J.R. Gomes

Effect of α-/β Si3N4-phase ratio and microstructure on the tribological behaviour up to 700°C

Abstract The present work analyses the effect of mechanical and microstructural characteristics in the tribological behaviour of Si3N4 materials. Special attention was given to the α/β phase ratio and the coarseness of the microstructure. Wear experiments were performed in a pin-on-disc tribometer using self-mated Si3N4 pairs varying the sliding speed and the test temperature. In the case of fine and medium microstructures, fracture and delamination was the main wear mechanism while for heterogeneous coarse material deformation related to intergranular microfracture was prevalent. The latter present higher wear rates at room temperature and low sliding speed but showed the best wear resistance when either sliding speed or temperature were increased.

Tribological characteristics of self-mated couples of Si3N4–SiC composites in the range 22–700°C

Abstract The tribological behaviour of self-mated pairs of Si 3 N 4 , Si 3 N 4 /SiC P (platelets) and Si 3 N 4 /SiC N (nanosized) hot pressed materials was studied in a pin-on-disc configuration. Tests were performed without lubrication, at a fixed load of 5 N, using different temperatures (room temperature −700°C) and sliding speeds (0.5–2 m s −1 ). Friction coefficients were usually above 0.5, being almost independent of sliding speed and test temperature. For all test conditions, the Si 3 N 4 /SiC P composite had the lowest friction coefficient, due to the presence of SiC platelets with smooth cleavage planes oriented parallel to the sliding surface. Wear coefficients were above 10 −6 mm 3 N −1 m −1 denoting a generalised severe wear mode, controlled by surface microcracking. No net differences were found between the wear behaviour of monolithic and composite materials under this wear regime. Wear decreased with sliding speed due to the protective action of a coherent tribofilm resulting from debris aggregation. Conversely, wear coefficients showed a steep increase above room temperature, to values around 10 −4 mm 3 N −1 m −1 at 700°C.

Tribocorrosion Studies in Centrifugally Cast Al-Matrix SiCp-reinforced Functionally Graded Composites

The present work reports results obtained from a series of preliminary experiments aiming at complementing the current knowledge about the wear behaviour of centrifugally-cast FGM Al/SiCp composites, through concurrent corrosion processes. Precursor MMCs were prepared by rheocasting, using 118.8 µm SiC particles and an Al-10Si-2.2 Mg alloy. Those MMCs were then molten and centrifugally cast in order to produce cylindrical FGMMCs. Discs machined from the top surface of each sample were tested against nodular cast iron pins, using an inverted configuration pin-on-disc tribometer. Sliding tests took place at room temperature, over a 50000 m sliding distance, with a sliding speed of 0.3 m s -1 , under a 5 N normal load; both dry-sliding and water-lubricated tests were performed. In order to elucidate the mechanisms involved, the wear coefficients were calculated for each condition, and the samples were subjected to morphological characterization via SEM/EDS. Concurrently, in the case of the water-lubrication tests, the corrosion potential of the tribological pair was monitored. The results obtained show an increase in material loss for the water-lubricated cases, although variations are registered depending on reinforcing particle volume fraction. At the same time, the open circuit potential response of the tribological pair may be correlated with the events of formation/destruction of the tribolayers.

Tribological properties of AlNCeO2Si3N4 cutting materials in unlubricated sliding against tool steel and cast iron

Abstract Ceramic pins of the AlN-CeO 2 -Si 3 N 4 system were tested in a pindashondashdisc tribometer against discs of tool steel and grey cast iron, at room temperature, without lubrication, in different conditions of humidity and sliding speed. Ceramic samples were selected on the basis of their mechanical properties (hardness and fracture toughness), and microstructural characteristics, namely porosity, volume of intergranular phase and nitrogen content of the glass phase. Water vapour increased the weight loss of the nitride by promoting the tribo-oxidation wear mode and by weakening the adhesion of debris to the ceramic surfaces. In dry air, the adhering wear debris provided protection to the sliding surfaces and the wear coefficients of the ceramic converged to similar values for tests with both iron alloys. For the ceramic/tool steel tribopairs, the ceramic surfaces become more protected as the amount of the intergranular glassy layer of the nitride is increased, as the glassy phase gives enhanced debris adhesion. Microcracking of the ceramic surface was the dominant wear mode and the volumic wear rate was found to be dependent on the inverse of hardness and fracture toughness. In humid environments, the effect of roughness of the grey cast iron worn surface surmounted the dependence of the wear rate on microstructural and mechanical properties of the nitride, which was found in the ceramic/steel tribopairs. The sliding speed has a strong effect on the wear behaviour. At low speeds, no protective plates of debris were detected on the worn surfaces. When the speed was increased above 0.5 ms −1 , the wear coefficient values fell down almost one order of magnitude. The wear coefficients of porous nitride materials of relative open porosity close to 20%, tested against cast iron, were unexpectedly lower than the values obtained for dense materials of same composition (K ≈ × 10 −15 Pa −1 for porous samples and K ≈ 1.5 × 10 −14 Pa −1 , for fully dense samples). SEM observations showed an extensive coating by the metallic rich debris, that performed a solid lubricant action.

Wear Mechanisms in Functionally Graded Aluminium Matrix Composites: Effect of the Presence of an Aqueous Solution

Functionally graded aluminium matrix composites reinforced with SiC particles are attractive materials for a broad range of engineering applications whenever a superior combination of surface and bulk mechanical properties is required. In general, these materials are developed for the production of high wear resistant components. Also, often this kind of mechanical part operates in the presence of aggressive environments, such as marine atmospheres. In this work, aluminium composites with functionally graded properties, obtained by centrifugal cast, are characterised by reciprocating pin-on-plate sliding wear tests against nodular cast iron. Three different volume fractions of SiC reinforcing particles in each functionally graded material were considered. Sliding experiments were performed with and without the presence of a lubricant (3% NaCl aqueous solution). In the case of the lubricated tests, electrochemical parameters (corrosion potential) were monitored during sliding. Friction values were in the order of 0.42 for unlubricated conditions, but varied between 0.22 and 0.37 when the aqueous solution was present. For all test conditions, relatively high wear rates (over 1×10-6 gm-1) were obtained, particularly for the cast iron pin. The volume fraction of SiC particles exerted a net effect on the tribological response of the composites, although conditioned by the presence or absence of the aqueous solution. The worn surface morphology of the composites indicated that the presence of the aqueous solution modifies the protective action promoted by the combined effect of the presence of reinforcing particles as load bearing elements and the formation of adherent iron-rich tribolayers. The evolution of the corrosion potential during the sliding action is in accordance to the degradation mechanisms proposed for these systems.

Effect of Functionally Graded Properties on the Tribological Behaviour of Aluminium-Matrix Composites

In this work, aluminium based matrix composites with functionally graded properties are tested against cast iron in a pin-on-disc tribometer. SiC particulate reinforced F3S-20S aluminium matrix composite (Duralcan) was melted and centrifugally cast in order to obtain a gradient regarding the ceramic particle area fraction and mean particle diameter. Three different cross sections of the functionally graded material were considered for tribological characterisation. Friction and wear tests were performed at room temperature, constant sliding speed (0.5 ms -1 ) and at 5 N of normal load. The morphological features of the sliding surfaces were analysed by SEM/EDS in order to understand the prevailing wear mechanisms. The wear coefficient was relatively low for both matting surfaces (in the order of 10 -6 mm 3 N -1 m -1 ), with the composites presenting the lowest wear values due to the combined effect of reinforcing particles as load bearing elements and the formation of protective adherent iron-rich tribolayers. A comparative analysis is established between the functionally graded properties of the aluminium based composites and the tribological response at different cross sections. Results suggests that there is a critical area fraction of SiC particles above which severe wear, attributed to lack in fatigue strength, is observed.

Regeneration of the peripheral nerve — Development and evaluation of guide tubes of biodegradable polymer

Damage to peripheral nerve fibers results in axonal loss and demyelination followed by regeneration and remyelination under optimal conditions with the possibility of some functional recovery. The experimental challenge is to accelerate axonal regeneration to promote reinnervation and improve functional recovery after peripheral nerve injury. In the past few decades, different types of biological or artificial guide tubes have been developed to bridge the gap of a sectioned nerve, to limit the fibrosis process and to orient the regenerating fibers towards the distal stump. Chitosan is widely used for biomedical applications, including crosslinked with other materials. In this work, chitosan guide tubes were produced and implanted using the rat sciatic nerve animal model. Functional tests were performed as well as a mechanical and structural characterization of the guide tubes.

The structural analysis of chitosan tubes using meshless methods

Chitosan is a natural polymer widely studied and often considered as a suited biomaterial for the construction of nerve guidance channels for peripheral nerve injuries. These tubes can take the shape of hollow tubes that bridge the gap between the distal and proximal nerve stumps, creating optimal conditions towards the regeneration of the peripheral nerve. In this work, several geometric models of chitosan tubes were constructed and numerically simulated using advanced discretization techniques. The results show the structural response of these scaffolds when submitted to flexion.

In-Situ Friction Monitoring of Self-Mated CVD Diamond Coatings Using Acoustic Emission

In-situ measurements of acoustic emission (AE) in self-mated tribological pairs of CVD diamond coated silicon nitride (Si3N4) were made with the purpose of investigating the relationship between AE signal and friction events. A good correlation is found between the energy dissipation/emission processes, therefore enabling the possibility of monitoring the different friction regimes occurring during the sliding contact of microcrystalline diamond (MCD) coatings. Deposition of MCD on flat and ball-shaped Si3N4 samples was accomplished using microwave plasma assisted chemical vapour deposition (MPCVD) with H2/CH4 gas mixtures. The friction behaviour of self-mated MCD coatings was assessed using a reciprocating ball-on-flat geometry. The tests were run in ambient atmosphere without lubrication, the frequency (1Hz) and stroke (6mm) were kept constant while the applied normal load varied in the range 10-80N. The microstructure, surface topography and roughness of the MCD coatings were characterised by SEM and AFM techniques. The diamond quality was assessed from micro-Raman spectroscopy. The friction evolution was characterised by a short running-in period where the main feature is a sharp peak reaching values as high as approximately 0.6 followed by a steady-state regime with very low values in the range 0.03-0.04.

Tribological Characterization of Si3N4-Bioglass Biocomposites in Self-Mating Experiments and Dissimilar Tests against UHMWPE

The tribological behaviour of a new bioceramic material, a Si3N4-bioglass composite, is investigated. The experiments were performed in a pin-on-disc tribometer with constant sliding speed (0.1 ms -1 ) at two distinct loads (0.5N and 5N), in dry and wet (Hank’s balanced salt solution, HBSS) conditions. In self-mated dry experiments, the 20wt% bioglass composite presents the lowest wear coefficient (K=3.7x10 -6 mm 3 N -1 m -1 ) due to its better intrinsic mechanical characteristics. Fine scale abrasion at low loads and extensive fracture at high loads were the main wear mechanisms. For lubricated experiments in HBSS media, both the wear and the friction coefficients decrease comparing to the dry sliding tests, namely under an applied load of 5N, that promotes the retention of a protective tribolayer. The best tribological system was the dissimilar pair Si3N4-20wt.% bioglass/UHMWPE, for which K=1.2x10 -6 mm 3 N -1 m -1 and K=7.1x10 -7 mm 3 N 1 m -1 , respectively, and f=0.1, exhibiting a considerable potential for prosthetic applications.