Modelling, production and characterisation of duplex coatings (HVOF and PVD) on Ti-6Al-4V substrate for specific mechanical applications

Titanium and its alloys are extensively used in aerospace and mechanical application because of their high specific strength and high fracture toughness. On the other hand, titanium alloys often show low hardness, very low load bearing capacity and poor resistance to sliding wear, so that surface properties improvement is in many cases recommended, often by PVD processes.Present work deals with design, production and experimental characterisation of a duplex coating for Ti-6Al-4V components, consisting of a thick WC-Co interlayer deposited by High Velocity Oxygen Fuel (HVOF), followed by a Ti/TiN multilayer (two layer pairs, including the Ti bond layer) deposited by Cathodic Arc Evaporation (CAE) PVD.Before deposition, a preliminary coating design was carried out, based on finite element simulation of residual stress fields on the PVD coating for a range of configurations of its multilayered structure (Ti buffer layer position and thickness).Morphological properties of the produced coatings (thickness, grain size, surface defect size distribution, roughness) were measured by means of Digital Optical (DOM), Scanning Electron (SEM), Atomic Force (AFM) and Focused Ion Beam (FIB) microscopy techniques.Coatings mechanical properties were investigated by micro-scratch testing, Rockwell C adhesion test, nano-indentation techniques, Vickers/Knoop micro-hardness testing and composite hardness modelling.Results showed that the use of a CAE-PVD multilayer Ti/TiN top layer, whose thicknesses and Ti distribution were suggested by finite element modelling optimisation, leads to a significant increase (45%) in adhesion of PVD coating to the HVOF layer and load bearing capacity of the coated system, compared to monolayered TiN, without reduction in superficial hardness and load bearing capacity.     

Event-driven optimization-based control of hybrid systems with integral continuous-time dynamics

In this paper we introduce a class of continuous-time hybrid dynamical systems called integral continuous-time hybrid automata (icHA) for which we propose an event-driven optimization-based control strategy. Events include both external actions applied to the system and changes of continuous dynamics (mode switches). The icHA formalism subsumes a number of hybrid dynamical systems with practical interest, e.g., linear hybrid automata. Different cost functions, including minimum-time and minimum-effort criteria, and constraints are examined in the event-driven optimal control formulation. This is translated into a finite-dimensional mixed-integer optimization problem, in which the event instants and the corresponding values of the control input are the optimization variables. As a consequence, the proposed approach has the advantage of automatically adjusting the attention of the controller to the frequency of event occurrence in the hybrid process. A receding horizon control scheme exploiting the event-based optimal control formulation is proposed as a feedback control strategy and proved to ensure either finite-time or asymptotic convergence of the closed-loop.     

Observability and controllability of piecewise affine and hybrid systems

We prove, in a constructive way, the equivalence between piecewise affine systems and a broad class of hybrid systems described by interacting linear dynamics, automata, and propositional logic. By focusing our investigation on the former class, we show through counterexamples that observability and controllability properties cannot be easily deduced from those of the component linear subsystems. Instead, we propose practical numerical tests based on mixed-integer linear programming.     

A Lyapunov method for stability analysis of piecewise-affine systems over non-invariant domains

This paper analyses stability of discrete-time piecewise-affine systems, defined on possibly non-invariant domains, taking into account the possible presence of multiple dynamics in each of the polytopic regions of the system. An algorithm based on linear programming is proposed, in order to prove exponential stability of the origin and to find a positively invariant estimate of its region of attraction. The results are based on the definition of a piecewise-affine Lyapunov function, which is in general discontinuous on the boundaries of the regions. The proposed method is proven to lead to feasible solutions in a broader range of cases as compared to a previously proposed approach. Two numerical examples are shown, among which a case where the proposed method is applied to a closed-loop system, to which model predictive control was applied without a-priori guarantee of stability.     

F-substituted hydroxyapatite nanopowders: Thermal stability,sintering behaviour and mechanical properties

Fluorine-substituted hydroxyapatites are considered promising materials for bone scaffolding. In this study a systematic investigation on F-half substituted hydroxyapatite (F-HAp, Ca₁₀(PO₄)₆OHF) obtained by precipitation is reported. Results on composition analysis, thermal behaviour, and sinterability are presented for a comparison with the respective pure hydroxyapatite. Samples were characterised by electron microscopy, induced coupled plasma-atomic emission spectroscopy, thermal analysis, infrared spectroscopy, N₂ adsorption measurements, X-ray diffraction and dilatometry. A semicrystalline F-substituted hydroxyapatite powder made up of needle-like nanoparticles was obtained. Notwithstanding that it was thermally stable up to 1300 °C, it was sintered at relatively low temperatures obtaining a very fine microstructure with a relevant nano-porosity, beneficial in view of cell adhesion and in-growth.Hardness, elastic modulus and toughness of produced samples were investigated by Vickers–Knoop microhardness testing. Significant improvements of the mechanical properties were observed for the F-substituted hydroxyapatite, in comparison with pure hydroxyapatite.     

Optimal control of continuous-time switched affine systems

This paper deals with optimal control of switched piecewise affine autonomous systems, where the objective is to minimize a performance index over an infinite time horizon. We assume that the switching sequence has a finite length, and that the decision variables are the switching instants and the sequence of operating modes. We present two different approaches for solving such an optimal control problem. The first approach iterates between a procedure that finds an optimal switching sequence of modes, and a procedure that finds the optimal switching instants. The second approach is inspired by dynamic programming and identifies the regions of the state space where an optimal mode switch should occur, therefore providing a state feedback control law.     

The Vortex Path Model Analysis of the Field Angle Dependence of the Critical Current Density in Nanocomposite YBa2Cu3 O 7−x – BaZrO3 Films Obtained by Low Fluorine Chemical Solution Deposition

In the present paper, we analyze the role of in situ grown BaZrO₃ (BZO) inclusions in YBa₂Cu₃O_7?x (YBCO) thin films prepared by chemical solution deposition using a low fluorine coating solution, on the field angle dependence of the critical current density, J _c (??), data using the vortex path model. In order to form a coherent picture on the BZO doping influence on the pinning properties of the YBCO matrix, detailed structural analyses performed by X-ray diffraction techniques and microstructural evaluation by transmission electron microscopy are also presented. The evaluation of different contributions to the overall, J _c , permitted us to prove the effectiveness of the BZO inclusions acting as isotropic pinning centers, reflected in a uniform component of high relative value with respect to other components. For the studied 10 mol % BZO doping concentration, a threefold increase in the critical current density, J _c , of the YBCO host is measured, in self-field at 77 K, corresponding to a value of J _c =2.9MA/cm², whereas a factor 10 is measured at 1 T (J _c =0.35 MA/cm²).     

Analysis of the instabilities related to the multiselective injection and withdrawal procedures of the advanced solar pond

This paper concerns the development of instabilities stemming from the multiselective injection and withdrawal procedures which create the stratified thermal layer of the advanced solar pond (ASP). A cascade of thin plates models the Entrance Unit, and a line sink models the Exit Unit. Two flow patterns are identified: A transient pattern and a steady state. The transient pattern may destroy the steady state flow. However, proper operations of the ASP injection and withdrawal ports may reduce or cancel the potentially destructive flow instabilities. Other topics concerned in this paper refer to the decay of turbulence in the ASP stratified layers, and differences between types of instability developed in the ASP subject to field conditions, and those developed in the ASP laboratory simulator.     

Theoretical and experimental comparison of conventional and advanced solar pond performance

Numerical and physical experiments were carried out to compare the performance of two solar pond systems: (a) a conventional salt gradient solar pond (CSP) and (b) a salt gradient pond operated as an “advanced solar pond” (ASP). The main differences in the ASP, as originally proposed by Osdor[1], are an increase in overall salinity and the introduction of a stratified flowing layer near the bottom of the gradient zone. The increased salinity is meant to reduce evaporative heat loss and make up water requirements, while the additional flowing layer allows extra heat extraction and possibly higher temperatures to develop in the lower convective zone. A numerical study was performed to evaluate the salinity effect and the results show only a minor effect of increased salinity on heat collection efficiency. However, slightly higher collection temperatures are obtained, which may provide some benefit for heat engine efficiency. Physical experiments were performed to test the feasibility of constructing and maintaining the necessary flow system for the ASP and also to compare the performance of the ASP and the CSP under similar laboratory conditions. These tests showed that a stable stratified flowing layer could be maintained and that the ASP configuration produced higher efficiencies.     

Nonlinear control of constrained linear systems via predictivereference management

A method based on conceptual tools of predictive control is described for solving set-point tracking problems wherein pointwise-in-time input and/or state inequality constraints are present. It consists of adding to a primal compensated system a nonlinear device, called command governor (CG), whose action is based on the current state, set-point, and prescribed constraints. The CG selects at any time a virtual sequence among a family of linearly parameterized command sequences, by solving a convex constrained quadratic optimization problem, and feeds the primal system according to a receding horizon control philosophy. The overall system is proved to fulfill the constraints, be asymptotically stable, and exhibit an offset-free tracking behavior, provided that an admissibility condition on the initial state is satisfied. Though the CG can be tailored for the application at hand by appropriately choosing the available design knobs, the required online computational load for the usual case of affine constraints is well tempered by the related relatively simple convex quadratic programming problem