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Tribological study of NiCrBSi coating obtained by different processes
Tribology International 36 (2003) 181–187 www.elsevier.com/locate/triboint
Tribological study of NiCrBSi coating obtained by different processes J.M. Miguel ∗, J.M. Guilemany, S. Vizcaino CPT (Thermal Spray Centre). Materials Engineering. Dept. d’Enginyeria Quı´mica i Metallu´rgia. Universitat de Barcelona. C/Martı´ i Franque`s, 1. E-08028 Barcelona, Spain Received 21 February 2002; received in revised form 12 September 2002; accepted 13 September 2002
Abstract Thermal spraying offers a wide range of coatings with very different composition and properties. NiCrBSi is a Ni-base superalloy widely used to obtain high wear resistant coatings. This coating is usually heat treated after thermal spraying to improve their tribological properties. In this work a tribological comparison between NiCrBSi coatings obtained by spray&fuse and as-sprayed coatings obtained by Atmospheric Plasma Spraying (APS) and High Velocity Oxy Fuel (HVOF) spraying is carried out. Ball on Disk (BOD) tests are performed with a martensitic plain steel as counterface and wear parameters are calculated by means of Scanning White Light Interferometry (SWLI). Main wear mechanisms are investigated by the characterisation of the coating wear track and debris using Scanning Electron Microscopy (SEM). It is observed that different wear mechanisms take place in the coatings obtained by the diverse processes. 2002 Elsevier Science Ltd. All rights reserved. Keywords: NiCrBSi; Thermal spraying; Tribological properties
1. Introduction Thermal spray is a technique that produces a wide range of coatings for diverse applications. The principle of thermal spray is to melt material feedstock (wire or powder), to accelerate the melt to impact on a substrate where rapid solidification and deposit build-up occurs [1]. Thermal spraying uses two principal energy sources, chemical energy of the combusting gases that power the flame spray torches (e.g. HVOF spraying), and electric currents providing energy for the plasmatons (e.g. Atmospheric Plasma spraying) [2]. There are important differences between APS and HVOF spraying with respect to the temperature and velocity of flame. While the flame velocity can be boosted in modern HVOF equipment to values near Mach 5, the temperatures achievable are limited to approximately 2800–3300 K. On the other hand, temperatures as high as 14,000 K and
∗
Tel.: +34-93-402-13-02; fax: +34-93-402-16-38. E-mail address: [email protected]. (J.M. Miguel).
velocity at the nozzle exit of 800 m/s are found in the plasma spraying [3]. Ni based coatings are used in applications when wear resistance combined with oxidation or hot corrosion resistance is required. Nickel-base self-fluxing alloys are currently and mainly used in the chemical industry, petrol industry, glass mould industry and for valves, hot working punches, fan blades, mud purging elements in cement factories. Their advantages are especially related to coating large size components such as piston rods, earth-working machines, etc [4]. The largely employed Ni based powder belongs to the Ni-B-Si system with the addition of other alloying elements. Chromium promotes the oxidation and corrosion resistance at elevated temperatures and increases the hardness of the coating by the formation of hard phases. Boron depresses the melting temperature and contributes to the formation of hard phases. Silicon is added to increase the self-fluxing properties. Carbon produces hard carbides with elevated hardness that promotes the wear resistance of the coating [5,6]. The wear resistance of NiCrBSi coatings can be increased by adding different hard precipitates and several papers on
0301-679X/02/$ - see front matter 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 3 0 1 - 6 7 9 X ( 0 2 ) 0 0 1 4 4 - 5
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J.M. Miguel et al. / Tribology International 36 (2003) 181–187
this issue have been written. It is reported that the addition of WC can improve wear resistance [7,8]. Similar improvements are achieved using TiC and B13C2 [9,10]. The spray&fuse process is a technique that involves thermal spraying to apply a coating of special self-fluxing alloys based on cobalt or nickel. The coatings are submitted to a post thermal treatment at a temperature between the solidus and liquidus of the alloy, when important diffusion processes take place. This temperature is usually between 1200–1400 K. Fusing is typically carried out by manual fusing using an oxyacetylene torch or in a furnace. The coatings obtained by this process show an improvement of strength of the bond between coating and substrate. Nevertheless, the fusing process has some inherent problems. It is not possible to carry out this thermal treatment with all substrates because of the possibility of coating detachment with unsuitable substrates [11]. In addition, at high temperatures phase transformation may be produced in the substrate worsening its properties. The thermal cycle has to be controlled; if not, residual stresses produced by the phase transformations could lead to the cracking of the coatings. Spray& fused NiCrBSi coatings involve two different processes, so the price of the final product is higher than as-sprayed coatings. Thus, sometimes the fuse post thermal treatment enhances some tribological properties, but in some cases may be not a good choice. For this reason this work analyses some tribological properties of spray& fused coatings and compare them to the as-sprayed coatings.
2. Experimental procedure The powder selected to obtain the coatings was a commercial NiCrBSi Amdry 4727, manufactured by atomisation (Fig. 1). The composition of the powder is (% wt) 73.28% Ni, 14.80% Cr, 4.28% Si, 3.7% Fe, 3.21% B, 0.73% C. The mean particle size, measured using a laser diffractometer (Microtrac model X100/SRA 150) was 36 µm. Mild steel was used as substrate. Before spraying substrates were cleaned and grit blasted with Al2O3 grade 24. Preliminary tests were carried out with different spray parameters. The best spray parameters in terms of lower porosity and lower amount of cold particles are listed in Table 1. High Velocity Oxy Fuel (HVOF) coatings were obtained using CDS 100 equipment. Plasma sprayed coatings were obtained using Plasma-Technik A-3000 equipment with an F4 plasma torch in the CPT (Thermal Spray Center). Thermal treatments were carried out in a conventional furnace operating under argon stream to avoid the coating oxidation. Plasma sprayed samples were heated for 1 h at 1373 K followed by a low cooling rate process in the furnace.
Fig. 1. Structure of the NiCrBSi powder. Precipitates have darker tonality.
Table 1 Spraying conditions for the NiCrBSi coatings Used gases (l/min) HVOF sprayed coating Plasma sprayed coating
Spray distance
420 l/min C3H6, 60 l/min O2 300 mm 55 l/min Ar, 15 l/min H2 (I=650 A)
140 mm
The hardness measures were carried out at 100 g using a Matzusawa MXT-OX microhardness tester in the cross section of the coatings. Roughness analysis was done randomly in all the samples. The roughness value Ra (standard deviation with respect to the mean value) was obtained by averaging five sets of data. Friction tests were carried out using a Ball-on-Disk (BOD) machine according to ASTM G99-90 procedure. A martensitic steel ball with a hardness HVN0.3 ⫽ 585 and 11 mm in diameter was selected as counterface. This counterface was selected because steel slides against NiCrBSi coatings in many applications. The environmental conditions were held constant during the test, being the relative humidity and temperature Hr ⫽ 15–20% and 20°C, respectively. The sliding distance was kept constant for all the tests at s ⫽ 1000 m. A track diameter of d ⫽ 16 mm, sliding speed v ⫽ 0.11 m·s⫺1, load F ⫽ 15 N was used. Samples were ground and polished until achieve a final coating roughness of Ra ⫽ 0.2–0.4 µm. The wear tracks produced in the coating were studied by SEM (Jeol JSM-5310), the damage produced in the coating was evaluated using SWLI (Zygo NewView 100) and the results of coating volume loss are reported.
J.M. Miguel et al. / Tribology International 36 (2003) 181–187
Fig. 2.
General view of the plasma sprayed coating.
The porosity of the coating was measured by image analysis.
3. Results and discussion 3.1. Structure of the coatings Fig. 2 shows the plasma sprayed coating. A low quantity of unmelted particles is shown. Coatings have a dense structure with the porosity (black areas) randomly distributed in the coating. Porosity value in the HVOF coatings ( ⬍ 1%) is lower than the obtained in the plasma sprayed coatings (3%) (Table 2). Adherence between substrate and coating seems to be good with a low presence of either cracks or voids in the interface. Porosity is completely eliminated after the fuse process. As-sprayed coatings have low size precipitates that also exist in the powder feedstock, and high magnification is necessary to observe them. Coatings were etched with a dissolution of 1HCl:10HNO3:10H2O that eliminates the matrix preferentially, making the precipi-
183
Fig. 3. Structure of the precipitates in the as-sprayed coatings. A cold particle in the middle of the micrography and surrounding melted particles (some of them without precipitates) can be observed.
tates observation easier (Fig. 3). Certain areas of the coatings do not show these precipitates because they have been dissolved during thermal spray. Precipitate dissolution is promoted by elevated flame temperature and a high time of particle residence in flame. EPMA measurements carried out in these precipitates revealed that they are chromium borides and carbides. Fuse treatments promote precipitate growth (Fig. 4), achieving several micrometers in size. Compositional analysis of the precipitates indicates that they are Cr7C3 and CrB. The presence of small quantities of oxygen in the furnace chamber produces the oxides formation (black, rounded areas). The matrix is mainly composed by Ni and Ni3B. Roughness parameters of the coatings are listed in Table 2. Plasma spraying produces higher roughness than HVOF spraying. Highest velocity of the powder particles combined to a perfect matrix melting during HVOF spraying enhances its better lamellae deposition,
Table 2 Properties of the NiCrBSi coatings
NiCrBSi plasma sprayed NiCrBSi HVOF sprayed Spray&fused NiCrBSi
Porosity
Roughness (Ra, µm)
Microhardness Thickness (µm) (HVN100g)
3%
13.0
611±50
250±12
⬍1%
6.8
819±78
270±7
0
1.4
667±142
241±4
Fig. 4. Structure of the spray&fused coating. Note the diverse phases: Precipitates are CrB (A) and Cr7C3 (B) whereas the matrix is composed by Ni (E) and Ni3B (D). Oxides present a spherical shape (C).
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decreasing, for this reason, coating roughness. Coating melted by the post fuse process strongly decreases the roughness parameters achieving a final soft surface. 3.2. Hardness values Microhardness values are reported in Table 2. Important differences are found among them. It is believed that two different processes are responsible for these differences, the quantity and size of the precipitates as well as the coating cohesion. During indentation tests small cracks growing parallel to the coating surface (interlamellae cracking) may be produced in the plasma sprayed NiCrBSi, decreasing the hardness value. Cracks grow preferentially in this direction due to the anisotropy of the as-sprayed coatings [12]. In addition, a high proportion of the precipitates has been degraded during thermal spray contributing to the hardness decrease. HVOF sprayed NiCrBSi shows the highest microhardness because it combines coating cohesion with a high quantity of small precipitates perfectly distributed in the coating. For this reason the hardness value is more enhanced than in the fused coating that contains worse distributed precipitates of bigger size (mechanism of disperse phase precipitation hardening). The low dissolution of the precipitates during the HVOF spraying could lead to a decrease in the microhardness value, but this negative effect is not so important as the benefits given by the good dispersion of the precipitates. 3.3. Wear resistance and wear mechanisms Table 3 lists the results of the wear track parameters calculated by SWLI. The values indicate that the fuse process promotes the sliding wear resistance. Fig. 5 shows the diverse wear tracks of the tested samples. The sliding wear behaviour of HVOF sprayed and spray& fused coatings show few differences. Nevertheless, the wear resistance of the plasma sprayed coating differs significantly from the rest of the coatings, being much lower. Main wear mechanisms of the samples, the wear track, the coating section and the debris are studied by SEM. HVOF sprayed and spray&fused coatings are studied together because they have similar wear mechanisms. The coating worn surfaces show a great amount of Table 3 Wear parameters for the diverse coatings Volume loss (mm3) NiCrBSi plasma sprayed NiCrBSi HVOF sprayed Spray&fused NiCrBSi
0.480 0.053 0.035
Depth wear track (µm) 18.6 2.0 1.05
Fig. 5. Wear track of the different coatings. ‘A’ corresponds to the plasma sprayed coating, ‘B’ HVOF sprayed coating, ‘C’ spray&fused coating. Dimensions of the images are identical.
adhered material layers located onto the whole wear track (Figs. 6 and 7). EDS analysis confirm that the adhered material is mainly composed of Fe with a low content of O, indicating that these layers are material transferred from the ball to the coating. Otherwise, the wear tracks also show a high quantity of parallel scratches. The cross section of the HVOF sprayed coating shows low subsurface cracking that mostly does not reach the surface. The spray&fused NiCrBSi also have cracks but its quantity is very low and are difficult to find (Fig. 8). Material of several micrometers in thickness transferred to the coating surface is clearly distinguished in Fig. 8, as well. The debris produced in both cases consists of low size particles with dimensions around the micrometer that are mainly composed by Ni,Cr,Si and Fe (Fig. 9). Thus, this debris comes from the worn ball and coating.
J.M. Miguel et al. / Tribology International 36 (2003) 181–187
Fig. 6. Wear track of the spray&fused coating. The arrow indicates material adhered on the coating surface.
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Fig. 8. Section of the spray&fused coating. Note the presence of material adhered on the coating surface indicated by an arrow. Cracks are shown in the coating subsurface.
Fig. 7. Wear track of the HVOF sprayed coating. Darkest areas correspond to the adhered material. Note the high quantity of parallel scratches.
Fig. 9. Debris produced by the wear of the spray&fused coating. Fine particles compose the debris.
The analysis of the HVOF sprayed and spray&fused coatings after BOD tests allows us to state that three could be the most important wear mechanisms, having each of them a different relatively importance: the adhesive wear mechanism, the abrasive wear mechanism and the fatigue wear mechanism. The adhesive wear mechanism causes the material transference. This wear mechanism is promoted by the high mutual solubility of the Fe and Ni (main element of the coatings) as well as the marginal lubricated conditions chosen to carry out the tests. Scratches in the coating surface indicate that abrasion has occurred. This abrasive wear mechanism may be produced by the steel ball (two-body abrasive wear) or by the debris (three-body abrasive wear). The hardness of both coatings is higher than the obtained by the steel ball. It is stated that a transition from ‘soft’ to
‘hard’ abrasive behaviour occurs when the ration of the hardness of the abrasive to that of the abraded material (Ha/Hm) is higher than 1.2 [13,14]. For this reason we may consider the two-body abrasion of these coatings to occur in the soft abrasive mode, producing relatively low damage. Three-body abrasion by the steel debris (that may suffer work hardening promoting its hardness) or by the coating debris can also damage the coating. The fatigue wear mechanism could also have a relevant role in the coating wear because cracks that are observed in the coating subsurface may lead to the coating delamination. The cracks do not reach the surface in most cases. The main wear processes in the delamination wear are subsurface deformation, crack nucleation and propagation. This wear mechanism leads to the formation of big particles debris. Nevertheless, the debris in the
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Fig. 10. Wear track of the plasma sprayed coating. Note the smooth cavities.
HVOF sprayed and spray&fused coatings is completely composed of fine particles. Thus, fatigue is of minor importance in these coatings, wear, though it could be more important at a higher time of testing or in other testing conditions when cracks reach the surface. The plasma sprayed coating is characterised by a rough wear track that contains smooth cavities with dimensions around 50–100 µm. It is possible to note the existence of an important network of cracks located in the entire wear track (Fig. 10). Neither scratches nor material adhered is observed in the worn coating. The cross section shows high cracking propagation in the coating subsurface (Fig. 11). These cracks are widely distributed and some reach the surface. The debris produced during the tribological test is mainly formed by flake-like particles with a size over the 50 µm (Fig. 12). EDS analysis revealed that these big particles are mainly composed by Ni, Cr, Si, therefore they come from the
Fig. 12. Debris produced by the wear of the plasma sprayed coating. The debris is mainly composed by flake-like particles (note the big size of the showed particles).
coating damage. Fine debris mainly composed by Fe from the ball and Ni, Cr, Si also exists but its quantity is much lower. The study carried out on the plasma sprayed coating indicates that fatigue is the main wear mechanism. Some authors call this process ‘splat delamination’ and it is commonly produced in thermal sprayed coatings with poor cohesion energy [15,16]. This mechanism involves intersplat crack propagation until splats are no longer attached and are easily removed. The high porosity of this coating, which can be mainly distributed in the splats boundaries, make it easy for cracks to propagate along boundaries. The main difference between the mechanisms of splat delamination and delamination wear proposed by Suh and co-workers [17] is that the first one takes place among splats due to their low cohesion. For this reason many cracks are shown in the coating surface and subsurface. The smooth cavities of Fig. 10 are caused by the removal of material situated above them following this fatigue mechanism. Debris produced by this mechanism has high dimensions because it comes from the detachment of single splats, though in other cases it could be generated by the elimination of bits of coating containing more than one splat. Abrasive and adhesive wear mechanisms may also have an important role but their relative importance is much lower than the fatigue due to its high velocity of fatigue wear mechanism, for this reason only signs of fatigue processes are shown.
4. Conclusions Fig. 11. Section of the plasma sprayed coating. Important crack propagation is shown in the coating.
The following conclusions can be drawn from this study:
J.M. Miguel et al. / Tribology International 36 (2003) 181–187
1. The fuse process produces precipitate growth as well as an improvement of the coating cohesion. Its microhardness value is lower than that obtained by the HVOF sprayed coating because this coating presents low size precipitate that is well distributed, which promotes the hardness value. 2. As sprayed coatings suffer dissolution during the thermal spraying that may produce a decrease of their tribological properties. 3. Plasma sprayed NiCrBSi shows the worst sliding wear resistance. The analysis of debris and wear track indicates that splat delamination is the main wear mechanism. This wear mechanism produces debris composed by large flake-like particles and big cavities of similar size in the wear track. 4. Spray&fused coating has the best sliding wear resistance (similar to the HVOF sprayed NiCrBSi). The main wear mechanism is adhesion, but abrasion and delamination also take place. The fatigue wear process (intersplat delamination) in the HVOF sprayed coating is not so important as in the plasma sprayed coating due to the best mechanical bonding among splats. Acknowledgements This work was done with the support of the Generalitat de Catalunya (2001SGR00145) and the Ministerio de Ciencia y Tecnologı´a. J.M.Miguel and S. Vizcaino wish to give thanks to the University of Barcelona by the concession of an economic support. References [1] Herman H, Sampath S. In: Stern KH, editor. Thermal spray coatings, metallurgical and ceramic protective coatings. London, UK: Chapman & Hall; 1996. p. 261–89.
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[2] Heimann Robert B. Plasma-spray coatings. Weinheim, Germany: VCH, 1996. [3] Pawlowski Lech. The science and engineering of thermal spray coatings. UK: John Wiley & Sons, 1995. [4] Rosso M, Bennani A, Studies of new applications of Ni-based powders for hardfacing processes. PM World Congress Thermal Spraying/Spray Forming, 1998, p. 524–530. [5] Otsubo F, Era H, Kishitake K. Structure and phases in nickel-base self-fluxing alloy coating containing high cromium and boron. J. Therm. Spray Techn 2000;9(1):107–13. [6] Lebaili S, Hamar-thibault S. Equilibres liquide-solide dans le systeme Ni-B-Si dans la region riche en nickel. Acta Metall 1987;35(3):701–10 in French. [7] Wang H, Xia W, Jin Y. A study on abrasive resistance of Nibased coatings with a WC hard phase. Wear 1996;195:47–52. [8] Franch R, Lorenzana C, Miguel JM, Guilemany JM. Recubrimientos de Cermet/NiCrBSi resistences al desgaste obtenidos por proyeccio´ n te´ rmica de alta velocidad (HVOF). Soldadura y tecnologı´as de unio´ n 2001;67:9–14 in Spanish. [9] Shieh Y, Wang J, Shih H, Wu S. Allowing and post-heat treatment of thermal sprayed coatings of self-fluxing alloys. Surf. Coat. Techn 1993;58:73–7. [10] Betancourt-Dougherty LC, Smith RW. Effect of load and sliding speed on the wear behaviour of plasma sprayed TiC-NiCrBSi coatings. Wear 1998;217:147–54. [11] Grainger S, Blunt J. Engineering coatings, design and applications. Cambridge, UK: Abington Publishing, 1995. [12] Lopez Cantera E, Mellor BG. Fracture toughess and crack morfologies in eroded WC-Co-Cr thermally sprayed coatings. Mater. Lett 1998;37(4-5):201–10. [13] Jia K, Fisher TE. Abrasion resistance of nanostructured and conventional cemented carbides. Wear 1996;200:206–14. [14] Hutchings IM. Tribology friction and wear of engineering materials. UK: Edward Arnold, 1992. [15] Hauschmann L, Siegi J, Karlik J, Kunes I, Chraska P. Plasma sprayed coatings under cyclic contact pressure on a small area. In: Proceedings of the International Thermal Spray Conference & Exposition, Orlando, USA. 1992. p. 723–7. [16] Hartfield-Wu¨ nsch SE, Tung SC. The effect of microstructure on the wear behaviour of thermal spray coatings. In: Proceedings of the 7th National Spray Conference, Boston, MA. 1994. p. 19–24. [17] Suh NP. The delamination theory of wear. Wear 1977;44:1–16.
Tribological study of NiCrBSi coating obtained by different processes J.M. Miguel ∗, J.M. Guilemany, S. Vizcaino CPT (Thermal Spray Centre). Materials Engineering. Dept. d’Enginyeria Quı´mica i Metallu´rgia. Universitat de Barcelona. C/Martı´ i Franque`s, 1. E-08028 Barcelona, Spain Received 21 February 2002; received in revised form 12 September 2002; accepted 13 September 2002
Abstract Thermal spraying offers a wide range of coatings with very different composition and properties. NiCrBSi is a Ni-base superalloy widely used to obtain high wear resistant coatings. This coating is usually heat treated after thermal spraying to improve their tribological properties. In this work a tribological comparison between NiCrBSi coatings obtained by spray&fuse and as-sprayed coatings obtained by Atmospheric Plasma Spraying (APS) and High Velocity Oxy Fuel (HVOF) spraying is carried out. Ball on Disk (BOD) tests are performed with a martensitic plain steel as counterface and wear parameters are calculated by means of Scanning White Light Interferometry (SWLI). Main wear mechanisms are investigated by the characterisation of the coating wear track and debris using Scanning Electron Microscopy (SEM). It is observed that different wear mechanisms take place in the coatings obtained by the diverse processes. 2002 Elsevier Science Ltd. All rights reserved. Keywords: NiCrBSi; Thermal spraying; Tribological properties
1. Introduction Thermal spray is a technique that produces a wide range of coatings for diverse applications. The principle of thermal spray is to melt material feedstock (wire or powder), to accelerate the melt to impact on a substrate where rapid solidification and deposit build-up occurs [1]. Thermal spraying uses two principal energy sources, chemical energy of the combusting gases that power the flame spray torches (e.g. HVOF spraying), and electric currents providing energy for the plasmatons (e.g. Atmospheric Plasma spraying) [2]. There are important differences between APS and HVOF spraying with respect to the temperature and velocity of flame. While the flame velocity can be boosted in modern HVOF equipment to values near Mach 5, the temperatures achievable are limited to approximately 2800–3300 K. On the other hand, temperatures as high as 14,000 K and
∗
Tel.: +34-93-402-13-02; fax: +34-93-402-16-38. E-mail address: [email protected]. (J.M. Miguel).
velocity at the nozzle exit of 800 m/s are found in the plasma spraying [3]. Ni based coatings are used in applications when wear resistance combined with oxidation or hot corrosion resistance is required. Nickel-base self-fluxing alloys are currently and mainly used in the chemical industry, petrol industry, glass mould industry and for valves, hot working punches, fan blades, mud purging elements in cement factories. Their advantages are especially related to coating large size components such as piston rods, earth-working machines, etc [4]. The largely employed Ni based powder belongs to the Ni-B-Si system with the addition of other alloying elements. Chromium promotes the oxidation and corrosion resistance at elevated temperatures and increases the hardness of the coating by the formation of hard phases. Boron depresses the melting temperature and contributes to the formation of hard phases. Silicon is added to increase the self-fluxing properties. Carbon produces hard carbides with elevated hardness that promotes the wear resistance of the coating [5,6]. The wear resistance of NiCrBSi coatings can be increased by adding different hard precipitates and several papers on
0301-679X/02/$ - see front matter 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 3 0 1 - 6 7 9 X ( 0 2 ) 0 0 1 4 4 - 5
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J.M. Miguel et al. / Tribology International 36 (2003) 181–187
this issue have been written. It is reported that the addition of WC can improve wear resistance [7,8]. Similar improvements are achieved using TiC and B13C2 [9,10]. The spray&fuse process is a technique that involves thermal spraying to apply a coating of special self-fluxing alloys based on cobalt or nickel. The coatings are submitted to a post thermal treatment at a temperature between the solidus and liquidus of the alloy, when important diffusion processes take place. This temperature is usually between 1200–1400 K. Fusing is typically carried out by manual fusing using an oxyacetylene torch or in a furnace. The coatings obtained by this process show an improvement of strength of the bond between coating and substrate. Nevertheless, the fusing process has some inherent problems. It is not possible to carry out this thermal treatment with all substrates because of the possibility of coating detachment with unsuitable substrates [11]. In addition, at high temperatures phase transformation may be produced in the substrate worsening its properties. The thermal cycle has to be controlled; if not, residual stresses produced by the phase transformations could lead to the cracking of the coatings. Spray& fused NiCrBSi coatings involve two different processes, so the price of the final product is higher than as-sprayed coatings. Thus, sometimes the fuse post thermal treatment enhances some tribological properties, but in some cases may be not a good choice. For this reason this work analyses some tribological properties of spray& fused coatings and compare them to the as-sprayed coatings.
2. Experimental procedure The powder selected to obtain the coatings was a commercial NiCrBSi Amdry 4727, manufactured by atomisation (Fig. 1). The composition of the powder is (% wt) 73.28% Ni, 14.80% Cr, 4.28% Si, 3.7% Fe, 3.21% B, 0.73% C. The mean particle size, measured using a laser diffractometer (Microtrac model X100/SRA 150) was 36 µm. Mild steel was used as substrate. Before spraying substrates were cleaned and grit blasted with Al2O3 grade 24. Preliminary tests were carried out with different spray parameters. The best spray parameters in terms of lower porosity and lower amount of cold particles are listed in Table 1. High Velocity Oxy Fuel (HVOF) coatings were obtained using CDS 100 equipment. Plasma sprayed coatings were obtained using Plasma-Technik A-3000 equipment with an F4 plasma torch in the CPT (Thermal Spray Center). Thermal treatments were carried out in a conventional furnace operating under argon stream to avoid the coating oxidation. Plasma sprayed samples were heated for 1 h at 1373 K followed by a low cooling rate process in the furnace.
Fig. 1. Structure of the NiCrBSi powder. Precipitates have darker tonality.
Table 1 Spraying conditions for the NiCrBSi coatings Used gases (l/min) HVOF sprayed coating Plasma sprayed coating
Spray distance
420 l/min C3H6, 60 l/min O2 300 mm 55 l/min Ar, 15 l/min H2 (I=650 A)
140 mm
The hardness measures were carried out at 100 g using a Matzusawa MXT-OX microhardness tester in the cross section of the coatings. Roughness analysis was done randomly in all the samples. The roughness value Ra (standard deviation with respect to the mean value) was obtained by averaging five sets of data. Friction tests were carried out using a Ball-on-Disk (BOD) machine according to ASTM G99-90 procedure. A martensitic steel ball with a hardness HVN0.3 ⫽ 585 and 11 mm in diameter was selected as counterface. This counterface was selected because steel slides against NiCrBSi coatings in many applications. The environmental conditions were held constant during the test, being the relative humidity and temperature Hr ⫽ 15–20% and 20°C, respectively. The sliding distance was kept constant for all the tests at s ⫽ 1000 m. A track diameter of d ⫽ 16 mm, sliding speed v ⫽ 0.11 m·s⫺1, load F ⫽ 15 N was used. Samples were ground and polished until achieve a final coating roughness of Ra ⫽ 0.2–0.4 µm. The wear tracks produced in the coating were studied by SEM (Jeol JSM-5310), the damage produced in the coating was evaluated using SWLI (Zygo NewView 100) and the results of coating volume loss are reported.
J.M. Miguel et al. / Tribology International 36 (2003) 181–187
Fig. 2.
General view of the plasma sprayed coating.
The porosity of the coating was measured by image analysis.
3. Results and discussion 3.1. Structure of the coatings Fig. 2 shows the plasma sprayed coating. A low quantity of unmelted particles is shown. Coatings have a dense structure with the porosity (black areas) randomly distributed in the coating. Porosity value in the HVOF coatings ( ⬍ 1%) is lower than the obtained in the plasma sprayed coatings (3%) (Table 2). Adherence between substrate and coating seems to be good with a low presence of either cracks or voids in the interface. Porosity is completely eliminated after the fuse process. As-sprayed coatings have low size precipitates that also exist in the powder feedstock, and high magnification is necessary to observe them. Coatings were etched with a dissolution of 1HCl:10HNO3:10H2O that eliminates the matrix preferentially, making the precipi-
183
Fig. 3. Structure of the precipitates in the as-sprayed coatings. A cold particle in the middle of the micrography and surrounding melted particles (some of them without precipitates) can be observed.
tates observation easier (Fig. 3). Certain areas of the coatings do not show these precipitates because they have been dissolved during thermal spray. Precipitate dissolution is promoted by elevated flame temperature and a high time of particle residence in flame. EPMA measurements carried out in these precipitates revealed that they are chromium borides and carbides. Fuse treatments promote precipitate growth (Fig. 4), achieving several micrometers in size. Compositional analysis of the precipitates indicates that they are Cr7C3 and CrB. The presence of small quantities of oxygen in the furnace chamber produces the oxides formation (black, rounded areas). The matrix is mainly composed by Ni and Ni3B. Roughness parameters of the coatings are listed in Table 2. Plasma spraying produces higher roughness than HVOF spraying. Highest velocity of the powder particles combined to a perfect matrix melting during HVOF spraying enhances its better lamellae deposition,
Table 2 Properties of the NiCrBSi coatings
NiCrBSi plasma sprayed NiCrBSi HVOF sprayed Spray&fused NiCrBSi
Porosity
Roughness (Ra, µm)
Microhardness Thickness (µm) (HVN100g)
3%
13.0
611±50
250±12
⬍1%
6.8
819±78
270±7
0
1.4
667±142
241±4
Fig. 4. Structure of the spray&fused coating. Note the diverse phases: Precipitates are CrB (A) and Cr7C3 (B) whereas the matrix is composed by Ni (E) and Ni3B (D). Oxides present a spherical shape (C).
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decreasing, for this reason, coating roughness. Coating melted by the post fuse process strongly decreases the roughness parameters achieving a final soft surface. 3.2. Hardness values Microhardness values are reported in Table 2. Important differences are found among them. It is believed that two different processes are responsible for these differences, the quantity and size of the precipitates as well as the coating cohesion. During indentation tests small cracks growing parallel to the coating surface (interlamellae cracking) may be produced in the plasma sprayed NiCrBSi, decreasing the hardness value. Cracks grow preferentially in this direction due to the anisotropy of the as-sprayed coatings [12]. In addition, a high proportion of the precipitates has been degraded during thermal spray contributing to the hardness decrease. HVOF sprayed NiCrBSi shows the highest microhardness because it combines coating cohesion with a high quantity of small precipitates perfectly distributed in the coating. For this reason the hardness value is more enhanced than in the fused coating that contains worse distributed precipitates of bigger size (mechanism of disperse phase precipitation hardening). The low dissolution of the precipitates during the HVOF spraying could lead to a decrease in the microhardness value, but this negative effect is not so important as the benefits given by the good dispersion of the precipitates. 3.3. Wear resistance and wear mechanisms Table 3 lists the results of the wear track parameters calculated by SWLI. The values indicate that the fuse process promotes the sliding wear resistance. Fig. 5 shows the diverse wear tracks of the tested samples. The sliding wear behaviour of HVOF sprayed and spray& fused coatings show few differences. Nevertheless, the wear resistance of the plasma sprayed coating differs significantly from the rest of the coatings, being much lower. Main wear mechanisms of the samples, the wear track, the coating section and the debris are studied by SEM. HVOF sprayed and spray&fused coatings are studied together because they have similar wear mechanisms. The coating worn surfaces show a great amount of Table 3 Wear parameters for the diverse coatings Volume loss (mm3) NiCrBSi plasma sprayed NiCrBSi HVOF sprayed Spray&fused NiCrBSi
0.480 0.053 0.035
Depth wear track (µm) 18.6 2.0 1.05
Fig. 5. Wear track of the different coatings. ‘A’ corresponds to the plasma sprayed coating, ‘B’ HVOF sprayed coating, ‘C’ spray&fused coating. Dimensions of the images are identical.
adhered material layers located onto the whole wear track (Figs. 6 and 7). EDS analysis confirm that the adhered material is mainly composed of Fe with a low content of O, indicating that these layers are material transferred from the ball to the coating. Otherwise, the wear tracks also show a high quantity of parallel scratches. The cross section of the HVOF sprayed coating shows low subsurface cracking that mostly does not reach the surface. The spray&fused NiCrBSi also have cracks but its quantity is very low and are difficult to find (Fig. 8). Material of several micrometers in thickness transferred to the coating surface is clearly distinguished in Fig. 8, as well. The debris produced in both cases consists of low size particles with dimensions around the micrometer that are mainly composed by Ni,Cr,Si and Fe (Fig. 9). Thus, this debris comes from the worn ball and coating.
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Fig. 6. Wear track of the spray&fused coating. The arrow indicates material adhered on the coating surface.
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Fig. 8. Section of the spray&fused coating. Note the presence of material adhered on the coating surface indicated by an arrow. Cracks are shown in the coating subsurface.
Fig. 7. Wear track of the HVOF sprayed coating. Darkest areas correspond to the adhered material. Note the high quantity of parallel scratches.
Fig. 9. Debris produced by the wear of the spray&fused coating. Fine particles compose the debris.
The analysis of the HVOF sprayed and spray&fused coatings after BOD tests allows us to state that three could be the most important wear mechanisms, having each of them a different relatively importance: the adhesive wear mechanism, the abrasive wear mechanism and the fatigue wear mechanism. The adhesive wear mechanism causes the material transference. This wear mechanism is promoted by the high mutual solubility of the Fe and Ni (main element of the coatings) as well as the marginal lubricated conditions chosen to carry out the tests. Scratches in the coating surface indicate that abrasion has occurred. This abrasive wear mechanism may be produced by the steel ball (two-body abrasive wear) or by the debris (three-body abrasive wear). The hardness of both coatings is higher than the obtained by the steel ball. It is stated that a transition from ‘soft’ to
‘hard’ abrasive behaviour occurs when the ration of the hardness of the abrasive to that of the abraded material (Ha/Hm) is higher than 1.2 [13,14]. For this reason we may consider the two-body abrasion of these coatings to occur in the soft abrasive mode, producing relatively low damage. Three-body abrasion by the steel debris (that may suffer work hardening promoting its hardness) or by the coating debris can also damage the coating. The fatigue wear mechanism could also have a relevant role in the coating wear because cracks that are observed in the coating subsurface may lead to the coating delamination. The cracks do not reach the surface in most cases. The main wear processes in the delamination wear are subsurface deformation, crack nucleation and propagation. This wear mechanism leads to the formation of big particles debris. Nevertheless, the debris in the
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Fig. 10. Wear track of the plasma sprayed coating. Note the smooth cavities.
HVOF sprayed and spray&fused coatings is completely composed of fine particles. Thus, fatigue is of minor importance in these coatings, wear, though it could be more important at a higher time of testing or in other testing conditions when cracks reach the surface. The plasma sprayed coating is characterised by a rough wear track that contains smooth cavities with dimensions around 50–100 µm. It is possible to note the existence of an important network of cracks located in the entire wear track (Fig. 10). Neither scratches nor material adhered is observed in the worn coating. The cross section shows high cracking propagation in the coating subsurface (Fig. 11). These cracks are widely distributed and some reach the surface. The debris produced during the tribological test is mainly formed by flake-like particles with a size over the 50 µm (Fig. 12). EDS analysis revealed that these big particles are mainly composed by Ni, Cr, Si, therefore they come from the
Fig. 12. Debris produced by the wear of the plasma sprayed coating. The debris is mainly composed by flake-like particles (note the big size of the showed particles).
coating damage. Fine debris mainly composed by Fe from the ball and Ni, Cr, Si also exists but its quantity is much lower. The study carried out on the plasma sprayed coating indicates that fatigue is the main wear mechanism. Some authors call this process ‘splat delamination’ and it is commonly produced in thermal sprayed coatings with poor cohesion energy [15,16]. This mechanism involves intersplat crack propagation until splats are no longer attached and are easily removed. The high porosity of this coating, which can be mainly distributed in the splats boundaries, make it easy for cracks to propagate along boundaries. The main difference between the mechanisms of splat delamination and delamination wear proposed by Suh and co-workers [17] is that the first one takes place among splats due to their low cohesion. For this reason many cracks are shown in the coating surface and subsurface. The smooth cavities of Fig. 10 are caused by the removal of material situated above them following this fatigue mechanism. Debris produced by this mechanism has high dimensions because it comes from the detachment of single splats, though in other cases it could be generated by the elimination of bits of coating containing more than one splat. Abrasive and adhesive wear mechanisms may also have an important role but their relative importance is much lower than the fatigue due to its high velocity of fatigue wear mechanism, for this reason only signs of fatigue processes are shown.
4. Conclusions Fig. 11. Section of the plasma sprayed coating. Important crack propagation is shown in the coating.
The following conclusions can be drawn from this study:
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1. The fuse process produces precipitate growth as well as an improvement of the coating cohesion. Its microhardness value is lower than that obtained by the HVOF sprayed coating because this coating presents low size precipitate that is well distributed, which promotes the hardness value. 2. As sprayed coatings suffer dissolution during the thermal spraying that may produce a decrease of their tribological properties. 3. Plasma sprayed NiCrBSi shows the worst sliding wear resistance. The analysis of debris and wear track indicates that splat delamination is the main wear mechanism. This wear mechanism produces debris composed by large flake-like particles and big cavities of similar size in the wear track. 4. Spray&fused coating has the best sliding wear resistance (similar to the HVOF sprayed NiCrBSi). The main wear mechanism is adhesion, but abrasion and delamination also take place. The fatigue wear process (intersplat delamination) in the HVOF sprayed coating is not so important as in the plasma sprayed coating due to the best mechanical bonding among splats. Acknowledgements This work was done with the support of the Generalitat de Catalunya (2001SGR00145) and the Ministerio de Ciencia y Tecnologı´a. J.M.Miguel and S. Vizcaino wish to give thanks to the University of Barcelona by the concession of an economic support. References [1] Herman H, Sampath S. In: Stern KH, editor. Thermal spray coatings, metallurgical and ceramic protective coatings. London, UK: Chapman & Hall; 1996. p. 261–89.
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[2] Heimann Robert B. Plasma-spray coatings. Weinheim, Germany: VCH, 1996. [3] Pawlowski Lech. The science and engineering of thermal spray coatings. UK: John Wiley & Sons, 1995. [4] Rosso M, Bennani A, Studies of new applications of Ni-based powders for hardfacing processes. PM World Congress Thermal Spraying/Spray Forming, 1998, p. 524–530. [5] Otsubo F, Era H, Kishitake K. Structure and phases in nickel-base self-fluxing alloy coating containing high cromium and boron. J. Therm. Spray Techn 2000;9(1):107–13. [6] Lebaili S, Hamar-thibault S. Equilibres liquide-solide dans le systeme Ni-B-Si dans la region riche en nickel. Acta Metall 1987;35(3):701–10 in French. [7] Wang H, Xia W, Jin Y. A study on abrasive resistance of Nibased coatings with a WC hard phase. Wear 1996;195:47–52. [8] Franch R, Lorenzana C, Miguel JM, Guilemany JM. Recubrimientos de Cermet/NiCrBSi resistences al desgaste obtenidos por proyeccio´ n te´ rmica de alta velocidad (HVOF). Soldadura y tecnologı´as de unio´ n 2001;67:9–14 in Spanish. [9] Shieh Y, Wang J, Shih H, Wu S. Allowing and post-heat treatment of thermal sprayed coatings of self-fluxing alloys. Surf. Coat. Techn 1993;58:73–7. [10] Betancourt-Dougherty LC, Smith RW. Effect of load and sliding speed on the wear behaviour of plasma sprayed TiC-NiCrBSi coatings. Wear 1998;217:147–54. [11] Grainger S, Blunt J. Engineering coatings, design and applications. Cambridge, UK: Abington Publishing, 1995. [12] Lopez Cantera E, Mellor BG. Fracture toughess and crack morfologies in eroded WC-Co-Cr thermally sprayed coatings. Mater. Lett 1998;37(4-5):201–10. [13] Jia K, Fisher TE. Abrasion resistance of nanostructured and conventional cemented carbides. Wear 1996;200:206–14. [14] Hutchings IM. Tribology friction and wear of engineering materials. UK: Edward Arnold, 1992. [15] Hauschmann L, Siegi J, Karlik J, Kunes I, Chraska P. Plasma sprayed coatings under cyclic contact pressure on a small area. In: Proceedings of the International Thermal Spray Conference & Exposition, Orlando, USA. 1992. p. 723–7. [16] Hartfield-Wu¨ nsch SE, Tung SC. The effect of microstructure on the wear behaviour of thermal spray coatings. In: Proceedings of the 7th National Spray Conference, Boston, MA. 1994. p. 19–24. [17] Suh NP. The delamination theory of wear. Wear 1977;44:1–16.