Showing papers by "Yang-Tse Cheng published in 2006"

Effects of micro- and nano-structures on the self-cleaning behaviour of lotus leaves

Abstract: When rain falls on lotus leaves water beads up with a high contact angle. The water drops promptly roll off the leaves, collecting dirt along the way. This self-cleaning ability or lotus effect has, in recent years, stimulated much research effort worldwide for a variety of applications ranging from self-cleaning window glasses, paints, and fabrics to low friction surfaces. What are the mechanisms giving rise to the lotus effect? Although chemical composition and surface structure are believed important, a systematic experimental investigation of their effects is still lacking. By altering the surface structure of the leaves while keeping their chemical composition approximately the same, we report in this study the influence of micro- and nano-scale structures on the wetting behaviour of lotus leaves. The findings of this work may help design self-cleaning surfaces and improve our understanding of wetting mechanisms.

Tribological behavior of diamond-like-carbon (DLC) coatings against aluminum alloys at elevated temperatures

Abstract: Diamond-like-carbon (DLC) coatings have a low friction coefficient and aluminum tends not to stick with it, which makes it a promising candidate tool coating for aluminum dry machining and hot forming applications. In this work, we compared the tribological properties of hydrogenated and non-hydrogenated DLC coatings made by unbalanced magnetron sputtering. Their friction, wear, and tendency of adherence at room and elevated temperatures were studied by sliding against 319 aluminum pins. In ambient air, aluminum did not stick to hydrogenated or non-hydrogenated DLC. At elevated temperature, aluminum adhered to the non-hydrogenated DLC. Hydrogenated DLC maintained its non-sticking nature at elevated temperature but its wear resistance deteriorated. The wear mechanism of hydrogenated-DLC at elevated temperature was investigated using thermo-gravimetric analysis (TGA), elastic recoil detection (ERD), nano-indentation, and Raman analysis. The accelerated wear of hydrogenated DLC at elevated temperature was attributed to the graphitization process.

Elevated temperature tribological behavior of non-hydrogenated diamond-like carbon coatings against 319 aluminum alloy

Abstract: Diamond-like carbon (DLC) coatings are promising candidates for dry machining of aluminum alloys since in ambient conditions aluminum has much less tendency to adhere to the DLC coating surfaces compared to other hard coatings such as TiN, TiAlN and CrN. In an attempt to better understand the tribological properties of the DLC coatings for cutting tool coating applications, non-hydrogenated DLC coatings were produced by magnetron sputtering and their elevated temperature friction and wear behavior were studied. DLC coated M2 tool steel discs were tested against 319 Al pins using a high-temperature tribometer. Counterface materials of tungsten carbide (WC) and sapphire (Al2O3) balls were also tested for comparison. Tests were done at 25, 120, 300 and 400 °C in air. The wear resistance of the DLC coating has been found to be poor at elevated temperatures. The mechanisms that provide the high wear resistance of the coating in ambient temperature cease to operate at temperatures as low as 120 °C. Annealing at elevated temperatures in air causes oxidation of the coating. Although much softer than WC and sapphire, the 319 Al alloy causes the most severe wear to the DLC coating. A two- and three-body abrasive wear model is proposed to explain this surprising observation.

Shape memory surfaces

Abstract: Temperature-controlled reversible surface protrusions can be made on NiTi shape memory alloys and thin films as a result of indentation induced two-way shape memory effect. First, spherical indents or scratches are made on the surface of a NiTi alloy in its martensite phase. Second, the indented or scratched surface is planarized to restore a flat surface. Reversible circular and line protrusions are produced by altering the temperature to drive the martensite to austenite phase transformation. This phenomenon can be exploited for a wide range of optical, tribological, and microelectromechanical device applications.

Nonlinear Analysis of Oscillatory Indentation in Elastic and Viscoelastic Solids

Abstract: Determining the mechanical properties at micro- and nanometer length scales using nanoindentation or atomic force microscopy is important to many areas of science and engineering. Here we establish equations for obtaining storage and loss modulus from oscillatory indentations by performing a nonlinear analysis of conical and spherical indentation in elastic and viscoelastic solids. We show that, when the conical indenter is driven by a sinusoidal force, the square of displacement is a sinusoidal function of time, not the displacement itself, which is commonly assumed. Similar conclusions hold for spherical indentations. Well-known difficulties associated with measuring contact area and correcting thermal drift may be circumvented using the newly derived equations. These results may help improve methods of using oscillatory indentation for determining elastic and viscoelastic properties of solids.

Methods of Obtaining Instantaneous Modulus of Viscoelastic Solids Using Displacement-Controlled Instrumented Indentation with Axisymmetric Indenters of Arbitrary Smooth Profiles

Abstract: We derive a relationship between the initial unloading slope, contact depth, and the instantaneous relaxation modulus for displacement-controlled indentation in linear viscoelastic solids by a rigid indenter with an arbitrary axisymmetric smooth profile. While the same expression is well known for indentation in elastic and in elastic-plastic solids, we show that it is also true for indentation in linear viscoelastic solids, provided that the unloading rate is sufficiently fast. When the unloading rate is slow, a "hold" period between loading and unloading can be used to provide a correction term for the initial unloading slope equation. Finite element calculations are used to illustrate the methods of fast unloading and "hold-at-the-maximum-indenter-displacement" for determining the instantaneous modulus using spherical indenters.

Sliding wear of non-hydrogenated diamond-like carbon coatings against magnesium

Abstract: Magnesium alloys are of increasing interest to the automobile industry for their potential in reducing vehicle weight, and in turn, improving fuel economy and lowering emissions. It follows that improving the magnesium manufacturing processes will promote greater use of magnesium alloys in automobiles. In recent years, diamond-like carbon (DLC) coatings have attracted attention for their low coefficient of friction (COF) and wear rates. The implementation of DLC coatings on tool and die surfaces may help increase the efficiency of magnesium component manufacturing processes (such as cutting and sheet forming) and also improve the surface quality of the finished products. In this study, the dry sliding wear behaviour of magnetron sputtered non-hydrogenated DLC coatings against Mg (> 99.9 wt.%) was investigated using a vacuum pin-on-disc tribometer. Tests were performed in ambient air (28% RH) and in argon, under a constant load of 5 N and at a sliding speed of 0.12 m/s. In argon, the non-hydrogenated DLC coatings showed a very low COF of 0.05 after an initial running-in period. Carbonaceous material transfer from the non-hydrogenated DLC to the contact surface of the Mg pin was observed in argon. Changing the test atmosphere from argon to ambient air increased the COF to 0.40, which was accompanied by the formation of oxidized Mg debris and an increased wear rate. The friction and wear mechanisms of non-hydrogenated DLC coatings against Mg are proposed to rationalize experimental observations.

Wear resistant self-healing tribological surfaces by using hard coatings on NiTi shape memory alloys

Abstract: Shape memory materials are among several smart materials being investigated for a wide range of applications, including “self-healing” tribological surfaces. Although shape memory alloys have recently been demonstrated as self-healing tribological surfaces, the wear resistance of the shape memory materials alone is usually not as good as conventional hard coatings. In this paper, the synergetic effects of combining hard coatings with shape memory alloys as wear resistant self-healing tribological surfaces are explored. Specifically, the wear resistance and shape recovery ability of uncoated and CrN coated NiTi alloys are investigated using pin-on-disc, indentation and scratch test. It is found that CrN hard coating can significantly improve the wear resistance of the NiTi shape memory alloy while maintaining its shape recovery ability. This example demonstrates that hard coatings on shape memory alloys can be used as wear resistant self-healing surfaces for tribological applications.

Two-way indent depth recovery in a NiTi shape memory alloy

Abstract: Controlled reversible changes in surface texture and topography are of interest to many applications, including information storage, optical communication devices, micro-fluidic instruments for drug delivery, and smart tribological surfaces for friction and wear control. Here, we demonstrate a method of inducing two-way reversible changes in an indentation made on the surface of a shape memory NiTi alloy. The two-way indent shape change is accomplished by thermomechanical training of spherical indents in NiTi shape memory alloy. After training, spherical indents exhibit two-way depth recovery: Shallow indent depth at high temperature and deep indent depth at low temperature. The reversible depth change is about 45% of the total indent depth and is stable over many cycles. The reversible depth change can have a wide range of engineering applications.

Relationship Between Contact Stiffness, Contact Depth, and Mechanical Properties for Indentation in Linear Viscoelastic Solids Using Axisymmetric Indenters

Abstract: We derive a relationship between the initial unloading slope, contact depth, and the instantaneous relaxation modulus for indentation in linear viscoelastic solids by a rigid indenter with an arbitrary axisymmetric smooth profile. Although the same expres

The influence of superelastic NiTi interlayers on tribological properties of CrN hard coatings

Abstract: The tribological performance of hard-coatings such as chromium nitride depends intimately on the mechanical properties of the support structure beneath the coating. For soft substrates, poor support characteristics severely degrade hard coating performance. The present work examines the potential for sputtered thin films of superelastic NiTi to improve hard-coating performance on soft substrates by limiting asperity contact stresses and mitigating the effects of compliance and thermal expansion coefficient mismatches between base metal and coating. NiTi thin films were deposited on 6061-T6 aluminum substrates using dc magnetron sputtering. CrN hard-coatings were then deposited by unbalanced reactive magnetron sputter deposition. Reversible stress-induced martensite transformations and mechanical properties of the NiTi films were investigated by temperature-controlled X-ray diffraction and nano-indentation experiments. The tribological properties of CrN coatings, with and without superelastic NiTi interlayers, were measured by ball-on-disk wear and scratch tests at different temperatures. It is shown that reversible martensite transformations in NiTi interlayers can exert a significant influence on the tribological properties of CrN coatings.

Effect of tribological media on tribological properties of multilayer Cr(N)/C(DLC) coatings

Abstract: In this research, Cr(N)/C(DLC) multilayered coatings were deposited on M2 steel substrates by an unbalanced magnetron sputtering technique. By varying the substrate rotation speed, four multilayered coatings with different bilayer thickness were produced. The bilayer thickness and structure of multilayered coatings were characterized by low-angle XRD, XPS depth profile, and cross-section TEM observation. The tribological investigation focused on the effect of layer thickness and tribological media on tribological property of the multilayer coatings against Al counterface. A pin-on-disc tribometer with a tribological medium container was used to investigate the wear behaviours of the four multilayered coatings under dry and wet (distilled water and S500 coolant) sliding. A stylus surface profilometer was used to measure wear rate. The investigation of wear tracks and wear mechanism was performed using Scanning Electron Microscopy (SEM). The research results showed that coatings with different bilayer period performed different tribological behaviour. The proper tribological media applied in the wear tests could improve the wear properties of multilayer coatings.

Information storage device

Abstract: An information storage device includes a substrate and a shape memory alloy film established on the substrate. The shape memory alloy film may receive, supply, and store digital information. One or more thermoelectric modules is/are nanoimprinted between the substrate and the shape memory alloy film. The thermoelectric module(s) is adapted to selectively erase at least some of the digital information from the shape memory alloy film.

Improving method for managing fuel cell water pipe

Abstract: PROBLEM TO BE SOLVED: To provide a method for improving manageability of a water pipe of a fuel cell. SOLUTION: The method includes a process to provide fuel cell components with surfaces with hydrophilic property or weak hydrophobic property, a process to improve hydrophobic property of at least one of hydrophilic surfaces and a process to make a fuel cell by assembling the fuel cell components. COPYRIGHT: (C)2006,JPO&NCIPI