Method to improve the thermal properties of a resistance element embedded in an alumina deposit on a surface of a substrate and application of said method

Abstract

A method for improving the heat resistance of a resistive element embedded in an alumina deposit covering a surface of a substrate, in which the alumina deposit includes a surface portion and a deep portion which is sandwiched between the surface portion and the surface of the substrate and in which the resistive element is located, is provided. The method includes a densification of the surface portion of the alumina deposit.

Claims

1. A method for improving the heat resistance of a resistive element embedded in an alumina deposit covering a surface of a substrate, the alumina deposit comprising a surface portion and a deep portion which is sandwiched between the surface portion and the surface of the substrate and in which the resistive element is located, the method comprising: densificating the surface portion of the alumina deposit, the densificating including a) impregnating said surface portion by a solution comprising alumina particles and an aluminium phosphate; b) drying the surface portion so impregnated; and c) applying a heat treatment to the surface portion so dried.

2. A method according to claim 1, wherein the solution used in a) is an aqueous solution which comprises 20% to 45% by mass of aluminium phosphate.

3. A method according to claim 1, wherein b) is achieved at ambient temperature for a period of 1 hour to 4 hours.

4. A method according to claim 1, wherein c) comprises in succession: heating the surface portion of the alumina deposit to a temperature T.sub.1 of between 90° C. and 100° C. for a period of 1 hour to 3 hours; heating the surface portion of the alumina deposit to a temperature T.sub.2 of between 240° C. and 280° C. for a period of 1 hour to 3 hours; and heating the surface portion of the alumina deposit to a temperature T.sub.3 of between 350° C. and 390° C. for a period of 1 hour to 3 hours.

5. A method according to claim 1, wherein a thickness of the surface portion of the alumina deposit is between 20% and 30% of a total thickness of the alumina deposit.

6. A method according to claim 1, wherein the resistance element is a strain gauge, and the substrate is a turbine blade.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) FIG. 1 is a scanning electron microscopy image (1,000 times enlargement) which partially shows a test piece consisting of a substrate made of a metal alloy covered with a layer of alumina, the surface portion of which has been densified by the method of the invention.

(2) FIGS. 2A and 2B represent the spectra obtained by energy-dispersive analysis at the points noted A and B in FIG. 1.

(3) FIG. 3 is a scanning electron microscopy image (2,000 times enlargement) which partially shows another test piece consisting of a substrate made of a metal alloy covered with a layer of alumina, the surface portion of which has also been densified by the method of the invention.

(4) FIGS. 4A, 4B and 4C represent the spectra obtained by energy-dispersive analysis at the points noted A, B and C in FIG. 3.

DETAILED DESCRIPTION OF PARTICULAR EMBODIMENTS

(5) Test pieces are produced consisting of a layer of alumina on a substrate made of an Ni53/Fe19/Cr19/Nb/Mo/Ti alloy (Inconel™ 718) by flame spraying of alumina rods (Rokide™). The layer of alumina is 500 μm thick.

(6) A paint brush is then used to apply, on to the surface of the layer of alumina of these test pieces, a layer 50 to 200 μm thick of an aqueous solution, called below an “impregnation solution”, which has been previously obtained by dilution of the product Ceramacoat™ 503-VFG-C (available from the company Polytec) with distilled water, at a proportion of 0.15 g of distilled water for 1 g of product.

(7) After drying at ambient temperature for 4 hours, the test pieces were subjected to a heat treatment comprising a first treatment lasting 2 hours at 95° C., followed by a second treatment lasting 2 hours at 260° C., and finally a third treatment lasting 2 hours at 370° C.

(8) They were then subjected to analyses by scanning electron microscopy (SEM) and to energy-dispersive analyses (EDS).

(9) The results of these analyses are illustrated in FIGS. 1, 2A and 2B for a first test piece, and in FIGS. 3, 4A, 4B and 4C for a second test piece.

(10) FIG. 1, which is an image taken using SEM (1,000 times enlargement) on the first test piece, shows a difference of appearance between the surface portion of the layer of alumina (at the top of the image), which is the portion which was impregnated by the impregnation solution and densified, and the underlying portion of this layer which was not.

(11) FIG. 2A, which represents the spectrum obtained by EDS at the point noted A in FIG. 1, shows the presence of phosphorus (through the presence of the phosphorus peak) at this point, and enables it to be concluded that the layer of alumina was impregnated by the impregnation solution at least as far as said point A.

(12) FIG. 2B, which represents the spectrum obtained by EDS at the point noted B in FIG. 1, shows the presence of the different components of the impregnation solution (through the presence of the alumina peak and of the phosphorus peak) at this point.

(13) FIG. 3, which is an image taken using SEM (2,000 times enlargement) on the second test piece, also shows a difference of appearance between the surface portion of the layer of alumina (at the top of the image), which is the portion which was impregnated by the impregnation solution and densified, and the underlying portion of this layer which was not.

(14) FIG. 4A, which represents the spectrum obtained by EDS at the point noted A in FIG. 3, shows a phosphorus peak and an alumina peak, marking the presence of phosphorus and, therefore, the impregnation of the layer of alumina by the impregnation solution at this point.

(15) FIG. 4B, which represents the spectrum obtained by EDS at the point marked B in FIG. 3, shows an alumina peak corresponding to the alumina deposit, whereas FIG. 4C, which represents the spectrum obtained by EDS at the point noted C in FIG. 3, shows the presence of the different components of the impregnation solution (through the presence of the alumina peak and of the phosphorus peak) at this point.