Hongtu He

Tribology of Si/SiO2 in humid air: transition from severe chemical wear to wearless behavior at nanoscale.

Wear at sliding interfaces of silicon is a main cause for material loss in nanomanufacturing and device failure in microelectromechanical system (MEMS) applications. However, a comprehensive understanding of the nanoscale wear mechanisms of silicon in ambient conditions is still lacking. Here, we report the chemical wear of single crystalline silicon, a material used for micro/nanoscale devices, in humid air under the contact pressure lower than the material hardness. A transmission electron microscopy (TEM) analysis of the wear track confirmed that the wear of silicon in humid conditions originates from surface reactions without significant subsurface damages such as plastic deformation or fracture. When rubbed with a SiO2 ball, the single crystalline silicon surface exhibited transitions from severe wear in intermediate humidity to nearly wearless states at two opposite extremes: (a) low humidity and high sliding speed conditions and (b) high humidity and low speed conditions. These transitions suggested that at the sliding interfaces of Si/SiO2 at least two different tribochemical reactions play important roles. One would be the formation of a strong hydrogen bonding bridge between hydroxyl groups of two sliding interfaces and the other the removal of hydroxyl groups from the SiO2 surface. The experimental data indicated that the dominance of each reaction varies with the ambient humidity and sliding speed.

Humidity Dependence of Tribochemical Wear of Monocrystalline Silicon

The nanowear tests of monocrystalline silicon against a SiO2 microsphere were performed using an atomic force microscope in air as a function of relative humidity (RH=0%-90%) and in liquid water at a contact pressure of about 1.20 GPa. The experimental results indicated that RH played an important role in the nanowear of the Si/SiO2 interface. In dry air, a hillock-like wear scar with a height of ∼0.4 nm was formed on the silicon surface. However, with the increase of RH, the wear depth on the silicon surface first increased to a maximum value of ∼14 nm at 50% RH and then decreased below the detection limit at RH above 85% or in water. The transmission electron microscopy analysis showed that the serious wear on the silicon surface at low and medium RHs occurred without subsurface damage, indicating that the wear was due to tribochemical reactions between the Si substrate and the SiO2 counter surface, rather than mechanical damages. The RH dependence of the tribochemical wear could be explained with a model involving the formation of Si-O-Si chemical bonds (bridges) between two solid surfaces. The suppression of tribochemical wear at high RHs or in liquid water might be attributed to the fact that the thickness of the interfacial water layer is thick enough to prevent the solid surfaces from making chemical bridges. The results may help us understand the nanowear mechanism of silicon that is an important material for dynamic microelectromechanical systems.

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 1u202fmin in human saliva. The ASP formed in vitro within 1u202fmin was studied using a nanoindenter. The results show that the thickness of the inner layer of ASP is approximately 18u202fnm. 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

OBJECTIVEnTo determine the thickness and nanomechanical properties of salivary pellicle formed on tooth enamel.nnnMETHODSnIn 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.nnnRESULTSnThe 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.nnnCONCLUSIONSnIt 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.nnnCLINICAL SIGNIFICANCEnThe 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.