Leigh Fleming

The influence of bone cement type on production of fretting wear on the femoral stem surface: A preliminary study

BACKGROUND It has been reported that bone cement correlates with survivorship of cemented total hip replacement. However, little research has been published to investigate the influence of bone cement type on production of fretting wear on the femoral stem. METHODS In the present study, we performed six in vitro wear simulations using the same type of femoral stem (polished Exeter V40™) and three different bone cements (Simplex P, Palacos R, and CMW 3). FINDINGS Fretting wear was consistently reproduced on the stem surface and the wear locations compared well with the results of retrieval studies. Selected 3D surface parameters were utilised to quantitatively evaluate fretting wear and no significant difference was identified in terms of fretting wear severity between these simulations. The bone cements were all badly damaged in those sites contacting the fretting wear areas on the femoral stem. Additionally, there were plenty of wear debris present on the cement surface, and the energy dispersive X-ray analysis confirmed that it was just cement particles for Simplex P bone cement, whilst it included metallic particles for Palacos R and CMW 3 bone cements. INTERPRETATION This preliminary study shed some light on the influence of bone cement type on production of fretting wear on the femoral stem surface but further research is needed to gain a better understanding on this issue.

Correlation of micro and nano–scale defects with WVTR for aluminium oxide barrier coatings for flexible photovoltaic modules

This paper seeks to establish a correlation between surface topographical defects and water vapour transmission rate (WVTR) measured under laboratory conditions for aluminium–oxide (Al2O3) barrier film employed in flexible photovoltaic (PV) modules. Defects in the barrier layers of PV modules causing high WVTR are not well characterised and understood. A WVTR of ~10−1 g/m2/day is sufficient for the most packaging applications, but ≤10−6 g/m2/day is required for the encapsulation of long–life flexible PV modules (Carcia et al., 2010a, 2010b). In this study, surface metrology techniques along with scanning electron microscopy (SEM) were used for a quantitative characterisation of the barrier film defects. The investigation have provided clear evidence for the correlation of surface defect density and the transmission of water vapour through the barrier coating layer. The outcomes would appear to suggest that small numbers of large defects are the dominant factor in determining WVTR for these barrier layers.

Implementation of in Process Surface Metrology for R2R Flexible PV Barrier Films

Thin functional barrier layers of aluminum oxide (Al2O3) that are used particularly in photovoltaic (PV) modules to prevent the possibility of water vapor ingress should be applied over the entire PV surface without any defects. However, for barrier layer thicknesses within the sub-micrometer range (up to 50 nm) produced through the atomic layer deposition (ALD) method, it is common for defects to occur during the production process. To avoid defective barriers from being incorporated in the final PV unit, defects need to be detected during the barrier production process. In this paper, the implementation of in process inspection system capable of detecting surface defects such as pinholes, scratches, or particles down to a lateral size of 3 μm and a vertical resolution of 10 nm over a 500 mm barrier width is presented. The system has a built-in environmental vibration compensation capability, and can monitor ALD-coated films manufactured using roll-to-roll (R2R) techniques. Ultimately, with the aid of this in process measurement system, it should be possible to monitor the coating surface process of large-area substrates, and if necessary, carry out remedial work on the process parameters.

The Use of Feature Parameters to Asses Barrier Properties of ALD coatings for Flexible PV Substrates

This paper reports on the recent work carried out as part of the EU funded NanoMend project. The project seeks to develop integrated process inspection, cleaning, repair and control systems for nano-scale thin films on large area substrates. In the present study flexible photovoltaic films have been the substrate of interest. Flexible PV films are the subject of significant development at present and the latest films have efficiencies at or beyond the level of Si based rigid PV modules. These flexible devices are fabricated on polymer film by the repeated deposition, and patterning, of thin layer materials using roll-to-roll processes, where the whole film is approximately 3um thick prior to encapsulation. Whilst flexible films offer significant advantages in terms of mass and the possibility of building integration (BIPV) they are at present susceptible to long term environmental degradation as a result of water vapor transmission through the barrier layers to the CIGS (Copper Indium Gallium Selenide CuInxGa(1-x)Se2) PV cells thus causing electrical shorts and efficiency drops. Environmental protection of the GIGS cell is provided by a thin (40nm) barrier coating of Al2O3. The highly conformal aluminium oxide barrier layer is produced by atomic layer deposition (ALD) where, the ultra-thin Al2O3 layer is deposited onto polymer thin films before these films encapsulate the PV cell. The surface of the starting polymer film must be of very high quality in order to avoid creating defects in the device layers. Since these defects reduce manufacturing yield, in order to prevent them, a further thin polymer coating (planarization layer) is generally applied to the polymer film prior to deposition. The presence of surface irregularities on the uncoated film can create defects within the nanometre-scale, aluminium oxide, barrier layer and these are measured and characterised. This paper begins by reporting the results of early stage measurements conducted to characterise the uncoated and coated polymer film surface topography using feature parameter analysis. The measurements are carried out using a Taylor Hobson Coherence Correlation Interferometer an optical microscope and SEM. Feature parameter analysis allows the efficient separation of small insignificant defects from large defects. The presence of both large and insignificant defects is then correlated with the water vapour transmission rate as measured on representative sets of films using at standard MOCON test. The paper finishes by drawing conclusions based on analysis of WVTR and defect size, where it is postulated that small numbers of large defects play a significant role in higher levels of WVTR.

Further development of surface metrology methods for predicting the functional performance of flexible photovoltaic barrier films

Surface topography analysis plays a very significant role in determining the functional performance for many engineering surfaces. In this paper, feature characterisation techniques, based on the ‘Wolf pruning’ method are implemented to characterise micro and nano-scale features which have a dominant effect on the functional lifespan of flexible Photovoltaic (PV) modules. The densities and dimensions of the potential significant features are calculated by means of the feature “characterisation toolbox”. The outcome of this study has shown the potential of areal feature segmentation for detecting functionally significant defects present in Atomic Layer Deposition (ALD) barrier coatings of Al2O3 on polymer films. The analysis provides the basis for the development in process metrology for Roll-to-Roll (R2R) production of barrier coatings as applied to flexible PV arrays and is a first step in the demonstration of in-process use of feature parameters.

High fidelity replication of surface texture and geometric form of a high aspect ratio aerodynamic test component

This paper details, assesses and validates a technique for the replication of a titanium wind tunnel test aerofoil in polyurethane resin. Existing resin replication techniques are adapted to overcome the technical difficulties associated with casting a high aspect ratio component. The technique is shown to have high replication fidelity over all important length-scales. The blade chord was accurate to 0.02%, and the maximum blade thickness was accurate to 2.5%. Important spatial and amplitude areal surface texture parameter were accurate to within 2%. Compared to an existing similar system using correlation areal parameters the current technique is shown to have lower fidelity and this difference is discussed. The current technique was developed for the measurement of boundary layer flow‘laminar to turbulent’ transition for gas turbine compressor blade profiles and this application is illustrated.

Defect detection in thin-film photovoltaics; Towards improved efficiency and longevity

The Photovoltaic (PV) industry is seeking to increase efficiency and functional lifetime of PV modules manufactured on polymer substrates. High resolution and high speed surface inspection for the quality control of the manufacture of large area flexible PV modules are necessary to guarantee maximum quality, longer lifetime and enhanced product yield. Flexible PV films are the newest development in the renewable energy field and the latest films have efficiencies at or beyond the level of Si-based rigid PV modules. However, they are at present highly susceptible to long term environmental degradation as a result of water vapor transmission through the protective encapsulation to the active layer. To reduce the water vapor transmission rate (WVTR) the PV encapsulation includes a barrier layer of amorphous Al2O3 on a planarised polymer substrate. This highly conformal barrier layer is produced by atomic layer deposition (ALD). Nevertheless water vapour transmission is still facilitated by the presence of micro and nano-scale defects in these barriers which results in decreased cell efficiency and reduced longevity. The main aim of this research paper is to use surface metrology techniques including: White Light Scanning Interferometry (WLSI), Atomic Force Microscopy (AFM) and Scanning Electron Microscopy (SEM) to characterise the water vapor barrier defects which are seemed to be responsible for the water vapor permeation. A real surface texture parameter analysis allows the efficient separation of small insignificant features from significant defects. This parametric analysis is then correlated with the water vapour transmission rate as measured on typical sets of films using standard MOCON test. The paper finishes by drawing conclusions based on analysis of WVTR and defect size, density and distribution, where it is postulated that small numbers of large features have more influence on the deterioration of water vapor transmission rates than large numbers of small features. This result provides the basis for developing roll-to-roll in process metrology devices for quality control.

Chemical Imaging by Dissolution Analysis: Localized Kinetics of Dissolution Behavior to Provide Two-Dimensional Chemical Mapping and Tomographic Imaging on a Nanoscale

A new approach to achieving chemical mapping on a nanoscale is described that can provide 2D and tomographic images of surface and near-surface structure. The method comprises dissolving material from the surface of the sample by applying a series of aliquots of solvent, then analyzing their contents after removing them; in between exposures, the surface is imaged with atomic force microscopy. This technique relies on being able to compensate for any drift between images by use of software. It was applied to a blend of two polymers, PMMA and PS. The analytical data identified the material that was dissolved, and the topography images enabled the location of the various materials to be determined by analyzing local dissolution kinetics. The prospects for generalizing the approach are discussed.

Metrology of Al2O3 Barrier Film for Flexible CIGS Solar Cells

Flexible Cu (In, Ga) Se2 (CIGS) solar cells are very attractive renewable energy sources because of their high conversion efficiencies, their low cost potential and their many application possibilities. However, they are at present highly susceptible to long term environmental degradation as a result of water vapor ingress through the protective encapsulation layer to the absorber (CIGS) layer. The basic methodology to prevent the water vapor permeation is to combine an oxide layer (e.g. AlOx) coating with suitable polymer substrates. Nevertheless, micro and nano-scale defects can appear at any stage of the coating process thus affecting the module efficiency and lifespan. The main aim of this research paper is to use surface metrology techniques including: White Light Scanning Interferometry (WLSI), Atomic Force Microscopy (AFM) and Environmental Scanning Electron Microscopy (ESEM) to characterise the aluminum oxide (Al2O3) barrier film defects, which appear to be directly responsible for the water vapor permeability. This paper reports on the development of a characterisation method for defect detection based on “Wolf Pruning†method and then correlates this with measured water vapor transmission rates (WVTRs) using standard MOCON® test. The results presented in this paper provided a detailed knowledge of the nature of micro and nano-scale defects on the Al2O3 barrier films which are responsible for water vapor and oxygen ingress. This result can then be used to provide the basis for developing roll-to-roll in process metrology devices for quality control of flexible PV module manufacture.

In Vitro Experimental Investigation of the Integrity of the Stem–Cement Interface

The rationale behind failure of cemented total hip replacement is still far from being well understood in a mechanical and molecular perspective. In the present study, the integrity of the stem–cement interface was investigated through an in vitro experiment monitoring fluid flow along this interface. The results indicated that a good mechanical bonding formed at the stem–cement interface before debonding of this interface was induced by physiological loadings during the in vivo service of the hip prosthesis.