SiCf/SiC composites tolerance to high temperature combustion atmosphere and post ageing mechanical and microstructural characterisation

This article reports the activities carried out under WP4, coordinated by ENEA, within the CEM-WAVE European project, about experimental study on SiCf/SiC composites tolerance to high temperature combustion atmosphere. Water corrosion phenomena are known to affect mechanical properties of SiC-based CMC, due to the formation of a silica, volatile in certain condition. The study will simulate combustion conditions, aiming at using more environmentally friendly alternatives to methane, such as hydrogen, coke oven gas or biomethane, in steel production. It is essential to determine, in each case, if corrosion happens in active (that means continuous degradation) or passive (silica forms a protective “scale” on the component) regime. Since fibre-to-matrix interface can also be degraded, it is important to determine post-ageing mechanical flexural strength (by 4point bending tests). Water corrosion ageing was performed by means of direct flame exposure. When the oxidizing species becomes CO2, instead of O2, the formation rate of silica passivating layer is expected to be lower than the volatilization rate, and consequently active oxidation results in continuous weight loss. Ageing tests of SiCf/SiC composites were carried out at high temperature in a CO2/H2O/N2 gaseous environment at atmospheric pressure (reference conditions: 1200°C, 10/20/70 v/v). Bending tests up to failure are carried out at room temperature on these aged samples after different exposure times. Relationships are established between the evolution of mechanical properties, the ageing conditions, and the rates of oxidation reactions.


Introduction
The EU CEM-WAVE project (G.A. 958170, www.cem-wave.eu)aims to introduce the use of ceramic matrix composites (CMCs) for high temperature applications in some energy-intensive processes, such as the production of steel.This need is a consequence of the ongoing transition from burning methane to hydrogen and biomethane, produced from renewable energy sources, in order to reduce carbon emissions and fossil fuel exploitation.Conventional metallic radiant tubes cannot withstand increased corrosion associated with the use of hydrogen and biomethane, so that CMC-based tubes seem an alternative to be validated under the expected operating conditions, characterized by very high temperatures, water vapour and acid impurities, promoting "active oxidation" and progressive erosion.
In order to overcome the main technical limits of the CMCs production, namely the high costs production times and low density, CEM-WAVE is developing Microwave-assisted Chemical Vapor Infiltration technology (MW-CVI) in theory a lot faster than conventional Isothermal/Isobaric CVI.Starting from these considerations, this paper presents the preliminary investigations on the mechanical properties of new SiCf/SiC composites developed within the European CEM-WAVE project, unlocking wider adoption of renewable energy sources in energy demanding productions.

SiC based Ceramic Matrix Composites (CMCs)
Because of their outstanding performance, silicon carbide fibre reinforced silicon carbide ceramic matrix composites (SiCf/SiC) are widely used in high temperature application, including high specific strength/modulus, excellent high-temperature resistance, and low density [1][2][3][4][5][6][7].However, problems related to hydrothermal corrosion have to be carefully considered, since the combustion of methane or hydrogen could result in a condition in which SiC is consumed continuously, progressively reducing both the mechanical properties and the functionality of the material [8,9].SiC water corrosion at high temperature can occur in two ways [10-12]: • Passive corrosion.In which SiC reacts with the oxidative environment leading to the production of an oxidized phase (SiO2) that settles on the surface of the composite.• Active corrosion.Rapid recession of mechanical properties of SiCf/SiC composites by forming gaseous reaction products such as Si(OH)4 by the presence of water vapor.Figure 1 shows the behaviour of the SiCf/SiC CMC obtained under oxidation in N2/CO2/H2O environment.As it can be seen, the first stage of mass uptake is rather fast; however, after some time, the kinetics tend towards the expected parabolic rate law, associates to the formation to a surface oxide scale [15].Several examples of volatilization of free SiO2 deposited on SiC CVD are described in literature.Among those, Kim and Readey [16] and Narushima et al [17] report on the oxidation/volatilization of SiC when H2/H20 or CO/CO2 gas mixtures are used.It is, therefore, necessary to identify the operating conditions that lead to the transition from passive to active corrosion, in order to obtain a final product able to meet industrial requirements.On the other hand, CEM-WAVE project also proposes the application of an EBC (Environmental Barrier Coating) on the surface of SiCf/SiC CMC, in order to protect SiC from oxidation [12,18].Besides microstructural observations and weight changes, it is important to assess the mechanical performance upon ageing in post combustion environment, by performing mechanical tests, according to the main international standards [19][20][21][22][23].These experimental activities allow the identification of the limit of the operating conditions to avoids active oxidation of the composite material, ensuring a longer reliability.For this purpose, ENEA is carrying out a testing campaign to assess the actual loss of mechanical properties of the SiCf/SiC composite, with and without EBC.

Thermomechanical characterisation
The thermomechanical tests were carried out in ENEA Laboratories using two servo-hydraulic MTS testing machines, with different full scales (50 kN and 67 kN).
In order to set up the mechanical testing campaign, the testing procedure have been before developed using a Mersen C/Cf composites.These experimental activities are summarised below: • machining of the Cf/C test samples using a grinding machine that lock in place the material through freezing.
• determination of flexural and interlaminar shear properties of CMCs according to the international standards [19-23].Interlaminar shear tests (Figure 2) were performed in displacement control mode, using the MTS 67 kN servo-hydraulic testing machine, in accordance with the international standards ASTM C1292 and UNI 20].N. 10 samples were tested and, considering the fractographic analysis, n. 9 tests were valid.The average value obtained for the Interlaminar Shear Strength is 19.5±5.9MPa.Four-point bending tests [21][22][23] were carried out at room temperature (RT) in displacement control mode, using the 67 kN MTS servo-hydraulic testing machine, using two different outer span/thickness ratio (respectively 1:13 and 1:26).The Mode of Failure observed during tests, pointed out that the results obtained for 1:13 ratio samples should be considered invalid (shear failure, Figure 3a), while for 1:26 ratio valid (tensile failure, Figure 3b).Therefore, based on these results, both high temperature and room temperature flexural tests on CMCs that will be developed during CEM-WAVE project, should be

SiCf/SiC thermomechanical characterisation
Three SiCf/SiC composite slabs were produced by ATL (www.cvd.co.uk), partner of CEM-WAVE project, and prepared using CVD/CVI process.The densification degree of these samples was kept intentionally low (generally between 1650 and 1720 kg/m 3 , compared to theoretical density of SiC around 3200 kg/m 3 ), to obtain a composite material that is highly subject to degradation in water corrosion conditions.ATL densified Nicalon 1 st generation SiC fibre preforms, so thermal ageing was performed around 1050 °C and not exceeding 1100 °C.In the case the applied EBC is able to avoid mechanical degradation in these unfavourable conditions of low densification, the long-term reliability on dense SiCf/SiC can also be considered achieved.From these slabs (named P1, P2 and P3 respectively), ENEA cut bending samples (in 0° and 90° direction), with sizes 10 mm width x 85 mm length x 3 mm thickness, using the same grinding machine reported above.In order to assess the mechanical properties, samples from all three slabs were analysed.The acronym PX_Y identifies the bending specimen No. Y obtained from the machined slab X.The samples were tested at RT by fourpoint bending configuration (outer span 75 mm and inner span 25 mm) according to the international standards [21][22][23].The tests were performed in displacement control mode using the 50kN MTS servohydraulic testing machine.The results are summarised in Table 1 in terms of maximum load, maximum flexural strength, modulus of elasticity and failure typical mode.In Figure 4 the Flexural Strength with respect to Geometric Density is shown.This plot pointed out that the strength of samples cut in 90° direction is greater than samples cut in 0° direction, by a factor of 2. This trend can be related to the lower density of samples obtained in the 0° direction of the fabric, but also an intrinsic asymmetry in the warp and weft weaving of the textile, resulting in a decrease of mechanical performance.Subsequently, the characterisation of the SiCf/SiC composite focused on the evaluation of the loss of performance following thermal ageing.These investigations were performed on the same slabs developed by ATL, and used to characterise the as-it-is material at room temperature.Flame treatment was performed by torch flame exposure, in the ICB Laboratory (University Bourgogne Franche-Comté) under the same operating conditions, which radiant tubes should work continuously for years.The use of low density SiCf/SiC results in an acceleration of mechanical properties decrease, probably involving a degradation in the fibre-matrix interface.Specifically, two different corrosive environments were considered: a) CH4/O2; b) CH4/O2 + H2O/50 ppm H2SO4.Table 2 summarises the actual mass loss of the samples assessed after the thermal ageing test.After flame treatments, four-point bending test was conducted to determine the actual mechanical properties loss.Table 3 shows the results in terms of maximum load, maximum flexural strength, modulus of elasticity and failure typical mode.Figure 5 shows the comparison at RT, before and after thermal ageing.Comparing the results summarized in Table 1 and 3, it can be noticed that, for samples cut in direction 90°, the Flexural Strength after ageing is, in terms of average values, about 33% lower than before ageing.A similar consideration can be done for the Modulus of Elasticity, that is the average value after ageing is about 9% lower than before ageing.These decrease of mechanical properties can be related to the presence of a severe oxidative environment and acidic impurities (H2SO4) that lead to a higher degradation of SiC for samples P1_14 and P2_14.Particularly for sample P2_14, a decrease of about 52% in term of Flexural Strength, and 13.5% in term of Modulus of Elasticity were determined after thermal ageing.

Conclusion
One of the purposes of this paper was to set up the most suitable mechanical test methods and optimize them, simulating the operating conditions of CEM-WAVE industrial application, in order to obtain a reliable estimate of the mechanical properties of the developed CMCs.For this reason, ENEA carried out a preliminary mechanical testing campaign on a Mersen Cf/C composite: (i) setting up and developing the CMC material machining method, by using a grinding machine, aiming to realize samples for mechanical tests; (ii) setting up and validating the interlaminar shear and bending test methods.
Considering the results of Cf/C testing campaign, the characterisation on SiCf/SiC CMCs, realized by ATL was performed, allowed to assess the actual loss of composite performances when subjected to an environment simulating the operating conditions for the required application (radiant tubes used for steel production).For this purpose, the as-it-is material was characterized at RT, by four-point bending test, comparing samples obtained cutting the slabs in different direction (0° and 90°).Following thermal ageing, carried out by the Laboratory ICB (University Bourgogne Franche-Comté), a four-point bending test campaign at RT was again carried out on samples cut in 90° direction, in order to have a comparison.The results showed that the SiCf/SiC mechanical properties decrease after ageing (average values of about 33% for Flexural Strength and 9% for Modulus of Elasticity), particularly for the samples subjected to the presence of acid impurities (P2_14 presented a decrease of about 52% for Flexural Strength and 13.2% for Modulus of Elasticity).In the light of these mechanical properties, the application of an EBC become fundamental to preserve the functionality of the material.Therefore ENEA, in the next steps of the activity, will continue the mechanical characterisation of SiCf/SiC composite (i) before, (ii) after torch flame exposure thermal ageing, and perform new mechanical characterisation (iii) after ageing in tubular furnace at various temperatures (and the same controlled atmosphere) comparing the effect of the EBC on the SiCf/SiC.

Figure 2
Figure 2 Test fixture for ISS measurements

Figure 4
Figure 4 Dependence of flexural strength on the geometric density at RT

Figure 5
Figure 5 Dependence of flexural strength on the geometric density at RT -Comparison before and after thermal ageing at RT (machining direction 90°)

Table 1
Results of four-point bending tests at RT for ATL SiCf/SiC composite

Table 2
Mass loss recorded after flame test *The operative conditions indicated in Table2refer to the oxidative environments indicated in the paragraph: a) CH4/O2; b) CH4/O2 + H2O/50 ppm H2SO4.

Table 3
Results of four-point bending tests at RT for SiCf/SiC after thermal ageing for [90°] samples