•Aluminum phosphate sealing can dramatically increase wear resistance of Al2O3 coating.•Aluminum phosphate sealing can increase the load bearing capacity of Al2O3 coating.•There are three regimes for the APS sprayed Al2O3 coating in sliding wear process.•Fatigue crack and micro-grind are two main sliding wear mechanisms for Al2O3 coating.In this investigation, aluminum phosphate was used to seal an air plasma sprayed alumina coating, the hardness, porosity, phase composition, microstructure, and the sliding wear performance of alumina coatings before and after sealing treatment were examined. The results showed that the aluminum phosphate can penetrate the interface between the alumina coating and the substrate. The microstructural features of the coatings such as pores, cracks, and gaps between splats were found to be filled with the sealant and the cross-sectional hardness of the sealed coating was increased from 962.7 ± 77.2HV0.3 to 1299.3 ± 70.2HV0.3. The critical load and sliding wear resistance of alumina coating can be greatly increased by aluminum phosphate sealing treatment.

•A digital image technique for monitoring strain in coated laminates is discussed.•The technique can identify the onset of surface cracking during uniaxial loading.•Coatings undergo brittle fracture after yielding of the steel substrates.•Different coating systems show variable strain to failures, critical for design.Overlay coatings are widely used in engineering components to impart a range of surface functionalities including thermal, wear, and corrosion protection, as well as material reclamation. In most surface engineering applications, the coating's role is restricted to the surface, with limited integration with the underlying substrate. However, the situation is changing: there is an emerging need for so-called structurally integrated coatings, where the coating and substrate are intimately bonded, resulting in a coupled system. Of interest are the emerging applications of thermal spray and cold spray overlay coatings applied on loaded engineering components such as landing gear, heavy machinery hydraulics, and steel infrastructure. These coatings, even metals or cermets, respond in a brittle manner associated with their layered processing and ultra-fine grain structures resulting from rapid quenching. As such, it is of importance to understand their coupled mechanical response, especially stress-strain behavior and strain to fracture.In this study, an approach involving strain monitoring of coated steel via digital image correlation has been developed. Both elastic response and strain beyond the yield point of the system are assessed to examine load transfer between the coating and substrate and onset of cracking. Three different coating materials (Ni, WC-CoCr and Al2O3) were deposited to near full density via high velocity thermal spray. The results point to a powerful new approach for understanding mechanical behavior of heterogeneous composites using advanced imaging techniques.Download high-res image (142KB)Download full-size image

•Damage tolerant Al2O3 templates produced via thermal spray, with post deposition epoxy infiltrated.•Presence of epoxy in the sprayed architecture shows improvements in mechanical properties.•Enhancement in the abrasion performance was observed in both the epoxy treated structures.•Epoxy infiltrated brick-&-mortar structure presented greater impact damage character compared to disordered structure.Design and fabrication of ceramic materials that simultaneously provide adequate strength and toughness is of importance in engineering applications. Nature has demonstrated it is feasible to achieve this duality in properties through a combination of materials and microstructural engineering. The nacreous layer within the abalone shell is perhaps the most notable demonstration of such combined properties through unique combination of ceramic and polymeric material in an ordered, layered assembly. Recent studies have shown that through appropriate control of thermal spray processes and post-spray polymer infiltration of the deposited ceramic, it is possible, to some extent, to harness nacre's microstructural attributes resulting in the material's dramatic improvement in both strength and toughness. This paper seeks to build upon previous work of layered, natural inspired design seen in nacreous materials, with an emphasis on abrasion and contact damages. Two distinct thermal spray ceramic templates, one nacreous analogue and one standard thermal spray template, have been reproduced here and evaluated with regards to their functional performance e.g. abrasive wear and particle impact. The results from this study confirm the mechanical property improvements after polymer infiltration, and further show that it is indeed feasible to achieve improved surface properties from natural design principles and processing innovations.

Thermal spray (TS) coatings have been extensively utilized for various surface modifications such as enhancing wear/erosion resistance and thermal protection. In the present study, a new function of TS material is explored by studying its load-carrying capability. Due to the inherent microstructures containing voids and interfaces, it has been presumed TS materials were not suitable to bear loads. However, the recent advances in TS technology to manufacture near fully dense TS coatings have expanded their potential applications. In the current experiments, TS nickel coatings are deposited onto metallic substrates, and their mechanical behaviors are closely examined. Based on the measured data, the estimated elastic modulus of TS Ni is about 130 GPa (35% less than bulk value), and the maximum tensile strength is about 500 MPa (comparable to bulk value). It was found that such a high value is attainable because the coating is deposited onto a substrate, enabling a load-transfer mechanism and preventing coating failure at a much lower stress level. Three distinct deformation stages are identified to describe this behavior. Such a clarification is critical for enabling TS process to restore structural parts as well as to additively manufacture load-bearing components.

The advent of user-friendly in-flight process diagnostic tools has significantly improved our understanding of thermal spray processes. This paper examines the critical attributes of these diagnostic measurements and the applicability of the nondimensional group parameters as a mapping strategy for data visualization. Specifically, first-order process maps (process-particle interactions) have been addressed by converting the temperature (T)-velocity (V) of particles obtained via diagnostics into nondimensional group parameters [Melting Index (MI)-Reynolds number (Re)]. This approach provides an improved description of the thermal and kinetic energy of particles and allows for cross comparison of diagnostic data within a given process for different materials, comparison of a single material across different thermal spray processes, and detailed assessment of the melting behavior through recourse to analysis of the distributions. An additional group parameter, Oxidation Index (OI), has been applied to relatively track the oxidation extent of metallic particles under different operating conditions.

Thermal spray offers a variety of sub-sets of processing approaches to produce coatings. The various processes are classified based on the thermal spray source (from low velocity combustion spray to high temperature plasma jets) and method of material injection (in the form of powder, wire or rod). However, it is this intrinsic versatility which sets-up variations in characteristics of the applied coatings. Properties of thermally sprayed coatings, including process induced residual stress, are controlled by various parameters of the spraying process. This study examines three thermal spraying techniques with significantly different particle temperatures and velocities. They are air plasma spraying (APS), twin wire-arc spraying (TWA) and high velocity oxy-fuel (HVOF) spraying. For comparison purposes the recently developed cold spray processed materials were included in the study. For each method, in-flight particle diagnostics was performed; Ni–5 wt.%Al splats and deposits were fabricated and analyzed. Porosity, elastic modulus and thermal conductivity of the deposits were evaluated and correlated to the process variables. Using indentation at different loads and analysis of the indented region, stress–strain relationships for these coatings were obtained. Surprising differences in the properties were observed and were explained based on the fundamental variations in microstructure development. Through-thickness residual stress profiles in Ni–5 wt.%Al coatings on steel substrates were determined non-destructively by neutron diffraction. The stresses range from highly tensile in the APS coating to compressive in the HVOF coating. Various stress generation mechanisms—splat quenching, peening and thermal mismatch—are discussed with respect to process parameters and material properties.

•Design and fabrication of TEGs for waste heat application.•Scalable and additive manufacturing demonstrated using thermal spray.•Multi-layer deposits composed of distinct functionality.•TEG produces 2.43 mW electrical power with a max efficiency of 0.85% (1.52 V).Traditional thermoelectric modules are acquired as separate components and then integrated by mechanical attachment into the engineering systems. There is, however, an interest and opportunity to manufacture thermoelectric device and basic electronics directly onto engineering structures. Recent studies have shown that plasma spray synthesized sub-stoichiometric titanium oxide (TiO2−x) deposits show reasonable thermoelectric figure-of-merit, are capable of operating at relatively high temperatures (∼500 °C), and can be easily and cost effectively deposited onto both planar and cylindrical substrates over large areas with the capability to produce patterned and multilayer assemblies to optimize the power harvesting. This study demonstrates the fabrication and performance of such thermoelectric generators based on n-type TiO2−x and Ni as the surrogate p-type and interconnect structures embedded within ceramic deposits. Up to 72 thermocouple modules were prepared incorporating both series and parallel connections to augment the performance resulting in a max efficiency of 0.85% for the couple and electric power of 2.43 mW at temperature of 723 K. Preliminary experiments were conducted with Li:Co3O4 as the p-type material with significant performance improvement. The methodologies described in this paper represents a potential pathway for large scale synthesis and fabrication of thermoelectric system directly in waste heat systems over large areas.