Extensive research has been carried out in order to study the feasibility of various methods for evaluating the adhesion strength of thin hard coatings, produced by a wide variety of coating methods, to steels. Arai, T., Fujita, H. and Watanabe, M., 1987. Evaluation of adhesion strength of thin hard coatings. Thin Solid Films, 154(1-2), pp.387-401. It appears that the most straightforward technique is microscopic observation. Among the methods based on the use of a physical measurement, acoustic emission detection is the most effective. The dynamics ratio between the signals below and above the critical load for the acoustic emission (much greater than 100) is well above that obtained with the normal, tangential and lateral forces (from less than 5 to 10). Finally, the fields of application of the scratch test as an adhesion test are discussed. Valli, J., Mäkelä, U., Matthews, A. and Murawa, V., 1985. TiN coating adhesion studies using the scratch test method. Journal of Vacuum Science & Technology A: Vacuum, Surfaces, and Films, 3(6), pp.2411-2414. Scratch test is mainly used to study mechanical properties of materials near their surface. Understanding of this test is of great interest to both academic and industrial communities. The scratch hardness and surface deformation mechanisms of materials depend in particular on the rheology of the material, the friction at the interface and the indenter geometry. Optical observations of the residual groove and the measure of the apparent coefficient of friction, μ0=Ft/W, the ratio of ploughing force, Ft, to normal force, W, are important to understand what it occurs during a scratch test ( Fig. 1). Bucaille, J.L., Felder, E. and Hochstetter, G., 2001. Mechanical analysis of the scratch test on elastic and perfectly plastic materials with the three-dimensional finite element modeling. Wear, 249(5), pp.422-432. 3.2 Project specification what is needed from you in this project. And how do you need to make it exactly. What are the requirements. 1. Market research 2. Research methodology What have you done for the research? and how? 3. Background research/literature research 3.1 Substrate 3.2 Coatings A coating is a film layer covering a surface for various purposes. Mainly the purposes of coatings are to protect the actual surface underneath the coating. Coatings are used in numerous applications including; packaging, biomedical and engineering et cetera. In engineering coatings enhance the durability of the component. This includes lowering friction in tribological applications, resisting of wear and abrasion in mechanical applications. Coatings also protect substrates and act as a barrier to chemical applications. [1] coatings are critical to engineering applications. ‘The functional behaviour of coatings is critical to its adhesion to the substrate’ (Kuiry, 2012, p.3) [2] Since coatings are critical in engineering applications, it is necessary they are tested accordingly to ensure their durability. There are different methods of testing durability of coatings, however the most used method is ‘Scratch Testing of Film Coatings’ [3] n the development of modern materials, the functionality is often improved by combining materials of different properties into composites. Many classes of composites exist, most of which are addressing improved mechanical properties such as stiffness, strength, toughness and resistance to fatigue. Coating composites (i.e. surface engineered materials) are designed to specifically improve functions such as tribological, electrical, optical, electronic, chemical and magnetic, see Fig. 1. It is thus natural to select the bulk of a component to meet the demands for stiffness, strength, formability, cost, etc. and then modify or add an other material as a thin surface layer. This surface layer or coating is the carrier of virtually all other functional properties. Application of coatings on tools and machine elements is, therefore, a very efficient way of improving their friction and wear resistance properties. Attar, F. and Johannesson, T., 1996. Adhesion evaluation of thin ceramic coatings on tool steel using the scratch testing technique. Surface and Coatings Technology, 78(1-3), pp.87-102. 3.3 Adhesion and cohesion Bonding is the joining of two substrates using an adhesive. According to DIN EN 923, an adhesive is defined as a non-metallic binder that acts via adhesion and cohesion. ASTM D907-06 defines an adhesive as "a substance capable of holding materials together by surface attachment". Overview of Adhesion and Cohesion Adhesion is the adhering of similar or different types of materials to each other. Cohesion is the inner strength of a material, such as the adhesive in this case. The adhesive interactions between an adhesive and a substrate not only concern the actual area of contact (adhesion zone) of the adhesive and substrate but also concern the state of the adhesive in the vicinity of the surface of the substrate (transition zone). • In the cohesion zone, the adhesive is present in its normal state. • In the adhesion zone, the adhesive has a modified structure and composition due to its adhesion to the surfaces of the substrates. This structure and composition is different from the state in the cohesion zone. As a result, the macroscopic properties of the adhesive in the adhesion zone are also altered. • The structure, composition and macroscopic properties of the adhesive continuously change in the transition zone between the adhesion zone and the cohesion zone. There may for example be separation of the components of the adhesive due to diffusion of the small components of the adhesive into surface pores. The optimum composition of the adhesive is hence adversely affected. http://www.adhesives.org/adhesives-sealants/science-of-adhesion reference 8 on the paper ADHESIVES.ORG (n.d.) Science of adhesion- What is bonding? [online] Adhesives.org. Available from: http://www.adhesives.org/adhesives-sealants/science-of-adhesion [ Accessed 22/02/17] Adhesion and cohesion are terms that are often confused although these subjects are discussed in many standard texts in dental biomaterials science [1–3]. There are also many excellent texts and monographs on adhesion, cohesion, and interfacial reactions [4–6] together with a comprehensive treatment in the on-line encyclopedia, Wikipedia. Since adhesion and cohesion play a very important role in the use of luting agents, an in-depth discussion is appropriate in view of the communications presented in this issue. The Merriam-Webster dictionary has several definitions of the word “adhesion” but the most apposite here is the molecular attraction exerted between the surfaces of bodies in contact. This dictionary likewise has several definitions of the word “cohesion” but the most pertinent here is the molecular attraction by which the particles of a body are united throughout the mass. In other words, adhesion is any attraction process between dissimilar molecular species, which have been brought into direct contact such that the adhesive “clings” or binds to the applied surface or substrate. The postsurgical complication of adhesions, involving soft tissues, will not be discussed here. von Fraunhofer, J.A., 2012. Adhesion and cohesion. International journal of dentistry, 2012. 3.4 Coating scratch-testing Scratching has been a well known tool for obtaining a material hardness since 1812 when the German mineralogist Friedrich Mohs put forth his scale of the mineral hardness. This scale contains ten minerals (1. talc, 2. gypsum, 3. calcite, 4. fluorite, 5. apatite, 6. orthoclase feldspar, 7. quartz, 8. topaz, 9. corundum, 10. diamond), ordered from the softest to the hardest mineral. This scale is based on the simple idea that harder material can visibly scratch another material, but not contrarily. Yet the scale itself might be sufficient to acquire basic idea of minerals hardness it does not meet the requirements of industrial practice. Hardness is defined as an ability of material to resist penetration or abrasion by other materials. In agreement with this definition can be performed various tests that differ from the technique and value of the hardness. Evaluating of the hardness can be divided into the three main types: rebound hardness, indentation and scratch. Rebound hardness is evaluated by measuring of the bounce of hammer dropped from the fixed height onto the material. Indentation hardness is evaluated according to the dimensions of indent left by the indenter. On the field of engineering is the most common Vickers’s, Brinnel’s and Rockwell’s test. Scratch hardness is often assessed in the case of surface films or as a comparative method. [1] [2] Hardness can be evaluated on the basis of three different scales: macro, micro and nano scale. Specimen tested on the macro-scale usually undergoes test load higher than 10 N and in this scale are also included aforementioned Vickers’s, Brinnel’s and Rockwell’s tests. Development of the micro and nano-scale tests has been driven by the need of material science to test samples on the smaller scale e.g. hard thin coats, separately test elements of composed material or when only a limited amount of material is available. Macro-scale test would not be possible to perform under such circumstances. Interestingly, it has been observed that due to flaws occurred in a material, hardness measured on the micro and nano-scale is higher than hardness measured on the macro-scale. This paper is focused on the discussion over the various types of scratch tests for different purposes and is divided into the two sections. In the first section will be discussed scratch test method itself and scratch tests for different purposes according to the tested material. Second part will be devoted to the practical evaluation of the scratch test In accordance with aforesaid scales of scratch tests, there are corresponding testers that are used for given range of load. These high sophisticated devices are produced by the specialized manufacturers such as CSM, Tribotechnic, Anton Paar or Sheen. For the sake of illustration see Tab. 1 where are mentioned different types of CSM scratch testers including their specifications. CSM SCRATCH TESTERS [Online]. Available at: www.csm-instruments.com. (Reference for above table) 1.2 SCRATCH TEST In the case of nano and micro-scratch test the surface of the specimen must the flattened and polished. On that scale any disturbance on the surface might cause fluctuation in both force and depth of scratch which might make further correct evaluation complicated or impossible. When ready, the specimen is inserted into the chamber, which during the test separate specimen from external surroundings as it can cause inaccuracy during the test. In the next step the location for the scratch test is chosen and the surrounding area is scanned. 3 Scratch test itself usually consists of three stages, which follow the same trajectory over the surface: First - So called prescan is used for measuring the surface. It is performed under the lowest possible pressure so that no permanent damage is made on the surface. Second - Scratch is performed according to preset conditions (speed, force, depth) Third – Postscan is performed in order to measure residual topograpfy of the damaged area. Similarly to the first stage, it is performed under the lowest possible pressure. When the specimen is subjected to the load test, crucial parameters of the test are measured such as vertical and horizontal force, depth and length of the scratch (see Fig. 1). Additionally, surroundings of the scratch test can be mapped and scaled so that precise geometry including the width of the scratch test can be measured. According to the vertical load, scratch test can be performed in three types: Constant, progressive, incremental, see Fig. 1. During the scratch test can occur three types of failure: elastic, plastic and fracture, whilst each is studied for the different purpose. Technical standards of procedures and application methods linked to the indentation and scratch tests are also developed by ASTM organization (American Society for Testing and Materials). 3] CSM SCRATCH TESTERS, [Online]. Available at: www.csm-instruments.com. 4] Scratch Testing [Online]. Available at: http://www.qualitymag.com/articles/89229- scratch-test (Reference above is for the image) http://ksm.fsv.cvut.cz/~nemecek/teaching/dmpo/clanky/2015/Kadlicek_zav%20prace_Scratch%20test.pdf (reference 7 on the paper) KADLICEK, T. (2015) Micro mechanics and microstructural description of materials. Published thesis (PhD.), Czech technical University. scratch testing is one of the widely and effective ways of obtaining the critical load on a coating by linearly moving an indenter into a coating surface. Load is applied on the indenter where the indenter then creates a scratch into the surface. The scratch is viewed with an optical microscope to determine the critical load Constant Load scratch test provides better differentiation of damage levels. Such test requires more specimens area and test time, it is used for research and process development of coatings. Progressive Load scratch test covers full load range with a single test without any gap, therefore, it is faster than Constant Load test. It is the most popular test for R&D and QC of coating. https://www.bruker.com/fileadmin/user_upload/8-PDF-Docs/SurfaceAnalysis/TMT/Webinars/Tribology_101_3-Advanced_Scratch_Testing_Applications-06182013.pdf (reference 5 on the paper) BRUKER NANO SERVICES DEVISION (2013) Tribology 101- Advanced scratch testing applications. Karlsruhe: Bruker. (a)To obtain scratch hardness (b)To evaluate resistance against deterioration of surface quality during handling and in real-life applications. For Coating: (a)To estimate practical adhesion strength of coating for various applications (protective, decorative, tribological, optical, biological, etc.) (b)To understand failure modes of coating and coating-substrate interface https://www.bruker.com/fileadmin/user_upload/8-PDF-Docs/SurfaceAnalysis/TMT/Webinars/Tribology_101_3-Advanced_Scratch_Testing_Applications-06182013.pdf (reference 5 on the paper) BRUKER NANO SERVICES DEVISION (2013) Tribology 101- Advanced scratch testing applications. Karlsruhe: Bruker. The scratch test for adhesion is reviewed as the only method currently available for testing thin, hard and well-adhering coatings such as TiC on steel or cemented carbide substrates. The critical load, mode of coating removal and acoustic signals are discussed. It is found that the combination of acoustic signal with microscopic observations can indicate whether failure occurs following a cohesive or an adhesive mode. The critical loads increase with increasing coating thickness in a manner which is a characteristic of the coating-substrate combination being studied. Critical loads are higher for harder tougher substrate materials; they also appear to depend on the elastic modulus and the coefficient of friction of the coating itself. http://www.sciencedirect.com/science/article/pii/0040609083900196 reference 4 on paper PERRY, A.J. (1983) Scratch adhesion testing of hard coatings. Thin solid films 107 (2) pp.167-180 Microhardness and scratch adhesion testing are the most commonly used techniques for assessing the mechanical properties of thin surface coatings such as physical vapour-deposited TiN. Both of these test methods utilize single-point contacts, a diamond pyramid for microhardness testing and usually a Rockwell C 120° diamond cone for scratch testing, and both induce plastic deformation in the substrate and coating. Clearly, static microhardness and scratch adhesion testing will have common features since, in both, the plastic deformation processes are likely to be similar. An analysis based on elastic-plastic identation theory has been developed that allows both the prediction of the hardness of a given coating-substrate system and the estimation of shear strains developed at the coating-substrate interface which, for weakly adhered films, leads to delamination of the coating around an indentation. The ideas embodied in the volume law-of-mixtures hardness model have also been applied to the scratch test, allowing estimates to be made of the interfacial shear strains present during testing since these contribute to the measured “critical load” Lc for coating failure. The role of substrate plastic deformation and other factors affecting Lc (such as friction, internal stress and coating thickness) will be discussed. Research paper: The Relationship Between Hardness and Scratch Adhesion. Available from: https://www.researchgate.net/publication/248309122_The_Relationship_Between_Hardness_and_Scratch_Adhesion [accessed Apr 19, 2017]. (reference 2 on the paper) BURNETT, P.J. AND RICKERBY, D.S. (1987) the relationship between hardness and scratch adhesion. Thin solid films, 152 (1-2) pp.403-416 When performing a progressive load test, the critical load is defined as the smallest load at which a recognizable failure occurs. In the case of a constant load test, the critical load corresponds to the load at which a regular occurrence of such failure along the track is observed. In the case of bulk materials, the critical loads observed are cohesive failures, such as cracking, or plastic deformation or the material. In the case of coated samples, the lower load regime results in conformal or tensile cracking of the coating which still remains fully adherent (which usually defines the first critical load). In the higher load regime, further damage usually comes from coating detachment from the substrate by spalling, buckling or chipping. Fig. 2 illustrates the principle of scratch testing. (Make sure to reference this picture)!!!!!!!!! http://nanovea.com/App-Notes/coating-failure-scratch.pdf Li, D., 2013. UNDERSTANDING COATING FAILURES USING SCRATCH TESTING Comments on the critical load The scratch test gives very reproducible quantitative data that can be used to compare the behavior of various coatings. The critical loads depend on the mechanical strength (adhesion, cohesion) of a coating-substrate composite but also on several other parameters: some of them are directly related to the test itself, while others are related to the coating-substrate system. The parameters that determine the critical loads are summarized in Table 1. 4 Test specific parameters Sample specific parameters Loading rate Scratching speed Indenter tip radius Indenter material Friction coefficient between surface and indenter Internal stresses in the material for bulk materials Material hardness & roughness for coating-substrate systems Substrate hardness and roughness Coating hardness and roughness Coating thickness Table 1: List of parameters that determine the critical loads. http://nanovea.com/App-Notes/coating-failure-scratch.pdf Li, D., 2013. UNDERSTANDING COATING FAILURES USING SCRATCH TESTING According to ISO/WD 27307 there are generally two types of failure that can occur during the test (Fig. 9): the cone-shaped fracture at the substrate/coating interface (indication of the coating adhesion) and the cone-shaped fracture in the coating (indication of the coating cohesion). As expected the cone-shaped fracture occurred on all the examined coatings (Fig. 10). Using the OM images after the scratch test it is very easy to indicate which type (adhesion/cohesion) of bond strength is critical for the observed coating. For all coatings the cone-shaped fracture occurs just inside the coatings, which reveals that the failure is due to a problem of cohesion within the coating itself (Table 4). Only on coating 505 small cracks were observed around the scratch, which decreased the values of the cohesive strength. These cracks are more obvious on the SEM image of the coating 505 scratch path (Fig. 11). http://www.sciencedirect.com/science/article/pii/S0301679X1100096X (reference 3 on the paper) VENCL, A ET AL ( 2011) Evaluation of adhesion/cohesion bond strength of the thick plasma spray coatings by scratch testing on coating cross-sections. Tribology international, 44 (11), pp.1281-1288 The scratch test is an instrumented complex technique for the assessment of both adhesive and cohesive properties and tribological characteristics of thin films. During the scratch test a diamond indenter is straightforwardly pulled over the investigated surface at a constant velocity as can be seen from Figure 1. The normal force is applied either in steps or more often continuously (usually linear increase). The latter case is called progressive load scratch test or ramped scratch test. The level of forces used for investigation of thin films usually ranges from several tens to hundreds of milinewtons while the corresponding displacements are typically from several nanometers to hundreds of nanometers [6]. The most commonly used type of indenter is the diamond conical Rockwell indenter. However, sharp indenters like Berkovich or cube-corner are also used in some special cases [7,8]. The stress field generated under the moving indenter leads to the occurrence of different failure modes like coating detachment, through-thickness cracking, plastic deformation and cracking in the coating and/or substrate. These failure modes arise as a result of cohesion failure of coating and/or substrate or due to failure of coatingsubstrate adhesion. The goal of the test is to find so called critical loads (LC) at which characteristic failures occur. These results then provide information about coating quality, its adhesive and cohesive properties and tribological resistance [9]. http://www.epj-conferences.org/articles/epjconf/pdf/2013/09/epjconf_OAM2012_00027.pdf reference number 1 on the paper TOMASTIC, J and CTVRLIK, R (2013) Nano scratch test – A tool for evaluation of cohesive and adhesive properties of thin films and coatings. In: EPJ Web of Conferences, Olomouc, May 2013. EDP Sciences, pp.1-4 The typical course of the most often used progressive load scratch test is as follows: Before the test itself only minor force is loaded on the tip so it scans the surface. This data are used for surface roughness estimation and/or for compensation of the sample tilt. Then the load is gradually applied to the moving tip. At the beginning, surface deforms only elastically with no residual pattern on the surface. With increasing load the coating starts to deform plastically and the residual scratch on surface appears. Further increase of normal force results in creation of small cracks inside and at the edges of the scratch groove followed by more pronounced cracking and finally complete delamination of the coating. The onsets of failures, which appear regularly along the track, represent the critical loads, which can be considered as quantitative output of the scratch test method. The nature of tribological contact of two surfaces in relative motion is very complex, which makes any simulation or prediction very difficult. Complexity of the system further increases if coating is introduced to the surface. Stress field resulting from the indenter movement can be understood as a sum of different contributions. Penetration of indenter into the surface cause bending of coating where both compressive and tensional stresses appear. The friction force between surface and sliding tip causes compressive stress in front of moving tip and tensile stress behind the tip. Another important factor is a magnitude of residual stresses stored in coating- substrate system during deposition process. Extensive analysis of these contributions can be seen in Holmberg [2]. http://www.epj-conferences.org/articles/epjconf/pdf/2013/09/epjconf_OAM2012_00027.pdf (on the blank paper reference number 1) TOMASTIC, J and CTVRLIK, R (2013) Nano scratch test – A tool for evaluation of cohesive and adhesive properties of thin films and coatings. In: EPJ Web of Conferences, Olomouc, May 2013. EDP Sciences, pp.1-4 The principal parameter predetermining response during the scratch test is considered coating and substrate hardness. In general, four possible situations can be distinguished: • Soft coating on soft substrate – plastic deformation dominates and residual groove is created by ploughing. Crack formation is not present except at very high loads. • Soft coating on hard substrate – plastic deformation dominates again. Coating gradually thins until the substrate is uncovered. • Hard coating on soft substrate – deformation of substrate dominates. Coating bends into the track created by the plastic deformation of substrate. As a result, coating deforms plastically or cracks. • Hard coating on hard substrate – Only minor or insignificant plastic deformation appears. Cracking of coating occurs, which can spread to the substrate [18]. Soft coatings generally tend to fail by ductile manner and are usually characterized by smaller cracks that are confined around residual groove. In contrast, hard coatings are rather brittle and their cracks often extend beyond the edges of the track. http://www.epj-conferences.org/articles/epjconf/pdf/2013/09/epjconf_OAM2012_00027.pdf reference number 1 on the paper TOMASTIC, J and CTVRLIK, R (2013) Nano scratch test – A tool for evaluation of cohesive and adhesive properties of thin films and coatings. In: EPJ Web of Conferences, Olomouc, May 2013. EDP Sciences, pp.1-4 icroscopic observation This is the most reliable method to detect surface damage. This technique is able to differentiate between cohesive failure within the coating and adhesive failure at the interface of the coating-substrate system. Tangential (frictional) force recording This enables the force fluctuations along the scratch to be studied and correlated to the failures observed under the microscope. Typically, a failure in the sample will result in a change (a step, or a change in slope) in coefficient of friction. Frictional responses to failures are very specific to the coating-substrate system in study. Acoustic emission (AE) detection Detection of elastic waves generated as a result of the formation and propagation of microcracks. The AE sensor is insensitive to mechanical vibration frequencies of the instrument. This method of critical load determination is mostly adequate for hard coatings that crack with more energy. Depth Sensing Sudden change in the depth data can indicate delimitation. Depth information pre and post scratch can also give information on plastic versus elastic deformation during the test. 3D Non-Contact imaging such as white light axial chromatism technique and AFMs can be useful to measure exact depth of scratch after the test. (Detailed description of scratch testing. Benefits of scratch testing and the importance. How would it be if there were no scratch testing machines. (Show figures and reference the figures) http://nanovea.com/App-Notes/coating-failure-scratch.pdf Li, D., 2013. UNDERSTANDING COATING FAILURES USING SCRATCH TESTING. 3.5 Parts 3.5 stepper motor (explain stepper motor and explain why stepper motor. Why not brush motor or any other motor?) Stepper motors are DC motors that move in discrete steps. They have multiple coils that are organized in groups called "phases". By energizing each phase in sequence, the motor will rotate, one step at a time. With a computer controlled stepping you can achieve very precise positioning and/or speed control. For this reason, stepper motors are the motor of choice for many precision motion control applications. What are stepper motors good for? Positioning – Since steppers move in precise repeatable steps, they excel in applications requiring precise positioning such as 3D printers, CNC, Camera platforms and X,Y Plotters. Some disk drives also use stepper motors to position the read/write head. Speed Control – Precise increments of movement also allow for excellent control of rotational speed for process automation and robotics. Low Speed Torque - Normal DC motors don't have very much torque at low speeds. A Stepper motor has maximum torque at low speeds, so they are a good choice for applications requiring low speed with high precision. What are their limitations? Low Efficiency – Unlike DC motors, stepper motor current consumption is independent of load. They draw the most current when they are doing no work at all. Because of this, they tend to run hot. Limited High Speed Torque - In general, stepper motors have less torque at high speeds than at low speeds. Some steppers are optimized for better high-speed performance, but they need to be paired with an appropriate driver to achieve that performance. No Feedback – Unlike servo motors, most steppers do not have integral feedback for position. Although great precision can be achieved running ‘open loop’. Limit switches or ‘home’ detectors are typically required for safety and/or to establish a reference position Round or "D" Shaft: These are available in a variety of standard diameters and there are many pulleys, gears and shaft couplers designed to fit. "D" shafts have one flattened side to help prevent slippage. These are desirable when high torques are involved. Now we come to the most important part: making sure that your motor and driver are compatible. Mismatched motors and drivers can result in disappointing performance. Or worse: damage to the motor and/or controller. https://learn.adafruit.com/all-about-stepper-motors/what-is-a-stepper-motor (reference 6 on the paper) EARL, B. (2014) what is a stepper motor?[online] Adafruit. Available from: https://learn.adafruit.com/all-about-stepper-motors/what-is-a-stepper-motor [Accessed 17/11/16]