Genesis of solid self lubricating film analyzed by along stroke roughness.

Eftimie, Lucian

1. INTRODUCTION

The materials wear behaviour can be studied by tribometer test rigs, in respect to real functioning condition. A specially designed tribometer was made to test reciprocating movement as a practical solution, to transform the alternate linear movement to circular alternate one, in respect to crank slide move equation, exactly the same movement which acts piston compressors. The rig allows applying different loads on the sliding surface in order to reach the real contact pressure, and allows modifying the alternate stroke and instant speed value.

To obtain higher results accuracy and to diminish the experiment's duration, it is necessary to process simultaneously more than one test at once. In the present study, FcA2, FcX250 and OLC45 industrial base materials have been chosen to compare their roughness one against each other, in contact with filled PEEK.

2. SOLID LUBRICATING FILM MEASURE

Due to the piston's reciprocating motion, measurements had to be realized at both end strokes, and between them.

The stroke is divided in four equal parts as represented in fig. 1. Zero degrees marks the middle of the stroke and--135deg and +135deg mark the maxima of the stroke. The opposite mark to the middle of the stroke was chosen as reference for base material roughness, because the stroke doesn't act on this point.

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

The sliding active faces of the discs were machined by grinding, in the same conditions, and their roughness is similar one to each other--around Ra=0.5 [micro]m.

Sliding test parameters were developed under normal force of 257.579N acting on 16 [mm.sup.2] square sliding face, against cylindrical face of metallic base material. Oscillatory movements of base material performs reciprocating stroke of 0.563m, acting by 71 double strokes each minute.

The solid lubricating film transferred from the filled plastic PEEK, is 6 mm wide, the rest will be clear base material--3mm each side of the film trace--fig. 2.

The roughness measurement along the stroke implies the palpation of a 120mm radius curve, corresponding to the discs diameter. Due to its construction, the Surtronic 25 roughness checker allows measuring, without errors, lengths of 0.8mm, 4mm and 8mm (fig. 3.).

[FIGURE 3 OMITTED]

[FIGURE 4 OMITTED]

[FIGURE 5 OMITTED]

Checking Ra and Rz roughness of the solid lubricating film from the metallic support was revealed end stroke non-uniform distribution of the film particles along the stroke (fig. 4.). It was established that the wear is mainly due to the film discontinuities to the end stroke, caused by stick-slip phenomenon produced by rapid decrease of speed under load.

The stick-slip phenomenon appears both at translational and rotational move. At low sliding velocities, the thin lubricant layer brakes and for a short period a high adherence is produced. Meanwhile, the forces transmissions are elastically deformed, until the adhesion friction force is exceeded.

When overriding the stick-slip action, the sudden relaxation appears and the systems instantaneous changes to the sliding friction. The adherent friction phenomenon repeats after relaxation and the stick-slip process--relaxation cycle goes on many times.

The end stroke acts as specific condition to perform stick-slip motion. The speed is finishing at zero and starts under load slowly from zero. The reverse stroke changes bending direction of gripped flakes, destroying by extraction the film's continuity.

3. RESULTS ANALYSIS

Performing sliding wear tests of filled PEEK composite against metallic support as FcA2, FcX250 and OLC45, there were established the comparative roughness diagrams.

Analyzing the roughness behaviour diagrams along the sliding direction, it can be established that the roughness increases to the stroke ends, comparing to middle stroke, in both Ra and Rz measurements.

The wear developed during the reciprocating movement is preferentially created by solid lubricating film damages, close to stroke ends.

Comparing film roughness against the base material roughness is evident that film face is smoother than base material. We can accept that the flakes bending direction is coherent--left or right--because the films roughness is constantly smaller than base material (fig. 5.).

[FIGURE 6 OMITTED]

[FIGURE 7 OMITTED]

The roughness behaviour across the stroke shows that the flakes anchorage is discontinuous more interrupted to both sides of the film track--less smooth.

In the case of high plasticity materials--OLC45--the grinded surface asperities remove continuously particles from the test sample, regenerating immediately the lubricating film.

4. CONCLUSION

Comparing Ra roughness of the lubricating solid film generated on each of the three base materials, we observe that the roughness of the film laid-down on FcX250 is smaller than the ones laid-down on FcA2 or OLC45 base materials (fig. 6.).

Analysing the Rz roughness of the base material relative to the lubricating solid film Rz roughness, we observe that in all three tested situations, Rz roughness of the film is smaller than metallic base material roughness (fig. 7.).

As a conclusion, it can be sustained that the self lubricating film decreases the roughness of the sliding surface.

5. REFERENCES

Bloch, H. & Hoefner, J. (1996). Reciprocating Compressors: Operation and Maintenance, Gulf Publishing Company, Book Division, ISBN:0-88415-525-0, USA, TX, Houston

Burris, D. & Sawyer, G. (2006). A low friction and ultra low wear rate PEEK/PTFE composite, Elsevier B.V., Department of Mechanical and Aerospace Engineering, University of Florida, Gainesville, FL 32611, U.S., Available from: http://nersp.nerdc.ufl.edu/~wsawyer/ wearMaterialX2006.PDF Accessed: 2007-02-15

Chesa, I.; Lascu, N. S. & Nedelcu, C. (1984). Alegerea si utilizarea otelurilor, Ed. Tehnica, Romania, Bucharest

Cozma, R. (2004). Elemente fundamentale de tribologie, frecare-ungere, Ed. Matrix Rom, ISBN 973-685-861-8, Romania, Bucharest

Mccook, N.L.; Hamilton, M.A.; Burris, D.L. & Sawyer, W.G. (2006). Tribological results of PEEK nanocomposites in dry sliding against 440C in various gas environments, Elsevier B.V., Department of Mechanical and Aerospace Engineering, University of Florida, Gainesville, FL 32611, United States, Available from: http://nersp.nerdc.ufl.edu/ ~wsawyer/PeekWear2007.PDF Accessed: 2008-06-20

Stachowiak, G.W. & Batchelor, A.W. (2001). Engineering Tribology, Second Edition, Butterworth-Heinemann Publications, ISBN 0-7506-7304-4, USA, Woburn