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C/C–HfC–SiC composites were prepared by reactive melt infiltration using Hf–Si alloy (Hf, more than 95 wt%) with a density of 3.86 g/cm3 and an open porosity of 8.34%. The microstructure, mechanical properties, and ablation behavior at high temperatures were studied in detail. SiC played a crucial role in alleviating the thermal mismatch between HfC and PyC, which formed at the interface between the carbon matrix and the HfC matrix. The flexural strength and modulus of C/C–HfC–SiC composites were 237 MPa and 37.6 GPa, respectively. The C/C–HfC–SiC composites exhibited excellent ablation resistance with a linear ablation rate of 8.9 × 10−3 mm/s and maintained a surface temperature above 2925°C during ablation. During this process, HfO₂ remained in a molten state with high viscosity and served as a thermal barrier, while the volatilization of SiO₂ effectively removed heat, protecting the composites from further ablation. [ABSTRACT FROM AUTHOR]

We studied the behavioral characteristics of a newly developed dual-layer ablator, which uses carbon-phenolic as a recession layer and silica-phenolic as an insulating layer. The ablator specimens were tested in a 0.4 MW supersonic arc-jet plasma wind tunnel, employing two different shapes (flat-faced and hemispherical-faced) and varying thicknesses of the carbon-phenolic recession layer. The specimens underwent two test conditions, namely, stationary tests (7.5 MW/m2, ~40 s) and transient tests simulating an interplanetary spacecraft re-entry heat flux trajectory (6.25↔9.4 MW/m2, ~108 s). During the stationary tests, stagnation point temperatures of the specimens were measured. Additionally, internal temperatures of the specimens were measured at three locations for both stationary and transient tests: inside the carbon-phenolic recession layer, inside the silica-phenolic insulating layer, and at the recession layer–insulating layer intersection. The hemispherical-faced specimen surface temperatures were about 3000 K, which is about 350 K higher than those of flat-faced specimens, resulting in higher internal temperatures. The recession layer internal temperatures rose more exponentially when moved closer to the specimen stagnation point. Layer interaction and insulating layer internal temperatures were found to be dependent on both the recession layer thickness and the exposed surface shape. The change in exposed surface shape increased mass loss and recession, with hemispherical-faced specimens showing ~1.4-fold higher values than the flat-faced specimens. [ABSTRACT FROM AUTHOR]

We developed and tested two carbon–phenolic-based ablators for future Korean spacecraft heat shield applications. The ablators are developed with two layers: an outer recession layer, fabricated from carbon–phenolic material, and an inner insulating layer, fabricated either from cork or silica–phenolic material. The ablator specimens were tested in a 0.4 MW supersonic arc-jet plasma wind tunnel at heat flux conditions ranging from 6.25 MW/m2 to 9.4 MW/m2, with either specimen being stationary or transient. Stationary tests were conducted for 50 s each as a preliminary investigation, and the transient tests were conducted for ~110 s each to stimulate a spacecraft's atmospheric re-entry heat flux trajectory. During the tests, each specimen's internal temperatures were measured at three locations: 25 mm, 35 mm, and 45 mm from the specimen stagnation point. During the stationary tests, a two-color pyrometer was used to measure specimen stagnation-point temperatures. During the preliminary stationary tests, the silica–phenolic-insulated specimen's reaction was normal compared to the cork-insulated specimen; hence, only the silica–phenolic-insulated specimens were further subjected to the transient tests. During the transient tests, the silica–phenolic-insulated specimens were stable, and the internal temperatures were lower than 450 K (~180 °C), achieving the main objective of this study. [ABSTRACT FROM AUTHOR]

Ceramic matrix composite is widely applied in thermal protection system (TPS) of space vehicles to resist ultrahigh temperature aerodynamic heating. In this work, a novel carbon fiber reinforced hafnium carbide matrix composite (C f /HfC) is fabricated, and its potential serving as TPS material is studied inside an inductively coupled plasma wind tunnel. The oxidation mechanism of the as-fabricated C f /HfC is thus revealed in the dissociated air flows with different heat fluxes, based on the temperature profiles and the microstructural evolutions of HfO 2 oxide scale. The results suggest a transition in the oxidation activity of C f /HfC at 1650–1700 °C in reduced pressure (<10 kPa), from reaction-control to diffusion-control, due to the accumulation of porous HfO 2 oxide scale. When its thickness beyond a critical value, inner diffusions of oxidized species dominate the oxidation activity of C f /HfC. The thicker the HfO 2 is, the weaker the oxidation of C f /HfC can be in dissociated air plasmas. The oxidation mechanism endows C f /HfC with an excellent thermal protection property at ultrahigh temperatures (≥2000 °C). However, when the oxidation temperature ≥2700 °C, due to strong melt and evaporation, the continuity of HfO 2 is interrupted, which results in degradation of the oxidation blockage ability. As a result, C f /HfC is strongly ablated in the dissociated air plasmas. This infers that C f /HfC can be applied in TPS by preventing strong melt and dissipation of HfO 2 , and it is suggested to control the surface temperature of C f /HfC lower than 2700 °C in reduced pressures. This work may forward the advance of the novel material in the TPS of future space vehicles. [ABSTRACT FROM AUTHOR]

Since March 2014, all ESA satellites and launcher upper stages which will be disposed of by atmospheric re-entry at the end of their operational life must demonstrate that the risk from fragments surviving the re-entry and causing casualties on ground is less than 1 in 10,000. This casualty risk is calculated by re-entry tools simulating the uncontrolled re-entry event using a computer aided design model of the spacecraft. The uncertainties on several parameters such as the aerothermodynamics fluxes model, the structural interfaces model, the materials model, and the level of detail of the spacecraft architecture will have an impact on the re-entry event simulation and the associated casualty risk calculations. To better understand the uncertainties associated to material modelling, five materials often used on space missions were tested in Plasma Wind Tunnels, mimicking atmospheric re-entry environment. Thermo-physical properties, thermo-optical properties and mechanical properties at high temperature were also characterized. Analysis of the samples after plasma wind tunnel tests was performed. A database compiling the materials properties measured and the plasma wind tunnel test results was created. The material properties characterised and generated during the activities will serve as inputs for the re-entry simulation events at equipment and system level. [ABSTRACT FROM AUTHOR]

Abstract This work employed an inductively coupled plasma wind tunnel to study the dynamic oxidation mechanisms of carbon fiber reinforced SiC matrix composite (Cf/SiC) in high-enthalpy and high-speed plasmas. The results highlighted a transition of passive/active oxidations of SiC at 800–1600 °C and 1–5 kPa. Specially, the active oxidation led to the corrosion of the SiC coating and interruption of the SiO2 growth. The transition borders of active/passive oxidations were thus defined with respect to oxidation temperature and partial pressure of atomic O in the high-enthalpy and high-speed plasmas. In the transition and passive domains, the SiC dissipation was negligible. By multiple dynamic oxidations of Cf/SiC in the domains, the SiO2 thickness was not monotonously increased due to the competing mechanisms of passive oxidation of SiC and dissipation of SiO2. In addition, the mechanical properties of the SiC coating/matrix and the Cf/SiC were maintained after long-term dynamic oxidations, which suggested an excellent thermal stability of Cf/SiC serving in thermal protection systems (TPSs) of reusable hypersonic vehicles.

The emissivity of typical SiC-based thermal protection materials was measured in-situ at a wide temperature range (800 ~ 2300°C) inside a plasma wind tunnel that was capable of simulating hypersonic environments on-ground. Based on it, the evolution mechanism dominated by dynamic oxidation was discussed. The results suggest an emissivity of Cf/SiC 0.84 ~ 0.88 at 858 ~ 1502°C, prior to “temperature jump”. If “temperature jump” emerged, the emissivity was decreased rapidly to ≈0.76. The emissivity drop was explained by the microstructural transition of the oxidized surfaces that were triggered by the dissipation of SiO2 oxide scale at 1600 ~ 1900°C. Similar emissivity evolution was observed in SiCf/SiC after “temperature jump”. The effect of temperature on the emissivity of ZrB2-SiC was more pronounced. It was increased from ≈0.73 to ≈0.98 at 1009 ~ 1297°C, and was plateaued at 1298 ~ 1497°C, ≈0.98. This was a consequence of the formation of higher percentage SiO2-rich layers. However, due to the dissipation of SiO2 and B2O3, the emissivity of ZrB2-SiC was declined at higher temperatures, from ≈0.98 (≈1497°C) to ≈0.85 (≈1768°C).

Abstract Carbon fiber reinforced silicon carbide-hafnium carbide (C/SiC-HfC) composite was prepared by precursor infiltration and pyrolysis process. Then, ablation behavior of C/SiC-HfC was evaluated in plasma wind tunnel. It was found that oxide layer formed during ablation significantly influenced the surface temperature. Formation of dense HfO2-SiO2 layer under low heat flux led to stable surface temperature. Silica (SiO2) on the surface was gradually consumed when heat flux increased, resulting in conversion of HfO2-SiO2 on the surface to HfO2. Converted HfO2 with high catalytic coefficient absorbed more energy, causing gradual increase in the surface temperature. Formed oxide layer was destroyed at high heat flux and high stagnation point pressure. After carbon fiber lost the protection of HfO2-SiO2 layer, it burned immediately, leading to surface temperature jump.

The response of C/SiC composites prepared via precursor impregnation and pyrolysis was investigated in a 1 MW plasma wind tunnel. Under a considerable aero heating of up to 26.2 MJ/kg of specific total enthalpy, the samples were exposed to heat fluxes exceeding 5.7 MW/m2 and low pressures of 4.5–6.6 kPa. The samples were able to withstand low heat fluxes and low stagnation pressures, and their carbon-rich nature improved the thermal conductivity, presenting a low steadystate surface temperature. However, a spontaneous jump in the surface temperature at around 1700 °C was observed at high heat fluxes and high stagnation pressures. The jump temperature was lower compared with that reported in previous studies, and was found to increase rapidly to temperatures above 2000 °C. This low-temperature jump phenomenon was associated with the evolution of microstructure during testing, and the underlying mechanism was revealed through the use of thermodynamics analysis. [ABSTRACT FROM AUTHOR]

• The static and dynamic oxidation of SiC were investigated in air and in plasma wind tunnels, respectively. • The activation energies for static oxidation were approximately ten times higher than the value of dynamic oxidation. • The observed Si-O-C transition layer located at the SiO 2 /SiC interface after both static and dynamic oxidation. SiC has extensive applications in high-temperature oxidation environments. However, few studies have investigated the differences between the static and dynamic oxidation behaviour. In this study, the static and dynamic oxidation of SiC were investigated in air and in plasma wind tunnels, respectively. The results demonstrated that the activation energy of static oxidation was ∼68.02 kJ/mol at 1300–1600 ℃, which was approximately ten times that of dynamic oxidation ∼7.05 kJ/mol at 1290–1534 ℃. The observed Si-O-C transition layer located at the SiO 2 /SiC interface, and its thickness after dynamic oxidation for 300 s was thicker than that after static oxidation for 30 h. In dynamic oxidation, high-speed flowing atomic oxygen reacted directly with SiC, whereas molecular oxygen needed extra energy to break the O O bond and react with SiC in static oxidation. Atomic oxygen also migrated easier in the amorphous SiO 2 coating, contributing to a thicker Si-O-C layer and lower activation energy. [ABSTRACT FROM AUTHOR]

• Active and passive oxidations of SiC fiber/matrix occurred at 900∼1600 °C in high-enthalpy plasmas and reduced pressure. • Fiber/matrix interphase is prone to oxidize in high-enthalpy plasmas, leading to penetration of oxidized species. • Ablation was triggered at ≈1600 °C (≈4.5 kPa) in high-enthalpy plasmas due to abrupt changes in surface microstructure. A plasma wind tunnel was utilized to explore the dynamic oxidation mechanism of SiC fiber reinforced SiC matrix composite in high-enthalpy plasmas. The results suggest the occurrences of active and passive oxidations on SiC fiber/matrix with atomic oxygen at 900∼1600 °C and 1∼6 kPa. Increasing plasma pressure could retard the active oxidation and promote the passive oxidation. Severe corrosion of SiC fibers due to active oxidation is highlighted. The as-formed SiO 2 layers cannot fully seal the open porosities and the interfacial gaps formed by oxidation of pyrocarbon interphases. This led to penetration of oxidized species and failure of fiber bundles directly exposed to heat flows. In addition, the spatial heat flux and temperature distributions were not homogeneous on the oxidized surface, which triggered early 'temperature jump' at ≈1600 °C (≈4.5 kPa) and severe localized ablation. If sealing the composite surface with SiC coating, the ablation resistance was significantly improved. [ABSTRACT FROM AUTHOR]

It is significant to compare the ablative and compressive mechanical behavior of different PICA-like materials in the engineering applications. The plasma wind tunnel ablation tests with high-entropy air and CO2 atmospheres, and compressive experiments in the ambient and 150°C, were conducted for three kinds of PICA-like materials (CF/PR-Si, CBCF/PR-SiOC and NQF/PR-Si composites). The traditional carbon/phenolic (C/PR) braided composites were also used for comparison. PICA-like materials have the better thermal insulation than traditional C/PR composite, especially for CBCF/PR-SiOC composite. The ablation behavior of CF/PR-Si and CBCF/PR-SiOC PICA-like materials in the CO2 atmosphere can produce a large amount of SiO2 in the form of coatings, oxide layers and skeletons on the ablated surface, which are greatly different from that in the air atmosphere. The compressive behavior of PICA-like material is greatly depended on the fiber fabrics, and exhibits the large discrete characteristics. The longer fiber in the PICA-like materials plays the role in maintaining the material integrity, while it may increase the thermal conductivity. [ABSTRACT FROM AUTHOR]

The ablation behaviors of an as-annealed aluminized Ir (IrAl/Ir) coating sample and two aluminized Ir (IrAl 2.7(3) /IrAl/Ir) coating samples were investigated in a plasma wind tunnel for up to 2000s. The IrAl/Ir and IrAl 2.7(3) /IrAl/Ir coating samples showed quite different thermal responses, with ablation resistance limits of ~3.35 MW/m2 and ~3.75 MW/m2, respectively, due to their different oxidation mechanisms. The failure mechanisms of both Ir Al intermetallic coating samples were closely related to the melting of Al 2 O 3 coating due to over-temperature. The results indicated that the Ir Al is a promising non-ablative thermal protection coating material used in non-equilibrium oxidizing environment up to 2000 °C. Unlabelled Image • The ablation resistance of Ir Al coating was investigated in a plasma wind tunnel. • Ir Al intermetallic coating is a non-ablative coating for up to 2000 °C. • The oxidation mechanism of Ir Al coating is determined by the contents of Al. • The failure of Ir Al coating is resulted from the melting of Al 2 O 3 coating. [ABSTRACT FROM AUTHOR]

Abstract This article deals with the aerothermodynamic analyses of the hypersonic diffuser of GHIBLI Plasma Wind Tunnel at CIRA (Centro Italiano Ricerche Aerospaziali). This diffuser presented an unexpected behavior not ensuring nozzle under-expanded flow in test chamber, even in several free jet conditions. This experimental evidence was investigated following two approaches. First, a literature review pointed out that the design rules, generally recommended in supersonic diffusers design, were not observed in the case of GHIBLI diffuser. Second, a simplified numerical model was introduced and validated with experimental results, performing Computational Fluid Dynamics analyses of GHIBLI diffuser. Finally, in order to enhance the pressure recovery and to obtain the desired under-expanded conditions, two new candidate designs compliant with the literature recommended values have been proposed and analyzed via CFD. [ABSTRACT FROM AUTHOR]

Plasma-based CO 2 conversion is a promising pathway towards greenhouse gas recycling. In the corresponding research field, various types of plasma reactors are applied for carbon dioxide dissociation. So far, spatial inhomogeneities of the specific energy (SEI) distribution in plasma generators, e.g., induced by non-uniform heating or an inhomogeneous mass distribution, are not the focus of the investigations. In this work, the spatial inhomogeneity of mass-specific enthalpy in the plasma jet of the inductive plasma generator IPG4 at the Institute of Space Systems (IRS) is examined. For this purpose, the mean mass-specific enthalpy as well as the radial distribution of the local enthalpy are measured using plasma probes. Moreover, the influence of the determined specific enthalpy inhomogeneity on the CO 2 splitting performance is quantified. It is shown that an inhomogeneous radial distribution of the specific energy can significantly lower the carbon dioxide conversion, compared to a homogeneous case. With regards to IPG4, the performance reduction is 16%. • Measurement of mean and local mass-specific enthalpy in a high-power CO 2 plasma. • Determination of mass flux distribution in a plasma jet. • Spatial inhomogeneity of specific energy input lowers CO 2 splitting performance in plasma. [ABSTRACT FROM AUTHOR]

Abstract The ablation and surface heating behaviors of iridium and Ir-Al coatings on graphite substrates under non-equilibrium conditions were investigated in a plasma wind tunnel. The results showed that the iridium coating had a much higher thermal response than the Ir-Al coating in non-equilibrium conditions. The iridium coating melted at a heat flux of 2.03 MW/m2 with the surface temperature up to 2494 °C, while the surface temperature of the Ir-Al coating was only 1330 °C at this condition. When the heat flux rose to 3 MW/m2, the surface temperature of the Ir-Al coating stayed at ~1520 °C. After continuous ablation at this condition for 500 s, the Ir-Al coating remained intact. It can be concluded that the Al alloying modification could largely improve the ablation resistance of an iridium coating. The present research proposed a novel idea for ultra-high temperature coating for hypersonic applications. Highlights • Pure iridium coating had a heat response of over 2440°C at a heat flux of 3MW/m2 in plasma wind tunnel • Ir-Al coating could withstand a heat flux of more than 3 MW/m2 in plasma wind tunnel • Surface temperature of Ir-Al was about 1000°C lower than iridium coating at the same condition [ABSTRACT FROM AUTHOR]

Silicon nitride ceramics were prepared by means of a hot-press sintering method. The ablation behavior was studied by plasma wind tunnel tests. The data suggested that, under low heat flux and stagnation pressure conditions, the ablation process was controlled by the atomic oxidation of Si3N4, leading to the elimination of Si3N4. By contrast, the erosion was mainly produced by the decomposition of Si3N4 under high heat flux and stagnation pressure conditions. Under these conditions, a fraction of Si phase, formed upon decomposition of Si3N4, was volatilized. The remained Si had melted due to high temperatures and scoured away from stagnation point area, generating mushroom-shaped samples. [ABSTRACT FROM AUTHOR]

Novel hybrid thermal protection material (TPM), consisting of TaSi 2 -based coating on carbon-bonded fiber (CBCF) composites and mullite fiber insulation tiles, is investigated using a plasma wind tunnel. Mullite fiber insulation tile is ceramically bonded to coated-CBCF composites. Results show that surface temperatures range from 400 °C°C to 1200 °C°C at low heat flux and dynamic pressure. However, at high heat flux and dynamic pressure, the surface temperature rapidly reaches 1400 °C°C, and the damage to the TaSi 2 -based coating is also observed. The cold-wall temperature of the hybrid structure remains at room temperature during ablation. In addition, TPM is invalid because of the consumption of the TaSi 2 -based coating. [ABSTRACT FROM AUTHOR]

Analysis of plasma flows at hypersonic velocity over blunt bodies is quite complex and challenging as it involves complex flow physics and carries several uncertainties. Simultaneous simulation of all the parameters as existing in re-entry flight puts constraints on most of the ground based experiments. Numerical simulations, on the other hand, require modelling of ionisation and real gas effects and prove to be computationally costly. This paper highlights the development of unstructured, cell centred second order accurate parallel version of in-house computational fluid dynamics (CFD) solver where high temperature equivalent properties used from Hansen's 7 species model and establishment of a simplified procedure for estimation of heat flux over wedge models tested in Plasma Wind Tunnel facility, Vikram Sarabhai Space Centre. Numerical simulations were carried out for Plasma tunnel initially to get the flow properties inside the tunnel when operated without any model. A simplified CFD based approach is established for computing the heat flux over the bodies tested inside the tunnel and compared with the measured data. The comparison of numerical and measured values shows that the proposed methodology captures the flow physics and various parameters with acceptable levels of accuracy. [ABSTRACT FROM AUTHOR]

Thermal plasma wind tunnels with power of 0.4 MW and 2.4 MW have been constructed at Chonbuk National University (CBNU) in Korea. This facility is capable of producing a heat flux greater than 10 MW/m2, a level that is relevant for testing thermal protection materials that are used for re-entry vehicles in space transportation. A segmented arc plasma torch was adopted as a plasma source; this was designed to have high thermal efficiency and long life, and to produce a supersonic plasma flow with enthalpy greater than 10 MJ/kg. We investigated the characteristics of the supersonic plasma flow using intrusive and non-intrusive diagnostic systems. Ablation characteristics of potential thermal protection materials such as carbon/carbon composites and graphite were investigated with the plasma wind tunnel. Cracks and pores in the materials accelerated the erosion. For carbon/carbon composites, the pores grew and the cracks which occurred at the interfaces between the carbon fibres and the matrix propagated, while for the graphite, the erosion started at the pores and peeled off the surface. [ABSTRACT FROM PUBLISHER]

Aim of this work is to analyze the response of an ultra-high temperature ceramic at typical heat flux conditions of thermal protection systems of a re-entry spacecraft. In particular, a ZrB 2 –SiC based ultra-high temperature advanced ceramic sharp leading edge demonstrator (1 mm nominal radius of curvature) was manufactured and tested in a non-equilibrium high enthalpy supersonic airflow, 20 MJ/kg of peak total enthalpy, by using an arc-jet ground facility. The surface temperature of the leading edge was monitored by infrared thermo-cameras coupled to a two-color pyrometer. The ultra-refractory advanced ceramic leading edge withstood stressful thermo-chemical loads successfully, without obvious failure. Ad-hoc computational fluid dynamics simulations rebuilt the adopted set-up and related experiment conditions: the numerical outputs matched fairly well the experimental in-situ determinations. [ABSTRACT FROM AUTHOR]

An arc-heated plasma wind tunnel with power of 0.4 MW has been constructed at Chonbuk National University (CBNU) in Korea. A segmented arc plasma torch was adopted as the plasma source and was designed to produce a supersonic plasma flow with enthalpy greater than 270 kJ/mol at a velocity of Mach 3. The characteristics of the plasma flow were investigated for gas flow rates ranging from 5 × 10 5 to 8 × 10 5 sccm and input current ranging from 200 to 350 A. The facility is capable of producing ion flux greater than 10 24 m −2 s −1 , as well as heat flux greater than 10 MW/m 2 , levels that are relevant for the study of plasma material interactions under fusion reactor conditions. [ABSTRACT FROM AUTHOR]

The operation of plasma wind tunnels requires an excessive pressure ratio owing to the excess total pressure loss in the hypersonic environment. This loss increases when a large blockage model is mounted on the test section. Therefore, designing a diffuser that can alleviate this total pressure loss by compressing the hypersonic flow efficiently is essential. However, the flow inside a plasma wind tunnel diffuser exhibits a low Reynolds number and a high Mach number, rendering the prediction of the flow and performance of a diffuser difficult. Furthermore, limited literature exists on the design of plasma wind tunnel diffusers. In the study, we propose an efficient plasma wind tunnel diffuser capable of testing despite a large blockage model. We carried out flow analysis and a parametric study on the shape of the diffuser and numerically investigated the influence of a large blockage model. Additionally, a performance analysis was conducted to estimate the pressure ratio required for the operation of the wind tunnel. Throughout the study, the newly proposed parameter, mass flow rate weighted average of the total pressure, was identified as the primary observation value because it was found helpful for improving diffuser efficiency in the design stage. Furthermore, several design considerations are recommended for the enhancement of plasma wind tunnel diffusers. The inflow Mach number and total pressure at the nozzle exit were 7 and 21.9 bar, respectively, corresponding to conditions of MW class large plasma wind tunnels. With limited related studies in current literature, we expect this study to serve as a reference when designing a plasma wind tunnel diffuser. [ABSTRACT FROM AUTHOR]

• Short duration heat flux gauge calibrations can be extrapolated for use at longer timescales. • Surface junction thermocouples, with appropriate insulation, can be used in plasma facilities. • Fast-response heat flux gauges can measure a heat flux profile in a single plasma traverse. A fast response heat transfer gauge has been developed to measure the radial distribution and the fluctuations of heat flux in a supersonic arcjet plasma wind tunnel. The heat flux gauge consists of 3.2 mm diameter coaxial surface junction thermocouple which was positioned behind a thin layer of PTFE and mounted in a ESA standard 50 mm diameter flat faced copper probe head. A non-dimensional impulse response for the heat flux gauge was identified using pulsed optical power deposition from a laser. The impulse response was used in combination with reference measurements from a calorimeter at a single location to determine the distribution and fluctuations in heat flux. Eight traverses of the PWK4 jet at the University of Stuttgart confirmed a symmetric Gaussian-like heat flux distribution with a centreline heat flux consistent to within ± 6 %. These distributions were measured in less than one second, representing a significant gain in productivity when compared to calorimeter-based heat flux measurements where probes must be held stationary at a number of locations in order to resolve spatial distributions. At this particular PWK4 operating condition, which had a flow stagnation enthalpy of approximately 15 MJkg − 1 , heat flux fluctuations of up to ± 140 kWm-2 (corresponding to a relative variation of 15 % of the centreline heat flux) were identified near the vicinity of the nozzle centreline for frequencies from 4 Hz to 1 kHz. [ABSTRACT FROM AUTHOR]

A novel Ir-Hf diffusion coating was prepared on iridium through modified pack cementation to improve the non-equilibrium ablation property while maintaining the oxygen diffusion resistance. The Ir-Hf coating consisted of Ir 3 Hf, IrHf and IrHf 2 sublayers with a total thickness of ∼20 μm. During ablation testing in a plasma wind tunnel, the Ir-Hf coating remained intact after 60 s ablation at a heat flux of 4.23 MW/m2 when the surface temperature reached 2320 °C. The coating also passed a 2000 s ablation test at a heat flux of 4.03 MW/m2 without damage. Through comparison, the Ir-Hf coating has a higher maximum tolerable heat flux than pure iridium, Ir-Al coating and C/SiC-HfC composite, showing a promising potential for ultra-high temperature thermal protection applications. Image 1 • Ir-Hf coating was prepared through modified pack cementation method. • The coating could withstand a heat flux of 4.03 MW/m2 for 2000 s in the plasma wind tunnel. • Ir-Hf coating improved the ablation property of pure iridium and Ir-Al coating. • The emissivity of Ir-Hf during ablation has been measured. [ABSTRACT FROM AUTHOR]