Corinne Bagoulla

Patrice Guillotreau
email: patrice.guillotreau@univ-nantes.fr

Lemna

Maritime transport in the French economy and its impact on air pollution: An input-output analysis

Keywords: maritime transport, input-output analysis, air pollution, France maritime transport, input-output analysis, air pollution, France

niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Acknowledgements:

The authors would like to thank the Ministère de la Transition Écologique et Solidaire (MTES), Direction générale des infrastructures, des transports et de la mer (DGITM), Mission de la Flotte de Commerce (MFC), and the Centre d'études et d'expertise sur les risques, l'environnement, la mobilité et l'aménagement (CEREMA) for their financial support and expertise, in particular Marie Douet and Romuald Lacoste from CEREMA who were part of the initial study. Any mistake or omission found in this article remains ours.

Introduction

The European integrated maritime policy was first launched in 2007 1 following the recommendations of the Green Book (2006) and the Blue Book (2007), and later adopted in 2012 2 by the Ministers in charge of the maritime policy and the European Commission. It emphasized the potential contribution of maritime sectors to the European economic growth and employment, inviting Member States to define their own "blue growth" strategy for the sustainable development of their sea-related activities.

With 12,840 km of coastline 3 and 11 km² of Exclusive Economic Zone (EEZ), France possesses the second largest EEZ in the world, just behind the USA. Historically more focused on land activities, France has recently realised that the "blue economy" represents a tremendous growth potential 4 . It encompasses a large number of interconnected sub-sectors: marine renewable energy, extraction of minerals from the seabed, offshore oil and gas production, ports, shipbuilding and repair, short and long-distance shipping, fishing, aquaculture, marine bioprospecting, etc. Maritime transport is a key player of the blue economy in France. In 2016, exports and imports represented around 30% of the French GDP 5 , 40% of French exports and 52% of French imports being seaborne trade 6 . Marseille-Fos and Le Havre are two important ports in Europe and the French Group CMA CGM is the fourth largest shipping firm worldwide. However, even if the French maritime industry has achieved good results in 2016, much more could be done to increase its competitiveness.

Promoting a sustainable blue economy first requires good empirical knowledge about its economic importance and linkages. This may provide policy-makers with accurate information about the potential of this industry for future economic growth. One of the main objectives of this research is therefore to assess the impact of water transport services on the French economy over the period 2010-2016, by using an input-output (I-O) approach. This has never been done so far for the French domestic economy, compared with similar studies applied to other countries. Some recent articles have measured the economic role of maritime sectors in China [START_REF] Wang | The role of the marine industry in China's national economy: an inputoutput analysis[END_REF], South Korea [START_REF] Jun | Impact of the smart port industry on the Korean national economy using input-output analysis[END_REF][START_REF] Lee | The role of transportation sectors in the Korean national economy: An input-output analysis[END_REF][START_REF] Kwak | The role of the maritime industry in the Korean national economy: an input-output analysis[END_REF], Ireland [START_REF] Morrissey | The potential for an Irish maritime transportation cluster: An input-output analysis[END_REF], Spain [START_REF] Fernández-Macho | Measuring the maritime economy: Spain in the European Atlantic Arc[END_REF] or Taiwan [START_REF] Chang | The Role of the Taiwan Marine Sectors in the National Economy: An Input-output Analysis[END_REF]. Kwak et al. [START_REF] Kwak | The role of the maritime industry in the Korean national economy: an input-output analysis[END_REF] studied the economic impact of five maritime industries 7 in South Korea using an I-O longitudinal approach between 1975 and 1998. They have pointed out that the Korean maritime industry had high backward linkage effects, low forward linkage effects and a high employment-inducing effect. Lee and Yoo [START_REF] Lee | The role of transportation sectors in the Korean national economy: An input-output analysis[END_REF] also used a comprehensive I-O model for Korea in a longitudinal approach to analyse the economic weight of four transportation industries 8 on the domestic economy. The economic impact was found higher for rail and road transportation than for the two other transportation sectors (water and air). Morrissey and O'Donogue [START_REF] Morrissey | The potential for an Irish maritime transportation cluster: An input-output analysis[END_REF] adopted a similar approach and reached identical conclusions as Kwak et al. [START_REF] Kwak | The role of the maritime industry in the Korean national economy: an input-output analysis[END_REF] for Ireland, but they 3 disaggregated the Irish I-O table to include ten additional marine sectors for one year only, using both public and non-public data. They showed that the water transport industry presented the highest backward linkage of the ten marine sectors included in the article. Chang et al. [START_REF] Chang | The Role of the Taiwan Marine Sectors in the National Economy: An Input-output Analysis[END_REF] distinguished nine marine sectors 9 in the Taiwan I-O Table (554 sub-sectors) over the period 1996-2011. They reported that the magnitude of the Taiwanese marine sectors in the domestic economy had increased significantly during the covered period. Transportation support services appeared to have one of the highest forward and backward linkages, and the strongest wealth-inducing and employment-inducing effects. Since 2014, the European Community Shipowners' Associations (ECSA), with the assistance of Oxford Economics, regularly publishes a report concerning the economic value of the EU shipping industry, still using an I-O approach. According to this literature, maritime industries, and particularly maritime transport [START_REF]The economic value of the EU shipping industry[END_REF][START_REF]The economic value of the EU shipping industry[END_REF][START_REF]The economic value of the EU shipping industry[END_REF], have major economic impacts for the studied countries. China has also a fast-growing maritime industry across many activities (aquaculture and fisheries, shipbuilding, transportation, offshore oil and gas, marine engineering…) and authors recently reported an impressive number of 46 million jobs (i.e. 6% of the working population) directly or indirectly linked with marine and marine-related industries [START_REF] Wang | The role of the marine industry in China's national economy: an inputoutput analysis[END_REF]. The authors also used a mixed I-O model, making marine sectors exogenous, and have been looking at production-inducing, sectoral supply-shortage and employment-inducing effects with a disaggregation in 12 marine sectors. They found that the three key production-inducing marine sectors were coastal tourism, maritime transport and shipbuilding. Any $1 investment in the marine industry creates a $1.65 effect which is distributed among various marine industries (tourism, shipping, shipbuilding, fisheries, oil & gas, offshore mining, marine chemicals, marine pharmaceutical, offshore engineering…). The backward linkage effect was also found to be far greater than the forward linkage effect in their Chinese study.

A comprehensive synthesis proposed by Jacobsen et al. [START_REF] Jacobsen | A global synthesis of the economic multiplier effects of marine sectors[END_REF] has surveyed a great number of economic multipliers (808 in overall) of maritime sectors in a meta-analysis. These authors have reported data issued by the oceaneconomics.org website on eight marine or maritime sectors (aquaculture, fisheries, marine renewable energy, fish processing, shipping, recreational fisheries, shipbuilding and tourism) in more than 180 countries, resulting in 189 income multipliers, 164 job multipliers, 90 GDP multipliers, etc. The estimated average output multiplier was 1.82, meaning that any US Dollar created by the maritime industry would increase the domestic output by an additional amount of 82 cents into the rest of the economy. In this study, the output (or Leontief) multipliers were found higher than others such as employment, GDP or income multipliers. In other terms, it was shown that marine sectors have many backward linkages across their production process, purchasing inputs from many different suppliers, but they somehow do not create as much added value or as many jobs as other industries, presumably because they remain capital-intensive activities.

Interestingly, we have not found any study using the same I-O basis to calculate the environmental impacts of maritime transport. However, this is of major importance at a time when the International Maritime Organization (IMO) attempts to reduce the greenhouse gas impact of shipping which has been left outside the Paris agreement on Climate Change, by imposing emission control areas [START_REF] Endresen | Emission from international sea transportation and environmental impact[END_REF][START_REF] Cullinane | Emission control areas and their impact on maritime transport[END_REF].

The present research aims at providing a basis for comparison with the magnitude of water transport multipliers in the French economy throughout several years (2010-2016) to account for potential interannual variability. Our approach is original by splitting up the water transport service activity in two separate branches: freight and passenger transport, showing distinct results in terms of employmentinducing effects. Moreover, on the basis of environmental data issued from the National Accounting Matrix including Environmental Accounts (NAMEA), we introduce environmental multipliers of maritime transport for CO2, SO2, NOx, CH4 emissions and suspended particulate matter (PM10 and PM2.5) to stress the major role played by shipping and seaborne trade on air pollution emissions. Our study therefore contributes to the literature by applying a standard I-O model for the first time to the French maritime transport industry, and by extending this empirical work to several types of air pollution multipliers and effects by using the European database NAMEA. After some stylized facts reported in Section 2, the I-O model is introduced in Section 3, including the development of environmental multipliers. Section 4 presents the major results before drawing some conclusions in Section 5.

Current status of maritime transport in France: some stylized empirical facts

In 2015, the EU shipping industry directly employed 640,000 people and generated a 57 billion euro contribution to GDP. Maritime freight accounted for 57% of the EU shipping industry's total contribution to GDP [START_REF]The economic value of the EU shipping industry[END_REF].

In France, in 2013, some 450,000 people, equivalent to 2% of overall national employment are employed in the maritime economy as a whole (from fishing to ocean biotechnologies, [START_REF]L'économie maritime : des activités diverses et localisées[END_REF]) and produce an output value of €104 billion for an added value of €35 billion according to Ifremer [START_REF]Données de l'économie française maritime[END_REF]. Coastal tourism accounts for half of the jobs (227,600). Public works, maritime and river transport and seafood products represent 19%, 11% and 10% of French maritime jobs, respectively (Fig 1). In its contribution to GDP, tourism represented 43%, offshore oil & gas 17%, maritime transport 13%, seafood (fisheries, aquaculture, fish processing) 10%, shipbuilding 10%, and the remaining contribution to GDP (7%) being shared between ports, offshore equipment, R&D, renewable energy, telecommunications and biotechnology [START_REF] Cullinane | Emission control areas and their impact on maritime transport[END_REF]. In overall, the blue economy contributed to 1.7% of the domestic GDP in 2013. The share of maritime transport services is fairly low and largely dominated by one single company (CMA-CGM). Being a global carrier, this company employs international seafarers, orders ships in Korea or China, and purchases inputs from all over the world (bunkering, shipchandling, stevedoring, manning, etc.). The estimation of the domestic share of added value created by such global companies remains a challenge for economists [START_REF] Miossec | Le conteneur et la nouvelle géographie des océans et des rivages de la mer[END_REF].

Material and methods

The Input-Output mixed model

The I-O approach proposes a multi-linear, inter-sectoral model that displays explicitly the relationships between the productive industries of a given economy [START_REF] Leontief | Input-Output Economics[END_REF]. In the standard open demand model, the direct and indirect output effects of an economy are determined by the exogenous final demand [START_REF] Leontief | Input-Output Economics[END_REF]. However, if one wishes to estimate the impact of a change in one industry's output on the economy as a whole, it is necessary to consider this output as exogenous, and include it in the final demand vector [START_REF] Kwak | The role of the maritime industry in the Korean national economy: an input-output analysis[END_REF][START_REF] Morrissey | The potential for an Irish maritime transportation cluster: An input-output analysis[END_REF]. The so-called mixed model allows to assess the effects of the change in the production of water transport activities on all other industries without considering the endogenous effects of these industries on the water transport activity itself [START_REF] Hambÿe | Analyse entrées-sorties, modèles, Multiplicateurs, Linkages[END_REF]. The basic balance equations of the I-O model consisting of N industry sectors can be expressed as:

= ∑ + = ∑ + (Eq. 1)
where is the total output in industry i = 1, 2, …, N. represents the final demand vector for products i and = , are the technical coefficients that give the proportion of input i needed to produce one unit of product j. denotes the total output vector for industry j.

The abbreviated matrix form of the model is

= + (Eq. 2) = - (Eq. 3) = - (Eq. 4)
In the mixed model, the first K branches of the model have an exogenous final demand and an endogenous production. The production the (n-k) exogenous (e.g. maritime) industries needs to be treated exogenously and left into the final set of industries. Eq. 1 must thus be rewritten as:

= ∑ + ∑ ̅ + = ∑ + ∑ + (Eq. 5)
In an abbreviated matrix form of the mixed model, it gives:

= ̅ (Eq. 6) With = 1 - - - 1 - 0 0 0 0 - , - , -" -" -1 0 0 -1 # = 1 0 0 1 , " , " 0 0 0 0 -1 - , ," ", -1 -"" # (Eq. 7-8) And ̅ = $ % % & ̅ ̅ " ' ( ( ) (Eq. 9)
Using the Leontief matrix of technical coefficient (A), we obtain:

= * -+ 0 + -1 , -. = * + 0 -1 -++ , (
Eq.10)

Output multipliers and linkage indices

Output multipliers are used to estimate the effects of a change of an exogenous variable (household, firms' investment, external or public consumption, or even production in water transport services) on the rest of the economy. The algebraic expression of the output multiplier is:

/ = ∑ 0 (Eq.11)
Where, 0 are the elements of the modified reverse matrix ( . These effects can be decomposed considering the direct output effect (i.e. the intermediate inputs purchased by the maritime industries to their suppliers ( ∑ " ), and the indirect output effects (i.e. purchases of the maritime industries' suppliers to their own suppliers ( ∑ 0 " ∑ -

).

In the framework of an I-O model, two indices of economic effects stemming from the maritime industries and directed toward other industries can be calculated: the Backward Linkage Index (BLI)

(1 = " ∑ 2 34 5 3 " 6 ∑ ∑ 2 34 5 3 5 4
) and the Forward Linkage Index (FLI) (1 = " ∑ 2 34 ). The BLI means that an increasing activity in water transport services may induce greater use of other industries' products as intermediate inputs. FLI shows the extent to which water transport services are used as inputs by other industries for their own production. These two effects are then useful to assess the overall impact of water transport activities onto the domestic economy.

Employment multipliers

In the paper we also seek to assess the effects of maritime industries' activities on the employment of the associated industries. In order to integrate the differences in employment structures, we have split up the water transport industry into two branches, freight and passenger water transport. First, the employment diagonal vector (7 8 ) has been constructed:

7 8 = 9 : ; : ⋯ 0 ⋮ ⋱ ⋮ 0 ⋯ 9 5 ; 5 # (Eq. 12)
Where ? is the number of jobs in industry i and , the output of industry i. Multiplying 7 8 by the modified reverse matrix ( = , we obtain the employment multipliers generated by a change in maritime industries' final uses of exogenous industries and productions of endogenous industries (∆ :

∆9 3 ∆; A = B7 8 * - D (Eq.13)

Environmental multipliers

For the period 2007-2012, the International Maritime Organization (IMO) estimated that shipping is responsible for approximately 3.1% of annual global CO2 and 2.8% of annual Greenhouse gas (GHG, including CO2, CH4 and N2O) emissions on a CO2-equivalent basis [START_REF] Imo | Third IMO Greenhouse Gas Study. International Maritime Organization (IMO)[END_REF]. It represents nearly 1 billion tonnes of GHG emissions. This is a major concern for IMO while striving to mitigate the impact of shipping on climate change, since CO2 emissions are expected to grow by 50% to 250% in 2050 according to some IMO scenarios [START_REF] Imo | Third IMO Greenhouse Gas Study. International Maritime Organization (IMO)[END_REF]. Many authors have studied the global CO2 emissions of the shipping industry in order to stress the overall reduction of emissions [START_REF] Bouman | State-of the-art technologies, measures, and potential for reducing GHG emissions from shipping-A review[END_REF][START_REF] Olmer | Greenhouse gas emissions from global shipping, 2013-2015[END_REF] or the expected impact of some specific mitigating measures [START_REF] Cariou | Towards low carbon global supply chains: a multi-trade analysis of CO2 emission reductions in container shipping[END_REF]. Most of them are doing so by estimating the fuel consumption of ships through different variables: the total number of active ships by size and type, the annual number of days at sea, the time spent by ships at various speeds, the travelled distance, the time at port calls, the main and auxiliary engine power, the Specific Fuel Oil Consumption (SFOC, in grams per kW), etc. Fuel consumption is converted thereafter into CO2-equivalent emissions by using emission factors (e.g. in g per tonne of fuel; IMO 2014).

One may consider that all GHG emissions are not included in such estimations which do not account for the indirect (backward and forward effects) of the shipping industry on the whole economy. Input-Output analysis can be useful in that regard by integrating environmental extensions to form what is called an environmentally-extended input-output (EE-IO) approach [START_REF] Gajos | National Accounting Matrix with Environmentally Extended IO analysis[END_REF]. The basic equation (Eq. 14) underlying EE-IO models has been defined by the United Nations [START_REF]Handbook on Supply, Use and Input-Output Tables with extensions and applications[END_REF]:

? EFE = 1 G * - * H G (Eq.

14)

Where ? EFE is the matrix of direct and indirect GHG emissions, 1 G is the diagonal vector of emission intensity coefficients (GHG emission per unit of output, industry by industry), I is the unit matrix, AM is the matrix of input coefficients obtained from the mixed model and H G is the diagonal vector for final use.

This approach allows assessing the domestic direct and indirect carbon dioxide emissions for the production of shipping services and other related activities. However, it should be noticed that such a result is far from estimating the comprehensive carbon footprint of this production, since it does not include input emissions imported by the domestic industry, nor extracted raw materials which remain unused by the production process. More global EE-IO tables through inter-country input-output (ICIO) models must be used to take global emissions into consideration [START_REF] Tukker | EXIOPOL and CREEA: Towards a global Multi-Regional Economic and Physical Supply and Use tables with Environmental Extensions (ESA/STAT/AC.238[END_REF], although this type of modelling goes beyond the scope of the present research.

In the above emission equation, the components of the 1 G -vector are obtained from a Physical supplyuse table which describes the magnitude (measured in physical units such as Mega-grams, i.e. tonnes, or Giga-grams, i.e. thousand tonnes) and the nature of residuals flowing between the economy and nature (UN 2018), following the international standards of the UN System of Environmental-Economic Accounting (SEEA) Central Framework (2012) 10 . Since the 1990s, within the SEEA framework, the European Statistical Office (EUROSTAT) has developed its own environmental accounting system called NAMEA (national accounting matrix with environmental accounts), which is a conceptual tool linking conventional (monetary) national accounts and environmental (physical) accounts [START_REF] Gajos | National Accounting Matrix with Environmentally Extended IO analysis[END_REF]. The physical satellite accounts include the energy consumption broken down by industry and household categories, as well as waste and air, soil and water polluting emissions. Most EU countries, including France, have now built their own NAMEA-air pollution data collection system 11 .

Data

The symmetrical input-output tables (IOT) product by product published by EUROSTAT between 2010 and 2014 were used in order to build the I-O model 12 . The input-output tables (IOT) combine both supply and use into a single table with identical classification of industries, applied to both rows and columns. Employment data are provided by the national account scheme and published by the National Institute of Statistics and Economic Studies [START_REF]L'économie maritime : des activités diverses et localisées[END_REF].

The physical air emission data are available in France for 14 chemical substances between 2008 and 2014 13 , including those usually associated with shipping: carbon dioxide (CO2), methane (CH4), sulphur dioxide (S2O), Nitrous oxides (NOx), suspended particulate matter (PM2.5 or PM10, with diameters of 2.5 or 10 micron, respectively). They can also be extracted from the same Eurostat source 14 . By contrast, the forward linkage (Table 2) of water transport services is the lowest of all sectors. When the economy is growing, water transport services are therefore less stimulated than other industries by the rest of the economy.

Results

Backward and forward linkage effects

Industry Value Rank V R V R V R V R V R V

Output-inducing effects

Table 3 shows the twenty domestic industries with the highest output multipliers and therefore showing the inter-industrial impact of water transport services. In 2016, the output multiplier of water transport services was 2.16 (the second highest). This number shows that a 1 million euro increase of the final demand for water transport services would generate a 1.16 million increase of output value (in addition to the water transport output itself) in the whole economy 15 . The total output-inducing impact result from both direct and indirect effects. Actually, a change in the final demand for water transport services generates a rise in production of related industries (direct effects) which in turn buy more from their own suppliers (indirect effects) and so on and so forth.

The output multiplier of water transport services sector remains relatively stable between 2011 and 2016, with a much lower value in 2010 when the impact was found lower that particular year (1.88) due to a slow economic recovery, thus showing the strong sensitivity of the shipping sector to international business conditions.

Industry Value Rank V R V R V R V R V R V R
Insurance, reinsurance and pension funding services

2

Employment-inducing effects

Maritime industries offer many employment opportunities. In 2016, the world maritime economy has generated 31 million jobs according to OECD [START_REF]The ocean economy in 2030[END_REF]. In France, the admitted figure is around 400,000 jobs [START_REF] Colas | Les emplois de l'économie maritime -données[END_REF].

In 2014, the Freight water transport industry had a job multiplier of 7, but the value of the passenger water transport industry was nearly twice higher (13) 17 . In other words, per million euro of additional demand for water transport services, 7 jobs are created in freight and 13 jobs in passenger transport. This difference makes sense when the distinct nature of jobs in these two activities is observed: more services (catering, accommodation, gift shops,…) are included onboard passenger ferries crossing the English channel or sailing to Corsica or Algeria for instance. These two branches rank respectively 19th and 53rd among the 64 branches of the economy by their multiplier, and could be considered as poorly job-creating. However, in 2014, some 105,259 jobs were directly or indirectly created by the final demand for water transport services within the whole domestic economy (of which 18,862 jobs concentrated in the two maritime transport industries). The Warehousing and support services for transportation industry is another industry taking advantage of the demand for water transport (Figure 2).

Environmental effects

Environmental multipliers and total effects of air chemical pollutions have been computed for the French shipping industry. As explained in the methodological section, multipliers are obtained by the product of the B diagonal vector of emission intensity coefficients (i.e. the physical amount of substance per M€ of output; CO2 is expressed in thousand tonnes per M€, the other chemical compounds in tonnes per M€) and the inverse Leontief Matrix of the mixed model (M -1 *N). The total effects are obtained by the matrix product of the emission multipliers and the diagonal vector of final demand. By doing so, the air polluting emissions are re-allocated throughout their indirect backward effects on upstream activities. For instance, water transport is not only responsible for its own CO2 emissions, but also for the CO2 emissions of the whole chain of suppliers, of which those emitted by the coke and refined petroleum product industry.

Total effect (Rank/64)

3,342 (16) 4,935 (34) 71,445 (2) 6,683 (5) 6,040 (4) 37,543 (1)
Note: Multiplier units are in thousand tonnes per M€ for CO2 and in tonnes per M€ for other compounds. Total emission effects are measured in thousand tonnes for CO2 and in tonnes for other compounds. The figure in brackets represents the rank out of the 64 French industries, i.e. by decreasing order of polluting emissions.

By its final use magnitude, the water transport industry (€13.5 Bn) is only ranked at the 42 nd level out of the 64 national industries in 2016. As a result, this activity contributes to only 1% of the domestic CO2 emissions, i.e. less than the worldwide average share which is above 3% [START_REF] Cariou | Is slow steaming a sustainable means of reducing CO2 emissions from container shipping?[END_REF]. However, the role played by shipping in the domestic air pollution is far from being negligible with regard to several chemical compounds. The intensity of emission per monetary unit of final use reaches 260 tonnes per M€ for carbon dioxide, 4 tonnes for nitrogen oxides and 1.9 tonnes for sulphur dioxide. For the two latter pollutants, water transport is even ranked fourth and first out of 64 in terms of total pollution effects, with 54,335 tonnes and 25,244 tonnes in 2016, respectively.

Interestingly, a comparison between two close dates (2014 and 2016) show how different multipliers per monetary unit can be. For sulphur dioxide (SO2), one of the major pollutants emitted by the shipping industry, a significant gap is observed between 2.60 and 1.87 tonnes per M€, representing a 28% decline between the two dates. The same observation goes for fine particulate matters, whose multipliers have decreased by 30% for both PM10 and PM2.5. Demand and the domestic supply have slightly decreased over this short period of time (-6%), nonetheless causing a significant decline of the major pollutants (-24% for NOx, -33% for SO2, -36% for PM10 and PM2.5). Because the multipliers are set in relative terms, the lower emissions cannot be fully explained by the decreasing demand for transport in France, but may simply mean that companies are now paying greater attention to environmental issues. When the oil price skyrocketed in 2007-08, before the financial bubble burst out, shipping companies have selected slow steaming strategies by reducing their commercial speed so as to gain a double dividend with lower bunker costs and GHG emissions [START_REF] Cariou | Is slow steaming a sustainable means of reducing CO2 emissions from container shipping?[END_REF]. The price of Brent oil was more than halved since the crisis and particularly during the 2014-16 period (falling from €80 to €40-50 per barrel). Saving fuel costs was no longer a priority but companies have also invested in new technologies and fuels (scrubbers, LNG, SCR,…) to mitigate their impact on the environment and anticipate the changing regulations.

NOx and SO2 are responsible for ocean acidification and eutrophication of waters. In particular, sulphur dioxide is produced by the diesel engines of ships and represents a major risk for human health [START_REF] Pandey | Health risks of NO2, SPM and SO2 in Delhi (India)[END_REF].

The International Maritime Organization has set a new limit for sulphur oxides in fuel oil used by ships outside Emission Control Areas defined by the MARPOL Convention (Annex VI) that comes into force from 1 January 2020 18 . The past limit is 3.50% m/m (mass by mass) and becomes 0.50% m/m since then, which should result in a decreasing pollution effect from the shipping industry. Suspended particulate matter is also a major risk for human health, especially fine particles like PM2.5 [START_REF] Pandey | Health risks of NO2, SPM and SO2 in Delhi (India)[END_REF][START_REF] Englert | Fine particles and human health-a review of epidemiological studies[END_REF].

Water transport stands in the top five most polluting industries concerning particulate matter effects which have major harmful consequences for lung and heart diseases.

Some conclusions and policy implications of the results

France is one of the most important maritime countries globally by its large Exclusive Economic Zone which is the second largest in the world with 11 million km². However, the maritime economy is still largely ignored by the French government and the economic interest of developing the blue economy has only started to be considered very recently. For the first time, this study estimates the role and relative influence that maritime transport has on the French economy by using an input-output approach. Such an analysis offers the interest to produce some results that can be compared with previous works achieved in other countries [START_REF] Wang | The role of the marine industry in China's national economy: an inputoutput analysis[END_REF][START_REF] Jun | Impact of the smart port industry on the Korean national economy using input-output analysis[END_REF][START_REF] Lee | The role of transportation sectors in the Korean national economy: An input-output analysis[END_REF][START_REF] Chang | The Role of the Taiwan Marine Sectors in the National Economy: An Input-output Analysis[END_REF][START_REF] Kwak | The role of the maritime industry in the Korean national economy: an input-output analysis[END_REF][START_REF] Morrissey | The potential for an Irish maritime transportation cluster: An input-output analysis[END_REF][START_REF] Fernández-Macho | Measuring the maritime economy: Spain in the European Atlantic Arc[END_REF], of which many were published in Marine Policy and grounded in a similar conceptual (Input-Output) framework. Obviously, the output multipliers estimated in the present study for one specific maritime industry are found smaller than national studies cumulating several sectors [START_REF] Wang | The role of the marine industry in China's national economy: an inputoutput analysis[END_REF][START_REF] Kwak | The role of the maritime industry in the Korean national economy: an input-output analysis[END_REF][START_REF] Morrissey | The potential for an Irish maritime transportation cluster: An input-output analysis[END_REF]. In particular, the employment multipliers [5 jobs per M€ of final demand on average] is far below the same value found for Ireland (29 jobs per M€ created by the 10 marine industries). However, the BLI and FLI are very consistent with those shown by these research works, with a much greater than average backward index and a much lower than average forward index, this latter low value being caused by the FOB registration of exports. With one major global carrier (CMA-CGM) ranking at the third or fourth position of the worldwide top liner shipping companies, France appears as an important maritime country, which is also revealed by the direct and indirect employment depending on the water transport activity. However, France is not amongst the major European nations involved in this activity (Germany, Denmark, United Kingdom…), although the output multiplier is found greater than the average European figure of 1.6 for maritime transport [START_REF]The economic value of the EU shipping industry[END_REF][START_REF]The economic value of the EU shipping industry[END_REF][START_REF]The economic value of the EU shipping industry[END_REF].

Another interest of this study relates to the concept of 'blue growth' and the need to follow the development of the European maritime industry by referring to a well-established initial point. Indeed, we now have a starting basis for the number of jobs or the wealth-inducing effects of maritime transport in France. As long as this country, through its national Blue growth strategy 19 , claims for greater investment in maritime sectors and the development of water transport, it is now possible to evaluate the outcomes of such a blue growth policy from the dynamics of economic effects produced by the maritime industry in the years to come in France.

From the results, we can see that passenger water transport services are more labour-intensive than maritime freight services and should be encouraged for their employment-inducing effects. Water transport services appear to be relatively less job-creating than many other French industries, particularly the freight services. For any million euro spent on the demand side, the freight industry creates 7 full-time equivalent (FTE) jobs, meanwhile the passenger transport activity creates 13 FTE jobs. However, because the demand for freight (€13 bn) exceeds by far that of passenger services (€1.4 bn in 2014), the combined direct and indirect effects of the demand for water transport (for both passenger and freight) in the economy result in a cumulated creation of 105,259 jobs in 2014 (of which 18,862 jobs for the mere water transport), i.e. 0.4% of total domestic employment. Although the ferry lines are not always profitable for private companies, they may produce significant spillover effects for the economy through the number of direct and indirect jobs which are created, not even mentioning the induced impact on local economies through the household expenditures 20 for the workers of this sector. Actually, sailors working aboard of passenger ferries are rather nationals and live near the domestic harbours, unlike the international market of seafarers where multinational freight companies hire their crew members through global manning agencies, resulting in a negative remittance balance because wages are usually spent abroad [START_REF] Bagoulla | Seafarers: an international labour market in perspective, ERC project Human Sea[END_REF].

The results of inter-industry linkages indicate that water transports services have more power in consuming services and products of related upstream industries rather than being used as an essential input by other downstream activities. Indeed, the water transport industry plays a significant role in the French economy by boosting other industries through its backward linkages. With an output multiplier of 2.16 in 2016 (the second highest among 64 industries), water transport services have a significant production-inducing effect in the economy, well above the average multiplier of 1.82 reported in a meta-analysis of maritime multipliers all over the world [START_REF] Jacobsen | A global synthesis of the economic multiplier effects of marine sectors[END_REF]. Three sectors associated as major suppliers of the water transport services really matter: warehousing and support services for transportation, legal and accounting services, coke and refined petroleum products. Altogether with the branch of water transport itself, these 4 sectors cumulate 68% of all backward economic effects of the shipping industry. If the last branch is poorly job-creating domestically and would rather tend to increase the trade deficit of the country, the first two sectors remain interesting to develop a public policy supporting the use of maritime transport because their job basis remains local and weakly redeployable internationally.

As far as the forward linkage index is concerned, the results are just opposite, water transport being ranked at the very bottom of the 64-industry list. This can be considered as a statistical artefact caused by the FOB registration of national exports. For this reason, whatever the domestic or foreign incurrence of freight costs, the sales of domestic water transport services are reported in the export column (i.e. as though all services were purchased by the rest of the world), i.e. representing a final use rather than an intermediate input consumed by other domestic industries. More information is needed to make a clearer interpretation of the results.

Finally, when compared to the strand of research works based on I-O models and dealing with the marine and maritime industries, this study contributes to the empirical IO literature applied to the maritime industry by introducing an original look at the direct and indirect air pollution effects that shipping produces on the environment, in connection with the ongoing discussion about the reduction of greenhouse gas emissions by the International Maritime Organisation (IMO 2014). Considering the environmental impact of water transport on air pollution, we show that water transport stands in the top five most polluting industries out of 64 concerning several pollutants (PM2.5, NOx and SO2). In terms of emission intensities (tonnes per M€ of final use), CO2 emissions is also a major concern, water transport belonging to the top-12 most polluting industries, despite the fact that shipping has only the 42 nd most important final use level out of the 64 national industries.

Since 2015, IMO and thereafter the European Parliament stipulated that all ships in SECA (Sulphur Emission Control Areas, set in 2005 and 2012) should use low sulphur marine fuel not exceeding 0.1% [START_REF] Imo | Third IMO Greenhouse Gas Study. International Maritime Organization (IMO)[END_REF]. It will be interesting to monitor the intensity coefficients shown and total direct and indirect emissions of NOx and SO2 in this article after the new reduction adopted on January 1 st 2020 beyond the SECA limits, where a standard of 0.50% sulphur rate is now imposed instead of the past 3.50% (MARPOL Annex VI). A debate has followed the announcement of the new rule to compare the disadvantage of affected companies which have to comply with stringent regulation compared to their competitors in non-SECA who are not subject to this regulation. Two years after the IMO regulation, the effect has not been as intense as originally expected for both shipowners and ports [START_REF] Cullinane | Emission control areas and their impact on maritime transport[END_REF][START_REF] Olaniyi | Towards EU2020: an outlook of SECA regulations implementation in the Baltic Sea region[END_REF]. However, the new sulphur reduction that is imposed will have a larger impact and push companies to invest in scrubbers, SCR or LNG technology or take other mitigating measures. From our results, most of the GHG emissions, whatever the compound, are concentrated in the water transport activity itself, and more weakly in other industries through the cascading effects. Policy efforts should therefore focus on this particular activity to further reduce the overall emissions rather than spreading the efforts over various industries [START_REF] Bouman | State-of the-art technologies, measures, and potential for reducing GHG emissions from shipping-A review[END_REF]. However, because forward linkages are imperfectly measured (due to the FOB registration of exports), it should be outlined that other industries might also be partly responsible for such emissions because of their long and global logistic supply chains consuming intermediate services of transport, which are not visible in the present domestic I-O framework [START_REF] Cariou | Towards low carbon global supply chains: a multi-trade analysis of CO2 emission reductions in container shipping[END_REF]. Nonetheless, the same EE-IO exercise as the one developed in this article remains an interesting preliminary approach to assess and follow throughout time the coming achievements regarding the reduction of GHG emissions by the shipping industry.

Fig. 1 .

 1 Fig. 1. Structure of employment in the French maritime industry

Figure 2 .

 2 Figure 2. Main industries benefitting from an increased demand for water transport in 2014 (in terms of employment); own calculation from authors

Table 1 and

 1 Table2show the backward and forward linkage indices of the top ten national industries ranked by decreasing order of Backward Linkage Index (BLI) in 2016. From Table1, one can see that the backward linkage of water transport services is between 1.21 and 1.30 in regular years. The significantly lower value at the beginning of the period is clearly caused by the decreasing demand which has followed the financial crisis of 2008. In 2016, water transport services reached the second highest backward linkage of all 64 industries(1.30). In other words, the maritime transport industry displays a certain capacity to pull other industries upwards by its high level of intermediate consumptions.

Table 1 . Backward Linkage Index (BLI).

 1

		2010	2011	2012	2013	2014	2015	2016
	Industry	Value	Rank	Value	Rank	Value	Rank	Value	Rank	Value	Rank	Value	Rank	Value	Rank
	Insurance,	1.34	1	1.32	1	1.36	1	1.34	1	1.37	1	1.37	1	1.37	1
	reinsurance and
	pension funding
	services
	Water transport	1.11	13	1.27	3	1.21	5	1.23	4	1.21	5	1.27	3	1.30	2
	services
	Wood and of	1.24	5	1.23	5	1.22	4	1.22	5	1.27	2	1.22	5	1.27	3
	products of wood
	and cork,
	Food, beverages	1.26	3	1.27	4	1.27	3	1.27	2	1.27	3	1.26	4	1.27	4
	and tobacco
	products
	Travel agency, tour	1.27	2	1.30	2	1.29	2	1.26	3	1.25	4	1.30	2	1.24	5
	operator and other
	reservation services

11

https://ec.europa.eu/eurostat/web/environment/emissions-of-greenhouse-gases-and-air-pollutants/air-emissionsaccounts

12

Table [naio_10_cp1700] extracted from https://ec.europa.eu/eurostat/fr/data/database 13 https://www.citepa.org/fr/activites/inventaires-des-emissions/namea

14

Table [env_ac_ainah_r2] extracted from https://ec.europa.eu/eurostat/fr/data/database.

Table 2 . Forward Linkage Index (FLI)

 2

	2010	2011	2012	2013	2014	2015	2016

  R Consequently, even if the water transport industry seems to be poorly supportive of other industries by the forward linkage effects, it plays a significant role in the French economy by boosting other activities through its backward linkage effects. This result is totally in line with what has been observed in other domestic economies[START_REF] Wang | The role of the marine industry in China's national economy: an inputoutput analysis[END_REF][START_REF] Kwak | The role of the maritime industry in the Korean national economy: an input-output analysis[END_REF][START_REF] Morrissey | The potential for an Irish maritime transportation cluster: An input-output analysis[END_REF].

	estimated.
	Legal and accounting	2.80	1	2.91	1	2.94	1	3.03	1	3.10	1	3.14	1	3.11	1
	services; services of head
	offices; management
	consult. services
	Wholesale trade services,	2.47	2	2.40	2	2.42	2	2.45	2	2.47	2	2.47	2	2.47	2
	except of motor vehicles
	and motorcycles
	Financial services, except	2.14	3	2.03	4	2.09	4	2.13	3	2.21	3	2.17	3	2.21	3
	insurance and pension
	funding
	Electricity, gas, steam and	2.11	4	2.09	3	2.11	3	2.08	4	2.03	4	1.98	4	2.03	4
	air conditioning
	Security and investigation	1.92	5	1.87	5	1.88	5	1.86	5	1.77	5	1.70	5	1.78	5
	services; services to
	buildings and landscape;
	office admin., etc.
	Warehousing and support	1.73	7	1.61	7	1.62	7	1.63	7	1.65		1.65	7	1.67	6
	services for transportation
	Real estate services	1.59	6	1.67	6	1.69	6	1.67	6	1.65	6	1.66	6	1.65	7
	excluding imputed rents
	Rental and leasing services	1.35	8	1.33	8	1.34	8	1.32	8	1.33	7	1.33	9	1.33	8
	Services auxiliary to	1.28	11	1.26	11	1.30	10	1.28	10	1.31	8	1.34	8	1.31	9
	financial services and
	insurance services
	Food, beverages and	1.24	13	1.26	12	1.27	11	1.29	9	1.30	9	1.27	10	1.30	10
	tobacco products

Table 3 . Output multipliers (direct + indirect impacts)

 3

	2010	2011	2012	2013	2014	2015	2016

Table 5 . Air pollution multipliers and total emissions from water transport

 5

	a) in 2014
		CO2	CH4	NOx	PM10	PM2.5	SO2
	Multiplier	0.30	0.34
	(Rank/64)	(9)

Classified in three sectors: marine fishery, marine contraction and marine transportation.

https://unstats.un.org/unsd/envaccounting/seearev/seea_cf_final_en.pdf

In 2016, the direct output effect was 9.6 billion euros and the indirect effect was 20.3 billion euros.

The employment effects have been calculated after splitting the maritime transport industries into two subsectors, freight water transport services and passenger water transport services.

http://www.imo.org

http://www.dirm.sud-atlantique.developpement-durable.gouv.fr/IMG/pdf/decret_2017-222_du_23fevrier2017-snml.pdf

[START_REF] Olmer | Greenhouse gas emissions from global shipping, 2013-2015[END_REF] Such an impact on regional consumptions stems from the Keynesian (income) multiplier effect which has not been estimated in the present analysis but could represent an interesting extension of this work.

We hope that other studies will attempt to replicate a similar methodology for some other countries. We also believe that this type of research would benefit from being developed in a multi-regional or inter-country input-output framework for such a global industry [25]. By doing so, the GHG footprint and other types of footprints (e.g. water, energy, materials, residuals…) would be re-affected to the countries where the demand for shipping services originates, instead of estimating an inventory based on the domestic output only. Such a study would probably require an international research endeavour to re-allocate the shipping services between importing and exporting countries.

/maritimeaffairs/policy/documents/ 2 Declaration of the European Ministers responsible for the Integrated Maritime Policy and the European Commission, on a Marine and Maritime Agenda for growth and jobs the "Limassol Declaration", 7 Oct. 2012, https://ec.europa.eu/maritimeaffairs/sites/maritimeaffairs/files/docs/body/limassol_en.pdf

trade database. http://ec.europa.eu/eurostat/web/international-trade

Because input coefficients can fluctuate from year to year, a Monte Carlo approach based on random values drawn from a Normal law (with a standard-deviation of 20%, which is tremendous) was used for the main 13 input coefficients. With 1000 random trials, the output multiplier of water transport has a 90% probability of being found between 1.90 and 2.13 over the full period).

Note: sectors are ranked by decreasing order of output multipliers in 2016.

The final demand of water transport services industry has grown from 12 billion euros in 2010 to 14 billion euros in 2016. Consequently, the total output effect created by the domestic maritime transport industry has increased from €23 billion in 2010 to €30 billion in 2016 16 (Table 4). Only four industries cumulated 68% of the maritime transport's output-inducing effects, of which primarily the branch itself with 49%. Warehousing and support services for transportation, legal and accounting services and the sector of coke and refined petroleum products have also benefitted from the boom of maritime transport, but to a much lesser extent. This indicates that the costs of these four industries make the largest share of production costs for the maritime transport industry.


C Fellah

J Braun

C Sauder

F Sirotti

M.-H Berger

Clémentine Fellah

Influence of the carbon interface on the mechanical behavior of SiC/SiC composites

Keywords: Ceramic-matrix composites (CMCs), Interface/Interphase, Microstructural analysis

SiC/SiC composites reinforced with 3 rd generation SiC fibers (Hi-Nicalon S and Tyranno SA3 fibers) are promising candidates for thermomechanical applications in high technology industries. Both composites exhibited a pseudo-ductile mechanical behavior but the HNS/PyC/SiC composite reaches higher failure strains than TSA3/PyC/SiC ones. The mechanical behavior of SiC/SiC composites is linked to the way PyC is bonded to the fiber surface. Analyses have shown that these interactions and the Fiber/Matrix debonding behavior depend strongly on the nature of the carbon on the SiC fiber surface, which is different according to the SiC fiber. In order to understand the mechanism governing the chemical adhesion at the PyC/SiC fiber interface, the formation, the chemistry and the structure of the surface carbon layer were studied. Understanding the origin of this carbon has allowed elucidating the local interaction mechanisms of the studied SiC/SiC composites.

Introduction

Ceramic matrix composite materials have aroused interest for aircraft engines as well as for the improvement of the nuclear reactors safety, especially for core materials such as cladding and structural materials [START_REF] Bansal | Ceramic Matrix Composites: Materials, Modeling and Technology[END_REF]- [START_REF] Krenkel | Ceramic Matrix Composites: Fiber Reinforced Ceramics and Their Applications[END_REF]. Among them, SiC fiber-reinforced SiC matrix composites (SiC/SiC) are promising candidates to replace metallic alloys for these applications. These thermostructural composites have to bear high temperatures and strong mechanical stresses. However, both SiC fibers and SiC matrix are brittle ceramics and their combination in SiC/SiC composites find technological applications only if the composite is damage tolerant and can acquire a pseudo-ductile mechanical behavior [START_REF] Naslain | SiC-Matrix Composites: Nonbrittle Ceramics for Thermo-Structural Application[END_REF]- [START_REF] Naslain | The design of the fibre-matrix interfacial zone in ceramic matrix composites[END_REF].

The ability of SiC/SiC composites to sustain damage is controlled by the fiber/matrix (F/M) coupling mode [START_REF] Grathwohl | Fibrereinforced SiC-matrix composites: Microstructure, interfaces and mechanical properties[END_REF]. The F/M coupling should neither be too strong to prevent matrix cracks propagation through the fibers, nor too weak to allow the load transfer from the matrix to the fibers. In this way, an interphase is deposited on the fibers prior to the matrix infiltration to control this fiber/matrix coupling and provide the damage tolerance of SiC/SiC composites [START_REF] Inghels | Caractérisation de l'endommagement de composites tissés à matrice SiC[END_REF].

This layer can be made of pyrolytic carbon (PyC) [START_REF] Katoh | Mechanical properties of advanced SiC fiber composites irradiated at very high temperatures[END_REF], [START_REF] Oberlin | Carbonization and graphitization[END_REF]. The pyrocarbon can provide the suitable bonding strength level to allow load transfer and its structure acts as a fuse regarding the crack propagation. In this way, crack deflection at the SiC/SiC composites interface is favored [START_REF] Naslain | The concept of layered interphases in SiC/SiC[END_REF]. Two 3 rd generation SiC fibers are chosen for this study, the Hi-Nicalon S (HNS) and the Tyranno SA3 (TSA3) fibers (resp. manufactured by NGS Advanced Fibers and Ube Industries) [START_REF] Hinoki | The effect of high dose/high temperature irradiation on high purity fibers and their silicon carbide composites[END_REF], [START_REF] Katoh | Effect of neutron irradiation on tensile properties of unidirectional silicon carbide composites[END_REF]. From these fibers two composites have been manufactured by CVI (Chemical Vapor Infiltration) using the same process parameters to deposit first a pyrocarbon interphase around the fibers and then the SiC matrix. These SiC/SiC composites have two distinct mechanical behaviors and F/M couplings depending on the nature of their reinforcement [START_REF] Buet | Influence of chemical and physical properties of the last generation of silicon carbide fibres on the mechanical behaviour of SiC/SiC composite[END_REF]. The HNS/PyC/SiC composites exhibit a high damage tolerance, a pseudo-ductile mechanical behavior, resulting from a balanced F/M coupling. On the contrary, the TSA3/PyC/SiC composites have a too strong F/M coupling and are less resistant to mechanical loading that prohibits their use for the aimed applications [START_REF] Sauder | Mechanical Properties of Hi-NICALON S and SA3 Fiber Reinforced SiC/SiC Minicomposites[END_REF]. However, the TSA3 fibers provide better thermal properties to the composite by their thermal conductivity, justifying the interest in the TSA3-based composites [START_REF] Hasegawa | Critical issues and current status of SiC/SiC composites for fusion[END_REF].

The distinct F/M coupling strengths of SiC/SiC composites were attributed to differences in the SiC fibers surface characteristics. The intensity of the F/M coupling could be influenced by the fiber surface roughness. The TSA3 fiber surface roughness is three times superior to the HNS fibers one [START_REF] Buet | Influence of chemical and physical properties of the last generation of silicon carbide fibres on the mechanical behaviour of SiC/SiC composite[END_REF]. Matrix cracks generate frictions at the F/M interface with sliding and debonding. A high roughness would cause a strong interfacial sliding resistance [START_REF] He | Crack deflection at an interface between dissimilar elastic materials: Role of residual stresses[END_REF]. However, previous studies have shown that the SiC fiber surface morphology is not the only key factor controlling the F/M coupling intensity [START_REF] Buet | Influence des caractéristiques de surface des fibres de type Hi-Nicalon S et Tyranno SA3 sur le comportement mécanique des composites SiC/SiC[END_REF]. The interfacial interactions in SiC/SiC composites between the pyrocarbon and the fiber surface must also be influenced by the microstructure and the chemistry of the SiC fiber surfaces [START_REF] Sauder | Influence of Interface Characteristics on the Mechanical Properties of Hi-Nicalon type-S or Tyranno-SA3 Fiber-Reinforced SiC/SiC Minicomposites: Influence of Interface Characteristics on the Mechanical Properties[END_REF]. The local interaction mechanisms are still not well understood. Previous works have identified differences in the structure and chemistry of the carbon rich layer in the first nanometers below the surfaces of the two fibers [START_REF] Buet | Influence of chemical and physical properties of the last generation of silicon carbide fibres on the mechanical behaviour of SiC/SiC composite[END_REF].

Several hypotheses about the origin of the surface carbon have been discussed. This carbon could be formed during the manufacturing of the fibers. The carbon excess in HNS and TSA3 fibers is due to the side methyl groups found in polycarbosilane precursors used for these SiC fibers. Carbon Basic Structural Units (0.7 nm) are observed within a Si-C-(O) amorphous phase at the initial stages of the ceramization [START_REF] Bodet | Tensile creep behaviour of a silicon carbide-based fibre with a low oxygen content[END_REF]. During the crystallisation and growth of SiC grains, the free carbon grows by epitaxy on the SiC grains facets to form turbostratic carbon layers with their (002) planes parallel to the {111} planes of β-SiC grains. This turbostratic carbon is found within the fibers (at the SiC grain boundaries) and also on their extreme surface, i.e. in the near-surface first nanometers, covering SiC surface grains.

The surface carbon could also be formed by the pyrolysis of the sizing polymer, first step of the composites processing. The as received commercial fibers are coated by a thin polymer layer to protect them and ease the textile shaping processes. A thermal treatment is performed to remove the sizing on the fibers before the manufacturing steps of the SiC/SiC composites. However, carbonaceous residues could remain at the fiber surface after this treatment and contribute to the carbon layer rich extreme surface [START_REF] Thomason | The use of XPS for characterisation of glass fibre coatings[END_REF].

This work aims to elucidate the local bonding modes governing the F/M coupling and damage mechanisms at the F/M interface of these two composites. Firstly, the thermal decompositions of the sizing polymers have been studied using thermogravimetric analysis and X-Ray Photoelectron Spectroscopy to investigate its contribution to the formation of the carbon rich surfaces on the two SiC fibers. Then the interfacial regions have been characterized using High Resolution Transmission Electronic Microscopy (HRTEM) and their structural modifications after mechanical loading up to failure have been followed. In addition composite without PyC interfaces (SiC/SiC) have been prepared for comparison, analyzed by HRTEM before and after mechanical loading. The understanding of the origin, the role and the formation mechanisms of the carbon on the fiber surface opens up the possibility to design a surface treatment to improve the mechanical behavior of the TSA3-based SiC/SiC composites.

Experimental procedures

Materials

The two nearly stoichiometric commercial fibers, the Hi-Nicalon S (HNS) and Tyranno SA3 (TSA3) have mean diameters of 13 µm and 10 µm, C/Si atomic ratios of 1.02 and 1.08 and low oxygen levels of 0.2 and 0.3% at., respectively. Table 1 summarizes the main characteristics of the two fibers, as given by the manufacturers. These fibers are highly crystalline and the excess carbon forms turbostratic intergranular pockets [START_REF] Bunsell | Fine diameter ceramic fibres[END_REF], [START_REF] Dong | Characterization of nearly stoichiometric SiC ceramic fibres[END_REF]. The HNS and TSA3 fibers have high Young's moduli (373 and 368 GPa respectively), a tensile strength ranging from 2.24 to 3.01 GPa and a failure strain about 0.7% [START_REF] Buet | Influence of chemical and physical properties of the last generation of silicon carbide fibres on the mechanical behaviour of SiC/SiC composite[END_REF], [START_REF] Sauder | Influence of Interface Characteristics on the Mechanical Properties of Hi-Nicalon type-S or Tyranno-SA3 Fiber-Reinforced SiC/SiC Minicomposites: Influence of Interface Characteristics on the Mechanical Properties[END_REF].

The HNS fibers are characterized by a homogeneous chemical composition from the core to the surface except at the extreme surface where a carbon enrichment is observed [START_REF] Sauder | Ceramic Matrix Composites: Nuclear Applications[END_REF]. On the contrary, the TSA3 fibers contain more free carbon in the core than at the surface (from 16% at. to 4% at., respectively). This gradient originates from the synthesis of the TSA3 fibers derived from a Si-Al-C-O precursor [START_REF] Ichikawa | Development of High Performance SiC Fibers Derived from Polycarbosilane Using Electron Beam Irradiation Curing-A Review[END_REF], [START_REF] Yang | New tyranno-SA fiber reinforced CVI-SiC/SiC composite[END_REF]. The chemistry of the TSA3 precursor also induces the presence of about 1000 ppm of aluminum and 3000 ppm of bore in the ceramic fiber [START_REF] Berger | Fine Ceramic Fibers[END_REF], [START_REF] Ishikawa | High-strength alkali-resistant sintered SiC fibre stable to 2,200[END_REF]. The TSA3 grain size ranges from 200 to 400 nm, whereas the HNS one lays between 10 and 50 nm. The higher surface roughness of TSA3 fibers is directly related to its higher grain size. It also explains its higher thermal conductivity compared to the HNS fibers (65 against 18 W.m.K -1 for HNS fibers at 25°C). This makes TSA3 fiber more attractive for thermal applications in extreme thermal conditions [START_REF] Hasegawa | Critical issues and current status of SiC/SiC composites for fusion[END_REF]. Both SiC fibers are protected by a polymer layer: the poly(vinyl alcohol) (PVA) for HNS fibers and the polyoxyethylene (POE) for TSA3 fibers.

Properties

Hi-Nicalon S (HNS) Tyranno SA3 (TSA3) Batch 520226 S1F08PX

Fiber diameter (µm) [START_REF] Inghels | Caractérisation de l'endommagement de composites tissés à matrice SiC[END_REF]

Thermal analysis

The thermal decomposition of the fiber sizing was followed by a thermogravimetric analyzer (SETSYS-Setaram Evolution Analytical). The experimental parameters for these analyses were close to the desizing ones and were carried out between 10 -3 -10 -5 mbar with a thermal ramp of 2.5°C/min up to 900°C followed by a 2 hours' isotherm at this temperature. The sizing decomposition was also analyzed with a thermal cycle including a 48 h isotherm at 900°C to detect any further weight evolution at longer time scales. The weight changes obtained during thermogravimetric analysis were determined relatively to the theoretical weight of the sizing initially covering the HNS and TSA3 fibers. These tests were repeated five times for each fiber type to ensure the reproducibility of the measurements. Changes in the morphology of the fiber surfaces after these heat treatments were observed by SEM (SEM FEI Nova NanoSEM 450) coupled to an EDS detector at 5 keV.

Chemical analysis

High Resolution X-Ray Photoemission Spectroscopy was used to study the chemistry and bonding configuration of the HNS and TSA3 fibers surface. These analyses were performed on the TEMPO beamline (Time resolved Experiments on Materials with Photoelectron Spectroscopy) of SOLEIL synchrotron [START_REF] Polack | TEMPO: a New Insertion Device Beamline at SOLEIL for Time Resolved Photoelectron Spectroscopy Experiments on Solids and Interfaces[END_REF]. The synchrotron beam had a diameter of 100 µm for the study and its energy could be tuned at 400, 700 (baseline energy for the experiment) and 1000 eV to probe any changes in chemistry in the near surface region from 2 to 6 nm below the surface. This probing depth is considered to be three times the inelastic mean free path of the photoelectrons [START_REF] Jablonski | NIST Electron Inelastic-Mean-Free-Path Database[END_REF]. Prior to these analyses, the desized fibers were heat treated at about 900°C within the TEMPO line to get closer to the surface configuration after desizing.

Carbon to silicon ratio was deduced from XPS spectra recorded in the 0-600 eV binding energy range and high-resolution spectra of carbon core level (C1s peak) were recorded and fitted using XPS CASA software. The XPS lineshape was represented by a symmetrical Gaussian-Lorentzian function. The Csp 2 component was fitted by an asymetrical model (Doniach-Sunjic function). A Shirley background substraction was applied for the high-resolution spectra.

Mechanical tests

Room-temperature tensile tests were carried out on SiC/SiC plates using an Instron 2404 tensile testing machine. Plates were 55 mm length, 8 mm width and 1 mm thick. The gauge length was 25 mm. Aluminium end tabs were glued (3M Scotch-Weld TM 9323 B/A), on both sides of the specimens. A 25 kN load cell was used, crosshead displacement rate was 0.05 mm/min to 0.2 mm/min and specimen elongation was measured with an extensometer fixed onto the plates (Instron 2620-603). The displacement speed was raised during loading-unlodaing tensile tests to limit the duration of tests. There was no influence of displacement rate in this range (0.05 to 0.2mm/min). The fracture surfaces were examined by scanning electron microscopy (SEM FEI Nova NanoSEM 450).

Microstructural analysis

The SiC fiber extreme surfaces and the interphase regions of SiC/ (PyC/) SiC composites were After mechanical tests of SiC/SiC composites, many cracks have propagated close to the surface of the fibers. Using the FIB has allowed to extract a region where cracks have propagated between the fiber and the matrix (figure 2). The samples were previously mounted with epoxide resin and polished. The mounting resin has impregnated even within cracks from mechanical tests. The thin foil is extracted from the sample following the lift-out method [START_REF] Giannuzzi | A review of focused ion beam milling techniques for TEM specimen preparation[END_REF] and thinned down to 100 nm using the gallium ion beam.

Characterization of the fibers surface

Thermal and chemical analysis

The temperature profile and the weight changes of the sized fibers during the thermogravimetric analysis are represented on Figure 3a.

Successive weight losses were identified on the weight change profile of the HNS fibers and could be correlated to the thermal decomposition of their sizing polymer (PVA) [START_REF] Holland | The thermal degradation of poly(vinyl alcohol)[END_REF]- [START_REF] Pogue | Evaluation of High Molecular Weight Species Produced during Thermal Degradation of Poly(Vinyl Alcohol)[END_REF]:

-From 150°C to 230°C for a weight loss of 25 %. The first mass loss refers to the dehydration of PVA. Water molecules are removed to lead to the formation of conjugated polyene macromolecules around 200°C [START_REF] Gilbert | Carbonization of polymers I-Thermogravimetric analysis[END_REF].

-From 240°C to 450°C for a weight loss of 50 %. The C-C bonds scission of the PVA main chain results in the formation of terminal carbonyls groups between 300°C and 500°C [START_REF] Tsuchiya | Thermal decomposition products of polyvinyl chloride[END_REF].

The final weight loss was equivalent to 55 % of the sizing amount present initially on the HNS fibers (1.13 %wt of HNS fibers). The sizing was not completely decomposed and evaporated on the HNS fibers at 900°C on contrary to the TSA3 fibers. The final weight loss for the TSA3 fibers was 93 % relative to the initial amount of the sizing (0.59 %wt of TSA3 fibers). The decomposition of the TSA3 sizing polymer (POE) occurred in a single step between 290°C and 360°C [START_REF] Choukourov | Vacuum Thermal Degradation of Poly(ethylene oxide)[END_REF]- [START_REF] Madorsicy | Thermal degradation of polyethylene oxide and polypropylene oxide[END_REF].

The thermal decompositions of the HNS and TSA3 fiber sizings were followed thereafter with a thermal cycle including an isotherm at 900°C during 48 hours (Figure 3b.). This extended isotherm revealed if the mass loss, obtained at the end of the previous thermogravimetric test, continued to evolve or was stabilized. Based on figure 3b, this thermal cycle applied the HNS fiber sizing did not allow to decrease below 50 % the side products of the sizing and to decompose and evaporate completely the sizing, whereas the POE was well eliminated from the TSA3 fibers. The fibers surface chemistries were analysed after desizing. Photoemission spectra obtained at 700 eV are shown in figure 4a. and the evolution of the C/Si ratio according to the beam energy in figure 4b. These analyses indicated that the carbon was the predominant element on the extreme surface of both desized fibers. Oxygen, silicon and calcium were also detected (from O1s, Si2p and Ca2p peaks). Calcium originated from the aqueous bath in which the sizing was added. The surface composition of the HNS fibers was similar from a probed region to another one for a given beam energy. However, the carbon concentration, and so the C/Si, decreased with the probed thickness (i.e. higher incident energy) due to the contribution below the surface of the silicium carbide. The C/Si ratio of the TSA3 fibers surface was lower than the HNS one. The C1s peak deconvolution (figure 5) revealed different contribution of carbon bonds (Csp 2 , Csp 3 , C-O, C-Si, SiCxOy) for the HNS and TSA3 fibers as shown in table 2. The sp 2 hybridized C-C was the major C bonding mode for both SiC fiber surfaces. This contribution is even higher for the TSA3 fibers (~ 80 % of C at.) than for the HNS fibers (~ 70 % of C at.), this could be associated with the higher concentration of turbostratic carbon on the TSA3 fiber surface [START_REF] Bianco | All in the graphene family -A recommended nomenclature for two-dimensional carbon materials[END_REF].

The fraction of sp 3 hybridized carbon was higher on the HNS fiber surface (19 % of the carbon bonds). This carbon layer was therefore not completely composed of graphitic units and could be closer to the structure of some amorphous carbons [START_REF] Robertson | Amorphous carbon[END_REF]- [START_REF] Beghi | Bonding and mechanical properties of ultrathin diamond-like carbon films[END_REF].

In the HNS fiber, around 3.5% of near surface carbon was engaged in a silicon oxycarbide phase (SiCxOy), whereas silicon carbide (SiC) did not contribute to the extreme surface composition probed. On the contrary, for the TSA3 fiber, the contribution of the Si-C bonds had to be added for the deconvolution of its C1s peak. The Si-C bonds and SiCxOy were observed on all scales of depth of these analyses, ranging from 4 to 8 nm. The carbon rich layer on TSA3 fiber surface was thinner than the HNS fiber one and so, SiC grains were closer the extreme fiber surface. The C-O bonds of both SiC fiber surfaces were hydrocarbon pollutants that could be deposited when the fibers are exposed to air [START_REF] Barr | Nature of the use of adventitious carbon as a binding energy standard[END_REF].

Microstructural analysis

The desized HNS fibers observed by SEM, exhibited many surface heterogeneities (figure 6a.) that could explain the incomplete weight loss obtained at the end of the previous thermogravimetric analyses and the photoemission results with ~19% of sp 3 carbon bonds.

These non-spherical clusters could have dimensions going hundreds of nanometers to one micrometer. EDX analysis at 5 keV made on the largest one revealed mainly carbon. Their spatial distribution was rather heterogeneous. When observed in TEM (Figure 6b.), and HRTEM (Figure 6c.), these carbon particles exhibited an amorphous structure. An amorphous outer carbon layer, approximately ten nanometers thick, was also noticed (Figure 6d.). The carbon layer and clusters had a similar structural organization responsible for the sp 2 /sp 3 carbon detected by photoelectron spectroscopy. The HRTEM of fig 7 shows a few carbon stacks exceeding three graphene layers, located solely on the surface of SiC grains. This carbon originated from the excess carbon of the fiber before its sizing. The carbon attributed to the desizing process showed some stacks of 2 to 3 few graphene sheets associated with a lateral lenght (La) not exceeding 0.8 nm. They corresponded to the early stage of carbon organization, described by M. Guigon [START_REF] Guigon | Relation entre la microtexture et les propriétés mécaniques et électriques des fibres de carbone ex-polyacrylo-nitrile[END_REF] as basic structural units (BSU) from 8 to 12 Å of lateral length and at least two fringes.

Figure 7. HRTEM image of HNS desized fiber

No clusters of amorphous carbon were observed on the desized TSA3 fiber surface (Figure 8a.).

Discontinuous stripes of turbostratic carbon were found on the surface of some SiC grains, made of ~ 10 graphen layers (Figure 8b.). These observations confirm the high sp 2 carbon contribution of C1s peak obtained by high resolution photoemission spectroscopy. The nucleation of the carbon on the TSA3 fiber surface has not occurred on all the SiC grain facets.

This explains why the Si-C bonds were highlighted even for the lowest probed depths.

Microstructural analysis of the F/M interface of SiC/SiC and SiC/PyC/SiC composites 3.2.1. SiC/SiC and SiC/PyC/SiC composites reinforced by HNS fibers

When the desized HNS fiber was incorporated in a SiC matrix without PyC interphase, the two structures of carbon described above for the desized fiber could be recognized at the fiber/matrix interface (figure 9a.): the carbon rich amorphous layer and the turbostratic carbon clusters associated to the free carbon which developped preferentially parallel to the {111} plans of the SiC grains. When a PyC interphase was added on the desized HNS fiber before the matrix infiltration, the interphase was deposited on the amorphous carbon layer previously described. Most of the PyC stacks were parallel to the HNS fiber axis (figure 9b.) except locally for the very first layers deposited close to the amorphous carbon layer. The HNS/SiC and HNS/PyC/SiC composites have equivalent Young's moduli, the PyC contribution on the total stiffness of these composites being insignificant. The HNS/PyC/SiC composites have higher failure strain and stress compared to HNS/SiC due to the PyC interphase. More surprisingly HNS/SiC composites also exhibit a pseudo-ductile mechanical behavior. Such behavior was previously attributed to the PyC interphase in HNS/PyC/SiC composites [START_REF] Sauder | Influence of Interface Characteristics on the Mechanical Properties of Hi-Nicalon type-S or Tyranno-SA3 Fiber-Reinforced SiC/SiC Minicomposites: Influence of Interface Characteristics on the Mechanical Properties[END_REF], [START_REF] Bertrand | SiC/SiC minicomposites with nanoscale multilayered fibre coatings[END_REF], [START_REF] Lamon | Microcomposite Test Procedure for Evaluating the Interface Properties of Ceramic Matrix Composites[END_REF]. Thus, contrary to what was generally presupposed for CMC reinforced by SiC fibers of the 3 rd generation, the deposition of an interphase is not a necessary condition for the development of micromechanisms leading to pseudoductility. The PyC interphase, however, has a positive impact on the reduction of the dipersion in the failure strains. The failure strain of TSA3/PyC/SiC composites reached 0.22% while the TSA3/SiC one did not exceed 0.05%, corresponding to the failure strain of monolithic SiC [START_REF] Snead | Handbook of SiC properties for fuel performance modeling[END_REF]. The presence of the PyC allowed a pseudo-ductile behavior to be obtained for the TSA3/PyC/SiC composites although the failure strain and stress of TSA3/PyC/SiC composites were lower than those of the HNS/PyC/SiC composites. The dispersion of the principal mechanical characteristics was important for the TSA3/PyC/SiC composite. No fiber pull-out were observed on the fracture surfaces in the TSA3/SiC composite and pull out was limited in the TSA3/PyC/SiC composite (figures 12b. and 12c.), leading to macroscopic flat failure surfaces, which is consistent with the observed mechanical behaviour.

Discussion

The carbon-rich extreme surfaces of the HNS and TSA3 fibers have different compositions and type of carbon bonds. These differences are not only due to the variations of fiber microstructures. The composition of their sizing polymers and their thermal decomposition mechanisms must be also considered.

Carbonaceous residues, products of PVA pyrolysis, are observed on the HNS fibers surface after desizing. A pure PVA polymer should decompose completely by heat treatment. These residues and incomplete weight loss observed during thermogravimetric analyses suggest that the chemical composition of the sizing on HNS fibers was probably not that of the pure polymer.

The size disparities of these carbon residues result from heterogeneities in the thickness of the deposited PVA, linked to the sizing methodology. The HNS fiber surface is also covered by carbon layer having a similar structure than that of the carbon residues. Not being observed on HNS sized fiber surfaces, this carbon layer originates from the desizing step. The surface carbon is mainly engaged in sp 2 bonds but also for ~19 % of the atoms in sp 3 bonds. Its structure is close to that frequently observed for amorphous carbon obtained by the thermal decomposition of a polymer at high temperature and is distinct from that of the turbostratic carbon of the fiber or the PyC of the interphase.

The thermal decomposition of POE (sizing of TSA3 fibers) does not produce carbon residues on the contrary to the HNS fiber sizing. A turbostratic carbon layer is observed on the desized TSA3 fibers surface with a majority of sp 2 hybridized C-C bonds detected by photoelectron spectroscopy. The carbon layer is not continuous leaving SiC grains uncovered on the extreme surface. This carbon rich surface is already present before the desizing and originates from the chemistry of the SiC fibers precursor. The -CH3 groups of the polycarbosilane precursor are responsible for the free carbon that develops in turbostractic clusters by epitaxy on the {111} sides of the SiC grains. This turbostratic carbon is also detected on the SiC grains surface of the HNS fibers, covered by the amorphous carbon after desizing, but the pyrolysis in hydrogen atmosphere during the last step of the HNS fibers manufacturing reduces its amount. In addition the larger SiC grains size of the TSA3 fibers promotes the growth of a larger turbostratic carbon layer on their wider facets.

Two composites reinforced by HNS and TSA3 fibers were produced using the same 2.5D

weaving texture and infiltration parameters of PyC interphase and SiC matrix. In the two cases the very first PyC planes are difficult to localize. For the HNS, these planes are bonded to an amorphous carbon layer covering the HNS fiber, with a possible transition toward the highly anisotropic PyC. In the TSA3 they are mainly bonded to the turbostractic carbon ribbons covering the SiC surface grains and they follow their orientations from the first PyC layers. This study has revealed the major role played by a weakly organized and homogeneous carbon phase in the damage tolerance of the composites. A surface treatment of TSA3 fibers will be undertaken to obtain this amorphous carbon layer similar to that found on the HNS fibers. This layer is expected to screen the carbon turbostratic on TSA3 fibers surface, which induced strong interfacial interactions with the pyrocarbon, and to lead to an ideal F/M coupling for the TSA3/PyC/SiC composites.

Conclusion

We have studied the origin, the composition and the microstructure of the carbon-rich extreme The properties of the surface carbon are then essential and depend on the nature of the reinforcement. In one case, the desizing of HNS fibers leads to carbonaceous residues, giving rise to an amorphous carbon layer covering completely the fibers. The TSA3 fibers surface is composed of turbostratic carbon formed during the fiber ceramisation step.

The amorphous layer, left by the thermal decomposition of PVA on HNS fibers, has a crucial role in the optimization of the mechanical behavior. Indeed, the matrix cracks are always deflected at the carbon surface.

Therefore, the weaker interface of these composites is between the carbon layer and either the PyC interphase or the SiC matrix. The damage tolerance of the composites reinforced by HNS is not only brought by the PyC interphase, but also by the amorphous carbon. The bond between amorphous carbon and PyC plays the role of the mechanical fuse whereas the pyrocarbon decrease the fiber/matrix friction.

The presence of PyC interphase is mandatory for composite reinforced by the TSA3 to bring some damage tolerance but without reaching that of HNS/PyC/SiC composite or even HNS/SiC. The strong bonding between PyC and turbostratic carbon explains this specific behaviour. We show that a weakly organized carbon layer is the key for higher damage tolerance of SiC/SiC composites and it is expected that a chemical treatment on the TSA3 would result in an improved pseudo-ductile behavior for TSA3/PyC/SiC composites.

Figure 1 .

 1 Figure 1. TEM cross-section of the interface of SiC/PyC/SiC composites reinforced by HNS fibers (a.) and TSA3 fibers (b.)

  investigated by Transmission Electron Microscopy (TEM, FEI Tecnai F20 ST operating at 200 kV). Carbon (002) lattice fringe images were recorded to investigate the structural organization of the carbon-rich surfaces on the fibers and the composite interface regions before and after mechanical loading. Thin foils of the surface of the desized SiC fibers were prepared by Focused Ion Beam (MEB-FIB Zeiss Auriga 40) and those of SiC/SiC composites by Ion Slicer (Jeol IS).

Figure 2 .

 2 Figure 2. SEM image of FIB thin foil around an extracted fiber in a SiC/SiC composite

Figure 3 .

 3 Figure 3. Thermogravimetric analysis of the HNS and TSA3 sized fibers with an isotherm at 900°C during 2 hours (a.) and 48 hours (b.)

Figure 4 .

 4 Figure 4. Overview spectra of the surface of the HNS and TSA3 desized fibers at 700 eV (a.) and C/Si ratio of the HNS and TSA3 desized fibers relative to the energy beam (b.)

Figure 5 .

 5 Figure 5. High-resolution spectra of carbon (C1s peak) of the surface of the HNS (a.) and TSA3 (b.) desized fibers at 700 eV

Figure 6 .

 6 Figure 6. SEM image of a desized HNS fiber (a.), TEM cross-section of the surface of a HNS desized fiber (b.), HRTEM image of the carbon cluster (c.) and the carbon layer (d.) on the surface

Figure 8 .

 8 Figure 8. SEM image of TSA3 desized fibers (a.), HRTEM image of the surface of TSA3 desized fiber (b.)

Figure 9 .

 9 Figure 9. HRTEM images of the F/M interface of the HNS/SiC composite (a.) and of the HNS/PyC/SiC composite (b.)

Figure 10 .

 10 Figure 10. HRTEM image of the F/M interface of the TSA3/SiC composite (a.) and of the TSA3/PyC/SiC composite (b.)

Figures

  Figures 11b. and 11c. display SEM images of the entire fracture surfaces of the HNS/SiC and HNS/PyC/SiC composites after tensile loading. Numerous HNS fibers pull-outs were observed for both composites, confirming the pseudo-ductile mechanical behavior for both HNS reinforced composites, behavior previously attributed to the PyC in HNS/PyC/SiC composites.

Figure 11 .Figure 12 .

 1112 Figure 11. Tensile stress-strain curves for HNS reinforced composites (a.), fracture surfaces of HNS/SiC (b.) and HNS/PyC/SiC (c.) by SEM

Figure 13 .

 13 Figure 13. Damage mechanisms of HNS/SiC observed by TEM (a.) and HRTEM (b.) and damage mechanisms of HNS/PyC/SiC observed by TEM (c.) and HRTEM (d.) after tensile test

Figure 14 .

 14 Figure 14. Damage mechanisms of TSA3/PyC/SiC observed by TEM (a.) and HRTEM (b.) after tensile test and HRTEM image of PyC interphase damage (c.)

  One of the most interesting results of this study concerns the mechanical behavior of the HNS fibers-reinforced SiC/SiC composites. The HNS/SiC and HNS/PyC/SiC composites both exhibit pseudo-ductile mechanical behavior, moderate F/M coupling and similar mechanical behavior up to a deformation of 0.2%. Fracture analysis showed that damage mode at the F/M interface is equivalent, i.e. the cracks deflection occurs on the outer surface of the amorphous carbon on the HNS fibers. Therefore, the local damage mechanisms of the HNS-reinforced composites with and without PyC interphase are governed by the bond of amorphous carbon to SiC or PyC that has a crack-deflecting function. At ε > 0.2 %, the damage, in HNS/PyC/SiC composites, could be reduced by the presence of PyC interphase still bounded to the matrix.The action of this interphase improved the mechanical behavior of HNS/PyC/SiC composites compared to HNS/SiC composites and limited the dispersion in the failure stresses. However, the crack deflection is firstly governed by the weaker bonds at the PyC/amorphous carbon interface that allows the interfacial debonding. The structure of the amorphous carbon, and more particularly the carbon residues of the sizing polymer (PVA), is seen to play an important role on the damage mechanisms.The damage mechanisms of TSA3 reinforced composites differ from those of HNS reinforced ones. For the TSA3/SiC composites, fiber pull out is not possible and matrix cracks propagate directly across TSA3 fibers leading to a fragile behavior. The presence of turbostratic carbon aggregates covering the SiC grains on TSA3 surface and the preferred orientation of the first SiC planes deposited on this carbon with {111}SiC//(002)C during the CVI process, explain the stronger F/M coupling and the brittle behavior in TSA3/SiC composites. The PyC interphase on the TSA3 fiber increases the damage tolerance without reaching the one of the HNS/SiC or HNS/PyC/SiC composites. The damage mecanisms of TSA3/PyC/SiC composites are still difficult to explain. No net crack deflection in F/M interface is observed for these materials. The F/M bonds are very strong which mainly results in damage to the PyC interphase. The PyC has a preferred orientation on the turbostratic carbon of the TSA3 fibers surface. A reorientation and damage of PyC stacks strongly bounded to the turbostratic carbon of TSA3 fibers surface occur before the failure of these bonds. The TSA3/PyC/SiC composites can display a pseudoductile behavior because the PyC interface keeps its cracks deflection function and the F/M debonding is observed although with shorter pull out length than HNS/PyC/SiC or even HNS/SiC composites.

  surface of two SiC fibers. By understanding the chemical adhesion at the PyC/SiC fiber interface, we explain its effect on the Fiber/Matrix coupling and the resulting local damage mechanisms of SiC/SiC composites under mechanical loading. The surface characteristics of SiC fibers influence the bonding mode of the first planes of PyC interphase with the fibers, determine the intensity of the F/M coupling and the localization of the F/M debonding, control the damage mode and the macroscopic behavior.

10 Number of fibers per tow 500 800 Tex (g/10 3 m) 199 175 Density (g/cm 3 )

		3.05	3.01
	Sizing amount (%wt)	1.13	0.59
	Tensile strength (GPa)	3.01	2.24
	Tensile modulus (GPa)	373	368
	Failure strain (%)	0.81	0.61
	Oxygen content (%wt)	0.2	0.3

Table 1 . Main characteristics of Hi-Nicalon Type S and Tyranno SA3 fibers

 1 SiC matrix were achieved by CVI. A final 30 -40 % fiber volume fraction in the composite was reached. The PyC texture was highly anisotropic to optimize fiber/matrix coupling[START_REF] Bouchard | Pyrocarbon Deposits on a Graphite Surface Observed by STM[END_REF]. This interphase was ~ 50 nm thick (figures 1a. and 1b.). The SiC/SiC composites in this study had a porosity rate between 7.5% and 9%.

	Interlock 2.5D woven fabrics with 0° and 90° fiber orientations were produced. The
	decomposition of the sizing was obtained after the weaving by thermal treatment of the fibers
	at 900°C under vacuum for 2h. Then infiltration a pyrocarbon interphase (PyC) and SiC matrix

Table 2 . C1s peak contributions of HNS and TSA3 desized fibers relative to the energy beam

 2

	1)

Table 3 . Main mechanical characteristics of HNS and TSA3 reinforced composites after tensile test

 3

	HNS/SiC	E (GPa) 278 (± 11)	σr (MPa) 225 (±38)	εr (%) 0.37 (± 0.10)
	HNS/PyC/SiC TSA3/SiC TSA3/PyC/SiC	268 (± 4.1) 318 (± 14) 315 (± 28)	286 (± 16.8) 0.45 (± 0.03) 141 (± 8.7) 0.05 (± 0.02) 275 (± 28) 0.22 (± 0.03)

: E (GPa), σr (MPa), εr (%)

Acknowledgements

The authors are very grateful to S. Poissonnet for the FIB preparation and scientific discussions, G. Brabant for his help in SEM images and G. Loupias for mechanical tests.