Optical method and arrangement for measuring residual stresses, in particular in coated objects

Abstract

The present invention relates to a method and an apparatus for establishing residual stresses in objects, in particular in coated objects, and to a method and an apparatus for coating objects. The method comprises: impinging a surface (8) of the object (5) with laser light and generating a hole or a pattern of holes and/or locally heated points in the object (5); establishing the surface deformations by an optical deforming measuring method after the object (5) is impinged by laser light; establishing the residual stresses present in the object (5) from the measured surface deformations, wherein the generation of the hole pattern is carried out by an optical scanning apparatus which comprises an optical deflection and/or modulation arrangement for controllable deflection and/or modulation of the laser light, and/or a focusing arrangement for controllable focusing of the laser light.

Claims

1. A method for determining residual stresses of an object comprising: applying, by a laser exposure system, laser light to a surface of the object; creating a pattern of holes or a pattern of locally heated points in the object; after applying the laser light to the object, using an optical deformation measurement to determine one or more surface deformations; determining, based on the one or more surface deformations, one or more residual stresses in the object; and adapting a shape of the pattern of holes or of the pattern of locally heated points to at least one of: a topography of the surface of the object, the determined one or more residual stresses, or a gradient of the one or more residual stresses in the object.

2. The method of claim 1, wherein the pattern of holes or the pattern of locally heated points is generated sequentially or in parallel.

3. The method of claim 1, further comprising: adapting a geometry of individual holes of the pattern of holes or the pattern of locally heated points to at least one of a topography of the surface of the object, the determined one or more residual stresses, or a gradient of the one or more residual stresses in the object.

4. The method of claim 1, wherein the pattern of holes or the pattern of locally heated points comprises at least one of: a plurality of lines at least approximately perpendicular to a direction of a largest local curvature of the surface of the object, at least one spiral, wherein the center of the spiral is positioned at location of the largest local curvature of the surface of the object, a circle positioned at the location of the largest local curvature of the surface of the object, or a plurality of concentric circles positioned at the location of the largest local curvature of the surface of the object.

5. The method of claim 1, wherein the pattern of holes or the pattern of locally heated points comprise at least one of a line, a line grid, a matrix, a rosette, a slit, or a cross.

6. The method of claim 1, wherein creating the pattern of holes or the pattern of locally heated points in the object further comprises: generating, by a controllable spatial light modulator, the pattern of holes or the pattern of locally heated points in real time.

7. The method of claim 1, further comprising: obtaining data relating to at least one of a geometry of individual holes or the pattern of holes on the surface of the object or the pattern of locally heated points, a spatial position of the individual holes or the pattern of holes or the pattern of locally heated points, a topography of the pattern of holes or the pattern of locally heated points, a positioning of the pattern of holes or the pattern of locally heated points, a shape of the object, or a shape of the surface of the object.

8. A device for determining residual stresses of an object, the device comprising: a laser exposure system comprising: at least a first laser; and an optical scanning device to apply laser light to a surface of the object and to generate a pattern of holes or a pattern of locally heated points in the object, wherein the optical scanning device comprises: an optical deflection and/or modulation arrangement to controllably modulate the laser light; or a focusing arrangement to controllably focus the laser light; and an optical measuring system that determines a deformation of the surface of the object, wherein the optical scanning device is further programmed and adapted to: adapt a shape of the pattern of holes or of the pattern of locally heated points to at least one of a topography of the surface of the object, one or more residual stresses in the object, or a gradient of the one or more residual stresses in the object; or adapt a position of the pattern of holes or of the locally heated points to at least one of a topography of the surface of the object, the determined one or more residual stresses, or a gradient of the one or more residual stresses in the object.

9. The device of claim 8, wherein the optical scanning device is further programmed and adapted to: generate the pattern of holes or the pattern of locally heated points, sequentially or in parallel.

10. The device of claim 8, wherein the optical scanning device is further programmed and adapted to: vary a geometry of at least one of the holes of the generated pattern of holes or the locally heated points of the generated pattern of locally heated points, a shape of the generated pattern of holes or of the locally heated points, or a position of the generated pattern of holes or of the locally heated points; and adapt the geometry to at least one of a topography of the surface of the object, the determined residual stresses, or gradients of the residual stresses in the object.

11. The device of claim 8, wherein the optical scanning device is further programmed and adapted to generate at least one of: a pattern with a plurality of lines, wherein the plurality of lines are at least approximately perpendicular to a direction of the largest local curvature of the surface of the object, a pattern comprising at least one spiral, a circle, or a plurality of concentric circles, wherein the center of the spiral, the center of the circle, or the center of the concentric circles is positioned at a location of the largest local curvature of the surface of the object, or a pattern comprising at least one of a line, a line grid, a rosette, a matrix, a slit, or a cross.

12. The device of claim 8, further comprising: an optical 3D measuring arrangement that is programmed and adapted to obtain three-dimensional data relating to at least one of a geometry of individual holes, a geometry of the pattern of holes or the pattern of locally heated points, a spatial position of the individual holes, a spatial position of the pattern of holes or the pattern of locally heated points, a topography of the pattern of holes or the pattern of locally heated points, a positioning of the pattern of holes on the surface of the object or the pattern of locally heated points, a shape of the object, or a shape of the surface of the object.

13. The device of claim 8, wherein the optical modulation arrangement further comprises: a controllable light modulator, wherein the controllable light modulator comprises at least one of a liquid crystal light modulator, a DMD light modulator, or an acousto-optical modulator; or a rotating wedge plate.

14. The method of claim 1, wherein the creating the at least one of the hole, the pattern of holes, or the pattern of locally heated points in the object is performed by an optical scanning device, wherein the optical scanning device comprises: an optical deflection and/or modulation arrangement to controllably modulate the laser light; or a focusing arrangement to controllably focus the laser light.

15. The method of claim 1, wherein creating the pattern of holes or the pattern of locally heated points in the object further comprises: generating, by a laser device, the pattern of holes or the pattern of locally heated points, wherein the laser device is also used to determine the surface deformations.

16. A method for determining residual stresses of an object comprising: applying, by a laser exposure system, laser light to a surface of the object; creating a pattern of holes or a pattern of locally heated points in the object; after applying the laser light to the object, using an optical deformation measurement to determine one or more surface deformations; determining, based on the one or more surface deformations, one or more residual stresses in the object; and adapting a position of the pattern of holes or of the locally heated points to at least one of a topography of the surface of the object, the determined one or more residual stresses, or a gradient of the one or more residual stresses in the object.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Further objects, features, and advantages of the present invention will become apparent from a detailed description of preferred embodiments of the present invention with reference to the following drawings, which show:

(2) FIG. 1 an exemplary device for measuring residual stresses in coated objects;

(3) FIG. 2 exemplary exposure arrangements;

(4) FIG. 3 a further exemplary device for measuring residual stresses in coated objects;

(5) FIG. 4 exemplary patterns on a coated cylindrical surface;

(6) FIG. 5 exemplary patterns on a coated spherical surface;

(7) FIG. 6 exemplary patterns on a coated freeform surface.

(8) The terms measuring radiation, exposure radiation, and light as used in the context of the present application relate to electromagnetic radiation from the deep UV via the VIS, NIR, MIR, FIR to the terahertz range.

DETAILED DESCRIPTION

(9) FIG. 1 shows an exemplary device for measuring residual stresses in coated objects. The device comprises an arrangement for exposing a coated object 5 (detail A) and an optical measuring system for measuring 3D deformations or 3D surface deformations, which are created by the exposure of the coated object (detail B).

(10) The object may be an arbitrary coated object, including a strongly curved object. Non-limiting examples include coatings for corrosion protection, protective layers for electrical insulation and/or the thermal protection. The coating may have a thickness in the range of 10 micrometers to 500 micrometers. The number and the arrangement of the layers in the coating may vary.

(11) Different exposure arrangements (laser exposure systems) can be used for the exposure of the coated object 5. Three exemplary exposure arrangements are shown in FIGS. 2a, 2b and 2c. FIG. 2.a shows an exemplary exposure arrangement with a simple construction. The exposure arrangement comprises a pulse laser 1.1, which emits a sequence of short laser pulses 1.2. This may be both in the pico and in the micro-seconds range, in the embodiment here with pulse lengths in the 10-nanosecond range. The exposure radiation emitted by the pulse laser 1.1 is focused onto the object 5 having a coating 6 by a lens 1.3. The color filter 3, shown in FIG. 1, detail A, transmits the light for exposure. The power of the laser pulses 1.2 of the pulsed laser 1.1 may be suitably selected depending on the object and/or coating being examined. The intensity density on the surface 8 of the object 5 may be at least 10.sup.8 W/cm.sup.2, so that material is removed from the surface 8 of the object 5 and a hole 7 having a circular shape 7.1 is formed.

(12) FIG. 2.b shows an exemplary exposure arrangement with a rotating wedge plate 1.4. The wedge plate 1.4 changes the laser beam direction by its rotation. With a rotating wedge plate 1.4, a sequence of laser pulses 1.2 reaches this wedge plate 1.4. Here, each pulse is focused onto another point of the surface 8 of the object 5 by the lens 1.3. This allows material to be removed along a fine circular-ring-shaped line 7.2.

(13) FIG. 2.c shows an exemplary exposure arrangement, in which the light beam is first reflected by a mirror 1.5 in the direction of a spatial light modulator 1.6. Two-dimensional (2D) patterns in the form of holograms, i.e. light-diffracting gratings, are inscribed into the spatial light modulator 1.6. These holograms can be selected within broad limits, so that the incident light beam, as a result of the diffraction, forms a desired pattern 7.3 with an almost arbitrarily programmable form, for example in the form of a ring, in the form of an X, in the form of a cross or also in the form of a double cross (#), and with a depth, which can be selected within broad limits, on the surface 8 of the object 5. If the laser power is sufficiently high per object surface area, material can also be removed at different points at the same time on the surface 8 of the object 5. On the one hand, the lens 1.3 in FIG. 2.c allows focusing of the diffracted laser beam onto the surface 8 of the object 5. On the other hand, however, focusing may also be created by the spatial light modulator 1.6 itself. In this case, the lens 1.3 is not necessary. The removal of material from the surface 8 of the object 5 creates a three-dimensional (3D) deformation of the surface 8 in the immediate vicinity of the hole 7.

(14) Detail B of FIG. 1 shows an exemplary optical measuring system 2 for measuring the surface deformation, which is based on digital holography. A laser 2.1 emits a laser beam of rays that is divided into two partial beams of rays by the first beam splitter 2.2. One partial beam of rays is coupled into an optical monomode fiber 2.4 by the lens 2.3. The light is guided through the monomode fiber 2.4 and the output of the monomode fiber 2.4 opens into a small hole 2.5, so that the light, which in this case represents the reference beam or rays 2.6 for the holographic measuring arrangement, reaches a CCD or CMOS detector 2.7.

(15) The other partial beam of rays from the first beam splitter 2.2 is split again into at least 3 partial light beams of rays by the second beam splitter 2.8. FIG. 1 shows four partial light beams of rays 4.1, 4.2, 4.3 and 4.4. These partial light beams of rays 4.1, 4.2, 4.3 and 4.4 illuminate the surface 8 of the object 5 from at least three independent directions. This illumination can be carried out simultaneously with all partial light beams of rays 4.1, 4.2, 4.3 and 4.4 or also one after the other. The light scattered from the surface 8 of the object 5 is reflected by the color filter 3 in the direction of the measuring system 1. The lens 2.9 images the surface 8 of the object 5 onto the detector 2.7. The aperture stop 2.10 determines the lateral resolution of the imaging system. The light scattered from the surface 8 of the object 5 interferes with the reference beam of rays 2.6. The interference pattern is recorded by the detector (2.7) and then represents a digital hologram. Prior to the exposure of the object 5, several holograms with different illuminations from at least three independent directions are registered. After the exposure, further holograms with different illuminations are registered. After evaluation of the holograms, the 3D deformation between the non-exposed and the exposed state is determined. This process can be repeated, so that different exposure states are created and thus the 3D deformation is determined as a function of the exposure.

(16) The geometry (depth, shape) of the hole formed by the laser exposure and/or the topography of the resulting pattern is measured by a measuring arrangement for the 3D shape 9. Said measuring arrangement may be a confocal microscope or a system based on stripe projection or digital holography, the latter being based on the two-wavelength method, for example.

(17) The 3D deformation in connection with the measurement of the geometry (depth, shape) of the holes and/or the topography or depth profile of the pattern, which arise as a result of the laser exposure, and material parameters of the object 5 and the coating 6 are evaluated and the residual stresses present in the coating are determined, for example by a finite element method. Further methods for determining the residual stresses are known from the prior art.

(18) FIG. 3 shows an exemplary device for measuring residual stresses in coated objects, in which a pulsed laser having two wavelengths (10), .sub.1 and .sub.2, is used both for exposure of the object and for optical measurement. The color splitter 2 reflects the beam with the wavelength .sub.2 and transmits the beam with the wavelengths .sub.1. The beam with the wavelength .sub.1 is used for the laser exposure system 13. The beam with the wavelength .sub.2 is used for the optical measuring system 14.

(19) FIG. 4 shows a coated cylindrical surface 15 onto which line patterns (radial 16, axial 17), cross patterns 18 and/or elliptical or ring-shaped patterns 19 for the local removal of material are inscribed. FIG. 5 shows a coated spherical surface 20 onto which the spirals 21, double or multi-spirals 22 for the local removal of material are inscribed. FIG. 6 shows a coated free-form surface 30 onto which adapted line patterns for the local removal of material are inscribed.

(20) The exemplary methods and devices for determining residual stresses may be integrated into a coating process as described above, for example. On the basis of the determined residual stresses, the parameters of the coating process (such as cooling or heating of the substrate, application rate and temperature of the coating, layer thickness, kinematics, etc.) can be controlled in-line (i.e. during the coating process). In this way, it is possible to create coatings with high quality and low residual stresses in a fast and efficient manner.

(21) Throughout the Figures reference numbers are used to denote same or similar elements. Moreover, a list of reference numerals and corresponding explanations are provided in Table I.

(22) TABLE-US-00001 TABLE 1 List of Reference Numerals with Designations Reference numeral Designation 1 laser exposure system (exposure arrangement) 1.1 pulsed laser 1.2 laser pulses 1.3 lens 1.4 rotating wedge plate 1.5 mirror 1.6 spatial light modulator (SLM) 2 optical measuring system for determining the surface deformations (e.g. on the basis of digital holography) 2.1 laser 2.2 first beam splitter 2.3 lens for coupling light into a fiber 2.4 optical monomode fiber 2.5 small hole for fiber 2.6 reference beam of rays 2.7 detector (CCD or CMOS) 2.8 second beam splitter 2.9 imaging lens 2.10 aperture stop 3 color splitter 4.1 to 4.4 partial light beam of rays for object illumination for deformation measurement 5 coated object 6 coating 7 laser-drilled hole on the coated object 7.1 laser-drilled circular hole on the coated object 7.2 laser-drilled ring on the coated object 7.3 laser-drilled patterns on the coated object 8 surface of the object 5 or 3D deformation of the surface of the object 5 9 optical 3D measuring arrangement for 3D measurement or detection of the surface of the object 10 pulsed laser with two wavelengths, .sub.1 and .sub.2 11 color splitter 12 mirror 13 optical system for laser exposure system (without laser) (as part of the scanning device) 14 measuring system for digital holography (without laser) 15 cylindrical surface 16 line pattern (radial) 17 line pattern (axial) 18 cross pattern 19 ring-shaped pattern 16.a multi-line pattern (radial) 17.a multi-line pattern (axial) 18.a multi-cross pattern 19.a multi-ellipse-pattern or multi-ring pattern 20 spherical surface 21 spiral on spherical surface 21.a double spiral on spherical surface 30 free-form surface 31 adapted patterns (adjusted)