Ceramic Matrix Composite Materials Research Articles

Investigation of the film cooling performance of ceramic matrix composites plate with different braided structures

The ceramic matrix composite (CMC) materials have been gradually applied in the high temperature components, due to its excellent heat resistance and mechanical performance. The CMC component still need cooling to protect its safe operation in the high temperature environment of aero engine, such as film cooling structure. This study focuses on the influence of braided structure on the film cooling effect over a three-dimensional braided CMC plate. The full-size calculation model reflecting the internal mesoscale structure of CMC plate was established. The thermal properties of fibers and matrix are also introduced in mesoscale. The geometric parameter which is braided angle of braided structure is changed in different models, to analyze its impacts on the comprehensive film cooling effect. The results show that the fiber bundles inside the CMC plate are the main heat transfer channel, due to its relatively higher thermal conductivity. The different braided angles affect the anisotropic thermal conductivity of CMC on the three main directions. There exists an optimal braided angle near 40° that maximizes the heat dissipation effect in the region near the film cooling holes. While in the downstream region away from holes, the influence of braided angle is weak.

Ceramic matrix composite based on silicon carbide and nanostructured nitrogen-doped carbon for supercapacitor electrodes

The results of studies on the production of a porous ceramic-matrix composite material C–N/SiC from silicon carbide and nitrogen-doped nanostructured carbon for subsequent use as supercapacitor electrodes are presented. The material is formed by pressing silicon carbide micropowder (1 µm) and impregnating with a solution of carbamide (nitrogen source) in phenol-formaldehyde varnish (carbon source), curing and pyrolysis in a nitrogen atmosphere. The maximum concentration of carbamide was obtained in the solution (16 wt.%) at 50 ºС with a viscosity of 134.3 mPa⋅s. Thermogravimetric analysis in nitrogen of the cured solution revealed multistage decomposition with a residual mass of C–N of 48 % at 1000 ºС. Studies of the elemental composition showed a nitrogen content of 1.4 wt.% in C–N/SiC composite (up to 7 % of C–N active mass). In the composite structure, the C–N carbon-nitrogen layer (up to 12 wt.%) distributed inside the matrix pores and covering the SiC grains is X-ray amorphous has a complex nanoscale relief with an average pore size of 1.0–1.5 nm. According to electrochemical studies, the specific capacitance of the C–N/SiC material and the C–N active layer is 16.84 and 153.2 F/g respectively, and the equivalent resistance of the test supercapacitor cell with C–N/SiC electrodes is 0.567 Ohm for samples with maximum doping. The electrodes operate according to the sorption-desorption mechanism of charge accumulation and release, which is typical for a classic supercapacitor based on a double electric layer without the presence of redox reactions on the electrodes. The influence of technological regimes of pyrolysis on the electrophysical parameters of the cell is revealed: lower values of the pyrolysis temperature and nitrogen pressure in the chamber lead to an increase of the material specific capacitance and reduction of the cell equivalent resistance. The obtained results demonstrate the possibility of utilizing C–N/SiC material for the manufacture of supercapacitor electrodes.

Density Functional Theory Insight into Chemical Vapor Infiltration

Chemical Vapor Infiltration (CVI) has proven remarkably successful in producing strong and lightweight ceramic matrix composite materials. This technology has matured to regular industrial use. However, two fundamental problems remain, and those are the formation of pores and depositing of weaker material than silicon carbide (SiC), namely, Si. Definitive knowledge of the molecular mechanism would catalyze an advance in the chemical precursors used in CVI. In this work, the CVI reaction is modeled using density functional theory (DFT) calculations. The DFT calculations here use the Bayesian Error Estimation Functional with van der Waals correction (BEEF-vdW). The main findings begin with C deposition determining the rate of solid SiC growth due to Si being far more reactive. Therefore, increasing the C content of the precursor is a logical CVI strategy. Methane (CH4) is more reactive than ethane (C2H6) and ethylene (C2H2) and would be effective as an additive to the chemical precursor. Increasing the deposition rate of C has the benefit of decreasing pure Si deposits. Si melts at 1410 °C and CMCs are used in high-temperature settings beyond this melting point, including in aeroengines and nuclear fuel cladding.

Process parameter analysis of ultrasonic vibration-assisted polishing of SiCp/Al ceramics based on optimized Hilbert trajectories

As an important ceramic matrix composite material, SiCp/Al ceramics have found extensive applications across several domains. However, the intricate nature of their material properties has resulted in a scarcity of scholarly investigations pertaining to the topic of ultrasonic vibration-assisted polishing (UVAP) of SiCp/Al ceramics. To solve this problem, a theoretical derivation of the material removal model (MRM) for UVAP SiCp/Al ceramics is presented in this paper. The selection of UVAP feeding speed is analyzed and discussed. The process parameters of UVAP of SiCp/Al ceramics are optimized based on the improved Hilbert's trajectory characteristics. According to the experimental results, high-quality machined surfaces can be obtained and the “fish scale” phenomenon can be avoided when the feeding speed is 0.015 m/s. Compared with the traditional trajectory, the improved Hilbert trajectory is easier to achieve uniform surface polishing. The introduction of ultrasonic amplitude (UA) can increase the material removal rate (MRR) by more than 21.6 % compared with the conventional polishing method. The maximum removal depth (MRD) increased by more than 27.76 % and the surface roughness (SR) is reduced by more than 76.45 %. The research results have an important application value for realizing efficient and high-quality polishing of SiCp/Al ceramics.

3D printed crack-free SiOC(Fe) structures with pyrolysis-induced carbon nanowires for enhanced wave absorption performance

High-performance SiOC(Fe) wave-absorbing ceramics, containing a large number of carbon nanowires, were successfully prepared using a combination of photopolymerization 3D printing technology and the polymer-derived ceramic pyrolysis method. By employing an optimized segmented slow heating scheme with extended holding time, the pyrolysis of SiOC(Fe) ceramics at 1000 °C facilitated the growth of carbon nanowires, Fe3C and SiO2 grains. These carbon nanowires were interlaced and interconnected within the samples, forming abundant conductive networks. This highly conducive network efficiently converted electromagnetic energy into thermal energy, effectively dissipating electromagnetic waves, and consequently enhancing the microwave absorption performance of ceramics. Moreover, this approach not only reduced ceramic cracks but also improved the dielectric loss performance of the materials, achieving a minimum reflectivity value of −35.72 dB. The SiOC(Fe) ceramics added with 5 wt% VcFe effectively enhanced the magnetic loss of the material, reduced the difference between the relative complex permeability (μr) and the relative complex dielectric constant (εr), and improved the impedance matching between the material surface and air, thereby further improving its microwave absorption performance. This resulted in an increase in the maximum effective absorption bandwidth of the material to 12.7 GHz at 5 mm. This study offers a promising solution for the preparation of ceramic matrix composite materials incorporating carbon nanowires, magnetic particles and ceramic precursors, which would be potentially valuable for radar detection and sensor applications.

Ceramic Matrix Composites for Aero Engine Applications—A Review

Ceramic matrix materials have attracted great attention from researchers and industry due to their material properties. When used in engineering systems, and especially in aero-engine applications, they can result in reduced weight, higher temperature capability, and/or reduced cooling needs, each of which increases efficiency. This is where high-temperature ceramics have made considerable progress, and ceramic matrix composites (CMCs) are in the foreground. CMCs are classified into non-oxide and oxide-based ones. Both families have material types that have a high potential for use in high-temperature propulsion applications. The oxide materials discussed will focus on alumina and aluminosilicate/mullite base material families, whereas for non-oxides, carbon, silicon carbide, titanium carbide, and tungsten carbide CMC material families will be discussed and analyzed. Typical oxide-based ones are composed of an oxide fiber and oxide matrix (Ox-Ox). Some of the most common oxide subcategories are alumina, beryllia, ceria, and zirconia ceramics. On the other hand, the largest number of non-oxides are technical ceramics that are classified as inorganic, non-metallic materials. The most well-known non-oxide subcategories are carbides, borides, nitrides, and silicides. These matrix composites are used, for example, in combustion liners of gas turbine engines and exhaust nozzles. Until now, a thorough study on the available oxide and non-oxide-based CMCs for such applications has not been presented. This paper will focus on assessing a literature survey of the available oxide and non-oxide ceramic matrix composite materials in terms of mechanical and thermal properties, as well as the classification and fabrication methods of those CMCs. The available manufacturing and fabrication processes are reviewed and compared. Finally, the paper presents a research and development roadmap for increasing the maturity of these materials allowing for the wider adoption of aero-engine applications.

Woven ceramic matrix composite surrogate model based on physics-informed recurrent neural network

• Recurrent neural network-based surrogate model for nonlinear, non-monotonic CMC constitutive response presented. • Physics-informed constraints employed through regularization to enforce requisite physics. • Surrogate model accurately represents non-monotonic, included reversed loading, behavior of multiple textile CMC material systems. • Efficiency over multiscale numerical approach improved several orders of magnitude. A recurrent neural network (RNN) based surrogate model is developed to emulate the nonlinear constitutive behavior of woven ceramic matrix composites (CMCs) driven by matrix damage at multiple length scales. Physics-informed constraints are introduced into the surrogate model through regularization to ground the prediction in physics and improve its predictive capabilities. Training data is generated using the multiscale generalized method of cells (MSGMC) approach coupled with a matrix damage model. This coupling permits simulating the nonlinear behavior of woven CMCs based on constituent response at the micro-, meso-, and macroscales. The multiscale repeating unit cell is loaded under non-monotonic conditions including multiple load / unload cycles and tension / compression. The fiber volume fraction as well as the intra- and intertow void volume fractions are also varied in the generation of training data. Therefore, the RNN-based surrogate model is tasked with predicting, as a function of variable input strain sequence and fiber and void volume fractions, the resulting stress versus strain response while satisfying physical constraints such as positive semi-definiteness of the tangent stiffness matrix and linear elastic unloading. The trained surrogate model effectively matches the stress versus strain response and successfully predicts the tangent modulus throughout the loading regime. Neural network based surrogate models can offer efficient alternatives to running computationally intensive multiscale material models to simulate the nonlinear response of large structural models. Therefore the presented work provides evidence towards the feasibility of developing, training, and running such models for CMCs with complex architectures, nonlinear multiaxial material response, and under non-monotonic loading conditions.

Investigations on an All-Oxide Ceramic Composites Based on Al2O3 Fibres and Alumina–Zirconia Matrix for Application in Liquid Rocket Engines

High performance ceramics, particularly Ceramic Matrix Composite (CMC) materials found their way into liquid rocket engines. Yet, so far, mainly carbide or nonoxide CMCs have been of interest. This paper explores the potential and challenges of oxide–oxide ceramic matrix composites (OCMCs) for application in rocket thrust chambers. Therefore, strength, leakage and hot gas tests are conducted with material samples. A particular focus lies on the application of coatings to seal the permeability inherent to the material. Furthermore, prototypes in the form of flame tubes, ceramic chambers with nozzles and ceramic chambers with graphite inlays are developed and investigated experimentally in test firings. The results show that a recrystallised glass of a Y-Al-Si-O compound can successfully create an impermeable coating of the OCMC without affecting its damag-tolerant behaviour. However, the prototype developments show that it is still very challenging to manufacture even slightly complex structures without critical failures. Nevertheless, OCMC structures of relatively simple geometries showed promising results in hot firings and could be used as a lightweight housing, while the inner contour of the chamber and nozzle are realised, e.g., by a graphite inlay of appropriate quality.

Preparation and water-vapour corrosion behaviour of BSAS environmental barrier coatings fabricated on ceramic matrix composites

Environmental barrier coatings (EBCs) are commonly utilized to protect ceramic matrix composite components from water vapour attacks in commercial gas turbine engines. In this work, Si/3Al2O3·2SiO2 (Mullite)/BaO-SrO-Al2O3-SiO2 (BSAS) tri-layered EBCs were fabricated on SiC-based ceramic matrix composites (CMC) substrate via atmospheric plasma spraying technique. The phase composition, microstructure, and bond strength of the environmental barrier coatings were studied by scanning electron microscope, energy dispersive spectrometer, X-ray diffraction analyzer, and mechanical test machine. The water vapour resistance of the BSAS coating and CMC was evaluated with a self-assembled water-vapour corrosion test device. The results indicate that the as-sprayed BSAS coating with a porosity of (5.9 ± 0.8) % mainly comprises the monoclinic celsian BSAS phase, hexacelsian BSAS phase, and SiO2 phase. The consumption velocity of the BSAS coating is about 12 % that of the naked CMC material in a 1300 °C water vapour environment. After corrosion for 300 h, the average flexural strength of naked samples was (481 ± 9) MPa, which was about 64 % of the coated ones. In addition, the bond strength of the as-sprayed coating is (18.7 ± 1.6) MPa. Even after 300 h of water vapour corrosion, the bond strength of the coating does not decay. Evidently, BSAS coating can protect CMC materials from water vapour corrosion at 1300 °C.

A Comprehensive Study of Ceramic Matrix Composites for Space Applications

Ceramic matrix composites (CMCs) have grown in popularity as a material for a range of high as well as protection components, increasing the need to better understand the impacts of multiple machining methods. It is primarily composed of ceramic fibers embedded in the matrix. Ceramic materials, especially carbon fibers and carbon were used to create the matrix and fibers. These ceramics include a huge variety of non-metallic inorganic materials that are regularly utilized under high temperatures. The aircraft industry became revolutionized by this unique combination of materials, which made parts better resistant under extreme conditions as well as lighter than the earlier technology. The development, properties, and production of ceramic matrix composites, as well as space applications, are discussed in this article. Ceramic materials have an interesting set of properties, including great strength and stiffness under extremely high temperatures, chemical inertness, low density, etc. In CMC, ceramics are used in the matrix as well as reinforcement. The matrix material keeps things running smoothly while the reinforcement delivers unique special properties. Ceramic matrix composites are developed for applications that required high thermal and mechanical characteristics, which include nuclear power plants, aircraft, chemical plants, space structures, and transportation services. Even though advanced aircraft relies on high-performance propulsion systems, improving the total impulses over the total mass ratio for rocket engines becomes essential for improving their performance that demands reduced engine structural weight as well as higher component heat resistance. The evolution of new ultra-high-temperature composites having high-temperature resistance as well as low density that a substitute super alloy and refractory metal material has become so essential and laid the foundation for high-performance engine design. The benefits of continuous fiber- reinforced CMC with high-temperature engine designs have long been recognized as a better measure of a country’s ability to design and produce spacecraft, modern aircraft, and weapons. Ceramic matrix composites materials are used in various aircraft type engines, aircraft brake disks, high-temperature gas turbines components, slide bearing components, hot gas duct, flame holders and components for burners are made by using oxide CMCs.

Parametric Influences of Geometric Dimensions on High Temperature Mechanical Behaviors and Damage Mechanisms of Ceramic Matrix Composite and Superalloy Double Bolted Joints

Given multiple material performance advantages, ceramic matrix composite (CMC) material has become one of the most promising hot structural materials used for thermal protection system in hypersonic vehicles. Under harsh thermal exposure of vehicles in flight, the design of connection structure would be a critical issue in improving load-carrying efficiency and ensuring service safety of aircraft structures in service environments. However, little attention was paid on mechanical behavior and its factors affecting the mechanical property of CMC joining at elevated temperature. To address this concern, a 3D finite element model coupled with progressive damage analysis is carried out to predict high temperature tensile properties and failure behavior of single-lap, double-bolt CMC/superalloy joints assembled by two serial protruding-head bolts. In the implementation of progressive damage analysis of 2D plain-woven C/SiC composites, a user-defined subroutine UMAT including a nonlinear constitutive model, 3D Alvaro failure criterion and Tan’s material degradation rule were embedded into the general package ABAQUS® through Fortran program interface. A parametric study considering geometries of joints was performed to evaluate their resultant influence on high temperature tensile behavior and the associated damage mechanisms for the CMC/superalloy double-bolt joint. New findings were provided for full exploitation of high performance through geometric design of ceramic matrix composite hot structure for hypersonic aircraft.

Assessment of Performance Boundaries and Operability of Low Specific Thrust GUHBPR Engines for EIS2025

Abstract This paper aims to develop a robust design process by approaching the performance boundaries and evaluating the operability of the pursued geared turbofan (GTF) engine with low specific thrust for entry-into-service (EIS) 2025. A two-spool direct-drive turbofan (DDTF) engine of EIS 2000 was improved according to aircraft specifications and technology boundaries in 2025. A series of optimized engines with consecutive fan diameters were established to seek the ideal engine by balancing specific fuel consumption (SFC), weight, and mission fuel burn. The fan diameter was proved to be a decisive factor for lowering SFC and energy usage. The cycle design optimization process achieved a thermal efficiency of approximately 52%, and a propulsive efficiency of 79.5%, which is 8.19% increase in propulsive efficiency by enlarging fan diameter from 1.6 m to 1.9 m. Meanwhile, the 1.9 m-fan diameter engine achieved a reduction in SFC and fuel burn of 7.47% and 6.58%, respectively, which offers an overall reduction of 30.82% in block fuel burnt and CO2 emission compared to the DDTF engine. A feasibility check verified the viability of the designed optimum engine in terms of fan tip speed, stage loading, and AN2. Dynamic simulation offered a deep understanding of transient behavior and fundamental mechanism of the GTF engine. An important aspect of this paper is the use of advanced ceramic matrix composite (CMC) materials, which led to an improvement of 4.92% in block fuel burn and 2.93% in engine weight.

An orthotropic damage model with internal sliding and friction for masonry-like material

Quasi-brittle materials like masonry exhibit mechanical properties and develop nonlinearities that are mainly driven by their joints. In the present work, a Continuum Damage Mechanics point of view is considered to describe the macroscopic behavior of masonry. Based on damage models developed for ceramic matrix composite materials, a fixed directional damage approach is proposed. From this formulation, unilateral effect, as well as internal sliding and friction coupled with damage, are introduced. Numerical examples of the response of the model for different loading cases involving cyclic and non-proportional loadings are carried out and compared to experimental results.

Hole characteristics and surface damage formation mechanisms of C /SiC composites machined by abrasive waterjet

Abrasive waterjet (AWJ) has a significant impact on the dimensional accuracy and machining damage of C f /SiC composites. Thus, this paper aims to investigate the hole characteristics and formation mechanisms of surface damages (e.g. crack damage, tearing damage, delamination damage and burr damage). The results show that the diameters of hole entrance and hole exit varies with the traverse speed, jet pressure, standoff distance, and abrasive flow rate within certain parameters range. The over-machined phenomenon has the greatest impact on the hole dimensional accuracy, which can be effectively enhanced by improving the jet trajectory. The formation mechanisms of various damages are also obtained. These results will guide realizing high-precision and low-damage hole machining for ceramic matrix composite material.

Experimental and numerical study of a woven oxide/oxide ceramic matrix composite adhesive joint using four-point bending test

Oxide/oxide ceramic matrix composite materials are now used for applications in hot part of engines. To manufacture some components bonding techniques could be used. In order to understand the possible damage mechanisms of adhesive joint with CMC materials, experimental and numerical analysis using a four-point bending test is presented here. The numerical studies are performed using a continuum damage model for the behavior of the substrate and a cohesive zone model for the behavior of the interface between the composite and the bond. The results of the simulations are presented and discussed.

Picosecond laser machining of ceramic matrix composite

In recent years, there has been increased interest in using silicon carbide (SiC) based ceramic matrix composites (CMC) materials for aero-engine applications; however, both conventional and non-conventional machining of SiC CMC is challenging. Ultrashort pulse laser machining is evolving as an attractive option for materials like SiC CMC; however, the current understanding is based only on low-power lasers, whose removal rate is not in-line with the industrial requirements. This paper presents the results of 300 W picosecond laser machining of SiC CMC. The laser machining characteristics of SiC CMC were investigated at high average power and discussed in detail. A 300 W picosecond laser can be used to achieve a material removal rate of ∼230 mm3/min (0.76 mm3/minW) for a machining depth of 2 mm in SiC CMC, however, irrespective of the laser parameters a wall taper of ∼7° was observed over the sides of the slots that need to be addressed.

Development and study of the properties of a ceramic-matrix composite material for the production of ultra-thin large-format ceramic granite slabs

The article analyzes the advantages and disadvantages of large-format ultra-thin ceramic granite slabs, shows the need and identifies ways to increase their strength. The composition of the ceramic matrix has been developed, provides the maximum level of sintering (W ≤ 0.5 %) in accordance with the requirements for ceramic plates of class ВІа, and the choice of the reinforcing filler has also been substantiated. It has been experimentally established that mullite-silica fibers have a positive effect on the physical and mechanical properties of the slabs, which makes it possible to reduce their thickness by three times, and therefore significantly reduce the weight of products. The fractional composition of the grinding products of mullitesilica fibers was determined depending on the grinding conditions, and the influence of the quantitative and qualitative characteristics of the reinforcing filler on the main properties that determine the quality of ceramic granite slabs was investigated. The conditions for the formation of densely sintered composite materials with a complex of high-performance properties have been determined. It was shown the possibility of increasing the samples bending strength up to 57 MPa by introducing 4 wt. % of finely dispersed mullitesilica fibers, most of which (~ 60 %) have a length up to 45 microns, which is achieved by 3 min grinding in a planetary mill at a speed of 300 rpm. The structural-phase features of the samples obtained under the high-speed firing conditions, which consist in the increased amount of the mullite phase formation in the presence of mullite-silica fibers, have been established. The latter not only reinforce the ceramic matrix, but also act as a precursor of new mullite formations, the crystals of which, in the presence of a melt, acquire an isometric shape and contribute to an increase in the products strength. The research results indicate the promising obtain large-format ultra-thin ceramic granite slabs using composite materials technology. The combination of excellent technical qualities, large dimensions, and reduced slabs thickness opens up the new possibilities for using composite ceramic granite in design of furniture and large rooms due to the possibility of creating a continuous seamless coating and a significant reduction in the products weight.

Study on the Influence of Amplitude on Ultrasonic Assisted Grinding of Hard and Brittle Materials

With the development of advanced manufacturing technology, ceramic matrix composite materials, a typically hard and brittle material, have been widely used in high-tech fields such as aerospace manufacturing. Due to the anisotropy of materials, the quality of conventional processing workpieces is poor, and the processing accuracy is difficult to guarantee. In this experiment, ceramic matrix composite materials are machined by ultrasonic vibration grinding with the CBN grinding rods. The influence of amplitude on the grinding force and the surface quality of the workpiece in the grinding process are analyzed by a series of experiments on ceramic matrix composites. The results show that, compared with the conventional grinding process, in the ultrasonic vibration-assisted grinding process, the grinding force is reduced by about 60%, and the surface quality is also improved significantly,the surface roughness Sa is reduced by about 25%.

Damage tolerance evaluation of a C/C-SiC composite body flap of a re-entry vehicle

This paper deals with the delamination propagation in Ceramic Matrix Composites (CMC) for spacecraft applications. Presently, Ceramic Matrix Composites (CMC) are increasingly adopted for application where heat resistance and structural stiffness requirements need to be fulfilled at the same time. In particular, in aerospace applications, where the lightweight requirement is of primary concern, CMC materials seem to be the most fascinating solution for effective and performant design. One of the possible limits in the use of such materials for aerospace applications is related to the onset and evolution of damages, in particular by delamination under service loading conditions. Indeed, commonly used rules and regulations on delamination propagation, which are well-established for carbon fibre reinforced plastic, are not straight applicable for CMC materials due to the specific nature of this ceramic materials. Moreover, methodologies for fracture toughness evaluation are still in a preliminary stage for these materials.In this work, a preliminary CMC material model calibration for Cohesive elements numerically has been performed at coupon level (DCB and ENF tests) and validated by experimental literature data. Secondly, a sensitivity analysis on delamination position and size has been carried out on a C/C-SiC body flap for a reusable re-entry vehicle.To comply with a step by step gradually increased model complexity-based damage tolerance approach, preliminary analyses have been performed on plates with embedded circular delamination, comparing the delamination onset stress level with respect to the stress state corresponding to the most critical body flap service load.

High-Temperature Performance of Next-Generation Silicon Carbide Fibers for CMCs

Abstract Single-fiber (monofilament, not twisted tow) high-temperature creep performance of refractory ceramic compositions is a fundamental material property that must be examined and understood for the successful deployment of ceramic matrix composite material systems in demanding applications. A novel creep testing approach was devised utilizing an innovative approach to fiber sample attachment and used to evaluate the creep behavior of Free Form Fibers’ LP-30SC and Nippon Carbon-based Hi-Nicalon Type S (HNS) silicon carbide fiber products. The testing apparatus, analytical basis for the creep calculations, including the sample gage length, and collected data are presented. Test conditions include a range of peak hold temperatures, from 1,300°C to 1,500°C, gas environments, and hold time patterns at the peak temperature. Clear differences in resistance to creep degradation were evident between the LP-30SC and HNS fibers, as the LP-30SC demonstrated strong resiliency to high temperature exposure while the HNS showed performance degradation consistent with oxidation attack.