Comparison of experiments and computations for cold gas spraying through a mask. Part 2

This paper presents experimental and simulation results of cold spray coating deposition using the mask placed above the plane substrate at different distances. Velocities of aluminum (mean size ~ 30 μm) and copper (mean size ~ 60 μm) particles in the vicinity of the mask are determined. It was found that particle velocities have angular distribution in flow with a representative standard deviation of 1.5–2 degrees. Modeling of coating formation behind the mask with account for this distribution was developed. The results of model agree with experimental data confirming the importance of particle angular distribution for coating deposition process in the masked area.     

冷动力喷涂法制备钨和钨合金涂层 及其涂层的计算模拟

采用冷动力喷涂法以纯钨和钨-镍-铁合金为原料在铜合金基体上制备了钨涂层和钨-镍-铁涂层. 研究了冷喷涂过程中钨粉粒径、喷涂距离等因素对涂层性能的影响. 用扫描电子显微镜分析了涂层的表面、断面微观结构, 并用原子力显微镜测量了涂层的粗糙度. 此外, 计算了冷喷涂过程中粉末颗粒的实际速度, 并采用有限元分析软件ANSYS/LS-DYNA模拟了冷喷涂过程中颗粒撞击基体时的变形情况.     

Contributions of suppression and excitation to simultaneous masking: effects of signal frequency and masker-signal frequency relation

This study investigated the contributions of suppression and excitation to simultaneous masking for a range of masker frequencies both below and above three different signal frequencies (750, 2000, and 4850 Hz). A two-stage experiment was employed. In stage I, the level of each off-frequency simultaneous masker necessary to mask a signal at 10 or 30 dB sensation level was determined. In stage II, three different forward-masking conditions were tested: (1) an on-frequency condition, in which the signals in stage I were used to mask probes of the same frequency; (2) an off-frequency condition, in which the off-frequency maskers (at the levels determined in stage I) were used to mask the probes; and (3) a combined condition, in which the on- and off-frequency maskers were combined to mask the probes. If the off-frequency maskers simultaneously masked the signal via spread of excitation in stage I, then the off-frequency and combined maskers should produce considerable forward masking in stage II. If, on the other hand, they masked via suppression, they should produce little or no forward masking. The contribution of suppression was found to increase with increasing signal frequency; it was absent at 750 Hz, but dominant at 4850 Hz. These results have implications for excitation pattern analyses and are consistent with stronger nonlinear processing at high rather than at low frequencies.     

Nanostructured TiN coating prepared by reactive plasma spraying in atmosphere

In the present study, the nanostructured TiN coatings are obtained by means of reactive plasma spraying in the air. The XRD analysis shows the coating was mainly composed of TiN (max. 86.3%) and a small quantity of Ti₃O. Their microstructures were observed by SEM and TEM. The TEM pictures show the TiN coatings were composed of the nanoscaled grains (range from 70 to 90 nm), and the Scherrer equation analysis on the average grain size corresponded with that result. The effects on the average grain size also were studied by changing the spraying power and the spraying distance.     

Study of hafnium-oxide coatings by means of nuclear backscattering spectrometry

Single-layer coatings made of hafnium oxide and double-layer structures with an intermediate nickel layer, the total thickness of which is 70 μm, are obtained via plasma spraying with a supersonic jet in a rarefied atmosphere. A nozzle extension capable of implementing a Prandtl–Meyer expansion fan is used to generate nanostructured coatings. The coatings are investigated via the nuclear backscattering of spectrometry 7.6 MeV protons, scanning electron microscopy, X-ray microanalysis, and X-ray diffraction. The studies of the surface and transverse microsection of the coatings indicate that they comprise not only deformed particles of the sprayed powder with sizes of greater than 20 μm but also layers and conglomerates of nanoparticles with sizes of 30–60 nm. Depth profiling of the elemental composition performed by means of the nuclear backscattering spectrometry of protons demonstrates that transition layers exist at the interface between the substrate and coating layers characterizing the average size of the coating microparticles. A comparison of thicknesses defined by two methods allows estimation of the overall porosity of the hafnium-oxide layer. In accordance with X-ray diffraction data, the cubic and monoclinic phases of hafnium oxide with a high fraction of the amorphous component are formed in the coating.     

The effect of auditory feedback on phonation threshold pressure measurement

The effect of auditory feedback on phonation threshold pressure (Pth) measurement was investigated in 14 females with normal, untrained voices. Two measurement systems (Glottal Enterprises MS 100--circumferentially vented mask and Kay Elemetrics Aerophone II--non-circumferentially vented mask) were examined under three conditions: (1) masked, (2) no mask, and (3) masked with enhanced auditory feedback-acoustic signal placed at ears through headphones. Masked with enhanced auditory feedback, in addition to subject training, significantly lowered Pth values regardless of mask design. The amount of auditory feedback provided by different mask designs was investigated and revealed a significant difference. Clinical significance of different auditory feedback levels provided by the two mask designs was investigated. Direct comparison of the mean values between systems was not possible because of each system's design and calibration. Comparisons were accomplished by subtracting means of select-paired conditions (masked/no mask; masked/masked plus masked with enhanced auditory feedback) within each system and then comparing these difference scores from the same paired conditions between each system. No clinical significance in difference scores was revealed because of varying amounts of auditory feedback provided by the masks. Results support the use of enhanced auditory feedback, in addition to subject training, when measuring Pth.     

Deposition characteristics of Al-12Si alloy coating fabricated by cold spraying with relatively large powder particles

In this paper, the microstructure, microhardness and adhesive strength of Al-12Si coating produced by cold spraying were investigated. It is found that a thick, dense and well bonded Al-12Si coating could be produced by cold spraying with a relatively large powder through the control of spray conditions. The critical velocity for large Al-12Si particles was lower than that of small Al-12Si particles. The as-deposited Al-12Si coating had the same crystal structure as Al-12Si powder. The localized interface melting occurred resulting from both the adiabatic shearing upon impact and the thermal effect of hot gas. Some fine Si particles precipitated in α-Al matrix because of the thermal effect of hot gas during coating deposition. The dispersed Si particles in Al-12Si coating improved the coating microhardness.     

Measurement of particle velocity and characterization of deposition in aluminum alloy kinetic spraying process

Particle velocity is a very important parameter in kinetic spraying (or cold gas dynamic spraying). It is difficult to measure the velocity of a particle with supersonic speed at low temperature (lower than 500 °C). Thus, in many investigations only estimated values are used for evaluating coating processes. In this paper, the modeling of particle acceleration was reviewed, and the measurement of in-flight particle velocity in a kinetic spraying process was performed. Particle velocity and flux distributions from different process gas temperatures and pressures were investigated. The influences of process gas temperature and pressure on particle velocity were discussed. Characteristic of Al-Si feedstock deposition onto a mild steel substrate was described by comparing coatings structures with the in-flight particle conditions. The deposition behavior showed two critical particle velocities for Al-Si powder deposition onto a substrate and for particle-particle bonding.     

Effect of plasma spraying regimes on structure and properties of Ni<Subscript>3</Subscript>Al coatings

The structure and tribological properties of coatings made of PN85YU15 powder were studied. The coatings were deposited on the mild steel blanks by the technology of air-plasma spraying using a unit of annular input and gas-dynamic powder focusing. Efficiency of heating and acceleration of powder particles was studied preliminarily. Measurement results on temperature and velocity distributions of particles at a certain spraying distance by the method of spectral pyrometry and time-of-flight method are presented. The effect of plasmatorch arc current and amount of propane-butane in the plasma flow on the structure and properties of coatings is analyzed in this paper. It is determined that the phase composition of coatings and initial powder is the same: the main phase is Ni₃Al compound; moreover, the structure contains Ni₅Al₃ phase. It is shown that an increase in the amount of propane-butane increases coatings porosity. The densest coatings (5.77%) were obtained at the plasmatorch arc current of 200 A with the reduced amount of propane-butane. The coatings obtained at the minimal arc current of 100 A with an increased amount of propane-butane are characterized by maximal porosity (20.38%). The results of tribological testing of the coatings under the conditions of sliding friction with a lubricant by the disc-plane scheme are presented. From the standpoint of obtaining the densest coatings with high performance, the optimal regimes of plasma spraying of PN85YU15 powder are the current from 140 A to 200 and using the air and propane-butane mixture only as the shielding gas (anode curtain).     

Critical thicknesses of electrostatic powder coatings from inside

A simple electrostatic model is applied to the charge powder coating of a grounded conductor eventually covered by insulating layers. The electric field inside the powder coating and its evolution during the process are established with also the corresponding evolution in the dielectric layer and some practical consequences are also discussed. The thickness of the charged powder layer is limited by two types of process: a self-limiting process related to the vanishing field in the air gap and a field strength process occurring on one of the two sides of the coating–dielectric interface. Inside the powder coating, the electric field induces an increased electrostatic pressure on the powder grains at the substrate–coating interface and a vanishing pressure on the grains at the coating–air interface. This internal field is amplified into air bubbles and it may be responsible for the back ionisation process and of the formation of moon craters via the ionisation of the air molecules followed by the pressure exerted by the ions pushed in the direction of the free surface. For each of these limits, analytical expressions are established and they permit to identify the role of physical properties of the deposited powder (particle size and dielectric constant) as well as the role of thickness, structure, and dielectric constant of the insulating substrate. The present approach explains the difficulty in obtaining thick coatings on thick insulating substrates or thick coatings from the use of too fine powder grains. Finally, different behaviours as a function of the size of the deposited powder grains are deduced from the contribution of the electric field to the velocity of the particles sticking to the surface. Then and for the first time, the present contribution underlines the important role of the subsurface composition and the need to characterize the structure, dimension and dielectric constant of this subsurface for various applications concerning electrostatic spraying and powder painting of plastics and of insulating coatings on metallic work pieces.     

Effects of pre-placed coating thickness on thermal fatigue resistance of cast iron with biomimetic non-smooth surface treated by laser alloying

In order to enhance the thermal fatigue resistance of gray cast iron with biomimetic non-smooth surface further, studies on laser alloying of Cr powder with different pre-placed coating thickness were performed to change both the composition and the microstructure of non-smooth unit. Additionally, the optimization of coating thickness was done based on the content of Cr in alloyed zone and the thermal fatigue behaviors of non-smooth samples. The results indicated that there was a critical coating thickness which corresponded to the increase of Cr content in alloyed zone under a definitive laser processing conditions, and the critical thickness was 0.3 mm in this paper. Any coating thicker than 0.3 mm would lead to the waste of alloying powder. The thermal fatigue resistance of non-smooth samples was better than that of smooth sample. In addition among all the non-smooth samples, the sample which was treated by the laser alloying of Cr had superior resistance to thermal fatigue compared with laser melting treated samples. And the thermal fatigue resistance increased with increasing of Cr content in alloyed zone which was caused by pre-placed coating thickening.     

火焰喷涂重熔Ni基WC复合涂层的耐磨性能试验研究

利用火焰喷涂重熔方法制备了Ni基WC复合涂层，并进行了耐磨性能试验研究.通过扫描电子显微镜观察涂层磨损后的表面形貌，分析了WC体积分数、颗粒分布均匀性、包裹粉颗粒尺寸对涂层耐磨性的影响.结果表明:涂层硬度和耐磨性随着WC体积分数增加而提高，当WC体积分数过高时，降低了涂层致密性，其硬度和耐磨性反而有所下降；聚乙二醇(PEG)400+PEG2000+无水乙醇混粉方式的WC颗粒分布最均匀，降低了涂层磨损量；加入相同体积分数的亚微米级WC所制备的涂层耐磨性较微米级WC所制备的涂层耐磨性好. 关键词： 火焰喷涂 显微组织 颗粒尺寸 耐磨性能     

Analysis and optimization of gas-thermal spray process in terms of condensed phase velocity and temperature

The paper presents the experimental results for velocity and tempertaure disibutions of the condensed phase in a plasma jet from the plasmatorch PNK-50 (design from ITAM SB RAS, Novosibirsk). The plasma jet is used at different operational modes for thermal spraying of nickel alloy powder PR-NKh16SR3. The measurements for average velocity (230–280 m/s) and temperature (2290–2410 K) of sprayed particles were matched to data on microhardness (630–710 HV) and porosity (1.7–13.5 %) of samples. Results were transferred into coating properties maps plotted in coordinates “arc current vs. torch offset” and “particle velocity vs. particle temperature”. Experiments demonstrated the change in parameters of condensed phase in the jet after performimg of maintenance job for the plasmatorch. We propose the method for adjusting the operational parameters of thermal spraying equipment using optical methods of control for particle velocity and particle temperature. The exemplary apparatus function of the plasmatorch was plotted; an approach is proposed for optimization and transfer of spraying technology of coatings with specified properties between equipment from different manufacturers, different class and power.     

The effects of powder properties on in-flight particle velocity and deposition process during low pressure cold spray process

In cold spray process, impacting velocity and critical velocity of particles dominate the deposition process and coating properties for given materials. The impacting velocity and critical velocity of particles depend on the powder properties and cold spray conditions. In the present study, the in-flight particle velocity of copper powder in low pressure cold spraying was measured using an imaging technique. The effects of particle size and particle morphology on in-flight particle velocity and deposition efficiency were investigated. The critical velocity of copper powder was estimated by combining the in-flight particle velocity and deposition efficiency. The effect of annealing of feedstock powder on deposition and critical velocity was also investigated. The results showed that the irregular shape particle presents higher in-flight velocity than the spherical shape particle under the same condition. For irregular shape particles, the in-flight velocity decreased from 390 to 282 m/s as the particle size increases from 20 to 60 μm. Critical velocities of about 425 m/s and more than 550 m/s were estimated for the feedstock copper powder with spherical and irregular shape morphology, respectively. For the irregular shape particles, the critical velocity decreased from more than 550 to 460 m/s after preheating at 390 °C for 1 h. It was also found that the larger size powder presents a lower critical velocity in this study.     

Influences of different powders on the characteristics of particle charging and deposition in powder coating processes

This experimental study investigated the influences of two different powder systems (coarse and ultrafine) on particle charging and deposition characteristics during electrostatic powder coating processes. Results disclosed that, despite their differences in particle sizes, the two powders behave similarly in deposition process, commonly resulting in a cone-shaped deposited pattern in the inner portion of the substrate and an increase of deposited particles in the fringe region. However, their different properties lead to the discrepancies in their deposition efficiencies, which account for a higher efficiency with the coarse powder. The study further revealed that the coarse powder is superior to the ultrafine powder in charging in-flight particles, which directly contributes to its higher deposition efficiencies. Furthermore, it was disclosed that the two powders exhibit distinct characteristics in charging deposited particles. Compared to the coarse powder, the ultrafine powder is more uniform in charging deposited particles, mainly owing to its greater particle number and higher specific surface area but less mass. In particular, the charging efficiency of overall deposited particles decreases for the ultrafine powder but increases for the coarse powder with increased charging voltage, closely related to their particle properties. However, both powders decrease in charging efficiency of deposited particles with extended spraying duration due to back corona intensifying with spraying.     

Kinetics of the irreversible propagation of a thermal instability in the presence of a nonuniform temperature distribution over the cross section of a superconducting composite

The conditions for the irreversible propagation of a normal zone along a composite superconductor are investigated within the model of a continuous medium with consideration of its transverse thermal conductivity under the assumption of a uniform distribution of the current over the cross section of the wire. The numerical experiments performed for a currentcarrying element of circular cross section with variation of the cooling rate and the transverse dimensions are compared with known results of the one-dimensional theory. It is shown that the one-dimensional theory, as opposed to the two-dimensional theory, leads to underestimated values of the velocity of a thermal instability. The size effect modifies the propagation conditions of a normal zone to the greatest extent as the heat-transfer coefficient increases. This law is based on an increase in temperature at the center of the wire with a simultaneous decrease on its surface, as a result of which the mean temperature over the cross section of the composite increases as its radius increases. Zh. Tekh. Fiz. 68, 53–57 (January 1998)     

The effect of powder preparation method on the corrosion and mechanical properties of TiN-based coatings by reactive plasma spraying

TiN-based composite coatings with and without the addition of Cr were deposited by reactive plasma spraying (RPS) in air. Both sintered and mixed powder of Ti and B₄C were used for the RPS process. A thermodynamic model was firstly used to estimate the complicated phase composition of composite coatings prepared by RPS. The phase composition, structures and properties of TiN-based coatings were investigated using XRD, SEM and a Vickers microhardness tester. The results show that the phases in TiN-based coatings do not generate according to priority of Gibbs free energy value due to non-equilibrium reactive course during thermal spraying. The coating deposited using sintered Ti and B₄C powder is composed of two main phases (TiN and TiN_0.3), two minor phases (Ti₂O₃ and TiB₂), and a small fraction of TiC phase. The composition of the coating deposited using the mixed powder with Cr added is predominantly in the TiN and TiB₂ phases, a smaller phase fraction of Ti₂O₃ and TiO₂, and some unreacted Cr. The Vickers microhardness of the coating deposited using sintered powder is higher than that of using mixed powder. The composite coating deposited using mixed powder with the addition of Cr shows superior corrosion resistant to that using sintered powder when tested in 3.5 wt.% NaCl electrolytic solution.     

Transverse superresolution and focal shift with rotational tunable phase mask

This paper reports the imaging characteristics of an optical system can be modified by our designed polarization pupil mask. The novel rotational symmetric polarization pupil mask design based on combination of half-wave and quarter-wave plates is introduced for realizing the focal shift and extending focal depth of an optical system and the procedure for designing is presented. Numerical results show when an appropriate rotational symmetric polarization pupil mask is used as an apodizer in the optical imaging system, it not only can effectively achieve the continuously focal shift in a small range and extend focal depth of the optimized system, but also can evidently increase the transverse resolution of the optimized system at the genuine focal plane.     

Formation of a high hydrophilic/hydrophobic contrast surface on PET substrates by ECR generated sulfur hexafluoride plasma

High hydrophilic/hydrophobic contrast surfaces on polyethylene terephthalate (PET) substrates were formed by shadow mask technique in electron cyclotron resonance generated sulfur hexafluoride plasma atmosphere. The X-ray photoelectron spectroscopy (XPS) analyses indicate that the unmasked PET surfaces contained a high proportion of the CF₂-CF₂ groups, and therefore were hydrophobic with large water contact angle. However, the surface wettability was found to increase drastically on the masked PET surfaces. This could be resulted from a mass of COF (acid fluoride) compounds observed by XPS on the masked film surfaces. The COF compounds could react with atmospheric moisture to form -COOH groups, which in turn increased the surface wettability. In addition, the surface wetting property of the masked areas was found to change significantly with the plasma treatment time, the mask-to-substrate distance and the storage time after the treatment. The best contract in water contact angle obtained from the treated PET samples was larger than 100° after 168 h of storage.     

On the structure of the combustion wave of nitroglycerine-based propellants

The parameters of the temperature distribution in the combustion wave of nitroglycerin-based propellants N and NB are analyzed and compared. The aim of the study is to explain the known experimental fact that the size of hotspots and the critical quenching diameter for propellant NB (more rapidly burning) are larger than those for propellant N. It is demonstrated that, at a given burning rate, the burning surface temperature, heat conduction zone thickness, temperature gradient near the burning surface, and the dark zone temperature for propellants N and NB are the same, but the fizz zone thickness for NB is approximately twice as wide as that for N. The dependence of the ratio of the hotspot size to the fizz zone thickness is described by a single power law for both propellants. It is also shown that the hotspot size can be defined as the distance between two consecutive transverse waves, which, in turn, is determined by the delay in the initiation of each following wave.