Today, parts and tools are increasingly made of composite materials. Realization of specific connection between basic material and coating is very important. The quality of coating on products, in terms of wear and resistance to destruction, has a large impact on productivity and reliability of production processes, in particular their life. In this paper, based on experimental investigations, the effect of specific relationship between the base material and coating on tribological and protective features of the product is analyzed.Danas se dijelovi i alati sve češće izrađuju iz kombiniranih materijala. Ostvarenje specifične veze između osnovnog materijala i prevlake je izuzetno značajno. Kvaliteta prevlake na proizvodima, sa aspekta habanja i otpornosti na razaranje, ima velik utjecaj na produktivnost i pouzdanost proizvodnih procesa, a osobito na njihov vijek trajanja. U radu je na temelju eksperimentalnih istraživanja analiziran utjecaj specifične veze između osnovnog materijala i prevlake na tribološke i zaštitne karakteristike proizvoda

B. Sovilj

D. Ješić

I. Sovilj-Nikić

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21METALURGIJA 51 (2012) 1, 21-24B. SOVILJ, I. SOVILJ –NIKIĆ, D. JEŠIĆTHE EFFECT OF SPECIFIC RELATIONSHIP BETWEEN MATERIAL AND COATING ON TRIBOLOGICAL AND PROTECTIVE FEATURES OF THE PRODUCTReceived – Prispjelo: 2010-12-30Accepted – Prihvaćeno: 2011-05-15Original Scientifi c Paper – Izvorni znanstveni radISSN 0543-5846METABK 51(1) 21-24 (2012)UDC – UDK 669.131.6/.7+669.14]:[620.169.1+620.178=111Today, parts and tools are increasingly made of composite materials. Realization of specifi c connection between basic material and coating is very important. The quality of coating on products, in terms of wear and resistance to destruction, has a large impact on productivity and reliability of production processes, in particular their life. In this paper, based on experimental investigations, the eff ect of specifi c relationship between the base material and coat-ing on tribological and protective features of the product is analyzed.Key words: materials, tribology, coatings, scratch, adhesionUtjecaj specifi čne veze između materijala i prevlake na tribološke i zaštitne karakteristike proizvoda. Danas se dijelovi i alati sve češće izrađuju iz kombiniranih materijala. Ostvarenje specifi čne veze između osnovnog materi-jala i prevlake je izuzetno značajno. Kvaliteta prevlake na proizvodima, sa aspekta habanja i otpornosti na razaranje, ima velik utjecaj na produktivnost i pouzdanost proizvodnih procesa, a osobito na njihov vijek trajanja. U radu je na temelju eksperimentalnih istraživanja analiziran utjecaj specifi čne veze između osnovnog materijala i prevlake na tribološke i zaštitne karakteristike proizvoda.Ključne riječi: materijal, tribologija, prevlake, skreč, adhezijaINTRODUCTIONThe most important prerequisite for improving the scientifi c and technological progress is the fundamental reconstruction and development of engineering com-plex. Technology of machining and forming technology are the basis technologies for material processing. Tools have always been the base of industrial production. For making almost all products made from various materi-als that we use in everyday life it is necessary to have the right tool [1, 2]. Widespread application of coated tools is caused by its high effi ciency, thanks to the coat-ing strength of tool increases signifi cantly tools. Until now, there is no evidence of correlation between the tool strength tools and coating features, and questions of optimization of the properties of coating and tool base complex are still unsolved.Persistence of tools and machine parts can be incre a-sed by different surface engineering processes such as the preparation of surface and changing composition of the surface layer using appropriate method, or by applying to the surface a thin layer of another material having the desired mechanical and physical-chemical characteris-tics. The best protection against tool wear is a few micro-meters thick layer of hard coating on the base material.The tendency of tool materials development and technologies of their surface reinforcement is condi-tioned by the need aimed at improving the physical and B. Sovilj, I. Sovilj-Nikić, Faculty of Technical Sciences, Novi Sad, Ser-bia, D. Ješić, Tribotehnik, Rijeka, Croatiamechanical properties of tool materials, including the strength, heat stability, resistance, brittleness long been considered an important, while other properties are con-sidered less important. Ignoring the importance of cer-tain properties of tool material is one of the main rea-sons for lack of understanding of the main functions of durable coating on the tool.It is necessary, for each tribological system, to decide on appropriate methods of protecting the surface of its ele-ments based on the prevailing wear mechanism [3 - 5].Development of theoretical concepts in the fi eld of materials surface resistance greatly contributes to the quick improvement of surface coating technology. SPECIFIC CONNECTIONS BETWEEN THE BASE MATERIAL AND COATINGHard coatings were initially used only for the pro-tection of cutting tools, and later for the protection of forming tools and other type of tools. In addition, hard coatings are increasingly being used for protection of machine parts [6]. Adhesion is defi ned as a situation in which two sur-faces stick together due to chemical or mechanical con-nections (Figure 1). The opposite of cohesion, where the connection happens in one material, adhesion is a connection between the two materials. Adhesion is not a fundamental physical quantity, it is the set many fea-tures. Adhesion at the atomic level is well defi ned, but the connection to the macroscopic adhesion, which is 22  METALURGIJA 51 (2012) 1, 21-24B. SOVILJ et al.: THE EFFECT OF SPECIFIC RELATIONSHIP BETWEEN MATERIAL AND COATING ON TRIBOLOGICAL...technologically interesting, is still unclear. Condition for successful use of protective coating is the good ad-hesion. It is possible to provide by adequate pretreat-ment of tool surface, which includes sand blasting, chemical cleaning, et al. Adhesion is improved by ap-plying a thin middle layer, which provides a chemical connection to the substrate. Adhesion can be improved by ion bombardment during the coating process (ion coverage), because high energy parts of the plasma al-low the appearance of pseudo-diffusion zones. If work is positive, the base and layer attract (adhe-sion) and if it is negative then they reject. Great adhe-sion, i.e. great work of adhesion can be achieved if the two materials with a large specifi c surface energy are in connection. Balanced growth of a thin layer with a large number of nucleation centers, indicating strong chemi-cal connections between the base and layer, and that the adhesion of resulting system will be good. In contrast, it can expect weak connection and the possible formation of pores on the boundary if the growth is spot; therefore the adhesion will be weak. Scheme of tribological sys-tem and four different types of boundary between the base and coating are given in Figure 1.The thickness of the coating signifi cantly affects the effi ciency of tool and it is very important data for the technologists. Based on series of investigations it can be concluded that there are optimal coating thicknesses for the specifi c conditions. The difference between the val-ues of coeffi cients of linear expansion of the base mate-rial and deposited layer infl uences the appearance of residual stresses in increasing of thickness of the coat-ing on the boundary of base-coating. Stresses grow and they tend to break adhesive connections between coat-ing and the base. Long-term effects of internal stresses on the adhesive connections lead to its gradual destruc-tion, with the adhesive resistance decreases. The opti-mum coating thickness depends on the technology of coating, which determines the quality of its adhesion, the presence of defects and stresses, as well as the con-ditions of tool exploitation.EXPERIMENTAL TESTS OF SPECIFIC CONECTION BETWEEN THE BASE MATERIAL AND COATINGFrom the microscopic point of view the adhesion represents strength of chemical connections between atoms of base and coating. Directly measuring the strength of these connections is expensive and unsuita-ble for industrial use. Mechanical methods are of pri-mary importance. Mechanical methods of adhesion measuring are oriented towards the easier practical use, not to the physical background. For measuring the ad-hesion of hard coatings the scratch test and the test with print are mostly applied.In this study the measurement of adhesion hardness was performed. For adhesion of hard coatings, where exists a few micrometers thick layer of high hardness on medium-hard basis, scratch test can be applied. Scratch test is performed with a diamond tip of a standard form (type Rockwell C, tip radius is 0,2 mm) on the surface of coating. Size of adhesion is determined by the minimum force that through the diamond needle can scratch into coating. There are two ways of measuring. The fi rst method is the measurement at a constant load, where it must do more scratching; every following must have a greater load. Another way of measuring is done by increasing the load linearly from 0 N to the desired max-imum load on the same scratching phase. Important terms of scratching are the following: load range (0-150 N), scratching speed (10 m·min–1), load speed (100 N·min–1).During the process of scratching two parameters are observed. One is a scratching force or force in the tangen-tial direction which is at low load equivalent to the force of friction. Coeffi cient of scratching force and the load is called the scratching coeffi cient. It is a dimensionless size, which is at low load equivalent to the coeffi cient of fric-tion. The second parameter is the acoustic emissions, whi-ch is released during the formation of micro cracks. At the end of scratching, a scratch is observed with an optical microscope. The analysis is based on a decision of critical loads for the certain type of load. They are indicated by the abbreviation Lcn, where n is the order number. Unfor-tunately the use of these marks is not unique. Standard (ENV 1071-3, 1994) provides only critical loads Lc1 and Lc2, and Lc(AE). The good correlation is between the crit-ical force Lc(Tt) and Lc5 as well as between Lc(AE) and Lc3. Selected critical forces in N are measures of adhe-sion. Acceptable adhesion is for the Lc5 > 60 N. The fi rst condition for a correct result is a suffi cient number of mea-surements. On each sample it should be made at least three scratching and the measuring device should periodica-lly calibrate to the reference sample. Conditions of meas-uring are determined by measuring standard (ENV 1071-3, 1994).Figure 1  Scheme of tribological system: 1-workpiece, 2-hard coating, 3-base, 4-tribological contact (lubricant), 5-adhesive joint between the coating and base: a)keen boundary, b)connected boundary, c)diff usion boundary, d)mechanical boundary23METALURGIJA 51 (2012) 1, 21-24B. SOVILJ et al.: THE EFFECT OF SPECIFIC RELATIONSHIP BETWEEN MATERIAL AND COATING ON TRIBOLOGICAL...Quality of coating, from the tribological point of view, today is determined by the measuring instrument known as Scratch tester. Measuring instruments differ in the level at which the measurement is done. In indus-try and research institutions measurements are carried out at the micro level and in laboratories where they perform fundamental researches measurements are car-ried out at the nano level. To determine the exact micro strength of persistent coatings it is necessary that its thickness is three times larger than the depth of print. Accurate micro hardness of coating, on the basis of TiN at the given loads, can be measured if the thickness of the coating is not less than 0,5 micrometers. Superior adhesion of the coating re-duces the likelihood of forming a free area on the bound-ary under the coating, which leads to an increase of the value of micro strength. During the penetration of the measuring tool in the material or coating, it suffers not only in volume in which a crater forms, but on its edges too. Therefore, in determining the strength it is consider that weight must be several times larger than the depth of print. Only under these conditions specifi ed values of strength adequately characterize the material resistance to penetration of the measuring tool. When adhesion of coating is good, plastic deformation of coating and base takes place. If the adhesion is poor, on the coating-base boundary under the load, layering takes place and plas-tic deformation is not spread to the base (Figure 2).Base and its hardness infl uence on the ratio of pen-etration depth of measuring tool (h) and coating thick-ness (H). This ratio ranges from 1/3 to 1/6. Method for evaluating quality of adhesion of coating with tool basis is only suitable in the case of control fl at surface forms. To control the quality of adhesion of coatings on com-plex profi led tool or element it is necessary to use the fl at models. Coating is applied simultaneously to the tool and the model so the conditions are identical to conditions of the tool coating. Development of methods for reliable quality control of coating on tool is an im-portant task that requires a operating solution. During the experiment the normal load, frictional force, friction coeffi cient μ and acoustic emission (AE) are measured.Contemporary design of these devices has a sensor for monitoring the depth of notches during the test.The duration of the test is determined by selection the changes of the normal force per unit time / N·min–1 and their maximum size. The length of the notch is de-termined by choice of sliding speed v/ m·min–1 . Basic characteristics of scratch tester ST99 are given in [7].Measuring the shape and depth of notch is done after the experiment on measuring device Sytem Talysurf Form No.6 Taylor Hobson, which is used for identifi ca-tion of the surface topography. The base material to which the coating applied is 100Cr6. This material is mostly used for making forming tools (forging) and elements of ball bearings. Thickness of TiN coating on the samples is from 1,8 to 6 micrometers. TiN layers are used as hard, decorative coatings, coatings for corrosion protection, coatings for protection of wear and they may be used as coatings for diffuse barrier.The grain size in the layers varies depending on the deposition conditions, a negative bias on the substrate leads to a decrease of grain in the layer. Monitoring the changes of the friction force and the coeffi cient of friction for the change of normal load from 0 to 150 N during the realization of contact en-abled the identifi cation of critical size of normal force Fnc at which the coating is destroyed. Changes of the friction force and the coeffi cient of friction for the nor-mal force changes were observed on the computer mon-itor by using appropriate software and printed on a printer DESK JET 840C. The records of the measure-ments of samples 100Cr6/TiN-1, 100Cr6/TiN-2 and 100Cr6/TiN-3 are given in the Figures 3, 4 and 5.Experimental determination of heights of the surface roughness of 100Cr6/TiN-1 sample is shown in Figure 6.In the Figure 7 diagrams of changes of the friction coeffi cient for 100Cr6/TiN-1, 100Cr6/TiN-2 and 100Cr6/TiN-3 samples are given.Complete coating distortion for 100Cr6/TiN-1 is at normal load of 83 N, for 100Cr6/TiN-2 is at normal force of 87,5 N and for 100Cr6/TiN-3 at normal force of 50 N.One can see that the coeffi cient of scratching is the smallest for 100Cr6/TiN-2 sample (Figure 4). The adhesion of coating to the base is very depen-dent on the technology of previous preparation of the sample surface. This is one of the most important ques-tions of persistent coating technology, which requires detailed analysis.The experiments show that the optimal coating thick-ness is in the range of 3-5 micrometers. At lower thickness coating does not provide a suffi cient degree of durability Figure 2  Measurement of adhesion using scratch test and scheme of propagation of distortions under measuring palpus in good and poor adhesion Figure 3 Scratch test 100Cr6/TiN-124  METALURGIJA 51 (2012) 1, 21-24B. SOVILJ et al.: THE EFFECT OF SPECIFIC RELATIONSHIP BETWEEN MATERIAL AND COATING ON TRIBOLOGICAL...and at higher thickness the adhesion of the coating be-comes worse and the likelihood of its failure is increased. CONCLUSIONSurface strengthening is useful for the surface of machine elements and tools.The quality of the coating from tribological aspects and in terms of resistance to destruction is necessary in the process of their application to the contact surfaces of solid elements, and in the process of their selection for specifi c contact conditions. TiN coatings have high strength and durability, which is 2-8 times higher than the durability of the base material. Durability depends on the base material and coating process which is indi-cated by the results shown in the test diagrams.An important task placed in front of the science of materials at the modern level of development of persis-tent coating technology is to search for new and composi-tions that would guarantee highly effi cient tools and opti-mal durability of elements of the tribological systems.Acknowledgements – this work was supported by grant TR -14059 from Ministry of Science of Republic of SerbiaFigure 4 Scratch test 100Cr6/TiN-2Figure 5 Scratch test 100Cr6/TiN-3Figure 6  Roughness heights to the length of the profi le 100Cr6/TiN-1Figure 7  Scratching coeffi  cient for 100Cr6/TiN-1, 100Cr6/TiN-2 and 100Cr6/TiN-3 REFERENCES[1] I. Kolev, Rjazane na materialite, Preč baza na RU “A. Kičev”, Ruse, 2009, pp. 23-33.[2] I. Kolev, Naučni trudove, RU “A. Kičev”, 49, (2010) 2, 114-118.[3] M. Hebdi, A.V. Čičinadze, Spravočnik po tribotehnike - Teoretičeskie osnovi, Mašinostoenie, Moskva, 1989, pp 15-45.[4] B. Sovilj, D. Ješić, I. Sovilj-Nikić, Savremena poljo-privredna tehnika, 36, (2010) 3, 295-305.[5] B. Sovilj, I. Sovilj-Nikić, D. Ješić, Metalurgija, 50, (2011) 2, 107-111.[6] P. Panjan, M. Čekada, Zaščita orodij s trdimi PVD-prevle-kami, Institut Jožef Stefan, Ljubljana, 2005, pp. 9-11, 102-104, 144-146.[7] B. Ivković, In Proceedings, INTERTRIBO’02, Slovak University of Technology in Bratislava, Bratislava, 2002, pp. 251-254.List of symbols and acronymsAE Acoustic emissionμ Friction coeffi cienth Penetration depth of measuring toolH Coating thicknessFnc Critical normal loadFn Normal loadFt Tangent loadLc1 Critical load for chippingLc2 Critical load for tearingLc3 Critical load for fl akingLc5 Critical load for total delaminationLc(AE) Critical load for starting of acoustic emissionLc(Ft) Critical load for increasing of scratch forceNote:  The responsible translator for English language Elisabeth Sal-more, Faculty of Philosophy, Novi Sad, Serbia

Utjecaj specifične veze između materijala i prevlake na tribološke i zaštitne karakteristike proizvoda

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