Fatigue resistance of PECVD coated steel alloy

International Journalof Fatigue

International Journal of Fatigue 29 (2007) 1832–1838

www.elsevier.com/locate/ijfatigue

Fatigue resistance of PECVD coated steel alloy S. Baragetti a, F. Tordini a

b,*

Dipartimento di Progettazione e Tecnologie, Universita` degli Studi di Bergamo, Viale Marconi 5, 24044 Dalmine (BG), Italy b Facolta` di Ingegneria, Universita` degli Studi di Ferrara, Via Saragat 1, 44100 Ferrara, Italy Received 27 July 2006; received in revised form 31 January 2007; accepted 2 February 2007 Available online 20 February 2007

Abstract The thin hard coating deposition techniques CVD and PVD have been used for a long time in industry. Such coatings prove very effective in improving the tribological and corrosive resistant properties of the substrate. It was shown that the compressive residual stresses are introduced on the surface layer from the PVD deposition process that helps to increase the fatigue limit of coated structural components. The aim of this work is to evaluate the effect of a SiOx coating, deposited by means of PECVD technique, on the fatigue resistance of a quenched and tempered alloy steel (39NiCrMo3). Rotating bending fatigue tests were carried out to assess its fatigue limit and characterize any possible variation between the coated and the uncoated material. Fracture surface observations were made using SEM on fracture surfaces, and scratch tests were performed on samples to assess the coating-substrate interface delamination. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: PECVD; Thin hard coatings; Stair-case fatigue; Residual stresses

1. Introduction Thin hard coatings deposited on structural mechanical components by means of PVD and CVD processes allow to enhance the resistance properties of wear and corrosion [1–3], and in some cases the fatigue resistance [4–12]. These coatings are used in aeronautical and automotive industry [13,14], and for cutting tools and dies [15]. The main benefits in using thin hard coatings versus thick hard coatings and soft coatings are:  high hardness and wear resistance without affecting the dimensional tolerances of coated components,  suitability for coating small components,  good wear protection of particular geometries (e.g. sharp edges),  process capability to coat complex-shaped components.

*

Corresponding author. Tel.: +39 035 2052382; fax: +39 035 2052077. E-mail address: [email protected] (F. Tordini).

0142-1123/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijfatigue.2007.02.008

The surface residual stresses introduced by the coating process are probably one of the factors which most affects the fatigue resistance. Their effect will be beneficial for compressive stresses [4–9] with an un-cracked coating. The literature presents a broad range of PVD techniques and their benefits on the fatigue behaviour of coated components [4,7–10]. However, we find that no significant studies have been made on SiOx thin coatings deposited by means of the plasma enhanced chemical vapor deposition (PECVD) technique for fatigue applications. Nowadays the deposition of thin films by means of this technique represents one of the most popular industrial applications of plasmas. Cold plasmas, like the one used for the PECVD process, are obtained at extremely low pressures (from a few hundred to ten thousand times lower than ambient pressure) and at low temperatures (a few tens of Celsius degrees). Using this PECVD technique many kinds of materials can be treated without degrading the properties from thermal effects. The main applications are in particular those requiring a high resistance to scratches and corrosion or a barrier for gas permeation, at both high and low temperatures. The deposed thicknesses are very thin (even

S. Baragetti, F. Tordini / International Journal of Fatigue 29 (2007) 1832–1838

1833

samples in order to evaluate their adhesion to the substrate.

lower than 1 lm) in order to reduce the processing time (with a compact film there is no need for a thick layer for corrosion resistance [16–19]) and to guarantee a good film elasticity. The PECVD/SiOx thin coatings can be applied on steels used for applications where fatigue stresses are significant. The intrinsic brittleness of the film under consideration was thought to be partially countered with a certain level of deformability guaranteed by the very small thickness. In particular, we considered the cases in which contact fatigue is not present. Assuming this hypothesis, our aim was to verify whether the coating deposition could determine either the same or a higher fatigue limit with respect to the uncoated material with, moreover, the benefit of a good corrosion resistance [16–19]. The experimental results were slightly lower than expected, but, however, they were considered rather satisfying. Pursuant to applicable standards (ISO 1143 and UNI 3964), standard hourglass-shaped specimens in quenched and tempered alloy steel (39NiCrMo3), both coated and uncoated, were studied in order to test their fatigue behaviour in rotating bending machine (R = 1). Before the coating process, cleaning operations were carried out on the surface of the specimens to remove contaminants like dusts, grease and oxides and therefore to guarantee the uniformity and good adhesion of the film. The coating-substrate interface is an extremely critical area, where physical and mechanical discontinuities occur. For example, if a surface is not properly prepared, the coating may delaminate with cracks. The tests performed allowed to evaluate, by means of a statistical method (according to UNI 3964 and [20]), the fatigue limit for the coated and the uncoated material. The fracture surfaces of the specimens were examined under the scanning electron microscope to evaluate the possible mechanisms in fatigue damage in coated and uncoated samples. Scratch tests were also performed on coated

2. The test machine In order to carry out the rotating bending fatigue tests, an improved version of a traditional machine was designed (Fig. 1). The bending moment (R = 1) is applied by means of a worm gear screwjack. During the tests the load was constantly kept under control by means of a load cell. It is also possible to take advantage of the automatic correction of the applied moment, instant by instant, as well as of the automatic suspendability of the test in the case of a significant variation in the bending moment applied with respect to the earlier one. On the steel platform (1) two mandrel holder supports (2 and 3) are mounted, which are connected to the corresponding mandrels (4 and 5) where two journals are perpendicular to the longitudinal–vertical plane. With this arrangement the system ensures the autoalignment of the specimen (6) during the assembly. A brushless motor (11) (200 W) is mounted on the opposite side with respect to the specimen end and transmits the main rotary motion to the one of the two mandrels (4), while the other (5) is being driven. The two collets (12) can hold a circular–cross section specimen. The rotation of the mandrels around the above-mentioned journals allows the specimen to be subjected to a uniform bending moment over its whole gage length. The value of the bending moment on which the test is going to be set is reached by operating the worm gear screwjack (10), which is controlled by the load cell (9). Also several specimen lengths (for a possibly gage length between 30 and 90 mm) and diameters (6–12 mm) can be used by changing the position of the slide (7) and therefore translating the driven mandrel (5). The blocking system of the slide is made of two levers (8) at the base of the support of the mandrel.

410-480 4

30-90

12

180

480

6

5

12

100

11

2

3 10

9

7

1

950 Fig. 1. Rotating bending test machine: fundamental components.

8

1834

S. Baragetti, F. Tordini / International Journal of Fatigue 29 (2007) 1832–1838

The machine allows for a maximum bending moment of 75 Nm and a maximum rotational speed of 3000 rpm. A computer equipped with a data acquisition card with a serial communication port and a suitable electric control panel can control the machine, by setting selected values for all the test parameters which can be monitored during each test. The applied bending moment is detected at each time interval by the load cell. In the case there are variations beyond a set threshold, the control software will interrupt the test. The automatic correction of the load may also be used during the tests in order to constantly keep it at the preset value with very low oscillations. By means of strain gage tests, a very low (0.2%) average error was for the preset bending moment with respect to the value for the feedback by the load cell.

Table 1 Chemical analysis of the alloy steel Detected chemical composition, % C

Mn

Si

P

S

Cr

Ni

Mo

0.395

0.700

0.240

0.018

0.031

0.740

0.880

0.190

be concluded that this steel was not previously subjected to any surface treatment. A wet-chemical analysis (testing method ASTM E35095 (2005) [21]) was carried out on samples of material in order to assess its alloy composition, as shown in Table 1. The material composition indicates that the material is a quenched and tempered alloy steel 39NiCrMo3 UNI EN 10083-1:1993. 4. Fatigue tests

3. Characterization of the material Before proceeding with the fatigue tests, the chemical composition and the microstructure of the steel under examination were analysed and its mechanical characteristics were defined. Tensile tests were carried out on standard cylindrical specimens according to the standard UNI EN 10002-1. The resulting value for the ultimate tensile strength (Rm) was 1050 MPa, while the yield strength (Rp0.2) was 910 MPa. The metallographic etching (a nital, 96% ethyl alcohol and 4% HNO3 solution) on polished specimens allowed to define the microstructure of the material, which was made of very fine grains with a high level of carbides (Fig. 2), showing a tempered martensitic structure due to quenching and tempering process. The hardness average value was 322 HV5, which satisfactorily meets the previously reported data on the material resistance. Furthermore, similar hardness values were observed at the core and on the surface. It could therefore

Pursuant to the procedures prescribed by the applicable standards (UNI 3964 and ISO 1143), rotating bending tests (R = 1) were carried out. Standard hourglass-shaped specimens (ISO 1143) were produced in order to carry out the fatigue tests. The specimens (Fig. 3) are characterised by a wide fillet radius and a smooth surface finishing in the gage length. The average surface roughness was below the value of 0.25 lm by polishing. The final polishing operations were performed lengthwise leaving no scratches along the circumferential direction. The average roughness value was about 0.20 lm. The technological parameters of the deposition process are listed in Table 2. Fatigue sample were loaded in the range 400–560 MPa. The pitch chosen for the load variation was 40 MPa, while

110

Ø6

R34

Ø12

41

0.20

Fig. 3. Nominal dimensions of the specimens used in the fatigue tests.

Table 2 Parameters of the PECVD deposition process

Fig. 2. Optical microstructure of the alloy steel.

Polymeric product deposited

SiOx

Plasma generation frequency Reagents Thickness deposited (estimation) Temperature Pressure

13.56 MHz (radiofrequency) O2 + HMDSO (hexamethyldisiloxane) 61 lm Room 0.1 mbar

S. Baragetti, F. Tordini / International Journal of Fatigue 29 (2007) 1832–1838

the upper limit of fatigue life was set at 10 million cycles. The speed of rotation of the machine used in the tests was 2000 rpm. The statistical analysis procedure was used to calculate the fatigue limit, in terms of the number of cycles to failure for each level of imposed bending stress, using the stair-case method [20]. This method defines the average value of the fatigue limit with a reliability of 50%. Each test should be repeated with at least 15 speci-

Table 3 Fatigue test results for the uncoated specimens Test no.

Applied stress (MPa)

Cycles to failure

1 2 3 4 5 6 7 8 9 10 11

400 440 480 520 560 520 560 520 560 520 560

>107 >107 >107 >107 875,000 >107 1,194,000 >107 676,000 >107 526,000

Table 4 Fatigue test results for the coated specimens Test no.

Applied stress (MPa)

Cycles to failure

1 2 3 4 5 6 7 8

560 520 480 520 480 520 480 520

< 107a 227,000 >107 269,000 >107 319,000 >107 286,000

a

Not recorded value, in any case lower than 107 cycles.

800

R = -1

Applied stress (MPa)

700

Uncoated

Coated 600

1835

mens. Due to the smaller number of samples tested a reduced stair-case method [20] was applied to evaluate an average value for the fatigue limit. In this case, such a method was chosen for the regularity it showed in the trend of the numerical data resulting from the tests. Tables 3 and 4 present the results of the tests obtained for the coated and uncoated specimens. Based on experimental data (Table 3 and Fig. 4), for the uncoated material a fatigue limit value rD(50%) of 538 MPa was calculated. A standard error of 21 MPa was estimated in the value of the fatigue limit. The same procedure was set up for the coated specimens. Table 4 and Fig. 4 show the results. A fatigue limit of 503 MPa was calculated, with a standard error of 23 MPa. The data show small difference between the coated and the uncoated specimens. The difference between the average values of the fatigue limits was about 10%. The fatigue behaviour, from the point of view of the material resistance, did not therefore seem to be significantly affected by the presence of the PECVD coating. 5. Laboratory analyses and discussion In order to test the adhesion quality of the coating to the substrate steel, scratch tests were carried out with a Rockwell indenter on coated flat samples. The samples were first subjected to two successive tests with a Rockwell indenter having a radius of 0.8 mm, without any damage both applying an incremental load from 0.1 to 10 N, and with a load from 0.1 to 20 N. The tests were then repeated with an indenter of 0.2 mm, and the coating failed for both load ranges (Table 5). By observing the traces (Fig. 5 shows two of those made with the indenter of 0.2 mm) some elasticity of the film resulted. The behaviour of the coating during the tests was observed to be rather uniform, and its adhesion to the substrate was reasonable. The fracture surfaces produced from the fatigue tests were analysed, for the coated and the uncoated specimens, using SEM. The objective was to check whether different fatigue crack propagation mechanisms had occurred between the two kinds of specimen. In other words, if the PECVD coating had affected the crack initiation part of fatigue. Fig. 6a shows the fracture surface of one of the

(4) 500

400

300 1.0E+04

(3)

Uncoated 39NiCrMo3 Coated 39NiCrMo3

1.0E+05

1.0E+06

Table 5 Results of the scratch tests with a Rockwell indenter with a radius of 0.2 mm

Run outs

1.0E+07

1.0E+08

Cycles to failure Fig. 4. Applied stress vs. cycle to failure graph for the coated and uncoated specimens.

I test

II test

Incremental load 0.1–10 N First delamination (N) Complete delamination (N)

3.0 5.5

4.2 7.3

Incremental load 0.1–20 N First delamination (N) Complete delamination (N)

4.4 5.0

4.0 5.2

1836

S. Baragetti, F. Tordini / International Journal of Fatigue 29 (2007) 1832–1838

Fig. 5. Results of scratch tests (radius 0.2 mm, load 0.1–10 N): (a) first delamination and (b) complete delamination.

Fig. 6. (a) Fracture surface of an uncoated specimen and (b) at higher magnification of C.

uncoated specimens. The areas of the classic stable propagation of the cracks, as fatigue cycles repeat, are shown (on the left side of Fig. 6a) as well as that of the overload final failure (on the right). In the specific area of the specimen shown in Fig. 6, the coalescence of two cracks was observed. Their possible initiations were located in the areas A and C, which by propagating generated a ‘‘step’’ (B) because of the offset of the corresponding crack planes. By observing the fracture surfaces of the coated specimens, we observed that the mechanism of nucleation and fatigue crack propagation was the same as in uncoated specimens. The presence of the thin coating was therefore not affecting significantly the fatigue crack propagation. Fig. 7 shows the fracture surface of one of the coated specimens, with the direction of crack propagation shown by an arrow. The surface defects, where fatigue cracks are generated, were more than 10 lm deep, while the coating has a thickness of less than 1 lm. One of the reasons for the similar

behaviour of the coated and the uncoated specimens could be due to this. The initiation and propagation points were considered superimposable with respect to the position and number between the coated and the uncoated specimens. 6. Conclusions The fatigue tests performed showed that the PECVD/ SiOx coating did not significantly affect the fatigue behaviour of the base material. Furthermore, the fatigue crack propagation mechanism in coated specimens seems similar to the one occurring in uncoated specimens, as examined by the fracture surfaces. The fatigue limit for the coated material diminished by less than 10% in the average value with respect to the uncoated material. We concluded that the thermal expansion of the substrate induced by the low temperature used in the coating deposition process should not have been so relevant as to induce high residual stresses on the substrate surface.

S. Baragetti, F. Tordini / International Journal of Fatigue 29 (2007) 1832–1838

1837

Fig. 7. (a) Fracture surface of a coated specimen; higher magnifications of possible crack initiations, (b) B and (c) A.

This was considered a concrete reason for the unbeneficial contribution of the studied thin coating to fatigue resistance. Acknowledgements The authors thank the company Moma srl, represented by Mr. Roberto Canton and Mr. Roberto Bellini for the deposition of the PECVD coating and for their technical support, the Universita` di Modena e Reggio Emilia for the scratch tests and Marta Daniele. References [1] Baragetti S, Gelfi M, La Vecchia GM, Lecis N. Fatigue resistance of CrN thin films deposited by arc evaporation process on H11 tool steel and 2205 duplex stainless steel. Fatigue Fract Eng M 2005;28(7): 615–21. [2] Barata A, Cunha L, Moura C. Characterisation of chromium nitride films produced by PVD techniques. Thin Solid Films 2001;398:501–6. [3] Sathiyanarayanan S, Rajagopal G, Palaniswamy N, Raghavan M. Corrosion protection by chemical vapor deposition: a review. Corros Rev 2005;23(4–6):355–70. [4] Kim KR, Suh CM, Murakami RI, Chung CW. Effect of intrinsic properties of ceramic coatings on fatigue behaviour of Cr–Mo–V steels. Surf Coat Technol 2003;171:15–23.

[5] Stewart S, Ahmed R. Rolling contact fatigue of surface coatings – a review. Wear 2002;235:1132–44. [6] Inoue K, Lyu S, Deng G, Kato M. Fracture mechanics based evaluation of the effects of the surface treatments on the strength of carburized gears. Proc VDI Berichte 1996;1320:357–69. [7] Baragetti S, La Vecchia GM, Terranova A. Fatigue behaviour and FEM modelling of thin-coated components. Int J Fatigue 2003;25:1229–38. [8] Baragetti S, La Vecchia GM, Terranova A. Variables affecting the fatigue resistance of PVD-coated components. Int J Fatigue 2005;27(10–12):1541–50. [9] Su YL, Yao SH, Wei CS, Wu CT, Kao WH. Evaluation on wear, tension and fatigue behaviour of various PVD coated materials. Mater Lett 1998;35:255–60. [10] Gelfi M, La Vecchia GM, Lecis N, Troglio S. Relationship between through thickness residual stress of CrN-PVD coatings and fatigue nucleation sites. Surf Coat Technol 2005;192: 263–8. [11] Ejiri S, Sasaki T, Hirose Y. X-ray stress measurement for TiN films evaporated by PVD. Thin Solid Films 1997;307:178–82. [12] Murotani T, Hirose H, Sasaki T, Okazaki K. Study on stress measurement of PVD-coating layer. Thin Solid Films 2000;377– 378:617–20. [13] Merlo AM. The contribution of surface engineering to the product performance in the automotive industry. Surf Coat Technol 2003;174–175:21–6. [14] Vetter J, Barbezat G, Crummenauer J, Avissar J. Surface treatment selections for automotive applications. Surf Coat Technol 2005;200: 1962–8.

1838

S. Baragetti, F. Tordini / International Journal of Fatigue 29 (2007) 1832–1838

[15] Su YL, Yao SH, Wei CS, Kao WH, Wu CT. Comparison of wear, tensile, and fatigue properties of PVD coated materials. Mater Sci Technol 1999;15(1):73–7. [16] Angelini E, d’Agostino R, Fracassi F, Grassini S, Rosalbino F. Surface analysis of PECVD organosilicon films for corrosion protection of steel substrates. Surf Interface Anal 2002;34:155–9. [17] Angelini E, Grassini S, Rosalbino F, Fracassi F, Laera S, Palombo F. PECVD coatings: analysis of the interface with the metallic substrate. Surf Interface Anal 2006;38:248–51. [18] He JL, Chu CH, Wang HL, Hon MH. Corrosion protection by PECVD-SiOx as a top coating on TiN-coated steel. Surf Coat Technol 1994;63(1):15–23.

[19] Moretti G, Guidi F, Canton R, Battagliarin M, Rossetto G. Corrosion protection and mechanical performance of SiO2 films deposited via PECVD on OT59 brass. Anti-Corros Method Mater 2005;52(5):266–75. [20] Dixon WJ, Massey Jr FJ. Introduction to statistical analysis. New York: McGraw-Hill; 1983. pp. 386–94. [21] ASTM E350-95 (2005). Standard test methods for chemical analysis of carbon steel, low-alloy steel, silicon electrical steel, ingot iron, and wrought iron. Annual Book of ASTM Standards, vol. 03.05. Philadelphia: American Society for Testing and Materials; 2005.