This paper reviews moving mechanical assembly (MMA) and component technology shortfalls imposed by demanding ground, naval and aerospace system requirements of the future. The tribology development needs for specific harsh environment applications such as high temperature engines, chemical reactant turbopumps, precision acquisition, tracking and pointing MMAs surveillance systems and laser communications components are identified. Development goals for lightweight bearing and gear alloys and ceramics, ultra low friction and wide temperature range solid lubricants, self lubricating composites and real time health monitoring devices are translated into a rationale for mechanistic, more fundamental research approaches.

A survey was conducted of existing technologies for moving mechanical assemblies used in spacecraft applications. The purpose was to identify areas where future requirements for lifetimes in excess of ten years with anticipated speeds, loads, and temperatures might not be satisfied. Some specific mechanisms, such as momentum/reaction wheels, high-speed turbines, pointing and tracking mechanisms, despin mechanisms, and gimbal mechanisms, were identified as areas for potential application of existing but unused technologies. Two major problem areas identified involve boundary-regime lubrication and lubricant supply (active or passive) for long life. Areas where substantial, near-term improvements appear practical include the use of hybrid bearings, new synthetic fluid lubricants, new bearing retainer materials, and properly designed solid-film lubricants.

The development of space tribology and its application in the design of spacecraft moving mechanical assemblies (MMA's) in Europe has followed a different course from that in the USA. This is probably because of the distinct preponderance of 3-axis stabilised space vehicles - used for communications and for earth observation over Europe - together with the preference for solid lubricants in such devices as solar array drives and some types of scanner and de-spin mechanism. The paper reviews the current status and discernible future trends in European space tribology. Examples of current MMA types are given where evolved lubricants and lubrication techniques have been applied.

Tribology, the science and engineering of solid surfaces in moving contact, is a field that encompasses many disciplines: solid state physics, chemistry, materials science, and mechanical engineering. In spite of the practical importance and maturity of the field, the fundamental understanding of basic phenomena has only recently been attacked. In this paper we will attempt to define some of these problems and indicate some profitable directions for future research. We make three broad classifications: (1) fluid properties--compression, rheology, additives and particulates; (2) material properties of the solids--deformation, defect formation and energy loss mechanisms; and (3) interfacial properties--adhesion, friction chemical reactions, and boundary films. Research in the categories has traditionally been approached by considering macroscopic material properties. Recent activity has shown that some issues can be approached at the atomic level: that is, the atoms in the materials can be manipulated both experimentally and theoretically, and can produce results related to macroscopic phenomena. This experimental and theoretical activity is reviewed and related to the traditional research areas.

We describe experiments involving the static or sliding contact of a 'single asperity' and a flat surface. Plastic deformation and possibly junction growth can occur even at zero applied load, as a consequence of surface forces. The conditions for this to occur, and the different modes of separation of an adhesive contact, are clarified with the help of 'maps': use of the macroscopic concepts involved, such as yield stress and work of adhesion, is sometimes questionable and evidence of the importance of nanometre-scale stepped topography is presented. Strong adhesion between hard non-metals can required thermal activation. Sliding contacts at the sub-micrometre level have shown behaviour ranging from 'unstable ploughing' to adhesive peeling, but the frictional mechanisms involved are often unclear.

This paper is a part of a long-term study of the physics of boundary lubrication. In this regime, the shape and pressure of the film are controlled by two phenomena: first, the elastic or plastic deformation of the substrate, and second, the physical state of the film. Three physical states can be detected: the condensed solid, granular and colloidal states. A new piece of apparatus, allowing the continuous and simultaneous measurement of static and dynamic forces, displacement and current voltage between a sphere and a plane, is used to determine surface forces and rheological properties of the film. The viscous and elastoplastic behavior of the interface is given by a transfer function. The lubricant we studied was calcium dialkylbenzenesulfonate solution containing encapsulated calcium carbonate and dissolved in pure dodecane (volume fraction 5.10-). While a pure dodecane solution behaves like a Newtonian fluid, with its bulk viscosity, the colloidal solution shows an increase of the viscosity near the steel surfaces suggesting adsorption. After 8 hours, each steel surface is covered with a layer, which includes an almost complete “monolayer” of calcium carbonate particles, with some incomplete layers on top of it. During indentation of the layer the repulsive forces vary exponentially with the thickness of the layer. This result is consistent with two effects: the consolidation of the layer during the process and scaling factors such as the Hill number (a/D).

New experimental techniques are described for measuring both the static and dynamic interactions of very thin liquid films between two molecularly smooth surfaces as they are moved normally or laterally relative to each other. Film thickness can be measured and controlled to IA. Results are presented of how the properties of liquid films only one, two, or three molecular layers thick differ from those of bulk liquids. When close together, two molecularly smooth surfaces slide (shear) past each other while separated by a discrete number of liquid layers. The critical shear stress required to initiate motion for simple spherical molecules is “quantized” with the number of layers. Another important characteristic of molecular thin liquid films undergoing shear is the phenomenon of “stick-slip” or “intermit-tent” motion. Both of these phenomena have been studied as a function of driving velocity, area of contact, and number of molecular layers.

The universal relation between binding energy and interatomic separation was originally discovered for adhesion at bimetallic interfaces involving the simple metals Al, Zn, Mg, and Na. We show here that the same universal relation extends to adhesion at transition metal interfaces. Adhesive energies have been computed for the low index interfaces of Al, Ni, Cu, Ag, Fe, and W using the Equivalent Crystal Theory (ECT) and keeping the atoms in each semi-infinite slab fixed rigidly in their equilibrium positions. These adhesive energy curves can be scaled onto each other and onto the universal adhesion curve. We have also investigated the effect of tip shape on the adhesive forces in the Atomic Force Microscope (AFM) by computing energies and forces using the ECT. While the details of the energy-distance and force-distance curves are sensitive to tip shape, all of these curves can be scaled onto the universal adhesion curve. This means that although absolute magnitudes of adhesive forces in the AFM cannot be determined without knowing the geometry of the tip, the shape of the force-distance curve is universal, at least for distances changing normal to the substrate.

Basic understanding of the structure and dynamics of materials and their response to external perturbations requires knowledge on the microscopic level, of the underlying energetics and atomic dynamics, whose consequences we observe and measure. Coupled with the above is the everlasting quest to observe and understand natural phenomena on refined microscopic scales, which provides the impetus for the development of experimental and theoretical techniques for the interrogation of materials with refined spatial and temporal resolution. In this paper we review the development of molecular dynamics simulations for studies of the energetics and dynamical response of materials to external mechanical perturbations. Applications to investigations of solid and liquid interfacial systems under stress and to studies of the consequences of tip-substrate interactions in atomic force microscopy are demonstrated.

By borrowing ideas from control theory, Nonequilibrium Molecular Dynamics incorporates temperature, stress, and heat flux directly into atomistic, time-reversible, deterministic equations of motion. We are applying this technique to studies of surface indentation, surface cutting, friction, ablation, and condensation. Here we describe simulations of the indentation and cutting processes using two-dimensional crystals composed of a few thousand particles.

The method of molecular tribometry is described. Ultrathin films of liquids are confined between parallel plates, usually of atomically smooth muscovite mica, such that the apparent area of contact equals the true area of contact. This paper consists of 5 sections. First, the phenomenon of solid lubrication by liquids is described. Second, the alternative viscous response of ultrathin liquid lubricants is described; the microviscosity is considerably enhanced over that of the isotropic liquid. Third, the critical shear strength is shown to have an extreme dependence on the normal pressure. Fourth, the materials requirements for molecular tribometry are discussed. Finally, a discussion is offered of the strengths and limitations of this new tool.

The changes in friction and wear properties (i.e., tribology) of steels due to thin surface alloys formed by ion beams are surveyed, and considered in light of their microstructures. Amorphous alloys containing both Ti and C are found to give superior performance in comparison to alloys with N, B, C, P or Ti. All the amorphous layers reduce friction, but the duration of benefits and wear resistance at high loads vary with composition. By examining similar amorphous alloys with Ti alone or C alone, the critical roles played by each in resisting wear are demonstrated. Amorphous alloys with Ti and C appear to be quite hard.

Despite the advantages of laser processing for the production of wear resistant materials, laser surface melting results in tensile stresses because the melted layer shrinks during resolidification. These tensile stresses may lead to severe cracking of the material and to deleterious effects on the wear behaviour. Our basic idea presented in this paper is to convert the high tensile stresses in the laser melted surface into a compressive state after implantation. In general, neon implantation is not very effective in the reduction of wear rates. However, neon implantation into laser melted steel turns out to reduce the wear rate substantially.

A modified pin-on-disc wear test technique and conversion electron M6ssbauer spectroscopy (CEMS) have been used to characterize the tribological and microstructural properties, respectively, of pure Fe implanted with N and Ar ions at high current densities. CEMS measurements were made before and after wear testing. For the lubricated, mild adhesive wear conditions used here, no evidence was found for iron-nitride dissolution or N atom migration. Disordering of γ'-Fe4N did occur as a result of the wear process. All Nimplanted surfaces were tribologically superior to the Ar-implanted surface and an extremely wear-resistant surface layer about 30-50 nm thick was produced with a dose of 8x1016 N atoms/cm2, at a dose rate of 100 μA/cm2. However, once this layer was worn away the wear rate returned sharply to that of unimplanted pure Fe. A high retained N dose has been observed even for a dose rate of 750 μA/cm2 during which the sample reached 280°C.

A study has been made on friction and hardness of boron and nitrogen ion implanted Fe-Cr alloys. Ion implantation has been carried out with doses ranging from 5 wt% up to 20 wt% at energies of 50, 100 and 150 keV at room temperature. Reciprocal traces for friction coefficient measurements were performed by using a Bowden-Leben tester at a low speed without lubricant. Hardness was measured by using a micro-Vickers tester at a load of 2gf. Hardness of the specimen increases after the implantation with either of ion species. Friction measurements with reciprocal sliding show that a life-time of decreased friction coefficient depends on implanted ion species, Cr concentrations and acceleration energies. For B+- implantation, the lower the chromium contents are, the longer a decreased friction coefficient lasts. Meanwhile, the friction reduced by nitrogen implantation lasts longer for higher chromium content substrates. Dependence of friction coefficient on an acceleration energy is discussed for B+-implantation.

The influence of the growth conditions on the composition, microstructure, and mechanical properties of TiN formed with low energy Ar+ ion bombardment during reactive physical vapor deposition was studied. The ratios of the incident ion and atom fluxes at the growth surface were varied systematically and were found to have a strong influence on both the composition and the microstructure of the films. These changes correlated well with measured changes in mechanical and optical properties. The films were examined using Auger electron spectroscopy, transmission electron microscopy, and their mechanical properties were studied using hardness, friction, and wear testing.

The method of Ion Beam Enhanced Deposition, (IBED), was used to produce hard films of i-BN, i-SixNy and i-TixNy. The friction, hardness, adhesion, and wear behavior of these nitride coatings were examined using a ball-ondisc friction/wear tester. The friction and wear results were sensitive to stoichiometry and the presence of impurities (e.g., hydrogen) in the film as well as the choice of ball material (400C stainless steel or silicon nitride).

Cross section transmission electron microscopy was used to study the microstructure and interface structure of Cr films deposited by ion-assisted deposition as a function of the incident ion energy. High-energy ion bombardment (1 keV) was found to enhance the adhesion of the deposited film owing to the formation of an intermixed layer, whereas deposition with lowenergy ions (100 eV) was found to reduce or eliminate grain boundary porosity. A tailored Cr film with excellent adhesion and no grain boundary porosity was deposited by combining highand low-energy ion bombardment.

Multilayered Fe-Ti-C films consisting of eleven sublayers were vacuum deposited onto an AISI 304 stainless steel substrate and subsequently mixed using either 400 keV Xe ions or an excimer laser operating at a wavelength of 308 nm. Ion mixing was accomplished in a two step process: the multilayers were first irradiated with 1xl017 Xe/cm2 at 520 °C, after which half of the sample was irradiated with 5x1015 Xe/cm2 at 0 °C. Laser mixing was carried out at both 1.1 and 1.7 J/cm17 with the number of pulses varied between 1 and 10. Pin-on-disc studies revealed only slight differences between the two kinds of ion beam mixed samples, whereas the dry sliding properties of laser mixed samples were strongly dependent on the total fluence used. In the optimum conditions, similar friction coefficients were obtained on both kinds of samples.

We have investigated the properties of surface alloys of Ti formed by excimer laser melting and mixing of Ti overlayers on AISI 304 stainless steel substrates.The thickness of the initial Ti surface layers and the fluence and number of pulses used to mix the surface were varied toinvestigate a broad range of processing conditions. Samples were uniformly translated at different speeds under repetitive pulses of homogenized laser radiation at 308 nm to vary the number of pulses incident on the sample. We investigated the surface structure and composition, the wear and friction behavior, and the surface hardness of the resulting alloys.

The extent of mixing is observed by Rutherford backscattering spectroscopy to be proportional to the number of pulses incident on the sample. Amorphous surface layers are formed at some mixing levels with a composition which indicates that Ti substitutes for Fe in the stainless steel on a one to one basis. Incorporation of C from surface and interface contamination is observed but at a level much below that found in comparable ion implanted materials.

Friction coefficient measurements indicate that there exists an optimum level of mixing for each layer thickness. Excessive mixing degrades both the coefficient of friction and the wear performance in dry sliding tests.

As with the tribological measurements, the surface hardness showed a dependence on amount of mixing. The surface hardness of the as deposited samples was higher than that of the laser mixed samples, initially decreased with mixing, and finally increased with further mixing.