High Velocity Oxy-Fuel (HVOF) as a materials test method for simulating aerothermal effects

High Velocity Oxygen-Fuel (HVOF) is a member of the thermal spray process family in which a mixture of oxygen, fuel, and compressed air are continuously combusted and the resulting flame is accelerated through a nozzle to achieve high gas velocities. HVOF is most commonly used to heat (to the point of melting) and accelerate powder particles towards a substrate to apply coatings. Here, HVOF is presented as a method for simulating high-speed aerothermodynamic effects. With gas velocities in the low Mach regime and gas temperatures exceeding 2000°C in an oxidative environment, HVOF is a promising low- cost test method for screening high temperature materials and components. Three examples of experiments based on using HVOF as a test method are presented, including low thermal conductivity thermal barrier coatings, thermal protection system materials for solid rock motor exhaust environments, and evaluation of high temperature ceramics and glass-ceramic composites

Synthesis and corrosion behavior of cold sprayed dual nanoparticle T reinforced Al coatings

The article of record as published may be found at https://doi.org/10.1016/j.surfcoat.2020.126280This study investigates the synergistic and individual effects of nano boron carbide (nB4C) and boron nitride nanoplatelets (BNNP) on the corrosion behavior of cold spray aluminum matrix composite (MMC) coatings. Cryomilling and cold spraying were used to synthesize four novel Al MMC coatings with reinforcements consisting of either 2 vol.% nB4C, 2 vol.% BNNP, or dual nanoparticle reinforcement consisting of 1 vol.% nB4C plus 1 vol.% BNNP. Cold sprayed coatings were evaluated using microhardness testing which revealed an 11.7% increase in hardness in the dual nanoparticle reinforced composite coating. Coatings were subjected to 500 h and 2000 h corrosion tests in a salt fog chamber consisting of 3.5% NaCl to simulate an aggressive marine environment. Coatings were evaluated post-salt fog chamber by mass and thickness change measurements, optical microscopy, scanning electron microscopy, and energy dispersive spectroscopy. Following 500 h corrosion testing, each composite coating experienced mild to severe pitting throughout a significant portion of the coating. Following 2000 h exposure, in addition to severe pitting, all coatings experienced galvanic corrosion at the coating/substrate interface.AN acknowledges support from the Naval Postgraduate School thru the Research Initiation Program (RIP) and from the NPS Foundation thru a SEED Phase II grant.AN acknowledges support from the Naval Postgraduate School thru the Research Initiation Program (RIP) and from the NPS Foundation thru a SEED Phase II grant

Evaluation of the Fatigue Behaviour and Failure Mechanisms of 52100 Steel Coated with WIP-C1 (Ni/CrC) by Cold Spray

Cold spray technique has been major improved in the last decades, for studying new properties for metals and alloys of aluminum, copper, nickel, and titanium, as well as steels, stainless steel and other types of alloys. Cold sprayed Ni/CrC coatings have the potential to provide a barrier as well as improved protection to steels. Fatigue characteristics of 52100 steel coated with Ni/Chrome-Carbide (Ni/CrC) powder mixture by using cold gas dynamic spray are investigated. Fatigue samples were subjected to symmetrically alternating, axially applied cyclic fatigue loading until failure. The test was stopped if a sample survived more than 5 × 106 cycles at the applied stress. Fracture surfaces for each sample were examined to investigate the behaviour of the coating both at high stress levels and at a high number of stress cycles. Scanning electron microscopy was used to assess the damage in the interface of the two materials. Good fatigue behaviour of the coating material was observed, especially at low stresses and a high number of cycles. Details of the crack initiation region, the stable crack propagation region and the sudden crack expansion region are identified for each sample. In most of the samples, the initiation of the crack occurred on the surface of the base material and propagated into the coating material. The possible effects of coatings on the initial deterioration of the base material and the reduction of the lifespan of the coated system were also investigated. The aim of the paper was to study the interface between the base material and the coating material at the fatigue analysis for different stresses, highlighting the appearance of cracks and the number of breaking cycles required for each sample

Evaluation of the Fatigue Behavior and Failure Mechanisms of 4340 Steel Coated with WIP-C1 (Ni/CrC) by Cold Spray

Fatigue behavior of standardized 4340 steel samples uniformly coated with WIP-C1 (Ni/CrC) by cold spray was investigated. In particular, when a crack appeared at the interface between the base material and the coating, the cause of it as well as its shape and size were investigated. Fatigue loading was applied by alternating symmetrical cycles. Scanning electron microscopy was used to study the onset of failure and the subsequent propagation of cracks. The interface between the two materials performed well—in all samples, the initial crack propagation occurred on the surface of the base material, continuing into the coating material and in the interior of the base material. The fatigue durability curve of stress vs. number of cycles (S-N) presented a conventional form for a metallic alloy and the coating material had an influence only on the damage on the surface of the base material