The invention relates to a device for determining the bonding strength of a coating on a substrate, wherein a hole extending up to the interface between substrate and coating is formed in the substrate and a fluid is introduced into said hole under pressure for the purpose of determining the bonding strength, and wherein the device comprises means for fixing the substrate on a supporting plate forming part of the device, in such a manner that the hole present in the substrate is in open communication at its open end remote from said coating with a channel formed in said supporting plate, which channel is in communication with a source of pressurized fluid during operation.

The principle of determining the bonding strength of a coating on a substrate, wherein a hole extending up to the interface between substrate and coating is formed in the substrate, to which hole a pressurized fluid can be supplied via a channel formed in a supporting plate which supports the substrate, is known from US patent No. 4,899,581.

From said publication no clear indication can be derived, however, with regard to the construction of the device being used and with regard to the manner in which the fluid is pressurized.

US patent No. 5,575,868 diagrammatically shows a device for determining the bonding strength of a coating on a substrate. Said device uses an unspecified source of pressurized fluid for generating the pressure in the hole formed in the substrate, which source is connected, via a line, to a channel which is provided in the supporting plate which supports the substrate. A pressure regulator is disposed in the line between said channel and said source of pressurized fluid, by means of which the pressure can be adjusted. It will not be possible with such a construction to achieve a gradual increase of the pressure, which is required for precisely determining the bonding strength, since the pressure that occurs will be influenced not only by the setting of the pressure regulator, but also by possible fluctuations of the pressure at the source for pressurized fluid.

According to the invention, the channel is in open communication with a cylinder space forming part of the device, in which a piston or plunger being capable of movement in the longitudinal direction

of said cylinder space is present.

By using the construction according to the invention, a device of simple construction can be obtained, by means of which a very precise control and gradual build-up of the pressure can be effected by moving the piston or plunger in the cylinder space. The gradual build-up of the pressure is important in order to be able to achieve a correct determination of the pressure at which the coating becomes detached from the substrate.

Consequently, another aspect of the invention relates to a method for determining the bonding strength of a coating on a substrate, wherein a hole extending to the interface between substrate and coating is formed in the substrate and a fluid is introduced into said hole under pressure for the purpose of determining the bonding strength, and wherein the substrate is fixed on a supporting plate, in such a manner that the hole present in the substrate is in open communication at its open end remote from said coating with a source of pressurized fluid.

According to the invention, the channel is placed in open communication with a cylinder space, in which a piston or plunger being capable of movement in the longitudinal direction of the cylinder space is present, which piston or plunger is gradually moved within the cylinder space for building up the pressure of the fluid.

The measurement of the bonding strength can be carried out in a quick and simple manner thereby.

The device and/or the method can be used on dry substrates comprising a coating, for example a coat of paint or lacquer, after full drying and curing of the coating, as well as on substrates which have been immersed in water for some time, for example a few days, after drying and curing of the coating.

The invention will be explained in more detail hereafter with reference to the accompanying figures.

Figure 1 is a diagrammatic sectional view of a substrate and a coating applied thereto.

Figure 2 is a diagrammatic sectional view of an embodiment of the device according to the invention.

Figure 1 shows a substrate 1 on which a coating 2 is present. Said substrate may be a metal substrate, for example, although the invention is not limited thereto, on which a coating, which may or

may not be an anticorrosive coating, is present. Metal parts which are used in structures are frequently provided with a coating, for example consisting of paint or the like, which is intended to prevent corrosion of the metal parts. In order to be able to form a judgement on the corrosion resistance that is obtained when using coatings on metals, it is important to have an understanding of the bonding strength of a coating on the metal (substrate). In order to determine the bonding strength, a hole or bore 3 is formed in the substrate, as is also described in the aforesaid publications, which hole or bore extends up to the interface between substrate 1 and coating 2. The diameter of said bore is preferably equal to or smaller than the thickness of the coating. Pressurized fluid in introduced into said bore, so that a pressure is applied to the coating, as is indicated by arrows P in Figure 1. When the pressure is increased, the part of the coating which extends over bore 3, initially parallel to the interface between substrate and coating, will at a certain point be forced upwards in the form of a blister to the position of said part of the coating present over bore 3 which is illustrated in full lines, and subsequently the coating will become detached, whereby the increased pressure will generally cause the blister that has formed to break at the edge and/or at the top, depending on the thickness and the composition of the coating. The magnitude of the pressure at which this phenomenon occurs, provides an indication with regard to the bonding strength of the coating 2 on substrate 1.

The device which is shown in Figure 2 is used for determining said magnitude.

In the illustrated embodiment, said device is built up of two housing parts 4 and 5, which are joined along a joint face 6.

A recess is formed in the upper surface of housing part 4, seen in Figure 2, into which recess a substrate 1 comprising a coating on the upper side thereof can be placed.

Substrate 1 can be clamped down in housing part 4, which forms a supporting plate, by means of a cover 7, which can be secured to housing part 4 by means of bolts, for example, or other fasteners. Cover 7 is thereby provided with a projecting part, which fits in the recess formed in housing part 4, in which substrate 1 is received. As will furthermore be apparent from Figure 2, sealing rings 8, preferably so- called 0-rings, are provided in the recess in housing part 4 or in the

part of cover 7 that is inserted in said recess, which function to provide a seal surrounding the bore 3 in substrate 1, which seal will prevent leakage of fluid past said sealing rings 8.

Cover 7 is furthermore provided with a central hole 9, which is covered on its upper side by a transparent cover plate 10.

Hole 9 gradually tapers off in the direction of the substrate, and it terminates in an end which has a diameter which is larger than that of the bore 3 formed in a substrate 1, so that the above-mentioned blister can form whilst a proper support of the substrate can still be realized.

A channel 11 is formed in housing 4, which channel extends perpendicularly to the boundary surface of the recess formed in housing part 4 on which substrate 1 will be positioned. Channel 11 thereby opens into a chamber 12 near said boundary surface, whereby the diameter of said chamber will be larger than that of a bore 3 formed in a substrate 1. In this manner it can be ensured that the bore 3 formed in a substrate will be in open communication with chamber 12 at all times, even if bore 3 is not provided exactly centrically in substrate 1.

A pressure gauge 13 is connected to the bottom end of channel 11, seen in Figure 2.

Channel 11 is furthermore in open communication with a further channel 14 formed in housing part 4, which extends from channel 11 to boundary surface 6, and which is in open communication at that point with a channel 15 formed in housing part 5, which is in line with channel 14. The mouths of channels 14 and 15 are surrounded by a sealing packing 16 near boundary surface 6.

The end of channel 15 remote from channel 14 opens into the bottom end of a bore or cylinder space 17 formed in housing part 5.

Present in said cylinder space is the end of a piston or plunger 18, which is surrounded by a number of sealing rings or 0-rings 19. In order to enable an efficient manufacture and an effective pressure build-up during operation, the diameters of cylinder space 17 and the piston or plunger 18 accommodated therein have been selected considerably larger than the diameters of channels 11,14 and 15.

When using the above-described device, plunger or piston 18 will gradually be moved downwards in the direction indicated by arrow A after a substrate 1 comprising a coating present thereon has been placed in the device in the above-described manner, all this in such a manner

that a gradual and precisely controlled movement of piston or plunger 18 takes place in cylinder space 17. This may be effected by means of a so- called pressure bench, for example, whereby piston or plunger 18 can be moved in the direction indicated by arrow A in cylinder space 17 by press- on means which can be mechanically moved with great precision. The pressure in cylinder space 17, channel 11 and chamber 12, which are in communication with each other, and which can be considered as communicating vessels, will gradually increase thereby, which pressure increase can be detected by means of pressure gauge 13. When the blister forms in the part of coating 2 that extends over bore 3, the pressure will decrease a little when the blister is comparatively large and the fluid volume is small, which pressure decrease can likewise be established by means of pressure gauge 13. As already known, the pressure at which the formation of the blister in coating 3 takes place, provides an indication as to the bonding strength of coating 2 on substrate 1.

From the above it will be apparent that the bonding strength of a coating on a substrate can be determined in an effective manner by means of a device of simple construction. It is possible thereby to realize a compact construction of the device by having channel 11 and cylinder space 17 extend parallel to each other.

Since the device comprises the two housing parts 4 and 5, which can be joined together by means of bolts, for example, the passages to be provided in said housing parts can be formed in a simple manner.

It will be apparent that the device can be disposed in a position in which cylinder space 17 extends horizontally, or in a position in which said cylinder space extends vertically, or in any other intermediate position. Generally it will be preferred, however, to use the vertical position of the cylinder space, since there is a chance that fluid will run out of the cylinder space in the horizontal position if a blister that has formed in the coating over bore 3 in the substrate breaks.

In the above, a substrate wherein the coating is made up of a coat of paint or lacquer has been discussed. The coating may also be a tape adhering to a substrate, a film of a material applied to a substrate by galvanic or electrolytic means, for example, a layer of glue or a top coat applied to another coating on the substrate.