Part made of composite material, having controlled creep

Abstract

A part made of coated composite material, includes a substrate made of ceramic matrix composite material; a tie-coat layer covering the substrate; and a protective coating on the tie-coat layer and defining an environmental barrier, the protective coating including a rare-earth silicate and including a first outer region including an outer surface of the protective coating opposite to the substrate and having a first working creep, having deformation of less than or equal to 0.07% when a compressive stress of at least 50 MPa is applied for a duration of 10 hours at a temperature of between 1050 C. and 1300 C. The first region includes a grain growth inhibitor; and a second, inner, environmental barrier region, including an interface of the protective coating with the tie-coat layer and having a second working creep.

Claims

1. A part made of coated composite material comprising: a substrate made of ceramic matrix composite material; a tie-coat layer covering the substrate; and a protective coating that is on the tie-coat layer and that defines at least one environmental barrier, the protective coating comprising at least one rare-earth silicate, which is selected from the group consisting of ytterbium disilicate (Yb.sub.2Si.sub.2O.sub.7), yttrium disilicate (Y.sub.2Si.sub.2O.sub.7) and any combination thereof, and comprising at least: a first outer region comprising an outer surface of the protective coating opposite to the substrate and having a first working creep, having deformation of less than or equal to 0.07% when a compressive stress of at least 50 MPa is applied for a duration of 10hours at a temperature of between 1050 C. and 1300 C., said first region comprising at least one grain growth inhibitor in a sufficient quantity to obtain this first creep; and a second, inner, environmental barrier region, comprising at least one interface of the protective coating with the tie-coat layer and having a second working creep, having deformation of at least 0.01% when a compressive stress of at least 50 MPa is applied for a duration of 10 hours at a temperature of between 900 C. and 1000 C., wherein the grain growth inhibitor is selected from the group consisting of zirconium oxide (ZrO.sub.2), titanium oxide (TiO.sub.2) and hafnium oxide (HfO.sub.2) and any combination thereof, and wherein the content by mass of grain growth inhibitor in the first region is between 0.05% and 10%.

2. The part according to claim 1, wherein a thickness of the second region can be between 20 m and 300 m.

3. The part according to claim 1, wherein the tie-coat layer comprises silicon (Si).

4. The part according to claim 1, wherein the protective coating comprises at least a first layer defining the first outer region and comprising a first rare earth silicate, and a second layer defining the second, inner, region and comprising a second rare earth silicate different from the first rare earth silicate.

5. The part according to claim 4, wherein the first layer is an abradable layer and the second layer an environmental barrier.

6. The part according to claim 5, wherein the first layer comprises an yttrium disilicate (Y.sub.2Si.sub.2O.sub.7) and said at least one grain growth inhibitor, and the second layer comprises an ytterbium disilicate (Yb.sub.2Si.sub.2O.sub.7).

7. The part according to claim 1, wherein the protective coating further comprises an intermediate buffer region present between the first and second regions, this intermediate region having a composition distinct from those of the first and second regions.

8. The part according to claim 1, wherein said part is a turbine ring sector and wherein the first region is abradable.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) FIG. 1 schematically shows the change over time in the stress observed during one temperature cycle, with a thermal gradient applied on a coated ceramic matrix composite, at the hottest point on the surface of the coating for a first layer of a coating according to the invention and a first layer of a coating not according to the invention.

(2) FIG. 2 shows the change over time in grain size, observed with and without growth inhibitor for a thermal cycle representative of the stabilisation treatment for a coating according to the invention and a coating not according to the invention.

DESCRIPTION OF THE EMBODIMENTS

(3) The invention is now described by means of particular embodiments and figures, which are provided for illustrative purposes and should not be interpreted as limiting.

(4) Within the meaning of the invention, a rare earth silicate should be understood as a compound comprising silicon Si, oxygen O and a rare earth denoted RE. it will be noted, in particular, that a rare earth RE monosilicate of general formula RE.sub.2SiO.sub.5, and a rare earth RE disilicate of general formula RE.sub.2Si.sub.2O.sub.7, both fall within the definition retained for the invention of a rare earth silicate.

(5) As described above, the coatings of the prior art are consistent with the second creep. If no special provision is made, known environmental or thermal barriers comprising one or more rare earth silicates are consistent with the second creep. For this reason, the presence of the grain growth inhibitor is important for enabling a region having the first creep to be obtained.

(6) Within the meaning of the invention, creep has its usual definition in the art, namely the deformation that a material can undergo when subjected for a prolonged period to a stress or a stress gradient. Thus, the creep is expressed as a deformation given as a function of the stress, the time and the applied temperature.

(7) In general, the first region with the first creep comprises a first rare earth silicate and the grain growth inhibitor, and the second region with a second creep comprises a second rare earth silicate that is identical to or different from the first rare earth silicate.

(8) Reference is now made to FIGS. 1 and 2.

(9) These figures show the differences in behaviour between a coating according to the invention and a coating not according to invention. The measurements presented in these figures are obtained from a protective coating according to the invention and a protective coating of the prior art, for which the compositions are similar to the presence of the grain growth inhibitor in the outer layer.

(10) For the measurements of FIGS. 1 and 2, the coated part according to the invention comprises the following layers: a substrate made of ceramic matrix composite material; a tie-coat layer of silicon (Si); a layer defining the environmental barrier comprising ytterbium disilicate (Yb.sub.2Si.sub.2O.sub.7); and an abradable layer defining the first region and comprising yttrium disilicate (Y.sub.2Si.sub.2O.sub.7) and a content by mass of 1% zirconium oxide (ZrO.sub.2) as grain growth inhibitor.

(11) The coated part not according to the invention comprises the same layers defining the same regions, but no grain growth inhibitor in its first abradable layer.

(12) In the embodiment of these examples, it will be noted that the first (and respectively second) layer defines the first (and respectively second) region. It is recalled that this is just a descriptive embodiment and that it does not exclude the case where the layers do not exactly define the regions.

(13) FIG. 1 illustrates the change over time in the stress observed during one temperature cycle, with a thermal gradient applied to a coated ceramic matrix composite material, at the hottest point on the surface of the coating for a first layer of a coating according to the invention and a first layer of a coating not according to the invention.

(14) FIG. 1 shows the change over time in the stress 12 in the layer, expressed in MPa, as a function of time 11 expressed in seconds.

(15) The change over time in the stress in each of the first layers is evaluated during a heating and then cooling cycle. The heating is not homogeneous and creates a thermal gradient in the coating. During the period 101, the coating is heated to 1300 C., then the coating is left in the air to cool for the period 102.

(16) The curves 13, 14 in FIG. 1 show that a part with a first layer according to the invention 13 or not according to the invention 14 have, during the heating 101, compression stresses, the stresses being negative.

(17) It is however remarkable that during the cooling 102, a first layer according to the invention 13 has no large tensile stress. On the other hand, a first layer not according to the invention 14 has a large tensile stress.

(18) It would appear on looking at FIG. 1 that a first layer according to the invention 13 can avoid an excessive tensile stresses, thus avoiding surface cracking of a coating.

(19) More specifically, in the coating not according to the invention 14, the creep can accommodate the compression stresses created during heating, which is seen in curve 14 by the reduction in absolute value of the compressive stress appearing during heating 101. The first layer not according to the invention is then under tension during cooling 102, but can no longer accommodate this tensile stress by creep, because the temperature is then too low and, as a consequence, a crazing of the layer not according to the invention is observed.

(20) By contrast, a layer according to the invention 13 does not creep during heating 101, and it can be seen in FIG. 1 that effectively the stress is not reduced. Since the layer according to the invention has not undergone creep, cooling 102 enables it to return to a state of stress close to the initial state without passing through an excessively high tensile stress state.

(21) FIG. 2 shows the change over time in the average size of the grains 22, expressed in m, in the first layers of the two parts, one according to the invention, the other not according to the invention, during sintering at 1400 C., the duration of which 21 is expressed in hours This treatment is representative of the stabilisation treatment applied in order to form the first layer.

(22) FIG. 2 shows the change over time in the observed grain size, for a coating according to the invention and a coating not according to the invention, during a thermal cycle representative of a stabilisation treatment of the coating. It illustrates that the presence of the grain growth inhibitor in a first layer of a coating according to the invention, curve 201, can prevent the growth in size of the grains, which then limits the creep in a first region according to the invention. On the other hand, in a first region of a coating not according to the invention, curve 202, the grain size changes linearly with the duration of the stabilisation treatment, which is associated with significant creep.

(23) In an embodiment, the inhibitor is present in the form of precipitates, located at the grain boundaries, and the average size of which can be between 100 nm and 1 m.

(24) In a particular embodiment, the first region can comprise yttrium disilicate (Y.sub.2Si.sub.2O.sub.7) and zirconium oxide (ZrO.sub.2) as grain growth inhibitor. For example, the content by mass of yttrium disilicate (Y.sub.2Si.sub.2O.sub.7) is greater than or equal to 90%, for example between 90% and 99.95%, and the content by mass of zirconium oxide (ZrO.sub.2) is between 0.05% and 10%.

(25) In another particular embodiment, the first region can comprise yttrium disilicate (Y.sub.2Si.sub.2O.sub.7) and hafnium oxide (HfO.sub.2) as grain growth inhibitor. For example, the content by mass of yttrium disilicate (Y.sub.2Si.sub.2O.sub.7) is greater than or equal to 90%, for example between 90% and 99.95%, and the content by mass of hafnium oxide (HfO.sub.2) is between 0.05% and 10%.

(26) In another particular embodiment, the first region can comprise yttrium disilicate (Y.sub.2Si.sub.2O.sub.7) and titanium oxide (TiO.sub.2) as grain growth inhibitor. For example, the content by mass of yttrium disilicate (Y.sub.2Si.sub.2O.sub.7) is greater than or equal to 90%, for example between 90% and 99.95%, and the content by mass of titanium oxide (TiO.sub.2) is between 0.05% and 10%.

(27) In an embodiment, the second region can comprise ytterbium disilicate (Yb.sub.2Si.sub.2O.sub.7), for example in a content by mass of ytterbium disilicate (Yb.sub.2Si.sub.2O.sub.7) greater than or equal to 90%.

(28) For example, a protective coating as has just been described can be obtained by a method known per se of thermal spraying of a powder mixture and sintering. In the embodiment described, it is again provided that a single layer of the coating defines a region.

(29) In an embodiment, the composition of the powder used in a thermal spraying method of powder in order to create the second region can be an ytterbium disilicate (Yb.sub.2Si.sub.2O.sub.7) powder. In an alternative embodiment, the thermally sprayed powder can comprise an ytterbium (Yb) powder and a silica (SiO.sub.2) powder, in proportions which will make it possible to obtain ytterbium disilicate (Yb.sub.2Si.sub.2O.sub.7) after sintering.

(30) In an embodiment, the composition of the powder used in a powder thermal spraying method in order to create the first region can be an yttrium disilicate (Y.sub.2Si.sub.2O.sub.7) powder and a grain growth inhibitor powder, for example a zirconium oxide (ZrO.sub.2) powder. In an alternative embodiment, the thermally sprayed powder can comprise a silica (SiO.sub.2) powder and an yttrium (Y) powder, in proportions making it possible to obtain, after sintering, a layer of yttrium disilicate (Y.sub.2Si.sub.2O.sub.7), and a grain growth inhibitor powder, for example a zirconium oxide (ZrO.sub.2) powder.

(31) Of course, the grain growth inhibitor powder is chosen such that the quantity of grain growth inhibitor obtained is suitable for producing the desired creep limitation.

(32) Once the one or more powders are deposited on the surface of the composite material part, the coating can be obtained by sintering.

(33) Alternatively, the protective coating can be obtained by depositing powders in the form of slurries and sintering. The protective coating can again be obtained by plasma torch or by liquid method, for example by electrophoresis or by dip coating. The coating can also be obtained by chemical vapour deposition.