This review critically examines the current understanding of calcia–magnesia–alumina–silicate (CMAS) degradation mechanisms and mitigation approaches in thermal and environmental barrier coatings. First, the review introduces case studies of field returned engine components exposed to CMAS attack, followed by fundamental aspects of CMAS-induced degradation. Understanding CMAS adhesion, infiltration, spallation mechanics, and thermochemical attack mechanisms is crucial to designing materials approaches to mitigate CMAS attack. CMAS mitigation strategies have focused on reactive approaches aimed at crystallising molten CMAS at the earliest stage possible to inhibit infiltration. Promising approaches are presented, starting with fundamental reaction kinetics studies, followed by the effects of microstructure in actual coatings systems. Salient results on coating systems tested in various burner rigs and a full engine test are presented to benchmark the success of various mitigation strategies. Lastly, several key future research areas are presented in order to provide a roadmap towards ‘sandphobic’ thermal and environmental barrier systems.

Magnesium is a highly desired material for multiple applications including automotive, aircraft, and biomedical components due to its high strength to weight ratio, non-toxicity, and its good machinability. The main drawback of magnesium use is its poor resistance to corrosion. The goal of this experiment was to determine the effect of surface roughening treatments on the rate of corrosion and to characterize the resultant magnesium oxide (MgO) and brucite (Mg(OH)₂) corrosion products. Three samples of AZ31 alloy with varying initial surface roughness were exposed to simulated marine conditions using a salt fog chamber for 672 h. The salt fog experiments used a 3.5 wt.% NaCl solution and were conducted in accordance with the ASTM B117 test protocols. One sample was tested in an as-received state, served as the control, another sample was roughened using a grit blaster, and another was polished to a mirror finish. Formed oxide scales were characterized using scanning electron microscopy, X-ray diffraction, and energy dispersive spectroscopy. The samples with a higher roughness underwent the severest corrosion and ensuing formation of the thickest oxide layers. The results indicate that the initial surface roughness has a profound effect on the resulting severity of pitting corrosion in Mg alloys and that mechanical polishing can substantially reduce the propensity for uniform and pitting corrosion to occur.

Properties, such as morphology, particle size, and hardness affect the ability of a powder to flow and bond to a surface in additive manufacturing (AM) applications. The effects of high energy ball milling on spherical copper and stainless steel powders were evaluated. Morphology of both stainless steel and copper powders, quantifiable by aspect ratio, showed larger changes due to ball-to-powder ratio (BPR, 2:1–1:10) compared to the total milling time (2–60 min). Hardness of copper increased from 53 HV_0.01 in the as-received condition to 96 HV_0.01 after milling for 60 min with a BPR of 1:1 or 2:1. Hardness of steel increased from 336 HV_0.01 in the as-received condition to 523 HV0.01 after milling for 60 min with a BPR of 2:1. Hardness of both powders was insensitive to milling times at low BPR (1:10). At high BPR (2:1), hardness of steel increases after just 2 min of milling, while Cu changed significantly only after 60 min. Hardness was influenced more by BPR than by milling time. It is shown that a broad range of milling parameters exist where metallic powders can be processed with minimal changes to their morphology, while controlling for hardness.

Extensive research efforts have been devoted to understand the complex mechanisms of wear with the aim to minimize wear in sliding systems. Improvements in the instruments used for the characterization of the wear phenomenon are required to enhance the effectiveness of research method. In this paper, we report the design of an experimental platform that enables in-situ observation of the surface topography evolution during the evaluation of the tribological behavior of surfaces in dry and lubricated conditions. Use of state-of-the-art components for surface topography measurement, planar positioning, and force sensing allowed for the improvement of sensitivity and resolution compared with the previously reported systems. The effectiveness of the tribotester was demonstrated through friction and wear tests performed using a stainless-steel ball and a silicon wafer coated with SiO₂. It was found that transition of the wear mechanism from adhesive to abrasive wear took place when a significant amount of wear debris was formed as evidenced by the in-situ observation of removal of the coating and exposure of the Si substrate. The in-situ observation of wear phenomena enabled a robust and in-depth elucidation of wear mechanisms.