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Thanks to the low working temperature, less product size limitations and one order of magnitude higher deposition rates compared to the established additive manufacturing techniques, more attention has been brought to the potential of cold spraying for additive manufacturing . As a process dealing with deformation of solid particles possibly leaving non-bonded interfaces and causing work hardening, any optimization should (i) adjust spray parameters to obtain high performance as-sprayed parts and (ii) tune ductility and internal stresses by post-treatments. The present study first deals with strategies to optimize spray parameters for fabrication of high performance steel 316 L deposits. Next, the performances of deposits are further adjusted by various heat treatments. The structural strength of the freestanding samples before and after the heat treatments is evaluated under static and cyclic axial loading and supported by fatigue crack growth rate analysis. The results highlight the feasibility of obtaining high quality steel 316 L deposits using N2 as process gas, rather than the costly He that is commonly suggested. This study demonstrates the potential of cold spraying to be used for deposition of freeform structural components with a static strength comparable to that of bulk and laser-based additive manufactured materials and a fatigue strength similar to that of bulk cast material.

The method of crack growth rate measurement in miniature single edge notched (SEN) cantilever specimen at stress ratio R ∼ −1 is presented. The resonance circuit is formed by the cantilever specimen with inertial yoke at its free end excited by alternating electromagnetic field. The parameters of the harmonic excitation are controlled by the phase shift technique. The crack length is estimated from specimen compliance added by the crack opening. The stress intensity factor is computed from the applied bending moment. Linear elastic fracture mechanics with standard shape functions corresponding to the particular specimen geometry and applied loading are used. The analysis of signal from single piezoelectric accelerometer provides data for the estimation of crack length and applied stress intensity factor, making the method very simple and inexpensive, yet reasonably accurate. The crack length measurement was calibrated using fractographic marking technique. The presented method was validated by performing crack growth rate tests in miniature 4 × 3 × 32 mm beam specimens with 150° chevron notch. 7075 aluminium alloy and 4043 high strength steel were tested at load frequency of 110 and 160 Hz respectively. The crack growth curves were measured in both load increase and load reduction mode and the resulting crack compare well with the baseline data available for standard central crack (CCT) specimens.

A methodology that allows joint determination of fundamental engineering properties of materials/components vital to their efficient design and a safe service lifetime operation is presented. Using a combination of recently developed techniques, a simultaneous assessment of mechanical properties, fatigue crack growth rates, as well as fracture toughness is described. All these are performed using unified-geometry samples that are simple and easy to produce. The mechanical properties are measured in tensile and compressive modes, while the combination of developed bending test methods (quasi-static four-point bending and fatigue bending) enables to cover the fatigue loading from very low loads corresponding to crack propagation threshold values up to the high loads corresponding to a static fracture of the fracture toughness test. To validate and complement the bending data, additional resonant ultrasonic measurements of the corresponding properties are carried out. The methodology is partially suitable for testing of cold sprayed deposits. Therefore, Al, Cu, Ni and Ti deposits are tested and compared to cold-rolled sheet reference samples. The results are quantified in the form of NASGRO equation, widely used in software for prediction of fatigue lives. Together, the obtained set of data may be readily used for modeling of the performance of cold sprayed parts. Finally, fractographic analysis of the failed specimens is presented to describe the mechanisms leading to failure. At low loads, the reference sheets and the cold sprayed deposits exhibit similar behavior, where the cracks grow in a trans-crystalline mode without any significant interaction with deposit particle boundaries. Contrary to this, the behavior changes at higher loads: the particle interfaces in the cold sprayed deposits become the weak point and the cracks grow at high rates by inter-particle decohesion, while the sheet materials generally fail by striation mechanism at much lower rates.

Three sets of hydroxyapatite and rutile-TiO₂ coatings were plasma sprayed onto metallic substrates. The spray parameters of the sets were modified so as to obtain different in-flight temperatures and velocities of the powder particles within the plasma jet (ranging from 1778 to 2385 K and 128 to 199 ms⁻¹, respectively). Fatigue endurance of the coated specimens was then tested. The samples were subjected to a symmetric cyclical bend loading, and the crack propagation was monitored until it reached a predefined cross-section damage. The influence of the coating deposition was evaluated with respect to a noncoated reference set and the in-flight characteristics. Attributed to favorable residual stress development in the sprayed samples, it was found that the deposition of the coatings generally led to a prolongation of the fatigue lives. The highest lifetime increase (up to 46% as compared to the noncoated set) was recorded for the coatings deposited under high in-flight temperature and velocity. Importantly, this was achieved without significantly compromising the microstructure or phase composition of the deposited HA and TiO₂ layers. The experimental study was supported through Czech Science Foundation Grant GB14-36566G “Multidisciplinary research centre for advanced materials.” Financial support by the European Regional Development Fund in the frame of the project Centre of Advanced Applied Sciences (No. CZ.02.1.01/0.0/0.0/16_019/0000778) is gratefully acknowledged.

Ceramic Al2O3-ZrO2-SiO2 coatings with near eutectic composition were plasma sprayed using hybrid water stabilized plasma torch (WSP-H). The as-sprayed coatings possessed fully amorphous microstructure which can be transformed to nanocrystalline by further heat treatment. The amorphous/crystalline content ratio and the crystallite sizes can be controlled by a specific choice of heat treatment conditions, subsequently leading to significant changes in the microstructure and mechanical properties of the coatings, such as hardness or wear resistance. In this study, two advanced methods of surface heat treatment were realized by plasma jet or by high energy laser heating. As opposed to the traditional furnace treatments, inducing homogeneous changes throughout the material, both approaches lead to a formation of gradient microstructure within the coatings, from dominantly amorphous at the substrate&ndash, coating interface vicinity to fully nanocrystalline near its surface. The processes can also be applied for large-scale applications and do not induce detrimental changes to the underlying substrate materials. The respective mechanical response was evaluated by measuring coating hardness profile and wear resistance. For some of the heat treatment conditions, an increase in the coating microhardness by factor up to 1.8 was observed, as well as improvement of wear resistance behaviour up to 6.5 times. The phase composition changes were analysed by X-ray diffraction and the microstructure was investigated by scanning electron microscopy.

Our previous experiments with low-cost steel substrates confirmed that individual steps of conventional thermal barrier coating (TBC) deposition may influence fatigue properties of the coated samples differently. In the presented study, testing was carried out for TBC samples deposited on industrially more relevant Hastelloy X substrates. Samples were tested after each step of the TBC deposition process: as-received (non-coated), grit-blasted, bond-coated (NiCoCrAlY), and bond-coated + top-coated yttria-stabilized zirconia (YSZ). Conventional atmospheric plasma spraying (APS) was used for deposition of bond coat and top coat. In addition, for one half of the samples, dual-layer bond coat was prepared by combination of high-velocity air-fuel (HVAF) and APS processes. Samples were tested in the as-sprayed condition and after 100 hours annealing at 980 °C, which simulated application-relevant in-service conditions. Obtained results showed that each stage of the TBC manufacturing process as well as the simulated in-service heat exposure may significantly influence the fatigue properties of the TBC coated part. HVAF grit-blasting substantially increased the fatigue performance of the uncoated substrates. This beneficial effect was suppressed by deposition of APS bond coat but not by deposition of dual-layer HVAF + APS bond coat. All heat-treated samples showed again enhanced fatigue performance.

This paper presents a summary of the research on the influence of plasma and cold spray deposition of bio-grade Ti powder on the high-cycle fatigue properties of Ti6Al4V substrates. Four sets of flat specimens (as-received, grit-blasted, plasma and cold sprayed) were prepared and subjected to cantilever–beam cyclic bend loading with a constant deflection of the free end. It was found that the grit-blasting procedure significantly increased the fatigue lives of the specimens compared to the as-received set (1.81x increase). The deposition of the coatings onto grit-blasted specimens led to deterioration of fatigue properties. The average fatigue lives of the plasma sprayed and cold sprayed samples reached 1.16x and 0.91x of the as-received specimens, respectively. In order to understand the positive effect of the grit-blasting procedure and the adverse effect of the coatings deposition on fatigue lives, chemical analyses, fractographic analysis, microstructural investigations as well as layers elastic moduli estimation were carried out. It was found that the cold spray deposition retained the composition of the powder feedstock while a complete transformation of Ti into nitrides and oxides was detected in the plasma deposited layers. Owing to the different coating build-up principles and their respective different porosity levels, the moduli of the coatings were found to vary (7.2 GPa for plasma sprayed and 36.7 GPa for cold sprayed layers). The fatigue crack initiation sites and propagation directions in the plasma and cold sprayed specimens were found to differ substantially. Based on the obtained results, two explanations of the fatigue loading results are suggested in the paper. Tento článek představuje shrnutí výsledků výzkumu vlivu plazmového a kinetického nanášení biologických vrstev Ti na únavové vlastnosti substrátů Ti6Al4V. Čtyři vyrobené série vzorků (původní, otryskané, s plazmově nanesenou vrstvou, s vrstvou nanesenou studenou kinetizací) byly vystaveny cyklickému ohybu s krakorcovým upnutím a konstantní výchylkou volného konce. Bylo zjištěno, že procedura tryskání významně prodlužuje únavové životy vzorků ve srovnání s původními tělesy (nárůst až 1.81x). Depozice vrstev na tyto tryskané povrchy vedla ke zhoršení únavových charakteristik. Průměrný únavový život vzorků s plazmově nanesenými vrstvami dosáhl 1.16x a vzorků s vrstvami nanesenými studenou kinetizací 0.91x hodnot původních netryskaných vzorků. Pro pochopení kladného efektu technologie tryskání a negativního efektu depozice nástřiků na únavové životy byla uskutečněna řada analýz (chemické, fraktografické, mikrostrukturální) a byly kalkulovány elastické moduly vrstev. Bylo zjištěno, že nástřik pomocí studené kinetizace nezměnil složení původního Ti prášku, kdežto u vrstev deponovaných plazmovým nanášením byla pozorována kompletní transformace na nitridy a oxidy. Vzhledem k rozdílným principům vzniku vrstev a jejich odlišné porozitě byly dále zjištěny zásadní rozdíly v jejich elastických modulech (7.2 GPa u plazmově a 36.7 GPa u kineticky nanesených vrstev). bylo prokázáno, že místa iniciací únavových trhlin a směry jejich šíření v plazmově a kineticky nanesených vrstvách se výrazně odlišují. Na základě získaných poznatků byly v textu navrženy dvě výklady výsledků únavového zatěžování.