Microstructural analysis and mechanical behaviour of oxide/oxide CMCs prepared by continuous tow impregnation
Topic(s) : Material science
Co-authors :
Zoé BORIUS (FRANCE), Antoine DEBARRE (FRANCE), Marc SINGLARD (FRANCE), Thierry CUTARD , Aurélie JULIAN-JANKOWIAKAbstract :
Oxide/oxide ceramic matrix composites (CMC) are considered for continuous thermomechanical applications at temperatures between 800 and 1000°C, under moderate mechanical loads, in oxidizing atmosphere. Indeed, they could allow the required increase in operating temperatures in aeronautical propulsion systems to answer new environmental standards. The main challenge is to control the heterogeneous microstructures of these materials, and to automate their production processes.
Classical production methods include liquid impregnation with an aqueous slurry, which is an easy and cost-effective process. Used mainly on 2D fabrics, it has enabled microstructural and mechanical characterizations of alumina/alumina CMC developed at Onera [1], [2]. In this work, a continuous fibres tow impregnation is assessed to reduce the elaboration costs, automate the impregnation and improve the homogeneity of the impregnated tows (tow-pregs). Thus, tow-pregs are laid up before autoclaving and sintering steps. This study involves adapting a slurry to continuous fibre tow impregnation for oxide/oxide CMC manufacturing and studying the mechanical behaviour of the produced CMCs in relation to the process and their microstructures.
First of all, a study of the slurry composition was carried out, to produce tow-pregs of good quality and compatible with the manufacturing of composites, i.e. flexible and with some tackiness. Two hygroscopic organic additives [3] (sorbitol and glycerol) were evaluated as plasticizers, as well as a gelling powder boehmite [4]. Slurries were formulated with various amount either of sorbitol, glycerol or a mixture of glycerol and boehmite. Thus, the kinetic modelling of their thermal degradation was carried out in order to favour their elimination during autoclave curing.
Regarding slurries with sorbitol, high amounts are necessary to manufacture CMCs. Indeed, delamination occurs for sorbitol contents lower than 38 wt.%. However, such high amounts induce a high porosity level after sintering (33 vol.%). Glycerol, which is more hygroscopic, made possible to produce CMCs from tow-pregs with lower plasticizer contents (20 wt.%), of about 32 vol.% porosity. Then, thanks to its gel-forming properties, boehmite improves the surface impregnation, promoting inter-ply cohesion. Indeed, boehmite-containing tow-pregs produced CMCs with better morphology [5]: around 24 vol.% of porosity and between 52 and 60 vol.% of fibres, close to the specifications of the study. Spatial distribution of pores was studied through x-ray tomography and SEM observations.
Then, mechanical behaviour of the plates was characterized with four-point bending and tensile tests, in relation to the microstructures. Flexural properties are slightly weaker (E = 129 ± 3 GPa), but still comparable to those obtained on 2D fabrics based plates (E = 146 ± 10 GPa) [2]. Furthermore, continuous tow impregnation allows the manufacture of CMCs without matrix-rich areas between plies.
Classical production methods include liquid impregnation with an aqueous slurry, which is an easy and cost-effective process. Used mainly on 2D fabrics, it has enabled microstructural and mechanical characterizations of alumina/alumina CMC developed at Onera [1], [2]. In this work, a continuous fibres tow impregnation is assessed to reduce the elaboration costs, automate the impregnation and improve the homogeneity of the impregnated tows (tow-pregs). Thus, tow-pregs are laid up before autoclaving and sintering steps. This study involves adapting a slurry to continuous fibre tow impregnation for oxide/oxide CMC manufacturing and studying the mechanical behaviour of the produced CMCs in relation to the process and their microstructures.
First of all, a study of the slurry composition was carried out, to produce tow-pregs of good quality and compatible with the manufacturing of composites, i.e. flexible and with some tackiness. Two hygroscopic organic additives [3] (sorbitol and glycerol) were evaluated as plasticizers, as well as a gelling powder boehmite [4]. Slurries were formulated with various amount either of sorbitol, glycerol or a mixture of glycerol and boehmite. Thus, the kinetic modelling of their thermal degradation was carried out in order to favour their elimination during autoclave curing.
Regarding slurries with sorbitol, high amounts are necessary to manufacture CMCs. Indeed, delamination occurs for sorbitol contents lower than 38 wt.%. However, such high amounts induce a high porosity level after sintering (33 vol.%). Glycerol, which is more hygroscopic, made possible to produce CMCs from tow-pregs with lower plasticizer contents (20 wt.%), of about 32 vol.% porosity. Then, thanks to its gel-forming properties, boehmite improves the surface impregnation, promoting inter-ply cohesion. Indeed, boehmite-containing tow-pregs produced CMCs with better morphology [5]: around 24 vol.% of porosity and between 52 and 60 vol.% of fibres, close to the specifications of the study. Spatial distribution of pores was studied through x-ray tomography and SEM observations.
Then, mechanical behaviour of the plates was characterized with four-point bending and tensile tests, in relation to the microstructures. Flexural properties are slightly weaker (E = 129 ± 3 GPa), but still comparable to those obtained on 2D fabrics based plates (E = 146 ± 10 GPa) [2]. Furthermore, continuous tow impregnation allows the manufacture of CMCs without matrix-rich areas between plies.