PEEK (Polyether-ether-ketone) Based Cervical Total Disc Arthroplasty: Contact Stress and Lubrication Analysis

This paper presents a theoretical analysis of the maximum contact stress and the lubrication regimes for PEEK (Polyether-ether-ketone) based self-mating cervical total disc arthroplasty. The NuNec® cervical disc arthroplasty system was chosen as the study object, which was then analytically modelled as a ball on socket joint. A non-adhesion Hertzian contact model and elastohydrodynamic lubrication theory were used to predict the maximum contact stress and the minimum film thickness, respectively. The peak contact stress and the minimum film thickness between the bearing surfaces were then determined, as the radial clearance or lubricant was varied. The obtained results show that under 150 N loading, the peak contact stress was in the range 5.9 – 32.1 MPa, well below the yield and fatigue strength of PEEK; the calculated minimum film thickness ranged from 0 to 0.042 µm and the corresponding lambda ratio range was from 0 to 0.052. This indicates that the PEEK based cervical disc arthroplasty will operate under a boundary lubrication regime, within the natural angular velocity range of the cervical spine

Corrosion and tribological performance of quasi-stoichiometric titanium containing carbo-nitride coatings

Zr, Nb and Si doped TiCN coatings, with (C+N)/(metal + Si) ratios of approximately 1, were deposited on stainless steel and Si wafer substrates using a cathodic arc technique in a mixture of N2 and CH4 gases. The coatings were comparatively analysed for elemental and phase composition, adhesion, anticorrosive properties and tribological performance at ambient and 250 °C. Zr, Nb and Si alloying contents in the coatings were in the range 2.9–9.6 at.%. All the coatings exhibited f.c.c. solid solution structures and had a 〈1 1 1〉 preferred orientation. In the adhesion tests conducted, critical loads ranged from 20 to 30 N, indicative of a good adhesion to substrate materials. The Ti based coatings with Nb or Si alloying elements proved to be resistant to corrosive attack in 3.5% NaCl and of these coatings the TiNbCN was found to have the best corrosion resistance. TiCN exhibited the best tribological performance at 250 °C, while at ambient temperatures it was TiNbCN. Abrasive and oxidative wear was found to be the main wear mechanism for all of the coatings. Of the tested coatings, TiNbCN coatings would be the most suitable candidate for severe service (high temperature, corrosive, etc.) applications

The influence of variations of geometrical parameters on the notching stress intensity factors of cylindrical shells

The modern approach of Virtual Engineering allows one to detect with some accuracy the residual life of components especially free of cracks. The life estimation becomes cumbersome when the components contain a crack. A straightforward formulation requires a parameter that considers geometrical constraints and materials properties. The magnitude of the stress singularity developed by the tip of a crack, needs to be expressed by the Stress Intensity Factors (SIF). In order to prove the validity of the results, calibration by experimental and/or analytical technique is required. To have a better understanding of this parameter, in the first part of this paper an analytical model to compute the SIF connected to crack propagation into Mode I has been implemented. The case study displays a pipeline component with a crack defect submitted to internal pressure. Therefore, an appropriate correlation between the analytical approach and numerical simulation has been established embedded

Multifunctional Ti based carbonitride coatings for applications in severe environments

In this work, the influence of NbZr and ZrSi addition to TiCN coatings are studied, aiming for their use as protective layers for parts subjected to severe corrosion and wear. The coatings with C/N ratios ranging from 0.4 to 2.5 were deposited using the cathodic arc technique in a mixture of N2 and CH4 gases, on 316 stainless steel discs and Si (111) wafers. All the coatings exhibited residual compressive stresses, with values ranging from approximately-2.4 GPa to −3.5 GPa. The addition of Si led to an increase in hardness, regardless of the C/N ratio. All coatings with high C/N ratio (~2.5) presented slightly lower stress values and superior performance in 3.5% NaCl corrosive solutions, the best performance being obtained for the TiSiZrCN coating, which exhibited the highest protective efficiency to corrosion (97.8%), due to its low corrosion current density (1.734 μA/cm2) and high polarization resistance (31.775 kΩ). The tribological tests, performed at 23 °C and 250 °C, indicated that abrasion and oxidation were the predominant wear mechanism for all coatings. At 23 °C, the friction coefficients of the coated specimens were significantly lower than those of the uncoated samples. When the collective performance across all of the experimental parameters was assessed, the coatings with C/N of about 2.5 proved to be the most suitable candidates to be used in severe service conditions