REDUCTION OF FRICTION AND WEAR OF AUTO- MOTIVE COMPONENTS BY GALVANIC COATINGS

Transcription

1 REDUCTION OF FRICTION AND WEAR OF AUTO- MOTIVE COMPONENTS BY GALVANIC COATINGS W. SIEGERT Collini Skolnik GmbH, Lohnergasse 4, 1210 Vienna, AUSTRIA; R. PASCHITZKY The Austrian Tribology Society, Floragasse 7/2, 1040 Vienna, AUSTRIA; SUMMARY Different materials were investigated in oscillating, sliding motion offering potential tribological improvements and technological innovations for automotive components. A well proven test rig (TOG apparatus) was chosen to determine basic tribological and technical parameters. The following galvanic coatings were investigated: Glatox, Triflon, Tricoat and hard chrome. The combinations Triflon, Ck 45 and Tricoat on Glatox and Tricoat and Ck 45 on Tricoat showed good performance. They offer a potential for new possibilities in sliding guidances design in general and in engine and gearing design in particular. Presumably some manufacturing processes can be substituted or made more effective. However, the results encourage further investigations. Keywords: corrosion resistance, wear protection, surface treatments, aluminium oxide layers, dispersion coatings, surface characteristics, wear, friction 1 INTRODUCTION Limited metal resources and the need of energy saving led to use of light weighted materials in automotive design. The extensive use of aluminium and magnesium started with the development of energy saving "two litre cars". Aluminium and magnesium offer outstanding advantages for light weight components. Availability is high, mechanical processing as well as recycling methods are well known. They have, however, to provide the same mechanical properties as conventional materials and to be resistant to corrosion and wear. Due to their nature- they oxidise easily, magnesium cast is porous- special surface treatments is necessary to achieve the demanded quality. Recently developed processes allow to control roughness of aluminium oxide layers on aluminium alloys by process parameters. Further properties of those coatings are electrical insulation, high hardnesses and outstanding tribological qualities. Dispersion coatings containing PTFE as dry lubricant in a hard nickel-phosphorus matrix provide wear resistance and excellent tribological characteristics. Special pretreatment is necessary to obtain the necessary adhesion of the deposit on the base metal. The aim of this work is general comparison of various surface treatments for specific automotive applications focusing on aluminium. component kind of wear type of motion contact brake piston adhesion/abrasive/corrosion oscillating surface shock absorber adhesion/abrasive/corrosion oscillating surface/line motor valves adhesion oscillating surface clutch pistons adhesion oscillating surface/line piston cylinder adhesion/abrasion oscillating surface/line Table 1: Examples of automotive applications materials used for time static components bearing bushes oscillating shaft (rings) cast iron, steel sintered steel/cu-alloys hard chrome plated steel ?? aluminium alloys sintered steel/cu-alloys hard chrome plated steel 2004-?? Al-, Mg-alloys directly plated (anodised) "Functional Surfaces Table 2: History of automotive tribological applications

2 2 EXPERIMENTAL Investigated in this work are linear oscillating motions at moderate parameters. Wear mechanisms are to be expected as complex systems of abrasion-, adhesive wear and corrosion wear. The large number of rotating automotive components is not looked at in this paper. Their tribological parameters are completely different. 2.1 Test rig The TOG test rig ("translatoric oscillating apparatus) was used, modelling the kinematics of sliding automotive components as close as possible. To determine the wear were used: a high precision scale for specimen blocs a profilometer for specimen plates The pyramidical shape of blocs and the resulting declining specific load was taken into account. Figure 2: Geometry of specimen blocks Figure 3: Typical wear profile on plate Figure 1: Principle of test rig contact situation: plain-plain; nominal contact surface at test start: 3 x 8 mm specific loads p: 1,6 N/mm², 3 N/mm², 20 N/mm² (nominal) environment temperature T 1 : 22 ± 2 C; T 2 : 100 ± 10 C environment humidity: % kinematics: translatoric oscillating, amplitude 10 mm frequency f: approx. 1 Hz average speed v: 2 m/s cycles: 2000 interface: technical dry (no lubricant except the PTFE incorporated in Triflon ) Table 3: Test parameters 2.2 Description of Layers In commonly used galvanic coating processes, higher plating thicknesses at edges (higher current densities) occur in general. This makes manufacturing more complicated and therefore expensive Hard chrome Since the beginning of manufacturing of combustion engines hard chrome has been used. Finish (grinding), however, is necessary to obtain final tolerances and required mechanical properties. Another disadvantage are micro cracks in the surface causing corrosion and possibly uneven wear. For comparison, hard chrome coatings have been investigated too Glatox Glatox is a galvanic aluminiumoxide coating with excellent properties for machine components: tolerances are very accurate making mechanical finish (honing, grinding, polishing,...) unnecessary. Pore size and roughness- which tribological and corrosion properties depend on- can be controlled by process parameters.

3 Electrical deposition of the layer starts in areas of high current density (edges etc.). With the increasing thickness of the deposited electrical insulating aluminium oxide layer, the plating process shifts to areas of now lower electrical resistance and therefore higher electrochemical activity. Thus the thickness of the layer grows very steadily, building up volume. Al 2 O 3 Al Figure 4: Layer built up from original surface Figure 6: Comparison: hard chrome / Tricoat on a valve lifter Figure 5: Hexagonal Structure of eloxal cell: pores nm const; d: cell size surface property: Gmat.R max. + 0,5-1 µm thickness Up to 200 µm (Al Alloys) 70 µm (GD highly alloyed) thickness built up: 50 % tolerances ± 3 % (depending on alloy) hardness up to 500 HV o,o5 corrosion Resistance minimum 1000 hours (20µm) DIN NSS electrical Insulation max. 35 V/µm specific heat elongation 20 % of base material. temperature resistance up to 1950 C at short burden colour depend on alloy (greenbrown-grey-black) structure micro porous (can be coloured or impregnated) Table 4: Properties of Glatox Triflon Triflon is deposited electroless from aqueous nickel solutions. The process itself is well known within the metal finishing branch. But in the automotive and precision mechanical industries this technology and its advantages in tribological aspects are not well known. The following should make it more popular. The solution mainly consists of nickel sulphate, sodium hypophosphite as catalyst and organic / inorganic stabilisers. The reaction produces a plain parallel nickelphosphor alloy deposit whose outstanding properties are wear resistance, hardness and corrosion resistance. A big advantage is the possibility to coat complex shaped structures (e. g. blind holes). No mechanical finish is necessary as usual in the past. Special processes allow NiP coatings containing PTFE particles to be used as tribological surfaces. Those coatings combine the advantages of the wear resistance of a NiP matrix with the lubrication properties of PTFE. When working, the PTFE contained in the matrix builds up a lubricating layer on the surface, offering low friction at moderate loads. specific weight 6,5-7,5 g/cm³ hardness without tempering HV after tempering at 250 C, 4h HV corrosion resistance ( DIN SS) 12 µm roughness Ra 1µm < 480 h 25 µm roughness Ra 1µm < 960 h critical load of acoustic scratch test 3 N Table 5: Properties of Triflon

4 specific weight 7-8 g/cm³ thickness 20 µm (+5 µm interface) hardness KHN (dep. on heat treatment) critical load of acoustic > 25 N scratch test Table 6: Properties of Tricoat Figure 7: Triflon is a NiP -dispersion coating containing PTFE dispersed in the nickelmatrix Tricoat Tricoat is a electroless (autocatalytic-chemical) coating containing high percentage of cobalt. The advantages are wear resistance (at higher loads compared with Triflon ) and corrosion resistance. According to made experiences smooth, noiseless sliding can be expected. Tricoat has been introduced to several applications in the US market, for example as plating of compressor components. Figure 8: Example: Tricoat coatings on Al-parts of a compressor [7] The technology has been introduced by Collini in Europe during the past year and has met good acceptance within industry. A couple of applications is just before kick-off. Figure 9: Taber wear index depending on alloy and temperature; alloy composition: Co %; P 4-6 %; Ni rest; 3 RESULTS In a larger number of tests the test plates coated with Glatox and Tricoat showed the best results demonstrated here. The specimen blocks (base metal: Ck 45) had coatings of Glatox, Tricoat and hard chrome and the original surface. 3.1 Plate coating Glatox The result being discussed in the following are valid for environment temperatures of 22 C. The coefficient of friction of Triflon specimen blocks on the Glatox plate is excellent low, slowly increasing and almost constant as long as load stays moderate. Tricoat blocks on the Glatox plate show low wear while friction is high but almost constant. Lubricants could reduce the coefficient of friction. Remarkable results can be observed with Ck 45 specimen on the Glatox plate. The coefficient of friction is not as low as the results of Triflon, but wear, both of the plate as well as of the blocks, is very low. Wear stays low even when increasing the load. Again suitable lubricants could reduce friction. A large number of new automotive applications can be imagined for all of these material combinations. Commonly used hard chrome failed in combination with Glatox. The results, however, encourage further investigations.

5 loss of mass [mg] 2 18,0 16,0 14,0 12,0 1 8,0 6,0 4,0 1,6 N/mm² 3 N/mm² mass of coating+interface Coefficient Reibzahl of [-] friction time Zeit [s] Figure 12: : Tricoat on Glatox (load 1,6 N/mm²) average depth of wear track on plate [µm] 2, Triflon Tricoat Ck 45 hard chrome plate coating Glatox thickness of the plate coating -90 1,6 N/mm² 3,0 N/mm² -100 Figure 10: Loss of mass of the specimen blocks; depth of wear track on specimen plate coefficient of friction f Figure 13: : Ck 45 on Glatox (load 1,6 N/mm²) 0,9 0,7 0,5 0,3 0,1 failure: B: ca Triflon 1,6_N/mm² failure: P: ca ; K: ca Al - Ck45 Al - Triflon Triflon 3,0_N/mm² Tricoat 1,6_N/mm² Tricoat 3,0_N/mm² Ck45 1,6_N/mm² Ck45 3,0_N/mm² Glatox - Ck45 failure: B: ca. 40 har chrome 1,6_N/mm² Al - hard chrome failure: P: ca. 40 hard chrome 3,0_N/mm² Figure 14: Range of coefficients of friction on Glatox Figure 11: : Triflon on Glatox (load 1,6 N/mm²) 3.2 Plate coating Tricoat The result being discussed in the following are valid for environment temperatures of 22 C. Tricoat blocks on the Tricoat plate show low wear at moderate loads while friction is high but almost constant. Suitable lubricants could reduce the coefficient of friction and make accessible a large territory of new automotive applications profiting from the excellent wear properties. To estimate the possibilities of this material further investigation is necessary. Ck 45 specimen on the Tricoat plate show good results, considering wear, as well. Friction, however, is even higher than friction of Tricoat on Tricoat. The stated above is applicable here too. In general, loads of 20 N/mm² are definitely too high for all coatings investigated here: wear increases and the surface is being destroyed.

6 loss of mass [mg] average depth of wear track on plate [µm] 2 18,0 16,0 14,0 12,0 1 8,0 6,0 4,0 2, N/mm² 20 N/mm² mass of coating+interface Triflon Tricoat Ck 45 hard chrome 3,0 N/mm² 2 N/mm² plate coating Tricoat thickness of the plate coating (left column: coating; right column: coating + interface) Figure 15: : Loss of mass of the specimen blocks; depth of wear track on specimen plate Figure 16: Tricoat on Tricoat (load 3 N/mm²) Figure 16: Ck 45 on Tricoat (load 3 N/mm²) coefficient of frictionf 0,9 0,7 0,5 0,3 0,1 Tricoat - Ck45 Tricoat - Interface(Triflon) Tricoat - Triflon Triflon 3,0_N/mm² Triflon 2_N/mm² Tricoat 3,0_N/mm² > ; grooves failure: P: ca Tricoat 2_N/mm² Ck45 3,0_N/mm² Ck45 2_N/mm² > ; grooves > ; grooves hard chrome 3,0_N/mm² failure: P: ca ; B hard chrome 2_N/mm² Figure 17: Range of coefficient of friction on Tricoat 4 CONCLUSIONS The industrial galvanic surface coatings Glatox and Triflon can be adopted for a number of new tribological applications reducing wear and friction. Especially automotive components could benefit from the outstanding properties. The recently introduced electroless nickel coating Tricoat shows low wear. This process bears the potential to substitute technologies like nitrideing, carbon hardening or hard chrome coating. The results encourage further tribological investigations. 5 REFERENCES [1] Hutchings, I. M., Tribology: Friction and Wear of Engineering Materials. Pub. Edward Arnold, London, 1992, P [2] Franek, F., Braunstorfer E., Tribologische Eigenschaften von Chemisch Nickel Schichten, Bericht, Österreichische Tribologische Gesellschaft, Wien, 1994, P.12 [3] Weiner, R., Die Galvanische Verchromung, Eugen G. Leuze Verlag, Saalgau, 1974 [4] Keszthelyi, Z., Polster K., Eigenschaften und Anwendungen der verschleißfesten Triflon- und Glatoxschichten, ÖTG Symposium, Österreichische Tribologische Gesellschaft, Wien 1999, [5] Mallory, G.O., Haydu J.B. (Eds.) Electroless Plating: Fundamentals and Applications, American Electroplaters and Surface Finishers Society, Orlando, 1990, P. 111 ff. [6] Fischer, C.T., Siegert W., Verschleiß- und Gleitverhalten von NiP-, Hartchrom- und NiP/PTFE- Schichten, Metalloberfläche, Carl Hanser Verlag, München 12/94, 48. Jahrgang [7] Zitko, M.W., Electroless Nickel-Cobalt-Phosphorus, A New Wear Resistant Coating, AESF (American Electroplating and Surface Finishing Society), Sur-Fin 1996, Cleveland, Ohio