PERFORMANCE OF THERMAL SPRAY COATINGS UNDER DRY ABRASIVE WEAR CONDITIONS

Transcription

1 published at the THE Coatings 2004, Erlangen, D, April 5 th - 7 th 2004 PERFORMANCE OF THERMAL SPRAY COATINGS UNDER DRY ABRASIVE WEAR CONDITIONS A. Wank, B. Wielage, G. Reisel, T. Grund, E. Friesen Institute of Composite Materials, Chemnitz University of Technology, GERMANY Abstract The performance under dry abrasive wear conditions is studied for HVOF sprayed cermet coatings in comparison to APS Cr 2 O 3 and cermet as well as electroplated hard chromium coatings in Taber Abraser tests. Investigations on the wear behavior depending on the use of different abrasive wheels as well as on the influence of hard phase size, microhardness and chemical composition of the metallic matrix alloy in HVOF sprayed cermet coatings are carried out. In general cermet coatings show superior wear resistance compared to APS Cr 2 O 3 and electroplated hard chromium coatings in Taber Abraser tests with H-22 abrasive wheels. The chemical composition of the metallic matrix of HVOF cermet coatings proves to be decisive concerning optimized wear resistance. The size of hard phases shows only minor influence. Coatings produced with coarse carbides make the highest wear resistance accessible, but the wear behavior depends much more severely on an optimized processing compared to coatings with fine carbides. Keywords: thermal spraying, cermets, Taber Abraser wear test 1 Introduction Besides corrosion protection the main application field of thermal spray coatings is wear protection. Such protective coatings are applied for original part production as well as for restoration purposes. Already several decades ago self fluxing NiCrBSi coatings produced by powder flame spraying or wire flame sprayed molybdenum coatings have been introduced in many fields of industries. Due to the extremely high processing temperatures atmospheric plasma spraying (APS) is capable of refractory oxide ceramic coating production. Commonly applied coatings based on Cr 2 O 3, Al 2 O 3, TiO 2 and their mixtures show very high hardness. Due to very high gas jet and therefore also very high particle velocities the high velocity oxy

2 fuel (HVOF) spraying process, which was introduced in industries in the early 80 s of the past century permits manufacturing of cermet coatings featuring nearly theoretical density. By use of advanced spraying guns, especially liquid fuel systems, melting of the cermet powders can be avoided. Thereby produced WC-Co based coatings show only negligible content of brittle mixed carbide phases. For some wear stress conditions increasing resistance with decreasing size of hard phases in cermets containing equal metallic binder content has been proved. For example in ASTM B-611 tests a decrease by one order of magnitude is observed for reduction of the average WC size from 5.1 µm down to 0.6 µm [1]. Also for sliding and abrasion wear conditions improved resistance with decreasing carbide size is reported for APS WC-Co [2] and HVOF WC-Co or Cr 3 C 2 -NiCr coatings [3] respectively. Contrarily in erosion wear tests applying coarse abrasive particles impinging at an angle of 30 relative to the surface plane nanostructured Ni/Al 2 O 3 composite coatings do not provide advantages in comparison to mild steel. In frequently used ASTM G65 tests using corundum or quartz abrasives with different particle size fractions even increased wear rate is observed for HVOF WC-Co coatings containing fine carbides compared to coatings with standard carbide size [5]. As wear resistance is no material property but depends on the tribological system, the development of optimized protective coatings requires knowledge concerning the integral stress conditions (wear mechanism, load, environment, ) and use of according wear tests. This research work aims for the development of optimized wear protective coatings for components subject to dry abrasive wear. 2 Experimental Dry abrasive wear stress is applied on the tested coatings in Taber Abraser tests corresponding to ASTM F1978 [6]. Pre-investigations on the influence of the used abrasive wheels applying the maximum load of 10 N per wheel show that H-10 (d 50 = 75 µm, SiC, Al 2 O 3 based ceramic binder) and S-35 (sintered hard metal, 1626 ± 35 HV30, inclined notches) wheels cause negligible linear wear rate of HVOF WC-Co coatings [7]. Use of abrasive wheels H-22 (d 50 = 165 µm, SiC, Al 2 O 3 based ceramic binder) results in significantly stronger wear. In order to permit detection of microstructural influences all further tests are conducted using H-22 wheels. The tests are carried out at ambient temperature of 25 C and humidity of % applying the maximum load of 1000 g per abrasive wheel. The coatings are tested 50,000 cycles or up to reaching the minimum permissible wheel radius. All coatings are tested with polished surfaces state exclusively. The mass loss is determined in several intervals by a laboratory scale with an accuracy of ± 10-3 g. Average values of three weight measurements are recorded.

3 HVOF WC-CoCr and 75 Cr 3 C 2-25 Ni20Cr coatings are manufactured by use of the liquid fuel spraying gun Tafa JP5000. For comparison additional WC-CoCr coatings produced by another liquid fuel HVOF gun (Thermico CJS) and by the triple cathode plasma torch Northwest Mettech Axial III are tested. All cermet coatings are manufactured using spray dried and sintered powders. APS Cr 2 O 3 coatings are sprayed with GTV F6 torch. Finally industrial standard electroplated hard chromium coatings are tested. The influence of microstructural features on the wear resistance is studied by use of WC-CoCr feedstock with fine (FC, d 50 = 0.8 µm), standard (SC, d 50 = 2 µm) and coarse carbides (CC, d 50 = 5 µm) at the example of HVOF coatings produced by Tafa JP5000 gun. The composite powder size fraction is 20 µm < d < 45 µm. Additionally for these three different powders the influence of the coatings microhardness, which is an integral feature comprising phase contents, sizes and distribution as well as residual stresses and porosity, is analyzed. Microhardness values of the tested wear protective coatings are comprised in Tab. 1. Results of detailed microstructural investigations by means of optical and electron microscopy, EDXS and XRD as well as thermal properties (thermal diffusivity, thermal expansion behavior and thermal contact resistance) are published elsewhere [8]. Tab. 1: Microhardness of tested coatings coating material production method microhardness HV0.3 WC-CoCr , FC s HVOF Tafa JP WC-CoCr , FC m HVOF Tafa JP5000 1,080 WC-CoCr , FC h HVOF Tafa JP5000 1,200 WC-CoCr , SC s HVOF Tafa JP WC-CoCr , SC m HVOF Tafa JP5000 1,180 WC-CoCr , SC h HVOF Tafa JP5000 1,420 WC-CoCr , CC s HVOF Tafa JP WC-CoCr , CC m HVOF Tafa JP5000 1,160 WC-CoCr , CC h HVOF Tafa JP5000 1,330 WC-CoCr , NC HVOF Thermico CJS 1,040 WC-CoCr , NC APS Mettech Axial III 1,380 WC-Co HVOF Tafa JP5000 1,130 (HV1.0) 75 Cr 3 C 2-25 Ni20Cr HVOF Tafa JP5000 1,170 Cr 2 O 3 APS GTV F6 1,500 hard chromium electroplating Results Even the use of H-22 abrasive wheels causes only extremely low mass loss for some types of WC-CoCr coatings. Therefore specimen weighing needs to be conducted very carefully. The mass loss within the first 2,000 cycles can roughly correspond to the weight

4 gain by water absorption of specimens, which are exposed to 65 % air humidity after storage in an exicator. 3.1 Influence of hard phase size and microhardness Generally the tested specimens show a relatively high initial mass loss followed by a linear wear characteristic as a first order approximation. The wear behavior depending on the average carbide size of HVOF WC-CoCr coatings with the highest obtained microhardness is shown in Fig. 1. Especially the mass loss curves of coatings containing fine or coarse carbides are subject to only small deviations. Both initial mass loss and linear wear rate show increasing tendency with decreasing carbide size. Coarse carbide coatings show a linear wear rate of g/cycle, while for coatings produced with standard or fine carbide powders the linear wear rates amounts to g/cycle and g/cycle respectively. Fig. 1: Wear behavior of WC-CoCr coatings with optimized microhardness depending on the average carbide size of the applied feedstock powder The use of powders containing coarse carbides permits production of coatings exhibiting the best protective function against dry abrasive wear. However, the performance of coatings with coarse carbides depends much stronger on an optimized processing in comparison to coatings containing fine carbides (Fig. 2). The linear wear rate of soft coatings is nearly two orders of magnitude higher compared to the hard ones. In contrast for FC coatings only an increase by factor 2 is observed. The low linear wear rate of a single tested soft SC coating is regarded unreliable.

5 Fig. 2: Linear wear rate of HVOF WC-CoCr coatings depending on carbide size and microhardness 3.2 Influence of spraying equipment As each spraying process and even each spraying gun has its own characteristics powder feedstock is developed specifically and so it is not sensible to compare coatings produced with the same powder in order to evaluate the capability of spraying equipment. Fig. 3 shows the wear behavior of SC coatings with optimized microhardness produced by Tafa JP5000 gun in comparison to coatings with the same chemical composition produced by another liquid fuel HVOF spraying gun (Thermico CJS) and the triple cathode plasma torch Northwest Mettech Axial III. Powders, which have been optimized for each gun, and industrial standard parameters are used. The HVOF coatings produced with the two different spraying guns show comparable wear behavior. While the initial mass loss of Thermico CJS coatings is a little stronger, their linear wear rate ( g/cycle) is lower in comparison to coatings produced by Tafa JP5000 ( g/cycle). The initial mass loss of the plasma sprayed coatings scatters significantly stronger than for the HVOF coatings. For one specimen it is comparable to the best performing HVOF coating and for the other one the worst value of all considered specimens in this comparison is observed. The average linear wear rate ( g/cycle) is the same like for Tafa JP5000 coatings.

6 Fig. 3: Wear behavior of WC-CoCr coatings produced by different spraying guns 3.3 Influence of coating type When comparing the wear performance of the WC based cermet coatings to HVOF 75 Cr 3 C 2-25 Ni20Cr, APS Cr 2 O 3 and electroplated hard chromium coatings, the strong differences in density need to be taken into account, as the volumetric wear loss is the relevant criterion. As the coatings density is not easily accessible the theoretical density of the coating materials is considered. This procedure appears permissible, as for the used materials and processes no substantial change of the chemical composition of spraying materials during the spraying process is observed. However, for APS Cr 2 O 3 coatings neglecting porosity may result in slightly too high volumetric wear values. The results of the volume losses depending on the amount of wear cycles are shown in Fig. 4. Generally the resistance of cermet coatings to dry abrasive wear is superior to that of APS Cr 2 O 3 and electroplated hard chromium coatings. While HVOF WC-Co and 75 Cr 3 C 2-25 Ni20Cr coatings, which contain different hard phases, show a similar wear behavior the wear resistance of WC-CoCr coatings is significantly better. The difference in wear behavior depending on the carbide size in WC-CoCr coatings is negligible compared to the difference to coatings with pure cobalt matrix. Since the volumetric WC content of these cermet coatings is similar, the matrix composition must take a decisive influence on the wear characteristics.

7 Fig. 4: Wear behavior of different wear protective thermal spray coatings in comparison to electroplated hard chromium coatings 4 Summary and Conclusions The high potential of thermal spray coatings for protection against dry abrasive wear is proved in Taber Abraser tests. HVOF and APS sprayed cermet coatings exhibit higher wear resistance in comparison to APS Cr 2 O 3 and electroplated hard chromium coatings. Among the cermet materials WC-CoCr prove to be best performing. The use of coarse carbide feedstock for HVOF spraying permits manufacturing of coatings with highest wear resistance. The best performing fine carbide coatings show 140% higher linear wear rate. However, coatings produced with coarse carbide feedstock are very sensitive to process parameter variations leading to drastically increased wear rate with decreased coating microhardness. The performance of APS WC-CoCr coatings produced by the triple cathode Northwest Mettech Axial III torch is comparable to that of HVOF coatings produced with liquid fuel spraying guns. Cr 3 C 2 -Ni20Cr and WC-Co show comparable resistance to dry abrasive wear, however, significantly inferior to WC-CoCr coatings. Thus, for the applied wear conditions the matrix material of the cermet coatings influences the wear behavior in a decisive way. Though there is an obviously rising wear rate with decreasing carbide size, the effect of this parameter is only second order in comparison to the matrix influence.

8 5 Acknowledgements The authors gratefully acknowledge funding of this research work by the German Federal Ministry of Economics and Technology BMWA (AiF B) with support by the German Welding Society DVS. Further thanks for provision of coatings apply to Sulzer Metco Coatings GmbH, Weißenborn, Germany (Thermico CJS WC-CoCr), to GTVmbH, Luckenbach, Germany (APS Cr 2 O 3 ) and to Thermico GmbH, Castrop-Rauxel, Germany (Northwest Mettech Axial III WC-CoCr). 6 References [1] O`Quigley, D.; Luyck, S.; James, M.: An empirical ranking of a wide range of WC-Co grades in terms of their abrasion resistance measured by ASTM standard B test, Int. J. Refractory Met. Mater. 15 (1997) [2] Zhu, Y.-C.; Yukimura, K.; Ding, C.-X.; Zhang, P.-Y.: Tribological properties of nanostructured and conventional WC-Co coatings deposited by plasma spraying, Thin Solid Films 388 (2001) [3] Li, C.-J.; Wang, Y.-Y.; Ji, G.-C.; Ohmori, A.: Relation between Abrasive Wear and Microstructure of HVOF Cermet Coatings. In: Moreau C., Marple B. (Edtrs.): Proc. of the ITSC, May 5-8, 2003, Orlando, Florida, USA, [4] Lugscheider, E.; Zwick, J.; Zhang, H.: HVOF Coatings of Ball-Milled Al 2 O 3 /NiCr Dispersion-Strengthened Powders. In: Moreau C., Marple B. (Edtrs.): Proc. of the ITSC, May 5-8, 2003, Orlando, Florida, USA, [5] Steward, D.; Shipway, P.; McCartney, D.: Abrasive wear behaviour of conventional and nanocomposite HVOF-sprayed WC-Co coatings, Wear (1999) [6] ASTM F 1978, Standard Test Method for Measuring Abrasion Resistance of Metallic Thermal Spray Coatings by Using the Taber Abraser [7] Wank, A.; Reisel, G.; Wözel, M.; Grund, T.; Wielage, B.: Tribologische Eigenschaften von thermisch gespritzten Verschleißschutzschichten. In: Wielage B. (Edtr.): Proc. of the OWT, Sept , 2003, Chemnitz, Germany, (in German) [8] Wielage, B.; Wank, A.; Reisel, G.: Entwicklung auf Wärmedurchgang optimierter Schichtsysteme für tribologisch hoch beanspruchte Bauteile. Final report, AiF project No B (in German),