High-velocity air fuel coatings for steel for erosion-resistant applications

High-velocity air fuel (HVAF) coating processes have advantages over conventional high-velocity oxygen fuel (HVOF) processes, resulting in coatings with superior properties. The present review first provides a concise overview of HVAF coatings, highlighting their advantages over HVOF coatings. Then, the fundamentals of solid particle, slurry, and cavitation erosion are briefly introduced. Finally, the performance of HVAF coatings for erosion-resistant applications is discussed in detail. The emerging research consistently reports HVAF-coatings having higher erosion resistance than HVOF-coatings, which is attributed to their elevated hardness and density and improved microstructural features that inhibit the surface damages caused by erosion. The dominant wear mechanisms are mainly functions of particle impact angle. For instance, the removal of the binder phase at high impact angles causes the accumulation of plastic strain on hard particles (e.g., WC particles) in the matrix, forming micro-cracks between the hard particles and the matrix, eventually decreasing the erosion resistance of HVAF coatings. The binder phase of HVAF-coatings significantly affects erosion resistance, primarily due to their inherent mechanical properties and bearing capacity of hard particles. Optimizing spraying parameters to tailor the microstructural characteristics of these coatings appears to be the key to enhancing their erosion resistance. The relationship between microstructural features and erosion mechanisms needs to be clarified to process coatings with tailored microstructural features for erosion-resistant applications

The Effects of Shot Peening parameters on the subsurface properties of AA7075 T6 aluminum alloy

Shot peening (SP) is an important mechanical surface treatment widely used to improve the fatigue life of mechanical components operating under dynamic loads which have been specifically used in automotive and aerospace industries. The enhancement of the fatigue life is achieved by preventing the crack formation and slowing crack propagation by improving the mechanical properties and the microstructure of subsurface via SP. In this study, it is aimed to investigate the variation of the subsurface microstructure and the subsurface mechanical properties of AA7075 T6 aluminum alloy shot-peened under different parameters. The subsurface hardness and microstructural variations were investigated in relation to depth depending on the shot size and the peening pressure. SP was conducted under specially designed SP system by stainless steel shots of 0.1-0.3 mm and 0.4-0.9 mm diameters under 2 and 4 bar peening pressures. Subsurface hardness values improved significantly by increasing pressure and shot size, and the depth of the hardness increased regions was increased. The depth of the microstructural region affected by SP also increased for both shot sizes. The formation of deep and large pits and surface microcracks were determined on the surface due to the excessive plastic deformation that occurs with increases in pressure and shot size

SURFACE, SUBSURFACE AND TRIBOLOGICAL PROPERTIES OF TI6AL4V ALLOY SHOT PEENED UNDER DIFFERENT PARAMETERS

Ti6Al4V alloy was shot peened by using stainless-steel shots with different sizes (0.09–0.14 mm (S10) and 0.7–1.0 mm (S60)) for two durations (5 and 15 min) using a custom-designed peening system. The shot size was the main parameter modifying the roughness (0.74 µm for S10 vs. 2.27 µm for S60), whereas a higher peening time slightly increased roughness. Hardness improved up to approximately 35% by peening with large shots, while peening time was insignificant in hardness improvement. However, longer peening duration with large shots led to an unwanted formation of micro-cracks and delamination on the peened surfaces. After dry sliding wear tests, the mass loss of peened samples (S60 for 15 min) was 25% higher than that of un-peened samples, while the coefficient of friction decreased by 12%. Plastically deformed regions and micro-scratches were observed on the worn surfaces, which corresponds to mostly adhesive and abrasive wear mechanisms. The present study sheds light on how surface, subsurface and tribological properties of Ti6Al4V vary with shot peening and peening parameters, which paves the way for the understanding of the mechanical, surface, and tribological behavior of shot peened Ti6Al4V used in both aerospace and biomedical applications

Modification of Surface and Subsurface Properties of AA1050 Alloy by Shot Peening

AA1050 Al alloy samples were shot-peened using stainless-steel shots at shot peening (SP) pressures of 0.1 and 0.5 MPa and surface cover rates of 100% and 1000% using a custom-designed SP system. The hardness of shot-peened samples was around twice that of unpeened samples. Hardness increased with peening pressure, whereas the higher cover rate did not lead to hardness improvement. Micro-crack formation and embedment of shots occurred by SP, while average surface roughness increased up to 9 µm at the higher peening pressure and cover rate, indicating surface deterioration. The areal coverage of the embedded shots ranged from 1% to 5% depending on the peening parameters, and the number and the mean size of the embedded shots increased at the higher SP pressure and cover rate. As evidenced and discussed through the surface and cross-sectional SEM images, the main deformation mechanisms during SP were schematically described as crater formation, folding, micro-crack formation, and material removal. Overall, shot-peened samples demonstrated improved mechanical properties, whereas sample surface integrity only deteriorated notably during SP at the higher pressure, suggesting that selecting optimal peening parameters is key to the safe use of SP. The implemented methodology can be used to modify similar soft alloys within confined compromises in surface features

Solid particle erosion behavior of thermal barrier coatings produced by atmospheric plasma spray technique

Thermal barrier coatings (TBCs) are commonly applied specifically for aerospace applications in which they are subjected to air-borne particles. Therefore, solid particle erosion behavior of all coating layer has been an important phenomenon and erosion behavior of various TBCs has been widely investigated in literature. In the present study, CoNiCrAlY and yttria stabilized zirconia (ZrO2 + 8\% Y2O3) powders were deposited on Inconel 718 nickel based super alloy substrate. Atmospheric plasma spraying technique was applied for the deposition of the metallic bond coat and the ceramic top coats. Erosion tests were carried out under various particle impingement angles with an air jet erosion tester. Afterwards, eroded surfaces of the specimens were investigated with a three-dimensional (3D) optical surface profilometer (noncontact) and scanning electron microscope. The erosion rates, the areal surface roughness values, the 3D surface topographies, and the surface morphology of the specimens were evaluated based on the particle impingement angle to understand the solid particle erosion behavior of the produced coatings. The maximum erosion rates occurred at 60 degrees impingement angle which is an indication of semi-ductile/semi-brittle erosion behavior. Furthermore, the surface roughness values and surface topographies also dramatically varied depending on the impingement angle. Deeper and wider erosion craters formed at 60 degrees impact angle and the erosion craters were visualized by profilometer analysis

3D Imaging of Indentation Damage in Bone

Bone is a complex material comprising high stiffness, but brittle, crystalline bio-apatite combined with compliant, but tough, collagen fibres. It can accommodate significant deformation, and the bone microstructure inhibits crack propagation such that micro-cracks can be quickly repaired. Catastrophic failure (bone fracture) is a major cause of morbidity, particularly in aging populations, either through a succession of small fractures or because a traumatic event is sufficiently large to overcome the individual crack blunting/shielding mechanisms. Indentation methods provide a convenient way of characterising the mechanical properties of bone. It is important to be able to visualise the interactions between the bone microstructure and the damage events in three dimensions (3D) to better understand the nature of the damage processes that occur in bone and the relevance of indentation tests in evaluating bone resilience and strength. For the first time, time-lapse laboratory X-ray computed tomography (CT) has been used to establish a time-evolving picture of bone deformation/plasticity and cracking. The sites of both crack initiation and termination as well as the interconnectivity of cracks and pores have been visualised and identified in 2D and 3D