ABRASIVE WEAR MECHANISMS OF GRAY CAST IRON APPLICATION TO CYLINDER BORE ABRASION
Luiz Alberto Franco (a)*, Amilton Sinatora (a)
a) Surface Phenomena Laboratory, Polytechnic School, University of São Paulo São Paulo, Brazil
*[email protected] ABSTRACT
A significant part of the friction losses in the automotive engine results from the abrasive action of particles formed in the interface between cylinder and piston/rings.
Those particles are responsible for axial grooves that are observed in the liners of used engines. The objective of this work was to get a better understanding of the wear mechanisms related to liner/bore grooving and to identify a laboratory testing setup that might reproduce them under controlled conditions. Specimens of Gray Cast Iron (GCI) and of AISI 1070 steel with matrix hardness close to that found in GCI (≈200HV30) were submitted to scratch tests in a tribometer. It was found that scratches performed under 20-50 mN indenter load were similar to grooves observed in cylinder liners. No sharp transition between abrasion mechanisms was observed.
Calculation of material removal factor fab from optical profilometry resulted in values with a large dispersion; they could not be associated with different abrasion mechanisms.
KEYWORDS: abrasion, gray cast iron, cylinder, liner, graphite 1. INTRODUCTION
Approximately 11% of the total energy supplied by the fuel is actually used to move the engine. Of those, 45% (5% of the total) are spent in the contact between cylinder and piston/rings as indicated in Figure 1.
Figure 1 – Energy consumption of typical automotive engine [1]
Part of that energy is consumed by the abrasive action of particles that form in the interface between cylinder liner and piston as a result of combustion and metal wear.
30%
ENGINE
Bearings and Seals Pumps+
Viscous Losses Valve train
Piston / Piston rings
10%
15%
45%
Those particles are responsible for axial grooves that are observed in the liners [2] as shown in Figure 2.
Figure 2 – Grooving of cylinder liner by abrasive particles [2]
Naturally those figures call for a good understanding of the wear mechanisms involved as well as means for estimating what part of the energy losses could be attributed to the cylinder liner abrasion.
Scratch tests were a natural choice since it seemed that they could simulate what happens when an abrasive particle is caught between piston and engine block or cylinder liner. It was not clear how the indenter would interact with the graphite lamellae in GCI. Scratch tests of the AISI steel would provide a reference, considering that for the scale under consideration its microstructure might be considered homogeneous.
Besides the values of the apparent coefficient of friction (COF) directly provided by the tribometer, geometric parameters of the scratches – width, depth, average profile and material removal factor fab – were analyzed and scanning electronic microscope (SEM) images provided additional information on the abrasion mechanisms.
2. MATERIALS AND METHODS
In order to obtain some information on the effect of surface finishing half the samples were ground and half were polished. Table 1 summarizes the basic information on the specimens:
Table 1 – Specimen Characteristics Material Origin Hardness
HV30
Dimensions
(mm) Surface
finish Roughness Sa (μm) GCI Engine
block 212 ± 2 20 x 12 x 3 Ground 0.22 ± 0.01 Engine
block 207 ± 3 20 x 12 x 3 Polished 0.043 ± 0.06 AISI
1070 Round bar 198 ± 4 18 x 10 x 6 Ground 0.20 ± 0.01 Round bar 190 ± 5 18 x 10 x 6 Polished 0.011 ± 0.01 Scratch tests were performed on a Hysitron 950 Triboindenter with a diamond conical
o
material/finishing combination (a total of twelve combinations, half of them ground and half polished). All the scratches were done under constant load, and for each load level two scratches were done on each specimen. Seven load levels were used:
20/50/75/100/125/150/200 mN. The Hysitron program consists of three main phases of indenter displacement:
Indenter displacement under extremely low load in order to obtain original surface profile;
Indenter return under specified load making the scratch;
Indenter under light load measuring scratch depth.
Files generated by the equipment (.txt) provided force values as well as vertical position of indenter carriage and directly calculated COF values.
SEM images provided important information related to the role played by the the graphite lamellae and to the abrasion micromechanisms.
Geometric parameters of the scratches were measured using a Taylor Hobson optical 3D profilometer CCI-MP as indicated in Figure 3:
Figure 3 – Scratch parameters measured with 3D optical profilometry (polished GCI 50 mN)
In the example of Figure 3 the instrument provided scratch width measured between peaks of the pile up (lp ≈ 4.9 μm) and at the reference plane level (lr ≈ 3.1 μm), maximum average depth (p ≈ 0,9 μm) and areas of the scratch cross section (Ss=1.489 μm2) and of the pile up material cross section (Sp1 + Sp2= 1.118 μm2). The latter would yield a value of fab = 0.25 through use of equation (1) [3]:
(1)
3. RESULTS
At light loads strong correlation can be observed between indenter depth and coefficient of friction. Superposition of SEM image and depth/COF curves indicates
lp lr
Ss
Sp 1
Sp 2
that when the indenter crosses a graphite lamella the scratch depth increases and there is a sudden COF increase as indicated in Figure 4.
Figure 4 – Correlation between indenter depth, COF and graphite crossing (polished GCI 20 mN)
The above results agree with models seen in the literature [4].
Abrasion mechanisms are highly dependent on the microstructure and scratch width may vary accordingly. Steadite presence seems to be responsible for width reduction as seen in Figure 5:
Figure 5 – Scratch width reduction in the vicinity of steadite
Figure 6 presents scratch depths produced under indenter loads of 20 and 50 mN;
values are close to those observed in cylinder liners and indicated in the literature [2],
[5] and [6]. Corresponding scratch widths remained below 6 μm. Both depth and width were lower for the ground specimens.
Figure 6 – Scratch depth x Indenter load (GCI)
COF at both the lower and higher load regions of the Gray Cast Iron and AISI 1070 graphs were quite similar as indicated in Table 2 and Figure 7 a) and b).
Table 2 – Coefficient of Friction x Load AVERAGE COEFFICIENT OF FRICTION
Gray Cast Iron ISI 1070
Load (mN) Polished Ground Polished Ground
20 0.142 0.136 0.210 0.174
50 0.270 0.153 0.512 0.284
75 0.605 0.192 0.803 0.725
100 0.750 0.375 0.874 0.823
125 0.875 0.741 0.949 0.909
150 0.937 0.887 1.011 0.894
200 1.028 1.014 1.035 0.981
Error bars indicated in the graphs represent the standard deviation of the mean.
a) GCI – COF x Load b) AISI 1070 – COF x Load
Figure 7 – Friction coefficient as a function of indenter load
R² = 0,9816
R² = 0,9518
0,0 0,5 1,0 1,5 2,0 2,5 3,0 3,5 4,0
0 20 40 60 80 100 120 140 160 180 200
μm
LOAD (mN)
GCI - SCRATCH DEPTH
Polished Ground
0,00 0,20 0,40 0,60 0,80 1,00 1,20
0 20 40 60 80 100 120 140 160 180 200
COF
LOAD (mN)
1070 - AVERAGE FRICTION COEFFICIENT
Polished Ground
It should be noted that a high COF around 1.0 was attained, similar to literature values [7], but it was not possible to correlate it to high values of fab [3] as the latter measurements were highly dispersed for both GCI and 1070.
Tests with samples that had been ground presented lower COF values than polished samples. While for steel the difference might not be statistically significant, that was not the case of GCI where differences exceeded the standard deviation of the means. One possible explanation in the case of GCI ground samples might be the presence of graphite particles appearing as "background noise" in the Carbon lines obtained with the scanning electronic microscope. Figure 8 shows a striking difference between the two carbon lines:
a) ground GCI specimen b) polished GCI specimen
Figure 8 – Carbon lines taken along the bottom of the scratch 4. CONCLUDING REMARKS
• It was possible under laboratory conditions to reproduce grooves similar to those observed in cylinder liners.
• Material removal factor fab could not be used to identify abrasion mechanisms.
• COF values should allow initial estimates of energy consumption by abrasion.
• Contrarily to what is indicated in the literature, no sharp transition between abrasion mechanisms was observed.
• Further studies of the effect of surface finishing should be undertaken.
5. REFERENCES
[1] Holmberg, K. et al, Global Energy Consumption due to Friction in Passenger Cars, Tribology International, 47, 2012.
[2] Obara, R, Analysis of vertical abrasion grooves by means of fab (in Portuguese), Internal LFS Report, 2014.
[3] Zum-Gahr, KH, Microstructure and wear of materials, Elsevier, 1987
[4] Nakamura, R and Iwabuchi, A., Role of graphite in cast iron on tribological behavior in nano-scratch test, J. of Adv. Mech. Design, Systems & Manuf., vol. 6, Np.
7, 2012.
[5] Dimkovski, Z., Surfaces of Honed Cylinder Liners, PhD Thesis, Chalmers University of Technology, Halmstad, 2011.
[6] Santos Fo, D., Alterações metalúrgicas e topográficas do cilindro de bloco de motor de combustão interna flex-fuel, Master Degree Thesis (in Portuguese), Escola Politécnica, USP, 2013.
[7] Hokkirigawa, K and Kato, K., An Experimental and Theoretical Investigation of Ploughing, Cutting and Wedge Formation during Abrasion Wear, Tribology International, 21, 1988.