Desiderio Kovar

Purpose Previous studies have shown it is possible to initiate selective laser flash sintering (SLFS) by applying an electric field while scanning a laser. Chemical bonding between particles results without the need for polymer binders and at much lower temperatures and faster times than with traditional laser sintering. However, these studies were not successful in consistently producing crack-free parts using direct-current electric fields. The purpose of this study is to study the use of alternating current (AC) electric fields to produce crack-free parts. Design/methodology/approach AC fields were used and the effects of laser power and field frequency on the initiation of SLFS were studied using single line scans. Findings It was found that, for the scan conditions studied, a processing window existed with AC fields where SLFS was initiated at temperatures as low as 320 °C and cracking was not visible until temperatures reached 430 °C. Experiments also showed that pairs of scan lines produced regions without cracking using open-loop control to maintain an approximately constant temperature from line-to-line. Research limitations/implications The research focused on single-layer scanning. To produce 3D parts, it must be expanded to larger scanned hatches and multiple layers. Practical implications Implications of these results toward the binder-free additive manufacturing of multi-layer ceramic parts and potential methods for expanding the processing window are discussed. Originality/value This paper demonstrates that it is possible to bond particles by interparticle neck formation while producing crack-free scanned regions using open-loop control. This is a key step toward developing SLFS for ceramics.

Abstract Engineering design optimization of complex systems often involves varying operating conditions, e.g., the same design of a cold-spray nozzle under different manufacturing configurations with varying pressures and stand-off distance. This requires robust design methods to simultaneously ensure high performance and the stability of such a high performance. This paper presents a constrained Bayesian Optimization (BO) framework for robust design, aimed at optimizing performance and stability across varying operation conditions or configurations. The proposed methodology leverages condition-specific Gaussian Process (GP) submodels to estimate the nonlinear relationships between design parameters and performance using a newly developed robust Expected Improvement (EI) acquisition function. This function aggregates GP submodels from different configurations accounting for the worst-case scenario, a strategy that selects the minimum posterior mean and maximum posterior variance across the submodels to prioritize the most challenging scenario from all operating conditions. In addition, a penalty term based on the variance of the posterior mean is added to the new EI acquisition function to improve the consistency of the design performance. To demonstrate its effectiveness, we applied the framework to the design optimization of cold spray nozzles, where the objective is to find dimension variables of the nozzle that maximize particle impact velocity while ensuring a stable performance under four operating conditions. The results demonstrate that the proposed approach achieves high performance, with a 13% improvement over the original design. Furthermore, the performance variance is reduced from 204.97 (m/s)2 to 43.06 (m/s)2, significantly improving the design consistency. These results highlight its potential for broader applications in robust engineering design optimization.

Abstract A noncontact laser ultrasonics method for determining the onset temperature and the early stage sintering state is studied. Because this technique measures properties near the surface in selected regions on the sample, it is particularly well‐suited to parts produced by additive manufacturing routes such as selective laser flash sintering, where local variations in sintering state along one dimension result from the laser scan pattern. We demonstrate the ability to measure very small changes in interparticle neck size by measuring Rayleigh wave speeds. The changes in wave speed result initially from rapid changes in Young's modulus that occur at the onset and in the early stages of sintering, before significant densification is observed. This measurement method is demonstrated using alumina pellets that were partially sintered at different temperatures to produce parts with a range of neck sizes and relative densities. Using the laser ultrasonics technique, the onset of sintering is detected at temperatures between 520°C and 650°C, which is significantly below the onset sintering temperatures detectable using traditional methods.

Although dry aerosol deposition methods (aerosol deposition, micro cold spray, and vacuum kinetic spraying) for producing films typically utilize fine powders that are agglomerated or aggregated, to date these processes have been modeled using single particle impacts. In this study, molecular dynamics simulations were conducted to study the more realistic scenario where aggregates of ZnO were impacted at a high velocity. For these simulations, aggregates containing six primary particles with a diameter of 10 nm were first annealed at three temperatures (1000 °C, 1300 °C, and 1500 °C) to induce growth of interparticle necks with varying sizes and therefore indirectly affect the strengths of these interparticle necks. The annealed aggregates were then impacted to observe the deformation mechanisms that contribute to film formation. The results suggest that deformation within the aggregates is driven by viscous flow that occurs after solid state amorphization or melting and is driven by large, localized pressures and heating. The size/strengths of the necks, impact velocity, and aggregate orientations affect how deformation is concentrated and whether the aggregate can dissipate sufficient kinetic energy to deposit onto the substrate without fracturing the interparticle necks. • The strength of aggregates is varied by annealing at a range of temperatures. • Higher annealing temperatures strengthen aggregates so that they resist fracture upon impact. • Effects of impact orientation on deposition are more pronounced for weaker aggregates. • Aggregates can be deposited at a much lower velocity compared to single particles. • Deformation mechanisms for aggregate impacts are consistent with single particles.

Micro-Cold Spray (MCS), also known as the Aerosol Deposition Method (ADM), is a solidstate that enables the formation of dense, thick films at room temperature. One challenge is that precise geometric control remains difficult due to complex deposition dynamics, similar to cold spray and thermal spraying. This limitation hinders the advancement of MCS as a direct-write additive manufacturing (AM) method, particularly for micro-electromechanical systems applications. A critical yet underexplored factor in achieving geometric control is the spray path. While preliminarily studied in traditional cold spray, a systematic investigation in MCS is lacking. This paper presents the first experimentally validated study of how hatch distance and spray pattern, specifically normal and overlapping serpentine paths, affect MCS film morphology. By analyzing film adhesion and surface roughness, we offer new insights into deposition control, advancing the potential of MCS as a viable AM technology.

Abstract Selective laser flash sintering utilizes a scanning laser as a heat source to locally initiate flash sintering in the regions scanned by the laser. A key challenge towards utilizing this process for additive manufacturing (AM) is the induced electrical current that arises during flash sintering. With traditional scan patterns conducted with static electric fields, electrical and thermal runaway and associated thermal shock cracking occur. In this study, several open‐loop control strategies with static and dynamic applied electric fields were utilized to control peak currents. These strategies were shown to be effective in reducing peak currents to below 1.0 µA and reducing the severity of, but not completely eliminating cracking. Alternative strategies are suggested that could lead to complete elimination of cracks.

Cold spray offers several advantages for material deposition, including the ability to deposit a wide range of materials without the need for high temperatures, minimizing heat-related distortion and maintaining material properties. Additionally, cold spray is a highly efficient and environmentally friendly process, as it reduces waste and energy consumption compared to traditional thermal spray methods. Lithium iron phosphate (LFP) has been gaining popularity as a cathode material for lithium-ion batteries due to its stable cycling performance, lack of oxygen generation, thermal stability, and use of environmentally friendly iron over nickel or cobalt. In this study, LFP was deposited by vacuum cold spray onto aluminum foil substrate to investigate the viability of cold spray as a possible method of manufacturing all-solid-state batteries. Results were obtained through in-situ X-ray diffraction (XRD), scanning electron microscopy (SEM), and in-situ transmission electron microscopy (TEM). Electrochemical results were obtained by assembling cold spray deposited LFP into half-cells with Li-metal anode and liquid electrolyte before being tested on electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV), and C-Rate. Due to the high impact velocities during cold spray, crystalline LFP was observed to undergo changes to an amorphous phase. Using in-situ X-ray diffraction (XRD), scanning electron microscopy (SEM), and in-situ transmission electron microscopy (TEM) the crystallographic and morphological changes to cold spray deposited LFP were investigated across various temperatures. In-situ XRD initially showed a decrease in crystallinity with crystallinity being restored with elevated temperatures, with in-situ TEM further supporting these findings. Investigation with SEM showed LFP particles undergo deformation upon impact with a mix of intact particles interspersed throughout to create a dense layer. Electrochemical impedance spectroscopy (EIS) of assembled half-cells show low resistance of 90Ω with further reduction to 60Ω upon cycling of the cell. In terms of cycling performance, initial discharge capacities showed 80 mAh/g and 20mAh/g at 0.1C and 1C respectively. These capacities dropped further with repeated cycling. Future work will be focused on improving capacity, annealing post deposition to restore crystallinity, and incorporation of solid electrolyte.

Abstract The use of ultrafine powders in the micro-cold spray (MCS) process, also referred to as the aerosol deposition method, typically results in porous and/or poorly adhering films because the particles do not impact at a high enough velocity for sufficient plastic deformation and interparticle bonding to occur. Under typical operating conditions, particles < 100 nm accelerate to high velocities but then are slowed by the stagnant gas in the bow shock that forms just upstream of the substrate. Using larger particles reduces particle slowing, but large particles can cause erosion of the film at high impact velocity, decreasing deposition efficiency. In this study, a pressure relief channel nozzle using helium as a carrier gas is proposed such that high-velocity deposition of yttria-stabilized zirconia particles as small as 10 nm in diameter is possible. This is well below the size range of powders previously used for MCS. The proposed nozzle design increases impact velocities for 10, 20, and 50 nm particles by ~ 880, 560, and 160 m/s, respectively, when compared to a conventional nozzle. Experimental deposition of ultrafine 8YSZ powder shows that the pressure relief channel nozzle results in lower porosity and more uniform deposits, with a ∼ 186% increase in deposition efficiency.

Micro-cold spray is typically conducted by impacting 200-2000 nm particles at velocities of ~500 m/s. Molecular dynamics simulations have been used to study micro cold spray, but the particles that can be simulated using conventional potentials are much smaller than those used experimentally. The viewgraphs taken from molecular dynamics simulations and shown below are cross-sections of a 50 nm Ta particle with a 3 nm oxide layer impacting at 500 m/s using a machine learning enabled DeePMD potential. Using more expensive reactive interatomic potentials, the previous largest particle size for a metal/oxide impact simulation was 12 nm.

These images show the strain states for a 50 nm ceramic particle impacting a ceramic substrate at 1200 m/s as a function of time.

Micro-cold spray (MCS) is a thermal spray process that can be used to apply thick ceramic films (1-100 μm) using kinetic energy to consolidate aerosolized powders into a dense film. While this process is compatible with many substrates due to the low working temperatures, the fundamental mechanisms that enable MCS and shape the operational window are poorly understood.

Abstract The impact behavior of a single yttria (Y 2 O 3 ) nanoparticle onto a Y 2 O 3 substrate is studied as a function of particle velocity (300–1200 m/s) and diameter (12–50 nm) using molecular dynamics simulations. To analyze the results, a strain density function is developed to provide both quantitative and qualitative information about the deformation mechanisms that contribute to the final state of the system. This function provides both clear evidence of shear localization and insight into the conditions for which localization occurs during the impact of Y 2 O 3 particles as they approach experimentally relevant sizes. Further analysis shows that localization and fragmentation only occur for Y 2 O 3 impacts with diameters of at least 50 nm, while particles with diameters of 25 nm and smaller deform primarily by amorphization and viscous flow. Implications for the deposition of films of Y 2 O 3 and other rare‐earth oxides via the micro cold spray process are discussed.

Abstract Previous studies have shown that selective laser flash sintering (SLFS) can be initiated in dielectrics that exhibit ionic or electronic conduction at high temperature. These materials required high laser powers to reach the temperatures where electrical conduction is sufficient to initiate SLFS. In this study, SLFS in lanthanum chromite (LC), an intrinsic electronic conductor with high conductivity, and lanthanum strontium chromite (LSC), which is doped to further increase electronic conductivity, were investigated with a focus on understanding the initiation mechanisms. Results show that the initiation of SLFS in LC and LSC occurs when electronic charge carriers are activated and flow to the electrode where the current is measured. A combination of carriers produced at the electrode, temperature‐activated intrinsic charge carriers, and extrinsic charge carriers present in LSC due to doping are responsible for the facile initiation of SLFS.

Abstract Molecular dynamics simulations of particle impact have been conducted for a ceramic with mixed ionic-covalent bonding. For these simulations, individual zinc oxide (ZnO) nanoparticles (NPs) were impacted onto a ZnO substrate to observe the effects of impact velocity (1500–3500 m s −1 ) and particle diameter (10, 20, and 30 nm) on particle deformation and film formation mechanisms that arise during the micro-cold spray process for producing films. The study shows that a critical impact velocity range exists, generally between 1500 and 3000 m s −1 , for sticking of the NP to the substrate. Results suggest that solid-state amorphization-induced viscous flow is the primary deformation mechanism present during impact. Decreasing particle diameter and increasing impact velocity results in an increased degree of amorphization and higher local temperatures within the particle. The impact behavior of mixed ionic-covalent bonded ZnO is compared to the behavior of previously studied ionic and covalent materials.

Abstract Selective laser flash sintering (SLFS) is a variation of flash sintering where the only external heat source is a scanning laser. The scanning laser locally heats a region of the sample between two electrodes, initiating measured current flow at a threshold electric field and laser energy density. This study focuses on understanding how charge transport occurs during stage I SLFS in 8 mol% yttria stabilized zirconia. Two potential charge transport mechanisms are considered: (1) a continuous current moving along a hot line between electrodes and (2) charge transported in a discrete bundle within a hot spot, localized to the area under the laser beam. Two laser scan patterns are employed to experimentally determine how charge is transported during stage I SLFS. Numerical modeling is used to estimate the temperatures at which SLFS initiates, which allows the calculation of charge carrier densities and mobilities relevant for the onset of SLFS. Results demonstrate that both a discrete bundle of charges and continuous flow of charge carriers contribute to the current at the onset of SLFS.

Abstract not provided.