Coating processes – Measuring – testing – or indicating
Reexamination Certificate
2001-04-20
2003-03-25
Bareford, Katherine A. (Department: 1762)
Coating processes
Measuring, testing, or indicating
C427S446000, C118S666000, C118S712000, C118S302000
Reexamination Certificate
active
06537605
ABSTRACT:
BACKGROUND
1. Field of the Invention
The invention relates to a method for coating high-temperature components by means of plasma spraying, in particular gas turbine components. The invention also relates to a coating device having an infrared camera.
2. Related Art
In addition to other thermal coating methods, because of its flexible use options and a good economic balance, plasma spraying is of great importance in the production of coatings for protecting components, for example against corrosion by hot gases. Vacuum plasma spraying (VPS), low-pressure plasma spraying (LPPS) and atmospheric plasma spraying, inter alia, are among the various known methods.
In plasma spraying technology, a coating is produced by directing a very hot plasma jet onto the substrate to be coated while feeding material which is to be applied. The coating material is present in this case mostly as powder or wire and is fused during transport by the plasma jet before striking the substrate. It is therefore possible in principle to produce the most varied layer thicknesses using very different coating materials and substrate materials. It is possible to use metal powder and ceramic powder in the most varied mixtures and grain sizes as long as the starting material has a defined melting point. An MCrAlY layer, M standing for the metals Ni and Co, is used, for example, to coat gas turbine buckets with a layer protecting against corrosion by hot gases.
The type and quality of the layer is influenced, inter alia, by the pore content, the oxide and nitride content and by its adhesive properties. In addition to the roughness of the surface, the mutual diffusion of the different materials or chemical reactions are important adhesion mechanisms. It is frequently necessary to apply an adhesion promoter layer before applying the actual protection layer, in particular whenever there is a need to balance different coefficients of thermal expansion.
Various methods are applied to monitor the quality of the coating. Preference is to be given in this case to nondestructive tests such as are provided by ultrasonic or infrared technology, for example. In the case of the first-named methods, it is frequently disadvantageous that the inspection instruments touch the surface of the workpiece, thereby limiting the use options, for example to specific component geometries. Furthermore, errors frequently occur owing to surface contamination and surface irregularities or other surface anomalies. The inspection of the component consists in observation over a large area and in an averaging fashion.
Many of these disadvantages are eliminated in the case of infrared technologies. They are based on the fact that, in a fashion correlated with the temperature of the component, each material absorbs and emits electromagnetic radiation which is recorded by infrared detectors. The infrared methods can be used quickly and flexibly and can be applied without difficulty with controlling and regulating systems.
An infrared thermography method represented in U.S. Pat. No. 5,111,048 can be used to detect cracks which arise, for example, due to stresses in the layers. In this case, laser radiation is used to produce contrast between the fault positions and the remainder of the surface. By contrast with the undisturbed surface,fault positions exhibit other absorption or emission properties of electromagnetic radiations. It is disadvantageous, inter alia, that this method cannot be used in a coating chamber during coating, and that the radiation must firstly be excited by external radiation means independently of the heating.
A device and a method for inspecting the thickness and the faults of the coating by means of an infrared technique is described in GB 2 220 065. In this case, the coated component is irradiated by a short infrared pulse and the response beam is recorded by an infrared camera. The region to be inspected is illuminated in this case more homogeneously than in the method described above. It is disadvantageous, inter alia, that at higher process temperatures the infrared radiation of the heated component and of the flash lamp overlap in a way which is difficult to separate for the purpose of detection and evaluation provided in the measurement method.
The monitoring methods set forth above and others, as well, are generally carried out after fabrication of the coating. However, it is desirable to carry out online monitoring as early as during the coating, in order to intervene for control purposes, if required, and/or to control the method with the aid of the results. Moreover, monitoring and control, associated therewith, of the method parameters is indicated during the process in order to ensure the quality and to improve the method.
A method for online monitoring of the coating during the coating operation is described in U.S. Pat. No. 5,047,612. An infrared detector is used to determine the position of the jet spot of the plasma jet on the component to be coated, and the application of the coating is influenced during the coating by controlling the powder flow and the carrier gas of the powder. It is disadvantageous in this case that the setting of process parameters is performed essentially independently for each component. The control of the powder distribution does not, moreover, constitute per se a sufficient condition for a reliable adhesion of the coating which satisfies the operating requirements.
By contrast, the surface temperature of the component to be coated is of fundamental importance for forming the various protective functions of the coating. The abovementioned MCrAlY layers achieve their protective function by, for example, forming aluminum oxide or chromium oxide layers. Attack by oxidation, in particular, is thereby prevented in the base material. The oxide layers are formed differently depending on the surface temperature of the component. In accordance with recent results, the surface temperature of the substrate and the temperature gradient on the component surface are likewise to be accorded greater importance for the adhesion of different metal/ceramic layers in the plasma spraying process (see, for example, Proc. Int. Therm. Spr. Conf. 1998, Nice, France, pages 1555 ff.).
Pyrometers are frequently used at a point on the surface of the component which is to be freely defined for the purpose of temperature measurement during plasma spraying. However, these supply only point measurements, and in the event of a movement of the bucket during the conduct of the process there is a risk that pyrometric temperature measurement will be carried out at differing locations on the bucket surface. The temperature measured in this way is therefore subject to large fluctuations which cannot be calculated.
It is therefore the object of the present invention to improve the initially mentioned method/the initially mentioned device such that the quality of the layers produced can be observed and set reliably and reproducibly during the coating method
SUMMARY OF THE INVENTION
An area-wide overview of the component surface is obtained in real time by means of measuring the thermal distribution of a surface region of the component with the aid of an infrared camera for the purpose of the present invention. Measurement of the thermal radiation with the aid of an infrared camera has certainly already been used to monitor the application of powder during plasma coating, for example in the abovenamed known method according to U.S. Pat. No. 5,047,612. By contrast, in the present invention the exact absolute temperature distribution of the overall component surface or of selected, predetermined sections of the component surface is determined exactly and as a function of time. An infrared camera according to the invention corresponds to an infrared-sensitive CCD array with optical systems for imaging the component on the CCD array, and to intensity- or frequency-dependent evaluation devices. The temperature distribution is determined from the thermal distribution by comparing the thermal radiation of the component surface measu
Kirchner Franz
Raake Dieter
Reymann Helge
Bareford Katherine A.
Eckert Seamans Cherin & Mellott , LLC
Siemens Aktiengesellschaft
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