Ceramic coatings are commonly used to improve the wear or heat resistance of many technical components, but due to their application process, e.g. plasma or high velocity oxygen fuel spraying, rather high residual stresses can build up within the coating and underneath. The reason for that are differences in the coatings and substrates expansion coefficients, inhomogeneous distributed temperature during the process and the quenching of splats1. The hole drilling technique can be used for the detection of residual stresses in coatings. This is a quasi-non-destructive method with the advantage of time/cost efficient application and possibility of in situ measurements, without neglecting a good depth resolution and quantitative accuracy2-5. The residual stresses are locally released due to the controlled material removal, which leads to a deformation of the surface around the hole. These deformations, measured as relaxed strains through strain gauges rosettes, in combination with appropriate calibration curves (separately determined by simulation for the layer composite), allows the quantitative determination of the residual stress depth profiles. The disadvantage of the strain gauges is that they can only be used on flat and relatively smooth surfaces, where the rosette is applied. Furthermore they measure only in-plane strains (2-dimensional). Since the distance between the hole-centre and the strain gauge is always relatively large, there are limitations both in terms of spatial resolution and practicality. The accuracy of the measurements depends as well on the asymmetries of the hole and the positioning of the strain gauge rosette. We propose an approach to avoid the mechanical drilling operation and the application of strain gauges, where a pulsed laser is used for the object machining (ablation process) leading to 3D residual deformation by stress relaxation which are measured by an optical system based on digital holographic interferometry. With the proposed method the fieldwise distribution and the shape of the holes as well can be controlled simultaneously by beam conditioning using a SLM. For the validation of the method, test plates were prepared, where aluminium/titan oxide coatings are deposited by atmospheric plasma spraying technique on aluminium substrates.
The experimental setup for residual stress analysis (Fig.1) can be divided into two parts one for the machining of the object and the other for the measurement of the resulting 3D deformations. The harmonic separator (HS), transmits the infrared light for the laser machining (wavelength: 1064 nm) and reflects the visible green light (wavelength: 532 nm), for the deformation measurement, allowing at the same time machining and deformation measurements. Laser pulses with a power density higher than 109 W/cm2 are used for the ablation of material, in order to obtain such density the laser beams having pulse length of some nanosecond are focused by a lens on the sample surface. Complex structures are machined by using a spatial light modulators (SLM), where a given light distribution is produced by writing a phase/amplitude pattern (computer generated hologram) on the SLM. The release of residual stresses by the laser machining system produces 3D deformations that are measured by the system based on digital holography shown in the bottom part of Fig. 1. Light from a laser is divided into two beams by the beam splitter BS, one is coupled into a single mode optical fibre and serves as the reference beam and the other one is further divided into four beams illuminating the object sequentially from four different directions. The phase of the wavefront scattered by the object changes as a function of the deformation6, 7, and by processing holograms recorded by different illumination it is possible to measure the 3D deformation around the machined surface.
MEASUREMENT PROCEDURE AND RESULTS
The SLM based system was used for machining structures with different shape and depth on the coated surface. Figure 2.a shows a milled horizontal bar obtained after 6.4000 laser pulses, the depth of the machined structure is 130 μm. Figures 2. b-g show the wrapped phase and the corresponding 3D deformations produced by the milling. By incremental loading structures (bars, crosses, rings) having different depth are produced and the resulting 3D deformations are measured. The residual stresses at different depth of the coating are calculated from the deformations together with the profile (shape, depth) of the machined surface and the material parameters. The coating used for the investigations shown in Fig. 2 had thickness of 70 μm, at this depth the residual stress was -250 MPa.
The following procedure is applied for the measurements:
1. the object is illuminated sequentially from 4 directions and 4 holograms are recorded (it is possible to illuminate the object simultaneously from four directions and record the information about the 3D deformation in a single shot, but in this case, the quality of the measurements will be reduced due to the recording of four superimposed holograms in one frame),
2. a pulsed laser is used for machining the surface,
3. the object is illuminated again from 4 directions and 4 holograms are again recorded,
4. from the holograms recorded before and after the laser processing, the 3D deformations due to the surface machining (relaxation of residual stresses) are calculated.
The steps 2-4 are repeated M times, allowing the measuring of 3D deformations as a function of the incremental machining (thus, measuring residual surface strain/drilling depth profiles). The surface shap and deformation is measured by digital holography6, 7. Based on a suitable FEM model considering the material and load parameters the residual stresses are calculated by a combined and model based strategy8.
A system using a pulsed laser for machining of a surface and digital holography suitable for in-process measurement of the resulting 3D deformations that are due to relaxation of residual stresses in the material, was described. The obtained deformations together with the profile (shape, depth) of the surface machining pattern and the material parameters can be used to determine the residual stresses at different depths below the surface of a bulk material or a coating. The accuracy of the deformation measurement is ±8 nm, and thus, well suited to retrieve relaxation strains from which residual stresses can be obtained. For the particular case of drilling a hole with a laser, the residual stresses were calculated for coated sample materials and are in agreement with the results obtained by using the standard, mechanical hole drilling strain gauge method. Investigations that should allow the calculation of residual stresses at different depths from the deformation produced by laser milling of complex shapes (bars, rings, crosses) are still in progress. Compared with the standard hole drilling technique, the method presented in this paper has the advantage that the machining is done without mechanical contact by using a pulsed laser. Furthermore, the use of a SLM modulator allows more flexibility in the machining, producing not only circular holes but also more complex structures of arbitrary shape. As a further advantage, measurement of the surface deformations is done optically and does not require the application of devices (such as strain gauges, which require a high degree of accuracy during application and which are not applicable on every surface) or manipulations on the surface (like polishing, lacquering or similar). Due to the fact that machining and measurement are achieved without contact, the method permits (in principle) in-process measurement and can thus improve the hole drilling residual stress analysis, allowing not only for residual stress detection in static condition after component processing, but also the in-process measurement of process-induced, transient stresses (thermal stresses, coating solidification stresses etc.). The next step of our investigations will be to implement a system using laser machining as well as deformation and shape measurement during the coating application process. This would lead to an increased understanding of residual stress evolution, especially a weighting of the different contributing phenomena to the final state of residual stresses, and could enable the manufacturing of coated parts with a residual stress state that lies within narrow limits of application oriented, predefined residual stress specifications. The method was developed for the machining and measurement of coated surfaces, but it could be used as well for other kind of applications where residual stresses inside an object (close to its surface) need to be determined, i.e. hard materials, where a mechanical drilling is not possible or small objects, on whom strain gauge rosettes cannot be placed.
This work was supported by the German Research Foundation (DFG) under Grant Nos. GA746/10, OS111/37 and Schm 746/120.