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8 INŻYNIERIA MATERIAŁOWA MATERIALS ENGINEERING ROK XXXIX

A comparative research on mechanical properties of monolayer, gradient and multi-module coatings

on CrN(C) baseKatarzyna Mydłowska1, Laure Libralesso2, Łukasz Szparaga1, Adam Gilewicz1,

Jerzy Ratajski1*

1Faculty of Technology and Education, Koszalin University of Technology, Koszalin, Poland, 2CRM asbl, Liege, Belgium; *[email protected]

Chromium has been used for decades for the production of relatively thick, hard, wear resistant coatings in a wide range of industries — including the automotive and aircraft industry. Whilst traditional hard chromium finished coatings have no known health hazards, the chemicals, used in the process of ap-plication, comprise hexavalent chromium compounds (Cr6+ ions) and chromium in this state is extremely toxic. Due to the European directive on chemicals (REACH), hard chromium coatings, produced from highly toxic and carcinogenic baths, must be replaced by “green” solutions. In certain applications, the coatings obtained by PVD method can be the alternative to hard chromium electroplating where superior wear resistance to that of hard chromium electro-plate is required — especially if a thin coating is suitable. Among PVD coatings, the chrome nitride coatings have demonstrated to possess excellent wear resistance properties. In this article was presented a fragment of research on this subject, carried out as part of an international research project CORNET, whose aim was to develop technologies of anti-wear and anti-corrosion coatings as replacements for hard chromium and cadmium coatings for applications in the aerospace and automotive industries. In particular, the article contains a description of the investigation of the mechanical properties of monolayer, gradient and multi-module coatings on CrN(C) base deposited with use of the cathodic arc evaporation CAE-PVD method on nitrided steel 42CrMo4 and additionally in the case of gradient and multi-module coatings on substrates of HS6–5–2 steel. The investigation includes the adhesion analysis by scratch test and by the Rockwell C indentation test (VDI — Verein Deutscher Ingenieure indentation test) tribomechanical analysis by pin–on–disc, measurement of friction coefficient and nanoindentation tests.

Key words: PVD coatings, mechanical properties, monolayer coatings, multi-module coating, gradient coating.

Inżynieria Materiałowa 1 (221) (2018) 8÷14DOI 10.15199/28.2018.1.2

MATERIALS ENGINEERING

1. INTRODUCTION

Chromium is used for decades for producing relatively thick, hard, anti-wear coatings in a variety of industries, including automotive and aerospace industries. However, the chemicals used for the dep-osition of these coatings contain hexavalent chromium (Cr6+ ions), and chromium in this state is extremely toxic. Therefore, in relation to the European directive concerning chemicals (REACH), hard chrome coatings, produced in toxic and carcinogenic baths, must be replaced by “green” solutions. After this period, the application of these technologies will be significantly limited.

In the aerospace industry, in some applications, coatings pro-duced by Physical Vapour Deposition (PVD) methods may be an alternative to hard chrome coatings [1÷3]. However, it should be noted that current practice in the designing of safe aircraft structures is the assumption, that the design should be resistant both to dam-age at a certain level (damage tolerant design) and have a degree of redundancy that will prevent the catastrophe in the case of total dysfunction of any part (fail-safe design). For this reason, in cases where it is necessary to take into account the resistance to acciden-tal impacts, or impacts resulting from way of functioning of specific mechanisms, on parts subjected to such operations, are typically created composite surface layers.

The best known and most widely used technology of surface treatment, capable of producing a composite layer, is a combination of gas or plasma nitriding process with deposition of hard, anti-wear coatings by PVD methods (duplex technology). The result of such configured technology is composite layer composed of nitrided layer and formed directly on its surface PVD coating. One of the first flagship works, indicating the synergistic effects resulting from sequential combination of nitriding processes with PVD deposition of thin coatings is the work of Sun and Bell [4], which contains the

results showing relatively significant reduction in wear of EN40B steel samples after being subjected to the mentioned surface treat-ments.

The effectiveness of the duplex technology depends on the properties of the formed nitrided layer as well as PVD coating. For example, during tests of coating resistance by impact tests, it was found that the coating of the predominant columnar structure is resistant to larger deformation than a coating of higher density and not characterized by columnar structure [5, 6]. Deformability of columnar structure that is more tolerant to deformation, allows coatings with such structure to withstand more impacts without vis-ible cohesive and adhesive damage. During impact tests perform well also multilayer coatings having a relatively high strength [7, 8]. However, during impact tests in the analysis of the influence of coatings structure on their degradation, mainly should be taken into account the deformation of the substrate, which occurs during impact. Then, in order to reduce stress in the coating, around the crater produced by impact, it starts to create a net of microcracks causing damage to the brittle coating [5]. Thus, the resistance of the substrate/coating system should be optimized by selecting both the kind and the structure of coatings, which must be sufficiently elastic to adopt a deformation of the substrate and by an increase in load bearing capacity of the substrate. Hence there are still so great expectations in relation to optimization of duplex technology.

During the last decade has seen a huge increase in the use of CrN coatings, among other characterized by relatively low value of residual stress and good resistance to oxidation at high tem-perature [9÷11]. There are various modifications of these coatings such as multilayer coatings and multi-module Cr/CrN [12÷15], CrN/CrCN [16÷18] or gradient CrN/Cr(CN) coatings [19÷22]. This type of coating is also known as a intelligent coating because their structure, among other Cr to CrN thickness ratio in multi-module

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Cr/CrN coating, determines the intensity of their wear depending on the operating conditions. For example, the main feature of the multi-module Cr/CrN coatings is their relatively high resistance to erosion [3]. Thus, depending on the operating conditions, it is important to produce coatings of proper structure, for example, in the case of multi-module coatings it is important already mentioned thickness ratio of Cr to CrN or CrN to CrCN layer [23]. On the other hand, in the case of gradient CrN/CrCN coatings, the shape of the carbon concentration profile in the transition CrCN layer, determines the mechanical properties of the coating.

In this article was presented a fragment of research on this sub-ject, carried out as part of an international research project COR-NET, whose aim was to develop technologies of anti-wear and anti-corrosion coatings as replacements for hard chromium and cadmium coatings for applications in the aerospace and automo-tive industries. In particular, in this article were compared results of the mechanical properties of monolayer CrN coatings with dif-ferent thicknesses and multi-module CrN/CrCN as well as gradient CrN/CrCN coatings. All coatings were deposited on the nitrided substrates of 42CrMo4 steel, and additionally in the case of gradient and multi-module coatings on substrates of HS6–5–2 steel.

2. SCOPE OF RESEARCH

2.1. Preparation of samples

The chemical composition of the steel from which the samples were produced and the parameters of the heat treatment carried out on samples is given in Table 1. The sample roughness after grinding was Ra = 0.3 µm.

2.2. Nitriding process

Nitriding of samples was carried out in a laboratory furnace equipped with a quartz tube. The process temperature was 833 K (560°C), and was controlled with a resolution of 1°C at the loca-tion of the samples. Nitriding atmosphere consisted of ammonia (99.9 vol. %) and hydrogen (99.999 vol. %). The gas flow rate was controlled by Bronkhorst’s flow meters. The linear flow rate of the gas atmosphere through the quartz tube was 1.4 cm/s.

2.3. PVD process

Coatings were deposited in the device TINA 900, by cathodic arc evap-oration method (CAPVD). The working chamber is equipped with six sources which allow operation with cathodes made from different ma-terials. As a result, this allows for the deposition of monolayer as well as multilayer coatings of varying chemical composition.

In all tested samples, directly on the steel substrate, was de-posited a thin layer of chromium, so called adhesive layer, having a thickness of 0.1 µm, and then the actual coatings, which consist-ed of monolayer CrN coatings having a thickness of 4.0 µm and 10.0 µm, gradient CrN/CrCN coating with variable carbon concen-tration in CrCN layer (Fig. 1a) and multi-module coating consisting

of 6, bilayer, CrN/CrCN modules with total thickness of the module 660 nm (Fig. 1b). The thickness ratio of layers in the bilayer mod-ule was 1:1. Thickness of gradient and multi-module coatings was amounted to 4.0 µm.

The process of CrN layers deposition was conducted with sub-strate bias of –70 V, and arc current equal to 80 A in N2 atmosphere at a pressure of 1.8 Pa. In order to obtain CrCN layers, additionally acetylene was introduced into the working chamber to allow gen-eration of a step change in carbon concentration (Fig. 1b). Varying carbon concentration in the CrCN layer (Fig. 1a) was obtained by changing the flow rate of acetylene by 2 cm/s at five time steps

Table 1. The chemical composition of the steel from which the samples were produced and parameters of heat treatmentTabela 1. Skład chemiczny stali, z której wykonano próbki oraz przepro-wadzona obróbka cieplna

Grade of steel

Heat treatment before nitriding

Chemical composition, wt %

C Si Cr Mn Mo P

42CrMo4annealing 1118 K,quenching in oil,tempering 893 K

0.40 0.25 0.95 0.80 0.25 <0.035

HS6–5–2 annealing 1123 K, tempering 843 K 0.87 1.00 4.00 <0.40 5.00 <0.003

Fig. 1. Gradient CrN/CrCN coating (a) and multi-module CrN/CrCN coating (b)Rys. 1. Powłoka gradientowa CrN/CrCN (a) oraz powłoka wielomoduło-wa CrN/CrCN (b)

a)

b)


lasting two minutes. The result is a monotonic (power law) modifi-cation of the carbon concentration in the range of 0÷10% at. causing also the monotone (power law) change in physico-chemical proper-ties of CrCN layer.

2.4. Characterization of coatings

The phase composition of nitrided layer was determined using X-ray diffraction, applying CoKα radiation (diffractometer DRON 2.0). The thickness of layers in coating was determined by spherical abration — the Calotest method. The adhesion of the coatings was evaluated based on the scratch test (Revetest®) and by a Rockwell C (test VDI — indentation test). Diamond stylus in the scratch test was moved across a coating at a constant speed of 1 mm/s and at a constantly increasing load of the stylus in a range 0÷100 or 200 N. In this test the adhesion of the coating to the substrate was deter-mined by three parameters: – Lc1 – indenter load at which cracks are generated in the test-

ed coating, – Lc2 – indentation load at which occurs the local loss of adhesion

in an area of scratch and its edge, – Lc3 – indentation load at which occurs removing of the coating

from the entire surface of a scratch.The evaluation of adhesion by analysing an indentation crater caused by the diamond Rockwell’s C indenter with a load of 1500 N was based on radial and lateral cracks. In this method, the shape and size of damages is closely related with the adhesion of the coating to the substrate.

Friction coefficient measurements were performed using ball–on–disc method with a load of 30 N and a speed of 60 mm/s under

the dry friction on the sliding distance 1000 m. Counterspecimen was an alumina ball of 10 mm diameter and Ra < 0.03 µm. The measurements were performed in air atmosphere with humidity about 50% at ambient temperature. Profile of the sample wear (wear track) was measured using a Hommel Werke profilograph T2000. Wear rate was determined as the volume of the material removed during the friction test in relation to the product of sliding distance and applied load [24].

3. RESULTS AND DISCUSSION

3.1. Adhesion tests using scratch test method

All coatings were deposited on nitrided substrates of 42CrMo4 steel. Parameters of nitriding process were chosen so that the ni-trided layer was built exclusively of the diffusion zone [25, 26]. Results of tests concerning adhesion of each coating to the substrate are given in Figure 2, and for more readable comparison, in Table 2.

A comparison of the value of critical force Lc3, indicating a loss of adhesion of the coating to the substrate, shows that the best ad-hesion characterizes gradient coating (Lc3 = 95 N), and the worst monolayer CrN coating having a thickness of 4.0 µm (Lc3 = 75 N). For a monolayer coating having a thickness of 10.0 µm and mul-ti-module coating was measured the same critical force value (Lc3 = 90 N). It is also interesting to compare Lc1 forces, at which first damage (spallation) of a coating appears, what shows that the greatest value of the critical force is measured for a monolayer coat-ing with a thickness of 10.0 µm. This coating is also characterized by the highest Lc2 force value, at which first local delamination of the coating occurs.

Fig. 2. Dependence of the friction coefficient on normal force in scratch test: a) multi-module coating, d = 4.0 µm, b) gradient coating, d = 4.0 µm, c) monolayer coating 10.0 µm, d) monolayer coating 4.0 µmRys. 2. Zależność współczynnika tarcia od siły normalnej w próbie zarysowana: a) powłoka wielomodułowa, g = 4,0 µm, b) powłoka gradientowa, g = 4,0 µm, c) powłoka jednowarstwowa, g = 10,0 µm, d) powłoka jednowarstwowa, g = 4,0 µm

a)

c)

b)

d)


Table 2. Summary of the critical forces for tested coatingsTabela 2. Zestawienie wartości sił krytycznych dla badanych powłok

Lc1, N Lc2, N Lc3, N

Multi-module, 4.0 µm 24 45 90

Gradient, 4.0 µm 26 56 95

Monolayer, 10.0 µm 40 63 90

Monolayer, 4.0 µm 25 40 75

3.2. Tests of wear and friction coefficient

In the case of testing wear rate and friction coefficient (Fig. 3 and 4) monolayer CrN coating having a thickness of 10.0 µm is character-ized by the highest values of these parameters. For the rest of the tested coatings, ie. monolayer CrN having a thickness of 4.0 µm, multi-module and gradient of thickness 4.0 µm, values of these pa-rameters are practically the same. Definitely higher wear rate char-acterizes coating of hard chromium. For this coating also the largest friction coefficient was determined.

3.3. Nanohardness measurements

Nanohardness measurements [27] presented in Figure 4 indicate that the gradient coating has the highest hardness. Slightly smaller hardness characterizes multi-module coating, then monolayer with a thickness of 4.0 µm. The lowest hardness among coatings based on chromium nitride has a monolayer with a thickness of 10.0 µm. The hardness of the hard chromium coating is 10 GPa and it is the minimum value of hardness which characterizes tested coatings.

3.4. Adhesion assessment using Rocwell C test

Based on the comparison of the results of all performed tests, best properties characterize gradient coating with the power law (para-bolic) change in the carbon concentration in CrCN layer. However, not much smaller differences in the measured values occur for mul-ti-module coating. These two coatings were selected for independ-ent evaluation of the coating adhesion to the substrate by Rockwell C test. With this destructive test two properties of substrate/coat-ing systems can be clearly distinguished, ie. an interphase adhe-sion as well as cohesion and coating embrittlement. In these studies, coatings were deposited on nitrided 42CrMo4 steel, in analogy to previous studies and additionally on HS6–5–2 steel. In the case of nitrided substrates of 42CrMo4 steel, adhesion of the gradient and multi-module coating is practically the same (Fig. 5) and is in 1–2

Fig. 3. Wear rate and friction coefficient of tested coatingsRys. 3. Wskaźnik zużycia oraz współczynnik tarcia badanych powłok

1234

1

2

34


` Fig. 4. Nanohardness of tested coatingsRys. 4. Nanotwardość badanych powłok

HS6-5-2 – multi-module coating

HS6-5-2 – parabolic gradient coating

42CrMo4 – multi-module coating

42CrMo4 – parabolic gradient coating

Fig. 5. Indentation craters caused by the diamond Rockwell C indenter with a load of 1500 NRys. 5. Ślady zagłębienia spowodowane diamentowym wgłębnikiem Rockwella C pod obciążeniem 1500 N


adhesion class [28, 29]. Differences in adhesion occur in case of coatings deposited on HS6–5–2 steel. It has been observed in these studies, that improved adhesion measured in this test, characterizes multi-module coating compared to the gradient coating. In particu-lar, for multi-module coating radial cracks, and almost no delami-nation around the crater show a strong adhesion of the coating but also its brittleness. In case of gradient coating small fragments of delamination observed near the crater indicating weaker interphase adhesion (coating-substrate) compared to the multi-module coating.

4. CONCLUSIONS

1. All tested coatings based on chromium nitride have a higher hardness and lower wear compared to hard chromium coatings.

2. Among the four coatings, ie. multi-module, gradient, 10.0 µm monolayer and monolayer having a thickness of 4.0 µm, best mechanical properties: adhesion, nanohardness, friction coeffi-cient and wear rate characterize multi-module and gradient coat-ing.

3. On the basis of the adhesion test using analysis of indentation crater caused by Rockwell C diamond indenter at a load of 1500 N (VDI test), it was shown that the gradient and multi-module coating have good adhesion in the case of deposition onto the nitrided 42CrMo4 steel substrate, and in the case of dep-osition on HS6–5–2 steel better adhesion of the multi-module coating was observed.

4. Based on the performed research it was demonstrated that there is practically no difference between the mechanical properties of the gradient and multi-module coatings. Similar values in the measurement of hardness, critical Lc2 forces during scratch tests and wear rates were obtained. There were however differences in the evaluation of adhesion of these coatings in the VDI test. This is an important argument to predict that the multi-layer coating structure will provide better durability in operating conditions characterized by periodic loads, because the individual layers act as an effective barrier to the propagation of micro-cracks. In the case of gradient coatings should be expected longer life with stable loads.

ACKNOWLEGMENTS

This research was supported by a grant from The National Cen-tre for Research and Development in frame of COllective Research NETworking (CORNET).

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Badania właściwości mechanicznych jednowarstwowych, wielomodułowych oraz gradientowych

powłok na bazie CrN(C)Katarzyna Mydłowska1, Laure Libralesso2, Łukasz Szparaga1, Adam Gilewicz1,

Jerzy Ratajski1*

1Wydział Technologii i Edukacji, Politechnika Koszalińska, Koszalin, 2CRM asbl, Liege, Belgium; *[email protected]

Inżynieria Materiałowa 1 (221) (2018) 8÷14DOI 10.15199/28.2018.1.2

MATERIALS ENGINEERING

Słowa kluczowe: stale typu HSLA, obróbka cieplno-plastyczna, struktura, właściwości mechaniczne.

1. CEL PRACY

W związku z europejską dyrektywą dotyczącą chemikaliów (RE-ACH) twarde powłoki chromowe, produkowane w toksycznych i rakotwórczych kąpielach, muszą być zastąpione przez rozwią-zania „zielone”. W niektórych zastosowaniach, między innymi w przemyśle lotniczym, powłoki otrzymywane metodą PVD mogą być alternatywą dla powłok z twardego chromu. Wokół tego proble-mu zogniskowano badania, których fragment opisano w artykule. Zasadniczym celem tych badań było porównanie właściwości tri-bomechanicznych powłok jednowarstwowych CrN oraz wielomo-dułowych i gradientowych CrN/CrCN osadzanych na azotowanych podłożach ze stali 42CrMo4 oraz dodatkowo w przypadku powłok wielomodułowych i gradientowych na podłożach ze stali HS6–5–2.

2. MATERIAŁ I METODYKA BADAŃ

Skład chemiczny stali, z której wykonano próbki oraz parametry przeprowadzonej obróbki cieplnej próbek zamieszczono w tabeli 1. Azotowanie próbek przeprowadzono w piecu laboratoryjnym wypo-sażonym w rurę kwarcową. Powłoki przeciwzużyciowe Cr/CrN, Cr/CrN/CrCN osadzano w urządzeniu TINA 900 metodą katodowego rozpylania łukowego (CAPVD). Skład fazowy warstwy azotowa-nej określano za pomocą dyfrakcji RTG, stosując promieniowanie CoKα. Grubość warstw powłoki wyznaczano metodą wytarcia ku-listego — metodą Calotest. Adhezję powłok oceniano na podstawie próby zarysowania (Revetest®) oraz za pomocą próby Rockwella C. Pomiary współczynnika tarcia wykonano w układzie kula–tarcza.

3. WYNIKI I ICH DYSKUSJA

Wyniki badań adhezji poszczególnych powłok do substratu, wy-znaczane metodą zarysowania, zamieszczono na rysunku 2 oraz dla porównania zestawiono je w tabeli 1. Z porównania wartości siły krytycznej Lc3 świadczącej o utracie adhezji powłoki z substra-tem wynika, że najlepszą adhezją charakteryzuje się powłoka gra-dientowa (Lc3 = 95 N), a najgorszą powłoka jednowarstwowa CrN o grubości 4,0 µm (Lc3 = 75 N). W przypadku badań wskaźnika zużycia oraz współczynnika tarcia (rys. 3 i 4) powłoka jednowar-stwowa CrN o grubości 10,0 µm charakteryzuje się największymi wartościami tych parametrów. Dla pozostałych badanych powłok, tj. jednowarstwowej CrN o grubości 4,0 µm, wielomodułowej oraz gradientowej o grubościach 4,0 µm, wartości tych parametrów są praktycznie takie same. Zdecydowanie największym wskaźnikiem zużycia charakteryzuje się powłoka twardego chromu. Dla tej po-włoki wyznaczono też największy współczynnik tarcia.

Na podstawie porównania wyników wszystkich przeprowadzonych badań najlepszymi właściwościami charakteryzuje się powłoka gra-

dientowa z potęgową (paraboliczną) zmianą stężenia węgla w warstwie CrCN. Jakkolwiek niewiele mniejsze różnice w wartościach zmierzo-nych parametrów występują dla powłoki wielomodułowej. Te dwie powłoki wybrano do niezależnej oceny adhezji powłoki do podłoża za pomocą próby Rockwella C. W przypadku azotowanych podłoży ze stali 42CrMo4 adhezja powłoki gradientowej oraz wielomodułowej jest prak-tycznie taka sama (rys. 5) i mieści się w klasie 1–2. Różnice w adhezji występują natomiast w przypadku powłok osadzanych na stali HS6–5–2. Zaobserwowano, że lepszą adhezją ocenianą w tej próbie charakteryzu-je się powłoka wielomodułowa w porównaniu z powłoką gradientową. Promieniowe pęknięcia i praktycznie brak rozwarstwień wokół śladu od-cisku w powłoce wielomodułowej wskazują na silne przyleganie powło-ki, ale jednocześnie na jej kruchość. W przypadku powłoki gradientowej obserwuje się małe fragmenty delaminacji w sąsiedztwie śladu odcisku wskazujące na słabszą adhezję międzyfazową (powłoka–substrat) tej po-włoki w porównaniu z powłoką wielomodułową.

4. PODSUMOWANIE

1. Wszystkie badane powłoki na bazie azotku chromu wykazują większą twardość oraz mniejsze zużycie w porównaniu z powło-kami z twardego chromu.

2. Spośród czterech powłok, tj. wielomodułowej, gradientowej, jednowarstwowej o grubości 10 µm i jednowarstwowej o grubo-ści 4,0 µm, najlepszymi właściwości mechanicznymi: adhezją, nanotwardością, zużyciem oraz współczynnikiem tarcia charak-teryzują się powłoka gradientowa oraz wielomodułowa.

3. Na podstawie badań adhezji za pomocą analizy śladu zagłębienia wykonanego diamentowym wgłębnikiem Rockwella C pod ob-ciążeniem 1500 N (próba VDI) wykazano, że powłoka gradien-towa oraz wielomodułowa cechują się dobrą adhezją w przypad-ku osadzania na azotowane podłoża ze stali 42CrMo4, natomiast w przypadku osadzania na stal HS6–5–2 zaobserwowano lepszą adhezję dla powłoki wielomodułowej.

4. Analiza otrzymanych wyników wskazuje, że nie ma praktycznie różnic pomiędzy mechanicznymi właściwościami powłok gra-dientowych i wielomodułowych. Otrzymano zbliżone wartości w pomiarach twardości, sił krytycznych Lc2 podczas próby za-rysowania oraz wskaźników zużycia. Wystąpiły natomiast róż-nice w ocenie adhezji tych powłok w próbie VDI. Jest to waż-ny argument do prognozowania, że wielowarstwowa struktura powłok zapewni większą trwałość w warunkach eksploatacji charakteryzujących się periodycznymi obciążeniami, ponieważ indywidualne warstwy działają jako efektywne bariery w propa-gacji mikropęknięć. W przypadku powłok gradientowych należy spodziewać się większej trwałości przy stabilnych obciążeniach.

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