A method for testing the residual stress of a coating

Abstract

The present invention discloses a method for testing the residual stress of a coating, which relates to the technical field of non-destructive detection of residual stress. The testing method of the present invention comprises the following steps: (1) determining the phases and corresponding crystal structures in the coating by using the X-ray diffraction method; (2) obtaining a diffraction pattern by using the grazing incidence method; (3) performing whole-spectrum fitting by using the Rietevield method, and calculating the residual stress of the coating through the peak position deviation value obtained by fitting. The testing method of the present invention is simple, and can analyze a complex stress state with relatively high accuracy.

Description

Technical Field

[0001] The present invention relates to the technical field of non-destructive testing of residual stress, and particularly to a method for testing the residual stress of a coating. Background Art

[0002] The theory of XRD for testing the residual stress of crystalline materials has been proposed for decades. According to actual needs, the measurement methods include co-tilting, side-tilting, grazing incidence, etc. However, according to research, in addition to theoretical and experimental studies, there are few actual cases of applying the detection method to coating stress detection for improving the production process.

[0003] The main reasons are as follows: when the coating is thin, the degree of crystallization is poor, or the crystal grains are small, it is difficult to distinguish the specific peak positions of the phases with similar crystal structures; the matrix phase will also interfere with the measurement process, and the diffraction peaks may overlap with the phases to be analyzed; it is difficult to calculate the actual stress through the peak positions. In addition, with the change of the coating preparation process, solid solution phases with different lattice constants may be generated; since the XRD test method infers the stress by studying the peak position shift caused by the change of the lattice constant of the phase under different stress states, if the phase changes, the measured peak position shift is not only caused by the stress change, and the calculated result is likely to be meaningless. Summary of the Invention

[0004] The purpose of the present invention is to overcome the deficiencies of the above-mentioned prior art and provide a method for testing the residual stress of a coating.

[0005] To achieve the above purpose, the technical solution adopted by the present invention is as follows:

[0006] A method for testing the residual stress of a coating, the coating is attached to a substrate, and the test method includes the following steps:

[0007] (1) Using X-ray diffraction method to determine the phases and corresponding crystal structures in the coating;

[0008] (2) Obtaining a diffraction pattern by grazing incidence method;

[0009] (3) Performing whole pattern fitting by Rietevield method, and calculating the residual stress of the coating through the peak position deviation value obtained by fitting.

[0010] The present invention proposes to use the Rietevield method to replace the traditional single peak analysis method to characterize the analysis results of the grazing incidence method, which can avoid the influence of overlapping peaks, and the measurement results have high accuracy, good repeatability and stability, and RSD < 5%.

[0011] First, the present invention uses X-ray diffraction method (B-B geometry, θ-θ or θ-2θ scan) to determine the phases and corresponding crystal structures in the coating and determine the analysis object, so as to avoid misinterpreting the peak position deviation caused by overlapping peaks or precipitated phases as the peak position shift caused by stress, which affects the accuracy of the results. Secondly, the crystallinity of the phases is quickly judged through the spectrum to analyze whether the method described in the present invention can be used for stress measurement. If the crystallinity is too low, accurate stress measurement cannot be carried out. Thirdly, the penetration depth of the grazing incidence method is shallow and it is insensitive to the texture, which is suitable for measuring the coating stress.

[0012] Preferably, in the step (1), according to the spectrum obtained by the X-ray diffraction method, through the analysis software EVA, the phases are matched with known elements to determine the types of phases and the corresponding crystal structures, so as to prepare for the full-spectrum fitting by the Rietevield method.

[0013] Preferably, in the step (2), a Cu target is selected for testing, the scanning angle is 20-150°, and the incident angles are 1°, 3°, 5°, and 16° respectively.

[0014] Preferably, in the step (3), the CIF card corresponding to the determined crystal structure is imported into the analysis software TOPAS, and the Rietevield method is used for full-spectrum fitting; the residual stress A of the coating and the residual stress B of the substrate are calculated respectively, and the residual stresses corresponding to the incident angles of 1°, 3°, 5°, and 16° are marked as A1, A2, A3, A4, B1, B2, B3, B4 respectively.

[0015] The measurement result is closely related to the crystal structure of the measurement object. To obtain an accurate residual stress measurement result, a corresponding detection method and target material need to be adopted according to the crystal structure characteristics of the measurement object. The inventors of the present application found through multiple experiments that more accurate test results can be obtained by using the above parameter conditions for testing.

[0016] Preferably, it further includes step (4): measuring the residual stress of the coating by the side-rotation method, and the average stress measured at a ψ angle of 0-45° is marked as σ1, and the average stress measured at a ψ angle of 0-70° is marked as σ2;

[0017] When (σ1 - σ2) / σ1 ≥ 5%, if A1, A2, A3, A4 are all less than or equal to 0, then select the A value with the same incident angle as the B value greater than or equal to 0 in B1, B2, B3, B4 as the residual stress value of the coating; if A1, A2, A3, A4 are all greater than or equal to 0, then select the A value with the same incident angle as the B value less than or equal to 0 in B1, B2, B3, B4 as the residual stress value of the coating;

[0018] When (σ1 - σ2) / σ1 < 5%, select A1 or A2 as the residual stress value of the coating.

[0019] The side tilt method can be used to determine the stress state of the phase to be analyzed, which has a guiding role in stress measurement. Usually, when the stress curve is approximately a straight line, ψ can be taken as 45°; however, when there are situations such as texture and stress gradient in the material, as large a ψ angle as possible can weaken the influence of texture and reflect the stress change trend more completely; but if the ψ angle is too large, the discrete phenomenon will be more serious, easily leading to peak shape distortion, and here the maximum value of ψ is 70°.

[0020] Preferably, the coating includes a first coating and a second coating, the first coating is attached to the substrate, and the second coating is attached to the first coating.

[0021] Preferably, when performing full-spectrum fitting using the Rietevield method, calculate the residual stress C of the second coating and the residual stress D of the first coating respectively, and the residual stresses corresponding to the incident angles of 1°, 3°, 5°, and 16° are marked as C1, C2, C3, C4, D1, D2, D3, D4 respectively.

[0022] Preferably, it further includes step (4): Measure the residual stress of the coating using the side tilt method, the average stress measured at a ψ angle of 0 - 45° is marked as σ1, and the average stress measured at a ψ angle of 0 - 70° is marked as σ2;

[0023] When (σ1 - σ2) / σ1 ≥ 5%, if C1, C2, C3, C4 are all less than or equal to 0, then select the D value greater than or equal to 0 among D1, D2, D3, D4 as the residual stress value of the first coating, and select the C value with the same incident angle as the D value greater than or equal to 0 as the residual stress value of the second coating; if C1, C2, C3, C4 are all greater than or equal to 0, then select the D value less than or equal to 0 among D1, D2, D3, D4 as the residual stress value of the first coating, and select the C value with the same incident angle as the D value less than or equal to 0 as the residual stress value of the second coating;

[0024] When (σ1 - σ2) / σ1 < 5%, select C1 or C2 as the residual stress value of the second coating, and select D1 or D2 as the residual stress value of the first coating.

[0025] Preferably, when using the side tilt method for testing, the φ angle is taken as 0° and 180°, the ψ angle ranges from 0 to 70°, and 5 - 15 points are taken at equal intervals to measure the average stress σ1 and the average stress σ2.

[0026] The ψ angle is the angle between the normal line of the specimen surface and the normal line of the diffraction crystal plane (the angle of specimen rotation in the vertical direction), and the φ angle is the angle of specimen rotation in the horizontal direction.

[0027] Preferably, the coating is selected from at least one of TiAlN, TiCN, and Al2O3, and the substrate is a cemented carbide substrate.

[0028] Compared with the prior art, the beneficial effects of the present invention are as follows: By using the Rietevield method to perform fitting analysis on the diffraction pattern obtained by the grazing incidence method, the present invention can solve the problems of texture and overlapping peaks, and the reliability of the test results is very high. Detailed implementation manners

[0029] To better illustrate the purpose, technical solution, and advantages of the present invention, the present invention will be further described below in conjunction with specific embodiments.

[0030] Four samples were used to verify the coating residual stress test method described in Example 1. The four samples were respectively labeled as Sample 1#, Sample 2#, Sample 3#, and Sample 4#. The matrixes of the four samples were prepared from 95 wt.% WC and 5 wt.% Co. An INNOVA coating furnace was used to prepare a coating with residual stress: First, arc discharge was initiated by an arc initiation device, an electric double layer was formed near the cathode and near the surface of the target material, and the target material was heated and evaporated by the action of the electric field. Under the action of the negative bias voltage of the workpiece, it was deposited on the matrix at high speed to form a dense coating structure.

[0031] The bias voltage applied during the coating deposition process of Sample 1# was 40 V, the coating composition formed by deposition was TiAlN, and the coating thickness was 3 μm;

[0032] The bias voltage applied during the coating deposition process of Sample 2# was 80 V, the coating composition formed by deposition was TiAlN, and the coating thickness was 3 μm;

[0033] During the coating deposition process of Sample 3#, a bias voltage of 40 V was first applied to form a TiAlN coating with a thickness of 1.67 μm, and then a bias voltage of 80 V was applied to form a TiAlN coating with a thickness of 3.33 μm;

[0034] During the coating deposition process of Sample 4#, a bias voltage of 40 V was first applied to form a TiAlN coating with a thickness of 3.68 μm, and then a bias voltage of 80 V was applied to form a TiAlN coating with a thickness of 0.62 μm.

[0035] The same phase is used as the same coating.

[0036] Example 1

[0037] An embodiment of the method for testing the residual stress of the coating described in the present invention. In this embodiment, samples 1#, 2#, 3#, and 4# are tested. The testing method includes the following steps:

[0038] (1) Use an X-ray diffractometer (B-B geometry, θ-θ scanning method) to determine the phase composition of the TiAlN coating attached to the cemented carbide substrate. The scanning angle is 10-90°, and the target is a Cu target. Analyze by elements Ti / Al / N / W / C / Co to determine that the phase composition is the cubic phase of TiAlN and the hexagonal phase of the matrix WC, and AlN hexagonal phase is precipitated.

[0039] (2) Obtain the diffraction pattern by the grazing incidence method. Select a Cu target for testing. The scanning angle is 20-150°, and the incident angles are 1°, 3°, 5°, and 16° respectively.

[0040] (3) Use the Rietevield method for full-spectrum fitting. Calculate the residual stress A of the coating and the residual stress B of the substrate respectively. The residual stresses corresponding to the incident angles of 1°, 3°, 5°, and 16° are marked as A1, A2, A3, A4, B1, B2, B3, and B4 respectively. Record the results in Table 1.

[0041] The fitting process is to import the CIF cards of the corresponding crystal structures of WC, AlN, and TiAlN into the analysis software TOPAS for fitting, adjust the corresponding structure parameters, and obtain the fitting pattern closest to the actual peak shape. At this time, the stress value is calculated according to the following formula:

[0042] s = -2Rad·Tan(θ)·((v + 1)·Sin(θ0) 2 -2·v) / E / △2θ;

[0043] Among them, s - stress, θ - diffraction angle, v - Poisson's ratio, θ0 - Bragg angle of the diffraction peak of the stress-free specimen, E - elastic modulus.

[0044] (4) The test object of the side tilt method is the (200) crystal plane of TiAlN. When testing, the φ angle is taken as 0° and 180°, the ψ angle ranges from 0 to 70°, and 11 points are taken at equal intervals. Measure the average stress σ1 measured at ψ angles of 0-45° and the average stress σ2 of 0-70°;

[0045] When (σ1 - σ2) / σ1 ≥ 5%, if A1, A2, A3, and A4 are all less than or equal to 0, then select the A value with the same incident angle as the B value greater than or equal to 0 among B1, B2, B3, and B4 as the residual stress value of the coating; if A1, A2, A3, and A4 are all greater than or equal to 0, then select the A value with the same incident angle as the B value less than or equal to 0 among B1, B2, B3, and B4 as the residual stress value of the coating;

[0046] When (σ1 - σ2) / σ1 < 5%, select A1 or A2 as the residual stress value of the coating.

[0047] The test results of σ1 and σ2 are shown in Table 1, and the calculation formula is as follows:

[0048]

[0049] Where σ is stress, K is the stress constant, θ is the diffraction angle, ψ is the angle between the normal of the specimen surface and the normal of the diffracting crystal plane, E is the elastic modulus, v is the Poisson's ratio, and θ0 is the Bragg angle of the diffraction peak of the stress-free specimen.

[0050] Three samples were used to verify the coating residual stress test method described in Example 2. The three samples were respectively labeled as Sample 5#, Sample 6#, and Sample 7#. The matrix of the three samples was prepared from 94 wt.% WC and 6 wt.% Co. Using an IHI BernexAG CVD 530L BPXpro coating furnace, a coating with MT-TiCN and Al2O3 as the main functional layers was deposited by chemical vapor deposition. First, 8.5 μm of MT-TiCN was deposited on the cemented carbide substrate, and then 7 μm of Al2O3 was deposited.

[0051] Sample 5# was post-treated with a sandblasting pressure of 2 bar and a sandblasting time of 1.8 s;

[0052] Sample 6# was post-treated with a sandblasting pressure of 2 bar and a sandblasting time of 2.6 s;

[0053] Sample 7# was post-treated with a sandblasting pressure of 2.5 bar and a sandblasting time of 4.5 s.

[0054] Example 2

[0055] An embodiment of the test method for the coating residual stress of the present invention. In this embodiment, Samples 5#, 6#, and 7# were tested. The test method includes the following steps:

[0056] (1) Use an X-ray diffractometer (B-B geometry, θ-θ scanning method) to determine the phase composition of the TiCN and Al2O3 coatings attached to the cemented carbide substrate. The scanning angle is 10 - 90°, and the target is a Cu target; Analyze by elements Ti / Al / C / N / O / W / Co to determine that the phase composition is the cubic phase of Al2O3, the cubic phase of TiCN, and the hexagonal phase of the matrix WC.

[0057] (2) Obtain the diffraction pattern by the grazing incidence method. Use a Cu target for testing. The scanning angle is 20 - 150°, and the incident angles are 1°, 3°, 5°, and 16° respectively.

[0058] (3) The Rietevield method is used for full-spectrum fitting to calculate the residual stress C of the Al2O3 coating and the residual stress D of the TiCN coating respectively. The residual stresses corresponding to the incident angles of 1°, 3°, 5°, and 16° are marked as C1, C2, C3, C4 and D1, D2, D3, D4 respectively. Record the results in Table 2.

[0059] The fitting process is to import the CIF cards of Al2O3 and TiCN into the analysis software TOPAS for fitting, adjust the corresponding structural parameters, and obtain the fitting pattern closest to the actual peak shape. At this time, the stress value is calculated according to the following formula.

[0060] s = -2Rad·Tan(θ)·((v + 1)·Sin(θ0) 2 - 2·v) / E / △2θ;

[0061] Among them, s - stress, θ - diffraction angle, v - Poisson's ratio, θ0 - Bragg angle of the diffraction peak of the stress-free specimen, E - elastic modulus.

[0062] (4) The test object of the side-inclination method is the Al2O3 (116) crystal plane. When testing, the φ angle is taken as 0° and 180°, the ψ angle ranges from 0 to 70°, and 11 points are taken at equal intervals. Measure the average stress σ1 measured at ψ angles from 0 to 45° and the average stress σ2 from 0 to 70°;

[0063] When (σ1 - σ2) / σ1 ≥ 5%, if C1, C2, C3, and C4 are all less than or equal to 0, then select the D value greater than or equal to 0 among D1, D2, D3, and D4 as the residual stress value of the TiCN coating, and select the C value with the same incident angle as the D value greater than or equal to 0 as the residual stress value of the Al2O3 coating; if C1, C2, C3, and C4 are all greater than or equal to 0, then select the D value less than or equal to 0 among D1, D2, D3, and D4 as the residual stress value of the TiCN coating, and select the C value with the same incident angle as the D value less than or equal to 0 as the residual stress value of the Al2O3 coating;

[0064] When (σ1 - σ2) / σ1 < 5%, select C1 or C2 as the residual stress value of Al2O3, and select D1 or D2 as the residual stress value of TiCN.

[0065] The test results of σ1 and σ2 are shown in Table 2. The calculation formula is as follows:

[0066]

[0067] Among them, σ is stress, K is stress constant, θ is diffraction angle, ψ is the angle between the normal of the specimen surface and the normal of the diffracting crystal plane, E is elastic modulus, v is Poisson's ratio, and θ0 is the Bragg angle of the diffraction peak of the stress-free specimen.

[0068] Table 1

[0069]

[0070]

[0071] Note: When the stress value is close to the subsequent deviation value, the residual stress is usually considered approximately 0.

[0072] Table 2

[0073]

[0074] Note: When the stress value is close to the subsequent deviation value, the residual stress is usually considered approximately 0.

[0075] As can be seen from Table 1, for samples 1# and 2#, (σ1 - σ2) / σ1 < 5%, so the results measured at the incident angles of 1° and 3° are used as the residual stress values of the coating. The surface stress of sample 1# is -2500 to -3000 MPa; the surface stress of sample 2# is -4900 to -5600 MPa; at this time, the test results are similar to those of the side-rolling method. For samples 3# and 4#, (σ1 - σ2) / σ1 ≥ 5%, and A1 to A4 are all less than 0. Select the A value with the same incident angle as the B value greater than or equal to 0 as the residual stress value. Then, the surface stress of sample 3# is -6600 to -9500 MPa, and the surface stress of sample 4# is -3800 to -4600 MPa.

[0076] Theoretically, as the bias voltage increases, the coating stress should increase. Therefore, the stress of sample 2# should be greater than that of sample 1#; the coating formed on sample 3# with an 80V bias voltage is thicker than the coatings formed on samples 2# and 4# with an 80V bias voltage, and its surface stress should be greater than that of samples 2# and 4#. In addition, as the incident angle increases, the stress of samples 1# and 2# both increase; however, the coating stress of sample 3# first increases and then decreases, and the coating stress of sample 4# first decreases and then increases. The reason for this result is that both samples 3# and 4# have two bias voltages applied, and the coating formed on sample 4# with a high bias voltage is thinner. When the incident angle is 3°, the proportion of the coating formed with a 40V bias voltage measured is larger. The stress trend reflected by the above test results is consistent with the conventional understanding in this field, and this result indicates that the test method described in the present invention has high accuracy.

[0077] As can be seen from Table 2, for Samples 5#, 6#, and 7#, (σ1 - σ2) / σ1 ≥ 5%; among them, for Sample 5#, C1 to C4 are all greater than 0. The C value with the same incident angle as the D value less than 0 is selected as the residual stress value of the Al2O3 coating, which is 153 ± 14 MPa; for Sample 6#, the residual stress C4 measured at an incident angle of 16° is 18 ± 15 MPa, approximately 0. Its C1 to C4 are all less than or equal to 0. The C value with the same incident angle as the D value greater than or equal to 0 is selected as the stress value of the Al2O3 coating, which is 0; for Sample 7#, C1 to C4 are all less than 0. The C value with the same incident angle as the D value greater than or equal to 0 should be selected as the residual stress of the Al2O3 coating. And D4 measured at an incident angle of 16° is -22 ± 29 MPa, approximately 0, then the C value is -257 ± 23 MPa.

[0078] Shot peening can change the stress state of the coating surface; the degree of shot peening for Sample 5# is relatively light, and the surface of the coating remains the original tensile stress; the degree of shot peening for Sample 6# is relatively heavy, offsetting the tensile stress on the surface of the sample; the degree of shot peening for Sample 7# is further increased, forming a compressive stress on the coating surface. The above test results are consistent with the conventional understanding in this field, indicating that the test method for the residual stress of the coating described in the present invention has high accuracy.

[0079] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention rather than to limit the protection scope of the present invention. Although the present invention has been described in detail with reference to the preferred embodiments, those of ordinary skill in the art should understand that the technical solutions of the present invention can be modified or equivalently replaced, but they do not depart from the essence and scope of the technical solutions of the present invention.

Claims

1. A method for testing the residual stress of a coating, wherein the coating is attached to a substrate, characterized in that, It includes the following steps: (1) Using X-ray diffraction method to determine the phases and corresponding crystal structures in the coating; (2) Obtaining diffraction patterns by the grazing incidence method; wherein, a Cu target is selected for testing, the scanning angle is 20~150°, and the incident angles are 1°, 3°, 5°, 16° respectively; (3) Using the Rietevield method for whole spectrum fitting, and calculating the residual stress of the coating through the peak position deviation value obtained by fitting; wherein, when using the Rietevield method for whole spectrum fitting, the residual stress A of the coating and the residual stress B of the substrate are calculated respectively, and the residual stresses corresponding to the incident angles of 1°, 3°, 5°, 16° are marked as A1, A2, A3, A4, B1, B2, B3, B4 respectively; (4) Measuring the residual stress of the coating by the side tilt method, and the average stress measured at ψ angle of 0~45° is marked as σ1, and the average stress measured at ψ angle of 0~70° is marked as σ2; the ψ angle is the angle between the normal of the specimen surface and the normal of the diffraction crystal plane, that is, the angle of rotation of the specimen along the vertical direction; When (σ1 - σ2) / σ1 ≥ 5%, if A1, A2, A3, A4 are all less than or equal to 0, then select the A value with the same incident angle as the B value greater than or equal to 0 among B1, B2, B3, B4 as the residual stress value of the coating; if A1, A2, A3, A4 are all greater than or equal to 0, then select the A value with the same incident angle as the B value less than or equal to 0 among B1, B2, B3, B4 as the residual stress value of the coating; when (σ1 - σ2) / σ1 < 5%, select A1 or A2 as the residual stress value of the coating.

2. The method for testing the residual stress of the coating according to claim 1, wherein The coating is selected from at least one of TiAlN, TiCN, Al2O3, and the substrate is a cemented carbide substrate.

3. The method for testing the residual stress of the coating according to claim 1, characterized in that In the step (1), according to the spectrum obtained by the X-ray diffraction method, through analysis software, match the phases according to known elements to determine the types of phases and the corresponding crystal structures.

4. A method for testing the residual stress of a coating, wherein the coating is attached to a substrate, characterized in that, It includes the following steps: (1) Using X-ray diffraction method to determine the phases and corresponding crystal structures in the coating; the coating includes a first coating and a second coating, the first coating is attached to the substrate, and the second coating is attached to the first coating; (2) Obtaining diffraction patterns by the grazing incidence method; wherein, a Cu target is selected for testing, the scanning angle is 20~150°, and the incident angles are 1°, 3°, 5°, 16° respectively; (3) Using the Rietevield method for whole spectrum fitting, and calculating the residual stress of the coating through the peak position deviation value obtained by fitting; wherein, when using the Rietevield method for whole spectrum fitting, the residual stress C of the second coating and the residual stress D of the first coating are calculated respectively, and the residual stresses corresponding to the incident angles of 1°, 3°, 5°, 16° are marked as C1, C2, C3, C4, D1, D2, D3, D4 respectively; (4)The residual stress of the coating is measured by the side inclination method. When measuring the residual stress of the coating by the side inclination method, the average stress measured at a ψ angle of 0 to 45° is denoted as σ1, and the average stress measured at a ψ angle of 0 to 70° is denoted as σ2; the ψ angle is the angle between the normal of the specimen surface and the normal of the diffraction crystal plane, that is, the angle of rotation of the specimen in the vertical direction. When (σ1 - σ2) / σ1 ≥ 5%, if C1, C2, C3, and C4 are all less than or equal to 0, then the D value greater than or equal to 0 among D1, D2, D3, and D4 is selected as the residual stress value of the first coating, and the C value with the same incident angle as the D value greater than or equal to 0 is selected as the residual stress value of the second coating; if C1, C2, C3, and C4 are all greater than or equal to 0, then the D value less than or equal to 0 among D1, D2, D3, and D4 is selected as the residual stress value of the first coating, and the C value with the same incident angle as the D value less than or equal to 0 is selected as the residual stress value of the second coating. When (σ1 - σ2) / σ1 < 5%, C1 or C2 is selected as the residual stress value of the second coating, and D1 or D2 is selected as the residual stress value of the first coating.

5. The method for testing the residual stress of the coating according to claim 1 or 4, characterized in that, When measuring by the side inclination method, the φ angle is taken as 0° and 180°, the ψ angle ranges from 0 to 70°, and 5 to 15 points are taken at equal intervals to measure the average stress σ1 and the average stress σ2; the φ angle is the angle of rotation of the specimen in the horizontal direction.

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