W.M. Rainforth

Precipitation of NBC in a model austenitic steel

A model Fe–30 wt% Ni, 0.1 C, 1.61 Mn, 0.1 Nb microalloyed steel, that simulates conventional microalloyed C–Mn steels, but does not transform from the austenite phase on cooling, is reported. Plane strain compression testing was undertaken at 950°C at a constant true strain rate of 10 s−1. Samples were deformed in a two stage process. An initial true strain of 0.25–0.45 was followed by unloading, a hold of 1–1000 s and a final deformation to a total true strain of 0.5–0.9. A single deformation was undertaken under identical conditions, but to the total true strain of the double deformation tests. Electron spectroscopic imaging (ESI) in the TEM was used to determine precipitate size and distribution. A 1 s hold time between equal strains of ϵ=0.25 was sufficient for appreciable strain induced precipitation, although 40% static recrystallisation occurred during the hold time. Precipitation occurred entirely on dislocations, present principally as microband walls but also as a rudimentary cell structure within the microbands. No evidence was found for NbC precipitation in the matrix, which therefore remains supersaturated with Nb. NbC particle diameter was in the range 2.5–15 nm, with a density of 3.8×1021 particles/m3 for a 100 s delay period between two strains of ϵ=0.45 at 950°C. Both the size and number density are consistent with those observed in conventional microalloyed C–Mn steels. The behaviour of the model microalloyed Fe–30 Ni steel is discussed in relation to the data on conventional microalloyed steels.

Deformation structures induced by sliding contact

Abstract The deformed microstructure generated by the sliding contact of a low carbon 316 stainless steel against a ceramic counterface has been investigated by transmission electron microscopy. Deformation occurred by twinning followed by intense shear banding. Once formed, the shear bands were shown to accommodate a large strain by crystallographic slip. The structure and the texture observed indicated that the deformation could be best described by simple shear (torsion) which led to a saturation flow stress. Contact was shown to be plastic over the load range 2-55N(pin)−1. Wear debris was apparently created within freshly formed shear bands at the outermost surface, coinciding with the maximum strain. The implications of the microstructural evolution are discussed in terms of wear theories and behaviour of metals at high strains.

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.

The role of trace additions of alumina to yttria–tetragonal zirconia polycrystals (Y–TZP)

Abstract We demonstrate, using high spatial resolution electron microscopy and surface science techniques, the segregation of Al 3+ and relative increase in the Y 3+ concentration at grain boundaries in a 5.2 wt.% yttria stabilised TZP doped with 0.15 wt.% Al 2 O 3 ; free of any intergranular glassy film.

Tribological investigation of TiAlCrN and TiAlN/CrN coatings grown by combined steered-arc/unbalanced magnetron deposition

The dry sliding wear of monolayer TiAlCrN and multilayer TiAlN/CrN coatings has been investigated against a BM2 tool steel counterface. The coatings were deposited on a BM2 tool steel substrate by combined steered-arc/unbalanced-magnetron deposition. Increasing either contact load or sliding speed led to a reduction in friction coefficient, typically from 1.1 to 0.2. Increasing load resulted in an increase in wear rate for both TiAlCrN and TiAlN/CrN (e.g. from 7×10-6 mm3/m at 22 N to 4×10-5 mm3/m at 189 N for the TiAlCrN monolayer coating, and from 7×10-6 mm3/m at 22 N to 2.5×10-5 mm3/m at 189 N for TiAlN/CrN multilayer). The wear rate for all coatings was at least an order of magnitude lower than the uncoated BM2 steel. The wear rate of the TiAlCrN coating tended to decrease with an increase in sliding speed (from 7.4×10-5 mm3/m at 0.2 m/s to 1.3×10-5 mm3/m at 1.1m/s) while the wear rate of the TiAlN/CrN was approximately constant as a function of sliding speed (∼1.5×10-5 mm3/m).

Deceleration of hydrothermal degradation of 3Y-TZP by alumina and lanthana co-doping

Zirconia has been used as an orthopaedic material since 1985 and is increasingly used in dental applications. One major concern with the use of zirconia is the significant loss in mechanical properties through hydrothermal degradation, with the uncontrolled transformation of tetragonal to monoclinic (t→m) zirconia. We report on the addition of alumina and lanthana as dopants to an yttria-stabilized tetragonal zirconia polycrystal ceramic as an effective strategy to significantly decelerate the hydrothermal degradation kinetics, without any loss of mechanical properties, in particular, fracture toughness. Hydrothermal degradation was studied on the exposed surface as well as in the sub-surface region using Raman microspectroscopy, atomic force microscopy and cross-sectional transmission electron microscopy, providing a comprehensive insight into the mechanism of propagation of the t→m transformation. The addition of dopants resulted in the reduction of monoclinic zirconia nucleation rate at the surface and a substantial deceleration of the overall transformation kinetics, in particular a greatly reduced propagation of the transformation into the bulk and decreased grain boundary microcracking. High-resolution transmission electron microscopy analysis showed that the co-dopant cations segregate to the grain boundaries where they play a key role in the stabilization of the zirconia tetragonal phase.

The effects of notch width on the SENB toughness for oxide ceramics

Abstract The fracture toughness of a range of oxide ceramics, namely aluminas with various grain sizes, Y2O3-stabilized tetragonal zirconia polycrystals (Y-TZP), MgO-partially stabilized zirconias (Mg-PSZ) and zirconia-toughened ceramics (ZTC), has been measured by the single edge notch beam (SENB) technique using machined notches of varying widths from 150 to 1800 μm. It was found that the response of each material was critically dependent on its microstructure. Experimental results show that where ‘dynamic’ toughening mechanisms occur, such as transformation toughening, SENB toughness depends strongly on the notch width. For example, the SENB toughness of Y2O3-stabilized tetragonal zirconia polycrystals and MgO-partially stabilized zirconia ceramics, which are typical of transformation toughened ceramics, increases significantly with increasing notch width when the notch width is less than 1 mm. A further increase in the notch width results in only a slight increase in the SENB toughness. Zirconia-toughened alumina (ZTA) ceramics show similar behaviour, although the increase in SENB toughness is smaller than that for Y-TZP ceramics. The notch width dependence of SENB toughness for the transformation toughened ceramics is related to the transformation zone at the notch tip, induced by diamond blade machining. In contrast, the SENB toughness of alumina ceramics shows very little response to the variation in notch width. A discussion on the relationships between SENB toughness values measured and the operative toughening mechanisms is presented.

Wear mechanisms of monolithic and multicomponent nitride coatings grown by combined arc etching and unbalanced magnetron sputtering

Polycrystalline nitride coatings TiAlCrN and TiAlCrYN have been grown by combined steered arc etching and unbalanced magnetron sputtering. The multicomponent coatings were found to exhibit superior wear resistance compared with commercial TiN and CrN coatings in dry-sliding conditions at low load, but similar wear rates at higher loads. An understanding of the wear mechanism was obtained by analytical transmission electron microscopy and scanning electron microscopy of the worn surfaces. The sliding wear was dominated by a surface film of Fe-based oxides, formed through material transfer from the tool steel counterface. For the TiN, this layer contained a significant contribution from the coating, while for the TiAlCrYN there was only a minimal content. Below this, a thin layer of plastically deformed material was observed. Pre-existing peaks on the surface, arising from deposition defects, were preferentially removed during the early stages of wear, resulting in locally more severe surface deformation and consequent delamination. At higher load or low speed, more severe wear modes were observed associated with (a) cracks due to friction traction; (b) delamination sheets resulting from coalescence of cracks perpendicular and parallel to the worn surface; (c) cohesive cracking and detachment of the film due to substrate deformation; and (d) more severe tribo-oxidation than found in the mild wear regime.

A comparison of domain images obtained for nanophase alloys by magnetic force microscopy and high resolution Lorentz electron microscopy

The study of domain images in advanced magnetic materials is central to their further development and exploitation. For nanophase exchange-coupled magnetic materials, high spatial resolution is required for both topographic and magnetic imaging. In this paper we make a direct comparison of magnetic force microscopy and Lorentz microscopy, in order to establish the degree of complementarity and agreement between the two techniques. Examples will be drawn from both hard (Nd-Fe-B) and soft (Fe-based) nanophase magnetic materials.

Electron energy-loss spectroscopy (EELS) studies of an yttria stabilized TZP ceramic

Abstract Electron energy-loss spectroscopy (EELS) was used to examine the distribution of zirconia polymorphs within a sintered 3Y–TZP ceramic prepared by co-milling via detailed analysis of the energy-loss near-edge structure (ELNES) at the oxygen K-edge. Measurement of the peak positions and the overall shape of the fine structure in the oxygen K ELNES clearly facilitates the differentiation between the cubic, tetragonal and monoclinic zirconia phases within the bulk material. Examination of t –ZrO 2 : t –ZrO 2 grain–boundary interfaces revealed significant co-segregation of the cation dopants Al 3+ and Y 3+ . However, the ELNES at the oxygen K-edge exhibited characteristics which suggest that the oxygen coordination at the boundary interface is similar to that of the bulk tetragonal grains for the volume sampled by the electron beam.