Tribological investigations of LIGA-microstructures

 The performance and lifetime of micro electro mechanical systems (MEMS) is strongly affected by friction and wear. We therefore analysed the friction and wear characteristics of microstructures produced by the LIGA- process. The measurements were carried out in a special designed microtribometer, which is capable to work inside a scanning electron microscope (SEM), and hence offering the possibility of a high resolution in-situ observation of the microscopic tribological mechanism during operation of the system. The material combinations investigated were chosen in order to represent the most important tribological junctions occurring in the ‘micro-motor’ and ‘micro-turbine’ currently produced by the LIGA-technique. We studied the tribological properties of nickel–nickel micro components (LIGA-sidewall/LIGA-sidewall and LIGA-bottom side/LIGA -top side), as well as nickel micro components (bottom side)– Al₂O₃-ceramic substrates (contact between rotor-base). The results indicate a strong influence of the relative humidity level (RHL) on the friction and wear properties of the metal/ceramic as well as of the metal/metal contacts appearing in the LIGA-MEMS, showing the lowest level of friction and wear at high RHL-values. We also found out that the friction and wear parameters were highly influenced by the applied surface pressure. Changing the surface pressure results in the formation of different kinds of wear debris accumulating or smearing out in the wear track. The metal oxides produced hereby in the wear track can lower adhesion and thus reduce the friction of the system. Received: 30 October 1995/Accepted: 18 December 1995     

Submicron coatings for micro-tribological applications

The miniaturisation of mechanical components and machines enables innovative future products. However, for the improvement of functionality, reliability and lifetime of those micro systems, micro tribological coatings with thicknesses in the sub-micron range are needed. To cover these needs, we investigated different submicron coatings with the aim to develop wear and friction optimized thin films for this application. The basis of this work has been the state of the art know how of well established macroscopic coatings which in general are in a thickness range of a few micrometers. It turned out, that particularly the surface topography and the substrate material influence the properties of very thin films. For investigations with single asperity contact, the coefficient of friction (COF) was reduced by lowering the tip radius and the hardness of the substrate material. In contrast, for larger contact radii (pin-on-disc), an increase of the COF with decreasing substrate Youngs modulus was found. With respect to wear, it turned out that the wear depth increased dramatically with increasing initial surface roughness (R_a).The authors gratefully acknowledge the financial support of the German Research Foundation by grant SFB 516. We also thank T. Staedler and A. Wortmann for their contributions through their respective PhD Theses on this topic.     

Fabrication, metallographic and tribologic investigations of LIGA-microstructures made of nickelphosphorous alloys

 The performance and lifetime of micro electro mechanical systems (MEMS) is strongly affected by friction and wear. Because of the restrictions due to the fabrication process the variety of materials used for micro systems is not very manifold. This results often is very poor tribological properties, since the tribological pairings are disadvantageous with regard to friction and especially wear. We therefore investigated materials which can be fabricated by the process currently used for LIGA-microstructures, and have the potential of better properties concerning friction and wear. The results of our tribologic experiments showed, that nickel phosphorous alloys are a promising material for microsystems suffering wear, especially at high surface pressures (high loading). Their absolute values of the wear intensity, are at least one order of magnitude lower than those of nickel and copper, which are the materials mostly used today. Received: 27 June 1996/Accepted:12 July 1996     

Sub-micron coatings with low friction and wear for micro actuators

 This paper deals with the fabrication and characterisation of friction and wear reduced nano-films for micro actuator applications. For this investigation films of diamond-like-carbon (DLC), metal doped Me-DLC, carbon-nitride CN _x , boron-nitride BN and alumina Al₂O₃ films have been applied in the thickness range of 20–500 nm. The hardness of those coatings varied between 10 and 60 GPa. Micromechanical and microtribological properties of nano-coatings have been characterized by a modified scanning probe microscope. Besides this a novel micro tester for abrasive wear measurement of nano coatings was used. Friction of micro-contact areas was measured by use of a pin-on-disc tester. It turned out, that friction was – besides other parameters – dependent on determination method and load. Friction determined at areal DLC/DLC contact zones was generally much higher (μ_DLC 0.1) than diamond tip versus DLC (∼μ _DLC 0.06). Received: 19 June 2001/Accepted: 15 September 2001     

Tribology of microsystems

In this work the possibilities for the reduction of friction and wear in micro electro-mechanical systems (MEMS) are investigated. An improvement of the tribological behaviour of microsystems can be realized by optimizing the contact condition and by application of special coatings with low friction and low wear rates. For optimizing the contact condition a defined topography and surface profile is generated by photolithography. Furthermore tribological coatings with low friction and low wear rates are developed and investigated using nanoindentation and micro scratch experiments. Also novel results on micro structured surfaces coated with a-C:H and a-C films will be discussed. The results show the great potential of carbon-based coatings in combination with an optimized geometrical surface design.     

Characterisation and testing of micro specimen

Currently for the design of micro components often no reliable material data is available. Therefore in the German Collaborative Research Centre (SFB) 499 [1] metallic and ceramic micro specimen that are manufactured by different micro moulding techniques as micro casting, slip casting or micro powder injection moulding are investigated with respect to their microstructure, mechanical and tribological properties. As metal was investigated a AuAgCu-alloy named Stabilor G and as ceramic was investigated ZrO₂. Additionally the wear behaviour of mould inserts made of different materials as X38CrMoV5–1, Ni and Cu63Zn37 and the modifications of near surface microstructures due to micro milling in X38CrMoV5–1 in different heat treatment states are presented.Abbreviations R_b Bending strength of one specimen - ₀ Characteristic beinding strength of a batch after Weibull - m_corr Weibull exponent corrected by the Maximum Likelihood Method - P_F Probability of failure - Friction coefficient - W Linear amount of wear - f DeflectionThe authors are grateful for the support of the research work in the SFB499 Design, production and quality assurance of moulded micro components made of metallic and ceramic materials by the Deutsche Forschungsgemeinschaft.     

Nanotribological characterization of perfluoroalkylphosphonate self-assembled monolayers deposited on aluminum-coated silicon substrates

Aluminum-coated silicon substrates are commonly used for various micro/nanooptoelectromechanical systems (MOEMS/NOEMS) including Digital Micromirror Devices (DMD^®). For efficient and failure proof operation of these devices, ultra-thin lubricant films of self-assembled monolayers (SAMs) are increasingly being employed. Fluorinated molecules are known to exhibit low surface energy, adhesion, and friction, desirable for tribological applications. In this study, we investigate contact angle, surface energy, friction, adhesion, and wear properties of a perfluoroalkylphosphonate SAM and compare them with those of alkylphosphonate SAMs. The influence of relative humidity, temperature, and sliding velocity on the friction and adhesion behavior is studied. Failure mechanisms of SAMs are investigated by wear tests. These studies are expected to aid in the design and selection of proper lubricants for MOEMS/NOEMS.     

Pest Control Expert System for Tomato (PCEST)

This paper presents a real-life pest control expert system for tomato. The system involves two main subtasks, namely: ‘diagnose’ and ‘treat’. The ‘diagnose’ subtask finds out the causes of the growers' complaints, while the ‘treat’ subtask finds out a treatment plan for these causes. CommonKADS methodology has been used to develop the system. Dependency network is used as one of our knowledge representation schemes in both subtasks. An expert system evaluation methodology has been suggested and applied to the developed system. Received May 1998 / Revised January 1999 / Accepted in revised form May 1999     

A numerical method for prediction of curved rail wear

Important published papers on rail wear in the past were reviewed. A numerical method was put forward to predict curved rail wear during a railway vehicle curving. The numerical method was discussed in detail. It considered a combination of Kalker’s non-Hertzian rolling contact theory, rail material wear model, the coupling dynamics of the vehicle and track, and the three-dimensional contact geometry analysis of wheel-rail. In its numerical implementation, the dynamical parameters of all the parts of the vehicle and track, such as normal loads and creepages of the wheels and rails, were firstly obtained through the curving dynamics analysis. The wheel-rail contact geometry calculation gave the wheel-rail contact geometry parameters, which were used in the wheel-rail rolling contact calculation with Kalker’s non-Hertzian rolling contact theory modified. The friction work densities on the contact areas of the wheels and rails were obtained in the rolling contact calculation, and were used to predict the rail running surface wears caused by the multiple wheels of the vehicle simultaneously with the rail material wear model. In the rail material wear model, it was assumed that the mass loss of each unit area was proportional to the frictional work density in the contact area. A numerical example was present to verify the present method. The numerical results of the example are reasonable, and indicate that the high rail wear of the curved track caused by the leading wheelset is much more serious than those caused by the other three wheels of the same bogie.     

Mechanical structuring,surface treatment and tribological characterization of steel mould inserts for micro powder injection moulding

Manufacturing of ceramic and metallic micro components in micro powder injection moulding (μPIM) requires mould inserts offering high wear resistance and a sufficient demoulding behaviour. Within the frame of this research μPIM mould inserts made from low and high alloyed tool steel were structured by micro milling and finished by micro peening and ultrasonic wet peening. Influence of surface condition on wear and demoulding behaviour of the steels in μPIM with ceramic feedstock was characterized using a laboratory tribotester simulating powder injection moulding and a specially adapted static friction tester. Results indicate that performance of mould inserts in micro powder injection moulding depends not only on hardness, surface condition and homogeneity of the mould insert materials but also is strongly influenced by the characteristics of the feedstock, like composition of the binder or amount and hardness of the ceramic particles.     

Tribological characterization and numerical wear simulation of microcomponents under sliding and rolling conditions

Increasing requirements for higher power densities and further miniaturisation in microsystem technology result in highly loaded micromechanical systems. Regarding these applications, the friction and wear behaviour of self-mated Si₃N₄ ceramic and WC-Co hardmetal has been characterized in laboratory micro tribometers. Experiments under unidirectional sliding and rolling conditions were carried out in air of 50% relative humidity as well as lubricated with water. Results from the model tests were used as input dataset for a numerical simulation tool Global Incremental Wear Model (GIWM) to predict volumetric wear as a function of operating conditions. The laboratory tests indicated that WC-Co as well as Si₃N₄ are applicable for microsystems running in water, due to their high wear resistance and low friction coefficient. Furthermore it was shown, that the numerical simulation can help to reduce the experimental effort by reliable predicting wear under different operating conditions, provided that the active wear mechanisms are comparable.     

Roll-to-roll hot embossing of microstructures

In this paper we present a new roll-to-roll embossing process allowing the replication of micro patterns with feature sizes down to 0.5 μm. The embossing process can be run in ‘continuous mode’ as well as in ‘discontinuous mode’. Continuous hot embossing is suitable for the continuous output of micro patterned structures. Discontinuous hot embossing has the advantage that it is not accompanied by waste produced during the initial hot embossing phase. This is because in ‘discontinuous mode’, embossing does not start before the foil has reached the target temperature. The foil rests between two parallel heating plates and foil movement and embossing starts only after the part of the foil resting between the heating plates has reached a thermal steady state. A new type of embossing master is used which is based on flexible silicon substrates. The embossing pattern with sub-μm topographic resolution is prepared on silicon wafers by state of the art lithography and dry etching techniques. The wafers are thinned down to a thickness of 40 μm, which guarantees the mechanical flexibility of the embossing masters. Up to 20 individual chips with a size of 20 × 20 mm2 were assembled on a roller. Embossing experiments with COC foils showed a good replication of the silicon master structures in the foil. The maximum depth of the embossed holes was about 70% of the master height.     

DSM as a knowledge capture tool in CODE environment

A design structure matrix (DSM) provides a simple, compact, and visual representation of a complex system/ process. This paper shows how DSM, a system engineering tool, is applied as a knowledge capture (acquisition) tool in a generic NPD process. The acquired knowledge (identified in the DSM) is provided in the form of Questionnaires, which are organized into five performance indicators of the organization namely ‘Marketing’, ‘Technical’, ‘Financial’, ‘Resource Management’, and ‘Project Management’. Industrial application is carried out for knowledge validation. It is found form the application that the acquired knowledge helps NPD teams, managers and stakeholders to benchmark their NPD endeavor and select areas to focus their improvement efforts (up to 80% valid).     

Towards a natural language semantics without functors and operands

The paper sets out to offer an alternative to the function/argument approach to the most essential aspects of natural language meanings. That is, we question the assumption that semantic completeness (of, e.g., propositions) or incompleteness (of, e.g., predicates) exactly replicate the corresponding grammatical concepts (of, e.g., sentences and verbs, respectively). We argue that even if one gives up this assumption, it is still possible to keep the compositionality of the semantic interpretation of simple predicate/argument structures. In our opinion, compositionality presupposes that we are able to compare arbitrary meanings in term of information content. This is why our proposal relies on an ‘intrinsically’ type free algebraic semantic theory. The basic entities in our models are neither individuals, nor eventualities, nor their properties, but ‘pieces of evidence’ for believing in the ‘truth’ or ‘existence’ or ‘identity’ of any kind of phenomenon. Our formal language contains a single binary non-associative constructor used for creating structured complex terms representing arbitrary phenomena. We give a finite Hilbert-style axiomatisation and a decision algorithm for the entailment problem of the suggested system.     

Witnessing and re-enacting in Cambodia: reflection on shifting testimonies

Thirty years after the collapse of the Khmer Rouge regime (1975–1979) how do Cambodians cope with the traumatic legacy of Pol Pot’s reign of terror? What forms does witnessing take on in post-socialist and transitional Cambodia as senior Khmer Rouge leaders await prosecution at the Cambodian Tribunal? The paper examines aspects of witnessing in today’s Cambodia, expressing each in its own way the idea of the ‘shifting’ of witnessing: the transformation of testimonies due to time passing and contrasted systems of justice through a comparison of testimonies in the trial of the ‘Pol Pot/Ieng Sary clique’ (1979) and the current Cambodian Tribunal; the complex forms of witnessing emerging from participatory projects developed with Western authors in ‘We want (u) to know’ (documentary movie made by an international film crew with the inhabitants of the village of Thnol Lok in 2009) and ‘Breaking the silence’ (theatre play realised by the Dutch dramaturge Annemarie Prins that premiered in Phnom Penh in 2009 and toured Cambodia in the following years); the relationship between documentary and legal forms of witnessing through the example of Vann Nath, a survivor of S-21/Tuol Sleng, the prison where the Khmer Rouge tortured and killed thousands of their fellow countrymen. The paper analyses the difficulty Western organisers of participatory projects experienced in applying the hybrid model of transitional justice to sociocultural contexts of witnessing. Nevertheless it points out their contribution to processes of ‘recognition beyond recognition’ in which cultural differences in coming to terms with historical trauma are expressed and recorded.     

Reporting computing projects through structured abstracts: a quasi-experiment

Previous work has demonstrated that the use of structured abstracts can lead to greater completeness and clarity of information, making it easier for researchers to extract information about a study. In academic year 2007/08, Durham University’s Computer Science Department revised the format of the project report that final year students were required to write, from a ‘traditional dissertation’ format, using a conventional abstract, to that of a 20-page technical paper, together with a structured abstract. This study set out to determine whether inexperienced authors (students writing their final project reports for computing topics) find it easier to produce good abstracts, in terms of completeness and clarity, when using a structured form rather than a conventional form. We performed a controlled quasi-experiment in which a set of ‘judges’ each assessed one conventional and one structured abstract for its completeness and clarity. These abstracts were drawn from those produced by four cohorts of final year students: two preceding the change, and the two following. The assessments were performed using a form of checklist that is similar to those used for previous experimental studies. We used 40 abstracts (10 per cohort) and 20 student ‘judges’ to perform the evaluation. Scored on a scale of 0.1–1.0, the mean for completeness increased from 0.37 to 0.61 when using a structured form. For clarity, using a scale of 1–10, the mean score increased from 5.1 to 7.2. For a minimum goal of scoring 50% for both completeness and clarity, only 3 from 19 conventional abstracts achieved this level, while only 3 from 20 structured abstracts failed to reach it. We conclude that the use of a structured form for organising the material of an abstract can assist inexperienced authors with writing technical abstracts that are clearer and more complete than those produced without the framework provided by such a mechanism.     

Decision Making and Management of Dynamic Risk

Managing dynamic environments often requires decision making under uncertainty and risk. Two types of uncertainty are involved: uncertainty about the state and the evolution of the situation, and ‘openness’ of the possible actions to face possible consequences. In an experimental study on risk management in dynamic situations, two contrasted ‘ecological’ scenarios – transposed from effective situations of emergency management – were compared in order to identify the impact of their ‘openness’ in the subjects’ strategies for decision making. The ‘Lost Child’ scenario presented qualitative and irreversible consequences (child’s death) and high uncertainty; it exerted high demands both in risk assessment (risk representation) and action elaboration and choice. A less open situation (‘Hydrocarbon Fire’) required a main choice between two contrasted actions, with quantitative computable consequences. The strategies of ‘experimental subjects’ (university students) and ‘operative subjects’ (professional fire-fighter officers) were compared in order to evaluate the ecological validity of experimental research in this field, from the point of view of the subjects themselves. The two scenarios appeared to be independent, so that quite different models of decision making have to be hypothesised, differing by the importance of assessing risk and defining possible actions on the one hand, and by the process of choice on the other. ‘Experimental’ subjects dramatically differed from ‘operative’ subjects when confronted with the same scenario, particularly for the less technical but more demanding scenario. It is hypothesised that three components might account for the effect of the situations and for the differences between and within groups of subjects: importance of situation assessment, spatial abilities, and global orientation of activity in managing dynamic risk.     

The Education of the Classical Architect from Plato to Vitruvius

Plato divided science (episteme) into ‘science of action’ (praktike) and ‘science of mere knowing’ (gnostike). His argument is the first known attempt to distinguish what is now recognised as technology, as distinct from more purely rational science. Aristotle coined the compound term technologia and thereby established this new department of science within the general system of knowledge. Plato did not develop his novel characterisation of the architect any further, for the ancient Greeks did not consider architecture a fine or estimable art. The best available source of Greek architectural pedagogy is the Roman Vitruvius. Graham Pont discusses Vitruvius’s distinction between the ‘practical’ side of architecture (fabrica) and the ‘theoretical’ (ratiocinatio), and examines the mathematical preparation of ancient Greek and Roman architects

相似文献   

Three-dimensional, one-point collision with friction

This paper deals with one-point collision with friction in three-dimensional, simple non-holonomic multibody systems. With Keller’s idea regarding the normal impulse as an independent variable during collision, and with Coulomb’s friction law, the system equations of motion reduce to five, coupled, nonlinear, first order differential equations. These equations have a singular point if sticking is reached, and their solution is ‘navigated’ through this singularity in a way leading to either sticking or sliding renewal in a uniquely defined direction. Here, two solutions are presented in connection with Newton’s, Poisson’s and Stronge’s classical collision hypotheses. One is based on numerical integration of the five equations. The other, significantly faster, replaces the integration by a recursive summation. In connection with a two-sled collision problem, close agreement between the two solutions is obtained with a few summation steps.     

Scenario Development and Practical Decision Making under Uncertainty: Application to Requirements Engineering

In this paper, we address the question of how flesh and blood decision makers manage the combinatorial explosion in scenario development for decision making under uncertainty. The first assumption is that the decision makers try to undertake ‘robust’ actions. For the decision maker a robust action is an action that has sufficiently good results whatever the events are. We examine the psychological as well as the theoretical problems raised by the notion of robustness. Finally, we address the false feeling of decision makers who talk of ‘risk control’. We argue that ‘risk control’ results from the thinking that one can postpone action after nature moves. This ‘action postponement’ amounts to changing look-ahead reasoning into diagnosis. We illustrate these ideas in the framework of software development and examine some possible implications for requirements analysis.