Thermal protection coating comprising a fiber reinforced main layer and an insulative sublayer

A thermal protection coating for a surface having a reinforcement embedded in a thermally insulative material as a main layer. This layer is formed from a succession of refractory fibrous reinforcements parallel to each other and inclined at an angle to the surface being protected. Insulative layers are located between the reinforcements. An insulating sublayer is located between the surface and the main layer and is compatible with the main layer and the surface.

The invention concerns thermal protection materials designed to cover 
objects which have to withstand a very high thermal flux for a relatively 
short time during which they must retain good mechanical strength 
characteristics. 
It is directed to a coating for thermally protecting a surface, a method, 
in particular an automated method, for coating at least the exterior of a 
body of revolution (cylinder, cone or otherwise) with such thermal 
protection and an apparatus for implementing this method. 
This kind of thermal protection is usually made from materials comprising a 
sandwich or winding of textile (braid, cut woven fabric, tape, fringes, 
etc) reinforcement layers of silica, glass, carbon or other refractory 
materials bonded by an elastomer or a resin (thermosetting or 
thermoplastic). 
The thermal insulation function is then essentially provided by the binder 
(or matrix) while mechanical strength is provided by the layers of textile 
reinforcement which are degraded only at very high temperatures but which, 
on the other hand, are better conductors of heat than the binder. The 
binder is very quickly destroyed by heat whereas the reinforcement passes 
through a softening phase before it melts completely. 
To preserve the integrity of the protection during thermal attack it is 
essential that part of the textile reinforcement is anchored in a part of 
the thermal protection coating that remains cool enough to prevent it 
becoming separated or uncovered prematurely. 
For this reason the layers are generally perpendicular or inclined to the 
surface, rather than parallel to it. Known materials of this kind include 
"CLINOSTRASIL" and "ORTHOSTRASIL". 
In some applications this type of material has an irredeemable defect, 
namely inadequate thermal insulation in the case of a moderate flux 
maintained for a relatively long time (example: 200 kW/m.sup.2, for four 
minutes) interrupted by very high amplitude peaks with very short 
durations (example: 1 to 2 MW/m.sup.2, for a few seconds). Because of the 
surface temperatures reached during the peaks the material must be 
mechanically strong at these temperatures, whereas because of the duration 
of the moderate flux thermal attack the material must be highly 
insulative. 
Conventional materials (including the previously mentioned "CLINOSTRASIL" 
and "ORTHOSTRASIL" materials) are characterised by a very high fibre 
coefficient (by definition, but also because this is obligatory given 
their manufacturing processes, as will be explained later). These 
materials are therefore too good as conductors of heat. 
To improve the insulation qualities the layers of reinforcement can be 
inclined, which artificially lengthens the path of the heat flow in the 
fibres. However, this solution is limited to cylindrical or very slightly 
conical parts because with relatively high cone angles the inclined layers 
slip during manufacture. 
Various fabrication methods are known for using these known materials. 
In a first method (sometimes referred to as "CLINO" or "ORTHO" winding), a 
woven fabric or tape preimpregnated with thermosetting resin is wound 
layer by layer onto a former or directly onto the object to be coated. The 
angle of inclination between the axis of revolution and the layers is 
defined initially by a wedge supporting the first layer. After 
polymerisation the coating is machined to the correct thickness. 
This method has a number of disadvantages: 
it is impossible to obtain materials with a very low reinforcement factor 
(which are therefore highly insulative) as the angle of inclination is 
controlled during winding by the laying of one layer onto another; if 
there were no reinforcement to reinforcement bearing relationship the 
binder would creep and the angle would be lost and, in the short term, the 
succeeding layers would slip: this could lead to insufficient and 
irregular thermal insulation properties combined with poor resistance to 
abolation due to incorrect location of the reinforcements; 
it is a fortiori impossible to make materials based on cold vulcanising 
silicon (RTV) elastomers with a relatively low reinforcement factor; this 
type of binder can only be used by direct impregnation; the fluidity of 
the binder gives rise to the same problem; 
it is impossible to wind strongly conical bodies because the layers of 
reinforcement slip along the generatrix; 
it is not possible to achieve good control over the position of the layers 
and of the interval between layers. 
In a second method precut rings of woven fabric are stacked on each other. 
This very simple method has the same defects as previously described, 
often much accentuated, and additionally: 
material is wasted because of the cutting, 
the method is time-consuming. 
In another method the body to be protected has wound onto it a fringed 
strip incorporating support fibres running along the surface to be 
protected, from which extend appropriately inclined short fibres. 
In a further method described in the patent FR-2.569.237 a fringed mesh is 
wound onto an intermediate former or onto the body to be protected itself, 
with the special feature that the fringed tape incorporates a mesh part 
directly exposed to the thermal flux. 
Despite their undoubted advantages, these improved solutions suffer from 
the same disadvantages. 
The invention is directed to alleviating the aforementioned disadvantages 
by proposing a thermal protection coating which represents an 
insulation/mechanical strength trade-off which is a much better match to 
actual requirements than has been achieved in the past, particularly a 
much better thermal insulator than known coatings, so meeting the 
requirements for thermal and mechanical strength. 
It is also directed to a method for the industrial scale manufacture of a 
coating of this kind and an installation for implementing this. 
It therefore proposes a thermal protection coating for a surface to be 
protected comprising a reinforcement embedded in a first thermally 
insulative material and characterised in that it includes a main layer 
formed from a succession of substantially parallel refractory fibrous 
reinforcement layers inclined to the surface to be protected, between 
which are interleaved insulative layers essentially made up of said first 
insulative material, said main layer being lined with at least one 
sublayer extending along the surface to be protected and essentially being 
made of a second insulative material compatible with said first insulative 
material. 
According to preferred features: 
the reinforcement layers represent less than 30% by volume of the main 
layer; 
the reinforcement layers represent between 10 and 20% by volume of the main 
layer; 
the sublayer is in contact with the surface to be protected; 
the coating has an axis of revolution and the reinforcement layers are 
inclined to this axis at an angle of inclination (by definition less than 
or equal to 90.degree.) which is less than 60.degree.; 
the angle of inclination is between 30.degree. and 60.degree.; 
the angle of inclination is between 40.degree. and 50.degree.; 
the coating has an axis of revolution and the reinforcement layers and the 
insulative layers are formed by interleaved continuous windings; 
the reinforcement layers comprise a woven silica tape; 
the reinforcement layers are substantially equidistant; 
the first insulative material of the insulative layers interleaved between 
the reinforcement layers is a lightened silicone type reference elastomer; 
the second insulative material is identical to that of the insulative 
layers; 
at least the second insulative material is a doped material adapted to 
provide, for example, protection against X-rays, as described in the 
patent FR-2.597.651; 
the second insulative material is a cellular material; 
the thickness of the sublayer is at least in the order of one tenth of a 
millimeter. 
The invention also proposes a method of manufacturing a thermal protection 
coating for a surface to be protected characterised in that: 
at least one sublayer of a thermally insulative material adapted to be 
parallel to the surface to be protected is deposited onto a support and 
polymerised if necessary; 
then, in a continuous manner so as to build up progressively a main layer: 
invariant and identical profile parallel grooves are machined into said 
sublayer; 
at least one refractory fibrous reinforcement layer is placed in each 
groove so that an edge portion thereof runs along a flank of said groove 
and is inserted thereinto as far as its bottom; 
an insulative layer of a first insulative material is deposited along each 
reinforcement layer to fill at least the groove; 
the thickness of said insulative layer parallel to the associated 
reinforcement layer is formed to the required value; and 
said insulative material is polymerised. 
According to preferred features: 
said insulative material is polymerised as it is applied; 
the combination of the sublayer and the main layer obtained in this way is 
machined to define its thickness perpendicular to the surface to be 
protected (this thickness may vary if necessary); 
if the surface to be protected has an axis of revolution, a continuous 
groove is machined around said axis and, as said groove is machined, there 
is applied to every point of said groove and before the latter has 
progressed 360.degree. from said point, a reinforcement tape forming a 
reinforcement layer, a bead of said first insulative material is spread 
along said tape and the thickness of the insulative layer obtained in this 
way is formed to the required value and the material is polymerised; 
the support is the surface to be protected; 
the support is an intermediate former. 
The invention also proposes an apparatus for implementing the method 
characterised in that it comprises: 
means for depositing at least an insulative sublayer; 
a cutting tool adapted to form at least one groove in the sublayer; 
an applicator member for applying at least one refractory fibrous 
reinforcement layer to each groove, against one flank thereof; 
an injection nozzle for depositing a layer of insulative material along 
each reinforcement layer; 
a tool for forming each insulative layer to the required thickness; 
a polymerising member for polymerising the insulative material; and 
a machining tool for obtaining a required final profile of the coating. 
Preferably: 
said tools are carried, angularly offset, by at least one tool-holder which 
is part of a rotary machine such as a copier lathe or a numerically 
controlled lathe; 
the machining tool is associated with a copying system different that a 
first copying system associated with the cutting tool, so that the 
thickness of the coating can be varied at will. The same result can of 
course be obtained with a numerically controlled machine.

The coating in FIG. 1 comprises a main layer 1 formed of a succession of 
refractory fibrous reinforcing layers 2 which are substantially parallel 
and in this instance equally spaced by a distance e and inclined at an 
angle .gamma. relative to the surface S of the body 50 to be protected. 
Thermally insulative layers 3 are interleaved between these reinforcement 
layers, which are preferably of a woven or fringed textile reinforcement. 
The main layer 1 is lined with an insulative sublayer 4 parallel to the 
surface S to be protected, in this instance in contact with it. In this 
instance this surface has an axis of revolution X--X. In this instance the 
body has a cone half-angle .alpha. and the inclination of the layers 2 
relative to this axis is .beta. so that .gamma.=.beta.-.alpha.. 
The coating is therefore made up of a reinforcement (a woven silica tape, 
for example) wound with a particular angle and sandwiched with a matrix 
forming a binder (of silicone elastomer, for example), the whole being 
bonded to an insulative sublayer. 
It is similar in principle to the "CLINOSTRASIL" type material described 
previously but it is characterised by: 
a very low reinforcement factor (preferably less than 30% by volume with 
respect to the main layer and more particularly between 10 and 20%); 
a highly insulative matrix (a lightened silicone elastomer, for example) 
deposited by direct impregnation; 
strongly inclined layers (the angle .beta. is advantageously between 
30.degree. and 60.degree.); it has been found that the angle .gamma. can 
take a very small value, possibly as low as 10.degree., representing a 
range of 0.degree. to 50.degree., preferably 20.degree. to 50.degree.; 
a pure matrix sublayer. 
All these characteristics guarantee very good thermal insulation 
properties. 
According to an essential characteristic of the invention a sublayer 4 is 
formed. 
This sublayer enables: 
anchoring of textile reinforcements during manufacture; 
bonding, usually between the surface S of the body to be protected and the 
combined thermal protection coating (1+4); 
improved thermal insulation characteristics. 
The insulative sublayer 4 may be made from the same material as the 
insulative layers 3. 
In some embodiments of the invention this sublayer may be made from a 
material different than that of the matrix 4 of the main thermal 
protection layer 1 (provided that these materials are compatible), in 
order to fulfil other functions, for example: 
protection against X-rays, 
additional insulation by virtue of it being a cellular material. 
Various stages of the process are shown in FIGS. 2 through 6. 
Referring to FIG. 2, the sublayer 4 is formed directly on the surface S of 
the body to be protected (after application of an adhesion-promoting 
primer) or on a former (with a mould release agent), in this instance by 
moulding a layer of a material that is optionally identical to the 
material of the matrix using an appropriate device of any known type 
schematically represented at 20. The thickness must be slightly greater 
than the required final thickness of the sublayer. 
The main layer 1 is then formed by a process that might be described as 
insertion winding. 
After the sublayer 4 is polymerised the following operations are carried 
out continuously so as to build up the main layer progressively, for 
example on a specially adapted rotary machine 10 (see FIG. 7), starting 
from the larger diameter end in the case of a conical shape and with an 
angle .beta.&lt;90.degree.: 
a continuous groove 5 is machined around the X--X axis with the required 
pitch by a cutting tool 11 (see FIG. 3); the groove must have one flank 5A 
inclined at the angle .beta.; the bottom of the groove must be at a 
distance d from the surface S of the body to be protected corresponding to 
the required thickness of the sublayer (minimum value: a few tens of 
millimeters), 
the textile reinforcement strip 3 (a woven tape in FIG. 4) is inserted and 
wound into the groove, using an applicator roller 12, with one edge 
located at the bottom of the groove; the reinforcement must be applied by 
a guide 12 at the angle .beta. and must bear against the previous 
insulative layer 3'; 
a bead 6 of binder (matrix) previously (or continuously) catalysed is 
deposited by an injection nozzle 13 into the groove 5 (see FIG. 5); 
a tool 14 is used to form the deposit to the required thickness to improve 
impregnation of the reinforcement by the binder and to eliminate any 
excess material (see FIG. 6). 
The main layer 1 is then polymerised, advantageously at the same time as it 
is formed to the required size and profile by means of a known tool 
schematically represented at 16 in FIG. 6. 
Note that to improve the quality of the deposit it is advantageous to 
employ a deformable textile reinforcement (especially for small 
diameters). 
The efficacy of this method results from the fact that the groove allows 
perfect anchoring of the textile reinforcement 3, which is placed exactly 
at the required location, and prevents any slipping, irrespective of the 
cone half-angle .alpha. of the body 50 and the angle .beta. of the 
reinforcement; as already explained, the angle .gamma. may be as small as 
10.degree.. What is more, the dimension machined at the angle .beta. 
assists with maintaining the inclination initially set by a support wedge. 
Finally, the presence of the groove facilitates the controlled deposition 
of the binder and limits runs. 
This method therefore makes it possible to manufacture a coating of the 
FIG. 1 type very quickly and automatically with very high accuracy in 
respect of: 
the reinforcement factor (related to the winding pitch), and 
the angle of inclination .beta. of the layers, irrespective of the profile 
of the body of revolution to be coated. 
FIG. 7 shows schematically a machine for implementing the process in the 
case of FIGS. 2 through 6 in which the body 50 whose surface S to be 
protected has an axis of revolution X--X. 
In FIGS. 2 through 6 arrows show relative movement of the body 50 and the 
various tools: simultaneous axial forward movement and rotation movement, 
representing a generally helical overall relative movement. 
However, it must be noted that the principle of the method from FIGS. 2 
through 6 is not limited to bodies with an axis of revolution. The general 
principle of the invention teaches the protection of a portion of surface 
by forming multiple parallel sections of grooves. There could likewise be 
a plurality of interleaved continuous grooves with a radii varying as a 
function of the shape of the body to be protected. 
Given the generally helical relative movement of each of the tools 11 
through 14 relative to the body, the invention teaches fastening these 
tools together in the angular direction (FIG. 7). 
In this example a rotary machine 100 (modified copying lathe) is equipped 
with a special carriage 101 implementing the machining, winding, 
impregnation and thickness forming functions while the body 50 to be 
protected is being rotated with no translation movement. 
The four functions implemented on the carriage are tracked along the 
generatrix of the workpiece to be coated by a radial advance system 
synchronised with the lathe profile copier. 
As can be seen from FIG. 7 the lathe 100 includes at least one tool-holder 
turret 101 carrying the angularly offset tools 11 through 14. The 
tool-holder 101 is mobile in translation along a rail 102. 
Near the applicator tool 12 is a spool 12A of reinforcement tape and near 
the injection nozzle 13 is a tank 13A of binder. 
Also provided on the tool-holder in this instance is a polymerisation tool 
15, in this instance an irradiating tool for polymerising the binder 
disposed between the tools 12 and 13. There could likewise be provided on 
the same tool-holder 101 a peripheral machining tool for optionally 
smoothing the beads 3A from FIG. 6. 
On the copying lathe is a copying groove 103 tracked by a roller carried by 
an angled arm 104 carrying one of the tools, in this instance the tool 12 
for applying the reinforcement 2. The arm 104 is mobile in the radial 
direction relative to the tool-holder turret 101 and meshes with a toothed 
wheel 105. The other tools are also attached to radially mobile arms and 
mesh with similar toothed wheels 106 through 109 which are synchronised in 
rotation by chains 110. 
It will be realised that the design of the machine represents a copying 
lathe correlating the local radius r of the body to be protected with the 
distance X between the copying groove 103 and the axis of revolution X--X. 
As an alternative (not shown) the polymerisation tool 15 and the final 
machining tool may be mounted on a second tool-holder which follows the 
first tool-holder at a distance of several rearward offset increments. 
The final machining tool could equally well be fixed to a tool-holder 
fastened to the turret with its radial position slaved to a shape copier 
different than the basic copier (if the thickness of the coating had to 
vary longitudinally, for example). 
The material was obtained during trial implementation of the method 
described previously with a reinforcement factor of approximately 15%, an 
angle .beta. of 45.degree. and an angle .gamma. varying from 10.degree. to 
20.degree.; the sublayer was approximately 1 mm thick for a total 
thickness varying continuously between 4 and 8 mm. 
It goes without saying that the previous description has been given by way 
of non-limiting example only and that numerous variations thereon may be 
put forward by those skilled in the art without departing from the scope 
of the invention.