PATENT DOCUMENT

Publication Number: US-9869623-B2
Application Number: US-201514678881-A
Country: US
Kind Code: B2

Title: Process for evaluation of delamination-resistance of hard coatings on metal substrates

Abstract:
This disclosure relates to rapid and repeatable tests that can be used to evaluate the interfacial adhesion of coatings to substrates. In particular embodiments, tests are used to assess the resistance of anodic oxides to delamination from aluminum substrates. The tests can be conducted using standard hardness test equipment such as a Vickers indenter, and yield more controlled, repeatable results than a large sample of life-cycle tests such as rock tumble tests. In particular embodiments, the tests involve forming an array of multiple indentations within the substrate such that stressed regions where the coating will likely delaminate are formed and evaluated.

Claims:
What is claimed is: 
     
       1. A method of determining an adhesion strength between a metal substrate and a coating that overlays the metal substrate, the method comprising:
 forming indentations extending through the coating and into the metal substrate by applying a pre-defined amount of impact force to a region of the coating using an impact agent, wherein at least two of the indentations are capable of generating residual stresses commensurate with a stress induced by the impact force at the region, and the residual stresses combine to form a detachment force that, when greater than the adhesion strength, is capable of causing a separation of the coating and the metal substrate at the region. 
 
     
     
       2. The method of  claim 1 , wherein the at least two indentations are separated by a pre-defined distance that is sufficient for physical communication with each other at the region. 
     
     
       3. The method of  claim 2 , wherein the pre-defined distance between the at least two indentations is less than three times an average diameter of the at least two indentations. 
     
     
       4. The method of  claim 1 , wherein the indentations are arranged according to an array of rows and columns. 
     
     
       5. The method of  claim 1 , wherein the pre-defined amount of impact force is constant. 
     
     
       6. The method of  claim 1 , wherein the pre-defined amount of impact force is individually adjustable. 
     
     
       7. The method of  claim 1 , further comprising:
 determining a portion of the coating that is separated from the metal substrate at the region. 
 
     
     
       8. The method of  claim 1 , wherein the region is defined by edges of the indentations. 
     
     
       9. The method of  claim 1 , wherein the impact agent includes an indenting tool having a symmetric shape that is capable of forming corresponding symmetrically shaped indentations. 
     
     
       10. The method of  claim 1 , wherein the impact agent has a mass that is between 500 grams to 50 kilograms. 
     
     
       11. The method of  claim 1 , wherein the metal substrate is at least one of aluminum or aluminum alloy, and the coating is aluminum oxide. 
     
     
       12. A method of determining an adhesion strength between a metal substrate and a coating disposed over the metal substrate, the method comprising:
 forming indentations within the coating and the metal substrate, wherein the indentations include at least a first indentation and a second indentation that are separated by a pre-defined distance that is sufficient to cause a first amount of strain generated by the first indentation to physically communicate with a second amount of strain generated by the second indentation to form a delamination force at a region of the coating, and wherein the coating delaminates from the metal substrate at the region when the delamination force is greater than the adhesion strength. 
 
     
     
       13. The method of  claim 12 , wherein the indentations are formed by an impacting agent having a mass that ranges between 500 grams to 50 kilograms. 
     
     
       14. The method of  claim 12 , wherein the first and second indentations are adjacent to each other. 
     
     
       15. The method of  claim 12 , wherein the indentations are formed by using an indentation tool arranged to deliver a pre-defined amount of impact force on the coating and the substrate. 
     
     
       16. The method of  claim 12 , wherein the indentations are formed within the coating and the metal substrate by an indentation tool that delivers a pre-defined amount of impact force in a direction that is generally perpendicular to a surface of the coating. 
     
     
       17. A system for determining an adhesion strength between a substrate and a coating that overlays the substrate, the system comprising:
 an indentation tool arranged to form indentations extending through the coating and the substrate by applying a pre-defined amount of impact force at a region of the coating, wherein at least two of the indentations formed by the indentation tool are capable of physically communicating with each other to form a detachment force that, when greater than the adhesion strength, is capable of causing a portion of the coating to separate from the substrate; and 
 an image processing component capable of determining the portion of the coating that separates from the substrate. 
 
     
     
       18. The system of  claim 17 , wherein the indentation tool delivers the pre-defined amount of impact force in a direction that is generally perpendicular to a surface of the coating. 
     
     
       19. The system of  claim 17 , wherein the indentation tool is capable of adjusting the pre-defined amount of impact force for the indentations. 
     
     
       20. The system of  claim 17 , wherein the indentations are arranged according to an array of rows and columns.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This is a continuation of International Application PCT/US15/24349, with an international filing date of Apr. 3, 2015, entitled “PROCESS FOR EVALUATION OF DELAMINATION-RESISTANCE OF HARD COATINGS ON METAL SUBSTRATES”, the disclosure of which is incorporated herein by reference in its entirety. 
    
    
     FIELD 
     The described embodiments relate generally to evaluating hard coatings on surfaces of articles. More particularly, the present embodiments relate to methods for evaluating the adhesion strength and delamination-resistance of a coating on a surface of an article. 
     BACKGROUND 
     Surface coatings are used on consumer devices to protect the surface and enhance the aesthetics and texture of the device. One example of such a coating is anodizing a metal surface. Anodizing a metal surface converts a portion of the metal surface into an anodic oxide, thereby creating an anodic oxide layer. The anodic oxide layer may be harder than the underlying metal substrate. 
     While a coating may be harder than the underlying substrate, a relatively stiff coating is susceptible to becoming detached from the substrate when the article is subjected to mechanical stresses. Thick, stiff coatings, on relatively compliant substrates are particularly susceptible to delamination, and since properties such as stiffness and thermal expansion are often strongly correlated, this scenario can be exacerbated by thermally induced strain. 
     Various mechanical tests exist for evaluating the interfacial strength and interfacial adhesion of the coatings. These include pull-off tests, thermal cycling and thermal shock, and techniques such as four-point bend delamination for propagating delamination under steady state and measuring interfacial adhesion energies. One common (though more qualitative) test for the adhesive strength of a coating of consumer products is the “rock tumble” test. This test is performed by tumbling the article having the coating for an extended time with items the article will typically encounter during its expected lifetime. However, these tests are random in nature, and must be repeated multiple times for each coating to be tested to extrapolate a statistically significant result. In addition, these tests may have inherent limitations, such as the strength of adhesives used for bonding in pull-off tests, or require very specific sample geometries, such as is the case in bend delamination tests. For these reasons, incremental improvements in layered structure strength and adhesion are difficult to evaluate. 
     SUMMARY 
     This paper describes various embodiments that relate to coatings and methods of testing the adhesion strength of these coatings. The methods described can be used to precisely and reliably evaluate the integrity and expected lifetime durability of a coating. 
     According to one embodiment, a method of testing an adhesion strength between a coating and a substrate is described. The method includes creating a pre-defined pattern of indentations using an impacting agent arranged to deliver a pre-defined impact force, and a corresponding pattern of applied stresses, to the coating and substrate at specified locations. When the adhesion strength is less than a delamination force and its corresponding pattern of applied stresses, at least a portion of the coating delaminates from the substrate at a stressed region of the coating defined in part by the specified locations and associated with the delamination force. 
     According to another embodiment, a method of method of testing an adhesion strength between a coating a substrate is described. The method includes forming a pattern of indentations using an indentation tool arranged to deliver a pre-defined impact force on the coating and the substrate. Forming the pattern includes creating indentations within the coating and the substrate by moving the indentation tool along a surface of the coating a pre-defined distance between the indentations such that the indentations are equidistantly spaced. A delamination force is formed within stressed regions between the indentations. The coating delaminates from the substrate at the stressed regions when the delamination force is greater than the adhesion strength. 
     According to a further embodiment, an apparatus for determining an adhesion strength between a coating and a substrate is described. The apparatus includes an indentation tool arranged to create a pattern of indentations within the coating and the substrate. The indentation tool includes an impactor arranged to form an indentation at a specified location on the surface of the coating by delivering a pre-defined impact force at the specified location on the coating and the substrate. When the adhesion strength is less than a delamination force, at least a portion of the coating delaminates from the substrate at a stressed region of the coating defined in part by the specified locations and associated with the delamination force. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The disclosure will be readily understood by the following detailed description in conjunction with the accompanying drawings, wherein like reference numerals designate like structural elements. 
         FIG. 1  shows a perspective view of an indentation tool and an indentation pattern on an article having two indentations and a resulting delamination of a coating in a stressed region formed between the indentations. 
         FIGS. 2A and 2B  show perspective views of an indentation pattern on an article having multiple indentations arranged in a grid. 
         FIG. 2C  shows optical and scanning electron microscope (SEM) images of five-by-five patterns of indentations. 
         FIG. 3  shows a perspective cross-sectional view of an indentation pattern where the indentations are partially spherical. 
         FIG. 4  shows a perspective view of an indentation pattern on an article with minimal spacing between indentations. 
         FIGS. 5A and 5B  show top views of indentations patterns with varied spacing between indentations. 
         FIG. 6  shows a perspective view of an indentation pattern on an article having multiple coatings. 
         FIG. 7  is a flow diagram depicting a method of testing a coating for adhesion integrity. 
         FIG. 8  is a block diagram of an electronic device suitable for use with the described embodiments. 
     
    
    
     DETAILED DESCRIPTION 
     Reference will now be made in detail to representative embodiments illustrated in the accompanying drawings. It should be understood that the following descriptions are not intended to limit the embodiments to one preferred embodiment. To the contrary, it is intended to cover alternatives, modifications, and equivalents as can be included within the spirit and scope of the described embodiments as defined by the appended claims. 
     The following disclosure relates to methods of testing the adhesion strength of coating on a surface of substrate. There is a need in the art of coatings to test the adhesion of a coating to an underlying substrate, particularly where the coating is relatively stiff in relation to the underlying substrate. For example, coatings on surfaces of consumer devices are normally subjected to various mechanical and thermal stresses during the lifetime of the consumer devices. Coating spallation can occur when a coating is subjected to these stresses, which results in an undesirable surface finish. 
     As used herein, the terms “adhesive failure” or “detachment” of a coating or indeed a system of multiple coatings (such as a multilayered stack of coatings) are sometimes also described as “spallation” or “delamination”. The latter term is used throughout this paper to refer to generally to failure of the interfacial adhesion of a coating. It should also be recognized that certain coatings or coating systems may fail at locations other than the immediate interface; for instance, due to crack propagation along an intermediate level, broadly parallel to the interface. Such failures, while not strictly interfacial, can have similar detrimental effects (such as the loss of a dye or seal layer) and are considered to lie within the scope of testing methods described herein. Thus, the terms “adhesive failure,” “detachment,” “spallation,” and “delamination” are used interchangeably in this paper, and can refer broadly to adhesion failure of a coating at or near the interface between the coating and underlying substrate. Likewise, the terms “detachment force” and “delamination force” are used interchangeably, and can refer broadly to a force created at the coating and/or substrate that can cause detachment of the coating at or near the interface. 
     While various mechanical tests exist to evaluate the adhesion strength of coatings, these tests are imprecise in their results and thus require multiple samples to be evaluated over an extended period of time to extrapolate statistically significant results. As an example, a “rock-tumble ” test that is routinely used requires multiple samples of a coating to be individually tumbled with various objects over an extended period of time to simulate the life expectancy of the coating. Due to the inherent randomness of this test, the results are unreliable and an improved method for testing the integrity of coatings is disclosed herein. 
     In some embodiments, testing the surface adhesion strength of a coating includes forming two or more indentations in the coating. In some cases the indentations extend through the coating and plastically deform the underlying substrate. The deformation of the substrate and the coating can induce a pattern of stresses in the coating, with stressed regions in the coating in areas between the indentations. This stressed state of the coating and/or substrate may exert a detachment or delamination force on the coating that can cause the coating to detach or delaminate from the substrate. The delamination force can be perpendicular to the substrate, coplanar to the substrate, or some combination of these directions. 
     In particular embodiments, the testing process involves applying a Vickers indenting tool normal to the surface of the coating a number of times to form a grid or array of indentations. The indentations are of such size as to produce substantial plastic deformation in the substrate material, and are spaced very closely such that the residual strain from each successive indentation interacts with each other. In some embodiments a square array of between three-by-three and five-by-five indentations are be used. Each indentation produces large interfacial shear strains between the coating and substrate, inducing controlled delamination. Subsequent, adjacent indentations help to promote spallation of the coating, and expose the substrate. 
     In other embodiments, the spacing of indents is not uniform, but is varied—either in progressively more widely spaced rows or columns, or with both row and column spacing progressively increasing. The applied force may be constant, or may also be progressively increased. Thus, a single pattern can produce multiple instances of various different stress states, and any observed pattern of coating spallation may be correlated to the pattern of applied stresses to determine a threshold for failure. 
     These and other embodiments are discussed below with reference to  FIGS. 1-8 . However, those skilled in the art will readily appreciate that the detailed description given herein with respect to these Figures is for explanatory purposes only and should not be construed as limiting. 
     The testing methods described herein can be used to rapidly, accurately, and controllably replicate conditions that induce delamination of a coating in-service, enabling rapid assessment of the relative delamination resistance of various different types of coatings. The testing methods can employ common laboratory equipment, and can be applicable to samples of any suitable geometry. The methods involve producing multiple indentations within the coating such that stressed regions are formed within coating. While single large indentations induce interfacial delamination, implementing a succession of adjacent of indentations with interacting stress fields form a pattern of spallation that can be analyzed. 
     In some embodiments, the process involves forming two or more indentations within a coated substrate.  FIG. 1  shows a perspective view  100  of indentation tool  108  forming two indentations  110  and  112  within sample or article  102  in accordance with some embodiments. Article  102  includes substrate  104  and coating  106 . An impacting agent or impactor, such as indentation tool  108  is pressed into the surface of article  102 , plastically deforming coating  106  and substrate  104  of article  102 , thus creating indentations  110  and  112 . The force applied to coating  106  and substrate  104  can be a pre-defined amount of impact force such that each of indentations  110  and  112  are formed using the same amount of impact force. In some embodiments, indentations  110  and  112  formed by pressing indentation tool  108  under constant force for a predetermined period of time. In some embodiments, the constant force is exerted for about 10 seconds. In other embodiments, a constant force is exerted on indentation tool  108  for between 1 and 30 seconds. The force applied by indentation tool  108  can vary depending on the hardness of coating  106  and substrate  104 , as well as a desired depth of indentations  110  and  112 . In some embodiments where coating  106  is an anodic oxide material and substrate is metal, the force used to form indentations  110  and  112  is in the range of 500 grams to 50 kilograms. In other embodiments, less force is used. For example, in a particular embodiment, the force used to form indentations  110  and  112  is about 10 kilograms. 
     When indentation  110  is formed, a corresponding residual stress is formed within article  102  proximate to indentation  110 . Likewise, when indentation  112  is formed, a corresponding residual stress is formed within article  102  proximate to indentation  112 . The residual strain proximate to each of indentations  110  and  112  is created when substrate  104  and coating  106  are plastically deformed, creating a large interfacial shear strain between substrate  104  and coating  106 . If indentations  110  and  112  are spaced close enough to each other, the residual strains associated with each of indentations  110  and  112  overlap to form stressed region  114  of coating  106 . In this way, coating  106  is placed in a stressed state. This creates a corresponding delamination force within stressed region  114 . That is, the residual stress associated with each of indentations  110  and  112  can cooperate to form stressed region  114  and a corresponding delamination force. If the delamination force is greater than an adhesion strength between coating  106  and substrate  104 , a portion of coating  106  delaminates from substrate  104  exposing portion  105  of substrate  104 . Put another way, when an adhesion strength between coating  106  and substrate  104  is insufficient to withstand the detachment or delamination force created by the stressed state, at least a portion of coating  106  detaches or delaminates from substrate  104  at stress region  114 . Delamination is most likely to occur at or near stressed region  114  since this is where most of the delamination force is concentrated. 
     It is important to note that this type of delamination is generally avoided in standard hardness testing (e.g., Vickers hardness testing) where indentations are typically spaced apart at least 5 to 10 times the dimension of the residual deformation as a separation between indentations in order to avoid strain interactions. That is, it is generally undesirable to form stressed region  114  in conventional hardness testing techniques. 
     The distance d between indentations  110  and  112 , as well as the amount of force applied to form indentations  110  and  112 , can be chosen so as to provide repeatable results across multiple samples. For example, distance d can be chosen so as to optimize overlap and cooperation of the residual stresses created by indentations  110  and  112 . In this way, controlled delamination and repeatable results can be achieved. In some embodiments, distance d is measured relative to diameter D of each of indentation  110  and indentation  112 . If indentations  110  and  112  are the same size, diameter D of indentations  110  and  112  are the same. If indentations  110  and  112  are of different sizes, diameter D can refer to an average diameter of indentations  110  and  112 . In some embodiments, diameter D measured from opposing corners of the indentations  110  and  112  and distance d between indentations  110  and  112  is measured from the center of each of indentations  110  and  112 . According to some embodiments, measurable and repeatable results are accomplished when distance d is less than three times the diameter D of indentations  110  and  112 . 
     The testing methods described herein are well suited for testing adhesion of anodic oxide layers since anodic oxides are generally stiffer than the corresponding underlying metal substrate, which is generally more compliant. Thus, in some embodiments substrate  104  is an anodizable metal material and coating  106  is a corresponding anodic oxide layer. For example, substrate  104  can be made of aluminum or aluminum alloy and coating  106  can be made of aluminum oxide. In some embodiments, article  102  corresponds to a consumer product having an anodized metal portion, such as a housing for an electronic device like a mobile phone, tablet device, laptop, or other computing device or electronic accessory. However, the testing methods described herein are not limited to use on anodic oxide layers and can be used to test adhesion strengths of any suitable type of coating. Thus, substrate  104  and coating  106  can be made of any suitable materials. For example, coating  106  can be made of a material that is formed using a physical vapor deposition (PVD) process. In other embodiments, the coating  106  is plated layer, such as a plated nickel, chrome, or other metal layer. In some embodiments, coating  106  includes multiple layers. 
     In some embodiments, indentation tool  108  is diamond indenter as part of a Vickers hardness testing apparatus. However, indentation tool  108  can be made of any suitable material harder than substrate  104  and coating  106 . Indentation tool  108  has a square-based pyramid shape; however, indentation tool  108  can have any suitable shape and size. In some embodiments, the tip of indentation tool  108  is pressed into the surface of the article  102  substantially perpendicular with respect the surface of the article  102  forming substantially symmetric indentations  110  and  112 . In some embodiments, the force used to form indentation  110  is equal to the force used to form indentation  112 . In other embodiments, the force used to form indentation  110  is larger or smaller than the force used to form indentation  112 . 
     The amount of delamination is associated with the area and number of exposed portions  105 , which can be evaluated visually or by optical microscopy (if there is sufficient optical contrast between coating  106  and the substrate  104 ), and/or by electrical continuity testing or electron microscopy (if there is limited optical contrast). For example, a dyed anodic oxide coating on an aluminum alloy substrate can generally be detectable using optical microscopy techniques. 
     In some embodiments, more than two indentations are formed in a substrate such that the indentations form a pattern in the substrate.  FIGS. 2A and 2B  show perspective views of an indentation pattern  203  on sample or article  201 , in accordance with some embodiments.  FIG. 2A  shows perspective view  200  of indention pattern  203  on an article  201 .  FIG. 2B  shows perspective cross-sectional view  209  of article  201  as indicated by reference line  207  in  FIG. 2A . Indentation pattern  203  is an array or grid of indentations  208  separated from each other by a pre-defined distance chosen to provide substantially the same delamination force at stressed regions  210  positioned between indentations  208 . One can also view stressed regions  210  as different portions of a combined stressed region formed within article  201  by indentation pattern  203 . In the instant case, indentation pattern  203  is an array or grid of indentations  208  arranged in five rows and five columns, which can be referred to as a five-by-five indentation pattern with indentations  208  substantially equidistantly spaced. In some embodiments, indentations  208  are arranged such that the corners of each indentation  208  are in contact with or proximate to the corners of adjacent indentations  208 . Variations of other patterns can be formed, such as three-by-three or four-by-four indentation patterns. In some embodiments, optimal results were obtained using arrays of three-by-three or more (e.g., four-by-four, five-by-five, or greater). In some embodiments, substrate  204  has a minimum thickness of 0.4 mm with indentations  208  arranged in an array of a minimum of three-by-three in order to provide repeatable results. 
     Indentations  208  can each be formed using an indenting tool such as indentation tool  108  described above. Indentations  208  plastically deform substrate  204  and coating  206  creating a pattern of stress in the substrate  204 , in the coating  206 , and at the interface between the substrate  204  and coating  206 . A number of similarly stressed regions  210  of coating and interface are created, defined by the edges of surrounding indentations  208 . As described above, a delamination force is formed within stressed regions  210  when stresses from adjacent indentations  208  overlap. When the delaminating force results in stresses that exceed an interfacial adhesion of coating  206  to substrate  204 , a portion of coating  206  delaminates from substrate  204  and exposes portions  205  of substrate  204  at or near stressed regions  210 . Note that stressed regions  210  are not directly pressed on by an indenter, yet experience a delamination force due to stresses from the adjacent, closely space indentations  208 . 
     In some embodiments, indentation pattern  203  is formed in a sequential manner. That is, each indentation  208  is formed one at a time. This can be achieved, for example, by moving an indentation tool relative to article  201  in rows of predetermined linear tool paths until indentation pattern  203  is formed. In particular, a first row of indentations  208  is formed by moving the indentation tool in a linear direction in one direction. Subsequent second, third, fourth and fifth rows can be formed similarly. In other embodiments, indentation pattern  203  is formed in one indentation event where the indentation tool includes multiple protrusions that form all indentations  208  at once. Movement of the indentation tool can be controlled such that corners of adjacent indentations  208  contact each other or are proximate to each other. Precise movement and applied force of the indentation tool can be controlled by an electronic system, such as an electronic system described below with reference to  FIG. 7 . In some embodiments, the indentation tool should be controllable to and accuracy of within about 5 micrometers. In some embodiments for testing aluminum oxide coatings on aluminum alloy substrates, the indentation tool should be capable of applying at least a 10 kilogram-force load. 
     The extent of delamination of coating  206  has been well-correlated with interfacial adhesion of coating  206  to substrate  204  as evaluated by more conventional controlled four-point bend delamination tests or pull-off tests. However, in contrast to four-point bend delamination or pull-off tests, the sample geometry is not as constrained, and little or no sample preparation is required for the indentation test described herein. Also, unlike four-point bend delamination or pull-off tests, there is no limit imposed by the strength of adhesives. Furthermore, unlike other more conventional mechanical tests, relatively high interfacial shear strains are readily attained under indentation with loads of just a few kilograms. The damage induced by described test procedures are also very localized, enabling an accurate and complete quantitative appraisal within a single, high-resolution optical image. 
       FIG. 2C  shows optical and scanning electron microscope (SEM) images of five-by-five patterns of indentations in accordance with described embodiments.  FIG. 2C  show images of articles with aluminum alloy substrates with aluminum oxide coatings. The top row of images (samples  211  and  213 ) shows light optical images using 400× magnification in a bright field with a camera. The bottom row of images (samples  215  and  217 ) shows SEM images using 60× magnification utilizing back scatter detection. The left column shows images of articles that experienced substantially no delamination after the five-by-five indentation test, and the right column shows images of articles that experienced significant delamination after the five-by-five indentation test. The diameters D of each indentation within the five-by-five patterns, as measured from centers of the indentations, are the same. The five-by-five pattern is arranged such that distance d between adjacent indentations is less than three times the diameter D. In some embodiments where the article is a 7000 series aluminum alloy (T5 or T6 temper), distance d between indentations is 350 micrometers±5 micrometers. In another embodiment where the article is a 6063 aluminum alloy (T5 or T6 temper), distance d between indentations is 420 micrometers±5 micrometers. 
     The images of  FIG. 2C  demonstrate that articles that experience delamination can be observed and distinguishable over articles that do not experience substantial delamination. In particular, samples  211  and  215  did not experience substantial delamination, whereas samples  213  and  217  did experience substantial delamination. As shown, delamination occurs mostly at or near stressed regions  220 . In some embodiments, the amount of delamination is quantified such that data from a number of articles can be collected and analyzed. For example, an analysis can be performed based on counting the number of stressed regions  220  within a five-by-five sample that have delamination. The counting can be accomplished visually by an operator or automatically using a computer. In other embodiments, the total area of exposed substrate is measured. 
     The methods described herein are not limited to indentations having any particular size or shape.  FIG. 3  shows a perspective cross-sectional view  300  of indentation tool  310  forming an indentation pattern  307  having indentations  308  within article  301  in accordance with some embodiments. Indentation tool  310  has spherical-based shape and thereby forms spherical-based or curved indentations  308 . As with previously described indentations, forming indentations  308  plastically deforms coating  304  on the substrate  302  causing residual stress in the vicinity of indentations  308 . The residual stresses overlap at stressed regions  306 , corresponding to regions of article  301  likely to experience delamination. Stressed regions  306  are defined by edges  309  of indentations  308 . In this case, stressed regions  306  have a rectangular shape, in some embodiments a square shape. Distances between indentations  308  (as measured from centers of indentations) can be chosen such that residual stresses cooperate to form a delamination force at stressed regions  306 . In some embodiments, stressed regions  306  are continuous and flow from a first interstitial location to a proximate interstitial location. Other suitable variations of indentation shapes can include triangular-based and hexagonal-based shapes. In some embodiments, the pattern of indentations includes indentations having different shapes. 
       FIG. 4  shows perspective view  400  of indentation tool  406  forming indentation pattern  407  on article  401  in accordance with another embodiment. Indentation pattern  407  includes indentations  410  with minimal spacing there between, indicating that orientation and spacings between indentations  410  can vary. In some embodiments, indentations  410  are arranged such that stressed regions  414  created by indentations  410  occupy edges  412  of adjacent indentations  410 . Delamination can occur at or near stressed regions  414  such that portions  418  of underlying substrate  402  are exposed through coating  404 . 
     In some embodiments, indentation spacing and indentation force may be varied to produce a stress pattern in the coating with varied stress states.  FIGS. 5A and 5B  show top views of indentation patterns having varied indentation spacing and/or indentation force.  FIG. 5A  shows an indentation pattern  501  arranged in an array where spacing between indentations  503  is varied by row. As shown, spacings m, n, o, p between rows  507   a ,  507   b ,  507   c ,  507   d ,  507   e  vary progressively, producing multiple instances of various different stress states in stressed regions  505  created by indentations  503 . Stress regions  505  between rows  507   a  and  507   b  can have similar stress states since they are separated by the same spacing m. Stress regions  505  between rows  507   b  and  507   c  can have similar stress states since they are separated by the same spacing n, but different than the stress states of stress regions  505  between rows  507   a  and  507   b . Similarly, stress regions  505  between rows  507   c ,  507   d ,  507   e  separated by spacings o and p produce corresponding stress regions that are different than stress regions  505  between rows  507   a  and  507   b  separated by spacings n and m. In this way, varying stress regions  505  having different stress states can be formed within a single indentation pattern  501 . Thus, indentation pattern  501  can be used to determine a stress threshold at which a coating will delaminate. For example, the detachment force created at stress regions between row  507   a  and  507   b  may be great enough to cause delamination; however, the detachment force created at stress regions between row  507   d  and  507   d  may not be great enough to cause delamination. In some embodiments, the force applied by the indentation tool can also be varied among indentations  503  within each of rows  507   a ,  507   b ,  507   c ,  507   d ,  507   e , further varying the stress states of stress regions  505  between indentations  503 . In other embodiments, spacings between columns are varied instead of between rows. 
       FIG. 5B  shows an alternative indentation pattern  511  where indentations  513  are arranged in an array with spacing between rows and columns of indentations  513  are both varied progressively. In particular, spacings m, n, o, p between rows  517   a ,  517   b ,  517   c ,  517   d ,  517   e  vary progressively and spacings m, n, o, p between columns  519   a ,  519   b ,  519   c ,  519   d ,  519   e  vary progressively. This arrangement creates more varied stress states at stress regions  515  between indentations  513  across pattern  511 . In some embodiments, the force applied by the indentation tool can also be varied among indentations  513  within indentations of each of rows  517   a ,  517   b ,  517   c ,  517   d ,  517   e  and/or columns  519   a ,  519   b ,  519   c ,  519   d ,  519   e . These variations can be used to further determine a stress threshold for coating delamination. Although the spacing between rows and columns shown in  FIGS. 5A and 5B  are varied progressively, any suitable arrangement may be used that varies the spacing between indentations. 
       FIGS. 3, 4, 5A and 5B  are shown to illustrate that the indentation methods described herein can use any suitable size, shape, orientation and distance (spacing) between indentations. These examples, however, are not meant to exclude other variations that may be implemented within the scope of the embodiments presented herein. In some embodiments, the shape, orientation, size and distance between indentations are chosen to result in providing repeatable results for a number of articles having similar or different coatings. 
       FIG. 6  shows a perspective view  600  of indentation pattern  603  on an article  601  having coating  604  with multiple layers. In particular, coating  604  includes first layer  604   a  and second layer  604   b . First layer  604   a  can correspond, for example, to a first material deposited onto substrate  602  using a first deposition process and second layer  604   b  can correspond to a second material deposited on first layer  604   a  using a subsequent second deposition process. In other embodiments, coating  604  includes more than two layers  604   a  and  604   b . Indentations  608  formed in article  601  can plastically deform the first layer  604   a , second layer  604   b , and the substrate  602  creating stressed region  610 . A delamination force at stressed region  610  can cause delamination of second layer  604   b , or both second layer  604   b  and first layer  604   a , at or near stressed region  610 . Delamination of both second layer  604   b  and first layer  604   a  is evidence by exposed portion  612  of underlying substrate  602 . If only some of second layer  604   b  is delaminated, a corresponding portion of underlying first layer  604   a  will be visible. 
       FIG. 7  is a flow diagram  700  depicting a method of testing a coating for adhesion integrity in accordance with some embodiments. At  702 , a pattern spacing of an indentation pattern is determined. Desired indentation shape, orientation, size, depth and spacing (distance between indentations) can depend on the hardness of the coating(s), the hardness of the substrate, the shape and size of the indenting tool, and the force used to form the indentations. Spacing between the indentations may depend on the size and shape of the indentations, in addition to the type and hardness of the coating and substrate. For example, in some applications a spacing that is less than about three times a diameter of substantially equally sized indentations provides optimal results. 
     At  704 , the determined indentation pattern is formed on an article. In some embodiments the surface of the article is substantially flat and the indenter is pressed into the surface of the article in a direction substantially perpendicular to the surface of the article. Maintaining a flat surface on the article and perpendicular force on the indenter can ensure that the indentations are symmetric in shape. In some embodiments, the indenting tool applies force for 10 seconds. An array or grid of symmetric indentations may ensure reliable and repeatable evaluations of a coating. Stressed regions are formed in the interstices between the indentations, which correspond to locations where delamination is likely to occur. 
     At  706 , the indentation pattern that is formed on the article is examined. In some embodiments an optical image of the indentation pattern is created. An optical image can show delamination of coating that optically contrasts with the substrate. In other embodiments, a scanning electron microscope (SEM) image is used. SEM may be required where there is little optical contrast, such as for some non-dyed or light-colored anodic oxide coatings. However, darker dyed anodic oxide coatings may have enough optical contrast with the underlying metal substrate to use optical imaging techniques. In some embodiments, multiple images are stitched together to provide a single image of the indentation pattern. 
     At  708 , an image is provided detailing the level of delamination. The image can be in the form of a picture or image displayed on a computer screen. The image can be analyzed, either by an operator or automatically using image analyzing techniques. In some embodiments, the number of stressed regions that experience delamination is counted and compared to similar articles to obtain objective results as to adhesion performance of different coatings. In some embodiments, a total area of delamination is determined as a measure of the extent of delamination. 
       FIG. 8  is a block diagram of electronic system  800  suitable for controlling some of the indentation testing processes described above. Electronic system  800  can represent a computing system as part of an indentation machine such as a Vickers hardness testing machine. Electronic system  800  includes a processor  802  that pertains to a microprocessor or controller for controlling the overall operation of electronic system  800 . Electronic system  800  contains instruction data pertaining to manufacturing instructions in a file system  804  and a cache  806 . The file system  804  is, typically, a storage disk or multiple disks. The file system  804  typically provides high capacity storage capability for the electronic system  800 . However, since the access time to the file system  804  can be relatively slow, electronic system  800  can also include a cache  806 . Cache  806  can be, for example, Random-Access Memory (RAM) provided by semiconductor memory. The relative access time to the cache  806  can be substantially shorter than for the file system  804 . However, cache  806  may not have the large storage capacity of the file system  804 . Further, file system  804 , when active, can consume more power than cache  806 . The power consumption is often a concern when the electronic system  800  is a portable device that is powered by a battery  824 . The electronic system  800  can also include a RAM  820  and a Read-Only Memory (ROM)  822 . ROM  822  can store programs, utilities or processes to be executed in a non-volatile manner. RAM  820  can provide volatile data storage, such as for cache  806 . 
     Electronic system  800  can also include a user input device  808  that allows a user of the electronic system  800  to interact with the electronic system  800 . For example, a user input device  808  can take a variety of forms, such as a button, keypad, dial, touch screen, audio input interface, visual/image capture input interface, input in the form of sensor data, etc. Still further, the electronic system  800  can include a display  810  (screen display) that can be controlled by the processor  802  to display information to the user. As described above, in some embodiments, display  810  provides images collected from an optical imaging tool and/or a scanning electron microscope. Data bus  816  can facilitate data transfer between at least the file system  804 , the cache  806 , the processor  802 , and a coder/decoder (CODEC)  813 . CODEC  813  can be used to decode and play multiple media items from file system  804  that can correspond to certain activities taking place during a particular manufacturing process. Processor  802 , upon a certain manufacturing event occurring, supplies the media data (e.g., audio file) for the particular media item to a CODEC  813 . CODEC  813  can then produce analog output signals for a speaker  814 . Speaker  814  can be a speaker internal to electronic system  800  or external to electronic system  800 . For example, headphones or earphones that connect to the electronic system  800  would be considered an external speaker. 
     Electronic system  800  can also include a network/bus interface  811  that couples to a data link  812 . Data link  812  can allow electronic system  800  to couple to a host computer or to accessory devices. Data link  812  can be provided over a wired connection or a wireless connection. In the case of a wireless connection, network/bus interface  811  can include a wireless transceiver. The media items (media assets) can pertain to one or more different types of media content. In one embodiment, the media items are audio tracks (e.g., songs, audio books, and podcasts). In another embodiment, the media items are images (e.g., photos). However, in other embodiments, the media items can be any combination of audio, graphical or visual content. Sensor  826  can take the form of circuitry for detecting any number of stimuli. For example, sensor  826  can include any number of sensors for monitoring a manufacturing operation such as for example a Hall Effect sensor responsive to external magnetic field, an audio sensor, a light sensor such as a photometer, and so on. 
     The various aspects, embodiments, implementations or features of the described embodiments can be used separately or in any combination. Various aspects of the described embodiments can be implemented by software, hardware or a combination of hardware and software. The described embodiments can also be embodied as computer readable code on a non-transitory computer readable medium for controlling manufacturing operations or as computer readable code on a non-transitory computer readable medium for controlling a manufacturing line. The non-transitory computer readable medium is any data storage device that can store data, which can thereafter be read by a computer system. Examples of the non-transitory computer readable medium include read-only memory, random-access memory, CD-ROMs, DVDs, magnetic tape, optical data storage devices, and carrier waves. The non-transitory computer readable medium can also be distributed over network-coupled computer systems so that the computer readable code is stored and executed in a distributed fashion. 
     The foregoing description, for purposes of explanation, used specific nomenclature to provide a thorough understanding of the described embodiments. However, it will be apparent to one skilled in the art that the specific details are not required in order to practice the described embodiments. Thus, the foregoing descriptions of the specific embodiments described herein are presented for purposes of illustration and description. They are not target to be exhaustive or to limit the embodiments to the precise forms disclosed. It will be apparent to one of ordinary skill in the art that many modifications and variations are possible in view of the above teachings.

Metadata:
Filing Date: 20150403
Publication Date: 20180116
Grant Date: 20180116
Priority Date: 20150403
Inventors: HAMANN ERIC W.
COUNTS WILLIAM A.
CURRAN JAMES A.
Assignee: APPLE INC
CPC Classifications: [{"code": "G01N19/04", "inventive": true, "first": false, "tree": "[]"}, {"code": "G01N3/42", "inventive": true, "first": false, "tree": "[]"}, {"code": "G01N2203/0286", "inventive": false, "first": false, "tree": "[]"}, {"code": "G01N3/48", "inventive": true, "first": true, "tree": "[]"}, {"code": "G01N2203/0286", "inventive": false, "first": false, "tree": "[]"}, {"code": "G01N19/04", "inventive": true, "first": false, "tree": "[]"}, {"code": "G01N3/48", "inventive": true, "first": true, "tree": "[]"}, {"code": "G01N19/04", "inventive": true, "first": true, "tree": "[]"}, {"code": "G01N2203/0286", "inventive": false, "first": false, "tree": "[]"}, {"code": "G01N3/42", "inventive": true, "first": false, "tree": "[]"}, {"code": "G01N3/42", "inventive": true, "first": false, "tree": "[]"}]
Family ID: 57004854