I. Apachitei

The effect of heat treatment on the structure and abrasive wear resistance of autocatalytic NiP and NiP–SiC coatings

Abstract A systematic study on the relationships between the structure and abrasive wear resistance of autocatalytic nickel–phosphorus coatings (particle-free and SiC composite) with different phosphorus contents (i.e. 2.5–10.2 wt.% P) and under different thermal treatments (i.e. 300, 400 and 500°C) has been performed. The phase structure, composition and properties of the coatings could be controlled by changing the phosphorus content of the nickel–phosphorus matrix and by performing thermal treatments. The improvement in abrasive wear behaviour of the nanocrystalline (i.e. ≤6.0 wt.% P) coatings with heat treatment temperature up to 400°C was related to (i) the formation of a metastable equilibrium phase and (ii) precipitation of Ni 3 P compound. At higher thermal treatments (500°C), a change in the deformation mechanisms (Orowan mechanism) determined by the coarsening of Ni 3 P precipitates was associated with the decrease in abrasive wear resistance of the coatings. In addition, for the NiP–SiC coatings after annealing at 500°C, Ni 3 Si was formed and the adhesion between the reinforcement and the matrix was enhanced.

Electroless Ni–P Composite Coatings: The Effect of Heat Treatment on the Microhardness of Substrate and Coating

A possibility to increase materials performance for different applications is to protect them by coatings. Electroless nickel deposition represents an alternative method to obtain coatings on various substrates (metallic and nonmetallic). The process is based on a redox reaction in which the reducing agent is oxidized and Ni ions are reduced on the substrate surface. Once the first layer of nickel is deposited it is acting as a catalyst for the process. As a result, a linear relationship between coating thickness and time, usually occurs [1]. If the reducing agent is sodium hypophosphite, the obtained deposit will be a nickel–phosphorus alloy. The as-deposited Ni–P alloys were reported to have a non-equilibrium phase structure [2]. It is generally accepted that a microcrystalline, amorphous or a co-existence of these two phases can be obtained depending on phosphorus content [2–6]. A recent advance in electroless Ni–P deposition is the co-deposition of solid particles within coatings. These solid particles can be hard materials (SiC, B4C, Al2O3, diamond [7–10]) and dry lubricants (PTFE, MoS2 and graphite [11–13]). By this method, composite layers with very good characteristics for specific applications can be produced. Electroless Ni–P–SiC coatings are recognized for their hardness and wear resistance and can replace “hard chromium” in aerospace industry [1, 10, 14]. Also, the electroless Ni–P–PTFE films present excellent self-lubricating properties [12]. One of the more increasingly used substrate materials in electroless nickel deposition is aluminium and its alloys [15]. However, aluminium is a very reactive metal which easily form a hard to remove oxide film during rinsing or exposure to air. This oxide film represents a disadvantage for electroless nickel deposition where a metal-metal bond is required. To overcome this problem, a suitable pretreatment sequence including a deoxidizing step followed by a double immersion in a modified alloy zincate (MAZ) solution can be applied [15, 16]. To increase the hardness and the abrasion resistance of electroless Ni–P deposits, heat treatments are performed. Studies carried out in this direction [7] showed that a maximum hardness can be achieved after a heat treatment at 400°C for 1 h, when the hardness of the deposit increased from 500–600 up to 1000–1100 HV100. In this study, the electroless Ni–P and Ni–P–X (X 5 SiC, Al2O3 and B) coatings deposited on a heat treatable Al alloy (6063-T6) were investigated. Coating structure investigated by XRD and DSC, and Pergamon Scripta Materialia, Vol. 38, No. 9, pp. 1347–1353, 1998 Elsevier Science Ltd Copyright

Size effect of PLGA spheres on drug loading efficiency and release profiles

Drug delivery systems (DDS) based on poly (lactide-co-glycolide) (PLGA) microspheres and nanospheres have been separately studied in previous works as a means of delivering bioactive compounds over an extended period of time. In the present study, two DDS having different sizes of the PLGA spheres were compared in morphology, drug (dexamethasone) loading efficiency and drug release kinetics in order to investigate their feasibility with regard to production of medical combination devices for orthopedic applications. The loaded PLGA spheres have been produced by the oil-in-water emulsion/solvent evaporation method following two different schemes. Their morphology was assessed by scanning electron microscopy and the drug release was monitored in phosphate buffer saline solution at 37°C for 550xa0h using high performance liquid chromatography. The synthesis schemes used produced spheres with two different and reproducible size ranges (20xa0±xa010 and 1.0xa0±xa00.4xa0μm) having a smooth outer surface and regular shape. The drug loading efficiency of the 1.0xa0μm spheres was found to be 11% as compared to just 1% for the 20xa0μm spheres. Over the 550xa0h release period, the larger spheres (diameter 20xa0±xa010xa0μm) released 90% of the encapsulated dexamethasone in an approximately linear fashion whilst the relatively small spheres (diameter 1.0xa0±xa00.4xa0μm) released only 30% of the initially loaded dexamethasone, from which 20% within the first 25xa0h. The changes observed were mainly attributed to the difference in surface area between the two types of spheres as the surface texture of both systems was visibly similar. As the surface area per unit volume increases in the synthesis mixture, as is the case for the 1.0xa0μm spheres formulation, the amount of polymer-water interfaces increases allowing more dexamethasone to be encapsulated by the emerging polymer spheres. Similarly, during the release phase, as the surface area per unit volume increases, the rate of inclusion of water into the polymer increases, permitting faster diffusion of dexamethasone.

Solid-state reactions in low-phosphorus autocatalytic NiP–SiC coatings

Abstract Composite NiP–SiC coatings with a nanocrystalline nickel matrix produced by autocatalytic deposition were subjected to phase transformations by isochronal and isothermal heating. Isochronal heating to 700°C (heating rate 10°C min−1) revealed three exothermic effects (reactions) occurring in the coatings. X-Ray diffraction showed that only the second and third thermal effects were associated with phase transformations. The first peak was associated with chemical and structural relaxation and a slight grain growth of the matrix. The second reaction was attributed to the nucleation and growth of Ni3P precipitates, while the third was related to the complete dissolution of SiC particles in the matrix with the formation of Ni3Si and carbon. A similar trend in the phase formation sequence was observed by isothermal heating. However, the formation of nickel silicides at the SiC/matrix interface occurred at lower temperatures (i.e. 500°C for 1 h). The formation of silicides appeared to be governed by the diffusion of nickel atoms into the SiC lattice, as indicated by transmission electron microscopy.

PARTICLES CO-DEPOSITION BY ELECTROLESS NICKEL

Laboratory of Materials Science, Delft University of Technology,Rotterdamseweg 137, 2628 AL Delft, The Netherlands(Received November 4, 1997)(Accepted in revised form January 23, 1998)IntroductionElectroless, as well as electrochemical co-deposition of inert particles, represents a method to obtainmetal matrix composite materials as thin foils (coatings) at low temperature (’90°C). Compared toelectrodeposition, electroless can be applied on different substrates (conductive and nonconductive) andthe obtained layer has a very homogeneous distribution regardless the substrate geometry [1].An electroless nickel composite coating consists of small inert particles, such as: oxides, carbides,nitrides polymers etc., uniformly dispersed into a nickel-based matrix. During the redox reactionbetween Ni

Biofunctional surfaces by plasma electrolytic oxidation on titanium biomedical alloys

The effects of substrate composition on oxide layer properties following plasma electrolytic oxidation under similar conditions have been evaluated for α-cpTi, α/β-Ti6Al7Nb, β-Ti35Zr10Nb and β-Ti45Nb alloys. All oxidised surfaces revealed enhanced wettability, surface free energy and roughness relative to the non-oxidised surfaces. Nevertheless, the resultant oxides differed with respect to average pore size, pores density, layer chemistry and phase composition. The β-titanium alloys developed oxides with a larger average pore size and lower pore density relative to the α-cpTi and α/β-Ti6Al7Nb substrates. Anatase dominated the oxide layer formed on α-cpTi and β-Ti45Nb alloys, a mixture of anatase and rutile was present on the oxidised α/β-Ti6Al7Nb surface, whereas Ti2ZrO6 was the only phase detected on the oxidised surface of the β-Ti35Zr10Nb alloy.

Effects of dexamethasone-loaded PLGA microspheres on human fetal osteoblasts

Integration of a drug delivery function into implantable medical devices enables local release of specific bioactives to control cells–surface interactions. One alternative to achieve this biofunctionality for bone implants is to incorporate particulate drug delivery systems (DDSs) into the rough or porous implant surfaces. The scope of this study was to assess the effects of a model DDS consisting of poly(D,L-lactide-co-glycolide) (PLGA) microspheres loaded with an anti-inflammatory drug, dexamethasone (DXM), on the response of Simian Virus-immortalized Human Fetal Osteoblast (SV-HFO) cells. The microspheres were prepared by the oil-in-water emulsion/solvent evaporation method, whereas cells response was investigated by Alamar Blue test for viability, alkaline phosphatase (ALP) activity for differentiation, and Alizarin Red staining for matrix mineralization. Cell viability was not affected by the presence of increased concentrations of polymeric microspheres in the culture media. Furthermore, in the cultures with DXM-loaded microspheres, ALP activity was expressed at levels similar with those obtained under osteogenic conditions, indicating that DXM released from the microsphere-stimulated cell differentiation. Matrix mineralization occurred preferentially around the DXM-loaded microspheres confirming that the released DXM could act as osteogenic supplement for the cells. These in vitro findings suggest that a particulate PLGA-DXM DDS may actually provide dual, anti-inflammatory and osteogenic functions when incorporated on the surface of bone implants.