Jiaxin Yu

Friction-induced nanofabrication on monocrystalline silicon

Fabrication of nanostructures has become a major concern as the scaling of device dimensions continues. In this paper, a friction-induced nanofabrication method is proposed to fabricate protrusive nanostructures on silicon. Without applying any voltage, the nanofabrication is completed by sliding an AFM diamond tip on a sample surface under a given normal load. Nanostructured patterns, such as linear nanostructures, nanodots or nanowords, can be fabricated on the target surface. The height of these nanostructures increases rapidly at first and then levels off with the increasing normal load or number of scratching cycles. TEM analyses suggest that the friction-induced hillock is composed of silicon oxide, amorphous silicon and deformed silicon structures. Compared to the tribochemical reaction, the amorphization and crystal defects induced by the mechanical interaction may have played a dominating role in the formation of the hillocks. Similar to other proximal probe methods, the proposed method enables fabrication at specified locations and facilitates measuring the dimensions of nanostructures with high precision. It is highlighted that the fabrication can also be realized on electrical insulators or oxide surfaces, such as quartz and glass. Therefore, the friction-induced method points out a new route in fabricating nanostructures on demand.

Effect of surface hydrophilicity on the nanofretting behavior of Si(100) in atmosphere and vacuum

With an atomic force microscopy, the effect of surface hydrophilicity on the nanofretting behavior of Si(100) against SiO2 microsphere was investigated under vacuum and atmosphere conditions, respectively. The surface hydrophilicity revealed a strong effect on the motion behavior, adhesion force, friction force, and nanofretting damage of Si(100)/SiO2 pairs. The increase in the hydrophilicity of Si(100) surface could expand the stick regime of Si(100)/SiO2 pairs into a higher value of displacement amplitude. While the nanofretting ran in atmosphere, both adhesion and friction forces in the initial cycle would be larger when the Si(100) surface was more hydrophilic. However, because of the in situ chemical modification of SiO2 tip in nanofretting, they might reveal a decrease with increasing nanofretting cycles. Either in vacuum or in atmosphere, the nanofretting damage was weaker when the Si(100) surface was more hydrophobic. Because of the lack of oxygen and vapor in vacuum, the nanofretting damage on th...

Investigation of silicon wear against non-porous and micro-porous SiO2 spheres in water and in humid air

Tribochemical wear, a method to achieve controlled material removal without residual damage on substrates, plays a very important role in super-smooth silicon surface manufacturing. By using non-porous SiO2 spheres and micro-porous SiO2 spheres, the wear of silicon substrates was comparatively investigated in DI water, humid air (50% RH) and dry air. The wear behaviors presented entirely different cases at the same load in DI water and humid air: (a) less material removal of silicon against non-porous SiO2 spheres than micro-porous SiO2 spheres in water and (b) more serious wear of silicon substrate against non-porous SiO2 spheres in humid air. When the wear tests were operated in dry air, no obvious damage was incurred on the silicon surface against the non-porous SiO2 spheres but slight wear was observed against micro-porous SiO2 spheres under the given conditions. Raman results revealed that a hydrolysis reaction was involved in the tribochemical wear of the silicon substrate and the micro pores in SiO2 spheres could accelerate this process. The corresponding analysis suggests an exponential dependence of wear rate on contact stress, which is consistent with the stress-assisted chemical kinetics model. Although with much lower elastic modulus, micro-porous SiO2 spheres caused a larger wear rate of silicon substrate than non-porous SiO2 spheres at the same contact pressure both in water and humid air. The results indicate that the micro-porous SiO2 spheres can promote the tribochemical reaction due to the storage of water molecules in micro pores.

Impact of strain rate on the hardness and elastic modulus of human tooth enamel

Human tooth enamel is a natural biocomposite consisting of mineral units surrounded by a soft protein shell. The mechanical behaviors of enamel are closely associated with its structure. In this paper, the strain-rate dependent mechanical properties of enamel were investigated with nanoindentation techniques. Five constant strain rates (0.01s-1, 0.03s-1, 0.05s-1, 0.1s-1, 0.3s-1) were used in this study. Results showed that the hardness and elastic modulus of enamel increased with increasing strain rate. These results indicate that the variation of hardness under different stain rates is associated with creep behavior of organic matrix in enamel. And indentation creep rate sensitivity of human enamel was measured with a value of 0.062. Moreover, the elastic module of enamel is dependent upon strain rate. Such rate dependence originates from the organic matrix which is sensitive to the strain rate. This behavior may be important in explaining the excellent toughness and energy absorption abilities of natural tooth structure.

Mechanical characterization of in vitro-formed short-term salivary pellicle

Human saliva consists of approximately 98% water and a variety of electrolytes and proteins. Those proteins can be selectively adsorbed onto the enamel surface. The cuticular material formed on the enamel surface is termed acquired salivary pellicle (ASP), which is critical for the health of oral mucosa and teeth. The ASP is composed of a inner layer and a outer layer. The lubricating properties of ASP are closely associated with the inner layer. The aim of this research is to characterize the structural and mechanical properties of the inner layer of ASP. In this paper, enamel specimens were immersed for 1 min in human saliva. The ASP formed in vitro within 1 min was studied using a nanoindenter. The results show that the thickness of the inner layer of ASP is approximately 18 nm. Moreover, the inner layer is a heterogeneous pellicle with a gradient in density. From the surface of the inner layer to the enamel surface, the density and mechanical properties gradually increase. The research results may be helpful to extend the understanding of mechanical properties of salivary pellicle and to the oral hygiene industry for diagnose oral diseases.

Towards a deeper understanding of nanoscratch-induced deformation in an optical glass

In this work, the factors affecting the nanoscratch-induced deformation of oxide glass in the completely ductile range are classified into three independent aspects, including plastic damage, densification, and chemistry-enhanced material-removal. Through comparing the deformation volume of a phosphate laser glass subject to different conditions, viz., in dry or humid air and before or after annealing, the contributions of the three factors to the total deformation were quantified and their evolution mechanisms were clarified for sustained scratching. Analyses indicate that the combined action of plastic damage and densification dominates the glass deformation in fewer-pass scratching, while plastic damage and chemistry-enhanced material-removal together determine the final deformation volume for multi-pass scratching in humid air. The results presented in this study provide further insights into the physics of glass deformation.In this work, the factors affecting the nanoscratch-induced deformation of oxide glass in the completely ductile range are classified into three independent aspects, including plastic damage, densification, and chemistry-enhanced material-removal. Through comparing the deformation volume of a phosphate laser glass subject to different conditions, viz., in dry or humid air and before or after annealing, the contributions of the three factors to the total deformation were quantified and their evolution mechanisms were clarified for sustained scratching. Analyses indicate that the combined action of plastic damage and densification dominates the glass deformation in fewer-pass scratching, while plastic damage and chemistry-enhanced material-removal together determine the final deformation volume for multi-pass scratching in humid air. The results presented in this study provide further insights into the physics of glass deformation.

On the thickness and nanomechanical properties of salivary pellicle formed on tooth enamel

OBJECTIVE To determine the thickness and nanomechanical properties of salivary pellicle formed on tooth enamel. METHODS In vitro adsorption experiments were conducted by immersing enamel samples in centrifuged saliva for 1min, and then the nanomechanical properties of the salivary pellicle/tooth enamel system were measured firstly using nanoindentation based on a continuous stiffness measurement technique. Finally, a model was proposed to obtain the thickness and the intrinsic nanohardness of this biofilm. RESULTS The composite nanohardness of salivary pellicle/tooth enamel system varied with indentation depth. The model can describe the experimental date at both shallow and deep indentation depths very well. The fitted average thickness of salivary pellicle was about 17nm, which was in good accord with the scanning probe microscopy experimental results. The intrinsic hardness of salivary pellicle and tooth enamel was about 0.52Gpa and 4.88Gpa respectively, which was consistent with previous studies. CONCLUSIONS It was convenient to extract intrinsic hardness and thickness of salivary pellicle from the indentation curve according to the model. Moreover, this model was applicable to plasticity-dominated behaviour of the soft film/hard substrate system. CLINICAL SIGNIFICANCE The research results may be helpful to extend the understanding of our lubricating and anti-caries behaviours of salivary pellicle and to the oral hygiene industry for diagnose oral diseases.

Nanofretting Wear of Monocrystalline Silicon (100) against Spherical SiO2 Tip in Vacuum

With an atomic force microscopy (AFM), the tangential nanofretting between spherical SiO2 tips and monocrystalline Si(100) surface was carried out at various displacement amplitudes (0.5∼250 ran) under vacuum condition. Similar to fretting, the nanofretting of Si(100)/SiO2 pair could also be divided into stick regime and slip regime upon the transition criterion. However, it was found that the energy ratio corresponding to the transition between two nanofretting regimes varied between 0.41∼0.63, which was higher than the normal value of 0.2 in fretting. One of the reasons may be attributed to the effect of adhesion force, since whose magnitude is at the same scale to the value of the applied normal load in nanofretting. During the nanofretting process of Si(100)/SiO2 pair, the adhesion force may induce the increase in the maximum static friction force and prevent the contact pair from slipping. The larger the curvature radius of spherical Si02 tip, the higher the applied load, or the higher the adhesion force is, the larger the transition displacement amplitude between two regimes in nanofretting will be. Different from fretting wear, the generation of hillocks was observed on Si(100) surface under the given conditions in nanofretting wear. With the increase in the displacement amplitudes in slip regime of nanofretting, the height of hillocks first increased and then attained a constant value. Compared to chemical reaction, the mechanical interaction may be the main reason responsible for the formation of silicon hillocks during the nanofretting in vacuum. The results in the research may be helpful to understand the nanofretting failures of components in MEMS/NEMS.

Tribological Behaviors of Self-Assembled Dual-Layer Films in Atmosphere and in Vacuum

Three kinds of self-assembled dual-layer films with various tail groups and chain length were prepared on APS film substrated by silicon wafer. Using an atomic force microscopy, the tribological behaviors of these films were detected both in atmosphere and in vacuum. Compared to those in atmosphere, the films revealed smaller adhesion and friction forces in vacuum. The more hydrophobic film was found to exhibit the less difference between the friction forces in vacuum and in atmosphere. The reason may be partly attributed to the adsorbed water layer on the samples, since which will show relatively weak effect on the friction force on the hydrophobic surface. No obvious damage was observed on the self-assembled films after the friction tests in vacuum at an applied load of 140 nN by a Si3N4 tip. As the initial stage of nanowear process by a diamond tip, a series of hillocks were observed on silicon surface along the scratching line. All the films can effectively enhance the antiwear ability of silicon surface and the self-assembled dual-layer film terminated by long chains (STA/APS) or −C6H5 groups (PAA/APS) performed much better than that terminated by short chains. Finally, the microwear abilities of the films were also examined on a universal micro-tribometer at the normal load from 50 to 200 mN. The wear life varied for different films and good antiwear performance was assigned to STA/APS and PAA/APS. This work can be indicative in the application of self-assembled films in MEMS/NEMS.