Abstract:
A thermal spray coating method for creating a coating layer on a surface of a substrate is monitored by determining characteristics of the thermal spray coating as it affects the quality of the coating layer by recording, controlling, and monitoring through a digital camera whose image is analyzed and characterized.

Description:
BACKGROUND AND SUMMARY OF THE INVENTION 
   This application claims the priority of German Patent Document 199 10 892.7, filed Mar. 11, 1999, the disclosure of which is expressly incorporated by reference herein. 
   The present invention concerns a thermal spray coating method for creating a coating layer on the surface of a substrate, wherein at least one characteristic of the thermal spray coating method affecting the quality of the coating layer is recorded, controlled, and/or monitored by a digital camera. The present invention also concerns a device for quality assurance when creating a coating layer on the surface of a substrate by thermal spray coating which comprises a digital camera for recording, controlling, and/or monitoring at least one characteristic of the thermal spray coating method affecting the quality of the coating layer. 
   In the thermal spray coating method, typically an additive is melted onto or applied in molten form with the aid of a gas or gas mixture to the surface of the substrate to be coated. 
   A method and a device of this kind are described in our German patent application 198 20 195.8. The starting point in that application was to guarantee reproducibility; achievement of quality demands; and adherence to prescribed quality requirements by recording, controlling, and/or monitoring the parameters in thermal spray coating. To do so, relevant process parameters are measured, controlled and perhaps also documented. Such parameters could be, for example, gas flows (carrier gas and/or perhaps fuel gas); current strengths; the spraying distance; the spraying angle (angle between the coating jet and the substrate surface); the velocity of the coating jet relative to the substrate surface; the quantity of additive taken up; the quantity of sprayed powder or the wire feed rate; and the like. 
   Within the scope of this present invention, all known variants of thermal spray coating would in principle be feasible as process variants, for example, autogenous flame coating, high velocity flame coating, plasma coating, electric arc coating, detonation coating or laser coating, and also the thermal coating variant known as cold gas coating, which is a type of further development of high velocity coating (for example, as described in the European patent specification EP 0 494 533 B1). In cold gas coating, an additive is in powder form in which the powder particles are not melted in the gas jet during cold gas coating. Instead, the temperature of the gas jet is kept below the melting point of the additive powder particles. 
   In the device as described in our German patent application 198 20 195.9, a digital camera is provided for recording, controlling, and/or monitoring at least one characteristic of the thermal spray coating method affecting the quality of the coating layer. The digital cameras could be either digital image cameras or digital video cameras. The required recording, controlling, and/or monitoring could therefore be achieved by single images and/or video images combined together as sequences to make a film. The boundary between single images on the one hand and film on the other hand is not sharply defined. The lower Iimit for the frame frequency can be regarded as approximately 16 images per second given the slow response of the human eye. 
   The diagnostic for recording, controlling, and/or monitoring of characteristics of the thermal spray coating method affecting the quality of the coating layer, as described in our German patent application 198 20 195.9, allows quality assurance of the thermal coating process with relatively little effort yet with exceptional efficiency. So, for example, in companies where thermal coating is used and at the same time frequent changing of coating applications arises, the reproducibility of the coating layer can be guaranteed, and consistent quality of the coating layers very quickly achieved by a diagnostic that evaluates quality-influencing characteristics or parameters and/or quantitatively measures the spray coating method using image standards. It is important, due to the purely optical approach used, that the recording, controlling, and/or monitoring of the quality characteristics in no way whatsoever adversely affects the thermal spray coating method or damages the coating layer in any way. On the other hand, for example, it can be guaranteed even after a longer period of time that the same application is coated with the same coating accuracy if, for example, the characteristics of the image in the melting zone are identical to the previous ones. 
   The recording, controlling, and/or monitoring by a digital camera can be used to control and, if necessary, optimize one or more parameters. With digital technology, it is completely unproblematic to display and/or evaluate, during the running spray coating process, the recordings made for the purpose of recording, controlling, and/or monitoring the quality of the coating layer so that optimized control of the spray coating parameters can take place. This optimization of the parameters contributes to the economic efficiency of the thermal spray coating method because an ineffective high consumption of one or more of the materials required by the thermal spray coating method (e.g., gas volumes, additives) is avoided, thus allowing savings to be achieved. 
   In doing so, advantage can be taken of the many display possibilities that digital technology provides. Depending on the individual case, the various display variants—in particular computer processing or encoding—can provide particular advantages. The images or video recordings can in principle be presented in black and white or color. Mixed forms with, for example, partial color representation are also possible. 
   The task of the present invention is to provide a method and a device as described at the outset wherein the computer processing and/or encoding has been further developed and improved. In particular, the volume of information upon which the diagnostic is based is to be kept as small as possible or reduced so as to simplify handling, speed, and/or data storage. 
   Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawing. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows the individual stages of an image processing method for quality assurance in thermal spray coating in accordance with the present invention as a series of images; 
       FIG. 2  is a system for generating the digital image processing according to the present invention; and 
       FIG. 3  is a flow chart detaching the image processing according to the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   In the images according to the method of the present invention, at least one area of equal intensity and/or at least one area within a particular intensity interval is assigned to one or more symmetric geometrical surface regions by computer processing and/or encoding. 
   In the device according to the present invention, means are provided for recording the one or more symmetric geometrical surface regions as a data record or records, based on the typical characteristics of the respective geometric shape, whereby at least one of the characteristics of the spray coating method affecting the quality of this coating layer can be stored, controlled, and/or monitored by means of this data record or these data records. 
   The symmetry of the geometrical surface regions includes, within the scope of the present invention, axial symmetry and rotational symmetry. 
   In designing the present invention, the one or more symmetric geometrical surface regions are recorded as a data record or data records based on the typical characteristics for the respective geometric shape and at least one of the characteristics of the thermal spray coating method affecting the quality of the coating layer is recorded, controlled, and/or monitored by this data record or data records. 
   The particular symmetric geometrical surface regions used are circles, squares, rectangles, parallelogram, and/or ellipses. Of those, ellipses are preferentially used because oval structures are formed as a rule which, because of their similar contour to ellipses, can be recorded easily and relatively precisely. 
   It is advantageous to record independent typical characteristics as a data record for the respective geometric shape. This helps to keep the quantity of data small and to obtain the most respective data possible. 
   The computer processing and/or encoding is preferably carried out by a contour detection algorithm; by a gradient steps representation; and/or by a gradient accentuating representation reduced to bit planes. 
   The at least one characteristic of the thermal spray coating method affecting the quality of the coating layer could relate to the spray coating method itself and/or the spraying device being used. 
   The symmetric geometrical surface regions or preferably their data records can be used to control and possibly optimize one or more parameters. 
   It is also possible for the symmetric geometrical surface regions or preferably their data records to be used to document one or more of the characteristics affecting the quality of the coating layer and/or the spray coating method itself. 
   The present invention enables quality assurance by a diagnostic on the basis of relatively (with regard to the large number of parameters in thermal spray coating) small quantities of data and based on representative and unique data for the spray coating method or the spraying device. 
   With regard to the spraying device (burner) the following conclusions can be drawn directly from the geometrical surface regions or preferably their data records:
         for the plasma burner example (plasma coating):
           state of the electrodes,   enthalpy changes in the free jet, and   the enthalpy distribution in the free jet.   
           for the HVOF burner example (high velocity flame coating):
           velocity of the discharged gas (separation of the ultrasonic nodes),   enthalpy changes in the free jet, and   the enthalpy distribution in the free jet.   
               

   The following characteristics of the spray coating method (i.e. the particular jet) can, for example, be recorded from the geometrical surface regions or preferably their data records: (1) melting behavior; (2) aperture; (3) center-of-mass; and (4) direction. 
   The present invention will now be described in more detail with the aid of an example.  FIG. 1  shows the individual stages of an image processing method for quality assurance in thermal spray coating in accordance with the present invention (e.g., plasma coating) as a series of images. The individual stages are: 
   1. Image recording, 
   2. Contour detection, 
   3. Ellipse fitting, 
   4. Ellipse characterization, and 
   5. Database. 
   The sequence of images is based on a computer processing and/or encoding method in accordance with the present invention. 
   Areas of equal intensity in the exposed image sections are marked by a contour detection algorithm, a gradient steps representation, or a gradient accentuating representation reduced to bit planes. This information, representative of both the state and the operation of the spraying device (burner) and of the state and progress of the spray coating method are then used as information carriers for further processing. 
   One or more ellipses are fitted to the resulting oval structures. In doing so, each ellipse is fully characterized by means of its five independent properties. The properties are:
         vertical position of the ellipse center-of-mass,   horizontal position of the ellipse center-of-mass,   length of semi-axis  1 ,   length of semi-axis  2 , and   angle of one of the semi-axes to the horizontal.       

     FIG. 2  illustrates an arrangement to accomplish the imaging and processing of the present invention. A digital camera records images of thermal spray coating of a substrate  80  by way of a sprayer  90 . The digital images captured by camera  70  are fed to a computer  30  having a microprocessor  40 , ROM  60  and a storage device  50  in the form of a CD, CD-ROM, floppy disk, or other media. A keyboard  20  and a monitor  10  complete the system. 
   The processing of the digital image from the camera  7  is detailed in  FIG. 3  wherein the digital image  110  is fed to a contour detector  120  which provides an outline of the geometric shape. The resulting contour is analyzed and compared to a series of ellipses at  130  to find the closest fit for particular portions of the region of the image. The resulting closest fitting chosen ellipses are then subjected to analysis at  140  to determine their characteristics including the five above discussed independent properties. 
   The computer processed or encoded image representation leads to a data record  150  of these independent properties of one or more ellipses which, with regard to quality assurance in thermal spray coating method (in a wider sense), characterizes both the spraying device and the spray coating method itself (in a narrower sense). 
   By this means the information of the entire spray coating method can be reduced to a small quantity of very representative, unique information that allows a simple, mathematically supported process diagnostic to be realized. 
   The foregoing disclosure has been set forth merely to illustrate the invention and is not intended to be limiting. Since modifications of the disclosed embodiments incorporating the spirit and substance of the invention may occur to persons skilled in the art, the invention should be construed to include everything within the scope of the appended claims and equivalents thereof.