THE MICRO-HARDNESS ANISOTROPY OF FLUOR-OLIGOMERIC AND Fe-Co-W COATINGS

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1 HARDMEKO 27 Recent Advancement of Teory and Practice in Hardness Measurement November, 27, Tsukuba, Japan THE MICRO-HARDNESS ANISOTROPY OF FLUOR-OLIGOMERIC AND Fe-Co-W COATINGS Vytautas Vasauskas, Juozas Padgurskas, Raimundas Rukuiza Department of Mecanical Engineering, Lituanian University of Agriculture, LT-561 Kaunor., Lituania Abstract Te influences of indenter micro-geometry on te micro-ardness performance in te film/substrate system are discussed. Experimental results verify te force required to produce ardness indents for tin films, te measurement of te size of plastic zone cross-sections of te indents, and te caracterisation of te indenters. It was found tat te deformation is igly localised beneat te indenter and failures in te corners of pyramidal indenters occurred. Keywords: micro-ardness, tin films, failure. 1. INTRODUCTION Te conventional ardness testing is used among oter metods in material researc to retrieve corresponding data and to caracterise specific materials [1]. Te advantages of tis metod include many factors, and one of tese, tat only small volumes of material are required. Hardness determination of tin films by indentation is complicated because of extension of te plastically deformed zone. It is commonly stated tat tis zone extends 7-1 times te dept of te indentation. Tis plastically deformed zone extends up to different depts and we can conclude on te average ardness contribution of individual layers. However te interpretation of suc results is difficult because practically we can separately determine te volumes of deformations of te two parts. Te traditional approac to te mecanical caracterisation of coatings is (1) a measurement of ardness in wic low loads and penetration depts (typically a tent of te film tickness) are used to extract a film ardness wile a iger loads are applied making an attempt to relate te film/substrate system response and (2) te measurement of tougness of film or interfacial fracture. Tese materials are often micro-structurally and mecanically eterogeneous due to local variations in termo-mecanical istory, wic arise during te processing. As penetration progresses, te average stress in te coat increases, and generalised yield (large plastic zone directly under te indenter tip) is observed at depts of penetration approacing 1 diameter aead of te tip face (fig. 1b). Micro-indentation tecnique generally used to investigate te film ardness on te surface (current practice standards) can be applied also for investigations troug cross-section of te layer and te study of fine-scale canges in ardness. Te aim of investigations is to present some explanations regarding te micro-ardness measuring of anisotropic solids in te tin film/substrate system. 2. TESTING THE MICRO-HARDNESS OF COATED SURFACES Hardness normally implies te resistance of te material to local deformation and it is obtained by dividing te maximum indentation load by te projected contact area. However above mentioned definition of ardness does not describe precisely te resistance of te material to local deformation. Some autors suggested recently [] tat since te deformation is volume process and it takes energy to induce it, te energy related definition of te ardness is more descriptive particularly in te case of micro-ardness measurement of film/substrate system were te volume of deformed material is very small and mixed [4]. Dissipated indentation energy is calculated out of wole load displacement data (F- curves) obtained during te testing by discrete integration of force being te function of displacement [5]. Tis approac considers tat te on-load maximum indentation dept ( max ) is te sum of te plastic ( p ) and te elastic ( e ) components of indentation. For a Vickers indenter (A = 24,5 r 2, were A is te projected area of indentation), te Meyer ardness values were derived from: HM F max A F max 2 d Were F max is te maximum load applied, d - caracteristic lengt scale of te resultant indentation impression, - geometrical parameter of used indenter. Te ratio between te penetration dept and te coat tickness (t) always as to be kept below 1 % ( max /t = 1/1) [5]. However tis correlation covers in our experiments only narrow range of te F max values. Volume of te indent is derived as an integral of te area of te indenter being function of penetration dept and ten corrected by factor of final dept. Previous experimental results of te fully plastic indentation regime [6] indicate tat for identical sperical and Vickers indentation depts (i.e. s = t), te (1)

2 corresponding ardness values are related by H s ( s ),7H v ( t ). Tis relationsip may be explained analytically relating te ardness wit te work done by te indenter and te plastic zone volume for bot types of indentation [7], and it is valid approximation at te end of te elastic-plastic indentation regime. Deformation model assumes tat tat te volume of impression is proportional to te kinetic energy W of te indenter but is independent of its sape or tip angle 2 [6]. Te volume of a rigid sarp indenter wit an apex angle of 2 is: were d is te diagonal, and: W V cot d, (2) 24 Wo V Wo cot d Wo k d, () 24 were W o is te assumedly constant kinetic energy absorbed per unit volume of deformed metal and k = cot /24 is a constant. Te total energy required to produce an indentation of dept is given by: W t Fd H, (4) k Tis is termed te work of indentation and, if measured (as could be readily acieved wit continuos recording indentation tecnique) can be used to define an effective value of H wic usefully describes te resistance to deformation over te penetration. Above a critical load, wen te substrate starts to make itself felt, it is found tat a wide range of films deforms by te sear of te coatings into te substrate (fig. 1). For te same reason H in (4) may be considered as te plastic ardness value of te given material. Film effects become negligible in te deformation of te film/substrate system for large indentations. All stress components in te region beneat te indenter are compressive and increase in magnitude wit contact angle. Te circumferential stress on te specimen surface (or radial stress along te indenter axis) attains a tensile peak at te elastic-plastic boundary. Te radius of te plastic zone is given by [8]: pl E H 1 R cot n res, (5) were R pl is radius of te plastic zone; - for Vickers indenter is equal,5; E - Youngs modulus; H - ardness; - effective alf-included angle; res - residual dept. Te radius R pl was measured by averaging te orizontal size and te vertical size of te plastic deformation zone. Te area of te imprint increases by nearly a factor of six wit deeper penetration wile te strain only increases by a factor of tree. a) b) PLASTIC ZONE SHEAR BANDS LOAD INDENT CRACK Fig. 1. Cross-sections of te ardness measurement specimens - evolution of sear band patterns on PMMA material (a) and plastic zone volume (b) in te film/substrate system wit load beneat te Vickers indentation. Moreover at small lengt scale additional factors like tip rounding or oter size effects migt influence te extension of te plastic zone. Depending on te strain limit, different sizes for te plastic zone can be defined. In tis case, te radius of te plastic zone R pl approximately equal: R pl a c, were is a constant ranging from zero to ~,5; a c - contact radius.. EXPERIMENTAL METHODOLOGY COAT PLASTIC ZONE Te fluor-oligomeric materials (FOM) - solutions of FOLEOX (,51% concentration) were used for creating polymer coat on te metal surface. Te specimen materials used are carbon steel utilised for macine structures (carbon content.45%). Cylindrical samples of x mm in size of medium carbon steel AISI 145 (DIN ) composition in wt % (Fe, (,7,44) C, (,6,9) Mn, (,19,2) Si) were cut, quenced and tempered state for various ardness levels (4<HRC<6). Te samples were grounded and mecanically polised to a surface rougness R a of,2 m. Tey were tan cleaned wit acetone before testing. After te surface is coated wit fluor-oligomeric coat te cemosorbic bindings are created between metal surface and te film. Te tickness of FOM layers was about,1,16 m. Altoug te films allow water vapour diffusion, te

3 films bot are ydropobic and are not wetted wen liquid waters contacts tese layers. Te Fe-Co-W coatings were produced on roller specimens wic could be adopted to tribological tests on according testing rig. Fe-Co-W containing films were electro-deposited on te mecanically polised tempered and non-tempered steels. Substrates were degreased and activated wit dilute sulfuric acid immediately before plating and a seed-layer of Ni was electroplated witin 1 min. from Woods type solution containing NiCl 2-6H 2 O 24 g/l + HCl 8 g/l at ma/cm 2. Ten te Fe-Co-Wcontaining alloys were electro-deposited from te citrateammonia bats. Te buffer capacity of te plating bat solution was determined by titration wit 15 M NaOH solution to avoid any dilution effects. Canges in ph were monitored by phmeter TermoOrion type 42. Buffer capacity values at certain ph were extracted after digital differentiation of titration curves. Te cross-sections of te samples were prepared by cutting, grinding, polising to te sane level of rougness revealed by etcing in aqua regia and observed by optical and SEM microscopy. Hardness tests were carried out wit a Vickers pyramid indenter, using a Fiscer HP 1 XY-PROS ultra-microardness tester [9]. A diamond indenter of standard geometry, typically a 16 square based pyramidal diamond (Vickers) is indented under a known load, into te surface of te sample. During tests, load indentation dept time data were recorded. Hardness measurements were performed under six different indentation loads, ranging from.1 to 1. N. At eac load level, at least 1 measurements were made on te coatings. However, besides te properties of te coatings, te testing metods can also affect te performance of te coatings. Firstly, te fact tat te response of te system depends significantly on te scale of contact, being dominated by te film ardness at small scales in comparison wit te coating tickness and by te substrate ardness at large scales. Secondly, te stress-strain situation during indentation is triaxial. 4. RESULTS AND DISCUSSION Te figure 2 sows te loading-unloading cycle obtained using an micro-indentation tester operating wit fluoroligomeric film in te ramp mode and corresponding Vickers contact geometry. Te load is incremented at constant speed up to te maximum load (F max ) and subsequently released at te same rate as in loading cycle. Continuos dept-sensing recording does not give values of absolute micro-ardness value directly, because te area of indentation is not measured explicitly. However te indentation data can be processed on te basis of wellknown assumptions [2]. Te results sow tat only FOM film sligtly reduced te micro-ardness value on te surface. It is likely attributed to te presence of pores in te coats and te reason could be Rebinder effect [1] and combination of factors: te real effect of native oxide at te surface or an effect of te coating procedure, and an artefact of te sape indenter tip for small indents. As te deformed pase-transformed fluor-oligomeric layer of material tries to pus in troug te surrounding bulk material, internal stresses may be developed in te micro-volumes and it would elp te relieving process of tis excess internal stresses. Some muc localised microcracking, particularly near te edges of te Vickers indenter, always occurs. In te case of FOM film large-scale brittle fracture wic induces material removal, is essentially absent in tese materials. Te increase of ardness is acieved by accumulation of impurities in area of dislocations. Te ductility increases wit te mobility of dislocations. Fluor-oligomeric layer contributes to acievement of bot effects un-deposited deposited loading unloading r p m Indentation dept,, m Fig. 2. Typical loading and unloading curves from a microindentation test cycle for fluor-oligomeric film. F is te loading force and is te indentation dept. Te dept of circle of contact p is obtained from te dotted extrapolation line. Te ardness of a material depends on several parameters suc as te yield stresses y, te Youngs modulus E and te flow rule (plastic stress-strain relation) [7]. For small ratios y/e, e.g. less tan,2, te ardness values increase wit increasing indentation angle. In fact in appears to depend on te evolution of plastic zone size wit respect to te contact radius and te elastic beaviour does not influence te plastic flow for coating. Tese results may be explained by assuming tat in present experiments te coat adjusts initially to te elasticplastic deformation of te underlying substrate during te indentation process until at a given deformation of substrate, te coat undergoes a coesive-adesive failure and tat leads to film detacment. According to tis interpretation for certain given film/substrate system caracteristics (i.e. material, tickness, microstructure etc.) te failure will always occur wen tis critical deformation is reaced, independently of te substrate ardness. SEM observations on indentation impressions demonstrate tat te radial cracks are straigt lines as seen in figure (b,c). Several factors may be responsible for te observed micro-cracking in Fe-Co-W coatings. It is igly probable, tat te ig interfacial friction between two adjacent surfaces and te deforming material may cause significant localised stress in te tip of indentation.

4 a) indentation apex along te edges of te pyramid indenter, and in te form circumferential cracks around te indentation. Plasticity and fracture for micro-ardness measurements must be considered separately, altoug te experimental results sow some convergence and may be expressed by one formula. If te coating as been bent (fig. c) to displacements of te order of its own tickness, we identified te local plastic deformation at te points of maximum bending strain. Tere te true ardness value may reflect some oter function of te yield stress of te coat rater tan its conventional ardness. Te results of micro-ardness measurements of Co-W coats using different substrate are presented in fig. 4. Tey sow ow different could be surface strengt if different substrate for coating is used. Te coating of specimens wit Co-W increases te micro-ardness of surface except wen initial micro-ardness of substrate is iger as average. b) Microardness HV, MPa Co-W coat Substrate c) Fig.. Cracks development around a Vickers indent on Fe-Co-W coats sowing general view of indentation (a), median (b) and lateral (c) vent cracks. Te lateral cracks sows a marked anisotropy arising from te structure. Te ardness is described by te square of te diagonal ratio. Terefore is more sensitive to te precise diagonal values. Stress concentration effects on te corners of te pyramidal indenter enance plastic deformation and failure and tis may influence te measured area. Te observations suggest tat wile plastic deformation usually occurs in bot coat and substrate (fig. 1b) - fracture also be possibly at larger loads (fig. a). Certain similarities also exist in te coat wen deformation is dominated by cracking (fig. b,c). It is clear tat tey can occur at te Fig. 4. Comparison of te micro-ardness of Co-W coats on different substrates: 1 mecanically polised copper; 2 brass mirror (te Cu-Zn alloy sprayed on te steel); steel Stal ; 4 cemically polised copper (micro-rougness); 5 steel Stal. 5. CONCLUSIONS Te results obtained suggest tat wen indentations are performed in te fully plastic regime it is possible to establis a representative correlation between energetically defined complex ardness value and te substrate ardness. Tis result may be explained by considering te indentation size effect index values for te used substrate. Surface modification by fluor-oligomeric film usually introduces a ardness gradient in te near surface region of te material. Te fluor-oligomeric treatment decreases te micro-ardness of solid body till 15% at,2, mm deep because of cemosorbic interaction wit steel. It is necessary to estimate te arising of failures at te measurements of ardness and anisotropy. Several factors may be responsible for te observed micro-cracking in Fe- C-W coatings. Stress concentration effects on te corners of te pyramidal indenter enance plastic deformation and failure and tis may influence te measured area. Film tickness and film ardness are relevant variables for determining te composite ardness and te deformation

5 beaviour of te film/substrate system at low indentation conditions. In furter investigations is necessary to understand wy tere is a difference between te calculated and measured ardness correction factors. REFERENCES [1] "Measurements & Testing Newsletter", vol. 6, no. 2,4 December [2] T.A. Venkates et al., "Determination of Elasto-Plastic Properties by Instrumental Sarp Indentation; Guidlines for Property Extraction", Scripta Materialia., vol. 42, pp. 8-89, 2. [] Y.T. Ceng, C.M. Ceng, "Wat is Indentation Hardness?", Surf. Coat. Tecnol., vol. 1-14, pp , 2. [4] A.M. Korsunsky et al., "On te ardness of coated systems", Surf. Coat. Tecnol., vol. 99, pp , [5] "Metallic materials - Instrumented indentation test for ardness and material parameters, Part 1: Test metod", ISO/DIS /2; April 2. [6] V. Vasauskas, "Geometry effect of indenters on dynamic ardness", IMEKO 2 XVI IMEKO World congress Trans., pp. 4-48, 2. [7] S.J. Bull, "Modelling te ardness response of bulk materials, single and multilayer coatings", Tin Solid Films, vol. 8-9, pp , 21. [8] K.L. Jonson, "Contact mecanics", Cambridge University Press, Cambridge, UK, [9] Evaluation Manual of Indentation Procedure, Helmut Fiscer GmbH + Co; Sindelfingen, Germany, 2. [1] P.A. Rebinder, V. Liktman. Proc. of 2 nd Int. Conf. Surface Activity, III ed. J.H.Sculman, p. 56, Academic Press, New York, Autor: Assoc. prof. dr. Vytautas Vasauskas, Prof. dr. Juozas Padgurskas, Assoc. prof. dr. Raimundas Rukuiza Department of Mecanical Engineering, Lituanian University of Agriculture, Studentu str. 15, Akademija, LT-561 Kauno r., Lituania Pone: , Fax: , vytautas.vasauskas@ktu.lt, raimundas.rukuiza@lzuu.lt,