METHODS FOR SURFACE EVALUATION

Abstract

Method of evaluation a performance of a surface of a material, the method comprising steps of: measuring a first surface profile of a material sample along a predetermined track by bringing a tip of a stylus into contact with a surface of the sample and traversing said tip across the surface of the sample in a direction tangential to the tip of the stylus while recording said first surface profile, subsequently; applying a normal force to the surface of the sample with the stylus, said normal force being in a direction substantially perpendicular to the surface of the sample, subsequently; moving said tip across the surface of the sample following at least part of said predetermined track while applying said normal force, subsequently; measuring a second surface profile of said sample along at least part of said predetermined track by bringing said tip into contact with the surface of the sample and traversing said tip across the surface of the sample along said track while recording said second surface profile, calculating a first residual depth profile as a difference of said first surface profile and said second surface profile.

According an aspect of the invention, the method comprises a step of determining a failure point of the surface of the sample by identifying when at least one value of a function of said first residual depth profile deviates from an expected magnitude value by at least a predetermined amount.

Claims

1-17. (canceled)

18. Method of evaluation a perform lance of a surface of a material, the method comprising steps of: measuring a first surface profile of a material sample along a predetermined track by bringing a tip of a stylus into contact with a surface of the sample and traversing said tip across the surface of the sample in a direction tangential to the tip of the stylus while recording said first surface profile, subsequently; applying a normal force to the surface of the sample with the stylus, said normal force being in a direction substantially perpendicular to the surface of the sample, subsequently; moving said tip across the surface of the sample following at least part of said predetermined track while applying said normal force, subsequently; measuring a second surface profile of said sample along at least part of said predetermined track by bringing said tip into contact with the surface of the sample and traversing said tip across the surface of the sample along said track while recording said second surface profile, calculating a first residual depth profile as a difference of said first surface profile and said second surface profile, characterised by determining a failure point of the surface of the sample by identifying when at least one value of a function of said first residual depth profile deviates from an expected magnitude value by at least a predetermined amount.

19. Method according to claim 18, wherein said function of said residual depth profile a first differential of said residual depth profile.

20. Method according to claim 18, wherein said normal force varies as a cyclical function.

21. Method according to claim 18, further comprising subsequent steps of: measuring a third surface profile of said sample along said predetermined track by bringing said tip into contact with the surface of the sample and traversing said tip across the surface of the sample along said track while recording said third surface profile, said third surface profile being measured at a predetermined time after said second profile, subsequently; calculating a second residual depth profile as a difference of said first surface profile and said third surface profile; subsequently determining a viscoelastic behaviour of the surface of the sample based on said first residual depth profile and said second residual depth profile.

22. Method according to claim 18, comprising a step of determining energy lost at said failure point based on said first surface profile and said residual depth profile.

23. Method according to claim 18, comprising a step of measuring a penetration depth profile of the tip into the sample during application of said normal force, and a step of determining a value of elastic modulus based on said penetration depth profile and said residual depth profile.

24. Method according to one of claim 18, wherein said normal force is predetermined, or is dependent upon a predetermined depth profile.

25. Method according to one of claim 18, wherein said sample comprises a substrate and a coating provided thereupon, said method further comprising a step of determining adhesion of said coating to said substrate based on said first residual depth profile.

26. Method of evaluation a performance of a surface of a material, the method comprising steps of: measuring a first surface profile of a material sample along a predetermined track by bringing a tip of a stylus into contact with a surface of the sample and traversing said tip across the surface of the sample in a direction tangential to the tip of the stylus, subsequently; applying a normal force to the surface of the sample with the stylus, said normal force being in a direction substantially perpendicular to the surface of the sample, subsequently; moving the tip across the surface of the sample following at least part of said predetermined track while applying said normal force; subsequently; measuring a second surface profile of said sample along at least part of said predetermined track by bringing said tip into contact with the surface of the sample and traversing said tip across the surface of the sample along said track while recording said second surface profile, calculating a first residual depth profile as a difference of said first surface profile and said second surface profile, characterised by after a predetermined period of time, measuring a third surface profile of said sample along at least part of said predetermined track by bringing said tip into contact with the surface of the sample and traversing said tip across the surface of the sample along said track while recording said third surface profile, calculating a second residual depth profile as a difference of said first surface profile and said third surface profile, and determining at least one parameter of a viscoelastic behaviour of the material of said sample based on a function of said first residual depth profile and said second residual depth profile.

27. Method according to claim 26, wherein said normal force varies as a cyclical function.

28. Method according to claim 26, wherein said normal force is predetermined, or is dependent upon a predetermined depth profile.

29. Method of evaluation a performance of a surface of a material, the method comprising steps of: measuring a first surface profile of a material sample along a predetermined track by bringing a tip of a stylus into contact with a surface of the sample and traversing said tip across the surface of the sample in a direction tangential to the tip of the stylus, subsequently; applying a normal force to the surface of the sample with the stylus, said normal force being in a direction substantially perpendicular to the surface of the sample, subsequently; moving the tip across the surface of the sample following at least part of said predetermined track while applying said normal force and measuring a penetration depth profile of the tip of the stylus into the surface of the sample, said penetration depth profile taking into account said first surface profile, subsequently; measuring a second surface profile of said sample along at least part of said predetermined track by bringing said tip into contact with the surface of the sample and traversing said tip across the surface of the sample along said track while recording said second surface profile, calculating a first residual depth profile as a difference of said first surface profile and said second surface profile, characterised by determining a value of elastic modulus based on a difference between said penetration depth and said first residual depth profile.

30. Method according to claim 29, wherein said normal force varies as a cyclical function.

31. Method according to claim 29, wherein said normal force is predetermined, or is dependent upon a predetermined depth profile.

32. Method according to claim 29, wherein said normal force varies as a step function.

33. Scratch test apparatus adapted to carry out the method of claim 18.

34. Scratch test apparatus adapted to carry out the method of claim 26.

35. Scratch test apparatus adapted to carry out the method of claim 29.

36. Computer program product directly loadable into the internal memory of a digital computer comprising software code portions for causing a scratch testing apparatus to perform the steps of claim 18 when said product is run on a computer controlling said scratch test apparatus.

37. Computer program product directly loadable into the internal memory of a digital computer comprising software code portions for causing a scratch testing apparatus to perform the steps of claim 26 when said product is run on a computer controlling said scratch test apparatus.

38. Computer program product directly loadable into the internal memory of a digital computer comprising software code portions for causing a scratch testing apparatus to perform the steps of claim 29 when said product is run on a computer controlling said scratch test apparatus

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0039] The invention will be further explained in the context of the following figures, which illustrate:

[0040] FIG. 1: a schematic diagram of a scratch testing apparatus;

[0041] FIG. 2: a schematic flow diagram of a method according to the invention;

[0042] FIGS. 3 and 4: graphs of scratch test data illustrating the functioning of the method of the invention;

[0043] FIG. 5: a schematic flow diagram of a further embodiment of a method according to the invention;

[0044] FIG. 6: a schematic flow diagram of a further embodiment of a method according to the invention; and

[0045] FIG. 7: a graph of scratch test data illustrating calculating an elastic modulus of a sample based on the method of FIG. 6.

MODE(S) FOR CARRYING OUT THE INVENTION

[0046] FIG. 1 shows schematically a scratch testing apparatus 1. As is commonly known, this apparatus 1 comprises a sample holder 3 upon which a sample 5 can be fixed, e.g. by clamping, bolting or similar. Sample 5 may optionally comprise a coating 5a. Sample holder 3 is typically displaceable in at least one direction in the plane XY of the sample (the Y direction being into the page), and normally also perpendicular thereto, i.e. according to the Z direction, by means of suitable actuators (not illustrated) as generally known.

[0047] The apparatus 1 also comprises a headstock 7, supporting a stylus 9. A distal end of the stylus 9 comprises a tip 9a facing the sample holder 3 and hence equally facing the sample 5. The tip 9a may be of any know shape (e.g. conical, hemispherical, pyramidal . . . ) and of any convenient material (e.g. steel, diamond, alumina . . . ), depending on the material of the surface of the sample to be tested.

[0048] Headstock 7 comprises various actuators 7a and sensors 7b, for applying and measuring normal force Fn (in the Z direction), tangential force Ft (in at least one direction in the XY plane, if desired), and displacement Dn normal to the surface of the sample 5, as is generally known. Alternatively, or additionally, sensors 3a for one or more forces and/or displacements may be comprised within the sample holder 3 and/or within a structure (not illustrated) which supports this latter.

[0049] The stylus 9 can be displaced with respect to the sample 5 by means of any combination of moving sample holder 3 and/or the headstock 7 and/or moving the stylus 9 with respect to the headstock 5, as is generally known.

[0050] A controller 4 controls all displacements and actuators, and receives all measurements from the sensors 7b, as is generally known. Controller 4 may for instance be a dedicated control unit comprising a dedicated computer integrated therein, or may be a general purpose programmable computer adapted to control the scratch testing apparatus 1.

[0051] According to an aspect of the present invention, the scratch testing apparatus is adapted to carry the method of FIG. 2.

[0052] This method incorporates the following steps.

[0053] After provision of a suitable scratch testing apparatus 1 and an appropriate sample 5 upon the sample holder 3 such that the upper face of the sample 5 is substantially situated in the XY plane, in step 101 a so-called pre-scan is carried out. It should be noted that it is very difficult to ensure that the upper face of the sample is in the XY plane to nanometre precision, and hence a tolerance of for instance 1 m??? per mm of scratch length (i.e. 1%) of slope along the predetermined track 6 is acceptable. As a general rule, perpendicular, normal and parallel should be understood as incorporating a tolerance of 1% as mentioned above.

[0054] The pre-scan involves measuring a first surface profile by bringing the tip 9a of the stylus 9 into contact with the surface of the sample 5 and tracking it along the surface along a predetermined track 6 while recording the first surface profile based on output from the various sensors 3a and/or 7b. This is carried out while applying insufficient force to substantially deform the surface. The exact amount of force required to measure the first surface profile without substantially deforming the surface depends on the material of the sample. The predetermined track 6 is typically a straight line of several millimetres length, although other shapes are possible. Furthermore, the predetermined track 6 may be as long as desired.

[0055] The first surface profile can thus be determined based on the output of the sensors 7b, particularly a sensor providing information relating to the Z-axis (the height axis).

[0056] In step 102, the tip 9a of the stylus 9 is positioned at a desired point along the predetermined track 6, and a normal force Fn is applied, i.e. a force substantially parallel to the Z-axis. It should be noted that the axial force can be predetermined, i.e. follow a desired development, or the normal force can be a dependent variable in dependence of a desired penetration depth of the tip 9a into the surface of the sample 5. For instance, it may be desired to cause the scratch to extend to a fixed depth or to a predetermined depth profile (ramp, waveform . . . ), which can be determined based on the first surface profile.

[0057] In step 103, the tip 9a is scratched along the predetermined track 6, as is generally known, while sensor data is collected from the various sensors 7a in the headstock 7 and/or in the sample holder 3 so as to output normal force, tangential force (if measured), displacement of the stylus 9 in the Z direction and hence parallel to the longitudinal axis of the stylus 9, and so on. During this step, the applied force may remain constant, or may be varied. In a classic scratch test, the normal force Fn is ramped linearly. However, normal force Fn may be varied in any other convenient manner. To this effect, it may follow a cyclical pattern, such as a sinusoid, a triangular wave, a saw-toothed wave or any other convenient waveform. Furthermore, a predetermined depth profile scratch test may also be carried out based on a cyclical depth pattern following e.g. a sinusoid, a triangular wave, a saw-toothed wave, or any other convenient waveform. Furthermore, penetration depth Pd of the tip 9a into the surface of the sample 5 can also be measured and recorded, if desired.

[0058] In step 104, the tip 9a of the stylus is re-scanned across at least the part of the predetermined track 6 affected by the scratch test carried out in steps 102 and 103, in order to determine and record a second surface profile,

[0059] In step 105, a residual depth profile is calculated based on the first and second surface profiles, typically by subtracting one from the other. This pre-scan/post-scan process is described in EP 1 092 142, herein incorporated by reference in its entirety.

[0060] In step 106, at least one critical point representing a fracture of the sample is determined based on the residual depth profile calculated in step 105. This determination can then be displayed automatically for the user, if desired.

[0061] This is carried out by determining when a function of the residual depth profile exceeds an expected magnitude value by a predetermined amount. This function may, for instance, be the first differential of the residual depth profile exceeding a threshold (which defines an expected magnitude) by a predetermined amount, indicating that a sudden change has occurred. This change could be separation of a coating from the underlying substrate, or cracking or spalling of the surface of the sample. Other functions are also possible, such as a higher-order differential (second or third order), or a departure from a linearly-approximated and projected trend, this approximated and projected trend representing a set of expected magnitude values.

[0062] FIGS. 3 and 4 illustrate graphs of results obtained from linearly-ramped scratch tests carried out on a 50 m diamond-like-carbon (DLC) coating on a steel substrate, synchronised with optical microscope images of the scratch. In these tests, the normal force Fn was ramped linearly, and the pre-scan profile Pf (corresponding to the first surface profile mentioned above), penetration depth Pd (measured during the application of the normal force Fn and calculated in function of the pre-scan profile Pf), the residual depth Rd as calculated in step 105, the acoustic emissions Ae, the tangential force Ft as measured and also the coefficient of friction as calculated are plotted. It should be noted that the second surface profile is not plotted, since it is only used for calculating Rd and is itself not useful information to display to an operator. It should also be noted that Pf is sloped, since as mentioned above it is very difficult, or even impossible, to ensure that the surface of the sample is absolutely perpendicular to the Z-axis on a scale of nanometres. However, since the Pd and Rd measurements are also on the scale of nanometres, this slight slope should be taken into account for maximum precision.

[0063] From these graphs, it is clear that a conventional acoustic-emission-based or friction-based measurement based on Ae, Ft and/or , does not precisely identify the first critical point CP1, identified by a vertical line. In fact, the first critical point identified by a significant change in Ae, Ft or (CP2) comes significantly after the first critical point CP1 which is measurable via the residual depth profile Rd. The surprising technical effect of the invention is thus a significantly more precise identification of at least the first critical failure point of the sample, which is not identified by prior art methods for all materials. Detection of further critical points can equally also be improved.

[0064] This improvement is particularly visible in the sample of FIG. 3, wherein CP1, which corresponds to delamination of the coating from its substrate without spalling, is extremely difficult to detect via changes in Ae, Ft or , as evidenced by the fact that the Ft and p curves do not exhibit any change at all at this point, and that the Ae curve only exhibits an insignificant change.

[0065] The residual depth profile Rd, and knowledge of the Young's and bulk moduli of the material, can then be used to determine the energy lost at each failure point identified above, or identified with any other convenient parameter.

[0066] FIG. 5 illustrates a method according to a further aspect of the invention. In this method, steps 101 to 105 are the same as in the method of FIG. 2, and step 106 is optional. Subsequent to step 105 or 106 (if executed), a second post-scan of the scratch is carried out after a predetermined period of time in step 107. This results in a third surface profile being obtained. In step 108, this third surface profile is subtracted from the first surface profile to obtain a second residual depth profile. On the basis of the first and second residual depth profiles, in step 109 the relaxation of the material of the sample is calculated, i.e. its viscoelastic properties and behaviour are obtained.

[0067] Steps 107 to 109 can be carried out repeatedly thereby generating further residual depth profiles in order to observe the relaxation of the material over a longer period. Further useful information regarding the relaxation of the sample over time can thereby be obtained.

[0068] The methods according to the invention permit also determining other properties. For instance, the adhesion of a coating to a substrate can be determined at points along the predetermined track 6 based on the first and/or second residual depth profile(s). This provides particularly useful data at points of delamination between the coating and the substrate, such as critical point CP1 mentioned above.

[0069] In the case of a multiphase material, the first and/or subsequent residual depth profile(s) can be analysed statistically in order to identify the individual phases of a multiphase material.

[0070] Furthermore, the elastic modulus of the sample can be determined based on comparison of the residual depth profile and penetration depth during the scratch test, since both the stress and the strain applied during the scratch test can be calculated from these profiles, with or without detection of critical failure points. This is illustrated in FIGS. 6 and 7.

[0071] The method of FIG. 6 differs from that of FIG. 1 in that step 106 is replaced by a step 110 of calculating a value of elastic modulus of the material of the sample. A further step 106 of identifying failure points may nevertheless additionally be incorporated, either before or after step 110, if desired.

[0072] FIG. 7 illustrates one way by which a value of elastic modulus can be determined. Other methods are of course possible within the scope of the invention, and the method described below is not to be construed as limiting since other methodologies are also possible.

[0073] This figure illustrates the sensor output from a scratch test carried out on a bulk PMMA sample, which has a known elastic modulus and Poisson's ratio. In this test, the normal force Fn was stepped up by discrete steps along the length of the scratch, and the indenter comprises a spherical tip.

[0074] Each step of the force curve can be considered as being similar to a quasi-static indentation test, the only difference being the movement of the sample. Hence, maximum penetration depth and residual depth can be used to calculate an elastic modulus based on standard indentation contact mechanics.

[0075] Contact stiffness, S is defined as follows when unloading the sample, where P is the load on the sample (corresponding to Fn in the graphs illustrated) and h is the displacement of the indenter (corresponding to Pd or Rd, as appropriate):

[00001]$S=\frac{\mathrm{dP}}{\mathrm{dh}}=\frac{2}{\sqrt{}}.Math.E_r.Math.\sqrt{A}$

[0076] If we assume that h.sub.max=P.sub.d (Maximum scratch penetration depth) and h.sub.min=R.sub.d (Residual scratch penetration depth), then:

[00002]$S=\frac{\mathrm{dP}}{\mathrm{dh}}=\frac{P_{\max }-P_{\min }}{P_d-R_d}$

[0077] and the Reduced Modulus (as generally defined in the field of indentation measurements), E.sub.r, can be calculated from:

[00003]$E_r=\frac{\sqrt{}}{2}.Math.\frac{S}{\sqrt{A}}$

[0078] where A is the area of the indenter in contact with the sample at maximum load P.sub.max. Therefore:

[00004]$\frac{1}{E_r}=\frac{\left(1-v_s^2\right)}{E_{\mathrm{IT}}}+\frac{\left(1-v_i^2\right)}{E_i}$

[0079] where E.sub.IT and v.sub.IT are Young's modulus and Poisson's ratio for the sample and E.sub.i and v.sub.i are the same parameters for the indenter.

[0080] If we assume that the indenter is extremely rigid (usually diamond) when compared to the sample material, then E.sub.i and 2v.sub.i can be ignored and thus the indentation modulus, E.sub.IT can be calculated from:

E.sub.IT=E.sub.r(1v.sub.s.sup.2)

[0081] Looking now at the results of FIG. 7, we can calculate E.sub.IT for load steps in which the penetration depth Pd ensures that the spherical part of the indenter is not exceeded, and hence the area A can be calculated based on the geometry of the indenter tip.

[0082] For example, for load increment L2, we have a measured P.sub.max of 0.00532 N, P.sub.min again of 0.0004, a Pd of 3.85 m, Rd of 670 nm, and an area A calculated as 1.8142710.sup.12 m.sup.2. Following the above equations, we have a contact stiffness of 1547 N/m, a reduced modulus E.sub.r of 1.017963 GPa and an E.sub.IT of 0.893263 GPa, assuming a Poisson's ration (v) of 0.35 for the PMMA substrate.

[0083] The E.sub.IT calculated for load increment L2 differs from the known elastic modulus of PMMA (typically 3-5 GPa), since a conventional indentation curve extrapolates only the linear portion of the unloading curve, whereas the present method assumes linearity of the curve over the entire depth change rebound from penetration depth Pd to residual depth Rd.

[0084] This method thus produces a lower modulus value for E.sub.IT than Young's modulus, which we can define as Scratch Modulus, E.sub.s.

[0085] The scratch modulus E.sub.s, even though it does not correspond directly to Young's modulus or bulk modulus, in any case provides a directly-calculable modulus derivable from a scratch test which permits direct comparison between different materials, and as such represents an extremely useful measurement not requiring a separate test to determine.

[0086] Computer-readable instructions for carrying out any of the above-mentioned methods can be provided as a computer program product, provided on a computer-readable storage medium or for download via the Internet or any other network. This computer program product comprises software code portions for causing a suitable scratch testing apparatus 1 to perform the steps the methods when the product is run on a computer 4 controlling the scratch test apparatus 1.

[0087] Although the invention has been described in terms of specific embodiments, these are not to be construed as limiting the scope of the invention, as variations thereto can be envisaged without departing from the invention as defined in the appended claims.