Liquid Droplet Erosion Research Articles

Fatigue‐based model for the droplet impingement erosion incubation period of metallic surfaces

AbstractDroplet impingement of metallic surfaces at high impact velocities results, after some time, in erosion of the surface due to fatigue. By extending our previously published analytical model to enable the use of experimental fatigue data (S‐N curves), here, for the first time, a wide range of experimental liquid droplet erosion incubation period test states for both ferrous (stainless steel AISI 316) and nonferrous (aluminium 6061‐T6) engineering metals have been investigated. To achieve this, the developed model includes additional surface hardening and a residual compressive stress state at the surface due to a water drop peening effect. As such, the interrelation of the physical and mechanical properties that follows from the model has been used to identify how changes in selected metal properties might enhance droplet impingement erosion incubation life. Model predictions for both metals, using fatigue data from S‐N curves from different literature sources, showed for the droplet impact velocity range of 140 to 400 m/s an excellent agreement with results from a multiregression equation as determined from an ASTM interlaboratory test program.

Surface topography parameters as a correlation factor for liquid droplet erosion test facilities

An evaluation of six different coatings was completed in a set of inter-laboratory tests, involving three rain erosion test facilities. Results from two whirling arm type test facilities and one water-jet test facility were compared in the test campaign. The materials and coatings that were investigated included clad aluminium alloy, paint coatings, polyurethane tape, plasma vapour deposited thin film and sol–gel coating. The surface topography of the test coupons was measured using a confocal laser scanning microscope. Using the primary profile of the surface, a comparison was made based on the incubation time or time to failure in the case of coatings. Correlation factors were estimated between the three test facilities. A comparison of the surface features and the surface topographical parameters lead to correlations for two different impact velocities at two of the facilities. Cross correlations between facilities and impact velocities were also shown with this method to within 20%. The incubation period was the main measure of the resistance of the coatings to rain erosion.

Cavitation erosion and jet impingement erosion mechanism of cold sprayed Ni–Al2O3 coating

Abstract A composite coating was deposited on Inconel 600 substrate by cold spray method using pure Ni powder (60 wt.%) blended with α-Al2O3 (40 wt.%) as feedstock. It is expected to be applied to repair the bellows eroded by the liquid droplet erosion (LDE). Microstructure of the coating was observed using optical microscope (OM) and scanning electron microscopy (SEM). Microhardness of the coating was determined by Vickers hardness tester. Cavitation erosion (CE) experiments were carried out in the distilled water. Jet impingement erosion (JIE) experiments were performed in slurry containing 1 wt.% quartz particle with the flow velocity of 15 m/s at impingement angles of 30°, 60° and 90°, respectively. Cumulative mass loss vs. testing time was used to evaluate the erosion rate of the coating. The erosion mechanism was analyzed by OM, SEM, X-ray diffraction (XRD) and the microhardness measurement. The results show that the composite coating has compact microstructure and relatively high hardness. The resistance to CE of the coating is not as good as that of Inconel 600 substrate due to the weak bonds of the Al2O3 particles. However, the results of the JIE test indicate that the slurry erosion resistance of the coating is better than that of Inconel 600 at the impact angles of 30° and 60°, but not at the normal impact angle.

Predicting the preferential sites to liquid droplet erosion of the bellows assemblies by CFD

Bellows assemblies are placed within extraction steam lines to absorb the differential displacement (driven primarily by thermal growth) between the turbine lower casing and the condenser shell in nuclear power plants. The thin wall thickness and service environmental conditions of the bellows predetermine their vulnerable endurance to the liquid droplet erosion (LDE). A three-dimensional computational fluid dynamics (CFD) bellows model proposed in this paper consists of a hydraulic model and a LDE model aiming to investigate the LDE on the external surface of the bellows assemblies. The result shows that the most prone site to LDE locates at the upwind side of the first bellows convolution (USFBC). The droplet diameter has significant effect on the LDE extent, rather than the LDE distribution. Moreover, the addition of the weld seam before the bellows can mitigate the LDE of the bellows downstream. In addition, the protecting cover (PC) effectively prevents the bellows from the LDE, but causes the droplet accumulation on the convolution, which possible leads to corrosive damage. The PC structure was designed optimization according to the requirements of the displacement compensation, the manufacture cost and the LDE resistance.

Three-Dimensional Modeling for Wet Compression in a Single Stage Compressor Including Liquid Particle Erosion Analysis

Gas turbine inlet fog/overspray cooling is considered as a simple and effective method to increase power output. To help understand the water mist transport in the compressor flow passage, this study conducts a 3D computational simulation of wet compression in a single rotor-stator compressor stage using the commercial code FLUENT. A sliding mesh scheme is used to simulate the stator-rotor interaction in a rotating frame. Eulerian–Lagrangian method is used to calculate the continuous phase and track the discrete (droplet) phase. Models to simulate droplet breakup and coalescence are incorporated to take into consideration the effect of local acceleration and deceleration on water droplet dynamics. Analysis on the droplet history (trajectory and size) with stochastic tracking is employed to interpret the mechanism of droplet dynamics under the influence of local turbulence, acceleration, diffusion, and body forces. A liquid-droplet erosion model is included. The sensitivity of the turbulence models on the results is conducted by employing six different turbulence models and four different time constants. The result shows that the local thermal equilibrium is not always achieved due to short residence time and high value of latent heat of water. Local pressure gradients in both the rotor and stator flow passages drive up the droplet slip velocity during compression. The erosion model predicts that the most eroded area occurs in the leading edge and one spot of the trailing edge of the rotor suction side.

Microstructure and tribological performance of nanocomposite Ti–Si–C–N coatings deposited using hexamethyldisilazane precursor

Thick nanocomposite Ti–Si–C–N coatings (20–30 μm) were deposited on Ti–6Al–4V substrate by magnetron sputtering of Ti in a gas mixture of Ar, N2, and hexamethyldisilazane (HMDSN) under various deposition conditions. Microstructure and composition of the coatings were studied using scanning electron microscopy, x-ray diffraction, and energy dispersive x-ray spectroscopy, while the mechanical and tribological properties of these coatings were studied using Rc indentation, and micro- and nanoindentations, solid particle erosion testing, and ball-on-disk wear testing. It has been observed that the Si concentration of these coatings is varied from 0% (TiN) to 15% (Ti–Si–C–N), while the structure of these coatings is similar to the nanocomposite Ti–Si–N coatings and consists of nanocrystalline B1 structured Ti(C,N) in an amorphous matrix of SiCxNy with the grain size of 5−>100 nm, depending on the coating preparation process. These coatings exhibit excellent adhesion when subjected to Rc indentation tests. The microhardness of these coatings varies from 1200 to 3400 HV25, while the nanohardness varies from 10 to 26 GPa. Both the microhardness and nanohardness are slightly lower than those of similar coatings prepared using trimethylsilane. However, the erosion test using a microsand erosion tester at both 30° and 90° incident angles shows that these coatings have very high erosion resistance and up to a few hundred times of improvement has been observed. These coatings also exhibit very high resistance to sliding wear with a low coefficient of friction of about 0.2 in dry sliding. There are a few advantages of using the HMDSN precursor to prepare the Ti–Si–C–N coatings over conventional magnetron sputtered deposition of Ti–Si–N coatings including composition uniformity, precursor handling safety, and high deposition rate. The coatings can be applied to protect gas turbine compressor blades from solid particle erosion and steam turbine blades from liquid droplet erosion, as well as other mechanical components that experience severe abrasion. These coatings may also be used in areas where both high wear resistance and low friction are required.

Comparison of Inconel 625 and Inconel 600 in resistance to cavitation erosion and jet impingement erosion

Liquid droplet erosion (LDE), which often occurs in bellows made of nickel-based alloys, threatens the security operation of the nuclear power plant. As the candidate materials of the bellows, Inconel 600 and Inconel 625 were both tested for resistance to cavitation erosion (CE) and jet impingement erosion (JIE) through vibratory cavitation equipment and a jet apparatus for erosion-corrosion. Cumulative mass loss vs. exposure time was used to evaluate the erosion rate of the two alloys. The surface and cross-sectional morphologies before and after the erosion tests were observed by scanning electron microscopy (SEM), the inclusions were analyzed by an energy dispersive spectroscopy (EDS), and the surface roughness was also measured by surface roughness tester to illustrate the evolution of erosion process. The results show that the cumulative mass loss of CE of Inconel 625 is about 1/6 that of Inconel 600 and the CE incubation period of the Inconel 625 is 4 times as long as that of the Inconel 600. The micro-morphology evolution of CE process illustrates that the twinning and hardness of the Inconel 625 plays a significant role in CE. In addition, the cumulative mass loss of JIE of Inconel 625 is about 2/3 that of Inconel 600 at impacting angle of 90°, and almost equal to that of the Inconel 600 at impacting angle of 30°. Overall, the resistance to CE and JIE of Inconel 625 is much superior to that of Inconel 600.

Nanotechnology Coatings for Erosion Protection of Turbine Components

Solid particle erosion (SPE) and liquid droplet erosion (LDE) cause severe damage to turbine components and lead to premature failures, business loss, and repair costs to power plant owners and operators. Under a program funded by the Electric Power Research Institute, TurboMet International and Southwest Research Institute (SRI) have developed hard erosion resistant nanocoatings and have conducted evaluation tests. These coatings are targeted for application in steam and gas turbines to mitigate the adverse effects of SPE and LDE on rotating blades and stationary vanes. Based on a thorough study of the available information, the most promising coatings, such as nanostructured titanium silicon carbonitride (TiSiCN), titanium nitride (TiN), and multilayered nanocoatings, were selected. State-of-the-art nanotechnology coating facilities at SwRI were used to develop the coatings. The plasma enhanced magnetron sputtering method was used to apply these coatings on various substrates. Ti–6Al–4V, 12Cr, 17-4PH, and custom 450 stainless steel substrates were selected based on the current alloys used in gas turbine compressors and steam turbine blades and vanes. Coatings with up to 30μm thickness have been deposited on small test coupons. Initial screening tests on coated coupons by solid particle erosion testing indicate that these coatings have excellent erosion resistance by a factor of 20 over the bare substrate. Properties of the coating, such as modulus, hardness, microstructural conditions including the interface, and bond strength, were determined. Tensile and high-cycle fatigue tests on coated and uncoated specimens indicate that the presence of the coatings has no negative effects but has a positive influence on the high-cycle fatigue strength at zero and high mean stresses.

Cavitation erosion behavior of nanocomposite Ti–Si–C–N and Ti/Ti–Si–C–N coatings deposited on 2Cr13 stainless steel using a Plasma Enhanced Magnetron Sputtering process

Abstract In the work presented in this paper, the liquid droplet erosion resistance of two nanocomposite Ti–Si–C–N single layer coatings (SMR1 and SMR3) and one multilayer structured nanocomposite Ti/Ti–Si–C–N coating (SMR2) on 2Cr13 stainless steel has been investigated by means of cavitation erosion. Three sets of 2Cr13 samples were deposited using a Plasma Enhanced Magnetron Sputtering (PEMS) process using different parameters to 35 µm, 68 µm and 59 µm thick. The cavitation erosion resistance of the coatings was evaluated with a vibratory cavitation apparatus in distilled water and in a 3 wt.% NaCl solution; the latter was used to simulate an aggressive environment. The surface and cross-sectional morphologies of the samples were studied using scanning electron microscopy (SEM) before and after the cavitation erosion tests. The results from the investigation indicate that the three coatings exhibited excellent cavitation erosion resistance in both media compared with the uncoated substrate. The cavitation erosion resistance of the thicker single layer coating SMR3 and the multilayer coating SMR2 was especially pronounced, while SMR1 was not so satisfactory because microcracks extended to the substrate due to its small thickness. The SMR2 coating effectively prevented the propagation of the microcracks in a manner of layer-by-layered spallation, owing to its multilayer structure. Even though the coatings improved the erosion resistance of the substrate, the SEM study revealed that there were many micro-droplets and V-shaped defects in the coatings, which formed the nucleation sites for the coating spallation. Therefore, if the size and the number density of the micro-droplets can be reduced and the coatings can be made more homogeneous, without V-shaped defects, the cavitation erosion resistance of coated 2Cr13 steel should be further improved.

Erosion Resistance of Thick Nitride and Carbonitride Coatings Deposited using Plasma Enhanced Magnetron Sputtering

In this paper, we report the microstructural and erosion test results of thick coatings deposited using a PEMS technique. Nitrides (TiN, CrN, and ZrN) and nanocomposite carbonitride (TiSiCN) were deposited on Ti-6Al-4V disk coupon and turbine blade samples. The samples were analyzed using SEM with energy dispersive EDS, nanoindentation, and XRD. Selected samples were subjected to alumina erosion tests separately using two incident angles (30° and 90°, respectively). The nanohardness of the nitride coatings was 30 GPa for carbonitride. It was observed that at 30° TiSiCN renders the best erosion resistance. At 90° incidence, the erosion damage becomes more severe. However, one TiSiCN sample still showed an order of magnitude improved erosion resistance over the commercial TiN while delamination occurred on other samples. The delamination is believed to be the result of high internal stress of the TiSiCN coatings. A multilayered TiSiCN/Ti coating is proposed to maintain the high erosion resistance while reducing the internal stress. It is believed that this technology, once fully developed, may be applied to protect turbine engine blades and vanes from both solid particle erosion (SPE) and liquid droplet erosion (LDE) as well as piston rings of heavy-duty diesel engines from wear.

Increased liquid droplet erosion resistance of steam turbine blades

AbstractAlloys used for the blades of steam turbines usually do not show a satisfactory resistance to erosion and require surface protection with suitable coatings. Starting from the results obtained with Stellite 6 coatings on AISI 420 stainless steel blades, this research has been developed with the aim to increase erosion resistance by modifying the chemical composition of the coatings. Several samples have been coated by laser cladding and exposed to liquid erosion tests. In particular, concentrations of carbide forming and solid solution strengthening elements have been varied in order to establish their effects against liquid droplet erosion. Results of the tests and EDX analyses on the samples are reported, with a discussion on the various effects on erosion resistance. Two directions have appeared as the most promising, either increasing the concentration of carbide forming elements, or increasing the concentration of nickel in the coatings.