Wear and friction of TiAlN/VN coatings against Al2O3 in air at room and elevated temperatures

TiAlN/VN multilayer coatings exhibit excellent dry sliding wear resistance and low friction coefficient, reported to be associated with the formation of self-lubricating V2O5. To investigate this hypothesis, dry sliding ball-on-disc wear tests of TiAlN/VN coatings on flat stainless steel substrates were undertaken against Al2O3 at 25 C, 300 C and 635 C in air. The coating exhibited increased wear rate with temperature. The friction coefficient was 0.53 at 25 C, which increased to 1.03 at 300 C and decreased to 0.46 at 635 C. Detailed investigation of the worn surfaces was undertaken using site-specific transmission electron microscopy (TEM) via focused ion beam (FIB) microscopy, along with Fourier transform infrared (FTIR) and Raman spectroscopy. Microstructure and tribo-induced chemical reactions at these temperatures were correlated with the coating’s wear and friction behaviour. The friction behaviour at room temperature is attributed to the presence of a thin hydrated tribofilm and the presence of V2O5 at high temperature

High resolution observations of friction-induced oxide and its interaction with the worn surface

A detailed transmission electron microscopy study of oxide and oxygen-containing phase formation during the sliding wear of metals, composites and coatings is provided. A wide range of different materials types are reported in order to compare and contrast their oxidational wear behaviour: a low carbon stainless steel, a H21 tool steel containing 7%TiC particles, a 17%Cr white iron,an Al–Si/30%SiC composite, an Al–alloy (6092)–15%Ni3Al composite and finally a 3rd generation TiAlN/CrN ‘superhard’ multilayer coating. For the ferrous alloys, nanoscale oxides and oxygen-containing phases were formed that exhibited excellent adhesion to the substrate. In all cases, an increase in oxide coverage of the surface was associated with a decrease in Lancaster wear coefficient. The oxide at the surface of the 316L and H21+7%TiC was found to deform with the substrate, forming a mechanically mixed layer that enhanced surface wear resistance. Evidence of oxidational wear is presented for the wear of the Al–Si–30%SiC composite, but this did not give a beneficial effect in wear, a result of the brittle nature of the oxide that resulted in detachment of fine (150nm) thick fragments. The worn surface of the Al–alloy (6092)–15%Ni3Al and TiAlN/CrN coating was characterized by reaction with the counterface and subsequent oxidation, the product of which enhanced wear resistance. The observations are related to the classical theory of oxidational wear

Dry sliding wear behaviour of powder metallurgy Al-Mg-Si alloy-MoSi2 composites and the relationship with the microstructure

The effect of the microstructure on the dry sliding wear of six aluminium alloy 6061 matrix composites reinforced with 15vol.% of MoSi2 particles and two monolithic 6061 alloys processed by powder metallurgy with and without ball milling has been studied. Wear testing was undertaken using pin-on-ring configuration against an M2 steel counterface at 0.94m/s and normal load of 42, 91 and 140N. The wear resistance of the aluminium alloys was significantly improved by ball milling and the addition of reinforcing MoSi2 particles due to a more stable and more homogeneous microstructure, which avoids the detachment of the mechanically mixed layer. Wear rate of materials in T6 decreases as solutionized hardness of the materials increases. This behaviour is rationalized by taking into account the precipitation state of the matrix. In addition, wear rate follows a Hall-Petch type relationship, showing that the reduction of matrix grain size plays an important role in the increase in the wear resistance of the composites. The results indicate that the present intermetallic reinforced composites can be considered potential substitutes for ceramic reinforced aluminium alloys in tribological applications. © 2011 Elsevier B.V.Peer Reviewe

The effect of (Ti + Al): V ratio on the structure and oxidation behaviour of TiAlN/VN nano-scale multilayer coatings

Nano-scaled multilayered TiAlN/VN coatings have been grown on stainless steel and M2 high speed steel substrates at U-B = - 85 V in an industrial, four target, Hauzer HTC 1000 coater using combined cathodic steered arc etching/unbalanced magnetron sputtering. X-ray diffraction (XRD) has been used to investigate the effects of process parameters (Target Power) on texture evolution (using texture parameter T*), development of residual stress (sin(2) psi method) and nano-scale multilayer period. The composition of the coating was determined using energy dispersive X-ray analysis. The thermal behaviour of the coatings in air was studied using thermo-gravimetric analysis, XRD and scanning electron microscopy. The bi-layer period varied between 2.8 and 3.1 nm and in all cases a {1 1 0} texture developed with a maximum value T* = 4.9. The residual stress varied between -5.2 and -7.4 GPa. The onset of rapid oxidation occurred between 628 and 645 degreesC depending on the (Ti+Al):V ratio. After oxidation in air at 550 degreesC AlVO4, TiO2 and V2O5 Phases were identified by XRD with the AlVO4, TiO2 being the major phases. The formation of AlVO4 appears to disrupt the formation of Al2O3 which imparts oxidation resistance to TiAlN based coatings. Increasing the temperature to 600 and 640 degreesC led to a dramatic increase in the formation of V2O5 which was highly oriented (0 0 1) with a plate-like morphology. At 640 degreesC there was no evidence of the coating on XRD. Increasing the temperature to 670 degreesC led to further formation of AlVO4 and a dramatic reduction in V2O5. (C) 2003 Elsevier B.V. All rights reserved

The use of Fe-30% Ni and Fe-30% Ni-Nb alloys as model systems for studying the microstructural evolution during the hot deformation of austenite

The development of physically-based models of microstructural evolution during thermomechanical processing of metallic materials requires knowledge of the internal state variable data, such as microstructure, texture, and dislocation substructure characteristics, over a range of processing conditions. This is a particular problem for steels, where transformation of the austenite to a variety of transformation products eradicates the hot deformed microstructure. This article reports on a model Fe-30wt% Ni-based alloy, which retains a stable austenitic structure at room temperature, and has, therefore, been used to model the development of austenite microstructure during hot deformation of conventional low carbon-manganese steels. It also provides an excellent model alloy system for microalloy additions. Evolution of the microstructure and crystallographic texture was characterized in detail using optical microscopy, X-ray diffraction (XRD), SEM, EBSD, and TEM. The dislocation substructure has been quantified as a function of crystallographic texture component for a variety of deformation conditions for the Fe-30% Ni-based alloy. An extension to this study, as the use of a microalloyed Fe-30% Ni-Nb alloy in which the strain induced precipitation mechanism was studied directly. The work has shown that precipitation can occur at a much finer scale and higher number density than hitherto considered, but that pipe diffusion leads to rapid coarsening. The implications of this for model development are discussed

Cross sectional TEM analysis of duplex HIPIMS and DC magnetron sputtered Mo and W doped carbon coatings

A FIB lift-out sample was made from a wear-resistant carbon coating deposited by high power impulse magnetron sputtering (HIPIMS) with Mo and W. TEM analysis found columnar grains extending the whole ∼1800 nm thick film. Within the grains, the carbon was found to be organised into clusters showing some onion-like structure, with amorphous material between them; energy dispersive X-ray spectroscopy (EDS) found these clusters to be Mo- and W-rich in a later, thinner sample of the same material. Electron energy-loss spectroscopy (EELS) showed no difference in C-K edge, implying the bonding type to be the same in cluster and matrix. These clusters were arranged into stripes parallel to the film plane, of spacing 7-8 nm; there was a modulation in spacing between clusters within these stripes that produced a second, coarser set of striations of spacing ∼37 nm

TEM-EELS study of low-friction superlattice TiAlN/VN coating: the wear mechanisms

A 20-50 nm thick tribofilm was generated on the worn surface of a multilayer coating TiAlN/VN after dry sliding test against an alumina counterpart. The tribofilm was characterized by applying analytical transmission electron microscopy techniques with emphasis on detailed electron energy loss spectrometry and energy loss near edge structure analysis. Pronounced oxygen in the tribofilm indicated a predominant tribo-oxidation wear. Structural changes in the inner-shell ionization edges of N, Ti and V suggested decomposition of nitride fragments

Bird-Like Anatomy, Posture, and Behavior Revealed by an Early Jurassic Theropod Dinosaur Resting Trace

BACKGROUND: Fossil tracks made by non-avian theropod dinosaurs commonly reflect the habitual bipedal stance retained in living birds. Only rarely-captured behaviors, such as crouching, might create impressions made by the hands. Such tracks provide valuable information concerning the often poorly understood functional morphology of the early theropod forelimb. METHODOLOGY/PRINCIPAL FINDINGS: Here we describe a well-preserved theropod trackway in a Lower Jurassic ( approximately 198 million-year-old) lacustrine beach sandstone in the Whitmore Point Member of the Moenave Formation in southwestern Utah. The trackway consists of prints of typical morphology, intermittent tail drags and, unusually, traces made by the animal resting on the substrate in a posture very similar to modern birds. The resting trace includes symmetrical pes impressions and well-defined impressions made by both hands, the tail, and the ischial callosity. CONCLUSIONS/SIGNIFICANCE: The manus impressions corroborate that early theropods, like later birds, held their palms facing medially, in contrast to manus prints previously attributed to theropods that have forward-pointing digits. Both the symmetrical resting posture and the medially-facing palms therefore evolved by the Early Jurassic, much earlier in the theropod lineage than previously recognized, and may characterize all theropods

Powder bed generation in integrated modelling of additive layer manufacturing of orthopaedic implants

This paper presents an original model of powder bed generation developed within the frame of an integrated modelling approach for studying the interaction of physical mechanisms in additive layer manufacturing (ALM) of orthopaedic implants. The model is based on cellular automata (CA) approach and describes the relationship between moving particles of different sizes during deposition on a surface in three dimensions. The surface is defined by the horizontal two-dimensional CA on which particles fall and irreversibly stick to a growing deposit. The model allows for consideration of different restructuring cases when particles are allowed to rotate as often as necessary until achievement of a local minimum position. Changes in the packing density of the powder bed have been investigated numerically depending on technological parameters, such as particle size distribution, deposition rate and sequence of powder deposition. The model has been developed with the aim of merging to the finite element (FE)-based integrated model and is applicable to a different ranges of materials including metals and also non-metals