A study on wear rates of 100Cr6 steel running against sintered steel surfaces under dry and starved lubrication

This paper investigates the tribological behavior of 100Cr6 steel pin running against sintered steel bearing material used in hermetic compressors. Tests were conducted under dry and starved lubrication sliding conditions in air at room temperature. Although porous structure acts as crack initiation sites thus limiting the wear resistance of sintered iron in dry sliding conditions under high contact stresses, it is believed to be beneficial in lubricated sliding conditions as it absorbs a large amount of lubricant. Wear tests without lubrication show that these pores are completely filled by abrasive particles in the initial stages of the test and no longer maintain their oil absorption capability. Initial results show that oxidation of frictional surfaces by flash temperature in dry conditions reduces weight loss volume by decreasing the coefficient of friction

Effect of R600a on tribological behaviour of sintered steel under starved lubrication

This study aims to develop and characterize wear resistant and low friction tribopairs that are compatible with new ozone-friendly Isobutane refrigerant to run at hermetic compressor bearings. The tribological behavior of 100Cr6 steel pin is investigated under starved lubrication condition in air and R600a environments when running against sintered steel with and without steam treatment. EDS and SEM are carried out on pin and plate samples after wear tests. The results indicate that durability distance is lower for the tests with R600a than those with air. The adverse effect of R600a on wear rate is linked to the change in the viscosity and foaming characteristics of the oil in the presence of R600a as well as the lack of oxides. Kemal Sariibrahimoglu1, Huseyin Kizil1*, Mahmut F. Aksit2, Ihsan Efeoglu3, and Husnu Kerpicci

Strategies to accelerate the degradation of injectable calcium phosphate-based composite materials for bone regeneration.

Contains fulltext : 127111.pdf (publisher's version ) (Open Access)Radboud Universiteit Nijmegen, 7 mei 2014Promotor : Jansen, J.A. Co-promotores : Wolke, J.G.C., Leeuwenburgh, S.C.G

Injectable biphasic calcium phosphate cements as a potential bone substitute

Item does not contain fulltextApatitic calcium phosphate cements (CPCs) have been widely used as bone grafts due to their excellent osteoconductive properties, but the degradation properties are insufficient to stimulate bone healing in large bone defects. A novel approach to overcome the lack of degradability of apatitic CPC involves the development of biphasic CPCs (BCPC) based on tricalcium phosphate (TCP) in both alpha- and beta-polymorphs. The aim of the current study was to prepare and analyze the physicochemical properties of BCPCs based on dual phase alpha/beta-TCP as obtained by heat treatment of pure alpha-TCP. The handling and mechanical characteristics of the samples as well as the degradation behavior under in vitro condition were investigated and compared with a standard monophasic alpha-TCP-based CPC. The results showed that different heat treatments of commercially available alpha-TCP allowed the formation of biphasic calcium phosphate powder with a variety of alpha/beta-TCP ratios. The use of biphasic powder particles as a reactant for CPCs resulted into increased setting and injectability times of the final BCPCs. During hardening of the cements, the amount of apatite formation decreased with increasing beta-TCP content in the biphasic precursor powders. The morphology of the monophasic CPC consisted of plate-like crystals, whereas needle-like crystals were observed for BCPCs. In vitro degradation tests demonstrated that dissolution rate and corresponding calcium release from the set cements increased considerably with increasing beta-TCP content, suggesting that apatitic CPCs can be rendered degradable by using biphasic alpha/beta-TCP as powder precursor phase. (c) 2013 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 102B: 415-422, 2014

Accelerated calcium phosphate cement degradation due to incorporation of glucono-delta-lactone microparticles

Item does not contain fulltextInjectable calcium phosphate cements (CPC) are frequently used for filling of bone defects due to their excellent osteocompatibility. Their poor degradability, however, limits complete regeneration of bone defects. Organic additives that produce acid by-products are particularly attractive to create macroporosity in situ since CPC degrade by acid dissolution. The aim of the current study was to investigate whether glucono-delta-lactone (GDL) can be used as acid-producing microparticles for incorporation into CPC without compromising its osteocompatibility. Characterization studies confirmed that CPCs containing either low or high amounts of GDL were injectable and self-setting, while a considerable amount of porosity was formed already within 1 day of incubation in phosphate buffered saline due to dissolution of GDL. Histomorphometrical evaluation after 2 weeks of implantation revealed that CPC containing 10% of GDL degraded faster and was replaced by more bone tissue than CPCs containing either Poly (lactic-co-glycolic acid) (PLGA) or gelatin microspheres. Summarizing, the current study showed that CPCs containing appropriate amounts of GDL display accelerated degradation and new bone formation compared with CPCs containing microparticles made of conventional polymers such as PLGA or gelatin

Comparison of micro- vs. nanostructured colloidal gelatin gels for sustained delivery of osteogenic proteins: Bone morphogenetic protein-2 and alkaline phosphatase

Item does not contain fulltextColloidal gels have recently emerged as a promising new class of materials for regenerative medicine by employing micro- and nanospheres as building blocks to assemble into integral scaffolds. To this end, physically crosslinked particulate networks are formed that are injectable yet cohesive. By varying the physicochemical properties of different particle populations, the suitability of colloidal gels for programmed delivery of multiple therapeutic proteins is superior over conventional monolithic gels that lack this strong capacity for controlled drug release. Colloidal gels made of biodegradable polymer micro-or nanospheres have been widely investigated over the past few years, but a direct comparison between micro- vs. nanostructured colloidal gels has not been made yet. Therefore, the current study has compared the viscoelastic properties and capacity for drug release of colloidal gels made of oppositely charged gelatin microspheres vs. nanospheres. Viscoelastic properties of the colloidal gelatin gels were characterized by rheology and simple injectability tests, and in vitro release of two selected osteogenic proteins (i.e. bone morphogenetic protein-2 (BMP-2) and alkaline phosphatase (ALP)) from the colloidal gelatin gels was evaluated using radiolabeled BMP-2 and ALP. Nanostructured colloidal gelatin gels displayed superior viscoelastic properties over microsphere-based gels in terms of elasticity, injectability, structural integrity, and self-healing behavior upon severe network destruction. In contrast, microstructured colloidal gelatin gels exhibited poor gel strength and integrity, unfavorable injectability, and did not recover after shearing, resulting from the poor gel cohesion due to insufficiently strong interparticle forces. Regarding the capacity for drug delivery, sustained growth factor (BMP-2) release was obtained for both micro- and nanosphere-based gels, the kinetics of which were mainly depending on the particle size of gelatin spheres with the same crosslinking density. Therefore, the optimal gelatin carrier for drug delivery in terms of particle size and crosslinking density still needs to be established for specific clinical indications that require either short-term or long-term release. It can be concluded that nanostructured colloidal gelatin gels show great potential for sustained delivery of therapeutic proteins, whereas microstructured colloidal gelatin gels are not sufficiently cohesive as injectables for biomedical applications. (C) 2012 Elsevier Ltd. All rights reserved

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Wear performance of titanium and niobium added MOS2 coatings under isobutane exposure

As conventional refrigerants phased out, Hydrocarbons such as Isobutane emerged as the main alternative in refrigeration industry. However, compatibility of these new ozone friendly refrigerants with compressor oils and bearing materials need to be evaluated. This study investigates wear performance of Titanium and Niobium added MoS2 coatings running against 100Cr6 steel under isobutane exposure. Wear performance is established under starved lubrication condition by comparing the maximum achieved sliding distance before the sudden increase and fluctuation of the friction curve. Sinter steel samples were coated with MoS2, MoS2-Ti, MoS2-Nb thin films by magnetron sputtering. The concept of durability distance has been defined as the distance until a sudden fluctuation of the friction curve is observed. It was found that addition of Titanium and Niobium adversely affects wear performance of the MoS2 coatings under isobutane exposure. This can be attributed to the possible degradation of MoS2 lamellar structure that acts as solid lubricant under such exposure