sliding contact under boundary lubrication

sliding contact under boundary lubrication

Journal of Universityof Science and Technology Beijing Volume 14, Number 2, April 2007, Page 167 Tribological properties of electroless Ni-P-Sic comp...

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Journal of Universityof Science and Technology Beijing Volume 14, Number 2, April 2007, Page 167

Tribological properties of electroless Ni-P-Sic composite coatings in rolling/sliding contact under boundary lubrication Wei-Long Liu", Shu-Hue Hsieh'), Shen-Jenn Hwang2),Ting-Kan Tsai", and Wen-Jauh Chen3) 1) Department of Materials Science and Engineering, Formosa University, Huwei, China Taipei

2) Department of Mechanical Design Engineering, Formosa University, Huwei, China Taipei

3) Department of Materials Engineering, Pingtung University of Science and Technology, Pingtung, China Taipei (Received 2006-06-16)

Abstract: Ni-P-SIC composite coatings were prepared under a given bath composition and operation parameters of electroless plating. The tribological properties of the Ni-P-SiC composite coatings after annealing at 4OOOC for 1 h were tested in rollinglsliding contact under boundary lubrication condition using a two-roller tribometer. The measurement contained friction coefficient, contact surface temperature, contact electrical resistance, and wear rate of the Ni-P-SiC composite coatings under various slide to roll ratios, loads, and rolling speeds. For the simultaneous examination of the effect of the chosen parameters on the tribological properties of the Ni-P-Sic composite coatings, an orthogonal regression experimental design method was used. Key words: Ni-P-Sic composite coating; rolling-sliding; friction; wear; two-roller test

1. Introduction Composite coatings are a class of materials which are mostly used for mechanical and tribological applications. The Ni-Sic composite coatings combine anticorrosion property because of the presence of Ni with mechanical and tribological performance because of the presence of SIC. The Ni-Sic composite coatings can be made by electrolytic plating [l-101or electroless plating [ 11-201. The coating by electroless plating can possess different properties from electrolytic plating by adjusting P content, crystal structure, and microstructure of coating. The rate of incorporation of S i c into the Ni matrix is roughly proportional to its concentration in the electrolyte. The incorporation of S i c into Ni is also governed by the degree of particle dispersion in solution, mass transport, adhesion to the growing surface, and the electro-crystallization of Ni around the Sic [17]. The number density of S i c in the coating increases with decreasing particle size [8]. The addition of ultra fine Sic particles into the Ni matrix apparently reduces the size of Ni grains during the plating process and inhibits the grain growth during heat treatment [4-51. The hardness of the coating increases with the increase of the Sic content in the coating and the wear resistance increases to a maximum and then reduces again with the increase of the size of the S i c particles in the coatCorrespondingauthor: Shen-Jenn Hwang, E-mail: [email protected]

ing [5, 8, 181. The hardness and wear resistance of composite coatings can be markedly increased by heat treatment [12], and the Ni-P-Sic composite coating kept at 400°C for 1 h has the highest hardness and wear resistance [7, 13, 151. Most tribological tests for composite coatings were carried out in dry or lubricant condition using a ball on disk tribometer. This kind of test can only evaluate the performance of the device with reciprocating movement, such as pistons. In this study, the Ni-P-Sic composite coatings were first made under a given bath composition and operation parameters of electroless plating. Then the tribological properties of this Ni-PS i c composite coating were tested in rolling/sliding contact under boundary lubrication condition using a two-roller tribometer. This test method can really estimate the performance of the device with rotary motion, such as bearings and gears.

2. Experimental In this study, the measurement of tribological properties of electroless Ni-P-Sic composite coatings was carried out in rolling/sliding contact under boundary lubrication condition with two rollers. Both rollers were made of quenched-and-tempered bearing steel (AISI52100) and with a hardness of HRC 60-62. After Also available online at www.sciencedirect.com

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heat treatment, the surfaces of rollers were finished by a grinder to remove the oxide layers on the surfaces and got a surface roughness of 0.2 pm Rq (root-meansquare). Fig. 1 is a schematic diagram showing the two rollers in contact in which one is a driving roller with 110 mm in diameter and 18 mm in thickness and the other is a driven roller with 30 mm in diameter and 18 mm in thickness. The driven roller has a running surface with a transverse radius of 1 0 0 mm and an electroless Ni-P-Sic composite coating is deposited only on the running surface. Normal load

Fig. 1. Schematic diagram of two-roller in contact with each other.

The composition of the plating bath and the operation parameters for electroless Ni-P-SIC composite coatings in the study are listed in Table 1. To keep the plating bath stable and get the fastest plating rate, the composition of the plating bath and the operation parameters were taken in such concentrations and values [21] as listed in Table 1. All the samples were heated at 400°C for 1 h in Nz atmosphere to make the Ni,P phase be precipitated and increase the hardness of the coatings. The top and cross section views and the composition of the electroess Ni-P-Sic composite coating before and after tribological tests were observed and analyzed by scanning electron microscope (SEM) (JEOL JXA-840) operated at 25 kV with wavelength dispersive X-ray spectrometer (WDS) and energy dispersive spectrometer (EDS) in SEM, respectively. The S i c distribution of Ni-P-Sic coatings was examined in cross-sections by both optical and scanning electron microscope (JOEL T330A). There were at least 5 specimens in the same experimental condition to be observed and evaluated for the mean area of S i c in Ni-P-Sic coatings. A two-roller test rig shown in Fig. 2 is designed to measure the friction coefficient, contact surface temperature, and contact electrical resistance of the electroless Ni-P-Sic composite coatings under various slide-to-roll ratios, loads, and rolling speeds. The main

drive train is driven by a 3 HPAC servomotor with a continuously variable speed controller. The different slide-to-roll ratios can be obtained by a set of gears which drive the upper (driven) roller. There are three different sets of gears used for providing slide-to-roll ratios of 0.84, 1.10, and 1.38, respectively. The slideto-roll ratio (SR)is defined as: S/R=sliding speed/mean rolling speed=(Ul-U2)/[(U,+ U2Y21, where U 1is the velocity of the point at the circumference of the upper (driven) roller, and U, is the velocity of the point at the circumference of the lower (drive) roller. The parameters related to the tribological properties of electroless Ni-P-Sic composite coatings measured in this study include friction coefficient, contact temperature, contact electrical resistance, and wear rate. The friction coefficient is calculated from the formulap=F/N, where F is the tangential force between the two roller measured by a load cell and N is the normal load applied to the roller by a loading lever. The contact temperature is measured by a thermocouple. Table 1. Composition of electroless Ni-P-SIC composite plating and operation parameters NiS0,.6 H,O / (g.L-') NaH,PO,.H,O / (g,L-') Na2C,H4O4.6H,O/ (gL-I) Pb+*/ ppm

0.5

PH

4.8

Plating temperature /"C SIC uowder (1.5-3 urn) I (PL-')

88-90

21

20 16

10

The wear rate is obtained from the formula K,=W,IF,S (mm3-N-'-rn-l), where FN is the normal force, N; S the sliding distance, m; and W, the worn volume determined by profilometry on four positions for each sample before and after the test, mm3. To evaluate the tribological properties of the electroless Ni-P-Sic composite coatings, the measurements were carried out under normal atmosphere, 25°C. Table 2 shows the magnitudes of the test parameters, load and sliding speed, which are adopted on the base of the orthogonal regression experimental design method. In Table 2 coding of the independent variables was done according to the following equation:

where x, and X i are the dimensionless and the actual values of the independent variable i , X o the actual value of the independent variable i at the central point, and AX the step change of X ,. .

W.L.Liu et aL, Tribological properties of electroless Ni-P-Sic composite coatings in. .. Before the two-roller tribometer started, two drops of lubricant oil were dropped on the surface of the Ni-

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P-Sic composite coating to create a boundary lubrication condition in the test process.

Upper roller I

Fig. 2. Two-roller test rig. Table 2. Experimental range and levels of code and actual levels of the independent variables Variables

Code

-1.414

-1

0

+1

+1.414

Load I N

XI

Sliding speed / (ms-I)

x2

78 0.61

98 0.71

147 0.95

196 1.18

216 1.27

3. Results and discussion Figs. 3(a)-(c) show the SEM image, X-ray mapping for Ni and Si elements of the cross section of the electroless Ni-P-Sic composite coating, respectively. In Fig. 3(a), the darker parts with irregular shape are S i c particles, which can be confirmed by the X-ray mapping for Si element in Fig. 3(c). The SIC particles are smaller than 3 pm in size and have about 10~01%in the

coating of about 15 pm thickness. It can be seen from Fig. 3(b) that Ni element distributes rather uniformly in the coating. Figs. 4(a) and (b) show the SEM plane views of the Ni-P-Sic composite coating before and after tribological test, respectively. In Fig. 4(b), there are many wear tracks and S i c particles which protrude above the surface of the Ni matrix to carry most of the load.

Fig. 3. Cross section of the electroless Ni-P-Sic composite coating: (a) SEM image; (b) X-ray mapping for Ni element; (c) X-ray mapping for Si element.

Fig. 5 displays the variations of contact temperature, friction coefficient, and electrical resistance with test time for the Ni-P-Sic composite coating under rollinghliding test with a load of 147 N and a sliding ve-

locity of 0.61 d s . The behaviors of these three responding properties can be divided into three stages. Initially both the contact temperature and the friction coefficient are increasing and the electrical resistance is

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almost constant, then the electrical resistance is increasing and both the contact temperature and the friction coefficient are decreasing, and at last the electrical resistance is increasing and both the contact temperature and the friction coefficient are almost constant. These results manifest that at the first stage the two rough surfaces of the Ni-P-Sic coating and the counter roller contact directly so that the electrical resistance is very low and both the contact temperature and the friction coefficient are rapidly increasing, at the second stage the Ni-P-Sic coating and the counter roller are in

the state of running-in and there is a very thin oil film created between the two surfaces so that the electrical resistance is rapidly increasing and both the contact temperature and the friction coefficient are decreasing, and at the third stage the two surfaces of the coating and the counter roller are tribologically fitted for each other and the wear tracks are created so that both the contact temperature and the friction coefficient get a steady value, respectively and the electrical resistance decreases and then increases.

Fig. 4. SEM plan view of the Ni-P-SIC composite coating: (a) before tribological test; (b) after tribological test.

r

.

1 60 - 1

1.0

c:

20 2.- 4000 - . 2 a 01. 0 V

rn

-2a

-g Y

-Y Lr:

K

k

0.4 L

Friction coefficient

0.2

8 0 g

2000 -0.2

-40

2000 Time I s

-0.4

4000

Fig. 5. Variations of contact temperature, friction coefficient and electrical resistance with test time for the Ni-P-Sic composite coating under rolling/sliding test with the load of 147 N and the sliding velocity of 0.61 m/s.

The results of friction coefficient, wear rate, and maximum contact temperature of the Ni-P-Sic composite coating for various loads and sliding speeds are summed up in Table 3. The friction coefficient, the wear rate, and the maximum contact temperature of the Ni-P-SIC coating under the load of 78-216 N and at the sliding speed of 0.61-1.27 m / s vary between 0.0482 and 0.0648, 2433x10" and 5 8 . 8 5 ~ 1 0mm3.N-'.m-', ~ and 33.58 and 46.96"C, respectively. Because the properties of Ni-P-SIC coatings are very sensitive to the parameters of manufacturing process and heat treatment, their tribological behaviors should differ much from each other. From the data in Table 3, three mathematical equations representing the friction

coefficient 01). the wear rate (K,) and the maximum contact temperature (T,) can be derived as the following:

t+ M=O.O52+0.00538~1-O.1546~2+0.00366~ 0.004035~-0.001425~,~,, K,,=3.588~ lo-'+ 1 . 0 2 5 ~ 1 0 - ~1.798~10-~x,+ ~,+ 3 . 1 5 7 104x ~ t +5.59x 104x + 2 . 6 51~0-7x,x2,

Tc=61.33+9.08x,+5.91x2+3.37x~ +1.78~;+O.lx1x2, where xl=load and x,=sliding velocity are expressed in terms of coded values. The relative extremum of the friction coefficient b),the wear rate (K,) or the maximum contact temperature (Tc),and the corresponding variables can be obtained by calculus from the above equations.

4. Conclusions (1) For the Ni-P-Sic composite coating annealed at 400°C for 1 h with S i c smaller than 3 pm in size and about 10%in volume fraction, the tribological behavior in rolling/sliding process under boundary lubrication condition can be divided into direct contact, running-in, and matching three stages. (2) In the first stage the Ni-P-Sic coating contacted directly with the counter roller so that the electrical resistance across the contact interface was very low and both contact temperature and friction coefficient were rapidly increasing. In running-in stage a very thin oil

W.L Liu eta& Tribological properties of electroless Ni-P-Sic composite coatings in...

film was created between the two surfaces so that the electrical resistance was rapidly increasing and both contact temperature and friction coefficient were decreasing. In the third stage the two surfaces of the NiP-Sic coating and the counter rollers were tribological

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fitted for each other and wear tracks were created so that both contact temperature and friction coefficient got a steady value and the electrical resistance decreased and then increased gradually.

Table 3. Experimental design and results __

Variable

Runs

1 2 3 4 5 6 7

8 9 10 11 12 13

Results

X1

x,

Friction coefficient

Wear rate / (lo4 mm3.N-'.m-' )

Maximum contact temperature, T, /OC

-1 1 -1 1

-1 -1 1 1 0 0

0.0574 0.0648 0.0564 0.0581 0.0482 0.0722

35.36 51.78 37.66 55.14 24.83

33.58 36.72 37.35 42.94 35.97

58.85

46.96

-Jz Jz

0.0626

43.62

34.39

0.0593 0.0515 0.0521 0.05 18 0.0529 0.0526

49.79

44.92 37.74 37.83 37.96 37.74 37.88

-Jz &

0 0 0 0 0 0 0

0 0 0 0 0

(3) The friction coefficient, the wear rate and the maximum contact temperature of the Ni-P-Sic coating under the load of 78-216 N and the sliding speed of 0.61-1.27 m/s varied between 0.0482 and 0.0648, 24.83~10-~ and 5 8 . 8 5 ~ 1 0mm3.N-'.m-', ~ and 33.58 and 46.96"C, respectively.

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