized light microscope. It was found that 180° domain-switching consisted of two 90° switching steps. The polarization reversal of domains took place when an antiparallel electric field was applied to the BaTiO3 single crystal. The domain-switching process was different under different field intensities. Under a low field, all domains switched 180° through two 90° domain-switching steps. However, under a high field, only some 90° domains were nucleated in the initial period and subsequently all domains switched to the direction of the external field quickly.

Delayed fracture of lead zirconate titanate ferroelectric ceramics under sustained electric field

In this letter, we report on delayed fracture of lead zirconate titanate ceramics with a Zr/Ti ratio of 52/48(PZT-5) ferroelectric ceramics in silicon oil under sustained electric field, and that in silicon oil or moist atmosphere under sustained mechanical load. The experimental results show that sustained electric fields may cause delayed fracture of PZT-5 ceramics and there is a threshold field for the delayed fracture. The threshold electric field is less than one third of the critical electric field to cause instant fracture and is also half of the coercive field of the PZT-5 ceramics.

Anisotropy of stress corrosion cracking of a PZT piezoelectric ceramics

Abstract Anisotropy of stress corrosion cracking (SCC) of a lead zirconate titanate piezoelectric ceramics in water and formamide has been investigated at constant load test using a single-edge notched tensile specimen. The results showed that SCC could occur in the two solutions, and the normalized threshold stress intensity factor of SCC, K ISCC , revealed anisotropy. The K a ISCC for poling direction parallel to the crack plane, K ISCC , was greater than that perpendicular to the crack plane, K b ISCC , similar to the anisotropy of fracture toughness K IC , however, the anisotropy factor of the K ISCC , which was K a ISCC / K b ISCC =1.8 (in formamide) and 2.1 (in water), was larger than that of K IC , which is K a IC / K b IC =1.4. The stress-induced 90° domain switching decreases the efficient stress intensity factor and then causes the anisotropy of apparent K IC and K ISCC , besides, the resistance of SCC exists also anisotropy.

Delayed crack propagation in barium titanate single crystals in humid air

Domain witching of ferroelectrics under mechanical or electric load in vacuum or dry air has been intensively studied. However, the effects of environments on the domain switching in ferroelectrics have not been well understood. Here, we demonstrate that domain configurations in BaTiO3 single crystal under sustained load can be significantly affected by the humidity due to the decrease in surface energy and electrostatic energy upon adsorption of polar water molecules. Consequently, the crack propagation behaviors of the ferroelectrics under sustained load can be remarkably altered.

Stress corrosion cracking of a PZT piezoelectric ceramics

Abstract Stress corrosion cracking (SCC) of a lead–zirconate–titanate (PZT) piezoelectric ceramics in water, methanol and formamide has been investigated at constant load test using a single-edge notched tensile specimen. The result showed that SCC or static fatigue fracture could occur in the three solutions, and the normalized threshold stress intensity factor of SCC was K ISCC /K IC =0.66 (in water), 0.73 (in methanol) and 0.75 (in formamide), respectively, where K IC =1.34±0.25 MPa m 1/2 is the fracture toughness.

Effect of proteins on the surface microstructure evolution of a CoCrMo alloy in bio-tribocorrosion processes.

Under tribological contact, the subsurface microstructure of CoCrMo alloys for artificial joint implants can be changed and affect the life and safety of such devices. As one of the most important and abundant components in the synovial fluid, proteins play a key role in affecting the bio-tribocorrosion behaviors of metal implants. The effect of proteins on the subsurface microstructure evolution of a CoCrMo alloy was investigated using a transmission electron microscope (TEM) in this study. The result shows that proteins have two main effects on the subsurfaces evolution: forming a multilayered structure and causing severer subsurface deformation. The tribo-film can protect the passive film from scrapping, and then the passive film can reduce or even suppress the stacking fault annihilation by blocking the access to the metal surface. It leads to the stacking fault being diffused towards the deeper area and a strain accumulation in the subsurface, before inducing a severer deformation. On the other hand, the effect of proteins results in the location changing from the top surface to be underneath the top surface, where the maximum frictional shear stress occurs. This can cause a deeper deformation.

The effect of hydride and martensite on the fracture toughness of TiNi shape memory alloy

The effect of hydride and hydrogen-induced martensite on the fracture toughness of a TiNi shape memory alloy has been investigated using a notched tensile specimen. The results show that the fracture toughness of the hydrogenated samples decreases linearly with increasing hydrogen concentration, and the relative fracture toughness loss is as high as 96%. The relative fracture toughness loss induced by the martensite, however, is about 1.8%, and is independent of hydrogen concentration. The decrease in the fracture toughness of hydrogenated samples is almost completely due to the hydride. The relative fracture toughness loss induced by the hydride, ΔKICTiNiH/KIC, increases with increasing hydride content, WTiNiH, i.e., ΔKICTiNiH/KIC(%) = 93[1−exp(−WTiNiH/9.5)]. Microcracks can generate along the hydrides during charging at , but do not further increase the relative fracture toughness loss induced by the hydride.

Local piezoelectric effect on single crystal ZnO microbelt transverse I-V characteristics

One-dimensional ZnO microbelts were prepared by chemical vapor deposition on Si substrates with sputtered Pt film. Using Pt-coated atomic force microscope (AFM) tip, the belts’ transverse I-V characteristics were measured under varying applied elastic loads. ZnO microbelt conductivity reduced with load, but gradually increased upon unloading. Transverse electrical conductivity decrease at higher loads is attributed to the depletion zone formation induced by local piezoelectric effect in ZnO single crystal belt with (0001¯) top surface indented by the AFM tip. The observed effect can be utilized in a nanoforce sensor device.

Martensite caused by passive film-induced stress during stress corrosion cracking in type 304 stainless steel

Abstract After a pre-crept specimen of type 304 stainless steel underwent stress corrosion cracking (SCC) at constant load in a 42% MgCl2 solution for 2 h, α′ martensite on the surface and over the entire specimen increased in volume fraction by 14 and 1.5%, respectively. If the specimen pre-crept in the silicon oil was immersed in the 42% MgCl2 solution at a cathodic potential of −600 mVSCE, in which no SCC occurs, the α′ martensite volume fraction increased by only 0.6%. The critical hydrogen concentration for the formation of hydrogen-induced α′ martensite was 50.9 wppm, which was larger than that which entered into the sample during SCC. The difference between the flow stress of the 304 steel before unloading and the yield stress of the same specimen extended in air after unloading and immersed in the 42% MgCl2 solution to form a passive film is defined as a passive film-induced stress. The passive film forming at the open-circuit potential generated a large tensile stress, and that at a cathodic potential of −600 mVSCE generated a small compressive stress. Therefore, the observed martensite transformation during SCC can be attributed to a passive film-induced tensile stress.

Release of metal ions from nano CoCrMo wear debris generated from tribo-corrosion processes in artificial hip implants

CoCrMo alloys have been widely used in metal-on-metal (MoM) hip replacements due to their superior wear and corrosion resistance properties. However, metal ions like Co2+ and Cr3+, or even Cr6+ released from CoCrMo hip prostheses can induce macrophage apoptotic vs. necrotic mortality and damage the surrounding tissues. Simultaneously, osteolysis induced by the wear debris can be a cause of failure. Nano wear debris is more active than the bulk material, due to its small size. In this study, to accurately analyse the fresh wear debris retrieved from the hip simulator and the interaction between the particles and tribocorrosion of CoCrMo, wear debris was observed without protein digest, using a combined experimental approach involving the employment of TEM and ICP-MS. The results suggest that nanoscale wear debris generated from a hip simulator in bovine serum albumin (BSA) lubrication was Cr-rich, containing crystalline and amorphous structures; meanwhile, without any proteins, the wear particles mostly had an hcp-Co crystalline structure.