Damage Accumulation Phenomena in Multilayer (TiAlCrSiY)N/(TiAlCr)N, Monolayer (TiAlCrSiY)N Coatings and Silicon upon Deformation by Cyclic Nanoindentation.

The micromechanism of the low-cycle fatigue of mono- and multilayer PVD coatings on cutting tools was investigated. Multilayer nanolaminate (TiAlCrSiY)N/(TiAlCr)N and monolayer (TiAlCrSiY)N PVD coatings were deposited on the cemented carbide ball nose end mills. Low-cycle fatigue resistance was studied using the cyclic nanoindentation technique. The obtained results were compared with the behaviour of the polycrystalline silicon reference sample. The fractal analysis of time-resolved curves for indenter penetration depth demonstrated regularities of damage accumulation in the coatings at the early stage of wear. The difference in low-cycle fatigue of the brittle silicon and nitride wear-resistant coatings is shown. It is demonstrated that when distinguished from the single layer (TiAlCrSiY)N coating, the nucleation and growth of microcracks in the multilayer (TiAlCrSiY)N/(TiAlCr)N coating is accompanied by acts of microplastic deformation providing a higher fracture toughness of the multilayer nanolaminate (TiAlCrSiY)N/(TiAlCr)N.

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

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.

Why can tiAicrsiYN-based adaptive coatings deliver exceptional performance under extreme frictional conditions?

Adaptive TiAlCrSiYN-based coatings show promise under the extreme tribological conditions of dry ultra-high-speed (500-700 m min-1) machining of hardened tool steels. During high speed machining, protective sapphire and mullite-like tribo-films form on the surface of TiAlCrSiYN-based coatings resulting in beneficial heat-redistribution in the cutting zone. XRD and HRTEM data show that the tribo-films act as a thermal barrier creating a strong thermal gradient. The data are consistent with the temperature decreasing from approximately 1100-1200 degrees C at the outer surface to approximately 600 degrees C at the tribo-film/coating interface. The mechanical properties of the multilayer TiAICrSiYN/TiA1CrN coating were measured by high temperature nanoindentation. It retains relatively high hardness (21 GPa) at 600 degrees C. The nanomechanical properties of the underlying coating layer provide a stable low wear environment for the tribo-films to form and regenerate so it can sustain high temperatures under operation (600 degrees C). This combination of characteristics explains the high wear resistance of the multilayer TiAlCrSiYN/TiAICrN coating under extreme operating conditions. TiAlCrSiYN and TiAlCrN monolayer coatings have a less effective combination of adaptability and mechanical characteristics and therefore lower tool life. The microstructural reasons for different optimum hardness and plasticity between monolayer and multilayer coatings are discussed.

Nanoindentation of advanced polymers under non-ambient conditions: creep modelling and tan delta.

The creep modelling approach described in this paper reveals a correlation between the strain rate sensitivity parameter determined from room temperature nanoindentation data and tan delta determined from conventional dynamic mechanical analysis (DMA). With recent advances in nanomechanical instrumentation it is possible to obtain reliable raw data under non-ambient conditions. Nanoindentation tests on polymeric films at elevated temperature provide support for the correlation, with high values of the high strain rate sensitivity parameter only observed in the vicinity of the glass transition temperature. Analysis of the shapes of the indentation loading curves at elevated temperature has revealed a sharp decrease in the loading curve exponent near the glass transition. The nanoindentation approach holds promise for assessment of actual components (e.g., micro-moulded components) where production of DMA test pieces is not feasible.

Elevated temperature nanoindentation and viscoelastic behaviour of thin poly(ethylene terephthalate) films.

A commercial nanoindentation system fitted with a heating stage and heated indenter has been used to investigate how the elevated temperature nanoscale mechanical properties of poly(ethylene terephthalate) films vary with their processing history and crystallinity over the temperature range 60-110 degrees C. Three additive-free thin films were tested; an undrawn amorphous film, a uniaxially drawn film, and a commercial biaxially drawn Melinex film. A sharp decrease in mechanical properties was observed between 70 and 80 degrees C on the undrawn and uniaxial film consistent with the presence of a glass transition over this temperature range in agreement with literature values for bulk materials. In contrast, a gradual decrease in properties was observed over the same temperature range on the biaxially oriented film. The high crystallinity of the biaxial film could be beneficial in extending the operating temperature of the film. There is a minimum in the elastic recovery parameter around 80 degrees C on both the amorphous and biaxial film. This indicates that the elastic recovery parameter may be more sensitive to changes in mechanical properties occurring at/near the glass transition region than the hardness or modulus alone. A recently introduced dimensionless parameter for creep, A/d(0), was also found to be a promising way to characterise the increased time-dependent deformation around the glass transition region.