Ben D. Beake

Hierarchical adaptive nanostructured PVD coatings for extreme tribological applications: the quest for nonequilibrium states and emergent behavior

Abstract Adaptive wear-resistant coatings produced by physical vapor deposition (PVD) are a relatively new generation of coatings which are attracting attention in the development of nanostructured materials for extreme tribological applications. An excellent example of such extreme operating conditions is high performance machining of hard-to-cut materials. The adaptive characteristics of such coatings develop fully during interaction with the severe environment. Modern adaptive coatings could be regarded as hierarchical surface-engineered nanostructural materials. They exhibit dynamic hierarchy on two major structural scales: (a) nanoscale surface layers of protective tribofilms generated during friction and (b) an underlying nano/microscaled layer. The tribofilms are responsible for some critical nanoscale effects that strongly impact the wear resistance of adaptive coatings. A new direction in nanomaterial research is discussed: compositional and microstructural optimization of the dynamically regenerating nanoscaled tribofilms on the surface of the adaptive coatings during friction. In this review we demonstrate the correlation between the microstructure, physical, chemical and micromechanical properties of hard coatings in their dynamic interaction (adaptation) with environment and the involvement of complex natural processes associated with self-organization during friction. Major physical, chemical and mechanical characteristics of the adaptive coating, which play a significant role in its operating properties, such as enhanced mass transfer, and the ability of the layer to provide dissipation and accumulation of frictional energy during operation are presented as well. Strategies for adaptive nanostructural coating design that enhance beneficial natural processes are outlined. The coatings exhibit emergent behavior during operation when their improved features work as a whole. In this way, as higher-ordered systems, they achieve multifunctionality and high wear resistance under extreme tribological conditions.

High-temperature nanoindentation testing of fused silica and other materials

Abstract This paper describes a new high-temperature stage for small-scale mechanical property testing. This allows the determination of the load-penetration curve of a diamond tip in a temperature range extending from room temperature to 400°C. Both sample and indenter can be heated separately. Indentation curves show that very low thermal drift can be achieved. Nanoindentation results are presented for gold, soda—lime glass, fused silica and a polyimide and compared with existing microscale and bulk mechanical property data where available. Results from fused silica show that its mechanical properties exhibit a completely different temperature dependence from those of soda-lime glass, as expected since fused silica is an anomalous glass.

Influence of mechanical properties on the nanoscratch behaviour of hard nanocomposite TiN/Si3N4 coatings on Si

A dual ion beam system has been used to produce hard nanocomposite TiN/Si3N4 coatings on silicon substrate. Mechanical properties have been determined by nanoindentation and tribological properties have been measured by nanoscratch testing. Nanoindentation showed that harder nanocomposites exhibited higher ratios of hardness to modulus (H/E). The dependence of the resistance to plastic deformation (H3/E2) on hardness was approximately linear. The H/E value influenced the nanoscratch behaviour. Coatings with higher H/E showed higher critical loads for elastic–plastic transition and also the total coating failure occurring in front of the probe. However, coatings with higher H/E also exhibited an unloading failure, occurring behind the probe at much lower load than the loading failure. Optimizing this stress-related unloading failure could be more important for tribological applications.

Modelling indentation creep of polymers: a phenomenological approach

The room temperature nanoindentation creep behaviour of a range of amorphous and semicrystalline polymers has been investigated. It was found that experimental data during the first 20 s at constant load were well fitted by a logarithmic equation relating the fractional increase in penetration depth during creep to [A/d(0)]ln(t/τ + 1) where A and τ are constants, d(0) is the initial penetration depth at the end of the loading period and t is the time at peak load. For samples of ultra-high molecular weight polyethylene and polypropylene, above their glass transition temperature at room temperature, it was found that the creep parameter A/d(0) increased at lower load and at higher loading rates when a Berkovich indenter was used. In contrast, samples of poly(methylmethacrylate) and poly(ethylene terephthalate), below their glass transition at the test temperature, showed no clear variation in A/d(0) with loading rate. The constant τ varied with loading rate (and hence loading time), following a power-law relationship. Fitting of creep data with the equation above enables prediction of the extent and rate of the creep occurring after any given combination of loading rate and maximum load with reasonable accuracy, particularly at higher load. Importance should be placed on the actual loading rate and time to reach peak load as this is shown to influence the initial creep kinetics dramatically.

Nanomechanical and nanotribological testing of ultra-thin carbon-based and MoST films for increased MEMS durability

Reliability of MEM (microelectromechanical) devices can be limited by stiction forces that develop in use. It is desirable to alter the mechanical and interfacial behaviour of the silicon surfaces by the application of very thin, low surface energy and low stress coatings. In this publication we report the nanomechanical and nanotribological characterization of a range of 5?150?nm thin films deposited on silicon by filtered cathodic vacuum arc (FCVA) and closed field unbalanced magnetron sputtering. A method of analysing nano-scratch data with spherical indenters is proposed. The method suggests the onset of non-elastic deformation in the nano-scratch test is due to substrate yield rather than film deformation on all but the softest films studied in this publication. The critical load for total film failure is a marked function of indenter radius, the ratio of hardness to modulus and the film thickness. The FCVA films were tested with probes of different radii (1.1, 3.1 and 9.0??m) and the critical load for film failure was found to vary strongly with probe radius. The deposition of <100?nm amorphous carbon films on Si could be a promising strategy for improving the reliability of Si-based MEMS devices as none of the very thin films tested underwent stress-related delamination failures that occur behind the indenter during the nano-scratch testing of thicker amorphous carbon films.

Nanocrystalline coating design for extreme applications based on the concept of complex adaptive behavior

The development of effective hard coatings for high performance dry machining, which is associated with high stress/temperatures during friction, is a major challenge. Newly developed synergistically alloyed nanocrystalline adaptive Ti0.2Al0.55Cr0.2Si0.03Y0.02N plasma vapor deposited hard coatings exhibit excellent tool life under conditions of high performance dry machining of hardened steel, especially under severe and extreme cutting conditions. The coating is capable of sustaining cutting speeds as high as 600 m/min. Comprehensive investigation of the microstructure and properties of the coating was performed. The structure of the coating before and after service has been characterized by high resolution transmission electron microscopy. Micromechanical characteristics of the coating have been investigated at elevated temperatures. Oxidation resistance of the coating has been studied by using thermogravimetry within a temperature range of 25–1100 °C in air. The coefficient of friction of the coatings ...

Evaluating the fracture properties and fatigue wear of tetrahedral amorphous carbon films on silicon by nano-impact testing

Abstract A repetitive contact technique, nano-impact testing, has been used to investigate the fracture properties of tetrahedral amorphous carbon (ta-C) thin films deposited on silicon by the filtered cathodic vacuum arc method. The impact test has shown clear differences in the resistance to impact wear of ta-C films with their thickness, for film thicknesses between 5 and 80 nm. The resistance to impact-induced fracture decreases as the film thickness increases. This may be due to the maximum shear stress being closer to the film–substrate interface, or reflect reduced toughness in the thicker films that are less able to deform as the substrate deforms plastically during the repetitive contact test. The mechanism of impact-induced failure on these thin films is (i) the initial impact stage where plastic deformation causes cracks to nucleate sub-surface, (ii) fatigue—further nucleation and growth of sub-surface cracks (with little or no change in probe depth) and (iii) crack coalescence and film fracture leading to a rapid change in probe depth as the film fails. The influence of impact load on fracture probability has been investigated. Fracture probability increases sharply as the impact load is increased from 100 to 300 μN. The greater load provides the stresses necessary to nucleate and propagate the sub-surface cracks; at low load the driving force for the cracks to coalesce and spall the coating is much reduced, so failure is less likely to occur within the test duration.

Improved nanomechanical test techniques for surface engineered materials

Abstract The development and implementation of a wide range of innovative nanomechanical test techniques to solve tribological problems in surface engineered systems are described in this review. By combining results with several different nanomechanical techniques, predictive design rules based on the elastic and plastic deformation energies involved in contact are proposed to optimise mechanical properties in the various contact situations that occur for different applications. Results are presented with the NanoTest platform for applications in biomedical devices, surface engineering of lightweight alloys, wear resistance of physical vapour deposition and chemical vapour deposition coatings as well as fracture fatigue resistance of diamond-like carbon coatings. Surface engineering to increase the ratio of hardness to elastic modulus (H/E) can be beneficial in a range of applications but care should be taken that, first, it be done without introducing too large intrinsic stress or stress discontinuities in mechanical contact loading, second, the severity of the contact results in high stresses and there is a requirement for some plasticity in contact to avoid fracture.

Nano-impact testing of TiFeN and TiFeMoN films for dynamic toughness evaluation

TiFeN and TiFeMoN films were deposited on silicon wafers by ion-beam-assisted deposition. Their mechanical properties were measured by nanoindentation (quasi-static) and nano-impact (dynamic) techniques. Nano-impact testing enabled assessment of their toughness and resistance to fatigue fracture under repetitive loading. At low impact forces, films with a higher resistance to plastic deformation (H3/E2) were much more resistant to the formation of cracks throughout the test. At higher impact forces, these films initially show impact resistance but with continued impacts they are unable to protect the Si substrate, performing as poorly as films with lower H3/E2 and suffer delamination from the Si substrate over a large area.

Micro-Impact Testing: A New Technique for Investigating Thin Film Toughness, Adhesion, Erosive Wear Resistance, and Dynamic Hardness

Abstract A small scale probe testing method has been developed for investigating the behaviour of thin films under dynamic loading conditions. The primary objective of the development was to produce quantifiable techniques that closely simulate the conditions that thin films experience in service. Variations of the technique allow measurements related to: impact wear and adhesion failure, erosive wear resistance, fracture toughness, work hardening, and dynamic hardness. The common element in each variant is the acceleration of a test probe (usually diamond) towards the specimen surface and the monitoring its instantaneous position before and after collision. The impact energy can be controlled and either single impacts or multiple impacts can be produced. For single impacts, the energy delivered to the contact point can be quantified, allowing calculation of a dynamic hardness number.