Browsing by Author "Corbett, John"
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Item Open Access Controlled porosity alumina structures for ultra-precision hydrostatic journal bearings(Blackwell Publishing Ltd., 2010-11-04T00:00:00Z) Durazo-Cardenas, Isidro; Stephenson, David J.; Corbett, JohnA series of fine-grade alumina powders has been used in combination with maize starch granules to produce porous structures for porous hydrostatic journal- bearing applications. A comprehensive series of tests was conducted to characterize porosity in terms of density, pore size, and permeability. Successful processing of quality journal-bearing components has been demonstrated for preferred combinations of alumina size and starch content, using fixed-processing parameters. The new porous ceramic bearings showed consistent and reproducible properties and are suitable for a wide range of higher precision engineering applications. The porous ceramic-bearing processing route has also proved to be low cost and environmentally sound.Item Open Access Controlled porosity alumina structures for ultra-precision hydrostatic journal bearings(American Ceramic Society, 2010-11-04) Durazo-Cardenas, Isidro; Stephenson, D. J.; Corbett, JohnA series of fine grade alumina powders has been used in combination with maize starch granules to produce porous structures for porous hydrostatic journal bearing applications. A comprehensive series of tests was conducted to characterize porosity in terms of density, pore size and permeability. Successful processing of quality journal bearing components has been demonstrated for preferred combinations of alumina size and starch content, using fixed processing parameters. The new porous ceramic bearings showed consistent and reproducible properties and are suitable for a wide range of higher precision engineering applications. The porous ceramic bearing processing route has also proved to be low cost and environmentally sound.Item Open Access Critical study of high efficiency deep grinding(Cranfield University, 2002) Johnstone, lain; Stephenson, David J.; Corbett, JohnThe recent years, the aerospace industry in particular has embraced and actively pursued the development of stronger high performance materials, namely nickel based superalloys and hardwearing steels. This has resulted in a need for a more efficient method of machining, and this need was answered with the advent of High Efficiency Deep Grinding (HEDG). This relatively new process using Cubic Boron Nitride (CBN) electroplated grinding wheels has been investigated through experimental and theoretical means applied to two widely used materials, M50 bearing steel and IN718 nickel based superalloy. It has been shown that this grinding method using a stiff grinding centre such as the Edgetek 5-axis machine is a viable process. Using a number of experimental designs, produced results which were analysed using a variety of methods including visual assessment, sub-surface microscopy and surface analysis using a Scanning Electron Microscope (SEM), residual stress measurement using X-Ray Diffraction (XRD) techniques, Barkhausen Noise Amplitude (BNA) measurements, surface roughness and Vickers micro-hardness appraisal. It has been shown that the fundamentals of the HEDG process have been understood through experimental as well as theoretical means and that through the various thermal models used, grinding temperatures can be predicted to give more control over this dynamic process. The main contributions to knowledge are made up of a number of elements within the grinding environment, the most important being the demonstration of the HEDG effect, explanation of the phenomenon and the ability to model the process. It has also been shown that grinding is a dynamic process and factors such as wheel wear will result in a continuous change in the optimum grinding conditions for a given material and wheel combination. With the significance of these factors recognised, they can be accounted for within an industrial adaptive control scenario with the process engineer confident of a more efficient use of time and materials to produce a higher quality product at lower cost.Item Open Access A design methodology for jigless aerospace manufacture(2005-03) Burley, Graham John; Corbett, JohnBetween 10% and 40% of the overall cost of an aerostructure can be directly attributed to the cost of type specific, hard-tooling. Hence, any attempt to reduce the cost of aerostructure must eventually focus on reducing the need for type specific hard-tooling. The tooling is one of the major investments made during the development phase of an aircraft programme, accounting for more than a third of the development cost of a civil aircraft and nearly a quarter of the development cost of a military aircraft. The development phase for civil aircraft typically lasts two to four years and the tooling cost represents a multi-million pound forward investment with a long pay back period. Consequently, reducing the need for type specific hard-tooling will significantly reduce development costs and the time from concept to market. This thesis describes a new design-for-assembly JAM Methodology that reduces the need to manufacture and build with type specific hard-tooling. The Methodology was developed by identifying, utilising and developing existing design methodologies, and by identifying and applying enabling technologies. The methodologies are integrated to create a design algorithm that identifies and draws on the enabling technologies to achieve minimal hard tooling assembly concepts. Included in the methodologies is an innovative systems analysis tool. This enables the comparison of alternative assembly concepts ahead of the detail design, and the prediction and control of the total assembly error at the design stage. The first part of the thesis describes the development of the JAM Methodology and explains the function and purpose of each step. The second part of the thesis demonstrates, and tests, two slightly different forms of the Methodology in the re-engineering of an existing aerospace structure, and in the designing of a new aerospace structure. Publications relating to this research thesis include eleven authored, and co-authored, refereed conference papers, one journal article, and two EPSRC final reports.Item Open Access The development of a porous ceramic water hydrostatic bearing for ultra high precision applications(Cranfield University, 1999-12) Almond , R. J.; Corbett, JohnThe principal objective of this research was to produce a series of extremely stiff and thermally stable porous ceramic bearings, and to expand the performance envelope of fluid film technology beyond that currently achievable with conventional oil hydrostatic bearings. The driving force for these developments came from recent advances in ultra- precision and high speed machining, which have placed severe demands on the accuracy and performance of spindle and guide-way bearing systems. A critical part of this research was to develop a material processing methodology, which enabled porous ceramic bearing structures to be manufactured with consistent permeability coefficients. Such control of the material microstructure was necessary in order to produce a useful and predetermined level of performance. The bearings were fabricated using bimodal blends of alumina powder, and vibratory packing into graphite tooling was used to achieve uniform green densities. Following this, the tooling was transferred directly to a hot isostatic press for capsule free high pressure sintering. The influence of temperature and pressure on sintering and permeability was studied, and optimum processing conditions were established. The operation of the water lubricated porous hydrostatic bearing was investigated on a highly instrumented journal test rig. This research has resulted in a bearing with greater stiffness, higher operating speed and lower power consumption than conventional oil hydrostatic technology could achieve. Significant savings were also shown regarding the energy required to drive the spindle assembly, with a reduction in both rotating frictional power and lubricant pumping power consumption.Item Open Access Development of porous ceramic air bearings(Cranfield University, 2001-02) Roach, Christopher James; Stephenson, David J.; Corbett, JohnPorous air bearings enjoy some important advantages over conventional air bearing types such as increased load carrying capacity, higher stiffness and improved damping. However, these types of bearings have yet to find widespread acceptance due to problems with obtaining materials with consistent permeability, instability issues relating to the volume of gas trapped at the bearing surface in the pores, and manufacturing the bearing without altering the permeability. Using a series of fine grades of alumina powder to minimise surface pore volume it has been demonstrated that it is possible to consistently and reproducibly manufacture porous bearings by injection moulding and slip casting. The relationship between powder size, processing conditions, porosity, mechanical properties and fluid flow characteristics were experimentally determined. The temperature of processing and the green density were found to be the controlling parameters in the resulting fluid flow properties for a given powder size, Test bearings were produced from the range of processing conditions investigated. It was found that the fine powder size bearings were stable over the entire range of test conditions irrespective of their initial manufacturing route. The most important consideration for the bearing performance was the quality of manufacture. The bearings were found to be sensitive to the flatness of their working surface and quality of fit in their test holder. The bearings were compared with published theories for load capacity and stiffness. A reasonable agreement was found with load carrying capacity once a correction for surface roughness was incorporated. Stiffness predictions provided a useful tool for the analysis and prediction of properties such as optimum values of permeability for a given geometry, if certain allowances are made.Item Open Access Development of porous-ceramic hydrostatic bearings(Cranfield University, 2003-10) Durazo-Cardenas, Isidro; Corbett, John; Stephenson, David J.Porous-ceramic hydrostatic bearings have been recently developed. These bearings have demonstrated an exceptional overall performance when compared with conventional technology bearings. However, despite all the benefits, porous-ceramic hydrostatic bearings have yet to find widespread acceptance due to the problems found in tailoring the bearings geometry and size to suit precision engineering applications, while producing porous-structures with consistent and reproducible permeability. Using a series of fine grades of alumina powders in combination with maize starch granules, a new method for the manufacture of porous-ceramic bearings has been developed, based on the starch consolidation technique. By employing this method, it has been demonstrated that is possible to manufacture bearings of different geometries and shapes, with consistent and reproducible properties. The new method also proved to be low cost and environmentally sound. The performance of the new journal bearings has been investigated in a highly instrumented test-rig, and a comparable performance to that of previous porous- ceramic journal research has been observed. In a direct performance comparison between a porous-ceramic hydrostatic journal bearing and a conventional hydrostatic bearing of the same size, the porous-ceramic bearing demonstrated a significant performance improvement in terms of stiffness, power consumption and thermal performance. In previous research, water lubrication proved to significantly improve the spindle thermal performance. However, water lubrication is feared to promote corrosion within the spindle components. In the present research, the effects of water lubrication in porous-ceramic bearing systems were investigated. As a result, it has been demonstrated that corrosion in typical machine-tool materials can be effectively controlled by using inhibitors and low cost surface coatings. On the other hand, it has been also demonstrated that undesirable foaming, air entrainment and microbial growth can potentially develop in water/inhibitors lubrication systems. In this sense, the use of low viscosity oils proved to offer a comparable performance.Item Open Access Enhanced surface finishing of bearing raceways(2000-10) Knight, Peter; Corbett, JohnBearing surfaces are highly engineered and high precision parts in regard to surface finish, waviness, form and dimensional size. Surface finishing techniques will not only affect the surface finish and texture but the mechanical properties of the surface and subsurface including surface stress state and hardness as well as chemical properties. The bearing surface is a functional surface influencing fatigue, wear, and friction, noise and lubrication regime. Bearings are low cost, high volume commodity products, which adds additional constraints to any manufacturing process of low cost, fast cycle time and automation. This thesis documents the development route for a superfinishing technique for bearing raceways. Initially work was performed on electrochemical machining (ECM) of raceways, but this was found to improve the surface finish too slowly and gave poor waviness as a result of non-conductive MnS inclusions becoming exposed on the bearing surface during ECM. Abrasive honing stones or superfinishing tapes were then combined with the ECM, a process called electrofinishing to remove these MnS peaks. A beneficial interaction between the ECM and the abrasive resulted in reduced loading of the abrasive and therefore more aggressive cutting and for longer. Using a fine grit (3- 4 pm) CBN vitreous bonded honing stone the surface finish of a ground ring (0.3 pm Ra) or a turned surface (1.2 pm Ra) could be improved to <0.02 pm Ra in around 6 seconds. These new surfaces showed low Rtm and Aq (slope), but also acceptable or very good levels of waviness. The material removal is dominated by the ECM process while the rate of surface finishing is controlled by the abrasive process. The process of electrofinishing gives the possibility of fast, high and controlled material removal capability combined with fine surface finishing, which presently must be achieved by throughfeed honing or two stage honing. The main difficulty to overcome is achieving a stable consistent honing needed for a production process. These difficulties arise from the incompatibility of the ECM electrolyte and honing fluid (usually an oil). Improvements were made by using a water soluble synthetic metalworking fluid mixed with the NaN03 electrolyte. Further improvements are possible by better choice of honing stone, better application of stones, while for superfinishing tapes, the use of tape feed and oscillation will give further improvements.Item Open Access Feature based design for jigless assembly(Cranfield University, 2004-06) Naing, Soe; Corbett, JohnThe work presented in this thesis was undertaken as part of the three-year ‘Jigless Aerospace Manufacture’ (JAM) project which was set-up to investigate and address the significant scientific, technological and economic issues to enable a new design, manufacture and assembly philosophy based on minimising product specific jigs, fixtures and tooling. The main goal of the JAM project at Cranfield was the development of appropriate jigless methods and principles, and the subsequent redesign of the JAM project demonstrator structure – a section of the Airbus A320 aircraft Fixed Leading Edge – to fully investigate and realise the capabilities of jigless methodologies and principles. The particular focus of research activity described in this thesis was the development of a methodology to design for jigless assembly and a process of selecting assembly features to enable jigless assembly. A review of the literature has shown that no methodologies exist to specifically design for jigless assembly; however, previous relevant research has been built upon and extended with the incorporation of novel tools and techniques. To facilitate the assembly feature selection process for jigless assembly, an Assembly Feature Library was created that broadened and expanded the conventional definition and use of assembly features. The developed methodology, assembly feature selection process and Feature Library have been applied and validated on the JAM project demonstrator structure to serve as a Case Study for the tools and techniques developed by the research. Additionally, a Costing Analysis was carried out which suggests that the use of the tools and techniques to enable jigless assembly could have a large and considerable impact on both the Non-Recurring and Recurring costs associated with the design, manufacture and assembly of aircraft.Item Open Access Grinding processes and their effects on surface integrity(Cranfield University, 2005-03) Comley, Paul; Corbett, John; Stephenson, David J.The introduction of high performance grinding machines in combination with the latest superabrasive technology has the potential to impact significantly on existing process chains. The aim of the research was to look at both the high and low rate removal grinding processes and their effects on the surface integrity, as a means to exploit the above technologies. A major objective was to determine the feasibility of High Efficiency Deep Grinding (HEDG) in cylindrical plunge grinding. HEDG is a high speed removal process which differs from conventional forms of grinding in that it uses large depths of cut together with high feedrates. Together, these changes affect the thermal energy partitioning within the work zone. Through this work an understanding of the process conditions enabled the development of this process, such that prevention of thermal damage to the finished workpiece surface is achievable. At the opposite extreme to the high material removal rates of HEDG, kvdrk was carried out in the high precision finish grinding regime. Developments *ere undertaken to look at the implementation of a modified path into the normal cylindrical plunge grinding action, in a process referred to as Superfinish Grinding. The aim of this process being to demonstrate an improvement to the surface texture primarily through a reduction in grinding directionality. Surface integrity is an important consideration in the development of any grinding process. Damage as a result of grinding is predominately of a thermal nature and results in changes to the material properties in the near surface region. One such change is the residual stress, which was measured using Barkhausen Noise intensity instrumentation, which provided a reliable early indication to a build up in thermal energy. Developments in thermal modelling supported by temperature measurements provided a better understanding of the HEDG regime. The model employed new energy partitioning theories together with circular arc modelling of conditions along the contact length. A model was derived to predict the surface finish produced with the Superfinish Grinding approach, this again provided an increased understanding of the grinding process. Industrial trials have shown how HEDG can be implemented on standard production machine tools for the cylindrical plunge grinding of crankshaft components. The process demonstrated the potential for improved surface integrity, whilst maintaining surface finish and form accuracy. The same grinding machine was also used to generate high quality surfaces using a Superfinish Grinding process. Roughness values of the order of 0.11um RQ were routinely obtained exhibiting reduced levels of grinding directionality. Thus, using a single machine tool and a single set-up, exceptionally high stock removal rates are achievable in a roughing cycle followed by superfinishing to generate the required surface characteristics and profile.Item Open Access Investigation into the grinding of titanium alloys(Cranfield University, 2000-07) Wang, S. H.; Corbett, John; Stephenson, David J.Titanium alloys are used extensively in the aerospace industry due to their high specific strength and excellent corrosion resistance. However, their poor thermal conductivity and high chemical reactivity with tool materials make the machining difficult, especially when grinding. During grinding, the excessive heat generated at the wheel-work piece interface may result in poor surface finisil,1_, a transformed surface layer, excessive plastic deformation, thermallyinduced residual tensile stress, burn and micro-cracking on the ground surface. The poor surface integrity and metallurgical changes of the surface and sub-surface may impair the surface sensitive properties such as fatigue life during service. ln order to overcome the thermal problems when grinding titanium alloys, a new cooling strategy, cryogenic grinding, was studied which involves the supply of liquid nitrogen into the grinding zone using a nozzle jet system. lt was found that cryo-cooling with conventional grinding wheel decreased surface roughness values, burn and plastic deformation of Ti-6Al-4V alloys and produced better a cutting mechanism than when using water-based coolant, especially at higher depths of cut. However, the thermal problems still introduced high residual tensile stress which degraded the fatigue life of ground specimens. Another approach to reducing the grinding temperature was to use a superabrasive wheel (diamond), because of its superior thermal conductivity, the integrity of the ground surface was improved and the fatigue life properties of the specimen were maintained at higher values than for conventional grinding wheels. An ultra stiff machine tool, Tetraform C, was also used to grind Ti-6Al-4V alloys and under selected conditions the ground surface reached a good surface finish and the fatigue properties were also retained at lower depth of cut. An ELID system on the Tetraform C was also investigated. It produced a rougher surface finish in this study. However, it is believed that reducing the wheel loading problem when grinding titanium alloys may contribute to maintaining the fatigue properties.Item Open Access Jig-less assembly for aerospace manufacture(2000) Snelling, Peter D.; Corbett, JohnDue to the high level of investment required to compete successively in the global aerospace and automotive markets, these industries are forced to form partnerships wherever possible and thereby share their resources appropriately. This in turn has brought about the requirement to provide a standardized flexible design and manufacturing capability in which interchangability and compatibility may take place. Current assembly practices and associated tooling can be traced back to the earliest days of aircraft production and have become relatively expensive and inflexible in today’s environment. The final assembly stage has been recognized to be a key area which has the potential to offer substantial returns as well as play a major role in any change management process within the organisation. Assembly tooling, jigs and fixtures, are required to support and maintain positional accuracy of components during assembly. Traditional jigs and fixtures make up for the short comings at the product design and manufacturing phases and add significantly to the final product costs and reduce flexibility in the production process. Jig-Less Assembly Concept (JAC) has been defined and researched with the aim to integrate and optimize various tools and techniques with which to reduce or eliminate the assembly tooling currently in use. The outcome of the research presents a comprehensive critique of the processes involved in and pertaining to the assembly of typical airframe assemblies. The thesis forms a platform from which to move forward towards the embodiment of the concept of jig-less assembly. Particular attention is drawn from the research to the need for appropriate organisational and management strategies as well as technical innovation in the adoption of a jig-less approach to airframe assembly. Together with BAe Airbus and Military this collaborative research seeks to define the scope of JAC by identifying and evaluating the issues and constraints, to enable the development of supportive techniques in unison with best practice engineering within a robust and sustainable manufacturing system. This commercially focused R & D required liaison and working at all levels within a variety of industrial sites using live case studies at Filton and Chester.Item Open Access Machining surfaces of optical quality by hard turning(Cranfield University, 2003-11) Knuefermann, Markus M. W.; Corbett, John; Shore, Paul; Stephenson, David J.The main aim of this work was the machining by hard turning of surfaces with optical surface quality. A numerical target had been set as a surface roughness Ra = 10nm. It has been shown that achieving roughness of that magnitude by hard turning is possible. Individual work pieces exhibited the desired surface properties for short lengths at a time, but it proved to be very difficult to achieve these surfaces consistently and over longer cuts. The factors influencing the surface roughness were identified as tool defects and machine vibration in addition to the standard cutting parameters and choice of cutting tool. A model of surface generation in hard turning has been developed and good correlation between simulated and experimentally determined surface roughnesses was achieved. By introducing a material partition equation which determines the proportional contribution of material removal mechanisms in the undeformed chip a comprehensive method for assessing the contributing factors in material removal was developed. While it has been shown that surfaces in hard turning are almost exclusively generated by chip removal and plastic deformation the developed model is versatile enough to include elastic deformation of the work piece. With the help of the model of surface generation in hard turning it has been possible to attribute magnitudes of the influencing factors with respect to the cutting parameters such as feed rate and tool corner radius, and the main disturbances - tool defects and machine vibration. From this conclusions were drawn on the requirements for machine tools and cutting tools, which will need to be realised to make ultra-precision hard turning of surfaces of optical quality a feasible manufacturing process.Item Open Access Modelling and evaluation of time-varying thermal errors in machine tool elements(Cranfield University, 1997-04) Gim, Taeweon; Corbett, John; Gee, A. E.This thesis addresses a comprehensive approach to understanding the time-varying thermal errors in machine tools. Errors in machine tools are generally classified as being time or spatial dependent. Thermal errors are strongly dependent on the continuously changing operating conditions of a machine and its surrounding environment. Uniform temperature rises or stable temperature gradients, which produce time-invariant thermal errors, are considered to be rare in ordinary shop floor environments. Difficulties in analysing time-varying thermal errors are that, first of all, the temperature distribution within the components of a machine should be evaluated, and secondly, the distribution is continuously changing with time. These difficulties can be overcome by introducing a point-wise description method with three thermal parameters. From the theoretical analysis of simple machine elements such as bars, beams and cylinders, and extensive finite-element simulation data for a straightedge subject to room temperature variations, three thermal parameters, i. e. time-delay, time-constant and gain, were identified to obtain a precise description of the thermal deformation of a point of a machine body. Time-delay is dependent largely on thermal diffusivity, and the heat transfer mechanism. The time-constant is governed by heat capacity, heat transfer mechanism and body size. Gain, on the other hand, is determined by the thermal expansion coefficient, heat transfer mechanism and mechanical constraint. The three thermal parameters, in turn, imply that thermal deformation of a point in a body can be described by a simple first- order differential equation. Regarding their dependence on the heat transfer mechanism, a more refined description requires a time-varying linear first-order differential equation. Such an equation can be applied to each point of interest of a machine body. The final form of modelling, using the parameters, is a state-space equation gathering the governing equations for the points of interest. By adopting the point-wise discrete modelling method, we can overcome the difficulty of the spatial distribution of the temperature. Indeed, the calibration of a machine tool is usually performed at discrete points. The completion of this approach was made by presenting the methods by which the three thermal parameters can be evaluated. The first method employs analytical tools based on simplifying assumptions about the shape and boundary conditions of machine components. The second method was to apply numerical techniques to complex machine components. Because there are many drawbacks in theoretical approaches, experimental techniques are essential to complement them. The three thermal parameters can be easily identified using popular parameter identification techniques which can be applied to time-varying cases by their recursive forms. The techniques described were applied to modelling the thermal errors in a single-point diamond turning research machine. It was found that the dominant error component was spindle axial growth. The predictive model for the time-constant was shown to be in agreement with both the machine and with the scaled physical model rig.Item Open Access Optimisation of the High Efficiency Deep Grinding Process with Fuzzy Fitness Function and Constraints.(2004-06-01T00:00:00Z) Jones, Phil; Tiwari, Ashutosh; Roy, Rajkumar; Corbett, JohnThis paper describes the application of two multi-objective optimisation techniques to the high efficiency deep grinding process. The process is modelled using a fuzzy expert system. This allows understanding gained through theoretical analysis to be combined with empirical data in a solitary model. The objective is to simultaneously minimise the surface temperature and specific grinding energy. A problem constraint is represented within the fuzzy model. It forms an objective representing the degree of feasibility of the solution.Item Open Access The performance of a porous ceramic hydrostatic journal bearing(Professional Engineering Publishing, 2010) Durazo-Cardenas, Isidro; Corbett, John; Stephenson, David J.The performance of a porous-ceramic hydrostatic journal bearing manufactured by the starch consolidation (SC) technique has been examined using a highly instrumented test rig. The results have been compared with those of a 5-recess hydrostatic bearing of the same size and comparable design, under the same testing conditions. The SC porous-ceramic bearing showed an improved performance over the conventional hydrostatic bearing. Static and rotational stiffness were 95 per cent and over 150 per cent higher, respectively. In addition, the porous ceramic bearing exhibited a more economic performance with a 64 per cent lower flowrate and pumping power than the hydrostatic bearing. In terms of heat generation, the porous ceramic bearing showed 50 per cent lower temperature riseItem Open Access Permeability and dynamic elastic moduli of controlled porosity ultra-precision aerostatic structures(Elsevier Science B.V., Amsterdam., 2014-03-03T00:00:00Z) Durazo-Cardenas, Isidro; Corbett, John; Stephenson, David J.Porous ceramic aerostatic bearings enable precise and smooth motion and improved stiffness compared with widely used orifice restrictor bearings. However, the processing techniques so far used are too complex or rely in lowering the sintering temperature to increase fluid flow. Preferred combinations of fine-grade alumina powders and starch granules were used to produce quality porous structures using fixed processing parameters. Component shrinkage, permeability, pore size and elastic properties were comprehensively characterised as a function of porosity. The new porous ceramic structures exhibited controllable and reproducible permeability and modulus, within the range required for ultra-precision porous aerostatic applicationsItem Open Access Processing and fluid flow characteristics of hot isostatically pressed porous alumina for aerostatic bearing applications(Cranfield University, 1996-12) Kwan, Y. B. P; Stephenson, David J.; Corbett, JohnDespite their well known superior load and stiffness characteristics, the wide-spread use of porous aerostatic bearings in preference to other bearing types has been hampered by difficulties in controlling the permeability of the porous material during manufacture and machining, in addition to instability problems caused by the additional volume of air trapped amongst the pores. Recent development in porous aerostatic bearings centres around the use of a thin dense surface layer to overcome the stability problem. The production of single and two-layered porous ceramic structures for aerostatic bearing applications have been investigated using the free-capsule hot isostatic pressing process, and in conjunction with slip and tape castings. The influence of various process parameters on open porosity, and the empirical relationships between porosity, particle size and the resulting fluid flow and structural properties were determined from experimental data. The measurement accuracy and uniformity of temperature within the furnace are identified as the most important factors affecting consistency and predictability of the permeability of the porous substrate. Prototype bearings were produced and tested, based on the above materials. The single-layer bearing was, not unexpectedly, found to be unstable over a wide operating range. The use of a two-layeredb earingm ateriale liminatedth e stabilityp roblem. Initial measurements of the slip coefficient of the porous material in air and at small gaps indicated significant deviation from the Beavers' theory. The effect of velocity slip was found to be significant in both test bearings, and was allowed for by the addition of an equivalent clearance to the bearing gap. The value of the equivalent clearance was deduced from experimental data. The static load characteristics and the pressure profile of both test bearings agreed well with published theories, once the above-mentioned correction for slip was applied.