Author
Jean-Pierre Celis
Other affiliations: Catholic University of Leuven
Bio: Jean-Pierre Celis is an academic researcher from Katholieke Universiteit Leuven. The author has contributed to research in topics: Tribology & Fretting. The author has an hindex of 54, co-authored 334 publications receiving 11409 citations. Previous affiliations of Jean-Pierre Celis include Catholic University of Leuven.
Papers published on a yearly basis
Papers
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TL;DR: In this article, the feasibility of processing B4C reinforced Al composite was investigated and a comparison was made with the other two composites by means of optical and scanning electron microscopy (SEM).
Abstract: Three aluminium metal matrix composites containing reinforcing particles of B4C, SiC and Al2O3 (0-20 vol. %) were processed. The stir-casting manufacturing route followed by hot extrusion was utilized, being one of the cost-effective industrial methods. In this study, the feasibility of processing B4C reinforced Al composite was investigated and a comparison was made with the other two composites. The microstructural distribution of reinforcing particles in all three composites was studied by means of optical and scanning electron microscopy (SEM). The distribution and chemical composition of the phases formed at matrix/particulate interface of the processed composites were also investigated by SEM and energy dispersive x-ray spectroscopy (EDX). A clear interfacial reaction product/layer was found at Al/SiC interface for composites held for a relatively long processing time (> 30 minutes). No reaction product was observed at Al/B4C and Al/Al2O3 interfaces at the resolution limit of the SEM used. On the other hand, two secondary phases (alumina and another phase containing aluminium, boron and carbon) were found in the aluminum matrix away from the interface in Al-B4C composites. From the fracture surface analysis, B4C reinforced Al composite seemed to exhibit a better interfacial bonding compared to the other two composites.
543 citations
01 Dec 2001
TL;DR: In this article, the feasibility of processing B4C reinforced Al composite was investigated and a comparison was made with the other two composites by means of optical and scanning electron microscopy (SEM).
Abstract: Three aluminium metal matrix composites containing reinforcing particles of B4C, SiC and Al2O3 (0-20 vol. %) were processed. The stir-casting manufacturing route followed by hot extrusion was utilized, being one of the cost-effective industrial methods. In this study, the feasibility of processing B4C reinforced Al composite was investigated and a comparison was made with the other two composites. The microstructural distribution of reinforcing particles in all three composites was studied by means of optical and scanning electron microscopy (SEM). The distribution and chemical composition of the phases formed at matrix/particulate interface of the processed composites were also investigated by SEM and energy dispersive x-ray spectroscopy (EDX). A clear interfacial reaction product/layer was found at Al/SiC interface for composites held for a relatively long processing time (> 30 minutes). No reaction product was observed at Al/B4C and Al/Al2O3 interfaces at the resolution limit of the SEM used. On the other hand, two secondary phases (alumina and another phase containing aluminium, boron and carbon) were found in the aluminum matrix away from the interface in Al-B4C composites. From the fracture surface analysis, B4C reinforced Al composite seemed to exhibit a better interfacial bonding compared to the other two composites.
505 citations
TL;DR: The hardness and Young's modulus of the successive layers across a resin-dentin bonding area were determined by nano-indentation for four commercially-available dentin adhesive systems, of which two were also applied with a different conditioning agent.
Abstract: The hardness and Young's modulus of the successive layers across a resin-dentin bonding area were determined by nano-indentation for four commercially-available dentin adhesive systems, of which two were also applied with a different conditioning agent. With a computer-controlled nano-indentation technique, minute triangular indentations were made within a small area of a few micrometers' diameter at a load of a few milli-Newtons. The load and displacement of the indenter were continuously monitored during the loading-unloading sequence, so hardness and Young's modulus could be computed as a function of the indenter geometry and the applied load. The hardness of the resin-dentin interdiffusion zone was significantly lower than that of unaltered dentin. A gradient of moduli of elasticity was observed from the rather stiff dentin over a more elastic resin-dentin interdiffusion zone and adhesive resin layer to the restorative composite. That gradient was more substantial in those systems that produced relatively thick adhesive resin layers or supplementally provided a filled low-viscosity resin as an intermediate layer between the adhesive resin and the bulk restorative composite. Such an elastic bonding area might have a strain capacity sufficient to relieve stresses between the shrinking composite restoration and the rigid dentin substrate, thereby improving the conservation of the dentin bond and, as a consequence, the marginal integrity and retention of the restoration.
413 citations
TL;DR: It can be concluded that the materials of choice for restoring posterior cavities at present are the Ultrafine Compact-Filled Composites because their intrinsic surface roughness, Young's modulus and, indirectly, their filler content, compressive strength, and surface hardness are comparable to the same properties of enamel and dentin.
Abstract: The on-going search for a biologically acceptable restorative material has brought a confusing variety of composites on the dental market. In the present study, commercially available composites are categorized as a function of their mean particle size, filler distribution, filler content, Young's modulus, surface roughness, compressive strength, surface hardness, and filler morphology. Out of this information, it can be concluded that the materials of choice for restoring posterior cavities at present are the Ultrafine Compact-Filled Composites because their intrinsic surface roughness, Young's modulus and, indirectly, their filler content, compressive strength, and surface hardness are comparable to the same properties of enamel and dentin. The Ultrafine Midway-Filled Composites seem to be very satisfactory materials for anterior use.
363 citations
TL;DR: A new model that overcomes several of the shortcomings of the previously proposed models is developed starting from a statistical approach of the incorporation of particles for codeposition of inert particles with metals.
Abstract: Over the last decade the general knowledge of the electrolytic codeposition of inert particles with metals has increased markedly, and a few models have been proposed. In this paper a new model that overcomes several of the shortcomings of the previously proposed models is developed starting from a statistical approach of the incorporation of particles. The validity of the new model is shown for two experimental codeposition systems, namely, from acidic sulfate baths and from additive‐free potassium cyanide baths.
301 citations
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TL;DR: The atomic force microscope (AFM) is not only used to image the topography of solid surfaces at high resolution but also to measure force-versus-distance curves as discussed by the authors, which provide valuable information on local material properties such as elasticity, hardness, Hamaker constant, adhesion and surface charge densities.
Abstract: The atomic force microscope (AFM) is not only a tool to image the topography of solid surfaces at high resolution. It can also be used to measure force-versus-distance curves. Such curves, briefly called force curves, provide valuable information on local material properties such as elasticity, hardness, Hamaker constant, adhesion and surface charge densities. For this reason the measurement of force curves has become essential in different fields of research such as surface science, materials engineering, and biology. Another application is the analysis of surface forces per se. Some of the most fundamental questions in colloid and surface science can be addressed directly with the AFM: What are the interactions between particles in a liquid? How can a dispersion be stabilized? How do surfaces in general and particles in particular adhere to each other? Particles and surfaces interactions have major implications for friction and lubrication. Force measurements on single molecules involving the rupture of single chemical bonds and the stretching of polymer chains have almost become routine. The structure and properties of confined liquids can be addressed since force measurements provide information on the energy of a confined liquid film. After the review of Cappella [B. Cappella, G. Dietler, Surf. Sci. Rep. 34 (1999) 1–104] 6 years of intense development have occurred. In 1999, the AFM was used only by experts to do force measurements. Now, force curves are used by many AFM researchers to characterize materials and single molecules. The technique and our understanding of surface forces has reached a new level of maturity. In this review we describe the technique of AFM force measurements. Important experimental issues such as the determination of the spring constant and of the tip radius are discussed. Current state of the art in analyzing force curves obtained under different conditions is presented. Possibilities, perspectives but also open questions and limitations are discussed.
3,281 citations
TL;DR: Further investigations of mechanical properties at the "materials level", in addition to the studies at the 'structural level' are needed to fill the gap in present knowledge and to achieve a complete understanding of the mechanical properties of bone.
Abstract: Detailed descriptions of the structural features of bone abound in the literature; however, the mechanical properties of bone, in particular those at the micro- and nano-structural level, remain poorly understood. This paper surveys the mechanical data that are available, with an emphasis on the relationship between the complex hierarchical structure of bone and its mechanical properties. Attempts to predict the mechanical properties of bone by applying composite rule of mixtures formulae have been only moderately successful, making it clear that an accurate model should include the molecular interactions or physical mechanisms involved in transfer of load across the bone material subunits. Models of this sort cannot be constructed before more information is available about the interactions between the various organic and inorganic components. Therefore, further investigations of mechanical properties at the 'materials level', in addition to the studies at the 'structural level' are needed to fill the gap in our present knowledge and to achieve a complete understanding of the mechanical properties of bone.
2,352 citations
TL;DR: In this paper, the surface chemistry of the trimethylaluminum/water ALD process is reviewed, with an aim to combine the information obtained in different types of investigations, such as growth experiments on flat substrates and reaction chemistry investigation on high-surface-area materials.
Abstract: Atomic layer deposition(ALD), a chemical vapor deposition technique based on sequential self-terminating gas–solid reactions, has for about four decades been applied for manufacturing conformal inorganic material layers with thickness down to the nanometer range. Despite the numerous successful applications of material growth by ALD, many physicochemical processes that control ALD growth are not yet sufficiently understood. To increase understanding of ALD processes, overviews are needed not only of the existing ALD processes and their applications, but also of the knowledge of the surface chemistry of specific ALD processes. This work aims to start the overviews on specific ALD processes by reviewing the experimental information available on the surface chemistry of the trimethylaluminum/water process. This process is generally known as a rather ideal ALD process, and plenty of information is available on its surface chemistry. This in-depth summary of the surface chemistry of one representative ALD process aims also to provide a view on the current status of understanding the surface chemistry of ALD, in general. The review starts by describing the basic characteristics of ALD, discussing the history of ALD—including the question who made the first ALD experiments—and giving an overview of the two-reactant ALD processes investigated to date. Second, the basic concepts related to the surface chemistry of ALD are described from a generic viewpoint applicable to all ALD processes based on compound reactants. This description includes physicochemical requirements for self-terminating reactions,reaction kinetics, typical chemisorption mechanisms, factors causing saturation, reasons for growth of less than a monolayer per cycle, effect of the temperature and number of cycles on the growth per cycle (GPC), and the growth mode. A comparison is made of three models available for estimating the sterically allowed value of GPC in ALD. Third, the experimental information on the surface chemistry in the trimethylaluminum/water ALD process are reviewed using the concepts developed in the second part of this review. The results are reviewed critically, with an aim to combine the information obtained in different types of investigations, such as growth experiments on flat substrates and reaction chemistry investigation on high-surface-area materials. Although the surface chemistry of the trimethylaluminum/water ALD process is rather well understood, systematic investigations of the reaction kinetics and the growth mode on different substrates are still missing. The last part of the review is devoted to discussing issues which may hamper surface chemistry investigations of ALD, such as problematic historical assumptions, nonstandard terminology, and the effect of experimental conditions on the surface chemistry of ALD. I hope that this review can help the newcomer get acquainted with the exciting and challenging field of surface chemistry of ALD and can serve as a useful guide for the specialist towards the fifth decade of ALD research.
2,212 citations
TL;DR: This paper critically appraises methodologies that focus on chemical degradation patterns of hydrolysis and elution of interface components, as well as mechanically oriented test set-ups, such as fatigue and fracture toughness measurements, to assess adhesion durability.
Abstract: The immediate bonding effectiveness of contemporary adhesives is quite favorable, regardless of the approach used. In the long term, the bonding effectiveness of some adhesives drops dramatically, whereas the bond strengths of other adhesives are more stable. This review examines the fundamental processes that cause the adhesion of biomaterials to enamel and dentin to degrade with time. Non-carious class V clinical trials remain the ultimate test method for the assessment of bonding effectiveness, but in addition to being high-cost, they are time- and labor-consuming, and they provide little information on the true cause of clinical failure. Therefore, several laboratory protocols were developed to predict bond durability. This paper critically appraises methodologies that focus on chemical degradation patterns of hydrolysis and elution of interface components, as well as mechanically oriented test set-ups, such as fatigue and fracture toughness measurements. A correlation of in vitro and in vivo data revealed that, currently, the most validated method to assess adhesion durability involves aging of micro-specimens of biomaterials bonded to either enamel or dentin. After about 3 months, all classes of adhesives exhibited mechanical and morphological evidence of degradation that resembles in vivo aging effects. A comparison of contemporary adhesives revealed that the three-step etch-and-rinse adhesives remain the 'gold standard' in terms of durability. Any kind of simplification in the clinical application procedure results in loss of bonding effectiveness. Only the two-step self-etch adhesives approach the gold standard and do have some additional clinical benefits.
1,778 citations