TRIBOLOGICAL BEHAVIOUR OF SYSTEMS WITH TWO THERMAL SPRAYED COATINGS R. POLAK Techn.Büro Dr.Reinhard Polak, Franzensbrückenstr.3, A-1020 Vienna, AUSTRIA; e-mail: r.polak@aon.at M.KIRCHGAßNER Castolin Ges.m.b.H, Brunnerstraße 69, A-1230 Vienna, AUSTRIA; e-mail: mkirchgassner@castolin.at E.STERGAR VA-Bergtechnik, A-8740 Zeltweg, AUSTRIA; e-mail: IWSS.Stergar@tor.twin.at SUMMARY Surface structures, typically formed by thermal spraying, can resist well in sliding regimes like rotary seals, where coatings are applied on both friction partners. Provided materials with good sliding properties and appropriate hardness are applied by thermal spray processes, which give a sufficient bond strength between the inter-structural lamellas, this work confirms their good characteristic in lubricated system. When identical coatings are mated in such regimes the best friction character were is found for carbide layers. Seals using different material applied with others than high-energy spray processes performed less good under conditions tested.

Keywords: friction, sliding, coating, thermal spraying, seal

1 INTRODUCTION Arising from problems with structure hardened rings used in mechanical face seals the objective of this work is to investigate the appropriateness of coatings for this application. Increasing the lifetime in such tribo-systems under extreme conditions, as for instance in the mining industry, is the most vital interest.

Thermal spraying produces coatings, which are well suited for friction. They often show well-balanced tribological properties in field applications. As of their flexibility in applying different coating materials the processes of thermal spraying are very common for designing surfaces with hard and anti-abrasive character. Various literatures deal with wear and/or friction related properties [1-6]

Although sliding regimes in tribosystems typically have only one sprayed coating staying in contact with a bulk metallic counterpart (e.g. steel), the actual work aims to mate two sprayed coatings in mechanical seals. Therefore the behaviour of two coatings in frictional contact was studied to understand, how overlays applied by thermal spraying perform in a sliding environment as anticipated.

Ring shaped specimens were coated and exposed to tests cycles in standard ring-ring friction units for their tribological characterisation. Instead of other lubricants only natural water was used. The most appropriate mating couples had to pass successfully laboratory tests, before implementing them in field machines.

2 COATINGS Thermal sprayed coatings, known for their general good sliding properties, were selected for the mating couples. With the background to withstand in abrasive environment, typical for mining applications, more preference was given to very hard layers.

2.1 Deposit materials The method of high velocity, plasma and arc spraying was taken for applying carbide, oxide and metallic coatings.

Powders fractions of -45µm/+15µm were used for the HVOF1 and Plasma-process. Four standard powders were selected for surfacing the substrates with the HVOF gun.

HVOF coatings: type material deposit H1 WC-12Co H2 Cr3C2-25NiCr H3 WC-10Co-4Cr H4 Ni15CrBSi

Good wear resistance in dry sliding regimes at very low sliding speed, is reported for coating type H1 and H2 [2], also under presence of abrasives. In lubricated systems such coatings are discussed as favourite candidates in replacing hard chrome plating. H3 is often used when wear and corrosion resistance is required simultaneously [4]. Low friction coefficients in wet regimes are stated for coatings of type H4 [2].

Known for good hardness and friction behaviour in lubricated systems, a chromium oxide layer was added for testing. This material was applied by atmospheric plasma spraying.

Plasma coating: type material deposit P1 Cr2O3

Two further materials applicable with the more efficient arc spraying were selected also. Arc spray deposits are formed of particles, which result from atomising molten material at the tips of wires, heated by an electrical arc burning between them.

1 HVOF = high velocity oxygen fuel

Arc- coatings: type material deposit A1 Mo A2 Fe30CrBSi

Beside its good friction properties Mo has high inter-lamella and substrate bond strength. High hardness and excellent abrasion resistance was the reason for selecting the chromium-steel material.

2.2 Coating combinations For studying the appropriateness of thermal sprayed coatings under sliding conditions, the following layers were mated for testing.

Couple Combination Materials CP1 H1 / H1 WC/Co – WC/Co CP2 H2 / H2 Cr3C2/NiCr - Cr3C2/NiCr CP3 A1 / H3 Mo – WC/CoCr CP4 H3 / A2 WC/CoCr - FeCrBSi CP5 H3 / P1 WC/CoCr - Cr2O3 CP6 H3 / H4 WC/CoCr - NiCrBSi CP7 H3 / H2 WC/CoCr - Cr3C2/NiCr CP8 A1 / A1 Mo - Mo

Substrate rings of carbon steel were coated in a thickness of 200 – 300 µm with the HVOF and plasma process, 500 µm with the arc-spray process.

3 TEST RESULTS

Figure 1: Test principle

A first characterisation of the sliding properties was done in a laboratory friction tester, operating the chosen couples under defined rotation speed and load force. Although speed and load force did not reach the values in real application, the tests allowed a selection of coatings.

Only coatings of couples passing the friction test successfully were taken for overlaying seal rings, to run in a ring-tester under conditions comparable to that in mining machines.

All tests were made at room temperature. Any friction induced heat input could be measured during the test cycles.

3.1 Preliminary characterising tests All rings were run-in for 3 hours with a load force of 600 N and a sliding velocity of 0,2 m/s.

For the selective tests all couples had to pass cycles of 20 min each with similar speed values, starting with a load force of 600 N (pressure p = 0,3 N/mm²). The load force then was increased in steps of 200 N unless serious irregularities were observed. A maximum of 1600 N (p = 0,8 N/mm²) was defined for best performing couples.

3.1.1 Dry sliding test Pre-selective tests with coating type H1 were performed using couple CP1 in dry contact. Some investigations state good performance for WC/Co in dry friction regimes [1, 3].

Testing conditions for CP1 were a sliding velocity of 0,13 m/s and a specific test load of 0,1 N/mm². In begin the friction coefficient was with f = 0,2 rather low, but increased during the test time to 0,6. Consequently the system temperature raised up to 150°C. Similar results were achieved passing a second test run with a sliding speed of 0,16 m/s.

Chattering effects producing unstable conditions in the system were recognised when rising the specific load beyond 0,2 N/mm². Consequently the decision was taken to focus tests an a lubricated system.

3.1.2 Sliding tests with water Seal rings in field conditions are typically sliding without any particular lubrication, but environmental mining water and oil is penetrating into the system.

All further characterisation tests were made in a water bath, simulating the wet environment in the real application. To avoid hydrodynamic lubrication effects, which would contradict the real conditions, a limitation of the sliding velocity was done.

The following test parameters were defined because of their appropriateness to reality:

sliding speed: 0,8 m/s specific load: 0,3 N/mm² gradually raised to

0,8 N/mm² environment: water temperature: 20°C (+5°C)

Tests were suspended at higher load forces for those couples, which did not perform in the anticipated way.

Best performing couple under the specified tribologic conditions is CP2. Cr3C2-NiCr coatings, sliding in direct contact, show a low friction coefficient f < 0,10 with only little influence due to increase of pressure. Visually the sliding areas reveal smoothing effects with very little wear. The remarkable decrease of friction, even at the highest load force, is interpreted with the appearance of hydrodynamic effects.

The WC/Co couple CP1 is only slightly disadvan-tageous compared with CP2. Its friction coefficient increases from f = 0,12 to 0,15 in line with the rising load force. A very smooth surface in the contact areas without any aggravating wear is detected, which corresponds with very stable sliding properties.

0,00

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400 600 800 1000 1200 1400 1600 1800Prüfkraft Fn [N]

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]20110/1-20100/220650/1-20650/220300/2-MO20300/3-95MXC20300/4-2040020300/5-Cr2O3/120300/1-Cr2O3/220300/6-20650/3Mo-Mo

Figure 2: Coefficient of friction for different couples

and load forces

All other couples with dissimilar coating materials do not reach the low friction of CP2 or CP1. For samples CP3 (Mo - WC/CoCr) and CP5 higher levels of friction coefficients are measured showing a strong correlation with the specific pressure. Unusual deteriorating sliding effect is observed for couple CP3 (Mo-WC/CoCr) at load levels of 0,5 N/mm². Chromium oxide commonly known for good sliding properties in engineering applications confirms in the test a good stability with very low chattering tendency, but gives typically higher friction coefficients.

3.2 Face seal simulator tests Coatings of type H2 and A1 were applied to face seal rings, to compare them with the standard mechanical seal, made of steel rings (material 100Cr6) with a hardness of 52 HRC. Therefore couples of thermally sprayed rings H2/H2 and A1/A1 were tested against standard rings under identical, field near conditions. Prior to installation all rings have been finished according to surface specifications.

Figure 3: Wear and scoring effects for A1 coatings

Each pair of rings were foreseen to pass operating cycles of 350 hours with a sliding speed v = 3,25 m/s and a specific pressure of 0,5 N/mm² with water used as lubricant in the tribologic regime. During the whole test cycle the system temperature was recorded.

Both sprayed couples have finished the full test cycle without any irregularities, whereas the standard steel rings failed after 21 hours due to seizure.

A visual inspection of the contact areas after 350 hours test cycle revealed more wear intensity and scoring effects for couple A1/A1 (see Fig.3). Cross-sections, subsequently made, confirmed for the Mo-layers a remarkable wear and coating degradation. In some areas of the contact zone almost the entire coating thickness was abraded. Segregation effects for single particles in the contact surface were identified as major superficial wear phenomena by SEM studies (Fig.4). Under operational test conditions a weakening of the cohesion forces between the structural lamellas resumed in a finally spalling of individual spray particles

Figure 4: Segregation of particles and scoring effect in

the contact area of A1 coatings

Similar wear phenomena as found for arc-sprayed molybdenum were not observed on chromium carbide coatings. They are more resistant. No other effects than surface smoothening are observed in the contact areas.

The denser structure of coating type H2, the higher hardness and better sliding property can withstand in the test regime more favourable. Superficial segregation tendencies can’t be perceived under the microscope.

3.3 Field tests Based on the findings of this study a pair of H2 coated rings with a diameter of 600 mm was assembled in a mechanical seal on a very critical position of a mining machine. A second seal with a couple of Mo-coated rings was installed for comparison. Fig. 5 reveals the constructive arrangement of the rings in the seal.

CP1

CP2

CP3

CP5 CP6

CP4

CP7

CP8

Figure 5: Seal and O-ring arrangement in real test

After working for 6 month in real environment with sliding velocities and pressures comparable to the simulation test both seals were dismounted for inspection.

Visually appeared the contact area of rings with Cr3C2-NiCr very smooth with minor scoring effect in the direction of motion. Both rings overlaid with Mo could not resist and all of the coating was worn off (Fig 6).

Figure 6: Rings coated with Mo compared with those

overlaid with H2 after field test

Cuts made for H2 rings confirmed their good visual results under the microscope. The principle findings of prior simulation results are confirmed and a good aptitude for the particular friction regime is verified. A total thickness loss of only 35 µm was measured for the Cr3C2-NiCr coating.

4 INTERPRETATION High velocity spraying has proved to produce deposits with strong inter-structural binding mechanisms between individual lamellae. Moving two thermally sprayed surfaces in a friction regime against each other needs lubrication to prevent chattering effects.

Cr3C2-NiCr powders thermally sprayed with HVOF produce a two-phase structure, which is characterised by hard particles distributed in a softer matrix. Stable

sliding properties can be achieved with a minimum of lubrication. High inter-particle cohesion strengths as typical for HVOF deposits prevent the mechanism of segregation of single particles in the contact surface. Those effects immediately deteriorate the sliding character and increase remarkable the wear phenomena, as seen for arc-spray coatings.

WC-Co layers give similar results with slightly higher coefficient of friction than Cr3C2-NiCr. In common the bond strength measured for thermal sprayed WC-Co coatings is superior to values for chromium carbide layers. Such tungsten carbide can be anticipated to be also a candidate in comparable regimes, even it was not tested in field near conditions.

All other couples mated for testing did not give sufficient results. Coating structures and/or inherent particle cohesion did not allow a good track record in the sliding regime tested.

5 SUMMARY It is shown that thermal spray coatings perform much better than structural hardened steel in sliding regimes as represented by face seals. When mating two coatings in direct frictional contact the major success criteria are the coating morphology, a sufficient internal cohesion and a minimum of lubrication. Otherwise functional and stable systems are not guaranteed. Especially for high velocity spray deposits with hard metallic carbides dispersed in a softer matrix an excellent friction character is found.

No good wear resistance is perceived when coatings with low structural cohesion are mated in such systems, regardless of their common frictional behaviour. The tendency of segregating individual spray particles between the surfaces in motion leads to accelerated wear phenomena.

6 REFERENCES [1] Antoszewski, B. et al: Development of scuffing phenomena in detonation sprayed coatings; UTSC ’99; 17 –19th May 99, Düsseldorf, p.627 [2] Steffens, H.D et al: Low friction Coating for Lubricant Free use in Rail Points; 8th NTSC; 11 – 15th Sept. 95, Ohio, p 677 [3] Jia,K., Fischer,T.E.: Sliding wear of conventional and nanostructured cemented carbides; Wear 203-204 (1997) 310-318 [4] Berger L.M., Vuoristo, P.: Microstructure and properties of WC-Co-Cr Coatings; Practical Solutions for Engineering Problems C.C.Berndt; ASM Int. Ohio,1996 [5] Berger L.M., Vuoristo, P., Lengauer,W.: Structure, Properties and Potentials of WC-Co, Cr3C2/NiCr and TiC-Ni-Based Hardmetal-like Coatings; Practical Solutions for Engineering Problems C.C.Berndt; ASM Int. Ohio,1996 [6] Hajashi, H. et al: Microstructure and properties of tungsten carbide coatings sprayed with various HVOF spray systems; 15th ITSC; 25-29th May 1998