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Scratch test of active screen low temperature plasma nitrided AISI 410 martensitic stainless steel
Wear 376-377 (2017) 30–36
Contents lists available at ScienceDirect
Wear journal homepage: www.elsevier.com/locate/wear
Scratch test of active screen low temperature plasma nitrided AISI 410 martensitic stainless steel L.A. Espitia a,n, Hanshan Dong b, Xiao-Ying Li b, C.E. Pinedo c, A.P. Tschiptschin d a
Engineering, Science and Technology Research Group, Mechanical Engineering Department, University of Córdoba, Cr 6 No. 76-103, Montería, Colombia School of Metallurgy and Materials, College of Engineering and Physical Sciences, The University of Birmingham, Edgbaston, Birmingham B15 2TT, United Kingdom c University of Mogi das Cruzes, Av. Dr. Candido Xavier de Almeida Souza, 200, ZIP 08780-911, Mogi das Cruzes, SP, Brazil d Metallurgical and Materials Engineering Department, University of São Paulo, Av. Prof. Mello Moraes 2463, 05508-030 São Paulo, SP, Brazil b
art ic l e i nf o
a b s t r a c t
Article history: Received 15 September 2016 Received in revised form 19 January 2017 Accepted 23 January 2017
A nitrided case composed of expanded martensite and small quantities of hexagonal ε-Fe24N10 iron nitrides was formed in a martensitic stainless steel by means of active screen plasma nitriding process. Nanoindentation tests were carried out in order to assess the mechanical properties and to obtain an energy dissipation coefficient defined as the ratio of plastic to total deformation energy. Friction coefficient, mechanical failure mode and critical load for damaging the nitrided case were determined using linear scratch tests performed at both linearly-increased normal force and constant normal force according to ASTM C1624 standard. The scratch test results showed that the groove features and the friction coefficient could be well correlated to the energy dissipation coefficient. The expanded martensite strongly decreased the friction coefficient in comparison to the non-nitrided martensitic stainless steel. The critical load was 14 N and tensile cracking was the mechanical failure mode of the nitrided case. & 2017 Elsevier B.V. All rights reserved.
Keywords: Expanded martensite Nanoindentation Scratch test Active screen plasma nitriding Mechanical failure mode
1. Introduction Scratch test has been widely used to provide a measure of coating/substrate adhesion as well as to identify a well-defined failure occurring at a specific load termed the critical load LC [1]. Several coatings have been assessed by using scratch tests, such as DLC coatings [2–5], CrN coatings, AlCrN coatings, TiN and TiC coatings produced by PVD [6–8] and plasma nitrided cases formed in pure titanium and Ti6Al4V alloys [9, 10]. Furthermore, the tribological behavior of plasma nitrided steels during scratch tests has been also reported. Paschke et al. [11] evaluated the influence of the nitriding temperature on the crack behavior of the plasma nitrided forging tool steel DIN-1.2367. Cracking was the mechanical failure mode occurring at a critical load of 46 N. It was concluded that the cracking behavior of the tool steel could be influenced positively by adequate plasma nitriding treatments. Kamminga et al. [12] performed scratch tests at constant forces on the plasma nitrided X38CrMoV5-1 steel, using several loads between 2 and 200 N. Tensile cracking was the failure mode reported by the authors and it was observed just for loads above 40 N. Yildiz n
Corresponding author. E-mail address: [email protected] (L.A. Espitia).
http://dx.doi.org/10.1016/j.wear.2017.01.091 0043-1648/& 2017 Elsevier B.V. All rights reserved.
et al. [10] investigated the scratch test behavior of a nitrided case composed of expanded austenite and CrN formed by plasma nitriding in an AISI 316L stainless steel. They also reported tensile cracking as the failure mode and a critical load of 48 N. In addition to scratch tests, nanoindentation allows characterizing the mechanical properties of films and coatings. Many attempts have been made to correlate mechanical properties with the tribological behavior of materials [13]. Specifically for films and coatings, hardness, Young modulus, elastic recovery and some ratios between these properties are being extensively used. A new approach to establish a relationship between mechanical properties and tribological behavior of coatings has been suggested by Recco et al. [8]. The approach defines an energy dissipation coefficient which is related to the irreversible losses of energy associated with plastic deformation mechanisms. The coefficient considers simultaneously the elastic and plastic responses of the coatings and its capability to store elastic energy. The outstanding cavitation erosion resistance exhibited by expanded martensite formed on active screen plasma nitrided AISI 410 martensitic stainless steel was reported in a previous work [14]. In this work, the mechanical and tribological properties of an expanded martensite layer, formed after active screen plasma nitriding, were assessed by means of nanoindentation and scratch
L.A. Espitia et al. / Wear 376-377 (2017) 30–36
31
tests. The results are discussed in terms of friction coefficient variations, mechanical failure mode and critical load for damaging the nitrided case. In addition, some relationships between groove features, elastic response and mechanical properties of expanded martensite are established.
2. Experimental procedure 2.1. Materials and treatments Table 1 shows the nominal and measured chemical compositions of the AISI 410 martensitic stainless steel used in this work. Optical emission spectroscopy (OES) was used to measure the chemical composition. Heat treatments were carried out in argon atmosphere, under 0.15 MPa pressure. AISI 410 specimens were austenitized for 1 h at 1000 °C and quenched in water. Afterwards, the quenched specimens were tempered at 600 °C, during 1 h and air cooled to room temperature. These heat treatments produced a tempered lath martensite microstructure in the AISI 410 martensitic stainless steel [14]. Plasma nitriding treatment was carried out in a Metal SA – Luxemburg unit, using active screen (AS) in order to avoid any edge effect. Prior to plasma treatment, the specimens were ground up to ASTM 1200 emery paper, cleaned in acetone and air-dried. The specimens were nitrided at 400 °C for 20 h with a gas mixture of 75% of nitrogen and 25% of hydrogen. A 28 mm thick nitrided case formed on the surface of the AISI 410 martensitic steel [14].
Fig. 1. Typical nanoindentation curve illustrating the maximum depth of penetration ( hmax ), residual depth ( hf ) and elastic ( Ee ) and plastic ( Ep ) deformation energies [8].
2.2. Microstructure characterization The specimens were analyzed by optical microscopy (OM) and scanning electron microscopy (SEM). Cu-Kα radiation, X-ray diffraction using the Bragg-Brentano θ–2θ configuration was used to identify the crystal structure of the phases present in the microstructures.
Fig. 2. Schematics of scratch width ( w ) and penetration depth (x) produced at the specimen during indenter displacement [18].
2.3. Micromechanical characterization Nanoindentation tests were carried out, according to the procedure proposed by Oliver and Pharr [15], in a Hysitron Triboscope nanoindenter coupled to a Shimadzu SP 9500J3 atomic force microscope. A Three-Faceted Berkovich Diamond Pyramid tip was used as indenter. The measurements were conducted on the surface of the nitrided and non-nitrided specimens using a loading/ unloading rate of 1400 mN/s up to a peak load of 7000 mN. The peak load was held constant for a period of 5 s. Hardness (H), Young modulus (E), maximum depth penetration ( hmax ) and residual depth after unloading ( hf ) were determined. Subsequently, the H/E ratio and the elastic recovery ( We ) were calculated, using Eq. (1):
⎡ ⎛ h ⎞⎤ We( %) = ⎢ 1 − ⎜ f ⎟⎥*100 ⎝ hmax ⎠⎦ ⎣
(1)
In addition, an energy dissipation coefficient ( Kd ) defined as the ratio of the plastic deformation energy to the total deformation Table 1 AISI 410 martensitic stainless steel nominal and measured chemical composition wt%.
Nominal Measured
C
Cr
P
S
0.15 max 0.13
11.5–13.0 12.20
0.04 max 0.025
0.002 max o 0.001
Fig. 3. X-ray diffraction patterns for nitrided and non-nitrided specimens [14].
energy was calculated as suggested by Recco et al. [8]. The plastic deformation energy ( Ep ) can be obtained, by the difference between total deformation energy ( Et ) and the elastic deformation energy ( Ee ). Et and Ee correspond to the areas under the loading and unloading curves, respectively. These areas were calculated integrating a second-degree polynomial function previously fitted to the loading and unloading curves. Fig. 1 indicates hmax , hf , Ee and
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L.A. Espitia et al. / Wear 376-377 (2017) 30–36
Fig. 4. Surface of the nitrided specimen without any metallographic preparation or etching. Surface relief resultant from nitrogen pick-up and lattice expansion during nitriding. OM image.
Fig. 6. Scratch results of test at linearly-increased normal force.
Fig. 7. First damage observed at around 3 mm of the track length of the nitrided specimen, OM image. Fig. 5. Nanoindentation curves of the nitrided and non-nitrided specimens.
Ep in a typical nanoindentation curve [8]. Each specimen was submitted to ten nanoindentation measurements. 2.4. Scratch test Scratch tests were conducted using a Rockwell C indenter with an apex angle of 120° and a spherical tip radius of 200 mm according to ASTM C1624 standard [16]. The indenter was drawn across the surfaces of nitrided and non-nitred specimens at a constant speed of 0.16 mm/s and a linearly-increased normal force from a preload of 1–50 N for a 10 mm length. Friction coefficients were calculated from the ratio between lateral and normal force. Critical load ( LC ) for damaging the nitrided case was determined by measuring the distance at which the first failure appeared along the scratch track and under a corresponding value of the
normal load. The failure identification and distance measurements were carried out using OM and SEM. The mechanical failure modes were identified according to the scratch atlas provided in the ASTM C1624 standard [16]. In addition, scratch tests at a constant normal force lower than LC were carried out at both specimens keeping the same values of speed, time and track length to determine the scratch hardness number ( HSP ) according to ASTM G171 standard [17]. This quantity characterizes the resistance of a solid surface to penetration by a moving indenter under constants normal force and scratching speed and is given by Eq. (2):
HSP =
8P πw 2
(2)
where:
Table 2 Hardness (H), Young modulus (E), H/E ratio, maximum depth of penetration (h max ), residual depth (hf ), elastic recovery ( We ) and energy dissipation coefficient ( Kd ) for both specimens, measured by nanoindentation. Specimen
Non-nitrided Nitrided
H
E
(GPa)
(GPa)
4.7 7 0.3 13.7 7 0.7
1897 6 191 77
H/E
h max (nm)
hf (nm)
We (%)
Kd (%)
0.03 7 0.001 0.0770.003
234 7 6 1487 4
198 7 7 817 4
167 1 45 72
83 71 53 72
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33
Fig. 8. Scratching track observed on the nitrided specimen evidencing cracks formation behind the indenter (3 and 4 mm), cracks encountering along grain boundaries (5 mm) and detachment of martensitic grains from the nitrided case (10 mm). SEM images.
HSP ¼ scratch hardness number, Pa P ¼normal force, N w ¼ scratch width, m. Scratch width ( w ) and penetration depth (x) are schematically represented in Fig. 2. These values were calculated using digital image analysis and Eq. (3) respectively [18]:
x=
w ctg α 2
(3)
Three tracks were made on each specimen in both scratch tests.
3. Results and discussion 3.1. Microstructure characterization Fig. 3 shows X-ray diffraction patterns for nitrided and nonnitrided specimens [14]. One can see that the original martensite peaks were shifted and broadened to lower 2θ angles, due to nitrogen pick-up and diffusion in the nitrided specimen. Nitrogen atoms were dissolved at interstitial sites of the BCC crystal structure, creating a nitrogen supersaturated phase known as expanded martensite ( αN´ ). In addition to the expanded martensite peaks, the nitrided specimen diffraction pattern showed hexagonal ε-Fe24N10 iron nitrides peaks. A deeper analysis regarding to expanded martensite formation, nitrogen content, microhardness measurements and X-ray diffraction results of the nitrided specimen assessed in this paper can be found in a previous work [14]. Fig. 4 shows the surface of a pre-polished nitrided specimen, after the nitriding treatment. It is worth noting that the specimen, which was neither metallographically prepared nor chemically etched, shows a surface relief resultant from nitrogen pick-up and lattice expansion, during nitriding. After the thermochemical treatment, lath martensite sheaves
delimiting former austenitic grains can be seen at the surface of the nitrided specimen. Nitrogen pick-up induced compressive residual stresses, producing heterogeneous deformation among the grains leading to relief on the surface revealing the microstructure of the nitrided specimen, as reported for low temperature plasma nitriding and carburizing treatments of stainless steels [19, 20]. 3.2. Micromechanical characterization Nanoindentation curves of the nitrided and non-nitrided specimens are presented in Fig. 5, while the mechanical properties and We and Kd parameters are listed in Table 2. One can see in Fig. 5 that the non-nitrided specimen exhibited the greater deformation during indentation achieving a hmax of 234 nm while the nitrided specimen showed a hmax of 148 nm. The residual depths ( hf ) were 198 and 81 nm for the non-nitrided and nitrided specimens, respectively. Furthermore, it can be seen that the area between the loading and unloading curves is greater for the non-nitrided specimen, evidencing that the nitrided specimen dissipates less plastic deformation energy. The elastic recovery of the expanded martensite case was 45% while just 16% for the nonnitrided specimen. Kd was also greater for the non-nitrided specimen, 83%, compared to 53% for the expanded martensite. From the results, it can be concluded that plastic deformation predominates during nanoindentation of the non-nitrided specimens, also suggesting that plasma nitriding increased the yield stress of AISI 410 martensitic stainless steel, leading to an increase on the elastic response of the expanded martensite nitrided case. An increase of the yield stress of the X38CrMoV5-1 steel from 2.7 to 7 GPa due to plasma nitriding has been reported by Kamminga et al. [12]. In addition, nitrogen expanded martensite formation increased the hardness almost three times, from 4.7 to 13.7 GPa without changing significantly the Young modulus. Expanded martensite also showed a higher H/E ratio. Greater H/E ratios are desirable because they denote higher elastic deformation before failure by plastic deformation or cracking.
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Fig. 10. Friction coefficient behavior of both specimens registered during scratch tests at a constant normal force of 10 N.
Fig. 9. Track of the nitrided specimen evidencing microploughing, extensive plastic deformation and wedge formation, SEM images.
3.3. Scratch tests The results of the scratch tests, carried out with linearly-increased normal forces are shown in Fig. 6. The friction coefficients of the nitrided and non-nitrided specimens were quite different. The friction coefficient of the nitrided specimen, was around 0.07 during the first 2.5 mm of track length. From this point on, the apparent friction coefficient continuously increased up to a final value of 0.20. In contrast, for the non-nitrided specimen no
stability was observed. The friction coefficient increased steadily from 0.06 to 0.28. Although the friction coefficients at the beginning of the test, for both specimens, were quite similar, the value of the coefficient at the end of the stability plateau for the nitrided specimen was much smaller. Cracking was the first damage observed at around 3 mm of the track length of the nitrided specimen, as can be seen in Fig. 7. Accordingly, from Fig. 6, the critical load LC for damaging the nitrided case, corresponding to that distance, was 14 N. Other researches had reported critical loads between 40 and 48 N for damaging the nitrided case formed by plasma nitriding on several steels [10–12]. However, none of those critical loads correspond to a plasma nitrided martensitic stainless steel and the phases that composed those nitrided cases are quite different of expanded martensite. In addition, the scratch response of a material depends on the complex interaction of testing parameters such as stylus properties and geometry, loading rate, displacement rate and the coating/substrate properties including hardness, fracture strength, Young modulus, damage mechanisms, microstructure, flaw population and surface roughness [16]. From this point on, other cracks parallel to each other and perpendicular to the scratching direction arose in the track. The distance between parallel cracks decreased with increasing normal loads. This cracks formed behind the indenter at places where the tensile stress exerted during the indenter displacement exceeded the fracture strength of expanded martensite. Beyond 4 mm of the track length, the cracks started to encounter each other following a zig-zag trajectory along grain boundaries. In addition, some pieces of martensitic grains were detached from the nitrided case. These features are shown in Fig. 8. The nitrided case showed a cohesive failure and the mechanical failure mode acting on the nitrided specimen was tensile cracking. This failure mode has been reported in CrN coatings deposited by PVD [6, 16] as well as in plasma nitrided steels [10–12]. On the other hand, non-nitrided specimen showed the typical behavior of a ductile material submitted to a hard particle movement. Microploughing, extensive plastic deformation and wedge formation, were observed as shown in Fig. 9. Fig. 10 shows the friction coefficient behavior of both specimens registered during scratch tests at a constant normal force of 10 N, lower than LC . Both friction coefficient remained almost constant during the tests. The nitrided specimen exhibited a value around 0.06–0.07, while for the non-nitrided specimen the friction coefficient was close to 0.16, nearly two and a half times greater than that of the nitride specimen. The decrease of friction coefficient observed in this experiment can be explained using the friction coefficient model
L.A. Espitia et al. / Wear 376-377 (2017) 30–36
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plasma nitriding may be also considered as an alternative to be evaluated in order to decrease friction coefficient at real components where metal – metal contact occurs, for instance, in the valves employed in the mining and oil industries.
4. Conclusions The nitrided case is formed by nitrogen supersaturated expanded martensite and hexagonal ε-Fe24N10 iron nitrides. Energy dissipation coefficient ( Kd ) was greater in the non-nitrided specimen with 83% and 53% for the expanded martensite; therefore, plastic deformation is the main deformation occurring in the non-nitrided specimen. Expanded martensite deforms mainly elastically, before cracking, during the scratch test. Furthermore, the increase on the elastic response of expanded martensite results in lower scratch depths in the nitrided specimen. The scratch resistance is controlled by the resistance to elastoplastic deformation of the specimens. The nitrided case showed a cohesive failure and the mechanical failure mode acting on the nitrided specimen was tensile cracking. The critical load LC for cracking the nitrided case was 14 N. Expanded martensite formation made possible an outstanding decrease on the friction coefficient in comparison to the non-nitrided AISI 410 martensitic stainless steel. The scratch test results showed that the groove features and the friction coefficient could be well correlated to the energy dissipation coefficient.
Acknowledgments Fig. 11. Tacks produced at constant load, a) non-nitrided specimen, b) nitride specimen, OM images.
The authors acknowledge the supports of CNPq processes n. 151653/2010-0 481918/2010-8 and 486104/2012-5, FAPESP, process n. 2012/50890-0 and COLCIENCIAS processes n. 497, Colombia.
Table 3 Track width ( w ), penetration depth (x) and scratch hardness number ( HS P ). Specimen
Non-nitrided Nitrided
Track width ( w )
Penetration depth (x)
lm
lm
HS P GPa
90 43
26 12
3.1 13.8
proposed by Bowden and Tabor [18]. The model states that the friction coefficient arises from an adhesive force involving the true area of contact and a ploughing force related to a projected contact area of the indenter. This area is dependent on the track width and on the penetration depth of the indenter. A decrease on the latter values lessens the projected contact area, therefore, decreasing the friction coefficient. Fig. 11 shows the tracks produced on both specimens at constant load, while Table 3 summarizes the track width measured by digital image analysis, the penetration depth and HSP calculated by using Eqs. (3) and (2), respectively. HSP of expanded martensite is at least 2.6 times higher than that reported for plasma nitrided titanium alloys [9]. The scratch test results suggest that expanded martensite deforms mainly elastically before cracking. Furthermore, it was loaded at the elastic zone during both the linearly-increased normal force test at loads lower than 14 N and the constant normal force test. The increase on the elastic response of expanded martensite results in lower scratch depths observed in the nitrided specimen. The scratch resistance is controlled by the resistance to elastoplastic deformation of the specimens. According to these results, active screen low temperature
References [1] S.J. Bull, E.G. Berasetegui, An overview of the potencial of quantitative coating adhesion measurement by scratch testing, Tribol. Int. 39 (2) (2006) 99–114. [2] P. Mutafov, J. Lanigan, A. Neville, A. Cavaleiro, T. Polcar, DLC-W coatings tested in combustion engine — frictional and wear analysis, Surf. Coat. Technol. 260 (2014) 284–289. [3] Shih-Hsien Chang, Te-Chien Tang, Kuo-Tsung Huang, Chung-Ming Liu, Investigation of the characteristics of DLC films on oxynitriding-treated ASP23 high speed steel by DC-pulsed PECVD process, Surf. Coat. Technol. 261 (2015) 331–336. [4] L. Vieira, F.L.C. Lucas, S.F. Fisssmer, L.C.D. dos Santos, M. Massi, P.M.S.C.M. Leite, C.A.R. Costa, E.M. Lanzoni, R.S. Pessoa, H.S. Maciel, Scratch testing for microand nanoscale evaluation of tribocharging in DLC films containing silver nanoparticles using AFM and KPFM techniques, Surf. Coat. Technol. 260 (2014) 205–213. [5] Eugenia L. Dalibón, Daniel Heim, Christian Forsich, Andreas Rosenkranz, Agustina Guitar, Sonia P. Brühl, Characterization of thick and soft DLC coatings deposited on plasma nitrided austenitic stainless steel, Diam. Relat. Mater. 59 (2015) 73–79. [6] R. Hoy, J.-D. Kamminga, G.C.A.M. Janssen, Scratch resistance of CrN coatings on nitrided steel, Surf. Coat. Technol. 200 (2006) 3856–3860. [7] Jaume Pujante, Montserrat Vilaseca, Daniel Casellas, Maria Dolors Riera, High temperature scratch testing of hard PVD coatings deposited on surface treated tool steel, Surf. Coat. Technol. 254 (2014) 352–357. [8] A.A.C. Recco, C.C. Viáfara, A. Sinatora, A.P. Tschiptschin, Energy dissipation in depth-sensing indentation as a characteristic of the nanoscratch behavior of coatings, Wear 267 (2009) 1146–1152. [9] S. Farè, N. Lecis, M. Vedani, A. Silipigni, P. Favoino, Properties of nitrided layers formed during plasma nitriding of commercially pure Ti and Ti–6Al–4V alloy, Surf. Coat. Technol. 206 (2012) 2287–2292. [10] F. Yildiz, A. Alsaran, Multi-pass scratch test behavior of modified layer formed during plasma nitriding, Tribol. Int. 43 (2010) 1472–1478. [11] H. Paschke, M. Weber, P. Kaestner, G. Braeuer, Influence of different plasma nitriding treatments on the wear and crack behavior of forging tools evaluated
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by Rockwell indentation and scratch tests, Surf. Coat. Technol. 205 (2010) 1465–1469. J.-D. Kamminga, P.F.A. Alkemade, G.C.A.M. Janssen, Scratch test analysis of coated and uncoated nitrided steel, Surf. Coat. Technol. 177–178 (2004) 284–288. H.C. Meng, K.C. Ludema, Wear models and predictive equations: their form and content, Wear 181–183 (1995) 443–457. L.A. Espitia, Hanshan Dong, Xiao-Ying Li, C.E. Pinedo, A.P. Tschiptschin, Cavitation erosion resistance and wear mechanisms of active screen low temperature plasma nitrided AISI 410 martensitic stainless steel, Wear 332-333 (2015) 1070–1079. W.C. Oliver, G.M. Pharr, A new improved technique for determining hardness and modulus using load and sensitive indentation experiments, J. Mater. Res. v7 (1992) 1564–1582. ASTM C1624 American society for testing and materials. Standard test method
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for adhesion strength and mechanical failure modes of ceramic coatings by quantitative single point scratch testing, Annual book of ASTM standards, 2005. ASTM G171 American society for testing and materials. Standard test method for scratch hardness of materials using a diamond stylus, Annual book of ASTM standards, 2009. Ian M. Hutchings, Tribology: Friction and Wear of Engineering Materials, First edition, Butterworth-Heinemann, 1992. L.A. Espitia, L. Varela, C.E. Pinedo, A.P. Tschiptschin, Cavitation erosion resistance of low temperature plasma nitrided martensitic stainless steel, Wear 301 (2013) 449–456. S.C. Gallo, H. Dong, EBSD, and AFM observations of the microstructural changes induced by low temperature plasma carburizing on AISI 316, Appl. Surf. Sci. 258 (2011) 608–613.
Contents lists available at ScienceDirect
Wear journal homepage: www.elsevier.com/locate/wear
Scratch test of active screen low temperature plasma nitrided AISI 410 martensitic stainless steel L.A. Espitia a,n, Hanshan Dong b, Xiao-Ying Li b, C.E. Pinedo c, A.P. Tschiptschin d a
Engineering, Science and Technology Research Group, Mechanical Engineering Department, University of Córdoba, Cr 6 No. 76-103, Montería, Colombia School of Metallurgy and Materials, College of Engineering and Physical Sciences, The University of Birmingham, Edgbaston, Birmingham B15 2TT, United Kingdom c University of Mogi das Cruzes, Av. Dr. Candido Xavier de Almeida Souza, 200, ZIP 08780-911, Mogi das Cruzes, SP, Brazil d Metallurgical and Materials Engineering Department, University of São Paulo, Av. Prof. Mello Moraes 2463, 05508-030 São Paulo, SP, Brazil b
art ic l e i nf o
a b s t r a c t
Article history: Received 15 September 2016 Received in revised form 19 January 2017 Accepted 23 January 2017
A nitrided case composed of expanded martensite and small quantities of hexagonal ε-Fe24N10 iron nitrides was formed in a martensitic stainless steel by means of active screen plasma nitriding process. Nanoindentation tests were carried out in order to assess the mechanical properties and to obtain an energy dissipation coefficient defined as the ratio of plastic to total deformation energy. Friction coefficient, mechanical failure mode and critical load for damaging the nitrided case were determined using linear scratch tests performed at both linearly-increased normal force and constant normal force according to ASTM C1624 standard. The scratch test results showed that the groove features and the friction coefficient could be well correlated to the energy dissipation coefficient. The expanded martensite strongly decreased the friction coefficient in comparison to the non-nitrided martensitic stainless steel. The critical load was 14 N and tensile cracking was the mechanical failure mode of the nitrided case. & 2017 Elsevier B.V. All rights reserved.
Keywords: Expanded martensite Nanoindentation Scratch test Active screen plasma nitriding Mechanical failure mode
1. Introduction Scratch test has been widely used to provide a measure of coating/substrate adhesion as well as to identify a well-defined failure occurring at a specific load termed the critical load LC [1]. Several coatings have been assessed by using scratch tests, such as DLC coatings [2–5], CrN coatings, AlCrN coatings, TiN and TiC coatings produced by PVD [6–8] and plasma nitrided cases formed in pure titanium and Ti6Al4V alloys [9, 10]. Furthermore, the tribological behavior of plasma nitrided steels during scratch tests has been also reported. Paschke et al. [11] evaluated the influence of the nitriding temperature on the crack behavior of the plasma nitrided forging tool steel DIN-1.2367. Cracking was the mechanical failure mode occurring at a critical load of 46 N. It was concluded that the cracking behavior of the tool steel could be influenced positively by adequate plasma nitriding treatments. Kamminga et al. [12] performed scratch tests at constant forces on the plasma nitrided X38CrMoV5-1 steel, using several loads between 2 and 200 N. Tensile cracking was the failure mode reported by the authors and it was observed just for loads above 40 N. Yildiz n
Corresponding author. E-mail address: [email protected] (L.A. Espitia).
http://dx.doi.org/10.1016/j.wear.2017.01.091 0043-1648/& 2017 Elsevier B.V. All rights reserved.
et al. [10] investigated the scratch test behavior of a nitrided case composed of expanded austenite and CrN formed by plasma nitriding in an AISI 316L stainless steel. They also reported tensile cracking as the failure mode and a critical load of 48 N. In addition to scratch tests, nanoindentation allows characterizing the mechanical properties of films and coatings. Many attempts have been made to correlate mechanical properties with the tribological behavior of materials [13]. Specifically for films and coatings, hardness, Young modulus, elastic recovery and some ratios between these properties are being extensively used. A new approach to establish a relationship between mechanical properties and tribological behavior of coatings has been suggested by Recco et al. [8]. The approach defines an energy dissipation coefficient which is related to the irreversible losses of energy associated with plastic deformation mechanisms. The coefficient considers simultaneously the elastic and plastic responses of the coatings and its capability to store elastic energy. The outstanding cavitation erosion resistance exhibited by expanded martensite formed on active screen plasma nitrided AISI 410 martensitic stainless steel was reported in a previous work [14]. In this work, the mechanical and tribological properties of an expanded martensite layer, formed after active screen plasma nitriding, were assessed by means of nanoindentation and scratch
L.A. Espitia et al. / Wear 376-377 (2017) 30–36
31
tests. The results are discussed in terms of friction coefficient variations, mechanical failure mode and critical load for damaging the nitrided case. In addition, some relationships between groove features, elastic response and mechanical properties of expanded martensite are established.
2. Experimental procedure 2.1. Materials and treatments Table 1 shows the nominal and measured chemical compositions of the AISI 410 martensitic stainless steel used in this work. Optical emission spectroscopy (OES) was used to measure the chemical composition. Heat treatments were carried out in argon atmosphere, under 0.15 MPa pressure. AISI 410 specimens were austenitized for 1 h at 1000 °C and quenched in water. Afterwards, the quenched specimens were tempered at 600 °C, during 1 h and air cooled to room temperature. These heat treatments produced a tempered lath martensite microstructure in the AISI 410 martensitic stainless steel [14]. Plasma nitriding treatment was carried out in a Metal SA – Luxemburg unit, using active screen (AS) in order to avoid any edge effect. Prior to plasma treatment, the specimens were ground up to ASTM 1200 emery paper, cleaned in acetone and air-dried. The specimens were nitrided at 400 °C for 20 h with a gas mixture of 75% of nitrogen and 25% of hydrogen. A 28 mm thick nitrided case formed on the surface of the AISI 410 martensitic steel [14].
Fig. 1. Typical nanoindentation curve illustrating the maximum depth of penetration ( hmax ), residual depth ( hf ) and elastic ( Ee ) and plastic ( Ep ) deformation energies [8].
2.2. Microstructure characterization The specimens were analyzed by optical microscopy (OM) and scanning electron microscopy (SEM). Cu-Kα radiation, X-ray diffraction using the Bragg-Brentano θ–2θ configuration was used to identify the crystal structure of the phases present in the microstructures.
Fig. 2. Schematics of scratch width ( w ) and penetration depth (x) produced at the specimen during indenter displacement [18].
2.3. Micromechanical characterization Nanoindentation tests were carried out, according to the procedure proposed by Oliver and Pharr [15], in a Hysitron Triboscope nanoindenter coupled to a Shimadzu SP 9500J3 atomic force microscope. A Three-Faceted Berkovich Diamond Pyramid tip was used as indenter. The measurements were conducted on the surface of the nitrided and non-nitrided specimens using a loading/ unloading rate of 1400 mN/s up to a peak load of 7000 mN. The peak load was held constant for a period of 5 s. Hardness (H), Young modulus (E), maximum depth penetration ( hmax ) and residual depth after unloading ( hf ) were determined. Subsequently, the H/E ratio and the elastic recovery ( We ) were calculated, using Eq. (1):
⎡ ⎛ h ⎞⎤ We( %) = ⎢ 1 − ⎜ f ⎟⎥*100 ⎝ hmax ⎠⎦ ⎣
(1)
In addition, an energy dissipation coefficient ( Kd ) defined as the ratio of the plastic deformation energy to the total deformation Table 1 AISI 410 martensitic stainless steel nominal and measured chemical composition wt%.
Nominal Measured
C
Cr
P
S
0.15 max 0.13
11.5–13.0 12.20
0.04 max 0.025
0.002 max o 0.001
Fig. 3. X-ray diffraction patterns for nitrided and non-nitrided specimens [14].
energy was calculated as suggested by Recco et al. [8]. The plastic deformation energy ( Ep ) can be obtained, by the difference between total deformation energy ( Et ) and the elastic deformation energy ( Ee ). Et and Ee correspond to the areas under the loading and unloading curves, respectively. These areas were calculated integrating a second-degree polynomial function previously fitted to the loading and unloading curves. Fig. 1 indicates hmax , hf , Ee and
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Fig. 4. Surface of the nitrided specimen without any metallographic preparation or etching. Surface relief resultant from nitrogen pick-up and lattice expansion during nitriding. OM image.
Fig. 6. Scratch results of test at linearly-increased normal force.
Fig. 7. First damage observed at around 3 mm of the track length of the nitrided specimen, OM image. Fig. 5. Nanoindentation curves of the nitrided and non-nitrided specimens.
Ep in a typical nanoindentation curve [8]. Each specimen was submitted to ten nanoindentation measurements. 2.4. Scratch test Scratch tests were conducted using a Rockwell C indenter with an apex angle of 120° and a spherical tip radius of 200 mm according to ASTM C1624 standard [16]. The indenter was drawn across the surfaces of nitrided and non-nitred specimens at a constant speed of 0.16 mm/s and a linearly-increased normal force from a preload of 1–50 N for a 10 mm length. Friction coefficients were calculated from the ratio between lateral and normal force. Critical load ( LC ) for damaging the nitrided case was determined by measuring the distance at which the first failure appeared along the scratch track and under a corresponding value of the
normal load. The failure identification and distance measurements were carried out using OM and SEM. The mechanical failure modes were identified according to the scratch atlas provided in the ASTM C1624 standard [16]. In addition, scratch tests at a constant normal force lower than LC were carried out at both specimens keeping the same values of speed, time and track length to determine the scratch hardness number ( HSP ) according to ASTM G171 standard [17]. This quantity characterizes the resistance of a solid surface to penetration by a moving indenter under constants normal force and scratching speed and is given by Eq. (2):
HSP =
8P πw 2
(2)
where:
Table 2 Hardness (H), Young modulus (E), H/E ratio, maximum depth of penetration (h max ), residual depth (hf ), elastic recovery ( We ) and energy dissipation coefficient ( Kd ) for both specimens, measured by nanoindentation. Specimen
Non-nitrided Nitrided
H
E
(GPa)
(GPa)
4.7 7 0.3 13.7 7 0.7
1897 6 191 77
H/E
h max (nm)
hf (nm)
We (%)
Kd (%)
0.03 7 0.001 0.0770.003
234 7 6 1487 4
198 7 7 817 4
167 1 45 72
83 71 53 72
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Fig. 8. Scratching track observed on the nitrided specimen evidencing cracks formation behind the indenter (3 and 4 mm), cracks encountering along grain boundaries (5 mm) and detachment of martensitic grains from the nitrided case (10 mm). SEM images.
HSP ¼ scratch hardness number, Pa P ¼normal force, N w ¼ scratch width, m. Scratch width ( w ) and penetration depth (x) are schematically represented in Fig. 2. These values were calculated using digital image analysis and Eq. (3) respectively [18]:
x=
w ctg α 2
(3)
Three tracks were made on each specimen in both scratch tests.
3. Results and discussion 3.1. Microstructure characterization Fig. 3 shows X-ray diffraction patterns for nitrided and nonnitrided specimens [14]. One can see that the original martensite peaks were shifted and broadened to lower 2θ angles, due to nitrogen pick-up and diffusion in the nitrided specimen. Nitrogen atoms were dissolved at interstitial sites of the BCC crystal structure, creating a nitrogen supersaturated phase known as expanded martensite ( αN´ ). In addition to the expanded martensite peaks, the nitrided specimen diffraction pattern showed hexagonal ε-Fe24N10 iron nitrides peaks. A deeper analysis regarding to expanded martensite formation, nitrogen content, microhardness measurements and X-ray diffraction results of the nitrided specimen assessed in this paper can be found in a previous work [14]. Fig. 4 shows the surface of a pre-polished nitrided specimen, after the nitriding treatment. It is worth noting that the specimen, which was neither metallographically prepared nor chemically etched, shows a surface relief resultant from nitrogen pick-up and lattice expansion, during nitriding. After the thermochemical treatment, lath martensite sheaves
delimiting former austenitic grains can be seen at the surface of the nitrided specimen. Nitrogen pick-up induced compressive residual stresses, producing heterogeneous deformation among the grains leading to relief on the surface revealing the microstructure of the nitrided specimen, as reported for low temperature plasma nitriding and carburizing treatments of stainless steels [19, 20]. 3.2. Micromechanical characterization Nanoindentation curves of the nitrided and non-nitrided specimens are presented in Fig. 5, while the mechanical properties and We and Kd parameters are listed in Table 2. One can see in Fig. 5 that the non-nitrided specimen exhibited the greater deformation during indentation achieving a hmax of 234 nm while the nitrided specimen showed a hmax of 148 nm. The residual depths ( hf ) were 198 and 81 nm for the non-nitrided and nitrided specimens, respectively. Furthermore, it can be seen that the area between the loading and unloading curves is greater for the non-nitrided specimen, evidencing that the nitrided specimen dissipates less plastic deformation energy. The elastic recovery of the expanded martensite case was 45% while just 16% for the nonnitrided specimen. Kd was also greater for the non-nitrided specimen, 83%, compared to 53% for the expanded martensite. From the results, it can be concluded that plastic deformation predominates during nanoindentation of the non-nitrided specimens, also suggesting that plasma nitriding increased the yield stress of AISI 410 martensitic stainless steel, leading to an increase on the elastic response of the expanded martensite nitrided case. An increase of the yield stress of the X38CrMoV5-1 steel from 2.7 to 7 GPa due to plasma nitriding has been reported by Kamminga et al. [12]. In addition, nitrogen expanded martensite formation increased the hardness almost three times, from 4.7 to 13.7 GPa without changing significantly the Young modulus. Expanded martensite also showed a higher H/E ratio. Greater H/E ratios are desirable because they denote higher elastic deformation before failure by plastic deformation or cracking.
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Fig. 10. Friction coefficient behavior of both specimens registered during scratch tests at a constant normal force of 10 N.
Fig. 9. Track of the nitrided specimen evidencing microploughing, extensive plastic deformation and wedge formation, SEM images.
3.3. Scratch tests The results of the scratch tests, carried out with linearly-increased normal forces are shown in Fig. 6. The friction coefficients of the nitrided and non-nitrided specimens were quite different. The friction coefficient of the nitrided specimen, was around 0.07 during the first 2.5 mm of track length. From this point on, the apparent friction coefficient continuously increased up to a final value of 0.20. In contrast, for the non-nitrided specimen no
stability was observed. The friction coefficient increased steadily from 0.06 to 0.28. Although the friction coefficients at the beginning of the test, for both specimens, were quite similar, the value of the coefficient at the end of the stability plateau for the nitrided specimen was much smaller. Cracking was the first damage observed at around 3 mm of the track length of the nitrided specimen, as can be seen in Fig. 7. Accordingly, from Fig. 6, the critical load LC for damaging the nitrided case, corresponding to that distance, was 14 N. Other researches had reported critical loads between 40 and 48 N for damaging the nitrided case formed by plasma nitriding on several steels [10–12]. However, none of those critical loads correspond to a plasma nitrided martensitic stainless steel and the phases that composed those nitrided cases are quite different of expanded martensite. In addition, the scratch response of a material depends on the complex interaction of testing parameters such as stylus properties and geometry, loading rate, displacement rate and the coating/substrate properties including hardness, fracture strength, Young modulus, damage mechanisms, microstructure, flaw population and surface roughness [16]. From this point on, other cracks parallel to each other and perpendicular to the scratching direction arose in the track. The distance between parallel cracks decreased with increasing normal loads. This cracks formed behind the indenter at places where the tensile stress exerted during the indenter displacement exceeded the fracture strength of expanded martensite. Beyond 4 mm of the track length, the cracks started to encounter each other following a zig-zag trajectory along grain boundaries. In addition, some pieces of martensitic grains were detached from the nitrided case. These features are shown in Fig. 8. The nitrided case showed a cohesive failure and the mechanical failure mode acting on the nitrided specimen was tensile cracking. This failure mode has been reported in CrN coatings deposited by PVD [6, 16] as well as in plasma nitrided steels [10–12]. On the other hand, non-nitrided specimen showed the typical behavior of a ductile material submitted to a hard particle movement. Microploughing, extensive plastic deformation and wedge formation, were observed as shown in Fig. 9. Fig. 10 shows the friction coefficient behavior of both specimens registered during scratch tests at a constant normal force of 10 N, lower than LC . Both friction coefficient remained almost constant during the tests. The nitrided specimen exhibited a value around 0.06–0.07, while for the non-nitrided specimen the friction coefficient was close to 0.16, nearly two and a half times greater than that of the nitride specimen. The decrease of friction coefficient observed in this experiment can be explained using the friction coefficient model
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plasma nitriding may be also considered as an alternative to be evaluated in order to decrease friction coefficient at real components where metal – metal contact occurs, for instance, in the valves employed in the mining and oil industries.
4. Conclusions The nitrided case is formed by nitrogen supersaturated expanded martensite and hexagonal ε-Fe24N10 iron nitrides. Energy dissipation coefficient ( Kd ) was greater in the non-nitrided specimen with 83% and 53% for the expanded martensite; therefore, plastic deformation is the main deformation occurring in the non-nitrided specimen. Expanded martensite deforms mainly elastically, before cracking, during the scratch test. Furthermore, the increase on the elastic response of expanded martensite results in lower scratch depths in the nitrided specimen. The scratch resistance is controlled by the resistance to elastoplastic deformation of the specimens. The nitrided case showed a cohesive failure and the mechanical failure mode acting on the nitrided specimen was tensile cracking. The critical load LC for cracking the nitrided case was 14 N. Expanded martensite formation made possible an outstanding decrease on the friction coefficient in comparison to the non-nitrided AISI 410 martensitic stainless steel. The scratch test results showed that the groove features and the friction coefficient could be well correlated to the energy dissipation coefficient.
Acknowledgments Fig. 11. Tacks produced at constant load, a) non-nitrided specimen, b) nitride specimen, OM images.
The authors acknowledge the supports of CNPq processes n. 151653/2010-0 481918/2010-8 and 486104/2012-5, FAPESP, process n. 2012/50890-0 and COLCIENCIAS processes n. 497, Colombia.
Table 3 Track width ( w ), penetration depth (x) and scratch hardness number ( HS P ). Specimen
Non-nitrided Nitrided
Track width ( w )
Penetration depth (x)
lm
lm
HS P GPa
90 43
26 12
3.1 13.8
proposed by Bowden and Tabor [18]. The model states that the friction coefficient arises from an adhesive force involving the true area of contact and a ploughing force related to a projected contact area of the indenter. This area is dependent on the track width and on the penetration depth of the indenter. A decrease on the latter values lessens the projected contact area, therefore, decreasing the friction coefficient. Fig. 11 shows the tracks produced on both specimens at constant load, while Table 3 summarizes the track width measured by digital image analysis, the penetration depth and HSP calculated by using Eqs. (3) and (2), respectively. HSP of expanded martensite is at least 2.6 times higher than that reported for plasma nitrided titanium alloys [9]. The scratch test results suggest that expanded martensite deforms mainly elastically before cracking. Furthermore, it was loaded at the elastic zone during both the linearly-increased normal force test at loads lower than 14 N and the constant normal force test. The increase on the elastic response of expanded martensite results in lower scratch depths observed in the nitrided specimen. The scratch resistance is controlled by the resistance to elastoplastic deformation of the specimens. According to these results, active screen low temperature
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