Search

Impact nanoindentation using a pendulum-based nanoindenter is an attractive technique for characterizing the high strain rate behavior of materials. However, pendulum-based systems typically allow the impacting indenter to strike multiple times as the pendulum rebounds and settles against the sample surface. This causes uncertainty as to whether the deformation is caused solely by the first impact or whether additional deformation occurs during the additional impacts. This work answers this question by using a modified system with a catching solenoid to prevent rebounding impacts. The difference in residual impressions made over a range of incident velocities between rebounding and single impacts is characterized using AFM. The rebounding impacts appear to be purely elastic and any additional deformation appears to be within the variability of the test.

Abstract: The sintering of free-standing plasma sprayed TBCs has been modeled, based on variational principles of free energy minimization and comparisons are made with experimental results. Predictions of through-thickness shrinkage and changing pore surface area are compared with the experimental data obtained by dilatometry and BET analysis, respectively. The sensitivity of the predictions to initial pore architecture and material properties is assessed. The model can be used to predict the evolution of the contact area between overlying splats. This is in turn related to the through-thickness thermal conductivity, using a previously developed analytical model (I.O. Golosnoy, et al. J. Therm. Spray Technol., 2005, 14(2), p 205-214).

Yttria-stabilized zirconia powders, containing different levels of SiO2 and Al2O3, have been plasma sprayed onto metallic substrates. The coatings were detached from their substrates and a dilatometer was used to monitor the dimensional changes they exhibited during prolonged heat treatments. It was found that specimens containing higher levels of silica and alumina exhibited higher rates of linear contraction, in both in-plane and through-thickness directions. The in-plane stiffness and the through-thickness thermal conductivity were also measured after different heat treatments and these were found to increase at a greater rate for specimens with higher impurity (silica and alumina) levels. Changes in the pore architecture during heat treatments were studied using Mercury Intrusion Porosimetry (MIP). Fine scale porosity (<∼50 nm) was found to be sharply reduced even by relatively short heat treatments. This is correlated with improvements in inter-splat bonding and partial healing of intra-splat microcracks, which are responsible for the observed changes in stiffness and conductivity, as well as the dimensional changes.

Abstract: It has been found that reducing the level of impurity oxides (particularly SiO2 and Al2O3) in 7YSZ, from about 0.2 wt% to below 0.1 wt% raises the sintering resistance and the phase stability of plasma-sprayed coatings. The implications for the usage of these coatings at elevated temperatures are examined. It is concluded that using relatively high-purity powder of this type is likely to confer substantial benefits in terms of the thermomechanical stability of the coatings under service conditions.

A brief analysis is presented of how heat transfer takes place in porous materials of various types. The emphasis is on materials able to withstand extremes of temperature, gas pressure, irradiation, etc., i.e. metals and ceramics, rather than polymers. A primary aim is commonly to maximize either the thermal resistance (i.e. provide insulation) or the rate of thermal equilibration between the material and a fluid passing through it (i.e. to facilitate heat exchange). The main structural characteristics concern porosity (void content), anisotropy, pore connectivity and scale. The effect of scale is complex, since the permeability decreases as the structure is refined, but the interfacial area for fluid–solid heat exchange is, thereby, raised. The durability of the pore structure may also be an issue, with a possible disadvantage of finer scale structures being poor microstructural stability under service conditions. Finally, good mechanical properties may be required, since the development of thermal gradients, high fluid fluxes, etc. can generate substantial levels of stress. There are, thus, some complex interplays between service conditions, pore architecture/scale, fluid permeation characteristics, convective heat flow, thermal conduction and radiative heat transfer. Such interplays are illustrated with reference to three examples: (i) a thermal barrier coating in a gas turbine engine; (ii) a Space Shuttle tile; and (iii) a Stirling engine heat exchanger. Highly porous, permeable materials are often made by bonding fibres together into a network structure and much of the analysis presented here is oriented towards such materials.

It is shown that extrusion of “FORMGRIP” precursor material prior to baking leads to accelerated foaming and finer, more uniform cell structures. This effect is correlated with microstructural observations made on the precursor materials and on the foams. It is thought that, while extrusion apparently does not extensively fracture the gas‐generating hydride particles, it does induce some damage to their protective surface oxide layers. Removal of the porosity normally present in as‐cast precursors may also be beneficial.

The impact response of laminated composites consisting of alternate layers of Al alloy foam and Al2O3was studied experimentally in low and intermediate velocity regimes. Low velocity impacts (1.2–2.8 m s-1) were conducted using an instrumented falling weight apparatus and were compared with static indentation tests (0.2×10-4m s-1). Intermediate velocity impacts were carried out by means of both Hopkinson bar (60 m s -1)and gas gun (200 m s-1) tests. Post­impact damage was assessed using X-ray radiography and microscopy. It was found that there is good correlation between low velocity impact and quasi­static responses. In both cases, penetration of the layered targets resulted in the formation of a distinctive plug. Increasing impact velocity (intermediate velocity range) switched the penetration mode from plugging to fragmentation, giving rise to an increase in the absorbed energy. In this range, impacts led to localisation of damage in the region under the projectile. Furthermore, a comparison has been made between the penetration response of foam laminates and dense metal laminates of equivalent areal density. Preliminary results suggest that the dense metal laminates are superseded by the foam laminates on an energy absorption basis.

Further development and applications of the analytical model, presented in Part 1 and Part 2, are presented in this paper. These involve extension of the analysis for bi-material systems to multi-layered cases and determination of the critical strain energy release rate from a knowledge of the point at which debonding occurs. Mention is also made of the possibility that, since relaxation of stresses parallel to the crack front cannot occur close to the crack tip, the associated strain energy may not contribute fully to the effective strain energy release rate. An outline is given of how the model can be used in conjunction with experimental data on specimen curvature to determine the deposition stress of the coating. Some suggestions are also made about how predicted residual stress distributions could be validated, using X-ray diffraction or layer removal techniques.

An analytical model has been developed to predict the residual stress distributions in progressively deposited coatings, such as those produced by thermal spraying. This is based on the concept of a misfit strain, caused by either the deposition stress (e.g. due to quenching of splats in thermal spraying) or by differential thermal contraction between substrate and coating during cooling. The deposition stress is introduced as the coating is formed layer-by-layer, with a specified layer thickness, such that the misfit strain is accommodated after each layer addition (rather than for the coating as a whole). From a knowledge of deposition temperatures, material properties and specimen dimensions, residual stress distributions can be predicted. The model is straightforward. It can be implemented with a simple customised computer program or by using a standard spreadsheet. Comparisons are presented between predictions from this model and from a numerical model for three plasma sprayed systems. Good agreement is observed and the predictions also serve to highlight various features of typical sprayed systems.

An analytical model is presented to predict the residual stress distributions in progressively deposited coatings with cylindrical substrates. Misfit strains, due either to the deposition stress or to differential thermal contraction, set up axial forces in the longitudinal direction and interfacial pressures in the radial direction. Use of stress-strain relationships in the longitudinal and hoop directions allows these forces and pressures to be evaluated. A layer-by-layer approach is employed to determine the stresses set up in the coatings during deposition. Predicted stress distributions are presented for three plasma sprayed systems. Comparisons with a numerical model, of the type presented in Part 1 for planar systems, have not been made, but the behaviour predicted is intuitively as expected. In addition, the model has been applied to predict the through-thickness stresses in specimens with planar substrates for the case when bending is not allowed, by artificially assigning a large inner radius to the substrate.

Not Available

Diamond deposition has been carried out on commercial purity (CP) titanium and Ti-6A1-4V substrates, using hot filament chemical vapour deposition. Interfacial reactions have been studied in detail using scanning electron microscopy, X-ray photoelcctron spectroscopy, optical microscopy and X-ray diffraction. Hydrogen suppresses diamond nuclealion by etching the diamond debris left on the substrate surface after polishing. Hydride precipitation was observed throughout the substrate. This caused embrittlement. TiC formation on the substrate surface started after 5 min. In the Ti-6A1-4V alloy and in the CP titanium, TiC formation occurred preferentially on the β-phase and on grain boundaries respectively. The growth of TiC competes with diamond formation for the available carbon. Relatively low temperatures inhibit TiC formation more than diamond formation, allowing the diamond nuclei to grow faster in the early stages. The formation of a thick TiC layer also appears to promote interfacial debonding and spallation of the diamond film. Prior bombardment with carbon ions appears to be beneficial as a result of allowing TiC growth to occur without removing as much atmospheric carbon, so that the diamond film can form in more intimate contact with the substrate.

The durability of diamond-like carbon (DLC) films, deposited by an r.f. plasma-assisted glow discharge chemical vapour deposition (CVD) technique onto metallic substrates (304 steel and Ti-6Al-4V), has been measured by a novel method in which the coating is bombarded with small solid particles in a carefully controlled manner. Two different types of particle, soft rounded glass beads and hard angular silica particles, were used to examine the effect of particle type on the wear behaviour. The durability was measured both before and after exposure of the specimens to various biological fluids, including distilled water, phosphate-buffered saline (PBS) solution and 40% bovine serum in sterile PBS. The specimens were exposed for 7 and 28 days at 37 °C in a humidified atmosphere containing 5% carbon dioxide. One set of experiments was carried out at 60 °C to determine the effect of temperature on the film stability. Environmental exposure had little direct effect on the film properties, but the fluids tended to degrade the interfacial adhesion when penetration to the interface was possible. This can occur via microscopic penetrations in the film or via the edges. The buffered saline solution appeared to be the most aggressive in this respect. A correlation between the damage mechanism during solid particle erosion and the type of particle used was found.

An experimental investigation is presented of a multi-stage purification process which utilizes fractional solidification in a way related to that employed in zone melting. The process, for which the term cascade purification has been suggested, is shown to be technically viable. The validity of a previous theoretical analysis of the expected solute redistribution has been broadly confirmed via radiatracer detection of tin and zinc in an aluminium base. Factors controlling the purification efficiency are outline, with particular reference to minimizing the solute-enriched boundary layer ahead of the growth front through control of the melt convection.

Hydrogenated diamond-like carbon (DLC) films have been prepared using 13.56 MHz capacitively coupled RF glow discharge in a methane atmosphere. Film structure was characterised by FTIR and Raman spectroscopy. Deposition temperatures, which were measured using irreversible self-stick thermax, varied from 40 to 182 °C. Stress levels were measured by monitoring the curvature changes of thin (125 μm) Ti foil after depositing thick (several μm) DLC films, using an Al bonding layer. This intrinsic (deposition) stress was compressive and varied from −0.3 to −2.0 GPa. It increased initially with the average energy of the bombarding ions, reached a maximum and then decreased. Debonding behaviour of DLC coatings on four metallic substrates, with the interfaces being prepared in various ways, has been investigated. Films with a range of thicknesses have been prepared and specimens have been subjected to changes in temperature. From a knowledge of the residual stress levels in these films as a function of temperature, the strain energy release rate for interfacial debonding has been monitored during deposition and subsequent temperature changes. The value of the strain energy release rate at a point when interfacial debonding occurred has been taken as the interfacial toughness (fracture energy). It is concluded from these observations that the interfacial toughness is low for DLC films deposited onto mechanically polished surfaces of titanium, mild steel or stainless steel, but quite high on aluminium. Precleaning by bombardment with energetic argon ions raises the interfacial toughness for the steels and for aluminium, but not for titanium. These observations have been related to the nature of the oxide films on these substrates. A thin sputtered interlayer of aluminium was found to raise the interfacial toughness significantly for the steels and for titanium.

Not Available

Not Available

Not Available

Not Available

Not Available

Composites have been produced by spraying titanium powder onto parallel arrays of tungsten monofilaments, using the vacuum plasma spray technique. The main variables studied were powder particle size, chamber pressure, fibre spacing and plasma gun operating variables. Porosity levels were measured by a gravimetric technique and also studied metallographically. It was found that relatively high porosity levels, largely located in the “shadow” of the fibres, persisted in all of the composites produced. These were measured as ranging from 10–15% to over 30%. Similar trends were apparent from metallographic sections, although the void contents appeared lower, 11 was found that lower porosity levels were favoured by larger particle sizes and lower chamber pressures, although the latter trend was not very clear-cut. Porosity increased sharply as the ratio of the gap between the fibres to the droplet diameter became less than a value of about 5. These results have been considered in terms of the mechanisms by which droplet impingement leads to envelopment of an array of fibres. It is concluded that it will be very difficult to identify a combination of operating parameters which will allow production of composite material with both fibre contents of commercial interest and suitably low porosity levels. The technique does not at present appear to represent an attractive option for commercial exploitation.

Density measurements and microstructural examinations have been used to follow the formation, growth and coalescence of cavities during tensile loading of metal matrix composites (MMCs). Finite element modelling has been used to predict the stress states around reinforcements of various shapes. The positions of peak hydrostatic and normal stress components at the interface have been compared with the microstructural sites where voids were found to form experimentally in aluminium reinforced with alumina particles or fibres. It is concluded that cavities form when a critical hydrostatic stress is reached.

The durability of diamond-like carbon (DLC) films, deposited by an r.f. plasma-assisted glow discharge chemical vapour deposition (CVD) technique onto metallic substrates (304 steel and Ti-6Al-4V), has been measured by a novel method in which the coating is bombarded with small solid particles in a carefully controlled manner. Two different types of particle, soft rounded glass beads and hard angular silica particles, were used to examine the effect of particle type on the wear behaviour. The durability was measured both before and after exposure of the specimens to various biological fluids, including distilled water, phosphate-buffered saline (PBS) solution and 40% bovine serum in sterile PBS. The specimens were exposed for 7 and 28 days at 37 °C in a humidified atmosphere containing 5% carbon dioxide. One set of experiments was carried out at 60 °C to determine the effect of temperature on the film stability. Environmental exposure had little direct effect on the film properties, but the fluids tended to degrade the interfacial adhesion when penetration to the interface was possible. This can occur via microscopic penetrations in the film or via the edges. The buffered saline solution appeared to be the most aggressive in this respect. A correlation between the damage mechanism during solid particle erosion and the type of particle used was found.

Hydrogenated diamond-like carbon (DLC) films have been deposited onto aluminium, titanium and silicon substrates using a 13.56 MHz capacitively-coupled RF glow discharge in a methane atmosphere. All of the DLC films, which were prepared with a negative bias voltage in the range 40 V to 345 V and a gas pressure in the range 1 Pa to 40 Pa. showed strong sp★-CH, FTIR absorbance peaks at 2900 cm★and a broad Raman peak at 1540 cm★. Deposition temperatures, which were measured using irreversible self-stick thermax, varied from 25°C to 182°C. Stress levels in the films were measured by monitoring the curvature changes of thin (125 μm) Ti foil after depositing thick (several μm) DLC films, using an AI bonding layer. This intrinsic (deposition) stress was compressive and varied from −0.3 GPa to −2.0 GPa. It increased initially with the energy of the bombarding ions, as implantation became more pronounced, reached a maximum and then decreased, as a consequence of thermally-activated structural relaxation. The experimental stress data agree well with predictions from a model proposed by Davis [C.A. Davis, A simple model for the formation of compressive stress in thin film by ion bombardment. Thin Solid Films. 226 (1993) 30–34].

DLC coatings have been deposited onto four metallic substrates, and also onto silicon, with the interfaces being prepared in various ways. Films with a range of thicknesses have been produced and specimens have been subjected to changes in temperature. From a knowledge of the residual stress levels in these films as a function of temperature (see Part I), the strain energy release rate for interfacial debonding has been monitored during deposition and subsequent temperature changes. The value of the strain energy release rate at a point when interfacial debonding occurred has been taken as the interfacial toughness (fracture energy). It is concluded from these observations that the interfacial toughness is low for DLC films deposited onto mechanically-polished surfaces of titanium, mild steel or stainless steel, but quite high on aluminium. Pre-cleaning by bombardment with energetic argon ions raises the interfacial toughness for the steels and for aluminium, but not for titanium. These observations have been related to the nature of the oxide films on these substrates. A thin sputtered interlayer of aluminium was found to raise the interfacial toughness significantly for the steels and for titanium. However, experiments with silicon substrate specimens showed that the good interfacial adhesion obtained with Al interlayers is severely impaired by heating in vacuum to temperatures of the order of 500°C. It has been established that this impairment of adhesion is a consequence of the release of Ar entrapped in the Al during sputter deposition, which promotes the formation of blisters and detachment of the film.