How do H/E and H3/E2 control coating system wear? - Insights gained from elevated temperature nanoindentation, scratch and impact tests

Please click Additional Files below to see the full abstract

Incipient plasticity in tungsten during nanoindentation: Dependence on surface roughness, probe radius and crystal orientation

The influence of crystallographic orientation, contact size and surface roughness effects on incipient plasticity in tungsten were investigated by nanoindentation with indenters with a range of end radius (150, 350, 720 and 2800 nm) in single crystal samples with the (100) and (111) orientations. Results for the single crystals were compared to those for a reference polycrystalline tungsten sample tested under the same conditions. Surface roughness measurements showed that the Ra surface roughness was around 2, 4, and 6 nm for the (100), (111) and polycrystalline samples respectively. A strong size effect was observed, with the stress for incipient plasticity increasing as the indenter radius decreased. The maximum shear stress approached the theoretical shear strength when W(100) was indented using the tip with the smallest radius. The higher roughness and greater dislocation density on the W(111) and polycrystalline samples contributed to yield occurring at lower stresses

Contact size effects on the friction and wear of amorphous carbon films

Since different properties of coating systems influence their friction and wear at different length scales contact size can play a critical role in microtribological experiments. In this study the behaviour of 3 different types of coating system which vary in terms of their thickness, substrate and mechanical properties has been investigated. The coatings were chosen for either their industrial relevance in automotive or MEMS applications, or as model coating systems. A wide range of nano/microtribological tests have been performed with different indenter geometries (tip sharpness), including single and repetitive scratch tests with unidirectional contact, and reciprocating wear tests, with depth and friction evolution monitored so that the relationships between failure mechanism and friction in coating systems with differing mechanical properties could be explored. The influence of surface topography on friction has been shown in ramped and constant load scratch tests. When fracture occurred resulting in a sudden increase in probe depth there was an abrupt decrease in friction which is ascribed to a contact area effect. In contrast, where deformation progressed through micro-wear a more gradual increase in depth can be associated with higher contact area and higher friction

Temperature dependence of indentation size effects, pile-up and strain rate sensitivity in polycrystalline tungsten from 25-950 C

Elevated temperature nanoindentation measurements were performed on polycrystalline tungsten to 950 ºC. Tests were carried out under high vacuum conditions as tungsten oxidizes in air at \u3e500 ºC. The temperature dependence of the hardness, elastic modulus, strain rate sensitivity, activation volume and the indentation size effect in hardness were investigated at 25, 750, 800, 850, 900 and 950 ºC. Thermal drift assessed from the last 60% of a hold period at 90% unloading was typically ~0.05 nm/s and it did not vary significantly with load or temperature [1]. The hardness measurements were in good agreement with previous determinations by non-depth sensing hot microhardness. Please click Additional Files below to see the full abstract

New instrumentation and analysis methodology for nano-impact testing

Nanoindentation testing has become increasingly popular for mechanical characterization of materials. This is motivated by the high versatility of the technique that allows testing of small volumes that could not be tested otherwise by macroscopic techniques, with minimal test preparation. The interest on nano-/microscale characterization of materials has been also extended to the study of high strain rate mechanical behaviour. One of the available techniques is nano-impact testing. It is carried out on a pendulum-based force-actuated, displacement-sensing device with the ability of performing energy-controlled impacts. The combination of conventional nanoindentation, for which a range of strain rates from 10-3 to 10-1 s-1 can be tested, with nano-impact provides a tool for materials characterization at the nano/microscale from 10-3 to 103 s-1. Regarding the analysis of nano-impact test results, there has been no consensus in literature over what material metrics to extract from the test. Several authors base the analysis of nano-impact test on the calculation of a dynamic hardness defined as change in kinetic energy throughout the impact divided by the residual volume of indentation [1-4]. However, there are two issues with the assumptions in which this equation is based. First, it only considers the change in kinetic energy and it neglects other important contributions like the work of impulse force. Then, it assumes that hardness is constant throughout the entire impact period. While for self-similar indenters this is true in the loading part, Cheng’s dimensional analysis shows that this is not the case in the unloading [5]. Therefore, the hardness calculated from this definition is not necessarily equal to the hardness under load commonly used in the instrumented indentation literature. To this end, an alternative analysis methodology is proposed. The analysis is based on the same definition of hardness under load commonly used in the instrumented indentation literature, computed as force divided by contact area. This way, the nano-impact hardness is directly comparable with results of conventional nanoindentation that use this definition. The instrumentation of the nanoindentation device with force-sensing capability was found crucial for the implementation of the analysis methodology. In addition, and in line with the nano-impact hardness definition in literature, an energy-based hardness is presented. The technique is assessed using finite element simulations and by testing six materials covering a wide range of mechanical behaviours. The FE simulations are used to assess the two energy-based definitions of hardness, the one in literature and the one proposed in this work. It was found that the literature definition leads to values that differ significantly from the ones obtained as force divided by contact area. On the other hand, the proposed energy-based definition provides values that match the ones obtained by force-approach. The experimental results are also in line with this conclusions. The literature energy-based hardness presents significant differences compared to the force-based hardness, which are higher for the more elastic materials. Furthermore, the force-based hardness computed from nano-impact results was compared with the hardness from conventional nanoindentation. A close match is found between both set of results. References [1] J.R. Trelewicz, C.A. Schuh, The Hall–Petch breakdown at high strain rates: Optimizing nanocrystalline grain size for impact applications, Appl. Phys. Lett. 93 (2008) 171916. [2] H. Somekawa, C.A. Schuh, High-strain-rate nanoindentation behavior of fine-grained magnesium alloys, Journal of Materials Research. 27 (2012) 1295–1302. [3] J.M. Wheeler, A.G. Gunner, Analysis of failure modes under nano-impact fatigue of coatings via high-speed sampling, Surface and Coatings Technology. 232 (2013) 264–268. [4] C. Zehnder, J.-N. Peltzer, J.S.K.-L. Gibson, S. Korte-Kerzel, High strain rate testing at the nano-scale: A proposed methodology for impact nanoindentation, Materials & Design. 151 (2018) 17–28. [5] Y.-T. Cheng, C.-M. Cheng, Scaling, dimensional analysis, and indentation measurements, Materials Science and Engineering: R: Reports. 44 (2004) 91–149. doi:10.1016/j.mser.2004.05.001

Micro-impact testing of AlTiN and TiAlCrN coatings

A novel micro-scale repetitive impact test has been developed to assess the fracture resistance of hard coatings under dynamic high strain rate loading. It is capable of significantly higher impact energies than in the nano-impact test. It retains the intrinsic depth-sensing capability of the nano-impact test enabling the progression of the damage process to be monitored throughout the test, combined with the opportunity to use indenters of less sharp geometry and still cause rapid coating failure. The micro-impact test has been used to study the resistance to impact fatigue of Al-rich PVD nitride coatings on cemented carbide. The impact fatigue mechanism has been investigated in nano- and micro-scale impact tests. Coating response was highly load-dependent. A Ti0.25Al0.65Cr0.1N coating with high H3/E2 performed best in the nano- and micro- impact tests although it was not the hardest coating studied. The role of mechanical properties, microstructure and thickness on impact behaviour and performance in cutting tests is discussed

Designing nanoindentation simulation studies by appropriate indenter choices: Case study on single crystal tungsten

Atomic simulations are widely used to study the mechanics of small contacts for many contact loading processes such as nanometric cutting, nanoindentation, polishing, grinding and nanoimpact. A common assumption in most such studies is the idealisation of the impacting material (indenter or tool) as a perfectly rigid body. In this study, we explore this idealisation and show that active chemical interactions between two contacting asperities lead to significant deviations of atomic scale contact mechanics from predictions by classical continuum mechanics. We performed a testbed study by simulating velocity-controlled, fixed displacement nanoindentation on single crystal tungsten using five types of indenter (i) a rigid diamond indenter (DI) with full interactions, (ii) a rigid indenter comprising of the atoms of the same material as that of the substrate i.e. tungsten atoms (TI), (iii) a rigid diamond indenter with pairwise attraction turned off, (iv) a deformable diamond indenter and (v) an imaginary, ideally smooth, spherical, rigid and purely repulsive indenter (RI). Corroborating the published experimental data, the simulation results provide a useful guideline for selecting the right kind of indenter for atomic scale simulations

Elevated temperature repetitive micro-scratch testing of AlCrN, TiAlN and AlTiN PVD coatings

In developing advanced wear-resistant coatings for tribologically extreme highly loaded applications such as high speed metal cutting a critical requirement is to investigate their behaviour at elevated temperature since the cutting process generates frictional heat which can raise the temperature in the cutting zone to 700–900 °C or more. High temperature micro-tribological tests provide severe tests for coatings that can simulate high contact pressure sliding/abrasive contacts at elevated temperature. In this study ramped load micro-scratch tests and repetitive micro-scratch tests were performed at 25 and 500 °C on commercial monolayer coatings (AlCrN, TiAlN and AlTiN) deposited on cemented carbide cutting tool inserts. AlCrN exhibited the highest critical load for film failure in front of the moving scratch probe at both temperatures but it was prone to an unloading failure behind the moving probe. Scanning electron microscopy showed significant chipping outside the scratch track which was more extensive for AlCrN at both room and elevated temperature. Chipping was more localised on TiAlN although this coating showed the lowest critical loads at both test temperatures. EDX analysis of scratch tracks after coating failure showed tribo-oxidation of the cemented carbide substrate. AlTiN showed improved scratch resistance at higher temperature. The von Mises, tensile and shear stresses acting on the coating and substrate sides of the interface were evaluated analytically to determine the main stresses acting on the interface. At 1 N there are high stresses near the coating-substrate interface. Repetitive scratch tests at this load can be considered as a sub-critical load micro-scale wear test which is more sensitive to adhesion differences than the ramped load scratch test. The analytical modelling showed that a dramatic improvement in the performance of AlTiN in the 1 N test at 500 °C could be explained by the stress distribution in contact resulting in a change in yield location due to the high temperature mechanical properties. The increase in critical load with temperature on AlTiN and AlCrN is primarily a result of the changing stress distribution in the highly loaded sliding contact rather than an improvement in adhesion strength

Probe geometry and surface roughness effects in microscale impact testing of WC-Co

Depth-sensing repetitive microimpact tests have been performed on cemented carbide cutting tool inserts with spheroconical diamond probes with end radii of 8, 20 and 100 µm. Results were strongly dependent on the probe radius and applied load. At higher load, there was a transition to a faster damage rate marking the onset of more variability in rate and in the residual depth of the impact crater when using 8 and 20 µm probes. SEM images show the breakup of the WC skeleton at the periphery of the contact zone. Lower surface roughness slowed the initial damage rate at a higher load but did not significantly influence the final crater depth. The load-dependent fatigue mechanism displayed by the cemented carbide also has implications for the study and optimization of coatings when these are deposite

High frequency acoustic emission monitoring in nano-impact of bulk ceramics

Acoustic Emission (AE) monitoring is proving a powerful technique in improving our understanding of deformation processes occurring at the nano- and micro-scale [1-3]. The recent development of advanced high frequency AE sensor systems and their integration to commercial nanomechanical test instrumentation has been a catalyst for research into damage processes in scratching thin films [2,3]. In recent studies AE detection revealed the onset of cracking otherwise undetectable but subsequently confirmed by FIB sectioning. In this current study the high frequency AE technique has been used to investigate the cracking behavior of bulk technical ceramics (MgO-partially stabilized zirconia (PSZ) and zirconia-toughened alumina (ZTA)) at low strain rate in nanoindentation and at higher strain rate in the nano-impact test. Although both ceramics were susceptible to indentation or impact-induced cracking it was more prevalent on PSZ. The influence of accelerating force and probe geometry were explored in the higher strain rate tests with cube-corner and 5 mm radius spherical diamond indenters. Large bursts of AE were observed at the abrupt displacement step in repetitive impact with a cube corner but were generally much smaller/absent during other impact events. The AE response with the 5 mm radius probe was quite different with smaller displacement events and AE bursts on most impacts at higher load. References [1] Indentation testing and its acoustic emission response: applications and emerging trends, N.H. Faisal, R. Ahmed and R.L. Reuben, International Materials Reviews 56 (2011) 98. [2] On the importance of combined scratch/acoustic emission test evaluation: SiC and SiCN thin films case study, J. Tomastik et al, Coatings 8 (2018) 196 [3] Effect of nitrogen doping and temperature on mechanical durability of silicon carbide thin films, J. Tomastik et al, Sci Rep 8 (2018) 10428